R. P. Robertazzi

Spin torque switching of perpendicular Ta∣CoFeB∣MgO-based magnetic tunnel junctions

Spin torque switching is investigated in perpendicular magnetic tunnel junctions using Ta∣CoFeB∣MgO free layers and a synthetic antiferromagnet reference layer. We show that the Ta∣CoFeB interface makes a key contribution to the perpendicular anisotropy. The quasistatic phase diagram for switching under applied field and voltage is reported. Low switching voltages, Vc 50 ns=290 mV are obtained, in the range required for spin torque magnetic random access memory. Switching down to 1 ns is reported, with a rise in switching speed from increased overdrive that is eight times greater than for comparable in-plane devices, consistent with expectations from a single-domain model.

Spin torque switching of 20 nm magnetic tunnel junctions with perpendicular anisotropy

Spin-transfer torque magnetic random access memory (STT-MRAM) is one of the most promising emerging non-volatile memory technologies. MRAM has so far been demonstrated with a unique combination of density, speed, and non-volatility in a single chip, however, without the capability to replace any single mainstream memory. In this paper, we demonstrate the basic physics of spin torque switching in 20 nm diameter magnetic tunnel junctions with perpendicular magnetic anisotropy materials. This deep scaling capability clearly indicates the STT MRAM device itself may be suitable for integration at much higher densities than previously proven.

Multilevel YBaCuO flux transformers with high Tc SQUIDs: A prototype high Tc SQUID magnetometer working at 77 K

We have constructed optimized high Tc YBa2Cu3O7 (YBCO) multiturn flux transformers using laser‐deposited films patterned with ion milling and only photolithographic masking. A five‐turn flux transformer with a 0.5×0.5 mm2 pickup loop was coupled to a YBCO superconducting quantum interference device (SQUID) made on a separate substrate to demonstrate a prototype high Tc SQUID magnetometer with all components working at 77 K. There was no additional noise associated with the input coil and transformer. The magnetic field sensitivity of the 0.5×0.5 mm2 magnetometer was 3.8 pT/√Hz at 77 K and 1 kHz.

Identifying the source of 1/f noise in SQUIDs made from high‐temperature superconductors

We compare model predictions for 1/f noise in dc SQUIDs with experimental data and show that the 1/f noise in the our devices results from fluctuations in the magnitude of the critical current of the Josephson junctions that form the SQUID. Operating the SQUID while the bias current direction was being alternated canceled the 1/f noise power from these fluctuations and reduced the output noise power by two orders of magnitude. Using this process, we have measured the 1/f noise in several SQUIDs to be 1×10−7 Φ02/Hz at 0.1 Hz and 77 K, the lowest value reported to date.

Wet etch process for patterning insulators suitable for epitaxial high Tc superconducting thin film multilevel electronic circuits

We describe a wet etch process for patterning insulators suitable for multilayer epitaxial highTc superconductor‐insulator‐superconductor structures down to micronmeter‐scale dimensions. A solution of 7% HF in water gives convenient etch rates for SrTiO3 and MgO insulators (about 1500 A/min for single crystals), and easily stops on thin high Tc superconducting layers, due to the high selectivity of this etchant between these insulators and the cuprate superconductors. Using entirely wet etching patterning processes, we have fabricated 5‐turn (20‐turn) coils with zero resistance at 89 K (79 K) and critical currents at 77 K of 2.5 mA (6 μA).

Switching distributions and write reliability of perpendicular spin torque MRAM

We report data from 4-kbit spin torque MRAM arrays using tunnel junctions (TJs) with magnetization perpendicular to the wafer plane. We show for the first time the switching distribution of perpendicular spin torque junctions. The percentage switching voltage width, σ(Vc)/<Vc> = 4.4%, is sufficient to yield a 64 Mb chip. Furthermore we report switching probability curves down to error probabilities of 5×10−9 per pulse which do not show the anomalous switching seen in previous studies of in-plane magnetized bits.

YBa2Cu3O7−δ thin‐film gradiometers: Fabrication and performance

We have fabricated and tested superconducting first‐order gradiometers made from YBa2Cu3O7−δ (YBCO) thin‐film flux transformers coupled to YBCO dc SQUIDs. The flux transformers consisted of 10‐turn input coils with 5‐μm linewidth and two pick‐up loops with 600‐μm linewidth, each of which surrounded an area of 2.1 by 2.8 mm. All levels were patterned by wet etching. A magnetic field gradient of 1 nT/m resulted in a flux of 5.8×10−4 Φ in the SQUID which corresponds to a responsivity of 1.21×10−9 m3. The 1/f noise level at 77 K and 10 Hz was 0.6 nT/m√Hz and was dominated by SQUID noise.

Size dependence of spin-torque induced magnetic switching in CoFeB-based perpendicular magnetization tunnel junctions (invited)

CoFeB-based magnetic tunnel junctions with perpendicular magnetic anisotropy are used as a model system for studies of size dependence in spin-torque–induced magnetic switching. For integrated solid-state memory applications, it is important to understand the magnetic and electrical characteristics of these magnetic tunnel junctions as they scale with tunnel junction size. Size-dependent magnetic anisotropy energy, switching voltage, apparent damping, and anisotropy field are systematically compared for devices with different materials and fabrication treatments. Results reveal the presence of sub-volume thermal fluctuation and reversal, with a characteristic length-scale of the order of approximately 40 nm, depending on the strength of the perpendicular magnetic anisotropy and exchange stiffness. To have the best spin-torque switching efficiency and best stability against thermal activation, it is desirable to optimize the perpendicular anisotropy strength with the junction size for intended use. It also...

Y1Ba2Cu3O7/MgO/Y1Ba2Cu3O7 edge Josephson junctions

Josephson edge junctions have been fabricated using thin sputtered films of MgO (8–32 A) to produce a weak connection between high Tc electrodes. The current voltage characteristics of these junctions can be well modeled by the resistively shunted junction (RSJ) equation and they possess excellent Josephson properties, exhibiting strong magnetic field modulation and microwave response at 10 and 100 GHz up to 82 K. The best high‐temperature (T≳70 K) operation has been obtained with devices which have critical current densities in excess of 105 A/cm2 at 4.2 K, and junction critical current resistance products IcRn≂0.15 mV at 60 K. Experiments indicate that improvements in device quality are expected as the base electrode junction surface is improved.

Materials and devices for reduced switching field toggle magnetic random access memory

Toggle magnetic random access memory (MRAM) has been proposed to solve the problems of small switching margins and half-select activated errors found in Stoner-Wohlfarth MRAM. However, it is widely acknowledged that the switching fields required for toggle MRAM are substantially larger than those needed for Stoner-Wohlfarth MRAM. Previously published reports on toggle switching use large toggle start fields around 75Oe. Here we examine, both experimentally and with a single-domain model, how both the toggle start and end fields vary with free layer intrinsic anisotropy, thickness, width, aspect ratio, and interlayer exchange coupling. By optimizing these parameters, we obtain 400nm width devices with toggle start fields below 30Oe.