Abstract
The magnetic insulator yttrium iron garnet can be grown with exceptional quality, has a ferrimagnetic transition temperature of nearly 600 K, and is used in microwave and spintronic devices that can operate at room temperature. The most accurate prior measurements of the magnon spectrum date back nearly 40 years, but cover only 3 of the lowest energy modes out of 20 distinct magnon branches. Here we have used time-of-flight inelastic neutron scattering to measure the full magnon spectrum throughout the Brillouin zone. We find that the existing models of the excitation spectrum fail to describe the optical magnon modes. Using a very general spin Hamiltonian, we show that the magnetic interactions are both longer-ranged and more complex than was previously understood. The results provide the basis for accurate microscopic models of the finite temperature magnetic properties of yttrium iron garnet, necessary for next-generation electronic devices.
Highlights
Yttrium iron garnet (YIG) is the ‘miracle material’ of microwave magnetics.[1]
The magnetic order in YIG is driven by exchange interactions, which are the result of the quantum mechanical requirement that the electronic states are antisymmetric under particle exchange
Models including anisotropic which would be expected to change very little exchange or Dzyaloshinskii–Moriya interactions on the 1st–4th neighbour bonds were tested, but such interactions were found to destabilise the magnetic structure for arbitrarily small perturbations
Summary
Yttrium iron garnet (YIG) is the ‘miracle material’ of microwave magnetics.[1]. Since its synthesis by Geller and Gilleo in 1957,2 it is widely acknowledged to have contributed more to the understanding of electronic spin–wave and magnon dynamics than any other substance.[3]. YIG (chemical formula Y3Fe5O12, crystal structure depicted in Fig. 1a) is a ferrimagnetic insulating oxide with the lowest magnon damping of any known material. Its exceptionally narrow magnetic resonance linewidth—orders of magnitude lower than the best polycrystalline metals—allows magnon propagation to be observed over centimetre distances This makes YIG both a superior model system for the experimental study of fundamental aspects of microwave magnetic dynamics[4] (and general wave and quasi-particle dynamics5,6), and an ideal platform for the development of microwave magnetic technologies, which have already resulted in the creation of the magnon transistor and magnon logic gates.[4,7,8]
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