Magnetic ordering in the perovskites Eu1−x CaxMnO3 (0⩻x⩻0.5)

The magnetic properties of Eu_1−x Ca_xMnO₃ have been investigated. As the calcium content increased up to x=0.2, the magnetization and the blocking temperature of the magnetic moments of clusters increased and the magnetic anisotropy decreased. As the calcium content increased further, the magnetization decreased, while the “freezing” temperature of the magnetic moments increased. Anomalies of the magnetic properties were observed in compositions with x=0.4 and 0.5 at T=40 K; these anomalies are attributed to a transition to the antiferromagnetic state in the charge-ordered phase. Fiz. Tverd. Tela (St. Petersburg) 39, 117–120 (January 1997)     

Magnetic and crystalline properties of GdxLa1−x FeSi compounds

The magnetic and crystalline properties of the polycrystalline compounds Gd_xLa_1−x FeSi (x=0, 0.1, 0.3, 0.45, 0.55, 0.65, 0.8, 1) and single crystal GdFeSi are studied. All the compounds have the CeFeSi tetragonal structure. The temperature dependences of the magnetization and initial susceptibility show that ferromagnetic behavior is characteristic of these compounds; the exception is the Pauli paramagnetic LaFeSi. Substitution of gadolinium by the nonmagnetic lanthanum leads to a sharp drop in the Curie temperature T _c, and to a reduction in the saturation magnetic moment μ ₀, effective magnetic moment μ _eff, and paramagnetic Curie temperature Θ_p. Measurements on single crystal GdFeSi imply that the easy magnetization axis of this compound is the c axis and the anisotropy field is H _a∼4 kOe. No magnetic moment was observed on the iron ion in any of these compounds. Fiz. Tverd. Tela (St. Petersburg) 39, 325–329 (February 1997)     

Influence of a nonmagnetic impurity on the properties of the quasi-one-dimensional antiferromagnet CsNiCl3

Various magnetic properties of the diluted quasi-one-dimensional antiferromagnet CsNi_1−x MgxCl₃ are investigated experimentally for several impurity concentrations. The antiferromagnetic resonance spectrum and the phase diagrams are found to depend significantly on the amount of added Mg. The field and temperature dependences of the static magnetization is measured for crystals with two different contents x. A substantial increase in the magnetization is observed at low temperature, where the additional susceptibility is approximately proportional to the concentration. The physical mechanisms underlying the observed strong influence of magnetic defects formed at breaks in the spin chains in a quasi-one-dimensional antiferromagnet on its magnetic properties in the ordered state and for T<T _N are discussed. Zh. éksp. Teor. Fiz. 112, 209–220 (July 1997)     

Magnetoresistance of HgSeS crystals at hydrostatic pressures of up to 1 GPa

Magnetoresistance (MR) of HgSe_1−x S_x crystals has been studied in the temperature range 4.2–300 K and in magnetic fields up to 12 T under hydrostatic pressures P exceeding the threshold P _t for the structural phase transition. Shubnikov-de Haas quantum oscillations in longitudinal and transverse MR were observed to occur in the original samples at T=4.2 K. For P>P _t, HgSeS crystals transferred to metastable states, which presumably incorporate the high-and low-pressure phases, and in which the oscillations vanished. At the same time the monotonic behavior of MR with magnetic field B, as well as the temperature dependences of resistivity ρ retained the shape characteristic of the original phases. The observed weakening of the dependences of ρ on B and T is attributed to an increase of the temperature independent contribution to ρ caused by phase inclusions and structural defects in the metastable states. It is the corresponding decrease of electron mobility that suppresses the oscillations. Fiz. Tverd. Tela (St. Petersburg) 39, 1717–1722 (October 1997)     

Magnetically two-phase state in Eu1−x AxMnO3 (A=Ca, Sr)

It is observed that low-temperature magnetic properties (dependence of the magnetization on the cooling conditions and the presence of a maximum in the initial magnetic susceptibility) of Eu_{1− x} A_xMnO₃ (A=Ca, Sr; x=0,0.3) samples are similar to those of spin glasses. However, there are also substantial differences: The magnetization depends on the cooling conditions right up to the maximum measurement fields H=45 kOe, and the temperature of T _N of the maximum of the initial magnetic susceptibility is independent of the frequency of the ac magnetic field in which the susceptibility is measured. The magnetization isotherms for T<T _N are a superposition of a linear part, characteristic for an antiferromagnet, and a small spontaneous part. For compositions containing Sr a maximum of the resistivity ρ (ρ_max∼10⁸ Ω ·cm) is observed near T _N; in a 120 kOe magnetic field this maximum is lowered by four orders of magnitude and the temperature of the maximum is two times higher. In compositions with x=0.3 the paramagnetic Curie point is much higher than for the composition with x=0: θ=110 K (A=Ca), 175 K (A=Sr), and −100 K (x=0). These characteristic features of the magnetic and electric properties are explained by the existence of a magnetically two-phase state in this system, consisting of ferromagnetic clusters, in which the charge carriers are concentrated, embedded in an insulating antiferromagnetic matrix. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 5, 375–380 (10 March 1999)     

Anomalous magnetic properties of ferrites of the CuFe2−x CrxO4 system with high Cr3+ content

The paper reports on a comprehensive study of magnetic properties of CuFe_2−x Cr_xO₄ ferrites (x=0.0, 0.2, 0.3, 1.0, 1.4, 1.6, and 2.0). The curves of magnetization versus temperature, σ _s(T), of the ferrites with x=1.0, 1.4, and 1.6 have anomalous shapes: the magnetization begins to fall off at lower temperatures than the Curie point TC. The experimental results and analysis of exchange interactions suggest that in ferrites with high Cr³⁺ content, magnetic transitions to either a frustrated magnetic structure or a clustered spin glass can take place. Zh. éksp. Teor. Fiz. 111, 1732–1737 (May 1997)     

Phase transitions and magnetic-transport phenomena in the system La2/3Ba1/3(Mn1−x Cox)O3

A study is performed of the crystalline structure, magnetization, and magnetotransport properties of the system La_2/3Ba_1/3(Mn_1−x Co_x)O₃ with perovskite structure. It is shown that cubic solid solutions exist over the entire range of cobalt concentrations 0⩽x⩽1. Compositions with x⩽0.2 are ferromagnets with maximum resistance near T _C . Compositions with 0.2<x<0.4 manifest properties of inhomogeneous ferromagnets. Measurements of magnetic properties indicate the absence of long-range magnetic order in compositions with 0.5⩽x⩽0.9, which are probably spin glasses. The spontaneous magnetization of cobaltate (2μ _B per formula unit) corresponds to ferromagnetic ordering of the moments of the Co³⁺ and Co⁴⁺ ions found in the intermediate spin state. It is conjectured that the magnetoresistance consists of an extrinsic and an intrinsic contribution. The first arises as a result of intergrain transport of spin-polarized charge carriers, and the second, as a result of magnetic ordering near T _C . The magnetoresistance is essentially independent of the spontaneous magnetization and decreases abruptly as the cobalt concentration is increased with a corresponding transition from long-range to short-range magnetic order. Zh. éksp. Teor. Fiz. 116, 604–610 (August 1999)     

Characteristics of magnetic order in perovskite manganites La1−x CaxMnO3

Solid solutions of the system La_1−x Ca_xMnO₃ are synthesized, and their magnetic and electrical properties are investigated. As x is increased, the crystal lattice changes symmetrically from an orthorhombic (x=0 and x=0.8) to a cubic structure (0<x⩽0.6 and x=1). Nonstoichiometric LaMnO₃ and compositions with a low Ca content (0<x<0.4) are collinear ferromagnets. A metal-insulator transition is observed in them near the Curie temperature. Compositions with 0.6⩽x⩽0.9 exhibit a semiconductor-type conductivity and an unusual temperature behavior of the magnetization and the susceptibility with very distinct Curie and Néel temperatures. The magnetic properties of the solid solutions are explained on the basis of the model of ferro-antiferromagnetic phase separation. Zh. éksp. Teor. Fiz. 116, 1664–1675 (November 1999)     

Quantum oscillations of resistance and magnetization in the degenerate semiconductor n-HgCr2Se4

In the magnetic field range ΔH=8–60 kOe we observed and studied the anomalous oscillations in the magnetic field dependence of the resistance and magnetization of single crystals of n-HgCr₂Se₄. The absence of periodicity in 1/H in the ΔH=8–20 kOe range can be explained by the non-Fermi-liquid behavior of the electron subsystem and agrees with the theory of the de Haas-van Alphen in systems with intermediate valence. In stronger fields, ΔH=20–60 kOe, the amplitude of the fundamental harmonic decreases, with the number and amplitude of the higher-order harmonics increasing. As a result, noise is superimposed on the signal as magnetic field strength grows. The temperature dependence of the magnetization is the sum of the monotonic spin-wave contribution and the oscillating part. Zh. éksp. Teor. Fiz. 113, 1877–1882 (May 1998)     

Shubnikov-de Haas effect in tricrystals of the system of alloys Bi1−x Sbx (x=0.1; 0.13) with n-type conductivity

The Shubnikov-de Haas effect has been investigated in tricrystals of the alloy system Bi_1−x Sb_x (x=0.1; 0.13) with n-type conductivity in stationary (up to 14 T) and pulsed (up to 40 T) magnetic fields. Reconstruction of the internal boundary of the tricrystals was observed, along with a number of new component oscillations of ρ(B) indicating a rotation of the constant-energy surfaces of the L electrons on the intercrystallite boundary at an angle of ∼74° in the binary-trigonal plane. Fiz. Tverd. Tela (St. Petersburg) 41, 991–993 (June 1999)     

Conductivity jumps and low-temperature magnetoresistance in the new semiconducting perovskites La1− x BixMnO3+δ

Abrupt jumps are observed on the temperature dependences of the electrical conductivity of La_{1− x} Bi_xMnO_3+δ (0.1<x<0.7) and La_0.3Bi_0.7Na_0.1MnO_3+δ ceramics. The temperatures at which the jumps take place can be reduced by several degrees by applying an external magnetic field of 15 kOe. It can be seen from the magnetization data that the poorly conducting low-temperature phase is not antiferromagnetic (it is likely ferromagnetic) and hence the real-space charge ordering mechanism is not sufficient for explaining the experimental results. An interpretation in terms of magnetic phase nonuniformity is proposed. All the samples studied exhibit high low-temperature magnetoresistance. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 4, 250–253 (25 August 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Formation of bound magnetic polarons in Cd1−x MnxTe crystals. Contribution of the average exchange field and thermodynamic fluctuations of the magnetization to the binding energy

It is shown that the temperature dependence of the binding energy of exciton-impurity complexes in the semimagnetic semiconductors Cd_1−x Mn_xTe (x≈0.05) is described well in the donor-electron model with an effective characteristic magnetic-polaron energy. The contributions of the average exchange field and of thermodynamic fluctuations of the magnetization to the binding energy and their variation with temperature with and without a weak magnetic field (H⩽3.5 kOe) are determined. How doping with scandium, vanadium, and cobalt effects the appearance of the magnetic-polaron in the experimental crystals was studied. Fiz. Tverd. Tela (St. Petersburg) 39, 527–535 (March 1997)     

Iron garnet films with enhanced thermal stability of their magnetic parameters

The ferromagnetic resonance in Y₃Fe_5−x Ga_xO₁₂ (0⩽x⩽0.63), iron garnet films obtained by liquid phase epitaxy on substrates of gadolinium gallium garnet is studied at temperatures of 213–353 K. It is found that in the case of liquid phase epitaxy the distribution coefficient of the gallium in the films varies from 2.2 to 4.0, depending on the composition and growth conditions. It is found that the resonance magnetic fields can be temperature stabilized through temperature-induced changes in the saturation magnetization and anisotropy field. Zh. Tekh. Fiz. 68, 46–50 (September 1998)     

Magnetic phases of FexV1−x S and their electronic structure

A study has been made of the Fe_xV_1−x S solid solutions with 0<x<0.5. For the compounds with x>0.1, x-ray diffraction analysis discloses a V₅S₈ superstructure. Samples with x>0.1 are magnetically ordered at room temperature. The concentration dependences of resistivity and magnetization exhibit sharp peaks for x=0.1 and x=0.2, respectively. The main features of the structure and electronic properties have been qualitatively explained in terms of the three-band exciton-insulator model, and the maxima in resistivity and magnetization are assigned to the formation of localized magnetic moments with S=1, which become delocalized with increasing x. Fiz. Tverd. Tela (St. Petersburg) 40, 1890–1893 (October 1998)     

Anomalies of the electronic properties of alloys in the presence of variation of the local magnetic moments

The concentration dependences of the electronic properties (residual electrical resistivity, diffusion thermoelectric power, normal Hall effect, and low-temperature specific heat) and the magnetic characteristics (magnetization and paramagnetic susceptibility) of quasibinary (Pd_xPt_1−x )₃Fe, Pt₃Mn_xFe_1−x , (Pd_xAu_1−x )₃Fe, and Sc_xTi_1−x Fe₂ alloys are investigated. A relationship is established between the anomalous behavior of the kinetic properties and the variation of the local magnetic moments. The absence of corresponding anomalies in the concentration dependence of the specific heat indicates that the density of states at the Fermi level does not change significantly and, therefore, that the conventional Mott two-band model cannot be used to describe the anomalies in the properties of the alloys in question. A single interpretation of the sum-total of the experimental results is given on the basis of the theory of local fluctuations of the electron spin density in metal magnets. Fiz. Tverd. Tela (St. Petersburg) 39, 1257–1262 (July 1997)     

Study of the magnetic properties of ferrites in the CuGaxAlxFe2−2x O4 system having a frustrated magnetic structure

This paper reports a first preparation of samples of the CuGa_xAl_xFe_2−2x O₄ system (x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7) and a study of temperature dependences of their spontaneous magnetization σ _s, coercive force H _c, and of the longitudinal, λ_∥, and transverse, λ_⊥, magnetostriction. Our experimental data have established that compositions with x>0.4 undergo two magnetic phase transitions, namely, from the paramagnetic to cluster spin-glass state at the Curie temperature T _C, and another transformation, at T _tr<T _C, from the cluster spin glass to a frustrated magnetic structure. It was found that the coercive force of ferrites with a frustrated magnetic structure is an order of magnitude lower than that of ferrimagnetically ordered ferrites. The behavior of the magnetostriction of frustrated ferrites was studied. Such ferrites were found to exhibit a considerable positive magnetostriction of the paraprocess λ _para. It was shown that the magnetostriction of spin-glass ferrites is lower by nearly an order of magnitude. Fiz. Tverd. Tela (St. Petersburg) 40, 1505–1509 (August 1998)     

Ferrimagnetic phase transition in the system Li0.5Fe2.52−x GaxO4 in the vicinity of the multicritical point of the x-T phase diagram

In connection with the problem of identifying magnetic states in the vicinity of x ₀ (the multicritical point of the x-T diagrams of spin-glass systems) a study has been made of properties that can be exploited to determine the presence of a thermodynamic phase transition at the Curie point T _C and the distinctive features of the transition, specifically, the temperature dependence of the magnetic part of the specific heat C _m(T), the temperature dependence of the low-field magnetization σ _H(T), and (with a view toward examining critical behavior in a magnetic field) the magnetization isotherms σ _H(T). The investigated object is the system of dilute ferrimagnetic spinels Li_0.5Fe_2.5−x Ga_xO₄, in which every type of magnetic state has spatially inhomogeneous cluster structures. The results obtained for a sample with x=1.45 indicate that the classical criteria of a ferrimagnetic second-order phase transition at T _C=(97±2) K occur for x∼x ₀. The results of similar investigations for a sample with x=1.6, which exists in the cluster spin-glass state for T<T _f=22 K and in an uncorrelated cluster state of the superparamagnetic type for T>T _f, are also given for comparison with the preceding case. Zh. éksp. Teor. Fiz. 114, 2065–2077 (December 1998)     

Magnetic and structural transitions in La1−x Sr x MnO3: T-x phase diagram

The electrical conductivity, magnetic susceptibility, magnetization, and submillimeter (v=5∓20 cm⁻¹) permittivity and dynamic conductivity of La_1−x Sr _x MnO₃ (0≤x≤ 0.45) single crystals are investigated. The anomalies in the temperature dependences of these quantities are identified with diverse magnetic and structural phase transformations, including antiferromagnetic and ferromagnetic ordering, structural transitions between strongly distorted (Jahn-Teller) and weakly distorted (pseudocubic) orthorhombic phases, structural transitions to a rhombohedral phase and unusual transitions to a polaron-ordering state. As a result, the complete T-x phase diagram of the system La_1−2x Sr _x MnO₃ is constructed in a wide interval of temperatures T=4.2∓1050 K and concentrations x=0−0.45. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 4, 331–336 (25 August 1998)     

Effect on the magnetic properties of small substitutions of iron and copper for nickel in YNi3 and Y2Ni7 intermetallics

Measurements of magnetization in static fields up to 60 kOe and magnetic susceptibility in weak ac fields of the Y(Ni_1−x M_x)₃ and Y₂(Ni_1−x M_x)₇ intermetallics (M=Fe and Cu, x _max=0.2), as well of the specific heat of these systems for some compositions are reported. It was found that these intermetallics are ferromagnetic at low temperature, and that their spontaneous magnetizations M _s at 4.2 K and Curie temperatures T _C decrease monotonically with increasing copper concentration. The magnetic susceptibility of the Y(Ni_1−x Fe_x)₃ system with 0.06≲x≲0.2 was observed to decrease rapidly with the temperature lowered below a characteristic temperature T _s. One of possible reasons for such an anomaly is shown to be the onset in this concentration region of the reentrant spin-glass state at low temperatures. The results obtained demonstrate that the tight-binding d-band model fails to explain the evolution of the magnetic properties of YNi₃ and Y₂Ni₇ in the case of small substitutions of iron and copper for nickel. Fiz. Tverd. Tela (St. Petersburg) 39, 1828–1830 (October 1997)