We consider magnetic systems where the magnetic sublattices can be unambiguously separated into sublattices of inducing and induced moments. The concrete numerical calculations are performed for half-metallic ferromagnetic Heusler compound NiMnSb and hexagonal phase of MnAs. In both systems, Mn atoms possess a robust atomic moment that is much larger than the induced moments of other atoms. It is shown that the treatment of the induced moments as independent variables of the Heisenberg Hamiltonian leads to artificial features in the spin-wave spectrum. We show that the artificial features of the model do not have a dramatic influence on the estimated value of the Curie temperature. This is demonstrated within both mean-field approximation and random-phase approximation. It is shown that the calculational scheme where the induced moments are assumed to fully adjust their values and directions to the adiabatic magnetic configurations of the inducing moments is free from the artificial feature in the spin-wave spectra. In this scheme, the exchange interaction between the inducing and induced moments appears as renormalization of the exchange interactions between inducing moments. It is shown that the redistribution of the exchange interactions has strong influence on the estimated value of the Curie temperature because of the decreased number of the degrees of freedom in the thermodynamic model. Different schemes of the mapping of the systems on the Heisenberg Hamiltonian are examined. The similarities and differences in the properties of NiMnSb and MnAs are discussed.

The great power of the cluster-expansion method for atomistic simulations of the ground state properties and the thermodynamic properties of multi-component solids is highlighted. In the first part of the paper some recently developed combinations of this method with other atomistic simulation techniques (N-body potentials and cluster functionals) are discussed. In the second part the ab-initio cluster expansion method is used to investigate the ground-state structures for bulk Fe x Co 1-x , the Ni90% -Al(110) surface and for adatom configurations on Mo(112), and the phase diagram for the system Ni -Fe -Al. Finally, an outlook for future applications of this calculational technique is given.

Usually, the adiabatic magnetic exchange-energy hypersurface is parametrized in terms of the bilinear Heisenberg interactions in pairs of atoms. For general magnetic configurations, this model is not complete even if it includes pairs with up to infinite interatomic distances. In contrast, the modeling by an in principle infinite spin-cluster expansion is complete for all conceivable magnetic configurations. In the present Letter, it is shown for bcc and fcc iron that a very accurate representation can be reached with a finite expansion with 20 terms which include biquadratic or multispin non-Heisenberg interactions.

Kuhnke, K.; Repetto, D.; Sessi, V.; Wahl, Peter; Buchsbaum, A.; Varga, P.; Gardonio, S.; Carbone, C.; Krishnakumar, S. R.; Gambardella, P.; Komelj, M.; Singer, R.; Fähnle, M.; Fauth, K.; Schütz, G.; Enders, Axel; and Kern, K., "Complex magnetic phase in submonolayer Fe stripes on Pt (997) Correlations between magnetism and morphology of iron nanostructures of monatomic height on Pt͑997͒ substrates are studied using x-ray magnetic circular dichroism as well as scanning tunneling microscopy and helium scattering. A drastic collapse of the average magnetization by more than a factor of 4 is observed when increasing the iron coverage from 0.1 to 0.2 ML. This effect goes along with a softening of the magnetic anisotropy energy and a gradual reorientation of the magnetic easy axis from in plane to out of plane. The experimental findings together with electronic density-functional calculations suggest the formation of a complex magnetic phase in corrugated rim regions of Fe islands, leading to both ferromagnetic and antiferromagnetic exchange couplings of Fe moments depending on their various local bonding configurations.