Spin-wave excitations in the ferromagnetic metallic and in the charge-, orbital-, and spin-ordered states in Nd1−xSrxMnO

H. Ulbrich

¹

,

Frank Krüger

²

,

A. A. Nugroho

³

et al.

Abstract

We report inelastic neutron scattering experiments on single crystals of Nd1−xSrxMnO3 with x=0.5 and x=0.49. The spin-wave dispersion in the charge, orbital, and spin ordered state in Nd0.5Sr0.5MnO3 exhibits a strongly anisotropic stiffness. The sign of the anisotropy is characteristic for the site-centered model for charge and orbital ordering in half-doped manganites. Within this model, linear spin-wave theory yields a perfect description of the experimental dispersion. In the ferromagnetic metallic state of Nd1−xSrxMnO3 with x=0.49 and x=0.50 magnetic excitations exhibit nearly the same magnon dispersion. High intense signals near the zone-boundary over a wide energy level overlap with a sharp spin-wave dispersion which can be described with a Heisenberg model including nearest-neighbor interactions.

Spin excitations used to probe the nature of exchange coupling in the magnetically ordered ground state ofPr0.5Ca0.5MnO3

“…We determined from our global best fit that J F 1 = 9.01 ± 0.02 meV. This value is comparable to that measured in PCSMO (11.39 ± 0.05 meV) 39 , La 0.5 Sr 1.5 MnO 4 (9.98 meV) 41 , the AFM CO phases of Nd 0.5 Sr 0.5 MnO 3 (6.76 meV) 42 , and a little larger than the typical values for the FM double exchange in metallic FM manganites 37,59 . In other words, it is consistent with its origin being due to double exchange.…”

“…However, calculation of the excitation spectra within such a framework is, though possible, far from straightforward 49,50 . In the absence of such calculations, we therefore use an effective Heisenberg Hamiltonian to model our data, just as was done for the half-doped manganites mentioned in the introduction 39,41,42 .The use of an effective Heisenberg Hamiltonian is a well-established procedure to analyze the spin wave spectra in magnetically ordered systems, and not just those with purely integer or halfinteger spins, nor even solely insulators. The exchange interactions in the model Hamiltonian that are required to reproduce the observed dispersion relations and intensities establish the strength, range and symmetries of the magnetic interactions.…”

“…1(a), since this then provides a natural explanation of how the signs of the nearest neighbor exchange interactions we consider come about. We also allow for the possibility of next-nearestneighbor interactions along the FM zig-zags, as has been required in previous studies of similar systems 39,41,42 and is taken to indicate a degree of electronic itinerancy along the zig-zag chains.…”

“…As discussed briefly above, in two earlier studies of the spin waves in CE-type half-doped manganites 41,42 , single-layered La 1/2 Sr 3/2 MnO 4 and pseudo-cubic Nd 0.5 Sr 0.5 MnO 3 , the lower part of the spin wave dispersion was measured and found to be well-explained by the Goodenough model including second neighbor exchange J F 2 along the FM zig-zags (see Fig. 1(a)).…”

“…First is the question of whether or not the Mn ions are paired in a dimer-like fashion in Pr 0.5 Ca 0.5 MnO 3 . On the basis of previous inelastic neutron scattering measurements of other half-doped manganites 39,41,42 we might expect the Goodenough model to provide a better description than models involving dimerization. However, since Pr 1−x Ca x MnO 3 is the material in which the possible existence of Zener polarons came to prominence, we believe that it is important to confirm whether or not this is the case.…”