Bradley N. Engel

Recent developments in magnetic tunnel junction MRAM

We summarize our progress on Magnetoresistive Random Access Memory (MRAM) based on Magnetic Tunnel Junctions (MTJ). We have demonstrated MTJ material in the 1-1000 k/spl Omega/-/spl mu/m/sup 2/ range with MR values above 40%. The switching characteristics are mainly governed by the magnetic shape anisotropy that arises from the element boundaries. The switching repeatability, as well as hard axis selectability, are shown to be dependent on both shape and aspect ratio. MTJ memory elements were successfully integrated with 0.6 /spl mu/m CMOS technology, achieving read and program address access times of 14 ns in a 256/spl times/2 MRAM.

Area dependence of high-frequency spin-transfer resonance in giant magnetoresistance contacts up to 300nm diameter

We measured high-frequency spin-transfer resonances from 26GHz excited by dc currents through giant magnetoresistance point contacts with diameters d from <50 to almost 300nm. The slope of resonance frequency versus current decreased with increased d and was fit best by a spin-transfer model where the effective d extends ⩾50nm past the contact edge into the surrounding magnetic film. An increased resonance critical current versus contact area was also fit well by this model including a surrounding ring of excited area. Spin-transfer resonance in large devices eliminates the need for electron-beam lithography in applications.

Magnetostatic interactions between sub-micrometer patterned magnetic elements

We have investigated the effect of the magnetostatic interaction field between submicrometer patterned magnetic elements in arrays (/spl sim/10/sup 8/ elements) using Alternating Gradient Force Magnetometry. Single layer NiFe elements were studied over a range of width (0.12 to 0.48 /spl mu/m), length to width aspect ratio (1.5 to 4) and thickness (30 /spl Aring/ to 60 /spl Aring/). The arrays were patterned using e-beam lithography in a rectangular lattice with bit separation equal to element width. Effective interaction fields were obtained using a novel application of /spl Delta/M plots to these patterned arrays. The /spl Delta/M plot is derived by subtracting remanent magnetization curves that had initial states of full magnetization from those that were initially demagnetized. Interaction fields were quantified using H/sub int/=/spl Delta/M//spl chi//sub irr/, where /spl Delta/M is the difference in these remanent loops and /spl chi//sub irr/ is the derivative of the remanent magnetization with respect to field. We found H/sub int/ to increase with element aspect ratio, thickness and inverse width. For structures with the largest aspect ratio, thickness and smallest width, H/sub int//spl ap/6 Oe. We calculated the actual dipolar interaction fields using micromagnetic simulation and used these fields in a simple 2-d Ising model to simulate the experiment. We found H/sub int/ that is a good measure of the difference in interaction fields tending to magnetize or demagnetize the sample.

The switching properties of patterned synthetic ferrimagnetic structures

We investigated the switching properties of patterned submicrometer synthetic ferromagnetic (SF) Ni65Fe15Co20(t1nm)∕Ru0.8nm∕Ni65Fe15Co20(t2nm) tri-layers. By changing t1 and t2, the shape anisotropy field, Hksh∝∣t1−t2∣, was changed from 36 to 18Oe, and the effective material anisotropy field, Hkeff,∝α=(t1+t2)∕∣t1−t2∣, was changed from 28 to over 60Oe. We found that a hard axis field, Hhd, is less effective at reducing the easy axis switching field, Hsw, as α is increased, with α=3.7 requiring twice the relative magnitude of Hhd for the same relative reduction in Hsw as a single magnetic layer. In addition, by repeating the basic SF tri-layer structure in circular elements, we demonstrated improved stability against thermal activation by a factor of 2 with no associated increase in Hsw.

MRAM Memory for Embedded and Stand Alone Systems

Magnetoresistive random access memory (MRAM) is based on magnetic tunnel junction devices integrated with standard CMOS resulting in high-speed read and write, unlimited endurance, and the highest reliability of any non-volatile memory. MRAM is a unique memory technology in that the module is inserted late in the manufacturing process, making MRAM highly compatible with advanced processing. The manufacturing flexibility of MRAM makes it an attractive choice for embedded and stand alone memory systems.

Toggle MRAM: A highly-reliable Non-Volatile Memory

Magnetoresistive Random Access Memory (MRAM) is based on magnetic tunnel junction devices integrated with standard CMOS, resulting in high-speed read and write, unlimited endurance, and the highest reliability of any non-volatile memory. The first commercially available MRAM product, Freescales 4Mb MR2A16A Toggle MRAM, was released for production in 2006 and is now in volume production. In this paper we provide an overview of Freescales MRAM technology and describe the performance and reliability attributes of the MR2A16A.

Switching Energy Barrier Study of Toggle MRAM Using a Novel Pulse Technique

We present a complete study of the influence of thermal activation on the DW mode, the toggle mode, and on the separate first and second pulses of the toggle sequence. To obtain these results, we developed a novel technique that employs a train of three-pulse packets. We found good agreement with a single energy barrier (Eb) thermal activation model for both the DW and toggle modes, indicating excellent bit switching quality. We also measured the Eb vs. bit size with no changes in the material stack, and found that Eb >70 kbT to below 0.1 um, which is sufficient stability for scaling toggle MRAM to beyond the 65 nm CMOS node.

Area-Dependence of High Frequency Spin-Transfer Resonance in GMR Contacts up to 300 nm Diameter

The area-dependence of spin-transfer resonance in giant magnetoresistance (GMR) contacts from 50-300 nm diameter d is measured in this paper. With increasing d , a decreasing slope df/dl of precession frequency vs. current and an increasing critical current Ic is found. The data is well fit by a model where the precessing region extends outside the contact by a ring of width delta~ 50 nm. A GMR film with a base electrode, a 20 nm Co81Fe19 fixed layer, a 6 nm Cu spacer layer, a 4.5 nm Ni80Fe20 free layer, and a cap. The point contacts were written by optical or e-beam lithography and formed through SiO2 or hardened PMMA. The inset shows an SEM image of a nominally 60 nm diameter contact. A dc current I is applied to the contact using a high-frequency bias tee and microwave probe and measured the GMR frequency spectrum. A magnetic field ~1 T normal to the film saturated the free moment out-of-plane at I=0 and tilted the fixed moment ~30deg out-of-plane.