ABSTRACT
Apart from being so far the only known binary multiferroic compound, CuO has a much higher transition temperature into the multiferroic state, 230 K, than any other known material in which the electric polarization is induced by spontaneous magnetic order, typically lower than 100 K. Although the magnetically induced ferroelectricity of CuO is firmly established, no magnetoelectric effect has been observed so far as direct crosstalk between bulk magnetization and electric polarization counterparts. Here we demonstrate that high magnetic fields of ≈ 50 T are able to suppress the helical modulation of the spins in the multiferroic phase and dramatically affect the electric polarization. Furthermore, just below the spontaneous transition from commensurate (paraelectric) to incommensurate (ferroelectric) structures at 213 K, even modest magnetic fields induce a transition into the incommensurate structure and then suppress it at higher field. Thus, remarkable hidden magnetoelectric features are uncovered, establishing CuO as prototype multiferroic with abundance of competitive magnetic interactions.
ABSTRACT
Reaction of 2,4-(2,2'-dipyridylamino)-6-(2-pyridylhydrazino)-1,3,5-triazine, abbreviated dppht (5), with copper(II) chloride in methanol yields a mixed-valent Cu(II)(4)Cu(I) compound (6), also involving the presence of a radical anion. The single-crystal structure determination for [Cu(5)(5(M))(5(MR))Cl(8)] (6) reveals that the original dppht ligand (5) has been monochlorinated at one of its pyridine rings and oxidized at its hydrazyl moiety, generating the ligand 2,4-(2,2'-dipyridylamino)-6-(2-(5-chloropyridyl)azo)-1,3,5-triazine (5(M)). Moreover, the molecular structure of 6 indicates that one of the two coordinated ligands 5(M) is in a radical-anion state, symbolized as 5(MR), characterized by a typical N-N bond length of about 1.33 A for a one-electron reduced azo group. The nature of this unique [Cu(II)(4)Cu(I)(radical)] assembly is corroborated by X-ray absorption spectroscopy, electron paramagnetic resonance, and magnetic susceptibility measurements. Density-functional theory calculations are consistent with the unprecedented structural features and support the spectroscopic data.
