Memory effect and super-spin-glass ordering in an aggregated nanoparticle sample

A system consisting of aggregated nonstoichiometric zinc ferrite nanoparticles has been studied using AC and DC magnetization measurements. A superparamagnetic– super-spin-glass phase transition at Tg has been identified. The relaxation time diverges at Tg and the nonlinear susceptibility shows an abrupt increase. The critical behavior vanishes when the nanoparticles are not in close contact. The observation of the memory effect identical to that which has been already discovered in canonical spinglass supports the existence of a true thermodynamic transition in agglomerated magnetic nanoparticles. There has been an increasing number of publications focusing on the magnetic properties of nanoparticle systems in the last few decades [1]. One of the most fascinating problems that researchers have focused on recently is the possible appearance of interparticle long-range magnetic ordering at low temperatures when the interactions between nanoparticles are sufficiently strong. Because of the size distribution of the particles and the randomness inherent to these systems, fingerprints of a possible superparamagnetic– super-spin-glass phase transition, in agreement with canonical spin-glasses, have been pursued and found: (i) the critical slowing down of the relaxation time τ on approaching the transition temperature Tg [2, 3 and 4]; (ii) the divergence of the nonlinear susceptibility χnl at Tg [5, 6 and 7]; (iii) the magnetic aging in the ordered phase [4, 8, 9 and 10]. So far, most of the investigations have been performed on nanoparticles dispersed in a diamagnetic matrix, which can be a solid or a liquid. The magnetic fluids offer great advantages because the interactions between the particles can be tuned by changing nanoparticle concentration. But the main drawback is that the possibility of forming aggregates in the fluid can never be completely discounted. In aggregated nanoparticle samples this is not a problem and, furthermore, the interactions between the particles are stronger making them suitable for observation of collective behaviors. Consequently, Dormann et al. [4] found that in aggregated samples the low-temperature regime is close to that of a canonical spin-glass. However, they did not put in evidence the critical divergence of the nonlinear susceptibility on approaching the transition temperature. Lately, the spectacular memory effect has been found in concentrated ferrofluids [10 and 11]. In a typical memory experiment the sample is cooled in zero external magnetic field. At a given temperature, Tw, the cooling process is halted for a certain time, tw, then the cooling is resumed. After cooling the AC magnetic response is measured during the warming up of the sample. An infinitesimal cusp appears at Tw when Tw is lower than the blocking temperature, TB, of the particles [11]. This cusp does not exist when the cooling process is not halted. The memory effect had been previously reported below the freezing temperature, Tg, in some spin-glasses [12]. This phenomenon can also be seen as a thermally hidden magnetic inscription. Indeed, at the lowest temperatures (far below Tw), the magnetization is strictly identical if the cooling has been paused or not. In other words, the system has memorized information but the reading is possible only at temperatures in the vicinity at which the information has been stored. In this paper it is shown that the relaxation time of a sample of aggregated nanoparticles of nonstoichiometric zinc ferrite exhibits a critical slowing down and that the nonlinear susceptibility increases strongly on approaching Tg. In addition, it was found that the sample does show the memory effect. These observations support the presence of interactions between the nanoparticles, which may be of dipolar origin and/or exchange. The synthesis and the physical characterizations of these nanoparticles are