Structural, magneto transport and magnetic properties of Ruddlesdenâ•fiPopper La2-2xSr1+2xMn2O7 (0.42â›¤xâ›¤0.52) layered manganites

The magneto transport of Ruddlesden–Popper, La2-2xSr1+2xMn2O7 (0.42≤x≤0.52), 2-dimensional bilayered manganites have been investigated in a broad temperature range. The samples have been synthesized using the solid-state reaction method. Rietveld refinement of the X-ray diffraction data indicates the tetragonal structure formation with I4/mmm space group. The resistivity curves of the samples present a general characteristic of metal-insulator (MI) transition at certain temperature (TMI). Besides, for samples with x = 0.48, 0.50, and 0.52, at a certain temperature (TCO) well below TMI the charge ordering is also evident. Furthermore, the samples display a shallow upturn in resistivity below Tmin due to the Kondo like spin scattering effect, weak localization and electron-phonon interaction. The high temperature semiconducting (T>TMI) region, the resistivity curve follows the 3D Mott’s variable hopping transport mechanism. The overall suppression of resistivity accounts for a substantial magnetoresistance by applying a magnetic field and the characteristic change in TMI, TCO, and Tmin are discussed. Temperature-dependent magnetization demonstrates the suppression of ferromagnetism and evident of antiferromagnetic nature with an apparent charge ordering with increasing concentration of Sr2+.


I. INTRODUCTION
Ruddlesden-Popper compounds having general formula (A) n+1 BnO 3n+1 {AO(ABO 3 )n} are a kind of perovskite structure consisting of a two-dimensional (2D) layered structure interspersed with cations (A =La, Sr, and B=Mn) and anions (Oxygen) and n represents the number of layers in perovskite-like array, stacked between rock-salt AO layers along the crystallographic c-axis. 1 In this array, the perovskite structure corresponds to n = ∞ and the K 2 NiF 4 structure to n = 1. The 2-dimensional character in the compounds can be introduced by switching from the perovskite (n = ∞) to the bilayer (n = 2) array. Subsequently, due to the reduction in number of next neighbour B cations around each metal transition from 6 to 5 causing anisotropic reduction in the bandwidth of one electron, which in turn substantially alters the electronic and transport properties. 2 The La-based Ruddlesden-Popper double layered manganite La 2-2x Sr 1+2x Mn 2 O7 have gained importance due to colossal magnetoresistance (CMR) [3][4][5] analogous to 3D perovskite manganite (La 1-x SrxMnO 3 ). However, in incongruity with the three-dimensional (3D) half doped perovskite manganite, which shows charge exchange (CE) spin/charge ordering (CO), 6 the A-type of antiferromagnetic (AFM) state is dominant in Ruddlesden-Popper half substituted layered manganite. 7,8 Mitchell et al. 9 studied the magnetic phase diagram of La 2-2x Sr 1+2x Mn 2 O7 studied using neutron diffraction for a series substitution of Sr 2+ at La 3+ site (x). Besides, the double exchange mechanism, ARTICLE scitation.org/journal/adv which basically accounts for intrinsic MR in 3D perovskite manganite. 10 Due to the high spin polarization of the conduction electron, the MR found in the 2D layered system is much larger than found in 3D. 3,11,12 Further, the electrical transport mechanism in theses 3D and 2D manganite has been reported on the accounts of various transport mechanism, 13-18 such as; low temperature upturn due to Kondo effect, weak localization effect, electron-electron, electron phonon scattering and High temperature semiconducting region on account of 2D Mott's variable range hopping model (2D M-VRH), 3D M-VRH, spin polarized conduction (SPC) etc.
In this study a comprehensive study of low temperature transport and high temperature semiconducting behavior in La 2-2x Sr 1+2x Mn 2 O7 (0.42≤x≤0.52) are investigated. It is observed that the resistivity below T min , shows T 1/2 dependence suggesting the incidence of weak localization. In the region T>T MI , electrical is expressed by Mott's variable hopping model. In addition, samples with x = 0.46, 0.48, and 0.50 shows significant ∼58% MR at 8T, measured at 5 K.

II. EXPERIMENTAL
The polycrystalline samples of Sr 2+ substituted La 2-2x Sr 1+2x Mn 2 O7 (0.42≤x≤ 0.52) were synthesized adopting a conventional high temperature solid state reaction method. The La 2 O 3 , SrCo 3 , and Mn 2 O 3 powders of 99.9% purity procured from sigma Aldrich were weighed stoichiometric ratio and grounded several times until the homogeneous distribution was achieved. The homogenous powders thus, obtained were sintered at 1000 ○ C for 48 hr with The obtained powder was pressed into pellets and at last sintered at 1350 ○ C for 30 hr, followed by furnace cooling in the air. Room temperature powder X-ray diffraction technique was carried out for the study of structural characterization and phase formation. A standard four probe set up equipped with 8T Oxford-superconducting magnet (at UGC-DAE-CSR, Indore Centre) was used for the measurement of temperature dependent DC electrical resistivity under different magnetic field.

III. RESULTS AND DISCUSSION
The X-ray diffraction profile of La 2-2x Sr 1+2x Mn 2 O7 (0.42≤x ≤ 0.52) shown in Fig. 1 (a), indicates that samples are formed in the single phase. The structure of the samples refined considering tetragonal unit cell, I4/mmm space group, shows excellent agreement between the experimental and the theoretical intensities. The refinement was carried out using Rietveld refinement technique 19 FullProf Program 20 and the lattice parameters taken for the refinement are tabulated in Table I. The decrease in c/a ratio observed, with an increasing concentration of Sr 2+ at La-site is in accordance with the results of Ling et al. 21 The temperature (T) dependent electrical resistivity (ρ) curve for the series of La 2-2x Sr 1+2x Mn 2 O7 (0.42≤x≤ 0.52) measured in the absence (0T) and the in the presence of the applied field (H) is shown in Fig 2 (a-e). It can be observed from these figs that the samples demonstrate the generic metal-insulator transition at a certain temperature (T MI ), which for x = 0.42, 0.46, 0.48, 0.50, and 0.52 is approximately 172, 182, 192, 203, and 199 K respectively. T MI is determined by the minimum in the dρ/dT curve. The shift of T MI first to the higher temperature side up to x = 0.48 and later shift to the lower temperature side with the increasing substitution of Sr 2+ at La 3+ , suggesting the first incitement of the double exchange (DE) mechanism 10 and later the inference of charge-ordering (marked as T CO in the fig) suppress the DE interaction in the samples with x = 0.50 and 0.52. Besides, for the samples, on the whole the resistivity decreases substantially and T MI falls to the high temperature region by applying a magnetic field. This proposes once more the eminence of DE due to the magnetic field, which leads to enhanced ferromagnetism and substantial magnetoresistance (MR) (shown in the inset of Fig. 1(f)) in the samples. The magnetoresistance ratio (MR %) was determined from the equation, MR% = (ρ0 − ρH)/ρH ) × 100, where, ρ0 and ρH are the resistivity without and with field respectively. The (-) MR% isotherms at 5 K shown in the inset of Fig 1(f), indicates that MR increases with the field and at 8T, it's comparable in the samples with x = 0.46, 0.48, and 0.50 (∼58%). Besides, the x = 0.46 sample shows a significant low field MR (∼18%, at 1T) compared to the other samples. It can also be observed from Fig 1 (b-f), that with the decreasing temperature, at a certain temperature (T min ), resistivity curve shows a shallow upturn and shifts to the higher temperature side with the increasing Sr 2+ . For sample with comparatively high Sr 2+ , T min gets suppressed with charge ordering in the samples and shits to the high temperature side. Besides, the application of magnetic field, T min gets suppressed and also shifts to the lower temperature side. Therefore, it is understood that the spin-dependent scattering must be coupled with the resistivity minimum which is eventually reduced by the applied magnetic field. The perovskite manganite shows strongly correlated electronic behavior and to understand the transport behavior we have adopted the model considering the suitable interactions. The resistivity behavior in the region T<T min , could be well fitted using equation using equation, 22 ρ I = ρ 0 − ρs ⋅ ln(T) + ρc ⋅ T 0.5 + ρp ⋅ T 5 and the region T min <T<T MI , followed, ρ M = ρ 0 + ρ T ⋅ T 2.5 + ρ R ⋅ T 4.5 . Hence, the overall resistivity data for the temperature T<T MI has been fitted using the combined equation given below (fitting shown along with the resistivity curve in Fig. 1 (b-f)); Here ρ 0−MI is the residual resistivity which depends on defects, grain boundary effects and magnetic domain boundary effects; ρs is the Kondo-like spin-dependent scattering; ρc is weak-localization effect in a 3D system, ρ T is the electron-electron interaction at low temperature and the metallic region respectively; and ρp is the electronphonon interaction, ρ T is the single magnon scattering and ρ R suggest two magnons scattering process. The best fit value using Eq. (1) is presented in Table I. The obtained best fit value, suggests Kondo effect certainly does not appear to be a potential source for the pragmatic up-turn in the resistivity with decreasing temperature for the double layered manganite and the weak localization is found to be dominant. The 3D T 0.5 dependence in 2D layered manganite is understood to arise from a residual of the carrier hopping between the bi-layers. 23 The electron-phonon interaction also found to contribute notably to the shallow upturn in the resistivity. Besides, in the mid temperature region T min <T< T MI , the two magnons scattering is dominant. The high temperature resistivity (T>T MI ) region follows 3D Mott's variable hopping mechanism (M-VRH) in agreement with Gupta et al., 23 explained that 3D M-VRH model is most suitable to describe the temperature dependent resistivity in the semiconducting region is given by; Here ρ 0 is the "pre-exponential factor" and T 0 is the "Mott characteristic temperature" related to the density of state in near Fermi energy N(E f ).
Here ξ is the localization length. The density of state N(E f ) can be determined from T 0 by the estimating value of ξ=0.45 nm -1 . Besides the resistivity behavior in the mid temperature region near T MI is identified to be percolative in nature and can be explained using two phase model (Ref. 24 and references therein); not a scope of present study. Fig. 2 shows the temperature dependent magnetization plot of samples with x = 0.42, 0.46, 0.48, 0.52 measured at 1000 Oe and x = 0.50 at 100 Oe. In the inset dM/dT vs. T curves are plotted to estimate paramagnetic (PM) to ferromagnetic (FM) temperature, defined as Curie temperature Tc, transition and the charge ordering (CO) temperature (T CO ). The estimated T C is 332, 316, 294, 276, 262 K for x = 0.42, 0.46, 0.48, 0.50, and 0.52 respectively and T CO is evident at 218, 232, 232, 236 K for x = 0.46, 0.48, 0.50, and 0.52 respectively. Dediu et al. 25 proposed that while the materials preserve the paramagnetic (PM) nature both above and below T CO , the magnetic interaction at T CO may change from ferromagnetic (FM) (T > T CO ) to antiferromagnetic (AFM) (T < T CO ) evident as a peak-like feature, making up a sample anisotropic antiferromagnetic, which is a characteristic feature of 2D layered manganites. 26 It is evident that x = 0.42, shows PM to FM transition with no evidence of charge ordering. Besides, with the increase of the concentration of Sr 2+ at La-site, the CO (growth in peak-like feature) is apparent by suppressing the FM behavior and progression of AFM nature. As discussed previously, the aforementioned behaviors of the samples are in resemblance with the ρ(T) characteristics. The reduction in T C and the enhancement of T CO could be understood due to the weakening of DE interaction with the increase of Mn 4+ /Mn 3+ ratio. The disparity between T MI and T C can be interpreted as a consequence of non-magnetic randomness and active hopping disorder in the DE mechanism, that have a vital role in the evaluation of anomalous transport characteristics and magnetism in manganites. 27

IV. CONCLUSION
In conclusion, we have presented the electrical transport behavior of La 2-2x Sr 1+2x Mn 2 O7 (0.42≤x≤0.52) in the absence and presence of a magnetic field. The common metal insulator transition at certain temperature (T MI ), which falls to higher temperature side on applying the field, is profound and understood owing to double exchange (DE) interactions. The low temperature resistivity upturn behavior ARTICLE scitation.org/journal/adv has been studied taking into account the various scattering processes, though the weak-localization effect is found to be dominant. Besides, the resistivity coefficients acquired considering numerous scattering processes are strongly dependent on the applied magnetic field. The metallic nature follows T 2.5 and T 4.5 , suggesting the contribution of both single magnon and two magnon scattering processes. The high temperature semiconducting nature satisfies the Mott's-variable range hopping mechanism in the wide temperature range. The temperature dependent magnetization behavior is in close agreement with the temperature dependent resistivity data. Besides, the difference between T MI and T C may be due to non-magnetic randomness and active hopping disorder in the DE mechanism.