Friday 12th August 2022

Electric-field control of spin dynamics during magnetic phase transitions

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Abstract

Controlling magnetization dynamics is imperative for developing ultrafast spintronics and tunable microwave devices. However, the previous research has demonstrated limited electric-field modulation of the effective magnetic damping, a parameter that governs the magnetization dynamics. Here, we propose an approach to manipulate the damping by using the large damping enhancement induced by the two-magnon scattering and a nonlocal spin relaxation process in which spin currents are resonantly transported from antiferromagnetic domains to ferromagnetic matrix in a mixed-phased metallic alloy FeRh. This damping enhancement in FeRh is sensitive to its fraction of antiferromagnetic and ferromagnetic phases, which can be dynamically tuned by electric fields through a strain-mediated magnetoelectric coupling. In a heterostructure of FeRh and piezoelectric PMN-PT, we demonstrated a more than 120% modulation of the effective damping by electric fields during the antiferromagnetic-to-ferromagnetic phase transition. Our results demonstrate an efficient approach to controlling the magnetization dynamics, thus enabling low-power tunable electronics.

INTRODUCTION

Damping, which is a fundamental parameter that defines the magnetization relaxation process, plays a crucial role in the performances of spintronics, magnetic sensors, and magnon devices (16). For example, the spin transfer torque magnetic random access memory (STT-MRAM) devices favor a low damping parameter for a small critical switching current, while a high damping parameter is also desired for reaching an ultrafast switching speed (3). To meet both criteria in STT-MRAM and to achieve tunable microwave and magnon devices, dynamic manipulation of magnetic damping is critical.

The current-induced spin-orbit torque generated by spin-Hall source materials can be used to modulate the effective magnetic damping parameter in ferromagnets (4, 7, 8). The more energy-efficient approach, namely, voltage control of magnetic damping, has been demonstrated with electric field effects (9, 10) and piezo strain effects (1114) by modulating the intrinsic part of the damping (Gilbert damping) in various material systems; however, their tunability has been limited. The extrinsic part of the damping can be contributed from the spin pumping in ferromagnets/spin sink bilayer structures. In such damping process, the angular momentum is transferred from the ferromagnet into the spin sink at ferromagnetic resonance (FMR) (2, 15). It has been shown recently that spin pumping can occur laterally in FeRh (16), a metallic alloy that has ferromagnetic and antiferromagnetic phases coexisting during the metamagnetic phase transition, where the spin current is transported from its ferromagnetic domain into the surrounding antiferromagnetic spin sink (as shown schematically in Fig. 1A). During the antiferromagnetic-to-ferromagnetic phase transition of FeRh, the damping would change drastically because the lateral spin pumping is sensitive to the fraction of the ferromagnetic phases. This suggests an approach to modulate the damping dynamically by small external perturbations. Here, we demonstrate the modulation of the lateral spin pumping in epitaxial thin films of FeRh on piezoelectric substrates by controlling the ferromagnetic phase fraction using the electric field–induced strain, which results in a reversible modulation of the effective magnetic damping of more than 120% during the phase transition. This is evidenced by the strong correlation between the electric-field–induced magnetic damping and the change of ferromagnetic domain fraction, both probed by the FMR. Our analytical model suggests that the spin pumping is the dominant contribution to the damping enhancement, demonstrating the role of mixed-phase coexistence, more importantly the coupling between the two phases, in achieving high tunability of damping. This study not only provides an efficient approach to control the magnetic damping that is useful for tunable magnetic devices but also demonstrates the FeRh/piezoelectric heterostructure to be a good platform to explore the spin dynamics and the interfacial spin transport.

<a rel="nofollow" href="https://advances.sciencemag.org/content/advances/6/40/eabd2613/F1.large.jpg?width=800&height=600&carousel=1" title="Tuning mechanism of the magnetic damping in FeRh/PMN-PT. (A) Schematic illustration of the FeRh/PMN-PT heterostructure where the antiferromagnetic (AFM) domains embedded in the ferromagnetic (FM) matrix of FeRh were driven into FMR by microwave magnetic fields hrf. The electric field (E) was applied across the thickness direction of the PMN-PT substrate to induce piezo strains. Bottom schematics show the isothermal growth of antiferromagnetic domains in the FeRh under relatively large electric fields. The curves in the blue region represents the profile of spin current density across the lateral direction of the antiferromagnetic domain, where |Js0| is the magnitude of spin current density at the ferromagnetic/antiferromagnetic interface; λ is the spin diffusion length in the antiferromagnetic domain. a.u., arbitrary units. (B) FMR spectra and the fittings of the FeRh/PMN-PT at 380 K during heating with applied electric fields of 0 (red) and 0.67 V μm−1 (blue), respectively. (C) Temperature dependence of the resonance linewidth μ0ΔH and the corresponding effective magnetic damping αeff with applied electric fields of 0 (red) and 0.67 V μm−1 (blue), respectively. (D) Temperature dependence of the ferromagnetic phase fraction with applied electric fields of 0 (red) and 0.67 V μm−1 (blue), respectively." class="fragment-images colorbox-load" rel="gallery-fragment-images-1320266724" data-figure-caption="

Fig. 1 Tuning mechanism of the magnetic damping in FeRh/PMN-PT.

(A) Schematic illustration of the FeRh/PMN-PT heterostructure where the antiferromagnetic (AFM) domains embedded in the ferromagnetic (FM) matrix of FeRh were driven into FMR by microwave magnetic fields hrf. The electric field (E) was applied across the thickness direction of the PMN-PT substrate to induce piezo strains. Bottom schematics show the isothermal growth of antiferromagnetic domains in the FeRh under relatively large electric fields. The curves in the blue region represents the profile of spin current density across the lateral direction of the antiferromagnetic domain, where |Js0| is the magnitude of spin current density at the ferromagnetic/antiferromagnetic interface; λ is the spin diffusion length in the antiferromagnetic domain. a.u., arbitrary units. (B) FMR spectra and the fittings of the FeRh/PMN-PT at 380 K during heating with applied electric fields of 0 (red) and 0.67 V μm−1 (blue), respectively. (C) Temperature dependence of the resonance linewidth μ0ΔH and the corresponding effective magnetic damping αeff with applied electric fields of 0 (red) and 0.67 V μm−1 (blue), respectively. (D) Temperature dependence of the ferromagnetic phase fraction with applied electric fields of 0 (red) and 0.67 V μm−1 (blue), respectively.

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Fig. 1 Tuning mechanism of the magnetic damping in FeRh/PMN-PT.

(A) Schematic illustration of the FeRh/PMN-PT heterostructure where the antiferromagnetic (AFM) domains embedded in the ferromagnetic (FM) matrix of FeRh were driven into FMR by microwave magnetic fields hrf. The electric field (E) was applied across the thickness direction of the PMN-PT substrate to induce piezo strains. Bottom schematics show the isothermal growth of antiferromagnetic domains in the FeRh under relatively large electric fields. The curves in the blue region represents the profile of spin current density across the lateral direction of the antiferromagnetic domain, where |Js0| is the magnitude of spin current density at the ferromagnetic/antiferromagnetic interface; λ is the spin diffusion length in the antiferromagnetic domain. a.u., arbitrary units. (B) FMR spectra and the fittings of the FeRh/PMN-PT at 380 K during heating with applied electric fields of 0 (red) and 0.67 V μm−1 (blue), respectively. (C) Temperature dependence of the resonance linewidth μ0ΔH and the corresponding effective magnetic damping αeff with applied electric fields of 0 (red) and 0.67 V μm−1 (blue), respectively. (D) Temperature dependence of the ferromagnetic phase fraction with applied electric fields of 0 (red) and 0.67 V μm−1 (blue), respectively.

FeRh with the ordered CsCl-type structure exhibits a first-order metamagnetic phase transition from an antiferromagnetic order with a G-type spin structure to a ferromagnetic order above the room temperature (17). The transport (18, 19), spin and orbital moment (2022), FMR (16, 23, 24), and the ultrafast dynamics (25, 26) have been investigated in FeRh thin films across the phase transition, in which the unique magnetic phase transition and the coexisting phases trigger tremendous interest in heat-assisted magnetic recording (2729), voltage-controlled magnetism (30), and antiferromagnetic spintronics (3136). The magnetic phase transition is accompanied by a volume expansion of 1%, suggesting strong coupling between the magnetic ordering and the unit cell structure (3741). By using piezoelectric substrates, electric-field control of the magnetic phase transition in FeRh has been realized (42), probed by the electric field dependence of magnetization (4347) and resistivity measurements (48, 49).

RESULTS

In this study, highly [001]-oriented FeRh thin films were grown on (001)-oriented PMN-PT (0.72PbMg1/3Nb2/3O3-0.28PbTiO3) substrates by DC magnetron sputtering (Materials and Methods), which is confirmed by the out-of-plane x-ray diffraction with the observation of only (001) and (002) reflections from the FeRh thin film (see the Supplementary Materials).The details of the film growth and structural characterizations can be found in Materials and Methods and the Supplementary Materials (48). The phase…

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