Chapter Three: Influence of Fe doping in Nd0.67Sr0.33MnO3 polycrystalline system Effect of Fe Substitution for Mn on the Structural, Magnetic and Transport Properties in Polycrystalline Manganite Nd0.67Sr0.33MnO3 System 3.1 Introduction The perovskite manganite Nd1-xSrxMnO3 compound can be labeled as a material having characteristics common to compounds in the “intermediate” bandwidth subset of manganese oxides [88] due to the presence of a stable charge-ordered phase at x = 0.5, state which can be easily destroyed by a magnetic field in a first order transition. In this chapter, a considerable emphasis will be given to the analysis of Nd0.67Sr0.33MnO3 (Nd1xSrxMnO3 with x = 0.33) compound. At this composition, the compound is found to exhibit the highest Curie temperature, Tc and the moments are close to the expected spinonly value. The compounds with Nd3+Mn3+O3 (x = 0) [89] and Sr2+Mn4+O3 (x = 1) [90] are antiferromagnetic and insulating in its ground state. However when the two compounds are mixed, ferromagnetic Nd0.67Sr0.33Mn 30+.67 Mn 04.+33 O3 compound is formed. The mixed valency of the manganates leads to strong ferromagnetic (FM) interactions arising from the exchange of electrons via Mn3+ – O – Mn4+ bonds as explained by the double-exchange (DE) mechanism [28]. At this composition, the DE interaction prevails over the other interactions (eg. superexchange interaction from Mn3+ – O – Mn3+ bonds) and one sees the occurrence of ferromagnetism, metal-to-insulator transition, and large magnetoresistance occurring at and around the ferromagnetic Curie temperature, Tc. The onset of ferromagnetism arises from the core spin S = 3/2 of both Mn3+ and Mn4+ due to half-filled crystal field split t2g orbital and a strong intra-atomic Hund’s rule coupling 63 Chapter Three: Influence of Fe doping in Nd0.67Sr0.33MnO3 polycrystalline system which spin aligns the electrons in the eg orbital and causes them to take part in the electronic transport. The metallic state in the manganites is thus caused by the ferromagnetic interactions. In addition to DE interaction, lattice distortions are believed to play an important role through strong electron-lattice coupling which arises from the Jahn-Teller distortion [91, 44] around the Mn3+ ions. What happens to the ferromagnetic order, metallic state and magnetoresistance when we mix a double exchange (DE) type ferromagnet such as the manganate with an antiferromagnetic insulator like ironate? To answer this question we have investigated the polycrystalline Nd0.67Sr0.33Mn1-xFexO3 (x = – 1) system. This is done in a systematic way where Fe ion is added in steps to Nd0.67Sr0.33MnO3 until it has completely substituted the Mn ions in the compound, Nd0.67Sr0.33FeO3. The main reason Fe is chosen in this study is because the Fe ion has identical ionic radius to that of the Mn ion, so we would expect the otherwise strong lattice effects to be maintained and the resultant effects are mainly due to changes in the electronic structure. However the transition metals (TM) in the middle of the row of the Periodic table such as Fe and Mn are expected to reflect the transitional behavior of the early and late TM oxides [92]. The magnetic and electronic nature in oxides of type Nd1xSrxMnO3 and Nd1-xSrxFeO3 are completely different [93, 94]. For example, the end member of the system, Nd0.67Sr0.33MnO3 (NSMO) undergoes a paramagnetic (PM) to ferromagnetic (FM) transition at Curie temperature Tc ≈ 274 K [95] arising from the mixed valency of both Mn3+ and Mn4+. The Mn ions with valency +3 and +4 exist in the high spin state with the electronic configurations t 23g e1g , S = and t 23g e g0 , S = / , respectively. Both the valence states carry spin and the conduction band arises from the eg electron which takes part in the DE interactions. On the other hand, Nd0.67Sr0.33FeO3 64 Chapter Three: Influence of Fe doping in Nd0.67Sr0.33MnO3 polycrystalline system (NSFO) has a ground state as an antiferromagnetic (AFM) insulator arising from bondings of Fe3+ - O2- - Fe3+, Fe4+ - O2- - Fe4+, and Fe3+ - O2- - Fe4+, which can be explained by a superexchange (SE) model. The electronic configuration of the Fe ions is more complicated due to the possibility of the existence of the Fe ions in the low spin and high spin states. The high spin Fe3+ ion has a configuration of t 23g e g2 , S = / and the low spin (LS) Fe3+ ion has a configuration of t 25g e g0 , S = / . The high spin (HS) state is energetically more favorable. The magnetic susceptibility measurements for the paramagnetic state, Mossbauer and powder neutron diffraction studies for the related Fe perovskites have proved Fe3+ to be in the HS state [96, 97]. The tetravalent Fe ion, according to ligand field theory, can assume either a HS t 23g e1g , S = configuration or a LS t 24g , S = configuration in an octahedral crystal field, with the latter being stabilized when the crystal field splitting is large. However, based on the measured magnitude of the magnetic moment with µ Fe4 + = 3.1µ B by neutron scattering, Takeda, Komura, and Watanabe [98] have suggested that the Fe4+ ions in SrFeO3 are to be in the HS state with three electrons filling the t2g band, with the remaining eg electron itinerant. Apart from that, Chul et al. [99] have pointed out convincingly in the Mossbauer spectra (MS) of the Nd1-xCaxFeO3-y that Fe4+ exists as the HS configuration in the octahedral environment. This finding was further supported by studies related to MS [100] as well as X-ray and UV photoemission spectroscopy [101] on La1-xSrxFeO3 and La1-xSrxMnO3 compounds. Hence, in our system we will consider Fe3+ and Fe4+ ions to be predominantly in the high spin state. However, we not rule out the possibility of Fe ion to being in a different spin state given its proximity to Mn ions. 65 Chapter Three: Influence of Fe doping in Nd0.67Sr0.33MnO3 polycrystalline system Thus, the correlation between the valence state and spin state of the TM ions is believed to influence strongly the competing interactions such as the magnetic and electrical transport properties of the rare earth TM oxides. Therefore, it is of interest to trace out the changes in their physical properties as the concentration of the oxides is shifted from pure manganate to pure ironate. The objectives of this work are to find the changes in structural composition in relation to the valence state of the Fe ions and at the same time the influence of Fe doping on the magnetic and electrical transport properties in the polycrystalline Nd0.67Sr0.33Mn1-xFexO3 (NSMFO) oxides. The FM behavior was observed for x < 0.1 compounds, while AFM nature was observed for x > 0.3 compounds. Intermediate composition with x = 0.1, 0.15 and 0.3 showed the presence of both FM and AFM behavior arising from the complex electronic structure in the system. 3.2 Experiments Polycrystalline Nd0.67Sr0.33Mn1-xFexO3 (NSMFO) ceramics in this work have been prepared by solid state reaction method as described before in Chapter with x = 0, 0.05, 0.1, 0.3, 0.4, 0.6, 0.8 and 1. The final products were characterized by a fine-step-mode xray diffraction (XRD), model Phillips Diffractometer with Cu Kα source at room temperature. An Oxford superconducting vibrating sample magnetometer (VSM) was used to measure the magnetic property of the samples. The magneto- and electrotransport properties were measured via the standard four-point probe on gold-coated surfaces. Xray photoelectron (XPS) measurements were carried out using a spectrometer equipped with Mg Kα source with hν = 1253.6 eV. 66 Chapter Three: Influence of Fe doping in Nd0.67Sr0.33MnO3 polycrystalline system 3.3 Experimental results and discussions 3.3.1 Structural Characterization Figure – presents the XRD patterns of polycrystalline samples of Nd0.67Sr0.33Mn1-xFexO3 (x = – 1) taken at room temperature. The patterns show clean single-phase patterns without any detectable impurity for all samples. All the samples were found to be crystalline in an orthorhombic structure obtained from the program DICVO91 [102]. No apparent macroscopic structural changes can be identified by small amount of Fe doping. However, the peaks shift slightly to lower two theta degree as Fe doping increases. Upon increasing the amount of Fe ions substituting Mn ions of different ionic radii, the observed shift in the XRD position in figure – 1(b) is expected (ionic radii of Mn3+ = 0.785, Mn4+ = 0.670, Fe3+ = 0.785 and Fe4+ = 0.725 [103]). Table – shows the room-temperature unit cell volumes and axis a, b and c, respectively. The lattice parameters for lower Fe-doped samples (x < 0.3) remain almost unaltered. The unit cell volume increases linearly as the iron concentration increases as depicted in figure – 2. The observations indicate that the MnO6 octahedra are strongly distorted by the introduction of Fe ions. The increase in the unit cell volume is hypothesized to be due to increasing amount of Mn ions being replaced by the Fe ions of variable valence state, such as Fe3+ and Fe4+ in the samples. 67 (213) (422) x =1 (312) (420) (311) (210) (a) (310) Chapter Three: Influence of Fe doping in Nd0.67Sr0.33MnO3 polycrystalline system x = 0.80 Intensity (arb. units) x = 0.60 x = 0.40 x = 0.30 x = 0.15 x = 0.10 x = 0.08 x = 0.05 x =0 20 30 40 50 o 2θ ( ) 60 (b) 70 80 x = 1.00 x = 0.80 Intensity (arb. Units) x = 0.60 x = 0.40 x = 0.30 x = 0.15 x = 0.10 x = 0.08 x = 0.05 x = 0.00 32 32.5 o 2θ( ) 33 33.5 Figure – (a) The XRD θ − 2θ patterns of polycrystalline Nd0.67Sr0.33Mn1-xFexO3 samples with x = 0, 0.05, 0.08, 0.1, 0.3, 0.4, 0.6, 0.8 and (b) A section of the XRD patterns of polycrystalline Nd0.67Sr0.33Mn 1− x Fe x O3 samples from 32o < 2θ < 33.5o. Note the systematic decrease in 2θ with increasing Fe doping as indicated by the vertical dotted line. 68 Chapter Three: Influence of Fe doping in Nd0.67Sr0.33MnO3 polycrystalline system Table – Unit cell parameters of the Nd0.67Sr0.33Mn1-xFexO3 (x = – 1) manganites. V(Å3) 228.58 228.83 228.99 228.94 229.33 229.80 230.15 232.68 234.21 a (Å) 7.69859 7.72231 7.70187 7.72387 7.72943 7.73240 7.73290 7.73270 7.73297 b (Å) 5.44940 5.45034 5.45277 5.45145 5.45345 5.46425 5.46460 5.47740 5.51237 c (Å) 5.44848 5.43686 5.45264 5.43721 5.44051 5.43874 5.44644 5.49352 5.49443 5.52 234 5.5 233 5.48 232 b a/√2 5.46 c 5.44 5.42 231 230 229 V 5.4 Volume Lattice parameter (Å) Composition Nd0.67Sr0.33MnO3 Nd0.67Sr0.33Mn0.95Fe0.05O3 Nd0.67Sr0.33Mn0.90Fe0.10O3 Nd0.67Sr0.33Mn0.85Fe0.15O3 Nd0.67Sr0.33Mn0.70Fe0.30O3 Nd0.67Sr0.33Mn0.60Fe0.40O3 Nd0.67Sr0.33Mn0.40Fe0.60O3 Nd0.67Sr0.33Mn0.20Fe0.80O3 Nd0.67Sr0.33FeO3 20 40 60 Fe at.% x 80 228 100 Figure – Dependence of the lattice parameters (left axis) and the volume (right axis) on Fe at. % x in Nd0.67Sr0.33Mn1-xFexO3 (x = – 1) manganites. 69 Chapter Three: Influence of Fe doping in Nd0.67Sr0.33MnO3 polycrystalline system 3.3.2 X-ray Photoelectron Spectroscopy (XPS) For the perovskite manganites, the Mn3+ content and the mole ratio of Fe4+ ions to the total Fe ions have important effects on the magnetic and transport properties. Ahn et al. [104] reported that doping with Fe causes a depletion of the Mn3+/Mn4+ ratio, the population of the electron hopping electrons, and the number of available hopping sites in the La1-xCaxMn1-yFeyO3 system. These results originate from the suppression of double exchange due to depopulation of hopping electrons by Fe doping, resulting in the reduction of ferromagnetism and metallic conduction. Early study by Jonker [105] has shown that the presence of Fe3+, Mn3+ and Mn4+ for x < 0.85 and Fe3+, Fe4+ and Mn4+ for 0.85 < x < 1.00 plays a crucial role on the conductivity of La0.85Ba0.15Mn1-xFexO3 as the widths and energies of their eg bands dictate the electron distribution of the Fe and Mn ions. From these, we can infer that the presence of the amount of Fe4+ and Mn3+ have close relation with the structural, magnetic and conductivity properties in the samples. Noting the importance of valence state of Mn and Fe ions, XPS measurements have been employed to further analyze the valency of Mn and Fe in the system. Fe 2p and Mn 2p XPS spectra of NSMFO are shown in figure – 3(a) and (b), respectively. In figure – 3(b), Mn 2p peaks can be resolved into two spin-orbit doublets, 2p3/2 and 2p1/2 of the Mn 2p core hole. They are Mn2O3 and MnO2 at binding energies of 641.5 and 643.4 eV, respectively. The valence state of Mn ions in NSMFO system can be attributed to a mixed valence state of +3 and +4 depending on Fe concentration. The average Mn valency of NSMO is ∼ +3.3. 70 Chapter Three: Influence of Fe doping in Nd0.67Sr0.33MnO3 polycrystalline system (a) Intensity (Arb. Units) x=1 x = 0.80 x = 0.60 x = 0.40 x = 0.30 x = 0.10 x = 0.08 x = 0.05 x=0 700 710 720 730 740 750 Binding Energy (eV) (b) Intensity (Arb. Units) x=0 x = 0.05 x = 0.08 x = 0.10 x = 0.30 x = 0.40 x = 0.60 x = 0.80 x=1 630 640 650 660 Binding Energy (eV) Figure – XPS spectra of (a) Fe 2p and (b) Mn 2p in NSMFO system for x = 0, 0.05, 0.08, 0.1, 0.3, 0.4, 0.6, 0.8 and 1. The shift is shown by the vertical line drawn. 71 Chapter Three: Influence of Fe doping in Nd0.67Sr0.33MnO3 polycrystalline system As iron content increases, the Mn peaks decrease in intensity. This implies an overall gradual decrease in the average Mn valency in the system. At x = 1, no peaks are observed. Thus the FM ground state is weakened by Fe substitution and this is followed by the onset of AFM ground state in x = sample. End member Nd 30+.67 Sr 02.+33 FeO (NSFO) is taken as a reference. An appreciable shift towards lower energy value (as shown by slanted dotted line) is observed for Fe 2p spectra in figure – 3(a). The shift in binding energy as Fe concentration increases is expected to be due to the sensitivity of the TM spectra to the symmetry of the ground state [106]. According to Abbate et al. [107], such chemical shift is due to a change in the electrostatic energy at the TM site. Thus, when an increasing amount of Fe ions replaces Mn sites, the electrostatic energy at the Mn sites changes due to a change in the 3d count. It is worth noting that up to x ≈ 0.1, the overall shapes of the multiplets hardly change, resembling the spin orbit doublet, 2p3/2 of the Fe 2p core hole. The binding energy of the Fe 2p peaks is very close to that in Fe2O3 (around 711.4 eV), which suggests the Fe ions are in the trivalent state. This indicates that at least in the first x = 0.1 series, the Fe ions essentially remain in a 3d5 (Fe3+) configurations. This is consistent with results from Mossbauer spectroscopy (MS) studies described earlier [108] with the detection of trivalent Fe ions at a low doping level (x < 0.1). For x > 0.1, we observe that the Fe 2p spectra grow and dominate the region resemble that of 2p / peaks of NSFO. At the intermediate range, the ground state has become a combination of both NSMO and NSFO depending on the doping concentration. For x = 1, the Fe 2p XPS results for NSFO correspond well to that of SrFe4+O3 [107, 109] and LaFe3+O3 [107] MS studies, based on the behavior of doped Ln1-xAxFeO3 (Ln = La, Pr, and Nd; A = Sr and Ca) oxides. These results have proved that Fe3+ ( t 23g e g2 , S = ) and 72 Chapter Three: Influence of Fe doping in Nd0.67Sr0.33MnO3 polycrystalline system Fe4+ ( t 23g e1g , S = ) are in the high spin states [94, 110, 111]. As compared to NSFO, the multiplets for the higher doped Fe content samples hardly shift in position resembling that of NSFO. However the only difference is in their intensity. It is observable that both figure – 3(a) and (b) complement each other such that when Fe substitution increases, the Fe 2p peak intensity in figure – 3(a) increases while the Mn 2p peak intensity in figure – 3(b) decreases. The results of Mn4+/Mn percentage are presented in Table – based on the chemical analysis from XPS data. It is found that Mn4+ content decreases from 31.8% for x = to 15.1% for x = 0.4 as the Fe doping level increases in the nominal composition of the samples. The decrement of Mn4+ contents correspond well with other related Al, Fe and In-doped La-based compounds measured by redox titration method [112 - 114]. However, we not exclude the possibility of the formation of oxygen vacancies by means of preserving the charge equilibrium in the sample. The above result also helps to explain our earlier experimental observation that the unit cell undergoes an increase in volume as a consequence of more Mn4+ ions of smaller ionic size being replaced by Fe ions of bigger ionic radii in higher Fe-doped compounds. 3.3.3 Magnetic Properties After zero-field cooling (ZFC) down to 77 K, the magnetization data were collected in a 100 Oe magnetic field during the warming process. Figure – 4(a) and (b) depict the temperature dependence of magnetization, M(T) for polycrystalline Nd0.67Sr0.33Mn 1− x Fe x O3 samples with x = 0, 0.05, 0.08, 0.1, 0.15 and x = 0.4, 0.6, 0.8 and 1, respectively. It is clearly shown that only samples from x = 0, 0.05, 0.08 and 0.1 in 73 Chapter Three: Influence of Fe doping in Nd0.67Sr0.33MnO3 polycrystalline system figure – 4(a) undergo a ferromagnetic (FM) to paramagnetic (PM) phase transition within the narrow temperature range. For Fe concentration x ≤ 0.05, the compositions are characterized by sharp transition from magnetically ordered states to paramagnetic states. In compositions with 0.08 ≤ x ≤ 0.15, the transition gradually disappears with increasing x because of the higher degree of structural disorder in the Mn lattice. For x ≥ 0.4, the FM to PM transition completely disappears. This is witnessed by the onset of the Neel temperature, TN at 140 K in the x = 0.4 sample. For 0.6 ≤ x ≤ 1, the magnetization values greatly reduce to less than 0.3 emu/g and continue to decrease with increasing temperature. For x = 1, the magnetization value is so insignificant that it is hardly noticeable over a wide temperature range. Based on MS studies, Chul et al. [94] reported the antiferromagnetic insulating behavior in Nd1-xSrxFeO3 (x = – 1) samples. The Curie temperature Tc is defined as the temperature corresponding to the minimum of dM(T)/dT. For x = 0, Tc is 270 K which is in agreement with the previous report [115]. In the range of x = – 0.1, Tc shifts to a lower temperature progressively with increasing amount of Fe doping. The values of Tc are 270, 190, 115, 88 K for x = 0, 0.05, 0.08 and 0.1, respectively. Thus it can be seen that Fe doping not only drives Tc to lower temperatures but also weakens the magnetization in the system. A 1% increase of Fe doping causes Tc to drop by approximately 18 K. It is interesting to compare this value with that of ∼12 K for Fe-doped La0.7Sr0.3Mn1-xFexO3 (LSMFO) system in Xianyu et al. [116], which leads us to conclude that Fe doping has a stronger effect on weakening the ferromagnetism in Nd- based systems than that in the La-based systems. 74 Chapter Three: Influence of Fe doping in Nd0.67Sr0.33MnO3 polycrystalline system (a) x = 0.0 Magnetization, M (emu/g) x = 0.05 x = 0.08 x = 0.10 x = 0.15 75 125 175 225 275 Temperature, T (K) (b) 0.004 0.0035 x =1 1.6 0.003 1.4 0.0025 x = 0.40 1.2 0.002 0.8 0.0015 0.6 0.001 0.4 x = 0.60 0.0005 0.2 x = 0.80 50 100 Magnetization, M (emu/g) Magnetization, M (emu/g) 1.8 150 200 Temperature, T (K) 250 300 Figure – (a) Temperature dependence of magnetization, M(T) for polycrystalline Nd0.67Sr0.33Mn1-xFexO3 samples with x = 0, 0.05, 0.08, 0.1 and 0.15 (b) M(T) Temperature dependence of magnetization, for polycrystalline Nd0.67Sr0.33Mn 1− x Fe x O3 samples with x = 0.4, 0.6, 0.8 and 1. 75 Chapter Three: Influence of Fe doping in Nd0.67Sr0.33MnO3 polycrystalline system As compared to La0.67Sr0.33MnO3 (LSMO) which has a Tc of 380 K, NSMO has a Tc of 270 K. The lower Tc as observed in the Nd-based system may be attributed to the fact that by replacing the bigger ionic radius La3+ (0.136 nm) by the smaller ionic radius Nd3+ (0.127 nm), a larger lattice distortion is introduced due to the narrower bandwidth of eg electrons in Nd-based system [117] than the La-based system. This makes the sample more susceptible to electron-phonon coupling and Coulomb correlation effects. Ferromagnetism of NSMO is due to double – exchange transfer of the itinerant eg electron between two partially filled d shells via the O2-, such as Mn3+ - O2- - Mn4+ as proposed by Zener [29]. However, it was reported that the Fe3+ moments are antiferromagnetically coupled to the ferromagnetic Mn-O network in lower Fe-doped compounds resulting in a decrease in magnetization. For x = 1, Chul et al. reported that by replacing Nd3+ ions with the Sr2+ ions, Fe4+ ions are produced. This effect interrupts the strong Fe3+- O- Fe3+ antiferromagnetic interaction as the empty eg orbital of Fe4+ ions enable electron transfer between the Fe3+ and Fe4+ ions [94]. 76 Chapter Three: Influence of Fe doping in Nd0.67Sr0.33MnO3 polycrystalline system M a g n e tiza tio n , M (e m u /g ) 80 M a g ne tiz atio n , M (e m u /g) 90 (a) x= 70 x = .0 60 x = .0 50 80 70 60 50 40 30 20 10 x = 0.0 x = 0.1 x = 0.1 10 A pplied Field,H (T) 40 x = .1 30 20 10 x = .1 0 A pplied F ield, H (T ) 12 (b) Magnetization, M (emu/g) 10 x = 0.4 x = 0.6 x = 0.8 x=1 0 Applied Field, H (T) Figure – (a) Field dependence of magnetization, M(H) for polycrystalline Nd0.67Sr0.33Mn1-xFexO3 samples with x = 0, 0.05, 0.08, 0.1 and 0.15 (b) Field dependence of magnetization, M(H) for polycrystalline Nd0.67Sr0.33Mn1-xFexO3 samples with x = 0.4, 0.6, 0.8 and 1. Inset shows the field dependence of magnetization, M(H) for polycrystalline Nd0.67Sr0.33Mn1-xFexO3 samples with x = 0.08, 0.1 and 0.15 at higher applied field. 77 Chapter Three: Influence of Fe doping in Nd0.67Sr0.33MnO3 polycrystalline system To get a clearer picture of the magnetic behavior, we measured the field, H dependence of magnetization, M(H) at 77 K for polycrystalline Nd0.67Sr0.33Mn1-xFexO3 samples with x = 0, 0.05, 0.08, 0.1, 0.15, 0.4, 0.6, 0.8 and as shown in figure – 5. The M – H curves of x = and 0.05 in figure – 5(a) show ferromagnetic behavior at 77 K, reaching saturation at a low field of 10 kOe. It is found that a magnetically soft NSMO ferromagnet has a magnetic moment of 4.2µB per formula unit at K [95]. This value is higher than the spin-only value of 3.67µB expected from the full alignment of Mn3+ and Mn4+ ions. The additional magnetic moment of ∼0.5µB may be attributed to the magnetic Nd3+ ions. Previously, Millange et al. [118] demonstrated using neutron diffraction method that the magnetic moment of Nd3+ in Nd0.7Sr0.3MnO3 reaches up to ∼ 0.8µB and points in the same direction as the Mn magnetic moments. However, at 77 K, the calculated effective magnetic moment of NSMO is only ∼ 3.4µB. Therefore we can conclude that some of the Nd and Mn spins not align ferromagnetically with each other at this high temperature, resulting in lower effective magnetic moment obtained. As iron doping increases, the effective magnetic moment in the overall compound decreases. For x = 0.08 and 0.1 compositions, larger field is needed for the samples to attain saturation. However, the M – H curves for x ≥ 0.15 in figure – 5(b) are quite different from that in figure – 5(a) for x = and 0.05. The magnetization of these compounds increase consistently with increasing applied field, without attaining full saturation even at high field. At low Fe content, it was proposed that Fe exists only as Fe3+ ion in the high spin state ( t 23g e g2 , S = ) in the compounds. Fe3+ ions cause a strong antiferromagnetic interaction of Fe3+-O- Fe3+ and Mn3+- O- Fe3+ couplings based on MS studies and as reported by Ahn et al. [119, 120]. However, in pure ironate (x = 1), the observed effective 78 Chapter Three: Influence of Fe doping in Nd0.67Sr0.33MnO3 polycrystalline system magnetic moment is ∼2.7µB at a field of T. The theoretical spin-only value from the full alignment of high spin states of Fe3+ and Fe4+ ions is approximately 4.67µB assuming that they are in the nominal ratio. The reduction of moments is most probably owing to the covalency (i.e. Fe3+ and Fe4+), possible formation of spin glass and misalignments of spins [121]. These experiments on the magnetic properties of NSMFO compounds give many interesting results. We now present the following hypothesis for discussion. Upon Fe doping, the effects caused directly by Fe are (1) change of the lattice parameters and thus the unit volumes, (2) modification of the internal magnetic environment of the Mn ions. Upon doping at Mn sites by Fe ions of variable valence state, the lattice parameters are changed due to the different ionic radii of the Fe ions from the Mn ions. This produces a distortion of the Mn-O-Mn bond length and a change in the Mn-O-Mn bond angle [122]. When this happen the regular octahedra buckles and the overlapping between the orbitals narrow, leading to weaker ferromagnetic coupling among the Mn ions. This is manifested by the fact that Tc is driven to a lower temperature and the magnetization exhibits a drop with Fe doping. At the same time, antiferromagnetic couplings among the Mn-O-Fe bonds are also induced. As a result, the effective magnetic moment decreases progressively. The contribution of Nd3+ ions which causes bandwidth narrowing effect may be another reason for faster drop in Tc with increasing Fe content in the system. 3.3.4 Electrical Transport Properties Figure – (a) shows the resistivity curves as a function of temperature for NSMFO (x = 0.0, 0.05, 0.08, 0.1, 0.15, 0.3, 0.4 and 1) compounds under zero field. 79 Chapter Three: Influence of Fe doping in Nd0.67Sr0.33MnO3 polycrystalline system (a) (b) Figure – 6(a) Zero-field resistivity ρ as a function of temperature for polycrystalline Nd0.67Sr0.33Mn1-xFexO3 samples with x = 0, 0.05, 0.08, 0.1, 0.15, 0.3, 0.4, 0.8 and 1. (b) ln(ρ) as a function of T-1/4 for x = 0, 0.05, 0.08, 0.1, 0.15, 0.3 and 0.4. 80 Chapter Three: Influence of Fe doping in Nd0.67Sr0.33MnO3 polycrystalline system Table – Mn4+, Curie temperature Tc, resistivity peak temperature Tp, Neel temperature TN and parameter T0 of VRH model for x = – 0.4 samples. Sample Mn4+ (%) Tc (K) Tp (K) TN (K) x=0 31.8 274 272 - x = 0.05 34.6 190 188 - x = 0.08 32.8 115 88 - x = 0.1 28.4 88 78 - x = 0.15 22.1 - x = 0.3 18 - x = 0.4 15.1 T0 (K) 7.37x106 6.52x107 5.22x107 9.95x107 1.36x108 2.60x108 3.07x108 140 Samples with x ≤ 0.1 display a metal-to-insulator transition at a resistivity peak temperature, Tp. The values of Tp are 272, 188, 88 and 78 K for x = 0.0, 0.05, 0.08 and 0.1, respectively. The changes in Tp are similar to those for Tc, which implies a close correlation between the magnetism and conductivity of the samples. When x exceeds 0.1, the samples behave like insulators without showing metal-to-insulator transition throughout the whole temperature range. The resistivity increases noticeably by about seven orders of magnitude with Fe doping. It should be noted that the resistivity for x > 0.3 is so large that it can only be measured from T > 100 K onwards. The Fe ions are expected to replace some of the Mn ions, thus blocking part of the transport channels. The high resistivity observed in our samples as compared to about only 100 Ωcm in La0.67Sr0.33Co0.7Fe0.3O3 [123] implies that there are more electrical transport channels in La0.67Sr0.33CoO3 than in Nd0.67Sr0.33MnO3. As mentioned earlier, with the prediction of Fe3+ in Fe 2p XPS spectra, the AFM SE interaction between Fe3+- O- Fe3+ in these compounds causes the decrease in magnetization, hence the increase in resistivity. Figure – 6(b) illustrates the relation of ln(ρ) as a function of T-1/4 for x = 0, 0.05, 0.08, 0.1, 81 Chapter Three: Influence of Fe doping in Nd0.67Sr0.33MnO3 polycrystalline system 0.15, 0.3 and 0.4 samples. It is found that the resistivity above Tp can be fitted well into T  Mott’s law of variable range hopping (VRH) model [40], ρ = ρ ∞ exp  T  parameter T0 is related to the localization length ξ by kT0 = 24 πN (E F )ξ 1/ . The where N (EF ) denotes the density of states at the Fermi level and k is the Boltzmann constant. From Table – 2, the value of T0 for x = 0.4 is almost 42 times of that for x = 0, thus the localization length decreases greatly in samples with higher Fe doping. Hence Fe doping on Mn sites increases the extent of disorder in the lattice so the carriers are trapped in the localized state by severe potential fluctuation. The carriers’ mobility greatly reduces with increasing Fe doping. This result is very similar to that observed in Fe-doped La0.67Sr0.33CoO3 [39] by Sun et al. As Fe doping further increases, some of the iron ions begin to transit to tetravalent state with only singly unoccupied eg orbital in the high spin state. The weak antiferromagnetic bonding of Fe3+- O- Fe4+ enhances the electrical conduction. For a pure NSFO compound, the estimated concentration of Fe3+:Fe4+ by XPS spectra is 2:1. The resistivity curves for highly Fe doped (x = 0.8 and 1) compounds not resemble those of the lowly Fe doped (x = 0.0, 0.05, 0.08, 0.1 and 0.15) compounds in terms of their shape. This is because as Fe3+ and Fe4+ become the majority carriers in the system, the different electronic state will form another type of transport paths for carrier motion to take place. The resistivity drops slightly below that of x = 0.4 with increasing amount of Fe4+ content. The electrical conductivity is enhanced by the easy transfer of eg electron of Fe3+ to the empty eg orbital of Fe4+ ion by the intervening oxygen ion. Though enhanced, the dominating Fe3+ in the system still plays a major role in the high resistivity observed in the sample. 82 Chapter Three: Influence of Fe doping in Nd0.67Sr0.33MnO3 polycrystalline system 90 .5 x = 0.08 .5 M R (% 80 .5 70 x = 0.10 .5 60 M R (% 120 50 40 170 220 270 T e m p e tu re , T (K ) x = 0.05 x = 0.00 30 20 10 50 100 150 200 T e m p e ture , T (K ) 250 300 Figure – MR (%) ratio versus temperature for polycrystalline Nd0.67Sr0.33Mn1xFexO3 samples with x = 0, 0.05, 0.08 and 0.1. Inset shows the MR ratio for polycrystalline Nd0.67Sr0.33Mn0.6Fe0.4O3 samples. .9 x = .1 x= .8 x = .0 M R (% .7 .6 .5 .4 x = .1 0 .3 .2 10 A p p lie d F ie ld , H (kO e ) 15 Figure – MR (%) ratio versus applied field, H for polycrystalline Nd0.67Sr0.33Mn1xFexO3 samples with x = 0, 0.05, 0.1 and 0.15. 83 Chapter Three: Influence of Fe doping in Nd0.67Sr0.33MnO3 polycrystalline system 3.3.5 Magnetoresistance In figure – 7, we show the temperature dependence of the magnetoresistance (MR) for Nd0.67Sr0.33Mn1-xFexO3 (x = 0, 0.05, 0.08 and 0.1) samples under an applied field of 10 kOe. Here, the MR is defined as MR = [ρ(H = 0) - ρ(H = 10 kOe)]/ ρ(H = 0) (3 – 1) where ρ(H = 0) is the zero-field resistivity and ρ(H = 10 kOe) is the resistivity in the 10 kOe applied field, respectively. For x = 0, a maximum MR, MRmax of about 33% was recorded at a temperature TMR of 273 K. With Fe doping, TMR is gradually reduced with increasing MRmax. The MRmax values for x = 0.05 and 0.08 are 43% and 82% at TMR = 181 K and 85 K, respectively. For x = 0.1, the MRmax value at 77 K was 65%. The TMR falls below the measured temperature range for the x = 0.1 sample. Therefore, the MR peak for x < 0.1 corresponds closely to both Tc and Tp. Previously, Xianyu et al. [116] reported that the MR values for Fe-doped LSMFO are 10% and 17% for x = 0.05 and 0.1 compositions under a field of 10 kOe. By comparison, we found that the same amount of Fe doping causes a greater MR enhancement in the NSMFO system. This can be correlated to our earlier findings that Fe doping is more effective in weakening the DE in Nd-based systems than in La-based systems. The larger weakening effect of ferromagnetism is caused by more disordered spins in Nd-based systems than in La-based systems. As a result, a higher MR enhancement was observed when those disordered spins are aligned under an external applied field. This is in contrast to Co substitution in La0.7Ca0.3Mn1-xCoxO3 systems [124]. Co substitution suppresses the magnetoresistance over the entire composition range for the lower Co-doped samples. The inset in figure – 84 Chapter Three: Influence of Fe doping in Nd0.67Sr0.33MnO3 polycrystalline system depicts the temperature dependence of MR for x = 0.4 sample. The MR drops so low that it is hardly noticeable for x > 0.4 samples. The strong spin-disordering and lattice distortion in these samples are discouraged for conduction even under the applied magnetic field. At a fixed temperature of 77 K, the MR in the x = and 0.05 samples exhibit two distinct regimes as a function of magnetic field up to 10 kOe as demonstrated in figure – 8. The low-temperature MR has a low-field and high-field component. For H < kOe, the MR drops rapidly in the low field region followed by a gradual change as H is increased further. At 5% Fe doping, it is found that the initial drop in MR at low field is reduced, but the MR slope under a high field increased. This is similar to that observed in the nano-grained LSMFO polycrystalline thin films [125]. For those low Fe-doped samples, the magnetization is saturated at 77 K, as shown in figure – 5. The conduction mechanism can be attributed to spin polarized tunneling (SPT), where MR effect arises from the alignment of the magnetization of the ferromagnetic grains under an applied field. In the SPT process, the spin polarization of the conduction electrons, P (related to M), and the spin-flip scattering ratio at grain boundaries (GB), γ, are the two important factors that determine the magnitude of the initial drop in MR at low field [126, 127]. For low Fe-doped samples, the decrease in MR at low field can be attributed to the reduced spin polarization and the increased spin-flip scattering as suggested by Huang et al. [125]. On the other hand, the high-field MR slope is correlated to the surface spin states of the magnetic grains and the weakened FM spin interactions at GBs are proposed to be responsible for the enhanced MR at high field region [125]. However, the above arguments not stand for the sample with x = 0.1 and 0.15. The MR observed in these 85 Chapter Three: Influence of Fe doping in Nd0.67Sr0.33MnO3 polycrystalline system samples is due to an intrinsic CMR effect. For these high Fe-doped samples, the effect of Fe doping on the resistivity can be much larger than that from the disorder at the GBs. Therefore, the effect of GB MR is negligible. Furthermore, Tc of the sample is almost located exactly at the measuring temperature of 77 K. Thus, the MR observed here originates from the field suppression of the spin fluctuations inside grains and lattice distortion in the sample itself due to self-trap electrons in the localized states. 3.4 Conclusions In summary, we have made a systematic investigation into the influence of Fe doping on the structural, magnetic and electrical properties of Nd0.67Sr0.33Mn1-xFexO3 (x = – 1) polycrystalline system. Although Mn and Fe are neighboring elements from the middle of the TM row, their contrasting behavior give rise to different effects that affect the magnetic and electrical transport properties in the materials. It is observable that the Curie temperature, metal-to-insulator transition, resistivity and magnetoresistance behaviors are strongly dependent on Fe doping level. The unit cell volume increases indicating that the lattice distortion in the MnO6 octahedra is enhanced as the Fe doping level increases. The type of magnetism formed in the Fe substituted compound changes from FM metallic behavior in NSMO compound to AFM insulator as the system comes close to the other end member of the series, NSFO. The influence on the resistivity is more drastic as the resistivity increases by orders of magnitude (nearly – orders) even with small amount of Fe doping. The resistivity in the paramagnetic region for x ≤ 0.4 compounds fits well into Mott’s law of variable range hopping model. Increasing Fe doping greatly reduces the carriers’ mobility so that the carriers are trapped in the 86 Chapter Three: Influence of Fe doping in Nd0.67Sr0.33MnO3 polycrystalline system localized state by severe potential fluctuation as witnessed by the increasing parameter, To. Since the long-range ferromagnetic order is weakened by the Fe substitution, the magnetoresistance is also affected. The MR increases as Fe content increases in the system for x < 0.15. Further increment of Fe content does not bring further increase of MR as is seen in x > 0.3 compounds. Therefore, it can be seen that, while Mn3+ and Mn4+ enhance the ferromagnetic DE interaction in the system, Fe3+ and Fe4+ enhance the antiferromagnetic SE interaction in the system, although Fe4+ increases electron transfer via the singly unoccupied eg orbital. The presence of a larger amount of Fe3+ in the system as compared to Fe4+ shifts the entire system from FM towards AFM as x increases. In AFM however, the electrical conductivity in the system is observed to increase with more Fe4+, which enables greater transfer of electrons. 87 [...]... magnetization of these compounds increase consistently with increasing applied field, without attaining full saturation even at high field At low Fe content, it was proposed that Fe exists only as Fe3 + ion in the high 3 2 spin state ( t 2 g e g , S = 5 ) in the compounds Fe3 + ions cause a strong antiferromagnetic 2 interaction of Fe3 +-O- Fe3 + and Mn3+- O- Fe3 + couplings based on MS studies and as reported... localized states 3. 4 Conclusions In summary, we have made a systematic investigation into the influence of Fe doping on the structural, magnetic and electrical properties of Nd0. 67Sr0. 33 Mn1-xFexO3 (x = 0 – 1) polycrystalline system Although Mn and Fe are neighboring elements from the middle of the TM row, their contrasting behavior give rise to different effects that affect the magnetic and electrical... narrow, leading to weaker ferromagnetic coupling among the Mn ions This is manifested by the fact that Tc is driven to a lower temperature and the magnetization exhibits a drop with Fe doping At the same time, antiferromagnetic couplings among the Mn-O -Fe bonds are also induced As a result, the effective magnetic moment decreases progressively The contribution of Nd3+ ions which causes bandwidth narrowing... the ferromagnetic Mn-O network in lower Fe- doped compounds resulting in a decrease in magnetization For x = 1, Chul et al reported that by replacing Nd3+ ions with the Sr2+ ions, Fe4 + ions are produced This effect interrupts the strong Fe3 +- O- Fe3 + antiferromagnetic interaction as the empty eg orbital of Fe4 + ions enable electron transfer between the Fe3 + and Fe4 + ions [94] 76 Chapter Three: Influence... for polycrystalline Nd0. 67Sr0. 33 Mn 1− x Fe x O3 samples with x = 0, 0.05, 0.08, 0.1, 0.15 and x = 0.4, 0.6, 0.8 and 1, respectively It is clearly shown that only samples from x = 0, 0.05, 0.08 and 0.1 in 73 Chapter Three: Influence of Fe doping in Nd0. 67Sr0. 33 MnO3 polycrystalline system figure 3 – 4 (a) undergo a ferromagnetic (FM) to paramagnetic (PM) phase transition within the narrow temperature range... Nd0. 67Sr0. 33 MnO3 As mentioned earlier, with the prediction of Fe3 + in Fe 2p XPS spectra, the AFM SE interaction between Fe3 +- O- Fe3 + in these compounds causes the decrease in magnetization, hence the increase in resistivity Figure 3 – 6(b) illustrates the relation of ln(ρ) as a function of T-1/4 for x = 0, 0.05, 0.08, 0.1, 81 Chapter Three: Influence of Fe doping in Nd0. 67Sr0. 33 MnO3 polycrystalline system... Increasing Fe doping greatly reduces the carriers’ mobility so that the carriers are trapped in the 86 Chapter Three: Influence of Fe doping in Nd0. 67Sr0. 33 MnO3 polycrystalline system localized state by severe potential fluctuation as witnessed by the increasing parameter, To Since the long-range ferromagnetic order is weakened by the Fe substitution, the magnetoresistance is also affected The MR increases... increases as Fe content increases in the system for x < 0.15 Further increment of Fe content does not bring further increase of MR as is seen in x > 0 .3 compounds Therefore, it can be seen that, while Mn3+ and Mn4+ enhance the ferromagnetic DE interaction in the system, Fe3 + and Fe4 + enhance the antiferromagnetic SE interaction in the system, although Fe4 + increases electron transfer via the singly... Figure 3 – 8 MR (%) ratio versus applied field, H for polycrystalline Nd0. 67Sr0. 33 Mn1xFexO3 samples with x = 0, 0.05, 0.1 and 0.15 83 Chapter Three: Influence of Fe doping in Nd0. 67Sr0. 33 MnO3 polycrystalline system 3. 3.5 Magnetoresistance In figure 3 – 7, we show the temperature dependence of the magnetoresistance (MR) for Nd0. 67Sr0. 33 Mn1-xFexO3 (x = 0, 0.05, 0.08 and 0.1) samples under an applied... that the resistivity for x > 0 .3 is so large that it can only be measured from T > 100 K onwards The Fe ions are expected to replace some of the Mn ions, thus blocking part of the transport channels The high resistivity observed in our samples as compared to about only 100 Ωcm in La0 .67Sr0. 33 Co0. 7Fe0 .3O3 [1 23] implies that there are more electrical transport channels in La0 .67Sr0. 33 CoO3 than in Nd0. 67Sr0. 33 MnO3 . has a ground state as an antiferromagnetic (AFM) insulator arising from bondings of Fe 3+ - O 2- - Fe 3+ , Fe 4+ - O 2- - Fe 4+ , and Fe 3+ - O 2- - Fe 4+ , which can be explained by a. ( t , 23 2 gg e 2 5 = S ) in the compounds. Fe 3+ ions cause a strong antiferromagnetic interaction of Fe 3+ -O- Fe 3+ and Mn 3+ - O- Fe 3+ couplings based on MS studies and as reported by Ahn. Chapter Three: Influence of Fe doping in Nd 0.67 Sr 0 .33 MnO 3 polycrystalline system 3 Effect of Fe Substitution for Mn on the Structural, Magnetic and Transport Properties in Polycrystalline