Satellite NMR study of the dynamic susceptibility of Fe in Cu above and below Tk

Abstract

2014 NMR relaxation times T1 of 63Cu satellites corresponding to 1st, 2nd and 3rd nearest neighbours of Fe in Cu-Fe dilute alloys have been measured in 55 kG for 1.2 K T 310 K. These data allow a direct determination of the relaxation rate 03C4-1 of the local magnetization of Fe. It is found to change from a constant value which characterizes the strong coupling state of Fe below TK to a Korringa like behaviour 03C4-1 03B1T for T ~ TK, as qualitatively expected from theoretical calculations on Kondo effect. LE JOURNAL DE PHYSIQUE LETTRES Classification Physics Abstracts 8 . 526 8 . 664 8 . 630 TOME 37, SEPTEMBRE 1976, It has been clear for a long time that spin flip processes and consequently the fluctuations of the local magnetization play a major role in the establishment of the Kondo state of transition metal impurities in noble metal matrices [1]. It has thus been a great challenge for experimentalists to measure the frequency dependence of the local susceptibility, or at least its spectral width, which in ideal cases can be identified with the inverse of a spin autocorrelation time T (or spin lattice relaxation time in spin resonance terminology). A great number of experimental techniques such as ESR [2], impurity [3] or host NMR [4, 5], Mossbauer effect [6], and neutron scattering [7] have been used to determine i-1 in various characteristic systems. However up to now such experiments were only successful for limited ranges of temperature, either r ~ TK or T ~ TK. In the classic CuFe Kondo system for which TK ~ 30 K, recent nuclear spin lattice’relaxation data for the near neighbour ~ 3Cu nuclei of Fe showed that for 7 ~ TK z has a constant value ’[-1 f’OoJ kB 7~/~ [5]. In this letter we present an extension of these measurements to room temperature, where for the first time the variation of i -1 could be accurately followed from well below to well above TK (30 K). Measurements have been carried out with pulse NMR techniques at about 60 MHz on the three 63CU satellites A, M, B [8] which correspond to the 1 st, (*) Laboratoire associe au C.N.R.S. 2nd and 3rd nearest neighbours to Fe. Full details of the experimental technique will be reported elsewhere. Here the physical significance of the nuclear T1 data which are summarized in figure 1 will be discussed. For satellite B data could be taken for a large range of concentration, showing that the measured T1 values are characteristic of neighbours of isolated impurities. T1-1 is proportional to T below 4.2 K, goes through a maximum ( ~ 50 K) then a minimum ( ~ 150 K) and finally approaches the pure copper Korringa rate (TiK T = 1.28 s K) at higher temperature. The shorter relaxation times and lower sensitivity make T1 measurements more difficult and less accurate for the first and second nearest neighbours. It was shown in ref. [5] that, at low T, for various satellites, the relaxation is dominated by impurity induced relaxation due to isotropic hyperfine coupling with the transverse fluctuations of the impurity magnetization. For a shell of neighbours at distance r from the impurity this extra relaxation rate is given by : where XT(W) is the transverse susceptibility of the impurity and WD the nuclear Larmor frequency. The isotropic transferred hyperfine coupling constant As(r) can be deduced from the differential Knight Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyslet:01976003709020500 L-206 JOURNAL DE PHYSIQUE LETTRES FIG.1. NMR relaxation rates of the 1 st (A), 2nd (M) and 3rd (B) shells of neighbours of Fe in Cu measured near 55 kG. The impurity induced relaxation rate (7~?)’~ is obtained by subtracting the Korringa rate (TIK) ’ of pure Cu. The dashed lines for satellites A and M are deduced from the solid line (which approximates the data for B), by assuming that (T’mP)-’ scales with (âK)2. shift of each shell of nuclei with respect to pure