Helimagnetic Structure Research Articles

A neutron powder diffraction study of the helimagnetic structure of TlCo 2Se 2− xS x

The magnetic layer structure of TlCo 2Se 2− x S x has been thoroughly re-investigated with neutron powder diffraction. The cobalt magnetic moments are ferromagnetically arranged within the layers, but the interlayer coupling differs profoundly with varying composition ( x): the spins in TlCo 2Se 2 form a helix along the c-axis with a turning-angle of ∼119° at 1.4 K. This kind of helical structure prevails for 0≤ x≤1.5 with a gradual decrease of the angle with increasing sulphur content, down to 34°, showing an almost linear relationship with the interlayer distance of Co–Co. For x≥1.75 the interlayer coupling changes to ferromagnetic. Unexpectedly, two helices were found to coexist at x=0.5 and x=1.0. The interaction between adjacent cobalt layers is there characterized by an incommensurate angle (106°, resp., 73°) together with a commensurate angle of 90°. The magnetic structures have been refined as two magnetic phases, each having a characteristic wave vector. A tentative model where the symmetry of the structure and the interlayer distance compete is considered for explaining the simultaneous occurrence of the two kinds of diffraction profile satellites.

LFe 6Sn 4Ge 2 (L = Dy, Ho, Er) studied by neutron diffraction and Mössbauer spectroscopy

The compounds LFe 6Sn 4Ge 2 (L = Dy, Ho, Er) have been studied by neutron diffraction and 119Sn Mössbauer spectroscopy. At room temperature the magnetic structure consists of ferromagnetic Fe (0 0 1) planes which are coupled antiferromagnetically along the stacking direction, with the moments aligned along the hexagonal c axis. The lanthanoid sublattice orders around T t = 21 and 11 K for the Dy and Ho compounds respectively. The erbium moment is not ordered at 1.7 K. Below T t, LFe 6Sn 4Ge 2 (L = Dy, Ho) display a helimagnetic structure with propagation vectors of (0, 0, 0.1262) and (0, 0, 0.1559) respectively. Ordering of the Dy and Ho moments causes a small distorsion of the high temperature AF structure of the iron sublattice and the development of a weak helimagnetic component (0.33 and 0.27 μ B respectively). The orientation of the iron component with respect to the L moments suggests that the main magnetic interaction between the two sublattice takes place between the L plane and the next nearest Fe planes.

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.

Polarized Raman scattering in helimagnetic β‐MnO2

AbstractA rutile β‐MnO2 film was grown on MgO substrate using plasma‐assisted molecular beam epitaxy (PAMBE) monitored by reflection high‐energy electron diffraction (RHEED). Polarized Raman spectra at various temperatures were obtained to investigate the influence of the helimagnetic structure on the vibrational modes of β‐MnO2. A red shift of Eg modes indicates a gradual formation of spin angles between neighboring Mn4+ ions. The intensities of the Eg and A1g modes with y‐polarized incidence increase remarkably below the Néel temperature. A new view as vibrational mode projection (VMP) indicates the interactions between the magnetic component of incident light and the helimagnetic structure. Copyright © 2008 John Wiley & Sons, Ltd.

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.

Hexagonal TmMn6Sn4Ge2 investigated by neutron diffraction and 119Sn Mössbauer spectroscopy

The hexagonal compound TmMn6Sn4Ge2 has been investigated by neutron diffraction and 119Sn Mössbauer spectroscopy. The location of Ge atoms on the 2(c) site has been confirmed. Above 230 K, an easy plane helimagnetic structure is stable and below 70 K an easy axis ferrimagnetic structure prevails. The magnetic moments at 2 K are μTm = 6.93 μB and μMn = 2.25 μB. In the intermediate temperature range, we observed a tilting of the spiral plane. The intermediate magnetic structure is clearly observed by the 119Sn local probe through a field distribution for the Sn(2e) site and a quadrupolar distribution for the Sn(2d) site. The change of the magnetocrystalline anisotropy of the Tm sublattice with respect to the easy plane observed in the parent stannide TmMn6Sn6 might be related either to a shortening of the Tm–Mn distances or to a deformation of coordination shell around the Tm atom, both being caused by the location of Ge atoms on the 2(c) site. The stabilisation of the ferrimagnetic structure at low temperature might be driven by stronger Tm–Mn interactions that result from the reduced Tm–Mn bond lengths.

Superexchange Interactions of (Ba1-xSrx)2Zn2Fe12O22 System Studied by Neutron Diffraction

Neutron diffraction experiments were carried out on single crystals of (Ba 1- x Sr x ) 2 Zn 2 Fe 12 O 22 at 8 K, where x is the Sr concentration ranging from 0 to 1.0. Competition of three superexchange interactions exists among Fe magnetic moments present in the 4th, 5th, and 8th layers of Sr-rich crystals. This competition leads to a distorted helimagnetic structure consisting of large and small ferrimagnetic bunches. The relations between exchange integrals J 45 , J 48 , and J 58 are determined by using the molecular field approximation as J 45 / J 58 =0.365 7 +0.019 0 x +0.061 7 x 2 and J 48 / J 58 =0.504 7 +0.010 4 x +0.011 6 x 2 at 8 K. These relations provide a clear interpretation for the magnetic structure of the (Ba 1- x Sr x ) 2 Zn 2 Fe 12 O 22 system. Zn ion distributions in the tetrahedral sites and a local lattice deformation around Sr ions are also discussed in detail.

Magnetic ordering in the HfFe 6Ge 6-type TbFe 6Sn 4Ge 2 compound

We have determined the magnetic structures of the Tb and Fe sublattices in TbFe 6 Sn 4 Ge 2 by combining powder neutron diffraction with Fe 57 and Sn 119 Mössbauer spectroscopy. TbFe 6 Sn 4 Ge 2 adopts the hexagonal HfFe 6 Ge 6-type ( P 6 / m m m ) crystal structure. The Fe sublattice orders antiferromagnetically below T N = 518 ( 3 ) K, with the moments parallel to c. At T t = 42 ( 1 ) K, the Tb sublattice orders helimagnetically, with a propagation vector Q = ( 0 , 0 , 0.0963 ) r.l.u. (at 2 K). This imposes helimagnetic order (with an identical propagation vector) on the Fe sublattice, indicating that the two sublattices are coupled for T < T t . The canting angle of the Fe moments from the c-axis in the helimagnetic structure is 10(1) ° .

μSR experiment on YMn2D0.5 and YMn2D3 deuterides

The Laves phase YMn2 exhibits a complex helimagnetic structure, which can be transformed by applying external pressure, chemical substitution or hydrogen absorption. The hydrides YMn2Hx can be obtained in a cubic structure above TC,N with a cell parameter increase up to x=4.3 H/f.u. For x≤3.4 H/f.u., both magnetic and hydrogen orderings appear below Tc. These transitions have been studied by μSR experiments in zero field and longitudinal geometry and in a transverse field to study spin fluctuations for x=0.5 and 3 D/f.u. down to 20 K. The correlation between the critical parameter γ and the deuterium concentration will be shown.

Neutron diffraction study of HfFe 6Ge 6-type TmMn 6Sn 6− xIn x compounds (0.4≤ x≤1.0)

The HfFe 6Ge 6-type TmMn 6Sn 5.8In 0.2, TmMn 6Sn 5.6In 0.4 and TmMn 6Sn 5In compounds have been studied by magnetization measurements and powder neutron diffraction experiements. TmMn 6Sn 5.8In 0.2 displays a Néel point at 332 K and undergoes a second transition at 62 K. Neutron diffraction study evidences a helimagnetic structure in the 62–300 K range and a conical one below 62 K. TmMn 6Sn 5.6In 0.4 orders ferrimagnetically at 320 K, displays antiferromagnetic behaviour in the 104–287 K temperature range and becomes again ferrimagnetic below 104 K. Neutron diffraction study confirms the collinear high and low temperature ferrimagnetic structures as well as the stabilization of a helimagnetic structure in the intermediate temperature range 104–287 K. TmMn 6Sn 5In displays ferrimagnetic order in its whole ordered range ( T C=313 K). All these compounds are characterized by an easy plane anisotropy in contrast with the easy axis anisotropy observed in the corresponding Ga-rich TmMn 6Sn 6− x Ga x compounds. These different behaviours might be related to the different In and Ga distributions on the Sn sites in the two series.

A neutron diffraction study of HfFe6Ge6-type ScMn6Ge6−xGax compounds and YMn6Ge6−xGax compounds (0.3≤x≤1.6)

The HfFe6Ge6-type ScMn6Ge6−xGax and YMn6Ge6−xGax solid solution (0.3≤x≤1.6) have been studied by magnetization measurements, microprobe analysis and powder neutron diffraction. The ScMn6Ge5.63Ga0.37 and YMn6Ge5.66Ga0.34 compounds display skewed helimagnetic structures in the whole 2–300 K studied range. The YMn6Ge5.35Ga0.65 compound displays a helimagnetic structure above 160 K and a ferromagnetic structure below 40 K. In the intermediate temperature range, both structures coexist. Finally, the ScMn6Ge5.34Ga0.66, ScMn6Ge5.12Ga0.88 and YMn6Ge5.08Ga0.92 display ferromagnetic structure in the whole 2–300 K range. The results are discussed and compared with those of all the RMn6Ge6−xGax compounds (R=Tb–Tm, Lu; 0≤x≤1.0) studied by neutron diffraction.

A neutron diffraction study of HfFe 6Ge 6-type ErMn 6Ge 6− xGa x compounds ( x=0.2, 0.4 and 1.0)

The HfFe 6Ge 6-type ErMn 6Ge 6− x Ga x compounds ( x=0.2, 0.4, 1.0) have been studied by powder neutron diffraction. The ErMn 6Ge 5.8Ga 0.2 compound orders antiferromagnetically at 460 K in a collinear easy axis magnetic structure characterized by a doubling of the c-axis. Below T t=270 K, a helimagnetic structure takes place and persists down to 50 K. Below this temperature, a ferrimagnetic structure with moments aligned along the c-axis is stable ( μ Mn=2.21(5) μ B and μ Er=8.60(8) μ B). The ErMn 6Ge 5.6Ga 0.4 compound orders antiferromagnetically at 394 K. From 300 to 120 K, it is characterized by a helimagnetic arrangement. In both cases, the normal to the c-axis is perpendicular to the [001] direction. Below 110 K, a ferrimagnetic structure takes place and the direction of the ferromagnetic component is aligned along the c-axis. The Mn and Er moments refined at 2 K are μ Mn=2.21(6) μ B and μ Er=9.06(11) μ B. The ErMn 6Ge 5Ga compound orders ferrimagnetically at 373 K and a moment reorientation takes place around 200 K. At high temperature the moments are close to the (001) plane whereas at low temperature they align along the c-axis. The Mn and Er moments refined at 2 K are μ Mn=2.23(6) μ B and μ Er=9.40(12) μ B. Evolutions of the moment magnitudes, magnetocrystalline anisotropy and interlayer couplings, as a function of the Ga content and the nature of the rare earth element, are discussed.

Metamagnetic behaviour and phase diagram of Lu 2Fe 17 under high pressure

The magnetic phase diagram of the Lu 2Fe 17 intermetallics was determined at temperatures below 300 K under hydrostatic pressure up to 1.2 GPa using a SQUID magnetometer. The Néel temperature T N decreases slowly under pressure; d T N/d p=−19 K/GPa. A range of stability of the helimagnetic structure below T N increases at the expense of the low-temperature ferromagnetic phase, which is depressed completely above 0.5 GPa. Existence of two helimagnetic phases under pressure is discussed.

A neutron diffraction study of HfFe 6Ge 6-type TbMn 6Ge 6− xGa x compounds ( x=0.1, 0.2 and 1.0)

TbMn 6Ge 6− x Ga x compounds ( x=0.1, 0.2, 1.0) were studied by magnetization measurements and neutron diffraction. The three compounds order ferrimagnetically at 449, 449 and 406 K, respectively, and display various transitions at lower temperatures. TbMn 6Ge 5.9Ga 0.1 displays a conical structure below 71 K ( μ Tb=8.71(9) μ B, μ Mn=2.24(5) μ B at 2 K) and a helimagnetic structure from 71 K to room temperature, where an additional ferrimagnetic component occurs. TbMn 6Ge 5.8Ga 0.2 is ferrimagnetic below 102 K and above 212 K, and displays a complex magnetic structure where commensurate and incommensurate components coexist in the intermediate temperature range. At 2 K, the moments are aligned along the c-axis ( μ Tb=8.5(1) μ B, μ Mn=2.24(5) μ B at 2 K) and are perpendicular to the c-axis at 300 K. TbMn 6Ge 5Ga is ferrimagnetic over the whole ordered range and displays a moment reorientation process at 214 K. At 2 K, the moments ( μ Tb=9.0(1) μ B, μ Mn=2.24(6) μ B) are aligned along the c-axis and they are close to the (001) plane at 300 K. The results are discussed and compared with the behavior of the other RMn 6Ge 6− x Ga x compounds (R=Ho, Tm, Lu).

Cation Distribution and Helimagnetic Structure of the Ba2(Zn1-xMgx)2Fe12O22System as Revealed by Magnetization Measurements and Neutron Diffraction

Magnetization measurements and neutron diffraction experiments were carried out on single crystals of Ba 2 (Zn 1- x Mg x ) 2 Fe 12 O 22 , where x is the Mg concentration ranging from x =0 to 1.0. The Fe ions present in the octahedral sites have a tendency to decrease with increasing x . The configuration of Fe magnetic moments in the Mg-rich crystal is a distorted helimagnetic one consisting of large and small ferrimagnetic bunches. We have found that there are competing exchange interactions among Fe magnetic moments lying on the forth, fifth and eighth layers of Mg-rich crystal. The exchange integrals J 45 , J 48 and J 58 were determined using the molecular field approximation: J 45 / J 58 =0.365±0.008 and J 48 / J 58 =0.503±0.016 at 9 K. These values provide a clear interpretation of the turn angle of the helix as well as the helimagnetic structure for the Ba 2 (Zn 1- x Mg x ) 2 Fe 12 O 22 system.

Multi spin-reorientation process in HfFe 6Ge 6-type HoMn 6Ge 6− xGa x compounds ( x=0.2, 0.4, 1.0)

The HoMn 6Ge 6− x Ga x compounds ( x=0.2, 0.4, 1.0) have been studied by magnetization measurements and neutron diffraction in the temperature range 2–300 K. The HoMn 6Ge 5.8Ga 0.2 compound orders antiferromagnetically at 415 K and displays a Curie point at 72 K. Neutron diffraction indicates a helimagnetic structure from 300 to 72 K and a cone structure at lower temperature ( μ Mn=2.23(6) μ B and μ Ho=9.46(12) μ B at 2 K). The HoMn 6Ge 5.6Ga 0.4 compound orders antiferromagnetically at 395 K. Neutron diffraction study indicates a helimagnetic structure between 250 and 300 K, a ferrimagnetic structure with moments almost aligned along the c-axis between 225 and 180 K, a cone structure in the temperature range 180–70 K and finally a ferrimagnetic structure below 70 K. At 2 K, the moment direction is at 43(1)° from the c-axis ( μ Mn=2.18(6) μ B and μ Ho=9.59(9) μ B. The HoMn 6Ge 5Ga compound is ferrimagnetic in its whole ordered range ( T C=378 K). At room temperature, the moment direction is at 60(4)° from the c-axis and rotates continuously towards the c-axis on cooling. In the 250–150 K temperature range, the moments are almost aligned along the c-axis and rotate again towards the (001) plane at lower temperature. At 2 K, the moment direction is at 46(1)° from the c-axis ( μ Mn=2.28(5) μ B and μ Ho=9.81(9) μ B at 2 K). The anomalous variation of the moment direction is discussed and related to the successive effect of the second and fourth order crystalline field parameters. The evolution of the magnetic order as a function of the Ga content is examined and comparisons are made with the RMn 6Ge 6− x Ga x compounds (R=Lu, Tm).

Partial frustration of the copper spins in RbCuCl 3

In order to investigate the magnetic order of the copper moments in the monoclinic (distorted hexagonal) compound RbCuCl 3 we carried out neutron diffraction experiments using single crystals and powder samples. Below the Néel temperature T N=19.3 K a helimagnetic structure with the propagation vector k mon=(0, 0.5985, 0) was observed. The magnetic moments lie in the monoclinic ab-plane and rotate along the b-axis with a turning angle of ϑ=108° between the adjacent planes (± 1/2 b). In the hexagonal representation the propagation vector corresponds to k hex=(0.2993, 0.2993, 0). This vector is close to k =(1/3, 1/3, 0) and therefore we deal with a distorted triangular antiferromagnet with partially released frustration. At 1.6 K the experimental magnetic moment of the copper atoms is μ exp=0.69(6) μ B. From susceptibility measurements with a SQUID magnetometer we derived the coupling constants of exchange interactions of the copper spins between ( J 0=25(1) K) and within ( J 1=−9(1) K) the stacked planes of triangles.

Neutron diffraction study of HfFe 6Ge 6-type TmMn 6Ge 6− xGa x compounds ( x=0.2, 0.4, 0.6, 0.8 and 1.0)

The TmMn 6Ge 6− x Ga x compounds ( x=0.2, 0.4, 0.6, 0.8 and 1.0) have been investigated by magnetization measurements and neutron diffraction. The TmMn 6Ge 5.8Ga 0.2 compound orders antiferromagnetically at 471 K in a colinear magnetic structure characterized by a doubling of the c axis. Below T t=220 K, a helimagnetic structure takes place. At 2 K, the spiral plane lies in the (101) plane with μ Mn=2.02(5) μ B and μ Tm=6.27(9) μ B. The TmMn 6Ge 5.6Ga 0.4 compound orders antiferromagnetically at 402 K. From 300 to 42 K, it is characterized by a helimagnetic arrangement with a helical plane lying in the (101) plane. Below 42 K, a conical structure takes place and the direction of the ferromagnetic component is almost aligned along the c axis. The Mn and Tm moments refined at 2 K are μ Mn=2.21(5) μ B and μ Tm=6.82(8) μ B. The TmMn 6Ge 5.4Ga 0.6 compound orders antiferromagnetically at 379 K. From 300 to 124 K, it is characterized by a helimagnetic arrangement with a helical plane close to the (001) plane. Below 124 K, this compound becomes ferrimagnetic with moments almost aligned along the c axis. The Mn and Tm moments refined at 2 K are μ Mn=2.19(4) μ B and μ Tm=6.89(7) μ B. The TmMn 6Ge 5.2Ga 0.8 and TmMn 6Ge 5Ga compounds order ferrimagnetically at 360 and 359 K, respectively, and a moment reorientation takes place at 73 and 56 K, respectively. At high temperature the moments are close to the (001) plane whereas at low temperature they almost align along the c axis in good agreement with the large coercive field reported at 4.2 K for this compound ( H c≈20 kOe). The Mn and Tm moments refined at 2 K are μ Mn=2.20(5)μ B [2.18(4)] and μ Tm=7.01(8) μ B [7.01(8)], respectively. The evolutions of the moment magnitudes, magnetocrystalline anisotropy and interlayer couplings, as a function of the Ga content, are discussed.

Helimagnetic structure ofYMn2 observed by means of nuclear magnetic resonance and neutrondiffraction

We approach the helimagnetic structure of the itinerant-electronmagnet YMn2, in which the Mn sublattice forms a geometricallyfrustrated corner-sharing tetrahedral lattice, by means of bothlow-energy neutron diffraction and precise nuclear magneticresonance (NMR) experiments, by modifying the magnetic state by Tbsubstitution (the introduction of lattice inhomogeneity), and byconsidering the origin of the tetragonal lattice distortion detectedbelow TN. The spin reorientation induced by the Tbsubstitution from the helical state with the propagation vectorQ⃗ = [τ01] (τ = 0.018) and with spins lying in the{100} plane to that with Q⃗ = [ττ1] (τ = 0.015) and {111}-plane rotation is interpreted as symmetrylowering from a coherent proper screw state with a high magneticsymmetry to an asymmetric but locally preferable configuration withthe easy-plane anisotropy perpendicular to the local symmetry axis.Treating the magnetic state as a weakly coupled assembly ofone-dimensional antiferromagnetic spin chains, which is realized asa result of the partial release of the frustration, and taking intoconsideration the lattice symmetry, the possibility of double-axialhelical structures arises and is discussed. In this study, all theinconsistency among previously reported NMR spectra is removed, andthe strong frequency dependence of the spin-echo decay time T2found unexpectedly is explained in terms of a phenomenological modeltaking into consideration the dynamic motion of electron spins inthe locally anisotropic magnetic system.

TOF neutron powder diffraction study of the helimagnetic structures in the Cr xFe 1− xVO 4-I system

The use of cold neutrons for long d-spacing time-of-flight neutron diffraction allows high-resolution data to be obtained for the solution and Rietveld refinement of complex magnetic structures. We have used the OSIRIS diffractometer at ISIS to collect data for Cr x Fe 1− x VO 4-I compounds with x=0.30, 0.35, 0.40 and 0.45 down to 2 K. The compounds order with a helimagnetic structure indexed by a propagation vector (0, k y, 1 2 ) , analogous to the previously reported x=0.25 and 0.50 samples. Our Rietveld refinements show that k y varies as a smooth function of x, which indicates that the structures are not locked into simple commensurate configurations. An alteration of the moment directions in the models reported for the x=0.25 and 0.50 magnetic structures was necessary in order to account for this high-resolution data.