Magnetic Losses in Thick Barium-Hexaferrite Films on MgO (111)
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Magnetic Losses in Thick Barium-Hexaferrite Films on MgO (111) S.D. Yoon1, S.A. Oliver2, P. Shi1,3, X. Zuo1 and C. Vittoria1 1 Department of Electrical and Computer Engineering, Northeastern University, Boston MA. 2 Center for Subsurface and Sensing and Imaging Systems, Northeastern University, Boston MA. 3 P. Shi is now with Philips Research Ins., Richardson TX. ABSTRACT Barium hexaferrite films having thickness from 3 to 30 µm were deposited onto 0.5 mm Magnesium Oxide (111) substrates, and were examined by vibrating sample magnetometry, torque magnetometry, x-ray diffraction, and scanning electron microscope measurements. Ferrimagnetic resonance measurements were taken with the magnetic field applied perpendicular to the film plane. The FMR linewidth values for as-produced 3 - 30 µm films was 0.45 - 0.70 kOe, and was reduced to 0.06 - 0.45 kOe after annealing at 1000 oC for 2 hours. Further annealing increased the linewidth. The FMR linewidth for the 30 µm thick film was also found to decrease upon mechanically removing 55 ~ 77 % of the substrate thickness. A 6-fold in-plane anistropy symmetry in the anisotropy energy was observed in torque magnetometry measurements for films thicker than 30 µm. These results can be interpreted as due to film inhomogeneity, stress, and the growth of small regions of non c-axis textured material. INTRODUCTION M-type barium ferrimagnetic materials (BaFe12O19 or BaM) have a large uniaxial anisotropy (HA=17.0 kOe) that makes this material important for microwave and millimeter wave applications. Indeed, we have integrated these films with silicon wafers to produce monolithic microwave integrated circuit (MMIC) ferrite devices.[1] Growth of 30 µm thick films with excellent c-axis orientation by pulsed laser ablation deposition (PLD) onto MgO (111) substrates has been demonstrated in previous work.[2] BaM films can be grown up to 30 µm thick with epitaxial quality, but the films exhibit significant magnetic loss as deduced from ferrimagnetic resonance (FMR) linewidths (∆H). Here, the ∆H for 30 µm thick BaM films which is smaller than oriented polycrystalline bulk BaM ceramic but are still high compared to the ∆H values found for single crystal of BaM platelets and submicron films. [3][4][5] The broadening of the FMR linewidth from its intrinsic values arises from film inhomogeniety in either the bulk or interfaces. Contributions to the film inhomogeniety include the presence of voids, defects in the crystalline lattice, the incorporation of crystallites that do not retain the film texture, and stress gradients caused by the film deposition and cooling where these stress gradients couple to the film magnetic properties through the magnetoelastic effects.[6] In addition, an oxygen pressure study for PLD hexaferrite films on c-axis oriented sapphire substrates suggests that higher oxygen pressure (300 mTorr) growth during the deposition produces near-intrinsic FMR ∆H for the sub-micron films.[4][5] However, low oxygen pressure is usually required to grow thick hexaferrite films (20 mTorr)
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