Non-collinear Spin Arrangement Research Articles

Electronic topological transition and noncollinear magnetism in compressed hcp Co

Recent experiments showed that Co undergoes a phase transition from ferromagnetic hcp phase to non-magnetic fcc one around 100 GPa. Since the transition is of first order, a certain region of co-existence of the two phases is present. By means of \textit{ab initio} calculations, we found that the hcp phase itself undergoes a series of electronic topological transitions (ETTs), which affects both elastic and magnetic properties of the material. Most importantly, we propose that the sequence of ETTs lead to the stabilisation of a non-collinear spin arrangement in highly compressed hcp Co. Details of this non-collinear magnetic state and the interatomic exchange parameters that are connected to it, are presented here.

Magnetic charges and magnetoelectricity in hexagonal rare-earth manganites and ferrites

Magnetoelectric (ME) materials are of fundamental interest and show broad potential for technological applications. Commonly the dominant contribution to the ME response is the lattice-mediated one, which is proportional to both the Born electric charge $Z^{\rm e}$ and its analogue, the dynamical magnetic charge $Z^{\rm m}$. Our previous study has shown that exchange striction acting on noncollinear spins induces much larger magnetic charges than those that depend on spin-orbit coupling. The hexagonal manganites $R$MnO$_3$ and ferrites $R$FeO$_3$ ($R$ = Sc, Y, In, Ho-Lu) exhibit strong couplings between electric, magnetic and structural degrees of freedom, with the transition-metal ions in the basal plane antiferromagnetically coupled through super-exchange so as to form a 120$^\circ$ noncollinear spin arrangement. Here we present a theoretical study of the magnetic charges, and of the spin-lattice and spin-electronic ME constants, in these hexagonal manganites and ferrites, clarifying the conditions under which exchange striction leads to an enhanced $Z^{\rm m}$ values and anomalously large in-plane spin-lattice ME effects.

Direct observation of finite size effects in chains of antiferromagnetically coupled spins.

Finite spin chains made of few magnetic ions are the ultimate-size structures that can be engineered to perform spin manipulations for quantum information devices. Their spin structure is expected to show finite size effects and its knowledge is of great importance both for fundamental physics and applications. Until now a direct and quantitative measurement of the spatial distribution of the magnetization of such small structures has not been achieved even with the most advanced microscopic techniques. Here we present measurements of the spin density distribution of a finite chain of eight spin-3/2 ions using polarized neutron diffraction. The data reveal edge effects that are a consequence of the finite size and of the parity of the chain and indicate a noncollinear spin arrangement. This is in contrast with the uniform spin distribution observed in the parent closed chain and the collinear arrangement in odd-open chains.

Unusual Temperature Dependence of Magnetization and Possible Magnetic Noncollinearity in Tm and Pr Clusters.

Rare-earth metals in their bulk form possess rather similar crystallographic structures, which is due to the very similar features of their outer electronic states. On the other hand, their magnetic properties are of rich variety, which is related to the specific form of the indirect magnetic exchange interaction between the inner electronic shells. In cluster form, this interplay may lead to very unusual magnetic structures. Here we show how the magnetic moments vary with size and temperature in Tm and Pr clusters. While in Pr clusters clear atom-by-atom oscillations indicate antiferromagnetic ordering, smooth variation and anomalous temperature behavior in Tm is representative for an essentially non-collinear spin arrangement. Their electric behavior is also very different, with a metallic-like behavior of Pr and localized electronic states in Tm.

Study on Mössbauer Signature, Hyperfine Interaction Parameters and Removal of Delafossite Phase of Al3+-Modified Copper Ferrite

The effect of Al3+ substitution on Mossbauer signature and hyperfine interaction parameters of copper ferrite (CuFe2O4) is studied at 300 K. The X-ray powder diffraction pattern analysis shows that all the four compositions of the ferrite system, CuAlxFe2−xO4 (x = 0.0, 0.2, 0.4, and 0.6), have single-phase fcc spinel structure. On the other hand, Mossbauer spectral analysis reveals that x = 0.0 and 0.2 compositions possess paramagnetic singlet of delafossite (CuFeO2) phase along with two magnetic sextets of spinel ferrite phase. On Al3+ substitution for Fe3+ in the system (x ≥ 0.4), formation of this secondary unwanted phase of CuFeO2 is found to be prevented. The hyperfine interaction parameters are determined and found to be influenced by Al3+ substitution. The noncollinear spin arrangement in the system is verified and the actual cationic distributions are suggested.

Magnetic-field-induced spin flop transition and magnetoelectric effect inCa2Fe2−xAlxO5

Magnetic-field-induced spin flop transition and magnetoelectric effect in noncentrosymmetric ${\text{Ca}}_{2}$${\text{Fe}}_{2\ensuremath{-}x}$${\text{Al}}_{x}$${\text{O}}_{5}$ (0.5 $\ensuremath{\le}$ $x$ $\ensuremath{\le}$ 1) single crystals have been investigated. Temperature dependence of magnetic susceptibility shows that the compounds have antiferromagnetic transition temperatures ${T}_{N}$ above room temperature. The spin easy axis varies from the $c$ axis to the $a$ axis with increasing Al content $x$. When a magnetic field is applied along the spin easy axis, a spin flop transition takes place, accompanied by anomalies in the electric polarization and dielectric constant. The anomalies can be ascribed to a noncollinear spin arrangement in the domain walls between two magnetic phases and/or the spin direction dependent modulation of the metal-ligand hybridization. Such a polarization change with a spin flop transition in polar antiferromagnets may provide a route to realize large magnetoelectric coupling at high temperatures.

Self-consistent determination of HubbardUfor explaining the anomalous magnetism of the Gd13cluster

functional theory and the linear response approach, we determine U self-consistently for the cluster and apply it to explain the anomalous magnetic properties of Gd13. We demonstrate that the interaction between core and shell atoms of the Gd13 cluster strongly depends on the Hubbard U .F orU = 0 eV magnetism is governed by a direct f-f electron interaction between core and shell atoms, while for U = 5.5 eV it is the indirect Ruderman-KittelKasuya-Yosida interaction that prevails. We also demonstrate that the noncollinear spin arrangement of each atom in the cluster strongly depends on the Hubbard U. Monte Carlo calculations further confirm that magnetic moments decrease with temperature, thus addressing a long-standing disagreement in experimental results.

Coupling of the local defect and magnetic structure of wüstite Fe1−xO

The local nuclear and magnetic structure of wustite, Fe1-xO, and the coupling between them, has been examined using reverse Monte Carlo refinements of variable-temperature neutron total scattering data. The results from this analysis suggest that the individual units in a tetrahedral defect cluster are connected along <110> vectors into a Koch-Cohen-like arrangement, with the majority of octahedral vacancies concentrated near these defects. Bond valence calculations indicate a change in the charge distribution on the cations with the charge on the tetrahedral interstitials increasing on cooling. The magnetic structure is more complex than previously thought, corresponding to a non-collinear spin arrangement described by a superposition of a condensed spin wave on the established type-II antiferromagnetic ordering. This leads to an architecture with four groups of cations each with different spin directions. The cations within the interstitial clusters appear to be weakly ferromagnetically coupled and their spins are correlated to the spins of the octahedral cations closest to them. This work not only provides further insight into the local structure of wustite but also a better understanding of the coupling between defect structures and magnetic and charge-ordering in complex materials.

Increase in initial permeability due to substitution of high spin cations in nanocrystalline Ni–Mg ferrites

In order to increase the permeability, high spin Mn2+ substituted nanocrystalline Ni0.50−xMnxMg0.5Fe2O4 were synthesized by the combustion technique. Mn2+ substitution acts as an accelerator of grain growth and uniform grain distribution in these chemical compositions. Both the real part of initial permeability and the saturation magnetization increase with increasing Mn content up to x=0.30 and they decrease for higher Mn2+ content indicating the possibility of a non-collinear spin arrangement between tetrahedral and octahedral sites. The maximum increase of initial permeability is 54% compared to that of parent composition. The weakening of exchange interaction may be confirmed by increasing lattice parameter and decreasing Néel temperature with increasing Mn2+ content.

Large magnetoresistance and charge transfer between the conduction and magnetic electrons in layered oxyselenide BiOCu0.96Se

The electrical and magnetic properties of slightly Cu-deficient BiOCu(0.96)Se have been investigated using neutron and X-ray diffraction, ac magnetic susceptibility, magnetization and electric resistivity measurements. The layered BiOCu(0.96)Se crystallizes into a tetragonal lattice with a P4/nmm symmetry. Thermal profiles of the electrical resistivity reveal a semiconductor type behavior, but depart from its course at low temperatures when antiferromagnetic coupling becomes thermally loosened at 140 K. Positive magnetoresistances are obtained at all temperatures studied. With an applied magnetic field of 0.5 kOe, the magnetoresistance reaches 235% at 2 K. It decreases with increasing temperature, but stabilizes to 70% above 60 K. Both ferromagnetic and antiferromagnetic coupling are detected between the Cu spins in the SeCu4 pyramidal blocks, which results in a non-collinear spin arrangement at low temperatures. The antiferromagnetic component becomes disordered above T(N) = 140 K, whereas the ferromagnetic moment persists up to T(C) = 300 K. Interlayer charge transfer between the conduction and magnetic electrons gives rise to an anomaly in the magnetic order parameter.

Chain length dependence of polyol synthesis of zinc ferrite nanoparticles: why is diethylene glycol so different?

Superparamagnetic ZnFe2O4 nanoparticles with size range of 28-38 nm were synthesized by polyol process based on use of varying chain length glycols as solvent. We have offered, for the first time, the plausible mechanism behind in situ formation of zinc ferric oxalate hydroxide hydrate [Fe2Zn(C2O4)2(OH)3](+)·4H2O complex from diethylene and polyethylene glycol. We are also reporting, the magnetic properties of above complexes. We have found a ferromagnetic ordering in precursor complex compounds. The intermediate hydrocarbon chain between the oxalato bridged metal cations plays a crucial role in obtaining anomalous magnetic behavior. ZnFe2O4 nanoparticles obtained after annealing the DEGylated precursor complex (precursor complex formed in diethylene glycol) showed the highest superparamagnetic (SPM) behavior (22.4 emu g(-1)) among others. The reasons for anomalous SPM behavior of ZnFe2O4 nanoparticles are explained on the basis of the degree of inversion of the spinel structure, high surface-to-volume ratio, which causes non-collinear spin arrangement in a surface layer and higher oxygen concentration on the surface of dead organic layer, which increases the unpaired valence electrons leading to uncompensated surface spins.

Synthesis and Characterization of the Crystal Structure and Magnetic Properties of the New Fluorophosphate LiNaCo[PO4

The new compound LiNaCo[PO(4)]F was synthesized by a solid state reaction route, and its crystal structure was determined by single-crystal X-ray diffraction measurements. The magnetic properties of LiNaCo[PO(4)]F were characterized by magnetic susceptibility, specific heat, and neutron powder diffraction measurements and also by density functional calculations. LiNaCo[PO(4)]F crystallizes with orthorhombic symmetry, space group Pnma, with a = 10.9334(6), b = 6.2934(11), c = 11.3556(10) Å, and Z = 8. The structure consists of edge-sharing CoO(4)F(2) octahedra forming CoFO(3) chains running along the b axis. These chains are interlinked by PO(4) tetrahedra forming a three-dimensional framework with the tunnels and the cavities filled by the well-ordered sodium and lithium atoms, respectively. The magnetic susceptibility follows the Curie-Weiss behavior above 60 K with θ = -21 K. The specific heat and magnetization measurements show that LiNaCo[PO(4)]F undergoes a three-dimensional magnetic ordering at T(mag) = 10.2(5) K. The neutron powder diffraction measurements at 3 K show that the spins in each CoFO(3) chain along the b-direction are ferromagnetically coupled, while these FM chains are antiferromagnetically coupled along the a-direction but have a noncollinear arrangement along the c-direction. The noncollinear spin arrangement implies the presence of spin conflict along the c-direction. The observed magnetic structures are well explained by the spin exchange constants determined from density functional calculations.

Synthesis and characterization of the crystal structure, the magnetic and the electrochemical properties of the new fluorophosphate LiNaFe[PO4

The new compound LiNaFe[PO(4)]F was synthesized by a solid state reaction route, and its crystal structure was determined using neutron powder diffraction data. LiNaFe[PO(4)]F was characterized by (57)Fe Mössbauer spectroscopy, magnetic susceptibility, specific heat capacity, and electrochemical measurements. LiNaFe[PO(4)]F crystallizes with orthorhombic symmetry, space group Pnma, with a = 10.9568(6) Å, b = 6.3959(3) Å, c = 11.4400(7) Å, V = 801.7(1) Å(3) and Z = 8. The structure consists of edge-sharing FeO(4)F(2) octahedra forming FeFO(3) chains running along the b axis. These chains are interlinked by PO(4) tetrahedra forming a three-dimensional framework with the tunnels and the cavities filled by the well-ordered sodium and lithium atoms, respectively. The specific heat and magnetization measurements show that LiNaFe[PO(4)]F undergoes a three-dimensional antiferromagnetic ordering at T(N) = 20 K. The neutron powder diffraction measurements at 3 K show that each FeFO(3) chain along the b-direction is ferromagnetic (FM), while these FM chains are antiferromagnetically coupled along the a and c-directions with a non-collinear spin arrangement. The galvanometric cycling showed that without any optimization, one mole of alkali metal is extractable between 1.0 V and 5.0 V vs. Li(+)/Li with a discharge capacity between 135 and 145 mAh g(-1).

Noncollinear magnetism in transition metal nanostructures: Exchange interaction and local environment effects in free and deposited clusters

A self-consistent tight-binding theory of noncollinear magnetism in transition metal nanostructures is presented and applied to free and deposited clusters. The electronic structure is calculated by using a realistic rotational invariant tight-binding Hamiltonian and a real-space recursive expansion of the local Green's functions. For free clusters with sizes $N\ensuremath{\leqslant}13$, results are given for the ground-state local magnetic moments, magnetic order, and average magnetic moments as a function of the Coulomb exchange integral $J$ and $d$-band filling. A variety of qualitatively different noncollinear spin arrangement solutions are obtained, which reveals the complex magnetic landscape in transition metal nanostructures. The onset of noncollinear magnetism is discussed by analyzing the stability of the various magnetic solutions. Structural effects are studied by considering representative compact and open geometries. The calculations are compared with available density-functional results. Substrate effects on the noncollinearity of the moments are discussed by considering small Fe clusters deposited on Pt(111). Extensions and limitations of our work are also pointed out.

Magnetic spin structure of pyroxene-typeMnGeO3

Polycrystalline MnGeO3 material was synthesized at 1473 K and ambient pressures using ceramicsintering techniques. Under these conditions the pyroxene-type compoundcrystallizes in the orthorhombic modification with space groupPbca, asdetermined from the refinement of the neutron diffraction data. The monoclinic modification, spacegroup C 2/c, was also present at a level of 8.8(4) wt% and the magnetic structures for the twopolymorphs at low temperatures have been found simultaneously. The monoclinicform orders magnetically below 34 K; the spin structure can be described usingk = (0, 0, 0) in the magneticspace group C 2′/c, having an antiferromagnetic spin arrangement within and betweenthe chains of M1 and M2 sites. The orthorhombic phase ofMnGeO3 transforms to a magnetically ordered state withk = (0, 0, 0) and magneticspace group Pb′c′a below 12 K. Spins on M1 sites are aligned along the crystallographica-axis with a slight non-collinear antiferromagnetic spin arrangement with and between theM1 chains. Spins on M2 sites are also antiferromagnetically coupled; however, one of thethree different M1–M2 superexchange pathways within the band of M1 and M2 sitesdisplays a ferromagnetic interaction, while the other two allow antiferromagneticinteraction.

ChemInform Abstract: Analysis of the Magnetic Structure and Ferroelectric Polarization of Monoclinic MnSb2S4 by Density Functional Theory Calculations.

Monoclinic MnSb 2 S 4 consists of MnS 4 chains made up of edge-sharing MnS 6 octahedra and adopts a (0, 0.369, 0) magnetic superstructure below 25 K. This ordered magnetic structure, in which the spins of each MnS 4 chain possess a helical spin arrangement, has C 2 ' symmetry. On the basis of density functional theory calculations, we explored the origin of the observed noncollinear spin arrangement of MnSb 2 S 4 by evaluating its spin exchanges to find that spin exchanges are frustrated not only within each MnS 4 chain but also between adjacent MnS 4 chains. Our analysis predicts that MnSb 2 S 4 is a multiferroic with a ferroelectric polarization of ~14 μC/m 2 along the chain direction, and a field-induced reversal of the ferroelectric polarization of MnSb 2 S 4 can occur by reversing the direction of the helical spin rotation in each MnS 4 chain.

Magnetic switch of polarization in epitaxial orthorhombic YMnO3 thin films

We report on the growth and characterization of epitaxial thin films of orthorhombic YMnO3. It is found that a ferroelectric polarization exists along c-axis (Pc∼90 nC/cm2) and a magnetic field induces the development of a ferroelectric polarization along a-axis (Pa) while suppressing Pc. This field-induced switching is consistent with the existence of a cycloidal magnetic order in YMnO3. This finding evidences that a noncollinear spin arrangement in thin films allows switchable electrical polarization and of tunability of their dielectric response by magnetic field.

Analysis of the Magnetic Structure and Ferroelectric Polarization of Monoclinic MnSb2S4 by Density Functional Theory Calculations

Monoclinic MnSb(2)S(4) consists of MnS(4) chains made up of edge-sharing MnS(6) octahedra and adopts a (0, 0.369, 0) magnetic superstructure below 25 K. This ordered magnetic structure, in which the spins of each MnS(4) chain possess a helical spin arrangement, has C(2)' symmetry. On the basis of density functional theory calculations, we explored the origin of the observed noncollinear spin arrangement of MnSb(2)S(4) by evaluating its spin exchanges to find that spin exchanges are frustrated not only within each MnS(4) chain but also between adjacent MnS(4) chains. Our analysis predicts that MnSb(2)S(4) is a multiferroic with a ferroelectric polarization of ∼14 μC/m(2) along the chain direction, and a field-induced reversal of the ferroelectric polarization of MnSb(2)S(4) can occur by reversing the direction of the helical spin rotation in each MnS(4) chain.

Magnetism of small Cr clusters: Interplay between structure, magnetic order, and electron correlations

The magnetic properties of small ${\text{Cr}}_{N}$ clusters $(N\ensuremath{\le}6)$ are investigated in the framework of density-functional theory. The interplay between electron correlations, cluster structure, and magnetic order is quantified by performing fully spin-unrestricted calculations allowing for noncollinear spin arrangements within both the local-spin-density approximation (LSDA) and the generalized-gradient approximation (GGA). The possible transition or saddle-point states are identified by determining the vibrational frequencies from diagonalizing the dynamical matrix. In agreement with previous studies, a dimer-based growth pattern is found in all considered low-lying isomers with very short equilibrium bond lengths (typically ${d}_{eq}^{\text{GGA}}=1.55--1.65\text{ }\text{\AA{}}$) alternating with relative long ones (typically ${d}_{eq}^{\text{GGA}}=2.75--2.85\text{ }\text{\AA{}}$) in the relaxed geometries. Strong local magnetic moments ${\stackrel{P\vec}{\ensuremath{\mu}}}_{i}$ are, in general, obtained for the relaxed geometries (e.g., $|{\ensuremath{\mu}}_{i}^{\text{GGA}}|\ensuremath{\simeq}2\text{ }{\ensuremath{\mu}}_{\text{B}}$ in ${\text{Cr}}_{4}$), which show a collinear magnetic order with antiparallel (parallel) alignment of the ${\stackrel{P\vec}{\ensuremath{\mu}}}_{i}$ along the short (long) bonds. In contrast to the GGA, the LSDA yields vanishing magnetization density in some cases ($|{\ensuremath{\mu}}_{i}^{\text{LSDA}}|=0\text{ }\ensuremath{\forall}\text{ }i$ for $N=2$ and 4). Despite quantitative differences, both LSDA and GGA functionals always yield collinear ground-state solutions for the fully relaxed structures. In fact, noncollinear spin arrangements are found only for particular symmetric (nondimerized) geometries. However, the structures are not local minima and involve considerably large excitation energies. The results clearly indicate that the magnetic frustration, which one would physically expect in compact antiferromagnetic spin systems, is solved by dimerization rather than by noncollinearity of the local moments.

Magnetic moment and local moment alignment in anionic and/or oxidized Fen clusters

First principles studies on the ground state structure, binding energy, spin multiplicity, and the noncollinearity of local spin moments in Fe(n) and Fe(n) (-) clusters and their oxides, viz., Fe(n)O(2) and Fe(n)O(2) (-) have been carried out within a density functional formalism. The ground states of Fe(n) and Fe(n) (-) clusters have collinear spins with a magnetic moment of around 3.0 micro(B) per atom. The O(2) molecule is found to be dissociatively absorbed and its most significant effect on spin occurs in Fe(2), where Fe(2)O(2) and Fe(2)O(2) (-) show antiferromagnetic and noncollinear spin arrangements, respectively. The calculated adiabatic electron affinity and the vertical transitions from the anion to the neutral species are found to be in good agreement with the available negative ion photodetachment spectra, providing support to the calculated ground states including the noncollinear ones.