Magnetoelectric effect inR2CuO4(R=Gd, Sm, and Nd)

Abstract

In strong magnetic fields up to 20 T the magnetoelectric (ME) effect has been measured in the tetragonal antiferromagnets ${\mathrm{R}}_{2}$${\mathrm{CuO}}_{4}$, R = Gd, Sm, and Nd, which are parent compounds for high-${\mathrm{T}}_{\mathrm{c}}$ superconductivity. For the ${\mathrm{Gd}}_{2}$${\mathrm{CuO}}_{4}$ and ${\mathrm{Sm}}_{2}$${\mathrm{CuO}}_{4}$ compounds a symmetry analysis yields that the ME effect is associated with the antiferromagnetically ordered rare-earth subsystem. From the magnetic field and temperature dependence of the ME effect in ${\mathrm{Gd}}_{2}$${\mathrm{CuO}}_{4}$ some very detailed conclusions can be drawn about the nature of the magnetic ordering which appear to be in agreement with the magnetic structure revealed in this compound. For ${\mathrm{Sm}}_{2}$${\mathrm{CuO}}_{4}$ in addition to the ME effect a distinct ferroelectric behavior has been observed which is only compatible with a lower crystal symmetry than it has been thought to be. It raises the question of whether superconductivity in Ce-doped ${\mathrm{Sm}}_{2}$${\mathrm{CuO}}_{4}$ coexists with ferroelectricity. The ME-effect data in high magnetic fields show evidence for magnetic phase transitions, associated with a rearrangement of the intrinsic rare-earth magnetic structure. From the obtained (H,T) phase diagrams of ${\mathrm{Gd}}_{2}$${\mathrm{CuO}}_{4}$ and ${\mathrm{Sm}}_{2}$${\mathrm{CuO}}_{4}$ critical exponents [\ensuremath{\mathrm{B}}(Gd)\ensuremath{\approx}0.4 and \ensuremath{\mathrm{B}}(Sm)\ensuremath{\approx}0.5] were found and it can be concluded that the intrinsic ordering of the rare-earth magnetic subsystem is of three-dimensional character. In the case of ${\mathrm{Nd}}_{2}$${\mathrm{CuO}}_{4}$, the adopted crystal and magnetic structure of this compound rules out any kind of ME effect. Nevertheless, we have observed some weak but distinct ME response, which suggests the existence of a spontaneous structural distortion within the ${\mathrm{CuO}}_{2}$ planes.