Monte Carlo simulations have been performed to study the magnetic relaxation of a small particle system with dipolar interaction. The simulated system is a simplified version of the real situation in a small particle system with a random distribution of anisotropy axes and long range dipolar interaction among the particles. This model consists on a one-dimensional Ising model with dipolar interaction and a distribution of uniaxial anisotropy strengths. The anisotropy axes were considered perpendicular to the line connecting the spins. These choices allow us to focus on the influence of the demagnetizing part of the dipolar interaction on the magnetic relaxation by taking into account the main features of the system. The Tln͑t/ 0 ͒ scaling variable is used to determine the effective distribution of energy barriers for the different interaction strengths showing an enhancement in the number of the lowest energy barriers as the interaction strength is increased. Moreover, the histograms of the energy barrier distribution as a function of the time are analyzed and this study leads to a deeper knowledge of the microscopic processes involved in the magnetic relaxation.

A numerical simulation of a two-dimensional XY model has been carried out to study the thermal behaviour of the magnetization process and the variation of the coercive field as a function of the random anisotropy amplitude. The existence of two different magnetic regimes is evidenced: a low-anisotropy regime, which is characterized at zero temperature by a power law increase of the coercive field as the anisotropy amplitude increases, and a regime for anisotropy values higher than D/zJ=0.5, for which the system behaves as an assembly of quasi-independent clusters of two or three atoms. In this regime, the coercive field increases linearly with the anisotropy strength. The numerical estimates of the power law exponent are in good agreement with the heuristic predictions of Imry and Ma (1975). The domain wall motion and the spin configuration for the two anisotropy regimes have also been studied, pointing out the differences in the pinning process between both regimes. The thermal dependence of the coercive field has been obtained and fitted to an exponential law.

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Spin crossover complexes have a very striking signature of a huge volume change coupled with low–high spin conversion around a critical temperature, which can be pressure tuned in a large temperature interval. Herein, the barocaloric effect is reported in the spin crossover complex {Fe[H2B(pz)2]2(bipy)} (bipy = 2,2′‐bipyridine) from theoretical and applied points of view. The experimental data reveal a giant barocaloric effect, through the isothermal entropy change (ΔST = 83 J kg−1 K−1) around T = 273 K, upon moderated hydrostatic pressure variation (ΔP = 2 kbar). The high and linear behavior in the pressure dependence of the phase transition temperature (19 K kbar−1) leads to a huge relative cooling power (RCP = 7296 J kg−1) upon ΔP = 3 kbar, which is discussed using Ericsson's cooling cycle. Theoretical results, obtained from a microscopic model, updated with the vibration modes from density functional theory (DFT) calculation, show remarkable agreement with the experimental data.