Magnetothermoelectric Power Research Articles

Anisotropic Fermi Surfaces, Electrical Transport, and Two-Dimensional Fermi Liquid Behavior in Layered Ternary Boride MoAlB

We report a study of fermiology, electrical anisotropy, and Fermi liquid properties in the layered ternary boride MoAlB, which could be peeled into two-dimensional (2D) metal borides (MBenes). By studying the quantum oscillations in comprehensive methods of magnetization, magnetothermoelectric power, and torque with the first-principle calculations, we reveal three types of bands in this system, including two 2D-like electronic bands and one complex three-dimensional-like hole band. Meanwhile, a large out-of-plane electrical anisotropy (ρbb /ρaa ∼ 1100 and ρbb /ρcc ∼ 500, at 2 K) was observed, which is similar to those of the typical anisotropic semimetals but lower than those of some semiconductors (up to 105). After calculating the Kadowaki–Woods ratio (KWR = A/γ 2), we observed that the ratio of the in-plane A a,c /γ 2 is closer to the universal trend, whereas the out-of-plane A b /γ 2 severely deviates from the universality. This demonstrates a 2D Fermi liquid behavior. In addition, MoAlB cannot be unified using the modified KWR formula like other layered systems (Sr2RuO4 and MoOCl2). This unique feature necessitates further exploration of the Fermi liquid property of this layered molybdenum compound.

Thermoelectric materials taking advantage of spin entropy: lessons from chalcogenides and oxides

ABSTRACT The interplay between charges and spins may influence the dynamics of the carriers and determine their thermoelectric properties. In that respect, magneto-thermoelectric power MTEP, i.e. the measurements of the Seebeck coefficient S under the application of an external magnetic field, is a powerful technique to reveal the role of magnetic moments on S. This is illustrated by different transition metal chalcogenides: CuCrTiS4 and CuMnTiS4 magnetic thiospinels, which are compared with magnetic oxides, Curie-Weiss (CW) paramagnetic misfit cobaltites, ruthenates, either ferromagnetic perovskite or Pauli paramagnet quadruple perovskites, and CuGa1-x Mn x Te2 chalcopyrite telluride and Bi1.99Cr0.01Te3 in which diluted magnetism is induced by 3%-Mn and 1%-Cr substitution, respectively. In the case of a ferromagnet (below TC) and CW paramagnetic materials, the increase of magnetization at low T when a magnetic field is applied is accompanied by a decrease of the entropy of the carriers and hence decreases. This is consistent with the lack of MTEP in the Pauli paramagnetic quadruple perovskites. Also, no significant MTEP is observed in CuGa1-x Mn x Te2 and Bi1.99Cr0.01Te3, for which Kondo-type interaction between magnetic moments and carriers prevails. In contrast, spin glass CuCrTiS4 exhibits negative MTEP like in ferromagnetic ruthenates and paramagnetic misfit cobaltites. This investigation of some chalcogenides and oxides provides key ingredients to select magnetic materials for which S benefits from spin entropy.

Doping effect on the thermoelectric transport properties of HfTe5

We studied the influence of doping HfTe5 with 5% Ti on electric (resistivity and the Hall effect) and thermoelectric transport properties (the Seebeck coefficient, magneto-thermoelectric power, and Nernst effect). The properties of 5% Ti-doped HfTe5 do not change much. Nernst coefficients larger than magneto-thermoelectric power were observed in a temperature range near the compensation temperature at which the Seebeck coefficient vanishes. This indicates that a two-carrier conduction model could describe our experimental results. Owing to the high thermoelectric performance, thermopiles were made on a printed circuit board based on doped and undoped HfTe5. A large Seebeck voltage was obtained at room temperature. It became even larger in a low temperature range and presented strong magnetic field dependence.

Electronic Transport and Magnetic Properties of Co/SiO2 Magnetic Nanocomposites

Electric, thermoelectric, and magnetic properties of the magnetic nanocomposite (MNC) Co/SiO2 consisting of nanometer‐size Co nanoparticles embedded in SiO2 dielectric matrix have been studied. MNC is obtained in the form of (0.8–2.3) μm thick films with Co concentration 31.7–65.4 at.% by the electron beam physical vapor deposition method on polycrystalline Al2O3 substrates. The temperature dependence of the resistivity follows the Mott law. The negative magnetothermoelectric power discovered in MNC Co/SiO2 can be explained by hopping conductivity through the magnetic localization centers in the SiO2 matrix or chemical interaction of Co with Si and O2, which results in a mixture of CoSi (ferromagnetic) and CoO (antiferromagnetic) phases. As the cobalt concentration increases, the particle size increases and the interparticle distances decrease. This is confirmed by the magnetic behavior of the sample with high Co concentration, which is dominated by large Co nanoparticles and strong interparticle magnetic interactions.

Nontrivial Phenomena in Magnetic Nanocomposites Co/Al2O3 and Co/SiO2

Magnetic nanocomposites (MNC), in which nanoparticles of ferromagnetic metals are distributed in wide-gap dielectric matrices (Al2O3 or SiO2), are prospective materials for electronics due to their amenability to technological control of the concentration and size of ferromagnetic nanoparticles. Co/Al2O3 and Co/SiO2 MNC layers with Co concentrations below the percolation threshold were deposited on polycor substrates using electron-beam deposition in a vacuum (EB-PVD). Scanning electron microscopy showed the presence of tightly packed Co grains of irregular shape with sizes of 5–50 nm in MNC. Low-temperature measurements of the magnetization for MNC Co/Al2O3 were made in the temperature range 4–300 K and magnetic fields up to 10 kOe. The ‘magnetic exchange bias’ has been detected, and it increases with a rise in Co concentration. By studying the electrical properties of the MNC Co/Al2O3 and Co/SiO2 in the temperature range of 77–280 K, the Mott type electron transport hopping mechanism was established. We first observed the effect of a giant positive thermoelectric power in a magnetic field in MNC Co/Al2O3 and the effect of a negative magnetothermoelectric power in a Co/SiO2.Magnetic nanocomposites (MNC), in which nanoparticles of ferromagnetic metals are distributed in wide-gap dielectric matrices (Al2O3 or SiO2), are prospective materials for electronics due to their amenability to technological control of the concentration and size of ferromagnetic nanoparticles. Co/Al2O3 and Co/SiO2 MNC layers with Co concentrations below the percolation threshold were deposited on polycor substrates using electron-beam deposition in a vacuum (EB-PVD). Scanning electron microscopy showed the presence of tightly packed Co grains of irregular shape with sizes of 5–50 nm in MNC. Low-temperature measurements of the magnetization for MNC Co/Al2O3 were made in the temperature range 4–300 K and magnetic fields up to 10 kOe. The ‘magnetic exchange bias’ has been detected, and it increases with a rise in Co concentration. By studying the electrical properties of the MNC Co/Al2O3 and Co/SiO2 in the temperature range of 77–280 K, the Mott type electron transport hopping mechanism was established. We...

Anomalous amplitude of the quantum oscillations in the longitudinal magneto-thermoelectric power

Longitudinal magneto-thermoelectric power Syy(y) of a pure bismuth single crystal was measured in magnetic fields up to 8T at several fixed temperatures between 1.4 and 15 K to investigate the magneto-phonon effect in the longitudinal magneto-thermoelectric power (MTP). The oscillation patterns of the longitudinal MTP was similar to that of the longitudinal Shubnikov-de Haas (SdH) effect, expectedly. However, the observed amplitude of oscillations showed a curious temperature dependence. That is, in the range of temperature T > 4.2 K, the amplitude has a maximum around 9K, which is well described by considering the inter-Landau level scattering of electrons. On the contrary, in the range of T < 4.2K, the observed amplitude is enhanced markedly although that of the longitudinal SdH oscillations becomes less pronounced by lowering temperature. This discrepancy may be attributed to the effect of the surface (wrapping) current and to the energy dependence of the electron relaxation time.

Anisotropic magnetothermoelectric power of ferromagnetic thin films

In this article, we report the measurements of the magnetothermoelectric power (MTEP) in metallic ferromagnetic thin films of Ni80Fe20 (Permalloy; Py), Co and CrO2 at temperatures in the range of 100K to 400K. In 25nm thick Py films and 50nm thick Co films both the anisotropic magnetoresistance (AMR) and MTEP show a relative change in resistance and thermoelectric power (TEP) of the order of 0.2% when the magnetic field is reversed, and in both cases there is no significant change in AMR or MTEP after the saturation field has been reached. Surprisingly, both Py and Co films have opposite MTEP behaviour although both have the same sign for AMR and TEP. The data on half metallic ferromagnet CrO2 films show a different picture. Films of thickness of 100nm were grown on TiO2 and on sapphire. The MTEP behavior at low fields shows peaks similar to the AMR in these films, with variations up to 1%. With increasing field both the MR and the MTEP variations keep growing, with MTEP showing relative changes of 1.5% with the thermal gradient along the b-axis and even 20% with the gradient along the c-axis, with an intermediate value of 3% for the film on sapphire. It appears that the low-field effects are due to the magnetic domain state, and the high-field effects are intrinsic to the electronic structure of CrO2 and intergarian tunnelling magnetoresistance that contributes to MTEP as tunnelling-MTEP. Our results will stimulate the research work in the field of spin dependent thermal transport in ferromagnetic materials to further develop spin-Caloritronics.

Extremely large magnetoresistance in the type-II Weyl semimetalMoTe2

We performed the angle dependent magnetoresistance (MR), Hall effect measurements, the temperature dependent magneto-thermoelectric power (TEP) S(T) measurements, and the first-principles calculations to study the electronic properties of orthorhombic phase MoTe2 (Td-MoTe2), which was proposed to be electronically two-dimensional (2D). There are some interesting findings about Td-MoTe2: (1) A scaling approach {\epsilon}{\theta}=(sin2{\theta}+{\gamma}-2cos2{\theta})1/2 is applied, where {\theta} is the magnetic field angle with respect to the c axis of the crystal and {\gamma} is the mass anisotropy. Unexpectedly, the electronically 3D character with {\gamma} as low as 1.9 is observed in Td-MoTe2; (2) The possible Lifshitz transition and the following electronic structure change can be verified around T~150 K and T~60 K, which is supported by the evidence of the slop changing of the temperature dependence of TEP, the carrier density extracted from Hall resistivity and the onset temperature of {\gamma} obtained from the MR measurements. The extremely large MR effect in Td-MoTe2 could originate from the combination of the electron-hole compensation and a particular orbital texture on the electron pocket, which is supported by the calculations of electronic structure. Our results may provide a general scaling relation for the anisotropic MR and help to recognize the origins of the MR effect in other systems, such as the Weyl semimetals and the Dirac ones.

Spin caloritronics, origin and outlook

Spin caloritronics refers to research efforts in spintronics when a heat current plays a role. In this review, we start out by reviewing the predictions that can be drawn from the thermodynamics of irreversible processes. This serves as a conceptual framework in which to analyze the interplay of charge, spin and heat transport. This formalism predicts tensorial relations between vectorial quantities such as currents and gradients of chemical potentials or of temperature. Transverse effects such as the Nernst or Hall effects are predicted on the basis that these tensors can include an anti-symmetric contribution, which can be written with a vectorial cross-product. The local symmetry of the system may determine the direction of the vector defining such transverse effects, such as the surface of an isotropic medium. By including magnetization as state field in the thermodynamic description, spin currents appear naturally from the continuity equation for the magnetization, and dissipative spin torques are derived, which are charge-driven or heat-driven. Thermodynamics does not give the strength of these effects, but may provide relationships between them. Based on this framework, the review proceeds by showing how these effects have been observed in various systems. Spintronics has become a vast field of research, and the experiments highlighted in this review pertain only to heat effects on transport and magnetization dynamics, such as magneto-thermoelectric power, or the spin-dependence of the Seebeck effect, the spin-dependence of the Peltier effect, the spin Seebeck effect, the magnetic Seebeck effect, or the Nernst effect. The review concludes by pointing out predicted effects that are yet to be verified experimentally, and in what novel materials the standard thermal spin effects could be investigated.

Magnetothermoelectric power in Co/Pt layered structures: Interface versus bulk contributions

The dependence of the longitudinal thermoelectric power on the orientation of magnetization in Pt/Co/Pt sandwiches is investigated. In the Co thickness range $\ensuremath{\le}6$ nm, where interface scattering is relevant, the thermoelectric power depends on the orientation of magnetization in the plane perpendicular to the temperature gradient. This behavior reveals the thermoelectric analog to the anisotropic interface magnetoresistance. It is shown that this interfacial magnetothermoelectric power fulfills Mott's formula, however, significant deviations from the bulk anisotropic magnetothermoelectric behavior are reported. The dependence of the Seebeck effect on magnetization orientation therefore provides experimental evidence of differences in the electronic states of the bulk and interface. We demonstrate that for the very same system the Seebeck coefficient does not show a one-to-one correspondence to the conductivity.

Simple Theoretical Analysis of the Magneto Thermoelectric Power in Semiconductor Quantum Dots of Heavily Doped Non-Parabolic Materials

Simple Theoretical Analysis of the Magneto Thermoelectric Power in Semiconductor Quantum Dots of Heavily Doped Non-Parabolic Materials

The angular magnetothermoelectric power of a charge density wave system

The angular dependence of the magnetothermopower of a charge transfer organic salt α-(ET)2KHg(SCN)4 below (4 K) and above (9 K) the phase transition temperature, Tp = 8 K, and under fields of 15 T and 25 T, below and above the ‘kinkfield’, has been studied. We find that for a longitudinal thermoelectric measurement both an interlayer thermopower (the Seebeck effect), Szz, and a transverse thermopower (the Nernst effect), Syz, exist in all three different B–T phases (the CDW 0, CDW x and metallic states) with large amplitude. Both thermoelectric effects display a resonant-like behavior without a sign reversal at the angles corresponding to angular magnetoresistance oscillation minima and maxima in this compound. The resonant behavior is most evident in the CDW0 state, indicating a mechanism involving the Fermi surface nesting. Angular dependences reveal different behaviors of the thermopower and Nernst effect in the high magnetic field (CDWx) state.

Spin caloritronics in magnetic tunnel junctions:Ab initiostudies

This Letter presents ab initio calculations of the magneto-thermoelectric power (MTEP) and of the spin-Seebeck coefficient in MgO based tunnel junctions with Fe and Co leads. In addition, the normal thermopower is calculated and gives for pure Fe and Co an quantitative agreement with experiments. Consequently, the calculated values in tunnel junctions are a good estimation of upper limits. In particular, spin-Seebeck coefficients of more than 100 \mu V/K are possible. The MTEP ratio exceed several 1000% and depends strongly on temperature. In the case of Fe leads the MTEP ratio diverges even to infinity at certain temperatures. The spin-Seebeck coefficient as a function of temperature shows a non-trivial dependence. For Fe/MgO/Fe even the sign of the coefficient changes with temperature.

Magnetotransport in iodine-doped single-walled carbon nanotubes

Magnetoresistance (MR) and magnetothermoelectric power (MTEP) of iodine-doped single-walled carbon nanotubes (I@SWNT) under magnetic fields up to 14 T are investigated from room temperature (300 K) down to 1.6 K. Our results on resistivity and thermoelectricpower (TEP) in a zero magnetic field are similar to those reported by Grigorian et al. [Phys. Rev. Lett. 80, 5560 (1998)] The positive sign of the TEP values indicates that the majority of the carriers in the I@SWNT are holes. The broad enhancement of TEP at temperatures of 30\char21{}200 K shows quasilinear temperature dependence and is consistent with sharply varying density of states near the Fermi level with additional contribution from the spin-orbit scattering in the normal metallic characteristics of the I@SWNT. For $Tl7\text{ }\text{K}$, MR is negative and it decreases with ${H}^{2}$ followed by the ${H}^{1/2}$ dependence at around $H=2\text{ }\text{T}$ which is characteristic for the weak localization. In the range $7\text{ }\text{K}lTl70\text{ }\text{K}$, MR is positive at low magnetic field and becomes negative at higher magnetic field. The negative MR in the high magnetic fields decreases linearly. At $T\ensuremath{\ge}\ensuremath{\sim}100\text{ }\text{K}$, MR is positive up to 14 T, which could be the result of spin-orbit scattering in the I@SWNT. The MTEP decreases under magnetic field at $Tl90\text{ }\text{K}$. The reduction in MTEP is originated from the delocalization of electron wave functions under the magnetic field. At $Tg90\text{ }\text{K}$, the thermal fluctuation dominates the effect of magnetic fields resulting the MTEP to be the same as the zero magnetic field TEP.

Temperature dependent conductivity and thermoelectric power of the iodine doped poly(vinyl alcohol)–Cu 2+ chelate

We have investigated the temperature dependence of electrical conductivity and thermoelectric power (TEP) at 1.7 K < T < 300 K in an organo metallic complex, the iodine doped poly(vinyl alcohol)–Cu 2+ chelate. We observed intrinsic metallic temperature dependence of resistivity from room temperature to 68 K with a broad minimum [ ρ(68 K)/ ρ(300 K) ∼0.75], which has not been observed previously in similar organo metallic complexes. There occurs an unusual metal-insulator transition at T ∼68 K and the resisitivity increases upon cooling below 68 K. However, the low temperature resistivity becomes finite (instead of going to infinity), [ρ(1.7 K)/ ρ(300 K) ∼0.98] indicating that a quantum mechanical tunneling conduction is dominant at this low temperature. It is remarkable that the resistivity at 1.7 K is as small as that of room temperature. Such unusual temperature dependence of conductivity could be understood as thermally assisted hopping conduction between metallic islands. However, the observed intrinsic metallic temperature dependence of resistivity implies that such hopping conduction barrier is not important at high temperature ( T > 68 K). The intrinsic metallic characteristics are confirmed by the quasi-linear temperature dependence of TEP for the whole measured temperature range (1.7 K < T < 300 K) with a small slope change at low temperature, T < 68 K, which is understood as an effect of variable range hopping (VRH) conduction at low temperature. The results of magneto resistance (MR) and magneto thermoelectric power (MTEP) are consistent with the above interpretation.

Thermoelectric power in the double exchange model

Employing the Monte Carlo method and the exact diagonalization, we have investigated the temperature dependence of the thermoelectric power (TEP) for the double exchange model in the dilute carrier concentration limit. We have found that the TEP follows the Heikes formula in the high temperature regime, whereas, in the intermediate temperature regime, the TEP is suppressed by the exchange coupling between itinerant electrons and local spins. In the low temperature regime, the TEP exhibits an anomalous peak and dip feature near the magnetic transition temperature TC which can be understood based on the magnetic polaron state. We have also found that the TEP, in the presence of the magnetic field, shows the positive magnetothermoelectric power near TC.

The magnetothermoelectric power in the Y- and Ho-doped La 0.9Te 0.1MnO 3

We present the results of a systematic study of thermoelectric power S ( T ) at zero field and applied magnetic field in the nonmagnetic Y- and magnetic Ho-doped manganites La 0.9− x Y (Ho) x Te 0.1MnO 3 ( 0.05 ≤ x ≤ 0.15 ). For x ≤ 0.10 samples, an additional positive magnetothermoelectric power (MTEP) is induced by Y and Ho doping below Curie temperature T C besides a large negative MTEP in the vicinity of T C . The Ho-doped compounds have larger resistivity ρ and larger S compared to the Y-doped compounds in the same concentration. The magnetic disorder introduced by Ho doping can play an important role in affecting the electronic and thermal transport properties of the sample.

Transport mechanism and magnetothermoelectric power of electron-doped manganites La0.85Te0.15Mn1−xCuxO3 (⩽x⩽0.20)

We present the results of a systematic study of the transport mechanism and magnetothermoelectric power (MTEP) of electron-doped manganites La0.85Te0.15Mn1−xCuxO3 (0⩽x⩽0.20). Two peaks are observed in thermoelectric power S(T) curves for x&amp;lt;0.10 samples. For x&amp;gt;0.10 samples, the very large S value with over 100μV∕K at low temperatures appears, which is attributed to the destruction of ferromagnetic (FM) order and the strong carrier localization at low temperatures due to Cu doping. In addition, a sign variation of S(T) for Cu-doped samples is also observed, which may originate from the narrowing of the concomitant σ(eg↑−2p) band. Particularly, an anomalous behavior of S(T) is observed in x=0.10 sample, which is suggested to be related to the contribution of spin polarization and phonon drag. Based on the results of resistivity ρ(T) and S(T), the transport mechanism in the high-temperature paramagnetic region for all the samples and low-temperature FM insulating region below TC for the samples with x⩽0.10 can be described by the variable-range-hopping model. However, in the intermediate-temperature FM metallic region below TC, ρ(T) and S(T) of the samples with x⩽0.10 are well fitted by the formula ρ=ρ0+ρ2.5T2.5 and S=S0+S3∕2T3∕2+S4T4, respectively, implying the importance of electron-magnon scattering. As to the MTEP, only a negative MTEP peak close to TC is observed in the whole measured temperature range for the samples with x&amp;gt;0.10, which is suggested to originate from the spin alignment induced by applied magnetic fields. However, for x⩽0.10 samples, an additional positive MTEP peak is induced by Cu doping below TC besides a large negative MTEP peak in the vicinity of TC, which is ascribed to the enhancement of electron-magnon interaction caused by the Cu doping and the decrease of magnetic entropy around TC, respectively.

Diamagnetism, transport, magnetothermoelectric power, and magnetothermal conductivity in electron-doped CaMn1−xVxO3 manganites

The effects of V doping on field-cooled magnetization MFC(T), zero-field-cooled magnetization MZFC(T), resistivity ρ, thermoelectric power S, and thermal conductivity κ in manganites CaMn1−xVxO3 (0.02⩽x⩽0.08) have been investigated systematically. As the V doping level exceeds 0.02, an anomalous “diamagnetism” has been observed. It is suggested that the force generated by the orbit rotation of eg electron in Mn3+O6 octahedron makes the spin tilt, as a result, the vector sum of individual spins may be along or opposite to the direction of the applied magnetic field, and macroscopically, the average magnetization exhibits positive or negative values. In addition, the transport mechanism in the high and low temperature ranges is dominated by the small polaron conduction and the variable-range-hopping conduction, respectively, according to the fitting analysis of the temperature dependence of Seebeck coefficient S(T) and resistivity ρ(T). Both S and κ peaks appearing at low temperature is gradually suppressed by V doping. Additionally, obvious magnetothermoelectric power and magnetothermal conductivity are observed in the temperature region which an anomalous diamagnetism appears. The results are discussed based on spin-orbital coupling and spin-phonon coupling induced due to V doping, respectively. Moreover, the large thermoelectric figure of merit Z=S2∕ρκ for the slightly V-doped sample suggests that the V-doped manganite may be a good candidate for thermoelectric materials.

Anisotropic magnetothermopower: Contribution of interband relaxation

Spin injection in metallic normal/ferromagnetic junctions is investigated taking into account interband relaxation and the consequences in terms of thermoelectric power. On the basis of a generalized two-channel model, it is shown that there is an interface resistance and thermoelectric power contribution due to anisotropic scattering, besides spin accumulation and giant magnetoresistance. The corresponding expression of the thermoelectric power is derived and compared with the expression accounting for the thermoelectric power produced by the giant magnetoresistance. Measurements of anisotropic magnetothermoelectric power are presented in electrodeposited Ni nanowires contacted with Ni, Au, and Cu. It is shown that a thermoelectric power is generated at the interfaces of the nanowire and that the experimental results strongly support the model.