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Anisotropic spin--spin correlations in Mn$_1$/X(111), with X = Pd, Pt, Ag and Au

We present a finite-temperature theory of the anisotropic spin-spin correlations in magnetic metallic monolayers, deposited on a suitable substrate. The `spins' are the local moments set up by the itinerant electrons, and the key concept is the relativistic disordered local moment state, which represents the paramagnetic state of a set of local moments. The spin-spin correlations between these local moments are then extracted using the linear response formalism. The anisotropy is included in a fully relativistic treatment, based on the Dirac equation, and has a qualitative impact on noncollinear magnetic states, by lifting their chiral degeneracy. The theory is applied to Mn monolayers on the hexagonal (111) surfaces of Pd, Pt, Ag and Au. The presence of competing exchange interactions is highlighted by choosing different substrates, which favour either the row-wise antiferromagnetic state or the chiral triangular Néel state. We correlate the electronic structure with the magnetic properties, by comparing filled with partially filled substrate d--bands, and low vs high atomic number. The disagreement between theory and experiment for Mn$_1$/Ag(111) is addressed, and the nature of the magnetic domains found experimentally is suggested to be chiral.