Can you imagine a laptop hard drive the size of a grain of rice? Skyrmion, a mysterious quasiparticle structure in the magnetic field, could make this seemingly unthinkable idea a reality, with more storage space and faster data transfer rates for this "grain of rice. So how to observe this strange particle structure? The CIQTEK Quantum Diamond Atomic Force Microscope (QDAFM), based on the nitrogen-vacancy (NV) center in diamond and AFM scanning imaging, can tell you the answer.
What is Skyrmion
With the rapid development of large-scale integrated circuits, the chip process into the nanometer scale, the quantum effect is gradually highlighted, and "Moore's Law" encountered physical limits. At the same time, with such a high density of integrated electronic components on the chip, the thermal dissipation problem has become a huge challenge. People urgently need a new technology to break through the bottleneck and promote the sustainable development of integrated circuits.
Spintronics devices can achieve higher efficiency in information storage, transfer, and processing by exploiting the spin properties of electrons, which is an important way to break through the above dilemma. In recent years, topological properties in magnetic structures and their related applications are expected to be the information carriers of next-generation spintronic devices, which is one of the current research hotspots in this field.
The skyrmion (hereafter referred to as a magnetic skyrmion) is a topologically protected spin structure with quasiparticle properties, and as a special kind of magnetic domain wall, its structure is a magnetization distribution with vortices. Similar to the magnetic domain wall, there is also a magnetic moment flip in the skyrmion, but unlike the domain wall, the skyrmion is a vortex structure, and its magnetic moment flip is from the center outward, and the common ones are Bloch-type skyrmions and Neel-type skyrmions.
Figure 1: Schematic diagram of the structure of skyrmion. (a) Neel-type skyrmions (b) Bloch-type skyrmions
The skyrmion is a natural information carrier with superior properties such as easy manipulation, easy stability, small size, and fast driving speed. Therefore, the electronic devices based on skyrmions are expected to meet the performance requirements for future devices in terms of non-volatile, high capacity, high speed, and low power consumption.
What are the Applications of Skyrmions
Skyrmion Racetrack Memory
Racetrack memory used magnetic nanowires as tracks and magnetic domain walls as carriers, with electric current driving the motion of the magnetic domain walls. In 2013, the researchers proposed the skyrmion racetrack memory, which is a more promising alternative. Compared to the drive current density of a magnetic domain wall, the skyrmion is 5-6 orders of magnitude smaller, which can lead to lower energy consumption and heat generation. By compressing the skyrmions, the distance between adjacent skyrmions and the skyrmion diameter can be in the same order of magnitude, which can lead to higher storage density.
Figure 2: Skyrmion-based Racetrack Memory
Skyrmions can also be used in the direction of transistors, opening up new ideas for semiconductor development. As shown in Figure 3, a skyrmion is generated at one end of the device using an MTJ (magnetic tunnel junction), followed by a spin-polarization current to drive the skyrmion toward the other end of the device. To achieve the switching state of the transistor, a gate is installed in the middle of the device. By applying a voltage to the gate, an electric field is generated, which can change the perpendicular magnetic anisotropy of the material and thus control the on/off of the skyrmion. When no voltage is applied, the skyrmion can pass through the gate to the other end of the device, and this state is defined as the on state; when an external electric field is applied, the skyrmion does not pass through the gate, and this state is defined as the off state.
Figure 3: Skyrmion Transistor
Skyrmion-based Unconventional Computing
Compared with conventional computing units, neuromorphic computing units have the advantages of low power consumption and large-scale computing in terms of neural networks. To manufacture neuromorphic computing units need to meet the requirements of nanometer size, non-volatility, and low power consumption. Skyrmion brings new possibilities for such devices. Skyrmion has controlled mobility, which can simulate biological nerves well, and at the same time, skyrmion can get rid of impurity pegging effect more easily, which makes it better robustness.
Figure 4: (a) Skyrmion-based neural computing device (b) Skyrmion-based stochastic computing device
Skyrmions can also be used in random computing devices. While mainstream computing techniques encode values in conventional binary format, random computing can continuously process a random stream of bits. Conventional semiconductor circuits use a combination of pseudo-random number generators and shift registers to generate signals, which has the disadvantage of high hardware cost and low energy efficiency. Researchers have recently discovered a thermally induced generation of skyrmions, both theoretically and experimentally, which provides the basis for skyrmion-based random computing devices.
CIQTEK Quantum Diamond Atomic Force Microscope in the Application of Skyrmion Research
The study of skyrmions cannot be carried out without suitable observation techniques, and the following techniques are commonly used to observe skyrmions in real space:
Lorentz transmission electron microscopy (LTEM), the principle of which is to use an electron beam to penetrate the sample and record the Lorentz force on the electrons; magnetic force microscopy (MFM), which uses a magnetic tip to record the magnetic field forces on the sample surface using atomic force microscopy techniques; X-ray microscopy, the principle of which is that the absorption rate of X-rays can reflect the magnetic field of the sample; and magneto-optical Kerr microscopy (Moke), which uses magneto-optical Kerr effect to measure the magnetization distribution. Each of these observational tools has its own limitations, such as the demanding sample size requirements of LTEM, the poor spatial resolution of Moke, and the magnetic properties of the MFM tip that can affect the imaging of skyrmions.
In recent years, the existence of a special defect structure in diamond, the Nitrogen-Vacancy (NV) center, has attracted the attention of researchers. The intensity of the magnetic field component in the NV axis can be obtained by manipulating and reading out the quantum state of the electron spin of the NV center by microwave and laser.
NV center scanning probe microscopy (SPM) is the integration of the NV center in diamond into the AFM probe tip, combined with the AFM scanning technique to obtain magnetic domain results on the sample surface, with the advantages of very high sensitivity (1 uT/Hz1/2), spatial resolution (10 nm) and non-invasiveness. The NV SPM is used to study a variety of magnetic structures of interest, such as scanning magnetic vortex heterodyne fields, enabling the determination of the polarity and chirality of magnetic vortex cores; measuring the conformation of magnetic domain walls, and observing the dynamics of domain walls under modulation.
Researchers aim to study new materials and to prepare skyrmion that is stable at room temperature with zero fields, small in size, and easy to manipulate. Diamond NV center SPM is well suited for high-resolution quantitative magnetic imaging of skyrmions at room temperature.
Currently, NV SPM has been quite successful in studying the magnetization structure of skyrmions and related physical processes. For example:
1) Reconstructing the magnetization structure based on the stray field distribution of the skyrmion.
Figure 5: NV scanning probe microscopy to resolve the magnetization structure of the skyrmion
(Scale bar: 500 nm)
2) Study of the structural morphology of the skyrmions. For example, Jacques group studied the skyrmion morphology in Pt/FM/Au/FM/Pt ferromagnetic multilayers.
Figure 6: NV scanning probe microscope for studying the skyrmion morphology
3) Observation of the intrinsic kinetic evolution of the skyrmion. For example, the Ania group studied the evolution of the skyrmion in the Ta/CoFeB/MgO system under the variation of the external magnetic field.
Figure 7: NV scanning probe microscope for studying the skyrmion under external magnetic field
4) Study of the kinetic process of current-driven skyrmions.
Figure 8: NV scanning probe microscope used to study the dynamics of current-driven skyrmions
CIQTEK NV scanning probe microscope - Quantum diamond atomic force microscope (QDAFM), has the unique advantages of being non-invasive, can cover a wide temperature range, large magnetic field measurement range. It can be applied to two-dimensional material magnetic imaging, nano-current imaging, superconducting vortex magnetic imaging, and cellular magnetic imaging, and has a wide range of applications in quantum science, chemistry, and materials science, as well as biological and medical research fields.
CIQTEK Quantum Diamond Atomic Force Microscope
(The ambient version and the cryogenic version)