Peter Greene

University of California, Davis, Davis, California, United States

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Publications (13)21.05 Total impact

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    ABSTRACT: Combined first order reversal curve (FORC) analyses of the magnetization (M-FORC) and magnetoresistance (MR-FORC) have been employed to provide a comprehensive study of the M-MR correlation in two canonical systems: a NiFe/Cu/FePt pseudo spin-valve (PSV) and a [Co/Cu]8 multilayer. In the PSV, due to the large difference in switching fields and minimal interactions between the NiFe and FePt layers, the M and MR show a simple one-to-one relationship during reversal. In the [Co/Cu]8 multilayer, the correlation between the magnetization reversal and MR evolution is more complex. This is primarily due to the similar switching fields of, and interactions between, the constituent Co layers. The FORC protocol accesses states with much higher spin disorders and larger MR than those found along the conventional major loop field-cycle. Unlike the M-FORC measurements, which only probe changes in the macroscopic magnetization, the MR-FORCs are more sensitive to the microscopic domain configurations, as those are most important in determining the resultant MR effect size. This approach is generally applicable to spintronic systems to realize the maximum spin-disorder and the largest MR.
    09/2014;
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    ABSTRACT: The tunability of Ar+ ion irradiation of Co/Pd multilayers has been employed to create depth-dependent perpendicular anisotropy gradients. By adjusting the Ar+ kinetic energy and fluence, the depth and lateral density of the local structural modification are controlled. First-order reversal curve analysis through X-ray magnetic circular dichroism and conventional magnetometry studies show that the local structural damage weakens the perpendicular anisotropy near the surface, leading to a magnetization tilting towards the in-plane direction. The ion irradiation method is complementary to, and may be used in conjunction with, other synthesis approaches to maximize the anisotropy gradient.
    Applied Physics Letters 08/2014; 105(7). · 3.79 Impact Factor
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    ABSTRACT: A vertically graded anisotropy profile has been proposed as an optimized balance of low coercivity and thermal stability for multilayers used in magnetic media. Deposition pressure is known to have a profound effect on the magnetic reversal properties of Co/Pd multilayers, making it an attractive control parameter for achieving an anisotropy gradient. We have used polarized neutron reflectometry to study the depth-dependent reversal behavior of "pressure-graded" Co/Pd, and observed pronounced gradients in the saturation magnetization and in the rate at which magnetization changes with field (the effective anisotropy). While the anisotropy gradient likely arises from a combination of factors intrinsic to deposition pressure, micromagnetic simulations indicate that the observed saturation magnetization gradient alone has a major effect on the resulting coercivity.
    03/2014;
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    ABSTRACT: Magnetization reversal mechanisms and depth-dependent magnetic profile have been investigated in Co/Pd thin films magnetron-sputtered under continuously varying pressure with opposite deposition orders. For samples grown under increasing pressure, magnetization reversal is dominated by domain nucleation, propagation, and annihilation; an anisotropy gradient is effectively established, along with a pronounced depth-dependent magnetization profile. However, in films grown under decreasing pressure, disorders propagate vertically from the bottom high-pressure region into the top low-pressure region, impeding domain wall motion and forcing magnetization reversal via rotation; depth-dependent magnetization varies in an inverted order, but the spread is much suppressed.
    Applied Physics Letters 01/2014; 104(15):152401-152401-5. · 3.79 Impact Factor
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    ABSTRACT: Dewetting of ultra-thin Ni films deposited on SiO2 layers was observed, in cross-section, by in situ scanning transmission electron microscopy. Holes were observed to nucleate by voids which formed at the Ni/SiO2 interface rather than at triple junctions at the free surface of the Ni film. Ni islands were observed to retract, in attempt to reach equilibrium on the SiO2 layer. SiO2 layers with 120nm thickness were found to limit in situ heating experiments due to poor thermal conductivity of SiO2. The formation of graphite was observed during the agglomeration of ultra-thin Ni films. Graphite was observed to wet both the free surface and the Ni/SiO2 interface of the Ni islands. Cr forms surface oxide layers on the free surface of the SiO2 layer and the Ni islands. Cr does not prevent the dewetting of Ni, however it will likely alter the equilibrium shape of the Ni islands.
    Ultramicroscopy 11/2013; 137C:55-65. · 2.47 Impact Factor
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    ABSTRACT: Ultrathin films of nickel deposited onto (1 0 0) Si substrates were found to form kinetically constrained multilayered interface structures characterized by structural and compositional gradients. The presence of a native SiO2 on the substrate surface in tandem with thickness-dependent intrinsic stress of the metal film limits the solid-state reaction between Ni and Si. A roughly 6.5 nm thick Ni film on top of the native oxide was observed regardless of the initial nominal film thickness of either 5 or 15 nm. The thickness of the silicide layer that formed by Ni diffusion into the Si substrate, however, scales with the nominal film thickness. Cross-sectional in situ annealing experiments in the transmission electron microscope elucidate the kinetics of interface transformation towards thermodynamic equilibrium. Two competing mechanisms are active during thermal annealing: thermally activated diffusion of Ni through the native oxide layer and subsequent transformation of the observed compositional gradient into a thick reaction layer of NiSi2 with an epitaxial orientation relationship to the Si substrate; and, secondly, metal film dispersion and subsequent formation of faceted Ni islands on top of the native oxide layer.
    Acta Materialia. 01/2012; 60:2668-2678.
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    ABSTRACT: Magnetization reversal and the effect of patterning have been investigated in full-film and dot arrays of Co/Pd multilayers, using the first-order reversal curve and scanning electron microscopy with polarization analysis techniques. The effect of patterning is most pronounced in low sputtering pressure films, where the size of contiguous domains is larger than the dot size. Upon patterning, each dot must have its own domain nucleation site and domain propagation is limited within the dot. In graded anisotropy samples, the magnetically soft layer facilitates the magnetization reversal, once the reverse domains have nucleated.
    Journal of Applied Physics 05/2011; · 2.21 Impact Factor
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    ABSTRACT: We report deposition-order-dependent, and depth-dependent, magnetization reversal in Co/Pd with graded anisotropy, which are technologically important as they address both writability and thermal stability challenges. Multilayers of [Co(0.4nm)/Pd(0.6nm)]60 have been deposited by sputtering, where the Ar pressure has been varied from 5 to 12 and 20 mtorr in type A samples and in the reverse order in type B samples. An extensive set of structural and magnetization reversal studies with depth-resolution has been performed using XRD, cross-sectional TEM, magnetometry, PNR and XMCD. In type A samples, due to the larger grain size, lower interfacial roughness and less disorder in the magnetically softer layer, magnetization reversal proceeds via domain nucleation, propagation, and annihilation. Type B samples show a more localized reversal. Layers grown at higher pressure contain more disorder and rougher interfaces, which is carried into the magnetically softer layers deposited on top, thus impeding domain movement.
    03/2011;
  • Microscopy and Microanalysis 01/2011; 17:1328-1329. · 2.50 Impact Factor
  • Microscopy and Microanalysis 08/2010; 16(S2):1462-1463. · 2.50 Impact Factor
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    ABSTRACT: Magnetic configurations in heterostructures are often difficult to probe when the magnetic entities are buried inside. In this study we have captured magnetic and magnetoresistance "fingerprints" of Co nanodiscs embedded in Co/Cu multilayered nanowires using a first-order reversal curve method. In 200nm diameter nanowires, the magnetic configurations can be tuned by adjusting the Co nanodisc aspect ratio. Nanowires with the thinnest Co nanodiscs exhibit single domain behavior, while those with thicker Co reverse via vortex states. A superposition of giant and anisotropic magnetoresistance is observed, which corresponds to the different magnetic configurations of the Co nanodiscs.
    Applied Physics Letters 02/2009; · 3.79 Impact Factor
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    ABSTRACT: Multilayered magnetic nanowires provide ideal platforms for nanomagnetism and spin-transport studies. They exhibit complex magnetization reversal behaviors as dimensions of the magnetic components are varied, which are difficult to probe since the magnetic entities are buried inside the nanowires. We have captured magnetic and magnetoresistance "fingerprints" of Co nanodiscs in Co/Cu multilayered nanowires as they undergo a single domain to vortex state transition, using a first-order reversal curve (FORC) method. The Co/Cu multilayered nanowires have been synthesized by pulsed electrodeposition into nanoporous polycarbonate membranes. In 50 nm diameter nanowires of [Co(5nm)/Cu(8nm)]400, a 10% magnetoresistance effect is observed at 300 K. In 200 nm diameter nanowires, the magnetic configurations can be tuned by adjusting the Co nanodisc aspect ratio. The thinnest nanodiscs exhibit single domain behavior. The thicker ones exhibit vortex states, where the nucleation and annihilation of the vortices are manifested as butterfly-like features in the FORC distributions. The magnetoresistance effect shows different characteristics, which correspond to the different magnetic configurations of the Co nanodiscs.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
    08/2008;
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    ABSTRACT: Multilayered magnetic nanowires provide ideal platforms for nanomagnetism and spin-transport studies. They exhibit complex magnetization reversal behaviors as dimensions of the magnetic components are varied, which are difficult to probe since the magnetic entities are buried inside the nanowires. We have captured magnetic and magnetoresistance "fingerprints" of Co nanodiscs in Co/Cu multilayered nanowires as they undergo a single domain to vortex state transition, using a first-order reversal curve (FORC) method. The Co/Cu multilayered nanowires have been synthesized by pulsed electrodeposition into nanoporous polycarbonate membranes. In 50 nm diameter nanowires of [Co(5nm)/Cu(8nm)]400, a 10% magnetoresistance effect is observed at 300 K. In 200 nm diameter nanowires, the magnetic configurations can be tuned by adjusting the Co nanodisc aspect ratio. The thinnest nanodiscs exhibit single domain behavior. The thicker ones exhibit vortex states, where the nucleation and annihilation of the vortices are manifested as butterfly-like features in the FORC distributions. The magnetoresistance effect shows different characteristics, which correspond to the different magnetic configurations of the Co nanodiscs.
    Proc SPIE 08/2008;