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Magnetic hysteresis in Fe-R-B powders
Journal of Magnetism and Magnetic Materials 71 (1988) 235-239 North-Holland, Amsterdam
235
MAGNETIC HYSTERESIS IN F e - R - B POWDERS G.C. H A D J I P A N A Y I S and C.N. C H R I S T O D O U L O U Department of Physics, Kansas State University, Cardwell Hal~ Manhattan, KS 66506, USA
Received 29 May 1986; in revised form 7 April 1987
The hard magnetic properties of Fe-R-B powders have been examined using various magnetic techniques. High coercivities were obtained in fine powders with a particle size around 1 )xm. A domain wall pinning process is suggested for the less anisotropie, Nd-containing samples where the domain walls are pinned at the particles surface. The initial magnetization curves, field dependence of coercivity and remanenee curves of thermally demagnetized samples change significantly after ac and dc demagnetization indicating a more uniform domain wail pinning process in the ae and de demagnetized samples. A magnetization rotation reversal process is suggested for the more anisotropie, heavy rare-earth containing powders.
1. Introduction The hard magnetic properties of F e - R - B alloys were first shown [1] on melt-spun samples where coercivities in excess of 15 kOe were obtained. These properties have been attributed to the highly anisotropic Fe14R2B phase [2] which has a tetragonal structure with the c-axis as the easy axis of magnetization for most F e - R - B alloys at r o o m temperature [3,4]. The potential of these alloys for permanent magnet development was soon exploited and presently commercial sintered magnets with energy products up to 50.6 M G O e are readily available [5,6]. Recently, we have also obtained high coercivities in F e - R - B powders [7] and their magnetic characteristics indicated a magnetization reversal process different from that of melt-spun alloys. In the present study we examine the hard magnetic properties of these powders in more details in an attempt to elucidate the origin of magnetic hardening.
2. Experimental As-cast samples with composition Fe77Nd15_ x TbxB s where 0 < x < 15 were prepared by arcmelting the pure constituents (at least 99.9%) un-
der argon atmosphere. Fine powders were prepared by first crushing the bulk samples to coarse powders and then ball-milling the powders under toluene using an attritor. Samples were chosen at different milling times and the coercivity was measured as a function of particle size. The average size of the particles was measured with a scanning electron microscope (SEM). The magnetic properties were measured with a vibrating sample magnetometer (VSM) in the temperature range of 4.2-800 K and in fields up to 18 kOe. The remanence curves M R ( H ) were obtained for samples with the following three different magnetic histories: (i) on thermally demagnetized samples, (ii) on samples first magnetized in the m a x i m u m forward field and then demagnetized by applying a field close to the remanent coercivity (de demagnetized) and (iii) on samples magnetized at the m a x i m u m field and then demagnetized by reducing the amplitude of an applied alternating field (ac demagnetized). The remanence curves were measured as follows: The initial samples with zero magnetization were subjected to an applied field which subsequently was reduced to zero and the remanence was measured. This was repeated for different higher fields up to the m a x i m u m field available. The same procedure was also applied for reversed fields and the whole remanence loop M R ( H ) was obtained.
0304-8853/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
236
(7,.C. Hadfipanayis, C.N. Christodoulou / Magnetic hysteresis in Fe- R - B powders
3. Results and discussion
Table 1 Size dependence of coercivity
3.1. Coercioity m e a s u r e m e n t s
Composition
M i l l i n g A v e r a g e Coercivity time (rain) particle alignedin wax size (pm) (kOe)
Fe77Nd15B8
0.00 2.50 55.00
30.00 4.50 0.60
0.70 1.50 4.70
Fe77NdxoTb5B8
0.00 3.00 30.00
30.00 4.00 2.80
3.50 9.30 12.40
Fe77NdsTb10B8
0.00 2.00 30.00
33.30 3.60 0.70
5.50 12.90 16.70
0.00
27.80 0.90 0.80
6.90 15.20 > 16.80
The coercivity of powders was found to increase with milling time, going through a maxim u m in some samples (fig. 1). This shows a strong dependence of coercivity on particle size (table 1) in contrast to previous studies [8]. The optimum coercivity was observed in powders with a particle size close to that of single domain. The single domain particle size for F e - N d - B is estimated [9] to be 0.3 ~ m while in F e - T b - B it is believed to be about 5 ixm. The decrease in coercivity for some of the very fine powders is attributed to oxidation problems and not to superparamagnetism. The superparamagn, etic particle size is estimated to be less than 35 A according to Livingston [10]. The oxidization problem has also been observed in
°aligned in wax • Pressed 48kPSi
86 ~4 2 I
I
J
40
0
I
I
80 t (rain)
l
4.00 30.00
sintered F e - R - B magnets where the coercivity of very small sample pieces was significantly reduced. The increase in coereivity with decreasing particle size could be explained with nucleation type processes where the density of defects decreases with the decreasing particle size. However, etching experiments (fig. 2) did show a decrease in H c and this rather indicates a domain wall pinning process at the surface of the particles. The possibility of oxidation of powders during etching is presently being examined. The domain wall pinning hypothesis is further suggested by the angle dependence of He (fig. 3). The coercivity of sampies measured at an angle to the easy axis is
Fe77 Ndl 5B8
8
Fe77TblsB8
f
120
20
E~hing of F..e..~%T% powder (3/o HNO3)
~15 0 e 77NdsTb10B8
10
b
10
~.
44r
~6 4 5 0
I
I
10
I
I
20
I
I
30 t (rnin)
I
I
t
I
2
40
Fig. 1. Coercivity as a function of milling time in Fe-R-B magnets; (i) less anisotropic, (ii) more anisotropic.
Etching time(rain) Fig. 2. Dependence of coercivity on etching time.
237
G.C. Hadjipanayis, C.N. Christodoulou / Magnetic hysteresis in F e - R - B powders
16
As $intered NEO 3OH
12
T I
3b 0 6b 9'0 n
Fig. 3. Angular dependence of coercivity in Fe77NdloTbsB8.
I
increased, consistent with the 1 / c o s 0 dependence of H c due to domain wall pinning [11]. For the heavy rare-earth rich samples, the particle size for maximum Hc is closer to that of single domain size and in this case a magnetization rotation process might be more predominant. In all cases the coercivity is found to increase with the amount of Tb which is consistent with the fact that the magnetic anisotropy constant of F e - T b - B is much higher than that of F e - N d - B [4]. For the Nd rich samples, the coercivities of pressed powdered samples are lower than those of powders aligned on wax (fig. la) because of magnetostatic interactions, H e = H 0 ( 1 - p ) , p is the packing fraction [12]. However, for the heavy rare-earth rich samples the opposite effect is observed (fig. lb). We do not know yet whether this effect is intrinsic or is due to sample preparation. It could be that for the finer particles the applied pressures used cause misalignment of grains and this would lead to higher coercivities as explained before. Magnetization measurements showed a decrease in remanence with applied pressure that is characteristic of grain misalignment. Further studies are in progress to clarify this issue. Fine powders of Neo 30H (a commercial sintered magnet with composition F e - N d ( D y ) - B ) were also prepared and their coercivity was measured (fig 4). The initial steep decrease in H~ is possibly related to stresses introduced in the grains during the grinding process. The subsequent increase in He with milling is due to the finer size of grains. This behavior clearly indicates two different magnetic hardening mechanisms for sintered magnets and powders. This difference is further suggested by the temperature dependence of Hc
o
20
,
I
I
40 60 Milling time(min)
80
Fig. 4. Size dependence of coercivity in Neo 30H.
which for powders is almost linear in contrast to t h e T 4/3 dependence observed in sintered magnets [13].
3.2. Initial magnetization curves- hysteresis loops The initial magnetization curves with the corresponding hysteresis loops are shown in figs. 5 and 6. It is obvious that in the thermally demagnetized samples the domain walls move freely inside the grains resulting in a drastic increase of magnetization at relatively low fields. However, when the samples are demagnetized after they are initially magnetized the "effective initial" curve is different and it lies below that of the thermally demagnetized sample. This indicates that after the appli-
ac dernagn.
~
~
,
Fig. 5. Initial curves and hysteresis loops of F e - R - B powders (Neo 35).
G.C. Hadjipanayis, C.N. Christodoulou / Magnetic hysteresis in F e - R - B powders
238
Fe77Nd1c)Tb5B8
,~'..-.----'-"-~,.,. ..... .'" - ' " ~
..... therrr~liy demclgrt/~ ,' ---dc derr~gn. / ~ / --ocderr~gn. / ~-50 i/ ~
15
.-'" // ~
// /
Fe77NdIoTbsB8 - - - C o a r s e Povvderl 12 ..... FinePowder ~AC dernogn. As Sintered J
'
/
e) 15 -50
t
8
4
o
Fig. 6. Initial curves and hysteresis loops of F e - R - B powders.
/
"'"
...... ::/; 4
8
.-/"
/
//
/"
12 Ha(KOe)
16
Fig. 7. Cocrcivity as a function of applied field in Fe-R-B magnets.
cation of a field the domain wall distribution changes and some of the walls ware engaged with different and stronger pinning sites making it more difficult to completely saturate the sample. It is interesting to note the kink which appears on the "effective initial" magnetization curve of the ac demagnetized samples. Neo 35 does not contain any heavy rare-earth elements and the effect is small (fig. 5). The kink becomes a shoulder in samples containing small amounts of heavy rareearths (fig. 6). This is an indication of two types of domain wall pinning processes and it is also reflected on the H¢(H~) curves as will be seen later. It appears that after the thermally demagnetized samples are subjected to an ac field a situation is reached which strongly resembles domain wall pinning.
pinning possibly inside the grains. Saturation of H c was observed only in samples where the coercivity was much less than the maximum available field. This is the case of F e - N d - B fine powders with n s a t = 4 kOe and F e - N d - T b - B coarse powders with H~at= 11 kOe (fig. 7). The field dependence of Hc in ac demagnetized samples is consistent with domain wall pinning processes. 3.4. R e m a n e n c e curves
The remanence curves for F e - N d - T b - B powder are shown in fig. 8. Again the dc and ac demagnetized sample remanence curves are below those of thermally demagnetized samples. The shape of the curves is also very different with the steeper curve corresponding to thermally demag-
3.3. FieM dependence of coercivity The coercivity of all samples is found to be strongly dependent on applied field. In thermally demagnetized samples H¢ increases initially slowly and then more rapidly with the applied field. In ac demagnetized samples a different behavior is observed (fig. 7). The coercivity is practically negligible until a critical field where it starts increasing rather drastically. This critical field is close to the field where the shoulder appears on the magnetization curve of ac demagnetized samples. The disappearance of the slow increase in H¢ observed in thermally demagnetized samples is indicative of the fact that this corresponds to domain wall
REMANENCE C
ES~--
Nota,ened~/ -~ -~" -4
--Alligned
E/
.--~t_.4-U' 0
4
8 12 Ha(KOe)
Fig. 8. Remanence curves of F e - R - B powders.
16
G.C Hadjipanayis, CN. Christodoulou / Magnetic hysteresis in Fe- R - B powders M r C H y M r (c°)
Fe77 Ndl0Ib5B8 - + .
• t h e m . demagn. • dc field ,, + ac
,,
Powder
"'0.4
,,
-o:8
-o14
o
OA
Q8
%(H)/M/=)
Fig. 9. Remanence curve fits to Wohlfarth relationship.
earth containing powders have a particle size greater than the single d o m a i n particle size, suggesting a d o m a i n wall pinning process. D o m a i n walls move easily inside the grains and are pinned at the surface of the grains. After ac and dc demagnetization the d o m a i n wall distribution changes to a situation which strongly resembles that of a m o r e uniform d o m a i n wall pinning. The nature of the pinning sites at the particle surface is not yet known. The heavy rare-earth containing powders have a particle size close to that of a single d o m a i n particle indicating a magnetization rotation reversal process.
netized samples (fig. 8). The thermally demagnetized Fe77Nd~sTbsBs p o w d e r samples had a m u c h smaller value of remanence because of the isotropic nature of the samples. A n attempt was m a d e to fit the remanence data to the relation [14,15],
Acknowledgements
M D ( H ) = MR(OO ) -- 2 M R ( n ) ,
References
where M R ( H ) is the initial remanence, MR(oo ) the saturation remanence and M D ( H ) the demagnetization remanence. This relation should be obeyed for non-interacting single d o m a i n particles. It should also hold in the case of d o m a i n wall pinning if the d o m a i n walls encounter the same types of pins in the initial forward state and in the demagnetized state (this is the case of uniform d o m a i n wall pinning). The remanence relationship is f o u n d to be o b e y e d for p o w d e r e d samples only in the thermally demagnetized state (fig. 9). This is expected because the particles behave independently of each other. D o m a i n wall pinning takes place possibly at the surface of the particle. The fit is worse in the ac and dc demagnetized samples. This is possibly due to the change of the magnetic d o m a i n structure of these samples. A closer analysis o f the data is in progress.
4. S u m m a r y W e have shown that high coercive fields can be achieved in fine F e - R - B powders. The light rare-
239
W e are grateful to the Office of N a v a l Research and US A r m y Research for their support of this work.
[1] G.C. Hadjipanayis, R.C. Hazelton and K.R. Lawless, Appl. Phys. Lett. 43 (1983) 797; J. Appl. Phys. 55 (1984) 2073. [2] J.F. Herbst, J.J. Croat, J.D. Pinkerton and W. Yelon, Phys. Rev. B 29 (1984) 4176. [3] D. Givord, H.S. Li and J.M. Moreau, Solid State Commun. 50 (1984) 497. [4] S. Sinnema, R.J. Radwanski, J.J.M. Franse, D.B. de Mooij and K.H.J. Bus*how, J. Magn. Magn. Mat. 44 (1985) 333. [5] K.S.V.L. Narasimhan, J. Appl. Phys. 57 (1985) 4081. [6] M. Sagawa, S. Hirosawa, H. Yamamoto,, Y. Matsuura, S. Fujimura, H. Tokunama and H. Hiraga, Intermag, Phoenix (1986). [7] K. Gudimetta, C.N. Christodoulou and G.C. Hadjipanayis, Appl. Phys. Lett. 48 (1986) 670. [8] H. Oesterreicher, 8th Intern. Workshop on Rare-Earth Magnets, Dayton (1985). [9] J.D. Livingston, J. Appl. Phys. 57 (1985) 4137. [10] J.D. Livingston, Proc. 8th Intern. Workshop on Rare-Earth Magnets and Their Applications, Dayton, Ohio (1985). [11] J.J. Becket, J. Appl. Phys. 38 (1967) 1015. [12] Introduction to Magnetic Materials, ed. B.D. Cullity (Addison-Wesley, Reading, MA, 1972) p. 387. [13] G.C. Hadjipanayis and K.R. Lawless, J. Appl. Phys. 5 (1985) 4097. [14] E.P. Wohlfarth, J. Appl. Phys. 29 (1958) 595. [15] R.A. McCurrie and P. Gaunt, Proc. Intern. Conf. on Magnetism, Nottingham (1964) 780.
235
MAGNETIC HYSTERESIS IN F e - R - B POWDERS G.C. H A D J I P A N A Y I S and C.N. C H R I S T O D O U L O U Department of Physics, Kansas State University, Cardwell Hal~ Manhattan, KS 66506, USA
Received 29 May 1986; in revised form 7 April 1987
The hard magnetic properties of Fe-R-B powders have been examined using various magnetic techniques. High coercivities were obtained in fine powders with a particle size around 1 )xm. A domain wall pinning process is suggested for the less anisotropie, Nd-containing samples where the domain walls are pinned at the particles surface. The initial magnetization curves, field dependence of coercivity and remanenee curves of thermally demagnetized samples change significantly after ac and dc demagnetization indicating a more uniform domain wail pinning process in the ae and de demagnetized samples. A magnetization rotation reversal process is suggested for the more anisotropie, heavy rare-earth containing powders.
1. Introduction The hard magnetic properties of F e - R - B alloys were first shown [1] on melt-spun samples where coercivities in excess of 15 kOe were obtained. These properties have been attributed to the highly anisotropic Fe14R2B phase [2] which has a tetragonal structure with the c-axis as the easy axis of magnetization for most F e - R - B alloys at r o o m temperature [3,4]. The potential of these alloys for permanent magnet development was soon exploited and presently commercial sintered magnets with energy products up to 50.6 M G O e are readily available [5,6]. Recently, we have also obtained high coercivities in F e - R - B powders [7] and their magnetic characteristics indicated a magnetization reversal process different from that of melt-spun alloys. In the present study we examine the hard magnetic properties of these powders in more details in an attempt to elucidate the origin of magnetic hardening.
2. Experimental As-cast samples with composition Fe77Nd15_ x TbxB s where 0 < x < 15 were prepared by arcmelting the pure constituents (at least 99.9%) un-
der argon atmosphere. Fine powders were prepared by first crushing the bulk samples to coarse powders and then ball-milling the powders under toluene using an attritor. Samples were chosen at different milling times and the coercivity was measured as a function of particle size. The average size of the particles was measured with a scanning electron microscope (SEM). The magnetic properties were measured with a vibrating sample magnetometer (VSM) in the temperature range of 4.2-800 K and in fields up to 18 kOe. The remanence curves M R ( H ) were obtained for samples with the following three different magnetic histories: (i) on thermally demagnetized samples, (ii) on samples first magnetized in the m a x i m u m forward field and then demagnetized by applying a field close to the remanent coercivity (de demagnetized) and (iii) on samples magnetized at the m a x i m u m field and then demagnetized by reducing the amplitude of an applied alternating field (ac demagnetized). The remanence curves were measured as follows: The initial samples with zero magnetization were subjected to an applied field which subsequently was reduced to zero and the remanence was measured. This was repeated for different higher fields up to the m a x i m u m field available. The same procedure was also applied for reversed fields and the whole remanence loop M R ( H ) was obtained.
0304-8853/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
236
(7,.C. Hadfipanayis, C.N. Christodoulou / Magnetic hysteresis in Fe- R - B powders
3. Results and discussion
Table 1 Size dependence of coercivity
3.1. Coercioity m e a s u r e m e n t s
Composition
M i l l i n g A v e r a g e Coercivity time (rain) particle alignedin wax size (pm) (kOe)
Fe77Nd15B8
0.00 2.50 55.00
30.00 4.50 0.60
0.70 1.50 4.70
Fe77NdxoTb5B8
0.00 3.00 30.00
30.00 4.00 2.80
3.50 9.30 12.40
Fe77NdsTb10B8
0.00 2.00 30.00
33.30 3.60 0.70
5.50 12.90 16.70
0.00
27.80 0.90 0.80
6.90 15.20 > 16.80
The coercivity of powders was found to increase with milling time, going through a maxim u m in some samples (fig. 1). This shows a strong dependence of coercivity on particle size (table 1) in contrast to previous studies [8]. The optimum coercivity was observed in powders with a particle size close to that of single domain. The single domain particle size for F e - N d - B is estimated [9] to be 0.3 ~ m while in F e - T b - B it is believed to be about 5 ixm. The decrease in coercivity for some of the very fine powders is attributed to oxidation problems and not to superparamagnetism. The superparamagn, etic particle size is estimated to be less than 35 A according to Livingston [10]. The oxidization problem has also been observed in
°aligned in wax • Pressed 48kPSi
86 ~4 2 I
I
J
40
0
I
I
80 t (rain)
l
4.00 30.00
sintered F e - R - B magnets where the coercivity of very small sample pieces was significantly reduced. The increase in coereivity with decreasing particle size could be explained with nucleation type processes where the density of defects decreases with the decreasing particle size. However, etching experiments (fig. 2) did show a decrease in H c and this rather indicates a domain wall pinning process at the surface of the particles. The possibility of oxidation of powders during etching is presently being examined. The domain wall pinning hypothesis is further suggested by the angle dependence of He (fig. 3). The coercivity of sampies measured at an angle to the easy axis is
Fe77 Ndl 5B8
8
Fe77TblsB8
f
120
20
E~hing of F..e..~%T% powder (3/o HNO3)
~15 0 e 77NdsTb10B8
10
b
10
~.
44r
~6 4 5 0
I
I
10
I
I
20
I
I
30 t (rnin)
I
I
t
I
2
40
Fig. 1. Coercivity as a function of milling time in Fe-R-B magnets; (i) less anisotropic, (ii) more anisotropic.
Etching time(rain) Fig. 2. Dependence of coercivity on etching time.
237
G.C. Hadjipanayis, C.N. Christodoulou / Magnetic hysteresis in F e - R - B powders
16
As $intered NEO 3OH
12
T I
3b 0 6b 9'0 n
Fig. 3. Angular dependence of coercivity in Fe77NdloTbsB8.
I
increased, consistent with the 1 / c o s 0 dependence of H c due to domain wall pinning [11]. For the heavy rare-earth rich samples, the particle size for maximum Hc is closer to that of single domain size and in this case a magnetization rotation process might be more predominant. In all cases the coercivity is found to increase with the amount of Tb which is consistent with the fact that the magnetic anisotropy constant of F e - T b - B is much higher than that of F e - N d - B [4]. For the Nd rich samples, the coercivities of pressed powdered samples are lower than those of powders aligned on wax (fig. la) because of magnetostatic interactions, H e = H 0 ( 1 - p ) , p is the packing fraction [12]. However, for the heavy rare-earth rich samples the opposite effect is observed (fig. lb). We do not know yet whether this effect is intrinsic or is due to sample preparation. It could be that for the finer particles the applied pressures used cause misalignment of grains and this would lead to higher coercivities as explained before. Magnetization measurements showed a decrease in remanence with applied pressure that is characteristic of grain misalignment. Further studies are in progress to clarify this issue. Fine powders of Neo 30H (a commercial sintered magnet with composition F e - N d ( D y ) - B ) were also prepared and their coercivity was measured (fig 4). The initial steep decrease in H~ is possibly related to stresses introduced in the grains during the grinding process. The subsequent increase in He with milling is due to the finer size of grains. This behavior clearly indicates two different magnetic hardening mechanisms for sintered magnets and powders. This difference is further suggested by the temperature dependence of Hc
o
20
,
I
I
40 60 Milling time(min)
80
Fig. 4. Size dependence of coercivity in Neo 30H.
which for powders is almost linear in contrast to t h e T 4/3 dependence observed in sintered magnets [13].
3.2. Initial magnetization curves- hysteresis loops The initial magnetization curves with the corresponding hysteresis loops are shown in figs. 5 and 6. It is obvious that in the thermally demagnetized samples the domain walls move freely inside the grains resulting in a drastic increase of magnetization at relatively low fields. However, when the samples are demagnetized after they are initially magnetized the "effective initial" curve is different and it lies below that of the thermally demagnetized sample. This indicates that after the appli-
ac dernagn.
~
~
,
Fig. 5. Initial curves and hysteresis loops of F e - R - B powders (Neo 35).
G.C. Hadjipanayis, C.N. Christodoulou / Magnetic hysteresis in F e - R - B powders
238
Fe77Nd1c)Tb5B8
,~'..-.----'-"-~,.,. ..... .'" - ' " ~
..... therrr~liy demclgrt/~ ,' ---dc derr~gn. / ~ / --ocderr~gn. / ~-50 i/ ~
15
.-'" // ~
// /
Fe77NdIoTbsB8 - - - C o a r s e Povvderl 12 ..... FinePowder ~AC dernogn. As Sintered J
'
/
e) 15 -50
t
8
4
o
Fig. 6. Initial curves and hysteresis loops of F e - R - B powders.
/
"'"
...... ::/; 4
8
.-/"
/
//
/"
12 Ha(KOe)
16
Fig. 7. Cocrcivity as a function of applied field in Fe-R-B magnets.
cation of a field the domain wall distribution changes and some of the walls ware engaged with different and stronger pinning sites making it more difficult to completely saturate the sample. It is interesting to note the kink which appears on the "effective initial" magnetization curve of the ac demagnetized samples. Neo 35 does not contain any heavy rare-earth elements and the effect is small (fig. 5). The kink becomes a shoulder in samples containing small amounts of heavy rareearths (fig. 6). This is an indication of two types of domain wall pinning processes and it is also reflected on the H¢(H~) curves as will be seen later. It appears that after the thermally demagnetized samples are subjected to an ac field a situation is reached which strongly resembles domain wall pinning.
pinning possibly inside the grains. Saturation of H c was observed only in samples where the coercivity was much less than the maximum available field. This is the case of F e - N d - B fine powders with n s a t = 4 kOe and F e - N d - T b - B coarse powders with H~at= 11 kOe (fig. 7). The field dependence of Hc in ac demagnetized samples is consistent with domain wall pinning processes. 3.4. R e m a n e n c e curves
The remanence curves for F e - N d - T b - B powder are shown in fig. 8. Again the dc and ac demagnetized sample remanence curves are below those of thermally demagnetized samples. The shape of the curves is also very different with the steeper curve corresponding to thermally demag-
3.3. FieM dependence of coercivity The coercivity of all samples is found to be strongly dependent on applied field. In thermally demagnetized samples H¢ increases initially slowly and then more rapidly with the applied field. In ac demagnetized samples a different behavior is observed (fig. 7). The coercivity is practically negligible until a critical field where it starts increasing rather drastically. This critical field is close to the field where the shoulder appears on the magnetization curve of ac demagnetized samples. The disappearance of the slow increase in H¢ observed in thermally demagnetized samples is indicative of the fact that this corresponds to domain wall
REMANENCE C
ES~--
Nota,ened~/ -~ -~" -4
--Alligned
E/
.--~t_.4-U' 0
4
8 12 Ha(KOe)
Fig. 8. Remanence curves of F e - R - B powders.
16
G.C Hadjipanayis, CN. Christodoulou / Magnetic hysteresis in Fe- R - B powders M r C H y M r (c°)
Fe77 Ndl0Ib5B8 - + .
• t h e m . demagn. • dc field ,, + ac
,,
Powder
"'0.4
,,
-o:8
-o14
o
OA
Q8
%(H)/M/=)
Fig. 9. Remanence curve fits to Wohlfarth relationship.
earth containing powders have a particle size greater than the single d o m a i n particle size, suggesting a d o m a i n wall pinning process. D o m a i n walls move easily inside the grains and are pinned at the surface of the grains. After ac and dc demagnetization the d o m a i n wall distribution changes to a situation which strongly resembles that of a m o r e uniform d o m a i n wall pinning. The nature of the pinning sites at the particle surface is not yet known. The heavy rare-earth containing powders have a particle size close to that of a single d o m a i n particle indicating a magnetization rotation reversal process.
netized samples (fig. 8). The thermally demagnetized Fe77Nd~sTbsBs p o w d e r samples had a m u c h smaller value of remanence because of the isotropic nature of the samples. A n attempt was m a d e to fit the remanence data to the relation [14,15],
Acknowledgements
M D ( H ) = MR(OO ) -- 2 M R ( n ) ,
References
where M R ( H ) is the initial remanence, MR(oo ) the saturation remanence and M D ( H ) the demagnetization remanence. This relation should be obeyed for non-interacting single d o m a i n particles. It should also hold in the case of d o m a i n wall pinning if the d o m a i n walls encounter the same types of pins in the initial forward state and in the demagnetized state (this is the case of uniform d o m a i n wall pinning). The remanence relationship is f o u n d to be o b e y e d for p o w d e r e d samples only in the thermally demagnetized state (fig. 9). This is expected because the particles behave independently of each other. D o m a i n wall pinning takes place possibly at the surface of the particle. The fit is worse in the ac and dc demagnetized samples. This is possibly due to the change of the magnetic d o m a i n structure of these samples. A closer analysis o f the data is in progress.
4. S u m m a r y W e have shown that high coercive fields can be achieved in fine F e - R - B powders. The light rare-
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W e are grateful to the Office of N a v a l Research and US A r m y Research for their support of this work.
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