Crystal and magnetic structures and physical properties of the Sm<Subscript>0.37</Subscript>Sr<Subscript>0.63</Subscript>MnO<Subscript>3</Subscript> manganite

The ¹⁵²Sm_0.37Sr_0.63MnO₃ manganite is investigated using neutron diffraction. The parameters of the crystal and magnetic structures of the manganite are determined. The diffraction data are compared with the transport and magnetic characteristics of this compound. A comparison is performed between the ¹⁵²Sm_0.37Sr_0.63MnO₃ and ¹⁵²Sm_0.45Sr_0.55MnO₃ manganites. Although these compounds differ insignificantly in the strontium doping level, are homogeneous antiferromagnets, and do not exhibit a colossal negative magnetoresistance, they have different crystal symmetries (tetragonal I4/mcm and orthorhombic Pnma), differ in the type of spin ordering (C-type antiferromagnetic and A-type antiferromagnetic ordering), are characterized by different orbital polarizations (\(d_{3z^2 - r^2 } \) and \(d_{x^2 - y^2 } \)), and possess one-and two-dimensional magnetic and transport properties, respectively. The critical concentration range in which samarium strontium manganites undergo a concentration structural transition from the orthorhombic to tetragonal crystal symmetry with a change in the type of orbital and magnetic order is revealed.     

Magnetic Structures and Magnetic Phase Transitions in Rare-Earth <Emphasis Type="Italic">R</Emphasis>Mn<Subscript>2</Subscript>Si<Subscript>2</Subscript> Intermetallic Compounds (<Emphasis Type="Italic">R</Emphasis> = Sm,Tb)

The magnetic structures that form in La_1–xR_xMn₂Si₂ (R = Sm, Tb) layered compounds with various concentrations x have been determined by magnetic neutron diffraction and magnetic measurements, and the magnetic phase diagrams have been built. It is shown that the formation of the magnetic structures is dependent not only on exchange interactions, but also on the type of the magnetic anisotropy of a rare-earth atom. It is found that, in La_1–xTb_xMn₂Si₂ compounds with 0.2 < x < 0.5, the competition of the Tb–Mn and Mn–Mn interlayer exchange interactions and the existence of a strong uniaxial magnetic anisotropy in the Mn and Tb sublattices leads to the frustrated magnetic state and prevents the formation of the long-range magnetic order in the Tb sublattice.     

Effect of structural defects on the magnetic properties of the EuBaCo<Subscript>1.90</Subscript>O<Subscript>5.36</Subscript> single crystal

The effect of structural defects in cobalt and oxygen sublattices with the constant average oxidation level 3+ of all cobalt ions on the magnetic properties of the EuBaCo_1.90O_5.36 single crystal has been studied. The magnetic properties of the single crystal and the polycrystalline sample of the corresponding composition are compared in the range T = 200–650 K. The results show that the cobalt-deficient EuBaCo_2–xO_5.5–δ samples demonstrate a three-dimensional XY ferromagnetic ordering of magnetic sublattices. The values of the effective magnetic moment at T > 480 K indicate the existence of the IS and HS states of Co³⁺ ions. The large difference of values of μ_eff of the EuBaCo_1.90O_5.36 single crystal and polycrystal can be due to that the magnetic ion spins lie in plane ab. The magnetic field directed along plane ab substantially influences the magnetic ordering at T < 300 K.     

Investigation of a Spin Transition in a LaCoO<Subscript>3</Subscript> Single Crystal by the Method of X-Ray Magnetic Circular Dichroism at the Cobalt <Emphasis Type="Italic">K</Emphasis>- and <Emphasis Type="Italic">L</Emphasis><Subscript>2,3</Subscript>-Edges

Spin transitions of cobalt ions in LaCoO₃ single crystals have been studied by the method of X-ray magnetic circular dichroism (XMCD) at the K- and L_2,3-edges of Co³⁺ ions. The orbital momentum of cobalt ions obtained for the K-edge at the 3d level in the region of the spin transition in the temperature range from 25 to 120 K increases by a factor of approximately 1.6, whereas the slope of the magnetization curve value in the same temperature range and magnetic field increases by a factor of more than 10. XMCD experiments at the cobalt L_2,3-edges demonstrate gradual growth of the ratio of the orbital momentum to the spin one L/S from 0.48 to 0.53 in the temperature range from 60 K to 120 K.     

Effect of a magnetic field on the thermal and kinetic properties of the Sm<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3.02</Subscript> manganite

The temperature and magnetic-field dependences of the heat capacity, thermal conductivity, thermopower, and electrical resistivity of the Sm_0.55Sr_0.45MnO_3.02 ceramic material are studied in the temperature range 77–300 K and in magnetic fields up to 26 kOe. It is revealed that the quantities under investigation exhibit anomalous behavior due to a magnetic phase transition at the Curie temperature T_C. An increase in the magnetic field strength H leads to an increase in the Curie temperature T_C and a jump in the heat capacity ΔC_p at T_C. The temperature dependences of the measured quantities are characterized by hystereses that are considerably suppressed in a magnetic field of 26 kOe and depend neither on the thermocycling range nor on the rate of change in the temperature. The thermal conductivity K at temperatures above T_C shows unusual behavior for crystalline solids (dK/dT>0) and, upon the transition to a ferromagnetic state, drastically increases as a result of a decrease in the phonon scattering by Jahn-Teller distortions. It is demonstrated that the hystereses of the studied properties of the Sm_0.55Sr_0.45MnO_3.02 manganite are caused by a jumpwise change in the critical temperature due to variations in the lattice parameters upon the magnetic phase transition.     

Spin-dimerization in rare-earth substituted La<Subscript>2</Subscript>RuO<Subscript>5</Subscript>

The Ru-Ru spin-singlet formation in La_2 ? x L n _x RuO₅ (Ln = Pr, Nd, Sm, Gd, Dy) was investigated by measurements of the specific heat and magnetic susceptibility. After subtraction of the lattice contribution from the specific heat (C _p ), similar excess entropy values were obtained for all compounds. These entropies can be explained by the formation of antiferromagnetic Ru-spin dimers at low temperatures and provide a lower estimate for the intradimer exchange strength. Pronounced changes in the transition temperatures and a broadening of the corresponding peak in C _p were observed. These changes depend on the rare-earth element and are due to local structural changes and heterogeneities caused by the substitution. The magnetic susceptibilities can be described by the sum of a rare-earth paramagnetic moment and the susceptibility of the unsubstituted La₂RuO₅. Density functional theory (DFT) calculations were performed for various compounds to investigate the origin of the magnetic transition and the relationship between structural changes and the spin-dimerization temperature. The combination of the present results with previous structural investigations supports the model of a spin-pairing of the Ru moments which occurs as a reason of the structural phase transition in La_2 ? x L n _x RuO₅.     

Adiabatic calorimetric study of the intense magnetocaloric effect and the heat capacity of (La<Subscript>0.4</Subscript>Eu<Subscript>0.6</Subscript>)<Subscript>0.7</Subscript>Pb<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript>

The temperature dependences of the intense magnetocaloric effect ΔT _AD(T, H) and the heat capacity C _p (T) of the (La_0.4Eu_0.6)_0.7Pb_0.3MnO₃ manganite are directly measured using adiabatic calorimetry. The experimental dependences ΔT _AD(T) are in satisfactory agreement with those calculated from the data on the behavior of the magnetization. The factors responsible for the absence of an anomaly in the experimental temperature dependence of the heat capacity C _p (T) in the range of the magnetic phase transition are discussed.     

On the correlation between supercooling,superheating and kinetic arrest in a magnetic glass Pr<Subscript>0.5</Subscript>Ca<Subscript>0.5</Subscript>Mn<Subscript>0.975</Subscript>Al<Subscript>0.025</Subscript>O<Subscript>3</Subscript>

We report a quantitative investigation of the magnetic field-temperature phase diagram by taking into account a simple phenomenological model arising out of the interplay of kinetic arrest and thermodynamic transitions in a magnetic glass Pr_0.5Ca_0.5Mn_0.975Al_0.025O₃, through magnetization measurements. Such studies are necessary as kinetic arrest plays an important role in the formation of “magnetic glasses”, which has been observed in systems undergoing first order magnetic phase transitions. It has been shown that disorder in a system results in the formation kinetic arrest (H _K ,T _K ) band, like supercooling (H ^*,T ^*) and superheating (H ^**,T ^**) band. Quantitative proofs are given to show that (H _K ,T _K ) band is anticorrelated with (H ^*,T ^*) and (H ^**,T ^**) bands, while the later two are correlated among themselves. Analysis of time dependence of magnetization at different temperatures is carried out to establish the fact that the kinetic arrested state is different from the supercooled state.     

Magnetic phase diagram and correlation between metamagnetism and superconductivity in Ru<Subscript>0.9</Subscript>Sr<Subscript>2</Subscript>YCu<Subscript>2.1</Subscript>O<Subscript>7.9</Subscript>

The magnetic superconductorRu_0.9Sr₂YCu_2.1O_7.9 (Ru-1212Y) has beeninvestigated using neutron diffraction under variable temperature and magnetic field. Withthe complementary information from magnetization measurements, we propose a magnetic phasediagram T-H for the Ru-1212 system. Uniaxialantiferromagnetic (AFM) order of 1.2μ _B /Ruatoms with moments parallel to the c-axis is found below the magnetictransition temperature at  ~140 K in the absence of magnetic field. In addition,ferromagnetism (FM) in the ab-plane develops below  ~120 K, butis suppressed at lower temperature by superconducting correlations. Externally appliedmagnetic fields cause Ru-moments to realign from the c-axis to theab-plane, i.e. along the ?1,1,0? direction, and induce ferromagnetismin the plane with  ~1μ _B at 60 kOe.These observations of the weak ferromagnetism suppressed by superconductivity and thefield-induced metamagnetic transition between AFM and FM demonstrate not only competingorders of superconductivity and magnetism, but also suggest a certain vortex dynamicscontributing to these magnetic transitions.     

Structural and magnetic phase transformations in the BiFeO<Subscript>3</Subscript>-CaFe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> system

The crystal structure and magnetic properties of the Bi_{1 ? x} Ca _x Fe_{1 ? x/2}Nb _x/2O₃ system were studied. It is shown that, at x ≤ 0.15, the unit-cell symmetry of solid solutions is rhombohedral (space group R3c). Solid solutions with x ≥ 0.3 have an orthorhombic unit cell (space group Pbnm). The rhombohedral compositions are antiferromagnetic, while the orthorhombic compositions exhibit a small spontaneous magnetization due to Dzyaloshinski?-Moriya interaction. In CaFe_0.5Nb_0.5O₃, the Fe³⁺ and Nb⁵⁺ ions are partially ordered and the unit cell is monoclinic (space group P2₁/n). In the concentration range 0.15 < x < 0.30, a two-phase state (R3c + Pbnm) is revealed.     

Magnetic properties of Pb<Subscript>2</Subscript>Fe<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>9</Subscript> single crystals

Single crystals of Pb₂Fe₂Ge₂O₉ have been grown. They were subjected to X-ray diffraction, magnetic, neutron diffraction, Mössbauer and spin resonance studies. It has been established that Pb₂Fe₂Ge₂O₉ is a weak ferromagnet with a Néel temperature T _N = 46 K, and the exchange and spin-flop transition fields have been estimated. It has been demonstrated that the weak ferromagnetic moment is actually the result of the single-ion anisotropy axes for the magnetic moments of different magnetic sublattices being not collinear.     

Magnetostriction of Hexagonal HoMnO<Subscript>3</Subscript> and YMnO<Subscript>3</Subscript> Single Crystals

We report on the magnetostriction of hexagonal HoMnO₃ and YMnO₃ single crystals in a wide range of applied magnetic fields (up to H = 14 T) at all possible combinations of the mutual orientations of magnetic field H and magnetostriction ΔL/L. The measured ΔL/L(H, T) data agree well with the magnetic phase diagram of the HoMnO₃ single crystal reported previously by other authors. It is shown that the nonmonotonic behavior of magnetostriction of the HoMnO₃ crystal is caused by the Ho³⁺ ion; the magnetic moment of the Mn³⁺ ion parallel to the hexagonal crystal axis. The anomalies established from the magnetostriction measurements of HoMnO₃ are consistent with the phase diagram of these compounds. For the isostructural YMnO₃ single crystal with a nonmagnetic rare-earth ion, the ΔL/L(H, T) dependences are described well by a conventional quadratic law in a wide temperature range (4–100 K). In addition, the magnetostriction effect is qualitatively estimated with regard to the effect of the crystal electric field on the holmium ion.     

Effect of the disorder parameter on the magnetic,electrical, galvanomagnetic,elastic, and magnetoelastic properties of <Emphasis Type="Italic">R</Emphasis><Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3</Subscript> manganites

This paper reports on a study of the magnetic, transport, magnetotransport, elastic, and magnetoelastic properties of the R_0.55Sr_0.45MnO₃ ceramics (R=Sm, Eu_0.40Nd_0.15, Tb_0.25Nd_0.30) with the same carrier concentration and identical tolerance factor but which differ in the cation disorder parameter σ². It was found that the Curie temperature T_C decreases linearly with increasing σ². An increase in σ² results in an increase in the maximum electrical resistivity and an increased jump in the temperature dependence of linear thermal expansion near T_C, as well as in a decrease in magnetoresistance and magnetostriction. For T>T_C, one observes an abrupt increase in magnetostriction, magnetization, and magnetoresistance in a critical FIeld H_C1 which grows with increasing temperature. The value of H_C1 determined at fixed T/T_C decreases with increasing σ².     

Spin dynamics and phase transitions in quasi-2D antiferromagnets <Emphasis Type="Italic">R</Emphasis><Subscript>2</Subscript>CuO<Subscript>4</Subscript> (<Emphasis Type="Italic">R</Emphasis>=Pr,Nd, Sm,Eu, Gd)

The spin and lattice dynamics of the R₂CuO₄ quasi-2D antiferromagnetic crystals (R=Pr, Nd, Sm, Eu, Gd) were studied in the millimeter-range electromagnetic wave band. Strong variations of the absorption coefficient were observed to occur at temperatures T≈T₀. Absorption lines of electrical nature due to lattice dynamics were also revealed near the T₀ temperatures. The observed anomalies are assumed to originate from phase transitions at T≈T₀, which entail changes in the structural and magnetic properties.     

Pressure-induced antiferromagnetism in La<Subscript>0.75</Subscript>Ca<Subscript>0.25</Subscript>MnO<Subscript>3</Subscript> manganite

The crystal and magnetic structures of La_0.75Ca_0.25MnO₃ manganite are studied under high pressures up to 4.5 GPa in the temperature range 12–300 K by the neutron diffraction method. At normal pressure and temperature T _C = 240 K, a ferromagnetic state is formed in La_0.75Ca_0.25MnO₃. At high pressures P ≥ 1.5 GPa and at temperatures T < T _N ≈ 150 K, a new A-type antiferromagnetic state appears. A further increase in pressure leads to an increase in the volume fraction of the antiferromagnetic phase, which coexists with the initial ferromagnetic phase. The effect of high pressure causes a considerable increase in T _C with the slope dT _C /dP ≈ 12 K/GPa. Calculations performed in the framework of the double exchange model with allowance for the electron-phonon interaction make it possible to explain this pressure dependence of T _C on the basis of experimental data.     

The role of magnetization compensation point for efficient ultrafast control of magnetization in Gd<Subscript>24</Subscript>Fe<Subscript>66.5</Subscript>Co<Subscript>9.5</Subscript> alloy

The ability of a femtosecond laser pulse to manipulate and reverse the magnetization in a ferrimagnetic Gd₂₄Fe_66.5Co_9.5 thin film was studied experimentally as a function of temperature. For a fixed energy of the laser pulse, the dynamics of magnetization showed different behavior depending on whether the sample temperature was below or above the magnetization compensation point (T _M ). The conditions for full ultrafast demagnetization and magnetization reversal were easily achieved below T _M , while the same laser excitation caused just 50% demagnetization above T _M . This interesting change in magnetization dynamics is qualitatively explained in terms of effective changes in the magnitudes of magnetizations of atomic sublattices.     

Anisotropy of magnetic properties of Sm<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3</Subscript> single crystals

Anisotropy of the magnetic properties of Sm_0.55Sr_0.45MnO₃ single crystals has been studied. A significant increase in the antiferromagnetic component of magnetization in the case of orientation of an external magnetic field H close to the c axis has been found. Magnetization for a field lying in the ab plane seems typical of a ferromagnet. Anisotropy of susceptibility reaches 2.2 in weak fields and nearly vanishes at H > 1 T.     

Magnetic circular dichroism and optical absorption in TmAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

The polarized spectra of absorption and magnetic circular dichroism in a TmAl₃(BO₃)₄ single crystal are studied in the region of ³ H ₆ → ³ F ₄, ³ H ₆ → ³ F ₃, and ³ H ₆ → ³ F ₂ electronic transitions in the Tm³⁺ ion. The structure of the spectra is interpreted qualitatively. It is shown that the magnetic circular dichroism of the ³ H ₆ → ³ F ₄ transition is determined by the contribution from the splitting of the ground state, whereas the magnetic circular dichroism of the ³ H ₆ → ³ F ₃ transition is governed by the contribution from the splitting of an excited state in a trigonal crystal field.     

X-ray study of [N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>]<Subscript>2</Subscript>ZnCl<Subscript>4</Subscript> at low temperatures

The unit cell parameters a, b, and c of [N(CH₃)₄]₂ZnCl₄ have been measured by x-ray diffraction in the temperature range 80–293 K. Temperature dependences of the thermal expansion coefficients α_a, α_b, and α_c along the principal crystallographic axes and of the unit cell thermal expansion coefficient α_V were determined. It is shown that the a=f(T), b=f(T), and c=f(T) curves exhibit anomalies in the form of jumps at phase transition temperatures T₁=161 K and T₂=181 K and that the phase transition occurring at T₃=276 K manifests itself in the a=f(T) and b=f(T) curves as a break. A slight anisotropy in the coefficient of thermal expansion of the crystal was revealed. The phase transitions occurring at T₁=161 K and T₂=181 K in [N(CH₃)₄]₂ZnCl₄ were established to be first-order.     

Manifestation of pseudogap features in structurally inhomogeneous optimally doped HTSC YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.92</Subscript>

Magnetization M(H,T) in magnetic fields H up to 90 kOe and at temperatures 2 K ≤ T < T _c (where Tc is the superconducting transition temperature), along with magnetic susceptibility χ(T) in the normal state T _c < T < 400 K for optimally oxygen-doped samples of YBa₂Cu₃O_6.92 with varying degrees of defects in the crystal structure, are studied to determine the influence of structural inhomogeneity on the electron systems characteristics of cuprate superconductors. It is shown that the existence of structural inhomogeneity of samples leads to the manifestation of peculiarities appropriate to pseudogap regime in their properties.