Effects of thermomagnetic prehistory in the behavior of magnetization of a powder system of synthetic nanoferrihydrite in the presence of magnetic interparticle interactions

The temperature dependences of the magnetization $M(T)$ of two powder systems of ferrihydrite nanoparticles with identical sizes of ferrihydrite particles (average particle size is $\approx$ 2.7 nm) and different intensities of magnetic interparticle interactions (MIPI) were studied. In addition to the commonly observed increase in the superparamagnetic blocking temperature TB (from 17 K to 50 K), MIPIs clearly manifest themselves under different conditions and regimes of thermomagnetic prehistory. It was found that the rate of pre-cooling in an external magnetic field (in the used range of 1–10 K/min) affects the magnitude of the magnetization of the system at low temperatures and the shape of the $M(T)$ dependence in the temperature range up to TB. This effect is significant for fairly weak external fields (up to $\sim$300 Oe), and when the field increases to $\sim$800 Oe, the cooling rate becomes insignificant for the magnitude of magnetization. In this case, for the range of external fields up to $\sim$300 Oe, the $M(T)$ dependences obtained during cooling in an external field and when heating the sample in the field (after pre-cooling) are different. For a system of ferrihydrite nanoparticles, in which the MIPIs are weakened, these effects are absent. Analysis of the results obtained allowed us to propose the following scenario for the implementation of the detected thermomagnetic effects. In the presence of MIPIs (in the temperature range below $T_\mathrm{B}$), the basic state of the structure of the magnetic moments of particles $\mu_\mathrm{P}$ is such that the vectors $\mu_\mathrm{P}$ of neighboring particles tend to be oriented predominantly against each other (anticollinearly). This takes place upon relatively “slow” cooling of the system (1 K/min), but upon “fast” cooling (10 K/min), i.e. “hardening” in an external field, the $\mu_\mathrm{P}$ vectors remain predominantly directed “along the field”, as in the temperature region of the superparamagnetic state (at $T >T_\mathrm{B}$). The range of magnetic fields in which the described effects are observed is determined by the competition between the MIPI energy and the Zeeman energy $\mu_\mathrm{P}$ $\cdot$ $H$.