  Localized laser heating creates temperature gradients in all directions and
thus leads to three-dimensional electron flux in metallic materials.
Temperature gradients in combination with material magnetization generate
thermomagnetic voltages. The interplay between these direction-dependent
temperature gradients and the magnetization along with their control enable to
manipulate the generated voltages, e.g. in magnetic nanodevices. We identify
the anomalous Nernst effect (ANE) generated on a nanometer length scale by
micrometer sized temperature gradients in magnetic tunnel junctions (MTJs). In
a systematic study, we extract the ANE by analyzing the influence of in-plane
temperature gradients on the tunnel magneto-Seebeck effect (TMS) in three
dimensional devices. To investigate these effects, we utilize in-plane
magnetized MTJs based on CoFeB electrodes with an MgO tunnel barrier. Due to
our measurement configuration, there is no necessity to disentangle the ANE
from the spin Seebeck effect in inverse spin-Hall measurements. The temperature
gradients are created by a tightly focused laser spot. The spatial extent of
the measured effects is defined by the MTJ size, while the spatial resolution
is given by the laser spot size and the step size of its lateral translation.
This method is highly sensitive to low voltages and yields an ANE coefficient
of $K_N\approx 1.6\cdot 10^{-8}\,\mathrm{V/TK}$ for CoFeB. In general, TMS
investigations in MTJs are motivated by the usage of otherwise wasted heat in
magnetic memory devices for read/write operations. Here, the additionally
generated ANE effect allows to expand the MTJs' functionality from simple
memory storage to nonvolatile logic devices and opens new application fields
such as direction dependent temperature sensing with the potential for further
downscaling.
