Multiple magnetic transitions, dynamical magnetic liquid and magnetic glass in La_1−x−yPr_yCa_xMnO₃ (x≈0.42, y≈0.40) thin films: A thickness dependent study

The influence of substrate induced strain and its relaxation on the evolution of the multiple magnetic transitions and ensuing modifications in the degree of phase separation, the nature of the dynamical magnetic liquid, the randomly frozen glass and insulator–metal transitions have been investigated in single crystalline La_1−x−yPr_yCa_xMnO₃ (x≈0.42, y≈0.40) in t~20–140 nm thick films deposited on LaAlO₃ (001) substrates. The ferromagnetic (FM) transition temperature (T_C) first decreases as the film thickness is increased from t~20 nm to t~60 nm and then increases with increasing film thickness. In contrast the charge ordering (CO), antiferromagnetic (AFM) and glass transition temperatures shift towards higher values with increasing film thickness. The field cooled cooling (FCC) and field cooled warming (FCW) magnetization (M–T) of films having t≥60 nm shows pronounced hysteresis and ΔT_C=T_C^FCW−T_C^FCC decreases concomitantly from 46 K to 35 K as the thickness increases from ~60 to ~140 nm. The thinnest film shows insulator to metal transitions (IMT) only at magnetic field H>40 kOe. Films with t≥T_C show sharp hysteretic IMT, with ΔT_IM=T_IM^W−T_IM^C decreasing from ~70 K to ~50 K as the thickness increases from ~60 nm to ~140 nm. Such strong hysteresis is a characteristic of first order phase transition and also a signature of magnetic liquid like phase created by the magnetic frustration created by the delicate balance between FM and AFM/CO phases. The H induced AFM/CO to FM transition reduces ΔT_IM and at higher fields the phase transition appears akin to the second order. The observed difference in the magnetic and transport properties have been explained in terms of the substrate induced strain at lower film thickness and its relaxation at higher thickness. - Highlights: • Different thickness La_1−x−yPr_yCa_xMnO₃ films grown on (001) LaAlO₃ by RF magnetron sputtering. • Relaxation of strain with increasing film thickness. • Thickness dependent multiple magnetic transitions and magneto-electric phase profile. • Magnetic frustration generated dynamical magnetic liquid extremely sensitive to film thickness. • Crossover from a higher temperature COI to lower temperature glassy state via AFMI and FMM phases