We study tunneling magnetothermopower (TMTP) in CoFeB/MgO/CoFeB magnetic tunnel junction nanopillars. Thermal gradients across the junctions are generated by an electric heater line. Thermopower voltages up to a few tens of μV between the top and bottom contact of the nanopillars are measured which scale linearly with the applied heating power and hence the thermal gradient. The thermopower signal varies by up to 10  μV upon reversal of the relative magnetic configuration of the two CoFeB layers from parallel to antiparallel. This signal change corresponds to a large spin-dependent Seebeck coefficient of the order of 100  μV/K and a large TMTP change of the tunnel junction of up to 90%.

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The emerging field of spintronics explores the many possibilities offered by the prospect of using the spin of the electrons for fast, nanosized electronic devices. The effect of magnetization acting on a current is the essence of giant or tunnel magnetoresistance. Although such spintronics effects already find technological applications, much of the underlying physics remains to be explored. The aim of this article is to demonstrate the importance of spin mixing in metallic nanostructures. Here we show that magnetic clusters embedded in a metallic matrix exhibit a giant magnetic response of more than 500% at low temperature, using a recently developed thermoelectric measurement. This method eliminates the dominating resistivity component of the magnetic response and thus reveals an intrinsic spin-dependent process: the conduction-electron spin precession about the exchange field as the electron crosses the clusters, giving rise to a spin-mixing mechanism with strong field dependence. This effect appears sensibly only in the smallest clusters, that is, at the level of less than 100 atoms per cluster. Spintronics seeks to exploit the interplay of spin-polarized conduction electrons and magnetization in nanostructures. Spin-dependent scattering leads to giant magnetoresistance [1][2][3][4][5] (GMR) and tunnelling magnetoresistance [6][7][8] , whereas the converse effect of a spin-polarized current on the magnetization 9-11 can be taken advantage of in magnetoresistive memory bits 12 and gigahertz oscillators 13 . GMR as a field sensing measurement of a resistivity ratio R/R is dominated by non-magnetic and spin-independent scattering processes determining R. Instead, the thermoelectric measurement protocol developed in our laboratory 14 depends on the first derivative of R with respect to the temperature and thus suppresses this resistive contribution. This allows us to fully reveal the otherwise negligible spin-mixing processes. In multilayers this mechanism is essentially a spin-dependent Peltier effect that roughly doubles the field sensitivity compared with GMR 14 . Here we have applied this measurement protocol to granular clusterassembled materials 15 , the geometry of which is not appropriate for a Peltier effect. Hence a clearly different microscopic mechanism takes a predominant role here. We invoke the predominance of spin mixing caused by a spin-precession effect 16 that is completely different in nature. Spin mixing was predicted to decrease GMR responses, as it scrambles the two spin channels of conduction. In our measurement scheme, on the contrary, it results in a 100-fold increase of the field response compared with GMR. The combined use of cluster-assembled materials and a novel measurement method thus reveal a different spin transport effect and may open a new route towards possible applications.Samples were prepared (see the 'Methods' section) according to the strategy of 'cluster-assembled materials' (Fig. 1). Briefly, the samples consist of thin films of copper in which well-defined cobalt clu...

We investigate the spin-dependent Seebeck coefficient and the tunneling magneto thermopower of CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJ) in the presence of thermal gradients across the MTJ. Thermal gradients are generated by an electric heater on top of the nanopillars. The thermo power voltage V TP across the MTJ is found to scale linearly with the heating power and reveals similar field dependence as the tunnel magnetoresistance.The amplitude of the thermal gradient is derived from calibration measurements in combination with finite element simulations of the heat flux. Based on this, large spindependent Seebeck coefficients of the order of (240 ± 110) µV/K are derived. From additional measurements on MTJs after dielectric breakdown, a tunneling magneto thermopower up to 90% can be derived for 1.5 nm MgO based MTJ nanopillars.

Spin-transfer-torque magnetic random access memory (STT-MRAM) is the most promising emerging non-volatile embedded memory. For most applications, a wide range of operating temperatures is required, for example −40 °C to +150 °C for automotive applications. This presents a challenge for STT-MRAM, because the magnetic anisotropy responsible for data retention decreases rapidly with temperature. In order to compensate for the loss of thermal stability at high temperature, the anisotropy of the devices must be increased. This in turn leads to larger write currents at lower temperatures, thus reducing the efficiency of the memory. Despite the importance of high-temperature performance of STT-MRAM for energy efficient design, thorough physical understanding of the key parameters driving its behavior is still lacking. Here we report on CoFeB free layers diluted with state-of-the-art non-magnetic metallic impurities. By varying the impurity material and concentration to modulate the magnetization, we demonstrate that the magnetization is the primary factor driving the temperature dependence of the anisotropy and thermal stability. We use this understanding to develop a simple model allowing for the prediction of thermal stability of STT-MRAM devices from blanket film properties, and find good agreement with direct measurements of patterned devices.