Fe-nanoparticle coated anisotropic magnet powders for composite permanent magnets with enhanced properties

Abstract

Utilizing the chemical reduction of Fe Cl ₂ with Na B H ₄ in the presence of 2:17 Sm–Co powders, we synthesized composite Sm ( Co _0.699 Fe _0.213 Cu _0.064 Zr _0.024)_7.4 /nano-Fe anisotropic hard magnetic powders. The average particle size of the hard magnetic core powder was 21 μ m while the soft magnetic Fe nanoparticles deposited uniformly on the core powder had a particle size smaller than 100 nm . Different reaction protocols, such as immersion of the hard magnetic core powder in each reagent, the use of microemulsion (micelle) technique, or doubling the weight ratio of Fe Cl ₂ to core powder, led to different degrees of magnetic coupling of the hard and soft magnetic components of the composite powder. A reaction time of 180 s led to deposition of 3.5 wt % Fe nanoparticles and improved magnetic properties of the composite powder compared to the uncoated Sm ( Co _0.699 Fe _0.213 Cu _0.064 Zr _0.024)_7.4 powder. The respective magnetic hysteresis parameters were 4πM_{18 kOe} =11.3 kG , 4πM_r=11 kG , and _iH_c≫20 kOe with a smooth demagnetization curve.

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Fe-nanoparticle coated anisotropic magnet powders for composite
permanent magnets with enhanced properties
M. Marinescua?and J. F. Liu
Electron Energy Corporation, Landisville, Pennsylvania 17538, USA
M. J. Bonder and G. C. Hadjipanayis
University of Delaware, Newark, Delaware 19716, USA
?Presented on 7 November 2007; received 21 September 2007; accepted 16 October 2007;
published online 29 January 2008?
Utilizing the chemical reduction of FeCl2with NaBH4in the presence of 2:17 Sm–Co powders, we
synthesized composite Sm?Co0.699Fe0.213Cu0.064Zr0.024?7.4/nano-Fe anisotropic hard magnetic
powders. The average particle size of the hard magnetic core powder was 21 ?m while the soft
magnetic Fe nanoparticles deposited uniformly on the core powder had a particle size smaller than
100 nm. Different reaction protocols, such as immersion of the hard magnetic core powder in each
reagent, the use of microemulsion ?micelle? technique, or doubling the weight ratio of FeCl2to core
powder, led to different degrees of magnetic coupling of the hard and soft magnetic components of
the composite powder. A reaction time of 180 s led to deposition of 3.5 wt % Fe nanoparticles and
improvedmagnetic properties ofthecomposite
Sm?Co0.699Fe0.213Cu0.064Zr0.024?7.4powder. The respective magnetic hysteresis parameters were
4?M18 kOe=11.3 kG, 4?Mr=11 kG, and
© 2008 American Institute of Physics. ?DOI: 10.1063/1.2830956?
powder comparedto theuncoated
iHc?20 kOe with a smooth demagnetization curve.
I. INTRODUCTION
The most important criteria for efficient exchange cou-
pling in nanocomposite permanent magnets are ?i? the size of
soft magnetic phase should be scaled to double domain wall
width of the hard magnetic phase1and ?ii? soft magnetic
phase should be uniformly distributed. So far, most of the
efforts to obtain such hard/soft nanocomposite magnets fol-
lowed “top down” metallurgical routes, i.e., development of
the nanostructure through rapid solidification or high-energy
mechanical milling. Both techniques provide certain degree
of control over the microstructure, but they produce crysta-
lographically isotropic materials with low maximum energy
product ?BH?max.2Recently, more attention has been given to
the “bottom up” approach, which assembles composite ma-
terial from anisotropic building blocks. There were a number
of attempts3–8to explore simple mechanical blending of Fe
nanoparticles with more coarse anisotropic hard magnetic
powders. Some of these efforts, in particular, blending of the
needle-shaped Fe particles with Sm–Co permanent magnet
powder,7yielded fairly good magnetic properties. In general,
however, mechanical blending of a relatively coarse powder
with an ultrafine one tends to result in agglomeration of the
latter5and the consequent reduction in exchange coupling.
On the other hand, coating of hard magnetic particles with a
layer of soft magnetic material may prevent agglomeration
of the latter and allow better control over its size. Liu et al.10
explored various coating techniques of depositing Fe on Nd–
Fe–B particles subsequently subjected to consolidation and
hot plastic deformation ?die upsetting? and reported signifi-
cant improvement in ?BH?maxcompared to the blending ap-
proach. Zeng et al.9published results on the coating of
Sm–Co particles with Co layers using electroless deposition.
This article presents our experimental results on the syn-
thesis and properties of composite anisotropic hard magnetic
Sm-Co/nano-Fe powders. These composite powders are can-
didate precursors for permanent magnets with enhanced
magnetic performance for high temperature applications.
II. EXPERIMENT
Composite Sm–Co/nano-Fe powders were synthesized
depositing Fenanoparticles
Sm?Co0.699Fe0.213Cu0.064Zr0.024?7.4 hard magnetic powder.
The hard magnetic powders were produced by jet milling the
consolidated aligned magnets. The magnets were not magne-
tized and the average particle size after jet milling was
21 ?m. The Fe nanoparticle synthesis employed wet chem-
istry according to the reaction
by onanisotropic
FeCl2+ NaBH4+ H2O → Fe?B? + NaCl+ H2+ H2O.
The hard magnetic powder was immersed in the reaction
solution and acted as a substrate onto which the Fe nanopar-
ticles formed and deposited. The amount of nanoparticle
coating was modified by varying the time allowed for the
reaction ?i.e., 33 and 180 s? and the mass fraction of FeCl2to
Sm–Co powders ?i.e., 1.5:6 and 3:6? while keeping the molar
ratio of the reaction reagents FeCl2:NaBH4at the optimal
1.7:1 for nanoparticle saturation magnetization.11The experi-
ments were done under different reaction protocols: ?i? im-
mersion of hard magnetic powder in each of the reagent so-
lutions and ?ii? microemulsion ?micelle? technique by adding
Si oil.12An idealized sketch of the core powder coated with
Fe nanoparticles through these deposition processes is given
?1?
a?Electronic mail: mmarinescu@electronenergy.com.
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in Fig. 1. After the reaction ceased, the powders were col-
lected with a magnet, washed, and sieved to remove un-
wanted by-products and free nanoparticles.
The composite powders were characterized by x-ray dif-
fraction ?XRD? using the Cu K? radiation, scanning electron
microscopy ?SEM? with a JEOL JSM-6330F instrument
equipped with an energy dispersive x-ray ?EDX? detector,
and magnetic measurements were done with a Lakeshore vi-
brating sample magnetometer ?VSM?. Alignment and mag-
netization of the uncoated and composite powder was done
by using pulse magnetizers with an applied field of 100 kOe.
The magnetic measurements ?hysteresis? were performed on
powders aligned and fixed in wax by using a demagnetiza-
tion factor of 0.12, derived from previous measurements on
Ni powder. The magnetization values in kilogauss unit were
calculated considering the ideal density.
III. RESULTS AND DISCUSSION
Sm?Co0.699Fe0.213Cu0.064Zr0.024?7.4 anisotropic powders
?not magnetized? were immersed in a FeCl2solution in eth-
anol, and upon the addition of a solution of NaBH4in dis-
tilled water, Fe precipitated within the whole volume, includ-
ing the surface of the Sm–Co powder, in accordance with Eq.
?1?. In contrast, the immersion of Sm–Co powder in the so-
lution of NaBH4in distilled water resulted in a substantial
corrosion that decreased dramatically the particle size and,
moreover,Fe nanoparticles
reaction ?1? did not attach to the Sm–Co powders.
Figure2 presentsthe
Sm?Co0.699Fe0.213Cu0.064Zr0.024?7.4/nano-Fe composite pow-
der synthesized by immersing the core powder in FeCl2so-
lution and allowing the reaction ?1? to take place, and then
washing in ethanol three times. One can see that the Fe nano-
particles with a particle size below 100 nm distribute fairly
uniform on the surface of the core powder. However, some
additional free clusters of Fe nanoparticles were still present.
The Fe nanoparticles on the core Sm–Co powder determine a
Gaussian profile in the XRD pattern of Fig. 3, which super-
imposes with the peaks for 2:17 Sm–Co phase obtained for
the uncoated core. EDX analyses of an area of 1200
?900 ?m2of compacted powder showed that the Fe nano-
particles, derived as the excess of Fe compared to the core
powder, were in the amount of 2.5 wt % for the composite
powder synthesized in a 33 s reaction time, 3.2 wt % for
generatedby the
SEM micrograph of
33 s reaction time with added Si oil, 3.5 wt % for a reaction
time of 180 s, and 3.5 wt % for double reactants protocol.
The demagnetization
Sm?Co0.699Fe0.213Cu0.064Zr0.024?7.4/nano-Fe powders synthe-
sized under different deposition protocols are compared in
Fig. 4 with the uncoated powder. The hysteresis magnetic
parameters of the uncoated powder are the following: rema-
nent magnetizatio 4?Mr=10.7 kG, magnetization at an ap-
plied field of 18 kOe 4?M18 kOe=10.9 kG, and intrinsic co-
ercivityiHc?20 kOe. We obtained that 4?M18 kOeincreases
with all Fe nanoparticle coating protocols, as expected, and
the amount of increase can be related with the amount of Fe
nanoparticles. For a 33 s reaction time, the composite par-
ticles have 4?Mr=10.7 kG, 4?M18 kOe=11.2 kG, andiHc
?20 kOe. By allowing a longer time ?180 s? for the reac-
tion, we obtained composite powder with 4?M18 kOe
=11.3 kG. The remanence increases also to 4?Mr=11 kG
and the smooth profile of the demagnetization curve is well
preserved. The microemulsion technique consists in the for-
mation of oil micelles within the reaction bath, and, there-
fore, a localized synthesis of Fe nanoparticles occurs within
curvesof composite
FIG. 1. ?a? Synthesis of Fe nanoparticles on the surface of the substrate
powder within the reaction volume. ?b? More uniform Fe-nanoparticle coat-
ing of the substrate powder by using the microemulsion technique with the
formation of micelles.
FIG. 2. SEM micrograph of a composite Sm?Co0.699Fe0.213Cu0.064Zr0.024?7.4/
nano-Fe powder. Inset shows Fe nanoparticles attached to the surface of the
substrate ?core? particle.
FIG. 3.
with
Sm?Co0.699Fe0.213Cu0.064Zr0.024?7.4/nano-Fe powder synthesized by coating
with a 33 s reaction time ?b?.
XRD of uncoated Sm?Co0.699Fe0.213Cu0.064Zr0.024?7.4 powder
average particle sizeof21 ?m
?a?
andcomposite
07E120-2 Marinescu et al. J. Appl. Phys. 103, 07E120 ?2008?
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Page 3

the micelles that incorporate the core magnet powder. This
technique produced composite powder with a slightly higher
4?Mrand 4?M18 kOeand a better profile of the demagneti-
zation curve compared to the “in volume” process for the
same reaction time. By doubling the weight ratio of FeCl2to
the core powder, compared to the above experiments,
4?M18 kOedoes not exceed the values obtained through a
longer reaction time, whereas the demagnetization curve
shows a pronounced kink. We noticed that this later type of
reaction produced ?as observed with SEM and not shown
here? a larger amount of free Fe nanoparticles not attached to
the powder substrate and not removed after washing. Ag-
glomeration of these free nanoparticles results in a lack of
magnetic coupling with the hard magnetic core and therefore
the kink on the demagnetization curve ?4? in Fig. 4.
One should mention that the only slight increase in the
magnetization for all the presented coating protocols is
strictly related to the small amount of Fe coating, determined
by the specific surface area of the core powder. Smaller par-
ticle size of the core powder will allow for a larger volume
fraction of the Fe nanoparticle coating.
A direct comparison in terms of the magnetic hysteresis
between Fe nanoparticle
Sm?Co0.699Fe0.213Cu0.064Zr0.024?7.4 powder and mechanical
blending of the same hard magnetic powder with Fe nano-
particles ?commercially available with average particle size
of 100 nm? is given in Fig. 5. The amount of added Fe was
3.5 wt % for both cases. Mechanical blending was done in
hexane assisted by a surfactant ?oleic acid? in order to dis-
perse more uniformly the powder particles and avoid ag-
glomeration. One can see that, even if, with the Fe addition,
4?M18 kOereaches the same value, the profile of the demag-
netization curve is deteriorated in the case of mechanical
blending.By increasingto
Fe nanoparticlesmechanically
Sm?Co0.699Fe0.213Cu0.064Zr0.024?7.4powder, a high 4?M18 kOe
coating of
10 wt % the amount
with
of
the blended
of 12 kG and 4?Mrof 11.3 kG were obtained although the
demagnetization curve shows a magnetically uncoupled sys-
tem.
IV. CONCLUSION
Anisotropic composite Sm?Co0.699Fe0.213Cu0.064Zr0.024?7.4
nano-Fe powders were synthesized by depositing on aniso-
tropic hard magnetic Sm?Co0.699Fe0.213Cu0.064Zr0.024?7.4core
powder, soft magnetic Fe nanoparticles produced by the
chemical reduction of FeCl2with NaBH4. Optimized depo-
sition conditions lead to improved magnetic properties com-
pared to the uncoated powder.
ACKNOWLEDGMENTS
This work was supported by DoD through AFOSR Con-
tract No. FA9550-07-C-0029. One of the authors ?M.M.? ac-
knowledges the contribution of Dr. A. Gabay of University
of Delaware with SEM and XRD investigations and Steve
Kodat of Electron Energy Corporation for the VSM measure-
ments.
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FIG. 4. Magnetic hysteresis for uncoated Sm?Co0.699Fe0.213Cu0.064Zr0.024?7.4
and composite Sm?Co0.699Fe0.213Cu0.064Zr0.024?7.4/nano-Fe powder synthe-
sized by: ?1? a 33 s reaction time ?2.5 wt % Fe nanoparticles?; ?2? idem, by
using the microemulsion technique with Si oil ?3.2 wt % Fe nanoparticles?;
?3? a 180 s reaction time ?3.5 wt % Fe nanoparticles?; ?4? multiplying by 2
the weightratio between FeCl2
nanoparticles?.
andcore powder
?3.5 wt % Fe
FIG. 5. Magnetic hysteresis for uncoated Sm?Co0.699Fe0.213Cu0.064Zr0.024?7.4
powder and ?1? composite Sm?Co0.699Fe0.213Cu0.064Zr0.024?7.4/nano-Fe pow-
der synthesized by a 180 s reaction time with 3.5 wt % of Fe coating; ?2?
Sm?Co0.699Fe0.213Cu0.064Zr0.024?7.4
powder
3.5 wt % Fe nanoparticles; and ?3? blended with 10 wt % Fe nanoparticles.
mechanicallyblendedwith
07E120-3Marinescu et al. J. Appl. Phys. 103, 07E120 ?2008?
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