Ca MnO3 Thin Films Research Articles

Formation mechanism and thermoelectric properties of CaMnO3 thin films synthesized by annealing of Ca0.5Mn0.5O films

A two-step synthesis approach was utilized to grow CaMnO3 on M-, R- and C-plane sapphire substrates. Radio-frequency reactive magnetron sputtering was used to grow rock-salt-structured (Ca, Mn)O followed by a 3-h annealing step at 800 °C in oxygen flow to form the distorted perovskite phase CaMnO3. The effect of temperature in the post-annealing step was investigated using x-ray diffraction. The phase transformation to CaMnO3 started at 450 °C and was completed at 550 °C. Films grown on R- and C-plane sapphire showed similar structure with a mixed orientation, whereas the film grown on M-plane sapphire was epitaxially grown with an out-of-plane orientation in the [202] direction. The thermoelectric characterization showed that the film grown on M-plane sapphire has about 3.5 times lower resistivity compared to the other films with a resistivity of 0.077 Ωcm at 500 °C. The difference in resistivity is a result from difference in crystal structure, single orientation for M-plane sapphire compared to mixed for R- and C-plane sapphire. The highest absolute Seebeck coefficient value is − 350 µV K−1 for all films and is decreasing with temperature.

Magnetism in CaMnO3 thin films

Epitaxial CaMnO3 thin films on (001) LaAlO3, (LaAlO3)0.3(Sr2AlTaO6)0.7, and SrTiO3 were synthesized in a range of strain states from coherently strained on LaAlO3 (1.6% strain) to fully relaxed on SrTiO3. We observed a magnetic ordering transition in CaMnO3 thin films suppressed from the Néel temperature of bulk CaMnO3 with the suppression increasing with increasing strain in the thin film. In contrast to bulk studies, our CaMnO3 films do not exhibit any sign of weak ferromagnetism according to field-dependent magnetization and x-ray magnetic circular dichroism measurements. Therefore, to within experimental resolution, all of our CaMnO3 films are antiferromagnetic with negligible oxygen off-stoichiometry.

Strain-controlled electronic properties and magnetorelaxor behaviors in electron-doped CaMnO3 thin films

We have fabricated epitaxial thin films of electron-doped manganite Ca1−xCexMnO3 (CCMO) with 0≤x≤0.08. The transport properties of CCMO films are very sensitive to substrate-controlled epitaxial strain. For the CCMO(x=0.05) film, the metallic transport characteristic is observed only on a nearly lattice-matched NdAlO3 (NAO) substrate, while tensilely and compressively stressed films are insulating. The CCMO(x=0.06) film on the NAO substrate shows a large magnetoresistance characteristic of a magnetorelaxor. This behavior can be explained in terms of the phase separation and the irreversible growth of the metallic domain in antiferromagnetic insulating matrix.

Substrate, annealing, and Mn excess effects on La–Ca–MnO3 thin films grown by metalorganic chemical vapor deposition: A way to room-temperature Tc

Oriented thin films of the La–Ca–MnO3 system with several Mn-excess compositions were prepared by aerosol-assisted metalorganic chemical vapor deposition. Depositions both on MgO (100) and LaAlO3 (100) substrates were performed at temperatures between 700 and 800 °C, with 30 min in situ oxygen annealing at the deposition temperature. The films deposited on LaAlO3 have much better structural quality with rocking curves of 0.3–0.7 full width at half maximum and higher Tc. For films with Mn excess, an ex situ annealing (800 °C for 1 h in flowing oxygen) increases Tc dramatically, from 180–190 K to values in the range 240–296 K, above the maximum Tc of the stoichiometric, optimally doped La0.67Ca0.33MnO3 composition. This behavior may be associated with the presence of La vacancies, which act as self-dopants, and whose role is enhanced after suitable annealing. The films present high magnetoresistance values in the vicinity of the critical temperatures: at 7 kOe, 32% for a film with Tc=279 K and 13% for a film with Tc=296 K. The magnetic properties of the films on MgO were degraded by annealing, probably due to Mg diffusion to the film.