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Séminaire MCBT

Mardi 24 février 2015 à 11h00
Salle Louis Weil, E424
Orateur : Evan Constable
"Terahertz spectroscopy of spin waves in the rare earth orthoferrite : NdFeO3"


With the optical excitation and control of spins in magnetic materials promising new avenues for devices that couple photonic and spintronic functionality, there is a desire to better understand the behaviour of microscopic magnetic interactions under optical excitation. Terahertz radiation has emerged as an effective method for the study of spin dynamics in magnetic materials. Its low energy in the 0.1-10 meV range is is comparable with the low-energy terms of the magnetic Hamiltonian such as magnetic anisotropy and the Dzyaloshinsky-Moriya interaction, providing a route to probe these directly. Terahertz radiation is therefore also complementary to established methods that probe these mechanisms such as inelastic neutron scattering. Furthermore, the electromagnetic nature of terahertz radiation means that the magnetic field component can couple directly to the ordered spin moments, while the electric field component may interact with lattice coupled magneto-electric excitations. However, owing to a lack of efficient sources and detectors, the terahertz frequency band has long been inaccessible for efficient experimentation. In recent years, developments in ultrafast laser and electronic technology has markedly improved terahertz accessibility.

In this presentation I will discuss some state-of-the-art techniques in terahertz spectroscopy available for studying magnetic materials. In particular, I will present results on the rare-earth orthoferrite compound NdFeO3. A canted S = 5/2 antiferromagnet, NdFeO3 features strong spin wave excitations in the terahertz range. At temperatures between the Fe and Nd magnetic ordering, a peculiar 90 degree spin reorientation is observed. Using THz spectroscopy, the temperature dependent magnetic anisotropy is extracted. A crossing of the anisotropy terms in the x and z directions is suggested as a possible mechanism for the spin reorientation.

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