- Email: [email protected]
NMR and local anisotropy in YCo4B
Journal of Magnetism North-Holland
and Magnetic
Materials
83 (1990) 153-154
NMR AND LOCAL ANISOTROPY Cz. KAPUSTA, Department
N. SPIRIDIS’and
153
IN YCo,B H. FIGIEL
of Solid State Physics, Academy of Mining and Metallurgy,
al. Mickiewicza
30, 30-059 Cracow, Poland
The 59Co NMR spin-echo spectra of YCo4B and NdCo,B were measured. The observed very low values of hyperfine field on both Co sites in the compounds indicate on large orbital contribution to Co hyperfine field and to local anisotropy of Co sites.
The polycrystalline samples of YCo_,B and NdCo,B were investigated by use of NMR spin-echo technique at 4.2 K within the frequency range from 20 up to 230 MHz. To obtain the spin-echo spectra the sequence l/30/2 l.ts of exciting pulses has been applied. For the peak frequencies the transverse relaxation times were measured. The compounds crystallize in the hexagonal CeCo, B type structure. The Co ions occupy two types of crystallographic sites: the axial 2c and the orthorhombic 6i site. The compounds reveal a significant magnetocrystalline anisotropy which in the case of YCo,B originates from Co sites only. The Y-based compound undergoes a change of easy magnetisation direction (e.m.d.) from c-axis to c-plane at 145 K [l]. At the temperature of our NMR measurements YCo,B possesses the easy c-plane anisotropy ( EA = -3.5 X 10’ J/m3) [2], whereas NdCo,B has the e.m.d. along the c-axis [3]. The obtained NMR spectra are shown in fig. 1. The resonance lines in the range 20-45 MHz were attributed to 59Co signals of the compounds. Also the traces of 59Co signals at 84 MHz originating from Co,B impurity phase were detected in both samples. In order to assign the resonance lines and corresponding effective fields B, to the Co sites of the compounds the intensities of resonant lines have been compared with the site abundances. The assignment marked in fig. 1. The corresponding Be are equal to 3.1 and 2.5 T for the 6i site and 4.0 and 3.8 T for the 2c site in YCo,B and NdCo,B, respectively. These values are more than twice smaller than in Co,B [4] having the same average value of the cobalt moment p = 0.7~~ per Co ion. To explain the reason for this discrepancy the following contributions to B, were considered: B,=B,,+&r,
where B, is the term originating from cores and conduction electron polarization by Co spin moment itself, B orb is the orbital field of 3d electrons expressed as: B orb
=
2(k)(‘-3h~
where (pr) is the mean value of the orbital contribution to Co magnetic moment, B, is the polarization contribution from magnetic neighbours via conduction electrons (of RKKY type), To analyse and compare the magnitudes of all terms contributing to B, in both compounds the directions of magnetic moments of Co ions have to be determined. The NMR signal of the compound with easy plane type anisotropy (YCo,B) comes out from nuclei of ions having their magnetic moments in the easy plane. In the case of NdCo_,B with e.m.d. along the c-axis the dominant NMR signal originate from the centers of 180” Bloch walls, where the ionic magnetic moments lie in the c-plane. The spectrum of NdCo,B requires a very 10
20
30
40
60
60
70
YCo4B
60 Ll -1 .
(1)
where B,, = B, + B, + Bdlp and B,: demagnetizing dipolar field.
(2) field,
B,:
Lorentz
field,
B,,=Bs+&,+B,, 0304-8853/90/$03.50 (North-Holland)
Bdlp:
(3) 0 Elsevier Science Publishers
B.V.
10
20
30 40 frequency
50
60 MHz
70
60
Fig. 1. The s9Co NMR spectra of YCo,B and NdCo,B at 4.2 K. The line assignment to Co sites and the transverse relaxation times T2 are marked.
154
Cz. Kapustn et al. / NMR and local anisotropy in YCo, B
low rf power level for optimum excitation conditions, indicating a large value of enhancement factor, as usually observed for signals arising from the middle of the domain walls. Also the short transverse relaxation times T2 of order 50 us are consistent with the above interpretation. So that the 59Co spectra of both compounds are interpreted as originating from Co sites with magnetic moments within the c-plane. The difference of B, between Y and Nd based compounds equals to 0.6 T for 6i site and 0.2 T for 2c site. Assuming the same values of Co moments in both compounds the above differences are caused by the change of B,, and B, with replacing of Nd for Y. The contribution of Nd and Co moments to Bdip at both Co sites were calculated. The values of pcO = 0.7~~ and pLNd= 3~~ taken from saturation magnetisation [3] were used. The summation was performed within spheres of 0.8 nm for the moments along a and b hexagonal axes, respectively. The Nd contribution reaches 0.2 T at the 2c site and 0.45 T with a splitting of 0.5 T between adjacent values at the 6i site. The corresponding Co contributions are about’two times smaller. The change of Nd for Y which of B, and B, with substitution increases two times the saturation magnetisation, may be evaluated to 0.2 T. So that the extra B, contribution may be at most of the order of B,,,r and the same limit for Co contribution to B, may be deduced. On the basis of the above analysis it may be stated that B,, Be only. and B, are a small part of the observed Treating the Co moment in the compounds as a pure spin moment one can calculate the expected value of B, contribution using relation: Bs = Asus
The authors wish to thank Dr. T.T. Dung for providing the sample. The work was supported by the Institute of Physics of the Polish Academy of Sciences in Warsaw (contract C.P.B.P. 01.04).
(5)
with the hyperfine spin coupling constant A, = -12 T/pa which is valid for variety of cobalt rich ferromagnetic compounds as Co,B, YCos, Y,Co,, and metallic Co as well. Taking for the average Co moment in YCo,B the value p = 0.7j.~a one gets B, = - 8.4 T. The experimental absolute values of Be are more than two times smaller, indicating the presence of a considerable orbital contribution, opposite in sign to the spin contribution: B orb ‘Al_PL~
where the hyperfine orbital coupling constant A, = 65 of the Co T/pa and uL is the orbital component magnetic moment. The value of /.tL strongly depends on the local symmetry of the sites and the orientation of their magnetic moments, following changes of the effective orbital momentum L of d-electrons participating in formation of local Co magnetic moment. The spin-orbit interaction expressed as E,, = XL . S gives the contribution to the local anisotropy energy of the Co sites. From the difference between the calculated B, and the experimental values of Be the values of Barb at both Co sites in the compounds may be derived. If the negative sign of Be is assumed, Barb may be evaluated as 4 T, which corresponds to the value of uL = 0.06ua. As the spin-orbit coupling constant h is negative for cobalt, the value of the orbital moment is larger for the local easy direction of the site, than for the local hard direction. The higher Be at the 2c site in both compounds suggest that the c-axis is an easy direction at this site, which is consistent with the prediction of ref. [l] and analogous to the preference of the 2c site in YCo, [5]. At the 6i site Co prefers the alignment of magnetic moments within the c-plane, resulting in the c-plane type anisotropy of YCo,B at 4.2 K.
(6)3
References
111T.T. Dung, N.P. Thuy, N.M. Hong and T.D. Hien, Phys. Stat. Sol. (a) 105 (1988) 536. R. Chevalier, J. Deportes, B. Kebe and R. Lemaire, J. Magn. Magn. Mat. 68 (1987) 190. [31 A.T. Pedziwiatr, S.Y. Jiang, W.E. Wallace, E. Burzo and V. Pop, J. Magn. Magn. Mat. 66 (1987) 69. Solid State Commun. 16 [41 B. Lemius and R. Kuentzler, (1975) 639. I51Quan Lu. Doctor Thesis, Univ. Grenoble (1981).
121 M.T. Averbuch-Pouchot,
and Magnetic
Materials
83 (1990) 153-154
NMR AND LOCAL ANISOTROPY Cz. KAPUSTA, Department
N. SPIRIDIS’and
153
IN YCo,B H. FIGIEL
of Solid State Physics, Academy of Mining and Metallurgy,
al. Mickiewicza
30, 30-059 Cracow, Poland
The 59Co NMR spin-echo spectra of YCo4B and NdCo,B were measured. The observed very low values of hyperfine field on both Co sites in the compounds indicate on large orbital contribution to Co hyperfine field and to local anisotropy of Co sites.
The polycrystalline samples of YCo_,B and NdCo,B were investigated by use of NMR spin-echo technique at 4.2 K within the frequency range from 20 up to 230 MHz. To obtain the spin-echo spectra the sequence l/30/2 l.ts of exciting pulses has been applied. For the peak frequencies the transverse relaxation times were measured. The compounds crystallize in the hexagonal CeCo, B type structure. The Co ions occupy two types of crystallographic sites: the axial 2c and the orthorhombic 6i site. The compounds reveal a significant magnetocrystalline anisotropy which in the case of YCo,B originates from Co sites only. The Y-based compound undergoes a change of easy magnetisation direction (e.m.d.) from c-axis to c-plane at 145 K [l]. At the temperature of our NMR measurements YCo,B possesses the easy c-plane anisotropy ( EA = -3.5 X 10’ J/m3) [2], whereas NdCo,B has the e.m.d. along the c-axis [3]. The obtained NMR spectra are shown in fig. 1. The resonance lines in the range 20-45 MHz were attributed to 59Co signals of the compounds. Also the traces of 59Co signals at 84 MHz originating from Co,B impurity phase were detected in both samples. In order to assign the resonance lines and corresponding effective fields B, to the Co sites of the compounds the intensities of resonant lines have been compared with the site abundances. The assignment marked in fig. 1. The corresponding Be are equal to 3.1 and 2.5 T for the 6i site and 4.0 and 3.8 T for the 2c site in YCo,B and NdCo,B, respectively. These values are more than twice smaller than in Co,B [4] having the same average value of the cobalt moment p = 0.7~~ per Co ion. To explain the reason for this discrepancy the following contributions to B, were considered: B,=B,,+&r,
where B, is the term originating from cores and conduction electron polarization by Co spin moment itself, B orb is the orbital field of 3d electrons expressed as: B orb
=
2(k)(‘-3h~
where (pr) is the mean value of the orbital contribution to Co magnetic moment, B, is the polarization contribution from magnetic neighbours via conduction electrons (of RKKY type), To analyse and compare the magnitudes of all terms contributing to B, in both compounds the directions of magnetic moments of Co ions have to be determined. The NMR signal of the compound with easy plane type anisotropy (YCo,B) comes out from nuclei of ions having their magnetic moments in the easy plane. In the case of NdCo_,B with e.m.d. along the c-axis the dominant NMR signal originate from the centers of 180” Bloch walls, where the ionic magnetic moments lie in the c-plane. The spectrum of NdCo,B requires a very 10
20
30
40
60
60
70
YCo4B
60 Ll -1 .
(1)
where B,, = B, + B, + Bdlp and B,: demagnetizing dipolar field.
(2) field,
B,:
Lorentz
field,
B,,=Bs+&,+B,, 0304-8853/90/$03.50 (North-Holland)
Bdlp:
(3) 0 Elsevier Science Publishers
B.V.
10
20
30 40 frequency
50
60 MHz
70
60
Fig. 1. The s9Co NMR spectra of YCo,B and NdCo,B at 4.2 K. The line assignment to Co sites and the transverse relaxation times T2 are marked.
154
Cz. Kapustn et al. / NMR and local anisotropy in YCo, B
low rf power level for optimum excitation conditions, indicating a large value of enhancement factor, as usually observed for signals arising from the middle of the domain walls. Also the short transverse relaxation times T2 of order 50 us are consistent with the above interpretation. So that the 59Co spectra of both compounds are interpreted as originating from Co sites with magnetic moments within the c-plane. The difference of B, between Y and Nd based compounds equals to 0.6 T for 6i site and 0.2 T for 2c site. Assuming the same values of Co moments in both compounds the above differences are caused by the change of B,, and B, with replacing of Nd for Y. The contribution of Nd and Co moments to Bdip at both Co sites were calculated. The values of pcO = 0.7~~ and pLNd= 3~~ taken from saturation magnetisation [3] were used. The summation was performed within spheres of 0.8 nm for the moments along a and b hexagonal axes, respectively. The Nd contribution reaches 0.2 T at the 2c site and 0.45 T with a splitting of 0.5 T between adjacent values at the 6i site. The corresponding Co contributions are about’two times smaller. The change of Nd for Y which of B, and B, with substitution increases two times the saturation magnetisation, may be evaluated to 0.2 T. So that the extra B, contribution may be at most of the order of B,,,r and the same limit for Co contribution to B, may be deduced. On the basis of the above analysis it may be stated that B,, Be only. and B, are a small part of the observed Treating the Co moment in the compounds as a pure spin moment one can calculate the expected value of B, contribution using relation: Bs = Asus
The authors wish to thank Dr. T.T. Dung for providing the sample. The work was supported by the Institute of Physics of the Polish Academy of Sciences in Warsaw (contract C.P.B.P. 01.04).
(5)
with the hyperfine spin coupling constant A, = -12 T/pa which is valid for variety of cobalt rich ferromagnetic compounds as Co,B, YCos, Y,Co,, and metallic Co as well. Taking for the average Co moment in YCo,B the value p = 0.7j.~a one gets B, = - 8.4 T. The experimental absolute values of Be are more than two times smaller, indicating the presence of a considerable orbital contribution, opposite in sign to the spin contribution: B orb ‘Al_PL~
where the hyperfine orbital coupling constant A, = 65 of the Co T/pa and uL is the orbital component magnetic moment. The value of /.tL strongly depends on the local symmetry of the sites and the orientation of their magnetic moments, following changes of the effective orbital momentum L of d-electrons participating in formation of local Co magnetic moment. The spin-orbit interaction expressed as E,, = XL . S gives the contribution to the local anisotropy energy of the Co sites. From the difference between the calculated B, and the experimental values of Be the values of Barb at both Co sites in the compounds may be derived. If the negative sign of Be is assumed, Barb may be evaluated as 4 T, which corresponds to the value of uL = 0.06ua. As the spin-orbit coupling constant h is negative for cobalt, the value of the orbital moment is larger for the local easy direction of the site, than for the local hard direction. The higher Be at the 2c site in both compounds suggest that the c-axis is an easy direction at this site, which is consistent with the prediction of ref. [l] and analogous to the preference of the 2c site in YCo, [5]. At the 6i site Co prefers the alignment of magnetic moments within the c-plane, resulting in the c-plane type anisotropy of YCo,B at 4.2 K.
(6)3
References
111T.T. Dung, N.P. Thuy, N.M. Hong and T.D. Hien, Phys. Stat. Sol. (a) 105 (1988) 536. R. Chevalier, J. Deportes, B. Kebe and R. Lemaire, J. Magn. Magn. Mat. 68 (1987) 190. [31 A.T. Pedziwiatr, S.Y. Jiang, W.E. Wallace, E. Burzo and V. Pop, J. Magn. Magn. Mat. 66 (1987) 69. Solid State Commun. 16 [41 B. Lemius and R. Kuentzler, (1975) 639. I51Quan Lu. Doctor Thesis, Univ. Grenoble (1981).
121 M.T. Averbuch-Pouchot,