Temperature dependence of magnetoresistance oscillations due to Coulomb staircases in a nanometer vertical ferromagnetic tunnel junction

Abstract

Magnetic random access memory is considered to be a promising candidate for a future nonvolatile memory. The size of the magnetic tunnel junction (MTJ) cell has been gradually decreasing and has reached the sub-100 nm level. As the size of the MTJ cell approaches the 10 nm level, we must consider the effects of the interaction between the spin-dependent tunneling and the Coulomb blockade, such as magneto-Coulomb oscillation. Spin-dependent tunneling in a nanometer ferromagnetic junction has been investigated in Coulomb blockade regime. The current-bias voltage (I-V) characteristics of NiO/Co/NiO/Co double junctions with a 30 nm contact hole fabricated by an electron beam direct process have been measured in magnetic fields. The temperature dependence of the I-V curve shows clear Coulomb staircases at temperatures below 50 K. At 12 K, the Coulomb threshold of the I-V curve depends on the magnetization configuration, which leads to a tunnel magnetoresistance (TMR) of over 500% around the Coulomb threshold. Although the origin of such a large TMR has not been well proven, spin accumulation in a nanometer metal island is considered. The bias dependence of TMR shows oscillatory behavior due to Coulomb staircases, and at higher temperatures, the TMR oscillations disappear. The TMR oscillation results from the enhancement of TMR at the steps of the Coulomb staircase associated with discrete charging effects and spin accumulations of a nanometer metal island.