Two-dimensional magnetic phase transition of ultrathin iron films on Pd(100)

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Abstract
Epitaxial Fe films grown on Pd(100) are used to study monolayer magnetism, critical behavior, and surface magnetic anisotropy, by means of in situ surface magneto-optical Kerr-effect measurements. Auxiliary LEED-Auger observations in 10−11 Torr vacuum are used to characterize the (1×1) epitaxy and the layer-by-layer film-growth mode. Ferromagnetic hysteresis loops were detected for all Fe thicknesses from 0.6–4 monolayers (ML) with the TC monotonically increasing with thickness, independent of the easy-axis orientation. The easy axis is perpendicular to the film plane below a critical thickness of 2.5 ML for 100-K film growth, and reorients in-plane above this thickness, and for all thicknesses for films grown at 300 K. The temperature dependence of the magnetization was obtained from the height of the Kerr loops in the remanent state and used to extract an effective magnetization exponent β for different film thicknesses and spin orientations. A value of β=0.127±0.004 is reported for a 1.2-ML Fe film with perpendicular spin orientation from a log-log plot for T>0.9TC, in agreement with the value of the theoretical two-dimensional Ising critical exponent βc=1/8=0.125.