M. Forker

Rare earth hyperfine interactions studied by perturbed angular correlations

This paper reviews the application of the perturbed angular correlation (PAC/D) technique to studies of rare earth hyperfine interactions. First, PAC/D measurements of the interaction of rare-earth nuclei with the large hyperfine fields of their own partially filled 4f electronic shell are discussed. It is shown that PAC/D spectroscopy is particularly useful for the investigation of 4f spin relaxation in liquid and solid environments, and for the study of the formation and stability of 4f magnetic moments in metallic hosts. The second part of the paper deals with the static hyperfine interaction of non-rare-earth impurity nuclei in the hexagonal, magnetically ordered rare-earth metals. The PAC/D contributions to the systematic investigation of impurity electric field gradients and magnetic hyperfine fields in rare earths are discussed. The importance of PAC/D as the main source of information on the temperature dependence of impurity hyperfine interactions in rare earths is stressed. The trends of the electric field gradients are compared to the behaviour of simpler non-cubic metal systems. The magnetic fields are discussed in the context of current theories.

The magnetic hyperfine field and the electric field gradient at181Ta impurities in Gd metal

The magnetic hyperfine fieldHhf and the electric field gradientVzz at181Ta impurties in metallic Gd were determined by time differential perturbed angular correlation measurements with the 133 keV K-conversion electron 482 keV γ-cascade of181Ta. The sources for these measurements were prepared by implantation of radioactive181Hf ions into Gd. The results are: |Hhf(TaGd; 77 K)|=285(14)kG, and |Vzz(TaGd; 330 K)|=5.32(15)·1017V/cm2. The value ofHhf fits well into the systematics for 5d impurities in Gd and indicates a positive core polarisation contribution, which is expected if the conduction electrons of Gd have to a large extent d-character. The electric field gradients of the 5d impurities in Gd are not consistent with a proportionality between the ionic and the electronic contribution.

Mössbauer study of the magnetic and electric hyperfine interaction of dilute57Fe impurities in rare earth metal hosts

The hyperfine interaction of dilute57Fe in the rare earth (RE)metals Gd to Lu was investigated by Mössbauer measurements with57Co doped RE sources. In all hosts well split, 2-lines spectra were observed at room temperature, with slight asymmetries of the line intensities in some cases. The quadrupole splitting eQVzz/2 increases from 0.29 mm/sec for Gd to 0.50 mm/sec for Tb, and decreases by less than 10 % between Tb and Lu. Only about 10 % of the corresponding electric fieldgradient (EFG) can be accounted for by the ionic EFG on a substitutional RE site. The temperature dependence of the EFG was measured in the case of Tb. No variation within 3 percent was found between 300 K and 700 K.Measurements of the magnetic hyperfine interaction at low temperatures were carried out in Tb. The saturation field of57Fe in this host is Hhf(FeTb;4.2 K)=25(2) KOe. The temperature dependence of the magnetic hyperfine field does not follow the host magnetization (Tc=220K) but vanishes at about 80 K. Similar anomalies of Hhf(T) have previously been observed for other transition element impurities in the RE ferromagnets.

Investigation of the magnetic hyperfine field at197Au impurities in the heavy Rare Earth metals

The Mössbauer effect has been used to measure the magnetic hyperfine field at the site of197Au impurities in the heavy Rare Earth metals Gd, Tb, Dy, Ho and Er at 4.2 K. The magnetic hyperfine field decreases in a non linear way with decreasing spin of the Rare Earth host. For197Au this decrease is stronger than for any other impurity investigated up to now. Possible reasons for this behaviour are discussed.

PAC study of nuclear relaxation in solids:181Ta in hafnium hydrides

The electric quadrupole interaction (QI) at the site of181Ta in different hafnium hydrides HfHx has been investigated as a function of temperature by TDPAC measurements. Below 300 K the QI is static within a 300 nsec time window. Above 300 K a dynamic QI related to the diffusion of hydrogen atoms leads to relaxation effects in the TDPAC spectra. The average residence time between hydrogen jumps, evaluated on the basis of the theory of stochastic perturbations, ranges from 100 nsec at 400 K in the ɛ-phase to 1 nsec at 700 K in the α-phase of the hafnium-hydrogen system. The activation energy for hydrogen jumps was found to be independent of the hydrogen concentration: Ea=0.53(12) eV.

Linear temperature dependence of the electric quadrupole interaction of111Cd impurities in holmium

The electric quadrupole interaction of111Cd impurities in metallic holmium has been investigated between 140 K and 655 K by time differential perturbed angular correlation measurements. The quadrupole frequency decreases with increasing temperature. This decrease is a linear function of temperature, in contrast to theT3/2 dependence usually observed in non-cubic metals. Possible reasons for the deviation from theT3/2 relation are discussed.

Anomalous temperature dependence of the magnetic hyperfine field of99Ru in gadolinium

The magnetic hyperfine field at dilute99Ru impurities in ferromagnetic Gd has been investigated as a function of temperature by time differential perturbed angular correlation (TDPAC) measurements. The saturation field at 11 K is Hhf(Ru Gd)=51(2) KOe. This value fits well into the systematics of 4d impurity hyperfine fields in Gd. The magnetic hyperfine field of RuGd does not follow the magnetization of the host (Tc=290 K) but vanishes abruptly at about 70 K. A similar behaviour has previously been observed for the 5d impurity Os in Gd. From first TDPAC measurements of the hyperfine interactions in the intermetallic phases of the Ru-Sc system it can not be completely ruled out that the observed collapse of the hyperfine field at 70 K is due to the formation of the intermetallic compound RuGd3.

The electric fieldgradient of the transition element impurities57Fe,99Ru,197Au in scandium and193Ir in beryllium

The electric fieldgradient of the transition element impurities57Fe,99Ru,197Au in Sc and193Ir in Be has been determined by Mössbauer measurements. The following values were obtained: (+)4.06(9)⋅1017 V/cm2 for57Fe in Sc, ±19.2(4.1)⋅1017 V/cm2 for99Ru in Sc, ±11.6(4)⋅1017 V/cm2 for197Au in Sc, and ±6.4(1.5)⋅1017 V/cm2 for193Ir in Be. The results for57Fe,99Ru and197Au in Sc are incompatible with the universal correlation between the electronic and the ionic fieldgradient observed in numerous other metals. The data suggest, that a strong contribution to the electric fieldgradient arises from electrons localized at the impurity, if the host is a transition metal and the impurity belongs to the second half of a transition series.

Investigation of the temperature and pressure dependence of the electric field gradient of 111Cd in β-Ce Nd and Y

The nuclear quadrupole interaction of dilute 111Cd impurities has been investigated in the rare-earth metals beta -Ce, Nd and Y by time differential perturbed angular correlation measurements. The quadrupole frequency nu Q has been determined in beta -Ce as a function of temperature between 290 K and 25 K, and in Nd and Y as a function of pressure at 290 K. The quadrupole frequency in beta -Ce, which at 290 K is nu Q (111Cd beta -Ce)=12.5(7) MHz, shows a linear temperature dependence between 25 K and 290 K with a slope of (d ln nu Q/dT)290 K=-13.3(9)*10-4 K-1. The pressure coefficient of nu Q of 111Cd in Nd and Y at 290 K is (d ln nu Q/dP)290 K=+6.8(2,5)*10-3 kbar-1 and +16(3)*10-3 kbar-1 respectively. The systematics of impurity quadrupole interactions in rare-earth metals show that the strength and the temperature and pressure dependence of the electric field gradients at impurities in the rare earth are strongly correlated to the number of 4f electrons of the rare-earth ions. Possible mechanisms behind the observed trends are discussed.

The temperature dependence of the hyperfine interaction of the transition metal osmium in a gadolinium host matrix

AbstractIntegral perturbed angular correlations of the 931-155keVγγ-cascade of188Os in Gd have been measured. With this technique the combined magnetic and electric hyperfine interaction of the 155 keV level of188Os as an impurity in a Gd host has been studied as a function of temperature. The result for the electric field gradient of Os in Gd at 300 K is:n