Electric-field modulation of perpendicular magnetic anisotropy and damping constant in MgO/Co/Pt trilayers

Abstract

Voltage controlled magnetic anisotropy (VCMA) has attracted much attention, since it can dramatically reduce the power consumption during the writing in magnetic random access memory (MRAM) [1]. The voltage-induced anisotropy change triggers the magnetization precession, and a stable magnetization switching is confirmed by the application of appropriate voltage pulses [1]. It is also reported that the stable switching is significantly influenced by the relaxation time of the magnetization precession [2], and thus the magnetization dynamics under the application of the electric field is an important topic to be understood [2]. In this study, we report the electric-field modulation of perpendicular magnetic anisotropy (PMA) and the magnetization dynamics in MgO/Co/Pt trilayers by using time-resolved magneto-optical Kerr effect (TRMOKE).ITO(20) /MgO(10)/Co(1.2)/Pt(1.6)/Ta(10)/oxidized Si substrate (thickness in nanometer) was prepared by using a magnetron spattering system. The film was microfabricated into a Hall-bar structure and a gate electrode of ITO(15 nm) was deposited on the junction. Gate voltage was applied from -3 V to 3 V corresponding to the electric field from -0.3 V/nm to 0.3 V/nm. To investigate PMA, we measured anomalous Hall effect (AHE). The magnetization dynamics was measured by TRMOKE using high-power fiber laser with λ=1040 nm, a pulse width of 500 fs, and a repetition frequency of 100 kHz. During the measurement, an external magnetic field Hext in the range of 5-14 kOe was applied in the direction of 73 deg from the film normal. The measured precession was fitted with the damped oscillation function, exp(-t/τ)sin(ωt), where τ and ω denote relaxation time and angular frequency of the precession. By fitting the Hext dependence ofω, we estimated effective magnetic anisotropy field Hk and g-factor.Figure 1 shows the electric field E dependence of Hk estimated from AHE and TRMOKE. Hk decreases linearly with increasing the electric field in both measurements. The variation of PMA with the electric field η obtained from AHE and TRMOKE were -32 fJ/Vm and -47 fJ/Vm, respectively. Figure 2 (a) shows the electric field dependence of the inverse of relaxation time 1/τ obtained at Hext=14 kOe. 1/τ is known to consist of four contributions; effective Gilbert damping αeff, anisotropy axis distribution ΔθH, anisotropy distribution ΔHk and two-magnon scattering (TMS). The four contributions depend on Hext and E, since Hk depends on E, and we fitted Hext and E dependences of 1/τ by assuming material-dependent parameters which are independent of Hext and E. The extracted four contributions at Hext=14 kOe are also shown in Fig. 2 (a). The results indicate a large electric-field modification of 1/τ is caused by the two contributions: effective Gilbert damping and ΔθH, which have the same trend as the electric-field modification of 1/τ. Figure 2 (b) shows the electric field dependence of αeff. αeff is found to decrease with increasing electric field with a ratio of -13 % per 1 V/nm. Okada et al. reported a ferromagnetic resonance (FMR) study on MgO/CoFeB/Ta trilayers, and they showed PMA variation of η=-60 fJ/Vm and α modulation of -21 % per 1 V/nm [3]. These results are similar to our results, however their samples have MgO(001)/bcc-CoFe interface. Our study indicates similar PMA and α modulations are found in MgO/Co/Pt trilayers having MgO(111)/fcc-Co interface. **