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International audience; Perpendicular shape anisotropy (PSA) offers a practical solution to downscale spin-transfer torque magnetoresistive random-access memory (STT-MRAM) beyond the sub-20 nm technology node while retaining thermal stability. However, our understanding of the thermomagnetic behavior of PSA-STT-MRAM is often indirect, relying on magnetoresistance measurements and micromagnetic modeling. Here, the magnetism of a NiFe PSA-STT-MRAM nanopillar is investigated using off-axis electron holography, providing spatially resolved magnetic information as a function of temperature. Magnetic induction maps reveal the micromagnetic configuration of the NiFe storage layer (∼60 nm high, ≤20 nm diameter), confirming the PSA induced by its 3:1 aspect ratio. In situ heating demonstrates that the PSA of the storage layer is maintained up to at least 250 °C, and direct quantitative measurements reveal a moderate decrease of magnetic induction. Hence, this study shows explicitly that PSA provides significant stability in STT-MRAM applications that require reliable performance over a range of operating temperatures.

Perpendicular shape anisotropy (PSA) and double magnetic tunnel junctions (DMTJ) offer practical solutions to downscale spin-transfer-torque Magnetic Random-Access Memory (STT-MRAM) beyond 20 nm technology nodes, whilst retaining their thermal stability and reducing critical currents applied. However, as these modern devices become smaller and three-dimensionally (3D) complex, our understanding of their functional magnetic behavior is often indirect, relying on magnetoresistance measurements and micromagnetic modelling. In this paper, we review recent work that was performed on these structures using a range of advanced electron microscopy techniques, focusing on aspects specific to the 3D and nanoscale nature of such elements. We present the methodology for the systematic transfer of individual SST-MRAM nano-pillars from large-scale arrays to image their magnetic configurations directly using off-axis electron holography. We show that improved phase sensitivity through stacking of electron holograms can be used to image subtle variations in DMTJs and the thermal stability of < 20 nm PSA-STT-MRAM nano-pillars during in-situ heating. The experimental practicalities, benefits and limits of using electron holography for analysis of MRAM devices are discussed, unlocking practical pathways for direct imaging of the functional magnetic performance of these systems with high spatial resolution and sensitivity., 22 pages, 7 figures

The concept of Perpendicular Shape Anisotropy (PSA) spin transfer torque (STT) MRAM has been recently proposed as a solution to achieve downsize scalability of MRAM below sub-10 nm technology nodes, down to 3–4 nm cell size lateral dimensions. In conventional p-STT-MRAM, at sub-20 nm diameters, the perpendicular anisotropy arising from the MgO/CoFeB interface becomes too weak to ensure sufficient stability of the storage layer magnetization. In addition, this interfacial anisotropy decreases rapidly with increasing temperature, resulting in a significant drawback for applications having to operate on a wide temperature range. Here, we combine both coercivity and electron holography measurements as function of temperature to show that in a PSA storage layer, the source of anisotropy is much more robust versus temperature compared to the interfacial anisotropy of conventional STT-MRAM stacks. This allows to considerably reduce the temperature dependence of coercivity. This property is quite beneficial for applications having to operate on an extended temperature range, such as automotive (−40°C to 150°C), or to fulfill solder reflow compliance requiring 1 minute retention at 260°C.

The concept of Perpendicular Shape Anisotropy STT-MRAM (PSA-STT-MRAM) has been recently proposed as a solution to enable the downsize scalability of STT-MRAM devices beyond the sub-20 nm technology node. For conventional p-STT-MRAM devices with sub-20 nm diameters, the perpendicular anisotropy arising from the MgO/CoFeB interface becomes too weak to ensure thermal stability of the storage layer. In addition, this interfacial anisotropy rapidly decreases with increasing temperature which constitutes a drawback in applications with a large range of operating temperatures. Here, we show that by using a PSA based storage layer, the source of anisotropy is much more robust against thermal fluctuations than the interfacial anisotropy, which allows considerable reduction of the temperature dependence of the coercivity. From a practical point of view, this is very interesting for applications having to operate on a wide range of temperatures (e.g. automotive -40 °C/+150 °C).

International audience; The fabrication of muti-gigabit magnetic random access memory (MRAM) chips requires the patterning of magnetic tunnel junctions at very small dimensions (sub-30 nm) and very dense pitch. This remains a challenge due to the difficulty in etching magnetic tunnel junction stacks. We earlier proposed a strategy to circumvent this problem by depositing the magnetic tunnel junction material on prepatterned metallic pillars, the junction being then naturally shaped during deposition. Once electrically contacted, the deposit on top of the pillars constitutes the magnetic storage element of the memory cell. However, in this process, magnetic material is also deposited in the trenches between the pillars that might affect the memory cell behaviour. Here we study the magnetic interactions between the deposit on top of the pillars and in the trenches by electron holography, at room temperature and up to 325 °C. Supplied by models, we show that the additional material in the trenches is not perturbing the working principle of the memory and even plays a role of flux absorber which reduces the crosstalk between neighboring dots. Besides, in the studied sample, the magnetization of the 1.4 nm thick storage layer of the dots is found to switch from out-of-plane to an in-plane configuration above 125 °C, while gradually decreasing with temperature. Electron holography is shown to constitute a very efficient tool for characterizing the micromagnetic configuration of the storage layer in MRAM cells.

International audience

Spintronic neurons that emit sharp voltage spikes are needed for neural-network hardware, for fast data processing with low power consumption. Conventional spintronic neurons, $i.e.$ spin-torque nano-oscillators (STNOs), emit sinusoidal waveforms, though. The authors suggest how to imitate neuron spiking in STNOs in which both magnetic layers are free and thin enough to be switched by current. Simulations show that the chaotic behavior of these frequency-tunable devices can be adjusted by the magnetic stack and current. The proposed spintronic relaxation oscillator is a promising building block for hardware neuromorphic chips, leveraging nonlinear dynamics for computing.

International audience

International audience; 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. The main building bloc of an MRAM is a trilayer FM1/I/FM2 (Fig. 1.a) named magnetic tunnel junction (MTJ) , 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 written by spin transfer torque (STT) , 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 kBT the thermal energy (Fig. 1.b). For practical applications, one aims for ∆ values from 60 to 100.

Spin-Transfer-Torque Magnetic Random Access Memories (STT-RAM) based on out-of-plane magnetized MTJ (pMTJ) are one of the most promising emerging non-volatile memory technologies as they combine a unique set of assets: quasi-infinite write endurance, high speed, low power consumption and scalability. Embedded STT-MRAM are about to enter in volume production for e-FLASH replacement. For this type of applications not requiring very high memory density, the preferred etching technique is still Ion Beam etching (IBE) [1]. However this technique is not appropriate for high density memory as it requires to etch the MTJ stack at specific angles to minimize the re-deposition on the sidewalls of the tunnel junctions. Etching at some angle leads to shadowing effects which give to the pillars a conical shape. This effect worsens as the memory pitch shrinks below typically 5F resulting in a poor control of the critical dimension at very dense pitch. Besides it is difficult to implement on large wafer with good uniformity [2]. Reactive ion etching (RIE) was also tried for MTJs with various gas but was found to be very complex due to the heterogeneous nature of the MTJ stacks and to cause corrosion of the magnetic materials [3]. Therefore, to be able to use STT-RAM as a dense working memory requires a new method for nanopatterning MTJ elements at small feature size $(< 20$ nm) and high pitch $( \sim 2\mathrm {F})$. In the approach we propose, the MTJ material is directly deposited on pre-patterned pillars (e.g Ta pillars prepared by RIE or Cu or W vias prepared by damascene process). The MTJ stack is then naturally patterned while being deposited thus not requiring any post-deposition etching. For the pre-patterned non-magnetic posts, we chose Ta as post-material since its reactive ion etching is very well controlled. In order to avoid the risk of electrical shorts between pillars due to the material deposited in the trenches between posts, the latter are given an undercut shape. Thanks to this shape, during the MTJ deposition, no metal gets deposited on the pillar sidewalls nor at the foot of the metallic posts. The process for fabricating the conducting non-magnetic Ta posts with undercut is depicted in Fig. 1. Ta is coated by Pt to form the top part of the post and protect the top Ta surface from oxidation. Then, following the formation of cylindrical Ta/Pt posts by an anisotropic RIE process, an isotropic RIE process is subsequently used in order to laterally trim the Ta part of the posts. Perpendicular MTJ stacks with MgO barrier were then deposited on these pre-patterned substrates. By depositing the MgO tunnel barrier at oblique incidence while rotating the substrate, it is possible to completely coat the MTJ bottom electrode with MgO and even to get thicker MgO deposit on the sidewall of the bottom electrode than on the horizontal part of the MTJ. This can help to concentrate the current away from the edge of the nano-patterned MTJ thus reducing the influence of possible edge defects. Similarly, lateral gradient of chemical composition can be induced in the storage or reference layers. Such gradient can be used to induce different properties at the edges and center of each dot in order for instance to reduce demagnetizing or nucleation effects at edges and improve STT switching efficiency. The magnetic properties of pMTJ stacks deposited on the pre-patterned substrate were evaluated by focused Kerr microscopy. Half-MTJ stacks were first deposited with bottom and top electrodes only to characterize the interfacial perpendicular anisotropy of the deposits on top of the posts CoFeB/MgO. The contribution of individual pillars and of the continuous deposit in the trenches could be distinguished. After optimizing the structural and magnetic properties, electrically characterization of these patterned MTJs were performed in terms of TMR and STT switching characteristics. The field-voltage switching phase diagram at room temperature was measured and is shown in Fig.2. At a constant applied field and even at zero field, the junctions can be switched from AP to P and from P to AP by STT. A switching voltage of 0.34 V for 100ns pulse was measured which is quite comparable or even slightly lower than similar MTJs patterned by IBE [4]. These electrical results demonstrate that functional patterned perpendicular MTJs can be obtained by this novel patterning process consisting in depositing the MTJ material on pre-patterned conducting posts. In conclusion, we have demonstrated a novel approach for nanopatterning MTJ down to sub-30nm dimensions at very narrow pitch by depositing the MTJ material on pre-patterned metallic posts. Remarkably, our approach allows to fabricate extremely dense arrays of very small size MTJs which is still impossible to achieve by IBE. This opens a possible route toward high density STT-MRAM application.

International audience; A new approach to increase the downsize scalability of perpendicular STT-MRAM is presented. It consists in significantly increasing the thickness of the storage layer in out-of-plane magnetized tunnel junctions (pMTJ) as compared to conventional pMTJ in order to induce a perpendicular shape anisotropy (PSA) in this layer. This PSA is obtained by depositing a thick ferromagnetic (FM) layer on top of an MgO/FeCoB based magnetic tunnel junction (MTJ) so that the thickness of the storage layer becomes of the order or larger than the diameter of the MTJ pillar. In contrast to conventional spin transfer torque magnetic random access memory (STT-MRAM) wherein the demagnetizing energy opposes the interfacial perpendicular magnetic anisotropy (iPMA), in these novel memory cells, both PSA and iPMA contributions favor out-of-plane orientation of the storage layer magnetization. Using thicker storage layers in these PSA-STT-MRAM has several advantages. Thanks to the PSA, very high and easily tunable thermal stability factors can be achieved, even down to sub-10 nm diameters. Moreover, low damping material can be used for the thick FM material thus leading to a reduction of the write current. The paper describes this new PSA-STT-MRAM concept, practical realization of such memory arrays, magnetic characterization demonstrating thermal stability factor above 200 for MTJs as small as 8nm in diameter and possibility to maintain thermal stability factor above 60 down to 4nm diameter.

International audience; STT-MRAM are attracting an increasing interest from microelectronics industry. They are about to enter in volume production with the first goal of replacing embedded Flash memory. To go towards high density STT-MRAM at sub-20nm nodes, two major issues have to be solved. One is the nanopatterning of the magnetic tunnel junctions at 1x feature size (F) and narrow pitch (pitch

A new concept to increase the downsize scalability of perpendicular spin transfer torque magnetic random-access memory (p-STT-MRAM), called perpendicular shape anisotropy (PSA) STT-MRAM is presented. This approach consists of significantly increasing the thickness of the storage layer in p-STT-MRAM to values comparable to the cell diameter so as to induce a PSA in this layer which comes on top of the MgO/FeCoB interfacial anisotropy. This PSA-STT-MRAM is provided by depositing a thick ferromagnetic (FM) layer on top of an MgO/FeCoB based magnetic tunnel junction (MTJ) so that the thickness of the storage layer becomes of the order or larger than the diameter of the MTJ pillar. In contrast to conventional spin transfer torque (STT) magnetic random access memory, wherein the demagnetizing energy opposes the interfacial perpendicular magnetic anisotropy (iPMA), in these novel memory cells, both PSA and iPMA contributions favor out-of-plane orientation of the storage layer magnetization. Using thicker storage layers in these PSA-STT-MRAM has several advantages. Thanks to this robust source of bulk anisotropy, PSA-STT-MRAM offers a greatly improved downsize scalability over conventional perpendicular p-STT-MRAM. Despite the large thickness of the storage layer, PSA-STT-MRAM cells can still be written by STT provided their thermal stability factor Δ is adjusted in the same range as in conventional p-STT-MRAM, i.e. Δ of the order of 60–100 depending on the memory capacity and required bit error rate. Moreover, a low damping material can be used for the thick FM material, thus leading to a reduction of the write current. Thanks to the PSA, very high and easily tunable thermal stability factors can be achieved, even down to sub-10 nm diameters. The paper describes this new PSA-STT-MRAM concept, practical realization of such memory arrays, magnetic characterization demonstrating thermal stability factor above 200 for MTJs as small as 8 nm in diameter and the possibility to maintain thermal stability factor above 60 down to 4 nm diameter. We also show that thanks to the increased thickness of the storage layer, the anisotropy and therefore the memory retention are much less sensitive on temperature than in conventional p-STT-MRAM. This is very interesting for applications operating on a wide range of temperatures (e.g. automotive −40 °C to +150 °C), as well as to fulfill solder reflow compliance.

Spin torque resonance has been used to simultaneously probe the dynamics of a magnetic domain wall and of magnetic domains in a nanostripe magnetic tunnel junction. Due to the large associated resistance variations, we are able to analyze quantitatively the resonant properties of these single nanoscale magnetic objects. In particular, we find that the magnetic damping of both the domains and the domain wall is doubled compared to the damping value of the host magnetic layer. We estimate the contributions to the damping arising from the dipolar couplings between the different layers in the junction and from the intralayer spin pumping effect, and find that they cannot explain the large damping enhancement that we observe. We conclude that the measured increased damping is intrinsic to large amplitudes excitations of spatially localized modes or solitons such as vibrating or propagating domain walls.

We present time-resolved measurements of the displacement of magnetic domain-walls (DWs) driven by vertical spin-polarized currents in track-shaped magnetic tunnel junctions. In these structures we observe very high DW velocities (600 m/s) at current densities below $10^7 A/cm^2$. We show that the efficient spin-transfer torque combined with a short propagation distance allows to avoid the Walker breakdown process, and achieve deterministic, reversible and fast ($\approx$ 1 ns) DW-mediated switching of magnetic tunnel junction elements, which is of great interest to the implementation of fast DW-based spintronic devices.

We study translational, breathing and twisting resonant modes of transverse magnetic domain walls pinned at notches in ferromagnetic nanostrips. We demonstrate that a mode's sensitivity to notches depends strongly on the characteristics of that particular resonance. For example, the frequencies of modes involving lateral motion of the wall are the ones which are most sensitive to changes in the notch intrusion depth (especially at the narrower, more strongly confined end of the domain wall). In contrast, the breathing mode, whose dynamics are concentrated away from the notches is relatively insensitive to changes in the notches' sizes. We also demonstrate a sharp drop in the translational mode's frequency towards zero when approaching depinning which is found, using a harmonic oscillator model, to be consistent with a reduction in the local slope of the notch-induced confining potential at its edge.