Antiferromagnetic structure of FePt₃ films studied by neutron scattering

The magnetism of antiferromagnetic FexPt1-x thin films (x=0.27 and 0.30) epitaxially grown onto MgO (110) and a-axis sapphire (α-Al2O3) substrates has been studied by elastic neutron and x-ray scattering. Bulk chemically ordered FePt3 exhibits an antiferromagnetic spin structure with a wave vector Q1=2π/a(1/2 1/2 0) below TN1 ∼ 160 K. For slightly Fe-rich alloys (x ≳ 0.26) a spin-reorientation transition to a second antiferromagnetic phase with a wave vector Q2=2π/a(1/200) occurs below TN2 ∼ 100 K at the expense of·Q1. For increased Fe content (x∼0.30) the Q1 phase is strongly suppressed with a dominant Q2 phase. For (111)-oriented films grown on a-axis sapphire the spin structure is the same as that found in the bulk. The x = 0.27 film exhibits transitions at TN1 and TN2. The film with x = 0.30 exhibits an almost completely suppressed Q1 phase and a dominant Q2 phase with an enhanced ordering temperature of TN2 ∼ 140 K. In contrast FePt3 (110) films grown onto MgO (110) exhibit only the Q1 phase for both compositions x = 0.27 and x = 0.30 with the onset of the Q2 phase suppressed. The distinct behavior of the films grown onto MgO from those grown onto a-axis sapphire and bulk FePt3 may be explained by higher strain and defect densities incorporated in the films grown onto MgO.