Study of nanostructured hard ferrites and their coupling with a soft layer

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física Aplicada. Fecha de Lectura: 22-06-202

Magnetic domains in SrFe12O19/Co hard/soft bilayers

ESRF (The european Synchrotron) User Meeting 2022, 7 - 9 February, 2022 . -- online meeting . -- https://www.esrf.fr/fr/home/events/conferences/2022/user-meeting-2022.html .-- Youtube access: https://www.youtube.com/playlist?list=PLsWatK2_NAmyYnkC-bXhvT70wsYaTmojqThe nature of the magnetic coupling between a SrFe12O19 particle (hard phase) and a Co layer grown on top (soft phase) has been studied by means of photoemission electron microscopy (PEEM) and spatially-resolved x-ray absorption (XAS) and magnetic circular dichroism (XMCD) at CIRCE, ALBA synchrotron (Spain). Our study reveals the soft metallic overlayer presents an in-plane magnetization despite the strong out-of-plane magnetocrystalline anisotropy of the hard platelet. Thus, the two phases show completely uncorrelated magnetic domain patterns. Micromagnetic simulations seem to indicate the degree of exchange-coupling is low or null, although the conditions for rigid coupling are a priori well met

Tuning the Néel temperature in an antiferromagnet: the case of NixCo1−xO microstructures

We show that it is possible to tune the Néel temperature of nickel(II)-cobalt(II) oxide films by changing the Ni to Co ratio. We grow single crystalline micrometric triangular islands with tens of nanometers thickness on a Ru(0001) substrate using high temperature oxygen-assisted molecular beam epitaxy. Composition is controlled by adjusting the deposition rates of Co and Ni. The morphology, shape, crystal structure and composition are determined by low-energy electron microscopy and diffraction, and synchrotron-based x-ray absorption spectromicroscopy. The antiferromagnetic order is observed by x-ray magnetic linear dichroism. Antiferromagnetic domains up to micrometer width are observedThis work is supported by the Spanish Agencia Estatal de Investigación (MCIU/AEI/FEDER, EU)) through Projects Nos MAT2015-64110-C2-1-P, MAT2015-64110-C2-2-P, RTI2018-095303-B-C51, and RTI2018-095303-B-C53, by the European Commission through Project H2020 No. 720853 (Amphibian) and by the Comunidad de Madrid through Project. NANOMAGCOST-CM P2018/NMT-4321. These experiments were performed at the CIRCE beamline of the ALBA Synchrotron Light Facility. A.M. acknowledges funding via a CSIC-Alba agreemen

Insights onto the magnetic coupling at hexaferrite-based hard/soft bilayer systems

IBERTRIVA 2019 X Iberian Conference on Tribology – IBERTRIB, XI Iberian Vacuum Conference - RIVA, Seville, Spain,June 26-28Magnets are used in a variety of applications, such as generators, magnetic recording media, components in RF and microwave devices. However, many of these magnets contain s rare earths, critical elements whose extraction is environmentally harmful and that present price volatility risks. Their replacement by cheaper and more environmentally friendly materials is therefore sought. In our case, we have focused on magnetically hard strontium hexaferrite (SrFe 12O19, SFO) as the base for alternative permanent magnets (Figure 1a). The atomic arrangement of this ferrite results in a high magnetocrystalline anisotropy and a coercive field, however, its magnetization is moderate (1). It is well known that the coupling between a magnetically hard and soft material improved magnetization while avoiding a high cost in coercitivity loss (2). However, results have been disappointing so far as structural and geometrical limitations make it extremely challenging to fabricate. In this work, we aim at further understanding the magnetic coupling at hard -soft interfaces involving ferrites, for which we have deposited soft iron and cobalt metals on top of SrFe12O19 thin films with controlled easy-axis of magnetization. SFO thin films have been obtained by RF magnetron sputtering at 260W followed by a subsequent annealing in air of 850ºC. Their structure and composition was characterized by Raman spectroscospy (Figure 1b), Mössbauer spectroscopy, X-ray photoemission spectroscopy and low-energy electron microscopy (LEEM). We have grown the magnetically soft layer by molecular-beam epitaxy and we have analyzed the resulting bilayer system through photoemission electron microscopy, LEEM and vibrating-sample magnetometry. References [1] R.C. Pullar, Hexagonal ferrites: a review of the synthesis, properties and applications of hexaferrite ceramics, Progress in Materials Science 57 (2012), pp 1191¿1334. [2] Eric E. Fullerton, J. S. Jiang, M. Grimsditch, C. H. Sowers, and S. D. Bader, Exchange-spring behavior in epitaxial hard/soft magnetic bilayers, Phys. Rev. B 58 (1998) 1219

Real space observation of the magnetic coupling between a Co film and a barium hexaferrite film

RIVA ONLINE 2021 – IBERIAN VACUUM ONLINE MEETING. The Iberian Vacuum Conference, (Reunión Ibérica de Vacío, RIVA) is a joint meeting of the Portuguese Vacuum Society (SOPORVAC) and the Spanish Vacuum Society (ASEVA), 2021 RIVA will take place ON-LINE from 4-6th October 2021. .-https://aseva.es/conferences/riva-online/Barium ferrite (BaFe12O19, BFO) is a hexagonal ferrite with applications as permanent magnet in many different devices due to its high magnetocrystalline anisotropy, high coercive field and low cost. However, the moderate saturation magnetization of BFO means that the energy product is orders of magnitude smaller than the one that rare-earth-based magnetic materials offer. To overcome this limitation, a commonly proposed strategy to enhance the energy product is exchange-coupling the magnetically hard component (BFO) with a soft phase in order to improve the combined remanent magnetization without a high loss in coercivity. Nonetheless, the results obtained in other hard/soft systems (SFO/Co bilayers) have pointed out the difficulty to take advantage of this rigid coupling magnetic regime1. In this research, we focus on two steps to investigate the Co/BFO coupling in a bilayer system: first, we sought to obtain BFO films with an in- plane magnetic easy axis to avoid shape anisotropy competition, and second, we deposit Co on top of such a BFO film while monitoring both the BFO and Co magnetic domains

Structure and magnetism of ultrathin nickel-iron oxides grown on Ru(0001) by high-temperature oxygen-assisted molecular beam epitaxy

We demonstrate the preparation of ultrathin Fe-rich nickel ferrite (NFO) islands on a metal substrate. Their nucleation and growth are followed in situ by low-energy electron microscopy (LEEM). A comprehensive characterization is performed combining LEEM for structural characterization and PEEM (PhotoEmission Electron Microscopy) with synchrotron radiation for chemical and magnetic analysis via X-ray Absorption Spectroscopy and X-ray Magnetic Circular Dichroism (XAS-PEEM and XMCD-PEEM, respectively). The growth by oxygen-assisted molecular beam epitaxy takes place in two stages. First, islands with the rocksalt structure nucleate and grow until they completely cover the substrate surface. Later three-dimensional islands of spinel phase grow on top of the wetting layer. Only the spinel islands show ferromagnetic contrast, with the same domains being observed in the Fe and Ni XMCD images. The estimated magnetic moments of Fe and Ni close to the islands surface indicate a possible role of the bi-phase reconstruction. A significant out-of-plane magnetization component was detected by means of XMCD-PEEM vector maps

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]

Study of nanostructured hard ferrites and their coupling with a soft layer

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física Aplicada. Fecha de Lectura: 22-06-2021. Tuning the Néel temperature in an antiferromagnet: the case of NixCo1−xO microstructures  Mandziak, Anna; D. Soria, Guiomar; Prieto, José Emilio; Prieto, Pilar; Granados-Miralles, Cecilia; Quesada, Adrian; Foerster, Michael; Aballe, Lucia; de la Figuera, Juan 2019-12-01 (http://hdl.handle.net/10261/191677) Adipose-derived mesenchymal stromal cells for the treatment of patients with severe SARS-CoV-2 pneumonia requiring mechanical ventilation. A proof of concept study  Sánchez-Guijo, Fermín; García-Arranz, Mariano; López-Parra, Miriam; Monedero, Pablo; Mata-Martínez, Carmen; Santos, Arnoldo; Sagredo, Víctor; Alvarez-Avello, José Manuel; Guerrero, José Eugenio; Pérez-Calvo, César; Sánchez-Hernández, Miguel Vicente; Del-Pozo, José Luis; Andreu, Enrique J.; Fernández-Santos, María Eugenia; Soria-Juan, Barbara; Hernández-Blasco, Luis M.; Andreu, Etelvina; Sempere, José M.; Zapata, Agustín G.; Moraleda, José M.; Soria, Bernat; Fernández-Avilés, Francisco; García-Olmo, Damián; Prósper, Felipe 2020-07-10 (http://hdl.handle.net/10261/83705) Self-assembly of 1D/2D hybrid nanostructures consisting of a Cd(II) coordination polymer and NiAl-layered double hydroxides  Abellán, Gonzalo; Amo-Ochoa, P.; Fierro, J.G.; Ribera, Antonio; Coronado, E.; Zamora, F. 2015-12-29 (http://hdl.handle.net/10261/137036)Peer reviewe

Reversible Bilayer-Monolayer Restructuring of ultrathin FeO on Ru(0001)

IBERTRIVA 2019 X Iberian Conference on Tribology – IBERTRIB, XI Iberian Vacuum Conference - RIVA, Seville, Spain,June 26-28Iron monoxide, FeO, has a simple cubic structure that along the direction is comprised of alternating hexagonal layers of iron and oxygen with an fcc sequence. In bulk form it is usually iron deficient, and it can be grown as an ultrathin film on a variety of metallic substrates, among them Ru(0001) [1,2]. Here we show how an incomplete ultrathin FeO filmon Ru(0001) can be reversibly restructured from bilayer to monolayer thickness by changing the background oxygen pressure at high temperature and thus the amount of oxygen adsorbed in the uncovered Ru areas. The growth and the evolution of the iron oxide film upon different treatments is followed in real time by low-energy electron microscopy (LEEM) in ultra-high vacuum. LEEM is a full-field technique where an electron beam is directed towards the surface under observation and the elastically backscattered electrons are used to form a magnified image by means of electrostatic and/or electromagnetic lenses. In this work we employ the two LEEM instruments available in Spain. Both of them have similar characteristics and are located at the Alba synchrotron [3] and at the Instituto de Química Fïsica ¿Rocasolano¿ respectively. References [1] B. Santos, E. Loginova, A. Mascaraque, A. Schmid, K. McCarty, J. de la Figuera, Structure and magnetism in ultrathin iron oxides characterized by low energy electron microscopy, J. Physics-Cond. Matt. 21 (2009) 314011. [2] I. Palacio, M. Monti, J.F. Marco, K.F. McCarty, J. de la Figuera, Initial stages of FeO growth on Ru(0001), J. Phys.: Condens. Matter. 25 (2013) 484001 [3] L. Aballe, M. Foerster, E. Pellegrin, J. Nicolas, S. Ferrer, The ALBA spectroscopic LEEM-PEEM experimental station: layout and performance, J Synchrotron Radiat. 22 (2015) 74

Understanding structural and magnetic properties of SrFe12O19 in platelets and thin film form

ACS Fall Meeting 2021. (Mössbauer Spectroscopy from Magnetic Nanoarchitectures to Environmental Science: A Symposium in honor of Dr. Jean-Marc Greneche), ACS Fall National Meeting in Atlanta, Georgia on August 22-26, 2021. https://www.agrodiv.org/event/2021-acs-fall-national-meeting/ DAY & TIME OF PRESENTATION: Wednesday, August 25, 2021 from 2:20 PM - 2:40 PM ROOM & LOCATION: Virtual Roo