Preferred Orientation and Magnetic Properties of Barium Hexaferrite Thin Films Devitrified from the Glass
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ABSTRACT Easy axis orientation of rf magnetron sputtered barium hexaferrite films plays an important role in their usefulness as recording media. Processing variables included substrate temperature, pressure, composition, and duration of sputtering, as well as secondary heat-treatment schedule. As-deposited films were primarily amorphous; after secondary heat-treatment, barium hexaferrite devitrified with a crystal size of -•200 nm. The c-axis of crystalline grains formed parallel to the heat-treated film surface when the substrate was at 620°C during deposition. A perpendicular c-axis orientation was measured using an argon atmosphere at 1.33 Pa and a substrate temperature of 720'C. This film showed the highest remnance (4.781 x 10- emu). The coercivity of such films ranged from 0.8 2 KOe.
INTRODUCTION Digital magnetic recording media are generally made up of magnetic particulates (e.g. Cr2 0 3 or a-Fe) embedded into a polymer dispersing phase. A recording head "writes" a bit onto a location of the media via an applied magnetic field, locally re-aligning the magnetic moments of exposed media with the field. Reading of impressed bit involves that region of the media near a coil in the head, where the changing magnetic field induces a voltage in a coil. For such applications, "hard" magnets are desirable, which have large, square hysteresis loops between applied magnetic field and magnetic induction. The reverse magnetic field intensity required to bring the magnetic induction to zero is referred to as the coercivity. Thus, hard magnets have a high coercivity, the significance of
which is that stray fields of significant strength would be required to change the orientation of the bit. The magnetic induction when externally applied fields are removed is referred to as the remnance, and it is desirable for this to be as large as feasible so that a strong signal may be measured during reading cycles. Barium hexaferrite (BaO.6Fe 2 0 3) adopts a complex hexagonal structure (magnetoplumbite) consisting of 64 atoms in the unit cell[l]. Fe3+ cations reside in three coordinations, tetrahedral, octahedral, and hexahedral (five surrounding oxygens). The unfilled inner orbitals of the iron ions result in a net magnetic moment, where nearby magnetic moments of iron ions oppose. Barium hexaferrite is ferrimagnetic, since these opposing magnetic moments are not equally matched and the unit cell has a net magnetization (a net of 8 Fe3 + per unit cell with aligned spin). These spin orientations are aligned with the c-direction so that spin moments have orientations of either [0001] or [0001] (uniaxial crystal anisotropy)[1]. These favorable orientations are referred to as the "easy axes". Magnets generally change their net polarizations by the movement of domain walls so that domains of favorable orientation to the applied field grow at the expense of others[2]. However, if the particle size is decreased below -1jpm, the particles can be single-domained[3, 4], and spin orientation can only be inverted by a higher energy mechanism
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