Ultra-Soft and High Magnetic Moment NiFe Films Prepared via Electrodeposition from A Cu2+ Contained Solution

Abstract

There is a growing need to reduce the size and cost of power converter, which was widely used in portable/wireless devices like mobile phones, palmtop terminals, organizers, and other versatile communication tools. [1,2] For this purpose, the use of high frequencies (e.g. > 100 MHz) combined with thin ultra-soft high magnetic moment (Bs) films is desirable. High moment soft magnetic films are also widely used in modem electromagnetic devices, such as a high-frequency field-amplifying component, read-rite heads for magnetic disk memories in computers, and magnetic shielding material in tuners. [3] Although there are a lot of soft magnetic films, such as Fe based Co, Si, N, B, Al, Ti, Cr, Zr, Hf, Nb, and Ta alloys that have been achieved currently, an ultra-soft (Hc < 0.3 Oester (Oe)) magnetic film with high magnetic moment (Bs > 1.0 Tesla (T)) is yet to be achieved.[3] Herein, we report a novel approach to fabricate ultra-soft magnetic NiFe films from a sulfate salt based solution containing a little quantity of Cu2+ additive via electrodeposition, which is well known as a cheap and simple way to prepare metal films. The measurements from atomic force microscopy (AFM) and alpha-stepper showed that the thickness of prepared films ranged from tens of nanometers to micrometers. The magnetic characterization performed using vibrating sample magnetometer (VSM) and quantum design superconducting quantum interfaces device magnetometer (SQID) showed that the film possessed very soft magnetic property with an easy axis coercivity (Hce) and hard axis coercivity (Hch) no bigger than 0.15, 0.04 Oe, respectively. The film also had a high saturation magnetization of almost 1.65 T and good anisotropy. The NiFe films were electrodeposited on either Si (100) or SiO2 (amorphous) wafers. A seed layer such as Au, Cu or NiFe with thickness of 20 -30 nm was sputtered on each wafer as electrical conducting layer for the electrodeposition. The permalloy NixFe100-x (x = 45 -82) films were fabricated at the temperature of 40 ± 2 o C from a solution of 0.2 mol/l NiSO4.6H2O, 0.025 -0.035 mol/l FeSO4.7H2O, 0.28 mol/l NH4Cl, 0.4 mol/l H3BO3, and low concentration of Cu2+ additive. The electroplating system used was 55 l Paddle-Cell with a DC power. The atom ratio of Ni to Fe in the electrodeposited permalloy films was determined by energy dispersive spectra. The electroplating current density was controlled in the range of 15 -25 mA/cm2. Table 1 shows that the little amount (< 0.001 mol/l) of Cu2+ additive (added as CuSO4.5H2O or CuCl2.2H2O), greatly decreased the coercivity of Ni55Fe45 permalloy films without decreasing their Bs. It is because the Cu and O components (which decrease the Bs value) were of very low content in the deposited film and thus could not be detected with EDS. The Hce decreased from 0.5 to 0.15 Oe while the Hch decreased from 0.3 to 0.03 Oe when Cu2+ concentration increased from 0.0 to to 0.0006 g/l in electroplating solution. Figure 1 shows that the film changed from not having to having anisotropy when Cu2+ concentrat...