Ferromagnetic Arrangement Research Articles

Magnetic Properties of the Ordered Double Perovskite Sr2MnTeO6

AbstractThe ordered double perovskite Sr2MnTeO6 was prepared by the freeze‐drying method. The nuclear and magnetic structures were determined by both X‐ray and neutron (D2B and D1B) powder diffraction. A distorted, double perovskite structure type described by the monoclinic P121/n1 space group is observed at room temperature and down to 4 K. The room temperature EPR spectrum shows an isotropic signal centered at a g value of 1.998 indicating that manganese ions are in the +2 oxidation state. At low temperature the EPR signal broadens as the exchange and dipolar interactions between Mn2+ cations increase. Below 25 K no signal is observed, indicating that the sample is magnetically ordered. The dc magnetic susceptibility shows the existence of antiferromagnetic interactions with an ordering temperature around 20 K. A sharp λ‐type anomaly observed at 19 K in the specific heat curve and refinement of low‐temperature neutron diffraction data confirm the presence of a three‐dimensional, antiferromagnetic ordered structure with an ordering temperature of ca. 23 K. The propagation vector of the magnetic structure is k = [0,0,0], yielding a type I antiferromagnetic ordering between Mn2+ (S = 5/2) cations. At 2 K, each manganese ion is antiferromagnetically aligned to 8 out of 12 nearest‐neighbors at an approximate distance of 5.6 Å. This magnetic structure can be viewed as being composed of ferromagnetic arrangement of Mn2+ moments in the ab plane, coupled antiferromagnetically along the c axis. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

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.

First-principles study of ultrathin magnetic Mn films on W surfaces.

The structural and magnetic properties of static compact thin Mn films deposed on W(100) and W(110) substrates have been investigated in the framework of the density functional theory. In good agreement with experiments, pseudomorphically adsorbed Mn films up to three monolayers (ML) are found to be stable despite the huge tensile strain induced by the epitaxial conditions. For $\mathrm{Mn}∕\mathrm{W}(110)$ our results demonstrate a $c(2\ifmmode\times\else\texttimes\fi{}2)$ in-plane antiferromagnetic order for thicknesses up to 3 ML, with a degenerated layered antiferromagnetism in the (110) planes for 3 ML thick $\mathrm{Mn}∕\mathrm{W}(110)$ films. For $\mathrm{Mn}∕\mathrm{W}(110)$ our calculations predict a ferromagnetic arrangement in 1 and 2 ML thick films stabilized by the hybridization with the substrate, with a transition to an antiferromagnetism layered in (100) planes at higher thickness. The effects induced by the reduced dimensionality are discussed based on the comparison with bulk Mn and free-standing Mn ML. The magnetic contribution to the adsorption is determined by comparing magnetic and nonmagnetic calculations.

Magnetic properties of MnN: Influence of strain and crystal structure

For manganese mononitride (MnN), the total energy versus lattice constant is obtained using the spin density functional theory. Instead of the tetragonally distorted NaCl structure, we study the zinc blende and wurtzite structures in which AlN, GaN, and InN crystallize. The ground state with nonmagnetic, antiferromagnetic (AFM), or ferromagnetic (FM) arrangement of spins depends on the polymorph of MnN and on the lattice constant. At equilibrium lattice constants, in zinc blende it is AFM in [100] direction, and in wurtzite it is FM. The zinc blende polytype of MnN under hydrostatic pressure at the InN lattice constant presents FM ground state. For the wurtzite polytype at the GaN and AlN lattice constants, the AFM is the ground state, but goes back to a FM ground state for the InN lattice constants. For both structures, the system presents a half-metallic state at InN lattice constants (with a total magnetic moment of 4μB per Mn atom) instead of the metallic state obtained for smaller lattice constants. Results indicate that the FM or the AFM state of Ga1−xMnxN and In1−xMnxN may be related to, relaxed, or strained, MnN incorporations or Mn-rich composition fluctuations.

Monatomic Fe and Co wires at the Pt surface step edge: theory of the unconventional magnetic anisotropy

We report a first-principles investigation of the anomalous magnetic anisotropy of quasi-one-dimensional ferromagnetic Fe and Co chains decorating the Pt(111) step edge. Our calculations for both Fe and Co wires show that the easy magnetization is always confined in the plane perpendicular to the wire. The symmetry breaking at the step leads to an easy magnetization axis of the Co wire at an odd angle of ∼20∘ towards the Pt step, in agreement with the experiment. This canting becomes more pronounced for the Fe wire, for which we obtain an easy axis at 30∘ towards the Pt step. Also, the Fe and Co spin and orbital moments become noncollinear, even in the case of a collinear ferromagnetic spin arrangement.

Incommensurate Magnetic Ordering inCu2Te2O5X2(X=Cl,Br) Studied by Neutron Diffraction

We present the results of the first neutron powder and single crystal diffraction studies of the coupled spin tetrahedra systems Cu2Te2O5X2 (X = Cl,Br). Incommensurate antiferromagnetic order with the propagation vectors kCl approximately [0.150,0.422,1/2], kBr approximately [0.158,0.354,1/2] sets in below TN = 18 K for X = Cl and 11 K for X = Br. No simple collinear antiferromagnetic or ferromagnetic spin arrangements within Cu2+ tetrahedra fit these observations. Fitting the diffraction data to more complex but physically reasonable models with multiple helices leads to a moment of 0.67(1)microB/Cu2+ at 1.5 K for the Cl compound. The reason for such a complex ground state may be geometrical frustration of the spins due to the intratetrahedral and intertetrahedral couplings having similar strengths. In neither compound has any evidence for a structural transition accompanying the magnetic ordering been found.

Influence of the structure on electric and magnetic properties of La 0.8Na 0.2Mn 1−xCo xO 3 perovskites

The influence of the cobalt substitution for manganese ions in the mixed valence perovskites La 0.8Na 0.2Mn 1− x Co x O 3 (0⩽ x⩽0.2) was investigated by X-ray, electric transport and magnetic measurements. The study carried out on sintered polycrystalline samples revealed the rhombohedral ( R 3 ¯ c ) structure and the insulator–metal transition connected with a ferromagnetic arrangement in the whole concentration range. Increasing concentration of cobalt ions leads to a gradual decrease of PM-FM and I–M transition temperatures. An influence of the cobalt ions on the observed behavior is attributed to charge compensation Mn 3+→Mn 4+ leading to the formation of stable couples Mn 4+–Co 2+. Therefore the double-exchange interactions Mn 3+–O 2−–Mn 4+ partly vanish and they are replaced by positive superexchange interactions Mn 4+–O 2−–Co 2+, but of a semiconducting character.

Neutron Scattering as a Powerful Tool in Solid State Chemistry. Examples from the Investigations in Crystal Structures and Properties of Metal Hydrogen and Metal Chalcogen Compounds

AbstractNeutron scattering experiments play a key role in the investigation in crystal structures and properties of solid compounds which is demonstrated with examples from the compound series AxMyXz (A ≙ alkali metal, M ≙ transition metal and X ≙ H, S or Se). Measurements on powdered samples provide as well the hydrogen positions within the crystal structures of ternary hydrides by using the analogous deuterium compounds as the lithium positions in the compounds with A ≙ Li. Ferromagnetic and antiferromagnetic arrangements are investigated by neutron diffraction which lead to the determination of the magnetic moments of transition metal atoms providing information about chemical bonding. Finally, it is demonstrated that inelastic neutron scattering is an independent method used in a complementary way to infrared or Raman spectroscopy, and, due to the interactions of neutrons with the magnetic states of transition metal atoms, to susceptibility or magnetisation.

Crystal structure, 57Fe Mössbauer spectroscopy and magnetization of U xFe 6Sn 6 (0≤ x≤0.6)

The U x Fe 6Sn 6 (0≤ x≤0.6) intermetallics were prepared as polycrystalline samples by arc melting followed by annealing. They crystallize in the P6/ mmm space group as a lacunary variant of the HfFe 6Ge 6-type, with an ordered insertion of U within the Sn 8 voids of the FeSn framework, and are isostructural with the R x Fe 6Sn 6 compounds (R=Sc, Zr, Tm, Yb, Lu). 57Fe Mössbauer spectroscopy shows that the Fe magnetic moments, μ Fe, are already ordered at room temperature. Anisotropic contributions to the total hyperfine field at the Fe site indicates that μ Fe are perpendicular to the crystallographic c axis in contrast to the Sc, Zr and rare-earth (except Yb) containing compounds, where μ Fe are directed along the c axis. The values deduced for the angles between the main axes of the electric field gradient and μ Fe further show that, within each (001) plane, μ Fe have a ferromagnetic arrangement similar to that in FeSn and Yb 0.6Fe 6Sn 6. In U x Fe 6Sn 6, the hyperfine fields increase with U content. Magnetization data in the 2–300 K show small ferromagnetic components for the U 0.4Fe 6Sn 6 and U 0.6Fe 6Sn 6 compositions.

Specific-heat study of the spin-structural change in pyrochloreNd2Mo2O7

By measurements of specific heat, we have investigated the magnetic field $(H)$ induced spin-structural change in ${\mathrm{Nd}}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{7}$ that shows intriguing spin-chirality-related magnetotransport phenomena. With increasing $H$, a broad peak around $2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ caused by the ordering of 2-in 2-out structure of the Nd moments shifts to the lower temperature $(T)$ up to around $3\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ and then to the higher $T$ above around $3\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ irrespective of the $H$-direction. This is due to the crossover from antiferromagnetic to ferromagnetic arrangement between the Nd and Mo moments. While the peak $T$ increases monotonically above $3\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ for $H\ensuremath{\Vert}[100]$, another peak emerges around $0.9\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ at $12\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ for $H\ensuremath{\Vert}[111]$, which is ascribed to the emergence of 3-in 1-out structure. For $H\ensuremath{\Vert}[110]$, a spikelike peak is observed at around $3\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, which is caused perhaps by some spin flip transition.

Electronic structure and exchange interactions inBaVS3

The electronic structure and spin exchange interactions of a quasi-one-dimensional sulfide ${\mathrm{BaVS}}_{3}$ have been studied within the density functional theory $(\mathrm{DFT})$ with the generalized gradient approximation $(\mathrm{GGA})$ using the projected augmented wave $(\mathrm{PAW})$ method. The on-site Coulomb repulsion has also been taken into account in the $\mathrm{GGA}+U$ approach to unravel the correlation effects on the electronic structure. We found that in the high-temperature hexagonal structure the partially filled ${t}_{2g}$ conduction bands including the broad $d(3{z}^{2}\ensuremath{-}{r}^{2})$ and two narrow $e({t}_{2g})$ orbitals contribute to the conductivity of ${\mathrm{BaVS}}_{3}$, suggesting a weak anisotropy in the electronic transport. Furthermore, our $\mathrm{GGA}+U$ band structure calculations predict an insulating ground state with a narrow gap of $0.15\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for ${\mathrm{BaVS}}_{3}$ in the low-temperature $(60\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ orthorhombic structure, thereby indicating the Mott-Hubbard mechanism for the low-temperature insulating behavior. Our total energy analyses of the ground-state spin configuration in the low-temperature phases show that the magnetic structure with an intrachain ferromagnetic spin arrangement and an interchain antiferromagnetic spin arrangement that is consistent with a superexchange mechanism through sulfur $3\phantom{\rule{0.3em}{0ex}}p$ orbitals is preferred. We have also estimated the values of exchange integrals by mapping the calculated total energies onto the Heisenberg spin model. It is found that the stronger exchange interaction is ferromagnetic intrachain coupling, while the weaker antiferromagnetic interchain coupling occurs along the $a$ axis. A substantial magnetic anisotropy is found and results in the observed quasi-1D magnetic character in ${\mathrm{BaVS}}_{3}$. This suggests the separation of the spin and charge degrees of freedom as a possible cause of the anomalous properties, e.g., the 3D electronic transport and the quasi-1D magnetism of ${\mathrm{BaVS}}_{3}$.

Anomalous ferromagnetism of a monatomic Co wire at the Pt(111) surface step edge

A first-principles investigation of the anomalous ferromagnetism of a quasi-one-dimensional Co chain at the Pt(111) step edge is reported. Our calculations show that the symmetry breaking at the step leads to an easy magnetization axis at an odd angle of $\sim20^{\circ}$ {\em towards} the Pt step, in agreement with experiment [P. Gambardella {\em et al.}, {\em Nature} {\bf 416}, 301 (2002)]. Also, the Co spin and orbital moments become noncollinear, even in the case of a collinear ferromagnetic spin arrangement. A significant enhancement of the Co orbital magnetic moment is achieved when modest electron correlations are treated within LSDA+$U$ calculations.

Magnetic field induced absorption in Pb xEu1-xTe magnetic semiconductors

We report an investigation of the optical absorption spectrum, using non-polarized light, in Pb xEu1-xTe, x=0 and x=0.095, epitaxial thick layers grown by molecular beam epitaxy (MBE). The absorption edge is described by a broad band, due to the electronic transitions from the 4f7 of Eu2+ to the states in 4f6 5d configuration, as seen previously in bulk Eu chalcogenides. When a magnetic field is applied, a narrow absorption band (full width ~50 meV) emerges from the broad one. The energy of this absorption peak red shifts when the magnetic field increases, and reaches saturation when the Eu2+ attain ferromagnetic arrangement. This behaviour can be described by a localized excitation model with d - f exchange interaction.

Frustrating interactions and broadened magnetic excitations in the edge-sharingCuO2chains inLa5Ca9Cu24O41

Inelastic neutron scattering measurements have been performed on ${\mathrm{La}}_{5}{\mathrm{Ca}}_{9}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41},$ which consists of edge-sharing ${\mathrm{CuO}}_{2}$ chains and shows an antiferromagnetic long-range order below ${T}_{N}=10.5\mathrm{K}$ with ferromagnetic arrangement within the chain. Although the intrachain coupling is expected to be ferromagnetic from the geometrical structure, it is found that the interactions are weakly antiferromagnetic both parallel and perpendicular to the chain and there exist frustrating interactions between them. It is also found that magnetic excitations are broadened significantly. The broadening is probably enhanced by the frustration and a disorder originating from a structural distortion and doped holes although the broadening at finite Q is expected even in the pure system without frustration and disorder.

Ferromagnetism in Mn-doped GaAs due to substitutional-interstitial complexes

While most calculations on the properties of the ferromagnetic semiconductor GaAs:Mn have focussed on isolated Mn substituting the Ga site (Mn$_{Ga}$), we investigate here whether alternate lattice sites are favored and what the magnetic consequences of this might be. Under As-rich (Ga-poor) conditions prevalent at growth, we find that the formation energies are lower for Mn$_{Ga}$ over interstitial Mn (Mn$_i$).As the Fermi energy is shifted towards the valence band maximum via external $p$-doping, the formation energy of Mn$_i$ is reduced relative to Mn$_{Ga}$. Furthermore, under epitaxial growth conditions, the solubility of both substitutional and interstitial Mn are strongly enhanced over what is possible under bulk growth conditions. The high concentration of Mn attained under epitaxial growth of p-type material opens the possibility of Mn atoms forming small clusters. We consider various types of clusters, including the Coulomb-stabilized clusters involving two Mn$_{Ga}$ and one Mn$_i$. While isolated Mn$_i$ are hole killers (donors), and therefore destroy ferromagnetism,complexes such as Mn$_{Ga}$-Mn$_i$-Mn$_{Ga}$) are found to be more stable than complexes involving Mn$_{Ga}$-Mn$_{Ga}$-Mn$_{Ga}$. The former complexes exhibit partial or total quenching of holes, yet Mn$_i$ in these complexes provide a channel for a ferromagnetic arrangement of the spins on the two Mn$_{Ga}$ within the complex. This suggests that ferromagnetism in Mn doped GaAs arises both from holes due to isolated Mn$_{Ga}$ as well as from strongly Coulomb stabilized Mn$_{Ga}$-Mn$_i$-Mn$_{Ga}$ clusters.

Influence of Co doping on properties of Pr 0.8Na 0.2Mn (1− y) Co yO 3 perovskites

The influence of the cobalt substitution for manganese ions in the series of the perovskites Pr 0.8Na 0.2Mn (1− x) Co x O 3 (0⩽ x⩽0.1) was investigated. The study of electric and magnetic properties was carried out on sintered polycrystalline samples. The composition of x=0.04 exhibits an insulator to metal-like (I–M) transition at ∼106 K, connected with a ferromagnetic arrangement. For x=0.1, however, an insulating behavior persists down to low temperatures in spite of the transition to the bulk ferromagnetism. The observed properties are related to an acting of the cobalt ions as point defects. They disturb the tendency to charge ordering and instead of the antiferromagnetic arrangement typical for x=0 ferromagnetic double-exchange interactions Mn 3+–O 2−–Mn 4+ and Mn 3.5+ δ –O 2−–Co 2+, decisive for the resulting behavior, arise.

Superconductivity in Non-adiabatic Ferromagnetic Alloys

The main equations of the theory of superconductivity in non-adiabatic systems with ferromagnetic arrangement of spins of impurity atoms are derived. The influence of non-adiabaticity on the temperature of superconducting transition TC as well as on the critical concentration of impurity is also studied. The behavior of the temperature of magnetic arrangement of spins Tk as function of concentration of impurity c in the superconducting state for the non-adiabatic systems is investigated as well. The phase diagram (T, c) is built and it is proved that the effects of non-adiabaticity favor a raise in the field with coexistence of superconductivity and ferromagnetism.

Insulator–metal transition in Nd 0.8Na 0.2Mn (1− x) Co xO 3 perovskites

The electric and magnetic properties of the perovskites Nd 0.8Na 0.2Mn (1− x) Co x O 3 (0⩽ x⩽0.2) prepared by the usual ceramic procedure were investigated. The insulator-to-metal-like (IM) transition, closely related to a ferromagnetic arrangement, was revealed for the composition of x=0.04 and a similar tendency was detected for x=0. The insulating behavior persists down to low temperatures for higher contents of cobalt ions in spite of the transition to the bulk ferromagnetism. The properties are interpreted in terms of the steric distortion, tilting of the Mn(Co)O 6 octahedra and the double-exchange interactions of the type Mn 3+–O 2−–Mn 4+and Mn 3.5+ δ –O 2−–Co 2+, respectively. Presence of antiferromagnetic domains in the ferromagnetic matrix for the most of cobalt-substituted samples is supposed.

Polarized neutron reflectometry studies of GaMnAs/GaAs superlattices

Polarized neutron reflectometry has been used to investigate details of spin ordering in ferromagnetic (FM) GaMnAs/GaAs superlattices. The reflectivity spectra measured below the Curie temperature reveal additional magnetic contributions to the structural superlattice Bragg peaks, clearly indicating the existence of FM interlayer correlations. Closer investigation of the magnetic reflectivity maxima using a full polarization analysis provides direct evidence that the FM order in the GaMnAs layers is truly long-range. Moreover, as shown by the data, the system exhibits a strong tendency of forming a single-domain FM arrangement, even when cooled through TC in zero external field.

Magnetic profile and interlayer exchange coupling in fccFen/Nim(001) superlattices

We have performed ab initio calculations to study the electronic and magnetic structures of fcc ${\mathrm{Fe}}_{n}/{\mathrm{Ni}}_{m}(001)$ superlattices. A ferromagnetic coupling between Fe and Ni atoms is always obtained at the interfaces for any value of $m$ or $n.$ The interlayer exchange coupling between adjacent ${\mathrm{Ni}}_{m}$ slabs through ${\mathrm{Fe}}_{n}$ spacer displays an oscillatory behavior which is strongly modified when mixed Fe-Ni interfaces are considered. The iron atoms close to the interface form an induced ferromagnetic arrangement whereas the atoms inside the fcc Fe film give rise to an antiferromagnetic order. The compositional ordering at the Fe/Ni interface introduces a $c(2\ifmmode\times\else\texttimes\fi{}2)$ type of antiferromagnetism in the Fe layers in the case of ferromagnetic interlayer exchange coupling.