On the origin of coercivity reduction in surface patterned magnetic thin films

Abstract

Spinel nickel ferrite (NiFe2O4) is a promising material for next generation high-frequency sensors, antennae and microwave devices. A key issue to be addressed is the magnetic loss, which is proportional to frequency and becomes substantial at frequencies above 1 GHz. Previously, we reported on coercivity reduction in NiFe2O4 through surface patterning (Rasic and Schwartz, MRS Commun. 3, 207 (2013) and IEEE Magn. Lett. 5, 1 (2014) 1, 2). Here, we report on the effects of varying the feature size on magnetic behavior, including loss, and explain the reduced coercivity theoretically. Nickel ferrite thin films were deposited on c-plane sapphire substrates using chemical solution deposition and patterned via nanoimprint lithography with patterning masters having feature sizes varying from 500 to 1500 nm in 200 nm increments. Atomic force microscopy showed good feature transfer for all samples. X-ray diffraction images showed all samples to be single-phase inverse spinel nickel ferrite with similar texture. All patterned samples showed coercivity reductions relative to the unpatterned samples. The effects of feature size on coercivity reduction showed opposite trends for in-plane and out-of-plane magnetization hysteresis measurements, whereas saturation magnetization was not affected by feature size changes. Magnetic force microscopy images confirmed the origin of coercivity reduction to be shape-anisotropy-forced alternating domain formation. The coercivity reduction phenomena observed in patterned NiFe2O4 thin films are explained theoretically. The effect of changing the film thickness and domain size on the equilibrium energy density was calculated. While the relative energy density showed a stable equilibrium, the energy increase for domain sizes equaling that of patterned films in this study was small. Finally, the new domain structure in patterned NiFe2O4 films was explained within micromagnetism theory.