P. James

Non-collinear states in magnetic sensors

Certain materials have an electrical conductivity that is extremely sensitiveto an applied magnetic field; this phenomenon, termed ‘giant magnetoresistance’, can be used in sensor applications. Typically, such a devicecomprises several ferromagnetic layers, separated by non-magnetic spacer layer(s)—aso-called ‘super-lattice’ geometry. In theabsence of a magnetic field, the ferromagnetic layers may be magnetized inopposite directions by interlayer exchange coupling, while an applied externalmagnetic field causes the magnetization directions to become parallel. Becausethe resistivity depends on the magnetization direction, an applied field thatchanges the magnetic configuration may be detected simply by measuring thechange in resistance. In order to detect weak fields, the energy differencebetween different magnetization directions should be small; this is usuallyachieved by using many non-magnetic atomic spacer layers. Here we show, usingfirst-principles theory, that materials combinations such as Fe/V/Co multilayerscan produce a non-collinear magnetic state in which the magnetization directionbetween Fe and Co layers differs by about 90°. This state is energeticallyalmost degenerate with the collinear magnetic states, even though the numberof non-magnetic vanadium spacer layers is quite small.

Calculated trends of the magnetostriction coefficient of 3d alloys from first principles

The general behavior of the magnetostriction coefficients for Fe, Co, and Ni and their alloys is reproduced by first-principles theory. It is shown that the low magnetostriction coefficient of soft magnetic materials, observed for specific alloy concentrations, such as permalloy, can be explained theoretically by employing relativistic first-principles calculations in combination with the rigid-band approximation. The connection between magnetostriction and magnetocrystalline anisotropy is discussed as well as the physical mechanisms behind soft magnetic materials. Theory is hence demonstrated to have the ability of predicting values of the magnetostriction coefficient of any alloy, and a microscopic understanding of this entity is starting to emerge.

A theoretical study of the pressure-induced structural phase transition in CdTe

We have performed first-principles, full-potential linear muffin-tin-orbital (FPLMTO) calculations to study the structural phase transition in CdTe under pressure. By calculating the total energies of different phases we have shown that CdTe shows the following structural sequence with increasing pressure; zinc-blende → cinnabar → Nacl → orthorhombic structure. This is in complete agreement with recent experiments. We have also looked for the possibility of an additional phase transition from the orthorhombic to the CsCl structure, which has been anticipated for the isoelectronic compound HgTe in recent experiments.

Structural Stability in Fe-Based Alloys

Experimentally it is found that the Fe-Co and Fe-Ni alloys undergo a structural transformation from the bcc structure to the hcp or fcc structures, respectively, with increasing number of valence electrons, while the Fe-Cu alloy is unstable at most concentrations. In addition to this some of the alloy phases show a partial ordering of the constituting atoms. One may wonder if this structural behaviour can be simply understood from a filling of essentially common bands or if the alloying implies a modification of the electronic structure and as a consequence also the structural stability. In this paper we try to answer this question and reproduce the observed structural behaviour by means of accurate alloy theory and total energy calculations.

Crystal Structure and Phase Stability in Fe 1-x Co x from AB Initio Theory

The crystal structure for many metals is well established, and early research discovered a pattern which was shown to depend on the chemical periodicity of these elements. Particularly for the non-magnetic metals in the 4d and 5d transition metal series a connection between the atomic number and the crystal structure was recognized1. The crystal structure sequence involved more (fcc and hcp) or less (bcc) close-packed structures with high symmetry. The fact that the rare-earth elements show a regular behavior of the crystal structure as a function of atomic number led to the proposal2that the occupation of the d states was the important parameter for the crystal structure for these metals. This could then explain the more dramatic behavior of the d transition metals compared to the rare-earths since for the latter the occupation of the d states does not change much over the series whereas for the transition metals the d band is successively being filled when proceeding through the series.