Direct observation of paramagnetic spin fluctuations in LaFe13−xSix

Tom Faske,Oliver Gutfleisch,Wolfgang Donner,Iliya A Radulov,Markus Hölzel

doi:10.1088/1361-648x/ab5a99

Abstract

Spin fluctuations are a crucial driving force for magnetic phase transitions, but their presence usually is indirectly deduced from macroscopic variables like volume, magnetization or electrical resistivity. Here we report on the direct observation of spin fluctuations in the paramagnetic regime of the magnetocaloric model system LaFe11.6Si1.4 in the form of neutron diffuse scattering. To confirm the magnetic origin of the diffuse scattering, we correlate the temperature dependence of the diffuse intensity with ac magnetic susceptibility and x-ray diffraction experiments under magnetic field. Strong spin fluctuations are already observable at 295 K and their presence alters the thermal contraction behavior of LaFe11.6Si1.4 down to the Curie temperature of the first-order magneto-structural transition at 190 K. We explain the influence of the spin fluctuation amplitude on the lattice parameter in the framework of the internal magnetic pressure model and find that the critical forced magnetostriction follows Takashi’s spin fluctuation theory for itinerant electron systems.

Highlights

The LaFe13−xSix system has been widely investigated in recent years because of the giant magnetocaloric effect (GME) that was observed for compositions 1.0 ⩽ x ⩽ 1.8 [1, 2]
Neutron and x-ray diffraction studies on LaFe11.6Si1.4 reveal that short-range magnetic correlations in the paramagnetic regime drive the first-order PM‒FM transition
These spin fluctuations are observable as neutron diffuse scattering and exist as far as 100 K above the Curie temperature TC = 190 K

Summary

Introduction

The LaFe13−xSix system has been widely investigated in recent years because of the giant magnetocaloric effect (GME) that was observed for compositions 1.0 ⩽ x ⩽ 1.8 [1, 2]. LaFe13−xSix is a ferromagnetic material with a composition-dependent Curie temperature TC. It is described as an itinerant-electron system in which the magnetic transition from the paramagnetic (PM) to a ferromagnetic (FM) state can be induced by either temperature or magnetic field, if applied just above TC [1, 7, 8]. The magnetic transition is of first-order for compositions x ⩽ 1.6 [9] and, if induced by a magnetic field, is referred to as an itinerant-electron metamagnetic (IEM) transition [1, 8]. The change in internal energy during the IEM transition results in the GME and is dominated by changes in the electronic structure [10]

Results

Discussion

Conclusion