Jason T. Haraldsen

Spin rotation technique for non-collinear magnetic systems: application to the generalized Villain model

This work develops a generalized technique for determining the static and dynamic properties of any non-collinear magnetic system. By rotating the spin operators into the local spin reference frame, we evaluate the zeroth, first, and second order terms in a Holstein-Primakoff expansion, and through a Greens functions approach, we determine the structure factor intensities for the spin-wave frequencies. To demonstrate this technique, we examine the spin-wave dynamics of the generalized Villain model with a varying interchain interaction. The new interchain coupling expands the overall phase diagram with the realization of two non-equivalent canted spin configurations. The rotational Holstein-Primakoff expansion provides both analytical and numerical results for the spin dynamics and intensities of these phases.

Spin-Wave Instabilities and Noncollinear Magnetic Phases of a Geometrically Frustrated Triangular-Lattice Antiferromagnet

This Letter examines the relation between the spin-wave instabilities of collinear magnetic phases and the resulting noncollinear phases for a geometrically frustrated triangular-lattice antiferromagnet in the high-spin limit. Using a combination of phenomenological and Monte Carlo techniques, we demonstrate that the instability wave vector with the strongest intensity in the collinear phase determines the wave vector of a cycloid or the dominant elastic peak of a more complex noncollinear phase. Our results are related to the observed multiferroic phase of Al-doped CuFeO2.

Importance of stacking to the collinear magnetic phases of the geometrically frustrated antiferromagnet CUFeO2

R.S. Fishman, F. Ye, J.A. Fernandez-Baca, J.T. Haraldsen, and T. Kimura Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee 37831, USA and Division of Materials Physics, Graduate School of Engineering Science, Osaka University, Toyanaka, Osaka, 560-8531, Japan

Spin-wave dynamics of magnetic heterostructures: application to Dy/Y multilayers

We examine the spin-wave (SW) dynamics of Dy/Y multilayers in order to separate the contribution of the Dy-Y interface from that of bulk Dy. The SW frequencies and intensities of bulk Dy are determined analytically. When the Dy layers in a multilayer geometry are decoupled, the SW dispersion relations are discontinuous with discrete excitations. With a Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction coupling through the Y spacer, the discrete excitations become dispersive and the main SW branches split due to the multilayer geometry. Regardless of the strength of the intermediate RKKY interaction, the dispersion signature of the bulk remains.

Critical anisotropies of a geometrically frustrated triangular-lattice antiferromagnet

This work examines the critical anisotropy required for the local stability of the collinear ground states of a geometrically-frustrated triangular-lattice antiferromagnet (TLA). Using a Holstein-Primakoff expansion, we calculate the spin-wave frequencies for the 1, 2, 3, 4, and 8-sublattice (SL) ground states of a TLA with up to third neighbor interactions. Local stability requires that all spin-wave frequencies are real and positive. The 2, 4, and 8-SL phases break up into several regions where the critical anisotropy is a different function of the exchange parameters. We find that the critical anisotropy is a continuous function everywhere except across the 2-SL/3-SL and 3-SL/4-SL phase boundaries, where the 3-SL phase has the higher critical anisotropy.

Spin dynamics in the multiferroic materials (invited)

We report high resolution inelastic neutron scattering measurements and spin dynamics calculations in two multiferroic materials: the geometrically frustrated triangular lattice CuFeO2 and mineral Hubnerite MnWO4. In un-doped CuFeO2 a low-T collinear spin structure is stabilized by long range magnetic interactions. When doped with a few percent of gallium, the spin order evolves into a complex noncollinear configuration and the system becomes multiferroic. Similarly, the ground state collinear spin order in pure MnWO4 results from delicate balance between competing magnetic interactions up to 11th nearest neighbors and can be tuned by substitution of Mn ions with magnetic or nonmagnetic impurities. The comprehensive investigation of spin dynamics in both systems help to understand the fundamental physical process and the interactions leading to the close interplay of magnetism and ferroelectricity in this type of materials.