Synthetic Ferrimagnetic Free Layer Research Articles

Spin valves with Conetic based synthetic ferrimagnet free layer

The detection of low magnetic field requires hysteresis-free magnetoresistive sensor with high sensitivity. Giant magnetoresistance spin valve (SV) sensors with CoFeB based synthetic antiferromagnetic (SAF) pinned layer and Conetic based synthetic ferrimagnet (SF) free layer are developed in this paper. Low free layer coercivity (HC) of 0.27 Oe is achieved through adapting synthetic antiferromagnetic pinned layer in SV with Conetic single free layer. In SV with SAF pinned layer and SF free layer, the MR ratio, HC, switching field (HSW) and offset field (HO) increase with the thickness of the Conetic alloy layer right above the Cu spacer. The HC, HO, and HSW show oscillatory behavior with the Ru spacer thickness in the SF free layer. Through introducing CoFeB/SF free layer, an increased sensitivity of 0.27%/Oe and a small coercivity of 0.3 Oe are attained, which holds promise for low field sensing applications.

Numerical analysis of thermally assisted spin-transfer torque magnetization reversal in synthetic ferrimagnetic free layers

The spin transfer torque magnetization reversal of synthetic ferrimagnetic free layers under pulsed temperature rise was numerically studied by solving the Landau–Lifshitz–Gilbert equation, taking into account the stochastic random fields, the temperature dependence of magnetic parameters, and the spin torque terms. The anti-parallel magnetization configuration was retained at the elevated temperature, due to interlayer dipole coupling. A significant thermal assistance effect, resulting in a 40% reduction in the switching current, was demonstrated during a nanosecond pulsed temperature rise up to 77% of the Curie temperature.

Large-amplitude, narrow-linewidth microwave emission in a dual free-layer MgO spin-torque oscillator

Synchronized magnetization motion among the several magnetic layers composing a spin-torque oscillator (STO) is considered an effective way to improve spectral purity. To utilize this scheme in a MgO-based STO, we have fabricated a dual free-layer STO composed of a CoFeB free layer (FL), a MgO barrier layer, and a CoFe/Ru/CoFeB synthetic ferrimagnet free layer (SyF). Unlike conventional MgO-based STOs, this structure does not have an antiferromagnetic layer that pins the SyF, leading to a large-amplitude oscillation of magnetization in the SyF. The dual free-layer STO exhibits coherent microwave emissions with power spectrum density beyond 800 nW/GHz and narrow spectral linewidth below 5 MHz (Q-factor ≈ 2000). Macrospin simulations confirm that the stable oscillations originate from the synchronized magnetization motion of the FL and the SyF through dynamical dipolar coupling.

Magnetization dynamics of an in-plane magnetized synthetic ferrimagnetic free layer submitted to spin-transfer torques and applied field

An equilibrium stability analysis of an in-plane magnetized synthetic ferrimagnetic free layer (SyF) submitted to multiple spin-transfer-torque (STT) influences and to applied field has been carried out. Analytical expressions for the frequency and linewidth of the excitations were derived, in the case of strong coupling between the two layers compared to the anisotropy field. The expression of the critical current for the onset of excitations in the SyF was then calculated. Above the critical current, the destabilized mode, acoustic or optical, is found to depend on the applied field direction, but also on the asymmetry of the two layers composing the SyF. A strong thickness asymmetry reduces the critical current at zero field, and allows to select the destabilized mode. The critical current of an in-plane SyF with mutual spin torque between the layers and without reference layer was computed. For a symmetric SyF, the optical mode is the only mode that can be destabilized by STT.

Critical switching current and thermal stability of magnetic tunnel junctions with uncompensated CoFeB/Ru/CoFeB synthetic free layers

The critical switching current and thermal stability parameter are investigated for magnetic tunnel junctions with uncompensated synthetic ferrimagnetic free layers. The parameters are obtained by analyzing the experimental results for the thermally activated magnetization switching probability as functions of both a bias current and an applied magnetic field. The analysis is greatly facilitated by the use of an analytical equation for the applied magnetic field dependence of the energy barrier. A figure of merit given by the ratio of the two parameters differs substantially depending on the direction of the magnetization switching.

Dependence of spin-transfer switching characteristics in magnetic tunnel junctions with synthetic free layers on coupling strength

Dependence of spin-transfer switching characteristics on the interlayer exchange coupling strength in the MTJs with synthetic free layers of Co90Fe10/Ru/Co40Fe40B20 with strong coupling strength was investigated. In the MTJ with the relatively weakly coupled synthetic ferrimagnetic free layer, larger thermal stability (Δ0) and lower intrinsic critical current density (Jc0) than those of the MTJ with the single free layer were observed. Meanwhile, in the MTJs with the strongly coupled synthetic ferri- or ferromagnetic free layers, very large Δ0 and high Jc0 were observed probably due to high effective magnetic energy barrier. It was found that the MTJ with the relatively weakly coupled synthetic ferrimagnetic free layer is suitable for the STTRAM application.

Spin transfer torque in MTJs with synthetic ferrimagnetic layers by the Keldysh approach

Based on the Keldysh Green’s function method within the tight-binding model, the spin transfer torque is analyzed in a magnetic tunnel junction with a synthetic ferrimagnetic (SyF) free layer in the ballistic regime. The spin transfer torque exerted on the magnetizations of ferromagnetic bilayers in the SyF free layer tends to rotate the magnetizations in the same direction as a combined motion of the SyF free layer regardless of the bias direction. This rotation causes a reduction of the critical current for the magnetization switching of the SyF free layer which is consistent with experimental observations.

Fabrication of Magnetic Tunnel Junctions with a Synthetic Ferrimagnetic Free Layer for Magnetic Field Sensor Applications

Magnetic tunnel junctions (MTJs) with a CoFeB/Ru/Ni80Fe20 synthetic ferrimagnetic free layer and an MgO barrier layer were fabricated. The effect of the shape and thickness of the free layer on the magnetic field sensor characteristics was systematically investigated. We achieved a high sensitivity of 4.8%/Oe in the MTJ with a 70-nm-thick Ni80Fe20 layer and an aspect ratio of 1.0. Here, sensitivity is defined as TMR/(2Hk), where TMR is tunnel magnetoresistance ratio in the MTJ and Hk is a magnetic anisotropy field of the free layer. Furthermore, we successfully increased the detection field range up to 230 Oe while keeping high sensitivity and linearity.

Fabrication of Magnetic Tunnel Junctions with a Synthetic Ferrimagnetic Free Layer for Magnetic Field Sensor Applications

Fabrication of Magnetic Tunnel Junctions with a Synthetic Ferrimagnetic Free Layer for Magnetic Field Sensor Applications

Effect of Interlayer Coupling of Synthetic Ferrimagnetic Free Layer on Current Induced Magnetization Switching in MTJs

In order to study the effect of the interlayer exchange coupling of synthetic ferrimagnetic (SyF)-free layer on the current induced magnetization switching (CIMS) in magnetic tunnel junctions (MTJs), in the thermal activation regime, we examine the magnetization dynamics of MTJs with the SyF-free layer based on a macrospin model, combining with the STT analysis. Compared with the MTJ with a single layer, the reduction of the critical current (Jc) is observed in the MTJ with the SyF-free layer. The considerable reduction of Jc is also observed in MTJs with the ferromagnetically coupled-free layer over that with the antiferromagnetically coupled free layer. In connection with CIMS in each SyF-free layer of the MTJ, we found two different types of CIMS dependent on Jex, that is, decoupled CIMS and coupled CIMS.

Magnetic dead layer in amorphous CoFeB layers with various top and bottom structures

The magnetic dead layer (MDL) in amorphous CoFeB layers is investigated for four different unit structures. These structures are relevant to the synthetic ferrimagnetic (SyF) free layer structure in magnetic tunnel junctions used for high density magnetic random access memory (MRAM). The MDL results for these unit structures are then converted to those for the constituent interfaces of the SyF free layer structure. These MDL results are critically tested by fabricating the synthetic ferrimagnetic free layer structures with various thickness asymmetries. The observed switching properties of these tested structures are in good agreement with those expected from the effective thicknesses after the MDL correction, confirming the accuracy of the present results for the MDLs at the constituent interfaces.

Angular dependence of spin transfer torque on magnetic tunnel junctions with synthetic ferrimagnetic free layer

Based on a spin-polarized free-electron model, spin and charge transports are analyzed in magnetic tunnel junctions with synthetic ferrimagnetic layers in the ballistic regime, and the spin transfer torque is derived. We characterize the synthetic ferrimagnetic free layer by extending an arbitrary direction of magnetizations of the two free layers forming the synthetic ferrimagnetic free layer. The synthetic ferrimagnetic configuration exerts the approximately optimum torque for small magnetization angle of the first layer relative to that of the pinned layer. For approximately anti-parallel magnetization of the first layer to that of the pinned layer, the parallel magnetization of two magnetic layers is favorable for magnetization reversal rather than the synthetic ferrimagnetic configuration.

Spin-transfer switching in magnetic tunnel junctions with synthetic ferri-magnetic free layer

We have fabricated the SyF structure with both high annealing stability and strong interlayer exchange coupling and investigated tunnelling magnetoresistance (TMR) and spin-transfer switching properties of magnetic tunnel junctions (MTJs) with developed SyF free layer. The fabricated SyF with structure of Ta/Ru/CoFe/Ru/CoFeB possessed high annealing stability of 400°C and strong interlayer exchange coupling. Consequently, a large TMR ratio of 122% has been observed after annealing at high temperature of 350°C. In addition, we have successfully observed spin-transfer switching by the net current density of 14 MA/cm2 and the large thermal stability factor of 62.

Coresonant Enhancement of Spin-Torque Critical Currents in Spin Valves with a Synthetic-Ferrimagnet Free Layer

It is experimentally shown that the critical current for onset of spin torque instability in current-perpendicular-to-plane spin valves can be strongly increased using "synthetic-ferrimagnet" free layers of form FM1/Ru/FM2 (FM=ferrromagnet). However, this increase occurs for only one polarity of bias current. A two-macrospin model is shown to reproduce the observations. The model suggests that this phenomenon is related to a polarity-dependent, spin torque-induced coresonance between the two natural dynamic modes of the FM1/FM2 couple. The coresonance condition facilitates energy transfer out of the spin-torque destabilized mode into the other stable mode whose effective damping (or line width) is strongly enhanced by spin torques.

High-output current-perpendicular to the plane giant magnetoresistance sensor with synthetic-ferrimagnet free layer and enhanced spin-torque critical currents

It is shown that the maximum stable output of a current-perpendicular to the plane giant magnetoresistance sensor is increased significantly by using a synthetic-ferrimagnet free layer, provided the electron current flows from free layer to reference layer. This free layer allows a larger magnetoresistance ratio for a given free layer magnetic moment, and in addition results in a greater than threefold increase in the critical current, above which spin-torque instability of the free layer occurs. In read heads with net free layer moments equivalent to only 4.5 nm of Ni80Fe20, this effect is shown to result in sustainable sense current densities above 2×108 A/cm2.

Evaluation of Characteristic Parameters of MgO Barrier Based Magnetic Tunnel Junctions with CoFeB/Ru/CoFeB Synthetic Ferrimagnetic Free Layers

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Current-Induced Magnetization Switching in MgO Barrier Magnetic Tunnel Junctions With CoFeB-Based Synthetic Ferrimagnetic Free Layers

We investigated the effect of using a synthetic ferrimagnetic (SyF) free layer in MgO-based magnetic tunnel junctions (MTJs) on current-induced magnetization switching (CIMS), particularly for application to spin-transfer torque random access memory (SPRAM). The employed SyF free layer had a Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">40</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">40</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> /Ru/Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">40</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">40</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> and Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> /Ru/Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> structures, and the MTJs (100 times (150-300) nm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) were annealed at 300 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">deg</sup> C. The use of SyF free layer resulted in low intrinsic critical current density ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c0</sub> ) without degrading the thermal-stability factor ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> , where <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i> , <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> , and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> are the energy potential, the Boltzmann constant, and temperature, respectively). When the two CoFeB layers of a strongly antiferromagnetically coupled SyF free layer had the same thickness, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c0</sub> was reduced to 2-4 times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . This low <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c0</sub> may be due to the decreased effective volume under the large spin accumulation at the CoFeB/Ru. The <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> was over 60, resulting in a retention time of over ten years and suppression of the write current dispersion for SPRAM. The use of the SyF free layer also resulted in a bistable (parallel/antiparallel) magnetization configuration at zero field, enabling the realization of CIMS without the need to apply external fields to compensate for the offset field.

Micromagnetic Simulation of Spin Valves with Synthetic Free and Pinned Layers and Detection of Magnetic Labels with AC Field

We present a micromagnetic simulation for detection of magnetic labels modulated by an ac magnetic field. As the sensing element, spin valves with synthetic ferrimagnetic (SF) free and pinned layers are optimized. At lower fields, the sensors with SF free layers show linear transfer curves, demonstrating a coherent switching due to the strong antiferromagnetic coupling inside the SF free layers. The sensor offset field can be reduced by tuning the effective thickness of SF free and pinned layers. Magnetic simulation for the detection of 250-nm magnetic labels with this sensor shows a linear giant magnetoresistive signal dependence on the number of particles.

Magnetic Tunnel Junctions for Spintronic Memories and Beyond

In this paper, recent developments in magnetic tunnel junctions (MTJs) are reported with their potential impacts on integrated circuits. MTJs consist of two metal ferromagnets separated by a thin insulator and exhibit two resistances, low (Rp) or high (Rap) depending on the relative direction of ferromagnet magnetizations, parallel (P) or antiparallel (AP), respectively. Tunnel magnetoresistance (TMR) ratios, defined as (Rap $Rp)/Rp as high as 361%, have been obtained in MTJs with Co40Fe40B20 fixed and free layers made by sputtering with an industry-standard exchange-bias structure and post deposition annealing at Ta = 400 degC. The corresponding output voltage swing DeltaV is over 500 mV, which is five times greater than that of the conventional amorphous Al-O-barrier MTJs. The highest TMR ratio obtained so far is 500% in a pseudospin-valve MTJ annealed at Ta = 475 degC, showing a high potential of the current material system. In addition to this high-output voltage swing, current-induced magnetization switching (CIMS) takes place at the critical current densities (JCO) on the order of 106 A/cm2 in these MgO-barrier MTJs. Furthermore, high antiferromagnetic coupling between the two CoFeB layers in a synthetic ferrimagnetic free layer has been shown to result in a high thermal-stability factor with a reduced JCO compared to single free-layer MTJs. The high TMR ratio enabled by the MgO-barrier MTJs, together with the demonstration of CIMS at a low JCO, allows development of not only scalable magnetoresistive random-access memory with feature sizes below 90 nm but also new memory-in-logic CMOS circuits that can overcome a number of bottlenecks in the current integrated-circuit architecture

Reduction of switching fields of submicrometer sized magnetic tunnel junction with NiFe-based synthetic ferrimagnetic free layer

We fabricated submicrometer sized magnetic tunnel junctions (MTJs) with soft NiFe-based synthetic ferrimagnet (SynF) free layers. An extremely low switching field of 13Oe was achieved when the stacking structure of the free layer was NiFe(2 nm)/CoFe(0.2 nm)/Ru(0.4 nm)/CoFe(0.2nm)/NiFe(2.1 nm) with 0.4μm cell. The switching field of the SynF structure was almost independent of the cell width. Small magnetic anisotropy of NiFe and enhanced antiferromagnetic coupling strength from insertion of 0.2-nm-thick CoFe were important attributes for the low switching field in the submicrometer sized MTJs.