The structural and magnetotransport properties of the solid solution CaxSr1-xMn0.5Ru0.5O3 were explored using chemical, epitaxial and physical pressure. The experimental procedure involved solid state synthesis and film growth using pulsed laser deposition (PLD). Extensive magnetotransport and structural characterization using x-ray diffraction and Rietveld refinements were carried out to understand the correlations between the structure and properties. Additionally, high pressure neutron powder diffraction (HPNPD) experiments were completed in order to investigate magnetic order as a function of physical pressure.
The magnetoresistance (MR) of several CaxSr1-xMn0.5Ru0.5O3 compositions in bulk and thin films were explored. In the bulk, three different regions were defined to help explain their MR properties. Region I was composed of the ferrimagnetic x = 1, 0.5 compositions which exhibited a sharp low field MR reminiscent of intergranular tunneling magnetoresistance (TMR). Region II contained the x = 0.25 two-phase composition which exhibited the largest MR of -67% MR at 25 K. The third region contained the antiferromagnetic x < 0.25 compositions which exhibited MR as high as -58% for the full Sr analogue. The MR properties of this region are influenced by the increasing size of the AFM domains and formation of magnetic clusters under applied magnetic fields. Additionally, exchange bias (EB) was observed in the MR loops of the Sr-containing compositions but not the ferrimagnetic Ca-end member. This is a good indication that the EB observed is highly dependent upon the ratio of the antiferromagnetic regions to the ferrimagnetic regions. In order to minimize grain boundary effects that can dominate the behavior of bulk materials, thin films of the x = 0, 0.25 and 1 compositions were grown and the magnetotransport data collected. The comparative data of the bulk and film materials illustrates that their MR and conductivity trends are similar. However, the x = 0.25 composition in the films exhibits the largest negative MR of the samples studied, -93% at low temperatures.
The sensitivity of CaMn0.5Ru0.5O3 and SrMn0.5Ru0.5O3 films to lattice mismatch induced grain boundaries effects and different growth atmospheres (respectively) was determined. CaMn0.5Ru0.5O3 films were grown on LSAT and SrTiO3 which have different lattice mismatches. From this study, the most bulk-like films were grown on LSAT, which has a smaller lattice mismatch than the SrTiO3. SrMn0.5Ru0.5O3 films grown in different levels of pO2 exhibited an increase in lattice parameters, but there were limited effects upon the conductivity, stoichiometry and magnetoresistive properties. This suggests that the properties of these materials may be relatively insensitive to moderate levels of oxygen vacancies.
The magnetic order of orbitally-ordered SrMn0.5Ru0.5O3 was monitored using HPNPD in order to determine the compression mechanism and any correlations with chemical pressure. These experiments were completed at pressures ranging from 0 GPa to 8 GPa and at temperatures above and below the magnetic ordering temperature of 200 K. Anisotropic compression of the axial bonds containing Jahn-Teller distorted Mn3+ ions dominated at pressures below 2 GPa. Above this pressure, an increase in octahedral tilting was observed. However, the increase in octahedral tilting at 8 GPa was not enough to completely destroy the orbital ordering and drive the magnetic order from antiferromagnetic to ferrimagnetic nor a phase transition to lower symmetry.