Muons and magnets

Abstract:

The F-μ-F state provides a unique signature in muon spin rotation data and is observed in many fluorides. It gives rise to a characteristic oscillatory muon polarization that results from entanglement between the spins of the muon and nearby fluorine nuclei. If the muon hops from site to site, then the relaxation of this oscillatory signal can increase. The usual method to treat muon hopping in fluorides assumes the strong-collision approximation, which is then used to dynamicize the standard F-μ-F relaxation function. This approach presupposes Markovian dynamics and so neglects the quantum entanglement between a muon and a neighbouring spin to which it may still be strongly coupled, even after a hopping event. Entanglement that persists between a hopping event becomes even more important to include in the case when a muon subsequently hops back to its initial position. This article demonstrates the effect of including these coherent effects which results in dynamics that depart from those that are calculated using the strong-collision approximation.

Abstract:

We report muon-spin relaxation (μ+SR) measurements of copper acetate [Cu(CH₃CO₂)₂·H₂O], a model spin-1/2 Heisenberg antiferromagnetic dimer chain with an alternation parameter α = 0.001. Zero-field μ⁺SR data collected from 2 to 200 K revealed an oscillatory asymmetry that was analyzed using a model based on the muon-stopping site determined by density-functional theory +μ calculations. Below 50 K, the fitted parameters capture spin dynamics characteristic of the singlet ground state, while at higher temperatures, an additional relaxation was observed due to the thermally populated triplet state, affecting the local magnetic field around the muon-stopping site. The temperature dependence of the fitting parameters was found to exhibit characteristics similar to those of a bipartite entanglement measure, “distance between the states,” obtained from the magnetic susceptibility data. Longitudinal-field μ⁺SR measurements reveal field-dependent relaxation at field values much lower than the field values required to close the spin singlet-triplet gap, emphasizing the importance of quantum fluctuations in the spin dynamics of the dimerized copper acetate.

Abstract:

The interplay between magnetism and charge transport is central to understanding colossal magnetoresistance (CMR), a phenomenon well studied in ferromagnets. Recently, antiferromagnetic (AFM) EuCd2P2 has attracted considerable interest due to its remarkable CMR, for which magnetic fluctuations and the formation of ferromagnetic clusters have been proposed as key mechanisms. Here we provide direct evidence that these effects originate from the formation and percolation of magnetic polarons. We employ a complementary set of sensitive probes that allows for a direct comparison of electronic and magnetic properties on multiple time scales revealing pronounced electronic and magnetic phase separation below T* ≈ 2TN. These measurements indicate an inhomogeneous, percolating electronic system below T* and well above the magnetic ordering temperature TN = 11 K. In applied magnetic fields, the onset of the pronounced negative MR in the paramagnetic regime emerges at a universal critical magnetization. The characteristic size of the magnetic polarons near the percolation threshold is estimated to be ~6−10 nm. Our results establish dynamic polaron percolation within an AFM matrix as the microscopic origin of CMR in EuCd2P2, providing a unified framework for magnetotransport in Eu-based correlated semiconductors.