Nicholas D. Rizzo

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.

Low-power switching in magnetoresistive random access memory bits using enhanced permeability dielectric films

We reduced the switching field (Hsw) in arrays of single-layer magnetoresistive random access memory elements using enhanced permeability dielectric (EPD) films. This reduction is due to an increased magnetic flux density produced at the bit by increasing the permeability μ of the surrounding dielectric. The authors produced EPD films by embedding superparamagnetic nanoparticles of various sizes in oxides of Al, Mg, or Si. For bits surrounded by EPD, Hsw decreased linearly as μ increased. Using this approach, we reduced Hsw by ≈40% for μ=3.5, without changing the energy barrier to magnetization reversal.

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.

ST-MRAM fundamentals, challenges, and applications

Magnetoresistive Random Access Memory (MRAM) technology emerged from research and development into volume production within the last decade in the form of Toggle MRAM. The latest Magnetic Tunnel Junction (MTJ) based memory technology, Spin-Torque MRAM, has reached the level of customer sampling, offering higher density and bandwidth. Spin-Torque MRAM enables new applications, offers a wide range of features for use in embedded memory, and has the potential to extend to technology nodes beyond the capability of DRAM. This paper describes the devices, fundamental circuit challenges, and applications of this evolving MTJ based memory.

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.

Toggle MRAM with CoFeB-Based Synthetic Antiferromagnet Free Layers

For toggle MRAM, the free layer is a synthetic antiferromagnet (SAF) that must have specific magnetic properties such as: low magnetostriction, a repeatable saturation field (Hsat) that can be adjusted within a specific range, and a well-defined and reproducible intrinsic anisotropy axis. The results from fully-functional Mb-scale MRAM circuits using CoFeB SAF free layers optimized for toggle switching is reported. A 40% improvement in useable MR in 4Mb MRAM circuits using optimized CoFeB SAF free layers is demonstrated, with switching comparable to standard NiFe material. The critical properties optimized for switching, electrical properties of the devices, and thermal endurance are described.