M.R. Wells

Giant magnetoresistance by exchange springs in DyFe2/YFe2 superlattices

Magnetization and magnetoresistance measurements are reported for antiferromagnetically coupled DyFe2/YFe2 multilayers in fields up to 23 T. It is demonstrated that the formation of short exchange springs ( ~20 A) in the magnetically soft YFe2 layers results in a giant magnetoresistance as high as 32% in the spring region. It is shown that both the magnitude of the effect and its dependence on magnetic field are in good agreement with the theory of Levy and Zhang for domain wall induced giant magnetoresistance.

Magnetic Properties of Antiferromagnetic GdAlO3

Specific‐heat and magnetic‐susceptibility measurements on GdAlO3 show that it undergoes a transition to antiferromagnetism at 3.69°K. Magnetization measurements on a single crystal at temperatures down to 0.6°K in fields up to 70 kOe show that the ordered state is a simple two‐sublattice structure with the orthorhombic b axis as the axis of alignment. The spin flop, exchange, and anisotropy fields have been found to be 11.2 kOe, 20 kOe to 25 kOe, and 2.3 kOe to 3.0 kOe, respectively. No parasitic ferromagnetism is observed.Optical absorption studies of Er3+ and Dy3+ in GdAlO3 observe the spin flop and confirm that in the ordered structure the distorted simple cubic lattice of Gd3+ ions divides into two sublattices with each ion surrounded by six nearest neighbors belonging to the opposite sublattice.

Negative coercivity in epitaxially grown (110) DyFe2/YFe2 superlattices

Molecular beam epitaxial methods have been used to grow single crystal Laves phase DyFe2/YFe2 superlattice samples with a (110) growth direction. Detailed magnetization curves have been obtained for YFe2 dominated multilayer samples [wDyFe2/4wYFe2]×16 with w=45, 50, and 55 A. In particular, it is shown that the formation of magnetic exchange springs in the magnetically soft YFe2 layers, can be used to engineer multilayer samples with a negative coercivity. Further, by using asymmetric field cycling procedures, we have investigated the irreversible parts of the M–B loop, associated with the switching of the DyFe2 multilayers.

Magnetic properties of epitaxial (110) multilayer films of DyFe2 and YFe2

Laves phase DyFe2/YFe2 multilayers have been grown epitaxially on a YFe2 seed layer, with a (110) growth direction. Magnetic measurements taken in applied fields of up to 12 T, and from 5 K to room temperature, show that short period multilayers (∼100 A) behave, collectively, as a single magnetic entity. As a result, it is possible to engineer magnetic compensation points, in a digital manner, by adjusting the thicknesses of the alternate DyFe2 and YFe2 layers. Nevertheless, the magnetic response of the DyFe2/YFe2 structure and that of the YFe2 seed layer are not completely independent of one another. Because of a mismatch in the Fe–Fe magnetic exchange at the multilayer/seed interface, a 180° magnetic soliton-like domain (“magnetic twister”) is set up in the top of the YFe2 seed layer. A semiquantitative model describing the properties of the magnetic twister is presented and discussed.

Structural changes to epitaxial (0001) holmium layers during hydrogen loading

HoHy exhibits dramatic changes in both structural and optical properties as y is varied from zero to three by hydrogen loading. This work reports on the effect of such loading upon epitaxial single-crystal Ho films grown by molecular beam epitaxy on (110) Nb (11 0) Al2 O3 substrates. Upon loading, the film undergoes a structural transition from hcp metal ( -phase, H in solid solution) to the fcc dihydride ( -phase). There is a further transformation between the dihydride phase and the hexagonal trihydride ( -phase). Three films of HoHy , where, nominally, y = 0,2,3, were studied by XRD, AFM, UFM, SEM and TEM. Triangular networks of features of width ~300 nm and height ~20-30 nm that align with the [0 1], [ 01] and [1 0] directions of the dihydride sample are seen on the surface of both the dihydride and the trihydride samples, but have a much greater density on the surface of the latter. Such features are not present on the as-grown metal layer. In situ controlled-environment transmission electron microscopy (CETEM) studied the effects of hydrogen loading within the dihydride phase, in cross sectional geometry. During loading, slip within the dihydride phase was observed. The hypothesis that the triangular features are due to the precipitation of the di- and trihydride compounds, with expanded lattices, on the slip planes within the - or -phase host crystal is discussed.

Misfit dislocations of epitaxial (110) niobium‖ (112̄0) sapphire interfaces grown by molecular beam epitaxy

Abstract High resolution electron microscopy, HREM, of (110)Nb‖(1120)Al2O3 interfaces grown by molecular beam epitaxy, MBE, has confirmed that the interface is semicoherent for the films investigated, and that for this orientation misfit dislocations occur at the interface both with and without stand-off from the interface. The dislocation networks have been identified by conventional transmission electron microscopy, CTEM, to be composed of misfit dislocations with a Burgers vector of 1/2〈111〉, which corresponds to the Burgers vector of bulk dislocations in Nb.

Interplay between superconductivity and magnetism in Gd/La superlattices

Abstract A [Gd 30 /La 10 ] 60 superlattice has been studied using SQUID magnetometry and polarised neutron reflectivity. Zero-field cooling results in the coexistence of an antiferromagnetic alignment of the ferromagnetic Gd blocks, and 3D superconductivity at low temperature. Field cooling from room temperature results in ferromagnetic coupling between the Gd blocks, and under these conditions, the superconducting transition in the La is suppressed.

Magnetic Properties of Gadolinium Ortho-Aluminate

An account is given of specific heat, magnetic susceptibility and magnetic moment measurements on gadolinium ortho-aluminate, GdAlO3, at temperatures in the liquid helium range. This compound orders antiferromagneticallv at 3.87 K. In the antiferromagnetic state, its properties can be accounted for satisfactorily as those of an array of S-state magnetic ions with isotropic magnetic properties, coupled together by exchange interaction between nearest neighbours and subject to a ‘single ion’ anisotropy force due to the crystalline electric field. Because the structure approximates closely to a simple cubic lattice of Gd3+ ions, magnetic dipole effects have little influence on the ordering. The exchange and anisotropy forces are calculated from the experimental data, and the measured properties are compared with various theoretical predictions for a simple two sublattice antiferromagnet. The variation of the specific heat in the critical region close to the Néel point is compared with that of other compounds undergoing magnetic transitions at low temperatures, and a consistent pattern of behaviour is found.

Engineering coercivity in epitaxially grown (110) films of DyFe2–YFe2 superlattices

Molecular beam epitaxial methods have been used to grow single crystal Laves phase DyFe2–YFe2 superlattice samples with a ~110! growth direction. It is shown that it is possible, in principle, to engineer a desired coercivity between the limits KDyFe2<K<` . This can be achieved by adjusting the relative thickness of the individual DyFe2 and YFe2 layers, in multilayer films This novel feature is illustrated, using the superlattice films @x A DyFe2 /(100-x) A YFe2#340, with x580, 60, 50, and 45. It is found that the measured coercivity is in semiquantitative agreement with a simple theoretical expression, for the nucleation fields in both bilayer and multilayer compounds. However, in practice, exchange spring penetration into the DyFe2 layers can set a limit to the maximum coercivity that can be achieved.

Probing the local electronic structure in the H induced metal - insulator transition of Y

The occupied acid unoccupied valence band states of yttrium (Y) metal, Y dihydride and Y trihydride were measured by soft x-ray emission and absorption spectroscopy. The experiments were complement ...