Magnetic Anisotropy and Stress-Dependent Epoxy Wetting in FeNi-Based Metal Amorphous Nanocomposites

Recently developed FeNi-based metal amorphous nanocomposites (MANCs) used in high-speed motors (HSMs) exhibit reduced eddy current losses while maintaining good mechanical properties and glass-forming abilities. Magnetic anisotropy in (Fe70Ni30)80Nb4B14Si2 amorphous magnetic ribbon (AMR) in the as-cast state and upon conventional (
$T_{\mathrm {CA}}$
) and strain (
$T_{\mathrm {SA}}$
) annealing heat treatment is investigated. From ribbon samples in as-cast condition, quenched in stress from planar flow casting (PFC) induced as-cast curvature derived uniaxial magnetic anisotropy. Stress relief by conventional furnace annealing at
$T_{\mathrm {CA}} >$
350 °C achieved isotropic properties in the bulk. Annealing about the primary crystallization temperature,
$T_{\mathrm {CA}} \sim $
450 °C, resulted in the formation of both face-centered cubic (FCC) and body-centered cubic (BCC) nanocrystallites and evolution to isotropic bulk magnetic properties confirming the random anisotropy model. In samples strain annealed at
$T_{\mathrm {SA}}$
= 440 °C at various tensions, relatively large controlled induced uniaxial anisotropy is achieved. The largest magnetic anisotropy occurs in annealing under the stress of 250 MPa yielding an anisotropy field of
$7.1\times 10^{4}$
A/m. Surface anisotropy observed by the magneto-optical Kerr effect (MOKE) differs from bulk anisotropy due to image contrast from closure domains. Epoxy coatings are important for improved bonding, mechanical properties, and resistivity in tape-wound MANC cores for HSMs. Using a sessile droplet method, the equilibrium contact angle of an epoxy droplet on a tensile stress-annealed MANC exhibits stress-dependent surface energies. Anisotropic wetting in FeNi-based MANC heat treated at
$T_{\mathrm {CA}}$
= 440 °C mimics surface magnetic anisotropy observed by MOKE.