Christian Heiliger

Ab initio studies of the spin-transfer torque in magnetic tunnel junctions.

We calculate the spin-transfer torque in Fe/MgO/Fe tunnel junctions and compare the results with those for all-metallic junctions. The spin-transfer torque is interfacial due to the half-metallic nature of the Fe Delta1 states. For samples with typical interfacial roughness, the in-plane torque varies linearly with bias and the out-of-plane torque varies quadratically, both in quantitative agreement with experiment. For ideal samples, we predict that the out-of-plane component of the torque varies linearly with bias and oscillates as a function of the ferromagnetic layer thickness.

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

Microscopic origin of magnetoresistance

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

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

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.

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

Tunneling magnetoresistance is one of the basic effects of spintronics with the potential for applications in sensors and IT, where the spin degree of freedom of electrons is exploited. Successful application requires control of the materials and processes involved on the atomic scale. To support experimental developments, predict new materials, and optimize the effect, first-principle electronic structure calculations based on density functional theory are the most powerful tool. The method gives an insight into the microscopic origin of spin-dependent tunneling. The main components of a planar tunnel junction – barrier, leads, and their interface – and their specific role for tunneling magnetoresistance are discussed for one of the standard systems, Fe/MgO/Fe.

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.

Auger recombination rates in ZnMgO from first principles

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.

Modeling of interface roughness in thermoelectric composite materials.

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.

Magnetic Sensor Devices Based on Ordered Planar Arrangements of MnAs Nanocluster

We investigate direct electron-electron-hole interband Auger recombination for wurtzite Zn1-xMgxO alloys in the range 0 ≤ x ≤ 1. Recombination rates are computed by interpolating the band structure and transition matrix elements from ab initio calculations of bulk ZnO, Zn0.5Mgn0.5O, and MgO primitive cells. We find that interband Auger recombination is most probable for Mg concentrations around 50%, where ZnMgO does not exist in a stable wurtzite phase. Since, for low Mg concentrations, the calculated Auger coefficients are far below 10−32 cm6/s, we do not expect significant nonradiative loss through direct interband recombination in wurtzite ZnMgO.