Magnetic Ordering Temperature Research Articles

Jahn–Taller distortion on exchange interactions between cations in Mn x Cu1−x Co2O4 system

Pure-phase Mn x Cu1−x Co2O4 (x = 0, 0.25, 0.5, 0.75 and 1) polycrystalline powders were prepared by using a low-temperature solid-phase reaction method. With doping Mn ion into the octahedral sites, the unit cell transforms from cubic phase to tetragonal phase, and the octahedral Co3+ ions move to the tetrahedral sites and become high spin Co2+ ions. When the doped Mn concentration is low, the sample is dominated by antiferromagnetic (AFM) interactions between tetrahedral Cu2+. As the Mn concentration increases, ferrimagnetic interactions between tetrahedral Co2+ and octahedral Mn3+ can be observed. When the Mn concentration is high enough, due to strong Jahn–Taller distortion, the e g energy level of Mn splits, which weakens the interactions between the tetrahedral and the octahedral cations, and enhances interactions between the octahedral cations. Ferromagnetic interactions between octahedral Mn3+ and AFM interactions between octahedral Mn2+ become obvious. The magnetic ordering temperature has been raised from 20 K to 93 K as the Mn concentration increases, and the maximum exchange bias field is observed to be 1.063 kOe.

Anisotropic magnetic property of single crystals RV6Sn6 (R=Y, Gd−Tm, Lu)

$R$V${_6}$Sn${_6}$ ($R$ = Y, Gd - Tm, Lu) single crystals are synthesized by Sn-flux method and their physical properties are characterized by magnetization, resistivity, and specific heat measurements. Powder X-ray diffraction patterns of all samples can be well indexed with the hexagonal HfFe$_6$Ge$_6$-type structure, where rare-earth atoms form hexagonal layers and vanadium atoms form Kagome layers. At high temperatures, magnetic susceptibility measurements of moment bearing rare-earths ($R$ = Gd - Tm) follow Curie-Weiss behavior. Effective moments estimated from the polycrystalline average of magnetic susceptibility curves are consistent with the values for free $R^{3+}$ ion. Strong magnetic anisotropy due to crystalline electric field effects is observed for moment bearing rare-earths, except GdV$_6$Sn$_6$. The easy magnetization direction is determined to be $c$-axis for $R$ = Tb - Ho and $ab$-plane for $R$ = Er, and Tm. The vanadium ions in $R$V${_6}$Sn${_6}$ possess no magnetic moment. The compounds for $R$ = Y and Lu exhibit typical characteristics of paramagnetic metals. At low temperatures, the magnetic ordering is confirmed from magnetization, specific heat, and resistivity: the highest $T_{N} = 4.9$~K for GdV$_6$Sn$_6$ and the lowest $T_{N} = 2.3$~K for HoV$_6$Sn$_6$. No magnetic ordering is observed down to 1.8~K for $R$ = Er and Tm. A slight deviation of the magnetic ordering temperature from the de Gennes scaling suggests the dominant Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction between rare-earth moments in metallic $R$V${_6}$Sn${_6}$ compounds.

Dimensional Transformation of Molecular Magnetic Materials.

Two-dimensional (2D) magnetic layered materials have revolutionized size dependent magnetism to manipulate spin-based devices. However, it has been challenging to artificially create 2D magnetic materials from three-dimensional (3D) crystal structures with a variety of material groups. Here, we present the dimensionality manipulation via cation exchange of a 3D Prussian blue analogue [RbMnFe(CN)6] toward a 2D magnetic sheet [(K,Rb)(V,Mn)(Cr,Fe)(CN)6] with the magnetic ordering temperature rising from 12 to 330 K. Such a 2D magnetic sheet achieves crystalline V-Cr coordination in the Prussian blue lattice with pronounced anisotropy and stimuli responsiveness. The pressure dependent magnetic tunability of such 2D networks is predicted using first-principles calculations and demonstrated using the phase transitions of the hydrogel. This previously unobserved phenomenon of dimensional manipulation of a bulk crystal structure provides a rational strategy to expand the diversity and chemical compositions of 2D molecular magnetic material libraries.

Pressure effect on magnetism and valence in ferromagnetic superconductor Eu(Fe0.75Ru0.25)2As2

Eu(Fe0.75Ru0.25)2As2 is an intriguing system with unusual coexistence of superconductivity and ferromagnetism, providing a unique platform to study the nature of such coexistence. To establish a magnetic phase diagram, time-domain synchrotron Mössbauer experiments in 151Eu have been performed on a single crystalline Eu(Fe0.75Ru0.25)2As2 sample under hydrostatic pressures and at low temperatures. Upon compression the magnetic ordering temperature increases sharply from 20 K at ambient pressure, reaching ∼49 K at 10.1 GPa. With further compression, the magnetic order is suppressed and eventually collapses. Isomer shift values from Mössbauer measurements and x-ray absorption spectroscopy data at Eu L 3 edge show that pressure drives Eu ions to a homogeneous intermediate valence state with mean valence of ∼2.4 at 27.4 GPa, possibly responsible for the suppression of magnetism. Synchrotron powder x-ray diffraction experiment reveals a tetragonal to collapsed-tetragonal structural transition around 5 GPa, a lower transition pressure than in the parent compound. These results provide guidance to further work investigating the interplay of superconductivity and magnetism.

Stable ferromagnetism and high Curie temperature in VGe2N4

The discovery of monolayer MA2Z4 (M = transition metals; A = IVA elements; Z = VA elements) [Hong et al 2020 Science 369 670] family has led another advance for facilitating and harnessing magnetism in low-dimensional materials. However, only Cr and V based MA2N4 compounds exhibit intrinsic magnetism yet with unsatisfied magnetic ordering temperature. Herein, we identify a stable ferromagnetic number of this family, i.e., VGe2Z4 monolayer, by means of first-principles calculations. It is found that the magnetic configuration sustains under both compression and tensile uniaxial in-plane strain, and the former can act as a positive modulator to enhance magnetic ordering temperature (T C). Electronic structure calculations reveal a large band gap in the spin down channel while band-gapless in the spin up channel, an impressive near-half-metallic character, which is a favorable candidate for spintronic device.

The interplay between the incommensurate spin correlations and superconductivity in Mn substituted La1.90Sr0.10CuO4

The Mn substitution effect on superconductivity and spin correlations in La1.90Sr0.10Cu1−y Mn y O4 was investigated by means of magnetic susceptibility and neutron scattering measurements. The superconducting (SC) transition temperature (T c) and the SC volume fraction are reduced by Mn substitution with y = 0.01. It was clarified that the peak-splitting of the incommensurate (IC) low-energy spin excitations narrows by 30% and the magnetic ordering temperature is significantly enhanced in the y = 0.01 sample. Thus, the SC state and the magnetic ordering state coexist at low temperature. Furthermore, incommensurability (δ), corresponding to half the distance between the IC peaks, and T c (∼ 18 K) of y = 0.01, both follow the linear T c−δ relation established for the pristine La2−x Sr x CuO4 (LSCO). Therefore, although Tc varies as a result of Mn substitution, spin correlations are closely coupled to superconductivity as is the case for LSCO.

Ferrihydrite nanoparticles produced by Klebsiella oxytoca: Structure and properties dependence on the cultivation time

Ferrihydrite nanoparticles were synthesized using Klebsiella oxytoca microorganisms under various cultivation conditions. The cultivation of bacteria was carried out under various lighting conditions, and the duration of cultivation varied from 3 to 56 days. Biogenic ferrihydrite nanoparticles were studied by Mössbauer spectroscopy, magnetometry, and small-angle X-ray scattering. The process of formation of ferrihydrite nanoparticles and the states arising during the cultivation of microorganisms have been investigated. The results of Mössbauer spectroscopy showed that, depending on the time of cultivation, three different states of ferrihydrite can be realized. States differ both in the ratio of defective and non-defective positions, and the size of the particle. Experimental results indicate that ferrihydrite nanoparticles are a system of variable composition and pass through several structural (or morphological) states during the cultivation of microorganisms. A model of the structure of ferrihydrite nanoparticles is proposed, which consists in the presence of an antiferromagnetic dense core with a high Néel temperature and a friable shell with a significantly lower temperature of magnetic ordering.

Gd(OH)F2: A Promising Cryogenic Magnetic Refrigerant.

Magnetic refrigerants with a large magnetocaloric effect (MCE) in a wide temperature range and low magnetic ordering temperature (To) in the sub-kelvin temperature region are not only crucial for adiabatic demagnetization refrigeration but also open up a broader parameter space for the optimal design of adiabatic demagnetization refrigerators. However, such magnetic refrigerants are extremely rare because they require magnetic materials to simultaneously satisfy three conditions: low To, weak magnetic interaction, and high magnetic density. Here, we report the syntheses, heat capacities, and magnetic properties of Gd(OH)3-xFx (1: x = 1, 2: x ≈ 1.5, and 3: x = 2), demonstrating for the first time that the introduction of fluoride anions into antiferromagnetic Gd(OH)3 can effectively regulate its To. Significantly, 3 not only has a To of 0.5 K but also exhibits a large MCE in the temperature range from 0.5 to 4 K, representing the best magnetic refrigerant reported to date in the temperature range of 0.5-4 K from the viewpoint of the MCE.

Role of Dy on the magnetic properties of orthorhombic DyFeO3

Orthoferrites are a class of magnetic materials with a magnetic ordering temperature above 600 K, predominant G-type antiferromagnetic ordering of the Fe-spin system and, depending on the rare-earth ion, a spin reorientation of the Fe spin taking place at lower temperatures. DyFeO3 is of particular interest since the spin reorientation is classified as a Morin transition with the transition temperature depending strongly on the Dy-Fe interaction. Here, we report a detailed study of the magnetic and structural properties of microcrystalline DyFeO3 powder and bulk single crystal using neutron diffraction and magnetometry between 1.5 and 450 K. We find that, while the magnetic properties of the single crystal are largely as expected, the powder shows strongly modified magnetic properties, including a modified spin reorientation and a smaller Dy-Fe interaction energy of the order of 10 {\mu}eV. Subtle structural differences between powder and single crystal show that they belong to distinct magnetic space groups. In addition, the Dy ordering at 2 K in the powder is incommensurate, with a modulation vector of 0.0173(5) c*, corresponding to a periodicity of ~58 unit cells.

Negative-chirality order in the S=12 kagome antiferromagnet CdCu3(OH)6(NO3)2·H2O

The neutron diffraction and nuclear magnetic resonance (NMR) measurements have been used to microscopically analyze the magnetic structure in the $S = 1/2$ kagome antiferromagnet CdCu$_{3}$(OH)$_{6}$(NO$_{3}$)$_{2}\cdot $H$_{2}$O. Below the magnetic ordering temperature $T_N\simeq 4$ K, magnetic Bragg reflections at (110) and (100) were found in the neutron diffraction pattern, which suggests a $q=0$ magnetic structure. Furthermore, the vector spin chirality for the $q=0$ structure was successfully identified from the internal field direction obtained by the $^{14}$N-NMR measurement. Our findings point to a chirality-ordered magnetic structure with negative vector chirality and $\langle 100 \rangle$ anisotropy.

Evolution of magnetic phase in two-dimensional van der Waals Mn1−x Ni x PS3 single crystals

Metal thio(seleno)phosphates MPX3 have attracted considerable attentions with wide spanned band gaps and rich magnetic properties. In this series, two neighboring members MnPS3 and NiPS3 differ in magnetic atoms, magnetic easy axes, spin anisotropy, as well as nearest-neighbor magnetic interactions. The competition between these components may cause intriguing physical phenomena. In this article, the evolution of magnetism of Mn1−x Ni x PS3 series is reported. Despite the incompatible antiferromagnetic orders of two end members, the antiferromagnetism persists as the ground state in the whole substitution region. The magnetic ordering temperature T N show nonmonotonic V-shape behavior, and the reentrant spin glass phase at x= 0.5 is observed. In addition, abnormal bifurcation of T N occurs at x = 0.75, which may be due to the temperature-dependent spin reorientation or phase separation. The evolution of magnetism is further confirmed semi-quantitatively by our density functional theory calculations. Our study indicates that exotic magnetism can be intrigued when multi-degrees of freedom are involved in these low-dimensional systems, which call for more in-depth microscopic studies in future.

Magnetic behavior of Ru substituted skyrmion metal MnSi

We report on the structural and magnetic properties of Ru substituted skyrmion metal MnSi i.e. MnRu x Si for the nominal compositions of . The composition–temperature (x–T) phase diagram illustrates the substitution-driven changes in the magnetic behavior. It is confirmed that the magnetic ordering temperature (para-to helimagnetic) and the effective magnetic moment decrease with increasing x. This indicates the suppression of magnetic order by the substitution of Ru in MnSi. However, the magnetic nature is sustained up to a concentration of about x = 0.1 above which the system exhibits spin-glass like nature as inferred from the negative Curie–Weiss temperature , reduced magnetic moment (of the order 10) and linear M–H (at 2 K) in x = 0.5. MnRu x Si is found to avoid the quantum phase transition and exhibits a composition-driven magnetic to spin-glass like transition.

Setting of the magnetic structure of chiral kagome antiferromagnets by a seeded spin-orbit torque.

The current-induced spin-orbit torque switching of ferromagnets has had huge impact in spintronics. However, short spin-diffusion lengths limit the thickness of switchable ferromagnetic layers, thereby limiting their thermal stability. Here, we report a previously unobserved seeded spin-orbit torque (SSOT) by which current can set the magnetic states of even thick layers of the chiral kagome antiferromagnet Mn3Sn. The mechanism involves setting the orientation of the antiferromagnetic domains in a thin region at the interface with spin currents arising from an adjacent heavy metal while also heating the layer above its magnetic ordering temperature. This interface region seeds the resulting spin texture of the entire layer as it cools down and, thereby, overcomes the thickness limitation of conventional spin-orbit torques. SSOT switching in Mn3Sn can be extended beyond chiral antiferromagnets to diverse magnetic systems and provides a path toward the development of highly efficient, high-speed, and thermally stable spintronic devices.

Understanding the structural behaviors and abrupt enhancement in magnetic ordering temperature of Fe-substituted Mn3O4

We report an increase in magnetic ordering temperature from 43 K to 425 K and a structural phase transition from tetragonal I41/amd (x = 0.00) to cubic Fd3¯m (x = 0.40) in (Mn1-xFex)3O4 (x = 0.00–0.40) systems. The phase diagram clearly demonstrates that lattice constants a√2 and c converge smoothly with Fe-substitution and become equal for x = 0.40, while the room temperature magnetic moments increase from 0.15 μB/f.u. (x = 0.10) to 1.1 μB/f.u. (x = 0.40). There are two possible cationic distribution scenarios for Fe cations; one leads Fe2+ to occupy the tetrahedral site, while the other leads Fe3+ to occupy the octahedral site. The octahedral site preference energy indicates that Fe3+ cations occupying the octahedral site have higher preference values. X-ray absorption spectra studies of MnL2,3-edge and FeL2,3-edge confirm that Fe3+ cations occupy octahedral sites by replacing Jahn-Teller active Mn3+ cations. OK-edge spectra revealed that Jahn-Teller distortion is reduced in the cubic x = 0.40 system. However, the higher structural symmetry is not the most significant factor in the abrupt enhancement of magnetic ordering temperature. The increase in magnetic ordering temperature could be understood on the basis of increased spin moment coupling between tetrahedral and octahedral cations due to substitution of Fe3+ cations at the octahedral site.

Longitudinal fluctuations of Co spin moments and their impact on the Curie temperature of the Heusler alloy Co2FeSi

The magnetism of the full Heusler alloy Co2FeSi with its high magnetic ordering temperature is studied on a first-principles basis employing the disordered local moment approximation, the magnetic force theorem and single-site spin fluctuation theory as formulated recently in the framework of the Local Spin Density Approximation. We find that the magnetic moments of Fe and Co in Co2FeSi exhibit a quite distinctive behavior at high temperatures. The Fe moments are well localized and keep their magnitude unchanged with temperature, whereas the Co moments are itinerant and show a temperature dependence. We find that the effects of magnetic disorder strongly renormalize the Fe-Co inter-atomic exchange interactions. Our results suggest a deficiency of the classical Heisenberg model with rigid localized atomic spin moments for the description of magnetism in Co2FeSi. An accurate estimation of the Curie temperature is obtained by taking into the account the thermal longitudinal fluctuations the Co moments.

Hydrostatic pressure effect on the Co-based honeycomb magnetBaCo2(AsO4)2

The honeycomb antiferromagnet ${\mathrm{BaCo}}_{2}{({\mathrm{AsO}}_{4})}_{2}$, in which small in-plane magnetic fields ($H1\ensuremath{\approx}0.26$ T and $H2\ensuremath{\approx}0.52$ T at $T=1.8$ $\mathrm{K}<{T}_{N}\ensuremath{\approx}5.4$ K) induce two magnetic phase transitions, has attracted attention as a possible candidate material for the realization of Kitaev physics based on the $3d$ element ${\mathrm{Co}}^{2+}$. Here, we report on the change in the transition temperature ${T}_{N}$ and the critical fields $H1$ and $H2$ of ${\mathrm{BaCo}}_{2}{({\mathrm{AsO}}_{4})}_{2}$ with hydrostatic pressure up to $\ensuremath{\sim}20$ kbar, as determined from magnetization and specific heat measurements. Within this pressure range, a marginal increase in the magnetic ordering temperature is observed. At the same time, the critical fields are changed significantly (up to $\ensuremath{\sim}25$--35%). Specifically, we find that $H1$ is increased with hydrostatic pressure, i.e., the antiferromagnetic state is stabilized, whereas $H2$, which was previously associated with a transition into a proposed Kitaev spin-liquid state, decreases with increasing pressure. We discuss to what extent these results are compatible with suggested models with sizable third-nearest-neighbor exchange. Overall, the results put constraints on the magnetic models that are used to describe the low-temperature magnetic properties of ${\mathrm{BaCo}}_{2}{({\mathrm{AsO}}_{4})}_{2}$.

First-principles study on giant magneto-optical Kerr effect in double perovskites Sr2BB′O6 (B = Cr, Mo, B′ = W, Re, Os)

Sr2CrB′O6 (B′ ​= ​W, Re, Os) are famous double perovskite oxides because of their high magnetic ordering temperature Tc and half metallic property. Based on them, Sr2MoB′O6 were designed, and were also reported to be half metals and possess Tc well above the room temperature. In this work, we systematically investigated the magneto-optical Kerr effect of Sr2BB′O6 (B = Cr, Mo, B′ ​= ​W, Re, Os) by the density functional theory. All these compounds exhibit remarkable magneto-optical Kerr effect in the infrared to visible spectral ranges. The maxima of Kerr rotation for half metals Sr2CrWO6, Sr2CrReO6, and Sr2MoOsO6 can reach up to −5.02°, −2.56°, and 4.45°, respectively. The characteristics of the Kerr spectra in lower and higher energy region are dominated by two different mechanisms, respectively. Our work provides a theoretical basis for designing new magneto-optical materials.

Magnetism, spin-phonon coupling and Kitaev interaction in Mott insulator La2ZnIrO6 single crystal oxide

Due to the coexistence of spin-orbital coupling (SOC) and electron correlation interactions, double perovskite iridates are considered ideal material to investigate the Mott insulating states and Kitaev interactions. In this work, we studied the spin-orbital coupling (SOC), spin-phonon coupling (SPC), and Kitaev magnetism in antiferromagnetic La2ZnIrO6 single crystal by magnetometer and Raman scattering spectroscopy measurements. Based on the temperature dependence of the magnetization under different magnetic fields, we found that all the initial temperatures of the paramagnetic-ferromagnetic transition were basically concentrated around 8.8 K, while the temperature of ferromagnetic-antiferromagnetic transition continued to decrease from 7 K to 2 K. Anomalous modes of frequency softening at magnetic-order temperature were detected from the polarized Raman scattering spectra, indicating the presence of strong spin-phonon coupling and Kitaev interaction in La2ZnIrO6.

Manipulation of Antiskyrmion Phase in Mn2+xNi1−xGa Tetragonal Heusler System

The inhomogeneous magnetization distribution in antiskyrmions helps in nullifying the unwanted skyrmion Hall effect generally found in the current driven motion of skyrmions. At present, the observation of room-temperature antiskyrmion phase is limited to only a very few materials. Here, we present the evidence of a tunable antiskyrmion phase in the ${D}_{2d}$-symmetry-based tetragonal Heusler system ${\mathrm{Mn}}_{2+x}{\mathrm{Ni}}_{1\ensuremath{-}x}\mathrm{Ga}$. With the help of dc magnetization, ac susceptibility, and topological Hall effect measurements, we demonstrate that the potential antiskyrmion phase can be tuned over a wide compositional range with magnetic ordering temperature of above 600 K. In addition, we find the existence of multiple topological phase transitions for a certain Mn/Ni ratio where the magnetic anisotropy attains its maxima. Our micromagnetic simulations suggest that the transition from antiskyrmionium and antiskyrmion pockets to the conventional antiskyrmion phase at the optimal value of magnetic anisotropy might be responsible for the observed multiple topological transitions in the present materials. The expected small size of antiskyrmions in the present low magnetic-moment-based ferrimagnetic system gives a great advantage over other skyrmion and antiskyrmion hosting materials for their potential application in racetrack-based memory devices.

Drastic enhancement of magnetic critical temperature and amorphization in topological magnet EuSn2P2 under pressure

High pressure is an effective tool to induce exotic quantum phenomena in magnetic topological insulators by controlling the interplay of magnetic order and topological state. This work presents a comprehensive high-pressure study of the crystal structure and magnetic ground state up to 62 GPa in an intrinsic topological magnet EuSn2P2. With a combination of high resolution X-ray diffraction, 151Eu synchrotron Mössbauer spectroscopy, X-ray absorption spectroscopy, molecular orbital calculations, and electronic band structure calculations, it has been revealed that pressure drives EuSn2P2 from a rhombohedral crystal to an amorphous phase at 36 GPa accompanied by a fourfold enhancement of magnetic ordering temperature. In the pressure-induced amorphous phase, Eu ions take an intermediate valence state. The drastic enhancement of magnetic ordering temperature from 30 K at ambient pressure to 130 K at 41.2 GPa resulting from Ruderman–Kittel–Kasuya–Yosida (RKKY) interactions likely attributes to the stronger Eu–Sn interaction at high pressure. These rich results demonstrate that EuSn2P2 is an ideal platform to study the correlation of the enhanced RKKY interactions, disordered lattice, intermediate valence, and topological state.