Magnetic Properties Hard-Soft SmCo<sub>5</sub>-FeNi and SmCo<sub>5</sub>-FeCo Composites Prepared by Electroless Coating Technique

doi:10.4236/ojcm.2012.24014

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Open Journal of Composite Materials, 2012, 2, 119-124

http://dx.doi.org/10.4236/ojcm.2012.24014 Published Online October 2012 (http://www.SciRP.org/journal/ojcm)

119

Magnetic Properties Hard-Soft SmCo5-FeNi and

SmCo5-FeCo Composites Prepared by Electroless Coating

Technique

M. Lamichanne1, B. K. Rai1, S. R. Mishra1*, V. V. Nguyen2, J. P. Liu2

1Department of Physics, The University of Memphis, Memphis, USA; 2Department of Physics, The University of Texas, Arlington,

USA.

Email: *[email protected]

Received May 25th, 2012; revised June 27th, 2012; accepted July 8th, 2012

ABSTRACT

Composites of SmCo5-FeNi and SmCo5-FeCo, hard-soft magnetic materials, have been synthesized via electroless

plating of magnetically hard SmCo5 powder particles with magnetically soft FeNi and FeCo, respectively. The influence

of coating thickness of soft magnetic layers on the structure and magnetic properties of the composite has been studied.

Overall FeNi coating was found to be less dense compared to FeCo for the same plating duration. Structurally the coat

ing was found to be nodular in morphology. These coating have dramatic effect on the overall magnetic property of the

composite. As compared to FeNi coated SmCo5 composite, two-fold increase in the saturation magnetization has been

observed upon coating SmCo5 (Ms ~ 28 emu/g) with FeCo to a value 56 emu/g. The coercivity of composite powder

was found to decrease with increasing the coating layer thickness. The absence of exchange spring behavior in the

hard-soft composite is attributed to magnetically soft layer thickness exceeding the theoretical length limit for ex-

change-spring coupling.

Keywords: Electroless Plating; Hard-Soft Composites; Exchange Spring

1. Introduction

Permanent magnetic materials are characterized by their

figure of merit, (BH)max energy product, which is the

maximum rectangular area under the second quadrant of

the hysteresis loop. The increase in (BH)max is possible

by increasing the coercivity (HC) and the saturation

magnetization (Ms). For materials with high HC values

(HC > 2Ms), the theoretical limit for the energy product

is limited only by Ms and is given by (BH)max  (2Ms)2.

Methods to overcome this limitation have focused on

developing high anisotropy materials with high Ms and

Curie temperatures (TC), materials which are based on

rare-earth (RE)-transition metal (TM) intermetallics such

as SmCo5, Sm2Fe17 and their boride or nitride com-

pounds such as Nd2Fe14B and Sm2Fe17Nx [1-3]. In spite

of the considerable improvement in the magnetocrystal

line anisotropy, these materials still exhibit lower Ms

than Co, Fe or Fe65Co35, which have 4Ms values of 18,

21, and 24 kG, respectively [1].

The concept of an “Exchange-Spring” or “Exchange-

hardened” magnet was proposed by Kneller and Hawig

[4] to overcome the limitations outlined above. The

spring magnet is essentially a composite of two ferro-

magnetic (FM) phases; hard and soft magnetic phases.

The soft phase, rich in transition metal, enhances satura

tion magnetization, while the hard phase provides the

required magnetic anisotropy and stabilizes the exchange

coupled soft phase from demagnetization. Most of the re-

ported literature on spring magnets is based on Nd2Fe14B

or Sm2Fe17 (hard)-α-Fe or Fe3B (soft) [5-7]. Insufficient

temperature stability and poor corrosion re sistance are

main factors limiting applications of Nd2Fe14B-based

magnets. However, the family of sa marium based alloys

such as SmCo5 and Sm2Fe17Nx could be an attractive

candidate material since it can be used as the hard phase

due to its high magnetization (SmCo5, Ms ~ 0.84 MAm−1,

Sm2Fe17Nx, Ms ~ 1.22 MAm−1) and high Curie tempera-

ture, TC, (SmCo5, TC ~ 720˚C, Sm2Fe17Nx, TC ~ 476˚C)

which are comparable to those of Nd2Fe14B (Ms ~ 1.28

MAm−1, TC ~ 312˚C, HC ~ 6.0 MAm−1) [8].

The exchange-spring magnets have been fabricated by

melt-spinning, hot-compaction [9], mechanical alloying

[10,11], however these techniques yields nanocomposites

with random dispersion and orientation of hard magnetic

phase and often having no control on the growth of soft-

phase [6,12]. Thus, if the soft phase size could be re-

*Corresponding author.

Magnetic Properties Hard-Soft SmCo5-FeNi and SmCo5-FeCo Composites Prepared by Electroless Coating Technique

120

duced and highly dispersed throughout the magnet, then

the interaction between hard-soft phases could be sig-

nificantly enhanced, leading to significant improvement

in (BH)max energy product. Various techniques such as

sputtering, pulse laser deposition (PLD), electrolytic

coating, and electroless coating [13] have been explored

to increase the interaction between hard-soft phases of

the composite.

Majority of electroless coating studies are limited to

Nd-Fe-B powder [14-16] and were carried out mainly

with a purpose of improving the corrosion resistance of

the magnet. However, the use of electroless plating

method has not been yet fully explored for the purpose of

developing hard-soft exchange-spring magnets [17].

Considering compatible magnetic properties of SmCo5 to

Nd-Fe-B, and high magnetization of FeNi and FeCo as

compared to



-Fe, the present study explores the synthe-

sis of hard-soft phase SmCo5-FeNi/FeCo composites via

electroless plating technique. Electroless plating method

is known to produce uniform adhesive coating on a wide

variety of substrate. In the electroless plating method, the

autocatalytic plating baths contain reducing agents ready

to react with the substrate in the presence of catalyst, a

metal which is to be deposited on the substrate. The

property of the coated film depends on time, temperature,

and catalyst concentration. In this article, magnetic prop-

erties of hard-soft phase composite are studied as a func-

tion of thickness of the soft magnetic layer.

2. Experiment

The hard-soft composites, with a choice of SmCo5 as a

hard phase and FeNi and FeCo as soft phase, were pre-

pared using modified electroless plating method [17].

SmCo5 powder of average grain size ~50 micron and

99.5% purity was obtained from Sigma Aldrich. SmCo5

was degreased in acetone solution and activated in 0.5 M

sodium hypoposhite at 90˚C for 10 min. An electroless

plating bath was prepared by using 0.09 mol of nickel

sulfate (NiSO4·6H2O) or cobalt sulfate (CoSO4·7H2O),

0.07 mol of iron sulfate (FeSO4·7H2O), 0.5 mol of so-

dium hypophosphite (NaH2PO2·H2O), 0.3 mol of sodium

citrate (Na3C6H5·2H2O), and 0.1 mol of ammonium sul-

fate ((NH4)2SO4). Activated SmCo5 powder was intro-

duced into the electroplating bath. The pH value of the

plating bath was maintained at 8 - 10 by using sodium

hydroxide. The plating was done at temperature 80˚C.

The electroless plating of FeNi was carried out for the

duration of 15, 30, and 60 minutes and for FeCo was

carried out for 30, 60, 90, and 120 min. Afterward elec-

troplated SmCo5 composite powder was rinsed with the

de-ionized water and dried in the presence of argon gas

at 90˚C for three hours. The X-ray diffraction (XRD)

patterns of the composite were obtained using Bruker D8

Advance diffractometer using Cu K



radiation. The sur-

face morphology and composition of the composites

were studied using scanning electron microscope (SEM)

and energy dispersive X-ray (EDX) spectroscopy, re-

spectively. The room temperature magnetic measure-

ments were made by the Alternating Gradient Magne-

tometer (AGM, Princeton Measurement Corp.) in the

maximum field of 14 kOe.

3. Results and Discussion

X-ray diffraction pattern of SmCo5 powder electoplatted

with FeNi and FeCo is shown in Figures 1(a) and (b),

respectively. Presence of strong FeNi and FeCo peaks is

recorded. With the electroless plating time, the peak in-

tensity of FeNi and FeCo increases and correspondingly

the peak intensity of SmCo5 decreases. Difference in the

growth of FeNi and FeCo is also evident. The presence

of broad FeNi peaks shows that initially FeNi layers are

amorphous. Furthermore, the presence of SmCo5 peaks

even after 60 minute of plating time shows the formation

of thin layer of FeNi. However, the full-width-at-half-

maximum (FWHM) of FeNi peak de creases with the

plating time. This decrease of FWHM of FeNi peaks

indicate a crystalline growth of FeNi, a growth which

results upon thickening of FeNi layers. On the other hand,

even for short plating duration ~15 min, thick and crystal-

line FeCo layers are formed as evident from the presence

of sharp FeCo diffraction peaks. Furthermore, gradual

reduction in SmCo5 peaks is noticed with the plating time;

which clearly indicates the thickening of FeCo layers

over SmCo5 particles.

Figures 2 and 3 show SEM images of FeNi and FeCo

coated SmCo5 composite powders, respectively. These

SEM images show that in all cases the coated layer is

neither smooth nor uniform. A nodule like coating struc-

ture appears. The EDX elemental analysis confirms that

both FeNi and FeCo are in 30:70 weight ratios. This is

further confirmed by the X-ray diffraction peaks indexed

by ICDD data base. The ICDD index number 71-8326

and 50-0795, for FeNi and FeCo, respectively show that

soft layers are iron deficient. Pres ence of trace amount

of phosphorous is also detected in EDX.

Figures 4(a) and (b) show the room temperature hys-

teresis loops of SmCo5-FeNi and SmCo5-FeCo compos-

ites, respectively. The hysteresis loops of electroless

plated SmCo5 particles are also compared with SmCo5

particles. As it can be seen from these plots, that FeNi

and FeCo both have different influence on the overall

magnetic properties of these composites. Table 1 lists the

room temperature magnetic parameters extracted from

the hysteresis loop of these composites.

The observed decrease in magnetization of FeNi

coated SmCo5 composite (15 minute sample) may result

partly from the increased weight of the sample and

amorphisity of the FeNi layer. With the increase in the

Magnetic Properties Hard-Soft SmCo5-FeNi and SmCo5-FeCo Composites Prepared by Electroless Coating Technique 121

Intensity ( a.u)

7060504030

2 (degree)

SmCo5

FeNi

SmCo

t = 15 min.

t = 30 min.

t = 60 min.

(a)

Intensity (a.u)

7060504030 2 (degree)

SmCo

FeCo

SmCo

t = 30 min.

t = 60 min.

t = 90 min.

t = 120 min.

(b)

Figure 1. (a) X-ray diffraction pattern of SmCo5 powder

coated with FeNi as a functi on of electroless plating time; (b)

X-ray diffraction pattern of SmCo5 powder coated with

FeCo as a function of electroless plating time.

plating time, the magnetization seems to increase be-

cause of the growth of FeNi crystals. This is also evident

from monitoring the FeNi peaks in the XRD results. On

the contrary, in case of SmCo5-FeCo composites, highly

crystalline FeCo coating enhances the saturation mag-

netization of the composite by almost a factor of two.

This is because FeCo has higher magnetization than

FeNi.

The coercivity of composites was observed to decrease

upon coating as depicted in inset Figure 4 and Table 1.

The observed decrease in coercivity in SmCo5-FeCo is

more than that of SmCo5-FeNi. The decrease in coercivi-

ity is schematically explained in Figure 5. Assuming

SmCo5 as a spherical particle coated with thick and

magnetically soft phase layer of FeCo or FeNi. Under the

(a)

(b)

(c)

Figure 2. SEM images of SmCo5-FeNi composites electroless

plated for (a) 15; (b) 30; and (c) 60 minutes.

Figure 3. SEM images of SmCo5-FeCo composites elec-

troless plated for (a) 30; (b) 60; (c) 90; and (d) 120 minutes.

external field the soft layer is magnetized quickly. If this

soft phase layer (shell) is thick and coats entirely the hard

phase sphere (core) then the core cannot see the external

field but only see the field created by the magnetization

Magnetic Properties Hard-Soft SmCo5-FeNi and SmCo5-FeCo Composites Prepared by Electroless Coating Technique

122

1600

1400

1200

1000

Coercivity (Oe)

6040200

Plating Time (minutes)

-10

-20

Magnetization (emu/g)

10x10

50-5-10

Field

(

)

SmCo

-FeNi

15 min.

30 min.

60 min.

(a)

1600

1200

800

400

Coercivity (Oe)

12080400

Plating Time ( minutes)

-20

-40

Magnetization (emu/g)

10x10

50-5-10

Field (Oe)

SmCo

-FeCo

30 min.

60 min.

90 min.

120 min.

(b)

Figure 4. (a) Room temperature hysteresis loops of SmCo5-

FeNi composites. Coercivity of composites as a function of

plating time is shown in the inset; (b) Room temperature

hysteresis loops of SmCo5-FeCo composites. Coercivity of

composites as a function of plating time is shown in the in-

set.

Table 1. Magnetic parameters obtained from hysteresis

loop measured at room temperature.

Coating Time

(min)

(emu/g) HC (Oe) Mr

(emu/g) Mr/Ms

SmCo5 -- 27.99 1770 10.480.37

SmCo5-FeNi15 15.83 1725 6.93 0.43

30 18.49 1325 7.60 0.41

60 20.96 825 6.70 0.31

SmCo5-FeCo30 49.98 550 10.680.21

60 55.38 475 13.660.24

90 56.71 301 8.98 0.15

120 55.97 275 8.82 0.15

H=0

Figure 5. A model of the magnetic behavior of the hard

phase sphere coated by the thick soft phase layer.

vector of the soft phase layer. When the Hext is applied,

the magnetization in the soft phase shell is saturated first;

the magnetization in the hard phase core has some mod-

erate value dependent on the Ms of the shell. When the

Hext is being reduced to zero, the M inside the shell is

rapidly approaches zero but the M inside the core de-

creases to some value. Next assume Hext goes on to the

value iHC which corresponds to the zero averaged value

of magnetization. Since in this case the M inside the core

is not zero (arrow in the core, right picture of Figure 5,

that means that this value iHC must be smaller than the

value of the coercivity of the core hard phase when no

shell covers it. Thus, thicker shell the smaller the iHC.

Overall exchange-spring hardening is not observed in

these composites. The absence of exchange spring phe-

nomenon in SmCo5-FeNi/FeCo composites may be at-

tributed to large thickness/grain size of soft FeNi and

FeCo layers, which are much larger than the domain wall

width of SmCo5 (



B ~ 3 - 6 nm) [18-20]. This cannot guar-

antee that the entire volume of FeNi or FeCo is exchange

hardened by the SmCo5 phase because the exchange

coupling requires grain sizes of the order of



B ~ 8 nm.

4. Conclusion

In summary we have successfully coated hard SmCo5

particles with soft-magnetic FeNi and FeCo via elec-

troless plating technique. The electroless coating resulted

in a thick uniform coating more so for FeCo than FeNi

for similar plating duration. Adding magnetically soft

phase into the SmCo5-FeNi/FeCo composite enhances its

magnetization but decreases its coercivity. This coerciv-

Magnetic Properties Hard-Soft SmCo5-FeNi and SmCo5-FeCo Composites Prepared by Electroless Coating Technique 123

ity reduction is explained on the bases of increased

amount of magnetic softness of the coated layer. An in-

dependently tuning the size and composition of the indi-

vidual building blocks of the composite is necessary in

order to enhance exchange coupling between hard-soft

phases of the composite. It is expected that composite

magnets of SmCo5-FeNi or FeCo could be obtained with

core-particles’ coercivity exceeding 10 kOe, which can

be attained either via ball milling technique [21-23] or

wet-chemical method [24]. It is also evident from the

present study that the growth rate of soft magnetic layer

needs to be better controlled in order to form a fine uni-

form coating of a few nanometer thicknesses on core

particles to meet the theoretical exchanges spring inter-

action length limit.

5. Acknowledgements

This work was supported by DMR NSF-EAGER, Grant

#0965801.

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