Antiferromagnetic Order Research Articles

Understanding Antiferromagnetic Coupling in Lead-Free Halide Double Perovskite Semiconductors.

Solution-processable semiconductors with antiferromagnetic (AFM) order are attractive for future spintronics and information storage technology. Halide perovskites containing magnetic ions have emerged as multifunctional materials, demonstrating a cross-link between structural, optical, electrical, and magnetic properties. However, stable optoelectronic halide perovskites that are antiferromagnetic remain sparse, and the critical design rules to optimize magnetic coupling still must be developed. Here, we combine the complementary magnetometry and electron-spin-resonance experiments, together with first-principles calculations to study the antiferromagnetic coupling in stable Cs2(Ag:Na)FeCl6 bulk semiconductor alloys grown by the hydrothermal method. We show the importance of nonmagnetic monovalence ions at the BI site (Na/Ag) in facilitating the superexchange interaction via orbital hybridization, offering the tunability of the Curie-Weiss parameters between -27 and -210 K, with a potential to promote magnetic frustration via alloying the nonmagnetic BI site (Ag:Na ratio). Combining our experimental evidence with first-principles calculations, we draw a cohesive picture of the material design for B-site-ordered antiferromagnetic halide double perovskites.

Theoretical investigation of the Co-occurrence of superconductivity and antiferromagnetism in iron-based high-temperature superconductors

In this research work, the Co-occurrence of superconductivity and antiferromagnetism in Ca0.74 1 La0.26 1 Fe1−xCox As2 iron-based high-temperature superconductors have been investigated. Based on the multi-band nature of Ca0.74 1 La0.26 1 Fe1−xCox As2 iron-based superconductors, a two-band model Hamiltonian that contains intra- and inter-bands is developed. By employing the matrix form of the temperature-dependent Green’s function formalism with the two-band Hamiltonian, the mathematical expressions of superconducting transition temperature as functions of the superconducting order parameter and antiferromagnetism translational temperature as a function of the magnetic order parameter are obtained, respectively. The plotted graph of the superconducting and magnetic temperature as a function of the magnetic order parameter indicates a clear possibility of Co-occurrence of superconductivity and the antiferromagnetism order parameter in Ca0.74 1 La0.26 1 Fe1−xCox As2 iron-based superconductors in the range of magnetic order parameters between 2.3 meV and 6.07 meV, which is in good agreement with experimental observations. This research contributes to understanding the complex behavior of high-temperature superconductors and provides valuable technological applications for other fields.

Field-free control and switching of perpendicular magnetization by voltage induced manipulation of RKKY interaction

Voltage control of magnetization offers substantial advantages in energy efficiency for the development of spintronics technology. However, achieving a complete 180° magnetization switching remains as a challenging task since the electric field cannot provide torques to turn the magnetic moment in the ferromagnetic material. To address this challenge, we explore the utilization of synthetic antiferromagnetic (sAFM) structure coupled by Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction in the two ferromagnetic (FM) Co layers separated by a suitable thickness Ru spacer layer. One of the FM layers was prepared to be in contact with the GdOx layer, where ionic motion of oxygen can be manipulated via an application of electric field. Depending on the oxidation state at the interface with GdOx, the RKKY coupling can be adjusted and achieves reversible transitions between antiferromagnetic (AFM) and FM orders of FM layers at room temperature. The transition is mediated by the migration and redistribution of oxygen ions, transforming the Co/Gd interface into Co/GdOx and vice versa. This method suggests a stable and electrical route for magnetization reversals without an external magnetic field.

Electric current driven insulator–metal transition and large electroresistance in single crystal Sr2Ir1–xRuxO4

A series of single crystal Sr2Ir1–xRuxO4 (0 ≤ x ≤ 0.55) have been synthesized, and their physical properties have been investigated. Ru-substitution at Ir-site can drastically reduce the resistivity, which is accompanied by serious suppression of the long range antiferromagnetic order. With the application of electric current, remarkable electroresistance exceeding 90% is obtained at x ∼ 0.2, which may contain significant contribution from the strong magnetic competitions. Interestingly, insulator–metal transition driven by electric current is observed (e.g., below 100 K), which can be ascribed to the lattice modulation. The findings indicate that the electric current can be an efficient route to tune the physical properties of the Jeff = 1/2 Mott insulator.

Ptychographic Nanoscale Imaging of the Magnetoelectric Coupling in Freestanding BiFeO3.

Understanding the magnetic and ferroelectric ordering of magnetoelectric multiferroic materials at the nanoscale necessitates a versatile imaging method with high spatial resolution. Here, soft X-ray ptychography is employed to simultaneously image the ferroelectric and antiferromagnetic domains in an 80nm thin freestanding film of the room-temperature multiferroic BiFeO3 (BFO). The antiferromagnetic spin cycloid of period 64nm is resolved by reconstructing the corresponding resonant elastic X-ray scattering in real space and visualized together with mosaic-like ferroelectric domains in a linear dichroic contrast image at the Fe L3 edge. The measurements reveal a near perfect coupling between the antiferromagnetic and ferroelectric ordering by which the propagation direction of the spin cycloid is locked orthogonally to the ferroelectric polarization. In addition, the study evinces both a preference for in-plane propagation of the spin cycloid and changes of the ferroelectric polarization by 71°between multiferroic domains in the epitaxial strain-free, freestanding BFO film. The results provide a direct visualization of the strong magnetoelectric coupling in BFO and of its fine multiferroic domain structure, emphasizing the potential of ptychographic imaging for the study of multiferroics and non-collinear magnetic materials with softX-rays.

Influence of the Rare Earth Cation on the Magnetic Properties of Layered 12R-Ba4M4+Mn3O12 (M = Ce, Pr) Perovskites.

Material design is increasingly used to realize desired functional properties, and the perovskite structure family is one of the richest and most diverse: perovskites are employed in many applications due to their structural flexibility and compositional diversity. Hexagonal, layered perovskite structures with chains of face-sharing transition metal oxide octahedra have attracted great interest as quantum materials due to their magnetic and electronic properties. Ba4MMn3O12, a member of the "12R" class of hexagonal, layered perovskites, contains trimers of face-sharing MnO6 octahedra that are linked by a corner-sharing, bridging MO6 octahedron. Here, we investigate cluster magnetism in the Mn3O12 trimers and the role of this bridging octahedron on the magnetic properties of two isostructural 12R materials by systematically changing the M4+ cation from nonmagnetic Ce4+ (f0) to magnetic Pr4+ (f1). We synthesized 12R-Ba4MMn3O12 (M= Ce, Pr) with high phase purity and characterized their low-temperature crystal structures and magnetic properties. Using substantially higher purity samples than previously reported, we confirm the frustrated antiferromagnetic ground state of 12R-Ba4PrMn3O12 below TN ≈ 7.75 K and explore the cluster magnetism of its Mn3O12 trimers. Despite being atomically isostructural with 12R-Ba4CeMn3O12, the f1 electron associated with Pr4+ causes much more complex magnetic properties in 12R-Ba4PrMn3O12. In 12R-Ba4PrMn3O12, we observe a sharp, likely antiferromagnetic transition at T2 ≈ 12.15 K and an additional transition at T1 ≈ 200 K, likely in canted antiferromagnetic order. These results suggest that careful variation of composition within the family of hexagonal, layered perovskites can be used to tune material properties using the complex role of the Pr4+ ion in magnetism.

Sm substitution induced spin reorientation and stabilization of double perovskite structure resulting in enhanced magnetoelectricity in LaYFe2O6

We report an enhanced magnetoelectric (ME) effect in spin–phonon coupled single-phase La1−xSmxYFe2O6 (0 ≤ x ≤ 1). The structural, electric, magnetic, and ME properties have been investigated to establish their interplay leading to magnetoelectricity. X-ray diffraction study suggests the facilitation of the P21nm phase (double perovskite lattice arrangements) formation and improved structural order due to the substitution of Sm in the lattice. Antiferromagnetic (AFM) transition ∼700 K along with a spin-reorientation transition around room temperature (RT) and below is observed in the thermomagnetic curve. The indication of short range ordering in the magnetization data in the form of a non-Griffiths-like phase (nGP) is observed. The short range ordering could be minimized along with consequent improvement in AFM ordering, due to Sm substitution. An enhanced (∼31% with respect to x = 0) RT first-order ME coupling coefficient ∼0.59 mV cm−1 Oe−1 in x = 0.75 composition is observed. The findings reported here open the door to exercise spin-reorientation transition in the spin–phonon coupled double perovskites for spintronic device applications.

A systematic study of oxygen vacancy mediated short range and long range magnetic ordering in BaTi1-xFexO3(x = 0.02)

A systematic study on the magnetic properties of the BaTi1-xFexO3-δ (x = 0.02) system with decrease in grain sizes is presented. The combined study of long range and short range magnetic orderings together was carried out. The samples were prepared in single tetragonal phase. The grains of the pristine sample were refined from micron to nanometer range by mechanical milling. Oxygen vacancies in different charge states gave rise to ferromagnetic and antiferromagnetic orderings among the Fe3+ ions whereas oxygen atoms acted as the medium for antiferromagnetic superexchange among the Fe3+ ions. Magnetization was found to increase significantly as a result of milling and eventually the saturation magnetization became four-fold in the sample with grain size around 18 nm. The increase in magnetization is attributed to the increase in oxygen vacancies as a consequence of increase in milling time. Clear sextet patterns in the Mössbauer spectra were observed in the sample wherein 57Fe–enriched iron was used in place of natural iron. These sextets were attributed to the presence of FM and AFM orderings among Fe3+ ions. Along with the sextet patters, we also observed a doublet pattern in Mössbauer spectra because of quadrupolar interaction of the isolated Fe3+ ions arranged in non-cubic environment and lacking any kind of magnetic interactions among them. Mössbauer Spectroscopic (MS) study revealed that FM ordering increased while AFM ordering disappeared after milling. Drastic fall of coercivity in samples with grain sizes below around 40 nm indicated that they became single magnetic domain.

Sequence of Structural and Magnetic Phase Transitions in (NO)Mn6(NO3)13.

New nitrosonium manganese(II) nitrate, (NO)Mn6(NO3)13, has been synthesized and structurally characterized. In the temperature range of 45-298 K, the crystal is hexagonal (centrosymmetric sp. gr. P63/m). Mn2+ ions are assembled into tubes along axis c with both NO3- filling and coating. The nitrosonium cation is located in the framework cavity and is disordered by a 3-fold axis. At the temperature TS1 = 190 K, a structural phase transition related to the libration of the intertube NO3 group and a small variation of Mn polyhedron is observed. Moreover, the anomalies in physical properties of (NO)Mn6(NO3)13 allow suggesting that ordering of NO+ units occurs at low temperatures. The antiferromagnetic ordering in this compound is preceded by the formation of a short-range correlation regime at about 25 K and takes place in two steps at TN1 = 12.0 K and TN2 = 8.4 K.

Magnetism, heat capacity, magnetocaloric effect, and magneto-transport properties of heavy fermion antiferromagnet CeGaSi

We synthesize high-quality single crystal of CeGaSi by a Ga self-flux method and investigate its physical properties through magnetic susceptibility, specific heat and electrical resistivity measurements as well as high pressure effect. Magnetic measurements reveal that an antiferromagnetic order develops below T m ∼ 10.4 K with magnetic moments orientated in the ab plane. The enhanced electronic specific heat coefficient and the negative logarithmic slope in the resistivity of CeGaSi indicate that the title compound belongs to the family of Kondo system with heavy fermion ground states. The max magnetic entropy change −ΔSMmax (μ 0 H⊥c, μ 0 H = 7 T) around T m is found to reach up to 11.85 J⋅kg−1⋅K−1. Remarkably, both the antiferromagnetic transition temperature and −lnT behavior increase monotonically with pressure applied to 20 kbar (1 bar = 105 Pa), indicating that much higher pressure will be needed to reach its quantum critical point.

Composite antiferromagnetic and orbital order with altermagnetic properties at a cuprate/manganite interface.

Heterostructures from complex oxides allow one to combine various electronic and magnetic orders as to induce new quantum states. A prominent example is the coupling between superconducting and magnetic orders in multilayers from high- cuprates and manganites. A key role is played here by the interfacial CuO2 layer whose distinct properties remain to be fully understood. Here, we study with resonant inelastic X-ray scattering the magnon excitations of this interfacial CuO2 layer. In particular, we show that the underlying antiferromagnetic exchange interaction at the interface is strongly suppressed to meV, when compared with meV for the CuO2 layers away from the interface. Moreover, we observe an anomalous momentum dependence of the intensity of the interfacial magnon mode and show that it suggests that the antiferromagnetic order is accompanied by a particular kind of orbital order that yields a so-called altermagnetic state. Such a 2D altermagnet has recently been predicted to enable new spintronic applications and superconducting proximity effects.

Spin-Cluster Glassy and Long-Range Ordered Magnetic States in Honeycomb-Layered Compositionally Complex Oxides Na3-xLixT2SbO6 (T = Cu1/3Ni1/3Co1/3).

The concept of high-entropy oxides has triggered extensive research of this novel class of materials because their numerous functional properties are usually not mere linear combinations of those of the components. Here, we introduce the new series of compositionally complex honeycomb-layered magnets Na3-xLixT2SbO6 (T = Cu1/3Ni1/3Co1/3). An unusual feature of the system is its nonmonotonous dependences of the monoclinic lattice parameters b and β on x. Rietveld refinement of the crystal structures of the Na and Li end members reveals apparent Sb-T site inversion in the former and considerable Li-Cu site inversion in the latter. The materials are characterized by measurements of specific heat Cp, magnetization M, and ac and dc magnetic susceptibility χ. Na3T2SbO6 exhibits sharp long-range antiferromagnetic order (TN = 10.2 K) preceded by noticeable correlation effects at elevated temperatures. The magnetic phase diagram of Na3T2SbO6 is established. Introduction of Li, just at x = 0.8, destroys AFM order, resulting in spin-cluster glass behavior attributed to Li/Cu inversion, with TG growing with x to 10.4 K at x = 3.

Competing itinerant and local spin interactions in kagome metal FeGe

The combination of a geometrically frustrated lattice, and similar energy scales between degrees of freedom endows two-dimensional Kagome metals with a rich array of quantum phases and renders them ideal for studying strong electron correlations and band topology. The Kagome metal, FeGe is a noted example of this, exhibiting A-type collinear antiferromagnetic (AFM) order at TN ≈ 400 K, then establishes a charge density wave (CDW) phase coupled with AFM ordered moment below TCDW ≈ 110 K, and finally forms a c-axis double cone AFM structure around TCanting ≈ 60 K. Here we use neutron scattering to demonstrate the presence of gapless incommensurate spin excitations associated with the double cone AFM structure of FeGe at temperatures well above TCanting and TCDW that merge into gapped commensurate spin waves from the A-type AFM order. Commensurate spin waves follow the Bose factor and fit the Heisenberg Hamiltonian, while the incommensurate spin excitations, emerging below TN where AFM order is commensurate, start to deviate from the Bose factor around TCDW, and peaks at TCanting. This is consistent with a critical scattering of a second order magnetic phase transition with decreasing temperature. By comparing these results with density functional theory calculations, we conclude that the incommensurate magnetic structure arises from the nested Fermi surfaces of itinerant electrons and the formation of a spin density wave order.

Identification of the optimal site for oxygen adsorption in chemisorbed and oxidized states on pseudomorphic Fe/Ru(0001) surface: A density functional theory computational investigation

The oxidation state and charge distribution of FexOy binaries, including strained monolayers on transition metal surfaces, is a topic of significant interest. The p(2 × 2) and c(4 × 2) superstructures are two stable surface structures of chemisorbed oxygen on the pseudomorphic Fe/Ru(0001) surface that may coexist at 0.25 ML oxygen coverage. Density functional theory calculations were used to investigate the possibility of these two structures occurring on the surface. The calculations considered the effect of magnetic ordering on the choice of adsorption site for oxygen. Paramagnetic ordering of the Fe monolayer favors oxygen adsorption at the hcp site, while antiferromagnetic ordering favors oxygen adsorption at the fcc site at 0.25 ML oxygen coverage. Interestingly, in the case of antiferromagnetic ordering, although the adsorption of 1 ML coverage of oxygen for the oxidation reaction energetically prefers the hcp site, the chemisorbed structures at 0.25 ML coverage are found to prefer the fcc site on the pseudomorphic Fe/Ru(0001) surface. The DFT calculations suggest that both the p(2 × 2) and c(4 × 2) structures of oxygen are exothermic, indicating the possibility of coexistence on the pseudomorphic Fe/Ru(0001) surface and occurrence at room temperature. However, the use of the Hubbard potential parameter, which is used to correctly describe the electronic band structure of FeO and other Mott insulators, also revealed that the oxidation reaction of the pseudomorphic Fe monolayer is endothermic on the Ru(0001) surface.

Growth and surface magnetism of ultrathin Cr(001) films

We investigate the growth of ultrathin Cr films on a Au(001) surface and observe that the growth of 1.5 nm thick Cr layers at 290 K, followed by post-annealing at 520 K, results in high-quality epitaxial Cr(001) films with atomically flat large terraces and distinct surface states. Subsequently, these optimized growth conditions are successfully applied to the growth of 1 nm and 3 nm thick Cr films. Magnetic imaging of 1 and 1.5 nm thick Cr(001) films prepared under the optimized growth conditions is performed using spin-polarized scanning tunneling microscopy. Distinct magnetic contrasts featuring a topological antiferromagnetic (TAF) order are observed in both films; however, spin frustration originating from the density of screw dislocations for both films shows a significant difference. The 1.0 nm thick Cr film, which exhibits a clear TAF order with the suppression of a large spin-frustrated area, is suitable for application to spin-electronic devices.

Topological aspects of multi-k antiferromagnetism in cubic rare-earth compounds

We advertise rare-earth intermetallics with high-symmetry crystal structures and competing interactions as a possible materials platform hosting spin structures with non-trivial topological properties. Focusing on the series of cubic RCu compounds, where R = Ho, Er, Tm, the bulk properties of these systems display exceptionally rich magnetic phase diagrams hosting an abundance of different phase pockets characteristic of antiferromagnetic order in the presence of delicately balanced interactions. The electrical transport properties exhibit large anomalous contributions suggestive of topologically non-trivial winding in the electronic and magnetic structures. Neutron diffraction identifies spontaneous long-range magnetic order in terms of commensurate and incommensurate variations of (ππ0) antiferromagnetism with the possibility for various multi- k configurations. Motivated by general trends in these materials, we discuss the possible existence of topologically non-trivial winding in real and reciprocal space in the class of RCu compounds including antiferromagnetic skyrmion lattices. Putatively bringing together different limits of non-trivial topological winding in the same material, the combination of properties in RCu systems promises access to advanced functionalities.

Tunable valley splitting in RuClF bilayer

Valleytronic devices are increasingly gaining attention for their unique capability to encode and process information by leveraging electronic valley degrees of freedom. In this study, we performed first-principle calculations to analyze the band dispersions of double-layer antiferromagnetic valley material RuClF subjected to an external electric field. Our findings show that the double-layered structure exhibits interlayer antiferromagnetic order and that the bilayer bands display opposite valley splitting. We further categorized the atomic contact surfaces of the top and bottom layers into three interface conditions: F/F, Cl/Cl, and Cl/F. Under F/F and Cl/Cl interface conditions, when the electric field changes by 0.1 V/nm, the valley splitting values change by 12.2 meV and 9.5 meV, respectively. However, under the Cl/F interface condition, the structure exhibited metallic characteristics and was as such excluded from our analysis. These findings substantiate the practicality of employing an electric field to regulate the valley splitting of the bilayer structure, offering a new control strategy for valleytronic devices.

Emergence of the isotropic Kitaev honeycomb lattice α− RuCl3 and its magnetic properties

We present a comprehensive investigation of the crystal and magnetic structures of the van der Waals antiferromagnet α− RuCl3 using single crystal x-ray and neutron diffraction. The crystal structure at room temperature is a monoclinic ( C2/m ). However, with decreasing temperature, a remarkable first-order structural phase transition is observed, leading to the emergence of a rhombohedral ( R3ˉ ) structure characterized by three-fold rotational symmetry forming an isotropic honeycomb lattice. On further cooling, a zigzag-type antiferromagnetic order develops below TN=6∼6.6 K. The critical exponent of the magnetic order parameter was determined to be β=0.11(1) , which is close to the two-dimensional Ising model. Additionally, the angular dependence of the magnetic critical field of the zigzag antiferromagnetic order for the polarized ferromagnetic phase reveals a six-fold rotational symmetry within the ab− plane. These findingsreflect the symmetry associated with the Ising-like bond-dependent Kitaev spin interactions and underscore the universality of the Kitaev interaction-dominated antiferromagnetic system.

Epitaxial growth and magnetic properties of kagome metal FeSn/elemental ferromagnet heterostructures

Binary kagome compounds TmXn (T = Mn, Fe, Co; X = Sn, Ge; m:n = 3:1, 3:2, 1:1) have garnered recent interest owing to the presence of both topological band crossings and flatbands arising from the geometry of the metal-site kagome lattice. To exploit these electronic features for potential applications in spintronics, the growth of high-quality heterostructures is required. Here, we report the synthesis of Fe/FeSn and Co/FeSn bilayers on Al2O3 substrates using molecular beam epitaxy to realize heterointerfaces between elemental ferromagnetic metals and antiferromagnetic kagome metals. Structural characterization using high-resolution x-ray diffraction, reflection high-energy electron diffraction, and electron microscopy reveals that the FeSn films are flat and epitaxial. Rutherford backscattering spectroscopy was used to confirm the stoichiometric window where the FeSn phase is stabilized, while transport and magnetometry measurements were conducted to verify metallicity and magnetic ordering in the films. Exchange bias was observed, confirming the presence of antiferromagnetic order in the FeSn layers, paving the way for future studies of magnetism in kagome heterostructures and potential integration of these materials into devices.

Incommensurate Spin Density Wave in Antiferromagnetic RuO_{2} Evinced by Abnormal Spin Splitting Torque.

Since the discovery of antiferromagnetism, metallic oxide RuO_{2} has exhibited numerous intriguing spintronics properties such as the anomalous Hall effect and anisotropic spin splitting effect. However, the microscopic origin of its antiferromagnetism remains unclear. By investigating the spin splitting torque in RuO_{2}/Py, we found that metallic RuO_{2} exhibits a spatially periodic spin structure which interacts with the spin waves in Py through interfacial exchange coupling. The wavelength of such structure is evaluated within 14-20nm depending on the temperature, which is evidence of an incommensurate spin density wave state in RuO_{2}. Our work not only provides a dynamics approach to characterize the antiferromagnetic ordering in RuO_{2}, but also offers fundamental insights into the spin current generation due to anisotropic spin splitting effect associated with spin density wave.