Antiferromagnetic Ground State Research Articles

On‐Surface Synthesis of Non‐Benzenoid Nanographenes Embedding Azulene and Stone‐Wales Topologies

AbstractThe incorporation of non‐benzenoid motifs in graphene nanostructures significantly impacts their properties, making them attractive for applications in carbon‐based electronics. However, understanding how specific non‐benzenoid structures influence their properties remains limited, and further investigations are needed to fully comprehend their implications. Here, we report an on‐surface synthetic strategy toward fabricating non‐benzenoid nanographenes containing different combinations of pentagonal and heptagonal rings. Their structure and electronic properties were investigated via scanning tunneling microscopy and spectroscopy, complemented by computational investigations. After thermal activation of the precursor P on the Au(111) surface, we detected two major nanographene products. Nanographene Aa−a embeds two azulene units formed through oxidative ring‐closure of methyl substituents, while Aa−s contains one azulene unit and one Stone‐Wales defect, formed by the combination of oxidative ring‐closure and skeletal ring‐rearrangement reactions. Aa−a exhibits an antiferromagnetic ground state with the highest magnetic exchange coupling reported up to date for a non‐benzenoid containing nanographene, coexisting with side‐products with closed shell configurations resulted from the combination of ring‐closure and ring‐rearragement reactions (Ba−a, Ba−s, Bs‐a and Bs−s). Our results provide insights into the single gold atom assisted synthesis of novel NGs containing non‐benzenoid motifs and their tailored electronic/magnetic properties.

Low-temperature cluster spin glass transition in the single-domain NiCr2O4 nanoparticles

NiCr2O4 nanoparticles with average particle size ∼15 nm, a single-domain size maintains the bulk canted antiferromagnetic ground state, were synthesized by a microwave combustion method. The magnetic behavior was carefully investigated by static and dynamic magnetic susceptibility measurements. In addition to a spin-glass-like behavior below paramagnetic-ferrimagnetic transition at T C, the NiCr2O4 nanoparticles demonstrate a low-temperature cluster spin glass transition below the spin canting transition T S, which manifests itself as a magnetic anomaly peak around ∼12 K (at 100 Oe) in the zero-field cooled magnetization with a relatively stronger field dependence in a ‘de Almeida-Thouless’ line for spin glasses. The AC susceptibility analyses in different approaches demonstrate a larger relative peak temperature variation per frequency decade and a longer characteristic relaxation time in the order of 0.04 and 10−7 s, against 0.01 and 10−9 s for the high-temperature blocking, indicating the slow spin dynamics for the low-temperature cluster glassy phase. A field-temperature magnetic phase diagram is proposed for the single-domain NiCr2O4 nanoparticles.

Observation of plaid-like spin splitting in a noncoplanar antiferromagnet.

Spatial, momentum and energy separation of electronic spins in condensed-matter systems guides the development of new devices in which spin-polarized current is generated and manipulated1-3. Recent attention on a set of previously overlooked symmetry operations in magnetic materials4 leads to the emergence of a new type of spin splitting, enabling giant and momentum-dependent spin polarization of energy bands on selected antiferromagnets5-10. Despite the ever-growing theoretical predictions, the direct spectroscopic proof of such spin splitting is still lacking. Here we provide solid spectroscopic and computational evidence for the existence of such materials. In the noncoplanar antiferromagnet manganese ditelluride (MnTe2), the in-plane components of spin are found to be antisymmetric about the high-symmetry planes of the Brillouin zone, comprising a plaid-like spin texture in the antiferromagnetic (AFM) ground state. Such an unconventional spin pattern, further found to diminish at the high-temperature paramagnetic state, originates from the intrinsic AFM order instead of spin-orbit coupling (SOC). Our finding demonstrates a new type of quadratic spin texture induced by time-reversal breaking, placing AFM spintronics on a firm basis and paving the way for studying exotic quantum phenomena in related materials.

Exchange Bias Modulated by Antiferromagnetic Spin-Flop Transition in 2D Van der Waals Heterostructures.

Exchange bias is extensively studied and widely utilized in spintronic devices, such as spin valves and magnetic tunnel junctions. 2D van der Waals (vdW) magnets, with high-quality interfaces in heterostructures, provide an excellent platform for investigating the exchange bias effect. To date, intrinsic modulation of exchange bias, for instance, via precise manipulation of the magnetic phases of the antiferromagnetic layer, is yet to be fully reached, owing partly to the large exchange fields of traditional bulk antiferromagnets. Herein, motivated by the low-field spin-flop transition of a 2D antiferromagnet, CrPS4, exchange bias is explored by modulatingthe antiferromagnetic spin-flop phase transition in all-vdW magnetic heterostructures. The results demonstrate that undergoing the spin-flop transition during the field cooling process, the A-type antiferromagnetic ground state of CrPS4 turns into a canted antiferromagnetic one, therefore, it reduces the interfacial magnetic coupling and suppresses the exchange bias. Via conducting different cooling fields, one can select the exchange bias effect switching among the "ON", "depressed", and "OFF" states determined by the spin flop of CrPS4. This work provides an approach to intrinsically modulate the exchange bias in all-vdW heterostructures and paves new avenues to design and manipulate 2D spintronic devices.

Magnetoelectric CoV2O6—Role of Fe inter-play in Co-based Ising spin chain interactions

CoV2O6, a Brannerite-type material, exists in both monoclinic (α) and triclinic (γ) forms. α- CoV2O6 exhibits quasi-1D ferromagnetic chains of octahedrally-coordinated Co2+ions in the higher spin state. Fe2+ doped α-CoV2O6 (3 mol%) single crystals were synthesized using high temperature melt method. The motivation is to investigate whether disruption of 1D Co2+ ferromagnetic chains by small Fe3+ substitution alters the antiferromagnetic ground state and lead to stronger spin-frustration. In this paper, we report the magnetic and magnetodielectric properties of Fe doped α-CoV2O6 in detail. The strongly anisotropic nature of magnetic and magnetodielectric characteristics were captured well in the data through orientation dependent measurements with H being parallel to a and c axes of the crystal. Relative dielectric permittivity (εr) exhibited sharp peaks coinciding with the plateau edges Hc1 ∼2.2 T and Hc2 ∼4.4 T in the magnetization curves (M-H) for applied H parallel to a-axis. For dielectric measurements under applied H parallel to c-axis, relative dielectric permittivity exhibited sharp peak around Hc2 ∼3.1 T, again coinciding with the M-H behavior. Such closely related magnetic field induced dielectric transitions reflected in both M(H) and εr(H) measurements is a rare phenomenon and representative of strong spin-lattice coupling in this phase.

Theoretical study of the complex antiferromagnetic Heusler alloys Pd[formula omitted]MnAl, Pd[formula omitted]MnIn, and Pt[formula omitted]MnAl

The complex antiferromagnetic (AFM) phases displayed by the Heuser alloys Pd2MnAl, Pd2MnIn, and Pt2MnAl, are theoretically studied. The crystalline structure, electronic, and magnetic properties are calculated within the ab initio density functional theory. The super unit cell considered to describe the various magnetic arrangements contains 128 atoms. We analyzed the ferromagnetic (FM) and the three different antiferromagnetic (AFM) phases and found that the Pd2MnAl crystalline structure is orthorhombic with dimensions 6.055, 6.461, 6.026 Å. The AFM state with the lowest energy is the AFM3 with a Mn magnetic moment of 3.813 μB. The Pd2MnIn crystallizes in an orthorhombic structure with lattice parameters 6.249, 6.763, 6.227 Å. The ground state antiferromagnetic arrangement is also of type 3 with a Mn magnetic moment of 3.932 μB. The Pt alloy crystallizes in an orthorhombic structure with lattice dimensions of 6.029, 6.906, and 5.683 Å. In this case, the antiferromagnetic ground state is of type 1 with a Mn magnetic moment of 3.716 μB. The Pd and Pt d states are deep in energy and do not interact much with the Mn d states. This produces, in Pd and Pt, no magnetic polarization.

Synthesis of Tridecacene by Multistep Single-Molecule Manipulation.

Acenes represent a unique class of polycyclic aromatic hydrocarbons that have fascinated chemists and physicists due to their exceptional potential for use in organic electronics. While recent advances in on-surface synthesis have resulted in higher acenes up to dodecacene, a comprehensive understanding of their fundamental properties necessitates their expansion toward even longer homologues. Here, we demonstrate the on-surface synthesis of tridecacene via atom-manipulation-induced conformational preparation and dissociation of a trietheno-bridged precursor on a Au(111) surface. The generated tridecacene has been investigated by scanning tunneling microscopy and spectroscopy (STM/STS), combined with first-principles calculations. We observe that the STS transport gap (1.09 eV) shrinks again following the gap reopening of dodecacene (1.4 eV). Spin-polarized density functional theory calculations confirm an antiferromagnetic open-shell ground-state electronic configuration for tridecacene in the gas phase. Interestingly, tridecacene's open-shell character is significantly reduced upon interaction with the Au(111) surface despite being only physisorbed. The interaction with the surface leads to a lowering of the magnetization of tridecacene, a reduced gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), compared to the gas phase, and a reduced relative energy to the nonmagnetic state, making it nearly isoenergetic. These observations show qualitatively that the influence of the Au(111) substrate on the properties of long acenes is significant, which is important for interpreting the measured STS transport gaps. Our work contributes to a fundamental understanding of the electronic properties of long acenes, confirming a nonmonotonous length-dependent HOMO-LUMO gap, and to the development of multistep tip-assisted synthesis of elusive compounds.

Giant low-field magnetocaloric effect in unstable antiferromagnetic Tm1–xErxNi2Si2 (x = 0.2, 0.4) compounds

Magnetic refrigeration (MR) technology is regarded as an ideal solution for cryogenic applications, relying on magnetocaloric materials which provide necessary chilling effect. A series of polycrystalline Tm1–xErxNi2Si2 (x = 0.2, 0.4) compounds was synthesized, and their magnetic properties, magnetic phase transition together with magnetocaloric effect (MCE) were studied. The Tm1–xErxNi2Si2 (x = 0.2, 0.4) compounds display a field-induced metamagnetic transition from antiferromagnetic (AFM) to ferromagnetism (FM) in excess of 0.2 T, respectively. Meanwhile, the AFM ground state is unstable. Under the field change of 0–2 T, the values of maximal magnetic entropy change (−ΔSMmax) and refrigerant capacity (RC) for Tm0.8Er0.2Ni2Si2 compound are 17.9 J/(kg·K) and 83.5 J/kg, respectively. The large reversible MCE under low magnetic fields (≤2 T) indicates that Tm0.8Er0.2Ni2Si2 compound can serve as potential candidate materials for cryogenic magnetic refrigeration.

First-principles calculations of inorganic metallocene nanowires.

Inspired by the recently synthesized inorganic metallocene derivatives Fe(P4)22-, we have identified four stable inorganic metallocene nanowires, MP4 (M = Sc, Ti, Cr and Fe) in configurations of either regular quadrangular prism (Q-type) or anticube (A-type), and further investigated their magnetic and electronic characteristics utilizing the first-principles calculation. It shows that CrP4 is a ferromagnetic metal, while other nanowires are semiconducting antiferromagnets with bandgaps of 0.44, 1.88, and 2.29 eV within the HSE06 level. It also shows that both ScP4 and TiP4 can be stabilized in the Q-type and A-type, corresponding to the antiferromagnetic and ferromagnetic ground states, respectively, indicating a configuration-dependent magnetism. The thermodynamic and lattice stabilities are confirmed by the ab initio molecular dynamics and phonon spectra. This study has unmasked the structural and physical properties of novel inorganic metallocene nanowires, and revealed their potential application in spintronics.

Angular-dependent magnetoresistance modulated by interfacial magnetic state in Pt/LSMO heterostructures.

The interfaces between heavy metals and antiferromagnetic materials have garnered significant attention due to their interesting physical properties. La0.35Sr0.65MnO3 (LSMO), as a typical manganite, exhibits an antiferromagnetic ground state that can be controlled through epitaxial strain and interfacial spin-orbit coupling. In this work, we reported the diverse magnetoresistance, influenced by the interfacial magnetic state, in Pt (3 nm)/LSMO (6-20 nm) heterostructures. The strong spin-orbit coupling of Pt and Dzyaloshinskii-Moriya interaction alter the spin structure and enhance the electron scattering at the Pt/LSMO interface, resulting in positive magnetoresistance. The interfacial angular-dependent magnetoresistance modulated by the interfacial magnetic states was also observed in the Pt/LSMO (20 nm) heterostructures. Our findings contribute to a broader understanding of interfacial properties between heavy metals and antiferromagnetic manganites.

Controlling the 2D Magnetism of CrBr3 by van der Waals Stacking Engineering.

The manipulation of two-dimensional (2D) magnetic order is of significant importance to facilitate future 2D magnets for low-power and high-speed spintronic devices. van der Waals stacking engineering makes promises for controllable magnetism via interlayer magnetic coupling. However, directly examining the stacking order changes accompanying magnetic order transitions at the atomic scale and preparing device-ready 2D magnets with controllable magnetic orders remain elusive. Here, we demonstrate the effective control of interlayer stacking in exfoliated CrBr3 via thermally assisted strain engineering. The stable interlayer ferromagnetic (FM), antiferromagnetic (AFM), and FM-AFM coexistent ground states confirmed by the magnetic circular dichroism measurements are realized. Combined with the first-principles calculations, the atomically resolved imaging technique reveals the correlation between magnetic order and interlayer stacking order in CrBr3 flakes unambiguously. A tunable exchange bias effect is obtained in the mixed phase of FM and AFM states. This work will introduce new magnetic properties by controlling the stacking order and sequence of 2D magnets, providing ample opportunities for their application in spintronic devices.

Spin-liquid behavior in the classical antiferromagnetic [formula omitted] spin system on the square-kagome lattice: Exact analysis within recursive-lattice approach

Magnetic and thermodynamic properties of the antiferromagnetic J1−J2−J3 spin system, i.e., the spin system with three different nearest-neighbor antiferromagnetic interactions, on the square-kagome lattice is investigated in the framework of the corresponding exactly solvable spin-1/2 Ising model on the recursive square-kagome lattice. The phase diagram of the model is found and it is shown that, depending on the model parameters, the model exhibits the existence of three long-range order (antiferromagnetic) phases, which are separated from the paramagnetic phase by the second order phase transitions. It is also shown that, in the zero temperature limit, the paramagnetic phase splits into three different ground states realized on the lines that separate antiferromagnetic ground states of the model. Due to the frustration, all these “paramagnetic” ground states are highly macroscopically degenerated with strict hierarchy of their residual entropies. Moreover, the high macroscopic degeneracy of the paramagnetic phase in the vicinity of these ground states together with the presence of strong correlations (given by the proximity of the second order phase transitions) allow one to consider the paramagnetic ground states as well as their immediate paramagnetic surroundings as regions with the classical spin-liquid behavior. It is also shown that the presence of the second order phase transitions in the vicinity of the paramagnetic ground states suppresses expected anomalous (Schottky-type) behavior of the specific heat related to the rapid entropy changes caused by the frustration.

Two-Dimensional Transition Metal Boride TMB12 (TM = V, Cr, Mn, and Fe) Monolayers: Robust Antiferromagnetic Semiconductors with Large Magnetic Anisotropy.

Currently, two-dimensional (2D) materials with intrinsic antiferromagnetism have stimulated research interest due to their insensitivity to external magnetic fields and absence of stray fields. Here, we predict a family of stable transition metal (TM) borides, TMB12 (TM = V, Cr, Mn, Fe) monolayers, by combining TM atoms and B12 icosahedra based on first-principles calculations. Our results show that the four TMB12 monolayers have stable antiferromagnetic (AFM) ground states with large magnetic anisotropic energy. Among them, three TMB12 (TM=V, Cr, Mn) monolayers display an in-plane easy magnetization axis, while the FeB12 monolayer has an out-of-plane easy magnetization axis. Among them, the CrB12 and the FeB12 monolayers are AFM semiconductors with band gaps of 0.13 eV and 0.35 eV, respectively. In particular, the AFM FeB12 monolayer is a spin-polarized AFM material with a Néel temperature of 125 K. Moreover, the electronic and magnetic properties of the CrB12 and the FeB12 monolayers can be modulated by imposing external biaxial strains. Our findings show that the TMB12 monolayers are candidates for designing 2D AFM materials, with potential applications in electronic devices.

Anisotropic and High‐Mobility Electronic Transport in a Quasi 2D Antiferromagnet NdSb2

AbstractAdvancements in low‐dimensional functional device technology heavily rely on the discovery of suitable materials which have interesting physical properties as well as can be exfoliated down to the 2D limit. Exfoliable high‐mobility magnets are one such class of materials that, not due to lack of effort, has been limited to only a handful of options. So far, most of the attention has been focused on the van der Waals (vdW) systems. However, even within the non‐vdW, layered materials, it is possible to find all these desirable features. Using chemical reasoning, it is found that NdSb2 is an ideal example. Even with a relatively small interlayer distance, this material can be exfoliated down to few layers. NdSb2 has an antiferromagnetic ground state with a quasi 2D spin arrangement. The bulk crystals show a very large, non‐saturating magnetoresistance along with highly anisotropic electronic transport properties. It is confirmed that this anisotropy originates from the 2D Fermi pockets which also imply a rather quasi 2D confinement of the charge carrier density. Both electron and hole‐type carriers show very high mobilities. The possible non‐collinear spin arrangement also results in an anomalous Hall effect.

First-principles investigation of the magnetoelectric properties of Ba7Mn4O15

Type-II multiferroics, in which the magnetic order breaks inversion symmetry, are appealing for both fundamental and applied research due their intrinsic coupling between magnetic and electrical orders. Using first-principles calculations we study the ground state magnetic behaviour of Ba7Mn4O15 which has been classified as a type-II multiferroic in recent experiments. Our constrained moment calculations with the proposed experimental magnetic structure shows the spontaneous emergence of a polar mode giving rise to an electrical polarisation comparable to other known type-II multiferroics. When the constraints on the magnetic moments are removed, the spins self-consistently relax into a canted antiferromagnetic ground state configuration where two magnetic modes transforming as distinct irreducible representations coexist. While the dominant magnetic mode matches well with the previous experimental observations, the second mode is found to possess a different character resulting in a non-polar ground state. Interestingly, the non-polar magnetic ground state exhibits a significantly strong linear magnetoelectric (ME) coupling comparable to the well-known multiferroic BiFeO3, suggesting strategies to design new linear MEs.

Rapid Suppression of Quantum Many-Body Magnetic Exciton in Doped van der Waals Antiferromagnet (Ni,Cd)PS3.

The unique discovery of the magnetic exciton in van der Waals antiferromagnet NiPS3 arises between two quantum many-body states of a Zhang-Rice singlet excited state and a Zhang-Rice triplet ground state. Simultaneously, the spectral width of photoluminescence originating from this exciton is exceedingly narrow as 0.4 meV. These extraordinary properties, including the extreme coherence of the magnetic exciton in NiPS3, beg many questions. We studied doping effects using Ni1-xCdxPS3 using two experimental techniques and theoretical studies. Our experimental results show that the magnetic exciton is drastically suppressed upon a few % Cd doping. All this happens while the width of the exciton only gradually increases and the antiferromagnetic ground state is robust. These results highlight the lattice uniformity's hidden importance as a prerequisite for coherent magnetic exciton. Finally, an exciting scenario emerges: the broken charge transfer forbids the otherwise uniform formation of the coherent magnetic exciton in (Ni,Cd)PS3.

Role of A site and B site ion substitutions on the low-temperature magnetic behaviour of Ca2Fe2O5

In the present paper, we report the structural and low-temperature magnetic properties of Sm-substituted Ca2Fe2O5 (Ca2(1-x)Sm2xFe2O5, x = 0.05, 0.1) and Co-substituted Ca2Fe2O5( Ca2Fe2(1-y)Co2yO5, y = 0.1, 0.2). The polycrystalline samples of Sm and Co-based compounds, synthesised by a solid-state reaction route, are found to be crystallised with space group Icmm and Pnma respectively. The gradual substitution/doping of Sm3+ (5 % and 10 %) replacing Ca shows a significant change in temperature dependant magnetic behaviour, assumed to originate from complex interactions of Sm and Fe ions. For Co-substituted samples, the magnetic study as a function of temperature suggests the signature of ferri/ferromagnetic interactions along with the probable signature of a glassy feature. Isothermal magnetisation characterisations indicate the pure antiferromagnetic interactions in Sm samples, whereas the onset of hysteresis in Co samples can be correlated with the ferri/ferromagnetic nature of samples with the antiferromagnetic ground state.

Effect of concentration-driven magnetic phase changes on adsorption and diffusion in VSe2 monolayers: Implications for lithium-ion batteries

A vanadium diselenide (VSe2) monolayer is a two-dimensional (2D) magnetic material that exhibits ferromagnetic ordering at room temperature and exceptional metal-ion storage capacity, making it useful in spintronics and energy storage applications. However, a robust correlation between the magnetic and electrochemical properties of VSe2 remains to be established. In this study, first-principles density functional theory calculations were performed to investigate the effect of increasing Li-ion concentrations on the magnetic properties, particularly the magnetic ground state of the VSe2 monolayer. The results indicate that, as the concentration of Li ions on the surface of VSe2 monolayer increases, magnetic phase transitions occur, leading to a shift from the intrinsic ferromagnetic (FM) state to antiferromagnetic (AFM) and non-magnetic ground states. Analyses of the diffusion properties of ferromagnetic and antiferromagnetic VSe2 monolayers revealed a considerable (∼71%) increase in the Li-ion diffusion energy barrier for the AFM state compared to the FM state. This implies that FM-VSe2 facilitates relatively faster diffusion of Li ions than AFM-VSe2. Therefore, the Li-ion concentration-induced phase change in the VSe2 monolayer leads to variable adsorption and diffusion characteristics, which will have significant implications for its use in Li-ion battery anodes.

Band engineering of Co1−xNixS2 with virtual crystal approximation: A first-principles calculations

In this paper, we have explored the electronic and magnetic properties of MS2([Formula: see text], Ni) using first-principles calculations. Our data show rather high tunability of the electronic and magnetic properties of the alloy Co[Formula: see text]NixS2[Formula: see text] with the emergence of half-metallicity that persisted up to the intermediate doping concentration. The half-metallic ground state is characterized by large spin polarization at the Fermi level ([Formula: see text]). Beyond the critical doping concentration [Formula: see text], we obtained a metallic solution followed by an antiferromagnetic ground state at a larger doping concentration. This study provides the underlying physics to understand the low-energy Hilbert space and reports the role of the Fermi surface in controlling the electron transport and thus elucidating the anomalous electronic and magnetic behavior of Co[Formula: see text]NixS2.

Magnetic and magnetocaloric properties of polycrystalline Pr[formula omitted]Ca[formula omitted]MnO[formula omitted] (x [formula omitted] 0.85, 0.90, 0.95) compounds: Emergence of large inverse and conventional magnetocaloric effects

Magnetic and magnetocaloric effect have been investigated for the electron doped polycrystalline Pr1−xCaxMnO3 (x ∼ 0.85, 0.90, 0.95) compounds. The experimental outcomes indicate that the nature of the ground state is very much sensitive with the doping concentration. Pr0.15Ca0.85MnO3 compound exhibits antiferromagnetic ground state. In contrast to that ferromagnetic nature was found for the Pr0.10Ca0.90MnO3 and Pr0.05Ca0.95MnO3 compounds. Although, for x∼0.90 ferromagnetic phase was stable. However for x∼ 0.95, the ferromagnetic state is quite unstable. Additionally for the doping concentration x∼ 0.95, the anomalous nature was found in temperature dependent coercivity. Magnetocaloric effect study indicate that antiferromagnetic Pr0.15Ca0.85MnO3 compound exhibits large inverse magnetocaloric effect (10.5 J/kg-K at H = 70 kOe magnetic field). On the other hand, ferromagnetic Pr0.10Ca0.90MnO3 and Pr0.05Ca0.95MnO3 compounds show conventional magnetocaloric effect. The magnetocaloric entropy change for x∼0.90 compound is significantly large (7.5 J/kg-K at H = 70 kOe magnetic field) and comparable with the other reported magnetic refrigerant materials. Universal master curve for magnetic entropy change have been constructed for all the materials having inverse and conventional magnetocaloric effect by proper scaling of temperature axis.