For centuries, mankind has been exploring magnetism and its related phenomena without pause. In the early days of electromagnetism and quantum mechanics, it was difficult for humans to imagine the attraction of magnets to iron, and the ability of birds, fish, or insects to navigate between destinations thousands of miles apart - amazing and interesting phenomena with the same magnetic origin. These magnetic properties originate from the moving charge and spin of elementary particles, which are as prevalent as electrons.
Two-dimensional magnetic materials have become a research hotspot of great interest, and they open up new directions for the development of spintronics devices, which have important applications in new optoelectronic devices and spintronics devices. Recently, Physics Letters 2021, No. 12, also launched a special feature on 2D magnetic materials, describing the progress of 2D magnetic materials in theory and experiments from different perspectives.
A two-dimensional magnetic material only a few atoms thick can provide the substrate for very small silicon electronics. This amazing material is made of pairs of ultra-thin layers that are stacked together by van der Waals forces, i.e. intermolecular forces, while the atoms within the layers are connected by chemical bonds. Although only atomically thick, it still retains physical and chemical properties in terms of magnetism, electricity, mechanics, and optics.
Two-dimensional Magnetic Materials
Image referenced from https://phys.org/news/2018-10-flexy-flat-functional-magnets.html
To use an interesting analogy, each electron in a two-dimensional magnetic material is like a tiny compass with a north and south pole, and the direction of these "compass needles" determines the magnetization intensity. When these infinitesimal "compass needles" are spontaneously aligned, the magnetic sequence constitutes the fundamental phase of matter, thus allowing the preparation of many functional devices, such as generators and motors, magnetoresistive memories, and optical barriers. This amazing property has also made two-dimensional magnetic materials hot. Although integrated circuit manufacturing processes are now improving, they are already limited by quantum effects as devices are shrinking. The microelectronics industry has encountered bottlenecks such as low reliability and high power consumption, and Moore's law, which has lasted for nearly 50 years, has also encountered difficulties (Moore's law: the number of transistors that can be accommodated on an integrated circuit double in about every 18 months). If two-dimensional magnetic materials can be used in the future in the field of magnetic sensors, random memory, and other new spintronics devices, it may be possible to break the bottleneck of integrated circuit performance.
We already know that magnetic van der Waals crystals carry special magnetoelectric effects, and therefore quantitative magnetic studies are an essential step in the research of two-dimensional magnetic materials. However, quantitative experimental studies on the magnetic response of such magnets at the nanoscale are still very lacking. Some existing studies have reported the realization of the detection of crystal magnetism at the micron scale, but these techniques not only do not yet provide quantitative information about the magnetization but also are highly prone to interfere with the magnetic signal that hinders ultrathin samples. Therefore, the update of detection techniques is a very urgent challenge for probing the magnetic properties of materials on the nanoscale.
To address this challenge, CIQTEK offers a new quantum precision measurement, the Quantum Diamond Atomic Force Microscope (QDAFM), the scanning NV microscope based on diamond NV-center and AFM scanning imaging techniques. By quantum manipulation and readout of the spins of nitrogen-vacancy (NV) center defects in diamonds, quantitative nondestructive imaging of magnetic properties can be achieved. With the high spatial resolution at the nanometer scale and ultra-high detection sensitivity of individual spins, it can be used to quantitatively detect key magnetic properties of van der Waals magnets and perform high spatial resolution magnetic imaging of their magnetization, local defects, and magnetic domains. It has the unique advantages of being non-invasive, covering a wide temperature region, and a large magnetic field measurement range. It has a wide range of applications in quantum science, chemistry, and materials science, as well as biological and medical research fields.
Magnetization Diagram of Two-dimensional Chromium Iodide
Image referenced from Probing magnetism in 2D materials at the nanoscale with single-spin microscopy（Science, 2019, DOI: 10.1126/science.aav6926）
In the following, we introduce the specific applications of QDAFM in nano-magnetic resonance imaging, superconducting magnetic resonance imaging, in situ imaging of cells, and topological magnetic structure characterization.
CIQTEK Quantum Diamond Atomic Force Microscope
(The ambient version and the cryogenic version)
01 Nano-Magnetic Resonance Imaging
For magnetic materials, determining their static spin distribution is an important problem in condensed matter physics and a key to the study of new magnetic devices. QDAFM provides a new method that enables high spatial resolution magnetic imaging with unique advantages such as non-invasiveness, coverage of a wide temperature region, and a large magnetic field measurement range.
Bloch-type Magnetic Domain Wall Imaging
Image referenced from Tetienne, J. P.et al. The nature of domain walls in ultrathin ferromagnets revealed by scanning nanomagnetometry. Nature Communications6, 6733(2015)
02 Superconducting Magnetic Resonance Imaging
Microscale studies of superconductors and their vortices can provide important information for understanding the mechanism of superconductivity. Using QDAFM operating at low temperatures, quantitative imaging studies of magnetic vortices of superconductors can be performed and extended to magnetic measurements of numerous low-temperature condensed matter systems.
Quantitative Imaging of Spurious Fields of Single Magnetic Vortices
Image referenced from Thiel, L.et al.Quantitativenanoscale vortex imaging using a cryogenic quantum magnetometer. Nature Nanotechnology 11,677- 681 (2016).
03 Cells In Situ Imaging
Achieving nanoscale molecular imaging in situ in cells is an important tool for biological research. Among the many imaging techniques, magnetic resonance imaging, which can rapidly and non-destructively acquire images of spin distribution in the sample body, has been widely used in several scientific fields. Especially in clinical medicine, it plays an important role in the mechanistic study, diagnosis, and treatment of diseases because it is almost non-invasive to living organisms. However, conventional magnetic resonance imaging techniques use magnetic induction coils as sensors with spatial resolution limits above microns and are unable to perform intracellular molecular-scale imaging.
Using the high spatial resolution properties of QDAFM, the researchers observed ferritin present in organelles inside cells with a resolution of 10 nm.
Nanomagnetic Imaging of Cells In Situ Ferritin Molecules
Image referenced from Wang, P. et al. Nanoscale magnetic imaging of ferritins in a single cell. Science advances 5, 8038 (2019).
04 Topological Magnetic Structure Characterization
Magnetic skyrmions are nanoscale vortex magnetic structures with topologically protected properties. Magnetic skyrmions exhibit a wealth of novel physics and provide a new platform for the study of topological spintronics, and also have potential applications in future high-density, low-energy, non-volatile computing, and memory devices. However, the detection of individual skyrmions at room temperature is still experimentally challenging. The high sensitivity and high-resolution features of QDAFM are powerful tools to solve this challenge, and the magnetic structure of skyrmions can be reconstructed by stray field measurements.
Skyrmions Magnetic Field Imaging
Image referenced from Dovzhenko, Y. et al. Magnetostatic twists in room-temperature skyrmions explored by nitrogen-vacancy center spin texture reconstruction. Nature Communications 9, 2712 (2018).
References:
1. Journal of Physics, Vol. 12, 2021, Special Topic on Two-dimensional Magnetic Materials
2. Two-dimensional magnetic crystals and emergent heterostructure devices (Science, 2019, DOI: 10.1126/science.aav4450)
3. https://phys.org/news/2018-10-flexy-flat-functional-magnets.html
4. Probing magnetism in 2D materials at the nanoscale with single-spin microscopy (Science, 2019, DOI: 10.1126/science.aav6926)