Uniaxial magnetic anisotropy of cobalt thin films on different substrates using CW-MOKE technique

Highlights

• Deposited cobalt thin films on different substrates and annealed at 300 °C.

• Characterized as-grown and annealed films by GIXRD, AFM and MOKE.

• Uniaxial magnetic anisotropy observed for all the samples.

• Decrease in anisotropy on annealing may be due to release of stress during deposition.

Abstract

Cobalt thin films were deposited on GaAs, Si and Glass substrates by RF-magnetron sputtering. The structure was studied using atomic force microscopy, X-ray reflectivity and grazing incidence X-ray diffraction. Magnetic properties were determined with the magneto-optic Kerr effect. The deposited films have in-plane uniaxial anisotropy and after annealing the anisotropy reduces. The reduction in anisotropy may be due to release of stress and the remaining anisotropy after annealing may be due to shape anisotropy of the particulates.

Introduction

Magnetic properties of thin films are inherently related to the structure and morphology of the films. From the application viewpoint, magnetic anisotropy is one of the most important properties of the magnetic materials [1]. Depending upon the type of application e.g. permanent magnets, storage media or magnetic cores in transformers and magnetic recording heads, materials with high, medium or low magnetic anisotropy are required. In addition, due to extensive development in semiconductor technology, the properties of magnetic metal films especially of nanometer scale structures on semiconductor substrates have gained more importance. The magnetic and interfacial properties of thin films of cobalt (Co) on different substrates (e.g. Pt, Ge, SiO₂, GaAs, glass, MgO, Si, Cu and Pd) have been reported in the literature, but the emphasis have been on to Cu, Pt and Si substrates because of the possibility of growth of FCC Co in the case of Cu, perpendicular anisotropy in the case of Pt or Pd substrates and studies of growth on semiconductors for spintronics devices [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. The effect of surface roughness, thickness dependence, oblique deposition etc. has been extensively studied [8], [10], [11], [12], [13], [14]. Co thin films have been deposited by metal organic chemical vapor deposition (MOCVD), DC and RF magnetron sputtering, thermal evaporation, e-beam evaporation and molecular beam epitaxy [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. Uniaxial magnetic anisotropy in Co thin films on glass, Si, MgO, etc. has also been observed [8], [9], [11], [12], [13], [14]. Effect of surface roughness on the magnetization reversal in Co thin films on Si has been studied and it has been reported that increasing the roughness changes the magnetization reversal process from the magnetization rotation to a combination of magnetization rotation and domain wall motion [11]. Similarly, for studies of Co films on rippled Si substrates, the strong uniaxial anisotropy has been reported with its easy axis along a direction normal to the ripple wave vector [7]. Uniaxial magnetic anisotropy has been studied in Co films on glass using MOKE technique and the shape of the substrate is reported as the probable cause for the rotation of the easy axis of the magnetization for amorphous or polycrystalline films [4]. The correlation between micro-structure and magnetic properties has been studied and roughness is found to lower the uniaxial anisotropy and raise the coercivity [12]. However, in spite of the significant research effort, much of the observed magnetic behavior remains unexplained, including a detailed understanding of the magnetic anisotropy and the change from perpendicular to in plane magnetization. The reason for this surely arises from the large sensitivity of the magnetic anisotropy to structural, morphological, and chemical details at the interfaces, which is at the origin of a variety of experimental results due to a lack of complete and accurate control of the fabrication parameters. Therefore, it becomes important to study the effect of substrate on the magnetic properties of the sample. To study the effect of substrate on the magnetic properties of the thin film, we deposited thin films on different substrates under same deposition conditions. The earlier studies are independent studies mostly with one substrate at a time and there are very few reports devoted to the magnetic properties of thin films on two or more substrates. The magneto-optic Kerr effect (MOKE) has become a standard tool for the study of magnetic ultrathin films and magnetic multi-layers due to its simple experimental setup, ability for making in-situ measurements and high sensitivity down to monolayer resolution [2], [4], [6], [8], [14]. In the present work, we have studied the in-plane magnetic anisotropy of cobalt thin film on different substrates by MOKE technique. We have deposited Co thin films of 50 nm thickness on different substrates simultaneously and have shown that anisotropy is due to stress during deposition and with annealing, the anisotropy decreases which may be due to release of stress and the remanant anisotropy is due to shape anisotropy of the particulates.