Enhanced magnetopiezoelectric effect at the Néel temperature in CaMn2Bi2

Abstract

We have experimentally studied a magnetopiezoelectric effect (MPE), i.e., dynamic distortion caused by AC electric currents, in the antiferromagnetic conductor ${\mathrm{CaMn}}_{2}{\mathrm{Bi}}_{2}$ at low temperatures down to $77\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ using laser Doppler vibrometry. When AC electric currents are applied to ${\mathrm{CaMn}}_{2}{\mathrm{Bi}}_{2}$, dynamic displacement signals which increase in proportion to the current amplitude are clearly observed at $77\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, but their magnitude and temperature dependence strongly depend on Joule heating effects caused by the applied electric currents. Especially, poor thermal contact of ${\mathrm{CaMn}}_{2}{\mathrm{Bi}}_{2}$ samples to the sample holder produces second-harmonic displacement signals owing to thermal expansion due to the Joule heating, and tends to affect the temperature dependence of the MPE signals. In a measurement run without any heating effects, a sharp enhancement of the MPE signal is observed at the N\'eel temperature of ${\mathrm{CaMn}}_{2}{\mathrm{Bi}}_{2}$, which suggests that fluctuations of itinerant Mn moments near the N\'eel temperature play an important role in the MPE. We speculate that electric currents produce nonequilibrium antiferromagnetic moments via the antiferromagnetic Edelstein effect and the induced antiferromagnetic momemts enhance the MPE signal with the help of a large magnetovolume coupling near the N\'eel temperature. The MPE efficiency as a piezoelectric response reaches 300--$500\phantom{\rule{0.16em}{0ex}}\mathrm{pC}/\mathrm{N}$ at a maximum, which is comparable to high values of piezoelectric coefficients detected in the piezoelectric (ferroelectric) ceramics.