Effect of grain size on magnetic and nanomechanical properties of Co60Fe20B20 thin films

In this investigation, CoFeB thin films were sputtered onto glass substrates with thicknesses (tf) from 100 to 500 Å under the following conditions; (a) substrate temperature (Ts) maintained at room temperature (RT), (b) post-annealing at heat annealing TA = 150 °C for 1 h, and (c) post-annealing at heat annealing TA = 350 °C for 1 h. X-ray diffraction (XRD) reveals that CoFeB films are nano-crystalline at RT, and become more crystalline with post-annealing treatment. To determine the grain size distribution, the plane-view microstructure was observed under a high-resolution transmission electron microscope (HRTEM). The selected-area-diffraction (SAD) patterns obtained using HRTEM support the XRD results. The X-ray diffraction peak and the electron diffraction pattern demonstrate that the CoFeB thin film had a nano-crystallization body-centered cubic (BCC) CoFeB (1 1 0) at RT. Following annealing treatment, the CoFeB BCC (1 1 0) structure was more crystalline. Increasing the post-annealing temperature from RT increases the grain size. Additionally, the grain size distributions under various conditions are determined using plane-view HRTEM. The magnetic remanence properties of the CoFeB thin films are sensitive to grain size. This result shows that grain size refinement reduces effective anisotropy, increasing the ferromagnetic exchange coupling, and thereby remanence. The coercivity (Hc) is also observed to increase, since the grain size distribution is enlarged. Based on the relationship between grain size and the nano-indentation results, the decline in the hardness and Young's modulus can be reasonably inferred to be associated with an enlarged grain size, consistent with the “Hall–Petch” effect and the grain refinement mechanism.