Conical Spin Research Articles

Magnetic-field control of electric polarization in a helimagnetic haxaferrite Ba2Mg2Fe12O22

A Y-type hexaferrite Ba2Mg2Fe12O22 undergoes a transition to a proper-screw type helimagnetic structure with a propagation vector k0 ∥ [001] at 195 K, below which the system shows field-induced successive transitions under magnetic fields up to 1 T. Magnetization measurements also indicate the presence of the conical spin structure at low magnetic fields (∼ 10 mT) below about 50 K. We report on the magnetic control of the electric polarization in Ba2Mg2Fe12O22 with using the linearly oscillating fields up to ± 1 T at 5 K. The polarization vector can be cyclically reversed by weak magnetic fields of ± 30 mT. In addition, the polarization vector is repeatedly reversed without significant decay even by applying fields of ± 1 T, which suggests that the sense of the spin helix is somehow conserved in the process of the field-induced phase transitions. We propose that the conical spin structure carries the polarization vector upon the reversal of magnetic field.

Influence of solubility of MoO 3 in SbVO 5 on the magnetic spin arrangement

The magnetic properties of the solid solutions of MoO 3 in SbVO 5 for 5, 10 and 15 mol% were investigated by the magnetic susceptibility χ σ ( T) and magnetization σ( B). With increasing MoO 3 in SbVO 5 from 5 to 10 mol% the antiferromagnetic order shifts to higher temperatures and a Néel temperature of T N ≈ 6 K accompanied with a negative increase of the Curie–Weiss temperature from θ CW ≈ −6 K to −58 K is observed, indicating a change in the magnetic structure from non-collinear of the Yafet–Kittel type to a collinear antiferromagnetism. For 15 mol% a conical magnetic spin arrangement is likely. The values of the electrical resistivity ρ( T) and of the thermopower S at high temperatures indicate that all solid solutions are thermally activated p-type semiconductors above 325 K.

Rotation of an Electric Polarization Vector by Rotating Magnetic Field in Cycloidal MagnetEu0.55Y0.45MnO3

To clarify the microscopic origin of the gigantic magnetoelectric effect in multiferroics, we have investigated the variation of an electric polarization (P) vector under a rotating magnetic field (H) for a cycloidal helimagnet Eu0.55Y0.45MnO3 as a canonical example. P rotates smoothly by rotating H around the magnetic propagation wave vector k, which can be well understood by the rotation of the conical spin structure around k. We also show that the rotation process of the conical spin structure under H is crucial for the retention or reversal of the spin helicity or equivalently of the direction of P.

Generation of Electric Polarization with Rotating Magnetic Field in Helimagnet ZnCr2Se4

Application of magnetic field ( H ) can produce the ferroelectric polarization ( P ) in a proper screw helimagnet, ZnCr 2 Se 4 . While rotating H , we observed the cyclic generation of polarization current arising from the change of P in the conical spin structure. Versatile variations of P in magnitude and sign are observed while changing the direction and magnitude of H . The behaviors are ascribed to the rotation of the spin cones and the conserving or reversing spin-helicity upon the flop of the screw propagation vector, as affected by the competition among the magnetic anisotropy, inclination of the cone axis to H , and the P domain-wall stability.

Low-Magnetic-Field Control of Electric Polarization Vector in a Helimagnet

The mutual control of the electric and magnetic properties of a solid is currently of great interest because of the possible application for novel electronic devices. We report on the low-magnetic-field (for example, B values of +/-30 milliteslas) control of the polarization (P) vector in a hexaferrite, Ba2Mg2Fe12O22, which shows the helimagnetic spin structure with the propagation vector k0 parallel to [001]. The B-induced transverse conical spin structure carries the P vector directing perpendicular to both B and k0, in accord with the recently proposed spin-current model. Then, the oscillating or multidirectionally rotating B produces the cyclic displacement current via the flexible handling of the magnetic cone axis.

Multi-frequency ESR in NaCu2O2

NaCu2O2 is regarded as an S = 1/2 zigzag chain with competing nearest neighbor and next nearest neighbor exchange interactions. The spin structure of NaCu2O2 below a Néel temperature TN = 13 K was reported to be helical with a propagation vector q = (0.5, 0.227, 0). To obtain further information on the ordered state of NaCu2O2, we performed multi-frequency electron spin resonance (ESR) and high field magnetization measurements on single crystal samples of NaCu2O2. We observed a large and some small ESR signals and a monotonical increase of magnetization with a change of the slope around 20 T. One of the ESR resonance modes from the large signal is roughly reproduced by one of the helical resonance modes calculated on the assumption of a conical spin structure.

A comparative study of the magnetoelastic properties of the YFe 10V 2 and NdFe 10V 2 compounds

The magnetoelastic properties of iron-rich REFe 10V 2 (RE=Nd, Y) compounds were studied via magnetostriction and thermal expansion measurements in the 5–300 K range of temperature in up to 6 T external fields. Results of thermal expansion analysis show that the spontaneous magnetostriction of the compounds mostly originates from itinerant magnetization. Besides, the small volume striction appearing in the thermal expansion of the Nd compound close to 50 K suggests the existence of a basal to conical spin re-orientation transition. The volume magnetostriction isotherms of both compounds take minimum values for external field corresponding to the anisotropy field. In addition, the anisotropic and the volume magnetostriction traces of the NdFe 10V 2 take marked maxima under low field, with a relatively large initial magnetostrictivity, again more pronounced at the conical–axial spin re-orientation transition ( T SR=130 K). Analysis of the anisotropic magnetostriction of the Nd compound leads to the conclusion that the contribution of Nd–Fe interactions is negligible. The temperature dependence of volume magnetostriction is in good agreement with prediction of a phenomenological model based upon a fluctuating local band theory. This analysis shows that the difference between the forced volume strictions of Y and Nd compounds below and above T SR originates from the Nd sublattice magnetization.

Magnetic Reversal of the Ferroelectric Polarization in a Multiferroic Spinel Oxide

Ferroelectric transition has been detected in a ferrimagnetic spinel oxide of CoCr2O4 upon the transition to the conical spin order below 25 K. The direction [110] of the spontaneous polarization is normal to both the magnetization easy axis [001] and to the propagation axis [110] of the transverse spiral component, in accord with the prediction based on the spin-current model. The reversal of the spontaneous magnetization by a small magnetic field (approximately 0.1 T) induces the reversal of the spontaneous polarization, indicating the clamping of the ferromagnetic and ferroelectric domain walls.

The reaction of F with N 3 studied using multiconfiguration wave functions

CASSCF (complete active space self-consistent field) and CASPT2 (multiconfiguration second-order perturbation theory) calculations with the cc-pVTZ and cc-pVQZ basis sets were carried out for exploring the mechanisms of the N 3 + F → NF( 1 Δ ) + N 2 (i) and N 3 + F → NF( 3 Σ − ) + N 2 (ii) reactions. The CASSCF and CASPT2 calculations indicate that channel (i) occurs in the 1 A ′ potential energy surface (PES) and along the reaction path there exists an intermediate [IM ( 1 A ′ )] followed by a transition state and that channel (i) is energetically feasible. The CASSCF calculations indicate that channel (ii) occurs in the 3 A″ PES and it has a single step with a transition state [TS( 3 A″ )], but the CASPT2 single-point calculations imply that TS( 3 A″ ) may not exist. The 1 A ′ and 3 A″ PES crossing was explored by performing conical intersection and spin–orbit coupling calculations at the CASSCF level and the calculations imply that the crossing may not cause predissociation of IM( 1 A ′ ) into the triplet ground state product.

Magnetic structure and the anomalous Hall effect of Cu1$minus;xZnxCr2Se4

Abstract To study what roles spin structures of ferromagnets play in determining their Hall resistivity ρH or to see if the spin chiral order really contributes to the unusual behavior of anomalous Hall resistivity, we have carried out transport and neutron diffraction studies on the spinel-type system Cu1−xZnxCr2Se4, which is a collinear ferromagnet at x∼ 0 and in the helimagnetic state at x=1.0. It is established that the sign change of ρH takes place, with decreasing temperature, along with the appearance of the conical spin structure in the region 0

Conifold transitions and five-brane condensation in M-theory on Spin(7) manifolds

We conjecture a topology-changing transition in M-theory on a non-compact asymptotically conical Spin(7) manifold, where a 5-sphere collapses and a ℂP2 bolt grows. We argue that the transition may be understood as the condensation of M5-branes wrapping S5. Upon reduction to ten dimensions, it has a physical interpretation as a transition of D6-branes lying on calibrated submanifolds of flat space. In yet another guise, it may be seen as a geometric transition between two phases of type IIA string theory on a G2 holonomy manifold with either wrapped D6-branes, or background Ramond–Ramond flux. This is the first non-trivial example of a topology-changing transition with only 1/16 supersymmetry.

Antiferromagnetic phase transitions of the spinels, Zn 1-x Cu x Cr 2 Se 4 ; x =0.50 and 0.60

Magnetic and crystallographic properties have been studied by neutron powder diffraction and measurements of magnetic properties for the spinels Zn1-xCuxCr2Se4; 0.50≤x≤1.0. It is found that the spinels with x=0.50 and 0.60 show an antiferromagnetic phase transition at about 24 K and 28 K (TN), respectively, in addition to a ferromagnetic phase transition at about 420 K. Zn1-xCuxCr2Se4 with x=0.50 and 0.60 shows satellite-like magnetic reflections having indices of (h±Q, k, l); Q∼0.470 and 0.493 at 2 K, respectively. It is considered that the intermediate ferromagnetic phase does not result from a conical spin order of Cr3+ ions.

Cohomogeneity one manifolds of Spin(7) andG2holonomy

In this paper, we look for metrics of cohomogeneity one in D=8 and D=7 dimensions with Spin(7) and G_2 holonomy respectively. In D=8, we first consider the case of principal orbits that are S^7, viewed as an S^3 bundle over S^4 with triaxial squashing of the S^3 fibres. This gives a more general system of first-order equations for Spin(7) holonomy than has been solved previously. Using numerical methods, we establish the existence of new non-singular asymptotically locally conical (ALC) Spin(7) metrics on line bundles over \CP^3, with a non-trivial parameter that characterises the homogeneous squashing of CP^3. We then consider the case where the principal orbits are the Aloff-Wallach spaces N(k,\ell)=SU(3)/U(1), where the integers k and \ell characterise the embedding of U(1). We find new ALC and AC metrics of Spin(7) holonomy, as solutions of the first-order equations that we obtained previously in hep-th/0102185. These include certain explicit ALC metrics for all N(k,\ell), and numerical and perturbative results for ALC families with AC limits. We then study D=7 metrics of $G_2$ holonomy, and find new explicit examples, which, however, are singular, where the principal orbits are the flag manifold SU(3)/(U(1)\times U(1)). We also obtain numerical results for new non-singular metrics with principal orbits that are S^3\times S^3. Additional topics include a detailed and explicit discussion of the Einstein metrics on N(k,\ell), and an explicit parameterisation of SU(3).

Magnetotransport properties of SrFeO 2.95 perovskite

The resistivity, magnetoresistance, and Mössbauer effect of the metallic spiral antiferromagnet SrFeO 2.95 were examined in the temperature range 4.5–300 K. Two peaks were found in the plot of the temperature derivative of resistivity (d ρ/d T) versus temperature. The temperature of the upper peak, T N, corresponding to the onset of antiferromagnetic order, decreases monotonically from 105 to 98 K as the applied field increases from 0 to 9 T. This is due to the reduced antiferromagnetic superexchange interaction, while the temperature of the second transition, T a, which corresponds to a peak at low temperatures and coincides with the weak anomaly in χ( T) curve, remains constant upto 3.8 T and shows a rapid increase in the range from 40 to 65 K, as the magnetic field increases further. This is caused by the enhancement of the disappearance of Fe 3+ δ paramagnetic domains and also by the enhancement of the helical–conical spin transformation for the applied field greater than 3.8 T.

Magnetization process of itinerant-electron-type helical-spin-glass reentrant magnet Cr 0.81Mn 0.19Ge observed on various spatial scales

The magnetization process of an itinerant-electron-type helical-spin-glass reentrant magnet Cr 1− x Mn x Ge was analyzed using magnetic measurement, neutron depolarization analysis and small-angle neutron scattering, each of which has a different spatial resolution. At temperatures ranging from ∼8 to 13 K, the helical magnetism having a wave vector parallel to the crystal axis is stable in a zero magnetic field. As the magnitude of the field increases, the conical spin structure having a screw axis parallel to the applied field appears, and the cone angle decreases continuously. This change in spin structure primarily determines the macroscopic magnetization process in low magnetic fields. Although a ferromagnetic structure is induced in higher applied fields, magnetization saturation is difficult to achieve due to the inter-domain interaction. At approximately 8 K, spin-glass-like singularity is confirmed by means of the above-mentioned experimental methods. The appearance of spin-glass-like behavior is accompanied by changes in spin configuration on a microscopic scale. This behavior can be interpreted in terms of a mean-field-type picture, based on the spatial scale of the interaction.

The temperature dependence of magnetization of B containing melt-spun TbDyFe alloys

We have studied the magnetic properties of rapidly quenched [(Tb 0.33Dy 0.67) 1Fe 2] 1− x B x ( x = 0.00, 0.02, 0.04, 0.06) alloys by an analysis of magnetization behavior. Abnormal behavior in the temperature dependence of magnetization measured at 10 kOe is observed in all the specimens. Magnetization is very small at low temperature and it initially increases with increasing temperature, reaches a maximum, and then decreases with the further increase of temperature. There exists a conical spin structure which is caused by the non-collinear magnetic couplings between Fe atoms, and between Tb and Dy atoms due to the structural disorder. Our result suggests that the addition of B seems to decrease the magnetic anisotropy.

The magnetization study of B containing TbFe thin films

We report the magnetic behaviour of TbFeB thin films fabricated by RF magnetron sputtering technique. In order to obtain the intrinsic magnetic property, the temperature dependence of magnetization was measured using a SQUID magnetometer and VSM during heating from 5 to 500 K, with an applied field of either 10 or 50 kOe. Abnormal behavior in the temperature dependence of magnetization measured at 10 kOe is observed in all the specimens. Magnetization is very small at low temperatures and it initially increases with increasing temperature, reaches a maximum, and then decreases with the further increase of temperature. This result can be explained in terms of the existence of the conical spin structure which is caused by the noncollinear magnetic couplings between Fe atoms, and between Tb atoms due to the structural disorder.

Some magnetic properties and exchange interactions in Fe80−xTmxB12Si8 amorphous alloys

The magnetization and Curie temperatures of amorphous Fe80−xTmxB12Si8 alloys (0 ≤ x ≤ 15) have been measured. The Tm moment is 6.2 μB at 4.2 K, which is lower than the theoretical value, implying a conical spin structure. μFe is found to be independent of the Tm concentration and is equal to 2.05 μB. Mean-field theory has been used to describe the temperature dependence of the magnetization. The exchange interactions between Fe-Fe, Fe-Tm and Tm-Tm atom pairs have also been evaluated.

High field magnetic and Mössbauer studies in amorphous Fe-Er-B-Si alloys

We describe the results of magnetization and Mössbauer studies under high magnetic field on amorphous Fe 80- x Er x B 12Si 8 alloys with 0 ≤ x ≤ 15. The Er moment is 8μ B at 4.2 K which indicates a conical spin structure with an average cone angle of 54°. The Fe moment decreases only slightly with increasing x. For x = 15, the field dependence of magnetization shows a discontinuity at H = 3 T and it begins to increase remarkably with H. This is attributed, in the light of Mössbauer experiments under fields, to the opening of the canted Fe spin structure. The results are discussed.

Spin Waves in Systems with Long Period Helical Spin Density Waves Due to the Antisymmetric and Symmetric Exchange Interactions

In order to elucidate the different behaviours of the helical spin density waves (HSDW) due to the antisymmetric and symmetric exchange interactions, spin waves in the conical spin density wave (CSDW) states and the induced ferromagnetic states (IFMS) under external magnetic fields are calculated by use of phenomenological Hamiltonians with and without the antisymmetric exchange interaction. It is found that the spin wave spectra for the two types of HSDW are strongly different from each other, especially in the magnetic field dependences, enough to be able to distinguish them. The ESR frequencies are also investigated theoretically for bothtypes. In the light of these results, the observed magnetic field dependencies of the ESR frequencies in MnSi and Fe 1- x Co x Si are discussed to prove the HSDW due to the antisymmetric exchange interaction in these compounds.