Hsin-wei Tseng

Spin-Torque Switching with the Giant Spin Hall Effect of Tantalum

Giant Spin Hall One of the primary challenges in the field of spin-electronics, which exploits the electrons spin rather than its charge, is to create strong currents of electrons with polarized spins. One way to do this is to use a ferromagnet as a polarizer, a principle used in magnetic tunnel junctions; however, these devices suffer from reliability problems. An alternative is the spin Hall effect, where running a charge current through a material generates a spin current in the transverse direction, but the efficiency of this process tends to be small. Liu et al. (p. 555) now show that the spin Hall effect in Tantalum in its high-resistance β phase generates spin currents strong enough to induce switching of the magnetization of an adjacent ferromagnet; at the same time, Ta does not cause energy dissipation in the ferromagnet. These properties allowed efficient and reliable operation of a prototype three-terminal device. Tantalum is found to generate strong spin currents that can induce switching of ferromagnets efficiently and reliably. Spin currents can apply useful torques in spintronic devices. The spin Hall effect has been proposed as a source of spin current, but its modest strength has limited its usefulness. We report a giant spin Hall effect (SHE) in β-tantalum that generates spin currents intense enough to induce efficient spin-torque switching of ferromagnets at room temperature. We quantify this SHE by three independent methods and demonstrate spin-torque switching of both out-of-plane and in-plane magnetized layers. We furthermore implement a three-terminal device that uses current passing through a tantalum-ferromagnet bilayer to switch a nanomagnet, with a magnetic tunnel junction for read-out. This simple, reliable, and efficient design may eliminate the main obstacles to the development of magnetic memory and nonvolatile spin logic technologies.

Spin transfer torque devices utilizing the giant spin Hall effect of tungsten

We report a giant spin Hall effect in β-W thin films. Using spin torque induced ferromagnetic resonance with a β-W/CoFeB bilayer microstrip, we determine the spin Hall angle to be |θSHβ-W|=0.30±0.02, large enough for an in-plane current to efficiently reverse the orientation of an in-plane magnetized CoFeB free layer of a nanoscale magnetic tunnel junction adjacent to a thin β-W layer. From switching data obtained with such 3-terminal devices, we independently determine |θSHβ-W|=0.33±0.06. We also report variation of the spin Hall switching efficiency with W layers of different resistivities and hence of variable (α and β) phase composition.

Central role of domain wall depinning for perpendicular magnetization switching driven by spin torque from the spin Hall effect

We study deterministic magnetic reversal of a perpendicularly magnetized Co layer in a Co/MgO/Ta nano-square driven by spin Hall torque from an in-plane current flowing in an underlying Pt layer. The rate-limiting step of the switching process is domain-wall (DW) depinning by spin Hall torque via a thermally-assisted mechanism that eventually produces full reversal by domain expansion. An in-plane applied magnetic field collinear with the current is required, with the necessary field scale set by the need to overcome DW chirality imposed by the Dzyaloshinskii-Moriya interaction. Once Joule heating is taken into account the switching current density is quantitatively consistent with a spin Hall angle {\theta}

Atomic-scale spectroscopic imaging of CoFeB/Mg–B–O/CoFeB magnetic tunnel junctions

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Interface and oxide quality of CoFeB/MgO/Si tunnel junctions

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BOTTOM PINNED SOT-MRAM BIT STRUCTURE AND METHOD OF FABRICATION

0.07 for 4 nm of Pt.

Spin transfer torque devices utilizing the spin Hall effect of tungsten

Atomic-scale electron spectroscopic imaging on sputtered magnetic tunnel junctions (MTJs) with a thin, <2 nm, MgO layer and B-alloyed electrodes reveals B diffusion into the MgO, resulting in a Mg–B–O tunnel barrier. This ∼2 nm thick interfacial layer forms due to oxidation of CoFeB during radio frequency sputtering of MgO and subsequent B diffusion into MgO during annealing. We measure a room-temperature tunneling magnetoresistance (TMR) of ∼200% in IrMn/CoFeB/Mg–B–O/CoFeB MTJs after annealing, demonstrating that thin Mg–B–O barriers can produce relatively high TMR.

SELF-RECOVERY MAGNETIC RANDOM ACCESS MEMORY UNIT

The use of boron-alloyed electrodes with the radio frequency (rf) sputter deposition of MgO yields magnetic tunnel junctions (MTJs) with Mg–B–O tunnel barriers. After annealing, such MTJs can exhibit very high tunneling magnetoresistance (TMR) in the thin (∼1.0 nm) barrier regime. Scanning tunneling spectroscopy of Mg–B–O layers reveals a better defined, but smaller band gap in comparison to that of thin MgO. We produced Fe60Co20B20/Mg–B–O/Ni65Fe15B20 MTJs where after a 350 °C annealing the Ni–Fe–B free electrode crystallizes into a highly textured (001)-normal body centered cubic (bcc) crystal structure and the MTJs achieve 155% TMR.

Fabrication of side-by-side sensors for MIMO recording

Magnetic tunnel junction (MTJ) has attracted great interest due to its high tunneling magnetoresistance (TMR) ratio,1 where sputter deposition of MgO between CoFeB electrodes is a strong candidate. The improvement in TMR is believed to result from B diffusion into the MgO to form a polycrystalline Mg-B-O layer with a shaper interface after annealing.2 Decrease in trap states can lead to smaller leakage currents and improvement in tunneling conductance. Therefore, a thorough electrical characterization on the CoFeB/Mg-B-O quality is crucial to model the TMR increase and associated reliability. Although the trap charge in the MTJ structure will change the tunneling path and cause serious parametric drift, it is difficult to directly measure its magnitude. Instead, we made CoFeB/MgO/Si MOS capacitors with process flow illustrated in Fig. 1, which can independently determine the interface traps, oxide charge and stress-induced leakage current (SILC) through conductance, high-frequency capacitance-voltage (HFCV) and IV measurements. We can then characterize the boron diffusion and annealing effects on Mg-B-O.