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We studied the effects of medium microstructure on the switching field (SF) and its distribution (SFD). Specifically, quasi-single crystalline and polycrystalline Co/Pt based multilayers, chosen for its high perpendicular anisotropy and high exchange coupling, have been prepared via sputtering technique. The quasi-single crystalline was achieved by utilizing an Ag underlayer. Using electron-beam lithography, films were patterned into arrays of small islands, ranging from 1 mum to 0.050 mum (=50 nm) in size. The hysteresis loop for the both types of film, as unpatterned, showed similar anisotropy field and coercivity whereas, as patterned, the quasi-single crystalline samples consistently exhibited a higher SF than that of the polycrystalline samples. However, the interpretation on the SFD was rather complex. Additionally, a critical size where remanent magnetization changes a multi domain to a single domain state appeared to be notably smaller for the polycrystalline samples than that for the quasi-single crystal samples, suggesting a different nucleation volume in the films.