Abstract
Barium hexaferrite (BaM) films with in-plane c-axis orientation are promising and technically important materials for self-biased magnetic microwave devices. In this work, highly oriented BaM films with different thickness and an in-plane easy axis (c-axis) of magnetization were grown on a-plane single-crystal sapphire substrates by direct current magnetron sputtering. A procedure involving seed layers, layer-by-layer annealing was adopted to reduce the substrate-induced strains and allow for the growth of thick (~3.44 μm) films. The epitaxial growth of the BaM film on sapphire was revealed by high-resolution transmission electron microscopy with dislocations being observed at the film-substrate interface. The orientation was also verified by X-ray diffraction and more notably, polarized Raman scattering. The magnetic properties and ferromagnetic resonant frequencies were experimentally characterized by a vibrating sample magnetometry and a frequency-swept ferromagnetic resonant flip-chip technique, respectively. The micron-thick BaM films exhibited a large remanence ratio of 0.92 along in-plane easy axis and a small one of 0.09 for the in-plane hard axis loop measurement. The FMR frequency was 50.3 GHz at zero field and reached 57.9 GHz under a magnetic field of 3 kOe, indicating that the epitaxial BaM films with strong self-biased behaviors have good electromagnetic properties in millimeter-wave range.
Highlights
~50 GHz at zero external field, about 45% higher than that of of-plane c-axis (OCA) ones[29]
Direct current (DC) magnetron sputtering has long been used to sputter metal materials, while oxide ceramics have always been prepared by radio frequency (RF) magnetron sputtering, which is subject to low sputtering rate compared with direct current (DC) sputtering
The microstructure and epitaxial orientation relationship between BaM and sapphire were comprehensively investigated based on transmission electron microscopy (TEM) that has seldom been reported for this system and Raman analysis that is efficient in probing structures and strains in small regions[40,41,42,43]
Summary
Unlike OCA BaM films, which have been fabricated on various substrates, like Al2O3 (0001)[16,20,24], MgO (111)[31], SiC (0001)[12,17,18], Si (111)[19], GaN (0001)[32] and Gd3Ga5O12 (GGG) (111)[13], successful fabrication of the IPCA BaM films has always been a challenge It was not until the year 1992 that Hlyton et al reported the c-axis aligned BaM films with in-plane anisotropy for the first time by reactive magnetron sputtering[33]. The industrial production cost is very high for PLD technique, whose deposition rate is limited by the laser power Both LPE and PLD are restricted to relatively small size substrates. The increasing of FMR frequency of the BaM film with increasing external magnetic field was observed, with the FMR frequency at zero field extrapolated to be 50.3 GHz, implying their potential in the millimeter-wave magnetic film device applications
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