Ferromagnetic Spin Arrangement Research Articles

Role of Competing Magnetic Exchange on Non-Collinear to Collinear Magnetic Ordering and Skyrmion Stabilization in Centrosymmetric Hexagonal Magnets.

Topological magnetic skyrmions with helicity state degrees of freedom in centrosymmetric magnets possess great potential for advanced spintronics applications and quantum computing. Till date, the skyrmion study in this class of materials mostly remains focused to collinear ferromagnets with uniaxial magnetic anisotropy. Here, we present a combined theoretical and experimental study on the competing magnetic exchange-induced evolution of noncollinear magnetic ground states and its impact on the skyrmion formation in a series of centrosymmetric hexagonal noncollinear magnets, MnFe1-xCoxGe. We show that by engineering the Fe/Co ratio, the system progressively transforms from a noncollinear magnetic state with in-plane antiferromagnetic ordering in MnFeGe to a collinear ferromagnetic (FM) spin arrangement in MnFe0.2Co0.8Ge. We utilize Lorentz transmission electron microscopy, neutron diffraction experiments, and micromagnetic simulations to demonstrate the role of competing magnetic exchange-induced in-plane magnetic moment in the formation of nontopological type-II magnetic bubbles in the system. However, skyrmions with degenerate helicity states are found to be stable magnetic entities in the case of the perfect uniaxial magnetic anisotropy system. The present study offers a great opportunity to tune the skyrmion phase in centrosymmetric magnets by intrinsically designing the magnetic ground state, thereby providing a unique platform for realizing skyrmion-based spintronics devices.

Quantum geometric spin frustration of antiferromagnetic CuFeO2 enables photocatalytic applications

Photocatalysis, containing a potential solution to the global crises of energy shortage and the greenhouse effect, has been studied extensively in recent years. The CuFeO2 (abbreviated as CFO) is a material with a Delafossite structure, and has also been widely studied because of its triangular lattice antiferromagnets. The triangular lattice composed of Fe3+ ions, along with strong antiferromagnetic spin coupling between the Fe3+ ions, leads to a geometric spin frustration. Furthermore, CFO is also a potential candidate for the photocatalytic material due to its narrow band gap of 1.1–1.6 eV, electrochemical stability in an aqueous environment, high charge carrier mobility, flat-band potential positioned at ∼ 1 eV, and contains an abundance of raw materials in the earth. In this study, we successfully grew the size and highly-quality CFO single crystals using the optical floating-zone technique. Powder x-ray diffraction (XRD) and Rietveld refinement indicate that the CFO is a rhombohedral structure with a space group of R3̅m, consisting of edge-shared FeO6 octahedron to form the FeO2 layer, and the Cu cations occupied between the two FeO2 layers. The core-level electron energy loss (EEL) spectra found the valence of Fe and Cu in CFO is +3 and +1, respectively. The low-loss EEL spectrum observed two volume plasmons at ∼5 eV and ∼23 eV contributed from the collective oscillation of Cu-O antibonding states σCu−O* and mixed nonbonding Cu 3dxy, σCu−O bonding states, and Fe-O, respectively, based on the density function theory calculation. The optical absorption spectrum measured the energy bandgap for CFO is about 1.5 eV. The Raman spectrum shows significant Eg and A1 g vibration modes corresponding to the in-plane Fe-O and out-of-plane Cu-O vibrations, respectively. Both magnetic susceptibility, χ(T), with the applied magnetic field along the c-axis and specific heat capacity reveal the two magnetic phase transition temperatures at low temperatures: TN1 = 13.5 K when transition from paramagnetic (PM) phase to partially disordered antiferromagnetic, and TN2 = 10.5 K when complete transition from partially disordered antiferromagnetic to antiferromagnetic phase. The additional calculation of magnetic entropy change is 0.15, which is roughly close to 0.2, meaning that only 1/5 of the Fe3+ spins are disordered when transforming from PM states to partially disordered at T < TN1 in this study. The presence of instantaneous ferromagnetic spin arrangement (↑↑↓) extends the residence time of H2O on the surface of CFO, facilitating electron exchange with CFO and promoting photocatalytic water splitting reactions.

Spin-orbit coupling and electronic properties in Pt2MnGa: an ab-initio study

The electronic and magnetic properties of full-Heusler alloy Pt2MnGa with and without the effect of spin–orbit coupling are studied. The calculations have been carried out using ab initio density functional theory. Both the magnetic spin orders of Pt2MnGa, ferromagnetic and antiferromagnetic, are considered. It is found that the ferromagnetic spin arrangement is the most stable spin order at the ground state, regardless of the incorporation of spin–orbit coupling. The density of states and band structure plots are used to validate the obtained ground state structure, which is further validated by the Bader charge analysis and the charge density distribution of the individual atoms. The obtained results of magnetocrystalline anisotropy energy, total magnetic moment, Curie temperature, and the spin-polarization hint at the possible application of this compound in spintronics devices such as bit-patterned media.

Out-of-plane weak ferromagnetism at room temperature in lattice-distortion non-collinear antiferromagnet of single-crystal Mn3Sn

Out-of-plane weak ferromagnetic (OWFM) spin arrangements with topological properties can realize a series of interesting physical properties. However, this spin structure tends to exist at low temperatures. The OWFM structure can also be induced at room temperature by hydrostatic pressure, whereas this isotropic approach tends to form helical AFM structures. We report the OWFM spin arrangement in single crystal Mn3Sn by an anisotropic strategy of high-stressconstrained compression deformation at room temperature. Both experimental and theoretical simulation results show that the alignment of the OWFM spin structure is due to the distortion of the atomic scale caused by the strain energy during deformation. The OWFM spin arrangement can significantly change the magnetic property of Mn3Sn. As a result, the remanent magnetization M r for the deformed sample (0.056 μ B/f.u.) is about eleven times that for the pre-deformed sample (0.005 μ B/f.u.), and the coercivity (H c) increases from 0 kOe (pre-deformed sample) to 6.02 kOe (deformed sample). Our findings provide a way to generate the OWFM spin structure at room temperature and may give fresh ideas for creating antiferromagnetic materials with excellent physical properties.

Non-chiral spin frustration versus highly degenerate ferromagnetic state with local chiral degrees of freedom of an exactly solvable spin-electron planar model of inter-connected trigonal bipyramids

The frustration phenomenon in an exactly solvable spin-electron planar model constituted by identical bipyramidal plaquettes is discussed within the Toulouse’s and dos Santos and Lyra’s frustration concepts. It is shown that the ground state of the model contains the unfrustrated spontaneously ordered quantum ferromagnetic phase with local chiral degrees of freedom in the electron sub-lattice and the disordered quantum one, where both the Ising and electron sub-lattices are frustrated. The frustration of the latter sub-lattice persists at finite temperatures, but only in the disordered region. It finally vanishes at a certain frustration temperature. The reentrant behaviour of the frustration in the electron sub-lattice with three consecutive frustration temperatures due to a competition with the unfrustrated ferromagnetic spin arrangement near the ground-state phase transition can also be observed.

Carrier-induced antisymmetric–symmetric tendencies of spin stiffness in zigzag graphene nanoribbons

The generalized Bloch theorem was applied to calculate the spin stiffness and to consider its tendencies when introducing the doping in zigzag graphene nanoribbons. To reach the intentions, two different flat spin spiral formations were constructed by fixing the ferromagnetic and antiferromagnetic spin arrangements at the two different edges by applying a constraint scheme method. A spin stiffness was then calculated by means of a quadratic order function, which maps the total energy difference in the self-consistent calculations onto the Heisenberg Hamiltonian. We found a very high spin stiffness as predicted previously by the supercell calculation. We also showed that the antisymmetric–symmetric tendencies of spin stiffness are induced by the hole–electron doping. The dependence of ribbon widths of zigzag graphene nanoribbon on the spin stiffness was also provided with similar tendencies when the doping is taken into account.

First-principle study of metal-organic frameworks of the 4d and 5d transition metal series with phthalocyanine and tetracyanobenzene

Abstract In the present work, the structural, electronic and magnetic properties of planar frameworks built of Tetracyanobenze (TCNB) and Phthalocyanine (Pc) with transition metals (TM) of the 4d and 5d series (TM = Mo, Ru, Rh, Pd, Ag, Cd, Re, Ir, Pt, and Hg) have been investigated by density functional theory (DFT) including Hubbard-like Coulomb correction terms. In that series, ferromagnetism has been found for Ru-Pc, Re-Pc, Mo-TCNB and Re-TCNB which all are also at least half-metallic with the exception of Mo-TCNB being insulating. The ferromagnetic state is very stable for Mo-TCNB, Re-TCNB, and especially Ru-Pc with an energy difference between ferromagnetic and antiferromagnetic spin arrangements surpassing 100 meV per magnetic ion for Ru-Pc. The prediction of ferromagnetic and half-metallic metal-organic frameworks provides new insight into Pc/TCNB-based nanostructures with potential applications in spintronics.

Dependence of characteristic interlayer vibration modes on interlayer spin arrangement in stacked graphene nanofragments

The freedom of interlayer spin arrangement could emerge when assembling small-sized graphene-based nanostructures through stacking. Its possible influence on interlayer vibrations could be significant for developing Raman probes towards small complexes and device design involving intermolecular vibronic process. In this paper, using density-functional-theory calculations on bilayer and trilayer rhombic graphene fragments, we compare finite-size analogues to interlayer shear and breathing modes regarding to ferromagnetic and antiferromagnetic interlayer spin arrangements. For bilayer units, the antiferromagnetic configuration further separate two shear mode components by around 10 cm−1, inducing a further Raman peak splitting. This response is robust regarding to slightly enlarging the fragment and increasing layer number to three. Layer breathing mode is spin-arrangement robust in bilayers but show different arrangement-sensitivity in ABA-like and ABC-like trilayers. Visualization of interface electron density reveals the dependence of anisotropic interface environment to interlayer spin arrangement and unambiguously correlates the arrangement style with shearing responses. Further application of this analysis clarifies the arrangement-based phase-cooperation in trilayer shearing. Results and analysis in this work may contribute to the characterization and design of graphene-based nanostructures as well as understanding the interfacial nature beneath their intermolecular varieties in dynamic processes.

Structural, magnetic, and electron-transport properties of epitaxial Mn2PtSn films

The growth of new magnetic materials on suitable insulating substrates is an important part of the development of spin-electronics devices for memory or information processing. Epitaxial thin films of Mn2PtSn were grown on a MgO [001] substrate by magnetron co-sputtering of the constituents. Structural, magnetic, and electron-transport properties were investigated. The epitaxial Mn2PtSn film has an inverse tetragonal structure with the c-axis aligned in the plane of the MgO substrate. The lattice constants determined using XRD and TEM analysis are c = 6.124 Å and a = b = 4.505 Å. The orientation of Mn2PtSn c-axis which is 45° away from the a-axis of MgO has resulted in a small lattice mismatch of about 2.8%. The measured saturation magnetization is 5.3 μB/f.u., which is smaller than the first-principles calculated value of 6.4 μB/f.u. for ferromagnetic spin arrangement. Magnetization measurements determined the bulk magnetocrystalline anisotropy constant Kv of about 11.3 Merg/cm3 (1.13 MJ/m3). The electron-transport behavior is similar to that of normal magnetic metals. These results indicate that Mn2PtSn may have promising applications in spintronic devices.

Site preference and compensation behavior in Co(Cr, Mn)2O4 system

Site preference of doped Mn ions in CoCr2−xMnxO4 (x = 0–2) series has been derived separately from structure and magnetic measurement. It shows that parts of the doped Mn ions occupy the A (Co) sites when x &amp;lt; 0.5. And then, it takes the two B (Cr) sites in turn before and after x = 1.3. This site preference behavior results in a role conversion of the magnetic contributors and, thus, leads to the composition dependent magnetic compensation. Temperature induced compensation and negative magnetization have also been found in several samples, which is attributed to the large energy barrier between the ferromagnetic and antiferromagnetic spin arrangement. A structure transition from cubic to tetragonal symmetry has been detected.

On the Nature of the Magnetism-Promoting States in Dilute Magnetic Semiconductor and Oxide Thin Films

Ferromagnetic (FM) ordering is observed in intermediate and wide gap dilute magnetic semiconductors as well as in oxides. While the interpretation of the experimental results is often clouded by the existence of non-homogeneous and non ideal nanostructures related to the fact that impurity concentration tends to far exceed the thermodynamic solubility limit, a general physical picture as to the physical origin of the FM interactions has emerged. We discuss the physical mechanism of ferromagnetism mediated by the carriers. We show that what stabilizes the FM spin arrangement is the energy-lowering due to interaction between partially occupied states in the band gap, localized on different transition atoms. These partially occupied states are hybrids between the d impurity band states and host vacancy orbitals, never host-like states as imagined in model Hamiltonian approaches. The theory uses both the model and first principle approach and can be applied to various types of systems such as dilute magnetic semiconductors [(Ga, Mn)As, (Ga, Mn)N, etc.] and oxides [(Ti, Co)O2, (Zn, Mn)O, etc.] as well as nanodevices prepared of these materials.

Spin, charge and lattice couplings in Cu-deficient oxysulphide BiOCu0.94S

The electrical and magnetic properties of slightly Cu-deficient BiOCu0.94S are investigated using neutron diffraction, ac magnetic susceptibility, magnetization and electrical resistivity measurements. The Cu spins order in a ferromagnetic arrangement below TC = 250 K. An antiferromagnetic component develops below 180 K when the crystalline unit cell experiences a sharp thermal contraction upon cooling, resulting in a canted ferromagnetic spin arrangement at low temperatures. In the magnetically ordered state the electrical transport can be described using three-dimensional variable range hopping conduction. An applied magnetic field can effectively reduce the hopping barrier. Spin–charge couplings are clearly revealed when the resistivity departs from the hopping conduction and begins to increase with increasing temperatures above 250 K where the Cu spins become disordered.

Crystal and magnetic structure of theR15Si9C compounds(R = Ho, Er, Tb)

The synthesis of the new compoundsR15Si9C withR = Sm, Gd–Er,Y and R15Ge9C with R = Ce, Pr and Nd has been recently reported; these compounds crystallize in the hexagonalLa15Ge9Fe structuretype, hP50-P63mc,Z = 2 (orderedsuperstructure of La5Ge3 (Mn5Si3-type, hP 16-P63/mcm,Z = 2)). Here we report the results of a neutron diffraction investigation thatwe have performed to study the crystal and magnetic structures of theR15Si9C compoundswith R = Tb, Ho and Er.All three compounds see the establishment of commensurate magnetic order with a predominantlyferromagnetic interaction. Details of mixed antiferro–ferromagnetic spin arrangements (κ = [000]) (forTb15Si9C andHo15Si9C) or of purelyferromagnetic ordering (Er15Si9C), and of their temperature dependence, are given and linked to the different coordination ofthe four dissimilar rare earth sites. In the Tb and Ho compounds the thermal evolution ofthe magnetic moment values strongly differs between the different R sites. The positionoccupied by the principal carbon has been determined (Wyckoff site 2b) and the existence of asecond position available for the interstitial carbon (Wyckoff site 2a) has been revealed forR = Ho, Tb. Moreover, in the Tb and Ho compounds the magnetic moment value of the rare earth siteR4, surrounding the second interstitial carbon site, is strongly reduced if compared to the valueon the other rare earth sites. The magnetic transition temperatures of all three compounds, i.e.TC = 130, 43 and45 K for Tb15Si9C, Ho15Si9C and Er15Si9C, are remarkably high compared to those of the parentR5Si3 compounds. The magnetic behaviour of the partly filledTb5Si3C0.25 is reported.

Interplay between the Crystalline and Magnetic Structures in BiOCu0.94S

We report on the results of neutron diffraction studies made on a slightly Cu-deficient BiOCu 0.94 S sample. The sample BiOCu 0.94 S crystallizes into a tetragonal P4/nmm symmetry with cell parameters of a = b =3.8591 A and c =8.5540 A at 250 K. The magnetic order parameter measurements reveal an inflection point at 250 K, signaling the ferromagetic ordering of the Cu spins. An antiferromagnetic component develops below 180 K, when the crystalline unit cell experiences a sharp thermal contraction upon cooling. This results in a canted ferromagnetic spin arrangement at low temperatures, with a saturated moment of =0.61(2) µ B at 3.7 K.

Magnetic interaction in pairwise Mn-doped Si nanocrystals

The electronic-structure and magnetic properties of hydrogenated silicon nanocrystals doped with pairs of manganese atoms are investigated using spin-density-functional theory. Formation energies and total magnetic moments sensitively depend on the two sites occupied by manganese. Usually pairs at interstitial and substitutional sites with small total moment are energetically favored. Pairs at sites with the same character tend to ferromagnetic spin arrangements which are, however, significantly influenced by their noncollinearity. The resulting magnetic ordering is clearly related to the impurity levels and their occupation. The magnetic coupling is distance dependent, antiferromagnetic for small distances, and almost ferromagnetic for larger Mn-Mn distances. A Rudderman-Kittel-Kasuya-Yoshida-type exchange mechanism may describe the distance dependence but simultaneously not its magnitude.

Magnetic and nuclear structure and thermal expansion of orthorhombic and monoclinic polymorphs of CoGeO3 pyroxene

CoGeO3 was synthesized at 1,273 and 1,448 K using ceramic sintering techniques in the monoclinic and orthorhombic modification, respectively. The two compounds were analysed by magnetic susceptibility measurements and neutron diffraction in order to study magnetic ordering and spin structures at low temperature. The monoclinic form of CoGeO3 has C2/c symmetry and orders magnetically below 36 K with a small negative paramagnetic Curie temperature θ P = −4.6 (2) K. The magnetic structure can be described with k = (1, 0, 0) in the magnetic space group C2′/c′ having a ferromagnetic spin arrangement within the chains of M1 sites, but a dominating antiferromagnetic coupling between the chains. At the M1 sites the magnetic spins are aligned within the a–c plane forming an angle of 120° with the +a-axis and they are not parallel to the spins at M2. Here spins are also ferromagnetically coupled within, but antiferromagnetically coupled between the M1/M2 site bands. The orthorhombic phase of CoGeO3 displays Pbca symmetry and transforms to an antiferromagnetically ordered state [θ P = −18.6(2) K] below 33 K. The magnetic spin structure can be described with k = (0, 0, 0) in space group Pbca′ and it is similar to the one of the C2/c phase except that it is non-collinear in nature, i.e. there are components of the magnetic moment along all three crystallographic axes. Small magneto-elastic coupling is observed in the orthorhombic phase.

Low temperature magnetic properties and spin structures of Ising Spin System with two-Spin and four-Spin interactions

The magnetic properties of the spin S (=3/2 and 2) Ising systems with the bilinear exchange interaction J1SizSjz and the four-site four-spin interaction J4SizSjzSkzSlz are discussed by making use of the Monte Carlo (MC) simulation for the magnetization <Sz> and the magnetic specific heat CM and spin structures. The ferromagnetic spin arrangement ordered by the positive interaction J1 is destroyed and changed into lots of complicated spin structures with low energy by introducing negative four-spin interaction J4. Near the phase transitions of J4/J1 = -2/9 and -1/8 for S = 3/2 and 2, respectively, the thermal averages <Sz>, <SizSjzSkzSlz> and the specific heat CM show characteristic temperature dependences in the low temperature region. The change of spin structures of the system with the variation of the values of J1 and J4 are also investigated by using the result of the MC simulation and by the comparison of energies of the possible ground state spin structures.

Influence of spin arrangement on thermopower in Zn xCu yCr zSe 4 spinels

The thermopower analysis carried out on Zn x Cu y Cr z Se 4 spinel single crystals show that the ferrimagnetic spin arrangement in the range 0.49 ≤ y ≤ 0.72 makes easier the carrier diffusion from one side and hinders both the spin wave propagation and the transfer of the phonon momentum to the electron gas from the other. The opposite behaviour for the ferromagnetic spin arrangement in the range 0.81 ≤ y ≤ 0.95 was observed. In particular, the maxima contributions to the total thermopower give a magnon component (9 μV/K) of ferromagnetic samples in the temperature range 25–100 K, a phonon component (20 μV/K) of ferromagnetic samples in the temperature range 150–200 K and a diffusion component (47 μV/K) of ferrimagnetic samples in the temperature range 250–400 K. The impurity component of thermopower does not significantly depend on a kind of spin coupling and it increases with increasing non-stoichiometry of a sample ( x + y + z ≠ 3). Its maximum value does not exceed 2.5 μV/K at 400 K.

Ultrafast Photoinduced Formation of Metallic State in a Perovskite-type Manganite with Short Range Charge and Orbital Order

Femtosecond reflection spectroscopy was performed on a perovskite-type manganite, Gd0.55Sr0.45MnO3, with the short-range charge and orbital order (CO/OO). Immediately after the photoirradiation, a large increase of the reflectivity was detected in the mid-infrared region. The optical conductivity spectrum under photoirradiation obtained from the Kramers-Kronig analyses of the reflectivity changes demonstrates a formation of a metallic state. This suggests that ferromagnetic spin arrangements occur within the time resolution (ca. 200 fs) through the double exchange interaction, resulting in an ultrafast CO/OO to FM switching.

Magnetism of a vanadium monolayer on Ag(100): Experiment versus theory

We have studied the magnetic and structural properties of a vanadium monolayer deposited on an Ag(100) surface. Several theoretical studies have predicted an enhanced magnetic moment on vanadium and ferromagnetic order for such a film. No magnetic signal has been detected by means of X-ray magnetic circular dichroism. This result rules out any possible ferromagnetic spin arrangement. We have used scanning tunnelling microscopy in order to find an origin for the discrepancy between theoretical and experimental results. STM images of a vanadium deposition corresponding to 1 monolayer thickness reveal that vanadium grows in an island mode and the monolayer structure cannot be stabilized. This is in clear contrast with theoretical calculations that has been performed assuming an ideal vanadium monolayer slab.