Magnetic properties of Mn-substituted GdCoxMn1−xO3 and LaCoxMn1−xO3

Partial substitution of manganese by cobalt in rare-earth perovskites REMnO₃ leads to unusual magnetic phenomena because of the simultaneous presence of Mn³⁺, Mn⁴⁺, Co²⁺ and Co³⁺ species. The magnetic nature of the RE cation plays a fundamental role in the magnetic properties. We present herein two specific families: for RE=La the magnetic behavior of the |Co+Mn| network is observed, while for Gd its strong magnetic moment interacts with the transition metals, leading to a spin reversal state. Magnetic interactions are maximized at x=0.50, as if two regimes exist: for x<0.5 Co substitutes Mn in the REMnO₃ manganite, and for x>0.5 Mn substitutes Co in the RECoO₃ cobaltite.     

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.     

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.     

Structure, magnetic and transport properties of La1−xBixMnO3

We have investigated the structural, magnetic and transport properties of La_1−xBi_xMnO₃ samples. As the Bi content increases, a structural transition from rhombohedral to pseudocubic and a magnetic phase transition from ferromagnetic ordering to cluster glass are identified. Metal–insulator (MI) transitions and large magnetoresistance (MR) effects are observed at low Bi doping levels, while insulating behavior of resistivity is found in the whole measured temperature range at high-doping levels. Two distinct ferromagnetic insulating (FI) states are found at low temperatures in this system. One can be suppressed and the other can be enhanced by applying magnetic fields. Possible reasons for the observed structural, magnetic phase transitions and changes of resistivity behavior with Bi doping are discussed.     

Anomalies of ultrasonic velocities, attenuation and elastic moduli in Nd1−xSrxMnO3 perovskite manganite materials

Nd_1−xSr_xMnO₃ perovskite manganite material with different compositions (x=0.31, 0.35, 0.37, 0.39 and 0.41) have been prepared employing solid-state reaction technique. The ultrasonic velocities and attenuation of the above samples have been measured employing through transmission method operated at a fundamental frequency of 5 MHz over wide range of temperatures. The temperature dependence of ultrasonic velocities, attenuation, relative percentage variation in velocities and elastic constants show an interesting anomaly in all compositions. The observed anomalies in ultrasonic parameters at T_c in all compositions have been revealed in terms of existence of ferromagnetic (FM) state. Similarly, the anomalies at T_co show the transition from FM to charge-ordered antiferromagnetic (AFM) state. The observed results have been used to explore the competitions between FM and AFM.     

Magnetic properties of Lu2Co17−xSix single crystals

The Co-sublattice anisotropy in Lu₂Co₁₇ consists of four competitive contributions from Co atoms at crystallographically different sites in the Th₂Ni₁₇-type of crystal structure, which result in the appearance of a spontaneous spin-reorientation transition (SRT) from the easy plane to the easy axis at elevated temperatures. In order to investigate this SRT in detail and to study the influence of Si substitution for Co on the magnetic anisotropy, magnetization measurements were performed on single crystals of Lu₂Co_17−xSi_x (x=0−3.4) grown by the Czochralski method. The SRT in Lu₂Co₁₇ was found to consist of two second-order spin reorientations, “easy-plane”–“easy-cone” at T_SR1≈680 K and “easy-cone”–“easy-axis” at T_SR2≈730 K. Upon Si substitution for Co, both SRTs shift toward the lower temperatures in Lu₂Co₁₆Si (T_SR1≈75 K and T_SR2≈130 K) with the further onset of the uniaxial type of magnetic anisotropy in the whole range of magnetic ordering for Lu₂Co_17−xSi_x compounds with x>1 due to a weakening of the easy-plane contribution from the Co atoms at the 6g and 12k sites to the total anisotropy.     

Magnetocaloric and magnetoresistance properties of La2/3Sr1/3Mn1−xCoxO3 compounds

Structural, magnetic, magnetoresistance and magnetocaloric studies on La_2/3Sr_1/3Mn_1−xCo_xO₃ compounds were reported. The samples were prepared by the conventional ceramic method. X-ray analysis showed the presence of one phase only, in all studied samples. From electrical resistance measurements it was found that the samples show large negative magnetoresistance behavior. The magnetic measurements were performed in a large temperature range, 4.2–750 K and external magnetic fields up to 5 T. The adiabatic magnetic entropy changes, |ΔS|, were determined from magnetization data. Large magnetocaloric effect (MCE) has been obtained in all studied samples.     

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.     

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 and optical properties of spinel FexCo3−xO4 thin films

Magnetic and optical properties of Fe_xCo_3−xO₄ thin films grown by sol–gel method have been investigated as the Fe composition (x  ) increases from 0 to 2. X-ray diffraction measurements revealed that the normal- and inverse-spinel phases coexist for 0.76?x?0.93$0.76?x?0.93$. The normal-spinel phase is dominant below x=0.76$x=0.76$ while the inverse-spinel phase above x=0.93$x=0.93$. The lattice constant of the inverse-spinel phase is found to be larger than that of the normal-spinel phase. For both phases the lattice constant increases with increasing x. The Fe_xCo_3−xO₄ films containing the inverse-spinel phase exhibit net magnetization that increases with increasing x  . Conversion electron Mössbauer spectrum measured on the x=0.93$x=0.93$ sample showed that Fe²⁺ ions prefer the octahedral sites, indicating the formation of the inverse-spinel phase. Analysis on the measured optical absorption spectra for the samples by spectroscopic ellipsometry indicates a dominance of the normal-spinel phase for low x in which Fe³⁺ ions mostly occupy the octahedral sites. Observation of a crystal-field transition at 1.6 eV originating from tetrahedral Fe³⁺ ion confirms the existence of the inverse-spinel phase for high x.     

Phase composition-dependent physical and mechanical properties of YbxZr1−xO2−x/2 solid solutions

The paper represents a detailed insight into the correlation between changes of the phase composition of crystalline Yb_xZr_1−xO_2−x/2 solid solutions and their structural, electrical, mechanical and optical properties. Particularly, the effect of the crystal growth conditions and stabilizer amount in the range of 1.5–13.8 mol% of Yb₂O₃ are studied in terms of Rietveld analysis of powder X-ray diffraction data, electrical conductivity measured by impedance spectroscopy, absorption coefficient and refractive index measurements, Vickers microhardness (classical technique) as well as the plastic microhardness and effective elastic modulus (DSI—depth-sensing indentation technique). Potential applications of the investigated systems are discussed in view of the results obtained.     

Direct evidence for charge ordering and electronic phase separation in BixSr1−xMnO3 at room temperature

We present a study of charge ordering and electronic phase separation (EPS) phenomenon in Bi_xSr_1−xMnO₃, for an exhaustive range of x (0.25x0.75), by STM/STS at room temperature (RT) and specific heat measurements at high temperatures (350–650 K). Atomically resolved STM images of the samples, in real space, show the presence of stripe-like charge-ordered (CO) phase coexisting with charge-disordered (CD) phase. The STM images further reveal that the fraction of CO phase increases with an increase in x. The conductance spectra of these phases measured at nano level by STS are discussed. The transition to CO phase above RT is corroborated by specific heat measurements in all samples, giving a T_CO(x) phase diagram for this system.     

Magnetic properties of FexCo1−x/CoyFe1−yFe2O4 composite under hydrothermal condition

The metal–ferrite composites Fe_xCo_1−x/Co_yFe_1−yFe₂O₄ are synthesized by using disproportion of Fe (II) and reduction of Co (II) by Fe⁰ under hydrothermal condition. The size of the particles of the composites decreases as the [KOH] decreasing. The composites are measured by TEM and it can be deduced that when [KOH] = 0.1, the size of the alloy body-centered cubic (BCC) in composites is 20 ± 7 nm, the size of the Cobalt ferrite (spinel) is 170 ± 50 nm. The maximal value of the saturation magnetization (Ms) of the composite is about 100.14 emu/g, which is synthesized under Co (II)/Fe (II) = 0.05, [KOH] = 1 N, T = 150 °C and t = 3 h. The value of Hc of the composite synthesized under Co (II)/Fe (II) = 0.5, t = 3 h, T = 150 °C and [KOH] = 10.2 mol/L is about 2878.19 Oe. The Fe–Co alloy is synthesized through a reduction reaction of the composites in a flowing gaseous mixture. There is a maximal value (302.9 emu/g) of the Ms for the alloys generated at 1000 °C, which is the Co_0.412Fe_0.588 alloy.     

Structure, ferroelectric and piezoelectric properties of (Bi1−x−yNa0.925−x−yLi0.075)0.5BaxSryTiO3 lead-free piezoelectric ceramics

Lead-free multi-component ceramics (Bi_1−x−yNa_{0.925−x−y}Li_0.075)_0.5Ba_xSr_yTiO₃ have been prepared by an ordinary sintering technique and their structure and electrical properties have been studied. All the ceramics can be well-sintered at 1100 °C. X-ray diffraction patterns shows that Li⁺, Ba²⁺ and Sr²⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a new solid solution with a pure perovskite structure, and a morphotropic phase boundary (MPB) is formed at 0.04 < x < 0.08. As compared to pure Bi_0.5Na_0.5TiO₃ ceramic, the coercive field E_C of the ceramics decreases greatly and the remanent polarization P_r of the ceramics increases significantly after the formation of the multi-component solid solution. Due to the MPB, lower E_C and higher P_r, the piezoelectricity of the ceramics is greatly improved. For the ceramics with the compositions near the MPB (x = 0.04–0.08 and y = 0.02–0.04), piezoelectric coefficient d₃₃ = 133–193 pC/N and planar electromechanical coupling factor k_P = 16.2–32.1%. The depolarization temperature T_d reaches a minimum value near the MPB. The temperature dependences of the ferroelectric and dielectric properties suggest that the ceramics may contain both the polar and non-polar regions at temperatures near/above T_d.     

Divergency of SDW and structure transition in Fe1−xNixSe0.82 superconductors

The effect of the Ni-substitution on the superconductivity of α-FeSe_0.82 system has been investigated. It is found that the superconductivity of α-FeSe_0.82 can be quickly suppressed when a small quantity of Fe is substituted by Ni element. The resistivity and moment of the sample at normal state increase with increasing Ni-substitution content. Moreover, it is found that the Ni-substitution does not change the structure transition temperature, but increases the spin density wave (SDW) order temperature. It is suggested that the quick suppression of superconductivity of α-FeSe_0.82 caused by the Ni-substitution is related to the increase of SDW order temperature. Therefore, Fe atom plays a key role for the appearance of the superconductivity in FeSe system.     

Magnetic and electronic phase transitions and magnetoresistance effect in the Pr0.5−xLaxSr0.5MnO3 (, 0.15) system

The electronic and magnetic phase transitions of Pr_0.5−xLa_xSr_0.5MnO₃ with x=0.10 and 0.15 were investigated. The M(T) and ρ(T) curves for these samples clearly show transitions from antiferromagnetic insulator to ferromagnetic semiconductor, ferromagnetic metal and finally to paramagnetic semiconductor as the temperature is increased from 5 to 300 K. Especially, two obvious protrudent peaks in the magnetoresistance curves MR(T) for these samples were clearly observed in the relative low magnetic field, 1 T. One peak appears at around the antiferromagnet-ferromagnet transition temperature T_N (150 K) with MR≈−23%, another occurs at around the ferromagnet-paramagnet transition temperature T_C(275 K ) with MR≈−8.2%. In addition, when the magnetic field was increased, the temperature corresponding to the MR peak at T_N shifts to lower temperature while the temperature corresponding to the MR peak at T_C is fixed.     

Multi-magnetic phases in Fe1−xNixTa2O6

X-ray, neutron diffraction and magnetic susceptibility are reported for Fe_xNi_1−xTa₂O₆ mixed oxides. X-ray refinement indicates homogeneous samples for all the reported concentrations. The neutron-diffraction measurements reveal magnetic structures with double propagation vectors. This system exhibits at least two bicritical points at about x=0.15 and 0.60. For these concentrations, at low temperatures, the system shows the coexistence of two magnetic structures. This bicritical behaviour is interpreted as induced by competition between the different magnetic structures.     

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.     

Structure and magnetocaloric properties of La1−xKxMnO3 manganites

A technology of obtaining the single-phase ceramic samples of La_1−xK_xMnO₃ manganites and the dependence of their structural parameters on the content of potassium has been described. Magnetocaloric effect (MCE) in the obtained samples has been measured by two independent methods: classical direct methodic and a method of magnetic field modulation. The values of MCE obtained by both methods substantially differ. The explanation of the observed divergences is given. The correlation between the level of doping and MCE value has been defined. The value of T_C determined by the MCE maximum conforms with the literature data obtained by other methods.     

Magnetic hyperthermia with biphasic gel of La1−xSrxMnO3 and maghemite

Magnetic hyperthermia experiments were carried out using a biphasic gel of La_1−xSr_xMnO₃(LSMO) and γ-Al_0.07 Fe_1.93O₃ with an AC magnetic field of amplitude 88 mT and a frequency of 108 kHz. Specific absorption rate (SAR) increases with the increased ratio of Al-substituted maghemite. The T_max value for the gels prepared by the mixture of LSMO and Al-substituted maghemite can be adjusted to suit therapeutic temperature. The time required to reach optimum temperature decreased with the increased ratio of later. Such biphasic gel could be very useful for magnetic hyperthermia with in vivo control of temperature.