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In order to launch spin transfer torque-based magnetoresistive random access memory into mass production below the 40 nm technology node, high performance perpendicularly magnetized magnetic tunnel junctions (pMTJs) are crucial. One of the key issues for pMTJs is to ensure compatibility with conventional back-end-of-line (BEOL) heating process, where thermal tolerances over 350 C for standalone memory and 400 °C for embedded memory are typically required. In this work, we successfully demonstrated high thermal tolerance in CoFeB-based pMTJs with a synthetic antiferromagnet (SAF) pinned structure while keeping stray fields on the free layer low. The annealing temperature (Tanneal) dependence of spin torque switching (STS) and TMR characteristics revealed that the reduction of Ic0, the increase of Δ, and the increase of TMR ratio were achieved by elevating Tanneal from 250 °C up to 380 °C for 30 min. Thus, we attained high STS efficiency (Ic0/Δ)=0.88 (Ic0=69 μA, Δ=78), TMR ratio=120%, RA =12 Ωμm2 at Tanneal=380 °C for the pMTJs with dimensions of 44-46 nm in diameter. Moreover, by using specially tailored pMTJs with a SAF pinned structure, we achieved high thermal tolerance up to 400 °C for 30 min while keeping Ic0/Δ=1.12 (Ic0=56 μA, Δ=50), TMR ratio=150% , RA=13 Ωμm2.

Lorentz electron microscopy offers a very useful technique by which the magnetic domain structure of a thin film can be observed. In order to develop GMR materials with high sensitivity, it is desirable to understand the magnetic domain structure of the material. In this study, the magnetic domain structure of a series of NiFe/Au multilayer films (MLFs) with Au layer thickness between 1 nm and 4nm has been observed using Lorentz microscopy. The MLFs showed a complicated domain structure, which contained twin walls and cross-tie walls in the MLFs. As the Au layer thickness increased so the ferromagnetic coupling between the NiFe layers decreased resulting in a more complex domain structure. In-situ magnetizing experiments showed that magnetic domain wall motion was the dominant mechanism for reversing the direction of magnetization. It is believed that reversal of the magnetization in the MLFs with thick Au layers was greatly affected by the structural defects in the films.

A novel nonvolatile memory utilizing spin torque transfer magnetization switching (STS), abbreviated spin-RAM hereafter, is presented for the first time. The spin-RAM is programmed by magnetization reversal through an interaction of a spin momentum-torque-transferred current and a magnetic moment of memory layers in magnetic tunnel junctions (MTJs), and therefore an external magnetic field is unnecessary as that for a conventional MRAM. This new programming mode has been accomplished owing to our tailored MTJ, which has an oval shape of 100 times 150 nm. The memory cell is based on a 1-transistor and a 1-MTJ (ITU) structure. The 4kbit spin-RAM was fabricated on a 4 level metal, 0.18 mum CMOS process. In this work, writing speed as high as 2 ns, and a write current as low as 200 muA were successfully demonstrated. It has been proved that spin-RAM possesses outstanding characteristics such as high speed, low power and high scalability for the next generation universal memory

In this paper, we study to reduce the switching dispersion and improve 0/1 separation of Magnetic Tunnel Junction (MTJ) elements in order to realize high density MRAM. Various kinds of MTJ sizes and shapes have been evaluated and conclude that in ellipse like shape pattern aspect ratio more than 2 is enough for reproducing and reliable switching characteristics. As regards the reading characteristics, the combination of the optimized MTJ pattern and process makes 21.4 sigma separation between high and low resistance states. In further study of the relation between MTJ shapes and switching distribution, we found a "Saturn" shaped MTJ has best switching behavior. Also the toggle mode MRAM is evaluated and its effectiveness for high speed programming is confirmed.

Summary form only given. Increasing output and decreasing switching field distribution are very important for the development of magnetic tunnel junction (MTJ) materials, in order to achieve a high capacity MRAM. We introduce an amorphous MTJ material with excellent characteristics, which is evaluated through the MRAM/MTJ test element groups (TEG) with a MOS circuit.

A calculation of the intrinsic spin transfer switching current Ic0 is realized in the magnetic tunnel junctions (MTJs). Ic0 is determined by the distribution of Ic in repeated measurements, assuming the linear decrease of the thermal stability by increasing the spin-polarized current. The spin transfer torque is found to be proportional to η(I)I with the applied current I and I-dependent spin polarization η(I). Even in case that I is limited less than Ic due to the tunnel barrier breakdown, Ic0 is calculated with an assisting external magnetic field and making better use of the magnetic phase diagram. In our MTJs, the average intrinsic switching current density was 6.4MA∕cm2.

The reliability in magnetoresistive random access memory (MRAM) write operation was investigated for both toggle and asteroid memory chips developed with 0.18μm CMOS process. Thermally activated magnetization reversal, being the dominant origin of the intrinsic write error, was studied theoretically and experimentally. For asteroid MRAM, the bit line or word line disturbing error on half selected bits was proved to have significant effect on the write operation margin, even in the ideal case free from bit-to-bit switching field distribution and the hysteresis loop shift. For toggle MRAM, on the other hand, the dominant origin of the error occurs for selected bits, although its impact is much smaller than in the case of asteroid MRAM. As was expected from the estimation based on the single domain model, more than 10mA operation margin with the error rate of

The magnetoresistive properties of current perpendicular to plane (CPP) spin valves (SV) were investigated for various film structures. CPP-SV elements were fabricated by photolithography and ion-beam etching. As an unexpected result, resistivity change-area product (dRA) improved up to 60% with increasing the PtMn layer thickness from 20 to 40 nm in the bottom synthetic spin valve structure, even though the PtMn layer cannot contribute to the spin dependent scattering. For the free layers, dRA improved with increasing the free layer thickness, although a saturation magnetization (Ms) and free layer thickness were restricted for the head application. Aiming at the enhancement of the spin dependent scattering on the interface, CoFe/NiFe multilayer is adopted as the free layer. Although NiFe improves dR, dRA was not improved with increasing the number of interfaces between CoFe and NiFe when total thickness is kept constant. It also becomes clear that NiFe is the appropriate material in terms of the bulk sc...