Observation of spin-wave characteristics in the two-dimensional ferromagnetic ordering of in-plane spins

ISIS, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK(Dated: February 2, 2008)The role of dipolar interactions and anisotropy are important to obtain, otherwise forbidden, ferro-magnetic ordering at finite temperature for ions arranged in two-dimensional (2D) arrays (monolay-ers). Here we demonstrate that conventional low temperature magnetometry and polarized neutronscattering measurements can be performed to study ferromagnetic ordering of in-plane spins in 2Dsystems using a multilayer stack of non-interacting monolayers of gadolinium ions. The spontaneousmagnetization is absent in the heterogenous magnetic phase observed here and the saturation valueof the net magnetization was found to depend on the applied magnetic field. The net magnetiza-tion rises exponentially with lowering temperature and then reaches saturation following a T ln(βT)dependence. These findings verify predictions of the spin-wave theory of 2D in-plane spin systemwith ferromagnetic interaction and will initiate further theoretical development.