Stacked Nanosheets of Pr1−xCaxMnO3 (x = 0.3 and 0.49): AFerromagnetic Two-Dimensional Material with SpontaneousExchange BiasAnustup 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 haveattracted 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 and0.49) were obtained by decomposition of carbon coated CaCO3/MnCO3 microsheets andPr2O2CO3 aggregates, the latter synthesized under pressure at 500−800 °C. The PCMOnanosheets had flatter Mn−O−Mn tilt angles in the MnO6 octahedra and ferromagnetic(FM) moments at 5 K. Spontaneous exchange bias (SEB) coupling between theantiferromagnetic (AF)/FM spins was observed at 5 K under zero-field cooling. High FMmoments and SEB were possible due to the long-range magnetic interactions in thestacked 2D arrangement of Mn3+/Mn4+ d-electron spins, where charge ordering wascompletely suppressed. The SEB behavior was dependent on the initial magnetizationprocess and the direction of rotation of the random spins at the AF/FM interface.

■ INTRODUCTIONThe perovskite family of metal oxides is particularly importantbecause of their wide range of properties such as colossalmagnetoresistance,1 superconductivity,2 ferroelectricity,3 andapplications in sensors,4 fuel cells,5 and memory devices.6

Among them, the doped rare-earth manganites of the generalformula A1−xA′xMnO3 (where A and A′ are trivalent rare- anddivalent alkaline-earth ions, respectively) are particularly inheavy demand for the electronics and energy storage devicesbecause of their tunable electrical, magnetic, and magneto-resistance properties.7−9 Significant progress has been achievedto develop novel structural morphologies of these materi-als.10−12 In Pr1−xCaxMnO3, the most studied Ca-doping level is0.3 ≤ x ≤ 0.5 where the electronic and magnetic properties areinfluenced by the interplay of charge, orbital, lattice, and spinordering. In PCMO, the divalent alkaline-earth Ca2+ ionsreplace the trivalent rare-earth Pr3+ ions at the lattice sites andthe concentration of Mn3+ decreases at the cost of Mn4+ ions tomaintain charge balance. The Jahn−Teller distortion at theMn3+ sites with the 2-fold degenerate eg level leads todistortions due to puckering of the MnO6 octahedra andcubic to orthorhombic transition.13 Partial localization of the egelectrons leads to the charge order phase, whereas delocaliza-tion leads to the FM-metallic phase by the double-exchangemechanism. In our last report on lightly doped PCMOnanoparticles, we have shown that the flattening of the Mn−O−Mn bond angles and reduced orthorhombic strainpromoted improved FM moments at 5 K.14

In most of these materials, the orientation of the spinschanges from being random at room temperature to either AF

or FM ordering at low temperatures. Because of the presence ofinhomogeneous magnetic phases at the same temperature, thepossibility of the coexistence of AF and FM phases increases,which results in exchange anisotropy interactions at the AF/FMinterfaces within the same material.15 When the size of theparticles is reduced to the nanometer scale, the basic magneticproperties such as saturation magnetization, coercive field,remanence, and nature of magnetic ordering become sizedependent. In nanoparticles, the high concentration of defectscan melt the AF charge order phase.16 For weakly interactingnanoparticles, superparamagnetism leads to unstable magneticorder, which can be overcome by exchange coupling of AF andFM spins. When the nanoparticles of the manganites interactstrongly with each other, they demonstrate slow dynamics,aging, and memory effects similar to a spin glass system.17 Infact, for practical applications, it is desirable that significantexchange bias coupling occurs at zero or minimum appliedcooling fields. Nanowires or nanorods with 1D morphologylead to shape anisotropy, useful to increase the overall magneticanisotropy of the system.16 The 2D morphology of nanosheetsor thin films offers 1D quantum confinement and enhancedanisotropy along the micrometer-length dimensions, whichdistinctly separates their physical properties from the conven-tional nanoparticles and bulk material.18−20 Nanosheets havethe advantage of direct implementation as potential buildingblocks for next-generation nanodevices because of their

Received: September 27, 2013Revised: November 22, 2013Published: November 22, 2013

Article

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

extended 2D network with rich electronic and magneticproperties.21 Although the 2D morphologies of other materialsare known,22,23 so far there has been no report on thenanosheet morphology of rare-earth manganites. Perovskitenanosheets are only known to be those obtained by exfoliationof layered perovskite metal oxides with the help of organicbases at room temperature.24−27 However, nanosheets of thenon-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 re-ports on Ca-doped PrMnO3 materials are few.16,33−35 Thedoped rare-earth manganite nanostructures were so farsynthesized by the sol−gel method to give nanoparticles,31,34,35

and nanowires,16,30 the hydrothermal technique,33 the moltensalt 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 “beaker-less” pressure synthesis route for obtaining the phase-purePr0.7Ca0.3MnO3 (PNS1) and Pr0.51Ca0.49MnO3 (PNS2) nano-sheets. It is well-known that metal carbonates decompose tooxides and CO2, when heated in air. We demonstrate here thatmetal oxide nanosheets can be obtained if the precursor metalcarbonate was synthesized with a planar morphology. Themetal carbonate sheets were synthesized by dry autoclavingwhich was also used for large scale synthesis of carbon coatednanocrystals,38−41 and carbon nano- and microstructures.42,43

The reactions occur inside a closed autoclave cell at high

temperature and autogenic pressure. The morphology of thenanostructures can be manipulated in the absence of solvent bycontrolling the built-in autogenic pressure, altering the amountof precursor solid inside the autoclave, heating rate, temper-ature, and time of reaction in this high yield, bottom-upprocess. The nanosheets showed superior ferromagneticmoments at 5 K and spontaneous exchange bias under zero-field cooling.

■ 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 withethanol and dried in air before the synthesis process. Thereactions were performed in a Carbolite wire-wound tubefurnace - single zone, model MTF 12/38/400.

Synthesis of Carbon Coated Microsheets. Pr-, Ca-, andMn-acetates were weighed and mixed in mole ratios of 0.7:0.3:1and 0.5:0.5:1 according to the formula Pr1−xCaxMnO3 (x = 0.3and 0.5), respectively. The acetate mixture was introducedinside the 2 mL stainless steel autoclave and capped tightly atboth ends. The filled autoclave was heated at 6 °C min−1 andmaintained at 500−900 °C for 6 h, followed by slow cooling to

Figure 1. (a) TEM image of PNS1. i−iv represent four stacked nanosheets. (inset) SAED pattern. (b) High-resolution TEM image of twonanosheets 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.

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room temperature. This resulted in the carbon coated metalcarbonate microsheets.Synthesis of PCMO Nanosheets. The carbon coated

sheets were ground in a mortar and heat treated on aluminaboats in air. The heating rate was maintained again at 6 °Cmin−1 and the samples were calcined at 1000 °C for 24 h,followed by normal cooling to room temperature. The yield ofthe 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. A4 mg portion of the sample was dissolved in 10 mL of 65%suprapure nitric acid (Merck KGaA) at 120 °C for 2 h. Thesolution was digested at 70 °C overnight and diluted to 100ppb concentration for the ICP-MS analyses. The X-raydiffraction (XRD) measurements were carried out with aRigaku (mini flex II, Japan) powder X-ray diffractometer havingCu Kα = 1.54059 Å radiation. Rietveld analysis of the XRDpatterns was performed by the General Structure AnalysisSystem (GSAS) software, Los Alamos National LaboratoryReport (2004). Field emission scanning electron microscope(FESEM) images were recorded in Carl Zeiss SUPRA 55VPFESEM. Energy dispersive analysis of X-ray (EDAX) studieswere performed with the Oxford Instruments X-Max withINCA software coupled to the FESEM. Transmission electronmicroscopy (TEM) images were obtained by employing aJEOL-JEM 2010 electron microscope with a 200 kVaccelerating voltage. The thermogravimetric analysis (TGA)data were collected on a Mettler Toledo STARe, under airatmosphere at 10 °C/min heating and cooling rates. TheFourier transform infrared (FTIR) measurements were carriedout with a Perkin-Elmer spectrum RX1 with KBr pellets. Eachpellet contained 3 mg of the sample and 200 mg of KBr (FTIRgrade). A LABRAM HR800 Raman spectrometer wasemployed using the 633 nm line of a He−Ne ion laser as theexcitation source to analyze the nanomaterials. Magneticproperties were studied using the MPMS-XL EvercoolQuantum Design SQUID magnetometer, in the temperaturerange 5−300 K and applied fields of 0−4000 mT. Thetemperature-dependent zero-field cooled (ZFC) magnetizationwas measured using a DC procedure. The samples were cooledto 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 thepresence of 10 mT applied field, and the data were recordedwhile warming up the samples in the presence of the field.Thermo-remnant magnetization (TRM) was measured by thefollowing protocol: a 0.01 T field was applied at 300 K, and thesample was field cooled to 5 K. At 5 K, after a wait time of 1800s (30 min), the magnetic field was switched off. Themagnetization (M) was measured as a function of time (t) ateach 30 s interval from 30 to 11 500 s.

■ RESULTS AND DISCUSSIONThe nanosheets of PNS1 and PNS2 crystallize in theorthorhombic phase with the Pnma space group. Theinductively coupled plasma mass spectroscopy (ICP-MS)experiments revealed the bulk composition of the final solidproducts with Ca2+/Pr3+ atomic ratios of 0.3 and 0.49 for PNS1and PNS2 samples, respectively. The transmission electronmicroscope (TEM) image in Figure 1a shows four stackedsheets (i−iv) of the representative PNS1 sample. The selectedarea electron diffraction (SAED) pattern (Figure 1a, inset)shows the characteristic reflections of the orthorhombic phase.

The lattice spacing was measured to be 0.272 nm, whichcorresponds to the (112) reflection (Figure 1b). The fieldemission scanning electron microscope (FESEM) image inFigure 1c reveals the thickness of the PNS1 nanosheets to be10−14 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 10different nanosheets provided the homogeneity profile of thesamples. The nanosheet morphology (Figure 1f), lattice fringeswith FFT (Figure 1g), and EDAX pattern (Figure 1h) of PNS2were found similar to those for PNS1. The EDAX resultsmatched close to that of ICP-MS as Ca2+/Pr3+ atomic ratio tobe 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

consideration. The lattice parameters along the a, b, and c axesdecreased on moving from PNS1 to PNS2 (Table 1). In PNS1and 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 ofdouble exchange of Mn3+/Mn4+ spins leading to ferromagnet-ism.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 calculatedpatterns; Obs (observed pattern); Calc (calculated pattern); and Bckgr(background plot). (Inset of a) Schematic showing Mn−O−Mn bondlengths and angles.

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b√2)] were calculated to be 0.0038 and 0.0015 for PNS1 and0.008 and 0.0014 for PNS2, respectively.14 The reduced strainswere due to the nm thickness of the sheets. The synthesis routeto the nanosheets was explained on the basis of the controlexperiments and the characterization techniques such asFESEM, EDAX, FTIR, and TGA. Initially, when the metalacetates were autoclaved, the majority of microsheets wereobtained under autogenic pressure from 500 to 800 °C for 6 hand a mixture of particles and sheets at 900 °C. The sheetswere coated by carbon, and hence, they could be separatedfrom each other. The reaction conditions were optimized at700 °C under autogenic pressure to obtain the precursors ofPNS1 and PNS2, the former considered as the representativesystem. The autoclaved 2−3 μm thick sheets consisted ofsmaller platelets (Figure 3a) and were covered by ∼200 nmthick carbon film (Figure 3b,c). The carbon coating was

predominantly graphitic in nature (Supporting Information,Figure S1). The autoclaved products were inhomogeneouswherein the sheets consisted of morphologies with lower Pr3+/Ca2+ ratio, as compared with the brighter aggregates under thein-lens electron beam of FESEM (Figure 3d). In the absence ofenough oxygen inside the closed autoclave cell, theorthorhombic manganite phases were not obtained. Theautoclaved products of both PNS1 and PNS2 consisted of amajority of metal carbonates (CaCO3 and MnCO3), oxy-carbonate (Pr2O2CO3), and a lesser fraction of oxides (Pr6O11,MnO, Mn2O3), as shown in the XRD pattern (Figure 4). Thepresence of metal carbonates was cross-checked by infrared(IR) spectroscopy (Figure S2, Supporting Information). The IRbands at 1473 and 855 cm−1, 1095 and 717 cm−1, and 614 and504 cm−1 correspond to the C−O asymmetric stretching, OCOsymmetric stretching of CO3

2− ions, and Mn−O/Pr−O

Table 1. XRD-Rietveld refinement parameters

sample [space group] lattice parameters (Å); angles (deg); cell volume (Å3) atomic positions (x, y, z) occupation number weighted profile (Rwp)

PNS1 [Pnma] a = 5.4542(1) Å Pr (0.0325, 0.2500, 0.008) Pr = 0.70 4.72%b = 7.6763(1) Å Ca (0.0325, 0.2500, 0.008) Ca = 0.30c = 5.4334(1) Å Mn (0.0000, 0.0000, 0.5000) Mn = 1.0α = β = γ = 90° O1 (0.4762, 0.2500, 0.065) O1 = 1.0V = 227.49 Å3 O2 (0.2775, 0.0380, 0.7424) O2 = 1.0bond distances: (Mn−O)c = 1.952(4) Å; (Mn−O)ab = 1.956(4) Åbond angles: Mn−Oc−Mn = 160.8°; Mn−Oab−Mn = 157.8°

PNS2 [Pnma] a = 5.4302(3) Å Pr (0.0355, 0.2500, 0.002) Pr = 0.51 4.88%b = 7.6273(1) Å Ca (0.0355, 0.2500, 0.002) Ca = 0.49c = 5.3866(3) Å Mn (0.0000, 0.0000, 0.5000) Mn = 1.0α = β = γ = 90° O1 (0.4854, 0.2500, 0.075) O1 = 1.0V = 223.10 Å3 O2 (0.2862, 0.0350, 0.7324) O2 = 1.0bond distances: (Mn−O)c = 1.941(1) Å; (Mn−O)ab = 1.943(2) Åbond angles: Mn−Oc−Mn = 159.5°; Mn−Oab−Mn = 158.6°

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

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

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vibrations, respectively.44 When the autoclaved carbon-coatedcomposite products were mechanically ground and heated in airat 1000 °C for 24 h, the orthorhombic phases of PNS1 andPNS2 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 trappedinside the autoclave and kinetically converted to elementalcarbon.39 Pr-, Ca-, and Mn-acetates dehydrated at 100−200 °C,and the metal carbonates formed around 260−490 °C (Figure

S3, Supporting Information). At ≤700 °C, Pr6O11, MnO, andMn2O3 could form except CaO. This was unlike the 900 °Cautoclave reaction where all the metal oxide phases werepredominant (Figure S4, Supporting Information). In theinhomogeneous autoclaved products, the Pr/Mn ratio of thesheets and the aggregates was 2−6 and >400 atom %,respectively (Figure 5a,b). In a control experiment, when theinitial Pr/Mn stoichiometry was maintained at 0.03, only sheetswere obtained without any aggregate (Figure 5c), whichimplied that sheets could only contain lesser Pr3+ ions.

Figure 5. FESEM images, EDAX spectra, and element % of (a) sheets and (b) aggregates in the autoclaved product of PNS1. Pr content wassignificantly lower on the sheets compared to the aggregates. (c) FESEM image and EDAX spectrum of the sheets with a Pr/Mn atomic ratio of3.2%.

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

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Reactions with Pr- and Mn-acetates (1:1) in turn gave amixture of flower-like aggregates and sheets at 600 °C, but at800 °C, solely aggregates were found (Figure S5, SupportingInformation). When Pr-, Ca-, and Mn-acetates were separatelyautoclaved, self-assembled sheets in a flower-like shape, stackedsheets, 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 controlexperiments point to the fact that CaCO3 platelets wereembedded inside the carbon coated 2−3 μm thick MnCO3sheets with 2−6% Pr2O2CO3 doped inside (Figure 6a). TheCa/Mn ratio was studied at different heights along the z-axis ofthe sheets by altering the accelerating voltage from 7.5 to 20 kVat a particular location in FESEM (Figure 6b). For example, 7.5kV probed only a few atomic layers below the surface, whereas20 kV probed the entire sheet. CaCO3 and MnCO3 can readilyform solid solutions,46 but in this case, the Ca2+ concentrationwas observed to vary along the z-axis of the autoclaved sheets.Thus, it was inferred that the CaCO3 platelets were probablyintercalated inside the distorted microcubes of MnCO3,providing a microsheet morphology. The Pr-oxycarbonatecould not “solubilize” inside the mixed carbonate sheets andthus segregated as aggregates. The autoclaved sheets weremetastable and broke into smaller platelets at temperaturesabove 700 °C (Figure S4, Supporting Information). When theautoclaved products were mechanically ground, the carboncoating ruptured and the sheets exfoliated laterally to mix withPr2O2CO3 aggregates (Figure 6a). At 1000 °C, PNS1 andPNS2 phases were formed by a typical solid state reaction ofthe metal carbonates/oxycarbonates and oxides. In fact, thesuccessful conversion of autoclaved sheets of metal carbonatesto different metal oxide nanosheets confirmed our approach tobe a generalized synthesis method (Figure 7). If the carbonatemicrosheets were decomposed at lower temperatures andshorter duration, the nanosheets consist of 100−200 nm longplatelets. The platelets join to form single crystalline nanosheetswith minimum defect concentration, if calcined at and above1000 °C for longer durations (Figure 1c,f). The singlecrystalline nature of the nanosheets reduces the dangling

bonds at the surface and facilitates the long-range interaction ofelectron spins.The samples showed a paramagnetic-like linear M−H

behavior at 300 K (Figure 8a),16 since the thermal energycould overcome the magnetic anisotropy barrier to randomlyflip the magnetic moments. Size reduction in one of thedimensions in the nanosheets induced the FM phase at lowtemperatures dominating the AF component (Figure 8b), asreported in other PCMO nanomaterials.16,34,47 The magneticmoments 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 amongthe reported nanostructures of 30 and 50% Ca-dopedPrMnO3

16,33,34 but are less than the Pr0.7Ca0.3MnO3/SrRuO3superlattices.48 The enhanced magnetization was primarily dueto the magnetic anisotropy along the x- and y-axes of thestacked nanosheets. The moments were however lower ascompared to the saturated Mn magnetic moment of 3.8 μB,which indicates the presence of AF spins. The AF componentcreated disorder in the long-range FM alignment of the spins,also evident from the unsaturated hysteresis behavior in Figure8b. In the zero-field-cooled (ZFC) and field-cooled (FC)magnetization plots at 10 mT applied field (Figure 8c), thereported AF charge ordering peak at ∼250 K16,33 wascompletely suppressed. Although ZFC/FC irreversibility wasnot observed in PNS2, the curves bifurcate at 110 K for PNS1.In PNS2, magnetic transition was observed at 120 K. Additionaltransitions 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, inthe negative field direction at 5 K even in the absence of appliedcooling field (Figure 8d), a behavior termed as spontaneousexchange bias (SEB). Exchange bias (EB) is usually observed inmaterials with AF/FM interfaces after field cooling from abovethe Neel temperature of the AF phase. Under ZFC, SEB wasrecently 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 La0 . 6 7Sr0 . 3 3MnO3/PbZr0 . 8Ti0 . 2O3/La0.67Sr0.33MnO3 structure.55 With the bulk and pristinenanoparticles of doped rare-earth manganites, EB was so farobserved only under FC,15,47,56−58 and to our knowledge, this isthe first report of SEB with doped rare-earth manganites at thenanoscale. The 2D morphology of PCMO nanosheetsfacilitated the long-range AF/FM interactions to demonstrateSEB, the magnitude of which is comparable to those reportedunder FC.47 It was demonstrated earlier that the initialmagnetization process determines the nature of hysteresisloop shift (Figure 8d).49 In PNS1, when the maximum externalmagnetic field was 1000 mT, SEB was absent and the hysteresisloop was symmetric with Hc = 60 mT. On sweeping themagnetic field in the reverse direction, i.e., 0→ −3000 mT→ 0→ +3000 mT → 0 → −3000 mT, Hc was 89 mT and thehysteresis loop shifted in the positive direction by 2 mT. Whencooling fields were applied, any additional loop shift was notobserved. These observations indicate that SEB was intrinsic tothe PCMO nanosheets and related to the AF/FM interfacesformed during initial magnetization of the Mn3+/Mn4+ d-orbitalspins. The asymmetry related to the SEB behavior and thus theunidirectional anisotropy field was dependent on the initialdirection 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 airheating the respective carbonate microsheets at 900 °C for 4 h,respectively.

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the ZFC/FC plots (Figure 8c) can be attributed to the freezingof the randomly oriented spins at the AF/FM interface.59,60

To understand the relaxation dynamics of the random spinsat 5 K, thermo-remnant magnetization (TRM) was plotted(Figure 9). TRM data fitted best to the equation61

τ= + − +M t M a t b( ) exp( / ) ln(t)0 (1)

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

± 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. TheDebye exponential function was due to the single barrieractivation mechanism, and the logarithmic term denotes amechanism involving distribution of energy barriers to therotation of the Mn3+/Mn4+ spins. The fitting to the equationinvolving the double exponential relaxation mechanismcombining an initial fast relaxation related to spin glass and aslow exponential decay related to a random ferromagnet62

τ τ= + − + −M t M a t b t( ) exp( / ) exp( / )0 1 2 (2)

resulted in higher relaxation parameters (τ1 = 288 ± 13 s and τ2= 6625 ± 130 s) than expected. The other fitted parameterswere 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. Asimplified schematic diagram of the evolution of SEB shows thepresence of FM domains within the AF matrix of thenanosheets below 50 K (Figure 10).At 300 K, the nanosheets consisted of magnetic moments

which were thermally randomized. Between 50 and 110 K, thespins underwent AF alignment. Below 50 K, due to rotation ofthe spins crossing the energy barrier, isolated FM domainsemerged and remain embedded in the AF matrix. However, thedirection of rotation of the random spins at the AF/FMinterface determined the direction of the hysteresis loop shiftalong the field axis. The applied magnetic field aligned all themetastable FM domains in the direction of the field and the FMdomain size increased, resulting in improved coupling between

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 ofmagnetization with temperature at an applied field of 10 mT. Open symbols, ZFC; closed symbols, FC; stars, PNS1; circles, PNS2. (d) Enlargedview 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 maximumapplied field of 1000 mT, and (v) reverse field sweep.

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

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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 wereextended in the x- and y-axis due to the 2D arrangement of theself-assembled μm-length nanosheets, and also in the z-axis,since the nanosheets were stacked. This resulted in large FMdomains at 5 K and hysteresis loop shifts in the absence ofcooling fields. The frozen disordered spins due to domain wallmovement between 10 and 40 K and the unidirectionalanisotropy resulted in the low temperature features in Figure8c.

■ CONCLUSIONIn summary, stacked metal carbonate microsheets weresuccessfully decomposed in air to the oxide nanosheets. Thissimple approach resulted in stoichiometric, self-assembled, andstacked 10−14 nm thick nanosheets of Pr1−xCaxMnO3 (x = 0.3,0.49) with the Pnma space group. The PCMO nanosheets havesuperior magnetic properties at low temperatures. The FMdomains within the AF matrix resulted in FM moments at 5 K,and spontaneous exchange bias was observed even in theabsence of cooling fields. The long-range magnetic interactionswere possible due to the highly anisotropic x- and y-axes of the2D stacked nanosheets. This two-step synthesis method couldbe applied for the synthesis of other manganite nanosheets.Moreover, this nanosheet morphology is particularly useful forpractical handling in nanomemory device fabrication and is apotential alternative to thin films.

■ ASSOCIATED CONTENT*S Supporting InformationCharacterization data and control experiments. This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: sayanb@iiserkol.ac.in. Phone: +91-9051167666.NotesThe authors declare no competing financial interest.

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

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

La0.67Ca0.33MnO3/Pr0.67Ca0.33MnO3 Superlattices: Comparison withIndividual (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 Solutionbetween (Ca4Al2O6)(Fe2As2) and (Ca4Al2O6)(Fe2P2) Superconduc-tors. 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 withBismuth-Based Supercell Structure. Adv. Mater. 2013, 25, 1028−1032.(4) Bukhari, S. M.; Giorgi, J. B. Ni doped Sm0.95Ce0.05FeO3−δPerovskite based Sensors for Hydrogen Detection. Sens. Actuators, B2013, 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 ConductingPerovskite Oxides for Solid Oxide Fuel Cell Anode Materials: A CaseStudy 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 onPerovskite SrTiO3. Adv. Funct. Mater. 2013, 23, 4977−4984.(7) Cohn, J. L. Electrical and Thermal Transport in PerovskiteManganite. 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 ofthe 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 SizeMismatch and Charge Interactions Drive Dopant Segregation at theSurfaces 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-MetallicManganite for Molecular Spintronics. ACS Nano 2012, 6, 8753−8757.(12) Rao, C. N. R.; Cheetham, A. K.; Mahesh, R. GiantMagnetoresistance and Related Properties of Rare-Earth Manganatesand 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 RamanSpectroscopy. 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 DopedPr1−xCaxMnO3 (x = 0.023, 0.036) Nanoparticles Synthesized byMicrowave Irradiation. Chem. Mater. 2012, 24, 3758−3764.(15) Niebieskikwiat, D.; Salamon, M. B. Intrinsic Interface ExchangeCoupling of Ferromagnetic Nanodomains in a Charge OrderedManganite. Phys. Rev. B 2005, 72, 174422.(16) Rao, S. S.; Bhat, S. V. Probing the Existing Magnetic Phases inPr0.5Ca0.5MnO3 (PCMO) Nanowires and Nanoparticles: Magnet-ization and Magneto-transport Investigations. J. Phys.: Condens. Matter2010, 22, 116004.(17) Markovich, V.; Fita, I.; Wisniewski, A.; Jung, G.; Mogilyansky,D.; Puzniak, R.; Titelman, L.; Gorodetsky, G. Spin-glass-like Propertiesof 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-DrivenPhotocatalytic 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 Dioxidewith 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 FMdomains (black arrows) within AF matrix (maroon arrows) of thenanosheets below T ≤ 50 K. Blue and red dotted lines represent theFM interactions between adjacent nanosheets and SEB, respectively. x,y, and z represent the axes of the nanosheets, and H is the externalapplied field.

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp4096375 | J. Phys. Chem. C 2013, 117, 26351−2636026358

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 Two-Dimensional Material with Ferromagnetic Charge-Density-WaveBehavior. 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. Two-Dimensional Nanosheets Produced by Liquid Exfoliation of LayeredMaterials. Science 2011, 331, 568−571.(23) Aksit, M.; Toledo, D. P.; Robinson, R. D. Scalable Nano-manufacturing 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 ThinPerovskite 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 ComplexMethod 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 NanosheetsPrepared 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 PerovskiteNanosheet 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 andMagnetic Properties. Chem. Mater. 2006, 18, 6088−6090.(30) Carretero-Genevrier, A.; Gazquez, J.; Idrobo, J. C.; Oro, J.;Arbiol, J.; Varela, M.; Ferain, E.; Rodríguez-Carvajal, J.; Puig, T.;Mestres, N.; et al. Single Crystalline La0.7Sr0.3MnO3 Molecular SieveNanowires 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. ParticleSize 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-temperatureElectrochemical Carbon Monoxide Sensor. Nanotechnology 2012, 23,305501.(33) Rao, S. S.; Anuradha, K. N.; Sarangi, S.; Bhat, S. V. Weakeningof Charge Order and Antiferromagnetic to Ferromagnetic Switch Overin 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 ofthe 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 Propertiesin Pr0.55Ca0.45MnO3. J. Phys. Chem. C 2009, 113, 15817−15823.(36) Levstik, A.; Filipic, C.; Bobnar, V.; Potocnik, A.; Arcon, D.;Drnovsek, S.; Holc, J. Ordering of Polarons in the Charge-disorderedPhase of Pr0.7Ca0.3MnO3. Phys. Rev. B 2009, 79, 153110.(37) Frontera, C.; García-Munoz, J. L.; Beran, P.; Bellido, N.;Margiolaki, I.; Ritter, C. Short- and Long-Range Orbital Order inPhase Separated Pr0.50Ca0.50Mn0.99Ti0.01O3: Its Role in ThermalHysteresis. 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+/CCore−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-potFabrication and Magnetic Studies of Mn-doped TiO2 Nanocrystalswith an Encapsulating Carbon Layer. Nanotechnology 2008, 19,495711-(1−8).(41) Bhattacharyya, S.; Gedanken, A. Synthesis, Characterization, andRoom-Temperature Ferromagnetism in Cobalt-Doped Zinc Oxide(ZnO:Co2+) Nanocrystals Encapsulated in Carbon. J. Phys. Chem. C2008, 112, 4517−4523.(42) Datta, A.; Dutta, P.; Sadhu, A.; Maiti, S.; Bhattacharyya, S.Single-step Scalable Conversion of Waste Natural Oils to CarbonNanowhiskers 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 CarbonMicrospheres 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 Kineticsand 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 Acetatesand Dicarboxylates. Can. J. Chem. 1968, 46, 257−265.(46) Lee, Y. J.; Reeder, R. J.; Wenskus, R. W.; Elzinga, E. StructuralRelaxation in the MnCO3-CaCO3 Solid Solution: A Mn K-edgeEXAFS Study. Phys. Chem. Miner. 2002, 29, 585−594.(47) Zhang, T.; Dressel, M. Grain-size Effects on the ChargeOrdering and Exchange Bias in Pr0.5Ca0.5MnO3: The Role of SpinConfiguration. 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-FieldCooling from an Unmagnetized State. Phys. Rev. Lett. 2011, 106,077203.(50) Wang, B. M.; Liu, M.; Xia, B.; Ren, P.; Wang, L. Large ExchangeBias Obtained through Zero-field Cooling from an UnmagnetizedState. 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 inBulk 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. SuperspinGlass Mediated Giant Spontaneous Exchange Bias in a Nano-composite of BiFeO3-Bi2Fe4O9. Phys. Rev. Lett. 2013, 110, 107201.(54) Chauhan, S.; Srivastava, S. K.; Chandra, R. Zero-field CooledExchange 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. MagneticProperties 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 ElectronicPhase Separation with Magnetic Field in Bulk and NanometerPr0.67Ca0.33MnO3 Particles. J. Phys. Chem. C 2011, 115, 19482−19487.(59) Nogues, J.; Schuller, I. K. Exchange Bias. J. Magn. Magn. Mater.1999, 192, 203−232.

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp4096375 | J. Phys. Chem. C 2013, 117, 26351−2636026359

(60) Gajbhiye, N. S.; Bhattacharyya, S. Exchange Bias and Spin-Glass-Like Ordering in ε-Fe3N−CrN Nanocomposites. Jpn. J. Appl. Phys.2007, 46, 980−987.(61) Sirena, M.; Steren, L. B.; Guimpel, J. Magnetic Relaxation inBulk 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: aLikely Candidate to show Spin-glass Behavior at Low Temperatures. J.Phys.: Condens. Mater 2011, 23, 106002.

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp4096375 | J. Phys. Chem. C 2013, 117, 26351−2636026360