Emergence of a hidden magnetic phase in LaFe11.8Si1.2 investigated by inelastic neutron scattering as a function of field and temperature

TL;DR

This study uncovers a hidden magnetic phase in LaFe11.8Si1.2 driven by spin fluctuations near the critical point, revealed through neutron scattering and thermodynamic measurements, indicating complex magnetic behavior around the transition.

Contribution

It provides new insights into the magnetic fluctuations and hidden phases in LaFe13-xSix, combining neutron scattering with thermometry to reveal persistent spin dynamics.

Findings

Presence of broad quasielastic scattering above Tc

Finite Q quasielastic peak near a specific wavevector

Evidence of a hidden competing magnetic phase

Abstract

The NaZn13 type itinerant magnet LaFe13-xSix has seen considerable interest due to its unique combination of large magnetocaloric effect and low hysteresis. Here we demonstrate, with a combination of magnetometry, bespoke microcalorimetry and inelastic neutron scattering that this is due to the presence of paramagnetic spin fluctuations, which build up as the critical point is approached. While thermal measurements show significant latent heat independent changes in the heat capacity, inelastic neutron scattering reveals the presence of broad quasielastic scattering that persists above Tc, in addition to a finite Q quasielastic peak at Q=0.52 A$^{-1}$ (close to a 100 Bragg reflection in this system at Q = 0.54 A$^{-1}$). This finite Q quasielastic peak appears only in the paramagnetic state and when in proximity to the itinerant metamagnetic transition. We associate these observations with a hidden competing phase and spin fluctuations close to the transition temperature and magnetic field, that persist across the magnetic transition.