The influence of magnetic and structural heterogeneity on thermopower,magnetothermopower, electrical resistivity,and magnetoresistivity of Nd<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript> manganite

The thermopower, S, magnetothermopower, ΔS/S, resistivity, ρ, and magnetoresistivity, Δρ/ρ, depending on the temperature T and magnetic field H, have been studied in an Nd_0.5Sr_0.5MnO₃ single crystal consisting of three types of clusters: an antiferromagnetic CE-type with charge-orbital ordering (below the Neel temperature T_NCE ~ 145 K) and an A-type with T_NA ~ 220 K; a ferromagnetic at 234 ≤ T ≤ 252 K, and a ferromagnetic metal phase below the Curie temperature T_C = 248 K. The thermopower was found to be negative, indicating the dominance of the electronic type of conductivity. In the S(T) curves, a sharp minimum is observed in the temperature range of 100 K ≤ T ≤ 133 K, close to T_NCE, where the absolute S value attains 53 μV/K. With a further increase in temperature, the absolute S value decreases rapidly; at 200 K it is equal to 7 μV/K. It then slightly increases, reaching its maximum value of 15 μV/K at a temperature of 254 K, which is close to T_C. The absolute thermopower decreased under the influence of the magnetic field; i.e., a negative magnetothermopower occurs. In {ΔS/S}(T) curves, a sharp minimum is observed at T = 130 K close to T_NCE, where the magnetothermopower reaches a huge value of ~45% at H = 13.23 kOe. A broad minimum in the {ΔS/S}(T) curves is observed near the Curie temperature and its value is also high, viz., ~15% in the maximum measuring magnetic field of 13.23 kOe. The extremely high magnetothermopower values mean that the charge-orbital ordered nanoclusters or ferron type make the main contribution to the thermopower of the entire sample. The behavior of the ρ(T) and {Δρ/ρ}(T) curves is similar to that of the S(T) and {ΔS/S}(T) dependencies, which is in agreement with this conclusion.     

Magnetic phase coexistence in Tb- and Sr-doped La0.7Ca0.3MnO3 manganite:A temperature-dependent neutron diffraction study

We report the results of the temperature-dependent neutron diffraction measurements on the nearly half-doped (La_0.325Tb_0.125)(Ca_0.3Sr_0.25)MnO₃ manganite sample. The simultaneous doping of magnetic Tb³⁺ and divalent Sr²⁺ in the La_0.7Ca_0.3MnO₃ system results into a large A-site size disorder. Rietveld refinement of neutron diffraction data reveal that the single phase sample crystallizes in a distorted orthorhombic structure. Increased 〈r_A〉 value affects the transport behavior that results into an insulating-like behavior of the sample. Under application of 1 T field sample exhibit insulating-like behavior while insulator-metal transition (T_IM) is exhibited under 5 and 8 T fields. Variable range hoping (VRH) mechanism of charge carriers is exhibited in the insulating region. Field cooled and zero field cooled magnetization measurement shows the Curie temperature (T_C)~47 K. The refinement of the ND data collected at various temperatures below 300 K shows that there is no structural phase transition in the compound. Around 100 K, a magnetic peak appears at lower angle that can be ascribed to the presence of the A-type antiferromagnetic (AFM) phase. Two more peaks are observed around 50 K at lower angles that can be fitted in CE-type antiferromagnetic phase. Splitting of the peaks at lower temperatures is the signature of orbital ordering in the presently studied nearly half-doped manganite system. Results of the detailed structural analysis of the temperature-dependent ND measurements on (LaTb)_0.45(CaSr)_0.55MnO₃ sample has been discussed in the light of coexisting A-type and CE-type antiferromagnetic phases present in the sample at low temperature.     

Magnetocaloric effect in manganites

The magnetocaloric effect (MCE) in La_{1 ? x} Sr _x MnO₃, Sm_0.55Sr_0.45MnO₃, and PrBaMn₂O₆ compounds is studied. The maximum values of MCE (??T _max) determined by a direct method in the second and third compositions and in La_0.9Sr_0.1MnO₃ are found to be much lower than those calculated from the change of the magnetic part of entropy in the Curie temperature (T _C) and the Néel temperature (T _N) range. The negative contribution of the antiferromagnetic (AFM) part of a sample in the La_{1 ? x} Sr _x MnO₃ system at 0.1 ?? x ?? 0.3 decreases ??T _max and changes the ??T(T) curve shape, shifting its maximum 20?C40 K above T _C. Lower values of ??T _max are detected in the range T _C = 130?142 K in polycrystalline and single-crystal Sm_0.55Sr_0.45MnO₃ samples cooled in air. If such samples were cooled in an oxygen atmosphere (which restores broken Mn-O-Mn bonds and, thus, increases the volume of CE-type AFM clusters), the maximum in the temperature dependence of MCE is located at T _N (243 K) for CE-type AFM clusters. A magnetic field applied to a sample during the MCE measurements transforms these clusters into a ferromagnetic (FM) state, and both types of clusters decompose at T = T _N. The PrBaMn₂O₆ composition undergoes an AFM-FM transition at 231 K, and the temperature dependence of its MCE has a sharp minimum at T = 234 K, where MCE is negative, and a broad maximum covering T _C. The absolute values of MCE at both extrema are several times lower than those calculated from the change in the magnetic entropy. These phenomena are explained by the presence of a magnetically heterogeneous FM-AFM state in these manganites.     

Pressure and field dependence of the magnetic transition in GdBa2Cu3O7-x

The influence of hydrostatic pressure and of magnetic field strenght is presented for the low temperature antiferromagnetic ordering temperature (T_N=2.3 K) of GdBa₂Cu₃O_7-x. Data are presented for both superconducting and normal samples, the superconducting sample having a sharp 95 K transition and the oxygen-depleted normal sample being a semiconductor. For both systems the Néel temperatures, extrapolated to zero measuring field, are identical: T_N = (2.33±0.03) K. The effect of pressure is to raise the transition temperature slightly for both samples, dT_N/dP=+0.03 K/kbar for the superconducting sample and +0.04 K/kbar for the normal sample. The temperature dependence of the heat capacity made in several fixed external magnetic fields and the isothermal magnetization for T<T_N provide a measure of the antiferromagnetic-paramagnetic phase boundary, which shows T_N approaching T=0 K at about 2.5 T.     

Magnetic properties of La0.70Sr0.30MnO2.85 anion-deficient manganite under hydrostatic pressure

The magnetic properties of La_0.70Sr_0.30MnO_2.85 anion-deficient manganite are studied experimentally under hydrostatic pressure. The results show that, in the whole pressure range under investigation (0–1 GPa), the sample is a spin glass with a smeared phase transition to the paramagnetic state. It is found that the spin glass state arises from the frustration of the exchange coupling of the ferromagnetic clusters embedded in the antiferromagnetic matrix. The fraction of the sample volume occupied by the ferromagnetic phase is found to be V _fer ～ 13%. Under hydrostatic pressure, the freezing temperature T _f of the magnetic moments of the ferromagnetic clusters increases at a rate of 4.30 K/GPa and the magnetic ordering temperature T _MO increases at a rate of 12.90 K/GPa. In addition, the ferromagnetic part of the sample increases by ΔV _fer ～ 5%. The enhancement of the ferromagnetic properties of La_0.70Sr_0.30MnO_2.85 anion-deficient manganite under hydrostatic pressure is explained by the redistribution of oxygen vacancies and a decrease in the unit-cell parameters.     

Spontaneous generation of electric voltage in a single crystal of Sm<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3</Subscript>

Spontaneous generation of electric voltage has been found in a Sm_0.55Sr_0.45MnO₃ single crystal grown by floating zone melting with cooling in oxygen. The maximum voltage of 60 μV has been observed in the region of temperatures corresponding, simultaneously, to destruction of the CE-type antiferromagnetic ordering and charge ordering in some clusters. The maximum voltage remains unchanged for 24 h and decreases by 45% in a magnetic field of 14.2 kOe. It has been shown that the spontaneous voltage is caused by the presence of regions with different electric charges in the sample.     

Long-range and short-range magnetic order in new compound NaVGe<Subscript>2</Subscript>O<Subscript>6</Subscript>

New metal oxide pyroxene compound NaVGe₂O₆ containing isolated edge-sharing VO₆ (S=1) chains undergoes transition into a long-range antiferromagnetic state at T _N =16 K. The broad maximum in the temperature dependence of magnetic susceptibility at T _M =26 K indicates the low-dimensional character of the magnetic subsystem. Even though the antiferromagnetic ordering is accompanied by a sharp peak of specific heat, significant magnetic entropy is released above the Néel temperature.     

Magnetothermopower of nanocomposites in the vicinity of the percolation threshold

The field dependences of the thermopower of composites with Co and Co₄₅Fe₄₅Zr₁₀ nanoparticles in the Al₂O _n insulator matrix are studied in magnetic fields up to 10 kOe at room temperature with compositions up to the percolation threshold (i.e., in the region where tunnel conductivity takes place). In composites obtained in argon, negative magnetothermopower (i.e., a decrease in the thermopower in strong magnetic fields) is observed, while positive magnetothermopower is observed in composites obtained in the atmosphere of argon and oxygen. It is shown that the theory developed for tunnel magnetothermopower in nanocomposites makes it possible to explain the results on a qualitative level in the case when the local density of electron states at the surface of nanoparticles depends on the sputtering conditions. Nanocomposites CoFeZr _x (Al₂O _n )_100?x ) obtained in argon and nitrogen display a strong asymmetry of magnetothermopower relative to the magnetic field direction; this anisotropy is associated with anisotropy of these nanostructures.     

Quantum oscillations of thermopower of gallium single crystals

We present experimental results of the transverse thermopower of high- purity gallium single crystals in magnetic fields up to 41.8kG. Giant quantum oscillations in the magnetothermopower appear for a magnetic field parallel to the c-axis and a temperature gradient ▽T along the b-axis. The results are compared with the existing data of the transverse magneto- resistivity, the de Haas-van Alphen effect as well as with the Fermi surface model of gallium due to Reed's calculation.     

Magnetic and thermal properties of CuFeS<Subscript>2</Subscript> at low temperatures

The magnetic moment M, the magnetic susceptibility χ, and the thermal conductivity of chalcopyrite CuFeS₂, which is a zero-gap semiconductor with antiferromagnetic ordering, have been measured in the temperature range 10–310 K. It has been revealed that the quantities χ(T) and M(T) increase anomalously strongly at temperatures below ∼100 K. The temperature dependence M(T) is affected by the magnetic prehistory of the sample. An analysis has demonstrated that the magnetic anomalies are associated with the presence of a system of noninteracting magnetic clusters in the CuFeS₂ sample under investigation. The formation of the clusters is most likely caused by the disturbance of the ordered arrangement of Fe and Cu atoms in the metal sublattice of the chalcopyrite, which is also responsible for the phase inhomogeneity of the crystal lattice. The inhomogeneity brings about strong phonon scattering, and, as a result, the temperature dependence of the thermal conductivity coefficient exhibits a behavior characteristic of partially disordered crystals.     

Electronic transport properties of stuffed compositions of ferromagnetic copper chromium telluride: Cu1+xCr2Te4 (x=0-1)

We present here a detailed study of electronic transport properties of the metallic-ferromagnetic compounds Cu_1+xCr₂Te₄, having excess Cu atoms with x=0-1, from 2 to 400 K. The stuffing of the copper atoms in the parent structure reduces the ferromagnetic ordering temperature T_C from 325 to 156 K, while for the entire range the dependence of the electrical resistance and the thermopower with temperature and the anomalies in them on the magnetic ordering remain similar. All the compounds show a magnon-drag contribution in thermopower as a positive maximum around T_C/3, and a T² - dependence of resistivity at low temperatures. The increasing effects of the short range magnetic ordering in the paramagnetic resistivity are seen with the increase in the stuffing of atoms in these compounds. The transport properties are explained by the current carriers —the holes in a wide energy band dominated by the p-state of Te-atoms, which are scattered by the spindisorder in the paramagnetic phase and from the magnons in the ferromagnetic phase.     

Magnetic properties of the Nd0.9MnOx cation-deficient manganites

This paper reports on the results of x-ray diffraction and magnetic studies of manganites in the Nd_0.9MnO_x system with an oxygen content varying in the range 2.84 < x < 2.93. A sample with an oxygen content x = 2.84 undergoes a first-order phase transition at a temperature close to T = 1050 K, whereas an anomaly observed in the elastic properties for a sample with x = 2.93 indicates a phase transition near T ≈ 500 K. It is assumed that these transitions are governed by cooperative ordering of Mn e _g orbitals of the same type as in stoichiometric NdMnO₃. The manganite at an oxygen content x = 2.85 is an antiferromagnet with a Néel temperature T _N = 85 K, whereas the magnetic properties of the manganites at x = 2.90 and 2.93 suggest that an antiferromagnetic component coexists with a ferromagnetic component. The magnetic interaction between the ferromagnetic and antiferromagnetic components manifests itself in the fact that the magnetic moment becomes opposite in direction to the external magnetic field. The properties of the samples are consistent with the hypothesis that part of the neodymium ions (up to 5%) can be substituted for manganese ions.     

Proton NMR study of α-MnH0.06

Proton nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates for the solid solution α-MnH_0.06 have been measured over the temperature range 11-297 K and the resonance frequency range 20-90 MHz. A considerable shift and broadening of the proton NMR line and a sharp peak of the spin-lattice relaxation rate are observed near 130 K. These effects are attributed to the onset of antiferromagnetic ordering below the Néel temperature T_N≈130 K. The proton NMR line does not disappear in the antiferromagnetic phase; this suggests a small magnitude of the local magnetic fields at H-sites in α-MnH_0.06. The spin-lattice relaxation rate in the paramagnetic phase is dominated by the effects of spin fluctuations.     

Neutron diffraction study of the magnetic structure of NiCO3 at various temperatures

Neutron diffraction experiments performed on a powdered sample of NiCO₃ show that from 4.5K up to T_a=21K, the antiferromagnetic vector is perpendicular to the trigonal axis. At T_b=25K and up to about 44K, short range magnetic ordering is observed. These results are in good agreement with recent magnetization measurements performed in our laboratory on the same powdered sample.     

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.     

Anomalous magnetic and superconducting properties in a Ru-based double perovskite

We have studied the double perovskite [1] structure Sr₂Y(Ru_1-x Cu_x)O₆ system. The parent compound is an antiferromagnetic insulator with Neel temperature ～ 26 K. Partially substituted the Ru ion by Cu the compounds increase their conductivity drastically and eventually become superconducting. More intriguingly is the observation of the coexistence of superconductivity and magnetic ordering. The superconducting transition temperature T _c and the magnetic ordering temperature T _m are of the same order. The observed magnetic structure and superconductivity of these compounds can be understood in terms of a plausible theoretical model based on the double exchange idea.     

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.     

Thickness dependence of antiferromagnetic phase transition in Heisenberg-type MnPS3

The behavior of 2-dimensional (2D) van der Waals (vdW) layered magnetic materials in the 2D limit of the few-layer thickness is an important fundamental issue for the understanding of the magnetic ordering in lower dimensions. The antiferromagnetic transition temperature T_N of the Heisenberg-type 2D magnetic vdW material MnPS₃ was estimated as a function of the number of layers. The antiferromagnetic transition was identified by temperature-dependent Raman spectroscopy, from the broadening of a phonon peak at 155 cm⁻¹, accompanied by an abrupt redshift and an increase of its spectral weight. T_N is found to decrease only slightly from ~78 K for bulk to ~66 K for 3L. The small reduction of T_N in thin MnPS₃ approaching the 2D limit implies that the interlayer vdW interaction is playing an important role in stabilizing magnetic ordering in layered magnetic materials.     

Phase transition study in α-Fe2WO6 compound by EPR

The temperature dependence of the EPR spectrum for the α-phase of iron tungstate has been investigated in the temperature range of 40–260 K. At temperatures betweenT ₁ ≈ 250 K andT ₂ ≈ 205 K where the antiferromagnetic phase transition occurs, a relatively narrow EPR line arising from the dominant iron(III) species has emerged, gaining intensity with the temperature increase. Its linewidth temperature evolution could be described by Huber equation, with T_N = 200 K, which is consistent with the peak seen in magnetic susceptibility measurements, while the correspondingg-factor shifts to higher fields reflecting the build-up of internal field emerging from increasing shortrange order in the spin system. At temperatures lower than T₂, a very broad and distorted EPR line with temperature dependentg-factor and linewidth has been observed reflecting the corresponding rise of the magnetic susceptibility below the antiferromagnetic phase transition, presumably arising from magnetic clusters embedded in the antiferromagnetic background.     

Spontaneous generation of the electrical voltage in charge-ordered manganite Pr0.6Ca0.4MnO3

Spontaneous generation of the electrical voltage in a Pr_0.6Ca_0.4MnO₃ single crystal has been found. The charge and orbital ordering in the crystal takes place at T _CO = 240 K, while the antiferromagnetic ordering occurs at T _N = 174 K. As the temperature lowers, spontaneous voltage U increases initially slowly (in the temperature range from 300 K to T _CO) and then more rapidly (in the range T _CO-T _N ). Starting from T _N , the voltage U increases exponentially and, at 85 K, reaches 115 mV (in the ab plane) and 6.5 mV (along the c axis). The magnetic field differently affects the voltage U in different temperature ranges: it decreases the value of U in the temperature range of 85–130 K and increases in the range of 130–240 K. It is assumed that the spontaneous voltage is associated with the existence of ferromagnetic and charge orbital ordered clusters of different topologies in the crystal.