Magnetoresistive Memory Research Articles

Differential type giant magnetoresistive memory using spin-valve film with a NiO pinning layer

The giant magnetoresistive (GMR) memory device of differential type is able to perform data readout without a magnetic field, and has many highly efficient properties, such as a nonvolatile, nondestructive readout, high-speed access, large output signal, and low electric power consumption. We have demonstrated data read/write of this memory by means of a spin-valve film with a NiO pinning layer. The spin-valve film was constructed of a multilayered structure [NiO/NiFeCo/Cu/NiFeCo], and exhibited the appropriate properties of a storage element, such as a MR ratio of 5%, a squareness ratio of unity, a writing field of 20 Oe, and a high corrosion resistance. The differential type memory device is constructed of a memory cell including two storage elements, in which the word currents of each element are in opposite directions. The information storage is performed by storing different resistance values in each element. The data readout of this memory is accomplished by sensing the differentiation of each element output, therefore magnetic field is not necessary for the readout process. The data read/write was established by observing a step-shape output signal: [plus or minus] against a three-step input signal: [zero/plus/zero/minus].

A superconductive magnetoresistive memory element using controlled exchange interaction

A memory device that can be switched between the normal state and superconducting state by an external magnetic field is proposed. The device consists of a superconducting/double magnetic (SM1M2) trilayer and is switched in a manner analogous to giant magnetoresistive memory devices. Using Usadel equations it is shown that the superconducting transition temperature of the device changes when the magnetic configurations of magnetizations of the two lower layers are switched between parallel and antiparallel. Appropriate design parameters are discussed and the materials issues analyzed.

Magnetic tunneling applied to memory (invited)

Random access magnetoresistive memories have been designed using anisotropic magnetoresistive (AMR) material and more recently giant magnetoresistive (GMR) material. The thin films in these memories have low sheet resistivities (about 10 Ω/sq), resulting in cell resistances of 10 to 100 Ω at competitive areal densities. High sense currents of a mA or more are required to get signals on the order of a few mV. Spin dependent tunneling (SDT) devices are intrinsically high impedance, with typical equivalent resistance values of 104–109 Ω for a square micron area. SDT cells have the potential for signals on the order of 10 mV with lower sense currents, and hence, faster access times than GMR memory. A GMR pseudospin valve memory concept is presented for comparison with SDT memory. Three different design approaches are discussed for SDT memory: (1) high-density memory arrays similar to those in AMR and GMR memories, (2) a transistor per cell approach similar to semiconductor dynamic random access memory, and (3) embedded SDT devices in a flip–flop cell similar to semiconductor static random access memory. The conclusions are: (1) SDT memory is potentially higher speed than GMR memory, (2) SDT memory has no area advantage compared with dense GMR memory, and (3) risks with SDT memory include (a) processing ultrathin dielectric layers uniformly and reliably that are compatible with integrated circuits and (b) attaining sufficiently low impedance levels to get a satisfactory signal-to-noise ratio in a small area cell.

Feasibility of ultra-dense spin-tunneling random access memory

Three-dimensional (3-D) finite element models have been utilized to simulate electromagnetic behaviors in spin-tunneling random access memory (STram). The most significant contributors have been identified. Compared with conventional current-in-plane (CIP) giant magneto-resistive (GMR) memory, whose signal level is inversely proportional to the square root of the storage density, these current-perpendicular-to-plane (CPP) STram elements provide an excellent readout property in that their signal level is independent of their cross-section area. This result is so attractive that the density of STram should not be limited by signal degradation. Moreover, a magnetic flux closure design was found to reduce the crossfeed by about a factor of five, compared with conventional keeperless design, which is the most favored approach for achieving 10/sup 9/ bits/cm/sup 2/ areal density. Although the storage mechanism described in this paper is made of STram, the flux-closure design could be generally applicable to other magnetic solid state memories.

Giant magnetoresistive memory effect in Nd0.7Sr0.3MnOz (abstract)

A novel giant magnetoresistance memory effect has been observed in epitaxial Nd0.7Sr0.3MnOz thin films which have previously been found to exhibit a linear increase in conductivity on first application of a magnetic field (B). Here we reported that the conductivity of the films depends not only on the applied field but also on the magnetic history. At T well below the temperature Tp where the zero-field resistivity has a peak, the film enters a high conductivity state [(ΔR/RB)≳103] upon application of a magnetic field which persists even when B is reduced to zero. The original ‘‘zero’’ field state is not recovered until the sample is warmed to T∼Tp. Surprisingly, the dc magnetization exhibits only a weak irreversibility while the magnetoconductivity is markedly hysteretic. That is, while the remanent magnetization is small the remanent magnetic resistivity is 10−3 times the initial zero-field-cooled resistivity. A possible explanation based on a two-component model of semiconducting matrix with embedded shunting paths of ferromagnetic material will be presented.

Analytical model in a weakly coupled sandwich for memory purposes

A simple analytical model explains giant magnetoresistive properties in a weakly coupled sandwich with two magnetic components. The model describes in detail a new storage mechanism in which the minor loop’s slope depends on its past magnetic histories. A new type of solid-state giant magnetoresistance memory based upon this mechanism has been realized by experiments. Furthermore, some important factors included in nondestructive readout capability are also analyzed by this model.

スピンバルブ膜を用いたメモリー素子

A new type of magnetoresistive memory was developed, using spin-valve multilayers composed of semi-hard magnetic layers and soft magnetic layers, separated by nonmagnetic layers. The fabricated memory element is composed of a word line of Au film and a magnetoresistive sense line of [ {CoPt / Cu / NiFeCo / Cu}]n multilayers and [NiFe/Cu/Co] sandwiches. In the [{CoPt/Cu/NiFeCo/Cu}]n multilayers, it was possible to read out from memory written to by applying an external field, however, it was impossible to write by applying a field with the word line, owing to the large size of the writing field. In [NiFe/Cu/Co] sandwiched film, a relatively small writing field made it possible to write to a memory and to read out from it. This spin-valve memory has non-volatile and non-destructive read-out properties.

Giant magnetoresistive memory effect in Nd0.7Sr0.3MnOz films

A novel giant magnetoresistance memory effect has been observed in epitaxial Nd0.7Sr0.3MnOz thin films which have previously been found to exhibit a linear increase in conductivity on first application of magnetic field B. The resistivity of the films depends not only on the instantaneous applied field but also on the magnetic history of the sample. At T well below the temperature Tp, where the zero-field resistivity has a peak, the film enters a high-conductivity state (upon application of a magnetic field) which persists even when B is reduced to zero. The original ‘‘zero’’ field state is not recovered until the sample is warmed to T∼Tp. Surprisingly, the dc magnetization exhibits only a weak irreversibility while the magnetoconductivity is markedly hysteretic. A possible explanation is proposed.

1-Mb memory chip using giant magnetoresistive memory cells

A 1-Mb nonvolatile, nondestructive readout M-R memory chip using elements with Giant Magnetoresistance Ratio (GMR) material has been designed. The chip employs dual redundancy, CMOS drive electronics with minimum gate lengths of 0.8 microns, two metal layers, and a 5-V /spl plusmn/10% power supply. The layout has an area of 0.9 cm sq, and approximately 50% of the chip area is devoted to the memory cell array. The memory chip is designed around 1.4 /spl mu/m/spl times/6.1 /spl mu/m, 80-/spl Omega/ elements using GMR material; the elements are spaced 1.4 /spl mu/m apart. The material is composed of two 50-/spl Aring/ ternary alloy layers separated by 30 /spl Aring/ of copper and has a nominal M-R coefficient of 6.0%. Minimum read signal is /spl plusmn/2.5 mV; sense current is 2.5 mA; work current is /spl plusmn/30 mA; and input and output is 4b wide. The memory employs a new read scheme in which two-phase sensing is employed. The scheme provides a sensitive, stable output and diminishes the array area by a factor of two, at the expense of read access time. The design contains over 700000 transistors and over 2 million memory cells; a prototype 64 K section of this design has been built, but the full design has yet to be constructed on silicon. The design demonstrates that with GMR memory cells, M-R memories can be designed with densities and speeds comparable to dynamic RAM's. >

Analysis of 0.1 to 0.3 micron wide, ultra dense GMR memory elements

A micro magnetic analysis, has been performed for ultra dense magnetoresistive memory cells employing giant magnetoresistive materials that have array densities from 2/spl times/10/sup 8/ to 10/sup 9/ elements per square cm. The analysis shows that as elements are made smaller, it is necessary to increase the effective anisotropy constant by increasing the demagnetizing factor in the long dimension of the element (or by finding GMR materials with large anisotropy fields). With diminishing cell size, exchange torque becomes progressively more important, and depending in detail on the thickness of the magnetic and separation layers, the layers ultimately act semi-coherently. Cell widths are eventually limited to values between 0.05 and 0.1 microns by thermal considerations because the smaller cells require progressively larger word and sense fields. The analysis assumed GMR elements made from magnetic layers 50 to 60 Angstroms thick of Ni, Fe, Co alloys with Cu, Ag, or Au separation layers of 15 to 30 Angstroms and with a magnetoresistive coefficient of 6 to 9%. >

A high output mode for submicron M-R memory cells

A mode which substantially increases the nondestructive read output from submicron magnetoresistive (MR) memory elements has been demonstrated. This mode increases the output for large cells by 50% and the outputs of cells less than a micron wide by 150% to 300%. In this mode, information is stored in the direction of the edge magnetization, and the body of the element serves as a sensor. Outputs of +or-1.2 mV were obtained with cells with an active area of 0.7*3.0 microns. By appropriate use of a step in the field oxide and a word line keeper, the demagnetizing field in the long dimensions of a memory element can be tailored to one desired value to optimize the response characteristics. The mode requires an additional drive strap to write the edge magnetization by coincident selection. For memory application the threshold for switching the edge magnetization must be tightly controlled. >

Magnetoresistive memory technology

Magnetoresistive random-access memory (MRAM) is an integrated combination of non-volatile thin film magnetic storage and semiconductor support circuits. MRAMs fill unique product niches and are useful today. The ultimate potential of MRAM is to be the “next dynamic random-access memory”, but it can only be realized with higher signals, and the giant magnetoresistance (GMR) effect offers a direct solution. GMR has been noted in thin film multilayers having ferromagnetic films interleaved with conductor layers. Composite thin films have demonstrated magnetoresistance on the order of ten times the anisotropic magnetoresistance of the present magnetic materials. A brief literature review, a simple model and some data on the “spin valve” effect are presented. These materials can offer improvements of 100 in read time and substantial simplification in circuitry when applied to MRAM. Thin magnetic films demonstrating GMR and future MRAM memory cells using films a few monolayers thick with limited cell diameters (1000 Å) pose very interesting fundamental problems relating to ferromagnetism and transport theory, and also offer challenges in fabrication and measurement.

A 512 K-bit magneto resistive memory with switched capacitor self-referencing sensing

A 512 K-bit nonvolatile magnetoresistive memory with a switched-capacitor self-referencing sensing scheme is reported. This memory is economical since it requires only one mask beyond a typical CMOS process and has rad hard, scalable, and no wear-out properties. This memory has a cell size of 2.0 mu m*10 mu m, logic buried under its cells, and a 0.3 cm/sup 2/ die. It is useful for disk caches and for replacing plated wire memories in aerospace applications.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Write stability of 2.0*20 mu m/sup 2/ M-R memory cells

Write stability tests have been performed on 2*20 mu m/sup 2/ magnetoresistive memory cells by performing continuous write operations at a 60 MHz rate for a period of 15 months; each cell was subjected to a total of 2.0*10/sup 15/ write operations. Within experimental error, no change in the element write thresholds occurred. As expected, element behavior was similar to that of plated wire or core cells; no wear-out phenomena such as occurs with ferroelectric cells were found. Tests were performed at room temperature, 23+or-2 degrees C. Annealing tests were conducted at 250 degrees C for 1 h in a 97%N/sub 2/3%H/sub 2/ environment to experimentally determine annealing effects on element thresholds. The perturbations on the switching thresholds were found to depend on the state of stored information in the bits. The shifts in the threshold curves were consistent with local skewing of the easy axis in the direction of the magnetization. >

Properties of 1.4*2.8 mu m/sup 2/ active area M-R elements

Densely packed magnetoresistive memory cells with active areas of 1.4*2.8 mu m/sup 2/ have been studied experimentally and compared to larger 2.0*12 mu m/sup 2/ cells. The experimental results show that the smaller cells, although having smaller output voltages (0.3 mV), have about the same statistical distributions for critical parameters as the larger cells. Using the current nominal 1.2 mu m lithography prototype process, which adds one extra mask to the semiconductor processing, cells with an area of 12.6 mu m/sup 2/ are achieved. By adding another extra mask, the area can be reduced to 5.6 mu m/sup 2/. >

M-R element width and thickness effects on reverse switching thresholds

During the reverse reading of a 'one' for a magnetoresistive memory element, one or two stable configurations of the magnetization can exist involving smaller and larger rotations of the magnetization. In hostile radiation environments, the two-state condition is undesirable because restoration after reading may occur by wall motion. An analysis was performed on the thickness, separation, and element width dependence of the field boundary between the one and two stable condition. The boundary sense field increased as an inverse square of element width and approximately as the product of layer thickness and separation. Elements can be designed to conveniently operate in the single state mode.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Magnetoresistive memories - analogies with ferroelectrics

Both Honeywell's Magnetoresistive Random Access Memory (MRAM) technology and its antecedent, a magnetoresistive memory invented by Len Schwee1, are fortunate to use common magnetic materials which are relatively stable and easy to fabricate. Magnetoresistive material properties, the MRAM cell operation (bit selection, writing and reading), general circuit strategies, and packaging are described. Analogies and comparisons with ferroelectric memory material and cell operation are discussed. Both ferroelectric memory and MRAM can fill important (and different) product niches, but both require material developments in order for them to achieve general-purpose usage: the ferroelectric material must overcome fatigue and the MRAM must overcome a small signal level. Other less significant materials developments are also needed. Once the proper material is available, a surprisingly long development cycle is needed to get products through design, process start-up, reliability tests, and (for rad hard products) radi...

Quadrupled nondestructive outputs from magnetoresistive memory cells using reversed word fields

A new operating mode has been discovered in transverse magnetoresistive (MR) memory cells which increases the usable nondestructive sense output by more than a factor of 4 over the original mode described, which employed a unipolar word field. The new mode allows for increased cell density, decreased memory access time, and increased margins. This new operating mode was examined experimentally and analytically for experimental cells ranging in size from 1.5×5 μm to 1.8×18 μm. The MR material used in the cells consisted of two 150-Å-thick magnetic layers (65% Ni, 15% Fe, 20% Co) separated by a 50-Å-thick reacted tantalum layer to break the exchange. In the new operating mode, a word field of one polarity is used for writing (parallel to the edge magnetization) and a reversed polarity is used for reading. During the reading process, 120°–225° paired ‘‘walls’’ are formed at the edges. The energy in the walls is sufficient to restore the elements to their original states even if switching occurs (in a direction opposite to the normal rotation during writing). The operating margins are improved because while reading only one and not both the word field and the sense field are in a direction to cause normal switching.

Reprogrammable logic array using M-R elements

A reprogrammable logic array has been designed with features such as rapid infinite reprogrammability with a single power supply, nonvolatile configuration storage, and radiation hardness using magnetoresistive (MR) memories. A prototype layout was made for simulation and analysis. The results indicated that all the above features could be achieved along with an approximately 20-ns delay time in the logic array structure using 2- mu m CMOS. The sense amplifier reliably detected simulated stored bits subjected to all errors and offsets arising from temperature and manufacturing inconsistencies. It was also discovered that the MR memory consumed only 10-15% of the die area. >

The reversed word field switching threshold for M-R memory elements

An experimental and analytical study was performed on the threshold properties of 1.5*5- mu m/sup 2/ and 1.8*18- mu m/sup 2/ magnetoresistive memory cells (MR) for rotational switching when a reverse word field (opposed to the edge magnetization) is applied. The results show that the threshold for reverse switching requires larger fields than normal because of the energy associated with the formation of walls at the element edges. In the case of reverse switching (for a stored 'one'), the wall energy is sufficient to restore the element to its original state when the word field is removed for the 1.5- and 1.8- mu m-wide elements. If the elements are made wide enough, the wall energy is not sufficient to cause return switching. The current analytical model adequately accounts for the normal and reverse switching threshold when demagnetization corrections are taken into account. The model does not adequately account for the return switching threshold as a function of sense current. >