FeAlSi thin films with low magnetic anisotropy for the free layer of magnetic tunnel junction based sensors

Abstract

Soft magnetic materials showing a high tunnel magnetoresistance (TMR) effect are needed for improving the sensitivity of magnetic tunnel junction (MTJ) based sensors. We focused the Sendust alloy (Fe85Al5.4Si9.6 wt.%, hereinafter, this is called “FeAlSi”), which has soft magnetic property in D03-ordered structure. FeAlSi electrode is also expected to show a high TMR ratio by Δ1 electron coherent tunneling through MgO barriers as well as Fe electrodes with similar crystal structure to FeAlSi [1]. In previous work [2], we firstly succeeded to fabricate epitaxial D03-FeAlSi films with thickness of 30 nm on MgO substrates. Additionally, we observed good soft magnetic properties comparable to other free layer materials such as NiFe [3]. The purpose of this study is fabrication of MTJs using FeAlSi free layer and investigation of their TMR properties. All films were deposited on Si/SiO2 substrates using magnetron sputtering. The stacking structure for the characterization of magnetic and structural properties was Si/SiO2/Ta(5)/CoFeB(5)/MgO(20)/FeAlSi(30)/Ta(5)(in nm). TMR properties were investigated in the MTJs with stacking structure of Si/SiO2/Ta(5)/CoFeB(5)/MgO(20)/FeAlSi(30)/MgO(2)/CoFeB(3)/Ru(0.85)/CoFe(5)/IrMn(10)/Ta(5)/Ru(10). Crystal structure, magnetic properties, and TMR properties were measured by X-ray diffraction (XRD), vibrating sample magnetometer (VSM), and 4-probe DC measurement. Fig. 1 shows the XRD patterns for FeAlSi films prepared by optimal condition. A fundamental peak of FeAlSi (004) and superlattice peak of FeAlSi (002) from B2-ordered structure were clearly observed. The XRD result indicates that the FeAlSi film grown on SiO2 substrates with an MgO underlayer has (001)-orientation and contains B2-ordered structure. Fig. 2 shows magnetoresistance curve for MTJs with FeAlSi bottom electrode. We firstly observed TMR effect of 18.1% at RT using FeAlSi electrode with relatively small switching field. Although the further improvement of TMR effect is needed, the prepared FeAlSi thin film with soft magnetic property is a promising candidate for the free layer of MTJ based sensors with high sensitivity.