Paramagnetic susceptibility and ultrasonic attenuation of itinerant antiferromagnetism above the Neel temperature

Abstract

Making use of the two-band model of the itinerant antiferromagnetism, we calculate the wave-vector-dependent paramagnetic susceptibility (i.e., the fluctuation propagator of the order parameter) and the ultrasonic attenuation coefficient above the Neel temperatureT _N .We distinguish two regions depending on the degree of “the nesting” between the electron and the hole bands. The degree of the nesting is characterized byH, which measures the difference of the Fermi radii of the electron and the hole bands. WhenH is less than the critical nestingH*, the fluctuation is of the diffusion type indicating the transition into the commensurate spin density wave (CSDW) state; the fluctuation propagator diverges atT=T _N forq=0 (the paramagnetic susceptibility diverges forq=G _i /2;G _i are the reciprocal lattice vectors) whereq is the wave number vector. WhenH>H*, on the other hand, the fluctuation propagator diverges atT=T _N ,on the sphere in the wave vector space with the radiusq ₀,signalling the transition into the incommensurate spin wave (ISDW) state; the paramagnetic susceptibility diverges now forq=(G _i /2)+q′, where |q′|=q ₀.The attenuation coefficient is much enhanced due to the spin fluctuation nearT _N . We find that the attenuation coefficient of the longitudinal sound wave diverges atT=T _N as η^−3/2 forH≤H* and η^−5/2 forH>H*, where η =(T/T _N )−1. The latter prediction is in reasonable agreement with the recent measurement of the attenuation coefficient in pure Cr by Imai and Sawada.