Low-dimensional Magnetism Research Articles

Triangular Arrangement of Ferromagnetic Iron Chains in the High-TC Ferromagnet TiFe1-x Os2+x B2.

Transition-metal borides (TMBs) containing Bn -fragment (n>3) have recently gained interest for their ability to enable exciting magnetic materials. Herein, we show that the B4 -containing TiFe0.64(1) Os2.36(1) B2 is a new ferromagnetic TMB with a Curie temperature of 523(2) K and a Weiss constant of 554(3) K, originating from the chain of M3 -triangles (M=64 %Fe+36 %Os). The new phase was synthesized from the elements by arc-melting, and its structure was elucidated by single-crystal X-ray diffraction. It belongs to the Ti1+x Os2-x RuB2 -type structure (space group P 2 m, no. 189) and contains trigonal-planar B4 boron fragments [B-B distance of 1.87(4) Å] interacting with M3 -triangles [M-M distances of 2.637(8) Å and 3.0199(2) Å]. The experimental results were supported by computational calculations based on the ideal TiFeOs2 B2 composition, which revealed strong ferromagnetic interactions within and between the Fe3 -triangles. This discovery represents the first magnetically ordered Os-rich TMB, thus it will help expand our knowledge of the role of Os in low-dimensional magnetism of intermetallics and enable the design of such materials in the future.

History-dependent domain and skyrmion formation in 2D van der Waals magnet Fe3GeTe2

The discovery of two-dimensional magnets has initiated a new field of research, exploring both fundamental low-dimensional magnetism, and prospective spintronic applications. Recently, observations of magnetic skyrmions in the 2D ferromagnet Fe3GeTe2 (FGT) have been reported, introducing further application possibilities. However, controlling the exhibited magnetic state requires systematic knowledge of the history-dependence of the spin textures, which remains largely unexplored in 2D magnets. In this work, we utilise real-space imaging, and complementary simulations, to determine and explain the thickness-dependent magnetic phase diagrams of an exfoliated FGT flake, revealing a complex, history-dependent emergence of the uniformly magnetised, stripe domain and skyrmion states. The results show that the interplay of the dominant dipolar interaction and strongly temperature dependent out-of-plane anisotropy energy terms enables the selective stabilisation of all three states at zero field, and at a single temperature, while the Dzyaloshinksii-Moriya interaction must be present to realise the observed Néel-type domain walls. The findings open perspectives for 2D devices incorporating topological spin textures.

Topological Defects Induced High-Spin Quartet State in Truxene-Based Molecular Graphenoids

Topological Defects Induced High-Spin Quartet State in Truxene-Based Molecular Graphenoids

Spin-lattice couplings in two-dimensional CrI3 from first-principles computations

Since thermal fluctuations become more important as dimensions shrink, it is expected that low-dimensional magnets are more sensitive to lattice distortions and phonons than bulk systems are. Here we present a fully relativistic first-principles study on the spin-lattice coupling, i.e. how the magnetic interactions depend on local lattice distortions, of the prototypical two-dimensional ferromagnet CrI$_3$. We extract an effective measure of the spin-lattice coupling in CrI$_3$ which is up to ten times larger than what is found for bcc Fe. The magnetic exchange interactions, including Heisenberg and relativistic Dzyaloshinskii-Moriya interactions, are sensitive both to the in-plane motion of Cr atoms and out-of-plane motion of ligand atoms. We find that significant magnetic pair interactions change sign from ferromagnetic (FM) to anti-ferromagnetic (AFM) for atomic displacements larger than 0.16 {\AA}. We explain the observed strong spin-lattice coupling by analyzing the orbital decomposition of isotropic exchange interactions, involving different crystal-field-split Cr$-3d$ orbitals. The competition between the AFM t$_{2g}$ - t$_{2g}$ and FM t$_{2g}$ - e$_{g}$ contributions depends on the bond angle formed by Cr and I atoms as well as Cr-Cr distance. In particular, if a Cr atom is displaced, the FM-AFM sign change when the I-Cr-I bond angle approaches 90$^\circ$. The obtained spin-lattice coupling constants, along with the microscopic orbital analysis can act as a guiding principle for further studies of the thermodynamic properties and combined magnon-phonon excitations in two-dimensional magnets.

Pseudo spin-ladder CaCu2O3 nanostructures as potential electrode material for asymmetric supercapacitors

Supercapacitors have attracted noteworthy attention due to their high power density and excellent cyclic stability than batteries, despite the limited energy density of the supercapacitors which discourages their practical application. Herein, we have demonstrated the application of a pseudo spin-ladder CaCu2O3 (2-leg) nanostructures as an efficient electrode for the fabrication of a high-performance asymmetric supercapacitor (ASC). CaCu2O3 has received a lot of attention due to its unique magnetic properties and its close relation to high temperature superconductors. The number of Cu-O chains (legs) found in the spin ladder structure of this compound is responsible for its low dimensional magnetism with the Neel temperature of ∼25 K. Electrochemical analysis has revealed that the specific capacitance (specific capacity) of CaCu2O3 based supercapacitor is 205.45 F g−1 (92.45 C g-1) at a current density of 1.5 A g−1 and furthermore CaCu2O3 nanostructures exhibited excellent cyclic stability with capacitance retention of 104% after 10000 continuous cycles. The assembled ASC device (CaCu2O3//AC (activated carbon)) delivered the energy density of 11.8 Wh kg-1 at the power density of 362.5 W kg-1. Evidently, the energy density of 2.21 Wh kg-1 is still retained even at a high power density of 3625 W kg-1. The obtained results demonstrated that the spin-ladder CaCu2O3 electrode materials are potential candidates for high-energy storage applications.

Structural and electronic properties of a new material --- SrOs-=SUB=-2-=/SUB=-O-=SUB=-6-=/SUB=--=SUP=-*-=/SUP=-

It was shown theoretically that a new material SrOs2O6 can exist; the crystal structure of SrOs2O6 is formed by the layers of OsO6 octahedra having common edges and forming a honeycomb type lattice. The structural and electronic properties of SrOs2O6 were investigated by the first-principal calculations. An antiferromagnetic structure was found to be energetically favorable for the studied compound. Keywords: ab initio calculations, low dimensional magnetism, band structure.

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The article gives a brief overview of works on the study of crystals containing metal ions with an unfilled f- or d-shell, carried out at the Institute of Spectroscopy of the Russian Academy of Sciences by the method of high-resolution Fourier spectroscopy. Among the main areas of research are studies of the hyperfine structure in spectra, isotopic effects, the role of random deformations, spectroscopy of phase transitions, low-dimensional magnets, coupled electron-phonon excitations.

Epitaxial growth and electronic properties of an antiferromagnetic semiconducting VI2 monolayer.

The van der Waals materials down to the monolayer (ML) limit provide a fertile platform for exploring low-dimensional magnetism and developing the novel applications of spintronics. Among them, due to the absence of the net magnetic moment, antiferromagnetic (AFM) materials have received much less attention than ferromagnetic ones. Here, by combining scanning tunneling microscopy and state-of-the-art first-principles calculations, we investigate the preparation, and electronic and magnetic properties of a vanadium(II) iodide (VI2) ML. Single-layer VI2 has been grown by molecular beam epitaxy on Au(111) surfaces. A band gap of 2.8 eV is revealed, indicating the semiconducting nature of the VI2 ML. Vanadium and iodine vacancy defects give rise to additional feature states within the bandgap. A typical 120° AFM spin ordering is maintained in the ML limit of VI2, as revealed by the first-principles calculations. Besides, the AFM coupling is greatly enhanced by slightly decreasing lattice constants. Our work provides an ideal platform for further studying two-dimensional magnetism with non-collinear AFM ordering and for investigating the possibility of realizing the spin Hall effect in the ML limit.

Single-Laser-Pulse-Driven Thermal Limit of the Quasi-Two-Dimensional Magnetic Ordering in Sr2IrO4

Upon femtosecond-laser stimulation, generally materials are expected to recover back to their thermal-equilibrium conditions, with only a few exceptions reported. Here we demonstrate that deviation from the thermal-equilibrium pathway can be induced in canonical 3D antiferromagnetically (AFM) ordered Sr$_2$IrO$_4$ by a single 100-fs-laser pulse, appearing as losing long-range magnetic correlation along one direction into a glassy condition. We further discover a `critical-threshold ordering' behavior for fluence above approximately 12 mJ/cm$^2$ which we show corresponds to the smallest thermodynamically stable $c$-axis correlation length needed to maintain long-range quasi-two-dimensional AFM order. We suggest that this behavior arises from the crystalline anisotropy of the magnetic-exchange parameters in Sr$_2$IrO$_4$, whose strengths are associated with distinctly different timescales. As a result, they play out very differently in the ultrafast recovery processes, compared with the thermal equilibrium evolution. Thus, our observations are expected to be relevant to a wide range of problems in the nonequilibrium behavior of low-dimensional magnets and other related ordering phenomena.

Magnetic properties of the S = 52 anisotropic triangular chain compound Bi3FeMo2O12

Competing magnetic interactions in low-dimensional quantum magnets can lead to the exotic ground state with fractionalized excitations. Herein, we present our results on an S = 5/2 quasi-one-dimensional spin system Bi3FeMo2O12. The structure of Bi3FeMo2O12 consists of very well separated, infinite zig-zag S = 5/2 spin chains. The observation of a broad maximum around 10 K in the magnetic susceptibility suggests the presence of short-range spin correlations. Magnetic susceptibility data do not fit to S=5/2 uniform spin chain model due to the presence of 2nd nearest-neighbor coupling (J2) along with the 1st nearest-neighbor coupling J1 of the zig-zag chain. The electronic structure calculations infer that the value of J1 is comparable with J2 (J2/J1~1.1) with a negligible inter-chain interaction (J'/J ~ 0.01), implying that Bi3FeMo2O12 is a highly frustrated triangular chain system. The absence of magnetic long-range ordering down to 0.2 K is seen in the heat capacity data, despite a relatively large antiferromagnetic Curie-Weiss temperature of -40 K. The magnetic heat capacity follows nearly a linear behavior at low temperatures indicating that the S = 5/2 anisotropic triangular chain exhibits the gapless excitations.

Phenomenon of Low-Dimensional Magnetism in CKα, NKα, and OKα Spectra of Carbonates and Nitrates Obtained by the Electron Probe Microanalysis (EPMA)

Phenomenon of Low-Dimensional Magnetism in CKα, NKα, and OKα Spectra of Carbonates and Nitrates Obtained by the Electron Probe Microanalysis (EPMA)

Observation of fractional edge excitations in nanographene spin chains.

Fractionalization is a phenomenon in which strong interactions in a quantum system drive the emergence of excitations with quantum numbers that are absent in the building blocks. Outstanding examples are excitations with charge e/3 in the fractional quantum Hall effect1,2, solitons in one-dimensional conducting polymers3,4 and Majorana states in topological superconductors5. Fractionalization is also predicted to manifest itself in low-dimensional quantum magnets, such as one-dimensional antiferromagnetic S = 1 chains. The fundamental features of this system are gapped excitations in the bulk6 and, remarkably, S = 1/2 edge states at the chain termini7-9, leading to a four-fold degenerate ground state that reflects the underlying symmetry-protected topological order10,11. Here, we use on-surface synthesis12 to fabricate one-dimensional spin chains that contain the S = 1 polycyclic aromatic hydrocarbon triangulene as the building block. Using scanning tunnelling microscopy and spectroscopy at 4.5 K, we probe length-dependent magnetic excitations at the atomic scale in both open-ended and cyclic spin chains, and directly observe gapped spin excitations and fractional edge states therein. Exact diagonalization calculations provide conclusive evidence that the spin chains are described by the S = 1 bilinear-biquadratic Hamiltonian in the Haldane symmetry-protected topological phase. Our results open a bottom-up approach to study strongly correlated phases in purely organic materials, with the potential for the realization of measurement-based quantum computation13.

Iron-Based Low-Dimensional Magnets

Iron-Based Low-Dimensional Magnets

Inducing high- Tc ferromagnetism in the van der Waals crystal Mn(ReO4)2 via charge doping: A first-principles study

Magnetic van der Waals (vdW) materials, which can serve as ideal platforms to study low-dimensional magnetism and possess potential applications in next-generation spintronic devices, have attracted intensive attention recently. Here, based on density functional theory calculations we predict that the electron-doped $\mathrm{Mn}{({\mathrm{ReO}}_{4})}_{2}$ (MRO) with an undulating layered structure is a ferromagnetic (FM) vdW material. Our calculations show that the magnetic interaction between two nearest-neighboring Mn atoms in the undoped MRO is very weak due to the long distance between the Mn atoms separated by the nonmagnetic ${\mathrm{ReO}}_{4}$ tetrahedra. With moderate electron doping from charge injection or K intercalation, the Re atoms become spin polarized. Thus, an effective FM coupling between the Mn atoms can be induced via the Mn-O-Re antiferromagnetic superexchange, while the calculated ${T}_{c}$ can exceed room temperature. The electron-doped MRO not only takes the advantage of strong FM interaction as double perovskites, but also avoids the shortage of the antisite disorder. The calculated small cleavage energy and stable phonon spectra indicate that MRO can be exfoliated to thin films, which facilitates its surface modification. Our paper on MRO, thus, provides a template to realize ferromagnetism in vdW materials with multitype transition metals.

New Materials and Effects in Molecular Nanomagnets

Molecular magnets are a relatively new class of purely organic or metallo-organic materials, showing magnetism even without an external magnetic field. This interdisciplinary field between chemistry and physics has been gaining increased interest since the 1990s. While bulk molecular magnets are usually hard to build because of their molecular structures, low-dimensional molecular magnets are often easier to construct, down to dot-like (zero-dimensional) structures, which are investigated by different scanning probe technologies. On these scales, new effects such as superparamagnetic behavior or coherent switching during magnetization reversal can be recognized. Here, we give an overview of the recent advances in molecular nanomagnets, starting with single-molecule magnets (0D), typically based on Mn12, Fe8, or Mn4, going further to single-chain magnets (1D) and finally higher-dimensional molecular nanomagnets. This review does not aim to give a comprehensive overview of all research fields dealing with molecular nanomagnets, but instead aims at pointing out diverse possible materials and effects in order to stimulate new research in this broad field of nanomagnetism.

Magnetism in quasi-two-dimensional tri-layer La2.1Sr1.9Mn3O10 manganite

The tri-layer La_{3-3x}Sr_{1+3x}Mn_{3}O_{10} manganites of Ruddlesden–Popper (RP) series are naturally arranged layered structure with alternate stacking of ω-MnO_2 (ω = 3) planes and rock-salt type block layers (La, Sr)_2O_2 along c-axis. The dimensionality of the RP series manganites depends on the number of perovskite layers and significantly affects the magnetic and transport properties of the system. Generally, when a ferromagnetic material undergoes a magnetic phase transition from ferromagnetic to paramagnetic state, the magnetic moment of the system becomes zero above the transition temperature (T _{C} ). However, the tri-layer La_{2.1}Sr_{1.9}Mn_{3}O_{10} shows non-zero magnetic moment above T _{C} and also another transition at higher temperature T ^{*} approx 263 K. The non-zero magnetization above T _{C} emphasizes that the phase transition in tri-layer La_{2.1}Sr_{1.9}Mn_{3}O_{10} not a ferromagnetic to paramagnetic state. We show here the non-zero magnetic moment above T _{C} is due to the quasi-two-dimensional nature of the tri-layer La_{2.1}Sr_{1.9}Mn_{3}O_{10} manganite. The scaling of the magnetic entropy change confirms the second-order phase transition and the critical behavior of phase transition has been studied around T_C to understand the low dimensional magnetism in tri-layer La_{2.1}Sr_{1.9}Mn_{3}O_{10}. We have obtained the critical exponents for tri-layer La_{2.1}Sr_{1.9}Mn_{3}O_{10}, which belong to the short-range two-dimensional (2D)-Ising universality class. The low dimensional magnetism in tri-layer La_{2.1}Sr_{1.9}Mn_{3}O_{10} manganite is also explained with the help of renormalization group theoretical approach for short-range 2D-Ising systems. It has been shown that the layered structure of tri-layer La_{2.1}Sr_{1.9}Mn_{3}O_{10} results in three different types of interactions intra-planer ( J_{ab} ), intra-tri-layer ( J_{c} ) and inter-tri-layer ( J' ) such that J_{ab}> J_{c}>> J' and competition among these give rise to the canted antiferromagnetic spin structure above T _{C} . Based on the similar magnetic interaction in bi-layer manganite, we propose that the tri-layer La_{2.1}Sr_{1.9}Mn_{3}O_{10} should be able to host the skyrmion below T _{C} due to its strong anisotropy and layered structure.

Controlling the anisotropy of a van der Waals antiferromagnet with light.

Van der Waals magnets provide an ideal playground to explore the fundamentals of low-dimensional magnetism and open opportunities for ultrathin spin-processing devices. The Mermin-Wagner theorem dictates that as in reduced dimensions isotropic spin interactions cannot retain long-range correlations, the long-range spin order is stabilized by magnetic anisotropy. Here, using ultrashort pulses of light, we control magnetic anisotropy in the two-dimensional van der Waals antiferromagnet NiPS3 Tuning the photon energy in resonance with an orbital transition between crystal field split levels of the nickel ions, we demonstrate the selective activation of a subterahertz magnon mode with markedly two-dimensional behavior. The pump polarization control of the magnon amplitude confirms that the activation is governed by the photoinduced magnetic anisotropy axis emerging in response to photoexcitation of ground state electrons to states with a lower orbital symmetry. Our results establish pumping of orbital resonances as a promising route for manipulating magnetic order in low-dimensional (anti)ferromagnets.

Crystal Growth of the Quasi-2D Quarternary Compound AgCrP2S6 by Chemical Vapor Transport

We report optimized crystal growth conditions for the quarternary compound AgCrP2S6 by chemical vapor transport. Compositional and structural characterization of the obtained crystals were carried out by means of energy-dispersive X-ray spectroscopy and powder X-ray diffraction. AgCrP2S6 is structurally closely related to the M2P2S6 family, which contains several compounds that are under investigation as 2D magnets. As-grown crystals exhibit a plate-like, layered morphology as well as a hexagonal habitus. AgCrP2S6 crystallizes in monoclinic symmetry in the space group P2/a (No. 13). The successful growth of large high-quality single crystals paves the way for further investigations of low dimensional magnetism and its anisotropies in the future and may further allow for the manufacturing of few-layer (or even monolayer) samples by exfoliation.

High-Pressure Neutron Diffraction Study of the Crystal and Magnetic Structure of Materials at the Pulsed Reactor IBR-2: Current Opportunities and Prospects

The experimental capabilities and results of recent studies of the crystal and magnetic structure of functional materials on the diffractometers DN-12 and DN-6 of upgraded high-flux pulsed reactor IBR-2 in wide ranges of high pressures and temperatures are reviewed. The combination of high neutron flux on a sample, wide-aperture multidetector systems, and focusing devices has made it possible to implement diffraction experiments on samples with record-low volumes under ultrahigh pressures (up to 35 GPa) in the temperature range of 5–300 K. The results of recent studies of the structural and magnetic phase transitions in ferroelectrics, multiferroics, low-dimensional magnets, and other materials are presented.

Magnetic Neutron Diffraction of Quasi-Two-Dimensional Magnets

This study is devoted to magnetic neutron diffraction analysis (elastic neutron scattering yielding information about spin ordering in magnetic materials). Experimental approaches to magnetism are intensely developed mainly due to the discovery of new functional materials with unusual properties. Nevertheless, magnetic neutron diffraction analysis is a unique method, which makes it possible to directly determine magnetic structures (i.e., values and mutual orientations of magnetic moments and their orientations relative to the crystallographic axes). Neutron diffraction proved to be a good instrument for studying both simple commensurate magnetic structures in bulk samples and more complex noncollinear spiral magnets. Magnetic neutron diffraction is widely applied for studying magnetic structures in nanoscale multilayer film materials. However, bulk samples with a layered crystal structure, which can be considered as natural quasi-two-dimensional objects due to the weakness of interlayer couplings, are of greater interest for physics of low-dimensional magnetism. Our neutron diffraction studies on quasi-two-dimensional magnets, transition-metal oxides A3M2SbO6 (A = Li or Na, M = Co or Ni) and rare-earth intermetallic compounds La1–xTbx(Ni1–xMnx)2Si2, are briefly considered.