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A NEW "TWIST" ON NEUTRON REFLECTOMETRY: IMAGING EXCHANGE-SPRING MAGNETS.

Scientists of the NIST Center for Neutron Research (NCNR) have developed a new method of using polarized neutron reflectometry (PNR) to extract the structure of buried magnetic spirals in magnetic films. This technique improves upon earlier methods by being particu-larly sensitive to the presence of magnetic twists vis-a-vis structures in which the magnetization direction does not vary appreciably. Tracking the formation and growth of twists may solve a number of puzzles that hamper the development of magnetic thin film devices.

In collaboration with scientists from industry, the technique has been applied to a thin-film exchange-spring magnet. The results confirmed that current theory regarding the behavior of such magnets may be violated. The film consists of the hard ferromagnet [Fe.sub.55][Pt.sub.45] topped by the soft ferromagnets [Ni.sub.80][Fe.sub.20]. It has been predicted that the combination of soft and hard ferromagnets in close proximity produces a composite which has a strong moment and does not readily demagnetize. As a side effect, when a small external magnetic field is opposed to the magnet, the portion of the soft ferromagnet farthest from the hard ferromagnet may twist into alignment with the field.

Techniques other than PNR typically measure only the average orientation of the magnetic spins and cannot readily distinguish a spiral from a structure in which all the spins are canted with respect to an external field. PNR can extract the depth-dependence of the structure. The new modification of the PNR method greatly enhances the contrast between various structures. First the reflectivity is measured with neutrons glancing off the front surface of the material, and then repeat with neutrons glancing off the back surface. Key features in the data immediately indicate the presence of a spiral, and by studying the film at a number of fields, it is possible to track the development of the spiral. Surprisingly, it is found that the spiral invades the hard ferromagnet even at extremely low fields, in contradiction to current theory.

With this new technique, NIST is now able to better characterize the magnetic properties of thin films, which can improve the capability and reliability of industrial devices for magnetic recording and sensing.
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Publication:Journal of Research of the National Institute of Standards and Technology
Geographic Code:1USA
Date:Mar 1, 2001
Words:364
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