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Crystal and magnetic structure of the permanent magnet materials Nd2Fe14−xCoxB (x = 0–14)
335
Journal of the Less-Common Metals, 162 (1990) 335-342
CRYSTAL AND MAGNETIC STRUCTURE OF THE PERMANENT MAGNET MATERIALS Nd,Fe,,_,Co,B (x= O-14) K. GIRGIS, M. KRAFT and U. WEIS
Instinctfiir Kristallographie und Petrographic, ETHZ, 8092 Ziirich (Switzerland) P. FISCHER
Laborfir Neutronenstreuung, ETHZ, 5232 Villigen PSI (Switzerland) M. SOSTARICH Institut fir Werksloffe der Elektrotechnik, Ruhr Universitdt, 4630 Bochum (F.R.G.) (Received February 5,199O)
Summary We have investigated the Nd,Fe,,_,Co,B system by means of X-ray and neutron diffraction, metallographic techniques and bulk magnetic measurements. The tetragonal structure of Nd*Fe,,B exists in the entire range x=0-14. The lattice constants decrease with increasing cobalt content. The replacement of iron by cobalt raises the Curie temperature. The magnetic structure of NdzFe&o,B has been investigated at 740 K (above the Curie temperature), at 293 K (room temperature) and at 30 K (low temperature). The magnetic moments are parallel to the c axis and do not show any reorientation tendency with temperature. For low cobalt concentrations, cobalt atoms are more probably statistically distributed on the iron point positions.
1. Introduction A promising group of permanent magnet materials R*Fe,,B (R is a rare earth metal) was discovered some years ago [ 11. The main limitation of these compounds is their relatively low Curie temperature (e.g. 585 K for Nd,Fe,,B). The aim of this work is ( 1) to study the rise in the Curie temperature, and (2) to study the correlation between the structure and magnetic properties of these materials. 2. Experimental details 2.1. Sample preparation and heat treatment The samples were prepared from the pure elements: neodymium 99.9%; Johnson-Matthey), iron (purity, 99.9995%; Koenig), cobalt 0022-5088/90/$3.50
Q Elsevier Sequoia/Printed
(purity, (purity,
in The Netherlands
336
99.998%; Johnson-Matthey), boron-l 1 (purity, 99.94%; Centronic), These were melted in an arc furnace in an atmosphere of pure argon and then remelted several times to obtain complete mixing of the components. In order to provide a choice of good samples, several samples of the same nominal composition were prepared. Annealing was performed at 900 “C for 14-21 days and the samples were finally quenched into water. During annealing the samples were wrapped in tantahtm foil and enclosed in quartz ampoules under vacuum. 2.2. ~~crost~c~re and microhardness Microstructure was studied by standard metallographic techniques. In order to identify the phases present and possible impurities, the results of these metahographic studies were combined with those of X-ray and neutron diffraction. The microhardness measurements were carried out to help in the identification of the different phases. Microprobe analysis was used to test the homogeneity and determine the composition of the samples at different points using a step-scan method. Differences in composition of one or more components were detected in this way and gave a measure of homogeneity of the treated phase. 2.3. X-ray studies Powder photographs were obtained using a Guinier focusing camera (Jagodzinski type) with silicon as the internal standard. A powder diffractometer was also used for intensity measurements.
Neutron diffraction measurements were carried out at a high temperature (740 K) above the Curie point, at room temperature (293 K) and at a low temperature (30 K), using the less absorbing boron-l 1 (98.6 at.% llB) isotope. The measurements were performed on the muhidetector powder diffractometer DMC at the reactor SAPHIR (PSI). 3. Phases present The NdrFer+_, Co,B series crystallizes in the tetragonal space group P4z/mnm (136) with 68 atoms per unit cell. The tetragonal phase exists over the whole range of composition X= O-14. Figure 1 shows the decrease in the lattice constants with increase in cobalt content. The decrease in c is relatively large compared with that of a. The decrease is smooth compared with the results of Herbst and Yelon [2]. A small amount of iron and traces of a Ndr +pehB, phase have been observed metahographically and by neutron diffraction, but not by X-rays. 4. Curie temperature The Curie temperature has been determined from thermal demagnetization curves taken with a vibrating sample magnetometer at a constant field strength of 6
337
12.20t 12.16 12.12 12.0
C
12.04 12.00 11.96 11.92 11.69 I\
$\ 2
a
4
6
6
X -
10 12 14 Co-content
Co- contenf
Fig. 1. The dependence
of the cell constants of Nd,Fe,, _ $o,B
Fig. 2. The dependence
of the Curie temperature
on cobalt-concentration
of Nd,Fe,,_,Co,B
(x = 0- 14).
on cobalt concentration.
kOe. Our Tc values (Fig. 2) are somewhat higher than those given in refs. 3 and 4, but agree reasonably well with the values measured by Fuerst et al. [5].
5. Nuclear and magnetic structure We used as start parameters those found by Herbst et al. [6] for Nd,Fe,,B. The rare earth atoms possess two point positions, whilst the iron and cobalt atoms are distributed over six point positions, and the boron atoms occupy one further point position. A total of 32 parameters are necessary for the refinement of the structure: 14 atomic parameters, eight magnetic moments, three isotropic temperature factors, three gaussian halfwidths, two cell constants, one zero point correction and one overall scale factor. In order to refine the distribution of the cobalt atoms on the different positions, we studied the structure at 740 K, i.e. above the Curie temperature. This enabled us to refine the positional parameters and their occupation with iron and cobalt without magnetic influence. The scattering lengths for iron and cobalt (0.954 x lo-l2 cm and 0.250 X lo-‘* cm respectively) are essentially different. The corresponding 12 occupation parameters were refined, so that the number of parameters refined was 36. Two hypotheses have been studied carefully: (1) statistical distribution of cobalt according to microprobe analysis, and (2) different occupation parameters of the point positions with iron and cobalt, taking the microprobe analysis into consideration.
TABLE
1
Nuclear structure of Nd,Fe,,CoB
at 740 K
Atom
Point position
X
Y
Ndl
4f
0.269( I )
Nd2 Fe1 Fe2 Fe3 Fe4
4g 16k 16k
0.147(l) 0.226( 1) 0.036( 1) 0.098( 1) 0.319(l)
0.269( -0.147(l) 0.568( 0.360( 0.098( 0.319(
Fe5 Fe6 co1 co2 co3 co4 co5 Co6 B
8j 8j 4e 4c 16k 16k 8j 8j 4e 4c 4g
0.500 0.000 0.226( 0.036( 0.098( 0.319(
1) 1) 1) 1)
0.500 0.000 0.375( 1)
1) 1) 1) 1) 1)
0.500 0.500 0.568( 1) 0.360( 1) 0.098( 1) 0.319(l) 0.500 0.500 -0.375(l)
Z
Occupationa (%)
0.000 0.000 0.128( 0.176( 0.206( 0.244(
100 100 91.6 91.6 91.6 91.6 91.6 91.6 8.4 8.4 8.4 8.4 8.4 8.4 100
1) 1) 1) 1)
0.116(l) 0.000 0.128( 1) 0.176( 1) 0.206( 1) 0.244( 1) 0.116(l) 0.000 0.000
a,b=8.7980(6)A,c=12.1995(10)A B,, = 0.49( 18). &co =0.74(08), B, =0.55(36) R, = 5.72, R,, = 12.05, Rex,,= 4.26 ‘Iron and cobalt statistically distributed, according to microprobe analysis.
TABLE 2 Nuclear structure of Nd,Fe,,CoB
Atom
Point posilion
at 740 K
X
Y
Z
Occupation’ (%)
0.270( 1) -0.147(l) 0.568( 1) 0.360( 1) 0.098( 1) 0.319(l)
0.000 0.000 0.128(l) 0.176( 1) 0.207( 1) 0.244( 1) 0.116(l) 0.000 0.128( 1) 0.176( 1) 0.207( 1) 0.244( 1) 0.116(l) 0.000 0.000
100 100 94.2( 9) 91.1(9) 85.0( 1.5) 95.9( 1.4) 92.0( 1.8) 87.1( 1.9) 5.8(9) 8.9( 9) 15.0(1.5) 4.1(1.4) 8.0( 1.8) 12.9( 1.9) 100
Ndl
4f
0.270( 1)
Nd2 Fe1 Fe2 Fe3 Fe4 Fe5 Fe6 co1 co2 co3 co4 co5 Co6 B
4g 16k 16k
0.147(l) 0.226( 1) 0.036( 1) 0.098( 1) 0.319(l) 0.500 0.000 0.226( 1 0.036( 1 0.098( 1 0.319(1 0.500 0.000 0.375( 1)
8j 8j 4; 4c 16k 16k 8j 8j 4e 4c 4g
a, b=8.7980(6)A,c=12.1988(lO)A B,, = 0.38, Bbe,co = 0.61, B, = 1.00 R, = 4.92, R,, = 11.74, R,, = 4.30 “Iron and cobalt distribution is refined.
0.500 0.500 0.568( 1 0.360( 1 ; 0.098( 1 0.319(1 i 0.500 0.500 -0.375( 1)
TABLE 3 Nuclear and magnetic structure of Nd,Fe,,CoB
Atom
Point position
X
Ndl Nd2 Fe1 Fe2 Fe3 Fe4 Fe5 Fe6 co1
4f
0.268( 0.144( 0.226( 0.036( 0.098( 0.317( 0.500 0.000 0.226(
1) 1) 1) 1) 1) 1)
0.036( 0.098( 0.317( 0.500 0.000 0.374(
1) 1) 1)
co2 co3 co4 co5 Co6 B
4g 16k 16k
8j 8j
4e 4c 16k 16k 8j 8j 4e 4c 4g
a, b=8.7926(2)&
at 293 K
Y
1)
1)
0.268( 1) -0.144(l) 0.568( 1) 0.360( 1) 0.098( 1 0.317( 1 0.500 0.500 0.568( 1 0.360( 1 0.098( 1 0.317(1 0.500 0.500 -0.374(l)
Z
Occupation a (“/d
Id%)
0.000 0.000 0.128( 1) 0.176( 1) 0.205( 1 0.246( 1 0.113(1 0.000 0.128( 1 0.176(1 0.205( I 0.246( I 0.113(l) 0.000 0.000
100 100 91.6 91.6 91.6 91.6 91.6 91.6 8.4 8.4 8.4 8.4 8.4 8.4 100
2.2(2) 2.8( 2) 3.5( 1) 3.3( 1)
Z
Occupation” (%)
c1(~B)
0.000
100 100 91.6 91.6 91.6 91.6 91.6 91.6 8.4 8.4 8.4 8.4 8.4 8.4 100
2.7(2) 3.1(2) 3.5( 1) 3.4( 1) 3.6(2) 4.8( 2) 2.6( 3) 2.9( 3) 3.5( 1) 3.4(l) 3.6(2) 4.8( 2) 2.6( 3) 2.9( 3)
3.6(2) 5.0(2) 2.7( 2) 3.1(3) 3.5( 1) 3.3( 1) 3.6( 2) 5.0( 2) 2.7( 2) 3.1(3)
c= 12.1707(7)A
B,,=O.l5(lO),&,c,
= 0.23(3), B, = 0.23(no refinement)
R, =3.53, R,= 3.88, R,,=
10.05, R,,,=3.72
aIron and cobalt statistically distributed, according to microprobe
analysis.
TABLE 4 Nuclear and magnetic structure of Nd,Fe,,CoB
Atom
Point position
X
Ndl Nd2 Fe1 Fe2 Fe3 Fe4 Fe5 Fe6 co1 co2 co3 co4 co5 Co6 B
4f
8j 8j 4e
0.269( 1) 0.143( 1) 0.224( 1) 0.037( 1) 0.097( 1) 0.317(l) 0.500 0.000 0.224( 1) 0.037( 1) 0.097( 1) 0.317( 1) 0.500
4C
0.000
4g
0.373( 1)
4g 16k 16k 8j 8j 4e 4c 16k 16k
at 30 K
Y
0.269( 1) -0.143(l) 0.567( 1) 0.359( 1) 0.097( 1) 0.317( 1) 0.500 0.500 0.567( 1) 0.359( 1) 0.097( 1) 0.317(l) 0.500 0.500 -0.373(l)
0.000 0.129( 1) 0.175( 1) 0.205( 1) 0.246( 1) 0.112(l) 0.000 0.129( 1) 0.175( 1) 0.205( 1) 0.246( 1) 0.112(l) 0.000 0.000
a,b=8.7946(2)&~=12.1563(7)A R,, = 0.14( l), &co = 0.30(3), B, = 0.30(no refinement) R,=3.44, R,=3.83, R,,=10.59, R,,,=3.64 “Iron and cobalt statistically distributed, according to microprobe
analysis.
340
The difference in the weighted profile R, values is only 0.4%. Therefore we conclude that statistical distribution of cobalt and iron in our sample NdzFe,,Co,B is more probable. This is in contradiction with Herbst and Yelon [2], but in agreement with Bolzoni et al. [ 71. The results of the structure re~nements are given in Tables l-4 and Figs. 3-5.
30. 0. ?? b V &
h ; 0 V
20.
E
-10.
E ii! % g
r I f
10. -20.
0. 0
10
20
30
.40 P-Theta
50
60
70
80
90
(Degrees)
Fig. 3. Observed and calculated neutron diffraction patterns of paramagnetic NdzFe,,CoB at 740 K.
F
P-Theta Fig. 4.
(Degrees)
Observed and calculated neutron diffraction patterns of ferromagnetic
Nd,Fe,,CoB
at 293 K.
341
0. -
abs
-10.
7 0 V 1 I
-20.
i i
,r
t" 10. -30.
0.
-r
IL-Theta
(Degrees)
Fig. 5. Observed and calculated neutron diffraction patterns of ferromagnetic
= [Al 6
I
w
Nd,Fe,,CoB
at 30 K.
e
8.85-
H
L
‘;
E si
P
z ::::/
s
L
= z 0.80
-12.15
1.382 0
1W
2W
300
600
5&l
&Xl
700 K
d
0
Temperature
Fig. 6. (a) The dependence of the cell constants of Nd,Fe&oB of the c/a ratio of Nd,Fe,,CoB on temperature.
I
la,&30oWOoSooX
K
Temperature
on temperature.
(b) The dependence
6. Discussion The neodymium moments increase with decreasing temperature, but only Nd, reaches the Nd3’ free ion value whereas the iron/cob~t moments remain almost constant. The moment of Fe4 in the Sj, position reaches practically the Fe3+-value (5 pB). In general the moments are higher than those observed by Herbst and Yelon [Z]. The magnetic moments are parallel to the c axis and do not show any tendency for reorientation as a function of temperature. This result is advantageous for the application of these materials as permanent magnets. The lattice constant a is almost temperature independent, whilst c and the c/a ratio increase strongly with temperature, as shown in Fig. 6.
342
Acknowledgments
The authors thank the workshop division of LNS for support, Dr. J. Schefer for fruitful discussions, and Mrs. M. Schiess and F. BIrtschi for technical assistance. This work has been supported by “Swiss National Science Foundation”.
References 1 L. Arthur, Science, 223 ( 1984) 920. 2 J. F. Herbst and W. B. Yelon, J. Appl. Whys., 6U (1986) 4224. 3 Y. Matsuura, S. Hirosawa, H. Yamamoto, S. Fujimura and M. Sagawa, Appl. Whys. Left., 16 (198.5) 308. 4 5 6 7
K. H. J. Buschow, H. M. van Noort and D. B. de Mooij, J. Less-Common Met, 109 (1985) 79. C. D. Fuerst, J. F. Herbst and E. A. Alson, J. Magn. Magn. Mater., M-57( 1986) 567. J. F. Herbst, J. J. Croat and W. B. Yelon, J. Appl. Whys., 57( 1985) 4086. F. Bolzoni, F. Leccabue, 0. Moze, L. Pareti, M. Solzi and A. Deriu, J. Appl. Whys., 61(1987) 5369.
Journal of the Less-Common Metals, 162 (1990) 335-342
CRYSTAL AND MAGNETIC STRUCTURE OF THE PERMANENT MAGNET MATERIALS Nd,Fe,,_,Co,B (x= O-14) K. GIRGIS, M. KRAFT and U. WEIS
Instinctfiir Kristallographie und Petrographic, ETHZ, 8092 Ziirich (Switzerland) P. FISCHER
Laborfir Neutronenstreuung, ETHZ, 5232 Villigen PSI (Switzerland) M. SOSTARICH Institut fir Werksloffe der Elektrotechnik, Ruhr Universitdt, 4630 Bochum (F.R.G.) (Received February 5,199O)
Summary We have investigated the Nd,Fe,,_,Co,B system by means of X-ray and neutron diffraction, metallographic techniques and bulk magnetic measurements. The tetragonal structure of Nd*Fe,,B exists in the entire range x=0-14. The lattice constants decrease with increasing cobalt content. The replacement of iron by cobalt raises the Curie temperature. The magnetic structure of NdzFe&o,B has been investigated at 740 K (above the Curie temperature), at 293 K (room temperature) and at 30 K (low temperature). The magnetic moments are parallel to the c axis and do not show any reorientation tendency with temperature. For low cobalt concentrations, cobalt atoms are more probably statistically distributed on the iron point positions.
1. Introduction A promising group of permanent magnet materials R*Fe,,B (R is a rare earth metal) was discovered some years ago [ 11. The main limitation of these compounds is their relatively low Curie temperature (e.g. 585 K for Nd,Fe,,B). The aim of this work is ( 1) to study the rise in the Curie temperature, and (2) to study the correlation between the structure and magnetic properties of these materials. 2. Experimental details 2.1. Sample preparation and heat treatment The samples were prepared from the pure elements: neodymium 99.9%; Johnson-Matthey), iron (purity, 99.9995%; Koenig), cobalt 0022-5088/90/$3.50
Q Elsevier Sequoia/Printed
(purity, (purity,
in The Netherlands
336
99.998%; Johnson-Matthey), boron-l 1 (purity, 99.94%; Centronic), These were melted in an arc furnace in an atmosphere of pure argon and then remelted several times to obtain complete mixing of the components. In order to provide a choice of good samples, several samples of the same nominal composition were prepared. Annealing was performed at 900 “C for 14-21 days and the samples were finally quenched into water. During annealing the samples were wrapped in tantahtm foil and enclosed in quartz ampoules under vacuum. 2.2. ~~crost~c~re and microhardness Microstructure was studied by standard metallographic techniques. In order to identify the phases present and possible impurities, the results of these metahographic studies were combined with those of X-ray and neutron diffraction. The microhardness measurements were carried out to help in the identification of the different phases. Microprobe analysis was used to test the homogeneity and determine the composition of the samples at different points using a step-scan method. Differences in composition of one or more components were detected in this way and gave a measure of homogeneity of the treated phase. 2.3. X-ray studies Powder photographs were obtained using a Guinier focusing camera (Jagodzinski type) with silicon as the internal standard. A powder diffractometer was also used for intensity measurements.
Neutron diffraction measurements were carried out at a high temperature (740 K) above the Curie point, at room temperature (293 K) and at a low temperature (30 K), using the less absorbing boron-l 1 (98.6 at.% llB) isotope. The measurements were performed on the muhidetector powder diffractometer DMC at the reactor SAPHIR (PSI). 3. Phases present The NdrFer+_, Co,B series crystallizes in the tetragonal space group P4z/mnm (136) with 68 atoms per unit cell. The tetragonal phase exists over the whole range of composition X= O-14. Figure 1 shows the decrease in the lattice constants with increase in cobalt content. The decrease in c is relatively large compared with that of a. The decrease is smooth compared with the results of Herbst and Yelon [2]. A small amount of iron and traces of a Ndr +pehB, phase have been observed metahographically and by neutron diffraction, but not by X-rays. 4. Curie temperature The Curie temperature has been determined from thermal demagnetization curves taken with a vibrating sample magnetometer at a constant field strength of 6
337
12.20t 12.16 12.12 12.0
C
12.04 12.00 11.96 11.92 11.69 I\
$\ 2
a
4
6
6
X -
10 12 14 Co-content
Co- contenf
Fig. 1. The dependence
of the cell constants of Nd,Fe,, _ $o,B
Fig. 2. The dependence
of the Curie temperature
on cobalt-concentration
of Nd,Fe,,_,Co,B
(x = 0- 14).
on cobalt concentration.
kOe. Our Tc values (Fig. 2) are somewhat higher than those given in refs. 3 and 4, but agree reasonably well with the values measured by Fuerst et al. [5].
5. Nuclear and magnetic structure We used as start parameters those found by Herbst et al. [6] for Nd,Fe,,B. The rare earth atoms possess two point positions, whilst the iron and cobalt atoms are distributed over six point positions, and the boron atoms occupy one further point position. A total of 32 parameters are necessary for the refinement of the structure: 14 atomic parameters, eight magnetic moments, three isotropic temperature factors, three gaussian halfwidths, two cell constants, one zero point correction and one overall scale factor. In order to refine the distribution of the cobalt atoms on the different positions, we studied the structure at 740 K, i.e. above the Curie temperature. This enabled us to refine the positional parameters and their occupation with iron and cobalt without magnetic influence. The scattering lengths for iron and cobalt (0.954 x lo-l2 cm and 0.250 X lo-‘* cm respectively) are essentially different. The corresponding 12 occupation parameters were refined, so that the number of parameters refined was 36. Two hypotheses have been studied carefully: (1) statistical distribution of cobalt according to microprobe analysis, and (2) different occupation parameters of the point positions with iron and cobalt, taking the microprobe analysis into consideration.
TABLE
1
Nuclear structure of Nd,Fe,,CoB
at 740 K
Atom
Point position
X
Y
Ndl
4f
0.269( I )
Nd2 Fe1 Fe2 Fe3 Fe4
4g 16k 16k
0.147(l) 0.226( 1) 0.036( 1) 0.098( 1) 0.319(l)
0.269( -0.147(l) 0.568( 0.360( 0.098( 0.319(
Fe5 Fe6 co1 co2 co3 co4 co5 Co6 B
8j 8j 4e 4c 16k 16k 8j 8j 4e 4c 4g
0.500 0.000 0.226( 0.036( 0.098( 0.319(
1) 1) 1) 1)
0.500 0.000 0.375( 1)
1) 1) 1) 1) 1)
0.500 0.500 0.568( 1) 0.360( 1) 0.098( 1) 0.319(l) 0.500 0.500 -0.375(l)
Z
Occupationa (%)
0.000 0.000 0.128( 0.176( 0.206( 0.244(
100 100 91.6 91.6 91.6 91.6 91.6 91.6 8.4 8.4 8.4 8.4 8.4 8.4 100
1) 1) 1) 1)
0.116(l) 0.000 0.128( 1) 0.176( 1) 0.206( 1) 0.244( 1) 0.116(l) 0.000 0.000
a,b=8.7980(6)A,c=12.1995(10)A B,, = 0.49( 18). &co =0.74(08), B, =0.55(36) R, = 5.72, R,, = 12.05, Rex,,= 4.26 ‘Iron and cobalt statistically distributed, according to microprobe analysis.
TABLE 2 Nuclear structure of Nd,Fe,,CoB
Atom
Point posilion
at 740 K
X
Y
Z
Occupation’ (%)
0.270( 1) -0.147(l) 0.568( 1) 0.360( 1) 0.098( 1) 0.319(l)
0.000 0.000 0.128(l) 0.176( 1) 0.207( 1) 0.244( 1) 0.116(l) 0.000 0.128( 1) 0.176( 1) 0.207( 1) 0.244( 1) 0.116(l) 0.000 0.000
100 100 94.2( 9) 91.1(9) 85.0( 1.5) 95.9( 1.4) 92.0( 1.8) 87.1( 1.9) 5.8(9) 8.9( 9) 15.0(1.5) 4.1(1.4) 8.0( 1.8) 12.9( 1.9) 100
Ndl
4f
0.270( 1)
Nd2 Fe1 Fe2 Fe3 Fe4 Fe5 Fe6 co1 co2 co3 co4 co5 Co6 B
4g 16k 16k
0.147(l) 0.226( 1) 0.036( 1) 0.098( 1) 0.319(l) 0.500 0.000 0.226( 1 0.036( 1 0.098( 1 0.319(1 0.500 0.000 0.375( 1)
8j 8j 4; 4c 16k 16k 8j 8j 4e 4c 4g
a, b=8.7980(6)A,c=12.1988(lO)A B,, = 0.38, Bbe,co = 0.61, B, = 1.00 R, = 4.92, R,, = 11.74, R,, = 4.30 “Iron and cobalt distribution is refined.
0.500 0.500 0.568( 1 0.360( 1 ; 0.098( 1 0.319(1 i 0.500 0.500 -0.375( 1)
TABLE 3 Nuclear and magnetic structure of Nd,Fe,,CoB
Atom
Point position
X
Ndl Nd2 Fe1 Fe2 Fe3 Fe4 Fe5 Fe6 co1
4f
0.268( 0.144( 0.226( 0.036( 0.098( 0.317( 0.500 0.000 0.226(
1) 1) 1) 1) 1) 1)
0.036( 0.098( 0.317( 0.500 0.000 0.374(
1) 1) 1)
co2 co3 co4 co5 Co6 B
4g 16k 16k
8j 8j
4e 4c 16k 16k 8j 8j 4e 4c 4g
a, b=8.7926(2)&
at 293 K
Y
1)
1)
0.268( 1) -0.144(l) 0.568( 1) 0.360( 1) 0.098( 1 0.317( 1 0.500 0.500 0.568( 1 0.360( 1 0.098( 1 0.317(1 0.500 0.500 -0.374(l)
Z
Occupation a (“/d
Id%)
0.000 0.000 0.128( 1) 0.176( 1) 0.205( 1 0.246( 1 0.113(1 0.000 0.128( 1 0.176(1 0.205( I 0.246( I 0.113(l) 0.000 0.000
100 100 91.6 91.6 91.6 91.6 91.6 91.6 8.4 8.4 8.4 8.4 8.4 8.4 100
2.2(2) 2.8( 2) 3.5( 1) 3.3( 1)
Z
Occupation” (%)
c1(~B)
0.000
100 100 91.6 91.6 91.6 91.6 91.6 91.6 8.4 8.4 8.4 8.4 8.4 8.4 100
2.7(2) 3.1(2) 3.5( 1) 3.4( 1) 3.6(2) 4.8( 2) 2.6( 3) 2.9( 3) 3.5( 1) 3.4(l) 3.6(2) 4.8( 2) 2.6( 3) 2.9( 3)
3.6(2) 5.0(2) 2.7( 2) 3.1(3) 3.5( 1) 3.3( 1) 3.6( 2) 5.0( 2) 2.7( 2) 3.1(3)
c= 12.1707(7)A
B,,=O.l5(lO),&,c,
= 0.23(3), B, = 0.23(no refinement)
R, =3.53, R,= 3.88, R,,=
10.05, R,,,=3.72
aIron and cobalt statistically distributed, according to microprobe
analysis.
TABLE 4 Nuclear and magnetic structure of Nd,Fe,,CoB
Atom
Point position
X
Ndl Nd2 Fe1 Fe2 Fe3 Fe4 Fe5 Fe6 co1 co2 co3 co4 co5 Co6 B
4f
8j 8j 4e
0.269( 1) 0.143( 1) 0.224( 1) 0.037( 1) 0.097( 1) 0.317(l) 0.500 0.000 0.224( 1) 0.037( 1) 0.097( 1) 0.317( 1) 0.500
4C
0.000
4g
0.373( 1)
4g 16k 16k 8j 8j 4e 4c 16k 16k
at 30 K
Y
0.269( 1) -0.143(l) 0.567( 1) 0.359( 1) 0.097( 1) 0.317( 1) 0.500 0.500 0.567( 1) 0.359( 1) 0.097( 1) 0.317(l) 0.500 0.500 -0.373(l)
0.000 0.129( 1) 0.175( 1) 0.205( 1) 0.246( 1) 0.112(l) 0.000 0.129( 1) 0.175( 1) 0.205( 1) 0.246( 1) 0.112(l) 0.000 0.000
a,b=8.7946(2)&~=12.1563(7)A R,, = 0.14( l), &co = 0.30(3), B, = 0.30(no refinement) R,=3.44, R,=3.83, R,,=10.59, R,,,=3.64 “Iron and cobalt statistically distributed, according to microprobe
analysis.
340
The difference in the weighted profile R, values is only 0.4%. Therefore we conclude that statistical distribution of cobalt and iron in our sample NdzFe,,Co,B is more probable. This is in contradiction with Herbst and Yelon [2], but in agreement with Bolzoni et al. [ 71. The results of the structure re~nements are given in Tables l-4 and Figs. 3-5.
30. 0. ?? b V &
h ; 0 V
20.
E
-10.
E ii! % g
r I f
10. -20.
0. 0
10
20
30
.40 P-Theta
50
60
70
80
90
(Degrees)
Fig. 3. Observed and calculated neutron diffraction patterns of paramagnetic NdzFe,,CoB at 740 K.
F
P-Theta Fig. 4.
(Degrees)
Observed and calculated neutron diffraction patterns of ferromagnetic
Nd,Fe,,CoB
at 293 K.
341
0. -
abs
-10.
7 0 V 1 I
-20.
i i
,r
t" 10. -30.
0.
-r
IL-Theta
(Degrees)
Fig. 5. Observed and calculated neutron diffraction patterns of ferromagnetic
= [Al 6
I
w
Nd,Fe,,CoB
at 30 K.
e
8.85-
H
L
‘;
E si
P
z ::::/
s
L
= z 0.80
-12.15
1.382 0
1W
2W
300
600
5&l
&Xl
700 K
d
0
Temperature
Fig. 6. (a) The dependence of the cell constants of Nd,Fe&oB of the c/a ratio of Nd,Fe,,CoB on temperature.
I
la,&30oWOoSooX
K
Temperature
on temperature.
(b) The dependence
6. Discussion The neodymium moments increase with decreasing temperature, but only Nd, reaches the Nd3’ free ion value whereas the iron/cob~t moments remain almost constant. The moment of Fe4 in the Sj, position reaches practically the Fe3+-value (5 pB). In general the moments are higher than those observed by Herbst and Yelon [Z]. The magnetic moments are parallel to the c axis and do not show any tendency for reorientation as a function of temperature. This result is advantageous for the application of these materials as permanent magnets. The lattice constant a is almost temperature independent, whilst c and the c/a ratio increase strongly with temperature, as shown in Fig. 6.
342
Acknowledgments
The authors thank the workshop division of LNS for support, Dr. J. Schefer for fruitful discussions, and Mrs. M. Schiess and F. BIrtschi for technical assistance. This work has been supported by “Swiss National Science Foundation”.
References 1 L. Arthur, Science, 223 ( 1984) 920. 2 J. F. Herbst and W. B. Yelon, J. Appl. Whys., 6U (1986) 4224. 3 Y. Matsuura, S. Hirosawa, H. Yamamoto, S. Fujimura and M. Sagawa, Appl. Whys. Left., 16 (198.5) 308. 4 5 6 7
K. H. J. Buschow, H. M. van Noort and D. B. de Mooij, J. Less-Common Met, 109 (1985) 79. C. D. Fuerst, J. F. Herbst and E. A. Alson, J. Magn. Magn. Mater., M-57( 1986) 567. J. F. Herbst, J. J. Croat and W. B. Yelon, J. Appl. Whys., 57( 1985) 4086. F. Bolzoni, F. Leccabue, 0. Moze, L. Pareti, M. Solzi and A. Deriu, J. Appl. Whys., 61(1987) 5369.