Local spin moments, valency, and long-range magnetic order in monocrystalline and ultrathin films of Y3Fe5O12 garnet

Abstract

We investigate and compare the electronic structure of a bulk single crystal of ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ garnet [YIG, a high-${T}_{C}$ (= 560 K) ferrimagnet] with that of an epitaxial ultrathin (3.3 nm) film of YIG with a reduced ferrimagnetic temperature ${T}_{C}=380\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, using bulk-sensitive hard x-ray photoelectron spectroscopy (HAXPES), x-ray absorption spectroscopy (XAS), and x-ray magnetic circular dichroism (XMCD). The Fe $2p$ HAXPES spectrum of the bulk single crystal exhibits a purely trivalent ${\mathrm{Fe}}^{3+}$ state for octahedral and tetrahedral sites. The Fe $3s$ spectrum shows a clear splitting which allows us to estimate the on-site Fe $3s\text{\ensuremath{-}}3d$ exchange interaction energy. The valence band HAXPES spectrum shows Fe $3d$, O $2p$, and Fe $4s$ derived features and a band gap of $\ensuremath{\sim}2.3\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ in the occupied density of states, consistent with the known optical band gap of $\ensuremath{\sim}2.7\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$. Fe $L$-edge XAS identifies the octahedral ${\mathrm{Fe}}^{3+}$ and tetrahedral ${\mathrm{Fe}}^{3+}$ site features. XMCD spectra at the Fe ${L}_{2,3}$ edges show that bulk single-crystal YIG exhibits antiferromagnetic coupling between the octahedral- and tetrahedral-site spins. The calculated Fe $2p$ HAXPES, Fe $L$-edge XAS, and XMCD spectra using full multiplet cluster calculations match well with the experimental results and confirm the full local spin moments. In contrast, HAXPES, XAS, and XMCD of the Pt/YIG (3.3 nm) ultrathin epitaxial film grown by a pulsed laser deposition method show a finite ${\mathrm{Fe}}^{2+}$ contribution and a reduced ${\mathrm{Fe}}^{3+}$ local spin moment. The ${\mathrm{Fe}}^{2+}$ state is attributed to a combination of oxygen deficiency and charge transfer effects from the Pt capping layer to the ultrathin film. However, the conserved XMCD spectral shape for the ultrathin film indicates that the 3.3-nm epitaxial film is genuinely ferrimagnetic, in contrast to recent studies on films grown by radio-frequency magnetron sputtering which have shown a magnetic dead layer of $\ensuremath{\sim}6$ nm. The presence of ${\mathrm{Fe}}^{2+}$ and the reduced local spin moment in the epitaxial ultrathin film lead to a reduced Curie temperature, quantitatively consistent with well-known mean-field theory. The results establish a coupling of the local Fe spin moments, valency, and long-range magnetic ordering temperature in bulk single crystal and epitaxial ultrathin-film YIG.