RF Pulse Response and NMR Signal Enhancement in Ferromagnets

Abstract

Ferromagnets are very important class of materials having wide variety of technological applications as memory elements, solid
state switches etc. The overall property is graetly influenced by local structure, dynamics and the processing variables of the material. The local structure and dynamics can be easily studied using Nuclear Magnetic Resonance (NMR).The method of NMR essentially involves placing the sample in an externally applied static magnetic field and then applying a radio frequency (rf) field at right angles. The local dynamics in ferromagnets may be studied using the so called zero field NMR spectroscopy where the strong field already present in the material give rise to splitting of the Zeeman levels. Excitations among these levels are achieved by applying radio frequency (rf) field of appropriate frequency and magnitude. The applied rf field and the observed Free Induction Decay (FID) signal get highly enhanced in ferromagnets. This enhancement is up to different extents for nuclei in domains and domainwalls. The enhancement and variation of NMR Free Induction Decay (FID) signal amplitude with the strength of the applied radio frequency
field has been explained by Stearns by invoking a model where it has been assumed that the domain-walls oscillate like the drumhead when subjected to NMR rf pulses.In this paper an alternative model is proposed and an attempt has been made to calculate the FID signal by taking into consideration the spiral orientation of the magnetization when one moves from one domain to the adjacent domain. We concentrate on spin I=1/2 system for simplicity as no quadrupolar effects are then involved.