2018 Publications

Quantitative magneto-optical investigation of superconductor/ferromagnet hybrid structures
Shaw, G., Brisbois, J., Pinheiro, L. B. G. L., Müller, J., Blanco Alvarez, S., Devillers, T., Dempsey, N. M., Scheerder, J. E.,
Van de Vondel, J., Melinte, S., Vanderbemden, P., Motta, M., Ortiz, W. A., Hasselbach, K., Kramer, R. B. G., & Silhanek, A. (2018)
Review of Scientific Instruments, 89, 23705.

We present a detailed quantitative magneto-optical imaging study of several superconductor/ferromagnet hybrid structures, including Nb deposited on top of thermo-magnetically patterned NdFeB, and permalloy/niobium with erasable and tailored magnetic landscapes imprinted in the permalloy layer. The magneto-optical imaging data is complemented with and compared to scanning Hall probe microscopy measurements. Comprehensive protocols have been developed for calibrating, testing, and converting Faraday rotation data to magnetic field maps. Applied to the acquired data, they reveal the comparatively weaker magnetic response of the superconductor from the background of larger fi elds and field gradients generated by the magnetic layer.

Bulk superconducting tube subjected to the stray magnetic field of a solenoid
Hogan, K., Fagnard, J.-F., Wera, L., Vanderheyden, B., & Vanderbemden, P. (2018)
Superconductor Science and Technology, 31(1) 015001

Hard type-II hollow superconductors are well suited for low frequency magnetic shielding. The properties and performances of superconducting magnetic shields subjected to homogeneous magnetic fields have been extensively discussed in the literature. In the present work, we investigate the magnetic shielding and the penetration of magnetic flux in a bulk high temperature superconducting tube subjected to the inhomogeneous fringe field of a solenoidal coil. Thanks to a bespoke microdisplacement measurement system, we measure the magnetic field distribution around the tube. We develop a full 3D finite element model based on an H formulation to understand the flux penetration mechanisms and predict the shape of the current loops. Using constitutive law parameters obtained from previous independent experiments, our model is found to be in excellent agreement with the measurements. We discuss how to assess the degree of inhomogeneity of the magnetic field and show that, in our case study, the field can be treated as the magnetic field of an equivalent magnetic dipole. We also show that some features of the flux penetration in inhomogeneous field can be also observed when the tube is subjected to an oblique homogeneous magnetic field, which offers a better understanding of the shielding current density distribution inside the shield. Finally, we discuss the magnetic field concentration occurring around the shield for different magnetic field configurations. In particular, we show that the extremities of the tube on the side not facing the magnetic field source experience the highest flux concentration.

Red mud as aluminium source for the synthesis of magnetic zeolite
C. Belviso, A. Kharchenko, E. Agostinelli, F. Cavalcante, D. Peddis, G. Varvaro, N.Yaacoub, S. Mintova (2018)
Microporous & Mesoporous Materials 270, 24–29

Zeolite synthesis typically requires batch systems (precursor mixture) in which aluminate and silicate solutions are first mixed and then subjected to hydrothermal treatment. In this study, FAU and GIS types zeolite with wool ball-like morphology were synthesized using colloidal silica and red mud as an alternative aluminium source. The addition of aluminium powder to the precursor mixture composed by colloidal silica and red mud resulted in the crystallization of pure GIS-type zeolite with a cactus-like morphology. The crystalline products (GIS and FAU zeolites) were thoroughly characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and nitrogen adsorption. In addition, the magnetic properties of the zeolites were studied since the red mud used as aluminium source is mainly containing a mixture of magnetic iron-based oxides. The magnetic zeolites will be further considered for wastewater treatment, as they can be easily separated using an external magnetic field.

Surface Effects in Ultrathin Iron Oxide Hollow Nanoparticles: Exploring Magnetic Disorder at the Nanoscale
F. Sayed, N. Yaacoub, Y. Labaye, R. Sayed Hassan, G. Singh, P.A. Kumar, J.M. Greneche, R. Mathieu, G. Hadjipanayis, E. Agostinelli, D. Peddis (2018)
The Journal of Physical Chemistry C, 122(13), 7516-7524

A detailed study of the structural and magnetic properties of polycrystalline hollow γ-Fe2O3 nanoparticles of ∼9.4 nm size was performed. High-resolution transmission electron microscopy images confirmed the crystalline structure and the presence of a ultrathin shell thickness of ∼1.4 nm, implying a very high surface/volume ratio. These hollow nanoparticles were investigated using zero-field and in-field 57Fe Mössbauer spectrometry. The zero-field hyperfine structure suggests some topological disorder, whereas the in-field one shows the presence of a comp magnetic structure that can be fairly described as two opposite pseudosperomagnetic sublattices attributed to octahedral and tetrahedral iron sites. Such an unusual feature is consistent with the presence of noncollinear spin structure originated from the increased surface due to the hollow morphology. Such a complex local spin structure evidenced from Mössbauer experiments was correlated with exchange bias coupling showing at low temperature by magnetization measurements. Monte Carlo simulations on a ferrimagnetic hollow nanoparticle unambiguously corroborate the critical role of the surface anisotropy on the noncollinearity of spin structure in our samples.

X-ray magnetic circular dichroism discloses surface spins correlation in maghemite hollow nanoparticles
V. Bonanni, M. Basini, D. Peddis, A. Lascialfari, G. Rossi, G. Panaccione, P. Torelli (2018)
Applied Physics Letters, 112(2), 022404

The spin-spin correlations in hollow (H) and full (F) maghemite nanoparticles (NPs) have been studied by X-ray magnetic circular dichroism (XMCD). An unexpected XMCD signal was detected and analyzed under the application of a small field (μ0H = 160 Oe) and at remanence for both F and H NPs. Clear differences in the magnitude and in the lineshape of the XMCD spectra between F and H NPs emerged. By comparing XMCD measurements performed with a variable degree of surface sensitivity, we were able to address the specific role played by the surface spins in the magnetism of the NPs.

Impact of magnetization and hyperfine field distribution on high magnetoelectric coupling strength in BaTiO3-BiFeO3 multilayers
J.K. Jochum, M. Lorenz, H.P. Gunnlaugsson, C. Patzig, T. Hoeche, M. Grundmann, A. Vantomme, K. Temst, M.J. Van Bael, V. Lazenka (2018)
Nanoscale, 10(12), 5574-5580

Correlations were established between the hyperfine field distribution around the Fe atoms, the multiferroic properties, and the high magnetoelectric coefficient in BaTiO3–BiFeO3 multilayer stacks with variable BiFeO3 single layer thickness, down to 5 nm. Of key importance in this study was the deposition of 57Fe – enriched BiFeO3, which enhances the sensitivity of conversion electron Mössbauer spectroscopy by orders of magnitude. The magnetoelectric coefficient αME reaches a maximum of 60.2 V cm−1 Oe−1 at 300 K and at a DC bias field of 2 Tesla for a sample of 15 × (10 nm BaTiO3–5 nm BiFeO3) and is one of the highest values reported so far. Interestingly, the highest αME is connected to a high asymmetry of the hyperfine field distribution of the multilayer composite samples. The possible mechanisms responsible for the strong magnetoelectric coupling are discussed.

Intrinsic magnetic properties of hydrided and non-hydrided Nd5Fe17 single crystals
D. Yu. Karpenkov, K. P. Skokov, M. B. Lyakhova, I. A. Radulov, T. Faske, Y. Skourski, O. Gutfleisch (2018)
Journal of Alloys and Compounds, 741, 1012-1020

We report on the spontaneous magnetization Ms, the exchange stiffness constant A and the magnetocrystalline anisotropy constants K1, K2, K3 and K4 of Nd5Fe17 and Nd5Fe17H16 single crystals. Field dependencies of magnetization M(H) were measured along a, b’ and c principal crystallographic directions within the temperature range of 10–600 K and magnetic fields up to 40 T. Large anisotropies of spontaneous magnetization and high-field susceptibility were revealed for both compounds. The exchange stiffness parameter A was determined using Bloch’s T3/2 law. In order to provide high accuracy detection of K1(T), K2(T), K3(T) and K4(T), we used two different approaches: the modified Sucksmith- Thompson technique and the Néel’s phase method.

Anisotropic local hardening in hot-deformed Nd-Fe-B permanent magnets
S. Sawatzki, T. Schneider, M. Yi, E. Bruder, S. Ener, M. Schönfeldt, K. Güth, B.-X. Xu, and O. Gutfleisch (2018)
Acta Materialia, 147, 176-183

The diffusion of low-melting Nd-Cu alloys is very effective to increase coercivity in hot-deformed Nd-Fe-B permanent magnets without the use of heavy rare earth and to study the local hardening mechanism, especially the role of the Nd-rich grain boundary on the magnetic decoupling of the Nd-Fe-B grains on the nanoscale. In this study, we found that for a Nd-Cu diffusion parallel to the texture axis the increase in is higher than for a diffusion perpendicular to it and strongly depends on the diffusion depth whereas remanence develops in an inverse manner. We note the following three observations to explain This behavior results from: a) a higher overall Nd and Cu concentration for the parallel diffusion revealed by global energy dispersive X-ray (EDX) maps leading to a distinct change in the broadness of the interaction domains visualized by Kerr microscopy, b) a higher degree of misalignment of the Nd2Fe14B grains observed by electron backscattered diffraction (EBSD), and c) a more effective local hardening on the macroscopic scale governed by dipolar and exchange interactions as modeled by micromagnetic simulations. The misalignment and the incorporation of Nd and Cu also lead to a volume expansion of the magnet of around 0.6–0.8% as proven by in-situ thermo-optical measurements (TOM).

In-situ magnetic force microscopy analysis of magnetization and demagnetization behavior in Al3+ substituted Sr-hexaferrite
F. Rhein, T. Helbig, V. Neu, M. Krispin, and O. Gutfleisch (2018)
Acta Materialia, 146, 85-96

The sintering temperature of an Al3+ substituted Sr-hexaferrite composite was systematically varied from 1180 °C to 1280 °C resulting in different microstructures. The grain size was found to range from a few hundred nanometers to several hundred micrometers depending on Al content and sintering temperature. Adding an Al substituted powder to a commercial powder increased the coercivity from 360 mT to 470 mT, at the same time, decreasing remanence from 350 mT to 305 mT. Magnetization and demagnetization processes from the thermally demagnetized state (TDS) and DC-demagnetized state (DCD) have been investigated systematically by in-situ magnetic force microscopy (MFM) under magnetic field. From the surface domain contrast a polarization was derived which quantitatively matches the global i.e. bulk polarization obtained by superconducting quantum interface device (SQUID) magnetometry. The shape of the initial polarization curve and the polarization from the DCD state were correlated with the in-situ MFM data revealing a distinctly different magnetization behavior depending on grain size. The presented results enable a better understanding of local nucleation mechanisms, global influences of pinning centers and further opportunities to improve rare earth (RE) free permanent magnets based on ferrites.

Evolution of anisotropy in bcc Fe distorted by interstitial boron
D. Gölden, H. Zhang, I. Radulov, I. Dirba, P. Komissinskiy, E. Hildebrandt, and L. Alff (2018)
Phys. Rev. B 97, 0144116

The evolution of magnetic anisotropy in bcc Fe as a function of interstitial boron atoms was investigated in thin films grown by molecular beam epitaxy. The thermodynamic nonequilibrium conditions during film growth allowed one to stabilize an interstitial boron content of about 14 at.% accompanied by lattice tetragonalization. The c/a ratio scaled linearly with the boron content up to a maximum value of 1.05 at 300∘C substrate growth temperature, with a room-temperature magnetization of. In contrast to nitrogen interstitials, the magnetic easy axis remained in-plane with an anisotropy of approximately −5.1×106 erg/cm3. Density functional theory calculations using the measured lattice parameters confirm this value and show that boron local ordering indeed favors in-plane magnetization. Given the increased temperature stability of boron interstitials as compared to nitrogen interstitials, this study will help to find possible ways to manipulate boron interstitials into a more favorable local order.