Electronic Transport Properties In Co₂FeSi Heusler Compound Films And Co₂FeSi-Al₂O₃Granular Films

We fabricated a series of Co2FeSi Heusler compound films with different crystallization degrees at quartz glass substrates at different temperatures by rf sputtering deposition method and a series of (Co2FeSi)x(Al2O3)1-x granular films at glass substrates at room temperature by co-sputtering method. The structures as well as the temperature behaviors of anomalous Hall resistivities, low-field Hall sensitivities, and longitudinal resistivities of those films were investigated experimentally.We found that the Co2FeSi Heusler compound films deposited at853,843,813,743,723,693,673K were crystallized in the disordered A2type structure, and L21and B2structures are present in the film deposited at873K. The Co2FeSi film deposited at room temperature and Co2FeSi-Al2O3granular films are all amorphous. Transmission electron microscopy results show that Co2FeSi metal granules with several nanometers are randomly embedded in Al2O3insulating matrix, and the metal particle sizes decrease with reducing metal volume fractions x. The temperature coefficient of film deposited at room temperature is negative at the overall temperature range from300down to2K, and temperature coefficient is positive for the films deposited at high temperatures. The classical percolation threshold xc for the Co2FeSi-Al2O3granular films has been determined to be0.48.For the Co2FeSi films, the saturated anomalous Hall resistivity ρAs scales with the longitudinal resistivity ρxx as pAs ρAs∝ρxxn, with n much larger than2. There is no scaling relation betweenρAs andρxx for the (Co2FeSi)x(Al2O3)1-x (0.58≤x≤1, x=1represents the amorphous Co2FeSi film deposited at room temperature) granular films. For the granular films, as x decreases from1to0.67, the magnitude of ρxx increases by a factor of～25, whereas the magnitude enhancement in ρAs is less than50%, which strongly suggests that the longitudinal and anomalous Hall transports in ferromagnetic granular films are governed by different mechanisms. Thus this observation provides a piece of experimental evidence for the validity of the theory proposed by Meier et al.The low-field magnetic sensitivities results reveal that KH is insensitive to the temperature for all the Co2FeSi compound films, and increases with decreasing grain sizes. Particularly, the low-field Hall sensitivities of (Co2FeSi)x(Al2O3)1-x granular films with x values of0.6and0.65are much larger than that of Si or Ge layers in Hallsensors, and temperature insensitivity from300down to～75K, which might makethem be good candidate materials for low-field Hall sensors.