Spin waves in configurational antiferromagnetic states: transitions to and from ferromagnetic states

We study collective spin waves in transversally magnetized chains of thin elliptical dots initially arranged into an antiferromagnetic (AF) configuration. The dispersion relations of the SW as well as the corresponding magnetization precession profiles (eigenvectors) are computed with the dynamical matrix method [1]. We report the peculiar features of AF optical and acoustical spin modes within the effective wavevector model [2], which has been adapted to the present geometry. We investigate the effects of the application of a transverse external field, leading the system from a perfect AF configuration to the ferromagnetic (FM) state, at a specific transition field. We discuss the mode localization and symmetry, and in particular, we focus on the AF soft mode, its symmetry and its relationship with the instability leading to AF-to-FM transition. We discuss the importance and potential application of the SW propagative properties suddenly changed at the switching from AF to FM. Finally, we present the calculations of spin dynamics concerning a FM-to-AF transition in a chain of ellipses with different width: in this case, the different shape anisotropy of the two ellipses results in different switching fields and hence in a field interval where the AF state is spontaneously attained. We show how this AF field interval can also be tuned by an effective uniaxial anisotropy of one out of two ellipses: this is indicative of any other source of anisotropy (like inverse-magnetostriction, particularly promising for voltage driven magnetic transitions in multiferroics): in this way, we investigate the AF-to-FM transition spin dynamics by varying the effective anisotropy constant of the material. Finally, we show and comment on thepeculiar and remarkable change of slope of the mode dispersion across the AF-to-FM transition, with its implications in the field of magnon-spintronics devices. References: [1] L. Giovannini, F. Montoncello, and F. Nizzoli, Phys. Rev. B 75, 024416 (2007). [2] F. Montoncello, S. Tacchi, L. Giovannini, M. Madami, G. Gubbiotti, G. Carlotti, E. Sirotkin, E. Ahmad, F. Y. Ogrin, and V. V. Kruglyak, Appl. Phys. Lett. 102, 202411 (2013)