Bad neighbour, good neighbour: how magnetic dipole interactions between soft and hard ferrimagnetic nanoparticles affect macroscopic magnetic properties in ferrofluids

Hard ferrimagnetic CoFe2O4 nanoparticles become softer in the presence of neighbouring soft ferrimagnetic MnFe2O4 nanoparticles.

Smart elastomeric hydrogel of polyacrylamide containing nanosized barium ferrite and graphene oxide

Smart elastomeric hydrogels based on polyacrylamide containing ferrimagnetic nanoparticles of BaFe12O19 alone or a combination of BaFe12O19 and graphene oxide were synthesized. An improvised sample preparation technique was evolved to design actuators, where the hydrogel samples in rolls form were inserted into cut pieces of hard plastic sleeve in order to fabricate a lab-level actuator. The magneto-responsive actuation behaviour was measured by applying an external magnetic field with the help of a hand-held square type of NdFeB magnet. While the sample containing only 20 wt% BaFe12O19 exhibited only 15 mm deflection at threshold magnetic field of 28 mT, the deflection was significantly enhanced up to 23 mm for the sample containing 1 wt% GO and 20 wt% BaFe12O19 at a threshold magnetic field of 36.7 mT. Up to 20 wt% BaFe12O19, the hydrogels showed a heterogeneous dispersion of large-sized BaFe12O19 clusters and addition of a small percentage, e.g. 1% of GO resulted in significant improvement in the homogeneity of dispersion as well as reduction of the scale of the BaFe12O19 clusters. The saturation magnetization value of the hydrogels was also improved by 50–75% due to the addition of 1 wt% GO with 20 wt% BaFe12O19.

Импульсное перемагничивание антиферромагнитных наночастиц ферригидрита

The dynamic magnetization switching of ferrihydrite nanoparticles has been investigated by a pulsed magnetometer technique in maximum fields H _max of up to 130 kOe with pulse lengths of 4, 8, and 16 ms. Ferrihydrite exhibits antiferromagnetic ordering and defects cause the uncompensated magnetic moment in nanoparticles; therefore, the behavior typical of magnetic nanoparticles is observed. The dynamic hysteresis loops measured under the above-mentioned conditions show that the use of pulsed fields significantly broadens the temperature region of existence of the magnetic hysteresis and the coercivity can be governed by varying the maximum field and pulse length. This behavior is resulted from the relaxation effects typical of conventional ferro- and ferrimagnetic nanoparticles and the features typical of antiferromagnetic nanoparticles.