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We show that confinement in small volumes affects the interplay of exchange and dipolar interactions and the magnetic phases of hard and soft spherical core-shell nanoparticles. Large variations in the magnetization of thin shells may occur due to the core dipolar field gradient within the shell. The reversal field is tunable by the trends imposed by the dipolar and core-shell interface exchange energies. We show, for instance, that the reversal field of a CoFe 2 O 4 (30 nm)@MnFe 2 O 4 (6 nm) particle ranges from 15.5 kOe for antiferromagnetic coupling down to 2.5 kOe for ferromagnetic coupling.

We report a theoretical study of the impact of finite size on the magnetocaloric effect (MCE) of thin Ho films. For strong external field strengths, the adiabatic temperature change ΔTad is comparable to the value found in bulk Ho, reaching about 12 K for an external field strength change ΔH = 50 kOe. For thicknesses below the helix period, there is a large enhancement. In this thickness range, the helical state does not form, leading to a giant MCE reaching ΔT/ΔH = 6.5 K/T for ΔH = 2 kOe.

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