Showing papers on "Magnetic structure published in 1968"

Representation analysis of magnetic structures

Abstract: In the analysis of spin structures a `natural' point of view looks for the set of symmetry operations which leave the magnetic structure invariant and has led to the development of magnetic or Shubnikov groups. A second point of view presented here simply asks for the transformation properties of a magnetic structure under the classical symmetry operations of the 230 conventional space groups and allows one to assign irreducible representations of the actual space group to all known magnetic structures. The superiority of representation theory over symmetry invariance under Shubnikov groups is already demonstrated by the fact proven here that the only invariant magnetic structures describable by magnetic groups belong to real one-dimensional representations of the 230 space groups. Representation theory on the other hand is richer because the number of representations is infinite, i.e. it can deal not only with magnetic structures belonging to one-dimensional real representations, but also with those belonging to one-dimensional complex and even to two-dimensional and three-dimensional representations associated with any k vector in or on the first Brillouin zone. We generate from the transformation matrices of the spins a representation Γ of the space group which is reducible. We find the basis vectors of the irreducible representations contained in Γ. The basis vectors are linear combinations of the spins and describe the structure. The method is first applied to the k = 0 case where magnetic and chemical cells are identical and then extended to the case where magnetic and chemical cells are different (k ≠ 0) with special emphasis on k vectors lying on the surface of the first Brillouin zone in non-symmorphic space groups. As a specific example we consider several methods of finding the two-dimensional irreducible representations and its basis vectors associated with k = ½ b2 = [0½0] in space group Pbnm (D162h). We illustrate the physical context of representation theory by constructing an effective spin Hamiltonian H invariant under the crystallographic space group and under spin reversal. H is even in the spins and automatically invariant under the (isomorphous) magnetic group. We show by the example of CoO that the invariants in H, formed with the help of basis vectors, give information on the nature of spin coupling as for instance isotropic (Heisenberg–Neel) coupling, vectorial (Dzialoshinski–Moriya) and anisotropic symmetric couplings. Magnetic structures, cited in the text to show the implications of the representation theory of space groups are ErFeO3, ErCrO3, TbFeO3, TbCrO3, DyCrO3, YFeO3, V2CaO4, β-CoSO4, Er2O3, CoO and RMn2O5 (R = Bi, Y or rare earth). Representation theory of magnetic groups must be considered when the Hamiltonian contains terms which are odd in the spins. The case may occur when the magnetic energy is coupled with other forms of energy as for instance in the field of magneto-electricity. Here again representation theory correctly predicts the couplings between magnetic and electric polarizations as shown on LiCoPO4 and (previously) on FeGaO3.

The intensity, velocity and magnetic structure of a sunspot region

Abstract: The observational set-up for a detailed study of the velocity, intensity and magnetic-field fine structure in and around a sunspot is described. On highly resolved spectra we detected in the vicinity of a sunspot a large number of points with strong magnetic fields (magnetic knots). The magnetic field in these knots causes a striking decrease of the line depth (or a ‘line gap’ after Sheeley, 1967). The properties of the magnetic knots are: (1) magnetic fields up to 1400 gauss; (2) diameter ≈ 1100 km; (3) coincidence with dark intergranular spaces; (4) generally downward material motion; (5) lifetime>30min; (6) estimated total number around an unipolar spot ⩾ 2000; (7) combined magnetic flux comparable to the sunspot flux; (8) coincidence with Ca+ plages. For the smallest sunspots (pores) we obtained magnetic fields >1500 gauss. Hence a magnetic field of about 1400–1500 gauss appears to be a rather critical level for pore and spot formation. We found a large number of small areas producing line gaps without measurable magnetic field. These ‘non-magnetic gap-regions’ coincide with bright continuum structures. Some aspects arising from the occurrence of hundreds of magnetic knots in an active region are discussed in the last section.

The magnetic structure and hyperfine field of goethite (α-FeOOH)

Abstract: The magnetic structure of goethite (?-FeOOH) has been determined by M?ssbauer and neutron powder diffraction techniques using both natural and synthetic samples. The antiferromagnetic structure, which exists below the N?el point of (130?2) ?c, is collinear and the magnetic and chemical unit cells are the same; a = 4?587, b = 9?937 and c = 3?015 ?, space group Pbnm. The iron atoms related by the b-glide plane are coupled ferromagnetically and are antiferromagnetically coupled to the remaining iron atoms. The spins lie parallel to the c axis, and the major axis of the electric field gradient was found to lie in the ab plane, making an angle of about 40? with the b axis. No evidence was found for the existence of more than a single hyperfine field in the temperature range between 4?2 ?K and room temperature: the value of 504 kG at low temperatures is discussed in relation to the covalency of a number of ferric compounds. The exchange interactions in the oxyhydroxide are related to the magnetic structure; the observed behaviour of the susceptibility above the N?el point is qualitatively explained by the existence of residual magnetic order.

Magnetic Structures and Phase Transformations in Mn‐Based CuAu‐I Type Alloys

Abstract: Investigations of the magnetic properties in the ordered CuAu‐I‐type manganese alloys are reviewed. The formation of the ordered phase and the possible magnetic structures are discussed. New experimental results for the Mn‐Ni, Mn‐Pd, and Mn‐Pt systems are reported.Neutron diffraction shows the basic antiferromagnetic structure to be the same for all the alloys investigated. In MnPt a magnetic‐structure transformation has been observed. Below 800°K the magnetic moments turn from the basal plane gradually to the tetragonal axis. The variation of the transition temperature in the Mn‐Pt system as well as that of the Neel temperature in the Mn‐Ni, Mn‐Pd, and Mn‐Pt systems with concentration were measured and are theoretically analyzed.The dependence of lattice parameters on concentration and temperature was determined. The lattice parameters suffer a significant but continuous change near the Neel temperature which is attributed to Mn‐Mn exchange interaction.Magnetic susceptibility measurements support the neu...

Magnetic Structure of Ca2Fe2O5

Abstract: Magnetic measurements indicate that Ca 2 Fe 2 O 5 is an antiferromagnet, with weak parasitic ferromagnetism The lattice, with the space group Pcmn -D 2 h 16 , can be considered as an oxygen deficient perovskite The magnetic structure as determined from a neutron diffraction study is of G -type, and Fe 3+ spins are directed approximately along c -axis

Neutron Diffraction Study of FeSn

Abstract: A neutron diffraction study on the powdered sample of FeSn has been made in order to determine the magnetic structure of this compound. The magnetic unit cell is twice as large as the chemical cell, being doubled along the c -axis. The moments of iron atoms are ferromagnetically coupled within a c -plane, while they are coupled antiferromagnetically to those on the adjacent c -planes. The moments lie in the c -plane. The atomic magnetic moment of Fe is obtained to be 1.5 5 ±0.1 µ B at liquid nitrogen temperature.

Magnetic and Chemical Order in Pd2MnAl in Relation to Order in the Heusler Alloys Pd2MnIn, Pd2MnSn, and Pd2MnSb

Abstract: The chemical and magnetic structures of the alloy Pd2MnAl, determined from magnetic and neutron diffraction measurements, are described, and compared with results recently obtained for the alloys Pd2MnIn, Pd2MnSn, and Pd2MnSb.Pd2MnSn and Pd2MnSb have the Heusler (L21) structure, and are ferromagnetic with Curie temperatures of 189°K and 247°K, and magnetic moments, confined to the Mn atoms, of 4.23μB and 4.40μB, respectively. Pd2MnIn may be obtained in ordered (L21) and partially disordered (B2) forms by slow‐cooling or quenching, respectively. The ordered alloy is antiferromagnetic with a Neel temperature of 142°K and a magnetic moment of 4.3μB per Mn atom. The magnetic moments are aligned in ferromagnetic (111) planes, with neighbouring planes oriented antiparallel as in NiO. In the partially disordered regions the Pd atoms remain ordered, but disorder occurs between the Mn and In atoms resulting in a B2, CsCl type, structure. The Mn atoms are still antiferromagnetically aligned, but on a magnetic unit ...

Neutron Diffraction Study of Antiferromagnetic Mn 2 N

Abstract: Crystal and magnetic structures of antiferromagnetic Mn 2 N were studied by means of neutron diffraction at 290°K, 230°K, and 120°K. An orthorhombic arrangement of nitrogen atoms in the octahedral interstices of hcp manganese lattice was found. The crystal structure is ζ-Fe 2 N type whose space group is D 2 h 14 - P b n a . The intensity analysis of the diffraction pattern at 120°K yields a magnetic structure with four sublattices having the same orthorhombic unit cell as the chemical one. The magnetic moment of manganese atom was found to be 1.4±0.4µ B at 230°K and 1.6±0.4µ B at 120°K.

Antiferromagnetism of Dilute Cr Alloys with Co and Ni

Abstract: The magnetic properties of single crystals of Cr alloys containing a small amount of Co and Ni have been studied by neutron diffraction and thermal expansion. The alloy with less than 1 at%Co has the same magnetic structure as that of pure Cr. With increasing Co concentration, the Neel temperature decreases, and the wave vector of the S.D.W. increases, while the magnetic moment remains constant. For 2 at%Co-Cr, the commensurable antiferromagnetic structure becomes stable. The Neel temperature slightly increases and the magnetic moment still remains constant. With the further increase of Co concentration, the antiferromagnetic structure tends to be destroyed by Co-Co pairs. The alloys with Ni have been found to show the magnetic behavior as the alloys with impurities which decrease the electron to atom ratio. All of wave vector, Neel temperature and magnetic moment decrease with an increase of Ni concentration.

Reinvestigation of Magnetic Structures of CoCr2O4 and MnCr2O4 Obtained by Neutron Diffraction

Abstract: Spinels CoCr2O4 and MnCr2O4 display a magnetic structure in which moments on octahedral sites split into two groups In one of these, transverse components are antiparallel and have the fcc symmetry of the chemical cell The transverse components of the other group together with those of the tetrahedral sites build a (3a, 3a, a) magnetic cell Axial components are along [001] for CoCr2O4 and along [110] for MnCr2O4, and are in good agreement with magnetization measurements Good agreement is also obtained between measured and calculated magnetic intensities These structures can be explained in a molecular‐field approximation Short‐range order is still present at low temperatures

Magnetic Structure of Cobalt Manganite by Neutron Diffraction

Abstract: Pure iron manganite FeMn2O4 was prepared at 1250°C and quenched. It crystallizes in the b.c. tetragonal system D4h19. Neutron diffraction measurements above the Curie temperature (120°C) show that the inversion parameter is 0.91 (nearly inverse spinel). The spontaneous magnetization at 4.2°K measured in high static and pulsed fields is 1.55 μB/mol. The magnetic moment was observed to increase linearly above 70 kOe and showed the presence of a strong anisotropy with easy axis near the [101] direction (unusual fact). Neutron diffraction measurements at 4.2°K lead to the determination of the magnetic structure of FeMn2O4. The tetrahedral sublattice moments (4.3 μB) are collinear in a (010) plane making an angle of − 172° with Ox [100]. Spins in octahedral sites (3.1 μB) are divided into four magnetic sublattices with a pyramidal arrangement whose axis of symmetry lies in the plane (010) and makes an angle of 14° with Ox [100]. At 55°K the spins of the octahedral sublattices become collinear, the easy magneti...

The magnetic structure of Mn2P

Abstract: Mn2P has been found to be an antiferromagnet with Neel temperature 103°K. The magnetic structure has been determined by neutron diffraction studies of single crystals. A model of the structure was obtained directly from the diffraction intensities by means of a spin-density Patterson synthesis. This model was refined by a least-squares procedure and gave very good agreement with the observed magnetic reflections. The structure consists of a spin modulation commensurate with the lattice propagating along three hexagonally equivalent directions. The spins are all perpendicular to the propagation directions and the mean values of magnetic moments at the two manganese sites are MnI: 0·01 ± 0·04μB, MnII: 0·84 ± 0·03μB. The possible existence of spin domains has been examined by cooling the crystal through the Neel point in a large magnetic field and remeasuring the magnetic reflections. However, no evidence for their existence was found.

Magnetic Structure and Metamagnetism of HgCr2S4

Abstract: HgCr2S4 is a normal cubic spinel whose metamagnetic properties were first reported by Baltzer et al. Our neutron diffraction studies show that the magnetic structure at 4.2°K is a simple spiral, identical with that proposed by Plumier for ZnCr2Se4. The propagation vector τ is parallel to the symmetry axis of the spiral and directed along a particular cube edge in a given domain. The moment of Cr3+ is 2.73 μB, in agreement with the magnetization measurements, and the wavelength of the modulation is ∼42 A. The wavelength increases with temperature, reaching a value of ∼90 A at 30°K, and shows very little further variation up to the Neel point (∼60°K). Application of a magnetic field along a cube edge produces no magnetization in that direction, but rather a growth of domains for which τ ‖ H. This process is complete at ∼4 kOe and is followed by a rapid collapse of the spiral into the field direction which saturates at ∼10 kOe. The growth of favorably oriented domains decreases progressively as the direction...

Magnetic Structure of DyC2

Abstract: The compound, DyC2, having the tetragonal body‐centered CaC2‐type structure has been shown by neutron diffraction to exhibit the magnetic spin alignment of a linear, transverse wave mode below the Neel temperature of 59°K. This static moment wave is propagating along the a axis and is polarized in the c‐axis direction. The root‐mean‐square and maximum saturation moments per Dy are 8.37 and 11.8 Bohr magnetons, respectively, the latter being considerably larger than the ordered moment of the free Dy3 + ion, 10.0 μB. The wavelength of the moment wave is about 1.3 times the a spacing and is practically temperature independent. An additional, coexisting spin alignment with a very small moment appears to take place below 31°K. The crystallographic parameters in the range 300°–5°K are also presented. The thermal‐neutron coherent scattering amplitude of Dy is established as (1.70 ± 0.01) × 10− 12 cm.

Spin‐Wave Relaxation in Ferromagnetic CdCr2Se4

Abstract: The rather simple magnetic structure of the ferromagnetic insulator CdCr2Se4 makes it an attractive substance for the study of magnetic relaxation mechanisms in general, but in particular those mechanisms of importance in the vicinity of the ordering temperature. The ferromagnetic relaxation has been studied here by means of both FMR and high‐power parallel‐pump techniques. Using the latter technique, the intrinsic spin‐wave linewidth was measured up to temperatures within a few degrees of the Curie temperature. Near the Curie temperature the linewidth diverges as (T0‐T)−1 with T0=133°K. This value of T0 is in close agreement with our measured value of the Curie temperature 130°K. At low temperatures, up to 30°K, the relaxation appears to result from two‐magnon coupling of the k=0 mode to modes of higher k, which relax by the three‐magnon confluence process.

Magnetic Structure of TbAg2

Abstract: Neutron‐diffraction measurements have revealed that TbAg2 having the tetragonal CaC2‐type structure becomes antiferromagnetic below the Neel temperature of 35°K. The ordered magnetic structure consists of the ferromagnetic sheets which are perpendicular to the a axis (face‐centered description), and the moment directions of the adjacent ferromagnetic sheets are opposite to one another. All moments are aligned in the direction of the c axis, and the saturation moment per Tb is 8.95 ± 0.05 Bohr magnetons which is essentially equal to the ordered moment of the free Tb3 + ion. This ordered structure is identical to the commensurable magnetic structure of TbAu2 at temperatures below 42.5°K, but the incommensurable transverse‐wavelike spin alignment of TbAu2 found in the 42.5°–55°K range was not detectable in TbAg2. Also, no detectable moment is observed for Ag in TbAg2.

Thin film magnetic information stores

Abstract: 1,224,495. Magnetic storage arrangements. COMPAGNIE INTERNATIONALE POUR L'INFORMATIQUE, and CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE. 22 March, 1968 [29 March, 1967], No. 14072/68. Heading H3B. [Also in Division H1] A magnetic storage structure comprises a layer of anisotropic ferromagnetic material magnetically coupled to a layer of an antiferromagnetic material. The antiferromagnetic material has its domains immediately adjacent the ferromagnetic material in such a direction as to lock its magnetic storage state, and so enables non-destructive read-out to be effected. The existing magnetic state may be changed by heating the antiferromagnetic material to above its temperature of atomic disorder (the Neel temperature) and applying a magnetic field in the required direction during the cooling process. An alternating magnetic field applying during cooling erases the information stored without writing new information. Suitable materials for the structure are stated to be cobalt or nickeliron (80%-20%), for the ferromagnetic material, and cobalt oxide, chromium oxide, or nickel-iron-manganese for the antiferromagnetic material. As shown in Fig. 4, the structure comprises adjacent thin layers of antiferromagnetic material 1 and ferromagnetic material 2 on a glass substrate 3, the metallic layers being either in contact, as shown, or separated by a thin layer of non-magnetic material, Fig. 3 (not shown). A second layer of ferromagnetic material 5 separated from the first layer 2 by a thin gold layer is optional. The structure is formed by deposition processes in the presence of a magnetic field, and in the case of a structure using nickel-iron as the anisotropic ferromagnetic material, a coating of manganese may be diffused into the surface of the ferromagnetic material by heat treatment so as to form an integral antiferromagnetic layer of nickel-iron-manganese. A semi-permanent matrix store is formed by a co-ordinate array of parallel conductors glued to the structure, Fig. 9 (not shown), the conductors being formed by selectively etching metallic layers coated on both sides of a thin insulating sheet. Crossing points of the conductors define discrete storage areas which are read out non-destructively by passing current through conductors extending along the easy axis of magnetization of the ferromagnetic material. Alternatively, current may be passed through conductors extending along the hard direction of magnetization at the same time as a hard-direction biasing field is applied. In a modification the conductors may be omitted and the information stored read out as shown in Fig. 12 by a displaceable scanning head comprising a light source 13 and a photo-electric detector 14. The emergent light is polarized at 33, and the light reflected from the portion of the magnetic structure at which it is directed is analysed at 34. Scanning by a displaceable head can be avoided by providing a mosaic of photoelectric detectors, one for each storage location, and either scanning with a polarized light beam or lighting the whole surface of the structure with polarized light. The stored information may be changed by using a heat source 20, Fig. 13, such as a ruby laser, to raise the temperature of those areas of the storage structure exposed by coded perforations in a mask 17, an orienting magnetic field for the exposed portions being applied to the storage structure 18. Alternatively a single heating spot may be selectively directed to required locations in a storage structure by moving the structure in two coordinates in accordance with a programme recorded on a magnetic or punched tape, Fig. 15 (not shown). Erasement of information stored without writing new information may be effected by cooling from above the disorder temperature in an alternating field. Read in at selected storage locations is then carried out by localized reheating in the presence of a unidirectional field.

Effects of Pressure and a Magnetic Field on Chromium Studied by Neutron Diffraction

Abstract: In addition to decreasing the first‐order antiferromagnetic‐to‐paramagnetic phase change near 312°K, pressure also decreases the spin‐flip transformation near 122°K, and appears to decrease the amplitudes of the static magnetization waves. By means of neutron diffraction we have determined the initial derivatives of these pressure variations using small pressures up to 100 atm. Magnetic fields applied to samples while cooling through the 312°K transition produce crystals whose magnetic structure can be ascribed to a single fundamental wavevector. This state, however, is not single domain. A model has been proposed previously by two of the authors (S.A.W. and A.A.) for this state in which the crystal spontaneously breaks up into many small domains in each of which the polarization axis is thermally excited. The predictions of this model have been checked by studying the field dependence (to 27 kG) of the mean directions of the polarizations and how this varies with temperature. The agreement between the ex...

Magnetization and Magnetic Structure of Mn-Zn and Mn-Zn-Ga Alloys of CsCl-Type Structure

Abstract: The β 1 phase of the CsCl-type structure in Mn-Zn system, which exhibits strong ferromagnetism at room temperature, was found to be stable below 175°C. Its homogeneity range extends from 50 to 56.5 at.% Mn. A canted spin structure, i. e. a coexistence of ferromagnetic and antiferromagnetic moments which are orthogonal to each other, was revealed to exist by the technique of neutron diffraction. Both moments decrease with increasing Mn content. An attempt to substitute a third element for Zn or Mn was unsuccessful except for Ga. The β 1 phase can contain as much as 10% Ga. The substitution of Ga for Zn gives rise to an increase in ferromagnetic moment and a decrease in antiferromagnetic moment.

Analysis of Mössbauer Hyperfine Structure in Thulium Metal below the Néel Temperature

Abstract: M\"ossbauer-effect measurements of the hyperfine structure of ${\mathrm{Tm}}^{169}$ below the N\'eel temperature are presented and analyzed. The interpretation is based on the neutron diffraction analysis of the magnetic structure, and assumes fast relaxation among the exchange-split levels of the lowest $J$ multiplet. The assumption of slightly incommensurate magnetic and crystal unit cells is essential to the successful interpretation of the data, since it produces a continuous distribution of hyperfine fields, as is observed.

Magnetic structure of Fe0.9Mn0.9Ge

Abstract: The magnetic structure of Fe0.9Mn0.9Ge was studied on a single crystal by neutron and X-ray diffraction. The crystal structure of the hexagonal B82 and the room temperature data indicated the distribution of metal atoms to be 0.83 Mn + 0.12 Fe + 0.05 hole in the 2(a) site and 0.78 Fe + 0.07 Mn + 0.15 hole in the 2(d) site. Low temperature data at 103°K were analyzed by assuming a general model, from which four possible structures with non-collinear spins were found. These gave the same net magnetic moment per molecule of 1.66μB. Magnetization measurements yielded the Curie point at 241°K and the moment of 1.39 μB.