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Register a new userAC susceptometry of 2D van der Waals magnets enabled by the coherent control of quantum sensors
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Precision magnetometry is fundamental to the development of novel magnetic materials and devices. Recently, the nitrogen-vacancy (NV) center in diamond has emerged as a promising probe for static magnetism in 2D van der Waals materials, capable of quantitative imaging with nanoscale spatial resolution. However, the dynamic character of magnetism, crucial for understanding the magnetic phase transition and achieving technological applications, has rarely been experimentally accessible in single 2D crystals. Here, we coherently control the NV centers spin precession to achieve ultra-sensitive, quantitative ac susceptometry of a 2D ferromagnet. Combining dc hysteresis with ac susceptibility measurements varying temperature, field, and frequency, we illuminate the formation, mobility, and consolidation of magnetic domain walls in few-layer CrBr3. We show that domain wall mobility is enhanced in ultrathin CrBr3, with minimal decrease for excitation frequencies exceeding hundreds of kilohertz, and is influenced by the domain morphology and local pinning of the flake. Our technique extends NV magnetometry to the multi-functional ac and dc magnetic characterization of wide-ranging spintronic materials at the nanoscale.
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Spontaneous magnetic order is a routine instance in three-dimensional (3D) materials but for a long time, it remained elusive in the 2D world. Recently, the first examples of (stand-alone) 2D van der Waals (vdW) crystals with magnetic order, either antiferromagnetic or ferromagnetic, have been reported. In this review, we describe the state of the art of the nascent field of magnetic 2D materials focusing on synthesis, engineering, and theory aspects. We also discuss challenges and some of the many different promising directions for future work.
We have synthesized unique colloidal nanoplatelets of the ferromagnetic two-dimensional (2D) van der Waals material CrI3 and have characterized these nanoplatelets structurally, magnetically, and by magnetic circular dichroism spectroscopy. The isolated CrI3 nanoplatelets have lateral dimensions of ~25 nm and ensemble thicknesses of only ~4 nm, corresponding to just a few CrI3 monolayers. Magnetic and magneto-optical measurements demonstrate robust 2D ferromagnetic ordering in these nanoplatelets with Curie temperatures similar to those observed in bulk CrI3, despite the strong spatial confinement. These data also show magnetization steps akin to those observed in micron-sized few-layer 2D sheets and associated with concerted spin-reversal of individual CrI3 layers within few-layer van der Waals stacks. Similar data have also been obtained for CrBr3 and anion-alloyed Cr(I1-xBrx)3 nanoplatelets. These results represent the first example of laterally confined 2D van der Waals ferromagnets of any composition. The demonstration of robust ferromagnetism at nanometer lateral dimensions opens new doors for miniaturization in spintronics devices based on van der Waals ferromagnets.
Magnetic skyrmions in 2D chiral magnets are in general stabilized by a combination of Dzyaloshinskii-Moriya interaction and external magnetic field. Here, we show that skyrmions can also be stabilized in twisted moire superlattices in the absence of an external magnetic field. Our setup consists of a 2D ferromagnetic layer twisted on top of an antiferromagnetic substrate. The coupling between the ferromagnetic layer and the substrate generates an effective alternating exchange field. We find a large region of skyrmion crystal phase when the length scales of the moire periodicity and skyrmions are compatible. Unlike chiral magnets under magnetic field, skyrmions in moire superlattices show enhanced stability for the easy-axis (Ising) anisotropy which can be essential to realize skyrmions since most van der Waals magnets possess easy-axis anisotropy.
The exfoliation of two naturally occurring van der Waals minerals, graphite and molybdenite, arouse an unprecedented level of interest by the scientific community and shaped a whole new field of research: 2D materials research. Several years later, the family of van der Waals materials that can be exfoliated to isolate 2D materials keeps growing, but most of them are synthetic. Interestingly, in nature plenty of naturally occurring van der Waals minerals can be found with a wide range of chemical compositions and crystal structures whose properties are mostly unexplored so far. This Perspective aims to provide an overview of different families of van der Waals minerals to stimulate their exploration in the 2D limit.
We investigated low-frequency current fluctuations, i.e. noise, in the quasi-two-dimensional (2D) van der Waals antiferromagnetic semiconductor FePS3 with the electronic bandgap of 1.5 eV. The electrical and noise characteristics of the p-type, highly resistive, thin films of FePS3 were measured at different temperatures. The noise spectral density was of the 1/f - type over most of the examined temperature range but revealed well-defined Lorentzian bulges, and increased strongly near the Neel temperature of 118 K (f is the frequency). Intriguingly, the noise spectral density attained its minimum at temperature T~200 K, which was attributed to an interplay of two opposite trends in noise scaling - one for semiconductors and another for materials with the phase transitions. The Lorentzian corner frequencies revealed unusual dependence on temperature and bias voltage, suggesting that their origin is different from the generation - recombination noise in conventional semiconductors. The obtained results are important for proposed applications of antiferromagnetic semiconductors in spintronic devices. They also attest to the power of the noise spectroscopy for monitoring various phase transitions.
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