Understanding High Anisotropic Magnetism by Ultrathin Shell Layer

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Understanding High Anisotropic Magnetism by Ultrathin Shell Layer Formation for Magnetically Hard–Soft Core–Shell Nanostructures Kwan Lee, Sangyeob Lee, and Byungmin Ahn Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03591 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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Chemistry of Materials

Understanding High Anisotropic Magnetism by Ultrathin Shell Layer Formation for Magnetically Hard–Soft Core–Shell Nanostructures Kwan Lee, †#* Sangyeob Lee, ‡ and Byungmin Ahn †‖* †Department

of Energy Systems Research, Ajou University, Suwon 16499, Republic of Korea of Materials Science and Engineering, Hanbat National University, Daejeon, 34158, Republic of Korea ‖Department of Materials Science and Engineering, Ajou University, Suwon 16499, Republic of Korea ‡Department

ABSTRACT: Magnetic core-shell nanostructures offer a viable solution for tunable magnetism via nanoscale exchange interactions in a single component unit. A typical synthetic approach for monodisperse bimagnetic ferrite core-shell nanostructures employs the seed-mediated growth method using the heating-up process. Understanding magnetic coreshell interface formation and their interactions is crucial, however, the magnetical persistence of the pristine core component during heating-up is unclear. Here, we elucidate the enhancement mechanism of magnetic anisotropy when the hard-soft core-shell nanostructures are formed with ultrathin shell layer. The heating-up effect on the core component exhibits the coordination change of ligand chemisorption with surface metal ions, which lead to a substantial increase in surface anisotropy due to enhanced spin-orbit couplings. We further demonstrate that the selection of metal precursors and surfactant for additional shell layer formation is important. The kinetic of shell formation rate by their thermolysis and atomic-scale surface etching by the surfactant led to the disordering of surface spins on the core parts. Our observations provide the underlying mechanism of high anisotropic magnetism while bimagnetic ferrite core-shell interface formation and the voyage of synthetic procedures for the additional shell layer is critical to an outcoming magnetism.

INTRODUCTION Magnetic exchange interactions at the nanoscaleinterface between magnetically different components have significantly attracted interests due to their importance for both fundamental science and technological applications.1– 5 The combinations of distinct magnetic materials as coreshell nanostructured-configuration enable the integration of various magnetic properties and functionalities into an individual unit and provide the creation of novel magnetism.6,7 For example, magnetically hard-soft coreshell nanostructures lead to an efficient heat induction via exchange couplings, and ferromagnetic-antiferromagnetic core-shell type provides effective anisotropic magnetic properties by exchange bias.5,8 Although the control of the shell volume fraction permits to linearly modulate the magnetic anisotropy via exchange coupling, however, the formation of low shell volume fraction has recently exhibited unprecedented magnetism.9,10 The apparent increase in coercivity was experimentally demonstrated higher than the original coercivity of the core materials. An enhanced interface spin canting by ultrathin shell layer undergoes the substantial enhancement of magnetic anisotropy, however, understanding the underlying mechanism is lacking. Monodisperse magnetic nanostructures have been synthesized using two typical wet chemical routes including (i) thermal decomposition of metal oleate complexes and (ii) reduction enhanced reaction with metal

acetylacetonate reagents and oleylamine using hightemperature boiling organic solvents.9–11 Furthermore, seed-mediated growth approaches enable additional shell formation leading to both (i) the size-controlled single component nanostructures and (ii) the formation of judicious core-shell hetero-architecture with controllable interfaces by changing size, shape and chemical composition.12–16 For the significantly improved performances, bimagnetic ferrite core-shell nanostructures have been synthesized by maximizing the defect-free interface between the two magnetic phases using the heating-up. The heating-up process is crucial approaches controlling burst decomposition of metalorganic complexes, which undergo homogeneous nucleation and monodisperse ferrite nanostructures.17–19 However, thermal decomposition of metal complexes in the presence of surfactants and organic solvent at reflux temperature showed a lack of reproducibility.20 Furthermore, chemisorption stability of ligands on the magnetic nanoparticles is a critical factor during the heating-up, however, solvent annealing could undergo the desorption of the ligand molecules from the nanostructure surface.21,22 Here, we demonstrate the heating-up process effect on the magnetism of the seed materials for additional shell layer formation, which induce magnetically anisotropic enhancement when forming an ultrathin shell layer. We study two platforms as the representatives in this study

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using both the single component magnetic nanoparticles (e.g. Fe3O4) and the magnetically hard-soft core-shell nanocubes (e.g. CoFe2O4‒MnFe2O4) for additional layer formation with ultrathin thickness via the heating-up approaches. We observed the coordination modification of surfactant chemisorption on the core components during the heating-up in an organic solvent. It led to the increase in the surface anisotropy by enhancing spin-orbit couplings. In addition, we observed the surfactant-induced surface etching of the ferrite seed materials at the reflux temperature of the organic solvent. We show that the selection of metal precursor reagents and the surfactant are critical factors to modulate the exchange interactions of CoFe2O4‒MnFe2O4 core-shell nanocubes at low shell volume fraction. The kinetic-controlled shell layer formation rate by metal precursors concentration and their thermolysis with surfactants led simultaneously to three phenomena including (i) chemisorption modification of ligand, (ii) the surface etching of pristine core components by surfactants, and (iii) formation of ultrathin shell layer during the heating-up for CoFe2O4‒MnFe2O4 core-shell nanocubes. They lead to the magnetism evolution of hard-soft core-shell platforms from the enhanced surface spin canting induced anisotropy enhancement regime to a typical exchange couplings regime by controlling shell volume fraction. We can define the ultrathin shell layer as the shell volume fraction in the regime of the enhanced surface spin canting in this study. Our understanding the underlying mechanism of unprecedented magnetism with ultrathin layer formation paves a way to design the synthetic procedure of the coreshell configuration, in which the resultant nanoscale interface magnetism is substantially determined by the shell layer synthesis procedures. EXPERIMENTAL SECTION Chemicals. All chemicals were used as received. Iron (III) acetylacetonate (Aldrich, ≥ 99.9%), cobalt (II) acetylacetonate (Aldrich, ≥ 97%), manganese (II) acetylacetonate (Aldrich, ≥ 99.9%), manganese (II) acetate (Aldrich, ≥ 99.9%), oleic acid (Aldrich, technical grade 90%), sodium oleate (Pub chem), benzyl ether (Aldrich, 98%) and 1-octadecene (Aldrich, 90%). All above chemicals were used as received without further processing. Synthesis of spherical Fe3O4 nanoparticles. We synthesized a series of spherical Fe3O4 nanoparticles using previously reported procedures 23. Monodisperse magnetite nanoparticles with the diameter of 13 nm were synthesized using a mixture of iron-oleate complexes (2 mmol), oleic acid (1 mmol) and 1-octadecene (10 mL). The controlled heating rate between 3 ℃/min and 15 ℃/min was applied to modulate the size and crystallinity of magnetite nanoparticles and kept at the reflux temperature of the organic solvent for 30 min under an environment of argon flow. The final solution was cooled naturally to room temperature. The synthesized nanoparticles were separated from the reaction solution by adding ethanol (Aldrich, 98%, 2 times of the reaction solution) as a precipitating agent with the centrifugation. The separated nanoparticles were dispersed in hexane

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(Aldrich, 90%) forming a room temperature dispersion with dark brown color. Solvent thermal treatment was performed under identical heating rate, organic solvent volume, and surfactants concentration, in which the same solvent (10 mL) and surfactants (1 mmol) for synthesizing the pristine nanoparticle was used. Synthesis of cubic CoFe2O4 nanoparticles. The mixture of iron (III) acetylacetonate (3 mmol), and cobalt (II) acetylacetonate (1.5 mmol), oleic acid (1.14 mmol), and benzyl ether (30 mL) was placed in a 100 ml threeneck round-bottom flask. The mixed solution was degassed for 60 min at room temperature, heated up to reflux condition (over 290 ℃) for 15 min and kept for 30 min at that temperature under an environment of argon flow. After finishing the reaction, the heating source was quickly removed and naturally cooled down to room temperature with continuous stirring. Addition of ethanol with 2 times volume of the resultant solution led to the formation of black precipitation under ambient condition. Centrifugation for isolation of the precipitation and redispersion in toluene was repeated to remove unreacted reagents, and the resultant nanocubes were finally stocked in toluene with stable dispersion. Solvent thermal treatment was performed as same method (30 mL of benzyl ether and 1.14 mmol of oleic acid) for synthesizing the pristine nanocubes without metal precursors. Synthesis of cubic CoFe2O4-MnFe2O4 core-shell nanoparticles. For cubic magnetically hard-soft core-shell nanoparticle synthesis, the typical seed-mediated growth method was used. The cubic core nanoparticles were mixed with iron (III) acetylacetonate, manganese (II) reagents with high temperature boiling solvent and surfactants. We used manganese (II) acetylacetonate (m.p. = ~250 ℃) and manganese (II) acetate (m.p. > 300 ℃) as Mn source, and oleic acid and sodium oleate were used as a surfactant in this study. The mixture solution was degassed at room temperature for 60 min, heated up to reflux condition of organic solvent (over 290 ℃) for 15 or 20 min under an environment of argon flow, and kept for 30 min at the identical temperature. The separation and cleaning process of core-shell nanocubes was identically performed as done with the core nanocubes. The resultant cubic core-shell nanoparticles were finally stocked in toluene with stable dispersion. As the previous report, the ultrathin shell layer thickness (2 nm) was formed using 0.2 mmol of iron (III) acetylacetonate and 0.16 mmol of manganese acetylacetonate.10 For 5 nm thick shell layer of MnFe2O4, 0.5 mmol of iron (III) acetylacetonate and 0.32mmol of manganese acetylacetonate was used. Structural and compositional characterization. The size, shape and chemical composition of as-synthesized, heated, core-shell nanoparticles were examined using a field-emission transmission electron microscopy (JEOL 2100F FE-TEM, Tokyo, Japan) and energy dispersive X-ray spectroscopy (EDS, INCA, Oxford Instruments). Samples for TEM characterization were prepared by adding one drop of the solution with product onto 300 mesh copper grids with carbon support film (Ted Pella #01820). X-ray diffraction (XRD, Bruker D8 Advance X-ray diffractometer, Karlsruhe, Germany) was employed for crystalline structure analysis of all synthesized magnetic


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nanoparticles. Samples for FTIR characterization were prepared by drop-casting concentrated nanostructures solution onto ZnSe windows, followed by drying in air for achieving films. FTIR spectra were acquired by Nicolet 870 FTIR spectrometer. Magnetic properties characterization. The magnetic properties of as-prepared nanoparticles are examined by measuring the field-dependent magnetization in the range of various maximum magnetic field strength at 5 K and room temperature using a vibrating-sample magnetometer (VSM) and a superconducting quantum interference device (SQUID, Quantum Design) with a simple correction method for the sample shape and radially offset effects were used. The blocking temperature of all magnetic nanoparticles (10 mg) with the applied magnetic field (50 Oe) was determined by the ZFC (Zero Field Cooling)-FC (Field Cooling) measurement. Mössbauer spectrometer (homemade system assembled with a superconducting magnet in split-coil geometry and liquid helium bath cryostat) was recorded under transmission geometry. Magnetic field was aligned the γ-ray propagation direction. A constant-acceleration driving mode using a 57Co source with 50 mCi activity in rhodium matrix was applied without vibration noise. RESULTS AND DISCUSSION High-temperature synthetic approach effect on magnetic anisotropy. Monodisperse magnetic nanoparticles with controlled size, shape, and chemical composition were prepared under the high-temperature synthetic condition.23,24 The heating-up process is a versatile approach to initiate burst nucleation separated with nanoparticle growth. Two steps synthetic method using seed materials allow for both sizes-controlled single component nanoparticles and core-shell heterostructure formation. Although a high-temperature process with controlled heating rate is a critical factor, however, the seed materials are inevitable from solvent thermal treatment during the heating-up. To understand the solvent heating effect on the pristine magnetic nanoparticles while it is heated up, we first synthesized monodisperse iron oxide (Fe3O4) nanoparticles with 13 nm in diameter through thermal decomposition method of iron oleate complexes at a reflux temperature condition of an alkene hydrocarbon solvent (e.g. 1-octadecene, 315 ℃). We performed thermal treatment for 30 min without surfactants (e.g. oleic acid) using an identical solvent for the synthesis of pristine nanoparticles with the heating-up rate of 15 ℃/min. In Figure 1, we study both the crystallinity and the magnetic properties of monodisperse Fe3O4 nanoparticles between the as-synthesized state (MNPpristine) and the solvent-heated counterpart (MNPheated). Large-scale transmission electron microscope (TEM) images in Figure 1a confirm that the thermal treatment at solvent reflux allows for preservation of identical spherical morphology and the size of both MNPpristine (black outline) and MNPheated (red outline) (Supporting Information Figure S1). In addition, the powder X-ray diffraction (XRD) in Figure 1b shows that the crystalline feature of both MNPpristine (black line) and MNPheated (red line) is identically evident as the

inverse spinel cubic structures. No detectable higher angle peaks and emergence of weak diffraction peaks corresponding to the (110), (111) and (211) planes is confirmed. We conclude no formation of γ-Fe2O3 (wurtzite) phase by solvent thermal treatment. Mössbauer spectra at 5K also confirmed the pure magnetite phase for both MNPpristine and MNPheated in Supporting Information Figure S2 and Table S1. We further conducted FTIR spectroscopy investigation of ligand-metal ions interactions on the nanoparticles surface. Figure 1c shows that the chemisorption variation of stabilized oleic acid surfactants on the Fe3O4 nanoparticles surface between MNPpristine and MNPheated comparing with the free oleic acid solution. No vibration stretching of C=O (1715 cm-1) indicates no detection of pure liquid oleic acid (blue line) for MNPpristine (black line) and MNPheated (red line). FTIR spectra of both nanoparticles show the presence of symmetric (2850 cm-1) and asymmetric (2918 cm-1) stretching vibrations of alkyl chain (‒CH2) modes of oleic acid (Supporting Information Figure S3). The oleate chemisorbed onto the nanoparticles surface is confirmed by the stretching vibration characteristics of the carboxylic group (COO‒) appeared as asymmetric (1545 cm-1) and symmetric (1411 cm-1) mode on MNPpristine. The ligand-metal ion coordination undergoes the shift of the carboxylate absorption from 1715 cm-1 (free oleic acid) to lower energy bands with the presence of doublet.25,26 Their separation of 130 cm-1 in the spectrum of MNPpristine represents the bidentate coordination of bridging type between the ligand and two metal ions. The characteristic bands of chemisorbed oleate on MNPheated show a weaken asymmetric mode of COO– with larger separation. It indicates the formation of monodentate coordination as schematically described in Figure 1a Thermal treatment can be used to desorb or remove surfactants on nanoparticles.21,22 We observed that the solution thermal treatment of magnetic nanoparticles lead to a weakened chemisorption of ligands-metal ions. To evaluate the post-heating process effect on magnetic properties of the MNPpristine, Figure 1d shows the magnetization-field (M-H) hysteresis characteristics at room temperature using a vibrating sample mode magnetometer (VSM, Quantum Design PPMS). MNPpristine provides the typical characteristics of superparamagnetic behavior with zero remanences and coercivity. In contrast, MNPheated is relatively unsaturated at the applied maximum field in this study and the evolution of the M-H hysteresis curve is discriminated with that of MNPpristine (minor M-H hysteresis curves in Supporting Information Figure S4). The behavior of monodisperse superparamagnetic nanoparticles can be described using Langevin equation (M(H)/MS = coth(αH) – 1/(αH), where α = µ0m/kBT in Supporting Information Figure S5) based on the ratio of field-dependent magnetic energy to thermal energy.27–29 Under the identical measurement condition, a variation of α in the equation modulates the evolution feature of M-H hysteresis curves from superparamagnetic nanostructures. The field-induced magnetization evolution at the low field in minor M-H hysteresis curves directly associate with the spontaneous magnetic moment (m) of a single particle,29 which is dependent on the volume ratio of the magnetic



dead layer by surface spin canting when using the identical size of nanoparticles. In this context, the discrimination of the M-H curve feature between MNPpristine and MNPheated is attributed to the disordering degree of surface spins. The fitting results using the Langevin equation on minor hysteresis curves at low field shows 21 % decrease of the magnetic moment of MNPheated comparing that of MNPpristine (Supporting Information Figure S5). It represents that the volume ratio between normal spins arrangement and surface spins disordering led to being decreased by the solvent heating process. The magnetic anisotropy of magnetic nanoparticles enables the blocking temperature distribution, in which a ZFC-FC curve provide two characteristic temperatures.30,31 The maximum peak in the ZFC curve represents the highest energy barrier between thermal energy and magnetic anisotropic energy. The split point between ZFC and FC curves serves as the average blocking temperature of the magnetic nanoparticles.30,31 The width of two characteristic temperatures in a ZFC-FC curve can be attributed to both the surface effect and dipole-dipole interactions.32–34 Recently, the theoretical study demonstrated that the dipole-dipole interactions among the monodisperse nanoparticles negligibly contribute to the separation between ZFC and FC curves. 30,31 The width of two blocking temperatures is critically responsible to the surface spin state of the magnetic nanoparticles when applying the realms of superparamagnetic behavior with the low dispersity (Supporting Information Figure S6). In Figure 1e, the normalized ZFC curves of MNPpristine (black square) and MNPheated (red square) exhibit the maximum peak discrimination under the identical external field of 50 Oe. The maximum peak (Tmax = 71 K) of MNPheated is shifted to higher temperature than that of the MNPpristine identifying blocking temperature value of 56 K. The effective magnetic anisotropy (Keff) was expressed by Keff = Kvol + 6Ksurface/D including volume anisotropy (Kvol), surface magnetic anisotropy (Ksurface), and nanoparticles diameter (D), in which the blocking temperature has a linear relationship with Keff.10 The effective anisotropy variation shows 21±2 % increase of MNPheated comparing MNPpristine using the blocking temperature from a maximum peak of ZFC-FC, which is mainly attributed to the surface anisotropy variation and matched well with Langevin fitting results using minor M-H hysteresis curves. In this context, our observations clarify that disordering degree of surface spins on MNPheated is higher than that of MNPpristine due to coordination change of ligand-metal ions. Low chemisorption coordination lead to the enhanced surface anisotropy of magnetic nanostructures by both higher spin-orbit coupling and lower crystal field splitting energy.35 Additional shell formation at high-temperature. Seed-mediated growth strategy performs the size increase of the Fe3O4 nanoparticles by additional shell formation at the high-temperature condition.12 For monodisperse sizecontrolled Fe3O4 nanoparticles synthesis with this twostep method, solvothermal decomposition of an iron complex precursor (e.g. iron oleate or iron (III) acetylacetonate) was mixed with the Fe3O4 seed nanoparticles. The shell thickness was modulated with a

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concentration of both iron precursor and surfactant. As shown in Figure 2a, we investigate three phases of Fe3O4 nanoparticles when ultrathin shell is formed using the seed-mediated growth method; (1) as-synthesized nanoparticles as core seeds (Phase-I), (2) intermediated phase during the heating-up process prior to additional shell formation (Phase-II) and (3) finally, shell-grown step of ultrathin shell layer (Phase-III). Comparing Phase-I as starting core nanoparticles, large-scale TEM images of Phase-II in Figure 2b shows the size reduction of Phase-I nanoparticles prior to additional layer formation via thermal decomposition of precursors due to the addition of oleic acid surfactants in the heating up process. We demonstrate the identical size of nanoparticles between Phase-I and Phase-III via seed-mediated growth synthesis (Supporting Information Figure S7). Contrary to the solvent heating condition as depicted in Figure 1, the surfactants were mixed with the metal ions reagents for shell formation. We observed in this study that the addition of oleic acid to the mixture of the seed nanoparticles and the solvent undergoes the atomic scale surface etching of the seed materials without chemical composition variation and the generation of irregular morphology depending on the concentration of surfactants and heating time (Supporting Information Figure S8-S10). In the heating-up process with oleic acid, thermal decomposition of metal precursors took place closely at the refluxing temperature and a heating rate enables the kinetic control of additional layers formation.18,36 Furthermore, the high affinity of surfactants to metal ions can be competitive between the surface of the seed materials and newly decomposed metal ions within the solvent medium.37 The surface etching of core component during the heating-up process have been concealed and it is difficult to distinguish the subtle size change by a low concentration of oleic acid. We can assume that the high affinity of surfactant to the decomposed metal ions and free protons from oleic acid dissociation enable dissolution of as-synthesized magnetic nanostructures or the formation of irregular morphology at high temperature.38 Ultrathin shell formation process with low concentration condition of precursors led to the size reduction of seed materials during the heating-up. The surface etching of nanoparticle by surfactant is induced when the shell growth rate is low, by which morphology of seed materials enable to be changed before decomposing the precursors to be grown at solvent refluxing temperature. Figure 2c shows M-H hysteresis curves of three different phases using SQUID measurement. The saturation magnetization (MS) of Phase-I and Phase-III is identically reaching at the high field up to 15 kOe due to identical size and shape, however, the field-induced magnetization evolution of seed-mediated grown nanoparticles (PhaseIII) diverge from that of Phase-I. Furthermore, the magnetic feature of Phase-II shows the reduced MS. We performed the minor hysteresis measurement between ± 150 Oe, in which the feature of M-H hysteresis curves of each phase clearly appears different as shown at the inset in Figure 2c. Using the Langevin equation for all phases, the magnetic moment of Phase-II and Phase-III shows the decrease of 33 % and 17 % compared with the value of


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Phase-I respectively (Supporting Information Figure S11). Although Phase-I and Phase-III exhibit identical size with a narrow distribution, it is worth to note that the history of synthetic route enables the change of the magnetic properties by the variation of disordering degree in surface spins as depicted in Figure 2d. Magnetic dead layer due to broken symmetry naturally induced surface spin canting, which is responsible for the reduced MS with respect to their bulk counterparts. As-synthesized Fe3O4 nanoparticles (Phase-I) exhibits disordering of surface spins (tilted grey arrow in Figure 2d). The reduced MS of Phase-II with respect to the Phase-I is associated with size decrease by surfactants-induced surface etching, which provides the enhanced disordering of surface spins during the heating-up within a mixture of oleic acid and an organic solvent (red arrow in Figure 2d). The ultrathin shell formation as Phase-III enables the recovery of Fe3O4 nanoparticles size, however, disordering of surface spin is irreversible (blue arrow as additional spin canting in Figure 2d) as the previous report,10 which undergo less recovery of magnetic anisotropy. Core-Shell nanocubes with ultrathin shell layer. We further study the heating-up effect on the magnetically hard-soft core-shell nanostructures by controlling shell volume fraction of the compositional platform of CoFe2O4– MnFe2O4 nanocubes in Figure 3 and Supporting information Figure S12. The CoFe2O4 core nanocubes were synthesized in an aromatic ether solvent (e.g. dibenzyl ether, BE) with oleic acid using the heating-up method.10,36 The shell layer of MnFe2O4 was formed through a seedmediated growth method. The structural morphology of TEM image and the chemical composition using STEM-EDS analysis confirm CoFe2O4–MnFe2O4 core-shell nanocubes with an edge length of 21 nm (variation coefficient less than 10 %) with 2 nm shell thickness (shell volume fraction = 0.41) as shown in Figure 3a (Supporting Information Figure S13 and Figure S14). We performed the solvent heating with the synthesized CoFe2O4 core nanocubes using 2 organic solvents of alkene hydrocarbons (1-octadecene) and aromatic ether (dibenzyl ether). The redox activity of organic solvent played a crucial role in determining the crystalline structural phase of the synthesized ferrite nanoparticles,39 in which alkene hydrocarbons showed reducing effect and aromatic ether provided oxidizing species for controlling the valence state of iron ions. We measured M-H hysteresis curves at room temperature and FTIR spectrometer of solvent-heated CoFe2O4 core nanocubes under reflux condition (Supporting Information Figure S15). The significant enhancement of coercivity value and negligible variation in MS from both organic solvent heating was shown. It is the identical feature of Fe3O4 spherical nanoparticles as shown in Figure 1, which implicate that the enhanced anisotropic magnetism characteristics through solvent heating are associated with coordination change of ligand chemisorption. Furthermore, we confirmed structural characterization using Mössbauer spectroscopy for as-synthesized CoFe2O4 core nanocubes and solvent-heated CoFe2O4 nanocubes. The spectra were obtained at 5 K under zero-field and in-field (5 T) paralleled to the γ-beam. The results were fitted with only

two magnetically ordered components of hyperfine fields and the hyperfine parameters were summaries in Supporting Information Figure S16. We observed the negligible variation of inversion degree by solvent heating treatment, which indicates that cationic distribution was not introduced during the heating step. In-field Mössbauer spectra at 5K provided the effective magnetic field information indicating that the increase of spin canting angles was mainly attributed to the enhanced surface spin disordering after solvent heating treatment. As shown in Figure 3b, we further investigated that the shell thickness dependence of MnFe2O4 on CoFe2O4 for tuning the magnetism of the hard-soft core-shell nanocubes. Shell thickness was controlled by metal precursors concentration. From magnetic properties characterization using the SQUID at 5 K, the solvent-heated core components (red curve) with a mixture of metal reagents and surfactant show the substantial increase of coercivity value with slightly reduced MS comparing assynthesized CoFe2O4 nanocubes (blue curve). We understand that the decrease of the MS value can be attributed to the atomic-scale surface etching of core nanocubes by oleic acid at high temperature as shown in Phase-II of Figure 2. In the case of ultrathin shell formation (orange curve), the increased coercivity shows negligible change, however, the MS is recovered to the identical value of as-synthesized core nanocubes. With a higher precursor concentration for 5 nm thick shell formation (green curve), the complete hard-soft exchange coupling is allowed with enhanced magnetization and decreased coercivity values. Figure 3c summarizes the variation of MS and coercivity value from each step for the synthesis of CoFe2O4–MnFe2O4 nanocubes. Theoretical results of the exchange coupling in hard-soft core-shell nanostructures expressed the change of the coercivity value.5,9 The increase of the magnetically soft shell volume fraction led to a linearly propositional decrease by HC = HH (1 – fS), where HH, HC, and fS represent the pure core coercivity, exchange coupled coercivity and volume fraction of soft layer respectively. Contrary to this expression, our results show that ultrathin shell formation step exhibit the anisotropy enhancement and MS reduction. We can define this regime as the enhanced surface spin canting effect, which is dependent on the shell formation condition. The evolution of magnetic interactions of hard-soft coreshell nanocubes is outlined in the schematic flowchart in Figure 3d. As-synthesized core nanoparticles (C) as a magnetically hard phase shows a typical M-H hysteresis. The solvent-heated core components (h-C) during heatingup exhibit the enhanced anisotropy with reduced magnetization due to the weakened coordination with capping agents and the surface etching of the core components. The saturation magnetization is recovered, but the enhanced magnetic surface anisotropy shows negligible change when forming an ultrathin shell layer (CS-1). With increasing the shell thickness, the exchange coupling of the hard-soft platform shows a substantial increase of magnetization with weakening anisotropy as theoretical simulation. We understand that shell formation kinetic rely on the concentration gradient of decomposed metal ions. The initial concentration of metal precursors



for shell formation enable the modulated diffusion rate under the identical synthetic condition.40 In this context, the shell formation rate of C-S-1 is lower than that of C-S-2, in which the solvent-thermal treatment effect on the core component for C-S-1 is substantial than C-S-2. The thermal process history with shell formation rate is critical to determining the surface magnetism of the core materials and to modulate the exchange coupling using the hard-soft core-shell nanostructure platforms. Metal-anion complexes effect. The decomposition temperature represents the bond cleavage kinetic between metal ions and anions complexes, which is substantial to modulating nanocrystal growth.17 The seed-mediated growth method using thermal decomposition enable monodisperse bimagnetic spinel ferrite nanostructures for core-shell configuration, resulting from diffusioncontrolled and surface reaction-limited growth under less different values of decomposition temperature and metal ion reduction potential.17,41 The concentration gradientassisted diffusion driving force of decomposed monomers determines the shell layer volume growth rate by Fick’s first law.40 The shell growth rate of nanocrystals was linearly proportional to the monomer concentration at the reaction temperature. In this context, the kinetic of decomposed monomers is crucial to controlling the shell formation, for which the selection of precursor enable the determining factor of shell growth rate control when ultrathin shell formed with a low concentration of precursors.42,43 We exploited the different metal complexes (e.g. acetylacetonate and acetate reagents) for ultrathin shell formation of MnFe2O4 on CoFe2O4 nanocubes. STEMEDS chemical compositional analysis shows that an identical shell layer was formed using metal acetate precursors with identical size (Supporting Information Figure S17). The two different manganese precursors show thermal decomposition temperature with the difference of 50 ℃ (Supporting Information Table S2). In Figure 4a, the M-H hysteresis curves of a cobalt ferrite core with ultrathin manganese ferrite shell at low-temperature condition (5 K) exhibit metal precursor dependence. Acetate precursors for MnFe2O4 shell layer formation lead to the decreased in saturated magnetization with slightly enhanced coercivity value (blue line). We understand that the metal-acetate precursors were decomposed at a higher temperature than that of metal-acetylacetonate precursors. The growth rate for the identical shell layer thickness using metal-acetate precursor can be lower than that of metal-acetylacetonate precursors. Slow shell formation allows for chemisorption modification of surfactant and surface etching on the core components during the heating-up. Although the M-H hysteresis curves measured at room temperature shows no noticeable discrimination of MS and coercivity by different metal precursors (Supporting Information Figure S18), however, the selection of metal precursor for kinetic-controlled shell formation is substantial to modulating the nanoscale exchange couplings effect of hard-soft core-shell nanostructures. Sodium oleate surfactant effect. We further elucidate the surfactants (oleic acid and sodium oleate) effect on magnetic exchange coupling of CoFe2O4–MnFe2O4 core-

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shell nanocubes when the ultrathin shell is formed. The selection of surfactants for magnetic nanoparticle synthesis is crucial to directing size and shape via a thermal decomposition method.12,44,45 The efficiency of dissociation between cations and oleate ions can change the activity of ligand adsorption/desorption equilibrium on the nanoparticle surface.37 An alkali metal oleate (e.g. sodium oleate) have demonstrated the substantial ability to modulate the shape of magnetic nanotubes due to the less reducing ability of sodium oleate than that of oleic acid.36,46 Ultrathin shell of manganese ferrite on cobalt ferrite nanocubes was synthesized using a mixture of two metal reagents (manganese (II) acetylacetonate and iron (III) acetylacetonate), sodium oleate, and dibenzyl ether. We confirmed the identical size and composition of core-shell nanocubes with less than 2 nm thick shell using transmission electron microscope regardless of surfactant selection between oleic acid and sodium oleate (Supporting Information Figure S19). Alkali metal oleate complexes at high temperature can lead to the chemical etching of metal iron nanostructures as molten salt corrosions mechanism, whereas no etching effect with oleic acid was observed from previous reports.47,48 Although high temperature over 300 ℃ enable the thermolysis of oleic acid and sodium oleate,47 however, we observed that solvent thermal treatment of CoFe2O4 core nanocubes with sodium oleate keep the morphology of initial CoFe2O4 nanocubes, which is contrary to the heating effect on the as-synthesized nanoparticles with mixture of organic solvent and oleic acid (Supporting Information Figure S10 and Figure S20). Furthermore, M-H hysteresis curves show the interfacial interaction effect of CoFe2O4‒MnFe2O4 core-shell nanocubes with ultrathin shell layer prepared using sodium oleate (orange curve) comparing CoFe2O4 core nanocubes (black curve) as shown in Figure 4b. The sodium oleate led to the less enhancement of magnetic anisotropy with MS increase than when using oleic acid (red curve). The selective adsorption using sodium oleate on high surface energy facets (e.g. {100} of CoFe2O4 nanocubes) enables less chemisorption modification between carboxylic groups and surface metal ions during the heating-up. The deprotonated carboxyl group of oleic acid forms a bond with metal ions on the surface when the ferrite nanoparticles synthesis. The concentration of the free proton (deprotonated H+ of oleic acid) in the reaction medium can be increased when the more deprotonated carboxy group form the bonding with metal ions on the nanoparticles surface.49 In this context, we understand that the sodium oleate provides less deprotonated proton than oleic acid during the heating-up, which enable less atomic scale surface etching of the core nanocubes. Country to the molten salt corrosion of iron metal nanostructures, the deprotonated proton concentration from oleic acid is critical to preserving the magnetic surface of ferrite nanoparticles. In addition, the selection of surfactants is a crucial factor to preserve the magnetic features of core nanocubes during the shell formation at high temperature. CONCLUSION


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Our results provide understanding the enhanced anisotropic magnetism when ultrathin shell layer is additionally formed on a magnetic core component using a seed-mediated growth method via the heating-up process. We demonstrate the heating-up effect on magnetic properties of ferrite nanoparticles using 2 thermal treatment conditions including (i) high boiling temperature solvent only and (ii) a mixture of surfactant and an organic solvent. Solvent thermal treatment effect on magnetic nanoparticles during the heating-up lead to the coordination change of ligand chemisorption with surface metal ions, which allow for a substantial increase in surface anisotropy due to large spin-orbit couplings. Furthermore, we observe that solvent heating-up with surfactant undergoes the atomic-scale surface etching of core components. The disordering of the surface spins on the core materials is enhanced with decreasing saturation magnetization. In this content, the magnetic surface change of core components was enabled during the seedmediated growth method for an additional shell formation via the heating-up. For bimagnetic ferrite hard-soft coreshell nanostructures, we demonstrate that the kinetic of shell formation rate determine the enhancement of magnetic anisotropy by controlling the concentration of metal precursors and their thermolysis. Slow shell formation using the heating-up process experiences the coordination change of ligand-metal ions and atomic scale surface etching simultaneously. Finally, our observation shows that the selection of surfactant is important for preserving the disordering degree of surface spins on the core part to reduce the heating-up process effect. Our study prominently demonstrates that magnetic surface variation of core component during additional layer formation is crucial to the resultant magnetic properties due to solvent-thermal treatment effect. These findings shine insight into the underlying nature of the synthetic process of hard-soft core-shell nanostructures, and the procedure voyage for the shell formation is critical to resolving the outcoming magnetism.

ASSOCIATED CONTENT Supporting Information. More characterization details using transmission electron microscopy, XRD, FTIR, M-H hysteresis, Mössbauer spectroscopy and ZFC-FC measurement are available in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (K.L.) and [email protected] (B.A.).

ORCID Kwan Lee: 0000-0002-8269-6579 Sangyeob Lee: 0000-0002-9957-990X Byungmin Ahn: 0000-0002-0866-6398

Present Addresses # School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798

K.L. and B.A. conceived the concept, designed/performed the experiments, analyzed data, and wrote the manuscript. S.L performed magnetic characterization.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF2018R1D1A1B07044481).

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Figure 1. The solvent heating-up effect on Fe3O4 spherical nanoparticles. (a) Typical TEM images and schematics of coordination mode change of oleate ligand-metal ions after solvent thermal treatment (MNPheated, red outlined TEM image) using as-synthesized magnetite nanoparticles with the diameter of 13 nm (MNPpristine, black outlined TEM image) via thermal decomposition of iron oleate complex. (b) Structural characterization of X-ray powder diffraction of MNPpristine (black) and MNPheated (red) showing the typical inverse spinel cubic structures. (c) FTIR results of oleic acid solution (blue), MNPpristine (black) and MNPheated (red) show coordination change of ligand chemisorption by the stretching vibration characteristics of the carboxylic group. Magnetic properties characterization using (d) M-H hysteresis curves and (e) the normalized ZFC curves from MNPpristine (black) and MNPheated (red) represent an enhanced surface anisotropy of MNPheated by solvent thermal treatment. Scale bar for TEM images is 20 nm.

Figure 2. The additional shell formation for the size controlled Fe3O4 spherical nanoparticles using the seed-mediated growth method via the heating-up process. (a) Schematic outline for three phases depending on the heating-up process steps as Phase-I (MNPpristine), Phase-II (intermediate step during the heating-up), and Phase-III (additional



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shell formation). (b) Typical TEM images of Phase-I (black outline), Phase-II (red outline), and Phase-III (blue outline) exhibit nanoparticle size variation regarding each phase with low dispersity (less than 10% standard deviation). (c) M-H hysteresis curves for the three phases shows the evolution of field-stimulated magnetization. (d) Schematics of disordering of surface spins regarding three phases during the heating-up process with metal precursors and surfactant, in which surface spin canting (grey arrow) in Phase-I is enhanced by two factors including thermal treatment and surfactant-induced surface etching by surfactant as Phase-II (red arrow). Ultrathin shell layer formation for the identical size with Phase-I shows recovering MS value with additionally enhanced surface disordering in Phase-III (blue arrow) comparing Phase-I. Scale bar for TEM images is 50 nm.

Figure 3. Magnetic anisotropy enhancement by ultrathin shell layer formation using CoFe2O4–MnFe2O4 core-shell nanocubes. (a) Typical TEM images of CoFe2O4–MnFe2O4 core-shell nanocubes with an edge length of 25 nm with low dispersity. (b) M-H hysteresis curves at 5 K with four different nanocubes including as-synthesized CoFe2O4 core nanocubes (blue), solvent heating-up treated CoFe2O4 core nanocubes (red), ultrathin shell layer of MnFe2O4 formed core-shell nanocubes (orange) and CoFe2O4–MnFe2O4 core-shell nanocubes showing the exchange couplings (green). (c) The variation of magnetization and coercivity values from four different nanocubes. (d) Magnetism evolution schematics of core-shell nanocubes depending on the synthetic steps and the shell thickness during the heating-up. Scale bars for TEM images are 20 nm and 50 nm (inset).

Figure 4. Kinetic-controlled shell layer formation by thermolysis and surfactant effect on anisotropy magnetism of CoFe2O4–MnFe2O4 core-shell nanocubes. (a) Metal precursors with different decomposition temperatures for MnFe2O4 shell layer modulated magnetic anisotropy. Metal acetate precursors (blue color) exhibit more enhanced anisotropic magnetism than synthesized using metal acetylacetonate precursors (red color) due to different ACS Paragon Plus Environment

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thermolysis temperature during the heating-up. (b) Surfactant effect on exchange interactions of CoFe2O4–MnFe2O4 core-shell nanocubes, in which sodium oleate surfactant (orange color) shows less enhanced anisotropic magnetism comparing oleic acid surfactant (red color) while the shell layer was formed.

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Understanding High Anisotropic Magnetism by Ultrathin Shell Layer

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