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We use a novel solution-based disassociation synthesis scheme of the ionic complex Fe(LiBH4)2 to form Fe nanoparticles. The complex was formed initially using a gentle mechanochemical process, and the Fe nanoparticles emerged after 4 h of ball milling in an air-free environment. Rietveld refinement of x-ray diffraction measurements in an air-free sample holder identified a Im3m α-Fe phase. A room temperature Mössbauer spectrum of the sample presented a six-line spectrum unique to Fe0 metal, and the Fe nanoparticles were extremely well crystallized. Magnetometry results presented a reduced saturation magnetization (e.g., Ms ∼ 85 emu/g at 50 K) that had a Bloch-like T2 temperature dependence, consistent with a gap in the magnon fluctuation spectrum due to finitesize effects. The Fe nanoparticles were magnetically soft, with a coercivity ranging from ∼10 to 20mT with decreasing temperature from 350 K. [ABSTRACT FROM AUTHOR]

A series of core/shell Fe-oxide/SiO2 nanoparticles were produced to form Fe-oxide (1:1 Fe3O4 and c-Fe2O3) cores with diameters of approximately 4.6 nm and shell thicknesses ranging from 3.6 to 5.4 nm. Hot press sintering of core/shell nanoparticles created a nanocomposite of Fe-oxide nanoparticles in a SiO2 matrix. The presence of an iron-orthosilicate at the core-shell interface defines the intrinsic magnetic properties of the nanoparticle systems, resulting in an increase in magnetic anisotropy with thicker SiO2 shell. We find that dipole-dipole interactions are mediated by the overall SiO2 shell, and that these interactions are coupling neighbouring particles' magnetization with increasing correlation lengths. When the nanoparticles from a composite material, packing significantly increases the interaction strengths, altering the overall magnetization of the system so that the iron-oxide cores present an approximate 25% increase in (saturation) magnetization to a bulk-like (∼80 emu/g) value. [ABSTRACT FROM AUTHOR]

We have examined the effects of core-shell intermixing on the dynamical magnetism of γ-Fe2O3/ MnO nanoparticles. The core and shell phases were identified using x-ray diffraction, and x-ray absorption spectroscopy identified Mn ions in both octahedral and tetrahedral sites, consistent with a significant amount of substitution at the core-shell interface to form an Fe/Mn-ferrite. The dynamical response was probed by Mössbauer spectroscopy, which decouples surface and core spins, and suggested a change in the relaxation behaviour among the spin populations within γ-Fe2O3/MnO relative to the γ-Fe2O3 seed particles. Interestingly, the magnetic relaxation effects at the atomic scale, measured via Möossbauer spectroscopy, were enhanced, indicating that the addition of an MnO shell and intermixing affected the dynamical freezing process which altered the surface magnetism of the γ-Fe2O3 core. Our results show that both the MnO shell and the interfacial intermixed layer are important in determining the core-shell nanoparticle magnetism. [ABSTRACT FROM AUTHOR]

Significant efforts towards understanding bi-magnetic core-shell nanoparticles are underway currently as they provide a pathway towards properties unavailable with single-phased systems. Recently, we have demonstrated that the magnetism of c-Fe2O3/CoO core-shell nanoparticles, in particular, at high temperatures, originates essentially from an interfacial doped iron-oxide layer that is formed by the migration of Co2+ from the CoO shell into the surface layers of the γ-Fe2O3 core [Skoropata et al., Phys. Rev. B 89, 024410 (2014)]. To examine directly the nature of the intermixed layer, we have used high-resolution transmission electron microscopy (HRTEM) and first-principles calculations to examine the impact of the core-shell intermixing at the atomic level. By analyzing the HRTEM images and energy dispersive spectra, the level and nature of intermixing was confirmed, mainly as doping of Co into the octahedral site vacancies of γ-Fe2O3. The average Co doping depths for different processing temperatures (150 °C and 235 °C) were 0.56 nm and 0.78 nm (determined to within 5% through simulation), respectively, establishing that the amount of core-shell intermixing can be altered purposefully with an appropriate change in synthesis conditions. Through first-principles calculations, we find that the intermixing phase of γ-Fe2O3 with Co doping is ferromagnetic, with even higher magnetization as compared to that of pure γ-Fe2O3. In addition, we show that Co doping into different octahedral sites can cause different magnetizations. This was reflected in a change in overall nanoparticle magnetization, where we observed a 25% reduction in magnetization for the 235 °C versus the 150 °C sample, despite a thicker intermixed layer. [ABSTRACT FROM AUTHOR]

A single precursor method is used for preparing [gamma]-[Fe.sub.2][O.sub.3] nanoparticles and the blocking temperature of the nanoparticles has increased with Fe precursor concentration. Transmission Mossbauer spectroscopy of the different [gamma]-[Fe.sub.2][O.sub.3] nanoparticle systems have shown that the local Fe environments in the nanoparticle vary considerably with Fe precursor concentration.

The local magnetism of [gamma]-[Fe.sub.2][O.sub.3] nanoparticles is examined as a function of temperature by using Mossbauer spectroscopy. The surface moment fluctuations have increased in frequency with warming.

We have investigated the magnetism of NiFe/CoO/Co trilayers with different CoO spacer thicknesses. The dependence of the coercivity (Hc) and exchange bias field (Hex) on the CoO thicknesses indicated that different pinning strengths from the CoO were acting on the top NiFe and bottom Co layers, respectively. DC susceptibility indicated the different interlayer coupling energies and showed that the anisotropy of CoO layer strongly affected the temperature dependence of the magnetization. [ABSTRACT FROM AUTHOR]

A single precursor technique was used to synthesize γ-Fe2O3 nanoparticles with diameters from 5 to 7 nm. The blocking temperature of the nanoparticles essentially increases with Fe precursor concentration. Transmission Mössbauer spectroscopy of the different γ-Fe2O3 nanoparticle systems reveal that the local Fe environments in the nanoparticle vary considerably with Fe precursor concentration. Not only does the nanoparticle size change, but the magnitude of the hyperfine field (a measure of the nanoparticle moment), the amount of chemical disorder in the nanoparticle, and the number of Fe moments in the octahedral and tetrahedral sites of the γ-Fe2O3 spinel structure vary with Fe concentration as well. [ABSTRACT FROM AUTHOR]

We have examined the local magnetism of 7 nm γ-Fe2O3 nanoparticles as a function of temperature using transmission Mössbauer spectroscopy. Spectra indicate that the nanoparticles exhibit two different spin populations: one is due to the core Fe moments, while the other is from the surface moments. We find that the uniaxial anisotropy of the core increases with temperature from 5 to 75 K, whereupon it remains constant. Simultaneously, the surface moment fluctuations increase in frequency with warming. [ABSTRACT FROM AUTHOR]

γ-Fe2O3 particles, surface modified with NiO crystallites, form a unique nanocomposite that points to how to tune strong interfacial exchange coupling. We find that Ni 2 + migrates into the octahedral sites of the γ-Fe2O3 nanoparticle surface, and this NiFe2O4-like layer permits effective magnetic coupling of Ni and Fe sites that strengthens the interface exchange. A large increase in coercivity coinciding with a loss of exchange bias is achieved by this strong interfacial coupling that results in a Ni2+ moment reversal in the NiO with the γ-Fe2O3. This work reveals the importance of intermixing in, and possibility to use, such an exchange coupling regime to alter substantially the coercivity and hence control an important property of exchange-coupled nanocomposite magnets. [ABSTRACT FROM AUTHOR]

The interaction of magnetic order and spontaneous polarization is a fundamental coupling with the prospect for the control of electronic properties and magnetism. The connection among magnetic order, charge localization and associated metal-insulator transition (MIT) are cornerstones for materials control. Materials that combine both effects are therefore of great interest for testing models that claim the occurrence of spontaneous polarization from magnetic and charge order. One class of materials proposed to combine these functionalities is the family of RNiO 3 (R: Lanthanide or Yttrium), whose members show a clear MIT and an antiferromagnetic ground state and for which an electric polarization has been predicted. Here, using resonant magnetic x-ray scattering with circular polarization and an applied electric field we show that YNiO 3 possess a magnetic structure containing domains of spin-rotations that are consistent with an electric polarization. We show a reversal of the magnetic structure with the applied electric field confirming that charge ordered RNiO 3 are magnetoelectric type II multiferroics with a MIT., Competing Interests: Competing interestsThe authors declare no competing interests., (© The Author(s) 2024.)

Quantifying the dynamics of normal modes and how they interact with other excitations is of central importance in condensed matter. Spin-lattice coupling is relevant to several sub-fields of condensed matter physics; examples include spintronics, high-T c superconductivity, and topological materials. However, experimental approaches that can directly measure it are rare and incomplete. Here we use time-resolved X-ray diffraction to directly access the ultrafast motion of atoms and spins following the coherent excitation of an electromagnon in a multiferroic hexaferrite. One striking outcome is the different phase shifts relative to the driving field of the two different components. This phase shift provides insight into the excitation process of such a coupled mode. This direct observation of combined lattice and magnetization dynamics paves the way to access the mode-selective spin-lattice coupling strength, which remains a missing fundamental parameter for ultrafast control of magnetism and is relevant to a wide variety of materials., (© 2023. The Author(s).)

Recently, a highly ordered Moiré dislocation lattice was identified at the interface between a SrTiO 3 (STO) thin film and the (LaAlO 3 ) 0.3 (Sr 2 TaAlO 6 ) 0.7 (LSAT) substrate. A fundamental understanding of the local ionic and electronic structures around the dislocation cores is crucial to further engineer the properties of these complex multifunctional heterostructures. Here, we combine experimental characterization via analytical scanning transmission electron microscopy with results of molecular dynamics and density functional theory calculations to gain insights into the structure and defect chemistry of these dislocation arrays. Our results show that these dislocations lead to undercoordinated Ta/Al cations at the dislocation core, where oxygen vacancies can easily be formed, further facilitated by the presence of cation vacancies. The reduced Ti 3+ observed experimentally at the dislocations by electron energy-loss spectroscopy is a consequence of both the structure of the dislocation itself and of the electron doping due to oxygen vacancy formation. Finally, the experimentally observed Ti diffusion into the LSAT around the dislocation core occurs only together with cation vacancy formation in the LSAT or Ta diffusion into STO.

In magnetic systems, spin and exchange disorder can provide access to quantum criticality, frustration, and spin dynamics, but broad tunability of these responses and a deeper understanding of strong limit disorder are lacking. Here, it is demonstrated that high entropy oxides present a previously unexplored route to designing materials in which the presence of strong local compositional disorder may be exploited to generate tunable magnetic behaviors-from macroscopically ordered states to frustration-driven dynamic spin interactions. Single-crystal La(Cr 0.2 Mn 0.2 Fe 0.2 Co 0.2 Ni 0.2 )O 3 films are used as a model system hosting a magnetic sublattice with a high degree of microstate disorder in the form of site-to-site spin and exchange type inhomogeneity. A classical Heisenberg model simplified to represent the highest probability microstates well describes how compositionally disordered systems can paradoxically host magnetic uniformity and demonstrates a path toward continuous control over ordering types and critical temperatures. Model-predicted materials are synthesized and found to possess an incipient quantum critical point when magnetic ordering types are designed to be in direct competition, this leads to highly controllable exchange bias behaviors previously accessible only in intentionally designed bilayer heterojunctions., (© 2022 The Authors. Advanced Science published by Wiley-VCH GmbH.)

Quantum materials (QMs) with strong correlation and nontrivial topology are indispensable to next-generation information and computing technologies. Exploitation of topological band structure is an ideal starting point to realize correlated topological QMs. Here, we report that strain-induced symmetry modification in correlated oxide SrNbO 3 thin films creates an emerging topological band structure. Dirac electrons in strained SrNbO 3 films reveal ultrahigh mobility (μ max ≈ 100,000 cm 2 /Vs), exceptionally small effective mass ( m * ~ 0.04 m e ), and nonzero Berry phase. Strained SrNbO 3 films reach the extreme quantum limit, exhibiting a sign of fractional occupation of Landau levels and giant mass enhancement. Our results suggest that symmetry-modified SrNbO 3 is a rare example of correlated oxide Dirac semimetals, in which strong correlation of Dirac electrons leads to the realization of a novel correlated topological QM.

In past few decades, there have been substantial advances in theoretical material design and experimental synthesis, which play a key role in the steep ascent of developing functional materials with unprecedented properties useful for next-generation technologies. However, the ultimate goal of synthesis science, i.e., how to locate atoms in a specific position of matter, has not been achieved. Here, we demonstrate a unique way to inject elements in a specific crystallographic position in a composite material by strain engineering. While the use of strain so far has been limited for only mechanical deformation of structures or creation of elemental defects, we show another powerful way of using strain to autonomously control the atomic position for the synthesis of new materials and structures. We believe that our synthesis methodology can be applied to wide ranges of systems, thereby providing a new route to functional materials., (Copyright © 2021 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).)

Magnetic insulators are important materials for a range of next-generation memory and spintronic applications. Structural constraints in this class of devices generally require a clean heterointerface that allows effective magnetic coupling between the insulating layer and the conducting layer. However, there are relatively few examples of magnetic insulators that can be synthesized with surface qualities that would allow these smooth interfaces and precisely tuned interfacial magnetic exchange coupling, which might be applicable at room temperature. In this work, we demonstrate an example of how the configurational complexity in the magnetic insulator layer can be used to realize these properties. The entropy-assisted synthesis is used to create single-crystal (Mg 0.2 Ni 0.2 Fe 0.2 Co 0.2 Cu 0.2 )Fe 2 O 4 films on substrates spanning a range of strain states. These films show smooth surfaces, high resistivity, and strong magnetic responses at room temperature. Local and global magnetic measurements further demonstrate how strain can be used to manipulate the magnetic texture and anisotropy. These findings provide insight into how precise magnetic responses can be designed using compositionally complex materials that may find application in next-generation magnetic devices.

The layered Ruddlesden-Popper crystal structure can host a broad range of functionally important behaviors. Here we establish extraordinary configurational disorder in a layered Ruddlesden-Popper (RP) structure using entropy stabilization assisted synthesis. A protype A 2 CuO 4 RP cuprate oxide with five cations on the A -site sublattice is designed and fabricated into epitaxial single crystal films using pulsed laser deposition. When grown on a near lattice matched substrate, the (La 0.2 Pr 0.2 Nd 0.2 Sm 0.2 Eu 0.2 ) 2 CuO 4 film features a T '-type RP structure with uniform A -site cation mixing and square-planar CuO 4 units. These observations are made with a range of combined characterizations using X-ray diffraction, atomic-resolution scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray absorption spectroscopy measurements. It is further found that heteroepitaxial strain plays an important role in crystal phase formation during synthesis. Compressive strain over ∼1.5% results in the formation of a non-RP cubic phase consistent with a Cu X 2 O 4 spinel structure. The ability to manipulate configurational complexity and move between 2D layered RP and 3D cubic crystal structures in cuprate and related materials promises to enable flexible design strategies for a range of functionalities, such as magnetoresistance, unconventional superconductivity, ferroelectricity, catalysis, and ion transport.

We report on the growth of stoichiometric, single-crystal YCrO 3 epitaxial thin films on (001) SrTiO 3 substrates using pulsed laser deposition. X-ray diffraction and atomic force microscopy reveal that the films grew in a layer-by-layer fashion with excellent crystallinity and atomically smooth surfaces. Magnetization measurements demonstrate that the material is ferromagnetic below 144 K. The temperature dependence of dielectric permittivity shows a characteristic relaxor-ferroelectric behavior at T C = 375-408 K. A dielectric anomaly at the magnetic transition temperature indicates a close correlation between magnetic and electric order parameters in these multiferroic YCrO 3 films. These findings provide guidance to synthesize rare-earth, chromite-based multifunctional heterostructures and build a foundation for future studies on the understanding of magnetoelectric effects in similar material systems.

Chiral interactions in magnetic systems can give rise to rich physics manifested, for example, as nontrivial spin textures. The foremost interaction responsible for chiral magnetism is the Dzyaloshinskii-Moriya interaction (DMI), resulting from inversion symmetry breaking in the presence of strong spin-orbit coupling. However, the atomistic origin of DMIs and their relationship to emergent electrodynamic phenomena, such as topological Hall effect (THE), remain unclear. Here, we investigate the role of interfacial DMIs in 3 d -5 d transition metal-oxide-based LaMnO 3 /SrIrO 3 superlattices on THE from a chiral spin texture. By additively engineering the interfacial inversion symmetry with atomic-scale precision, we directly link the competition between interfacial collinear ferromagnetic interactions and DMIs to an enhanced THE. The ability to control the DMI and resulting THE points to a pathway for harnessing interfacial structures to maximize the density of chiral spin textures useful for developing high-density information storage and quantum magnets for quantum information science., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).)

Artificial heterostructures composed of dissimilar transition metal oxides provide unprecedented opportunities to create remarkable physical phenomena. Here, we report a means to deliberately control the orbital polarization in LaNiO 3 (LNO) through interfacing with SrCuO 2 (SCO), which has an infinite-layer structure for CuO 2 . Dimensional control of SCO results in a planar-type (P-SCO) to chain-type (C-SCO) structure transition depending on the SCO thickness. This transition is exploited to induce either a NiO 5 pyramidal or a NiO 6 octahedral structure at the SCO/LNO interface. Consequently, a large change in the Ni d orbital occupation up to ~30% is achieved in P-SCO/LNO superlattices, whereas the Ni e g orbital splitting is negligible in C-SCO/LNO superlattices. The engineered oxygen coordination triggers a metal-to-insulator transition in SCO/LNO superlattices. Our results demonstrate that interfacial oxygen coordination engineering provides an effective means to manipulate the orbital configuration and associated physical properties, paving a pathway towards the advancement of oxide electronics.

Ferromagnetic insulators (FMIs) are one of the most important components in developing dissipationless electronic and spintronic devices. However, FMIs are innately rare to find in nature as ferromagnetism generally accompanies metallicity. Here, novel room-temperature FMI films that are epitaxially synthesized by deliberate control of the ratio between two B-site cations in the double perovskite Sr 2 Fe 1+ x Re 1- x O 6 (-0.2 ≤ x ≤ 0.2) are reported. In contrast to the known FM metallic phase in stoichiometric Sr 2 FeReO 6 , an FMI state with a high Curie temperature (T c ≈ 400 K) and a large saturation magnetization (M S ≈ 1.8 µ B f.u. -1 ) is found in highly cation-ordered Fe-rich phases. The stabilization of the FMI state is attributed to the formation of extra Fe 3+ Fe 3+ and Fe 3+ Re 6+ bonding states, which originate from the relatively excess Fe ions owing to the deficiency in Re ions. The emerging FMI state created by controlling cations in the oxide double perovskites opens the door to developing novel oxide quantum materials and spintronic devices., (© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Magnetic nanoparticles (MNPs) have become increasingly important in biomedical applications like magnetic imaging and hyperthermia based cancer treatment. Understanding their magnetic spin configurations is important for optimizing these applications. The measured magnetization of MNPs can be significantly lower than bulk counterparts, often due to canted spins. This has previously been presumed to be a surface effect, where reduced exchange allows spins closest to the nanoparticle surface to deviate locally from collinear structures. We demonstrate that intraparticle effects can induce spin canting throughout a MNP via the Dzyaloshinskii-Moriya interaction (DMI). We study ~7.4 nm diameter, core/shell Fe 3 O 4 /Mn x Fe 3-x O 4 MNPs with a 0.5 nm Mn-ferrite shell. Mössbauer spectroscopy, x-ray absorption spectroscopy and x-ray magnetic circular dichroism are used to determine chemical structure of core and shell. Polarized small angle neutron scattering shows parallel and perpendicular magnetic correlations, suggesting multiparticle coherent spin canting in an applied field. Atomistic simulations reveal the underlying mechanism of the observed spin canting. These show that strong DMI can lead to magnetic frustration within the shell and cause canting of the net particle moment. These results illuminate how core/shell nanoparticle systems can be engineered for spin canting across the whole of the particle, rather than solely at the surface.