Unusual antiferromagnetic resonance in a common iron oxide

What is it about?

The antiferromagnetic materials are crystals with spins of magnetic atoms oriented in a staggered order with half of spin oriented antiparallel to the other half. This ordering results in a zero net magnetic moment that makes it difficult to read a state of an antiferromagnet. However, the interaction responsible for this spin arrangement also causes spins to precess at rates orders of magnitude faster than in conventional ferromagnets. This precession is called the antiferromagnetic resonance and can be excited with electromagnetic radiation in the range of terahertz frequencies. This antiferromagnetic excitation could be used to read or even change a state of an antiferromagnet. For applications, it is not only important that the spin dynamics are fast, but also that they are very weakly damped. One antiferromagnet that has both of these qualities is a common iron oxide, hematite (alpha-Fe2O3). In our spectroscopy studies of a bulk crystals, we observed that the width of antiferromagnetic resonance in this material is below 1 GHz, while its frequency ranges from 200 to almost 500 GHz when the temperature changes.

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Photo by Roberto Sorin on Unsplash

Photo by Roberto Sorin on Unsplash

Why is it important?

It has a crucial importance for designing spintronic devices that would be based on hematite. We show that around room temperature, the resonance frequency of hematite rises with temperature, which is an atypical behavior to magnetic materials. At around 410 C, this resonance frequency reaches a maximum of about 485 GHz and then drops sharply when approaching the critical temperature of about 693 C. We characterized the width of this resonance above room temperature, and found that it grows exponentially with rising temperature.