Recently, several groups have reported spin-dependent thermoelectric effects in magnetic tunnel junctions. In this paper, we present a setup for time-resolved measurements of thermovoltages and thermocurrents of a single micro-to nanometer-scaled tunnel junction. An electrically modulated diode laser is used to create a temperature gradient across the tunnel junction layer stack. This laser modulation technique enables the recording of time-dependent thermovoltage signals with a temporal resolution only limited by the preamplifier for the thermovoltage. So far, time-dependent thermovoltage could not be interpreted. Now, with the setup presented in this paper, it is possible to distinguish different Seebeck voltage contributions to the overall measured voltage signal in the µs time regime. A model circuit is developed that explains those voltage contributions on different sample types. Further, it will be shown that a voltage signal arising from the magnetic tunnel junction can only be observed when the laser spot is directly centered on top of the magnetic tunnel junction, which allows a lateral separation of the effects.

We have prepared MgO based magnetic tunnel junctions which show up to 143% tunneling magnetoresistance ͑TMR͒ ratio at room temperature and 205% at 12 K. This TMR temperature dependence is mainly caused by a strong temperature dependence in the antiparallel magnetic state, while in the parallel state the change of condunctance is small. We found that a modified version of the magnon excitation model may be applied to these MgO magnetic tunnel junctions. If the thermal smearing of the tunneling electron's energy is included it is possible to fit the temperature dependence. We will show the results for our data and we have also tested our model successfully on data from other publications.

MgO-based magnetic tunnel junctions with up to 230% tunnel magnetoresistance ratio at room temperature and up to 345% at 13 K are prepared. The lower electrode is either exchange-biased or free, while the top electrode is free or an exchanged-biased artificial ferrimagnet, respectively. Additionally, a pseudo-spin-valve ͑hard-soft switching͒ design with two unpinned electrodes is used. Inelastic electron-tunneling spectra for each of these systems show a strong variation in the zero-bias anomaly with a reduced peak for some of the junctions. At voltages around 200 mV additional structures are found, which are not known from junctions with lower magneto resistance, such as alumina-based junctions. We discuss the spectra for the different electrode types and compare our findings with respect to barrier material and magnetoresistance ratio.

Interest in femtosecond demagnetization dynamics was sparked by Bigot's experiment in 1996, which unveiled the elementary mechanisms that relate the electrons' temperature to their spin order. Simultaneously, the application of fast demagnetization experiments has been demonstrated to provide key insight into technologically important systems such as high-spin-polarization metals, and consequently there is broad interest in further understanding the physics of these phenomena. To gain new and relevant insights, we performed ultrafast optical pump-probe experiments to characterize the demagnetization processes of highly spin-polarized magnetic thin films on a femtosecond time scale. Full spin polarization is obtained in half-metallic ferro-or ferrimagnets, where only one spin channel is populated at the Fermi level, whereas the other one exhibits a gap. In these materials, the spin-scattering processes is controlled via the electronic structure, and thus their ultrafast demagnetization is solely related to the spin polarization via a Fermi golden-rule model. Accordingly, a long demagnetization time correlates with a high spin polarization due to the suppression of the spin-flip scattering at around the Fermi level. Here we show that isoelectronic Heusler compounds (Co 2 MnSi, Co 2 MnGe, and Co 2 FeAl) exhibit a degree of spin polarization between 59% and 86%. We explain this behavior by considering the robustness of the gap against structural disorder. Moreover, we observe that CoFe-based pseudogap materials, such as partially ordered Co-Fe-Ge and Co-Fe-B alloys, can reach similar values of the spin polarization. By using the unique features of these metals we vary the number of possible spin-flip channels, which allows us to pinpoint and control the half-metals' electronic structure and its influence on the elementary mechanisms of ultrafast demagnetization.Since the discovery of ultrafast demagnetization processes on femtosecond time scales, the underlying mechanism has been under debate [1,2]. However, the last few years have seen the development of the first quantitative models, such as the microscopic three-temperature model [3], the stochastic Landau-Lifshitz-Bloch equation describing averaged spin ensembles [4,5], and stochastic atomistic descriptions [6]. These models suggest that the spin-scattering el-sp is related to the Gilbert damping parameter that describes the energy dissipation of the magnetic system in quasiequilibrium via the same elementary spin-flip processes [7]. The Gilbert damping tends to be small in half-metals where the elementary spin-flip processes are blocked [8]. Just recently, progress in the ab initio description of Gilbert damping has been made [9], shedding additional light onto a long-standing issue. By correlating the experimentally observed values of the Gilbert damping parameter to the coupling parameter of the magnetic system (magnons) and the electron temperature, the Landau-Lifshitz-Bloch model allows the quantitative description of ultrafast demagnetization versus time wi...

Magnetic tunnel junctions for spin-transfer torque switching were prepared to investigate the dielectric breakdown. The breakdown occurs typically at voltages not much higher than the switching voltages, a bottleneck for the implementation of spin-transfer torque Magnetic Random Access Memory. Intact and broken tunnel junctions are characterized by transport measurements and then prepared for transmission electron microscopy and energy dispersive x-ray spectrometry by focussed ion beam. The comparison to our previous model of the electric breakdown for thicker MgO tunnel barriers reveals significant differences arising from the high current densities.