We report an investigation into the magnetoresistance (MR) of La_0.8Ba_0.2MnO₃ ultrathin films with various thicknesses. While the 13 nm-thick film shows the commonly reported negative magnetoresistive effect, the 6 nm- and 4 nm-thick films display unconventional positive magnetoresistive (PMR) behavior under certain conditions. As well as the dependence on the film’s thickness, it has been found that the electrical resistivity and the PMR effect of the thinner films are very dependent on the test current. For example, the magnetoresistive ratio of the 4 nm-thick film changes from +46% to –37% when the current is increased from 10 nA to 100 nA under 15 kOe at 40 K. In addition, the two thinner films present opposite changes in electrical resistivity with respect to the test current, i.e., the electroresistive (ER) effect, at low temperatures. We discuss the complex magnetoresistive and ER behaviors by taking account of the weak contacts at grain boundaries between ferromagnetic metallic (FMM) grains. The PMR effect can be attributed to the breaking of the weak contacts due to the giant magnetostriction of the FMM grains under a magnetic field. Considering the competing effects of the conductive filament and local Joule self-heating at grain boundaries on the transport properties, the dissimilar ER effects in the two thinner films are also understandable. These experimental findings provide an additional approach for tuning the magnetoresistive effect in manganite films.

我们报告了对 La _0.8 Ba _0.2 MnO ₃的磁阻 (MR) 的研究各种厚度的超薄膜。虽然 13 nm 厚的薄膜显示出普遍报道的负磁阻效应，但 6 nm 和 4 nm 厚的薄膜在某些条件下显示出非常规的正磁阻 (PMR) 行为。除了对薄膜厚度的依赖性外，还发现较薄薄膜的电阻率和 PMR 效应非常依赖于测试电流。例如，当电流从 10 nA 增加到 100 nA 在 15 kOe 下 40 K 时，4 nm 厚膜的磁阻比从 +46% 变为 –37%。此外，两种更薄的膜呈现相反的变化相对于测试电流的电阻率，即低温下的电阻 (ER) 效应。我们通过考虑铁磁金属 (FMM) 晶粒之间晶界处的弱接触来讨论复杂的磁阻和 ER 行为。PMR 效应可归因于 FMM 晶粒在磁场下的巨磁致伸缩导致弱接触的断裂。考虑到导电丝和晶界局部焦耳自热对传输特性的竞争影响，两种较薄薄膜中不同的 ER 效应也是可以理解的。这些实验结果为调节锰矿薄膜中的磁阻效应提供了另一种方法。PMR 效应可归因于 FMM 晶粒在磁场下的巨磁致伸缩导致弱接触的断裂。考虑到导电丝和晶界局部焦耳自热对传输特性的竞争影响，两种较薄薄膜中不同的 ER 效应也是可以理解的。这些实验结果为调节锰矿薄膜中的磁阻效应提供了另一种方法。PMR 效应可归因于 FMM 晶粒在磁场下的巨磁致伸缩导致弱接触的断裂。考虑到导电丝和晶界局部焦耳自热对传输特性的竞争影响，两种较薄薄膜中不同的 ER 效应也是可以理解的。这些实验结果为调节锰矿薄膜中的磁阻效应提供了另一种方法。