MAGNETIC ORDER AND CRITICAL-BEHAVIOR AT SURFACES OF ULTRATHIN FE(100)P(1X1) FILMS ON PD(100) SUBSTRATES

The magnetic order and critical behavior at well-defined surfaces of ultrathin ( 1.9-2.1 ) monolayer Fe(100)p(1 X 1) films. deposited at room temperature on atomically flat Pd(100) substrate crystals, is investigated using electron-capture spectroscopy (ECS). Ferromagnetic order sets in at a surface Curie temperature T(Cs) = 613.1 K. At room temperature, the long-ranged electron-spin polarization (ESP) amounts to 34% and is oriented in-plane for well-annealed films. Near T(Cs), the ESP follows the power law (T(Cs)-T)beta, with a critical exponent beta = 0. 125 +/- 0.0 1 evaluated near T(Cs). This result is in good agreement with the exact value beta = 1/8 of the two-dimensional Ising model with short-ranged forces. For temperatures far from T(Cs), the ESP follows precisely the temperature dependence of the magnetization as predicted by Yang's exact solution of the two-dimensional Ising model including long-ranged forces.