Research

The cutting edge SPM techniques and the unique instrumentations allow us to carry out top notch research in materials science. Specifically, our research has made significant impacts on magnetoelectrics and multiferroics. The major research accomplishments of my group are in the following areas: (1) emergent properties of new topological defects in multiferroic hexagonal manganites, (2) direct observation of novel pancake-like magnetic domains in multiferroic LuFe₂O₄, (3) domain structure of hard magnet Fe_0.25TaS₂ with extremely large anisotropy, (4) strain-induced magnetism and ferroelectricity, (5) percolation of gapped topological surface states in Bi₂Se₃.

1. Emergent phenomena at the vortex domain walls in hexagonal manganites

Recently, we discovered interlocked structural anti-phase and ferroelectric domain walls forming vortex-like topological defects in multiferroic hexagonal manganites [1]. The realization of a 6-state vortex domain structure has a broad impact on various areas of condensed matter physics (e.g. clock model, graph theory) and even on cosmology model (the Kibble-Zurek mechanism which describes the possible formation of topological defects during a spontaneous symmetry breaking phase transition at the birth of the universe) [2-4]. Following our initial breakthrough, we also discovered collective uncompensated magnetic moments at cross-coupled domain walls in multiferroic hexagonal manganites by correlating room temperature piezoelectric-response force microscopy (PFM) images of ferroelectric domains and MFM images at low temperature at the same location. Our results open up the possibility of controlling nanoscale magnetic moments with an electric field [5]. To the best of our knowledge, this is the first direct observation of uncompensated moments at antiferromagnetic domain walls.

Besides local magnetism, we also investigated the intriguing local conduction and piezoelectric response of multiferroic hexagonal manganites. Our results provide direction observation of polarization modulated Schottky-like barrier on the (001) surface of HoMnO₃ [6]. Furthermore, we observed enhanced conduction at charged ferroelectric domain walls that are protected by the presence of topological defects [7], which open the possibility of creating a tunable conducting channel without chemical doping. Finally, my former undergraduate student Edward Lochocki discovered an intriguing out-of-plane piezoelectric-response at these charged domain walls [8].

2. Direct observation of pancakelike magnetic domains in LuFe₂O₄

Another interesting multiferroic that we have studied is hexagonal LuFe₂O₄ which has a giant magnetic coercivity (~9 T) at low temperature. In collaboration with colleagues (Kiryukhin and Cheong), my group discovered random packing of nanosize pancake-like Ising domains by a combination of magnetic imaging and neutron scattering [9]. The freezing of the random configuration of Ising pancakes was linked to enhancement of magnetic coercivity of LuFe₂O₄ at low temperature, suggesting the presence of strong magnetic disorders. Consistently, we also observed strong memory effect of magnetic domain pattern by performing magnetic imaging in high magnetic fields [10]. By correlating magnetic imaging results and structural imaging data from electron microscopy, my group and collaborators provided a microscopic insight on the origin of the strong magnetic disorder and the giant magnetoelectric coupling in multiferroic LuFe₂O₄ [10].

3. Domain observation of layered magnets with extremely large anisotropy

In addition to multiferroics, we studied domain structure in layered hard magnets Fe_0.25TaS₂, which is a model system to explore new hard magnets without heavy elements (e.g. rare earth). Due to its special crystal field environment around Fe ions, Fe_0.25TaS₂ hosts extremely large magnetic anisotropy at low temperatures, which is responsible for many anomalous physical behaviors. More interestingly, the density of defects (anitphase boundaries) that would pin magnetic domain walls can be controlled by annealing conditions [11]. Therefore, Fe_0.25TaS₂ is an ideal model system for studying the intriguing competition between domain wall pinning and nucleation in hard magnets. We have obtained numerous additional imaging results on magnetic domains and domain walls in Fe_0.25TaS₂ and a manuscript on these results is being prepared [12].

Correlation between antiphase domains (TEM) and magnetic domains (MFM)	ZFC warming MFM measurements (0.03 T). The magnetic domain pattern disappears at TC (~ 160 K)

4. Strain-induced magnetism and ferroelectricity

In collaboration with researchers from Argonne National Laboratory and University of California at Riverside, we have carried out magnetic imaging studies of strained and unstrained LaCoO₃ thin films [13]. Our results provide supporting evidence that the ferromagnetic coupling in LaCoO₃ is enhanced by epitaxial strain. In collaboration with researchers at multiple institutes (Cornell, Argonne and etc.), we also successfully applied our magnetic imaging technique to multiferroic EuTiO₃ thin film, where multiferroicity is induced by epitaxial strain [14]. Our MFM results demonstrated that the magnetic state is inhomogeneous, and likely due to strain inhomogeneity, which provides microscopic origin of the “missing saturation moment” issue [15]. These results clearly demonstrate that real space magnetic imaging characterization provides indispensible information for fine-tuning the novel properties of artificial materials for desired functionalities.

Induced Ferromagnetism in LaCoO3 thin film	Multiferroic EuTiO3 thin film

5. Percolation of topological surface states on In doped Bi₂Se₃

Topological insulators belong to a new class of electronic materials that have bulk band gap and conducting surface states with Dirac-like dispersion. It is believed that the Dirac surface states are robust against non-magnetic impurities due to topological protection, and may lead to novel emergent phenomena such as topological magnetoelectric effect and majorana fermions. Using low temperature scanning tunneling microscopy/spectroscopy (STM/STS), we have studied the topological-normal insulator phase transition in (Bi,In)₂Se₃. The STS results suggest that the topological states are locally destroyed by non-magnetic In dopants in topological insulator Bi₂Se₃. Furthermore, the inhomogeneity effectively removes the Dirac point by gapping the long wavelength surface states. These results bring in new perspective to the dynamic research of topological insulators, and may help to design/discover new topological materials.

References

1. Choi, T., et al., Insulating Interlocked Ferroelectric and Structural Antiphase Domain Walls in Multiferroic YMnO₃. Nat. Mater., 9, 253-258, (2010).

2. Chae, S.C., et al., Self-organization, condensation, and annihilation of topological vortices and antivortices in a multiferroic. P. Natl. Acad. Sci. USA, 107, 21366-21370 (2010).

3. Zurek, W.H., Cosmological experiments in superfluid helium? Nature, 317, 505-508, (1985).

4. Chae, S.C., et al., Direct observation of the proliferation of ferroelectric dislocation loops and vortex-antivortex pairs. Phys. Rev. Lett., 108, 167603, (2012).

5. Geng, Y., et al., Collective magnetism at multiferroic vortex domain walls Nano Letters, 12, 6055–6059 (2012).

6. Wu, W., et al., Polarization-Modulated Rectification at Ferroelectric Surfaces. Phys. Rev. Lett., 104, 217601, (2010).

7. Wu, W., et al., Conduction of topologically-protected charged ferroelectric domain walls. Phys. Rev. Lett., 108, 077203, (2012).

8. Lochocki, E.B., et al., Piezoresponse force microscopy of domains and walls in multiferroic HoMnO₃. Appl. Phys. Lett., 99, 232901, (2011).

9. Wu, W., et al., Formation of pancakelike Ising domains and giant magnetic coercivity in ferrimagnetic LuFe₂O₄. Phys. Rev. Lett., 101, 137203, (2008).

10. Park, S., et al., Pancakelike Ising domains and charge-ordered superlattice domains in LuFe₂O₄. Phys. Rev. B, 79, 180401 (R), (2009).

11. Choi, Y.J., et al., Giant magnetic coercivity and ionic superlattice nano-domains in Fe0.25TaS2. EuroPhys. Lett., 86, 37012, (2009).

12. Park, S., Y.J. Choi, S.-W. Cheong, and W. Wu, Irreversible domain states in colossal magnetic coercive Fe_0.25TaS₂. in preparation, (2012).

13. Park, S., et al., Microscopic evidence of a strain-enhanced ferromagnetic state in LaCoO₃ thin films. Appl. Phys. Lett., 95, 072508, (2009).

14. Lee, J.H., et al., A strong ferroelectric ferromagnet created by means of spin–lattice coupling. Nature, 466, 954-958, (2010).

15. Geng, Y., et al., Magnetic inhomogeneity in a multiferroic EuTiO₃ thin film. Phys. Rev. B, 87, 121109 (R), (2012).