Two-dimensional Ferromagnetic System Research Articles

Ferrovalleytricity in Two-Dimensional Antiferromagnetic Lattice

The control and manipulation of valley physics via ferrovalleytricity is highly desirable for advancing valleytronics. Current research primarily focuses on two-dimensional ferromagnetic systems, while antiferromagnetic counterparts are seldom explored. Here,...

Fe and Cr co-intercalation in 2H-NbS2 single crystals for realization of perpendicular magnetic anisotropy and large anomalous Hall effect

Recently, many interesting magnetic and electrical properties have been demonstrated in transition-metal intercalated $2\text{H-Nb}{\mathrm{S}}_{2}/\mathrm{Ta}{\mathrm{S}}_{2}$ layered single crystals. In this work, single crystals of Cr and Fe co-intercalated $2\text{H-Nb}{\mathrm{S}}_{2}$, with the compositions of ${({\mathrm{Cr}}_{1--x}{\mathrm{Fe}}_{x})}_{1/3}\mathrm{Nb}{\mathrm{S}}_{2} (x=0.0--1.0)$, are successfully grown and their magnetic and transport properties are systematically studied. With increasing Fe content, the magnetic transition temperature continually decreases from 117 to 37 K, and the magnetic coupling transforms from ferromagnetic to antiferromagnetic. Importantly, a rotation of the magnetic easy axis occurs in this process, with perpendicular magnetocrystalline anisotropy and considerable coercive field achieved in the $x=0.13--0.66$ compounds. Both $x=0.33$ and $x=0.50$ exhibit relatively low carrier concentration, stable anomalous Hall effect, and butterfly-shape magnetoresistance. Notably, large anomalous Hall conductivity is identified in $x=0.50$, whose origin is discussed from the calculated electronic structures. The realization of perpendicular anisotropy and large anomalous Hall conductivity in this two-dimensional ferromagnetic system can facilitate their future application in spintronic devices.

Origin and regulation of triaxial magnetic anisotropy in the ferromagnetic semiconductor CrSBr monolayer.

Magnetic anisotropy plays a vital role in stabilizing the long-range magnetic order of two-dimensional ferromagnetic systems. In this work, using the first-principles method, we systematically explored the triaxial magnetic anisotropic properties of a ferromagnetic semiconductor CrSBr monolayer, which is recently exfoliated from its bulk. Further analysis shows that the triaxial magnetic anisotropic properties originate from the coexistence of the magnetic dipole-dipole interaction (shape anisotropy) and the spin-orbit coupling interaction (magnetocrystalline anisotropy). Interestingly, the shape anisotropy, which has been neglected in most previous works, dominates over the magnetocrystalline anisotropy. Besides, the experimental Curie temperature of the CrSBr monolayer is well reproduced using Monte Carlo simulations. What is more, the easy magnetic axes and ferromagnetism in the CrSBr monolayer can be manipulated by strains and are relatively more susceptible to the uniaxial strain in the x direction. Our study not only explains the mechanism of triaxial magnetic anisotropy of the CrSBr monolayer, but also sheds light on how to tune the magnetic anisotropy and Curie temperature in ferromagnetic monolayers.

Two-dimensional ferromagnetic systems with finite driving

We study the effect of finite driving by the time-varying external magnetic field in the two-dimensional disordered ferromagnetic systems portrayed on the case of nonequlibrium athermal random field Ising model. The response of studied systems is rate-dependent exhibiting scaling and data collapsing above the dynamical critical point in the full range of driving rates provided that the system parameters satisfy both the finite-size and here introduced rate-dependent scaling conditions. These findings are based on extensive numerical simulations of large systems reliably demonstrating the observed behavior and could be relevant for real thin systems driven at finite rates.

Measurement of electronic structure in van der Waals ferromagnet Fe5–x GeTe2

As a van der Waals ferromagnet with high Curie temperature, Fe5–x GeTe2 has attracted tremendous interests recently. Here, using high-resolution angle-resolved photoemission spectroscopy (ARPES), we systematically investigated the electronic structure of Fe5–x GeTe2 crystals and its temperature evolution. Our ARPES measurement reveals two types of band structures from two different terminations with slight kz evolution. Interestingly, across the ferromagnetic transition, we observed the merging of two split bands above the Curie temperature, suggesting the band splitting due to the exchange interaction within the itinerant Stoner model. Our results provide important insights into the electronic and magnetic properties of Fe5–x GeTe2 and the understanding of magnetism in a two-dimensional ferromagnetic system.

Tuning Ginzburg–Landau theory to quantitatively study thin ferromagnetic materials

Along with experiments, numerical simulations are key to gaining insight into the underlying mechanisms governing domain wall motion in thin ferromagnetic systems. However, a direct comparison between numerical simulation of model systems and experimental results still represents a great challenge. Here, we present a tuned Ginzburg–Landau model to quantitatively study the dynamics of domain walls in quasi two-dimensional ferromagnetic systems with perpendicular magnetic anisotropy. This model incorporates material and experimental parameters and the micromagnetic prescription for thermal fluctuations, allowing us to perform material-specific simulations and at the same time recover universal features. We show that our model quantitatively reproduces previous experimental velocity-field data in the archetypal perpendicular magnetic anisotropy Pt/Co/Pt ultra-thin films in the three dynamical regimes of domain wall motion (creep, depinning and flow). In addition, we present a statistical analysis of the domain wall width parameter, showing that our model can provide detailed nano-scale information while retaining the complex behavior of a statistical disordered model.

Numerical Investigation of Magnetic Bubble Types in a Two-Dimensional Ferromagnetic System with Dipole–Dipole Interactions

Landau–Lifshitz equation for a two-dimensional anisotropic Heisenberg model with dipole–dipole interactions is solved numerically to investigate magnetic bubbles. Type I magnetic bubbles (magnetic ...

Fractal character of the phase ordering kinetics of a diluted ferromagnet

We study numerically the coarsening kinetics of a two-dimensional ferromagnetic system with aleatory bond dilution. We show that interfaces between domains of opposite magnetisation are fractal on every lengthscale, but with different properties at short or long distances. Specifically, on lengthscales larger than the typical domains’ size the topology is that of critical random percolation, similarly to what observed in clean systems or models with different kinds of quenched disorder. On smaller lengthscales a dilution dependent fractal dimension emerges. The Hausdorff dimension increases with increasing dilution d up to the value 4/3 expected at the bond percolation threshold d = 1/2. We discuss how such different geometries develop on different lengthscales during the phase-ordering process and how their simultaneous presence determines the scaling properties of observable quantities.

Magnetic symmetry dependent wall domain motion mechanisms in two-dimensional ferromagnetic systems

We report the systematic change of the domain wall (DW) motion mechanism depending on the ratio of composition x in two-dimensional Co1−xFex ferromagnetic systems (0.0 ≤ x ≤ 1.0) by means of the direct domain observation. Interestingly, as the Fe composition x increases, the DW motion mechanism becomes changed from the stochastic behavior with random DW sizes to the thermally activated behavior with specific DW sizes. This result could be understood by the change of the magnetic angular symmetry from a 2-fold one to a 4-fold one with increasing the Fe composition, responsible for the change of the DW type.

Dependence of Curie temperature on Pt layer thickness in Co/Pt system

The Pt thickness dependence of the Curie temperature of perpendicularly magnetized ultra-thin (Pt/)Co/Pt films has been investigated by magnetization measurements. The Curie temperature and the saturation magnetic moment increase with the Co layer thickness and even with the Pt layer thickness. The Curie temperature is found to have linear dependence on the total magnetic moment of the system and the coefficients of the linear fits are almost identical, regardless of whether the thicknesses of the ferromagnetic Co layer or the Pt layer are varied. The Curie temperature also increases with the magnetic anisotropy, but no systematic dependence is observed. These results suggest that the magnetic moment induced in the Pt layer by the ferromagnetic proximity effect plays a significant role in determining the Curie temperatures of such two-dimensional ferromagnetic systems.

Monte Carlo Simulation of Magnetization Recovery in Two-Dimensional Ferromagnetic System after Ultrafast Pump Pulses

The magnetization recovery of a two-dimensional ferromagnetic system after excitation by intensive ultrafast pump pulses is investigated by a Monte Carlo method with a focus on the evolution of domain structure and the magnetization in the equilibrium state. Our simulations can explain the effect of the pumping fluence on the recovery process. In particular, they reveal the importance of domain formation in the recently reported accumulation effect already found by the pump–probe magnetization hysteresis loop measurement of a metallic ferromagnetic film. In the framework of the model, it is predicted that by repeating pump pulses a sufficient number of times, the magnetization of the two-dimensional ferromagnetic system can be eliminated when the pump fluence is above a critical value.

Monte Carlo Simulation of Magnetization Recovery in Two-Dimensional Ferromagnetic System after Ultrafast Pump Pulses

The magnetization recovery of a two-dimensional ferromagnetic system after excitation by intensive ultrafast pump pulses is investigated by a Monte Carlo method with a focus on the evolution of domain structure and the magnetization in the equilibrium state. Our simulations can explain the effect of the pumping fluence on the recovery process. In particular, they reveal the importance of domain formation in the recently reported accumulation effect already found by the pump–probe magnetization hysteresis loop measurement of a metallic ferromagnetic film. In the framework of the model, it is predicted that by repeating pump pulses a sufficient number of times, the magnetization of the two-dimensional ferromagnetic system can be eliminated when the pump fluence is above a critical value.

Unusual ferromagnetic superexchange in CdVO3: The role of Cd

A microscopic magnetic model of the low-pressure modification of CdVO3 is established, based on density functional theory (DFT) band-structure calculations, magnetization measurements, and quantum Monte-Carlo simulations. This compound is a rare example of a quasi-one-dimensional spin-1/2 system showing exclusively ferromagnetic exchange. The spin lattice of CdVO3 entails zigzag chains with an effective intrachain coupling J ~ -90 K and interchain couplings of Jc ~ -18 K and Ja ~ -3 K. Quantum fluctuations are partially suppressed by the sizable interchain coupling Jc that leads to an intermediate regime between one-dimensional and two-dimensional ferromagnetic systems. Apart from the peculiar spin model, CdVO3 features an unusual mechanism of ferromagnetic superexchange. The couplings largely originate from Cd 5s states mediating hoppings between half-filled and empty 3d states of V+4.

Ferromagnetic transition in GaAs/Mn/GaAs/In x Ga1 − x As/GaAs structures with a two-dimensional hole gas

The transport properties of GaAs/Mn/GaAs/In x Ga1 − x As/GaAs structures with a layer that is separated from the quantum well and contains Mn impurities in the concentration range 4–10 at % corresponding to the reentrant metal-insulator transition observed in the bulk GaMnAs material [17] have been investigated. The hole mobility in the objects under investigation is more than two orders of magnitude higher than the known values for the GaMnAs semiconductor and GaMnAs-based magnetic heterostructures. This makes it possible to observe Shubnikov-de Haas oscillations, which confirm a two-dimensional character of the hole energy spectrum. The calculated Curie temperature for heterostructures with indirect exchange interaction through a two-dimensional hole channel is in good agreement with the position of the maximum (at 25–40 K) in the temperature dependences of the electrical resistance of the channel. This suggests that two-dimensional holes play an important role in ferromagnetic ordering of the Mn layer under these conditions. The observations of a negative spin-dependent magnetoresistance and an anomalous Hall effect, whose magnitude correlates well with the results of theoretical calculations for two-dimensional ferromagnetic systems based on III-Mn-V, also indicate a significant role of the two-dimensional channel in ferromagnetic ordering.

Localization corrections to charge and spin conductivity in ferromagnetic layered structures

Localization corrections to charge and spin conductivity of two-dimensional ferromagnetic systems with spin–orbit interaction are studied theoretically. The corrections lead to negative magnetoresistance — also in the presence of spin–orbit scattering. Magnitude of the corrections depends on the magnetization orientation with respect to the plane of the system. The corrections to spin conductivity are shown to be small.

Spin waves of lightly doped La 1− xSr xMnO 3

Neutron scattering has been used to study the spin dynamics of the lightly doped La 1− x Sr x MnO 3. As the dopant concentration increases from 0 to 0.09, the magnetic long-range order changes from a slightly to a strongly canted antiferromagnetic order, while the corresponding transition temperature is reduced from 140 to about 120 K. The spin waves show an anisotropic dispersion relation which can be described by a strong in-plane ferromagnetic coupling J ab and a weak inter-plane antiferromagnetic coupling J c . The spin-wave dispersion exhibits a finite energy gap which decreases with the dopant concentration from about 2.5 meV for x = 0 to about 1.5 meV for x = 0.09. Furthermore, while the ferromagnetic in-plane coupling is slightly enhanced upon doping, the inter-plane antiferromagnetic coupling is strongly reduced, leading to a two-dimensional ferromagnetic system near the concentration x = 0.09.

Spin dynamics of lightly doped La1−xSrxMnO3 (abstract)

Neutron scattering has been used to study the spin dynamics of the lightly doped La1−xSrxMnO3, with x between 0 and 18. We show that as the dopant concentration increases from 0 to 0.09, the magnetic long-range order progressively changes from a slightly to a strongly canted antiferromagnetic order, whereas the transition temperature is continuously reduced from about 140 K to about 120 K. The spin waves show a strongly anisotropic dispersion relation that can be described by a strong ferromagnetic in-plane coupling Jab and a weak antiferromagnetic out-of-plane coupling Jc. The spin wave dispersion exhibits a finite gap that continuously decreases with the dopant concentration from about 2.5 meV for x=0 to about 1.5 meV for x=0.09. Furthermore, while the ferromagnetic in-plane coupling is slightly enhanced from SJab∼1.8 meV for the undoped sample to SJab∼2.0 meV for 9% doping, the out-of-plane antiferromagnetic coupling is strongly reduced from SJc∼−1.2 meV to SJc∼−0.4 meV. In the same concentration range, the system remains orthorhombic with a continuously decreasing lattice distortion, which could indicate that part of the reduction in the out-of-plane antiferromagnetic coupling can be ascribed to steric effects, whereas the enhancement of the ferromagnetic coupling could be solely associated with electronic effects. Preliminary studies of the spin-wave linewidths in the two systems x=0 and x=0.09 show a significant increase in the damping of the in-plane ferromagnetic spin waves. These results clearly show that La1−xSrxMnO3 evolves into a two-dimensional ferromagnetic system near the concentration x=0.1.

Frequency dependent magnetic susceptibility of Ni(OH) 2 monolayer nanoclusters

The a.c. magnetic susceptibility χ ac was measured for Ni(OH) 2 monolayer nanoclusters (Ni-MNC) in the temperature region 4.2–20 K. The frequency dependence of χ ac was investigated between 98 Hz and 8 kHz with an a.c. field amplitude of 1.8 Oe. The magnetic freezing temperature T′ f was measured above 10 K using the real and imaginary parts of χ ac, ( χ′ 0, χ′' 0). The value of dynamic critical exponent zv was determined by applying a dynamic scaling law to the present results. It is considered that the finite two-dimensional ferromagnetic system of Ni-MNCs has magnetic order at 9.5 K, at which the non-linear a.c. magnetic susceptibility χ′ 2 has an anomalous dip.

Dipolar interactions and the magnetic behavior of two-dimensional ferromagnetic systems.

A simple model is presented for spin-wave excitations and the low-temperature magnetization of ferromagnetic ultrathin films. The films are assumed to be quasi-two-dimensional arrays of fixed magnetic moments and both exchange and dipolar interactions are taken into account. Discrete and continuous models are used to examine the wave-vector dependence of the spin-wave frequency in the long-wavelength limit. This spin-wave theory can be used to calculate the temperature dependence of the saturation magnetization in the limit of small fluctuations. The role of dipolar interactions is examined for a single monolayer and for thin films consisting of two or three atomic layers.

EPR in the impurity-doped two-dimensional ferromagnetic system K 2Cu cZn 1−cF 4

EPR measurements of the half-field sideband as well as of the main line are reported in the impure two-dimensional ferromagnetic system K 2Cu c Zn 1− c F 4. A drastic change of the decay rate of spin correlation near the percolation limit and direct evidence for a correlation between these two lines have been observed.