Ultrafast Switching in Synthetic Antiferromagnet with Bilayer Rare-Earth Transition-Metal Ferrimagnets

2025-05-06 0 0 682.34KB 9 页 10玖币

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Ultrafast Switching in Synthetic Antiferromagnet

with Bilayer Rare-Earth Transition-Metal

Ferrimagnets

Chung Ting Ma1,*, Wei Zhou1, and S. Joseph Poon1,2

1University of Virginia, Department of Physics, Charlottesville, Virginia, 22904, USA

2University of Virginia, Department of Materials Science and Engineering, Charlottesville, Virginia, 22904, USA

*ctm7sf@virginia.edu

ABSTRACT

In spintronics, it is important to be able to manipulate magnetization rapidly and reliably. Several methods can control

magnetization, such as by applying current pulses or magnetic ﬁelds. An applied current can reverse magnetization with

nanosecond speed through the spin torque effect. For faster switching, subpicosecond switching with femtoseconds laser pulse

has been achieved in amorphous rare-earth transition-metal ferrimagnets. In this study, we employed atomistic simulations

to investigate ultrafast switching in a synthetic antiferromagnet with bilayer amorphous FeGd ferrimagnets. Using a two-

temperature model, we demonstrated ultrafast switching in this synthetic antiferromagnet without external magnetic ﬁelds.

Furthermore, we showed that if we initially stabilize a skyrmion in this heterostructure, the ultrafast laser can switch the skyrmion

state using the same mechanism. Furthermore, this bilayer design allows the control of each ferrimagnetic layer individually

and opens the possibility for a magnetic tunnel junction.

Introduction

The ability to control magnetization is a critical component of designing memory and logical devices. In thin ﬁlms, mag-

netizations are commonly manipulated through current or external ﬁelds. In spintronic devices, currents are often used

to induce spin-transfer torque and spin-orbit torque to reliably switch magnetizations without magnetic ﬁelds

1–5

. Besides

electrical current, a laser pulse can also induce changes in magnetization. Subpicosecond demagnetization with femtosecond

laser pulse was ﬁrst observed in ferromagnetic nickel ﬁlm

. Since then, ultrafast manipulation of magnetization has drawn

considerable interest for its potential applications. In ferromagnets, a multistep procedure has been demonstrated to switch

magnetization

7–10

. For example, in FePt nanoparticles, magnetizations are ﬁrst thermally demagnetized, then re-magnetized

through the laser-induced inverse Faraday effect

. In antiferromagnets, optical switching of antiferromagnetic order is observed

in multiferroic TbMnO

at 18 K

. Furthermore, reliable all-optical switching of magnetization in easy-plane CrPt has been

proposed by unitizing the inverse Faraday effect

. Nonetheless, one-shot all-optical subpicosecond switching has only been

observed in ferrimagnets, such as rare-earth transition metal (RE-TM) ferrimagnets

13–20

and recently, Mn-based crystalline

alloys21.

Amorphous RE-TM ferrimagnetic ﬁlms are one of the more appealing materials for applications. They consist of two

antiferromagnetically coupled RE-TM sublattices, which align in an antiparallel direction. There exists a compensation

temperature (T

Comp

) where the magnetic moment of the two sublattices cancel each other and magnetization goes to zero

22,23

RE-TM ﬁlms contain several attractive properties, including perpendicular magnetic anisotropy (PMA)24,25 and high domain

wall velocity

26,27

. Furthermore, they are deposited at room temperature

and their composition can be tuned to adjust

magnetization and coercivity

23,28

. Recent experiments also observed skyrmions in RE-TM thin ﬁlms

27,29–31

. One of the

most intriguing properties of RE-TM ferrimagnet is the access to one-shot all-optical ultrafast switching

13–20

. In previous

studies, it is revealed that angular momentum exchange between the two different sublattices is a key ingredient in all-optical

switching

15,32,33

. The requirement of having two different sublattices makes ferrimagnets, such as RE-TM, one of the few

PMA materials to have this capability.

In this study, we investigate laser-induced ultrafast switching in a synthetic antiferromagnet (SAF) formed from a bilayer

RE-TM ferrimagnet . A schematic diagram of this heterostructure is shown in Figure 1. In this heterostructure, two different

compositions of 5 nm thick FeGd combine to form a 10 nm thick SAF, with one layer having T

Comp

above room temperature

and the other having T

Comp

below room temperature. Such control of T

Comp

in RE/TM ﬁlms has been achieved experimentally

by tuning composition of Fe and Gd

, where a higher T

Comp

was achieved by increasing rare-earth concentration. To elaborate,

arXiv:2210.14119v1 [cond-mat.mes-hall] 25 Oct 2022

this SAF arises from the cancellation of magnetization between the top and bottom FeGd layer. The magnetization in each layer

is designed to be opposite but equal in magnitude at room temperature. This is obtained by choosing the T

Comp

of the top layer

to be 350 K and the T

Comp

of the top layer to be 250 K. This heterostructure presents several advantages. Compared to SAF with

ferromagnet or multilayer RE/TM ﬁlms, SAF with RE-TM allows more ﬂexible tuning of each layer. The thickness

34,35

and

composition

of each layer can be varied while the net magnetization stays zero, and PMA remains robust. In contrast, SAF

with ferromagnet and multilayer RE/TM ﬁlms are limited in thickness and composition to maintain PMA

18,36

. Furthermore,

the use of thicker layers diminishes the relative strength of interface exchange on an individual layer. This opens the possibility

of switching each layer individually. In this study, we explored laser-induced ultrafast switching in SAF with RE-TM by using

a two-temperature model for laser irradiation

37,38

. We found deterministic spins switching in this heterostructure, like those

observed in single-layer RE-TM ﬁlms. More importantly, synchronized switching are found within the same sublattice in the

FeGd bilayer. Furthermore, we stabilized skyrmions in this heterostructure as initial states and found switching remains robust

with a laser pulse. These ﬁndings pave the way to employ SAF with a bilayer RE-TM for spintronics applications.

Figure 1.

A schematic diagram of a synthetic antiferromagnet used in this study. Two 5 nm thick FeGd combines to form the

synthetic antiferromagnet. In this study, layer 1 has T

Comp

at 350 K, above room temperature, and layer 2 has T

Comp

at 250 K,

below room temperature.

Results and Disscussions

Figure 2. (a) Time evolution of magnetic moment per atom with application of a 100-fs laser pulse with 30 mJ/m2ﬂuence

every 100 ps. (b) Time evolution of total magnetic moment of Gd and Fe sublattice in each FeCo layer with application of a

laser pulse every 100 ps.

Figure 2shows the results of ultrafast switching in SAF with FeGd after laser pulses. Initially, the spins of Fe sublattices

are pointing down (- z-direction) and the spins of Gd sublattices are pointing up (+ z-direction). The spins are initialized by

the application of a 0.01 T out-of-plane magnetic ﬁeld, and no external ﬁelds are applied after initialization and during the

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摘要：

UltrafastSwitchinginSyntheticAntiferromagnetwithBilayerRare-EarthTransition-MetalFerrimagnetsChungTingMa1,*,WeiZhou1,andS.JosephPoon1,21UniversityofVirginia,DepartmentofPhysics,Charlottesville,Virginia,22904,USA2UniversityofVirginia,DepartmentofMaterialsScienceandEngineering,Charlottesville,Virginia...

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Ultrafast Switching in Synthetic Antiferromagnet with Bilayer Rare-Earth Transition-Metal Ferrimagnets

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