Magnetoelectric effect and phase transitions in CuO in external magnetic fields

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.

Spin dynamics and exchange interactions in CuO measured by neutron scattering

“…A spin-flop transition has been reported at B = 10.4 T at low temperature for magnetic fields applied along the b axis 40,71,72 . The spin-flop field B sf depends on the exchange interactions and the axial anisotropy.…”

Multiferroic spin-superfluid and spin-supersolid phases in MnCr2S4

“…To align the FE domains, on cooling electric poling fields of the order 1 kV/cm were applied. Electric polarization at high fields was measured by a pyroelectric technique (40). In these experiments pyrocurrent was mainly measured along the [111] direction, supplemented by some experiments along [110].…”

Proof of the elusive high-temperature incommensurate phase in CuO by spherical neutron polarimetry

“…The spin arrangement (7)(8)(9)(10) in the so-called AF2 phase was already investigated 30 years ago, and it was notably the spherical neutron polarimetry (SNP) work by Brown et al (9) that allowed an unambiguous determination of the associated magnetic structure: an incommensurate oblique helix with spins rotating in a plane with slight inclination with regard to the plane perpendicular to the propagation vector q. This spin rotation plane or spin envelope is defined by two main axes b and , where the latter lies within the a-c plane, at 28° from the positive c toward the positive a axis (11). The electric polarization along the monoclinic axis b (space group C2/c) may result from the cycloidal component via the inverse Dzyaloshinskii-Moriya effect (11)(12)(13)(14), although the actual mechanism remains under debate (15)(16)(17)(18)(19) and several other alternative mechanisms, e.g., involving magnetic frustration and magnetostriction, have been proposed.…”

“…This spin rotation plane or spin envelope is defined by two main axes b and , where the latter lies within the a-c plane, at 28° from the positive c toward the positive a axis (11). The electric polarization along the monoclinic axis b (space group C2/c) may result from the cycloidal component via the inverse Dzyaloshinskii-Moriya effect (11)(12)(13)(14), although the actual mechanism remains under debate (15)(16)(17)(18)(19) and several other alternative mechanisms, e.g., involving magnetic frustration and magnetostriction, have been proposed. The direct relation between the chiral spin helix and the electric polarization was demonstrated in an SNP experiment by switching the chiral domains with an applied electric field (20).…”

“…CuO was argued to exhibit an inverted sequence of symmetrybreaking mechanisms with respect to other type II multiferroics. Ultrasound velocity (11,15) and thermal expansion (28) measurements indeed suggest a third magnetically ordered phase just below the Néel temperature with a temperature stability range of less than 0.5 K. Being close to the elevated Néel temperature, the ordered magnetic moment  is very small, and therefore no neutron diffraction study was able to clarify the existence and the nature of this phase. However, to understand the complete picture of CuO andespecially by comparing it to related compounds-type II multiferroics in general, the microscopic structure of this AF3 phase is of major importance since the sequence of magnetic phase transitions and their associated magnetic structures are a result of the competition between exchange interactions and single-ion anisotropy that govern the physics of the system.…”