New applications of nanocrystalline Fe(Co–Fe)–Hf–O magnetic films to micromagnetic devices

Abstract

New applications of high resistivity magnetic films, such as Fe–Hf–O and Co–Fe–Hf–O as thin film inductors and flux gate magnetic sensors, were investigated. These soft magnetic films with nanocrystalline structure and high resistivity were deposited on glass substrate by using the oxygen reactive sputtering method. Planar sandwich thin film inductors developed by using Fe–Hf–O or Co–Fe–Hf–O films had a higher quality factor than that of an amorphous Co–Ta–Hf film inductor. The dc–dc converter using the Fe(Co–Fe)–Hf–O film inductor had higher conversion efficiency. These high resistivity magnetic films were also applied to a flux gate magnetic sensor operated at high frequencies over 1 MHz. Both high frequency excitation and high permeability (owing to the low eddy current loss) enabled us to increase the sensitivity, e.g., 39 mV/Oe/turn at 5 MHz when using a 3 μm thick Fe–Hf–O film (12×6 mm2 area).

Novel nanostructure and magnetic properties of Co–Fe–Hf–O films

“…Hayakawa et al [10] first reported that Fe-M-O (M = Hf, Zr) magnetic films possessed a good magnetic softness owing to their nanostructure which consisted of ferromagnetic bcc nanocrystals (∼10 nm in size) embedded in an amorphous matrix containing large amounts of M and O elements. Soon after this work, several attempts were made to improve the high-frequency performance of these films [11][12][13][14][15][16][17][18][19]. It was found that the addition of Co to Fe-Hf-O films led to a significant improvement in the high-frequency magnetic response, mainly due to an increase in the crystalline anisotropy of the nanograins.…”

“…It was found that the addition of Co to Fe-Hf-O films led to a significant improvement in the high-frequency magnetic response, mainly due to an increase in the crystalline anisotropy of the nanograins. Sato et al [11,12] revealed that magnetic inductors using Co-Fe-Hf-O thin films had a higher quality factor and conversion efficiency than those using Fe-Hf-O and Co-Ta-Hf thin films. Since the Co-Fe-Hf-O thin films are insensitive to oxygen at interfaces, they could also be used as free layers for fabrication of spin-dependent tunnelling (SDT) junctions [16] and as electrode layers for tunnelling magnetoresistive (TMR) junction applications [17].…”

“…In this context, the recent findings of new magnetic oxide thin films are of practical importance in high-frequency applications because these materials can show a good highfrequency performance into the gigahertz range owing to their resistivity, which is approximately ten times higher than that of conventional magnetic alloy films [6,7,[10][11][12][13][14][15][16][17][18][19]. Hayakawa et al [10] first reported that Fe-M-O (M = Hf, Zr) magnetic films possessed a good magnetic softness owing to their nanostructure which consisted of ferromagnetic bcc nanocrystals (∼10 nm in size) embedded in an amorphous matrix containing large amounts of M and O elements.…”

“…Recently there has been an emerging demand for the use of magnetic thin films in high-frequency applications including magnetic recording write-heads and soft underlayers for perpendicular media and thin film wireless inductor cores [6,7,[10][11][12][13][14][15][16][17][18][19]. For these applications, magnetic thin films are required to have high electrical resistivity (ρ) to minimize energy loss due to eddy currents, and a large saturation magnetization (4π M s ) and hard-axis anisotropy field (H kH ) to increase the magnetic switching capacity at high frequencies [7,10].…”

Design of microfabricated inductors for microprocessor power delivery

“…Prototype microfabricated thin-film magnetic components for power conversion have been reported in [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. However, these designs are limited by low efficiency (often 60% or lower), low power density (often under 1 W/cm ¾ of substrate area), or both.…”