Nature of the low-temperature colossal magnetoresistance of La0.35Nd0.35Sr0.3MnO3 epitaxial films

The colossal negative magnetoresistance (approximately 12%) in a field of 8 .4 kOe over a wide range of temperatures below the Curie point T-C approxim ate to 240 K in a single-crystal La0.35Nd0.35Sr0.3MnO3 film on a single-cry stal (001)ZrO2(Y2O3) wafer substrate is discussed. Isotherms of the magneto resistance of this film reveal that its absolute value increases with the f ield, abruptly in the technical magnetization range and almost linearly in stronger fields. For three single-crystal films of the same composition on (001)LaAlO3, (001)SrTiO3, and (001)MgO substrates, colossal magnetoresistan ce only occurred near T-C approximate to 240 K and at T < T-C it increased weakly, almost linearly, with the field. In the film on a ZrO2(Y2O3) substr ate the electrical resistivity was almost 1.5 orders of magnitude higher th an that in the other three films. It is shown that this increase is attribu table to the electrical resistance of the interfaces between microregions h aving four types of crystallographic orientations, while the magnetoresista nce in the region before technical saturation of the magnetization is attri butable to tunneling of polarized carriers across these interfaces which co incide with the domain walls (in the other three films there is one type of crystallographic orientation). The reduced magnetic moment observed for al l four samples, being only 46% of the pure spin value, can be attributed to the existence of magnetically disordered microregions which originate from the large thickness of the domain walls which is greater than the size of the crystallographic microregions and is of the same order as the film thic kness. The colossal magnetoresistance near T-C and the low-temperature magn etoresistance in fields exceeding the technical saturation level can be att ributed to the existence of strong s-d exchange which is responsible for a steep drop in the carrier mobility (holes) and their partial localization a t levels near the top of the valence band. Under the action of the magnetic field the carrier mobility increases and they become delocalized from thes e levels. (C) 2000 MAIK "Nauka/Interperiodica".