Magnetothermal Transport Research Articles

Magneto-thermal transport indicating enhanced Nernst response in FeCo/IrMn exchange coupled stacks

We present an analysis of magneto-thermal transport data in IrMn/FeCo bilayers based on the Mott relation and report an enhancement of the Nernst response in the vicinity of the blocking temperature. We measure all four transport coefficients of the longitudinal resistivity, anomalous Hall resistivity, Seebeck effect, and anomalous Nernst effect, and we show a deviation arising around the blocking temperature between the measured Nernst coefficient and the one calculated using the Mott rule. We attribute this discrepancy to spin fluctuations at the antiferromagnet/ferromagnet interface near the blocking temperature. The latter is estimated by magnetometry and magneto-transport measurements.

Measurements of thermoelectric figure of merit based on multi-harmonic thermal analysis of thermographic images

We demonstrate a versatile measurement method for the thermoelectric figure of merit and related transport properties by means of a multi-harmonic thermal analysis of a thermographic movie. The method is based on the thermal analyses of the charge-current-induced temperature distribution generated by the Peltier effect at the first harmonic and by Joule heating at the second harmonic, measured with an infrared camera. This allows simultaneous estimation of the thermal diffusivity, thermal conductivity, volumetric heat capacity, and Peltier/Seebeck coefficient of conductors without attaching an external heater. The thermal analysis developed here is applicable to a system with the interfacial thermal resistance between the target conductor and reference material. Our method enables the measurements while applying an external magnetic field, opening the way for investigating the magnetic field and/or magnetization dependences of the figures of merit and associated properties for the magneto-thermoelectric effects. We demonstrate the usability of this method by estimating the figures of merit for the Peltier/Seebeck, magneto-Peltier/Seebeck, and Ettingshausen/Nernst effects in a Bi–Sb alloy as a promising material for thermoelectric applications. The multi-harmonic thermal analysis method will, thus, aid in developing highly efficient thermoelectric materials and further investigations of magneto-thermal and magneto-thermoelectric transport properties.

Berry curvature induced magnetotransport in 3D noncentrosymmetric metals

We study the magnetoelectric and magnetothermal transport properties of noncentrosymmetric metals using semiclassical Boltzmann transport formalism by incorporating the effects of Berry curvature (BC) and orbital magnetic moment (OMM). These effects impart quadratic-B dependence to the magnetoelectric and magnetothermal conductivities, leading to intriguing phenomena such as planar Hall effect, negative magnetoresistance (MR), planar Nernst effect and negative Seebeck effect. The transport coefficients associated with these effects show the usual oscillatory behavior with respect to the angle between the applied electric field and magnetic field. The bands of noncentrosymmetric metals are split by Rashba spin–orbit coupling except at a band touching point (BTP). For Fermi energy below (above) the BTP, giant (diminished) negative MR is observed. This difference in the nature of MR is related to the magnitudes of the velocities, BC and OMM on the respective Fermi surfaces, where the OMM plays the dominant role. The absolute MR and planar Hall conductivity show a decreasing (increasing) trend with Rashba coupling parameter for Fermi energy below (above) the BTP.

High-throughput imaging measurements of thermoelectric figure of merit

We demonstrate a method for the simultaneous determination of the thermoelectric figure of merit of multiple martials by means of the lock-in thermography (LIT) technique. This method is based on the thermal analyses of the transient temperature distribution induced by the Peltier effect and Joule heating, which enables high-throughput estimation of the thermal diffusivity, thermal conductivity, volumetric heat capacity, Seebeck or Peltier coefficient of the materials. The LIT-based approach has high reproducibility and reliability because it offers sensitive noncontact temperature measurements and does not require the installation of an external heater. By performing the same measurements and analyses with applying an external magnetic field, the magnetic field and/or magnetization dependences of the Seebeck or Peltier coefficient and thermal conductivity can be determined simultaneously. We demonstrate the validity of this method by using several ferromagnetic metals (Ni, Ni$_{95}$Pt$_{5}$, and Fe) and a nonmagnetic metal (Ti). The proposed method will be useful for materials research in thermoelectrics and spin caloritronics and for investigation of magneto-thermal and magneto-thermoelectric transport properties.

Magneto-thermal transport implies an incoherent Hall conductivity

We consider magnetohydrodynamics with an external magnetic field. We find that in general one must allow for a non-zero incoherent Hall conductivity to correctly describe the DC longitudinal and Hall thermal conductivities beyond order zero in the magnetic field expansion. We apply our result to the dyonic black hole, determining the incoherent Hall conductivity in that case, and additionally prove that the existence of this transport coefficient leads to a significantly better match between the hydrodynamic and AC thermo-electric correlators.

Thickness dependence of the anomalous Nernst effect and the Mott relation of Weyl semimetal Co2MnGa thin films

We report a robust anomalous Nernst effect in Co2MnGa thin films in the thickness regime between 20 and 50 nm. The anomalous Nernst coefficient varied in the range of -2.0 to -3.0 uV/K at 300 K. We demonstrate that the anomalous Hall and Nernst coefficients exhibit similar behavior and fulfill the Mott relation. We simultaneously measure all four transport coefficients of the longitudinal resistivity, transversal resistivity, Seebeck coefficient, and anomalous Nernst coefficient. We connect the values of the measured and calculated Nernst conductivity by using the remaining three magneto-thermal transport coefficients, where the Mott relation is still valid. The intrinsic Berry curvature dominates the transport due to the relation between the longitudinal and transversal transport. Therefore, we conclude that the Mott relationship is applicable to describe the magneto-thermoelectric transport in Weyl semimetal Co2MnGa as a function of film thickness.

Berry curvature induced thermopower in type-I and type-II Weyl semimetals

Berry curvature acts analogous to a magnetic field in the momentum-space, and it modifies the flow of charge carriers and entropy. This induces several intriguing magnetoelectric and magnetothermal transport phenomena in Weyl semimetals. %, including the planar Hall and planar Nernst effect. Here, we explore the impact of the Berry curvature and orbital magnetization on the thermopower in tilted type-I and type-II Weyl semimetals, using semiclassical Boltzmann transport formalism. We analytically calculate the full magnetoconductivity matrix and use it to obtain the thermopower matrix for different orientations of the magnetic field $(B)$, with respect to the tilt axis. We find that that the tilt of the Weyl nodes induces linear magnetic field terms in the conductivity matrix, as well as in the thermopower matrix. The linear-$B$ term appears in the Seebeck coefficients, when the $B$-field is applied along the tilt axis. Applying the magnetic field in a plane perpendicular to the tilt axis results in a quadratic-$B$ planar Nernst effect, linear-$B$ out-of-plane Nernst effect and quadratic-$B$ correction in the Seebeck coefficient.

Linear magnetochiral transport in tilted type-I and type-II Weyl semimetals

Berry curvature in Weyl semimetals results in very intriguing magneto-electric and magneto-thermal transport properties. In this paper, we explore the impact of the tilting of the Weyl nodes, which breaks the time-reversal symmetry, on the magneto-electric conductivity of type-I and type-II Weyl semimetals. Using the Berry curvature connected Boltzmann transport formalism, we find that in addition to the quadratic-B corrections induced by the tilt, there are also anisotropic and B-linear corrections in several elements of the conductivity matrix. For the case of magnetic field applied perpendicular to the tilt direction, we show the existence of finite B-linear transverse conductivity components. For the other case of magnetic field applied parallel to the tilt axis, the B-linear corrections appear in the longitudinal conductivity and will result in asymmetric lineshape in the magneto-resistence measurements. Unlike the quadratic-B corrections, these linear corrections are independent of the chemical potential, and are sensitive to the direction of the tilt axis and the applied magnetic field. Our systematic analysis of the full magneto-electric conductivity matrix, predicts several specific experimental signatures related to the tilting of the Weyl nodes in both type-I and type-II Weyl semimetals.

Spin phonon interactions and magneto-thermal transport behavior in p-Si

The spin-phonon interaction is the dominant process for spin relaxation in Si, and as thermal transport in Si is dominated by phonons, one would expect spin polarization to influence Si's thermal conductivity. Here we report the experimental evidence of just such a coupling. We have performed concurrent measurements of spin, charge, and phonon transport in p-doped Si across a wide range of temperatures. In an experimental system of a free-standing 2 μm p-Si beam coated on one side with a thin (25 nm) ferromagnetic spin injection layer, we use the self-heating 3ω method to measure changes in electrical and thermal conductivity under the influence of a magnetic field. These magneto-thermal transport measurements reveal signatures in the variation of electrical and thermal transport that are consistent with spin-phonon interaction. Raman spectroscopy measurements and first principle's calculations support that these variations are due to spin-phonon interaction. Spin polarization leads to softening of phonon modes, a reduction in the group velocity of acoustic modes, and a subsequent decrease in thermal conductivity at room temperature. Moreover, magneto-thermal transport measurements as a function of temperature indicate a change in the spin-phonon relaxation behavior low temperature.

Nanoscale measurement of Nernst effect in two-dimensional charge density wave material 1T-TaS2

Advances in nanoscale material characterization on two-dimensional van der Waals layered materials primarily involve their optical and electronic properties. The thermal properties of these materials are harder to access due to the difficulty of thermal measurements at the nanoscale. In this work, we create a nanoscale magnetothermal device platform to access the basic out-of-plane magnetothermal transport properties of ultrathin van der Waals materials. Specifically, the Nernst effect in the charge density wave transition metal dichalcogenide 1T-TaS2 is examined on nano-thin flakes in a patterned device structure. It is revealed that near the commensurate charge density wave (CCDW) to nearly commensurate charge density wave (NCCDW) phase transition, the polarity of the Nernst effect changes. Since the Nernst effect is especially sensitive to changes in the Fermi surface, this suggests that large changes are occurring in the out-of-plane electronic structure of 1T-TaS2, which are otherwise unresolved in just in-plane electronic transport measurements. This may signal a coherent evolution of out-of-plane stacking in the CCDW → NCCDW transition.

Giant thermal Hall effect in multiferroics

Multiferroics, in which dielectric and magnetic orders coexist and couple with each other, attract renewed interest for their cross-correlated phenomena, offering a fundamental platform for novel functionalities. Elementary excitations in such systems are strongly affected by the lattice-spin interaction, as exemplified by the electromagnons and the magneto-thermal transport. Here we report an unprecedented coupling between magnetism and phonons in multiferroics, namely, the giant thermal Hall effect. The thermal transport of insulating polar magnets (ZnxFe1-x)2Mo3O8 is dominated by phonons, yet extremely sensitive to the magnetic structure. In particular, large thermal Hall conductivities are observed in the ferrimagnetic phase, indicating unconventional lattice-spin interactions and a new mechanism for the Hall effect in insulators. Our results show that the thermal Hall effect in multiferroic materials can be an effective probe for strong lattice-spin interactions and provide a new tool for magnetic control of thermal currents.

Spin and magnetothermal transport in the S = 1/2 XXZ chain

We present a temperature and magnetic field dependence study of spin transport and magnetothermal corrections to the thermal conductivity in the spin S = 1/2 easy-plane Heisenberg chain, extending an earlier analysis based on the Bethe ansatz method. We critically discuss the low temperature, weak magnetic field behavior, the effect of magnetothermal corrections in the vicinity of the critical field and their role in recent thermal conductivity experiments in 1D quantum magnets.

Thermal drag in spin ladders coupled to phonons

We study the spin-phonon drag effect in the magnetothermal transport of spin-1/2 two-leg ladders coupled to lattice degrees of freedom. Using a bond operator description for the triplon excitations of the spin ladder and magnetoelastic coupling to acoustic phonons, we employ the time convolutionless projection operator method to derive expressions for the diagonal and off-diagonal thermal conductivities of the coupled two-component triplon-phonon system. We find that for magnetoelastic coupling strengths and diagonal scattering rates relevant to copper-oxide spin-ladders the drag heat conductivity can be of similar magnitude as the diagonal triplon heat conductivity. Moreover, we show that the drag and diagonal conductivities display very similar overall temperature dependences. Finally, the drag conductivity is shown to be rather susceptible to external magnetic fields.

Magnetothermal transport in spin-ladder systems

We study a theoretical model for the magnetothermal conductivity of a spin-1/2 ladder with low exchange coupling ($J\ll\Theta_D$) subject to a strong magnetic field $B$. Our theory for the thermal transport accounts for the contribution of spinons coupled to lattice phonon modes in the one-dimensional lattice. We employ a mapping of the ladder Hamiltonian onto an XXZ spin-chain in a weaker effective field B_{eff}=B-B_{0}$, where $B_{0}=(B_{c1}+B_{c2})/2$ corresponds to half-filling of the spinon band. This provides a low-energy theory for the spinon excitations and their coupling to the phonons. The coupling of acoustic longitudinal phonons to spinons gives rise to hybridization of spinons and phonons, and provides an enhanced $B$-dependant scattering of phonons on spinons. Using a memory matrix approach, we show that the interplay between several scattering mechanisms, namely: umklapp, disorder and phonon-spinon collisions, dominates the relaxation of heat current. This yields magnetothermal effects that are qualitatively consistent with the thermal conductivity measurements in the spin-1/2 ladder compound ${\rm Br_4(C_5H_{12}N)_2}$ (BPCB).

Ballistic magnetothermal transport in a Heisenberg spin chain at low temperatures

We study ballistic thermal transport in Heisenberg spin chain with nearest-neighbor ferromagnetic interactions at low temperatures. Explicit expressions for transmission coefficients are derived for thermal transport in a periodic spin chain of arbitrary junction length by a spin-wave model. Our analytical results agree very well with the ones from nonequilibrium Green's function method. Our study shows that the transmission coefficient oscillates with the frequency of thermal wave. Moreover, the thermal transmission shows strong dependence on the intrachain coupling, the length of the spin chain, and the external magnetic field. The results demonstrate the possibility of manipulating spin-wave propagation and magnetothermal conductance in the spin-chain junction by adjusting the intrachain coupling and/or the external magnetic field.

Multiple superconducting phases in heavy fermion compounds PrOs4Sb12 and CeCoIn5

In recently discovered heavy fermion compounds, quasi-two-dimensional CeCoIn5 and skutterudite PrOs4Sb12, multiple superconducting phases with different symmetries manifest themselves belowT c. The angle-resolved magnetothermal transport measurements revealed that in PrOs4Sb12 a novel change in the symmetry of the superconducting gap function occurs deep inside the superconducting state. The ultrasound velocity measurements revealed that in CeCoIn5 the Fulde-Ferrel-Larkin-Ovchinikov (FFLO) phase, in which the order parameter is spatially modulated and has planar nodes aligned perpendicular to the vortices, appears at low temperature and high field. These results open up a new realm for the study of the superconductivity with multiple phases.

Multiple superconducting phases in heavy fermion superconductors

We show that in recently discovered heavy fermion superconductors, quasi two-dimensional CeCoIn 5 and skutterudite PrOs 4Sb 12, multiple superconducting phases with different symmetries manifest themselves below T c. The ultrasound velocity measurements revealed that in CeCoIn 5 the Fulde–Ferrel–Larkin–Ovchinikov phase, in which the order parameter is spatially modulated and has planar nodes aligned perpendicular to the vortices, appears at low temperature and high field. The angle resolved magnetothermal transport measurements revealed that in PrOs 4Sb 12 a novel change in the symmetry of the superconducting gap function occurs deep inside the superconducting state. These results open up a new realm for the study of the superconductivity with multiple phases.

Superconducting gap function in antiferromagnetic heavy-fermion UPd2Al3 probed by angle-resolved magnetothermal transport measurements

The superconducting gap structure of heavy fermion $\mathrm{U}{\mathrm{Pd}}_{2}{\mathrm{Al}}_{3}$, in which unconventional superconductivity coexists with antiferromagnetic (AF) order with atomic size local moments, was investigated by the thermal conductivity measurements in a magnetic field $\mathbit{H}$ rotating in various directions relative to the crystal axes. The thermal conductivity displays distinct twofold oscillation when $\mathbit{H}$ is rotated in the plane orthogonal to the basal ab plane, while no oscillation was observed when $\mathbit{H}$ is rotated within the basal plane. These results provide strong evidence that the gap function $\mathrm{\ensuremath{\Delta}}(\mathbit{k})$ has a single line node orthogonal to the $c$ axis located at the AF Brillouin zone boundary, while $\mathrm{\ensuremath{\Delta}}(\mathbit{k})$ is isotropic within the basal plane. This gap structure indicates that the pairing interaction in neighboring planes strongly dominates over the interaction in the same plane. The determined nodal structure is compatible with the resonance peak in the dynamical susceptibility observed in neutron inelastic scattering experiments. Based on these results, we conclude that the superconducting pairing function of $\mathrm{U}{\mathrm{Pd}}_{2}{\mathrm{Al}}_{3}$ is most likely to be $d$-wave with a form $\mathrm{\ensuremath{\Delta}}(\mathbit{k})={\mathrm{\ensuremath{\Delta}}}_{0}\phantom{\rule{0.2em}{0ex}}\mathrm{cos}({k}_{z}c)$.

Magnetothermal Transport of Oriented Graphite at Low Temperatures

We have studied the magnetic field dependence of the thermal conductivity $\kappa(T,B)$ of highly oriented pyrolytic graphite samples at temperatures 0.2K$ \le T < 10$K and fields 0T$ \le B \le 9$T. The samples show clear deviations from the Wiedemann-Franz law with a kink behavior at fields $B \sim 0.1 $T near the metal-insulator transition observed in electrical resistivity measurements. We further show that the oscillations in the thermal conductivity at the quantum limit $B > 1 $T, which are correlated with the Landau quantization observed in Hall measurements, increase in amplitude with temperature following a $\sim T^3$ law at $T > 0.2 $K and show a maximum at $T \sim 6 $K, suggesting that they are phonons mediated.

Models for organic superconductors

Most of organic superconductors appear to be of non-s-wave. We present our analysis of the angular dependence of the superfluid density and the magnetothermal transport in d-wave and p-wave superconductors. We hope this will help to identify the pairing symmetry of the superconductivity in Bechgaard salts and κ-(BEDT-TTF) 2 salts.