SPIN-DEPENDENT TUNNELLING IN THE NATURAL PGM–GREIGITE PERCOLATION NETWORK: IMPEDANCE RESONANCES, MAGNETORESISTANCE AND THE EFFECT OF SILVER

The aim of this work was to establish the physical nature of electrical transport in quartz pebbles of the Takyr-Kaldzhir deposit (East Kazakhstan), which contain platinum-group metals (PGM) in nanoscale form, and to determine the material composition at which discrete conduction states are observed. Impedance spectroscopy (0.5–200 kHz, 300 K, zero field and 0.7 T), atomic absorption spectrometry (Au, Ag), inductively coupled plasma atomic emission spectrometry (Pt, Pd, Rh, Os, Ir) and Raman spectroscopy were applied. Plates cut from a single pebble show narrow dielectric-loss resonances at individual frequencies of 91.8, 116.6 and 135.5 kHz with quality factors of 57–135, one to two orders of magnitude above the values typical of Maxwell–Wagner relaxation. In a field of 0.7 T, plates of the same pebble exhibit magnetoresistance ranging from +97 to −39%; an effect of this magnitude at room temperature directly indicates spin-dependent transport. The resonance frequency and the sign of the magnetoresistance change consistently and are described by a single parameter, the mean tunnel-barrier thickness; in the field, the resonance amplitude increases in all plates regardless of the magnetoresistance sign, which is attributed to the suppression of superparamagnetic fluctuations of the core charges. Eight frequencies observed on five plates of a second pebble form three groups whose extreme-centre ratio (1.448) coincides with the resistance contrast of the first pebble (1.476) within 2%. Comparison of two pebbles from the same sample with natural silver contents of 13 and 236 g/t showed that the resonance structure is absent at high silver content; normalisation to iridium revealed selective (13.5-fold) silver enrichment with unchanged ratios of the other platinoids. The results are interpreted in terms of spin-dependent tunnelling through domain walls in a core–shell–bridge–shell–core link. A practical selection criterion is formulated: silver contents of the order of hundreds of grams per tonne preclude the observation of discrete states, which can be verified by routine chemical analysis.