3D Magnetic Ordering Research Articles

Defect-induced magnetism in TiO2: An example of quasi 2D magnetic order with perpendicular anisotropy

Magnetic order at room temperature induced by atomic lattice defects, like vacancies, interstitials, or their pairs, has been observed in a large number of different non-magnetic hosts, such as pure graphite, oxides, and silicon-based materials. High Curie temperatures and time-independent magnetic response at room temperature indicate the extraordinary robustness of this new phenomenon in solid-state magnetism. In this work, we review experimental and theoretical results of pure TiO2 (anatase), whose magnetic order can be triggered by low-energy ion irradiation. In particular, we discuss the systematic observation of an ultrathin magnetic layer with perpendicular magnetic anisotropy at the surface of this oxide.

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

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

Magnetic properties and structural anomalies observed in multiferroic NdFe3(BO3)4 by 57Fe Mossbauer spectroscopy

The results of studies of the NdFe3(BO3)4 by 57Fe Mossbauer spectroscopy in comparison with the data of single crystal X-ray diffraction measurements are presented. Scanning of the crystal cell parameters in a wide temperature range T = 15–500 K revealed a negative thermal expansion along the c axis and structural anomalies. The temperature dependences of the Mössbauer parameters of hyperfine interaction in the paramagnetic state of NdFe3(BO3)4 correlate well with the behavior of crystal cell parameters obtained by X-ray diffraction data. The temperature of the magnetic phase transition TN = 32.54(4) K is established, below which the iron ions form a 3D magnetic order of the Izing type. The magnetic transition of the iron subsystem from a commensurate to an incommensurate structure at a temperature of about T ≈ 15 K is discussed. The "Mössbauer" Debye temperature ΘM was estimated to be 485(2) K.

Strong Magnetocrystalline Anisotropy Arising from Metal–Ligand Covalency in a Metal–Organic Candidate for 2D Magnetic Order

Layered metal–organic frameworks are promising candidates for new two-dimensional (2D) magnets, as the synthetic programmability of these materials can provide a route to diverse structural and ele...

E_{8} Spectra of Quasi-One-Dimensional Antiferromagnet BaCo_{2}V_{2}O_{8} under Transverse Field.

We report ^{51}V NMR and inelastic neutron scattering (INS) measurements on a quasi-1D antiferromagnet BaCo_{2}V_{2}O_{8} under transverse field along the [010] direction. The scaling behavior of the spin-lattice relaxation rate above the Néel temperatures unveils a 1D quantum critical point (QCP) at H_{c}^{1D}≈4.7 T, which is masked by the 3D magnetic order. With the aid of accurate analytical analysis and numerical calculations, we show that the zone center INS spectrum at H_{c}^{1D} is precisely described by the pattern of the 1D quantum Ising model in a magnetic field, a class of universality described in terms of the exceptional E_{8} Lie algebra. These excitations are nondiffusive over a certain field range when the system is away from the 1D QCP. Our results provide an unambiguous experimental realization of the massive E_{8} phase in the compound, and open a new experimental route for exploring the dynamics of quantum integrable systems as well as physics beyond integrability.

Recent progress on 2D magnets: Fundamental mechanism, structural design and modification

The two-dimensional (2D) magnet, a long-standing missing member in the family of 2D functional materials, is promising for next-generation information technology. The recent experimental discovery of 2D magnetic ordering in CrI3, Cr2Ge2Te6, VSe2, and Fe3GeTe2 has stimulated intense research activities to expand the scope of 2D magnets. This review covers the essential progress on 2D magnets, with an emphasis on the current understanding of the magnetic exchange interaction, the databases of 2D magnets, and the modification strategies for modulation of magnetism. We will address a large number of 2D intrinsic magnetic materials, including binary transition metal halogenides; chalogenides; carbides; nitrides; oxides; borides; silicides; MXene; ternary transition metal compounds CrXTe3, MPX3, Fe-Ge-Te, MBi2Te4, and MXY (M = transition metal; X = O, S, Se, Te, N; Y = Cl, Br, I); f-state magnets; p-state magnets; and organic magnets. Their electronic structure, magnetic moment, Curie temperature, and magnetic anisotropy energy will be presented. According to the specific 2D magnets, the underlying direct, superexchange, double exchange, super-superexchange, extended superexchange, and multi-intermediate double exchange interactions will be described. In addition, we will also highlight the effective strategies to manipulate the interatomic exchange mechanism to improve the Curie temperature of 2D magnets, such as chemical functionalization, isoelectronic substitution, alloying, strain engineering, defect engineering, applying electronic/magnetic field, interlayer coupling, carrier doping, optical controlling, and intercalation. We hope this review will contribute to understanding the magnetic exchange interaction of existing 2D magnets, developing unprecedented 2D magnets with desired properties, and offering new perspectives in this rapidly expanding field.

57Fe and 151Eu M\xf6ssbauer studies of 3d-4f spin interplay in EuFe2\u2212xNixAs2

The EuFe2−xNixAs2 (with 0 ≤ x ≤ 0.4) compounds exhibiting 3d and/or 4f magnetic order were investigated by means of 57Fe and 151Eu Mössbauer spectroscopy. Additionally, results for EuNi2As2 are reported for comparison. It was found that spin-density-wave order of the Fe itinerant moments is monotonically suppressed by Ni-substitution. However, the 3d magnetic order survives at the lowest temperature up to at least x = 0.12 and it is certainly completely suppressed for x = 0.20. The Eu localized moments order regardless of the Ni concentration, but undergo a spin reorientation with increasing x from alignment parallel to the a-axis in the parent compound, toward c-axis alignment for x > 0.07. Change of the 4f spins ordering from antiferromagnetic to ferromagnetic takes place simultaneously with a disappearance of the 3d spins order what is the evidence of a strong coupling between magnetism of Eu2+ ions and the conduction electrons of [Fe2−xNixAs2]2- layers. The Fe nuclei experience the transferred hyperfine magnetic field due to the Eu2+ ordering for Ni-substituted samples with x > 0.04, while the transferred field is undetectable in EuFe2As2 and for compound with a low Ni-substitution level. It seems that the 4f ferromagnetic component arising from a tilt of the Eu2+ moments to the crystallographic c-axis leads to the transferred magnetic field at the Fe atoms. Superconductivity is not observed down to 1.8 K, although a comparison with 57Fe and 151Eu Mössbauer data for EuFe2As2-based superconductors indicates a similar magnetic structure.

3D to 2D Magnetic Ordering of Fe3+ Oxides Induced by Their Layered Perovskite Structure

The antiferromagnetic behavior of Fe3+ oxides of composition RE1.2Ba1.2Ca0.6Fe3O8, RE2.2Ba3.2Ca2.6Fe8O21, and REBa2Ca2Fe5O13 (RE = Gd, Tb) is highly influenced by the type of oxygen polyhedron around the Fe3+ cations and their ordering, which is coupled with the layered RE/Ba/Ca arrangement within the perovskite-related structure. Determination of the magnetic structures reveals different magnetic moments associated with Fe3+ spins in the different oxygen polyhedra (octahedron, tetrahedron, and square pyramid). The structural aspects impact on the strength of the Fe-O-Fe superexchange interactions and, therefore, on the Néel temperature (TN) of the compounds. The oxides present an interesting transition from three-dimensional (3D) to two-dimensional (2D) magnetic behavior above TN. The 2D magnetic interactions are stronger within the FeO6 octahedra layers than in the FeO4 tetrahedra layers.

Properties of axion insulator candidate layered Eu<sub>1–<i>x</i></sub>Ca<i><sub>x</sub></i>In<sub>2</sub>As<sub>2</sub>

The study of two-dimensional (2D) magnetic materials has driven the development of modern nano-electronic devices. Exploration of novel intrinsic layered materials with 2D magnetic order will provide a material candidate pool for fabricating 2D devices and searching for new quantum phases. Recently the layered antiferromagnetic (AF) topological insulators have aroused the great interest of researchers. As one of the proposed axion insulators, EuIn<sub>2</sub>As<sub>2</sub> exhibits a layered structure and 2D AF order. It is found that the parent compound EuIn<sub>2</sub>As<sub>2</sub> exhibits metallic behavior instead of the predicted insulating feature. To pursuit the predicted non-trivial topological state and novel feature, in this paper, we use various elements to dope the system to adjust the Fermi level. It is found that only Ca is successfully doped into the EuIn<sub>2</sub>As<sub>2</sub> system. The systematic transport and magnetization studies are performed on the single crystal of Eu<sub>1–<i>x</i></sub>Ca<i><sub>x</sub></i>In<sub>2</sub>As<sub>2</sub>. The long-range AF order is revealed to be similar to the parent compound. Above the AF transition, the magnetization violated Curie-Weiss behavior and magnetoresistance keeps negative, indicating the ferromagnetic order. With doping nearly 20% non-magnetic Ca, the magnetic properties of the system barely change, which is favorable to keeping the former predicted nontrivial topological properties in EuIn<sub>2</sub>As<sub>2</sub>. Although Ca shares the same valence with Eu, the carrier density of Eu<sub>1–<i>x</i></sub>Ca<i><sub>x</sub></i>In<sub>2</sub>As<sub>2</sub> is one order lower than that of EuIn<sub>2</sub>As<sub>2</sub>. The Ca doping brings electrons in and lifts the Fermi level. The results enrich the 2D magnetic material candidate pool and provide useful information for realizing the nontrivial topological state in the 2D AF system.

Near Degeneracy of Magnetic Phases in Two-Dimensional Chromium Telluride with Enhanced Perpendicular Magnetic Anisotropy.

The discovery of atomically thin van der Waals magnets (e.g., CrI3 and Cr2Ge2Te6) has triggered a renaissance in the study of two-dimensional (2D) magnetism. Most of the 2D magnetic compounds discovered so far host only one single magnetic phase unless the system is at a phase boundary. In this work, we report the near degeneracy of magnetic phases in ultrathin chromium telluride (Cr2Te3) layers with strong perpendicular magnetic anisotropy highly desired for stabilizing 2D magnetic order. Single-crystalline Cr2Te3 nanoplates with a trigonal structure (space group P3̅1c) were grown by chemical vapor deposition. The bulk magnetization measurements suggest a ferromagnetic (FM) order with an enhanced perpendicular magnetic anisotropy, as evidenced by a coercive field as large as ∼14 kOe when the field is applied perpendicular to the basal plane of the thin nanoplates. Magneto-optical Kerr effect studies confirm the intrinsic ferromagnetism and characterize the magnetic ordering temperature of individual nanoplates. First-principles density functional theory calculations suggest the near degeneracy of magnetic orderings with a continuously varying canting from the c-axis FM due to their comparable energy scales, explaining the zero-field kink observed in the magnetic hysteresis loops. Our work highlights Cr2Te3 as a promising 2D Ising system to study magnetic phase coexistence and switches for ultracompact information storage and processing.

Breakdown of the linear physical behavior in a solid solution of a halometallate molten salt, (dimim)[Fe(Cl1−xBrx)4] 0 ≤ x ≤ 1

Eighteen samples of the (dimim)[Fe(Cl1−xBrx)4] 0 ≤ x ≤ 1 solid solution were prepared in a one-pot synthesis. Single crystal and synchrotron X-ray powder diffraction studies show that all compounds are isomorphous with an orthorhombic structure (Ama2 space group) at room temperature. The crystal structure of these imidazolium molten salts can be described as layers of (dimim)+ cations (dimim = 1,3-dimethyl-imidazolium) extended in the bc-plane with an antiparallel stacking along the a-axis forming a herring-bond motive. The (FeX4)− (X = Cl and Br) anions form layers extended on the bc-plane that are intercalated between the organic layers and inverted with respect to the adjacent layer along the a direction. The evolution of the cell volume at this temperature follows the Vegard's law in the whole range of substitution with ∆V ≈ 115 Å3 between non-substituted phases, showing a non-linear axial expansion in the b and c directions. In addition, the melting point of the solid solution does not follow the expected linear physical behavior. Finally, the magnetic study indicates the existence of overall antiferromagnetic interactions as predominant, which are stronger with the increase in the (FeBr4)− ion substitution. However, it was found that the 3D magnetic ordering of the non-substituted phases vanishes or it is below the minimum temperature reachable by the magnetometer devices (1.8 K) for the 25–50% bromide substitutions. This is connected with the appearance of different crystal phases as function of bromide composition at low temperature.

Eye on China

The following topics are under this section: Open Source AI-Powered Tool by Tencent for COVID-19 Preliminary Self Evaluation The Effort to Measure Black Carbon in Antarctica Visualizing Core-Shell Formation by in situ Liquid Cell TEM Novel Strategy to Create 2D Magnetic Order Solid-State Photosensitizer for Singlet Oxygen Generation Warming Arctic Mitigates Global Warming Self-Activation Loop Maintains Stem Cells for Plant Branching Graphene-based Actuator Swarm Enables Programmable Complex Deformation Optimal Egg Consumption to Lower Risk of Cardiovascular Disease

Quantum properties and applications of 2D Janus crystals and their superlattices

Two-dimensional (2D) Janus materials are a new class of materials with unique physical, chemical, and quantum properties. The name “Janus” originates from the ancient Roman god which has two faces, one looking to the future while the other facing the past. Janus has been used to describe special types of materials which have two faces at the nanoscale. This unique atomic arrangement has been shown to present rather exotic properties with applications in biology, chemistry, energy conversion, and quantum sciences. This review article aims to offer a comprehensive review of the emergent quantum properties of Janus materials. The review starts by introducing 0D Janus nanoparticles and 1D Janus nanotubes, and highlights their difference from classical ones. The design principles, synthesis, and the properties of graphene-based and chalcogenide-based Janus layers are then discussed. A particular emphasis is given to colossal built-in potential in 2D Janus layers and resulting quantum phenomena such as Rashba splitting, skyrmionics, excitonics, and 2D magnetic ordering. More recent theoretical predictions are discussed in 2D Janus superlattices when Janus layers are stacked onto each other. Finally, we discuss the tunable quantum properties and newly predicted 2D Janus layers waiting to be experimentally realized. The review serves as a complete summary of the 2D Janus library and predicted quantum properties in 2D Janus layers and their superlattices.

Phosphine-free synthesis of FeTe2 nanoparticles and self-assembly into tree-like nanoarchitectures**Project supported by the National Natural Science Foundation of China (Grant No. 11874027) and the China Postdoctoral Science Foundation (Grant Nos. 2019T120233 and 2017M621198).

Manipulating the self-assembly of transition metal telluride nanocrystals (NCs) creates opportunities for exploring new properties and device applications. Iron ditelluride (FeTe2) has recently emerged as a new class of magnetic semiconductor with three-dimensional (3D) magnetic ordering and narrow band gap structure, yet the self-assembly of FeTe2 NCs has not been achieved. Herein, the tree-like FeTe2 nanoarchitectures with orthorhombic crystal structure have been successfully synthesized by hot-injection solvent thermal approach using phosphine-free Te precursor. The morphology, size, and crystal structure have been investigated using transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and powder x-ray diffraction (XRD). We study the formation process of tree-like FeTe2 NCs according to trace the change of the sample morphology with the reaction time. It was found that the FeTe2 nanoparticles show oriented aggregation and self-assembly behavior with the increase of reaction time, which is attributed to size-dependent magnetism properties of the samples. The magnetic interaction is thought to be the driving force of nanoparticle self-organization.

Two‐Dimensional Magnets: Forgotten History and Recent Progress towards Spintronic Applications

AbstractThe recent discovery of 2D magnetic order in van der Waals materials has stimulated a renaissance in the field of atomically thin magnets. This has led to promising demonstrations of spintronic functionality such as tunneling magnetoresistance. The frantic pace of this emerging research, however, has also led to some confusion surrounding the underlying phenomena of phase transitions in 2D magnets. In fact, there is a rich history of experimental precedents beginning in the 1960s with quasi‐2D bulk magnets and progressing to the 1980s using atomically thin sheets of elemental metals. This review provides a holistic discussion of the current state of knowledge on the three distinct families of low‐dimensional magnets: quasi‐2D, ultrathin films, and van der Waals crystals. It highlights the unique opportunities presented by the latest implementation in van der Waals materials. By revisiting the fundamental insights from the field of low‐dimensional magnetism, this review highlights factors that can be used to enhance material performance. For example, the limits imposed on the critical temperature by the Mermin–Wagner theorem can be escaped in three separate ways: magnetocrystalline anisotropy, long‐range interactions, and shape anisotropy. Several recent experimental reports of atomically thin magnets with Curie temperatures above room temperature are highlighted.

Flat-band spin dynamics and phonon anomalies of the saw-tooth spin-chain system Fe2O(SeO3)2

Fe$^{3+}$ $S = 5/2$ ions form saw-tooth like chains along the $a$ axis of the oxo-selenite Fe$_2$O(SeO$_3$)$_2$ and an onset of long-range magnetic order is observed for temperatures below $T_C = 105$ K. This order leads to distinct fingerprints in phonon mode linewidths and energies as resolved by Raman scattering. In addition, new excitations with small linewidths emerge below $T = 150$ K, and are assigned to two-magnon scattering processes with the participation of flat-band and high energy magnon branches. From this a set of exchange coupling constants is estimated. The specific ratio of the saw-tooth spine-spine and spine-vertex interactions may explain the instability of the dimer quantum ground state against an incommensurate 3D magnetic order.

Surface-Engineered MXenes: Electric Field Control of Magnetism and Enhanced Magnetic Anisotropy.

Controlling magnetism in two-dimensional (2D) materials via electric fields and doping enables robust long-range order by providing an external mechanism to modulate magnetic exchange interactions and anisotropy. In this report, we predict that transition metal carbide and nitride MXenes are promising candidates for controllable magnetic 2D materials. The surface terminations introduced during synthesis act as chemical dopants that influence the electronic structure, enabling controllable magnetic order. We show ground-state magnetic ordering in Janus M2XO xF2- x (M is an early transition metal, X is carbon or nitrogen, and x = 0.5, 1, or 1.5) with asymmetric surface functionalization, where local structural and chemical disorder induces magnetic ordering in some systems that are nonmagnetic or weakly magnetic in their pristine form. The resulting magnetic states of these noncentrosymmetric structures can be robustly switched and stabilized by tuning the interlayer exchange couplings with small applied electric fields. Furthermore, bond directionality is enhanced by Janus functionalization, resulting in improved magnetic anisotropy, which is essential to stable 2D magnetic ordering. The mixed termination-induced anisotropy leads to robust Ising ferromagnetism with an out-of-plane easy axis over the full range of relevant termination compositions for Janus Mn2N. Janus Cr2C, V2C, and Ti2C were found to be robustly antiferromagnetic. Our results provide a strategy for exploiting asymmetric surface functionalization to achieve room-temperature nanoscale magnetism under ambient conditions in MXenes with currently available synthesis techniques.

From order to randomness: Onset and evolution of the random-singlet state in bond-disordered BaCu2(Si1−xGex)2O7 spin-chain compounds

Heisenberg-type spin-chain materials have been extensively studied over the years, yet not much is known about their behavior in the presence of disorder. Starting from BaCu$_2$Si$_2$O$_7$, a typical spin-1/2 chain system, we investigate a series of compounds with different degrees of bond disorder, where the systematic replacement of Si with Ge results in a re-modulation of the Cu$^{2+}$ exchange interactions. By combining magnetometry measurements with nuclear magnetic resonance studies we follow the evolution of the disorder-related properties from the well-ordered BaCu$_2$Si$_2$O$_7$ to the maximally disordered BaCu$_2$SiGeO$_7$. Our data indicate that already a weak degree of disorder of only 5% Ge, apart from reducing the 3D magnetic ordering temperature $T_\mathrm{N}$ quite effectively, induces a qualitatively different state in the paramagnetic regime. At maximum disorder our data indicate that this state may be identified with the theoretically predicted random singlet (RS) state. With decreasing disorder the extension of the RS regime at temperatures above $T_\mathrm{N}$ is reduced, yet its influence is clearly manifest, particularly in the features of NMR relaxation data.

Excitations in the spin-1 trimer chain compound CaNi3P4O14 : From gapped dispersive spin waves to gapless magnetic excitations

Magnetic excitations and the spin Hamiltonian of the spin-1 J1-J1-J2 trimer chain compound CaNi3P4O14 have been investigated by inelastic neutron scattering. Experimental data reveal gapped dispersive spin wave excitations in the 3D long-range ordered magnetic state (TC = 16 K), and gapless magnetic excitations above the TC. Simulated magnetic excitations, by using the linear spin wave theory, for a model of coupled trimer spin-chains provide a good description of the observed experimental data. The analysis reveals both ferromagnetic J1 and J2 interactions within the chains, and an antiferromagnetic interchain interaction J3 between chains. The strengths of the J1 and J2 are found to be closer (J2/J1 ~ 0.81), and J3 is determined to be weaker (J3/ J1 ~ 0.69), which is consistent with the spin chain type crystal structure. Presence of a weak single-ion-anisotropy (D/J=0.19) is also revealed. The strengths and signs of exchange interactions explain why the 1/3 magnetization is absent in the studied spin-1 compound CaNi3P4O14 in contrast to its S= 5/2 counterpart Mn based isostructural compound. The signs of the exchange interactions are in agreement with that obtained from the reported DFT calculations, whereas, their strengths are found to be significantly different. The relatively strong value of the J3 in CaNi3P4O14 gives a conventional 3D type magnetic ordering below the TC, however, retains its 1D character above the TC. The present experimental study also reveals a sharp change of single-ion anisotropy across the TC indicating that the stability of the 3D magnetic ordering in CaNi3P4O14 is ascribed to the local magnetic anisotropy in addition to interchain interactions. The present study divulges the importance of full knowledge of the exchange interactions in trimer spin chain compounds to understand their exotic magnetic properties, such as 1/3 magnetization plateau.

3D long-range magnetic ordering in (C2H5NH3)2CuCl4 compound revealed by internal magnetic field from muon spin rotation and first principal calculation

We report the results of muon spin rotation measurement that reveal a 3D long-range magnetic ordering in the inorganic CuCl4 layers separated by (C2H5NH3) organic ligands. The measured temperature dependent internal magnetic field down to 300 mK shows the 3D long-range magnetic ordering with magnetic transition at the Néel temperature of TN = 10.06 K and critical exponent of 0.31. The ground state internal dipole field is calculated by considering muon zero-point motion effect at the muon stop position determined by first principal DFT calculation, in combination with magnetization density calculated on the basis of a specific magnetic structure model. The calculated result is shown in good agreement with the measured value of internal field at the ground state temperature, thereby justifying the magnetic structure model adopted for this system and explaining the 3D nature of magnetic ordering. This model may therefore be applicable to the study of other magnetic materials.