Journal of the Korean Physical Society, Vol 62, No 12, June 2013, pp 2133∼2138 Optical Spectra of the Colloidal Fe-doped Manganate CaMn1−x Fex O3 (x = 0, 0.01, 0.03, 0.05) Duc Huyen Yen Pham, Duc Tho Nguyen, Duc Thang Pham and Nam Nhat Hoang∗ Faculty of Technical Physics and Nanotechnology, Vietnam National University, University of Engineering and Technology, 144 Xuan Thuy, Cau Giay, Ha Noi, Viet Nam The Tan Pham Faculty of Basic Science, Hung Yen University of Technology and Education, Hung Yen, Viet Nam (Received 31 May 2012, in final form 19 December 2012) We report the optical behaviors of the Fe-doped CaMnO3 family of compounds at low doping concentrations x ≤ 5% The study aims at assisting the evaluation of the competition between ferroand antiferromagnetic orderings, which is believed to be a cause of many interesting properties of this class of compounds, including the magnetization reversal effect recently discovered The structural characterization showed a predominant orthorhombic phase with slightly increased cell constants due to doping The Raman spectra revealed changes associated with the Mn sites, and the IR absorption spectrum showed a characteristic Fe band at 1.2 eV, which should be accompanied by a change of spin The analysis of the magnetization data allowed us to predict that while the doping reduced the ferromagnetic coupling strength, and therefore the TC , the maximal doping concentration for the effective exchange to be zero was around 14% PACS numbers: 75.47.Lx; 75.50.Ee; 74.25.Fy; 75.30.Kz Keywords: Perovskite, Manganate, Structure, Optical DOI: 10.3938/jkps.62.2133 I INTRODUCTION The multiferroics based on doped CaMnO3 have attracted much attention from scientists because of their many potential applications in modern spintronics [1] Recently, Fe-doped CaMnO3 [2] was reported to show a magnetization reversal effect (MRE) in response to temperature at low fields and low doping contents This effect may be of extreme importance for temperaturecontrol devices Earlier reports indicated that this effect in ferrimagnets was found in YVO3 [3]; (the temperature at which the magnetization reversal response happened was around 80K, which is twice as large as that (40 K) in CaMn1−x Fex O3 , LaVO3 [4,5] and spinel Co2 VO4 [6] In doped CaMnO3 , the effect may also be observed in Gd1−x Cax MnO3 [7] and Dy1−x Cax MnO3 [8] The chrome-based perovskite compound La1−x Prx CrO3 [9] also revealed a magnetization reversal in response to temperature Unlike YVO3 whose MRE is associated with a first-order phase transition and is believed to originate from the different alignments of the spins of the sublattices according to temperature (from the presence of a so-called anti-symmetric Dzyaloshinsky-Moriya (DM) ∗ E-mail: namnhat@gmail.com interaction between two nearest-neighbor V ions), the MRE in Fe-doped CaMnO3 was not accompanied by structural changes and seemed to follow from a competition between antiferromagnetic (AFM) and ferromagnetic (FM) orderings within a unified structural frame Because the development of the FM interaction is important for understanding the physics of Fe-doped CaMnO3 multiferroics, we present here an investigation of the Curie temperature TC and the optical behaviors of this class of compound at low doping contents (x ≤ 0.05) The maximal doping content is below the critical value of x = 0.08 where the MRE has been reported [2], so the compounds should fall into a region where FM develops within the dominance of the AFM interaction Thus, clarifying the first stage of the FM formulation is important It is worthwhile noting that pure CaMnO3 possesses a G-type AFM ground state with a net Mn4+ magnetic moment of 2.46 µB [10] (close to the experimental value of 2.65 µB [11]) A theoretical study using density functional theory (DFT) showed that 2D and finite systems (nanoclusters) may exhibit FM ordering at surfaces that penetrates into the bulk with a penetration depth of 2.7 nm Thus, although FM is absent in bulk CaMnO3 , due to surface relaxation, it may appear and prevail in nanoclusters [10] In undoped CaMnO3 the FM interaction is a result of -2133- -2134- Journal of the Korean Physical Society, Vol 62, No 12, June 2013 an exchange between Mn4+ and Mn3+ ions, which occurs due to impurities or oxygen vacancies The occurrence of Mn3+ ions may also be induced by substitution, which in the case of Fe-doping brings a complicated picture of microscopic interactions because Fe is a multivalent ion The available studies of the CaFeO3 family (and derived compounds) show that Fe4+ ions may exhibit at low temperatures a so-called charge disproportionation (CD) phenomenon [12–14]; that is, the unpaired e1g electron of the high spin Fe4+ ions (t32g e1g ) is disproportionally located so that two Fe4+ ions are transformed to Fe3+ and Fe5+ : 2Fe4+ (t32g e1g ) → Fe3+ (t32g e2g ) + Fe5+ (t32g e0g ) An exact knowledge of the amount of the Fe4+ species is important in many cases because Fe-based perovskites are highly efficient catalysts in oxidation reactions of propane and ethanol [15] Therefore the possible interaction involves, except Mn3+ and Mn4+ pairs, the coupling pairs of Fe ions themselves (Fe4+ , Fe3+ , Fe5+ ) and of Fe ions and Mn ions (Mn3+ or Mn4+ ) At low doping concentration, if we were suggesting a doubleexchange mechanism (DE, according to Refs 16 and 17), and at high temperatures, where the CD phenomenon is absent, the two Fe ions may be considered as being far enough away from each other, for the possible FM interaction pairs to be reduced to Mn3+ and Mn4+ , Fe4+ (substituted for Mn4+ ) and Mn3+ , and Fe3+ (substituted for Mn3+ ) and Mn4+ Therefore, we will show here that if this scenario holds, then Fe-doping introduces a decrease in the overall FM strength and, together with this, a decrease in TC The conclusion may not hold for higher doping concentration because one cannot rule out the possible occurrence of Fe2+ , which may exhibit a high spin state (dxy , dyz )3 (dxy )1 (d1z2 )(dx2 −y2 )1 (S = 2) with magnetic moments of 3.1 and 3.6 µB at 293 and 10 K, respectively, as in SrFeO2 [18] For CaMn1−x Fex O3 one may expect a behavior similar to those of SrMn1−x Fex O3 [19] and of the oxygen-deficit SrMn1−x Fex O3−δ [20] These compounds possess antiferromagnetic ordering at low and heavy doping contents whereas intermediate substitution leads to a spin-glass behavior Near 50% doping, two types of ordering, AFM and FM, were found to co-exist The authors argued that the spin-glass behavior was a result of competing AFM and FM interactions between Mn4+ and observed Fe3+ and Fe5+ ions Note that SrMn1−x Fex O3 also exhibits a G-type AFM ground state as CaMnO3 and SrMnO3 At the other end, SrFeO3−σ also possesses an AFM structure (TN = 134 K) [21] However, a fully oxygenated SrFeO3 shows a metallic character (no static Jahn-Teller distortion inspite of the one unpaired electron in the Fe4+ e1g orbital) For the purpose of this work, which is to show that the overall FM strength decreases according to the doping, the correct determination of TC and the evolution of TC upon doping are of major importance II EXPERIMENTS The CaMn1−x Fex O3 (CMFO) bulk samples were prepared by using solid-state reaction technique with CaCO3 , MnO2 and Fe2 O3 powders as the precursors (Merck, 99.9%) As common for ceramic materials, the technological factors usually have strong influences on the properties of the materials prepared We utilized the following route: First, the parent oxides were dried to minimize the possible dampness; these were then weighted in the required molar proportions, mixed together, and ground for hours The mixture was ground again for more hours in ethanol and were then pressed into pellet (at the pressure of tons/cm2 , and without bonding colloid) with heights of mm and diameters of 10 mm The pellets were sintered at 700 ◦ C for hours This temperature was chosen to assure the burn out of organic substances and the dehydration of coordinated water The sintered pellets were ground again for hours and pressed; then, they were calcined at 1200 ◦ C for 24 hours in ambient conditions and at a constant ramping rate ◦ C/min The structures of the samples were characterized by using X-ray diffraction with a Bruker D5005 diffratometer The Raman scattering measurements were carried out by using a Renishaw Invia Microscope and using HeNe excited radiation (λ = 632.8 nm) The Curie temperatures of the samples were determined from the M(T) curves measured by using a vibrating sample magnetometer (DMS 880) having a sensitivity of about 10−6 emu/g III RESULTS AND DISCUSSION Structure Characterization Figure shows the XRD patterns of CaMn1−x Fex O3 (x = 0, 0.01, 0.03, 0.05) The data show no secondary phase except the one with the orthorhombic structure Pnma with (hkl) indices As seen, with increasing Fe concentration, the peaks shift to lower angles, signifying increases in the lattice constants (Table 1) Because Fe was substituted into the Mn sites, the increases in the lattice parameters and the volume may be interpreted in terms of the larger ionic radius of Fe ions in comparison with that of Mn ions (particularly, 0.585 ˚ A for A for Mn4+ ) Thus, the 6-coordinated Fe4+ and 0.530 ˚ observed lattice expansion caused by doping may serve as a signature of site occupation of Fe ions Indeed, if Fe ions are substituted in Ca sites (12-coordinated oxygen atoms) or as impurities in the grain boundaries, the developments of the lattice constants may be quite different In the orthorhombic symmetry with slight deformation as for our cases, the bond angles are not changed so much, so the expected values of the Optical Spectra of the Colloidal Fe-doped Manganate· · · – Duc Huyen Yen Pham et al -2135- Table Lattice parameters, Mn-O bond length and expected exchange integral Samples 0.00 0.01 0.03 0.05 a, c (˚ A) 5.2934, 5.279, 5.264 [22] 5.2935 5.2960 5.2980 b (˚ A) 7.4860, 7.448 [22] 7.4862 7.4890 7.4920 V (˚ A3 ) 209.76, 207.0 [22] 209.77 210.01 210.26 d (Mn-O) (˚ A) 1.8715, 1.895 [22] 1.8716 1.8720 1.8730 JMn−Mn (eV) 0.30 0.29 0.26 0.20 Fig (Color online) XRD patterns of CaMn1−x Fex O3 bulk samples exchange integrals (both super-exchange or double exchange) completely depend on the values of the bond lengths (Mn-O) As the lattice parameters are increased due to doping, one may expect a corresponding decrease in the strength of the exchange interaction Table gives the expected values of the double exchange integral JMnMn evaluated using first-principles calculation with the local density approximation (LDA) functional To compare our results with the results previously published, we list the structural parameters reported for undoped polycrystalline CaMnO3 , which were determined from the X-Ray and the Neutron scattering data given in Ref 22 The structure of the undoped CaMnO3 has also been studied at temperature from room temperature to 800 ◦ C by using high-resolution synchrotron X-ray powder diffraction The CaMnO3 structure was found to remain orthorhombic in the Pbnm space group over the entire temperature range [23] Raman Scattering Measurement To confirm the correct substitution of Fe into the Mn sites, we performed Raman scattering measurements for undoped CaMnO3 and doped CaFex Mn1−x O3 , and the results are shown in Fig According to the group Fig (Color online) Raman scattering spectra of (a) CaMnO3 [24] and (b) CaFex Mn1−x O3 theory, the vibrations of atoms in the lattice of CaMnO3 (Pnma) consist of 24 Raman active modes: ΓCaMnO3 = 7Ag + 5B1g + 7B2g + 5B3g +10B1u + 8B2u + 10B3u + 8Au (1) In comparison with the theoretical values from Bhattacharjee et al [24], the vibration modes may be assigned as follows: B2g (258 cm−1 ), Ag (280, 322, and 467 cm−1 ), B3g (433 cm−1 ), and B1g (489 cm−1 ) For various doping concentrations, the vibration modes may be extinguished (for example 489 cm−1 ) or enhanced (i.e., 280, 298, 376, 393, 466, 613, 632 and 736 cm−1 ) According to Ref 25, the vibration mode at 301 cm−1 belongs to a rotation around the x axis of the BO6 octahedron -2136- Journal of the Korean Physical Society, Vol 62, No 12, June 2013 Table Optical band-gaps and theoretical values x x = 0.0 x = 0.01 x = 0.03 x = 0.05 Eg (eV)∗ 0.45 [12] 0.36 0.34 0.38 Eg (eV)∗∗ 0.46 0.39 0.33 0.27 ∗ Extrapolated from the tangent of the absorption edge Calculated from first principles by using the LDA functional ∗∗ Fig (Color online) Infrared absorption spectra of CaMn1−x Fex O3 samples at room temperature The enhancement of this peak due to doping may be caused by the co-existence of FeO6 and MnO6 in a unit cell The peak at 319 cm−1 should belong to a Ca2+ oscillation, and the peak at 375 cm−1 to oscillations of both Ca2+ and O2− In the case of 1% doping, the separation of the peaks at 467 cm−1 and 489 cm−1 was relatively clear, but when the doping content was increased, the peak at 467 cm−1 was extinguished whereas the one at 489 cm−1 was enhanced It should be noticed here that strong enhancements due to doping were observed for the peaks at 613 cm−1 and 632 cm−1 The 613-cm−1 peak may belong to an impurity Another peak at 728 cm−1 , which corresponds to an asymmetric deformation of the BO6 octahedron, was also enhanced when the doping content of Fe was increased The changes in the spectroscopic data for Mn sites provide good experimental evidence for the correct substitution of Fe into the Mn sites Theoretically, such substitution may also lead to spectral shifts and widenings of spectral lines due to a change in (or weakening of) the metal-to-oxygen bonding force constants; unfortunately the accuracy obtained here (at a low doping concentration) was not enough to demonstrate these effects Infrared Absorption Measurement To reveal the change of allowed optical transitions due to Fe-doping and to determine the reduction of optical gaps, we performed IR absorption measurements Figure illustrates the result obtained for CaMn1−x Fex O3 at room temperature As seen, there are three major absorption peaks, 2.5, 4.6 and 5.7 eV For the doped samples, a new absorption band appears at 1.2 eV We found that the band gaps as calculated from first principles (using the LDA functional) were quite similar to the ones extrapolated from the absorption edges (see Table 2) A red-shift can be clearly observed with increasing Fig (Color online) Partial density of states (s, p, d) for the sample CaMn0.95 Fe0.05 O3 doping concentration The estimated band-gaps tended to decrease when the doping was increased To interpret the absorption spectrum, we calculated the density of states (DOS) for 5%-doped CaMn1−x Fex O3 (Fig 4) The time-dependent density functional theory should be required to evaluate all possible exited states (CI-Singles), but the DOS is adequate for qualitative interpretation In Fig 4, we found a clear correspondence between the absorption lines at 2.5 eV and 4.6 eV and the excitations of 3d electrons seen at about 2.5 eV and 4.6 eV The appearance of a peak at 1.2 eV is characteristic of Fe doping and should correspond to an antiferromagnetic exchange (with a change of spin) Evolution of TC According to Doping We have measured the M(T) curves in an applied field H = 500 G (field cooling (FC) mode) The obtained data are shown in Fig where one may easily estimate the values of TC by extrapolation; these values fall roughly around 140 K Table gives the estimates of TC from the minima of the dM/dT versus T curves For the perovskite manganates, the values of TC are directly linked to the coupling strength of the double-exchange (DE) Optical Spectra of the Colloidal Fe-doped Manganate· · · – Duc Huyen Yen Pham et al Fig (Color online) Magnetization versus temperature curves in a field of 500 Gauss for CaMn1−x Fex O3 samples Table Estimated TC , its decrease due to doping ∆TC = [TC (x) − TC (x = 0)]/TC (x = 0), and the expected decrease in the effective coupling strength ∆Jef f ≈ 1.5 ∆J x TC (K)∗ ∆TC [%] ∆Jef f [%] TC (K)∗∗ ∆TC [%]∗∗∗ 0.00 139.5 0 141.0 0.01 135.7 2.7 7.1 111.7 20.7 0.03 133.5 4.3 14.2 103.3 26.7 0.05 140.2 0.5 35.5 113.1 19.8 ∗ Determined from the minimum of the dM/dT versus T curve ∗∗ Determined from Eq (2) ∗∗∗ Calculated for the TC obtained from Eq (2) by using the optical gaps given in Table interaction [16] Because DE is a cause of ferromagnetism, its competition with the super-exchange (SE) antiferromagnetic ordering is a key factor in the development of TC As the final effective exchange strength Jef f is a sum of the double-exchange and super-exchange strengths JDE and JSE [26], estimating the interplay between Jef f and TC is desirable For the Fe-doped manganates, several important experimental results should be addressed For La0.67 Ca0.33 Mn0.097 Fe0.03 O3 , Przewo´znik et al [27] found that Jef f = 1.18 meV (for which JMn−Mn = 1.24 and JFe−Mn = 0.06 meV) The Fe doping in this compound until 12% reduced the TC and the magnetization but increased the magnetoresistance [28] Furthermore, the 57 Fe NMR data revealed that the Fe ions exhibited the oxidation state 3+ with anti-parallel spin in comparison with the spins of the neighboring Mn ions; that is, the Fe-Mn couplings were purely of SE character [29] Hence, the ferromagnetic domains should only be due to Mn4+ Mn3+ double-exchanged pairs [30] For this situation, if a one-unit increase in the Fe-doping concentration intro- -2137- duces a unit increase in −∆J on the SE side, then it has to induce a comparable decrease in k∆J on the DE side (k is a linear scaling constant), and the new effective coupling Jef f should be equal to (JSE − ∆J) + (JDE − k∆J) = Jef f − (1k)∆J The estimate for k may be taken from Refs 25-27 where −JFe−Mn ≈ 0.5JMn−Mn , thus implying k = 0.5 As a result, overall decrease in the effective coupling strength due to a unit increase in the doping content is ∆Jef f = Jef f − Jef f ≈ 1.5∆J As the 3% doping of Fe in La0.67 Ca0.33 MnO3 (TC = 344 K and JMn−Mn = 1.43 meV) leads to a 14.2% weakening of the JMn−Mn strength, a 1% increase in Fe content should weaken Jef f by about 7.1% The values of ∆Jef f calculated for our cases on the basis of k = 0.5 are given in Table These values predict that at 14% doping, the compound should be paramagnetic because at that concentration, ∆Jef f = 100% and Jef f = 0; that is, JMn−Mn = JFe−Mn For comparison, Table also gives the values of TC predicted according to Chong Der Hu’s relation [31] TC ≈ 0.027(1 − x)B , (2) where x is the substitution concentration and B is the band gap The relation is based on previous results obtained by de Gennes [17] and by Kubo and Ohata [32] for La1−x Ax MnO3 (which gave a one order larger TC of ≈ 4×B/15) By extrapolating the theoretical TC to x = 14%, we arrived at 60 K for the TC of the paramagnetic state IV CONCLUSION A small doping of Fe in CaMnO3 (≤ 5%) led to increases in the lattice constants and an observable decrease in Curie temperatures TC of all samples The analysis of the Raman scattering data showed an enhancement of BO6 resonance modes due to doping, and the IR absorption spectrum revealed the characteristic Fe band at 1.2 eV, which was due to the transition of electrons with a change of spin (SE interaction) The Fe doping induced an increase in the antiferromagnetic exchange, JFe−Mn , between Fe and Mn sites and a strong reduction of the ferromagnetic coupling, JMn−Mn , between Mn sites The analysis showed that the paramagnetic state, for which the effective exchange Jef f is zero because the ferro- and the antiferro- couplings cancel each other, should be present at a doping concentration of 14% and at TC of about 60 K ACKNOWLEDGMENTS The authors would like to thank the National Foundation for Science and Technology Development (NAFOSTED) of Vietnam, project code #103.02.19.09 “Nanofluid and Application” (2009-2013), for its financial supports -2138- Journal of the Korean Physical Society, Vol 62, No 12, June 2013 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DISCUSSION Structure Characterization Figure shows the XRD patterns of CaMn1−x Fex O3 (x = 0, 0.01, 0.03, 0.05) The data show no secondary phase except the one with the orthorhombic structure Pnma... and the results are shown in Fig According to the group Fig (Color online) Raman scattering spectra of (a) CaMnO3 [24] and (b) CaFex Mn1−x O3 theory, the vibrations of atoms in the lattice of