Structural transition and enhanced magnetization in Bi1−xYxFeO3

Bi_1−xY_xFeO₃ (x=0-0.2) powders were prepared to study the effect of Y substitution on their structural and magnetic properties. A structural symmetric breaking from the rhombohedral R3c to orthorhombic Pnma at around x=0.10 was identified across a ferroelectric-paraelectric phase. A parabolic dependence of the magnetization upon substitution was obtained with a maximum at the phase transition boundary and a switching behavior for x=0.20. The composition-driven magnetic structure evolution was proposed to account for the magnetic properties in Bi_1−xY_xFeO₃.     

Phase transitions in ferromagnetic Ni2+x Mn1−x Ga alloys with regard for the modulation order parameter

The phase diagram of ferromagnetic alloys Ni_2+x Mn_1?x Ga is reconstructed on the basis of temperature dependences of the resistance. It is seen from this diagram that for small x, structural transitions from the cubic to the tetragonal phase are preceded by structural transformations in the cubic phase. In the framework of the phenomenological Landau theory of phase transitions, phase diagrams of the structural and magnetic phase transitions in these alloys are analyzed with regard for the modulation order parameter. It is shown that premartensitic and postmartensitic phase transitions related to the appearance of the modulated structure can occur along with martensitic transformations. The strain and modulation order parameters substantially affect the magnetic phase transitions via the interaction with the magnetic order parameter.     

Magnetic phase transitions in PrMn2−xCrxGe2

The structural and magnetic properties of PrMn_2−xCr_xGe₂ (0⩽x⩽1.0) were studied by X-ray diffraction and magnetization measurements. The powder samples crystallize in the ThCr₂Si₂-type structure, and the lattice constants at room temperature show almost no variation as Cr substitutes Mn. The observed phase transitions are summarized in a proposed magnetic x−T phase diagram and compared with previous Moessbauer spectroscopy and neutron diffraction results for x=0.     

Acoustic anomalies near phase transitions in manganite

The behavior of the velocities and attenuation of ultrasonic waves propagating in La_1?x Sr_xMnO₃ (x=0.175) manganite at frequencies of 700–800 MHz is studied in the temperature interval from 320 to 180 K, and the effect of magnetic field on the acoustic parameters is investigated. The transformation of acoustic modes in the vicinity of the magnetic phase transition is observed. The changes in the acoustic parameters near the structural and magnetic phase transitions are shown to be related to the strong spin-phonon and electron-phonon interactions.     

Magnetic microstructure of strontium-substituted thulium ferromanganites Tm0.65Sr0.35Mn1 ? x Fe x O3 (x = 0.3?0.4)

The structural and magnetic properties of the Tm_0.65Sr_0.35Mn_{1 ? x} Fe _x O₃ (x = 0.3?0.4) have been studied by methods of magnetic resonance and differential thermomagnetic analysis. A magnetic phase separation has been revealed in structurally single-phase samples.     

Vanadium-induced decrease in the magnetization of a VxCo1−x alloy film on the W(110) surface

The magnetic and structural properties of a series of V_xCo_1?x alloy films prepared in a variety of compositions on a W(110) surface were investigated using spin-resolved photoemission spectroscopy (SRPES) and low-energy electron diffraction (LEED) measurements. The epitaxial structures of the films were preserved over all V concentrations, despite the observation of a structural phase transition (SPT: hcp/bcc at x = 0.38), which was apparent from the LEED images. A magnetic phase transition (MPT) from ferromagnetic to paramagnetic occurred in the Co-rich region at x = 0.38. The SPT (from hcp and bcc) and MPT (from ferromagnetic to paramagnetic) were clearly correlated at x = 0.38, possibly due to changes in the electronic and magnetic structures of the valence band, as monitored by SRPES and LEED. The magnetization in the V_xCo_1?x alloy films on a W(110) surface varied systematically with the V concentration, and correlations between the structural changes and magnetic properties were revealed using SRPES and LEED.     

The influence of magnetoelastic interaction on structural phase transitions in cubic ferromagnetics

In the framework of the Landau theory of phase transitions, the influence of the magnetoelastic interaction on structural transitions in cubic ferromagnetics with a positive first magnetic anisotropy constant is analyzed. It is shown that structural transitions are not accompanied by a reorientation of magnetization in this case. The phase diagrams of such ferromagnetics either contain a termination point of the structural transition or a critical point in which the first-order transition is replaced by a second-order one. Magnetoelastic interaction also leads to the appearance of an interval of the ferromagnetic parameters in which a coupled first-order structural-magnetic transition exists. The phase T?x diagram for Heusler Ni_2+x Mn_1?x Ga alloys is calculated, which is in good agreement with the experimental phase diagram of these alloys.     

Influence of hydrogen on structural and magnetic properties of the hexagonal Laves phase HoMn2

The HoMn₂ compound crystallizes in the cubic C15 or hexagonal C14 Laves phases depending on preparation. The effect of hydrogen absorption on structural and magnetic properties of HoMn₂H_x hydrides for the C14 phase has been investigated by XRD and AC/DC magnetometry in the temperature ranges of 75-380 K and 4-390 K, respectively. In addition to general features revealed by RMn₂H_x compounds (R=rare earth or Yttrium), unusual behavior of these hydrides was found. In particular, a transformation from the hexagonal to the monoclinic structure was detected, the same as that observed for cubic HoMn₂H_x compounds. The structural transformations are correlated to the magnetic behavior. The presented results are compared mainly with the properties of the cubic HoMn₂H_x hydrides as well as with those of other RMn₂H_x hydrides. Tentative magnetic and structural phase diagrams are proposed.     

Magnetic properties and Mössbauer studies in Y1−x Gd x Fe2

Alloys of Y_1???x Gd _x Fe₂B _y (x = 0, 0.25, 0.5, 0.75 and 1; y = 0, 0.1, 0.15 and 0.2) have been prepared and investigated for structural and magnetic properties. The compounds with x = 0 and 1 are found to form in single phase with C15-type cubic Laves phase structure, while those with x = 0.25, 0.5 and 0.75 are observed to form with small quantities of secondary (Y,Gd)Fe₃ phase. The lattice parameters, Curie temperature and the average Fe hyperfine field are found to increase with increasing x. The Gd–Gd and Gd–Fe interactions are attributed to be the main reason for the enhancement of magnetic properties. Boron was found to stabilize the (Y,Gd)Fe₂ phase without affecting the magnetic properties.     

The static and hyper-frequency magnetic properties of a ferromagnetic-ferroelectric composite

This paper reports the static and hyper-frequency magnetic properties, as well as their relationship with microstructure, of the ferromagnetic-ferroelectric co-fired composite ceramic, (1−x)Ba₂Zn_1.2Cu_0.8Fe₁₂O₂₂-xPb(Ni_1/3Nb_2/3)_0.8Ti_0.2O₃. The X-ray diffraction results did not detect any other phase in the co-fired ceramics, but found a crystal structural distortion of ferrite phase. Scanning electron microscopy photos showed that two phases’ grains matched well and stacked compactly and the hexagonal ferrite changed its grain morphology. The saturation magnetization increased with the reduction of magnetic phase in the range of 0<x<0.65 because of the stress-induced structural distortion. The permeability decreases monotonically with the reduction of magnetic phase in the whole composition range.     

Effect of Si/Ge ratio on resistivity and thermopower in Gd5SixGe4−x magnetocaloric compounds

The effect of Si/Ge ratio on resistivity and thermopower behavior has been investigated in the magnetocaloric ferromagnetic Gd₅Si_xGe_4−x compounds with x=1.7-2.3. Microstructural studies reveal the presence of Gd₅(Si,Ge)₄-matrix phase (5:4-type) along with traces of secondary phases (5:5 or 5:3-type). The x=1.7 and 2.0 samples display the presence of a first order structural transition from orthorhombic to monoclinic phase followed by a magnetic transition of the monoclinic phase. The alloys with x=2.2 and 2.3 display only magnetic transitions of the orthorhombic phase. A low temperature feature apparent in the AC susceptibility and resistivity data below 100 K reflects an antiferromagnetic transition of secondary phase(s) present in these compounds. The resistivity behavior study correlates with microstructural studies. A large change in thermopower of −8 μV/K was obtained at the magneto-structural transition for the x=2 compound.     

Magnetic-field-induced displacive phase transition in IV–VI compounds

The present work calculates the structural phase transition temperature (T_c) in a system with interband electron-phonon interaction in a strong magnetic field, assuming the temperature dependence of the soft-mode frequency to be due to anharmonicities. The results of the calculation are compared with the experiment on Pb_1?xGe_xTe.     

Influence of silicon and atomic order on the magnetic properties of (Fe80Al20)100– x Si x nanostructured system

Mechanically alloyed (Fe₈₀Al₂₀)_100???x Si _x alloys (with x?=?0, 10, 15 and 20) were prepared by using a high energy planetary ball mill, with milling times of 12, 24 and 36 h. The structural and magnetic study was conducted by X-rays diffraction and Mössbauer spectrometry. The system is nanostructured and presents only the BCC disordered phase, whose lattice parameter remains constant with milling time, and decreases when the Si content increases. We found that lattice contraction is influenced 39% by the iron substitution and 61% by the aluminum substitution, by silicon atoms. The Mössbauer spectra and their respective hyperfine magnetic field distributions show that for every milling time used here, the ferromagnetism decreases when x increases. For samples with x?≥?15 a paramagnetic component appears. From the shape of the magnetic field distributions we stated that the larger ferromagnetic phase observed in the samples alloyed during 24 and 36 h is a consequence of the structural disorder induced by mechanical alloying.     

Structural stability and magnetic properties of Bi1−xLa(Pr)xFeO3 solid solutions

In this work, X-ray diffraction data taken on Bi_1−xLa_xFeO₃ solid solutions are used to verify the following structural phase transitions: “polar rhombohedral-antipolar orthorhombic” at x≈0.16 and “commensurate-incommensurate” within the orthorhombic phase at x≈0.18. In contrast, in the Bi_1−xPr_xFeO₃ series, the polar rhombohedral phase transforms into an antipolar orthorhombic one at x≥0.13. The polar rhombohedral phase near the morphotropic phase boundary exhibits an isothermal transformation into an antipolar orthorhombic phase, though the transformation occurs much faster in the case of La-doped compounds. The incommensurate structural phase was not detected in Bi_1−xPr_xFeO₃ solid solutions. The ternary structural phase diagram is constructed for (Bi,La,Pr)FeO₃ systems. In addition, the polar rhombohedral phase exhibits a magnetic field-induced transition from the modulated antiferromagnetic state into a homogeneous weak ferromagnetic state whereas the antipolar phase is a weak ferromagnetic state in the absence of an external field.     

Structural and magnetic properties of Ga-substituted Co2−W hexaferrites

Precursor powders of BaCo₂Fe_16-xGa_xO₂₇ with 0.0 ≤ x ≤ 0.8 were prepared using high-energy ball milling, and the effects of chemical composition on the structural and magnetic properties of the powders sintered at 1300 °C were investigated using x-ray diffractometer (XRD), scanning electron microscopy (SEM), and vibrating sample magnetometry (VSM). XRD patterns of all samples indicated crystallization of pure BaCo₂−W (BaCo₂Fe₁₆O₂₇) hexaferrite phase. SEM measurements revealed large step-like formations with hexagonal crystallites. The magnetic data revealed small fluctuations of the saturation magnetization below the value 72.56 emu/g corresponding to the unsubstituted sample. The coercive field H_c of all samples ranged between 70 Oe and 130 Oe, indicating soft magnetic phase. Curie temperature determined from the thermomagnetic curves of the samples decreased from 485 °C at x = 0.0 down to 451 °C at x = 0.6. Also, the thermomagnetic curves revealed the presence of a minority magnetic phase with enhanced superexchange interaction, and the occurrence of complex magnetic phase transitions associated with spin reorientation transitions above room temperature.     

Hyperfine magnetic interactions in Tb(Fe1 − x Al x )2 alloys

The structural and magnetic characteristics of phase transformations in Tb(Fe_{1 ? x} Al _x )₂ alloys with concentrations x = 0?0.9 have been measured in the temperature range from 90 to 450 K. The temperature dependences of the hyperfine magnetic fields for each of local configurations of the nearest environment of iron atoms upon substitution of aluminum atoms for iron atoms have been found using Mössbauer spectroscopy.     

Absorption of light by exchange-coupled ions in a 2D antiferromagnet

The optical absorption spectra of Rb₂Mn_xCd_1?xCl₄ crystals are experimentally studied in the vicinity of a magnon sideband of the exciton band at a manganese content x ranging from 1.0 to 0.4. Additional absorption bands are observed with an increase in the magnetic structural disorder upon replacement of manganese ions by cadmium ions. An analysis of the evolution of the additional absorption bands in a magnetic field during the spin-flop phase transition and the change in the intensity with variations in the manganese content x demonstrates that these bands are associated with the excitation of the exchange-coupled pairs of manganese ions located in different environments in a plane square lattice. The phase boundary between the antiferromagnetic and spin-flop phases is constructed using the results of optical measurements. The manganese content corresponding to the magnetic percolation point is evaluated.     

Structural,thermal and magnetic investigations of heavily Mn-doped ZnO nanoparticles

We present a systemic study on the structural, thermal and magnetic properties of Zn_1?xMn_xO nanoparticles synthesized by a combustion method with heavily Mn doping concentrations x=0.05, 0.15 and 0.25. The structural evolutions in relation to the possible variation of the Mn oxidation state and dopant induced tiny Zn₂MnO₄/Mn₂O₃ and ZnMnO₃/MnO₂ impurities, which have not been detected by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM), were investigated by means of X-ray photoelectron spectroscopy (XPS), Raman scattering spectra, thermo-gravimetric analysis (TGA) and magnetic measurements. It is evidenced that the optimal Mn concentration x in ZnO to grow single phase Zn_1?xMn_xO should be below the cation percolation threshold x_p (about 0.125), which is the basis to form real diluted magnetic semiconductors (DMSs).     

Effect of annealing conditions on the magnetism of Ni-doped La0.4Sr0.6TiO3−δ powders

La_0.4Sr_0.6Ti_1−xNi_xO_3−δ powders with x=0 and 2% were synthesized by the sol-gel method and characterized by a variety of techniques. We mainly investigate the effect of annealing conditions on the structural and magnetic properties of La_0.4Sr_0.6Ti_1−xNi_xO_3−δ powders. X-ray diffraction and transmission electron microscopy studies indicate all samples annealed at high temperature (T>873 K) are single phase and no secondary magnetic phase in this study. By the commercial physical properties measurements system (PPMS), the sample with x=0 exhibits diamagnetic behavior, whereas all the samples with x=2% are found to be ferromagnetic even at room temperature. With the increase of annealing temperature, the ferromagnetism of the samples increases initially, and then it decreases after reaching the maximum value. The structural defects such as oxygen vacancies mediated mechanism is reasonable explanation of the magnetic properties in the present results.     

Martensitic transformations and magnetic properties of nonstoichiometric alloys of the Ni-Mn-In system

Martensitic transformations and magnetic properties of Ni_89-x Mn _x In₁₁ (42 ≤ x ≤ 44) alloys have been investigated. Critical temperatures of magnetic and structural phase transitions in the studied alloy system have been determined. It has been shown that the martensitic transformation induced by the magnetic field is observed in all alloys. Temperature dependences of the spontaneous magnetization of austenite and martensite as well as the magnitude of the critical field, in which martensitic transformation occurs, have been determined.