First Steps toward Tuning the Magnetocaloric Effect in Fe₃O₄ Nanoparticles: Unraveling the Role of the Superparamagnetic Moment

Attempts have long been made to correlate the magnetocaloric effect (MCE) exhibited by nanosized materials in the superparamagnetic regime with their corresponding magnetic susceptibility curves. However, these two curves often display different profiles. In this study, two kinds of magnetite nanoparticles with distinct microstructures (singlecore and multicore) were synthesized. Interestingly, despite their microstructural differences, both types exhibited similar magnetic susceptibility curves characterized by broad maxima. A similar magnetocaloric effect was therefore expected for both types of sample; however, this was not the case. Their different microstructures lead to different evolutions of the superparamagnetic (SP) moment with the temperature (T), despite illustrating both similar susceptibility curves. We found that the profile of the SP moment versus T curve seems to be the key factor explaining the MCE in the SP regime. This is reasonable, as within this temperature range a characteristic disordered-to-ordered phase transition occurs due to the long-range alignment of the net magnetic moments of the particles when a magnetic field is applied. This finding opens the possibility of designing magnetic nanoparticles with specific magnetocaloric properties by preparing them with a certain microstructure.