Surface alloying and mixing at the Mn/Fe(001) interface: Real-time photoelectron spectroscopy and modified embedded atom simulations

Structural and magnetic properties of thin Mn films on the Fe(001) surface have been investigated by a combination of photoelectron spectroscopy and computer simulation in the temperature range 300 K < ∼T < ∼750 K. Room-temperature as deposited Mn overlayers are found to be ferromagnetic up to 2.5-monolayer (ML) coverage, with a magnetic moment parallel to that of the iron substrate. The Mn atomic moment decreases with increasing coverage, and thicker samples (4-ML and 4.5-ML coverage) are antiferromagnetic. Photoemission measurements performed while the system temperature is rising at constant rate (dT/dt ∼ 0.5 K/s) detect the first signs of Mn-Fe interdiffusion at T = 450 K, and reveal a broad temperature range (610 K < ∼T < ∼680 K) in which the interface appears to be stable. Interdiffusion resumes at T > ∼680 K. Molecular dynamics and Monte Carlo simulations allow us to attribute the stability plateau at 610 K < ∼T < ∼680 K to the formation of a single-layer MnFe surface alloy with a 2 × 2 unit cell and a checkerboard distribution of Mn and Fe atoms. X-ray-absorption spectroscopy and analysis of the dichroic signal show that the alloy has a ferromagnetic spin structure, collinear with that of the substrate. The magnetic moments of Mn and Fe atoms in the alloy are estimated to be 0.8μ_B and 1.1μ_B, respectively.