Enhanced ferromagnetism from electron-electron interactions in double-exchange-type models

Abstract

The magnetic properties of a variety of materials with promise for technological applications have been described by models in which fermions are coupled to local moment spins. Monte Carlo studies of such models usually ignore electron-electron interactions, even though the energy scale corresponding to these interactions may be comparable to or larger than other relevant energy scales. In this work we add on-site interactions between fermions to the double exchange model which we study with a Monte Carlo scheme in which temporal fluctuations of local moment spins are fully accounted for and electron-electron interactions are treated at a mean field level. We show that when the number of fermions is considerably less than the number of local moments even moderate interactions can lead to significant enhancement of ferromagnetism and the Curie temperature.The magnetic properties of a variety of materials of fundamental interest and technological promise, such as collosal magnetoresistance (CMR) manganites [1], rare earth hexaborides [2], and diluted, magnetic semiconductors (DMS) [3] have been described using double exchange (DE) type models in which fermions are coupled to local moment spins. In the limit that the number of fermions is considerably less than the number of local moments, such models generally display ferromagnetism [4].In manganites it is well established that electronelectron interactions are at least as large as the Hund coupling, and may be the largest energy scale in the problem [1,[5][6][7]. The importance of interactions has also been stressed for DMS [8-10] and hexaborides [11]. The combination of disorder and electron-electron interactions may also play an important role in nanoscale electronic phase separation [12], which has been argued to be important for CMR in manganites [13]. It is hence important to develop accurate techniques that can account for the effects of electron-electron interactions in DE type models and to study their effects on magnetic properties of these models.Here we introduce a method to study the effects of electron-electron interactions in DE type models when the energy scale for interactions is in the experimentally relevant regime of no more than a few times the Hund coupling. We use this method to show that even moderate interactions can enhance ferromagnetism and the Curie temperature T c significantly when the number of fermions is considerably less than the number of local moments.We combine a Hartree-Fock treatment of electronelectron interactions with a Monte Carlo scheme for fermions coupled to classical local moment spins. Through the use of exact diagonalizations on small systems we have determined regions of parameter space where this technique should be most accurate. We calculate the magnetization as a function of temperature for a variety of interaction strengths and determine the effects of interactions on the Curie temperature T c . Electronelectron interactions can lead to ferromagnetism in the absence of Hund coupling [14], so the enhancement in...