Atomic layer deposition of yttrium iron garnet thin films

Magnetic nanostructures with nontrivial three-dimensional (3D) shapes enable complex magnetization configurations and a wide variety of new phenomena. To date predominantly magnetic metals have been considered for nontrivial 3D nanostructures, although the magnetic and electronic transport responses are intertwined in metals. Here we report the first successful fabrication of the magnetic insulator yttrium iron garnet $({\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}, \mathrm{YIG})$ via atomic layer deposition (ALD) and show that conformal coating of 3D objects is possible. We utilize a supercycle approach based on the combination of subnanometer thin layers of the binary systems ${\mathrm{Fe}}_2{\mathrm{O}}_3$ and ${\mathrm{Y}}_2{\mathrm{O}}_3$ in the correct atomic ratio with a subsequent annealing step for the fabrication of ALD-YIG films on ${\mathrm{Y}}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$ substrates. Our process is robust against typical growth-related deviations, ensuring a good reproducibility. The ALD-YIG thin films exhibit a high crystalline quality as well as magnetic properties comparable to samples obtained by other deposition techniques. We show that the ALD-YIG thin films are conformal. This enables the fabrication of 3D YIG nanostructures once appropriate nonmagnetic, 3D templates are developed. Such 3D YIG structures build the groundwork for the experimental investigation of curvature-induced changes on pure spin currents and magnon transport effects.

Figure 1

A schematic of combining two binary ALD processes via the supercyle approach for the deposition of a ternary compound is given in panel (a). Binary process I describes the atomic layer deposition of ${\mathrm{Y}}_2{\mathrm{O}}_3$, binary process II the one of ${\mathrm{Fe}}_2{\mathrm{O}}_3$. By alternately depositing thin layers of the two materials, a multilayer is achieved. In panel (b) the crystal structure of YIG is depicted. Panel (c) shows the color change observed in ALD deposited nanolaminates on heat treating them at $700{}^{\circ }\mathrm{C}$ for $4\mathrm{h}$ in 3 mbar ${\mathrm{O}}_2$.

Figure 2

XRD spectra showing that for annealing temperatures higher than $600{}^{\circ }\mathrm{C}$, the nanolaminates are converted into crystalline YIG layers [panel (a)]. Panel (b) shows XRD measurements of nanolaminate samples featuring different nominal $Y:Fe$ ratios. The values are up to $\pm 30\%$ off from the optimal ratio of $3:5=0.6$. The light gray arrow denotes the increase of Fe content for decreasing atomic ratio. XRD measurements on samples with different nominal thickness of the ${\mathrm{Y}}_2{\mathrm{O}}_3$ layer (1 to 5 monolayers) are depicted in panel (c). The thickness of the ${\mathrm{Fe}}_2{\mathrm{O}}_3$ layer was adapted accordingly to yield an atomic ratio of 0.6. The samples of all series were annealed for 4 h under an oxygen pressure of 3 mbar. Panel (d) shows a cross-sectional TEM image of an ALD-YIG layer on a YAG substrate as well as the FFTs obtained from the image prior to the annealing step. The cross-sectional TEM image and the FFTs of the YAG substrate and the ALD-YIG layer after the annealing are shown in panel (e).

Figure 3

The evolution of the magnetic moment of the sample annealed at $800{}^{\circ }\mathrm{C}$ for the external magnetic field applied (ip and oop is given in panel (a). To reveal the coercive fields, panel (b) shows the same data around ${\mu }_{0}H=0\mathrm{T}$.

Figure 4

The real and imaginary part of $d_D{S}_{21}$ for an external magnetic field of ${\mu }_{0}H=0.29\mathrm{T}$ applied in the film plane are given in panel (a) as well as fits to the data following Ref. [28]. False color plots of the real part of $d_{\mathrm{D}}{S}_{21}$ as a function of ${\mu }_{0}H$ for ip and oop alignment of the magnetic field as well as the Kittel fits [36] to the respective data sets are depicted in panel (b).

Figure 5

A schematic of a cut through the pre- and postpolished sample is given in panel (a). The field-dependent magnetic moment of the sample pre- and postpolishing recorded at $300\mathrm{K}$ with $H$ within the sample plane is shown in panel (b). In panel (c) possible geometries for future magnetic structures fabricated by ALD are given where the ALD-YIG layer is indicated in orange.