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See detailMagnetic structure factor of correlated moments in small-angle neutron scattering
Honecker, Dirk UL; Fernández Barquín, Luis; Bender, Philipp Florian UL

in Physical Review. B, Condensed Matter and Materials Physics (2020), 101(13), 134401

The interplay between structural and magnetic properties of nanostructured magnetic materials allows one to realize unconventional magnetic effects, which results in a demand for experimental techniques ... [more ▼]

The interplay between structural and magnetic properties of nanostructured magnetic materials allows one to realize unconventional magnetic effects, which results in a demand for experimental techniques to determine the magnetization profile with nanoscale resolution. Magnetic small-angle neutron scattering (SANS) probes both the chemical and magnetic nanostructure and is thus a powerful technique, e.g., for the characterization of magnetic nanoparticles. Here, we show that the conventionally used particle-matrix approach to describe SANS of magnetic particle assemblies, however, leads to a flawed interpretation. As a remedy, we provide general expressions for the field-dependent two-dimensional magnetic SANS cross section of correlated moments. It is shown that for structurally disordered ensembles the magnetic structure factor is in general, and contrary to common assumptions, (i) anisotropic also in zero field and (ii) that even in saturation the magnetic structure factor deviates from the nuclear one. These theoretical predictions explain qualitatively the intriguing experimental, polarized SANS data of an ensemble of dipolar-coupled iron oxide nanoparticles. [less ▲]

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See detailMagnetic Guinier law
Michels, Andreas UL; Malyeyev, Artem UL; Titov, Ivan UL et al

in IUCrJ (2020), 7

Small-angle scattering of x-rays and neutrons is a routine method for the determination of nanoparticle sizes. The so-called Guinier law represents the low-q approximation for the small-angle scattering ... [more ▼]

Small-angle scattering of x-rays and neutrons is a routine method for the determination of nanoparticle sizes. The so-called Guinier law represents the low-q approximation for the small-angle scattering curve from an assembly of particles. The Guinier law has originally been derived for nonmagnetic particle-matrix-type systems, and it is successfully employed for the estimation of particle sizes in various scientific domains (e.g., soft matter physics, biology, colloidal chemistry, materials science). An important prerequisite for it to apply is the presence of a discontinuous interface separating particles and matrix. Here, we introduce the Guinier law for the case of magnetic small-angle neutron scattering (SANS) and experimentally demonstrate its applicability for the example of nanocrystalline cobalt. It is well- known that the magnetic microstructure of nanocrystalline ferromagnets is highly nonuniform on the nanometer length scale and characterized by a spectrum of continuously varying long-wavelength magnetization fluctuations, i.e., these systems do not manifest sharp interfaces in their magnetization profile. The magnetic Guinier radius depends on the applied magnetic field, on the magnetic interactions (exchange, magnetostatics), and on the magnetic anisotropy-field radius, which characterizes the size over which the magnetic anisotropy field is coherently aligned into the same direction. In contrast to the nonmagnetic conventional Guinier law, the magnetic version can be applied to fully dense random-anisotropy-type ferromagnets. [less ▲]

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See detailSupraferromagnetic correlations in clusters of magnetic nanoflowers
Bender, Philipp Florian UL; Honecker, Dirk UL; Fernández Barquín, Luis

in Applied Physics Letters (2019), 115

Magnetic nanoflowers are densely packed aggregates of superferromagnetically coupled iron oxide nanocrystallites, which excel during magnetic hyperthermia experiments. Here, we investigate the nature of ... [more ▼]

Magnetic nanoflowers are densely packed aggregates of superferromagnetically coupled iron oxide nanocrystallites, which excel during magnetic hyperthermia experiments. Here, we investigate the nature of the moment coupling within a powder of such nanoflowers using spin-resolved small-angle neutron scattering. Within the powder, the nanoparticles are agglomerated to clusters, and we can show that the moments of neighboring nanoflowers tend to align parallel to each other. Thus, the whole system resembles a hierarchical magnetic nanostructure consisting of three distinct levels, i.e., (i) the ferrimagnetic nanocrystallites as building blocks, (ii) the superferromagnetic nanoflowers, and (iii) the supraferromagnetic clusters of nanoflowers. We surmise that such a supraferromagnetic coupling explains the enhanced magnetic hyperthermia performance in the case of interacting nanoflowers. [less ▲]

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See detailMagnetic ordering of the martensite phase in Ni-Co-Mn-Sn-based ferromagnetic shape memory alloys
Sarkar, Sudip Kumar; Ahlawat, Sarita; Kaushik, S. D. et al

in Journal of Physics: Condensed Matter (2019)

The magnetic state of low temperature martensite phase in Co-substituted Ni-Mn-Sn-based ferromagnetic shape memory alloys (FSMAs) has been investigated, in view of numerous conflicting reports of ... [more ▼]

The magnetic state of low temperature martensite phase in Co-substituted Ni-Mn-Sn-based ferromagnetic shape memory alloys (FSMAs) has been investigated, in view of numerous conflicting reports of occurrences of spin glass (SG), superparamagnetism (SPM) or long range anti- ferromagnetic (AF) ordering. Combination of dc magnetization, ac susceptibility and small angle neutron scattering (SANS) studies provide a clear evidence for AF order in martensitic phase of Ni45Co5Mn38Sn12 alloy and rule out SPM and SG orders. Identical studies on another alloy of close composition of Ni44Co6Mn40Sn10 point to presence of SG order in martensitic phase and absence of SPM behavior, contrary to earlier report. SANS results do show presence of nanometre-sized clusters but they are found to grow in size from 3 nm at 30 K to 11 nm at 300 K, and do not correlate with magnetism in these alloys. <P /> [less ▲]

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See detailCritical size limits for collinear and spin spiral magnetism in CoCr$_2$O$_4$
Zákutná, Dominika; Alemayehu, Adam; Vlček, Jan et al

in Physical Review. B, Condensed Matter and Materials Physics (2019)

The multiferroic behavior of CoCr$_2$O$_4$ results from the appearance of conical spin-spiral magnetic ordering, which induces electric polarization. The magnetic ground state has a complex size dependent ... [more ▼]

The multiferroic behavior of CoCr$_2$O$_4$ results from the appearance of conical spin-spiral magnetic ordering, which induces electric polarization. The magnetic ground state has a complex size dependent behavior, which collapses when reaching a critical particle size. Here, the magnetic phase stability of CoCr$_2$O$_4$ in the size range of 3.6 - 14.0 nm is presented in detail using the combination of neutron diffraction with XYZ polarization analysis and macroscopic magnetization measurements. We establish critical coherent domain sizes for the formation of the spin spiral and ferrimagnetic structure and reveal the evolution of the incommensurate spin spiral vector with particle size. We further confirm the presence of ferroelectric polarization in the spin spiral phase for nanocrystalline CoCr$_2$O$_4$. <P /> [less ▲]

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See detailCritical size limits for collinear and spin-spiral magnetism in $CoCr_2\mathrmO_4$
Zákutná, D.; Alemayehu, A.; Vlček et al

in Physical Review. B, Condensed Matter and Materials Physics (2019), 100

The multiferroic behavior of CoCr2O4 results from the appearance of conical spin-spiral magnetic ordering, which induces electric polarization. The magnetic ground state has a complex size-dependent ... [more ▼]

The multiferroic behavior of CoCr2O4 results from the appearance of conical spin-spiral magnetic ordering, which induces electric polarization. The magnetic ground state has a complex size-dependent behavior, which collapses when reaching a critical particle size. Here the magnetic phase stability of CoCr2O4 in the size range of 3.6–14.0 nm is presented in detail using the combination of neutron diffraction with XYZ polarization analysis and macroscopic magnetization measurements. We establish critical coherent domain sizes for the formation of the spin spiral and ferrimagnetic structure and reveal the evolution of the incommensurate spin spiral vector with particle size. We further confirm the presence of ferroelectric polarization in the spin spiral phase for nanocrystalline CoCr2O4. [less ▲]

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See detailTransverse and longitudinal spin-fluctuations in INVAR Fe0.65Ni0.35.
Stewart, J. Ross; Giblin, Sean R.; Honecker, Dirk UL et al

in Journal of physics. Condensed matter : an Institute of Physics journal (2019), 31(2), 025802

The presence of spin-fluctuations deep within the ordered state of ferromagnetic [Formula: see text] alloy [Formula: see text] has long been suspected but seldom directly observed. Inhomogeneities of one ... [more ▼]

The presence of spin-fluctuations deep within the ordered state of ferromagnetic [Formula: see text] alloy [Formula: see text] has long been suspected but seldom directly observed. Inhomogeneities of one type or another have been cited as important in stabilizing [Formula: see text] behaviour-either longitudinal spin-fluctuations associated with the [Formula: see text]-state (local environment) model or transverse magnetisation arising from non-collinear spin structures. In this study we employ small-angle neutron scattering with neutron polarization analysis to distinguish between the two possibilities. Surprisingly we in fact find evidence of dominant but uncorrelated longitudinal spin-fluctuations coexisting with transverse magnetisation which exists in short-range clusters of size ~[Formula: see text]. This finding supports recent first principles calculations of [Formula: see text] in which both longitudinal spin-fluctuations and magnetic short-range order are identified as important ingredients in reproducing the equilibrium [Formula: see text] lattice. [less ▲]

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See detailSpin-wave stiffness of the Dzyaloshinskii-Moriya helimagnet compounds Fe1−xCoxSi studied by small-angle neutron scattering
Grigoriev; Pschenichnyi; Altynbaev et al

in Physical Review. B, Condensed Matter (2019)

The spin wave stiffness was measured by small-angle neutron scattering method in the Dzyaloshinskii-Moriya helimagnet compounds Fe1−xCoxSi with x=0.25,0.30,0.50. It has been shown that the spin wave ... [more ▼]

The spin wave stiffness was measured by small-angle neutron scattering method in the Dzyaloshinskii-Moriya helimagnet compounds Fe1−xCoxSi with x=0.25,0.30,0.50. It has been shown that the spin wave dispersion in the fully polarized state is anisotropic due to Dzyaloshinskii-Moriya interaction. It is reflected in the neutron scattering pattern as two circles for neutrons obtaining and losing the magnon energy, respectively. The centers of the circles are shifted by the momentum transfer oriented along the applied magnetic field H and equal to the wave vector of the spiral ±ks. The radius of the circles is directly related to the stiffness of spin waves and depends on the magnetic field. We have found that the spin-wave stiffness A change weakly with temperature for each individual compound. On the other hand, the spin-wave stiffness A increases linearly with x in contrast to the x dependences of the critical temperature Tc and the low-temperature ordered moment. Experimentally obtained values of the stiffness A approve quantitative applicability of the Bak-Jensen model for the compounds under study. [less ▲]

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See detailMagnetic ordering of the martensite phase in Ni-Co-Mn-Sn-based ferromagnetic shape memory alloys
Sarkar, Sudip Kumar; Ahlawat, Sarita; Kaushik, Som Datta et al

in Journal of Physics: Condensed Matter (2019)

The magnetic state of low temperature martensite phase in Co-substituted Ni-Mn-Sn-based ferromagnetic shape memory alloys (FSMAs) has been investigated, in view of numerous conflicting reports of ... [more ▼]

The magnetic state of low temperature martensite phase in Co-substituted Ni-Mn-Sn-based ferromagnetic shape memory alloys (FSMAs) has been investigated, in view of numerous conflicting reports of occurrences of spin glass (SG), superparamagnetism (SPM) or long range anti-ferromagnetic (AF) ordering. Combination of DC magnetization, AC susceptibility and small angle neutron scattering (SANS) studies provide a clear evidence for AF order in martensitic phase of Ni45Co5Mn38Sn12 alloy and rule out SPM and SG orders. Identical studies on another alloy of close composition of Ni44Co6Mn40Sn10 point to presence of SG order in martensitic phase and absence of SPM behavior, contrary to earlier report. SANS results do show presence of nanometre-sized clusters but they are found to grow in size from 3 nm at 30 K to 11 nm at 300 K, and do not correlate with magnetism in these alloys. [less ▲]

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See detailUsing the singular value decomposition to extract 2D correlation functions from scattering patterns
Bender, Philipp Florian UL; Zákutná, Dominika; Disch, Sabrina et al

in Acta Crystallographica. Section A, Foundations and Advances (2019), A75

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See detailPhase-Transfer and Stabilization of Highly Monodisperse Ferrite Nanoparticles into Polar Solvents by Ligand Exchange Synthesis.
Zakutna, Dominika; Honecker, Dirk UL; Disch, Sabrina

in Journal of nanoscience and nanotechnology (2019), 19(8), 5048-5051

Cobalt ferrite nanoparticles with controlled particle size are transferred from nonpolar into polar solvent by exchange of the as-synthesized oleic acid ligand shell with polyacrylic acid (PAA). The ... [more ▼]

Cobalt ferrite nanoparticles with controlled particle size are transferred from nonpolar into polar solvent by exchange of the as-synthesized oleic acid ligand shell with polyacrylic acid (PAA). The nanoparticles are highly monodisperse (sigmalog < 6%), and the ligand exchange synthesis has no measurable effect on particle size, shape, or size distribution. The stability of the aqueous particle dispersion without significant interparticle correlations as observed using small-angle X-ray scattering confirms the successful phase-transfer. [less ▲]

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See detailMultiple-q noncollinear magnetism in an itinerant hexagonal magnet
Takagi, R.; White, J. S.; Hayami, S. et al

in SCIENCE ADVANCES (2018), 4(11),

rticle Figures & Data Info & Metrics eLetters PDF Abstract Multiple-q spin order, i.e., a spin texture characterized by a multiple number of coexisting magnetic modulation vectors q, has recently ... [more ▼]

rticle Figures & Data Info & Metrics eLetters PDF Abstract Multiple-q spin order, i.e., a spin texture characterized by a multiple number of coexisting magnetic modulation vectors q, has recently attracted attention as a source of nontrivial magnetic topology and associated emergent phenomena. One typical example is the triple-q skyrmion lattice state stabilized by Dzyaloshinskii-Moriya interactions in noncentrosymmetric magnets, while the emergence of various multiple-q states of different origins is expected according to the latest theories. Here, we investigated the magnetic structure of the itinerant polar hexagonal magnet Y3Co8Sn4, in which several distinctive mechanisms favoring multiple-q states are allowed to become active. Small-angle neutron-scattering experiments suggest the formation of incommensurate triple-q magnetic order with an in-plane vortex-like spin texture, which can be most consistently explained in terms of the novel four-spin interaction mechanism inherent to itinerant magnets. The present results suggest a new route to realizing exotic multiple-q orders and that itinerant hexagonal magnets, including the R3M8Sn4 family with wide chemical tunability, can be a unique material platform to explore their rich phase diagrams. [less ▲]

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See detailDirect evidence for cycloidal modulations in the thermal-fluctuation-stabilized spin spiral and skyrmion states of GaV 4 S 8
White, J. S.; Butykai, A.; Cubitt, R. et al

in Physical Review B (2018), 97(2), 020401

We report small-angle neutron scattering studies of the lacunar spinelGaV4S8,which reveal the long-wavelength magnetic phases to be cycloidally modulated. Upon cooling, these modulated phases, including a ... [more ▼]

We report small-angle neutron scattering studies of the lacunar spinelGaV4S8,which reveal the long-wavelength magnetic phases to be cycloidally modulated. Upon cooling, these modulated phases, including a recently proposed Néel-type skyrmion phase, transform into a simple ferromagnetic state. These results indicate the modulated phases in GaV4S8 gain their stability from thermal fluctuations, while at lower temperatures the ferromagnetic state emerges in accord with the strong easy-axis magnetic anisotropy. Crucially, our study provides microscopic evidence that the skyrmions in GaV4S8 indeed display a Néel-type helicity. More generally, our approach can be applied to evidence the helicity of any bulk skyrmion phase. [less ▲]

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See detailTransparent-to-dark photo- and electrochromic gels
Gonzalez, Laura; Liu, Chung; Dietrich, Bart et al

in COMMUNICATIONS CHEMISTRY (2018), 1

Smart windows in which the transmittance can be controlled on demand are a promising solution for the reduction of energy use in buildings. Windows are often the most energy inefficient part of a building ... [more ▼]

Smart windows in which the transmittance can be controlled on demand are a promising solution for the reduction of energy use in buildings. Windows are often the most energy inefficient part of a building, and so controlling the transmittance has the potential to significantly improve heating costs. Whilst numerous approaches exist, many suitable materials are costly to manufacture and process and so new materials could have a significant impact. Here we describe a gel-based device which is both photo- and electrochromic. The gel matrix is formed by the self-assembly of a naphthalene diimide. The radical anion of the naphthalene diimide can be formed photo or electrochemically, and leads to a desirable transition from transparent to black. The speed of response, low potential needed to generate the radical anion, cyclability of the system, temperature stability and low cost mean these devices may be suitable for applications in smart windows. [less ▲]

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See detailAligning self-assembled perylene bisimides in a magnetic field.
Draper, Emily R.; Wallace, Matthew; Honecker, Dirk UL et al

in Chemical communications (Cambridge, England) (2018), 54(78), 10977-10980

Photoconductive self-assembled 1D structures can be formed by perylene bisimides. These structures are generally randomly orientated, limiting their applications as conductive wires. Here, we show that ... [more ▼]

Photoconductive self-assembled 1D structures can be formed by perylene bisimides. These structures are generally randomly orientated, limiting their applications as conductive wires. Here, we show that magnetic fields can be used to create highly aligned, directionally-dependent thin films. This approach leads to well-aligned structures over large areas. [less ▲]

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See detailNoncollinear magnetism in nanosized cobalt chromite
Zakutna, D.; Vlcek, J.; Fitl, P. et al

in PHYSICAL REVIEW B (2018), 98(6), 064407

Using a combination of neutron diffraction with XYZ polarization analysis and magnetization measurements, the noncollinear magnetism in nanosized cobalt chromite, a potential multiferroic material, is ... [more ▼]

Using a combination of neutron diffraction with XYZ polarization analysis and magnetization measurements, the noncollinear magnetism in nanosized cobalt chromite, a potential multiferroic material, is revealed. For noninteracting 26.9(1) nm nanoparticles, a bulklike behavior is identified, including a ferrimagnetic Curie temperature of 99 K and a transition to the spin spiral magnetic phase at 27 K with a temperature-dependent, incommensurate propagation vector. A lock-in transition towards a commensurate propagation vector is not observed. Much smaller, 3.1(2) nm, nanoparticles reveal a strong cluster glass behavior, characterized by ferrimagnetic behavior below the Curie temperature of 43 K and a transition to asperomagnetic behavior at 18 K, with the absence of any magnetic reflections at a base temperature of 5 K. [less ▲]

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See detailRelating Magnetic Properties and High Hyperthermia Performance of Iron Oxide Nanoflowers
Bender, Philipp Florian UL; Fock, Jeppe; Frandsen, Cathrine et al

in JOURNAL OF PHYSICAL CHEMISTRY C (2018), 122(5), 3068-3077

We investigated, in depth, the interrelations among structure, magnetic properties, relaxation dynamics and magnetic hyperthermia performance of magnetic nanoflowers. The nanoflowers are about 39 nm in ... [more ▼]

We investigated, in depth, the interrelations among structure, magnetic properties, relaxation dynamics and magnetic hyperthermia performance of magnetic nanoflowers. The nanoflowers are about 39 nm in size, and consist of densely packed iron oxide cores. They display a remanent magnetization, which we explain by the exchange coupling between the cores, but we observe indications for internal spin disorder. By polarized small-angle neutron scattering, we unambiguously confirm that, on average, the nanoflowers are preferentially magnetized along one direction. The extracted discrete relaxation time distribution of the colloidally dispersed particles indicates the presence of three distinct relaxation contributions. We can explain the two slower processes by Brownian and classical Néel relaxation, respectively. The additionally observed very fast relaxation contributions are attributed by us to the relaxation of disordered spins within the nanoflowers. Finally, we show that the intrinsic loss power (ILP, magnetic hyperthermia performance) of the nanoflowers measured in colloidal dispersion at high frequency is comparatively large and independent of the viscosity of the surrounding medium. This concurs with our assumption that the observed relaxation in the high frequency range is primarily a result of internal spin relaxation, and possibly connected to the disordered spins within the individual nanoflowers. [less ▲]

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See detailDirect evidence for cycloidal modulations in the thermal-fluctuation-stabilized spin spiral and skyrmion states of GaV4S8
White, J. S.; Butykai, A.; Cubitt, R. et al

in PHYSICAL REVIEW B (2018), 97(2),

We report small-angle neutron scattering studies of the lacunar spinelGaV4S8,which reveal the long-wavelength magnetic phases to be cycloidally modulated. Upon cooling, these modulated phases, including a ... [more ▼]

We report small-angle neutron scattering studies of the lacunar spinelGaV4S8,which reveal the long-wavelength magnetic phases to be cycloidally modulated. Upon cooling, these modulated phases, including a recently proposed Néel-type skyrmion phase, transform into a simple ferromagnetic state. These results indicate the modulated phases in GaV4S8 gain their stability from thermal fluctuations, while at lower temperatures the ferromagnetic state emerges in accord with the strong easy-axis magnetic anisotropy. Crucially, our study provides microscopic evidence that the skyrmions in GaV4S8 indeed display a Néel-type helicity. More generally, our approach can be applied to evidence the helicity of any bulk skyrmion phase. [less ▲]

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