Toggle MRAM

First principles Modeling of Magnetoresistance in Magnetic Memory devices KURT STOKBRO, MORTEN STILLING, Atomistix Inc, KARSTEN FLENSBERG, Copenhagen University -We have performed first principles calculations of the zero-bias conductance and TMR for crystalline Fe-MgO-Fe MTJs, and studied the effects of different oxide layers in the Fe/MgO interface, and the effects of structural "disorder" in the device. We find that such "defects" in the atomic structure have strong effects on the conductance. We use the result of the calculations to rationalize recent experimental findings. The simulations have been done with the software package Atomistix ToolKit (ATK), which is based on density functional theory (DFT) and non-equilibrium Green's functions (NEGFs).

In the development of magneto-resistance random access memory (MRAM), the theoretical studies are now concentrated in various parameters improving. Some important parameters are the range of the operating field and the switching time. The analysis presented in this paper is concentrated to a synthetic antiferromagnet (SAF). Recently a considerable effort has been expended by Fujiwara and co-workers to the analysis of the switching mode suggested by Savtchenko et al., called also toggle MRAM, and a number of conditions were found to maximize the operating field margin; most of these studies were based on a critical-curve approach. For uniaxial anisotropy, an analytical approach is possible and simple results and critical curves are presented. For the more general case, only the micromagnetic simulation can offer results. The studies are made for different coupling constants between the two ferromagnetic layers and the results are discussed.

For toggle magnetic random access memory, the saturation field represents the exterior limit of the work region. Exterior magnetic fields higher than the saturation field lead to an irremediable loss of the stored information because the antiferromagnetic state of the synthetic antiferromagnetic (SAF) structure becomes a ferromagnetic state and data stored in the magnetic memory cell are altered. At the saturation point, the two ferromagnetic-layer magnetizations became parallel oriented, in a direction close to that of the applied field. In this paper, we present a method to find an analytical formula for the magnetic saturation field that depends on the angle between the applied magnetic field related to the easy anisotropy axis, on the amplitude of the antiferromagnetic coupling field between the ferromagnetic layers, on the geometrical characteristics of the SAF structure, and on the second term of the series expansion of the anisotropy energy of the system. This method allows us to obtain an analytical formula for saturation field that depends only on the controllable parameters of the SAF structure. This analytically obtained critical saturation curve perfectly matches with the critical saturation curve reported in previous papers.

The interaction of two magnetic particles separated by an interlayer is illustrated through the "astroid" curves that represent regions in the magnetic field plane where different numbers of minima associated with stable or metastable states may exist. For a single particle, we describe the astroid curves of the Stoner-Wohlfarth model. The case of two particles is then examined and found to be much more complicated. The energy landscape of the two-particle system contains ferromagnetic, antiferromagnetic and canting states that emerge in response to the level of applied magnetic field. Because of this, up to four energy minima can exist in the system, depending upon the strength of the magnetic field and the material properties of the particles.

For almost last three years computer memory system use volatile, high speed memory technology like SRAM for cache, DRAM for main memory, flash memory for low power consumption applications. The fast access to memory is the major bottleneck for CPU performance. The emerging memory technology could improve the performance of CPU. The focus of this paper is to summarize the emerging technology of memory system design, presents these new memory technologies and survey the recent trends for memory technologies to advance memory hierarchies.

Magnetic tunnel junctions, i.e. the combination of two ferromagnetic electrodes separated by an ultrathin tunnel oxide barrier, are core elements in a large variety of spin-based devices. We report on the use of combined chemical vapor and atomic layer deposition processes for the synthesis of magnetic tunnel junctions with no vacuum break. Structural, chemical and morphological characterizations of selected ferromagnetic and oxide layers are reported, together with the evidence of tunnel magnetoresistance effect in patterned Fe/MgO/Co junctions.

One of the biggest challenges in magnetic memories is represented by operating field margin precision determination. Operating field region for classical Stoner-Wohlfarth model based memories is limited by neighbor cells interactions and half selected disturbs problems. Toggle magnetic memories eliminate many of these interactions related problems. A synthetic antiferromagnetic (SAF), formed by two antiferromagnetically coupled magnetic layers, is used as a free layer. To determine the toggle mode operation margin is important to take into consideration the strength of antiferromagnetic interactions between ferromagnetic layers, the geometric characteristics of the memory ceil, the relative thickness of the ferromagnetic layers and the magnetocrystalline anisotropy energy of ferromagnetic materials of the layers. These parameters control and determine the active domain for toggle MRAM cell.

The magnetization switching in Stoner-like magnetic particles is one of the fundamental issues in magnetic data storage. In develop of magnetoresistance random access memory (MRAM), the theoretical studies are now concentrated in various parameters improving. Some important parameters are the range of the operating field and the switching time. The writing mode proposed by Savtchenko and co-workers is known as the toggle MRAM or toggle write mode, is essentially based on a sequence of fields applied at 45° with respect to the easy axis of an antiferromagnetically coupled system of two single domain particles. We have recently analyzed systematically systems of two magnetic moments coupled with magnetostatic interactions. In this paper we are presenting the effect of the anisotropy of each material on the switching of a SAF in a toggle mode. For the uniaxial anisotropy type analytical approach is possible and simple results and critical curves are presented. For the more general case only the micromagnetic simulation can offer results. The studies are made for different coupling constants between the two ferromagnetic layers and the results are discussed.

In the development of magneto-resistance random access memory (MRAM), the theoretical studies are now concentrated in various parameters improving. Some important parameters are the range of the operating field and the switching time. The analysis presented in this paper is concentrated to a synthetic antiferromagnet (SAF). Recently a considerable effort has been expended by Fujiwara and co-workers to the analysis of the switching mode suggested by Savtchenko et al., called also toggle MRAM, and a number of conditions were found to maximize the operating field margin; most of these studies were based on a critical-curve approach. For uniaxial anisotropy, an analytical approach is possible and simple results and critical curves are presented. For the more general case, only the micromagnetic simulation can offer results. The studies are made for different coupling constants between the two ferromagnetic layers and the results are discussed.

For toggle magnetic random access memory, the saturation field represents the exterior limit of the work region. Exterior magnetic fields higher than the saturation field lead to an irremediable loss of the stored information because the antiferromagnetic state of the synthetic antiferromagnetic (SAF) structure becomes a ferromagnetic state and data stored in the magnetic memory cell are altered. At the saturation point, the two ferromagnetic-layer magnetizations became parallel oriented, in a direction close to that of the applied field. In this paper, we present a method to find an analytical formula for the magnetic saturation field that depends on the angle between the applied magnetic field related to the easy anisotropy axis, on the amplitude of the antiferromagnetic coupling field between the ferromagnetic layers, on the geometrical characteristics of the SAF structure, and on the second term of the series expansion of the anisotropy energy of the system. This method allows us to obtain an analytical formula for saturation field that depends only on the controllable parameters of the SAF structure. This analytically obtained critical saturation curve perfectly matches with the critical saturation curve reported in previous papers.

In this paper, we propose a method for a synthetic antiferromagnet structure's critical curve determination. The method is based on reversible susceptibility's singularities detection, as the magnetic field is swept along easy axis, in both positive and negative direction, while a hard axis bias field is also applied. By performing susceptibility measurements with different values of the bias field, the critical curve can be determined. Knowing the critical curve of a synthetic antiferromagnetic structure is essential for devices such as magnetic random access memories.

In the development of magneto-resistance random access memory (MRAM), the theoretical studies are now concentrated in various parameters improving. Some important parameters are the range of the operating field and the switching time. The analysis presented in this paper is concentrated to a synthetic antiferromagnet (SAF). Recently a considerable effort has been expended by Fujiwara and co-workers to the analysis of the switching mode suggested by Savtchenko et al., called also toggle MRAM, and a number of conditions were found to maximize the operating field margin; most of these studies were based on a critical-curve approach. For uniaxial anisotropy, an analytical approach is possible and simple results and critical curves are presented. For the more general case, only the micromagnetic simulation can offer results. The studies are made for different coupling constants between the two ferromagnetic layers and the results are discussed.

In this paper we present a study of switching characteristics of a series of synthetic antiferromagnet ͑SAF͒ structures using reversible susceptibility experiments. Three series of SAF samples were considered in our study with ͑t1, t2͒, the thickness of the FeCoB layers of ͑80 nm, 80 nm͒, ͑50 nm, 50 nm͒, and ͑80 nm, 20 nm͒ and with the interlayer of Ru ranging from 0 to 2 nm. A vector vibrating sample magnetometer was used to measure the hysteresis loops along the different directions in the plane of the samples. The reversible susceptibility experiments were performed using a resonant method based on a tunnel diode oscillator. We showed that the switching peaks in the susceptibility versus field plots obtained for different orientations of the applied dc field can be used to construct the switching diagram of the SAF structure. The critical curve constitutes the fingerprint of the switching behavior and provides information about micromagnetic and structural properties of SAF which is an essential component of modern magnetic random access memories.

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Magnetic random access memory (MRAM) is an emerging technology with potential to become the universal on-chip memory. Among existing MRAM technologies, thermally assisted switching (TAS)-MRAM technology offers several advantages compared with other technologies: selectivity, single magnetic field, and high-integration density. In this paper, we analyze the impact of resistive-open defects on TAS-MRAM behavior. Electrical simulations were performed on a hypothetical 16 word TAS-MRAM architecture enabling any combination of read and write operations. Results show that read and write sequences may be affected by resistive-open defects that may induce single and double-cell faulty behaviors. As a next step, we will exploit the analyses results to guide the test phase by providing effective test algorithms targeting faults related to actual defects affecting TAS-MRAM architectures. Index Terms-Fault modeling, nonvolatile memories (NVM), resistive-open defects, spintronics, thermally assisted switching (TAS)-MRAM, test. 1063-8210

The magnetization switching in Stoner-like magnetic particles is one of the fundamental issues in magnetic data storage. In develop of magnetoresistance random access memory (MRAM), the theoretical studies are now concentrated in various parameters improving. Some important parameters are the range of the operating field and the switching time. The writing mode proposed by Savtchenko and co-workers is known as the toggle MRAM or toggle write mode, is essentially based on a sequence of fields applied at 45° with respect to the easy axis of an antiferromagnetically coupled system of two single domain particles. We have recently analyzed systematically systems of two magnetic moments coupled with magnetostatic interactions. In this paper we are presenting the effect of the anisotropy of each material on the switching of a SAF in a toggle mode. For the uniaxial anisotropy type analytical approach is possible and simple results and critical curves are presented. For the more general case only the micromagnetic simulation can offer results. The studies are made for different coupling constants between the two ferromagnetic layers and the results are discussed.

For toggle magnetic random access memory, the saturation field represents the exterior limit of the work region. Exterior magnetic fields higher than the saturation field lead to an irremediable loss of the stored information because the antiferromagnetic state of the synthetic antiferromagnetic (SAF) structure becomes a ferromagnetic state and data stored in the magnetic memory cell are altered. At the saturation point, the two ferromagnetic-layer magnetizations became parallel oriented, in a direction close to that of the applied field. In this paper, we present a method to find an analytical formula for the magnetic saturation field that depends on the angle between the applied magnetic field related to the easy anisotropy axis, on the amplitude of the antiferromagnetic coupling field between the ferromagnetic layers, on the geometrical characteristics of the SAF structure, and on the second term of the series expansion of the anisotropy energy of the system. This method allows us to obtain an analytical formula for saturation field that depends only on the controllable parameters of the SAF structure. This analytically obtained critical saturation curve perfectly matches with the critical saturation curve reported in previous papers.

The magnetic switching behavior of Co/ SiO 2 multilayers has been studied using reversible susceptibility experiments performed along different orientations in the sample's plane. A sensitive method for critical curve determination of two-dimensional magnetic systems was proposed. It was shown that this method, based on reversible susceptibility's singularities detection, is general and can be applied independent of the expression of free energy describing the magnetic system under study. It is found that as the Co/ SiO 2 ratio increases in the samples, the switching mechanism is governed by a noncoherent rotation mechanism.

In this paper, we present an analytical method to study the magnetization processes of two magnetic particles with ferromagnetic or antiferromagnetic coupling by using a modified expression for the anisotropy free energy. By considering a system consisting of two uniaxial particles, coupled by a simple exchange interaction, we describe analytically: the particles orientations, switching points and hysteresis loops for different ferromagnetic or antiferromagnetic couplings and for different orientations of the applied field.

The influence of various physical parameters on magnetizations processes of two interacting ferromag-netic particles with significance for synthetic antiferromagnetic structures as a component of the magnetic random access memory are presented. Because the real structures are not perfectly symmetric, the alterations of the perfect structure reflected in magnetization processes especially on critical curves are analyzed. The effects of the coupling constant, magnetostatic interactions and anisotropy are also analyzed.

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In this paper, we present an analytical method to study the magnetization processes of two magnetic particles with ferromagnetic or antiferromagnetic coupling by using a modified expression for the anisotropy free energy. By considering a system consisting of two uniaxial particles, coupled by a simple exchange interaction, we describe analytically: the particles orientations, switching points and hysteresis loops for different ferromagnetic or antiferromagnetic couplings and for different orientations of the applied field.