Ferromagnetism in Lightly Doped Pr1âxCaxMnO3 (x = 0.023, 0.036

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Ferromagnetism in Lightly Doped Pr1−xCaxMnO3 (x = 0.023, 0.036) Nanoparticles Synthesized by Microwave Irradiation Anustup Sadhu,† Thilo Kramer,‡ Abheek Datta,† Stefanie Anna Wiedigen,‡ Jonas Norpoth,‡ Christian Jooss,‡ and Sayan Bhattacharyya†,* †

Department of Chemical Sciences, Indian Institute of Science Education and Research, Kolkata, Mohanpur - 741252, Nadia, W.B., India ‡ Institute of Materials Physics, University of Göttingen, Friedrich-Hund-Platz 1, 37077 Goettingen, Germany ABSTRACT: Although the magnetic phase diagrams of bulk and thin film samples of Pr1−xCaxMnO3 (x ≤ 0.5) are widely explored, few works have been published on the magnetic properties of nanoparticles, especially in the lightly doped regime. In this paper, microwave irradiation was used to synthesize the Pr0.977Ca0.023MnO3 and Pr0.964Ca0.036MnO3 Manganite phases with Pnma space group in the form of anisotropic nanoparticles. The phase identification, structural characteristics, and formation of the nanostructures were analyzed by Rietveld analysis of the X-ray diffraction patterns, electron microscopy, and combining thermal analysis with infrared spectroscopy, respectively. Transport measurements on the annealed samples revealed the insulating nature, and electrical conduction occurs through thermally activated hopping of small polarons. The Mn−Oc−Mn tilt angles in the MnO6 octahedra show considerable flattening (160.9° and 167.6° for x = 0.023 and x = 0.036, respectively), possibly enhancing the electronic double exchange and promoting ferromagnetism. Ferromagnetic ordering of Mn spins was indeed observed below 109 K, and interestingly, the magnetic moment for x = 0.036 was 3.92 μB/f.u. at 5 K, which is higher than the saturated Mn magnetic moment (3.8 μB). The enhanced magnetization is attributed to ordering of the Pr spins. KEYWORDS: manganites, nanoparticles, microwave synthesis, X-ray Rietveld refinement, ferromagnetism number for reports on PCMO nanostructures are scarce.4,5a All these studies deal with calcium doping concentrations of 0.3 ≤ x ≤ 0.5, since in this range PCMO exhibits rich physics due to the interplay of different kinds of ordering (charge, orbital, lattice, and spin). Nevertheless, PCMO in the doping range x ≤ 0.25 is equally interesting since spontaneous magnetization is the highest especially for x = 0.2 in the Pr1−xCaxMnO3 (0 ≤ x ≤ 1) series and the low temperature magnetization is largely influenced by ordered praseodymium moments.7 The above fact motivated us to study the Ca doping concentrations at the minimum level close to the parent compound PrMnO3, that is, Pr0.977Ca0.023MnO3 (P1) and Pr0.964Ca0.036MnO3 (P2) phases, in the form of nanoparticles and to examine the magnetic and other physical properties when the size is reduced to few tens of nanometers. Most of the manganite nanostructures including PCMO were until now synthesized by the solution phase synthesis like the sol−gel method and hydrothermal routes followed by calcination to yield nanoparticles and nanowires.4b,c,6a,8 We have used microwave irradiation which yields better homogeneity and efficient doping in the final products within the shortest possible duration. The basic principle involves the conversion of microwave energy to heat, resulting in selective heating in different zones due to the different constituents,

1. INTRODUCTION Doped perovskite manganite compounds of the general formula A1−xA′xMnO3 (where A and A′ are trivalent rareand divalent alkaline-earth ions, respectively) have been of great interest for nearly two decades due to their technologically important electrical, magnetic, and magnetoresistance properties and their subtle relation to structure.1 The complexity of these systems due to the characteristic lattice distortions in the whole range of doping concentrations is directly responsible for their tunability, and the balance between different competing phases can produce significant changes in the physical properties. Mostly studied as single crystals2 and thin films,3 the hole-doped Pr1−xCaxMnO3 (PCMO) phases experience lattice distortion involving the Jahn−Teller effect of Mn3+ ions leading to distortions and puckering of the Mn3+O6 octahedra. The formula Pr3+1−xCa2+x[Mn3+1−xMn4+x]O2−3 is obtained when the concentration of Mn3+ is formally decreased by Ca2+ substitution. As for the nanoscale doped perovskite manganites, the major focus has been devoted to the weakening/suppression of the charge order phase and evolution of a size-induced ferromagnetism (FM) in place of the antiferromagnetic (AFM) phase, although the root cause for such a change is still under intense debate.4 Moreover, due to the high surface-to-volume ratio of the nanoparticles, a large percentage of uncompensated spins are present at the surface, which can result in a spin-glass-like behavior at lower temperatures.5 There are a good number of articles on nanocrystalline manganites in general;6 however, the © XXXX American Chemical Society

Received: July 26, 2012 Revised: September 10, 2012

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dx.doi.org/10.1021/cm3018924 | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

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giving rise to hot spots and heat transfer between the domains. Thermal equilibrium is quickly achieved at the cost of these short-lived nonisothermal microscopic hot spots, leading to high reaction rates. In fact, microwave reaction can be extended to create porous three-dimensional manganite systems, where the organic moieties in inorganic oxides act as templates and the porosity being lost only after calcination in air.9 The microwave synthesized products were calcined in air to obtain the well-defined crystallographic phase, as analyzed by Rietveld analysis of the X-ray diffraction (XRD) patterns and further annealed to remove defects. The formation of the nanostructures is discussed based on the thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR) data. Electrical transport properties were studied on the annealed samples and fitted with small polaron hopping conduction models. Magnetic measurements revealed low temperature ferromagnetism with high magnetic moments even with Ca doping levels below 4%, compositionally close to the antiferromagnetic PrMnO3.

2. RESULTS AND DISCUSSION 2.1. Stoichiometry and Formation of the Nanostructures. The inductively coupled plasma atomic emission spectroscopy (ICP-AES) experiments revealed the bulk composition of the final solid products with Ca:Pr atomic ratios of 0.023 and 0.036 for samples named P1 and P2, respectively. The initial synthesis step, that is, microwave irradiation, allows short reaction durations due to significant rate enhancements of the heterogeneous reaction mixture comprising Pr-, Ca-, and Mn-nitrates in a polar solvent (water). During the 15 min programmed microwave reaction, temperature of the reaction did not reach beyond 115 °C, since the solvent water boils at 100 °C. This resulted in amorphous products, which had to be calcined to give crystalline P1 and P2. In both the reactions to P1 and to P2, the hydrogen ion activity inside the reaction vessel was maintained at pH 12, which resulted in precipitation of Ca2+ ions close to the initial Ca:Pr molar ratios of 0.03 and 0.05 in the solution phase. Part of the calcium was washed away in the supernatant during the centrifugation step finally yielding the concentrations given above. Inside the microwave chamber, proper crystallization of the products was hindered due to the low reaction temperatures and the leftover presence of inorganic functional groups. This was confirmed from FTIR studies (Figure 1A) where the microwave product of P1 shows the presence of hydroxyl and nitro groups due to the formation of the M−OH bond (M = Pr, Ca, Mn)10 and leftover metal nitrate precursors,11 respectively. The Mn−O−Mn stretching band originated from the amorphous manganite phase.12 When the microwave product of P1 was calcined at 750 °C, the metal hydroxides were converted to manganite oxides; however, nitro groups were still present. The OH stretching band was from the absorbed moisture in the KBr pellets. P1 was further calcined up to 850 °C to crystallize PCMO with Pnma space group, where only the Mn−O stretching bands were present, neglecting the −OH bands from absorbed moisture. The reason why P2 needed a calcination temperature of 1150 °C, 300 °C higher than that needed for P1, was explained from the thermogravimetric (TGA) analyses (Figure 1B). The TGA analysis of the microwave product of P1 was carried out in air, whereas that of P2 was performed both in air and in nitrogen atmosphere. Among the common features in these three TGA

Figure 1. (A) FTIR spectra of P1 after different treatments: (a) microwave irradiation, calcination at (b) 750 °C and (c) 850 °C. The bands are (i) −OH stretching at 3372 cm−1 from metal hydroxides in the microwave product, (ii) −OH stretching at 3349−3390 cm−1 from absorbed water in the KBr pellets, (iii) −OH bending at 1634 cm−1, (iv) NO3− stretching at 1384−1400 cm−1, from leftover metal nitrates, (v) Mn−O−Mn stretching in MnO6 at 580−630 cm−1, (vi) Mn−O− Mn bending at < 500 cm−1. (B) TGA plots of the samples P1 in air and P2 in air and nitrogen atmosphere.

profiles are the weight loss at 200−208 °C, due to removal of absorbed water and weight loss up to around 500 °C, due to the decomposition of Mn−OH, Pr−OH (200−400 °C),13 and Ca−OH (400−500 °C), to the mixed oxide phases. An interesting feature is the weight loss of 0.72%, 1.45%, and 1.27% at 767, 797, and 760 °C for P1 (in air), P2 (in air), and P2 (in N2), respectively. The above trend is indicative of the loss of oxygen from the lattice of Pr1−xCaxMnO3−δ, δ being the extent of oxygen nonstoichiometry. At the above-mentioned temperatures, δ is 0.1, 0.2, and 0.18 for P1 (in air), P2 (in air), and P2 (in N2), respectively. Second, the weight loss is also indicative of the loss of nitro groups, since from FTIR studies, the 1400 cm−1 band was present in the 750 °C calcined product but absent when treated at 850 °C. When TGA of the microwave product of P2 was carried out in air, 0.76% weight increase was observed from 882 to 950 °C and such peculiar weight increase was absent when the TGA analysis was performed in nitrogen. This clearly points out the uptake of oxygen until ∼1090 °C. Such oxygen nonstoichiometry is common for the manganite systems.14 The instrument did not allow the measurement to continue beyond 1100 °C. Hence, P2 was calcined 60 °C higher (i.e., at 1150 °C) than the B



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nonstoichiometry point (1090 °C) to obtain the desired welldefined Pr0.964Ca0.036MnO3 phase. 2.2. X-ray Rietveld Analysis. Figure 2 shows the powder XRD patterns of P1 and P2 recorded with Cu Kα radiation in

unit cell into consideration. The determined parameters such as lattice parameters, interaxial angles, cell volume, atomic positions and occupation number of the lattice sites are given in Table 1. The Rietveld analyses show the formation of a single phase and absence of impurities. The best fits with lowest standard deviations were obtained with space group Pnma. The Rietveld analysis of the patterns shows that the unit cell is orthorhombic and yields lattice parameters of P1 as a = 5.4956(4) Å, b = 7.7170(1) Å, and c = 5.4665(4) Å and for P2 a = 5.4903(1) Å, b = 7.7286(1) Å, and c = 5.4681(1) Å. These parameters differ significantly from the bulk lattice parameters of PrMnO3, namely, a = 5.4562(7) Å, b = 7.672(1) Å, and c = 5.5914(7) Å (cell volume 234.06 Å3).14a On moving from P1 to P2, there is a slight decrease of the unit cell parameter along the a axis, whereas the b and c axes are found to increase. The calculated cell volumes of P1 and P2 are 231.83 Å3 and 232.02 Å3, respectively. As reported earlier, the cell volume decreases with the decrease in particle size from bulk to the nanoscale.4c,15 The Mn−O−Mn bonding distances and the tilt angles are also tabulated in Table 1. The regular MnO6 octahedra undergo distortion, as evident by the (Mn−O)c/ (Mn−O)ab ratios of 0.99 and 0.98 for P1 and P2, respectively. Interestingly, the Mn−O−Mn tilt angles for P1 and P2 are much higher as compared to the bulk PrMnO3 [Mn−Oc−Mn: 154° and Mn−Oab−Mn: 149°].7a This significant flattening of the Mn−O−Mn bond angles strongly supports the enhancement of the double exchange, evident from the ferromagnetic properties of P1 and P2 (Section 2.5). Furthermore, the orthorhombic strains (OS∥ and OS⊥) were calculated. OS∥ is the strain in the ac plane [OS∥ = 2 (c − a)/(c + a)] and OS⊥ [= 2 (a + c − b√2)/(a + c + b√2)] is the strain along the b-axis.16 In bulk Pr0.9Ca0.1MnO3,7a strains of OS∥ = 0.031 and OS⊥ = 0.024 have been reported. In P1 we found OS∥ = 0.0053 and OS⊥ = 0.0044, and in P2, OS∥ = 0.0041 and OS⊥ = 0.0026. The reduction in size has thus reduced the orthorhombic strains, as compared to bulk samples, again mirroring the observed magnetic moments in P1 and P2 (Section 2.5). 2.3. TEM Analysis. The transmission electron micrographs (TEM) in Figure 3 present a representative overview of the shape and size of the particles. The particles in the microwave products are in the range of 6−10 nm (not shown). Calcination results in removal of the hydroxyl and nitro groups present in the microwave products and aggregation of the smaller amorphous particles into larger polycrystalline PCMO

Figure 2. Rietveld analysis of the X-ray diffraction patterns of P1 and P2 according to the parameters listed in Table 1. The legends indicate dif f (blue line; difference plot between the observed and calculated observed patterns; Obs (plus symbols; observed pattern); Calc (black line; calculated pattern); and bckgr (gray line; background plot).

the 2θ scan range from 20 to 90° with step size of 0.02° and Rietveld refinement performed on these patterns taking one Table 1. XRD Rietveld Refinement Parameters of P1 and P2 sample [space group] P1 [Pnma]

P2 [Pnma]

lattice parameters (Å); angles (dego); cell volume (Å3)

atomic positions (x, y, z)

a = 5.4956(4) Å Pr (0.0331, 0.2500, −0.0094) b = 7.7170(1) Å Ca (0.0331, 0.2500, −0.0094) c = 5.4665(4) Å Mn (0.0000, 0.0000, 0.5000) α = β = γ = 90° O1 (0.4776, 0.2500, −0.0691) V = 231.83 Å3 O2 (0.2755, 0.0350, 0.7284) bond distances: (Mn−O)c = 1.965(1) Å; (Mn−O)ab = 1.969(3) Å bond angles: Mn−Oc−Mn = 160.9°; Mn−Oab−Mn = 156.7° a = 5.4903(1) Å Pr (0.0315, 0.2500, −0.0112) b = 7.7286(1) Å Ca (0.0315, 0.2500, −0.0112) c = 5.4681(1) Å Mn (0.0000, 0.0000, 0.5000) α = β = γ = 90° O1 (0.4862, 0.2500, 0.0365) V = 232.02 Å3 O2 (0.2875, 0.04500, 0.7514) bond distances: (Mn−O)c = 1.943(4) Å; (Mn−O)ab = 1.973(3) Å bond angles: Mn−Oc−Mn = 167.6°; Mn−Oab−Mn = 158.0° C

occupation number

weighted profile (Rwp)

Pr = 0.977 Ca = 0.023 Mn = 1.0 O1 = 1.0 O2 = 1.0

6.77%

Pr = 0.964 Ca = 0.036 Mn = 1.0 O1 = 1.0 O2 = 1.0

5.94%



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and the corresponding samples are named P1A and P2A in the following sections. Energy dispersive X-ray spectroscopy (EDS) coupled with TEM again reveals homogeneous phase formation without clustering (Figure 3e). The Ca:Pr atomic ratios are consistent throughout the samples and equal the numbers obtained by ICP-AES analysis within error limits. The Cu signal in both the spectra stems from the copper grid on which the sample was dispersed. The homogeneity of the individual elements throughout the nanoparticles was further ascertained from elemental mapping (Figure 3f). 2.4. Electrical Transport Studies. The introduction of a large concentration of defects and grain boundaries in the nanostructured materials is reflected in its electrical transport properties. A distinct difference is the few orders of magnitude higher resistivity compared to the bulk samples.4c Figure 4 shows the resistivity of the annealed samples as a function of T, measured under zero magnetic field with a constant applied

Figure 3. TEM images of the calcined products (a) P1 and (b) P2, showing ensembles of anisotropic nanoparticles; high resolution TEM images of (c) P1 and (d) two ellipsoidal particles of P2. (e) EDAX analysis of the products. (f) Elemental mapping of the selected image of P1.

products. The TEM image in Figure 3a shows an ensemble of anisotropic and irregularly shaped nanoparticles of P1 with a range of sizes of 30−80 nm. Figure 3b shows an average particle size of 35−55 nm for P2. In both samples, some particles consists of multiple grains which have sizes of about 30−50 nm for P1 and 15−25 nm for P2. The high resolution TEM images (Figure 3c,d) reveal interplanar spacing typical for low-indexed lattice planes. The particles are not separated and rather highly interlinked resulting in large interparticle interactions which govern their physical properties. Both samples were postannealed at 1150 °C for 2 h to reduce the defect concentration,

Figure 4. Temperature dependences of the resistivity of the annealed samples P1A and P2A, without any applied magnetic field. The arrows indicate the corresponding axes of the plotted data. Analysis of the data in the temperature interval 90 K ≤ T ≤ 300 K, measured at a constant voltage of U = 1 V. The ln ρ (T) over T−1 plot is compared with the models of thermally activated hopping (TAP) of small polarons and variable range hopping (VRH). D



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Figure 5. Plots of magnetization as a function of magnetic field for P1, P2, and P2A at (a) 300 K and (b) 5 K. (c) Variation of magnetization with temperature increase at applied fields of 0.1 and 1.0 T. (d) Enlarged view of the low temperature region of the M−T curves of P2 and P2A; the dotted lines are guides to the eye to show the presence of an additional moment below 50 K. (Inset) The temperature dependence of magnetization dM/dT curves at 0.1 T for P1, P2, and P2A, where two ferromagnetic transitions are clearly observed for P2 and P2A.

paramagnetic (linear) behavior (Figure 5a). Figure 5b depicts the M−H curves of P1, P2, and P2A samples at 5 K, as obtained by sweeping the magnetic field from 0 → 4 T → 0 → −4 T → 0 → 4 T. All the curves exhibit a ferromagnetic (FM) hysteresis behavior with unsaturation even at field amplitudes of 4 T, which might be a signature of disorder in the long-range ferromagnetic alignment of the spins. Coercivity and magnetic moments read 37.7, 14.8, and 14.4 mT and 1.95, 3.92, and 3.83 μB/f.u. for P1, P2, and P2A, respectively, whereas the saturated Mn magnetic moment is 3.8 μB. The temperature dependence of the magnetization at two different magnetic fields 0.1 and 1.0 T is shown in Figure 5c. No irreversibility was observed between the zero-field-cooled (ZFC) and field-cooled (FC) magnetization plots (not shown). The temperature dependence of the differential magnetization dM/dT for P1 and P2 at 0.1 T is given in the inset of Figure 5d. The respective minima indicate the paramagnetic to FM transition temperature Tc, namely, 49 K, 109 K, and 119 K for P1, P2, and P2A, respectively. A second magnetic transition was observed below 50 K for P2 and P2A at 42.4 and 45.1 K, respectively (Figure 5d). We attribute this additional low temperature contribution to ordering of the Pr spins, since the ordered Pr moment (0.50 ± 0.03 μB) can only be envisaged at very low temperatures. The low temperature features in Figure 5d are likely due to exchange coupling between the Pr 4f electrons localized below the ordering temperature of manganese and the neighboring

voltage of 1 V. Data was analyzed in terms of small-polaron hopping conductivity models, namely, thermally activated hopping (TAP) and variable range hopping (VRH):17 TAP:

ρ(T ) = ρ0 + T exp(EA /kT )

(1)

VRH:

ρ(T ) = ρ∞ exp(T0/T )0.25

(2)

Both models are only valid above half the Debye temperature of PCMO, and the fit range was restricted to 160 K ≤ T ≤ 300 K. For both samples P1A and P2A, the fitting yields reasonable agreement with the experimental data, thus confirming the supposed conduction mechanism. In contrast to our thin film data on Pr0.7Ca0.3MnO3,17c where the TAP model gives much better fitting results, both TAP and VRH exhibit similar fit quality, which is an expression of a higher degree of disorder in the defect prone nanomaterials. The obtained fit parameters are activation energy (EA) = 170 meV (TAP), ρ∞ = (2.0 ± 0.1) × 10−15 mΩ·m (VRH), and T0 = (5.0 ± 0.02) × 108 K (VRH), respectively, moderately enhanced compared to typical thin film values for x = 0.3.17d For the unannealed samples P1 and P2, the resistance could not be measured since the data exceeded the limit of the instrument. 2.5. Magnetic Properties. Detailed magnetic measurements have been performed on the powders tightly packed inside Teflon tape. The magnetization (M) as a function of magnetic field (H) ranging from −4 to +4 T at 300 K shows a E



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itinerant Mn 3d spins.7b Moreover, superparamagnetic fractions due to the existence of smaller particles are likely to be present at low temperatures, and these spins are unblocked at even very high fields (4 T). The unsaturation in the M−H curves (Figure 5b) is a probable signature to this effect, and the transition temperatures observed below 50 K (Figure 5d inset) may have partial contribution from the transition between the superparamagnetic and the blocked state of the nanoparticles. The observation of low temperature FM in our lightly doped nanoparticles is in contrast to the antiferromagnetic (AFM) phase commonly reported in bulk or thin film samples.4b Similar trends of a size-induced FM phase dominating the AFM component have been published for the half-doped compound.4b,c Actually, in the lightly doped manganites, the FM insulator phase coexists with the charge ordered AFM phase, before the onset of the FM ordering.7b PrMnO3 is AFM with a magnetic moment 3.35 μB, lower than the spin-only value for Mn3+, whereas in the range 0.1 ≤ x < 0.2, the magnitude of the moments at the He temperature was reported to be 3.35−3.75 μB, a combination of ferromagnetic ordering along the [110] direction and the AFM component.7a We did not find any magnetic data on the doping concentrations below x = 0.05, and herein we show that PCMO is largely FM with suppressed AFM component in the case of nanoparticles. However, the magnetic moment of P1 (x = 0.023) was observed to be 1.95 μB/f.u., where the reduction of the moment is attributed to a distinct phase separation between AFM and FM fractions. This could be expected since pristine PrMnO3 is purely AFM; therefore, lowering the Ca content from P2 to P1 should increase the AFM tendencies. This is also reflected in the determined Mn−O−Mn bonding angles (Table 1), which are considerably flatter for P2 compared to P1, thus indicating a differing impact of the magnetic double exchange.

were performed in a Carbolite wire-wound tube furnacesingle zone, model MTF 12/38/400. The ICP-AES measurements were carried out in a Perkin-Elmer Optima 2100 DV instrument. The XRD spectra were recorded with a Rigaku SmartLab powder diffractometer having Cu Kα = 1.54059 Å radiation. Rietveld analysis of the XRD patterns was performed by the General Structure Analysis System (GSAS) software, Los Alamos National Laboratory Report (2004). High resolution transmission electron microscopy (HRTEM) images were obtained by employing a Phillips CM30 instrument with 300 kV accelerating voltage. Energy dispersive X-ray spectroscopy (EDS) was performed with the Oxford Instruments X-Max Si-drift detector with 130-eV resolution coupled to TEM Phillips CM12. The dispersion of the samples in dry ethanol was spread onto carbon coated Cu grids and dried overnight. The EDS spectra were recorded at five to six different locations to ascertain the homogeneity of Ca incorporation in the nanoparticles. The thermogravimetric analysis (TGA) data were collected on a Mettler Toledo STARe, under N2 and air atmosphere at 10 °C/min heating or cooling rates. The Fourier transform infrared (FTIR) measurements were carried out with a Perkin-Elmer spectrum RX1 with KBr pellets. Each pellet contained 3 mg of the sample and 200 mg of KBr (FTIR grade). The temperature profiles of electrical resistance of the sample were measured with a Keithley electrometer. Magnetic properties were studied using the MPMS-XL Evercool Quantum Design SQUID magnetometer, in the temperature range of 5−300 K and applied fields of 0−4 T. The temperature-dependent susceptibility was measured using a DC procedure. The sample was cooled to 5 K under zero magnetic field. Low magnetic fields (0.1 and 1.0 T) were applied, and data were collected from 5 to 300 K. 4.2. Synthesis of the Products. Microwave Irradiation. For the synthesis of the precursors to samples P1 and P2, microwave irradiation was performed. Required stoichiometric amounts of Pr(NO3)3·6H2O, Ca(NO3)2·xH2O, and Mn(NO3)2·xH2O were mixed according to the Ca:Pr molar ratios of 0.03 and 0.05 (in the solution phase) and dissolved in 150 mL of double-distilled water followed by vigorous stirring for 1 h. The pH of the reaction mixture was increased to 12 by dropwise addition of NH4OH solution. The stirred solutions turned gray and turbid, and stirring was continued for another 1 h at room temperature inside the reaction vessel fitted to a reflux condenser within the microwave oven chamber. The microwaveassisted reactions were performed under stirring for 15 min with the total power fixed at 900 W. The microwave heating under air atmosphere was effectively pulsed by an automated on/off program so as to reduce the risk of superheating the solvent and prevent the violent “bump” boiling of the solvent. After cooling the reaction vessel, the microwave products were centrifuged once at 7500 rpm with the mother liquid to separate the solid mass from the liquid and then washed a few times with water and ethanol at 25 °C and 7500 rpm. The products were dried overnight in the oven at 80 °C. Calcination. The microwave irradiated products were ground well and heat treated at 850 °C for 3 h and 1150 °C for 10 h in air to obtain P1 and P2 samples, respectively. Annealing. The P1 and P2 samples were further annealed in air at 1150 °C for 2 h to get the annealed samples P1A and P2A.

3. CONCLUSIONS The present work can be summarized point-wise as follows: (i) Pr1−xCaxMnO3 (x = 0.023, 0.036) anisotropic nanoparticles with Pnma space group were synthesized by microwave irradiation followed by calcination. (ii) Even at doping levels of x < 0.04, the nanoparticles are FM at low temperatures, in contrast to the AFM nature of the close parent PrMnO3 and apart from the conventional knowledge that ferromagnetism sets in only at x > 0.1. (iii) The enhanced magnetization below 50 K can be attributed to ordering of the Pr 4f spins localized in a narrow region below the ordering temperature of the Mn spins. (iv) Electrical transport properties on the annealed samples revealed the insulating nature of the nanostructures with thermally activated hopping of small polarons, and the results provide a signature of the presence of disorder in the system. (v) Reduction in the particle size allows flattening of the Mn−O−Mn tilt angles and decrease of the orthorhombic strains, which in turn via double exchange enhances the ferromagnetic contribution.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

4. EXPERIMENTAL SECTION

Notes

4.1. Materials and Characterization. All reagents were of the highest purity. Praseodymium(III) nitrate hexahydrate (Pr(NO3)3.6H2O; Aldrich 99.9%), calcium(II) nitrate hydrate (Ca(NO3)2·xH2O; Aldrich 99.997%), and manganese(II) nitrate hydrate (Mn(NO3)2·xH2O; Aldrich 99.99%) were used without further purification. The microwave-assisted reactions were carried out in a Sineo MASII-1000W commercial microwave oven with a wellequipped refluxing system. The calcination and annealing experiments

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

A.S. and A.D. thank University Grants Commission (UGC), New Delhi, for their fellowship. Research funding from IISER Kolkata is duly acknowledged. F



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REFERENCES

(1) (a) Coey, J. M. D.; Viret, M.; Molnár, S. v. Adv. Phys. 2009, 58, 571−697. (b) Salamon, M. B.; Jaime, M. Rev. Mod. Phys. 2001, 73, 583−628. (2) (a) Jooss, C.; Wu, L.; Beetz, T.; Klie, R. F.; Beleggia, M.; Schofield, M. A.; Schramm, S.; Hoffmann, J.; Zhu, Y. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 13597−13602. (b) Chi, S.; Ye, F.; Dai, P.; Fernandez-Baca, J. A.; Huang, Q.; Lynn, J. W.; Plummer, E. W.; Mathieu, R.; Kaneko, Y.; Tokura, Y. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 10796−10801. (3) (a) Chen, Q. P.; Yang, J. J.; Zhao, Y. G.; Zhang, S.; Wang, J. W.; Zhu, M. H.; Yu, Y.; Yang, B.; Xie, D.; Ren, T. L. Appl. Phys. Lett. 2011, 98, 172507. (b) Ghosh, B.; Das, K.; Raychaudhuri, A. K. J. Appl. Phys. 2011, 109, 083934. (4) (a) Chai, P.; Wang, X.; Hu, S.; Liu, X.; Liu, Y.; Lv, M.; Li, G.; Meng, J. J. Phys. Chem. C 2009, 113, 15817−15823. (b) Rao, S. S.; Bhat, S. V. J. Phys.: Condens. Matter 2010, 22, 116004. (c) Sarkar, T.; Mukhopadhyay, P. K.; Raychaudhuri, A. K.; Banerjee, S. J. Appl. Phys. 2007, 101, 124307. (d) Rao, S. S.; Anuradha, K. N.; Sarangi, S.; Bhat, S. V. Appl. Phys. Lett. 2005, 87, 182503. (e) Jirák, Z.; Hadová, E.; Kaman, O.; Knížek, K.; Maryško, M.; Pollert, E.; Dlouhá, M.; Vratislav, S. Phys. Rev. B 2010, 81, 024403. (5) (a) Jammalamadaka, S. N.; Rao, S. S.; Vanacken, J.; Stesmans, A.; Bhat, S. V.; Moshchalkov, V. V. AIP Adv. 2011, 1, 042151. (b) Gajbhiye, N. S.; Bhattacharyya, S. Mater. Chem. Phys. 2008, 108, 201−207. (6) (a) Epherre, R.; Pepin, C.; Penin, N.; Duguet, E.; Mornet, S.; Pollert, E.; Goglio, G. J. Mater. Chem. 2011, 21, 14990−14998. (b) Phan, M. H.; Chandra, S.; Bingham, N. S.; Srikanth, H.; Zhang, C. L.; Cheong, S. W.; Hoang, T. D.; Chinh, H. D. Appl. Phys. Lett. 2010, 97, 242506. (7) (a) Jirák, Z.; Krupička, S.; Šimša, Z.; Dlouhá, M.; Vratislav, S. J. Magn. Magn. Mater. 1985, 53, 153−166. (b) Dho, J.; Chi, E. O.; Hur, N. H.; Lee, K. W.; Oh, H. S.; Choi, Y. N. Solid State Commun. 2002, 123, 441−444. (8) Wang, Y.; Fan, H. J. Appl. Phys. Lett. 2011, 98, 142502. (9) (a) Bhattacharyya, S.; Gedanken, A. J. Phys. Chem. C 2008, 112, 659−665. (b) Bhattacharyya, S.; Gedanken, A. J. Phys. Chem. C 2008, 112, 13156−13162. (10) Chen, H.; He, J. J. Phys. Chem. C 2008, 112, 17540−17545. (11) Hussein, G. A. M.; Balboul, B. A. A.; Warith, M. A. A.; Othman, A. G. M. Thermochim. Acta 2001, 369, 59−66. (12) Liang, S.; Teng, F.; Bulgan, G.; Zong, R.; Zhu, Y. J. Phys. Chem. C 2008, 112, 5307−5315. (13) (a) Keshavaraja, A.; Jacob, N. E.; Ramaswamy, A. V. Thermochim. Acta 1995, 254, 267−275. (b) Treu, B. L.; Fahrenholtz, W. G.; Okeefe, M. J. Inorg. Mater. 2011, 47, 974−978. (14) (a) Cherepanov, V. A.; Barkhatova, L. Y.; Petrov, A. N.; Voronin, V. I. J. Solid State Chem. 1995, 118, 53−61. (b) Atsumi, T.; Ohgushi, T.; Namikata, H.; Kamegashira, N. J. Alloys Compd. 1997, 252, 67−70. (c) Zuev, A. Y.; Tsvetkov, D. S. Solid State Ionics 2010, 181, 557−563. (15) Zhang, T.; Dressel, M. Phys. Rev. B 2009, 80, 014435. (16) Sarkar, T.; Ghosh, B.; Raychaudhuri, A. K.; Chatterji, T. Phys. Rev. B 2008, 77, 235112. (17) (a) Holstein, T. Ann. Phys. 1959, 8, 343. (b) Prohorov, V. G.; Kaminsky, G. G.; Flis, V. S. Low Temp. Phys. 1999, 25, 792. (c) Westhauser, W.; Schramm, S.; Hoffmann, J.; Jooss, C. Eur. Phys. J. B 2006, 53, 323−331. (d) Schramm, S.; Hoffmann, J.; Jooss, C. J. Phys.: Condens. Matter 2008, 20, 395231.

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