S. E. Barnes

Current-Spin Coupling for Ferromagnetic Domain Walls in Fine Wires

The coupling between a current and a domain wall is examined. In the presence of a finite current and in the absence of a potential which breaks the translational symmetry, there is a perfect transfer of angular momentum from the conduction electrons to the wall. As a result, the ground state is in uniform motion and this remains the case even when relaxation is included. This is described by, appropriately modified, Landau-Lifshitz-Gilbert equations. The results for a simple pinning model are compared with experiment.

New method for the Anderson model

A new method is described which permits the use of temperature ordered diagrams for the Schrieffer-Wolff limit of the Anderson model. The propagators, self energies and the static susceptibilities are calculated. The structure of a magnetic impurity is discussed.

Electromotive force and huge magnetoresistance in magnetic tunnel junctions

The electromotive force (e.m.f.) predicted by Faraday’s law reflects the forces acting on the charge, –e, of an electron moving through a device or circuit, and is proportional to the time derivative of the magnetic field. This conventional e.m.f. is usually absent for stationary circuits and static magnetic fields. There are also forces that act on the spin of an electron; it has been recently predicted that, for circuits that are in part composed of ferromagnetic materials, there arises an e.m.f. of spin origin even for a static magnetic field. This e.m.f. can be attributed to a time-varying magnetization of the host material, such as the motion of magnetic domains in a static magnetic field, and reflects the conversion of magnetic to electrical energy. Here we show that such an e.m.f. can indeed be induced by a static magnetic field in magnetic tunnel junctions containing zinc-blende-structured MnAs quantum nanomagnets. The observed e.m.f. operates on a timescale of approximately 102–103 seconds and results from the conversion of the magnetic energy of the superparamagnetic MnAs nanomagnets into electrical energy when these magnets undergo magnetic quantum tunnelling. As a consequence, a huge magnetoresistance of up to 100,000 per cent is observed for certain bias voltages. Our results strongly support the contention that, in magnetic nanostructures, Faraday’s law of induction must be generalized to account for forces of purely spin origin. The huge magnetoresistance and e.m.f. may find potential applications in high sensitivity magnetic sensors, as well as in new active devices such as ‘spin batteries’.

Universality classes for domain wall motion in the ferromagnetic semiconductor (Ga,Mn)As

Magnetic domain wall motion induced by magnetic fields and spin-polarized electrical currents is experimentally well established. A full understanding of the underlying mechanisms, however, remains elusive. For the ferromagnetic semiconductor (Ga,Mn)As, we have measured and compared such motions in the thermally activated subthreshold, or “creep,” regime, where the velocity obeys an Arrhenius scaling law. Within this law, the clearly different exponents of the current and field reflect different universality classes, showing that the drive mechanisms are fundamentally different.

New method for the Anderson model. II. The U=0 limit

For pt.I see ibid., vol.6, p.1375 (1976). This new method is shown to give the exact result for U=0. It is implied that the same expansion in Vkd will converge in both the large U, magnetic regime, and the small U, nonmagnetic regime.

Magnetic memory and current amplification devices using moving domain walls

A moving magnetic domain wall produces an electromotive force (emf). It is therefore possible to read the state of a magnetic memory device via the emf it produces when subjected to an interrogation pulse. It is also possible to amplify currents in pulse circuits, opening up the possibility of all magnetic logic circuits.

Continuous Generation of Spinmotive Force in a Patterned Ferromagnetic Film

We study, both experimentally and theoretically, the generation of a dc spinmotive force. By exciting a ferromagnetic resonance of a comb-shaped ferromagnetic thin film, a continuous spinmotive force is generated. Experimental results are well reproduced by theoretical calculations, offering a quantitative and microscopic understanding of this spinmotive force.

Unusual behavior of the Gd ESR in single crystals of GdyY1−yBa2Cu3O6+x with x=0.1−0.8 and y=0.03−0.06: Evidence for magnetic interaction in the superconductors

The ESR of small concentration of Gd 0.03<y<0.06 substituting for Y in single crystals of GdyY1−yBa2Cu3O6−x has been measured. In the insulating compound, with x∼0.1, and the superdconducting materials with 30 K < Tc < 80 K, the measurements were performed at X-band, 9.3 GHz, and Kα-band, 36 GHz, over a large temperature range above Tc. Angular dependence measurements exhibit a spectrum which is fully resolved in certain directions, but only partially resolved, because of exchange narrowing, in other directions. Comparisons between the spectra in the insulating and superconducting compounds shows similar angular dependent behavior. This seems to indicate that the origin of the exchange narrowing is the same in both compounds. Since this narrowing in the insulating compound arises from interaction with, or via, the Cu magnetic system, it is implied that there is a similar, perhaps fluctuating, system in the superconducting state. Preliminary measurements of the temperature dependence of the line widths may indicate the presence of spin pairing at about 110 K, above the actual Tc of 70 K. The crystal field parameters are D = 3B02 = 1307 MHz, B04 = 3.014 MHz and B44 = -11.43 MHz, for the semiconducting sample. The g-value is 1.989 0.005. These values change only slightly in the superconducting crystals.

Spin waves in a spin glass

It is shown that, in an isotropic three-dimensional spin glass with an axial anisotropy energy, three spin wave branches exist in the limit k to O. The two transverse branches have an identical gap in this limit while the longitudinal branch does not. The transverse branches are split by an external static field. Dispersion is initially quadratic for the transverse branches but linear for the longitudinal branch. For vanishingly small anisotropy all the branches are linear.

Electrical measurements of the polarization in a moving magnetic vortex

We propose that the polarization of the moving magnetic vortex core can be detected by the electro- and spin-motive forces acting on the spin-polarized conduction electrons. With parameters appropriate to Permalloy, we have simulated the dynamics of a magnetic vortex core resulting from an applied oscillating magnetic field. We show that the polarization of the moving core can be detected by a simple electrical measurement.