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  • 1
    Electronic Resource
    Electronic Resource
    Dipolar ferromagnets and glasses (invited) : The 5th Joint MMM−Intermag Conference (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 5946-5951 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: What is the ground state and what are the dynamics of 1023 randomly distributed Ising spins? We have attempted to answer these questions through magnetic susceptibility, calorimetric, and neutron scattering studies of the randomly diluted dipolar-coupled Ising magnet LiHoxY1−xF4. The material is ferromagnetic for dipole concentrations at least as low as x=0.46, with a Curie temperature obeying mean-field scaling relative to that of pure LiHoF4. In the dilute spin limit, an x=0.045 crystal shows very unusual glassy properties characterized by decreasing barriers to relaxation as T→0. Its properties are consistent with a single low degeneracy ground state with a large gap for excitations. A slightly more concentrated x=0.167 sample, however, supports a complex ground state with no appreciable gap, in accordance with prevailing theories of spin glasses. The underlying causes of such disparate behavior are discussed in terms of random clusters as probed by neutron studies of the x=0.167 sample. In addition to tracing the evolution of the glassy and ferromagnetic states with dipole concentration, we investigate the effects of a transverse magnetic field on the Ising spin glass, LiHo0.167Y0.833F4. The transverse field mixes the eigenfunctions of the ground-state Ising doublet with the otherwise inaccessible excited-state levels. We observe a rapid decrease in the characteristic relaxation times, large changes in the spectral form of the relaxation, and a depression of the spin-glass transition temperature with the addition of quantum fluctuations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.350087
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  • 2
    Electronic Resource
    Electronic Resource
    Very large magnetoresistance in electrodeposited single-crystal Bi thin films (invited) (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 4659-4664 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Single-crystal bismuth thin films, fabricated by electrodeposition and suitable annealing, exhibit very large magnetoresistance of 400 000% at 5 K and 300% at 300 K, as well as pronounced Shubnikov–de Haas oscillations. A hybrid structure demonstrates the potential for field sensing with sensitivities of 35%/Oe at 5 K and 0.2%/Oe at 300 K. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.373123
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  • 3
    Electronic Resource
    Electronic Resource
    Giant positive magnetoresistance of Bi nanowire arrays in high magnetic fields (1999)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 85 (1999), S. 6184-6186 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have studied the magnetoresistance of electrodeposited Bi wires with diameters between 200 nm and 2 μm in magnetic fields up to B=55 T. In zero field, the resistance increases with decreasing temperature, indicating that the mean free path is strongly influenced by the nanowire geometry. The high-field magnetoresistance shows strong dependence on field orientation; typically 200% for B parallel to the wires, and 600%–800% for B perpendicular to the wires. The perpendicular magnetoresistance is well described by a modified two-current model which suggests that the high-field response of the arrays is fairly insensitive to the wire diameter, and is dominated by bulk properties of Bi. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.370215
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  • 4
    Electronic Resource
    Electronic Resource
    Large magnetoresistance and finite-size effect in electrodeposited bismuth lines (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 7206-7208 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Finite-size effects in Bi lines of cross section 3 μm×w, where w=8–120 μm, have been studied. The lines were made by optical lithography followed by electrodeposition. At T=5 K, the magnetoresistance (MR) decreases monotonically with decreasing line width of the Bi films, from 44 000% at w=120 μm to 4000% at w=8 μm in a field of 50 kOe. The decrease of the MR with decreasing linewidth is due to a decrease of the size-limited effective-mean-free path. At low temperatures, both the resistivity and the magnetoresistivity show a strong dependence on w, whereas at room temperature, they are independent of w. The MR at room temperature remains at about 230% for linewidths w=8–120 μm at 50 kOe. This demonstrates that microstructured Bi lines can be made for field-sensing applications without compromising their MR characteristics. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1357115
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  • 5
    Electronic Resource
    Electronic Resource
    Imaging and magnetotransport in superconductor/magnetic dot arrays (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 7478-7480 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Magnetoresistance and scanning Hall probe microscopy studies of Nb-film/Ni-dot structures are reported. The dots act as pinning sites for superconducting vortices. The transport measurements focus on the effects of introducing disorder into the positions of the pinning lattice near the superconducting critical temperature Tc in structures with 250 nm diameter Ni dots randomized about an ideal square lattice with lattice constant a=560 nm. Features observable in the ordered arrays at higher multiples of the matching field H0=Φ0/a2 are washed out in the disordered arrays, but those at H0 remain. Scanning Hall probe microscope images were taken of the vortex configurations at fields up to 1.2H0 in ordered arrays of 1-μm-diameter dots on a 5.2 μm×4 μm rectangular lattice. These show that despite the relatively weak pinning of the magnetic dots, ordering commensurate with the dot lattice occurs even for fields below H0. Both transport and imaging studies point to the importance of interstitial vortices in determining the properties of superconductor/magnetic dot arrays. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1357858
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  • 6
    Electronic Resource
    Electronic Resource
    Finite temperature effects in the S=1/2 Heisenberg chain Cu(C6D5COO)2⋅3D2O (abstract) : The 40th annual conference on magnetism and magnetic materials (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 5391-5391 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Quasi-one-dimensional magnetic materials provide important test beds for rigorous models of interacting many-body systems. Nevertheless, there have been few studies of finite temperature effects on spin correlations in low-spin systems, though these are most likely to exhibit quantum effects. Copper benzoate, Cu(C6D5COO)2⋅3D2O, is a good realization of the S=1/2 Heisenberg antiferromagnetic chain (HAFC). We have performed ac susceptibility and inelastic neutron scattering experiments on this material which establish the Hamiltonian and an excitation spectrum which is consistent with the spinon continuum ansatz of Müller et al. We present inelastic neutron scattering measurements of the temperature evolution of the dynamic spin–spin correlation function Szz(π,ω). Comparison will be made to the classical prediction and to a finite temperature field theory of Schulz as applied to this S=1/2 HAFC. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.362314
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  • 7
    Electronic Resource
    Electronic Resource
    Cu2(1,4-diazacycloheptane)2Cl4: Spin-ladder system with competing interactions (abstract) : The 41st annual conference on magnetism and magnetic materials (1997)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 81 (1997), S. 4614-4614 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The material Cu2(1,4-diazacycloheptane)2Cl4 consists of well-separated ladderlike chains of Cu atoms that undergo spontaneous dimerization leading to a singlet ground state. Previous measurements12 have shown that for T≤0.7 K, this singlet ground state exists up to a field HC=6.7. Here we report on heat capacity, Cp(H,T), and neutron scattering studies of this material. For T〈8 K, Cp(H=0,T)∝T−3/2 exp{−Δ/T} with an activation energy Δ=9.8 K. Cp(H,T) shows a finite-T transition for H〉HC, presumably to a canted state. Inelastic neutron scattering on powder samples at H=0 shows a gap of 0.8 meV and a bandwidth for the excitations of 0.6 meV. These excitations exhaust the total moment sum rule, and hence the measured triplet band dominates the magnetic fluctuation spectrum. Comparison of the data with exact diagonalization of finite spin chains shows that the system appears to be close to a spin ladder, but with an additional, weak interrung interaction. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.365179
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