CPM Seminar

Magnetic hyperfine interactions on ¹⁴⁰Ce in rare-earth magnetic compounds

Artur Carbonari

Instituto de Pesquisas Energéticas e Nucleares (IPEN-CNEN/SP)

The investigation of magnetism from an atomic view in intermetallic compounds containing rare-earth elements is useful to understand the wide variety of interesting physical phenomena, especially different magnetic behaviors, associated to these materials. Measurements of the magnetic hyperfine field (B_hf) at nuclear sites in magnetically ordered compounds are a source of information on the exchange interactions leading to spontaneous magnetic order, on the order of magnetic phase transitions, on spin wave excitations, on relaxation processes, and other parameters characterizing a magnetic system. Magnetic compounds involving rare-earth (R) elements are attractive systems because one finds a large number of isostructural series of compounds that, for different R constituents, differ only slightly in the crystallographic properties, but strongly in the magnetic properties and thus offer very favorable conditions for the separation of the magnetic from other solid-state parameters. B_hf can be measured by techniques based on resonance which use the constituent R element nucleus itself as a probe. However, as rare-earth ions have the 4f shell uncompleted, contributions to B_hf from the orbital angular momentum or core polarization of the probe atom itself are usually rather large and may be different for each rare-earth ion. As a consequence, in a systematic investigation where a series of compounds with different rare-earth elements are being studied and the interaction with the magnetic neighbor is the object of interest, it is not convenient to use different probes in such investigation once the large 4f contribution in each one would mask contributions from the neighborhood. Perturbed gamma-gamma angular correlation (PAC) spectroscopy is a hyperfine interaction technique in which radioactive probe nuclei, which emit gamma rays in cascade, are introduced into the samples with very low concentrations and the time dependence of the gamma-rays emission pattern are measured. In PAC spectroscopy it is therefore possible to use the same probe nucleus to investigate a series of compounds with different rare-earth elements. ¹⁴⁰Ce is a PAC probe that can be used to investigate the local magnetism in rare-earth compounds once it substitutes rare-earth ions. However, as a rare-earth,Ce⁺³ ions have one 4f electron and consequently a localized moment is found in such ions, which also create a contribution from the probe itself to the magnetic hyperfine field. In the present talk we will show how it is possible to separate, from experimental measurements, the contributions from the host and the Ce probe to the B_hf in two series of compounds: RAg and RRh₂Si₂. Moreover, we will also show how PAC can provide useful information on another very interesting and yet not well understood problem: the anomalous temperature dependence of B_hf at probe nuclei of magnetic rare-earth ions diluted in magnetic intermetallic compounds.