Magnetic irreversibility in Fe/Cu multilayers

The magnetic behaviour of Fe/Cu multilayers produced by electron-beam evapo ration has been investigated in the range 5 to 400 K and in magnetic fields up to 1 kOe using SQUID magnetometry. The multilayers exhibit an irreversi ble behaviour below a characteristic temperature Ti,,. Low-field DC magneti c moment measurements on samples cooled in zero field revealed the usual si gnature of a spin-glass state. Time relaxation of the remanence and frequen cy-shift of the peak of the imaginary part of the AC magnetic moment were a lso observed. The irreversibility line was fitted to a de Almeida-Thouless line with a crossover critical exponent phi = 3.6. We have interpreted thes e results as being due to the presence of some interdiffusion between Fe an d Cu at the interfaces. The surprisingly high characteristic irreversibilit y temperatures indicate that diffused Fe atoms aggregate in clusters at the interfaces.