Microwave Conductivity Dependence of Ferromagnetic Resonance Linewidth in Magnetic Semiconductors

Abstract

Ferromagnetic and paramagnetic resonance linewidth ΔB has been measured in single crystals of as-grown and annealed EuSe in the liquid helium temperature range. A standard microwave spectrometer operating at K-band frequencies was used. The polished spherical samples were oriented so that the (110) plane contained both the mw and static magnetic fields. The measured large paramagnetic linewidth ∼ 1400 G, on which annealing had no effect, is explained solely on the basis of spin-spin interactions. A further conclusion is that the non-classical dipolar and quadrupolar mechanisms are not important in EuSe.In the critical temperature range 4-12 K an exchange narrowing of ΔB, typical for ferromagnets, was found in annealed samples, which were electrically insulating. On the other hand, in the as-grown samples, which had a high microwave conductivity, the ferromagnetic linewidth showed the opposite behaviour, i.e., it increased largely with decreasing temperature being approximately 3000 G at 2.8 K. This exceptionally broad line-width can be explained by an indirect spin-lattice relaxation of magnons due to charge carriers. This effect is proportional to the microwave conductivity σmw of the sample and hence it has the same annealing dependences of ΔB and the dependence on the sample size. In annealed samples surface pit scattering of magnons and 3-magnon processes dominate at low temperatures.