Materials whose layers are bound together by these van der Waals forces can therefore be easily exfoliated as a result of the weak forces that are present between them. The possible exfoliation of these materials.
Heterostructures are combinations of several layers of different two-dimensional (2D) atomic layers that are stacked on top of each other. While weak in nature, van der Waals forces are strong enough to hold these stack of layers together.
Heterostructures, or more specifically van der Waals crystals, have been previously understood to possess inherent ferromagnetic properties that have allowed for their application in a wide variety of applications including spintronic devices, nanoscale memory chips and magnetic sensors.
To further extend the ability of these heterostructures to positively impact these applications, a recent discover conducted by a team of scientists from the U.S. Department of Energy (DOE) in conjunction with the Lawrence Berkeley National Laboratory have discovered an intrinsic magnetic property that exists in the 2D material they studied.
The strength of a material’s magnetic field is entirely dependent upon the direction of its magnetic moment, which is a dipole moment of the electrons in which they will either “spin up” or “spin down.1” While all materials possess some type of magnetism, some are much stronger than others in their ability to allow for the physical phemoena of attracting or repelling other objects to occur. For example, iron, a permanent magnet, is one of the strongest in its category as a result of a magnetic property known as ferromagnetism.
In permanent magnetism, the magnetic spin of the electrons exhibits a uniform parallel alignment in the absence of an applied external magnetic field or electric current. If the temperature of the magnetic increases, the spin of the electrons will become disoriented, resulting in the loss of the material’s magnetic properties to occur.
Curie temperature describes the temperature that can cause materials to undergo this disorientation, thereby causing the materials to lose their intrinsic magnetic properties as a direct result of a change affecting its magnetic movements1.
When the magnetic movements within a material undergo such disorganization, which is also referred to as paramagnetism, the application of an external magnetic field is often able to realign the magnetic moments to their previous parallel design. Ferromagnetism, a property that determines the strength of the magnet, describes a material’s ability to realign the magnetic spins following its exposure to an external magnetic field.
Magnetic anisotropy is an inherent magnetic property that describes the ordered directional alignment of the electron spins within the material, which occurs as a result of a property known as spontaneous magnetism.
Materials that exhibit this anisotropic effect, such as 2D materials, often have a very disordered magnetic moment, unless an external magnetic field is applied. In fact, prior to the work conducted by Berkely Laboratories, it was previously thought that if 2D materials lose their magnetic anisotropy, they would completely lose their magnetic ability as well2.
To further investigate the magnetic properties of 2D structures that are not well documented, the team of researchers led by principal investigator and University of California Berkeley professor Xiang Zhang, a bulk material of chromium, gernmanium and telluride (CGT) was utilized. CGT is a well-known material used in semiconductor devices as a result of its intrinsic ferromagnetic properties.
The atomically thin layers of CGT were prepared by mechanical exfoliation by use of Scotch tape2. These atomic layers were prepared on silicon dioxide/silicon chips, while bulk CGT van der Waals crystals were reported to be ferrogmanetic under a temperature of 61 Kelvin (K).
While monolayers were found to undergo rapid degradation and become invisible over time, the researchers attempted to peel the layers off without causing any visible degradation during a 90-minute experimental period under ambient atmospheric conditions. The thickness of these layers was then determined by atomic force microscopy and optical contrast.
To measure the magnetism of the nanometer-thick and macrometer-sized CGT flakes, nondestructively scanning Kerr microscopy, otherwise known as the magneto-optic Kerr effect. This type of Kerr microscopy is capable of detecting the rotation of an applied linearly polarized light as it interacts with the electron spins that are present within a magnetic material3.
As a result of the low anisotropic property that is associated with this 2D single atom-thick layered material, the researchers were able to easily manipulate its magnetic moment by introducing small magnetic fields to the material. The extremely small nature of this material also allowed the researchers to easily control the Curie temperature that often causes it to lose its ferromagnetism. What was discovered was that the Curie temperature of a material is not just an innate material property, but is something that can be mechanically manipulated and change for a given purpose.
This discovery is a significant advancement in the field of 2D structures as it provides important information on how to manipulate certain materials such as iron, cobalt and nickel, which are often used for thin film products.
By understanding this flexible magnetic nature of 2D structures, which is extremely different as compared to its 3D counterparts, the team of Berkeley researchers are hopeful that future magneto-electric and magneto-optical devices will be much more resistant to possible deformations in their magnetic abilities.
- “What Is Magnetism?” - LiveScience
- “Berkeley Lab Scientsts Discover New Atomically Layered, Thin Magnet” – Berkeley Lab
- “Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals.” C. Gong, L. Li, et al. Nature. (2017). DOI: 10.1038/nature22060