Ordering Temperature Tc Research Articles

Deformation behavior and magnetic properties of equiatomic FeNi single crystals

An equiatomic Fe–Ni alloy in its disordered A1 structure is a soft magnetic alloy. On ordering to the L10 ordered phase, it shows significant magneto-crystalline anisotropy and a strong permanent magnet behavior and is of interest as a rare earth-free permanent magnet. However, synthesis of an L10 phase in a bulk form remains a challenge due to its low critical ordering temperature Tc and consequent extremely slow ordering kinetics. This phase is present in asteroids, and how it was formed remains unclear. The likely mechanism is enhanced diffusion kinetics due to extreme dislocation densities and vacancy concentrations produced by deformation during asteroid collisions and the presence of S. Prior to examining extreme deformation in an FeNi alloy comparable to that in asteroid collisions, low strain rate deformation behavior, magnetic properties, and the structure of undoped and S-doped [100]-oriented FeNi single crystals were carried out. Controlled deformation at a strain rate of 1 × 10−5/s showed the yield point to be 89 MPa, and the critical resolved shear stress was 25.7 MPa. The dislocation densities obtained were ∼1017/m2. The saturation magnetization value was ∼147–151 emu/g both before and after deformation, comparable to NdFeB magnets. Coercivity increased slightly from ∼0.04–0.4 to ∼5 Oe after deformation due to an increase in dislocation density. The x-ray diffraction scan of S-doped and deformed single crystals after annealing at 300 °C, just below Tc, showed no evidence of L10 order. These data serve as a baseline for extreme strain rate deformation where much higher dislocation densities and vacancy concentrations can be obtained to facilitate L10 order.

Strain enhanced magnetism of V-implanted CrI3 monolayer

In recent years, one of the urgent issues for two dimensional (2D) magnetic materials is to find efficient ways in enhancing the magnetic ordering temperature Tc. It is believed that an in-plane (IP) compressive strain can greatly enhance the interatomic interactions by shortening the chemical bond length if at all possible, leading to the enlarged spin exchange and possibly higher Tc. However, a large compressive strain usually favors antiferromagnetic (AFM) ordering due to growing dominance of the Pauli exclusion principle, in contradiction with the common requirement of nonzero magnetization. In compromise, ferrimagnetic (FiM) ordering can be alternated by synthesizing artificial 2D compound with two magnetic sublattices. In this work, we propose a V-implanted CrI3 monolayer, short for V-(CrI3)2, and study its FiM ordering under a series of IP biaxial strains using the first-principles calculations and Monte Carlo simulations. It is found that the V-(CrI3)2 monolayer may evolve from the stripy-type AFM insulator toward the FiM half-metal with net magnetic moment of 5.0 μB/f.u. aligned in parallel to the ab-plane upon increasing the IP biaxial strain up to ∼−3% (compressive strain) and beyond. As the IP biaxial strain increases up to ∼−5%, the Tc of the FiM state may be raised to room temperature. This work suggests that the IP strain engineering combined with spin implantation can be an alternative strategy for enhancing 2D magnetism.

Combined pressure and magnetic field induced caloric effects in Fe7Se8 single crystals doped with Ni and Co ions

The influence of Co and Ni ion doping on the crystal structure, phase transitions, and magnetic properties of the Fe7Se8 system has been investigated. The compounds studied are (Fe0.987Ni0.013)7Se8, (Fe0.955Ni0.045)7Se8, (Fe0.915Ni0.085)7Se8, (Fe0.89Ni0.11)7Se8, (Fe0.79Ni0.21)7Se8, (Fe0.975Co0.025)7Se8, (Fe0.951Co0.049)7Se8, and (Fe0.91Co0.09)7Se8 as well as the parent compound Fe7Se8. The crystals are grown using a modified Bridgman method and exist as a hexagonal NiAs-like structure with an ordered distribution of Fe vacancies that determine their magnetic properties. The substitution effect, inducing a systematic decrease in the unit-cell volume due to the ionic radii of doping ions being smaller than the radii of Fe2+ ions, is a source of chemical pressure, acting on the magnetic subsystem and determining the magnetic ordering temperature TC and the spin-reorientation temperature (TSRT). As the most important result of this paper, it was shown that hydrostatic and chemical pressures are strongly correlated in the investigated system. The character of magnetic phase transitions has been designated in the vicinity of TC and TSRT temperatures, and the magnetic entropy change ΔSm and refrigeration potential magnitudes have been determined for this temperature range. These magnetocaloric parameters were shown to depend strongly on chemical pressure. A phenomenological model developed based on the experimental measurements demonstrated that the magnetic properties of the system depend strongly on the crystal field acting on Fe2+ ions and the ratio of the numbers of Fe2+ and Fe3+ ions.

Temperature dependent partially compensated to nearly fully compensated magnetic state in half-metallic full Heusler alloy, Mn1.2Fe1.18V0.62Al

We report experimental and theoretical studies of magnetization compensation phenomenon in quaternary Heusler alloy Mn1.2Fe1.18V0.62Al by employing dc magnetization, neutron depolarization, neutron diffraction, specific heat, electrical resistivity, and electronic structure calculation. In this ferrimagnetic alloy system with ordering temperature TC = 360 K, the asymmetric thermal variations of the site magnetic moments provide an evidence of the partially compensated magnetic state of the system down to ∼ 48 K. However, below 48 K, the system exhibits its magnetic ground state with the negligibly small ordered moment of 0.05(5) μB/f.u. at 10 K, as obtained from the neutron diffraction study. Nearly full recovery of neutron beam polarization at 4 K in the neutron depolarization study also infers a nearly fully compensated magnetization state in this Heusler alloy. Temperature dependent specific heat and resistivity measurements also corroborate with the dc magnetization study representing 48 K as a turning point at which the system reaches its ground state. The electronic structure calculations, using the SPR-KKR Green’s function approach, verify the half-metallic nature of the present system. The net magnetic moment (0.06 μB/f.u.) obtained from the theoretical calculations as well as neutron diffraction experiments (0.05 μB/f.u.) is in good agreement with the Slater-Pauling rule (0.06 μB/f.u.). This small magnetic moment of the system (Mn0.82Fe1.18)(Mn0.38V0.62)Al proclaims to be a nearly compensated ferrimagnetic system with 23.94 valence electrons. Such magnetically compensated systems with finite spin polarization could be favorable for spintronics applications.

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.

Mixed Ionic–Electronic Conduction and Magnetoelectric Coupling in Li0.5Fe2.5–xCrxO4(x= 1.0, 1.1, 1.3, 1.5, and 1.6) Involving Magnetization Compensation Phenomenon

The correlations between the crystal structural, magnetic, and electrical properties of Li0.5Fe2.5–xCrxO4 (x = 1.0, 1.1, 1.3, 1.5, and 1.6) have been investigated by neutron diffraction, DC magnetization, and impedance spectroscopy. Crystal structural analysis reveals mixed cation (Li, Fe, and Cr) distributions over both tetrahedral and octahedral crystallographic sites and an enhancement of the site disorder with increasing Cr concentration. The magnetization study yields ferrimagnetic ordering for all the compounds below the respective TC (640 K for x = 1.0 to 470 K for x = 1.6), and magnetization compensation at the respective TComp (540 K for x = 1.0 to 290 K for x = 1.6). The comprehensive analyses of the frequency and temperature-dependent impedance data in terms of DC and AC conductivities reveal the presence of mixed ionic and electronic conductions. The ionic conduction in the compounds is anticipated due to the presence of Li+ ions over the tetrahedral and octahedral crystallographic sites, whereas electron migration through a small polaron hopping mechanism is responsible for electronic conductivity. The relative contributions of electronic and ionic conductions are determined by charge transference number analysis. Almost equal fractions of the ionic and electronic conductivities for the composition x = 1.5 over the entire temperature range of 300 to 600 K reveal the character of a nearly ideal mixed conductor. We have shown highly tunable relative contributions of the ionic and electronic conductions with Cr substitution at room temperature. In addition, a strong coupling between magnetic and electric degrees of freedoms has been demonstrated by the changes in the slopes of the temperature-dependent DC and AC conductivities at both the ferrimagnetic ordering temperature TC and at the magnetization compensation temperature TComp. The role of cation distributions over the tetrahedral and octahedral sites on the conduction as well as magnetic properties has been discussed.

A study on magnetic and spin polarized electric transport properties of cation doped LaCaMnO3perovskite ceramic system

Thepoly-crystallineLa0.5Nd0.15Ca0.35MnO3 has been synthesized by a conventional solid reaction method through the cation doping in LaCaMnO3. The analysis of the powder XRD of the composites revealed the formation of an orthorhombic structure in Pbnm space group. The cell parameters of the formed system has obtained and confirmed through Reitveld analysis and it is found to be a=5.4505,b=5.4457,c=7.6876.The lattice parameters and the spin ordering temperature TC of this system are decreasing due to Nd substitution. The ac and dc magnetization studies and resistivity measurements in ZFC and FC mode gives good understanding to the magnetic and electric transport properties of the doped system. The magnetization studies show a paramagnetic to ferromagnetic transition at a temperature below the critical temperature TC.The resistivity measurements reveals semiconducting to metallic transition below the temperature TMS. The temperature dependent transport nature shown in this system is one type of the correlated polaron hopping mechanism and it can be modeled by an adiabatic small polaron hopping(SPH) mechanism.

Frustration-driven magnetic fluctuations as the origin of the low-temperature skyrmion phase in Co7Zn7Mn6

In chiral cubic helimagnets, phases of magnetic skyrmions—topologically protected spin whirls—are stabilized by thermal fluctuations over a narrow region directly below the magnetic ordering temperature Tc. Due to often being touted for use in applications, there is a high demand to identify new ways to stabilize equilibrium skyrmion phases far below Tc where they may display an enhanced robustness against external perturbation due to a larger magnetic order parameter. Here, from quantum beam experiments on the chiral magnet Co7Zn7Mn6, we unveil a direct correlation between the stability of its second skyrmion phase-stable far from Tc, and a concomitant enhancement of an underlying magnetic fluctuation rate that is driven by geometric magnetic frustration. The influences of other leading skyrmion stability mechanisms, such as those derived from thermal fluctuations and low T cubic anisotropies, are shown to be weak in this system. We therefore advance the existence of a fundamental mechanism for stabilizing topological skyrmions in Co7Zn7Mn6 chiral magnet that draws upon magnetic frustration as the key ingredient.

Magnetization distribution in Cu0.6Mn2.4Ge2 ferromagnet from polarized and non-polarized neutron powder diffraction aided by density-functional theory calculations

The crystal structure and magnetic properties of Cu0.6Mn2.4Ge2 have been re-investigated by a combination of extensive magnetic measurements and neutron scattering experiments, aided by electronic structure calculations. The material is found to be a soft ferromagnet with the ordering temperature TC = 316 K. The magnetocaloric effect evaluated from field-dependent magnetization isotherms is equal to 1.2 J/(kg·K) and 2.5 J/(kg·K) under the maximum applied magnetic field of 2 T and 5 T, respectively. The compound crystallizes in the hexagonal space group P63/mmc. A complex structural disorder necessitated testing of several disorder models against the results of non-polarized and polarized neutron scattering experiments and magnetization measurements. Simulations at the density-functional theory level were also performed to identify the most robust solution that properly described the data observed. The final magnetic structure model reveals non-equal magnetic moments on the Mn1 and Mn2 atoms (2.29(9) µB and 2.7(1) µB, respectively) and the presence of vacancies and minor Cu substitution defects in both Mn sites. The work demonstrates how the non-polarized and polarized neutron scattering methods can be combined with electronic structure calculations to establish the microscopic structure of magnetic materials with complex crystallographic disorder.

Structural analysis of high-pressure phase for skyrmion-hosting multiferroic Cu2OSeO3

Cu2OSeO3 is known as a unique example of insulating multiferroic compounds with skyrmion spin texture, which is characterized by the chiral cubic crystal structure at ambient pressure. Recently, it has been reported that this compound shows pressure-induced structural transition with large enhancement of magnetic ordering temperature Tc. In the present study, we have investigated the detailed crystal structure in the high pressure phase, by combining the synchrotron X-ray diffraction experiment with the diamond anvil cell and the analysis based on the genetic algorithm. Our results suggest that the original pyrochlore Cu network is sustained even after the structural transition, while the orientation of SeO3 molecule as well as the position of oxygen in the middle of Cu tetrahedra are significantly modified. The latter features may be the key for the reported enhancement of Tc and associated stabilization of skyrmion phase at room temperature.

Dramatic impact of intermediate-valence impurity on induced magnetism in singlet ground state system PrNi

We studied the influence of La (nonmagnetic) and Ce (intermediate valence) impurities on the ferromagnetic ordering temperature Tc of compounds RE1-x(La,Ce)xNi (RE = Pr, Gd). The ordering temperature Tc was measured in a wide range of impurity concentration. Crystal electric field (CEF) splitting for the PrNi and Pr0.25Ce0.75Ni was defined by neutron spectroscopy measurements. The strong difference in the influence of La and Ce on Tc of PrNi and GdNi based systems has been obtained. The introduction of a spin-fluctuating intermediate valence impurity in PrNi leads to a dramatic increase in the area of the ordered state on the phase diagram, due to an increase in Tc, compared to the impact of La-impurity over the entire concentration range. The dependence of Tc on the Ce concentration for the Pr1-xCexNi was calculated within the framework of the model of microscopic states. The essential modification of the CEF splitting by the Ce impurity appears to be the decisive factor affecting Tc wherein the Ce ion carries zero static magnetic moments similar to the La case. The latter explains the identical dependencies of the ordering temperature on the impurity concentration observed in systems with a long-range order of local magnetic moments Gd1-xLaxNi and Gd1-xCexNi.

Observation of planar Hall effect in the ferromagnetic Weyl semimetal Co3Sn2S2

We report detailed magneto-transport measurements on single crystals of the magnetic Weyl semi-metal Co3Sn2S2. Recently a large anomalous Hall effect and chiral anomaly have been reported in this material which have been suggested to be related to the large Berry curvature between the Weyl points (Liu et al., Nature Physics (2018).). Another effect expected to result from the topological band structure of Weyl materials is the planar Hall effect (PHE). In this work we indeed observe PHE in single crystals of Co3Sn2S2. However, PHE is observed for temperatures ⩽the ferromagnetic ordering temperature Tc=175 K. Therefore we conclude that most likely the PHE can’t be connected to the Topological character of Co3Sn2S2. We also did not observe the Chiral anomaly induced negative longitudinal MR.

Above-Room-Temperature LiNbO3-Type Polar Magnet Stabilized by Chemical and Physical Pressure

LiNbO3 (LN)-type polar magnets are technologically important but require stringent and costly high-pressure synthesis with very limited sample yields. We develop a chemical strategy to reduce the physical synthesis pressure. LN-type polar magnets require 7 GPa to stabilize in the high-pressure Mn2FeNbO6 (MFNO) phase. Here, MFNO was successfully stabilized in the isostructural LN matrix at intermediate physical pressure (below 5 GPa) at gram levels for each run by dilution with LN according to (Li1–xMnx)(Fex/2Nb1–x/2)O3 (x = 0.18, 0.33, 0.46, 0.57). LN-diluted MFNO demonstrates ferromagnetism above room-temperature (magnetic ordering temperature TC ≈ 516–554 K) and has large estimated spontaneous polarization (PS ≈ 18–63 μC/cm2). Irreversible c-axis near-zero thermal expansion stemming from magnetostriction was observed around the magnetic transition temperature region upon heating at ambient pressure, which irreversibly elongates the distance between the face-sharing (Li/Mn) and (Fe/Nb) octahedral centroi...

Correlation between magnetic and transport properties of rare earth doped perovskite manganites La0.6R0.1Ca0.3MnO3 (R = La, Nd, Sm, Gd, and Dy) synthesized by Pechini process

Here, we report a comparative study of rare earth doped La0.6R0.1Ca0.3MnO3 (R = La, Nd, Sm, Gd, and Dy) samples synthesized by sol-gel Pechini process. All the compounds are single-phased and crystallize in Pbnm space group with orthorhombic symmetry. Temperature dependent magnetization measurements revealed that all our compounds exhibit a paramagnetic (PM)–ferromagnetic (FM) transition with decrease in both ordering temperature TC and the freezing temperature TF with decrease in ionic radii of lanthanide ions. The compounds display metal-semiconductor transition and the temperatures (TMS) obtained from the electrical resistivity measurements are consistently higher than those of the Curie temperatures TC. The transport properties are dominated by the Variable-range hopping (VRH) model above TMS, showing an increase in characteristic VRH temperature with decreasing size of rare-earth ion. The estimated hopping distance (RH) in all the samples is close to nearest neighbor Mn–O–Mn distance (~5 Å) in the ab plane.

Magnetic phase transition in La0.8Sr0.2Mn0.9Sb0.1O3 manganite under pressure

The high-pressure evolution of structural and magnetic properties of the single valence Mn3+ manganite La0.8Sr0.2Mn0.9Sb0.1O3 was studied by means of neutron powder diffraction up to 4.5 GPa in the temperature range of 10–300 K. At ambient pressure, a ferromagnetic ground state with the magnetic ordering temperature TC ≈ 175 K is stabilized. Application of high pressure suppresses the initial FM phase and leads to the appearance of a new A-type antiferromagnetic AFM phase with TN ~ 130 K at P = 2.2 GPa. Under compression, both TC and TN almost linearly increases with the pressure coefficient of 1.9(2) and 0.9(3) K/GPa, respectively. It is assumed that the appearance of the A-type AFM state is mediated by the strong anisotropic distortions of MnO6 octahedra, coupled to the egdx2-z2 orbital polarization at high pressures.

Femtosecond Optical and Magneto-Optical Spectroscopy Study of Magnetic and Electronic Inhomogeneities in a Pd0.94Fe0.06 Thin Film

The results of a femtosecond optical and magneto-optical spectroscopy study of a thin epitaxial film of a low-temperature magnetically soft Pd0.94Fe0.06 alloy on a ({dy001})-MgO substrate are reported. The photoinduced demagnetization and magnetization recovery times are determined. The latter increases critically at approaching the ferromagnetic ordering temperature TC = 190 K from below. It is shown that the reflectivity dynamics after a photoexcitation pulse evolves from a two-exponential in the paramagnetic phase to a four-component at 80 K < T < TC, simplifying to a three-component at T < 50 K. According to our interpretation, such an evolution, along with the manifestation of an additional increasing component in the magnetic response at 80 K < T < TC, indicates a magnetic and electronic inhomogeneity of the film associated with the distribution of local iron concentrations. The fraction of small-scale inclusions of the paramagnetic phase is estimated as ≈10 vol %.

Ab initio study on Pb2FeOsO6 and possible half metallicity via Na doping

Abstract Pb2FeOsO6, with an Fe-Os antisite occupancy of about 16%, was synthesized recently and measured to be a cubic ferrimagnetic semiconductor with a high magnetic ordering temperature (Tc) of 280 K. We performed an investigation on Pb2FeOsO6 by using the density functional theory. Our calculated results indicate that Pb2FeOsO6 crystallizes into tetragonal symmetry and is a C-type antiferromagnet. After doping with Na, the system experiences a transition from C-type antiferromagnetic state to ferrimagnetic state, and the half-filled Os5+ t2g orbitals cross the Fermi level owning to the injection of holes. Therefore, the Na-doped Pb2FeOsO6 exhibits a half metallic nature. Moreover, the Tc raises significantly by Na doping. Therefore, we expect that the Na-doped Pb2FeOsO6 would be promising candidates as spintronic material.

Induced magnetism and “magnetic hole” in singlet ground state system PrNi

Abstract The phenomenon of “induced magnetic ordering” for the local magnetic moments with singlet ground state in a periodical lattice is considered for the prototype intermetallic rare-earth compound PrNi ordered ferromagnetically at temperature TC = 21 K. Within the framework of a realistic model the energy dispersion relations are obtained for the magnetic excitations above Tc along the high symmetry directions. The “soft mode” behaviour observed for the acoustic magnetic excitation along (q 0 0) direction is reproduced. The influence of the substitution of isovalent nonmagnetic La for Pr1−xLaxNi on the temperature of ferromagnetic ordering is investigated. The linear decrease of the ordering temperature TC down to zero value in increasing of the La concentration up to x = 0.5 is observed and described by the model.

Nonequilibrium critical dynamics of low-dimensional frustrated magnets and multilayer structures

A Monte Carlo simulation of the non-equilibrium behavior of multilayer magnetic nanostructure is carried out. Equilibrium properties, critical temperatures Tc and hysteresis effects were studied for various thicknesses of ferromagnetic layers. It is shown that, in contrast to bulk magnetic systems, the aging effects in nanostructure arise not only at the ferromagnetic ordering temperature Tc but also within a wide temperature range at T ≤ Tc. Simulation of transport properties in multilayer structure permitted to calculate non-equilibrium values of magnetoresistance.

Manifestation of aging in giant magnetoresistance of the Co/Cu/Co nanostructure

A Monte Carlo simulation of the non-equilibrium behavior of multilayer magnetic nanostructure Co/Cu(100)/Co consisting of alternating magnetic and nonmagnetic nanolayers is carried out. Analysis of calculated two-time autocorrelation function for structure relaxing from both high-temperature and low-temperature initial states reveals aging characterized by a slowing down of correlation characteristics with increase of the waiting time. It is shown that, in contrast to bulk magnetic systems, the aging effects in nanostructure arise not only at the ferromagnetic ordering temperature Tc but also within a wide temperature range at T ≤ Tc. For evolution from high-temperature initial state, the study of dependence of aging characteristics on thickness N of cobalt films reveals a weakening of the aging with increasing N at the critical temperatures Tc(N) and an opposite tendency at temperatures T < Tc(N) with strengthening of aging with increasing N for considered N ≤ 9 ML. This phenomenon is connected with increasing correlation and relaxation times in nanostructures when temperature is decreased. For case of the low-temperature initial state, it is shown that correlation times are two-three orders of magnitude smaller than those in the evolution from a high-temperature initial state at the same tw values. In this case, time behavior of the autocorrelation function doesn’t depend considerably on temperature for Ts ≤ Tc and thickness N of cobalt films. Simulation of transport properties in Co/Cu(100)/Co structure permitted to calculate temperature dependence of its equilibrium magnetoresistance values. For the first time, it was revealed influence of non-equilibrium behavior on the magnetoresistance with demonstration of nontrivial aging effects. It has been shown that the magnetoresistance reaches plateau in asymptotic long-time regime with values , which depend on type of initial state, thickness of cobalt films, and temperature.