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The existence of the V3+ ion orbital moment is the open issue of the nature of magnetism in the van der Waals ferromagnet VI3. The huge magnetocrystalline anisotropy in conjunction with the significantly reduced ordered magnetic moment compared to the spin-only value provides strong but indirect evidence of a significant V orbital moment. We used the unique capability of X-ray magnetic circular dichroism to determine the orbital component of the total magnetic moment and provide for the first time a direct proof of an exceptionally sizable orbital moment of the V3+ ion in VI3. Our ligand field multiplet simulations of the XMCD spectra in synergy with the results of DFT calculations agree with the existence of two V sites with different orbital occupations and therefore different OM magnitudes in the ground state.

We use the synergy of infrared, terahertz, and Raman spectroscopies with DFT calculations to shed light on the magnetic and lattice properties of VI

We use the synergy of infrared, terahertz, and Raman spectroscopies with DFT calculations to shed light on the magnetic and lattice properties of VI3. The structural transition at TS1 = 79 K is accompanied by a large splitting of polar phonon modes. Below TS1, strong ferromagnetic fluctuations are observed. The variations of phonon frequencies at 55 K induced by magnetoelastic coupling enhanced by spin-orbit interaction indicate the proximity of long-range ferromagnetic order. Below TC = 50 K, two Raman modes simultaneously appear and show dramatic softening in the narrow interval around the temperature TS2 of the second structural transition associated with the order-order magnetic phase transition. Below TS2, a magnon in the THz range appears in Raman spectra. The THz magnon observed in VI3 indicates the application potential of 2D van der Waals ferromagnets in ultrafast THz spintronics, which has previously been considered an exclusive domain of antiferromagnets., Accepted in Journal of Physical Chemistry Letters

We grew high-quality single crystals of the 5f electron van der Waals compound UI$_3$ and investigated them by measurements of specific heat and magnetization as functions of temperature and magnetic field. UI$_3$ behaves as an antiferromagnet with a first-order magnetic phase transition at the Ne\'el temperature $T_N$ = 2.65 K. It is characterized by a sharp symmetric specific-heat peak which is gradually shifted to lower temperatures by increasing magnetic field applied along either the a- or b-axis. The behavior in magnetic fields reveals orthorhombic magnetocrystalline anisotropy with the hard magnetization direction along the c-axis. The 2-K a- and b-axis magnetization curves exhibit a metamagnetic transition at the critical field $H_c$ = 3 T and 1.8 T, respectively. In higher fields, when the long-range antiferromagnetism is suppressed by a metamagnetic transition, signs of short-range magnetic ordering of antiferromagnetic correlations in the paramagnetic state show up both in specific heat and magnetization. The anomalous S-shape field dependence of a-axis magnetization well above $H_c$ can be understood as the crossover from the correlated paramagnetic regime to the high-field polarized paramagnet. The magnetic phase diagrams for the magnetic field applied along the a- and b-axis, respectively, designed using the mentioned experimental results are presented. We have also found that the UI$_3$ crystals are easily cleavable which predisposes this material for direct investigation of 5f electron magnetism in the 2D limit on exfoliated atomically thin samples., Comment: Submitted to JALCOM

Most transition-metal trihalides are dimorphic. The representative chromium-based triad, CrCl3, CrBr3, CrI3, is characterized by the low-temperature (LT) phase adopting the trigonal BiI3-type while the structure of the high-temperature (HT) phase is monoclinic of AlCl3 type (C2/m). The structural transition between the two crystallographic phases is of the first-order type with large thermal hysteresis in CrCl3 and CrI3. We studied crystal structures and structural phase transitions of vanadium-based counterparts VCl3, VBr3, and VI3 by measuring specific heat, magnetization, and x-ray diffraction as functions of temperature and observed an inverse situation. In these cases, the HT phase has a higher symmetry while the LT structure reveals a lower symmetry. The structural phase transition between them shows no measurable hysteresis in contrast to CrX3. Possible relations of the evolution of the ratio c/a of the unit cell parameters, types of crystal structures, and nature of the structural transitions in V and Cr trihalides are discussed.

We present an extensive powder and single-crystal neutron scattering investigation of the crystallographic structure and magnetic order of the frustrated metallic $f$-electron magnet $\mathrm{U}{\mathrm{Ni}}_{4}\mathrm{B}$. We carry out a full refinement of the crystallographic structure and conclude that the low-temperature lattice symmetry is orthorhombic [space group Pmm2; cell parameters: $a=6.963(4)$, $b=14.793(9)$, $c=17.126(8)$ \AA{}]. We determine the magnetically ordered structure, concluding that below ${T}_{\mathrm{N}}=19.5\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, the material undergoes a transition into a partially ordered antiferromagnetic state. The magnetic structure is consistent with the existence of toroidal order in this material. We further test the proposal of a second magnetic transition occurring at $330\phantom{\rule{0.28em}{0ex}}\mathrm{mK}$, concluding that the thermodynamic anomalies observed at these temperatures do not reflect modifications of the magnetic structure. Our study provides a consistent picture of the interrelationship of structural and magnetic properties in the frustrated magnet $\mathrm{U}{\mathrm{Ni}}_{4}\mathrm{B}$ previously unresolved.

Evolution of magnetism in single crystals of the van der Waals compound $\mathrm{V}{\mathrm{I}}_{3}$ in external pressure up to 7.3 GPa studied by measuring magnetization and ac magnetic susceptibility is reported. Four magnetic phase transitions, at ${T}_{1}=54.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}, {T}_{2}=53\phantom{\rule{0.16em}{0ex}}\mathrm{K}, {T}_{\mathrm{C}}=49.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and ${T}_{\mathrm{FM}}=26\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, respectively, have been observed at ambient pressure. The first two have been attributed to the onset of ferromagnetism in specific crystal-surface layers. The bulk ferromagnetism is characterized by the magnetic ordering transition at Curie temperature ${T}_{\mathrm{C}}$ and the transition between two different ferromagnetic phases ${T}_{\mathrm{FM}}$, accompanied by a structure transition from monoclinic to triclinic symmetry upon cooling. The pressure effects on magnetic parameters were studied with three independent techniques. ${T}_{\mathrm{C}}$ was found to be almost unaffected by pressures up to 0.6 GPa whereas ${T}_{\mathrm{FM}}$ increases rapidly with increasing pressure and reaches ${T}_{\mathrm{C}}$ at a triple point at \ensuremath{\approx} 0.85 GPa. At higher pressures, only one magnetic phase transition is observed moving to higher temperatures with increasing pressure to reach 99 K at 7.3 GPa. In contrast, the low-temperature bulk magnetization is significantly reduced by applying pressure (by more than 50% at 2.5 GPa) suggesting a possible pressure-induced reduction of vanadium magnetic moment. First-principles calculations of $\mathrm{V}{\mathrm{I}}_{3}$ under pressure allow us to ascribe the evolution of ${T}_{\mathrm{C}}$ with pressure to the reduction of interplanar distance, including the observed slope change at 0.6 GPa. These calculations also describe the associated band gap closing, showing that with a modest compression the material would become metallic. Overall, the large pressure range covered corresponds to a significant change of interplanar interactions. The obtained data thus allow us to shed light on how does the transition between the three-dimensional (3D) and quasi-2D system affect magnetic interactions in the system.

We present a detailed study of the magnetic and electronic properties of ${\mathrm{U}}_{2}{\mathrm{Rh}}_{3}{\mathrm{Si}}_{5}$, a material that has been demonstrated to exhibit a first-order antiferromagnetic phase transition. From a high-magnetic-field study, together with extensive experiments in moderate fields, we establish the magnetic phase diagrams for all crystallographic directions. The possibility of an electronic phase in a narrow interval above the N\'eel temperature as a precursor of a magnetic phase is discussed.

We present acoustic signatures of the electric quadrupolar degrees of freedom in the honeycomb-layer compound UNi$_4$B. The transverse ultrasonic mode $C_{66}$ shows softening below 30 K both in the paramagnetic phase and antiferromagnetic phases down to $\sim0.33$ K. Furthermore, we traced magnetic field-temperature phase diagrams up to 30 T and observed a highly anisotropic elastic response within the honeycomb layer. These observations strongly suggest that $\Gamma_6$(E$_{\rm 2g}$) electric quadrupolar degrees of freedom in localized $5f^2$ ($J = 4$) states are playing an important role in the magnetic toroidal dipole order and magnetic-field-induced phases of UNi$_4$B, and evidence some of the U ions remain in the paramagnetic state even if the system undergoes magnetic toroidal ordering., Comment: 6 pages, 4 figures + Supplemental Materials (11 pages, 9 figures)

Magnetization isotherms of the $5f$-electron ferromagnets URhGa, UCoGa, and $\mathrm{U}{\mathrm{Co}}_{0.98}{\mathrm{Ru}}_{0.02}\mathrm{Al}$ were measured at temperatures in the vicinity of their Curie temperature to investigate the critical behavior near the ferromagnetic phase transition. These compounds adopt the layered hexagonal ZrNiAl-type structure and exhibit huge uniaxial magnetocrystalline anisotropy. The critical \ensuremath{\beta}, \ensuremath{\gamma}, and \ensuremath{\delta} exponents were determined by analyzing Arrott-Noakes plots, Kouvel-Fisher plots, critical isotherms, scaling theory, and Widom scaling relations. The values obtained for URhGa and UCoGa can be explained by the results of the renormalization group theory for a two-dimensional (2D) Ising system with long-range interactions similar to URhAl reported by other investigators. On the other hand, the critical exponents determined for $\mathrm{U}{\mathrm{Co}}_{0.98}{\mathrm{Ru}}_{0.02}\mathrm{Al}$ are characteristic of a three-dimensional (3D) Ising ferromagnet with short-range interactions suggested in previous studies also for the itinerant $5f$-electron paramagnet UCoAl situated near a ferromagnetic transition. The change from the 2D to the 3D Ising system is related to the gradual delocalization of $5f$ electrons in the series of the URhGa, URhAl, and UCoGa to $\mathrm{U}{\mathrm{Co}}_{0.98}{\mathrm{Ru}}_{0.02}\mathrm{Al}$ and UCoAl compounds and appears close to the strongly itinerant nonmagnetic limit. This indicates possible new phenomena that may be induced by the change of dimensionality in the vicinity of the quantum critical point.

Single crystals of solid solutions of ferromagnetic superconductors UCoGe and URhGe were investigated by magnetization and specific-heat measurements. These compounds behave as anisotropic ferromagnets with the easy-magnetization direction along the orthorhombic $c$ axis. The maximum Curie temperature ${T}_{C}=\phantom{\rule{0.16em}{0ex}}20\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ has been observed in $\mathrm{UC}{\mathrm{o}}_{1\ensuremath{-}x}\mathrm{R}{\mathrm{h}}_{x}\mathrm{Ge}$ for $x=0.4$. The main interest of the study was the intriguing behavior in magnetic fields applied along the $b$ axis. The temperature dependence of the $b$-axis magnetic susceptibility exhibits a maximum at a characteristic temperature ${T}_{\mathrm{max}}$. For low Rh concentrations $(xl0.4){T}_{\mathrm{max}}g{T}_{C}$ and the magnetic entropy ${S}_{\mathrm{mag}}$ at ${T}_{C}$ is very low, whereas ${T}_{\mathrm{max}}={T}_{C}$ and the ${S}_{\mathrm{mag}}$ at ${T}_{C}$ is much higher for $x\ensuremath{\ge}0.4$. A metamagneticlike anomaly in magnetization is observed in all compounds, at a critical field which appears on a similar energy scale with the value of the corresponding ${T}_{\mathrm{max}}$. The character of three paramagnetic regimes which can be distinguished in the T-$x$ and H-$x$ phase diagrams, normal, correlated, and field polarized, is discussed in relation to the field-induced reentrant superconductivity in the parent compounds.

Pressurizing strategically selected compositions of the $\mathrm{EuC}{\mathrm{u}}_{2}{(\mathrm{G}{\mathrm{e}}_{1\ensuremath{-}x}\mathrm{S}{\mathrm{i}}_{x})}_{2}$ series affords an opportunity for gaining microscopic insight into the ground-state properties and interplay between magnetism and valence fluctuations across a quantum critical point. This is investigated by way of systematic $^{151}\mathrm{Eu}$ M\"ossbauer spectroscopy measurements on $x=0$ and $x=0.5$ compositions in the series, pressurized up to 7 GPa including variable temperature scans in the range 300--4.2 K. In $\mathrm{EuC}{\mathrm{u}}_{2}\mathrm{G}{\mathrm{e}}_{2}$ the temperature and pressure dependences of the hyperfine interaction parameters indicate that both the magnetic and divalent state, $\mathrm{E}{\mathrm{u}}^{\ensuremath{\nu}+}$ where $\ensuremath{\nu}=2$, are stable up to 6--7 GPa, thus serving as a useful reference. Whereas in the $x=0.5$ composition which initially involves $\mathrm{E}{\mathrm{u}}^{2+}$, collapse of the magnetically ordered state is onset at \ensuremath{\sim}1.3 GPa and there is emergence of a nonmagnetic intermediate valence state coexisting with the magnetically ordered state. This regime of mixed states is a precursor of a quantum phase transition to a nonmagnetic homogeneous intermediate valence state $\ensuremath{\nu}\ensuremath{\sim}2.45$, across a quantum critical point at 3.6 GPa, suggesting a first-order phase transition. X-ray-diffraction pressure studies at 300 K up to 6 GPa of the $x=0.5$ composition indicate there is no change in lattice symmetry from the tetragonal $\mathrm{ThC}{\mathrm{r}}_{2}\mathrm{S}{\mathrm{i}}_{2}$-type structure. There are also no obvious discontinuities in pressure dependences of the lattice parameters upon evolving through the quantum critical point at 3.6 GPa. Increasing pressure changes the starting $\mathrm{E}{\mathrm{u}}^{2+}$ valence monotonically, until the mean valence attains its largest value $\ensuremath{\nu}\ensuremath{\sim}2.45$ indicative of enhanced charge fluctuations at the quantum critical point and plateaus thereafter. High-pressure resistance measurements at low temperatures down to 40 mK near the quantum critical point reveal no evidence for superconductivity.

Doping ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ by magnetic ions represents an interesting problem since it may break the time-reversal symmetry needed to maintain the topological insulator character. Mn dopants in ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ represent one of the most studied examples here. However, there is a lot of open questions regarding their magnetic ordering. In the experimental literature different Curie temperatures or no ferromagnetic order at all are reported for comparable Mn concentrations. This suggests that magnetic ordering phenomena are complex and highly susceptible to different growth parameters, which are known to affect material defect concentrations. So far theory has focused on Mn dopants in one possible position, and neglected relaxation effects as well as native defects. We have used ab initio methods to calculate the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ electronic structure influenced by magnetic Mn dopants, and exchange interactions between them. We have considered two possible Mn positions, the substitutional and interstitial one, and also native defects. We have found a sizable relaxation of atoms around Mn, which affects significantly magnetic interactions. Surprisingly, very strong interactions correspond to a specific position of Mn atoms separated by a van der Waals gap. Based on the calculated data we performed spin dynamics simulations to examine systematically the resulting magnetic order for various defect contents. We have found under which conditions the experimentally measured Curie temperatures ${T}_{\mathrm{C}}$ can be reproduced, noticing that interstitial Mn atoms appear to be important here. Our theory predicts the change of ${T}_{\mathrm{C}}$ with a shift of Fermi level, which opens the way to tune the system magnetic properties by selective doping.

The results of a single-crystal x-ray-diffraction study of the evolution of the crystal structures of $\mathrm{V}{\mathrm{I}}_{3}$ with temperature with an emphasis on phase transitions are presented. Some related specific-heat and magnetization data are included. The existence of the room-temperature trigonal crystal structure $R\overline{3}$ (148) has been confirmed. Upon cooling, $\mathrm{V}{\mathrm{I}}_{3}$ undergoes a structural phase transition to a monoclinic phase at ${T}_{\mathrm{s}}\ensuremath{\sim}79\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. ${T}_{\mathrm{s}}$ is reduced in magnetic fields applied along the trigonal $c$ axis. When $\mathrm{V}{\mathrm{I}}_{3}$ becomes ferromagnetic at ${T}_{\mathrm{FM}1}\ensuremath{\sim}50\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, magnetostriction-induced changes of the monoclinic-structure parameters are observed. Upon further cooling, the monoclinic structure transforms into a triclinic variant at 32 K which is most likely occurring in conjunction with the previously reported transformation of the ferromagnetic structure. The observed phenomena are preliminarily attributed to strong magnetoelastic interactions.

KTm(MoO 4 ) 2 belongs to the family of binary alkaline rare-earth molybdates. This compound can be considered to be an almost ideal quantum two-level system at low temperatures. Magnetocaloric properties of KTm(MoO 4 ) 2 single crystals were investigated using specific heat and magnetization measurement in the magnetic field applied along the easy axis. Large conventional magnetocaloric effect (–Δ S M ≈ 10.3 J/(kg K)) was observed in the magnetic field of 5 T in a relatively wide temperature interval. The isothermal magnetic entropy change of about 8 J/(kgK) has been achieved already for the magnetic field of 2 T. Temperature dependence of the isothermal entropy change under different magnetic fields is in good agreement with theoretical predictions for a quantum two-level system with Δ ≈ 2.82 cm −1 . Investigation of magnetocaloric properties of KTm(MoO 4 ) 2 suggests that the studied system can be considered as a good material for magnetic cooling at low temperatures.

The experimental and theoretical study of magnetocaloric effect and magnetic relaxation of the powder sample of CsGd(MoO4)2 were performed. The large conventional magnetocaloric effect was found around 2 K with − Δ S max ≈ 26.5 J/(kg K) for B = 7 T. AC susceptibility measurement revealed multiple-time scale magnetic relaxation effects on different time scales. Slowest relaxation effect was attributed to the direct process with a bottleneck effect and two faster relaxation processes are effectively temperature independent, probably as a result of averaging in the powder sample.

We report the single crystal properties of CePd2Zn3 and CePd2Ga3 compounds. The compounds were prepared by Bridgman method in high-frequency induction furnace. Both compounds adopt the hexagonal PrNi2Al3-type structure with a = 5.3914(2) A, c = 4.3012(2) A for CePd2Zn3 and a = 5.4106(8) A, c = 4.2671(8) A for CePd2Ga3, respectively. CePd2Zn3 orders antiferromagnetically below T N = 1.9 K. Magnetoresistance measurements revealed a crossover at B c = 0.95 T. CePd2Ga3 orders ferromagnetically at T C = 6.7 K. Applied hydrostatic pressure reduces the value of the Curie-temperature (rate ∂ T C / ∂ p = 0.9 K GPa − 1 ) down to 3.9 K at 3.2 GPa. Both compounds display a strong magnetocrystalline anisotropy with easy axis of magnetization perpendicular to the c-axis in the hexagonal lattice.

A pressure-induced valence transition has attracted much attention in Eu-compounds. Among them, EuRh2Si2, EuNi2Ge2, and EuCo2Ge2 reveal the valence transition around 1, 2, and 3 GPa, respectively. We have succeeded in growing single crystals of EuT2X2 (T: transition metal, X: Si, Ge) and studied electronic properties under pressure. EuRh2Si2 indicates a first-order valence transition between 1 and 2 GPa, with a large and prominent hysteresis in the electrical resistivity. At higher pressures, the first-order valence transition changes to a cross-over regime with an intermediate valence state. Tuning of the valence state with pressure is reflected in a drastic change of the temperature dependence of the electrical resistivity in EuRh2Si2 single crystals. Effect of pressure on the valence states on EuRh2Si2, EuIr2Si2, EuNi2Ge2, and EuCo2Ge2, as well as an isostructural related compound EuGa4, are reviewed.

Single crystals of ThBe13 and LuBe13 have been prepared using aluminum flux method. The crystals structure of both compounds was confirmed to be cubic of the NaZn13-type with the lattice parameters a = 10.183(6) A and a = 10.395(3) A for LuBe13 and ThBe13, respectively. The low temperature measurements of electrical resistance and ac susceptibility have revealed a BCS-type of superconductivity in ThBe13 below Tc = 125 mK. LuBe13 has been found to be superconducting below Tc = 630 mK.

We report on the measurements of the magnetization and heat capacity of the tetragonal heavy-fermion compound ${\mathrm{Ce}}_{2}{\mathrm{RhIn}}_{8}$, dependent on the direction of the magnetic field applied perpendicular to the $c$ axis. A notable anisotropic magnetization component having a fourfold symmetry has been observed in the antiferromagnetic state in low fields. The angular dependence of the heat capacity does not indicate any additional phase which implies that the observed magnetization anisotropy is an intrinsic property of ${\mathrm{Ce}}_{2}{\mathrm{RhIn}}_{8}$. A clear angular dependence of magnetic phase boundaries on the applied magnetic field has been also revealed. A possible interpretation of these results is discussed in the context of previously reported studies of magnetic structures in ${\mathrm{Ce}}_{2}{\mathrm{RhIn}}_{8}$.

Two conflicting ground states, i.e., an antiferromagnetic ground state at 3.8 K and a ferromagnetic ground state at 5.1 K, have been reported for $\mathrm{CePtG}{\mathrm{e}}_{2}$, which has two nonequivalent crystallographic Ce sites. In this study, we have prepared polycrystalline and single-crystalline $\mathrm{CePtG}{\mathrm{e}}_{2}$ samples and measured their magnetization, specific heat, resistivity, and Seebeck coefficient. The observed bend in resistivity, peak in magnetic susceptibility, and \ensuremath{\lambda}-type anomaly in specific heat at ${T}_{N}=3.8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and the metamagnetic transition in this $\mathrm{CePtG}{\mathrm{e}}_{2}$ suggest antiferromagnetism at low temperatures. No evidence of ferromagnetic order was found in the studied temperature range, indicating possible crystal polymorphism with ferromagnetic character. The large susceptibility around ${T}_{N}$ and the Schottky-type specific heat indicate a well-localized $f$-electron state. However, the large residual \ensuremath{\gamma} and \ensuremath{\chi}(0) indicate an itinerant heavy $f$-electron state. These findings suggest the coexistence of localized and heavy itinerant states corresponding to $f$ electrons at the two Ce sites in this antiferromagnetic $\mathrm{CePtG}{\mathrm{e}}_{2}$. The field dependence of magnetization indicates a sudden increase in magnetization at weak magnetic fields $(\ensuremath{\sim}0.2\phantom{\rule{0.16em}{0ex}}\mathrm{T})$, followed by a plateau at fields above 0.2 T, resulting in a possible different polarized metamagnetic state via interaction between localized and itinerant $f$ electrons.

The electrical resistance, Hall resistance, and thermoelectric power of the Ising-like antiferromagnet $\mathrm{UIrS}{\mathrm{i}}_{3}$ were measured as functions of temperature and magnetic field. We have observed that the unequivocally different characters of first-order and second-order magnetic phase transitions lead to distinctly different magnetotransport properties in the neighborhood of corresponding critical temperatures and magnetic fields, respectively. The magnetic contributions to the electrical and Hall resistivity in the antiferromagnetic state, and the polarized and normal regimes of paramagnetic state are driven by different underlying mechanisms. Results of detailed measurements of magnetotransport in the vicinity of the tricritical point reveal that the Hall-resistivity steps at phase transitions change polarity just at this point. The jumps in field dependences of specific heat, electrical resistivity, Hall resistivity, and Seebeck coefficient at the first-order metamagnetic transitions indicate a Fermi surface reconstruction, which is characteristic of a magnetic-field-induced Lifshitz transition. The presented results emphasize the usefulness of measurements of electrical- and thermal-transport properties as sensitive probes of magnetic phase transformations in antiferromagnets sometimes hardly detectable by other methods.

Magnetotransport constitutes a useful probe to understand the interplay between electronic band topology and magnetism in spintronic devices. A recent theory of Lu and Shen [Phys. Rev. Lett. 112, 146601 (2014)] on magnetically doped topological insulators predicts that quantum corrections $\mathrm{\ensuremath{\Delta}}\ensuremath{\kappa}$ to the temperature dependence of conductivity can change sign across the Curie transition. This phenomenon has been attributed to a suppression of the Berry phase of the topological surface states at the Fermi level, caused by a magnetic energy gap. Here, we demonstrate experimentally that $\mathrm{\ensuremath{\Delta}}\ensuremath{\kappa}$ can reverse its sign even when the Berry phase at the Fermi level remains unchanged. The contradictory behavior to theory predictions is resolved by extending the model by Lu and Shen to a nonmonotonic temperature scaling of the inelastic scattering length showing a turning point at the Curie transition.

A thorough investigation is presented of the magnetic and structural properties of Mn-doped Bi2Se3 topological insulators grown by molecular beam epitaxy on top of insulating BaF2 (111) substrates. The magnetic properties have been studied in the temperature range from 2 K to 300 K in magnetic fields up to 7 T. The systems were further characterized by means of high-resolution X-ray diffraction, electron-microprobe analysis, and X-ray photoemission spectroscopy. Samples with the atomic concentration of Mn up to about 0.06 exhibit an almost perfect crystalline structure while, for higher Mn concentrations, diffuse scattering from defects is observed. Photoemission results suggest a localized non-metallic Mn 3d5 ground state which is weakly or intermediately coupled to the Bi2Se3 environment. The exchange interaction between the Mn moments leads to a ferromagnetic phase at low temperatures with a roughly linear relation between the Curie temperature and the atomic concentration of Mn.

Surface magnetic domains of a UCoGa single crystal during magnetization/demagnetization processes in increasing/decreasing magnetic fields were investigated by means of magnetic-force-microscopy (MFM) images at low temperatures. The observed domain structure is typical for a ferromagnet with strong uniaxial anisotropy. The evolution of magnetic domains during cooling of the crystal below TC has also been manifested by MFM images. Analysis of the available data reveals that the high uniaxial magnetocrystalline energy in combination with the relatively small ferromagnetic exchange interaction in UCoGa gives rise to the formation of very narrow domain walls formed by the pairs of the nearest U neighbor ions with antiparallel magnetic moments within the basal plane. Since the very high anisotropy energy is a common feature of the majority of the uniaxial U ferromagnets, analogous domain-wall properties are expected for all these materials.

TmB4 - the metallic tetraboride with a geometrically frustrated Shastry-Sutherland lattice (SSL) has attracted recently a lot of attention. It exhibits a strong Ising-like anisotropy along the c-axis and a rather complex phase diagram of the ordered magnetic phase below TN ≈ 11.7 K in which besides the main magnetization (M) plateau also small fractional plateaus can be observed. To learn more about the properties of fractional plateaus we have investigated the dynamics of magnetization processes in TmB4 by M vs. H (applied magnetic field) measurements at different temperatures and different field sweep rates (swr) 1.8 K. In addition, experiments of the charge carrier mobility and magnetostriction as a function of temperature and magnetic field were carried out to specify the interplay between magnetic structure and the charge dynamics, and the role of the magneto-elastic interaction in this compound, respectively. It was shown that the magnetization values of fractional plateaus become elevated with the increase of temperature at which magnetization takes place and with the decrease of magnetic field sweep rate, which points to inconsistency with theoretical predictions based on Glauber dynamics. Moreover, it turned out that the transitions between magnetic phases during magnetization processes are accompanied by an anomalous behavior of charge mobility and distinctive variations of magnetostriction.

Thermal expansion, magnetostriction, and magnetization measurements under magnetic field and hydrostatic pressure were performed on a ${\mathrm{UAu}}_{2}{\mathrm{Si}}_{2}$ single crystal. They revealed a large anisotropy of magnetoelastic properties manifested by prominent length changes, leading to a collapse of the unit-cell volume accompanied by breaking the fourfold symmetry (similar to that in ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$ in the hidden-order state) in the antiferromagnetic state as consequences of strong magnetoelastic coupling. The magnetostriction curves measured at higher temperatures confirm a bulk character of the $50\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ weak ferromagnetic phase. The large positive pressure change of the ordering temperature predicted from Ehrenfest relation contradicts the more than an order of magnitude smaller pressure dependence observed by the magnetization and specific heat measured under hydrostatic pressure. A comprehensive magnetic phase diagram of ${\mathrm{UAu}}_{2}{\mathrm{Si}}_{2}$ in magnetic field applied along the $c$ axis is presented. The ground-state antiferromagnetic phase is suppressed by a field-induced metamagnetic transition that changes its character from second to first order at the tricritical point.

The metamagnetic transition between the antiferromagnetic and paramagnetic state in UIrGe has been studied at various temperatures by magnetization, heat capacity and magnetocaloric-effect measurements on a single crystal in static and pulsed magnetic fields applied along the orthorhombic b-axis. A first-order transition is observed at temperatures below 13 K and a second-order one at higher temperatures up to the N\'eel temperature (TN = 16.5 K). The first-order transition is accompanied by a dramatic increase of the Sommerfeld coefficient. Magnetization measurements extended to the paramagnetic range revealed an anomalous S-shape (inflection point at a magnetic field Hm) in magnetization isotherms at temperatures above 13 K and a temperature dependence of susceptibility with a maximum at Tmax well above TN. The lines representing the temperature-induced evolution of Hm and field-induced evolution of Tmax, respectively, are bound for the point in the magnetic phase diagram at which the order of metamagnetic transition changes. A tentative scenario explaining these anomalies by antiferromagnetic correlations or short-range order in the paramagnetic state is discussed., Comment: Accepted to PRB

YFeO

In a cubic ferromagnet, small spontaneous lattice distortions are expected below the Curie temperature, but the phenomenon is usually neglected. This study focuses on such an effect in the ${\mathrm{U}}_{4}{\mathrm{Ru}}_{7}{\mathrm{Ge}}_{6}$ compound. Based on DFT calculations, we propose a lattice distortion from the cubic $Im\text{\ensuremath{-}}3m$ space group to a lower, rhombohedral, symmetry described by the $R\text{\ensuremath{-}}3m$ space group. The strong spin-orbit coupling of the uranium ions plays an essential role in lowering the symmetry, giving rise to two different U sites (U1 and U2). Using polarized neutron diffraction in applied magnetic fields of 1 and $9\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ in the ordered state ($1.9\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) and in the paramagnetic state ($20\phantom{\rule{0.16em}{0ex}}\mathrm{K}$), we bring convincing experimental evidence of this splitting of the U sites, with different magnetic moments. The data have been analyzed both by maximum entropy calculations and by a direct fit in the dipolar approximation. In the ordered phase, the ${\ensuremath{\mu}}_{\mathrm{L}}/{\ensuremath{\mu}}_{\mathrm{S}}$ ratio of the orbital and spin moments on the U2 site is remarkably lower than for the free ${\mathrm{U}}^{3+}$ or ${\mathrm{U}}^{4+}$ ion, which points to a strong hybridization of the U $5f$ wave functions with the $4d$ wave functions of the surrounding Ru. On the U1 site, the ${\ensuremath{\mu}}_{\mathrm{L}}/{\ensuremath{\mu}}_{\mathrm{S}}$ ratio exhibits an unexpectedly low value: the orbital moment is almost quenched, like in metallic $\ensuremath{\alpha}$-uranium. As a further evidence of the $5f\text{\ensuremath{-}}4d$ hybridization in the ${\mathrm{U}}_{4}{\mathrm{Ru}}_{7}{\mathrm{Ge}}_{6}$ system, we observe the absence of a magnetic moment on the Ru1 site, but a rather large induced moment on the Ru2 site, which is in closer coordination with both U positions. Very similar results are obtained at $20\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ in the ferromagnetic regime induced by the magnetic field of $9\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. This shows that applying a strong magnetic field above the Curie temperature also leads to the splitting of the uranium sites, which further demonstrates the intimate coupling of the magnetic ordering and structural distortion. We propose that the difference between the magnetic moment on the U1 and U2 sites results from the strong spin-orbit interaction with different local point symmetries.

We report on the single crystal properties of the novel U$_{2}$RhIn$_{8}$ compound studied in the context of parent URhIn$_{5}$ and UIn$_{3}$ systems. The compounds were prepared by In self-flux method. U$_{2}$RhIn$_{8}$ adopts the Ho$_{2}$CoGa$_{8}$-type structure with lattice parameters a $= 4.6056(6)$ \AA\ and c $= 11.9911(15)$ \AA. The behavior of U$_{2}$RhIn$_{8}$ strongly resembles that of the related URhIn$_{5}$ and UIn$_{3}$ with respect to magnetization, specific heat and resistivity except for magnetocrystalline anisotropy developing with lowering dimensionality in the series UIn$_{3}$ vs. U$_{2}$RhIn$_{8}$ and URhIn$_{5}$. U$_{2}$RhIn$_{8}$ orders antiferromagnetically below T$_{\textrm{N}}$ $= 117$ K and exhibits a slightly enhanced Sommerfeld coefficient $\gamma = 47$ mJ$\cdot$mol$^{-1}\cdot$K$^{-2}$. Magnetic field leaves the value of N\'{e}el temperature for both URhIn$_{5}$ and U$_{2}$RhIn$_{8}$ unaffected up to 9 T. On the other hand, T$_{\textrm{N}}$ is increasing with applying hydrostatic pressure up to 3.2 GPa. The weak temperature dependence of $\chi(T)$ in all studied compounds might be attributed to the mainly itinerant nature of 5f electrons. The character of uranium 5f electron states of U$_{2}$RhIn$_{8}$ was studied by first principles calculations based on the density functional theory. The overall phase diagram of U$_{2}$RhIn$_{8}$ is discussed in the context of magnetism in the related URhX$_{5}$ and UX$_{3}$ (X = In, Ga) compounds.

A new quantum simulation approach has been applied in the present work to the two-dimensional (2D) ferromagnetic and antiferromagnetic Ising lattices to calculate their magnetic structures, magnetizations, free energies and specific heats in the absence of an external magnetic field. Surprisingly, no size effects could be observed in our simulations performed for the Ising lattices of different sizes. Most importantly, our calculated spontaneous thermally averaged spins for the two kinds of systems are exactly same as those evaluated with quantum mean field theory, and the magnetic structures simulated at all chosen temperatures are perfectly ferromagnetic or antiferromagnetic, verifying the correctness and applicability of our quantum model and computational algorithm. On the other hand, if the classical Monte Carlo (CMC) method is applied to the ferromagnetic 2D Ising lattice with S=1, it is able to generate correct magnetization well consistent with Onsager's theory; but in the case of S=1/2, the computational results of CMC are incomparable to those predicted with the quantum mean field theory, giving rise to very much reduced magnetization and considerably underestimated Curie temperature. The difficulty met by the CMC method is mainly caused by its improperly calculated exchange energy of the randomly selected spin in every simulation step, especially immediately below the transition temperature, where the thermal averages of spins are much less than 1/2, however they are assigned to ±1/2 by CMC to evaluate the exchange energies of the spins, such improper manipulation is obviously impossible to lead the code to converge to the right equilibrium states of the spin systems.

The use of paramagnetic molecules as cryogenic coolants usually requires relatively large fields to obtain a practical cooling effect. Thus, research into magnetic molecular materials with larger MCEs in fields of ≤2 T is the main focus in this area. In this work, the crystal structure, magnetic susceptibility and isothermal magnetization for the inorganic framework material GdF3 were measured, and the isothermal entropy change was evaluated up to 9 T. Thanks to the combination of the large isotropic spin of Gd3+, the dense structure and weak ferromagnetic interaction, an extremely large −ΔSm for GdF3 was observed up to 528 mJ cm−3 K−1 for Δμ0H = 9 T, proving it to be an exceptional cryogenic magnetic coolant.

Magnetocaloric effect in the Haldane-gap $S=1$ spin-chain $[\mathrm{Ni}{({\mathrm{C}}_{2}{\mathrm{H}}_{8}{\mathrm{N}}_{2})}_{2}{\mathrm{NO}}_{2}]({\mathrm{BF}}_{4})$ was investigated at low temperatures $({k}_{B}T\ensuremath{\ll}J)$ and magnetic fields up to nominally $0.1{B}_{\mathrm{sat}}$, where $J$ and ${B}_{\mathrm{sat}}$ denote an exchange interaction and saturation magnetic field, respectively. Magnetic entropy was found to scale as $exp(\mathrm{\ensuremath{\Delta}}/{k}_{B}T)$ in zero magnetic field and ${T}^{0.67}$ for $B={B}_{c}$, in which the Haldane gap should be closed, in reasonable agreement with the theoretical predictions. The existence of the inverse magnetocaloric effect for $Bl{B}_{c}$ was confirmed. The occurrence of normal magnetocaloric response at lowest temperatures and magnetic fields was attributed to $S=1/2$ degrees of freedom arising from the ends of chain segments.

We compare the magnetic and electronic configuration of single Mn atoms in molecular beam epitaxy (MBE) grown Bi2Se3 thin films, focusing on electron paramagnetic (ferromagnetic) resonance (EPR and FMR, respectively) and superconducting quantum interference device (SQUID) techniques. X-ray diffraction (XRD) and electron backscatter diffraction (EBSD) reveal the expected increase of disorder with increasing concentration of magnetic guest atoms, however, Kikuchi patterns show that disorder consists majorly of μm-scale 60° twin domains in the hexagonal Bi2Se3 structure, which are promoted by the presence of single unclustered Mn impurities. Ferromagnetism below TC ~ (5.4±0.3) K can be well described by critical scaling laws M ( T ) ~ ( 1 − T / T C ) β with a critical exponent β = ( 0.34 ± 0.2 ) , suggesting 3D Heisenberg class magnetism instead of e.g. 2D-type coupling between Mn-spins in van der Waals gap sites. From EPR hyperfine structure data we determine a Mn2+ (d5, S = 5/2) electronic configuration with a g-factor of 2.002 for −1/2 → +1/2 transitions. In addition, from the strong dependence of the low temperature FMR fields and linewidth on the field strength and orientation with respect to the Bi2Se3 (0001) plane, we derive magnetic anisotropy energies of up to K1 = −3720 erg/cm3 in MBE-grown Mn-doped Bi2Se3, reflecting the first order magneto-crystalline anisotropy of an in-plane magnetic easy plane in a hexagonal (0001) crystal symmetry. We observe an increase of K1 with increasing Mn concentration, which we interpret to be correlated to a Mn-induced in-plane lattice contraction. Across the ferromagnetic-paramagnetic transition the FMR intensity is suppressed and resonance fields converge the paramagnetic limit of Mn2+ (d5, S = 5/2).

Strong magnetocrystalline anisotropy (MA) is a well-known property of uranium compounds. The almost isotropic ferromagnetism in ${\mathrm{U}}_{4}{\mathrm{Ru}}_{7}{\mathrm{Ge}}_{6}$ reported in this paper represents a striking exception. We present results for magnetization, ac susceptibility, thermal expansion, specific heat, and electrical resistivity measurements performed on a ${\mathrm{U}}_{4}{\mathrm{Ru}}_{7}{\mathrm{Ge}}_{6}$ single crystal at various temperatures and magnetic fields; we discuss the results in relation to first-principles electronic structure calculations. ${\mathrm{U}}_{4}{\mathrm{Ru}}_{7}{\mathrm{Ge}}_{6}$ behaves as an itinerant $5f$-electron ferromagnet (${T}_{\mathrm{C}}=10.7\phantom{\rule{0.16em}{0ex}}\mathrm{K}, {\ensuremath{\mu}}_{\mathrm{S}}=0.85\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$ at $1.9\phantom{\rule{0.16em}{0ex}}\mathrm{K}$). The ground-state easy magnetization direction is along the [111] axis of the cubic lattice. The anisotropy field ${\ensuremath{\mu}}_{0}{H}_{\mathrm{a}}$ along the [001] direction is only about $0.3\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, which is at least three orders of magnitude smaller than for other U ferromagnets. At ${T}_{r}=5.9\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ the easy magnetization direction changes to [001], and remains [001] up to ${T}_{\mathrm{C}}$. This transition is due to a change in magnetic symmetry, and is quite apparent in the low-field magnetization, ac susceptibility, and thermal expansion data, whereas only weak anomalies are observed at ${T}_{r}$ in the temperature dependence of the specific heat and electrical resistivity. The magnetoelastic interaction induces a rhombohedral (tetragonal) distortion of the paramagnetic cubic crystal lattice in the case of the [111] ([001]) easy magnetization direction. The rhombohedral distortion is connected with two crystallographically inequivalent U sites. Our density functional theory calculations, including spin-orbit interaction (SOI) of the U $5f$ electrons, also produces two inequivalent U sites, because SOI leads to a reduction of the symmetry of the former cubic structure. The calculated ground state is in agreement with the experimentally observed [111] easy magnetization direction. The first excited state has moments along the [001] direction, which agrees with the moment orientation for $Tg{T}_{\mathrm{r}}$. The energy of the first excited state is $0.9\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ above the ground state, which is comparable to the value of $0.51\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$, corresponding to ${k}_{\mathrm{B}}{T}_{\mathrm{r}}$. We propose that weak MA of the ${\mathrm{U}}_{4}{\mathrm{Ru}}_{7}{\mathrm{Ge}}_{6}$ compound is due to the lack of direct overlap of the $5f$ orbitals of the nearest U ions, which is screened out by the closed Ru and Ge cuboctahedra coordinating each U ion.

Magnetic structure of tetragonal UAu$_2$Si$_2$ was investigated by single-crystal neutron diffraction experiments. Below $T_{\rm N}$ = 20 K it orders antiferromagnetically with a propagation vector of $k = (2/3, 0, 0)$ and magnetic moments of uranium ions pointing along the tetragonal $c$-axis. Weak signs of the presence of a ferromagnetic component of magnetic moment were traced out.Taking into account a group theory calculation and experimental results of magnetization and $^{29}$Si-NMR, the magnetic structure is determined to be a squared-up antiferromagnetic structure, with a stacking sequence ($+ + -$) of the ferromagnetic $ac$-plane sheets along the $a$-axis. This result highlights similar magnetic correlations in UAu$_2$Si$_2$ and isostructural URu$_2$Si$_2$., Comment: 7 pages, 7 figures

We investigate the discontinuous ferromagnetic phase diagram near tricritical point in UCo 1-x Ru x Al compounds by electrical resistivity measurements. Separation of phases in UCo 0.995 Ru 0.005 Al at ambient pressure and in UCo 0.990 Ru 0.010 Al at pressure of 0.2 GPa and disappearance of ferromagnetism at 0.4 GPa is confirmed. The exponent of temperature dependence of electrical resistivity implies change from Fermi liquid behavior to non-Fermi liquid at 0.2 GPa and reaches minimum at 0.4 GPa. Our results are compared to results obtained on the pure UCoAl and explanation for different exponents is given.

Two new quantum simulation methods, which we developed recently based on the Metropolis and self-consistent algorithms defined as QMC and SCA approaches respectively, were employed to investigate the magnetic properties of an antiferromagnetic nanoparticle with strong surface anisotropy. All simulations were started from a random magnetic configuration and carried out from a temperature well above the phase transition stepwise down to very low temperatures as other researchers have been doing in classical Monte Carlo (CMC) simulations. It turns out that the magnetic structures, magnetizations, total (free) energy, magnetic entropy and specific heat calculated by means of the two approaches are well consistent with each other, thereby verifying their correctness mutually.

The new quantum simulation model and the self-consistent algorithm (SCA) for magnetic nanosystems, that we proposed 2 years ago, were extended to study the magnetic properties of a nanowire consisting of 3d ions which are coupled ferromagnetically. To test the applicability of the algorithm, our simulations in the present work were started from a magnetic structure in which all spins in the whole nanosample were randomly oriented (defined as the random magnetic configuration for later use) as other authors have been doing with Monte Carlo or micromagnetism method, and such calculated results were all reasonable. Especially, the free energies evaluated at the chosen temperatures were found to attenuate spontaneously and quickly, as the program ran, towards the minima according to the principle of lowest free energy as expected. This suggests that the computational algorithm is able to lead the code to converge rapidly to the equilibrium state automatically without the need to minimize the total (free) energy of the system elaborately that must be done if the Monte Carlo or micromagnetism method is used, demonstrating the great power of natural laws.

The transformation of Mn(II) glycolates (glc) between the three-dimensional coordination polymer [Mn(glc)2]n (1) and discrete mononuclear phase [Mn(glc)2 (H2O)2] (2) can be reversibly switched by water molecules, which dramatically change the magnetocaloric effect (MCE) of Mn(II) glycolates from the maximum of 6.9 J kg(-1) K(-1) in 1 to 60.3 J kg(-1) K(-1) in 2. This case example reveals that the effect of magnetic coupling on MCE plays a dominant role over that of other factors such as magnetic density for 3d-type magnetic refrigerants.

[Cu(dmbpy)(H2O)2SO4] in the form of small blue crystals was prepared from the aqueous-ethanolic reaction mixture formed of CuSO4 and dmbpy (dmbpy = 4,4′-dimethyl-2,2′-bipyridine). Its crystal structure consists of chains in which hexacoordinated Cu(II) atoms are linked by (μ2-SO4)2− anions. The Cu(II) atom exhibits elongated tetragonal bipyramidal coordination with one chelate bonded dmbpy and two aqua ligands placed in the equatorial plane while the axial positions are occupied by bridging sulfato ligands. The study of electron spin resonance and specific heat enabled to identify the studied material as an S = 1/2 Heisenberg antiferromagnetic chain with weak intrachain exchange interaction, 2J/kB = −1.12 K and a small anisotropy of g – factor, Δg/g ≈ 0.1. The analysis of the magnetic entropy revealed pronounced release of the entropy at the saturation magnetic field, which for spin chains induces quantum phase transition, namely Bsat = 1.56 T. The maximum isothermal change of the entropy ΔSM = 2.86 J/Kmol is comparable to that released in a critical region for materials with magnetic phase transitions. The obtained results suggest that the existence of a quantum critical point significantly influences finite-temperature properties.