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Register a new userSpin-dependent Seebeck effect and spin caloritronics in magnetic graphene
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Added by
Babak Zare Rameshti
Publication date
2014
fields
Physics
and research's language is
English
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We investigate the spin-dependent thermoelectric effects in magnetic graphene in both diffusive and ballistic regimes. Employing the Boltzmann and Landauer formalisms we calculate the spin and charge Seebeck coefficients (thermopower) in magnetic graphene varying the spin splitting, temperature, and doping of the junction. It is found that while in normal graphene the temperature gradient drive a charge current, in the case of magnetic graphene a significant spin current is also established. In particular we show that in the undoped magnetic graphene in which different spin carriers belong to conduction and valence bands, a pure spin current is driven by the temperature gradient. In addition it is revealed that profound thermoelectric effects can be achieved at intermediate easily accessible temperatures when the thermal energy is comparable with Fermi energy $k_BTlesssim mu$. By further investigation of the spin-dependent Seebeck effect and a significantly large figure of merit for spin thermopower $mathcal{Z}_{rm sp}T$, we suggest magnetic graphene as a promising material for spin-caloritronics studies and applications.
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The interplay between spin and heat currents at magnetic insulator|nonmagnetic metal interfaces has been a subject of much scrutiny because of both fundamental physics and the promise for technological applications. While ferrimagnetic and, more recently, antiferromagnetic systems have been extensively investigated, a theory generalizing the heat-to-spin interconversion in noncollinear magnets is still lacking. Here, we establish a general framework for thermally-driven spin transport at the interface between a noncollinear magnet and a normal metal. Modeling the interfacial coupling between localized and itinerant magnetic moments via an exchange Hamiltonian, we derive an expression for the spin current, driven by a temperature difference, for an arbitrary noncollinear magnetic order. Our theory reproduces previously obtained results for ferromagnetic and antiferromagnet systems.
We report time-resolved magneto-optic Kerr effect measurements of the longitudinal spin Seebeck effect driven by an interfacial temperature difference between itinerant electrons and magnons. The measured time-evolution of spin accumulation induced by laser-excitation indicates transfer of angular momentum across Au/Y$_3$Fe$_5$O$_{12}$ and Cu/Y$_3$Fe$_5$O$_{12}$ interfaces on a picosecond time-scale. The product of spin-mixing conductance and interfacial spin Seebeck coefficient determined is of the order of $10^8$ A m$^{-2}$ K$^{-1}$.
We theoretically investigate the spin-dependent Seebeck effect in an Aharonov-Bohm mesoscopic ring in the presence of both Rashba and Dresselhaus spin-orbit interactions under magnetic flux perpendicular to the ring. We apply the Greens function method to calculate the spin Seebeck coefficient employing the tight-binding Hamiltonian. It is found that the spin Seebeck coefficient is proportional to the slope of the energy-dependent transmission coefficients. We study the strong dependence of spin Seebeck coefficient on the Fermi energy, magnetic flux, strength of spin-orbit coupling, and temperature. Maximum spin Seebeck coefficients can be obtained when the strengths of Rashba and Dresselhaus spin-orbit couplings are slightly different. The spin Seebeck coefficient can be reduced by increasing temperature and disorder.
We theoretically propose a nonreciprocal spin Seebeck effect, i.e., nonreciprocal spin transport generated by a temperature gradient, in antiferromagnetic insulators with broken inversion symmetry. We find that nonreciprocity in antiferromagnets has rich properties not expected in ferromagnets. In particular, we show that polar antiferromagnets, in which the crystal lacks the spatial inversion symmetry, exhibit perfect nonreciprocity --- one-way spin current flow irrespective of the direction of the temperature gradient. We also show that nonpolar centrosymmetric crystals can exhibit nonreciprocity when a magnetic order breaks the inversion symmetry, and in this case, the direction of the nonreciprocal flow can be controlled by reversing the magnetic domain. As their representatives, we calculate the nonreciprocal spin Seebeck voltages for the polar antiferromagnet $alpha$-Cu$_2$V$_2$O$_7$ and the honeycomb antiferromagnet MnPS$_3$, while varying temperature and magnetic field.
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.
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