Antiferromagnetic Chromium Research Articles

Van der Waals polarity-engineered 3D integration of 2D complementary logic

Vertical three-dimensional integration of two-dimensional (2D) semiconductors holds great promise, as it offers the possibility to scale up logic layers in the z axis1–3. Indeed, vertical complementary field-effect transistors (CFETs) built with such mixed-dimensional heterostructures4,5, as well as hetero-2D layers with different carrier types6–8, have been demonstrated recently. However, so far, the lack of a controllable doping scheme (especially p-doped WSe2 (refs. 9–17) and MoS2 (refs. 11,18–28)) in 2D semiconductors, preferably in a stable and non-destructive manner, has greatly impeded the bottom-up scaling of complementary logic circuitries. Here we show that, by bringing transition metal dichalcogenides, such as MoS2, atop a van der Waals (vdW) antiferromagnetic insulator chromium oxychloride (CrOCl), the carrier polarity in MoS2 can be readily reconfigured from n- to p-type via strong vdW interfacial coupling. The consequential band alignment yields transistors with room-temperature hole mobilities up to approximately 425 cm2 V−1 s−1, on/off ratios reaching 106 and air-stable performance for over one year. Based on this approach, vertically constructed complementary logic, including inverters with 6 vdW layers, NANDs with 14 vdW layers and SRAMs with 14 vdW layers, are further demonstrated. Our findings of polarity-engineered p- and n-type 2D semiconductor channels with and without vdW intercalation are robust and universal to various materials and thus may throw light on future three-dimensional vertically integrated circuits based on 2D logic gates.

Long-distance magnon transport in the van der Waals antiferromagnet CrPS4

We demonstrate the potential of van der Waals magnets for spintronic applications by reporting long-distance magnon spin transport in the electrically insulating antiferromagnet chromium thiophosphate ($\mathrm{Cr}\mathrm{P}{\mathrm{S}}_{4}$) with perpendicular magnetic anisotropy. We inject and detect magnon spins nonlocally by Pt contacts and monitor the nonlocal resistance as a function of an in-plane magnetic field up to 7 T. We observe a nonlocal resistance over distances up to at least a micron below the N\'eel temperature (${T}_{\mathrm{N}}=38$ K) close to magnetic field strengths that saturate the sublattice magnetizations.

Very high-frequency, gate-tunable CrPS4 nanomechanical resonator with single mode.

Two-dimensional (2D) antiferromagnetic semiconductor chromium thiophosphate (CrPS4) has gradually become a major candidate material for low-dimensional nanoelectromechanical devices due to its remarkable structural, photoelectric characteristics and potentially magnetic properties. Here, we report the experimental study of a new few-layer CrPS4 nanomechanical resonator demonstrating excellent vibration characteristics through the laser interferometry system, including the uniqueness of resonant mode, the ability to work at the very high frequency, and gate tuning. In addition, we demonstrate that the magnetic phase transition of CrPS4 strips can be effectively detected by temperature-regulated resonant frequencies, which proves the coupling between magnetic phase and mechanical vibration. We believe that our findings will promote the further research and applications of the resonator for 2D magnetic materials in the field of optical/mechanical signal sensing and precision measurement.

Monolayer chromium dichloride: An anti-ferromagnetic semiconductor with a high Néel temperature above room temperature

Two-dimensional anti-ferromagnetic materials with room-temperature magnetism are highly desirable for the applications of anti-ferromagnetic spintronic devices. Here, a monolayer anti-ferromagnetic chromium dichloride (CrCl2) with a 1 T′ structure is reported based on first-principle calculations. The structural dynamic stability can be ensured by the lattice distortion of dimerization and the 1 T′ CrCl2 is an anti-ferromagnetic semiconductor with a bandgap of 2.03 eV. Importantly, The dimerization also leads to that the nearest Cr atoms exhibit high anti-ferromagnetic coupling. 1 T’ CrCl2 has a Néel temperature of 386 K according to estimations from a Monte-Carlo simulation using the Heisenberg model. This work provides a two-dimensional material with intrinsic anti-ferromagnetism above room temperature, which is one of the promising candidates for the future nanoscale spintronics.

Constructing semilocal approximations for noncollinear spin density functional theory featuring exchange-correlation torques

We present a semilocal exchange-correlation energy functional for noncollinear spin density functional theory based on short-range expansions of the spin-resolved exchange hole and the two-body density matrix. Our functional is explicitly derived for noncollinear magnetism, is U(1) and SU(2) gauge invariant, and gives rise to nonvanishing exchange-correlation torques. Testing the functional for frustrated antiferromagnetic chromium clusters, the exchange part is shown to perform favorably compared with the far more expensive Slater and optimized effective potentials, and a delicate interplay between exchange and correlation torques is uncovered.

Infrequent Cubane-Like Chromium Sulfide Cluster with σ-Donor Ligands with Efficient Electrocatalytic Property Toward Hydrogen Evolution Reaction.

The exploitation of efficient and economical electrocatalysts for hydrogen evolution reaction (HER) is of exceeding interest in renewable clean-energy technologies. Herein, the facile solvothermal reaction of S and chromic acetate in ethylenediamine (en) achieved a novel organic hybrid chromium sulfide [Cr4(μ3-S)4(en)4(SH)4]·0.25H2O (1), which offers a new type of antiferromagnetic cubane-like chromium sulfide cluster with σ-donor en ligands. 1 was utilized in combination with Ni nanoparticles and porous Ni foam (NF) to fabricate a Ni/1/NF electrode as an efficient cathodic catalyst, indicating excellent electrocatalytic property toward HER.

Resistive Memory Based on the Spin-Density-Wave Transition of Antiferromagnetic Chromium

Spin density waves (SDWs) are antiferromagnetic ground states characterized by real-space spin modulation. When an electronic system undergoes a paramagnetic-SDW transition, the translational symmetry is spontaneously broken and energy gaps are developed near the Fermi level, which offers potential for constructing various SDW components. Here we report a prototype resistive memory device based on a prototypical SDW metal, antiferromagnetic chromium. Transport and magnetic measurements show that the paramagnetic-SDW transition, i.e., the SDW antiferromagnetism, can be effectively suppressed by the electric-field-generated piezoelectric strain in epitaxial $\mathrm{Cr}/0.7\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_{3}$--$0.3{\mathrm{Pb}\mathrm{Ti}\mathrm{O}}_{3}$ (PMN-PT) heterostructures. This enables a large electroresistance effect for metallic systems as the SDW band gaps can be intentionally controlled to vanish or develop. Combining this electroresistance effect with the different remanent piezoelectric strain of PMN-PT after poling by electric-field pulses of opposite polarity, we obtain two nonvolatile resistance states differing by about 1.8% and stable against a magnetic field of 3 T at room temperature. Our work unveils the electric-field controllability of the SDW transitions in thin films and the consequent wide application prospects of SDW materials.

Antiferromagnetic chromium thin films as piezoresistive sensor materials

Sputter-deposited thin films of pure chromium and of chromium with small amounts of nitrogen are characterized regarding their electrical resistivity and strain dependence, i.e., piezoresistivity. They show a temperature dependent piezoresistive effect with gauge factors ranging approximately from 10 to 20. Related to this effect, they exhibit signs of a paramagnetic–antiferromagnetic transition at temperatures of 420 K and higher. For characterization, resistivity is measured at different strain levels: in a bending setup with a fixed radius and in a four-point bending system with reference strain gauges. Several parameter series of the sputter deposition of pure Cr films show that the higher gauge factor is correlated to a higher temperature coefficient of resistivity (TCR). The addition of nitrogen extends the range of TCR toward negative values, with gauge factors still in the same range as pure Cr. A Cr–N strain gauge is characterized and shows a linear, low-hysteresis strain–resistivity effect as well as a relatively large transverse sensitivity. Resistivity and gauge factor of one Cr and one Cr–N sample are measured from room temperature up to 600 K. These films have a resistivity anomaly indicating an antiferromagnetic ordering temperature TN that is much higher than in bulk Cr. The gauge factor has a maximum near TN and falls to small values at higher temperatures. The results indicate that the piezoresistivity of Cr and Cr-rich films is coupled to their spin-density wave (SDW) antiferromagnetism. Since the SDW state is known to be tunable through alloying, internal stress, and crystallinity, it appears that piezoresistivity can be influenced by these parameters as well.

Temperature dependence of Fano resonances in CrPS4.

A Fano resonance, as often observed in scattering, absorption, or transmission experiments, arises from quantum interference between a discrete optical transition and a continuous background. Here, we present a temperature-dependent study on Fano resonances observed in photoluminescence from flakes of the layered semiconductor antiferromagnet chromium thiophosphate (CrPS4). Two Fano resonances with a distinctly different temperature dependence were identified. The continuous background that is responsible for the Fano resonances is attributed to the d-d transition of the optically active Cr3+ center, predominantly the spin-forbidden 2Eg → 4A2g transition with contributions of the broad-band 4T2g → 4A2g transition. The discrete states that interfere with this continuous background are suggested to arise from localized atomic phosphorus. A model idea for explaining the individual temperature dependence of the Fano resonances is presented.

Magnetic Phase Transitions and Magnetoelastic Coupling in a Two-Dimensional Stripy Antiferromagnet.

Materials with a quasi-one-dimensional stripy magnetic order often exhibit low crystal and magnetic symmetries, thus allowing the presence of various energy coupling terms and giving rise to macroscopic interplay between spin, charge, and phonon. In this work, we performed optical, electrical and magnetic characterizations combined with first-principles calculations on a van der Waals antiferromagnetic insulator chromium oxychloride (CrOCl). We detected the subtle phase transition behaviors of exfoliated CrOCl under varying temperature and magnetic field and clarified its controversial spin structures. We found that the antiferromagnetism and its air stability persist down to few-layer samples, making it a promising candidate for future 2D spintronic devices. Additionally, we verified the magnetoelastic coupling effect in CrOCl, allowing for the potential manipulation of the magnetic states via electric field or strain. These virtues of CrOCl provide us with an ideal platform for fundamental research on spin-charge, spin-phonon coupling, and spin-interactions.

Exchange Coupling in Synthetic Anion‐Engineered Chromia Heterostructures

AbstractControl of magnetic states by external factors has garnered a mainstream status in spintronic research for designing low power consumption and fast‐response information storage and processing devices. Previously, magnetic‐cation substitution was the conventional approach to induce ferromagnetism in an intrinsic antiferromagnet. Theoretically, anion doping is proposed to be another means to change magnetic ground states. Here, the authors demonstrate the synthesis of high‐quality single‐phase chromium oxynitride thin films using in‐situ nitrogen doping. Unlike antiferromagnetic monoanionic chromium oxide and nitride phases, chromium oxynitride exhibits a robust ferromagnetic and insulating state, as demonstrated by the combination of multiple magnetization probes and theoretical calculations. With increasing the nitrogen content, the crystal structure of chromium oxynitride transits from trigonal (Rc) to tetragonal (4 mm) phase and its saturation magnetization reduces significantly. Furthermore, they achieve a large and controllable exchange bias field in the chromia heterostructures by synthetic anion engineering. This work reflects the anion engineering in functional oxides towards potential applications in giant magnetoresistance and tunnelling junctions of modern magnetic sensors and read heads.

Mechanism of highly sensitive strain response in antiferromagnetic chromium

We studied a possible mechanism for the highly sensitive response of electrical resistivity to strain in metal Cr by means of theoretical calculation and experimental measurement. First-principles calculations based on density functional theory were performed for antiferromagnetic Cr in the spin-density wave (SDW) state. The calculation succeeded to reproduce a significant magnetovolume effect by hydrostatic pressure observed in Cr, and the obtained result revealed that the electronic structure and magnetic properties in the SDW state are sensitive to uniaxial strain. The magnetic moment of Cr changed more than 5% with a strain of 1%. We estimated the gauge factor (GF), which denotes the sensitivity of resistance to strain, from the local density of states. The obtained GF value of Cr in the SDW state was found to be approximately 10, whereas that of Cr in the nonmagnetic state, Fe in the ferromagnetic state, and V in the nonmagnetic state was around 1. This result was consistent with our experimental measurement of the GF of Cr, Fe, and V thin films. We found that the large GF is related to a significant magnetovolume effect in Cr. The volume variation accompanying uniaxial strain influences both the magnetic state and electrical conduction of Cr through sensitive changes of the electronic structure in the SDW state.

Dynamics of the electric-field induced magnetization in antiferromagnetic chromium oxide observed by Faraday rotation

We observed the dynamics of the electric-field induced magnetization in antiferromagnetic chromium oxide (Cr2O3) by the Faraday-rotation measurement using a continuous-wave probe light in the millisecond region and a pulse probe light in the nanosecond region. It was found that the Faraday-rotation amplitude linearly depends on the electric field, decreases with increasing temperature, and disappears above the Néel temperature. In the nanosecond region, nanosecond rise of the electric-field induced Faraday-rotation signal was observed.

Chromium plasmonic polarizer for high intensity light

In this work, we investigate a thin-film polarizer for a high intensity of the electromagnetic (EM) beam based on Cr nano wire arrays. Commonly used thin-film polarizing components are very sensitive for high power of EM waves and can be easily damaged by focused beams. The solution to this problem could be the thin-film polarizer based on metallic subwavelengths structures. This type of optical element has huge resistance comparing to typical thin-film polarizers. However, designing such an optical element for proper wavelength of EM wave and transmissions is not easy task. In this paper we present numerical as well as experimental results for specially designed chromium thin-film polarizer for wavelength 532nm Full Text: PDF ReferencesW. Zhou, K. Li, C. Song, P. Hao, M. Chi, M. Yu and Y. Wu, "Polarization-independent and omnidirectional nearly perfect absorber with ultra-thin 2D subwavelength metal grating in the visible region", Opt. Express 23, 11 (2015). CrossRef W. L. Barnes, A . Dereux, and T. W. Ebbesen, "Surface plasmon subwavelength optics", Nature 424, 824-830 (2003). CrossRef C. Lee, E. Sim, D. Kim, "Blazed wire-grid polarizer for plasmon-enhanced polarization extinction: design and analysis", Opt. Express 25, 7 (2017). CrossRef A. Lehmuskero, Metallic thin film structures and polarization shaping gratings (University of Eastern Finland 2010).Y. Leroux, J. C. Lacroix, C. Fave, V. Stockhausen, N. Felidj, J. Grandm, A. Hohenau, J. R. Krenn, "Active plasmonic devices with anisotropic optical response: a step toward active polarizer", Nano Lett. 5, 9 (2009). CrossRef R. T. Perkins, D. P. Hansen, E. W. Gardner, J. M. Thorne, A. A. Robbins, Broadband wire grid polarizer for the visible spectrum, US 6122103 (2000). DirectLink D. M. Sullivan, Electromagnetic simulation using the FDTD method, New York: IEEE Press Series (2000). CrossRef J. P. Berenger, Perfectly Matched Layer (PML) for Computational Electromagnetics, Morgan & Claypool Publishers (2007). CrossRef Yu, W., and R. Mittra, "A conformal FDTD software package modeling antennas and microstrip circuit components", IEEE Antennas Propagat. Magazine 42, 28 (2000) . CrossRef L. W. Bos, D. W. Lynch, "Optical Properties of Antiferromagnetic Chromium and Dilute Cr-Mn and Cr-Re Alloys", Phys. Rev. Sect. B, 2, 4267 (1970). CrossRef

Role of anti-ferromagnetic Cr nanoparticles in CuTl-1223 superconducting matrix

Anti-ferromagnetic chromium (Cr) nanoparticles were added to Cu0.5Tl0.5Ba2Ca2Cu3O10−δ (CuTl-1223) superconducting matrix to synthesize (Cr)x/CuTl-1223 (x = 0, 0.5, 0.75, and 1.0 wt.%) nano-superconductor composites. Different experimental techniques were used to study the structural, morphological, compositional and superconducting transport properties of these composites. It was observed that the addition of Cr nanoparticles had not altered the crystal structure of the host CuTl-1223 superconducting matrix, which indicates the presence of these nanoparticles at the grain-boundaries. Suppression of superconducting transport properties can be attributed to two factors (i) spins scattering by net magnetization due to uncompensated spins on the surface of these anti-ferromagnetic Cr nanoparticles and (ii) oxygen imbalance due to formation of Cr2O3 surface layers on these nanoparticles. The prominent reduction in the diamagnetic signal was also observed, which had indicated the decrease of superconducting volume fraction after the inclusion of Cr nanoparticles in the composites. Superconducting microscopic parameters such as coherence length {ξc(0)}, inter-layer coupling (J), Fermi velocity (VF) and Fermi energy (EF) of the carriers were determined as a function of Cr content in (Cr)x/CuTl-1223 nano-superconductor composites by excess conductivity analyses in three dimensional (3D), two dimensional (2D) and short wave dimensional (0D) fluctuation regions.

First-principles study of layered antiferromagnetic CuCrX2 (X = S, Se and Te)

We present detailed electronic band-structure calculations for antiferromagnetic chromium compounds, CuCrX2 (X = S, Se or Te), carried out using spin-polarized density functional theory within the generalized-gradient approximation (GGA). A narrow-band semiconductor-to-metal transition is observed upon replacement of S or Se by Te. The indirect bandgap is found at 0.58 eV and 0.157 eV for CuCrS2 and CuCrSe2, respectively. The results for our theoretical calculations are well in line with the electronic transport properties experimentally observed for CuCrS2 and CuCrSe2.

Quantum gapped spin excitations in theS=32zigzag ladder compoundβ-CaCr2O4

We report on the spin dynamics in antiferromagnetic (${T}_{N}=21$ K) chromium oxide $\ensuremath{\beta}$-CaCr${}_{2}$O${}_{4}$, which has been proposed recently as a good experimental realization of a spin-$3/2$ strongly frustrated zigzag ladder. Inelastic neutron-scattering experiments evidence unconventional magnetic excitations, reminiscent of a spin-(pseudo)gap behavior above ${T}_{N}$. We interpret this result as related to strong one-dimensional quantum fluctuations over a broad temperature range extending up to $~$80 K. Below ${T}_{N}$, spin dynamics are compatible with a model of weakly ferromagnetically coupled frustrated ladders.

Diffraction line-shapes, Fermi surface nesting, and quantum criticality in antiferromagnetic chromium at high pressure (invited)

We explore the behavior of the nested bandstructure of chromium as a function of temperature and pressure to the point where magnetism disappears. X-ray diffraction measurements of the charge order parameter suggest that the nesting condition is maintained at high pressure, where the spin density wave ground state is destabilized by a continuous quantum phase transition. By comparing diffraction line-shapes measured throughout the temperature-pressure phase diagram we are able to identify and describe three regimes: thermal near-critical, weak coupling ground state, and quantum critical.

Magnetoelectric Cr2O3 and relativity theory

Relativity theory is useful for understanding the phenomenology of the magnetoelectric effect of the antiferromagnet chromium sesquioxide Cr_2 O_3 in two respects: (i) One gets a clear idea about the physical dimensions of the electromagnetic quantities involved, in particular about the dimensions of the magnetoelectric moduli that we suggest to tabulate in future as dimensionless relative quantities; (ii) one can recognize and extract a temperature dependent, 4-dimensional pseudoscalar from the data of magnetoelectric experiments with Cr_2 O_3. This pseudoscalar piece of Cr_2 O_3 is odd under time reflections and parity transformations and is structurally related ("isomorphic") to the gyrator of electric network theory, the axion of particle physics, and the perfect electromagnetic conductor of electrical engineering.

On the calculation of the magnetic structure of surfaces, near-surface layers, and interfaces of 3d metals

A microscopic approach to the calculation of the distribution of magnetic moments in the vicinity of surfaces, interfaces, and other spatially inhomogeneous 3d metal systems is proposed in the framework of model Hamiltonians for itinerant electrons. The method of Green’s functions in the configuration space is used. The equations for Green’s functions are solved by the recursion method. The poles of the Green’s functions of d electrons are numerically determined, and then, analytical expressions are derived for the density of states and the occupation numbers of electronic states. The magnetic moments in the vicinity of surfaces and interfaces with different orientations relative to crystallographic axes are calculated for ferromagnetic iron and antiferromagnetic chromium. The dependences of the magnitude of the magnetization on the distance to the surface for both metals proved to be very similar to each other, even though the densities of states and the types of their magnetic ordering are different. Compared to massive samples, the magnetic moments at Fe/Cr interfaces increase for chromium and decrease for iron, and the magnetization profile depends on the interface orientation. The results of the calculation make it possible to explain the experimental Mossbauer data for Cr/Sn and Fe/Cr/Sn multilayer systems.