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  • 1990-1994  (13)
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  • 1
    Electronic Resource
    Electronic Resource
    Langevin approach to hysteresis and Barkhausen jump modeling in steel : 37th Annual conference on magnetism and magnetic materials (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 5848-5850 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: To represent the multiplicity of metastable states in a bulk ferromagnetic material with domain-wall pinning effects, fluctuations in the domain-wall energies are described in terms of an ensemble of stochastic Langevin functions. The model parameters used are a correlation length, a rms value for the amplitude of the fluctuations in the domain-wall energy gradient, and a "demagnetizing factor.'' The model generates both hysteresis loops and Barkhausen effect (BE) jump size distributions. Jump size distributions were determined experimentally for low-carbon rolled sheet steel with the field applied both parallel and perpendicular to the rolling direction. Both the model and the experimental BE jump size distributions show a power-law behavior for small jumps and a rapid cutoff at large jump sizes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.353547
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  • 2
    Electronic Resource
    Electronic Resource
    The magnetocaloric effect: The role of magnetic anisotropy : 37th Annual conference on magnetism and magnetic materials (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 6507-6509 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In a ferromagnet, the magnetic entropy change induced by the application of a magnetic field is greatest in the temperature regime near the Curie point. In the paramagnetic temperature regime, the magnitude of the magnetocaloric effect is expected to rise monotonically with the size of the individual moments that make up the material. The magnetic properties of such materials are relevant to devices employing magnetic refrigeration. One aspect of the problem, namely the impact of uniaxial magnetic anisotropy on the magnetocaloric effect, is explored. The results of Monte Carlo simulations for classical Heisenberg ferromagnets on a fcc lattice with anisotropy are presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.352596
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  • 3
    Electronic Resource
    Electronic Resource
    Effect of twin boundaries on flux pinning in YBa2Cu3O7−x at low and intermediate magnetic fields : The 5th Joint MMM−Intermag Conference (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 5739-5741 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Magnetic measurements on a single crystal of YBa2Cu3O7−x in the twinned and detwinned states have demonstrated that flux pinning by twin boundaries is strongly dependent upon temperature and the magnitude and orientation of the applied field with respect to the crystallographic orientation. For applied magnetic fields perpendicular to the c axis, no effect of twin boundaries on flux pinning was found at any temperature. In contrast, for fields up to 400 Oe aligned parallel to the c axis, the hysteresis loops at 77 K clearly showed increased pinning due to twin boundaries. At 40 K and low fields (H〈500 Oe) the flux pinning was nearly the same for the twinned and detwinned states, whereas at higher fields (up to 40 kOe), pinning was higher in the twinned state. At this temperature, a tendency toward flux depinning occurred in the detwinned state at about 10 kOe, and both the twinned and detwinned states exhibited a more rigid flux structure at 50 kOe than at 10 kOe.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.350145
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  • 4
    Electronic Resource
    Electronic Resource
    Enhanced magnetocaloric effect in Gd3Ga5−xFexO12 : 37th Annual conference on magnetism and magnetic materials (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 6946-6948 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The working refrigerant material in the majority of magnetic refrigerators has been Gd3Ga5O12 (GGG) which has an upper temperature limit near 15 K. In this paper we report on the field-induced adiabatic magnetic entropy change, ΔSm(H,T), of a series of iron-substituted gadolinium garnets (GGIG) Gd3Ga5−xFexO12 which have the potential to increase the working temperature range or to reduce the field requirements of cryogenic magnetic refrigeration. Depending on Fe concentration, x, the entropy change of these materials at applied fields of 0.9 and 5.0 T is much greater than that of GGG, especially at temperatures above 15 K. At low Fe concentrations, the results are consistent with formation of magnetically ordered clusters of spins at low temperatures. Room temperature electron paramagnetic resonance measurements show that Fe3+ ions mediate exchange interactions which are responsible for clustering at low temperatures.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.352443
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  • 5
    Electronic Resource
    Electronic Resource
    Magnetic and optical properties of γ-Fe2O3 nanocrystals (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 5109-5116 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: γ-Fe2O3 nanocrystals have been synthesized in a polymer matrix by an ion exchange and precipitation reaction. The particles are crystalline with a mean diameter of 85 A(ring). The unit cell volume is 1735.2 A(ring)3 corresponding to a unit cell compression of 0.35%. Magnetization and susceptibility data from experiment and computer simulations indicate that the system is superparamagnetic. The calculated magnetic anisotropy for the particles is two orders of magnitude larger than that of bulk γ-Fe2O3 . The optical absorption edge is red shifted with respect to that of an epitaxially grown single-crystal film of γ-Fe2O3 . The red shift is attributed to lattice strain in the small particles.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.353784
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  • 6
    Electronic Resource
    Electronic Resource
    Parametric excitation of magnetostatic modes in thin yttrium iron garnet films (invited) : 35th annual conference on magnetism and magnetic materials (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 5425-5429 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Recently, equations of motion for the normal modes of normally magnetized thin films have been successful in predicting experimental auto-oscillation frequencies and onset pumping powers in circular yttrium iron garnet films under ferromagnetic resonance. These results have now been extended to include "hidden'' modes, modes that do not couple directly to the pumping field. Using the equations of motion derived for this system, hidden modes are shown to be parametrically excited through processes analogous to the second-order Suhl instability. Through excellent agreement with experimental results, parametrically excited hidden modes are shown to be involved in features of the domain of auto-oscillation that were not previously understood. Numerical studies of the effects of degeneracy breaking in pairs of hidden modes show how small perturbations can fix the phases of parametrically excited modes. The random-phase approximation is shown to have a significant effect in simulations of the experiment.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.348000
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  • 7
    Electronic Resource
    Electronic Resource
    Nonlinear dynamics of magnetoexchange modes in YIG films (abstract) (1990)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 67 (1990), S. 5642-5642 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The nonlinear dynamics of magnetoexchange modes in two films of yttrium iron garnet (YIG) with similar low-power spectra, but different mode densities have been investigated at perpendicular resonance. In this configuration, the resonances are not degenerate with a large number of short wavelength spin-wave modes as in bulk samples, and the density of modes near the resonances can be varied by adjusting the sample dimensions. Domains of auto-oscillation in applied field–pumping field parameter space appear to arise as fingers from each mode in the low-power spectrum. Differences between the behavior of the two samples are attributed to excitation of hidden modes from a degenerate exchange branch, exited through a process analogous to a second-order Suhl instability. A model of this system is proposed in which nonlinear interactions between modes are solely due to the shape demagnetization field. The resulting equations of motion have been integrated numerically using parameters determined from low-power FMR and from knowledge of the sample shape. Including only the modes from the main branch of the magnetoexchange spectrum that couple directly to a uniform pumping field, these calculations predict the frequency of auto-oscillation of the first finger to within 15% and predict the power at onset to within 1 dB. Hidden modes from the main branch may play an important role in the dynamics of the other fingers. We believe this to be the most accurate prediction of an auto-oscillation frequency in a magnetic system yet published.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.345911
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  • 8
    Electronic Resource
    Electronic Resource
    Method for determining both magnetostriction and elastic modulus by ferromagnetic resonance : 38th Annual Conference on Magnetism and Magnetic Materials (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 5650-5652 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The magnetostriction constant λs includes a combination of both elastic constants cij and magnetoelastic coupling constants Bi. To provide information on both cij and Bi, a method for measurement of both λs and the elastic modulus Y of thin films has been developed using ferromagnetic resonance (FMR). As in prior FMR schemes for measuring λs, a strain is produced in the film by bending the substrate, and the resulting anisotropy field is measured. However, by using a triangular polymer beam as a substrate, three important advantages are obtained (i) the triangular beam shape has uniform strain along its surface, (ii) the polymer material supports large strains ((approximately-greater-than)10−3) without breaking, and (iii) the low elastic modulus of the beam allows measurement of the sample elastic modulus through variation of the sample aspect ratio. Measurements on Ni foils yield values of λs and Y which fall within the scatter of literature data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.355625
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  • 9
    Electronic Resource
    Electronic Resource
    Giant magnetoresistance peaks in CoNiCu/Cu multilayers grown by electrodeposition : The 6th joint magnetism and magnetic materials (MMM)-intermag conference (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 6519-6521 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Giant magnetoresistance (GMR) of CoNiCu/Cu multilayers grown by electrodeposition was measured as a function of the copper layer thickness and effects of the order of 14% were obtained. The copper layer thickness ranged from 0.7 to 3.5 nm. Two peaks in the magnetoresistance were observed. One was centered at a copper thickness of ∼1.0 nm and the second was centered at ∼2.3 nm. Comparison of the field dependence of the magnetoresistance with the field dependence of the magnetization, as determined by vibrating-sample magnetometer, suggests that the saturation field for GMR and the magnetization are similar for the larger copper thicknesses, but are strikingly different near 1.0 nm copper thickness. This observation suggests that the GMR is affected by different factors depending on the thickness of the copper layer.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.358250
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  • 10
    Electronic Resource
    Electronic Resource
    Demagnetized-state dependence of Henkel plots. I. The Preisach model : 38th Annual Conference on Magnetism and Magnetic Materials (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 5689-5691 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The interpretation of interaction through Henkel plots and Δm plots has become popular in recent years; however, the demagnetized state is often not specified. In this paper, the demagnetized state dependence of Henkel plots is calculated using the classical Preisach model, the moving model, and the complete-moving-hysteresis model. In the calculation of the virgin remanence curve, ac, dc+, and dc− demagnetized states were used. The resulting collection of Henkel plots contains examples of "up'' and "down'' deviations from the Wohlfarth line by changing the demagnetized state only.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.355638
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