Magnetic and structural study of Fe doped tin dioxide

The structural and magnetic properties of Fe 10 at% doped SnO₂ powders milled for different times have been investigated. XAS results demonstrate the dilution of Fe atoms in the rutile structure after 5 h of milling. Fe atoms have almost one oxygen vacancy near neighbour. At RT the sample presents the superposition of paramagnetic and ferromagnetic behaviours. When temperature decreases a progressive blocking process was observed. Below 100 K a field shift of hysteresis loops is evident indicating magnetic coupling between ferromagnetic/antiferromagnetic phases.     

Structures and magnetic behaviours of TiO2-Mn-TiO2 multilayers

The TiO2-Mn-TiO2 multilayers are successfully grown on glass and silicon substrates by alternately using radio frequency reactive magnetron sputtering and direct current magnetron sputtering. The structures and the magnetic behaviours of these films are characterised with x-ray diffraction, transmission electron microscope (TEM), vibrating sample magnetometer, and superconducting quantum interference device (SQUID). It is shown that the multi-film consists of a mixture of anatase and rutile TiO2 with an embedded Mn nano-film. It is found that there are two turning points from ferromagnetic phase to antiferromagnetic phase. One is at 42 K attributed to interface coupling between ferromagnetic Mn3O4 and antiferromagnetic Mn2O3, and the other is at 97 K owing to the interface coupling between ferromagnetic Mn and antiferromagnetic MnO. The samples are shown to have ferromagnetic behaviours at room temperature from hysteresis in the M-H loops, and their ferromagnetism is found to vary with the thickness of Mn nano-film. Moreover, the Mn nano-film has a critical thickness of about 18.5 nm, which makes the coercivity of the multi-film reach a maximum of about 3.965×10 2 T.     

Room temperature ferromagnetism behavior of Sn1−xMnxO2 powders

In this paper, we have investigated Mn-doped SnO₂ powder samples prepared by solid-state reaction method. X-ray diffraction showed a single phase polycrystalline rutile structure. The atomic content of Mn ranged from ∼0.8 to 5 at%. Room temperature M-H loops showed a ferromagnetic behavior for all samples. The ferromagnetic Sn_0.987Mn_0.013O₂ showed a coercivity H_c=545 Oe, which is among the highest reported for dilute magnetic semiconductors. The magnetic moment per Mn atom was estimated to be about 2.54 μ_B of the Sn_0.9921Mn_0.0079O₂ sample. The average magnetic moment per Mn atom sharply decreases with increasing Mn content, while the effective fraction of the Mn ions contributing to the magnetization decreases. The magnetic properties of the Sn_1−xMn_xO₂ are discussed based on the competition between the antiferromagnetic superexchange coupling and the F-center exchange coupling mechanism, in which both oxygen vacancies and magnetic ions are involved.     

Magnetic structure and phase diagram of Sr<Subscript>5</Subscript>Rh<Subscript>4</Subscript>O<Subscript>12</Subscript> frustrated Ising magnet

The magnetic structure of Sr₅Rh₄O₁₂ is based on Ising chains of rhodium ions with a variable valence, Rh³⁺-Rh⁴⁺. The ordering in the chains is assumed to be ferromagnetic. It has been shown that the magnetic structure and phase diagram of Sr₅Rh₄O₁₂ are well described in a model taking into account weak antiferromagnetic interactions between the nearest and next-nearest neighbors on the triangular lattice of ferromagnetic Ising chains. The ground state at low temperatures is the two-sublattice stripe phase; this phase in the magnetic field is transformed to the ferrimagnetic phase and, then, to the ferromagnetic phase. Small plateaus can be observed in the region of the transition from the ferrimagnetic phase to the ferromagnetic one.     

Room‐Temperature Insulating Ferromagnetic (Ni,Co)1+2xTi1−xO3 Thin Films

Insulating uniaxial room‐temperature ferromagnets are a prerequisite for commonplace spin wave‐based devices, the obstacle in contemporary ferromagnets being the coupling of ferromagnetism with large conductivity. It is shown that the uniaxial A_{1 + 2x}Ti⁴⁺_{1 ? x}O₃ (ATO), A = Ni²⁺,Co²⁺, and 0.6 < x ≤ 1, thin films are electrically insulating ferromagnets already at room temperature. The octahedra network of the ATO and the corundum and ilmenite structures are the same yet different octahedra‐filling proved to be a route to switch from the antiferromagnetic to ferromagnetic regime. Octahedra can continuously be filled up to x = 1, or vacated (?0.24 < x < 0) in the ATO structure. TiO‐layers, which separate the ferromagnetic (Ni,Co)O‐layers and intermediate the antiferromagnetic coupling between the ferromagnetic layers in the NiTiO₃ and CoTiO₃ ilmenites, can continuously be replaced by (Ni,Co)O‐layers to convert the ATO‐films to ferromagnetic insulator with abundant direct cation interactions.     

Interlayer coupling between out-of-plane magnetized multilayers across a thin antiferromagnetic spacer

The interlayer exchange coupling between Co/Pt perpendicular-to-plane magnetized layers across a thin IrMn spacer layer was experimentally studied. In contrast to earlier studies on interlayer coupling through antiferromagnetic NiO, which revealed an oscillatory coupling behavior as a function of NiO thickness, a ferromagnetic coupling was observed here in the range of IrMn thickness between 0.6 and 1.5 nm and antiferromagnetic between 1.5 and 2.5 nm. The antiferromagnetic coupling is attributed to an orange peel magnetostatic mechanism whereas the ferromagnetic coupling is attributed to an out-of-plane polarization of the antiferromagnetic IrMn layer induced by the interfacial exchange interaction with the adjacent out-of-plane ferromagnetic layers. Measurements of hysteresis loops versus temperature show that the coupling vanishes at 510 K for t_IrMn=1 nm. This critical temperature is far below the Néel temperature of bulk IrMn, but above the blocking temperature of IrMn/Co bilayers at such thickness. Using a one-dimensional model describing a partial domain wall in the antiferromagnet, we explain the coupling in terms of an out-of-plane tilt of the Mn moments at the IrMn/(Co/Pt) interfaces yielding a weak net polarization of the IrMn. Finally, the non-oscillatory decay of the coupling was attributed to the compensated spin structure of the IrMn in the parallel to the interfaces.     

Dzyaloshinsky-Moriya antisymmetric exchange coupling in cuprates: Oxygen effects

We reconsider the conventional Moriya approach to the Dzyaloshinsky-Moriya antisymmetric exchange coupling for a single Cu₁-O-Cu₂ bond in cuprates using a perturbation scheme that provides an optimal way to account for intra-atomic electron correlations, low-symmetry crystal field, and local spin-orbital contributions with a focus on the oxygen term. The Dzyaloshinsky vector and the corresponding weak ferromagnetic moment are shown to be a superposition of comparable and, sometimes, competing local Cu and O contributions. We predict the effect of oxygen staggered spin polarization in the antiferromagnetic edge-shared CuO₂ chains due to the uncompensated oxygen Dzyaloshinsky vectors. The polarization is perpendicular to both the main chain antiferromagnetic vector and the CuO₂ chain normal. The intermediate ¹⁷O NMR is shown to be an effective tool to inspect the effects of Dzyaloshinsky-Moriya coupling in an external magnetic field. In particular, we argue that the puzzling planar ¹⁷O Knight shift anomalies observed in the paramagnetic phase of the generic Dzyaloshinsky-Moriya antiferromagnetic cuprate La₂CuO₄ can be assigned to the effect of the field-induced staggered magnetization. Finally, we revisit the effects of symmetric spin anisotropy, in particular, those directly induced by the Dzyaloshinsky-Moriya coupling. The perturbation scheme generalizes the well-known Moriya approach and presents a basis for reliable quantitative estimates for the symmetric partner of the Dzyaloshinsky-Moriya coupling. In contrast to the conventional standpoint, the parameters of the effective two-ion spin anisotropy are shown to incorporate the contributions of a single-ion anisotropy for two-hole configurations at both Cu and O sites. The text was submitted by the author in English.     

Sr2FeMoO6: A Prototype to Understand a New Class of Magnetic Materials

We propose a new mechanism to explain the magnetic structure of a recently discovered magnetoresistive double perovskite oxide system, Sr₂FeMoO₆, with the help of detailed experimental and theoretical results. This model, based on a strong antiferromagnetic coupling between the local moment and the charge carriers arising from local hopping interactions, can give rise to ferromagnetic metallic as well as ferromagnetic insulating ground states. The relevance of this mechanism in understanding the magnetism in dilute magnetic semiconductors such as Ga_{1 − x} Mn _x As, is also discussed.     

Hydrogen-controlled interlayer exchange coupling in Fe/LaHx multilayers

Magneto-optic Kerr magnetometry and neutron reflectometry reveal that Fe layers exhibit magnetic exchange coupling through LaH_x spacer layers. Ferromagnetic and antiferromagnetic coupling is observed on multilayers of these materials depending on the thickness of the hydride layers, but without oscillatory behavior. Starting from metallic La dihydride spacer layers the effect of dissolving increasingly more hydrogen was examined. Sign and value of the coupling depend crucially on the hydrogen content x. The coupling can be inverted from antiferromagnetic to ferromagnetic and vice versa. These alterations are due to modifications of the electronic structure of the hydride. When the hydrogen absorption saturates the hydride layers become insulating and the exchange coupling is likely to disappear. In this final state the multilayers are always characterized by a very soft ferromagnetic rectangular hysteresis curve. Upon removal of the hydrogen to the initial concentration the original magnetic structure is restored.     

Investigations of a quasi two-dimensional Heisenberg antiferromagnetic system. Nondeuterated and deuterated alkyl-ammonium-tetrachloromanganate

By neutron diffraction and susceptibility measurements the crystallographic and magnetic structures of (CH₃NH₃)₂MnCl₄ and (CD₃ND₃)₂MnCl₄ have been investigated. These compounds belong to a system of quasi two-dimensional Heisenberg antiferromagnets. At 96K a structural first order phase transition from the tetragonal high-temperature structure to an orthorhombic low-temperature structure was found. A magnetic phase transition from a two-dimensional antiferromagnetic preorder to a three-dimensional magnetic order occurs at 44.5K. The three-dimensional magnetic structure is characterized by antiferromagnetic (MnCl₄)^2- — layers perpendicular to the c-axis, with a ferromagnetic coupling between interacting next nearest (MnCl₄)^2- layers. The magnetic moments of the Mn-ions lie in the antiferromagnetic planes.     

Pressure-induced weak moment in the two-dimensional antiferromagnet (C2H5NH3)2CuCl4

Abstract

The two-dimensional Heisenberg antiferromagnet (C₂H₅NH₃)₂CuCl₄ has the ferromagnetic intralayer exchange interaction, while the extremely weak interlayer exchange interaction is antiferromagnetic. Neutron scattering experiments under high pressures have been performed on this compound. We confirm that the spin structure changes around 1～2 GPa from the collinear alignment along the a-axis to a spin-canting one. The weak moment due to the canting is parallel to the c-axis. The results indicate that the ferromagnetic intralayer and the antiferromagnetic interlayer exchange interactions are maintained up to 1～2 GPa. Why the weak ferromagnetic moment along the c-axis occurs is due to a lowering of crystal symmetry by pressure.     

The electronic structure of polymeric complex Co(thiazole)2Cl2 studied by first-principle calculation

One-dimensional dihalide-bridged polymer Co(thiazole)₂Cl₂ has been studied with the self-consistent full-potential linearized augmented plane wave method (FP_LAPW) based on the density functional theory (DFT). Spin distributions in ferromagnetic and antiferromagnetic states of it have been obtained by the calculation. The electronic structure and magnetic coupling between tow cobalt (II) ions along chain are discussed.     

Stable atomic structure and magnetism of Pt–Cr binary surface alloys on Pt(0 0 1): First-principle calculations

The possibility of Pt–Cr surface alloys formation on Pt(0 0 1) was investigated and their magnetism was calculated by the full-potential linearized augmented plane wave (FLAPW) method with eight different atomic configurations. The most stable structure was calculated to be the Pt-segregated L1₂ ferromagnetic surface alloy. A₃B types (L1₂ or D0₂₂) were more stable compared to AB types (L1₀). It implies that the A₃B type surface alloys may be formed when depositing a monolayer of Cr on Pt(0 0 1). It was found from the total energy calculations that there exists a strong tendency of the Pt segregation. The segregation further stabilizes the surface alloy significantly. The work function of the most stable surface alloy was calculated to be 6.02 eV and the magnetic moment of the surface Cr was much enhanced to 3.3 μ_B. It is a quite interesting finding that the coupling between Cr and Pt atoms on the surface plane is ferromagnetic in the Pt-segregated L1₂ ferromagnetic surface alloy, while the coupling is antiferromagnetic in the bulk.     

Magnetic properties of zig-zag ladders

We analyze the phase diagram of a system of spin-1/2 Heisenberg antiferromagnetic chains interacting through a zig-zag coupling, also called zig-zag ladders. Using bosonization techniques we study how a spin-gap or more generally plateaux in magnetization curves arise in different situations. While for coupled XXZchains, one has to deal with a recently discovered chiral perturbation, the coupling term which is present for normal ladders is restored by an external magnetic field, dimerization or the presence of charge carriers. We then proceed with a numerical investigation of the phase diagram of two coupled Heisenberg chains in the presence of a magnetic field. Unusual behaviour is found for ferromagnetic coupled antiferromagnetic chains. Finally, for three (and more) legs one can choose different inequivalent types of coupling between the chains. We find that the three-leg ladder can exhibit a spin-gap and/or non-trivial plateaux in the magnetization curve whose appearance strongly depends on the choice of coupling. Received 11 February 1999 and Received in final form 16 June 1999     

Magnetic structure of a CaMnO2.75 crystal with ordered oxygen vacancies

The magnetic state of a CaMnO_{3 ? δ} crystal with ordered oxygen vacancies (for δ = 0.25, when the numbers of Mn⁴⁺ and Mn³⁺ ions in the manganite are equal to each other) is studied using neutron diffraction. Magnetic scattering in the CaMnO_2.75 crystal in the ground state is determined by the wave vector (1/2, 1/2, 1/2)2π/a _c (G-type antiferromagnetic order). In the crystal, long-range magnetic order disappears at the temperature T _N = 116 K, whereas short-range magnetic order is retained up to 240 K. It is shown that the instability of the G-type structure in the temperature range 60 K < T < T _N is associated, in many respects, with the formation of the C′ antiferromagnetic phase in the bulk of the crystal. The structure of the C′ antiferromagnetic phase involves chains with Mn³⁺-Mn⁴⁺ ferromagnetic interaction. A comparison of the results of the neutron diffraction investigations with the experimental data on the magnetic characteristics and electrical resistivity demonstrates that the specific features revealed in the spin system of the CaMnO_2.75 crystal are governed directly by the competition of the Mn³⁺-Mn⁴⁺ ferromagnetic double exchange with the antiferromagnetic superexchange between manganese ions.     

Constant coupling approximation for antiferromagnets with mixed ferro- and antiferromagnetic interactions

The constant-coupling approximation is extended to an antiferromagnetic spin-$\text{1}\text{2}$ system with two distinct anisotropic exchange interactions. The thermodynamic properties such as the transition temperature, magnetization, susceptibility and specific heat are discussed for three special cases: (i) ferro- and antiferromagnetic Ising interactions, (ii) isotropic ferro- and antiferromagnetic Heisenberg interactions; and (iii) isotropic ferromagnetic Heisenberg interactions and antiferromagnetic Ising interactions, allowing in each case for two different nearest-neighbour coupling constants. Numerical calculations have been performed for a layer structure with z = 6 intraplanar and z' = 6 interplanar nearest neighbours and the results are compared with those obtained in other approximations. Applying the theory to FeCl₂, the exchange constants are evaluated. It is shown that the calculated magnitudes of the interactions strongly depend upon the exchange-interaction model assumed.     

First-principles study on electronic,magnetic properties and optical absorption of vanadium doped rutile TiO2

The electronic, magnetic properties and optical absorption of vanadium (V) doped rutile TiO₂ have been studied by the generalized gradient approximation GGA and GGA+U (Hubbard coefficient) approach respectively. On the one hand, we consider the influence of vanadium with different doping concentration on the electronic structure. On the other hand, we study double V atoms doped TiO₂, mainly study four V-doped TiO₂ configurations, and find the magnetic ground states are ferromagnetic state. For the TiO₂@V-V1, TiO₂@V-V3 and TiO₂@V-V4 configurations without O ion as bridge between V-V atoms, there will have a metastable state of antiferromagnetic configurations, while, for the TiO₂@V-V2 configurations with an O ion as bridge between V-V atoms, due to the existence of superexchange between V-O-V, there will only exist the ground state of ferromagnetic state and there are no other metastable configurations. Furthermore, the optical properties of V-doped TiO₂ are calculated. The results show that the V-doped TiO₂ has strong infrared light absorption and visible light absorption.     

Mössbauer and magnetic studies of orthorhombic FeSiO3

Magnetic properties of orthoferrosilite FeSiO₃ have been examined using susceptibility, magnetization measurements and Mössbauer spectroscopy. From magnetic and Mössbauer measurements, one obtains close values of the magnetic ordering temperature, T_N=39±1 K and T_N=41±1 K, respectively. The magnetic order is characterized by strong ferromagnetic coupling of Fe²⁺ moments within the ribbons and a weak antiferromagnetic coupling of the moments between adjacent ribbons. The 4.2 K Mössbauer spectra can be fitted with two different hyperfine magnetic fields H_hf=68 kOe and H_hf=314 kOe which can be assigned to Fe²⁺ in the octahedrally coordinated M1 and M2 sites, respectively, of the FeSiO₃ structure.     

Magnetic ordering in GdI2

GdI₂ powder samples have been investigated by means of a.c. susceptibility and magnetisation measurements indicating magnetic ordering below T_C = 313 K. The structure consists of strongly coupled ferromagnetic layers of Gd³⁺ moments aligned perpendicular to the hexagonal crystal axis. The inter-layer coupling is much weaker and probably antiferromagnetic suggesting that GdI₂ is to a very good approximation a two-dimensional Heisenberg system.     

Calculated magnetic moment of δ-manganese

Spin-density-functional theory is used to calculate the magnetic moment of δ-Mn whose ground state is assumed to be either antiferromagnetic or ferromagnetic. The band structure is given for paramagnetic, antiferromagnetic and ferromagnetic δ-Mn. The magnetic moment of antiferromagnetic δ-Mn is found to be 3μ_B while that of ferromagnetic δ-Mn is 2.7 μ_B. The total energy favors the antiferromagnetic ground state by about 0.3 eV.