Showing papers on "Magnetic structure published in 2005"

Geometric, electronic, and magnetic structure of Co2FeSi: Curie temperature and magnetic moment measurements and calculations

Abstract: In this work a simple concept was used for a systematic search for materials with high spin polarization It is based on two semiempirical models First, the Slater-Pauling rule was used for estimation of the magnetic moment This model is well supported by electronic structure calculations The second model was found particularly for ${\mathrm{Co}}_{2}$ based Heusler compounds when comparing their magnetic properties It turned out that these compounds exhibit seemingly a linear dependence of the Curie temperature as function of the magnetic moment Stimulated by these models, ${\mathrm{Co}}_{2}\mathrm{FeSi}$ was revisited The compound was investigated in detail concerning its geometrical and magnetic structure by means of x-ray diffraction, x-ray absorption, and M\"ossbauer spectroscopies as well as high and low temperature magnetometry The measurements revealed that it is, currently, the material with the highest magnetic moment $(6{\ensuremath{\mu}}_{B})$ and Curie temperature (1100 K) in the classes of Heusler compounds as well as half-metallic ferromagnets The experimental findings are supported by detailed electronic structure calculations

Spin disorder on a triangular lattice

Abstract: As liquids crystallize into solids on cooling, spins in magnets generally form periodic order. However, three decades ago, it was theoretically proposed that spins on a triangular lattice form a liquidlike disordered state at low temperatures. Whether or not a spin liquid is stabilized by geometrical frustration has remained an active point of inquiry ever since. Our thermodynamic and neutron measurements on NiGa2S4, a rare example of a two-dimensional triangular lattice antiferromagnet, demonstrate that geometrical frustration stabilizes a low-temperature spin-disordered state with coherence beyond the two-spin correlation length. Spin liquid formation may be an origin of such behavior.

Second-harmonic generation as a tool for studying electronic and magnetic structures of crystals: review

Abstract: Second-harmonic generation (SHG) in magnetically ordered crystals is reviewed. The symmetry of such crystals is determined by the arrangement of both the charges and the spins, so their contributions to the crystallographic and the magnetic structures, respectively, must be distinguished. Magnetic SHG is introduced as a probe for magnetic structures and sublattice interactions. The specific degrees of optical experiments - including spectral, spatial, and temporal resolution - lead to the observation of novel physical effects that cannot be revealed by other techniques of probing magnetism. These include local or hidden phase transitions, interacting magnetized and polarized sublattices and domain walls, and magnetic interfaces. SHG in various centrosymmetric and noncentrosymmetric crystal classes of antiferromagnetic oxides such as Cr2O3, hexagonal RMnO3(R=Sc,Y,In,Ho-Lu), magnetic garnet films, CuB2O4, CoO, and NiO, is discussed.

Electric polarization rotation in a hexaferrite with long-wavelength magnetic structures.

Abstract: We report on the control of electric polarization (P) by using magnetic fields (B) in a hexaferrite having magnetic order above room temperature (RT). The material investigated is hexagonal Ba0.5Sr1.5Zn2Fe12O22, which is a nonferroelectric helimagnetic insulator in the zero-field ground state. By applying B, the system undergoes successive metamagnetic transitions, and shows concomitant ferroelectric order in some of the B-induced phases with long-wavelength magnetic structures. The magnetoelectrically induced P can be rotated 360 degrees by external B. This opens up the potential for not only RT magnetoelectric devices but also devices based on the magnetically controlled electro-optical response.

Transport and magnetic properties of Gd Ba Co 2 O 5 + x single crystals: A cobalt oxide with square-lattice Co O 2 planes over a wide range of electron and hole doping

Abstract: Single crystals of the layered perovskite GdBaCo_{2}O_{5+x} (GBCO) have been grown by the floating-zone method, and their transport, magnetic, and structural properties have been studied in detail over a wide range of oxygen contents. The obtained data are used to establish a rich phase diagram centered at the "parent'' compound GdBaCo_{2}O_{5.5} -- an insulator with Co ions in the 3+ state. An attractive feature of GBCO is that it allows a precise and continuous doping of CoO_{2} planes with either electrons or holes, spanning a wide range from the charge-ordered insulator at 50% electron doping (x=0) to the undoped band insulator (x=0.5), and further towards the heavily hole-doped metallic state. This continuous doping is clearly manifested in the behavior of thermoelectric power which exhibits a spectacular divergence with approaching x=0.5, where it reaches large absolute values and abruptly changes its sign. At low temperatures, the homogeneous distribution of doped carriers in GBCO becomes unstable, and both the magnetic and transport properties point to an intriguing nanoscopic phase separation. We also find that throughout the composition range the magnetic behavior in GBCO is governed by a delicate balance between ferromagnetic (FM) and antiferromagnetic (AF) interactions, which can be easily affected by temperature, doping, or magnetic field, bringing about FM-AF transitions and a giant magnetoresistance (MR) phenomenon. An exceptionally strong uniaxial anisotropy of the Co spins, which dramatically simplifies the possible spin arrangements, together with the possibility of continuous ambipolar doping turn GBCO into a model system for studying the competing magnetic interactions, nanoscopic phase separation and accompanying magnetoresistance phenomena.

Spin structure and magnetic frustration in multiferroic RMn2O5 (R=Tb,Ho,Dy)

Abstract: We have studied the crystal and magnetic structures of the magnetoelectric materials RMn2O5 (R=Tb,Ho,Dy) using neutron diffraction as a function of temperature. All three materials display incommensurate antiferromagnetic ordering below 40 K, becoming commensurate on further cooling. For R=Tb,Ho, a commensurate-incommensurate transition takes place at low temperatures. The commensurate magnetic structures have been solved and are discussed in terms of competing exchange interactions. The spin configuration within the ab plane is essentially the same for each system, and the radius of R determines the sign of the magnetic exchange between adjacent planes. The inherent magnetic frustration in these materials is lifted by a small lattice distortion, primarily involving shifts of the Mn3+ cations and giving rise to a canted antiferroelectric phase.

Method and system for cleaning magnetic artifacts using a carbonyl reactive ion etch

Abstract: A method and system for providing a magnetic structure that includes at least one magnetic material is disclosed The method and system include defining the magnetic structure The magnetic structure also includes a top layer that is insensitive to an istroropic carbonyl reactive ion etch The defining of the magnetic structure results in at least one artifact The method and system further includes cleaning the at least one artifact using at least one isotropic carbonyl reactive ion etch

Ground-state and finite-temperature signatures of quantum phase transitions in the half-filled Hubbard model on a honeycomb lattice

Abstract: We investigate ground state and finite temperature properties of the half-filled Hubbard model on a honeycomb lattice using quantum Monte Carlo and series expansion techniques. Unlike the square lattice, for which magnetic order exists at $T=0$ for any nonzero $U$, the honeycomb lattice is known to have a semimetal phase at small $U$ and an antiferromagnetic one at large $U$. We investigate the phase transition at $T=0$ by studying the magnetic structure factor and compressibility using quantum Monte Carlo simulations and by calculating the sublattice magnetization, uniform susceptibility, spinwave, and single hole dispersion using series expansions around the ordered phase. Our results are consistent with a single continuous transition at ${U}_{c}∕t$ in the range 4--5. Finite-temperature signatures of this phase transition are seen in the behavior of the specific heat, $C(T)$, which changes from a two-peaked structure for $Ug{U}_{c}$ to a one-peaked structure for $Ul{U}_{c}$. Furthermore, the $U$ dependence of the low temperature coefficient of $C(T)$ exhibits an anomaly at $U\ensuremath{\approx}{U}_{c}$.

Revealing antiferromagnetic order of the Fe monolayer on W(001): spin-polarized scanning tunneling microscopy and first-principles calculations.

Abstract: We prove that the magnetic ground state of a single monolayer Fe on W(001) is $c(2\ifmmode\times\else\texttimes\fi{}2)$ antiferromagnetic, i.e., a checkerboard arrangement of antiparallel magnetic moments. Real space images of this magnetic structure have been obtained with spin-polarized scanning tunneling microscopy. An out-of-plane easy magnetization axis is concluded from measurements in an external magnetic field. The magnetic ground state and anisotropy axis are explained based on first-principles calculations.

Magnetic order and spin fluctuations in the spin liquid Tb$_2$Sn$_2$O$_7$.

Abstract: We have studied the spin liquid Tb$_2$Sn$_2$O$_7$ by neutron diffraction and specific heat measurements. Below about 2 K, the magnetic correlations change from antiferromagnetic to ferromagnetic. Magnetic order settles in two steps, with a smeared transition at 1.3(1) K then an abrupt transition at 0.87(2) K. A new magnetic structure is observed, not predicted by current models, with both ferromagnetic and antiferromagnetic character. It suggests that the spin liquid degeneracy is lifted by dipolar interactions combined with a finite anisotropy along $ $ axes. In the ground state, the Tb$^{3+}$ ordered moment is reduced with respect to the free ion moment (9 $\mu_{\rm B}$). The moment value of 3.3(3) $\mu_{\rm B}$ deduced from the specific heat is much smaller than derived from neutron diffraction of 5.9(1) $\mu_{\rm B}$. This difference is interpreted by the persistence of slow collective magnetic fluctuations down to the lowest temperatures.

Theoretical study of structural, electronic, and magnetic properties of Au n M + clusters ( M=Sc , Ti, V, Cr, Mn, Fe, Au; n⩽9 )

Abstract: We investigate the element- and size-dependent electron stability of Aun M+ clusters (M=Sc, Ti, V, Cr, Mn, Fe, Au; n≤9) by means of first-principles density functional calculations. The interplay between the cluster atomic arrangements and their electronic and magnetic structure is studied for the few lowest energy isomeric states and their dependence on the dopant atom and its environment in the host cluster. Our total energy calculations provide a clear explanation of the abundance peaks observed recently in photofragmentation experiments. The magnetic and geometrical configurations are strongly correlated. The local magnetic moment of the dopant atom shows a pronounced odd-even oscillation with the number of Au atoms, and decreases when the cluster size increases. © 2005 The American Physical Society.

Ordering in the pyrochlore antiferromagnet due to Dzyaloshinsky-Moriya interactions

Abstract: The Heisenberg nearest neighbour antiferromagnet on the pyrochlore (3D) lattice is highly frustrated and does not order at low temperature where spin-spin correlations remain short ranged. Dzyaloshinsky-Moriya interactions (DMI) may be present in pyrochlore compounds as is shown, and the consequences of such interactions on the magnetic properties are investigated through mean field approximation and monte carlo simulations. It is found that DMI (if present) tremendously change the low temperature behaviour of the system. At a temperature of the order of the DMI a phase transition to a long range ordered state takes place. The ordered magnetic structures are explicited for the different possible DMI which are introduced on the basis of symmetry arguments. The relevance of such a scenario for pyrochlore compounds in which an ordered magnetic structure is observed experimentally is dicussed.

A new numerical scheme for structures of rotating magnetic stars

Abstract: We have developed a new numerical scheme for obtaining structures of rapidly rotating stars with strong magnetic fields. In our scheme, both poloidal and toroidal magnetic fields can be treated for stars with compressibility and infinite conductivity. By introducing the vector potential and its integral representation, we can treat the boundary condition for the magnetic fields across the surface properly. We show structures and distributions of magnetic fields as well as the distributions of the currents of rotating magnetic polytropic stars with polytropic index N = 1.5. The shapes of magnetic stars are oblate as long as the magnetic vector potential decreases as 1/r when r →∞ .F or extremely strong magnetic fields, equilibrium configurations can be of toroidal shapes. Ke yw ords: stars: magnetic fields ‐ stars: rotation.

Search for half-metallic ferrimagnetism in V-based Heusler alloys Mn$_2$VZ (Z$=$Al, Ga, In, Si, Ge, Sn)

Abstract: Using a state-of-the-art full-potential electronic structure method within the local spin density approximation, we study the electronic and magnetic structure of Mn$_2$V-based full Heusler alloys: Mn$_2$VZ (Z=Al, Ga, In, Si, Ge, and Sn). We show that small expansion of the calculated theoretical equilibrium lattice constants restores the half-metallic ferrimagnetism in these compounds. Moreover a small degree of disorder between the V and Z atoms, although iduces some states within the gap, it preserves the Slater-Pauling behaviour of the spin magnetic moments and the alloys keep a high degree of spin-polarisation at the Fermi level opening the way for a half-metallic compensated ferrimagnet.

Local structure of LiNiO2 studied by neutron diffraction

Abstract: The nature of the magnetic state of $\mathrm{Li}\mathrm{Ni}{\mathrm{O}}_{2}$ has been controversial. In this compound Ni spins $(S=1∕2)$ form a triangular lattice with the possibility of magnetic frustration, but the exact state of spin correlation has not yet been known in spite of the extensive research work. A factor that complicates understanding of the magnetic state is the orbital state of ${\mathrm{Ni}}^{3+}$ which is a Jahn-Teller (JT) ion. While there is no signature of long-range Jahn-Teller distortion, local JT distortion has been suspected. We have performed neutron diffraction and atomic pair-density function analyses up to unprecedented large distances to discover a number of unusual features, such as anomalous peak broadening, local JT distortion, sharp oxygen-oxygen distance correlations, and inverted temperature dependence of medium range correlation. These observations are best explained by local orbital ordering of ${\mathrm{Ni}}^{3+}$ ions into three sublattices. This orbital ordering, however, cannot develop into long-range order because of the strain field it generates, and domains of about $10\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in size are formed. Domains are susceptible to random pinning by impurities (site disorder) resulting in the loss of structural long-range order. We suggest that this local orbital ordering is the basis for the complex magnetic properties observed in this compound.

Spin collinearity and thermal disorder in ε-Fe2O3

Abstract: Studies of magnetoelectric e-Fe2O3 nanoparticles (30nm) in silica by Fe57 Mossbauer spectrometry (4.2–600K,0–6T) are reported. They indicate a magnetic/structural transition at ∼130K, a high-temperature collinear ferrimagnetic spin structure similar to FeGaO3 and FeAlO3, and the Curie temperature TC at 585K. Reduced hyperfine field versus T∕TC is (S=5∕2) Brillouin-like or Langevin-like according to the structural site. The magnetization at 300K is estimated to 0.25μB∕Fe in accord with published data.

Angle-dispersive neutron diffraction under high pressure to 10 GPa

Abstract: We present a method which allows high-quality powder neutron diffraction patterns to be obtained under pressure by angle-dispersive diffraction to at least 10GPa. This technique uses a new type of Paris–Edinburgh press in conjunction with sintered boron nitride anvils. As an example, we show NiO diffraction patterns obtained under purely hydrostatic pressures up to 10GPa. These data were collected within a few hours, and are free from any contaminating signal from the pressure cell. High-resolution nuclear and magnetic structural information can be readily extracted by Rietveld refinements, without additional data correction. This technique will allow powder neutron diffraction at elevated pressures to become a standard tool on continuous neutron facilities.

Crystal structure and magnetic ordering of the LaCo1?xFexO3 system

Abstract: The LaCo1−xFexO3 compounds have been investigated by means of neutron powder diffraction (NPD), x-ray powder diffraction (XPD) and magnetization measurements. The NPD and XPD patterns were successfully refined as rhombohedral (x≤0.5) and orthorhombic (x≥0.6). The temperature-induced transition from the rhombohedral phase into the orthorhombic one is characterized by a two-phase crystal structure state. Magnetization and neutron powder measurements have revealed that compounds with x<0.4 exhibit a paramagnetic-like behaviour, whereas for x≥0.4 samples a weak ferromagnetic component was observed. The NPD patterns were successfully refined by admitting a Gz spatial orientation of the antiferromagnetic vector. The magnetic properties of the LaCo1−xFexO3 samples can be explained assuming a low spin state of the Co3+ ions, whereas antiferromagnetism is caused by magnetic interactions between the Fe3+ ions. Based on the obtained data the combined crystal and magnetic phase diagram has been constructed.

Preparation, crystal and magnetic structures of two new double perovskites: Ca2CoTeO6 and Sr2CoTeO6

Abstract: Ca2CoTeO6 and Sr2CoTeO6 double perovskites have been prepared as polycrystalline powders by solid state reaction, in air. These materials have been studied by X-ray, neutron powder diffraction (NPD) and magnetic measurements. At room temperature, the crystal structure is monoclinic, space group P21/n for both compounds with a = 5.4569(2) A, b = 5.5904(2) A, c = 7.7399(2) A, β = 90.239(2)° and a = 5.6417(2) A, b = 5.6063(2) A, c= 7.9234(2) A, β = 90.117(4)° for Ca2CoTeO6 and Sr2CoTeO6 respectively. The perovskite lattices contain an almost completely ordered array of CoO6 and TeO6 octahedra. The monoclinic distortion is larger in Ca2CoTeO6 than in Sr2CoTeO6, which is associated with the tilting of the CoO6 and TeO6 octahedra, displaying tilting angles φ = 4.8° for Sr2CoTeO6 and φ = 15.1° for Ca2CoTeO6. The low temperature magnetic structures were determined by NPD, selected among the possible magnetic solutions compatible with the P21/n space group, according with the group theory representation. The propagation vectors for the Ca and Sr compounds are k = 0 and k = (½, ½, 0), respectively. A canted antiferromagnetic structure is observed for Ca2CoTeO6 below TN = 10 K, which remains stable down to 2.2 K, with an ordered magnetic moment of 2.37(3) μB for Co2+ cations. For Sr2CoTeO6, an antiferromagnetic ordering is displayed below TN = 15 K, that remains stable down to 2.9 K, with an ordered magnetic moment for Co2+ of 2.15(4) μB.

Magnetic and Electrical Properties of NpTGa5 (T=Fe, Rh and Ni)

Abstract: We grew high-quality single crystals of NpTGa 5 (T = Fe, Rh, and Ni) with a tetragonal structure by the Ga-flux method and measured their electrical resistivity, specific heat, magnetic susceptibility, and magnetization. All the investigated compounds undergo magnetic ordering. NpFeGa 5 orders antiferromagnetically at T N =118 K and shows another magnetic transition at T * =78 K. A relatively large electrical resistivity indicates that NpFeGa 5 might be a low-carrier compound. NpRhGa 5 also orders antiferromagnetically at T N1 =36 K. Below another magnetic transition at T N2 =32 K, the antiferromagnetic easy-axis is most likely changed from [001] to the (001) plane. On the other hand, NpNiGa 5 undergoes ferromagnetic ordering at T C =30 K, where the magnetic moment of Np is directed along the [001] direction. Furthermore, below another magnetic transition at T * =18 K, the ordered moment is discontinuously enlarged and a change of the magnetic structure occurs. The electronic specific heat coefficients ar...

Magnetic Phase Diagram of the Triangular Lattice Antiferromagnet CuFe1-xAlxO2

Abstract: We have performed magnetic susceptibility, neutron diffraction and specific heat measurements on the triangular lattice antiferromagnet CuFe 1- x Al x O 2 for x ≤0.050. A magnetic phase diagram for temperature vs x was determined. The various magnetically ordered phases compete with each other on the x – T magnetic phase diagram, strongly suggesting that the quasi-Ising orderings are stabilized in CuFeO 2 with delicate balance of competing interactions. In the ground state for 0.014 < x < 0.030 [low-temperature (LT) phase], the magnetic structure consists of two magnetic modulations with slightly different wave numbers. Moreover, we observed the change in magnetic diffraction patterns in the LT phase, which corresponds to a crystallographic distortion from “hexagonal” to “orthorhombic”, suggesting that the Ising anisotropy in CuFeO 2 is closely related to the crystallographic distortion.