Magnetic properties and domain structure of ultrathin yttrium iron garnet/Pt bilayers

We report on the structure, magnetization, magnetic anisotropy, and domain morphology of ultrathin yttrium iron garnet (YIG)/Pt films with thickness ranging from 3 to 90 nm. We find that the saturation magnetization is close to the bulk value in the thickest films and decreases at low thickness with a strong reduction below 10 nm. We characterize the magnetic anisotropy by measuring the transverse spin Hall magnetoresistance as a function of applied field. Our results reveal strong easy-plane anisotropy fields of the order of 50–100 mT, which add to the demagnetizing field, as well as weaker in-plane uniaxial anisotropy ranging from 10 to 100 $\mu \mathrm{T}$. The in-plane easy-axis direction changes with thickness but presents also significant fluctuations among samples with the same thickness grown on the same substrate. X-ray photoelectron emission microscopy reveals the formation of zigzag magnetic domains in YIG films thicker than 10 nm, which have dimensions larger than several 100 $\mu \mathrm{m}$ and are separated by achiral Néel-type domain walls. Smaller domains characterized by interspersed elongated features are found in YIG films thinner than 10 nm.

Figure 1

(a–c) Structural characterization of GGG/YIG(111) films of different thickness by $\theta -2\theta$ scans using XRD. (d, e) Reciprocal space maps around the (664) and (486) diffraction peaks. The oscillations in the $\theta -2\theta$ and $q_{\mathrm{z}}$ scans are thickness oscillations. The thickness derived from these measurements agrees with that measured using x-ray reflectivity. (f, g) High-resolution STEM image of the two interfaces in the stack GGG/YIG(72)/Pt(9). The inset shows a cross section of the full stack. (h, i) Elemental profiles across the YIG/Pt and YIG/GGG interfaces measured by EDS.

Figure 2

Comparison of the Fe $L_{2,3}$ spectra measured by EELS across the YIG/Pt interface. The spectra, shown after background subtraction, have been averaged on 3-nm-long line scans for each region of interest, as labeled in the inset with colored dots. The probe beam diameter is 0.15 nm.

Figure 3

(a) AFM image of YIG(28.5)/Pt(3). (b) Root-mean-square roughness as a function of YIG thickness measured by AFM.

Figure 4

(a) $M_{\mathrm{s}}$ as a function of YIG thickness measured by SQUID for sample series SMR 1 (red) and SMR 2 (grey). (b) Magnetic hysteresis of YIG(9)/Pt(3) as a function of in-plane magnetic field. (c) X-ray absorption spectra of YIG(3.7,12.4,86.7)/Pt(1.9). Each line represents the sum of two spectra acquired with positive and negative circular x-ray polarization. The spectra are shifted vertically by a constant offset for better visibility. (d) XMCD spectra for the thickest and thinnest samples obtained by taking the difference between two absorption spectra acquired with positive and negative circular x-ray polarization. The spectra were acquired on homogeneously magnetized domains using the XPEEM setup.

Figure 5

(a) Schematic of the Hall bar and coordinate system. OOP field scans for the (b) thickest (90 nm) and (c) thinnest (3.4 nm) YIG thickness from series SMR 1. From these scans, the out-of-plane saturation field $B_{\text{s}}$ was determined as indicated by the dashed line and summarized in (d) as a function of YIG thickness. (e) Easy-plane anisotropy field $B_{{\text{K}}_1}=B_{\text{s}}-{\mu }_0{M}_{\mathrm{s}}$. Data shown for sample series SMR 1 (red) and SMR 2 (grey).

Figure 6

Transverse resistance $R_{\text{xy}}$ as a function of azimuthal orientation of the external magnetic field, ${\phi }_{\mathrm{B}}$, for different thicknesses in (a–c). The data are shown by black symbols, the macrospin simulations by red solid lines. (d) Easy-axis orientation, ${\phi }_{\mathrm{EA}}$, relative to the $\left[1\overline{1}0\right]$ crystal axis. (e) Effective uniaxial anisotropy field $B_{{\text{K}}_2}$ and (f) in-plane uniaxial energy $M_{\text{s}}{B}_{{\text{K}}_2}$ as a function of YIG thickness. Black and red points show the results for two different Hall bars patterned on the same chips of series SMR 1; grey triangles are results obtained on Hall bars of series SMR 2.

Figure 7

(a) Schematics of the direction of the incoming x-ray beam (red arrow) relative to the crystal axes in the XPEEM setup. (b–d) Domain structure of YIG/Pt(1.9) bilayers with $t_{\text{YIG}}=$ 12.4, 8.7, and 3.7 nm observed by XPEEM. The gray scale contrast corresponds to different in-plane orientations of the magnetization.

Figure 8

XPEEM images of domain walls in (a) 86.7-, (b) 28.5-, and (c) 12.4-nm-thick YIG. The arrows indicate the x-ray incidence direction. (d–f) Line profiles of the walls corresponding to the dashed lines in (a–c). (g, h) Details of the apices of the zigzag domains in 86.7- and 28.5-nm-thick YIG. The samples are oriented with the crystal axes defined in Fig. 7.