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The Dzyaloshinskii–Moriya interaction (DMI), i.e., the antisymmetric part of the exchange coupling tensor, favors the perpendicular arrangement of magnetic moment, thus inducing canting in otherwise collinear structures. The DMI is the prerequisite for the emergence of weak ferromagnetism in antiferromagnets, but can stabilize twisted magnetic textures, such as spin spirals, soliton lattices, and magnetic skyrmions. While the magnitude of the DMI determines the canting angle of adjacent spins, its sign dictates the sense of the spin rotation. Based on focused polarized neutron diffraction (PND) study, combined with symmetry analysis, we determine the sign of the DMI in the unconventional multiferroic Ba 2 CoGe 2 O 7 and reveal its detailed magnetic structure in magnetic fields applied in the tetragonal plane. As PND gives unique access to the scattering contribution from the phase-sensitive nuclear-magnetic interference, it is a valuable tool for a straightforward DMI sign determination in bulk materials and allows to disclose even very weak magnetic moments. Remarkably, the sign of the DMI could be determined from the PND measurement of a single reflection, which is demonstrated to be reliable for a large range of applied magnetic field directions and values.

We have carried out a comprehensive investigation of the magnetic properties of the intercalated Lu2Fe3O7 by the means of dc magnetization, polarized neutron scattering, and x-ray magnetic circular dichroism (XMCD) measurements. The results obtained by these techniques support a spin order in each of the Fe-O bilayers that is the same as observed in the nonintercalated LuFe2O4. In the Fe-O single layers, a magnetic moment appears to be induced by the application of a magnetic field, paramagnetic-like in the first approximation. Furthermore, a spin-charge coupling was revealed by the XMCD measurements

Physical review / X 11(1), 011060 (2021). doi:10.1103/PhysRevX.11.011060, Published by American Physical Society, College Park, Md.

50th IFF Spring School, Jülich, Germany, 11 Mar 2019 - 22 Mar 2019; Jülich : Forschungszentrum Jülich GmbH, Zentralbibliothek, Verlag, Schriften des Forschungszentrums Jülich. Schlüsseltechnologien 190, 1 Band (nicht paginiert) : Illustrationen (2019)., Published by Forschungszentrum Jülich GmbH, Zentralbibliothek, Verlag, Jülich

Nanoscale horizons 5(7), 1065 - 1072 (2020). doi:10.1039/D0NH00117A, The self-assembly of nanoparticles into highly ordered crystals is largely influenced by variations in the size and shape of the constituent particles, with crystallization generally not observed if their polydispersity is too large. Here, we report on size selectivity in the self-assembly of rounded cubic maghemite nanoparticles into three-dimensional mesocrystals. Different X-ray scattering techniques are used to study and compare a nanoparticle dispersion that is used later for self-assembly, an ensemble of mesocrystals grown on a substrate, as well as an individual mesocrystal. The individual μm-sized mesocrystal is isolated using a focused-ion-beam-based technique and investigated by the diffraction of a micro-focused X-ray beam. Structural analysis reveals that individual mesocrystals have a drastically smaller size dispersity of nanoparticles than that in the initial dispersion, implying very strong size selectivity during self-assembly. The small size dispersity of the nanoparticles within individual mesocrystals is accompanied by a very narrow lattice parameter distribution. In contrast, the lattice parameter distribution within all mesocrystals of an ensemble is about four times wider than that of individual mesocrystals, indicating significant size fractionalization between mesocrystals during self-assembly. The small size dispersity within each mesocrystal has important implications for their physical properties., Published by Royal Society of Chemistry, Cambridge

Single crystalline magnetite Fe 3 O 4 was investigated at low temperatures in the charge ordered state by electric measurements and time-resolved diffraction with voltage applied in-situ. Dielectric spectroscopy indicates relaxor ferroelectric characteristics, with polarization switching observably only at sufficiently low temperatures and in a suitably chosen time-window. PUND measurements with a ms time scale indicate a switchable polarization of about 0 . 6 &mu, C / cm 2 . Significant switching occurs only above a threshold field of about 3 kV / mm , and it occurs with a time delay of about 20 &mu, s . The time-resolved diffraction experiment yields, for sufficiently high voltage pulses, a systematic variation by about 0 . 1 % of the intensity of the ( 2 , 2 &macr, 10 &macr, ) Bragg reflection, which is attributed to structural switching of domains of the non-centrosymmetric C c structure to its inversion twins, providing proof of intrinsic ferroelectricity in charge ordered magnetite.

We report the growth of YFe2O4-�� single crystals by the optical floating zone method, showing for the first time the same magnetization as highly stoichiometric (��= 0.00) powder samples and sharp superstructure reflections in single crystal x-ray diffraction. The latter can be attributed to three dimensional long-range charge ordering. Resonant x-ray diffraction at the Fe K-edge with full linear polarization analysis was used for the investigation of the possibility of orbital order., 6 pages, 8 figures

The pressure-temperature-electric field phase diagram of Fe2OBO3 is studied under ambient and high pressure using resistivity and thermoelectric power measurements. The onset of the incommensurate charge order at T-CO=340 K does not depend on pressure up to at least 2 GPa. The temperature of the transition to the commensurate charge order is increased by similar to 10 K/GPa. High pressure stabilizes the commensurate phase. We find evidence for resistive switching in the incommensurate phase, which may be linked to the dynamics of the charge-order domain boundaries.

Dissertation, RWTH Aachen University, 2021; Aachen : RWTH Aachen University 1 Online-Ressource : Illustrationen (2021). = Dissertation, RWTH Aachen University, 2021, This thesis is dedicated to the elucidation of the charge order (CO) realized in the intercalated rare earth ferrites RFe2O4(RFeO3)n. Rare earth ferrites have attracted a lot of attention as proposed multiferroics. In particular, LuFe2O4 was considered a clear example of ferroelectricity from CO of Fe2+ and Fe3+ in the Fe-O bilayers, though recently this was contradicted. YbFe2O4 was found by structural refinement and bond-valance-sum (BVS) analysis to contain polar bilayers, though with anti-polar stacking. In order to modify the CO, one can focus on the interactions between different bilayers. These can be tuned by inserting single Fe-O layers, increasing the distance between the bilayers. The CO within individual bilayers of intercalated rare earth ferrites is expected to be very similar as in YbFe2O4, with the intercalation possibly rendering the anti-polar stacking to a polar. Deducing the CO pattern realized in the intercalated compounds requires the growth of single crystals. These should be of high quality with the proper oxygen stoichiometry, which is critical for the establishment of 3D CO as already noted for not intercalated rare earth ferrites. However, the more complex crystal structure makes the synthesis of high quality single crystals more difficult. With the controlled growth using mixed gas flow of CO2:CO, single crystals of intercalated layered R1+nFe2+nO4+3n−δ (n=1,2) with different oxygen stoichiometries δ are fabricated. For the first time crystals sufficiently stoichiometric to exhibit superstructure reflections in X-ray diffraction attributable to charge ordering were obtained. The estimated correlation lengths tend to be smaller in Lu2Fe3O7 and larger in Lu3Fe4O10 compared to the not intercalated LuFe2O4. For both compounds, two different superstructures were observed in different crystals, one an incommensurate zigzag pattern, the other an apparently commensurate pattern with (1/3 1/3 0)-propagation. This propagation vector is similar to what was found in LuFe2O4 and in YbFe2O4, however, without doubling of the cell in c-direction. The magnetic properties of both compounds were studied using macroscopic magnetization measurements and X-ray magnetic circular dichroism (XMCD). Additionally, polarized neutron scattering and ac-susceptibility measurements were performed on Lu2Fe3O7. Macroscopic magnetization measurements performed on the most stoichiometric Lu2Fe3O7 crystal show no sharp features attributable to phase transitions, suggesting reduced magnetic correlations with the presence of the ferrimagnetic state and the absence of an antiferromagnetic state like those observed in LuFe2O4 and YbFe2O4. In contrast to this, similar measurements performed onLu3Fe4O10 suggested a first order meta-magnetic transition between a high-field ferrimagnetic and a low-field antiferromagnetic state similar to LuFe2O4. Polarized neutron scattering on Lu2Fe3O7 reveals diffuse magnetic scattering along (1/31/3 ℓ) and frequency-dependence is observed in the ac-susceptibility, consistent with glassy freezing rather than long-range spin order (SO). The appearance of 3D CO but not 3D SO in a Lu2Fe3O7 crystal indicates that the SO is more fragile with respect to oxygen off-stoichiometry. All magnetic properties are consistent with the SO within individual bilayers of Lu2Fe3O7 essentially being the same as in LuFe2O4, though with a less well-established order. This is indicated in particular by XMCD measurements, which support the same SO in the individual bilayer of Lu3Fe4O10 as well. Moreover, XMCD confirms also the same CO in the individual bilayers. In the additional Fe-O single layers, a magnetic mont seemems induced by the application of a magnetic field, i.e. the single layers act as a paramagnetic-like additional contribution. The availability of stoichiometric crystals with commensurate CO facilities the refinement of CO. For Lu2Fe3O7, symmetry analysis based on the (1/3 1/3 0)-propagation vector led to the same likely CO configurations as discussed for LuFe2O4: either charged bilayers or polar bilayers stacked with the same or alternating polarizations. For Lu3Fe4O10, only polar bilayers stacked with the same polarizations can be realized. The refinement of the average structure disregarding the modulation for the data collected from a commensurate CO Lu2Fe3O7 crystal by single crystal synchrotron X-ray diffraction at 100 K, manifests symptoms of the CO represented in the splitting of some of the Lu-atom positions. This splitting seems to result from the positional modulation of the Lu that accompanies the CO as observed in LuFe2O4. Surprisingly, the refinement of the superstructure in all symmetries still exhibits the Lu-splitting. The best results are achieved for the polar Cmc21 structure. However, the performed BVS analysis reveals an intermediate valance on 8 Fe-sites indicating an incomplete CO in the bilayer. These sites correlate with the Lu-position splitting. This is very likely due toa superposition of CO configurations of different symmetries. The formation of these structure polytypes can be understood as resulting from the inter-bilayer interactions being much weaker due to the intercalation, which makes CO with different stacking closer in energy. In conclusion, the results obtained within this study indicate strongly that both compounds are polar, validating the intercalation approach to producing ferroelectricity from CO in the rare earth ferrites., Published by RWTH Aachen University, Aachen

Dissertation, RWTH Aachen University, 2020; Aachen : RWTH Aachen University 1 Online-Ressource (180 Seiten) : Illustrationen, Diagramme (2020). = Dissertation, RWTH Aachen University, 2020, The main focus of this thesis was dedicated to uncovering the true intrinsic magnetic and charge order properties of YbFe2O4, with a large emphasis on the crystal structure determination at 200 and 90 K. Previous research on its isostructural neighbor LuFe2O4, uncovered what was thought to initially be; ferroelectricity through valence ordering". The high temperature RFe2O4 structure (space group R-3m) can be described as Fe2+ and Fe3+ valence mixed bilayers, separated by R3+-O mono-layers. The concept of ferroelectricity through valence ordering was later suspended after the refinement on a highly stoichiometric LuFe2O4 single crystal, which resulted ina lower symmetry C2/m space group, and charged rather than polar bilayers. This sparked new interest in this series of compounds and provided the inspiration for this thesis; to determine if highly stoichiometric YbFe2O4 exhibits ferroelectricity through charge ordering. The charge order investigated in this thesis exhibits the same incommensurate phase at room temperature, but in contrast to LuFe2O4, a second phase transition to a commensurate charge order was also observed. This commensurate charge order was refined in the P-1 space group using single crystal x-ray diffraction data measured at 200 and 90 K. This space group is representative of all temperatures until above the 3D charge order phase, at which point the charge order becomes diffuse and the structure can be refined in the R-3m space group. For the first time, the magnetic phase diagram for highly stoichiometric YbFe2O4 has been established, and shows a ferrimagnetic state even in zero field, in the temperature range 240-260 K, which was not observed in LuFe2O4. From the analysis of neutron diffraction data measured on YbFe2O4, the same ferri (fM) and antiferromagnetic (AFM) spin structures intrinsic to LuFe2O4 are present in YbFe2O4, and deduced from almost identical intensity ratios of reflections measured along ( 1/3 , 1/3 , ℓ), for the three magnetic domain populations. The two magnetic phases are newly described in the proper magnetic space groups; P-1 (fM phase) and P-1’ (AFM phase) in contrast to the previous refinement of LuFe2O4, where the observed reflections could not be refined in a proper monoclinic magnetic space group. The final charge order structure, determined through bond valence sum analysis, provides polar bilayers with an anti-polar stacking in the P-1 space group. This charge order is characterized by three modes; Y1, Y2 and T1+, and results from a combination of 2 irreducible representations, namely a Y1 and Y2 propagation vector. This was not considered before, as it is very unlikely for a first order charge order phase transition to be described by 2 irreducible representations. It was the successful refinement in the P-1 space group, which led to the understanding that a combination of 2 irreducible representations is required, as it is this that reduces the space group symmetry. The final correct spin-charge structure was determined through x-ray magnetic circular dichroism measurements, which yielded the same spin arrangement as that observed for LuFe2O4, except it is described in the lower symmetry P-1 space group, in contrast to the original refinement in the C2/m symmetry., Published by RWTH Aachen University, Aachen

Dissertation, RWTH Aachen University, 2018; Aachen 1 Online-Ressource (VI, 140 Seiten) : Illustrationen (2018). = Dissertation, RWTH Aachen University, 2018, The main part of this thesis is dedicated to the determination of the charge order crystallographic structures of YFe2O4−δ at 160 K and 200 K. YFe2O4−δ is above the charge ordering transition isostructural to LuFe2O4−δ with the space group R-3m. The structure is characterized by an alternating stacking of triangular Yttrium layers and Fe bilayers each triangular, with a mean iron valence of 2.5. The arrangement of the Fe2+ and Fe3+ ions on the triangular lattice leads to frustration which is resolved at lower temperates through charge order. LuFe2O4−δ was long time the primary example for a charge order driven ferroelectric material but was found to be not ferroelectric. The isostructural YbFe2O4−δ is indicated to show ferroelectricity by one recent report, although much weaker than predicted for LuFe2O4−δ . In the series of the RFe2O4−δ ferrites YFe2O4−δ is of special interest because it is the upper end member in regard of the ionic radius of the rare earth ion, which determines the intra and inter Fe bilayer spacing and therefore interaction strengths and ultimately the charge order. The charge order of YFe2O4−δ at 160K is described by a propagation vector of (1/4 1/4 3/4) with a symmetry reduction from R-3m to P-1. The charge order is found to be bimodal in contrast to previous results. The charge order at 200 K is incomplete with a seven times enlarged supercell based on a propagation vector of (2/7 2/7 3/7) again with spacegroup P-1 in contrast to a previously suggested monoclinic cell. Based on this charge ordered cell possible magnetic structures are evaluated using single crystal neutron diffraction. In contrast to the observation of well separated phases in X-ray diffraction, in neutron diffraction at 200 K and 160 K a superposition of the two CO-phases is observed, although the phases observed in X-ray diffraction are dominant. This is most likely due to the larger sample volume and different cooling times. The magnetic cell based on the 200 K charge-order structure is characterized by a propagation vector of (5/14 5/14 18/14)_hex . The magnetic cell based on the 160 K charge order cell is based on a propagation vector of (0 1/2 1/2) CO. Both cells have the magnetic spacegroup Ps-1. The magnetic phases show no metamagnetic transitions up to 24 T in contrast to LuFe2O4−δ. The smaller part of the thesis is dedicated to another mechanism of multiferroicity, a vortex like arrangement of spins having a toroidal moment breaks both space and time inversion and is intrinsically magnetoelectric. Such an toroidal moment can be induced in specific materials by the application of crossed electric and magnetic fields. Ni0.42Mn0.58TiO3 is of particular interest because it is so far the only material in which a toroidal moment can be induced in absence of long range magnetic order, since it is an XY-spin glass. Single crystals of Ni0.42Mn0.58TiO3 were grown by the optical floating zone method and the the Ni/Mn-ratio was confirmed by powder and single crystal X-ray diffraction. The spin glass behavior is proved by a magnetic memory test and the observation of a frequency shift in the AC-susceptibility. A magnetoelectric effect as reported in the literature could not be reproduced after magnetoelectric field cooling. Neutron diffraction shows diffuse magnetic scattering along (0 0 l) with enhanced intensity at the positions expected for NiTiO3 and MnTiO3. Polarization analysis confirms the spin direction in the c_hex-plane but an evaluation of the toroidal state could not be performed due to experimental problems., Published by Aachen

Dissertation, RWTH Aachen, 2016; Jülich : Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag 150 p. (2016). = Dissertation, RWTH Aachen, 2016, In this thesis I will describe investigations of two ferrite systems:(1) A detailed ferroelectric study of magnetite (Fe3O4)Multiferroics, consisting of both ferroelectric and ferromagnetic phases, have attractedscientic and technological interest due possible magnetoelectric coupling between thephases. Such materials are very rare though, as conventional ferroelectricity requires anempty d-shell, preventing the presence of magnetism. Among unconventional mechanismsleading to ferroelectricity, multiferroicity due to charge ordering (CO) is a strongcandidate for practical applications. However, proven examples are very rare as of yet.The 120 K Verwey transition (TV ) in magnetite, reported in 1939, is the classical examplefor charge ordering. Despite controversies regarding the existence of CO, magnetite hasbeen proposed as one of the CO-based multiferroics. Although early experiments alreadyindicated for example a magnetoelectric eect, those studies were mainly focused oncomplex low temperature structure rather than possible multiferroicity.In order to study the ferroelectric properties of magnetite by dielectric spectroscopy, anew dielectric measurement set-up was built at the institute. After an introduction andthe description of experimental techniques, this thesis begins with the presentation ofour newly built dielectric set-up and of the performed test experiments to standardizemeasurements of the dielectric constant.The Verwey transition is very sensitive to oxygen stoichiometry. The oxygen stoichiometrywas tuned by appropriate gas mixtures of CO2 and CO or Ar(H2)4%. I rst investigatedappropriate ratios of CO2/Ar(H2)4% at high temperature on polycrystalline samples andconrmed the phase purity by x-ray diraction. Verwey transition was characterizedprimarily by thermo-remanent magnetization and specic heat. The results obtainedfrom the basic macroscopic analysis were used for the growth of high quality crystals byoptical oating zone method. Proposed low temperature relaxor ferroelectric property ofmagnetite was studied by neutron and high energy X-ray diuse scattering experiments.The observed weak diuse scattering by neutron diraction, which was absent in highenergy X-ray studies, indicated that it is magnetic in origin.For the rst time, a time resolved X-ray diraction technique has been implemented totest the switchabilty of the polar structure by application of an electric eld in magnetite.The observed change in the intensity of the Bragg reection to its Friedel mate (reectionrelated by inversion symmetry) constitutes to the rst microscopic proof of ferroicbehavior of classical magnetite.(2) Study of various physical properties of oxygen decient strontiumferrite (SrFeO3􀀀).Colossal magnetoresistance eect, i.e., the huge change in the electrical resistance bythe application of magnetic eld is a key to the next generation of magnetic memorydevices. The oxygen decient strontium ferrite (SrFeO3􀀀, =0{0.5) system exhibitsvarious types of magnetoresistance eect depending on the presence of dierent magneticphases. Oxygen decient SrFeO3􀀀 crystals with = 0.27 and = 0.35 (as determinedby infrared absorption) were grown by optical oating zone method using dierentgrowth conditions. This oxide system contains a mixture of Fe ions in tetravalentand trivalent states. Anomalies around 70 K, 230 K and 130 K observed bymagnetization measurements indicated the presence of a tetragonal, orthorhombic andcubic phase respectively. Presence of these phases were conrmed by further microscopicmeasurements by neuron scattering. Two new magnetic phases at the propagationvector k = (0:25 0:25 0:25)c and (0:25 0 0:15)c were observed by our detailed neutrondiraction experiments with polarization analysis. Results of xyz- polarization indicatedthat majority of the spins lies in the ab-plane. For the rst time CO superstructurereection was observed at (2 2 32 )t position, which indicates the doubling of the c-axis.The observation of diuse scattering around the magnetic Bragg reection indicated thepresence of short range spin correlations in the system. Observed frequency dependentac-susceptibility and the presence of memory eect from magnetization indicated thepresence of glassy state below 60 K in the system., Published by Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag, Jülich