J. Van Cauteren

High resolution TDPAC work using CsF and BaF2 fast detectors

Abstract Barium fluoride and cesium fluoride inorganic scintillators offer excellent timing, reasonable energy resolution and good gamma ray detection efficiencies. These characteristics are very desirable for accurate time differential perturbed angular correlation/distribution (TDPAC/D) experiments and enables one to extend the TDPAC/D techniques for the study of many interesting solid state aspects such as near neighbour interactions, conduction electron polarization, short and long range ordering etc. The performance of these detectors is compared with the commonly used ones and their potential for new solid state applications is demonstrated by means of some illustrative examples.

On the ferromagnetism in the Au82Fe18 alloy

Abstract Perturbed angular correlation on the 111 Cd nucleus as a probe in the Au 82 Fe 18 alloy observes only the spin-glass freezing process but no ferromagnetic spin correlation. This result strongly supports the idea that no long range ferromagnetism and thus no double magnetic phase transition (paramagnet-ferromagnet-spinglass) exists in this alloy.

A TDPAC study of the lattice location of implanted indium in silicon

Abstract The annealing behaviour of indium implanted silicon (doses 1011−1013 atoms/cm2) was studied by time differential perturbed angular correlation spectroscopy. Besides the substitutional site two types of defect configurations were observed. In the as-implanted condition the substitutional In fraction remains below 20% even at the lowest dose. This result is in accordance with the prediction of the track amorphization model.

External-field influence on the magnetic hyperfine fields in alpha-manganese

We report a study on the magnetic nature of the two hyperfine fields observed in α-Mn by PAC. Both of the field sites behave completely differently under the influence of an external field ranging from zero to 4 Tesla. The so-called high field originates from the local moment antiferromagnetism of the matrix, while the low field should be associated with an itinerant magnetism.

PAC study of spin glass freezing in AuFe alloys

We introduce perturbed angular correlation (PAC) on radioactive isotopes as a new and feasible technique to sense the hyperfine field distribution in randomly disordered alloys. This technique is similar to μSR in the sense that it is a nuclear spin precession measurement, but more widely applicable in view of the limited instrumentation. We present our first results obtained using 111Cd as a typical PAC probe in Au–Fe alloys of different magnetic impurity concentration. Our data clearly show the onset of the spin glass freezing and allows one to derive the mean field value as a function of temperature as well as magnetic impurity concentration.

Internal field distribution around spin-glas freezing temperature in AuFe-alloys

We demonstrate an application of PAC to the study of magnetic ordering in random alloys. Using111Cd as a probe the spin-glass freezing inAuFe-alloys of different concentrations could be observed. In the Au82Fe18 alloy no indication of ferromagnetism above the freezing temperature was found, due to the fact that the111In probe nuclei are located in a Au rich environment.

Near-neighbour interactions in iron based alloys

Time differential perturbed angular correlation experiments were carried out using111Cd probe nuclei to measure the change in conduction electron polarization at different lattice sites due to scattering around an impurity atom.

Hyperfine interaction in Ni79Mn21 and Ni76Mn24

We report on PAC measurements revealing very different magnetic behaviour in two nearby alloys. The Ni79Mn21 alloy orders at TC=280(5) K, but a two field sites interpretation is needed. Both hyperfine fields (hff) are in the ratio 1∶2 and their distributions have the same absolute width, the relative abundance of the high field site is 30%. A slope change in the hff (T) curve at low temperature may indicate the transition into the mixed state. In the Ni76Mn24 alloy we observe magnetic ordering at T=95(3) K followed by a smooth increase of the effective hff upon cooling but with an anomalous minimum around T=20 K.

Impact of baf2 detectors on nuclear solid-state investigations

High resolution time differential angular correlation, now achievable at low cost by the fast and efficient BaF2 detectors, will be introduced from some examples recently worked out. The results obtained manifest a new impetus for this technique in its extended application to particular problems in nuclear solid state research.

Satellite hyperfine fields in low concentration NiMn alloys

Concentration- and temperature dependent measurements were made on NiMn-alloys with the perturbed angular correlation technique (PAC). At room-temperature, samples of 1.3; 2.4; 3.6; 5.6; 6.9 and 8.3 at%Mn were examined. Field values on both sides of the Ni-value were observed with a pronounced concentration dependence above 3 at%Mn. The 3.6 at%Mn-sample was examined as function of temperature between 284 K and 573 K. Each of the fields has the same temperature-behaviour, resulting in the same Curie-temperature, which is however smaller than the Curie-temperature for pure Ni.