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. 2018 Jun 1;4(6):eaas8660.
doi: 10.1126/sciadv.aas8660. eCollection 2018 Jun.

Dirac surface state-modulated spin dynamics in a ferrimagnetic insulator at room temperature

Chi Tang  1 , Qi Song  2   3 , Cui-Zu Chang  4   5 , Yadong Xu  1 , Yuichi Ohnuma  6 , Mamoru Matsuo  6   7 , Yawen Liu  1 , Wei Yuan  2   3 , Yunyan Yao  2   3 , Jagadeesh S Moodera  4   8   9 , Sadamichi Maekawa  5 , Wei Han  2   3 , Jing Shi  1
Affiliations

Dirac surface state-modulated spin dynamics in a ferrimagnetic insulator at room temperature

Chi Tang et al. Sci Adv. .

Abstract

This work demonstrates markedly modified spin dynamics of magnetic insulator (MI) by the spin momentum-locked Dirac surface states of the adjacent topological insulator (TI), which can be harnessed for spintronic applications. As the Bi concentration x is systematically tuned in 5-nm-thick (Bi x Sb1-x )2Te3 TI films, the weight of the surface relative to bulk states peaks at x = 0.32 when the chemical potential approaches the Dirac point. At this concentration, the Gilbert damping constant of the precessing magnetization in 10-nm-thick Y3Fe5O12 MI films in the MI/TI heterostructures is enhanced by an order of magnitude, the largest among all concentrations. In addition, the MI acquires additional strong magnetic anisotropy that favors the in-plane orientation with similar Bi concentration dependence. These extraordinary effects of the Dirac surface states distinguish TI from other materials such as heavy metals in modulating spin dynamics of the neighboring magnetic layer.

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Figures

Fig. 1
Fig. 1. FMR measurement principle and TI properties.
(A) Schematic drawing of magnetization dynamics in YIG interfaced with TI in which the spin of the surface state electron is locked to momentum. (B) Room temperature sheet resistance of (BixSb1−x)2Te3 with different Bi concentrations. (C) Temperature dependence of the sheet resistance of seven YIG (10 nm)/(BixSb1−x)2Te3 (five QLs) heterostructures.
Fig. 2
Fig. 2. YIG FMR spectra with and without TI.
(A) Definition of polar angles θH and θM, in FMR measurements. (B) FMR derivative absorption spectra of YIG/Sb2Te3 and YIG reference sample at a frequency of 9.6 GHz with magnetic field applied in-plane (θH = 90°). The solid lines are the best fits to extract the resonance field Hres and peak-to-peak linewidth ΔHpp. a.u., arbitrary units. (C) FMR derivative absorption spectra of YIG/Sb2Te3 and YIG reference sample with the polar angle θH ranging from 0° (out-of-plane) to 90° (in-plane) at 300 K. The extra peak-like feature on the high-field side of the resonance at 0° and 10° is also observed in some other samples, which could be caused by minor inhomogeneity change in YIG due to the presence of the TI layer.
Fig. 3
Fig. 3. Extracted 4πMeff from FMR polar angle dependence fitting.
(A) Polar angle θH dependence of FMR resonance field Hres for all seven YIG (10 nm)/(BixSb1−x)2Te3 (five QLs) samples and YIG reference sample. Solid curves are the best fits. (B) Bi concentration dependence of extracted effective anisotropy field 4πMeff obtained from fitting in (A) for all seven YIG (10 nm)/(BixSb1−x)2Te3 (five QLs) samples. The gray dashed line is the 4πMeff value for the YIG reference sample.
Fig. 4
Fig. 4. Extracted Gilbert damping from FMR linewidth fitting.
(A) FMR transmission spectra S21 for YIG/Sb2Te3 for different chosen frequencies: 4, 6, 8, 10, and 12 GHz at 300 K after background subtraction. (B) Normalized FMR spectra S21 at a fixed frequency of 10 GHz with an applied in-plane static field for YIG/(BixSb1−x)2Te3 samples with different Bi concentrations and YIG reference sample. (C) Frequency dependence of FMR linewidth for all seven YIG/TI samples and YIG reference sample. The resonance peak height is reduced in samples with increased damping constant, which causes poor Lorentzian fitting and, consequently, large apparent noise in extracted linewidth. (D) Bi concentration (x) dependence of the Gilbert damping constant α extracted from the slope of the straight lines in (C).
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