No abstract

Magnetic anisotropy phenomena in bimetallic antiferromagnets Mn2Au and MnIr are studied by first-principles density functional theory calculations. We find strong and lattice-parameter dependent magnetic anisotropies of the ground state energy, chemical potential, and density of states, and attribute these anisotropies to combined effects of large moment on the Mn 3d shell and large spin-orbit coupling on the 5d shell of the noble metal. Large magnitudes of the proposed effects can open a route towards spintronics in compensated antiferromagnets without involving ferromagnetic elements.

No abstract

It has been found experimentally that the order of the magnetic phase transitions in RCo 2 compounds (R standing for rare-earth metals) at T c changes from second order for the lightrare-earth series up to TbCo 2 to first order for the heavier-rare-earth compounds DyCo 2 , HoCo 2 and ErCo 2 . On the basis of results of fixed-spin-moment band-structure calculations for the isostructural compound YCo 2 at different lattice constants, we propose an explanation for this behaviour. In contrast to the widely accepted Inoue-Shimizu theory for this class of compounds, our explanation also includes Pr, Nd which were thought to behave differently due to the influence of crystal-field effects. We show that an itinerant-electron metamagnetic transition in these compounds can occur only over a certain range of lattice constants and that the possibility of a first-order phase transition is connected to features of the electronic structure rather than to the magnitude of the transition temperature as conjectured earlier. The influence of the latter is only important if the transition takes place at elevated temperatures, where effects of spin fluctuations can suppress a first-order transition.

On the basis of ab initio calculations employing density functional theory ͑DFT͒ we investigate half metallic ferromagnetism in zinc-blende and wurtzite compounds composed of group I/II metals as cations and group V elements as anions. We find that the formation of ferromagentic order requires large cell volumes, high ionicity and a slight hybridization of anion p and cation d states around the Fermi energy. Our calculations show that a ferromagnetic alignment of the spins is energetically always more stable than simple AF arrangements, which makes these materials possible candidates for spin injection in spintronic devices. To clarify the conditions for the flat p-band carrying the magnetism, we present results of a tight binding analysis.

We show that the large negative magnetic contribution to the thermal expansion in disordered Fe-Pt alloys can be understood within the disordered local moment (DLM) approach. On the basis of first principles calculations we quantitatively describe the spontaneous volume magnetostriction for various Pt concentrations. It is found that the Invar effect in these alloys is entirely related to the state of thermal magnetic disorder modeled by the DLM states. We also show that the experimentally observed anomaly in the temperature dependence of the magnetization is due to a spontaneous reduction of the local magnetic moments rather than to "hidden excitations."

The micromagnetic exchange stiffness is a critical parameter in numerical modeling of magnetization dynamics and reversal processes, yet the current literature reports a wide range of values even for such simple and widely used material as Cobalt.With use of ab-intio estimated Heisenberg parameters we calculate the low temperature micromagnetic exchange stiffness parameters for hexagonal-close-packed (HCP) and face-centred cubic Cobalt without previous and our own. For HCP Co they are slightly different in the directions parallel and perpendicular to the c-axis. We establish the exchange stiffness scaling relation with magnetization A(m) ∼ m 1.8 valid for all sets of parameters for a wide range of temperatures. For HCP Co we find an anisotropic domain wall width in the range 24 − 29 nm which increases slowly with temperature. The results form a critical input for large-scale temperature-dependent micromagnetics simulations and demonstrate the importance of correct parameterization for accurate simulation of magnetization dynamics.

We put forward a scheme to study the anisotropic magnetic couplings in Sr 2 IrO 4 by mapping fully relativistic constrained noncollinear density functional theory including an on-site Hubbard U correction onto a general spin model Hamiltonian. This procedure allows for the simultaneous account and direct control of the lattice, spin and orbital interactions within a fully ab initio scheme. We compute the isotropic, single site anisotropy and Dzyaloshinskii-Moriya (DM) coupling parameters, and clarify that the origin of the canted magnetic state in Sr 2 IrO 4 arises from the interplay between structural distortions and the competition between isotropic exchange and DM interactions. A complete magnetic phase diagram with respect to the tetragonal distortion and the rotation of IrO 6 octahedra is constructed, revealing the presence of two types of canted to collinear magnetic transitions: a spin-flop transition with increasing tetragonal distortion and a complete quenching of the basal weak ferromagnetic moment below a critical octahedral rotation.In weak ferromagnetic materials the subtle interplay among different types of magnetic interactions can cause the formation of complex canted spin structures involving the so-called Dzyaloshinskii-Moriya (DM) effect, arising from the coupling between the spin and orbital angular momenta [1,2]. Intense research was done in this field in the last few years, motivated by the foreseeable applications in storage technology and by the air of mystery enveloping the quantum-mechanical origin of DM structures [3][4][5][6]. A crucial aspect of the DM systems is the entanglement between structural distortions and magnetism, which could be exploited as a way to tune the spin texture by modifying the structure upon external stimuli such as pressure and strain [4,7,8].The cross coupling between the different electronic, lattice and spin degrees of freedom is particulary rich in iridates. Here, the spin-orbit coupling (SOC), electron-electron correlations, and spin-exchange interactions operate with comparable strengths and gives rise to a large variety of exotic states [9][10][11][12][13][14][15]. The most striking example of this class of materials is the layered perovskite Sr 2 IrO 4 , characterized by a novel relativistic Mott insulating state [10-12, 16, 17] and an unusual in-plane canted antiferromagnetism (AFM) with a weak net ferromagnetic (FM) component [4,18]. The small electronic gap (≈ 0.3 eV [17]) is opened by modest Hubbard interactions (U ≈ 1.5-2 eV [19]) and by the strong spin-orbit coupling (ξ soc ≈ 0.5 eV [20]) which effectively narrows the d orbital bandwidth and give rise to an ideal J eff =1/2-like state [10,21,22]. This is considered to be robust despite the presence of noncubic structural distortions [23]. Neutron diffraction experiments indicate that the IrO 6 octahedra are rotated by α =11.5• and elongated in the c direction (c/a ≈ 1.04) [24], generating the enlarged √ 2a ×2c I4 1 /acd tetragonal cell shown in Fig. 1(a). The spins, coupled with the orbital mome...