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Temperature dependence of magnetic properties of sintered Nd-Fe-B magnets modified with Dy
Journal
of Magnetism
and Magnetic
Materials
199
83 (1990) 199-200
North-Holland
TEMPERATURE DEPENDENCE OF MAGNETIC MAGNETS MODIFIED WITH Dy A. HANDSTEIN, Zentralinstitut
J. SCHNEIDER,
ftir Festkarperphysik
R. KREWENKA,
OF SINTERED
Nd-Fe-B
K.-H. MiiLLER
und Werkstofforschung
R. GRbSINGER
TU Wien, Institut ftir Experimentalphysik,
PROPERTIES
der AdW der DDR, 8027 Dresden, German Dem. Rep.
and H.R. KIRCHMAYR A-1040
Vienna, Austria
The coercivity ,H, and anisotropy field H, in sintered NdFeB and Dy-modified magnets were investigated as function of temperature. Dy does not only change H, and the magnetization but also the microstructure. None of present models describes well the observed complex changes of nucleation field and local stray fields. ,H,
in nucleation
type
magnets
like
I
NdFeB-based
on the local values of Ha and saturation magnetization .I, as well as on the character of structural inhomogeneities. The dependence of JHc on temperature T and composition c has been mainly analyzed within a model for pinning or nucleation in an infinite extended planar perturbed region of a certain thickness d (see e.g. refs. [1,2]). Other structural features like grain size, nature of the particle surface, volume portion and spatial distribution of additional phases are not included in this model. The influence of the degree of imperfection of particle surface and grain size D has been recently examined within a statistical model [3]. In the following the T- and c-dependence of JHC in Dy-modified sintered NdFeB-based magnets will be analysed using the relation:
materials
depends
CL~J% = w$&
(1)
-NJ,.
The
relevance of different models for describing JHC(c, T) and for relating (Y, N to intrinsic properties J,, H, and microstructural features, respectively, is considered.
1.5 t
0.9 4
200
300
It00
500
TIKI Fig. 2. Temperature
dependence
of the magnetization
J,.
Fig. 1 gives the T-dependence of H, determined by the SPD technique (sample A: Ndr,Fe,,B,, sample B: (Nd0.szDy0.0s)17Fe7sAlIB7, sample C: (Nda.s,Dya.r,)r,Fe,,B,). The anomaly near 290 K is attributed to impurities dissolved in the grains of the main phase.
0 200
300
coo
500
600 TIKI
TlKl Fig. 1. Temperature 0304-8853/90/$03.50 (North-Holland)
dependence
of the anisotropy
0 Elsevier Science Publishers
field H,. B.V.
Fig. 3. Temperature
dependence
of the coercive
force ,H,.
200
A. Handstein et al. / Sintered Nd-Dy-Fe-B
magnets
changes in a variation of d (thickness of a planar perturbed region), it appears to be rather artificial ((Y = a(6/d), 6 - wall width, see refs. [1,2]). An analysis of the dependence of ,H, on the logarithm of the square of the average grain size D, using the model in ref. [3], does not give a linear dependence. According to ref. [3], this points to a variation of the number of defects per unit area h upon changing the average size D(cu = a(XS); S = S(D) - surface area of grain [3]). Unfortunately, there are no experimental studies for h. The effect of microstructural changes on local fluctuation of the internal stray field (parameter N in eq. (1)) is not regarded in refs. [l-3]. This may be done writing
2.5
1.5 “3
c
==x i 0.5
N=1/3+cif((S/D). Fig. 4. Dependence of pLoJ H,/J,
on p,, H, /Js
Figs. 2 and 3 show J, vs. T and JHc vs. T, respectively. Although there are quite different values of JHc for different systems, the T-dependence of ,I%, is practically the same. Plots of pLoJHc/JS versus poH,/Js are given in fig. 4. These plots are more or less straight lines with exception of the high temperature region, where pLoH,/Js is small. The values of a and N determined according eq. (1) are listed in table 1. As can be seen, Dy increases CI and N. The effect of D on JHc for sintered NdFeB magnets is discussed in ref. [4] (fig. 2). Similar results are obtained for NdDyFeB magnets. One finds a remarkable increase of , H, with decreasing D, as expected from general consideration on the influence of D on critical field of nucleation H,, the number of defects on grain surface and wall energy [5]. By substituting Dy for Nd the value of p,,H,/J, increases much less than that of poJHc/Js (see also ref. [6]). This points to changes of microstructural features, by which (Y and N may be altered. While H, and J, may be changed by the chemical composition of the alloy, the parameters CYand N are influenced by the chemical composition of the alloy as well as by technological conditions during preparation (see also refs. ~2,781). Although there are some hints for the nature of the microstructural changes [7,8], the quantitative description of the influence of these structural features on ,H, is rather incomplete. To reflect all the microstructural
Table 1
Values of a and N for samples A, B and C Sample
a
N
A B C
0.2 0.346 0.315
0.78 1 S72 1.454
(2)
where a^ is a normalized mean field fluctuation at H = -J H, (see also ref. [4]). In summary it appears, that for such a complex analysis a more detailed structural characterization is necessary, even the experimental methods have to be developed in some cases. Furthermore, a more comprehensive model is necessary. This may be done in a simple way starting from formula (23) in ref. [3]: kH, = a, - ln( a,XD’), where a, expression
and
as
(3)
are parameters.
Taking
k = I/a,H,.
for k the
(4)
with a3 I 1, it follows CL,&=~&,
- ln( azAD2))poH,
For N the expression
- NJ,.
(5)
in eq. (2) may be used.
References
[II H. Kronmtiller,
K.D. Durst and M. Sagawa, J. Magn. Magn. Mat. 74 (1988) 291. [21 K.D. Durst, H. Kronmtiller and G. Schneider, in: Proc. 5th Intern. Symp. on Magnetic Anisotropy and Coercivity in REPTM Alloys (Bad Soden, Fed. Rep. Germany, 1987) p. 209. J. Appl. Phys. 64 (1988) [31 R. Ramesh and K. Srikrishna, 6406. P. Nothnagel and [41 K.-H. Mtiller, D. Eckert, A. Handstein, J. Schneider, J. Magn. Magn. Mat. 83 (1990) 195. D. Eckert and P. [51J. Schneider, K.-H. Mtiller, A. Handstein, Nothnagel, Proc. 7th Intern. Sem. on Magnetism (Rathewalde, German Dem. Rep., 1989, in press). [61 B.M. Ma and R.F. Krause, in ref. [2], p. 141. Perma[71 M. Sagawa and S. Hirosawa, in: High Performance nent Magnetic Materials, eds. S.G. Sankar, J.F. Herbst and N.C. Koon (Materials Research Center, Pittsburgh, 1987) p. 161. IEEE Trans. Magn. (Proc. INTERMAG, PI S. Hirosawa, Washington, USA, 1989).
of Magnetism
and Magnetic
Materials
199
83 (1990) 199-200
North-Holland
TEMPERATURE DEPENDENCE OF MAGNETIC MAGNETS MODIFIED WITH Dy A. HANDSTEIN, Zentralinstitut
J. SCHNEIDER,
ftir Festkarperphysik
R. KREWENKA,
OF SINTERED
Nd-Fe-B
K.-H. MiiLLER
und Werkstofforschung
R. GRbSINGER
TU Wien, Institut ftir Experimentalphysik,
PROPERTIES
der AdW der DDR, 8027 Dresden, German Dem. Rep.
and H.R. KIRCHMAYR A-1040
Vienna, Austria
The coercivity ,H, and anisotropy field H, in sintered NdFeB and Dy-modified magnets were investigated as function of temperature. Dy does not only change H, and the magnetization but also the microstructure. None of present models describes well the observed complex changes of nucleation field and local stray fields. ,H,
in nucleation
type
magnets
like
I
NdFeB-based
on the local values of Ha and saturation magnetization .I, as well as on the character of structural inhomogeneities. The dependence of JHc on temperature T and composition c has been mainly analyzed within a model for pinning or nucleation in an infinite extended planar perturbed region of a certain thickness d (see e.g. refs. [1,2]). Other structural features like grain size, nature of the particle surface, volume portion and spatial distribution of additional phases are not included in this model. The influence of the degree of imperfection of particle surface and grain size D has been recently examined within a statistical model [3]. In the following the T- and c-dependence of JHC in Dy-modified sintered NdFeB-based magnets will be analysed using the relation:
materials
depends
CL~J% = w$&
(1)
-NJ,.
The
relevance of different models for describing JHC(c, T) and for relating (Y, N to intrinsic properties J,, H, and microstructural features, respectively, is considered.
1.5 t
0.9 4
200
300
It00
500
TIKI Fig. 2. Temperature
dependence
of the magnetization
J,.
Fig. 1 gives the T-dependence of H, determined by the SPD technique (sample A: Ndr,Fe,,B,, sample B: (Nd0.szDy0.0s)17Fe7sAlIB7, sample C: (Nda.s,Dya.r,)r,Fe,,B,). The anomaly near 290 K is attributed to impurities dissolved in the grains of the main phase.
0 200
300
coo
500
600 TIKI
TlKl Fig. 1. Temperature 0304-8853/90/$03.50 (North-Holland)
dependence
of the anisotropy
0 Elsevier Science Publishers
field H,. B.V.
Fig. 3. Temperature
dependence
of the coercive
force ,H,.
200
A. Handstein et al. / Sintered Nd-Dy-Fe-B
magnets
changes in a variation of d (thickness of a planar perturbed region), it appears to be rather artificial ((Y = a(6/d), 6 - wall width, see refs. [1,2]). An analysis of the dependence of ,H, on the logarithm of the square of the average grain size D, using the model in ref. [3], does not give a linear dependence. According to ref. [3], this points to a variation of the number of defects per unit area h upon changing the average size D(cu = a(XS); S = S(D) - surface area of grain [3]). Unfortunately, there are no experimental studies for h. The effect of microstructural changes on local fluctuation of the internal stray field (parameter N in eq. (1)) is not regarded in refs. [l-3]. This may be done writing
2.5
1.5 “3
c
==x i 0.5
N=1/3+cif((S/D). Fig. 4. Dependence of pLoJ H,/J,
on p,, H, /Js
Figs. 2 and 3 show J, vs. T and JHc vs. T, respectively. Although there are quite different values of JHc for different systems, the T-dependence of ,I%, is practically the same. Plots of pLoJHc/JS versus poH,/Js are given in fig. 4. These plots are more or less straight lines with exception of the high temperature region, where pLoH,/Js is small. The values of a and N determined according eq. (1) are listed in table 1. As can be seen, Dy increases CI and N. The effect of D on JHc for sintered NdFeB magnets is discussed in ref. [4] (fig. 2). Similar results are obtained for NdDyFeB magnets. One finds a remarkable increase of , H, with decreasing D, as expected from general consideration on the influence of D on critical field of nucleation H,, the number of defects on grain surface and wall energy [5]. By substituting Dy for Nd the value of p,,H,/J, increases much less than that of poJHc/Js (see also ref. [6]). This points to changes of microstructural features, by which (Y and N may be altered. While H, and J, may be changed by the chemical composition of the alloy, the parameters CYand N are influenced by the chemical composition of the alloy as well as by technological conditions during preparation (see also refs. ~2,781). Although there are some hints for the nature of the microstructural changes [7,8], the quantitative description of the influence of these structural features on ,H, is rather incomplete. To reflect all the microstructural
Table 1
Values of a and N for samples A, B and C Sample
a
N
A B C
0.2 0.346 0.315
0.78 1 S72 1.454
(2)
where a^ is a normalized mean field fluctuation at H = -J H, (see also ref. [4]). In summary it appears, that for such a complex analysis a more detailed structural characterization is necessary, even the experimental methods have to be developed in some cases. Furthermore, a more comprehensive model is necessary. This may be done in a simple way starting from formula (23) in ref. [3]: kH, = a, - ln( a,XD’), where a, expression
and
as
(3)
are parameters.
Taking
k = I/a,H,.
for k the
(4)
with a3 I 1, it follows CL,&=~&,
- ln( azAD2))poH,
For N the expression
- NJ,.
(5)
in eq. (2) may be used.
References
[II H. Kronmtiller,
K.D. Durst and M. Sagawa, J. Magn. Magn. Mat. 74 (1988) 291. [21 K.D. Durst, H. Kronmtiller and G. Schneider, in: Proc. 5th Intern. Symp. on Magnetic Anisotropy and Coercivity in REPTM Alloys (Bad Soden, Fed. Rep. Germany, 1987) p. 209. J. Appl. Phys. 64 (1988) [31 R. Ramesh and K. Srikrishna, 6406. P. Nothnagel and [41 K.-H. Mtiller, D. Eckert, A. Handstein, J. Schneider, J. Magn. Magn. Mat. 83 (1990) 195. D. Eckert and P. [51J. Schneider, K.-H. Mtiller, A. Handstein, Nothnagel, Proc. 7th Intern. Sem. on Magnetism (Rathewalde, German Dem. Rep., 1989, in press). [61 B.M. Ma and R.F. Krause, in ref. [2], p. 141. Perma[71 M. Sagawa and S. Hirosawa, in: High Performance nent Magnetic Materials, eds. S.G. Sankar, J.F. Herbst and N.C. Koon (Materials Research Center, Pittsburgh, 1987) p. 161. IEEE Trans. Magn. (Proc. INTERMAG, PI S. Hirosawa, Washington, USA, 1989).