Showing papers on "Magnetic structure published in 2007"

Chiral magnetic order at surfaces driven by inversion asymmetry

Abstract: Chirality is a fascinating phenomenon that can manifest itself in subtle ways, for example in biochemistry (in the observed single-handedness of biomolecules) and in particle physics (in the charge-parity violation of electroweak interactions). In condensed matter, magnetic materials can also display single-handed, or homochiral, spin structures. This may be caused by the Dzyaloshinskii-Moriya interaction, which arises from spin-orbit scattering of electrons in an inversion-asymmetric crystal field. This effect is typically irrelevant in bulk metals as their crystals are inversion symmetric. However, low-dimensional systems lack structural inversion symmetry, so that homochiral spin structures may occur. Here we report the observation of magnetic order of a specific chirality in a single atomic layer of manganese on a tungsten (110) substrate. Spin-polarized scanning tunnelling microscopy reveals that adjacent spins are not perfectly antiferromagnetic but slightly canted, resulting in a spin spiral structure with a period of about 12 nm. We show by quantitative theory that this chiral order is caused by the Dzyaloshinskii-Moriya interaction and leads to a left-rotating spin cycloid. Our findings confirm the significance of this interaction for magnets in reduced dimensions. Chirality in nanoscale magnets may play a crucial role in spintronic devices, where the spin rather than the charge of an electron is used for data transmission and manipulation. For instance, a spin-polarized current flowing through chiral magnetic structures will exert a spin-torque on the magnetic structure, causing a variety of excitations or manipulations of the magnetization and giving rise to microwave emission, magnetization switching, or magnetic motors.

Magnetic vortex oscillator driven by d.c. spin-polarized current

Abstract: Transfer of angular momentum from a spin-polarized current to a ferromagnet provides an efficient means to control the magnetization dynamics of nanomagnets. A peculiar consequence of this spin torque, the ability to induce persistent oscillations in a nanomagnet by applying a d.c. current, has previously been reported only for spatially uniform nanomagnets. Here, we demonstrate that a quintessentially non-uniform magnetic structure, a magnetic vortex, isolated within a nanoscale spin-valve structure, can be excited into persistent microwave-frequency oscillations by a spin-polarized d.c. current. Comparison with micromagnetic simulations leads to identification of the oscillations with a precession of the vortex core. The oscillations, which can be obtained in essentially zero magnetic field, exhibit linewidths that can be narrower than 300 kHz at ∼1.1 GHz, making these highly compact spin-torque vortex-oscillator devices potential candidates for microwave signal-processing applications, and a powerful new tool for fundamental studies of vortex dynamics in magnetic nanostructures.

Lanthanide contraction and magnetism in the heavy rare earth elements

Abstract: The heavy rare earth elements crystallize into hexagonally close packed (h.c.p.) structures and share a common outer electronic configuration, differing only in the number of 4f electrons they have. These chemically inert 4f electrons set up localized magnetic moments, which are coupled via an indirect exchange interaction involving the conduction electrons. This leads to the formation of a wide variety of magnetic structures, the periodicities of which are often incommensurate with the underlying crystal lattice. Such incommensurate ordering is associated with a 'webbed' topology of the momentum space surface separating the occupied and unoccupied electron states (the Fermi surface). The shape of this surface-and hence the magnetic structure-for the heavy rare earth elements is known to depend on the ratio of the interplanar spacing c and the interatomic, intraplanar spacing a of the h.c.p. lattice. A theoretical understanding of this problem is, however, far from complete. Here, using gadolinium as a prototype for all the heavy rare earth elements, we generate a unified magnetic phase diagram, which unequivocally links the magnetic structures of the heavy rare earths to their lattice parameters. In addition to verifying the importance of the c/a ratio, we find that the atomic unit cell volume plays a separate, distinct role in determining the magnetic properties: we show that the trend from ferromagnetism to incommensurate ordering as atomic number increases is connected to the concomitant decrease in unit cell volume. This volume decrease occurs because of the so-called lanthanide contraction, where the addition of electrons to the poorly shielding 4f orbitals leads to an increase in effective nuclear charge and, correspondingly, a decrease in ionic radii.

Multiferroic phases of Eu 1-x Y x MnO 3

Abstract: We report on structural, magnetic, dielectric, and thermodynamic properties of (Eu:Y)MnO3 for Y doping levels 0 <= x < 1. This system resembles the multiferroic perovskite manganites RMnO3 (with R= Gd, Dy, Tb) but without the interference of magnetic contributions of the 4f-ions. In addition, it offers the possibility to continuously tune the influence of the A-site ionic radii. For small concentrations x <= 0.1 we find a canted antiferromagnetic and paraelectric groundstate. For higher concentrations x <= 0.3 ferroelectric polarization coexists with the features of a long wavelength incommensurate spiral magnetic phase analogous to the observations in TbMnO3. In the intermediate concentration range around x = 0.2 a multiferroic scenario is realized combining weak ferroelectricity and weak ferromagnetism, presumably due to a canted spiral magnetic structure.

Direct transition from a disordered to a multiferroic phase on a triangular lattice.

Abstract: We report the first direct transition from a paramagnetic and paraelectric phase to an incommensurate multiferroic in the triangular lattice antiferromagnet $\mathrm{RbFe}({\mathrm{MoO}}_{4}{)}_{2}$. Ferroelectricity is observed only when the magnetic structure has chirality and breaks inversion symmetry. A Landau expansion of symmetry-allowed terms in the free energy demonstrates that chiral magnetic order can give rise to a pseudoelectric field, whose temperature dependence agrees with experiment.

Competition between Magnetic Structures in the Fe-Rich FCC FeNi Alloys

Abstract: We report on the results of a systematic ab initio study of the magnetic structure of Fe rich fcc FeNi binary alloys for Ni concentrations up to 50 at. %. Calculations are carried out within densit ...

Pyroxenes: a new class of multiferroics

Abstract: Pyroxenes with the general formula AMSi2O6 (A =?mono-?or divalent metal, M =?di-?or trivalent metal) are shown to be a new class of multiferroic materials. In particular, we have found so far that NaFeSi2O6 becomes ferroelectric in a magnetically ordered state below ~6?K. Similarly, magnetically driven ferroelectricity is also detected in the Li homologues, LiFeSi2O6 (TC~18?K) and LiCrSi2O6 (TC~11?K). In all these monoclinic systems the electric polarization can be strongly modified by magnetic fields. Measurements of magnetic susceptibility, pyroelectric current and dielectric constants (and their dependence on magnetic field) are performed using a natural crystal of aegirine (NaFeSi2O6) and synthetic crystals of LiFeSi2O6 and LiCrSi2O6 grown from melt solution. For NaFeSi2O6 a temperature versus magnetic field phase diagram is proposed. Exchange constants are computed on the basis of ab initio band structure calculations. The possibility of a spiral magnetic structure caused by frustration to be the reason for the origin of the multiferroic behaviour is discussed. We propose that other pyroxenes may also be multiferroic, and that the versatility of this family offers an exceptional opportunity to study general conditions for and mechanisms of magnetically driven ferroelectricity.

Structural investigation and electronic properties of the nickel ferrite NiFe2O4: a periodic density functional theory approach

Abstract: Periodic density functional theory (DFT) calculations using plane-wave basis sets were performed in order to study the bulk of nickel ferrite NiFe2O4. The local spin density approximation (LSDA) and the generalized gradient approximation (GGA) formalism were used, and it appeared that the LSDA failed to describe the magnetic structure of this compound. However, the GGA formalism gave reliable results in good agreement with experimental data for the lattice parameters, the electronic properties and the bulk modulus. In addition, the calculated density of states of the metallic species d block as well as their local magnetic moments were correlated to the crystal-field theory. Then, a charge deformation map was computed and, as expected from the electronegativity scale, the electron excess is localized around oxygen atoms along the bond axes. The formation energies of metallic vacancies are in good agreement with the inverse spinel structure experimentally observed.

Pyroxenes: A new class of multiferroics

Abstract: Pyroxenes with the general formula $AM$Si$_2$O$_6$ ($A$ = mono- or divalent metal, $M$ = di- or trivalent metal) are shown to be a new class of multiferroic materials. In particular, we have found so far that NaFeSi$_2$O$_6$ becomes ferroelectric in a magnetically ordered state below $\simeq 6$ K. Similarly, magnetically driven ferroelectricity is also detected in the Li homologues, LiFeSi$_2$O$_6$ ($T_C \simeq 18$ K) and LiCrSi$_2$O$_6$ ($T_C \simeq 11$ K). In all these monoclinic systems the electric polarization can be strongly modified by magnetic fields. Measurements of magnetic susceptibility, pyroelectric current and dielectric constants (and their dependence on magnetic field) are performed using a natural crystal of aegirine (NaFeSi$_2$O$_6$) and synthetic crystals of LiFeSi$_2$O$_6$ and LiCrSi$_2$O$_6$ grown from melt solution. For NaFeSi$_2$O$_6$ a temperature versus magnetic field phase diagram for NaFeSi$_2$O$_6$ is proposed. Exchange constants are computed on the basis of {\it ab initio} band structure calculations. The possibility of a spiral magnetic structure caused by frustration as origin of the multiferroic behaviour is discussed. We propose that other pyroxenes may also be multiferroic, and that the versatility of this family offers an exceptional opportunity to study general conditions for and mechanisms of magnetically driven ferroelectricity.

Single-crystal and powder neutron diffraction experiments on Fe P S 3 : Search for the magnetic structure

Abstract: The long-accepted magnetic structure of $\mathrm{Fe}\mathrm{P}{\mathrm{S}}_{3}$ has recently been refuted through extensive powder neutron diffraction studies. Single-crystal neutron diffraction, using both quasi-Laue and monochromatic techniques, has now been conducted to reveal more information about the nature of the magnetic ordering in this compound. The magnetic unit cell was found to be twice as large as the crystallographic cell in both the $a$ and $b$ directions and around three times as large along the $c$ direction, giving a propagation vector of $[\frac{1}{2}\phantom{\rule{0.3em}{0ex}}\frac{1}{2}\phantom{\rule{0.3em}{0ex}}0.34]$. There is incomplete long-range magnetic order along the ${c}^{*}$ direction due to weak Fe-Fe interactions and large anisotropy in that direction. The variation of the spontaneous moment with temperature closely resembles that for an antiferromagnetic two-dimensional Ising model on a honeycomb lattice.

Remanence of Ni nanowire arrays: Influence of size and labyrinth magnetic structure

Abstract: The influence of the macroscopic size of the Ni nanowire array system on their remanence state has been investigated. A simple magnetic phenomenological model has been developed to obtain the remanence as a function of the magnetostatic interactions in the array. We observe that, due to the long range of the dipolar interactions between the wires, the size of the sample strongly influence the remanence of the array. On the other hand, the magnetic state of nanowires has been studied by variable field magnetic force microscopy for different remanent states. The distribution of nanowires with the magnetization in up or down directions and the subsequent remanent magnetization has been deduced from the magnetic images. The existence of two short-range magnetic orderings with similar energies can explain the typical labyrinth pattern observed in magnetic force microscopy images of the nanowire arrays.

Magnetic structure of Cd-doped CeCoIn5

Abstract: The heavy-fermion superconductor CeCo In5 is believed to be close to a magnetic instability, but no static magnetic order has been found. Cadmium doping on the In site shifts the balance between superconductivity and antiferromagnetism to the latter with an extended concentration range where both types of order coexist at low temperatures. We investigated the magnetic structure of nominally 10% Cd-doped CeCo In5, being antiferromagnetically ordered below TN ≈3 K and superconducting below Tc ≈1.3 K, by elastic neutron scattering. Magnetic intensity was observed only at the ordering wave vector QAF = (1 2, 1 2, 1 2) commensurate with the crystal lattice. Upon entering the superconducting state, the magnetic intensity seems to change only little. The commensurate magnetic ordering in CeCo (In1-x Cdx) 5 is in contrast to the incommensurate antiferromagnetic ordering observed in the closely related compound CeRh In5. Our results give insights into the interplay between superconductivity and magnetism in the family of CeT In5 (T=Co, Rh, and Ir) based compounds. © 2007 The American Physical Society.

Magnetic structure, magnetic interactions and metamagnetism in terbium iron borate TbFe3(BO3)4: a?neutron diffraction and magnetization study

Abstract: Magnetization, susceptibility and neutron scattering measurements were performed on the terbium iron borate TbFe3(BO3)4. Structural and magnetic phase transitions were obtained as a function of external magnetic field and temperature. A metamagnetic transition of the terbium spins and a spin-flop transition of the iron sublattice are obtained at an external magnetic field 35?kOe

Kondo effect in a semiconductor quantum dot coupled to ferromagnetic electrodes

Abstract: Using a laterally fabricated quantum-dot (QD) spin-valve device, we experimentally study the Kondo effect in the electron transport through a semiconductor QD with an odd number of electrons (N). In a parallel magnetic configuration of the ferromagnetic electrodes, the Kondo resonance at N=3 splits clearly without external magnetic fields. With applying magnetic fields (B), the splitting is gradually reduced, and then the Kondo effect is almost restored at B=1.2T. This means that, in the Kondo regime, an inverse effective magnetic field of B∼1.2T can be applied to the QD in the parallel magnetic configuration of the ferromagnetic electrodes.

Structural transition of ferromagnetic Ni2MnGa nanoparticles

Abstract: We report here that the ball-milling process induces the phase transformation from the tetragonal structure to the disordered face-centered-cubic structure in Ni2MnGa ferromagnetic shape-memory alloys. The in situ high-energy x-ray diffraction analyses reveal that an intermediate phase, which is characterized by amorphous structure, controls the transformation kinetics during the postannealing process. Completely different from their coarse-grained counterparts, the ferromagnetic Ni2MnGa nanoparticles undergo various sequences of structural transitions that are tailored by the crystallite size, atomic order, and intrinsic magnetic structure.

Intrinsic ferromagnetism versus phase segregation in Mn-doped Ge

Abstract: We report on a detailed study of structural, magnetic, and electronic properties of MnxGe1−x single crystals (0

Relationship between Magnetic Structure and Ferroelectricity of LiVCuO4

Abstract: Neutron scattering studies and measurements of the dielectric susceptibility and ferroelectric polarization P have been carried out in various magnetic fields H for single-crystal samples of the multiferroic system LiVCuO4 with quasi one-dimensional spin 1/2 Cu2+ chains formed of edge-sharing CuO4 square planes, and the relationship between the magnetic structure and ferroelectricity has been studied. The ferroelectric polarization is significantly suppressed by the magnetic field H above 2 T applied along a and b axes. The helical magnetic structure with the helical axis parallel to c has been confirmed in H=0, and for H//a, the spin flop transition takes place at H=2 T with increasing H, where the helical axis changes to the direction parallel to H. The ferroelectric polarization along a at H=0 is found to be proportional to the neutron magnetic scattering intensity, indicating that the magnetic order is closely related to the appearance of the ferroelectricity. The relationship between the magnetic structure and ferroelectricity of LiVCuO4 is discussed by considering the existing theories.

Absence of a spiral magnetic order in Li 2 CuO 2 containing one-dimensional CuO 2 ribbon chains

Abstract: On the basis of first principles density functional theory electronic structure calculations as well as classical spin analysis, we explored why the magnetic oxide Li2CuO2, consisting of CuO2 ribbon chains made up of edge-sharing CuO4 squares, does not exhibit a spiral-magnetic order. Our work shows that, due to the next-nearest-neighbor interchain interactions, the observed collinear magnetic structure becomes only slightly less stable than the spin-spiral ground state, and many states become nearly degenerate in energy with the observed collinear structure. This suggests that the collinear magnetic structure of Li2CuO2 is a consequence of order-by-disorder induced by next-nearest-neighbor interchain interactions.

Map of metastable states for thin circular magnetic nano-cylinders

Abstract: Nano-magnetic systems of artificially shaped ferromagnetic islands, recently became a popular subject due to their current and potential applications in spintronics, magneto-photonics and superconductivity. When the island size is close to the exchange length of magnetic material (around 15 nm), its magnetic structure becomes markedly different. It determines both static and dynamic magnetic properties of elements, but strongly depends on their shape and size. Here we map this dependence for circular cylindrical islands of a few exchange lengths in size. We outline the region of metastability of "C"-type magnetic states, proving that they are indeed genuine and not a result of pinning on particle imperfections. A way to create the smallest particles with guaranteed magnetic vortex state at zero field becomes evident. It is expected that the map will help focus the efforts in planning of experiments and devices.

Spin Noncollinearlity in Multiferroic Phase of Triangular Lattice Antiferromagnet CuFe1-xAlxO2

Abstract: We report a neutron diffraction study of the magnetic field- and impurity-induced ferroelectric states of the triangular lattice antiferromagnet CuFe 1- x Al x O 2 . The magnetic structure of the ferroelectric phase was elucidated to be not a cycloidal structure, which can successfully lead to the electric polarization through the formula P ∝ e i j ×( S i × S j ) [H. Katsura et al. : Phys. Rev. Lett. 95 (2005) 057205], but a proper helical structure. Nevertheless, the fact that the helical magnetic ordering appears only in the ferroelectric phase among various magnetically ordered phases of CuFe 1- x Al x O 2 strongly suggests that a spin noncollinearlity is relevant to the multiferroic nature in CuFe 1- x Al x O 2 .

Superexchange Interactions of (Ba1-xSrx)2Zn2Fe12O22 System Studied by Neutron Diffraction

Abstract: Neutron diffraction experiments were carried out on single crystals of (Ba 1- x Sr x ) 2 Zn 2 Fe 12 O 22 at 8 K, where x is the Sr concentration ranging from 0 to 1.0. Competition of three superexchange interactions exists among Fe magnetic moments present in the 4th, 5th, and 8th layers of Sr-rich crystals. This competition leads to a distorted helimagnetic structure consisting of large and small ferrimagnetic bunches. The relations between exchange integrals J 45 , J 48 , and J 58 are determined by using the molecular field approximation as J 45 / J 58 =0.365 7 +0.019 0 x +0.061 7 x 2 and J 48 / J 58 =0.504 7 +0.010 4 x +0.011 6 x 2 at 8 K. These relations provide a clear interpretation for the magnetic structure of the (Ba 1- x Sr x ) 2 Zn 2 Fe 12 O 22 system. Zn ion distributions in the tetrahedral sites and a local lattice deformation around Sr ions are also discussed in detail.

Incommensurate magnetic structure in the orthorhombic perovskite ErMnO3

Abstract: By combining dielectric, specific heat, and magnetization measurements and high-resolution neutron powder diffraction, we have investigated the thermodynamic and magnetic and structural properties of the metastable orthorhombic perovskite ErMnO3 prepared by high-pressure synthesis. The system becomes antiferromagnetically correlated below 42 K and undergoes a lock-in transition at 28 K with propagation wave vector (0,k(b),0), which remains incommensurate at low temperature. The intercorrelation between the magnetic structure and electric properties and the role of the rare earth moment are discussed.

Magnetic structure of free cobalt clusters studied with Stern-Gerlach deflection experiments

Abstract: We have studied the magnetic properties of free cobalt clusters in two semi-independent Stern-Gerlach deflection experiments at temperatures between 60 and $307\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. We find that clusters consisting of 13--200 cobalt atoms exhibit behavior that is entirely consistent with superparamagnetism, though complicated by finite-system fluctuations in cluster temperature. By fitting the data to the Langevin function, we report magnetic moments per atom for each cobalt cluster size and compare the results of our two measurements and all those performed previously. In addition to a gradual decrease in moment per atom with increasing size, there are oscillations that appear to be caused by geometrical shell structure. We discuss our observations in light of the two competing models for Langevin-like magnetization behavior in free clusters, superparamagnetism and adiabatic magnetization, and conclude that the evidence strongly supports the superparamagnetic model.

Switching from ferro- to antiferromagnetism in A2CrSbO6 (A = Ca, Sr) double perovskites: a neutron diffraction study

Abstract: Double perovskites Sr2CrSbO6 and Ca2CrSbO6 have been prepared by a solid-state procedure. The crystal and magnetic structures have been studied from X-ray (XRD) and neutron powder diffraction (NPD) data. Rietveld refinements show that the room-temperature crystal structure is monoclinic (space group P21/n), and contains an almost completely ordered array of alternating CrO6 and SbO6 octahedra sharing corners, tilted along the three pseudocubic axes according to the Glazer notation a−a−b+. The monoclinic distortion is larger in Ca2CrSbO6 than in Sr2CrSbO6, which is associated with the tilting of the CrO6 and SbO6 octahedra, displaying tilting angles φ = 13.5° and φ = 5.5°, respectively. Magnetization measurements and low-temperature NPD data show that Sr2CrSbO6 is an antiferromagnet with a Neel temperature of 12 K with an ordered magnetic moment of 1.64(4) μB per Cr3+. The propagation vector is k = 0. Ca2CrSbO6 exhibits ferromagnetic long-range order below TC = 16 K, with a saturation magnetization of 2.36 μB at 5 K. In the ferromagnetic arrangement, the Cr3+ spins are aligned approximately along the [110] direction with an ordered magnetic moment of 2.6(2) μB. To our knowledge, this is the first example of a ferromagnetic double perovskite containing a non-magnetic element in the B positions of the perovskite structure.

Principal interactions in the magnetic system Fe 1-x Co x Si : Magnetic structure and critical temperature by neutron diffraction and SQUID measurements

Abstract: The compound ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Si}$ is a good representative for a cubic magnet with Dzyaloshinskii-Moriya interaction. On the basis of the neutron diffraction and superconducting quantum interference device measurements, we built the $H\text{\ensuremath{-}}T$ phase diagram for the compound with different $x$ from 0.1 to 0.7. The same set of parameters governs the magnetic system for different $x$. These parameters are well interpreted in the framework of the recently developed theory [S. V. Maleyev, Phys. Rev. B 73, 174402 (2006)]. As a result, the spin-wave stiffness, the Dzyaloshinskii constant, the anisotropic exchange constant, and the spin-wave gap caused by the Dzyaloshinskii interaction have been obtained and plotted as a function of $x$. The changes of the magnetic structure with $x$ can be well interpreted on the basis of our findings.