Low-temperature Magnetic Phase Research Articles

Spin flop transitions and complex magnetic behaviour in CoMoO4 powders and [formula omitted]-CoMoO4 thin films

Magnetic measurements were made on α-CoMoO4 powder and β-CoMoO4 thin film samples. α-CoMoO4 powder showed antiferromagnetic ordering below 13 K and two sharp spin flop transitions with complete magnetic saturation after the second transition (>5 T). A magnetic phase diagram for α-CoMoO4 was constructed, revealing three low-temperature magnetic phases: an antiferromagnetic phase at low field, an intermediate spin-flop phase, and a paramagnetic phase at high field. Two well-defined magnetisation plateaus were observed either side of the spin-flop phase: the first occurred with a moment per Co ion of 1/3 of spin saturation (where Co moments are completely aligned to the applied magnetic field), whereas the second occurred at a moment per Co ion matching calculations for spin saturation (3 μB per Co). No geometric origin for this factor of 1/3 could be found. Hysteresis was observed around the first spin-flop transition, indicating the presence of spin canting or magnetic domains under an applied field. The first successful fabrication of CoMoO4 thin films was achieved via RF magnetron sputtering, yielding β phase films which remain in the β phase upon cooling. The β-CoMoO4 thin films demonstrated antiferromagnetic ordering below 10 K. While two spin-flop transitions are seen in the β phase films (with a hysteresis around the first), spin saturation is not reached in fields up to 6 T, suggesting the existence of a third spin reorientation at higher field. A magnetic phase diagram was also constructed for the β-CoMoO4 film, which demonstrated the same three low-temperature magnetic phases, though with lower critical fields and transition temperatures. Comparisons with isostructural Ni and Mn molybdates showed that the key differences in the α and β phases arise from the different coordination of the Mo tetramers, indicating differences in magnetic properties in both phases arise from variations in the inter-tetramer exchange coupling.

Intrinsic magnetic properties of the layered antiferromagnet CrSBr

van der Waals magnetic materials are an ideal platform to study low-dimensional magnetism. Opposed to other members of this family, the magnetic semiconductor CrSBr is highly resistant to degradation in air, which, in addition to its exceptional optical, electronic, and magnetic properties, is the reason the compound is receiving considerable attention at the moment. For many years, its magnetic phase diagram seemed to be well-understood. Recently, however, several groups observed a magnetic transition in magnetometry measurements at temperatures of around 40 K that is not expected from theoretical considerations, causing a debate about the intrinsic magnetic properties of the material. In this Letter, we report the absence of this particular transition in magnetization measurements conducted on high-quality CrSBr crystals, attesting to the extrinsic nature of the low-temperature magnetic phase observed in other works. Our magnetometry results obtained from large bulk crystals are in very good agreement with the magnetic phase diagram of CrSBr previously predicted by the mean-field theory; A-type antiferromagnetic order is the only phase observed below the Néel temperature at TN = 131 K. Moreover, numerical fits based on the Curie–Weiss law confirm that strong ferromagnetic correlations are present within individual layers even at temperatures much larger than TN.

Correlating structural and magnetic ground states of 3d-5d based double perovskites: Evidence of two magnetic phase transitions

We present a detailed study on both the structural and magnetic properties of the Sr2RuMn1-xIrxO6double perovskite oxides with four different concentrations (x = 0, 0.25, 0.5 and 0.75). The main objective of the work is to study the effect of Ir substitution for Mn on the high-temperature crystallographic and low-temperature magnetic phases of the compounds. Thephase purity was confirmed by the Rietveld analysis of the X-ray diffraction (XRD)pattern. The high temperature crystallographic information of the compounds was investigated by the help of high temperature X-ray diffraction and Raman spectroscopy. Signature of two structural phase transitions for x = 0.75 and one structural phase transition for x = 0 was seen. FESEM (Field emission scanning electron microscopy) with EDS (energy dispersive spectroscopy) of Ir-01 (x = 0.25) confirmed the presence of all the elements in the sample. The magnetic structure of all the samples have been studied at low temperature (5 K). The existence of two magnetic transitions for x = 0.5 and x = 0.75 are evident from the temperature dependent magnetization behavior.

Field direction dependent skyrmion crystals in noncentrosymmetric cubic magnets: A comparison between the point groups (O,T) and Td

We investigate the instability toward a skyrmion crystal (SkX) in noncentrosymmetric cubic magnets with an emphasis on a comparison between point groups $(O,T)$ and ${T}_{\mathrm{d}}$. By constructing low-temperature magnetic phase diagrams under an external magnetic field for three directions based on numerically simulated annealing, we find that the system under the point groups $(O,T)$ exhibits different two types of SkXs depending on the field direction, while that under ${T}_{\mathrm{d}}$ does not show such an instability. The difference between them is understood from the difference in the momentum-dependent Dzyaloshinskii-Moriya interaction under each point group. Meanwhile, we show that the system under ${T}_{\mathrm{d}}$ leads to the SkX instability by considering an additional effect of the uniaxial strain, which lowers the symmetry to ${D}_{2\mathrm{d}}$. We obtain two different SkXs: N\'eel-type SkX and anti-type SkX, the former of which is stabilized in the presence of the interactions at the three-dimensional ordering wave vectors. The present results provide rich topological spin textures in the three-dimensional systems, which are sensitive to the magnetic-field direction and point-group symmetry.

Synthesis and magnetic properties of ε-AlxFe2-xO3 nanoparticles

ε-AlxFe2-xO3 (x = 0, 0.125, 0.167, 0.222 and 0.4) nanoparticles were manufactured by a new kind of mesoporous template-compacted fumed silica. The effect of Al3+ composition on the purity and magnetic performances of the ε-AlxFe2-xO3 samples were investigated. The structural and magnetic performances of the samples were characterized by X-ray diffraction, transmission electron microscope and SQUID magnetometer. The results indicate that the doping of Al3+ can decrease the dimension of ε-Fe2O3 nanoparticles and effectively reduce the formation of α-Fe2O3 impurity in the samples. And the magnetic phase transition temperature of the ε-AlxFe2-xO3 samples decreases with the increase of Al3+. And the low temperature magnetic phase transition finally vanishes at x = 0.222. The ε-AlxFe2-xO3 samples exhibit large coercivity. With the increase of Al3+ content, the coercivity of ε-AlxFe2-xO3 increases firstly to a maximum and then decreases.

Influence of Ga-substitution to the structural and magnetic properties of (Mn,Fe)-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=- bixbyite

To study the dependence of the properties of ternary oxides (Mn,Fe,Ga)2O3 with the bixbyite structure on the composition, the temperature dependences of the magnetization and ac magnetic susceptibility of two single-crystal samples of different compositions obtained using the flux method were analyzed. A detailed study of the structure was carried out using single-crystal X-ray diffraction analysis, and the changes in structural parameters depending on the composition were analyzed. The dc magnetization and ac magnetic susceptibility of Fe1.1Mn0.76Ga0.14O3 and Fe0.65Mn1.1Ga0.26O3 bixbyites have been studied. Despite the qualitatively similar behavior of the magnetic properties, significant differences were also found, despite a small difference in the Mn/Fe/Ga ratio in the samples under study. It is shown that both compounds experience two successive low-temperature magnetic phase transitions from the paramagnetic phase at T=20-32 K as the temperature is lowered. Calculations of the Mydosh parameter for the detected phase transitions showed different degrees of ordering in the compounds under study. Keywords: transition metal oxides, magnetic phase transitions, spin glass state, bixbyite.

Влияние Ga-замещения на структурные и магнитные свойства биксбиита (Mn,Fe)-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=-

To study the dependence of the properties of ternary oxides (Mn,Fe,Ga)2O3 with the bixbyite structure on the composition, the temperature dependences of the magnetization and ac magnetic susceptibility of two single-crystal samples of different compositions obtained using the flux method were analyzed. A detailed study of the structure was carried out using single-crystal X-ray diffraction analysis, and the changes in structural parameters depending on the composition were analyzed. The dc magnetization and ac magnetic susceptibility of Fe1.1Mn0.76Ga0.14O3 and Fe0.65Mn1.1Ga0.26O3 bixbyites have been studied. Despite the qualitatively similar behavior of the magnetic properties, significant differences were also found, despite a small difference in the Mn/Fe/Ga ratio in the samples under study. It is shown that both compounds experience two successive low-temperature magnetic phase transitions from the paramagnetic phase at T = 20–32 K as the temperature is lowered. Calculations of the Mydosh parameter for the detected phase transitions showed different degrees of ordering in the compounds under study.

High temperature crystallographic and low temperature magnetic phase transitions in Fe doped Sr2RuMnO6

Fe doped Sr2RuMnO6 (SRMO) double perovskites (Sr2RuMn1-xFexO6, x = 0, 0.1, 0.2 and 0.3) were prepared by solid-state route. Both x-ray diffraction and Raman spectroscopy were performed to investigate the crystal structure of the synthesized double perovskites. Rietveld refinement of the x-ray diffraction patterns confirmed a phase transition from tetragonal to cubic space group as a function of doping concentration of iron. Raman spectroscopy at room temperature and group theory analysis revealed the phonon modes associated with the space group of the samples. The temperature dependent Raman spectroscopy showed an anharmonic behaviour of the phonon modes of the Fe doped SRMO samples. The temperature evolution of the phononic modes in the range of 300 K–620 K is predominantly influenced by the lattice degrees of freedom. The presence of several oxidation states Mn (2+, 3+ and 4+) and Fe (3+ and 4+) was confirmed by an X-ray photoemission spectroscopy analysis of the highest doped sample (x = 0.3). The magnetic properties measurements showed that the samples were completely paramagnetic at room temperature. The samples exhibit antiferromagnetism at very low temperatures and we conclude that they exhibit ferrimagnetic ground state in the mid temperature region.

Skyrmion crystal under D3h point group: Role of out-of-plane Dzyaloshinskii-Moriya interaction

The Dzyaloshinkii-Moriya (DM) interaction that originates from relativistic spin-orbit coupling in noncentrosymmetric magnets is a source of topological spin textures. We theoretically investigate the possibility of a skyrmion crystal by focusing on the role of the out-of-plane DM interaction that is different from the polar- and chiral-type DM interaction. By performing the simulated annealing for a spin model on a triangular lattice belonging to the $D_{3h}$ point group, we construct low-temperature magnetic phase diagrams in an applied magnetic field. As a result, we find the instability toward different skyrmion crystals under in-plane and out-of-plane magnetic fields, where the key ingredients to stabilize the SkX are different according to the magnetic field direction; the SkX is stabilized under the out-of-plane DM interaction for the in-plane magnetic field, while it is stabilized by additionally introducing an easy-axis anisotropy for the out-of-plane magnetic field. We also discuss the possible multiple-$Q$ states other than the skyrmion crystals in the presence of the out-of-plane DM interaction. Our result indicates that noncentrosymmetric magnetic materials with the out-of-plane DM interaction are potential candidates to host the skyrmion crystals.

Influence of the Chemical Pressure on the Magnetic Properties of the Mixed Anion Cuprates Cu2OX2 (X = Cl, Br, I)

In this study, we theoretically investigate the structural, electronic and magnetic properties of the Cu2OX2 (X = Cl, Br, I) compounds. Previous studies reported potential spin-driven ferroelectricity in Cu2OCl2, originating from a non-collinear magnetic phase existing below TN∼70 K. However, the nature of this low-temperature magnetic phase is still under debate. Here, we focus on the calculation of J exchange couplings and enhance knowledge in the field by (i) characterizing the low-temperature magnetic order for Cu2OCl2 and (ii) evaluating the impact of the chemical pressure on the magnetic interactions, which leads us to consider the two new phases Cu2OBr2 and Cu2OI2. Our ab initio simulations notably demonstrate the coexistence of strong antiferromagnetic and ferromagnetic interactions, leading to spin frustration. The TN Néel temperatures were estimated on the basis of a quasi-1D AFM model using the abinitioJ couplings. It nicely reproduces the TN value for Cu2OCl2 and allows us to predict an increase of TN under chemical pressure, with TN = 120 K for the dynamically stable phase Cu2OBr2. This investigation suggests that chemical pressure is an effective key factor to open the door of room-temperature multiferroicity.

Slow spin dynamics in a CoM2O4 A-site spinel (M=Al, Ga, and Rh)

We report thermomagnetic properties in magnetically frustrated A-site spinel magnets of CoAl2O4, CoRh2O4, and cluster glass CoGa2O4 to comprehensively show the proximity effects of the Néel-to-spin-spiral (NSS) transition, which is predicted theoretically to occur in a magnetically frustrated A-site spinel antiferromagnet. The low-temperature magnetic phase remains controversial since the magnetic state of CoAl2O4 is in the vicinity of the NSS transition derived by the inherent magnetic frustrated interaction for an A-site spinel magnet and is also considerably sensitive to crystallographic disorder. The antiferromagnetic and spin-glass transitions were detected by measuring the direct-current and alternating-current susceptibilities and also thermoremanent magnetization (TRM) developing below the magnetic transitions at the Néel point (T N) and spin-glass transition temperature (T SG). The relaxation rate and the temperature derivative of TRM were both enhanced at T N and T SG, and they decayed rapidly above and below the transitions. We succeeded in extracting the relaxation time τ and other characteristic parameters from the isothermal relaxation of TRM, which was well fitted with a non-exponential relaxation form formulated by Weron based on purely stochastic theory in order to describe the originally dielectric relaxation. For the typical CoRh2O4 normal spinel (inversion-free A site) antiferromagnet and the CoGa2O4 random spinel spin-glass, the temperature variations of these parameters can distinguish the magnetic states. In contrast to the cases for CoRh2O4 and CoGa2O4, an enhancement of the relaxation rate of TRM for CoAl2O4 is indicated at low temperatures, which is probably related to the suppression of long-range antiferromagnetic order revealed by neutron diffraction studies.

The effect of high pressure on the electrical and transport properties of the InSb-MnSb magnetic eutectic composition

In this work we studied transport properties of eutectic InSb-MnSb crystals with oriented MnSb needle-like inclusions under a hydrostatic pressure up to P = 8 GPa and after its application. We show that the pressure-induced variation of the samples resistivity and transport parameters at room temperature can be interpreted in the terms of crystal phase transition of the InSb matrix. We argue that the irreversibility of this transition, deduced from transport measurements, may stem from the composite nature of the crystals studied. The resistivity of the samples subjected to high pressures increases upon cooling, which can be a consequence of the increased disorder accompanying the irreversible transition. Magnetotransport data for these samples suggest that charge carriers interact with a low-temperature magnetic phase. The latter can be formed by isolated Mn atoms in the InSb matrix.

Structural and magnetic alteration of Cu2GaBO5 forced by Mn3+ doping

To study the gradual change of the structure and the magnetic state of the ludwigite Cu2GaBO5 upon Mn3+ doping, single crystals of Cu2Ga1−xMnxBO5 (x = 0.55, 0.7, 0.8) with the size up to 3 × 3 × 10 mm3 were grown using the flux technique. The phase homogeneity and crystal structure of the obtained compounds were investigated by the powder and single crystal X-ray diffraction. All the samples possessed the monoclinic-distorted ludwigite structure with the P21/c space group. The study of the actual Cu/Ga/Mn composition by the EDX (energy-dispersive X-ray spectroscopy) technique revealed the lower Mn content in all the samples and the refined formulas were Cu2Ga0.47Mn0.53BO5, Cu1.92Ga0.5Mn0.58BO5 and Cu2Ga0.32Mn0.68BO5, respectively. Despite the high manganese content, the concentration transition (from Cu2GaBO5 to Cu2MnBO5) and change of the monoclinic angle did not occur, but strong Me-O octahedra distortions exceeding those both in the parent ludwigites Cu2GaBO5 and Cu2MnBO5 were found. The study of the thermodynamic and magnetic properties revealed the low-temperature magnetic phase transition inherited from the parent Cu2GaBO5 in all the samples. However, the nature and ordering type for the compounds with different Mn content were different: there was a complex transformation of the magnetic state from the partially ordered AFM (antiferromagnetically) in Cu2Ga0.47Mn0.53BO5, through the spin glass state, to the combined spin glass/ordered state in Cu2Ga0.32Mn0.68BO5 with the appearance of magnetic anisotropy. The evident dependence of Tc (phase transition temperature) on the magnetic field was found as well as its decrease at the nonzero magnetic field in the samples with x = 0.53 and 0.68.

Multiple pseudo-plateaux states and antiferromagnetic pair selection in the XY model in the highly frustrated honeycomb lattice

We present a study of the low temperature magnetic phases of the classical XY model with third nearest neighbor interactions on the honeycomb lattice at the maximally frustrated point under an external magnetic field by extensive Monte Carlo simulations. We focus on the characterization of the emergent low temperature phases, which are a direct consequence of the unusually high numbers of spins per plaquette in the model. Specifically, we show that, since thermal fluctuations partially lift the ground-state degeneracy and select the most collinear states, the selected states are those with the highest number of ‘antiferromagnetic pairs’ (AFp) compatible with the external magnetic field. These AFp are formed in such a way that they maximize the degeneracy of the selected submanifold of ground states. Moreover, two collinear pseudoplateaux emerge at M = 1/3 and M = 2/3. To characterize the magnetization process, we employ Monte Carlo simulations and calculate relevant order parameters to construct the complete temperature vs magnetic field phase diagram.

Heavy Fermion Metal CeB6 in SubTHz and THz Range: The Electron Spin Resonance and Neutron Scattering Studies

Cerium hexaboride attracts attention of solid-state community for almost 50 years. Nowadays, this material is considered as a unique example of a system with orbital ordering resulting in formation of low-temperature magnetic phase with quadrupolar order, the so-called antiferroquadrupole phase. In crystal electric field 4f 1 state of Ce3+ splits into Γ7 doublet and Γ8 quartet, the latter is believed to be the ground state, as long as just Γ8 allows quadrupolar effects. An additional tool for studying rich physics of CeB6 appeared since the discovery in this material the ESR, which is specific to antiferroquadrupole (AFQ) phase and is missing in the paramagnetic phase. In the present review, we have brought together electron spin resonance and inelastic neutron scattering data, which allowed obtaining most detailed structure of the magnetic resonance modes in the AFQ phase in a wide-frequency range up to 0.6 THz. It is shown that apart paramagnetic resonance main mode with homogeneous precession of the magnetization, there are antiferromagnetic resonance modes as well, some of them existing in the AFQ phase. Analysis of the g factors demonstrates that available theories based on Γ8 ground state are facing difficulties with accounting experimental data, thus providing a challenge for developing of a theory correctly describing dynamic magnetic properties of this interesting material.

Unraveling the nature of Fe-doping mediated inter- and intra-chain interactions in Ca3Co2O6

The structural and magnetic properties of quasi-one-dimensional (1D) spin-chain compounds Ca3Co2-xFexO6 (x = 0, 0.1, 0.2 and 0.3) synthesized by a sol-gel method have been systematically studied by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), neutron powder diffraction (NPD), dc and ac magnetization measurements. The samples adopt a rhombohedral crystal structure with the space group R-3c in a temperature range of 5–100 K. Fe ions are found to locate at the trigonal prism Co2 crystallographic sites. Fe doping converts some of low-spin Co3+ ions at Co1 octahedral sites into Co2+ ions. For the high-temperature paramagnetic phase, the Curie paramagnetic temperature θp changes its sign from positive to negative at x = 0.2, implying that the dominant magnetic interaction is driven from ferromagnetic (FM) to antiferromagnetic (AFM) upon Fe substitution. A partial low-spin to high-spin state crossover of the Co1 ions is observed at high Fe-doping level x = 0.3. All the samples exhibit a long-range spin-density wave (SDW) AFM ordering below TN, followed by a spin-glass-like transition at Tf. These low-temperature magnetic phases were significantly suppressed upon Fe doping. The Fe substitution in Ca3Co2-xFexO6 weakened both intrachain and interchain magnetic interactions, caused by enhanced magnetic disorder due to the different magnetic characters of Ising Co and Heisenberg Fe spins.

Cryostat for cooling samples in the study of low-temperature structural and magnetic phase transitions by neutron diffraction

A vertical cryostat was developed built around a Sumitomo RDK-101D cold head. The cryostat is intended for investigating the structure and phase transformations of different materials via neutron diffraction experiments on samples under high vacuum of up to 10−5 mbar. A working temperature of the sample close to 2 K was reached. The drift diameter of the head is 54 mm, making it possible to cool samples with a height of 55 mm and 15 mm in diameter. Test measurements were conducted and illustrative spectra were recorded on the example of a sample of HoFeO3. The cryostat was designed and manufactured at the Frank Laboratory of Neutron Physics of the Joint Institute for Nuclear Research.

Effects of Dzyaloshinskii–Moriya Interactions in Volborthite: Magnetic Orders and Thermal Hall Effect

Volborthite offers an interesting example of a highly frustrated quantum magnet in which ferromagnetic and antiferromagnetic interactions compete on anisotropic kagome lattices. A recent density functional theory calculation has provided a magnetic model based on coupled trimers, which is consistent with a broad \(\frac{1}{3}\)-magnetization plateau observed experimentally. Here we study the effects of Dzyaloshinskii–Moriya (DM) interactions in volborthite. We derive an effective model in which pseudospin-\(\frac{1}{2}\) moments emerging on trimers form a network of an anisotropic triangular lattice. Using the effective model, we show that for a magnetic field perpendicular to the kagome layer, magnon excitations from the \(\frac{1}{3}\)-plateau feel a Berry curvature due to the DM interactions, giving rise to a thermal Hall effect. Our magnon Bose gas theory can explain qualitative features of the magnetization and the thermal Hall conductivity measured experimentally. A further quantitative comparison with experiment poses constraints on the coupling constants in the effective model, promoting a quasi-one-dimensional picture. Based on this picture, we analyze low-temperature magnetic phase diagrams using effective field theory, and point out their crucial dependence on the field direction.

Effect of Mn-doping on the low temperature magnetic phase transitions of BiFeO3

In the present work, we have carried out a comparative study of the low temperature phase transitions using dc magnetization (M (T)) and ac susceptibility (χ (ω, T)) measurements on undoped and 0.3 wt% MnO2 doped BiFeO3 samples in the 2–300 K temperature range. It is shown that MnO2 doping increases the resistivity and decreases the dielectric loss as a result of reduced oxygen vacancy concentration as confirmed by iodometry and x-ray photoelectron spectroscopy (XPS) studies. A comparative study of the dc M (T) and ac χ (ω, T) results on two types of samples reveal that the transitions around 25 K, 110 to 150 K and 260 K are intrinsic to BiFeO3. The widely reported transition at 50 K is argued to be defect induced, as it is absent in the doped samples. We also show that the spin-glass transition temperature (TSG) ∼20 K, determined from an analysis of χ (ω, T), is less than the spin-glass freezing temperature (Tf) ∼25 K in marked contrast to TSG > Tf reported by earlier workers.

Deep learning approach for image classification of magnetic phases in chiral magnets

Chiral magnets have been vastly studied in the last years due to their potential applications as information carrying devices. In this work, it was studied the influence of the magnetocrystalline anisotropy term on the magnetic properties of two-dimensional chiral magnets, by means of Monte Carlo simulations. On the obtained magnetic arrangements, a Deep Learning algorithm based on computer vision techniques, was used for the phase recognition in each sample and their later classification. In this way, the low-temperature phase diagrams were built for a given set of anisotropy constants. It was observed that the skyrmion phase can be stabilized by tuning the magnetocrystalline anisotropy constant; in particular, the skyrmion region in the low-temperature magnetic phase diagrams can be increased by using an easy-plane anisotropy term. The machine learning algorithm described in this work can be easily extended to real-world applications, in the analysis and classification of magnetic force or Lorentz transmission electron microscopy images.