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Experimental studies of the magnetoelectric effect in the antiferromagnetic crystals Cr20 3, Gd2CuO4 and Sm2CuO 4 have been performed in strong magnetic fields up to 20 T. The magnetoelectric effect is fully determined by the symmetry of the magnetically ordered material and yields hence valuable information about magnetic ground states. When the magnetic symmetry is changed at a magnetic field induced phase transition the magnetoelectric effect exhibits anomalies at the phase transition owing to the fact that the magnetoelectric signal is related to the antiferromagnetic order parameter L. Even at temperatures close to the Nrel temperature TN the obtained experimental data resolve the magnetic properties very well, giving the possibility to study magnetic phase transitions in the critical temperature range.

The magnetoelectric (ME) efFect is observed in the tetragonal antiferromagnet GdqCu04 in magnetic fields up to 7 T. This is an observation of the ME effect in the class of tetragonal crystals with the general formula RqCu04 (R =rare earth), which are parent compounds for high-T, superconductivity. The ME effect is shown to exist only below the ordering temperature of the rare-earth magnetic subsystem at T~(Gd)= 6.5 K ln accordance with results based on a symmetry analysis.The field and temperature dependence of the effect confirm the magnetic structure revealed recently in this compound.The magnetic properties of the R~Ce Cu04 (R=rare-earth ion) compounds, which belong to the family of recently discovered high -T, superconductors with electron -type conductivity, have been intensively investigated during the last few years, following the suggestion that the antiferromagnetism of the Cu02 planes plays an important role in the superconductivity of these compounds. In fact, the rare-earth subsystem is also of importance in the magnetism of these materials. Unlike in R-Ba-Cu-0 compounds where the substitution of trivalent rare earths had little effect on their superconductivity, in electron-doped R2 Ce Cu04 compounds the rare-earth subsystem affects strongly both magnetic and superconducting properties. According to Refs. 3 -6, the superconductivity of R2 Ce Cu04 is observed only for compounds containing light rare-earth ions (R =Nd, Sm, Eu, and Pr), and no superconductivity is observed in compounds with heavier rare-earth ions, starting &om Gd. Also investigations of the upper critical Geld H, 2 of Smq 85Ceo i5Cu04 give indications for an infiuence of the magnetic ordering of the rare-earth ions on the superconducting state. ' It is therefore of main concern to elucidate the role of the rare-earth ions in forming the electronic properties of R2 Ce Cu04 and of the parent compound R2Cu04.Gd2Cu04 occupies a unique place in the family of electron doped high -T superconducting materials due to a number of unusual magnetic properties.In addition, a ferromagnetic ordering of the rare-earth spins within the (a, b) layers, which are parallel to the Cu02 layers, has been detected in Gd~Cu04 by means of neutron scattering. Considering the possible correlation between nonsuperconducting and magnetic properties of Gd2Cu04 we looked for an independent method of studying the magnetic properties of the Gd spin system in Gd2Cu04. Therefore, we have undertaken a systematic search for the magnetoelectric effect in this material, the existence of which is directly related to the symmetry properties of the magnetic structure in Gd2Cu04. In this paper we present experimental results of the magnetic 6eld induced electric polarization in Gd2Cu04, which, to our knowledge, magnetoelectric effect has not been observed before in any compound with the general formula R2Cu04.The magnetic structure of Gd2Cu04 includes two magnetic subsystems (copper and rare earth) with essentially different magnetic properties. The Cu spins become antiferromagne...