epl draft Remanence effects in the electrical resistivity of spin glasses

We have measured the low temperature electrical resistivity of Ag : Mn mesoscopic spin glasses prepared by ion implantation with a concentration of 700 ppm. As expected, we observe a clear maximum in the resistivity ρ(T ) at a temperature in good agreement with theoretical predictions. Moreover, we observe remanence effects at very weak magnetic fields for the resistivity below the freezing temperature Tsg: upon Field Cooling (fc), we observe clear deviations of ρ(T ) as compared with the Zero Field Cooling (zfc); such deviations appear even for very small magnetic fields, typically in the Gauss range. This onset of remanence for very weak magnetic fields is reminiscent of the typical signature on magnetic susceptibility measurements of the spin glass transition for this generic glassy system. Spin glasses are one of the most intriguing states of matter, and have been the subject of intense theoretical as well as experimental works over the last decades [1, 2]; this is due to the fact that they can be considered as the most generic example of a glassy phase [3] but also because the very nature of the ground state of these systems is still heavily debated and may eventually consist in a very peculiar state of matter [4,5]. In spin glasses (sg), magnetic moments occupying random positions in a metallic host interact via rkky interactions; the randomness in the position of these moments leads to a randomness in both the sign and the amplitude of their mutual interaction. Below the spin glass transition temperature Tsg, the interplay between disorder and frustration will freeze the magnetic spins in a complex way far from being well understood. Most of the experimental studies on spin glasses has been carried out on macroscopic systems: only few papers addressed the question of mesoscopic spin glasses, either theoretically [6–8] or experimentally [9–11]. Moreover, most of the measurements concerned thermodynamic quantities, mainly magnetic susceptibility [12,14,15]; very few experiments addressed the question of the transport properties of mesoscopic spin glasses [17–19] and their interpretation remains a theoretical challenge [20–22]. The most convincing signature that the spin glass transition may actually be a real phase transition is the cusp which appears in the low field magnetic susceptibility at the transition temperature [12]. On the other hand, one of the most fascinating properties of spin glasses, which makes this state of matter a very peculiar one, is the onset of remanence effects for very weak magnetic fields below Tsg: the dc susceptibility χdc of a spin glass depends strongly on the way the experiment is performed, in particular if the cooling through Tsg has been made under a small magnetic field (Field Cooled) or without magnetic field (Zero Field Cooled). Experimentally, χdc is much larger after Field Cooling than after Zero Field Cooling [12, 13]. We would like to stress that the magnetic fields involved in these experiments are very small, typically in the Gauss range, i.e. μBB ≪ kBTsg, with μB p-1 ha l-0 05 37 11 4, v er si on 1 17 N ov 2 01 0 Author manuscript, published in "Europhysics Letters (EPL) 93, 2 (2011) 27001" DOI : 10.1209/0295-5075/93/27001