C. Chappert

Nanoscale Magnetic Domains in Mesoscopic Magnets

The basic magnetic properties of three-dimensional nanostructured materials can be drastically different from those of a continuous film. High-resolution magnetic force microscopy studies of magnetic submicrometer-sized cobalt dots with geometrical dimensions comparable to the width of magnetic domains reveal a variety of intricate domain patterns controlled by the details of the dot geometry. By changing the thickness of the dots, the width of the geometrically constrained magnetic domains can be tuned. Concentric rings and spirals with vortex configurations have been stabilized, with particular incidence in the magnetization reversal process as observed in the ensemble-averaged hysteresis loops.

High Speed, High Stability and Low Power Sensing Amplifier for MTJ/CMOS Hybrid Logic Circuits

Densely embedding Magnetic Tunnel Junctions (MTJ) in CMOS logic circuits is considered as one potentially powerful solution to bring non volatility, instant on/off and low standby power in todays programmable logic circuits, in order to overcome major drawbacks while preserving high operation speed. A critical issue in this process is the integration of MTJ electric signal to CMOS electronics, in particular the requirement of ldquozerordquo read/write error for logic applications. In this paper, we propose a new sense amplifier circuit, called Pre-Charge Sense Amplifier (PCSA). This circuit, comprising 7 CMOS transistors at minimum size, is able to read the magnetic configuration of a pair of magnetic tunnel junctions with opposite configurations at high speed (about 200 ps), with very low power and error rate compared to previously proposed solutions. Simulations using a ST Microelectronics 90 nm design kit and a compact model of MTJ demonstrate the performances of PCSA.

Compact Modeling of Perpendicular-Anisotropy CoFeB/MgO Magnetic Tunnel Junctions

Magnetic tunnel junctions (MTJs) composed of ferromagnetic layers with perpendicular magnetic anisotropy (PMA) are of great interest for achieving high-density nonvolatile memory and logic chips owing to its scalability potential together with high thermal stability. Recent progress has demonstrated a capacity for high-speed performance and low power consumption through current-induced magnetization switching. In this paper, we present a compact model of the CoFeB/MgO PMA MTJ, a system exhibiting the best tunnel magnetoresistance ratio and switching performance. It integrates the physical models of static, dynamic, and stochastic behaviors; many experimental parameters are directly included to improve the agreement of simulation with experimental measurements. Mixed simulation based on the 65-nm technology node of a magnetic flip-flop validates its relevance and efficiency for MTJ/CMOS memory and logic chip design.

Current-driven vortex oscillations in metallic nanocontacts.

We present experimental evidence of subgigahertz spin-transfer oscillations in metallic nanocontacts that are due to the translational motion of a magnetic vortex. The vortex is shown to execute large-amplitude orbital motion outside the contact region. Good agreement with analytical theory and micromagnetics simulations is found.

Strain-controlled magnetic domain wall propagation in hybrid piezoelectric/ferromagnetic structures

The control of magnetic order in nanoscale devices underpins many proposals for integrating spintronics concepts into conventional electronics. A key challenge lies in finding an energy-efficient means of control, as power dissipation remains an important factor limiting future miniaturization of integrated circuits. One promising approach involves magnetoelectric coupling in magnetostrictive/piezoelectric systems, where induced strains can bear directly on the magnetic anisotropy. While such processes have been demonstrated in several multiferroic heterostructures, the incorporation of such complex materials into practical geometries has been lacking. Here we demonstrate the possibility of generating sizeable anisotropy changes, through induced strains driven by applied electric fields, in hybrid piezoelectric/spin-valve nanowires. By combining magneto-optical Kerr effect and magnetoresistance measurements, we show that domain wall propagation fields can be doubled under locally applied strains. These results highlight the prospect of constructing low-power domain wall gates for magnetic logic devices.

Chemical order induced by ion irradiation in FePt (001) films

We demonstrate that the long-range order parameter S of sputtered FePt (001) films may be improved by using postgrowth He ion irradiation. This was demonstrated both on disordered (S∼0) and partially ordered (S∼0.4) films in which S was increased up to 0.3 and 0.6, respectively. X-ray diffraction analysis showed that these changes are due to irradiation-induced chemical ordering. The changes in the magnetic hysteresis loops correlate with the expected perpendicular magnetic anisotropy increase. This method may find applications in ultrahigh-density magnetic recording.

Spin wave contributions to the high-frequency magnetic response of thin films obtained with inductive methods

The high-frequency magnetic response of Permalloy thin films have been measured using network-analyzer ferromagnetic resonance. We demonstrate that the excitation of spin waves by the coplanar wave-guide modify the magnetic response appreciably, in particular, by causing a frequency shift and broadening of the resonance peak. An analytic theory is presented to account for the experimental observations and provides a quantitative tool to accurately determine the Gilbert damping constant.

Sub-50 nm planar magnetic nanostructures fabricated by ion irradiation

He+ ion irradiation of Co–Pt multilayers through a silica mask obtained by a combination of high resolution lithography and reactive ion etching can produce an optical contrast-free, entirely planar, sub-50 nm magnetically patterned array. Furthermore, the specificity of magnetization reversal in such arrays leads to a weak dispersion of coercive forces. The technique holds promise for both present hard disk technology and future near field magneto-optical recording.

Spin transfer torque (STT)-MRAM--based runtime reconfiguration FPGA circuit

As the minimum fabrication technology of CMOS transistor shrink down to 90nm or below, the high standby power has become one of the major critical issues for the SRAM-based FPGA circuit due to the increasing leakage currents in the configuration memory. The integration of MRAM in FPGA instead of SRAM is one of the most promising solutions to overcome this issue, because its nonvolatility and high write/read speed allow to power down completely the logic blocks in “idle” states in the FPGA circuit. MRAM-based FPGA promises as well as some advanced reconfiguration methods such as runtime reconfiguration and multicontext configuration. However, the conventional MRAM technology based on field-induced magnetic switching (FIMS) writing approach consumes very high power, large circuit surface and produces high disturbance between memory cells. These drawbacks prevent FIMS-MRAMs further development in memory and logic circuit. Spin transfer torque (STT)-based MRAM is then evaluated to address these issues, some design techniques and novel computing architecture for FPGA logic circuits based on STT-MRAM technology are presented in this article. By using STMicroelectronics CMOS 90nm technology and a STT-MTJ spice model, some chip characteristic results as the programming latency and power have been calculated and simulated to demonstrate the expected performance of STT-MRAM based FPGA logic circuits.

Study of the dynamic magnetic properties of soft CoFeB films

We study the magnetization damping in ion-beam deposited Co72Fe18B10 thin films as a function of film thickness and crystalline state. As-deposited amorphous layers showed low damping (αapp=0.006) that is thickness independent. 40nm Co80Fe20 with no boron content exhibited a value twice higher (αapp=0.013). Crystallization in Co72Fe18B10, triggered by annealing at 280°C, results in increased magnetization as well as a strong increase in damping, by a factor of 5 for 40nm films. For lower thicknesses the damping increase upon annealing is less pronounced. The exchange stiffness constant for amorphous films is deduced from perpendicular standing spin waves to be 28.4×10−12J∕m. The annealing dependence of damping should have consequences for the spin-transfer switching in CoFeB∕MgO∕CoFeB magnetic tunnel junctions.