Imaging of Hybrid-Multiferroic and Translation Domains in a Spin-Spiral Ferroelectric
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1199-F02-07
Imaging of Hybrid-Multiferroic and Translation Domains in a Spin-Spiral Ferroelectric Meier D.1, Leo N.1, Lottermoser Th.1, Becker P.2, Bohatý L.2 and Fiebig M.1 1 Helmholtz-Institut für Strahlen- und Kernphysik, Universität Bonn, Nussallee 14-16, 53115 Bonn, Germany 2 Institut für Kristallographie, Universität zu Köln, Zülpicher Str. 49b, 50674 Köln, Germany ABSTRACT Two fundamentally different types of domains were resolved in multiferroic MnWO4 by optical second harmonic generation (SHG). Hybrid-multiferroic (absolute) domains reflect the magnetic chirality coupled 1:1 to the spontaneous polarization because of the magnetic origin of the ferroelectric order. Magnetic translation (relative) domains reflect discontinuities in the progression of the magnetic spin spiral. SHG topography is the only experimental method so far allowing one to image both types of domains. The imaging procedure and the SHG contributions involved are therefore discussed in detail. INTRODUCTION Materials with coexisting magnetic and electric order attract tremendous attention, because these so-called multiferroics often show pronounced magnetoelectric (ME) interactions between the spin and charge subsystems. The ME coupling enables magnetic phase control by application of an electric voltage or, vice versa, the switching of a spontaneous polarization by a magnetic field. The ME interactions are intrinsically strong in magnetically induced ferroelectrics, where magnetic long-range order in the form of spirals (or cycloids) breaks the inversion symmetry of the system and induces a spontaneous polarization. Recently, spin-spiral multiferroics such as TbMnO3, Ni3V2O8, and MnWO4 have been shown to develop particularly strong ME interactions [1−3]. In these systems the non-centrosymmetric spin arrangement induces a spontaneous polarization P ∝ eij × (Si× Sj),
(1)
with the unit vector eij connecting neighboring spins at sites i and j and Si× Sj as the vector chirality [4,5]. Since the direction and magnitude of P are determined by the magnetic order only, a unique coupling between magnetism and ferroelectricity is observed. Although the spontaneous polarization is usually small, its robustness renders spin-spiral multiferroics interesting for future applications [6,7]. The magnetic and electric domains in spin-spiral multiferroics are particularly interesting because, at its root, any ME interaction originates from their correlation. However, although domains are known to be present in these systems, they cannot be imaged in a straightforward way. For the antiferromagnetic order no macroscopic magnetic moments exist that can be exploited to image the magnetic domain structure, whereas the small value of the spontaneous electric polarization (1 − 10 nC/cm²) prohibits the application of established imaging techniques such as piezoresponse force microscopy (PFM) for mapping the ferroelectric domains. Thus, nearly nothing is known about the domain topology of these systems.
Here, we report how the different multiferroic domains in a
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