Magnetic and electrical properties of amorphous CoFeB films

CoFeB films were deposited on glass substrate by the sputtering method. From x-ray-diffraction and electron-diffraction-ring patterns, the major phase in the as-deposited CoFeB film is amorphous (or nanocrystalline). However, we could also identify a minor CoFe(110) crystalline phase in the film. We have tried to suppress this crystalline phase by changing the Ar partial pressure (P-Ar) during deposition and found that the optimal condition is P-Ar=5x10(-3) Torr. Because the electrical resistivity value (rho) of the film is in general larger than 100 mu Omega cm, it also indicates that the amorphous phase is dominant. From the temperature coefficient of resistance measurement, we learn that the amorphous phase in the CoFeB film crystallizes in succession at two higher temperatures (T-cr1 and T-cr2) than the room temperature (RT). Besides the electrical properties, the film thickness (t(f)) dependence of saturation magnetization (M-s), saturation magnetostriction (lambda(s)), and coercivity (H-c) has also been discussed. From the Auger-depth profile analysis, it is found that there is one CoOx (with 0.4 <= x < 1) oxide layer, about 15 angstrom in thickness, lying on the top surface of the CoFeB film, and another CoOx oxide layer, about 20 angstrom, lying near the CoFeB/glass interface. At RT CoOx is supposed to be paramagnetic. However, due to the proximity effect between CoOx and CoFeB, the CoOx layers may become ferromagnetic with the average magnetization M-ox. By fitting the M-s data as a function of (1/t(f)), we can show that the last conjecture is correct, and M-ox is not zero. The CoOx layer plays an important role on M-s,lambda(s), and H-c of the CoFeB films with t(f) ranging from 50 to 503 angstrom.