Christian Franz

Coherent ultrafast spin-dynamics probed in three dimensional topological insulators

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

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

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

MgO

On/off switching of bit readout in bias-enhanced tunnel magneto-Seebeck effect

based tunnel junctions. We use \textit{ab initio} alloy theory to consider different

Thermal spin-transfer torque in magnetic tunnel junctions (invited)

Fe_xCo_{1-x}

Band Structure and Effective Masses of Zn 1-x Mg x O

alloys for the ferromagnetic layer. Even a small change of the composition leads to strong changes in the magnitude or even in the sign of the tunneling magneto-Seebeck effect. This can explain differences between recent experimental results. In addition, changing the barrier thickness from six to ten monolayers of

Band Structure and Effective Masses of Zn1-xMgxO

MgO

Implementation of non-equilibrium vertex corrections in KKR: transport through disordered layers

leads also to a non-trivial change of the temperature dependence. Our results emphasize that the tunneling magneto-Seebeck effect depends very crucially and is very sensitive to material parameters and show that further experimental and theoretical investigations are necessary.

Band Structure and Effective Masses of ZnMgO

The dependence of tunneling magnetoresistance and spin-transfer torque in FeCo/MgO/FeCo tunnel junctions on the Co concentration and the bias voltage are investigated ab initio. We find that the tunneling magnetoresistance decreases with the Co concentration in contradiction with previous calculations but in agreement with recent experiments. This dependence is explained from bulk properties of the alloys. By using a realistic description of the disorder in the alloys we can show that even small amounts of disorder lead to a drastic drop in the tunneling magnetoresistance. This provides a quantitative explanation of the difference between calculated and measured values. The spin-transfer torque shows a linear voltage dependence for the in-plane component and a quadratic for the out-of-plane component for all concentrations at small bias voltages. In particular, the linear slope of the in-plane torque is independent of the concentration. For high bias voltages the in-plane torque shows a strong nonlinear deviation from the linear slope for high Co concentrations. This is explained from the same effects which govern the tunneling magnetoresistance.

Perpendicular magnetic anisotropy in CoFe/MgO/CoFe magnetic tunnel junctions by first-principles calculations

Thermoelectric effects in magnetic tunnel junctions are promising to serve as the basis for logic devices or memories in a ”green” information technology. However, up to now the readout contrast achieved with Seebeck effects was magnitudes smaller compared to the well-established tunnel magnetoresistance effect. Here, we resolve this problem by demonstrating that the tunnel magneto-Seebeck effect (TMS) in CoFeB/MgO/CoFeB tunnel junctions can be switched on to a logic “1” state and off to “0” by simply changing the magnetic state of the CoFeB electrodes. This new functionality is achieved by combining a thermal gradient and an electric field. Our results show that the signal crosses zero and can be adjusted by tuning a bias voltage that is applied between the electrodes of the junction; hence, the name of the effect is bias-enhanced tunnel magneto-Seebeck effect (bTMS). Via the spin- and energy-dependent transmission of electrons in the junction, the bTMS effect can be configured using the bias voltage with much higher control than the tunnel magnetoresistance and even completely suppressed for only one magnetic configuration. Moreover, our measurements are a step towards the experimental realization of high TMS ratios without additional bias voltage, which are predicted for specific Co-Fe compositions.