Fabrication of L10-MnAl thin films with high perpendicular magnetic anisotropy for STT-MRAM.

Abstract

Magnetic tunnel junctions with perpendicularly magnetized ferromagnetic materials $(p$-MTJs) have great potential to realize the ultra-high-density STT-MRAM. The switching current density $( J_{\mathrm{co}})$ in STT-MRAM is directly related to saturation magnetization $( M_{\mathrm{s}})$ and Gilbert damping constant $( \alpha )$ of the ferromagnetic free layer of MTJs [1]. In order to achieve high thermal stability and low switching current density in $p$-MTJs, ferromagnetic materials with large perpendicular magnetic anisotropy energy $( K_{\mathrm{u}})$, small $M_{\mathrm{s}}$ and low $\alpha $ are required. Here, we focus on a$L 1 _{0} -$MnAl alloy, which exhibits small $M_{s}$ and high $K_{u}$ [2, 3]. In our previous works, we obtained large $K_{u}$ in $L 1 _{0}$-MnAl films prepared at high substrate temperature [4]. However, high-substrate-temperature can cause increasing roughness of the films and atomic diffusion between the MnAl films and their buffer layers. In this work, we systematically investigated substrate and annealing temperature dependences of structural and magnetic properties in the MnAl thin films. The film stacking structure was MgO(001)-sub./CrRu(40)/MnAl(50)/Ta(5) (in nm). All the films were prepared by a magnetron sputtering system. The Mn-Al alloy target composition was Mn 46 Al 54.The substrate temperature $( T_{s})$ during deposition was varied from $200 ^{0}\mathrm {C}$ to $400 ^{0}\mathrm {C}$ and the post-annealing temperature $( T_{a})$ was varied from $200 ^{0}\mathrm {C}$ to $500 ^{0}\mathrm {C}$. The crystal structure of MnAl(50nm) films was investigated by an X-ray diffraction (XRD). The magnetic properties and surface morphology of the films were measured by superconductive quantum interference device (SQUID), vibrating sample magnetometer (VSM), and atomic force microscope (AFM). We confirmed that CrRu buffer layers had good structural property and very smooth surface morphology after annealing at $650 ^{\circ}\mathrm {C}$. Fig. 1showsXRD patterns of the films at $T_{s} \quad = 250 ^{\circ}\mathrm {C}$ with different annealing temperature. In the XRD patterns,(001) and (002) peaks of $L 1 _{0} -$MnAl were observed. This result indicates that both $L 1 _{0} -$ordered and (001)-oriented MnAl films were successfully fabricated. The peak intensity of $L 1 _{0}$-MnAl was improved with increasing both substrate and annealing temperature. However, surface roughness drastically increased above $T_{s} \quad = 300 ^{\circ}\mathrm {C}$. The annealing temperature dependence of magnetic properties was systematically investigated in MnAl films with $T_{s} \quad = 250 ^{\circ}\mathrm {C}$. A very high $K_{u}$ was obtained at $T_{a} \quad = 350 ^{\circ}\mathrm {C}$ as shown in $M-H$ curve in Fig. 2.We finally obtained a${L1}_{0}$-ordered MnAl film with high $K_{u}$ of 13.0 Merg/cc, relatively low $M_{s}$ of 497 emu/cc and small roughness $( R_{a})$ of 0.3 nm in the condition of $T_{s} = 250 ^{\circ}\mathrm {C}$ and $T_{a} = 350 ^{\circ}\mathrm {C}$. The optimized MnAl film will be greatly useful to realize the high-density STT-MRAM. This work was part of a research and development project for ICT key technology to realize future societies.