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  • 1
    Electronic Resource
    Electronic Resource
    Sputter deposited Co/CoO composite materials : The 40th annual conference on magnetism and magnetic materials (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 5072-5074 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Magnetic composites consisting of ultrafine particles of Co and CoO were prepared by dc reactive magnetron sputtering in a Ar+O2 gas mixture. The films were composed of 60–140 A(ring) particles of hcp Co and fcc CoO. The magnetization, coercivity, and particle size were found to be dependent on the O2 partial pressure, such that the magnetization decreased and the coercivity increased to a maximum value of 1000 Oe with increasing O2 partial pressure. A shift in the hysteresis loop was observed for several samples, indicating exchange coupling between the Co and CoO. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.361578
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  • 2
    Electronic Resource
    Electronic Resource
    High field irreversibility in NiFe2O4 nanoparticles (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. 5071-5071 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Several anomalous magnetic properties of organic coated NiFe2O4 nanoparticles have been reported previously by Berkowitz et al. These properties included low magnetization with a large differential susceptibility at high fields, shifted hysteresis loops after field cooling below 50 K, and Mössbauer spectra indicating a fraction of the material being magnetically ordered but not responsive to applied fields. It was suggested that a "strongly anisotropic phase'' was present on the particles surface, and that the organic coating was responsible for producing high local anisotropy fields on the surface atoms. The present study extends the original work in several ways. We find that the lack of saturation in high fields is accompanied by irreversibility up to 20 T, in some cases. We have confirmed the previously reported behavior, in addition to observing similar behavior in samples prepared without the organic surfactant. This implies that we are observing a finite size effect. We have recently reported time dependence of the remanent magnetization which persists down to 0.4 K and does not appear to follow a thermally activated law. Instead, the viscosity becomes temperature independent below 2 K. The model we propose for the high field irreversibility as well as the previously reported behaviors, is that there is a layer of spins at the surface which are spin glass-like, and that these spins can be reoriented irreversibly by a field or by thermal activation. The effect of surface spin interactions on magnetization relaxation will be discussed. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.361577
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  • 3
    Electronic Resource
    Electronic Resource
    Optimizing the giant magnetoresistance of symmetric and bottom spin valves (invited) : The 40th annual conference on magnetism and magnetic materials (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 5277-5281 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have attempted to optimize the values of the giant magnetoresistance in symmetric spin valves of the type NiO/Co/Cu/Co/Cu/Co/NiO (achieving 23.4%) and in bottom spin valves of the type Co/Cu/Co/NiO (achieving 17.0%), the largest values ever reported for such structures. The key elements in this achievement are improved vacuum conditions and careful attention to the film thicknesses. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.361352
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  • 4
    Electronic Resource
    Electronic Resource
    Unusual magnetic behavior in Nd–Fe–B alloy powder compacts : The 40th annual conference on magnetism and magnetic materials (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 5504-5506 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report an unusual magnetic behavior in powders prepared by spark-eroding (in liquid Ar) alloy electrodes containing approximately equal weights of Fe and Nd2Fe14B in an effort to prepare composite permanent magnets. Magnetization exhibits reproducible thermal hysteresis, peaking in all applied fields near 520 °C when warming, but increasing monotonically when cooling to room temperature from 700 °C and above. Mössbauer spectroscopy was used to show that the behavior is due to the metastability of Fe1−xO produced in the powders by partial oxidation in Ar gas flow. This compound, paramagnetic at room temperature, decomposes only slowly below 570 °C into ferromagnetic Fe and ferrimagnetic Fe3O4. The reverse reaction occurs readily at higher temperatures. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.362292
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  • 5
    Electronic Resource
    Electronic Resource
    Propagation of antiferromagnetic order across paramagnetic layers in CoO/NiO superlattices : 37th Annual conference on magnetism and magnetic materials (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 6898-6900 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Using neutron-diffraction techniques, we have probed the magnetic structure of nickel-oxide/cobalt-oxide superlattices grown by reactive sputtering. Collinear antiferromagnetic order propagates through several bilayers despite the short-range nature of the spin interaction in the components. The magnetic order for 36 A(ring) period superlattices develops simultaneously in the NiO and CoO layers at a Néel temperature between the values for bulk CoO and NiO. When the superlattice periodicity is increased to 72 A(ring), the Ni and Co spins appear to order separately at temperatures shifted from their bulk TN. The magnetic coherence length decreases as the temperature is increased, but remains greater than the width of a single bilayer above TN for the CoO interlayers. Mean-field calculations demonstrate that exchange coupling at the interfaces is responsible for the anomalous magnetic behavior near the phase transition.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.352427
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  • 6
    Electronic Resource
    Electronic Resource
    Heat capacity measurements of NiO/CoO superlattices (invited) (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. 5919-5919 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We present heat capacity measurements of NiO/CoO superlattices grown by reactive sputtering. Neutron diffraction studies of similar superlattices have shown antiferromagnetic ordering through several bilayers despite the short-range nature of the spin interaction in the constituent materials. Specific heat measurements were made using a unique thin film microcalorimeter capable of measuring the heat capacity of thin films from 1.5 K to well above room temperature. We examine the effect of exchange coupling at the interfaces by varying the thickness of the bilayers. For thin bilayers (26 A(ring)), we observe a single broad heat capacity peak similar to a Ni0.5Co0.5O alloy. This peak is at a temperature which corresponds to the superlattice magnetic blocking temperature. For thicker bilayers ((approximately-greater-than)50 A(ring)), two broad maxima occur which approach the individual Néel temperatures of CoO and NiO with increasing bilayer thickness. The ordering temperature of the NiO layers is more suppressed than expected, indicating a more pronounced size effect compared to the CoO layers. The magnetic entropy S=R(ln 3+ln 2)/2 for the superlattices, within the uncertainties of the measurement, is conserved. We compare the temperature dependences of the specific heats to various models. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.361838
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  • 7
    Electronic Resource
    Electronic Resource
    CoO-NiO superlattices: Interlayer interactions and exchange anisotropy with Ni81Fe19 (invited) : 37th Annual conference on magnetism and magnetic materials (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 6892-6897 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A strong interlayer exchange interaction is observed in polycrystalline antiferromagnetic CoO-NiO thin-film superlattices. This was determined by measuring the exchange field He resulting from coupling these superlattices with Ni81Fe19. The temperature above which He is zero (the blocking temperature) is taken as a measure of the ordering temperature of the superlattice. In CoO-NiO superlattices with short repeat distances the CoO and NiO layers order at a single temperature that is between the ordering temperatures of bulk CoO and NiO. These ordering temperatures are the same as for CoxNi1−xO alloy films with the same overall composition. The temperature dependence of He in some of the superlattice exchange couples deviates from the linear behavior expected for cubic antiferromagnets. In addition, the exchange anisotropy induced by some CoO-NiO superlattices is greater than that induced by CoxNi1−xO. The higher He and nonlinear temperature dependence suggest that the interlayer coupling has a strong effect on the magnetocrystalline anisotropy of the layers within the superlattice. Thick-NiO/thin-CoO/Ni81Fe19 trilayers were produced to investigate the thickness dependence of the oxide-oxide interaction. When the CoO layers are sufficiently thin (≤20 A(ring)), the CoO ordering temperature approximates the NiO value, indicating quite strong coupling throughout the CoO layer. The effect of the coupling is much weaker for thicker CoO layers (≥30 A(ring)).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.352426
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  • 8
    Electronic Resource
    Electronic Resource
    Giant magnetic field dependent impedance of amorphous FeCoSiB wire (1994)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 64 (1994), S. 3652-3654 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The impedance of an amorphous Fe4.3Co68.2Si12.5B15 wire (100 μm diameter) exhibits an extraordinarily large frequency dependent resistance in addition to the previously reported frequency dependent wire reactance. The frequency response of both the resistance and reactance is almost entirely suppressed by an axial magnetic field HA〈150 Oe, resulting in a typical magnetoresistance for frequencies f〈1 MHz of the order of the dc wire resistance. The magnetoresistance at f=1 MHz is ΔR/Rsat=370%. As the bulk of the magnetic response occurs for HA〈5 Oe, this system shows great technological promise. We give a quantitative analysis of the phenomenon, which is rooted in classical electrodynamics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.111170
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  • 9
    Electronic Resource
    Electronic Resource
    Magnetoresistance of magnetite (1998)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 72 (1998), S. 734-736 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The magnetoresistance behavior of Fe3O4 in polycrystalline thin film, powder compact, and single-crystal form are compared. Negative magnetoresistance with peaks at the coercive field, observed in thin films and powder compacts but not in the single crystal, is due to field-induced alignment of the magnetization of contiguous grains. The effect is associated with intergranular transport of spin-polarized electrons. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.120859
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  • 10
    Electronic Resource
    Electronic Resource
    Magnetoresistance values exceeding 21% in symmetric spin valves (1995)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 78 (1995), S. 273-277 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report values of the giant magnetoresistance (GMR) effect exceeding 21% in symmetric spin valves, the highest values ever reported for such structures. The key elements in this achievement are the use of a Co/Cu/Co/Cu/Co multilayer in which the center Co layer is substantially thicker than the outer Co layers and the use of the antiferromagnetic insulator NiO at the top and bottom to pin the adjacent Co layers magnetically. The relative Co layer thicknesses suggest that some specular scattering of conduction electrons may occur at the metal/insulator interfaces and may enhance the GMR. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.360692
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