Investigation of Amorphous SmFe2 Thin Films With Giant Negative Magnetostriction and Perpendicular Magnetic Anisotropy

Abstract

Further reduction of the consumption energy for the magnetization switching of a free layer of magnetic tunnel junctions (MTJs) is required in a spin transfer torque magnetoresistive random access memory (STT-MRAM). Using ferromagnetic materials with small magnetic anisotropy for the free layer can reduce the consumption energy but it will also cause the loss of thermal stability. Our group has proposed strain-assisted magnetization reversal (SAMR) using inverse magnetostriction (IMS) MTJs to reduce magnetic anisotropy only during the switching [1]. The proposed IMS-MTJs consist of an MTJ with a magnetostrictive material for the free layer and a piezoelectric material surrounding the MTJ pillar to apply pressure to the free layer effectively. Since lowering magnetic anisotropy caused by the inverse magnetostrictive effect, which results in a reduction of the switching current, occurs only while magnetization switching, thermal stability of the free layer will be kept. Ferromagnetic materials for the free layer of IMS-MTJs are required to have both a large, negative magnetostriction constant λ and perpendicular magnetic anisotropy (PMA). SmFe 2 is promising as such materials since SmFe 2 is well-known materials with large negative magnetostriction constant [2]. In addition, SmFe 2 films are expected to have PMA since some amorphous rare earth transition metal (RE-TM) alloys have been reported to show PMA [3]. In this paper, we systematically investigated magnetic anisotropy and magnetostriction of sputtered SmFe 2 thin films, and found that amorphous SmFe 2 films showed PMA and large, negative λ. All the films were prepared with a facing targets sputtering system. The stack structure was quartz-substrate/W (20nm)/Sm-Fe (100nm)/W (10nm). The substrate temperature $T_{\mathrm {S}}$ during sputtering varied from RT to 400°C. The post annealing temperature $T_{\mathrm {A}}$ also varied from 300°C to 600°C. The chemical composition of deposited Sm-Fe film was determined to be 1.05:2 by inductively coupled plasma-optical emission spectrometer (ICPOES). X-ray diffraction (XRD) analysis showed that all of the fabricated SmFe 2 thin films had amorphous structure. Figure 1(a) shows M-H curves for a Sm 1.05 Fe 2 film formed at $T_{\mathrm {S}} = 200 ^{\circ}C$ and post-annealed at $T_{\mathrm {A}} = 500 ^{\circ}C$ measured by vibrating sample magnetometer (VSM) with perpendicular and in-plane magnetic fields, clearly indicating that the Sm 1.05 Fe 2 film had PMA. ΔK, difference between perpendicular and in-plane magnetic anisotropy energy density, was determined to be 0.17 Merg/cc. Figure 2(b) shows ΔK of the Sm 1.05 Fe 2 thin films with various $T_{\mathrm {S}}$ and $T_{\mathrm {A}}$. PMA were observed only in the Sm 1.05 Fe 2 films formed at low $T_{\mathrm {S}}$ and annealed at high $T_{\mathrm {A}}$. One of the origin of this PMA in the amorphous Sm 1.05 Fe 2 films could be anisotropic, short range order similar to other amorphous RE-TM alloys [4]. Then, IMS effect of the Sm 1.05 Fe 2 thin film with $T_{\mathrm {S}} = 200 ^{\circ}C$ and $T_{\mathrm {A}} = 500 ^{\circ}C$ exhibiting PMA were analyzed. Pressures were applied to the sample by sandwiching it with two sample holders having the same curvature radius. In this experiment, a 0.03-mm thick quartz substrate was used to prevent it from cracking while bending. Figure 2(a) shows the demagnetization curves in the first quadrant under various applied pressures. The sample qualitatively exhibited negative λ as magnetization energy reduced (the demagnetization curve shifted in the upward direction) by compressive stress and increased by tensile stress. The change, $\Delta K_{{\vert {\sigma }} {\vert }}$, of the magnetization energy density of the Sm 1.05 Fe 2 film by stress was quantitatively evaluated from the demagnetization curves. Figure 2(b) shows $\Delta K_{{\vert {\sigma }} {\vert }}$ as a function of induced stress. The slope is equivalent in −3λ and λ was determined to be −920 ppm. From these results, we concluded that the Sm 1.05 Fe 2 film formed at $T_{\mathrm {S}} = 200 ^{\circ}C$ and $T_{\mathrm {A}} = 500 ^{\circ}C$ exhibited both PMA (ΔK = 0.17 Merg/cc) and large, negative magnetostriction (λ = −920 ppm). In conclusion, we found that Sm 1.05 Fe 2 thin films formed at low $T_{\mathrm {S}}$ and annealed at high $T_{\mathrm {A}}$ showed PMA. One of the possible origins of this PMA is the anisotropic, short range order of Sm and Fe atoms. The Sm 1.05 Fe 2 film with $T_{\mathrm {S}} = 200 ^{\circ}C$ and $T_{\mathrm {A}} = 500 ^{\circ}C$ exhibiting PMA also showed large, negative magnetostriction $\lambda = -920$ ppm. This result indicates that Sm 1.05 Fe 2 thin films are promising as the free layer of IMS-MTJ for ultra-low energy STT-MRAMs.