Synthesis of yttrium iron garnet/bismuth quantum dot heterostructures with localized plasmon enhanced magneto-optical performance

doi:10.1016/j.jmst.2020.03.025

Abstract

Abstract:

Interactions between light and magnetic matter attracted great attention lately due to their potential applications in nanophotonics, spintronics, and high-accuracy sensing. Here, we grew bismuth quantum dots (Bi-QDs) with strong spin-orbit coupling on a magnetic insulator yttrium iron garnet (YIG) via molecular beam epitaxy. The YIG/Bi-QDs material shows an enhanced magneto-optical Kerr rotation up to 130% compared with that of a bare YIG film. The Bi-QDs were also introduced onto a lutetium-bismuth co-doped YIG film to form a hybrid system with remarkably enhanced Kerr rotation (from 1626 to 2341 mdeg). Ferromagnetic resonance measurements showed an increased effective magnetization as well as interfacial spin-orbit field in the YIG/Bi-QD heterostructures. Localized plasmons were mapped using electron energy loss spectroscopy with high spatial resolution, revealing enhanced plasmon intensity at both the Bi-QD surface and YIG/Bi-QD interface. Introducing Bi-QDs onto the YIG film enhanced Kerr rotation owing to the attenuated optical reflection and increased effective magnetization. The Bi-QD-enhanced magneto-optical effect enables development of efficient nanoscale light switching, spintronics, and even plasmonic nano-antennas.

Key words: Electron energy loss spectroscopy, Surface plasmon resonance, Magneto-optical materials, Bismuth quantum dot, Ferromagnetic resonance

Lichuan Jin, Caiyun Hong, Dainan Zhang, Peng Gao, Yiheng Rao, Gang Wang, Qinghui Yang, Zhiyong Zhong, Huaiwu Zhang. Synthesis of yttrium iron garnet/bismuth quantum dot heterostructures with localized plasmon enhanced magneto-optical performance[J]. J. Mater. Sci. Technol., 2020, 51: 32-39.

Figures/Tables 7

Fig. 1. (a) STEM image of Bi-QDs grown on a YIG film surface; the scale bar is 100 nm. (b) Bright-field (BF) image of a single self-assembled Bi-QD on a YIG film; the scale bar is 10 nm. (c) Expanded view of the BF image indicated by the square in (a); the scale bar is 5 nm. (d-g) EDX elemental mapping of bismuth, yttrium, iron, and oxygen, respectively; each scale bar is 10 nm.

Table 1 Size and distribution of different samples as determined by atomic force microscopy and STEM. Each number in parentheses denotes the Bi-QD growth time (time, min).

Sample	Number per μm²	Mean height (nm)	Mean diameter (nm)
YIG/Bi-QD(10)	4.9 ± 0.1	18.1 ± 0.1	29.8 ± 0.1
YIG/Bi-QD(20)	6.6 ± 0.1	31.9.±0.1	36.2 ± 0.1
YIG/Bi-QD(30)	12.7 ± 0.1	43.2 ± 0.1	55.1 ± 0.1
YIG/Bi-QD(40)	25.8 ± 0.1	45.8 ± 0.1	57.7 ± 0.1
YIG/Bi-QD(50)	38.5 ± 0.1	47.9 ± 0.1	60.4 ± 0.1

Fig. 2. (a) Schematic of the ferromagnetic resonance measurement. (b) H dependence of S21 in the YIG/Bi-QD specimen; the excitation frequency is 11 GHz, and the crystal orientation of the YIG substrate is (111). (c) Schematic of the angular-dependent FMR measurement. (d) Bi-QD growth-time dependence of the magnetic anisotropy field Hk for the crystal orientations (100) and (111) in YIG/Bi-QDs (marked by red dots and blue squares, respectively); the inset shows the resonance field HFMR as a function of azimuthal angle ΦH for YIG/Bi-QD (40) samples with substrates YIG (100) (red circles) and YIG (111) (blue squares). (e) Bi-QD growth-time dependence of the extracted inhomogeneous broadening ΔH0 (black squares), effective damping parameter αeff (blue diamonds), and effective magnetization 4πMeff (red dots) in YIG(111)/Bi-QDs.

Fig. 3. (a) Raman spectra of the vibrational modes of the YIG/Bi-QD specimen; the red fitting lines show the YIG modes for a laser wavelength of 514 nm and laser power of 2 mW. (b) Raman spectra for the (111) orientation of the GGG substrate. Bi-QD growth-time dependence of the Raman intensity for two A1g modes in GGG at (c) 355 cm-1 and (d) 741 cm-1.

Fig. 4. (a) Fourier transform infrared spectroscopy of the YIG/Bi-QD system; the break in the spectra omits a featureless region. (b) Fluorescence spectra of the YIG/Bi-QD system obtained under 220 nm laser excitation; the inset shows Bi-QD growth-time dependence of the fluorescence peak in each sample.

Fig. 5. Localized plasmon mapping. (a) Cross-sectional STEM image of the YIG/Bi-QD hybrid system. (b) High-angle annular dark-field image of an electron beam transmitted from the vacuum into a single Bi-QD and into the YIG film (the route of the incident electron beam is shown using a yellow arrow). (c) Spatially distributed EEL spectra of the YIG/Bi-QD system with an incident electron beam energy of 60 keV. (d) Position dependences of plasmon intensity and energy with line intensity profiles taken along the line X-X′ in (b); the distance from the vacuum to the Bi-QD is ～7 nm, and the height of a single Bi-QD is ～25 nm. EEL probability maps of the YIG/Bi-QD hybrid system corresponding to different electron energy intervals: (e) 3-7 eV; (f) 16-23 eV; (g) 28-32 eV.

Fig. 6. (a) Schematic of the enhanced polar magneto-optical Kerr effect in a YIG/Bi-QD hybrid system. (b) Enhanced magneto-optical Kerr rotation was observed with the introduction of Bi-QDs. (c) Histogram of Bi growth-time dependence of the polar Kerr signal (red triangles mark reflections); the inset is a schematic of the enhanced magneto-optical Kerr effect in a YIG/Bi-QD hybrid system. (d) Polar magneto-optical Kerr rotation curves for a LBIG/Bi-QD hybrid system with varying Bi-QD growth time.

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