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NMR investigation of Y-Co and Gd-Co compounds
NMR INVESTIGATION OF Y - C o AND G d - C o C O M P O U N D S H. F I G I E L and M. JASZCZEWSKI Solid State Physics Dept. I. M. of Academy of Mining and Metallurgy, 30-059 Krak{Tw, Poland
The NMR spin echo measurements for Y2Col7 and Gd2COl7 are discussed and analysed taking into account the anisotropy of dipolar fields. It allows an explanation of the positions of observed Co resonance lines as caused by dipolar fields reaching 12.3 kO¢ in Gd2Col7.
Recently many N M R investigations of R - C o compounds were reported. Continuing our investigations of Y - C o [1] we have also investigated G d - C o compounds by use of N M R spin echo technique. As is well known YCo s compounds possess a large uniaxial anisotropy ( K ~ - 7.5 × 107 erg/cma). Assuming an influence of this anisotropy on hyperfine fields, Streever [2] explained the large split of resonance lines for YCo s and SmCo 5. R2Col7 compounds exhibit an easy plane ( a - b plane) anisotropy with anisotropy constants much smaller than RCo s [3] which means that Streevers's interpretation does not apply. In such a case the influence of dipolar fields would be observed. Because of an easy plane anisotropy, dipolar fields Hdip were calculated for magnetic moments lying in the a - b plane by the use of the formula
/./dip = ~ [ ~ i
R3
_
3
?i
)"
As an example, the results of calculations with
~tcoll/ for Y2Co17 and Gd2COl7 in a hexagonal (Th2Nil7 type) structure are presented in table I. In each case about 50 neighbouring atoms were taken into consideration. As is clear from the above data, some anisotropy of the dipolar fields is obtained. Let us consider only H x, since it influences the effective hyperfine field, i.e. resonance line position. It is seen that for site CoI there are two positions giving AH x ffi H~,o - Hxb -- - 1.42 kOe for Y2Col7 and AHx ffi - 3.23 kOe for Gd2COlT. Comparable values of AH~ are obtained for ColI, whereas for ColII the largest split (AHx ffi - 5.54 kOe for Y2Col7 and AH~ f f i - 12.34 kOe for Gd2COl7 ) takes place. The ColV site exhibits the best symmetry-small and only one value of Hd~v. Such a big AHx for ColII finds confirmation in experimental N M R spectra. The observed Co lines
are tabulated in table 2. The differences AH = H ( B 2 ) - H ( B 3 ) - - 5 kOe for Y2Co17 and A H - 14.5 kOe for Gd2Col7 agree with calculated differences of dipolar fields for CoIII. From the results of Nagai et al. [5] for (Yl_xGdx)2COl7 it is clear that according to growth of Gd content the distance between B 2 and B 3 increases whereas line C shifts nearer to B a. This allows us to conclude that B 2 and B 3 correspond to the same CoIII site with B 2 resulting from b, and B 3 from a positions. The relative intensities of the lines are in agreement with this interpretation. Line C has relatively low intensity, and this, together with its small width, suggests combining it with CoIV since it has Hdip not split and the largest magnetic moment. The A line in Y2Col7 has the same position as the line in YCo 5 [1, 9]. However, if there were precipitates of YCo 5 in the sample, it would be detected by X-ray analysis and, in addition, its intensity would be diminished because of large uniaxial anisotropy of YC05. If there existed only local environments typical for YCo 5 within Y2Col7 structure, the Co atoms should possess a magnetic moment typical for Y2Cot7 that gives p=
P~(Y2CoI7) ?~(YCos) VVCo,ffi 115 MHz,
and in fact one. can find a weak maximum at this position in our spectra [1, 8]. The A line observed for Gd2Co17 was not detected for GdCo s. Since that line is broad, with intensity exceeding half that for B lines, correlation with CoI site seems reasonable. For both compounds a line D at 216-225 MHz range is detected, and Inomata [4] reports also such a line for Pr and Tb compounds, which does not change its position according to changes of rare earth, contrary to other observed lines. At the same
Journal of Magnetism and Magnetic Materials 15-18 (1980) 673-675 ©North Holland
673
674
H. Figiel and M. Jaszczewski / N M R investigation of Y - C o and G d - C o compounds TABLE 1 Components of dipolar fields (kOe) Compound
Hx
Hy
a
- 0.80
0.0
0.0
b a b c a b
0.62 2.09 1.51 0.36 - 4.40 1.10
_ 0.82 _+0.66 _ 1.00 _ 0.34 0.0 ± 3.18
+ 2.78 0.0 0.0 0.0 0.0 -+ 1.33
- 0.46
0.0
0.0
0.0 -+(0.84 -+ 1.03) -+ (0.67 -+
Site
H~
(A H x)max
Col (6g) Y2Col7 (hex) gco ~ 1.64 [/.tB]
Coil (12j) ColII (12k) ColV
- 1.42
1.73
- 5.50
(40
Gd2COl7 (hex)
CoI (6g)
a b
- 2.07 1.16
Coil
a
5.88
0.0 -+(2.83 -+ 4.89)
-3.23
0.0
2.44) (12j) /~co ~ 1.67
b
4.55
c
-0.94
/aoa ~ 7.1
CoIII
a
- 9.42
[ ~B]
(12k) CoIV (4f)
b
2.90 - 2.20
-+(1.02 -+ 2.86) -+(0.34 -+ 0.43) 0.0 -+(3.23 -+ 3.88) 0.0
0.0
6.82
0.0 0.0 -+(1.35 -+ 2.26.)
- 12.32
0.0
TABLE 2 59Co resonance lines (MI-lz)
A 96a
Y2C°l 7
74.8 Gd2COl7
B1
B2
B3
171 a
174a
179a
C
D 216.8
176 177 175 181
200.8(rh) a 204.4(h) 200 200 198 198
170 171 168 168 168 169 167
184 184 184
194.5 194 194
Co-met.
219
219.2 216.8 225 216.8, t 220, 227 216,221 217,221228 t
Note: All measurements performed at 77 K except a - taken at 4.2 K.
Ref. 1, 8, this work 4 5 6 7 this work 5 4 this work 10
H. Figiel and M. Jaszczewski/ NMR investigation of Y-Co and Gd-Co compounds
frequency interval the lines typical for pure Co are observed [9, 10], which leads to the supposition that in all cases the signal f r o m an excess of C o precipitated at grain boundaries [11, 12] is detected. In such a case the position of C o i l would be at B , overlapping the split C o l I I line. The calculated dipolar fields have c o m p o n e n t s perpendicular to the direction of the magnetic m o ment (table 1). This means that these fields pull magnetization out of the easy plane, i.e. the anisotr o p y in R2Co~7 c o m p o u n d s is not an exact easy plane anisotropy. Therefore large a n d differentiated dipolar fields are p r o b a b l y one of the reasons leading to changes of easy direction by substitutions of 3-d metals a n d rare earths in p s e u d o b i n a r y R2BI7 c o m p o u n d s . The authors thank the Physics Institute of the Polish A c a d e m y of Science, Warsaw, a n d the
Alexander von H u m b o l d t financial support.
675
F o u n d a t i o n for their
References [1] H. Figiel et al., Physica 86-88B (1977) 77.
[2] R. L. Streever, Phys. Lett. 65A (1978) 360. [3] H. R. Kirchmayr and C. A. Poldy, J. Magn. Magn. Mat. 8 (1978) 1. [4] K. Inomata, J. Phys. Soc. Jap. 41 (1976) 1890. [5] H. Nagai ¢t al., J. Phys. Soc. Jap. 41 (1976) 1907. [6] H. Yoshie J. Phys. Soc. Jap. 43 (1977) 862. [7] J. Depor~s and A. Tsuimura, C. R. Acad. Sci. Paris B277 (1973) 333. [8] H. Figiel, E. Dormann and A. Oppelt, Proc. 10th Polish NMR Seminar, Krak6w (1978) p. 243. [9] C. W. Searle, Phys. Rcv. B. 15 (7) (1977) 3305. [10] M. Kawakami et al., J. Phys. Soc. Jap. 33 (1972) 1591. [11] K. H. J. Buschow, J. Less Common Metals 31 (1973) 359. [12] K. H. J. Buschow, F. Den Broeder, J. Less Common Metals 33 (1973) 191.
The NMR spin echo measurements for Y2Col7 and Gd2COl7 are discussed and analysed taking into account the anisotropy of dipolar fields. It allows an explanation of the positions of observed Co resonance lines as caused by dipolar fields reaching 12.3 kO¢ in Gd2Col7.
Recently many N M R investigations of R - C o compounds were reported. Continuing our investigations of Y - C o [1] we have also investigated G d - C o compounds by use of N M R spin echo technique. As is well known YCo s compounds possess a large uniaxial anisotropy ( K ~ - 7.5 × 107 erg/cma). Assuming an influence of this anisotropy on hyperfine fields, Streever [2] explained the large split of resonance lines for YCo s and SmCo 5. R2Col7 compounds exhibit an easy plane ( a - b plane) anisotropy with anisotropy constants much smaller than RCo s [3] which means that Streevers's interpretation does not apply. In such a case the influence of dipolar fields would be observed. Because of an easy plane anisotropy, dipolar fields Hdip were calculated for magnetic moments lying in the a - b plane by the use of the formula
/./dip = ~ [ ~ i
R3
_
3
?i
)"
As an example, the results of calculations with
~tcoll/ for Y2Co17 and Gd2COl7 in a hexagonal (Th2Nil7 type) structure are presented in table I. In each case about 50 neighbouring atoms were taken into consideration. As is clear from the above data, some anisotropy of the dipolar fields is obtained. Let us consider only H x, since it influences the effective hyperfine field, i.e. resonance line position. It is seen that for site CoI there are two positions giving AH x ffi H~,o - Hxb -- - 1.42 kOe for Y2Col7 and AHx ffi - 3.23 kOe for Gd2COlT. Comparable values of AH~ are obtained for ColI, whereas for ColII the largest split (AHx ffi - 5.54 kOe for Y2Col7 and AH~ f f i - 12.34 kOe for Gd2COl7 ) takes place. The ColV site exhibits the best symmetry-small and only one value of Hd~v. Such a big AHx for ColII finds confirmation in experimental N M R spectra. The observed Co lines
are tabulated in table 2. The differences AH = H ( B 2 ) - H ( B 3 ) - - 5 kOe for Y2Co17 and A H - 14.5 kOe for Gd2Col7 agree with calculated differences of dipolar fields for CoIII. From the results of Nagai et al. [5] for (Yl_xGdx)2COl7 it is clear that according to growth of Gd content the distance between B 2 and B 3 increases whereas line C shifts nearer to B a. This allows us to conclude that B 2 and B 3 correspond to the same CoIII site with B 2 resulting from b, and B 3 from a positions. The relative intensities of the lines are in agreement with this interpretation. Line C has relatively low intensity, and this, together with its small width, suggests combining it with CoIV since it has Hdip not split and the largest magnetic moment. The A line in Y2Col7 has the same position as the line in YCo 5 [1, 9]. However, if there were precipitates of YCo 5 in the sample, it would be detected by X-ray analysis and, in addition, its intensity would be diminished because of large uniaxial anisotropy of YC05. If there existed only local environments typical for YCo 5 within Y2Col7 structure, the Co atoms should possess a magnetic moment typical for Y2Cot7 that gives p=
P~(Y2CoI7) ?~(YCos) VVCo,ffi 115 MHz,
and in fact one. can find a weak maximum at this position in our spectra [1, 8]. The A line observed for Gd2Co17 was not detected for GdCo s. Since that line is broad, with intensity exceeding half that for B lines, correlation with CoI site seems reasonable. For both compounds a line D at 216-225 MHz range is detected, and Inomata [4] reports also such a line for Pr and Tb compounds, which does not change its position according to changes of rare earth, contrary to other observed lines. At the same
Journal of Magnetism and Magnetic Materials 15-18 (1980) 673-675 ©North Holland
673
674
H. Figiel and M. Jaszczewski / N M R investigation of Y - C o and G d - C o compounds TABLE 1 Components of dipolar fields (kOe) Compound
Hx
Hy
a
- 0.80
0.0
0.0
b a b c a b
0.62 2.09 1.51 0.36 - 4.40 1.10
_ 0.82 _+0.66 _ 1.00 _ 0.34 0.0 ± 3.18
+ 2.78 0.0 0.0 0.0 0.0 -+ 1.33
- 0.46
0.0
0.0
0.0 -+(0.84 -+ 1.03) -+ (0.67 -+
Site
H~
(A H x)max
Col (6g) Y2Col7 (hex) gco ~ 1.64 [/.tB]
Coil (12j) ColII (12k) ColV
- 1.42
1.73
- 5.50
(40
Gd2COl7 (hex)
CoI (6g)
a b
- 2.07 1.16
Coil
a
5.88
0.0 -+(2.83 -+ 4.89)
-3.23
0.0
2.44) (12j) /~co ~ 1.67
b
4.55
c
-0.94
/aoa ~ 7.1
CoIII
a
- 9.42
[ ~B]
(12k) CoIV (4f)
b
2.90 - 2.20
-+(1.02 -+ 2.86) -+(0.34 -+ 0.43) 0.0 -+(3.23 -+ 3.88) 0.0
0.0
6.82
0.0 0.0 -+(1.35 -+ 2.26.)
- 12.32
0.0
TABLE 2 59Co resonance lines (MI-lz)
A 96a
Y2C°l 7
74.8 Gd2COl7
B1
B2
B3
171 a
174a
179a
C
D 216.8
176 177 175 181
200.8(rh) a 204.4(h) 200 200 198 198
170 171 168 168 168 169 167
184 184 184
194.5 194 194
Co-met.
219
219.2 216.8 225 216.8, t 220, 227 216,221 217,221228 t
Note: All measurements performed at 77 K except a - taken at 4.2 K.
Ref. 1, 8, this work 4 5 6 7 this work 5 4 this work 10
H. Figiel and M. Jaszczewski/ NMR investigation of Y-Co and Gd-Co compounds
frequency interval the lines typical for pure Co are observed [9, 10], which leads to the supposition that in all cases the signal f r o m an excess of C o precipitated at grain boundaries [11, 12] is detected. In such a case the position of C o i l would be at B , overlapping the split C o l I I line. The calculated dipolar fields have c o m p o n e n t s perpendicular to the direction of the magnetic m o ment (table 1). This means that these fields pull magnetization out of the easy plane, i.e. the anisotr o p y in R2Co~7 c o m p o u n d s is not an exact easy plane anisotropy. Therefore large a n d differentiated dipolar fields are p r o b a b l y one of the reasons leading to changes of easy direction by substitutions of 3-d metals a n d rare earths in p s e u d o b i n a r y R2BI7 c o m p o u n d s . The authors thank the Physics Institute of the Polish A c a d e m y of Science, Warsaw, a n d the
Alexander von H u m b o l d t financial support.
675
F o u n d a t i o n for their
References [1] H. Figiel et al., Physica 86-88B (1977) 77.
[2] R. L. Streever, Phys. Lett. 65A (1978) 360. [3] H. R. Kirchmayr and C. A. Poldy, J. Magn. Magn. Mat. 8 (1978) 1. [4] K. Inomata, J. Phys. Soc. Jap. 41 (1976) 1890. [5] H. Nagai ¢t al., J. Phys. Soc. Jap. 41 (1976) 1907. [6] H. Yoshie J. Phys. Soc. Jap. 43 (1977) 862. [7] J. Depor~s and A. Tsuimura, C. R. Acad. Sci. Paris B277 (1973) 333. [8] H. Figiel, E. Dormann and A. Oppelt, Proc. 10th Polish NMR Seminar, Krak6w (1978) p. 243. [9] C. W. Searle, Phys. Rcv. B. 15 (7) (1977) 3305. [10] M. Kawakami et al., J. Phys. Soc. Jap. 33 (1972) 1591. [11] K. H. J. Buschow, J. Less Common Metals 31 (1973) 359. [12] K. H. J. Buschow, F. Den Broeder, J. Less Common Metals 33 (1973) 191.