Size dependences of magnetic properties and switching behavior in FePt L1₀ nanoparticles

We have prepared epitaxial FePt L1₀ (001) nanoparticles covered with Ag and Pt overlayers and investigated their magnetic behaviors by means of anomalous Hall resistance measurements. The particle shapes are thin oblate spheroids with the aspect ratio (height/diameter) of 1/5. The size is ranging from 1 to 2.5 nm in height and from 5 to 30 nm in diameter. FePt L1₀ nanoparticles show extremely large coercivity H_k of about 70 kOe at 10 K, which is close to the anisotropy field H_k of highly ordered FePt L1₀. This verifies that the very strong magnetic anisotropy K_u of FePt L1₀ remains even in the size of several atomic layers along the c axis. For a particle diameter of D_m<20 nm, all the magnetic properties, such as the angular dependence of irreversible switching field, the magnitude of H_c, and their temperature dependence, are fully explained by the coherent rotation model, taking the thermal relaxation into account. Although both Ag- and Pt-coated particles follow the coherent rotation model, the latter always exhibits smaller H_c than the former. Such a decrease in H_c can be explained by assuming an enhancement of the effective magnetic moment caused by ferromagnetic polarization of Pt atoms at the Pt/FePt interface. As the particle size D_m exceeds 20 nm, the magnetic behaviors deviate from the ideal coherent rotation model, suggesting that the magnetization reversal mode changes from coherent to incoherent rotation. The critical diameter D_m∼20 nm at which the reversal mode changes is in good agreement with the critical diameter predicted by the micromagnetic theory.