Anomalous ferromagnetic resonance in manganese- and aluminum-doped germanium layers deposited from the laser plasma

Ferromagnetic resonance (FMR) with an anomalous angular dependence has been observed in the Ge:(Mn, Al)/GaAs nanolayers deposited from laser plasma at a reduced temperature of 150°C. The resonance is associated with the needle-like inclusions of a high-temperature ferromagnetic phase with the Curie temperature T _C > 293 K. Such a magnetic anisotropy is confirmed by the atomicforce and magneticforce microscopy of a side chip. A low-temperature ferromagnetic phase with normal FMR and T _C < 212 K is formed between the needle-like inclusions. This phase manifests itself in the anomalous Hall effect at 77 K and probably is a solid solution of manganese in germanium.

     

An inhomogeneous paramagnetic state of the LaMnO<Subscript>3 + δ</Subscript> nanoceramics prepared by shock-wave loading

The structural (at T = 300 K) and magnetic properties of LaMnO_{3 + δ} nanoceramic materials prepared by shock-wave loading have been investigated in the paramagnetic region. The samples contain a mixture of the orthorhombic and rhombohedral phases in different ratios. The Curie-Weiss law is satisfied in the temperature range T > 440 K > 2T _C, and magnetic polarons are generated in the vicinity of defects at temperatures in the range 300 K < T < 440 K. An increase in the concentration of Mn⁴⁺ ions leads to a decrease in the Curie temperature T _C due to the decrease in the total number of Mn ions, the size effects of small particles, and the long-range elastic stresses.     

Effect of electron irradiation on the paramagnetic state of La0.67Ca0.33MnO3

The effect of point defects on the magnetic properties of La_0.67Ca_0.33MnO₃ polycrystals and single crystals has been studied. The magnetic susceptibility χ _dc of the initial samples and samples irradiated by electrons to the maximum dose F = 9 × 10¹⁸ cm^?2 has been measured in the temperature region 80 K < T < 650 K. Local variations of Mn-O-Mn bond angles and lengths result in a nonmonotonic dose dependence of the Curie temperature T _C. At high doses of electron irradiation, F ≥ 5 × 10¹⁸ cm^?2, the temperature of the transition from the ferromagnetic to polaron state in a single crystal is found to increase. In the paramagnetic region close to T _C, ferromagnetically ordered polarons are observed to exist, while at T > 1.2T _C, localization of e _g electrons initiates formation of paramagnetic polarons with a higher magnetic moment. Electron irradiation stimulates persistence of magnetic polarons up to higher temperatures T > 2T _C.     

Paramagnetic-ferromagnetic and insulator-metal phase transitions in La<Subscript>0.88</Subscript>MnO<Subscript>2.95</Subscript>

We present the results a study of structure by neutron diffraction and data on the magnetic properties (linear and nonlinear (second and third order) susceptibilities) of polycrystalline La_0.88MnO_2.95. This compound exhibits an insulator-metal (IM) phase transition at T _IM ≈ 253 K (above the Curie temperature, T _C ≈ 244 K) and reveals colossal magnetoresistance. The crystal structure is found to be rhombohedral, and the space group is R3c. Analysis of magnetic properties shows that at T* ≈ 258 K > T _C , isolated paramagnetic clusters occur in the paramagnetic matrix; their concentration increases upon cooling. We observed no noticeable differences between the temperature evolution of the clustered state of this manganite with its insulator-metal transition and in the insulator La_0.88MnO_2.91. Possible scenarios of the paramagnet-ferromagnet and I-M transitions in a self-organized clustered structure are discussed.     

Thermal physical properties of the La0.825Sr0.175MnO3 single crystals

The heat capacity (C _P), the thermal diffusion (η), the thermal conductivity (κ), and the electrical resistance of the La_0.825Sr_0.175MnO₃ single crystal have been measured in the temperature range 80–350 K in magnetic fields to 40 kOe. Dependences C _P(T), κ(T), and η(T) have anomalies near T _C, which are suppressed in magnetic field. The minima in dependences κ(T) and η(T) near T _C are explained by the phonon scattering on fluctuations of the magnetic order parameter. Dependences κ(T) and η(T) have anomalies near T _S = 200 K related to the structural transition from the rhombohedral (R) to the orthorhombic (O*) phase.

     

Anomalous ferromagnetic resonance in manganese- and aluminum-doped germanium layers deposited from the laser plasma

Ferromagnetic resonance (FMR) with an anomalous angular dependence has been observed in the Ge:(Mn, Al)/GaAs nanolayers deposited from laser plasma at a reduced temperature of 150°C. The resonance is associated with the needle-like inclusions of a high-temperature ferromagnetic phase with the Curie temperature T _C > 293 K. Such a magnetic anisotropy is confirmed by the atomicforce and magneticforce microscopy of a side chip. A low-temperature ferromagnetic phase with normal FMR and T _C < 212 K is formed between the needle-like inclusions. This phase manifests itself in the anomalous Hall effect at 77 K and probably is a solid solution of manganese in germanium.     

Magnetic susceptibility and inelastic electrical resistivity of GdZnxCu1−x alloys

The deviation from the Nordheim-Kurnakov rule and the anomalous behavior of spin-disordered electrical resistivity in quasi-binary GdZn (T _C =268 K)-GdCu (T _N =142 K) solid solutions is explained in effective medium approximation within percolation theory for the case of three phases, viz., ferro-, antiferro-, and paramagnetic. The strong increase of ρ at zinc concentrations x∼0.45 is attributed to the closeness of the system to the percolation threshold. The phase volumes calculated for the random-distribution case fit well to the concentration dependence of magnetic susceptibility. Fiz. Tverd. Tela (St. Petersburg) 40, 974–979 (June 1998)     

Intrinsic inhomogeneities of low-doped lanthanum manganites in the paramagnetic temperature range

The nature of the electrical resistivity for low-doped lanthanum manganites is elucidated. The electrical resistivity is described by the Efros-Shklovskii law (lnρ √ (T ₀/T)^−1/2, where T ₀ √ 1/R _ls) in the temperature range from T* ≈ 300 K ≈ T _C (T _C is the Curie temperature for conducting manganites) to their T _C and is explained by the tunneling of carriers between localized states. The magnetoresistance is explained by a change in the size of localized states R _ls in a magnetic field. The patterns of change in R _ls with temperature and magnetic field strength determined from magnetotransport properties are satisfactorily described in the model of phase separation into small-radius metallic droplets in a paramagnetic matrix. The sizes R _ls and their temperature dependence have been estimated through magnetic measurements. The results confirm the existence of a Griffith phase. The intrinsic inhomogeneities produced by thermodynamic phase separation determine the electrical resistivity and magnetoresistance of lanthanum manganites.     

The nonlinear magnetic properties of the pseudocubic Nd0.77Ba0.23MnO3 single crystal in the critical paramagnetic region and phase separation

The second magnetization harmonic was studied for a moderately doped Nd_0.77Ba_0.23MnO₃ neodymium manganite single crystal in parallel constant and harmonic magnetic fields in the critical paramagnetic region. According to the neutron and X-ray diffraction data, the crystal was crystallographically single-phase and had a pseudocubic structure both at room temperature and below the Curie point T _C=124.1 K. Although the specific resistance of this compound had a singularity near T _C and exhibited giant magnetoresistance, it remained an insulator in the ferromagnetic state. Nonlinear response measurements in the T _C<T<T _*≈146.7 K paramagnetic region were indicative of the existence of two magnetic phases. Above T _*, the crystal was magnetically single-phase, and its critical behavior was well described by dynamical similarity theory for isotropic 3D ferromagnets. The unexpected appearance of a new magnetic phase in the structurally homogeneous crystal was discussed based on phase separation ideas; such a phase separation could occur in moderately doped cubic manganites experiencing orbital ordering.     

Magnetostriction and the two-spin correlation function in MnF2

The longitudinal magnetostriction along the [001] axis of MnF₂ was measured at temperatures 64 < T < 300 K in magnetic fields, H, up to 130 kOe. This magnetostriction is proportional to H² at low H, exhibits a λ anomaly near the Néel temperature T_N, and shows the field-induced transition from the antiferromagnetic phase to the paramagnetic phase for T just below T_N. The results are well described by a model which relates the magnetostriction to the two-spin correlation function.     

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 correlations in EuMnO and BiMnO crystals in the paramagnetic temperature range

The magnetic and dielectric properties of EuMn₂O₅ and BiMn₂O₅ crystals are investigated over a wide range of temperatures 4.2–250 K, including the range T≫T _N ≃40 K. Significant departures from the Curie-Weiss law are observed in both crystals for the magnetic susceptibility in the paramagnetic range; they are attributed to the presence of correlated domains of magnetic order over a wide range of temperatures. Anomalies in the dielectric properties of the crystals are observed in the same temperature range T>T _N and, as in the case T<T _N , are correlated with the magnetic properties. Zh. éksp. Teor. Fiz. 112, 284–295 (July 1997)     

Magnetocaloric effect in the Sm<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3</Subscript> manganite

The magnetocaloric effect ΔT has been studied by a direct method in two samples of the manganite Sm_0.55Sr_0.45MnO₃, namely, a single crystal (sample A) and a ceramic sample (sample C). The temperature dependences of the ΔT effect of both samples exhibit a maximum at T _max = 143.3 K for the sample A and T _max = 143 K for the sample C. In these maxima, the values of the ΔT effect are 0.8 and 0.4 K in the magnetic field H = 14.2 kOe for the samples A and C, respectively. In addition, the ΔT(T) curve of the sample A has a minimum at T _min = 120 K, in which ΔT = −0.1 K. The maximum value of the ΔT effect increases with an increase in the magnetic field H in the range of magnetic fields up to 14.2 kOe, and the rate of this increase at H > 8 kOe is higher than that at H < 8 kOe. These features of the ΔT effect are explained by the presence of ferromagnetic and antiferromagnetic A- and CE-type clusters in the samples.     

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)     

Low-temperature properties of CePd2In

We report measurements of the specific heatC _p(T), electrical resistivity ϱ(T) and magnetic susceptibility ξ(T) of hexagonal CePd₂In, at low temperatures. Anomalies inC _p(T), χ(T) and ϱ(T) atT=1.23 K, indicate a phase transition, most likely to an antiferromagnetically-ordered phase. The electronic entropy reachesR ln2 per mole Ce at 9.2K, suggesting that the phase transition involves a doublet state. The ordered phase coexists with moderately correlated itinerant electrons.     

Magnetic field and temperature-induced first-order transition in Gd5(Si1.5Ge2.5): a study of the electrical resistance behavior

Magnetic field (0–4 T) and temperature dependencies (4.2–320 K) of the electrical resistance of Gd₅(Si_1.5Ge_2.5), which undergoes a reversible first-order ferromagnetic↔paramagnetic phase transition, have been measured. The electrical resistance of Gd₅(Si_1.5Ge_2.5) indicates that the magnetic phase transition can be induced by both temperature and magnetic field. The temperature dependence of the electrical resistance, R(T), for heating at low temperatures in the zero magnetic field has the usual metallic character, but at a critical temperature of T_cr=216 K the resistance shows a 20% negative discontinuity due to the transition from the low-temperature high-resistance state to the high-temperature low-resistance state. The R(T) dependence for cooling shows a similar but positive 25% discontinuity at 198 K. The isothermal magnetic field dependence of the electrical resistance from 212T224 K indicates the presence of temperature-dependent critical magnetic fields which can reversibly transform the paramagnetic phase into the ferromagnetic phase and vice versa. The critical magnetic fields diagram determined from the isothermal magnetic field dependencies of the electrical resistance of Gd₅(Si_1.5Ge_2.5) shows that the FM↔PM transition in zero magnetic field on cooling and heating occurs at 206 and 213 K, respectively. The full isothermal magnetic filed hysteresis for the FM↔PM transition is 2 T, and the isofield temperature gap between critical magnetic fields is 7 K.     

Characteristic features of the magnetic ordering of Dy3+ ions in a monoclinic RbDy(WO4)2 single crystal

The investigation of the specific heat of a RbDy(WO₄)₂ single crystal at temperatures 0.2–2.5 K and in magnetic fields up to 2 T are reported. The temperature dependence of the specific heat near T _N=0.818 K is compared with the predictions for different models. The 2D Ising model describes satisfactorily C(T) below T _N, while for T>T _N none of the theoretical models agree with the behavior of C(T) of RbDy(WO₄)₂. The H-T phase diagram for H∥c is complicated and possesses a triple point, where regions of existence of three magnetic phases converge. The magnetic ordering is analyzed from the standpoint of the Jahn-Teller nature of the structural phase transitions occurring in RbDy(WO₄)₂ at higher temperatures. It is shown that the form of the phase diagram depends on the direction of the vector H, for the general case of an arbitrary direction of H, two phase transitions can occur with increasing field. Fiz. Tverd. Tela (St. Petersburg) 41, 491–496 (March 1999)     

Nonlinear properties and paramagnet-to-ferromagnet transition in Nd<Subscript>0.7</Subscript>Ba<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> single crystal with metallic ground state

We present the results from studying the magnetic properties (linear and nonlinear susceptibilities and the depolarization of polarized neutrons) of Nd_{1 − x} Ba _x MnO₃ manganite, x = 0.3, with Curie temperature T _C ≈ 140 K and dielectric-to-metal transition temperature T _DM ≈ 129 K. Its critical behavior corresponds to that of an isotropic 3-D ferromagnet at temperatures above T*≈ 144 K, but a strong nonlinear response in weak magnetic fields and depolarization are observed at temperatures below T*. It is shown that this nontraditional behavior is related to the generation of ferromagnetic clusters in the paramagnetic matrix that form a conducting percolative network at temperatures near T _DM.     

Study of crystal and magnetic structures of manganite Pr<Subscript>0.7</Subscript>Ba<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> at high pressure

The crystal and magnetic structures of manganite Pr_0.7Ba_0.3MnO₃ have been studied at high pressures of up to 5.1 GPa and temperatures from 10 to 300 K by means of the neutron diffraction. At normal pressure and a temperature T _C = 200 K, a ferromagnetic state forms in Pr_0.7Ba_0.3MnO₃. At high pressures P ≥ 1.9 GPa and T < T _N ≈ 153 K, a new antiferromagnetic state of A-type have been observed. Under high pressure, the Curie temperature T _C increases with the characteristic quantity dT _C/dP ≈ 2.4 K/GPa. A possible reason for the appearance of an A-type antiferromagnetic phase in Pr_0.7Ba_0.3MnO₃ at high pressures may be anisotropic uniaxial compression of oxygen octahedra along the b axis of the orthorhombic structure.     

Transition De Phase Metamagnetique Du Bromure Ferreux

In a magnetic field parallel to the magnetization axis of an antiferromagnetic Fe Br₂ single crystal, a caracteristic metamagnetic behaviour is observed. The transition from an antiferromagnetic phase to a paramagnetic phase is studied by help of magnetization measurements in a steady field (H < 60 kOe). The measurement precision has allowed a detailed study of the magnetization isotherms, caracteristic of a first order magnetization phase transition (T < T_c = 4, 7 K) and of a second order phase transition (T_c < T < T_N = 14, 2 K).We have observed an original phase diagram. In a certain temperature and field range, the ordered phase is stable on the high temperature side of the transition point. Some theoretical studies in an Ising model, or in the hypothesis of a strong magnetoelastic coupling forecast the existence of such a magnetic phase diagram.At present, we proceed to a theoretical study, in a molecular field approximation, of the magnetic phase diagram of compounds similar to Fe Br₂ where we take into account the relative values of parameters J₁, J₂ and D associated with ferromagnetic and antiferromagnetic interactions and crystalline anisotropy.