Invited Paper MI-WeA9
Concepts based on Magnetoelectrics and Half-Metals for Spintronic Applications

Wednesday, November 2, 2011, 4:40 pm, Room 105

I will discuss two new concepts of interest for voltage-controlled magnetism and for efficient high-current spin injection in semiconductors, as well as our computational studies of magnetic materials that may be employed in such devices.

First, I will explain the concept of a boundary magnetization carried by a magnetoelectric antiferromagnet, which can be used to induce a switchable equilibrium exchange bias in a proximate ferromagnet. This effect, demonstrated experimentally using magnetoelectric Cr₂O₃, can be utilized in non-volatile magnetoelectric memory and spin field-effect transistors. However, for these purposes the Cr₂O₃ Néel temperature of 307 K is too low. Using first-principles calculations, we predict that it can be increased by introducing boron as a substitutional dopant in the anion sublattice, whereas transition-metal dopants are detrimental. Compressive in-plane strain was also found to be favorable.

Next, I will discuss the possibility of Ohmic spin injection in semiconductors without using Schottky or tunnel barriers. Usually such a high-resistance interfacial barrier is used to overcome the conductivity mismatch problem, but this barrier limits the injected current density. A half-metal used as a spin injector overcomes this problem at zero temperature, but the situation at finite temperatures is nontrivial. I will argue that the two-current model is inapplicable to half-metals, and that barrier-free spin injection from a half-metal may be possible even at finite temperatures. I will present an intuitive model summing up multiple scatterings at the interface, as well as direct calculations of the spin injection efficiency in a simple tight-binding model with averaging over thermal spin fluctuations.

There is much interest in Gd-doped EuO as a half-metal that could be used as a spin injector. Gd doping (and, more controversially, O deficiency) sharply enhances the Curie temperature from 69 K up to as much as 170 K. I will report the results of first-principles studies of exchange interaction in Gd-doped EuO. In the virtual crystal approximation the indirect exchange through the conduction band qualitatively explains the observed doping dependence of the Curie temperature. We also considered EuO supercells with one or more substitutional Gd atom, as well as with an oxygen vacancy, and found deviations from the virtual-crystal behavior that can be associated with local lattice relaxations.

Time permitting, I will also show how first-principles calculations for large supercells with noncollinear spins can be used to analyze the spin-disorder contribution to the electric resistivity of magnetic metals.