Biquadratic magnetic coupling in NiFe/Ag multilayers

Magnetoresistance, magnetization, and polarized neutron experiments have been performed on ${\mathrm{Ni}}_{81}$ ${\mathrm{Fe}}_{19}$/Ag multilayers. The results give evidence for both biquadratic and bilinear contributions to the total coupling energy. The strong temperature dependence of the biquadratic term leads to a crossover from a canted state at low temperature to an antiferromagnetic one beyond 100 K. We suggest that the biquadratic term could arise from local concentration fluctuations in the chemically disordered NiFe layers.