Spin-polarized metastable He study of surface magnetic order in Ho thin films

A spin-polarized metastable helium beam operating in magnetic fields of up to 5 T has been used to investigate the spin polarization at the surface of thin films of Ho(0001) grown on W(110) substrates. The extreme surface sensitivity of this technique allows a direct probe of the various magnetic structures that form as a function of film thickness, temperature, and applied magnetic field. Evidence for a transition from a conical ferromagnetic structure to a helical antiferromagnetic (AFM) phase is observed at low temperatures with a further transition to paramagnetism at close to the AFM ordering temperature, T-N. In addition, the spin polarization of the 6s CE that mediate the exchange interaction between localized Ho 4f moments (the RKKY interaction) is found to be positive, supporting earlier observations. (C) 2011 American Institute of Physics. [doi:10.1063/1.3556958]