Professor Stephen Blundell

Abstract:

High-field magnetoresistance measurements have been performed on single-crystal samples of the charge-transfer salt α-ET2KHg (SCN)4 [where ET is bis(ethylenedithio)tetrathiafulvalene] using fields of up to 30 T. We examine the character of the Shubnikov-de Haas oscillations for a range of samples, temperatures, and field regimes to understand the role played by spin splitting and magnetic breakdown effects. Angle-dependent magnetoresistance oscillations (AMRO) in magnetic fields below the kink transition (22 T) indicate that the low-magnetic-field band structure is dominated by a one-dimensional (1D) open section of Fermi surface (FS) inclined at ∼21°to the crystallographic b*c plane; above the kink, the AMRO change in character, indicating that the FS now possesses a 2D closed section in the form of a distorted cylinder. The data are discussed in terms of a recent model of the low-field band structure and give definitive proof that the kink is the signature of a magnetic-field-induced change in the FS. © 1995 The American Physical Society.

Abstract:

Recent results of magnetization, susceptibility and muon spin rotation (mu SR) measurements and crystal structure determinations of the organic radicals based on cl-nitronyl nitroxide are described. Temperature variation of mu SR and a.c. susceptibility reveals that the organic radicals, which are found to exhibit ferromagnetic intermolecular interactions, undergo magnetic phase transitions at around 0.1 similar to 0.7 K. The organic radicals have not only a simply ferromagnetic ordered state but also canted spin magnetic orders. Microscopic views of the magnetically ordered states in the organic radicals are discussed by taking account of the molecular arrangements in the crystals.

Abstract:

A technique for engineering micron and submicron scale structures from magnetic films of transition metals has been developed using a combination of electron- and ion-beam lithography enabling high-quality arrays of submicron magnetic Fe wires to be fabricated. This process can be used to fabricate novel devices from a variety of metal combinations which would not be possible by the usual liftoff metallization method. The structure and magnetic properties are reported of an epitaxial 25 nm Fe(001)/GaAs(001) film and the wire gratings which are fabricated from it. The width of the wires in the grating is 0.5 μm for all structures studied, but the separation of each wire is varied in the range 0.5 to 16 μm. An artificially induced shape anisotropy field of around 1 kG, consistent with a magnetostatic calculation, was observed for all separations studied. The field dependence of the magneto-optic Kerr effect and magnetoresistance (MR) data is consistent with a twisted magnetization configuration across the width of the sample beneath saturation for transverse applied fields. In this case, the detailed form of the field dependence of the MR is strikingly modified from that observed in the continuous film and is consistent with coherent rotation of the magnetization.

Abstract:

Using a transfer matrix method, the band structure is calculated for a spin-dependent generalization of the Kronig-Penney model which consists of a one-dimensional array of delta function potentials whose strengths depend on the relative orientation of the electron spin and a vector located at each delta function site, the direction of which is helically modulated. This is compared with a spin-independent Kronig-Penney model in which only the amplitude of each delta function is modulated. The period of the modulation can be incommensurate with the periodicity of the delta function array. The fractal nature of the band structure in the spin-independent case is shown to be quenched by the additional symmetry in the spin-dependent model.