Theory of transition temperature of magnetic double perovskites

Abstract

We formulate a theory of double perovskite coumpounds such as Sr2FeReO6 and Sr2FeMoO6 which have attracted recent attention for their possible uses as spin valves and sources of spin polarized electrons. We solve the theory in the dynamical mean field approximation to find the magnetic transition temperature Tc. We find that Tc is determined by a subtle interplay between carrier density and the Fe-Mo/Re site energy difference, and that the non-Fe same-sublattice hopping acts to reduce Tc. Our results suggest that presently existing materials do not optimize Tc.Identification of a ferromagnet with high spin polarization at room temperature and stable surface properties is an important goal in the field of magnetic materials. Such a system would allow, for example the fabrication of 'spin-valve' devices of greatly improved efficiency for magnetic field sensing [1], the development of new magnetic recording media [2], and perhaps the construction of improved sources of spin-polarized electrons for 'spintronic' applications [3]. One promising family of materials are the 'double perovskites' [4][5][6]. These are compounds of chemical formula ABB ′ O 6 , with A an alkaline earth such as Sr, Ca or Ba, and B, B ′ two different transition metal ions. Double perovskites in which B is F e and B ′ is M o or Re are of particular recent interest because they seem [5] to be metallic ferrimagnets with very high magnetic transition temperatures and highly spin-polarized conduction bands. However, neither the physics nor the materials science of these compounds is yet well understood. The magnetic transition temperature and whether the ground state is metallic or insulating vary as A is changed from Ba to Sr to Ca [7]. Mis-site (B − B ′ ) disorder has a pronounced effect [8].In this paper we take a step towards a theoretical understanding for these materials. We derive a many-body Hamiltonian, using band theory calculations [5] to fix important parameters. We calculate the magnetic transition temperature T c , and determine how different material parameters affect it. Our results should provide guidance in attempts to design double perovskite materials with improved properties, and an appropriate starting point for calculations of other properties.Double perovskites have a crystal structure which generalizes the familiar ABO 3 perovskite structure by hav-ing two B-site ions, which in the ideal structure alternate in a simple two sublattice pattern. The band theory has been determined [5]. The conduction bands are derived from transition metal B-site t 2g d-orbitals, in agreement with quantum chemical considerations [7]. There are six conduction bands per spin direction per unit cell; roughly, one triplet arises mainly from the d xy,xz,yz orbitals on the F e and the other from the same orbitals on Mo/Fe. The occupied bands are fully polarized at T = 0.Because the near-fermi-surface bands are derived from transition metal d-orbitals, we argue that a simple tightbinding parametrization of the band theory is adequate. We therefore mod...

Bulk-sensitive photoemission spectroscopy ofA2FeMoO6double perovskites(Kang1, Kim2, Sekiyama3 et al. 2002Phys. Rev. B683140View full textAdd to dashboardCite

“…In the metallic SFMO and BFMO systems, the DE-like interaction becomes operative due to the degenerate Fe and Mo t 2g ↓ states near E F . In the DE mechanism, the magnetic transition temperature T C is proportional to the hopping strength, i.e., the bandwidth of the itinerant t 2g ↓ states [26]. The observed correlation between T C and the estimated bandwidth in metallic double perovskites provides evidence for the operation of the DE-like interaction [18].…”

Theory of metallic double perovskites with spin-orbit coupling and strong correlations: Application to ferrimagnetic Ba2FeReO6

“…For a general direction of the Fe moment, F = (sin θ cos φ, sin θ sin φ, cos θ) at a given site, we must project the added electrons onto the allowed direction to satisfy Pauli exclusion, locally setting f ↑ = sin θ 2 e −iφ/2 f and f ↓ = − cos θ 2 e iφ/2 f , effectively "stripping" the electron of its spin degree of freedom. Such models have been proposed for other DP materials, [3][4][5][6][7][8]10,11 and shown to capture the phenomenology of Sr 2 FeMoO 6 including thermal phase transitions and disorder effects. [37][38][39] However, most of these previous studies, with the notable exception of Ref.…”

“…[3][4][5][6][7][8][9][10][11] Insulating variants where only the B'-site ion is magnetic, such as Ba 2 YMoO 6 and La 2 LiMoO 6 , provide material examples of quantum mechanical moments living on the geometrically frustrated face-centered cubic lattice. [12][13][14][15][16] Metallic DPs, such as Sr 2 FeMoO 6 , are also of significant technological importance, being room temperature ferrimagnets with half-metallic band structures and a large spin polarization which is useful for spintronic applications.…”

Electronic structure of half-metallic double perovskites

“…9 It has been suggested that the magnetic temperature T c , and whether the ground state is metallic or insulating, vary as A is changed from Ba to Sr to Ca. 10 Also mis-site disorder may have pronounced effect on the magnetic properties of these compounds. 11,12 Recent experiments 13 have established in Sr 2 FeMoO 6 an electronic structure and configuration of five localized majority electrons (high spin state) on Fe and one delocalized electron shared between Mo and the other sites.…”