We investigate the automotive grade 1 temperature response of different MTJ stack designs based on the state-of-the-art GlobalFoundries 22FDX® Embedded-MRAM technology. An anomalous reference system reversal occurred in some devices as a response to external magnetic fields at operating temperatures as high as 150 °C. The strength of the external magnetic field, temperature and degree of magnetic moment mismatch of the synthetic antiferromagnet (SAF) reference system all affect the severity of the reference system’s instability. By careful stack optimization of the SAF reference system, the anomalous reversal was successfully suppressed in the investigated external magnetic field and temperature regime.

Embedded STT-MRAM has emerged to high-volume ramp stage. To sustain yield and to guarantee reliable operation of the memory arrays, a solid line of defense strategy is required. The unprecedented complexity of the MTJ, acting as the memory element in this new technology, can be compensated by an equally unprecedented reservoir of versatile characterization methodologies allowing for the prediction of yield and reliability critical properties of the final memory cell. To enhance the turnaround time of tool monitoring and to enable fast film level and patterned MTJ metrology, the transition of classical lab-based methodologies, such as perpendicular magneto-optical Kerr effect, current-in-plane tunneling and ferromagnetic resonance, to full on-chip capability is essential. The complementary use of these magneto-electrical and magneto-optical techniques for drift partitioning as well as their correlation to the final wafer electrical test and reliability are key for establishing a strong line of defense.

Temperature plays an important role in spin torque switching of magnetic tunnel junctions causing magnetization fluctuations that decrease the switching voltage but also introduce switching errors. Here we present a systematic study of the temperature dependence of the spin torque switching probability of state-of-the-art perpendicular magnetic tunnel junction nanopillars (40 to 60 nm in diameter) from room temperature down to 4 K, sampling up to a million switching events. The junction temperature at the switching voltage---obtained from the thermally assisted spin torque switching model---saturates at temperatures below about 75 K, showing that junction heating is significant below this temperature and that spin torque switching remains highly stochastic down to 4 K. A model of heat flow in a nanopillar junction shows this effect is associated with the reduced thermal conductivity and heat capacity of the metals in the junction.

The various memory applications such as embedded nonvolatile memory, embedded static random-access memory, and standalone dynamic random-access memory have different requirements for the performance of the perpendicular magnetic tunnel junction (pMTJ). However, in conventional pMTJ structures, the write current and stability of the free layer are directly coupled, leading to undesirable tradeoffs in performance specifications such as endurance and data retention. With the introduction of the precessional spin-current (PSC) structure, we demonstrate a significant gain in the spin transfer torque (STT) efficiency. The PSC structure shows similar critical current values as the conventional pMTJ structure. But we observe a significant increase in thermal stability, which is largely responsible for the enhancement in the STT efficiency. Smaller device dimensions show higher efficiency increase. We confirm the higher data retention time in a thermally activated switching experiment at 180 degrees C. The PSC structure exhibits more than two decades lower thermal decay rates within 100 h of wait over the conventional pMTJ. These results demonstrate that it is possible to achieve the necessary retention times at high-temperature operation without increasing the power consumption or higher current in normal operation. Furthermore, we demonstrate that even for short write pulsewidth, the switching currents of the PSC structure remain similar to the pMTJ structure while still having better retention.

Spin-transfer magnetic random access memory is of significant interest for cryogenic applications where a persistent, fast, low-energy consumption and high device density is needed. Here we report the low-temperature nanosecond duration spin-transfer switching characteristics of perpendicular magnetic tunnel junction (pMTJ) nanopillar devices (40 to 60 nm in diameter) and contrast them to their room temperature properties. Interestingly, at fixed pulse voltage overdrive the characteristic switching time decreases with temperature, in contrast to macrospin model predictions, with the largest reduction in switching time occurring between room temperature and 150 K. The switching energy increases with decreasing temperature, but still compares very favorably to other types of spin-transfer devices at 4 K, with < 300 fJ required per switch. Write error rate (WER) measurements show highly reliable (WER <= 5x10^-5 with 4 ns pulses at 4 K) demonstrating the promise of pMTJ devices for cryogenic applications and routes to further device optimization.

Perpendicularly magnetized CoFeB layers with ultrathin nonmagnetic insertion layers are very widely used as the electrodes in magnetic tunnel junctions for spin-transfer magnetic random access memory devices. Exchange interactions play a critical role in determining the thermal stability of magnetic states in such devices and their spin torque switching efficiency. Here, the exchange constant of free layers incorporated in full magnetic tunnel junction layer stacks, specifically CoFeB free layers with W insertion layers, is determined by magnetization measurements in a broad temperature range. A significant finding is that the exchange constant decreases significantly and abruptly with W insertion layer thickness. The perpendicular magnetic anisotropy shows the opposite trend; it initially increases with the W insertion layer thickness and shows a broad maximum for approximately one monolayer (0.3 nm) of W. These results highlight the interdependencies of magnetic characteristics required to optimize the performance of magnetic tunnel junction devices.

Miniaturized magnetic tunnel junctions enable single-shot time-resolved studies of magnetization reversal driven by spin currents that can test basic physical models, particularly as recent technological advances enable the study of sub-100 nm diameter junctions. The junctions consist of two magnetic layers with uniaxial magnetic anisotropy separated by a thin insulating layer that enables electron tunneling and thus transfer of spin-angular momentum in the tunneling process. One magnetic layer is free to reverse in response to the spin current while the other is fixed. Pulsed current measurements show the stochastic nature of the reversal process on nanosecond time scales and provide evidence for nonuniform magnetization reversal processes in junctions that are larger than 50 nm in diameter.

Spin-orbit interaction derived spin torques provide a means of reversing the magnetization of perpendicularly magnetized ultrathin films with currents that flow in the plane of the layers. A basic and critical question for applications is the speed and efficiency of switching with nanosecond current pulses. Here, we investigate and contrast the quasistatic (slowly swept current) and pulsed current-induced switching characteristics of micrometer scale Hall crosses consisting of very thin (<;1 nm) perpendicularly magnetized CoFeB layers on β-Ta. While complete magnetization reversal occurs at a threshold current density in the quasistatic case, short duration (≤10 ns) larger amplitude pulses (≏10 times the quasistatic threshold current) lead to only partial magnetization reversal and domain formation. We associate the partial reversal with the limited time for reversed domain expansion during the pulse.

We present the switching characteristics of a spin-transfer device that incorporates a perpendicularly magnetized spin-polarizing layer with an in-plane magnetized free and fixed magnetic layer, known as an orthogonal spin transfer spin valve device. This device shows clear switching between parallel (P) and antiparallel (AP) resistance states and the reverse transition (AP → P) for both current polarities. Further, hysteretic transitions are shown to occur into a state with a resistance intermediate between that of the P and AP states, again for both current polarities. These unusual spin-transfer switching characteristics can be explained within a simple macrospin model that incorporates thermal fluctuations and considers a spin-polarized current that is tilted with respect to the free layer's plane, due to the presence of the spin-transfer torque from the polarizing layer.

The scaling of the energy barrier to magnetization reversal in thin-film nanomagnets with perpendicular magnetization as a function of their lateral size is of great interest and importance for high-density magnetic random access memory devices. Experimental studies of such elements show either a quadratic or linear dependence of the energy barrier 2 3 on element diameter. I will discuss a theoretical model we developed to determine the micromagnetic configurations that set the energy barrier for thermally activated reversal of a thin disk with perpendicular magnetic anisotropy as a function of disk diameter 4. We find a critical length in the problem that is set by the exchange and effective perpendicular magnetic anisotropy energies, with the latter including the size dependence of the demagnetization energy. For diameters smaller than this critical length, the reversal occurs by nearly coherent magnetization rotation and the energy barrier scales with the square of the diameter normalized to the critical length (for fixed film thickness), while for larger diameters, the transition state has a domain wall, and the energy barrier depends linearly on the normalized diameter. Simple analytic expressions are derived for these two limiting cases and verified using full micromagnetic simulations with the string method. Further, the effect of an applied field is considered and shown to lead to a plateau in the energy barrier versus diameter dependence at large diameters. Based on these finding I discuss the prospects and material challenges in the scaling of magnetic memory devices based on thin films with strong perpendicular magnetic anisotropy.

We present the results of zero temperature macrospin and micromagnetic simulations of spin transfer switching of thin film nanomagnets in the shape of an ellipse with a spin-polarization tilted out of the layer plane. The perpendicular component of the spin-polarization is shown to increase the reversal speed, leading to a lower current for switching in a given time. However, for tilt angles larger than a critical angle, the layer magnetization starts to precess about an out-of-plane axis, which leads to a final magnetization state that is very sensitive to simulation conditions. As the ellipse lateral size increases, this out-of-plane precession is suppressed, due to the excitation of spatially non-uniform magnetization modes.

In this work we report on time resolved magnetization reversal driven by spin transfer torque in an orthogonal spin transfer (OST) magnetic tunnel junction device. We focus on the transitions from parallel (P) to antiparallel (AP) states and the reverse transitions (AP to P) under the influence of 10 ns voltage pulses. The electrical response is monitored with a fast real-time oscilloscope and thus timing information of the reversal process is obtained. The P to AP transition switching time decreases with increasing current and shows only direct switching behavior. The AP to P transition shows similar behavior, but has a broader distribution of switching times at high currents. The rare AP to P switching events that occur at later times are due to the occurrence of a pre-oscillation, which could be identified in time resolve voltage traces. A possible origin of these pre-oscillations is seen in micromagnetic simulations, where they are associated with the breakdown of the uniform precession of the magnetization, and lead to reversal of the magnetization at later times.

We present the quasi-static and dynamic switching characteristics of orthogonal spin-transfer devices incorporating an out-of-plane magnetized polarizing layer and an in-plane magnetized spin valve device at cryogenic temperatures. Switching at 12 K between parallel and anti-parallel spin-valve states is investigated for slowly varied current as well as for current pulses with durations as short as 200 ps. We demonstrate 100% switching probability with current pulses 0.6 ns in duration. We also present a switching probability diagram that summarizes device switching operation under a variety of pulse durations, amplitudes, and polarities.

The increasing prevalence of antibiotic resistance requires new approaches also for the treatment of infectious keratitis. Photodynamic Inactivation (PDI) using the photosensitizer (PS) Chlorin e6 (Ce6) was investigated as an alternative to antibiotic treatment. An in-vitro cornea model was established using porcine eyes. The uptake of Ce6 by bacteria and the diffusion of the PS in the individual layers of corneal tissue were investigated by fluorescence. After removal of the cornea’s epithelium Ce6-concentrations < 1 mM were sufficient to reach a penetration depth of 500 μm. Liquid cultures of microorganisms were irradiated using a specially constructed illumination chamber made of Spectralon(R) (reflectance: 99 %), which was equipped with high power light emitting diodes (λ = 670 nm). Clinical isolates of Staphylococcus aureus (SA) and Pseudomonas aeruginosa (PA) from keratitis patients were tested in liquid culture against different concentrations of Ce6 (1 - 512 μM) using 10 minutes irradiation (E = 18 J/cm2 ). This demonstrated that a complete inactivation of the pathogen strains were feasible whereby SA was slightly more susceptible than PA. 3909 mutants of the Keio collection of Escherichia coli (E.coli) were screened for potential resistance factors. The sensitive mutants can be grouped into three categories: transport mutants, mutants in lipopolysaccharide synthesis and mutants in the bacterial SOS-response. In conclusion PDI is seen as a promising therapy concept for infectious keratitis.

We present temperature dependent switching measurements of the Co/Ni multilayered free element of 75-nm-diameter spin-valve nanopillars. Angular dependent hysteresis measurements as well as switching field measurements taken at low temperature are in agreement with a model of thermal activation over a perpendicular anisotropy barrier. However, the statistics of switching (i.e. the mean switching field and the variance of the switching field distribution) from 20 up to 400 K are in disagreement with a Neel-Brown model that assumes a temperature independent barrier height and anisotropy field. We introduce a modified Neel-Brown model that fits the experimental data in which we attribute a T-3/2 dependence to the barrier height and the anisotropy field due to the temperature dependent magnetization and anisotropy energy.