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COMMENTS ON NUCLEAR ORIENTATION EXPERIMENTS IN DILUTE AuCo ALLOYS :
COMPARISON WITH NMR AND MAGNETISATION MEASUREMENTS
J. Boysen, W. Brewer, J. Flouquet
To cite this version:
J. Boysen, W. Brewer, J. Flouquet. COMMENTS ON NUCLEAR ORIENTATION EXPERIMENTS IN DILUTE AuCo ALLOYS : COMPARISON WITH NMR AND MAGNETISATION MEASURE- MENTS. Journal de Physique Colloques, 1974, 35 (C1), pp.C1-5-C1-6. �10.1051/jphyscol:1974103�.
�jpa-00215478�
JOURNAL DE PHYSIQUE Colloque C I, s~rpplirner~t au no 1 , Torne 35, Janliier 1974, page 5
COMMENTS ON NUCLEAR ORIENTATION EXPERIMENTS IN DILUTE AuCo ALLOYS :
COMPARISON WITH NMR AND MAGNET1 SATION MEASUREMENTS
J. BOYSEN and W. D. B R E W E R
Fachbereich Physik der Freien Universitat, Berlin-W, Germany J. F L O U Q U E T
Laboratoire de Physique des Solides, Facultt des Sciences, F-91405 Orsay, France
Resume. - Nous presentons, sur I'exemple de I'A-o, les renseignements extraits de la compa- raison entre les experiences classiques d'aimantation et les expkriences hyperfines (ON, N M R ) . Abstract. - The usefulness of combining classical magiletisation measurements and hyperfine measurements (e. g. N M R or Nuclear Orientation (NO)) on dilute alloys is discussed. Recent measurements on dilute $Co are presented as an example.
The comparison between classical magnetisation measurements and hyperfine interaction experiments ( N M R , NO) in dilute alloys is complicated by several factors. Even in systems like C u M n o r CuCr [I], where the magnetism is solely >ue to s p F e f f e c t s , it is necessary to carry out the experiments a t approxi- mately the same temperature since the exact tempera- ture dependence of the impurity magnetisation m l ( H , T) is unknown for T < T,, (spin-fluctuation o r K o n d o temperature) [2]. However, practical considerations restrict the temperature ranges of the various methods so that this condition can only be fulfilled when T,, is large, so that both the magnetisa- tion a n d hyperfine ~neasurements can be considered t o be in the low temperature limit :
Furthermore, the use of impurity N M R is restricted to cases with high T,, because otherwise the nuclear relaxation times T,, T, become too fast [3], [4] (").
The dilute alloy system AuCo provides ;I nearly unique case for such a comparison, since T,, is large, while the Knight shift K = (H,,, - H)/I H I is also sufficiently large to permit accurate N O measure- ments [ 5 ] , [8]. Thus a combination of NO, N M R and susceptibility measurements is possible. As has been pointed out [6], orbital effects rnay play a signifi- cant role in K, s o the separation of spin and orbital contributions is important. This cannot be done with the susceptibility measurements alone, since they y~eld
(*) Pulscd N M R at very low ternlxr.itul.cs ( T g 100 m K ) may.
in fact, be feasible even in systems with low T.I..
a single quantity x ( H , T). However, combination of K and [8], [9] o r of K and T I [7], allows the separa- tion (if spin and orbital effects are assumed t o act independently). The Knight shift is then given by :
where the R's are the proportionality constants bet- ween the local atomic moments and the resulting hyperfine fields. Since R,,, > 0 and R,,;,, < 0, a measurement of the sign of H,,, is essential to an interpretation of experimental results [S]. 111 the case of AuCo, an analysis assuming only spin moments l e a d s t o a n erroneous result (see [8]). The values for h.
determined from the two hyperfine methods S ~ O M
excellent agreement :
indicating proportionality of the individual contribu- tions to the hyperfine field to the spin and orbital moments. A measurement of the sign of H,,, can be made by observing /)-particle symmetries from oriented nuclei [8] o r by using a polarized rf field i n N M R [7].
The quantities K, T, (and T2) can thus be obtained from N M R subject to the limitation T
<
T,, alread!.mentioned. Nuclear orientation. by contrast, i \ applicable also when T > Ts, (e. g. AuMn, Rose- Gorter alignment case [lo]). ~ u r t l ~ e r m o r e ; even when K is too snlall to be determined nccuratcly by NO.
T , can still be measured by the pitlsed-heating methoti (see [I], ref. [20]). Also, in the case o f strongly inter- acting groups of atoms (e. g. the m;~gnetic triplet\
in - AuCo). - the N M R signal is rcmovcd by the wipe-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1974103
CI-6 J. BOYSEN, W. D. BREWER AND J. FLOUQUET out effect [7], while NO can still be used to determine
the sign and magnitude of H,,, [I I].
Magnetisation measurements in all parts of tlie temperature range suffer from tlie difficulty of separat- ing m , from interaction effects. When T
<
T,,,the single impurity signal is small so that higher concen- trations must be used and n l , deduced by an analysis of concentration effects [9], [12]. In the case T 2 T,,, interaction effects dominate the measurement, which must then be made at higher temperatures and then
extrapolated to find the low temperature limit, a process which is hampered by lack of a theoretical temperature dependence function [13], [14].
In summary, the NO method has two advantages : the ability to work in the extreine dilution limit where interaction effects are unimportant, a n d the possibility of making measurements in all parts of the temperature range relative to T,,. In the cases where a detailed comparison has been possible (AuCo [8], PtCo [12], [15]), a proportionality of H,,, a n d r n , has b& found.
References
[l] FLOUQUET, J., AIIIIIS cle Plzys. (to be published).
[2] KONDO, J., Solid Stote Pl~j~sics, vol. XXIII (Academic Press), 1969.
[3] WALSTEDT, R. F. and NARATH, R., Phjs. Rev. B 6 (1972) 4118.
[4] SPANJAARD, D., HARTMANN, F. (to be published).
[5] HOLLIDAY, R. J. and WEYHMANN, W., PIIJ's. Re\'. Lett. 25 (1970) 243.
[6] DWORIN, L. and NARATH, A,, Phys. Rev. Lett. 23 (1970) 1287.
[7] NARATH, A. and BARHAM, B., Phys. Rev. B 7 (1973) 2195.
[8] BOYSEN, J., BREWER, W. D., FLOUQUET, J., Solid State Cotlltnrtrl. 12 (1973) 1095.
[9] THOLENCE, D. L. and TOURNIER, R., Phjs. Rev. Lett. 25 (1970) 13.
[lo] GORTER, C. J., Physicn 14 (1948) 504.
[ I l l BOYSEN, J., BREWER, W. D., KLEIN, E. (to be published).
[12] TISSIER, B. and TOURNIER, R., Solid Slate Commun. 11 (1972) 895.
[13] HURD, C. M., J. Phys. & Chem. Solids 30 (1969) 539.
[14] MANHES, B., Thesis (Grenoble, 1971).
[I51 ALI, M., BREWER, W. D., FLOUQUET, J., GALLOP, J. C., KLEIN, E. (to be published).
