Ordering Temperature Research Articles

Recent Progress in Exploring the Magnetic Solids with Room‐Temperature Skyrmionic Spin Configurations

Magnetic materials with skyrmionic spin configurations have been widely investigated in recent years owing to their promising applications for magnetic memory storage and logic circuits. Considering their practical applications, room‐temperature skyrmion‐hosting magnetic materials shall be investigated comprehensively. The characteristics of magnetic skyrmions are closely associated with the physical properties of the host magnetic materials. The symmetry of the crystal structure, magnetic order temperature, and geometric shape of magnetic materials significantly affect the formation and annihilation of magnetic skyrmions. In addition to the intrinsic properties of materials, detection methods are vital to the investigation of new materials. Herein, the techniques for detecting magnetic skyrmions are briefly summarized, and the results of room‐temperature magnetic skyrmion hosting magnetic solids (the artificial materials are not included) with different topological characteristics from recent years are presented.

Study of temperature dependent phonon properties of SbxGe1-xSe (x = 0.00 and 0.15) crystals

A detailed study of phonon properties of SbxGe1-xSe (x = 0.00 and 0.15) crystals is demonstrated here. DVT technique was employed for crystal growth. Powder XRD confirmed the phase-singularity and revealed the structural minutiae of the grown materials. The surface morphology of the crystals was studied under a high-resolution microscope. Room temperature Raman spectroscopy revealed a blue shift in the out-of-plane mode (B3g) and in-plane modes (Ag(1) and Ag(2)) of GeSe with Sb mixing. Further, the temperature-dependent Raman spectroscopy showed a monotonous red-shift and softening of modes as the temperature increases from 103 K to 298 K. The first order temperature coefficients for peaks B3g and Ag(2) are calculated. Shifting of modes pertaining to laser-power is also recorded for room temperature, and thermal conductivity of the crystals is determined. A strong anisotropy is found in the thermal conductivity for out-of-plane and in-plane modes for both crystals.

Indium segregation in Gd5(Si, Ge)4 magnetocaloric materials

Chemical substitution is one of the most efficient tools to tune and optimize magnetic and magnetocaloric properties of the giant magnetocaloric materials. In particular, Indium substitutions could be useful both for tuning properties of these interesting intermetallic materials and to unveil their local-scale behavior across the magnetostructural transition via hyperfine techniques. Hence, in order to investigate the effect of Indium additions on the crystal structure, micro-structure, magnetic and magnetocaloric properties, a series of In-containing samples derived from the base Gd5Si1.2Ge2.8 stoichiometry were prepared. The major findings are that while In is insoluble in the 5: 4 phase, it will instead promote the emergence of the impurity 5: 3 phase and segregates into this phase. Hence, In leads to major crystallographic changes, which enhance atomic disorder and disrupt the Si to Ge ratio in the 5: 4 phase. Subsequently, a higher 5: 4 unit cell volume and a lower magnetic ordering temperature are found in the In-substituted samples. Finally, the magnetocaloric properties of the In-substituted samples reveal a detrimental effect on the maximum magnetic entropy change.

Strain‐Induced Magnetism in MSi2N4 (M = V, Cr): A First‐Principles Study

AbstractThe discovery of ferromagnetic 2D van der Waals (vdW) materials has opened up opportunities for exploring and harnessing magnetism in low‐dimensional limit, and developing innovative paper‐like spintronic devices. Stress engineering demonstrates unique advantages in regulating material properties, especially for 2D materials. With the aid of first‐principles calculations, the effectiveness of strain engineering for modulating the electronic and magnetic properties of MSi2N4 (M = V, Cr) monolayers from a new family of layer‐structured 2D vdW materials is explored. Their magnetic configurations are effectively tuned via the uniaxial and biaxial tensile strain of in‐plane lattice vectors. VSi2N4 exhibits half‐metallic ferromagnetic character, whereas CrSi2N4 is prone to be antiferromagnetic (AFM), which is rationalized from the competition between the direct metal–metal AFM interaction and the 900 ferromagnetic superexchange. More intriguingly, VSi2N4 demonstrates a high magnetic ordering temperature of 285 K, which is close to room temperature. The predicted appealing properties of VSi2N4 under strain await experimental confirmation.

Effect of Indirect Correlations on the Critical Temperature of Ordering in a Binary А0.5В0.5 System in the Bulk Phase and Limited Cubic Domains

Effect of Indirect Correlations on the Critical Temperature of Ordering in a Binary А0.5В0.5 System in the Bulk Phase and Limited Cubic Domains

Frustration enhanced by Kitaev exchange in a j̃eff=12 triangular antiferromagnet

Triangular Heisenberg antiferromagnets are prototypes of geometric frustration, even if for nearest-neighbor interactions quantum fluctuations are not usually strong enough to destroy magnetic ordering: stronger frustration is required to stabilize a spin-liquid phase. On the basis of static magnetization and electron spin resonance measurements, we demonstrate the emergence of ${\tilde{j}_{\text{eff}}=\frac12}$ moments in the triangular-lattice magnet Na$_2$BaCo(PO$_4$)$_2$. These moments are subject to an extra source of frustration that causes magnetic correlations to set in far above both the magnetic ordering and Weiss temperatures. Corroborating the $\tilde{j}_{\text{eff}}=\frac12$ ground state, theory identifies ferromagnetic Kitaev exchange anisotropy as additional frustrating agent, altogether putting forward Na$_2$BaCo(PO$_4$)$_2$ as a promising Kitaev spin-liquid material.

Polar Structure and Two-Dimensional Heisenberg Antiferromagnetic Properties of Arylamine-Based Manganese Chloride Layered Organic-Inorganic Perovskites.

The breaking of inversion symmetry can enhance the multifunctional properties of layered hybrid organic–inorganic perovskites. However, the mechanisms by which inversion symmetry can be broken are not well-understood. Here, we study a series of MnCl4-based 2D perovskites with arylamine cations, namely, (C6H5CxH2xNH3)2MnCl4 (x = 0, 1, 2, 3), for which the x = 0, 1, and 3 members are reported for the first time. The compounds with x = 1, 2, and 3 adopt polar crystal structures to well above room temperature. We argue that the inversion symmetry breaking in these compounds is related to the rotational degree of freedom of the organic cations, which determine the hydrogen bonding pattern that links the organic and inorganic layers. We show that the tilting of MnCl6 octahedra is not the primary mechanism involved in inversion symmetry breaking in these materials. All four compounds show 2D Heisenberg antiferromagnetic behavior. A ferromagnetic component develops in each case below the long-range magnetic ordering temperature of ∼42–46 K due to spin canting.

Hole and Electron Doping of Topochemically Reduced Ni(I)/Ru(II) Insulating Ferromagnetic Oxides

LaxSr2-xNiRuO6, LaxSr4-xNiRuO8, and LaxSr3-xNiRuO7 are, respectively, the n = ∞, 1, and 2 members of the (Lax/2Sr1-(x/2))nSr(Ni0.5Ru0.5)nO3n+1 compositional series. Reaction with CaH2, in the case of the LaxSr2-xNiRuO6 perovskite phases, or Zr oxygen getters in the case of the LaxSr4-xNiRuO8 and LaxSr3-xNiRuO7 Ruddlesden-Popper phases, yields the corresponding topochemically reduced (Lax/2Sr1-(x/2))nSr(Ni0.5Ru0.5)nO3n-1 compounds (LaxSr2-xNiRuO4, LaxSr4-xNiRuO6, and LaxSr3-xNiRuO5), which contain Ni and Ru cations in square-planar coordination sites. The x = 1 members of each series (LaSrNiRuO4, LaSr3NiRuO6, and LaSr2NiRuO5) exhibit insulating ferromagnetic behavior at low temperature, attributable to exchange couplings between the Ni1+ and Ru2+ centers they contain. Increasing the La3+ concentration (x > 1) leads to a reduction of some of the Ru2+ centers to Ru1+ centers and a suppression of the ferromagnetic state (lower Tc, reduced saturated ferromagnet moment). In contrast, increasing the Sr2+ concentration (x < 1) oxidizes some of the Ru2+ centers to Ru3+ centers and enhances the ferromagnetic coupling (increased Tc, increased saturated ferromagnet moment) for the n = ∞ and n = 2 samples but appears to have no influence on the magnetic ordering temperature of the n = 1 samples. The magnetic couplings and influence of doping are discussed on the basis of superexchange and direct exchange couplings between the square-planar Ni and Ru centers.

Revisiting the influence of Fe excess in the synthesis of BaFe2S3

BaFe$_2$S$_3$ is a quasi-one-dimensional antiferromagnetic insulator that becomes superconducting under hydrostatic pressure. The magnetic ordering temperature, $T_N$, as well as the presence of superconductivity have been found to be sample dependent. It has been argued that the Fe content may play a decisive role, with the use of 5%mol excess Fe being reportedly required during the synthesis to optimize the magnetic ordering temperature and the superconducting properties. However, it is yet unclear whether an Fe off-stoichiometry is actually present in the samples, and how it affects the structural, magnetic and transport properties. Here, we present a systematic study of compositional, structural and physical properties of BaFe$_{2+\delta}$S$_3$ as a function of the nominal Fe excess $\delta$. As $\delta$ increases, we observe the presence of an increasing fraction of secondary phases but no systematic change in the composition or crystal structure of the main phase. Magnetic susceptibility curves are influenced by the presence of magnetic secondary phases. The previously reported maximum of $T_N$ at $\delta$=0.1 was not confirmed. Samples with nominal $\delta$=0 present the lowest $T_N$ and the resistivity anomaly at the highest temperature $T^*$ while, for $\delta \geq 0.05$, both quantities and the transport gap are seemingly $\delta$-independent. Finally, we show that crystals free of ferromagnetic spurious phases can be obtained by remelting samples with nominal $\delta$=0.05 in a Bridgman process.

Coexistence of exchange bias and memory effect in nanocrystalline CoCr2O4

This article reports exchange bias (EB) and memory effect in nanocrystalline CoCr2O4 with a detailed understanding of the origin of the observed effect. In order to investigate the EB effect in nanocrystalline CoCr2O4, two samples are synthesized with particle sizes 12 and 7 nm via sol-gel route (the second one is embedded in amorphous silica host). For both the samples, despite usual core-shell interaction or ‘surface’ domination, conventional EB effect is observed where the EB field sharply decreases with temperature and vanishes over the spiral ordering temperature (TS) of the samples. A bulk sample of the same composition, with particle size greater than 150 nm, reproducing similar characteristics, rules out the origin of EB due to surface effect. Results indicate that the exchange bias is due to inherent magnetic inhomogeneity: interaction between spiral spin order and collinear ferrimagnetic order below TS. The ac-susceptibility measurement points towards absence of surface glassiness in the sample with 12 nm particle size. However, the sample with particle size 7 nm reveals the characteristics of superspin-glass (SSG), confirmed by magnetic memory effects both in time (t) and temperature (T) dependent magnetization protocols.

Massive Dirac fermions and strong Shubnikov–de Haas oscillations in single crystals of the topological insulator Bi2Se3 doped with Sm and Fe

Topological insulators (TIs) are emergent materials with unique band structure, which allow the study of quantum effect in solids, as well as contribute to high-performance quantum devices. To achieve the better performance of TIs, here, we present a codoping strategy using synergistic rare-earth (RE) Sm and transition-metal Fe dopants in ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ single crystals, which combine the advantages of both a transition-metal-doped TI [high ferromagnetic ordering temperature and observed quantum anomalous Hall effect (QAHE)], and a RE doped TI (large magnetic moments and significant spin-orbit coupling). In the as-grown single crystals, clear evidences of ferromagnetic ordering were observed. The angle-resolve photoemission spectroscopy indicates the ferromagnetism opens a \ensuremath{\sim}44 meV band gap at the surface Dirac point. Moreover, the mobility of the carriers at 3 K is $\ensuremath{\sim}7400\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{Vs}$, and we thus observed an ultra-strong Shubnikov--de Haas oscillation in the longitudinal resistivity, as well as the Hall steps in transverse resistivity 14 T. Our transport and angular-resolved photoemission spectroscopy results suggest that the RE and transition metal codoping in the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ system is a promising avenue to implement the QAHE, as well as harnessing the massive Dirac fermion in electrical devices.

Synthesis, characterization and multiferroic properties of the doubly-ordered polar perovskite Na[formula omitted]NiWO[formula omitted] ([formula omitted] = Er, Tm, and Lu)

The doubly-ordered perovskite NaLnNiWO6 (Ln = Lanthanide) have been prepared at high-pressure and high-temperature conditions. Analysis of powder X-ray diffraction data reveals that these compounds crystallize in a polar monoclinic (P21) structure. In addition to the commonly observed rock salt ordering of B-site cations, an unusual layered order of A-site cations occurs in these compounds. Highly charged second-order Jahn–Teller (SOJT) active cation (W6+) at the B-site stabilizes the layered ordering and the overall crystal structure. Magnetization and heat capacity measurements confirm the antiferromagnetic order of these compounds in the temperature (TN) range 18–21 K. Dielectric anomaly, and dc-bias signal at the magnetic ordering temperature (TN) reveal the multiferroic properties of these compounds.

Weak ferromagnetic – Antiferromagnetic transition in MnFe0.25Sb

Magnetic properties of intermetallic MnFe0.25Sb through magnetization and heat capacity measurements with the support of powder XRD patterns and electronic structure calculations are presented in this paper. Fe atoms occupy the vacant 2d sites in MnSb and as a result, the a-parameter increases and the c-parameter decreases. Both the magnetic ordering temperature and the saturation magnetization are smaller than those of MnSb. An A-type antiferromagnetic behaviour, where the Mn moments order ferromagnetically in the ab-plane (intra-layer) and the interlayer (along the c-direction) exchange is antiferromagnetic, was observed below 25 K. Specific heat data too revealed a small change in entropy at 28 K, consistent with the observations from the magnetization data. Furthermore, at 241 K a ferrimagnetic to paramagnetic transition was observed. Electronic structure calculations showed that MnFe0.25Sb stabilized in an A-type antiferromagnetic (AFM) like ground state. In this state, the exchange interactions between the moments of (i) Mn atoms along the c direction and (ii) Mn and Fe atoms are antiferromagnetic and is ferromagnetic between the moments of Mn atoms in the ab-plane. The competition between these interactions leads to magnetic frustration, which also emphasizes the near-degeneracy of different magnetic states.

Pressure Dependence of the Magnetic Ordering Temperature (Tc) for the Na2Mn[Mn(CN)6] Noncubic Prussian Blue Analogue.

The pressure dependence of the magnetic properties of rhombohedral Na2Mn[Mn(CN)6] up to 10 kbar has been studied. The magnetic ordering temperature, Tc, for Na2Mn[Mn(CN)6] reversibly increases with increasing applied hydrostatic pressure, P, by 9.0 K (15.2%) to 68 K at 10 kbar with an average rate of increase, dTc/dP, of 0.86 K/kbar. The magnetization at 50 kOe and remanent magnetization, Mr(H), remain constant with an average value of 13,100 ± 200 and 8500 ± 200 emuOe/mol. The coercive field Hcr increases by 12% from 13,400 to 15,000 Oe. The increase and rate of increase of Tc for rhombohedral Na2Mn[Mn(CN)6] are reduced with respect to monoclinic A2Mn[Mn(CN)6] (A = K and Rb), but they are still greater than those of cubic Cs2Mn[Mn(CN)6]. This is attributed to the compression of the MnNC framework bonding without decreasing ∠MnII-N≡C, maintaining the unit cell in accord with cubic A = Cs at lower applied pressures, and not due to reduction in ∠MnII-N≡C, which correlates with increasing Tc that is reported for A = K and Rb as well as Cs at higher applied pressures.

Systematic Study of Monolayer to Trilayer CrI3: Stacking Sequence Dependence of Electronic Structure and Magnetism

In recent years, two-dimensional magnetic materials have attracted a lot of attention due to their unique magnetic properties, which can be manipulated by various degrees of freedom in a van der Waals stacked heterostructure, where different interlayer stackings affect the properties in different ways by exploiting weak interlayer interactions. An interesting example is CrI3, where the interlayer magnetic coupling can be determined by the stacking sequence. Here we present a systematic study of the magnetic properties of monolayer, bilayer, and trilayer CrI3 by using density functional theory and Monte Carlo simulations. The effects of different stacking sequences and the number of layers on the magnetic interaction in multilayer CrI3 have been critically analyzed. We have found that the antiferromagnetic interlayer coupling occurs with specific interlayer shifting, while intralayer exchange interactions increase with the number of layers. It is noteworthy that interlayer interactions are independent of the number of layers, whereas the magnetic ordering temperature rises with the increasing number of layers. Moreover, we present the Curie temperatures obtained from Monte Carlo simulations and adiabatic magnon spectra for all the stacking sequences considered here.

Ab-initio study of the electronic structure and magnetic properties of Ce2Fe17

The Ce2Fe17 intermetallic compound has been studied intensely for several decades; its low-temperature state is reported experimentally either as ferromagnetic or antiferromagnetic by different authors, with a measured ordering temperature ranging within a hundred Kelvin. The existing theoretical investigations overestimate the experimental total magnetic moment of Ce2Fe17 by 20–40% and predict a ferromagnetic ground state. By means of first-principle electronic structure calculations, we show that the total magnetic moment of Ce2Fe17 can be reproduced within the Local Density Approximation while functionals based on the Generalized Gradient Approximation fail. Atomistic spin dynamics simulations are shown to capture the change in the magnetic state of Ce2Fe17 with temperature, and closely replicate the reported helical structure that appears in some of the experimental investigations.

Impact of stacking order and annealing temperature on properties of CZTS thin films and solar cell performance

In this study, Cu2ZnSnS4 (CZTS) thin films were synthesized by a two-stage process. In the first stage, CuSn/Zn/Cu (E-type) and CuSn/ZnS/Cu (B-type) stacked films were formed using the sputtering method. In the second stage, precursor films were annealed in sulfur atmosphere utilizing various annealing temperatures (500, 525, 550 and 575 °C) employing the Rapid Thermal Processing (RTP) method. The EDX measurements demonstrated that almost all the samples had Cu-poor and Zn-rich compositions, as targeted. The XRD patterns of all the CZTS samples were dominated by diffraction peaks of the kesterite CZTS phase. In addition to CZTS phase, Cu–S/Sn–S based secondary phases in all E-type CZTS thin films and some B-type CZTS samples annealed at lower temperatures (500 and 525 °C) were observed. The samples annealed at above 525 °C revealed purer crystal structure in terms of secondary phases and they have more promising crystallite size in both types of CZTS thin films. The Raman spectroscopy measurements confirmed the formation of kesterite CZTS phase and distinguished the formation of Cu2SnS3 (CTS) phase for some samples. The samples annealed at 550 °C presented purer structure for potential solar cell application. The SEM surface and cross-section images of all CZTS samples displayed dense and polycrystalline structures but samples annealed at 550 °C presented a larger-grained surface and cross-section structure in both types of CZTS films. PL spectra of the B-type CZTS samples exhibited a purer band structure with respect to E type CZTS samples according to their PL band values. The best solar cell performance was achieved with CZTS thin film prepared using CuSn/ZnS/Cu stack annealed at 550 °C temperature with 252 mV, 32 mA/cm2, and 3.79% parameters.

Reduced order modeling for coupled thermal-hydraulics and reactor physics problems

The development of a new generation of reactor presents several modelling challenges that cannot be effectively addressed by traditional tools used for Light Water Reactors. Multi-physics approaches allow for a comprehensive modeling of reactor cores, as they intrinsically couple the involved physics (i.e. fluid-dynamics, heat transfer, neutronics, thermo-mechanics), but require intense computational efforts that preclude their use in reactor control applications. This work aims at applying a Reduced Order Model (ROM) technique to multi-physics modelling. Such objective is achieved through a ROM technique previously used for Navier-Stokes equations and thermal-hydraulics problems (POD-FV-ROM). The technique used in this article, is built on a modeling framework developed ad-hoc for the Finite Volume (FV) scheme and relies on the Proper Orthogonal Decomposition (POD) and the Method of Snapshots. The reduced order multi-physics approach outlined in this work has been tested with success on a Lid-Driven-Cavity-based homogeneous reactor model. Reduced-order simulations have reproduced accurately the full order velocity, temperature, neutron flux and precursor concentration fields, resulting in relative L2 norms of the differences between full order and reduced order simulations below 1 % for all the considered fields.

Observation of structural anomaly and low-field magnetocaloric effect in Cu2OSeO3

We report here the detailed investigation on structural and magnetic behavior of the polycrystalline sample of Cu2OSeO3. The Mott insulator Cu2OSeO3 is a fascinating material showing skyrmionic state and magneto-electric coupling. We observe clear signature of thermal hysteresis between field-cooled and field-cooled heating data in the low-field magnetization versus temperature curve just below the magnetic ordering temperature of TC ~ 60 K. Temperature dependent powder x-ray diffraction data indicate the presence of lattice anomaly below TC. The sample also shows reasonable value of conventional magnetocaloric effect for small value of applied magnetic field, which can be related to the change in the magnetic structure with field.

Magnetic-order-induced ferroelectric polarization in the polar antiferromagnets RFeWO6 ( R = Sm, Gd, Er, and Tm)

We report the synthesis, structure, and magnetism-induced multiferroic properties of the polar magnets $R\mathrm{FeW}{\mathrm{O}}_{6}$ (R = Tm, Sm, Gd, and Er). All these compounds crystallize in the orthorhombic structure with the polar symmetry $Pna{2}_{1},$ which results from the ordering of ${\mathrm{Fe}}^{3+}$ and ${\mathrm{W}}^{6+}$ ions at different crystallographic sites. DC magnetization and specific heat measurements confirm the antiferromagnetic order of ${\mathrm{Fe}}^{3+}$ spins at ${T}_{\mathrm{N}1}=14\ensuremath{-}18\phantom{\rule{4pt}{0ex}}\mathrm{K}$ and magnetic ordering of R ions at low temperatures. The magnetic order of ${\mathrm{Fe}}^{3+}$ ions in these compounds is accompanied by a dielectric anomaly and a change in electric polarization. Intriguingly, a second ferroelectric transition occurs at the magnetic ordering temperature (${T}_{\mathrm{N}2}=5.5\phantom{\rule{4pt}{0ex}}\mathrm{K}$) of ${\mathrm{Tm}}^{3+}$ ions in $\mathrm{TmFeW}{\mathrm{O}}_{6}$. The magnetic field dependent behavior of electric polarization varies with R ion, indicating the coupling between $4f\ensuremath{-}3d$ spins. The emergence of change in ferroelectric polarization at the magnetic ordering temperatures demonstrates the multiferroic nature of the polar magnets $R\mathrm{FeW}{\mathrm{O}}_{6}$ (R = Tm, Sm, Gd, and Er). Our study indicates that the aeschynite type family of compounds with polar symmetry can be an excellent platform to understand the role of $4f\ensuremath{-}3d$ coupling on multiferroicity.