W. J. Gallagher

Exchange-biased magnetic tunnel junctions and application to nonvolatile magnetic random access memory (invited)

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.

Microstructured magnetic tunnel junctions (invited)

We have used a simple self-aligned process to fabricate magnetic tunnel junctions down to submicron sizes. Optical and electron-beam lithographies were used to cover a range of areas spanning five orders of magnitude. The bottom magnetic electrodes (Co or permalloy) in our junctions were exchange biased by an antiferromagnetic layer (MnFe). The top electrodes were made of soft magnetic materials (Co or permalloy). We have consistently obtained large magnetoresistance ratios (15%–22%) at room temperature and in fields of a few tens of Oe. The shape of the field response of the magnetoresistance was varied from smooth to highly hysteretic by adjusting the shape anisotropy of one junction electrode.

Observation of large low‐field magnetoresistance in trilayer perpendicular transport devices made using doped manganate perovskites

We report on the fabrication of a new class of trilayer epitaxial thin film devices based on the doped perovskite manganates La–Ca–Mn–O and La–Sr–Mn–O. We show that large resistance changes, up to a factor of 2, can be induced by a moderate applied magnetic field below 200 Oe in these trilayers supporting current‐perpendicular‐to‐plane transport. These results show that low‐field spin‐dependent transport in manganates can be accomplished, the magnitude of which is suitable for magnetoresistive field sensors.

High‐resolution scanning SQUID microscope

We have combined a novel low temperature positioning mechanism with a single‐chip miniature superconducting quantum interference device (SQUID) magnetometer to form an extremely sensitive new magnetic microscope, with a demonstrated spatial resolution of ∼10 μm. The design and operation of this scanning SQUID microscope will be described. The absolute calibration of this instrument with an ideal point source, a single vortex trapped in a superconducting film, will be presented, and a representative application will be discussed.

A 10 ns read and write non-volatile memory array using a magnetic tunnel junction and FET switch in each cell

Magnetic random access memory (MRAM) offers an alternative approach to fast low-power non-volatile VLSI memory. MRAM has been pursued for more than 10 years as a robust non-volatile memory for space applications. The magnetic tunnel junction (MTJ) MRAM is dramatically different and achieves four orders of magnitude better bandwidth to sense power ratio by utilizing a high resistance and high magneto-resistance (MR) MTJ and including a FET switch in each cell. Non-volatile storage and 10 ns performance are demonstrated in 1 kb arrays. Read and write on-chip power at 2.5 V and 100 MHz are 5 mW and 40 mW respectively.

Lanthanum gallate substrates for epitaxial high‐temperature superconducting thin films

We demonstrate that lanthanum gallate (LaGaO3) has considerable potential as an electronic substrate material for high‐temperature superconducting films. It provides a good lattice and thermal expansion match to YBa2Cu3O7−x, can be grown in large crystal sizes, is compatible with high‐temperature film processing, and has a reasonably low dielectric constant (e≂25) and low dielectric losses. Epitaxial YBa2Cu3O7−x films grown on LaGaO3 single‐crystal substrates by three techniques have zero resistance between 87 and 91 K.

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.

Design considerations for MRAM

MRAM (magnetic random access memory) technology, based on the use of magnetic tunnel junctions (MTJs) as memory elements, is a potentially fast nonvolatile memory technology with very high write endurance. This paper is an overview of MRAM design considerations. Topics covered include MRAM fundamentals, array architecture, several associated design studies, and scaling challenges. In addition, a 16-Mb MRAM demonstration vehicle is described, and performance results are presented.

Colossal magnetoresistance of 1 000 000‐fold magnitude achieved in the antiferromagnetic phase of La1−xCaxMnO3

Record values of colossal magnetoresistance (CMR) have been achieved in the antiferromagnetic phase of the La1−xCaxMnO3 system. At 125 K, the CMR of the La0.5Ca0.5MnO3 reaches nearly 1 000 000%. It increases exponentially to 100 000 000% at 57 K. While the ground state is primarily an antiferromagnet, application of a magnetic field induces a ferromagnetic alignment of spins that is highly beneficial to the electron conduction. Other ferromagnetic samples exhibit very sharp magnetic phase transitions, with which the magnetotransport is closely correlated.

Growth and giant magnetoresistance properties of La‐deficient LaxMnO3−δ (0.67≤x≤1) films

Epitaxial thin films of lanthanum‐deficient LaxMnO3−δ (0.67≤x≤1) have been grown on (100) SrTiO3 substrates by pulsed laser deposition. The as‐deposited films exhibit a ferromagnetic transition at temperatures ranging from 115 to 240 K, with the transition temperature (Tc) increasing with higher La deficiency. A sharp drop in resistivity and negative magnetoresistance is observed close to Tc, a behavior similar to that observed in divalent substituted La1−xMxMnO3−δ (M=Ba, Sr, Ca, Pb) films. Postannealing the films in O2 reduces the resistivity and raises the Tc to values close to room temperature. A magnetoresistance value of 130% (Δρ/ρH) has been obtained at 300 K at 4 T for a post‐annealed film with x=0.75.