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

Structure and magnetic properties of W-type hexaferrites

W-type hexaferrites (WHFs) (SrMe2Fe16O27, Me = Mg, Co, Ni and Zn) are hard magnetic materials with high potential for permanent magnet applications owing to their large crystalline anisotropy and high cation tunability. However, little is known with regards to their complex structural and magnetic characteristics. Here, the substitution of metals (Me = Mg, Co, Ni and Zn) in WHFs is described and their crystal and magnetic structures investigated. From joined refinements of X-ray and neutron powder diffraction data, the atomic positions of the Me atoms were extracted along with the magnetic dipolar moment of the individual sites. The four types of WHFs exhibit ferrimagnetic ordering. For Mg, Ni and Zn the magnetic moments are found to be ordered colinearly and with the magnetic easy axis along the crystallographic c axis. In SrCo2Fe16O27, however, the spontaneous magnetization changes from uniaxial to planar, with the moments aligning in the crystallographic ab plane. Macromagnetic properties were measured using a vibration sample magnetometer. The measured saturation magnetization (Ms) between the different samples follows the same trend as the calculated Ms extracted from the refined magnetic moments of the neutron powder diffraction data. Given the correlation between the calculated Ms and the refined substitution degree of the different Me in specific crystallographic sites, the agreement between the measured and calculated Ms values consolidates the robustness of the structural and magnetic Rietveld model

In-depth investigations of size and occupancies in cobalt ferrite nanoparticles by joint Rietveld refinements of X-ray and neutron powder diffraction data

[EN] Powder X-ray diffraction (PXRD) and neutron powder diffraction (NPD) have been used to investigate the crystal structure of CoFe2O4 nanoparticles prepared via different hydro­thermal synthesis routes, with particular attention given to accurately determining the spinel inversion degrees. The study is divided into four parts. In the first part, the investigations focus on the influence of using different diffraction pattern combinations (NPD, Cu-source PXRD and Co-source PXRD) for the structural modelling. It is found that combining PXRD data from a Co source with NPD data offers a robust structural model. The second part of the study evaluates the reproducibility of the employed multipattern Rietveld refinement procedure using different data sets collected on the same sample, as well as on equivalently prepared samples. The refinement procedure gives reproducible results and reveals that the synthesis method is likewise reproducible since only minor differences are noted between the samples. The third part focuses on the structural consequences of (i) the employed heating rate (achieved using three different hydro­thermal reactor types) and (ii) changing the cobalt salt in the precursors [aqueous salt solutions of Co(CH3COOH)2, Co(NO3)2 and CoCl2] in the synthesis. It is found that increasing the heating rate causes a change in the crystal structure (unit cell and crystallite sizes) while the Co/Fe occupancy and magnetic parameters remain similar in all cases. Also, changing the type of cobalt salt does not alter the final crystal/magnetic structure of the CoFe2O4 nanoparticles. The last part of this study is a consideration of the chemicals and parameters used in the synthesis of the different samples. All the presented samples exhibit a similar crystal and magnetic structure, with only minor deviations. It is also evident that the refinement method used played a key role in the description of the sample.Support from the Carlsberg Foundation (grant Nos. CF16-0084, CF18-0519 and CF19-0143). CG-M acknowledges financial support from MICINN through the ‘Juan de la Cierva’ Program (FJC2018-035532-I). JVA acknowledges financial support from Nordforsk (project No. 106874). We thank the Danish Agency for Science,Technology and Innovation for funding the instrument center DanScatt (7129-00003B

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.The authors thank financial support from the European Commission through the NANOPYME (FP7-SMALL-310516) and AMPHIBIAN (H2020-NMBP-2016-720853) projects. Financial support from the Danish National Research Foundation (Center for Materials Crystallography, DNRF-93) and the Danish Center for Synchrotron and Neutron Science (DanScatt) is gratefully acknowledged. Parts of this research were carried out at PETRA III at DESY, a member of the Helmholtz Association (HGF). We thank Jozef Bednarcik for assistance in using beamline P02.1. This work is based on experiments performed at the Swiss spallation neutron source SINQ, Paul Scherrer Institute, Villigen, Switzerland. This project has received funding from the European Union’s Seventh Framework Programme for research, technological development, and demonstration under the NMI3-II Grant 283883.Peer reviewe

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 <i>in situ</i> using synchrotron powder X-ray diffraction (PXRD). Sequential Rietveld refinements of the <i>in situ</i> 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 <i>ex situ</i> 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

AMPHIBIAN CSIC experimental data insitu exsitu PXRD NPD [Dataset]

The different datasets are divided in folders as follows: 1. In situ experiments. The article is based on 7 different in situ experiments: a. 400C_5mLmin_fr10; b. 400C_10mLmin_fr23; c. 400C_20mLmin_fr6; d. 400C_30mLmin_fr12; e. 300C_10mLmin_fr4; f. 350C_10mLmin_fr18; g. 500C_10mLmin_fr9. (Due to space limitations, only the data corresponding to the experiment c were uploaded to this repository. The remaining data are accessible through private communication with Cecilia Granados-Miralles, [email protected]). The folder “c_400C_20mLmin_fr6” contains in situ PXRD data (time resolution=5 s) collected during reduction at the temperature and gas flow indicated in the folder name (i.e. 400C_20mLmin stands for 400 °C and 20 mL/min). The temperature indicated in the file names is the set temperature but the sample temperature is the indicated in the folder name (i.e. Tset430 led to a sample temperature of 400 °C). Heating started on the frame indicated in the folder name (i.e. fr6 stands for frame 6). This folder contains 2D-diffraction images in *.tif format, which may be opened with the Python-based program for on-the-fly data processing and exploration of two-dimensional X-ray diffraction area detector data, available for free download at http://www.clemensprescher.com/programs/dioptas.2. Ex situ experiments: a. PXRD_Cu 1D-diffraction patterns in *.dat format. 3 space-/tab-separated columns: 2theta angle (degree), Diffracted Intensity (arbitrary units), σ (arbitrary units); b. PXRD_Co Same as previous; c. NPD_DMC 1D-diffraction patterns in *.dat format. The first line indicates: narrowest 2theta angle measured, measurement step, widest 2theta angle. The following values are the diffracted intensities recorded for each 2theta step; d. NPD_HRPT; Same as previous e. VSMHysteresis loops: Magnetic moment as a function of a variable applied field in *.dat format. Several comma-separated columns of which the relevant are the 4th, 5th, and 6th columns, which contain: Magnetic Field (Oe), Moment (emu), M. Std. Err. (emu).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.European Comission through FP7 program, NANOPYME project, grant agreement no. 310516. European Comission through H2020 program, AMPHIBIAN Project, grant agreement no. 720853. Danish National Research Foundation through Center for Materials Crystallography, DNRF-93. Danish Center for Synchrotron and Neutron Science through DanScatt. European Comission through FP7 program, NMI3-II project, grant agreement no. 283883.Peer reviewe

Enhanced intrinsic saturation magnetization of ZnxCo1-xFe2O4 nanocrystallites with metastable spinel inversion

The magnetic properties of spinel ferrites (MFe2O4, M=Mn, Fe, Co, Ni, Zn, etc.) are largely determined by the type of divalent cation, M2+ and cation distribution between the tetrahedral and octahedral sites in the structure. Partial substitution of Zn2+ into the thermodynamically preferred tetrahedral coordination in ferrites produces an increase in magnetic saturation at room temperature. However, nanosized crystallites are known to adopt different structures compared to their bulk equivalents. Consequently, reliable characterization of the atomic structure of nanosized ferrites is essential for understanding and tailoring their magnetic properties. Here, we present a meticulous study of the crystal-, magnetic- and micro-structures of mixed ZnxCo1-xFe2O4 spinel ferrite nanocrystallites in the entire composition range (x=0.0-1.0 in steps of 0.1). Gram-scale nanoparticle preparation was performed via the widely used hydrothermal method. Eight compositions were selected to study the effect of 4 hours vacuum annealing at 823 K. Combined Rietveld refinement of powder X-ray and neutron diffraction data along with Mössbauer analysis reveal how the as-synthesized nanocrystallites adopt metastable cation inversions, different from the well-established and thermodynamically stable inversions of the bulk equivalents. The annealing treatment causes the structure of the crystallites to relax towards a more bulk-like cation distribution. For all compositions, the smaller as-synthesized nanocrystallites with metastable cation inversion exhibit a higher saturation magnetization compared to the annealed samples. The demonstrated control over the spinel ferrite cation distribution is a key step on the way to designing cheap magnetic materials with tunable properties optimized for specific applications.This work was financially supported by the Danish National Research Foundation (Center for Materials Crystallography, DNRF93), Innovation Fund Denmark (Green Chemistry for Advanced Materials, GCAM-4107-00008B), Independent Research Fund Denmark (Small and Smart Magnet Design) and the Danish Center for Synchrotron and Neutron Science (DanScatt). Affiliation with the Center for Integrated Materials Research (iMAT) at Aarhus University is gratefully acknowledged. The authors are grateful for the obtained beamtime at the DMC beamline, SINQ, PSI, Villigen, Switzerland