Modulated Magnetic Structure Research Articles

Low-temperature heat transport, specific heat, and magnetic properties of the hexagonal TmMnO3single crystals

We study the low-temperature heat transport, as well as the magnetization and the specific heat, of TmMnO_3 single crystals to probe the transitions of magnetic structure induced by magnetic field. It is found that the low-T thermal conductivity (\kappa) shows strong magnetic-field dependence and the overall behaviors can be understood in the scenario of magnetic scattering on phonons. In addition, a strong "dip"-like feature shows up in \kappa(H) isotherms at 3.5--4 T for H \parallel c, which is related to a known spin re-orientation of Mn^{3+} moments. The absence of this phenomenon for H \parallel a indicates that the magnetic-structure transition of TmMnO_3 cannot be driven by the in-plane field. In comparison, the magnetothermal conductivity of TmMnO_3 is much larger than that of YMnO_3 but smaller than that of HoMnO_3, indicating that the magnetisms of rare-earth ions are playing the key role in the spin-phonon coupling of the hexagonal manganites.

Magnetic properties and structural transitions of fluorite-related rare earth osmates Ln3OsO7 (Ln=Pr, Tb)

Ternary rare-earth osmates Ln3OsO7 (Ln=Pr, Tb) have been prepared. They crystallize in an ortho-rhombic superstructure of cubic fluorite with space group Cmcm. Both of these compounds undergo a structural phase transition at 130K (Ln=Pr) and 580K (Ln=Tb). These compounds show complex magnetic behavior at low temperatures. Pr3OsO7 exhibits magnetic transitions at 8 and 73K, and Tb3OsO7 magnetically orders at 8 and 60K. The Os moments become one-dimensionally ordered, and when the temperature is furthermore decreased, it provokes the ordering in the Ln3+ sublattice that simultaneously becomes three-dimensionally ordered with the Os sublattice.

Determining the magnetic ground state of TbNi5 single crystal using polarized neutron scattering technique

The TbNi5 compound shows an interesting magnetic phase transition with an incommensurate structure below 23K, whose true nature remains unresolved. In order to solve this question, we have carried out polarized neutron diffraction experiments by measuring temperature and field dependence of the intensities of satellites and Bragg reflections. From the temperature dependence of both satellite peaks and Bragg reflection, we demonstrated that it has only one magnetic structure at a given temperature. Furthermore, unlike previous reports, we found that both ferromagnetic and modulated components of the Tb ion magnetic moments should be collinear to each other. Our data also show strong depolarisation effects that are most likely to arise from domain structure of ferromagnetic component. A critical field, which destroyers a modulated magnetic structure is found to be lower than a field value to saturate the ferromagnetic component, in which the intensity of Bragg ferromagnetic reflections reaches saturation.

Ab Initio Study of Electronic and Magnetic Structure and Structural Phase Transition of (Fe1 - xMnx)2P1 - yGeyAlloys

Ab Initio Study of Electronic and Magnetic Structure and Structural Phase Transition of (Fe_{1 - x}Mn_x)₂P_{1 - y}Ge_yAlloys

Investigation of magnetic properties of non-magnetic ion (Al, Ga, In) doped Ba2Mg0.5Co1.5Fe12O22

Ba2Mg0.5Co1.5Fe12O22 (non-doped) and non-magnetic ion (Al, Ga, In) doped Ba2Mg0.5Co1.5(Fe0.99M0.01)12O22 (M-doped) polycrystalline samples were prepared by the solid-state reaction method. The crystal structures and magnetic properties of samples were investigated with x-ray diffractometer (XRD), vibrating sample magnetometer (VSM), and Mössbauer spectroscopy. The crystal structures of non-doped and M-doped samples were determined as hexagonal structures with R-3m space group by the Rietveld refinement. The unit cell volume (Vu) of non-doped sample was Vu = 1298.0 Å3, while those of M-doped samples (M = Al, Ga, In) were Vu = 1295.7, 1296.7, and 1299.9 Å3, respectively. From the magnetic hysteresis curves at room temperature, the saturation magnetization (Ms) and coercivity (Hc) of non-doped sample were found to be Ms = 28.0 emu/g and Hc = 255.4 Oe, but those of M-doped samples (M = Al, Ga, In) were Ms = 24.7, 28.4, 28.6 emu/g, and Hc = 222.0, 215.9, 190.7 Oe, respectively. From the temperature dependence of magnetization curves under 100 Oe between 4.2 and 740 K, all samples showed magnetic structure transitions. In addition Mössbauer spectra of all samples were recorded at various temperatures ranging from 4.2 to 750 K. Based on the isomer shift (δ) values of all samples, the charge states were found to be Fe3+ state at 295 K.

Evolution of spin-modulated magnetic structure in multiferroic compound Bi(1−x)SrxFeO3

BiFeO3 is a perovskite-like compound with magnetic and electrical long-range order with antiferromagnetic transition temperature about 670 K. It has space modulated magnetic structure of cycloidal type with the period λ=620±20Å incommensurate with the lattice parameter leading to specific NMR line shape. Substitution of strontium for bismuth atoms destroys the space modulated structure. Several samples with substitution of strontium for bismuth Bi1−xSrxFeO3 were studied by means of zero field NMR and Mössbauer spectroscopy. A significant broadening of 57Fe NMR spectrum was observed at x=0.10 while Mössbauer spectroscopy demonstrates this effect at x=0.07. This phenomena evidence for the destruction of the spatial modulated magnetic structure at x∼0.07–0.10 by the heterovalent substitution of strontium for bismuth without the destruction of crystal rhombohedral structure.

Magnetic excitations in the longitudinally amplitude modulated magnetic structure of PrNi2Si2

Interest in PrNi2Si2 has been renewed recently because the magnetic structure remains longitudinally amplitude modulated down to low temperatures as inferred from single–crystal neutron diffraction. Based on a dynamical matrix diagonalisation (DMD) formalism considering exchange and crystal field interactions, we have calculated the dispersion of the magnetic excitations of PrNi2Si2 below the Néel temperature. The prediction of a low-energy soft-mode excitations around the scattering vector (0,0, Qz) within the first Brillouin zone, where Qz ≡ kz = 0.875 is the value of the propagation vector of the magnetic structure, is in good agreement with the experimental results obtained from inelastic neutron scattering.

Crystal structure and magnetic properties of R5Sn4 alloys, where R is Tb, Dy, Ho, and Er

Crystal structure and magnetic properties of R5Sn4 alloys with R = Tb, Dy, Ho, and Er have been studied. R5Sn4, R11Sn10, and R5Sn3 phases coexist in the annealed alloys and the content of 11:10 and 5:3 phases varies between 9 and 17 wt.%. The R5Sn4 major phase has Sm5Ge4-type orthorhombic structure with space group Pnma. Tb5Sn4 has a complex magnetic structure, spin re-orientation and ferrimagnetic-paramagnetic phase transitions occur at ∼54 and ∼84 K, respectively. For Dy5Sn4, Ho5Sn4, and Er5Sn4, the antiferromagnetic-paramagnetic phase transitions occur at about 22, 15, and 8 K, respectively. The magnetic entropy changes (−ΔSM) of all alloys are negative at low temperature and changes to positive at higher temperatures, which could be attributed to the change of magnetic states.

Neutron Diffraction Investigations of Magnetism in BiFeO3 Epitaxial Films

AbstractThe recovery of a modulated magnetic structure in epitaxial BiFeO3 thin films as revealed by neutron diffraction is reported. The magnetic structure in thin films is found to strongly depend on substrate orientation. The substrate orientation causes different strain–relaxation processes resulting in different thin‐film crystal structures. The (110) oriented film with a monoclinic structural phase has a single‐domain modulated magnetic structure where the magnetic moment lies in the HHL plane. For the (111) oriented film that has a rhombohedral structure, a modulated structure superimposed on the G‐type antiferromagnetic order is found. These results indicate that slight structural modifications in the BiFeO3 thin film cause drastic changes in the magnetic structure.

ChemInform Abstract: Magnetic Properties and Structural Transitions of Orthorhombic Fluorite‐Related Compounds Ln3MO7 (Ln = Rare Earths, M = Transition Metals).

AbstractThe compounds Ln3MoO7 (Ln: La—Gd) are synthesized using the corresponding rare earth oxides, MoO2, and MoO3 as starting materials (1200 °C, 12 h).

Structural phase transition and magnetic properties of Co50Fe50-xSix alloys

The structural and the magnetic properties of Co50Fe50-xSix alloys are investigated by carrying out experimental measurements and electron structure calculation. The alloy series exhibits the highly chemical ordering structure, showing a strong covalent bonding effect. The lattice parameters, molecular moment and Curie temperature linearly decrease with the increase of the Si content. The deviation of the structure is attributed to the atomic size effect. Based on Stearns theories, the decrease of the itinerant 3d (di) electron results in the decrease of Curie temperature due to the substitution of Si for Fe. The change of molecular moment follows the Slater-Pauling rule, but the calculation indicates that the change of the atomic moments is not linear. It implies that the covalent bonding effect is responsible for the magnetic changes. The molecular moment and Curie temperature of Heusler alloy Co2FeSi are measured to be 5.92 B and 777 ℃, respectively. The calculation results also indicate that the half-metallic property of Co2FeSi may not be so perfect as reported, which requires a new design of energy band for the practical application. The investigation also shows that the structural transition and the magnetic structure transition accumulate in a narrow interval of Si content, which may become a good object for studying the interaction between the magnetization and the structure.

Magnetocaloric effect, magnetic domain structure and spin-reorientation transitions in HoCo 5 single crystals

A study on the magnetocaloric effect (MCE) has been carried out on the HoCo 5 single crystal using a direct method in the temperature range 78–300 K and in a magnetic field of 1.3 T at various orientations of the single crystal with respect to the direction of external magnetic field. Our research has shown that magnetic anisotropy strongly affects magnitude and sign of the MCE in the regions of spin-reorientation phase transitions. In addition, the magnetic domain structure of the HoCo 5 single crystal in (0 0 1) plane in the temperature range from room temperature down to 4.2 K was investigated.

Magnetic properties and structural transitions of orthorhombic fluorite-related compounds Ln3MO 7 ( Ln=rare earths, M=transition metals)

Magnetic properties and structural transitions of ternary rare-earth transition-metal oxides Ln 3 MO 7 ( Ln=rare earths, M=transition metals) were investigated. In this study, we prepared a series of molybdates Ln 3MoO 7 ( Ln=La–Gd). They crystallize in an orthorhombic superstructure of cubic fluorite with space group P2 12 12 1, in which Ln 3+ ions occupy two different crystallographic sites (the 8-coordinated and 7-coordinated sites). All of these compounds show a phase transition from the space group P2 12 12 1 to Pnma in the temperature range between 370 and 710 K. Their magnetic properties were characterized by magnetic susceptibility measurements from 1.8 to 400 K and specific heat measurements from 0.4 to 400 K. Gd 3MoO 7 shows an antiferromagnetic transition at 1.9 K. Measurements of the specific heat for Sm 3MoO 7 and the analysis of the magnetic specific heat indicate a “two-step” antiferromagnetic transition due to the ordering of Sm magnetic moments in different crystallographic sites, i.e., with decreasing temperature, the antiferromagnetic ordering of the 7-coordinated Sm ions occur at 2.5 K, and then the 8-coordinated Sm ions order at 0.8 K. The results of Ln 3MoO 7 were compared with the magnetic properties and structural transitions of Ln 3 MO 7 ( M=Nb, Ru, Sb, Ta, Re, Os, or Ir).

Erratum: Dielectric response to the low-temperature magnetic defect structure and spin state transition in polycrystallineLaCoO3[Phys. Rev. B79, 125105 (2009)

Erratum: Dielectric response to the low-temperature magnetic defect structure and spin state transition in polycrystalline<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi mathvariant="normal">La</mml:mi><mml:mi mathvariant="normal">Co</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math>[Phys. Rev. B<b>79</b>, 125105 (2009)

Unusual Electronic Structure and Observation of Dispersion Kink in CeFeAsO Parent Compound of FeAs-based Superconductors

We report the first comprehensive high-resolution angle-resolved photoemission measurements on CeFeAsO, a parent compound of FeAs-based high temperature superconductors with a magnetic-structural transition at ∼150 K. In the magnetic-ordering state, four holelike Fermi surface sheets are observed near Γ(0,0), and the Fermi surface near M(±π,±π) shows a tiny electronlike pocket at M surrounded by four strong spots. The unusual Fermi surface topology deviates strongly from the band structure calculations. The electronic signature of the magnetic-structural transition shows up in the dramatic change of the quasiparticle scattering rate. A dispersion kink at ∼25 meV is observed for the first time in the parent compound of Fe-based superconductors.

ChemInform Abstract: Oxides: Neutron and Synchrotron X-Ray Diffraction Studies

We shall review some results from several areas of oxide science in which neutron scattering and X-ray synchrotron scattering exercise a complement- ary role to high-resolution transmission electron microscopy. The very high- resolution time-of-flight neutron diffraction technique and its role in studies of the magnetic structure of oxides will especially be reviewed. The selected topics of structural studies for the chosen oxides are: crystal and magnetic structure of the so-called cellular random systems, magnetic structure and phase transitions in ferrites and the behaviour of water in non-stoichiom etric protonic conductors and in the opal silica-water system.

Interplay between magnetism and superconductivity in HoNi2B2C revisited

In this work the magnetic and superconducting properties of HoNi2B2C were investigated by using powder neutron diffraction and the specific heat and upper critical field (Hc2) measurements as a function of temperature. Below T=8 K, three distinct anomalies at the temperatures TN=5.2 K, TH1=5.6 K and TM=6.0 K were observed in the specific heat of HoNi2B2C, as reported in the literature. Our neutron data confirm the transitions to the Néel structure (qN=c*) at TN and to the modulated structure (qM=0.586a*) at TM. The peak at TH1=5.7 K in the specific heat data, whose exact nature was not known hitherto, is now attributed to the onset of a qH1=0.905c* magnetic helical structure as seen in our neutron data. Comparison between the thermal evolution of the magnetic structures and the temperature dependence of the upper critical field confirms that the first Hc2(T) depression at 6.1 K arises from the qM=0.586a* modulated magnetic structure. The second depression in Hc2(T) below 5.7 K can be ascribed to the qH1=0.905c* magnetic helical structure.

NMR study of the high-field magnetic phase ofLiCuVO4

We report on NMR studies of the quasi--1D antiferromagnetic $S=1/2$ chain cuprate LiCuVO$_4$, focusing on the high--field spin--modulated phase observed recently in applied magnetic fields $H > H_{\rm c2}$ ($\mu_0H_{\rm c2} \approx 7.5$ T). The NMR spectra of $^7$Li and $^{51}$V around the transition from the ordered to the paramagnetic state were measured. It is shown that the spin--modulated magnetic structure forms with ferromagnetic interactions between spins of neighboring chains within the {\bf ab}--plane at low temperatures 0.6 K $ < T < T_{\rm N}$. The best fit provides evidence that the mutual orientation between spins of neighboring {\bf ab}--planes is random. For elevated temperatures $T_{\rm N} < T \lesssim 15$ K, short--range magnetic order occurs at least on the characteristic time scale of the NMR experiment.

Kinetic magnetism and orbital order in iron telluride

Iron telluride (FeTe), a relative of the iron-based high-temperature superconductors, displays unusual magnetic order and structural transitions. Here, we explore the idea that strong correlations may play an important role in these materials. We argue that the unusual orders observed in FeTe can be understood from a picture of correlated local moments with orbital degeneracy, coupled to a small density of itinerant electrons. A component of the structural transition is attributed to orbital, rather than magnetic ordering, introducing a strongly anisotropic character to the system along the diagonal directions of the iron lattice. Double exchange interactions couple the diagonal chains leading to the observed ordering wave vector. The incommensurate order in samples with excess iron arises from electron doping in this scenario. The strong anisotropy of physical properties in the ordered phase should be detectable by transport in single domains. Predictions for ARPES, inelastic neutron scattering and hole/electron doping studies are also made.

Magnetic-field-induced transformation in FeMnGa alloys

A kind of ferromagnetic shape memory alloy with off-stoichiometric composition of Heusler alloy Fe2MnGa has been synthesized. By optimizing composition, the martensitic transformation has been modified to occur at about 163 K accompanying spontaneous magnetization, which enables a magnetic field-induced structural transition from a paramagnetic parent phase to a ferromagnetic martensite with high magnetization of 93.8 emu/g. The material performs a quite large lattice distortion through the transformation, (c−a)/c=33.5%, causing a shape memory strain upto 3.6%. Such large lattice distortions strongly influence the electron structures, and thus some special physical behavior related to the transport and conductive properties is investigated.