Magnetic and electrical properties of amorphous CoFeB films

Abstract

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 (PAr) during deposition and found that the optimal condition is PAr=5×10−3Torr. Because the electrical resistivity value (ρ) of the film is in general larger than 100μΩcm, 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 (Tcr1 and Tcr2) than the room temperature (RT). Besides the electrical properties, the film thickness (tf) dependence of saturation magnetization (Ms), saturation magnetostriction (λs), and coercivity (Hc) 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 Å in thickness, lying on the top surface of the CoFeB film, and another CoOx oxide layer, about 20 Å, 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 Mox. By fitting the Ms data as a function of (1∕tf), we can show that the last conjecture is correct, and Mox is not zero. The CoOx layer plays an important role on Ms,λs, and Hc of the CoFeB films with tf ranging from 50 to 503 Å.