Discovery of New Magnon Modes in an Antiferromagnetic Thin Film
Scientists demonstrate how to directly measure higher magnon modes in material
February 8, 2023
Top: RIXS spectrum of a-Fe2O3 showing magnon modes DSz=1, 2, 3, 4, and 5, corresponding to different, final spin state Sz (middle panel). Bottom: RIXS process and interactions needed to access multi-magnons with DSz>2. Image credit: J. Li, Y. Gu, Y. Takahashi, K. Higashi, T. Kim, Y. Cheng, F. Yang, J.Kunes, J. Pelliciari, A. Hariki, V. Bisogni. PRX 13, 011012 (2023).
Scientists discovered a new set of magnon modes in a thin film of hematite (Fe2O3) and demonstrated that resonant inelastic x-ray scattering (RIXS) can directly measure them.
Antiferromagnetic (AFM) materials such as hematite offer potential for applications in information transfer via magnons. However, the fundamental magnetic properties of materials in thin film form are unknown. This study discovers a new method for studying magnetic excitations in hematite thin films, revealing new modes carrying spin momentum up to DSz = 5.
To meet the demand for faster and lower-power electronic devices, researchers are working on novel technologies such as antiferromagnetic spintronics. These technologies aim to exploit the robust and fast spin-transfer properties of these materials. However, to build such devices, researchers need to understand the fundamental spin excitations - magnons - in the material itself. From a fundamental perspective, higher-order magnon excitations have been inaccessible to direct measurement methods. Furthermore, from a technological perspective, the device fabrication relies on the use of materials in thin film form, which are more challenging and experimentally much less understood than bulk-like systems.
Hematite is a well-known antiferromagnet, with a total spin momentum S=5/2. In thin film form, several intriguing properties have been discovered in this material, ranging from long-range spin transport to electrical-switching capabilities. In this study, a team of researchers demonstrated for the first time the ability to directly measure higher-order magnon excitations in hematite thin films, using resonant inelastic x-ray scattering (RIXS) at the Soft Inelastic X-ray Scattering (SIX) beamline of the National Synchrotron Light Source II (NSLS-II) at the U.S. Department of Energy's Brookhaven National Laboratory.
Their results revealed an unexpected magnon spectrum with modes of progressively increasing spin momentum from DSz=1 up to 5 (corresponding to 2S). While the first order mode (DSz=1) is the same as in bulk materials, the subsequent higher-order modes (DSz >1 ) behave differently, representing an important discovery for building devices using hematite in various forms. The team worked with a group of theorists to confirm their discovery with spin calculations. These calculations agree well with the experimental findings, both in terms of the energies of the different modes and their spectral intensities.
In addition, this work also demonstrates that RIXS is a viable probe for studying magnon modes that carry a spin momentum of DSz>2, opening the way to further new discoveries in the field of antiferromagnetic materials with large values of S.
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National Synchrotron Light Source II, Brookhaven National Laboratory
J. Li, Y. Gu, Y. Takahashi, K. Higashi, T. Kim, Y. Cheng, F. Yang, J. Kunes, J. Pelliciari, A. Hariki, V. Bisogni. Single- and Multimagnon Dynamics in Antiferromagnetic α-Fe2O3 Thin Films. PRX 13, 011012 (2023). DOI: 10.1103/PhysRevX.13.011012
The authors thank M. P. M. Dean and T. Uozumi for fruitful discussions. This work is primarily supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences, Early Career Award Program (Brookhaven National Laboratory) and under Grant No. DE-SC0001304 (The Ohio State University). This research uses the beamline 2-ID of the National Synchrotron Light Source II, a DOE Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. A.H. is supported by JSPS KAKENHI Grants No. 21K13884 and No. 21H01003. The computations are performed at the Vienna Scientific Cluster.
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