What is antiferromagnetic material?
Figure 1: Magnetic Moment Arrangement in Antiferromagnets
The common iron properties are ferromagnetism, ferroelectricity, and ferroelasticity. Materials with two or more iron properties at the same time are called multiferroic materials. Multiferroics usually have strong iron coupling properties, i.e., one iron property of the material can modulate another iron property, such as using an applied electric field to modulate the ferroelectric properties of the material and thus affect the ferromagnetic properties of the material. Such multiferroic materials are expected to be the next generation of electronic spin devices. Among them, antiferromagnetic materials have been widely studied because they exhibit good robustness to the applied magnetic field.
Antiferromagnetism is a magnetic property of a material in which the magnetic moments are arranged in an antiparallel staggered order and do not exhibit a macroscopic net magnetic moment. This magnetically ordered state is called antiferromagnetism. Inside an antiferromagnetic material, the spins of adjacent valence electrons tend to be in opposite directions and no magnetic field is generated. Antiferromagnetic materials are relatively uncommon, and most of them exist only at low temperatures, such as ferrous oxide, ferromanganese alloys, nickel alloys, rare earth alloys, rare earth borides, etc. However, there are also antiferromagnetic materials at room temperature, such as BiFeO3, which is currently under hot research.
Application Prospects of Antiferromagnetic Materials
The knowledge of antiferromagnetism is mainly due to the development of neutron scattering technology so that we can "see" the arrangement of spins in materials and thus confirm the existence of antiferromagnetism. Maybe the Nobel Prize in physics inspired researchers to focus on antiferromagnetic materials, and the value of antiferromagnetism was gradually explored.
Antiferromagnetic materials are less susceptible to ionization and magnetic field interference and have eigenfrequencies and state transition frequencies several orders of magnitude higher than typical ferromagnetic materials. Antiferromagnetic ordering in semiconductors is more readily observed than ferromagnetic ordering. These advantages make antiferromagnetic materials an attractive material for spintronics.
The new generation of magnetic random access memory uses electrical methods to write and read information to ferromagnets, which may reduce the immunity of ferromagnets and is not conducive to stable data storage, and the stray fields of ferromagnetic materials can be a significant obstacle for highly integrated memories. In contrast, antiferromagnets have zero net magnetization, do not generate stray fields, and are insensitive to external fields. Therefore, antiferromagnet-based memory perfectly solves the problem of ferromagnetic memory and becomes a very attractive potential memory material.
Figure 2: Magnetic Random Access Memory (Image from the Internet)
Observation of Antiferromagnetic Domains
The study of antiferromagnetic domains is inseparable from the observation techniques. The common means of observing magnetic domains are magnetic force microscopy (MFM), which uses a magnetic needle tip to record the magnetic field force on the sample surface using atomic force microscopy techniques; X-ray microscopy, which is based on the principle that the absorption rate of X-rays can reflect the magnetic field of the sample; and magneto-optical Kerr microscopy (Moke), which uses the magneto-optical Kerr effect to measure the magnetization distribution. Although the techniques of each imaging method have been developed to perfection, these means are insufficient in terms of sensitivity to reach single-spin detection due to the weak antiferromagnetic magnetism, and it is difficult to observe the magnetic domain structure of antiferromagnets.
In recent years, a special defect structure in diamonds, the Nitrogen-Vacancy (NV) center, has attracted the attention of many researchers. The NV-center scanning probe microscope integrates the NV center in diamond into the tip of the AFM probe and combines the AFM scanning technique to obtain the magnetic domain results on the surface of the sample, which has the advantages of high sensitivity (1 T/Hz1/2), spatial resolution (10 nm) and non-invasiveness. resolution (10 nm) and non-invasiveness.
Bismuth ferrite BiFeO3 (BFO) belongs to a class of multiferroic materials with ferroelectricity and antiferromagnetism accompanied by weak ferromagnetism and is one of the current hot spots in the research of multiferroic materials. High-resolution neutron diffraction studies have revealed that BFO has a spatial magnetic structure with a period of 64 nm. In 2017, I. Gross et al. observed the antiferromagnetic sequence in BFO films at room temperature with the help of NV-center scanning probe microscopy, and the experimental results observed a spin pendulum magnetic structure with a period of about 70 nm, as shown in Figure 3.
Figure 3: BFO periodic magnetic structure observed by I. Gross et al. using NV-center scanning probe microscopy
(Image Source: I.Gross et al. Real-space imaging of non-collinear antiferromagnetic order with a single-spin magnetometer, Nature, 2017, 549:252)
Figure 4: Observation of antiferromagnetic structures and skyrmions using NV-center scanning probe microscopy by F. Aurore et al.
(Image Source: F. Aurore et al. Imaging non-collinear antiferromagnetic textures via single spin relaxometry, Nature communications, 2012, 12:767)
Furthermore, in 2021, F. Aurore et al. similarly used NV-center scanning probe microscopy to observe magnetic structures such as magnetic domain walls and skyrmions in synthetic antiferromagnets, as shown in Figure 4. The results of this experiment suggest that the NV-center scanning probe microscopy technique can be extended to other antiferromagnets, providing new opportunities to study magnetic local spin waves.
Figure 5: NV-center scanning probe microscopy study of CuMnAs antiferromagnetic domains
(Image Source: M. S. Wörnle et al. Current-induced fragmentation of antiferromagnetic domains arXiv:2019, 1912.05287)
M. S. Wörnle used NV-center scanning probe microscopy to study the effect of current pulses on the structural configuration of CuMnAs antiferromagnetic domains, showing that large resistance changes are associated with nanoscale fragmentation of the magnetic domains induced by writing current pulses. The current-induced changes in magnetic domain structure are further demonstrated to be inhomogeneous by imaging the current density distribution in CuMnAs microdevices with crossed geometries.
Figure 6: NV-center scanning probe microscopy for antiferromagnetic Cr2O3
(Image Source: W. S. Huxter et al. Scanning gradiometry with a single spin quantum magnetometer, arXiv:2202.09130v1)
In addition, Cr2O3 is an early-reported multiferroic material that is antiferromagnetic at room temperature. In 2022, W. S. Huxter et al. measured images of static magnetic field distributions on the order of micro-Tesla on atomic steps on the surface of Cr2O3, using the gradient scanning technique of NV-center scanning probe microscopy.
The advantages of NV-center scanning probe microscopy are the high sensitivity of a single spin (1uT/Hz1/2) and the nanoscale spatial resolution (10 nm). In addition, the diamond NV-centers are very light and thermally stable, and bio-friendly, allowing quantitative and non-destructive magnetic imaging. It also works under a wide range of conditions, especially at room temperature, and is well-suited as a magnetic imaging tool for life sciences.
CIQTEK has launched a commercial NV-center scanning probe microscope, Quantum Diamond Atomic Force Microscope (NV-AFM or QDAFM), which has the advantages of being non-invasive, covering a wide temperature range and 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.
Figure 7: CIQTEK Quantum Diamond Atomic Force Microscope (NV-AFM or QDAFM)
(The ambient version and the cryogenic version)
