Depth-Dependent Magnetization Profiles of Hybrid Exchange Springs

Abstract

We report on the magnetization depth profile of a hybrid exchange spring system in which a Co/Pd multilayer with perpendicular anisotropy is coupled to a CoFeB thin film with in-plane anisotropy. The competition between these two orthogonal anisotropies promotes a strong depth dependence of the magnetization orientation. The angle of the magnetization vector is sensitive both to the strength of the individual anisotropies and to the local exchange constant, and is thus tunable by changing the thickness of the CoFeB layer and by substituting Ni for Pd in one layer of the Co/Pd stack. The resulting magnetic depth profiles are directly probed by element specific x-ray magnetic circular dichroism (XMCD) of the Co, Fe, and Ni layers located at different average depths. The experimental results are corroborated by micromagnetic simulations.PACS numbers: 75.30. Gw, 75.30.Et, 78.20.Ls, 75.70.Cn a Author to whom correspondence should be addressed. Electronic mail: anhntn@kth.se. 2The phenomenon of spin transfer torque (STT) [1][2][3], in which a spin-polarized current transfers angular momentum to a magnetic layer, has brought about novel applications such as spin torque oscillators (STOs) [4][5][6][7] and spin transfer torque magnetoresistive random access memory (STT-MRAM) [8][9][10]. While initially based on magnetic materials with in-plane (IP) magnetic anisotropy, the realization that such materials lead to unnecessarily high STT-MRAM switching currents, poor memory retention, poor scalability [11], and high-field operation of STOs [12], there is now a rapidly growing interest in fabricating STT devices based on perpendicular magnetic anisotropy (PMA) materials. Recent tailoring of PMA materials and their interfaces have demonstrated low switching currents, high switching speed, good thermal stability, future scalability [9,[13][14][15], and low-to zero-field operation of STOs [16][17][18][19][20].Building upon these successes, the natural extension of using PMA materials is to also investigate the potential of devices in which the magnetization is tilted with respect to the surface normal. Such materials allow for additional control of the magnetization dynamics in magnetic nanostructures [17,[21][22][23], and hint at yet improved STT-MRAM switching behavior and thermal stability [24][25][26][27][28][29]. For STOs, tilted materials offer a route to improve their microwave generation properties, both in terms of higher output power and low-to zero-field operation [17,[21][22][23]28,[30][31][32].Recently, tilted materials have also been shown to have potential for current-driven domain wall motion [33]. The influence of a tilted anisotropy is stronger than simply tilting the applied field [34] as a mere 5 degree misalignment between the free and the fixed layer in magnetic tunnel junctions (MTJs) can reduce the switching current by 36%, the switching time by 30%, and improve the switching current distribution [35].Materials with tilted anisotropies have been realized using collimated oblique sputtering [36], depos...

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