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Browsing by Author "Lu, Zhihong"

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    Controlling magnetic anisotropy in epitaxial FePt(001) films
    (AVS American Institute of Physics, 2009-06-30) Lu, Zhihong; Walock, M. J.; LeClair, P.; Butler, W. H.; Mankey, G. J.; University of Alabama Tuscaloosa
    Epitaxial equiatomic Fe(50)Pt(50) thin films with a variable order parameter ranging from 0 to 0.9 and Fe(100-x)Pt(x) thin films with x ranging from 33 to 50 were deposited on MgO (001) substrates by dc sputtering. A seed layer consisting of nonmagnetic Cr (4 nm)/Pt (12 nm) was used to promote the crystallinity of the magnetic films. The crystal structure and magnetic properties were gauged using x-ray diffraction and magnetometry. The magnetic anisotropy can be controlled by changing the order parameter. For Fe(100-x)Pt(x) films, the increase in Fe composition leads to an increase in coercivity in the hard axis loop and causes a loss of perpendicular anisotropy.
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    Magnetic anisotropy graded media and Fe-Pt alloy thin films
    (University of Alabama Libraries, 2009) Lu, Zhihong; Butler, W. H.; University of Alabama Tuscaloosa
    Anisotropy graded media is promising to overcome the writability problem in achieving ultrahigh areal density for magnetic recording media. To more conveniently study and compare various media with regard to a particular figure of merit, a new energy landscape method of analysis is suggested. Using this method, the theoretical limit of the figure of merit for a graded medium is found to be 4. This limit can be approached by a graded medium with anisotropy quadratically increasing from zero to its maximum value. In order to characterize the anisotropy distribution of a graded medium, hard axis loops of graded media with various anisotropy profiles are simulated and analyzed. It is found that the second derivative of the hard axis loop can give useful information on the anisotropy distribution in a graded medium. Fe₅₀Pt₅₀ with the L1₀ structure, as one of the magnetically hardest materials, has great potential for media application. By using a first-principles calculation method, the magnetic and electronic structures of L1₀ structured Fe₅₀Pt₅₀ have been studied. These calculations show that although the ferromagnetic phase is the most stable phase for Fe₅₀Pt₅₀ with the L1₀ structure, there is a competition between the antiferromagnetic and the ferromagnetic phases when the ratio of lattice constants, c/a, decreases. Experimental investigations of Fe₅₀Pt₅₀ films with graded order parameter fabricated by varying the growth temperature during deposition demonstrate that these films have much smaller switching field than fully ordered Fe₅₀Pt₅₀, which suggests it is possible to make graded media by using this kind of films. Fe₁₀₋ₓPtₓ films with compositional gradient were also studied; however, the large easy axis dispersion in these films makes them unsuitable for the fabrication of graded media. Films with [FePt₃ordered)/FePt₃ (disordered)]n superlattices were deposited on MgO substrates and sapphire substrates. It was found that the exchange bias in superlattices deposited on MgO substrate show higher exchange bias field. Polarized neutron reflectivity results show that ferromagnetic layers on MgO substrates contain more antiferromagnetic component than those on sapphire substrates. The larger exchange bias of the superlattice on MgO substrate is hypothesized to be due to larger exchange bias in its ferromagnetic layers.
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    Reducing the writing field of L1(0)-FePt by graded order parameter
    (American Institute of Physics, 2013-02-21) Lu, Zhihong; Guo, Junbo; Gan, Zhanghua; Liu, Yong; Xiong, Rui; Mankey, G. L.; Butler, W. H.; Wuhan University of Science & Technology; Wuhan University; University of Alabama Tuscaloosa
    The dependence of the magnetic properties of epitaxial Fe50Pt50 films on order parameter (S) was investigated. It was demonstrated that the magnetic anisotropy could be tuned by controlling S which can be controlled by the growth temperature. Based on this result, two kinds of multilayered structures, each with a 13 nm fully ordered Fe50Pt50 layer as the bottom layer, were built: (1) 4-layered structure with S decreasing layer by layer from bottom to top; (2) graded structure with S changing more continuously along the thickness. The magnetic properties of the films were characterized using vibrating sample magnetometry. It was found that both structures have their easy axis perpendicular to the film; the anisotropy fields of the 4-layered film and the graded film are 53 kOe and 37 kOe, respectively. These values are much lower than that of the fully ordered uniform Fe50Pt50 film (about 73 kOe). The results suggest that it may be possible to reduce the writing field of Fe50Pt50 by gradually changing the order parameter. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4791583]