Abstract

A periodic magnetic stripe array has been studied with a combination of real and reciprocal space methods: Kerr microscopy and polarized neutron reflectivity. The basic features of our experimental neutron data are well reproduced by a theoretical model using the distorted Born wave approximation and providing a set of parameters quantifying the magnetization arrangement in the stripe array system. While the specular neutron reflectivity measures a mean value of the optical potential averaged over a number of structural elements within the neutron coherence length, Bragg diffraction filters out magnetic correlation effects in the system of individual magnetic units within this length scale. Off-specular diffuse scattering probes correlations of magnetization fluctuations on a scale smaller than the coherence length. This altogether gives access to a detailed understanding of the magnetization arrangement which appears to be quite complex and hardly accessible by other methods.

Keywords: Polarized neutron reflectometry; Magnetic properties of nanostructures; Magnetic domains; Kerr microscopy

PACS: 1.12.Ha; 75.75.+a; 75.70.Kw

Article Outline

References

(20K)
Fig. 1. Surface topography of an array of Co_0.7Fe_0.3 stripes obtained with an atomic force microscope shown in a three-dimensional surface view. The displayed area is .

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Fig. 2. Sketch of the scattering geometry for neutron reflectivity studies on patterned structures. The final state polarization analysis of the reflected neutrons is performed at the initial polarization directed either along, with, or opposite to the y-axis pointing along the external magnetic field. The latter is displayed within the film surface, but tilted with respect to the stripes so that the vector of the magnetic induction B makes the angle θ relative to the y-axis. For specular reflection the scattering vector q is parallel to the z-axis normal to the film surface, and α_i and α_f refer to the incident and exit angles of the neutrons to the sample surface. The sample rotation is expressed by the angle χ between the easy axis and the applied field.

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Fig. 3. KM images taken for two angles between magnetic field and sample: easy axis of χ=0 taken at (left) and χ=75 taken at (right). The plane of incidence results in a top-down magneto-optical sensitivity axis perpendicular to the stripes.

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Fig. 4. KM image taken at χ=75 and with a low resolution. The plane of incidence results in a top-down magneto-optical sensitivity axis perpendicular to the stripes.

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Fig. 5. Polarized neutron reflectivity measurements for sample rotation angles χ=0 (left column) and χ=75 (right column) (a) and (c). Field dependence of the NSF and SF reflectivities is compared to simulations (see text). Plotted is the difference of the NSF reflectivities (measurements: open squares, simulation: solid line), and the SF reflectivities R^+-=R^-+ (measurements: open circles, simulation: solid line); (b) and (d) field dependence of the mean projection of the normalized magnetization onto the easy axis as determined from MOKE (circles) and PNR (line) data.

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Fig. 6. Sketch for the model used in simulations for χ=0. The dark and light gray shaded stripes represent the magnetic stripes with the ripple domains. The arrows indicate magnetization vectors within domains of individual stripes. The dashed line marks the neutron coherence volume, over which the average of the magnetization fluctuations are taken. For simplicity only two stripes within the coherence range are drawn, the actual size being much bigger. In the lower part of the figure the dashed arrows indicate the mean magnetization direction averaged over a coherence volume. γ is the angle of the mean magnetization vector with respect to the stripe orientation. Δγ is the transverse fluctuation about the mean value. For further details see text.

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Fig. 7. Experimental maps of the polarized neutron intensity on a logarithmic scale from a periodic stripe array measured at a magnetic field of 310 Oe and plotted as a function of the angles of incidence α_i, and the scattering angles α_f. The left intensity map was measured with spin-up and the right with spin-down neutrons.

References

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