IEEE TRANSACTIONS ON MAGNETICS, VOL. 55, NO. 2, FEBRUARY 2019 2000704 Magnetic Anisotropy and Ferromagnetic Resonance in Nitrogen-Incorporated NiFe 2 O 4 Thin Films K. B. Anoop Baby 1,2 , Kodam Ugendar 3 , A. Subrahmanyam 2 , and G. Markandeyulu 1 1 Advanced Magnetic Materials Laboratory, Department of Physics, IIT Madras, Chennai 600 036, India 2 Semiconductor Laboratory, Department of Physics, IIT Madras, Chennai 600 036, India 3 Department of Applied Physics, Jabalpur Engineering College, Jabalpur 482011, India The nitrogen-incorporated nickel ferrite (N-NFO) thin films deposited by radio-frequency magnetron sputtering were found to exhibit interesting magnetic properties upon heat treatment. The in-plane (IP) and out-of-plane (OP) magnetizations of the heat-treated N-NFO thin films measured at 20 K were seen not to saturate even at higher fields, indicating a large perpendicular magnetic anisotropy (PMA). The coercive field value was found to decrease upon heat treatment, due to the weakened pinning of domain walls upon decrease in grain boundary volume. The IP magnetization exhibits a tendency to saturate at high fields, whereas the OP magnetization does not, indicative of the existence of easy magnetization direction in the plane of the film. The room temperature ferromagnetic resonance (FMR) spectra in both IP and OP configurations indicated the increase of FMR linewidth upon nitrogenation, revealing the increased distribution of magnetic anisotropy induced by nitrogen in the lattice. An increase in the resonance field value was observed for the N-NFO film grown at 100 W compared with that of NFO in the IP configuration, while a decrease in the value was observed for OP, which is attributed to an enhanced IP magnetic anisotropy. The films grown at 120 and 140 W have shown decrease in a resonance field value in the IP configuration and increase in OP, which is attributed to the increase in thickness-dependent PMA. Index Terms— Ferromagnetic resonance (FMR), magnetic anisotropy, nickel ferrite (NFO), nitrogenation, thin film. I. I NTRODUCTION N ICKEL ferrite (NiFe 2 O 4 or NFO) is a soft mag- netic material having a variety of applications, such as high-frequency devices in magneto-electronics and magneto- optics, sensors, memory devices, and so on [1], [2]. In particu- lar, nickel ferrite (NFO) thin films are widely used due to their high saturation magnetization, high magnetic permeability, high resistivity, and low losses that make them promising for high-frequency applications [3]–[6]. Nanostructured NiFe 2 O 4 (NFO) thin films with low eddy current losses have been proven to be excellent candidates for magnetic components, such as resonators, phase shifters, tunable signal filters, spin- tronic devices, and sensors [7]. Besides these applications, some properties of Ni ferrite are of fundamental interest. For example, the stoichiometry and size effects can fine-tune its electrical, electronic, and magnetic properties [1], [2], [6]–[8]. Furthermore, the effects of cationic interchange, crystal defects, and oxygen vacancies lead to interesting magnetic properties when these materials are fabricated as thin films and nanoparticles [9], [10]. However, the magnetic and electronic properties of ferrite thin films are very sensitive to growth conditions and the method of fabrication [7], [8]. Several methods are being used for the deposition of NFO thin films of which the radio-frequency (RF) magnetron sputtering is well known due to its high efficiency and ease of growth. Under certain specific growth conditions, ferrite thin films prepared by sputtering have been reported to be nanocrystalline, and their magnetic properties are entirely different from those of their bulk counterparts [11], [12]. Manuscript received June 4, 2018; revised July 11, 2018 and July 25, 2018; accepted August 2, 2018. Date of publication September 26, 2018; date of current version January 18, 2019. Corresponding author: G. Markandeyulu (e-mail: mark@iitm.ac.in). Digital Object Identifier 10.1109/TMAG.2018.2864313 Doping ferromagnetic materials with nitrogen has been reported to cause the increase of Curie temperature as well as coercivity values [13]–[16]. Babu et al. [17] from our group have reported the increase of saturation magnetization and decrease of coercivity in Fe–N thin films with the increase of film thickness and have reported that the films exhibited in-plane (IP) magnetocrystalline anisotropy, induced by the presence of nitrogen in the films. Incorporation of nitrogen into Ni ferrite lattice has been reported, by our laboratory, to modify the structural, microstructural, and magnetic properties of the base material [18], [19]. Bharathi et al. [10], [20] have reported the increase in grain size of dysprosium-doped NFO thin films upon heat treatment that lead to the increase of coercivity, increase of optical band gap, and decrease of electrical conductivity. Sui et al. [21] have studied heat-treated barium hexaferrite thin films, and have observed a preferential grain growth in the basal plane of the crystal (with a preferred growth direction along the c-axis and perpendicular to the film plane), which resulted in the uniaxial perpendicular anisotropy in the films. The above-mentioned reports point toward the existence of perpendicular magnetic anisotropy (PMA) in heat-treated ferrite films. Magnetization studies of nitrogen-incorporated NFO (N-NFO) thin films revealed superparamagnetic-like behavior due to the presence of a large perpendicular anisotropy in the films [18]. This led to the study of the effect of heat treatment on the magnetic properties of these films and the results are presented in this paper. Ferromagnetic resonance (FMR) study on these films is of significant interest as it is one of the key techniques to observe the magnetic anisotropies involved. Torres et al. [22] have investigated the frequency dependence of FMR linewidths of polycrystalline NFO, and the effects due to the porosity, 0018-9464 © 2018 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.