Magnetoresistance and collective Coulomb blockade in superlattices of ferromagnetic CoFe nanoparticles

Abstract

We report on the magnetotransport properties of chemically synthesized magnetic artificial solids consisting of millimeter-size superlattices of CoFe nanoparticles (NPs) separated by a thin organic insulating layer. Electrical measurements highlight the richness of the interaction between transport and magnetic field in three-dimensional networks of magnetic NPs, especially in the Coulomb blockade regime: (i) Resistance-temperature characteristics follow $R=R_0\text{exp}\lfloor {\left(T_0/T\right)}^{1/2}\rfloor$, as generally observed in NP arrays displaying charge or structural disorder. (ii) Low-temperature current-voltage characteristics scale according to $I\propto {\left[\left(V-V_{\text{T}}\right)/V_{\text{T}}\right]}^{\zeta }$ with $\zeta$ ranging from 3.5 to 5.2. For a sample with a very large size distribution of NPs, a reduced exponent down to $\zeta =1$ is found, the origin of which remains unclear. (iii) A large high-field magnetoresistance displaying a strong voltage dependence and a scaling versus the magnetic field/temperature ratio is observed in a limited temperature range (1.8–10 K). The most likely interpretation is related to the presence of paramagnetic centers at the surface or between the NPs. (iv) Below 1.8 K, concomitantly to the collapse of this high-field MR, a low field inverse tunneling magnetoresistance grows up with a moderate amplitude not exceeding 1%. (v) Below a critical temperature of 1.8 K, abrupt and hysteretic transitions between two well-defined conduction modes—a Coulomb blockade regime and a conductive regime—can be triggered by the temperature, electric, and magnetic fields. Huge resistance transitions and magnetoresistance with amplitude as high as a factor 30 have been observed in this regime. We propose that these transport features may be related to collective effects in the Coulomb blockade regime resulting from the strong capacitive coupling between NPs. They may correspond to the soliton avalanches predicted by Sverdlov et al. [Phys. Rev. B 64, 041302 (R), 2001] or could also be interpreted as a true phase transition between a Coulomb glass phase to a liquid phase of electrons. The origin of the coupling between magnetic field and transport in this regime is still an open question.