Strontium hexaferrite platelets: a comprehensive soft X-ray absorption and Mössbauer spectroscopy study

IBERMÖSS-2019, Bilbao, 30-31 may 2019. --https://www.ehu.eus/es/web/ibermossmeetingStrontium ferrite (SFO, SrFe12O19) is a ferrite employed for permanent magnets due to its high magnetocrystalline anisotropy. Since its discovery in the mid-20th century, this hexagonal ferrite has become an increasingly important material both commercially and technologically, finding a variety of uses and applications. Its structure can be considered a sequence of alternating spinel (S) and rocksalt (R) blocks. All the iron cations are in the Fe3+ oxidation state and it has a ferrimagnetic configuration with five different cationic environments for the iron (three octahedral sites, a tetraedrical site and a bipiramidal site)[1,2]. We have studied the properties of SrFe 12O19 in the shape of platelets, up to several micrometers in width, and tens of nanometers thick, synthesized by a hydrothermal method. We have characterized the structural and magnetic properties of these platelets by Mössbauer spectroscopy, x-ray transmission microscopy (TMX), transmission electron microscopy (TEM), x-ray diffraction (XRD), vibrating-sample magnetometry (VSM), x-ray absorption spectroscopy (XAS), x-ray circular magnetic dichroism (XMCD) and photoemission electron microscopy (PEEM). To the best of our knowledge this is the first time that the x-ray absorption spectra at the Fe L 2,3 edges of this material in its pure form have been reported. The Mössbauer results recorded from these platelets both in the electron detection and transmission modes have helped to understand the iron magnetic moments determined by XMCD (Fig.1). The experimental results have been complemented with multiplet calculations aimed at reproducing the observed XAS and XMCD spectra at the Fe L 2,3 absorption edge, and by density functional theory (DFT) calculations to reproduce the oxygen K- absorption edge. Finally the domain pattern measured in remanence is in good agreement with micromagnetic simulations [3]

Strontium hexaferrite platelets: a comprehensive soft X-ray absorption and Mössbauer spectroscopy study

Platelets of strontium hexaferrite (SrFe12O19, SFO), up to several micrometers in width, and tens of nanometers thick have been synthesized by a hydrothermal method. They have been studied by a combination of structural and magnetic techniques, with emphasis on Mössbauer spectroscopy and X-ray absorption based-measurements including spectroscopy and microscopy on the iron-L edges and the oxygen-K edge, allowing us to establish the differences and similarities between our synthesized nanostructures and commercial powders. The Mössbauer spectra reveal a greater contribution of iron tetrahedral sites in platelets in comparison to pure bulk material. For reference, high-resolution absorption and dichroic spectra have also been measured both from the platelets and from pure bulk material. The O-K edge has been reproduced by density functional theory calculations. Out-of-plane domains were observed with 180° domain walls less than 20 nm width, in good agreement with micromagnetic simulationsThis work is supported by the Spanish Ministry of Economy and Competitiveness through Projects MAT2015-64110-C2-1-P, MAT2015-64110-C2-2-P, MAT2015-66888-C3-1-R and by the European Commission through Project H2020 No. 720853 (Amphibian). These experiments were performed at the CIRCE, MISTRAL and BOREAS beamlines of the ALBA Synchrotron Light Facility. G.D.S. acknowledges the European Youth Employement Initiative and the Autonomous Community of Madrid for a one-year fellowship. Slovenian Research Agency is acknowledged for funding the research program Ceramics and complementary materials for advanced engineering and biomedical applications (P2-0087), CEMM, JSI for the use of TE

Development of irradiation tolerant tungsten alloys for high temperature nuclear applications.

Development of refractory metals for application as plasma-facing armour material remains among priorities of fusion research programmes in Europe, China and Japan. Improving the resistance to high temperature recrystallization, enhancing material strength to sustain thermal fatigue cracking and tolerance to neutron irradiation are the key indicators used for the down selection of materials and manufacturing processes to be applied to deliver engineering materials. In this work we investigate the effect of neutron irradiation on mechanical properties and microstructure of several tungsten grades recently developed. Neutron irradiation campaign is arranged for screening purposes and therefore is limited to the fluence relevant for the ITER plasma facing components. At the same time, the neutron exposure covers a large span of irradiation temperatures from 600 up to 1000 degrees C. Four different grades are included in the study, namely: fine-grain tungsten strengthened by W-carbide (W-4wt.% W2C), fine-grain tungsten strengthened by Zr carbides (W-0.5% ZrC), W alloyed with 10 at.% chromium and 0.5 at.% yttrium (W-10Cr-0.5Y) and technologically pure W plate manufactured according to the ITER specification by Plansee (Austria). The strengthening by W2C and ZrC particles leads to an enhanced strength, moreover, the W-0.5ZrC material exhibits reduced DBTT (compared to ITER specification grade) and is available in the form of thick plate (i.e. high up-scaling potential). The W-10Cr-0.5Y grade is included as the material offering the self-passivation protection against the high temperature oxidation

Magnetic performance of SrFe12O19–Zn0.2Fe2.8O4 hybrid magnets prepared by spark plasma sintering

[EN] In the last few years, significant effort has again been devoted to ferrite-based permanent magnet research due to the so-called rare-earth crisis. In particular, a quest to enhance ferrites maximum energy product, BHmax, is underway. Here, the influence of composition and sintering conditions on the microstructure and consequently magnetic properties of strontium ferrite-based hybrid composites was investigated. The powder mixtures consisted of hydrothermally synthesised Sr-ferrite with hexagonally shaped platelets with a diameter of 1 μm and thickness up to 90 nm, and a soft magnetic phase in various ratios. Powders were sintered using a spark plasma sintering furnace. The crystal structure, composition and microstructure of the starting powders and hybrid magnets were examined. Their magnetic properties were evaluated by vibrating sample magnetometer, permeameter and by single-point-detection measurements.This work is supported by the European Commission through Project H2020 No. 720853 (AMPHIBIAN) and Slovenian Research Agency is acknowledged for funding the research program Ceramics and complementary materials for advanced engineering and biomedical applications (P2-0087), CEMM, JSI is acknowledged for the use of EM.Peer reviewe

Magnetic performance of SrFe12O19-Zn0.2Fe2.8O4 hybrid magnets prepared by Spark Plasma Sintering

[EN] In the last few years, significant effort has again been devoted to ferrite-based permanent magnet research due to the so-called rare-earth crisis. In particular, a quest to enhance ferrites maximum energy product, BHmax, is underway. Here, the influence of composition and sintering conditions on the microstructure and consequently magnetic properties of strontium ferrite-based hybrid composites was investigated. The powder mixtures consisted of hydrothermally synthesised Sr-ferrite with hexagonally shaped platelets with a diameter of 1 μm and thickness up to 90 nm, and a soft magnetic phase in various ratios. Powders were sintered using a spark plasma sintering furnace. The crystal structure, composition and microstructure of the starting powders and hybrid magnets were examined. Their magnetic properties were evaluated by vibrating sample magnetometer, permeameter and by single-point-detection measurements.This work is supported by the European Commission through Project H2020 No. 720853 (AMPHIBIAN) and Slovenian Research Agency is acknowledged for funding the research program Ceramics and complementary materials for advanced engineering and biomedical applications (P2-0087), CEMM, JSI is acknowledged for the use of EM.Peer reviewe

Uncorrelated magnetic domains in decoupled SrFe12O19/Co hard/soft bilayers

8 pags., 5 figs.Composites of magnetically hard and soft phases are present in multiple and diverse applications, ranging from bulk permanent magnets in motors and generators to state-of-the-art recording media devices. The nature of the magnetic coupling between the hard and soft phases is of great technological relevance, as the macroscopic properties of the functional composite material ultimately depend on the atomic-scale interactions between phases. In this work, the hard/soft bilayer system SrFeO/Co has been studied based on photoemission electron microscopy combined with x-ray absorption and magnetic circular dichroism. Our experiments show that the magnetization of the hard magnetic oxide has a direction perpendicular to the layer plane, whereas the magnetization of the soft metallic overlayer remains in-plane. As a consequence, the magnetic domain patterns observed for the hard and soft phases are very different and completely uncorrelated to one another, indicating that no soft spins align with the hard phase by pure magnetodipolar arguments. The results are understood as the consequence of an absence of exchange-coupling between phases, in a scenario in which the shape anisotropy of the soft layer overcomes the Zeeman energy of the perpendicular magnetic field generated by the hard ferrite. Micromagnetic simulations of our system predict that low degrees of exchange-coupling effectively prevent substantial softening of the composite and lead to the alignment of soft and hard magnetic moments. A strategy thus emerges for the development of future hard-soft magnets, based on minimizing the degree of exchange-coupling while avoiding complete uncoupling.This work is supported by the Spanish Ministerio de Economía y Competitividad (MINECO) through Projects no. MAT2017-86450-C4-1-R, RTI2018-095303-B-C51, RTI2018-095303-B-C53, RTI2018-095303-A-C52, and FIS2017-82415-R and by the European Comission through Project H2020 no. 720853 (AMPHIBIAN). The work is funded as well by the Regional Government of Madrid through Project S2018/NMT-4321 (NANOMAGCOST). C.G.-M. acknowledges financial support from Spanish Ministerio de Ciencia e Innovación (MICINN) through the 'Juan de la Cierva' Program (FJC2018-035532-I). Spanish Ministerio de Economía y Competitividad (MINECO)

Strontium hexaferrite platelets: a comprehensive soft X-ray absorption and Mössbauer spectroscopy study

Platelets of strontium hexaferrite (SrFeO, SFO), up to several micrometers in width, and tens of nanometers thick have been synthesized by a hydrothermal method. They have been studied by a combination of structural and magnetic techniques, with emphasis on Mössbauer spectroscopy and X-ray absorption based-measurements including spectroscopy and microscopy on the iron-L edges and the oxygen-K edge, allowing us to establish the differences and similarities between our synthesized nanostructures and commercial powders. The Mössbauer spectra reveal a greater contribution of iron tetrahedral sites in platelets in comparison to pure bulk material. For reference, high-resolution absorption and dichroic spectra have also been measured both from the platelets and from pure bulk material. The O-K edge has been reproduced by density functional theory calculations. Out-of-plane domains were observed with 180° domain walls less than 20 nm width, in good agreement with micromagnetic simulations.This work is supported by the Spanish Ministry of Economy and Competitiveness through Projects MAT2015-64110-C2-1-P, MAT2015-64110-C2-2-P, MAT2015-66888-C3-1-R and by the European Commission through Project H2020 No. 720853 (Amphibian). These experiments were performed at the CIRCE, MISTRAL and BOREAS beamlines of the ALBA synchrotron Light Facility. G.D.S. acknowledges the European Youth Employement Initiative and the Autonomous Community of Madrid for a one-year fellowship. Slovenian Research Agency is acknowledged for funding the research program Ceramics and complementary materials for advanced engineering and biomedical applications (P2-0087), CEMM, JSI for the use of TEM

Strontium hexaferrite platelets: a comprehensive soft X-ray absorption and Mössbauer spectroscopy study

MECAME / GFSM 2019, Montpellier, 19 to 23 may 2019 .-- In honour of Dr Jean-Claude Jumas (Institut Charles Gerhardt, CNRS, University of Montpellier, France). -- https://mecame-gfsm2019.irb.hr/Strontium ferrite (SFO, SrFe12O19) is a ferrite employed for permanent magnets due to its high magnetocrystalline anisotropy. Since its discovery in the mid-20th century, this hexagonal ferrite has become an increasingly important material both commercially and technologically, finding a variety of uses and applications. Its structure can be considered a sequence of alternating spinel (S) and rocksalt (R) blocks. All the iron cations are in the Fe3+ oxidation state and it has a ferrimagnetic configuration with five different cationic environments for the iron (three octahedral sites, a tetraedrical site and a bipiramidal site)[1,2]. We have studied the properties of SrFe 12O19 in the shape of platelets, up to several micrometers in width, and tens of nanometers thick, synthesized by a hydrothermal method. We have characterized the structural and magnetic properties of these platelets by Mössbauer spectroscopy, x-ray transmission microscopy (TMX), transmission electron microscopy (TEM), x-ray diffraction (XRD), vibrating-sample magnetometry (VSM), x-ray absorption spectroscopy (XAS), x-ray circular magnetic dichroism (XMCD) and photoemission electron microscopy (PEEM). To the best of our knowledge this is the first time that the x-ray absorption spectra at the Fe L 2,3 edges of this material in its pure form have been reported. The Mössbauer results recorded from these platelets both in the electron detection and transmission modes have helped to understand the iron magnetic moments determined by XMCD (Fig.1). The experimental results have been complemented with multiplet calculations aimed at reproducing the observed XAS and XMCD spectra at the Fe L 2,3 absorption edge, and by density functional theory (DFT) calculations to reproduce the oxygen K- absorption edge. Finally the domain pattern measured in remanence is in good agreement with micromagnetic simulations [3]

AMPHIBIAN CSIC experimental data SrFe12O19 platelets SciRep2018. AMPHIBIAN CSIC simulated data SrFe12O19 platelets SciRep2018. [Dataset]

[EN] Platelets of strontium hexaferrite (SrFeO, SFO), up to several micrometers in width, and tens of nanometers thick have been synthesized by a hydrothermal method. They have been studied by a combination of structural and magnetic techniques, with emphasis on Mössbauer spectroscopy and X-ray absorption based-measurements including spectroscopy and microscopy on the iron-L edges and the oxygen-K edge, allowing us to establish the differences and similarities between our synthesized nanostructures and commercial powders. The Mössbauer spectra reveal a greater contribution of iron tetrahedral sites in platelets in comparison to pure bulk material. For reference, high-resolution absorption and dichroic spectra have also been measured both from the platelets and from pure bulk material. The O-K edge has been reproduced by density functional theory calculations. Out-of-plane domains were observed with 180° domain walls less than 20 nm width, in good agreement with micromagnetic simulations.UE, programa H2020, Proyecto AMPHIBIAN n º 720853Peer reviewe