Interrelation of magnetic and Jahn-Teller orderings in weakly doped lanthanium-strontium

The mutual influence of ferromagnetic and Jahn-Teller ordering in weakly doped manganites with the content La_{1 ? x} Sr _x MnO₃ (0.17 < x < 0.175) is considered. A phenomenological explanation of colossal magnetic resistance based on suppression of the local Jahn-Teller structure of distorted MnO₆ octahedrons by means of an applied magnetic field is suggested.     

Influence of the Jahn—Teller distortion on magnetic ordering in TbMn1-xFexO3

The Jahn-Teller distortion plays an important role in determining the exchange interaction in rare-earth manganites.In this work we study the influence of the Jahn-Teller distortion on the magnetic structures of TbMn1-xFexO3（x = 0,0.02,0.05,0.10,and 0.20） single crystals in the basal MnO2 plane.The decrease in the quadruple splitting with the increasing Fe doping indicates the reduction of the Jahn-Teller distortion,which makes the nearest neighboring（NN） FM interaction dominant over the next nearest neighbor（NNN） AFM interaction.This alteration is favorable for the development of A-type AFM ordering instead of the spiral magnetic ordering,which collapses when x ≥ 0.05.The analysis of dielectric data indicates that the ferroelectricity is arising from the peculiar spiral magnetic ordering.     

Influence of magnetic ordering on the structural properties of dilute copper ferrites

The x-ray structural properties of samples in the CuGa_xAl_xFe_2?2x O₄ (x = 0?0.7) and CuGa_xAl_2x Fe_2?3x O₄ (x = 0?0.5) systems are studied. It is found that magnetic ordering in dilute copper ferrites affects their structural properties. It is concluded that the frustration of magnetic coupling leads to suppression of the cooperative Jahn-Teller effect in dilute copper ferrites with a frustrated magnetic structure.     

Lattice,electrical, and magnetic effects in lanthanum manganites La1−x Sr x MnO3 (x = 0.125, 0.150, 0.175)

The temperature dependences of the velocities of transverse and longitudinal hypersound in lanthanum strontium manganites of the composition La_1−x Sr _x MnO₃ (x = 0.125, 0.150, 0.175) have been measured at frequencies of 0.5–0.7 GHz. The structural phase transitions have been revealed, and their positions have been confirmed by data on the electrical resistance and magnetic measurements performed using the same samples. The results obtained have been analyzed in the framework of the model of competing Jahn-Teller distortions and magnetic ordering. The anomalies observed in the behavior of the velocities of longitudinal hypersound have been attributed to the local Jahn-Teller distortions, and their suppression due to the magnetic ordering has been considered a possible factor responsible for the colossal magnetoresistance.     

Scattering of charge carriers on a high spin-low spin mixture in MnAs1−xPx crystals

The specific resistivity anomaly between the magnetic ordering temperature T_N and the displacive phase transition temperature T_t of MnAs_1?xP_x crystals with 0.03 < x < 0.15 seems to be connected with the existence of a high spin-low spin mixture which has been inferred recently from specific heat data.     

Local structure and magnetic properties of Mn substituted manganites studied by EXAFS and Dc magnetic measurements

We report extended X-ray absorption fine structure (EXAFS) measurements at the Mn K edge and magnetic measurements performed on (La_1−xCa_x)(Mn_1−yM_y)O₃ samples (M=Cr or Ni; x=0.37 and 0.75 and y=0.03 or 0.08). The Mn substitution produces important effects on both the sides of the LaMnO₃-CaMnO₃ phase diagram. For x<0.5 the ferromagnetic-metallic phase maintains its main character even after Mn substitution, but both the doping species (Ni or Cr) lower T_C and broaden the magnetic transition, and the EXAFS study evidences two Mn-O distances, suggesting the presence of zones of distorted insulating phase. For x>0.5, after the doping with Cr, the charge ordered phase persists but on a shorter scale, whereas the Jahn-Teller distortion is weakened as indicated by EXAFS measurements, and the formation of ferromagnetic clusters is evidenced by magnetic measurements.EXAFS and magnetic measurements are in mutual agreement, thus confirming the correlation between the local disorder determined by charge localization and magnetic degrees of freedom.     

Ferromagnetism of (Pb,Sn, Mn) Te under high pressure

Results of measurements of the magnetic susceptibility in the p-type diluted magnetic semiconductor [(PbTe)_1-x(SnTe)_x]_1-y(MnTe)_y with x = 0.72, y = 0.03 are presented. The temperature of the ferromagnetic ordering T_c increases (for hole concentration 3×10²⁰ cm^-3) < p < 7 × 10²⁰ cm^-3 or decreases (p > 7 × 10²⁰ cm^-3) as hydrostatic pressure is applied. The pressure causes an increase in the concentration of carriers responsible for the interactions between Mn ions. The observed shift of T_c with pressure is interpreted on the basis of the RKKY interaction mechanism and points out the importance of the band structure behavior in consideration of magnetic properties of the semiconductor.     

Complex competition between ferromagnetism and antiferromagnetism in the CMR manganites Pr1−x Ca x MnO3

The systematic investigation of the magnetic susceptibility of the CMR manganites Pr_1?x Ca _x MnO₃ versus temperature has been performed for 0.25 ≤ x ≤ 0.50. Due to the similar size of calcium and praseodymium, these results show the important role of the mixed valence of manganese upon the complex magnetic behaviour of these compounds. They demonstrate that the appearance of antiferromagnetism coincides with charge ordering, T _N = T _CO varying with x, from 250K for x = 0.50 to 225K for x = 0.35. A strong competition between ferromagnetism and antiferromagnetism is observed for 0.35 ≤ x < temperature (T > 170 K) and going through canted antiferromagnetic or weak ferromagnetic states for intermediate temperatures (70 K < T < 170 K). For 0.25 ≤ x ≤ 0.30, a strong ferromagnetic state is observed for 95 K ≤ T ≤ 150 K, with a transition to a spin glass like state below 95–110 K.     

Synthesis, structure, and magnetic properties of anion-substituted manganese chalcogenides

The anion-substituted solid solutions of the MnSe_{1 t x} Te _x system have been synthesized. The crystal structure and magnetic properties of the synthesized solid solutions have been investigated. It has been shown that, in the concentration range 0 ?? x ?? 0.4, the solid solutions have a face-centered cubic structure. It has been revealed that an increase in the concentration of the substituting element in the MnSe_{1 ? x} Te _x system leads to an increase in the coefficient of thermal expansion of the sample. The investigation of the magnetic properties has been carried out at temperatures in the range 80 K < T < 1000 K in a magnetic field up to 8.6 kOe. It has been experimentally found that the type of antiferromagnetic order (the second type of ordering) remains unchanged over the entire concentration range up to x = 0.4 and that the paramagnetic Curie temperature and the Néel temperature decrease within the limits of 20%. Theoretical calculations have been performed using the Monte Carlo method, and the model of nanoclusters with an uncompensated antiferromagnetic moment has been proposed.     

Magnetic phase diagram of a random mixture of Jahn-Teller ions

A magnetic model of a random mixture containing Jahn-Teller ions, in which several kinds of exchange interactions between the Jahn-Teller ions and between the Jahn-Teller and non-Jahn-Teller ions are assumed, is proposed and studied by using the distribution function method. It is shown that a spin glass phase occurs when these exchange interactions have different signs. Our model would be applicable to a mixture Rb₂Cr_xMn_1-xCl₄ in which the spin glass phase has been observed.     

Magnetic ordering in EuxSr1−xSySe1−y

Specific heat and ac-susceptibility measurements are reported for Eu_xSr_1?xS_ySe_1?y samples. The critical concentrat ion for long-range magnetic ordering x_c increases with decreasing y, for y = 0.1, x_c = 1.0 holds. For samples with x < x_c a broad peak in the specific heat and a peak in the susceptibility at lower temperatures indicates a spin glass type of magnetic ordering.     

Magnetic properties of MnxMg1-xS solid solutions

The static magnetic properties of the polycrystalline solid solutions Mn_xMg_1-xS(0<x?1) are reported. The experimental data reveal the appearance of a critical concentration x_c=0.13±0.01 for long-range magnetic ordering, which is in good agreement with the result of a series expansion of the mean cluster size for a face-centered cubic lattice with exchange interactions up to 12 nearest neighbours and 6 next-nearest neighbours. At concentrations far below x_c the existence of long-range exchange interactions is demonstrated.     

Carbon concentration dependence of the superconducting transition temperature and structure of MgCxNi3

The crystal structure of the superconductor MgC_xNi₃ is reported as a function of carbon concentration determined by powder neutron diffraction. The single-phase perovskite structure was found in only a narrow range of carbon content, 0.88<x<1.0. The superconducting transition temperature was found to decrease systematically with decreasing carbon concentration. The introduction of carbon vacancies has a significant effect on the positions of the Ni atoms. No evidence for long-range magnetic ordering was seen by neutron diffraction for carbon stoichiometries within the perovskite phase stability range.     

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.     

Magnetic phase diagram of the Bi<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript>x</Subscript>MnO<Subscript>3</Subscript> manganites

The magnetic and elastic properties of the Bi_1-xCa_xMnO₃ manganites are studied. The phase transformations revealed are ferromagnet-spin glass (x≥0.15) and spin glass-charge-ordered antiferromagnet (x≥0.25). The ferromagnetic state is characterized by ordering of the Mn³⁺d _x ²-y orbitals. It is suggested that thespin glass state originates from local static Jahn-Teller distortions. The antiferromagnetic charge-ordered and the spin-glass disordered phases coexist in samples with 0.25<x<0.32, which may be due to the charge order-disorder phase transformation being martensitic in character. The magnetic phase diagram is constructed.     

Investigation of the solid solution series 2(MnX)-CuInX2 (X=S, Se)

(2MnX)_x(CuInX₂)_1−x with X=S and Se were prepared by solid state reaction from the end members α-MnS, β-MnS and CuInS₂ in the range 0<x≤0.2 (≤0.6 for β-MnS) as well as MnSe and CuInSe₂ in the range 0<x≤0.1. Mixed crystals with 0≤x≤0.1 crystallize in the tetragonal chalcopyrite type structure, (2α-MnS)_x(CuInS₂)_1−x samples with 0.1<x≤0.2 and (2β-MnS)_x(CuInS₂)_1−x samples up to x=0.6 consist of two phases, occuring as tetragonal domains (x∼0.1 for X=S) within a cubic matrix with zinc-blende type structure (x∼0.4 for X=S), indicating a miscibility gap. For tetragonal single phase samples the band gap energy, the lattice constants and the anion parameter have been determined. The first and the latter ones show a different composition dependent behaviour caused by the modification of the MnS (α-MnS with NaCl type structure, β-MnS with zinc-blende type structure) used during the synthesis. Additionally a CuMn_xIn_1−xS₂ powder sample, in which Mn substitutes the M^III site, was investigated. The SQUID measurements revealed a well-distinct magnetic transition between 15 and 16 K as well as ferromagnetic-like hysteresis loops pronounced for temperatures below the transition temperature. Below this temperature a clear splitting between the zero field cooling (ZFC) and the field cooling (FC) curves indicate to the existence of a long-range magnetic ordering phenomenon. This behaviour was not found in the other samples were Mn substitutes both sites M^I as well as M^III.     

Phase transitions in crystals with competitive Jahn-Teller subsystems

A statistical thermodynamic model of phase transformations caused by the cooperative Jahn-Teller effect in spinels containing two types of Jahn-Teller cations has been proposed. It has been shown that the specific features of the phase diagrams of compounds similar to Cu_{1 ? x} Ni _x Cr₂O₄, such as the presence of one orthorhombic and two anti-isostructural tetragonal phases, are associated with the competition between orderings of the subsystems of tetrahedra distorted due to the Jahn-Teller effect. The degree and character of the ordering of each subsystem of the solid solution have been determined. The main factors responsible for the thermodynamic properties and phase diagrams, in particular, the conditions for splitting of an isolated critical point into two triple points, have been investigated.     

Reply to the comment on: ‘Magnetoresistance in La1−xCaxMnO3(0≤x<0.4)’

The author of the comment objects to the characterization and the interpretation of magnetoresistance (MR) effects observed by us in La_1−xCa_xMnO₃ (0≤x<0.4) samples. In this reply, arguments are used to show that the samples' characterization and explanation of the MR by considering the role of the short-charge ordering (CO) regions and magnetic domains are reasonable and acceptable.     

Spin freezing properties in LaNi5-xFex

We report magnetic and transport measurements in the metallic pseudobinary compound LaNi_5-xFe_x, which show a Fe spin freezing. The magnetic phase diagram in the x-T plane is derived in the range 0 ⩽ x 1.2, 20 K ⩽ T ⩽ 300 K. Above a critical Fe concentration x_c ≈ 0.3, the system undergoes a superparamagnetic to long range ferromagnetic ordering at a finite temperature T. At lower concentrations, no long range spin ordering is observed, which suggests that x is the percolation threshold. Instead, a transition to a phase characterized by strong irreversibilities is observed at temperature T_f(x). Very strong ferromagnetic coupling interactions between Fe atoms are observed at low concentrations (x < x_c), which contrast with small values of T observed for x ∼ x_c. The results, together with the nature of the spin freezing are discussed in relation to the 2d character of the magnetic lattice.     

Magnetic study of phase separation and charge ordering in La1−xSrxMnO3 near x=0.5

We have investigated the magnetic phase diagram of polycrystalline and single-crystal La_1−xSr_xMnO₃ near 0.46≤x≤0.50. It turns out that for x<0.48, the polycrystalline material is ferromagnetic (FM), but for x≥0.48, incipient charge ordering takes place along with antiferromagnetism. At x=0.48, the ferromagnetic-antiferromagnetic phase transition in ceramics occurs at less than 85 kOe but requires significantly larger field for increasing x. These observations are in contrast to what is found in the single crystals, which are all FM.