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) direct visualization of magnetoelectric domains; (2) emergent properties of new topological defects in multiferroic hexagonal manganites, (3) direct observation of novel pancake-like magnetic domains in multiferroic LuFe2O4, (4) domain structure of hard magnet Fe0.25TaS2 with extremely large anisotropy, (5) strain-induced magnetism and ferroelectricity.

 1. Direct visualization of magnetoelectric domains

The magnetoelectric effect originates from the cross-coupling between the magnetic and electric dipoles in insulating magnets. It holds promise for conceptually novel electronic devices such as electric field controlled magnetic memory and compact magnetic field sensors. However, the existence of domains and defects in these ferroic materials strongly influences their macroscopic responses, which calls for development of local probes of the magnetoelectric effect. We have developed a novel Magnetoelectric Force Microscopy (MeFM) technique that combines Magnetic Force Microscopy (MFM) with in-situ modulating high electric fields [1]. This microscopy technique enables direct visualization of the magnetoelectric response of the domain patterns in multiferroic materials (e.g. hexagonal manganites) [2]. The interesting observation of the sign change of magnetoelectric response at each structural domain wall is nicely explained by Craig Fennie at Cornell and Maxim Mostovoy at Groningen using symmetry analysis and phenomenological modeling, which provides compelling evidence for the mechanism of lattice-mediated magnetoelectric coupling [3]. Furthermore, the MeFM results revealed a divergent magnetoelectric effect near the tri-critical point of spin re-orientation transition, suggesting a possibility to enhance magnetoelectric couplings by harnessing critical fluctuations of spins.  The detection of magnetoelectric response at mesoscopic scales not only allows direct visualization of magnetoelectric domains, but also opens up explorations of exciting emergent phenomena in multifunctional materials with multiple coupled orders.

[1] Yanan Geng, Weida Wu, “Magnetoelectric force microscopy based on magnetic force microscopy with modulated electric field”, Rev. Sci. Instrum., in press (2014).

[2] Yanan Geng, Hena Das, Aleksander L. Wysocki, Xueyun Wang, S-W. Cheong, M. Mostovoy, Craig J. Fennie, and Weida Wu, “Direct visualization of magnetoelectric domains”, Nature Mater., 13, 163-167 (2014).

[3]  Hena Das, Aleksander L. Wysocki, Yanan Geng, Weida Wu and Craig J. Fennie, “Bulk magnetoelectricity in the hexagonal manganites and ferrites”, Nature Comm., 5, 2998 (2014).

2. 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 ofthe 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.

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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].

[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", PNAS, 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).


3. 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 [1]. 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 [2]. 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 [2].

      

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

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


4. Domain observation of layered magnets with extremely large anisotropy

In addition to multiferroics, we studied domain structure in layered hard magnets Fe0.25TaS2, 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, Fe0.25TaS2 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 [1]. Therefore, Fe0.25TaS2 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 Fe0.25TaS2.

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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)


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




5.
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 [1]. 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 [2]. 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 [3]. 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.

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Induced Ferromagnetism in LaCoO3 thin film

Multiferroic EuTiO3 thin film


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

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

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