Published: July 11, 2011

r 2011 American Chemical Society 14673 dx.doi.org/10.1021/jp204765e | J. Phys. Chem. C 2011, 115, 14673–14677

ARTICLE

pubs.acs.org/JPCC

Ferromagnetic Behavior of Ultrathin Manganese NanosheetsSreemanta Mitra,†,‡ Amrita Mandal,†,‡ Anindya Datta,§ Sourish Banerjee,‡ and Dipankar Chakravorty*,†

†MLS Prof’s Unit, Indian Association for the Cultivation of Science, Kolkata-700032, India§University School of Basic and Applied Science (USBAS), Guru Gobind Singh Indraprastha University, New Delhi-110075, India‡Department of Physics, University of Calcutta, Kolkata-700009, India

’ INTRODUCTION

Manganese (Mn) is probably one of the most importantdopants for semiconductors for creating a ferromagnetic res-ponse.1�3,6�9 In addition, dilute solutions of Mn-based systemsexhibit a wide variety of magnetic properties.4,5 However, Mn byitself is not ferromagnetic, even though Mn is a d-shell-basedtransition metal in its bulk form. This fact gives rise to a funda-mental question about Mn's ground state, which is still not veryclearly understood.

The unique properties of manganese among the first-rowtransition metal elements as atoms, clusters, or crystals probablyarise because of its atomic configuration. Manganese has anexactly half-filled 3d orbital and a fully filled 4s orbital, with anelectron configuration of 3d54s2. The energy required to changethe electronic configuration from 3d54s2 to 3d64s1 is high enough(∼2.4 eV)10 to keep the former as its ground-state configuration.If two Mn atoms are brought closer together, the 3d and 4sorbitals split into bonding and antibonding states, so the mag-netic ground state (ferro- or antiferromagnetic) depends on theenergy of the splitting and the exchange interaction.10 Calcula-tions have shown that the ferromagnetic ground state is notenergetically stable for Mn.11 There is a great deal of debateamong different theoretical studies, even for the simplest dimermolecule Mn2, with regard to ferromagnetic and antiferromag-netic ground states.12�20 Spin density functional calculationsof manganese nanostructures such as nanowires or nanorodsshowed these structures to be in high-moment states, withmagnetic moments per atom having values in the range of 2.96�3.79 μB depending on the morphology of the nanostructure.21

Experimentalists have shown that small manganese clustersexhibit complex magnetic behavior with the signature of super-paramagnetism.22�25 According to theoretical understanding

and experimental findings, the magnetic property fluctuatesbetween ferromagnetic and antiferromagnetic ground states asthe cluster size changes from 40 to 80 atoms.39 Beyond a clustersize of 80 atoms, the structures slowly converge to that of thebulk, and the magnetic ordering becomes antiferromagnetic.However, some magnetic deflection experiments showed non-zero magnetic moments for manganese clusters of 11�99 atoms,indicating ferromagnetic ordering of atomic spins, which had alower limit on the number of atoms in a cluster than thatmentioned earlier.22 Thus, manganese exhibits contradictorymagnetic ground states from the standpoint of theory andexperiments. Induced ferromagnetism has been observed inmanganese on clean ferromagnetic substrates,26�28 whereas fora nonmagnetic substrate, monolayer deposition ofMn resulted inantiferromagnetic ordering.29 One-thousand-atom Mn clusterswith heights of 10 nm and diameters of 15�25 nm on Si(111)and Si(112) were found to exhibit temperature-dependentferromagnetic-like behavior below 10 K, as a result of somesurface orientation effect that might be correlated with thesurface dangling-bond density or cluster shape.30 However, forMn thin films, the reported ground state is antiferromagnetic innature.31,40 Although two-dimensional Mn crystalline systemswould be more stable compared to small clusters, their ferro-magnetic response remains illusive to date. Hence, the search formagnetic behavior in nonferromagnetic transition metals hasoften focused on the effect of reduced dimensions.22 To inves-tigate this effect, we have synthesized manganese nanosheetsinside the two-dimensional crystal channels of Na-4 mica, with a

Received: May 23, 2011Revised: June 16, 2011

ABSTRACT: Ferromagnetic behavior has been observed experimentally for thefirst time in nanostructured manganese. Ultrathin (∼0.6-nm) manganese nano-sheets were synthesized inside the two-dimensional channels of sol�gel-derivedNa-4 mica. The magnetic properties of the confined system were measuredwithin the 2�300 K temperature range. The confined structure was found toshow a ferromagnetic behavior with a nonzero coercivity value. The coercivityvalue remained nonzero throughout the entire temperature range of measure-ment. The experimental variation of the susceptibility as a function oftemperature can be satisfactorily explained on the basis of a two-dimensionalsystem with a Heisenberg Hamiltonian involving direct exchange interaction.

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very simple synthesis method. In this article, we report on theirferromagnetic behavior.

’SYNTHESIS AND CHARACTERIZATION

Na-4 mica (Na4Mg6Al4Si4F4O20 3 xH2O) template was syn-thesized by the usual sol�gel technique, taking aluminumnitrate, magnesium nitrate (used as obtained from E Merck),tetraethylorthosilicate (TEOS), and ethanol as precursors. Inbrief, to prepare 1 g of Na-4 mica powder, 2.186 g of aluminumnitrate and 2.241 g of magnesium nitrate were dissolved inethanol, and then the solution was stirred vigorously for 1 h toobtain a homogeneous mixture. To this mixture was added1.291 cm3 of TEOS to achieve the target composition. Nitricacid (0.1 N) was also added as a catalyst. This solution wasstirred for 3 h and placed in an air oven at 333 K for 3 days. Thedried gel was crushed and calcined at 748 K for 12 h. Equalamounts of gel powder and crystalline sodium fluoride (NaF)were mixed thoroughly and heated at 1163 K for 18 h in aplatinum crucible under ordinary atmosphere. This step wasneeded for the dissolution of the gel in NaF. The reactionproduct was washed thoroughly with saturated boric acidseveral times to remove the water-insoluble fluoride salts.The powder was then washed again with 1 M NaCl solutionthree times to completely saturate all exchange sites with Na+

ions. The resultant product was then washed with deionizedwater several times and dried at 333 K in an air oven to yieldpure Na-4 mica powder.

The unit cell of Na-4 mica has a layered structure with aninterlayer spacing of 0.6 nm.32�34,38 A 2.36 g sample ofNa-4micapowder was then subjected to the ion-exchange reaction 2Na+SMn2+ by soaking it in a mixture of manganese nitrate [Mn-(NO3)2] and dextrose (C6H12O6) in aqueous solution at anelevated temperature (368 K) and pressure inside a Teflon-coated autoclave cell for 5 days. The pH of the solution was keptneutral throughout the ion-exchange process. The latter couldoccur only in the case of ions that were mobile. As such, in thepresent case, ion-exchange reactions with the other species,namely, Al, Mg, and Si, are ruled out. The resultant powderwas removed from the autoclave and washed thoroughly withdeionized water several times to ensure that no manganesenitrate molecules were present on the surface of the Na-4 micapowder. This was confirmed by a simple chemical group testanalysis: Sodium carbonate was added to the filtrate, and nowhite precipitate of manganese carbonate was obtained. Thewashed powder was then put in an alumina boat and placed in amuffle furnace at 675 K under ordinary atmosphere for 2 h. Thecarbon of the dextrose molecules reduced the Mn ions into Mnmetal while generating carbon dioxide. X-ray diffraction of thematerial was performed using a Bruker D-8 SWAX X-raydiffractometer with a Cu KR monochromatic source of wave-length 0.15408 nm. To study the microstructure, Mn nanosheetswere extracted from the mica channels by etching the compositesample with 10% HF aqueous solution and centrifuged in aSORVALL RC 90 ultracentrifuge at 30000 rpm for 30 min. Theresultant residue was washed thoroughly with deionized waterand then dispersed in acetone. From that dispersion, a drop wastaken and investigated in a JEOL 2010 transmission electronmicroscope operated at 200 kV. The magnetic properties of thecomposite were studied with an MPMS SQUID magnetometer(Quantum Design) in the temperature range of 2�300 K.

’RESULTS AND DISCUSSION

Figure 1 shows the X-ray diffraction pattern of the compositesample in the range 2θ = 5�80o. The presence of manganese wasconfirmed by the standard JCPDS value (file number 17-0910).Only the (200) plane of manganese was found to grow inside thetwo-dimensional mica nanochannels. The rest of the XRD linesoriginated from the basic structure of Na-4mica.35,36 The c axis ofNa-4 mica was found to be perpendicular to the (200) plane ofMn, allowing growth along this direction only. Figure 2a shows atransmission electron micrograph of the randomly assembledpartially etched nanocomposites containing Mn nanosheets.Figure 2b shows an enlarged view of the same assembly. Thenanosheets formed from manganese nanodisks of circular andrhombus-like structures. As the manganese nanosheets formedwithin the layers of Na-4 mica, their thickness was limited by thechannel thickness (i.e., 0.6 nm). Figure 2c shows a high-resolu-tion lattice image of one of the nanosheets in which the (200)plane of Mn can be observed along with the lattice planescorresponding to Na-4 mica. The interplanar spacing was found

Figure 1. XRD pattern of manganese and Na-4 mica composite.Numbers indicate the corresponding interplanar spacings (in

�A).

Figure 2. (a) Transmission electron micrograph of manganese nano-sheets, (b) enlargement of a portion of the image in panel a, (c) high-resolution lattice image of one manganese nanosheet, and (d) selected-area electron diffraction pattern of the region in panel b.

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to be 0.133 nm. As a result, the interatomic distance was0.266 nm. From the TEM images, the sizes of the nanosheetswere found to be around 70�150 nm. Figure 2d shows aselected-area electron diffraction (SAED) pattern of one of thenanosheets. As the sample was partially etched, some spots in theSAED pattern correspond to the Na-4 mica structure, whereasspots due to the (200) plane of manganese also arose, asexpected. The results are summarized in Table 1. To determinethe thickness of the manganese nanosheets synthesized in thepresent work, atomic force microscopy (Veeco model CP IImicroscope) was used. The height profiles of the nanosheetswere obtained by etching the composite powder in 10% HFaqueous solution for 4 days and dispersing it on a freshly cleavedatomically flat mica (SPI Supplies, West Chester, PA) surface. Atypical profile is shown in Figure 3. The heights measured wereeither 0.6 nm or an integral multiple thereof, which confirms thatthe manganese nanosheets were indeed grown within the nano-channels of the Na-4 mica structure. The AFM images gave athickness value equal to the thickness of the interlayer space inNa-4 mica reported in the literature.32�34 These facts led us toconclude that the original films were no thicker than 0.6 nm and,hence, dissolution of manganese during etching did not takeplace. Magnetic measurements were carried out for the composite

powder in the temperature range of 2�300 K. Figure 4 showsthe variation of the magnetization as a function of tempera-ture measured at an applied magnetic field 5 mT under bothzero-field-cooled (ZFC) and field-cooled (FC) conditions. Theabsence of any local maxima in the ZFC magnetization�temperature curve indicates ferromagnetic coupling betweenthe spins. This is borne out by the magnetization�magneticfield hysteresis curvemeasured at 2 K, which is shown in Figure 5.In the inset of Figure 5, the enlargement of the hysteresiscurve near zero magnetization shows a nonzero coercivity value(∼60 Oe). The magnetization�magnetic field hysteresis curvefor the composite at 300 K is also shown in Figure 6. A finitecoercivity present here (as observed from the inset) also indicatesferromagnetic behavior even at room temperature. The satura-tion magnetization was not achieved even at maximum field,indicating that all spins cannot become parallel to the magneticfield because of the thermal energy. Figure 7 shows the magne-tization as a function of magnetic field in the case of pure Na-4mica, which exhibits diamagnetic characteristics. This contribu-tion was subtracted from the results presented herein. Regardingthe Curie temperature of the ultrathinMn films, we believe that itis above room temperature in view of the fact that magnetichysteresis was observed at room temperature. The precise nature

Table 1. Interplanar Spacings Estimated from Electron Dif-fraction Data and JCPDS Filea.)

observed (nm) Na-4 mica (nm) manganese (nm)

0.30 0.303 (023) �b

0.26 0.263 (201) �0.20 0.202 (006) �0.18 0.186 (205) �0.17 0.173 (205) �0.16 0.166 (135) �0.141 0.143 (007) �0.133 � 0.1336 (200)

aNumbers in parentheses describe the Miller indices of the correspond-ing lattice planes. b Indicates that no planes with these interplanarspacings exist in the corresponding phases.

Figure 3. AFM images of manganese nanosheets and the correspond-ing height profile.

Figure 4. Variation of magnetization (M) with temperature (T) underboth field-cooled (FC) and zero-field-cooled (ZFC) conditions mea-sured at 5 mT.

Figure 5. Variation of magnetization (M) with magnetic field (H)measured at 2 K. Inset: Enlargement of the region near zeromagnetization.

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of the effect of size on the magnetic behavior cannot be inferredfrom our experimental data because the latter pertain to only onethickness of Mn nanosheet, namely, 0.6 nm. Also, the EDAX data(measured in a JEOL JSM-6700F field-emission scanning elec-tron microscope) in Figure 8 show that no magnetic impurity ispresent in the nanocomposite under investigation. The EDAXdata show that the amount of oxygen atoms present is justsufficient to fulfill the stoichiometric needs of the aluminum,magnesium, and silicon present in the Na-4 mica phase. Hence, itcan be safely concluded that there is no possibility for theformation of oxides of manganese in our system. This is alsoconsistent with the fact that the Mn nanosheets were formed bysubjecting Mn to reduction treatment at 675 K.

To determine the possible origin of this ferromagnetism, ofthe two exchange interactions possible (viz., direct and indirect),we ruled out the possibility of an indirect exchange interaction,as the hopping integral decays exponentially with the dis-tance between magnetic centers, and considered only thedirect exchange interaction.20,21 We applied the HeisenbergHamiltonian

H ¼ � 2X

ijh iJijSi 3 Sj ð1Þ

based variation of susceptibility with temperature for a two-dimensional ferromagnet37

χ ≈ expðβ=TÞ ð2Þwith

β ¼ ð4πJS2Þ=kB ð3Þwhere J is the exchange coupling constant, S is the spin quantumnumber, and kB is the Boltzmann constant. The calculation wasperformed by taking S = 5/2 for manganese and J/kB as aparameter. The fitting looks comprehensive, and the positiveJ value found indicates ferromagnetic coupling. Both the experi-mental data and the fitted curve are shown in Figure 9.

A previous theoretical investigation did arrive at the conclu-sion that monolayers and bilayers of Mn grown on the tungsten-(100) plane have a ferromagnetic ground state.41 Consideringthe thickness of 0.6 nm for our manganese films, our experi-mental results on their ferromagnetic behavior are consistentwith the above-mentioned theoretical calculations. Also, theore-tical predictions show that the obtained interatomic distance ofthe manganese atoms ensures a ferromagnetic response.15

’CONCLUSIONS

In summary, we have synthesized manganese nanosheets witha thickness of 0.6 nm in the nanochannels of layered Na-4 mica

Figure 6. Variation of magnetization (M) with magnetic field (H)measured at 300 K. Inset: Enlargement of the region near zeromagnetization.

Figure 7. Variation of magnetization (M) with magnetic field (H) forNa-4 mica.

Figure 8. EDAX analysis of the nanocomposite.

Figure 9. Variation of susceptibility (χ) with temperature (T) (opencircles, experimental data; solid line, theoretically fitted curve).

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by a simple process. These nanosheets are made up of nanodiskswith sizes of about 70�150 nm. The nanodisks consist of the(200) planes of Mn, aligned parallel to the nanochannel itself,as confirmed both by X-ray diffraction and selected-area electrondiffraction experiments. This confined Mn structure exhibitsferromagnetic behavior in its ultrathin configuration, as is evi-dent from the magnetization measurements. Calculation of thevariation in magnetic susceptibility as a function of temperatureon the basis of the Heisenberg ferromagnetic model involvingdirect exchange interaction matches the experimental datasatisfactorily.

’AUTHOR INFORMATION

Corresponding Author*E-mail: mlsdc@iacs.res.in.

’ACKNOWLEDGMENT

The work was supported by a grant awarded by NanoMissionCouncil, Department of Science and Technology, New Delhi,India. S.M. and A.M. thank the University Grants Commission,New Delhi, India, for Junior Research Fellowships. D.C. thanksthe Indian National Science Academy for awarding an HonoraryScientist’s position. S.M. thanks Dr. Molly De Raychaudhuryfor fruitful discussions. Support was partly derived from agrant received from Department of Science and Technology,New Delhi, India, under an Indo-Australian Project on Nano-composites.

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