- Email: [email protected]
Partial substitution of cobalt by rare-earths (Gd or Sm) in cobalt ferrite: Effect on its microstructure and magnetic properties
Journal Pre-proof Partial substitution of cobalt by rare-earths (Gd or Sm) in cobalt ferrite: Effect on its microstructure and magnetic properties A. Tijerina-Rosa, J.M. Greneche, A.F. Fuentes, J. Rodriguez-Hernandez, J.L. Menéndez, F.J. Rodríguez-González, Sagrario M. Montemayor PII:
S0272-8842(19)32161-3
DOI:
https://doi.org/10.1016/j.ceramint.2019.07.335
Reference:
CERI 22450
To appear in:
Ceramics International
Received Date: 26 April 2019 Revised Date:
4 July 2019
Accepted Date: 29 July 2019
Please cite this article as: A. Tijerina-Rosa, J.M. Greneche, A.F. Fuentes, J. Rodriguez-Hernandez, J.L. Menéndez, F.J. Rodríguez-González, S.M. Montemayor, Partial substitution of cobalt by rare-earths (Gd or Sm) in cobalt ferrite: Effect on its microstructure and magnetic properties, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.07.335. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Title: “Partial Substitution of Cobalt by Rare-Earths (Gd or Sm) in Cobalt Ferrite: Effect on its Microstructure and Magnetic Properties” Author names and affiliations: A. Tijerina-Rosaa,b,1, J. M. Grenechec, A. F. Fuentesd, J. Rodriguez-Hernandeza, J. L. Menéndeze, F. J. Rodríguez-Gonzáleza, Sagrario M. Montemayora* a
Departamento de Materiales Avanzados, Centro de Investigación en Química Aplicada, Blvd. Enrique Reyna No. 140, Saltillo, Coahuila, México. b
Facultad de Ciencias Químicas, Universidad Autónoma de Coahuila, V. Carranza esq. J. Cárdenas s/n, Saltillo, Coahuila, 25280, México. c
Institut des Molécules et Matériaux du Mans-IMMM UMR CNRS 6283, Université du Maine, Avenue Olivier Messiaen, 72085 Le Mans cedex France. d
CINVESTAV-Unidad Saltillo, Ave. Industrial Metalúrgica 1062, Parque Industrial Saltillo-Ramos Arizpe, Ramos Arizpe, Coahuila, 25900, México. e
Centro de Investigación en Nanomateriales y Nanotecnología, CINN-CSIC. Principado de Asturias-Consejo superior de Investigaciones Científicas (CSIC)Universidad de Oviedo (UO). Avda. de la Vega 4-6, 33940 San Martín del Rey Aurelio (Asturias), Spain. 1
Tecnológico Nacional de México, Instituto Tecnológico de Saltillo, Blvd. Venustiano Carranza #2400, Colonia Tecnológico, Saltillo, Coahuila, México, C.P. 25280, División de Estudios de Posgrado e Investigación.
Corresponding author: Sagrario M. Montemayor
Contact details:
E-mail address: [email protected] or [email protected]
Postal mail address: Blvd. Enrique Reyna Hermosillo No. 140, Col. San José de los Cerritos, C.P. 25294, Saltillo, Coahuila, México.
Telephone: (+52) 844 4389830 ext.1421
Fax number: (+52) 844 4389839
Partial Substitution of Cobalt by Rare-Earths (Gd or Sm) in Cobalt Ferrite: Effect on its Microstructure and Magnetic Properties
Abstract: Microstructure and magnetic properties of partially substituted cobalt ferrite, with small amounts of gadolinium or samarium, were investigated. Co1-xGdxFe2O4+δ (x = 0, 0.02, 0.04, 0.06, 0.08, 0.1, and 0.15) and Co1-xSmxFe2O4+δ (x = 0.04, and 0.08) were obtained through the citrate precursors route. X-ray powder diffraction patterns, of samples treated at temperatures between 400 and 1000 °C, are consistent with a cubic spinel-type structure. Substituted ferrites, with x = 0.04 and 0.08 (600 °C/2 h), show the combination of maximum enhancement of crystallinity and minimum change of the lattice parameters. At this temperature and composition range the crystallite size decreases up to a 41 %. Rietveld analyses of Co1-x(Gd/Sm)xFe2O4+δ (x = 0, 0.04, and 0.08), carried out considering that Gd3+ (or Sm3+) occupies Co2+ positions, give rise to a good quality of refinement and cell parameters which are slightly smaller than those provided in the crystallographic database of CoFe2O4. Raman spectra of Gd3+ or Sm3+ substituted ferrites show the characteristic Raman modes expected in inverse spinel ferrites without additional signals. TEM micrographs reveal the presence of small agglomerates of nanoparticles. SAED patterns and HRTEM micrographs, are characteristic of polycrystalline powders while the interplanar distances well correspond to those of CoFe2O4. EDS analyses confirm the presence of Gd or Sm in substituted ferrites. Magnetic hysteresis loops of samples suggest a ferrimagnetic behavior and their squareness ratio values decrease up to 36 % in Co1-xSmxFe2O4+δ (x = 0.04). Mössbauer studies reveal the
presence of only Fe3+ ions, distributed in a proportion of 60 and 40 % through octahedral and tetrahedral sites, respectively and a canted ferrimagnetic structure. Although the formation of the mixed spinel, rather than different substances (Sm or Gd oxide), fall in the sensitivity limits of the XRD technique all the results demonstrate that the replacement of Co2+ by Gd3+ or Sm3+ ions, in CoFe2O4, can tailor the crystal structure, microstructure, and magnetic properties of cobalt ferrite.
Keywords: Rare-earth substituted cobalt ferrite, (Co,Gd)Fe2O4, (Co,Sm)Fe2O4, Microstructure, Magnetic properties
I. Introduction Cobalt ferrite is a ceramic compound of the group of spinel ferrites (MFe2O4, M= Co2+, Cu2+, Mn2+, Ni2+, Zn2+, etc.). Its magnetic properties and well-established applications in high-density magnetic storage devices, electrical power transformer cores, and rod antennas, have attracted, for long time (~80 years), the attention of scientists and technologists [1]. Recently, the ability of tailoring the physical properties of ferrites, to make them useful in modern applications, has renewed the interest toward nanocrystalline ferrites. Some fields where, lately, spinel ferrites have been especially successful include: catalysis (energy devices) [2-5], biomedicine (early detection and treatment of cancer) [6, 7], electronic (spintronics) [8, 9], and environmental remediation (wastewater treatment) [10, 11]. To understand the wide range of physical properties and applications of spinel ferrites it is necessary to be informed about their structural characteristics, previously determined and reported [12, 13]. Briefly, CoFe2O4 (sometimes expressed as
CoO.Fe2O3) crystallizes in the spinel-type structure, with a cubic symmetry and a space group Fd-3m (JCPDS 22-1086) [14]. The unit cell of spinel ferrite contains 8 formula units (M(2+)8Fe(3+)16O32) where the cubic close-packed arrangement of the 32 oxygen ions generates 96 interstices, 64 tetrahedral and 32 octahedral units, available to be occupied by only 24 cations (8 M2+ and 16 Fe3+). The distribution of the di- and trivalent cations through the tetrahedral and octahedral sites varies, depending on several factors as ionic radii and valency of the specific ion, experimental conditions of synthesis, particle size, etc. [15], resulting in a normal, inverse or mixed spinel ferrite. In a normal-spinel, divalent ions occupy tetrahedral sites and trivalent ions occupy octahedral positions. In an inverse-spinel, divalent ions occupy octahedral sites and a half of trivalent ions occupy tetrahedral positions and the other half occupies octahedral sites. Since tetrahedral and octahedral sites are usually represented by parentheses and brackets, respectively, an inverse-spinel ferrite would be represented as (Fe)[MFe]O4. Mixed-spinel ferrites are intermediate cases, between normal and inverse, in which the inversion degree is designated as “x” in (M1-xFex)[MxFe2-x]O4. In addition to these singular structural features, spinel ferrites have a rich redox chemistry, associated to the solid state redox couples M2+/M+ or M3+/M2+ and Fe3+/Fe2+ usually present. The possibility of tailoring the magnetic (and electric, catalytic, etc.) properties of spinel ferrites resides in altering its structure or/and its microstructure, changing their chemical composition [16], cationic distribution [17], crystallite/particle size [18], morphology [19], etc. One way to achieve this is to vary experimental conditions used during the synthesis. Another way is through the partial substitution of cations in MFe2O4. Although there have been successful improvements in the electric and magnetic response of spinel ferrites, through the partial substitution of di- or trivalent
ions by rare earth ions [20-24], the reported investigations are, at least, controversial. This may be due to several factors such as, in example, the large size of rare earth ions, which becomes hard the process of diffusion through the crystalline network to distribute regularly in octahedral sites. Not to mention that different conditions of synthesis can lead to different degrees of diffusion. Another important factor is that, in rare earth doped spinel ferrites, there are complex multiple types of electronic interactions. Those between transition metals (3d-3d) and those between rare earthstransition metal (4f-3d). Depending on the specific rare earth ion and transition metal, and their structural positions, the spin coupling 4f-3d may be parallel or antiparallel, which affects the electric and magnetic response of these materials [25]. To improve the knowledge about how structure and microstructure affect the properties of spinel ferrites, partially substituted by rare earths, is necessary to pursue systematic research. This investigation studies the synthesis, through the citrate precursors route, of partially substituted cobalt ferrite (Co1-xGdxFe2O4+δ or Co1-xSmxFe2O4+δ), replacing cobalt ions (Co2+) by gadolinium (Gd3+) or samarium (Sm3+). In addition, the effect on its microstructure and magnetic properties is reported.
II. Methods The chemical reagents used in this work were citric acid (C6H8O7), ethylene glycol (C2H6O2), cobalt nitrate (Co(NO3)2·6H2O), gadolinium nitrate (Gd(NO3)3·6H2O), samarium nitrate (Sm(NO3)3·6H2O), and iron nitrate (Fe(NO3)3·9H2O). All of them were purchased from Aldrich with reagent grade (> 98 %) and used without further purification.
2.1 Synthesis of Co1-xGdxFe2O4+δ and Co1-xSmxFe2O4+δ The appropriate amounts of citric acid (1 mol) and ethylene glycol (4 mol) were mixed and stirred at 50 °C to obtain a transparent solution. Then, stoichiometric amounts of cobalt (1-x mol), gadolinium or samarium (x mol), and iron (2 mol) nitrates, were added. Once the mixture achieved complete solubility, the temperature was increased up to 80 °C. After approximately 10 minutes, the solution became more viscous and a brown-reddish gas was produced with no visible formation of turbidity. This gel was quickly poured into a Teflon coated Petri dish and heated at 80 °C during 24 hours in an oven. The product had a brown-reddish glassy aspect. Then, it was easily grounded into powder in an agate mortar. The powder was placed into a porcelain crucible and fired at temperatures between 400 and 1000 °C for 2 hours. The reaction scheme and the flowchart of the experimental procedure are illustrated in Figure 1. The identification of samples is related to the molar ratio between cations Co2+ and Gd3+ or Sm3+. For example, the pure cobalt ferrite (CoFe2O4) is named 1:0, a gadolinium substituted cobalt ferrite, with a 4 % Co0.96Gd0.04Fe2O4.02),is named 0.96:0.04Gd, and a samarium substituted cobalt ferrite, with an 8 % (Co0.92Sm0.08Fe2O4.04), is named 0.92:0.08Sm.
Figure 1. Reaction scheme (a) and flowchart of the experimental procedure (b).
2.2 Characterization of Co1-xGdxFe2O4+δ and Co1-xSmxFe2O4+δ Crystalline phases identification of all products was performed by Powder X-Ray Diffraction using a Philips X´Pert PW3040 diffractometer with a Ni-filtered CuKα radiation (λ=1.54183 Å) in a range from 10 to 80 °(2θ) and using a step size of 0.035 °(2θ) for 2 s. For the structural refinement of selected samples it was used a Rigaku Ultima IV diffractometer with CuKα radiation (λ=1.54183 Å), in a range from 5 to 105 ° (2θ), using a step size of 0.02 °(2θ) for 10 s and a scintillation counter. Rietveld analysis of XRD patterns were carried out using the FullProf software and the starting structural parameters provided by T.A.S. Ferreira et al [26] (considering only the
oxygen positions as free parameters). Room temperature Raman spectra were recorded in a Jobin–Yvon Horiba XploRA Raman Spectrometer equipped with a 50x objective and the samples were excited using an argon laser beam of 638 nm (using the 50 % of its power). Morphological and structural studies of selected samples were carried out using a Titan 80-300 FEI Transmission Electron Microscope. Magnetic properties were studied in a Vibrating Sample Magnetometer (Physical Property Measurement System, from Quantum Design). 57Fe transmission Mössbauer spectra were recorded at 300 and 77K by means of a conventional constant acceleration vibrating electromagnetic transducer using a 57Co/Rh source. In addition, in-field spectra were obtained using a cryomagnetic device with an external magnetic field oriented parallel to the gamma-beam. The powdered samples consist of 5 mg Fe·cm-2 while the isomer shift values are quoted to that of α-Fe standard at 300K.
III. Results and discussion The diffractograms of fired samples 1:0, at 400, 500, 600, 700, 800, 900, and 1000 °C for 2 hours, correspond to a single crystalline phase, identified as the cubic spineltype structure of CoFe2O4. The crystallinity and the crystallite size (calculated using the Scherrer equation for 311 planes), increases with the rising of the temperature of firing, as a consequence of a grain boundary enlargement which minimize the total surface energy. The smallest value of crystallite size is 13 nm for CoFe2O4 treated at 400 °C and the largest value is 45 nm for CoFe2O4 treated at 1000 °C (350 % bigger). The growth rate of crystallite size, throughout the temperatures tested, shows that the growth of crystallite is notoriously increased from 700 °C. While
crystallites of samples treated at 500 and 600 °C are 1.5 and 1.7 times larger than those obtained at 400 °C, crystallites of samples treated at 700-1000 °C are 2.9-3.5 times larger. Figure 2 shows the XRD experimental pattern of the CoFe2O4 treated at 600 °C compared to the reported pattern in the card 22-1086 of the ICDD-Powder Diffraction File database (bottom) and the evolution of crystallite size as a function of firing temperature (top).
Figure 2. Comparison of experimental and reported XRD patterns of CoFe2O4 (bottom). Crystallite size(temperature) graph (top).
The partial substitution of Co2+ by Gd3+ during the synthesis of Co1-xGdxFe2O4+δ, x= 0.02,0.04,0.06,0.08,0.1, and 0.15, leads to crystalline powders with cubic spinel-type structure close to that of CoFe2O4 (PDF 22-1086); however, their intensity of peaks and their sharpness decrease as the gadolinium content increases, see Figure 3a. Another effect of the increasing of Gd3+ amount is an evident decrease of the crystallite size of nanoparticles, see Figure 3b. There are some possible causes for this effect, among them: the strain of the lattice induced either by the difference of ionic radii [25], or by the difference of valence [27], or by the decrease of the nucleation energy [28]. The comparison of crystallite sizes, calculated using Scherrer equation, of CoFe2O4 with samples with x from 0.02 to 0.15 shows a decrease ranging from 23 to 48 %, respectively.
Figure 3. a) XRD patterns of 1:0, 0.96:0.04Gd, and 0.92:0.08Gd and b) Crystallite size versus gadolinium content.
To carry out more detailed microstructural and structural analyses of the substitution of Co2+ by Gd3+ or Sm3+, 1:0, 0.96:0.04Gd, 0.92:0.08Gd, 0.96:0.04Sm, and 0.92:0.08Sm powdered samples were characterized by XRD, under conditions
described in Section 2.2. The results of the refinement of the XRD patterns of these samples, using the crystallographic data reported in card number 98553 from the ICSD database, are reported in Figure 4. Figures 4a, 4c, and 4d show both experimental (x symbol) and calculated (line) diffractograms, reported Bragg position to cobalt ferrite (bars), and the difference between experimental and calculated data (bottom) of Co1-xGdxFe2O4+δ with x= 0, 0.04, and 0.08, respectively. All diffraction peaks are assigned to those in PDF 22-1086, which suggests the complete incorporation of Gd3+ ions in the spinel-type host lattice (within the sensitivity limits of the technique). The increase of gadolinium content leads to an obvious decrease in the intensity of signals and to a subtle shift, towards lower 2θ values. For example, the most intense signals (311) of 1:0, 0.96:0.04Gd, and 0.92:0.08Gd, in d-spacing values, are in 2.5267(2), 2.5271(3), and 2.5304(4) Å, respectively. These results support the previous statement, which could be the cause of the detriment of crystallinity as the gadolinium content is increased. The table in Figure 4b shows the crystallite sizes and lattice parameters obtained of the whole-pattern fitting (using the Halder-Wagner and Rietveld methods, respectively). The values of crystallite size of substituted samples, 0.96:0.04Gd (15 nm) and 0.92:0.08Gd (11 nm), are in good agreement with those values obtained using the Scherrer equation, which show a tendency to obtaining smaller coherent crystalline domains as x is larger in Co1xGdxFe2O4+δ.
Some probable reasons of this behavior are mentioned above. By
contrast, lattice parameters “a” remain almost Gd/Sm content independent.
Figure 4. XRD experimental and Rietveld-calculated patterns of 1:0 (a), 0.96:0.04Gd (c), and 0.92:0.08Gd (d) samples. (b) Crystallite sizes and “a” lattice parameters.
Figure 5 shows the experimental XRD patterns of 1:0, 0.96:0.04Sm, and 0.92:0.08Sm. All the peaks in these diffractograms correspond to the cubic spineltype structure associated to CoFe2O4 (PDF 22-1086). In this case, as in Co1xGdxFe2O4+δ,
peak intensity and sharpness decrease with increasing samarium
content. Another similitude found is in the crystallite size behavior of Co1xSmxFe2O4+δ,
which diminishes from 22 nm (x= 0) to 17 nm (x= 0.04) and 15 nm (x=
0.08). It is probable that the causes of this effect, in both cases (Gd or Sm
substitution), are the same. The whole-pattern fitting (to calculate the crystallite sizes and lattice parameters using the Halder-Wagner and Rietveld methods, respectively) of 0.96:0.04Sm and 0.92:0.08Sm show a similar behavior to that observed in cobalt ferrite doped with gadolinium. The lattice parameters are: 8.3764(3) and 8.3758(3) Å, and their crystallite sizes are: 18 and 13 nm, to 0.96:0.04Sm and 0.92:0.08Sm, respectively.
Figure 5. a) XRD patterns of 1:0, 0.96:0.04Sm, and 0.92:0.08Sm and b) Crystallite size values and “a” lattice parameters.
Room-temperature Raman spectra of 1:0, 0.96:0.04Sm, and 0.92:0.08Sm show five absorption bands at ~176, 286, 447, 583, and 656 cm-1 (in the region 100-1000 cm-
1
), in all samples, see Figure 6. On one hand, all these signals could be associated
to Raman active modes of the cubic inverse spinel-type structure of CoFe2O4 [29]. On the other hand, the absence of other signals discards the presence of secondary phases, as samarium oxide (Sm2O3) or hematite (α-Fe2O3, typically found in ferrites). The most obvious difference between pure (1:0) and doped (0.96:0.04Sm and 0.92:0.08Sm) samples is the small signal at ~583 cm-1, which is normally associated to the vibrations of the metal in the tetrahedral positions of the spinel-type structure [29]. It suggests that the inversion grade of the spinel is modified when small amounts of Co2+ are substituted by Sm3+ or Gd3+, in Co1-xSmxFe2O4+δ or Co1xGdxFe2O4+δ,
respectively. These results confirm that a complete incorporation of
Sm3+ ions is occurring in the spinel-type host lattice, previously suggested by Rietveld to Gd or Sm-doped cobalt ferrites.
Figure 6. Raman spectra of 1:0, 0.96:0.04Gd, and 0.92:0.08Gd samples.
Morphology and microstructural characteristics of Co0.92Gd0.08Fe2O4.04 and Co0.96Sm0.04Fe2O4.02 were analyzed using TEM and High-Resolution TEM (HRTEM) techniques. Figures 7a and 7b illustrate selected images of Co0.92Gd0.08Fe2O4.04 and Co0.96Sm0.04Fe2O4.02, respectively. TEM-images of both samples show small agglomerates formed of particles with different shapes and sizes. The average of particle sizes of Co0.92Gd0.08Fe2O4.04 and Co0.96Sm0.04Fe2O4.02 coincides with the crystallite size previously determined by XRD, ~13 (Co0.92Gd0.08Fe2O4.04) and ~17 nm (Co0.96Sm0.04Fe2O4.02). HRTEM-images and selected-area electron-diffraction (SAED) patterns show interplanar distances that can be associated to those previously reported to CoFe2O4, although they are slightly shifted. A probable cause of this displacement could be the strain of the lattice, induced by the partial substitution of Co2+ by Gd3+ or Sm3+. SAED ring patterns are characteristic of polycrystalline powders. Energy-dispersive X-ray spectroscopy (EDS) corroborate the presence of Gd3+ or Sm3+ in Co0.92Gd0.08Fe2O4.04 or Co0.96Sm0.04Fe2O4.02, respectively (see Figure 8).
Figure 7. TEM and HRTEM images of Co0.92Gd0.08Fe2O4.04 and Co0.96Sm0.04Fe2O4.02.
Figure 8. EDS spectra of Co0.92Gd0.08Fe2O4.04 and Co0.96Sm0.04Fe2O4.02.
All materials synthesized in this investigation, Co1-xGdxFe2O4+δ (x = 0, 0.02, 0.04, 0.06, 0.08, 0.1, and 0.15) and Co1-xSmxFe2O4+δ (x = 0.04, and 0.08), behave as ferrimagnets, typically associated to spinel cubic ferrites. As reported in Figure 9, the room-temperature magnetization curves and saturation magnetization (Ms), remanence (Mr), squareness ratio (R), and coercivity (Hc) values to Co1-xGdxFe2O4+δδ (a) and Co1-xSmxFe2O4+δδ (b) with x = 0, 0.04, and 0.08. Ms, Mr, and Hc of doped samples (Co1-x(Gd/Sm)xFe2O4+δ) decrease in comparison to those obtained to pure cobalt ferrite, although there is not a regular trend (see Figure 10). On one hand, magnetization of inverse spinel cubic ferrites is strongly associated to the net ionic moment of the antiparallel arrangement of their tetrahedral (Fe) and octahedral [M,Fe] sublattices. When the synthesis conditions lead to the thermodynamic equilibrium, the spinel ferrites could reach the antiparallel arrangement of their sublattices and exhibit the theoretically calculated saturation magnetization; however, it is not frequently reached. Three factors that probably alter the net ionic moment of Co1-x(Gd/Sm)xFe2O4+δ are: i) The more complex electronic interactions 4f-3d (than those 3d-3d), associated to the partial substitution of Co2+ (with 3 unpaired electrons in 3d orbitals) by Gd3+ (with 7 unpaired electrons in 4f orbitals) or Sm3+ (with 5 unpaired electrons in 4f orbitals), ii) the change in cationic preferences, induced by the partial substitution, which modifies the extent of anti-site defects, and iii) the reduction of the average particle size observed as x increases in Co1x(Gd/Sm)xFe2O4+δ,
associated to a bigger surface area of the particles (which lacks
the antiparallel arrangement) and becomes very large with respect to their volume. On the other hand, coercivity of nanocrystals of spinel cubic ferrites is determined by magnetic domain wall motion [30]; however, in Co1-x(Gd/Sm)xFe2O4+δ, it could vary because of changes in its magnetocrystalline anisotropy [31, 32] or its crystallite size
[33]. Indeed, if the system is composed, as it is observed in the TEM images recorded in this investigation, by nanoparticles below the single magnetic domain, the coercivity is only governed by their particle size. In order to improve the anisotropy of Co1-x(Gd/Sm)xFe2O4+δ, to increase its coercivity (as the rare-earth elements are introduced in the system), it would be necessary to increase their particle size. This can be achieved increasing the time or temperature of the thermal treatments, which gives place to bigger particles.
Figure 9. Hysteresis loops and Ms, Mr, R, and Hc values of Co1-xGdxFe2O4+δδ (a) and Co1-xSmxFe2O4+δδ (b) with x= 0, 0.04, and 0.08.
Figure 10. Variation of saturation magnetization (a) and coercive field (b) as a function of gadolinium content in Co1-xGdxFe2O4+δδ.
The 57Fe Mössbauer spectra, recorded at 300 K, consist of magnetic sextets with broadened lines resulting from the disorder induced by the presence of Gd3+ or Sm3+ in Co1-x(Gd/Sm)xFe2O4+δ, as it is illustrated in Figure 11. They must be decomposed into several magnetic components, but different fitting models can be considered because of the complex hyperfine structure. At this stage one can conclude to the presence of only Fe3+ species, taking into account of the mean value of isomer shift. Figure 12 depicts representative Mössbauer spectra of 0.92:0.08Gd and 0.92:0.08Sm under different conditions. At 77 K (Figure 12-top), the spectra exhibit a better resolved hyperfine structure which can be easily and perfectly described, a priori, by means of two magnetic components. The values of isomer shift confirm only the presence of Fe3+ species and allow to discriminate between two sites, tetrahedrally and octahedrally coordinated, as usually expected in the case of ferrites. In order to improve the resolution of the hyperfine structure and to calculate
more accurately the proportions of Fe3+ in those two sites (from the relative absorption area), the samples were studied in-field Mössbauer measurements at low temperature, at 20 K and under the effect of an applied magnetic field of 8T (Figure 12-middle). Indeed, the ferrimagnetic structure expected from Fe network favors a splitting of the magnetic sextet into two magnetic sextets unambiguously attributed to Fe cations occupying octahedral and tetrahedral sites, when the sample is submitted to a large applied external magnetic field (see a review in [34] and references therein). The main component has to be described by means of at least two subcomponents, while a single magnetic sextet with Lorentzian lines is sufficient to reproduce the other component (red and blue lines in Figure 12-middle, respectively). The refined values of the hyperfine parameters of 0.96:0.04Gd, 0.92:0.08Gd, 0.96:0.04Sm, and 0.92:0.08Sm, obtained at 300 and 77 K and of 0.92:0.08Gd and 0.92:0.08Sm obtained at 20 K and 80 T are listed in Table 1. The respective values of isomer shift allow the main (minor) component to the octahedral (tetrahedral) Fe3+ sites to be unambiguously assigned. It is important to emphasize that the respective proportions, 60 and 40 % respectively, calculated from the in-field measurements significantly differ from those estimated at 77K; consequently, the 77 K zero-field spectrum was remodeled by means of three magnetic components with Lorentzian lines, giving rise to a nice agreement with the respective proportions of Fe species obtained from the in-field experiments (blue rows in Table 1). The use of two magnetic components to describe the octahedral Fe sites suggests that their surroundings are more distorted than those of tetrahedral Fe sites, associated to the presence of Gd3+ or Sm3+ ions in the octahedral positions of the Co1x(Gd/Sm)xFe2O4+δ
lattice. According to the in-field Mössbauer, the presence of Gd or
Sm orthoferrites must be excluded: Indeed, their antiferromagnetic behaviour would
give rise to a component that can be easily observed on the field spectra (effective field quite similar to the hyperfine field characteristic of orthoferrite, reported in [35], and large intermediate line resulting from the perpendicular orientation of the magnetic moments with respect to the external field, i.e. gamma ray).
Figure 11. 57Fe Mössbauer spectra, recorded at 300 K, of Co1-x(Gd/Sm)xFe2O4+δ.
The second feature is related to the intensities of outer, middle, and inner lines of the Mössbauer spectra; those present in the spectra recorded at 77 K show the ratio 3:2:1 (respectively), characteristic in polycrystalline powdered materials (Figure 12-
top/bottom). The study of 0.92:0.08Gd and 0.92:0.08Sm, under the effect of an applied magnetic field of 8 T at 20 K, reveals that the intensity of the middle lines is smaller than that of those present in the spectra recorded at 77 K; it is thus possible to estimate the angle (θ) between the gamma-beam (parallel to the applied magnetic field) and the Fe magnetic moment, for both tetrahedral and octahedral Fe species. Then the total effective field at the nucleus results from the vectorial sum of the hyperfine field and the applied field, one can establish the following expression:
B2hf = B2eff + B2app - 2BeffBapp cosθ
which allows the hyperfine field to be estimated, as listed in Table 1. The canting of Fe magnetic moments is due to the non colinear magnetic structure resulting from both the frustrated nature of some magnetic interactions and the local anisotropy of Co2+ cations. In addition, the Mössbauer studies accomplished seems to reveal that there is not a significant difference between the octahedral or tetrahedral occupancy of Fe3+ cations when the amount of Gd3+ or Sm3+ increases from 0.04 to 0.08 in Co1x(Gd/Sm)xFe2O4+δ.
Figure 12. 57Fe Mössbauer spectra of 0.92:0.08Gd and 0.92:0.08Sm recorded at 77 K (top), and at 20 K and 80 T (middle), and at 77K (bottom) with the second fitting model (see text).
Table 1. Refined values of the hyperfine parameters of 0.96:0.04Gd, 0.92:0.08Gd, 0.96:0.04Sm, and 0.92:0.08Sm (see text).
Sample
0.96:0.04Gd
T (K) ±1 300 77 300 77 st
1 model
0.92:0.08Gd
20 8T 77 nd
2 model
0.96:0.04Sm
300 77 300 77 st
0.92:0.08Sm
1 model
20 8T 77 nd
2 model
IS (mm/s) ± 0.01 <0.33> 0.49 0.39 <0.32> 0.49 0.40 <0.50> 0.33 <0.48> 0.37 <0.33> 0.51 0.40 <0.30> 0 .50 0.29 <0.50> 0.36 0.48 0.37
2ε (mm/s) ± 0.01 <0.00> 0.01 -0.01 <0.01> 0.04 -0.02 <-.01> 0.03 <0.00> 0.00 <0.03> 0.04 -0.01 <-0.02> 0.03 -0.01 <-0.02> -0.01 <-0.01> 0.00
Beff (T) ± 0.5
θ (°) ±5
<47.1> 58.9
<26> 16
<47.2> 59.5
<31> 21
Bhf (T) ± 0.5 <47.8> 53.9 50.9 <47.1> 53.9 50.8 <54.4> 51.2 <53.0> 50.3 <47.8> 54.0 50.9 <47.4> 53.9 50.9 <54.2> 52.1 <52.9> 50.6
Absorption area ±1 100 40 60 100 35 65 61 39 60 40 100 40 60 100 37 63 62 38 61 39
Fe Site Mixed Octa Tetra Mixed Octa Tetra Octa Tetra Octa Tetra Mixed Octa Tetra Mixed Octa Tetra Octa Tetra Octa Tetra
IV. Conclusions Co1-xGdxFe2O4+δ (x = 0, 0.02, 0.04, 0.06, 0.08, 0.1, and 0.15) and Co1-xSmxFe2O4+δ (x = 0.04, and 0.08), with a cubic spinel-type structure, were synthesized through the citrate precursors route at temperatures as low as 400 °C. Their crystallite size decreases, as a function of Gd3+ or Sm3+ content, up to a 41 %. Rietveld refinement and Raman spectra reveal that Gd3+ (or Sm3+) ions, in Co1-x(Gd/Sm)xFe2O4+δ (x = 0.04, and 0.08), occupy preferentially Co positions in inverse spinel ferrites. TEM and HRTEM corroborates polycrystalline nanometric particles, with different shapes and sizes, of Co1-x(Gd/Sm)xFe2O4+δ. Saturation magnetization and coercivity values are lower than those obtained to CoFe2O4 probably due to the reduction of the average
particle size observed as x is larger in Co1-x(Gd/Sm)xFe2O4+δ, more complex electronic interactions 4f-3d (than those 3d-3d), and changes in its magnetocrystalline anisotropy. Mössbauer studies reveal the presence of only Fe3+ ions, distributed in a proportion of 60 and 40 % through octahedral and tetrahedral sites, respectively. Although the formation of the mixed spinel, rather than different substances (Sm or Gd oxide), fall in the sensitivity limits of the XRD technique all the results demonstrate that the replacement of Co2+ by Gd3+ or Sm3+ ions, in CoFe2O4, can tailor the crystal structure, microstructure, and magnetic properties of Co1x(Gd/Sm)xFe2O4+δ.
Acknowledgments The authors thank to Giberto Hurtado, Rosario Rangel, Mónica Ceniceros and Enrique Díaz for recording VSM analysis, Raman spectra, EDS spectra and TEM images, respectively. This project has been financially supported by CIQA (grant numbers: 6238 and 6311) and CONACyT (grant number: PEI 2018-250174). A. Tijerina-Rosa thanks to CONACYT the economic support provided.
References [1] Valenzuela R., Magnetic ceramics, Cambridge University Press: New York; 1994. [2] K.K. Kefeni, T.A.M. Msagati, B.B. Mamba, Ferrite nanoparticles: Synthesis, characterisation and applications in electronic device, Mater. Sci. Eng. B 215 (2017) 37-55. https://doi.org/10.1016/j.mseb.2016.11.002. [3] N.M. Sánchez-Padilla, S.M. Montemayor, F.J. Rodríguez-Varela, An easy route to synthesize novel Fe3O4@Pt core/shell nanostructures with high electrocatalytic activity, J New Mat Electr Sys, 15 (2012) 171-179. https://doi.org/10.14447/jnmes.v15i3.62. [4] N.M. Sánchez-Padilla, S.M. Montemayor, A. Torres, F.J. Rodríguez-Varela, Fast Synthesis and Electrocatalytic Activity of M@Pt (M=Ru, Fe3O4, Pd) Core-Shell Nanostructures for the Oxidation of Ethanol and Methanol, Int J Hydrogen Energy, 38 (2013) 12681-12688. https://doi.org/10.1016/j.ijhydene.2012.11.026. [5] N.M. Sánchez-Padilla, D. Morales-Acosta, M.D. Morales-Acosta, S.M. Montemayor, F.J. Rodríguez-Varela, Catalytic Activity and Selectivity for the ORR of Rapidly Synthesized M@Pt (M=Pd, Fe3O4, Ru) Core-Shell Nanostructures, Int J Hydrogen Energy, 39 (2014) 16706-16714. https://doi.org/10.1016/j.ijhydene.2014.03.223. [6] M.L. Martins, M.F. Calabresi, C. Quini, J.F. Matos, J.R.A. Miranda, M.J. Saeki, H.N. Bordallo, Enhancing the versatility of alternate current biosusceptometry (ACB) through the synthesis of a dextrose-modified tracer and a magnetic muco-adhesive
cellulose gel, Mater. Sci. Eng. C, 48 (2015) 80-85. https://doi.org/10.1016/j.msec.2014.11.059. [7] A. Makridis, I. Chatzitheodorou, K. Topouridou, M.P. Yavropoulou, M. Angelakeris, C. Dendrinou-Samara, A facile microwave synthetic route for ferrite nanoparticles with direct impact in magnetic particle hyperthermia, Mater. Sci. Eng. C, 63 (2016) 663-670. https://doi.org/10.1016/j.msec.2016.03.033. [8] V.R. Mudinepalli, P.C. Chang, C.C. Hsu, F.Y. Lo, H.W. Chang, W.C. Lin, Electricfield effects on magnetism of Fe/NCZF/PZT composite thin film, J Magn Magn Mater, 432 (2017) 90-95. https://doi.org/10.1016/j.jmmm.2016.12.141. [9] X. Yang, Z. Zhou, T. Nan, Y. Gao, G.M. Yang, M. Liu M, et al, Recent advances in multiferroic oxide heterostructures and devices. J Mater Chem C 4 (2016) 234-243. http://dx.doi.org/10.1039/C5TC03008K. [10] Reddy DHK, Yun YS. Spinel ferrite magnetic adsorbents: Alternative future materials for water purification? Coordination Chemistry Reviews 2016, 315:90-111. [11] D.H.K. Reddy, S.M. Lee, Magnetic biochar composite: Facile synthesis, characterization, and application for heavy metal removal, Colloids Surf A Physicochem Eng Asp, 454 (2014) 96-103. https://doi.org/10.1016/j.colsurfa.2014.03.105. [12] W.H. Bragg, The structure of the spinel group of crystals, Philos Mag, 30 (1915) 305-315. https://doi.org/10.1080/14786440808635400. [13] S. Nishikawa, Structure of Some Crystals of Spinel Group, Proceedings of the Tokyo Mathematico-Physical Society, 8 (1915) 199–209. https://doi.org/10.11429/ptmps1907.8.7_199.
[Dataset] [14] McMurdie HF, Morris MC, Evans EH, Paretzkin B, Wong-Ng W, Ettlinger L, et al. Standard X-Ray Diffraction Powder Patterns from the JCPDS Research Associateship, Powder Diffraction 1 (1986) 64-77. [15] M. Siddique, N.M. Butt, Effect of particle size on degree of inversion in ferrites investigated by Mossbauer spectroscopy, Physica B, 405 (2010) 4211-4215. https://doi.org/10.1016/j.physb.2010.07.012. [16] M. Yehia, S.M. Ismail, A. Hashhash, Structural and Magnetic Studies of RareEarth Substituted Nickel Ferrites, J Supercond Nov Magn, 27 (2014) 771–774. https://doi.org/10.1007/s10948-013-2340-z. [17] D. Peddis, N. Yaacoub, M. Ferretti, A. Martinelli, G. Piccaluga, A. Musinu, et al. 2011. Cationic distribution and spin canting in CoFe2O4 nanoparticles. J Phys Condens Matter. 23, 426004. https://doi.org/10.1088%2F09538984%2F23%2F42%2F426004. [18] H. Shokrollahi, L. Avazpour, Influence of intrinsic parameters on the particle size of magnetic spinel nanoparticles synthesized by wet chemical methods, Particuology, 26 (2016) 32–39. https://doi.org/10.1016/j.partic.2015.10.004. [19] A. Manikandan, R. Sridhar, S.A. Antony, S. Ramakrishna, A simple aloe vera plant-extracted microwave and conventional combustion synthesis: Morphological, optical, magnetic and catalytic properties of CoFe2O4 nanostructures, J Mol Struct, 1076 (2014) 188–200. https://doi.org/10.1016/j.molstruc.2014.07.054. [20] P. Kumar, G. Rana, G. Dixit, A. Kumar, V. Sharma, R. Goyal, et al., Structural, electrical and magnetic properties of dilutely Y doped NiFe2O4 nanoparticles, J Alloys Compd, 685 (2016) 492-497. https://doi.org/10.1016/j.jallcom.2016.05.248.
[21] A. Ditta, M.A. Khan, M. Junaid, R.M.A. Khalil, M.F. Warsi, Structural, magnetic and spectral properties of Gd and Dy co-doped dielectrically modified Co-Ni (Ni0.4Co0.6Fe2O4) ferrites, Physica B, 507 (2017) 27–34. https://doi.org/10.1016/j.physb.2016.11.030. [22] E.E. Ateia, G. Abdelatif, M.A. Ahmed, M.A.A. Mahmoud, Effect of Different Gd3+ Ion Content on the Electric and Magnetic Properties of Lithium Antimony Ferrite, J Inorg Organomet Polym, 26 (2016) 81–90. https://doi.org/10.1007/s10904-015-02835. [23] S.S. Laha, E. Abdelhamid, M.P. Arachchige, A. Kumar, A. Dixit, Ferroic ordering and charge-spin-lattice order coupling in Gd-doped Fe3O4 nanoparticles relaxor multiferroic system, J Am Ceram Soc, 100 (2017) 1534–1541. https://doi.org/10.1111/jace.14739. [24] D. Padalia, U.C. Johri, M.G.H. Zaidi, Effect of cerium substitution on structural and magnetic properties of magnetite nanoparticles, Mater Chem Phys 169 (2016) 89-95. https://doi.org/10.1016/j.matchemphys.2015.11.034. [25] C. Wang, Y. Wang, A. Zhang, Y. Cheng, F. Chi, et al., The influence of ionic radii on the grain growth and sintering-resistance of Ln2Ce2O7 (Ln= La, Nd, Sm, Gd), J Mater Sci 48 (2013) 8133–8139. https://doi.org/10.1007/s10853-013-7625-x. [26] T.A.S. Ferreira, J.C. Waerenborgh, M.H.R.M. Mendonça, M.R. Nunes, F.M. Costa, Structural and morphological characterization of FeCo2O4 and CoFe2O4 spinels prepared by a coprecipitation method, Solid State Sci, 5 (2003) 383–392. https://doi.org/10.1016/S1293-2558(03)00011-6.
[27] D. Chen and Y. Wang, Impurity doping: a novel strategy for controllable synthesis of functional lanthanide nanomaterials, Nanoscale 5 (2013) 4621-4637. http://dx.doi.org/10.1039/C3NR00368J. [28] C. Xu, M. Ma, L. Yang, S. Zeng, Q. Yang, Lanthanide doping-facilitated growth of ultrasmall monodisperse Ba2LaF7 nanocrystals with excellent photoluminescence, J Colloid Interface Sci, 368 (2012) 49-55. https://doi.org/10.1016/j.jcis.2011.10.072. [29] S.R. Naik and A.V. Salker, Change in the magnetostructural properties of rare earth doped cobalt ferrites relative to the magnetic anisotropy, J Mater Chem, 22 (2012) 2740-2750. http://dx.doi.org/10.1039/C2JM15228B. [30] Y. Matsuura, N. Kitai, S. Hosokawa, J. Hoshijima, Relation between the alignment dependence of coercive force decrease ratio and the angular dependence of coercive force of ferrites, J Magn Magn Mater, 411 (2016) 1-6. https://doi.org/10.1016/j.jmmm.2016.03.007. [31] M. Colarieti-Tosti, S.I. Simak, R. Ahuja, L. Nordstrom, O. Eriksson, M.S.S. Brooksa, Theory of the magnetic anisotropy of Gd metal, J Magn Magn Mater, 272– 276 (2004) e201-e202. https://doi.org/10.1016/j.jmmm.2003.12.655. [32] S. Thankachan, B.P. Jacob, S. Xavier, E.M. Mohammed, Effect of samarium substitution on structural and magnetic properties of magnesium ferrite nanoparticles, J Magn Magn Mater, 348 (2013) 140-145. https://doi.org/10.1016/j.jmmm.2013.07.065. [33] H. Yangy, Z. Wangy, L. Songy, M. Zhaoy, J. Wangz, H. Luoz, A study on the coercivity and the magnetic anisotropy of the lithium ferrite nanocrystallite, J Phys D, 29 (1996) 2574-2578. https://doi.org/10.1088%2F0022-3727%2F29%2F10%2F008.
[34] J.M. Greneche, The Contribution of 57Fe Mössbauer Spectrometry to Investigate Magnetic Nanomaterials, in: Y. Yoshida and G. Langouche (Eds), Mössbauer Spectroscopy, Springer, Berlin Heidelberg, 2013, pp.187-241. https://doi.org/10.1007/978-3-642-32220-4_4. [35] M. Eibschutz, S. Shtrikman and D. Treves, Mössbauer studies of 57Fe in orthoferrites, Phys. Rev. 156 (1967) 562. https://doi.org/10.1103/PhysRev.156.562.
Figure Captions Figure 1. Reaction scheme (a) and flowchart of the experimental procedure (b). Figure 2. Comparison of experimental and reported XRD patterns of CoFe2O4 (bottom). Crystallite size(temperature) graph (top). Figure 3. a) XRD patterns of 1:0, 0.96:0.04Gd, and 0.92:0.08Gd and b) Crystallite size versus gadolinium content. Figure 4. XRD experimental and Rietveld-calculated patterns of 1:0 (a), 0.96:0.04Gd (c), and 0.92:0.08Gd (d) samples. (b) Crystallite sizes and “a” lattice parameters. Figure 5. a) XRD patterns of 1:0, 0.96:0.04Sm, and 0.92:0.08Sm and b) Crystallite size values and “a” lattice parameters. Figure 6. Raman spectra of 1:0, 0.96:0.04Gd, and 0.92:0.08Gd samples. Figure 7. TEM and HRTEM images of Co0.92Gd0.08Fe2O4.04 and Co0.96Sm0.04Fe2O4.02. Figure 8. EDS spectra of Co0.92Gd0.08Fe2O4.04 and Co0.96Sm0.04Fe2O4.02. Figure 9. Hysteresis loops and Ms, Mr, R, and Hc values of Co1-xGdxFe2O4+δδ (a) and Co1-xSmxFe2O4+δδ (b) with x= 0, 0.04, and 0.08. Figure 10. Variation of saturation magnetization (a) and coercive field (b) as a function of gadolinium content in Co1-xGdxFe2O4+δδ. Figure 11. 57Fe Mössbauer spectra, recorded at 300 K, of Co1-x(Gd/Sm)xFe2O4+δ. Figure 12. 57Fe Mössbauer spectra of 0.92:0.08Gd and 0.92:0.08Sm recorded at 77 K (top), and at 20 K and 80 T (middle), and at 77K (bottom) with the second fitting model (see text).
Table Captions
Table 1. Refined values of the hyperfine parameters of 0.96:0.04Gd, 0.92:0.08Gd, 0.96:0.04Sm, and 0.92:0.08Sm (see text).
S0272-8842(19)32161-3
DOI:
https://doi.org/10.1016/j.ceramint.2019.07.335
Reference:
CERI 22450
To appear in:
Ceramics International
Received Date: 26 April 2019 Revised Date:
4 July 2019
Accepted Date: 29 July 2019
Please cite this article as: A. Tijerina-Rosa, J.M. Greneche, A.F. Fuentes, J. Rodriguez-Hernandez, J.L. Menéndez, F.J. Rodríguez-González, S.M. Montemayor, Partial substitution of cobalt by rare-earths (Gd or Sm) in cobalt ferrite: Effect on its microstructure and magnetic properties, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.07.335. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Title: “Partial Substitution of Cobalt by Rare-Earths (Gd or Sm) in Cobalt Ferrite: Effect on its Microstructure and Magnetic Properties” Author names and affiliations: A. Tijerina-Rosaa,b,1, J. M. Grenechec, A. F. Fuentesd, J. Rodriguez-Hernandeza, J. L. Menéndeze, F. J. Rodríguez-Gonzáleza, Sagrario M. Montemayora* a
Departamento de Materiales Avanzados, Centro de Investigación en Química Aplicada, Blvd. Enrique Reyna No. 140, Saltillo, Coahuila, México. b
Facultad de Ciencias Químicas, Universidad Autónoma de Coahuila, V. Carranza esq. J. Cárdenas s/n, Saltillo, Coahuila, 25280, México. c
Institut des Molécules et Matériaux du Mans-IMMM UMR CNRS 6283, Université du Maine, Avenue Olivier Messiaen, 72085 Le Mans cedex France. d
CINVESTAV-Unidad Saltillo, Ave. Industrial Metalúrgica 1062, Parque Industrial Saltillo-Ramos Arizpe, Ramos Arizpe, Coahuila, 25900, México. e
Centro de Investigación en Nanomateriales y Nanotecnología, CINN-CSIC. Principado de Asturias-Consejo superior de Investigaciones Científicas (CSIC)Universidad de Oviedo (UO). Avda. de la Vega 4-6, 33940 San Martín del Rey Aurelio (Asturias), Spain. 1
Tecnológico Nacional de México, Instituto Tecnológico de Saltillo, Blvd. Venustiano Carranza #2400, Colonia Tecnológico, Saltillo, Coahuila, México, C.P. 25280, División de Estudios de Posgrado e Investigación.
Corresponding author: Sagrario M. Montemayor
Contact details:
E-mail address: [email protected] or [email protected]
Postal mail address: Blvd. Enrique Reyna Hermosillo No. 140, Col. San José de los Cerritos, C.P. 25294, Saltillo, Coahuila, México.
Telephone: (+52) 844 4389830 ext.1421
Fax number: (+52) 844 4389839
Partial Substitution of Cobalt by Rare-Earths (Gd or Sm) in Cobalt Ferrite: Effect on its Microstructure and Magnetic Properties
Abstract: Microstructure and magnetic properties of partially substituted cobalt ferrite, with small amounts of gadolinium or samarium, were investigated. Co1-xGdxFe2O4+δ (x = 0, 0.02, 0.04, 0.06, 0.08, 0.1, and 0.15) and Co1-xSmxFe2O4+δ (x = 0.04, and 0.08) were obtained through the citrate precursors route. X-ray powder diffraction patterns, of samples treated at temperatures between 400 and 1000 °C, are consistent with a cubic spinel-type structure. Substituted ferrites, with x = 0.04 and 0.08 (600 °C/2 h), show the combination of maximum enhancement of crystallinity and minimum change of the lattice parameters. At this temperature and composition range the crystallite size decreases up to a 41 %. Rietveld analyses of Co1-x(Gd/Sm)xFe2O4+δ (x = 0, 0.04, and 0.08), carried out considering that Gd3+ (or Sm3+) occupies Co2+ positions, give rise to a good quality of refinement and cell parameters which are slightly smaller than those provided in the crystallographic database of CoFe2O4. Raman spectra of Gd3+ or Sm3+ substituted ferrites show the characteristic Raman modes expected in inverse spinel ferrites without additional signals. TEM micrographs reveal the presence of small agglomerates of nanoparticles. SAED patterns and HRTEM micrographs, are characteristic of polycrystalline powders while the interplanar distances well correspond to those of CoFe2O4. EDS analyses confirm the presence of Gd or Sm in substituted ferrites. Magnetic hysteresis loops of samples suggest a ferrimagnetic behavior and their squareness ratio values decrease up to 36 % in Co1-xSmxFe2O4+δ (x = 0.04). Mössbauer studies reveal the
presence of only Fe3+ ions, distributed in a proportion of 60 and 40 % through octahedral and tetrahedral sites, respectively and a canted ferrimagnetic structure. Although the formation of the mixed spinel, rather than different substances (Sm or Gd oxide), fall in the sensitivity limits of the XRD technique all the results demonstrate that the replacement of Co2+ by Gd3+ or Sm3+ ions, in CoFe2O4, can tailor the crystal structure, microstructure, and magnetic properties of cobalt ferrite.
Keywords: Rare-earth substituted cobalt ferrite, (Co,Gd)Fe2O4, (Co,Sm)Fe2O4, Microstructure, Magnetic properties
I. Introduction Cobalt ferrite is a ceramic compound of the group of spinel ferrites (MFe2O4, M= Co2+, Cu2+, Mn2+, Ni2+, Zn2+, etc.). Its magnetic properties and well-established applications in high-density magnetic storage devices, electrical power transformer cores, and rod antennas, have attracted, for long time (~80 years), the attention of scientists and technologists [1]. Recently, the ability of tailoring the physical properties of ferrites, to make them useful in modern applications, has renewed the interest toward nanocrystalline ferrites. Some fields where, lately, spinel ferrites have been especially successful include: catalysis (energy devices) [2-5], biomedicine (early detection and treatment of cancer) [6, 7], electronic (spintronics) [8, 9], and environmental remediation (wastewater treatment) [10, 11]. To understand the wide range of physical properties and applications of spinel ferrites it is necessary to be informed about their structural characteristics, previously determined and reported [12, 13]. Briefly, CoFe2O4 (sometimes expressed as
CoO.Fe2O3) crystallizes in the spinel-type structure, with a cubic symmetry and a space group Fd-3m (JCPDS 22-1086) [14]. The unit cell of spinel ferrite contains 8 formula units (M(2+)8Fe(3+)16O32) where the cubic close-packed arrangement of the 32 oxygen ions generates 96 interstices, 64 tetrahedral and 32 octahedral units, available to be occupied by only 24 cations (8 M2+ and 16 Fe3+). The distribution of the di- and trivalent cations through the tetrahedral and octahedral sites varies, depending on several factors as ionic radii and valency of the specific ion, experimental conditions of synthesis, particle size, etc. [15], resulting in a normal, inverse or mixed spinel ferrite. In a normal-spinel, divalent ions occupy tetrahedral sites and trivalent ions occupy octahedral positions. In an inverse-spinel, divalent ions occupy octahedral sites and a half of trivalent ions occupy tetrahedral positions and the other half occupies octahedral sites. Since tetrahedral and octahedral sites are usually represented by parentheses and brackets, respectively, an inverse-spinel ferrite would be represented as (Fe)[MFe]O4. Mixed-spinel ferrites are intermediate cases, between normal and inverse, in which the inversion degree is designated as “x” in (M1-xFex)[MxFe2-x]O4. In addition to these singular structural features, spinel ferrites have a rich redox chemistry, associated to the solid state redox couples M2+/M+ or M3+/M2+ and Fe3+/Fe2+ usually present. The possibility of tailoring the magnetic (and electric, catalytic, etc.) properties of spinel ferrites resides in altering its structure or/and its microstructure, changing their chemical composition [16], cationic distribution [17], crystallite/particle size [18], morphology [19], etc. One way to achieve this is to vary experimental conditions used during the synthesis. Another way is through the partial substitution of cations in MFe2O4. Although there have been successful improvements in the electric and magnetic response of spinel ferrites, through the partial substitution of di- or trivalent
ions by rare earth ions [20-24], the reported investigations are, at least, controversial. This may be due to several factors such as, in example, the large size of rare earth ions, which becomes hard the process of diffusion through the crystalline network to distribute regularly in octahedral sites. Not to mention that different conditions of synthesis can lead to different degrees of diffusion. Another important factor is that, in rare earth doped spinel ferrites, there are complex multiple types of electronic interactions. Those between transition metals (3d-3d) and those between rare earthstransition metal (4f-3d). Depending on the specific rare earth ion and transition metal, and their structural positions, the spin coupling 4f-3d may be parallel or antiparallel, which affects the electric and magnetic response of these materials [25]. To improve the knowledge about how structure and microstructure affect the properties of spinel ferrites, partially substituted by rare earths, is necessary to pursue systematic research. This investigation studies the synthesis, through the citrate precursors route, of partially substituted cobalt ferrite (Co1-xGdxFe2O4+δ or Co1-xSmxFe2O4+δ), replacing cobalt ions (Co2+) by gadolinium (Gd3+) or samarium (Sm3+). In addition, the effect on its microstructure and magnetic properties is reported.
II. Methods The chemical reagents used in this work were citric acid (C6H8O7), ethylene glycol (C2H6O2), cobalt nitrate (Co(NO3)2·6H2O), gadolinium nitrate (Gd(NO3)3·6H2O), samarium nitrate (Sm(NO3)3·6H2O), and iron nitrate (Fe(NO3)3·9H2O). All of them were purchased from Aldrich with reagent grade (> 98 %) and used without further purification.
2.1 Synthesis of Co1-xGdxFe2O4+δ and Co1-xSmxFe2O4+δ The appropriate amounts of citric acid (1 mol) and ethylene glycol (4 mol) were mixed and stirred at 50 °C to obtain a transparent solution. Then, stoichiometric amounts of cobalt (1-x mol), gadolinium or samarium (x mol), and iron (2 mol) nitrates, were added. Once the mixture achieved complete solubility, the temperature was increased up to 80 °C. After approximately 10 minutes, the solution became more viscous and a brown-reddish gas was produced with no visible formation of turbidity. This gel was quickly poured into a Teflon coated Petri dish and heated at 80 °C during 24 hours in an oven. The product had a brown-reddish glassy aspect. Then, it was easily grounded into powder in an agate mortar. The powder was placed into a porcelain crucible and fired at temperatures between 400 and 1000 °C for 2 hours. The reaction scheme and the flowchart of the experimental procedure are illustrated in Figure 1. The identification of samples is related to the molar ratio between cations Co2+ and Gd3+ or Sm3+. For example, the pure cobalt ferrite (CoFe2O4) is named 1:0, a gadolinium substituted cobalt ferrite, with a 4 % Co0.96Gd0.04Fe2O4.02),is named 0.96:0.04Gd, and a samarium substituted cobalt ferrite, with an 8 % (Co0.92Sm0.08Fe2O4.04), is named 0.92:0.08Sm.
Figure 1. Reaction scheme (a) and flowchart of the experimental procedure (b).
2.2 Characterization of Co1-xGdxFe2O4+δ and Co1-xSmxFe2O4+δ Crystalline phases identification of all products was performed by Powder X-Ray Diffraction using a Philips X´Pert PW3040 diffractometer with a Ni-filtered CuKα radiation (λ=1.54183 Å) in a range from 10 to 80 °(2θ) and using a step size of 0.035 °(2θ) for 2 s. For the structural refinement of selected samples it was used a Rigaku Ultima IV diffractometer with CuKα radiation (λ=1.54183 Å), in a range from 5 to 105 ° (2θ), using a step size of 0.02 °(2θ) for 10 s and a scintillation counter. Rietveld analysis of XRD patterns were carried out using the FullProf software and the starting structural parameters provided by T.A.S. Ferreira et al [26] (considering only the
oxygen positions as free parameters). Room temperature Raman spectra were recorded in a Jobin–Yvon Horiba XploRA Raman Spectrometer equipped with a 50x objective and the samples were excited using an argon laser beam of 638 nm (using the 50 % of its power). Morphological and structural studies of selected samples were carried out using a Titan 80-300 FEI Transmission Electron Microscope. Magnetic properties were studied in a Vibrating Sample Magnetometer (Physical Property Measurement System, from Quantum Design). 57Fe transmission Mössbauer spectra were recorded at 300 and 77K by means of a conventional constant acceleration vibrating electromagnetic transducer using a 57Co/Rh source. In addition, in-field spectra were obtained using a cryomagnetic device with an external magnetic field oriented parallel to the gamma-beam. The powdered samples consist of 5 mg Fe·cm-2 while the isomer shift values are quoted to that of α-Fe standard at 300K.
III. Results and discussion The diffractograms of fired samples 1:0, at 400, 500, 600, 700, 800, 900, and 1000 °C for 2 hours, correspond to a single crystalline phase, identified as the cubic spineltype structure of CoFe2O4. The crystallinity and the crystallite size (calculated using the Scherrer equation for 311 planes), increases with the rising of the temperature of firing, as a consequence of a grain boundary enlargement which minimize the total surface energy. The smallest value of crystallite size is 13 nm for CoFe2O4 treated at 400 °C and the largest value is 45 nm for CoFe2O4 treated at 1000 °C (350 % bigger). The growth rate of crystallite size, throughout the temperatures tested, shows that the growth of crystallite is notoriously increased from 700 °C. While
crystallites of samples treated at 500 and 600 °C are 1.5 and 1.7 times larger than those obtained at 400 °C, crystallites of samples treated at 700-1000 °C are 2.9-3.5 times larger. Figure 2 shows the XRD experimental pattern of the CoFe2O4 treated at 600 °C compared to the reported pattern in the card 22-1086 of the ICDD-Powder Diffraction File database (bottom) and the evolution of crystallite size as a function of firing temperature (top).
Figure 2. Comparison of experimental and reported XRD patterns of CoFe2O4 (bottom). Crystallite size(temperature) graph (top).
The partial substitution of Co2+ by Gd3+ during the synthesis of Co1-xGdxFe2O4+δ, x= 0.02,0.04,0.06,0.08,0.1, and 0.15, leads to crystalline powders with cubic spinel-type structure close to that of CoFe2O4 (PDF 22-1086); however, their intensity of peaks and their sharpness decrease as the gadolinium content increases, see Figure 3a. Another effect of the increasing of Gd3+ amount is an evident decrease of the crystallite size of nanoparticles, see Figure 3b. There are some possible causes for this effect, among them: the strain of the lattice induced either by the difference of ionic radii [25], or by the difference of valence [27], or by the decrease of the nucleation energy [28]. The comparison of crystallite sizes, calculated using Scherrer equation, of CoFe2O4 with samples with x from 0.02 to 0.15 shows a decrease ranging from 23 to 48 %, respectively.
Figure 3. a) XRD patterns of 1:0, 0.96:0.04Gd, and 0.92:0.08Gd and b) Crystallite size versus gadolinium content.
To carry out more detailed microstructural and structural analyses of the substitution of Co2+ by Gd3+ or Sm3+, 1:0, 0.96:0.04Gd, 0.92:0.08Gd, 0.96:0.04Sm, and 0.92:0.08Sm powdered samples were characterized by XRD, under conditions
described in Section 2.2. The results of the refinement of the XRD patterns of these samples, using the crystallographic data reported in card number 98553 from the ICSD database, are reported in Figure 4. Figures 4a, 4c, and 4d show both experimental (x symbol) and calculated (line) diffractograms, reported Bragg position to cobalt ferrite (bars), and the difference between experimental and calculated data (bottom) of Co1-xGdxFe2O4+δ with x= 0, 0.04, and 0.08, respectively. All diffraction peaks are assigned to those in PDF 22-1086, which suggests the complete incorporation of Gd3+ ions in the spinel-type host lattice (within the sensitivity limits of the technique). The increase of gadolinium content leads to an obvious decrease in the intensity of signals and to a subtle shift, towards lower 2θ values. For example, the most intense signals (311) of 1:0, 0.96:0.04Gd, and 0.92:0.08Gd, in d-spacing values, are in 2.5267(2), 2.5271(3), and 2.5304(4) Å, respectively. These results support the previous statement, which could be the cause of the detriment of crystallinity as the gadolinium content is increased. The table in Figure 4b shows the crystallite sizes and lattice parameters obtained of the whole-pattern fitting (using the Halder-Wagner and Rietveld methods, respectively). The values of crystallite size of substituted samples, 0.96:0.04Gd (15 nm) and 0.92:0.08Gd (11 nm), are in good agreement with those values obtained using the Scherrer equation, which show a tendency to obtaining smaller coherent crystalline domains as x is larger in Co1xGdxFe2O4+δ.
Some probable reasons of this behavior are mentioned above. By
contrast, lattice parameters “a” remain almost Gd/Sm content independent.
Figure 4. XRD experimental and Rietveld-calculated patterns of 1:0 (a), 0.96:0.04Gd (c), and 0.92:0.08Gd (d) samples. (b) Crystallite sizes and “a” lattice parameters.
Figure 5 shows the experimental XRD patterns of 1:0, 0.96:0.04Sm, and 0.92:0.08Sm. All the peaks in these diffractograms correspond to the cubic spineltype structure associated to CoFe2O4 (PDF 22-1086). In this case, as in Co1xGdxFe2O4+δ,
peak intensity and sharpness decrease with increasing samarium
content. Another similitude found is in the crystallite size behavior of Co1xSmxFe2O4+δ,
which diminishes from 22 nm (x= 0) to 17 nm (x= 0.04) and 15 nm (x=
0.08). It is probable that the causes of this effect, in both cases (Gd or Sm
substitution), are the same. The whole-pattern fitting (to calculate the crystallite sizes and lattice parameters using the Halder-Wagner and Rietveld methods, respectively) of 0.96:0.04Sm and 0.92:0.08Sm show a similar behavior to that observed in cobalt ferrite doped with gadolinium. The lattice parameters are: 8.3764(3) and 8.3758(3) Å, and their crystallite sizes are: 18 and 13 nm, to 0.96:0.04Sm and 0.92:0.08Sm, respectively.
Figure 5. a) XRD patterns of 1:0, 0.96:0.04Sm, and 0.92:0.08Sm and b) Crystallite size values and “a” lattice parameters.
Room-temperature Raman spectra of 1:0, 0.96:0.04Sm, and 0.92:0.08Sm show five absorption bands at ~176, 286, 447, 583, and 656 cm-1 (in the region 100-1000 cm-
1
), in all samples, see Figure 6. On one hand, all these signals could be associated
to Raman active modes of the cubic inverse spinel-type structure of CoFe2O4 [29]. On the other hand, the absence of other signals discards the presence of secondary phases, as samarium oxide (Sm2O3) or hematite (α-Fe2O3, typically found in ferrites). The most obvious difference between pure (1:0) and doped (0.96:0.04Sm and 0.92:0.08Sm) samples is the small signal at ~583 cm-1, which is normally associated to the vibrations of the metal in the tetrahedral positions of the spinel-type structure [29]. It suggests that the inversion grade of the spinel is modified when small amounts of Co2+ are substituted by Sm3+ or Gd3+, in Co1-xSmxFe2O4+δ or Co1xGdxFe2O4+δ,
respectively. These results confirm that a complete incorporation of
Sm3+ ions is occurring in the spinel-type host lattice, previously suggested by Rietveld to Gd or Sm-doped cobalt ferrites.
Figure 6. Raman spectra of 1:0, 0.96:0.04Gd, and 0.92:0.08Gd samples.
Morphology and microstructural characteristics of Co0.92Gd0.08Fe2O4.04 and Co0.96Sm0.04Fe2O4.02 were analyzed using TEM and High-Resolution TEM (HRTEM) techniques. Figures 7a and 7b illustrate selected images of Co0.92Gd0.08Fe2O4.04 and Co0.96Sm0.04Fe2O4.02, respectively. TEM-images of both samples show small agglomerates formed of particles with different shapes and sizes. The average of particle sizes of Co0.92Gd0.08Fe2O4.04 and Co0.96Sm0.04Fe2O4.02 coincides with the crystallite size previously determined by XRD, ~13 (Co0.92Gd0.08Fe2O4.04) and ~17 nm (Co0.96Sm0.04Fe2O4.02). HRTEM-images and selected-area electron-diffraction (SAED) patterns show interplanar distances that can be associated to those previously reported to CoFe2O4, although they are slightly shifted. A probable cause of this displacement could be the strain of the lattice, induced by the partial substitution of Co2+ by Gd3+ or Sm3+. SAED ring patterns are characteristic of polycrystalline powders. Energy-dispersive X-ray spectroscopy (EDS) corroborate the presence of Gd3+ or Sm3+ in Co0.92Gd0.08Fe2O4.04 or Co0.96Sm0.04Fe2O4.02, respectively (see Figure 8).
Figure 7. TEM and HRTEM images of Co0.92Gd0.08Fe2O4.04 and Co0.96Sm0.04Fe2O4.02.
Figure 8. EDS spectra of Co0.92Gd0.08Fe2O4.04 and Co0.96Sm0.04Fe2O4.02.
All materials synthesized in this investigation, Co1-xGdxFe2O4+δ (x = 0, 0.02, 0.04, 0.06, 0.08, 0.1, and 0.15) and Co1-xSmxFe2O4+δ (x = 0.04, and 0.08), behave as ferrimagnets, typically associated to spinel cubic ferrites. As reported in Figure 9, the room-temperature magnetization curves and saturation magnetization (Ms), remanence (Mr), squareness ratio (R), and coercivity (Hc) values to Co1-xGdxFe2O4+δδ (a) and Co1-xSmxFe2O4+δδ (b) with x = 0, 0.04, and 0.08. Ms, Mr, and Hc of doped samples (Co1-x(Gd/Sm)xFe2O4+δ) decrease in comparison to those obtained to pure cobalt ferrite, although there is not a regular trend (see Figure 10). On one hand, magnetization of inverse spinel cubic ferrites is strongly associated to the net ionic moment of the antiparallel arrangement of their tetrahedral (Fe) and octahedral [M,Fe] sublattices. When the synthesis conditions lead to the thermodynamic equilibrium, the spinel ferrites could reach the antiparallel arrangement of their sublattices and exhibit the theoretically calculated saturation magnetization; however, it is not frequently reached. Three factors that probably alter the net ionic moment of Co1-x(Gd/Sm)xFe2O4+δ are: i) The more complex electronic interactions 4f-3d (than those 3d-3d), associated to the partial substitution of Co2+ (with 3 unpaired electrons in 3d orbitals) by Gd3+ (with 7 unpaired electrons in 4f orbitals) or Sm3+ (with 5 unpaired electrons in 4f orbitals), ii) the change in cationic preferences, induced by the partial substitution, which modifies the extent of anti-site defects, and iii) the reduction of the average particle size observed as x increases in Co1x(Gd/Sm)xFe2O4+δ,
associated to a bigger surface area of the particles (which lacks
the antiparallel arrangement) and becomes very large with respect to their volume. On the other hand, coercivity of nanocrystals of spinel cubic ferrites is determined by magnetic domain wall motion [30]; however, in Co1-x(Gd/Sm)xFe2O4+δ, it could vary because of changes in its magnetocrystalline anisotropy [31, 32] or its crystallite size
[33]. Indeed, if the system is composed, as it is observed in the TEM images recorded in this investigation, by nanoparticles below the single magnetic domain, the coercivity is only governed by their particle size. In order to improve the anisotropy of Co1-x(Gd/Sm)xFe2O4+δ, to increase its coercivity (as the rare-earth elements are introduced in the system), it would be necessary to increase their particle size. This can be achieved increasing the time or temperature of the thermal treatments, which gives place to bigger particles.
Figure 9. Hysteresis loops and Ms, Mr, R, and Hc values of Co1-xGdxFe2O4+δδ (a) and Co1-xSmxFe2O4+δδ (b) with x= 0, 0.04, and 0.08.
Figure 10. Variation of saturation magnetization (a) and coercive field (b) as a function of gadolinium content in Co1-xGdxFe2O4+δδ.
The 57Fe Mössbauer spectra, recorded at 300 K, consist of magnetic sextets with broadened lines resulting from the disorder induced by the presence of Gd3+ or Sm3+ in Co1-x(Gd/Sm)xFe2O4+δ, as it is illustrated in Figure 11. They must be decomposed into several magnetic components, but different fitting models can be considered because of the complex hyperfine structure. At this stage one can conclude to the presence of only Fe3+ species, taking into account of the mean value of isomer shift. Figure 12 depicts representative Mössbauer spectra of 0.92:0.08Gd and 0.92:0.08Sm under different conditions. At 77 K (Figure 12-top), the spectra exhibit a better resolved hyperfine structure which can be easily and perfectly described, a priori, by means of two magnetic components. The values of isomer shift confirm only the presence of Fe3+ species and allow to discriminate between two sites, tetrahedrally and octahedrally coordinated, as usually expected in the case of ferrites. In order to improve the resolution of the hyperfine structure and to calculate
more accurately the proportions of Fe3+ in those two sites (from the relative absorption area), the samples were studied in-field Mössbauer measurements at low temperature, at 20 K and under the effect of an applied magnetic field of 8T (Figure 12-middle). Indeed, the ferrimagnetic structure expected from Fe network favors a splitting of the magnetic sextet into two magnetic sextets unambiguously attributed to Fe cations occupying octahedral and tetrahedral sites, when the sample is submitted to a large applied external magnetic field (see a review in [34] and references therein). The main component has to be described by means of at least two subcomponents, while a single magnetic sextet with Lorentzian lines is sufficient to reproduce the other component (red and blue lines in Figure 12-middle, respectively). The refined values of the hyperfine parameters of 0.96:0.04Gd, 0.92:0.08Gd, 0.96:0.04Sm, and 0.92:0.08Sm, obtained at 300 and 77 K and of 0.92:0.08Gd and 0.92:0.08Sm obtained at 20 K and 80 T are listed in Table 1. The respective values of isomer shift allow the main (minor) component to the octahedral (tetrahedral) Fe3+ sites to be unambiguously assigned. It is important to emphasize that the respective proportions, 60 and 40 % respectively, calculated from the in-field measurements significantly differ from those estimated at 77K; consequently, the 77 K zero-field spectrum was remodeled by means of three magnetic components with Lorentzian lines, giving rise to a nice agreement with the respective proportions of Fe species obtained from the in-field experiments (blue rows in Table 1). The use of two magnetic components to describe the octahedral Fe sites suggests that their surroundings are more distorted than those of tetrahedral Fe sites, associated to the presence of Gd3+ or Sm3+ ions in the octahedral positions of the Co1x(Gd/Sm)xFe2O4+δ
lattice. According to the in-field Mössbauer, the presence of Gd or
Sm orthoferrites must be excluded: Indeed, their antiferromagnetic behaviour would
give rise to a component that can be easily observed on the field spectra (effective field quite similar to the hyperfine field characteristic of orthoferrite, reported in [35], and large intermediate line resulting from the perpendicular orientation of the magnetic moments with respect to the external field, i.e. gamma ray).
Figure 11. 57Fe Mössbauer spectra, recorded at 300 K, of Co1-x(Gd/Sm)xFe2O4+δ.
The second feature is related to the intensities of outer, middle, and inner lines of the Mössbauer spectra; those present in the spectra recorded at 77 K show the ratio 3:2:1 (respectively), characteristic in polycrystalline powdered materials (Figure 12-
top/bottom). The study of 0.92:0.08Gd and 0.92:0.08Sm, under the effect of an applied magnetic field of 8 T at 20 K, reveals that the intensity of the middle lines is smaller than that of those present in the spectra recorded at 77 K; it is thus possible to estimate the angle (θ) between the gamma-beam (parallel to the applied magnetic field) and the Fe magnetic moment, for both tetrahedral and octahedral Fe species. Then the total effective field at the nucleus results from the vectorial sum of the hyperfine field and the applied field, one can establish the following expression:
B2hf = B2eff + B2app - 2BeffBapp cosθ
which allows the hyperfine field to be estimated, as listed in Table 1. The canting of Fe magnetic moments is due to the non colinear magnetic structure resulting from both the frustrated nature of some magnetic interactions and the local anisotropy of Co2+ cations. In addition, the Mössbauer studies accomplished seems to reveal that there is not a significant difference between the octahedral or tetrahedral occupancy of Fe3+ cations when the amount of Gd3+ or Sm3+ increases from 0.04 to 0.08 in Co1x(Gd/Sm)xFe2O4+δ.
Figure 12. 57Fe Mössbauer spectra of 0.92:0.08Gd and 0.92:0.08Sm recorded at 77 K (top), and at 20 K and 80 T (middle), and at 77K (bottom) with the second fitting model (see text).
Table 1. Refined values of the hyperfine parameters of 0.96:0.04Gd, 0.92:0.08Gd, 0.96:0.04Sm, and 0.92:0.08Sm (see text).
Sample
0.96:0.04Gd
T (K) ±1 300 77 300 77 st
1 model
0.92:0.08Gd
20 8T 77 nd
2 model
0.96:0.04Sm
300 77 300 77 st
0.92:0.08Sm
1 model
20 8T 77 nd
2 model
IS (mm/s) ± 0.01 <0.33> 0.49 0.39 <0.32> 0.49 0.40 <0.50> 0.33 <0.48> 0.37 <0.33> 0.51 0.40 <0.30> 0 .50 0.29 <0.50> 0.36 0.48 0.37
2ε (mm/s) ± 0.01 <0.00> 0.01 -0.01 <0.01> 0.04 -0.02 <-.01> 0.03 <0.00> 0.00 <0.03> 0.04 -0.01 <-0.02> 0.03 -0.01 <-0.02> -0.01 <-0.01> 0.00
Beff (T) ± 0.5
θ (°) ±5
<47.1> 58.9
<26> 16
<47.2> 59.5
<31> 21
Bhf (T) ± 0.5 <47.8> 53.9 50.9 <47.1> 53.9 50.8 <54.4> 51.2 <53.0> 50.3 <47.8> 54.0 50.9 <47.4> 53.9 50.9 <54.2> 52.1 <52.9> 50.6
Absorption area ±1 100 40 60 100 35 65 61 39 60 40 100 40 60 100 37 63 62 38 61 39
Fe Site Mixed Octa Tetra Mixed Octa Tetra Octa Tetra Octa Tetra Mixed Octa Tetra Mixed Octa Tetra Octa Tetra Octa Tetra
IV. Conclusions Co1-xGdxFe2O4+δ (x = 0, 0.02, 0.04, 0.06, 0.08, 0.1, and 0.15) and Co1-xSmxFe2O4+δ (x = 0.04, and 0.08), with a cubic spinel-type structure, were synthesized through the citrate precursors route at temperatures as low as 400 °C. Their crystallite size decreases, as a function of Gd3+ or Sm3+ content, up to a 41 %. Rietveld refinement and Raman spectra reveal that Gd3+ (or Sm3+) ions, in Co1-x(Gd/Sm)xFe2O4+δ (x = 0.04, and 0.08), occupy preferentially Co positions in inverse spinel ferrites. TEM and HRTEM corroborates polycrystalline nanometric particles, with different shapes and sizes, of Co1-x(Gd/Sm)xFe2O4+δ. Saturation magnetization and coercivity values are lower than those obtained to CoFe2O4 probably due to the reduction of the average
particle size observed as x is larger in Co1-x(Gd/Sm)xFe2O4+δ, more complex electronic interactions 4f-3d (than those 3d-3d), and changes in its magnetocrystalline anisotropy. Mössbauer studies reveal the presence of only Fe3+ ions, distributed in a proportion of 60 and 40 % through octahedral and tetrahedral sites, respectively. Although the formation of the mixed spinel, rather than different substances (Sm or Gd oxide), fall in the sensitivity limits of the XRD technique all the results demonstrate that the replacement of Co2+ by Gd3+ or Sm3+ ions, in CoFe2O4, can tailor the crystal structure, microstructure, and magnetic properties of Co1x(Gd/Sm)xFe2O4+δ.
Acknowledgments The authors thank to Giberto Hurtado, Rosario Rangel, Mónica Ceniceros and Enrique Díaz for recording VSM analysis, Raman spectra, EDS spectra and TEM images, respectively. This project has been financially supported by CIQA (grant numbers: 6238 and 6311) and CONACyT (grant number: PEI 2018-250174). A. Tijerina-Rosa thanks to CONACYT the economic support provided.
References [1] Valenzuela R., Magnetic ceramics, Cambridge University Press: New York; 1994. [2] K.K. Kefeni, T.A.M. Msagati, B.B. Mamba, Ferrite nanoparticles: Synthesis, characterisation and applications in electronic device, Mater. Sci. Eng. B 215 (2017) 37-55. https://doi.org/10.1016/j.mseb.2016.11.002. [3] N.M. Sánchez-Padilla, S.M. Montemayor, F.J. Rodríguez-Varela, An easy route to synthesize novel Fe3O4@Pt core/shell nanostructures with high electrocatalytic activity, J New Mat Electr Sys, 15 (2012) 171-179. https://doi.org/10.14447/jnmes.v15i3.62. [4] N.M. Sánchez-Padilla, S.M. Montemayor, A. Torres, F.J. Rodríguez-Varela, Fast Synthesis and Electrocatalytic Activity of M@Pt (M=Ru, Fe3O4, Pd) Core-Shell Nanostructures for the Oxidation of Ethanol and Methanol, Int J Hydrogen Energy, 38 (2013) 12681-12688. https://doi.org/10.1016/j.ijhydene.2012.11.026. [5] N.M. Sánchez-Padilla, D. Morales-Acosta, M.D. Morales-Acosta, S.M. Montemayor, F.J. Rodríguez-Varela, Catalytic Activity and Selectivity for the ORR of Rapidly Synthesized M@Pt (M=Pd, Fe3O4, Ru) Core-Shell Nanostructures, Int J Hydrogen Energy, 39 (2014) 16706-16714. https://doi.org/10.1016/j.ijhydene.2014.03.223. [6] M.L. Martins, M.F. Calabresi, C. Quini, J.F. Matos, J.R.A. Miranda, M.J. Saeki, H.N. Bordallo, Enhancing the versatility of alternate current biosusceptometry (ACB) through the synthesis of a dextrose-modified tracer and a magnetic muco-adhesive
cellulose gel, Mater. Sci. Eng. C, 48 (2015) 80-85. https://doi.org/10.1016/j.msec.2014.11.059. [7] A. Makridis, I. Chatzitheodorou, K. Topouridou, M.P. Yavropoulou, M. Angelakeris, C. Dendrinou-Samara, A facile microwave synthetic route for ferrite nanoparticles with direct impact in magnetic particle hyperthermia, Mater. Sci. Eng. C, 63 (2016) 663-670. https://doi.org/10.1016/j.msec.2016.03.033. [8] V.R. Mudinepalli, P.C. Chang, C.C. Hsu, F.Y. Lo, H.W. Chang, W.C. Lin, Electricfield effects on magnetism of Fe/NCZF/PZT composite thin film, J Magn Magn Mater, 432 (2017) 90-95. https://doi.org/10.1016/j.jmmm.2016.12.141. [9] X. Yang, Z. Zhou, T. Nan, Y. Gao, G.M. Yang, M. Liu M, et al, Recent advances in multiferroic oxide heterostructures and devices. J Mater Chem C 4 (2016) 234-243. http://dx.doi.org/10.1039/C5TC03008K. [10] Reddy DHK, Yun YS. Spinel ferrite magnetic adsorbents: Alternative future materials for water purification? Coordination Chemistry Reviews 2016, 315:90-111. [11] D.H.K. Reddy, S.M. Lee, Magnetic biochar composite: Facile synthesis, characterization, and application for heavy metal removal, Colloids Surf A Physicochem Eng Asp, 454 (2014) 96-103. https://doi.org/10.1016/j.colsurfa.2014.03.105. [12] W.H. Bragg, The structure of the spinel group of crystals, Philos Mag, 30 (1915) 305-315. https://doi.org/10.1080/14786440808635400. [13] S. Nishikawa, Structure of Some Crystals of Spinel Group, Proceedings of the Tokyo Mathematico-Physical Society, 8 (1915) 199–209. https://doi.org/10.11429/ptmps1907.8.7_199.
[Dataset] [14] McMurdie HF, Morris MC, Evans EH, Paretzkin B, Wong-Ng W, Ettlinger L, et al. Standard X-Ray Diffraction Powder Patterns from the JCPDS Research Associateship, Powder Diffraction 1 (1986) 64-77. [15] M. Siddique, N.M. Butt, Effect of particle size on degree of inversion in ferrites investigated by Mossbauer spectroscopy, Physica B, 405 (2010) 4211-4215. https://doi.org/10.1016/j.physb.2010.07.012. [16] M. Yehia, S.M. Ismail, A. Hashhash, Structural and Magnetic Studies of RareEarth Substituted Nickel Ferrites, J Supercond Nov Magn, 27 (2014) 771–774. https://doi.org/10.1007/s10948-013-2340-z. [17] D. Peddis, N. Yaacoub, M. Ferretti, A. Martinelli, G. Piccaluga, A. Musinu, et al. 2011. Cationic distribution and spin canting in CoFe2O4 nanoparticles. J Phys Condens Matter. 23, 426004. https://doi.org/10.1088%2F09538984%2F23%2F42%2F426004. [18] H. Shokrollahi, L. Avazpour, Influence of intrinsic parameters on the particle size of magnetic spinel nanoparticles synthesized by wet chemical methods, Particuology, 26 (2016) 32–39. https://doi.org/10.1016/j.partic.2015.10.004. [19] A. Manikandan, R. Sridhar, S.A. Antony, S. Ramakrishna, A simple aloe vera plant-extracted microwave and conventional combustion synthesis: Morphological, optical, magnetic and catalytic properties of CoFe2O4 nanostructures, J Mol Struct, 1076 (2014) 188–200. https://doi.org/10.1016/j.molstruc.2014.07.054. [20] P. Kumar, G. Rana, G. Dixit, A. Kumar, V. Sharma, R. Goyal, et al., Structural, electrical and magnetic properties of dilutely Y doped NiFe2O4 nanoparticles, J Alloys Compd, 685 (2016) 492-497. https://doi.org/10.1016/j.jallcom.2016.05.248.
[21] A. Ditta, M.A. Khan, M. Junaid, R.M.A. Khalil, M.F. Warsi, Structural, magnetic and spectral properties of Gd and Dy co-doped dielectrically modified Co-Ni (Ni0.4Co0.6Fe2O4) ferrites, Physica B, 507 (2017) 27–34. https://doi.org/10.1016/j.physb.2016.11.030. [22] E.E. Ateia, G. Abdelatif, M.A. Ahmed, M.A.A. Mahmoud, Effect of Different Gd3+ Ion Content on the Electric and Magnetic Properties of Lithium Antimony Ferrite, J Inorg Organomet Polym, 26 (2016) 81–90. https://doi.org/10.1007/s10904-015-02835. [23] S.S. Laha, E. Abdelhamid, M.P. Arachchige, A. Kumar, A. Dixit, Ferroic ordering and charge-spin-lattice order coupling in Gd-doped Fe3O4 nanoparticles relaxor multiferroic system, J Am Ceram Soc, 100 (2017) 1534–1541. https://doi.org/10.1111/jace.14739. [24] D. Padalia, U.C. Johri, M.G.H. Zaidi, Effect of cerium substitution on structural and magnetic properties of magnetite nanoparticles, Mater Chem Phys 169 (2016) 89-95. https://doi.org/10.1016/j.matchemphys.2015.11.034. [25] C. Wang, Y. Wang, A. Zhang, Y. Cheng, F. Chi, et al., The influence of ionic radii on the grain growth and sintering-resistance of Ln2Ce2O7 (Ln= La, Nd, Sm, Gd), J Mater Sci 48 (2013) 8133–8139. https://doi.org/10.1007/s10853-013-7625-x. [26] T.A.S. Ferreira, J.C. Waerenborgh, M.H.R.M. Mendonça, M.R. Nunes, F.M. Costa, Structural and morphological characterization of FeCo2O4 and CoFe2O4 spinels prepared by a coprecipitation method, Solid State Sci, 5 (2003) 383–392. https://doi.org/10.1016/S1293-2558(03)00011-6.
[27] D. Chen and Y. Wang, Impurity doping: a novel strategy for controllable synthesis of functional lanthanide nanomaterials, Nanoscale 5 (2013) 4621-4637. http://dx.doi.org/10.1039/C3NR00368J. [28] C. Xu, M. Ma, L. Yang, S. Zeng, Q. Yang, Lanthanide doping-facilitated growth of ultrasmall monodisperse Ba2LaF7 nanocrystals with excellent photoluminescence, J Colloid Interface Sci, 368 (2012) 49-55. https://doi.org/10.1016/j.jcis.2011.10.072. [29] S.R. Naik and A.V. Salker, Change in the magnetostructural properties of rare earth doped cobalt ferrites relative to the magnetic anisotropy, J Mater Chem, 22 (2012) 2740-2750. http://dx.doi.org/10.1039/C2JM15228B. [30] Y. Matsuura, N. Kitai, S. Hosokawa, J. Hoshijima, Relation between the alignment dependence of coercive force decrease ratio and the angular dependence of coercive force of ferrites, J Magn Magn Mater, 411 (2016) 1-6. https://doi.org/10.1016/j.jmmm.2016.03.007. [31] M. Colarieti-Tosti, S.I. Simak, R. Ahuja, L. Nordstrom, O. Eriksson, M.S.S. Brooksa, Theory of the magnetic anisotropy of Gd metal, J Magn Magn Mater, 272– 276 (2004) e201-e202. https://doi.org/10.1016/j.jmmm.2003.12.655. [32] S. Thankachan, B.P. Jacob, S. Xavier, E.M. Mohammed, Effect of samarium substitution on structural and magnetic properties of magnesium ferrite nanoparticles, J Magn Magn Mater, 348 (2013) 140-145. https://doi.org/10.1016/j.jmmm.2013.07.065. [33] H. Yangy, Z. Wangy, L. Songy, M. Zhaoy, J. Wangz, H. Luoz, A study on the coercivity and the magnetic anisotropy of the lithium ferrite nanocrystallite, J Phys D, 29 (1996) 2574-2578. https://doi.org/10.1088%2F0022-3727%2F29%2F10%2F008.
[34] J.M. Greneche, The Contribution of 57Fe Mössbauer Spectrometry to Investigate Magnetic Nanomaterials, in: Y. Yoshida and G. Langouche (Eds), Mössbauer Spectroscopy, Springer, Berlin Heidelberg, 2013, pp.187-241. https://doi.org/10.1007/978-3-642-32220-4_4. [35] M. Eibschutz, S. Shtrikman and D. Treves, Mössbauer studies of 57Fe in orthoferrites, Phys. Rev. 156 (1967) 562. https://doi.org/10.1103/PhysRev.156.562.
Figure Captions Figure 1. Reaction scheme (a) and flowchart of the experimental procedure (b). Figure 2. Comparison of experimental and reported XRD patterns of CoFe2O4 (bottom). Crystallite size(temperature) graph (top). Figure 3. a) XRD patterns of 1:0, 0.96:0.04Gd, and 0.92:0.08Gd and b) Crystallite size versus gadolinium content. Figure 4. XRD experimental and Rietveld-calculated patterns of 1:0 (a), 0.96:0.04Gd (c), and 0.92:0.08Gd (d) samples. (b) Crystallite sizes and “a” lattice parameters. Figure 5. a) XRD patterns of 1:0, 0.96:0.04Sm, and 0.92:0.08Sm and b) Crystallite size values and “a” lattice parameters. Figure 6. Raman spectra of 1:0, 0.96:0.04Gd, and 0.92:0.08Gd samples. Figure 7. TEM and HRTEM images of Co0.92Gd0.08Fe2O4.04 and Co0.96Sm0.04Fe2O4.02. Figure 8. EDS spectra of Co0.92Gd0.08Fe2O4.04 and Co0.96Sm0.04Fe2O4.02. Figure 9. Hysteresis loops and Ms, Mr, R, and Hc values of Co1-xGdxFe2O4+δδ (a) and Co1-xSmxFe2O4+δδ (b) with x= 0, 0.04, and 0.08. Figure 10. Variation of saturation magnetization (a) and coercive field (b) as a function of gadolinium content in Co1-xGdxFe2O4+δδ. Figure 11. 57Fe Mössbauer spectra, recorded at 300 K, of Co1-x(Gd/Sm)xFe2O4+δ. Figure 12. 57Fe Mössbauer spectra of 0.92:0.08Gd and 0.92:0.08Sm recorded at 77 K (top), and at 20 K and 80 T (middle), and at 77K (bottom) with the second fitting model (see text).
Table Captions
Table 1. Refined values of the hyperfine parameters of 0.96:0.04Gd, 0.92:0.08Gd, 0.96:0.04Sm, and 0.92:0.08Sm (see text).