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.

The properties of quantum systems interacting with their environment, commonly called open quantum systems, can be affected strongly by this interaction. Although this can lead to unwanted consequences, such as causing decoherence in qubits used for quantum computation, it can also be exploited as a probe of the environment. For example, magnetic resonance imaging is based on the dependence of the spin relaxation times of protons in water molecules in a host's tissue. Here we show that the excitation energy of a single spin, which is determined by magnetocrystalline anisotropy and controls its stability and suitability for use in magnetic data-storage devices, can be modified by varying the exchange coupling of the spin to a nearby conductive electrode. Using scanning tunnelling microscopy and spectroscopy, we observe variations up to a factor of two of the spin excitation energies of individual atoms as the strength of the spin's coupling to the surrounding electronic bath changes. These observations, combined with calculations, show that exchange coupling can strongly modify the magnetic anisotropy. This system is thus one of the few open quantum systems in which the energy levels, and not just the excited-state lifetimes, can be renormalized controllably. Furthermore, we demonstrate that the magnetocrystalline anisotropy, a property normally determined by the local structure around a spin, can be tuned electronically. These effects may play a significant role in the development of spintronic devices in which an individual magnetic atom or molecule is coupled to conducting leads.

Single magnetic atoms on surfaces are the smallest conceivable units for two-dimensional magnetic data storage. Previous experiments on such systems have investigated magnetization curves, the many-body Kondo effect and magnetic excitations in quantum spin systems, but a stable magnetization has not yet been detected for an atom on a non-magnetic surface in the absence of a magnetic field. The spin direction of a single magnetic atom can be fixed by coupling it to an underlying magnetic substrate via the exchange interaction, but it is then difficult to differentiate between the magnetism of the atom and the surface. Here, we take advantage of the orbital symmetry of the spin-polarized density of states of single cobalt atoms to unambiguously determine their spin direction in real space using a combination of spin-resolved scanning tunnelling microscopy experiments and ab initio calculations. By laterally moving atoms on our non-collinear magnetic template, the spin direction can also be controlled while maintaining magnetic sensitivity, thereby providing an approach for constructing and characterizing artificial atomic-scale magnetic structures.

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