Direct Exchange Interaction Research Articles

Structure development and photoluminescence properties of high-purity greenish-yellow emitting LaGaO3:Dy3+ phosphors

This study investigates the structure development and photoluminescence properties of high-purity greenish-yellow emitting LaGaO3 phosphors doped with dysprosium ions Dy3+. The phosphors were synthesized using a high-temperature solid-state reaction method. X-ray diffraction (XRD) confirmed the formation of a single-phase LaGaO3:Dy3+ structure and scanning electron microscopy (SEM) revealed uniform particle size and morphology. The phosphor exhibits the capability of being efficiently excited by ultraviolet 350 nm exhibiting the characteristic greenish-yellow emission with two strong peaks at 478 nm (cyan) and 572 nm (yellow), ascribing to the transitions of 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2, respectively. The electronic transition mechanism is direct exchange interaction between activators, contributing to the concentration quenching. The phosphor was able to retain 67 % of its intensity without significant color change at an elevated temperature up to 25 °C to 150 °C. The CIE chromaticity coordinates of the emitted light were calculated and found in the desired greenish-yellow region. The electroluminescence performance of the LaGaO3:0.06Dy3+ phosphor was investigated using 365 nm LED chips to check their applicability in the field of white light-emitting diodes (WLEDs). The synthesized phosphors demonstrated high thermal stability, making them suitable for white-LED applications. The study highlights the potential of LaGaO3:Dy3+ phosphors in enhancing white-LEDs' color quality and efficiency.

Exotic phases induced by RKKY interaction in the square lattice

We study a model of Ising spins in which direct exchange interactions compete with the Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling, which may arise effectively from itinerant electrons. We consider the model in the two-dimensional square lattice and focus on values of the RKKY coupling constant (JRKKY) and the Fermi momentum (kF) that induce strong frustration. We study the low-temperature magnetic field phase diagram using Monte Carlo simulations, considering several nearest neighbors in the RKKY interaction. Magnetic frustration yields a rich phase diagram with a variety of exotic phases. We compute the structure factors and define appropriate order parameters to describe the transitions. We also discuss the validity of the approximation made for long-range couplings and analytically demonstrate the change in the coexistence lines in the phase diagram by including up to the tenth nearest neighbors.

Air-Stable Wafer-Scale Ferromagnetic Metallo-Carbon Nitride Monolayer.

The pursuit of robust, long-range magnetic ordering in two-dimensional (2D) materials holds immense promise for driving technological advances. However, achieving this goal remains a grand challenge due to enhanced quantum and thermal fluctuations as well as chemical instability in the 2D limit. While magnetic ordering has been realized in atomically thin flakes of transition metal chalcogenides and metal halides, these materials often suffer from air instability. In contrast, 2D carbon-based materials are stable enough, yet the challenge lies in creating a high density of local magnetic moments and controlling their long-range magnetic ordering. Here, we report a novel wafer-scale synthesis of an air-stable metallo-carbon nitride monolayer (MCN, denoted as MN4/CNx), featuring ultradense single magnetic atoms and exhibiting robust room-temperature ferromagnetism. Under low-pressure chemical vapor deposition conditions, thermal dehydrogenation and polymerization of metal phthalocyanine (MPc) on copper foil at elevated temperature generate a substantial number of nitrogen coordination sites for anchoring magnetic single atoms in monolayer MN4/CNx (where M = Fe, Co, and Ni). The incorporation of densely populating MN4 sites into monolayer MCN networks leads to robust ferromagnetism up to room temperature, enabling the observation of anomalous Hall effects with excellent chemical stability. Detailed electronic structure calculations indicate that the presence of high-density metal sites results in the emergence of spin-split d-bands near the Fermi level, causing a favorable long-range ferromagnetic exchange coupling through direct exchange interactions. Our work demonstrates a novel synthesis approach for wafer-scale MCN monolayers with robust room-temperature ferromagnetism and may shed light on practical electronic and spintronic applications.

Disordered layers and dimerization in the crystal structure of TaOCl2

Understanding the properties of transition metal materials requires consideration of the competition between the various types of electrons that comprise the system, whether they be delocalized and metallic, localized and magnetic, or localized and bonded. Here we report the refined crystal structure determination and physical properties of the two-dimensional layered compound TaOCl2. Within each layer we find a strong in-plane dimerization of Ta atoms perpendicular to the Ta-O-Ta chains, with significant disorder in the positioning of dimers in adjacent layers, suggesting that the free electron of each Ta localizes in Ta-Ta bonds. This is supported by resistivity measurements (ρ300K = 4.4*108 Ω-cm) as well as featureless, temperature-independent magnetic susceptibility up to T = 750 K. Analysis of the phonon contribution to specific heat shows the presence of a localized Einstein mode with TE = 175 K, attributed to the localized vibrations of the Ta-Ta dimers. Density functional theory calculations show that the magnitude of the Ta displacements (0.24 Å) has a dramatic effect on the electronic structure, changing from a metal with a bandwidth of 3.2 eV to an insulator with a flat band just below EF. Our results show how direct exchange interactions between Ta atoms that are relatively far from each other can still play a central role in the behavior of 5d transition metal compounds, taking advantage of the extended 5d atomic orbitals, and produce localized dimers with a bond distance of approximately 2.84 Å.

Antiferromagnetic Semiconductor BaMnO3 Hexagonal Perovskite with a Direct Bandgap.

The unique properties of direct bandgap semiconductors make it important to search for semiconductors exhibiting this phenomenon in perovskite materials. In this study, we employed first-principles calculations to investigate the crystal structures, magnetic configurations, and electronic properties of hexagonal perovskite BaMnO3 in its 4H and 6H phases. The results indicate that both structures exhibit antiferromagnetic characteristics, in which the Mn-O-Mn superexchange plays the dominant role in the 4H phase, although there is a competition between the Mn-Mn direct exchange interaction and the Mn-O-Mn superexchange interaction. In contrast, these two interactions exhibit harmonious coexistence in the 6H phase, and the two antiferromagnetic transitions occurring in the experimental phase should be related to the synergistic effect between them. Despite their different internal arrangements, they exhibit the same charge combination of Ba2+Mn4+O2-3. More importantly, both phases exhibit semiconductor properties with a direct bandgap, making it suitable to serve as an alternative material for photovoltaic and optoelectronic devices. In particular, the band gap of the 4H phase is just the right size to absorb visible light, and the 6H phase should be a potential candidate to absorb light in the ultraviolet region.

A Route to Two-Dimensional Room-Temperature Organometallic Multiferroics: The Marriage of d-p Spin Coupling and Structural Inversion Symmetry Breaking.

Two-dimensional (2D) room-temperature multiferroic materials are highly desirable but still very limited. Herein, we propose a potential strategy to obtain such materials in 2D metal-organic frameworks (MOFs) by utilizing the d-p direct spin coupling in conjunction with center-symmetry-breaking six-membered heterocyclic rings. Based on this strategy, a screening of 128 2D MOFs results in the identification of three multiferroics, that is, Cr(1,2-oxazine)2, Cr(1,2,4-triazine)2, and Cr(1,2,3,4-trazine)2, simultaneously exhibiting room-temperature ferrimagnetism and ferroelectricity/antiferroelectricity. The room-temperature ferrimagnetic order (306-495 K) in these MOFs originates from the strong d-p direct magnetic exchange interaction between Cr cations and ligand anions. Specifically, Cr(1,2-oxazine)2 exhibits ferroelectric behavior with an out-of-plane polarization of 4.24 pC/m, whereas the other two manifest antiferroelectric characteristics. Notably, all three materials present suitable polarization switching barriers (0.18-0.31 eV). Furthermore, these MOFs are all bipolar magnetic semiconductors with moderate band gaps, in which the spin direction of carriers can be manipulated by electrical gating.

Observation of Topological Hall Effect in a Chemically Complex Alloy.

The topological Hall effect (THE) is the transport response of chiral spin textures and thus can serve as a powerful probe for detecting and understanding these unconventional magnetic orders. So far, the THE is only observed in either noncentrosymmetric systems where spin chirality is stabilized by Dzyaloshinskii-Moriya interactions, or triangular-lattice magnets with Ruderman-Kittel-Kasuya-Yosida-type interactions. Here, a pronounced THE is observed in a Fe-Co-Ni-Mn chemically complex alloy with a simple face-centered cubic (fcc) structure across a wide range of temperatures and magnetic fields. The alloy is shown to have a strong magnetic frustration owing to the random occupation of magnetic atoms on the close-packed fcc lattice and the direct Heisenberg exchange interaction among atoms, as evidenced by the appearance of a reentrant spin glass state in the low-temperature regime and the first principles calculations. Consequently, THE is attributed to the nonvanishing spin chirality created by strong spin frustration under the external magnetic field, which is distinct from the mechanism responsible for the skyrmion systems, as well as geometrically frustrated magnets.

Two-Dimensional Transition Metal Boron Cluster Compounds (MBnenes) with Strain-Independent Room-Temperature Magnetism.

Two-dimensional (2D) metal borides (MBenes) with unique electronic structures and physicochemical properties hold great promise for various applications. Given the abundance of boron clusters, we proposed employing them as structural motifs to design 2D transition metal boron cluster compounds (MBnenes), an extension of MBenes. Herein, we have designed three stable MBnenes (M4(B12)2, M = Mn, Fe, Co) based on B12 clusters and investigated their electronic and magnetic properties using first-principles calculations. Mn4(B12)2 and Co4(B12)2 are semiconductors, while Fe4(B12)2 exhibits metallic behavior. The unique structure in MBnenes allows the coexistence of direct exchange interactions between adjacent metal atoms and indirect exchange interactions mediated by the clusters, endowing them with a Néel temperature (TN) up to 772 K. Moreover, both Mn4(B12)2 and Fe4(B12)2 showcase strain-independent room-temperature magnetism, making them potential candidates for spintronics applications. The MBnenes family provides a fresh avenue for the design of 2D materials featuring unique structures and excellent physicochemical properties.

Magnetic phase transition regulated by an interface coupling effect in CrBr3/electride Ca2N van der Waals heterostructures.

Compared with ferromagnetic (FM) materials, antiferromagnetic (AFM) materials have the advantages of not generating stray fields, resisting magnetic field disturbances, and displaying ultrafast dynamics and are thus considered as ideal candidate materials for next-generation high-speed and high-density magnetic storage. In this study, a new AFM device was constructed based on density functional theory calculations through the formation of a CrBr3/Ca2N van der Waals heterostructure. The FM ground state in CrBr3 undergoes an AFM transition when combining with the electride Ca2N. In such a system, since the metal Ca atoms form the exposed layer in the electride, the heterostructure interface has a high binding energy and a large amount of charge transfer. However, for individual electron doping, the FM ground state in the CrBr3 monolayer is robust. Therefore, the main factor in magnetic phase transition is the interface orbital coupling caused by the strong binding energy. Furthermore, the interface coupling effect was revealed to be a competition between direct exchange and superexchange interactions. Additionally, different pathways of orbital hybridization cause a transition of the magnetic anisotropy from out-of-plane to in-plane. This work not only provides a feasible strategy for changing the ground state of magnetic materials on electride substrates but also brings about more possibilities for the construction and advancement of new AFM devices.

Rational design of a two-dimensional high-temperature ferromagnet from HCP cobalt.

Cobalt has the highest Curie temperature (Tc) among the elemental ferromagnetic metals and has a hexagonal close-packed (HCP) structure at room temperature. In this study, HCP Co was thinned to the thickness of several (n) unit cells along the c-axis and then passivated by halogen atoms, thus being named Co2nX2 (X = F, Cl, Br and I). For Co2X2 and Co3X2, all of them are not only kinetically but also thermodynamically stable from the viewpoint of the phonon spectra and molecular dynamics. Similar to HCP Co, two-dimensional (2D) Co2F2, Co2Cl2 and Co3X2 (X = Cl, Br and I) are still ferromagnetic metals within the Stoner model but Co2X2 (X = Br and I) is a ferromagnetic half-metal with the coexistence of the metallic behavior for one spin and the insulating behavior for the other spin. Taking into account the spin-orbital coupling (SOC), the easy-magnetization axis is within the plane where the magnetization is isotropic, making it look like a 2D XY magnet. Applying a critical biaxial strain could lead to an easy-magnetization axis changing from the in-plane to the out-of-plane direction. Finally, we use classical Monte Carlo simulations to estimate the Curie temperature (Tc) which is as high as 957 and 510 K for Co2F2 and Co2Cl2, respectively, because of the strong direct exchange interaction. Different from being obtained by mechanical or liquid exfoliation from van der Waals layered structures, our study opens up new possibilities to search for novel 2D ferromagnets from the elemental ferromagnets and provides opportunities for realizing realistic ultra-thin spintronic devices.

Carrier doping modulates the magnetoelectronic and magnetic anisotropic properties of two-dimensional MSi2N4 (M = Cr, Mn, Fe, and Co) monolayers.

Through extensive density functional theory (DFT) calculations, our investigation delves into the stability, electrical characteristics, and magnetic behavior of monolayers (MLs) of MSi2N4. Computational analyses indicate intrinsic antiferromagnetic (AFM) orders within the MSi2N4 MLs, as a result of direct exchange interactions among transition metal (M) atoms. We further find that CrSi2N4 and CoSi2N4 MLs with primitive cells (pcells) exhibit half-metallic properties, with respective spin-β electron gaps of 3.661 and 2.021 eV. In contrast, MnSi2N4 and FeSi2N4 MLs with pcells act as semiconductors, having energy gaps of 0.427 and 0.282 eV, respectively. When the SOC is considered, the CrSi2N4, MnSi2N4 and FeSi2N4 MLs are metals, while the CoSi2N4 ML is a semiconductor. Our findings imply the dynamics and thermodynamic stability of MSi2N4 MLs. We have also explored the influence of carrier doping on the electromagnetic attributes of MSi2N4 MLs. Interestingly, charge doping could transform CrSi2N4, MnSi2N4, and CoSi2N4 MLs from their original AFM state into a ferromagnetic (FM) order. Moreover, carrier doping transformed CrSi2N4 and CoSi2N4 MLs from spin-polarized metals to half-metals (HMs). It is of particular note that doping of CrSi2N4 MLs with +0.9 e per pcell or more holes caused a switch in the easy axis (EA) to the [001] axis. The demonstrated intrinsic AFM order, excellent thermodynamic and kinetic stability, adjustable magnetism, and half-metallicity of the MSi2N4 family suggest its promising potential for applications in the realm of spintronics.

Signature of elementary excitations in Se-substituted layered triangular-lattice antiferromagnet CuCrS2

Geometrical spin frustration in a layered triangular-lattice system prohibits the long-range magnetic order and could give rise to unusual spin-disordered states of matter in which spins are strongly correlated and highly entangled with low-energy excitations. Here we investigate the low-temperature magnetic and specific heat properties of the layered triangular-lattice system CuCr(S1-xSex)2 within the entire Se-for-S substitution range. In magnetization measurements, the x = 0 phase exhibits a sharp cusp-like antiferromagnetic transition at 38 K, while for all the Se-substituted samples a broad maximum along with a spin-glass-like freezing at low temperatures is seen. The Curie-Weiss temperature systematically increases from −122 K for x = 0 to + 15 K for x = 1. These observations suggest that the magnetic ground state is determined by the competition between antiferromagnetic direct exchange interactions and ferromagnetic super-exchange interactions. For x = 0, the magnetic contribution to heat capacity exhibits a sharp jump at the magnetic transition temperature indicative of 3D long-range magnetic order, while for the x > 0 samples it increases smoothly across the magnetic transition range following a power-law (∼T2.1) behavior. All these results are in line with an existence of unusual gapless spin-liquid like excitations originating from the spin frustrations and competing nearest-neighbor interactions in CuCr(S,Se)2.

Enhanced Direct Exchange Interaction and Hybridization by Single-Atom Linkers for High Curie Temperature and Superior Visible-Light Harvesting in Cr3(CN3)2.

Designing two-dimensional (2D) ferromagnetic (FM) semiconductors with elevated Curie temperature, high carrier mobility, and strong light harvesting is challenging but crucial to the development of spintronics with multifunctionalities. Herein, we show first-principles computation evidence of the 2D metal-organic framework Kagome ferromagnet Cr3(CN3)2. Monolayer Cr3(CN3)2 is predicted to be an FM semiconductor with a record-high Curie temperature of 943 K owing to the use of a single-atom linker (N), which results in strong direct d-p exchange interaction and hybridization between dyz/xz and pz of Cr and N, as well as excellent matching characteristics in energy and symmetry. The single-atom linker structural feature also leads to notable band dispersion and a relatively high carrier mobility of 420 cm2 V-1 s-1. Moreover, under the in-plane strain, 2D Cr3(CN3)2 can be tuned to possess a strong visible-light-harvesting functionality. These novel properties render monolayer Cr3(CN3)2 a distinct 2D ferromagnet with high potential for the development of multifunctional spintronics.

Indirect Exchange Interaction Leads to Large Lattice Contribution to Magnetocaloric Entropy Change.

Materials with a large magnetocaloric response are highly desirable for magnetic cooling applications. It is suggested that a strong spin-lattice coupling tends to generate a large magnetocaloric effect, but no microscopic mechanism has been proposed. In this Letter, we use spin-lattice dynamics simulation to examine the lattice contribution to the magnetocaloric entropy change in bcc iron (Fe) and hcp gadolinium (Gd) with exchange interaction parameters determined from abinitio calculations. We find that indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction in hcp Gd leads to longer-range spin-lattice coupling and more strongly influences the low-frequency long-wavelength phonons. This results in a higher lattice contribution toward the total magnetocaloric entropy change as compared to bcc Fe with short-range direct exchange interactions. Our analysis provides a framework for understanding the magnetocaloric effect in magnetic materials with strong spin-lattice couplings. Our finding suggests that long-range indirect RKKY-type exchange gives rise to a larger lattice contribution to the magnetocaloric entropy change and is, thus, beneficial for magnetocaloric materials.

Synthesis and Characterization of Octacyano-Fe-Phthalocyanine.

Octacyano-metal-phthalocyanine MPc(CN)8 is a promising n-type stable organic semiconductor material with eight cyano groups, including a strong electron-withdrawing group at its molecular terminals. However, a thorough investigation of MPc(CN)8 has not yet been conducted. Therefore, we synthesized FePc(CN)8 and investigated its crystal structure, chemical and electronic states, electrical properties, photocatalytic activity, and magnetic properties. In this paper, we discuss the various properties of MPc(CN)8 in comparison with those of FePc. X-ray diffraction measurements indicated that the crystal structure of FePc(CN)8 was strongly influenced by the cyano groups and differed from the α- and β-forms of FePc. The space group P4/mcc structure of FePc(CN)8 was similar to that of the x-form of LiPc. The ultraviolet-visible (UV-vis) absorption spectrum of FePc(CN)8 was observed at wavelengths longer than that of FePc. Density functional theory-based molecular orbital calculations indicated that the energy gap of FePc(CN)8 is smaller than that of FePc, which can lead to the observation of the Q-band in the UV-vis absorption spectrum of FePc(CN)8 at longer wavelengths than that of FePc. Because FePc(CN)8 has a wider optical absorption band in the visible region than FePc, its photocatalytic activity is approximately four times higher than that of FePc. The conductivity of FePc(CN)8 was also higher than that of FePc, which is due to the larger overlap of π-electron clouds of the molecules in the crystal structure of FePc(CN)8. Magnetic measurements revealed that FePc(CN)8 exists in an antiferromagnetic ground state. The magnetic properties of FePc(CN)8 are specific to its crystal structure, with direct exchange interactions between Fe2+ ions and π-electron-mediated interactions. In particular, the Pauli paramagnetic behavior at high temperatures and the antiferromagnetic behavior at low temperatures (Weiss temperature θ = -4.3 ± 0.1 K) are characteristic of the π-d system.

Stable antiferromagnetic property and tunable electronic structure of two-dimensional MnPX3 (X = S and Se) from pristine structure to Janus phase

Transition-metal phosphorus trichalcogenides have been considered as very promising two-dimensional (2D) magnetic candidates up-to-date. We performed a systematical first-principles study on the electronic structures and magnetic properties of pristine MnPX3 (X = S and Se) and Janus Mn2P2S3Se3 monolayers. All monolayers behave as a direct-band-gap semiconductor in antiferromagnetic ground state which is caused by strong direct and indirect exchange interactions. It is found that the electronic structures and magnetic properties can be manipulated by Janus phase. The calculated band gap is 2.44 eV, 1.80 eV and 1.86 eV for MnPS3, MnPSe3 and Mn2P2S3Se3 with a valley polarization with consideration of spin–orbital coupling (SOC), respectively. In particular, significant energy-splittings emerge in the SOC-band structures of Janus Mn2P2S3Se3 due to its broken-inversion-symmetry. Estimated by Monte Carlo simulations, the Néel temperature is 96 K, 71 K and 79 K based on Ising model while halved down to 41 K, 33 K and 36 K on the basis of XY model for MnPS3, MnPSe3 and Mn2P2S3Se3, respectively, indicating the XY model should be more reliable to describe the spin dynamics. Our research offers an insight into the magnetic mechanism and paves a feasible path to modulate the magnetism for 2D magnets in realistic applications on spintronics.

Two-dimensional ferromagnetism with strong magnetic interactions induced by Janus-type hydrogenation in FeSe2

In this paper, using first-principles calculations, we find that single-face (Janus-type) hydrogenation induces a magnetic transition from a ${120}^{\ensuremath{\circ}}$ noncollinear antiferromagnet (AFM) to a robust ferromagnet (FM) with high Curie temperature $({T}_{\mathrm{c}},\phantom{\rule{0.16em}{0ex}}\ensuremath{\sim}345\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ and large perpendicular magnetic anisotropy (MA, $\ensuremath{\sim}2$ meV) in two-dimensional (2D) $\mathrm{Fe}{\mathrm{Se}}_{2}$. This hydrogenation results in a shift of spin-down states of in-plane orbitals $({d}_{xy}/{d}_{{x}^{2}\ensuremath{-}{\mathrm{y}}^{2}})$ to the Fermi level, which is crucial in inducing the high ${T}_{\mathrm{c}}$ and large perpendicular MA. To clarify the strong FM, we proposed a mechanism, i.e., FM direct exchange interaction of spin-down half-filled in-plane orbitals. This interaction is different from the traditional direct exchange interactions in most 2D magnets, which tend to form AFM coupling. As the inversion symmetry is broken, a significant Dzyaloshinskii-Moriya interaction (DMI; $\ensuremath{\sim}2.6$ meV) is also induced by the Janus-type hydrogenation. It is found that tuning the exchange integral of orbitals can change the sign of its component of the DMI, providing an approach to manipulate the chirality of magnetic order.

Quintuple Function Integration in Two-Dimensional Cr(II) Five-Membered Heterocyclic Metal Organic Frameworks via Tuning Ligand Spin and Lattice Symmetry.

Two-dimensional (2D) semiconductors (SCs) integrated with two or more functions are the cornerstone for constructing multifunctional nanodevices but remain largely limited. Here, by tuning the spin state of organic linkers and the symmetry/topology of crystal lattices, we predict a class of unprecedented multifunctional SCs in 2D Cr(II) five-membered heterocyclic metal organic frameworks that simultaneously possess auxetic effect, room-temperature ferrimagnetism, chiral ferroelectricity (FE), electrically reversible spin polarization, and topological nodal lines/points. Taking 2D Cr(TDZ)2 (TDZ = 1.2.5-thiadiazole) as an exemplification, the auxetic effect is produced by the antitetra-chiral lattice structure. The high temperature ferrimagnetism originates from the strong d-p direct magnetic exchange interaction between Cr cations and TDZ doublet radical anions. Meanwhile, the clockwise-counterclockwise alignment of TDZ's dipoles results in unique 2D chiral FE with atomic-scale vortex-antivortex states. 2D Cr(TDZ)2 is an intrinsic bipolar magnetic SC where half-metallic conduction with switchable spin-polarization direction can be induced by applying a gate voltage. In addition, the symmetry of the little group C4 of the lattice structure endows 2D Cr(TDZ)2 with topological nodal lines and a quadratic nodal point in the Brillouin zone near the Fermi level.

Noncoplanar antiferromagnetism induced zero thermal expansion behavior in the antiperovskite Mn3Sn0.5Zn0.5Cx

Zero thermal expansion (ZTE) is one of the most fascinating phenomena in condensed-matter physics due to its strong correlation essence and great application prospect. However, the origin of ZTE remains elusive and it becomes an overwhelming challenge to realize the controllable design for which magnetic interaction plays a key role. In this work, with the help of neutron powder diffraction analysis, a noncoplanar $[\frac{1}{2},\frac{1}{2},\frac{1}{2}]$ antiferromagnetic (AFM) phase is experimentally obtained in the antiperovskite system ${\mathrm{Mn}}_{3}{\mathrm{Sn}}_{0.5}{\mathrm{Zn}}_{0.5}{\mathrm{C}}_{x}$. It shows ZTE behavior in a particularly wide temperature range from 10 to 162 K with linear thermal expansion coefficient 9.3 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}7}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$. Herein, the theoretical spin model is combined with the experimental results to well understand the strong spin-lattice correlation between AFM ordering and ZTE behavior. The ZTE in the noncoplanar antiferromagnetic ordered system is attributed to the fierce competition between the nearest AFM direct exchange and ferromagnetic superexchange interaction, which is effectively tuned by C occupation. Our work not only paves the way toward understanding the physical origin of ZTE, but it also provides a feasible strategy for the ZTE design, which is also of great significance for promoting the study of a strong correlation system.

Two-Dimensional Iron Silicide (FeSix) Alloys with Above-Room-Temperature Ferromagnetism.

Two-dimensional (2D) materials with intrinsic room-temperature ferromagnetism have gathered tremendous interest as promising candidates for next-generation spintronics. Here, on the basis of first-principles calculations, we report a family of stable 2D iron silicide (FeSix) alloys via dimensional reduction of their bulk counterparts. Our results demonstrate that 2D Fe4Si2-hex, Fe4Si2-orth, Fe3Si2, and FeSi2 nanosheets are lattice-dynamically and thermally stable, confirmed by the calculated phonon spectra and Born-Oppenheimer dynamic simulation up to 1000 K. 2D FeSix nanosheets are ferromagnetic metals with estimated Curie temperatures ranging from 547 to 971 K due to strong direct exchange interaction between Fe sites. In addition, the electronic properties of 2D FeSix alloys can be maintained on silicon substrates, providing an ideal platform for spintronics applications in the nanoscale.