Effect of phase separation induced supercooling on magnetotransport properties of epitaxial La5/8-yPryCa3/8MnO3 (y~0.4) thin film

Thin films of La5/8-yPryCa3/8MnO3 (y~0.4) have been grown on single crystal SrTiO3 (001) by RF sputtering. The structural and surface characterizations confirm the epitaxial nature of these film. However, the difference between the rocking curve of the (002) and (110) peaks and the presence of pits/holes in the step-terrace type surface morphology suggests high density of defect in these films. Pronounced hysteresis between the field cool cooled (FCC) and field cooled warming (FCW) magnetization measurements suggest towards the non-ergodic magnetic state. The origin of this nonergodicity could be traced to the magnetic liquid like state arising from the delicacy of the coexisting magnetic phases, viz., ferromagnetic and antiferromagnetic-charge ordered (FM/AFM-CO). The large difference between the insulator metal transitions during cooling and warming cycles (TIMC~64 K and TIMW~123 K) could be regarded as a manifestation of the nonergodicity leading to supercooling of the magnetic liquid while cooling. The nonergodicity and supercooling are weakened by the AFM-FM phase transition induced by an external magnetic field. TIM and small polaron activation energy corresponding the magnetic liquid state (cooling cycle) vary nonlinearly with the applied magnetic field but become linear in the crystalline solid state (warming cycle). The analysis of the low temperature resistivity data shows that electron-phonon interaction is drastically reduced by the applied magnetic field. The resistivity minimum in the lower temperature region of the self-field warming curve has been explained in terms of the Kondo like scattering in the magnetically inhomogeneous regime.


Introduction
Phase separation (PS) is believed to be the key ingredient of the physics of doped rare earth manganites. [1][2][3] Extensive experimental and theoretical investigations spread over the last two decades have established as the most dominant mechanism on the composition-temperature (x-T) diagram of intermediate and low bandwidth manganites like Nd1-xSrxMnO3, 3,4 Sm1-xSrxMnO3 5,6 and La1-x-yPryCaxMnO3. [7][8][9][10] Amongst these materials, La1-x-yPryCaxMnO3 has emerged as the prototypical among the phase separated manganites. Different compositional and structural variants like bulk single crystal, polycrystals and thin films of La1-x-yPryCaxMnO3 have been investigated. [7][8][9][10][11][12][13][14][15][16][17][18][19][20] The pronounced nature of the PS has been established by the observation of (i) strong divergence of the zero filed cooled (ZFC) and field cooled warming (FCW) magnetization, (ii) pronounced hysteresis between the field cooled cool (FCC) and FCW magnetization, and (iii) prominent thermomagnetic hysteresis in the temperature and magnetic field (H) dependent resistivity () measured in cooling-warming cycles. [7][8][9][10][11][12][13][14][15][16] The coexistence of sub-micrometer scale ferromagnetic metallic (FMM) and antiferromagnetic/charge ordered insulator (AFM/COI) clusters has been demonstrated by a study by Uehara et al. 7 Their study has also shown that the AFM/COI phase appears explicitly in magnetotransport measurements only at y≥0.3. The coexisting FMM and AFM/COI clusters, directly impact the electrical transport by making it percolative, which is evidenced by huge residual resistivity (0) for y0.4 in the metallic regime. 7 The study of Ghivelder and Parisi 8 on bulk-polycrystalline La5/8-yPryCa3/8MnO3 (y0.4) has shown that COI phase appears at TCO  230 K and subsequently undergoes transition to AFM and FM spin order at TN  180 K and TC  80 K, respectively. Large temporal relaxation in magnetization and resistivity has also been observed in this prototype PS system and has been attributed to the rapid spatial and temporal variations in the relative fraction of FMM and AFM phases. 8 Further, the theoretical study by these authors has predicted that interplay between temperature and separation of the system from equilibrium could create multiple blocked states. 8 Sharma et al. 9 studying a similar material have established the existence of a liquid like magnetic state in the phase separated regime, which transforms cooperatively to a randomly frozen glass like phase at low temperature. The frozen glass like phase (termed as strain glass) is believed to arise from the presence of martensitic accommodation strain. 9 Wu et al. 10 have demonstrated that in La5/8-yPryCa3/8MnO3 (y0.4) thin films the magnetic liquid like state exhibits a supercooled glass transition. This glass transition is believed to arise due to the presence of the accommodation strain caused by distinct structural symmetries of FMM and AFM/COI phases. 11 Their study has also provided evidence in favour of the non-ergodic nature of the magnetic liquid, which appears when the long range cooperative strain interactions hinder the cooperative dynamic freezing of the first-order AFM/COI-FMM transition. 9,10 In thin films an additional degree of freedom to play with and tune the magnetic and transport properties becomes available in form of substrate induced strain. Due to the delicate nature of the phase separated state in manganites of smaller bandwidths like Sm1-xSrxMnO3 5,6 and La1-x-yPryCaxMnO3 12 insulating phase with small phase separation while huge extrinsic disorder causes phase separation at larger length scales and shows an IMT in the absence of magnetic field at variance with the bulk compound. In (La0.4Pr0.6)0.67Ca0.33MnO3 thin films electric-field induced anisotropic transport has been observed in the FPS state by Jeen and Biswas. 19 According to them the main driving force for the anisotropy is the collective rearrangement of the FMM phase under electric fields.
Recently, Singh et al. 20 have investigated the tunability of the IMT in terms of the variation in the relative fraction of the coexisting FMM and AFM/COI phases in La5/8-yPryCa3/8MnO3 (y0.4) thin films (~42 nm). This study has clearly demonstrated that the supercooling transition temperature is non-unique and strongly depends on the magneto-thermodynamic path through which the low temperature state is accessed. In contrast, the superheating transition temperature remains invariant of the thermal cycling. However, the detail investigation of the impact of supercooling/superheating on the electrical transport in varying magnetic field has not been carried out. In the present paper we report detailed investigation on the structure, microstructure and magnetotransport properties of ~ 42 nm thin La5/8-yPryCa3/8MnO3 (y0.4) thin films grown by RF magnetron sputtering on (001) oriented SrTiO3 (STO) substrate. Our study reveals that the nature of the electrical transport in the PM regime remains unaffected by the thermal cycling. In contrast, in the lower temperature region different scattering mechanisms appear to acquire dominance during the cooling and warming cycles.
The parameters characterizing electrical transport are found to be non-linear in the supercooled regime and approach linearity in the superheated regime. (at−as) x100⁄ as where at and as are the lattice parameters of the bulk target and substrate,

Experimental Details
respectively.] and hence the strain is tensile. In order to achieve optimum oxygen content the films were annealed at ~ 900 C for 10 hr. in flowing oxygen. The film thickness was estimated from the X-ray reflectivity (XRR) measurements. The strcutructural and microstructural characteristics were probed by high resolution X-ray diffraction (HRXRD, PANalytical PRO X'PERT MRD, Cu-Kα1 radiation λ = 1.5406 Å) and atomic force microscopy (AFM, VEECO Nanoscope V), respectively. The temperature and magnetic field dependent magnetic and magnetotransport properties were measured by commercial MPMS and PPMS (both Quantum Design).

Results and Discussion
The experimental and simulated XRR curves are plotted in Fig In order to acquire qualitative idea about the degree of defects and hence to probe the structural quality of the film, in-plane and out-of-plane rocking curves ( scan) were measured.
As shown in Fig. 3 the out-of-plane rocking curve of (002) reflection) has full width at half maximum (FWHM) ≈ 0.63, which is higher than generally observed values for manganite films deposited by magnetron sputtering. 21 The in-plane rocking curve of (110) reflection has FWHM ≈ 0.99, which is considerably larger than the out-of-plane value. The symmetry of the two rocking curves suggests that the film is nearly strain free. The long duration oxygen annealing and larger mismatch between the substrate (a = 0.3905 nm) and bulk (a= 0.3842 nm) in plane lattice parameters could be regarded as the major influences causing relaxation of strain.
The broadening of the rocking curve is generally attributed to the presence of (i) strain, (ii) dislocation density, (iii) mosaic spread, and (iv) curvature. As pointed out above the rocking curve broadening due to strain is expected to be very small. Although small variation in the FWHM of the rocking curves was observed as a function of the beam size, no linear dependence could be established between them. This rules out the contribution from curvature induced broadening. Thus the rocking curve broadening in the present case is attributed mainly to the mosaic spread and dislocation density. Since the FWHM variation with the beam size was not appreciable we believe that the dominant contribution to the peak broadening comes from the presence of dislocation arrays/network. The large difference between the FWHMs of (002) and (110) rocking curves suggests that the density of defects and mosaicity are different along the different planes, that is, the density distribution is anisotropic. In this regard it appears that the substrate film interface could have higher density of dislocations as compared to the epitaxial layers above. In fact it is well known that the dislocation networks present at the The temperature dependent resistivity (-T) measured at different values of H was measured in cooling and warming cycles (Fig. 6). In the cooling cycle -T shows insulating behavior as shown by about six orders of magnitude rise in resistivity between 300 -65 K and IMT is observed at T C IM ≈ 64 K. As T is lowered further down the -T curve appears to saturate. In the warming cycle the -T curve remains reversible with the cooling cycle  (T) up to Tg 15 K. As T is increased further up  (T) decreases, approaching a minimum at TM  45 K (inset of Fig. 6). In the warming cycle the IMT shifts to a higher temperature and appears at T W IM ≈123 K. In the PMI regime -T curves overlaps with the one in cooling cycle. The two distinct transitions at TIM C and TIM W in cooling and warming cycles are separated by TIM59 K. The observed thermal hysteresis in  (T) is attributed to supercooling and superheating of the magnetic liquid consisting of FMM and AFM-COI phases and is an evidence of a first order phase transition. 20,23 The sharp drop in the -T during the cooling cycle could be regarded as manifestation of dynamical magnetic liquid behavior. The saturation and reversible behavior of -T at T<Tg is due to the crossover from the liquid like state to the strain glass regime. [8][9][10]23 During the warming cycle the -T minimum at TM  45 K has been regarded as a consequence of thermal devitrification of disordered SRG in to an ordered FMM. 16 Here, 'n' is the polaron concentration, 'a' is the site-to-site hopping distance, '' is the attempt frequency, and kB is the Boltzmann constant. EA is the activation energy, i.e., the height of the potential trap, and EA = EP/2 − t. In general, the overlap integral, 't' is so small that it could be neglected and then, EA ≈ EP/2, or the polaron binding energy EP ≈ 2EA.
The small polaron activation energy (EA) was calculated from the fitting parameter derived from the best fit (data not plotted here) to eq. (1) in the temperature range TIMT300 K.
Variation of EA with magnetic field is plotted in Fig. 8(a). In absence of external magnetic field (H=0 kOe) the estimated value of EA is 138 meV. Such high values of the activation energy are typical of the low bandwidth manganite films 29 and are significantly larger than that of the large and intermediate bandwidth compounds like La1-xSrxMnO3 33 , Nd1-xSrxMnO3. 24 Such high value of EA clearly suggests strong electron-lattice coupling through the J-T distortion. EA138 meV corresponds to TIM C /TIM W  64 K/123 K in cooling and warming cycles. As the magnetic field is increased the value of EA decreases nonlinearly and at H=50 kOe, EA112 meV corresponds to TIM C /TIM W ≈ 184 K/185 K in cooling and warming cycles. The variation of TIM C /TIM W with EA is plotted in Fig. 8(b). The sharp rise in TIM C at smaller magnetic field, that is, higher values of EA is a signature of the liquid like behaviour of the two phase mixture in the cooling cycle. This liquid like behaviour is not retraced in the warming cycle and hence TIM W -EA behaviour is nearly linear.
Deep in the FMM regime, the -T is characterized by (i) sharp decrease in the cooling cycle, (ii) appearance of a minimum at TM at H=0 kOe in the warming cycle which vanishes at H10 kOe, (iii) relatively slower increase in the warming cycle. The low temperature -T behaviour is not well studied in strongly phase separated manganites like the present compound. In order to explain the low temperature -T behaviour in manganites several mechanisms have been in taken into consideration. The first is the Kondo like scattering given by, ρ T nT. 35 The second mechanism considered is an elastic scattering correction term of the type ρ  T . , which accounts for the disorder enhanced strong Coulomb interaction between the carriers.
Such a term is generally observed in a disordered metallic system and it changes sign as a function of disorder in the system. 36 The third term is the electron-electron (e-e) scattering described by a contribution of the type ρ  T . 33,36 The fourth contribution is the electronphonon (e-ph) scattering, which at low temperatures acquires the form ρ  T and in view of the strong electron-lattice coupling through the J-T distortion is expected to be make significant contribution to the low temperature resistivity of low bandwidth manganites. 36 In the present case we tried to fit the cooling and warming cycle low temperature -T data measured at different magnetic fields using different combination of the above mentioned mechanisms. It is interesting to note that expect the zero magnetic field warming data, all the -T curves at T<<TIM were fitted very well by the following equation: Here, 0 is the residual resistivity arising due to elastic scattering like electron impurity scattering. The -T data of the zero field warming cycle could not be fitted with the equation 2. The appearance of the minimum in the H=0 warming cycle -T prompted us to consider the Kondo like scattering. It has been shown that in inhomogeneous magnetic systems, e. g., those exhibiting glassy behaviour like the present films, Kondo like transport may appear in the low temperature regime. 35 The best fit to this was obtained by replacing the e-e scattering by the Kondo term (ρ T nT ), that by the equation, However, the -T data corresponding to the glassy state (T<18 K) could not be fitted even by this equation. The representative low temperature data along with the fitted ones are presented in Fig. 9. The fitting parameters ρ and ρ have been found to depend on the thermal cycling as well as the applied magnetic field. The values of both, ρ and ρ are found to be higher in the cooling cycle and decrease under the influence of the magnetic field. However, the change in ρ as a function of thermal cycle and the magnetic field are much more pronounced. The variation of the coefficient ρ with the H in both the thermal cycles is plotted in Fig. 10.