Hao Meng

Spin transfer in nanomagnetic devices with perpendicular anisotropy

Spin momentum transfer in a nanomagnetic device with perpendicular magnetic anisotropy for both free and fixed magnetic layers is studied. The perpendicular anisotropy is induced by using CoFe∕Pt multilayer. The magnetoresistive loop shows that the perpendicular switching fields for the free and fixed layers are 170 and 380Oe, respectively, with ΔR∕R=0.47%. Resistance-current scanning clearly shows a full out-of-plane switching of the free layer magnetization under a sweeping current, which fully excludes the effect of switching by the magnetic field generated by the current. The critical current density is around 1.0×108A∕cm2, which could be tuned by changing the CoFe∕Pt multilayer structures.

A spintronics full adder for magnetic CPU

Spintronics devices are based on the up or down spin of the electrons rather than on electrons or holes as in the traditional semiconductor electronics devices. Magnetic processors using spintronics devices in principle are much faster and with the potential features of nonvolatile, lower power consumption and higher integration density compared with transistor-based microprocessor. Full adder is one of the most important basic units of the arithmetic/logic unit for any processors. The design of the full adder determines the speed and chip-density of a processor. In this paper, a novel spintronics full adder is proposed based on novel programmable spintronics logic devices. Only seven magnetic tunnel junction elements are needed for this full adder design.

Programmable spintronics logic device based on a magnetic tunnel junction element

A programmable spintronics logic device was designed and fabricated based on a single pinned magnetic tunnel junction (MTJ) element. In this work, a current input line C passing through the MTJ element itself was introduced. Two separated input current lines (A and B) could switch the magnetization of the pinned layer under the heat assistance from line C. Full logic functions (AND, OR, NAND, NOR, XOR, and XNOR) can be realized based on a normal pinned and a synthetic pinned MTJ element. A Wheatstone bridge was engineered to read this single MTJ element logic device. MTJ elements with 1μm2 and normal pinned structure: (Ta30A∕NiFe40A∕MnIr35A∕CoFe30A∕(Al7A)+oxidation∕CoFe30A∕NiFe40A∕Ta200A), have low resistance of 6.3Ω and high resistance of 7.2Ω, which gives the magnetoresistive (MR) ratio of 14%. Approximately a 3-mV output difference is obtained between logical 1 and 0.

Low critical current for spin transfer in magnetic tunnel junctions

We report a novel approach to reduce the critical current density for spin transfer in nanoscale magnetic tunnel junction (MTJ) structures. By integrating a spin valve and a MTJ structure with antiparallel pinned layers, the dc critical switching current density is reduced by one order of magnitude (2×106A∕cm2) at room temperature. The magnetoresistive (MR) properties of the MTJ + spin-valve device are dominated by the MTJ layers. The MR ratio is 15.8% with resistance area product of 4.5Ωμm2. This demonstration opens a window for high-density magnetic random access memory design.

Composite free layer for high density magnetic random access memory with lower spin transfer current

A magnetic tunnel junction (MTJ) structure with a composite free layer consisting of a nanocurrent-channel (NCC) layer sandwiched by two CoFe layers was proposed and investigated. The NCC layer increased the local spin current density inside the free layer and thus enhanced the writing capability for MTJ devices. In comparison with the conventional MTJ design with a single free layer, the intrinsic critical switching current density was reduced from 2.4×107to8.5×106A∕cm2 by using the composite free layer. On the other hand, the thermal stability factor of the composite free layer, KuV∕kBT, is around 149, which is almost the same as the value (159) for the MTJ device with a single free layer. The MTJ structure with the composite free layer is a candidate to solve the scaling problem for high density magnetic random access memory.

Spin transfer effect in magnetic tunnel junction with a nano-current-channel Layer in free layer

The current-induced magnetization switching (spin transfer effect) in a low resistance-area (RA) product magnetic tunnel junction (MTJ) device with critical current density of 1.4/spl times/10/sup 7/ A/cm/sup 2/ was demonstrated. The RA product of the MTJ is 4.2 /spl Omega//spl mu/m/sup 2/ and the magnetoresistance (MR) ratio induced by current is up to 16%. An MTJ structure with a novel nano-current-channel (NCC) layer inserted into the free layer for the current-induced magnetization switching by lower current density was proposed and prototyped. By using the current confined effect, the local current density in the integrated free layer was sufficiently high to switch the magnetization locally. Such local magnetization reversal helped to reverse the magnetic moments around together with the polarized current and spread out the switching of the entire free layer through the superparamagnetic nano-channels. The critical current density was reduced to 4.2/spl times/10/sup 6/ A/cm/sup 2/, which is only one quarter of that for a pure MTJ structure.

Low resistance spin-dependent magnetic tunnel junction with high breakdown voltage for current-induced-magnetization-switching devices

Spin-dependent magnetic tunnel junctions (MTJs) with pure AlOx barriers were fabricated by one-step and two-step natural oxidation processes, respectively (500mTorr 20min; 500mTorr 5min and 1Torr 10min). Preoxidized Al barrier thickness varies from 5to7A. In this work, a multilayer structure with a low resistance of 0.8Ω∕sq and rms of 1.54A was developed as the bottom electrode. MTJs with the following structure Ta(30A)∕NiFe (40A)∕MnIr (80A)∕CoFe (30A)∕Al+oxidation∕CoFe (30A)∕NiFe (40A)∕Ta (200A) were magnetically annealed at 230°C for 30min to set the exchange bias field in the MnIr∕CoFe bilayer. Resistance×area (RA) products varying from 0.5to13Ωμm2 were achieved with tunneling magnetoresistance ratios varying from 8% to 18%. Breakdown voltages higher than 450mV were obtained for a sample with RA 0.5Ω×μm2, which allows a current of 9×107A∕cm2 to flow through the MTJ without damaging the barrier. Current-induced magnetization switching based on spin transfer or spin torque effect with a current density o...

Improved current switching symmetry of magnetic tunneling junction and giant magnetoresistance devices with nano-current-channel structure

The asymmetry of the switching current in magnetic tunneling junction (MTJ) and giant magnetoresistance (GMR) spin torque transfer devices was reported in both theory and experiment. This is one of the key challenges for future magnetic random access memory applications. In this work, the switching symmetry was greatly improved by inserting a nano-current-channel (NCC) structure in both MTJ and GMR devices. With the NCC structure, the current induced magnetization switching is nonuniform with initiation cites induced by locally high current density. The critical switching current density in both switching directions was successfully reduced while the degree of switching asymmetry {[(JcP-AP−JcAP-P)∕JcAP-P]×100%} was improved as well.

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.

Asymmetric Spin Torque Transfer in Nano GMR Device With Perpendicular Anisotropy

Spin torque transfer behaviors in giant magnetoresistive (GMR) devices with perpendicular anisotropy were investigated. The critical switching current density could be effectively reduced by an inserted nano-current-channel layer through the current confined effect and the magnetic exchange coupling composite effect. Depending on the location of the nano-current-channel layer inside the device, the reduction of the critical switching current density could be symmetric (or asymmetric) for the switching processes between parallel and anti parallel configurations. With strong current confined and exchange coupling composite effects, the reduction is up to 52% for parallel rarr anti-parallel switching while it is only 31% for anti-parallel rarr parallel switching