Search

Cleavable metallic antiferromagnets may be of use for low-dissipation spintronic devices; however, few are currently known. Here we present orthorhombic MnRhBi3 as one such compound and present a thorough study of its physical properties. Exfoliation is demonstrated experimentally, and the cleavage energy and electronic structure are examined by density functional theory calculations. It is concluded that MnRhBi3 is a van der Waals layered material that cleaves easily between neighboring Bi layers, and that the Bi atoms have lone pairs extending into the van der Waals gaps. A series of four phase transitions are observed below room temperature, and neutron diffraction shows that at least two of the transitions involve the formation of antiferromagnetic order. Anomalous thermal expansion points to a crystallographic phase transition and/or strong magnetoelastic coupling. This work reveals a complex phase evolution in MnRhBi3 and establishes this cleavable antiferromagnetic metal as an interesting material for studying the interplay of structure, magnetism, and transport in the bulk and ultrathin limits as well as the role of lone pair electrons in interface chemistry and proximity effects in van der Waals heterostructures.

Magnetic skyrmion crystals are traditionally associated with non-centrosymmetric crystal structures; however, it has been demonstrated that skyrmion crystals can be stabilized by competing interactions in centrosymmetric crystals. To understand and optimize the physical responses associated with topologically-nontrivial skyrmion textures, it is important to quantify their magnetic interactions by comparing theoretical predictions with spectroscopic data. Here, we present neutron diffraction and spectroscopy data on the centrosymmetric skyrmion material GdRu$_2$Si$_2$, and show that the key spectroscopic features can be explained by the magnetic interactions calculated using density-functional theory calculations. We further show that the recently-proposed 2-$\mathbf{q}$ "topological spin stripe" structure yields better agreement with our data than a 1-$\mathbf{q}$ helical structure, and identify how the magnetic structure evolves with temperature., Comment: 6 pages, 3 figures

A codimension-two spiral spin-liquid is a correlated paramagnetic state with one-dimensional ground state degeneracy hosted within a three-dimensional lattice. Here, via neutron scattering experiments and numerical simulations, we establish the existence of a codimension-two spiral spin-liquid in the effective honeycomb-lattice compound Cs$_3$Fe$_2$Cl$_9$ and demonstrate the selective visibility of the spiral surface through phase tuning. In the long-range ordered regime, competing spiral and spin density wave orders emerge as a function of applied magnetic field, among which a possible order-by-disorder transition is identified., Comment: 21 pages, 23 figures

Here we present the discovery and characterization of the kagome metal TbTi$_3$Bi$_4$ in tandem with a new series of compounds, the Ln$_{2-x}$Ti$_{6+x}$Bi$_9$ (Ln: Tb-Lu) shurikagome metals. We previously reported on the growth of the LnTi$_3$Bi$_4$ (Ln: La-Gd$^{3+}$, Eu$^{2+}$, Yb$^{2+}$) family, a chemically diverse and exfoliable series of kagome metals with complex and highly anisotropic magnetism. However, unlike the La-Gd analogs, TbTi$_3$Bi$_4$ cannot be synthesized by our previous methodology due to phase competition with Ln$_{2-x}$Ti$_{6+x}$Bi$_9$ (x$\sim$1.7-1.2). Here we discuss the phase competition between the LnTi$_3$Bi$_4$ and Ln$_{2-x}$Ti$_{6+x}$Bi$_9$ families, helping to frame the difficulty in synthesizing LnTi$_3$Bi$_4$ compounds with small Ln species and providing a strategy to circumvent formation of Ln$_{2-x}$Ti$_{6+x}$Bi$_9$. Detailed characterization of the magnetic and electronic transport properties on single crystals of TbTi$_3$Bi$_4$ reveals a highly complex landscape of magnetic phases arising from an antiferromagnetic ground state. A series of metamagnetic transitions creates at least 5 unique magnetic phase pockets, including a 1/3 and 2/3 magnetization plateau. Further, the system exhibits an intimate connection between the magnetism and magnetotransport, exhibiting sharp switching from positive (+40%) to negative magnetoresistance (-50%). Like the LnTi$_3$Bi$_4$ kagome metals, the Ln$_{2-x}$Ti$_{6+x}$Bi$_9$ family exhibits quasi-2D networks of titanium and chains of rare-earth. We present the structures and some basic magnetic properties of the Ln$_{2-x}$Ti$_{6+x}$Bi$_9$ family alongside our characterization of the newly discovered TbTi$_3$Bi$_4$.

Using the framework of semi-classical Landau-Lifshitz dynamics (LLD), we conduct a systematic investigation of the temperature-dependent spin dynamics in the S = 1/2 Heisenberg square-lattice antiferromagnet (SqAF). By performing inelastic neutron scattering measurements on Zn2VO(PO4)2 (ZVPO) and corresponding finite-temperature spin dynamics simulations based on LLD, we present a comprehensive analysis that bridges quantum and classical spin dynamics over a broad temperature range. First, a remarkable agreement between experimental data and LLD simulations is found in the paramagnetic phase of ZVPO, demonstrating the capability of LLD in accurately determining the spin Hamiltonian of S = 1/2 systems and capturing the quantum-to-classical crossover of their spin dynamics. Second, by analyzing the discrepancies between the experimental data and the LLD simulations at lower temperatures, we determine the experimental temperature dependence of the quantum effects in the excitation spectrum of the S = 1/2 SqAF: the quantum renormalization factor for the magnon energies and the quantum continuum above the one-magnon bands. Notably, the emergence of each quantum effect is found to correlate with the formation of three-dimensional long-range order. This work demonstrates the utility of LLD in gaining experimental insights into the temperature-induced modifications of quantum spin dynamics and their convergence towards classical expectations at higher temperatures. This motivates further applications to more challenging quantum antiferromagnets dominated by stronger quantum fluctuations., Comment: 8 pages, 4 figures, Supplemental Material can be found on the journal's website

The S = 1/2 triangular lattice antiferromagnet (TLAF) is a paradigmatic example of frustrated quantum magnetism. An ongoing challenge involves understanding the influence of exchange anisotropy on the collective behavior within such systems. Using inelastic neutron scattering (INS) and advanced calculation techniques, we have studied the low and high-temperature spin dynamics of Ba2La2CoTe2O12 (BLCTO): a Co2+-based Jeff = 1/2 TLAF that exhibits 120{\deg} order below TN = 3.26 K. We determined the spin Hamiltonian by fitting the energy-resolved paramagnetic excitations measured at T > TN, revealing exceptionally strong easy-plane XXZ anisotropy. Below TN, the excitation spectrum exhibits a high energy continuum having a larger spectral weight than the single-magnon modes, suggesting a scenario characterized by a spinon confinement length that markedly exceeds the lattice spacing. We conjecture that this phenomenon arises from the proximity to a quantum melting point, even under strong easy-plane XXZ anisotropy. Finally, we highlight characteristic flat features in the excitation spectrum, which are connected to higher-order van Hove singularities in the magnon dispersion directly induced by easy-plane XXZ anisotropy. Our results provide a rare experimental insight into the nature of highly anisotropic S = 1/2 TLAFs between the Heisenberg and XY limits., Comment: 5 figures, Supplementary Information can be found on the journal's website

Square-lattice systems offer a direct route for realizing 2D quantum magnetism with frustration induced by competing interactions. In this work, the square-lattice materials YbBi2IO4 and YbBi2ClO4 were investigated using a combination of magnetization and specific heat measurements on polycrystalline samples. Specific heat measurements provide evidence for long-range magnetic order below TN = 0.21 K (0.25 K) for YbBi2IO4 (YbBi2ClO4). On the other hand, a rather broad maximum is found in the temperature-dependent magnetic susceptibility, located at Tmax = 0.33 K (0.38 K) in YbBi2IO4 (YbBi2ClO4), consistent with the quasi-2D magnetism expected for the large separation between the magnetic layers. Estimation of the magnetic entropy supports the expected Kramers' doublet ground state for Yb3+ and the observed paramagnetic behavior is consistent with a well-isolated doublet. Roughly two-thirds of the entropy is consumed above TN, due to a combination of the quasi-2D behavior and magnetic frustration. The impact of frustration is examined from the viewpoint of a simplified J1-J2 square lattice model, which is frustrated for antiferromagnetic interactions. Specifically, a high-temperature series expansion analysis of the temperature-dependent specific heat and magnetization data yields J2/J1 = 0.30 (0.23) for YbBi2IO4 (YbBi2ClO4). This simplified analysis suggests strong frustration that should promote significant quantum fluctuations in these compounds, and thus motivates future work on the static and dynamic magnetic properties of these materials., Comment: 8 pages, 5 figures

Pyrochlore materials are characterized by their hallmark network of corner-sharing rare-earth tetrahedra, which can produce a wide array of complex magnetic ground states. Ferromagnetic Ising pyrochlores often obey the "two-in-two-out" spin ice rules, which can lead to a highly-degenerate spin structure. Large moment systems, such as Ho$_2$Ti$_2$O$_7$ and Dy$_2$Ti$_2$O$_7$, tend to host a classical spin ice state with low-temperature spin freezing and emergent magnetic monopoles. Systems with smaller effective moments, such as Pr$^{3+}$-based pyrochlores, have been proposed as excellent candidates for hosting a "quantum spin ice" characterized by entanglement and a slew of exotic quasiparticle excitations. However, experimental evidence for a quantum spin ice state has remained elusive. Here, we show that the low-temperature magnetic properties of Pr$_2$Sn$_2$O$_7$ satisfy several important criteria for continued consideration as a quantum spin ice. We find that Pr$_2$Sn$_2$O$_7$ exhibits a partially spin-frozen ground state with a large volume fraction of dynamic magnetism. Our comprehensive bulk characterization and neutron scattering measurements enable us to map out the magnetic field-temperature phase diagram, producing results consistent with expectations for a ferromagnetic Ising pyrochlore. We identify key hallmarks of spin ice physics, and show that the application of small magnetic fields ($\mu_0 H_c \sim$0.75T) suppresses the spin ice state and induces a long-range ordered magnetic structure. Together, our work clarifies the current state of Pr$_2$Sn$_2$O$_7$ and encourages future studies aimed at exploring the potential for a quantum spin ice ground state in this system.

Here we present the family of titanium-based kagome metals of the form LnTi$_3$Bi$_4$ (Ln: La...Gd$^{3+}$, Eu$^{2+}$, Yb$^{2+}$). Single crystal growth methods are presented alongside detailed magnetic and thermodynamic measurements. The orthorhombic (Fmmm) LnTi$_3$Bi$_4$ family of compounds exhibit slightly distorted titanium-based kagome nets interwoven with zig-zag lanthanide-based (Ln) chains. Crystals are easily exfoliated parallel to the kagome sheets and angular resolved photoemission (ARPES) measurements highlight the intricacy of the electronic structure in these compounds, with Dirac points existing at the Fermi level. The magnetic properties and the associated anisotropy emerge from the quasi-1D zig-zag chains of Ln, and impart a wide array of magnetic ground states ranging from anisotropic ferromagnetism to complex antiferromagnetism with a cascade of metamagnetic transitions. Kagome metals continue to provide a rich direction for the exploration of magnetic, topologic, and highly correlated behavior. Our work here introduces the LnTi$_3$Bi$_4$ compounds to augment the continuously expanding suite of complex and interesting kagome materials.

Topological phases of matter are beyond the paradigm of Landau's symmetry breaking and have challenged our understanding of condensed matter systems. Here we report a new type of symmetry-protected topological phase of matter in the spin-1/2 coupled two-leg ladder antiferromagnet C9H18N2CuBr4, DLCB for short. In this two-sublattice antiferromagnet with a weak easy-axis anisotropy, we find no evidence of a conventional spin-flop transition in the magnetization with the magnetic field applied parallel to the easy axis at T=0.4 K, well below TN=2.0 K. Moreover, the temperature dependence of the gapped transverse excitations across TN indicates that they are not the conventional S=1 magnons associated with explicit symmetry breaking. Instead, the thermal renormalization of the gap energy shows a remarkable agreement with a calculation for the three-dimensional O(3) nonlinear sigma model. Accordingly, the spin gap in DLCB is not due to the spin anisotropy but to the separation between a spin singlet state and a triplet excited state. Since an antiferromagnetic spin-1/2 ladder systems can be mapped onto the spin-1 chain, the notion of the Haldane gap is proposed to explain the opening of the spin gap in DLCB. Therefore, the ground state of DLCB is best described as a quantum superposition of a Haldane phase and a Neel-ordered phase, which resembles the quantum state of a qubit in quantum computing. Our results indicate the presence of a symmetry-protected topological order coexisting with an antiferromagnetic order in this material., Comment: 4 figures in the main text and 9 figures in the Supplementary Material. Any comments are welcome

Magnetic van der Waals heterostructures provide a unique platform to study magnetism and spintronics device concepts in the two-dimensional limit. Here, we report studies of exchange bias from the van der Waals antiferromagnet CrSBr acting on the van der Waals ferromagnet Fe3GeTe2 (FGT). The orientation of the exchange bias is along the in-plane easy axis of CrSBr, perpendicular to the out-of-plane anisotropy of the FGT, inducing a strongly tilted magnetic configuration in the FGT. Furthermore, the in-plane exchange bias provides sufficient symmetry breaking to allow deterministic spin-orbit torque switching of the FGT in CrSBr/FGT/Pt samples at zero applied magnetic field. A minimum thickness of the CrSBr greater than 10 nm is needed to provide a non-zero exchange bias at 30 K.

The kagome lattice can host exotic magnetic phases arising from frustrated and competing magnetic interactions. However, relatively few insulating kagome materials exhibit incommensurate magnetic ordering. Here, we present a study of the magnetic structures and interactions of antiferromagnetic Na$_2$Mn$_3$Cl$_8$ with an undistorted Mn$^{2+}$ kagome network. Using neutron-diffraction and bulk magnetic measurements, we show that Na$_2$Mn$_3$Cl$_8$ hosts two different incommensurate magnetic states, which develop at $T_{N1} = 1.6$ K and $T_{N2} = 0.6$ K. Magnetic Rietveld refinements indicate magnetic propagation vectors of the form $\mathbf{q} = (q_{x},q_{y},\frac{3}{2})$, and our neutron-diffraction data can be well described by cycloidal magnetic structures. By optimizing exchange parameters against magnetic diffuse-scattering data, we show that the spin Hamiltonian contains ferromagnetic nearest-neighbor and antiferromagnetic third-neighbor Heisenberg interactions, with a significant contribution from long-ranged dipolar coupling. This experimentally-determined interaction model is compared with density-functional-theory simulations. Using classical Monte Carlo simulations, we show that these competing interactions explain the experimental observation of multiple incommensurate magnetic phases and may stabilize multi-$\mathbf{q}$ states. Our results expand the known range of magnetic behavior on the kagome lattice., Comment: 13 pages, 8 figures

Recently, it was argued that a ferromagnetic (FM) insulating phase can be induced by a novel {\it interorbital} hopping mechanism. Here, we study the stability range of this novel FM phase under modifications in the crystal fields and electronic correlation strength, constructing a theoretical phase diagram. A plethora of states is unveiled, including the FM Mott insulator (MI), a FM orbital-selective Mott phase (OSMP), several anferromagnetic (AFM) MI phases, an AFM metallic state, and a FM metal as well. Our most interesting result is that the FM regime, either in MI or OSMP forms, is shown to be stable in {\it large} portions of the phase diagram, at both intermediate and strong electronic correlations, respectively. Our results demonstrate via a detailed example that the recently proposed novel mechanism to stabilize FM insulators is not fragile but instead robust, and may enlarge substantially the relatively small family of known FM insulators., Comment: 10 pages, 5 figures

$\alpha$-RuCl$_3$, a well-known candidate material for Kitaev quantum spin liquid, is prone to stacking disorder due to the weak van der Waals bonding between the honeycomb layers. After a decade of intensive experimental and theoretical studies, the detailed correlation between stacking degree of freedom, structure transition, magnetic and thermal transport properties remains unresolved. In this work, we reveal the effects of a small amount of stacking disorder inherent even in high quality $\alpha$-RuCl$_3$ crystals. This small amount of stacking disorder results in the variation of the magnetic ordering temperature, suppresses the structure transition and thermal conductivity. Crystals with minimal amount of stacking disorder have a T$_N>$7.4\,K and exhibit a well-defined structure transition around 140\,K upon cooling. For those with more stacking faults and a T$_N$ below 7\,K, the structure transition occurs well below 140\,K upon cooling and is incomplete, manifested by the diffuse streaks and the coexistence of both high temperature and low temperature phases down to the lowest measurement temperature. Both types of crystals exhibit oscillatory field dependent thermal conductivity and a plateau-like feature in thermal Hall resistivity in the field-induced quantum spin liquid state. However, $\alpha$-RuCl$_3$ crystals with minimal amount of stacking disorder have a higher thermal conductivity that pushes the thermal Hall conductivity to be closer to the half-integer quantized value. These findings demonstrate a strong correlation between layer stacking, structure transition, magnetic and thermal transport properties, underscoring the importance of interlayer coupling in $\alpha$-RuCl$_3$ despite the weak van der Waals bonding.

Interatomic forces that bind materials are largely determined by an often complex interplay between the electronic band-structure and the atomic arrangements to form its equilibrium structure and dynamics. As these forces also determine the phonon dispersion, lattice dynamics measurements are often crucial tools for understanding how materials transform between different structures. This is the case for the mono-chalcogenides which feature a number of lattice instabilities associated with their network of resonant bonds and a large tunability in their functional properties. SnSe hosts a novel lattice instability upon above-bandgap photoexcitation that is distinct from the distortions associated with its high temperature phase transition, demonstrating that photoexcitation can alter the interatomic forces significantly different than thermal excitation. Here we report decisive time-resolved X-ray scattering-based measurements of the nonequlibrium lattice dynamics in SnSe. By fitting interatomic force models to the excited-state dispersion, we determine this instability as being primarily due to changes in the fourth-nearest neighbor bonds that connect bilayers, with relatively little change to the intralayer resonant bonds. In addition to providing critical insight into the nonthermal bonding origin of the instability in SnSe, such measurements will be crucial for understanding and controlling materials properties under non-equilibrium conditions.

Alloying indium into the topological insulator Sn1-xInxBi2Te4 induces bulk superconductivity with critical temperatures Tc up to 1.85 K and upper critical fields up to about 14 kOe. This is confirmed by electrical resistivity, heat capacity, and magnetic susceptibility measurements. The heat capacity shows a discontinuity at Tc and temperature dependence below Tc consistent with weak coupling BCS theory, and suggests a superconducting gap near 0.25 meV. The superconductivity is type-II and the topological surface states have been verified by photoemission. A simple picture suggests analogies with the isostructural magnetic topological insulator MnBi2Te4, in which a natural heterostructure hosts complementary properties on different sublattices, and motivates new interest in this large family of compounds. The existence of both topological surface states and superconductivity in Sn1-xInxBi2Te4 identifies these materials as promising candidates for the study of topological superconductivity.

Competition among exchange interactions is able to induce novel spin correlations on a bipartite lattice without geometrical frustration. A prototype example is the spiral spin liquid, which is a correlated paramagnetic state characterized by sub-dimensional degenerate propagation vectors. Here, using spectral graph theory, we show that spiral spin liquids on a bipartite lattice can be approximated by a further-neighbor model on the corresponding line-graph lattice that is non-bipartite, thus broadening the space of candidate materials that may support the spiral spin liquid phases. As illustrations, we examine neutron scattering experiments performed on two spinel compounds, ZnCr$_2$Se$_4$ and CuInCr$_4$Se$_8$, to demonstrate the feasibility of this new approach and expose its possible limitations in experimental realizations., Comment: 20 pages, 18 figures

Two-dimensional (2D) magnetic van der Waals materials provide a powerful platform for studying fundamental physics of low-dimensional magnetism, engineering novel magnetic phases, and enabling ultrathin and highly tunable spintronic devices. To realize high quality and practical devices for such applications, there is a critical need for robust 2D magnets with ordering temperatures above room temperature that can be created via exfoliation. Here the study of exfoliated flakes of cobalt substituted Fe5GeTe2 (CFGT) exhibiting magnetism above room temperature is reported. Via quantum magnetic imaging with nitrogen-vacancy centers in diamond, ferromagnetism at room temperature was observed in CFGT flakes as thin as 16 nm. This corresponds to one of the thinnest room-temperature 2D magnet flakes exfoliated from robust single crystals, reaching a thickness relevant to practical spintronic applications. The Curie temperature Tc of CFGT ranges from 310 K in the thinnest flake studied to 328 K in the bulk. To investigate the prospect of high-temperature monolayer ferromagnetism, Monte Carlo calculations were performed which predicted a high value of Tc ~270 K in CFGT monolayers. Pathways towards further enhancing monolayer Tc are discussed. These results support CFGT as a promising platform to realize high-quality room-temperature 2D magnet devices.

The tetragonal intermetallic compound EuAl$_{4}$ hosts an exciting variety of low temperature phases. In addition to a charge density wave below 140 K, four ordered magnetic phases are observed below 15.4 K. Recently, a skyrmion phase was proposed based on Hall effect measurements under a $c$-axis magnetic field. We present a detailed investigation of the phase transitions in EuAl$_{4}$ under $c$-axis magnetic field. Our dilatometry, heat capacity, DC magnetometry, AC magnetic susceptibility, and resonant ultrasound spectroscopy measurements reveal three magnetic phase transitions not previously reported. We discuss what our results reveal about the character of the magnetic phases. Our first key result is a detailed $H \parallel [001]$ magnetic phase diagram mapping the seven phases we observe. Second, we identify a new high-field phase, phase VII, which directly corresponds to the region were skyrmions have been suggested. Our results provide guidance for future studies exploring the complex magnetic interactions and spin structures in EuAl$_{4}$., Comment: 20 pages, 15 figures

The experimental realization of magnetic skyrmions in centrosymmetric materials has been driven by theoretical understanding of how a delicate balance of anisotropy and frustration can stabilize topological spin structures in applied magnetic fields. Recently, the centrosymmetric material Gd$_{2}$PdSi$_{3}$ was shown to host a field-induced skyrmion phase, but the skyrmion stabilization mechanism remains unclear. Here, we employ neutron-scattering measurements on an isotopically-enriched polycrystalline Gd$_{2}$PdSi$_{3}$ sample to quantify the interactions that drive skyrmion formation. Our analysis reveals spatially-extended interactions in triangular planes that are consistent with an RKKY mechanism, and large ferromagnetic inter-planar magnetic interactions that are modulated by the Pd/Si superstructure. The skyrmion phase emerges from a zero-field helical magnetic order with magnetic moments perpendicular to the magnetic propagation vector, indicating that the magnetic dipolar interaction plays a significant role. Our experimental results establish an interaction space that can promote skyrmion formation, facilitating identification and design of centrosymmetric skyrmion materials., Comment: 5 pages, 5 figures

Fe3-xGeTe2 is a layered magnetic van der Waals material of interest for both fundamental and applied research. Despite the observation of intriguing physical properties, open questions exist even on the basic features related to magnetism: is it a simple ferromagnet or are there antiferromagnetic regimes and are the moments local or itinerant. Here, we demonstrate that antiferromagnetic spin fluctuations coexist with the ferromagnetism through comprehensive elastic and inelastic neutron scattering and thermodynamic measurements. Our realistic dynamical mean-field theory calculations reveal that the competing magnetic fluctuations are driven by an orbital selective Mott transition, where only the plane-perpendicular a1g orbital of the Fe(3d) manifold remains itinerant. Our results highlight the multi-orbital character in Fe3-xGeTe2 that supports a rare coexistence of local and itinerant physics within this material., Comment: 16 pages, 3 main figures

In order to tune the magnetic properties of the cleavable high-Curie temperature ferromagnet Fe$_{5-x}$GeTe$_2$, the effect of increasing the electron count through arsenic substitution has been investigated. Small additions of arsenic (2.5 and 5%) seemingly enhance ferromagnetic order in polycrystalline samples by quenching fluctuations on one of the three magnetic sublattices, whereas larger As concentrations decrease the ferromagnetic Curie temperature ($T_{\rm C}$) and saturation magnetization. This work also describes the growth and characterization of Fe$_{4.8}$AsTe$_2$ single crystals that are structurally analogous to Fe$_{5-x}$GeTe$_2$ but with some phase stability complications. Magnetization measurements reveal dominant antiferromagnetic behavior in Fe$_{4.8}$AsTe$_2$ with a N\'{e}el temperature of $T_{\rm N}$ $\approx$42K. A field-induced spin-flop below $T_{\rm N}$ results in a switch from negative to positive magnetoresistance, with significant hysteresis causing butterfly-shaped resistance loops. In addition to reporting the properties of Fe$_{4.8}$AsTe$_2$, this work shows the importance of manipulating the individual magnetic sublattices in Fe$_{5-x}$GeTe$_2$ and motivates further efforts to control the magnetic properties in related materials by fine tuning of the Fermi energy or crystal chemistry., Comment: Accepted for publication in 2D Materials, with accepted version online

Van der Waals material Fe$_5$GeTe$_2$, with its long-range ferromagnetic ordering near room temperature, has significant potential to become an enabling platform for implementing novel spintronic and quantum devices. To pave the way for applications, it is crucial to determine the magnetic properties when the thickness of Fe5GeTe2 reaches the few-layers regime. However, this is highly challenging due to the need for a characterization technique that is local, highly sensitive, artifact-free, and operational with minimal fabrication. Prior studies have indicated that Curie temperature TC can reach up to close to room temperature for exfoliated Fe$_5$GeTe$_2$ flakes, as measured via electrical transport; there is a need to validate these results with a measurement that reveals magnetism more directly. In this work, we investigate the magnetic properties of exfoliated thin flakes of van der Waals magnet Fe$_5$GeTe$_2$ via a quantum magnetic imaging technique based on nitrogen vacancy diamond. Through imaging the stray fields, we confirm room-temperature magnetic order in Fe$_5$GeTe$_2$ thin flakes with thickness down to 7 units cell. The stray field patterns and their response to magnetizing fields with different polarities point to a perpendicular easy-axis anisotropy. Furthermore, we perform imaging at different temperatures and determine the Curie temperature of the flakes at Tc~300 K. These results provide the basis for realizing a room-temperature monolayer ferromagnet with Fe$_5$GeTe$_2$. This work also demonstrates that the imaging technique enables a rapid screening of multiple flakes simultaneously, thereby paving the way towards high throughput characterization of potential 2D magnets near room temperature.

Quantum spin liquid (QSL) state in rare-earth triangular lattice has attracted much attention recently due to its potential application in quantum computing and communication. Here we report the single-crystal growth synthesis, crystal structure characterizations and magnetic properties of $A$YbSe$_{2}$ ($A$= K, Rb) compounds. The X-ray diffraction analysis shows that $A$YbSe$_{2}$ ($A$ = K and Rb) crystallizes in a trigonal space group, $R$-3$m$ (No. 166) with Z = 3. $A$YbSe$_{2}$ possesses a two-dimensional (2D) Yb-Se-Yb layered structure formed by edged-shared YbSe$_6$ octahedra. The magnetic properties are highly anisotropic for both title compounds and no long-range order is found down to 0.4 K, revealing the possible QSL ground state in these compounds. The isothermal magnetization exhibits one-third magnetization plateau when the magnetic fields are applied in $ab$-plane. Heat capacity is performed along both $ab$-plane and $c$-axis, and features the characteristic dome for triangular magnetic lattice compounds as a function of magnetic fields. Due to the change of the interlayer and intralayer distance of Yb$^{3+}$, the dome shifts to low fields from KYbSe$_2$ to RbYbSe$_2$. All these results indicate the $A$YbSe$_2$ family presents unique frustrated magnetism close to the possible QSL and noncolinear spin states., Comment: 9 pages, 7 figures

The hierarchy of the coupling strengths in a physical system often engenders an effective model at low energies where the decoupled high-energy modes are integrated out. Here, using neutron scattering, we show that the spin excitations in the breathing pyrochlore lattice compound CuInCr$_4$S$_8$ are hierarchical and can be approximated by an effective model of correlated tetrahedra at low energies. At higher energies, intra-tetrahedron excitations together with strong magnon-phonon couplings are observed, which suggests the possible role of the lattice degree of freedom in stabilizing the spin tetrahedra. Our work illustrates the spin dynamics in CuInCr$_4$S$_8$ and demonstrates a general effective-cluster approach to understand the dynamics on the breathing-type lattices., Comment: 11 pages, 9 figures

Polar materials can host a variety of topologically significant magnetic phases, which often emerge from a modulated magnetic ground state. Relatively few noncentrosymmetric tetragonal materials have been shown to host topological spin textures and new candidate materials are necessary to expand the current theoretical models. This manuscript reports on the anisotropic magnetism in the polar, tetragonal material NdCoGe$_3$ via thermodynamic and neutron diffraction measurements. The previously reported $H$-$T$ phase diagram is updated to include several additional phases, which exist for both $H$ = 0 and with an applied field H$\perp$ c. Neutron diffraction data reveal that the magnetic structures below $T_{N1}$ = 3.70 K and $T_{N2}$ = 3.50 K are incommensurate, with a ground state magnetic order that is incommensurate in all directions with the propagation vector $\vec{k}$ = (0.494, 0.0044, 0.385) at 1.8 K. A unique magnetic structure solution is not achievable, but the possible single and multi-$\vec{k}$ spin models are discussed. These results demonstrate that NdCoGe3 hosts complicated magnetic order derived from modulated magnetic moments., Comment: 11 pages, 10 figures

Spin-1/2 chains with alternating antiferromagnetic (AF) and ferromagnetic (FM) couplings exhibit quantum entanglement like the integer-spin Haldane chains and might be similarly utilized for quantum computations. Such alternating AF-FM chains have been proposed to be realized in the distorted honeycomb-lattice compound Na$_2$Cu$_2$TeO$_6$, but to confirm this picture a comprehensive understanding of the exchange interactions including terms outside of the idealized model is required. Here we employ neutron scattering to study the spin dynamics in Na$_2$Cu$_2$TeO$_6$ and accurately determine the coupling strengths through the random phase approximation and density functional theory (DFT) approaches. We find the AF and FM intrachain couplings are the dominant terms in the spin Hamiltonian, while the interchain couplings are AF but perturbative. This hierarchy in the coupling strengths and the alternating signs of the intrachain couplings can be understood through their different exchange paths. Our results establish Na$_2$Cu$_2$TeO$_6$ as a weakly-coupled alternating AF-FM chain compound and reveal the robustness of the gapped ground state in alternating chains under weak interchain couplings., Comment: 9 pages, 7 figures

Fe5-xGeTe2 is a van der Waals material with one of the highest reported bulk Curie temperatures, $T_C$ ~ 310K. In this study, theoretical calculations and experiments are utilized to demonstrate that the magnetic ground state is highly sensitive to local atomic arrangements and the interlayer stacking. Cobalt substitution is found to be an effective way to manipulate the magnetic properties while also increasing the ordering temperature. In particular, cobalt substitution up to 30% enhances $T_C$ and changes the magnetic anisotropy, while approximately 50% cobalt substitution yields an antiferromagnetic state. Single crystal x-ray diffraction evidences a structural change upon increasing the cobalt concentration, with a rhombohedral cell observed in the parent material and a primitive cell observed for ~46% cobalt content relative to iron. First principles calculations demonstrate that it is a combination of high cobalt content and the concomitant change to primitive layer stacking that produces antiferromagnetic order. These results illustrate the sensitivity of magnetism in Fe5-xGeTe2 to composition and structure, and emphasize the important role of structural order/disorder and layer stacking in cleavable magnetic materials., Comment: This paper is a contribution to the joint Physical Review Applied and Physical Review Materials collection titled Two-Dimensional Materials and Devices

Fe$_{3-x}$GeTe$_2$ is a layered van der Waals magnetic material with a relatively high ordering temperature and large anisotropy. While most studies have concluded the interlayer ordering to be ferromagnetic, there have also been reports of interlayer antiferromagnetism in Fe$_{3-x}$GeTe$_2$. Here, we investigate the interlayer magnetic ordering by neutron diffraction experiments, scanning tunneling microscopy (STM) and spin-polarized STM measurements, density functional theory plus U calculations and STM simulations. We conclude that the layers of Fe$_{3-x}$GeTe$_2$ are coupled ferromagnetically and that in order to capture the magnetic and electronic properties of Fe$_{3-x}$GeTe$_2$ within density functional theory, Hubbard U corrections need to be taken into account.

Quantum interference on the kagome lattice generates electronic bands with narrow bandwidth, called flat bands. Crystal structures incorporating this lattice can host strong electron correlations with non-standard ingredients, but only if these bands lie at the Fermi level. In the six compounds with the CoSn structure type (FeGe, FeSn, CoSn, NiIn, RhPb, and PtTl) the transition metals form a kagome lattice. The two iron variants are robust antiferromagnets so we focus on the latter four and investigate their thermodynamic and transport properties. We consider these results and calculated band structures to locate and characterize the flat bands in these materials. We propose that CoSn and RhPb deserve the community's attention for exploring flat band physics., Comment: 12pg 11fig

The magnetic and transport properties of Fe-deficient Fe5GeTe2 single crystals (Fe5-xGeTe2 with x~0.3) were studied and the impact of thermal processing was explored. Quenching crystals from the growth temperature has been previously shown to produce a metastable state that undergoes a strongly hysteretic first-order transition upon cooling below ~100K. The first-order transition impacts the magnetic properties, yielding an enhancement in the Curie temperature T_C from 270 to 310K. In the present work, T_HT ~550K has been identified as the temperature above which metastable crystals are obtained via quenching. Diffraction experiments reveal a structural change at this temperature, and significant stacking disorder occurs when samples are slowly cooled through this temperature range. The transport properties are demonstrated to be similar regardless of the crystal's thermal history. The scattering of charge carriers appears to be dominated by moments fluctuating on the Fe(1) sublattice, which remain dynamic down to 100-120K. Maxima in the magnetoresistance and anomalous Hall resistance are observed near 120K. The Hall and Seebeck coefficients are also impacted by magnetic ordering on the Fe(1) sublattice. The data suggest that both electrons and holes contribute to conduction above 120K, but that electrons dominate at lower temperature when all of the Fe sublattices are magnetically ordered. This study demonstrates a strong coupling of the magnetism and transport properties in Fe5-xGeTe2 and complements the previous results that demonstrated strong magnetoelastic coupling as the Fe(1) moments order. The published version of this manuscript is DOI:10.1103/PhysRevMaterials.3.104401 (2019), Comment: Published in Physical Review Materials. https://doi.org/10.1103/PhysRevMaterials.3.104401

We present a comprehensive neutron scattering study of the breathing pyrochlore magnet LiGaCr4S8. We observe an unconventional magnetic excitation spectrum with a separation of high and low-energy spin dynamics in the correlated paramagnetic regime above a spin-freezing transition at 12(2) K. By fitting to magnetic diffuse-scattering data, we parameterize the spin Hamiltonian. We find that interactions are ferromagnetic within the large and small tetrahedra of the breathing pyrochlore lattice, but antiferromagnetic further-neighbor interactions are also essential to explain our data, in qualitative agreement with density-functional theory predictions [Ghoshet al.,npj Quantum Mater.4, 63 (2019)]. We explain the origin of geometrical frustration in LiGaCr4S8 interms of net antiferromagnetic coupling between emergent tetrahedral spin clusters that occupy a face-centered lattice. Our results provide insight into the emergence of frustration in the presence of strong further-neighbor couplings, and a blueprint for the determination of magnetic interactions in classical spin liquids., Comment: Supplemental information available upon request

In this paper we report the synthesis, magnetization and heat capacity of the frustrated magnets \emph{A}ErSe$_2$(\emph{A}=Na,K) which contain perfect triangular lattices of Er$^{3+}$. The magnetization data suggests no long-range magnetic order exists in \emph{A}ErSe$_2$(\emph{A}=Na,K), which is consistent with the heat capacity measurements. Large anisotropy is observed between the magnetization within the \emph{ab} plane and along the \emph{c} axis of both compounds. When the magnetic field is applied along \emph{ab} plane, anomalies are observed at 1.8 $\mu_B$ in NaErSe$_2$ at 0.2 T and 2.1 $\mu_B$ in KErSe$_2$ at 0.18 T. Unlike NaErSe$_2$, a plateau-like field-induced metamagnetic transition is observed for H$\|$\emph{c} below 1 K in KErSe$_2$. Two broad peaks are observed in the heat capacity below 10 K indicating possible crystal electric field(CEF) effects and magnetic entropy released under different magnetic fields. All results indicate that \emph{A}ErSe$_2$ are strongly anisotropic, frustrated magnets with field-induced transition at low temperature. The lack of signatures for long-range magnetic order implies that these materials are candidates for hosting a quantum spin liquid ground state., Comment: 7 pages, 4 figures

Co substituted Ni$_{1-x}$Co$_x$Br$_2$ (0 $\leq~x~\leq$ 1) single crystals were synthesized using vapor transport. The physical properties of the crystals were characterized by x-ray powder diffraction, magnetization, and specific heat measurements. Room temperature x-ray powder diffraction data indicate a change from the CdCl$_2$ structure type to the CdI$_2$ structure type occurs within 0.56 $<~x~<$ 0.76. NiBr$_2$ has a commensurate antiferromagnetic phase below $T_{\rm N} \approx$ 46 K and an incommensurate magnetic ground state below $T_{\rm IC} \approx$ 20 K. Both magnetic transitions are affected by cobalt substitution, and the incommensurate phase transition is present up to at least $x$ = 0.56. The evolution of magnetism has been studied as a function of cobalt content and is summarized in the temperature-composition phase diagram.

The first examples of single crystal epitaxial thin films of a high entropy perovskite oxide are synthesized. Pulsed laser deposition is used to grow the configurationally disordered ABO3 perovskite, Ba(Zr0.2Sn0.2Ti0.2Hf0.2Nb0.2)O3, epitaxially on SrTiO3 and MgO substrates. X-ray diffraction and scanning transmission electron microscopy demonstrate that the films are single phase with excellent crystallinity and atomically abrupt interfaces to the underlying substrates. Atomically-resolved electron energy loss spectroscopy mapping shows a uniform and random distribution of all B-site cations. The ability to stabilize perovskites with this level of configurational disorder offers new possibilities for designing materials from a much broader combinatorial cation pallet while providing a fresh avenue for fundamental studies in strongly correlated quantum materials where local disorder can play a critical role in determining macroscopic properties.

Frustrated magnetic systems exhibit extraordinary physical properties but quantification of their magnetic correlations poses a serious challenge to experiment and theory. Current insight into frustrated magnetic correlations relies on modelling techniques such as reverse Monte Carlo methods, which require knowledge about the exact ordered atomic structure. Here we present a method for direct reconstruction of magnetic correlations in frustrated magnets by three-dimensional difference pair distribution function analysis of neutron total scattering data. The methodology is applied to the disordered frustrated magnet bixbyite, (Mn1-xFex)2O3, which reveals nearest-neighbor antiferromagnetic correlations for the metal sites up to a range of approximately 15 {\AA}. Importantly, this technique allows for magnetic correlations to be determined directly from the experimental data without any assumption about the atomic structure.

The crystallographic and magnetic properties of the cleavable 4d3 transition metal compound a-MoCl3 are reported, with a focus on the behavior above room temperature. Crystals were grown by chemical vapor transport and characterized using temperature dependent x-ray diffraction, Raman spectroscopy, and magnetization measurements. A structural phase transition occurs near 585 K, at which the Mo-Mo dimers present at room temperature are broken. A nearly regular honeycomb net of Mo is observed above the transition, and an optical phonon associated with the dimerization instability is identified in the Raman data and in first-principles calculations. The crystals are diamagnetic at room temperature in the dimerized state, and the magnetic susceptibility increases sharply at the structural transition. Moderately strong paramagnetism in the high-temperature structure indicates the presence of local moments on Mo. This is consistent with results of spin-polarized density functional theory calculations using the low- and high-temperature structures. Above the magnetostructural phase transition the magnetic susceptibility continues to increase gradually up to the maximum measurement temperature of 780 K, with a temperature dependence that suggests two-dimensional antiferromagnetic correlations.

The magnetism in Mn$_3$Si$_2$Te$_6$ has been investigated using thermodynamic measurements, first principles calculations, neutron diffraction and diffuse neutron scattering on single crystals. These data confirm that Mn$_3$Si$_2$Te$_6$ is a ferrimagnet below a Curie temperature of $T_C$ approximately 78K. The magnetism is anisotropic, with magnetization and neutron diffraction demonstrating that the moments lie within the basal plane of the trigonal structure. The saturation magnetization of approximately 1.6$\mu_B$/Mn at 5K originates from the different multiplicities of the two antiferromagnetically-aligned Mn sites. First principles calculations reveal antiferromagnetic exchange for the three nearest Mn-Mn pairs, which leads to a competition between the ferrimagnetic ground state and three other magnetic configurations. The ferrimagnetic state results from the energy associated with the third-nearest neighbor interaction, and thus long-range interactions are essential for the observed behavior. Diffuse magnetic scattering is observed around the 002 Bragg reflection at 120K, which indicates the presence of strong spin correlations well above $T_C$. These are promoted by the competing ground states that result in a relative suppression of $T_C$, and may be associated with a small ferromagnetic component that produces anisotropic magnetism below $\approx$330K., Comment: Main article plus Supplemental

The behavior of matter near a quantum critical point (QCP) is one of the most exciting and challenging areas of physics research. Emergent phenomena such as high-temperature superconductivity are linked to the proximity to a QCP. Although significant progress has been made in understanding quantum critical behavior in some low dimensional magnetic insulators, the situation in metallic systems is much less clear. Here we demonstrate that NiCoCrx single crystal alloys are remarkable model systems for investigating QCP physics in a metallic environment. For NiCoCrx alloys with x = 0.8, the critical exponents associated with a ferromagnetic quantum critical point (FQCP) are experimentally determined from low temperature magnetization and heat capacity measurements. For the first time, all of the five critical exponents ( gamma-subT =1/2 , beta-subT = 1, delta = 3/2, nuz-subm = 2, alpha-bar-subT = 0) are in remarkable agreement with predictions of Belitz-Kirkpatrick-Vojta (BKV) theory in the asymptotic limit of high disorder. Using these critical exponents, excellent scaling of the magnetization data is demonstrated with no adjustable parameters. We also find a divergence of the magnetic Gruneisen parameter, consistent with a FQCP. This work therefore demonstrates that entropy stabilized concentrated solid solutions represent a unique platform to study quantum critical behavior in a highly tunable class of materials., Comment: 17 pages, 7 figures

Identifying and characterizing systems with coupled and competing interactions is central to the development of physical models that can accurately describe and predict emergent behavior in condensed matter systems. This work demonstrates that the metallic compound CuFe$_2$Ge$_2$ has competing magnetic ground states, which are shown to be strongly coupled to the lattice and easily manipulated using temperature and applied magnetic fields. Temperature-dependent magnetization $M$ measurements reveal a ferromagnetic-like onset at 228(1)\,K and a broad maximum in $M$ near 180\,K. Powder neutron diffraction confirms antiferromagnetic ordering below $T_{\textrm{N}}\approx$175\,K, and an incommensurate spin density wave is observed below $\approx$125\,K. Coupled with the small refined moments (0.5-1\,$\mu_B$/Fe), this provides a picture of itinerant magnetism in CuFe$_2$Ge$_2$. The neutron diffraction data also reveal a coexistence of two magnetic phases that further highlights the near-degeneracy of various magnetic states. These results demonstrate that the ground state in CuFe$_2$Ge$_2$ can be easily manipulated by external forces, making it of particular interest for doping, pressure, and further theoretical studies., Comment: Combines main article and supplemental information

The anharmonic phonon properties of SnSe in the Pnma phase were investigated with a combination of experiments and first-principles simulations. Using inelastic neutron scattering (INS) and nuclear resonant inelastic X-ray scattering (NRIXS), we have measured the phonon dispersions and density of states (DOS) and their temperature dependence, which revealed a strong, inhomogeneous shift and broadening of the spectrum on warming. First-principles simulations were performed to rationalize these measurements, and to explain the previously reported anisotropic thermal expansion, in particular the negative thermal expansion within the Sn-Se bilayers. Including the anisotropic strain dependence of the phonon free energy, in addition to the electronic ground state energy, is essential to reproduce the negative thermal expansion. From the phonon DOS obtained with INS and additional calorimetry measurements, we quantify the harmonic, dilational, and anharmonic components of the phonon entropy, heat capacity, and free energy. The origin of the anharmonic phonon thermodynamics is linked to the electronic structure., Comment: 14 pages, 12 figures

Strontium titanate (SrTiO3, STO) is a critically important material for the study of emergent electronic phases in complex oxides, as well as for the development of applications based on their heterostructures. Despite the large body of knowledge on STO, there are still many uncertainties regarding the role of defects in the properties of STO, including their influence on ferroelectricity in bulk STO and ferromagnetism in STO-based heterostructures. We present a detailed analysis of the decay of persistent photoconductivity in STO single crystals with defect concentrations that are relatively low but significantly affect their electronic properties. The results show that photo-activated electron transport cannot be described by a superposition of the properties due to independent point defects as current models suggest but is, instead, governed by defect complexes that interact through dynamic correlations. These results emphasize the importance of defect correlations for activated electronic transport properties of semiconducting and insulating perovskite oxides.

Quantum critical behavior has been associated with some of the most exotic emergent states of matter including high-temperature superconductivity. Much of the research into quantum critical point (QCP) physics has been hampered by the lack of model systems simple enough to be analyzed by theory. Here, we show that the concentrated solid solution fcc alloys, including the so-called high-entropy alloys, are ideal model systems to study the effects of chemical disorder on emergent properties near a quantum critical region. The face centered cubic (fcc) alloy NiCoCrx with x near 1 is found to be close to the Cr concentration where the ferromagnetic transition temperature, Tc, goes to 0. Near this composition these alloys exhibit a resistivity linear in temperature to 2 K, a linear magnetoresistance, an excess -TlnT contribution to the low temperature heat capacity and excess low temperature entropy. All of the low temperature electrical, magnetic and thermodynamic properties of the alloys with compositions near x near 1 are not typical of a Fermi liquid and suggest strong magnetic fluctuations associated with a quantum critical region. The limit of extreme chemical disorder in these simple fcc materials thus provides a novel and unique platform to study quantum critical behavior in a highly tunable system.