| Abstract Scope |
In order to increase the storage density of magnetic memory while maintaining the requirements of magnetic recording technology – high signal to noise ratio, robust thermal stability, and low switching field (or current) – new types of recording media and technologies have been proposed. Graded media is one such approach, where thin films have a gradually changing anisotropy strength from top to bottom.[1,2] In such systems the thermal stability is maintained by the high anisotropy region, while the write field is reduced by the low anisotropy region. Although (Ga,Mn)As is not ideal for magnetic data storage due to its low Curie Temperature Tc (below 200 K), the ability to read and write using (Ga,Mn)As nanostructures through tunneling anisotropic magnetoresistance (TAMR) and spin-orbital coupling [3] make it a very interesting model system for studying the graded media approach, provided that the magnetic anisotropy of (Ga,Mn)As depends strongly on Mn concentration. In this work we have successfully fabricated (Ga,Mn)As films with vertically graded Mn concentration, and studied them with x-ray diffraction (XRD), superconducting quantum interference device (SQUID) magnetometry, and polarized neutron reflectometry (PNR). To achieve directional control of magnetic anisotropy, the films were grown either directly on a GaAs substrate (in-plane anisotropy), or on top of an (In,Ga)As buffer layer (perpendicular anisotropy). XRD clearly shows a wide shoulder on the left or right side of GaAs (400) peak, indicating the variation of Mn concentration in the films (see Fig. 1). SQUID results in Fig. 2 show the cubic anisotropy to be dominant for both in-plane and perpendicular anisotropy films at low temperature. The hard axis hysteresis loops feature a convex profile, suggesting a non-uniform anisotropy profile. At higher temperature, an unusual linear temperature dependence of magnetization is observed, suggesting a depth variation of Tc. PNR measurements are sensitive to the nuclear and magnetic depth profiles of thin films and multilayers. Figure 3a shows model-fitted spin-down and spin-up reflectivity data plotted as spin asymmetry (spin-up – spin-down / spin-up + spin-down) for a 200 nm thick Mn-graded film on an (In,Ga)As buffer layer, at 6 K in a 100 mT (saturating) in-plane field. The solid (dashed) line in Fig. 3a is a best (constrained) fit corresponding to a non-uniform (uniform) magnetization depth profile shown in Fig. 3b. While this result confirms an inhomogeneous saturation magnetization profile, more detailed PNR measurements are planned to characterize the anisotropy and Tc profiles. The results will be compared with the Mn concentration profile determined by secondary ion mass spectroscopy measurements. |