In situ high temperature x-ray reflectivity and grazing incidence x-ray diffraction measurements in the energy dispersive mode are used to study the ordered face-centered tetragonal (fct) L 1(0) phase formation in [Fe(19 Å)/Pt(25 Å)]( × 10) multilayers prepared by ion beam sputtering. With the in situ x-ray measurements it is observed that (i) the multilayer structure first transforms to a disordered FePt and subsequently to an ordered fct L 1(0) phase, (ii) the ordered fct L 1(0) FePt peaks start to appear at 320 °C annealing, (iii) the activation energy of the interdiffusion is 0.8 eV and (iv) ordered fct FePt grains have preferential out-of-plane texture. The magneto-optical Kerr effect and conversion electron Mössbauer spectroscopies are used to study the magnetic properties of the as-deposited and 400 °C annealed multilayers. The magnetic data for the 400 °C annealed sample indicate that the magnetization is at an angle of ∼50° from the plane of the film.