Spectroscopy of Magnetic Excitations in Magnetic Superconductors Using Vortex Motion

Abstract

In magnetic superconductors a moving vortex lattice is accompanied by an ac magnetic field which leads to the generation of spin waves. At resonance conditions the dynamics of vortices in magnetic superconductors changes drastically, resulting in strong peaks in the dc I-V characteristics at voltages at which the washboard frequency of vortex lattice matches the spin wave frequency ωs(g), where g are the reciprocal vortex lattice vectors. We show that if washboard frequency lies above the magnetic gap, peaks in the I-V characteristics in borocarbides and cuprate layered magnetic superconductors are strong enough to be observed over the background determined by the quasiparticles.The coexistence of magnetism and superconductivity was observed in many crystals, such as RMo 6 S 8 , RRh 4 B 4 , RBa 2 Cu 3 O 7−δ and (R,A)CuO 4−δ (A=Sr, Ce) with the temperatures of magnetic ordering T M much smaller than the superconducting critical temperature T c , and also in borocarbides RT 2 B 2 C and ruthenocuprate RuSr 2 GdCu 2 O 8 with T M of the same order as T c . Here R is the rare earth element, while T=Ni, Ru, Pd, Pt. In such crystals f -electrons of ions R give rise to localized magnetic moments, while conducting electrons exhibit the Cooper pairing. In all these crystals, except HoMo 6 S 8 and ErRh 4 B 4 , magnetic moments order antiferromagnetically below T M . Such magnetic ordering coexists with superconductivity without strong interference because spin density varies on the scale much smaller than the superconducting correlation length and net magnetic moment vanishes, for review see Refs. 1, 2, 3.In this Letter we consider interplay between magnetic and superconducting excitations in magnetic superconductors, particularly interaction between a moving vortex lattice and spin waves via the ac magnetic field induced by moving vortices. The energy transfer from vortices to the magnetic system leads to dissipation which is additional to that caused by quasiparticles. This results in strong current peaks in the dc I-V characteristics at voltages at which the washboard frequency of vortex lattice [4] matches the spin wave frequency ω s (k) and k matches a reciprocal vortex lattice vector g.First we consider slightly anisotropic superconductors, i.e. all systems mentioned above except SmLa 1−x Sr x CuO 4−δ and RuSr 2 GdCu 2 O 8 crystals, and probably also Sm 2−x Ce x CuO 4−δ .The latter are layered superconductors with intrinsic Josephson junctions [5,6,7,8].We assume, for simplicity, a uniaxial crystal structure with the principal axis along z. The dc magnetic field is applied along the z-axis and we assume that the magnetic induction B(r), r = x, y, inside the superconductor corresponds to the ideal Abrikosov square vortex lattice (such a lattice is realized in clean borocarbide crystals in field B c in some field intervals [1]). The sublattice magnetization in the case of antiferromagnetic ordering is assumed to be oriented in the (x, y) plane. The dc transport current with the density j is along the y-axis which, due t...

Magnetic excitations of the 2-D Sm spin layers in

“…While this fact could be used to extract the magnetic spectrum via transport measurements [9], it may also have a significant effect on the Sm-Sm exchange interaction as observed previously in several other cuprates containing rare-earth elements within the charge reservoir layers [10]. …”

Band structure of magnetic excitations in the vortex phase of a ferromagnetic superconductor

“…7(a)]. This peculiar behavior the first band is a consequence of the nonmonotonicity of the unperturbed spectrum (11) and is connected with a commensurability effect: The peak appears when ω 0 (0) = ω 0 (G 1 ).…”

Effects of magnetic excitations on the I–V characteristic of magnetic superconductors

“…To introduce spectroscopy of magnetic excitations in moderately anisotropic superconductors we follow [3] in this section. We assume, for simplicity, a uniaxial crystal structure with the principal axis along z.…”

“…Then moving vortices excite spin waves and energy transfer from vortices to the magnetic system leads to additional dissipation relative to that caused by quasiparticles. This results in strong current peaks in the dc I-V characteristics at voltages at which the washboard frequency of vortex lattice [3,4] matches the spin wave frequency ω s (k) and k matches the reciprocal vortex lattice vector g.…”