HIGH-RESISTIVITY SOFT MAGNETIC THIN FILMS USING CoFe METAL/NATIVE OXIDE MULTILAYERS (INVITED)

Magnetic thin films suitable for gigahertz operation must simultaneously achieve soft properties, high saturation magnetization, and a high resistivity. The recently-introduced metal/native oxide multilayer (MNOM) composite meets these requirements. This system consists of nanogranular high-moment CoxFe100-x layers separated by ultrathin magnetic native oxide layers that isolate the metallic layers electrically while, in contrast to nonmagnetic oxides, coupling them magnetically and augmenting the volume-averaged saturation magnetization. The MNOM system delivers high permeability with a bandwidth of several GHz for most of the CoxFe100-x alloy composition range. This paper will review the soft magnetic properties of the MNOM system and describe the magnetic properties of the buried native oxide layers. INTRODUCTION There is strong industrial demand for new high-permeability thin films with GHz operating frequencies. Applications include write poles and shields in magnetic recording heads, soft underlayers for perpendicular media, and integrated thin film inductor cores. One challenge to meeting this demand is the necessity of achieving a higher resistivity than conventional materials, to limit eddy current screening, while maintaining or exceeding the high permeability and saturation magnetization of those materials. Among the more successful approaches to the problem are granular metal/insulator composites, in which high-moment metallic nanoparticles o are embedded in a nonmagnetic insulating matrix, such as Al2O3 [1]-[3]. Although such films have demonstrated soft magnetic properties, large magnetizations, and high resistivities, there are important limitations to simultaneously optimizing these properties. Resistivity comes at the expense of the volume-averaged saturation magnetization, which decreases in proportion to increasing oxide fraction. Furthermore, soft magnetic properties are only obtained for metal volume fractions approaching metallic percolation, in order to allow interparticle “exchange averaging” to “smooth” local variations in anisotropy magnitude and direction. A promising new approach [4], using CoxFe100-x metal/native oxide multilayers (MNOM) [shown schematically in Fig. 1(a)] aimed to replace the nonmagnetic oxide phase with a magnetic oxide. A magnetic oxide offers the possibility of isolating metallic regions electrically ICF09 Abstract ID: 10601 while coupling them magnetically. In addition, a moment-bearing oxide detracts less from the net magnetization, permitting a higher magnetization for a given metallic fraction. The CoxFe100-x MNOM system has demonstrated ideal soft magnetic properties [4], a large magnetization due in part to a significant contribution by the oxide [5], [6], and excellent highfrequency characteristics with bandwidths of several GHz [7]. This paper will review the technologically-significant soft magnetic properties and high-frequency characteristics of the MNOM system and discuss the magnetic properties of the buried native CoxFe100-x oxide layers. The oxide has a net moment comparable to the bulk ferrites, but its spin structure is significantly more complex. The magnetism of these ultrathin buried oxide layers, in which all spins are either directly or indirectly coordinated by metallic (Co)Fe, is supported by the metal and collapses in the absence of the metal. FABRICATION, MICROSTRUCTURE, AND RESISTIVITY The MNOM structure [see Fig. 1(a)], consists of alternating nanocrystalline CoxFe100-x metal and native oxide layers. The metal layers (typically a few nm) were dc sputter-deposited from stoichiometric CoxFe100-x alloy targets. Following deposition of each layer, the sputtering source was shuttered and the layer was exposed in situ to ~8×10 Torr of O2 for 10 s. Each exposure was followed by a 60 s pause to allow the O2 partial pressure to drop to its background level before deposition of the subsequent metal layer. The notation [CoxFe100-x(t0)/oxide]N describes a MNOM with N metal/native oxide bilayers, each formed by oxidizing a CoxFe100-x metal layer of nominal thickness t0. All depositions were performed in the presence of a dc magnetic field (~100 Oe), establishing a uniaxial anisotropy, and were capped with a 50-100 Å SiO2 layer. The cross-sectional transmission electron micrograph (TEM) of Fig. 1(b) shows the microstructure of a [Co50Fe50(20 Å)/oxide]50 film. The layers are nanocrystalline with lateral grain sizes on the order of a few nm. The metal and oxide are distinguished by dark and light contrast, respectively. Lattice fringes are visible in both metal and oxide and span several layers in some regions, indicating locally-epitaxial metal/oxide and oxide/metal growth. In other regions, the oxide layers appear to disrupt the metallic grain growth. The oxide layer thickness is passivation-limited [8], and is very reproducible, independent of t0. Mössbauer and EXAFS studies of this system [5] find ~8.4 Å of metal to oxidize in each layer for the pure Fe MNOM system, while somewhat less metal oxidizes under the same conditions for CoxFe100-x-based films (~7.1 Å for x = 50 and ~6.3 Å for x = 90). The in-plane resistivity ρ of MNOM films is substantially larger than for bulk CoxFe100-x. As t0 decreases from 20 Å to 10 Å, ρ increases from ~120 to ~600 μΩ-cm for Co50Fe50-based films [Fig. 1(c)] and reaches 12 mΩ-cm for Fe-based MNOMs. For thicker t0, the metal layers are percolated in the plane, evidenced metallic ρ(T), but as t0 is reduced below ~16 Å, the conductivity becomes thermally activated, as seen in the inset of Fig. 1(c) for Co50Fe50 MNOMs. (c) 10 12 14 16 18 20 0.1 1 T=296 K Co50Fe50 MNOM ρ (m Ω -c m ) t0 (Å) 50 100 150 200 250 0.8 0.9 1.0 12 Å