Measuring Magnetically-Tuned Ferroelectric Polarization in Liquid Crystals

This method can help answer key questions in the field of condensed matter physics and materials science on the room-temperature operation of the magnet electric effect. The main advantage of this technique is that it tests the magnet electric effect in liquid crystal with high fluidity. Begin with two glass substrates to create each liquid crystal cell.

Start with clean substrates with ITO on one side, typically one square centimeter. Make a scratch on the edge of each substrate to help orient them in later steps. At a spin coater, drip polyimide solution onto the ITO-coded side of the substrates before spin coating.

When done, bake the substrates at 200 degrees Celsius for one hour. Then take the substrates to an XZ stage that is under a velvet roller that can rotate. Orient a substrate ITO-side up and with its marked edge toward the roller, and fix it on the stage.

Start the roller in the translation stage so the substrate moves back and forth under the roller five times. The substrate should experience soft, uniform pressure when in contact with the roller. Then stop the roller in translation stage to retrieve the substrate.

When done, place the substrate in isopropyl alcohol for one minute. Then dry the substrates at 80 degrees Celsius for a few minutes. Now work with two rubbed substrates.

In addition to the substrates, have flat resin film for a spacer. Place one substrate with its ITO and polyimide side up, and deposit glue at its edges. Next, orient the second substrate over the first with its coded side down.

Orient the marked edges to be on opposite sides of the cell. Shift the two slightly before adhering them. Use the regions created by shifting the substrates to glue conducting wires with silver paste to the coded sides of the substrates.

Bake the assembly at 150 degrees Celsius for one hour. Now use a setup similar to this to perform measurements of the cell gap. Place the cell so that it is perpendicular to a white light source and in front of an optical spectrometer for measuring the transmission spectrum.

The measured transmittance as a function of wavelength allows determination of the cell gap, which is about 12 micrometers for this cell. To proceed, have the liquid crystal mixture in a cell ready. The liquid crystal mixture is comprised of these two pyrimidine compounds.

75 milligrams of compound one, along with 25 milligrams of compound two. Put the prepared cell on a hot stage at 80 degrees Celsius. With a spatula, introduce the liquid crystal mixture to the cell, and rely on capillary action.

Maintain the cell at 80 degrees Celsius for half an hour before cooling it to room temperature. The next step is to characterize the mixture. At a microscope, place a prepared sample in a hot stage between two crossed polarizers that are in line with the light source.

Focus the microscope to observe the textures in the cell. Heat the sample at a rate of 5 degrees Celsius per minute from room temperature up to 80 degrees Celsius, and identify the liquid crystal phases from the polarization micrographs. Use the observations to determine the transition temperatures.

The measurements require a superconducting magnet capable of a 9-Tesla field. Prepare an insert for the magnet that has a rod to house coaxial cables. At its head, there should be a connector terminal.

At its end, there should be a sample space capable of holding a cell. Glue the cell on the sample space of the insert so the magnetic field is parallel to the substrate surfaces. Solder the cell's wires to the high and low terminals.

Also place a thermometer on the widest length of the cell. When ready, introduce the insert into the superconducting magnet. With coaxial cables, connect the terminals of the insert to measurement instruments and gather data.

Here is the electric polarization as a function of temperature, found by integrating the displacement current measured with an electrometer. In the smectic C-star phase, a finite polarization develops on applying a magnetic field at about 328 Kelvin. As the system undergoes a phase transition to the smectic A-star phase, the polarization disappears.

These data are for the dielectric constant measured at 100 hertz using a quasi four-terminal method with an LCR meter. In the smectic C-star phase, it is suppressed as the magnetic field increases. This can be ascribed to the change in the molecular orientation state, coupled with the electric dipole moment.

In the smectic A-star phase, the dielectric constant is independent of the magnetic field. Fixing the temperature allows for a clear demonstration of magnetoelectric activity. At 300 Kelvin, the sample is in the smectic C-star phase.

Here the electric polarization varies with the changing magnetic field. The dielectric constant taken at 100 hertz shows similar behavior. Measurements performed at 335 Kelvin when the liquid crystal is in the smectic A-star phase do not depend on the magnetic field.

So this method can provide insight into magnetically tunable electric polarization in liquid crystals. It can also be applied to other measurements, such as electrically controllable magnetism, and lighting optical effects in liquid crystals. After each development, this technique paves the way for researchers in the field of condensed matter physics and materials science, magnetoelectric coupling in liquid crystals.