Deposition and Properties of Epitaxial Ferrite Thin Films
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JULIA M. PHILLIPS, R. B. VAN DOVER, E. M. GYORGY, AND J. H. MARSHALL AT&T Bell Laboratories, Murray Hill, NJ
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ABSTRACT We have grown thin films and multilayers of several ferrite materials with the spinel structure by pulsed laser deposition of stoichiometric targets. Epitaxial films can be grown on a variety of substrates including MgO, A1203, MgAl204, Y-stabilized ZrO2 (YSZ) and SrTiO 3 . Films on Mg- and Al-containing substrates have a low saturation magnetization, Ms, while films on YSZ and SrTiO3 exhibit bulk values of Ms. The anisotropy can be lowered by a post growth anneal, resulting in a film with a permeability of 28. INTRODUCTION Integrated circuits operating at microwave frequencies and other high frequency applications can potentially be improved if magnetic materials are incorporated in their designs, although this places stringent requirements on the magnetic materials. Ferrites with the spinel structure are one class of materials that is worth investigating. Previous work on nickel ferrite films has generally concerned polycrystalline or amorphous layers and has concentrated on the structural properties. 1 -7 The few measurements of magentic properties have been disappointing. 8 A recent report of pulsed laser deposited Mnl.xZnxFe204 films has reported interesting magnetic properties only in films deposited at 8000C. 9 In the present work we have concentrated on (Mn. 5Zn.5)Fe2O4 (MZFO) and NiFe2 04 (NFO). Both of these materials can have a low magnetocrystalline anisotropy. MZFO can have a very low magnetostriction, Xs while NFO has a relatively high value (Xs - 20 X 10-6). Since vapor deposited thin films are generally highly strained, this can lead to a large magnetostricture contribution to the magnetic anisotropy. However, the dielectric constant of MZFO can be high even at microwave frequencies 10 which may render it unsuitable for some applications. The lower resistivity of MZFO, which
conventionally rules out this material for high frequency applications, is nevertheless sufficient to avoid eddy-current loss in thin films, where the cross sectional area is very small. 589 Mat. Res. Soc. Symp. Proc. Vol. 317. ©1994 Materials Research Society
EXPERIMENTAL CONDITIONS Ferrite films are grown by pulsed laser deposition using a KrF excimer laser (248 nm). The energy density on the target is 4 J/cm 2 , and the pulse repetition rate is 10 Hz. The deposition chamber has a base pressure of 10-7 Torr. During growth, the substrate can be heated to a temperature as high as 700oC. Typical values of the substrate temperature are between 400 and 700oc. During growth, oxygen or a mixture of 1% oxygen in nitrogen (or argon) can be bled into the chamber. Typically, the ambient is 1 mTorr of 1% oxygen in nitrogen [p(O2) = 0.01 mTorrj, though pressures as high as 100 mTorr of pure oxygen have been investigated. RESULTS AND DISCUSSION Table 1 summarizes our results on NFO films on a wide variety of substrates. Although the epitaxial quality of the ferrite films on substrates such as MgO, A120 3 , and MgA
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