Strain - induced thermal cycle effect in two prototypical phase separated manganites

JOAQUIN SACANELL; M. QUINTERO; J. CURIALE; R. S. FREITAS; L. GHIVELDER; C. ACHA; G. POLLA; G. LEYVA; F. PARISI; P. LEVY

Trieste, Italia

Workshop; Workshop on "Intrinsic Multiscale Structure and Dynamics in Complex Electronic Oxides"; 2002

International Center for Theoretical Physics

Arising from the interplay of lattice, spin, charge and orbital degrees of freedom, the nontrivial coexistence of submicrometer ferromagnetic (FM) regions and charge/orbitally ordered (COO) ones in manganese-oxide-based compounds, known as phase separation (PS), gives rise to a rich variety of physical phenomena [1]. Besides, microstructural characteristics, as grain size [2,3] and connectivity [4], are emerging as  important tools to tailor transport and magnetic properties. In these PS compounds, the FM phase fraction f, its spatial distribution [5] and its softness to enlarge when an external H is applied, determine the overall magnetic and transport properties [6]. We report on a study of the effect of thermal cycles, performed by cooling from room temperature,  on transport and magnetic properties of polycrystalline samples of the phase separated compounds La1/2Ca1/2MnO3 (LCMO) and La5/8-yPryCa3/8MnO3 (y=0.3) (LPCMO). Samples were synthesized by the sol gel technique, thermal treatments were performed at different temperatures around 1000 C to obtain pellets with different grain size. We measured resistivity r (standard 4 probe technique) and magnetization  M (extraction QD PPMS magnetometer) as a function of temperature. Data obtained on LCMO with different low temperature FM fractions shows that r below 200 K is dependent on the number of thermal cycles previously performed on the sample, namely r at 30 K increases by about a 10 to 100% in each cycle, depending on the sample. Concomitantly, M below 200 K decreases with consecutive thermal cycles, unambiguously signaling a decrease in the amount of the FM fraction after each cycle. A similar result is found on a polycrystalline sample of LPCMO, notwithstanding that the differences between the phase separated states of both systems produces different temperature ranges to be affected. We suggest that this memory effect is related to a microstuctural training process related to strain induction, which favors the increase of the insulating phase when these regions were already formed in a previous thermal cycle. This strain-induced effect may be originated in the martensitic-like transformations [3] exhibited by these compounds. The thermal behavior of the PS compounds LCMO and LPCMO allows to delineate the following scenario: 1)  On cooling from room temperature these compounds undergo a sequence of phase transitions, including a first order one which involves structural changes. Strain originated in these transitions gives rise to the global irreversibility of the compounds; 2) The effect of a thermal cycle is always towards an increase of the amount of the COO phase in the next cycle, irrespective of the actual ground state of the sample (COO for LCMO, FM for LPCMO);3) Changes induced by thermal cycling can be partially erased by applying magnetic field or hydrostatic pressure, this last result enforcing the image of the structural nature of the effect; 4) A qualitative account of the r vs cycle behavior can be obtained considering a model in which the FM phase decrease at cycle n proportionally to the amount it did at cycle n-1. [1] E. Dagotto et al., Phys. Rep. 344 (2001)[2] P. Levy, et al., Phys. Rev. B 62, 6437 (2000)[3] V. Podzorov et al., Phys. Rev. B 64, 140406 (2001)[4] A. De Andres et al., Phys. Rev. B 60, 7328 (1999)[5] Uehara, Mori, Chen, Cheong, Nature, 399, 560 (1999)[6] J. Sacanell et al., Phys. B, In Press 2002