Lien-Chang Wang

Spin-transfer torque switching in magnetic tunnel junctions and spin-transfer torque random access memory

We present experimental and numerical results of current-driven magnetization switching in magnetic tunnel junctions. The experiments show that, for MgO-based magnetic tunnelling junctions, the tunnelling magnetoresistance ratio is as large as 155% and the intrinsic switching current density is as low as 1.1 ? 106?A?cm?2. The thermal effect and current pulse width on spin-transfer magnetization switching are explored based on the analytical and numerical calculations. Three distinct switching modes, thermal activation, dynamic reversal, and precessional process, are identified within the experimental parameter space. The switching current distribution, write error, and read disturb are discussed based on device design considerations. The challenges and requirements for the successful application of spin-transfer torque as the write scheme in random access memory are addressed.

Spin transfer switching in dual MgO magnetic tunnel junctions

Dual magnetic tunnel junction (MTJ) structures consisting of two MgO insulating barriers of different resistances, two pinned reference layers aligned antiparallel to one another, and a free layer embedded between the two insulating barriers have been developed. The electron transport and spin dependent resistances in the dual MTJ structures are accounted for by sequential tunneling with some spin-flip relaxation in the central electrode (the free layer). With a tunneling magnetoresistance ratio of 70%, a switching current density Jc (at 30ms) of 0.52MA∕cm2 is obtained, corresponding to an intrinsic value of Jc0 (at 1ns) of 1.0MA∕cm2. This value of Jc0 is 2–3 times smaller than that of a single MgO insulating barrier MTJ structure and results from improvements in the spin-transfer torque efficiency. The asymmetry between JcAP→P and JcP→AP is significantly improved, which widens the read-write margin for memory device design. In addition, the experimental results show that the switching current density can...

Spin-transfer switching in MgO-based magnetic tunnel junctions (invited)

We present spin-transfer switching results for MgO-based magnetic tunneling junctions (MTJs) with large tunneling magnetoresistance ratio of up to 150% and low intrinsic switching current density (Jc0) of (2–3)×106A∕cm2. The low intrinsic switching current density is attributed to high tunneling spin polarization (TSP) in MgO-based MTJs. The current switching data are discussed based on a qualitative study of TSP in MgO-based MTJs. Additional film stack modification needed to decrease the switching current to meet the requirement of advanced magnetoresistive random access memory application is also discussed.We present tight-binding calculations of the spin torque in non-collinear magnetic tunnel junctions based on the non-equilibrium Green functions approach. We have calculated the spin torque via the effective local magnetic moment approach and the divergence of the spin current. We show that both methods are equivalent, i.e. the absorption of the spin current at the interface is equivalent to the exchange interaction between the electron spins and the local magnetization. The transverse components of the spin torque parallel and perpendicular to the interface oscillate with different phase and decay in the ferromagnetic layer (FM) as a function of the distance from the interface. The period of oscillations is inversely proportional to the difference between the Fermi-momentum of the majority and minority electrons. The phase difference between the two transverse components of the spin torque is due to the precession of the electron spins around the exchange field in the FM layer. In absence of applied bias and for a relatively thin barrier the perpendicular component of the spin torque to the interface is non-zero due to the exchange coupling between the FM layers across the barrier.

Structure, materials and shape optimization of magnetic tunnel junction devices : Spin-transfer switching current reduction for future magnetoresistive random access memory application

We present a systematic study of spin transfer switching in magnetic tunneling junctions (MTJs). Several ways to decrease the switching current density through material and stack engineering and MTJ element shape optimization are explained in detail. The data are presented for switching on MgO-based MTJ with high tunnel magnetoresistance (TMR) of 150% and low intrinsic switching current density Jc0 of (2–3)×106 A/cm2. Micromagnetic modeling is used to study the spin transfer switching mechanism in nanosecond regime for spin transfer torque random access memory (STT-RAM) pillar. The importance of current-induced Oersted field on the initial onset of precession is discussed.