Looking for quantum effects in magnetic nanoparticles using the molecular nanomagnet approach

In an effort to build a unitary view of the properties of magnetic nanoparticles (MNPs) and molecular nanomagnets (MNMs), we investigated with different techniques MNPs of spinel-type iron oxide of approximately 3.5 and 8 nm mineralized in the internal cavity of ferritin-type proteins. In particular, we used electron magnetic resonance (EMR) and static and dynamic magnetic measurements and took advantage of the capacity of the protein shells to control the size of the MNPs. A signal at half-field in the EMR spectrum is observed for MNPs as big as 8 nm mineralized in protein cavities. This feature has been interpreted as the signature of the discrete structure of the energy levels and, therefore, of the quantum nature of the system. The EMR behavior of the MNPs is compared with that of two large MNMs: one containing 19 Fe(III) and a second containing 19 Mn(II) centers. The ideal structure of the latter is used as a model to show the structuring of the energy levels. In particular, the analysis of MNP behavior based on the MNM approach helps to shed light on the role of the different energy terms that govern the MNP properties.

Figure 1

Size distributions of the MNPs in LiDps (dashed line) and PfFt (continuous line) solutions obtained by AUC.

Figure 2

Hysteresis loops recorded at 1.8 K. The low-field behavior is evidenced in the inset.

Figure 3

Reduced remnant magnetization, $M_R$, directly evaluated from the hysteresis loops measured as a function of the temperature (open symbols) and from the fraction of unblocked MNPs, $x$($T$), as described in the main text (full symbols) for LiDps (circles) and PfFt (squares).

Figure 4

ZFC (full symbols) and FC (open symbols) magnetization curves for PfFt (squares) and LiDps (circles). The inset shows the normalized blocking temperature distributions.

Figure 5

$X$-band EMR spectra of PfFt (left) and LiDps (right) suspensions frozen in zero field acquired at various temperatures.

Figure 6

EMR spectra of PfFt frozen in field and acquired at θ $=$ 0° (blue line) and θ $=$ 90° (red line) and of the suspension frozen in zero field (black line).

Figure 7

Deconvolution of the $X$-band EMR spectra of field frozen PfFt acquired at θ $=$ 0° (left panel) and θ $=$ 90° (right panel). Black: experimental spectrum; red: simulation; magenta: simulation of signal B; green: simulation of signal A; blue: simulation of signal C.

Figure 8

Deconvolution of the $X$-band EMR spectra of PfFt acquired at θ $=$ 90° and 0° in correspondence of the $B_0$/2 transitions. Simulation includes two Gaussians for each orientation: an orientation-independent contribution and a second one that is orientation-dependent. The latter is indicated by the dashed line for the two orientations.

Figure 9

Scheme of the growth of brucite-type clusters. Sphere of different colors corresponding to different generations. The red spheres represent oxygen atoms.