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Considerable attention has been paid recently to $\mathrm{Fe}{\mathrm{Te}}_{2}{\mathrm{O}}_{5}\mathrm{Cl}$ due to reduced dimensionality and frustration in the magnetic subsystem, succession of phase transitions, and multiferroicity. The efforts to grow its selenite sibling resulted in the mixed halide compound $\mathrm{Fe}({\mathrm{Te}}_{1.5}{\mathrm{Se}}_{0.5}){\mathrm{O}}_{5}\mathrm{Cl}$, which was found crystallizing in a different structural type and possessing properties drastically different from those of a parent system. Its magnetic subsystem features weakly coupled ${\mathrm{Fe}}^{3+}\ensuremath{-}{\mathrm{Fe}}^{3+}$ dimers showing the regime of short-range correlations at ${T}_{\mathrm{M}}\ensuremath{\sim}70\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and long-range order at ${T}_{\mathrm{N}}=22\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. In a magnetically ordered state, sizable spin-orbital interactions lead to a small canting of ${\mathrm{Fe}}^{3+}$ moments. Magnetic dipole-dipole interactions contribute significantly to the experimentally observed orientation of magnetization easy axis in the $ac$ plane. The first principles calculations of leading exchange interactions were found in agreement with measurements of thermodynamic properties and Raman spectroscopy.

The manganese tellurate ${\mathrm{Li}}_{2}{\mathrm{MnTeO}}_{6}$ consists of trigonal spin lattices made up of ${\mathrm{Mn}}^{4+}$ (${d}^{3}$, $S=3/2$) ions. The magnetic properties of this compound were characterized by several experimental techniques, which include magnetic susceptibility, specific-heat, dielectric permittivity, electron-spin-resonance, nuclear magnetic resonance (NMR), and neutron powder-diffraction measurements, and by density functional theory calculations. The magnetic susceptibility $\ensuremath{\chi}(T)$ demonstrates very unusual behavior. It is described by the Curie-Weiss law at high temperature with Curie-Weiss temperature of $\mathrm{\ensuremath{\Theta}}=\ensuremath{-}74\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and exhibits no obvious anomaly indicative of a long-range magnetic ordering at low magnetic fields. At high magnetic fields, however, the character of $\ensuremath{\chi}(T)$ changes showing a maximum at about 9 K. That this maximum of $\ensuremath{\chi}(T)$ reflects the onset of an antiferromagnetic order was confirmed by specific-heat measurements, which exhibit a clear $\ensuremath{\lambda}$-type anomaly at ${T}_{\mathrm{N}}\ensuremath{\approx}8.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ even at zero magnetic field, and by $^{7}\mathrm{Li}$ NMR and dielectric permittivity measurements. The magnetic structure of ${\mathrm{Li}}_{2}{\mathrm{MnTeO}}_{6}$, determined by neutron powder-diffraction measurements at 1.6 K, is described by the ${120}^{\ensuremath{\circ}}$ noncollinear spin structure with the propagation vector $\mathbit{k}=(1/3,\phantom{\rule{0.28em}{0ex}}1/3,\phantom{\rule{0.28em}{0ex}}0)$. Consistent with this finding, the spin-exchange interactions evaluated for ${\mathrm{Li}}_{2}{\mathrm{MnTeO}}_{6}$ by density functional calculations are dominated by the nearest-neighbor antiferromagnetic exchange within each triangular spin lattice. This spin lattice is strongly spin frustrated with $f=|\mathrm{\ensuremath{\Theta}}|/{T}_{\mathrm{N}}\ensuremath{\approx}8$ and exhibits a two-dimensional magnetic character in a broad temperature range above ${T}_{\mathrm{N}}$.

An orthorhombic compound, $\mathrm{NaMnSb}{\mathrm{O}}_{4}$, represents a square net of magnetic $\mathrm{M}{\mathrm{n}}^{2+}$ ions residing in vertex-shared oxygen octahedra. Its static and dynamic magnetic properties were studied using magnetic susceptibility, specific heat, magnetization, electron spin resonance (ESR), nuclear magnetic resonance (NMR), and density-functional calculations. Thermodynamic data indicate an establishment of the long-range magnetic order with ${T}_{\mathrm{N}}\ensuremath{\sim}44\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, which is preceded by a short-range one at about 55 K. In addition, a nontrivial wasp-waisted hysteresis loop of the magnetization was observed, indicating that the ground state is most probably canted antiferromagnetic. Temperature dependence of the magnetic susceptibility is described reasonably well in the framework of two-dimensional square-lattice model with the main exchange parameter $J=\ensuremath{-}5.3\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, which is in good agreement with density-functional analysis, NMR, and ESR data.

We report that $\mathrm{C}{\mathrm{u}}_{9}{\mathrm{O}}_{2}{(\mathrm{Se}{\mathrm{O}}_{3})}_{4}\mathrm{C}{\mathrm{l}}_{6}$ is a new multiferroic compound. Comprehensive studies of this compound have been carried out on single crystals as well as polycrystalline samples. Magnetic susceptibility $\ensuremath{\chi}$ and specific heat $C$ with $H\ensuremath{\parallel}c$ axis both reveal an anomaly at ${T}_{N}=37\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ associated with the long-range antiferromagnetic order. However, no signature of this phase transition is seen with the $H\ensuremath{\parallel}b$ axis, providing evidence of the anisotropic nature of its magnetic properties. The results of $\ensuremath{\chi}$ measurements are consistent with the temperature evolution of the intensity of magnetic reflections from the neutron diffraction experiments. The magnetic structure derived from neutron scattering measurements shows that half of the Cu(5) ions carry no moments. Furthermore, an anomaly in the dielectric constant near ${T}_{N}$ and observable magnetoelectric coupling below ${T}_{N}$ are found. At high temperatures, a dielectric peak at ${T}_{E}=267\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ is identified with no measurable spontaneous polarization below ${T}_{E}$, thereby suggesting an antiferroelectric order. A steplike anomaly seen in $\ensuremath{\chi}(T)$ at ${T}_{E}$ hints the weak spin-lattice coupling. Concomitantly, high-resolution temperature-dependent synchrotron x-ray diffraction experiments elucidate a local structural distortion at ${T}_{E}$.

We report studies of the Seebeck coefficient $S$ of the quasi-one-dimensional compound $\mathrm{Nb}{\mathrm{S}}_{3}$, together with the temperature dependence of its specific conductivity ${\ensuremath{\sigma}}_{\mathrm{s}}$ and heat capacity ${c}_{p}$. The monoclinic phase $(\mathrm{Nb}{\mathrm{S}}_{3}$-II) is studied over the temperature range $T=80--400$ K, which covers two charge density wave (CDW) transitions at ${T}_{\mathrm{P}1}=360$ K (CDW-1) and ${T}_{\mathrm{P}2}=150$ K (CDW-2). The $S(T)$ curves show features in the vicinities of both CDW transitions and appear to be correlated with the value of ${\ensuremath{\sigma}}_{\mathrm{s}}(300\phantom{\rule{0.28em}{0ex}}\mathrm{K})$: The increase of $S$ below ${T}_{\mathrm{P}1}$ in the high-Ohmic samples reveals a complete dielectrization of the electronic spectrum, while in the low-Ohmic samples $S$ decreases below ${T}_{\mathrm{P}1}$ and even becomes negative below ${T}_{\mathrm{P}2}$. The magnitude of $S$ in low-Ohmic samples at $Tl{T}_{\mathrm{P}2}$ is well below ${k}_{\mathrm{B}}/e\ensuremath{\approx}86\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{V}/\mathrm{K},{k}_{B}$ being the Boltzmann constant and $e$ the elementary charge, which is surprisingly low for a usual CDW semiconducting state. Our results suggest that at ${T}_{\mathrm{P}1}$ the main electronic band with $p$-type carriers becomes gapped, while some $n$-type carriers can remain in a separate band with low density of states. These carriers, whose concentration is defined by the compositional doping, are gapped at ${T}_{\mathrm{P}2}$. We also report $S$ for the triclinic dielectric phase of $\mathrm{Nb}{\mathrm{S}}_{3}\phantom{\rule{4pt}{0ex}}(\mathrm{Nb}{\mathrm{S}}_{3}\text{\ensuremath{-}}\mathrm{I})$. Its low absolute value, $\ensuremath{\sim}{k}_{\mathrm{B}}/e$, and the anomalous temperature dependence demonstrate that $\mathrm{Nb}{\mathrm{S}}_{3}\text{\ensuremath{-}}\mathrm{I}$ is neither a semiconductor nor a CDW conductor in the usual sense.

We report the evolution of charge density wave states under pressure for two NbS3 phases: triclinic (phase I) and monoclinic (phase II) at room temperature. Raman and x-ray diffraction (XRD) techni ...

${\mathrm{Fe}}^{3+} S=5/2$ ions form saw-tooth-like chains along the $a$ axis of the oxoselenite ${\mathrm{Fe}}_{2}\mathrm{O}{({\mathrm{SeO}}_{3})}_{2}$ and an onset of long-range magnetic order is observed for temperatures below ${T}_{C}=105$ K. This order leads to distinct fingerprints in phonon mode linewidths and energies as resolved by Raman scattering. In addition, new excitations with small linewidths emerge below $T=150$ K, and are assigned to two-magnon scattering processes with the participation of flat-band and high energy magnon branches. From this a set of exchange coupling constants is estimated. The specific ratio of the saw-tooth spine-spine and spine-vertex interactions may explain the instability of the dimer quantum ground state against an incommensurate 3D magnetic order.

Using Wannier function formulation and total energy calculations by first-principles density functional theory (DFT), we derive the underlying spin model of a recently synthesized compound, ${\mathrm{Cs}}_{2}{\mathrm{CuAl}}_{4}{\mathrm{O}}_{8}$, having zeolitelike network structure. The computed magnetic interactions show that the interchain Cu-Cu interactions are negligibly small compared to intrachain Cu-Cu interactions, thus characterizing ${\mathrm{Cs}}_{2}{\mathrm{CuAl}}_{4}{\mathrm{O}}_{8}$ as a prototypical one-dimensional (1D) spin-$\frac{1}{2}$ system. Interestingly, the DFT-derived 1D spin model features a combination of alternating ferromagnetic-antiferromagnetic interactions, together with the presence of both nearest- and next-nearest-neighbor interactions, making it an unprecedented case. The solution of the derived spin model of the compound using the quantum Monte Carlo technique shows reasonably good agreement with the experimental susceptibility data, measured in the presence of the magnetic field. The presence of spin gap is suggested by quantum Monte Carlo simulations in the zero-field condition, which is cross-checked by a more rigorous exact diagonalization study. Motivated by the intricacy of the derived spin model, we further examine the ground-state properties of this model in the parameter space of exchange interactions, which shows the possibility of driving quantum phase transition between gapped and gapless spin excitation. Our study is expected to shed light on the fascinating world of 1D quantum spin systems.

We investigated transport and magnetic properties of single crystal ${\mathrm{NdFeAsO}}_{0.9}{\mathrm{F}}_{0.1}$ under hydrostatic pressures up to 50 GPa. The ambient pressure superconductivity at ${T}_{c}\ensuremath{\sim}$ 45.4 K was fully suppressed at ${P}_{c}\ensuremath{\sim}21$ GPa. Upon a further increase of pressure, ferromagnetism associated with the order of the rare-earth subsystem was induced at the border of superconductivity. Our finding is supported by the hysteresis in the magnetization $M(H$) loops and the strong increase in the field cooled data $M(T$) toward low temperatures. We also show that the temperature evolution of the electrical resistivity as a function of pressure is consistent with a crossover from a Fermi liquid to non-Fermi liquid to Fermi liquid. The Hall measurements suggest that the multiband electronic structures have changed with pressure, which should also affect the resistivity behavior. These results give access to the high-pressure side of the superconducting phase diagram in the 1111 type of materials.

Static and dynamic magnetic properties of P$\overline{3}$1$m$-phase ${\mathrm{MnSb}}_{2}{\mathrm{O}}_{6}$ have been studied by means of muon-spin relaxation ($\ensuremath{\mu}\mathrm{SR}$), high-frequency electron-spin resonance (HF-ESR), specific heat, and magnetization studies in magnetic fields up to 25 T. The data imply onset of long-range antiferromagnetic order at ${T}_{\mathrm{N}}$ = 8 K and a spin-flop-like transition at ${B}_{\mathrm{SF}}$ $\ensuremath{\approx}$ 0.7--1 T. Below ${T}_{\mathrm{N}}$, muon asymmetry exhibits well-defined oscillations indicating a narrow distribution of the local fields. A competing antiferromagnetic phase appearing below ${T}_{\mathrm{N}2}$ = 5.3 K is evidenced by a step in the magnetization and a slight kink of the relaxation rate. Above ${T}_{\mathrm{N}}$, both $\ensuremath{\mu}\mathrm{SR}$ and HF-ESR data suggest short-range spin order. HF-ESR data show that local magnetic fields persist up to at least $12{T}_{\mathrm{N}}\ensuremath{\approx}100$ K. Analysis of the antiferromagnetic resonance modes and the thermodynamic spin-flop field suggest zero-field splitting of $\mathrm{\ensuremath{\Delta}}\ensuremath{\approx}18$ GHz which implies small but finite magnetic anisotropy.

We report on the Andreev spectroscopy and specific heat of high-quality single crystals of ${\mathrm{BaFe}}_{1.9}{\mathrm{Ni}}_{0.1}{\mathrm{As}}_{2}$. The intrinsic multiple Andreev reflection spectroscopy reveals two anisotropic superconducting gaps ${\mathrm{\ensuremath{\Delta}}}_{L}\ensuremath{\approx}3.2\ensuremath{-}4.5\phantom{\rule{4pt}{0ex}}\mathrm{meV}$, ${\mathrm{\ensuremath{\Delta}}}_{S}\ensuremath{\approx}1.2\ensuremath{-}1.6\phantom{\rule{4pt}{0ex}}\mathrm{meV}$ (the ranges correspond to the minimum and maximum value of the coupling energy in the ${k}_{x}{k}_{y}$ plane). The $25%\ensuremath{-}30%$ anisotropy shows the absence of nodes in the superconducting gaps. Using a two-band model with $s$-wave-like gaps ${\mathrm{\ensuremath{\Delta}}}_{L}\ensuremath{\approx}3.2\phantom{\rule{4pt}{0ex}}\mathrm{meV}$ and ${\mathrm{\ensuremath{\Delta}}}_{S}\ensuremath{\approx}1.6\phantom{\rule{4pt}{0ex}}\mathrm{meV}$, the temperature dependence of the electronic specific heat can be well described. A linear magnetic field dependence of the low-temperature specific heat offers further support of $s$-wave type of the order parameter. We find that a $d$-wave or single-gap BCS theory under the weak-coupling approach cannot describe our experiments.

${\mathrm{Rb}}_{3}{\mathrm{Ni}}_{2}{({\mathrm{NO}}_{3})}_{7}$ was obtained by crystallization from anhydrous nitric acid solution of rubidium nitrate and nickel nitrate hexahydrate. The crystal structure determined on single crystals implies isolated spin-1 two-leg ladders of ${\mathrm{Ni}}^{2+}$ ions connected by (${\mathrm{NO}}_{3}{)}^{\ensuremath{-}}$ groups as basic elements. Magnetic susceptibility, specific heat in magnetic fields up to 9 T, magnetization, and high-frequency electron spin resonance studies performed on powder samples show the absence of long-range magnetic order at $T\ensuremath{\ge}2$ K. Electronic structure calculations and the detailed analysis of the experimental data enable quantitative estimates of the relevant parameters of the $S=1$ ladders in ${\mathrm{Rb}}_{3}{\mathrm{Ni}}_{2}{({\mathrm{NO}}_{3})}_{7}$. The rung coupling ${J}_{1}=10.16$ K, the leg coupling ${J}_{2}=1.5$ K, and the Ising-type anisotropy $|A|=8.6$ K are obtained. The scenario of a valence-bond solidlike quantum ground state realized in the two-leg ${\mathrm{Ni}}^{2+}$ ladders is further corroborated by model simulations of the magnetic susceptibility.

We report on magneto-optical imaging and the temperature dependency of the upper critical fields ${H}_{c2}^{c}(T)$ parallel to the $c$ axis and ${H}_{c2}^{ab}(T)$ parallel to the $ab$ plane in ${\mathrm{Ba}}_{2}{\mathrm{Ti}}_{2}{\mathrm{Fe}}_{2}{\mathrm{As}}_{4}\mathrm{O}$ single crystals. These data were inferred from the measurements of the temperature-dependent resistance in static magnetic fields up to 14 T and magnetoresistance in pulsed fields up to 60 T. ${H}_{c2}$ values are found to be 52 and 50 T for $H\ensuremath{\parallel}ab$ and $H\ensuremath{\parallel}c$, respectively. These values are 1.2--1.35 times larger than the weak-coupling Pauli paramagnetic limit $({H}_{p}\ensuremath{\sim}1.84{T}_{c})$, indicating that enhanced paramagnetic limiting is essential and this superconductor is unconventional. Our observations of strong bending in the ${H}_{c2}^{ab}(T)$ curves and a nearly isotropic maximum upper critical field ${H}_{c2}^{ab}(0)\phantom{\rule{0.16em}{0ex}}\ensuremath{\approx}\phantom{\rule{0.16em}{0ex}}{H}_{c2}^{c}(0)$ support the presence of a strong Pauli paramagnetic effect. We show that the Werthamer-Helfand-Hohenberg (WHH) formula that includes the spin-orbit scattering can effectively describe the ${H}_{c2}^{ab}(T)$ curve, whereas ${H}_{c2}$ deviates from the conventional WHH theoretical model without considering the spin paramagnetic effect for the $H\ensuremath{\parallel}c$ and $H\ensuremath{\parallel}ab$ directions. For $H\ensuremath{\parallel}c$, a two-band model is required to fully reproduce the behavior of ${H}_{c2}$, while for $H\ensuremath{\parallel}ab$ the spin paramagnetic effect is responsible for the behavior of ${H}_{c2}$. The anisotropy of ${H}_{c2}$ is close to 3 near ${T}_{c}$ and decreases rapidly at lower temperatures.

We present the synthesis and the experimental and theoretical study of the new member of the francisite family, $\mathrm{C}{\mathrm{u}}_{3}\mathrm{Yb}{(\mathrm{Se}{\mathrm{O}}_{3})}_{2}{\mathrm{O}}_{2}\mathrm{Cl}$. The compound reaches an antiferromagnetic order at ${T}_{\mathrm{N}}=36.7\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and experiences first-order spin-reorientation transition to weakly ferromagnetic phase at ${T}_{\mathrm{R}}=8.7\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ evidenced in specific heat ${C}_{\mathrm{p}}$ and magnetic susceptibility \ensuremath{\chi} measurements. Distinctly different magnetization loops in $Tl{T}_{\mathrm{R}}$ and ${T}_{\mathrm{R}}lTl{T}_{\mathrm{N}}$ temperature ranges reflect the interplay of rare-earth and transition-metal subsystems. At low temperatures, the saturation magnetization ${M}_{\mathrm{s}}\ensuremath{\sim}5.2\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$ is reached in pulsed magnetic-field measurements. The electron spin resonance data reveal the complicated character of the absorption line attributed to response from both copper and ytterbium ions. Critical broadening of the linewidth at the phase transitions points to quasi-two-dimensional character of the magnetic correlations. The spectroscopy of $\mathrm{Y}{\mathrm{b}}^{3+}$ ions evidences splitting of the lowest-energy Kramers doublet of ${}^{2}{F}_{5/2}$ excited multiplet at ${T}_{\mathrm{R}}lTl{T}_{\mathrm{N}}$ while the ground Kramers doublet splits only at $Tl{T}_{\mathrm{R}}$. We describe the magnetic properties both above and below the spin-reorientation transition in the framework of a unified approach based on the mean-field approximation and crystal-field calculations.

Specific heat $C$, thermal conductivity \ensuremath{\kappa}, dielectric permittivity \ensuremath{\epsilon}, electric polarization $P$, and Raman scattering experiments are performed on $\mathrm{C}{\mathrm{u}}_{3}\mathrm{Bi}{(\mathrm{Se}{\mathrm{O}}_{3})}_{2}{\mathrm{O}}_{2}X$ ($X=\mathrm{Br},\phantom{\rule{0.28em}{0ex}}\mathrm{Cl}$) single crystals. The Cl compound undergoes a structural phase transition at ${T}^{*}\ensuremath{\sim}115\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ evident in $C(T)$, $\ensuremath{\varepsilon}(T)$, and $\ensuremath{\kappa}(T)$ and accompanied by the appearance of unique phonon lines in Raman scattering. No evident structural changes are detected in the Br compound. At $Tl{T}^{*}$, a very weak polarization loop with a $P\ensuremath{\parallel}c$ axis is observed in the Cl compound. Both compounds order antiferromagnetically at comparable temperatures ${T}_{\mathrm{N}}\ensuremath{\sim}25\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ marked by sharp \ensuremath{\lambda}-type singularities. For $Tl{T}_{\mathrm{N}}$, an intensive mode of magnetic origin appears in both compounds. At the lowest temperatures, the energy of this mode is in good agreement with both the magnon excitation observed in infrared spectroscopy and the spin gap found recently in inelastic neutron scattering of the Cl compound.

We present a resonant x-ray diffraction study of the magnetic order in NdFe$_3$(BO$_3$)$_4$ and its coupling to applied electric fields. Our high-resolution measurements reveal A coexistence of two different magnetic phases, which can be triggered effectively by external electric fields. More in detail, the volume fraction of the collinear magnetic phase is found to strongly increase at the expense of helically ordered regions when an electric field is applied. These results confirm that the collinear magnetic phase is responsible for the ferroelectric polarization of NdFe$_3$(BO$_3$)$_4$ and, more importantly, demonstrate that magnetic phase coexistence provides an alternative route towards materials with a strong magnetoelectric response.

In this combined experimental and theoretical paper, we study the properties of ${\mathrm{Cu}}_{3}\mathrm{Sm}{({\mathrm{SeO}}_{3})}_{2}{\mathrm{O}}_{2}\mathrm{Cl}$ belonging to the francisite family of compounds, which are novel frustrated layered compounds. ${\mathrm{Cu}}_{3}\mathrm{Sm}{({\mathrm{SeO}}_{3})}_{2}{\mathrm{O}}_{2}\mathrm{Cl}$ is synthesized through a solid state reaction. Characterizations through measurements of magnetization, specific heat, X-band electron spin resonance, and rare-earth optical spectroscopy, establish that the compound orders antiferromagnetically at ${T}_{\mathrm{N}}=35\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and undergoes a spin-reorientation phase transition at ${T}_{\mathrm{C}}=8.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ due to the interplay of anisotropies in transition metal and rare-earth subsystems. The ground state Kramers doublet of Sm is found to split only at $Tl{T}_{\mathrm{C}}$ when the exchange field is aligned along the nonzero component of its $g$ factor. Prior to full saturation with ${M}_{\mathrm{s}}\ensuremath{\le}3\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$, which is reached under application of moderate magnetic field, the magnetization is found to exhibit complicated hysteresis behavior related to field-induced metamagnetic phase transitions. The first principles calculations reveal the hierarchy of exchange interaction parameters in the kagome-type magnetic subsystem of ${\mathrm{Cu}}_{3}\mathrm{Sm}{({\mathrm{SeO}}_{3})}_{2}{\mathrm{O}}_{2}\mathrm{Cl}$.

We present scanning tunneling microscopy and spectroscopy measurements on ${\mathrm{FeSe}}_{1\ensuremath{-}x}{\mathrm{S}}_{x}$ single crystals with $x=0$, $0.04$, and $0.09$. The S substitution into the Se site is equivalent to a positive chemical pressure, since S and Se have the same valence and S has a smaller ionic radius than Se. The subsequent changes in the electronic structure of FeSe induce a decrease of the structural transition temperature and a small increase in the superconducting critical temperature. The evolution of the gaps with increasing S concentration suggests an increase of the hole Fermi surface. Moreover, the vortex core anisotropy, that likely reflects the Fermi surface anisotropy, is strongly suppressed by the S substitution.

We present a comprehensive experimental and theoretical study of the electronic and magnetic properties of two quasi-two-dimensional (2D) honeycomb lattice monoclinic compounds ${A}_{3}\mathrm{N}{\mathrm{i}}_{2}\mathrm{Sb}{\mathrm{O}}_{6}\phantom{\rule{0.16em}{0ex}}(A=\mathrm{Li},\phantom{\rule{0.16em}{0ex}}\mathrm{Na})$. Magnetic susceptibility and specific heat data are consistent with the onset of antiferromagnetic long-range order with N\'eel temperatures of \ensuremath{\sim}14 and 16 K for $\mathrm{L}{\mathrm{i}}_{3}\mathrm{N}{\mathrm{i}}_{2}\mathrm{Sb}{\mathrm{O}}_{6}$ and $\mathrm{N}{\mathrm{a}}_{3}\mathrm{N}{\mathrm{i}}_{2}\mathrm{Sb}{\mathrm{O}}_{6}$, respectively. The effective magnetic moments of $4.3\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.\phantom{\rule{0.16em}{0ex}}(\mathrm{L}{\mathrm{i}}_{3}\mathrm{N}{\mathrm{i}}_{2}\mathrm{Sb}{\mathrm{O}}_{6})$ and $4.4\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.\phantom{\rule{0.16em}{0ex}}(\mathrm{N}{\mathrm{a}}_{3}\mathrm{N}{\mathrm{i}}_{2}\mathrm{Sb}{\mathrm{O}}_{6})$, where f.u. is formula units, indicate that $\mathrm{N}{\mathrm{i}}^{2+}$ is in a high-spin configuration ($S=1$). The temperature dependence of the inverse magnetic susceptibility follows the Curie-Weiss law in the high-temperature region and shows positive values of the Weiss temperature, $\ensuremath{\sim}8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ ($\mathrm{L}{\mathrm{i}}_{3}\mathrm{N}{\mathrm{i}}_{2}\mathrm{Sb}{\mathrm{O}}_{6}$) and \ensuremath{\sim}12 K ($\mathrm{N}{\mathrm{a}}_{3}\mathrm{N}{\mathrm{i}}_{2}\mathrm{Sb}{\mathrm{O}}_{6}$), pointing to the presence of nonnegligible ferromagnetic interactions, although the system orders antiferromagnetically at low temperatures. In addition, the magnetization curves reveal a field-induced (spin-flop type) transition below ${T}_{\mathrm{N}}$ that can be related to the magnetocrystalline anisotropy in these systems. These observations are in agreement with density functional theory calculations, which show that both antiferromagnetic and ferromagnetic intralayer spin exchange couplings between $\mathrm{N}{\mathrm{i}}^{2+}$ ions are present in the honeycomb planes, supporting a zigzag antiferromagnetic ground state. Based on our experimental measurements and theoretical calculations, we propose magnetic phase diagrams for the two compounds.

The synthetic mineral urusovite $\mathrm{CuAl}(\mathrm{As}{\mathrm{O}}_{4})\mathrm{O}$ was prepared through the wet chemistry route and characterized over a wide temperature range in terms of studies of magnetization, specific heat, and X-band electron spin resonance. The basic structural units of the compound are distorted square pyramids $\mathrm{Cu}{\mathrm{O}}_{5}$ assembled into corrugated honeycomb layers separated by $\mathrm{As}{\mathrm{O}}_{4}$ and $\mathrm{Al}{\mathrm{O}}_{4}$ tetrahedrons. Both thermodynamic and resonant measurements indicate that $\mathrm{CuAl}(\mathrm{As}{\mathrm{O}}_{4})\mathrm{O}$ is a spin gap system with a gap of $\ensuremath{\sim}350\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. The electronic structure calculations performed within the framework of density functional theory suggest a weakly interacting dimer model with antiferromagnetic signs for both intradimer and interdimer superexchange interactions. This establishes the valence bond solid as the quantum ground state of the title compound. The pronounced discrepancy between experimental data and calculations within the weakly interacting dimer model at elevated temperatures is ascribed in part to the steep increase of the intradimer exchange interaction parameter driven by the thermal expansion effects.

We report a detailed study of the magnetic properties of the buckled kagome compound ${\mathrm{Cu}}_{3}{\mathrm{Y}(\mathrm{SeO}}_{3}{)}_{2}{\mathrm{O}}_{2}\mathrm{Cl}$ using heat capacity, magnetization, powder neutron diffraction, electron spin resonance, and first-principles calculations. The crystal structure is confirmed to be isotypic with the mineral francisite, with orthorhombic space group symmetry Pmmn throughout the temperature range 5--300 K. Magnetization, heat capacity, and neutron diffraction confirm long range magnetic order below ${T}_{N}=35$ K. The electron spin resonance spectra reveal the presence of two modes corresponding to two different crystallographic Cu positions. The principal $g$ values of the $g$ tensor of Cu1 sites were found to be ${g}_{1}=2.18(4),{g}_{2}=2.10(6)$, and ${g}_{3}$ = 2.05(9), while the effective $g$ factor of Cu2 sites is almost isotropic and is on average $g=2.09(5)$. At low temperatures, ${\mathrm{Cu}}_{3}{\mathrm{Y}(\mathrm{SeO}}_{3}{)}_{2}{\mathrm{O}}_{2}\mathrm{Cl}$ undergoes a metamagnetic transition, with a critical field ${B}_{C}=2.6$ T at 2 K, due to the suppression of the interplane exchange interactions and saturates in modest magnetic field ${B}_{S}$ \ensuremath{\leqslant} 8 T. The first-principles calculations allow an estimation of both intraplane and interplane exchange interactions. The weakness of the interplane exchange interaction results in low values of the critical fields for the metamagnetic transition, while the competition between intraplane exchange interactions of different signs results in a similarly low value of the saturation field.

Both spin-liquid and magnetically ordered phases of both half-integer and integer low-spin quantum magnets are of interest, since the magnetic structures found in the latter case usually have no classical counterparts. Such a magnetic structure was found in a combined experimental and theoretical study of the integer spin system $\mathrm{Ni}{(\mathrm{NO}}_{3}{)}_{2}$. Our thermodynamic measurements have revealed a magnetically ordered phase with small spontaneous magnetization at ${T}_{\mathrm{C}} = 5.5\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. The magnetization saturation of about $2{\ensuremath{\mu}}_{\mathrm{B}}$ at low temperatures corresponds to the high-spin state ($S = 1$) of ${\mathrm{Ni}}^{2+}$ ions evidenced in ${L}_{2,3}$ edges in x-ray absorption spectroscopy spectra. We show that a consistent description of the available data is possible within a noncollinear umbrella-type ferrimagnetic ground state model for which both intra- and interlayer magnetic interactions should be antiferromagnetic. Such a scenario is suggested by the first-principles and model calculations.

It is well established that long-range magnetic order is suppressed in magnetic systems whose interactions are low dimensional. The prototypical example is the $S\ensuremath{-}\frac{1}{2}$ Heisenberg antiferromagnetic chain ($S\ensuremath{-}\frac{1}{2}$ HAFC) whose ground state is quantum critical. In real $S\ensuremath{-}\frac{1}{2}$ HAFC compounds interchain coupling induces long-range magnetic order although with a suppressed ordered moment and reduced N\'eel temperature compared to the Curie-Weiss temperature. Recently, it was suggested that order can also be suppressed if the interchain interactions are frustrated, as for the Nersesyan-Tsvelik model. Here, we study the new $S\ensuremath{-}\frac{1}{2}$ HAFC, $(\mathrm{NO})[\mathrm{Cu}({\mathrm{NO}}_{3}{)}_{3}]$. This material shows extreme suppression of order which furthermore is incommensurate revealing the presence of frustration consistent with the Nersesyan-Tsvelik model.

An antiferromagnetic ordering in Pb3TeCo3V2O14 takes place through formation of short range correlation regime with T* ~ 10.5 K and succession of second order phase transition at TN1 = 8.9 K and first order phase transition at TN2 = 6.3 K. An external magnetic field rapidly destroys magnetic structure at T < TN2 and influences the magnetic order at TN2 < T < TN1 resulting in complex magnetic phase diagram of Pb3TeCo3V2O14 as derived from magnetization and specific heat measurements. The first principles calculations indicate that in variance with layered crystal structure the magnetic subsystem of Pb3TeCo3V2O14 is quasi-one-dimensional and highly unusual consisting of weakly coupled triangular tubes.

We investigate the temperature dependence of the lower critical field Hc1(T) of a high-quality FeSe single crystal under static magnetic fields H parallel to the c axis. The temperature dependence of the first vortex penetration field has been experimentally obtained by two independent methods and the corresponding Hc1(T) was deduced by taking into account demagnetization factors. A pronounced change in the Hc1(T) curvature is observed, which is attributed to anisotopic s-wave or multiband superconductivity. The London penetration depth λab(T) calculated from the lower critical field does not follow an exponential behavior at low temperatures, as it would be expected for a fully gapped clean s-wave superconductor. Using either a two-band model with s-wave-like gaps of magnitudes � 1 = 0.41 ± 0.1 meV and � 2 = 3.33 ± 0.25 meV or a single anisotropic s-wave order parameter, the temperature dependence of the lower critical field Hc1(T) can be well described. These observations clearly show that the superconducting energy gap in FeSe is nodeless.

Polarized Raman-scattering spectra of superconducting, single-crystalline FeSe evidence pronounced phonon anomalies with temperature reduction. A large 6.5% hardening of the B_1g(Fe) phonon mode is attributed to the suppression of local fluctuations of the iron spin state with the gradual decrease of the iron paramagnetic moment. The ab-initio lattice dynamic calculations support this conclusion. The enhancement of the low-frequency spectral weight above the structural phase transition temperature T_s and its change below T_s is discussed in relation with the opening of an energy gap between low (S=0) and higher spin states which prevents magnetic order in FeSe. The very narrow phonon line widths compared to observations in FeTe suggests the absence of intermediate spin states in the fluctuating spin state manifold in FeSe., 9 pages, 5 figures

A. Vasiliev,1 O. Volkova,1 E. Zvereva,1 M. Isobe,2 Y. Ueda,2 S. Yoshii,3 H. Nojiri,3 V. Mazurenko,4 M. Valentyuk,4,5 V. Anisimov,4,6 I. Solovyev,4,7 R. Klingeler,8 and B. Buchner9 1Low Temperature Physics and Superconductivity Department, Lomonosov Moscow State University, 119991 Moscow, Russia 2Materials Design and Characterization Laboratory, Institute for Solid State Physics, University of Tokyo, Kashiwa 277-8581, Japan 3Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan 4Theoretical Physics and Applied Mathematics Department, Ural Federal University, 620002 Ekaterinburg, Russia 5Institute for Theoretical Physics, University of Hamburg, 20355 Hamburg, Germany 6Institute of Metal Physics, Russian Academy of Sciences, 620990 Ekaterinburg, Russia 7Computational Materials Science Unit, National Institute for Materials Science, Tsukuba 305-0047, Japan 8Kirchoff Institute for Physics, Heidelberg University, 69120 Heidelberg, Germany 9Leibniz Institute for Solid State and Materials Research, IFW Dresden, 01171 Dresden, Germany (Received 11 October 2012; revised manuscript received 25 March 2013; published 11 April 2013)

The low-dimensional s=1/2 compound (NO)[Cu(NO3)3] has recently been suggested to follow the Nersesyan-Tsvelik model of coupled spin chains. Such a system shows unbound spinon excitations and a resonating valence bond ground state due spin frustration. Our Raman scattering study demonstrates phonon anomalies as well as the suppression of a broad magnetic scattering continuum for temperatures below a characteristic temperature, T, Comment: 5 pages, 6 figures

We present the thermodynamic properties and the magnetic phase diagram of Gd${}_{2}$BaNiO${}_{5}$ investigated by means of specific heat and magnetization measurements performed along the main crystallographic axes of the single crystal. Our data imply antiferromagnetic ordering at ${T}_{\mathrm{N}}$ = 55 K and a spontaneous spin-reorientation transition at ${T}_{\mathit{SR}}$ = 24 K. Upon application of external magnetic fields, a spin-flop transition is observed at $T$ ${T}_{\mathit{SR}}$ in magnetic fields $B$ \ensuremath{\parallel} $b$ axis and at ${T}_{\mathit{SR}}$ $T$ ${T}_{\mathrm{N}}$ in magnetic fields $B$ \ensuremath{\parallel} $a$ axis. The magnetic phase diagram in Gd${}_{2}$BaNiO${}_{5}$ is established from these data. Considering Gd-Gd and Gd-Ni interactions allows estimating the magnetic contribution of the gadolinium subsystem to the magnetization and the specific heat. The Gd-Gd interaction is organized via Ni chains being therefore sensitive to the angle between Gd and Ni magnetic moments. The analysis of the experimental data shows that the behavior of the nickel subsystem differs from that of a classical antiferromagnet.

The Ba${}_{3}$Cu${}_{3}$In${}_{4}$O${}_{12}$ stands for unique topology of the magnetic subsystem. It consists of rotated by 90\ifmmode^\circ\else\textdegree\fi{} relative to each other ``paper-chain'' columns made of vertex-sharing Cu${}^{\mathrm{I}}$O${}_{4}$ and Cu${}^{\mathrm{II}}$O${}_{4}$ planar units. The overall pattern of the copper ions is that of a three-dimensional Shastry-Sutherland network. At high temperatures, the magnetic susceptibility follows the Curie-Weiss law with positive Weiss temperature indicating strong predominance of ferromagnetic coupling. At low temperatures, however, this compound exhibits a long-range antiferromagnetically ordered state that reaches saturation magnetization by a nontrivial succession of two spin-flop and two spin-flip transitions already in modest magnetic fields. We show that the ground state in Ba${}_{3}$Cu${}_{3}$In${}_{4}$O${}_{12}$ may be a three-dimensional orthogonal arrangement of the Cu${}^{2+}$ (S $=$ 1/2) magnetic moments forming three virtually independent antiferromagnetic subsystems. In this arrangement, favored by anisotropic exchange interactions, the quantum fluctuations provide the coupling between three mutually orthogonal magnetic subsystems resulting in an impressive ``order by disorder'' effect.

The chromium(II) orthophosphate $\ensuremath{\alpha}$-Cr${}_{3}$(PO${}_{4}$)${}_{2}$ is a weak ferrimagnet with the Curie temperature ${T}_{C}$ $=$ 29 K confirmed by a $\ensuremath{\lambda}$-type peak in specific heat. Dominant antiferromagnetic interactions in this system are characterized by the Weiss temperature $\ensuremath{\Theta}$ $=$ 96 K, indicating an intermediate frustration ratio $|$$\ensuremath{\Theta}$$|$/${T}_{C}$ \ensuremath{\sim} 3. In its magnetically ordered states $\ensuremath{\alpha}$-Cr${}_{3}$(PO${}_{4}$)${}_{2}$ exhibits a remarkable sequence of temperature-induced magnetization reversals sensitive to the protocol of measurements, i.e., either field-cooled or zero-field-cooled regimes. The reduction of the effective magnetic moment 4.5 $\ensuremath{\mu}$${}_{\mathrm{B}}$/Cr${}^{2+}$, as compared to the spin-only moment 4.9 $\ensuremath{\mu}$${}_{\mathrm{B}}$/Cr${}^{2+}$, cannot be ascribed to the occurrence of the low-spin state in any crystallographic site of the Jahn-Teller active 3${d}^{4}$ Cr${}^{2+}$ ions. X-ray absorption spectra at the $K$ edge indicate divalent chromium and unravel the high-spin state of these ions at the ${L}_{2,3}$ edges. Weak ferrimagnetism and multiple magnetization reversal phenomena seen in this compound could be ascribed to incomplete cancellation and distortion of partial spontaneous magnetization functions of Cr${}^{2+}$ in its six crystallographically inequivalent positions.

The double manganite CaCu${}_{3}$Mn${}_{4}$O${}_{12}$ doped with ${}^{119}$Sn atoms (\ensuremath{\sim}1 at.$%$ with respect to manganese atoms) was studied by use of M\"ossbauer spectroscopy. Formally tetravalent Sn${}^{4+}$ ions substitute for isovalent manganese ions in the octahedral (Mn${}^{4+}$O${}_{6}$) polyhedra. The covalency effects on the magnetic interactions like superexchange in Cu${}^{2+}$-O-Mn${}^{4+}$ and Mn${}^{4+}$-O-Mn${}^{4+}$ bonds and supertransferred hyperfine interactions of the ${}^{119}$Sn probe atoms in the manganite structure are discussed. Using a semiquantitative nearest-neighbor cluster model relating the hyperfine magnetic field on the ${}^{119}$Sn nuclei (${H}_{\mathrm{Sn}}$ $=$ 105 kOe at $T$ $=$ 77 K) to covalency parameters and angle characterizing the Sn-O-$M$ ($M$ $=$ Cu, Mn) bonds, it has been shown how such an analysis of supertransferred hyperfine interactions of tin probe ions can get fruitful information about strength and sign of the superexchange interactions between Mn${}^{4+}$ and Cu${}^{2+}$ magnetic ions. A consistent description of the results in the framework of the Weiss molecular field model considering the specific local environment of tin atoms has made it possible to estimate exchange integrals: ${J}_{\mathrm{CuMn}}$ $=$ \ensuremath{-}51.1 \ifmmode\pm\else\textpm\fi{} 0.3 K and ${J}_{\mathrm{MnMn}}$ $=$ \ensuremath{-}0.6 \ifmmode\pm\else\textpm\fi{} 0.2 K.

In this work, we study the interplay between the crystal structure and magnetism of the pyroarsenate $\ensuremath{\alpha}$-Cu${}_{2}$As${}_{2}$O${}_{7}$ by means of magnetization, heat capacity, electron spin resonance, and nuclear magnetic resonance measurements as well as density functional theory (DFT) calculations and quantum Monte Carlo (QMC) simulations. The data reveal that the magnetic Cu-O chains in the crystal structure represent a realization of a quasi-one-dimensional (1D) coupled alternating spin-1/2 Heisenberg chain model with relevant pathways through nonmagnetic AsO${}_{4}$ tetrahedra. Owing to residual 3D interactions, antiferromagnetic long range ordering at ${T}_{\mathrm{N}}\ensuremath{\simeq}10$ K takes place. Application of the external magnetic field $B$ along the magnetically easy axis induces the transition to a spin-flop phase at ${B}_{\mathrm{SF}}\ensuremath{\sim}1.7$ T (2 K). The experimental data suggest that substantial quantum spin fluctuations take place at low magnetic fields in the ordered state. DFT calculations confirm the quasi-one-dimensional nature of the spin lattice, with the leading coupling ${J}_{1}$ within the structural dimers. QMC fits to the magnetic susceptibility evaluate ${J}_{1}=164$ K, the weaker intrachain coupling ${J}_{1}^{\ensuremath{'}}/{J}_{1}=0.55$, and the effective interchain coupling ${J}_{\mathrm{ic}1}/{J}_{1}=0.20$.

Hard x-ray scattering experiments with a photon energy of 100 keV were performed as a function of temperature and applied magnetic field on selected compounds of the $R{\text{Fe}}_{3}{({\text{BO}}_{3})}_{4}$ family. The results show the presence of several diffraction features, in particular, nonresonant magnetic reflections in the magnetically ordered phase and structural reflections that violate the diffraction conditions for the low-temperature phase $P{3}_{1}21$ of the rare-earth iron borates. The temperature and field dependence of the magnetic superlattice reflections corroborate the magnetic structures of the borate compounds obtained by neutron diffraction. The detailed analysis of the intensity and scattering cross section of the magnetic reflection reveals details of the magnetic structure of these materials such as the spin domain structure of ${\text{NdFe}}_{3}{({\text{BO}}_{3})}_{4}$ and ${\text{GdFe}}_{3}{({\text{BO}}_{3})}_{4}$. Furthermore we find that the correlation length of the magnetic domains is around $100\text{ }\text{\AA{}}$ for all the compounds and that the Fe moments are rotated $53\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}3\ifmmode^\circ\else\textdegree\fi{}$ off from the hexagonal basal plane in ${\text{GdFe}}_{3}{({\text{BO}}_{3})}_{4}$.

The topology of the magnetic interactions of the copper spins in the nitrosonium nitratocuprate $(\text{NO})[\text{Cu}{({\text{NO}}_{3})}_{3}]$ suggests that it could be a realization of the Nersesyan-Tsvelik model [A. A. Nersesyan and A. M. Tsvelik, Phys. Rev. B 67, 024422 (2003)], whose ground state was argued to be either a resonating valence-bond state or a valence-bond crystal. The measurement of thermodynamic and magnetic resonance properties reveals a behavior inherent to low-dimensional spin $S=\frac{1}{2}$ systems and provides indeed no evidence for the formation of long-range magnetic order down to 1.8 K.

We have studied the magnetodielectric and magnetoelastic coupling in ${\text{TbFe}}_{3}{({\text{BO}}_{3})}_{4}$ single crystals by means of capacitance, magnetostriction, and Raman spectroscopy measurements. The data reveal strong magnetic field effects on the dielectric constant and on the macroscopic sample length which are associated to long-range magnetic ordering and a field-driven metamagnetic transition. We discuss the coupling of the dielectric, structural, and magnetic order parameters and attribute the origin of the magnetodielectric coupling to phonon mode shifts according to the Lyddane-Sachs-Teller relation.

We have investigated the magnetic and magnetoelectric properties of LiCrGe2O6 and reinvestigated the magnetic properties of LiCrSi2O6. Using superconducting quantum interference device magnetometry, electrical polarization, and neutron diffraction, we give evidence for the presence of magnetic frustration in the magnetoelectric pyroxenes LiCrX2O6 (X=Si,Ge). While pyroxene materials have been widely investigated for their low dimensional properties, we suggest that the magnetic frustration is likely to play a more important role into the nature of the magnetic ground state. The existence and possible interplay of low dimensionality and magnetic frustration resulting in multiferroic and/or magnetoelectric properties in the pyroxenes will probably open new avenues to tune and investigate the richness of the physic in this family.

We investigated the magnetic, magnetoelectric, and crystal structures of the pyroxene ${\text{NaCrSi}}_{2}{\text{O}}_{6}$ by superconducting quantum interference device (SQUID) magnetometry, electrical polarization measurement, and powder neutron diffraction. Magnetic exchange couplings extracted from magnetization measurements are found to be ${J}_{intra}/{k}_{b}=\ensuremath{-}0.48(4)\text{ }\text{K}$ and ${J}_{inter}/{k}_{b}=0.24(8)\text{ }\text{K}$. This is in perfect agreement with the antiferromagnetic order determined below ${T}_{N}=2.8(2)\text{ }\text{K}$ by neutron diffraction. Corroborating the determined magnetic structure (magnetic symmetry $\text{C-}{1}^{\ensuremath{'}}$), the magnetic field dependence of electrical polarization evidences a clear magnetoelectric effect below ${T}_{N}$. An induced magnetic field transition toward a ferromagnetic state (magnetic symmetry $\text{C}{2}^{\ensuremath{'}}/{\text{c}}^{\ensuremath{'}}$) is observed in the SQUID data and confirmed by neutron diffraction.

Low-temperature $(2\text{ }\text{K}\ensuremath{\le}T\ensuremath{\le}350\text{ }\text{K})$ heat capacity and room-temperature shear modulus measurements $(\ensuremath{\nu}=1.4\text{ }\text{MHz})$ have been performed on bulk ${\text{Pd}}_{41.25}{\text{Cu}}_{41.25}{\text{P}}_{17.5}$ in the initial glassy, relaxed glassy, and crystallized states. It has been found that the height of the low-temperature Boson heat capacity peak strongly correlates with the changes in the shear modulus upon high-temperature annealing. It is this behavior that was earlier predicted by the interstitialcy theory, according to which dumbbell interstitialcy defects are responsible for a number of thermodynamic and kinetic properties of crystalline, (supercooled) liquid, and solid glassy states.

We investigated the magnetic and structural properties of the ferromagnetic pyroxene ${\text{NaCrGe}}_{2}{\text{O}}_{6}$ by superconducting quantum interference device magnetometry and powder neutron diffraction. This material is the only ferromagnetic member of the recently reported multiferroic pyroxene family $AM{\text{X}}_{2}{\text{O}}_{6}$ ($A=\text{Li},\text{Na}$; $M=\text{Fe},\text{Cr}$; $X=\text{Si},\text{Ge}$). Below ${T}_{C}=6\text{ }\text{K}$, the magnetic structure is characterized by one-dimensional magnetic chains with spins aligned along the $c$ axis of the monoclinic cell. The magnetic moment of ${\text{Cr}}^{3+}$ is significantly reduced by about 25%. We show that this is likely the result of the low dimensionality of the system. The associated magnetic space group is $C{2}^{\ensuremath{'}}/{c}^{\ensuremath{'}}$. This symmetry does not allow a linear magnetoelectric effect. No structural phase transition was observed down to 1.8 K.

Current-voltage characteristics of Andreev contacts in ${\text{LaFeAsO}}_{0.9}{\text{F}}_{0.1}$ have been measured using the break-junction technique. The contacts were prepared at $T=4.2\text{ }\text{K}$ from polycrystalline samples and exhibit superconductor/normal conductor/superconductor-type behavior due to multiple Andreev reflections. Two sets of subharmonic gap structures were detected indicating the existence of two distinct superconducting gaps: ${\ensuremath{\Delta}}_{L}=(5.5\ifmmode\pm\else\textpm\fi{}1)\text{ }\text{meV}$ and ${\ensuremath{\Delta}}_{S}=(1\ifmmode\pm\else\textpm\fi{}0.2)\text{ }\text{meV}$.

We investigated the magnetic and crystal structures of the recent reported magnetoelectric system LiCrSi2O6 by powder neutron diffraction. Below TN = 11.5 K, an antiferromagnetic order appears. It is characterized by an antiferromagnetic coupling within the CrO6 octahedra chains and a ferromagnetic coupling between the chains. The magnetic order is commensurate with the lattice with k =0. The associated magnetic space group is P2 1 /c. This symmetry is in agreement with the reported magnetoelectric effect. We show that the magnetic frustration in this system is small. Finally, we discuss our results using a Landau phenomenological model and in the light of the literature.

A comprehensive study of the adiabatic temperature change $\ensuremath{\Delta}{T}_{\text{ad}}$ in the vicinity of a first-order magnetostructural phase transition has been carried out on a ${\text{Ni}}_{2.19}{\text{Mn}}_{0.81}\text{Ga}$ Heusler alloy. It was found that the directly measured $\ensuremath{\Delta}{T}_{\text{ad}}\ensuremath{\sim}1\text{ }\text{K}$ is of order of magnitude smaller than that expected from reported in the literature isothermal magnetic entropy change and specific-heat data. A new feature of the adiabatic temperature change in materials with giant magnetocaloric effect, specifically an irreversible character of $\ensuremath{\Delta}{T}_{\text{ad}}$ when the sample is subjected to repeatable action of magnetic field at a constant temperature, has been observed. This effect has been attributed to the irreversible magnetic-field-induced structural transformation. It has been shown that the small value of $\ensuremath{\Delta}{T}_{\text{ad}}$ in ${\text{Ni}}_{2.19}{\text{Mn}}_{0.81}\text{Ga}$ is not due to the kinetics of the transformation but originates from other factors which are intrinsic to first-order magnetic phase transitions. Relevance of these factors to other giant magnetocaloric materials has been outlined.

Ferromagnetic Heusler alloys exhibiting martensitic transformations are known to change their shape in an external magnetic field. Magnetization, electric resistance, and specific heat as a function of temperature are examined in ${\mathrm{Ni}}_{54}{\mathrm{Fe}}_{19}{\mathrm{Ga}}_{27}$ single crystal. Structural transition appears as sharp anomaly in these dependencies. This points to an avalanchelike character of martensitic transformation. The jump in resistivity at the structural phase transition and the lower density of states at the Fermi level in the martensite phase supports the hypothesis of the Jahn-Teller origin of the martensitic transformation. Magnetic measurements show that transformation to the martensitic phase is accompanied by the increase of spontaneous magnetization and an increase of magnetocrystalline anisotropy. Magnetization increase is due to different Curie temperatures of austenite and martensite. These were determined from critical behavior using Arrott plot. Additional analysis of magnetic behavior indicates ferrimagnetic ordering in this nonstoichiometric compound. Intrinsic properties of the compound are analyzed with respect to both of the actuation modes possible in magnetic shape-memory alloys. However, neither a magnetically induced martensite transformation nor a magnetically induced reorientation of variants has been observed.