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Origin of Magnetic Anomalies below the Ne´el Temperature in Nanocrystalline LuMnO3 Raja Das, Adhish Jaiswal, Suguna Adyanthaya, and Pankaj Poddar* Physical & Materials Chemistry DiVision, National Chemical Laboratory, Pune-411 008, India ReceiVed: April 5, 2010; ReVised Manuscript ReceiVed: June 13, 2010

Rare earth manganites crystallize in distorted orthorhombic perovskite or hexagonal structures and exhibit quite interesting optical and magnetic properties dictated by the size of the rare earth ion. Many of these materials might exhibit both ferroelectric and magnetic ordering as well as magnetoelectric coupling. However, their physical properties at reduced particle sizes remain underexplored due to the challenges associated with their synthesis with a proper control over the crystalline phase. Here, we report the wet-chemical synthesis of the hexagonal phase of nanocrystalline LuMnO3 with an average crystallite size of ∼32 nm. The roomtemperature Raman spectroscopy data are consistent with the calculated values of isomorphous hexagonal RMnO3 (R ) rare earth atom) compounds with P63cm symmetry. The UV-vis-NIR spectra recorded in the diffused reflectance mode at room temperature show electronic transitions at 1.7 eV (729 nm), 2.3 eV (539 nm), and 5 eV (258 nm). The magnetization measurements show that the Ne´el temperature for the LuMnO3 is situated at around 89 K, which is in close proximity to the reported value of the bulk phase. We also observed two unique and field-dependent magnetic anomalies that were predicted earlier but never reported experimentally. The first anomaly is observed as a sharp bifurcation in the ZFC-FC curves below 44 K at a 100 Oe applied field, which is accompanied with a sudden rise in the coercivity and magnetization. A second transition is observed at 12 K as a sharp peak in the ZFC curves, which is accompanied with a dip in coercivity. We attribute the transition at 44 K to the reorientation of the Mn3+ ions due to the Dzyaloshinskii-Moriya interaction, and the transition at 12 K is explained by weak antiferromagnetic coupling between Mn-O-Mn in the ab plane, which becomes dominant at lower temperatures. 1. Introduction The binary and ternary oxides have been investigated for their rich optical, ferroelectric, and magnetic properties.1-16 Several of the compounds from the rare earth manganite, chromite, and ferrite RBO3 (R ) rare earth ion, B ) Cr, Mn, Fe) family are believed to show multiferroic properties and adopt two types of structures depending upon the radius of the rare earth atoms and synthesis conditions, etc.1,2,5,17,18 In the case of R ) La, Ce-Dy, the RMnO3 compounds crystallize in the distorted orthorhombic perovskite structure.17,19,20 On the other hand, for R ) Ho-Lu, Y, Sc, a hexagonal structure is formed, where Mn3+ ions are coordinated in trigonal bipyramid geometry to surrounding O2- ions and form a pseudolayered structure by corner sharing of the trigonal basal plane O2- ions.17,20,21 These MnO5 bipyramids tilt and shift with respect to the R3+ cations to form a noncentrosymmetric structure, resulting in the ferroelectric polarization along the c axis.1,19,21 For example, in YMnO3, anomalies in the dielectric constant values were observed near the Ne´el temperature, showing the signature of magnetoelectric coupling.22 It is also possible to crystallize these materials in the orthorhombic phase (for R ) Ho-Lu, Y) either at very high pressures23,24 or by using special low-temperature soft chemical synthesis routes25 and epitaxial thin-film growth processes.26 The Ne´el temperature (TN) in rare earth manganites is highly dependent on the ionic radii of rare earth ions and decreases from 141 K for LaMnO3 to 40 K for TbMnO3.25 In hexagonal rare earth manganites, in-plane magnetic interactions between Mn3+ ions are prominent in comparison to the adjacent triangular planes via Mn-O-Mn antiferromagnetic superex* To whom correspondence should be addressed. E-mail: p.poddar@ ncl.res.in.

change interaction due to a relatively shorter in-plane Mn-Mn distance (3.5 vs 6 Å for the adjacent planes). The overall magnetic interaction in the in-plane Mn ions is decided by the size and magnetic property of the rare earth ions present in the noncoplanar layer (between two adjacent planes).27 In additional to the ordering of Mn spins, some of these rare earth manganites show an additional antiferromagnetic ordering transition much below the Ne´el temperature due to relatively weaker superexchange interactions between rare earth spins.17 Among these manganites, the LuMnO3 is quite interesting as it shows magnetic as well as ferroelectric dipolar coupling simultaneously below the Ne´el temperature where the geometrical frustration of Mn spins leads to a different triangular lattice arrangement at lower temperatures (far below their respective Ne´el temperatures).27 Such a type of long-range magnetic ordering depends not only on the temperature and magnetic field but also on factors, such as the size of the rare earth ion, interatomic distances, and angles in the structure.27 Smaller rare earth ions lead to more than one spin reorientation transition due to relatively enhanced geometrical frustration. With a decrease in the size of the rare earth ion, oxygen atoms readjust to maintain connectivity with rare earth and manganese ions, leading to overall tilting of the MnO5 planes. As the temperature goes below the Ne´el temperature, distortion in the lattice increases; such an enhanced geometrical frustration in LuMnO3 at low temperatures arises because of trimerization of Mn ions, which leads to the displacement of various atoms, resulting in the tilting of the MnO5 bipyramid. This is the reason that manganites with smaller rare earth ions, such as LuMnO3, were predicted to show magnetic ordering at much lower temperatures, whereas InMnO3 does not show such magnetic ordering at low temperatures.27,28 The spin reorientation transi-

10.1021/jp103037r  2010 American Chemical Society Published on Web 06/28/2010

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Figure 1. Comparison between (A) the powder X-ray diffraction pattern of LuMnO3 nanocrystals with (B) the data from JCPDS Card No. 200650 for the orthorhombic phase and (C) JCPDS Card No. 140030 for the hexagonal phase. The comparison shows that LuMnO3 crystallizes in the hexagonal phase.

tion as well as ordering at further lower temperatures for LuMnO3 was predicted but never reported experimentally so far. In this paper, we have revisited this interesting material by synthesizing it using a simple wet-chemical technique to explore the field- and temperature-dependent magnetic behavior. Below, we present a simple route to synthesize nanocrystalline LuMnO3 in the hexagonal phase. We will discuss the magnetic and optical properties of nanocrystalline h-LuMnO3. 2. Results and Discussion Hexagonal nanocrystalline LuMnO3 was prepared using a simple modified hydrothermal process, followed by annealing at 750 °C.5,9 In this reaction, stoichiometric amounts of lutetium (III) nitrate hydrate (Lu(NO3)3 · xH2O, Aldrich, 99.9% purity) and manganese (II) nitrate x hydrate (Mn(NO3)2 · xH2O, Aldrich, 99.9% purity) and an equal amount of citric acid (C6H8O7, metal/ citric acid molar ratio ) 1/1, Merck, 99.5% purity) were dissolved in deionized water. The transparent solution was stirred for 6 h, followed by the dropwise addition of ammonia solution (28 wt %) to neutralize the unreacted citric acid as well as to raise the pH value of the solution near 9.2, resulting into a sol formation. Further stirring for 3 h resulted in a brick red solution that was transferred into an 80 mL capacity autoclave with a Teflon liner. After the hydrothermal treatment at 150 °C for 20 h, the precipitate was, in turn, filtered, washed with deionized water, and finally dried at 100 °C. Finally, the powder sample was calcined at 750 °C for 6 h and used for further study reported in this paper. Figure 1 shows an X-ray diffraction pattern (XRD) of LuMnO3 obtained by a PANalytical X’PERT PRO instrument using iron-filtered Cu KR radiation (λ ) 1.5406 Å) in the 2θ range of 10-80° with a step size of 0.02°. We compared the experimental data with the reference data for hexagonal (JCPDS file no. 140030) and orthorhombic (JCPDS file no. 200650) phases of bulk LuMnO3. The XRD pattern matches nicely with the hexagonal form of LuMnO3 with a small variation in the relative peak intensity due to its shape anisotropy. The diffraction peaks are quite broad, showing the formation of LuMnO3 nanocrystallites with a mean crystallite size calculated as ∼32 nm. Additionally, our XRD results did not find any signature of Mn3O4 or any other impurity phase. To further investigate the microstructure and morphology of the as-synthesized material, we used the FEI (model Tecnai F30) high-resolution transmission electron microscope (HRTEM) equipped with a

Figure 2. Transmission electron micrograph of LuMnO3 at different scales taken from various parts of the same TEM grid (A, B) and selected area diffraction pattern of LuMnO3 nanoparticles (C).

field emission source operating at 300 kV to image the LuMnO3 nanocrystals on carbon-coated copper TEM grids. The TEM images of varied resolution shown in Figure 2A,B suggest that particles are with a platelike morphology and nanocrystalline in nature despite sintering due to the calcinations required to get the desired hexagonal phase. In panel C (Figure 2), we have shown the selected area electron diffraction pattern (SAED) that again confirms the formation of a hexagonal phase consistent with the powder XRD profile. We also recorded the Raman spectra at room temperature on an HR 800 Raman spectrophotometer (Jobin Yvon Horiba, France) using monochromatic radiation emitted by a He-Ne laser (633 nm), operating at 50 mW and with a spectral resolution of 0.3 cm-1. The Raman results are shown in Figure 3A. In the absence of any previous report on the roomtemperature Raman spectra on LuMnO3 in the literature, we compared our results with another closely related structure, YMnO3,29 which crystallizes in a similar P63cm space group symmetry. From the group theoretical analysis for the Γ-point

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Das et al. TABLE 1: Comparison between the Room-Temperature Raman Mode Positions (cm-1) from Our Present Study on LuMnO3 Nanoparticles (Figure 3A) and Reported Data on the Isomorphic Structure YMnO3 by Iliev et al. and Calculated Data on the Same Symmetric Structure (Iliev et al.)29

Figure 3. (A) Room-temperature Raman spectra of nanocrystalline LuMnO3. The arrows representing Raman shifts are tabulated in Table 1. (B) UV-vis-NIR spectra of LuMnO3 at room temperature in diffused reflectance mode. The inset shows the zoom-in view of part of the same curve. (C) X-ray photoelectron spectroscopy spectra of Mn 2p. The lines represent the deconvoluted peaks.

phonon modes of hexagonal RMnO3, 38 (9A1 + 14E1 + 15E2) modes are Raman-active. In Table 1, we have compared our results with the reported values on YMnO3 and calculated value for P63cm. The experimental values are in reasonable agreement with the calculated values. Figure 3B shows the UV-vis-NIR spectra in diffuse reflectance mode at room temperature. The measurements were done using a Jasco UV-vis-NIR spectrometer (model V570) operated at a resolution of 2 nm. We observed three peaks situated at around 1.7 eV (729 nm), 2.3 eV (539 nm), and 5 eV (248 nm) for the hexagonal phase of LuMnO3. The peak situated around 729 nm is attributed to the intersite optical transition from the hybrid occupied state with dxy/dx2-y2 orbital symmetry to the unoccupied Mn d3z2-r2 state, and the peak around 539 nm is attributed to the intersite optical transition from the hybrid occupied state with dyz/dzx orbital symmetry to the unoccupied Mn d3z2-r2 state.30 The 248 nm peak is due to the charge transfer transition from the hybridized oxygen 2p level to the Mn d3z2-r2 level.30 It should be noticed that these peaks are only observed for the hexagonal phase of RMnO3 compounds. On the other hand, for the orthorhombic phase, these peaks are either very weak in intensity or entirely absent, which is in accordance to the selection rules for the on-site Mn d-d transitions in the hexagonal and (near) cubic symmetries.30 The electronic transition spectra, therefore, are consistent with the powder XRD observations. In Figure 3C, we have shown the room-temperature X-ray photoelectron spectroscopy (XPS) results taken at high resolution (0.1 eV) using a VG Microtech, model number ESCA 3000, equipped with an ion gun (EX-05) for cleaning the surface spectra. We used a Shirley algorithm for background correction, and chemically distinct species were resolved using a nonlinear least-squares fitting procedure. The core-level binding energies were aligned with the carbon binding energy of 285 eV. The XPS spectrum of

Raman modes

our study

Iliev et al.

calculated

A1 A1 A1 A1 A1 A1 A1 A1 A1 E1 E1 E1 E1 E1 E1 E1 E1 E1 E1 E1 E1 E1 E1 E2 E2 E2 E2 E2 E2 E2 E2 E2 E2 E2 E2 E2 E2 E2

146 192 257 297

148 190 257 297

229 490

433 459

685 114

681

147 204 222 299 388 423 492 588 662 117 147 158 212 233 250 353 390 410 459 492 559 586 635 71 108 136 161 212 241 245 336 382 407 458 515 557 580 638

159

389 412

376 408

559 636 72 105 136 173 215 233 245 338 375 514 581

632 135 215 302

Mn3+ 2p shows two prominent peaks at 642.3 eV (2p3/2) and 654.2 eV(2p1/2). The slight hump centered at ∼646.4 eV is quite weak in intensity and can be considered as a satellite peak due to a shakeup process. The Mn3+ cations in R-Mn2O3 show peaks at 641.9 and 653.5 eV (within the error of 0.2 eV).31 In our case, the shift in Mn3+ peaks by 0.4 eV is probably due to the change in the crystal field environment in comparison to R-Mn2O3. It worth mentioning here that our XPS results, which are highly sensitive, did not find any trace of Mn2+, thereby ruling out any possibility of a Mn3O4 impurity. Next, to check the possibility of a predicated magnetic transition in LuMnO3 below the Ne´el temperature, we performed dc magnetic susceptibility versus temperature and magnetic field versus magnetization measurements using a Physical Property Measurement System (PPMS) from Quantum Design Inc., San Diego, CA., equipped with a 7 T superconducting magnet and a vibrating sample magnetometer operating at 40 Hz. Temperature-dependent M-H loop data were collected in a field sweep from -50 to +50 kOe at a rate of 25 Oe/s by, first, demagnetizing the sample by heating it at 250 K in zero field before cooling it to the desired temperature. Figure 4A shows that M-H curves at higher temperatures are almost linear, indicating the existence of a paramagnetic phase; however,

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Figure 4. (A) M-H loops for nanocrystalline LuMnO3 shown at various temperatures. (B) Initial magnetization curves at different temperatures, showing a nonlinear behavior below 90 K. (C) Zoom-in view of the M-H curves. (D) Variation of coercivity with temperature; the arrow shows the anomalies that were also seen in the M-T curves.

below 90 K, we observed the onset of a small nonlinear behavior (also evident from initial magnetization vs applied magnetic field curves in Figure 4B) with zero coercivity, indicating the onset of a Ne´el transition. In Figure 4C, we have shown the zoom-in view of M-H loops, and 4D shows coercivity versus temperature curve. The nonlinear behavior is followed by a sharp increase in the coercivity value below ∼45 K, which opens up further at lower temperatures along with an increasing nonlinear behavior. At 14 K, the M-H loop showed a dip in the coercivity from 90 Oe at 45 K to 78 Oe at 15 K. Below 14 K, the coercivity again shows an increasing trend. We will discuss the possible origin of these anomalies later. It can also be noticed that, at any of the measured temperatures, the magnetization does not saturate up to a 50 kOe magnetic field due to paramagnetic or antiferromagnetic character of the material. There is measurable remance magnetization (Mr) below TN, which may be due to the presence of surface defects/unsatisfied surface spins that develop in the crystals during the crystallization process as well as the contribution of nonmagnetic ions (Lu3+), which act as a domain wall, pinning centers and results in Mr.32 It is observed that Mr decreases as the temperature increases. The possible reason might be the flux trapped by the defects in the LuMnO3 nanocrystals that get excluded as the temperature increases.33 We also performed the dc magnetization versus temperature measurements in various field conditions in a broad temperature range from 3 to 350 K with cooling and heating rates of 2 K/min. In Figure 5A, we have shown the magnetic susceptibility (χ) and the inverse of magnetic susceptibility (1/χ) versus temperature for the LuMnO3 nanocrystals at a 100 Oe applied field. Here, an anomaly was observed below 89 K (shown as TN), which is in the close proximity to the Ne´el temperature observed for bulk LuMnO3.34 Below this temperature, the magnetic susceptibility curves deviated from the ideal Curie-Weiss behavior as Mn3+ sublattice antiferromagnetic ordering starts to set in through the superexchange interaction. Above this transition temperature, both ZFC and FC curves follow the Curie-Weiss law (as shown by the straight line). Below the Ne´el transition, we see two more sharp magnetic anomalies

marked by T1 and T2 in Figure 5A. The anomaly at T1 ) 44 K is witnessed by a sharp bifurcation between ZFC-FC curves and a sudden rise in the magnetization with decreasing temperature. Another anomaly has been observed in the ZFC curve at T2 ) 12 K. To see the field-dependent response of these curves, we measured the ZFC-FC curves at various field values from 50 Oe to 20 kOe (shown in Figure 5B,C). Here, we observe that the antiferromagnetic transition at ∼89 K does not depend on the applied external field up to the highest applied field of 20 kOe, probably due to the relatively strong antiferromagnetic exchange forces between Mn3+ ions. It is worth pointing out here that these anomalies at 44 and 12 K observed in our coercivity versus temperature and magnetization versus temperature curves were reported in its isostructural counterpart InMnO3.28 The earlier research efforts where the ZFC-FC measurements were done at much higher applied fields showed only the Ne´el transition in the form of a deviation in the ZFC-FC curve below 90 K.17,34 We believe that higher applied magnetic fields might have masked these relatively weaker magnetic anomalies, as seen by us in the coercivity and magnetization. Earlier reports on LuMnO3 did not attempt to check the coercivity variation with temperature. We attributed the bifurcation observed at 44 K to reorientation transition of Mn3+ spins, resulting into a sharp jump in the coercivity and net magnetization. For InMnO3 and ScMnO3, which have the same P63cm symmetry, this type of transition is seen at around 43 K.28 Our field-dependent ZFC-FC studies revealed that this anomaly T1 is highly field-dependent and shifts from 42 to 52 K as the field value decreases from 1000 to 25 Oe and disappears completely at 20 kOe (shown in Figure 5C). The origin of the spin reorientation can be explained by the onset of Dzyaloshinskii-Moriya interactions (DM) with deceasing temperature.35 The canting/reorientation of spins due to DM interaction results in the rise in coercivity and magnetization below T1. Due to an 4f14 outermost electronic configuration in Lu3+ ions, S ) 0, thereby denying it any direct role in magnetism through an exchange interaction on the Mn3+ spins, but the size of Lu atoms of course plays an important role in deciding the atomic

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Das et al. commented that such magnetoelectric coupling is observed at the Ne´el temperature, but they failed to mention that this magnetoelastic coupling starts from 40 to 50 K in proximity to the magnetic reorientation transition due to DM interactions.18 The other new anomaly observed at 12 K (marked as T2), which was reflected as a sharp peak, was also not reported previously for LuMnO3.17 We believe that this transition is of antiferromagnetic in origin, which is also accompanied with a decrease in the coercivity at 12 K (as discussed above). We also observed that, unlike the TN seen at ∼90 K, this rather weak antiferromagnetic superexchange coupling (T2) between Mn-O-Mn in the ab plane highly depends on the applied magnetic field and disappears at 1000 Oe. In hexagonal manganites, the in-plane Mn3+ ions interact much more strongly (through an antiferromagnetic Mn-O-Mn superexchange interaction) than adjacent triangular planes due to the relatively shorter distance of in-plane Mn-Mn ions (3.5 Å) than in the adjacent planes (∼6 Å).27 The geometrical distortion due to the size of rare earth ions creates differences in the Mn-O-Mn bond lengths and angles in the ab plane, leading to the appearance of various triangular ordered antiferromagnetic arrangements in LuMnO3 at low temperatures. 3. Conclusion We successfully synthesized h-LuMnO3 nanoparticles using the hydrothermal method. The XRD, UV-vis, and Raman spectra confirm the hexagonal phase with P63cm symmetry. The magnetic measurements revealed two new field-dependent transitions at 44 and 12 K. These transitions are attributed to Mn3+ ion reorientation and weak antiferromagnetic coupling between Mn-O-Mn in the ab plane. Low-temperature Raman and dielectric investigations will further confirm the origin of these transitions and possible phonon-magnon coupling.

Figure 5. (A) χ and 1/χ vs temperature curves at 100 Oe. The curves show three anomalies at temperatures TN (deviation of the 1/χ from a linear behavior), T1 (bifurcation temperature of the ZFC-FC curve), and T2 (sharp peak in the ZFC curve). (B) ZFC-FC curves at 50 and 125 Oe applied fields. (C) ZFC-FC curves at 1000 Oe and 20 kOe applied fields. The inset shows the variation of ordering temperature, T1, with applied field.

arrangement, thereby affecting the magnetic properties significantly. Unlike LuMnO3, the rare earth manganites with a larger ionic radius of rare earth ions do not show any further longrange magnetic ordering below the Ne´el temperature.28 The antiferromagnetic ordering in LuMnO3 results in a large atomic displacement around the Ne´el temperature, where the lattice constant values change in such a way that the overall symmetry remains unchanged. Such kinds of isostructural transitions (which are rarely observed) are also the driving force for the gigantic magnetoelastic coupling in the manganites, as seen in YMnO3.18 Lee et al. have reported that, in YMnO3, below the Ne´el temperature, the positions of Mn ions drastically change to make an ordered triangular arrangement in the ab plane.18 Such an abnormally large shift in the Mn3+ position starts from 40-50 K to the Ne´el temperature at ∼89 K.18 This large atomic displacement produces a coupling between the electric and magnetic dipoles from 40 to 89 K. However, Lee et al

Acknowledgment. P.P. acknowledges the financial support from the “Department of Science and Technology, India (DST)”, through Grant No. SR/S5/NM-104/2006 under the “Nano Mission” program and ARMREB/MAA/2008/104 by ARMREB, DRDO, India. A.J. acknowledges the support from the Council of Scientific and Industrial Research, India (CSIR), for providing the Senior Research Fellowship. R.D. acknowledges funding for his project assistantship from DST and ARMREB, DRDO. References and Notes (1) Kimura, T.; Goto, T.; Shintani, H.; Ishizaka, K.; Arima, T.; Tokura, Y. Nature 2003, 426, 55–58. (2) Tokunaga, Y.; Iguchi, S.; Arima, T.; Tokura, Y. Phys. ReV. Lett. 2008, 101, 097205. (3) Anderson, P. W. Phys. ReV. 1950, 79, 350–356. (4) Verwey, E. J. W. Nature 1939, 144, 327–328. (5) Jaiswal, A.; Das, R.; Vivekanand, K.; Maity, T.; Abraham, P. M.; Adyanthaya, S.; Poddar, P. J. Appl. Phys. 2010, 107, 013912. (6) Poddar, P.; Fried, T.; Markovich, G. Phys. ReV. B 2002, 65, 172405. (7) Poddar, P.; Fried, T.; Markovich, G.; Sharoni, A.; Katz, D.; Wizansky, T.; Millo, O. Europhys. Lett. 2003, 64, 98–103. (8) Woods, G. T.; Poddar, P.; Srikanth, H.; Mukovskii, Y. M. J. Appl. Phys. 2005, 97, 10C104. (9) Jaiswal, A.; Das, R.; Vivekanand, K.; Abraham, P. M.; Adyanthaya, S.; Poddar, P. J. Phys. Chem. C 2010, 114, 2108–2115. (10) Kumar, U.; Shete, A.; Harle, A. S.; Kasyutich, O.; Schwarzacher, W.; Pundle, A.; Poddar, P. Chem. Mater. 2008, 20, 1484–1491. (11) Swaminathan, R.; McHenry, M. E.; Poddar, P.; Srikanth, H. J. Appl. Phys. 2005, 97, 10G104. (12) Poddar, P.; Srikanth, H.; Morrison, S. A.; Carpenter, E. E. J. Magn. Magn. Mater. 2005, 288, 443–451. (13) Poddar, P.; Gass, J.; Rebar, D. J.; Srinath, S.; Srikanth, H.; Morrison, S. A.; Carpenter, E. E. J. Magn. Magn. Mater. 2006, 307, 227–231. (14) Bansal, V.; Poddar, P.; Ahmad, A.; Sastry, M. J. Am. Chem. Soc. 2006, 128, 11958–11963.

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