High-pressure magnetism and XMCD

François Baudelet, Synchrotron SOLEIL, St. Aubin

Applying pressure is one of the most effective means to probe solid-state properties. Its application to magnetism suggested early this century by Néel1 is useful in understanding magnetic exchange interaction mechanisms. The modification of the inter-atomic distances may lead to a large variety of magnetic transitions. Nevertheless pressure can also induce more subtle changes in crystallographic structures like bond angle variations or in electronic structures like in the electronic density at the Fermi level, leading to dramatic changes in the magnetic properties.
XAS (X-ray Absorption Spectroscopy) and XMCD (X-ray Magnetic Circular Dichroism) performed simultaneously give the structural and the magnetic properties on the same compound in rigorously the same thermodynamic condition. The high-pressure iron bcc to hcp phase transition has been studied with these combined techniques. The magnetic and structural transitions are sharp. Both are of first order and occur at the same time, i.e. in the same pressure domain, as theoretically predicted. The pressure domain of the structural transition is found around 2.4 ±0.2 GPa, narrower than usually described in the literature. No room temperature ferromagnetic order has been found in the iron hcp phase close to the transition pressure. The magnetic transition occurs in a more narrow domain and slightly precedes the structural one.