Low-frequency alternative-current magnetic susceptibility of amorphous and nanocrystalline Co60Fe20B20 films

Abstract

This investigation experimentally studies the low-frequency alternating-current magnetic susceptibility (χac) of amorphous and nanocrystalline CoFeB films by measuring the magnetic field established by passing currents of various frequencies through such films of various thicknesses (tf). A CoFeB film is sputtered onto a glass substrate with tf from 100Å to 500Å under the following conditions: (a) As-deposited films were maintained at room temperature (RT) and (b) films were post-annealed at TA=150°C for 1h. The samples thus obtained are analyzed in a magnetic field that was generated by an alternating current (AC) at various frequencies from 10Hz to 25,000Hz. The experimental results demonstrate that the χac declines as the thickness of the as-deposited sample and the post-annealed sample (TA=150°C) increases because the lower coercivity (Hc) of thinner CoFeB films is similar to a soft magnetic characteristic and is associated with a higher χac value. The best χac value is obtained at a thickness of 100Å under both conditions. The χac value of the post-annealed sample exceeds that of the RT sample at thicknesses from 100Å to 500Å because the magneto crystalline anisotropy of the post-annealed sample yields the highest χac value at the optimal resonance frequency (fres), at which the spin sensitivity is maximal. The X-ray diffraction patterns (XRD) of as-deposited CoFeB films reveal their amorphous structure. The XRD results for the post-annealed films include a main peak at 2θ=44.7° from the body-centered cubic (BCC) nanocrystalline CoFe that indicated a (110) textured structure. Post-annealing treatment caused that the amorphous structure to become more crystalline by a thermally driven process, such that the χac value of the post-annealed sample exceeded that of the RT sample. This experimental result demonstrates that the χac value decreased as the thickness of the thin film increased. Finally, the CoFeB thin films had the best χac at low frequency (<50Hz) following post-annealing treatment. The results obtained under the two conditions indicate that the maximum χac value and the optimal fres of a 100Å-thick CoFeB thin film were 1.6 and 30Hz, respectively, following post-annealing at TA=150°C for 1h, suggesting that a 100Å-thick CoFeB thin film that has been post-annealed at TA=150°C can be utilized as a gage sensor and in transformer applications at low frequencies.