Near-Curie magnetic anomaly at the Ni/C interface observed by electron holography

A Nickel film, coated with Carbon, has been observed with electron holography. The magnetization has been measured from room temperature to 720 K, crossing the nominal Curie temperature Tc=635 K. An anomaly is revealed, resulting (a) in a 10% increase of Tc; (b) in a hump of the magnetization (largely exceeding the residual magnetiztion due to the weak external field) above Tc. The anomaly is interpreted as due to a strain-induced spin reorientation triggered by the differential thermal expansion of Ni and C. A mean- field model is presented that quantitatively reproduces the the main features of the observed signal. The model is developed in terms of an anisotropic exchange coupling between the Ni moments at the C interface, that favors vertical alignment at low temperatures and in-plane reorientration at high temperatures.