Haut PDF Magnetization reversal driven by spin-transfer-torque in perpendicular shape anisotropy magnetic tunnel junctions

Magnetization reversal driven by spin-transfer-torque in perpendicular shape anisotropy magnetic tunnel junctions

Magnetization reversal driven by spin-transfer-torque in perpendicular shape anisotropy magnetic tunnel junctions

Index Terms— Micromagnetism, Perpendicular Shape Anisotropy, Spin-Transfer-Torque, Transverse domain wall I. I NTRODUCTION The spin-transfer-torque magnetic random-access memory (STT-MRAM) is one of the most promising emerging non- volatile memory technologies [1]-[4]. It combines non- volatility with a quasi-infinite write endurance, high speed, low power consumption and scalability [5]-[7]. These properties are making STT-MRAM about to enter in mass production for replacing e-FLASH and L3 SRAM [7]-[11]. While initial STT- MRAM devices used an in-plane (IP) magnetization, it has been shown that a perpendicular orientation of the magnetization leads to a better tradeoff between thermal stability factor Δ (related with the memory retention time) and switching current. These devices called perpendicular STT-MRAM (p-STT- MRAM) use the interfacial perpendicular magnetic anisotropy (iPMA) originated at the FeCoB layer and MgO tunnel barrier interface [1], [5], [12]. Nonetheless, there are still some major challenges, predominately when the MTJ goes to sub-20 nm diameters. As the device lateral size shrinks, there is a decrease in Δ due to a decrease in the storage layer volume. This decrease significantly reduces the retention time of the memory [13]- [15]. This limitation can be understood considering that at these small sizes the reversal of the magnetic volume is almost coherent, and so Δ is proportional to the layer volume. In addition, as the surface area shrinks, the iPMA decreases proportionally to the area, until a point where it becomes too weak to stabilize the magnetization perpendicularly. A proposal to counter this decrease is the use of a double FeCoB/MgO interface, by doubling the iPMA [16], [17]. Still, it is very challenging to keep Δ > 60 at sub-20 nm diameters. A promising solution to this problem is a novel concept that takes advantage of the shape anisotropy of the storage layer by increasing its thickness (L) to values of the order or larger than
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Spin transfer torque magnetic random-access memory: Towards sub-10 nm devices

Spin transfer torque magnetic random-access memory: Towards sub-10 nm devices

Contact email: bernard.dieny@cea.fr ___________________________________________________________________________ Magnetic Random-Access Memory (MRAM) is a non-volatile class of solid-state storage device where the information is stored in the magnetic state of a ferromagnetic layer. Microelectronic industry has recently shown a strong interest for MRAM as they are very promising for embedded RAM applications and particularly embedded FLASH replacement 1– 6 . The main building bloc of an MRAM is a trilayer FM1/I/FM2 (Fig. 1.a) named magnetic tunnel junction (MTJ) 7 , where FM1(2) refer to ferromagnetic layers and I to a thin insulating tunnel barrier ( ~ 1.2 nm). Nowadays researches are mainly focused on perpendicularly (p) magnetized tunnel junctions 8–11 written by spin transfer torque (STT) 12,13 , where the perpendicular axis is defined by the growth direction. FM1 is called the reference layer, its magnetization is pinned in one specific direction (for example, the up direction). FM2 is called the storage layer or free layer, its magnetization is free to be moved between its two stable states (up and down). The resistance of the stack depends on the relative orientation of the two magnetizations. The parallel (P) state (up/up) is a low resistance state while the antiparallel (AP) state (up/down) is a high resistance state, thus respectively coding the “0” and “1” of the binary logic. The tunneling magnetoresistance (TMR) is defined by the ratio = . The P and AP states are separated by an energy barrier Eb, used to define the thermal stability of the memory ∆= E /k T with k BT the thermal energy (Fig. 1.b). For practical applications, one aims for ∆ values from 60 to 100 14,15 .
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Respective influence of in-plane and out-of-plane spin-transfer torques in magnetization switching of perpendicular magnetic tunnel junctions

Respective influence of in-plane and out-of-plane spin-transfer torques in magnetization switching of perpendicular magnetic tunnel junctions

CEA, INAC-SPINTEC, F-38000 Grenoble, France; CNRS, SPINTEC, F-38000 Grenoble, France (Received 3 June 2015; revised manuscript received 26 August 2015; published 28 September 2015) The relative contributions of in-plane (damping-like) and out-of-plane (field-like) spin-transfer torques (STT) in the magnetization switching of out-of-plane magnetized magnetic tunnel junctions (pMTJ) has been theoretically analyzed using the transformed Landau-Lifshitz-Gilbert (LLG) equation with the STT terms. It is demonstrated that in a pMTJ structure obeying macrospin dynamics, the out-of-plane torque influences the precession frequency, but it does not contribute significantly to the STT switching process (in particular to the switching time and switching current density), which is mostly determined by the in-plane STT contribution. This conclusion is confirmed by finite temperature and finite writing pulse macrospin simulations of the current field switching diagrams. It contrasts with the case of STT switching in in-plane magnetized magnetic tunnel junction (MTJ) in which the field-like term also influences the switching critical current. This theoretical analysis was successfully applied to the interpretation of voltage field STT switching diagrams experimentally measured on pMTJ pillars 36 nm in diameter, which exhibit macrospin behavior. The physical nonequivalence of Landau and Gilbert dissipation terms in the presence of STT-induced dynamics is also discussed.
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Spin transfer torque magnetization reversal in a hard/soft composite structures

Spin transfer torque magnetization reversal in a hard/soft composite structures

All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/ ). https://doi.org/10.1063/1.5009589 It is now well established that due to spin transfer torque (STT), a polarized current can induce magnetization switching or precession. Those effects are promising technologies for two different application STT-MRAM (magnetic random access memories) and STT-NO (nano-oscillators). One of the present challenges to implement STT-MRAM lies on the reduction of the critical current whereas STT-NO depends on the maximum output power and the narrower frequency bandwidth of the oscillations possible. To improve these two technologies many studies have been performed to find optimal materials and geometries. For instance perpendicular magnetic anisotropy (PMA) materials have shown to improve both switching current for STT-MRAM 1 , 2 and the output power for STT-NO. 3 , 4 However up to now, most of the studies have considered free layers with uniform magnetization.
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A highly thermally stable sub-20 nm magnetic random-access memory based on perpendicular shape anisotropy

A highly thermally stable sub-20 nm magnetic random-access memory based on perpendicular shape anisotropy

Main Introduction A magnetic random-access memory (MRAM) is a non-volatile memory wherein one bit of information is stored by the magnetic state of a ferromagnetic (FM) layer. Microelectronic industry has recently shown a strong interest for MRAM as they are very promising for embedded RAM applications and particularly embedded FLASH replacement 1–6 . Nowadays most of the development are focused on MRAM based on out-of-plane magnetized tunnel junctions written by spin transfer torque (STT) 7–12 . The p-MTJ essentially consists of an MgO tunnel barrier (1-1.5 nm thick) sandwiched between two thin perpendicularly magnetized FeCoB layers (1-2.5 nm thick), namely the reference and storage layer. The magnetization of the reference is pinned in one specific direction by exchange coupling with a synthetic antiferromagnet (SAF). The magnetization of the storage layer can be switched between the up and down states, respectively coding the “0” and “1” of the binary logic. The state of the cell is read thanks to the tunneling magnetoresistance effect (TMR). The parallel (P) relative configuration between the magnetization of the reference and the storage layer leads to a low resistance state while the antiparallel one, to a high resistance state. TMR above 200% at room temperature are nowadays obtained in highly optimized MTJs deposited in state of the art sputtering deposition tools 13 . The energy E B required to switch the memory between these two
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Influence of spin-transfer torque on thermally activated ferromagnetic resonance excitations in magnetic tunnel junctions

Influence of spin-transfer torque on thermally activated ferromagnetic resonance excitations in magnetic tunnel junctions

B. Magnetic fluctuations in the presence of spin torque Now we consider a multilayer structure with two ferro- magnetic layers separated by a spacer. This structure corre- sponds to the active part of a spin valve or a magnetic tunnel junction. The magnetization of one of the ferromagnetic layer is supposed to be fixed 共reference layer兲, whereas the magnetization of the other can move freely 共free layer兲. The thin ferromagnetic layer that we considered in Sec. II A is now the free layer and is subjected to a perpendicular-to- plane current. A negative current corresponds to the situation where electrons flow from the reference layer toward the free layer. In this case, the electron spins are first polarized by the reference layer and may exert a torque on the free layer magnetization through a direct transfer of angular momen- tum. Inversely, a positive current corresponds to electrons flowing from the free layer. Thus electrons that have their polarization opposite to the reference layer one are mostly reflected at the interface and are responsible for the torque on the free layer. This spin-transfer torque is phenomenologi- cally written as the usual following expression: 15
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Route towards efficient magnetization reversal driven by voltage control of magnetic anisotropy

Route towards efficient magnetization reversal driven by voltage control of magnetic anisotropy

charge depletion and accumulation at the ferromagnet(FM)/oxide(Ox) interface, in the case of purely electronic effects [11] and/or ii) oxygen migration in the case of ionic mechanisms [12], [13]. Another very likely source of electric-field control of the magnetization is the Rashba spin- orbit mechanism, where the underlayer in contact with the FM layer is a heavy metal (HM) [14]. In some cases, the HM/FM interface can have a greater contribution to the VCMA effect than the FM/Ox interface [14]. Many first principle studies have been dedicated to this topic, generally for systems composed of a heavy metal, a 3d ferromagnet and a dielectric, typically MgO [15] [16] [17] [18]. Simultaneously, experimental evidence of voltage controlled magnetic anisotropy (VCMA) has emerged in a wide range of structures, and static and dynamic measurements have been performed in order to explore its limits [19]. However, from the micromagnetic and macrospin simulations point of view, this field is not explored enough and a deep understanding of the behavior of magnetization in this framework and of its governing parameters is still needed. Some studies on VCMA using the micromagnetic framework aim at explaining the magnetization reversal process under the influence of an electric field, with some design approaches being proposed [20] [21] [22]. A broad variety of macrospin studies exist, oriented towards the optimization of system operation through pulse shape [23] [24], thermal stability [25] and towards decreasing of the write-error-rate (WER), some of which accompanied by experimental studies [19] [26] [27] [25]. Other macrospin studies focus on the association between VCMA and spin transfer torque (STT) methods of magnetization manipulation [28] or on the VCMA and a Rashba field which is used as an in-plane bias magnetic field [26]. A detailed fundamental study of the dynamics is dedicated to a design where an assisting magnetic field is absent [29] and the switching relies purely on the fine tuning of the voltage pulse duration. However, a reliable fast switching process is conditioned by extremely short pulses of precise length. Therefore, we considered it appropriate to search for a less restrictive regime from an application point of view.
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Domain wall motion in nanopillar spin-valves with perpendicular anisotropy driven by spin-transfer torques

Domain wall motion in nanopillar spin-valves with perpendicular anisotropy driven by spin-transfer torques

The time evolution of the magnetization after application of a 1 mA current shows that in about 120 ns the system reaches a new equilibrium state that, in contrast to the previous case, is a DW at the same position, but with its internal structure modified. Indeed, the magnetic moments inside the DW rotate in the plane of the layer and its structure becomes closer to a Bloch wall. Note that the transition from a N´eel to a Bloch wall profile is plausible given that the quality factor of the free layer Q = 0.94 is close to 1. Therefore, it appears that the spin-transfer torque does not depin or even move the domain wall, but only modifies its internal micromagnetic structure. This is consistent with the observation that the spin-transfer torque has virtually no effect on the field-current state diagram or on the mean dwell time of the DW state.
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Perpendicular magnetic tunnel junctions with a synthetic storage or reference layer: A new route towards Pt- and Pd-free junctions

Perpendicular magnetic tunnel junctions with a synthetic storage or reference layer: A new route towards Pt- and Pd-free junctions

The discovery of very large TMR amplitude in in-plane magnetized magnetic tunnel junctions (MTJ) with a crystalline MgO barrier has been a major breakthrough 1,2 . However, for memory applications, the interest rapidly evolved towards out-of-plane magnetized systems. Indeed, using MTJs with Perpendicular Magnetic Anisotropy (PMA) is interesting in several respects: i) it allows increasing the density of memory cells on a wafer since no elliptical shape is required to stabilize the anisotropy direction, contrary to the planar systems, ii) PMA energy is usually much larger than shape anisotropy that can be obtained in planar MTJs, allowing longer memory reten- tion at small size; iii) for a given retention time, the critical current density to write information by Spin Transfer Torque (STT) switching is strongly reduced, provided the Gilbert damping remains low enough 3 .
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Enhanced annealing stability and perpendicular magnetic anisotropy in perpendicular magnetic tunnel junctions using W layer

Enhanced annealing stability and perpendicular magnetic anisotropy in perpendicular magnetic tunnel junctions using W layer

[ http://dx.doi.org/10.1063/1.4983159 ] Spin transfer torque magnetic random access memories (STT-MRAM) are the focus of intense research and develop- ment efforts due to their non-volatility, low-energy con- sumption, high-speed performance, quasi-infinite endurance, and reliability. 1 – 3 Especially, perpendicular magnetic tunnel junctions (pMTJs) used for STT-MRAM applications offer better downsize scalability and trade-off between retention and write power consumption than their in-plane magnetized counterparts. 2 – 5 The core of pMTJ is the FeCoB (reference)/ MgO/FeCoB (storage layer)/cap layer stack. To achieve high tunnel magnetoresistance (TMR) and high perpendicu- lar magnetic anisotropy (PMA), a post-deposition annealing is carried out. This anneal yields an improvement in the crys- tallinity of the MgO barrier, a crystallization of the FeCoB by gettering B out of the magnetic layers, 6 , 7 and a sharpening of the bcc (100) MgO/FeCo interface. 8 In general, the higher the annealing temperature, the better the crystallinity. 5 – 7 However, it was observed that above a certain annealing temperature, the TMR and PMA deteriorates 9 due to diffu- sion of non-magnetic species toward the tunnel barrier, espe- cially of Ta when this material is inserted next to the FeCoB layer. This diffusion mechanism limits the thermal budget that a pMTJ stack can tolerate. 10 , 11 Using the Ta cap layer, it is difficult to obtain pMTJs with a high PMA storage layer (SL) and back-end-of-line (BEOL) annealing tolerance which is 400  C for 30 min. Hence, a detailed investigation of magnetic properties of the top storage layer as a function of annealing temperature was carried out using the W mate- rial in the cap layer. Previous works reported on improve- ment in PMA of FeCoB with W and Mo buffer or cap layers associated with increasing annealing temperature up to
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Increased energy efficiency spin-torque switching of magnetic tunnel junction devices with a higher order perpendicular magnetic anisotropy

Increased energy efficiency spin-torque switching of magnetic tunnel junction devices with a higher order perpendicular magnetic anisotropy

We study the influence of a second order magnetic anisotropy on magnetization reversal by spin transfer torque in perpendicularly magnetized magnetic tunnel junctions (pMTJs). Using a macrospin model to describe the dynamics of the free layer, analytical solutions for the switching voltage and the voltage threshold for precession are determined as a function of the first and second order magnetic anisotropies. To compare the spin-transfer-torque energy efficiency to that of a classical pMTJ, a junction without the second order anisotropy term, we compare these cases at a fixed energy barrier to thermally activated reversal. We show that the critical voltage for switching can be reduced by a factor 0.7 when the ratio of the second to the first order magnetic anisotropy is 1/3. Importantly, the switching time can be reduced by nearly a factor of two for this magnetic anisotropy ratio. These results highlight an important and practical method to increase the spin-torque efficiency, while reducing the energy dissipation and switching time in magnetic random access memory devices.
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Asymmetric Magnetization Switching in Perpendicular Magnetic Tunnel Junctions: Role of the Synthetic Antiferromagnet’s Fringe Field

Asymmetric Magnetization Switching in Perpendicular Magnetic Tunnel Junctions: Role of the Synthetic Antiferromagnet’s Fringe Field

DOI: 10.1103/PhysRevApplied.11.034058 I. INTRODUCTION Current-induced manipulation of magnetization has been a subject of great interest since the prediction of spin transfer torque (STT) by Berger [ 1 ] and Slonczewski [ 2 ] in 1996. It has led to an innovative magnetic memory technology, namely spin-transfer torque magnetic ran- dom access memories (STT MRAM). Such devices are composed of magnetic tunnel junctions with electrodes exhibiting a strong perpendicular magnetic anisotropy (pMTJs) [ 3 – 6 ]. In these devices, the switching current is directly proportional to the energy barrier for thermally activated magnetization reversal [ 6 , 7 ]. Indeed, devices composed of materials with large perpendicular magnetic anisotropy (PMA) have been shown to combine both a good thermal stability and exhibit efficient current-induced switching [ 6 – 8 ]. To further improve a STT MRAM device, the switching energy and the switching time still need to be reduced [ 9 , 10 ], both of which depend on the energy barrier. The study and the characterization of the energy barrier is, therefore, crucial for improving the performance of STT MRAM.
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Heating asymmetry induced by tunneling current flow in magnetic tunnel junctions

Heating asymmetry induced by tunneling current flow in magnetic tunnel junctions

do tunnel through the barrier. As a result of this heating asymmetry, for the same power density, this results in a higher temperature rise in the receiving electrode. This higher temperature then propagates to the adjacent layers through usual heat diffusion. Data show that this effect is quite significant since the same temperature rise can be achieved in the FeMn antiferromagnet with a power density that can be reduced by 10% by applying the appropriate volt- age polarity. The applied heating current densities are between 1.7 and 3.1  10 6 A/cm 2 . The spin transfer torque effects are expected to play a minor role at these current den- sities in these structures because of their relatively thick stor- age layer. To confirm that spin transfer torque effect is not significant, the same experiments were performed with a reset condition that sets a negative exchange bias field in the storage layer, prior to each switching heating pulse. This sets the initial tunnel junction magnetization in the parallel (P) configuration, low resistance state. As shown in Fig. 3 , a lower write power density continues to be observed for the positive pulse polarity (electrons tunnelling from reference layer to storage layer). This would not be the case if spin transfer torque was responsible for the observed write power density reduction. The same voltage polarity would not help the storage layer reversal both from AP to P configuration and from P to AP. This seems to exclude any spin torque related effects in the observed influence of voltage polarity/ current direction. Fig. 3 nevertheless shows that the required current densities for switching from positive to negative exchange bias are lower than for switching from negative to positive exchange bias whatever the current direction. This observation may be ascribed to a positive stray field from the reference layer acting on the storage layer which would favor negative exchange bias of the storage layer.
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Field-free magnetization reversal by spin-Hall effect and exchange bias

Field-free magnetization reversal by spin-Hall effect and exchange bias

1 Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands 2 DSM/IRAMIS/SPEC, CNRS UMR 3680, CEA Saclay, 91191 Gif sur Yvette cedex,France Magnetic random-access memory (MRAM) driven by spin-transfer torque (STT) 1,2 is a major contender for future memory applications 3,4,5 . The energy dissipation involved in writing remains problematic 6 , even with the advent of more efficient perpendicular magnetic anisotropy (PMA) devices 7 . A promising alternative switching mechanism employs spin-orbit torques 8 and the spin-Hall effect 9,10 (SHE) in particular, but additional symmetry breaking is required to achieve deterministic switching in PMA devices.
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Description of current-driven torques in magnetic tunnel junctions

Description of current-driven torques in magnetic tunnel junctions

Another group presented at tight-binding model (TB) of a MTJ, giving more realistic band structures than the usual free electron model [ 26 , 27 ]. These studies showed that spin torque should present an important bias asymmetry and the dissipative part of IEC (also called current-induced effective field) should be of the same order of magnitude than STT with a quadratic dependence on the bias voltage [ 26 ]. Finally, we note that in the same spirit as Ref. [ 28 ], Levy and Fert studied the role of hot electrons-induced magnons on STT in MTJ [ 29 ]. In recent experiments, the important relative amplitude of current-induced effective field compared to the spin torque term has been verified [ 30 , 31 , 32 ] but the role of magnons is still under investigation (in the first experiment the current-induced magnetization reversal occured while the TMR was quenched by magnons emissions [ 20 ]). These specific features show that tunnelling transport has a strong influence on spin transfer torque characteristics.
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State diagram of nanopillar spin valves with perpendicular magnetic anisotropy

State diagram of nanopillar spin valves with perpendicular magnetic anisotropy

These peaks are compatible with the magnetization preces- sions predicted by the theory. Such precessions are commonly recorded in spin valves with at least one magnetization in-plane because they generate an alternative voltage due to the angular dependence of the giant magnetoresistance. However, in all these perpendicular spin valves, a uniform precession of the magnetization of the free layer around the out-of-plane axis does not affect the angle between the magnetizations of the free layer and of the polarizer. As a consequence, no alternative voltage can be generated in the first approximation. These precessions have to be detected indirectly thanks to differential measurements and a lock-in technique. Unfortunately these methods cannot guarantee that every measured peak is the consequence of magnetization precessions. Note that another method using GHz microwave irradiation has been developed to enhance and detect spin-torque driven magnetization preces- sion in nanopillars with magnetic perpendicular anisotropy. 55 The borders determined by the switching fields or currents evolve linearly over a large range of current and field, however, around the zero current switching fields a strong deviation from this linearity occurs. Experimentally it seems that the magnetization reversal becomes virtually independent of the
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Spin-dependent transport in antiferromagnetic tunnel junctions

Spin-dependent transport in antiferromagnetic tunnel junctions

[ http://dx.doi.org/10.1063/1.4896291 ] Antiferromagnets (AFs)-based spintronics is a branch of science that explores spin dependent transport devices using AFs instead of ferromagnets (F). 1 , 2 It is currently considered as a significant exploratory topic in spintronics 3 – 6 since AFs exhibit no stray fields, which is beneficial for ultimate down- size scalability. In particular, a first theoretical toy model showed AF spin transfer torque (STT) and giant magnetore- sistance (GMR) for metallic AF/PM/AF multilayers, 7 where PM stands for a paramagnetic metallic spacer. The authors considered crystalline uncompensated F monolayers with staggered AF order (i.e., two alternating F sublattices with opposite magnetizations). Furthermore, unlike the pioneering theoretical works on STT in F multilayers, 8 , 9 which predict torques exerted by a spin polarized current close to the inter- face between a F and a nonmagnetic metal, STT is expected to act cooperatively through the entire volume of the AF electrodes. This feature together with the absence of shape anisotropy in AFs explain that lower critical currents for magnetization switching are predicted for epitaxial AFs compared to the typical values for Fs.
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Field-current phase diagrams of in-plane spin transfer torque memory cells with low effective magnetization storage layers

Field-current phase diagrams of in-plane spin transfer torque memory cells with low effective magnetization storage layers

Applied Materials SSG CTO, 974 E Arques avenue, Sunnyvale, California 94085, USA (Presented 6 November 2013; received 23 September 2013; accepted 24 October 2013; published online 28 January 2014) Field-current phase diagrams were measured on in-plane anisotropy Co 60 Fe 20 B 20 magnetic tunnel junctions to obtain the spin transfer torque (STT) field-current switching window. These measurements were used to characterise junctions with varying free layer thicknesses from 2.5 down to 1.1 nm having a reduced effective demagnetizing field due to the perpendicular magnetic anisotropy at CoFeB/MgO interface. Diagrams were obtained with 100 ns current pulses, of either same or alternating polarity. When consecutive pulses have the same polarity, it is possible to realize the STT switching even for conditions having a low switching probability. This was evidenced in diagrams with consecutive pulses of alternating polarity, with 100% switching obtained at 4.7 MA/cm 2 , compared to the lower 3.4 MA/cm 2 value for same polarity pulses. Although the low level of the current density window is higher in alternating polarity diagrams, the field window in both diagrams is the same and therefore independent of the pulse polarity sequence. V C 2014 AIP Publishing LLC. [ http://dx.doi.org/10.1063/1.4862842 ]
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SPICE modelling of magnetic tunnel junctions written by spin-transfer torque

SPICE modelling of magnetic tunnel junctions written by spin-transfer torque

3.5. SPICE modeling implementation This STT-based MTJ model has been implemented in C-language, compiled with the compiled-model interface (CMI) provided by Cadence Design Systems. This model supports 9 nodes, including 4 external nodes and 5 internal nodes. The MTJ simulation cell view is shown in figure 4. Two inputs BL0 and BL1 represent the two electrodes of the junction which is modeled by its tunneling conductance and capacitance. One field line, with two nodes FL0 and FL1, is used for the analysis of magnetization dynamics in presence of external field. In order to ease the understanding of the device behaviour, internal nodes such as device temperature (th) and magnetic state (my) can also be monitored and plotted. This model includes 38 parameters, among which the geometrical parameters such as the shape, the size and the initial magnetic state of the MTJ are user-defined to perform the intended simulations. In contrast, the other physical and technological parameters are defined in a corner file and cannot be modified by users since they are provided by the manufacturer.
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Second order anisotropy contribution in perpendicular magnetic tunnel junctions

Second order anisotropy contribution in perpendicular magnetic tunnel junctions

From practical point of view, the easy cone anisotropy can be used to significantly improve the writing per- formances of pMTJ-based STT-MRAM elements 29,30 . In a standard pMTJ system, the magnetic moments of both free and reference layers are aligned parallel or antiparallel in standby regime. Upon writing, when the write current starts flowing through the MTJ, the initial STT-torque is zero and only thermal fluctuations or micromag- netic distortions provide the non-collinearity required to trigger the reversal of the storage layer magnetization. Both effects are generally undesirable in STT-MRAM technology. Indeed, thermal fluctuations are stochastic by nature and therefore the write pulse duration and intensity must be overdesigned to reach the specified write error rate. As for micromagnetic distorsions, the latter induce non-uniform switching process which can result in the need for higher switching current and variability in the switching process. An easy cone regime in the free layer and the easy axis configuration in the reference one would be the optimal configuration for a STT-MRAM mem- ory element. Unique features of easy cone regime is that it allows for a canted state and at the same time conserves the axial symmetry so essential for effective transfer of the STT torque into the angular motion.
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