0018-9464 (c) 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TMAG.2017.2731970, IEEE Transactions on Magnetics CP-15 0018-9464 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. (Inserted by IEEE.) 1 Large negative uniaxial magnetic anisotropy in epitaxially strained nickel ferrite films Mitsuharu Matsumoto 1 , Sonia Sharmin 1 , Jun-ichiro Inoue 1 , Eiji Kita 1 , and Hideto Yanagihara 1 1 University of Tsukuba, Tsukuba, Ibaraki 305, Japan We investigated the magnetic properties of NiFe2O4(NFO) epitaxial films grown on MgAl2O4(MAO) substrates by a reactive RF magnetron sputtering method. The films were found to be coherently distorted to at least 61 nm thickness because of the lattice mismatch between MAO and NFO. The NFO(001) films exhibited large negative uniaxial anisotropy that can be quantitatively explained by the magneto-elastic theory despite the lattice distortion being as much as 3%. We also discovered magnetic anomalies in both the saturation magnetization and anisotropy of the thinnest film, which may because of the reconstruction of the electronic structures at NFO interfaces. Index Terms—NiFe2O4 epitaxial thin film, magneto-elastic effect, magnetic uniaxial anisotropy. I. INTRODUCTION agnetic anisotropy is one of the most important properties of ferromagnetic materials, generally originating from low symmetry of local or entire structure. In a form of epitaxial thin films, a uniaxial lattice distortion can be easily introduced into the film through a lattice misfit. Therefore, some volume of the film adjacent to a substrate suffers from compressive or tensile stress depending on a sign of the lattice misfit. The lattice strain gives rise to an anisotropic magnetoelastic effect [1][2]. Thus, epitaxial mismatch is a promising way to induce a large uniaxial magnetic anisotropy in epitaxial films. Spinel ferrites are magnetic oxides which have been actively investigated from the viewpoints of both fundamental research and application[3][4]. Since the crystal symmetry of a spinel structure is cubic, large magnetic anisotropy is not expected for most spinel ferrites in bulk form. The chemical formula of spinel ferrite is described as MFe2O4 where M stands for metal ions such as Fe, Co, Ni, Zn etc. M and Fe ions occupy two crystallographically inequivalent sites. In the case of nickel ferrite(M=Ni; NFO), the octahedral sites (B-sites) are randomly occupied by equal numbers of Ni 2+ and Fe 3+ . On the other hand, the tetrahedral sites (A-sites) are occupied by only Fe 3+ . The Fe 3+ ions have electronic configuration of d 5 meaning that the electron distribution is spherical and therefore the magnetic properties are isotropic. In the case of Ni 2+ (d 8 ), since the three t2g levels of the minority spin band are fully occupied, large magnetic anisotropy cannot be expected. However, once the crystal lattice is deformed via uniaxial strain such as an epitaxial distortion, uniaxial magnetic anisotropy can be potentially induced[5]. Recently, we have discovered that NFO(001) films grown on MgAl2O4 (MAO) substrate by reactive sputtering exhibit a saturation magnetization comparable to that of bulk NFO[5] . MAO has a similar spinel crystal structure with a smaller lattice constant (a = 0.808 nm) than that of NFO (a = 0.834 nm) and therefore both suppression of anti-phase domain density[6] and large compressive epitaxial stress are to be expected if NFO thin films are grown on a substrate of an MAO single crystal. In this paper, we report that negative uniaxial magnetic anisotropy ( ܭ ୙ ) is induced by epitaxial strain for NFO film grown on MAO(001). The absolute value for the largest induced magnetic anisotropy exceeds 10 Merg/cm 3 . II. EXPERIMENT All films were grown by using a high-vacuum planar-type magnetron sputtering system. Reactive sputtering was performed by introducing both Ar and O 2 in a radio frequency (RF) sputtering mode. The sputtering target was a Fe-Ni alloy with an atomic composition of Fe/Ni=2:1[5]. Prior to the sputtering process, polished MAO(001) substrates were annealed at 1200°C for 6 hours in an atmosphere. After the substrates were introduced into a growth chamber, the substrates were annealed at 400°C for approximately 20 minutes in vacuum. Growth temperature of NFO epitaxial films were 300°C. Thickness of the fabricated films ranged from 4 nm to 61 nm. We confirmed the epitaxial growth and the crystal structure of the NFO films by using reflected high energy electron diffraction (RHEED) technique and an X-ray diffraction (XRD) method. Some of the XRD experiments were performed at BL- 4C of Photon Factory in KEK. The film thicknesses were determined by X-ray reflectivity (XRR). Surface morphology of the films was observed by atomic force microscopy (AFM). Magnetization and magnetic torque measurements were performed at room temperature using respectively a SQUID magnetometer and a PPMS with a torque magnetometer option. M Fig. 1. AFM images of (a) MAO(001) and (b)-(f) NFO(001) with different thicknesses.