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We report a theoretical study of the impact of dipolar interactions on the room temperature magnetic phases of superparamagnetic nanoparticles confined in spherical and ellipsoidal clusters. We consider Fe3O4 nanoparticles with size ranging from 9 nm to 12 nm, arranged with uniform density in hundred nanometer-sized clusters. We show that one may have a large enhancement of the initial susceptibility for ellipsoidal clusters of high eccentricity, as required for most biomedical applications. Spherical clusters display a reduction of the initial susceptibility, due to the early nucleation of new magnetic phases. In densely packed systems, the dipolar interaction may lead to thermal stabilization of the individual nanoparticle moments, while keeping the cluster superparamagnetic, with a vanishingly small magnetic moment in the absence of an external field. The theoretical model is used to discuss recent findings on quasi-one-dimensional arrays of superparamagnetic Fe and Co nanoparticles, and on spherical clusters of superparamagnetic Fe3O4 nanoparticles.

We report a theoretical study of the vortex profile of in-plane magnetized PyTM nano-cylinders subjected to the stray field of perpendicular anisotropy Co nano-cylinders. We consider 6 nm thick PyTM cylinders dipolar coupled to 60 nm thick Co cylinders, at distances from 1.5 nm to 30 nm, with diameters (D) ranging from 45 nm to 105 nm. We find considerable reduction of critical diameter for stable PyTM magnetic vortices and spiral-vortex phases, as well as vortex core diameters twice as large as the bulk value.

We report a theoretical study of the magnetic phases of core-shell nanocylinders, consisting of a Py cylindrical core, dipolar coupled to a coaxial Fe cylindrical shell. A few nanometers thick nonmagnetic cylindrical layer separates the core from the shell, and controls the magnitude of the core-shell dipolar interaction. New magnetic phases emerge from the dipolar interaction, and may consist of either the combination of the intrinsic magnetic phases or new phases that are not seen in isolated cylinders and shells. We discuss typical examples. The magnetic phases of a 21 nm-height nanocylinder composed of a 57 nm-diameter Py core coupled to a 12 nm-thick Fe shell may be set to be a Py vortex with the same chirality of the Fe shell circular state, or a Py uniform domain coupled to a pair of domain walls of the Fe shell onion state. A magnetic vortex may be stabilized in a 6 nm-height, 42 nm-diameter Py cylinder coupled to a 6 nm-thick Fe shell.

We report a theoretical study of the magnetic phases of core-shell nanocylinders, consisting of a Py cylindrical core, dipolar coupled to a coaxial Fe cylindrical shell. A few nanometers thick nonmagnetic cylindrical layer separates the core from the shell, and controls the magnitude of the core-shell dipolar interaction. New magnetic phases emerge from the dipolar interaction, and may consist of either the combination of the intrinsic magnetic phases or new phases that are not seen in isolated cylinders and shells. We discuss typical examples. The magnetic phases of a 21 nm-height nanocylinder composed of a 57 nm-diameter Py core coupled to a 12 nm-thick Fe shell may be set to be a Py vortex with the same chirality of the Fe shell circular state, or a Py uniform domain coupled to a pair of domain walls of the Fe shell onion state. A magnetic vortex may be stabilized in a 6 nm-height, 42 nm-diameter Py cylinder coupled to a 6 nm-thick Fe shell.