L’étude des alliages dilués par orientation nucléaire

Nuclear orientation studies on dilute alloys

Laboratoire de Physique des Solides, Bât. 510 — F 91 405 - Orsay, France.

Résumé

Nous décrivons les renseignements atteints dans les expériences d’orientation nucléaire effectuées sur les alliages dilués en insistant surtout sur les effets à une impureté (Effet Kondo). Nous citons un certain nombre d’exemples caractéristiques : — l’Au Mn : comportement d’une impureté paramagnétique pratiquement libre — le Cu Mn : premier cas de déviation à ce comportement — le Pt CO : cas d’un fort couplage résonnant avec les électrons de conduction — le Pd CO : problèmes posés par la non- détection par O. N. du moment géant de l’impureté, l’Au CO: décomposition du magnétisme orbital et de spin — les alliages de terre rare (impuretés de Ce et d’Yb) : influence du champ cristallin et du couplage résonnant. La présence d’interactions entre impuretés se manifeste, suivant le mode de couplage, par des effets géométriques de désalignement macroscopique des noyaux.

Nous précisons les possibilités de l’orientation nucléaire par rapport aux autres techniques expérimentales : chaleur spécifique — aimantation — effet Mõssbauer — RMN. Nous profitons de la nécessité d’expliquer les tentatives infructueuses de détecter la RMN de l’impureté par destruction de l’anisotropie γ pour aborder le problème mal connu de l’étude dynamique des fluctuations Kondo.

Abstract

The aim of this article is to describe nuclear orientation (N. O.) experiments which have been performed on dilute magnetic alloys. Although N. O. has been frequently employed to measure the hyperfine field on magnetic or non-magnetic impurities in ferromagnetic metal hosts, its use for the study of dilute magnetic alloys is more recent and is related to the general interest in this field since the explanation by Kondo of the resistivity minimum phenomenon. By N. O. one can study at almost 0° K the appearance and behaviour of magnetism for an isolated impurity as a function of an applied magnetic field. The experimenter can hope to measure in this way the localized magnetisation; we will discuss the difficulties associated with relating the γ anisotropy which is actually measured to the impurity magnetisation.

Chapter 1. The principle of N. O. is described simply so that non-specialists can appreciate the physical concepts behind the method. A brief comparison with other hyperfine techniques is given.

Chapter 2. The study of a paramagnetic impurity in a normal metal is split into two parts; first, weak resonant coupling or weak correlation corresponds to an isolated magnetic moment submitted essentially to relaxation by the conduction electrons; secondly, in strong resonant coupling or strong correlation we have a situation which is more complicated than one of relaxation, as there is a correlation between the impurity and the conduction electron which depends on the previous history of scattering of other electrons. The transition between these two regimes can be characterized by a temperature θ which is related to the Kondo temperature T_K or the spin fluctuation temperature T_sf. For simplicity we will identify θ with T_K. In the case of rare earth impurities, we emphasize the validity of an ionic approach even for strong resonant coupling.

Chapter 3. Low field N. O. experiments show by the behaviour of the hyperfine field that we pass from a situation T > 7_K, where the local moment S is strongly correlated with the nuclear spin 1 to a situation T ⪡ T_K where S is mainly correlated with the spins of the conduction electrons.

In this second situation, N. O. can be described using an effective field H_ceff proportional to the average local magnetisation. Three experimental examples illutrate this effect: Au Mn (T ⪢ T_K, Cu Mn (T ~ T_K) Pt (T ⪡ T_K).

Chapter 4. The results on 3d impurities are presented and show three main points:

• i)

  Information obtained from the saturation hyperfine field H_n (sat) and the ratio R of the hyperfine field to the impurity magnetisation.

• ii)

  The different examples of strong and weak coupling and possible comparisons with direct magnetisation and NMR measurements.

• iii)

  The anomalous behaviour of Pd Co (H_eff > 0, and apparent absence of induced moment).

Chapter 5. N. O. measurements on rare earth impurities consist essentially of experiments on Ce and Yb because these are liable to show Hondo effects. We emphasize the importance of the crystal field (Au Ce, Ag Ce, La Ce) and the variation of the apparent 7V with measuring temperature.

Chapter 6. The RKKY coupling at non-zero impurity concentration tends to produce incomplete alignment of impurities with respect to the external field. This leads to an apparent reduction in H_eff when N. O. results are expressed in terms of equivalent H_eff. Experimental examples are Pd_1—x Co_x (x < 0,002) and Cu Mn. Pt Co alloys have a different behaviour as at low concentrations the dominant effect is that of the drop of T_K and at high concentrations impurity interactions are ferromagnetic.

Chapter 7. N. O. iscompared with other techniques — magnetisation, NMR, Mossbauer effect, in particular for cases where different measurements on the same alloys can be compared directly. We discuss Pt Co, Au Co, Cu Mn, Pd Co, Pt Co and Pd Fe. It is shown for instance that in the alloy Pt Co the moment induced on the neighbours is proportional to that on the impurity at all fields, and that in Au Co the NMR and N. O. techniques give comparable results. We point out the possibilities offered by the low temperature Mossbauer technique.

Chapter 8. This chapter is concerned with discussing the possibilities for dynamic observations on the impurity. After recalling that the collisions of electrons with spin Rip show up above T_K as a reinforcement of mutuel spin fluctuations of the local moment and of the electron gas we show the limits of the classical EPR and NMR methods and show what information can be obtained by N. O. The failure of attempts to detect resonant destruction of N. O. on impurities with low T_K is discussed. Other radioactive methods are more suitable because the criteria for relaxation detection are different. The Mossbauer effect observation of a Hondo anomaly in Au Yb (Gonzales + Imbert) is the first experimental proof of a deviation from Horringa law due to higher order Kondo terms.

Appendix. We mention the influence of the hyperfine coupling on very low temperature susceptibility measurements.