LIQUID CRYSTAL MAGNET
Although magnetism is a well-known phenomenon in solids, it has only been observed in two liquid systems – in one of the phases of superfluid helium at temperatures approaching absolute zero  and in undercooled liquid ferromagnetic metal alloys at around 1,000K .
Over 40 years ago, Nobel Prize winner Pierre de Gennes and his colleague Françoise Brochard-Wyart suggested that a suspension of magnetic nanoparticles in liquid crystals may have magnetic properties at room temperatures . In such a system, the orientational order of a liquid crystal could cause magnetic calamitic (rod-like) particles or platelets to be ordered orientationally and, should magnetic coupling between particles be strong enough, magnetic ordering may occur. Despite attempts by many researchers, it is only recently that this type of liquid crystals magnet was successfully developed .
One of the properties of magnetic substances is that they can be magnetised. If we place a piece of iron inside a magnetic field and then turn off the magnetic field, the piece of iron will act as a magnet, i. e. a magnetic field source. Substances with this property are called ferromagnets (after ferrum, the Latin word for iron), or magnets for short. The polarity of the magnet is defined as the direction of the magnetic field it produces and is marked by an arrow, the magnetic moment. Even a non-magnetised piece of iron has areas much larger than the elementary agents of magnetism, i.e. atoms acting as magnets. These areas contain the magnetic moments of iron atoms, which may be visualised as small magnets all pointing in the same direction. These areas are known as domains and are separated by domain walls. A non-magnetised piece of iron contains a large number of such microscopic domains. Because they are randomly oriented, their total magnetic field equals zero. Magnetisation involves orienting these domains in the direction of an external magnetic field.
LIQUID CRYSTALS AND MAGNETIC LIQUIDS
Liquid crystals are liquids composed of calamitic molecules that become ordered so as to point in the same general direction at a certain temperature. The ordering direction can easily be altered by voltage and since the ordering direction also determines the propagation of light through a liquid crystalline layer, the voltage allows us to control the transmission of light. That is the basic working principle behind modern LCD TVs and computer screens. Conventional nematic crystals do not react to weak magnetic fields. One of the possible ways to increase the responsiveness of liquid crystals to magnetic fields is to combine the properties of liquid crystals with those of ferromagnetic fluids, also known as ferrofluids.
Ferrofluids are stable suspensions of monodomain magnetic nanoparticles in an isotropic fluid. Their strong reaction to magnetic fields results in spectacular normal-field instability (Figure 1). Magnetic nanoparticles in a liquid can be visualised as nano magnets freely spinning in the liquid in the absence of an external magnetic field. These nano magnets are randomly oriented, meaning the magnetic liquid is not a liquid magnet. Even a small external magnetic field causes the nano magnets to order.
Figure 1: A ferrofluid in a magnetic field showing normal-field instability (source)
Liquid crystals can be used to order nano magnets instead of an external magnetic field. Calamitic particles and platelets will order themselves in a certain direction inside the liquid crystal. Their direction depends on the ordering of liquid crystal molecules on the surface of the particle. If molecules on the surface are ordered along the surface, calamitic particles will follow the ordering of the liquid crystal while the axis of platelet particles will run perpendicular to it. If, however, the molecules are ordered perpendicularly to the surface, the calamitic particles will be perpendicular while the symmetrical axis of platelets will follow the ordering of the molecules (Figure 2).
Figure 2: Platelets (red, side view) in a liquid crystal have their axes pointing in the same direction as liquid crystal molecules (blue). A linear defect is created on the edges of the platelets (blue dots represent the cross-section of the defect). The magnet moments of the platelets (red arrows) point in the same direction, meaning the material is magnetic. Magnetic field lines are shown in orange.
That was Brochard and De Gennes’ idea, one that researchers unsuccessfully attempted to turn into reality for many years. At low concentrations, particles did not order magnetically, which meant their effect on the magnetic properties of the mixture was relatively small, and at higher concentrations the particles lumped together despite the use of surfactants.
Experiments were mostly performed using calamitic particles. Two problems arise with these. Similarly to two bar magnets, two magnetic calamitic particles tend to latch onto each other with the opposite poles touching and cancelling out each other’s magnetic fields. The other issue is that the elastic forces caused by the liquid crystal are weak and the attractive magnetic force prevails, causing lumps.
IT’S ALL ABOUT SHAPE
Slovene researchers resolved these issues by using barium ferrite platelets measuring approximately 5 nm in width and 70 nm in diameter . The particles were coated with a surfactant that ensured the liquid crystal was ordered perpendicularly to the surface of the particles.
Figure 3: Magnetic scandium-doped barium ferrite nanoplatelets captured by a TEM (D. Lisjak)
In order to understand why platelets are the superior option, let us look at some of the properties of liquid-crystal elastic forces. Around particles, the ordering of the liquid crystal becomes deformed, which is felt by other particles as an attractive or repulsive elastic force. The symmetry of the deformation determines how liquid crystal particles are arranged in space, i. e. whether they will form chains or shift diagonally. Figure 2 illustrates the arrangement of platelets with a surface that perpendicularly orders liquid crystal molecules. The elastic force mediated by the deformation of the liquid crystal causes nearby particles to shift diagonally. The magnetic force, however, causes the platelets to attempt to stack onto one another with their magnetic moments pointed in the same direction. The opposing effect of both forces stops the particles from lumping together and their magnetic moments remain ordered, making the system function as a magnetic liquid crystal (Figure 2).
In order to prove its magnetic properties, the mixture of the liquid crystal and nanoparticles was injected between two sheets of glass the surface of which ensured optimal parallel ordering of the liquid crystal. The walls of the glass cell were approximately 20 microns apart. Because of the low particle density, the cells appeared transparent.
Within the cells, areas of several dozen microns in size emerged where the magnetic moments of particles pointed along the ordering of the liquid crystal, with adjacent areas pointing in the opposite direction. This can be seen clearly with the use of polarisation microscopy (Figure 4). In the absence of a magnetic field, the image of the sample under the polarisation microscope between two crossed polarisers appears dark. By turning on an external magnetic field along the ordering of the liquid crystal, light and dark areas can be seen. In the dark areas, the liquid crystal remains ordered, with the magnetic moments of the platelets ordered along the direction of the magnetic field. In light areas, however, the original magnetic moments of the platelets face in the opposite direction. The magnetic field caused the platelets to turn, deforming the order of the liquid crystal, which can be seen as light spots on the images. If the sign of the magnetic field is switched, originally dark areas turn light and vice versa. This behaviour indicates the presence of two types of domains in the suspension.
Figure 4: Photographs of the sample under a polarisation microscope. In the absence of a magnetic field, the image is dark (upper left side), indicating the liquid crystal is correctly ordered. By turning on an external magnetic field along the ordering of the liquid crystal, light and dark areas can be seen (upper right and lower left side). If the sign of the magnetic field is switched, originally dark areas turn light and vice versa. The lower right side illustrates the sample as seen from the side. The width of the photograph corresponds to 840 μm. P and A designate the directions of the polariser and the analyser, while n marks the directional ordering of the liquid crystal.
A monodomain sample can be obtained by cooling it to a liquid crystal phase in the presence of an external magnetic field. The magnetisation curve of such a sample exhibits a hysteresis (Figure 5). The sum of particle magnetic moments divided by the volume of the sample equals sample magnetisation. If the magnetic field points to the same direction as the field used in creating the sample, the magnetisation of the system essentially does not change with the field. If the sign of the field is switched, magnetisation begins to decrease at a certain critical level and is completely reversed in larger fields. If the field is gradually reduced, the magnetisation remains constant, indicating that magnetisation has been reversed within the entire sample. Observing this phenomenon under a polarisation microscope reveals that the complete reversal of magnetisation occurs with the movement of surface domain walls.
Figure 5: The magnetisation curve of a monodomain sample with a field along a director. The photograph illustrates the process of magnetisation reversal on the surface of the sample, which occurs with the movement of surface domain walls, here seen as white lines. The width of the photograph corresponds to 440 μm.
CONCLUSION AND PROSPECTS
It was demonstrated that the system bears the hallmarks or a ferromagnetic substance: it contains domains and domain walls that can move; a monodomain sample can be obtained by being cooled in the presence of an external magnetic field; it has a magnetic hysteresis, and its magnetisation can be reversed by switching the sign of the magnetic field.
The most important practical property of this new composite material is its strong magneto-optic effect. This means that like regular liquid crystals, where the transmission of light can be regulated by an electric field, the transmission of light in this substance can be controlled by a weak magnetic field.
The material is also highly relevant to basic science, as it is the first example of an ordered liquid that is simultaneously a magnet. It is also a new kind of a multiferroic since its ordering can be changed by voltage as with regular liquid crystals, which allows us to indirectly alter its magnetic properties.
Sources: D. N. Paulson and J. C. Wheatley, Phys. Rev. Lett. 40, 557 (1978).  T. Albrecht, C. Bührer, M. Fähnle, K. Maier, D. Platzek, and J. Reske, Appl. Phys. A 65, 215 (1997).  F. Brochard and P. G. de Gennes, J. Phys. 31, 691 (1970).  A. Mertelj, D. Lisjak, M. Drofenik, and M. Čopič, Nature 504, 237 (2013).  D. Lisjak and M. Drofenik, Cryst. Growth Des. 12, 5174 (2012).
Translated by: Urša Klinc.