Effect of the Cation Distribution and Microstructure on the Magnetic

Article pubs.acs.org/IC

Effect of the Cation Distribution and Microstructure on the Magnetic Behavior of the CoMn2O4 Oxide Jasminka Popović,*,† Marijana Jurić,† Damir Pajić,‡ Martina Vrankić,† Janez Zavašnik,§ and Jelena Habjanič†,∥ †

Ruđer Bošković Institute, Bijenička cesta 54, 10000 Zagreb, Croatia Department of Physics, Faculty of Science, University of Zagreb, Bijenička cesta 32, 10000 Zagreb, Croatia § Center for Electron Microscopy and Microanalysis, Jožef Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia ∥ Department of Chemistry, University of Zürich, Rämistrasse 71, CH-8006 Zürich, Switzerland ‡

ABSTRACT: The sizes of CoMn2O4 nanoparticles can easily be tuned, from 40 to 8 nm, depending on the temperature of decomposition of the single-source molecular precursor {[Co(bpy)3][Mn2(C2O4)3]·H2O}n. The structural features of the CoMn2O4 spinel are also affected by the heat treatment temperature, showing a pronounced expansion of unit cell parameters as a consequence of thermally induced cation redistribution between tetrahedral and octahedral sites. Moreover, the magnetic behavior of CoMn2O4 was successfully tailored as well; depending on the heat treatment, it is possible to switch between the superparamagnetic and ferrimagnetic ordering and to tailor the magnetic transition temperatures, i.e., the boundaries between the hard and soft magnetic behavior.

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INTRODUCTION Complex metal oxides, especially those crystallizing in the spinel-type family AB2O4, represent an important class of functional materials that as a result of their unique chemical, electric, magnetic, and mechanical properties have a wide range of potential applications ranging from energy storage and conversion to magnetism, electronics, and catalysis.1 Abundance, nontoxicity, and multiple oxidation states enthroned the manganese-based spinel materials as an attractive research platform for new applications including magnetic materials and catalysts.2 Among the large number of different spinel materials, cobalt manganite (CoMn2O4) attracted great attention as a new advanced anode material for lithium-ion batteries (LiBs),3 an electrocatalyst for oxygen reduction/ evolution reactions,4 and a catalyst in CO oxidation.5 The majority of the recent work appears to be strongly focused on the electrochemical properties, i.e., applications of the CoMn2O4 oxide as high-capacity and high-performance LiB anodes,6−8 while magnetic studies have been scarce in spite of a few papers reporting on the very intriguing and complex but still poorly understood magnetic behavior.9 High magnetic anisotropy usually appears in spinels, especially in the cobaltsubstituted ferrites. A large coercive field was recently observed in the CoFe2O4 nanoparticles, but this enhancement is not fully understood because of very complex influences of the particle size, morphology, inversion, etc.10 Generally, the spinel structure AB2O4 consists of a cubicclose-packed array of anions, with one-eighth of all tetrahedral voids being occupied by divalent A cations, while trivalent B © 2017 American Chemical Society

cations are distributed over half of all octahedral sites. The unit cell of a spinel is face-centered-cubic (Fd3̅m); however, the presence of metal cations experiencing a pronounced Jahn− Teller effect, such as Mn3+ in an octahedral coordination, is known to lower its point symmetry to 2/m (tetragonal bipyramid) by the elongation of two axial bonds, (B−O)ax. When a critical amount of octMn3+ cations is present, tetragonal distortion yields a large-scale effect, a transition from cubic (Fd3̅m) to tetragonal (I41/amd) symmetry.11 The structure of the cobalt manganite spinel is described with tet(Co1−xMnx)oct[CoxMn2−x]O4, where parentheses and square brackets denote the cation residing at tetrahedral (A) and octahedral (B) sites, respectively. As shown by the formula, spinels will usually demonstrate some degree of cation disorder over the two crystallographic sites represented with x, the socalled parameter of an inversion. A literature survey led to some contradictive results regarding the cation distribution in the CoMn2O4 spinel; Bordeneuve et al. reported CoyMn3−yO4 (y = 0.93) in the form of a dense ceramic having only 1% of the Co atoms residing on an octahedral site at room temperature (RT), in spite of the fact that the samples were quenched from high temperatures (1160−1300 °C) and one could expect that a rapid cooling will quench-in the high-temperature disordered states.12 On the other hand, Tamura found that quenching from 1000 °C to RT led to a cobalt manganite structure with the degree of inversion x = 33%, thus indicating that, if the Received: December 20, 2016 Published: March 17, 2017 3983

DOI: 10.1021/acs.inorgchem.6b03104 Inorg. Chem. 2017, 56, 3983−3989

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Inorganic Chemistry

(Jeol JEM-2100, LaB6, Japan) operating at 200 kV. Samples for TEM were dispersed in absolute ethanol, sonicated to prevent agglomeration, and transferred on commercially available carbon-coated copper grids. Basic crystallographic parameters were calculated using selectedarea electron diffraction (SAED) patterns, collected from numerous particles. For reliable results, TEM was conducted prior to analysis calibrated with a MAG*I*CAL reference standard (No. 1769, Tehnoorg Linda, Hungary), while in diffraction mode, the camera constant (λL) was calibrated using a monocrystal of the Si[110] sample. Metal analysis of the samples was investigated by energydispersive X-ray spectrometry (EDS), while because of a small volume of the investigated particles, the quantitative ratios were calculated using a Cliff−Lorimer approximation for thin films. Magnetization Study. The magnetization M was measured using a MPMS-5 commercial superconducting quantum interference device magnetometer. The temperature dependence of magnetization, M(T), of all prepared oxides was measured in different magnetic fields, in the temperature range of 2−330 K. Two modes of measurement were applied: after cooling in a zero field (ZFC) and after cooling in a magnetic field (FC), in which measurement was performed during heating. The field dependence of magnetization, M(H), i.e., magnetic hysteresis loops, was measured at several stable temperatures in fields up to 50 kOe. The measured magnetic moments of the samples were corrected against the sample-holder contribution. The molar magnetic susceptibility χ = M/H was calculated for measurement in fields of 100 and 1000 Oe, being the same in the paramagnetic phase.

cation reordering is slow enough, the high-temperature disordering may be preserved at RT.13 There is also a report revealing the presence of lower-oxidation-state ions (Mn2+) residing on the tetrahedra, while the higher-oxidation-state cations (Mn4+, Co3+, and Co4+) were found in the octahedral surroundings.14 Obviously, the cation distribution in the CoMn2O4 lattice is a very complex function of the synthesis conditions, especially thermal treatment. From the literature, it can be noted that traditional methods for the preparation of spinel materials, such as a solid-state reaction requiring high temperatures and prolonged process times, have been abandoned and the focus has been switched to the low-cost preparation methods at moderate temperatures with enhanced reaction kinetics.15−22 Recently, we reported several novel heterobimetallic oxalate complexes that can be utilized as the single-source precursors for the preparation of various mixed-metal oxides via thermal decomposition, providing excellent control of the stoichiometry and crystallite size in the nanoregime.23−25 In this work, we present an effective way for tuning the structural and microstructural properties of CoMn2O4 by simple alternations in the recently developed route for the synthesis of CoMn2O423 and, furthermore, the correlation of these effects to the magnetic behavior of the CoMn2O4 phase. The same approach was proven to be very successful in tuning the phase composition and microstructure of the γ-Ba4Nb2O9 polymorph.26 One has to highlight that only elucidation of an interplay between the preparation conditions and the structure−microstructure opens the opportunity for controlling and forecasting the magnetic features and characteristics, such as coercivity and remanence, magnetic transition temperatures, and saturation magnetization.

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RESULTS AND DISCUSSION Structural and Microstructural Studies. Prepared samples S500−S1000 did not show the presence of any impurity phases besides the targeted oxide−tetragonal spineltype CoMn2O4 (I41/amd), as evidenced from the PXRD patterns and EDS results. Besides phase identification, a modelfree whole pattern decomposition method and the Rietveld refinement on PXRD data collected on the samples S500− S1000 at RT were carried out in order to follow the fine changes in the unit-cell parameters, inversion parameters, and bond lengths induced by different temperatures of the heat treatment. The inversion parameter has been quantified from the refined occupancy parameter of the tetrahedral (4a) and octahedral (8d) sites; however, because of the small difference in the X-ray scattering powers between the Co and Mn cations, we will rather discuss the temperature dependence of a cation distribution based on the changes of the unit-cell parameters and interatomic distances because they represent a more reliable indicator for tracking the fine structural changes within the crystal lattice. The changes of the unit-cell parameters a and c, as obtained by the whole pattern decomposition method implemented in the HighScore Plus program, are given in Figure 1. The increase of both lattice parameters has been observed. This increase of the unit-cell parameters was a first indication that the heat treatment induced significant structural changes within the lattice, specifically in the cation distribution between octahedral and tetrahedral sites. The observed increment was caused by the thermally induced substitution of the smaller Mn3+ cation (r = 0.645 Å) by the larger Co2+ cation (r = 0.745 Å) at the octahedral site, which consequently led to the increment of the octahedral bond lengths, which, being parallel to the crystallographic axes a and c, directly influence the unitcell parameters (Figure 2). The possibility of Co3+ (or even higher valence) incorporation at the octahedral site, as suggested from the literature,14 has also been considered; however, substitution of the larger Mn3+ cation (r = 0.645 Å) by the smaller Co3+ cation (r = 0.61

EXPERIMENTAL SECTION

Materials. A single-source molecular precursor, {[Co(bpy)3][Mn2(C2O4)3]·H2O}n (1), was prepared by following a previously described procedure. 23 Finely ground crystalline powders of compound 1 were heated in a thermal analyzer (Shimadzu DTG60H) from RT to 500, 700, 800, and 1000 °C in a stream of synthetic air (20.5% O2 and 79.5% N2) at a heating rate of 10 °C min−1. The resulting materials, marked as S500, S700, S800, and S1000, respectively, were cooled to RT at a cooling rate of 10 °C min−1. After decomposition, the resulting oxide products were characterized by the powder X-ray diffraction (PXRD) at RT. PXRD. Structural changes of the cobalt manganite samples caused by different temperatures of a heat treatment have been studied in detail by PXRD at RT using a Philips MPD 1880 counterdiffractometer with Cu Kα radiation. Data were collected in the 2θ range of 10−100° (step 0.02°) with a fixed counting time of 10 s step−1. Refinement was carried out by using the split-type pseudoVoigt profile function and the polynomial background model. Structure refinement was carried out by the Rietveld method within the program HighScore Plus, version 4.5 (PANalytical 2016). Isotropic vibrational modes were assumed for all atoms. During the refinement, zero shift, scale factor, half-width parameters (U, V, and W), asymmetry, and peak-shape parameters were simultaneously refined. The occupancies of different cations residing on the 4a and 8d sites were constrained to 1. Isotropic thermal displacement parameters for cations sharing the same crystallographic site were constrained to change in unison. Microstructural information, volume-weighted crystallite domain lengths, and an average maximum lattice strain were obtained in the course of the Rietveld refinement. Silicon was used as an instrumental broadening standard. Diffraction profiles for the sample and standard were described by the pseudo-Voigt function. Transmission Electron Microscopy (TEM). The microstructures of the samples were studied by a transmission electron microscope 3984

DOI: 10.1021/acs.inorgchem.6b03104 Inorg. Chem. 2017, 56, 3983−3989

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Inorganic Chemistry

standard deviation in the third decimal place. The influence of the temperature increment on the octahedral and tetrahedral bond lengths is summarized in Figure 3. We observed an increase in the octahedral bond lengths, d(Moct−O), while the interatomic distances in the tetrahedral coordination, d(Mtet− O), decreased. A decrease of the metal−oxygen interatomic distances within the tetrahedra, d(Mtet−O), from 1.9562(1) Å (for sample S500) to 1.9401(1) Å (for sample S1000), resulted from thermally enhanced substitution of Co2+ by smaller Mn3+ cations at the A site (4a) of the spinel structure. On the other hand, the octahedral B site (8d) becomes partially occupied by Co2+ as a consequence of transferred Mn3+ cations from the octahedral site to the tetrahedral site. The substitution of smaller Mn3+ cations by larger Co2+ led to an increase of the octahedral bond lengths from d(Moct−O)eq = 1.9678(2) Å and d(Moct−O)ax = 2.236(1) Å when the temperature was 500 °C to d(Moct−O)eq = 1.9781(3) Å and d(Moct−O)ax = 2.245(2) Å when the temperature of the heat treatment approached 1000 °C. Our results showed that CoMn2O4 can experience a wide range of different cation distributions, pointing to considerable flexibility of the spinel structures, while, on the other hand, a high space group symmetry of the spinel should not allow bond distance distortions and one might expect that the structure should behave very rigidly. Although those facts merged together sound like a contradiction, they can, in fact, be explained by taking into consideration that the optimal cation partition related to cation radii can compensate for the intrinsic structural rigidity as well as the effect of the crystal-field stabilization energy (CFSE).27 The larger stabilization energy of Co2+ in octahedral coordination, compared to the stabilization energy at the tetrahedral site, could be the driving force

Figure 1. Changes in the unit-cell parameters a and c for samples S500−S1000 as a function of the heat-treatment temperature.

Å) at the octahedral site would decrease the octahedral bonds and, consequently, result in a smaller unit cell. Therefore, the Rietveld structure refinement for samples S500−S1000 was carried out by assuming the nonzero inversion structural model: tet(Co2+1−xMn3+x)oct[Co2+xMn3+2−x]O4. To avoid a possible correlation between the parameters, the isotropic thermal displacement parameters and site occupation parameters were refined separately. The structure of CoMn2O4, prepared by thermal treatment of an oxalate complex at 1000 °C, is shown in Figure 2, together with the Rietveld refinements performed on samples S500 and S1000, while the refined structure data for all samples are summarized in Table 1. The stoichiometric factors in the formula summation were determined from refined occupancy parameters having a

Figure 2. Graphical result of the final Rietveld refinement of RT data for samples S500 (a) and S1000 (b). The green vertical marks represent the position of the CoMn2O4 diffraction lines. The experimental data are shown as the red line, the calculated pattern is represented by the blue line, and the difference curve is given below in red. The structure of CoMn2O4 obtained by the heat treatment of the single-source precursor 1 at 1000 °C is shown in the bc (c) and ac (d) planes. 3985

DOI: 10.1021/acs.inorgchem.6b03104 Inorg. Chem. 2017, 56, 3983−3989

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Inorganic Chemistry Table 1. Results of the Whole Pattern Decomposition Method and Rietveld Refinement for Samples S500−S1000 unit cell (Å) S500 S600 S800 S1000

site occupancy

Biso (Å2)

oxygen coordinates

a

c

A site

B site

y

z

A

B

O

Rwp

5.7221(3) 5.7229(5) 5.7235(3) 5.7244(5)

9.2663(4) 9.2679(5) 9.2691(4) 9.2701(4)

Co0.94Mn0.06 Co0.90Mn0.10 Co0.85Mn0.15 Co0.81Mn0.19

Co0.06Mn1.94 Co0.10Mn1.90 Co0.15Mn1.85 Co0.19Mn1.81

0.2357(1) 0.2367(2) 0.2376(1) 0.2382(2)

0.3839(1) 0.3835(1) 0.3832(2) 0.3829(1)

1.73(9) 1.85(7) 1.77(8) 1.48(9)

1.4(1) 1.58(9) 2.32(8) 2.38(9)

3.84(6) 3.0(1) 3.96(8) 3.2(1)

7.99 6.77 5.67 5.71

far, the most essential terms for intracrystalline disorder in the spinels.28 Changes in the thermal treatment, besides influencing the structural features within the crystal lattice, also had a strong impact on the microstructural properties of the prepared samples. Even before any calculations, the obvious difference in the breadths of diffraction lines points out the significant changes in the crystallite sizes, as can be noted in Figure 2. Size−strain analysis performed during the Rietveld refinement showed that the average crystallite size of CoMn2O4 can easily be tuned in the range of 8−40 nm [8(1), 18(2), 30(2), and 40(1) nm for samples S500, S700, S800, and S1000, respectively] depending on the temperature of annealing. Additionally, we investigated sample S500 by TEM to further confirm the sizes of the crystallites as obtained by the PXRD line-broadening analysis. At first glance, the particles appear to be stable under the electron beam. Larger agglomerates consist of many small particles with random orientation (Figure 4a). At a closer look, we observed no amorphous layer present on the surface of the individual particles (Figure 4b), and their outer morphology appears mostly round, while some of the crystal faces already start to appear. The size of the individual crystallites, measured from the bright-field conventional TEM micrographs, is approximately 10 nm and appears to be uniform throughout the entire sample. The SAED ring pattern (Figure 4b, inset) analysis confirms a uniform, single-phase composition, corresponding to CoMn2O4 [Inorganic Crystal Structure Database (ICSD), FIZ Karlsruhe, under no. 429387]. Magnetic Study. Magnetization (M) of the CoMn2O4 samples (S500, S700, S800, and S1000) is investigated in the broad temperature (T) and field (H) ranges. All prepared samples show the paramagnetic linear M(H) curves at temperatures of 200 K and above, and the values of the

Figure 3. Changes in the octahedral and tetrahedral bond lengths during the heat treatment.

responsible for the partial incorporation of Co2+ at the octahedral site; however, additionally, the steric requirement has to be met; i.e., the structure has to have the ability for octahedral expansion in order to allow the partial incorporation of larger cations at the octahedral site. On the other hand, our PXRD structural study showed significant transfer of Mn3+ from the octahedral site to the tetrahedral site (similar to what Bosi et al.27 found for MgMn2O4) despite the fact that Mn3+ cations are expected to have a strong tendency to order at the octahedral site based on CFSE arguments. It seems that the observed cation distribution is caused by the pronounced preference of the spinel lattice to incorporate smaller cations at the tetrahedral site. This conclusion is also in accordance with computational studies that show that the importance of CFSE is overestimated and that coulomb and repulsive terms are, by

Figure 4. (a) Low-magnification TEM micrograph of sample S500 obtained by the heat treatment of a single-source precursor 1 at 500 °C (brightfield conditions). (b) Sample S500, part of a larger agglomerate, showing randomly oriented individual particles with an even size distribution. The insets show the SAED pattern, recorded over the numerous particles in image b and indexed as CoMn2O4 (ICSD 429387). The Feret mean size of the particles is 9.5 nm, while the histogram represents the particle size distribution, calculated from the same image (Feret mean/modus size of approximately 10 nm). 3986

DOI: 10.1021/acs.inorgchem.6b03104 Inorg. Chem. 2017, 56, 3983−3989

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Inorganic Chemistry susceptibility at 200 K determined from the M(H) slopes are presented in Table 2.

consequently, leads to an increase of the effective magnetic interactions. The temperature where the maximum at the ZFC M(T) curve appears, denoted as T2, is very reminiscent of the blocking temperature TB of an ensemble of magnetic nanoparticles.29 Namely, slightly above TB (denoted here as T2) the ZFC and FC curves coincide, showing that the system is in equilibrium due to the fast fluctuations of the magnetic moments of the nanoparticles. In contrast, below TB, the magnetic moments are blocked on the time scale of observation, and there is a large irreversibility between the ZFC and FC measuring processes. Obviously, T2 increases with an increase in the particle size, but strong quantitative agreement between the volume of the particles and T2 is lacking because of the nonuniform magnetization of the whole particles as well as their mutual interactions, which make them far from the ideal ordered and noninteracting particles. An additional confirmation of the blocking scenario comes from the magnetic field influence on the ZFC and FC M(T) curves. While the transition at T1 is independent of an applied field, T2 is strongly shifted toward lower temperatures as the applied field is increased. Also, the splitting between the ZFC and FC curves becomes smaller and smaller, which could be explained with the lowering of the anisotropy barriers with the magnetic field. This effect, characteristic for every ensemble of nanoparticles, is shown in Figure 6 for sample S1000 and appears to be similar in samples S800 and S700.

Table 2. Parameters of the Magnetic Properties: Curie Constant C Obtained from the M/H Slope at 200 K, Temperature of the Higher Magnetic Transition (T1), Temperature of the Maximum on the ZFC Curve (T2), and Temperature of the Lower Magnetic Transition (T3) for Samples S500−S1000 sample

C (emu K mol−1 Oe−1)

T1 (K)

T2 (K)

T3 (K)

S500 S700 S800 S1000

2.54 2.86 2.12 1.56

167.5 177.7 183.4

134.5 156.8 164.5

87.1 86.3 82.3 78.9

Curie−Weiss analysis of the magnetic susceptibility χ(T) = C/(T − θ) above 200 K gives the Curie constant C, in an agreement with the values from the M/H slope. The Weiss parameters range from −50 to −100 K, which points to the predominantly antiferromagnetic interactions, but because of the complex magnetic interactions in this spinel structure, they cannot be referred to quantitatively as a measure of some specific interaction. The Curie constant is in agreement with the Mn3+ and Co2+ ionization states, with spins 2 and 3/2, respectively. The presence of Mn2+ and/or Co3+ is firmly discarded, even in small amounts, because it would considerably increase the value of the Curie constant. Below the temperature of the paramagnetic regime, the magnetic behavior becomes complex and different for every sample. The temperature dependence of magnetization measured in the 1000 Oe field for all samples is shown in Figure 5. Three

Figure 6. Temperature dependence of magnetization for sample S1000 measured in different applied fields.

Sample S500 does not manifest the transition at T1 and blocking at T2 because of the very small particles that prevent long-range ordering at higher temperatures. Additionally, the barriers are much lower, and blocking would be suppressed only at much lower temperatures. Below T3, another ferrimagnetic state is established, characterized with a strong increase of the magnetization and very large difference between the ZFC and FC curves. The value of T3 decreases with the temperature of the heat treatment (Figure 5). Because the octahedra are connected in an edge-sharing manner (Figure 2), this decrease in T3 can be explained with an increase of the octahedral bond lengths, both axial and equatorial, with an increase in the temperature of the heat treatment of the molecular precursor, as established from the PXRD data (Figure 3), leading to a decrease of the spin interactions between the metal centers. The peak on the ZFC curves at T3 does not shift to lower temperatures when the magnetic field is increased, thus representing the intrinsic

Figure 5. Temperature dependence (ZFC, lower branches; FC, upper branches) of magnetization for all samples measured in the 0.1 T magnetic field.

characteristic temperatures describing the magnetic behavior of this complex system can be identified. A transition from the paramagnetic state to the magnetically ordered state appears at the temperature T1. Although the Weiss parameters are negative, the magnetic moments are much bigger than those in an antiferromagnetic state but smaller than those in the case of parallel spin alignment, showing that the ferrimagnetic state is characteristic of the system. The value of the magnetization from the FC curve is in agreement with the ferrimagnetic CoMn2O4 particles of similar sizes.9 The particle sizes increase for samples S500−S1000, which decreases the relative amount of surface atoms and, 3987

DOI: 10.1021/acs.inorgchem.6b03104 Inorg. Chem. 2017, 56, 3983−3989

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CONCLUSIONS Changes in the preparation conditions, namely, changes in the temperature of the heat treatment, cause thermally induced cation redistribution (x = 6−19% for the samples S500−S1000, respectively) within the spinel lattice, an increase of the particle sizes (from 8 nm in the case of sample S500 to 40 nm for sample S1000), and switching between the superparamagnetic and ferrimagnetic behavior of the same system. This work shows that it is even possible to tailor the characteristic magnetic transition temperatures, i.e., the boundaries between the hard and soft magnetic behavior, by a simple alteration of the preparation conditions.

magnetic transition, which is not connected with the microstructural features of prepared nanoparticles. Prepared oxides are very hard magnetic materials, as is observed from the hysteresis loops shown in Figure 7, especially below T3.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: + 385 1 4561120. Mobile: +385 91 9574191. ORCID

Jasminka Popović: 0000-0003-0800-2249 Damir Pajić: 0000-0002-4907-7290

Figure 7. Magnetic hysteresis loops of all samples at 70 K.

Notes

The authors declare no competing financial interest.

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Sample S800 has a very large coercive field, being almost 2 T at 5 K, but it is impossible to measure it exactly because of an extremely high irreversibility field (much higher than the available 5 T). Sample S800 has a coercive field of 0.43 T at liquid-nitrogen temperature, and the remnant magnetization corresponds to the 0.16−0.18 μB per formula unit for samples S700−S1000. Such ferrimagnetic hysteresis loops survive up to T3 and then become much narrower and lower, disappearing above T2 and becoming linear above T1. Sample S500 has different behavior because of the significantly smaller nanoparticles and is reminiscent of superparamagnetic hysteresis below the blocking temperature, while near T2, they become reversible and S-shaped, corresponding to the unblocked superparamagnetic regime. There are previous reports in which the studied CoMn2O4 nanoparticles were somewhat smaller and hard ferrimagnetic order was not observed because of the superparamagnetic overbalance.14 Among all prepared samples, the dominant superparamagnetic regime was achieved only for sample S500, having the smallest particles, whereas samples S700−S1000 showed two transitions characteristic for this material and placed the superparamagnetic behavior between those temperatures because of the suitable anisotropy barrier.13,9 Similarly, a high magnetic hardness was recently observed in the CoFe2O4 nanoparticles but is still not fully understood.10 The performed experiments correlated the changes in the structure and particle size with the magnetic behavior; hysteresis loops can be easily transformed from very hard to very soft by changing the temperature of the heat treatment during the synthesis of CoMn2O4, with remanence staying high and their large permeability being very useful (when used at small fields). The behavior is, of course, very complex because of the changes of the cation redistribution and changes in the particle sizes, both occurring simultaneously. In summary, changes in the applied temperature during the thermal treatment of the single-molecular precursor allow one to switch between the superparamagnetic and ferrimagnetic behavior of the same system and even to tailor the characteristic magnetic transition temperatures, i.e., the boundaries between the hard and soft magnetic behavior.

ACKNOWLEDGMENTS This research was financed by the Croatian Science Foundation project under Grants IP-2014-09-4079 and UIP-2014-09-8276.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b03104 Inorg. Chem. 2017, 56, 3983−3989

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DOI: 10.1021/acs.inorgchem.6b03104 Inorg. Chem. 2017, 56, 3983−3989