Nanostructure, composition and magnetic properties in soft and hard Co-Ni nanoparticles: The effect on the magnetic anisotropy

The magnetic properties of Co-Ni alloys nanoparticles with composition Co(80)Ni(20) and Co(50)Ni(50) and dispersed in a silica matrix are investigated. The effective magnetic anisotropies are evaluated from the analysis of the temperature dependence of the zero-field-cooling magnetizations and from the hysteresis loops at 3 K considering the coherent rotation model. The results show that, on the contrary of what expected, Co(80)Ni(20) nanoparticles have smaller anisotropy than the Co(50)Ni(50) one showing that the magnetic properties of the system can be tuned from soft to hard by simply adjusting the alloy composition. This behaviour can be explained on the basis of the different size and composition of the two systems. Moreover we found that the evaluation of the anisotropy constant depends on the experimental method adopted due to the modifications by interparticle dipolar interactions of the reversal process.