Abstract
Nanoscale magnetic tunnel junctions play a pivotal role in magnetoresistive random access memories. Successful implementation depends on a simultaneous achievement of low switching current for the magnetization switching by spin transfer torque and high thermal stability, along with a continuous reduction of junction size. Perpendicular easy-axis CoFeB/MgO stacks possessing interfacial anisotropy have paved the way down to 20-nm scale, below which a new approach needs to be explored. Here we show magnetic tunnel junctions that satisfy the requirements at ultrafine scale by revisiting shape anisotropy, which is a classical part of magnetic anisotropy but has not been fully utilized in the current perpendicular systems. Magnetization switching solely driven by current is achieved for junctions smaller than 10 nm where sufficient thermal stability is provided by shape anisotropy without adopting new material systems. This work is expected to push forward the development of magnetic tunnel junctions toward single-digit nm-scale nano-magnetics/spintronics.
Highlights
Nanoscale magnetic tunnel junctions play a pivotal role in magnetoresistive random access memories
We employ a commonly available material system FeB/MgO with the double-interface structure, but this time increase the thickness of FeB so that the shape anisotropy emerges as a dominant factor to keep the easy axis along the perpendicular direction
Where μ0 is the permeability in free space, Kb and Ki are the bulk and interfacial-anisotropy energy densities, respectively, and MS, t, and D are the spontaneous magnetization, thickness, and diameter of the ferromagnetic layer, respectively. δN is the difference in dimensionless demagnetization coefficient, or the shape anisotropy coefficient, between the perpendicular and in-plane directions, which is close to 1 when D ≫ t, as has been the case so far
Summary
Nanoscale magnetic tunnel junctions play a pivotal role in magnetoresistive random access memories. Since the theoretical prediction of spin transfer torque (STT) 1,2 and early experimental demonstration of STT-induced magnetization switching[3,4,5,6,7,8,9], its application to writing scheme in magnetoresistive random access memories (STT-MRAMs) has been a focus of spintronics research for the last two decades[10,11,12,13,14,15,16,17,18,19] To make it a viable technology, the magnetic tunnel junctions (MTJs), the heart of the STT-MRAMs, is required to simultaneously show high tunnel magnetoresistance (TMR) ratio, low switching current (density), and high thermal stability factor Δ (≡ E/kBT, where E is the energy barrier between two possible states, kB the Boltzmann constant, and T the absolute temperature). The fabricated MTJs exhibit a high Δ of more than 80, a sufficiently high value for most of the applications, and yet can be switched by STT at sizes smaller than 10 nm
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