Abstract
Insulating helimagnetic Cu2OSeO3 shows sizeable magnetoelectric effects in its skyrmion phase. Using magnetization measurements, magneto-current analysis and dielectric spectroscopy, we provide a thorough investigation of magnetoelectric coupling, polarization and dielectric constants of the ordered magnetic and polar phases of single-crystalline Cu2OSeO3 in external magnetic fields up to 150 mT and at temperatures below 60 K. From these measurements we construct a detailed phase diagram. Especially, the skyrmion phase and the metamagnetic transition of helical to conical spin order are characterized in detail. Finally we address the question if there is any signature of polar order that can be switched by an external electric field, which would imply multiferroic behaviour of Cu2OSeO3.
Highlights
We have provided a detailed study of the magnetization, pyrocurrent, electric polarization and dielectric constants of this system as function of temperature and external magnetic fields, focusing on the SkX phase
From the polarization and magnetization measurements, we have quantified the magnetoelectric coupling in all magnetic phases
The complexity of the magnetic phase diagram, the cancellation of ferroelectric polarization averaged over the domains of the multiple q-domain structure of the helical phase and the magnetoelectric effects, arising from the d-p hybridization, result in a complex magnetic-field dependence of the polarization and of the magnetoelectric coupling
Summary
Dielectric spectroscopy in external magnetic fields is an appropriate technique to identify the polar ground state of materials and to quantify magnetoelectric coupling[2,3,4,6,33,34,35]. The dielectric constant is ε′ ~ 12.5 at low temperatures and it slightly increases by about 0.1% at the magnetic phase transition. It is almost independent of magnetic field and only weakly dependent on frequency (inset of Fig. 4). The small increase of the dielectric constant in passing the magnetic phase transition probably is due to magnetostriction effects[37]. For zero external magnetic fields, this observation agrees with the absence of a polar state in Cu2OSeO3 discussed above [cf Fig. 3(a)]. Reveals a slightly enhanced dielectric constant ε′ in the vicinity of Tc as measured at 25 mT. The observed enhancement is only slightly beyond experimental uncertainty
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