First-order ferromagnetic transitions of lanthanide local moments in divalent compounds: An itinerant electron positive feedback mechanism and Fermi surface topological change

TL;DR

This paper proposes an electronic mechanism involving Fermi surface topology change and positive feedback between itinerant electrons and local moments that drives first-order ferromagnetic transitions in divalent lanthanide compounds, explaining recent magnetocaloric observations.

Contribution

It introduces a purely electronic, topological Fermi surface mechanism for first-order magnetic transitions in divalent lanthanide compounds, distinct from magnetostructural coupling.

Findings

Identifies a positive feedback loop between itinerant electrons and local moments.

Explains the giant non-hysteretic magnetocaloric effect in Eu$_2$In.

Links divalency of Eu to the electronic transition mechanism.

Abstract

Around discontinuous (first-order) magnetic phase transitions the strong caloric response of materials to the application of small fields is widely studied for the development of solid-state refrigeration. Typically strong magnetostructural coupling drives such transitions and the attendant substantial hysteresis dramatically reduces the cooling performance. In this context we describe a purely electronic mechanism which pilots a first-order paramagnetic-ferromagnetic transition in divalent lanthanide compounds and which explains the giant non-hysteretic magnetocaloric effect recently discovered in a Eu$_2$In compound. There is positive feedback between the magnetism of itinerant valence electrons and the ferromagnetic ordering of local $f$-electron moments, which appears as a topological change to the Fermi surface. The origin of this electronic mechanism stems directly from Eu's divalency, which explains the absence of a similar discontinuous transition in Gd$_2$In.