Transport-magnetism correlations in the ferromagnetic oxide La0.7Ca0.3MnO3

Abstract

We present results of temperature and magnetic field dependent resistivity ρ(H,T) and bulk magnetization M(H,T) measurements on post-annealed La0.7Ca0.3MnO3 thin films that were grown via pulsed-laser deposition. Both the resistivity and the anomalously large negative magnetoresistance peak near the ferromagnetic ordering temperature (Tc=250 K), with Δρ/ρ0=−85% at 50 kOe. A clear correlation is found between ρ and M that is described by the phenomenological expression ρ(H,T)∝exp[−M(H,T)/M0]. This correlation reflects the important interplay between transport and magnetism in this system, and suggests that the transport below Tc involves polaron hopping.

Disorder effects in epitaxial thin films of (La,Ca)MnO3

“…Disorder and strain effects can also be influenced by postanneal procedures. Hundley et al 3 showed that with proper annealing at high temperature ͑950°C͒ and oxygen pressure (10 5 Pa͒, the values for T c , T p , and the MRR can become equal to the powder values quoted above. The precise morphology changes induced by the annealing are not exactly known.…”

“…In this letter we study transport, magnetization, and film 3 with a nominal composition of xϭ0.33 on STO substrates, in a pure oxygen atmosphere of 300 Pa, with substrate and source in line and about 2 cm apart. The high pressure causes a very low growth rate of the order of 0.9 nm/min.…”

“…The precise morphology changes induced by the annealing are not exactly known. Among them is most probably strain relief, since the film surface usually changes from atomically smooth to rough on a scale of several nm, 3 which would be due to the formation of relaxed grains. However, postannealing is often undesirable if the goal is to combine films with different physical properties ͑e.g., CMR and high-T c materials͒, nor is it a prerequisite for producing epitaxial and smooth films.…”

“…7 With respect to strain effects, it should be remarked that the unit cell volume is not conserved. Assuming an in-plane lattice parameter of 0.3905 nm, the volume is 5.84ϫ10 Ϫ2 ͑nm͒ 3 , compared to 5.75 ϫ10 Ϫ2 (nm͒ 3 for the bulk unit cell. The ͑negative͒ equivalent hydrostatic pressure for this volume change can be estimated at 5 GPa, 8 which could lead to ⌬T c ӍϪ20 K, 9 too small to explain the experimental value.…”

Exchange interaction and magnetoresistance in La2/3Ca1/3MnO3: experiment and models

“…The nature of this phenomenon is being studied intensively; the main directions of the research are outlined in the above-mentioned reviews. We note here also the following original papers, references to which will be made below [7][8][9][10][11][12][13][14].…”