Michael Czerner

Spin caloritronics in magnetic tunnel junctions: Ab initio studies

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, 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

Implementation of a nonequilibrium Green’s function method to calculate spin-transfer torque

100 \ensuremath{\mu}\mathrm{V}/\mathrm{K}

Thermal Transport and Nonequilibrium Temperature Drop Across a Magnetic Tunnel Junction.

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 nontrivial dependence. For Fe/MgO/Fe even the sign of the coefficient changes with temperature.

Coherent ultrafast spin-dynamics probed in three dimensional topological insulators

We present an implementation of the steady state Keldysh approach in a Green’s function multiple scattering scheme to calculate the nonequilibrium spin density. This density is used to obtain the spin-transfer torque in junctions showing the magnetoresistance effect. We use our implementation to study the spin-transfer torque in metallic Co∕Cu∕Co junctions.

PARAMETER SPACE FOR THERMAL SPIN-TRANSFER TORQUE

In the field of spin caloritronics, spin-dependent transport phenomena are observed in a number of current experiments where a temperature gradient across a nanostructured interface is applied. The interpretation of these experiments is not clear as both phonons and electrons may contribute to thermal transport. Therefore, it still remains an open question how the temperature drop across a magnetic nanostructured interface arises microscopically. We answer this question for the case of a magnetic tunnel junction (MTJ) where the tunneling magneto-Seebeck effect occurs. Our explanation may be extended to other types of nanostructured interfaces. We explicitly calculate phonon and electron thermal conductance across Fe/MgO/Fe MTJs in an ab initio approach using a Green function method. Furthermore, we are able to calculate the electron and phonon temperature profile across the Fe/MgO/Fe MTJ by estimating the electron-phonon interaction in the Fe leads. Our results show that there is an electron-phonon temperature imbalance at the Fe-MgO interfaces. As a consequence, a revision of the interpretation of current experimental measurements may be necessary.

Magnetic Sensor Devices Based on Ordered Planar Arrangements of MnAs Nanocluster

Topological insulators are candidates to open up a novel route in spin based electronics. Different to traditional ferromagnetic materials, where the carrier spin-polarization and magnetization are based on the exchange interaction, the spin properties in topological insulators are based on the coupling of spin- and orbit interaction connected to its momentum. Specific ways to control the spin-polarization with light have been demonstrated: the energy momentum landscape of the Dirac cone provides spin-momentum locking of the charge current and its spin. We investigate a spin-related signal present only during the laser excitation studying real and imaginary part of the complex Kerr angle by disentangling spin and lattice contributions. This coherent signal is only present at the time of the pump-pulses’ light field and can be described in terms of a Raman coherence time. The Raman transition involves states at the bottom edge of the conduction band. We demonstrate a coherent femtosecond control of spin-polarization for electronic states at around the Dirac cone.

Ab initiostudies of the tunneling magneto-Seebeck effect: Influence of magnetic material

Thermal spin-transfer torque describes the manipulation of the magnetization by the application of a heat flow. The effect has been calculated theoretically by Jia et al. in 2011. It is found to require large temperature gradients in the order of Kelvins across an ultra thin MgO barrier. In this paper, we present results on the fabrication and the characterization of magnetic tunnel junctions with three monolayer thin MgO barriers. The quality of the interfaces at different growth conditions is studied quantitatively via high-resolution transmission electron microscopy imaging. We demonstrate tunneling magnetoresistance ratios of up to 55% to 64% for 3 to 4 monolayer barrier thickness. Magnetic tunnel junctions with perpendicular magnetization anisotropy show spin-transfer torque switching with a critical current of 0.2 MA/cm2. The thermally generated torque is calculated ab initio using the Korringa–Kohn–Rostoker and nonequilibrium Greens function method. Temperature gradients generated from femtosecond laser pulses were simulated using COMSOL, revealing gradients of 20 K enabling thermal spin-transfer-torque switching.

Influence of the magnetic material on tunneling magnetoresistance and spin-transfer torque in tunnel junctions:Ab initiostudies

We propose planar magneto-electronic devices based on accurately positioned and shaped ferromagnetic MnAs nanoclusters in NiAs structure. The prototype discussed consists of a small hexagon-shaped cluster sandwiched between two elongated clusters with fixed magnetic orientations. The magnetization of the center cluster is free and can be rotated by an external magnetic field. Our ab initio calculations yield a strong dependence of the conductance as a function of the direction of the external magnetic field which exhibits an in-plane symmetry of 360 degrees.

Thermoelectric transport in Bi2Te3/Sb2Te3 superlattices

We found a strong influence of the composition of the magnetic material on the temperature dependence of the tunneling magneto-Seebeck effect in

Magnetic order in geometrically constrained domain walls

MgO