Voltage controlled exchange stiffness and magnetic anisotropy in CoFeB characterized by spin waves excitation in magnetic tunnel junction

Abstract

Voltage control of magnetic properties in metallic ferromagnets is attracting much attention because of its potential to be a key technology in the production of ultralow energy consumption spintronic devices. Voltage control of the exchange stiffness constant, Aex, was first predicted by a theory1 to explain an experimental result, which shows voltage control of the Curie temperature2. The voltage control of the Aex was also evaluated from observations about voltage dependent domain size3, 4. However, those results may include contradictions. Therefore, here we propose an independent and more precise measurement technique about Aex using a spinwave mode observations5, and provides reliable experimental data about the voltage modulation of the Aex in the magnetic tunnel junction with perpendicular magnetization. In this study, a 65-nm-diameter magnetic tunnel junction (MTJ) with 1.5-nm-thick CoFeB free layer which has perpendicular magnetic anisotropy was prepared by electron-beam lithography to investigate voltage modulation in Aex and magnetic anisotropy (Ks). The thermally excited ferromagnetic resonance (TE-FMR) measurements were employed to measure the voltage effect on spinwaves in the free layer of the MTJ. Typical TE-FMR spectra which were taken at 6.0 GHz as a function of the external magnetic field under a dc bias of ± 0.52 V as shown in Fig. 1. From voltage effects on the resonance frequency, voltage modulation of Aex and Ks can be determined. The voltage modulation of Aex is determined as 0.17 ± 0.02 pJ/m with a 1 V application through a 1.2 nm-thick-MgO layer. This corresponds to d(Aex)/dE = 0.20 ± 0.02 ×10-21 J/V for the 1.5 nm CoFeB film. The voltage modulation of Aex corresponds to 7 % change under 1 V/nm electric field. In addition, we determined voltage controlled magnetic anisotropy as -28 ± 9 fJ/Vm. The effect can be useful to control the Curie temperature, domain size, and other magnetic properties by a voltage application. This work was partially supported by ImPACT program, MI2I program and JSPS Research Fellow (No. P16362).