1 Current -induced perpendicular effective magnetic field in magnetic heterostructures Qianbiao Liu1 Lijun Zhu12

2025-04-28 0 0 999.57KB 11 页 10玖币

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Current-induced perpendicular effective magnetic field in magnetic heterostructures

Qianbiao Liu,1 Lijun Zhu1,2*

1. State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of

Sciences, Beijing 100083, China

2. College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences,

Beijing 100049, China

*Email: ljzhu@semi.ac.cn

Abstract: Generation of perpendicular effective magnetic field or perpendicular spins (σz) is central for the

development of energy-efficient, scalable, and external-magnetic-field-free spintronic memory and computing

technologies. Here, we report the first identification and the profound impacts of a significant effective

perpendicular magnetic field that can arise from asymmetric current spreading within magnetic microstrips and

Hall bars. This effective perpendicular magnetic field can exhibit all the three characteristics that have been widely

assumed in the literature to “signify” the presence of a flow of σz, i.e., external-magnetic-field-free current

switching of uniform perpendicular magnetization, a sin2φ-dependent contribution in spin-torque ferromagnetic

resonance signal of in-plane magnetization (φ is the angle of the external magnetic field with respect to the current,

and a φ-independent but field-dependent contribution in the second harmonic Hall voltage of in-plane

magnetization. This finding suggests that it is critical to include current spreading effects in the analyses of various

spin polarizations and spin-orbit torques in magnetic heterostructure. Technologically, our results provide

perpendicular effective magnetic field induced by asymmetric current spreading as a novel, universally accessible

mechanism for efficient, scalable, and external-magnetic-field-free magnetization switching in memory and

computing technologies.

Keywords: Spin current, spin-orbit torque, perpendicular spins, ferromagnetic resonance

INTRODUCTION

Development of high-density magnetic memory and computing technologies requires energy-efficient and scalable

electrical switching of perpendicular magnetization. Microscopically, perpendicular magnetization can be

switched by a current of transverse spins (σy) under the assist of an in-plane effective magnetic field along current

(𝐻𝑥

eff, to overcome the Dzyaloshinskii–Moriya interaction or to break the switching symmetry)1, or by an anti-

damping spin torque exerted by high-density current of perpendicular spins (σz)2-10, or by a strong perpendicular

effective magnetic field (𝐻𝑧

eff)11-15. Since the first method (σy+𝐻𝑥

eff) is hardly energy-efficient or scalable, searching

of σz or 𝐻𝑧

eff in magnetic heterostructures becomes a very hot topic.

Experimentally, it is widely assumed that the presence of σz could be concluded from a small but sizable sin2φ

dependent contribution in spin torque ferromagnetic resonance (ST-FMR)2,7,8,16-18 or a φ independent second

harmonic Hall voltage response (HHVR)5,19,20 of an in-plane magnetization (φ is the angle of external magnetic

field with respect to the current). Presence of σz is also claimed from the occurrence of external-field-free current

switching of a uniform perpendicular magnetization4,9-15 because the polarization and fieldlike spin-orbit torque

(SOT) field of σz, if any, are along the film normal.

In this work, we show that 𝐻𝑧

eff can arise from asymmetric current spreading and thus can widely exist in ST-

FMR bars and Hall bars of magnetic heterostructures. While being microscopically distinct from σz, this current-

spreading-induced 𝐻𝑧

eff shows all the three characteristics that were widely assumed in the literature to signify

the presence of σz, i.e., it can enable external-field-free current switching of perpendicular magnetization and

contributes to HHVR and ST-FMR signals of in-plane magnetization in analogue to the fieldlike SOT of σz.

Neglect of asymmetric current spreading can lead to erroneous analyses of various spin polarizations and SOTs.

Sample Characterizations

For this work, a series of Pt 4/Py 3.3-9.4, Pt 4/FeCoB 2.8-9.6, Pt 4/Ni 2.4-9.2, Pt 4/Co 1.7, and Pt 4/ FeTb 7.5

bilayers are sputter-deposited on oxidized Si substrates (the numbers are layer thicknesses in nanometer, FeCoB

= Fe60Co20B20, Py = Ni81Fe19, FeTb = Fe65Tb35). Each sample is protected by a MgO 1.6 /Ta 1.6 bilayer that is fully

oxidized upon exposure to the atmosphere21. The samples are patterned into microstrips and Hall bars by

photolithography and ion milling, followed by deposition of Ti 5/Pt 150 as the contacts for ST-FMR, HHVR, and

switching measurements.

Effective Perpendicular Magnetic Field within Magnetic Strips

We first perform ST-FMR measurements on the Pt 4/Py 9.4 microstrips using the nominally symmetric 3-

terminal contact configuration in Fig. 1(a). The symmetric (S) and anti-symmetric (A) components of the ST-FMR

responses for the Pt 4/Py 9.4 are plotted in Figs. 1(b) and 1(c) as a function of φ (see Ref. 28 and Supplementary

materials for the method how to determine the S and A). Considering a magnetic strip interacting with a spin

current with arbitrary spin polarization σ = (σx, σy, σz), the S and A values should vary with φ following2,22,23:

S =SDL,y sin2φcosφ + SDL,x sin2φsinφ + SFL,z sin2φ + SSPsinφ, (1)

A = AFL,y sin2φ cosφ + AFL,x sin2φ sinφ + ADL, z sin2φ. (2)

The four terms of Eq. (1) are contributions of the dampinglike SOT of σy, the dampinglike SOT of σx, 𝐻𝑧

eff

(fieldlike SOT of σz and others), and spin pumping, respectively. Equation (2) includes the contributions from the

sum of the fieldlike SOT of σy and the transverse Oersted ﬁeld, the fieldlike SOT of σx, and the dampinglike SOT

of σz. The fits of the S and A data to Eqs. (1) and (2) untangle different contributions and yield the values of SDL,y,

SDL,x, SFL,z, SSP, AFL,y, AFL,x, and ADL,z. As expected, there is a predominant contribution from σy (SDL,y, AFL,y) but no

indication of σx (SDL,x = 0, AFL,x = 0, see Figs. 1(b) and 1(c)).

However, it is striking that such heavy metal/ferromagnet (HM/FM) samples exhibit a non-negligible SFL,z term

and thus a non-zero 𝐻𝑧

eff, in the case of the 3-terminal geometry (Fig. 1(a)). This 𝐻𝑧

eff is unlikely to arise from

σz because we only measure a negligible dampinglike SOT of σz (i.e., ADL,z ≈ 0). This is reasonable because the

sputter-deposited polycrystalline bilayers do not have a long-range lateral crystal or magnetic symmetry breaking.

Fig. 1. ST-FMR results of the Pt 4/Py 9.4 strip measured using the nominally symmetric 3-terminal contact

configuration (W=10 μm, L=20 μm, the rf power is 15 dBm). (a) Schematic of the 3-terminal ST-FMR

measurement configuration. φ dependence of (b) S and (c) A from the 3-terminal ST-FMR measurements. The

solid curves in (b) represent the SFL,z, SDL,y, and Ssp components as determined from fit of the data to Eq.(1). The

solid curves in (c) represent the ADL,z and AFL,y components as determined from fit of the data to Eq.(2).

We also find no obvious correlation between SFL,z and the interfacial SOC strength for the Pt/FM samples. First,

we find a strong SFL,z component in control samples Cu 2/Py 9.4/Cu 2 and Cu 2/FeCoB 9.6/Cu 2 with negligible

SOC (Figs. 2(a) and 2(b)). As we show in Fig. 2(c), the SFL,z/SDL,y ratios for the Pt/Py, Pt/FeCoB, and Pt/Ni samples,

with similar layer thicknesses, do not scale with the interfacial perpendicular magnetic anisotropy energy density

(Ks) of the Pt/FM interfaces (as determined in Supplementary Materials), with Ks reflecting the interfacial SOC

strength24,25. These observations indicate that the 𝐻𝑧

eff here is not induced by any SOC-related precession/rotation

scattering4,5. The SFL,z/AFL,y ratio of a given device is found to be largely independent of the power of the rf current

(Fig. 2(d)), excluding any relevance of the 𝐻𝑧

eff to thermal effects. Note that the “hidden” vertical symmetry

breaking that leads to bulk SOT of σy in ferromagnets21,26 and ferrimagnets27 cannot explain the 𝐻𝑧

eff because the

latter requires a lateral rather than vertical asymmetry.

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1Current-inducedperpendiculareffectivemagneticfieldinmagneticheterostructuresQianbiaoLiu,1LijunZhu1,2*1.StateKeyLaboratoryforSuperlatticesandMicrostructures,InstituteofSemiconductors,ChineseAcademyofSciences,Beijing100083,China2.CollegeofMaterialsScienceandOpto-ElectronicTechnology,UniversityofChine...

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1 Current -induced perpendicular effective magnetic field in magnetic heterostructures Qianbiao Liu1 Lijun Zhu12

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