Calculation of neutron depolarization in a uniaxial ferromagnet

We consider a ferromagnetic system which is divided into domains with magnetization directions perpendicular to the plane of the sheet. This system is traversed by a polarized thermal neutron beam with the direction of the polarization vector perpendicular to the magnetization directions of the domains. On the basis of a few assumptions, we calculate the depolarization as a function of the direction of the neutron beam with respect to the plane of the sheet and in terms of the distribution function of the domain sizes as seen in a direction in the plane of the sheet. The result of this calculation is compared to a computer simulation of the same problem and to an experimental result published earlier.     

Study of ferromagnetic bulk domains by neutron depolarization in three dimensions

The polarization change of a polarized neutron beam after transmission through a partly magnetized ferromagnetic material can be described by a (3×3) depolarization matrix. A theory has been developed to interpret this matrix in terms of well-known magnetic domain quantities such as the reduced mean magnetizationm, the mean domain size δ and the mean square direction cosines γ _x , γ _y and γ _z of the inner magnetization within the domains. In order to do this it was necessary to make some simplifying assumptions about ferromagnetic domain structures. The influences of these assumptions on the quantities derived have been discussed. Finally the theory has been applied to depolarization measurements in nickel foils with the magnetic field and mechanical stress as parameters.     

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.     

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.     

Magnetization distribution in magnetic films studied with Larmor-encoding

Additional information about the magnetization distribution in magnetic films is obtained with a 3D-polarimetry set-up. A pilot experiment was performed with the neutron polarization aligned perpendicular to the surface of a Fe-film in a magnetic field parallel to its surface. The Larmor-precession in the magnetic field between two current sheets was used to adjust the neutron polarization perpendicular to the sample surface. This new polarization-magnetization configuration was probed with a Fe-film in specular and off-specular scattering. The off-specular scattering is created by the magnetic domain structure of the Fe-film in remanence. The results of specular and off-specular scattering are reproduced by calculations for the configuration of the incoming neutron polarization parallel to the sample surface and the magnetic field and for the configuration of the incoming neutron polarization perpendicular to the sample surface and the magnetic field.     

On the effect of an elastic shear stress upon the magnetization of ferromagnetic sheets

When an elastic shear stress and a cyclical magnetic field, parallel to each other, are applied in the plane of a ferromagnetic sheet, magnetization changes perpendicular to the field are induced in the sample. “Transverse” hysteresis loops, i.e. transverse magnetization plotted as a function of the longitudinal field, were studied in various materials. The characteristic shape of the loop and the differences in sign and magnitude of the “transverse” magnetization have been qualitatively explained in terms of magnetic domain theory. Work supported by G.N.S.M. (CNR).     

Magnetization distribution in submicrometer iron-nickel antidot arrays patterned by a focused ion beam

Antidots of size 0.5 μm are prepared by patterning iron-nickel films with a focused ion beam. The magnetization distribution in antidot arrays is examined with Lorentz transmission electron microscopy. It is shown that one side of the array makes an angle of about 20° with the easy magnetic axis of the film. Magnetization reversal in the direction close to the easy magnetic axis starts with domain nucleation at the antidot edges that are perpendicular to the applied field and adjacent to the unpatterned region of the film, and propagates as the domain walls move. Magnetization reversal in the direction close to the hard magnetic axis starts with magnetization rotation outside the patterned region at the antidot edges and propagates as the domain walls execute a complicated motion. It is demonstrated that some areas between the edges of adjacent antidots can carry information bits. Results obtained are explained in terms of competition between the demagnetizing energy, energy of internal anisotropy, and misorientation effect. The feasibility of such structures as high-density storage elements is discussed.     

Depolarization of a neutron beam in Laue diffraction by a noncentrosymmetric crystal

The depolarization of a neutron beam executing Laue diffraction in a thick (～3.5 cm) noncentrosymmetric α-quartz crystal is observed. This effect was predicted by us earlier and suggested for measuring the electric dipole moment (EDM) of a neutron. The effect is due to an interaction of the magnetic moment of a moving neutron with a strong crystal electric field, as a result of which the neutron spin rotates in opposite directions for waves of two types excited in the crystal. The effect is studied for neutron diffraction by a system of crystallographic (110) planes at Bragg angles close to π/2, up to 87°. It is shown that, for a crystal of thickness L=3.5 cm, a direct beam initially polarized along the reciprocal lattice vector becomes depolarized upon diffraction, irrespective of the value of Bragg angle, whereas the beam polarized perpendicular to the diffraction plane retains its polarization. The Eτ value determining the sensitivity of the method to EDM is experimentally estimated.     

Transport of polarized neutrons through magnetic noncoplanar layered systems

The transport of neutrons through noncoplanar systems is shown to have a partly unidirectional character. To this end, the transmission of a three-layer magnetic film with a noncoplanar magnetization in the absence of applied magnetic field and the transmission of a two-layer magnetic film with a noncollinear magnetization in the presence of a magnetic field are calculated. The calculations demonstrate that, if the losses in the films are neglected, the transmissions in two opposite directions differ only for processes with and without spin flip. If these losses are taken into account, the total transmissions of a nonpolarized neutron beam in two opposite directions are also different. The consequences of these specific features of noncoplanar systems are discussed.     

Magnetic properties of ThFe5

The easy direction of magnetization of the intermetallic compound ThFe₅ has been determined by means of the Mössbauer effect and X-ray diffraction. From a resemblance between the results of these techniques, magnetization, neutron depolarization and magnetic neutron diffraction it can be concluded that ThFe₅ has a complex ferrimagnetic moment arrangement with its easy axis of magnetization in the basal plane.     

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.     

MFM study of magnetic vortex cores in circular permalloy dots: behavior in external field

In a circular dot of permalloy with an appropriate size, a vortex structure with perpendicular (turned-up) magnetization at the core is realized. The existence of the perpendicular magnetization spot has been confirmed and the direction of the magnetization, up or down, has been determined by magnetic force microscopy (MFM) for permalloy dots with the diameter of 0.1–1 μm. The switching field of turned-up magnetization is determined by applying external fields perpendicularly and in tilted directions to the plane. By comparing the MFM results and the magnetization curves measured by a SQUID magnetometer, the switching process of turned-up magnetization is argued.     

抽运-检测型非线性磁光旋转铷原子磁力仪的研究

报道了一种抽运-检测型的非线性磁光旋转铷原子磁力仪.其原理是线偏振光通过处于外磁场环境中被极化的原子介质后,由于原子对线偏振光中左、右圆偏成分不同的吸收和色散,导致光的偏振方向会产生与磁场相关的转动.分析了该磁力仪的工作原理,并测试了它对不同磁场大小的响应.测试结果表明,磁力仪测量范围为100—100000 nT,极限灵敏度为0.2 p T/Hz~(1/2),磁场分辨率为0.1 p T.进一步研究了不同磁场下原子系综极化态的横向弛豫时间,讨论了原子磁力仪高磁场采样率的获得方法.本文的原子磁力仪在5000—100000 n T的磁场测量范围内磁场采样率可实现1—1000 Hz范围内可调,能够测量低频的微弱交变磁场.本文的研究内容为大磁场测量范围、高灵敏度、高磁场采样率的原子磁力仪研制提供了重要参考.     

Magnetization anisotropy of Ni dots with several tens of nanometer diameter

We have studied the magnetization of Ni dot with 50 to 70 nanometer diameter and 12 nanometer thickness using a magnetic force microscopy with an in-plane magnetic field. The Ni dots were prepared using self-assembled dot patterns with poly (styrene-b-methyl mathacrylate) diblock copolymers on Ni film and ion etching. It was found that the remanent magnetization direction of the dot was perpendicular to the plane as prepared. From the vibrating sample magnetometer measurement, a hysteresis loop was found in the perpendicular magnetization. When an in-plane external magnetic field was applied, the magnetization was rotated into a horizontal direction with low coercivity along the field direction.     

Neutron depolarization theory in the larmor and the scattering approach

The neutron depolarization theory in the Larmor and the scattering approach is presented. An extension of the former is proposed, as a result of which both approaches result in basically the same formulas. The intrinsic anisotropy, which is the phenomenon that for magnetically isotropic media the depolarization is still dependent on the mutual orientation of the polarization vector and the propagation direction of the neutron beam, is discussed furthermore. It is shown that this phenomenon is caused by local stray fields around magnetic inhomogeneities as well as eventual correlations between the shape of these inhomogeneities and the orientation of the local magnetization.     

Experimente mit dem neutronenspinecho mit langwelligen neutronen aus einem polarisierenden neutronenleiter

We describe an inexpensive method to obtain a highly polarized neutron beam of slow neutrons (λ = 1 – 15 Å) [1]. The high necessary field to saturate the applied soft magnetic material in the surface acting as a polarizing mirror indicates a behaviour of the magnetization in the surface different from that in the bulk material [2]. We also describe the realization of an apparatus proposed by F. Mezei [3], the behaviour of the polarized neutrons in this apparatus, and its use to get a spin echo with a neutron spectrum containing 3 – 9 Å wavelength neutrons. This spin echo is not destroyed by transmission through a nickel sheet, only the number of precessions is increased which appear in a displacement of the echo. This displacement can be used to measure the saturation magnetization of this sheet.     

Magnetization process of Fe layers in Fe/Nd multilayered films investigated by57Fe Mössbauer spectroscopy

The magnetization process of Fe and Nd layers at 5K in Fe/Nd multilayered films with strong perpendicular magnetic anisotropy is elucidated from a comparison of⁵⁷Fe Mössbauer spectra in the presence of the external field applied parallel to the film plane with total magnetization. At zero external field, the film has a magnetic multi-domain structure. The Nd layer moment is perpendicular to the film plane and the Fe layer moment points in the out-of-plane direction. The Fe layer moment monotonically rotates to the in-plane direction with increasing external field parallel to the film plane, while the Nd layer moment is oriented to the film normal direction up to the external field of 10kOe, above which the Nd layer moment gradually turns to the direction of the external field.     

Thin ferromagnetic nanodisk in transverse magnetic field

The magnetization of a ferromagnetic nanodisk is studied using micromagnetic modeling. It is demonstrated that, under an external magnetic field applied perpendicular to the disk surface, magnetic phase transitions can occur between uniform states, between uniform and vortex states, and between vortex states with different directions of polarization. A simple variation model is proposed describing the observed magnetic states quantitatively.     

On neutron depolarization in magnetized media

The neutron depolarization in magnetized samples is considered as a result of scattering in the case of arbitrary orientations between the incident polariztion, the sample field and the direction of polarization analysis. It is shown that the depolarization is determined by the small-angle magnetic scattering within the angular beam width and by an additional contribution arising from the spin-dependent scattering outside the passing beam. The latter follows from the consideration of the interference between the scattered and the transmitted beams. Special attention is paid to the analysis of several measuring regimes for the depolarization and its dependence on neutron wavelength. The depolarization in multidomain ferromagnets is considered.