Coexistence of High Magnetization and Anisotropy with Non

Article pubs.acs.org/JPCC

Coexistence of High Magnetization and Anisotropy with Nonmonotonic Particle Size Effect in Ferromagnetic PrMnO3 Nanoparticles Anustup Sadhu,† Hemant G. Salunke,‡ and Sayan Bhattacharyya*,† †

Department of Chemical Sciences and Centre for Advanced Functional Materials, Indian Institute of Science Education and Research (IISER) Kolkata Mohanpur 741246, India ‡ Technical Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India S Supporting Information *

ABSTRACT: Instances of the coexistence of high ferromagnetic magnetization with large anisotropy are scarce in the rare-earth manganite family. In manganites, high magnetizations are compromised with small coercivity and vice versa. Using nonaqueous sol−gel techniques, the undoped PrMnO3 nanoparticles with oxygen nonstoichiometry were rendered with exceptional ferromagnetic character. While ∼40 nm sized nanoparticles had magnetization of 84 emu/g and coercivity of 885 Oe with 50 kOe sweeping field, the bulk 2 μm sized particles showed a magnetization of 51 emu/g albeit with a higher coercivity of 2000 Oe. These parameters are so far the highest among manganite systems with similarly sized particles. The competition between the ferromagnetic and antiferromagnetic phases both at the particle core and at the grain boundaries resulted in a non-monotonous trend of magnetic properties between 20, 40, and 2 μm particles. The sudden increase of coercivity toward lower temperatures was a result of the freezing of random spins at the surface of the strongly interacting nanoparticles which also increased the magnetic anisotropy. These results are of prime significance since the coexistence of such a large magnetization with high coercivity was rarely observed in pristine or doped manganites.

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FM interface but reduce the magnetization considerably.12,13 Manganites with La3+ as the A-site cation has the highest magnetization and lowest anisotropy. From La3+ to Lu3+, anisotropy increases due to the lower cationic size, which decreases the bandwidth and tolerence factor leading to enhancement of AFM character. Furthermore, AFM contribution rapidly increases with more charge ordering present in the system, leading to higher anisotropy.14,15 The ideal source of an induced anisotropic SG phase at the NP surface is the interconnection of NPs through their grain boundaries.16 In fact, strong interparticle interactions help to overcome the anisotropic barrier of superparamagnetic “small” single domain NPs and align the randomized spins of an ensemble of NPs. The interconnected NPs with high crystallinity and large grain boundary fractions can be stabilized by a nonaqueous sol−gel synthesis method. This method among several other synthesis methodologies has been shown to be beneficial for obtaining temperature-dependent magnetic phase transitions, high FM moments, and large magnetoresistance.6,17 The nonaqueous solvent limits the NP size by C−O bond breaking and making of new M−O bonds.18 The

INTRODUCTION Manganite nanoparticles (NPs) are the subject of intense research owing to their unique magnetic properties which differ significantly from their bulk counterparts.1−4 The lesser studied antiferromagnetic (AFM) PrMnO3 may be converted to its ferromagnetic (FM) phase at reduced dimensions especially due to oxygen excess in the lattice.5,6 Upon exposure to air, oxygen diffuses through the NP surface incorporating additional oxide ions, which in turn oxidize a few Mn3+ ions into Mn4+ to create the Mn3+−O−Mn4+ units. The FM character is a result of double exchange between different valence states of Mn through the intermediate O2−. In a majority of manganite systems, the FM moments are, however, accommodated at the expense of reduced magnetic anisotropy, which is reflected in their low coercivity and remnant magnetization.5,7,8 The randomized surface atoms of NPs partly overcome this disadvantage since the surface dangling bonds behave as hard centers which are the originating sites for high anisotropic features like spin-glass-like ordering, exchange bias coupling, etc.8,9 In fact, the presence of a low-temperature spin glass (SG) phase in manganites is deprived of any significance because SG either improves the coercivity marginally or decreases the saturation magnetization considerably.10,11 On the other hand, AFM contributions in FM manganites help in increasing the anisotropy and coercivity by exchange coupling at the AFM/ © 2017 American Chemical Society

Received: July 20, 2017 Revised: August 30, 2017 Published: August 30, 2017 21029

DOI: 10.1021/acs.jpcc.7b07145 J. Phys. Chem. C 2017, 121, 21029−21036

Article

The Journal of Physical Chemistry C

crystal structures were obtained using VESTA 3 software. The magnetic measurements were performed using the Quantum Design PPMS system with VSM magnetometer in the temperature range of 5−300 K and applied fields of 0−50 kOe. The temperature-dependent zero-field-cooled (ZFC) magnetization was measured by cooling the samples to 5 K under zero magnetic field, and after 100 Oe field was applied, the data were collected from 5 to 300 K. The field-cooled (FC) measurements were performed by cooling the samples in the presence of the 100 Oe applied field. The real part of the ac magnetic susceptibility (χ′) measurements was performed over the temperature range 5−300 K at 1 Oe applied dc magnetic fields over the ac magnetic frequency range from 53 to ∼3 kHz.

absence of water molecules prevents hydrolysis, which further retards particle agglomeration in solution. During gelation, the high-density solvents like ethylene glycol reduce the proximity of these NPs, preventing them from agglomeration. The organic residues over the NP surface in turn help to induce high connectivity and thicker grain boundaries during calcination of the as-prepared NPs.17 At high calcination temperatures, the removal of organic moieties attached to the NP surface leads to increased interparticle connectivity and reduces the grain boundary thickness. Because of the intervening grain boundaries, the small size of the NPs can be stabilized under optimized conditions. Herein, three different particle sizes of PrMnO3 powder were obtained by nonaqueous sol−gel synthesis followed by calcination. This method led us to achieve “small” NPs with high crystallinity and large interconnections between the particles, which gives enhanced FM ordering with high anisotropy without charge-ordering transition. The nonmonotonous alteration of magnetic properties with increasing particle size is beyond the conventional understanding in manganite systems. The uniqueness of this work lies in the improved magnetic properties of PrMnO3 NPs as compared to previous reports with similar compositions.

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RESULTS AND DISCUSSION Morphological Characterization. Parts a−c of Figure 1 show the FESEM images of P1, P2 NPs and P3 bulk particles,

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EXPERIMENTAL SECTION Materials. All reagents were of analytical grade purity. Praseodymium(III) acetate hydrate (Pr(CH3COO)3·xH2O; Alfa Aesar 99.9%), manganese(II) acetate tetrahydrate (Mn(CH3COO)2·4H2O; Merck ≥99.5%), and ethylene glycol (OHCH2CH2OH; Merck ≥99%) were used without further purification. Synthesis. In order to maintain the stoichiometry, the hydrated acetate salts were converted to anhydrous acetates by removing the water of crystallization as follows: Pr(CH 3 COO) 3 ·xH 2 O was heated at 210 °C, and Mn(CH3COO)2·4H2O was heated at 170 °C for 30 min at 5 °C/min. This is followed by the addition of 130 mL of ethylene glycol to 0.55 g of anhydrous Pr(CH3COO)3 under sonication at 70 °C for 1 h. When a clear transparent solution was obtained, 0.3 g of anhydrous Mn(CH3COO)2 was added. The solution was vigorously stirred at 80 °C for 1 h, followed by refluxing at 120 °C for 12 h. The deep brown solution was transferred into three beakers and heated at 110 °C until all of the solvents evaporated. At this step, the sols turned into gels and finally into brown powders. The brown powders were calcined at 700, 900, and 1200 °C for 4 h each at 6 °C/min to obtain PrMnO3.18 NPs (P1), PrMnO3.17 NPs (P2), and bulk PrMnO3.15 (P3), respectively. Methods. Field emission scanning electron microscope (FESEM) images and energy-dispersive analysis of X-ray (EDAX) were recorded in Carl Zeiss SUPRA 55VP FESEM. The transmission electron microscope (TEM) images were obtained by UHR-FEG-TEM, JEOL, JEM 2100 F model using a 200 kV electron source. The final stoichiometry of the products was obtained by inductively coupled plasma−mass spectrometry (ICP−MS) analyses performed in a Thermo Scientific X-series with Plasma lab software. The XRD measurements were performed with a Rigaku (mini flex II, Japan) powder X-ray diffractometer having Cu Kα = 1.54059 Å radiation with a rate of 1°/min. Rietveld analysis of the diffraction patterns was performed with the General Structure Analysis System (GSAS) software, Los Alamos National Laboratory Report (2004). The three-dimensional views of

Figure 1. FESEM images of (a) P1 NPs, (b) P2 NPs and (c) P3 particles. TEM images of (d) P1 NPs, (e) P2 NPs and (f) P3 particles with high-resolution view of the selected area (open rectangle) and SAED patterns.

respectively. The images depict high interconnectivity between the individual NPs of P1 and P2. The inside-out growth and stacking of the NPs to create the bulk P3 is evident from Figure 1c. The sizes of the particles are determined from the TEM images and are found to be ∼20 nm, ∼40 nm, and ∼2 μm for P1, P2, and P3, respectively (Figure 1d−f). It is noteworthy that despite calcination temperatures as high as 900 °C for P2, the particle size could be constrained to only 40 nm, suggesting the advantage of nonaqueous sol−gel route. However, the NPs are nonspherical and irregularly shaped. The HRTEM and selected area electron diffraction (SAED) patterns in the inset of Figure 1d−f depict the crystalline nature of the particles from nano to bulk. The interplanar distances of (121) reflection in P1, P2, and P3 vary as 2.74, 2.74, and 2.73 Å, respectively. In all three samples, the Pr/Mn stoichiometric ratio is found to be 1:1 by EDAX analysis (Figure S1), further ascertained by ICPMS measurements. The consistent homogeneity of the 21030


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Figure 2. Rietveld analysis of the XRD patterns and corresponding unit cells based on the obtained parameters. Legends: diff (blue line; difference plot between the observed and calculated patterns), Obs (plus symbols; observed pattern), Calc (red line; calculated pattern), and bckgr (green line; background plot).

(reduced χ2) and weighted profile. With an increase in particle size from P1 to P2, the cell volume and lattice parameters a and c increase, whereas b remains almost constant. The structural changes from 40 nm P2 to 2 μm P3 is quite drastic, whereby cell volume and lattice parameter a increase and lattice parameters b and c decrease. The ⟨Mn−O−Mn⟩ bond angle first increases from 160° (P1) to 164.6° (P2) and then decreases to 158.7° for P3. The increased flattening of ⟨Mn− O−Mn⟩ bond angle corresponds to a favorable doubleexchange interaction and in this case P2 is supposed to be have more FM character than P1 and P3.2,3,17,20 It is to be noted that the increase in particle size has a greater influence on Mn−Ob−Mn bond than Mn−Oac−Mn with lesser variation for the latter. The orthorhombic strain (OS) increases slightly with particle size from P1 (OS∥ = 0.0082, OS⊥ = 0.0054) to P2 (OS∥ = 0.0112, OS⊥ = 0.0074) but increases rapidly to P3 (OS∥ = 0.026, OS⊥ = 0.021). The lower OS of the NPs is a clear indication that NPs are supposed to have higher magnetization as compared to bulk PrMnO3.17,21 Magnetic Properties. The temperature-dependent magnetization curves at 100 Oe of P1, P2, and P3 (Figure 3) show the paramagnetic to FM transition below ∼100 K. The point of rise of ZFC curves is denoted as the Curie temperature (Tc) and is found to be 100, 115, and 105 K for P1, P2, and P3,

individual A-site and B-site cations is also confirmed by elemental mapping (Figure S2). Structural Characterization. The oxygen nonstoichiometry in PrMnO3+δ is estimated by gravimetric analyses under reduced atmosphere (see discussion S1 in the Supporting Information), and δ is found to be 0.18 ± 0.01, 0.17 ± 0.01, and 0.15 ± 0.01 for P1, P2, and P3, respectively. The calculated oxygen nonstoichiometry also helps to elucidate the oxidation states of the Mn ion in each sample. In P1, P2, and P3, the Mn4+ fractions are 44, 42, and 38%, respectively. Therefore, the Mn3+/Mn4+ ratio does not alter drastically with increasing particle size from ∼20 nm to ∼2 μm; rather, this ratio is consistent with the FM region of Ca-doped PrMnO3.4 Hence, the variation of magnetic properties with increasing particle size is more due to the influence of finite size effects, surface, and grain boundary features than solely due to the alteration of electronic environment at the particle core.3,19 The XRD Rietveld refinement patterns and corresponding crystal structures of P1, P2, and P3 at room temperature are displayed in Figure 2. The samples crystallize in an orthorhombic structure with a Pnma space group. The refinement parameters at 300 K are tabulated in Table 1. The structural parameters obtained from the Rietveld analysis of the XRD patterns have minimum error with decent goodness of fit 21031


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The Journal of Physical Chemistry C Table 1. XRD-Rietveld Refinement Parameters of P1, P2, and P3 sample [space group] P1 [Pnma]

P2 [Pnma]

P3 [Pnma]

lattice parameters (Å); angles (deg); Cell volume (Å3)

occupation no.

atomic positions (x, y, z)

a = 5.5032(3) Å Pr (0.0346, 0.2500, −0.0057) Pr = 1.0 b = 7.7091(2) Å Mn (0.0000, 0.0000, 0.5000) Mn = 1.0 c = 5.4585(2) Å O1 (0.4806, 0.2500, −0.0471) O1 = 1.0 α = β = γ = 90° O2 (0.2977, 0.0390, 0.7291) O2 = 1.0 V = 231.575 Å3 bond distances: (Mn−O)b = 1.9472 (2) Å; (Mn−O)ac = 1.9785(3) Å bond angles: Mn−Ob−Mn = 163.5°; Mn−Oac−Mn = 156.5° a = 5.5226(1) Å Pr (0.03311, 0.2500, −0.0093) Pr = 1.0 b = 7.7092(4) Å Mn (0.0000, 0.0000, 0.5000) Mn = 1.0 c = 5.4613(2) Å O1 (0.4818, 0.2500, 0.031) O1 = 1.0 α = β = γ = 90° O2 (0.2755, 0.03541, 0.7284) O2 = 1.0 V = 232.514 Å3 bond distances: (Mn−O)b = 1.9373(4) Å; (Mn−O)ac = 1.9688(4) Å bond angles: Mn−Ob-Mn = 168.3°; Mn−Oac−Mn = 160.9° a = 5.6030(2) Å Pr (0.0421, 0.2500, 0.0064) Pr = 1.0 b = 7.6555(2) Å Mn (0.0000, 0.0000, 0.5000) Mn = 1.0 c = 5.4584(3) Å O1 (0.5089, 0.2500, −0.074) O1 = 1.0 α = β = γ = 90° O2 = 1.0 V = 234.13 Å3 O2 (0.3037, 0.0245, 0.7484) bond distances: (Mn−O)b = 1.9570 (4) Å; (Mn−O)ac = 1.9802(3) Å bond angles: Mn−Ob−Mn = 161.7°; Mn−Oac−Mn = 155.8°

weighted profile (Rwp) (%)

goodness of fit (reduced χ2)

2.83

1.246

3.35

1.281

3.96

1.082

state to the FM blocked state.22 TB generally increases with increasing particle size according to the equation

TB =

KV 25k

(1)

where K = anisotropy constant, V = volume of the particle, and k = Boltzmann constant. However, from Figure 3, it is evident that TB increases drastically from 33 K for P1 to 78 K for P2. Apart from particle size, the anisotropy constant K arising from interparticle interactions dominates the numerator of eq 1 and determines the TB of PrMnO3 samples. The ZFC-FC bifurcation temperature (Tirr) is 82, 105, and 95 K for P1, P2, and P3, respectively. While Tirr is closer to Tc for all samples, Tirr and TB are closer to each other for P2, while they are far apart in P1. In P3, the ZFC peak at 73 K relates to the emergence of FM and AFM domains and a phase competition between these two magnetic phases. Below Tirr, the difference in magnetization between the ZFC and FC loops is a measure of interparticle interactions and the extent of magnetic phase competition leading to FM moments.23−25 The largest ZFC-FC divergence in P2 is a signature of its highest FM character

Figure 3. Plots of magnetization (M) as a function of temperature (T) at an applied field of 100 Oe.

respectively. Tc is a measure of particle size and stoichiometrydependent FM interactions and is the highest for ∼40 nm P2, with a Pr/Mn ratio similar to that of P1 and P3. The maximum in the ZFC plot gives the blocking temperature (TB), which denotes the temperature of transition from superparamagnetic

Figure 4. Plots of (a) magnetization (M) as a function of magnetic field (H) at 5 K and (b) enlarged view of the M−H loop that shows the coercive fields and remnant magnetization. 21032


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Figure 5. (a−c) Plots of magnetization (M) as a function of magnetic field (H) at different temperatures: 5, 25, 50, 75, 100, and 150 K. (d−f) Enlarged view of M−H loops at 5, 25, 50, and 75 K. Plots of coercivity (Hc) and remnant magnetization (Mr) versus temperature (T) of (g) P1, (h) P2, and (i) P3.

system, the above coercive fields are the highest for any manganite nanoparticulate system with comparable size, whereas that of P3 is the highest comparable to any other polycrystalline bulk manganite system. In addition, the remnant magnetization of P2 is the highest for any manganite system to date. Another aspect to note is the behavior of electron spins, which is found to be quite different on either side of 25 K. The full M−H loops from 150 to 5 K are shown in Figure 5a−c. The straight line loops at 150 K indicate paramagnetic behavior. Below 150 K, the curves start showing a curvature which increases with lowering the temperature of measurement. From the enlarged view of the M−H loops near the origin (Figure 5d−f) it is evident that the curves start to broaden abruptly below 25 K for the NPs in P1 and P2. The result of this broadening is the sudden rise of coercivity and remnant magnetization below 25 K (Figure 5g−i). Such a change in magnetic properties is reminiscent of unusual increase of magnetic anisotropy below 25 K. The increased anisotropy is also evident from a consequential bifurcation between ZFC and FC curves of P1 NPs, starting from below 50 K, even at a high applied field of 10 kOe (Figure S3). For bulk P3 sample on the other hand, the pattern of rise of coercivity and remnant magnetization from 75 to 5 K is quite uniform. This brings us to the conjecture of the significance of frozen disordered surface spins of NPs in increasing the magnetic anisotropy

followed by P3, the least being P1. The thicker grain boundaries in P1 hinder interparticle spin polarization leading to its lowest magnetization. The plots of magnetization (M) as a function of sweeping magnetic field (H) are shown in Figure 4a. The presence of AFM fractions within a dominant FM spin ordering at 5 K leads to unsaturation of the M−H loops even with 50 kOe sweeping field. The magnetizations at 50 kOe are 55, 84, and 51 emu/g for P1, P2, and P3, respectively. It is noteworthy to mention here that the magnetization of P2 is the highest for any Pr−Mn−O system at least with a similar particle size.2,5,6,10,21,26−30 Furthermore, the non-monotonic change in magnetization with particle size is untraditional in manganite systems. Whereas in La-based pristine or doped manganites it is well-known that magnetization increases with increase in particle size,17,31,32 Pr-based systems behave differently. In a few reports, due to the increasing AFM character of Pr1−xDxMnO3 (D = dopant), the magnetization decreases with increasing particle size,15,26 whereas in a few other cases, the opposite trend was observed.5 On increasing the calcination temperature from 700 to 900 °C the crystallinity of the NPs improves, which is directly reflected in the jump in magnetization from 55 to 84 emu/g. In turn, at 5 K the coercivity values are 1073, 885, and 2000 Oe and remnant magnetizations are 9, 20.8, and 12.6 emu/g for P1, P2, and P3, respectively (Figure 4b). To highlight the magnetic properties of our 21033


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Figure 6. Plots of real part of AC susceptibility (χ′) as a function of temperature measured at 1 Oe and different ac frequencies for (a) P1, (b) P2, and (c) P3. (d) Fit to Volger−Fulcher law according to eq 3 for P1 and P2 (inset).

Table 2. Fitting Parameters of AC Susceptibility Plots According to Volger−Fulcher Law sample P1 P2

Ea/k (K) 117 172

Tf (K) 34.4 74.1

T0 (K) 29.4 68.3

(Tf − T0)/Tf 0.145 0.078

τ0 (s)

χ2 −12

1.14 × 10 2.14 × 10−15

R2 −8

4.9 ×10 3.2 × 10−8

0.999 0.999

while in P3 the shift is negligible. These features may be attributed either to the SG transition or the interacting NPs with superparamagnetic behavior. A frequency (ν) dependent empirical parameter γ is considered where37

below 25 K as compared to the bulk phase where the contribution of surface spin disorder is expected to be minimal. The smaller the NPs, the higher the surface contribution with dangling bonds and disordered spins. In ∼20 nm NPs of P1, the rise of coercivity is steeper (Figure 5g) in comparison to ∼40 nm NPs of P2 (Figure 5h). These disordered spins contribute to the grain boundaries of interconnecting NPs. While crystallinity improves the parallel spin alignment giving rise to better FM character, the frozen disordered spins result in spin-glass-like ordering. The SG phase becomes prominent on lowering the temperature from 25 to 5 K in P1 and P2. The high coercivity of P3 can be attributed to the strong AFM coupling which are common in bulk manganites.33 The AFM domains present within the FM matrix can couple with the FM spins in the presence of applied magnetic field.34−36 Unlike in earlier reports where SG affects the magnetization,11 in our PrMnO3 system the high crystallinity of the NPs helps in maintaining high magnetizations. The SG induced disorder is likely confined at the NP surface where the differently aligned spins exchange couple with the FM spins and increase the magnetic anisotropy. Dynamic Properties. The low-temperature magnetic-phase transitions were investigated by AC susceptibility measurements. The real (χ′) part of AC susceptibility is plotted in Figure 6a−c. At the lowest frequency of 53 Hz, a distinct peak (Tf) is observed at 34.4, 74.1, and 74.2 K for P1, P2, and P3, respectively. With the increase of frequency from 53 to 2993 Hz, the peaks shift toward higher temperature. The observed shift (ΔTf) is by 2.6 to 37 K for P1 and by 1.5 to 75.6 K for P2,

γ = ΔTf /(Tf Δlog v)

(2)

It is well-known that in conventional SG systems γ is in the range 0.005−0.015,37,38 whereas for coupled granules, γ is 0.05−0.13.37,38 Estimated for Δν between 53 and 593 Hz from eq 2, in the case of P1, γ is 0.016, whereas for P2, γ is 0.0064, both accommodated within the range of conventional SG systems. The slightly higher γ in P1 might be due to the interconnected “smaller” NPs and presence of frozen disordered spins at the NP surface in the proximity of thicker grain boundaries of P1 than in P2. To gain further insight into the time scale of spin reversal, data fitting according to the Volger−Fulcher law was applied, according to eq 339−41 τ = τ0 exp[Ea /k(Tf − T0)]

(3)

where τ = 1/f, f = frequency of measurement, τ0 = 1/f 0, f 0 = a characteristic frequency, Ea = activation energy, and T0 = a fitting parameter which formally represents a transition temperature to an equilibrium state and is related to an interaction energy of the system. The fitting curves with low χ2 values are presented in Figure 6d, and the fitted parameters are tabulated in Table 2. The resultant τ0 of the order of 10−12− 10−15 is in good agreement with the time scale of spin reversal 21034


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The Journal of Physical Chemistry C in the SG regime. In addition, (Tf − T0)/Tf are 0.145 and 0.078 for P1 and P2, respectively, which remain within 0.07−0.15, the range of SG systems.37,38 Overall, the SG ordering in P2 fits conventionally more than P1, which has a significant contribution from the disordered surface spins. Both P1 and P2 are composed of interconnected crystalline granular NPs separated by grain boundaries, as discussed previously. In P1, the grain boundaries are expected to be thicker due to the lower calcination temperature. The situation is somewhat different in P2, and the thinner grain boundaries in the latter allows reversal of the randomly oriented spins within the grain boundaries with a time scale similar to the conventional SG. Since freezing of the disordered surface spins due to dangling bonds preserve higher magnetic anisotropy, the contribution from surface SG phase is more than that of the randomized spins at the NP core.11,17,42 The small hump in the FC curve at and below 25 K exclusively for P1 (Figure 3) attests to this fact. Both temperature-dependent M−H loops (Figure 5) and AC susceptibility measurements (Figure 6) confirm that 25 K is the onset of SG ordering in P1, whereas 75 K for P2, though it effectively emerges below 25 K. With minimal surface spin disorder, the SG ordering is absent in P3, evident from a nearly uniform alteration in coercivity and remanent magnetization (Figure 5f,i) and negligible peak shift in AC susceptibility plots.

Notes

The authors declare no competing financial interest.

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REFERENCES

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CONCLUSIONS PrMnO3 NPs with oxygen nonstoichiometry were synthesized by a nonaqueous sol−gel technique followed by calcination at 700, 900, and 1200 °C. The low-temperature magnetic properties show a non-monotonic trend with increasing particle size. The 700 °C calcined ∼20 nm NPs possess a magnetization 55 emu/g at 5 K. The 900 °C calcined ∼40 nm NPs demonstrate a remarkably high magnetization of 84 emu/g. The 1200 °C calcination results in bulk ∼2 μm particles with magnetization 51 emu/g, which is unusual considering that bulk PrMnO3 particles show AFM character. This nontraditional behavior can be related to the nonaqueous sol−gel synthesis method which can retain grain boundaries between interconnected particles even when calcined at higher temperatures. The competition between AFM and FM domains results in high exchange anisotropy (while maintaining large magnetization), which in turn enhances the coercive field, the highest being observed is 2000 Oe for bulk PrMnO3. The magnetic characteristics below 25 K are due to the freezing of disordered spins majorly at the NP surface, giving rise to a glassy type localized spin ordering. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b07145. EDAX, elemental mapping, oxygen nonstoichiometry calculations, and ZFC−FC at 10 kOe applied field for P1 (PDF)

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ACKNOWLEDGMENTS

The financial support from Department of Science and Technology − Science and Engineering research Board (DST-SERB) under sanction No. EMR/2016/001703 is duly acknowledged. AS thanks University Grants Commission (UGC) New Delhi for his fellowship.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +91-33-6634-0000 ext 1275. Fax: +91-33-25873020. ORCID

Sayan Bhattacharyya: 0000-0001-8074-965X 21035


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