Stacked Nanosheets of Pr1âxCaxMnO3 (x = 0.3 and 0.49): A

Article pubs.acs.org/JPCC

Stacked Nanosheets of Pr1−xCaxMnO3 (x = 0.3 and 0.49): A Ferromagnetic Two-Dimensional Material with Spontaneous Exchange Bias Anustup Sadhu and Sayan Bhattacharyya* Department of Chemical Sciences, Indian Institute of Science Education and Research, Kolkata, Mohanpur - 741252, Nadia, W.B., India S Supporting Information *

ABSTRACT: Two-dimensional (2D) planar architectures of inorganic materials have attracted great attention owing to their fascinating physical properties. However, nanosheets of the technologically relevant doped rare-earth manganites are still elusive. Stacked 10−14 nm thick nanosheets of phase pure Pr1−xCaxMnO3 (PCMO; x = 0.3 and 0.49) were obtained by decomposition of carbon coated CaCO3/MnCO3 microsheets and Pr2O2CO3 aggregates, the latter synthesized under pressure at 500−800 °C. The PCMO nanosheets had flatter Mn−O−Mn tilt angles in the MnO6 octahedra and ferromagnetic (FM) moments at 5 K. Spontaneous exchange bias (SEB) coupling between the antiferromagnetic (AF)/FM spins was observed at 5 K under zero-field cooling. High FM moments and SEB were possible due to the long-range magnetic interactions in the stacked 2D arrangement of Mn3+/Mn4+ d-electron spins, where charge ordering was completely suppressed. The SEB behavior was dependent on the initial magnetization process and the direction of rotation of the random spins at the AF/FM interface.

■

INTRODUCTION The perovskite family of metal oxides is particularly important because of their wide range of properties such as colossal magnetoresistance,1 superconductivity,2 ferroelectricity,3 and applications in sensors,4 fuel cells,5 and memory devices.6 Among them, the doped rare-earth manganites of the general formula A1−xA′xMnO3 (where A and A′ are trivalent rare- and divalent alkaline-earth ions, respectively) are particularly in heavy demand for the electronics and energy storage devices because of their tunable electrical, magnetic, and magnetoresistance properties.7−9 Significant progress has been achieved to develop novel structural morphologies of these materials.10−12 In Pr1−xCaxMnO3, the most studied Ca-doping level is 0.3 ≤ x ≤ 0.5 where the electronic and magnetic properties are influenced by the interplay of charge, orbital, lattice, and spin ordering. In PCMO, the divalent alkaline-earth Ca2+ ions replace the trivalent rare-earth Pr3+ ions at the lattice sites and the concentration of Mn3+ decreases at the cost of Mn4+ ions to maintain charge balance. The Jahn−Teller distortion at the Mn3+ sites with the 2-fold degenerate eg level leads to distortions due to puckering of the MnO6 octahedra and cubic to orthorhombic transition.13 Partial localization of the eg electrons leads to the charge order phase, whereas delocalization leads to the FM-metallic phase by the double-exchange mechanism. In our last report on lightly doped PCMO nanoparticles, we have shown that the flattening of the Mn− O−Mn bond angles and reduced orthorhombic strain promoted improved FM moments at 5 K.14 In most of these materials, the orientation of the spins changes from being random at room temperature to either AF © 2013 American Chemical Society

or FM ordering at low temperatures. Because of the presence of inhomogeneous magnetic phases at the same temperature, the possibility of the coexistence of AF and FM phases increases, which results in exchange anisotropy interactions at the AF/FM interfaces within the same material.15 When the size of the particles is reduced to the nanometer scale, the basic magnetic properties such as saturation magnetization, coercive field, remanence, and nature of magnetic ordering become size dependent. In nanoparticles, the high concentration of defects can melt the AF charge order phase.16 For weakly interacting nanoparticles, superparamagnetism leads to unstable magnetic order, which can be overcome by exchange coupling of AF and FM spins. When the nanoparticles of the manganites interact strongly with each other, they demonstrate slow dynamics, aging, and memory effects similar to a spin glass system.17 In fact, for practical applications, it is desirable that significant exchange bias coupling occurs at zero or minimum applied cooling fields. Nanowires or nanorods with 1D morphology lead to shape anisotropy, useful to increase the overall magnetic anisotropy of the system.16 The 2D morphology of nanosheets or thin films offers 1D quantum confinement and enhanced anisotropy along the micrometer-length dimensions, which distinctly separates their physical properties from the conventional nanoparticles and bulk material.18−20 Nanosheets have the advantage of direct implementation as potential building blocks for next-generation nanodevices because of their Received: September 27, 2013 Revised: November 22, 2013 Published: November 22, 2013 26351

dx.doi.org/10.1021/jp4096375 | J. Phys. Chem. C 2013, 117, 26351−26360

The Journal of Physical Chemistry C

Article

Figure 1. (a) TEM image of PNS1. i−iv represent four stacked nanosheets. (inset) SAED pattern. (b) High-resolution TEM image of two nanosheets oriented in different directions. (c) FESEM image of PNS1. Arrows indicate the thickness of the nanosheet. (d) EDAX pattern of PNS1. (e) Homogeneity profile of Ca2+ doping on 10 nanosheets. (f) FESEM, (g) TEM (inset: FFT), and (h) EDAX pattern of PNS2.

temperature and autogenic pressure. The morphology of the nanostructures can be manipulated in the absence of solvent by controlling the built-in autogenic pressure, altering the amount of precursor solid inside the autoclave, heating rate, temperature, and time of reaction in this high yield, bottom-up process. The nanosheets showed superior ferromagnetic moments at 5 K and spontaneous exchange bias under zerofield cooling.

extended 2D network with rich electronic and magnetic properties.21 Although the 2D morphologies of other materials are known,22,23 so far there has been no report on the nanosheet morphology of rare-earth manganites. Perovskite nanosheets are only known to be those obtained by exfoliation of layered perovskite metal oxides with the help of organic bases at room temperature.24−27 However, nanosheets of the non-layered rare-earth manganites were not reported to date. The doped manganites exist in various nanostructures such as nanoparticles/nanowires/nanorods,28−32 although the reports on Ca-doped PrMnO3 materials are few.16,33−35 The doped rare-earth manganite nanostructures were so far synthesized by the sol−gel method to give nanoparticles,31,34,35 and nanowires,16,30 the hydrothermal technique,33 the molten salt route,29 the electrospinning method,32 ball milling,36 microwave irradiation,14 solid state calcination of oxides,37 and pulsed laser deposition.28 Herein we employed a “beakerless” pressure synthesis route for obtaining the phase-pure Pr0.7Ca0.3MnO3 (PNS1) and Pr0.51Ca0.49MnO3 (PNS2) nanosheets. It is well-known that metal carbonates decompose to oxides and CO2, when heated in air. We demonstrate here that metal oxide nanosheets can be obtained if the precursor metal carbonate was synthesized with a planar morphology. The metal carbonate sheets were synthesized by dry autoclaving which was also used for large scale synthesis of carbon coated nanocrystals,38−41 and carbon nano- and microstructures.42,43 The reactions occur inside a closed autoclave cell at high

■

EXPERIMENTAL METHODS Materials. All reagents were of the analytical grade purity. Praseodymium(III) acetate hydrate (Pr(OOCCH3)3·xH2O; Alfa Aesar 99.9%), calcium(II) acetate hydrate (Ca(OOCCH3)2·xH2O; Alfa Aesar 99.9965%), and manganese(II) acetate tetrahydrate (Mn(OOCCH3)2·4H2O; Merck ≥99.5%) were used without further purification. Ham-Let Union (SS316) 3/8″ stainless steel autoclaves were rinsed with ethanol and dried in air before the synthesis process. The reactions were performed in a Carbolite wire-wound tube furnace - single zone, model MTF 12/38/400. Synthesis of Carbon Coated Microsheets. Pr-, Ca-, and Mn-acetates were weighed and mixed in mole ratios of 0.7:0.3:1 and 0.5:0.5:1 according to the formula Pr1−xCaxMnO3 (x = 0.3 and 0.5), respectively. The acetate mixture was introduced inside the 2 mL stainless steel autoclave and capped tightly at both ends. The filled autoclave was heated at 6 °C min−1 and maintained at 500−900 °C for 6 h, followed by slow cooling to 26352



Article

room temperature. This resulted in the carbon coated metal carbonate microsheets. Synthesis of PCMO Nanosheets. The carbon coated sheets were ground in a mortar and heat treated on alumina boats in air. The heating rate was maintained again at 6 °C min−1 and the samples were calcined at 1000 °C for 24 h, followed by normal cooling to room temperature. The yield of the metal oxide nanosheets was 70−80 wt %. Characterization. The ICP-MS measurements were carried out in a Thermo Scientific X-series with Plasma lab software. A 4 mg portion of the sample was dissolved in 10 mL of 65% suprapure nitric acid (Merck KGaA) at 120 °C for 2 h. The solution was digested at 70 °C overnight and diluted to 100 ppb concentration for the ICP-MS analyses. The X-ray diffraction (XRD) measurements were carried out with a Rigaku (mini flex II, Japan) powder X-ray 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). Field emission scanning electron microscope (FESEM) images were recorded in Carl Zeiss SUPRA 55VP FESEM. Energy dispersive analysis of X-ray (EDAX) studies were performed with the Oxford Instruments X-Max with INCA software coupled to the FESEM. Transmission electron microscopy (TEM) images were obtained by employing a JEOL-JEM 2010 electron microscope with a 200 kV accelerating voltage. The thermogravimetric analysis (TGA) data were collected on a Mettler Toledo STARe, under air atmosphere at 10 °C/min heating and 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). A LABRAM HR800 Raman spectrometer was employed using the 633 nm line of a He−Ne ion laser as the excitation source to analyze the nanomaterials. Magnetic properties were studied using the MPMS-XL Evercool Quantum Design SQUID magnetometer, in the temperature range 5−300 K and applied fields of 0−4000 mT. The temperature-dependent zero-field cooled (ZFC) magnetization was measured using a DC procedure. The samples were cooled to 5 K under zero magnetic field. A 10 mT field was applied, and data were collected from 5 to 300 K. The field cooled (FC) measurements were done by cooling the samples in the presence of 10 mT applied field, and the data were recorded while warming up the samples in the presence of the field. Thermo-remnant magnetization (TRM) was measured by the following protocol: a 0.01 T field was applied at 300 K, and the sample was field cooled to 5 K. At 5 K, after a wait time of 1800 s (30 min), the magnetic field was switched off. The magnetization (M) was measured as a function of time (t) at each 30 s interval from 30 to 11 500 s.

The lattice spacing was measured to be 0.272 nm, which corresponds to the (112) reflection (Figure 1b). The field emission scanning electron microscope (FESEM) image in Figure 1c reveals the thickness of the PNS1 nanosheets to be 10−14 nm. The nanosheet surface spans over 500−600 nm, and on average, 10−12 nanosheets remain stacked together. Energy dispersive X-ray analysis (EDAX) (Figure 1d,e) on 10 different nanosheets provided the homogeneity profile of the samples. The nanosheet morphology (Figure 1f), lattice fringes with FFT (Figure 1g), and EDAX pattern (Figure 1h) of PNS2 were found similar to those for PNS1. The EDAX results matched close to that of ICP-MS as Ca2+/Pr3+ atomic ratio to be 30.0 ± 0.5 and 49.0 ± 0.5 atom % for PNS1 and PNS2, respectively. Figure 2 shows the XRD-Rietveld refinement patterns of PNS1 and PNS2 taking one orthorhombic unit cell into

RESULTS AND DISCUSSION The nanosheets of PNS1 and PNS2 crystallize in the orthorhombic phase with the Pnma space group. The inductively coupled plasma mass spectroscopy (ICP-MS) experiments revealed the bulk composition of the final solid products with Ca2+/Pr3+ atomic ratios of 0.3 and 0.49 for PNS1 and PNS2 samples, respectively. The transmission electron microscope (TEM) image in Figure 1a shows four stacked sheets (i−iv) of the representative PNS1 sample. The selected area electron diffraction (SAED) pattern (Figure 1a, inset) shows the characteristic reflections of the orthorhombic phase.

consideration. The lattice parameters along the a, b, and c axes decreased on moving from PNS1 to PNS2 (Table 1). In PNS1 and PNS2, the MnO6 octahedra were distorted, since the (Mn−O)c/(Mn−O)ab ratios were 1.019 and 1.006, respectively (Figure 2a, inset). The considerable flattening of the Mn−O− Mn bond angles as compared to the AF PrMnO3 [Mn−Oc− Mn, 154°; Mn−Oab−Mn, 149°]14 provided the opportunity of double exchange of Mn3+/Mn4+ spins leading to ferromagnetism. The orthorhombic strains in the ac plane [OS|| = 2(a − c)/(a + c)] and along the b-axis [OS⊥ = 2(a + c − b√2)/(a + c +

Figure 2. XRD-Rietveld analysis patterns of (a) PNS1 and (b) PNS2. The legends: dif f (difference plot between observed and calculated patterns; Obs (observed pattern); Calc (calculated pattern); and Bckgr (background plot). (Inset of a) Schematic showing Mn−O−Mn bond lengths and angles.

■

26353



Article

Table 1. XRD-Rietveld refinement parameters sample [space group] PNS1 [Pnma]

PNS2 [Pnma]

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

atomic positions (x, y, z)

a = 5.4542(1) Å Pr (0.0325, 0.2500, 0.008) b = 7.6763(1) Å Ca (0.0325, 0.2500, 0.008) c = 5.4334(1) Å Mn (0.0000, 0.0000, 0.5000) α = β = γ = 90° O1 (0.4762, 0.2500, 0.065) V = 227.49 Å3 O2 (0.2775, 0.0380, 0.7424) bond distances: (Mn−O)c = 1.952(4) Å; (Mn−O)ab = 1.956(4) Å bond angles: Mn−Oc−Mn = 160.8°; Mn−Oab−Mn = 157.8° a = 5.4302(3) Å Pr (0.0355, 0.2500, 0.002) b = 7.6273(1) Å Ca (0.0355, 0.2500, 0.002) c = 5.3866(3) Å Mn (0.0000, 0.0000, 0.5000) α = β = γ = 90° O1 (0.4854, 0.2500, 0.075) V = 223.10 Å3 O2 (0.2862, 0.0350, 0.7324) bond distances: (Mn−O)c = 1.941(1) Å; (Mn−O)ab = 1.943(2) Å bond angles: Mn−Oc−Mn = 159.5°; Mn−Oab−Mn = 158.6°

occupation number

weighted profile (Rwp)

Pr = 0.70 Ca = 0.30 Mn = 1.0 O1 = 1.0 O2 = 1.0

4.72%

Pr = 0.51 Ca = 0.49 Mn = 1.0 O1 = 1.0 O2 = 1.0

4.88%

b√2)] were calculated to be 0.0038 and 0.0015 for PNS1 and 0.008 and 0.0014 for PNS2, respectively.14 The reduced strains were due to the nm thickness of the sheets. The synthesis route to the nanosheets was explained on the basis of the control experiments and the characterization techniques such as FESEM, EDAX, FTIR, and TGA. Initially, when the metal acetates were autoclaved, the majority of microsheets were obtained under autogenic pressure from 500 to 800 °C for 6 h and a mixture of particles and sheets at 900 °C. The sheets were coated by carbon, and hence, they could be separated from each other. The reaction conditions were optimized at 700 °C under autogenic pressure to obtain the precursors of PNS1 and PNS2, the former considered as the representative system. The autoclaved 2−3 μm thick sheets consisted of smaller platelets (Figure 3a) and were covered by ∼200 nm thick carbon film (Figure 3b,c). The carbon coating was

predominantly graphitic in nature (Supporting Information, Figure S1). The autoclaved products were inhomogeneous wherein the sheets consisted of morphologies with lower Pr3+/ Ca2+ ratio, as compared with the brighter aggregates under the in-lens electron beam of FESEM (Figure 3d). In the absence of enough oxygen inside the closed autoclave cell, the orthorhombic manganite phases were not obtained. The autoclaved products of both PNS1 and PNS2 consisted of a majority of metal carbonates (CaCO3 and MnCO3), oxycarbonate (Pr2O2CO3), and a lesser fraction of oxides (Pr6O11, MnO, Mn2O3), as shown in the XRD pattern (Figure 4). The presence of metal carbonates was cross-checked by infrared (IR) spectroscopy (Figure S2, Supporting Information). The IR bands at 1473 and 855 cm−1, 1095 and 717 cm−1, and 614 and 504 cm−1 correspond to the C−O asymmetric stretching, OCO symmetric stretching of CO32− ions, and Mn−O/Pr−O

Figure 3. (a) FESEM image of autoclaved sheets, precursor to representative PNS1. (b) A detached carbon layer; arrows indicate the thickness of the layer. (c) Elemental line scan showing the presence of a partial carbon coating on an autoclaved sheet of PNS1. (d) Elemental line scan showing Pr-rich aggregate (1) and Pr-deficient sheet (2). (e) FESEM image of the autoclaved product of PNS2.

Figure 4. XRD patterns of the autoclaved product of (a) PNS1 and (b) PNS2. 26354



Article

Figure 5. FESEM images, EDAX spectra, and element % of (a) sheets and (b) aggregates in the autoclaved product of PNS1. Pr content was significantly lower on the sheets compared to the aggregates. (c) FESEM image and EDAX spectrum of the sheets with a Pr/Mn atomic ratio of 3.2%.

Figure 6. (a) Simplified schematics depicting the formation mechanism of PNS1 and PNS2 nanosheets. (b) Plots of Ca/Mn ratio versus accelerating voltage at different locations of the microsheets in the autoclaved products of PNS1 and PNS2.

S3, Supporting Information). At ≤700 °C, Pr6O11, MnO, and Mn2O3 could form except CaO. This was unlike the 900 °C autoclave reaction where all the metal oxide phases were predominant (Figure S4, Supporting Information). In the inhomogeneous autoclaved products, the Pr/Mn ratio of the sheets and the aggregates was 2−6 and >400 atom %, respectively (Figure 5a,b). In a control experiment, when the initial Pr/Mn stoichiometry was maintained at 0.03, only sheets were obtained without any aggregate (Figure 5c), which implied that sheets could only contain lesser Pr3+ ions.

vibrations, respectively.44 When the autoclaved carbon-coated composite products were mechanically ground and heated in air at 1000 °C for 24 h, the orthorhombic phases of PNS1 and PNS2 were formed. During autoclaving, the metal acetate hydrates transformed to anhydrous acetates which further decomposed to Pr2O2CO3, CaCO3, and MnCO3.45 All the greenhouse gases were trapped inside the autoclave and kinetically converted to elemental carbon.39 Pr-, Ca-, and Mn-acetates dehydrated at 100−200 °C, and the metal carbonates formed around 260−490 °C (Figure 26355



Article

Reactions with Pr- and Mn-acetates (1:1) in turn gave a mixture of flower-like aggregates and sheets at 600 °C, but at 800 °C, solely aggregates were found (Figure S5, Supporting Information). When Pr-, Ca-, and Mn-acetates were separately autoclaved, self-assembled sheets in a flower-like shape, stacked sheets, and microcubes of the metal carbonates resulted, respectively (Figure S6, Supporting Information). At 700 °C, Ca- and Mn-acetates resulted in stoichiometric microcubes (Figure S7, Supporting Information). All these control experiments point to the fact that CaCO3 platelets were embedded inside the carbon coated 2−3 μm thick MnCO3 sheets with 2−6% Pr2O2CO3 doped inside (Figure 6a). The Ca/Mn ratio was studied at different heights along the z-axis of the sheets by altering the accelerating voltage from 7.5 to 20 kV at a particular location in FESEM (Figure 6b). For example, 7.5 kV probed only a few atomic layers below the surface, whereas 20 kV probed the entire sheet. CaCO3 and MnCO3 can readily form solid solutions,46 but in this case, the Ca2+ concentration was observed to vary along the z-axis of the autoclaved sheets. Thus, it was inferred that the CaCO3 platelets were probably intercalated inside the distorted microcubes of MnCO3, providing a microsheet morphology. The Pr-oxycarbonate could not “solubilize” inside the mixed carbonate sheets and thus segregated as aggregates. The autoclaved sheets were metastable and broke into smaller platelets at temperatures above 700 °C (Figure S4, Supporting Information). When the autoclaved products were mechanically ground, the carbon coating ruptured and the sheets exfoliated laterally to mix with Pr2O2CO3 aggregates (Figure 6a). At 1000 °C, PNS1 and PNS2 phases were formed by a typical solid state reaction of the metal carbonates/oxycarbonates and oxides. In fact, the successful conversion of autoclaved sheets of metal carbonates to different metal oxide nanosheets confirmed our approach to be a generalized synthesis method (Figure 7). If the carbonate microsheets were decomposed at lower temperatures and shorter duration, the nanosheets consist of 100−200 nm long platelets. The platelets join to form single crystalline nanosheets with minimum defect concentration, if calcined at and above 1000 °C for longer durations (Figure 1c,f). The single crystalline nature of the nanosheets reduces the dangling

bonds at the surface and facilitates the long-range interaction of electron spins. The samples showed a paramagnetic-like linear M−H behavior at 300 K (Figure 8a),16 since the thermal energy could overcome the magnetic anisotropy barrier to randomly flip the magnetic moments. Size reduction in one of the dimensions in the nanosheets induced the FM phase at low temperatures dominating the AF component (Figure 8b), as reported in other PCMO nanomaterials.16,34,47 The magnetic moments were 2.6 and 1.2 μB/f.u., and coercive fields (Hc) were 42.6 and 19.9 mT for PNS1 and PNS2 at 5 K, respectively. These are the highest magnetic moments among the reported nanostructures of 30 and 50% Ca-doped PrMnO316,33,34 but are less than the Pr0.7Ca0.3MnO3/SrRuO3 superlattices.48 The enhanced magnetization was primarily due to the magnetic anisotropy along the x- and y-axes of the stacked nanosheets. The moments were however lower as compared to the saturated Mn magnetic moment of 3.8 μB, which indicates the presence of AF spins. The AF component created disorder in the long-range FM alignment of the spins, also evident from the unsaturated hysteresis behavior in Figure 8b. In the zero-field-cooled (ZFC) and field-cooled (FC) magnetization plots at 10 mT applied field (Figure 8c), the reported AF charge ordering peak at ∼250 K16,33 was completely suppressed. Although ZFC/FC irreversibility was not observed in PNS2, the curves bifurcate at 110 K for PNS1. In PNS2, magnetic transition was observed at 120 K. Additional transitions were observed at 10 and 40 K (PNS1-ZFC), 25 K (PNS1-FC), and 30 K (PNS2-ZFC/FC). The most fascinating observation was the hysteresis loop shift of 22.7 and 6.2 mT for PNS1 and PNS2, respectively, in the negative field direction at 5 K even in the absence of applied cooling field (Figure 8d), a behavior termed as spontaneous exchange bias (SEB). Exchange bias (EB) is usually observed in materials with AF/FM interfaces after field cooling from above the Néel temperature of the AF phase. Under ZFC, SEB was recently explained in bulk ternary metal alloys of Ni−Mn−In, Ni−Mn−Sn, and Mn2PtGa,49−51 LaFeO3 nanoparticles,52 BiFeO3−Bi2Fe4O9 nanocomposites,53 YMnO3 nanoparticles,54 and sandwiched La 0 . 6 7 Sr 0 . 3 3 MnO 3 /PbZr 0 . 8 Ti 0 . 2 O 3 / La0.67Sr0.33MnO3 structure.55 With the bulk and pristine nanoparticles of doped rare-earth manganites, EB was so far observed only under FC,15,47,56−58 and to our knowledge, this is the first report of SEB with doped rare-earth manganites at the nanoscale. The 2D morphology of PCMO nanosheets facilitated the long-range AF/FM interactions to demonstrate SEB, the magnitude of which is comparable to those reported under FC.47 It was demonstrated earlier that the initial magnetization process determines the nature of hysteresis loop shift (Figure 8d).49 In PNS1, when the maximum external magnetic field was 1000 mT, SEB was absent and the hysteresis loop was symmetric with Hc = 60 mT. On sweeping the magnetic field in the reverse direction, i.e., 0 → −3000 mT → 0 → +3000 mT → 0 → −3000 mT, Hc was 89 mT and the hysteresis loop shifted in the positive direction by 2 mT. When cooling fields were applied, any additional loop shift was not observed. These observations indicate that SEB was intrinsic to the PCMO nanosheets and related to the AF/FM interfaces formed during initial magnetization of the Mn3+/Mn4+ d-orbital spins. The asymmetry related to the SEB behavior and thus the unidirectional anisotropy field was dependent on the initial direction of the external applied field. The ≤40 K transitions in

Figure 7. FESEM images of (a) Pr6O11, (b) CaO, (c) PrMnO3, (d) CaMnO3, (e) La2O3, and (f) CeO2 nanosheets synthesized by air heating the respective carbonate microsheets at 900 °C for 4 h, respectively. 26356



Article

Figure 8. Plots of magnetization (M) as a function of magnetic field (H) for PNS1 and PNS2 at (a) 300 K and (b) 5 K. (c) Variation of magnetization with temperature at an applied field of 10 mT. Open symbols, ZFC; closed symbols, FC; stars, PNS1; circles, PNS2. (d) Enlarged view of the M−H loops showing SEB. (i) PNS1 at 5 K, (ii) PNS2 at 5 K, PNS1 at 5 K (iii) after field cooling with 1000 mT, (iv) with a maximum applied field of 1000 mT, and (v) reverse field sweep.

± 0.0007 emu/g, a = 0.0092 ± 0.0007 emu/g, τ = 14 258 ± 2693 s, b = 0.0034 ± 0.0001 emu/g, and χ2 = 7.06 × 10−8. The Debye exponential function was due to the single barrier activation mechanism, and the logarithmic term denotes a mechanism involving distribution of energy barriers to the rotation of the Mn3+/Mn4+ spins. The fitting to the equation involving the double exponential relaxation mechanism combining an initial fast relaxation related to spin glass and a slow exponential decay related to a random ferromagnet62

the ZFC/FC plots (Figure 8c) can be attributed to the freezing of the randomly oriented spins at the AF/FM interface.59,60 To understand the relaxation dynamics of the random spins at 5 K, thermo-remnant magnetization (TRM) was plotted (Figure 9). TRM data fitted best to the equation61 M(t ) = M 0 + a exp(−t /τ ) + b ln(t)

(1)

indicating magnetic moment rotation and domain wall movement at 5 K. The fitted parameters were M0 = 2.6097

M(t ) = M 0 + a exp( −t /τ1) + b exp(−t /τ2)

(2)

resulted in higher relaxation parameters (τ1 = 288 ± 13 s and τ2 = 6625 ± 130 s) than expected. The other fitted parameters were M0 = 2.5789 ± 0.0001 emu/g, a = 0.0118 ± 0.0007 emu/ g, b = 0.0180 ± 0.00009 emu/g, and χ2 = 3.43 × 10−8. A simplified schematic diagram of the evolution of SEB shows the presence of FM domains within the AF matrix of the nanosheets below 50 K (Figure 10). At 300 K, the nanosheets consisted of magnetic moments which were thermally randomized. Between 50 and 110 K, the spins underwent AF alignment. Below 50 K, due to rotation of the spins crossing the energy barrier, isolated FM domains emerged and remain embedded in the AF matrix. However, the direction of rotation of the random spins at the AF/FM interface determined the direction of the hysteresis loop shift along the field axis. The applied magnetic field aligned all the metastable FM domains in the direction of the field and the FM domain size increased, resulting in improved coupling between

Figure 9. Relaxation of magnetization at 5 K for PNS1. The red and green lines are fits to eqs 1 and 2, respectively. 26357



Article

La0.67Ca0.33MnO3/Pr0.67Ca0.33MnO3 Superlattices: Comparison with Individual (La1−yPry)0.67Ca0.33MnO3 films. J. Appl. Phys. 2013, 113, 17D701. (2) Shirage, P. M.; Kihou, K.; Lee, C. −H.; Takeshita, N.; Eisaki, H.; Iyo, A. Disappearance of Superconductivity in the Solid Solution between (Ca4Al2O6)(Fe2As2) and (Ca4Al2O6)(Fe2P2) Superconductors. J. Am. Chem. Soc. 2012, 134, 15181−15184. (3) Chen, A.; Zhou, H.; Bi, Z.; Zhu, Y.; Luo, Z.; Bayraktaroglu, A.; Phillips, J.; Choi, E. −M.; MacManus-Driscoll, J. L.; Pennycook, S. J.; et al. A New Class of Room-Temperature Multiferroic Thin Films with Bismuth-Based Supercell Structure. Adv. Mater. 2013, 25, 1028−1032. (4) Bukhari, S. M.; Giorgi, J. B. Ni doped Sm0.95Ce0.05FeO3−δ Perovskite based Sensors for Hydrogen Detection. Sens. Actuators, B 2013, 181, 153−158. (5) Suthirakun, S.; Xiao, G.; Ammal, S. C.; Chen, F.; Loye, H. −C. z.; Heyden, A. Rational Design of Mixed Ionic and Electronic Conducting Perovskite Oxides for Solid Oxide Fuel Cell Anode Materials: A Case Study for Doped SrTiO3. J. Power Sources 2014, 245, 875−885. (6) Bera, A.; Peng, H.; Lourembam, J.; Shen, Y.; Sun, X. W.; Wu, T. A Versatile Light-Switchable Nanorod Memory: Wurtzite ZnO on Perovskite SrTiO3. Adv. Funct. Mater. 2013, 23, 4977−4984. (7) Cohn, J. L. Electrical and Thermal Transport in Perovskite Manganite. J. Supercond. Novel Magn. 2000, 13, 291−304. (8) Kimura, T.; Tokura, Y. Layered Magnetic Manganites. Annu. Rev. Mater. Sci. 2000, 30, 451−474. (9) Mukherjee, A.; Cole, W. S.; Woodward, P.; Randeria, M.; Trivedi, N. Theory of Strain-Controlled Magnetotransport and Stabilization of the Ferromagnetic Insulating Phase in Manganite Thin Films. Phys. Rev. Lett. 2013, 110, 157201. (10) Lee, W.; Han, J. W.; Chen, Y.; Cai, Z.; Yildiz, B. Cation Size Mismatch and Charge Interactions Drive Dopant Segregation at the Surfaces of Manganite Perovskites. J. Am. Chem. Soc. 2013, 135, 7909− 7925. (11) Tatay, S.; Barraud, C.; Galbiati, M.; Seneor, P.; Mattana, R.; Bouzehouana, K.; Deranlot, C.; Jacquet, E.; Forment-Aliaga, A.; Jegou, P.; et al. Self-Assembled Monolayer-Functionalized Half-Metallic Manganite for Molecular Spintronics. ACS Nano 2012, 6, 8753−8757. (12) Rao, C. N. R.; Cheetham, A. K.; Mahesh, R. Giant Magnetoresistance and Related Properties of Rare-Earth Manganates and Other Oxide Systems. Chem. Mater. 1996, 8, 2421−2432. (13) Dediu, V.; Ferdeghini, C.; Matacotta, F. C.; Nozar, P.; Ruani, G. Jahn-Teller Dynamics in Charge-Ordered Manganites from Raman Spectroscopy. Phys. Rev. Lett. 2000, 84, 4489−4492. (14) Sadhu, A.; Kramer, T.; Datta, A.; Wiedigen, S. A.; Norpoth, J.; Jooss, C.; Bhattacharyya, S. Ferromagnetism in Lightly Doped Pr1−xCaxMnO3 (x = 0.023, 0.036) Nanoparticles Synthesized by Microwave Irradiation. Chem. Mater. 2012, 24, 3758−3764. (15) Niebieskikwiat, D.; Salamon, M. B. Intrinsic Interface Exchange Coupling of Ferromagnetic Nanodomains in a Charge Ordered Manganite. Phys. Rev. B 2005, 72, 174422. (16) Rao, S. S.; Bhat, S. V. Probing the Existing Magnetic Phases in Pr0.5Ca0.5MnO3 (PCMO) Nanowires and Nanoparticles: Magnetization and Magneto-transport Investigations. J. Phys.: Condens. Matter 2010, 22, 116004. (17) Markovich, V.; Fita, I.; Wisniewski, A.; Jung, G.; Mogilyansky, D.; Puzniak, R.; Titelman, L.; Gorodetsky, G. Spin-glass-like Properties of La0.8Ca0.2MnO3 Nanoparticles Ensembles. Phys. Rev. B 2010, 81, 134440. (18) Guan, M.; Xiao, C.; Zhang, J.; Fan, S.; An, R.; Cheng, Q.; Xie, J.; Zhou, M.; Ye, B.; Xie, Y. Vacancy Associates Promoting Solar-Driven Photocatalytic Activity of Ultrathin Bismuth Oxychloride Nanosheets. J. Am. Chem. Soc. 2013, 135, 10411−10417. (19) Yao, T.; Liu, L.; Xiao, C.; Zhang, X.; Liu, Q.; Wei, S.; Xie, Y. Ultrathin Nanosheets of Half-Metallic Monoclinic Vanadium Dioxide with a Thermally Induced Phase Transition. Angew. Chem., Int. Ed. 2013, 52, 7554−7558. (20) Xu, T. −G.; Zhang, C.; Shao, X.; Wu, K.; Zhu, Y. −F. Monomolecular-Layer Ba5Ta4O15 Nanosheets: Synthesis and Inves-

Figure 10. Simplified schematic diagram showing the evolution of FM domains (black arrows) within AF matrix (maroon arrows) of the nanosheets below T ≤ 50 K. Blue and red dotted lines represent the FM interactions between adjacent nanosheets and SEB, respectively. x, y, and z represent the axes of the nanosheets, and H is the external applied field.

the FM domains. With the increase in FM domain size, the AF/ FM coupling at the interface (SEB) was also enhanced. At 5 K, the FM interdomain interaction and AF/FM coupling were extended in the x- and y-axis due to the 2D arrangement of the self-assembled μm-length nanosheets, and also in the z-axis, since the nanosheets were stacked. This resulted in large FM domains at 5 K and hysteresis loop shifts in the absence of cooling fields. The frozen disordered spins due to domain wall movement between 10 and 40 K and the unidirectional anisotropy resulted in the low temperature features in Figure 8c.

■

CONCLUSION In summary, stacked metal carbonate microsheets were successfully decomposed in air to the oxide nanosheets. This simple approach resulted in stoichiometric, self-assembled, and stacked 10−14 nm thick nanosheets of Pr1−xCaxMnO3 (x = 0.3, 0.49) with the Pnma space group. The PCMO nanosheets have superior magnetic properties at low temperatures. The FM domains within the AF matrix resulted in FM moments at 5 K, and spontaneous exchange bias was observed even in the absence of cooling fields. The long-range magnetic interactions were possible due to the highly anisotropic x- and y-axes of the 2D stacked nanosheets. This two-step synthesis method could be applied for the synthesis of other manganite nanosheets. Moreover, this nanosheet morphology is particularly useful for practical handling in nanomemory device fabrication and is a potential alternative to thin films.

■

ASSOCIATED CONTENT

S Supporting Information *

Characterization data and control experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91-9051167666. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The Council of Scientific and Industrial Research (CSIR), India, is duly acknowledged for the financial support under sanction no. 01(2689)/12/EMR-II. A.S. thanks University Grants Commission (UGC), New Delhi, for his fellowship. The authors thank Dr. Shiv Prakash Singh for FESEM imaging of the metal oxide nanosheets.

■

REFERENCES

(1) Hühn, S.; Jungbauer, M.; Michelmann, M.; Massel, F.; Koeth, F.; Ballani, C.; Moshnyaga, V. Modeling of Colossal Magnetoresistance in 26358



Article

tigation of Photocatalytic Properties. Adv. Funct. Mater. 2006, 16, 1599−1607. (21) Xu, K.; Chen, P.; Li, X.; Wu, C.; Guo, Y.; Zhao, J.; Wu, X.; Xie, Y. Ultrathin Nanosheets of Vanadium Diselenide: A Metallic TwoDimensional Material with Ferromagnetic Charge-Density-Wave Behavior. Angew. Chem., Int. Ed. 2013, 52, 10477−10481. (22) Coleman, J. N.; Lotya, M.; O’Neill, A.; Bergin, S. D.; King, P. J.; Khan, U.; Young, K.; Gaucher, A.; De, S.; Smith, R. J.; et al. TwoDimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials. Science 2011, 331, 568−571. (23) Aksit, M.; Toledo, D. P.; Robinson, R. D. Scalable Nanomanufacturing of Millimetre-length 2D NaxCoO2 Nanosheets. J. Mater. Chem. 2012, 22, 5936−5944. (24) Osada, M.; Akatsuka, K.; Ebina, Y.; Funakubo, H.; Ono, K.; Takada, K.; Sasaki, T. Robust High-κ Response in Molecularly Thin Perovskite Nanosheets. ACS Nano 2010, 4, 5225−5232. (25) Maeda, K.; Eguchi, M.; Youngblood, W. J.; Mallouk, T. E. Calcium Niobate Nanosheets Prepared by the Polymerized Complex Method as Catalytic Materials for Photochemical Hydrogen Evolution. Chem. Mater. 2009, 21, 3611−3617. (26) Ida, S.; Ogata, C.; Eguchi, M.; Youngblood, W. J.; Mallouk, T. E.; Matsumoto, Y. Photoluminescence of Perovskite Nanosheets Prepared by Exfoliation of Layered Oxides, K2Ln2Ti3O10, KLnNb2O7, and RbLnTa2O7 (Ln: Lanthanide Ion). J. Am. Chem. Soc. 2008, 130, 7052−7059. (27) Ozawa, T. C.; Fukuda, K.; Akatsuka, K.; Ebina, Y.; Sasaki, T. Preparation and Characterization of the Eu3+ Doped Perovskite Nanosheet Phosphor: La0.90Eu0.05Nb2O7. Chem. Mater. 2007, 19, 6575−6580. (28) Jiang, J.; Henry, L. L.; Gnanasekhar, K. I.; Chen, C.; Meletis, E. I. Self-Assembly of Highly Epitaxial (La,Sr)MnO3 Nanorods on (001) LaAlO3. Nano Lett. 2004, 4, 741−745. (29) Tian, Y.; Chen, D.; Jiao, X. La1‑xSrxMnO3 (x = 0, 0.3, 0.5, 0.7) Nanoparticles Nearly Freestanding in Water: Preparation and Magnetic Properties. Chem. Mater. 2006, 18, 6088−6090. (30) Carretero-Genevrier, A.; Gázquez, J.; Idrobo, J. C.; Oró, J.; Arbiol, J.; Varela, M.; Ferain, E.; Rodríguez-Carvajal, J.; Puig, T.; Mestres, N.; et al. Single Crystalline La0.7Sr0.3MnO3 Molecular Sieve Nanowires with High Temperature Ferromagnetism. J. Am. Chem. Soc. 2011, 133, 4053−4061. (31) Zhou, S.; Guo, Y.; Zhao, J.; He, L.; Wang, C.; Shi, L. Particle Size Effects on Charge and Spin Correlations in Nd0.5Ca0.5MnO3 Nanoparticles. J. Phys. Chem. C 2011, 115, 11500−11506. (32) Zhi, M.; Koneru, A.; Yang, F.; Manivannan, A.; Li, J.; Wu, N. Electrospun La0.8Sr0.2MnO3 Nanofibers for a High-temperature Electrochemical Carbon Monoxide Sensor. Nanotechnology 2012, 23, 305501. (33) Rao, S. S.; Anuradha, K. N.; Sarangi, S.; Bhat, S. V. Weakening of Charge Order and Antiferromagnetic to Ferromagnetic Switch Over in Pr0.5Ca0.5MnO3 Nanowires. Appl. Phys. Lett. 2005, 87, 182503. (34) Sarkar, T.; Mukhopadhyay, P. K.; Raychaudhuri, A. K.; Banerjee, S. Structural, Magnetic, and Transport Properties of Nanoparticles of the Manganite Pr0.5Ca0.5MnO3. J. Appl. Phys. 2007, 101, 124307. (35) Chai, P.; Wang, X.; Hu, S.; Liu, X.; Liu, Y.; Lv, M.; Li, G.; Meng, J. Particle Size-Dependent Charge Ordering and Magnetic Properties in Pr0.55Ca0.45MnO3. J. Phys. Chem. C 2009, 113, 15817−15823. (36) Levstik, A.; Filipič, C.; Bobnar, V.; Potočnik, A.; Arčon, D.; Drnovšek, S.; Holc, J. Ordering of Polarons in the Charge-disordered Phase of Pr0.7Ca0.3MnO3. Phys. Rev. B 2009, 79, 153110. (37) Frontera, C.; García-Muñoz, J. L.; Beran, P.; Bellido, N.; Margiolaki, I.; Ritter, C. Short- and Long-Range Orbital Order in Phase Separated Pr0.50Ca0.50Mn0.99Ti0.01O3: Its Role in Thermal Hysteresis. Chem. Mater. 2008, 20, 3068−3075. (38) Bhattacharyya, S.; Estrin, Y.; Rich, D. H.; Zitoun, D.; Koltypin, Y.; Gedanken, A. Luminescent and Ferromagnetic CdS:Mn2+/C Core−Shell Nanocrystals. J. Phys. Chem. C 2010, 114, 22002−22011. (39) Bhattacharyya, S.; Zitoun, D.; Estrin, Y.; Moshe, O.; Rich, D. H.; Gedanken, A. A One-step, Template-free Synthesis, Characterization,

Optical and Magnetic Properties of Zn1−xMnxTe Nanosheets. Chem. Mater. 2009, 21, 326−335. (40) Bhattacharyya, S.; Pucci, A.; Zitoun, D.; Gedanken, A. One-pot Fabrication and Magnetic Studies of Mn-doped TiO2 Nanocrystals with an Encapsulating Carbon Layer. Nanotechnology 2008, 19, 495711-(1−8). (41) Bhattacharyya, S.; Gedanken, A. Synthesis, Characterization, and Room-Temperature Ferromagnetism in Cobalt-Doped Zinc Oxide (ZnO:Co2+) Nanocrystals Encapsulated in Carbon. J. Phys. Chem. C 2008, 112, 4517−4523. (42) Datta, A.; Dutta, P.; Sadhu, A.; Maiti, S.; Bhattacharyya, S. Single-step Scalable Conversion of Waste Natural Oils to Carbon Nanowhiskers and their Interaction with Mammalian Cells. J. Nanopart. Res. 2013, 15, 1808 -(1−15).. (43) Datta, A.; Sadhu, A.; Sen, B.; Kaur, M.; Sharma, R.; Das, S. C.; Bhattacharyya, S. Analysis of the Acid, Base and Air Oxidized Carbon Microspheres Synthesized in a Single Step from Waste Engine Oil. Corros. Sci. 2013, 73, 356−364. (44) Rodriguez-Blanco, J. D.; Shaw, S.; Benning, L. G. J. The Kinetics and Mechanisms of Amorphous Calcium Carbonate (ACC) Crystallization to Calcite, via Vaterite. Nanoscale 2011, 3, 265−271. (45) Patil, K. C.; Chandrashekhar, G. V.; George, M. V.; Rao, C. N. R. Infrared Spectra and Thermal Decompositions of Metal Acetates and Dicarboxylates. Can. J. Chem. 1968, 46, 257−265. (46) Lee, Y. J.; Reeder, R. J.; Wenskus, R. W.; Elzinga, E. Structural Relaxation in the MnCO3-CaCO3 Solid Solution: A Mn K-edge EXAFS Study. Phys. Chem. Miner. 2002, 29, 585−594. (47) Zhang, T.; Dressel, M. Grain-size Effects on the Charge Ordering and Exchange Bias in Pr0.5Ca0.5MnO3: The Role of Spin Configuration. Phys. Rev. B 2009, 80, 014435. (48) Ziese, M.; Vrejoiu, I.; Pippel, E.; Nikulina, E.; Hesse, D. Magnetic Properties of Pr0.7Ca0.3MnO3/SrRuO3 Superlattices. Appl. Phys. Lett. 2011, 98, 132504. (49) Wang, B. M.; Liu, Y.; Ren, P.; Xia, B.; Ruan, K. B.; Yi, J. B.; Ding, J.; Li, X. G.; Wang, L. Large Exchange Bias after Zero-Field Cooling from an Unmagnetized State. Phys. Rev. Lett. 2011, 106, 077203. (50) Wang, B. M.; Liu, M.; Xia, B.; Ren, P.; Wang, L. Large Exchange Bias Obtained through Zero-field Cooling from an Unmagnetized State. J. Appl. Phys. 2012, 111, 043912. (51) Nayak, A. K.; Nicklas, M.; Chadov, S.; Shekhar, C.; Skourski, Y.; Winterlik, J.; Felser, C. Large Zero-Field Cooled Exchange-Bias in Bulk Mn2PtGa. Phys. Rev. Lett. 2013, 110, 127204. (52) Ahmadvand, H.; Salamati, H.; Kameli, P.; Poddar, A.; Acet, M.; Zakeri, K. Exchange Bias in LaFeO3 Nanoparticles. J. Phys. D: Appl. Phys. 2010, 43, 245002. (53) Maity, T.; Goswami, S.; Bhattacharya, D.; Roy, S. Superspin Glass Mediated Giant Spontaneous Exchange Bias in a Nanocomposite of BiFeO3-Bi2Fe4O9. Phys. Rev. Lett. 2013, 110, 107201. (54) Chauhan, S.; Srivastava, S. K.; Chandra, R. Zero-field Cooled Exchange Bias in Hexagonal YMnO3 Nanoparticles. Appl. Phys. Lett. 2013, 103, 042416. (55) Mao, H. J.; Song, C.; Cui, B.; Wang, G. Y.; Xiao, L. R.; Pan, F. Room temperature Spontaneous Exchange Bias in (La,Sr)MnO3/ PbZr0.8Ti0.2O3/(La,Sr)MnO3 Sandwich Structure. J. Appl. Phys. 2013, 114, 043904. (56) Ziesse, M.; Bern, F.; Vrejoiu, I. Exchange Bias in Manganite/ SrRuO3 Superlattices. J. Appl. Phys. 2013, 113, 063911. (57) Markovich, V.; Jung, G.; Wisniewski, A.; Mogilyansky, D.; Puzniak, R.; Kohn, A.; Wu, X. D.; Suzuki, K.; Gorodetsky, G. Magnetic Properties of Electron-doped La0.23Ca0.77MnO3 Nanoparticles. J. Nanopart. Res. 2012, 14, 1119. (58) Zhang, T.; Wang, X. P.; Fang, Q. F. Evolution of the Electronic Phase Separation with Magnetic Field in Bulk and Nanometer Pr0.67Ca0.33MnO3 Particles. J. Phys. Chem. C 2011, 115, 19482−19487. (59) Nogués, J.; Schuller, I. K. Exchange Bias. J. Magn. Magn. Mater. 1999, 192, 203−232. 26359



Article

(60) Gajbhiye, N. S.; Bhattacharyya, S. Exchange Bias and Spin-GlassLike Ordering in ε-Fe3N−CrN Nanocomposites. Jpn. J. Appl. Phys. 2007, 46, 980−987. (61) Sirena, M.; Steren, L. B.; Guimpel, J. Magnetic Relaxation in Bulk and Film Manganite Compounds. Phys. Rev. B 2001, 64, 104409. (62) Banerjee, R.; Banerjee, M.; Majumdar, A. K.; Mookerjee, A.; Sanyal, B.; Hellsvik, J.; Eriksson, O.; Nigam, A. K. Fe3.3Ni83.2Mo13.5: a Likely Candidate to show Spin-glass Behavior at Low Temperatures. J. Phys.: Condens. Mater 2011, 23, 106002.

26360


Stacked Nanosheets of Pr1âxCaxMnO3 (x = 0.3 and 0.49): A

Recommend Documents

Stacked Nanosheets of Pr1âxCaxMnO3 (x = 0.3 and 0.49): A