Softened magnetic excitations in the s = 3/2 distorted triangular antiferromagnet alpha-CaCr2O4

TL;DR

This study investigates the magnetic excitations of alpha-CaCr2O4, revealing soft roton-like modes and complex spin interactions that influence its magnetic properties, using Raman spectroscopy and ESR techniques.

Contribution

It provides new insights into the spin dynamics of a distorted triangular s=3/2 antiferromagnet, highlighting the role of complex interactions and soft modes in its magnetic behavior.

Findings

Two-magnon excitations show multi-maximum structure with low-energy dominance.

ESR linewidth exhibits critical broadening with a small exponent, indicating soft modes.

Complex spin topology with multiple interactions causes magnetic instability.

Abstract

The spin dynamics and magnetic excitations of the slightly distorted triangular s = 3/2 system alpha-CaCr2O4 are investigated by means of Raman spectroscopy and electron spin resonance (ESR) to elucidate its peculiar magnetic properties. Two-magnon excitations in circular RL symmetry show a multi-maximum structure with a dominant spectral weight at low energies. The temperature dependence of the ESR linewidth is described by a critical broadening DeltaHpp(T) ~ (T - T_N)^{-p} with the exponent p = 0.30(3) - 0.38(5) for temperatures above T_N = 42.6 K. The exponent is much smaller than that of other s = 3/2 triangular lattices. This is ascribed to soft roton-like modes, indicative of the instability of a helical 120{\deg} phase. As an origin we discuss a complex spin topology formed by four inequivalent nearest neighbor and sizable next-nearest neighbor interactions.