Observation of low frequency spin dynamics in INVAR Fe$_{65}$Ni$_{35}$

TL;DR

This study uses neutron spin echo and muon spin relaxation techniques to reveal slow spin dynamics in the ordered phase of Fe$_{65}$Ni$_{35}$ INVAR alloy, suggesting complex magnetic behavior linked to the INVAR effect.

Contribution

It provides the first combined neutron and muon spin relaxation analysis of slow spin dynamics in INVAR Fe$_{65}$Ni$_{35}$, identifying two distinct timescales and their possible origins.

Findings

Evidence of slow spin dynamics in the ordered phase.

Identification of two separate timescales of spin fluctuations.

Possible connection of slow dynamics to cluster fluctuations or disordered moments.

Abstract

We present ferromagnetic neutron spin echo measurements and muon spin relaxation ($\mu$SR) measurements of the spin dynamics in the ordered phase of the metallic alloy INVAR Fe$_{65}$Ni$_{35}$. Both techniques display evidence of slow spin dynamics in the ordered phase which could be associated with the INVAR effect. Zero field (ZF) and longitudinal field $\mu$SR measurements suggest the presence of two separate timescales of slow spin-dynamics in the ordered phase; a fast component corresponding to that observed in our neutron-spin-echo measurements and a second slow component which may be related to the "cluster" spin-fluctuations or to the dynamics of disordered local moments or both as suggested by previous neutron scattering measurements and recent theoretical studies.