Microstructures and Soft Magnetic Properties of High Saturation Magnetization Fe-Co-N alloy Thin Films

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Microstructures and Soft Magnetic Properties of High Saturation Magnetization Fe-Co-N alloy Thin Films N. X. Sun and S. X. Wang Department of Material Sciences and Engineering, Stanford University Stanford, CA 94305-4045, USA Chin-Ya Hung, Chester X. Chien and Hua-Ching Tong Read-Rite Corporation, 44100 Osgood Road, Fremont, CA 94539 ABSTRACT High saturation magnetization soft magnetic materials are required for future high-density recording heads as well as high frequency inductors. In this work, (Fe0.7Co0.3)1-xNx (or in short FeCoN) alloy films were synthesized with a high saturation magnetization of 24.5 kG, a hard axis coercivity of 5 Oe, an easy axis coercivity of 18 Oe, and a resistivity of 55 µΩ⋅cm. The FeCoN film sandwiched between two permalloy layers (5 nm) shows very good magnetic softness, a low hard axis coercivity of 0.6 Oe, an easy axis coercivity of 7.8 Oe, an excellent inplane uniaxial anisotropy with an anisotropy of about 20 Oe, an initial permeability of 1000, and a roll-off frequency of 1.5 GHz. In order to understand the effect of the permalloy layers on the FeCoN layer, we fabricated four film structures: single layer FeCoN film; FeCoN film sandwiched between two permalloy layers on both sides; FeCoN film with one permalloy layer as the underlayer; and FeCoN film with one permalloy layer as caplayer. All these film structures were both magnetically and structurally characterized and compared. Structural characterization shows that there is no significant difference in the grain size of the FeCoN single layer and the FeCoN layer sandwiched between two permalloy layers. The four film structures have almost the same amount of compressive stress, about -300 MPa; and their saturation magnetostriction constants are also very close, in the range of 39.6×10-6 to 44.3×10-6. Difference in the crystallographic textures was observed in the pole figures for the FeCoN single layer and FeCoN film with permalloy underlayer. INTRODUCTION Soft magnetic thin films with high saturation magnetization are highly desired in a wide range of applications from magnetic write heads to high frequency inductors. However, soft magnetic thin-film materials with a saturation magnetization of > 21 kG and a low coercivity of less than 1 Oe are still not available, although magnetic materials with saturation magnetization higher than 21 kG are readily obtainable in the binary Fe1-xCox alloys [1]. Discovering new soft magnetic materials with very high saturation magnetization remains to be a major challenge. Magnetic softness of polycrystalline thin films is an extrinsic property of the magnetic materials, and is closely related to the structural and compositional characteristics in the thin films, such as grain size [2], crystallographic texture [3], strain and stress state [4]. These characteristics can also affect anisotropy and magnetostriction, which are in turn correlated with the magnetic softness. However, it is still not well established how these structural characteristics affect the magnetic softness.

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