From vanishing interaction to superferromagnetic dimerization: Experimental determination of interaction lengths for embedded Co clusters

Magnetic nanoparticles are of great interest in a wide range of disciplines, including magnetic fluids, catalysis, biotechnology/biomedicine, magnetic resonance imaging, data storage, and environmental remediation. Successful applications of such magnetic nanoparticles in the areas listed above are highly dependent on the stability of the particles under a range of different conditions. In particular, these nanomagnets might be used as magnetic media in future high-density magnetic storage devices with ultimate recording bits (i.e., single nanoparticles). Reading and writing of such a system requires to know perfectly its magnetic properties in particular its anisotropy constant. It is then crucial to be able to characterize the magnetic properties of nanoparticles, and to be able to separate the intrinsic behavior from other effects coming from interparticle interactions in an assembly. In this study [6], we present magnetic measurements of Co clusters (around 2.5 nm diameter) embedded in different matrices: carbon and two metallic matrices (Au and Cu). We will first show that by using highly diluted samples prepared by low energy cluster deposition, we can reach a situation where no interactions are detected. The intrinsic magnetic properties of the particles can then be accurately determined thanks to a “global” fitting procedure (see figure 1) relying on the theoretical description of various magnetometry measurements [1]-[5]: low-temperature(hysteresis) and high-temperature (superparamagnetic) m(H) loops, zero- field cooled (ZFC)/field cooled (FC) susceptibility curves, and isothermal remanent magnetization (IRM) curves. We show how both the magnetic size and magnetic anisotropy energy (MAE) can be impacted by the nature of the matrix. Then, by considering nanoparticle assemblies of increasing concentrations (still remaining in a diluted range, lower than 10% in volume), we discuss the different effects of interactions between particles on the magnetic measurements (see figure 2). The evolution of Δm curves (deduced from remanence curves) is found to be very different from that of susceptibility curves. In order to account for the observed evolution of the measurements, we propose a simple model where magnetic dimers are formed for distances lower than a given interaction length [6]. This super-ferromagnetic correlation, which can be consistently inferred for each matrix, thus modifies the magnetic size distribution which has a drastic effect (in particular on ZFC/FC curves) as soon as particles are close enough from each other. The deduced interaction length (of the order of one nanometer) is found to be larger for metallic matrices and could be ascribed to RKKY interactions between neighboring magnetic nanoparticles.