Observation of coupled magnetic and electric domains

Physical Review B

Improper ferroelectrics are described by two order parameters: a primary one, driving a transition to long-range distortive, magnetic or otherwise non-electric order, and the electric polarization, which is induced by the primary order parameter as a secondary, complementary effect. Using lowtemperature scanning probe microscopy, we show that improper ferroelectric domains in YMnO3 can be locally switched by electric field poling. However, subsequent temperature changes restore the as-grown domain structure as determined by the primary lattice distortion. The backswitching is explained by uncompensated bound charges occuring at the newly written domain walls due to the lack of mobile screening charges at low temperature. Thus, the polarization of improper ferroelectrics is in many ways subject to the same electrostatics as in their proper counterparts, yet complemented by additional functionalities arising from the primary order parameter. Tailoring the complex interplay between primary order parameter, polarization, and electrostatics is therefore likely to result in novel functionalities specific to improper ferroelectrics.

Acta Crystallographica Section B Structural Science, Crystal Engineering and Materials, 2013

Nature Materials, 2004

164 nature materials | VOL 3 | MARCH 2004 | www.nature.com/naturematerials F erroelectric materials have a spontaneous electric polarization that can be switched by an applied electric field.They are used in a wide range of applications, including data storage and as capacitors, transducers and actuators,as a result of their wide variety of physical and electronic properties 1,2 . Most technologically important ferroelectrics are perovskite structure oxides,but there is increasing interest in the class of ferroelectric hexagonal manganites as non-volatile memory materials 3 ,as gate ferroelectrics in field-effect transistors 4 and because of their coupled magnetic and ferroelectric behaviour 5 .In addition to their technological relevance,the fundamental physics of ferroelectrics is rich and fascinating. In particular, many ferroelectrics undergo a phase transition from a high-temperature,high-symmetry phase that behaves as an ordinary dielectric, to the spontaneously polarized phase at low temperature, and a great deal of research has focused on understanding the nature of,and driving force for,these ferroelectric phase transitions.

Arxiv preprint arXiv: …, 2011

We performed magnetic and ferroelectric measurements, first principle calculations and Landau theory analysis on hexagonal YMnO3. The polarization and the AFM order parameter were found to present different temperature dependence at TN. A linear coupling between these two order parameters is thus forbidden in the Landau theory and P63cm cannot be the magnetic group. The only compatible magnetic group is P6'3. In this group however, Landau theory predicts the possibility of a ferromagnetic component and of a linear coupling between the dielectric constant and the AFM order parameter. On one hand we performed dielectric constant measurements under magnetic field that clearly exhibit a metamagnetic transition, and thus confirm these predictions. On the other hand careful magnetization measurements show a small by non null FM component along the c-axis direction. Finally the Landau analysis within the P6'3 magnetic group shows that only the polarization square is coupled to the magnetic orders and thus neither the magnetization nor the AFM order can be reversed by an applied electric field.

Physical Review B, 2012

We discuss the relative roles played by the magnetic inversion symmetry breaking and the ferroelectric (FE) atomic displacements in the multiferroic state of YMnO 3 . For these purposes we derive a realistic low-energy model, using results of first-principles electronic structure calculations and experimental parameters of the crystal structure below and above the FE transition. Then, we solve this model in the mean-field Hartree-Fock approximation. We argue that the multiferroic state in YMnO 3 has a magnetic origin, and the centrosymmetric P bnm structure is formally sufficient for explaining the main details of the noncentrosymmetric magnetic ground state. The relativistic spin-orbit interaction lifts the degeneracy, caused by the frustration of isotropic exchange interactions in the ab plane, and stabilizes a twofold periodic noncollinear magnetic state, which is similar to the E state apart from the spin canting. The noncentrosymmetric atomic displacements in the P 2 1 nm phase reduce the spin canting, but do not change the symmetry of the magnetic state. The effect of the P 2 1 nm distortion on the FE polarization P a , parallel to the orthorhombic a axis, is twofold: (i) It gives rise to ionic contributions, associated with the oxygen and yttrium sites; (ii) it affects the electronic polarization, mainly through the change of the spin canting. The relatively small value of P a , observed in the experiment, is caused by a partial cancellation of the electronic and ionic contributions, as well as different contributions in the ionic part, which takes place for the experimental P 2 1 nm structure. The twofold periodic magnetic state competes with the fourfold periodic one and, even in the displaced P 2 1 nm phase, these two states continue to coexist in a narrow energy range. Finally, we theoretically optimize the crystal structure. For these purposes we employ the LSDA + U approach and assume the collinear E-type antiferromagnetic alignment. Then, we use the obtained structural information again as the input for the construction and solution of the low-energy model. We have found that the agreement with the experimental data in this case is less satisfactory and | P a | is largely overestimated. Although the magnetic structure can be formally tuned by varying the Coulomb repulsion U as a parameter, apparently LSDA + U fails to reproduce some fine details of the experimental structure, and the cancellation of different contributions in P a does not occur.

AIP Advances, 2011

Multiferroic materials such as YMnO 3 , which uniquely exhibit ferroelectricity and magnetism simultaneously, have been extensively studied for spintronic device applications. However, the origin of multiferroicity remains poorly understood. In this study, the structural phases of YMnO 3 ceramics and their lattice distortions after careful annealing were investigated to explain the origins of their multiferroicity. A structural transition from the orthorhombic to the hexagonal phase was observed when the annealing temperature reached around 1100 • C. This structural transformation also results in a magnetic transition from 3D Mn-O-Mn to 2D Mn-O-Mn superexchange coupling. The ferroelectricity was enhanced by escalation of the structural distortion caused by the rising annealing temperature. The annealing effect also results in the re-hybridization of the electronic structure of YMnO 3. X-ray absorption near-edge spectra suggest that there is charge transfer from the Y-O T (apical oxygen) bonds of Y 4d-O 2p hybridized states to the O T-Mn bonds of Mn 3d-O 2p hybridized states, which is responsible for the enhanced ferroelectricity. This approach could be used to probe the origin of the ferroelectricity and multiferroic properties in rare-earth manganites.

Physical Review Letters, 2003

The structure of antiferromagnetic (AFM) domain walls and their interaction with lattice strain are derived taking the multiple-order-parameter compound YMnO 3 as a model example. Contrary to the conviction that AFM domain walls are energetically unfavorable, their interaction with lattice strain lowers the total energy of the system and leads to a piezomagnetic clamping of the electric and magnetic order parameters in good agreement with the available experimental data.

2005

We have identified, for the first time, the change in symmetry at the ferroelectric transition TFE near 1023K of the ferroelectromagnet YMnO3. This transition takes place 300K below the transition to the centrosymmetric state at TIP. Single crystal synchrotron diffraction coupled to a group theoretical analysis show that the paraelectric intermediate phase between TIP and TFE has P63/mcm symmetry. This

Nature Communications

Magnetically induced ferroelectrics exhibit rigidly coupled magnetic and electric order. The ordering temperatures and spontaneous polarization of these multiferroics are notoriously low, however. Both properties can be much larger if magnetic and ferroelectric order occur independently, but the cost of this independence is that pronounced magnetoelectric interaction is no longer obvious. Using spatially resolved images of domains and density-functional theory, we show that in multiferroics with separately emerging magnetic and ferroelectric order, the microscopic magnetoelectric coupling can be intrinsically strong even though the macroscopic leading-order magnetoelectric effect is forbidden by symmetry. We show, taking hexagonal ErMnO3 as an example, that a strong bulk coupling between the ferroelectric and antiferromagnetic order is realized because the structural distortions that lead to the ferroelectric polarization also break the balance of the competing superexchange contrib...