Ferromagnetic Planes Research Articles

Influence of dipolar coupling on the magnetic phase diagram of few (111)-monolayer eute

We demonstrate the rich variety of magnetic phases in 2- and 3-layer (111) EuTe, a classical Heisenberg antiferromagnet with staggered ferromagnetic (111) lattice planes. Because of the reduction of the spatial dimension from 3D to 2D in few-layer samples, the long-range dipolar coupling within and between the (111)-planes induces a strong shape-anisotropy. Thus the possible mutual magnetic orientations of the intrinsic ferromagnetic lattice planes strongly depend on the magnetic field direction. Even numbers of (111)-monolayers form a compensated antiferromagnet, odd numbers an (uncompensated) ferrimagnet. Correspondingly distinct spin phases (antiferromagnet, ferrimagnet, spin-flop, and ferromagnet) are observed occurring at rather different magnetic fields and dependent on the number of layers. The magnetic ground state at T = 0 K is analyzed analytically taking into account 2D interlayer lattice sums for the dipolar interaction. For finite temperatures up to the bulk Neel temperature (TN < 10 K) the (H, T)-phase diagram is analyzed by a numerical mean-field technique, investigating the transition from the isotropic nondipolar model to the highly anisotropic “full” dipolar model.

Evolution of magnetic structures in the UNi 2 Si 2 -UPd 2 Si 2 system

The influence of Pd-Ni substitution on the formation of magnetic phases in the tetragonal U(Ni1-x, Pdx)2Si2 system and the concentration magnetic phase diagram are presented. A series of different substitutions was prepared and detailed studies by powder neutron diffraction were performed for x=0.25, 0.5 and 0.75. All compounds order antiferromagnetically and form ferromagnetic basal planes stacked along the c axis (q=(0,0,qz) propagation). The ground-state phase (AF3) of UNi2Si2 is an uncompensated AF structure (++- stacking (qz=2/3)). In UPd2Si2 the ground-state phase corresponds to the simple AF structure AF2 (+-+-(qz=1)). In solid solutions, no traces of the AF3 phase were found for x=0.25 and the ground-state powder patterns correspond to AF2 for x≥0.25.

Transport anisotropy in biaxially strainedLa2/3Ca1/3MnO3thin films

Due to the complex interplay of magnetic, structural, electronic, and orbital degrees of freedom, biaxial strain is known to play an essential role in the doped manganites. For coherently strained ${\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{MnO}}_{3}$ thin films grown on ${\mathrm{SrTiO}}_{3}$ substrates, we measured the magnetotransport properties both parallel and perpendicular to the substrate and found an anomaly of the electrical transport properties. Whereas metallic behavior is found within the plane of biaxial strain, for transport perpendicular to this plane an insulating behavior and nonlinear current--voltage characteristics (IVCs) are observed. The most natural explanation of this anisotropy is a strain induced transition from an orbitally disordered ferromagnetic state to an orbitally ordered state associated with antiferromagnetic stacking of ferromagnetic manganese oxide planes.

Magnetic phase diagram and critical scattering of UNi 2Si 2

In this paper, we present the results of a neutron-scattering study on a single crystal of UNi 2Si 2. This material crystallizes in the tetragonal ThCr 2Si 2-type structure and orders antiferromagnetically (AF) at T N=125 K. Below T N, it exhibits three different AF phases in zero magnetic field. The phases are strongly anisotropic with the U moment parallel to the c-axis, forming ferromagnetic basal planes. Between T N and 108 K, the incommensurate AF1 phase with q=(0,0,q z ) is established ( q z∼0.74). The intermediate phase AF2, between 108 and 40 K, is of the AF-I type with q=(0,0,1) and the ground-state phase AF3 is an uncompensated squared-up AF structure with (+ + −) stacking of the basal planes along the c-axis ( q=(0,0,2/3) ). Both the 108 and 40 K transitions are of the first order. The complex magnetic phase diagram of UNi 2Si 2 was studied in horizontal magnetic fields parallel to the c-axis up to 6 T. Above T N, the critical scattering was studied in zero field.

Monte Carlo simulation of the magnetization of a ferromagnet with antiphase boundaries

The magnetic properties of a ferromagnetic cylinder containing an antiphase boundary with antiferromagnetic interactions are studied using atomic Monte Carlo simulation. The approach to saturation shows a reduced magnetization compared with the completely ferromagnetic sample, even in huge applied fields. The magnetization profiles are studied as a function of the number of ferromagnetic planes in the cylinder with 80 atoms per plane and also as function of the ratio of the bulk-to-antiphase exchange coupling. Hysteresis appears after removing the applied field for relatively low antiphase boundary densities. It is shown that the antiferromagnetic coupling across the antiphase boundary leads to a progressive rotation of spins as the two half “domain walls” at the boundary with opposite chirality narrow with increasing applied field.

Orbital-lattice polarons in ferromagneticLaMnO3

We investigate the combined influence of superexchange and electron-phonon interactions on the excitation spectra and on hole propagation in a ferromagnetic plane of LaMnO 3 . The Jahn-Teller effect modifies the energies of both orbital and phonon excitations and can lead to modes with mixed character, and for strong Jahn-Teller interaction even to a structural instability due to profound softening of one of the phonon modes. Solving the problem of hole propagation in the self-consistent Born approximation, we found that the quasiparticle states at the bottom of the spectrum are accompanied by broad structures, representing vibrational side bands which result from the hole-lattice coupling.

Statistical Mechanics of Image Restoration by the Plane Rotator Model

On the basis of statistical mechanics formulation for problems of image restoration and error-correcting codes, we propose a new technique of image restoration for a binary image using the plane rotator model. In our formulation, the restored image is obtained from the equilibrium state of a ferromagnetic plane rotator model under a random field which consists of the corrupted image at finite temperature. The validity of our technique is evaluated by the dependence of overlap on the hyperparameters using the replica symmetric theory for the infinite-range model. The theory shows that our technique achieves the same optimal performance with that by the Ising spins. This statement is qualitatively confirmed by Monte Carlo simulations for two-dimensional images. Furthermore we estimate the dynamics of our technique by using Monte Carlo simulations. The simulations reveal that the convergence to the restored image is faster than that by the Ising model at low temperature.

Quasiparticles and the structure of orbital polarons in ferromagneticLaMnO3

We investigate the spectral functions and orbital-correlation functions relative to the position of an e g hole moving in the ferromagnetic plane of orbitally ordered LaMnO 3 . Solving this problem in a self-consistent Born approximation, we analyze a hole motion as a function of uniaxial pressure, the Jahn-Teller interaction, and the polarization of orbitals in the vicinity of the hole. We report a large redistribution of the spectral weight as compared with the free hole dispersion (t), strongly dependent on the type of the alternating orbital order. Polarization of orbitals around the hole (Δ) can lead to a very narrow quasiparticle band and to large incoherent spectral weight, indicating a confinement of holes in a lightly doped LaMnO 3 insulator. It thus strongly modifies the shape of the orbital polaron, and favors the localization of small orbital polarons in the regime of Δ > t.

Magnetic and calorimetric studies on 6H-perovskite Ba 3TbRu 2O 9

Magnetic and calorimetric properties of Ba 3TbRu 2O 9 are reported. Neutron diffraction measurements show that this compound adopts the 6H-perovskite structure with space group P6 3/ mmc, in which the cation sites within the face-sharing octahedra are occupied by Ru ions and those within the corner-sharing octahedra are occupied by Tb ions. The magnetic susceptibility and specific heat measurements indicate that an antiferromagnetic transition occurred at 9.5 K. Neutron diffraction data collected at 2 K show that Ba 3TbRu 2O 9 has a long-range antiferromagnetic ordering of Tb 4+ ions. The moments of Tb 4+ ions order ferromagnetically in the c planes, and these ferromagnetic planes are stacked antiferromagnetically along the c-axis. The direction of ordered moments is parallel to the c-axis. The ordered magnetic moment of Tb 4+ ions is 6.84(4) μ B.

Magnetic hysteresis and superantiferromagnetism in ferritin nanoparticles

The magnetic behaviour of nanoparticles of antiferromagnetic ferritin has been investigated by means of magnetic measurements. First, using a combination of low-field susceptibility and magnetisation measurements (up to 5.5×10 4 Oe) in the superparamagnetic regime (30–250 K), we propose a method to estimate the Néel temperature of ferritin, by which we obtained T N≃500 K. The hysteresis loop at 2.5 K has been measured, and interpreted with a simple model, where the irreversibilities are due to the switching of the uncompensated moments. The magnetisation curve at 2.5 K up to 30×10 4 Oe has also been measured, and we show that it can be interpreted in terms of superantiferromagnetism, which is a finite size effect predicted by Néel in 1961, but which has not been previously experimentally evidenced. Superantiferromagnetism occurs in small antiferromagnetic particles with an even number of ferromagnetic reticular planes, and results in an enhancement of the antiferromagnetic susceptibility and in a non-linear field dependence of the magnetisation at higher fields.

Synthesis, Magnetic and Electronic Properties of Single Crystals of EuMn2P2

Large single crystals of EuMn2P2 were grown from a tin flux whose melt composition and reaction temperature profile were optimized to avoid impurity phases. The crystal structure of EuMn2P2 is of the CaAl2Si2 structure type. The structure was determined by single-crystal X-ray crystallography, with the unit cell in the trigonal lattice, a=4.1294(3) Å and c=6.9936(8) Å, at T=90 K (Z=1, R1=0.0239, wR2=0.0632) and belongs to the P3m1 (#164) space group. Temperature-dependent magnetic susceptibility, resistivity, and neutron diffraction measurements on single crystals indicate that EuMn2P2 is an antiferromagnetic insulator with a Neél temperature of 16.5±0.25 K. The Eu spins order in a simple magnetic structure where the spins in the a–b plane are aligned ferromagnetically; these ferromagnetic planes are stacked antiferromagnetically along the c-axis.

A study of the new Yb ∼0.6Fe 6Sn 6 compound by neutron diffraction, [formula omitted]Fe and [formula omitted]Sn Mössbauer spectroscopy experiments

We have stabilized a new ytterbium-based ternary derivative of the CoSn-type (P6/mmm) FeSn compound whose chemical composition is Yb ∼0.6Fe 6Sn 6. The crystal structure of this compound can be viewed as a lacunary isotype of the SmMn 6Sn 6 type of structure (P6/mmm), a partly disordered variant of the HfFe 6Ge 6-type. Powder neutron diffraction experiments indicate that Yb ∼0.6Fe 6Sn 6 adopts an antiferromagnetic arrangement (+−+−) built upon ferromagnetic (0 0 1) Fe planes, with the iron moments in the basal plane (μ Fe=1.87(2) μ B at 2 K). No magnetic ordering of the ytterbium sublattice is detected down to 2 K. These findings are corroborated by 57 Fe and 119 Sn Mössbauer spectroscopy experiments. The local orthorombic symmetry of the iron site allows observing anisotropic contributions to the total iron hyperfine field. As a consequence of the ytterbium under-stoichiometry, the magnitude of the transferred hyperfine field on Sn(2e) nuclei is found to be slightly distributed with a mean value of 〈 H〉=12.5 T. The results are related and discussed to previously published data concerning other R 1− δ Fe 6Sn 6 representatives as well as the binary FeSn parent compound.

Orbital polarons in LaMnO3

We investigate the spectral functions and orbital correlation functions relative to the position of an eg hole moving in the ferromagnetic plane of orbitally ordered LaMnO3. Polarization of orbitals around the hole leads to a very narrow quasiparticle band and to a confinement of the hole in an orbital polaron with the shape strongly depending on the type of the orbital order.

Crystal structures and magnetic properties of the 6H-perovskites Ba3LnRu2O9(Ln = Ce, Pr and Tb)

Magnetic properties of quaternary oxides Ba3LnRu2O9 (Ln = Ce, Pr and Tb) are reported. These compounds adopt the 6H-perovskite structure with space group P63/mmc, in which the cation sites within the face-sharing octahedra are occupied by Ru ions and those within the corner-sharing octahedra are occupied by Ln ions. The oxidation states of both Ru and Ln ions were found to be tetravalent. These compounds are semiconductors over the temperature range 100–400 K. Measurements of the magnetic susceptibility and specific heat were carried out. The Ln4+ antiferromagnetic transition occurred at 9.5 K for Ln = Tb and 10.5 K for Ln = Pr. Powder neutron diffraction measurements for Ba3TbRu2O9 were performed at 2 and 15 K, and at room temperature. It was found that Ba3TbRu2O9 has a long range antiferromagnetic ordering of Tb4+ ions at 2 K. The moments of the Tb4+ ions order ferromagnetically in the c plane, and these ferromagnetic planes are stacked antiferromagnetically along the c axis. The direction of the ordered moments is parallel to the c axis, and the ordered magnetic moment of the Tb4+ ions is 6.84(4) μB.

Static Theory for Planar Ferromagnets and Antiferromagnets

Here we generalize the "BBH"-asymptotic analysis to a simplified mathematical model for the planar ferromagnets and antiferromagnets. To develop such a static theory is a necessary step for a rigorous mathematical justification of dynamical laws for the magnetic vortices formally derived in [1] and [2].

Effect of biquadratic exchange on phase transitions of a planar classical Heisenberg ferromagnet

Effect of biquadratic exchange on phase transitions of a planar classical Heisenberg (or XY) ferromagnet on a stacked triangular lattice is investigated by standard Monte Carlo and histogram Monte Carlo simulations in the region of a bilinear to biquadratic exchange interaction ratio J 1/ J 2⩽1. The biquadratic exchange is found to cause separate second-order phase transitions in a strong biquadratic exchange limit, followed by simultaneous dipole and quadrupole ordering, which is of first order for an intermediate range of the exchange ratio and changes to a second-order one again as J 1/ J 2 is further increased. Thus, a phase diagram featuring both triple and tricritical points is obtained. Furthermore, a finite-size scaling analysis is used to calculate the critical indices for both dipole and quadrupole kinds of ordering.

Accumulating evidence for the two-step magnetic ordering in the borocarbides DyNi 2B 2C and DyCo 2B 2C

Accumulating evidence for a two-step magnetic ordering in the borocarbide DyNi 2B 2C is summarized, including earlier overlooked evidence for the initial magnetic transition and a recent magnetization study of polycrystalline samples. The two-step ordering involves initial two-dimensional ferromagnetic ordering in the DyC (basal) planes at T N (=16.3 K), gradual build-up of the three-dimensional (3D) alternate stacking of ferromagnetic planes, and a final 3D ordering in the AF–I-related structure at a lower temperature T o (=10.4 K), depicting a first-order transition. Supporting evidence for the two-step magnetic ordering in DyNi 2B 2C comes from point-contact spectroscopy measurements in the normal state for DyNi 2B 2C–Ag contact, and from similar behaviour of PrNi 2B 2C and (Pr 0.91Dy 0.09)Ni 2B 2C. In the isostructural borocarbide DyCo 2B 2C the two magnetic transitions (at 7.8 and 2.6 K) deduced from the specific-heat measurements are also attributed to a two-step magnetic ordering.

Magnetic features of the (U 0.50Tm 0.50)Ni 2B 2C solid solution

The quaternary borocarbides TmNi 2B 2C and UNi 2B 2C crystallize in the body-centered tetragonal LuNi 2B 2C-type structure, and exhibit antiferromagnetic order below 1.53 and 218 K, respectively. The magnetic structure of TmNi 2B 2C is characterized by incommensurate, long-range, spin-density-wave in the basal plane, with Tm moments along the c-axis, and does not involve ferromagnetic basal planes, while that of UNi 2B 2C is assumed to contain U moments along the c-axis in ferromagnetic basal planes. In the system (U,Tm)Ni 2B 2C the different magnetic structures of the end compounds result in a frustration of magnetic moments, which is not just directional. Indeed, the newly-prepared intermediate solid solution (U 0.50Tm 0.50)Ni 2B 2C is paramagnetic down to 2 K, for both as-cast and annealed samples, with no indication for any cooperative phenomenon. The paramagnetic values of the annealed sample [ θ=−5.6(5) K, μ eff=5.4(1) μ B] are compatible with those of the end compounds.

Importance of anharmonic terms in the analysis of the specific heat ofUNi2Si2

The tetragonal compound ${\mathrm{UNi}}_{2}{\mathrm{Si}}_{2}$ in zero magnetic field exhibits three different antiferromagnetic phases below ${T}_{N}=124 \mathrm{K}.$ They are characterized by the U moments parallel to the c axis, forming ferromagnetic basal planes and by a propagation vector $\mathit{q}{=(0,0,q}_{z}).$ Consequently, below ${T}_{N}$ two order-order magnetic phase transitions are observed; namely, at ${T}_{1}=108 \mathrm{K}$ and ${T}_{2}=40 \mathrm{K}.$ The later two transitions are of the first order type. All three magnetic phase transitions are reflected by the pronounced anomalies of bulk thermodynamic properties such as thermal expansion, resistivity, susceptibility, and specific heat. Here we present the analysis of specific heat data going beyond the harmonic approximation. Realistic values of magnetic specific heat and magnetic entropy were obtained by this approach. An additional exponential contribution to the specific heat and resistivity below ${T}_{2}$ is considered. It is attributed to the anisotropy induced gap in the magnetic excitation spectrum.

Photoemission spectra of LaMnO3 controlled by orbital excitations

We investigate the spectral function of a hole moving in the orbital-ordered ferromagnetic planes of LaMnO3, and show that it depends critically on the type of orbital ordering. While the hole does not couple to the spin excitations, it interacts strongly with the excitations of e(g) orbitals (orbitons), leading to a new type of quasiparticle with a dispersion on the orbiton energy scale and with strongly enhanced mass and reduced weight. Therefore we predict a large redistribution of spectral weight with respect to the bands found in local density approximation (LDA) or in LDA+U.