Anomalous transmission of microwave power at FMR through nickel crystals at 4.2°K

Abstract

We have observed an unexpected increase in the transmission of 24 GHz microwave power through thin pure, nickel slabs at liquid helium temperatures, and at an applied static magnetic field value corresponding to ferromagnetic resonance (FMR). This increase in power at FMR was particularly unexpected because our specimens were very thick compared with the anomalous skin depth at FMR, d/δ∼100 to 1000. The nickel crystals were prepared from a boule having a residual resistance ratio in excess of 2000. The transmission amplitude ratio was found to be ca. 10−6 for a specimen 8.2 μm thick, and having the static field oriented along the hard axis (100). The transmission ratio was approximately six times smaller when the static field was oriented along the easy axis (111). The anomalous transmission amplitude decreased rapidly with increasing temperatures, and was no longer observable at 25 °K in a specimen 8.2 μm thick. Numerical calculations using a non-local conductivity and the assumption of specular scattering of the current carries at the specimen surfaces did not show any field dependence of the transmission at FMR. The origin of this effect is unknown, but is clearly related to the large mean free path at low temperatures (l∼d), since it was not observed in polycrystalline specimens having a residual resistance ratio ∼16.