Magnetoresistance of granular ferromagnets—observation of a magnetic proximity effect?
ABSTRACT We have observed a superparamagnetic to ferromagnetic transition in films of isolated Ni grains covered by non-magnetic overlayers. The magnetoresistance (MR) of the films was measured as a function of the overlayer thickness. Initially, the granular Ni films exhibited negative MR curves peaked at H=0. As different materials were deposited onto the grains hysteresis developed in the MR. This behavior is ascribed to an increase of the typical domain size because of the magnetic coupling between grains. The strength of the inter-grain coupling is found to correlate with the magnetic susceptibility of the overlayer material. We discuss possible mechanisms for this coupling and suggest that the data may reflect the existence of a magnetic proximity effect (analogous to the well-known effect in superconductivity) in which a ferromagnetic moment is induced in the metallic non-magnetic medium.
Full-textDOI: · Available from: A. Frydman, Apr 24, 2015
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ABSTRACT: RF-sputtered CoFe-NM granular alloys (NM=Ag, Cu) with CoFe volume content, xv, ranging from 0.10 to 0.45 have been studied. These two series of samples show similar features depending on the synthesis conditions and post-deposition annealing treatments, revealing the strong dependence of magnetotransport properties on microstructure. Three different regimes have been observed as xv is increased: the classical giant magnetoresistance (GMR) regime at low ferromagnetic contents; at intermediate xv, a domain structure appears, and GMR and anisotropic magnetoresistance (AMR) together with domain wall scattering are observed; and a third regime at xv close but below the volume percolation threshold, where the two latter contributions still coexist, while the GMR contribution has been suppressed by strong magnetic correlations. The role of the metallic matrix is crucial to determine the crossover ferromagnetic contents between these three regimes, which depend on the relative immiscibility of CoFe either in the Ag or Cu matrices and the diffusivity of Ag and Cu. Moreover, the metallic matrix settles the degree of CoFe segregation, sample crystallisation and texture, which are responsible for the magnetotransport properties.Journal of Magnetism and Magnetic Materials 02/2000; 210(1-3-210):295-301. DOI:10.1016/S0304-8853(99)00601-0 · 2.00 Impact Factor
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ABSTRACT: The effect of dipolar and exchange interactions on the magnetic behaviour of sputtered CoFe-Ag(Cu) granular alloys is examined through the study of remanence curves and M plots as a function of the ferromagnetic volume content, xv , and annealing temperature. As-deposited samples, with either a random distribution of weakly interacting fine magnetic particles (3 nm in size) at low xv or with strong dipolar and exchange interactions (leading to a long range out-of-plane stripe-like domain structure) at xv 0.25, display negative M values. In the former, this is attributed to dipolar interactions being dominant in a random distribution of well separated particles. In the latter, M <0 is attributed to the flux closure between antiparallel neighbouring domains, which is strongly demagnetizing. Annealing causes phase segregation and particle growth, while particle clusterization occurs. These facts lead to an increase of the direct exchange through the surface of neighbouring grains in the same particle clusters, at the expense of dipolar interactions, which decrease as the clusters become more separated. Consequently, at high xv , annealed samples display positive M values.Journal of Physics D Applied Physics 03/2000; 33(6):609. DOI:10.1088/0022-3727/33/6/304 · 2.52 Impact Factor
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ABSTRACT: Microstructural effects on the magnetic behavior of rf-sputtered CoFe–AgCu granular alloys are examined through the study of the in-plane remanence-to-saturation magnetization ratio, Mr/Ms, as a function of temperature, ferromagnetic volume content, xv, and annealing temperature. At low ferromagnetic contents (xv⩽0.25), the MrMs ratio in as-deposited samples tends towards 0.5 at low temperature, as expected from the uniaxial perpendicular anisotropy displayed by all samples, which is magnetoelastic in nature and arises from the axial distortion of the CoFe face-centered-cubic cells. In as-deposited samples with xv>0.25 (well below the volume percolation threshold, xp∼0.5–0.55), a collective magnetic behavior develops due to magnetic correlations among particles. Consequently, a domain structure perpendicular to the film plane appears, which results in a remanence breakdown: Mr/Ms is about 0.2. Besides, magnetic correlations prevents the thermal decay of Mr/Ms, which is almost constant between 5 and 300 K, even for ferromagnetic particles no more than 3 nm in size. The axial distortion disappears with annealing, the cubic symmetry is recovered, the out-of-plane magnetic structure is lost and, therefore, the Mr/Ms ratio at low temperature tends towards 0.8 for highly annealed samples. Consequently, the magnetic properties of granular alloys depend on the interplay between anisotropy, exchange, and dipolar interactions, which in turn depend crucially on the microstructure. © 2000 American Institute of Physics.Journal of Applied Physics 07/2000; 88(3):1576-1582. DOI:10.1063/1.373857 · 2.19 Impact Factor