Colossal room-temperature magnetoresistance in thin La<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ag<Subscript>y</Subscript>MnO<Subscript>3</Subscript> epitaxial films

The first thin La_1?xAg_yMnO₃ epitaxial films (_y ≤ x) were grown on SrTiO₃ (110) substrates with silver present in the ionized state (Ag⁺) only. The Curie temperatures T_C of the compositions with x = y = 0.05, x = y = 0.1, and x = 0.3 and y = 0.27 crystallizing in the hexagonal structure \(R\bar 3c\) above or close to room temperature. The temperature dependences of electrical resistivity ρ and of magnetoresistance ¦Δρ/ρ/¦ = ¦(ρ_H ? ρ _H = 0)/ρ_H=0¦ pass through maxima near T_C, with the magnetoresistance being negative and reaching colossal values of ～7–20% in a magnetic field H = 8.2 kOe not only at T_C but also at room temperature. The magnetic moment per formula unit as derived from the saturation magnetization at T = 5 K is substantially smaller than expected for complete ferromagnetic ordering. The magnetization in fields of up to 6 kOe depends on the actual sample cooling conditions, and the hysteresis loop of a field-cooled sample is displaced along the H axis by ΔH. The above properties can be accounted for by the fact that the films are in a two-phase magnetic (ferromagnetic-antiferromagnetic) state induced by strong s-d exchange. The maximum value of Δ H was used to calculate the energy of exchange coupling between the ferromagnetic and antiferromagnetic parts of a sample.     

Metal-insulator transition in Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>S crystals

Results of an experimental study of MnS, FeS, and Fe _x Mn_1?x S single crystals are presented. The phase composition, the lattice parameters, and the state of paramagnetic ions in Fe _x Mn_1?x S have been determined by x-ray diffraction analysis and Mössbauer spectroscopy. A sequence of transitions have been found in iron manganese sulfide with x = 0.29 at temperatures T ₁ ≈ 25–50 K, T ₂ ≈ 125 K, and T ₃ ≈ 190 K with a change in kinetic properties and the formation of a metallic state at low temperatures T ≈ 2 K. The possibility of a Mott-Hubbard transition in Fe _x Mn_1?x S sulfides with variation of the composition and the temperature is discussed.     

Properties of InGaAs/GaAs quantum wells with a δ〈Mn〉-doped layer in GaAs

A method of formation of two-dimensional structures containing a δ〈Mn〉-doped layer in GaAs and an In_xGa_1?x As quantum well (QW) separated by a GaAs spacer of thickness d = 4–6 nm is developed using laser evaporation of a metallic target during MOS hydride epitaxy. It is shown that, up to room temperature, these structures have ferromagnetic properties most likely caused by MnAs clusters. At low temperatures (T _m ～ 30 K), the anomalous Hall effect is revealed to occur. This effect is related to hole scattering by Mn ions in GaAs and to the magnetic exchange between these ions and QW holes, which determines the spin polarization of the holes. The behavior of the negative magnetoresistance of these structures at low temperatures indicates the key role of quantum interference effects.     

Thermoelectric properties and ferromagnetism of diluted magnetic semiconductors Sb<Subscript>2 − <Emphasis Type="Italic">x</Emphasis></Subscript>Cr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript>3</Subscript>

Thermoelectric properties of single crystals of a new dilute magnetic semiconductor p-Sb_{2 ? x} Cr _x Te₃ are studied in the temperature interval 7–300 K. The temperature dependences of the thermal conductivity are measured. The Seebeck coefficient S is found to increase upon doping with Cr. At low temperatures, a ferromagnetic phase with Curie temperature T _C ≈ 5.8 K exists for a Cr concentration x = 0.0215, its easy magnetization axis being parallel to the crystallographic axis C ₃. At T = 4.2 K, a negative magnetoresistance and anomalous Hall effect are observed; in strong magnetic fields, the Shubnikov-de Haas effect is manifested.     

Transport properties and ferromagnetism of Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>S sulfides

We have studied the resistivity and thermoelectromotive force (thermo emf) in a temperature range of T = 80–1000 K, the magnetic susceptibility and magnetization in a temperature range of T = 4.2–300 K at an external magnetic field of up to 70 kOe, and the structural characteristics of Co _x Mn_1?x S sulfides (0 ≤ x ≤ 0.4). Anomalies in the transport properties of these compounds have been found in the temperature intervals ΔT ₁ = 200–270 K and ΔT ₂ = 530–670 K and at T ₃ ～ T _N. The temperature dependences of the magnetic susceptibility, magnetization, and resistivity, as well as the current-voltage characteristics, exhibit hysteresis. In the domain of magnetic ordering at temperatures below the Néel temperature (T _N), the antiferromagnetic Co _x Mn_1?x S sulfides possess a spontaneous magnetic moment that is explained using a model of the orbital ordering of electrons in the t _2g bands. The influence of the cobalt-ion-induced charge ordering on the transport and magnetic properties of sulfides has been studied. The calculated values of the temperatures corresponding to the maxima of charge susceptibility, which are related to a competition between the on-site Coulomb interaction of holes in various subbands and their weak hybridization, agree well with the experimental data.     

Transport and ferromagnetism in InGaAs quantum well structures delta-doped with Mn

Samples containing InGaAs quantum wells with p-type conductivity delta-doped by Mn were synthesized and studied. Magnetic moment measurements on a SQUID magnetometer revealed the presence of ferromagnetism at temperatures T from 4.2 to 400 K. Anomalous Hall effect caused by additional sample magnetization was observed at temperatures of from about 30 to 80 K. The Shubnikov-de Haas effect was recorded at 4.2 K. Negative magnetoresistance changed sign for positive at T ≈ 30 K as the temperature increased.     

Magnetic polarons,clusters, and their effect on the electric properties of weakly doped lanthanum manganites

The resistivity, magnetoresistance, thermopower, and magnetic susceptibility of La_1?xA_xMnO₃(A≡Ca,Sr;x=0.07–0.1) single crystals are investigated in the temperature range from 77 to 400 K. Sharp changes in the properties (the resistivity activation energy ΔEρ, its temperature coefficient γ, the thermopower activation energy ΔE _S , the magnetoresistance, and the appearance of spontaneous magnetization) of these crystals occur near a temperature of 275±25 K, which is approximately twice as high as their Curie point T_C and approximately half of the structural transition temperature. The results are explained by the phase separation: the formation of ferromagnetic clusters. The phase separation occurs through the coalescence of small-radius unsaturated magnetic polarons, in which only two or three magnetic moments of Mn are polarized, into a large-radius ferromagnetic polaron (a cluster about 10–12 Å in size) with several charge carriers. As a result, the short-range order occurs in the cluster at a temperature of about 275 K, which is close to T _C of conducting doped manganites. The results of the experimental studies of the resistivity and the magnetoresistance as functions of temperature and magnetic field and the estimates agree well with the cluster model.     

Thermodynamic study of compounds of the Nd-Ba-Cu-O system

Standard enthalpies of formation for solid solutions of composition Nd_{1 + x} Ba_{2 ? x} Cu₃O _y (x = 0–0.8, y = 6.65–7.24) from oxides were determined by solution calorimetry. The heat capacity of NdBa₂Cu₃O_6.87 phase was measured in the range 5–320 K by low-temperature adiabatic calorimetry. The absolute entropy S ^o(T), the difference of enthalpies H ^o(T)-H ^o(0 K), and the reduced Gibbs energy Φ^o(T) = S ^o(T)–[H ^o(T)–H ^o(0)]/T were calculated on the basis of smoothed dependence C _p (T) in the 0–320 K range. An assessment was made for the heat capacities and the absolute entropies of solid solutions Nd_1+x Ba_2?x Cu₃O _y . The obtained set of thermodynamic parameters can be used for the calculation of phase equilibria in the Nd-Ba-Cu-O system.     

Anisotropic low-temperature in-plane magnetoresistance in electron doped Nd<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Ce<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CuO<Subscript>4+δ</Subscript>

Nominally electron doped antiferromagnetic tetragonal nonsuperconducting Nd_2?xCe _x CuO_4+δ(x=0.12) has been shown to manifest strong angular dependence of the in-plane magnetoresistance on the orientation of the external magnetic field within the ab plane in many aspects similar to that observed in hole doped YBa₂Cu₃O_7?δ and La_2?xSr_xCuO₄. Specific fourfold angular magnetoresistance anisotropy amounting to several percents was observed in oxygen annealed films at low temperatures and in an external magnetic field up to 5.5 T. The strong temperature dependence and fourfold symmetry observed in our sample points to a specific role of rare-earth (Nd) ions in magnetoresistance anisotropy. At low temperature T = 1.4 K, we observed the unusual transformation of magnetoresistance response with increasing the external magnetic field, which seems to be a manifestation of a combined effect of a crossover between first and second order spin-flop transitions and a field-dependent rare-earth contribution to quasiparticle magnetotransport.     

A Raman study of lattice vibrations in II–VI semiconductors doped with 3<Emphasis Type="Italic">d</Emphasis> elements

Room-temperature Raman spectra were obtained for powder samples of Zn_1?xNixSe and Zn_1?yCr_ySe compounds and for a single-crystal Zn_1?xNi_xSe (x = 0.0025) sample in the temperature range 5–140 K. The results obtained are interpreted in terms of large-scale lattice shear strains induced by 3d elements in these solid solutions.     

Role of structure imperfection in the formation of the magnetotransport properties of rare-earth manganites with a perovskite structure

The structure, the structure imperfection, and the magnetoresistance, magnetotransport, and microstructure properties of rare-earth perovskite La_0.3Ln_0.3Sr_0.3Mn_1.1O_3–δ manganites are studied by X-ray diffraction, thermogravimetry, electrical resistivity measurement, magnetic, ⁵⁵Mn NMR, magnetoresistance measurement, and scanning electron microscopy. It is found that the structure imperfection increases, and the symmetry of a rhombohedrally distorted R3?c perovskite structure changes into its pseudocubic type during isovalent substitution for Ln = La³⁺, Pr³⁺, Nd³⁺, Sm³⁺, or Eu³⁺ when the ionic radius of an A cation decreases. Defect molar formulas are determined for a real perovskite structure, which contains anion and cation vacancies. The decrease in the temperatures of the metal–semiconductor (T _ms) and ferromagnet–paramagnet (T _C) phase transitions and the increase in electrical resistivity ρ and activation energy E _a with increasing serial number of Ln are caused by an increase in the concentration of vacancy point defects, which weaken the double exchange 3d ⁴(Mn³⁺)–2p ⁶(O^2–)–3d ³(Mn⁴⁺)–V ^(a)–3d ⁴(Mn³⁺). The crystal structure of the compositions with Ln = La contains nanostructured planar clusters, which induce an anomalous magnetic hysteresis at T = 77 K. Broad and asymmetric ⁵⁵Mn NMR spectra support the high-frequency electronic double exchange Mn³⁺(3d ⁴) ? O^2–(2p ⁶) ? Mn⁴⁺(3d ³) and indicate a heterogeneous surrounding of manganese by other ions and vacancies. A correlation is revealed between the tunneling magnetoresistance effect and the crystallite size. A composition–structure imperfection–property experimental phase diagram is plotted. This diagram supports the conclusion about a strong influence of structure imperfection on the formation of the magnetic, magnetotransport, and magnetoresistance properties of rare-earth perovskite manganites.     

Low-temperature thermal-expansion anomaly in Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>CuO<Subscript>6</Subscript>

Single-crystal samples of the Bi_{2 + x}Sr_{2 ? x ? y}Cu_{1 + y}O_{6 + δ} system revealed anomalous (negative) thermal expansion in the temperature range 10–20 K. Magnetic fields of 1–3 T were found to strongly affect the position and width of the anomaly region. A thermal-expansion singularity was detected at temperatures T≈30–50 K, which may be related to the formation of a pseudogap.     

Microwave magnetoresistance and electron spin resonance in Ge:Mn thin films and nanowires

Resonant and nonresonant absorption of microwave radiation is found to occur in germanium films implanted with manganese at concentrations of 2, 4, and 8 at %. Electron spin resonance is observed in two temperature ranges: (i) in the vicinity of the phase transition of Mn₅Ge₃ clusters to the ferromagnetic state at T = 295 K; and (ii) in the range of temperatures below 60 K, at which collective ordering of Mn spins in the crystal lattice and spin-wave resonance take place. The dependence of the nonresonant signal of the microwave magnetoresistance on the magnetic field exhibits a nonmonotonic behavior identical for the X and K microwave bands. An analysis of the field dependence of the microwave magnetoresistance makes it possible to separate two components of the derivative of the magnetoresistance: the quasi-linear Lorentzian component observed in strong fields and the negative exponential anisotropic component determined by spin-dependent scattering of charge carriers from magnetic impurities. The length of the phase relaxation of charge carriers is estimated to be 350 nm at T = 2 K and exceeds the thickness of the film (120 nm) and the sizes of clusters and precipitates (3–5 nm). In quasi-one-dimensional nanowires of the composition Ge:Mn at the same impurity concentrations, microwave magnetoresistance is absent. These facts suggest that conduction in thin films has a quasi-two-dimensional character and that the measured microwave magnetoresistance is associated with charge carriers in the crystal lattice rather than with impurity clusters.     

Neutron investigations of the magnetic properties of Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>S under pressure up to 4.2 GPa

Fe _x Mn_1?x S belongs to the group of strong electron correlations compounds MnO. We present here experimental results for the antiferromagnetic iron–manganese sulfide system, based on X-ray and neutron diffraction studies. The neutron diffraction investigations were carried out at ambient conditions and at hydrostatic pressures up to 4.2 GPa in the temperature range from 65 to 300 K. Our results indicate that the Néel temperature of α-MnS increases up to room temperature by applying chemical (x _Fe) or weak hydrostatic pressure P. In Fe_0.27Mn_0.73S, the Néel temperature increases from 205(2) K (P = 0 GPa) to 280(2) K (P = 4.2 GPa) and the magnetization at 100 K decreases by a factor of 2.5 when the hydrostatic pressure increases from 0 to 4.2 GPa.     

Peculiarities of magnetic field-induced ferromagnetic ordering in narrow-band manganites

The temperature dependences of the residual magnetization in narrow-band manganites (Pr_0.67Ca_0.33MnO₃, Sm_0.55Sr_0.45Mn¹⁸O₃, Sm_0.55Sr_0.45Mn¹⁶O₃, and (NdEu)_0.55Sr_0.45Mn¹⁸O₃) have been studied. All compounds studied are characterized by a fairly high residual magnetization M _R (about 0.5 μ_B/Mn) at 4.2 K, which vanishes upon sample heating to the temperature T _RE ≈ 30–35 K, which is much lower than the temperature T _C of the ferromagnetic transition. However, upon magnetization of the samples at T _RE < T < T _C , the residual magnetization (smaller in magnitude) remains up to T _C . For the composition (NdEu)_0.55Sr_0.45Mn¹⁸O₃, the residual magnetization remains at T < T _C , independent of the temperature of magnetization. The disappearance of the residual magnetization found at intermediate temperatures is apparently related to the destruction of the magnetic field-induced ferromagnetic ordering (which contains an additional contribution of the rare-earth sublattice).     

EPR study of the spin dynamics of the (La<Subscript>1−<Emphasis Type="Italic">y</Emphasis></Subscript>Pr<Subscript>y</Subscript>)<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> system

The EPR of Mn ions in the (La_1?yPr_y)_0.7Ca_0.3MnO₃ system has been studied within a broad range of temperatures (4<T<600 K) and Pr concentrations (0≤y≤1), as well as under isotope substitution of ¹⁸O for ¹⁶O. All compositions were shown to undergo transitions to a magnetically ordered state with decreasing temperature. Magnetic phase diagrams were constructed for systems with different oxygen isotopes. The diagrams include paramagnetic, ferromagnetic, and antiferromagnetic regions. In the paramagnetic region, at temperatures not too close to the phase transition points, the Mn ion linewidth ΔH _pp (T) is related to the magnetic susceptibility χ(T) through the relation ΔH _pp (T) = [χ₀/χ(T)]ΔH _pp (∞) + ΔH₀, where ΔH _pp (∞) is the width of the exchange-narrowed line in the high-temperature approximation, χ₀ ∝ 1/T is the susceptibility of noninteracting ions, and ΔH₀ is the residual width originating from the sample porosity and resonance-field scatter in unoriented grains of a powder sample. An analysis of the data on ΔH _pp (∞), ΔH₀, and χ(T) made it possible to estimate the symmetric and antisymmetric exchange interaction of Mn ions and of the noncubic crystal-field component of the oxygen ions. These parameters were found to be independent of the oxygen isotope species to within experimental error.     

The Nernst-Ettingshausen coefficient in the normal phase of doped HTSCs of the YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>y</Subscript> system

The temperature dependence of the Nernst-Ettingshausen coefficient Q(T) in the normal phase of doped HTSCs of the yttrium system was studied. The main features characterizing the behavior of this coefficient were revealed, and the character and mechanism of the effect that various nonisovalent substituents exert on the Q(T) dependence were analyzed. It is shown that the narrow-band model permits one not only to describe all the specific features observed in the Q(T) curves but also to perform a simultaneous quantitative analysis of the temperature dependences of four kinetic coefficients (the electrical resistivity and the Seebeck, Hall, and Nernst-Ettingshausen coefficients) with the use of a common set of model parameters characterizing the band structure and carrier system in the normal phase of an HTSC. This approach was employed to determine the carrier mobilities and the asymmetry of the dispersion curve in the systems studied (YBa₂Cu₃O_y, y = 6.37–6.91; YBa₂Cu_3?xCo_xO_y, x = 0–0.3; Y_1?xCa_xBa₂Cu₃O_y, x = 0–0.25; Y_1?xCa_xBa_2?xLa_xCu₃O_y, x = 0–0.5) and to analyze the effect of the substitutions involved on the variation of these parameters.     

Galvanomagnetic properties of Heusler alloy Co<Subscript>2</Subscript><Emphasis Type="Italic">Y</Emphasis>Al (<Emphasis Type="Italic">Y</Emphasis> = Ti,V, Cr,Mn, Fe,and Ni)

The Hall effect and the magnetoresistance of ferromagnetic Heusler alloys Co₂ YAl, where Y = Ti, V, Cr, Mn, Fe, and Ni have been studied at T = 4.2 K in magnetic fields H ≤ 100 kOe. Normal R ₀ and anomalous R _S Hall coefficients are shown to be maximal in magnitudes in the middle of the 3d period of the periodic table of elements. Coefficient R ₀ changes the negative sign to positive sign in going from weak (Y = Ti, V) to strong (Y = Cr, Mn, Fe, and Ni) ferromagnetic alloys. Constant R _S is positive and proportional to ρ^2.9 in all the alloys. The magnetoresistance of the alloys is not higher than several percent and its magnitude is changed fairly significantly in the dependence on the number of valence electrons z; the magnetoresistance signs vary arbitrarily.     

Anomalous Hall effect in highly Mn-Doped silicon films

The transport and magnetic properties of Mn _x Si_{1 ? x} films with a high (x ≈ 0.35) content of Mn produced by laser deposition at growth temperatures of 300–350°C have been studied in a temperature range of 5–300 K in magnetic fields of up to 2.5 T. The films exhibit a hole-type metallic conductivity and a relatively weak change of magnetization in a temperature range of 50–200 K. An anomalous Hall effect with an essentially hysteretic behavior from 50 K up to ≈230 K has been discovered. The properties of the films are explained by the two-phase model, in which ferromagnetic clusters containing interstitial Mn ions with a localized magnetic moment are embedded in the matrix of a weak band MnSi_{2 ? x} (x ≈ 0.3) type ferromagnet with delocalized spin density.     

Anomalous Hall effect and ferromagnetism in the new diluted magnetic semiconductor Sb<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Cr<Subscript>x</Subscript>Te<Subscript>3</Subscript>

The magnetic and galvanomagnetic properties of single crystals of the new diluted magnetic semiconductor p-Sb_2?xCr_xTe₃ (0 ≤ x ≤ 0.02) have been studied in the temperature range 1.7–300 K. A ferromagnetic phase with the Curie temperature T_c ≈ 5.8 K and the maximum Cr content x = 0.0215 has been revealed. The easy magnetization axis is parallel to the C₃ crystallographic axis. In the presence of strong magnetic fields, the Shubnikov-de Haas effect has been observed. Analysis of this effect shows that doping with chrome reduces the concentration of holes. Negative magnetoresistance and the anomalous Hall effect are observed at liquid helium temperature.