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.
Highlights
Local moments formed on atoms from strongly correlated f -electrons interact with each other to produce a plethora of magnetic phases in lanthanide compounds [1]
We describe a purely electronic mechanism which pilots a first-order paramagnetic-ferromagnetic transition in divalent lanthanide compounds and which explains the giant nonhysteretic magnetocaloric effect recently discovered in a Eu2In compound
There is a 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
Summary
Local moments formed on atoms from strongly correlated f -electrons interact with each other to produce a plethora of magnetic phases in lanthanide compounds [1]. We show with an ab initio theory that the divalency causes the Fermi energy EF of Eu2In to be positioned so that there is a positive feedback from the itinerant valence electron subsystem on the magnetic interactions among the f -electron magnetic moments This results in a first-order PM-FM transition which is devoid of any magnetostructural coupling. In Eu2In, the change of FS topology is brought about by the spin polarization of these states produced by the magnetic field which is set up by the alignment of Eu moments as ferromagnetic order develops. II, we describe the purely electronic mechanism driving the discontinuous magnetic phase transition of Eu2In using a first-principles disordered local moment theory These results are compared with those for trivalent Gd2In in order to illustrate how the divalent state of the rare earth underpins the first-order character.
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