Jahn-Teller transition in Al doped LiMn2O4 spinel

Al-doped lithium manganese spinels, with starting composition Li_1.02Al_xMn_1.98−xO₄ (0.00<x≤0.06), are investigated to determine the influence of the Al³⁺ doping on the Jahn-Teller (J-T) cooperative transition temperature T_J-T. X-ray powder diffraction (XRPD), nuclear magnetic resonance, electron paramagnetic resonance, conductivity and magnetic susceptibility data are put into relation with the tetrahedral and octahedral occupancy fraction of the spinel sites and with the homogeneous distribution of the Al³⁺ ions in the spinel phase. It is observed that Al³⁺ may distribute between the two cationic sublattices. The J-T distortion, associated with a drop of conductivity near room temperature in the undoped sample, is shifted towards lower temperature by very low substitution. However, for x>0.04 T_J-T it increases with increasing x, as clearly evidenced in low temperature XRPD observations. A charge distribution model in the cationic sublattice, for Al substitution, is proposed to explain this peculiar behavior.     

Probing local magnetic cluster development in (CuZr)93−xAl7Gdx bulk metallic glasses by Al NMR

We report magnetization properties of (CuZr)_93−xAl₇Gd_x bulk metallic glasses from temperature dependent ²⁷Al nuclear magnetic resonance spectroscopy and magnetic susceptibility measurements. Significant non-linear line broadening of ²⁷Al spectra commencing at high temperatures is attributed to the development of a local magnetic susceptibility distribution that prevails over a finite temperature range. Magnetization measurements confirm the linewidth enhancement due to strong frustrated magnetic short-range order. This study provides insight into the nature of magnetic development and frustration in paramagnetic systems.     

Effect of Al doping on the magnetic and electrical properties of layered perovskite La1.3Sr1.7Mn2−xAlxO7

The effect of Al substitution for Mn site in layered manganese oxides La_1.3Sr_1.7Mn_2−xAl_xO₇ on the magnetic and electrical properties has been investigated. It is interesting that all the samples undergo a similar and complex transition with lowing temperature; they transform from the two-dimensional short-range ferromagnetic order at T*, then enter the three-dimensional long-range ferromagnetic state at T_C, at last they display the canted antiferromagnetic state below T_N. T*, T_C and T_N are all reduced with Al content. Resistivity increases sharply with increasing Al concentration, and the metal-insulator transition disappears when x reaches 10%. Additionally, magnetoresistance (MR) effect is weakened. Al substitution dilutes the magnetic active Mn-O-Mn network and weakens the double exchange interaction, and further suppresses FM ordering and metallic conduction. Owing to the anisotropic interaction in the layered perovskite, the magnetic and electrical properties are more sensitive to Al doping level than those in ABO₃-type perovskite.     

Observation of higher magnetization on the substitution of Al for Co in La2MnCoO6

Upon substitution of non-magnetic Al³⁺ for diamagnetic, low-spin, Co³⁺ in ferromagnetic La₂MnCoO₆, the ferromagnetic moment, measured at 82 K and 15 kOe, is found to increase initially with Al content and then decreases, though the magnetic transition temperature decreases continuously on increasing x in La₂MnCo_1−xAl_xO₆.     

Neutron diffraction studies of the magnetic properties of Pr0.5Sr0.5Co1 ? x Mn x O3 solid solutions

The crystal structure and magnetic properties of a system of Pr_0.5Sr_0.5Co_{1 ? x} Mn _x O₃ solid solutions were studied by neutron diffraction and magnetization measurements. It is shown that, at a low manganese concentration, the structure can be described by the I/2a monoclinic space group; with increasing substitution level x the structure becomes orthorhombic. For x > 0.9 the crystal structure is tetragonal at high temperatures and the symmetry is lowered to orthorhombic with lowering the temperature. The substitution of cobalt for manganese leads to the destruction of long-range ferromagnetic order near x ?? 0.25. A transition from the high-temperature ferromagnetic phase to the A-type low-temperature antiferromagnetic phase is observed at x ?? 0.93 in the temperature range 110?C160 K.     

Preparation of nano-sized Al-substituted yttrium iron garnets by the mechanochemical method and investigation of their magnetic properties

The polycrystalline Y₃Fe_5−xAl_xO₁₂ compounds with x=0.5, 1.0, 1.5 and 2.0 were prepared by the mechanochemical method. The samples were milled for 40 h in a high-energy planetary mill and then calcined at different temperatures from 1300 to 1100 °C. The minimum calcination temperature to get a single phase garnet decreases by increasing Al concentration. X-ray diffraction patterns reveal that the structures of nano-powders are bcc and the garnet phase has been obtained after calcining. Also, the lattice constant of the samples decreases by increasing Al concentration ,which is discussed based on the substitution of smaller aluminum ions instead of iron ions. The average crystallite sizes are in the range 24-35 nm using Scherrer's formula. The Curie temperature of single phase samples was found to decrease by increasing Al concentration, which can be discussed upon the reduction of magnetic interactions per magnetic ion. When more Al³⁺ is added, the magnetization is reduced because of the reduction of superexchange interactions in crystal lattice.     

Doping strategies for increased performance in BiFeO3

Investigation of crystal structure, dielectric, magnetic and local ferroelectric properties of the diamagnetically substituted Bi_1−xA_xFeO_3−x/2 (A=Ca, Sr, Pb, Ba; x=0.2, 0.3) polycrystalline samples has been carried out. It has been shown that the heterovalent A²⁺ substitution result in the formation of oxygen vacancies in the host lattice. The solid solutions have been found to possess a rhombohedrally distorted perovskite structure described by the space group R3c. Piezoresponse force microscopy has revealed signs of existence of the ferroelectric polarization in the samples at room temperature. Magnetization measurements have shown that the magnetic state of these compounds is determined by the ionic radius of the substituting elements. A-site substitution with the biggest ionic radius ions has been found to suppress the spiral spin structure of BiFeO₃ giving rise to the appearance of room-temperature weak ferromagnetism.     

Magnetic clusters in Fe3−x V x Al alloys with DO3 type structure forx≤1.1

Polycrystalline Fe_3?x V _x Al alloys with the DO₃ type structure were investigated by magnetostatic and Mössbauer methods. In alloys with 0.2<x<0.5 the ferro- and superpara-magnetic phases coexist. For alloys withx≥0.5 these investigations showed that in spite of the lack of magnetic order, they are characterized by strong nonlinear variations of the magnetization in a magnetic field over a wide temperature range. This indicates the presence of magnetic clusters of Fe atoms with very large magnetic moments. Analysis of the magnetization isotherms indicates that in these alloys a distribution of magnetic moments of clusters (μ_c) from 10 μ_B to 10⁴ μ_B exists.     

Structural and magnetic characterisation of the perovskite oxides La0.7Ca0.3−x Na x MnO3

X-ray powder diffraction (XRD) and magnetic measurements were performed in order to investigate the effect of Na⁺ ion substitution for Ca²⁺ ions on the crystallographic structure, the character of magnetic ordering, and the effect of transition temperature in La_0.7Ca_0.3−x Na _x MnO₃ manganites series (0 ⩽ × ⩽ 0.2). All samples crystallise in an orthorhombic structure with the Pnma space group. We have found a strong dependence of structural and magnetic properties on the cation-size disorder parameter σ ². The temperature dependence of magnetization of all samples obeys the Bloch T ^3/2 law. The values of the spin wave constant at low temperature B increase with the increase of x and the Curie temperature decreases. It is concluded that the substitution of Ca by Na⁺ ions causes a decrease in total exchange integral Aof the samples.      

A Mössbauer effect and X-ray diffraction study of Fe–Ga–Al thin films prepared by combinatorial sputtering

A combinatorial thin film library of the composition Fe_100−yGa_y−xAl_x was prepared by magnetron sputtering. The composition was determined by electron microprobe and showed that the Fe content was reasonably constant across the library and the Al to Ga ratio varied linearly with position. The crystallographic structure was determined with X-ray diffraction and found to be bcc. ⁵⁷Fe Mössbauer spectroscopy was used to investigate short-range order within the film. It was shown that non-magnetic atoms tend to cluster in these alloys, but that the substitution of Al for Ga reduced this tendency. This behavior may be, at least partially, responsible for the decrease in saturation magnetostriction between Fe–Ga and Fe–Al alloys.     

The influence of Al substitution on the phase transitions and magnetocaloric effect in Ni43Mn46Sn11−xAlx alloys

The effects of Al substitution on the phase transitions and magnetocaloric effect of Ni₄₃Mn₄₆Sn_11−xAl_x (x=0-2) ferromagnetic shape memory alloys were investigated by X-ray diffraction and magnetization measurements. With the increase of Al content, the cell volume decreases due to the smaller radius of Al, and the martensitic transformation temperature increases rapidly, while the Curie temperature of austenitic phase shows a small increase. A large positive and a negative magnetic entropy change were observed near the first-order martensitic transition and the second-order magnetic transition, respectively. The magnetic entropy changes, hysteresis behavior, and refrigerant capacity near the two transitions are compared.     

Spectral studies of Co substituted Ni-Zn ferrites

The spinel ferrites Zn_0.35Ni_0.65−xCo_xFe₂O₄, 0≤x≤1, have been prepared using the standard ceramic technique. Room temperature Mössbauer, X-ray and infrared IR spectra were used for carrying out this study. X-ray patterns reveal that all the samples have single-phase cubic spinel structure. The Mössbauer spectra of the samples show a paramagnetic phase for x=0 and a six-line magnetic pattern and a central paramagnetic phase for x≥0.1. They are analyzed and attributed to two magnetic subpatterns and two quadrupole doublets due to Fe³⁺ ions at the tetrahedral A-sites and octahedral B-sites. Four absorption bands are observed in IR spectra. They confirm the spinel structure of the samples and existence of Fe³⁺ ions in the sample sublattices. The deduced hyperfine interactions, lattice parameters, absorption band positions and intensities and force constant are found to be dependent on the substitution factor x, where the cation distribution is estimated. The hyperfine magnetic fields, magnetization and lattice resonant frequency are found to be dependent on the interionic distance.     

Effect of Fe substitution on magnetocaloric effect in La0.7Sr0.3Mn1−xFexO3 (0.05≤x≤0.20)

We have studied the effect of Fe substitution on magnetic and magnetocaloric properties in La_0.7Sr_0.3Mn_1−xFe_xO₃ (x=0.05, 0.07, 0.10, 0.15, and 0.20) over a wide temperature range (T=10-400 K). It is shown that substitution by Fe gradually decreases the ferromagnetic Curie temperature (T_C) and saturation magnetization up to x=0.15 but a dramatic change occurs for x=0.2. The x=0.2 sample can be considered as a phase separated compound in which both short-range ordered ferromagnetic and antiferromagnetic phases coexist. The magnetic entropy change (−ΔS_m) was estimated from isothermal magnetization curves and it decreases with increase of Fe content from 4.4 J kg⁻¹ K⁻¹ at 343 K (x=0.05) to 1.3 J kg⁻¹ K⁻¹ at 105 K (x=0.2), under ΔH=5 T. The La_0.7Sr_0.3Mn_0.93Fe_0.07O₃ sample shows negligible hysteresis loss, operating temperature range over 60 K around room temperature with refrigerant capacity of 225 J kg⁻¹, and magnetic entropy of 4 J kg⁻¹ K⁻¹ which will be an interesting compound for application in room temperature refrigeration.     

Electronic structure and magnetic properties of RCo5−xMx (R=Y,Pr and M=Al,Si) system

Detailed theoretical and experimental investigations of the electronic and magnetic properties of the RCo_5?xM_x compounds (R=Y, Pr and M=Si, Al) have been performed. All theoretical investigations of the electronic and magnetic properties of the system have been done using the Korringa–Kohn–Rostoker (KKR) band structure method. The Si for Co substitution in RCo₅ does not change the magnetic ordering: the RCo_5?xSi_x with R=Y, Nd and Pr is ferromagnetic, whilst the heavy rare-earth containing compounds are ferrimagnetic. The important modifications induced by this substitution concerns the magnetic properties of the system: the Curie temperature and the magnetic moments of Co decrease with Si content, indicating the weakening of the Co–Co exchange interaction. The band structure calculations evidence the hybridization between the 3d electronic states of Co and the 3p states of Si as possible reason for the diminishing of Co–Co exchange interaction. Also, the volume effect on the magnetic properties of the YCo₄Si was investigated using theoretical methods. The results are compared with the experimental measurements in order to distinguish the origin of magnetization reduction in YCo₄Si compared with YCo₄Al.     

Local spin reversal and associated magnetic responses in Ga-substituted Pb-hexaferrites

We have studied magnetic structure and properties of Ga-substituted Pb-hexaferrites having the stoichiometry of PbFe_12−xGa_xO₁₉ with x=6 (i.e., Fe:Ga=1:1). According to the neutron diffraction results, this compound is characterized by a collinear spin structure below its Curie temperature (∼325 K). Analysis of the neutron diffraction patterns further indicates that the magnetic-moment direction of Fe³⁺ ions located at the octahedral 2a sublattice is downward while that of the unsubstituted PbFe₁₂O₁₉ is upward at room temperature. With decreasing temperature, the Fe³⁺ magnetic moment at the octahedral 2a sublattice undergoes a reorientation to the upward direction while that of the unsubstituted PbFe₁₂O₁₉ remains upward down to 5 K. This selective local spin reversal at the 2a sublattice of PbFe₆Ga₆O₁₉ was attributed to the weakening of the superexchange interaction between the octahedral 2a site and the tetrahedral 4f_IV site upon the preferential substitution of Ga ions for Fe ions at these two neighboring sites. Comparison of the neutron diffraction results with dc magnetization responses and ac susceptibilities further indicates that the paramagnetic–ferrimagnetic transition at ∼325 K (T_c) is followed by the local spin reversal at lower temperatures.     

Sr2Fe1－xCoxMoO6体系中Co的元素替代效应

在成功制备具有双钙钛矿结构Sr₂Fe_1-xCo_xMoO₆系列样品的基础上，对其结构、输运性质和磁性质进行了系统研究.结果发现，随着Co替代浓度x值的增加，样品的电阻率-温度关系由半金属行为转变为半导体行为，其室温电阻率从3.9×10^-5Ω·cm增大到6.0×10^-1Ω·cm；样品由亚铁磁体转变成反铁磁体，其磁相变温度T_N值也随之下降; Co对Fe的部分替代使其磁电阻效应受到抑制.基于对其电子结构的分析，其磁电阻效应的起源以及Co的元素替代效应也在文中进行了讨论. 关键词： 双钙钛矿结构 2FeMoO₆')" href="#">Sr₂FeMoO₆ 磁电阻     

Novel structural and magnetic properties of Mg doped copper nanoferrites prepared by conventional and wet methods

Nanoferrites of the general formula Cu_1−xMg_xFe₂O₄ with 0≤x≤0.6 were prepared by standard ceramic and wet methods. The structure was studied by X-ray diffraction and IR spectroscopy. The density and lattice constant were calculated and reported. The particle size of the prepared nanoferrites ranged from 8.7 to 41.1 nm. It was found that the lattice parameter decreases with increasing cation substitution of Mg²⁺ due to the difference of ionic radius and atomic mass. The dc magnetic susceptibility was measured out using Faraday's method. The magnetic hysteresis measurement was performed using a vibrating sample magnetometer. Magnetic constants such as Curie temperature, effective magnetic moment, saturation magnetization, remanent magnetization and corecivicty were obtained and reported. The magnetic constants decrease with increasing Mg²⁺, except the remanent magnetization which increased.     

Structure, spin reorientation and M?ssbauer effect studies of Tb0.3Dy0.7(Fe1?xAlx)1.95 alloys

The effect of Al substitution for Fe on crystal structure, magnetostriction and spontaneous magnetostriction, anisotropy and spin reorientation of a series of polycrystalline Tb_0.3Dy_0.7(Fe_1−x Al_x)_1.95 alloys (x = 0, 0.05, 0.1, 0.15, 0.20, 0.25, 0.30, 0.35) at room temperature and 77 K was investigated systematically. It was found that the primary phase of Tb_0.3Dy_0.7(Fe_1−x Al_x)_1.95 is the MgCu₂-type cubic Laves phase structure when x < 0.4 and the lattice constant a of Tb_0.3Dy_0.7(Fe_1−x Al_x)_1.95 increases approximately and monotonically with the increase of x. The substitution of Al leads to the fact that the magnetostriction λ inceases slightly in a low magnetic field (H ⩽ 40 kA/m), but decreases sharply and is easily close to saturation in a high applied field as x increases, showing that a small amount of Al substitution is beneficial to a decrease in the magnetocrystalline anisotropy. It was also found that the spontaneous magnetostriction λ ₁₁₁ decreases greatly with x increasing. The analysis of the M?ssbauer spectra indicated that the easy magnetization direction in the {110} plane deviates slightly from the main axis of symmetry with the changes of composition and temperature, namely spin reorientation. A small amount of non-magnetic phase exists for x = 0.15 in Tb_0.3Dy_0.7(Fe_1−x Al_x)_1.95 alloys and the alloys become paramagnetic for x > 0.15 at room temperature, but at 77 K the alloys still remain magnetic phase even for x = 0.2. At room temperature and 77 K, the hyperfine field decreases and the isomer shifts increase with Al concentration increasing.     

Magnetocaloric effect of GdCo2−xAlx compounds

The structure, magnetic property and magnetocaloric effect of GdCo_2−xAl_x (x=0, 0.06, 0.12, 0.18, 0.24, 0.4) compounds have been investigated by X-ray diffraction (XRD) and magnetic measurement techniques. The experimental results show that the GdCo_2−xAl_x (x≤0.4) compounds are single phase with a Laves-phase MgCu₂-type structure. The Curie temperature T_c initially increases, and then decreases with increasing Al content. The maximum value of T_c, 418 K, is reached for the compound with x=0.06. The magnetic entropy change, which is determined from the temperature and field dependence of the magnetization by the Maxwell relation, decreases almost linearly with increasing Al content.     

Structural and magnetic properties of Mn3−xCdxTeO6 (x=0, 1, 1.5 and 2)

Mn₃TeO₆ exhibits a corundum-related A₃TeO₆ structure and a complex magnetic structure involving two magnetic orbits for the Mn atoms [Ivanov et al., 2011 [3]]. Mn_3−xCd_xTeO₆ (x=0, 1, 1.5, and 2) ceramics were synthesized by solid state reaction and investigated using X-ray powder diffraction, electron microscopy, and calorimetric and magnetic measurements. Cd²⁺ replaces Mn²⁺ cations without greatly affecting the structure of the compound. The Mn and Cd cations were found to be randomly distributed over the A-site. Magnetization measurements indicated that the samples order antiferromagnetically at low temperature with a transition temperature that decreases with increasing Cd doping. The nuclear and magnetic structure of one specially prepared ¹¹⁴Cd containing sample: Mn_1.5¹¹⁴Cd_1.5TeO₆, was studied using neutron powder diffraction over the temperature range 2-295 K. Mn_1.5¹¹⁴Cd_1.5TeO₆ was found to order in an incommensurate helical magnetic structure, very similar to that of Mn₃TeO₆ [Ivanov et al., 2011 [3]]. However, with a lower transition temperature and the extension of the ordered structure confined to order 240(10) Å.