We investigate and compare the electronic structure of a bulk single crystal of Y3Fe5O12 garnet [YIG, a high-TC (= 560 K) ferrimagnet] with that of an epitaxial ultrathin (3.3 nm) film of YIG with a reduced ferrimagnetic temperature TC=380K, 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 Fe3+ state for octahedral and tetrahedral sites. The Fe 3s spectrum shows a clear splitting which allows us to estimate the on-site Fe 3s-3d exchange interaction energy. The valence band HAXPES spectrum shows Fe 3d, O 2p, and Fe 4s derived features and a band gap of ∼2.3eV in the occupied density of states, consistent with the known optical band gap of ∼2.7eV. Fe L-edge XAS identifies the octahedral Fe3+ and tetrahedral Fe3+ site features. XMCD spectra at the Fe L2,3 edges show that bulk single-crystal YIG exhibits antiferromagnetic coupling between the octahedral- A nd 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 Fe2+ contribution and a reduced Fe3+ local spin moment. The Fe2+ 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 ∼6 nm. The presence of Fe2+ 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.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics