The heavy rare earth elements crystallize into hexagonally close packed (h.c.p.) structures and share a common outer electronic configuration, differing only in the number of 4f electrons they have. These chemically inert 4f electrons set up localized magnetic moments, which are coupled via an indirect exchange interaction involving the conduction electrons. This leads to the formation of a wide variety of magnetic structures, the periodicities of which are often incommensurate with the underlying crystal lattice. Such incommensurate ordering is associated with a 'webbed' topology of the momentum space surface separating the occupied and unoccupied electron states (the Fermi surface). The shape of this surface-and hence the magnetic structure-for the heavy rare earth elements is known to depend on the ratio of the interplanar spacing c and the interatomic, intraplanar spacing a of the h.c.p. lattice. A theoretical understanding of this problem is, however, far from complete. Here, using gadolinium as a prototype for all the heavy rare earth elements, we generate a unified magnetic phase diagram, which unequivocally links the magnetic structures of the heavy rare earths to their lattice parameters. In addition to verifying the importance of the c/a ratio, we find that the atomic unit cell volume plays a separate, distinct role in determining the magnetic properties: we show that the trend from ferromagnetism to incommensurate ordering as atomic number increases is connected to the concomitant decrease in unit cell volume. This volume decrease occurs because of the so-called lanthanide contraction, where the addition of electrons to the poorly shielding 4f orbitals leads to an increase in effective nuclear charge and, correspondingly, a decrease in ionic radii.

We propose a simplified version of self-interaction corrected local spin-density (SIC-LSD) approximation, based on multiple scattering theory, which implements self-interaction correction locally, within the KKR method. The multiple scattering aspect of this new SIC-LSD method allows for the description of crystal potentials which vary from site to site in a random fashion and the calculation of physical quantities averaged over ensembles of such potentials using the coherent potential approximation (CPA). This facilitates applications of the SIC to alloys and pseudoalloys which could describe disordered local moment systems, as well as intermediate valences. As a demonstration of the method, we study the well-known α-γ phase transition in Ce, where we also explain how SIC operates in terms of multiple scattering theory.

An ab initio study of magnetic exchange interactions in antiferromagnetic and strongly correlated 3d transition metal monoxides is presented. Their electronic structure is calculated using the local self-interaction correction approach, implemented within the Korringa-Kohn-Rostoker band structure method, which is based on multiple scattering theory. The Heisenberg exchange constants are evaluated with the magnetic force theorem. Based on these the corresponding Néel temperatures TN and spin wave dispersions are calculated. The Néel temperatures are obtained using mean field approximation, random phase approximation and Monte Carlo simulations. The pressure dependence of TN is investigated using exchange constants calculated for different lattice constants. All the calculated results are compared to experimental data.

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From the basis of ab initio electronic structure calculations which include the effects of thermally excited magnetic fluctuations, we predict Mn-stabilized cubic zirconia to be ferromagnetic above 500 K. We find this material, which is well known both as an imitation diamond and as a catalyst, to be half-metallic with the majority and minority spin Mn impurity states lying in zirconia's wide gap. The Mn concentration can exceed 40%. The high-Tc ferromagnetism is robust to oxygen vacancy defects and to how the Mn impurities are distributed on the Zr fcc sublattice. We propose this ceramic as a promising future spintronics material.

A combined experimental and theoretical study on the inelastic transfer of spin momentum between a spin-polarized tunneling current and a ferromagnetic electrode is presented. Using inelastic tunneling spectroscopy across a vacuum gap at 4 K we show that high-energy magnons are efficiently excited in inelasticscattering events and that the asymmetry of magnon excitation for tunneling into and out of the ferromagnet is proportional to the spin polarization of the tunneling current. We discuss the size of the resulting spin torque and explain the energy distribution of the excited magnons on basis of spin scattering mediated by the itinerant exchange interaction.

Low-temperature measurements of the magnetocrystalline anisotropy energy K in (Fe 1−x Co x ) 2 B alloys are reported, and the origin of this anisotropy is elucidated using a first-principles electronic structure analysis. The calculated concentration dependence K(x) with a maximum near x = 0.3 and a minimum near x = 0.8 is in excellent agreement with experiment. This dependence is traced down to spin-orbital selection rules and the filling of electronic bands with increasing electronic concentration. At the optimal Co concentration, K depends strongly on the tetragonality and doubles under a modest 3% increase of the c/a ratio, suggesting that the magnetocrystalline anisotropy can be further enhanced using epitaxial or chemical strain.Magnetocrystalline anisotropy (MCA) of a magnetic material is one of its key properties for practical applications, large easy-axis anisotropy being favorable for permanent magnets.1 Intelligent search for new materials requires understanding of the underlying mechanisms of MCA. This can be particularly fruitful for substitutional alloys whose properties can be tuned by varying the concentrations of their components. The analysis is often relatively simple in insulators, where MCA is dominated by single-ion terms which can be deduced from crystalfield splittings and spin-orbital (SO) selection rules. In contrast, in typical metallic alloys the band width sets the largest energy scale, and MCA depends on the details of the electronic structure.The (Fe 1−x Co x ) 2 B solid solution 2-6 (space group I4/mcm 7 ) is a remarkable case in point. Fe 2 B has a fairly strong easy-plane MCA, and Co 2 B, at low temperatures, a small easy-axis MCA. However, the alloy has a substantial easy-axis MCA around x = 0.3, 2 making it a potentially useful rare-earth-free 8 permanent magnet. At x ≈ 0.5 the MCA again turns easy-plane, peaks at x = 0.8, and then turns easy-axis close to x = 1. These three spin reorientation transitions must be related to the continuous evolution of the electronic structure with concentration. The goal of this Letter is to elucidate the origin of this rare phenomenon.First, we report the results of experimental measurements at low temperatures.Single crystals of (Fe 1−x Co x ) 2 B were grown from solution growth out of an excess of (Fe,Co) which was decanted in a centrifuge. 9The single crystals were grown as tetragonal rods which were cut using a wire saw to give them the shape of a rectangular prism. The demagnetization factor was calcua) This article has been accepted by Applied Physics Letters.After it is published, it will be found at http://scitation.aip.org/content/aip/journal/apl . lated using Ref. 10. Field-dependent magnetization measurements were performed in a Quantum Design MPMS at 2 K in fields up to 5.5 T. The MCA energy K was determined as the area between the two magnetization curves, with the field parallel and perpendicular to the c axis, taken at the same temperature.6 The results shown in Fig. 1 Density-functional calculations using several different m...

We describe an ab initio theory of finite temperature magnetism in strongly-correlated electron systems. The formalism is based on spin density functional theory, with a self-interaction corrected local spin density approximation (SIC-LSDA). The self-interaction correction is implemented locally, within the KKR multiplescattering method. Thermally induced magnetic fluctuations are treated using a meanfield 'disordered local moment' (DLM) approach and at no stage is there a fitting to an effective Heisenberg model. We apply the theory to the 3d transition metal oxides, where our calculations reproduce the experimental ordering tendencies, as well as the qualitative trend in ordering temperatures. We find a large insulating gap in the paramagnetic state which hardly changes with the onset of magnetic order.