Magnetothermoelectric Power Research Articles

The Nernst-Ettingshausen coefficient in hole-doped manganites

A systematic study of the Nernst-Ettingshausen coefficient (NEC) in hole-doped manganites of the LaMnO3 and SmMnO3 systems was carried out at temperatures both above and below the point of transition to the magnetically ordered state (Tc). The results obtained for T > Tc suggest that conduction is mediated here by small-radius polarons. For all the compositions studied, the Nernst mobilities at T = 300 K are small (of the order of 0.1–2 cm2/V s) and the carrier relaxation time at T > Tc increases with carrier energy. At temperatures below Tc, the NEC exhibits an anomalous behavior. The giant NEC effect was observed, which consists in a strong dependence of the NEC on applied magnetic field and the presence of a peak in the temperature dependence of the NEC at T ≈ Tc. Near the transition to the magnetically ordered state, the NEC follows a behavior similar to that of colossal magnetoresistance and giant magnetothermoelectric power. A possible origin of the anomalous NEC behavior at T < Tc is discussed.

MAGHETOTHERMO POWER OF ZnO FILMS

The Magnetothermoelectric and thermoelectric power of nano- ZnO films was investigated. The ZnO films in this study were prepared by DC reactive sputtering using a Zn target (99.99%) containing AL of 1.5%. The films obtained were characterized by SEM, x-ray diffraction, optical and electrical measurements. It was found that the sputtering ZnO films were highly orientation growth with the c-axis perpendicular to the substrate surface. The measurements showed that there was a striking seebeck effect in the ZnO films, and their thermoelectric power was linearly increased with increasing temperature. The experimental results were also demonstrated that the thermoelectric power was degraded under the magnetic field. This finding may ascribe to the magneto resistive effect.

Development of pulsed magnetic field and study of magnetotransport properties of K-doped La 1– xCa x– yK yMnO 3 CMR materials

Temperature-dependent magnetization, magnetoresistance and magneto-thermoelectric power of the K doped La 1− x Ca x− y K y MnO 3 type samples with x=0.3 and 0⩽ y⩽0.15 has been studied. All the samples exhibit sharp metal–insulator transition (MIT) around T p accompanied by a ferromagnetic (metallic) to paramagnetic (semiconducting) phase transition with a well-defined Curie temperature T C (almost equal to T p). Doping of monovalent K in the divalent Ca site of La 1− x Ca x− y K y MnO 3 drives the system from a high resistivity regime with lower T p to a lower resistivity regime with higher T p. Systematic increase of Curie temperature with increase of K doing is observed from the magnetization measurement down to 5 K. Low temperature resistivity ( ρ) and thermoelectric power (Seebeck coefficient, S) data well fit the relations ρ= ρ 0+ ρ 2 T 2 and S= S 0+ S 3/2 T 3/2+ S 4 T 4, respectively, signifying the importance of electron–magnon scattering process ( ρ 2 T 2 and S 3/2 T 3/2 term). On the other hand, the high temperature ( T> T p upto 320 K) conductivity data satisfy the variable range hopping (VRH) model. For T>320 K small polaron hopping model is more appropriate than the VRH model. High temperature thermoelectric power (TEP) data also indicates the formation of thermally activated small polarons. Even with very small change of y, the density of states at the Fermi level N( E F) changes considerably. The magnetotransport properties have been measured under pulsed magnetic field of microsecond duration. The decay time of the magnetic pulse within the sample ( ι) varies with field strength, which indicates that with change of magnetic field, ordering of the spin in the ferromagnetic regime changes.

Magnetothermoelectric power ofPr0.5Sr0.5MnO3with Ru substitution at the Mn site

The effects of Ru substitution at the Mn site for the manganite series of ${\mathrm{Pr}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Ru}}_{x}{\mathrm{O}}_{3}$ $(0.0l~xl~0.1)$ have been studied using magnetothermoelectric power (TEP) along with resistivity and magnetization. Systematic changes of TEP were observed with variation of Ru concentration: suppression of the large negative peak at low temperatures, decrease of curvature with ${T}_{C}$ enhancement at intermediate temperatures, and increase of the value of TEP at high temperatures. These changes of TEP with Ru substitution and an applied magnetic field are analyzed in terms of broad-band metallic transport $(Tl170\mathrm{K}),$ magnon scattering processes $(170\mathrm{K}lTl{T}_{C}),$ and polaronic transport $(Tg{T}_{C}).$ From these results, it was found that the ferromagnetism is more strongly induced by Ru substitution than by an applied magnetic field, suggesting the important role of the Ru d orbitals in the charge transport of the ${\mathrm{Mn}}^{3+}\ensuremath{-}\mathrm{O}\ensuremath{-}{\mathrm{Mn}}^{4+}$ network.

Estimation of the charge carrier localization length from Gaussian fluctuations in the magneto-thermopower ofLa0.6Y0.1Ca0.3MnO3

The magneto-thermoelectric power (TEP) $\Delta S(T,H)$ of perovskite type manganise oxide $La_{0.6}Y_{0.1}Ca_{0.3}MnO_3$ is found to exhibit a sharp peak at some temperature $T^{*}=170K$. By approximating the true shape of the measured magneto-TEP in the vicinity of $T^{*}$ by a linear triangle of the form $\Delta S(T,H)\simeq S_p(H)\pm B^{\pm}(H)(T^{*}-T)$, we observe that $B ^{-}(H)\simeq 2B ^{+}(H)$. We adopt the electron localization scenario and introduce a Ginzburg-Landau (GL) type theory which incorporates the two concurrent phase transitions, viz., the paramagnetic-ferromagnetic transition at the Curie point $T_C$ and the "metal-insulator" (M-I) transition at $T_{MI}$. The latter is characterized by the divergence of the field-dependent charge carrier localization length $\xi (T,H)$ at some characteristic field $H_0$. Calculating the average and fluctuation contributions to the total magnetization and the transport entropy related magneto-TEP $\Delta S(T,H)$ within the GL theory, we obtain a simple relationship between $T^{*}$ and the above two critical temperatures ($T_{C}$ and $T_{MI}$). The observed slope ratio $B ^{-}(H)/B ^{+}(H)$ is found to be governed by the competition between the electron-spin exchange $JS$ and the induced magnetic energy $M_sH_0$. The comparison of our data with the model predictions produce $T_{C}=195K$, $JS=40meV$, $M_0=0.4M_s$, $\xi_0=10\AA$, and $n_e/n_i=2/3$ for the estimates of the Curie temperature, the exchange coupling constant, the critical magnetization, the localization length, and the free-to-localized carrier number density ratio, respectively.

Probing the field-induced variation of the chemical potential in Bi2Sr2CaCu2Oy via magneto-thermopower measurements

Approximating the shape of the magneto-thermoelectric power (TEP) ΔS(T,H) measured in Bi2Sr2CaCu2Oy by an asymmetric linear triangle of the form ΔS(T,H)≃S p (H)±B ±(H)(T c −T) with positive B −(H) and B +(H) defined below and above T c , we observe that B +(H) ≃2B −(H). To account for this asymmetry, we explicitly introduce the field-dependent chemical potential μ(H) of holes into the Ginzburg-Landau theory and calculate both an average ΔS av(T,H) and fluctuation contribution ΔS fl(T,H) to the total magneto-TEP ΔS(T,H). As a result, we find a rather simple relationship between the field-induced variation of the chemical potential in this material and the above-mentioned magneto-TEP data around T c , viz. Δ μ(H)∝S p (H).

Magneto thermoelectric power of the doped polyacetylene

Abstract Thermoelectric power (TEP) and electrical conductivity of the metallic polyacetylene (PA) doped with AuCl 3 and I 2 are measured under high magnetic field up to 17 Tesla. We have observed the following two remarkable behaviors in the TEP results; (1) the temperature dependence of TEP for the iodine and metal-halide doped polyacetylene show clear difference at T

Sign of thermoelectric power in Co/Cu and Fe/Cr multilayers

We analyze the thermoelectric power (TEP) of Co/Cu and Fe/Cr multilayers in terms of the Mott formula, which relates TEP and energy dependent electric conductivity. The conductivity of the multilayers is treated using the model proposed by Tsymbal and Pettifor, Phys. Rev. B 54, 15 314 (1996). The model is based on a quantum-mechanical formulation for the electron transport within a realistic spd tight-binding approximation for the band structure and spin-independent disorder in the on-site atomic energy levels. We find that for a disorder, producing a realistic resistivity of the multilayers, the thermoelectric power is positive for Fe/Cr multilayers, but negative for Co/Cu multilayers, which is consistent with experiments. The difference in the sign of TEP is explained in terms of the difference in the electronic band structures for Co/Cu and Fe/Cr multilayers. We also show that the model accounts for the experimentally observed sign of the magnetothermoelectric power at room temperature, when the magnetizations of the consecutive ferromagnetic layers change their alignment.

The Sign of Magneto-Thermoelectric Power of Magnetic Granular Alloys

The thermoelectric power S and magneto-thermoelectric power δ S = S ( H )- S (0) were measured for Fe–Cu and Fe–Ag granular alloys. The sign of both S and δ S of the measured samples was confirmed to be negative, similar to that of all the other granular alloys, contrary to the theories proposed hitherto. We point out the importance of the Kondo effect of the magnetic impurities between the magnetic and non-magnetic clusters, and we show we can understand successfully the sign of both S and δ S from this standpoint.

On the Sign of Thermoelectric Power of GMR Multilayers

The thermoelectric power S of artificial magnetic multilayers of Fe/Au has been measured as functions of temperature T as well as magnetic field H . The magneto-thermoelectric power (MTEP) was found to have a negative sign. In addition, data of S of magnetic multilayers of Co/Cu hitherto published were reexamined in order of clarify its sample dependence, and MTEP for these multilayers was ascertained to have also a negative sign. The negative sign of MTEP of the two systems are discussed in relation to the theory developed by Inoue et al .

Transport properties in granular Co-Ag alloys

We have investigated the field dependence of the magnetoresistance, Hall effect, thermal conductivity and thermoelectric power on Co-Ag granular films made by electron gun evaporation under ultrahigh-vacuum conditions. A large field dependence correlated with the giant magnetoresistance is observed in all the transport properties. The dependence is anisotropic for 42at.% Co-Ag between parallel and perpendicular fields. At zero field, a resistivity minimum is found that varies with annealing temperature. The behaviours of the magnetothermal conductivity and the magnetothermoelectric power are generally consistent with previous studies. The extraordinary Hall resistivity exhibits an anomalous field dependence and changes sign with temperature.

Magnetothermal transport properties of granular Co-Ag solids.

Thermal conductivity and thermoelectric power have been measured between 2 K and 300 K on granular ${\mathrm{Co}}_{20}$${\mathrm{Ag}}_{80}$ solids. As in ordinary metals, thermal conductivity is largely dominated by the electronic contribution. The Wiedemann-Franz law is found to be satisfied over the whole temperature range investigated, which is mainly ascribed to dominant large-angle elastic scattering processes. Giant magnetothermal conductivity has been observed in accordance with the giant magnetoresistance effect. The magnetothermoelectric power is negative and increases with temperature.

The anomalies of the longitudinal magnetothermoelectric power of metal in the vicinity of electronic topological transition

The longitudinal magnetothermoelectric power (MTEP) is calculated for metal in magnetic field and in the vicinity of electronic topological transition (ETT). The giant oscillations of MTEP as a function of applied magnetic field are found. These oscillations originate from the energy dependence of electron relaxation time in magnetic field and they are peculiar to any normal metal independently of the shape of its Fermi surface. Nevertheless the ETT essentially changes the form and magnitude of such oscillations. The results of recent experimental investigation of MTEP in cadmium monocrystals under the pressure and in magnetic field are discussed from the point of view of the presented theory.

Space-time symmetry restrictions on the form of transport tensors. II. Thermomagnetic effects

For pt.I see abstr. A50562 of 1975. The forms of the magnetothermoelectric power tensor alpha ij(B) for each of the 32 point groups have been obtained by use of the transformation law for field-dependent tensors in conjunction with Neumann's principle. These forms provide a fundamental base for studies of the thermomagnetic effects; the even and odd terms of alpha ij(B) in magnetic induction B are defined as the magneto-Seebeck and Nernst effects respectively. The symmetry restrictions imposed in current expressions for the phonon-drag and diffusion thermopowers are shown to produce different forms for alpha ij(B); these two contributions to the magnetothermoelectric power can be separated by a detailed study of certain components of alpha ij(B). An an example, the form taken by alpha ij(B) for bismuth is discussed. It is shown from both phenomenological and band-structure considerations that the appearance of the Umkehr effect in alpha ij(B) is not anomalous but is a direct result of crystal anisotropy.

Low-temperature magnetothermoelectric power of aluminum

Measurements of the low-temperature magnetothermoelectric power of aluminum and a number of aluminum alloys show that the electron-diffusion component S d increases and then saturates with increasing magnetic field. The difference ΔS d between the high- and zero-field values of S d is observed to be independent of the type of impurity in the aluminum and has the value (2.2±0.2) T×10 −8 V/ K. These observations are interpreted in terms of a realistic semiclassical analysis of the thermopower. This treatment gives an excellent account of the qualitative behavior of the thermopower in a magnetic field but yields a value of ΔS d which is 30% lower than the observed value. Possible reasons for this descrepency are discussed.

Magnetic symmetry and transport properties of crystals

The forms of the galvanomagnetic and thermomagnetic property tensors appropriate to the 122 generalized crystal classes are discussed in the light of Kleiner's (1966-67) modified Onsager relations. The forms of the tensors of fourth rank are derived and discussed. The schemes for the grey groups are identical with those of the photoelastic tensor of the corresponding groups while the schemes for the single colour groups are similar to those of the magnetothermoelectric power tensor for the corresponding groups. 21 of the 58 two colour groups which contain the symmetry element 1 have schemes identical with those of the corresponding grey groups. The schemes for the remaining 37 groups which do not contain the symmetry element 1 are derived and presented here.

Electron effective-mass values in GaxIn1−xSb alloys

Room-temperature measurements of Faraday rotation, magneto–thermoelectric power, and infrared reflectance have been made on homogeneous coarsely polycrystalline n-type samples of GaxIn1−xSb alloys produced by the horizontal Bridgman technique. Using these data and a Kane equation for the (000) conduction band, values of the bottom of the band effective mass m00* have been determined over the composition range 0 &lt; x &lt; 0.85, i.e. the range in which the subsidiary [Formula: see text] minima make no contribution. The results from the three different techniques show very good agreement. From the data, values are obtained also for the square of the matrix element P2, the high-frequency dielectric constant ε∞, and the scattering parameter s for the various alloy specimens.

Free-carrier Faraday rotation in InAsxSb1−x alloys

Room-temperature free-carrier Faraday rotation measurements in the wavelength range 6–25 μ and Hall-coefficient measurements have been made on polycrystalline n-type samples of InAsxSb1−x alloys of carrier concentration ~1017/cm3. Using these data and a Kane equation for the conduction band, values of the bottom of the band effective mass m0* have been determined over the whole alloy range. The results are compared with similar data from magnetothermoelectric power measurements and also with previously calculated data. From the m0* results, values of the square of the matrix element P2 have been calculated as a function of x.

Magneto-Thermoelectric Power and Magnetostriction of FeNi3 Single Crystals

The change in thermoelectric power due to magnetization of ferromagnetic substance, i. e. the magneto-thermoelectric power, and linear magnetostriction at given temperatures are measured on single crystals of Fe-Ni alloy (about 24% Fe). The single crystal behaviour of magneto-thermoelectric power in these f. c. c. crystals are similar to that of magnetostriction. The constants (Em)100, (Em)111 and λ100, λ111 which specify magneto-thermoelectric power and linear magnetostriction in the [100] and [111] directions, respectively, have been determined. The magneto-thermoelectric power depends on the orientation of the crystal axes and is always negative. The numerical values obtained are as follows: (Em)100=-0.84×10-6 volt/degree, (Em)111=-0.53×10-6 volt/degree and λ100=8.4×10-6, λ111=1.3×10-6.

Galvano- and Thermomagnetic Effects in α-Ag2Te

Nernst coefficient, magnetoresistance, and magnetothermoelectric power together with electrical conductivity, Hall coefficient, and thermoelectric power of α-Ag 2 Te are measured as functions of the e.m.f., E of the galvanic cell Ag|AgI|Ag 2 Te|Pt, where E may be considered to represent the relative position of the Fermi level in the Ag 2 Te specimen. The experimental results exhibit consistent agreement with the Lorentz-Sommerfeld theory for degenerate two-band semiconductors with energy independent mean free paths of carriers. Relative thermal conductivity is also measured, showing minimum at a E value expected from the theory.