2D Ferromagnets Research Articles

Frustrated ferromagnetic transition in AB-stacked honeycomb bilayer

In two-dimensional (2D) ferromagnets, anisotropy is essential for the magnetic ordering as dictated by the Mermin-Wagner theorem. But when competing anisotropies are present, the phase transition becomes nontrivial. Here, utilizing highly sensitive susceptometry of scanning superconducting quantum interference device microscopy, we probe the spin correlations of ABC-stacked CrBr3 under zero magnetic field. We identify a plateau feature in susceptibility above the critical temperature (TC) in thick samples. It signifies a crossover regime induced by the competition between easy-plane intralayer exchange anisotropy versus uniaxial interlayer anisotropy. The evolution of the critical behavior from the bulk to 2D shows that the competition between the anisotropies is magnified in the reduced dimension. It leads to a strongly frustrated ferromagnetic transition in the bilayer with fluctuation on the order of TC, which is distinct from both the monolayer and the bulk. Our observation demonstrates unconventional 2D critical behavior on a honeycomb lattice.

Single Skyrmion Generation via a Vertical Nanocontact in a 2D Magnet-Based Heterostructure.

Skyrmions have been well studied in chiral magnets and magnetic thin films due to their potential application in practical devices. Recently, monochiral skyrmions have been observed in two-dimensional van der Waals magnets. Their atomically flat surfaces and capability to be stacked into heterostructures offer new prospects for skyrmion applications. However, the controlled local nucleation of skyrmions within these materials has yet to be realized. Here, we utilize real-space X-ray microscopy to investigate a heterostructure composed of the 2D ferromagnet Fe3GeTe2 (FGT), an insulating hexagonal boron nitride layer, and a graphite top electrode. Upon a stepwise increase of the voltage applied between the graphite and FGT, a vertically conducting pathway can be formed. This nanocontact allows the tunable creation of individual skyrmions via single nanosecond pulses of low current density. Furthermore, time-resolved magnetic imaging highlights the stability of the nanocontact, while our micromagnetic simulations reproduce the observed skyrmion nucleation process.

Origin of room temperature ferromagnetism in optically transparent 2D graphene/Co-doped ZnO/graphene

Despite extensive efforts to find room temperature ferromagnetic (FM) two-dimensional (2D) materials, only a few 2D FM materials have been experimentally synthesized. Particularly, it is a still challenging task to find optically transparent room temperature 2D FM systems. Recently, room temperature ferromagnetism was confirmed in 2D graphene/Co doped ZnO/graphene multilayer structure. However, in our calculations, the Co doped ZnO (ZCO) monolayer showed an antiferromagnetic (AFM) ground state. Moreover, both ZCO/graphene bilayer and graphene/ZCO/graphene trilayer systems also exhibited an AFM state. Thus, we investigated the possibility of hydrogen-induced ferromagnetism. After the introduction of the H atom (H-ZCO), we found a ferromagnetic (FM) ground state in both H-ZCO monolayer and graphene/H-ZCO/graphene trilayer systems. We obtained that the FM state appeared only when the H atom stayed on the same ZCO layer. Thus, we propose that the ferromagnetism in the graphene/ZCO/graphene originates from the H effect. Also, we obtained a Curie temperature (TC) of 500 K in H-ZCO and graphene/H-ZCO/graphene trilayer systems. Moreover, we find that the graphene/H-ZCO/graphene shows optically transparent behavior in the visible frequencies.

Two-dimensional Cr-based ferromagnetic semiconductor: Theoretical simulations and design

Two-dimensional (2D) material is the promising for next-generation information technology. The recently discovered intrinsic magnetic crystals have simulated a renaissance in 2D spintronics, which provides an ideal platform for exploring novel physical phenomena. However, current experimental trial-and-error methods in discovering new spintronic material are still very expensive and challenging. In contrast, based on well-developed first-principles calculations, computationally designing the spintronic materials provides a more efficient way for exploring new ferromagnetic (FM) materials and understanding the nature of magnetic properties. Several predictions, such as CrI3 monolayer, CrGeTe3 bilayer, CrSBr monolayer, FeCl2 monolayer, and Fe3GeTe2 monolayer have been confirmed by experiments, showing the great performance of computational approaches. This minireview article attempts to give a brief of discovering intrinsic 2D spintronics from theoretical aspect, and in particular, we emphasize roles played by calculation based on first-principles methods in designing 2D FM materials and devices. The current challenges and proposals on future developments of 2D spintronics are also discussed.

Hybrid states of a cavity-photon–vortex coupled system in a superconductive cavity

As the Abrikosov vortex lattice has recently been found in van der Waals heterostructures constructed by a two-dimensional (2D) ferromagnet and a superconductor, we propose the realization of cavity-photon–vortex coupling in a superconductive cavity to construct a new hybrid quantum system in this paper. We study the corresponding hybrid states therein, including the exceptional lines (ELs) in the parameter space. Considering that the parameters of our system are adjustable by external magnetic field and temperature, our system and the ELs are much easier to be realized in experiments. Furthermore, the numerical results show that the corresponding hybrid states can be switched by tuning the source of AC, which makes this hybrid system more advantageous to realize hybrid quantum computing in the future. Moreover, for practical use in detecting hybrid states and the vortex dynamics, the transmission amplitude of an external transverse electric wave through the cavity is also studied.

Unconventional magnetoresistive behavior near magnetic compensation temperature in ferrimagnetic Mn2.21Ru0.86Ga films

Ferrimagnets with magnetic compensation temperature (Tcomp) around room temperature are desirable due to their potential applications in low-energy consuming and high-frequency spintronic devices. In this study, the Tcomp of ferrimagnetic Mn2.21Ru0.86Ga (MRG) is tuned to near room temperature by strain. Moreover, we observed unconventional magnetoresistance behaviors for MRG-based Hall bar devices near Tcomp. First-principles calculations suggest two kinds of Mn moments, which lead to two anomalous Hall channels with opposite signs and consequently correspond to the peak structure and triple loops of the anomalous Hall effect loops. The unconventional temperature dependence of longitudinal resistivity is caused by the combined effects of two types of Mn moments and the anisotropic magnetoresistance of the MRG film. Interestingly, the spontaneous Hall angle of the MRG film is calculated to be ∼2.2%, which is one order of magnitude larger than those of other 3d ferromagnets. Therefore, our study demonstrates MRG to be a ferrimagnet with the Tcomp near room temperature, which enables its potential applications in spintronic devices.

Importance of magnetic shape anisotropy in determining magnetic and electronic properties of monolayer VSi2P4

Two-dimensional (2D) ferromagnets have been a fascinating subject of research, and magnetic anisotropy (MA) is indispensable for stabilizing the 2D magnetic order. Here, we investigate magnetic anisotropy energy (MAE), magnetic and electronic properties of by using the generalized gradient approximation plus U approach. For large U, the magnetic shape anisotropy (MSA) energy has a more pronounced contribution to the MAE, which can overcome the magnetocrystalline anisotropy (MCA) energy to evince an easy-plane. For fixed out-of-plane MA, monolayer undergoes ferrovalley (FV), half-valley-metal (HVM), valley-polarized quantum anomalous Hall insulator (VQAHI), HVM and FV states with increasing U. However, for assumptive in-plane MA, there is no special quantum anomalous Hall (QAH) state and spontaneous valley polarization within considered U range. According to the MAE and electronic structure with fixed out-of-plane or in-plane MA, the intrinsic phase diagram shows common magnetic semiconductor, FV and VQAHI in monolayer . At representative U = 3 eV widely used in references, can be regarded as a 2D-XY magnet, not Ising-like 2D long-range order magnets predicted in previous works with only considering MCA energy. Our findings shed light on importance of MSA in determining magnetic and electronic properties of monolayer .

Interaction of excitons with magnetic topological defects in 2D magnetic monolayers: localization and anomalous Hall effect

Novel 2D material CrI3 reveals unique combination of 2D ferromagnetism and robust excitonic response. We demonstrate that the possibility of the formation of magnetic topological defects, such as Néel skyrmions, together with large excitonic Zeeman splitting, leads to giant scattering asymmetry, which is the necessary prerequisite for the excitonic anomalous Hall effect. In addition, the diamagnetic effect breaks the inversion symmetry, and in certain cases can result in exciton localization on the skyrmion. This enables the formation of magnetoexcitonic quantum dots with tunable parameters.

Exchange Bias State at the Crossover to 2D Ferromagnetism.

The inherent malleability of 2D magnetism provides access to unconventional quantum phases, in particular those with coexisting magnetic orders. Incidentally, in a number of materials, the magnetic state in the bulk undergoes a fundamental change when the system is pushed to the monolayer limit. Therefore, a competition of magnetic states can be expected in the crossover region. Here, an exchange bias state is observed at the crossover from 3D antiferromagnetism to 2D ferromagnetism driven by the number of monolayers in the metalloxene GdSi2. The material constitutes a stack of alternating monolayers of Gd and silicene, the Si analogue of graphene. The exchange bias manifests itself as a shift of the hysteresis loop signifying coupling of magnetic systems, as evidenced by magnetization studies. Two features distinguish the phenomenon: (i) it is intrinsic, i.e. it is detected in an individual compound; (ii) the exchange bias field, 1.5 kOe, is unusually high, which is conducive to applications. The results suggest magnetic derivatives of 2D-Xenes to be prospective materials for ultracompact spintronics.

Surface Oxygen Passivation-Driven Large Anomalous Hall Conductivity in Early Transition Metal-Based Nitride MXenes: Can AHC Be a Tool to Determine Functional Groups in 2D Ferro(i)magnets?

Identifying the existence of specific functional groups in MXenes is a difficult topic that has perplexed researchers for a long time. At the same time, the realization of intrinsic ferromagnetism, which is important for two-dimensional (2D) materials’ families, is one of the fascinating properties that MXenes offer. Here, using first-principle calculations, we have examined the actual magneto-transport phenomena in transition metals and nitride-based-functionalized MXenes. We demonstrate that intrinsic anomalous Hall conductivity (AHC) can be used to identify the presence of more probable functional groups. The maximum AHC at Fermi energy, EF, is found in the case of Mn2NO2 (470 S/cm), which is attributed to the presence of avoided band crossing and a larger density of states. Together, when considering all the studied systems, the AHC can be above 2500 S/cm within EF± 0.25 eV. Our findings could be useful not only in guiding the experimentalists by considering AHC as a possible tool in determining the most probable functional groups in 2D ferro(i)magnets but also in designing memory devices with negligible stray fields.

Topological magnons in one-dimensional ferromagnetic Su–Schrieffer–Heeger model with anisotropic interaction

Topological magnons in a one-dimensional (1D) ferromagnetic Su–Schrieffer–Heeger (SSH) model with anisotropic exchange interactions are investigated. Apart from the intercellular isotropic Heisenberg interaction, the intercellular anisotropic exchange interactions, i.e. Dzyaloshinskii–Moriya interaction and pseudo-dipolar interaction, also can induce the emergence of the non-trivial phase with two degenerate in-gap edge states separately localized at the two ends of the 1D chain, while the intracellular interactions instead unfavors the topological phase. The interplay among them has synergistic effects on the topological phase transition, very different from that in the two-dimensional (2D) ferromagnet. These results demonstrate that the 1D magnons possess rich topological phase diagrams distinctly different from the electronic version of the SSH model and even the 2D magnons. Due to the low dimensional structural characteristics of this 1D topological magnonic system, the magnonic crystals can be constructed from bottom to top, which has important potential applications in the design of novel magnonic devices.

Large and Tunable Magnetoresistance in Cr1−xTe/Al2O3/Cr1−xTe Vertical Spin Valve Device

AbstractThe recent discovery of 2D ferromagnetic materials provides new opportunities for fabricating 2D ferromagnets‐based spin valve devices and exploring related novel physics. However, up to now, almost all works adopt a spin valve configuration by inserting different types of 2D materials into the gap between two 2D ferromagnetic electrodes as barrier spacer, rather than applying traditional tunneling barrier, such as Al2O3 films grown by atomic layer deposition (ALD), probably attributed to the instability and incompatibility for the widely explored 2D ferromagnets (CrI3, Fe3GeTe2) to the ALD growth process. Here, Cr1−xTe, an air‐stable 2D ferromagnetic metal grown by chemical vapor deposition, show excellent compatibility to ALD process of depositing Al2O3 films. The nonencapsulated Cr1−xTe/Al2O3/Cr1−xTe vertical spin valve devices demonstrate high magnetoresistance ratio of ≈28% and large spin polarization of 0.36 at 2 K. Furthermore, a gradual evolution from tunneling to metallic spin‐valve behavior is found upon decreasing Al2O3 spacer thickness. The work is constructive and illuminating for connecting the 2D ferromagnetic electrodes with traditional tunneling spacers for fundamental research and device applications.

Three-Dimensional Printable Magnetic Microfibers: Development and Characterization for Four-Dimensional Printing.

This study proposes a novel and simple fabrication method of magnetic microfibers, employing filament stretching three-dimensional (3D) printing, and demonstrates the capacity of four-dimensional (4D) printing of the proposed magnetic microfibers. A ferromagnetic 3D printing filament is prepared by the mixture of neodymium-iron-boron (NdFeB) and polylactic acid (PLA), and we investigate the characteristics of the ferromagnetic filament by mixing ratio, magnetic properties, mechanical properties, and rheological properties through experiments. By thermal extrusion of the ferromagnetic filament through a 3D printer nozzle, various thicknesses (80-500 μm) and lengths (less than ∼5 cm) of ferromagnetic microfibers are achieved with different printing setups, such as filament extrusion amount and printing speed. The printed ferromagnetic microfibers are magnetized to maintain a permanent magnetic dipole moment, and 4D printing can be achieved by the deformations of the permanently magnetized microfibers under magnetic fields. We observe that the mixing ratio, the thickness, and the length of the magnetized microfibers provide distinct deformation of the microfiber for customization of 4D printings. This study exhibits that the permanently magnetized microfibers have a great potential for smart sensors and actuators. Furthermore, we briefly present an application of our proposed magnetic microfibers for bionic motion actuators with various unique undulating and oscillating motions.

Tunable magnetic anisotropy in two-dimensional CrX3/AlN (X=I,Br,Cl) heterostructures

Controlling the magnetic anisotropy of two-dimensional (2D) ferromagnets (FMs) is of great importance for the development of next generation spintronics. Here, by combining the 2D FM chromium trihalides $\mathrm{Cr}{X}_{3} (X=\mathrm{I},\mathrm{Br},\mathrm{Cl})$ monolayer (ML) with the fully hydrogenated bilayer AlN (AlN-2L) together, the electronic and magnetic properties, especially the magnetic anisotropies, of $\mathrm{Cr}{X}_{3}\text{/}\mathrm{AlN}$ heterostructures ($\mathrm{Cr}{X}_{3}$ HSs) are systematically investigated based on first-principles calculations. When the polarization direction of AlN-2L reverses, the magnetic easy axis of $\mathrm{Cr}{X}_{3}$ MLs can be switched between out-of-plane and in-plane directions. Particularly the magnetic anisotropic energy is tuned by a maximum of 200% variation in $\mathrm{Cr}{\mathrm{I}}_{3}$/AlN. It is shown that the different effects of AlN-2L on the modulation of the magnetic easy axis of $\mathrm{Cr}{X}_{3}$ MLs are related to their different interfacial charge transfer/redistribution across their interfaces. In addition, a transition from FM semiconductor to half-metal is found in all $\mathrm{Cr}{X}_{3}$ MLs when the polarization direction points toward $\mathrm{Cr}{X}_{3}$ MLs. Our results suggest a feasible avenue for the design of van der Waals HSs to realize the control of magnetic anisotropy in 2D FMs.

Localization effects and anomalous hall conductivity in a disordered 3D ferromagnet

We have prepared the ferromagnetic Heusler alloy CoFeV0.5Mn0.5Si in bulk form via arc melting. The longitudinal resistivity exhibits a minimum at 150 K, which is attributable to competition between quantum interference corrections at low temperatures and inelastic scattering at higher temperatures. The magnetoresistance (MR) is positive and nearly linear at low temperatures and becomes negative at temperatures close to room temperature. The positive MR in the quantum correction regime is evidence of the presence of the enhanced electron interaction as a contributor to the longitudinal resistivity. Hall effect measurements indicate a carrier concentration of the order of 1022 cm−3, which is nearly 3 orders of magnitude higher than that found in the “parent” material CoFeMnSi. The higher carrier concentration is consistent with the predicted half metallicity of CoFeV0.5Mn0.5Si. The anomalous Hall conductivity of CoFeV0.5Mn0.5Si is temperature independent for temperatures below the resistivity minimum, which is strong evidence of the absence of quantum interference effects on the anomalous Hall conductivity in a 3D ferromagnet.

Temperature-dependent magnetic order of two-dimensional ferromagnetic Cr2Ge2Te6 single crystal

The vigorous development of two-dimensional materials has attracted much attention in fields including physics, chemistry, materials, and energy in recent years. As an important part among the two-dimensional (2D) material family, 2D ferromagnets provide unique physical properties and potential applications. However, the similarities and differences between two-dimensional ferromagnetic materials and traditional ferromagnetic materials in terms of magnetic properties have not been studied thoroughly. To this end, combining the ferromagnetic resonance (FMR) and superconducting quantum interference vibrating sample magnometer (SQUID VSM), we systematically study the changes in the internal magnetic order of the 2D van der Waals crystal Cr2Ge2Te6 (CGT). As the temperature rising to Curie Temperature (TC), the in-plane and out-of-plane resonance fields of CGT gradually merge into a same field, indicating the changes of anisotropic. The amplitude of ferromagnetic resonance shows a peak when the temperature is below TC, which may be due to the competition between damping progress and magnetization degradation during the heating process. This work paves the way for a better understanding of the mechanism of two-dimensional ferromagnetism.

Magnetic Properties of Layered Hybrid Organic‐Inorganic Metal‐Halide Perovskites: Transition Metal, Organic Cation and Perovskite Phase Effects

AbstractUnderstanding the structural and magnetic properties in layered hybrid organic‐inorganic metal halide perovskites (HOIPs) is key for their design and integration in spin‐electronic devices. Here, a systematic study is conducted on ten compounds to understand the effect of the transition metal (Cu2+, Mn2+, Co2+), organic spacer (alkyl‐ and aryl‐ammonium), and perovskite phase (Ruddlesden‐Popper and Dion‐Jacobson) on the properties of these materials. Temperature‐dependent Raman measurements show that the crystals’ structural phase transitions are triggered by the motional freedom of the organic cations as well as by the flexibility of the inorganic metal‐halide lattice. In the case of Cu2+ HOIPs, an increase of the in‐plane anisotropy and a reduction of the octahedra interlayer distance is found to change the behavior of the HOIP from that of a 2D ferromagnet to that of a quasi‐3D antiferromagnet. Mn2+ HOIPs show inherent antiferromagnetic octahedra intralayer interactions and a phenomenologically rich magnetism, presenting spin‐canting, spin‐flop transitions, and metamagnetism controlled by the crystal anisotropy. Co2+ crystals with non‐linked tetrahedra show a dominant paramagnetic behavior irrespective of the organic spacer and the perovskite phase. This study demonstrates that the chemical flexibility of HOIPs can be exploited to develop novel layered magnetic materials with tailored magnetic properties.

Nernst coefficient measurements in two-dimensional materials

The discovery of two-dimensional (2D) ferromagnets and antiferromagnets with topologically nontrivial electronic band structures makes the study of the Nernst effect in 2D materials of great importance and interest. To measure the Nernst coefficient of 2D materials, the detection of the temperature gradient is crucial. Although the micro-fabricated metal wires provide a simple but accurate way for temperature detection, a linear-response assumption that the temperature gradient is a constant is still necessary and has been widely used to evaluate the temperature gradient. However, with the existence of substrates, this assumption cannot be precise. In this study, we clearly show that the temperature gradient strongly depends on the distance from the heater by both thermoelectric transport and thermoreflectance measurements. Fortunately, both measurements show that the temperature gradient can be well described by a linear function of the distance from the heater. This linearity is further confirmed by comparing the measured Nernst coefficient to the value calculated from the generalized Mott’s formula. Our results demonstrate a precise way to measure the Nernst coefficient of 2D materials and would be helpful for future studies.

Spontaneous spin-valley polarization in NbSe2 at a van der Waals interface

A proximity effect at a van der Waals (vdW) interface enables creation of an emergent quantum electronic ground state. Here we demonstrate that an originally superconducting two-dimensional (2D) NbSe2 forms a ferromagnetic ground state with spontaneous spin polarization at a vdW interface with a 2D ferromagnet V5Se8. We investigated the anomalous Hall effect (AHE) of the NbSe2/V5Se8 magnetic vdW heterostructures, and found that the sign of the AHE was reversed as the number of the V5Se8 layer was thinned down to the monolayer limit. Interestingly, the AHE signal of those samples was enhanced with the in-plane magnetic fields, suggesting an additional contribution to the AHE signal other than magnetization. This unusual behavior is well reproduced by band structure calculations, where the emergence of the Berry curvature along the spin-degenerate nodal lines in 2D NbSe2 by the in-plane magnetization plays a key role, unveiling a unique interplay between magnetism and Zeeman-type spin-orbit interaction in a non-centrosymmetric 2D quantum material.

High-Temperature Ferromagnetism in a Two-Dimensional Semiconductor with a Rectangular Spin Lattice

Atomically thin two-dimensional (2D) ferromagnetic (FM) semiconductors with a high Curie temperature (TC) are essential and highly desired for nanoscale spintronic devices. However, the recently discovered 2D FM semiconductors show rather low TC (≤45 K), which limits their practical application. An important reason for the low TC is the lack of effective spin interactions. Here, we reveal that increasing the number of effective spin interactions by constructing a rectangular spin–lattice can significantly improve the TC of FM semiconductors. Based on this mechanism, we design a rectangular lattice of the CrO2 monolayer (denoted as R-CrO2) by performing global structural optimizations. The first-principles calculations show that the R-CrO2 is energetically more stable than the previously reported H- and T-CrO2. As expected, the R-CrO2 monolayer is an intrinsic FM semiconductor with TC up to ∼370 K, which is more than twice that of the T-CrO2 monolayer (∼150 K). This is because the R-CrO2 monolayer possesses four effective spin interactions between adjacent Cr sites, more than that of the T-CrO2 monolayer (3). These findings give rise to a new and efficient design principle for realizing high-temperature 2D FM semiconductors.