Per-Anker Lindgård

Calculation of the Ruderman-Kittel interaction and magnetic ordering in copper

Abstract Using first principles energy bands and wave functions we find the Ruderman-Kittel interaction having a more predominant nearest neighbor coupling than expected for free electrons. Using the correlation theory and including dipolar interactions we find the most probable structure to be a simple type I antiferromagnet. For smaller values of the interaction strength the structure should be incommensurate with Q along ΓK.

Renormalization of long wavelength spin waves in the 2D ferromagnet Rb2CrCl4

Abstract Rb2CrCl4 is a nearly 2d-ferromagnetic, optically transparent insulator isomorphous with K2CuF4. High resolution neutron scattering data for temperatures below Tc = 52.4 K of the low energy long wavelength spin waves are presented and a Hartree-Fock analysis yields Hamiltonian parameters and accounts for the renormalization. No evidence of a Bose condensate is found. A spin canting angle θ ≈ 2° is predicted.

Theory of spin waves in a sinusoidal phase

Abstract A theory of spin waves in a sinusoidal phase is developed by considering δQ = Q - Qc as a small parameter relative to the closest commensurate wave vector Qc, for which the spin wave spectrum can be solved using sublattices and the random phase approximation. A second order differential equation can then be formulated and solved to yield the dispersion relation and an energy width of the modes in any incommunsurate phase. The theory is relevant for experiments on dynamics of incommensurate systems like Nd, Pr and CeAl2.

Calculation of the Ruderman-Kittel interaction and the nuclear magnetic ordering in silver

Using first-principles energy bands and wave functions we have calculated the Ruderman-Kittel interaction between the nuclear spins of I = 12 in silver. We find, similar to copper, that the interaction is dominated by a nearest neighbor interaction and decreases more rapidly with distance than expected for the free electron model, further the dipolar interaction is relatively small. This makes the ideal fcc antiferromagnetic silver system particularly interesting. The uniform susceptibility and the Curie temperature is calculated using correlation theory and found in agreement with experimental observations. The calculated transition temperature. TN is somewhat higher than the quoted experimental value of 560 pK.

Theory of field induced incommensurability: CsFeCl3

Abstract Using correlation theory for the singlet-doublet magnet CsFeCl 3 in a magnetic field, a field induced incommensurate ordering along K-M is predicted without invoking dipolar effects. A fully self-consistent RPA theory gives H c =44 kG in agreement with experiments at T =1.3 K. Correlation and dipolar effect are discussed.

Correlation theory of crystal field and anisotropic exchange effects

Abstract A general theory for including correlation effects in static and dynamic properties is presented in terms of Raccah or Stevens operators. It is explicitly developed for general crystal fields and anisotropic interactions and systems with several sublattices, like the rare earth compounds. The theory gives explicitly a temperature dependent renormalization of both the crystal field and the interactions, and a damping of the excitations and in addition a central park component. The general theory is illustrated by a discussion of the singlet-doublet system. The correlation effects on the susceptibility, the first and second moment frequencies and the line shape are calculated self-consistently.

Theory and simulation of antiferomagnetic nano-particles

Abstract The structural behavior of ideal, bulk FCC antiferromagnets in a field is known to be very complex (Phys. Rev. Lett. 53 (1984) 1692; Phys. Rev. Lett. 61 (1988) 629, Phys. Rev. Lett. 64 (1990) 1421). Kodoma and Berkowitz (Phys. Rev. B 59 (1999) 6321) found that for example even small NiO particles had a non-colinear structure with several sublattices. This is confirmed by Monte Carlo simulations, which are well suited for studying the temperature and field dependence of magnetic nano-particles. It is further found that during the magnetization reversal in a field a given, complicated tour is taken from the 4- q ground state among various multi- q states. The energy differences between the non-colinear states are small, and they are all populated at room temperature. The relaxation behavior of a simple, ideal system is studied and discussed. It is found that the multi- q ordering in FCC antiferromagnets in a field severely limits the exchange biasing potential since a flipping of the excess moment requires only a flipping of a small fraction of the individual spins.

Theory of a new type of antiferromagnetism in the ideal fcc system Cu

In Cu the competition between the dipolar and Ruderman-Kittel interaction results in a simple type-I-antiferromagnetic ordering with Q = (2 π/ a )(1, 0, 0), which is nearly degenerated with a ΓK ordering with Q = (2π/ a )(0, η, η) for 0.6

Theory of singlet-doublet systems showing a soft mode and a central peak

Abstract The dynamics of a singlet-doublet system is investigated for T ≧ T c . The transition is shown to be initiated by a soft mode Ω Q ( T ) and a central peak with the intensity X q / Ω 2 q ≈ Ω −4 Q ( T ). This exhibits diffusional behavior and critical slowing down. The theory explains the observed soft mode and central peak with respect to the dependence on wavevector, energy, temperature and pressure in Pr.

Correlation theory of static and dynamic properties

Abstract A simple and practical Greens function theory, including correlations by the memory function technique, is developed for a general magnetic Hamiltonian yielding the exact results at T → 0 and T → ∞ and giving results for any q , ω and T which are considerably more accurate than obtained by the RPA theory.