Yunfei Ding

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 and spin polarization in magnetic tunnel junctions with MgO and AlOx barriers

We present spin transfer switching results for MgO based magnetic tunneling junctions (MTJs) with large tunneling magnetoresistance (TMR) ratio of up to 150% and low intrinsic switching current density of 2–3×106A∕cm2. The switching data are compared to those obtained on similar MTJ nanostructures with AlOx barrier. It is observed that the switching current density for MgO based MTJs is 3 to 4 times smaller than that for AlOx based MTJs, and that can be attributed to higher tunneling spin polarization (TSP) in MgO based MTJs. In addition, we report a qualitative study of TSP for a set of samples, ranging from 0.22 for AlOx to 0.46 for MgO based MTJs, and that shows the TSP (at finite bias) responsible for the current-driven magnetization switching is suppressed as compared to zero-bias tunneling spin polarization determined from TMR.

Spin transfer switching current reduction in magnetic tunnel junction based dual spin filter structures

Spin-transfer-driven magnetization switching was studied in single magnetic tunneling junctions (MTJ: Ta∕PtMn∕CoFe∕Ru∕CoFeB∕Al2O3∕CoFeB∕Ta) and dual spin filters (DSF: Ta∕PtMn∕CoFe∕Ru∕CoFeB∕Al2O3∕CoFeB∕spacer∕CoFe∕PtMn∕Ta) having resistance-area (RA) product in the range of 10–30Ωμm2 and tunnel magnetoresistance (TMR) of 15%–30%. The intrinsic critical current density (Jc0) was estimated by extrapolating experimentally obtained critical current density (Jc) versus pulse width (τ) data to a pulse width of 1ns. Jc, extrapolated to τ of 1ns (∼Jc0), was 7×106 and 2.2×106A∕cm2, respectively, for the MTJ and improved DSF samples having identical free layers. Thus, a significant enhancement of the spin transfer switching efficiency is seen for DSF structures compared to the single MTJ case.Spin-transfer-driven magnetization switching was studied in single magnetic tunneling junctions (MTJ: Ta∕PtMn∕CoFe∕Ru∕CoFeB∕Al2O3∕CoFeB∕Ta) and dual spin filters (DSF: Ta∕PtMn∕CoFe∕Ru∕CoFeB∕Al2O3∕CoFeB∕spacer∕CoFe∕PtMn∕Ta) having resistance-area (RA) product in the range of 10–30Ωμm2 and tunnel magnetoresistance (TMR) of 15%–30%. The intrinsic critical current density (Jc0) was estimated by extrapolating experimentally obtained critical current density (Jc) versus pulse width (τ) data to a pulse width of 1ns. Jc, extrapolated to τ of 1ns (∼Jc0), was 7×106 and 2.2×106A∕cm2, respectively, for the MTJ and improved DSF samples having identical free layers. Thus, a significant enhancement of the spin transfer switching efficiency is seen for DSF structures compared to the single MTJ case.

Critical current distribution in spin-transfer-switched magnetic tunnel junctions

In this paper, the switching current distribution data within a cell is presented. Current switching in the magnetic tunneling junctions (MTJ) is measured in DC pulse mode with pulse widths between 3 ms to 1 s for 25 times or more for each cell. RA of the films is in 10 to 20 /spl Omega/-/spl mu/m/sup 2/ range and TMR values between 18 to 30% are obtained using alumina barrier. It is seen, both from the distribution data as well as by evaluating the analytical expression derived for critical current distribution, that the thermal factor is the most important parameter determining the current distribution within a cell.

Spin-transfer switching current distribution and reduction in magnetic tunneling junction-based structures

Spin transfer switching current distribution within a cell and switching current reduction were studied at room temperature for magnetic tunnel junction-based structures with resistance area product (RA) ranged from 10 to 30 /spl Omega/-/spl mu/m/sup 2/ and TMR of 15%-30%. These were patterned into current perpendicular to plane configured nanopillars having elliptical cross sections of area /spl sim/0.02 /spl mu/m/sup 2/. The width of the critical current distribution (sigma/average of distribution), measured using 30 ms current pulse, was found to be 3% for cells with thermal factor (KuV/k/sub B/T) of 65. An analytical expression for probability density function p(I/I/sub c0/) was derived considering a thermally activated spin transfer model, which supports the experimental observation that the thermal factor is the most significant parameter in determining the within-cell critical current distribution. Spin-transfer switching current reduction was investigated through enhancing effective spin polarization factor /spl eta//sub eff/ in magnetic tunnel junction-based dual spin filter (DSF) structures. The intrinsic switching current density (J/sub c0/) was estimated by extrapolating experimental data of critical current density (J/sub c/) versus pulse width (/spl tau/), to a pulse width of 1 ns. A reduction in intrinsic switching current density for a dual spin filter (DSF: Ta/PtMn/CoFe/Ru/CoFeB/Al2O3/CoFeB/spacer/CoFe/PtMn/Ta) was observed compared to single magnetic tunnel junctions (MTJ: Ta/PtMn/CoFe/Ru/CoFeB/Al2O3/CoFeB/Ta). J/sub c/ at /spl tau/ of 1 ns (/spl sim/J/sub c0/) for the MTJ and DSF samples were 7/spl times/10/sup 6/ and 2.2/spl times/10/sup 6/ A/cm/sup 2/, respectively, for identical free layers. Thus, a significant enhancement of the spin transfer switching efficiency is seen for DSF structure compared to the single MTJ case.

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.

[CoFe∕Pt]×n multilayer films with a small perpendicular magnetic anisotropy

The effects of CoFe thickness, Pt thickness, and number of CoFe∕Pt bilayers on the anisotropy and coercivity of [CoFe∕Pt]×n multilayer films have been studied. These parameters are varied in an attempt to deposite [CoFe∕Pt]×n multilayer films with well-defined small perpendicular magnetic anisotropies. Best results were obtained in a [CoFe3A∕Pt10A]×5 film with coercivity Hc=42Oe, perpendicular anisotropy Hk=2200Oe, and easy-axis remanence Mr∕Ms=1. Large Pt thickness tends to cause well-defined interfaces thus larger surface anisotropy. Large CoFe thickness and more number of bilayers tend to cause bow-tie shaped easy-axis loops and multiple domain structures.

Fabrication of current-induced magnetization switching devices using etch-back planarization process

A patterning process for nanoscale current-perpendicular-to-plane magnetic devices was developed. Spin valve and magnetic tunnel junction (MTJ) pillars are patterned using electron-beam lithography and subsequent hard mask deposition and ion milling. Photoresist etch-back method is used to planarize the insulation layer, deposited on top of the spin valve/MTJ pillars, prior to top lead deposition. This method allows for a reduction of shadowing effect associated with the use of resist mask for ion milling. Critical switching current of ∼6×107A∕cm2 was observed for spin valve nanopillars with clear field dependence of the switching current, which is comparable to the reported value for metallic system.