Structural, vibrational and magnetic properties of NiO-(Mg,Ti) powders: The effect of reduction reaction

Journal of Magnetism and Magnetic Materials 494 (2020) 165784

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Structural, vibrational and magnetic properties of NiO-(Mg,Ti) powders: The effect of reduction reaction

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Aneeta Manjari Padhan, Bhagaban Kisan, Perumal Alagarsamy

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Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, India

ARTICLE INFO

ABSTRACT

Keywords: Transition metal oxides Reduction reaction Mechanical alloying Antiferromagnetism Ferromagnetism

We report a detailed structural, vibrational, and magnetic properties of mechanically alloyed NiO-(Mg,Ti) (x at. %) [xMg = 0–50 and xTi = 0–35] powders prepared using high-energy planetary ball mill under dry milling condition. Structural studies not only reveal a large crystallite size reduction during milling, but also illustrate an in-situ mechanochemical reduction of NiO with the reaction kinetics changing from gradual to self-sustaining type upon increasing xMg and xTi. This yields a maximum NiO reduction of 95% and 73% for xMg = 50 and xTi = 35 with the formation of nanocomposites of NiO-Ni-MgO and NiO-Ni-TiO2, respectively. As a result, antiferromagnetic behavior of pure un-milled NiO transforms into ferromagnetic one with increasing x. The progress of NiO reduction is demonstrated from both the structural studies and the change in magnetization at room temperature and temperature dependent magnetic properties at high temperatures.

1. Introduction In recent times, the artificial engineering of transition-metal-oxides into nanostructured composite materials has untied novel opportunities to tune physical properties of nanoscale materials and nanocomposites [1,2]. Among various nanoengineered materials, Nickel Oxide (NiO) is found to be promising due to its tuning possibilities through reduction reaction, which forms a combination of ferromagnetic (FM) – antiferromagnetic (AFM) system and finds applications in gas sensor, catalysis, battery, resistive random access memory [3–10], etc. The conventional metallothermic reduction is pertained to the direct physical contact amongst the feed and the metallic reducing agent and is primarily based on the kinetic pathways followed by the initial reaction under a thermally actuated environment [11]. Among various processing routes employed for not only reducing NiO, but also understanding the nature of reduction [10–16], high energy ball milling [17] has been found to be one of the inexpensive methods for studying the mechanochemical (MC) reduction process based on the different reaction kinetics and its industrial applicability. The ball milling method is not only used to synthesize nanocrystalline materials, amorphous alloys, solid solutions and intermetallic phases [18–20], but also identified as the process of reactive milling or MC reduction due to solid-solid and solid-liquid reactions, respectively [21]. The MC route is predominantly suitable for the synthesis of advanced materials, composites, nanoparticles and nanostructured materials [18,22,23] depending on the

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milling condition dependent reaction kinetics [19,24]. The relevant kinetic studies reported on NiO have mostly been displacement reactions with a redox response between a metal oxide and a more reactive metallic agent [25,26]. Some of these significant observations have been procured as: H2 based NiO MC reduction reported by Doppiu et al. [27], complete carbothermal reduction of NiO stated by Yang et al. [28] and Jagtap et al. [29], Al activated NiO reduction via ball milling technique reported by Matteazzi and LaCaer [30], Udhayabanu et al. [31] and Oleszak [32] as well as the Zn and Mg actuated NiO reduction using high-energy ball milling reported by Setoudeh et al. [33–35]. These studies revealed that the understanding of the reduction of NiO and the resulting structural and magnetic properties of nanoscale Ni in oxide matrix remain challenges. Although several methods and different reaction kinetics have been reported to study MC reaction of NiO, a detailed analysis of the reduction mechanism as a function of metallic reducing agents and the evolution of the physical properties of NiO based in-situ nanocomposite has not been well reported. Hence, we report here a systematic investigation of structural, vibrational and magnetic properties of ball-milled NiO-Mg and NiO-Ti powders to understand the effect of Mg and Ti contents individually on the type of MC reduction of NiO powders. 2. Material and methods Commercial NiO, Mg and Ti powders procured from Sigma-Aldrich,

Corresponding author. E-mail address: [email protected] (P. Alagarsamy).

https://doi.org/10.1016/j.jmmm.2019.165784 Received 31 May 2019; Received in revised form 2 September 2019; Accepted 2 September 2019 Available online 03 September 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.

Journal of Magnetism and Magnetic Materials 494 (2020) 165784

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India were used as the starting materials. The powders of NiO-Mg and NiO-Ti mixtures were taken to tune the composition of the powders, as NiO-Mg (x = 0–50 at. %) and NiO-Ti (x = 0–35 at. %), up to the stoichiometric composition to satisfy the reaction equations

NiO + Mg = Ni + MgO

(1)

2NiO + Ti = 2Ni + TiO2

(2)

corresponding to Mg. This is mainly due to dissolution of Mg into the NiO matrix, which increases the peak broadening and shifts the position of the peaks due to atomic disorder. As depicted in Fig. 1b, NiO(2 0 0) peak becomes asymmetric by nature and splits into multiple peaks with increasing xMg from 15 to 40. The careful analysis using multiple peak fitting procedures, as displayed in Fig. 1d–f, reveals the development of additional peaks at 2θ = 44.5° and 43.02°, which correspond to Ni (1 1 1) and MgO(2 0 0), respectively. Furthermore, the Ni(2 0 0) peak at 2θ = 52° becomes more prominent with increasing xMg, indicating a progressive increase of Ni in the milled powders due to gradual reduction of NiO by Mg and the formation of NiO-Ni-MgO nanocomposites. Remarkably, for xMg = 50 powder, the Mg activated reduction of NiO at its higher proximity shows highly intense peaks of Ni(1 1 1), Ni (2 0 0) and Ni(2 2 0) at 2θ = 44.5°, 51.8° and 76.4°, respectively and the development of additional peaks at 2θ = 37.05°, 43.07°, 62.46°,74.76° and 78.75° due to the formation of fcc MgO at the expense of nearly complete NiO reduction. Fig. 2a depicts the XRD patterns of pure as-mixed and 30 h milled NiO-Ti (x at.%) powders with xTi = 0–35. All peaks in as-mixed NiO-Ti powders can be indexed to fcc NiO and hcp Ti. It is noticed that with adding xTi = 5%, the milled NiO-Ti powder shows Bragg peaks of fcc NiO and a very low intense peak of Ti (1 0 1) reflection. This can be correlated to the dissolution of Ti into the NiO matrix. With increasing xTi ≥ 10, as similar to the case of NiO-Mg powders, NiO(2 0 0) peak becomes asymmetric (Fig. 2b) and then splits into multiple peaks for xTi up to 35. The evolution of peaks corresponding to new phases is clearly displayed in Fig. 2d–f. This reveals the presence of additional peaks at 2θ = 41.23° and 44.5° corresponding to TiO2(1 1 1) and Ni(1 1 1) respectively. In addition, the Ni(2 0 0) peak at 2θ = 52° becomes more protruding with increasing xTi due to the gradual increase of Ni as a resultant of NiO reduction and the formation of in-situ NiO-Ni-TiO2 nanocomposites. Interestingly, the signs of weak Nickel Titanate (NiTiO3) phases are also observed for xTi = 25 and 30 powders due to chemical reaction. However for xTi = 35, due to the maximum NiO reduction, highly intense peaks of Ni(1 1 1), Ni(2 0 0) and Ni(2 2 0) at 2θ = 44.5°, 51.8° and 76.4° and the marginal peaks at 2θ = 27.4°, 35.95°, 54.27°, 64.1°, 68.94° and 76.42° corresponding to the TiO2 rutile (1 1 0), (1 0 1), (2 1 1), (3 1 0), (3 0 1) and (2 0 2) peak positions, respectively are observed. These structural transformations in milled NiO-(Mg,Ti) powders confirm the occurrence of partial reduction process in NiO. In both the cases, no other peaks corresponding to any impurity phases were observed within the resolution limit of highpower X-ray diffractometer. Here it may be noted that MC displacement reactions follow two entirely different reaction paths depending upon the milling conditions and the nature of the as-mixed powders, i.e., (i) gradual reaction which progresses in a steady state manner through a very small volume during each collision of the milling balls and (ii) a self-sustaining reaction, if the reaction enthalpy is very high for the reaction to proceed itself. The reaction enthalpies, ΔH, in both the reaction modes are found to be substantially negative. In this context, if the reaction is highly exothermic one, a mechanically induced self-sustaining type reaction can 0 0 CP298K ) > 2000 K , where H298 be initiated, if the ratio ( H298 K is K )/ ( the enthalpy of reaction and CP298K is the total heat capacity of the products at 298 K [42]. Thermodynamic calculations for stoichiometric 0 composition of NiO-Mg and NiO-Ti mixtures show that H298 K is negative at room temperature with the value of −361 kJ/mole and −462 kJ/mole for the reactions given in Eqs. (1) and (2), respectively. 0 CP298K ) turns out to be around Accordingly, the ratio of ( H298 K )/ ( 5500 K and 4000 K for NiO-Mg and NiO-Ti mixtures [43]. Thus, both the NiO-(Mg,Ti) reactions are highly exothermic in nature and hence a mechanically induced self-sustaining reaction can be anticipated thermodynamically during the ball milling if abundant amount of reactants are availed in the as-mixed powders. Therefore, the reduction reaction initially follows a progressive/gradual reaction type due to the presence of an insufficient amount of reactants. Upon attaining the mixtures

The in-situ NiO-Mg (x = 0–50 at. %) and NiO-Ti (x = 0–35 at. %) based nanocomposites were prepared by high-energy ball milling under controlled argon gas environment. The Mg and Ti contents in both cases were chosen carefully depending on the possible reaction kinetics and tunable magnetic properties of the powders. Dry milling was carried out in a high-energy planetary ball-mill (Insmart, India) using hardened steel vial and 8 mm diameter hardened steel balls at a constant speed of 500 rpm with a milling period of 30 h. Throughout the process, the ballto-powder weight ratio was maintained constantly at 10:1. The milling was carried out in cycles of 15 min followed by 15 min of idling to avoid overheating of the vials from prolong milling periods. The optimization of the milling parameters was done mainly by analyzing the variations in structure, type of reduction reaction and magnetic properties of the milled powders with different Mg and Ti contents. The structural evolution of the milled powders was analyzed from Xray diffraction (XRD) patterns obtained using a high-power X-ray diffractometer (Rigaku TTRAX III 18 kW) with Cu-Kα radiation (λ = 1.541 Å) at a slow scan rate of 0.005°/s. Surface morphologies were characterized using field-emission scanning electron microscope (FESEM, Zeiss Sigma) with the compositional study done by energy dispersive X-ray analysis (EDAX) unit attached to FESEM. X-ray photoelectron spectroscopy (XPS) analyses were carried out in a standard ultrahigh vacuum surface science chamber consisting of a PSP Vacuum Technology electron energy analyzer (angle integrating ± 10°) and a dual anode X-ray source with an Mg-Kα source (1253.6 eV) at a base pressure of 2 × 10−10 mbar and energy resolution at full width at half maximum (FWHM) is about 0.8 eV. The spectrometer was calibrated using Au 4f7/2 at 83.9 eV. Mössbauer spectra were recorded at room temperature using a standard constant acceleration Mössbauer spectrometer in the transmission geometry. Detailed microstructural analysis were characterized by transmission electron microscopy (TEM, JEOL2100) technique. Room temperature Raman spectra were obtained using micro-Raman spectroscopy (LabRam HR800, Jobin Yvon) with an excitation wavelength of 633 nm. The isothermal magnetic measurements at room temperature were carried out using physical property measurement system (PPMS, Dynacool) at a maximum applied field of ± 50 kOe and the thermomagnetic (M−T) properties at high temperature (from 300 K to 1200 K) were measured by vibrating sample magnetometer (VSM, Lakeshore 7410, USA) at a heating rate of 4 °C/ min under the applied magnetic field of 2 kOe. 3. Result and discussion Fig. 1a shows the room temperature XRD patterns of pure as-mixed and 30 h milled NiO-Mg (x = 0–50 at.%) powders. As-mixed NiO-Mg powder exhibits sharp diffraction peaks of face-centered cubic (fcc) NiO and hexagonal-close-packed (hcp) Mg. Contrariwise, the 30 h milled pure NiO displays broad peaks and peak shift to lower angles [36]. The peak broadening in milled powders is generally ascribed to crystallite size refinement and increased strain whereas the peak shift happens due to change in lattice parameters [37]. Interestingly, the color of the pure un-milled NiO powder changes from pale green to dark green after milling. This can be correlated to the presence of non-stoichiometry in NiO ensued by the defects, crystallite size reduction [38] and Ni2+ to Ni3+ oxidization [39] due to breaking of Ni2+-O2−-Ni2+ super-exchange symmetry [40,41]. On the other hand, with increasing xMg up to 10%, the milled powders display Bragg peaks of fcc NiO without any peaks 2

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Fig. 1. Room temperature XRD patterns of (a) pure as-mixed and 30 h milled NiO-Mg (xMg at.%) powders with xMg = 0–50, (b) expanded XRD patterns in the range of 2θ = 40°–48° and the multiple peak fittings of NiO(2 0 0) peak for xMg = 0 (c), 30 (d), 40 (e) and 50 (f) powders.

close to stoichiometric compositions, the gradual reaction transforms into self-sustaining reaction revealing a well-described reaction kinetics due to MC activation of NiO-(Mg,Ti) powders. To quantify the fraction of NiO reacted with increasing Mg and Ti, we have calculated the change in integrated intensity of NiO(2 0 0) peak [31] by taking into account the relative integrated intensity of NiO (2 0 0) in un-milled and milled powders. Fig. 3 depicts the variation of NiO reduction as a function of xMg and xTi. In NiO-Mg powders, though the MC reduction gets initiated at xMg = 15, due to a lesser fraction of NiO being reduced and the difficulties in the meticulous deconvolution of the peaks, the percentage of reduction could not be calculated precisely. Hence, the reduction of NiO can be truly accounted at xMg = 20

and a maximum of 95% reduction is observed for xMg = 50 powder. However, two different reaction rates are observed, i.e., for xMg = 20–40, NiO reduction happens to be of 9–15% with a gradual reaction rate of 0.31% per xMg. But for xMg > 40%, the mechanically induced self-sustaining reaction enhances the reduction process and proximate to 95% due to the maximum reduction of NiO. Meanwhile, the NiO-Ti powders initiate a gradual reaction kinetics at xTi = 20 with 9% NiO reduction followed by a maximum reduction of 73% for xTi = 35 powder as attributable to the self-sustaining reduction reaction at the presence of higher Ti content. It has been observed that, due to the higher conversion of NiO to Ni and the simultaneous formation of in-situ nanocomposites for both NiO-Mg and NiO-Ti powders, the dark 3

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Fig. 2. Room temperature XRD patterns of (a) pure as-mixed and 30 h milled NiO-Ti (xTi at.%) powders with xTi = 0–35, (b) expanded XRD patterns in the range of 2θ = 40°–48° and the multiple peak fittings of NiO(2 0 0) peak for xTi = 5 (c), 20 (d), 30 (e) and 35 (f) powders.

green color of the non-stoichiometric milled NiO transforms into black color reinforcing the formation of Ni nanoparticles in the milled samples. To study the effect of NiO reduction on the structural properties, the lattice parameters (aNiO and aNi) and average crystallite size (DNiO and

DNi) of NiO and Ni, respectively were calculated for both NiO-Mg and NiO-Ti powders after the removal of instrumental peak broadening and depicted as a function of xMg and xTi in Fig. 4. The bulk values of aNiO and aNi, taken as 4.17 Å, and 3.521 Å, respectively, are shown in the figure for a better comparison with the obtained results. The as-milled 4

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Fig. 3. Percentage reduction of 30 h milled NiO-(Mg,Ti) powders as a function of (a) xMg and (b) xTi.

Fig. 4. Variations of lattice parameters [aNiO, aNi] and crystallite size [DNiO, DNi] as a function of xMg and xTi for 30 h milled NiO-(Mg,Ti) powders.

pure NiO exhibits a slight increase in aNiO (Fig. 4a & e) due to negative interface pressure activated by the strain during milling process [37]. For NiO-Mg powders, the value of aNiO is obtained to be 4.169 Å for xMg = 5% which increases to 4.178 Å at xMg = 10% and then tends to saturate up to 30% of Mg content. The value of aNiO shows a continuous decrease as xMg is increased up to 45%. This could be reasoned to the reduction of NiO into NiO-Ni-MgO nanocomposites at higher Mg content. Similarly, the addition of Ti up to 5% in the NiO-Ti powders shows aNiO to be 4.178 Å (Fig. 4e). This is attributed to the introduction of defects during the dissolution of larger atomic sized Ti in the NiO matrix to form the NiO-Ti solid solution. However with further increasing xTi beyond the solid solubility limit, aNiO decreases gradually down to 4.132 Å for xTi = 35 due to the gradual reduction of NiO by Ti to form NiO-Ni-TiO2 nanocomposite. Moreover, the value of aNi approaches towards the lattice parameter of bulk Ni with increasing x in both the powders suggesting to the growth of stable Ni phases to form in NiO-Ni-MgO and NiO-Ni-TiO2 nanocomposites. The NiO crystal size (DNiO) decreases significantly from about 50 nm for pure un-milled NiO to 14.5 nm for the 30 h milled NiO powder [36]. With increasing xMg (Fig. 4c), DNiO increases slightly for xMg = 5 and then decreases progressively down to 11.8 nm for x = 25. On further increasing xMg, DNiO

increases considerably up to 17 nm for xMg = 45. It may be noted that the error in obtaining DNiO increases with increasing Mg due to difficulties in exact deconvolution of NiO peak. On the other hand, the value of DNiO is found to decrease largely with increasing Ti content and reaches to about 5 nm for xTi = 35 powder (Fig. 4g). In contrast, DNi is found to be increasing significantly to around 27 nm and 20 nm for xMg = 50 and xTi = 35 powders, respectively. The large increase in DNi can be attributed to the increased crystallinity of Ni in the resulting in-situ nanocomposites. Moreover, the growth of Ni in the nanocomposites leads to aNi towards bulk value. These variations in the structural parameters clearly indicate a decrease in DNiO with the simultaneous increase in DNi due to the increased NiO reduction as a function of Mg and Ti contents in the nanocomposites. To elucidate the morphological changes with the reduction process, FESEM images of milled NiO-Mg and NiO-Ti powders as a function of xMg and xTi were obtained and shown in Fig. 5. Un-milled pure NiO has clear particle morphology with an approximate particle size ranging between 1 and 4 μm (not shown here). The dissolution of Mg and Ti in the NiO matrix refines the particle sizes by 100–150 nm with heavy agglomeration. Note that these aggregates are typical characteristics of ball-milled powders resulting in from repetitive cold welding and 5

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Fig. 5. FESEM micrographs of 30 h milled NiO-Mg powders with xMg = 5 (a), 10 (b), 40 (c); and NiO-Ti powders with xTi = 5 (d), 10 (e), 35 (f).

Fig. 6. BF-TEM images and SAED patterns of 30 h milled NiO-Mg powders with xMg = 10 (a), 20 (b), 40 (c), 50 (d); and NiO-Ti powders with xTi = 10 (e), 20 (f), 30 (f), 35 (g).

fracture during the milling process. However with increasing xMg and xTi, the nature of surface morphology depicts strongly agglomerated spherical particles and can be correlated to the increased NiO reduction and the formation of in-situ nanocomposites with high Ni content. We have observed a particle size refinement down to 50–60 nm and 40–50 nm for xMg = 40 and xTi = 35 powders, respectively. This clearly indicates the substantial role of ball milling on the particle size refinement with the nanostructure formation and the MC reduction process is a key factor on the modification of structural morphology of the milled powders. The composition analysis using EDAX confirms the presence of only Ni, Mg, Ti and O elements in the milled powders. To understand the evolution of nanocomposite in details and validate the XRD findings, the milled NiO-Mg and NiO-Ti powders were analyzed using TEM technique. Fig. 6 displays the bright-field TEM (BF-TEM) image and selected area electron diffraction (SAED) patterns for NiO(Mg,Ti) powders. The BF-TEM image shows fine nanocrystalline

microstructure with irregular particle morphology and the associated SAED patterns shown in the insets having concentric rings confirm the polycrystalline nature of the samples. This is in close agreement with the XRD results. With increasing xMg < 20, the SAED pattern evidences only the presence of NiO phases with finer and roughly nanometer sized crystals of 10–13 nm without any evidence of NiO reduction. The variations of TEM crystallite size show almost similar trend as XRD results, but with a broader size distribution and irregular particle morphology. With increasing xMg from 20 to 40%, a similar nanocrystalline behavior with irregular morphology is observed. However, the SAED patterns show a quite continuous and diffusive rings corresponding to Ni(1 1 1) and NiO(2 0 0) phases, respectively. Nevertheless, the presence of MgO (2 0 0) phase could not be resolved clearly due to mere overlap of MgO (2 0 0) rings with NiO(2 0 0) ring. On further increasing xMg to 50, diffraction rings corresponding to Ni and MgO could markedly be observed confirming the formation of NiO-Ni-MgO nanocomposite. 6

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Fig. 7. Room temperature Raman spectra of un-milled NiO (a), 30 h milled NiO (b); NiO-Mg (xMg at.%) powders with xMg = 5 (c), 10 (d), 20 (e) and 40 (f); and NiO-Ti (xTi at.%) powders with xTi = 5 (g), 10 (h), 20 (i) and 35 (j).

Concurrently, in the milled NiO-Ti powders, for xTi ≤ 20 (Fig. 6e & f), the SAED patterns show only the NiO phases with fine nanosized crystals ranging between 12 and 15 nm without the presence of any other phases. With further increasing xTi from 20 to 35, analogous nanocrystalline behavior with irregular morphology is observed. Here, the average crystallite size decreases considerably to 7–9 nm and the SAED patterns display continuous and diffusive rings corresponding to Ni(1 1 1) and NiO(2 0 0) phases along with the diffraction ring corresponding to TiO2 phases suggesting the plausible formation of NiO-NiTiO2 nanocomposite. These microstructural analyses are in close agreement with the XRD results and show no evidence of any other impurity phases in the as-milled powders. To study the defect structure in the milled powders, Raman spectra were obtained for un-milled pure NiO, milled NiO, and milled NiO(Mg,Ti) powders and presented in Fig. 7. Raman spectra were prudently probed by Lorentzian curve-fitting with various combinations of bands to find peak position, peak width and area under the curves. The AFM ordered NiO not only shows one-phonon (1P) transverse optical (TO) at 380–410 cm−1, 1P longitudinal optical (LO) at 520–550 cm−1, twophonon (2P) TO at 730–780 cm−1, 2P LO + TO at 900–950 cm−1, 2P LO at 1050–1110 cm−1, but also shows a peak at 1450–1500 cm−1 due

to two-magnon (2 M) scattering of bulk NiO involving Brillouin zoneedge magnons, which is associated with Ni2+-O2−-Ni2+ super-exchange symmetry [44,45]. However for 30 h milled NiO, the bands in the Raman spectra show varying features as (i) the 2 M band disappears due to the breaking of Ni2+-O2−-Ni2+ super-exchange symmetry, (ii) 1P LO mode becomes well pronounced and broadened, (iii) 2P TO + LO mode disappears after milling, (iv) the 2P TO and 2P LO modes broaden with the peak positions shifting towards lower wavenumber, and (v) the integrated intensity ratio between 1P LO and 2P LO modes (I1PLO/I2PLO) increases from 0.31 to 0.34, indicating the induced defects, surface effects and imperfections in particle shape and size caused by the ball milling process. The inclusion of Mg in NiO changes the Raman spectra drastically, i.e., I1PLO/I2PLO increases largely from 0.34 to 2.9 and the weakening of 2P TO mode largely as Mg content is increased to 5%. This could be attributed to dissolution of Mg into NiO. With increasing xMg > 5, I1PLO/I2PLO decreases progressively to 1.06, 2P TO peak disappears and only 1P LO and 2P LO modes appear in the Raman spectra with reduced intensity. This could be accredited to the gradual reduction process of NiO into NiO-Ni-MgO nanocomposite with enhanced fraction of Ni. On the other hand, the influence of Ti addition up to 10% 7

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alters the vibrational bands in NiO significantly. Upon the dilution of Ti at 5%, the spectral intensity ratio, i.e., I1PLO/I2PLO increases from 0.34 to 7.35 due to dissolution and this value is observed to be much larger as compared to the 5% Mg addition in NiO. However, the 2P TO mode becomes more pronounced with increasing Ti content whereas a relative decrease in intensity of 2P LO spectral band is observed. This is described by means of local symmetry lowering at Ni2+sites, actuated by chemical substitution and off-centre displacement of Nickel ions [44]. However with increasing xTi to 20, concerning the NiO-Ni-TiO2 nanocomposite formed due to the reduction process, Raman bands are observed at around 172 cm−1, 226 cm−1, 277 cm−1, 330 cm−1 and 477 cm−1 corresponding to the oxygen deficient TiO2 (TiO2-δ) phases [46]. Along with this, the Raman spectrum of anatase TiO2 at 143 cm−1 and 205 cm−1 corresponding to Eg(1) and Eg(2) phases and rutile TiO2 at around 448 cm−1 corresponding to Eg phase is observed [47]. Furthermore, Raman peaks of NiTiO3 at around 305 cm−1 and 750 cm−1 active modes are also observed. These findings not only show good agreement with the results of XRD analysis, but also confirms the formation of no other impurity phases in the as-milled powders. To understand the effects of reduction of NiO and structural refinement on the magnetic properties, room temperature initial magnetization (IM) curves, magnetic hysteresis (M−H) loops and temperature dependent magnetization data were measured. Fig. 8 illustrates the IM curves, M−H loops and extracted values of saturation magnetization (MS) and coercivity (HC) for as-mixed and milled NiO(Mg,Ti) powders. It is seen that (i) un-milled NiO and as-mixed NiO-Mg powders show a weak and linear response to the applied field due to its AFM nature, as supported by the Raman study, and therefore the loop passes through the origin. (ii) 30 h milled NiO powder, on the other hand, shows significant change in the magnetization curve, where the moment experiences an eloquent increase at lower applied field and then progressively varies at the higher field region. This leads to a clear M−H loop confirming the presence of room temperature ferromagnetism (RTFM) with magnetization at 50 kOe, MS, of 1.96 emu/g at 50 kOe and HC of 155 Oe. (iii) With increasing xMg up to 10%, though the M−H curves show a similar feature as that of milled NiO, a significant decrease in MS from 1.96 emu/g to 0.73 emu/g is observed. (iv) On further increasing xMg ˃ 10, MS increases gradually to 4.48 emu/g for xMg up to 40%. (v) However, a rapid change in MS to 10.9 and 20.7 emu/g is observed with increasing xMg to 45 and 50, respectively. HC decreases from 155 Oe to 13.5 Oe with increasing xMg from 0 to 25, respectively and then rises again as xMg is increased from 30 to 50%. Similarly from Fig. 8b, it is observed that (i) As-mixed NiO-Ti powders exhibit similar weak and linear magnetic response to the applied field with zero HC due to the AFM nature of NiO. (ii) For xTi = 5, the M−H curve shows a cognate feature as that of milled NiO but a significant decrease in MS from 1.96 emu/g to 1.07 emu/g is abided (iii) For xTi ˃ 5, MS increases progressively and reaches a maximum of 13.7 emu/g for xTi = 35 powder. The rate of increase in magnetization increases for xTi ˃ 20. (iv) On the other hand, HC decreases initially from 155 Oe to 71 Oe with increasing x from 0 to 15 followed by a gradual increase from xTi = 20 to 35 and sweeps to 147 Oe for xTi = 35 powder. In order to correlate the effect of Mg/Ti substitution and the NiO reduction on the magnetic behavior, the structural and magnetic properties of the in-situ nanocomposites are compared in detail. Unmilled NiO having AFM nature due to the existence of Ni2+-O2−-Ni2+ super-exchange symmetry exhibits no M−H loop and the curve passes through origin with zero HC [48]. In contrary, the 30 h milled NiO undergoes drastic size reduction from 50 nm to 14.5 nm along with the formation of defects, lattice distortion, broken Ni-O bonds and oxidization of Ni2+ to Ni3+ ions. This leads to a larger surface to volume ratio and higher number of misaligned surface spins due to breaking of the Ni2+-O2−-Ni2+ super-exchange symmetry [39–41,44,49,50]. As a result, the alignment of particle’s net moment at lower field region and non-saturated RTFM with moderate MS of 1.96 emu/g and HC of 155 Oe are observed. With increasing xMg up to 10, MS and HC decrease to

0.73 emu/g and 98 Oe, respectively due to the dissolution process, which reduces induced FM behavior of NiO-Mg powder. On further increasing xMg up to 25%, the Mg stimulated MC reduction process of NiO gets initiated and confirms the formation of Ni with size smaller than 10 nm. Hence, HC decreases largely from 98 Oe to 13.5 Oe upon increasing xMg from 10 to 25%. As displayed in Fig. 4, the average crystal size of Ni (DNi) increases due to enhanced reduction of NiO when xMg is increased above 25 and therefore MS and HC increase largely to 20.7 emu/g and 103 Oe, respectively for xMg = 50 powder. Similarly, with adding xTi up to 5%, MS and HC decrease to 1.07 emu/g and 92 Oe respectively due to the reasonable dissolution of non-magnetic Ti, which dilutes the induced FM properties of milled NiO-Ti powder. On further increasing xTi up to 15, due to the MC reduction of NiO, HC decreases largely to 71 Oe. Furthermore for xTi ≥ 20, due to the significant increase in Ni nanocrystals with increasing xTi in the NiO-NiTiO2 nanocomposite, MS and HC increases largely to 13.7 emu/g and 147 Oe, respectively for xTi = 35. This increased magnetization with the Mg and Ti substitution in NiO and the subsequent milling process could only be ascribed to a stronger magnetic interaction between nanocrystalline FM Ni particles and AFM NiO matrix in the NiO-Ni-MgO and NiO-Ni-TiO2 nanocomposites [50,51]. Nonetheless by taking into account the AFM nature of NiO, it is apparent that the all-inclusive hysteresis behavior in the milled powders and the variation of MS can be accredited to the formation of FM Ni from the NiO reduction. Hence, the percentage of Ni fraction is quantified explicitly by correlating MS of milled powders with respect to the MS of bulk Ni (~55 emu/g) [52]. Fig. 9 illustrates the relation between the attained Ni fraction and the percentage of NiO reduction. It can be observed that milled NiO powder has about 3.56 wt% Ni enriched regions due to the residual magnetic moments of misaligned surface spins as compared to the AFM particle core. This is in accordance with the previous reports on the hydrogenated NiO, where the saturation magnetization increases at a rate of about 0.6 emu/g per Ni % [53]. The percentage of Ni increases with increasing the percentage of NiO reduction and all the data fall into almost linear variation. The straight line fit to the data reveals that Ni % for NiO-Mg and NiO-Ti powders increases at a rate of 0.4 and 0.09% per NiO % reduction, respectively. Furthermore, the correlation between the structural and magnetic properties reveals that the in-situ nanocomposites exhibit RTFM even though the crystallite size of fine Ni crystals varies between 11 and 27 nm for xMg and 8–20 nm for xTi powders. It may be noted that the size of the Ni crystals is well below the critical size (~34 nm) of spherical Ni particles for exhibiting single domain behavior at room temperature. Hence, the persistence of RTFM ordering below the critical size is possible only if the total magnetic anisotropy (Keff) is larger than the bulk Ni equivalent [54]. Note that the major contribution to the Keff in nanoscale magnetic particles can be defined as,

K eff = Kmc + Ksh + Ksur + Kst + K ex

(3)

where, Kmc – magnetocrystalline anisotropy, Ksh – shape anisotropy, Ksur – surface anisotropy, Kst – strain anisotropy and Kex – exchange anisotropy. The microstructural studies reveal that the milled powders display quite irregular morphology and therefore Ksh is contemplated to play a dominant role as illustrated by earlier reports on smaller nanoparticles with highly anisotropic shapes [55,56]. It is also known that for smaller particles of nanometer range, Ksur becomes more prominent with size refinement and thereby plays a key role in increasing Keff [57,58]. As the milled powders endure severe fracture and cold welding during milling, Kst is expected to be an inevitable part of Keff. In addition, due to the evolution of FM Ni from the AFM NiO matrix, the presence of FM-AFM interfaces enhances Kex in the in-situ nanocomposites [51,56,59]. Keff is determined by fitting the IM curves at high-field region using the law of approach of the magnetization to saturation (LAS) for cubic materials [60] and depicted in Fig. 10 as a function of xMg and xTi. The estimated value of Keff of bulk Ni has also been shown in the 8

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Fig. 8. Room-temperature magnetic hysteresis (M−H) loops [initial magnetization (IM) curves with magnetization in logarithmic scale as insets] for (a) NiO-Mg (xMg at.%); (b) NiO-Ti (xTi at.%) powders, (c) variations of magnetization (MS) and (d) coercivity (HC) as a function of xMg and xTi. M−H loop of pure NiO without milling is also shown for the comparison.

analyzed using 57Fe Mössbauer spectroscopy, XPS, and EDAX, respectively. However, the presence of any impurity phases corresponding to the iron could not be detected at least to the detection limits of these techniques. Besides, the Ni 2p XPS spectra reveal the existence of mixed valency state 2+ and 3+ in the milled NiO powders due to the nonstoichiometry caused by the breaking of Ni2+-O2−-Ni2+ super-exchange symmetry due to size reduction with the irregular shapes and induced defects. This is not only in good agreement with the earlier reports [61], but also well-supported from the results of Raman analysis given in Fig. 7. Furthermore, the detailed annealing studies of the asmilled NiO powders in air atmosphere [37,62] reduced the magnetic moment of the milled powders largely and the rate of decrease of magnetic moment strongly dependent on the milling conditions. This was attributed to the release of strain (defects), reduction of Ni3+ to Ni2+ [63] and increase in average crystallite size with annealing. The above results along with the present milling conditions apparently rules out the possible contribution from the iron impurity phases on the magnetic properties and therefore the observed moment in the milled powders is mainly originated from the intrinsic properties. In order to study the FM stability in details, high temperature

figure for a better comparison with the milled powders. It has been observed that the values of Keff vary between 0.68–5.54 × 104 J/m3 and 0.65–4.3 × 104 J/m3 for NiO-Mg and NiO-Ti powders, respectively. This high Keff in milled powders as compared to bulk Ni (0.5 × 104 J/ m3) are in accordance with the earlier mentions by Zhang et al. [59]. Such a high value of Keff in the ball-milled powders comprises the combined effects of various factors such as shape, surface, strain, exchange and magnetocrystalline anisotropy of the fine Ni nanoparticles. To see probable correlation between Keff and MS due to the formation of Ni nanocrystallites from the NiO reduction process, the variation of Keff is also plotted as a function of MS. All the data almost fall into linear behavior and the straight line fitting between Keff and MS reveals that Keff increases at a rate of 2.54 kJ/m3 and 3.13 kJ/m3 per emu/g for the NiO-Mg and NiO-Ti powders. This anomalous enhancement of Keff originates to the FM thermal stability of milled powders below the single domain and thereby shifts the blocking temperature above room temperature regime. In order to understand the role of unexpected impurity contribution on the magnetic properties, the detailed magnetic structure, chemical bonding and chemical composition of the milled powders were 9

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Fig. 9. Relation between percentage of Ni and the percentage of NiO reduction for 30 h milled NiO-Mg (a) and NiO-Ti (b) powders.

thermomagnetic (M−T) measurements were carried out under an external field of 2 kOe over the temperature range between 300 K and 1050 K. Fig. 11 illustrates the normalized M−T curves of the un-milled pure NiO, milled NiO-Mg and NiO-Ti powders. To demonstrate the correlative high temperature magnetic variation close to zero, the same curves are plotted in logarithmic scales in the inset. The magnetization of un-milled NiO progressively increases with increasing temperature up to 525 K followed by a gradual decrease and thus forms a broad peak in the M−T curve. As bulk NiO exhibits AFM ordering at room-

temperature, the Neel temperature is determined from this peak maximum as 525 K [64,65]. Contradictorily, 30 h milled NiO displays a nearly constant magnetization in the vicinity of the room-temperature regime and then almost proximate to vanishing values above 900 K. The thermal derivative of the M−T curve shows two magnetic transitions: one near to TN due to the presence of AFM NiO core and the other at 780 K as a consequence of plausible magnetic phase transition (TC) of induced FM phase due to the misaligned surface spins of the magnetically disordered shell. Although the magnetic phase transition can be

Fig. 10. Variations of Keff as a function of xMg, xTi and MS for 30 h milled NiO-(Mg,Ti) powders. 10

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majorly due to the enhanced NiO reduction with increased Ni fraction at higher Mg and Ti contents to form nanocomposites. The observed results demonstrate a systematic correlation amongst the structural, vibrational, magnetic and thermomagnetic properties of NiO-(Mg,Ti) powders. 4. Conclusions We have systematically demonstrated the effect of mechanically activated reduction reaction of NiO via high-energy ball milling approach under dry-milling conditions using different Mg and Ti contents on the structural, vibrational and magnetic properties of resulting insitu nanocomposites. The milling process effectively reduced the average crystal size of pure NiO from about 50 nm to 14.5 nm without any change in the fcc structure and a simultaneous lattice expansion was observed. As a consequence, the antiferromagnetic behavior of bulk NiO changed into weakly induced ferromagnetic with a magnetization of 1.96 emu/g for the milled powder caused by the formation of 3.56 wt% Ni enriched spatial regions due to non-stoichiometry in NiO, presence of defects, size reduction and oxidation of Ni. The milled NiOMg and NiO-Ti powders formed solid solution for xMg up to 10% and xTi of 5% respectively. The NiO-Mg/Ti solid solution decreased the magnetization effectively. A gradual reduction of NiO into NiO-Ni-MgO and NiO-Ni-TiO2 nanocomposites was observed due to the mechanical activation of Mg and Ti reduction process with the NiO matrix. The increase of further xMg and xTi increased magnetization to 20.7 emu/g and 13.7 emu/g with a maximum reduction of 95% and 73% for xMg = 50 and xTi = 35, respectively. The structural, vibrational and magnetic properties exhibited a strong dependence on the relative changes in the NiO and Ni phases in the in-situ nanocomposites. Interestingly, the milled NiO-Mg and NiO-Ti powders showed strong room temperature ferromagnetism due to higher total magnetic anisotropy despite having fine Ni crystals below the critical size. High temperature thermomagnetization data confirmed a strong dependence of the magnetic phase transition temperature on the relative content of NiO and Ni phases. The observed results have been explained on the basis of effective mechanical activation on Mg and Ti stimulated reduction reaction of NiO powders. Fig. 11. Temperature dependent magnetization (M−T) curves for 30 h milled (a) NiO-Mg and (b) NiO-Ti powders. Inset shows the same data, but plotted in logarithmic scale. M−T curve of pure un-milled NiO is also shown for comparison.

Declaration of Competing Interest We have no conflict of interest to declare. Acknowledgements

correlated to the increased Ni content due to the increased number of disordered surface spins, TC is quite high than that of the bulk Ni (~630 K). This is strongly correlated to the stress induced due to the mechanical treatment of the powders during milling or the strain generated due to the interfacial Ni-NiO lattice mismatch and the induced FM phase-AFM core contending exchange interaction [66]. The nonsmooth decrease of magnetization in M−T curve evidences the presence of stress which acts more like a hydrostatic one and thereby increases TC [67]. But, the milled NiO-(Mg,Ti) powders exhibit a large decline in magnetization with increasing temperature and have two different magnetic transitions: (1) a substantial drop in magnetization at 640 K with a broad magnetic transition having peak temperature (TP) at 690 K and (2) a gradual decline in magnetization up to 900 K. While the first magnetization drop could be accredited to the magnetic phase transition of Ni due to the increased Ni content, the later one is ascribed to the induced FM phase transition [38]. With increasing xMg and xTi > 20, due to the formation of enriched Ni as seen evidently from the structural studies, M−T curves exhibit a sharp magnetic transition at around 645 K. This matches exactly with TC of bulk Ni. Interestingly, with increasing xMg and xTi, the magnitude of magnetization dropping at 645 K increases largely at the expense of magnetization decreasing gradually at higher temperatures. This is

We would like to acknowledge Council of Scientific and Industrial Research for the financial support vide Project no. 03(1337)/15/EMRII, New Delhi. Infrastructure facilities provided by the Department of Science and Technology, India vide Project no: [SR/S2/CMP-19/2006 and SR/FST/PII-020/2009; SR/FST/PSII-037/2016] and Central Instruments facility (CIF), IIT Guwahati are gratefully acknowledged. References [1] S. Singamaneni, V.N. Bliznyuk, C. Binek, E.Y. Tsymbal, Magnetic nanoparticles: recent advances in synthesis, self-assembly and applications, J. Mater. Chem. 21 (2011) 16819–16845, https://doi.org/10.1039/C1JM11845E. [2] U. Wiedwald, K. Fauth, M. Heßler, H.-G. Boyen, F. Weigl, M. Hilgendorff, M. Giersig, G. Schütz, P. Ziemann, M. Farle, From colloidal Co/CoO core/shell nanoparticles to arrays of metallic nanomagnets: surface modification and magnetic properties, ChemPhysChem 6 (2005) 2522–2526, https://doi.org/10.1002/cphc. 200500148. [3] B. Janković, B. Adnadević, S. Mentus, The kinetic analysis of non-isothermal nickel oxide reduction in hydrogen atmosphere using the invariant kinetic parameters method, Thermochim. Acta 456 (2007) 48–55, https://doi.org/10.1016/j.tca.2007. 01.033. [4] R. Chai, P. Chen, Z. Zhang, G. Zhao, Y. Liu, Y. Lu, Thin-felt NiO-Al2O3/FeCrAl-fiber catalyst for high-throughput catalytic oxy-methane reforming to syngas, Catal. Commun. 101 (2017) 48–50, https://doi.org/10.1016/j.catcom.2017.07.023.

11

Journal of Magnetism and Magnetic Materials 494 (2020) 165784

A.M. Padhan, et al. [5] S. Song, S. Yao, J. Cao, L. Di, G. Wu, N. Guan, L. Li, Heterostructured Ni/NiO composite as a robust catalyst for the hydrogenation of levulinic acid to γ-valerolactone, Appl. Catal. B: Environ. 217 (2017) 115–124, https://doi.org/10.1016/j. apcatb.2017.05.073. [6] B. Kucharczyk, W. Tylus, J. Okal, J. Checmanowski, B. Szczygieł, The Pt-NiO catalysts over the metallic monolithic support for oxidation of carbon monoxide and hexane, Chem. Eng. J. 309 (2017) 288–297, https://doi.org/10.1016/j.cej.2016.10. 032. [7] D. Xu, C. Mu, B. Wang, J. Xiang, W. Ruan, F. Wen, X. Du, Z. Liu, Y. Tian, Fabrication of multifunctional carbon encapsulated Ni@NiO nanocomposites for oxygen reduction, oxygen evolution and lithium-ion battery anode materials, Sci. China Mater. 60 (10) (2017) 947–954, https://doi.org/10.1007/s40843-017-9094-5. [8] S. Ni, T. Li, X. Lv, X. Yang, L. Zhang, Designed constitution of NiO/Ni nanostructured electrode for high performance lithium ion battery, Electrochim. Acta 91 (2013) 267–274, https://doi.org/10.1016/j.electacta.2012.12.113. [9] W. Lee, I. Kim, H. Choi, K. Kim, Synthesis of Ni/NiO core-shell nanoparticles for wet-coated hole transport layer of the organic solar cell, Surf. Coat. Technol. 231 (2013) 93–97, https://doi.org/10.1016/j.surfcoat.2012.01.024. [10] J.Y. Son, Y.-H. Shin, H. Kim, H.M. Jang, NiO resistive random access memory nanocapacitor array on graphene, ACS Nano 4 (5) (2010) 2655–2658, https://doi. org/10.1021/nn100234x. [11] K.B. Gerasimov, A.A. Gusev, E.Y. Ivanov, V.V. Boldyrev, Tribochemical equilibrium in mechanical alloying of metals, J. Mater. Sci. 26 (1991) 2495–2500, https://doi. org/10.1007/BF01130201. [12] M. Alagiri, S. Ponnusamy, C. Muthamizhchelvan, Synthesis and characterization of NiO nanoparticles by sol–gel method, J. Mater. Sci.: Mater. Electron. 23 (2012) 728–732, https://doi.org/10.1007/s10854-011-0479-6. [13] N. Rinaldi-Montes, P. Gorria, D. Martínez-Blanco, A.B. Fuertes, L.F. Barquín, J.R. Fernandez, I. de Pedro, M.L. Fdez-Gubieda, J. Alonso, L. Olivi, G. Aquilanti, J.A. Blanco, Interplay between microstructure and magnetism in NiO nanoparticles: breakdown of the antiferromagnetic order, Nanoscale 6 (2014) 457–465, https:// doi.org/10.1039/C3NR03961G. [14] C. Dueso, M. Ortiz, A. Abad, F. García-Labiano, L. de Diego, P. Gayán, J. Adánez, Reduction and oxidation kinetics of nickel-based oxygen-carriers for chemicallooping combustion and chemical-looping reforming, Chem. Eng. J. 188 (2012) 142–154, https://doi.org/10.1016/j.cej.2012.01.124. [15] S.-L. Che, K. Takada, K. Takashima, O. Sakurai, K. Shinozaki, N. Mizutani, Preparation of dense spherical Ni particles and hollow NiO particles by spray pyrolysis, J. Mater. Sci. 34 (1999) 1313–1318, https://doi.org/10.1023/ A:1004546014867. [16] A. Ainabayev, M. Arkhipov, A. Baideldinova, K. Omarova, G. Ksandopulo, Out-offurnace synthesis of high-temperature ceramic materials in the revolving reactor, IOP Conf. Ser.: Mater. Sci. Eng. 47 (2013) 012044, https://doi.org/10.1088/1757899X/47/1/012044. [17] S.Z. Anvari, F. Karimzadeh, M.H. Enayati, Synthesis and characterization of NiAl–Al2O3 nanocomposite powder by mechanical alloying, J. Alloys Compd. 477 (2009) 178–181, https://doi.org/10.1016/j.jallcom.2008.10.043. [18] C. Suryanarayana, Mechanical alloying and milling, Prog. Mater. Sci. 46 (2001) 1–184, https://doi.org/10.1016/S0079-6425(99)00010-9. [19] M. Broseghini, L. Gelisioa, M. D’Incaua, C.L. Azanza Ricardo, N.M. Pugno, P. Scardi, Homogeneity of ball milled ceramic powders: effect of jar shape and milling conditions, J. Eur. Ceram. Soc. 36 (9) (2016) 2205–2212, https://doi.org/10.1016/j. jeurceramsoc.2015.09.032. [20] T.S. Ward, W. Chen, M. Schoenitz, R.N. Dave, E.L. Dreizin, A study of mechanical alloying processes using reactive milling and discrete element modeling, Acta Mater. 53 (2005) 2909–2918, https://doi.org/10.1016/j.actamat.2005.03.006. [21] P.M. Botta, R.C. Mercader, E.F. Aglietti, J.M. Porto Lopez, Synthesis of Fe–FeAl2O4–Al2O3 by high-energy ball milling of Al–Fe3O4 mixtures, Scripta Mater. 48 (2003) 1093–1098, https://doi.org/10.1016/S1359-6462(02)00630-9. [22] M. Rabiee, H. Mirzadeh, A. Ataie, Unraveling the effects of process control agents on mechanical alloying of nanostructured Cu-Fe alloy, J. Ultrafine Grained Nanostruct. Mater. 49 (1) (2016) 17–21, https://doi.org/10.7508/JUFGNSM.2016. 01.03. [23] Y.A. Sorkhe, H. Aghajani, T.A. Taghizadeh, Mechanical alloying and sintering of nanostructured TiO2 reinforced copper composite and its characterization, Mater. Des. 58 (2014) 168–174, https://doi.org/10.1016/j.matdes.2014.01.040. [24] J. Li, F. Li, K. Hu, Preparation of Ni/Al2O3 nanocomposite powder by high-energy ball milling and subsequent heat treatment, J. Mater. Process. Technol. 147 (2) (2004) 236–240, https://doi.org/10.1016/j.jmatprotec.2003.12.022. [25] A. Dolatmoradi, S. Raygan, H. Abdizadeh, Mechanochemical synthesis of W-Cu nanocomposites via in-situ co-reduction of the oxides, Powder Technol. 233 (2013) 208–214, https://doi.org/10.1016/j.powtec.2012.08.013. [26] Y. Jia, C. Sun, Y. Peng, W. Fang, X. Yan, D. Yang, J. Zou, S.S. Mao, X. Yao, Metallic Ni nanocatalyst in situ formed from a metal–organic-framework by mechanochemical reaction for hydrogen storage in magnesium, J. Mater. Chem. A 3 (2015) 8294–8299, https://doi.org/10.1039/C5TA00278H. [27] S. Doppiu, V. Langlais, J. Sort, S. Surińach, M.D. Baró, Y. Zhang, G. Hadjipanayis, J. Nogués, Controlled reduction of NiO using reactive ball milling under hydrogen atmosphere leading to Ni−NiO nanocomposites, Chem. Mater. 16 (2004) 5664–5669, https://doi.org/10.1021/cm048810n. [28] H. Yang, P.G. McCormick, Mechanically activated reduction of nickel oxide with graphite, Metall. Mater. Trans. B 29 (1998) 449–455, https://doi.org/10.1007/ s11663-998-0123-x. [29] S.B. Jagtap, B.B. Kale, A.N. Gokaran, Carbothermic reduction of nickel oxide: effect of catalysis on kinetics, Metall. Trans. B 23B (1992) 93–96, https://doi.org/10. 1007/BF02654042.

[30] P. Matteazzi, G. Le Caer, Synthesis of nanocrystalline alumina-metal composites by room-temperature ball-milling of metal oxides and aluminum, J. Am. Ceram. Soc. 75 (1992) 2749–2755, https://doi.org/10.1111/j.1151-2916.1992.tb05499.x. [31] V. Udhayabanu, N. Singh, B.S. Murty, Mechanical activation of aluminothermic reduction of NiO by high energy ball milling, J. Alloys Compd. 497 (2010) 142–146, https://doi.org/10.1016/j.jallcom.2010.03.089. [32] D. Oleszak, NiAl-Al2O3 intermetallic matrix composite prepared by reactive milling and consolidation of powders, J. Mater. Sci. 39 (2004) 5169–5174, https://doi.org/ 10.1023/B:JMSC.0000039204.08971.26. [33] N. Setoudeh, M.H. Paydar, M. Sajjadnejad, Effect of high energy ball milling on the reduction of nickel oxide by zinc powder, J. Alloys Compd. 623 (2015) 117–120, https://doi.org/10.1016/j.jallcom.2014.10.085. [34] N. Setoudeh, C. Zamani, M. Sajjadnejad, Formation of ZnO/Ni0.6Zn0.4O mixture using mechanical milling of Zn-NiO, Mater. Trans. 57 (9) (2016) 1597–1601, https://doi.org/10.2320/matertrans.M2015430. [35] N. Setoudeh, C. Zamani, M. Sajjadnejad, Mechanochemical synthesis of nanostructured MgXNi1-XO compound by Mg-NiO mixture, J. Ultrafine Grained Nanostruct. Mater. 50 (1) (2017) 51–59, https://doi.org/10.7508/jufgnsm.2017. 01.07. [36] A.M. Padhan, M. Sathish, P. Saravanan, A. Perumal, Mechanical activation on aluminothermic reduction and magnetic properties of NiO powders, J. Phys. D: Appl. Phys. 50 (2017) 21LT01, https://doi.org/10.1088/1361-6463/aa6cee. [37] B. Kisan, PhD Thesis, Indian Institute of Technology Guwahati, India, 2018. [38] B. Kisan, P.C. Shyni, S. Layek, H.C. Verma, D. Hesp, V. Dhanak, S. Krishnamurthy, A. Perumal, Finite size effects in magnetic and optical properties of antiferromagnetic NiO nanoparticles, IEEE Trans. Magn. 50 (2014) 2300704, https:// doi.org/10.1109/TMAG.2013.2278539. [39] T. Ahmad, K.V. Ramanujachary, S.E. Lofland, A.K. Ganguli, Magnetic and electrochemical properties of nickel oxide nanoparticles obtained by the reverse-micellar route, Solid State Sci. 8 (2006) 425–430, https://doi.org/10.1016/j. solidstatesciences.2005.12.005. [40] N. Mironova-Ulmane, A. Kuzmin, I. Steins, J. Grabis, I. Sildos, M. Pärs, Raman scattering in nanosized nickel oxide NiO, J. Phys. Conf. series 93 (2007) 012039, , https://doi.org/10.1088/1742-6596/93/1/012039. [41] A.C. Gandhi, C.Y. Huang, C.C. Yang, T.S. Chan, C.L. Cheng, Y.R. Ma, S.Y. Wu, Growth mechanism and magnon excitation in NiO nanowalls, Nanoscale Res. Lett. 6 (2011) 485, https://doi.org/10.1186/1556-276X-6-485. [42] L. Takacs, Self-sustaining reactions induced by ball milling, Prog. Mater. Sci. 47 (2002) 355–414, https://doi.org/10.1016/S0079-6425(01)00002-0. [43] HSC Chemistry for Windows, version 5.1. Outokumpu, Oy, 1994. [44] E. Cazzanelli, A. Kuzmin, G. Mariotto, N. Mironova-Ulmane, Study of vibrational and magnetic excitations in NicMg1−cO solid solutions by Raman spectroscopy, J. Phys.: Condens. Matter 15 (2003) 2045–2052, https://doi.org/10.1088/09538984/15/12/321. [45] D. Liu, D. Li, D. Yang, Size-dependent magnetic properties of branchlike nickel oxide nanocrystals, AIP Adv. 7 (2017) 015028, , https://doi.org/10.1063/1. 4974307. [46] Y. Wu, Q. Zhang, X. Wu, S. Qin, J. Liu, High pressure structural study of β-Ti3O5: Xray diffraction and Raman spectroscopy, J. Solid State Chem. 192 (2012) 356–359, https://doi.org/10.1016/j.jssc.2012.04.036. [47] O. Frank, M. Zukalova, B. Laskova, J. Kürti, J. Koltai, L. Kavan, Raman spectra of titanium dioxide (anatase, rutile) with identified oxygen isotopes (16, 17, 18), Phys. Chem. Chem. Phys. 14 (2012) 14567–14572, https://doi.org/10.1039/ C2CP42763J. [48] F. Shahzad, K. Nadeem, J. Weber, H. Krenn, P. Knoll, Magnetic behavior of NiO nanoparticles determined by SQUID magnetometry, Mater. Res. Express 4 (2017) 086102, , https://doi.org/10.1088/2053-1591/aa8674. [49] P. Ravikumar, B. Kisan, A. Perumal, Enhanced room temperature ferromagnetism in antiferromagnetic NiO nanoparticles, AIP Adv. 5 (2015) 087116, , https://doi.org/ 10.1063/1.4928426. [50] S.A. Makhlouf, H. Al-Attar, R.H. Kodama, Particle size and temperature dependence of exchange bias in NiO nanoparticles, Solid State Commun. 145 (2008) 1–4, https://doi.org/10.1016/j.ssc.2007.10.019. [51] K.M. Krishnan, Fundamentals and Applications of Magnetic Materials, first ed., Oxford University Press, Oxford, United Kingdom, 2016. [52] S.-H. Wu, D.-H. Chen, Synthesis and characterization of nickel nanoparticles by hydrazine reduction in ethylene glycol, J. Colloid Interface Sci. 259 (2003) 282–286, https://doi.org/10.1016/S0021-9797(02)00135-2. [53] L. Del Bianco, F. Boscherini, A.L. Fiorini, M. Tamisari, F. Spizzo, M.V. Antisari, E. Psicopiello, Exchange bias and structural disorder in the nanogranular Ni∕NiO system produced by ball milling and hydrogen reduction, Phys. Rev. B 77 (2008) 094408, , https://doi.org/10.1103/PhysRevB.77.094408. [54] T. Bala, S.D. Bhame, P.A. Joy, B.L.V. Prasad, M. Sastry, A facile liquid foam based synthesis of nickel nanoparticles and their subsequent conversion to NicoreAgshell particles: structural characterization and investigation of magnetic properties, J. Mater. Chem. 14 (2004) 2941–2945, https://doi.org/10.1039/B405335B. [55] S.J. Park, S. Kim, S. Lee, Z.G. Khim, K. Char, T. Hyeon, Synthesis and magnetic studies of uniform iron nanorods and nanospheres, J. Am. Chem. Soc. 122 (35) (2000) 8581–8582, https://doi.org/10.1021/ja001628c. [56] A.G. Roca, P.K. Grady, C.J. Serna, Structural and magnetic properties of uniform magnetite nanoparticles prepared by high temperature decomposition of organic precursors, Nanotechnology 17 (2006) 2783–2788, https://doi.org/10.1088/09574484/17/11/010. [57] R. Yanes, O. Chubykalo-Fesenko, H. Kachkachi, D.A. Garanin, R. Evans, R.W. Chantrell, Effective anisotropies and energy barriers of magnetic nanoparticles with Néel surface anisotropy, Phys. Rev. B 76 (2007) 064416, , https://doi.org/10.

12

Journal of Magnetism and Magnetic Materials 494 (2020) 165784

A.M. Padhan, et al. 1103/PhysRevB.76.064416. [58] R. Yanes, O. Chubykalo-Fesenko, R.F.L. Evans, R.W. Chantrell, Temperature dependence of the effective anisotropies in magnetic nanoparticles with Néel surface anisotropy, J. Phys. D: Appl. Phys. 43 (2010) 474009, , https://doi.org/10.1088/ 0022-3727/43/47/474009. [59] H.T. Zhang, J. Ding, G. Chow, M. Ran, J. Yi, Engineering magnetic properties of Ni nanoparticles by non-magnetic cores, Chem. Mater. 21 (2009) 5222–5228, https:// doi.org/10.1021/cm902114d. [60] H. Kronmuller, M. Fahnle, Micromagnetism and the Microstructure of Ferromagnetic Solids, first ed., Cambridge University Press, Cambridge, United Kingdom, 2003. [61] A.P. Grosvenor, M.C. Biesinger, R.S. Smart, N.S. McIntyre, New interpretations of XPS spectra of nickel metal and oxides, Surf. Sci. 600 (2006) 1771–1779, https:// doi.org/10.1016/j.susc.2006.01.041. [62] B. Kisan, P. Saravanan, S. Layek, H.C. Verma, D. Hesp, V. Dhanak, S. Krishnamurthy, A. Perumal, J. Magn. Magn. Mater. 384 (2015) 296–301, https:// doi.org/10.1016/j.jmmm.2015.02.065. [63] N.H. Hong, J. Sakai, N. Poirot, V. Brize, Room temperature ferromagnetism

[64] [65] [66]

[67]

13

observed in undoped semiconducting and insulting oxide thin films, Phys. Rev. B 73 (2006) 132404–132408, https://doi.org/10.1103/PhysRevB.73.132404. S. Thota, J.H. Shim, M.S. Seehra, Size-dependent shifts of the Néel temperature and optical band-gap in NiO nanoparticles, J. Appl. Phys. 114 (2013) 214307, , https:// doi.org/10.1063/1.4838915. N. Rinaldi-Montes, P. Gorria, D. Martínez-Blanco, A.B. Fuertes, I. Puente-Orench, L. Olivi, J.A. Blanco, Size effects on the Néel temperature of antiferromagnetic NiO nanoparticles, AIP Adv. 6 (2016) 056104, , https://doi.org/10.1063/1.4943062. P. Gorria, D. Martínez-Blanco, M.J. Pérez, J.A. Blanco, A. Hernando, M.A. LagunaMarco, D. Haskel, N. Souza-Neto, R.I. Smith, W.G. Marshall, G. Garbarino, M. Mezouar, A. Fernández-Martínez, J. Chaboy, L.F. Barquín, J.A.R. Castrillón, M. Moldovan, J.I.G. Alonso, J. Zhang, A. Llobet, J.S. Jiang, Stress-induced large Curie temperature enhancement in Fe64Ni36 Invar alloy, Phys. Rev. B 80 (2009) 064421, , https://doi.org/10.1103/PhysRevB.80.064421. J.M. Leger, C. Loriers-Susse, B. Vodar, Pressure effect on the curie temperatures of transition metals and alloys, Phys. Rev. B 6 (1972) 4250, https://doi.org/10.1103/ PhysRevB.6.4250.