An ab initio effective Hamiltonian for magnetism including longitudinal spin fluctuations

• We discuss the use of the magnetic force theorem using different reference states upon which the perturbative approach is based. Using a fixed spin disordered local moment state one finds good Curie (or Neel) temperatures, and good energetics for planar spin spirals in the 3d magnets Fe, fcc Co, Ni, Mn, and Cr, though worse agreement for small theta spin spirals. On the other hand, the ferromagnetic reference state provides excellent energetics for small theta spin spirals in Fe, fcc Co, and Ni, and by extension magnon energies under the assumption of adiabacity. However, planar spin spiral energetics and transition temperatures show worse agreement. The reasons for this, and for the case of fcc Co where both approaches work very well, are discussed. We further provide an extension of the mapping of the quantum problem to include longitudinal fluctuations within force theorem based approaches, and discuss the role they will play in magnetic phase transitions. This construction is tested using planar spin spirals where q is fixed but the moment is allowed to relax. It is demonstrated that results from this approach and directly calculated ab initio values agree very well.

Subject headings

NATURVETENSKAP  -- Fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences (hsv//eng)

Keyword

temperature

fe

co

ni

approximation

nickel

metals

waves

iron

Physics

Publication and Content Type

ref (subject category)

art (subject category)