Spin–phonon interaction and mode softening in NiF2

The temperature dependence of the four Raman-active phonons in NiF2 is investigated at temperatures above and below the antiferromagnetic ordering temperature of TN=73 K. All four modes exhibit distinct anomalies in their intensities and frequencies near TN due to spin–phonon coupling. The phonon linewidths also exhibit weak anomalies. From the temperature dependences of the phonon frequencies, estimates are made of the spin–phonon coupling coefficients. The B1g phonon exhibits anomalous mode softening with decreasing temperature from 300 K to TN.