Haibao Zhao

Switching current reduction using perpendicular anisotropy in CoFeB–MgO magnetic tunnel junctions

We present in-plane CoFeB–MgO magnetic tunnel junctions with perpendicular magnetic anisotropy in the free layer to reduce the spin transfer induced switching current. The tunneling magnetoresistance ratio, resistance-area product, and switching current densities are compared in magnetic tunnel junctions with different CoFeB compositions. The effects of CoFeB free layer thickness on its magnetic anisotropy and current-induced switching characteristics are studied by vibrating sample magnetometry and electrical transport measurements on patterned elliptical nanopillar devices. Switching current densities ∼4 MA/cm2 are obtained at 10 ns write times.

Low writing energy and sub nanosecond spin torque transfer switching of in-plane magnetic tunnel junction for spin torque transfer random access memory

This work investigated in-plane MgO-based magnetic tunnel junctions (MTJs) for the application of spin torque transfer random access memory (STT-RAM). The MTJ in this work had an resistance area product (RA) = 4.3 Ω·μm2, tunneling magnetoresistance ratio ∼135%, thermal stability factor Δ(H)=68 (by field measurement), and Δ(I) = 50 (by current measurement). The optimal writing energy was found to be 0.286 pJ per bit at 1.54 ns for antiparallel (AP) state to parallel (P) state switching, and 0.706 pJ per bit at 0.68 ns for P state to AP state switching. Ultra fast spin torque transfer (STT) switching was also observed in this sample at 580 ps (AP to P) and 560 ps (P to AP). As a result, 0.6–1.3 GHz was determined to be the optimal writing rate from writing energy consumption of view. These results show that in-plane MgO MTJs are still a viable candidate as the fast memory cell for STT-RAM.

Deep subnanosecond spin torque switching in magnetic tunnel junctions with combined in-plane and perpendicular polarizers

We show that adding a perpendicular polarizer to a conventional spin torque memory element with an in-plane free layer and an in-plane polarizer can significantly increase the write speed and decrease the write energy of the element. We demonstrate the operation of such spin torque memory elements with write energies of 0.4 pJ and write times of 0.12 ns.

Low Write-Energy Magnetic Tunnel Junctions for High-Speed Spin-Transfer-Torque MRAM

This letter presents energy-efficient MgO based magnetic tunnel junction (MTJ) bits for high-speed spin transfer torque magnetoresistive random access memory (STT-MRAM). We present experimental data illustrating the effect of device shape, area, and tunnel-barrier thickness of the MTJ on its switching voltage, thermal stability, and energy per write operation in the nanosecond switching regime. Finite-temperature micromagnetic simulations show that the write energy changes with operating temperature. The temperature sensitivity increases with increasing write pulsewidth and decreasing write voltage. We demonstrate STT-MRAM cells with switching energies of <;1 pJ for write times of 1-5 ns.

Structural and magnetic properties of a core-shell type L10 FePt/Fe exchange coupled nanocomposite with tilted easy axis

Structural and magnetic properties of core-shell type L10 FePt/Fe exchange coupled nanocomposites are studied systematically. Core-shell nanocomposites with FePt core and Fe shell are obtained by depositing Fe cap layers on granular L10 FePt films. Epitaxial growth is disclosed by x-ray diffraction. Coercivity decreases drastically for FePt/Fe with the thickness increase of Fe cap layers. The coercivity reduction is due to the much increased domain wall area pinned and compressed at the soft-hard interface, and the tilted effective easy axis because of the presence of demagnetized energy. L10 FePt/Fe with a 3 nm Fe layer has high thermal stability and gain factor for media applications.

Effect of resistance-area product on spin-transfer switching in MgO-based magnetic tunnel junction memory cells

We use ultrafast current-induced switching measurements to study spin-transfer switching performance metrics, such as write energy per bit (EW) and switching current density (Jc), as a function of resistance-area product (RA) (hence MgO thickness) in magnetic tunnel junction cells used for magnetoresistive random access memory (MRAM). EW increases with RA, while Jc decreases with increasing RA for both switching directions. The results are discussed in terms of RA optimization for low write energy and current drive capability (hence density) of the MRAM cells. Switching times <2 ns and write energies <0.3 pJ are demonstrated for 135 nm×65 nm CoFeB/MgO/CoFeB devices.

Fabrication of FePt type exchange coupled composite bit patterned media by block copolymer lithography

We fabricated exchange coupled composite (ECC) L10 ordered, (001) oriented FePt (5nm)/Fe (5 nm) bit patterned media over a large area by diblock copolymer lithography. We formed the dot arrays of the copolymer directly on the magnetic film and used them as the etching mask. The average size of the ECC FePt/Fe pillars was about 32 nm, with a center to center distance of about 35 nm and a size distribution of about 8%. The perpendicular coercivity (Hc) of the ECC FePt/Fe patterned structures was about 4.3 kOe. Both the coercivity and the saturation field of the ECC FePt/Fe patterned structure were reduced by about 50% due to the exchange coupling between FePt and Fe in the FePt/Fe patterned structure compared to the FePt patterned structure with similar dot size and distribution. The thermal stability and gain factor of the FePt/Fe ECC structure were about 260 kBT and 1.35, respectively.

Reduction of switching current density in perpendicular magnetic tunnel junctions by tuning the anisotropy of the CoFeB free layer

The spin torque switching behavior of perpendicular magnetic tunnel junctions consisting of a CoFeB free layer and a CoFeB/Ru/(Co/Pd)n exchanged coupled fixed layer is investigated. At first, the Ru and CoFeB layer thickness is tuned in the CoFeB/Ru/(Co/Pd)n structure to form a ferromagnetically exchange coupled structure with a strong PMA at an annealing treatment of 325 °C for 1 h. Then it is shown that that the CoFeB free layer thickness plays an important role in the switching current density. The switching current density decreases with the increase of the CoFeB free layer thickness. A minimum switching current density of 1.87 MA/cm2 is achieved for a device with 60 nm diameter. The mechanism involved in the switching current reduction with the decrease of CoFeB free layer thickness is also studied.

Fabrication and Characterization of FePt Exchange Coupled Composite and Graded Bit Patterned Media

Three methods to fabricate continuous FePt films with graded magnetic anisotropy for bit patterned media (BPM) were evaluated. Continuous FePt films with surface roughness of less than 0.3 nm were achieved in continuous FePt hard magnetic films, FePt/Fe exchange coupled composite (ECC) films and FePt/Fe based graded films. Depositing an Fe-rich film on FePt at high temperature was found to form large grains and cause the film surface very rough for BPM fabrication. Depositing Fe on FePt at room temperature and then annealing it to create graded anisotropy through the layer interdiffusion process was demonstrated to fabricate FePt/Fe based graded BPM. The continuous FePt films with hard layer only, ECC structure and graded magnetic anisotropy were patterned using a di-block copolymer self-assemble hard mask method with 25 nm dot size over 2-inch substrate. The switching field distribution (SFD) broadening and degradation of FePt BPM was studied. The reduction of SFD was achieved using a postannealing process. It was confirmed that the patterned graded BPM sample has smaller switching field and larger thermal energy barrier than the ECC sample.

Characterization of L10-FePt/Fe based exchange coupled composite bit pattern media

L10-FePt exchange coupled composite (ECC) bit patterned media has been considered as a potential candidate to achieve high thermal stability and writability for future high density magnetic recording. In this paper, FePt based ECC bit patterned structures with 31 nm bit size and 37 nm pitch size were fabricated using di-block copolymer lithography on 3 inch wafer. Remanant states were tracked using magnetic force microscopy (MFM). DC demagnetization (DCD) curves were plotted by counting the reversed bits in the MFM images. Magnetic domains in which the magnetizations of the neighboring bits were aligned to the same direction were observed in the MFM patterns. Thermal decay measurement was performed for the samples to obtain the thermal stability and gain factor. The thermal barrier was found around 210 kBT with a gain factor up to 1.57 for the bit patterned structure FePt(4 nm)/Fe(4 nm).