Theory of Half-Metallic Ferrimagnetism in Double Perovskites

Abstract

We present a comprehensive theory of the temperature-and disorder-dependence of half-metallic ferrimagnetism in the double perovskite Sr2FeMoO6 (SFMO) with Tc above room temperature. We show that the magnetization M (T ) and conduction electron polarization P (T ) are both proportional to the magnetization MS(T ) of localized Fe spins. We derive and validate an effective spin Hamiltonian, amenable to large-scale three-dimensional simulations. We show how M (T ) and Tc are affected by disorder, ubiquitous in these materials. We suggest a way to enhance Tc in SFMO without sacrificing polarization. Double perovskites (DPs) A 2 BB ′ O 6 are an important family of complex oxides, derived from the simple ABO 3 perovskite structure by a three-dimensional (3D) checkerboard ordering of B and B ′ ions. One of the best studied examples is Sr 2 FeMoO 6 (SFMO), a half-metallic ferrimagnet with T c ≃ 420K, well above room temperature [1][2][3]. Clearly SFMO, and other DPs, can have enormous technological impact if their stoichiometry and ordering can be controlled.From a theoretical point of view, we argue that DPs are simple systems for understanding metallic ferromagnetism, despite their apparent complexity. First, in contrast to iron, there is a clear separation of the localized (B) and itinerant degrees of freedom (coming from B ′ ) in the DPs. Second, in contrast to the manganites, DPs have neither Jahn-Teller distortions nor competing superexchange, given the large distance between B-sites. Third, in contrast to dilute magnetic semiconductors, disorder is not an essential aspect of the theoretical problem. Important early theoretical work on halfmetallic DPs includes T =0 electronic structure calculations [3], and model Hamiltonians analyzed using various mean-field theories [4-6] and two-dimensional (2D) simulations [7].In this Letter, we present a comprehensive theory that gives insight into the temperature-and disorderdependence of the magnetic properties of metallic DPs. We make detailed comparisons with and predictions for SFMO. Our main results are: (1) We show that both the total magnetization M (T ) and the conduction electron polarization P (T ) at E f are proportional to the magnetization M S (T ) of localized Fe spins. This result is significant because, while M (T ) is easy to measure, it is P (T ) that is of crucial importance for spintronic applications.(2) Our main theoretical advance is the derivation and validation of an effective classical spin Hamiltonian H eff [see eq. (2)] for DPs, which differs from both the Heisenberg and Anderson-Hasegawa models [8]. We show that H eff describes the full T -dependence of the magnetization M S (T ), and hence that of M (T ) and P (T ). (3) We present the results of simulations of H eff on large 3D lattices, including disorder effects, thus going beyond all previous theoretical calculations on SFMO. (4) We compute M (T ) and T c , using microscopic bandstructure parameters as input, and see how these are affected by deviations from stoichiometry and by anti-site (A...

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.…”

“…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. 11 have ignored spin-orbit coupling effects, which are expected to be extremely important for 5d transition metal oxides.…”

“…[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.…”

Theory of HighTcFerrimagnetism in a Multiorbital Mott Insulator

“…It is now wellunderstood that a generalized double exchange mechanism can explain ferromagnetism with the scale of T c set by the kinetic energy of itinerant electrons [15]. From this perspective, the observation of an even higher T c ≈ 725K, the highest T c amongst all perovskites with a net moment, in an insulator Sr 2 CrOsO 6 (SCOO) [17] is rather puzzling.…”

Geometrical lattice engineering of complex oxide heterostructures: a designer approach to emergent quantum states

“…Moreover, the ionic radii difference between those ions must be within a limited range often preventing synthesis of a variety interesting structures. [72] Consequently, there are formidable challenges for the bulk synthesis of many theoretically proposed ordered double-perovskite materials, which are expected to host many interesting phenomena [20,[77][78][79] With the recent advances in thin-film growth techniques, it was noticed that there is another way to overcome those limitations. [66,[80][81][82][83] As illustrated in Fig.…”