# Ultrasoft  and High Magnetic Moment CoFe Films Directly Electrodeposited from a B-Reducer Contained Solution

Authors: Baoyu Zong; Guchang Han; Jinjun Qiu; Zaibing Guo; Li Wang; Wee-Kay Yeo; Bo Liu
Journal: Research Letters in Physical Chemistry
 (2008)

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## Abstract

A methodology to fabricate ultrasoft CoFe nano-/microfilms directly via electrodeposition from a semineutral iron sulfate solution is demonstrated. Using boron-reducer as the additive, the CoFe films become very soft with high magnetic moment. Typically, the film coercivity in the easy and hard axes is 6.5 and 2.5 Oersted, respectively, with a saturation polarization up to an average of 2.45 Tesla. Despite the softness, these shining and smooth films still display a high-anisotropic field of
~45 Oersted with permeability up to104. This kind of films can potentially be used in current and future magnetic recording systems as well as microelectronic and biotechnological devices.


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## Body

## 1. Introduction

The ever expanding
demand of the world market leads to magnetic recording data storage devices
advancing toward much smaller exterior dimension and higher capacity [1, 2]. In order to achieve very high capacity and fast  recording data storage in a
miniature device, an ultrasoft and high magnetic moment material is required
for producing high-saturation flux density (Bs), so that the necessary
flux density can be preserved on reducing device dimensions, while
simultaneously achieving a low coercivity (Hc) to match the hard
magnetic media with high Hc, track density, and linear density [3]. Soft
magnetic films with high moment are also widely used in modern electromagnetic
devices, such as high-frequency field-amplifying components, versatile
communication tools, and magnetic shielding materials in tuners [4]. Although
numerous soft or high-Bs magnetic films have been achieved nowadays via
sputtering, evaporation, and casting, most of them cannot be applied to device
fabrication due to the following reasons: low-deposition rate (usually <8 Å/S) and high-internal stress films from sputtering [5], overly
thick films (commonly > 1 mm) from casting, and coarse-grained films from
evaporation [6]. In addition, most of the reported materials have limited Bs
of ≤2.1T (e.g., NiFe, FeCoNi films) [3]. Despite CoFe alloy possessing the
highest Bs of about 2.45T theoretically, the CoFe
nano/microfilms prepared always encounter two vital issues for application:
poor magnetic properties [such as poor anisotropy, high value of Hc
(always >4 Oerted (Oe))] and poor mechanical properties (such
as rough surface, and cracking film) [7]. Electrodeposition is a
fast and simple method to achieve various thicknesses of soft Fe-based
materials with lower stress [8, 9]. However, preventing Fe2+ from
oxidization into Fe3+ (which decreases Bs), and minimizing
the Hc and roughness of the metal films are challenges in
electrodeposition [3, 7]. So far, only the softest magnetic CoFe film, which
possesses an easy axis Hc(Hce) and hard
axis Hc⁢(Hch) of 15 and 4.8 Oe, respectively,
has been reported [7].Herein,
we demonstrate a novel approach to the fabrication of ultrasoft magnetic CoFe
films via electrodeposition from a sulfate salt-based solution containing
dimethylamine borane [(CH3)2NHBH3] (B-reducer,
Bayer AG, Germany).
In comparison with commonly used strong acidic solutions (corroding devices) which
contain S-additives (saccharin, Na Lauryl sulfate), the solution containing B-reducer is a
semineutral medium with larger process latitude. For instance, the pH range
(3.5–5.1) is much wider than that of the common plating solutions containing S-additives
(pH=2.3–2.5⁢or2.5–3.0)  [7, 9]. Furthermore, the plating solution does not require a salt bridge
to protect Fe2+ from oxidization. The thickness of the prepared
films measured through atomic force microscopy (AFM) ranges from tens of
nanometers to micrometers. The magnetic characterizations performed by using
vibrating sample magnetometer (VSM) and superconducting quantum interference
device (SQUID) show that the films are very soft magnets with good anisotropy and
high magnetic moment properties.


## 2. Method and Results

The CoFe films
were electrodeposited on Si(100) wafers. A seed layer, for example, Au, Cu, or CoFe, with thickness of 20–30 nm was sputtered onto each wafer surface as
an electrical conducting layer for the electrodeposition. TheFexCo100−x⁢(x=55–68)  films were fabricated at the temperature of 40±2∘C from a
solution of 0.07 mol/L CoSO4⋅7H2O, 0.10 mol/L FeSO4⋅7H2O,
0.5 mol/L NH4Cl, 0.5 mol/L H3BO3, and 2.8–3.2 g/L B-reducer additive. The electroplating system used was a Paddle cell with
pulse DC power. During the electrodeposition of CoFe film, a magnetic field of 280 Oe was applied parallel to the substrate surface. The atomic ratio of Co to Fe in
the electrodeposited films was determined by energy dispersive X-ray (EDX,
JSM-6340F, JEOL Asia). While measuring Hc with the VSM
possessing Helmoltz coils, high resolution of 0.01 Oe was obtained through
the field control mode. The maximum relative permeability (μm)
was calculated from easy axis M-H loops according to the formula μm=B(T)/H(T)≈4π   magnetic moment
(M, emu)/[magnetic field (H,Oe) × film volume (cm3)], where (M,H)
refers to the turning point in the lower branch of the easy axis M-H loop. It is noted that the effect of film sample shape (normally, 1.20 cm × 1.20 cm × ~0.6 μm) can be neglected due to the length and width being much larger
than the thickness [10, 11]. The Bs
values were calculated
from the formula Bs(T)=μ0Ms=4π×104M (emu)/film volume
(cm3), where M refers to the saturation moment in the
M-H loops. The Bs result is the average value obtained from a series of CoFe films having
different thickness (80 nm −2.5⁢μm). The dimension of the films was measured
on ADM-60 Micro-Dicer. The total error of Bs measurement is
0.01T.Table1 shows that the B-reducer dramatically induces the decrease of Hc and the increase of Bs with
its addition increasing from 0 to 100 mL/L.
It is due to the fact that despite the absence of salt bridge (leading
to a lower efficiency of plating current) to prevent Fe2+ from
oxidization in the solution, the
B-reducer can protect the as-synthesized CoFe film effectively from oxidization
since B-reducer,
acting as a reductant molecule, can
reduce Fe3+ (produced during plating) back into Fe2+:
(1)Fe2+−e→O2⁢oratanodeFe3+⁢(by-reaction)Fe3++e→B-reducerFe2+.Table 1
Property comparison of CoFe films (without annealing).
PropertyNo additiveB-reducer (1.5 g/L)B-reducer (3 g/L)S-additive7Hce(Oe)636.66.513Hch(Oe)535.82.54.8Hk(Oe)78264520.2μm7.8×1014.5×1045.8×104—Bs(T)1.422.312.452.45Despite the addition, the B-reducer
did not lead to too much boron doping
into the CoFe film. Hence, there is a very low content of oxygen and boron
present in the  deposited  CoFe  films,
in  which  the
low oxygen content
cannot be detected by EDX, while the boron content is less than 0.8% after XPS
analyzing. For theCo40Fe60 film deposited from a
current density of 6.0 mA/cm2, with the concentration of B-reducer
increasing from 0.0 to 3.0 g/L in the electroplating solution, the Hce and Hch values decrease from 63 and 53 to 6.5 and 2.5 Oe, respectively, while Bs increases from 1.4 to 2.45T in average. On the
other hand, the permeability (μm) of
CoFe film also increases from the order of 101 to 104 after the addition of B-reducer. Thus, the film becomes very soft due to the
effect of B-reducer. Figure 1  shows the magnetic moment–applied field curves (M-H loops) of the CoFe films
from the solutions containing different concentrations of B-reducer. It appears
that the loops along the easy and hard axes change
from being similar [such as in Figure 1(a)] to distinctly different [shown in
Figure 1(c)]. Hence, the ultrasoft and high magnetic moment of Co40Fe60 film possess a much larger anisotropy compared to the softest CoFe films
reported to date [7]. Further investigations revealed that although similar
effects of B-reducer had also been observed for other components of CoFe
films by varying the plating current density, only the films with formulas from
Co35Fe65 to Co43Fe57 could achieve a high Bs of
≥2.4T, and
such films were electrodeposited at a
current density ranging between 4.8 and 7.2 mA/cm2.TheM-H loops of the Co40Fe60 films electrodeposited (at
a current density of ~6.0 mA/cm2) from the solutions which contain B-reducer
of (a) 0.0, (b) 1.5, and (c) 3.0 g/L, respectively.
(a)(b)(c)More measurements were carried out to identify the mechanism for the effect
of B-reducer on the properties of CoFe films. In Figure2, the AFM and FESEM images
show the surface variations of the deposited Co40Fe60 films
with the increase of B-reducer concentration from 0 to 1.5, then to 3.0 g/L. The sizes of CoFe nanoparticles become much smaller (roughly from 80 to 10–30 nm) with the increase of B-reducer concentration in the electroplating
solution. Thus, for the films, Hc drops and μm increases drastically since the average
magnetocrystalline anisotropy and the exchange coupling range become lower and
wider, respectively, with the smaller ferromagnetic nanoparticles [10, 11]. In
addition, X-ray diffraction (XRD) analysis (recorded from a powder sample) in
Figure 3 shows that the added B-reducer eliminates the foreign Fe2O3-phase (viz. protects Fe2+ from oxidization), which leads to a texture-structural change from
CoFe(110) and Fe2O3(209) polycrystalline to CoFe(110)
single-crystalline for the Co40Fe60 films. As a result,
the film possesses very low   Hc [11] and high theoretical Bs value
(~2.45T) [7]. Further investigation results revealed
that too much of B-reducer (>10 g/L) led to an unstable plating solution
and a high content of boron doping (>2%) in the CoFe films. The films
possess an amorphous texture, a decreased Bs, and poor
anisotropy.The
AFM and FESEM (inset) images of theCo40Fe60 films
electrodeposited (at a current density of ~6 mA/cm2) from the
solutions which contain B-reducer of (a) 0, (b) 1.5, and (c)
3.0 g/L, respectively. The scale bar in the inset (a)–(c)
is 1 μm, 100, and 100 nm, respectively.
(a)(b)(c)The2θ XRD results of
~0.6 μm Co40Fe60 films on Si(100) wafer.
(a) and  (b) stand for without adding and adding B-reducer, respectively.
(a)(b)Apart from having excellent magnetic properties,
our characterizations shown in Table2 indicate that the CoFe films also
displayed good mechanical and other properties, such as very low
magnetostriction (λs)
and surface roughness (Ra, also depicted in Figure 1), and absence
of microcracking defects, which is a critical problem faced by current CoFe
electrodeposition [7, 12]. These properties also contribute to low Hc of
the films [10].Table 2
Properties of softCo40Fe60 films.
FilmHce(Oe)Hch(Oe)Bs(T)λs(×10−6)SurfaceRa (nm)Before anneal6.52.52.459.2Shining2.4After anneal5.61.82.433.4Shining2.2Note:
the annealing was conducted at 200∘C for 4 hours.


## 3. Conclusion

The two challenging issues
(poor magnetic
and mechanical properties), encountered in the preparation of high-Bs magnetic CoFe films [7], can simply be solved by using boron reducer in
electrodeposition. The boron reducer can greatly improve the softness, magnetic
moment, mechanical, and other properties of the CoFe films.  Thus, the as-prepared films have potential
applications in ultrahigh density and frequency magnetic recording system,
biotechnological, and microelectronic devices [2, 4].



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*Source: 342976-2008-06-12.xml*