Effect of substrate temperature driven magnetic anisotropies on the magnetostrictive behaviour of Tb-Fe-Co thin films

Rare earth transition metal based alloy thin films exhibit giant magnetostriction owing to the large magnetic anisotropy arising from the rare earth sub-lattice. Elaborate studies carried out on these films have shown that Tb-Fe-Co based thin films are promising candidates for magnetostrictive applications as their magnetic anisotropy and thermal stabilities are large [1, 2]. Tb-Fe-Co magnetic phase diagram shows that films with compositions Tb10(Fe, Co)90 and Tb50(Fe, Co)50 exhibit strong in-plane (IP) magnetic anisotropy which is an important mandate for realizing high magnetostriction [3, 4]. Recent studies carried out on Tb10(Fe, Co)90 films have shown that presence of competing IP and out-of-plane (OOP) magnetic anisotropies has been found to be detrimental for magnetostriction [4]. Hence, there is an interest to explore the magnetic properties of Tb50(Fe, Co)50 films. It has also reported recently, that a considerable improvement in the magnetostriction has been obtained for thin films processed with in-situ substrate heating during thin film deposition [5]. In this context, a study has been planned to investigate the magnetic and magnetostrictive behaviour of Tb50(Fe, Co)50 films grown with different substrate temperatures. Tb50(Fe, Co)50 films were grown on Si < 100 > substrates by dc magnetron sputtering employing an alloy target. Prior to deposition a base vacuum of 1×10⁻⁸ torr has been achieved in the sputtering chamber. During deposition films were grown with different substrate temperatures (Ts) {viz}., 30, 300, 400, 450, 500 and 550°C. Deposition parameters such as sputtering power (50 W) and gas pressure (5m.torr) were kept constant for all the depositions. Structural and microstructural investigations were carried out using X-ray diffraction (XRD) and scanning electron microscopy (SEM) respectively. Simultaneous atomic and magnetic force microscopy (AFM/MFM) studies were carried out for the as-deposited and films processed with various Ts to observe the topography and magnetic domains. IP and OOP magnetization curves were traced upto maximum magnetic field of 15 kOe. IP magnetostriction measurements were measured using a homemade optical setup. The thickness of the as-deposited and films processed with different substrate temperatures are found to be ~150 nm. Structural studies showed that the as-deposited Tb-Fe-Co film is amorphous in nature. Upon increase in Ts, formation of Tb2(Fe, Co)17 phase could be noticed. Surface roughness estimated from AFM studies indicated an increase in the surface roughness with increase in Ts. SEM studies exhibited a globular kind of surface morphology whose sizes increased with increase in Ts owing to the growth of Fe-Co grains. Magnetization studies showed an IP magnetic anisotropy for the as-deposited film with low coercivity. With increase in Ts, a spin reorientation transition (SRT) from IP to OOP direction (SRT1) could be noticed (Fig. 1). Accordingly, the OOP coercivity was found to increase with increase in Ts. With subsequent increase in Ts (>450°C) the film undergoes one more SRT from OOP to IP direction (SRT2) (Fig. 2). The possible reason for SRT1 could be attributed to the magnetoelastic anisotropy arising due to the interfacial stresses developed in the film owing to the difference in the thermal expansion co-efficient between the film and interface [6]. The large formation of randomly distributed Tb2(Fe, Co)17 phase induces SRT2 at higher Ts. MFM image of as-deposited film indicate weak domain contrast with no identifiable domain pattern. However, with increase in Ts the OOP magnetic contrast is found to be enhanced (Fig. 1). The magnetic phase difference (MPD) which is a figure of merit to quantify the strength of OOP magnetic components show a considerable increase with increase in Ts upto 400°C (Fig. 2). With subsequent increase in Ts the MPD decreases. Finally, the film grown at 550°C shows a weak domain contrast devoid of any domain pattern. The microscopy results further confirms the SRT transitions observed from magnetization studies. Magnetostriction estimated from tip deflection measurement has been found to decrease marginally with increase in Ts upto 300°C. Films deposited with higher substrate temperatures such as 400 and 450°C show small change in magnetostriction owing to strong OOP magnetic anisotropy [4]. With subsequent increase in Ts, the magnetostriction was found to increase considerably. The possible reason for the increase in magnetostriction at higher substrate temperatures (>450°C) can be attributed to the formation of crystalline Tb2 (Fe, Co)17 with enhanced grain sizes and relaxation of residual stresses. Based on the structural, microstructural (surface and magnetic), magnetization and magnetostriction studies a strong correlation between magnetic anisotropy and magnetostriction has been established.