Thursday 11 July 2013, Strathblane & Cromdale Halls, 16:30-18:30
Poster session C - Intermetallics, skyrmion systems and conventional magnets
P.141 Temperature evolution of the magnetic structure in Mn1−xFexGe compounds
E Altynbaev1, S Grigoriev1, N Potapova1, S-A Siegfrid2, V Dyadkin3, E Moskvin1, D Menzel4, Ch. Dewhurst4 and A
Tsvyashchenko5
1
Petersburg Nuclear Physics Institute, Russia, 2Helmholtz Zentrum Geesthacht, Germany, 3Technische Universitat
Braunschweig, Germany, 4Institute Laue-Langevin, France, 5Institute for High Pressure Physics, Russia
The cubic B20-type structure of MnGe and FeGe order below Tc in a one-handed spin helical structure with a small
propagation vector k ≈ 2.3 nm−1 for MnGe and k ≈ 0.09 nm−1 for FeGe. It is widely recognized that the helicity is
induced by an antisymmetric Dzyaloshinskii-Moriya (DM) exchange interaction. We have grown the series of the
mixed compounds Mn1−xFexGe, where Mn atoms are substituted by Fe atoms in B20-type of the structure. The SANS
experiments are aimed to study the temperature evolution of the spin structure of newly grown compounds.
In the low temperature range below Tc1 (105K for x=0; 60K for x=0.25; 130K for x=0.50) the single Bragg reflection
with the wavevector Q=k1 is observed. This reflection splits into two different reflections at Tc1 with the values of
wavevectors satisfying the ratio k1=2k2 in the whole temperature range up to T=Tc2 (235K for x=0; 180K for x=0.25;
200K for x=0.50). The absolute value of the wavevectors k1 and k2 decreases to 0 upon temperature increase from
Tc1 to Tc2. Above the temperature Tc2 two peaks are combined to one at Q=0, which is well described by the
Lorenzian function. This diffuse scattering disappears upon further temperature increase. The critical temperatures of
Tc1 and Tc2 coincide with the inflection points of the temperature dependence of magnetic susceptibility χ(T) for
these compounds.
P.142 Crystal and magnetic structure of Mn3Ni20P6 and Mn3(Pd1-xNix)20P6
Y Andersson1, V Höglin1, M Sahlberg1, P Nordblad1 and G André2
1
Uppsala University, Sweden, 2LLB, CEA-Saclay, France
The crystal and magnetic structure of Mn3Ni20P6 has been determined from X-ray and neutron powder diffraction
data. The crystal structure is of an ordered Cr23C6-type structure, space group Fm-3m, with the unit cell parameter a
= 11.0820(2)Å at 296 K.The temperature dependence of the susceptibility of Mn3Ni20P6 showed a pronounced
anomaly around 30 K, revealing a transformation of the magnetic structure. Between 26 K and 30 K a
commensurate anti-ferromagnetic ordering occurred with the propagation vector (0 0 ½). Below 26 K, a slightly
shorter propagation vector and an incommensurate magnetic ordering was determined. The iso-structural
compound Mn3Pd20P6 with the unit cell a= 11.956 Å is ferromagnetically ordered below 110 K [1]. A solid solution
Mn3(Pd1-xNix)20P6 was found for substitutions x < 0.10. The solid solutions are all ferromagnetic and exhibit a
lowering of the Curie temperature with increasing Ni content, combined with a decrease in the unit cell parameter.
Detailed results regarding the magnetic structures from neutron diffraction experiments will be presented.
[1]
T. Eriksson, M. Vennström, S. Ronneteg, Y. Andersson, P. Nordblad, J. Magn. Magn. Mater. 308 (2007)
203.
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P.143 Magnetic structures in TmTX (T – transition metal, X – p-electron element) intermetallics
S Baran1, A Hoser3and A Szytuła2
1
Jagiellonian University, Poland, 2M. Smoluchowski Institute of Physics, Jagiellonian University, Poland, 3HelmholtzZentrum Berlin für Materialien und Energie GmbH, Germany
This work presents the actual knowledge on magnetic structures in TmTX (T – transition metal, X – p-electron
element) intermetallics with 1:1:1 stoichiometry. These compunds crystallize in an orthorhombic crystal structure of
the TiNiSi-type (TmTGe for T = Ru, Rh, Ir; TmTGa for T = Ni, Rh) or a hexagonal one of the ZrNiAl-type (TmTIn for T =
Ni, Pd, Pt; TmAgX for X = Si, Ge) or the LiGeGa-type (TmCuSi; TmAuGe). No long-range magnetic order was found
down to 1.9 K in TmTGe (T = Ru, Ir, Au) and TmPdIn. The other compounds order antiferromagnetically with the Néel
temperatures not exceeding 6.5 K. The only exception is for TmCuSi where an antiferromagnetic order turns into a
ferromagnetic one with decreasing temperature and both magnetic phases were found to coexist within some
temperature range. Neutron diffraction data revealed the existence of commensurate magnetic phases (TmCuSi;
TmRhGe; TmAgX for X = Si, Ge; TmPtIn) as well as incommensurate ones (TmNiX for X = Ga, In; TmCuSi; TmRhGa).
Among magnetic structures there are both collinear ones (TmNiGa; TmCuSi; TmRhX for X = Ga, Ge) and frustrated
ones (TmNiIn; TmAgX for X = Si, Ge; TmPtIn).
P.144 Magnetic components in the skyrmion lattice in MnSi
E Blackburn1, E M Forgan1, J S White2, F Jonietz3, C Pfleiderer3 and P Boeni3
1
University of Birmingham, UK, 2EPFL, Switzerland, 3TU Munchen, Germany
The Dzyaloshinskii-Moriya interaction in the non-centrosymmetric compound MnSi results in a long-period spiral
magnetic ordering in zero applied magnetic field; however, three of these spirals couple to form a skyrmion lattice
[1] when a field of about 0.2 T is applied. We have used polarized small angle neutron scattering to measure the
ratios of neutron spin flip (SF) and non spin flip (NSF) scattering from the Bragg reflections of the skyrmion phase,
and also the low field helical magnetic phase. This provides additional information on the directions of the
components of magnetization density in the material. The SF signal is generated by magnetization perpendicular to
the neutron spin, whereas the NSF signal is generated by magnetization parallel to the neutron spin. Measurements
were performed over a range of temperatures in the helical phase, and through the skyrmion phase, varying both
the temperature and field.
In the helical phase, the SF/NSF ratio is 1, consistent with helices of circular cross-section. For the skyrmion lattice,
a ratio of approximately 1.05 is observed, with a possible increase with field, indicating larger components
perpendicular to the field, than parallel to it, i.e. the component helices acquire an elliptical cross section.
[1]
S. Muhlbauer et al., Science 323, 915 (2009).
P.145 High-resolution investigation of the magnetic excitations in Mn 3Si
G Brandl1, R Toft-Petersen2, A Bauer3, C Pfleiderer3 and P Böni3
1
Forschungsneutronenquelle Heinz Maier-Leibnitz, Technische Universität München, Germany, 2Helmholtz-Zentrum
Berlin, Germany, 3Physik-Department E21, Technische Universität München, Germany
Itinerant-electron antiferromagnets are a fascinating subject. In contrast to itinerant ferromagnets, the magnetic
excitations in even seemingly simple systems like elemental chromium are not well understood. In Cr, the
anomalously steep slope of the magnon dispersion, i.e. cSW = vF/31/2, where vF ~ 1940 meVÅ is the Fermi velocity
[1], leads to spin waves with extremely high energy when compared with the Néel temperature TN = 311 K.
Surprisingly, between the incommensurate positions of the magnetic Bragg peaks, low-energy excitations called the
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Fincher-Burke (FB) modes develop [2], whose origin is not understood [3]. Therefore, it is of high relevance if the FB
modes are a general feature of itinerant antiferromagnetism or are a singular property of Cr only.
Mn3Si undergoes a similar incommensurate ordering at TN = 25 K, an order of magnitude lower in temperature than
in Cr. Therefore, the associated energy scales are smaller and the magnetic excitations are more amenable to
neutron scattering techniques. Using thermal neutron spectroscopy, Tomiyoshi et al. [4] could show that there is a
dispersion of the spin waves emanating from the magnetic satellite reflections, which has the expected cone shape
and a steep slope of 37 meVÅ. However, the limited resolution made it impossible for them to resolve the excitation
spectrum at low energy. In our contribution we present high-resolution inelastic cold neutron scattering experiments
to search for FB modes in Mn3Si.
[1]
[2]
[3]
[4]
E. Fawcett, Rev. Mod. Phys. 60, 209 (1988).
S. K. Burke et al., Phys. Rev. Lett. 51, 494 (1983).
H. Hiraka et al., Phys. Rev. B 70, 144413 (2004).
S. Tomiyoshi et al., Phys. Rev. B 36, 2181 (1987).
P.146 Flip of the spin chiralitiy in the mixed compounds Mn1-x FexGe
S Grigoriev1, N Potapova1, S-A Siegfried2, E Altynbayev1, E Moskvin1, V Dyadkin1, D Menzel3, C Dewhurst4 and A
Tsvyashchenko5
1
Petersburg Nuclear Physics Institute, Russia, 2Helmholtz Zentrum Geesthacht, Germany, 3Technische Universitat
Braunschweig, Germany, 4Institute Laue-Langevin, France, 5Institute for High Pressure Physics, Russia
Magnetic susceptibility measurements have shown that the mixed compounds Mn1-xFexGe are magnetically ordered
through the whole range of concentrations x = [0.0 - 1.0]. The small angle neutron scattering reveals the helical
nature of the spin structure with the wavevector, which changes from its maximum for pure MnGe (k = 2.4 nm-1),
through its minimum k = 0 at xc = 0.75 - 0.80, to the value of k = 0.09 nm-1 for the pure FeGe. The macroscopic
magnetic measurements confirm the ferromagnetic nature of the compound with x = xc. Further on, X-ray diffraction
using synchrotron radiation and polarized neutron small angle diffraction have been used to evaluate the absolute
crystallographic structure and the spin helix chirality of single crystalline samples of FeGe. In accord to previous
observations for FeSi-based compounds, the FeGe compounds demonstrate: left-(right-) handed crystalline chirality
establishes right (left) handedness of the magnetic helix. It is opposite to the MnSi-related compounds: left-(right)
handed crystalline chirality establishes left (right) handedness of the magnetic helix. We conclude that the same
holds for the MnGe compounds. Finally, we argue that the observed tranformation of the helix structure to the
ferromagnet at x = xc is caused by the different signs of chirality for the MnGe and FeGe compounds.
P.147 Understanding the magnetic phases in Cu1-x NixMnSb series: A neutron diffraction study
M Halder1, K G Suresh1 and S M Yusuf2
1
Indian Institute Of Technology Bombay, India, 2Bhabha Atomic Research Centre, India
In recent years, Heusler and semi-Heusler alloys have been investigated both theoretically and experimentally due
to their various interesting physical properties. The semi-Heusler alloys XMnSb (X = 3d) belong to the class of
intermetallic compounds with large local magnetic moments at the Mn site. The semi-Heusler alloy NiMnSb
is ferromagnetic (FM) with Curie temperature TC of 750 K and crystallizes in the C1b structure [1]. CuMnSb alloy
also has the same crystal structure but it is antiferromagnetic (AFM), with Néel temperature TN = 55 K [1].
In this work, we have investigated in detail the magnetic transition from an AFM to a FM state in Cu1-xNixMnSb alloys
by neutron diffraction. We observe that for x < 0.05, Cu1-xNixMnSb is mainly in the AFM state. In the region 0.05 ≤ x
≤ 0.2, with decrease in temperature, there is a transition from the paramagnetic to a FM state, and below ~50 K,
both AFM and FM phases coexist [1]. In the region of phase coexistence, there is also the existence of short range
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magnetic correlation [2]. With increase in Ni substitution, the FM phase grows at the expense of the AFM phase, and
for x > 0.2, the system fully transforms to the FM phase. We have performed a quantitative analysis of both
magnetic phases and propose a magnetic phase diagram for the Cu1-xNixMnSb series. Furthermore, we have carried
out a detailed neutron diffraction study in the region of phase coexistence (for x = 0.15 sample) [2]. Our study gives
a microscopic understanding of the observed crossover from the AFM to FM ordering in the studied semi-Heusler
alloys.
[1]
[2]
M. Halder, S. M. Yusuf, et al. Phys. Rev. B 84,094435 (2011).
M. Halder, et al. (to be published).
P.148 Valence transitions and negative thermal expansion in YbMn2Ge2
M Hofmann1 and S Campbell2
1
TU München, FRM II, Germany, 2School of Physical, Environmental and Mathematical Sciences, The University of
New South Wales, Australia
Rare-earth intermetallic compounds containing ytterbium exhibit a wide range of interesting and unusual physical
and magnetic properties. This occurs mainly as a result of their mixed valence states (II/III) or changes from one
valence state to the other [1]. The YbMn2Si2-xGex system with the tetragonal ThCr2Si2–type structure is of particular
interest as it exhibits a wide variety of electronic and magnetic properties with evidence for valence instabilities in
compounds with high Ge concentration.
YbMn2Ge2 has a planar antiferromagnetic AFl structure below TNintra~510 K and transforms to a canted
antiferromagnetic AFmc below TNinter~185 K [2] and exhibits unusual variations in the a-lattice parameter over an
extended temperature range (~1.5-560 K). The lattice changes are found to correlate with magnetic and valence
transitions, indicating a strong link between the structural behavior and valence change for YbMn2Ge2.
In this contribution we establish the valence of YbMn2Ge2 and clarify its link with the structural and magnetic
transitions that occur over the temperature range ~10-723 K. The valence has been determined by X-ray Absorption
Fine Structure spectroscopy(~16-600 K) with the structural and lattice changes established by powder neutron
diffraction (~10-723 K). The pronounced changes in the a-lattice parameter (including negative expansion from
~550-350 K) and unit cell volume are well described by a model based on the valence changes. Magnetostrictive
effects also contribute to the lattice parameter changes and are shown to be significant around both TNintra~510 K
and TNinter~185 K.
[1]
[2]
J N. Grima, et al, Xjenza 11, 17 (2006)
M. Hofmann, et al , J Alloys Comp, 311, 137 (2000)
P.149 Magnetically induced phase gap in pseudoternary RMn2X2 compounds
S Kennedy1, J Wang2, S Campbell3, M Hofmann4 and S Dou5
1
ANSTO, Australia, 2University of Wollongong /ANSTO, Australia, 3University of NSW at ADFA, Australia, 4Technische
Universitat Munchen, Germany, 5University of Wollongong, Australia
< The RMn2X2 compounds (X= Si, Ge) display a fascinating array of magnetic phases due to the interplay between
the 3d and 4f magnetism and the strong dependence of magnetic exchange interaction on Mn–Mn near neighbor
distances. Our neutron and X-ray diffraction investigations of Pr1-yYyMn2Ge2-xSix reveal a clear separation into two
distinct magnetic phases, canted ferromagnetic and antiferromagnetic, over a broad concentration range with a
concommensurate phase gap in the crystalline lattice, due to spontaneous magnetostrictive distortion. This
remarkable magnetoelastic phenomenon is driven by a non-uniform atomic distribution on the R or X site which
produces subtle variations in the local lattice and abrupt changes in the Mn-Mn exchange interaction. The
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magnetoelastic coupling also makes it possible to shift the magnetostructural phase boundary toward ferromagnetic
or antiferromagnetic states with magnetic field or applied pressure respectively. Our results show that co-existence
of canted ferromagnetic and antiferromagnetic phases depend on pressure (chemical or applied) from the rare
earth and metalloid sites, on applied magnetic field and on local lattice strain distributions. We demonstrate that
these magnetostructural correlations act across the entire family of R1-yR'yMn2X2-xX'x compounds.
P.150 Fluctuation induced first-order transition in chiral helimagnets
J Kindervater1, W Häußler2, M Garst3, A Bauer1, M Janoschek4, H Berger5, S Mühlbauer2, U Gasser6, C Pfleiderer1 and
P Böni1
1
Technische Universität München, Germany, 2Technische Universität München, FRM II, Germany, 3Institute for
Theoretical Physics, Universiät zu Köln, Germany, 4Los Alamos National Laboratory, USA, 5EPFL Laussane,
Switzerland, 6Paul Scherrer Institut (PSI), Switzerland
Generic aspects of the magnetic phase transition in chiral magnets have recently been discussed controversially. On
the one hand, the unusual signatures observed just above the transition temperature have been attributed to an
intermediate skyrmion liquid [1,2] or a magnetic blue phase [3]. On the other hand, they have been interpreted in
terms of chiral paramagnons [4] that eventually drive the transition weakly first order [5,6]. Indeed, a detailed study
combining results from thermodynamic and neutron scattering experiments has revealed that the paramagnetic to
helimagnetic transition in MnSi is a fluctuation-induced first order transition belonging to the Brazovskii class [6].
We report results from small-angle neutron diffraction from the conducting intermetallic MnSi and the insulating
multiferroic Cu2OSeO3. Our comparative study demonstrates the universal aspects of the Brazovskii transition in
both compounds in zero and applied magnetic fields.
[1]
[2]
[3]
[4]
[5]
[6]
Rößler et al., Nature 442, (2006)
Pappas et al., PRL 102, (2009)
Grigoriev et al., PRB 72, (2005)
Bak and Jensen, Journal of Physics C 13, (1980)
Brasovskii, Zh. Eksp. Teor. Fiz. 68, (1975)
Janoschek et al., arXiv:1205.4780, (2012)
P.151 Signature of gap closure in the phonon spectra of FeSi
S Krannich1, R Heid1, D Lamago1, Y Sidis2, J-M Mignot2, P Steffens3, A Ivanov3 and F Weber1
1
Karlsruhe Institute of Technology, Germany, 2Laboratoire Léon Brillouin, France, 3Institute Laue Langevin, France
The nature of the unusual electronic properties of FeSi has attracted a lot of theoretical and experimental work over
the last decades. Recently, two different mechanisms for the gap formation at 300K have been reported, where
either thermal disorder effects define the temperature dependent gap formation [1] or the gap is filled due to strong
electronic correlations [2]. Here we report on a detailed study of the lattice dynamics over the whole Brillouin zone
and over a large temperature range ranging from 15K < T < 800K. Some remarkable effects can be observed for
many phonon branches with some notable exceptions. First, a strong softening occurs between 100K and 300K
going beyond thermal volume expansion as calculated by DFPT. However, using two different electronic densities of
states (eDOS) for phonon calculations, i.e. having a gapped eDOS and one without an electronic gap, we can
describe the observed softening within the quasi-harmonic approximation, i.e. without involving any thermal
disorder. The second observed effect is a significant broadening of certain phonon modes that occurs between
200K and 600K. This broadening coincides with the temperature activated magnetic fluctuations. This is interpreted
as a signature of spin-phonon coupling. Our results show that thermal disorder is not important in understanding the
softening of phonon frequencies and, most likely, electronic correlation are responsible for the gap closure at
relatively low temperatures.
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[1] O. Delaire et al., PNAS USA 108 (2011).
[2] J. M. Tomczak et al., PNAS USA 109 (2012).
P.152 Approaching quantum criticality in Mn[1-x]Fe[x]Si: SANS Pol study
E Moskvin1, S Grigoriev1, N Potapova1, V Dyadkin2, C Dewhurst3, S Siegfried4 and D Menzel5
1
Petersburg Nuclear Physics Institute, Russia, 2SNBL/ESRF, France, 3Institute Laue Lagevin, France, 4Helmholtz
Zentrum Geesthacht, Germany, 5TU Braunschweig, Germany
We studied the critical spin fluctuations and spin structure in Mn1-xFexSi, compounds in the vicinity of the quantum
phase transition at x ≈ 0.15. Compounds with x = 0.1, 0.12, 0.15, 0.16, 0.20 were studied by ac susceptibility and
polarized neutron small-angle scattering. In accord with our previous study [1] the compound with x = 0.10
undergoes the transition from the paramagnetic to helimagnetic phase at Tc ≈ 7 K through the well distinguishable
crossovers: (i) from paramagnetic to partially chiral and (ii) from partially chiral to fully chiral fluctuating state. The
compounds with x = 0.15 and 0.16 show enhancement of the criticality with lowering temperature to T = 1.7 K. We
obtained temperature and magnetic field dependencies of the inverse correlation length, κ, susceptibility, χ, and
magnetic structure wave vector, k0, for the above-mentioned compounds. No spin ordering was observed for
compounds with x > 0.1. Extrapolation to T = 0 verifies our assumption of their closeness to the quantum phase
transition. Compound with x = 0.20 does not exhibit any long-range order or fluctuations down to the lowest
measured T. It clearly means that this concentration is already above the critical value xc of quantum phase
transition.
[1] Sergey V. Grigoriev, Evgeny V. Moskvin, Vadim A. Dyadkin, Daniel Lamago, Thomas Wolf, Helmut Eckerlebe, and
Sergey V. Maleyev, Phys. Rev. B 83 (2011) 224411.
P.153 Spin Dynamic in INVAR
G B Pasquino1, R Stewart2, J Taylor2, S Giblin2, J P Goff1 and P Fouquet3
1
Royal Holloway University of London, UK, 2ISIS, UK, 3Institut Laue-Langevin, France
The origin of the Invar effect, whereby certain ferromagnetic alloys exhibit near zero thermal expansion around room
temperature, is still not understood. Various models have been put forward, including two-state models, canted
moments and longitudinal spin-fluctuations, but in all cases experimental support is lacking, if not opposed to the
theories. Motivated by the search for enhanced low-frequency spin-fluctuations in INVAR (Fe65Ni35) we have
investigated the spin dynamics using both μSR and ferromagnetic neutron-spin-echo (FNSE). As a function of
temperature the muon depolarization rate in zero-field, displays a broad peak at ~270 K for INVAR, while for the
non-Invar concentrations (Fe80Ni20 and Fe60Ni40) this peak is strongly suppressed. Further longitudinal field μSR field
measurements, on INVAR, showed that while the muon depolarization rate is reduced at fields > 1 T (where INVAR is
single domain) a peak still persists. FNSE data taken in a field of 1 T (required to preserve the neutron polarization)
display a tantalising glimpse of low frequency spin-dynamics at around Q = 0, on a timescale of the order of nanoseconds. This confirmation of the existence of spin-fluctuations in INVAR suggests that the muon data do indeed
indicate enhanced dynamics (as opposed to muon hopping) and further that these dynamics may be associated
with short-range FM clusters, as previously observed by polarized neutron diffraction.
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P.154 Modeling of the magnetism in ScFe4Al8 compound using MCMag and MCPhase programs
K Recko1, L Dobrzyński2, J Waliszewski1 and K Szymański1
1
University of Bialystok, Poland, 2National Centre for Nuclear Research, Poland
Polarized neutron diffraction and magnetization experiments carried out on ScFe4Al8 compound, disclosed doublemodulated magnetic structure with two phase transition temperatures. In the single crystal both neutron and high
resolution X-ray diffraction measurements indicate perfect crystallochemical ordering. Scandium alloy belongs to
ThMn12 type structure (I 4/mmm no.139). The scandium (2a) site seems magnetically inactive while the iron (8f)
site splits into two independent magnetic orbits. Below ~115 K two sets of magnetic reflections were found. Each of
them requires different wave vector representation: q1 = (0, 13, 0,13, 0) and q2 = (0,18, 0,18, 0) and discloses
different temperature behaviour. MCMag and MCPhase computer codes provide appropriate tools for simulation of
the magnetic structure when coupling constants are known or can be roughly estimated. The magnetism of
ScFe4Al8 has been simulated on a sample containing at minimum 3x3x3 cells the iron spins lie in the ab-plane and
the almost antiferromagnetically coupled moments are canted into b axis. In order to produce this feature, the
positive D anisotropy coefficients along c and b axes and a negative D coefficient along a axis have been assigned.
During further tests all of the exchange integrals have been modified up to values obtained from MCPhase. The
simulation starts at room temperature where spins are dynamically disordered. New orientations of spins are drawn
at random and adopted or rejected according to Boltzmann statistic. The input files contain a list of the magnetic
sites in the unit cell, a list of their neighbours and corresponding exchange integrals, anisotropy coefficients and
spin amplitudes. No symmetry elements are taken into account.
P.155 Neutron dark field imaging of domain structures in superconductors
T Reimann1, C Grünzweig2, S Mühlbauer3 and M Schulz4
1
TU München, Germany 2Paul-Scherrer-Institut, Switzerland, 3Forschungs-Neutronenquelle Heinz Maier-Leibnitz,
Germany, 4Forschungs-Neutronenquelle Heinz Maier-Leibnitz\Physik Department E21, Germany
In the intermediate mixed state (IMS) of a type II superconductor (SC), the sample splits up into field-free Meissner
domains and Shubnikov domains which carry the vortex lattice. The IMS is analog to the intermediate state (IS) of a
type-I superconductor with normal and superconducting domains. Experiments on the topology of both states show
a variety of different patterns including striped, dendritic and bubble phases, which represent typical domain
morphologies also seen in various other physical contexts. Seen from another viewpoint, the emergence of the IMS
reflects the crossover from repulsive to attractive vortex- vortex interaction. A detailed investigation of domain
patterns offers the possibility to study general characteristics of domain nucleation and morphology as well as the
physical properties of vortex matter. Domain structures in SC are typically investigated by surface sensitive
techniques such as magneto optical imaging, but flux pinning as well as Landau branching can significantly hamper
the deduction of bulk properties. In this talk we show how neutron grating interferometry (nGI) can be used as a tool
for the unambiguous identification of bulk properties. The capability of this unique technique will be demonstrated
on Pb and Nb single crystals, which are classical representatives of type I and type II SC respectively.
ICNS 2013 International Conference on Neutron Scattering
P.156 Field evolution of the magnetic structure in Mn1-x FexGe
S-A Siegfried1, N Potapova2, E Altynbaev2, E Moskvin2, V Dyadkin3, D Menzel4, C Dewhurst5, A Tsvyashchenko6, D
Lott1, A Schreyer1 and S Grigoriev2
1
Helmholtz-Zentrum Geesthacht, Germany, 2Petersburg Nuclear Physics Institute, Russia, 3Swiss-Norwegian
Beamlines at ESRF, France, 4Technische Universität Braunschweig, Germany, 5Institut Laue-Langevin,
France, 6Institute for High Pressure Physics Troitsk, Russia
The cubic B20 type transition-metal monogermanides belong to the P213 space group. These B20 systems order in
a helical spin structure which is induced by an antisymmetric Dzyaloshinskii-Moriya (DM) exchange interaction
caused by the non-centrosymmetric arrangement of the magnetic atoms in these compounds [1,2]. Polycrystalline
Mn1-xFexGe samples have been synthesized (x:[0.0,1.0]) by high pressure method. SQUID magnetization
measurements were used to establish the magnetic ordering temperature TC. From x = 0.0 to x = 0.5 the ordering
temperature decreases from 180 K to 160 K and increases to 278 K for further Fe doping. Small angle neutron
scattering measurements were carried out to determine the temperature and field evolution of the magnetic
structure in these compounds. For zero magnetic field the scattering maps show a typical powder like patterns,
indicating different spiral domains in the samples with randomly oriented helix wavevectors k. We have chosen the
sample with a concentration of x = 0.5 for further field dependent scans. Below a critical temperature TC ≈ 175 K
the field scan reveals transition from the ring-like pattern to the spot-like pattern with two spots (k1 || H) and two
other spots (k2 perp. H). Further increase of the field results in remarkable rise of k1, while two other spots with k2
perp. H disappear. For low temperatures (T < 100 K) only two spots with k1 || H arise in the field scan (from 0-2 T).
In conclusion, we have observed a nontrivial field transformation of the spin structure in the mixed Mn 1-xFexGe
compounds.
[1] I. E. Dzyaloshinskii, 1964 Zh. Eksp. Teor. Fiz. 46 1420.
[2] P. Bak, M. H. Jensen, 1980 J. Phys. C13 L881.
P.157 Nonreciprocal reflection of unpolarized thermal neutrons by pair of magnetic mirrors in external magnetic
field
D Tatarskiy1, A Petrenko2, V Rogov1, O Udalov1, N Gusev1, Y Nikitenko2 and A Fraerman1
1
IPM RAS, Russia, 2Joint institute for nuclear research, Russia
It is known the unpolarized neutron scattering in systems with noncoplanar magnetic induction spatial distribution
can be nonreciprocal [1]. In this work we investigate system consisting of two magnetic mirrors in external magnetic
field. The nonreciprocal effect in this system is expected to be about of 50% of the initial neutron beam intensity.
The mirrors prepared by magnetron sputtering of Co60Fe40 thin films on smooth glass substrates. Thickness and
roughness are controlled by x-ray small angle reflectometry and atomic force microscopy. Magnetic properties
investigated by the longitudinal magneto-optic Kerr effect and magnetic force microscopy. The thickness is in the
range 105-135 nm and roughness is about 0.7-1.1 nm. The coercive force of mirrors is 60-80 Oe. The remanent
magnetization is 720 Oe.
The mirrors are placed parallel to each other but their remanent magnetizations are perpendicular. The external
magnetic field of 10-100 Oe is perpendicular to mirrors surfaces. So that the magnetizations and the external field
are perpendicular to each other. In this system the mirrors are perpendicular "polarizer" and "analyzer"
correspondingly which detect the neutron beam spin polarization precession in the external field. The neutron
reflection experiment is made at the Spectrometer of polarized neutrons REMUR of the high flux pulsed IBR-2M
reactor of the Joint institute for Nuclear research [2].
It is shown the reflectivity of unpolarized neutrons in such system is strongly depends on the magnitude and
direction of the external magnetic field and reflection is nonreciprocal.
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[1] D.A. Tatarskiy, O.G. Udalov, A.A. Fraerman, JETP 115, #4, 626 (2012).
[2] V.L. Aksenov et al., JINR Comms. D13-2004-47 (2004).
P.158 The Bragg glass phase diagram in low-kappa vanadium
R Toft-Petersen1, M Laver2, A Bech Abrahamsen3 and S Balog2
1
Helmholtz-Zentrum Berlin, Germany, 2Paul Scherrer Institut, Switzerland 3Technical University of Denmark,
Denmark
We investigate the pair correlations between flux lines in low-κ vanadium by using a simulated annealing (SA)
technique to refine the structure of a large virtual vortex lattice. The translational correlators are found to decay as
power-laws with exponents similar to those predicted by Bragg glass (BG) theory in a large field interval. This powerlaw decay is found not to hold at larger distances; above a critical pair separation length, the translational correlator
drops off rapidly, indicating a multi-domain glass phase. The rocking width increases before the intensity drops to
zero well below Hc2, without any change in the hexagonal order. This short range ordered hexatic glass undergoes
a de-pinning close to Hc2 in a small but finite field interval, without showing any peak effect. A phase diagram for the
vortex lattice of a dirty low-κ conventional superconductor is suggested.
P.159 Study of flux pinning in large superconductors by polarized neutron radiography
W Treimer1, O Ebrahimi2, K Nursel3, D Indu2, A Ebrahimi3 and C Schramm3
1
University of Applied Sciences Berlin - Beuth Hochschule fuer Technik, Germany, 2Helmholtz Zentrum Berlin
Wannsee, Joint Research Group , Germany, 3University of Applied Sciences - Beuth Hochschule für Technik Berlin,
Germany
The physics of flux trapping and pinning in superconductors is still of principle interest. So far, there exists no unique
theory that explains the amount, shape, and distribution of trapped, pinned magnetic fields. Type-I superconductors
(SC) do not show the creation of flux lines as compared to type-II SC, on application of an external magnetic field,
but they do show an intermediate state comprising both, normal and superconducting regions. Recently, there
have been studies reported about an additional state (suprafroth) and quantum tunneling of interfaces in Pb (type-I
superconductor), for example. In the last two years, flux pinning in massive Pb samples has been studied by the
means of imaging with polarized neutrons. This allowed the probing of interior regions of the samples in their
superconducting states. For cylindrically shaped Pb samples different trapping/pinning behavior of magnetic fields
was found for crystalline and polycrystalline samples, despite the fact that they have exactly the same shape,
dimensions and high purity . The trapped field appeared squeezed for T < Tc and Happl = 0. However, the samples
under investigations had much larger volume (cm3), than the ones studied with SQUID magnetometers (thin plates,
a few mm3, polycrystalline) . We report the behavior of flux pinning in large, differently shaped Pb samples such as
cylinders, plates, cubes (crystalline, polycrystalline, high and low purity), and their dependencies on temperature,
external magnetic fields and about imaging of possible macroscopic quantum tunneling.
P.160 “Skew" scattering of cold unpolarized neutrons in ferromagnetic crystal
O Udalov
Institute for physics of microstructures RAS, Russia
The problem of neutron scattering by a single magnetic atom is theoretically considered in the second order
perturbation theory. It is demonstrated that the elastic scattering of unpolarized neutron by a magnetic atom is
skewed, i.e., it contains a term including the symmetry of a mixed product of the atom magnetic moment and the
wave vectors of incident and scattered neutrons ([k×k']·h). The problem of dynamical diffraction of unpolarized
ICNS 2013 International Conference on Neutron Scattering
neutrons by a perfect ferromagnetic crystal is investigated. We consider the case when the Bragg condition is
satisfied for two reciprocal lattice vectors. In this situation the neutron skew scattering manifests itself as a
dependence of the diffracted beam intensity on the sign of the crystal magnetization. The diffraction of unpolarized
neutrons by a Co crystal has been calculated. The change in the intensity through the magnetization reversal in this
case is estimated at 40%.
P.161 Electric field control of skyrmions in the chiral-cubic insulator Cu2OSeO3
J S White1, I Levatić2, A A Omrani3, N Egetenmeyer1, K Prša3, I Živković2, J L Gavilano1, M Bartkowiak1, A Magrez3, H
Berger3 and H M Rønnow3
1
Paul Scherrer Institut, Switzerland, 2Institute of Physics, Croatia, 3EPF Lausanne, Switzerland, 4Paul Scherrer
Institut, Switzerland
Skyrmions are topologically protected magnetic spin vortices that form a hexagonal 2D lattice arrangement in noncentrosymmetric magnets. Until last year, skyrmions had been observed only in metallic and semiconducting chiralcubic B20 compounds where, in MnSi in particular, it was shown that skyrmions can also be manipulated by
conduction electrons. The recent discovery of a skyrmion lattice (SkL) phase in the chiral-cubic insulator Cu2OSeO3
is therefore exciting, since it further evidences skyrmion formation as a more general phenomenon to be expected in
systems with non-vanishing chiral interactions. Since Cu2OSeO3 furthermore displays a magnetoelectric coupling, an
important open question was to learn how and if the skyrmion lattice can be manipulated by applied electric fields.
We report small-angle neutron scattering experiments that demonstrate successfully the manipulation of skyrmions
by applied electric fields in insulating Cu2OseO3. In an experimental geometry with magnetic fields || [1-10]
and electric fields || [111], we discover that the effect of applying the electric field is to controllably rotate the SkL
around the magnetic field axis in a manner dependent on both its size and sign. Our results provide the first
evidence for a new manifestation of the electric field control of magnetism in magnetoelectrics, and thus show the
electric field to be a new experimental parameter for studying the basic physics of skyrmions inside chiral-cubic
lattices.
P.162 Research on coercivity of Nd-Fe-B permanent magnet by microstructure and magnetic domain observations
M Yano1, K Ono2, M Harada3, A Manabe1, T Shoji1, A Kato1 and J Kohlbrecher4
1
Toyota Motor Corporation, Japan, 2High Energy Accelerator Research Organization (KEK), Japan, 3Toyota Central
R&D Labs. Inc. Japan, 4Paul Scherrer Institut, Switzerland
High maximum energy product of Nd-Fe-B permanent magnet enables to make electric motors or generators
efficient. The motor in hybrid or electric vehicles require high coercivity magnet to maintain their magnetization even
at operating temperature up to about 200 °C. To obtain high coercivity for Nd-Fe-B magnets, Dy is indispensable
element to enhance magnetic anisotropy field. However, Dy reduces magnetization due to the antiparallel coupling
of the magnetic moment between Dy and Fe. Therefore, coercivity enhancement without Dy is an important
approach to develop next generation magnets.
In general, coercivity of Nd-Fe-B magnet can be enhanced by reducing the grain size. However, the mechanism of
the coercivity enhancement has not been clarified yet. In order to approach the coercivity mechanism, we have
focused on the investigation of magnetic domain reversal behaviors during the magnetic reversal event. Due to high
transparency of neutron beam, magnetic domain structures in the bulk can be observed by small-angle neutron
scattering (SANS).
Obtained SANS data were analyzed by separating nuclear and magnetic scatterings to understand the relationship
between chemical and magnetic domain structures. The magnetic scattering patterns were fitted by a
phenomenological correlation length model to quantify the periodicity, density and morphology of magnetic
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domains. The result indicates that nano-crystalline magnet which has nm sized grains shows single domain like
reversals which was comparable to the direct magnetic domain observations such as X-ray microscopy.
ICNS 2013 International Conference on Neutron Scattering