Types Of Magnetic Structures Research Articles

Multilayer Co/Pd films with nanocrystalline and amorphous Co layers: Coercive force, random anisotropy, and exchange coupling of grains

The magnetic structure of nanocrystalline and amorphous ferromagnets is determined by their random magnetic anisotropy (RMA) and results from a competition of the ordering action of exchange interactions and the disordering effect of the random local anisotropy K breaking the long-range ferromagnetic order [1]. In such materials, the ferromagnetic order is established at a characteristic length R f = R c ( H ex / D 1/2 H a ) 2 , where R c is the random anisotropy correlation radius, H ex is the exchange field, D is a numerical coefficient of the symmetry (equal to 1/15 in the case of a uniaxial anisotropy), and M s is the saturation magnetization. The resulting magnetic structure can be described in terms of the ensemble of weakly coupled magnetic blocks [1, 2] with an average dimension of 2 R f , the average block anisotropy 〈 K 〉 = K / N 1/2 = K ( R c / R f ) 3/2 , and randomly oriented unit anisotropy vectors n . To the present, researchers engaged in the magnetic properties of such materials believed that this type of magnetic structure is just what accounts for the observed values of characteristics of the magnetically soft amorphous and nanocrystalline alloys [2‐4]. It should be noted that an analysis of the behavior of the coercive force H c within the framework of the RMA model used in [2‐4] is directed mostly to the study of H c as a function of the nanograin size, H c ( R c ), thereby assuming that the values of anisotropy K ( R c ), exchange coupling A ( R c ), and saturation magnetization M s ( R c ) are constant (or their changes can be ignored). However, now there are prerequisites for undertaking complex measurements of all the parameters entering into theoretical expressions for H c ( R c , H a , A , and M s ) and revealing the corresponding dependences. In particular, the RMA and magnetic block structure can be studied using small-angle neutron diffraction [5‐7] and the measurements of magnetization curves in the region of magnetizations close to saturation [8‐10]. The exchange coupling constants are widely determined using methods based on the spin- wave resonance (SWR) [11] and Bloch’s T 3/2 law [12]. The results of our preliminary investigation of the magnetization behavior M ( H ) in multilayer Co/Pd film structures [9] showed that these films are characterized by a two-dimensional inhomogeneity of the magnetic anisotropy. In particular, it was found that the experimental curve of M ( H ) measured in the range of field strengths from 5 to 25 kOe can be described by the expression

The crystal and magnetic structure relationship in Cu(W1-xMox)O4 compoundswith wolframite-type structure

The magnetic structures of Cu(W1-xMox)O4 compounds withwolframite-type structure at 1.5 K have been determined by neutronpowder diffraction for average composition ⟨x⟩ = 0.15, 0.25 and 0.35. For⟨x⟩ = 0.15the magnetic structure is antiferromagnetic with a magnetic unit cell doubled along thea-axis, k⃗ = (1/2, 0, 0),i.e. the same magnetic structure as for CuWO4. For ⟨x⟩ = 0.25 and 0.35two magnetic structures are observed: one is identical to that for ⟨x⟩ = 0.15,while the other is doubled with respect to thec-axis, k⃗ = (0, 0, 1/2),i.e. the same magnetic structure as for the high-pressure modificationCuMoO4 III. The coexistence of these two magnetic arrangements isinterpreted as reflecting a slightly inhomogeneous contribution of Mo andW in different crystallites together with a sharp transition between thestability ranges of the two types of magnetic structure with respect tox. Thespecific Mo:W distributions in the grains of the powdered samples were deduced froma profile analysis based on high-resolution synchrotron powder diffraction data.No additional, intermediate magnetic phase with k⃗ = (1/2, 1/2, 0)was found in Cu(W0.75Mo0.25)O4, in contrast to predictions in the frameworkof extended Hückel calculations based on the precise crystal structure.

Vortices and magnetic structures of the target type in a two-dimensional ferromagnet with anisotropic exchange

Various types of magnetic structure in a two-dimensional Heisenberg ferromagnet are considered in a continuum approximation. The effect of anisotropic exchange and single-ion anisotropy on the vortex structure is analyzed. A new type of static magnetic structure (“target”) is predicted and investigated in an easy-plane ferromagnet.

Magnetic structures and their determination using group theory

The determination of magnetic structures is a special area of condensed matter research. While being fundamental to the understanding of electronic structures and properties, it remains a subject that is treated with difficulty and is full of incorrect solutions. This article is based around two goals: • The explanation of the different possible types of magnetic structure. • The demonstration of how symmetry leads to their proper description, and can aid their solution. In content, the first part of this article is based on the practicalities of what an experimenter should know in order to understand and describe a magnetic structure. In the second part, symmetry arguments will be shown to reduce the otherwise arduous task of determining a magnetic structure, to the investigation of a handful of possible structures.

The elliptical undulator UE46 and its monochromator beam-line for structural research on nanomagnets at BESSY-II

The soft X-ray monochromator beamline installed at an elliptical undulator at BESSY-II is discussed. A technical description of the undulator and the main elements of the beamline is given. The APPLE-II type magnetic structure of the undulator is a pure permanent magnet design and allows four different modes of operation: circular polarization, horizontal and vertical polarization, and linear polarization under various angles. The beamline optics was designed to cover the soft X-ray energy range from 200 up to 2000 eV, and the plane grating monochromator has three different gratings which can be selected by a lateral translation inside the UHV vacuum.

Noncollinear amplitude-modulated magnetic order in Gd compounds

In the present work a model within the mean-field theory is developed in order to analyze the specific heat of magnetically ordered systems. This model allows to draw conclusions about the type of magnetic structure from the specific heat near the magnetic transition. The known description of collinear amplitude-modulated and equal-moment magnetism has been extended to account for noncollinear amplitude-modulated (NCAM) antiferromagnetic order by introducing an anisotropic exchange interaction. Experimental evidence for NCAM order is expected from measurements of the specific-heat anomaly at the ordering temperature and from magnetic scattering experiments. The specific heat of ${\mathrm{GdCu}}_{2}$ was measured and analyzed within the model and a good agreement is reached. Furthermore, the specific heat of other noncollinear Gd antiferromagnets near the ordering temperature has been calculated and is compared to available experimental data.

The magnetic structure of TbNiAlD 1.1

The magnetic structure of orthorhombic TbNiAlD 1.1 was determined by powder neutron diffraction at 3.8 and 10 K, below the antiferromagnetic ordering temperature T N=11 K. The unit cell is doubled in b and c directions compared to the nuclear unit cell (space group Amm2; at 3.8 K a=3.6554; b=12.3954; c=7.6316 Å), and the magnetic moments are confined to the bc plane with main component along c. The magnetic structure is different from TbNiAl and TbNiAlD 0.3 which both have ordered magnetic moments along the corresponding a axis. The magnetic moments of Tb1 and Tb2 at 3.8 K are 6.8 and 5.7 μ B, respectively. On heating to 10 K, the type of magnetic structure is not changed, however, μ(Tb) are decreased to 2.9 μ B (Tb1) and 5.4 μ B (Tb2).

Incommensurate Magnetic Structure of the Heavy Fermion Antiferromagnet Ce7Ni3

Magnetic structures of the heavy fermion antiferromagnet Ce 7 Ni 3 have been studied by neutron diffraction using a single crystal. It undergoes the two successive phase transitions at T N1 = 1.8 K and T N2 = 0.7 K in zero field. At T N1 ordering of a single- k type magnetic structure with an incommensurate modulation vector k IC ≃0.22 c * takes place. This structure is approximately a sinusoidally modulated structure of c -axis moments, which belongs to the Γ 2 and Γ 3 irreducible representations of the space group P 6 3 m c . Root mean squares of the moments at T = 1.4 K are 0.46±0.07, 0.70±0.03 and 0.10±0.05 µ B for the Ce1, Ce2 and Ce3 sites, respectively. Below T N2 the incommensurate modulation vector becomes temperature independent, and a commensurate structure with \({\mib k}_{\rm C} = \frac{1}{4} {\mib c}^{\ast}\) appears in addition to the incommensurate structure. The metamagnetic transition under the magnetic field of H ∥ c = 0.16 T at T = 1.5 K is a first order transition to a ferromagnetic s...

Magnetotail response during a strong substorm as observed by GEOTAIL in the distant tail

Abstract. Simultaneous energetic particle and magnetic field observations from the GEOTAIL spacecraft in the distant tail (XGSM≈ –150 Re) have been analysed to study the response of the Earth's magnetotail during a strong substorm (AE ≤ 680 nT). At geosynchronous altitude, LANL spacecraft recorded three electron injections between 0030 UT and 0130 UT, which correspond to onsets observed on the ground at Kiruna Ground Observatory. The Earth's magnetotail responded to this substorm with the ejection of five plasmoids, whose size decreases from one plasmoid to the next. Since the type of magnetic structure detected by a spacecraft residing the lobes, depends on the Z extent of the structure passing underneath the spacecraft, GEOTAIL is first engulfed by a plasmoid structure; six minutes later it detects a boundary layer plasmoid (BLP) and finally at the recovery phase of the substorm GEOTAIL observes three travelling compression regions (TCRs). The time-of-flight (TOF) speed of these magnetic structures was estimated to range between 510 km/s and 620 km/s. The length of these individual plasmoids was calculated to be between 28 Re and 56 Re. The principal axis analysis performed on the magnetic field during the TCR encountered, has confirmed that GEOTAIL observed a 2-D perturbation in the X-Z plane due to the passage of a plasmoid underneath. The first large plasmoid that engulfed GEOTAIL was much more complicated in nature probably due to the external, variable draped field lines associated with high beta plasma sheet and the PSBL flux tubes surrounding the plasmoid. From the analysis of the energetic particle angular distribution, evidence was found that ions were accelerated from the distant X-line at the onset of the burst associated with the first magnetic structure. Key words. Magnetospheric physics (magnetospheric configuration and dynamics; magnetotail).

Bandwidth-controlled magnetic and electronic transitions inLa0.5Ca0.5−xSrxMnO3(0<~x<~0.5)distorted perovskite

We have investigated the effect of the increase of the one-electron e g bandwidth (W) on the ferromagnetic and antiferromagnetic transitions in charge ordered La 0.5 Ca 0.5 MnO 3 by substituting bigger Sr 2+ ions for Ca 2+ ions through electrical resistivity and low-field ac-susceptibility measurements as a function of temperature. The system La 0.5 Ca 0.5-x Sr x MnO 3 is shown to undergo structural transitions with increasing x. For x=0.0 and 0.1, the structure is orthorhombic with space group Pnma. For x=0.2, 0.3, and 0.4, the structure is consistent with a monoclinic space group I2/a. The end member (x=0.5) has a tetragonal structure (I4/mcm). T C increases with x monotonically, consistent with the fact that the increase of W enhances double-exchange ferromagnetic interaction. On the other hand, in contrast to the expected monotonic suppression, T N increases initially with Sr concentration, followed by a decrease, finally disappearing for x=0.4. The nonmonotonous variation of T N may indicate a possible change in the type of magnetic structure in the antiferromagnetic state.

Magnetotail response during a strong substorm as observed by GEOTAIL in the distant tail

Simultaneous energetic particle and magnetic field observations from the GEOTAIL spacecraft in the distant tail (XGSM≈ –150 Re) have been analysed to study the response of the Earth's magnetotail during a strong substorm (AE ≤ 680 nT). At geosynchronous altitude, LANL spacecraft recorded three electron injections between 0030 UT and 0130 UT, which correspond to onsets observed on the ground at Kiruna Ground Observatory. The Earth's magnetotail responded to this substorm with the ejection of five plasmoids, whose size decreases from one plasmoid to the next. Since the type of magnetic structure detected by a spacecraft residing the lobes, depends on the Z extent of the structure passing underneath the spacecraft, GEOTAIL is first engulfed by a plasmoid structure; six minutes later it detects a boundary layer plasmoid (BLP) and finally at the recovery phase of the substorm GEOTAIL observes three travelling compression regions (TCRs). The time-of-flight (TOF) speed of these magnetic structures was estimated to range between 510 km/s and 620 km/s. The length of these individual plasmoids was calculated to be between 28 Re and 56 Re. The principal axis analysis performed on the magnetic field during the TCR encountered, has confirmed that GEOTAIL observed a 2-D perturbation in the X-Z plane due to the passage of a plasmoid underneath. The first large plasmoid that engulfed GEOTAIL was much more complicated in nature probably due to the external, variable draped field lines associated with high beta plasma sheet and the PSBL flux tubes surrounding the plasmoid. From the analysis of the energetic particle angular distribution, evidence was found that ions were accelerated from the distant X-line at the onset of the burst associated with the first magnetic structure. Key words. Magnetospheric physics (magnetospheric configuration and dynamics; magnetotail).

Characteristics of phase transitions in crystals having a triangular structure in a magnetic field

Analysis of the field dependence of the order parameters and comparison with experimental data on the behavior of the magnetic characteristics of iron phosphide compounds in the stability region of the metamagnetic phase provides a basis for identifying the type of magnetic structure that can exist in this case. The invariants responsible for first-order magnetic phase transitions in crystals having a triangular magnetic structure are identified from the entire rational basis of invariants.

Magnetic field induced changes in the critical exponent β of the chiral triangular antiferromagnet CsMnBr3 in the presence of an electric field (abstract)

The magnetic moments in the triangular antiferromagnetic CsMnBr3 order at TN=8.3 K with a frustration of the magnetic moments in a 120° type magnetic structure. The magnetic moments are either ordered in a clockwise or an anticlockwise direction resulting in a chirality ordering, which is characterized by the critical exponent of the magnetic sublattice magnetization β=0.25. It has been shown that an electric field applied along the [110] direction removes the chiral ordering and a linear Ising-like phase will be established; consequently, the value of β decreases. The subsequent application of a magnetic field of B=4 T along the [100] direction causes the critical exponent β to increase. The magnetic phase diagram of CsMnBr3 does not change due to the introduction of an electric field.

Magnetic field induced crossover to chiral ordering in the easy-axis triangular antiferromagnet CsNiCl3 (abstract)

The easy-axis triangular antiferromagnet CsNiCl3 undergoes a magnetic phase transition to a triangular chirally ordered 120° type magnetic structure in an applied magnetic field of 2 T directed along the c axis. We studied by means of elastic neutron diffraction the crossover to chiral ordering. At B=2 T, the exponent of the sublattice magnetization β=0.25, which confirms the creation of the chirally ordered state. Data taken at B=1.75 T indicate that the crossover may be smeared out in B or is due to a slight misorientation of the applied field. The induced change in the magnetization direction of the ordered moments was also studied by means of SQUID magnetometry.

57Fe Mössbauer effect in UPdSn

Previously, the transitions to the noncollinear antiferromagnetic structures accompanied by the structural distortions have been studied by a variety of X-ray, magnetic and neutron-diffraction methods. For this aim we have applied the Mössbauer effect by doping the 57Fe isotope to UPdSn. Below TN, except for the magnetic component, the presence of the paramagnetic doublet with the intensity diminishing with decreasing temperature have also been observed. As a possible reason for the occurrence of this doublet in Mössbauer spectra even far below TN, among others, the fluctuation model of two magnetic phases being in thermal equilibrium has been postulated. Moreover, we have calculated and discussed the dipolar hyperfine fields at the Pd sites (occupied also by the iron isotope) for various types of magnetic structures reported in the literature. On this basis, the so-called ‘body diagonal’ model of the magnetic structures of UPdSn has been found as being more probable. We have also shown that the Mössbauer parameters exhibit an anomalous behaviour at the critical points, connected with both the structural and magnetic transitions which thus coincide in this compound.

Devil's staircase in Kondo semimetals at low temperatures.

We present an effective Hamiltonian for cerium monopnictides CeX (X=P,As,Sb,Bi) to explain their complex magnetic phase diagrams in a unified way. The effective Hamiltonian consists of semimetallic conduction bands and localized f spins. Between the conduction electrons and the localized f spins two kinds of interactions work. One is the intraband mechanism which is the usual Kondo exchange interaction and the other is the interband spin-dependent exchange interaction specific to this model. Owing to the competition between the above two interactions, various types of magnetic structures are realized. The ground-state magnetic phase diagram of this model is investigated by a simple mean-field treatment in one dimension and it is shown that the essential features of the complex ground-state magnetic phase diagram of CeX are reproduced.

Neutron polarimetry

Zero-field neutron polarimetry is a technique which has recently been developed to allow spherical polarization analysis to be carried out in a neutron scattering experiment. The incident beam polarization can be set to any desired direction and the magnitude and direction of the polarization of the scattered beam determined. The polarimeter exploits the Meissner effect to separate the magnetic fields of the active elements. A precision of ca . 2° in the polarization direction can be obtained with the prototype. Expressions are given for the change in polarization to be expected in diffraction experiments on various types of magnetic structure and the effect of magnetic domains is discussed. Some examples of the application of the technique to magnetic structure determination are given. Effects due to breakdown of the kinematical approximation have been demonstrated. Some ideas for a second generation polarimeter are outlined and new areas of application proposed.

Modulated amplitude type of TbNiSn magnetic structure

Neutron diffraction of a polycrystalline sample of TbNiSn was performed at 40 and 12K. The compound has the orthorhombic Pnma crystal structure and presents a modulated amplitude type of magnetic structure with k = [0.313, 0.325, 0.0] and a spin axis parallel to b .

Towards the Magnetic Field of M 81

We present calculations showing how both transient effects associated with nonaxisymmetric seed fields, and an interaction with a companion, can produce BSS type magnetic structure in a spiral galaxy.

Magnetic aftereffect following first order magnetostructural phase transitions

The H- and T-induced magnetic aftereffect associated with first order magnetostructural phase transitions has been studied experimentally. The alloys La(Fe, Co, Al) 13, Fe 0.49Rh 0.51 doped with Ni and Pd, as well as the compound ζ-Mn 5Ge 2 have been investigated. These materials have various magnetic phase H-T diagrams, various magnetoactive ions, various types of magnetic structures before and after the transitions. However, common magnetic aftereffect relationships have been established for them. A relationship between such an aftereffect and the characteristics (shapes of branches) of quasistatic hysteresis loops (strain in our case) has been revealed. It is shown that the aftereffect is of fluctuational nature: on the steep parts of hysteresis loop branches one observes so called “anomalous” and “giant” fluctuational aftereffects for which the linear dependence of deformation vs. In t does not hold. The existence of a fluctuational aftereffect following a change of temperature has been observed for the first time. The theory of fluctuational aftereffect has been generalized for the case of strain aftereffect with first order magnetostructural phase transitions. Such a theory describes the experimental relationships quite satisfactorily.