Double perovskites with ferromagnetism above room temperature

Abstract

We review the structural, magnetic and transport properties of double perovskites (A 2 BB O 6 ) with ferromagnetism above room temperature. Ferromagnetism in these compounds is explained by an indirect B-O-B -O-B exchange interaction mediated by itinerant electrons. We first focus on the BB = FeMo-based double perovskites, with Sr 2 FeMoO 6 (T C = 420 K) being the most studied compound. These compounds show metallic behaviour and low magnetic coercivity. Afterwards, we will focus on B = Re compounds, where the significant orbital moment of Re plays a crucial role in the magnetic properties, for example in the large magnetic coercivity and magnetostructural coupling. More specifically, we first discuss the A 2 FeReO 6 series, with maximum T C = 520 K for Ca 2 FeReO 6 , which shows a tendency to semiconducting behaviour. Finally, we describe the Sr 2 (Fe 1−x Cr x )ReO 6 series, with maximum T C = 625 K for Sr 2 CrReO 6 , which is the highest T C in an oxide compound without Fe. This compound is metallic. We discuss the impact of these materials for spin electronics in the light of their high spin polarization at the Fermi level and metallicity. In particular, we focus on the large intergrain magnetoresistance effect observed in polycrystalline samples and the possible implementation of these materials as electrodes in magnetic tunnel junctions.

Inter-grain tunnelling in the half-metallic double-perovskites Sr2BB'O6(BB'= FeMo, FeRe, CrMo, CrW, CrRe)

“…Eq. (1) shows that the higher T o (relative to RT), the closer is inter-grain tunnelling to elastic tunnelling. Table II and figure 3(b) show that, for only 3 samples out of 19, T o > 1000 K, namely for two polycrystalline samples of SFMO (in figure 3(b) the two symbols coincide) and for one c.p.…”

“…Therefore their magnetic, electronic and in particular their magnetoresistive properties [1] have been investigated intensively over the past two decades. The grain boundaries in these materials act in most cases as tunnel barriers.…”

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

“…[72] In this family, the B and B ′ ions are naturally ordered by forming a rock salt sublattice with B-O-B ′ super-exchange that can generate rich magnetic and topological behaviors depending on the choice of the transition metal elements. Thus, the ordered double perovskites offer an excellent engineering playground for which potential functionalities can be predicted and explored [72][73][74][75][76] . From the synthesis perspective, in the bulk the formation of an ordered double perovskite requires a relatively large difference between the chemical valences of B and B ′ ions.…”

Spin-Electron-Phonon Excitation in Re-based Half-Metallic Double Perovskites

“…Some compounds, such as A 2 FeMoO 6 and A 2 MReO 6 (A = Sr, Ba, M = Fe, Cr), have been widely studied due to its spin-polarized conduction electrons arising from a half-metallic ferrimagnetic ground states with ordering transitions at T c above room temperature, making them potential materials for spintronics devices [1,2]. In these materials the minority-spin conduction electrons have a mixed M 3d and Mo 4d or Re 5d character and mediate a double-exchange-like ferromagnetic interaction between the localized 3d majority spins.…”

“…For the Re-based compounds, an additional ingredient that further enriches the physics is the partly localized character of the 5d moments. In fact, the high T c ∼ 520 K for the non-metallic Ca 2 FeReO 6 [3,4] indicates that alternative exchange mechanisms not involving conduction electrons are dominant, and the large coercive fields H c of several tesla as opposed to a few Oersted for Sr 2 FeMoO 6 have been regarded a clear manifestation of the large spin-orbit coupling of the Re 5d electrons [2]. Finally, these Re based double perovskites were reported to be at the frontier of a metalinsulator transition, driven by spin-orbit coupling and unexpectedly large Re 5d correlations [2][3][4][5][6][7].…”