Electric polarization memory effect in a magnetoelectric multiferroic CuFe1−xGaxO2

Magnetic oxide CuFeO₂ is a magnetoelectric multiferroic with new type of spin–polarization coupling different from that in the spin-current mechanism, where magnetic field-induced or nonmagnetic impurity-induced proper helical magnetic ordering generates a spontaneous electric polarization parallel to the helical axis. Using a CuFe_1−xGa_xO₂ sample with x=0.035, in which the single ferroelectric phase is realized below T_N8 K in zero magnetic field unlike CuFe_1−xAl_xO₂, we have performed pyroelectric current, thermally stimulated current (TSC) and polarized neutron diffraction measurements to study a memory effect that the electric polarization is partially preserved even for 2nd-cooling from above T_N without poling electric field. It was found that the charge trapped during 1st-cooling with poling electric field, which is released as TSC on heating, plays an important role in the memory effect.     

Magnetic phase diagrams of Ho1−xDyxNi2B2C

We performed the magnetization measurement on Ho_1−xDy_xNi₂B₂C single crystals (x=0.1, 0.2, 0.3, 0.4, and 0.6) with magnetic field applied perpendicular and parallel to the c-axis. But only for the magnetic field perpendicular to the c-axis, the increase of Dy³⁺ concentration affects the magnetically ordered states of HoNi₂B₂C compound and makes the phase diagram more complicated. The antiferromagnetic ordering state attributed to Dy³⁺ sublattice starts to appear from a case of x=0.2 and finally the magnetic phase diagram becomes analogous to that of DyNi₂B₂C as x is increased which is consistent with the neutron scattering result.     

Magnetocaloric effect in the La0.8Ce0.2Fe11.4−xCoxSi1.6 compounds

The effects of substitution of Co for Fe on the magnetic and magnetocaloric properties of La_0.8Ce_0.2Fe_11.4−xCo_xSi_1.6 (0, 0.2, 0.4, 0.6, 0.8 and 1.0) compounds have been investigated. X-ray diffraction shows that all compounds crystallize in the NaZn₁₃-type structure. Magnetic measurements show that the Curie temperature (T_C) can be tuned between 184 and 294 K by changing the Co content from 0 to 1. A field-induced methamagnetic transition occurs in samples with x=0, 0.2 and 0.4. The magnetic entropy changes of the compounds have been determined from the isothermal magnetization measurements by using the Maxwell relation.     

Magnetic anisotropy of Tb1−xGdxMn6Sn6 compounds

Magnetization curves of Tb_1−xGd_xMn₆Sn₆ compounds (0?x?1) have been measured for aligned powder samples in the temperature range 4.2–300 K in pulsed magnetic fields up to 30 T. Temperature and concentration dependences of the magnetocrystalline anisotropy constants K₁ and K₂ and concentration dependence of the temperature of spontaneous spin-reorientation transition have been determined. Using these data, we estimated the contribution of the manganese and terbium atoms to the magnetic anisotropy of Tb_1−xGd_xMn₆Sn₆ and analyzed the origin of the appearance of field-induced first-order magnetic phase transition in these compounds.     

Magnetic properties on shape memory alloys Ni2Mn1+xIn1−x

X-ray powder diffraction and magnetization measurements were done on the magnetic shape memory alloys Ni₂Mn_1+xIn_1−x. On the basis of the results, the magnetic phase diagram was determined for Ni₂Mn_1+xIn_1−x alloys. Magnetization measurements make clear that the excess Mn atoms, which substitute for In sites, are coupled ferromagnetically to the ferromagnetic manganese sublattices. A magnetic phase diagram of Ni₂Mn_1+xIn_1−x alloys is discussed qualitatively on the basis of the interatomic dependence of the exchange interactions.     

Structural, magnetic and transport properties of double perovskite compounds (Sr2−3xLa2xBax)FeMoO6

The crystal structure and magnetic properties of a series of ordered double perovskite oxides (Sr_2−3xLa_2xBa_x)FeMoO₆ (0x0.3) have been investigated. X-ray powder diffraction reveals that the crystal structure of the compounds changes from a tetragonal I4/m lattice to a cubic lattice around x=0.2. Though the nominal average size of the A site cation of (Sr_2−3xLa_2xBa_x)FeMoO₆ is designed to be almost independent of x, the refinements of the crystal structure show that the lattice constants increase with x in both the tetragonal and the cubic phase regions due to electron doping. As the x increases, the degree of cationic ordering on the B site is decreased pronouncedly, while the Curie temperature of the compounds is nearly unchanged. The saturation magnetization of the compounds decreases with x and shows a linear dependence on the degree of cation ordering. The resistivity of the parent compound shows a semiconducting behavior below room temperature, but those of the doped samples exhibit a metal–semiconductor transition. A correlation between the resistivity and metal-semiconducting transition temperature (T_M−S) is observed. The resistivity and T_M−S of the compounds decrease with x for x0.2 and increase for x0.2. Magnetoresistance of the compounds is reduced by the La/Ba doping. All these observations can be understood based on the interplay of the electron doping, change in bandwidth and the anti-site defect concentration.     

Magnetic properties of Nd1−xKxMnO3 compounds

Polycrystalline Nd_1−xK_xMnO₃ (x=0.10–0.20) compounds have been prepared in single phase form with Pbnm space group. The magnetic properties were studied by measuring dc magnetization and ac susceptibility. They exhibit paramagnetic to ferromagnetic transition with transition temperature ranging from 116 to 128 K. The magnetization data have been analyzed by using Brillouin function model and by taking into account the ferromagnetic interaction. The effective spin contribution towards ferromagnetic interaction and spin canting angle have been estimated. The spin canting angle is found to decrease with increase in doping. Magneto-caloric effect (MCE) has been studied and the maximum change in entropy was found to be 1.76 J/kg K for 1 T field. Metal–insulator transition and colossal magnetoresistance of the order of 60% for 1 T field have been observed for x=0.20 sample.     

Magnetic properties of the Pr1−xGdxCo4B compounds

In this work, we have investigated the effect of the substitution of Gd for Pr on the crystal structure and magnetic properties of the Pr_1−xGd_xCo₄B compounds for 0?x?1 using X-ray powder diffraction, magnetic measurements, and differential scanning calorimetry (DSC). These compounds have hexagonal CeCo₄B-type structure with the space group P6/mmm. The substitution of Gd for Pr leads to a decrease of the unit-cell parameters a and the unit-cell volume V, while the unit-cell parameter c increases slightly. Magnetic measurements indicate that all samples are ordered magnetically below room temperature. The Curie temperatures determined by DSC technique increase as Pr is substituted by Gd. The saturation magnetization at 5 K decreases upon Gd substitution up to x=0.6, and then increases again.     

Monte Carlo simulation of magnetic and magnetocaloric properties of binary alloy Gd1−xCx

The Potts-like model is utilized to describe an alloy Gd_1−xC_x with x=0, 0.025, 0.06, 0.09, and the magnetic and magnetocaloric properties are calculated by Monte Carlo method. The effect of the local distortion of the lattice due to adulterated C atom on the exchange interaction between Gd atoms can be considered. The spontaneous magnetization, specific heat, and magnetic susceptibility are calculated. It is found that the magnetization at low temperature decreases but phase transition temperature from ferromagnetic to paramagnetic increases, as the concentration of the C atom in the system increases. Moreover, the specific heat and the susceptibility exhibit peaks at the transition temperature. For two external magnetic field h/J=0.25 and 10.0, the magnitude of the isothermal magnetic entropy change in binary alloy is more than in pure Gd system. Furthermore, the range of temperature of half peak in the curve of the magnetic entropy change becomes wide and the refrigerant capacity increases in the alloy.     

Effect of Zn substitution on the magnetic properties of Mg1−xZnxFe2O4 ferrites

Polycrystalline Mg_1−xZn_xFe₂O₄ (x=0.0–0.6) ferrites have been prepared using solid-state reaction technique. The X-ray diffraction analysis revealed that the samples crystallize in a single-phase cubic spinel structure. The lattice parameter increases linearly with increase in zinc content obeying Vegard's law. The continuous decrease in Curie temperature (T_c) with an increase in Zn content is attributed to the weakening of A–B exchange interaction. Saturation magnetization (M_s) and magnetic moment are observed to increase up to x=0.4, and thereafter decrease due to the spin canting in B-sites. The initial permeability is found to increase with the addition of Zn²⁺ ions but the resonance frequency shifts towards the lower frequency.     

Synthesis, characterization and melt processing of Y1−x(Yb0.9Nd0.1)xBa2Cu3Oz superconductors

Effects of a combined substitute of Yb and Nd on Y site on the superconducting properties of YBa₂Cu₃O_y have been studied. We synthesized Y_1−x(Yb_0.9Nd_0.1)_xBa₂Cu₃O_z compound with x = 0.2, 0.4, 0.6, 0.8 and 1.0. Here, the ratio of Yb–Nd was fixed to be 9:1 for obtaining 123 phase without secondary phases. The melt processing thermal profiles for Y_1−x(Yb_0.9Nd_0.1)_xBa₂Cu₃O_z with x = 0.2 and 0.4 and the addition of 40 mol% {Y_1−x(Yb_0.9Nd_0.1)_x}₂BaCuO₅ and 0.5 wt% Pt in air were determined on the basis of the thermal analysis results. All samples showed a low grain growth rate, particularly for high x values, which may be partially ascribed to un-optimized thermal schedules. Although almost all the samples exhibited low J_c values, the sample with x = 0.2 exhibited T_c of 88.8 K and a relatively high J_c value of 16,000 A/cm² at 77 K for H//c-axis.     

Magnetic properties and specific heat of Dy1−xLaxNi2 compounds

Magnetic and specific heat measurements have been carried out on polycrystalline series of single-phase Dy_1−xLa_xNi₂ (0?x?1) solid solutions. The compounds have a Laves-phase superstructure (space group F4¯3m) with the lattice parameter gradually increasing with decreasing Dy content. The samples with x?0.8 are ferromagnetic with the Curie temperature below 22 K. At high temperatures, all solid solutions are Curie-Weiss paramagnets. The Debye temperature, phonon and conduction electron contributions as well as a magnetic contribution to the heat capacity have been determined from specific heat measurements. The magnetocaloric effect was estimated from specific heat measurements performed in a magnetic field of 0.42 and 4.2 T.     

Short- and long-range order in La1−xSrxCoO3 and La1−xBaxCoO3

The short- and long-range order correlations of the crystal structure in the distorted perovskites La_1−xSr_xCoO₃ and La_1−xBa_xCoO₃ (0.0?x?0.5) have been studied by the neutron powder diffraction (NPD) and the Co K-edge X-ray absorption spectroscopy (XAS) measurements. The results of XAS and NPD indicate a local distortion around the Co³⁺ ions in LaCoO₃ at room temperature. The substitution of the La³⁺ ions by the Sr²⁺(Ba²⁺) ions leads to a gradual increase of the Co-O-Co angle and is accompanied by an increase of the mean square relative displacement (MSRD) of the Co-O bond. These results correlate with an increase of the oxygen amplitude vibration in the direction perpendicular to the Co-O bond. The possible explanation of the observed changes of the crystal and electronic structures in the above-mentioned cobaltites is discussed.     

Nd crystal-field study of weakly doped Nd1−xCaxMnO3

Nd³⁺ crystal-field excitations in Nd_1−xCa_xMnO₃ (x=0.025, 0.05 and 0.1) single crystals are studied via infrared transmission as a function of temperature and external magnetic field. We report excitations associated with Nd³⁺ sites as detected in NdMnO₃ and excitations due to Ca doping. The latter reveal phase separation between the usual A-type antiferromagnetic states and the insulating canted (ferromagnetic) spin states in the vicinity of doped Ca²⁺ ions. Both Nd³⁺ crystal-field levels could be described using calculated parameters for NdMnO₃. Also, while oxygen stoichiometry and coherent Jahn–Teller distortions seem not to be affected by Ca doping, increased absorption bandwidths characterize the doped crystals.     

Electron-doped Sm1−xSrxMnO3 perovskite manganites: Crystal and magnetic structures and physical properties

We present the results of a study of electron-doped Sm_1−xSr_xMnO₃ (x>0.5) perovskite manganites by combining high-resolution neutron powder diffraction with measurements of resistivity, magnetization and magnetic susceptibility. Although investigated Sm_0.45Sr_0.55MnO₃ and Sm_0.37Sr_0.63MnO₃ compounds belonging to the same phase diagram area differ significantly in the strontium content, they are homogeneous antiferromagnetic (AF) insulators and do not exhibit CMR. They have different crystallographic symmetries (orthorhombic Pbnm and tetragonal I4/mcm, respectively) in the entire temperature range under study (1.5-288 K), differ in the type of spin ordering at low temperatures (AF-A and AF-C), are characterized by different orbital polarizations (d_x²_−y² and d_3z²_−r²), and possess two- and one-dimensional magnetic properties, respectively. The lack of magnetoresistance for these compositions is explained by the lack of coexisting magnetic phases involving double exchange ferromagnetism, in contrast to what is observed for the magnetoresistive Sm_1−xSr_xMnO₃ compounds, that is with x?0.52.     

Magnetic and transport properties in Sr1−xLaxFe1−xMnxO3

The magnetic and transport properties in the perovskite Sr_1−xLa_xFe_1−xMn_xO₃ have been explored. As x rises, the systemic ferromagnetism increases gradually and cluster-spin-glass state occurs in the low-temperature region. For 0.3?x?0.7, the ferromagnetic phase separation from the paramagnetic phase was observed from the results of electron-spin-resonance measurement. Although all samples show a semiconducting behavior, their transport properties are dominated by two different mechanisms, namely, the electronic transport of x?0.5 samples is realized by thermal activation but the variable-range hopping is applied in x?0.7 ones. The different transport mechanism can be understood from the Mn/Fe ions interaction.     

Magnetic properties of the Ce2Fe17−xMnx helical magnets up to high magnetic fields

Magnetic properties of the Ce₂Fe_17−xMn_x, x=0–2, alloys in magnetic fields up to 40 T are reported. The compounds with x=0.5–1 are helical antiferromagnets and those with 1<x?2 are helical ferromagnets or helical antiferromagnets at low and high T, respectively. Mn ions in the system carry average magnetic moment of 3.0±0.2 μ_B that couple antiparallelly to the Fe moments. Easy-plane magnetic anisotropy in the Ce₂Fe_17−xMn_x compounds weakens upon substitution of Mn for Fe. The absolute value of the first anisotropy constant in the Ce₂Fe_17−xMn_x helical ferromagnets decreases slower with increasing temperature than that calculated from the third power of the spontaneous magnetization. Noticeable magnetic hysteresis in the Ce₂Fe_17−xMn_x, x=0.5–2, helical magnets over the whole range of magnetic fields reflects mainly irreversible deformation of the helical magnetic structure during the magnetization of the compounds. A contribution from short-range order (SRO) magnetic clusters to the magnetic hysteresis of the helical magnets has been also estimated.     

Magneto-optical imaging of iron-oxypnictide SmFeAsO1−xFx and SmFeAsO1−y

We have prepared iron-oxypnictide SmFeAsO_1−xF_x by ambient-pressure technique and SmFeAsO_1−y by high-pressure technique, and characterized their bulk and local magnetic properties by using SQUID magnetometer and magneto-optical imaging. While the high-pressure samples have densities close to the theoretical value, the ambient-pressure samples have several small voids. Despite these structural differences between the two kinds of samples, they both have superconducting transition temperature above 50 K. In addition, magneto-optical images for both samples show similar kinds of inhomogeneities with large current concentrated in several grains and with small intergranular current. The estimated intragranular currents for both samples are over 10⁵ A/cm² at low temperatures and low fields.     

Magnetoresistive properties of Zn1−xCoxFe2O4 ferrites

The magnetic and magnetoresistive properties of spinel-type Zn_1−xCo_xFe₂O₄ (x=0, 0.2 and 0.4) ferrites are extensively investigated in this study. A large negative magnetoresistance (MR) effect is observed in Zn_1−xCo_xFe₂O₄ ferrites of spinel structure. These materials are either ferrimagnetic or paramagnetic at room temperature, and show a spin-(cluster) glass transition at low temperatures, depending on the chemical compositions. The MR curves as a function of magnetic fields, MR(H), are parabolic at all temperatures for paramagnetic polycrystalline ZnFe₂O₄. The MR for ZnFe₂O₄ at 110 K in the presence of 9 T applied magnetic field is 30%. On the other hand, MR(H) are linear for x=0.2 and 0.4 ferrimagnetic Zn_1−xCo_xFe₂O₄ samples up to 9 T. The MR effect is independent of the sintering temperatures, and can be explained with the help of the spin-dependent scattering and the Yafet–Kittel angle of Zn_1−xCo_xFe₂O₄ mixed ferrites.     

Magnetic phase diagram of diluted spinel Zn1−xCuxCr2Se4 system

By using mean field theory, we have evaluated the nearest-neighbour and the next-neighbour super-exchange J₁(x) and J₂(x), respectively, for Zn_1−xCu_xCr₂Se₄ in the range 0?x?1. The intraplanar and the interplanar interactions are deduced. High-temperature series expansions are derived for the magnetic susceptibility and two-spin correlation functions for a Heisenberg ferromagnetic model on the B-spinel lattice. The calculations are developed in the framework of the random phase approximation. The magnetic phase diagram is deduced. A spin glass phase is predicted for intermediate range of concentration. The results are comparable with those obtained by magnetic measurements. The critical exponents associated with the magnetic susceptibility (γ) and the correlation lengths (ν) have been deduced. The values are comparable to those of the 3D Heisenberg model, and are insensitive to the dilution x.