Nanocomposite structures composed of ferromagnetic particles dispersed in a matrix are systems in which the magnetic properties can be tailored by varying the size and spacing of the ferromagnetic particles. Nanocomposites of SmCo5 in a non-magnetic Nb0.33Cr0.67 matrix exhibit a wide variety of magnetic properties. SmCo5 powder is premilled prior to mechanical alloying. The premilliing results in a maximum coercivity of 16 kOe after 2 hours of milling, and an enhanced remanence ratio. Both features may be due to exchange anisotropy and/or exchange coupling between hard and soft ferromagnetic phases. The nanocomposite samples show that, when the SmCo5 particulates are small enough, the primary effect of alloying is to disperse them throughout the matrix with no further refinement of size

Axtell, Steven C.

Knight, John

Leslie-Pelecky, Diandra

Schalek, Richard L.

Sellmyer, David J.

English

DigitalCommons@University of Nebraska

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln David Sellmyer Publications Research Papers in Physics and Astronomy November 1995 Tailoring of the magnetic properties of SmCo5:Nb0.33Cr0.67 nanocomposites using mechanical alloying Richard L. Schalek University of Nebraska - Lincoln Diandra Leslie-Pelecky University of Nebraska - Lincoln, diandra2@unl.edu John Knight University of Nebraska - Lincoln David J. Sellmyer University of Nebraska-Lincoln, dsellmyer@unl.edu Steven C. Axtell University of Nebraska - Lincoln Follow this and additional works at: https://digitalcommons.unl.edu/physicssellmyer  Part of the Physics Commons Schalek, Richard L.; Leslie-Pelecky, Diandra; Knight, John; Sellmyer, David J.; and Axtell, Steven C., "Tailoring of the magnetic properties of SmCo5:Nb0.33Cr0.67 nanocomposites using mechanical alloying" (1995). David Sellmyer Publications. 93. https://digitalcommons.unl.edu/physicssellmyer/93 This Article is brought to you for free and open access by the Research Papers in Physics and Astronomy at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in David Sellmyer Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. 3112 IEEE TRANSACTIONS ON MAGNETICS, VOL. 31, NO 6, NOVEMBER 1995 of the Magnetic Properties of SmCO~:Nb0.33Cr0.67 Nanocomposites Using Mechanical Alloying Richard L. Schalek’, Diandra L. Leslie-Pelecky’, John Knight’, D.J. Sellmyer’ and Steven C. Axtell’ Center for Materials Research and Analysis, University of Nebraska, Lincoln, NE 68588-01 13 Abstract-Nanocomposite structures composed of ferromagnetic particles dispersed in a matrix are systems in which the magnetic properties can be tailored by varying the size and spacing of the ferromagnetic particles. Nanocompo$i~es of SmCos in a non-magnetic Nb0.dk8.67 matrix exhibit a wide variety of magnetic properties. SmCos powder is premilled prior to mechanical alloying. The premilliing results in a maximum coercivity of 16 kOe after 2 hours of milling, and an enhanced remanence ratio. Both features may be due to exchange anisotropy and/or exchange coupling between hard and soft ferromagnetic phases. The nanocomposite samples show that, when the SmCos particulates are small enough, the primary effect of alloying is to disperse them throughout the matrix with no further refinement of size. I. INTRODUCTION Mechanical alloying is a powefil fabrication technique that provides the ability to tailor the nanostructure of materials. The intricate linkage between nanostructure and magnetism in turn allows tuning of the magnetic properties of the material to a desired set of specifications by varying the magnetic particle size and spacing between magnetic particles, The first of these parameters is varied by premilling the magnetic powder; the second, by varying the concentration of magnetic material in the finished product. Surface effects are enhanced by the large fraction of interfaces in mechanically alloyed systems. The complex phase diagram of the Sm-Co system makes producing single-phase stoichiometric SmCos challenging without annealing subsequent to fabrication. [ 1,2] We chose to start by pre-milling commercially purchased crystalline SmCos powders (-100 mesh, 99.9% purity) in a SPEX 8000 mixedmill with WC-lined vials and three 12 g-WC balls. All sample handling, including the milling, was done in an argon atmosphere to reduce oxidation. Throughout this paper, the term ‘milling’ is used to describe processing of the SmCos powder and ‘alloying’ is reserved for reference to the mixing of the SmCos powder and the matrix material. Paper received February 16, 1995 * Also Department of Physics and Astronomy # Also Department of Mechanical Engineering Work supported in part by the National Science Foundation through grant OSR-9255225 and the Center for Materials Research and Analysis - n o  milling 1 h r  - 2 hr 3 h r  4 h r  - 30 40  5 0  6 0  7 0  8 0  9 0  2 0  (degrees)  Fig. 1. The X-Ray diEaction spectra of premilled SmCos. 11. PREMILLED S M C O ~  Prior to mechanically alloying with Nb and Cr, the SmCoS was premilled to reduce the magnetic particle size. Premilling times ranged from 15 minutes to 30 hours. Fig. 1 indicates that the SmCos quickly becomes amorphous on the length scales probed by x-ray diffraction after minimal milling times. Peaks appearing for milling times greater than 15 hours may be indexed to the high-temperature, cubic phase of cobalt. Diffracting crystallite size as a function of milling time (shown in Fig. 2) was obtained by a Fourier analysis that separates the broadening effects of strain from those due to particle size.[3] The diffracting crystallite size becomes too small for accurate estimation at milling times greater than 4 hours. 15 I I I I , I  1 2 3 4 Milling Time (Hours) Fig. 2. Diffracting crystallite size of premilled SmCoS as a hnction of milling time. 0018-9464/95$04.00 0 1995 IEEE 3113 Milled powders were mixed with paraffin, packed in polyethylene bags and sealed in an argon atmosphere to reduce oxidation during magnetic measurements. The paraffin prevents magnetization reversal via rotation of the entire particle. Samples were measured in both unaligned and aligned configurations. For the aligned configuration, the paraffin was melted, a magnetic field of 55 kOe was applied and the sample cooled in the field to solidify the paraffin. Hysteresis loops of premilled SmCos were studied as functions of premilling times to facilitate understanding of the effect of the nanocomposite structure relative to free nanoscale particles. Fig. 3 shows the coercivity, %, and remanence ratio, MJMs, for premilled SmCos as a function of milling time. Error bars (omitted for clarity) are smaller than the symbol size. Open symbols represent samples measured in an unaligned state while solid symbols represent measurements of aligned powders. H, rises rapidly with milling time, increasing by a factor of three after just 15 minutes of milling. The maximum coercivity of 16 kOe is achieved after 2 hours of milling. This corresponds to a diffracting crystallite size of about 6 nm. Coincident with the rapid increase in W, is a decrease in the remanence ratio from a high of 0.90 to approximately 0.70. H, does not significantly depend on whether the samples are measured in the aligned or unaligned states for the milled samples. The remanence shows a slight dependence on alignment, but is enhanced in both aligned and unaligned states for milling times up to 18 hours. In contrast, the remanence for aligned unmilled powders is much higher (0.80) than for unaligned unmilled powders (0.50). As shown in Fig. 3, three regimes are identifiable as a function of milling time. In the first (0-2 hours), W, increases and M m  decreases. In the second (2-16 hours), H, decreases while M,/Ms remains relatively flat, and in the third, H, continues a slow decrease and M,&& exhibits a near linear decay. The behavior of the premilled SmCo is complicated, but two likely mechanisms may be identified. (1) The hysteresis loops are shifted toward negative values on the magnetic field axis, with the magnitude of the shift exhibiting the same dependence on milling time as the coercivity. This shift is a signature of exchange anisotropy, usually due to the proximity of antiferromagnetic and ferromagnetic regions. [4] Although the material was handled carefully to avoid oxidation, there is the likelihood that a thin layer of surface oxidation (possibly COO) is present. The magnitude of the coercivities in these materials suggests that the oxidation is limited to the surface and that significant regions of ferromagnetic SmCos are present. The larger number of interfaces in mechanically milled materials may increase the effect of exchange coupling. Although we might expect this system to be similar to the Co/CoO system, the temperature dependence of the shift does not agree with the numerical forms reported in the literature. A shift upward on the positive magnetization axis is also observed at low temperatures. The hysteresis loop displays a large reversible component for fields less than the coercive field. This behavior is also seen in ‘exchange-spring’ magnets due to exchange coupling of regions of hard and soft ferromagnetic material[5]. X-Ray diffraction scans of samples milled for longer times indicate the presence of cubic Co. Smaller quantities of CO may be present at shorter milling times that would produce the hardsofl ferromagnetic regions necessary for exchange coupling. Exchange coupling would also explain the enhanced remanence at room temperature. Again, the larger number of interfaces would act to enhance this effect. (2) 111. NANOCOMPOSITE SMCO~:NB,, 3 3 c ~  67 2 0 , .  I . ,  . I . , .  I . ,  1.0 J (g 0.8 0.6 0.4 0.2 0.0 a c 4 L 5 I O  ? 5  20 25 30 Milling Time (Hrs) Fig 3: The remanence and coercivity as a function of milling time for SmCos powders Open symbols represent powders measured in the unaligned state, while solid symbols represent powders measured in an aligned state. Nanocomposites can be made with either non-magnetic or magnetic matrix materials, depending on the properties desired of the final material. By varying the magnetic character of the component materials, a wide range of final magnetic properties may be achieved. Our chosen matrix material is Nb33Cr67. Mechanical alloying of Nb33Cr67 for extended periods of time results in an amorphous structure which, upon annealing at a sufficiently high temperature, crystallizes as NbCr2. This crystallization temperature is expected to be higher than that of amorphous (or nanocrystalline) SmCo5, making it an ideal matrix material for annealing experiments. The SmCos is 6.25 vol.% of the final composite, which should result in well-isolated particles with limited magnetic interaction. Measurements of nanocomposites with two pre-milling times (2 hours and 15 hours) are reported here. The 2-hour premill has the maximum coercivity (16 kOe) observed; however, at some temperatures, the samples are not saturated at the 3114 maximum available field of 55 kOe and the prominence of the exchange anisotropy and/or exchange-spring effects may complicate interpretation of results. In contrast, the 15-hour premill (which has a coercivity of 3.4 kOe) was chosen due to the lack of a significant anisotropy shift and the presence of complete saturation of the magnetization. X-Ray diffraction measurements of the nanocomposites show only peaks from crystallites of Nb and Cr. Fig. 4 shows the percent change in the coercivity from the value of the premilled SmCos as a function of alloying time. %change = H, (premilled SmCo5 - Hc (nanocomposite) Hc (premilled SmCo5 ) Data from the 15-hour premilled samples are shown as squares, with solid symbols representing aligned measurements and open symbols representing unaligned measurements. Circular symbols represent the 2-hour premilled samples. The coercivity in the 15-hour premilled samples increases briefly before decreasing, whereas the &hour premilled samples show no initial increase. The rate of decrease in H, is less for both than for SmCoS samples premilled for a comparable total time. Comparison of SmCoS milled for 16 hours with the 15-hour pre-millll-hour alloy samples shows that the S m C ~ ~ : N b ~ ~ c r ~ ~  nano omposite has a slightly higher coercivity and remanence than the premilled SmCo5. For both samples, the remanence behaves analogously to the coercivity. The remanence decreases, but at a slower rate than if the SmCo5 had continued to be premilled by itself. The slow rate of decrease of the 15-hour premilled samples indicates that the primary effect of mechanical alloying in these samples is dispersion of the SmCoS particles in the matrix. The faster rate of decrease in 0 t o  0 U -60 I ? ( ,  I , I , f ! f , I  O 0 5 10 15 20 25 30 Alloying Time (Hours) Fig. 4. Percent change in the coercivity of nanocomposited samples as defined in the text 15-hour preinilled samples are shown as squares and 2-hour premilled samples as circles Aligned measurements are shown as solid symbols and unaligned measurements as open symbols. the &hour premilled samples suggests that some refinement of SmCos particle size is still occurring. The values of the coercivity and the remanence ratio for both types of samples are not sigmficantly affected by exposure to air prior to measurement. The saturation magnetization decreases to 90% of the value for samples not exposed to air. IV. CONCLUSION The coercivity of mechanically milled SmCos increases by a factor of five (to 16 kOe) during the first two hours of milling, folIowed by a slow decrease. The magnitudes of the coercivities indicate that signifkant regions of SmCo5 are present, despite the likelihood of small amounts of oxidation. An enhanced remanence ratio independent of particle alignment is also observed. These effects may be a result of exchange anisotropy due to ferromagnetic-antiferromagnetic interactions (most likely due to small amounts of COO) and/or exchange-spring phenomena due to exchange coupling of hard and soft ferromagnetic regions (SmCo/Co). The large number of surfaces present in mechanically alloyed materials would enhance both effects. Nanocomposites formed by mechanically alloying premilled SmCos with a powder mixture of Nb33Cr67 show that, when the ferromagnetic particles Are small enough, the primary effect of alloying is to disperse the particles throughout the matrix material. Judicious selection of matrix material, premilling time and magnetic particle concentration can be used to tailor the magnetic properties of the final material to the desired specifications. The NbCr matrix material used here allows handling in air without significantly compromising the magnetic properties of the ultrafine rare earth powder. Proper selection of matrix materials provides an additional variable for tailoring by allowing controlled annealing. REFERENCES [I] Yinong Liu, MP.  Dallimore, P.G. McCormick and T. Alonso, “High coercivity SmCoS synthesized by chemical reduction during mechanical alloying”, J. Magn. Magn. Mat. 116, L320 (1992) [2] J. Ding P G. McCormick and R. Street, “Structure and magnetic properties of mcchanicaily alloyed Sm,Co,.,”, J. Alloys and Compounds 191, 197 (1993) [3] P. Ganesan, H. K. Kuo, A. Saavedra and R.J DeAngelis, “Particle size distribution fimction of supported metal catalysts by x-ray diffraction” J. Catalysis 52, 310 (1978) [4] J.S. Jacobs and C.P. Bean, “Fine particles, thin films and exchange anisotropy”, in Mugneflsm, vol. 111, G T. Rad0 and H. Suhl, E& New York, Academic, 1963 pp 271-350 (and references therein) [Sf Eckhait F. KnelIer, “The exchange-spring magnet: A new material principle for permanent magnets”, IEEE Trans. Magn. 27, 3588 (1991) 

Tailoring of the magnetic properties of SmCo\u3csub\u3e5\u3c/sub\u3e:Nb\u3csub\u3e0.33\u3c/sub\u3eCr\u3csub\u3e0.67\u3c/sub\u3e nanocomposites using mechanical alloying

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University of Nebraska - Lincoln 
DigitalCommons@University of Nebraska - Lincoln 
David Sellmyer Publications Research Papers in Physics and Astronomy 
November 1995 
Tailoring of the magnetic properties of SmCo5:Nb0.33Cr0.67 
nanocomposites using mechanical alloying 
Richard L. Schalek 
University of Nebraska - Lincoln 
Diandra Leslie-Pelecky 
University of Nebraska - Lincoln, diandra2@unl.edu 
John Knight 
University of Nebraska - Lincoln 
David J. Sellmyer 
University of Nebraska-Lincoln, dsellmyer@unl.edu 
Steven C. Axtell 
University of Nebraska - Lincoln 
Follow this and additional works at: https://digitalcommons.unl.edu/physicssellmyer 
 Part of the Physics Commons 
Schalek, Richard L.; Leslie-Pelecky, Diandra; Knight, John; Sellmyer, David J.; and Axtell, Steven C., 
"Tailoring of the magnetic properties of SmCo5:Nb0.33Cr0.67 nanocomposites using mechanical alloying" 
(1995). David Sellmyer Publications. 93. 
https://digitalcommons.unl.edu/physicssellmyer/93 
This Article is brought to you for free and open access by the Research Papers in Physics and Astronomy at 
DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in David Sellmyer Publications 
by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. 
3112 IEEE TRANSACTIONS ON MAGNETICS, VOL. 31, NO 6, NOVEMBER 1995 
of the Magnetic Properties of SmCO~:Nb0.33Cr0.67 Nanocomposites Using 
Mechanical Alloying 
Richard L. Schalek’, Diandra L. Leslie-Pelecky’, John Knight’, D.J. Sellmyer’ and Steven C. Axtell’ 
Center for Materials Research and Analysis, University of Nebraska, Lincoln, NE 68588-01 13 
Abstract-Nanocomposite structures composed of 
ferromagnetic particles dispersed in a matrix are systems in 
which the magnetic properties can be tailored by varying the 
size and spacing of the ferromagnetic particles. 
Nanocompo$i~es of SmCos in a non-magnetic Nb0.dk8.67 
matrix exhibit a wide variety of magnetic properties. SmCos 
powder is premilled prior to mechanical alloying. The 
premilliing results in a maximum coercivity of 16 kOe after 
2 hours of milling, and an enhanced remanence ratio. Both 
features may be due to exchange anisotropy and/or exchange 
coupling between hard and soft ferromagnetic phases. The 
nanocomposite samples show that, when the SmCos 
particulates are small enough, the primary effect of alloying is 
to disperse them throughout the matrix with no further 
refinement of size. 
I. INTRODUCTION 
Mechanical alloying is a powefil fabrication technique 
that provides the ability to tailor the nanostructure of 
materials. The intricate linkage between nanostructure and 
magnetism in turn allows tuning of the magnetic properties 
of the material to a desired set of specifications by varying 
the magnetic particle size and spacing between magnetic 
particles, The first of these parameters is varied by 
premilling the magnetic powder; the second, by varying the 
concentration of magnetic material in the finished product. 
Surface effects are enhanced by the large fraction of 
interfaces in mechanically alloyed systems. 
The complex phase diagram of the Sm-Co system makes 
producing single-phase stoichiometric SmCos challenging 
without annealing subsequent to fabrication. [ 1,2] We chose 
to start by pre-milling commercially purchased crystalline 
SmCos powders (-100 mesh, 99.9% purity) in a SPEX 8000 
mixedmill with WC-lined vials and three 12 g-WC balls. 
All sample handling, including the milling, was done in an 
argon atmosphere to reduce oxidation. Throughout this 
paper, the term ‘milling’ is used to describe processing of 
the SmCos powder and ‘alloying’ is reserved for reference to 
the mixing of the SmCos powder and the matrix material. 
Paper received February 16, 1995 
* Also Department of Physics and Astronomy 
# Also Department of Mechanical Engineering 
Work supported in part by the National Science Foundation 
through grant OSR-9255225 and the Center for Materials Research 
and Analysis 
- 
n o  milling 
1 h r  - 
2 hr 
3 h r  
4 h r  - 
30 40  5 0  6 0  7 0  8 0  9 0  
2 0  (degrees)  
Fig. 1. The X-Ray diEaction spectra of premilled SmCos. 
11. PREMILLED S M C O ~  
Prior to mechanically alloying with Nb and Cr, the 
SmCoS was premilled to reduce the magnetic particle size. 
Premilling times ranged from 15 minutes to 30 hours. Fig. 1 
indicates that the SmCos quickly becomes amorphous on the 
length scales probed by x-ray diffraction after minimal 
milling times. Peaks appearing for milling times greater 
than 15 hours may be indexed to the high-temperature, cubic 
phase of cobalt. 
Diffracting crystallite size as a function of milling time 
(shown in Fig. 2) was obtained by a Fourier analysis that 
separates the broadening effects of strain from those due to 
particle size.[3] The diffracting crystallite size becomes too 
small for accurate estimation at milling times greater than 
4 hours. 
15 
I I 
I I , I  
1 2 3 4 
Milling Time (Hours) 
Fig. 2. Diffracting crystallite size of premilled SmCoS as a hnction of 
milling time. 
0018-9464/95$04.00 0 1995 IEEE 
3113 
Milled powders were mixed with paraffin, packed in 
polyethylene bags and sealed in an argon atmosphere to 
reduce oxidation during magnetic measurements. The 
paraffin prevents magnetization reversal via rotation of the 
entire particle. Samples were measured in both unaligned 
and aligned configurations. For the aligned configuration, 
the paraffin was melted, a magnetic field of 55 kOe was 
applied and the sample cooled in the field to solidify the 
paraffin. 
Hysteresis loops of premilled SmCos were studied as 
functions of premilling times to facilitate understanding of 
the effect of the nanocomposite structure relative to free 
nanoscale particles. Fig. 3 shows the coercivity, %, and 
remanence ratio, MJMs, for premilled SmCos as a function 
of milling time. Error bars (omitted for clarity) are smaller 
than the symbol size. Open symbols represent samples 
measured in an unaligned state while solid symbols 
represent measurements of aligned powders. H, rises rapidly 
with milling time, increasing by a factor of three after just 
15 minutes of milling. The maximum coercivity of 16 kOe is 
achieved after 2 hours of milling. This corresponds to a 
diffracting crystallite size of about 6 nm. Coincident with the 
rapid increase in W, is a decrease in the remanence ratio 
from a high of 0.90 to approximately 0.70. H, does not 
significantly depend on whether the samples are measured in 
the aligned or unaligned states for the milled samples. The 
remanence shows a slight dependence on alignment, but is 
enhanced in both aligned and unaligned states for milling 
times up to 18 hours. In contrast, the remanence for aligned 
unmilled powders is much higher (0.80) than for unaligned 
unmilled powders (0.50). 
As shown in Fig. 3, three regimes are identifiable as a 
function of milling time. In the first (0-2 hours), W, 
increases and M m  decreases. In the second (2-16 hours), 
H, decreases while M,/Ms remains relatively flat, and in the 
third, H, continues a slow decrease and M,&& exhibits a 
near linear decay. 
The behavior of the premilled SmCo is complicated, but 
two likely mechanisms may be identified. (1) The 
hysteresis loops are shifted toward negative values on the 
magnetic field axis, with the magnitude of the shift 
exhibiting the same dependence on milling time as the 
coercivity. This shift is a signature of exchange anisotropy, 
usually due to the proximity of antiferromagnetic and 
ferromagnetic regions. [4] Although the material was 
handled carefully to avoid oxidation, there is the likelihood 
that a thin layer of surface oxidation (possibly COO) is 
present. The magnitude of the coercivities in these materials 
suggests that the oxidation is limited to the surface and that 
significant regions of ferromagnetic SmCos are present. The 
larger number of interfaces in mechanically milled materials 
may increase the effect of exchange coupling. Although we 
might expect this system to be similar to the Co/CoO system, 
the temperature dependence of the shift does not agree with 
the numerical forms reported in the literature. A shift 
upward on the positive magnetization axis is also observed at 
low temperatures. 
The hysteresis loop displays a large reversible 
component for fields less than the coercive field. This 
behavior is also seen in ‘exchange-spring’ magnets due to 
exchange coupling of regions of hard and soft ferromagnetic 
material[5]. X-Ray diffraction scans of samples milled for 
longer times indicate the presence of cubic Co. Smaller 
quantities of CO may be present at shorter milling times that 
would produce the hardsofl ferromagnetic regions necessary 
for exchange coupling. Exchange coupling would also 
explain the enhanced remanence at room temperature. 
Again, the larger number of interfaces would act to enhance 
this effect. 
(2) 
111. NANOCOMPOSITE SMCO~:NB,, 3 3 c ~  67 2 0 , .  I . ,  . I . , .  I . ,  
1.0 
J (g 0.8 
0.6 
0.4 
0.2 
0.0 
a 
c 4 
L 
5 I O  ? 5  20 25 30 
Milling Time (Hrs) 
Fig 3: The remanence and coercivity as a function of milling 
time for SmCos powders Open symbols represent powders 
measured in the unaligned state, while solid symbols represent 
powders measured in an aligned state. 
Nanocomposites can be made with either non-magnetic 
or magnetic matrix materials, depending on the properties 
desired of the final material. By varying the magnetic 
character of the component materials, a wide range of final 
magnetic properties may be achieved. 
Our chosen matrix material is Nb33Cr67. Mechanical 
alloying of Nb33Cr67 for extended periods of time results in 
an amorphous structure which, upon annealing at a 
sufficiently high temperature, crystallizes as NbCr2. This 
crystallization temperature is expected to be higher than that 
of amorphous (or nanocrystalline) SmCo5, making it an ideal 
matrix material for annealing experiments. The SmCos is 
6.25 vol.% of the final composite, which should result in 
well-isolated particles with limited magnetic interaction. 
Measurements of nanocomposites with two pre-milling times 
(2 hours and 15 hours) are reported here. The 2-hour premill 
has the maximum coercivity (16 kOe) observed; however, at 
some temperatures, the samples are not saturated at the 
3114 
maximum available field of 55 kOe and the prominence of 
the exchange anisotropy and/or exchange-spring effects may 
complicate interpretation of results. In contrast, the 15-hour 
premill (which has a coercivity of 3.4 kOe) was chosen due 
to the lack of a significant anisotropy shift and the presence 
of complete saturation of the magnetization. X-Ray 
diffraction measurements of the nanocomposites show only 
peaks from crystallites of Nb and Cr. 
Fig. 4 shows the percent change in the coercivity from 
the value of the premilled SmCos as a function of alloying 
time. 
%change = 
H, (premilled SmCo5 - Hc (nanocomposite) 
Hc (premilled SmCo5 ) 
Data from the 15-hour premilled samples are shown as 
squares, with solid symbols representing aligned 
measurements and open symbols representing unaligned 
measurements. Circular symbols represent the 2-hour 
premilled samples. 
The coercivity in the 15-hour premilled samples 
increases briefly before decreasing, whereas the &hour 
premilled samples show no initial increase. The rate of 
decrease in H, is less for both than for SmCoS samples 
premilled for a comparable total time. Comparison of SmCoS 
milled for 16 hours with the 15-hour pre-millll-hour alloy 
samples shows that the S m C ~ ~ : N b ~ ~ c r ~ ~  nano omposite has 
a slightly higher coercivity and remanence than the 
premilled SmCo5. For both samples, the remanence behaves 
analogously to the coercivity. The remanence decreases, but 
at a slower rate than if the SmCo5 had continued to be 
premilled by itself. The slow rate of decrease of the 15-hour 
premilled samples indicates that the primary effect of 
mechanical alloying in these samples is dispersion of the 
SmCoS particles in the matrix. The faster rate of decrease in 
0 
t o  0 U 
-60 I ? ( ,  I , I , f ! f , I  O 
0 5 10 15 20 25 30 
Alloying Time (Hours) 
Fig. 4. Percent change in the coercivity of nanocomposited samples 
as defined in the text 15-hour preinilled samples are shown as 
squares and 2-hour premilled samples as circles Aligned 
measurements are shown as solid symbols and unaligned 
measurements as open symbols. 
the &hour premilled samples suggests that some refinement 
of SmCos particle size is still occurring. The values of the 
coercivity and the remanence ratio for both types of samples 
are not sigmficantly affected by exposure to air prior to 
measurement. The saturation magnetization decreases to 
90% of the value for samples not exposed to air. 
IV. CONCLUSION 
The coercivity of mechanically milled SmCos increases 
by a factor of five (to 16 kOe) during the first two hours of 
milling, folIowed by a slow decrease. The magnitudes of the 
coercivities indicate that signifkant regions of SmCo5 are 
present, despite the likelihood of small amounts of oxidation. 
An enhanced remanence ratio independent of particle 
alignment is also observed. These effects may be a result of 
exchange anisotropy due to ferromagnetic-antiferromagnetic 
interactions (most likely due to small amounts of COO) 
and/or exchange-spring phenomena due to exchange 
coupling of hard and soft ferromagnetic regions (SmCo/Co). 
The large number of surfaces present in mechanically 
alloyed materials would enhance both effects. 
Nanocomposites formed by mechanically alloying 
premilled SmCos with a powder mixture of Nb33Cr67 show 
that, when the ferromagnetic particles Are small enough, the 
primary effect of alloying is to disperse the particles 
throughout the matrix material. Judicious selection of matrix 
material, premilling time and magnetic particle 
concentration can be used to tailor the magnetic properties of 
the final material to the desired specifications. The NbCr 
matrix material used here allows handling in air without 
significantly compromising the magnetic properties of the 
ultrafine rare earth powder. Proper selection of matrix 
materials provides an additional variable for tailoring by 
allowing controlled annealing. 
REFERENCES 
[I] Yinong Liu, MP.  Dallimore, P.G. McCormick and T. Alonso, “High 
coercivity SmCoS synthesized by chemical reduction during mechanical 
alloying”, J. Magn. Magn. Mat. 116, L320 (1992) 
[2] J. Ding P G. McCormick and R. Street, “Structure and magnetic properties 
of mcchanicaily alloyed Sm,Co,.,”, J. Alloys and Compounds 191, 197 (1993) 
[3] P. Ganesan, H. K. Kuo, A. Saavedra and R.J DeAngelis, “Particle size 
distribution fimction of supported metal catalysts by x-ray diffraction” 
J. Catalysis 52, 310 (1978) 
[4] J.S. Jacobs and C.P. Bean, “Fine particles, thin films and exchange 
anisotropy”, in Mugneflsm, vol. 111, G T. Rad0 and H. Suhl, E& New York, 
Academic, 1963 pp 271-350 (and references therein) 
[Sf Eckhait F. KnelIer, “The exchange-spring magnet: A new material 
principle for permanent magnets”, IEEE Trans. Magn. 27, 3588 (1991) 


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Tailoring of the magnetic properties of SmCo\u3csub\u3e5\u3c/sub\u3e:Nb\u3csub\u3e0.33\u3c/sub\u3eCr\u3csub\u3e0.67\u3c/sub\u3e nanocomposites using mechanical alloying

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