The origin of the resistivity minimum observed in strongly phase separated manganites has been investigated in single crystalline thin films of LPCMO (x~0.42, y~0.40). The antiferromagnetic/charge ordered insulator (AFM/COI)-ferromagnetic metal (FMM)
phase transition, coupled with the colossal hysteresis between the field cool cooled and field cooled warming magnetization demonstrates strongly phase separated nature, which gives rise to non-equilibrium magnetic liquid state that freezes into a magnetic glass. The thermal cycling and magnetic field dependence of the resistivity unambiguously shows that the pronounced resistivity minimum observed during warming is a consequence non-equilibrium states resulting from the magnetic frustration created by the delicate coexistence of the FMM and AFM/COI phases. The non-equilibrium states and hence the resistivity minimum is extremely sensitive to the relative fraction of the coexisting phases and can be tuned by intrinsic and extrinsic perturbations like the defect density, thermal cycling and magnetic field.
Magnetic and magnetotransport properties of oriented polycrystalline Pr0.58Ca0.42MnO3 thin films prepared in flowing oxygen and air ambient has been investigated. The magnetic ground state of both the films is a frozen cluster glass. In the air annea
led film charge order (CO) is quenched and ferromagnetic (FM) transition, which appears at TC=148 K is followed by antiferromagnetic (AFM) transition at TN=104 K. This film shows self-field hysteretic insulator-metal transition (IMT) at TIMC=89 K and TIMW=148 K in the cooling and warming cycle, respectively. Application of magnetic field (H) gradually enhances TIMC and TIMW, reduces the thermoresistive hysteresis and TIM diminishes. In contrast, the film annealed in flowing oxygen shows a CO transition, which is followed by FM and AFM transitions. This film shows appreciably smaller magnetic moment and does not show IMT upto H=20 kOe. As H is increased to H=30 kOe, IMT having strong thermoresistive hysteresis and sharp resistivity jumps appears in the cooling and warming cycles. As H increases to higher values the thermoresistive hysteresis is reduced, resistivity jumps are observed to disappear and TIM decreases. In the lower temperature regime the resistivity first decreases slowly with H and then shows sharp drop. The virgin cycle is not recoverable in subsequent cycles. The decrement far more pronounced in the oxygen annealed film and occurs at much higher H suggesting that the frozen cluster glass state is more robust in this film. The microstructural analysis of the two set of films shows CO quenching, FM transition and self-field IMT in air annealed film is caused by higher density of microstructural disorder and lattice defects. The difference in growth ambience of the two films could give rise to such microstructural perturbations.
The magnetic and magnetotransport properties of single crystalline La1-x-yPryCaxMnO3 (x=0.42, y=0.40) thin films (~140 nm) deposited on (110) oriented LaAlO3 and SrTiO3 substrates exhibit a crossover from the high temperature antiferromagnetic-charge
ordered insulator (AFM-COI) phase (T>TN) to strain glass (T<Tg). At intermediate temperatures (Tg<T<TN) dynamical liquid having prominent thermal-magneto-resistive hysteresis dominates in the cooling cycle, while in the warming cycle it is preceded by ferromagnetic metal (FMM) phase. Magnetic field required to drive AFM-COI to FMM phase transition are higher than that for the strain glass. The magneto-electric nature and temperature span of the distinct magnetic regimes are sensitive to the thermal cycling and substrate induced strain.
Highly oriented polycrystalline SSMO thin films deposited on single crystal substrates by ultrasonic nebulized spray pyrolysis have been studied. The film on LAO is under compressive strain while LSAT and STO are under tensile strain. The presence of
a metamagnetic state akin to cluster glass formed due to coexisting FM and antiferromagnetic/charge order (AFM/CO) clusters. All the films show colossal magnetoresistance but its temperature and magnetic field dependence are drastically different. In the lower temperature region the magnetic field dependent isothermal resistivity also shows signature of metamagnetic transitions. The observed results have been explained in terms of the variation of the relative fractions of the coexisting FM and AFM/CO phases as a function of the substrate induced strain and oxygen vacancy induced quenched disorder.
