56 research outputs found
Expanding the tunability and applicability of exchange-coupled/decoupled magnetic nanocomposites
CoFe2O4/Co-Fe magnetic composites are usually prepared through partial
reduction of CoFe2O4, which often yields monoxides (i.e., FeO, CoO) as
secondary phases. Since these compounds are paramagnetic at ambient conditions,
the presence of a small amount of monoxide is generally downplayed in the
literature, and the possible effects on the magnetic properties are simply
ignored. However, the present study shows that even a low concentration of
monoxide results in decoupling of the soft and hard magnetic phases, which
inevitably leads to a deterioration of the magnetic properties. Additionally,
it is confirmed that a partial reduction of CoFe2O4 is a suitable method to
produce CoFe2O4/Co-Fe nanocomposites, provided that the treatment is well
controlled with respect to duration, temperature and flow of reductant. A
monoxide-free nanocomposite was produced and its magnetic properties evaluated
both at room and low temperature. Our model system exemplifies the potential of
exchange-coupling (and decoupling) as a tool to tune the magnetic properties of
a material within a relatively wide range of values, thus widening its spectrum
of potential applications
Approaching Ferrite-Based Exchange-Coupled Nanocomposites as Permanent Magnets
During the past decade, CoFe2O4 (hard)/Co-Fe alloy (soft) magnetic
nanocomposites have been routinely prepared by partial reduction of CoFe2O4
nanoparticles. Monoxide (i.e., FeO or CoO) has often been detected as a
byproduct of the reduction, although it remains unclear whether the formation
of this phase occurs during the reduction itself or at a later stage. Here, a
novel reaction cell was designed to monitor the reduction in situ using
synchrotron powder X-ray diffraction (PXRD). Sequential Rietveld refinements of
the in situ data yielded time-resolved information on the sample composition
and confirmed that the monoxide is generated as an intermediate phase. The
macroscopic magnetic properties of samples at different reduction stages were
measured by means of vibrating sample magnetometry (VSM), revealing a magnetic
softening with increasing soft phase content, which was too pronounced to be
exclusively explained by the introduction of soft material in the system. The
elemental compositions of the constituent phases were obtained from joint
Rietveld refinements of ex situ high-resolution PXRD and neutron powder
diffraction (NPD) data. It was found that the alloy has a tendency to emerge in
a Co-rich form, inducing a Co deficiency on the remaining spinel phase, which
can explain the early softening of the magnetic material
Expanding the tunability and applicability of exchange-coupled/decoupled magnetic nanocomposites
[EN] CoFe2O4/Co–Fe magnetic composites are usually prepared through partial reduction of CoFe2O4, which often yields monoxides (i.e., FeO, CoO) as secondary phases. Since these compounds are paramagnetic at ambient
conditions, the presence of a small amount of monoxide is generally downplayed in the literature, and the possible effects on the magnetic properties are simply ignored. However, the present study shows that even a
low concentration of monoxide results in decoupling of the soft and hard magnetic phases, which inevitably leads to a deterioration of the magnetic properties. Additionally, it is confirmed that a partial reduction of
CoFe2O4 is a suitable method to produce CoFe2O4/Co–Fe nanocomposites, provided that the treatment is well controlled with respect to duration, temperature and flow of reductant. A monoxide-free nanocomposite
was produced and its magnetic properties evaluated both at room and low temperature. Our model system exemplifies the potential of exchange-coupling (and decoupling) as a tool to tune the magnetic properties of a material within a relatively wide range of values, thus widening its spectrum of potential applications.C. G.-M. and A. Q. have contributed equally to this work. The authors would like to thank financial support from the European Commission through the AMPHIBIAN project (H2020-NMBP-2016-720853), the Danish National Research Foundation
(Center for Materials Crystallography, DNRF-93), and the Spanish Ministerio de Ciencia, Innovación y Universidades (RTI2018-095303-A-C52). C. G.-M. acknowledges financial support from the Spanish Ministerio de Ciencia, Innovación y
Universidades through the Juan de la Cierva Program (FJC2018- 035532-I). Authors from Aarhus University gratefully acknowledge affiliation with the Center for Integrated Materials Research (iMAT) at Aarhus University. We acknowledge support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).Peer reviewe
Hexaferrite-based permanent magnets with upper magnetic properties by cold sintering process via a non-aqueous solvent
The incessant technological pursuit towards a more sustainable and green
future depends strongly on permanent magnets. At present, their use is
widespread, making it imperative to develop new processing methods that
generate highly competitive magnetic properties reducing the fabrication
temperatures and costs. Herein, a novel strategy for developing dense sintered
magnets based on Sr-hexaferrites with upper functional characteristics is
presented. An innovative cold sintering approach using glacial acetic acid as
novelty, followed by a post-annealing at 1100 {\deg}C, achieves a densification
of the ceramic magnets of 92% with respect to the theoretical density and
allows controlling the particle growth. After the cold sintering process, a
fraction of amorphous SrO is identified, in addition to a partial
transformation to {\alpha}-Fe2O3 as secondary crystalline phase. 46 wt% of
SrFe12O19 remains, which is mostly recuperated after the post-thermal
treatment. These findings do not significantly modify the final structure of
ferrite magnets, neither at short- nor long-range order. The innovative process
has a positive impact on the magnetic properties, yielding competitive ferrite
magnets at lower sintering temperatures with an energy efficiency of at least
25%, which opens up a new horizon in the field of rare-earth free permanent
magnets and new possibilities in other applications
Tuning the N\'eel temperature in an antiferromagnet: the case of NixCo1-xO microstructures
We show that it is possible to tune the N\'eel 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 observed
CoSb3-based skutterudite nanocomposites prepared by cold sintering process with enhanced thermoelectric properties
We show here for the first time the use of a cold sintering process (CSP) to
sinter CoSb3-based thermoelectric materials. CSP at 150 {\deg}C for 90 min
under a uniaxial pressure of 750 MPa yields pieces with a relative density of
86 %, which is increased to around 92 % after a post-annealing at temperatures
> 500 {\deg}C in Ar atmosphere. The reported CSP produces Te doped-CoSb3
nanocomposites with similar morphological and structural characteristics to the
starting nanopowders obtained by ball milling in air atmosphere. The
post-thermal treatment induces grain coalescence and grain growth, crystallite
size growth as well as compositional changes in the nanocomposite, decreasing
the amount of the main phase, CoSb3, and increasing the weight of secondary
phase, CoSb2, up to a 30 wt% at 600 {\deg}C. Remarkably, the average valence
for the Co, Sb and Te absorbing atoms is neither transformed by the sintering
process nor by the subsequent heat treatment. The functional response of the
sintered thermoelectric nanocomposites exhibits a maximum figure of merit of
0.12(3) at room temperature for the nanocomposites sintered by CSP with a
subsequent post-annealing at 500 {\deg}C. This is mainly due to its low thermal
conductivity in comparison with similar powders sintered by other approaches,
and it is explained by the morphological and structural properties. These
findings represent an attractive alternative for obtaining efficient
thermoelectric skutterudites by a scalable and cost-effective route
Remanence Increase in SrFeO/Fe Exchange-Decoupled Hard-Soft Composite Magnets Owing to Dipolar Interactions
In the search for improved permanent magnets, fueled by the geostrategic and
environmental issues associated with rare-earth-based magnets, magnetically
hard (high anisotropy)-soft (high magnetization) composite magnets hold promise
as alternative magnets that could replace modern permanent magnets, such as
rare-earth-based and ceramic magnets, in certain applications. However, so far,
the magnetic properties reported for hard-soft composites have been
underwhelming. Here, an attempt to further understand the correlation between
magnetic and microstructural properties in strontium ferrite-based composites,
hard SrFeO (SFO) ceramics with different contents of Fe particles
as soft phase, both in powder and in dense injection molded magnets, is
presented. In addition, the influence of soft phase particle dimension, in the
nano- and micron-sized regimes, on these properties is studied. While Fe and
SFO are not exchange-coupled in our magnets, a remanence that is higher than
expected is measured. In fact, in composite injection molded anisotropic
(magnetically oriented) magnets, remanence is improved by 2.4% with respect to
a pure ferrite identical magnet. The analysis of the experimental results in
combination with micromagnetic simulations allows us to establish that the type
of interaction between hard and soft phases is of a dipolar nature, and is
responsible for the alignment of a fraction of the soft spins with the
magnetization of the hard. The mechanism unraveled in this work has
implications for the development of novel hard-soft permanent magnets
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
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 VacÃo, 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
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