PCCP View Article Online PAPER View Journal | View Issue Tunable particle-agglomeration and magnetic This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. coupling in bi-magnetic nanocomposites† Open Access Article. Published on 02 October 2023. Downloaded on 1/18/2024 3:40:20 PM. Cite this: Phys. Chem. Chem. Phys., 2023, 25, 27817 Pierfrancesco Maltoni, *a Miran Baričić, ‡b Gianni Barucca, cd Maria Chiara Spadaro,c Jordi Arbiol, ef Nader Yaacoub,g Davide Peddis *bd and Roland Mathieu *a A set of non-stoichiometric Zn–Co-ferrite nanoparticles (NPs) was prepared by thermal decomposition of metallic complexes, in the presence of oleic acid, and, after a ligand-exchange process, was coated by a hydrophilic surfactant: these NPs were used as seeds in a sol–gel self-combustion synthesis to prepare nanocomposites (NCs) with a fixed weight ratio. Our focus here is the development of an efficient synthetic approach to control the magnetic coupling between a hard-magnetic matrix (Sr-ferrite) and NPs. Received 2nd August 2023, The physico-chemical synthetic conditions (temperature, pH, colloidal stability) were optimized in order to Accepted 29th September 2023 tune their effect on the final particles’ agglomeration in the matrix. We demonstrate that our synthetic DOI: 10.1039/d3cp03689h approach is a novel way to produce strongly magnetically coupled NCs, where the final extrinsic properties could be tuned by controlling (i) the agglomeration of seeds in the matrix and (ii) their rsc.li/pccp elemental doping. Introduction magnets.2–6 The main challenge is to design advanced synthetic approaches to obtain homogeneously-dispersed systems at the Bi-magnetic nanocomposites (NCs) have gained a lot of interest nanoscale. Among the several techniques to synthesize nano- over the last years, due to the possibility to finely control and particles (NPs), sol–gel chemistry has emerged as an advantageous modify their features at the nanoscale by combining two way to prepare transition-metal oxides from homogeneous prototypical phases.1 The novel properties arise from interfacial solutions containing the precursors,7 and found application physical effects, owing to the coupling interaction between in a wide range of nanomaterials (e.g., thin-films,8 monodis- the oxides’ interfaces which can be tuned via structural and perse nanoparticles,9 nanocomposites10). The main advantage morphological modification. Thus, composites represent a new of the sol–gel method is to enable the complete conversion of class of materials, owing to the combination of electronic and precursors which is normally prevented in solid-state methods, magnetic properties, which find applications as multiferroics, a solid being limited by mass transport.11 A possible way to catalysts, magnetorheological materials and new permanent tune the sol–gel process, relies upon the choice of molecular species for the starting solution, as the key to controlling the a Department of Materials Science and Engineering, Uppsala University, Box 35, morpho-structural features of the products, and thus designing Uppsala, 751 03, Sweden. E-mail: pierfrancesco.maltoni@angstrom.uu.se, novel systems. In the search for alternate ways to reduce the Roland.Mathieu@angstrom.uu.se b size down to the nanoscale, and explore novel properties Dipartimento di Chimica e Chimica Industriale & INSTM, nM2-Lab, Università degli Studi di, Genova, Via Dodecaneso 31, Genova, 1-16146, Italy. (stemming from the large surface energy and size effects), the E-mail: Davide.Peddis@unige.it citrate–gel process is a well-established approach to prepare c Dipartimento di Scienze e Ingegneria della Materia dell’Ambiente ed Urbanistica – magnetic nanomaterials such as hexagonal- and spinel-like SIMAU, Università Politecnica delle Marche, Ancona 60131, Italy ferrites:12–14 the low-temperature treatment permits to retain d Consiglio Nazionale delle Ricerche, Istituto di Strttura della Materia, nM2-lab, the single-domain size of the resulting sintered agglomerates, Monterotondo Scalo (RM), 00015, Italy e Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, essential for technological applications.15–17 Furthermore, Campus UAB, Bellaterra, 08193, Barcelona, Catalonia, Spain combining two or several magnetic phases was demonstrated f ICREA, Pg. Lluı́s Companys 23, 08020, Barcelona, Catalonia, Spain to be an efficient way to achieve superior magnetic g Le Mans Université, Institut des Molécules et Matériaux du Mans, CNRS UMR- performances.5,18,19 Within this framework, the need for sharp 6283, Avenue Olivier Messiaen, Le Mans, 72085, France interfaces with large contact areas to maximise the magnetic † Electronic supplementary information (ESI) available. See DOI: https://doi.org/ 10.1039/d3cp03689h interactions between particles has required the development of ‡ Current address: Laboratoire Interfaces Traitements Organisation et Dynami- more advanced synthetic strategies to control particle agglom- que des Systèmes, Université Paris Cité, CNRS UMR-7886, 75013 Paris, France. eration, and thus morpho-structural features. This journal is © the Owner Societies 2023 Phys. Chem. Chem. Phys., 2023, 25, 27817–27828 | 27817 View Article Online Paper PCCP Recent studies have reported several ways to enhance the magnetic properties of ferrites via chemical composition,20–24 owing to the flexible crystal structure of such ferrites that easily enables the modification of the cationic distribution, and in turn the magnetic anisotropy; specifically, an increase of saturation magnetization for Zn2+ substituted spinel ferrites.25–27 Hence, in this study, we present a novel way to prepare a set of bi-magnetic NCs by the citrate–gel method by including pre-formed seed NPs (CoxZnyFe3 (x+y)O4) in a matrix (SrFe12O19), after annealing the This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. composite in air. A previously employed sol–gel self-combustion Open Access Article. Published on 02 October 2023. Downloaded on 1/18/2024 3:40:20 PM. synthesis was modified,5,28 by tuning the initial colloidal stabi- lity of the seeds via changing the pH, controlling the final size of the NPs in the resulting powder-like binary composite. A non- toxic natural catechol ligand (i.e., di-hydro caffeic acid, DHCA) Fig. 1 Synthesis path of nanocomposites (NCs); on the right, the ligand- exchange process is illustrated. The stability of particles’ suspension at which recently gained interest for biomedical functions,29,30 was pH = 10 compared to pH = 7 is shown in the bottom panel, using dynamic chosen to functionalize the surface of seeds obtained by thermal light scattering data. decomposition.31 The use of this hydrophilic coating enabled us to tune the stability of NPs by controlling the acid dissociation of DHCA through basis addition.32 We show that controlling the (see procedure described in Section 5 of ESI†).34 After dissol- agglomeration of seeds in the matrix by chemical synthesis is an ving the precursors of SFO in a Teflon beaker and adding the efficient way to tune the magnetic coupling in the resulting citric acid, the seeds NPs (previously stabilized in a basic NaOH composites. In this context, our main purpose is to improve the aqueous dispersion) are added to the dispersion while adjusting chemical synthesis and shed light on the strength of magnetic the pH to the final desired value (pH = 7 and 10 were considered coupling deriving from the partial agglomeration of seeds in the in this study). After the mass correction accounting for the NCs obtained in different pH conditions. An undoped Co–ferrite coating of the seeds by thermogravimetry (TG), the final esti- sample was prepared as a reference, and the change of magnetic mated weight fraction of seeds added to the synthesis with anisotropy in the composite is discussed according to the respect to SFO is around 7 wt%. Next, it is heated in a silicon elemental doping of seeds. oil bath to 80 1C for B4 h to form a gel, and then let it quickly dry for 1 h at 120 1C. Next, the dried gel is ground to guarantee the homogeneity of the phases and burnt in a pre-heated oven at Experimental 300 1C. The resulting powders are ground and annealed at 850 1C for 4 h under air with a ramp of 5 1C min 1. We refer Synthesis of CoxZnyFe3 (x+y)O4 seeds to the final nanocomposites as NC@CZFO@pH10 (with CZFO The synthesis of CoxZnyFe3 (x+y)O4 seeds NPs of 6(1) nm was seeds, pH = 10), NC@CZFO@pH7 (with CZFO seeds, pH = 7) and carried out using standard thermal decomposition of metallic NC@CFO@pH10 (with CFO seeds, pH = 10). complexes in a high-boiling solvent under an inert atmosphere in the presence of stabilizing surfactants (see Section 1.1 of ESI†).33 Physical, structural, and magnetic characterization The Fe/Co/Zn precursors ratio has been chosen to obtain Zn0.4(1)- The powder samples were characterized by using a Bruker Co0.5(1)Fe2.1(1)O4 and Co1.0(1)Fe2.0(1)O4 (referred to as CZFO and D8 Advance diffractometer (solid state rapid LynxEye detector, CFO, respectively). The amount of Zn was chosen since Co–Zn Cu Ka radiation, Bragg–Brentano geometry, DIFFRACT plus ferrites from thermal decomposition synthesis are expected to software) in the 101–1401 2y range with a step size of 0.0131 increase their saturation with Zn2+ up to Zn0.4 0.5Co0.6–0.5Fe2O4 (counting time was 4 s per step). The powder samples were (see also Fig. S2, ESI†).25,26 Table 1 reports the samples’ details. grounded in an agate mortar and suspended in ethanol. A Si substrate was covered with several drops of the resulting Synthesis of SrFe12O19 matrix suspension, leaving randomly oriented crystallites after drying. The strontium ferrite SrFe12O19 matrix (SFO) was prepared by a Rietveld analysis was performed on the X-ray powder diffraction sol–gel combustion process.12,28 The pH effect upon the final (XRPD) data by using the FULLPROF program.35 The diffraction compound was evaluated by preparing SFO at pH = 4, 7 and 10. peaks were described by a modified Thompson–Cox–Hastings (see Section 1.2 of ESI†). pseudo-Voigt function. A peak asymmetry correction was made for angles below 401 (2y). Background intensities were estimated Synthesis of bi-magnetic NCs by interpolating a set of points. In the model, seeds were The sol–gel self-combustion synthesis was slightly modified to described as cubic Fd3% m space group. For CFO, the tetrahedral include the seed NPs in the SFO matrix as follows (Fig. 1). (Td) and octahedral (Oh) sites were assumed to be fully Firstly, a ligand-exchange process has been performed to occupied, with 2/3 Fe3+ and 1/3 Co2+. Since Fe and Co are replace the hydrophobic coating (oleic acid) with a hydrophilic indistinguishable from XRPD (electron scattering density is layer (di-hydro caffeic acid, DHCA) using tetrahydrofuran (THF) the same), we assume a random distribution along tetrahedral 27818 | Phys. Chem. Chem. Phys., 2023, 25, 27817–27828 This journal is © the Owner Societies 2023 View Article Online PCCP Paper and octahedral sites. For CZFO, the cationic distribution was assumed to be (Zn0.32Fe0.68)Td[Zn0.08Co0.46Fe1.46]OhO4 (according to a recent study by some of the authors25). SFO was modelled as a hexagonal P63/mmc space-group. The composites were refined accordingly. A NIST LaB6 660b standard was measured under the same conditions as the samples to account for the instrumental contribution to the peak broadening. Structural and morphological investigations were performed via high-resolution transmission electron microscopy (HRTEM) This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. and scanning transmission electron microscopy (STEM) in Open Access Article. Published on 02 October 2023. Downloaded on 1/18/2024 3:40:20 PM. high-angle annular dark-field mode (HAADF). These studies Fig. 2 Co0.46Zn0.40Fe2.14O4 (CZFO) seeds: (a) STEM-HAADF image show- were achieved on a field emission gun FEI Tecnai F20 micro- ing the shape and size of nanoparticles; (b) high-resolution TEM image of a scope. STEM analysis was combined with electron energy loss nanoparticle revealing the atomic planes. Inset is the Fast Fourier Trans- spectroscopy (EELS) in the Tecnai microscope by using a form of the squared area demonstrating the spinel structure of the seeds. GATAN QUANTUM energy filter in order to obtain composi- tional maps. For electron microscopy measurements, samples, in powder form, were dispersed in ethylic alcohol and sub- 5T to +5T range, and the obtained magnetization values were mitted to ultrasonic agitation for one minute. Then, a drop of normalized by the weight of powders present in the sample and the suspension was deposited on a commercial holey carbon- expressed in A m2 kg 1. To get information about the irreversible coated TEM grid and kept in the air until complete alcohol processes, direct current demagnetization (DCD) remanence evaporation. curves were measured by applying and removing a progressively 57 Fe Mössbauer spectra were recorded using a 57Co/Rh g-ray higher DC reverse field to a sample previously saturated under a source mounted on an electromagnetic transducer with velocity (negative) field of 5T and by recording, for each step, the value of modulated according to a triangular waveform. The spectra the remanent magnetization, MDCD, which is then plotted as a were obtained at 300 and 77 K without external applied field. function of the reverse field. The corresponding switching field The hyperfine structure was modeled by means of a least- distributions (SFDs) were obtained from the first order derivative square fitting procedure involving Zeeman sextets composed of the MDCD curves. We refer to the average switching field as HSW. of Lorentzian lines. The isomer shift (IS) values were referred to that of a-Fe at 300 K. Dynamic Light Scattering (DLS) measurements were per- Results and discussion formed with a Malvern Zetasizer Nano ZSP equipped with a 10 mW He–Ne red laser (632.8 nm), operating in backscattered Tuning particle-agglomeration geometry (1731). The suspensions of NPs were analysed in The thermal decomposition method has allowed the preparation plastic disposable cuvettes after filtration and sonication. of monodisperse, highly uniform spherical CZFO NPs. A typical More information about the measurements is reported in STEM-HAADF image of the CZFO seeds is shown in Fig. 2(a). The Section 6 of ESI.† NPs have a spherical shape with a uniform diameter of about Elemental analysis was performed by X-ray fluorescence 5.4 nm  1.4 nm, in agreement with XRPD analysis (see Section 3 (XRF) on the selected composites, showing only slight devia- of ESI† for more details). The seeds are well crystallized as tions from the target composition. Stoichiometric SFO and CFO evidenced by HRTEM analysis showing the atomic planes extend- powder samples were adopted as standards for the analysis, by ing on the entire particle size, Fig. 2(b). The inset of Fig. 2(b) is the checking their atomic composition (Fe/Sr = 12 and Fe/Co = 2) Fast Fourier Transform (FFT) of the image and allows to identify by inductively coupled plasma optical emission spectroscopy the cubic spinel structure of the particle. The composition of the (ICP-OES). The estimation of spinel ferrite NPs weight fractions seeds was confirmed by STEM-EELS compositional maps, and a was carried out as reported in Section 7 of ESI.† The elemental uniform distribution of Zn was observed, Fig. S4 (ESI†). From the composition of seeds NPs was confirmed by ICP-OES. ZFC/FC curves (see Section 4.1 of ESI†), the extracted maximum Magnetic measurements were performed at room and low temperature, irreversible temperature, and average blocking tem- temperature (T = 300 and 5 K, respectively) by using a Quantum perature (Tmax, Tirr and Tb respectively) were found to decrease for Design superconducting quantum interference device (SQUID) CZFO, due to the lowering of anisotropy upon replacement of Co2+ and Physical Property Measurement System (PPMS) with Vibrat- with Zn2+, as reported in Table 1. This effect is indeed accom- ing Sample Magnetometer (VSM) option magnetometers, which panied by the corresponding decrease of the coercivity HC.26,27 can supply maximum fields of 5 T and 9 T, respectively. Mössbauer spectrometry confirmed the dynamic properties of To avoid any displacement of the nano-powders during the CZFO, highlighting the presence of sextets in the spectrum at measurements, the samples were immobilized in a chemical 77 K, which account for the larger fractions of blocked NPs (see neutral ethyl cyanoacrylate glue (no significant magnetic contri- Section 4.3 of ESI†), in comparison to the large singlet at 300 K bution from the glue was observed). Isothermal field-dependent typical of unblocked, i.e. superparamagnetic, NPs (or for particles magnetization loops were recorded by sweeping the field in the whose relaxation time is near the measurement time scale). This journal is © the Owner Societies 2023 Phys. Chem. Chem. Phys., 2023, 25, 27817–27828 | 27819 View Article Online Paper PCCP Table 1 Composition from ICP-OES analysis, maximum temperature (Tmax), irreversible temperature (Tirr), and average blocking temperature (Tb) from ZFC–FC curves, coercive field (m0HC), saturation magnetization (M5T 5T S ) and reduced remanent magnetization (MR/MS ) from magnetization (M) vs. temperature (T) at 5 K, and calculated anisotropy constant (Keff). See ESI for magnetic curves Sample CFO CZFO Composition Co1.04Fe1.96O4 Co0.46Zn0.40Fe2.14O4 Tmax (K) 256(7) 197(6) This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Tb (K) 178(5) 140(4) Tirr (K) 264(7) 208(6) Open Access Article. Published on 02 October 2023. Downloaded on 1/18/2024 3:40:20 PM. m0HC (T) 0.90(1) 0.53(1) M5T 2 1 S (A m kg ) 90(1) 100(1) 5T MR/MS 0.57(1) 0.60(1) Keff (J m 3) 7.8(4)  105 4.8(2)  105 The CZFO NPs underwent a ligand-exchange process which permitted their controlled partial agglomeration upon tuning the pH conditions of the starting aqueous suspension (given by the presence of citric acid as a chelating agent).36 This enabled us to exploit the chemical structure of DHCA in a specific range, pH = 7–10, for which citric acid is completely deprotonated.37 Replacing the hydrophobic coating (oleic acid) with the hydro- philic one (DHCA), did not only allow us to obtain stable suspensions of nanoparticles in water, but provided one more Fig. 3 Rietveld refinement analysis of the composites containing CZFO degree of freedom to tune the morphology of the seeds. DHCA obtained at different pH for starting solutions: (a) pH = 10 (NC@CZFO@ has three dissociation constant pKa (4.5, 9.4 and 11.7),38 allowing pH10) and (b) pH = 7 (NC@CZFO@pH10) For NC@CZFO@pH7, * indicate a possible impurity.41 STEM-HAADF image of (c) NC@CZFO@pH7 and (d) us to tune the degree of deprotonation by simply adjusting the NC@CZFO@pH10 showing the different sizes of the crystallites and their acidic/basic water suspension, which in turn affects the stability agglomeration. of NPs in water.38 Moving from a neutral (i.e., pH = 7) environ- ment to basic (i.e., pH = 10) environment, yields a larger deprotonation of DHCA, being well above the second acid pKa phases for the composite at pH = 7 (NC@CZFO@pH7), which is (with a corresponding total deprotonation of the carboxylic inducing the independent growth of SFO. In contrast, a more moiety, and more than 50% of the catechol), and a consequent homogeneous distribution of the CZFO phase prevents the larger electrostatic stability due to the higher repulsion between growth of both phases, demonstrated earlier.5,44,45 In addition, the ligand molecules, which are likely attached to the NPs the presence of additional impurities, in the pattern at pH = surface through both the carboxylic and the catechol functional 7,41 suggests an incomplete growth of SFO. groups.39 While at pH = 7 the lower stability favours a higher The morphology of the composites has been investigated by degree of agglomeration, with a consequent growth of larger STEM-HAADF revealing a perfect agreement with the XRPD crystallites during the annealing step, compared to those at pH = results. In particular, Fig. 3(c) and (d) respectively show typical 10. As a result, the partial agglomeration of CZFO NPs was images of NC@CZFO@pH7 and NC@CZFO@pH10: the two evidenced by DLS measurements, which highlight a multiple nanocomposites are constituted by crystallites, strongly agglom- distribution decay of the correlation function for pH = 7 (Fig. 1 erate in the sample obtained with pH = 7, while in that with pH = and ESI,† Section 6).40 The subsequent addition to the sol–gel 10 they give rise to a more porous structure. Furthermore, self-combustion route, during the pH adjustment-step, resulted comparing the two images, the crystallites in the pH = 7 in an effective way to control the initial stability of CZFO composite are on average larger than in the pH = 10 one, as particles and thus their final size in the composite after the also evidenced by XRPD measurements. In such images, it is not annealing step, which causes a different growth of this phase in easy to distinguish the spinel CZFO phase from the hexaferrite the SFO matrix. SFO one and for this reason, electron energy loss spectroscopy Rietveld analysis of XRPD patterns for NC@CZFO@pH10 (EELS) measurements were performed in STEM mode to analyse and NC@CZFO@pH7, Fig. 3(a) and (b) respectively, reveals that the elemental distribution. the average crystallite size of the spinel fraction increases Two typical STEM-EELS compositional maps are reported in to 32(4) nm and to 57(5) nm, respectively (see Table 2); inter- Fig. 4. In particular, in the top panel of Fig. 4 the composition estingly, the SFO fraction increases as well, although we have of the NC@CZFO@pH7 sample is shown, and in the bottom observed that the size of SFO (without seeds) seems rather panel of the same figure the composition of NC@CZFO@pH10 insensitive to the change of pH from 10 to 7 (see Section 3 of is reported. In both images it is possible to clearly distinguish ESI†).42,43 Therefore, it is likely to be the segregation of the two CZFO and SFO structures, observing that in NC@CZFO@pH7 27820 | Phys. Chem. Chem. Phys., 2023, 25, 27817–27828 This journal is © the Owner Societies 2023 View Article Online PCCP Paper Table 2 List of parameters: starting precursors’ solution pH; lattice parameters a, b and c and crystallite sizes d for hexagonal and spinel ferrites extracted by XRPD analysis; compositional analysis by XRF; corresponding saturation at 5T (M5T 5T S ), and reduced remanent (MR/MS ) magnetization, coercive field (m0HC) and switching/coercive fields ratio (HSW/HC) measured at 300 K and 5 K (in square brackets). *indicates the presence of an impurity (see main text for details) Hexagonal Spinel 300 K [5 K] wt% a=b=c d wt% M5T S Sample pH a = b; c (Å) dab; dc (nm) (XRF) (Å) (nm) (XRF) (A m2 kg 1) MR/M5T S m0HC (T) HSW/HC NC@CZFO@pH10 10 5.88047(2); 23.04622(5) 135(15); 92.8(1) 8.4251(1) 32(4) 7.2(2) 73(1) 0.48(1) 0.48(2) 1.13 107(10) [108(3)] [0.50(1)] [0.38(2)] [1.10] This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. NC@CZFO@pH7 7 5.88118(2); 23.04327(6) 166(28); 93.1(9)* 8.42248(2) 57(5) 6.9(3)* 64(1) 0.48(1) 0.51(3) 1.20 116(10) [89(1)] [0.49(1)] [0.39(2)] [1.22] Open Access Article. Published on 02 October 2023. Downloaded on 1/18/2024 3:40:20 PM. SFO@pH10 10 5.87879(2); 23.05107(5) 136(9); 114(5) 100 — — — 69(1) 0.49(1) 0.61(3) 1.09 [100(3)] [0.50(1)] [0.51(3)] [1.23] NC@CFO@pH10 10 5.88073(2); 23.04259(5) 116(13); 92(8) 91.5(9)* 8.40204(6) 36(2) 8.5(8)* 69(1) 0.46(1) 0.55(3) 1.21 [89(1)] [0.50(1)] [0.53(3)] [1.26] the CZFO is in partial contact with the SFO, while in the 77 K do not show any such doublet (Fig. 5(c) and (d)), as a result NC@CZFO@pH10 the CZFO is fully embedded in the SFO. of high degree of agglomeration. The latter ensures an optimal connection between the two The Rietveld analysis does not show any secondary impurity structures and consequently an excellent magnetic coupling. upon introduction of Zn2+ in the spinel-structure of CZFO, Furthermore, looking at the CZFO size, a reduced particle while the (a) lattice parameter increases, as expected for similar dimension of this phase is observed in pH = 10 (B35 nm) Zn-doped particles, up to 8.4091 Å (see Section 8 of ESI†).26,47 compared to pH = 7 (B100 nm) in agreement with XRPD However, the annealing step in air for the composites might measurements and with the effect of pH on the tendency of induce a cationic redistribution, as hinted by the change of CZFO seeds to agglomerate. lattice parameter (up to 8.4162 Å):48–50 this is also in agreement Indeed, as a further confirmation of the particle size’s with the observed segregation of hematite for a reference CZFO change, the Mössbauer spectrum of NC@CZFO@pH10 sample, whose resulting weight fraction (B11 wt%) after recorded at 300 K (in Fig. 5(a)) displays overlapping mixed annealing matches with the iron excess determined by ICP sextets due to SFO/CZFO particles with long relaxation time (B13 wt%). Interestingly, the extracted lattice parameter for the compared to the timescale of the technique (i.e., blocked CZFO phase in the composite increases even slightly further (up particles),46 but also a doublet due to a fraction of CZFO with to B8.4251 and 8.4225 Å at pH = 10 and 7, respectively) (see shorter relaxation times, whose relative area decreases upon ESI,† Section 8.3 for details). This increase in the composite decreasing temperature (see Fig. 5(b) and Section 8.4 of ESI†). might be due to Sr2+ substituting cations (Co2+ or Zn2+) at the In contrast, the spectra of NC@CZFO@pH7 at both 300 and interface with the spinel phase. Previous studies have observed that the addition of Sr2+ in CFO and CZFO nanoparticles can increase the lattice parameter,51,52 up to B8.42 Å for Sr2+ contents as low as 0.01 (1%) in CZFO samples with Co2+/ Zn2+composition close to our case (e.g., Co0.59Zn0.4Sr0.01Fe2O4). Additional strain effects induced by the contact between the two phases cannot be excluded. Magnetic coupling Fig. 6 shows the loops field dependence of magnetization of the nanocomposites, obtained at different pH, at T = 300 (a) and 5 K (b). The main parameters extracted from the loops (saturation magnetization, MS, reduced magnetization, MR/MS, coercive field, HC, and switching/coercive fields ratio, HSW/HC) are shown in Table 2. The loops show rather similar hysteretic behaviour, with a similar evolution of the coercivity as a function of Fig. 4 Left: STEM-HAADF images of NC@CZFO@pH7 (top panel) and temperature, which preserves large coercivities at 300 K, thus NC@CZFO@pH10 (bottom panel) nanocomposites showing the size and reflecting the dominant character of SFO over CZFO (SFO has an distribution of the CZFO phase with respect to the SFO one. Right panels: average size corresponding to a high coercivity range), with slight SEM-EELS maps showing all elements, all cations but Fe, and the individual variations presumably due to the morphological differences O K-edge at 532 eV (pink), Fe L-edge at 708 eV (green), Co L-edge at 779 eV (blue), Zn L-edge at 1020 eV (red) and Sr L-edge at 1940 eV (light together with the different size distributions of CZFO phase. blue) (scale bars = 100 nm for NC@CZFO@pH7; scale bars = 50 nm for Besides, some significant differences may be noticed: the NC@CZFO@pH10). remanent magnetization from direct current demagnetization This journal is © the Owner Societies 2023 Phys. Chem. Chem. Phys., 2023, 25, 27817–27828 | 27821 View Article Online Paper PCCP This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 02 October 2023. Downloaded on 1/18/2024 3:40:20 PM. Fig. 5 Mössbauer spectra of NC@CZFO@pH10 at 300 K and 77 K (a), (b) and of NC@CZFO@pH7 at 300 K and 77 K (c), (d), respectively. (DCD) experiments, MDCD, versus reverse magnetic field,53 at enhancement of remanent magnetization for NC@CZFO@pH10 pH = 7 (NC@CZFO@pH7) is crossing the field axis at values (B13%) in comparison to NC@CZFO@pH7, can be ascribed to HSW (i.e. switching field) larger than HC (at both 300 and 5 K) in the more homogenous distribution of CZFO in the host SFO, due comparison to pH = 10, hinting at the presence of irreversible to the higher colloidal stability at pH = 10, which maximize the phenomena due to the less uniform particle distribution. This magnetic coupling between the two phases, as a result of the can be confirmed by comparing the respective differentiated controlled increase of surface area at the interface.54 curve of MDCD with respect to H, i.e. switching field distribu- However, understanding the coupling mechanisms in this tions (SFDs) which represent the irreversible component of kind of sintered magnets is a nontrivial task: such an effect the susceptibility (wirr) (inset of Fig. 5): the relative difference might be hidden at room temperature, where the predominant ((HSW HC)/HC) corresponds to 22% for NC@CZFO@pH7, com- harder magnetic nature of the Sr ferrite could mask the reversal pared to 10% for NC@CZFO@pH10, as may be generally observed of the small fraction of seeds. It was shown that a coupled for NPs systems.54 On the other hand, the harder magnetic SFO system at room temperature might actually display features of a phase dominates the reversal of NC@CZFO@pH10, pointing out decoupled one at lower ones.59 To better highlight the presence its role in preventing the demagnetization of the softer Zn-doped of an enhanced magnetic coupling between the two phases in ferrite. We also point out that the irreversible component accounts NC@CZFO@pH10, owing to the optimized interfaces, we com- for the thermally blocked NPs, so some discrepancies may arise pare in Fig. 6(c) and (d) the hysteresis loops at 300 and 5 K for from the different timescales in the relaxation process.55 The the weighted sum (i.e., superposition)of SFO@pH10 and narrow and symmetric average SFD of NC@CZFO@pH10 suggests annealed CZFO seeds (with the corresponding weight ratio), that the magnetic interaction here (whether or not direct exchange, and the experimental loops for NC@CZFO@pH10 as a refer- i.e. dipolar-like) seems to be maximized in that system. The ence. We should keep in mind that we expect the anisotropy of asymmetric SFDs of NC@CZFO@pH7 suggest the presence of CZFO to increase as temperature is lowered, while the aniso- more magnetic components at play.56 In addition, the average tropy of SFO decreases,60 and thus the critical size for effective size of CZFO in NC@CZFO@pH7 is close to the single domain coupling changes significantly.61 Even if CZFO phases grow limit, above which the magnetization does not reverse through a differently whether in the composite or individually (see Sec- uniform coherent rotation (with the corresponding non- tion 8 of ESI†), at 5 K it is evident (despite the low concentration monotonic dependence of HC with size, observed at low tempera- of CZFO, 7 wt%) that the hysteresis for the superposition shows ture for the annealed CZFO, see Fig. S19, ESI†).57,58 Thus, the a double-loop response, typical of mixtures/phase-segregated 27822 | Phys. Chem. Chem. Phys., 2023, 25, 27817–27828 This journal is © the Owner Societies 2023 View Article Online PCCP Paper This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 02 October 2023. Downloaded on 1/18/2024 3:40:20 PM. Fig. 6 Magnetization M vs. magnetic field m0H curves for NC@CZFO@pH10 and NC@CZFO@pH7 and magnetization MDCD vs. reverse magnetic field H curves for the corresponding samples at (a) 300 K and (b) 5 K. In the insets, the associated SFDs. Normalized magnetization M vs. magnetic field m0H curves for (c) superposition of CZFO (annealed at the desired temperature) and SFO@pH10 and (d) NC@CZFO@pH10, at both 300 and 5 K. In the insets, the corresponding switching field distributions. systems.5,17 On the contrary, the composite shows an ordinary by XRF in ESI†). Nonetheless, our purpose here is to show that single-loop response at low temperatures, as a result of strong the strength of magnetic coupling, qualitatively evinced by magnetic interactions between the NPs. Interestingly, we point demagnetization experiments, can be evaluated from the cor- out that at 300 K even hysteresis curve for the superposition relation between the change of magnetic anisotropy of compo- (where no magnetic interactions are considered) displays a site and that of the seeds. single loop, demonstrating the need of low-temperature As shown in Fig. 7, the anisotropy of the composite changes measurement to fully characterize the reversal in those binary according to the anisotropy of the corresponding spinel phase. systems.61 At both 300 K and 5 K, the coercive field of NC@CFO@pH10 increases as expected for regular undoped CFO particles, while Effect of elemental doping on the anisotropy of NCs the saturation magnetization decreases. These results are con- In the previous section we have shown that the magnetic firming earlier observations reported in ref. 54 and 62. In the coupling between CZFO and the matrix can be tuned by the insets of Fig. 7, we note that the average switching field of effect of the initial partial agglomeration. To evaluate the role of NC@CZFO@pH10 is located at lower values than that of anisotropy of the seed phase on the overall composite magnetic NC@CFO@pH10, as the corresponding magnetic anisotropy behaviour, undoped Co–ferrite (CFO) NPs with the same size of CZFO is lower than that of CFO,25 thus highlighting the (6(1) nm) were synthesized using the same route and used to effective tuning of the magnetic anisotropy of the composite.27 prepare a composite (NC@CFO@pH10) at pH = 10 starting The effective magnetic anisotropy of the composites is calcu- conditions. XRPD analysis revealed a CFO phase of 36(2) nm, lated in the Section 8.5 of the ESI.† Interestingly, a much higher thus confirming the robustness of the pH-controlled agglom- value is obtained for NC@CFO@pH10 at 5 K, suggesting a eration, as observed for NC@CZFO@pH10. Lattice parameters strong effect produced by the CFO seeds, as also indicated by for SFO@pH10 are in good agreement with the other compo- the small secondary peak of the irreversible susceptibility at sites, while for CFO do not change significantly (see ESI†). The large fields (m0HSW B 1.2T).54 Indeed, the corresponding ani- only difference is the small hematite impurity accompanied by sotropy field for non-agglomerated (smaller) particles is much unreacted SrCO3 which resulted in a slightly lower SFO fraction larger. As also previously evidenced by Mössbauer spectrometry (and in turn a larger CFO weight fraction (B8.5 wt%) as shown for the NC@CZFO@pH10 case, we may expect to have smaller This journal is © the Owner Societies 2023 Phys. Chem. Chem. Phys., 2023, 25, 27817–27828 | 27823 View Article Online Paper PCCP This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 02 October 2023. Downloaded on 1/18/2024 3:40:20 PM. Fig. 7 Magnetization M vs. magnetic field m0H curves for nanocomposites with preformed CFO and CZFO NPs at pH = 10 (NC@CFO@pH10 and NC@CZFO@pH10) and magnetization MDCD vs. reverse magnetic field H curves for the corresponding samples at (a) 300 K and (b) 5 K. For comparison, panels for SFO synthesized at pH = 10 (SFO@pH10), and NC@CZFO@pH10 are reported at (c) 300 K and (d) 5 K. particles for NC@CFO@pH10 as well (owing to the colloidal preformed NPs via control over the colloidal stability inside a stability at pH = 10) that are not coupled with the matrix and magnetically hard matrix. The nanocrystallites’ sizes extracted thus magnetically reverse independently (inset of Fig. 7(b)), from XRPD analysis match the results from STEM-EELS com- since we observed a doublet assigned to the CZFO component positional maps, which evidence the inclusion of the NPs in the in the spectrum at 300 K, which drastically decreases at 77 K SFO. The observed enhancement of remanent magnetization (i.e. moving from thermally unblocked to blocked). In contrast, for NC@CZFO@pH10 is related to the homogenous distribu- such contribution is hidden for CZFO, as the magnetic aniso- tion of NPs in the composite, which favours a higher degree of tropy is expected to be lower. We should point out that Keff magnetic interactions between the two phases in contact (i.e., considered here is used to compare the anisotropy for the an increase of surface area at the interface). Furthermore, we composites. demonstrate that the change of magnetic anisotropy in the seed Furthermore, by comparing the MDCD curves, we point out (i.e., CZFO) is extended to the composites, since their aniso- that remarkably, a slight increase of remanence is produced for tropy changes accordingly. To conclude, we propose a simple NC@CZFO@pH10 compared to NC@CFO@pH10, revealing the synthetic route to obtain nanomaterials with controlled struc- possibility to enhance the remanence by coupling SFO to a tural and well-defined morphological properties. second prototypical particle, extremely important for technolo- gical applications, such as permanent magnets.63–66 Among other synthesis methods to produce composites with similar magnetic properties (see Section 9 of ESI†),67–69 this design is Author contributions essentially suitable for any type of particle, thus opening up the The paper and ESI† were written based on the contributions of way for potential application of this pH-controlled agglomera- all authors. P. M., D. P. and R. M. designed the experiments; tion in other systems. R. M. and D. P. coordinated the data analysis and discussion. M. B. and P. M. synthesized the nanoparticles and nanocom- Conclusions posites and characterized them by XRPD, XRF, DLS, TG and magnetometry techniques. G. B., M. C. S. and J. A. characterized We have designed a seed-mediated approach to synthesize bi- the systems and analysed the data by HRTEM, STEM-HAAD and magnetic composites by controlling the aggregation of the EELS. N. Y. performed Mössbauer spectrometry. All authors 27824 | Phys. Chem. Chem. Phys., 2023, 25, 27817–27828 This journal is © the Owner Societies 2023 View Article Online PCCP Paper contributed to the results discussion and revision of the article, 8 T. Ohya, M. Ito, K. Yamada, T. Ban, Y. Ohya and which was written mainly by P. M. and R. M. Y. Takahashi, Aqueous Titanate Sols from Ti Alkoxide- Hydroxycarboxylic Acid System and Preparation of Titania Films from the Sols, J. Sol-Gel Sci. Technol., 2004, 30(2), Conflicts of interest 71–81, DOI: 10.1023/B:JSST.0000034694.61859.78. 9 E. Scolan and C. Sanchez, Synthesis and Characterization of There are no conflicts to declare. Surface-Protected Nanocrystalline Titania Particles, Chem. Mater., 1998, 10(10), 3217–3223, DOI: 10.1021/cm980322q. 10 C. Cannas, A. Musinu, D. Peddis and G. Piccaluga, Synthesis This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Acknowledgements and Characterization of CoFe2O4 Nanoparticles Dispersed in Open Access Article. Published on 02 October 2023. Downloaded on 1/18/2024 3:40:20 PM. This work was financially supported by the Swedish Energy a Silica Matrix by a Sol–Gel Autocombustion Method, Chem. Agency (project number 46561-1) and Swedish Research Coun- Mater., 2006, 18(16), 3835–3842, DOI: 10.1021/cm060650n. cil (VR). ICN2 acknowledges funding from Generalitat de 11 M. Zayat and D. Levy, Blue CoAl2O4 Particles Prepared by the Catalunya 2021SGR00457. The authors thank support from Sol–Gel and Citrate–Gel Methods, Chem. 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