Magnetic memory effect in self-assembled nickel ferrite nanoparticles having mesoscopic void spaces

Abstract

Self-assembled magnetic nanoparticles, which are fascinating for their potential applications in high-density data storage, nanoscale electronics, sensors, and medicines, attracted significant attention over the years. Controlled self assembly of magnetic nanoparticles is an art of chemical synthesis, because self assembly does not correspond to the thermodynamically minimum energy state. In order to construct the self-assembled nanostructure an input of external energy is required to direct the self assembly. In this article we report a novel approach of fabricating nanocrystalline and mesoporous nickel ferrite of 5–9 nm size particles by using supramolecular assembly of lauric acid (anionic surfactant) as a structure directing agent under hydrothermal conditions. Thermal, field and time variation of magnetization as well as careful studies of ageing effect clearly demonstrates strong interparticle interaction of the nanoparticle assembly. Interestingly, the memory effect is observed at low temperature as a consequence of interparticle interaction of the nickel ferrite nanoparticle assembly.

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Introduction

Ferrite materials with general formula MFe₂O₄ (M = Co, Ni, Mn, Zn, Cu) have attracted wide-spread research interest over several decades for their exciting electrical and magnetic properties.¹ Among them, nickel ferrite (NiFe₂O₄) has been investigated intensively because of its high magnetization, magneto-optical properties, low magnetic coercivity and high chemical and thermal stability.² Soft magnetic nickel ferrite has an inverse spinel structure. The bulk compound shows ferrimagnetism that originates from the anti-parallel alignment between Fe³⁺ sub-lattice at the tetrahedral sites and Ni²⁺ sub-lattice at the octahedral sites.³ Over the years nickel ferrite based nanostructured materials have found many useful applications as magnetic recording media, magnetic resonance imaging (MRI), magnetic fluids, drug delivery and catalysis.⁴ The magnetic properties of the nanoferrites are strongly depended upon their chemical composition, crystallinity, particle size and morphology. Interestingly, magnetization is considerably enhanced with decreasing particle size.⁵ Several synthetic methods have been implanted to synthesize nickel ferrite nanostructures for better morphological and dimensional control such as sol–gel/hydrothermal process,⁶ co-precipitation,⁷ citrate precursor process,⁸ microwave synthesis⁹ and sol–gel auto-combustion reaction.¹⁰

Introduction of nanoscale porosity in the intrinsic magnetic matrix can significantly modify the magnetic property of the resulting material.¹¹ However, controlled synthesis of the self assembly of magnetic nanoparticles having a significant mesoporous structure is still a challenging issue for developing improved magnetic properties. Mesoporous magnetic nanomaterials can display unique material properties due to their high specific surface area, large pore volume, tuneable pore size and periodic pore arrangement which are completely absent in their corresponding bulk counterpart.¹² Various synthetic strategies have been explored to synthesize porous nickel ferrite nanomaterial including the “hard template method” by using porous silica as template¹³ and “soft template method” including micelles, reverse micelles, copolymer and chelating agents.¹⁴ However, the hard template process is time consuming and complicated. Furthermore, the morphology of final mesoporous material strongly depends on template matrix and it is very difficult to sustain the porous structure during template removal. Several attempts have been made to prepare mesoporous nickel ferrite nanomaterial by soft template method but still now no breakthrough has been achieved. We have developed a novel strategy for the synthesis of self-assembled mesoporous TiO₂ and Al₂O₃ materials with well-defined crystal morphologies and high BET surface area by using sodium salicylate as a template.¹⁵ Latter it is found that anionic surfactants like lauric acid can stabilize the metal oxide nanoparticles under the hydrothermal synthesis conditions.¹⁶

Herein, we first report the fabrication of crystalline mesoporous nickel ferrite nanomaterial with large surface area by using the supramolecular assembly of lauric acid (anionic surfactant) as a structure directing agent under hydrothermal conditions. Anionic surfactant assembly of lauric acid under alkaline pH can interact with the positively charged metal centres of nickel ferrite species via electrostatic interactions and which is supposed to stabilize the mesophase of the material.¹⁶ The mesoporous NiFe₂O₄ material has been characterized by powder XRD, high resolution transmission electron microscopy (HRTEM), N₂ sorption, UV-visible spectroscopy and in-depth analysis on the dc magnetic property of this porous nanostructure. Significant amount of work has been carried out on NiFe₂O₄ nanoparticles focusing diverse directions of magnetic properties.¹⁷ Here, we are interested in the magnetism of mesoporous NiFe₂O₄ nanoparticles, where surface spin is further enhanced by the mesophase. The results suggest that enhanced surface spin may cause superspin-glass state and magnetic memory effect is a finger print of cooperative superspin-glass state.

Experimental section

Chemicals

Lauric acid (99%, used as structure directing agent), nickel chloride hexahydrate, anhydrous iron(III) chloride and aqueous ammonia (25%, used to maintain the pH of the synthesis gel) purchased from Merck, India. All chemicals were used without any further purification.

Synthesis of nickel ferrite (NiFe₂O₄) nanoparticle

Self-assembled NiFe₂O₄ nanoparticles were synthesized by the following procedures: 0.7 g lauric acid was dissolved in mixture of 10 ml doubled distilled water and 10 ml alcohol. Then, few drops (2–3) of 25% aq. ammonia solution were added to this solution to make the transparent solution and the resultant solution was stirred for 30 minute at room temperature. Then 1.62 g of the Fe(III) precursor, FeCl₃ and 1.28 g of the Ni(II) precursor, NiCl₂·6H₂O was dissolved into 10 ml of distilled water separately and mixed together under stirring conditions. The synthesis gel was prepared by adding the highly acidic metal salt solution to the above-mentioned template solution. Then, the pH of the final solution was adjusted 10.0 via the dropwise addition of 25% aqueous ammonia solution and stirred for 5 h at room temperature. Than the reaction mixture was transferred in to a Teflon-lined stainless steel autoclave and hydrothermally treated at 393 K at for 48 h. The resultant solids were collected by filtration, washed with double distilled water and dried at room temperature. After this, the material was calcined at 723 K for 5 h through controlled heating from room temperature at a rate of 5 °C min⁻¹ to remove the template molecule. After calcination the mesoporous NiFe₂O₄ became highly magnetic and it can be separated from the aqueous dispersion through magnetic needle (Fig. S1†).

Characterization techniques

Powder X-ray diffraction (XRD) patterns of the samples were recorded on a Bruker Advance D-8 diffractometer operated at a voltage of 40 kV and a current of 40 mA, using Cu Kα (λ = 0.15406 nm) radiation. Transmission electron microscopic (TEM) images were recorded in a JEOL Model 2010 TEM operated at 200 kV. A JEOL Model JEM 6700F field-emission scanning electron microscopy (FESEM) system was used to determine the particle morphology. Nitrogen sorption isotherms were obtained using a Belsorp-HP surface area analyser at 77 K. Prior to the measurement, the samples were degassed at 393 K for 12 h. The magnetic measurements were performed on a vibrating sample magnetometer from Cryogenic Ltd UK, as well as on a Quantum Design superconducting quantum interference device (SQUID) magnetometer. In zero-field cooled (ZFC) protocol sample is cooled down to low temperature in zero-field and magnetization is measured in warming mode with a static magnetic field. Magnetic field is applied during cooling the sample in the field-cooled (FC) mode and measurement is performed in warming cycle like ZFC measurement.

Results and discussion

X-ray diffraction pattern

Small-angle powder XRD pattern of mesoporous NiFe₂O₄ is shown in Fig. 1. As seen from the figure one broad peak centred at 2θ = 1.78 signifying the average particle centre-to-particle centre correlation distance is observed. The inter-particle distance corresponding to this small angle peak is 4.96 nm, which suggests that material is composed of very tiny self-assembled nanoparticles. Thus, their assembly and interparticle void spaces are responsible for this mesoporosity.¹⁸ The wide angle powder XRD patterns of the peaks are indexed as NiFe₂O₄ phase with face-centered cubic (FCC) structure. The material shows major peaks at 2θ values of 18.29, 30.29, 35.70, 43.35, 53.80, 57.35 and 62.93 which correspond to NiFe₂O₄ (111), (220), (311), (400), (422), (511) and (440) crystal planes. No separate peaks of other impurities such as NiO and Fe₂O₃ has been observed. Rietveld refinement of the diffraction is depicted by the continuous curve, which further confirms absence of any detectable impurity phase. The bars below the diffraction pattern represent the diffraction peak positions and the difference plot is shown at the bottom of each panel. The reliability parameters of the refinements, R_w (∼1.07), R_exp (∼4.90) and σ (∼0.21) are reasonably small. The sample crystallizes in cubic Fd3̄m space group with lattice parameter a = 8.364 Å. Rietveld refined structural parameters are given in ESI in Table S1.†

Fig. 1 Small angle powder XRD pattern of mesoporous NiFe₂O₄.

Nanostructure analysis

Field emission SEM (FE-SEM) images of NiFe₂O₄ are shown in Fig. 3a. From the SEM images, it is found that material is composed of very tiny nanoparticles with spherical morphology and uniform arrangement of nanoparticles. The representative HRTEM images of the material are shown in Fig. 3b–f. From the TEM analysis of NiFe₂O₄ it is clearly observable that; the material is composed self-assembled spherical nanoparticles of dimension ca. 5–9 nm to form interparticle mesoscopic voids space and mesoporosity (pores are surrounded by red circles in Fig. 3d). The high-resolution TEM image (Fig. 3e) of the NiFe₂O₄ material showed the presence of lattice fringes and suggested the high crystallinity of the material. The inter-planar distance is in quite good agreement with the d spacing of (400) crystal plane of face-cantered cubic structure with a unit cell parameter a = 8.36 Å. The selected area electron diffraction pattern of the material is shown in Fig. 3f. The diffraction spots are indexed corresponding to face-centered cubic (FCC) structure of single-phase nickel ferrite. d values corresponding to this FCC phase agrees very well with the powder X-ray diffraction pattern (Fig. 2). The histogram displaying particle size distribution as obtained from the TEM analysis is shown in Fig. S2.† The elemental mapping analysis for NiFe₂O₄ is shown in Fig. S3.† It suggested the presence of iron, nickel and oxygen, and their homogenous distribution throughout the mesoporous NiFe₂O₄ material.

Fig. 2 Wide angle PXRD pattern of NiFe₂O₄.

Fig. 3 FE-SEM image of the material NiFe₂O₄ (a), HR TEM image of NiFe₂O₄ showing self-assembled nanostructure and pore (b–d), high resolution TEM image of the material showing the lattice fringes (e) and (f) selected area electron diffraction (SAED) pattern of mesoporous NiFe₂O₄.

N₂ adsorption–desorption study of nickel ferrite nanoparticles

The N₂ adsorption–desorption isotherms of mesoporous nickel ferrite nanoparticle are shown in Fig. 4. The Brunauer–Emmett–Teller (BET) surface area, pore volume and average pore dimension of the nickel ferrite material are estimated from the adsorption/desorption isotherms at 77 K. The isotherm can be classified typical type IV, which is characteristic for the mesoporous material together with H3 type hysteresis loop.¹⁹ The material showed a strong capillary uptake in the relative pressure range (P/P₀) from 0.54 to 0.95, suggesting the presence of wide range of mesoporosity in the material. The BET surface area of the material is 74 m² g⁻¹ with a pore volume of 0.0988 cc g⁻¹. The corresponding pore size distribution estimated by employing the non-local density functional theory (NLDFT)²⁰ model is shown in the inset of Fig. 4. The average pore size distribution showed peaks centred at 6.96 nm. Pore width obtained from N₂ sorption analysis is quite close agreement with those seen from the electron microscopic images (Fig. 3d).

Fig. 4 N₂ adsorption (●)-desorption (○)-adsorption isotherms of the material NiFe₂O₄ at 77 K. Corresponding NLDFT pore size distributions are shown in the inset.

Dc magnetization study

Thermal variation of zero-field cooled and field-cooled magnetization measured at 100 Oe is shown in Fig. 5. A broadened maximum is noticed around 220 K (T_max) in the zero-field cooled curve. The field-cooled magnetization deviates from the zero-field cooled magnetization much above T_max, indicating considerable interparticle interaction. Fig. 6 shows the isothermal magnetization curves at selective temperatures. The value of coercivity is 615 Oe at 20 K which nearly vanishes above T_max. At 300 K, the sample shows almost zero coercivity as expected for superparamagnetic behaviour. All the magnetization curves below and above T_max do not show a saturating trend upto 50 kOe. The value of magnetization at 50 kOe increases with decreasing temperature and approaches nearly 34 emu g⁻¹ at 20 K. The value is much below the theoretical value (71.2 emu g⁻¹). The considerably small value appears due to disordered surface spin.²¹ To characterize magnetic properties, the memory effect in both the zero-field cooled and field-cooled magnetization is investigated both in zero-field cooled and field-cooled protocols. The results for field-cooled protocol below T_max are depicted in Fig. 7. In the study sample was cooled down from 300 K in a static magnetic field of 100 Oe at a constant rate of 1 K min⁻¹ and magnetization is recorded as a function of temperature. During this cooling process sample temperature was arrested at three different temperatures, T_halt = 200, 150 and 100 K for 90 min each. At T_halt magnetic field was cut off for 90 min. After the completion of this ageing process, the measurements were resumed in field and the recorded curve is described as FC (waiting) curve as depicted in Fig. 7. The FC (waiting) curve consists of three sharp steps because of arrests at 200, 150 and 100 K. After reaching the base temperature at 20 K, the second cycle of measurement was carried out in the heating mode. Importantly, measurement in this cycle was carried out continuously at a rate of 1 K min⁻¹ which is defined as FCH (memory) curve as depicted in the figure. Although FCH (memory) curve is recorded continuously, signature of steps at T_halt is evident in the FCH (memory) curve as displayed in Fig. 7. The results clearly demonstrate manifestation of previous memory during cooling cycle and display the memory effect. Memory effect in the field-cooled magnetization often ascribes to the extrinsic experimental artifact.²² To overcome it, memory effect was further measured in case of zero-field cooled magnetization as depicted by the difference of magnetization (ΔM) as a function of temperature in Fig. 8. The measurement is analogous to the protocol used for the field-cooled memory effect. During cooling process sample was cooled in zero field and halted at 100 and 150 K (below T_max) for 180 min and then sample was further cooled to the base temperature at 40 K. In the heating cycle magnetization was measured in field continuously. An anomaly is still observed close to T_halt demonstrating memory effect, as evident by the anomalies in the ΔM plot with temperature in Fig. 8. The memory effect in zero-field cooling demonstrates consequence of the cooperative nature of spins with randomly frustrated interaction and is an intrinsic process which is typically observed in an atomic spin-glass system.²³ Magnetization relaxation processes were recorded at three different temperatures at 100, 150 and 200 K. After cooling the sample in field-cooled conditions with a 100 Oe field, magnetization was recorded with time after switching off the field to zero. The M(t) at 200 K is fitted with the modified stretched exponential function M(t) = M₀ − M_g[exp−(t/τ)^β], where M₀ and M_g are the ferromagnetic (FM) and exponential components of magnetization. β is an exponent which lies in the range 0 < β ≤ 1. τ is the relaxation time. Satisfactory fit using the above expression is displayed in Fig. 9 by the continuous curve. The values of M_g, M₀, τ and β are 0.4802 emu g⁻¹, −0.5395 emu g⁻¹, 1138 s, and 0.51, respectively, at 200 K. In the above expression, β < 1 indicates activation against multiple anisotropy barriers and is consistent with the spin-glass-like behaviour.²⁴ Existence of memory effect and relaxation dynamics fitted in the stretched exponential function with β < 1 suggest strong interparticle interactions and the interparticle interaction is tuned by the self assembly structure at the nanoscale. Magnetic behaviour below T_max is quite different from the magnetic behaviour below the superparamagnetic blocking temperature as observed for the non-interacting fine nanoparticles. Here, we observe intrinsic memory effect below T_max, pointing superspin-glass²² behaviour analogous to that observed for the atomic spin-glasses. The superspin-glass state further confirms strong interparticle interaction. Thus, T_max represents superspin-glass transition temperature in place of superparamagnetic blocking temperature. Large difference between theoretically calculated saturation of magnetization and the current value at 20 K and 50 kOe probably occurs due to the disordered surface spin, which is significantly enhanced by the mesophase. These disordered surface spins give rise to the superspin-glass state at low temperature.²⁵

Fig. 5 Temperature variation of ZFC and FC magnetizations for mesoporous NiFe₂O₄ measured at 100 Oe.

Fig. 6 Magnetic hysteresis loops of mesoporous NiFe₂O₄ at selected temperatures.

Fig. 7 Thermal variation of field-cooled (FC) magnetization in different modes for mesoporous NiFe₂O₄. The field was cutoff during temporary stops at 100, 150 and 200 K. Arrows display the measurements either in cooling or heating modes. Memory effect is demonstrated by the steps close to 100, 150 and 200 K in FCH (memory) curve.

Fig. 8 Thermal variation of difference plot (ΔM) between ZFC (memory) and ZFC (reference), displaying absence of convincing signature of memory effect in mesoporous NiFe₂O₄.

Fig. 9 Time (t) dependence of M at 200 K in zero field after cooling the sample in 100 Oe for mesoporous NiFe₂O₄.

Conclusions

We have synthesized crystalline mesoporous nickel ferrite nanomaterial with large surface area by using the supramolecular assembly of lauric acid as a structure directing agent under hydrothermal conditions. Negative charge of the lauric acid molecules under alkaline pH can interacts with the positively charged metal centres of nickel ferrite species via electrostatic interactions and thus to stabilize the mesophase. The dc magnetization results demonstrate evident manifestation of interparticle interaction. As a consequence of significant interparticle interaction memory effect is observed which is promising for technological applications in future.

Acknowledgements

VK and KD wishes to thank CSIR, New Delhi for Senior Research Fellowships. AB wishes to thank DST, New Delhi for instrumental supports through the DST Unit on Nanoscience and DST-SERB project grants.

References

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra05483h

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