INVAR systems show near zero thermal expansion over a discrete temperature range. The discovery of the INVAR effect by Guillaume in 1897 won him the Nobel prize in 1920. However the underlying mechanism behind this phenomenon still remains unclear. Suggestions include a high-spin low-spin transition, or effects due to non-colinearity or disorder – but no experimental evidence has yet supported any particular model. µSR is an excellent probe of disorder in magnetic systems and furthermore is sensitive to a timescale that is difficult for neutron experiments to access. Here we studied Fe1-xNix with x=0.35 (INVAR, Tc=450K) and x=0.2 (non-INVAR, Tc=400K). The INVAR system shows a dramatic change in muon relaxation rate compared with the non-INVAR case. The amplitude for this relaxational process is observed to decrease to zero for the INVAR system whereas for the non-INVAR material this component remains constant above 300K. In elemental Fe and Ni the relaxation remains static over this temperature range as expected for an itinerant ferromagnetic below Tc. Here then we present the first observation of a two state dynamical transition in the INVAR system Fe1-xNix. This is commensurate with the idea that the large negative magneto-striction that dominates the lattice expansion in INVAR systems is driven by a dynamic process in the MHz range.
J Taylor, AD Hillier (ISIS), JA Duffy, CA Steer, CS Taylor (University of Warwick), M Berry (University of Loughborough)
Research date: December 2006
Dr JW Taylor, [firstname.lastname@example.org]
Khmelevskyi et al., Phys. Rev. Lett. 91 (2003) 037201