Synthetic Antiferromagnetic Structure Research Articles

Tuning exchange coupling by replacing CoFe with amorphous CoFeB in the CoFe/Ru/CoFe synthetic antiferromagnetic structure

We studied exchange coupling in the CoFe/Ru/CoFe synthetic antiferromagnetic structure with systematical replacement of the crystalline CoFe with amorphous CoFeB. Antiferromagnetic exchange coupling intensity decreases with an increase in the replacement in the bottom magnetic layer, which indicates that exchange coupling intensity could be tuned by the replacement. The origin of weakening antiferromagnetic exchange coupling is attributed to the amorphous CoFeB replacement inducing incomplete crystallization and disordered orientation in the Ru layer.

High Level Oscillations With Narrow Linewidth in Magnetic Nano-Contact Spin Torque Oscillator With Synthetic AF Spin-Valve Structure

We investigated the current induced magnetization dynamics, so called spin torque oscillation, in magnetic nano-contact MR element with a synthetic antiferromagnetic type spin-valve structure under high in-plane applied magnetic field of 0.9-1 kOe. Very high level oscillation of 18 nVHz-1/2 with narrow FWHM of 12 MHz was observed in the condition of negative applied current where electron-spin is injected from free layer to reference one. Applied current dependency on an oscillation frequency is a blueshift with good linearlity, and same dependency on level and FWHM is almost constant in the range of -10 to -14 mA, while applied field dependency on frequency shows redshift with two different slopes, -2.5 MHz/Oe in less than 1 kOe and -8.7 MHz/Oe in more than 1 kOe which field is smaller than the spin-flop field for the synthetic antiferromagnet, besides very clear and reasonable peak and bottom for oscillation level and FWHM are observed, respectively. It is thought that these magnetization dynamics are originated from synthetic antiferromagnet.

Annealing-Induced Solid-Phase Epitaxy at Interfaces between CoFeB Layers and MgO Barrier-Layer in CoFeB/MgO/CoFeB Magnetic Tunnel Junctions and Their Magnetic Properties

The present paper describes influences of annealing conditions and chemical composition on grain-to-grain epitaxy, which is induced by annealing after sputter-deposition, in MgO based magnetic tunnel junctions (MTJs) with CoFeB ferromagnetic layers. Annealing-induced crystallization in the amorphous CoFeB layer (at as-deposited state) at the interface with the MgO layer in CoFeB/MgO/CoFeB MTJs is investigated by electron microscopy. Annealing conditions such as temperature and time as well as the chemical composition in the CoFeB layers strongly influence magnetoresistance (MR) ratio. Excess annealing (temperature and time) causes the break-down of synthetic antiferromagnet (SAF) structure in MTJs for practical applications, resulting in the decrease of MR ratio. Fe-rich CoFeB is more suitable to obtain better grain-to-grain epitaxy and higher MR ratio in view of lattice mismatch with the MgO layer. The annealing conditions and chemical composition are optimized to exhibit the highest MR ratio with keeping SAF structure.

Evaluation of distribution of exchange coupling in CoFe/Ru/CoFe synthetic antiferromagnetic structure after annealing

We investigated the exchange-coupling fields of CoFe/Ru (tRu)/CoFe synthetic antiferromagnetic (SyAF) structures with a Ru underlayer and capping layer before and after annealing at 300 and 350 °C. Exchange-coupling field Hex decreased after annealing and distribution increases in Hex were observed in the structure of smaller tRu, especially at tRu≤0.7 nm. Remanence magnetizations increased at smaller tRu. These results indicate that distribution is caused by the existence of the antiferromagnetic coupling of locally various coupling fields due to annealing. A larger exchange-coupling field after high temperature annealing can be obtained by suppressing the distribution of the exchange coupling in the SyAF structure.

Magneto-Transport Characteristics of Magnetic Tunnel Junction With a Synthetic Antiferromagnetic Amorphous CoFeSiB Free Layer

A synthetic antiferromagnet (SAF) structure comprising two ferromagnetic amorphous CoFeSiB layers was employed as a free layer in magnetic tunnel junctions (MTJs) to enhance magnetotransport and magnetization switching performance. In Si-SiO2/Ta 45/Ru 9.5/IrMn 10/CoFe 7/AlOx1.5/CoFeSiB t (t=3.5 , 4.0, 4.5, 5.0)/Ru 1.0/CoFeSiB 7-t/Ru 60 (nm) MTJ structures, the tunneling magnetoresistance (TMR) ratio, interlayer coupling field, and switching field all showed layer thickness dependence for 3.5nnm les t les 5nnm . When the CoFeSiB t layer (a lower ferromagnetic layer in the SAF structure) became thinner, a lower TMR ratio with a lower switching field was observed. Whereas, when the CoFeSiB t layer became thicker, a higher junction resistance with a lower interlayer coupling field was observed. This was resulted from the decrement of saturation magnetization and the smooth tunnel barrier/free-layer interface formation, respectively.

Electrically Tunable Thin Film Magnetic Core Using Synthetic Antiferromagnet Structure

We demonstrate an electrically tunable thin-film magnetic core using a synthetic antiferromagnet (SAF) structure. It consists of two ferromagnetic layers antiferromagnetically coupled together through an exchange coupling layer. Variation of susceptibility is achieved by leading a current through the structure. A tuning range (relative change in susceptibility) of 35% can be achieved at 800 Hz by using patterned NiFe(12 nm)/Ru(0.7 nm)/NiFe(12 nm) SAF structures. Different patterns of test samples are designed to investigate the influence on the tuning performance.

A Planar Magnetic Content Addressable Memory Cell

In this paper, we present a novel magnetic content addressable memory (MCAM) cell design. The proposed MCAM cell consists of a magnetic tunneling junction (MTJ), where both magnets are programmable. The synthetic antiferromagnetic (SAF) structure is used for the top magnet of MTJ, which can be programmed in toggle mode by externally applied field. The bottom MTJ magnet is programmed by the magnetic fringe field from a programming line underneath the bottom magnet. The domain wall motion in the programming line is driven by the spin-polarized current. The feasibility of the proposed operation has been demonstrated by numerical micromagnetic simulation. The simulation results show a 2times margins on both top and bottom magnets programming field/current in the proposed planar MCAM cell design.

Measurement of the critical curve of a synthetic antiferromagnet

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.

CoFeSiB/Ru/CoFeSiB 자유층을 갖는 자기터널 접합의 스위칭 자기장

비정질 <TEX>$Co_{70.5}Fe_{4.5}Si_{15}B_{10}$</TEX>층을 갖는 자기터널접합(magnetic tunneling junctions; MTJ)를 연구하였다. 비정질 자유층이 MTJ의 스위칭 특성에 미치는 영향을 중점적으로 이해하기 위하여 기존의 사용된 CoFe 그리고 NiFe층들을 대신하여 비정질 강자성체 CoFeSiB을 사용하였다. CoFeSiB은 CoFe과 NiFe보다 각각 낮은 포화자기장(<TEX>$M_s:\;560\;emu/cm^3$</TEX>)과 높은 자기이방성 상수(<TEX>$K_u:\;0.2800\;erg/cm^3$</TEX>)를 갖는다. CoFeSiB층들의 사이에 1.0 nm Ru층 삽입시 <TEX>$-0.003\;erg/cm^3$</TEX> 교환결합에너지(<TEX>$J_{ex}$</TEX>)를 나타내었다. <TEX>$Si-SiO_2-Ta$</TEX> 45/Ru 9.5/IrMn 10/CoFe 7/<TEX>$AlO_x$</TEX>/CoFeSiB 7 또는 CoFeSiB (t)/Ru 1.0/CoFeSiB (7-t)/Ru 60(in nm) MTJ 구조의 터널접합에 대하여 실험 및 시뮬레이션 결과를 통하여 낮은 <TEX>$J_{ex}$</TEX>에 기인하는 스위칭 자기장(switching field; <TEX>$H_{sw}$</TEX>)의 시료 크기 의존성이 나타나는 것을 알 수 있었다. CoFeSiB 합성형 반강자성 구조는 micrometer뿐만 아니라 submicrometer 시료 크기영역에서도 보자력(<TEX>$H_c$</TEX>)의 감소와 민감도를 증가 시킴으로써 자기 스위칭 특성에 유리한 것으로 확인 되었다. Magnetic tunnel junctions (MTJs), which consisted of amorphous CoFeSiB layers, were investigated. The CoFeSiB layers were used to substitute for the traditionally used CoFe and/or NiFe layers with an emphasis given on understanding the effect of the amorphous free layer on the switching characteristics of the MTJs. CoFeSiB has a lower saturation magnetization (<TEX>$M_s\;:\;560\;emu/cm^3$</TEX>) and a higher anisotropy constant (<TEX>$K_u\;:\;2800\;erg/cm^3$</TEX>) than CoFe and NiFe, respectively. An exchange coupling energy (<TEX>$J_{ex}$</TEX>) of <TEX>$-0.003\;erg/cm^2$</TEX> was observed by inserting a 1.0 nm Ru layer in between CoFeSiB layers. In the Si/<TEX>$SiO_2$</TEX>/Ta 45/Ru 9.5/IrMn 10/CoFe 7/<TEX>$AlO_x$</TEX>/CoFeSiB 7 or CoFeSiB (t)/Ru 1.0/CoFeSiB (7-t)/Ru 60 (in nm) MTJs structure, it was found that the size dependence of the switching field originated in the lower <TEX>$J_{ex}$</TEX> using the experimental and simulation results. The CoFeSiB synthetic antiferromagnet structures were proved to be beneficial for the switching characteristics such as reducing the coercivity (<TEX>$H_c$</TEX>) and increasing the sensitivity in micrometer size, even in submicrometer sized elements.

Study of Spin Valves With $L 1_{0}$-FePt Pinning Layer and Different Pinned Layers

L10 FePt pinned spin valves (SVs) with different pinning/pinned layers: FePt/CoFe, FePt/Ru/CoFe, and FePt/CoFe/Ru/CoFe, were fabricated and investigated. Samples with rigidly ferromagnetic coupled bilayer of FePt (6.0 nm)/CoFe (4.0 nm) show giant magnetoresistance (GMR) ratio up to 7.06%, which is similar to that in the conventional MnIr-based SVs. Whereas the free layer coercivity (H cf) of CoFe (3.0 nm)/NiFe (4.0 nm) is as high as 45 Oe. This is mainly due to the exchange coupling between the free layer and (111) textured FePt layer. Synthetic antiferromagnetic structures, FePt/Ru/CoFe and FePt/CoFe/Ru/CoFe, were proved to be very effective in reducing the Hcf value by forming a closed flux pathway in the bottom electrode. Compared with FePt/Ru/CoFe SVs, the FePt/CoFe/Ru/CoFe ones show higher pinned layer switching field. The optimized SV structure in this study is glass/FePt (6.0)/CoFe (2.0)/Ru (0.8)/CoFe (2.5)/Cu (2.4)/CoFe (3.0)/NiFe (4.0)/Pt (2.0) (in unit of nm), which exhibits a GMR ratio of 7.04%, a free layer Hcf of 22 Oe, and a pinned layer switching field of 1824 Oe

The reversible susceptibility tensor of synthetic antiferromagnets

In this work we propose to study the reversible susceptibility tensor (including the transverse susceptibility components) of a synthetic antiferromagnetic (SAF) structure. This study is motivated by the fact that knowing the magnetic anisotropy of SAF structure is essential for the device functioning and the transverse susceptibility is known as one of the best methods to determine magnetic anisotropy and switching properties of magnetic systems. The starting point of this work is the magnetization’s equation of motion Landau-Lifshitz-Gilbert for the coupled two layers. By solving the inhomogeneous system of linear equations on the magnetization’s spherical coordinate deviations, the susceptibility tensor of the SAF structure is obtained. Simplified equations for the very low frequency case are given. By comparing the plots of the susceptibility versus the applied field with the regular hysteresis loops one concludes that susceptibility experiments are more appropriate for switching investigations of the SAF systems.

Reversible susceptibility studies of magnetization switching in FeCoB synthetic antiferromagnets

In this paper we present a study of switching characteristics of a series of synthetic antiferromagnet (SAF) structures using reversible susceptibility experiments. Three series of SAF samples were considered in our study with (t1, t2), the thickness of the FeCoB layers of (80nm, 80nm), (50nm, 50nm), and (80nm, 20nm) and with the interlayer of Ru ranging from 0to2nm. 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.

Current-induced dynamics in asymmetric spin valves

Spin transfer in an asymmetric nanopillar spin valve is studied numerically in the diffusive transport regime. The system considered includes a pinned synthetic antiferromagnetic structure, which is separated from the sensing layer by a thin nonmagnetic film. Landau-Lifshitz-Gilbert equation, with the spin-transfer torque taken into account, is used to study current-induced magnetic dynamics in the macrospin model. It is shown that steady dynamical states can occur in the absence of magnetic field. The advantage of systems including a synthetic antiferromagnetic structure, compared to simple spin valves, is also discussed.

Spin Flop Switching of the Guided Synthetic Antiferromagnet MRAM

A magnetoresistance random access memory (MRAM) with a guided synthetic antiferromagnet (SAF) and direct writing scheme is presented to reduce the writing time. The MRAM cell includes not only the balanced SAF trilayers but also one more guiding layer, which is weakly exchange-coupled with one of the SAF layers. The two orthogonally oriented digital and word lines are aligned at 45deg with respect to the easy axis of the cell. With the exchange-coupling field from the guiding layer, the MRAM cell can be written directly by the combined field induced by current of the digital and word line. However, the cell cannot be switched by any single field induced by either current due to the balanced SAF structure before saturation. The switching field has been reduced and the writing operation margin has been improved with the use of the exchange-coupling field. Both simulation and the experiments have been carried out to verify this scheme. The write operation margin-H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">disturb</sub> /H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sw</sub> of more than 4.3 has been achieved for the magnetic tunnel junction MRAM cell

Thermal robustness in synthetic antiferromagnetic free layer for magnetic random access memory applications

We have investigated the magnetic switching properties of synthetic antiferromagnetic (SyAF) structures consisting of Co90Fe10∕Ru∕Co90Re10 structure with a Ru capping layer annealed at 300°C. After annealing, the coercivity Hc of the structures with a Co90Fe10 thickness below 5nm increased. On the other hand, the Hc was maintained below 70Oe in structures with a Co90Fe10 thickness above 6nm after annealing. Thus, a low switching field for a free layer can be achieved by using the SyAF structures with a Ru capping layer, even after annealing at 300°C, and the SyAF free layer is considered to be effective for downsized memory cells in magnetic random access memories.

Study of synthetic ferrimagnet-synthetic antiferromagnet structures for magnetic sensor application

There has been a growing interest in using both synthetic ferrimagnet (SF) free and synthetic antiferromagnet (SAF) pinned layers for head and memory applications. In particular, for linear sensor applications, these structures lower the magnetostatic fields present at the free layer through the reduction of its effective thickness (teffSF). This allows higher sensitivity but at the expense of an increased offset field H0(Néel coupling fieldHf+interlayer demagnetizing fieldHdSAF). In this work, results on a series of patterned 3×1 and 6×2μm2 top-pinned SF-SAF spin valves are analyzed and compared with a three-dimensional micromagnetic simulation in order to clarify the role of the different ferromagnetic layers in the overall offset field and sensitivity. H0 varies as 1∕teffSF[teffSF=(Mata−Mbtb)∕MeffSF]. The magnetostatic field acting on the SF coming from the SAF (HdSAF) can act as a biasing field, partially counterbalancing the Néel coupling field (Hf) leading to a reduction of H0. In this work the offset field was reduced from an initial value of 25 Oe in a quasicompensated SAF to a value of −6Oe, by unbalancing the SAF and consequently increasing its effective moment (teffSF=15Å).

Magnetoresistance and magnetization switching characteristics of magnetic tunnel junctions with amorphous CoFeSiB single and synthetic antiferromagnet free layers

To obtain low switching field (Hsw) we introduced amorphous ferromagnetic Co70.5Fe4.5Si15B10 single and synthetic antiferromagnet (SAF) free layers in magnetic tunnel junctions (MTJs). The switching characteristics for MTJs with structures Si∕SiO2∕Ta 45/Ru 9.5/IrMn 10/CoFe 7∕AlOx∕CoFeSiB 7 or CoFeSiB (t)∕Ru 1.0/CoFeSiB (7−t)∕Ru 60 (in nanometer) were investigated and compared to MTJs with Co75Fe25 and Ni80Fe20 free layers. CoFeSiB showed a lower saturation magnetization of 560emu∕cm3 and a higher anisotropy constant of 2800erg∕cm3 than CoFe and NiFe, respectively. An exchange coupling energy (Jex) of −0.003erg∕cm2 was observed by inserting a 1.0nm Ru layer in between CoFeSiB layers. In the CoFeSiB single and SAF free layer MTJs, it was found that the size dependence of the Hsw originated from the lower Jex experimentally and by micromagnetic simulation based on the Landau-Lisfschitz-Gilbert equation. The CoFeSiB SAF structures showed lower Hsw than that of NiFe, CoFe, and CoFeSiB single structures. The CoFeSiB SAF structures were proved to be beneficial for the switching characteristics such as reducing the coercivity and increasing the sensitivity in micrometer- to submicrometer-sized elements.

Magnetization switching of CoFeSiB free-layered magnetic tunnel junctions

Magnetic tunnel junctions (MTJs) comprising amorphous Co 70.5Fe 4.5Si 15B 10, possessing low saturation magnetization of 560 emu/cm 3, as a free layer have been investigated. The switching behaviours were confirmed for the micrometer-sized elements experimentally, using the scanning magneto-optical Kerr effect (scanning MOKE). A micromagnetic modelling study was also carried out for the submicrometer-sized elements. By using either a CoFeSiB single or a synthetic antiferromagnetic free-layer structure, the magnetization switching field became much lower than conventionally used CoFe free layered MTJs.

Reduction of switching field in spin-flop switching for high-density magnetic random access memory

A magnetization switching method for magnetic random access memory (MRAM), recently proposed by Savtchenko et al. [U.S. Patent No. 6,545,906 (2003)], is known to have an important advantage of a wide window for bit writing over the conventional method based on the asteroid curve, but it has a serious problem of high switching fields. In an effort to solve this problem, the effects of the thickness asymmetry and antiferromagnetic exchange coupling of the synthetic antiferromagnetic free-layer structure on the switching field have been investigated by micromagnetic computer simulation. At conditions relevant to high-density MRAM, magnetization switching in the direct write mode occurs at reasonably low values of word- and bit-line fields (below 100Oe), combined with a substantially wide window for bit writing. A much wider window is observed in the toggle mode, but the required switching fields are too high, being over 150Oe.

비정질 CoFeSiB 단일 및 합성형 반강자성 자유층을 갖는 자기터널접합의 자기저항 효과

To obtain low switching field () we introduced amorphous ferromagnetic single and synthetic antiferromagnet (SAF) free layers in magnetic tunnel junctions (MTJs). The switching characteristics for MTJs with structures 45/Ru 9.5/IrMn 10/CoFe 7/AlOx/CoFeSiB 7 or CoFeSiB (t)/Ru 1.0/CoFeSiB (7-t)/Ru 60 (in nm) were investigated and compared to MTJs with and free layers. CoFeSiB showed a lower saturation magnetization of and a higher anisotropy constant of than CoFe and NiFe, respectively. An exchange coupling energy () of was observed by inserting a 1.0 nm Ru layer in between CoFeSiB layers. In the CoFeSiB single and SAF free layer MTJs, it was frond that the size dependence of the originated from the lower experimentally and by micromagnetic simulation based on the Landau-Lisfschitz-Gilbert equation. The CoFeSiB SAF structures showed lower than that of NiFe, CoFe and CoFeSiB single structures. The CoFeSiB SAF structures were proved to be beneficial far the switching characteristics such as reducing the coercivity and increasing the sensitivity in micrometer to submicrometer-sized elements.