Surface magnetic disorder in nanostructured Ni0.5Zn0.5Fe2O4 particles

Nanostructured ferroxide particles with initial formula Ni_0.5Zn_0.5Fe₂O₄ are investigated. The aim was to explore the monodomain and the superparamagnetic states of the ferrospinel and the impact of the surface magnetic disorder on the magnetization processes. Mössbauer spectroscopy (MöS) demonstrated that the ion distribution follows the general formula (Zn_0.5Fe_0.5)_A[Ni_0.5Fe_1.5]_BO₄, where A is the tetrahedral and B, the octahedral sublattice. MöS in an external magnetic field (5 T) at 4.2 K shows non-collinearity of the sublattices’ magnetic moments and deviations in the hyperfine magnetic field that could be related to a canting effect. Magnetic measurements were applied to characterize the temperature behavior of the magnetic properties and the a.c. complex magnetic susceptibility.     

Site preference and magnetism of Fe3−xCrxAl0.5Si0.5

In order to gain better insight into the origin of the observed differences between Fe_3−xCr_xAl and Fe_3−xCr_xSi, alloys of Fe_3−xCr_xAl_0.5Si_0.5 (x=0, 0.125, 0.250, 0.375 and 0.5) were prepared and studied by means of X-ray and neutron diffraction as well as by magnetization measurements. Electronic structure calculations of these alloys have been performed by means of TB-LMTO-ASA method. It was expected, and experimentally verified, that the presence of silicon and aluminum atoms in 1:1 proportion will result in the independence of the lattice parameter on the iron/chromium concentration. All samples have been proved to be a single phase of the DO₃-type of structure. Theoretical and experimental results indicate that chromium atoms locate preferentially in B sublattice. Cr magnetic moments are oriented antiparallel to Fe magnetic moments. Neutron measurements show a linear dependence of the magnetic moments of Fe(A,C), Fe(B) and Cr(B) as a function of Cr concentration. However the calculated total magnetic moment decreases faster with chromium content than indicated by the experiment.     

Effect of Mo doping on magnetic and transport properties of La0.5Sr0.5CoO3

The substitutional effect of Mo on the magnetic and transport properties of double exchange ferromagnets, La_0.5Sr_0.5Co_1−x Mo_xO₃ (0?x?0.2) has been investigated. Substitution of 10% Mo at the Co-site of La_0.5Sr_0.5CoO₃ decreases the Curie temperature by ∼60 K than that of the parent compound and the long-range ferromagnetic ordering disappears for x?0.2. The Mo-doped samples, however, undergo a transition from the parent metallic state to the insulating state below T_c. The insulating state is found to obey Mott's variable range hopping of conduction. The effect of Mo substitution is attributed to the factors namely, (i) the dilution of magnetic Co sublattice, (ii) the reduction of Co⁴⁺/Co³⁺ ratio resulting in a reduced carrier concentration and (iii) disruption of the intermediate spin structure of Co, namely Co³⁺: t_2g⁵e_g¹.     

Magnetic-field-induced transition from metastable spin glass to possible antiferromagnetic-ferromagnetic phase separation in Cd0.5Cu0.5Cr2O4

Using ac susceptibility, dc magnetization and heat-capacity measurements, we have investigated the magnetic properties of Cd_0.5Cu_0.5Cr₂O₄. Cd_0.5Cu_0.5Cr₂O₄ has an extraordinary magnetic phase including a metastable spin-glass (SG) phase at zero field, a possible phase separation scenario of AFM/FM above ∼0.5 T field, and at intermediate fields, an apparent pseudo reentrant spin-glass (RSG) plateau is observed. These phenomena are closely correlated with the pinning effect of the Cu²⁺ sublattice on the frustrated lattice.     

Size-induced ferromagnetism and metallicity in Nd0.5Sr0.5MnO3 nanoparticles: A neutron diffraction study

The phenomenon of destabilization of antiferromagnetic insulating state into a ferromagnetic metallic one in Nd_0.5Sr_0.5MnO₃ with the variation of particle/grain size is critically investigated. Based on our neutron diffraction study, magnetic and transport experiments, we observe ferromagnetism and metallic behavior in Nd_0.5Sr_0.5MnO₃ (∼40 nm grain size) as against A-type antiferromagnetic order in the sample with the largest grain size (∼800 nm). The latter shows a systematic change in the lattice parameters with temperature, and an antiferromagnetic ground state similar to that of a bulk system. Interestingly, the sample with the smallest grain sizes exhibits insignificant structural changes (compared to the largest grain size sample) but a complete change in the magnetic state (ferromagnetic behavior) as revealed from the neutron diffraction study. Magnetic measurements also confirm a ferromagnetic state in the small-grained sample. Electronic transport measurements exhibit a metal-insulator transition in this sample. The effects are primarily attributed to enhanced surface disorder.     

半掺杂Sm0.5Ca0.5MnO3体系的电荷有序和再入型自旋玻璃行为

研究了半掺杂锰氧化物Sm_0.5Ca_0.5MnO₃体系的结构、输运和磁特性，结果表明，在半掺杂情况下，该体系呈现O′类正交结构，表明体系存在典型的Jahn-Teller效应畸变；输运结果在整个测量温区均呈现半导体导电行为，没有出现金属－绝缘体(M-I)转变和CMR效应；电荷有序转变发生在T=270K左右，反铁磁转变温度出现在200K附近，且表现出典型的再入型自旋玻璃(spin-glass)行为，自旋玻璃转变温度T_SG在4 关键词： 自旋玻璃 电荷有序 负磁化现象 多相竞争     

相分离Nd0.5Ca0.5MnO3体系的再入型自旋玻璃行为和电荷有序

研究了Nd_0.5Ca_0.5MnO₃体系的结构和输运特性. 结构 分析表明，在300K下，体系表现为O′型正交结构并存在典型的Jahn-Teller畸变.在8 T磁场 下，体系出现顺磁绝缘-铁磁金属的转变，庞磁电阻效应发生. 磁测量发现，样品的奈尔温 度T_N和电荷有序转变温度T_CO分别在150和240K左右，在41K左右出 现典型再入型自旋玻璃行为，同时观察到了负的磁化率异常. 结果表明，Nd关键词： 庞磁电阻 自旋玻璃态 负磁化强度 电荷有序     

First order paramagnetic-ferromagnetic phase transition in Tb doped La0.5Ca0.5MnO3 manganite

Effect of A-site disorder in half-doped La_0.5Ca_0.5MnO₃ created by the substitution of Tb at La site is studied through temperature dependent neutron diffraction, resistivity, and magnetization on La_0.375Tb_0.125Ca_0.5MnO₃ (LTC) to identify the evolution of phases as a function of temperature and magnetic field. Contrary to the parent system, the paramagnetic (PM) to ferromagnetic (FM) transition is found to be of first order but becomes observable only in fields above 1 T. Absence of this transition in zero and low fields along with the observation of a very intriguing and irreversible M-H curve at low temperature raises the question about the ground state of the system. It is identified from the specially designed measurement protocol that the ground state of the system becomes FM, contrary to the parent LCMO. However, this remains masked because of the hindrance to the transformation kinetics of the PM-FM first order transition in low fields. Significantly, though the thermal hysteresis of the first-order transition decreases with the measurement field, collapse of such a behavior to a critical point is not observed in LTC, unlike earlier observations in some other manganites.     

Glassy-ferromagnetic behavior in Eu0.5Sr0.5CoO3

Polycrystalline perovskite cobalt oxide Eu_0.5Sr_0.5CoO₃ was prepared by the conventional solid-state reaction method. X-ray powder patterns indicated the prepared samples are pure, cubic perovskite structure (Pm3?m), and with no evidence of any secondary phases. The dc magnetization and ac susceptibility measurements were carried out to investigate the magnetic properties of the sample, and which indicated that cluster-glasses properties are suppressed with the increasing of the coercive field. We denied the possibility of spin-glasses and the existence of the Hopkinson effect in Eu_0.5Sr_0.5CoO₃ through the temperature-dependent ac susceptibility measurements, and explained the magnetic behavior of Eu_0.5Sr_0.5CoO₃ with the competition between magnetic anisotropy and the external magnetic field.     

Research on charge-ordering state in La0.5Sr0.5MnO2.88 and La0.5Sr0.5Mn0.5Ti0.5O3 systems

The structural, magnetic and transport properties of La_0.5Sr_0.5MnO_2.88 and La_0.5Sr_0.5Mn_0.5Ti_0.5O₃ samples have been investigated systematically. Indeed, this series has been considered to understand the influence of physical parameters such as oxygen deficiency and titanium doping effect in undoped La_0.5Sr_0.5MnO₃ sample. Ceramic material based on La_0.5Sr_0.5MnO_2.88 exhibits interesting behaviours of charge-ordering (CO), ferromagnetic (FM) states and a good conductivity down to the lowest temperatures. The substitution of Ti for Mn destroyed drastically the CO, damaged the motion of itinerant e_g electrons and changed the local parameters of perovskite cell. A change of the structure from tetragonal to rhombohedral symmetry is observed causing a weakening of double-exchange interaction. The experiment results show that the suppression of the CO is sensitive to the variety of Mn³⁺/Mn⁴⁺ ratio. In a field of 8 T at 10 K, FM and CO phase can be evaluated to be ∼20:80 according to the μ_exp/μ_cal ratio for La_0.5Sr_0.5MnO_2.88, whereas the CO state is suppressed for La_0.5Sr_0.5Mn_0.5Ti_0.5O₃ sample, FM and anti-ferromagnetic (AFM) phase are coexisted and evaluated to be ∼54:46, respectively.     

Magnetic and structural studies of the alloy system REIn0.5Ag0.5

The structural and magnetic properties of the alloy system REIn_0.5Ag_0.5 [RE = Gd, Tb, Dy, Ho, Er, Tm and Yb] are reported. All these alloys (except that of Yb) crystallize in a cubic CsCl type structure at room temperature. Low temperature X-ray diffraction data does not reveal any structural phase transformation down to 8 K. On the basis of magnetic susceptibility data at a different temperature (3–300 K) and applied magnetic field (2 × 10⁵ to 8 × 10⁶ A m^-1, it has been concluded that GdIn_0.5Ag_0.5 is ferromagnetic (T_c = 118 K), TbIn_0.5Ag_0.5 and DyIn_0.5Ag_0.5 are meta magnetic (T_N = 66 and 30 K, respectively) and alloys involving Ho, Er, Tm and Yb are ferrimagnetic with Néel temperatures (T_N) equal to 24, 22, 21 and 20 K, respectively. The evaluated effective magneton number (p) is found to be slightly larger compared to theoretical values for tripositive ions of Gd, Tb and Dy and a bit smaller for Ho, Er, Tm and Yb. The results have been qualitatively explained using appropriate theories.     

Effects of large cation size-disorder on the magnetic properties of the rare earth cobaltates, Ln0.5Ba0.5CoO3

Magnetic and electrical properties of Ln_0.5Ba_0.5CoO_3−δ with Ln=Dy and Er have been investigated to examine the effects of large cation size-disorder. While the Dy compound shows the small magnetic anomaly around 290 K just as the Gd derivative, the Er compound is essentially paramagnetic due to the large cation size-disorder. Compositions with the same average A-site cation radii as Dy_0.5Ba_0.5CoO_2.91 and Er_0.5Ba_0.5CoO_2.9, but with smaller size-disorder, show progressive evolution of ferromagnetism and metallic properties with decreasing disorder.     

Structure and magnetization studies of Nd0.5−xPrxSr0.5MnO3 system

The structural and magnetic properties of Nd_0.5−xPr_xSr_0.5MnO₃ (x=0, 0.1, 0.2, 0.3, 0.4 and 0.5) system have been investigated. With the substitution of Pr in Nd_0.5Sr_0.5MnO₃, it shows a gradual structure transformation from the Imma orthorhombic symmetry to the tetragonal I4/mcm phase, and the crystallographic transition remains incomplete, even in Pr_0.5Sr_0.5MnO₃. A large bifurcation between zero-field-cooled (ZFC) and field-cooled (FC) susceptibility has been observed below Curie temperature (T_C), which is characteristic of coexistence of ferromagnetism (FM) and antiferromagnetism (AFM) at low temperature region. The magnetization of Pr_0.5Sr_0.5MnO₃ is larger than that of Nd_0.5Sr_0.5MnO₃, while Nd_0.5Sr_0.5MnO₃ with more CE-type AFM shows larger magnetization than Nd_0.3Pr_0.2Sr_0.5MnO_3, which mixed with CE-type (majority) and A-type (minority) AFM at low temperature, indicating that the magnetization of Nd_0.5−xPr_xSr_0.5MnO₃ system is affected by A-site disorder combined with orbital ordering of A-type AFM and CE-type AFM.     

Magnetic coupling between Gd and Pr ions and magnetocaloric effect in Gd0.5Pr0.5Al2 compound

In this work, we report the theoretical and experimental investigations on the magnetic and magnetocaloric properties for Gd_0.5Pr_0.5Al₂ compound in different magnetic fields. The magnetization features indicate that Gd_0.5Pr_0.5Al₂ is ferrimagnetic at low temperatures. We also present data from X-ray magnetic circular dichroism (XMCD) experiments for this compound, with which we have confirmed that the magnetic moments of the Pr ions are antiparallel to the magnetic moments of the Gd ions. The magnetocaloric parameters, ΔT_S and ΔS_T, were obtained from calorimetric data and both curves present normal and inverse magnetocaloric effect. A theoretical model for ferrimagnetic coupling, including the crystalline electrical field anisotropy, was used to describe the ΔT_S and ΔS_T experimental results.     

Fabrication and magnetic properties of Ni0.50.5Zn0.5Fe2O4 nanofibres by electrospinning

NiZn ferrite/polyvinylpyrrolidone composite fibres were prepared by sol–gel assisted electrospinning.Ni0.5Zn0.5Fe2O4 nanofibres with a pure cubic spinel structure were obtained subsequently by calcination of the composite fibres at high temperatures.This paper investigates the thermal decomposition process,structures and morphologies of the electrospun composite fibres and the calcined Ni0.5Zn0.5Fe2O4 nanofibres at different temperatures by thermogravimetric and differential thermal analysis,x-ray diffraction,Fourier transform infrared spectroscopy and field emission scanning electron microscopy.The magnetic behaviour of the resultant nanofibres was studied by a vibrating sample magnetometer.It is found that the grain sizes of the nanofibres increase significantly and the nanofibre morphology gradually transforms from a porous structure to a necklace-like nanostructure with the increase of calcination temperature.The Ni0.5Zn0.5Fe2O4 nanofibres obtained at 1000 C for 2 h are characterized by a necklace-like morphology and diameters of 100–200 nm.The saturation magnetization of the random Ni0.5Zn0.5Fe2O4 nanofibres increases from 46.5 to 90.2 emu/g when the calcination temperature increases from 450 to 1000 C.The coercivity reaches a maximum value of 11.0 kA/m at a calcination temperature of 600 C.Due to the shape anisotropy,the aligned Ni0.5Zn0.5Fe2O4 nanofibres exhibit an obvious magnetic anisotropy and the ease magnetizing direction is parallel to the nanofibre axis.     

Neutron diffraction study of magnetic ordering in Tm0.5Eu0.5Se

A single crystal of the magnetic semiconductor Tm_0.5Eu_0.5Se was studied by means of neutron diffraction in the temperature range from 1.8 to 293 K. Long-range magnetic order is detected at temperatures below T_c = (18.5±1) K. The measured ferromagnetic moment component of (2.12±0.05) μ_B per rare-earth ion at saturation in zero external magnetic field indicates approximately antiparallel alignment of Tm moment and Eu spin (mutual angle 134°). The experimentally determined neutron magnetic form factor confirms the divalent state of both Tm and Eu in Tm_0.5Eu_0.5Se.     

Dependence of magnetic properties and microstructure of mechanically alloyed Ni0.5Zn0.5Fe2O4 on soaking time

Possible soaking-time effects on the magnetic and microstructural properties of polycrystalline samples of Ni_0.5Zn_0.5Fe₂O₄ have been studied. Nanosize powder produced by mechanical alloying was sintered at 800 °C with various soaking times. All samples showed the signature peak of Ni_0.5Zn_0.5Fe₂O₄ even with one hour of soaking time. The size distributions show a slow growth of microstructural evolution related to density, porosity and also to the magnetic hysteresis loops. Within these distributions it is observed that the formation of multi-domains is not possible and probably there are the regions of superparamagnetic and single-domain grains. From the permeability studies, it is believed that the rise of the magnetic moment on the B sites give rise to the total saturation magnetization with increase of soaking time. The hysteresis loop of one-hour soaking time showed paramagnetic behavior dominating while longer soaking times showed ferromagnetic behavior starting to dominate. The coercivity was observed to increase with soaking time, signaling the increase of the anisotropy fields which was attributed to the shape anisotropy and also to the magnetocrystalline anisotropy. By correlating the morphology, phase analysis, permeability and hysteresis loops results, it is believed that there was an increase in number of crystalline-growth regions which together formed a total mass of mixed superparamagnetic and ferromagnetic grains with the latter starting to dominate the samples.     

Spin-glass-like behaviour and magnetic order in Mn[Cr0.5Ga1.5]S4 spinels

Magnetization and susceptibility were investigated as a function of temperature and magnetic field in polycrystalline Mn[Cr_0.5Ga_1.5]S₄ spinel. The dc susceptibility measurements at 919 Oe showed a disordered ferrimagnetic behaviour with a Curie-Weiss temperature θ_CW=−55 K and an effective magnetic moment of 5.96 μ_B close to the spin-only value of 6.52 μ_B for Cr³⁺ and Mn²⁺ ions in the 3d³ and 3d⁵ configurations, respectively. The magnetization measured at 100 Oe revealed the multiple magnetic transitions with a sharp maximum at the Néel temperature T_N=3.9 K, a minimum at the Yafet-Kittel temperature T_YK=5 K, a broad maximum at the freezing temperature T_f=7.9 K, and an inflection point at the Curie temperature T_C=48 K indicating a transition to paramagnetic phase. A large splitting between the zero-field-cooled (ZFC) and field-cooled (FC) magnetizations at a temperature smaller than T_C suggests the presence of spin-glass-like behaviour. This behaviour is considered in a framework of competing interactions between the antiferromagnetic ordering of the A(Mn) sublattice and the ferromagnetic ordering of the B(Cr) sublattice.     

Solvothermal one-step synthesis of MWCNTs/Ni0.5Zn0.5Fe2O4 magnetic composites

A magnetic multi-walled carbon nanotubes-based (MWCNTs-based) composite, MWCNTs/Ni_0.5Zn_0.5Fe₂O₄, was synthesized via a facile solvothermal approach. The composites were characterized by X-ray diffraction analysis, transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and vibrating sample magnetometry. The results confirmed that MWCNTs and Ni_0.5Zn_0.5Fe₂O₄ coexisted in the composites. The TEM and HRTEM results showed a thick layer of Ni_0.5Zn_0.5Fe₂O₄ was intimately connected to the surface of MWCNTs. The saturation magnetization value of the composites was 45.8 emu/g. Furthermore, the probable synthesis mechanism of the magnetic composites was also investigated based on the experimental results.