Correlations in ferromagnetic domain structures studied by means of the neutron depolarization technique

The polarization change of a polarized neutron beam after transmission through a partly magnetized ferromagnetic material can be described by a (3 × 3) depoalarization matrix. An earlier interpretation of this matrix in terms of domain quantities such as the reduced mean magnetization m, the mean domain size δ and the mean square direction cosines γ_x, γ_y and γ_z of the inner magnetization within the domain is extended by taking into account different correlations occuring in real domain structures, such as the correlation between neighbouring domain orientations expressed by the mean direction cosine K between these orientations and the correlation between the domain dimensions and the domain magnetization orientation of an arbitrary domain. The latter correlation can be expressed by means of the quantities Δ and I`, where Δ is a measure for the average shape anisotropy of the domains with respect to the direction of domain magnetization, and Γ is a measure for the movement of domain walls during the magnetization process of the sample. The possibilities and limitations of the resulting theory on neutron depolarization experiments are discussed.     

Magnetic properties of superconductors studied by neutron depolarization

A new method is described which allows the study of the magnetic properties of superconductors. Measurements on various niobium and lead samples were taken to study the influence of surface conditions and lattice defects on the formation of the mixed and intermediate states. A strong dependence of flux penetration on surface conditions is found. AboveH _C1 the results indicate the formation of a disturbed flux line lattice. The influence of external fields present when the superconductor is cooled throughT _c is discussed and evidence for the intermediate state of type II superconductors is presented. Finally a simple model is used to evaluate the mean angle of deviation from the ideal lattice.     

Magnetization reversal in amorphous ribbons studied by neutron depolarization

The magnetization process in an amorphous ribbon with an easy axis perpendicular to its plane has been studied with the three-dimensional neutron depolarization technique. A simplified model of three layers is used, describing the magnetization distribution in the bulk and the two closure domain structures at the surface. Our analysis with this model gives direct experimental evidence that the magnetization process due to an applied magnetic field in the ribbon plane is dominantly accounted for by rotation processes of the local magnetization in the bulk. The influence of different types of closure domain structures on the magnetization distribution during the reversal process are discussed and compared with the experimental results.     

电沉积法制备的泡沫镍的晶体结构与磁性能

 采用X射线衍射仪、振动样品磁强计分别测试了电沉积法制备的泡沫镍的晶体结构和磁性能，研究了电流密度、温度、占空比、脉冲频率对镍沉积层的晶体结构和磁性能的影响；并用扫描电子显微镜、透射电子显微镜观察了泡沫镍的组织结构与微观形貌。制备出的3维网络状泡沫镍密度为0.25 g/cm³，孔隙率为97.5%、孔径分布为400～500 μm。沉积层大颗粒粒径为1 μm，沉积层晶粒尺寸在10nm以下;泡沫镍为超顺磁材料，具有低的矫顽力和高的磁导率，其磁导率和饱和磁化强度随沉积层晶粒尺寸的增加而增大。     

A theory of neutron depolarization from random magnetic materials

We have developed a rigorous density matrix formalism to calculate the effects of neutron depolarization in transmission through random magnetic materials. This approach has the advantage of being general enough to be applicable to dilute alloys, random anisotropy systems etc. for which the conventional neutron depolarization theory may not be relevant.     

The magnetic behaviour of CeAl2 studied by neutron scattering

Diffuse neutron scattering experiments with unpolarized neutrons in zero field have been performed on a CeAl₂ single crystal in the temperature range 2.5 K to 300 K. At the lowest temperature we observe collective excitations in the system. At higher temperatures the energy spectra can be analysed in terms of quasielastic and inelastic lines due to the crystal field splitting of the Ce ions.     

Temperature dependence of magnetic neutron scattering by dislocations in nickel

Magnetic small-angle scattering of neutrons by dislocations in a plastically deformed nickel single crystal has been investigated between room temperature and 650 K. The results are in disagreement with calculations from the micromagnetic theory [1]. For scattering vectors Q with 1 × 10^?2 ? Q ? 7 × 10^?2 nm^?1 the intensity goes to zero much more rapidly with increasing temperature than predicted. Some possible explanations for this observation are discussed.     

Domain wall bowing in a FeSi picture frame crystal studied by time-dependent neutron depolarization

This letter presents a new detection technique, called dynamic neutron depolarization technique, to study the shape of a moving domain wall within the bulk. Using this technique the motion of a single 180° domain wall in a FeSi (3.5 wt % Si) picture frame crystal has been investigated.     

Antiferromagnetic domains in nickel oxide by magnetic neutron laue diffraction

Two kinds of domains had been observed in antiferromagnetic nickel oxide: T-domains, associated with the four 〈111〉 propagation vectors and S-domains, associated with the three equivalent antiferromagnetic directions within a T-domain. Taking advantage of the direct interaction of neutrons with the arrangement and direction of magnetic moments, the T and S antiferromagnetic domain distributions are investigated in several single crystal samples by magnetic neutron Laue diffraction (MNLD). The T-domain behaviour when a stress is applied is qualitatively investigated. In white beam a direct topographic observation and identification of T-domains are presented. By simultaneously measuring the relative intensities of magnetic Bragg reflections from a sample with preferential domain population it is possible to confirm the direction of sublattice magnetization which had been obtained by an optical method. The S-domain distribution is studied as a function of the magnetic field on the one hand, and as a function of time on the other. This last phenomenon is very similar to the well-known magnetic after-effect which occurs in most ferromagnets. It is concluded that MNLD could be a very valuable technique to investigate many topics of magnetism.     

Structural and magnetic phase transitions in TbPO4 studied by neutron diffraction

Neutron diffraction experiments on TbPO₄ single crystals have been performed in the temperature range from 1.35 to 294 K. We observe two phase transitions: the onset of antiferromagnetic ordering along the tetragonalc-axis at 2.28 K and tilting of the moments away from thec-axis below 2.15 K. The analysis of the measured reflection profiles shows that the tilting is connected with a distortion of the tetragonal zircon structure.     

On the possibility to determine weak hyperfine structure interaction constants by magnetic depolarization

A new method to measure weak hyperfine structure interaction constants (aτ ? 1) by magnetic depolarization, involving interaction between nuclear orientation and electronic alignment, was used on the 3 ¹P₁ level of ³He. It was found that a/Γ = 0.075 ± 0.010.     

Incommensurate magnetic structure of CeSi as observed by neutron diffraction

The magnetic ordering of the orthorhombic compound CeSi (Pama space group, FeB type of structure, Z = 4) T_n = 5.6 K was investigated by means of powder neutron diffraction in the temperatures 1.5 and 10 K. Our results show that the magnetic ordering is associated with a three dimensionally amplitude-modulated phase which is incommensurate with the crystal lattice with wave vector q = (q_χ,q_y,q_z). The ordered Ce³⁺ moments are confined to the plane (a,b) at an angle of 16.7° with the b direction. It follows from the present data analysis that the dominant wave vector component is q_z, which approximately corresponds to a transversal modulation, however the incommensurateness involves also the q_x and q_y components. The amplitude of the sine wave is μ⁰_j = 1.667μ_B at 1.5 K which corresponds to quite a reduced ordered moment value compared to the Ce³⁺ free ion value gJ = 2.14μ_B.     

Contribution of electrolyte pH and deposition potentials to the magnetic anisotropy of electrodeposited nickel films

The influence of electrolyte pH and cathode potential on the magnetic properties of single layer Nickel films electrodeposited on polycrystalline titanium substrates was studied. The films were deposited at the electrolyte pH=3.5±0.1, 2.5±0.1 and 2.1±0.1 by varying the deposition potentials (?1.2, ?1.5 and ?1.8 V vs saturated calomel electrode, SCE) applied in continuous waveform. The structural analysis by X-ray diffraction revealed that the films have face-centred cubic structure. Results of the magnetic measurements obtained by vibrating sample magnetometer (VSM) indicated that the magnetic properties were affected by the electrolyte pH and the cathode potentials in terms of magnetic anisotropy. At the highest pH the films deposited at the lowest potential had in-plane magnetic anisotropy. As the electrolyte pH decreased from the high (pH=3.5±0.1) to low (pH=2.5±0.1), which is aided by increasing the potentials, resulted in an almost magnetic isotropy in the films. However, isotropic magnetic behaviour was observed for the film deposited at the low pH combined with the high potential (?1.8 V vs SCE). Magnetic thickness profile of the samples obtained by VSM revealed that the isotropic films have a smoother magnetic variation across the film from one edge than the anisotropic ones. This is also verified with a microscopic observation by an optical microscope and the surface of the isotropic films is observed to be smoother than that of the anisotropic ones. Furthermore, all films were found to have planar magnetic anisotropy irrespective of the pH’s and the potentials.     

Neutron depolarization study on the magnetic correlation length of nickel ferrite with different packing densities

The magnetic correlation length of a mixed nickel ferrite powder was studied by a newly commissioned depolarized neutron beamline at the W3 port of Tsing Hua Open Pool Reactor (THOR). In this work, Ni ferrite powder samples with different packing densities were studied. The magnetic correlation lengths of the sample were observed to be 2 μm at virgin state and about 3.1 μm at remanent state from the packing density of 20–60%. This magnetic domain size is smaller than particle size. No significant change of domain size at this packing density implies the domain wall motion is hindered by the porosity effectively up to at least 60% of packing density.     

Stationary structure of the domain wall of the soft magnetic bubble domain

A simple method for calculating the barrel-like curvature of a bubble domain is developed. The results obtained are compared with the results of the Thiele theory.     

Effect of natural and magnetic convections on the structure of electrodeposited zinc-nickel alloy

The effects of a magnetic field applied in a direction parallel or perpendicular to the cathode substrate plane, during electrodeposition process of Zn-Ni alloy have been investigated by means of chronoamperometric measurements, X-ray diffraction and EDX analysis. The modification of crystal orientation of the alloy by the superimposition of a high magnetic field is discussed for alloys with a content of nickel range 6-13 at%. Whatever the phase composition obtained without magnetic field, either γ-Ni₅Zn₂₁ or a mixture of the γ and zinc phases, which depends on the concentration of Ni²⁺ in the electrolyte bath, the preferential orientation (1 0 1) of the zinc phase is always favoured with perpendicular and parallel magnetic field. There is no saturation of this effect with amplitude of B up to 8 T. A study of different geometric configurations of the cathode, which induce more or less natural convection, consolidates these results. The structural modifications of Zn-Ni alloy electrodeposits are thus probably due to a magnetohydrodynamic effect. An additional phenomenon is observed in presence of a perpendicular applied magnetic field since the (3 3 0) preferential orientation of the γ-Ni₅Zn₂₁ disappears with high values of B.     

Optical and neutron domain structure studies of amorphous ribbons

The domain configuration of amorphous ribbons with positive, vanishing and negative magnetostriction constant λ_s has been investigated by means of the longitudinal magneto-optical Kerr-effect and a three-dimensional neutron depolarization analysis. The influence of an external tensile stress on the domain arrangement depends on the sign and the magnitude of the magnetostriction constant and could be measured quantitatively in the depolarization experiment. With the aid of the optical observations domain models have been established; these models were tested and completed by means of an analytic calculation of the corresponding depolarization matrix and a comparison with experimental data. Domain reorientation processes within the ribbons under an imposed tensile stress were found to agree with theoretical predictions. The nearly nonmagnetostrictive alloys shows neither stripe domains nor simple wide domains but a more complicated structure with several layers of domains.     

Quantification of magnetic domain disorder and correlations in antiferromagnetically coupled multilayers by neutron reflectometry

The in-plane correlation lengths and angular dispersion of magnetic domains in a transition metal multilayer have been studied using off-specular neutron reflectometry techniques. A theoretical framework considering both structural and magnetic disorder has been developed, quantitatively connecting the observed scattering to the in-plane correlation length and the dispersion of the local magnetization vector about the mean macroscopic direction. The antiferromagnetic domain structure is highly vertically correlated throughout the multilayer. We are easily able to relate the neutron determined magnetic domain dispersion to magnetization and magnetoresistance experiments.