Epitaxial growth and magnetic behavior of NiFe 2 O 4 thin films
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Epitaxial growth and magnetic behavior of NiFe2 O4 thin films S. Venzke,a) R. B. van Dover,b) Julia M. Phillips, E. M. Gyorgy, T. Siegrist, C-H. Chen, D. Werder, R. M. Fleming, R. J. Felder, E. Coleman, and R. Opila AT&T Bell Laboratories, Murray Hill, New Jersey 07974 (Received 22 March 1995; accepted 27 January 1996)
Thin films of NiFe2 O4 were deposited on SrTiO3 (001) and Y0.15 Zr0.85 O2 (yttria-stabilized zirconia) (001) and (011) substrates by 90±-off-axis sputtering. Ion channeling, x-ray diffraction, and transmission electron microscopy studies reveal that ˚ diameter grains separated by thin regions films grown at 600 ±C consist of ,300 A of highly defective or amorphous material. The development of this microstructure is attributed to the presence of rotated or displaced crystallographic domains and is comparable to that observed in other materials grown on mismatched substrates (e.g., GaAsySi or Ba2 YCu3 O7yMgO). Postdeposition annealing at 1000 ±C yields films that are essentially single crystal. The magnetic properties of the films are strongly affected by the structural changes; unannealed films are not magnetically saturated even in an applied field of 55 kOe, while the annealed films have properties comparable to those of bulk, single crystal NiFe2 O4 . Homoepitaxial films grown at 400 ±C also are essentially single crystal. I. INTRODUCTION
Ferrite thin films with the spinel (MgAl2 O4 ) structure are potentially interesting scientifically and are also promising for high-frequency applications, where the low conductivity (compared to metal films) is an important asset, as is the high saturation magnetization (compared to garnet-structure materials). The techniques required to obtain high-quality ferrite films are more sophisticated than those used for metal films such as permalloy. Several approaches to the synthesis of thin-film spinel ferrite have proven successful, including pulsed-laser deposition (PLD),1,2 sputtering,3,4 and e-beam reactive evaporation.5 As always the choice of substrate profoundly affects the structure and properties of the films. Spinel ferrite films typically have been grown on MgO, which has the NaCl structure with a lattice constant closely matched to the half-unit-cell dimension of the film. However, we have found inferior magnetic properties and significant Mg contamination in NiFe2 O4 films grown on MgO, and have therefore avoided using MgO substrates.2 We found that SrTiO3 (STO) and Y0.15 Zr0.85 O2 (yttria-stabilized zirconia, or YSZ) substrates are chemically compatible up to at least 1200 ±C. Thin film growth on a substrate that has half the lattice constant of the film can lead to antiphase domain disorder, with deleterious effects on the magnetic and electrical properties. We have explored this aspect by growth on STO (where antiphase domain disorder is allowed), which is compared to growth on YSZ (001)
(where only rotational domains are possible) and to homoepitaxial growth on NiFe2 O4 . The substrates we have investig
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