Evidence of c (2x2) antiferromagnetic order of Mn in an ideal monolayer onAg(001)

We present a dynamical low-energy electron diffraction analysis of the atom ic and magnetic structure obtained by deposition of 0.9 Mn monolayers on Ag (001) held at 100 K. We find that the film grows in an essentially flat ide al monoatomic layer with the Mn located in fourfold hollow sites of the Ag( 001) substrate that undergoes no sizeable reconstruction of its topmost lay ers. The Mn-Ag interlayer distance d(12)=2.00 +/- 0.03 Angstrom is found to be much larger than similar to 1.85 Angstrom expected from simple addition of atomic radii in elemental a-Mn and face-centered-cubic (fcc) Ag. This i s direct evidence of the strong reduction in Mn 3d contribution to cohesion due to the atomiclike high-spin state of the Mn in the monolayer structure . Taking into account the exchange scattering by means of a local von Barth -Hedin exchange-correlation potential, we demonstrate that the weak intensi ty of the c(2x2) superstructure reflection intensities I(E) observed at low energies only (E less than or equal to 120 eV) can be quite well explained in terms of a two-dimensional antiferromagnetic arrangement of the Mn mome nts. Nonmagnetic origins of these reflections such as out of plane (bucklin g) or in plane displacive reconstructions of the Mn monolayer as well as su rface alloying with Ag or impurity related superstructures can be safely ru led out. Evidence of a c(2x2) antiferromagnetic to p(1x1) paramagnetic phas e transition is also presented.