ИСТИНА

Magnetic tunnel junctions (MTJ) based on perpendicular magnetic anisotropy systems are intensively studied for the development of the next generations of spin transfer torque magnetic random access memories. The stability field-voltage diagrams (VH diagrams) are useful tools to study the mechanisms and the properties of STT induced magnetization reversal in such devices. However, these diagrams are usually measured in a collinear geometry, i.e. with the external magnetic field applied parallel to the easy-axis of MTJ magnetic layers (free layer and polarizer) [1,2]. Considering the rising interest for non-collinear geometries, it is also interesting to study these phase diagrams once the field is applied at some angle with respect to the normal to the layers, introducing thus a non-collinearity in the static configuration of the storage and polarizer layers. Non-collinear configuration of magnetic electrodes influences the switching characteristics of the storage layer and its field dependence defines the phase boundaries of the stability diagram. In this study systematic experimental measurements were performed on MgO-based magnetic tunnel junctions with various diameters ranging from 50 nm to 150 nm. We found that the shape of the stability field-voltage diagrams depends strongly on the direction of the applied field while the bistable parallel (P) / antiparallel (AP) region preserves its symmetry around the origin (Fig.1a). We have found also a quite noticeable difference in the shape of measured and simulated hard-axis VH diagrams, H in the hard plane, when considering that the anisotropy is only of uniaxial form. However, a good agreement between experiments and simulations is recovered when a second order anisotropy contribution is introduced in the model [3,4,5]. The signature of this higher order anisotropy term is not always visible in the collinear geometry but clearly shows up once the field is applied away from the normal to plane of the sample. Furthermore, performing the stability analysis in linear approximation allowed us to analytically extract the critical switching voltage at zero temperature for in-plane field. This study indicates that in the non-collinear geometry investigations are suitable to detect the presence of second order term in the anisotropy. Such higher order anisotropy term can yield an easy-cone anisotropy which reduces the thermal stability factor but allows for faster and more reproducible spin transfer torque switching due to a reduced stochasticity of the switching onset. As a result, the energy per write event decreases much faster than the thermal stability factor as the second order anisotropy becomes more negative. For memory applications the second order anisotropy appears to be an interesting parameter to control and adjust, since it allows to write with lower energy and less stochastically while preserving a reasonable thermal stability. [1] A. A. Timopheev, R. Sousa, M. Chshiev, L. D. Buda-Prejbeanu and B. Dieny, Phys. Rev. B 92, 104430 (2015). [2] L. Cuchet, B. Rodmacq, S. Auffret, R. C. Sousa, I. L. Prejbeanu and B. Dieny, Scientific Reports 6, 21246 (2016). [3] A. A. Timopheev, R. Sousa, M. Chshiev, H. T. Nguyen and B. Dieny, Scientific Reports 6, 26877 (2016). [4] J. M. Shaw, H. T. Nembach, M. Weiler, T. J. Silva, M. Schoen, J. Z. Sun and D. C. Worledge, IEEE Magn. Lett. 6, 1 (2015). [5] J. Z. Sun, Phys. Rev. B 91, 174429 (2015).