High temperature finite-size effects in the magnetic properties of Ni nanowires

The magnetic response of nickel nanowires embedded in porous alumina has been investigated in a wide temperature range, from 5 K up to 700 K. Hysteresis loops and magnetization isotherms were measured on samples of Ni nanowires with different sizes and morphologies up to the Curie temperature. At room temperature, the magnetic response shows evidences of a particle-to-wire crossover above an aspect ratio L/D = 4.3. The magnetic coercivity of high aspect ratio Ni nanowires in the direction parallel to the nanowires has a maximum at T ≈ 400 K, while in the parallel direction Hc decreases continuously with increasing temperature. It is explained in terms of competing anisotropies, magnetocrystalline and magnetoelastic. The expansion of the aluminium support of the membrane plays a fundamental role in the temperature dependence of the coercive field. We find also that TC progressively decreases due to a finite-size effect as the wire's diameter decreases.