Sang-Wook Cheong (Rutgers): Multiferroic vortices

Sang starts by introducing the materials with multiple directional orders which are most known in the community by now: (Anti)ferromagnetics and (anti)ferroelectrics: TbMnO3, DyMnO3, TbMn2O5 and DyMn2O5.

The most recent example is Eu(Y)MnO3: magnetization is along c and ferroelectric moment P is along a direction, both orders compete, and the balance between these two phases can be tuned by the external time-dependent magnetic (or electric) field. Piers: the scale of the application of the magnetic field is hours, why so? Reply: it depends on the scale of coupling between ferroelectric and magnetic orders and is determined by the behavior of the domain walls which can be different for the magnetic and ferroelectric orders. Usually the timescale is shorter for the magnetic order: Canfield: for Delta M/H shown on the viewgraph: How large it is in percents to the susceptibility values. Reply: few percents.

Canfield is also amused by the millimeter paper which Cheong uses as a background for his sample shown on the photograph, Cheong refers to the Bell Lab resources back in 80-ies [general appreciation of the good old times].

Next Sang shows the picture from the conference (seemingly our conference as Piers is also there) with a banana which he argues has something to do with multiferroics. The analogy is P vs E hysteresis curve which to Cheong looks like a banana. He further remarks that multiferroics are usually good insulators and they NEED to be a good insulators otherwise they are bad ferroelectrics. On the next slide Cheong shows the switchable photodiode device based on the multiferroic BiFeO3 (published in Science) with a potential to be used in the applied science. The most interesting thing is that you find a finite current at zero voltage which, as Cheong explains, is due to ferroelectric component.

Next we move to the most recent trends in the multiferroics:
1) use non-zero d-electrons
2) employ interaction among d-electroncs - drives antiferromagnetic order
3) look for collective phenomena - should give colossal magnetoelectric effect
4) and what everyone likes - Mott-type charge gap

This directs us to the Doped Mott Ferroelectrics (blogger again suspects that this new keyword in multiferroics goes back to Cheong), new aspect of the field of multiferroics.

Next he moves to the main topic of the talk - multiferroic vortices.

the system in question is h-YMnO3 with a hexagonal (h) crystal structure, Tc~900K T_N=90K - ferroelectric insulator. There is additional transition at 1325 K (is that right!?) which is slightly below the melting point which again raises the comment by Canfield whether this transition is well separated from the melting point. Reply: well separated

Mn3+ ions are in S=2 state and the magnetic structure above and below T_N looks fairly complicated. An observation by second harmonic generation (SHG) is that various domain walls are formed with purely ferrolectric (FEL ~P) component, mixed FEL & AFM (~PI)and AFM (~I) [blogger thinks that I refers to the magnetic moment and/or interaction]. Origin of the ferroelectricity in YMnO3 (Van Aken et al., Nature 2004) is a trimerization of Mn ions. This is supported by the fact that there are also two separate transitions: first structural and then ferroelectric one. The consequence of this is a very subtle effect on the magnetic properties as there are 6 possible domain walls expected. On the next slide Sang provides its visualization by means of the high-resolution TEM, AFM and conductive (c)-AFM techniques. Combining these techniques Cheong argues that you may identify 6 domain wall structures. The visualization of these domains on the next slide looks fairly complicated. Cheong claims that this is easy to follow, blogger doubts that but continues with a fear of messing up things.

Basically Cheong suggests to look on the domain boundary as a topological defect. Canfield points out that if this were topological defects you could heat the sample and then cool it down again and by doing this you would see the annihilation and creation of vortices at different places on the surface. Cheong agrees but says they do not have a picture yet to show.

Then Cheong shows the I(V) characteristics of this system. Negative voltage is more conducting than a positive one (range is from +10 to -10 eV). The explanation is given in terms of a non-ohmic conductance where various effects can contribute: transport along the domain boundaries, Schottky barrier, and non-linear effects.

Next slides are concerned with the control of the domain boundaries by an external electric field. Showing maps of c-AFM at various fields there is a poling effect at Ec ~ 45kV/cm, however above this field you still find vortices, up to 70 kV/cm [blogger guesses that it is in favor of 6 domain boundaries to be the topological defect]. A word of caution is that these pictures are obtained for different samples thus nothing can be said on whether the vortex is moving in a field or not.

Again the SHG picture comes and Canfield raises an issue whether all the curves on the cartoon agree with the actual data shown in b/w maps. Somehow he is convinced by the reply while the blogger simply has nothing to say as some very experimental words has been said.

Then Cheong speculates that the topological defect has to have a phase of 2pi for the magnetism, and 6pi for the ferroelectric order. Next slide says something about the interpretation power of the TEM pictures where Cheong sees (a) vortex-antivortex pairs and (b) face of Marilyn Monro.

Some speculations are made on whether or not these vortex-ativortex pairs are good or bad for the technical applications. In liquid crystals they are not, in multiferroics these not yet clear as the applications are not yet well-spread.



Questions:

1) blogger: origin of asymmetry between I(V) curves in YMnO3? Answer: it is basically the non-linear effect and many features may affect the non-ohmic behavior
2) Piers: the model of the vortex you offer: is it equivalent to the 6-vortex model, then you may expect BKT transition in thin films ? Answer: Yes, indeed it might be very interesting
3) Canfield pointed out that the vortices can be pinned by the dislocations