Microstructure, Magnetic And Transport Properties Of Fe₃O₄ Based Heterostructures

The heterostructures based on half-metals and semiconductors is very promising in spin-injection, p-n heterostructures, and magnetoresistive random access memories. Recently, Fe₃O₄ has been widely investigated in spintronics as a half-metal of high curie temperature and being easy to be fabricated. In microelectronics, silicon is the most widely used and technologically mature semiconductor. So, there are tempting applications of the heterostructures based on Fe₃O₄ and silicon. On the other hand, the sign of magnetoresistance in the heterostructures involving half-metals and semiconductors always be different from that of the intrinsic magnetoresistance of the half-metal itself. The mechanisms of the magnetoresistance of heterostructures have been explained as two current model, interface, space charge effect and so on. But the main factors determining the magnetoresistance of Fe₃O₄-based heterostructures are still controversial. To investigate the effects of different barriers on magnetotransport properties, we deposited Fe₃O₄ films on single-crystal silicon and varied the barriers between Fe₃O₄ and silicon.Fe₃O₄/MgO, Fe₃O₄/MgO/Fe₃O₄, Fe₃O₄/Si, Fe₃O₄/MgO/Si, Fe₃O₄/SiO2/Si and Fe₃O₄/MgO/SiO2/Si heterostructures were prepared by facing–target reactive sputtering and radio frequency (RF) sputtering. The microstructure, magnetic properties, interface phases and transport properties of the heterostructures were studied systematically.The I-V curves of the Fe₃O₄/MgO and Fe₃O₄/MgO/Fe₃O₄ heterostructures are nonlinear but no rectifying effect was observed. Conducting mechanisms are tunneling at low bias and thermal emission/diffusion at high bias. Magnetoresistance reaches its extreme value at ¹20 K and is considered to be related with bias. The results indicate that the magnetoresistance is caused by the intrinsic magnetoresistance of Fe₃O₄.The magnetotransport properties of the Fe₃O₄/barrier/Si heterostructures is barrier dependent obviously. The magnetoresistance was found to be positive in the Fe₃O₄/Si and Fe₃O₄/SiO2/Si heterostructures, and negative in the Fe₃O₄/MgO(3 nm)/Si heterostructure. The negative magnetoresistance is related with Verwey transition and reaches its extreme value at ¹20 K, while the positive magnetoresistance does not correlate with the Verwey transition. Magnetoresistance in the Fe₃O₄/barrier/Si heterostructures is almost zero when the current is low, and it becomes saturated gradually as the current increases. The magnetoresistance increases with applied magnetic field, and the increasing rate becomes lower as the magnetic field goes up. The magnetotransport properties of the Fe₃O₄/barrier/Si heterostructures are mainly caused by the spin polarization of Fe₃O₄ at the interface. If the spin polarization is different in sign between the interfacial Fe₃O₄ and the inside of the Fe₃O₄ layer, the magnetoresistance is positive, and vice versa.