Field-free switching through bulk spin−orbit torque in<i>L</i>1<sub>0</sub>-FePt films deposited on vicinal substrates

doi:10.1007/s11467-022-1197-7

Front. Phys.

2022, Vol. 17

Issue (5) : 53511 https://doi.org/10.1007/s11467-022-1197-7

RESEARCH ARTICLE

Field-free switching through bulk spin−orbit torque inL1₀-FePt films deposited on vicinal substrates

Yongming Luo¹(

), Yanshan Zhuang¹, Zhongshu Feng¹, Haodong Fan¹, Birui Wu¹, Menghao Jin¹, Ziji Shao¹, Hai Li¹, Ru Bai¹, Yizheng Wu², Ningning Wang¹, Tiejun Zhou¹(

)

¹. Center for Integrated Spintronic Devices, Hangzhou Dianzi University, Hangzhou 310018, China
². State Key Laboratory of Surface Physics and Department of Physics, Fudan University, Shanghai 200433, China

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Abstract

L1₀-FePt distinguishes itself for its ultrahigh perpendicular magnetic anisotropy (PMA), enabling thermally stabile memory cells to scale down to 3 nm. The recently discovered “bulk” spin−orbit torques inL1₀-FePt provide an efficient and scalable way to manipulate the L1₀-FePt magnetization. However, the existence of an external field during the switching limits its practical application, and therefore field-free switching of L1₀-FePt is highly demanded. In this manuscript, by growing the L1₀-FePt film on vicinal MgO (001) substrates, we realize the field-free switching of L1₀-FePt. This method is different from previously established strategies as it does not need to add other functional layers or create asymmetry in the film structure. The dependence on the vicinal angle, film thickness, and growth temperature demonstrates a wide operation window for the field-free switching of L1₀-FePt. We confirm the physical origin of the field-free switching is due to the tilted anisotropy of L1₀-FePt induced by the vicinal surface. We also quantitatively characterize the spin-orbit torques in the L1₀-FePt films. Our results extend beyond the established strategies to realize field-free switching, and potentially could be applied to mass production.

Keywords spin−orbit torque vicinal substrates field-free switching

Corresponding Author(s): Yongming Luo,Tiejun Zhou

About author:

Tongcan Cui and Yizhe Hou contributed equally to this work.

Issue Date: 09 October 2022

Cite this article:

Yongming Luo,Yanshan Zhuang,Zhongshu Feng, et al. Field-free switching through bulk spin−orbit torque inL1₀-FePt films deposited on vicinal substrates[J]. Front. Phys. , 2022, 17(5): 53511.

URL:

https://academic.hep.com.cn/fop/EN/10.1007/s11467-022-1197-7
https://academic.hep.com.cn/fop/EN/Y2022/V17/I5/53511

Fig.1 (a) Atomic structure of the L1₀-FePt. The orange and gray spheres represent the Fe and Pt atoms, respectively, while the pink arrows represent the spin of the associated Fe atoms. (b) Schematic drawing of the FePt film grown on vicinal substrates.

α

denotes the vicinal angle of the substrates. (c)

θ

−2

θ

XRD spectra of the 6 nm L1₀-FePt films grown on flat (

α

0 °

) and vicinal (

α

7 °

) MgO (001) substrates. (d) Out-of-Plane normalized AHE loops of the 6 nm L1₀-FePt grown on flat and vicinal MgO (001) substrates.

Fig.2 AMR measurements for the L1₀-FePt films grown on flat and vicinal MgO substrates. (a) The schematic configuration of the AMR measurements. (b) The schematic drawing of the switching of magnetization when it rotates across the hard axis, for systems with different anisotropies. The left and right panels show switching processes with perpendicular (left) and tilted (right) anisotropies. (c) Angular dependence of

Δ R

for L1₀-FePt grown on different substrates. The field with different amplitudes is rotated from

0 °

360 °

(CCW), and then back from

360 °

0 °

(CW). The top and bottom panels represent the results for samples grown on flat (

α = 0 °

) and vicinal (

α = 7 °

) substrates respectively. (d) Simulation of the angular dependence of

m y 2

in systems with different directions of the easy axis. Fields with different amplitudes are rotated CCW, and then back CW. The top and bottom panels represent the results for samples with perpendicular (

φ = 0 °

) and tilted anisotropy (

φ = 8 °

), respectively. In (c) and (d), the results at different fields are offset for clarity.

Fig.3 Current-induced switching of the L1₀-FePt films grown on vicinal substrates. (a) Schematic drawing of the set-up for the current-induced magnetization switching of the L1₀-FePt films. (b) Current-induced magnetization switching of the

6 n m

L1₀-FePt grown on the vicinal substrate (

α = 7 °

). The

R H

at different

H x

are offset for clarity. (c) Plot of

Δ R H

as a function of

H x

, for L1₀-FePt films grown on substrates with different vicinal angles.

Δ R H

is marked by the green dash arrow in (b). The dependence of

H e f f

with different parameters (d) Vicinal angles of the substrate, (e) Growth temperature and (f) The thickness of the L1₀-FePt film. If not specified, the vicinal angle for the vicinal substrates is

7 °

, the growth temperature of the L1₀-FePt film is

700 ° C

, and

6 n m

in thickness.

Fig.4 Harmonic Hall voltage analysis of the L1₀-FePt films. (a) Schematic of the set-up for the Harmonic Hall voltage measurements, and illustration of the spin−orbit effective fields

Δ H L

and

Δ H T

. (b)−(d) are measurement results for the

6 n m

L1₀-FePt film grown on the vicinal substrate. (b) First harmonic Hall voltage as a function of the longitudinal field

H x

. Blue and pink squares are experimental data for the up and down magnetization directions, respectively. The solid lines are parabolic fits to the data. Second harmonic Hall voltages as a function of the transverse (

H y

) (c) and longitudinal (

H x

) fields (d). The solid lines are linear fits to the data. For the data in (b)−(d), the AC current was

3 m A

in amplitude. Dependence of spin-torque efficiencies (

β L (T)

) with (e) Film Thickness, (f) Growth temperature and (g) Vicinal angles of the substrates (left). We also show the dependence of

θ S H

with vicinal angles of the substrates in (g) (right). If not specified, the vicinal angle for the vicinal substrates is

7 °

, the growth temperature of the L1₀-FePt film is

700 ° C

, and

6 n m

in thickness.

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