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.