Role of grain boundaries in double exchange manganite oxides La1−xAxMnO3 (A = Ba, Ca)

Magnetic and electronic properties of the magnetoresistive perovskites La0.67−xBixCa0.33MnO3

“…Behavior similar to that observed here has been seen previously in single phase polycrystalline samples and has been discussed in the context of grain boundary effects by Ju et al [16,17]. Although these authors have given convincing arguments in favor of grain boundary effects, it is important to consider the reasons why this behavior is not seen in all polycrystalline samples.…”

“…Unlike the higher temperature peak, however, there is no clearly associated corresponding feature in the magnetic susceptibility or FMR measurements. In single crystal samples, it is known that the magnetoresistivity effect is insignificant below ¹ , although in polycrystalline samples there are several reports of substantial lowtemperature effects [4,[16][17][18]. On the basis of several reports, the possible origin of such behavior could be (1) very fine particles as suggested by Mahesh et al [18]; (2) the existence of a second magnetic phase (3) the appearance of a second magnetic phase transition or (4) the effect of grain boundaries.…”

“…It is known in general that the grain boundary resistivity is very high in several oxide systems. Ju et al [17] have calculated the grain and grain boundary resistivity and found that the grain boundary resistivity is much larger than the bulk grain resistivity and that the peak in the grain boundary resistivity occurs below ¹ .…”

“…For example, in single crystal La (Pb,Ca) MnO a magnetoresitivity effect of only 0.2% is observed at 5 K in a field of 5 T [15]. However, in many polycrystalline materials a substantial effect has been reported even at low temperatures [4,[16][17][18]. It is customary to intrepret the temperature of the highest temperature peak in the MRR in terms of the magnetic transition temperature.…”

MAGNETORESISTANCE AT ROOM TEMPERATURE IN THE A-SITE ORDERED Pr0.70Ba0.30MnO3+δ MANGANITES

“…Below T C the ferromagnetic order decreases via magnetoimpurity scattering of the charge carriers and the conductiv-ity shows a metallic behavior. The resistivity peak at T I ~ 128 K for the oxygen stoichiometric A-site disordered Pr 0.70 Ba 0.30 MnO 3 sample can not be explained by spin-dependent tunneling of the charge carriers between grains because the grains are in the micrometer range and the grain boundaries bring in a small contribution to the resistivity [14]. However, above-mentioned peak at T I ~ 128 K may be explained by magnetic phase separation in the ferromagnetic and paramagnetic clusters.…”