Structure and properties of double perovskite system la2co1−xfexmno6 - Pham The Tan

Communications in Physics, Vol. 24, No. 3S1 (2014), pp. 80-84 DOI:10.15625/0868-3166/24/3S1/5224 STRUCTURE AND PROPERTIES OF DOUBLE PEROVSKITE SYSTEM La2Co1−xFexMnO6 PHAM THE TAN Faculty of Basic Science, Hung Yen University of Technology and Education, Hung Yen, Vietnam PHAM HUYEN YEN AND HOANG NAM NHAT Faculty of Technical Physics and Nanotechnology, University of Engineering Technology, Vietnam National University, Hanoi NGUYEN QUANG HOA Faculty of Physics, Hanoi University of Science, Vietnam National University, Hanoi, Vietnam E-mail: phamthetansp@gmail.com Received 20 June 2014 Accepted for publication 20 August 2014 Abstract. Low Fe-doped insulating ferromagnets La2CoMnO6 were prepared and studied. The compounds crystal- lized in orthorhombic space group Pnma with slight changes in the lattice constants. We have observed a significant reduction of resistivity due to doping, together with an increase of magnetization and saturated magnetization as dop- ing level increased. The doping also reduced TC for both transitions at around 220 and 140 K which attribute for the different spin orderings of the magnetic ions. The small features were also seen at around 40 K and should correspond to the cluster glassy region with spin disorders. Keywords: Structure, Magnetism, Fe-doped, La2CoMnO6. I. INTRODUCTION The ferroelectric insulators of a form RE2AMnO6 where RE is a rear earth element and A a transition metal attract a lot of interests due to a co-existence of ferromagnetism and insulating state within a single structural frame [1,2]. Among the effects that were found in these double perovksites is the dependence of dielectric permitivity on magnetic field which promises direct application in spintronics as the magnetic field driven capacitors. The double perovskites inherits from the single ones a structural type that is built up from the connecting MnO6 polyhedrons, but here the polyhedrons are connected only in (x,y) plane and in the (z) direction one inserted AO2 layer intercepts the continuity of MnO6 network. For a particular case of La2CoMnO6 (RE = La, A = Co) there are clearly two sub-lattices of MnO6 and CoO6 which are alternating each other along z direction but are competing in (x, y) plane. Therefore, besides a ferromagnetic com- ponent that arises from a double exchange (DE) interactions between manganese ions (Mn3+- O c©2014 Vietnam Academy of Science and Technology PHAM THE TAN, PHAM HUYEN YEN, HOANG NAM NHAT, AND NGUYEN QUANG HOA 81 Mn4+), there are also components arisen from exchange interactions between cobaltate ions them- selves and between cobaltate and manganese ions. The ordering of magnetic ions within a lattice of RE2AMnO6 is influenced by various factors (preparation routes, thermal treatment conditions such as time, gradient, ramping, cooling etc) and certainly determines the integral magnetic prop- erties of compound. Thus it is not a surprise that La2CoMnO6 shows two major ferromagnetic transitions with Curie temperatures TC at around 220 and 140 K, together with a minor transi- tion at a lower temperature around 40 K [3–5]. While the first two transitions results caused by the ordering states of the high spin Co2+ ( t32ge 2 g ) and Mn4+ ( t32ge 0 g ) ions (TC ∼ 220 K), and of the intermediate spin Co3+ ( t32ge 2 g ) and high spin Mn3+ ( t32ge 1 g ) ions (TC ∼ 140 K), the nature of the third transition (TC ∼ 40 K) is not quite clear, although the evidences exist to demonstrate a reentrant glass behavior at low temperature [5, 6]. The available literatures point to a mere cor- respondence between the sintering temperature and TC: samples sintered at lower temperature tend to show higher TC whereas the ones sintered at higher temperature usually exhibit lower TC [5,6]. The reentrant glass behavior has been demonstrated for Sr2Fe1−xMnxMoO6 [7] in which the appearance of a random spin ordering was attributed to the competition between the nearest neighbor clusters. This competition was caused by the substitution of Fe which introduced the super-exchange (SE) antiferromagnetic pairs Fe3+ - O Mn3+. While the magnetic phase transi- tions in La2CoMnO6 appear complicated, the compound did not show any abrupt changes in its conduction mode, which may be expressed, with addequate experimental accuracy, as of variable range hopping (VRH) mode with thermal activation energy of order 150 - 180 meV [6]. Although the bulk compound La2CoMnO6 has been carefully studied, its doped compounds, especially the ones with Fe, still leave many questions that need to be clarified. First and foremost is a role of Fe and its effect on TC in a multiferroic with different transition temperatures. As known for single perovskites [8] the doping of Fe showed a monotoneous effect on magnetic properties only at low concentration where the charge disproportionation (CD) phenomenon did not occur, that is, when two Fe substitution sites were distanced from each other and the unpaired electrons of two Fe4+ ions were not disproportionately located so that 2Fe4+→ Fe3+ + Fe5+ [9]. The occurence of the multi-spin low valent Fe2+ and high valent Fe5+ ions at higher doping concentration usually com- plicate the final image of magnetic interactions, so we restricted our case to the doping of Fe upto a concentration of 3%. II. EXPERIMENTAL The La2Co1−xFexMnO6 (LCFM) bulk samples were prepared by traditional solid-state re- action method with highly purified CaCO3, MnO2, La2O3 and Fe2O3 (Merck, > 99.9%) powders as the precursors. The parent oxides were dried, then weighted to the required molar proportions and mixed together and ground for 4 hours. The mixture was ground again for the next 4 hours in ethanol then pressed into the pellets (5 tons/cm2, no bonding colloid was applied) of height 2 mm and diameter 10 mm. The pellets were sintered at 700˚C for 4 hours. The sintered pel- lets were ground again for 4h and pressed, then calcinated at 1250˚C for one week in open air and at constant ramping rate 2˚C/min. The structure characterization was taken on a Bruker’s D5005 diffractometer equipped with CuKα radiation source (λ = 1.54018A˚) and a monochroma- tor of slit size 0.5 mm. The step width was 0.02˚. The resistivity measurement was performed by using the standard 4-probe technique with straight alignment of electrodes on a Bio-Rad Deep 82 STRUCTURE AND PROPERTIES OF DOUBLE PEROVSKITE SYSTEM La2Co1−xFexMnO6 Level Transient Spectroscopy system. The magnetic measurement was taken on from 2 K to room temperature on a Physical Property Measurement System (PPMS) from Quantum Design. III. RESULTS AND DISCUSSION 3 reentrant glass behavior at low temperature [5, 6]. The available literatures point to a mere correspondence between the sintering temperature and TC: samples sintered at lower temperature tend to show higher TC whereas the ones sintered at higher temperature usually exhibit lower TC [5, 6]. The reentrant glass behavior has been demonstrated for Sr2Fe1- xMnxMoO6 [7] in which the appearance of a random spin ordering was attributed to the competition between the nearest neighbor clusters. This competition was caused by the substitution of Fe which introduced the super-exchange (SE) antiferromagnetic pairs Fe3+ - O - Mn3+. While the magnetic phase transitions in La2CoMnO6 appear complicated, the compound did not show any abrupt changes in its conduction mode, which may be expressed, with addequate experimental accuracy, as of variable range hopping (VRH) mode with thermal activation energy of order 150 - 180 meV [6]. Although the bulk compound La2CoMnO6 has been carefully studied, its doped compounds, especially the ones with Fe, still leave many questions that need to be clarified. First and foremost is a role of Fe and its effect on TC in a multiferroic with different Fig. 1. The XRD patterns for the undoped La2CoMnO6. The inset shows the packing coordinations for two different space groups. The doped samples with Fe contents x ≤ 3% show the similar patterns and are omitted for clarity. Fig. 1. The XRD patterns for t e undoped La2CoMnO6. The inset sh ws the packing co- ordinations for two different space groups. The doped samples with Fe contents x ≤ 3% show the similar patterns and are omitted for clarity. Table 1. Cell parameters, curie temperatures, and acti- vation energies for undoped and Fe-doped La2CoMnO6 (s.g. pnma, no.62) x Cell constants [A˚] TC [K] Eg a b c [meV] 0.00 5.525 5.476 7.773 215 155 0.01 5.525 5.482 7.775 175 122 0.02 5.524 5.479 7.778 153 79 0.03 5.526 5.471 7.777 130 54 Table 2. Atomic positions and isotropic thermal motion factor (BISO) for the undoped La2CoMnO6 (s.g. pnma, no.62). RW = 22%, RI = 8%. Atom x y z BISO La 0.52 1/2 1/4 0.71 Co 0 0 0 0.82 Mn 1/2 1/2 0 0.56 O1 0 0 1/4 1.21 O2 0.27 0.022 0.714 1.10 Similar to the cases of many sin- gle perovskites, our doped and undoped La2CoMnO6 preferrably crystallized in an orthorhombic space group Pnma (No.62) which is a lower symmetry than of the ideal face-center cubic lattice of perovskites. The lattice constants derived from Rietveld analysis of profiles are listed in Table 1 and the sample diffractogram for the un- doped La2CoMnO6 is featured in Fig. 1. Commonly, RE2AMnO6 may also be found in a tetragonal space group I4/mmm of higher symmetry than Pnma (which can be obtained from the orthorhombic sym- metry by acquiring a = b) and in a mon- oclinic space group P21/n of lower sym- metry (β 6= 90˚) [1]. From the orthorhom- bic lattice a reduced cell with Z = 2 may be deduced as given in Ref. [2], i.e. space group R3, a = b = c = 5.488A˚, α = β = γ = 60.72˚. This reduced cell contains ex- actly two MnO6 layers. For our doped sam- ples, the small doping concentrations did not lead to the visible changes in lattice constants (Table 1). The atomic positions and thermal factors as obtained for the un- doped sample from the Rieveld refinement of phases are given in Table 2. The R- factors for weighted profile (RW ) and for integral intensity (RI) are somehow higher than usual, 22 and 8% respectively, and this situation did not allow us to obtain the re- fined site occupation factors at Mn and Co sites (in fact, they were fixed at 1.0). Thus, there was a question arose about a correct substitution site of doped Fe atoms. Sto- ichimetrically given, Fe should be substituted for Co, but as the Co and Mn sites were crystal- lographically equivalent, the Fe atoms might substitute for both Co and Mn. Unfortunately, the accuracy achieved with X-ray diffraction measurements in our case did not allow to clarify this situation crystallographically. PHAM THE TAN, PHAM HUYEN YEN, HOANG NAM NHAT, AND NGUYEN QUANG HOA 83 4 transition temperatures. As known for single perovskites [8] the doping of Fe showed a monotoneous effect on magnetic properties only at low concentration where the charge disproportionation (CD) phenomenon did not occur, that is, when two Fe substitution sites were distanced from each other and the unpaired electrons of two Fe4+ ions were not disproportionately located so that 2Fe4+→Fe3+ + Fe5+ [9]. The occurence of the multi-spin low valent Fe2+ and high valent Fe5+ ions at higher doping concentration usually complicate the final image of magnetic interactions, so we restricted our case to the doping of Fe upto a concentration of 3%. II. EXPERIMENTAL The La2Co1-xFexMnO6 (LCFM) bulk samples were prepared by traditional solid-state reaction method with highly purified CaCO3, MnO2, La2O3 and Fe2O3 (Merck, > 99.9%) powders as the precursors. The parent oxides were dried, then weighted to the required molar proportions and mixed together and ground for 4 hours. The mixture was ground again for the next 4 hours in ethanol then pressed into the pellets (5 tons/cm2, no bonding colloid was applied) of height 2 mm and diameter 10 mm. The pellets were sintered at 700oC for 4 hours. Fig. 2. Dependence of resistivity on temperature from 50 to 300K. Fig. 2. Dependence of resistivity on tempera- ture from 50 to 300 K. 7 activation energies given in Table 1. These values are comparable to the ones obtained by the other authors [10,11,12]. The magnetization versus field measured at 20K is given in Figure 3. The inset in Figure 3 illustrates the possible spin exchanges mediated through oxygen 2p orbitals between t2g and eg electrons of the metals according to Ref. [1] (Kanamori-Goodenough rule). The FM state which prevails below the Curie temperature is a result of a half-fill M(t2g)-O(2p)-M(eg) interaction. The other possible pairs such as M(t2g) - O(ppi)-M(t2g) and M(eg)- O(pσ)-M(eg) are all antiferromagnetic. As seen in Figure 3, the Fe-doping increased both magnetization and saturated magnetization while lowered the TC accordingly (Table 1). The doping also reduced coercive forces HC but this effect could only be seen at low temoperature. Above the Curie temperature TC all magnetization curves yield HC ≈ 0. Besides, the magnetization decreased as temperature increased and at room temperature the magnetization was about 20% as of that at 5K. We have obtained 4.1 µB per unit cell saturated magnetization on the Fig. 3. Magnetization vs. field at 20K. The insets show the Kanamori-Goodenough rule according to Ref. [5] and the saturated magnetization obtained at 60 kOe. Fig. 3. Magne ization vs. field at 20 K. The insets show the Kanamori-Go denough rul according to Ref. [5] and the saturated mag- netization obtained at 60 kOe 8 undoped sample and 5.3 µB on doped one with x = 0.03. The values for TC (Table 1) were determined on basis of fitting the inversed susceptibility χ-1 according to the Curie-Weiss law (curve fits not shown). In Figure 4 we show the dependence of Field Cooling (FC) and Zero-Field Cooling (ZFC) curves on temperature (from which the extrapolation fits for TC may also be obtained). While the FC curves showed a continuous increase as T decreased, the ZFC curves did separate from the FC curves at the temperatures near TC and expressed the maxima (seen at 142, 137, 107, 75 K for x = 0, 0.1, 0.2 and 0.3 respectively). As discussed, the first transitions with TC = 215, 175, 153 and 130 K (Table 1), are the results of the ordering states of the high spin Co2+( 3 22g gt e ) and Mn4+ ( 3 02 g gt e ) ions, and the second transitions with TC = 142, 137, 107, 75 K are the results of the orderings of the intermediate spin Co3+ ( 3 12 g gt e ) and high spin Mn3+ ( 3 12 g gt e ) ions. There are also the small features near 35 K which may correspond to the Fig. 4. The development of Field Cooling (FC) and Zero-Field Cooling (ZFC) magnetization at a constant field of 200 Oe according to temperature from 10 to 300K. The inset shows the evolution of coercive field strength on doping content. Fig. 4. The development of Field Cooling (FC) and Zero-Field Cooling (ZFC) magne- tization at co stant field of 200 Oe accord- ing to temperature from 10 to 300K. The inset shows the evolution of coercive field strength on doping content. The substitution of Fe strongly affected the resistivities of samples as seen in Fig. 2. The dop- ing reduced the resistivity by orders of magnitude but in overall our samples showed the higher re- sistivities in comparison with that of other works [10,11,12]. As observed, the resistivities de- creased upon doping with the decreasing tendency as temperature lowers (unitl 50 K). However, there was no abrupt change in the ρ(T ) curves which might signify some phase transition. We fitted the ρ(T ) curves according to VRH model and list the obtained activation energies given in Table 1. These values are comparable to the ones obtained by the other authors [10–12]. The magnetization versus field measured at 20 K is given in Fig. 3. The inset in Fig. 3 illustrates the possible spin exchanges mediated through oxygen 2p orbitals between t2g and eg electrons of the metals according to Ref. [1] (Kanamori-Goodenough rule). The FM state which prevails below the Curie temperature is a result of a half-fill M(t2g)−O(2p)−M(eg) inter- action. The other possible pairs such as M(t2g)− O(ppi)−M(t2g) and M(eg)−O(pσ)−M(eg) are all antiferromagnetic. As seen in Fig. 3, the Fe- doping increased both magnetization and satu- rated magnetization while lowered the TC accord- ingly (Table 1). The doping also reduced coer- cive forces HC but this effect could only be seen at low temoperature. Above the Curie tempera- ture TC all magnetization curves yield HC ≈ 0. Be- sides, the magnetization decreased as temperature increased and at room temperature the m gneti- zation was about 20% as of that at 5 K. We have obtained 4.1 µB per unit cell saturated magnetiza- tion on the undoped sample and 5.3 µB on doped one with x = 0.03. The values for TC (Table 1) were determined on basis of fitting the inve se susceptibility χ−1 according to the Curie-Weiss law (curve fits not shown). In Fig. 4 we show the dependence of Field Cooling (FC) and Zero-Field Cooling (ZFC) curves on temperature (from which the extrapolation fits for TC may also be obtained). While 84 STRUCTURE AND PROPERTIES OF DOUBLE PEROVSKITE SYSTEM La2Co1−xFexMnO6 the FC curves showed a continuous increase as temperature decreased, the ZFC curves did sep- arate from the FC curves at the temperatures near TC and expressed the maxima (seen at 142, 137, 107, 75 K for x = 0,0.1,0.2 and 0.3, respectively). As discussed, the first transitions with TC = 215,175,153 and 130 K (Table 1), are the results of the ordering states of the high spin Co2+(t32ge 2 g) and Mn 4+(t32ge 0 g) ions, and the second transitions with TC = 142,137,107,75 K are the results of the orderings of the intermediate spin Co3+(t32ge 1 g) and high spin Mn 3+(t32ge 1 g) ions. There are also the small features near 35 K which may correspond to the transitions observed for the reentrant glassy cluster behavior of doped samples similar to the situation discussed in Ref. [5] for the undoped La2CoMnO6. IV. CONCLUSION The small doping of Fe in La2CoMnO6 (≤ 3%) led to the slight changes in the lattice constants but the symmetry of the lattice remained unchanged. The doping led to the observable decreases of the Curie temperatures TC (from 215 to 130 K for the first and from 142 to 75 K for the second transitions). However the doping also resulted in the enhancement of saturated magnetizations in all samples. The substitution of Fe induced a decrease of resistivity by orders of magnitude but did not alter the conduction mode which was believed to follow the variable range hopping model. ACKNOWLEDGMENT The authors would like to thank the supports from Vietnam National University project code # QG.12.47. One of authors (N. Q. Hoa) was appreciate to the support from the research project TN 12-12 from Vietnam National University, Hanoi. REFERENCES [1] Yuichi Shimakawa, Masaki Azuma and Noriya Ichikawa, Materials 4 (2011) 153-168. [2] Santu Baidy and T. Saha-Dasgupta, Phys. Rev. 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