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We demonstrate spin-transfer torque magnetoresistive random access memory (STT-MRAM) arrays achieving 2.8e-10 write error rate (WER) performance at 3 ns write duration at a magnetic tunnel junction (MTJ) diameter of 40 nm. The bit-to-bit distribution of the write voltage at a WER of 1e-6 is characterized by a relative standard deviation of 3.7% for W0 and 4.5% for W1, sufficient to meet the write voltage distribution requirement for last-level cache (LLC) applications at 1x nm nodes.

We report reliable 5 ns switching of spin-transfer torque magnetoresistive random-access memory devices of nominal size 43 nm and a resistance area product of 11 Ω·µm2. We measured 256 devices with a 100% write-error-rate (WER) yield at a WER floor of $10^{-6}$ and a steep WER slope as a function of voltage. A single device had a WER less than $10^{-10}$ for 5 ns write pulses. We show promising 3 ns switching performance, with a 94% WER yield at a $10^{-6}$ WER floor, for 64 devices of nominal size 50 nm.

We report for the first time reliable 2 ns switching of spin-transfer torque magneto-resistive random access memory (STT-MRAM) devices by demonstrating 100% write-error-rate (WER) yield at 1e-6 write-error floor of 254 devices with tight distributions and steep WER slope at a nominal size of 49 nm. A single device was demonstrated with 2 ns write pulses to have less than 1e-11 write-error rate, limited only by test time. We further demonstrate reliable 3 ns switching performance, with 99% WER yield at 1e-6 write-error floor of 256 devices with nominal size of 43 nm and a single device with less than 1e-11 write-error rate with a completely different free layer materials design. These two different free layer materials designs address one of the major remaining challenges for STT-MRAM to replace SRAM for last level cache (LLC) applications.

We report the impact of four key parameters on switching efficiency of STT-MRAM devices with perpendicular magnetic anisotropy: device size, device resistance-area product (RA), blanket film Gilbert damping constant (a), and process temperature. Performance degradation observed in 400°C-processed devices was eliminated by optimizing the perpendicular magnetic tunnel junction (p-MTJ) materials. Furthermore, 400°C-compatible double MTJs were developed for the first time and showed 1.5x improvement in switching efficiency compared to single MTJs with identical free layers.

With the promise of nonvolatility, practically infinite write endurance, and short read and write times, magnetic tunnel junction magnetic random access memory could become a future mainstream memory technology.

Exchange biased magnetic tunnel junction (MTJ) structures are shown to have useful properties for forming magnetic memory storage elements in a novel cross-point architecture. MTJ elements have been developed which exhibit very large magnetoresistive (MR) values exceeding 40% at room temperature, with specific resistance values ranging down to as little as ∼60 Ω(μm)2, and with MR values enhanced by moderate thermal treatments. Large MR values are observed in magnetic elements with areas as small as 0.17 (μm)2. The magnetic field dependent current–voltage characteristics of an MTJ element integrated with a silicon diode are analyzed to extract the MR properties of the MTJ element itself.

The use of lithography and electroplating to fabricate variable-reluctance, nearly planar, integrated minimotors with 6-mm-diameter rotors on silicon wafers is described. The motors consist of six electroplated Permalloy® horseshoe-shaped cores that surround the rotor. Copper coils are formed around each core. The Permalloy and copper electroplating baths, electroplating seed layers, and through-mask plating techniques are similar to those used to fabricate inductive thin-film heads. High-aspect-ratio optical lithography or X-ray lithography was used to form the various resist layers. The rotors were fabricated separately, released from the substrate, and then slipped onto the shaft, which was plated as part of the stator fabrication process. The fabrication processes for stator and rotor are described in this paper, and initial minimotor operation data are presented.

Although considerable work has been done investigating the properties of arrays of magnetic elements, there have been few investigations on the reverse geometry, i.e., an array of nonmagnetic regions defined within a magnetic thin film. The 10 Hz BH loops, 10–500 MHz permeability spectra, and domain patterns of homogeneous, single layer 100 nm radio frequency (rf) sputtered Ni81Fe19 thin films with arrays of 23-, 50-, and 100-μm-diam holes defined by laser ablation were measured. The holes were defined in a grid along the hard and easy axes of the sample. Letting (x,y) represent, respectively, the hole spacing parallel to the easy and hard axes, the point to point spacing of the ablated circular regions was varied from (5, 2 mm) to (0.1, 0.1 mm).

Infroducfion: Small-size, highefficiency motors, fabricated by low cost batch fabrication techniques, are expected to have application in storage, automotive, medical and other fields. We describe a new approach to fabrication of an integrated experimental variable reluctance magnetic mini-motor, based on lithography and electroplating techniques, similar to those used in inductive head fabrication. Strumre Fabrication: The mini-motor consists of six, nearly planar, horseshoe-shaped electromagnets which surround a rotor. The stator diameter is 17 mm, rotor radius 2.976 mm, core width 0.7 mm, coil width 60 pm, and there are 120 turns per core., The motor thickness is -5OOpm, with a substrate approximately 1.0 mm. The fabrication sequence consists of a series of lithography, electroplating, seed layer removal, dielectric fill, and planarization steps. The process for the stator begins by sputterdepositing a Cu seed layer on an oxidized Si wafer, lithographically forming the plate-thru mask for the bottom part of the windings, and electroplating Cu through the mask. After removing the seed layer, a dielectric is applied across the entire structure, which is then planarized, and a pattemed dielectric is formed on top of the Cu to insulate the windings from the magnetic core. The Permalloy core is plated, and the top of the windings are formed by similar process steps. This simplifies the fabrication process. The vias that complete each tun of the coil are formed of PennaIloy during the core plating step. The rotor is fabricated by thru-mask plating of Permalloy on a separate substrate, detaching it, using a sacrificial layer, and then slipping it onto the pin. 'The pin is formed during the same lithography and plating steps thar created the core and the winding. Seed Layers: The seed layers for Cu and Permalloy plating respectively are of sputtered Ti/Cu (I 000-3000A) and of 1000-2OOOA sputtered Permalloy (80120) NiFe).

Wide-field magneto-optic Kerr observation techniques are described for the imaging of magnetic recording heads and other small magnetic devices. The imaging problems encountered for these samples are quite different from those for continuous films and lead to a different set of experimental approaches. Observation techniques that preserve the magnetic state of the sample allow for the implementation of new image acquisition algorithms based on multiple additions and subtractions that progressively build up a weak Kerr contrast even under unfavorable experimental conditions. Examples of results are presented. >

The cause of the readback waveform distortion in polycrystalline ferrite metal-in-gap (MIG) heads was investigated by using a magnetic force microscope (MFM) and a polarized optical microscope capable of microellipsometry for grain contrast and Kerr microscopy for domain contrast. It was revealed that, for high-asymmetry MIG heads, relatively large leakage of magnetic flux associated with complex multiple domains exists at remanence in the leading-side ferrite surface adjacent to the gap. The strengths of the magnetic flux and the magnetic domain states change irreproducibly when coil current is repeatedly applied. Such a remanent flux associated with the complex remanent state of magnetization distribution in the ferrite can cause an inhomogeneous flux coupling between the head and the recorded track, resulting in readback waveform asymmetry. >

The magnetic domain structure at the media-facing surfaces of the single-crystal ferrite MIG heads is investigated using the magnetooptical Kerr effect. Domain behavior depends strongly on the stress state and surface treatment of the head. The pole tips display coercive zig-zag domain walls, which are nucleated at the gap by write current pulses. For the samples studied, nucleation and motion of these walls are different between heads which exhibit small secondary pulses in the read-back signal and heads which do not. >

A novel magnetooptic imaging technique was used to investigate the dynamics of magnetization response in thin-film head yokes. Completed head devices were excited with a sinusoidal current applied to the integrated coils. The amplitude (20 to 40 mA pp) and frequency (1 to 50 MHz) of excitation were chosen to simulate the write process. Pulsed laser illumination permitted stroboscopic observation of domain-wall and flux-flow dynamics with a time resolution limited only by the 5-ns pulse width (full width at half maximum). Results suggest that high-frequency write performance is degraded by two mechanisms not considered previously for thin-film heads: first, 180 degrees walls appear to impede the flux-flow across the plane of the wall and, second, the inhomogeneous rotational magnetization response observed is known to be much slower than the typically assumed coherent rotation. The effect of NiFe composition on dynamics was also investigated by comparing responses of two heads, one having positive and the other negative magnetostriction. Flux in the head with positive magnetostriction flows in a constricted path along yoke edges, rather than at its center, which is consistent with the significantly lower efficiency measured electrically for this head. >

Ferrite MiG heads intended for narrow track (≲10 μm) digital recording were recently investigated in the critical pole‐tip region at the air‐bearing‐surface using micro‐ellipsometry, Kerr microscopy, and electron back‐scatter diffraction from individual grains,1 and using magnetic force microscopy to detect air‐gap remanent fields.2 Comparison of these direct observations with readback‐after‐write waveforms from written test tracks, and consideration of granularity influences on bulk permeability and domain size, indicate that waveform instability and asymmetry from polycrystalline ferrite (PCF) heads would be diminished by suitable size and orientation of the grains.1 The use of single‐crystal ferrite3 (SCF) for advanced laser enhanced etch definition3 of narrow pole MiGs can avoid this type of distortion. However, secondary signals4 often appear as weak pulses separated in time from the main gap pulse. We have associated this effect with a zig‐zag shaped wall seen nucleated and propagated from the pole ...

Laminated Permalloy, with edge‐curling walls replacing closure domains, has been proposed to increase permeability and reduce wall noise in recording. However, in structures meeting the criteria for Slonczewski’s ‘‘easy‐axis’’ state, normal walls often coexist with edge‐curling walls. We have used our laser magneto‐optic microscope to study inductive‐head‐yoke shaped elements of two and four Permalloy layers separated by nonmagnetic, metallic spacers. In the four‐magnetic‐layer sample a state with a single wall, terminating at the edge‐curling regions and lying along the easy‐axis direction, is often observed on the top and bottom layers. Some elements may be driven into an easy‐axis state with no observed domain walls. The two‐magnetic‐layer sample also exhibited simultaneous one‐wall structures on the top and bottom layers. The other stable configuration was a no‐wall state on the top layer and a two‐wall (three‐domain) state on the lower layer. These ‘‘coupled’’ states were exceptionally stable in both...

A magnetooptic microscope was used to observe the time-averaged magnetization distribution in an inductive thin-film head excited by continuous sine waves. Domain activity in yokes driven with sinusoidal currents (1 to 20 MHz) was observed using the Kerr effect at video frame rates (0 to 30 Hz). Thus, the average location and shape of domains in the top yoke of the head could be recorded. It is shown that the domain pattern generally undergoes significant changes in a slow, repeatable evolution. Some changes lead to abrupt conversions of domain states. Although specific behavior varies from head to head, these conversions follow measurable curves having a common trend in the amplitude versus frequency space. Previous work on analysis of head response has not considered this type of dynamic response, although it appears to be common to many magnetic system. In addition to possible response at the excitation frequency, the wall network can also undergo large changes with a time scale much longer than the excitation period. Three possible mechanisms driving the domain conversions are outlined. >