Giant Magnetoresistance Spin Valves Research Articles

Feasibility of TEOS Coated CoFe2O4 Nanoparticles to a GMR Biosensor Agent for Single Molecular Detection

Magnetic properties of 200 nm ferrimagnetic CoFe2O4 nanoparticles before and after coating with TEOS were explored and compared to soft ferrimagnetic MgFe2O4 nanoparticles (200 nm) to evaluate the feasibility as an in-vitro GMR SV (giant magnetoresistance spin-valve) biosensor agent for single molecular detection (SMD). It was found that the magnetic degradation (or variation) of TEOS coated CoFe2O4 and MgFe2O4 nanoparticles are dominantly affected by the chemical dispersion process, which is carried out in the oleic acid (OA), oleylamine (OL), or OA+OL surfactant, before starting major coating process. In addition, the TEOS coating thickness controlled by TEOS concentration and pH level in the buffer solution prominently influenced on the magnetic degradation of TEOS coated nanoparticles. According to the experimental analysis results, the magnetic degradation of TEOS coated nanoparticles is mainly attributed to the variation of particle dipole interaction caused by the degree of particle aggregation depending on TEOS coating process conditions. The TEOS coated CoFe2O4 nanoparticles exhibited a higher magnetic stability for a GMR biosensor agent, e.g., small variation of remnant magnetization, saturation magnetization and magnetic coercivity, than that of MgFe2O4 nanoparticles at the different coating process conditions. The physical and chemical analysis confirmed that this is primarily due to its higher magnetic anisotropy. The experimentally verified high biocompatibility as well as the stably maintained magnetic properties of TEOS coated CoFe2O4 nanoparticles demonstrate that CoFe2O4 nanoparticles can be considered as one of the promising ferrimagnetic nanoparticle sensor agent for an SMD GMR SV biosensor.

Fabrication and Analysis of PM-biased Spin-valve Sensors

GMR spin-valve sensors are fabricated and analyzed, where a pair of CoPt permanent magnets (PM) are attached to two ends of the sensor elements to suppress the hysteresis. The size dependence of the spin-valve sensors is investigated by measuring GMR spin-valve sensing elements patterned with different lengths and widths. The results show that the GMR ratio reduces and the saturation field increases respectively with decreased length of the spin-valve sensor. And the saturation field also decreases with increasing width due to a stronger demagnetization effect. The experimental results are in good agreement with the theoretical prediction.

Giant magnetoresistance in IrMn based antiferromagnetic spin valve

According to the parameter-free first principles calculations, we investigate the spin polarized transport in antiferromagnetic spin valve (AFSV) based on noncollinear IrMn. The giant magnetoresistance (GMR) in Co/IrMn/Cu(111) with L12-type IrMn shows three-fold rotational symmetry, which is easy to be distinguished from the GMR of normal ferromagnetic spin valves. Moreover, GMR based on -phase IrMn with current-in-plane (CIP) structure shows that GMR is 7.7%, around two time larger than that in current-pendicular-to-plane (CPP) structure (3.4%). Our study demonstrates that the AFSV CIP structure possesses a larger GMR effect than the CPP structure, and the GMR effect in AFSV based on noncollinear antiferromagnetic structure is larger than that based on collinear antiferromagnetic structure such as FeMn.

Engineering spin propagation across a hybrid organic/inorganic interface using a polar layer

Spintronics has shown a remarkable and rapid development, for example from the initial discovery of giant magnetoresistance in spin valves to their ubiquity in hard-disk read heads in a relatively short time. However, the ability to fully harness electron spin as another degree of freedom in semiconductor devices has been slower to take off. One future avenue that may expand the spintronic technology base is to take advantage of the flexibility intrinsic to organic semiconductors (OSCs), where it is possible to engineer and control their electronic properties and tailor them to obtain new device concepts. Here we show that we can control the spin polarization of extracted charge carriers from an OSC by the inclusion of a thin interfacial layer of polar material. The electric dipole moment brought about by this layer shifts the OSC highest occupied molecular orbital with respect to the Fermi energy of the ferromagnetic contact. This approach allows us full control of the spin band appropriate for charge-carrier extraction, opening up new spintronic device concepts for future exploitation.

강자성층 사이에 초연자성 NiFeCuMo 중간층을 삽입한 3층 박막구조의 자기적 특성

초연자성을 띠는 코네틱(Conetic; NiFeCuMo) 박막을 상호 중간층으로 강자성체인 CoFe 또는 NiFe 박막 사이에 삽입한 코닝 유리(Corning glass)/Ta(5 nm)/[CoFe or NiFe(5 nm-t/2)]/NiFeCuMo(t = 0, 4, 6, 8, 10 nm)/[CoFe or NiFe(5 nm-t/2)]/Ta(5 nm) 3층 박막구조에 대한 자기적 특성을 조사하였다. CoFe와 NiFe 박막의 자기적 특성은 박막의 두께에 따라 크게 결정되므로 자화 곤란축과 자화 용이축으로 측정된 이방성 자기저항 곡선으로부터 얻은 보자력과 자화율을 각각 비교하였다. 특히 3층 박막구조에서 NiFe 박막 사이에 자유층으로 NiFeCuMo 박막을 삽입하면 높은 자기저항비를 유지하면서 향상된 자장감응도를 유지하는 고감도 바이오센서용 거대자기저항-스핀밸브(giant magnetoresistive-spin valves; GMR-SV) 및 자기터널접합(magnetic tunnel junction; MTJ) 소자로 활용할 수 있다. Two-layered ferromagnetic alloy films (NiFe, CoFe) with a Conetic (NiFeCuMo) intermediately soft magnetic layer of different thickness were investigated to correlate the coercivity values and magnetization process with the strength of saturation field of hard axis. Thickness dependence of the <TEX>$H_{EC}$</TEX> (coercivity of easy axis), <TEX>$H_{HS}$</TEX> (saturation field of hard axis.), and X (susceptibility) of NiFe and NiFeCuMo thin films for the glass/Ta(5 nm)/[CoFe or NiFe(5 nm-t/2)]/NiFeCuMo(t = 0, 4, 6, 8, 10 nm)/[CoFe or NiFe(5 nm-t/2)]/Ta(5 nm) films prepared by the ion beam deposition method was measured. The magnetic properties <TEX>$H_{EC}$</TEX>, <TEX>$H_{HS}$</TEX>, and X of two-layered ferromagnetic CoFe, NiFe films with a NiFeCuMo intermediately super-soft magnetic layer were strongly depended on the thickness of NiFeCuMo layer. The value of the coercivity and magnetic susceptibility of the NiFeCuMo film decreased by 25% and doubled relative to that of the NiFe film.

Numerical simulation of current density induced magnetic failure for giant magnetoresistance spin valve read sensors

It was numerically demonstrated that current-in-plane (CIP) and current-perpendicular-to-plane (CPP) Ir20Mn80 based giant magnetoresistance spin valve read sensors operating at an extremely high current density (J≥1×108 A/cm2) show completely different electrical and magnetic failure mechanisms: (1) CIP read sensors, electromigration-induced Cu spacer diffusion and correspondingly degraded interlayer coupling were primarily responsible for the failures; while, (2) CPP read sensors, the deterioration of exchange bias due to thermomigration-induced Mn interdiffusion at the Co80Fe20/Ir20Mn80 interface was found to be dominant. The different temperature and current distribution resulting in different mass-transport mechanisms are the main physical reasons for the failure.

Magnetization Reversal Process of Tri-Layer Readers for Ultrahigh Density Data Storage

We study the magnetization reversal process of tri-layer readers using micromagnetic simulations. The magnetoresistance response of tri-layer readers has been shown to be distinctive from the conventional spin valve giant magnetoresistance sensors. We look into the challenges for future high storage density when scaling down the read sensor size. The possible sensor reversal process control by engineering the sensor dimensions and material properties has been systematically investigated. We further observe an interesting dependence of magnetization states on the media bit transition length, which could be attributed to the small but finite physical separation between the two coupled ferromagnetic films. The findings in this work are useful for the design of tri-layer sensors for ultrahigh storage density in hard disk drives.

NiFe/Cu/NiFe/IrMn 스핀밸브 박막소자의 자화 용이축에 따른 형상 자기이방성

NiFe/Cu/NiFe/IrMn 스핀밸브 박막소자에 대해 자화 용이축에 의존하는 형상 자기이방성을 조사하였다. Ta(5 nm)/NiFe(8 nm)/Cu(2.3 nm)/NiFe(4 nm)/IrMn(8 nm)/Ta(2.5 nm) 다층박막을 증착할 때 350 Oe의 자기장으로 인가한 자화 용이축을 폭 방향 또는 길이 방향으로 설정하여 <TEX>$1\;{\mu}m\;{\times}\;18\;{\mu}m$</TEX>의 소자를 제작하였다. 2단자 방법으로 소자의 자기저항 곡선으로부터 자장감응도를 측정하고 자화 용이축에 따른 형상 자기이방성을 비교하였다. 측정한 소자 길이 방향의 센싱전류와 고정층의 자화 용이축이 소자 폭방향 각도가 GMR-SV 소자를 바이오센서로 활용하는데 중요한 요인임을 확인하였다. The GMR-SV (giant magnetoresistance-spin valve) device depending on the micro patterned features according to two easy directions of longitudinal and transversal axes has been studied. The GMR-SV multilayer structure was Ta(5 nm)/NiFe(8 nm)/Cu(2.3 nm)/NiFe(4 nm)/IrMn(8 nm)/Ta(2.5 nm). The applied anisotropy direction of the GMR-SV thin film was performed under the magnitude of 300 Oe using by permanent magnet during the deposition. The size of micro patterned device was a <TEX>$1\;{\times}\;18\;{\mu}m^2$</TEX> after the photo lithography process. In the aspects of the shape magnetic anisotropy effect, there are two conditions of fabrication for GMR-SV device. Firstly, the direction of sensing current was perpendicular to the magnetic easy axis of the pinned NiFe/IrMn bilayer with the transversal direction of device. Secondly, the direction of shape magnetic anisotropy was same to the magnetic easy axis of the free NiFe layer with the longitudinal direction of device.

Predicted Effects of Pinhole and Surface Roughness in Magnetoresistive Read Head

Magnetoresistive read heads based on giant magnetoresistance spin-valves or magnetic tunnel junctions have been widely used in magnetic recording. In the fabrication of these nano structures, the surface roughness is crucial to the performance of the devices, both electrically and magnetically. In this work, we theoretically study the effect of roughness and pinholes in magnetic tunnel junctions and nanowire systems. Results are compared to experiment.

A micron-sized GMR sensor with a CoCrPt hard bias

A GMR (giant magneto-resistive) spin valve sensor for magnetic recording has been designed in an attempt to solve the Barkhausen noise problem in small-sized GMR sensors. In this study, the GMR ratio of the top-pinned spin valve is optimized to a value of 13.2%. The free layer is magnetized perpendicular to the pinned layer by a CoCrPt permanent magnetic bias so that a linear magnetic field response can be obtained. An obvious improvement on performance is observed when the permanent magnetic bias is magnetized, while the GMR sensor has a steadier MR–H loop and a smaller coercive field.

Positive Giant Magnetoresistance in La0.67Sr0.33MnO3/Alq3/Co Organic Spin-Valves

We report a positive spin valve effect—low resistance for parallel electrodes configuration—in organic spin valves fabricated by vacuum thermal evaporation using half-metal perovskite manganites La0.67Sr0.33MnO3 as the bottom electrode, cobalt as the top electrode, and tris(8-hydroxyquinoline) aluminum (Alq3) as the organic semiconductor spacer. A positive giant magnetoresistance (GMR) of ∼12% has been observed at 100 K. The origination mechanism of negative and positive GMR in organic spin valves has been discussed in detail.

Effects of controlling Cu spacer inter-diffusion by diffusion barriers on the magnetic and electrical stability of GMR spin-valve devices

An ultra-thin Co or CoFe diffusion barrier inserted at the NiFe/Cu interfaces was revealed to effectively control the electrical and magnetic stability of NiFe/Cu/NiFe-based giant magnetoresistance (GMR) spin-valve spintronics devices (SVSDs) operating at high current density. It was found that the activation energy, E a, related to the electromigration (EM)-induced inter-diffusion process for the patterned NiFe(3)/Cu(2)/NiFe(3 nm) magnetic multi-layered devices (MMLD) was remarkably increased from 0.52±0.2 eV to 1.17±0.16 eV after the insertion of an ultra-thin Co diffusion barrier at the NiFe/Cu interfaces. The dramatically reduced “current shunting paths” from the Cu spacer to the NiFe thin films and the development of “self-healing process” resulted from the effectively restrained Cu inter-diffusion (intermixing with Ni atoms) due to the diffusion barriers were found to be primarily responsible for the improvement of electrical and magnetic stability. The further investigation on the effects of controlling Cu spacer inter-diffusion by diffusion barriers on the EM and thermomigration (TM)-induced magnetic degradation was carried out for the NiFe/(Co or Co 90Fe 10)/Cu/(Co or Co 90Fe 10)/NiFe/FeMn top exchange-biased GMR (EBGMR) SVSDs electrically stressed under the applied DC current density of J=2.5×10 7 A/cm 2 ( I=16.5∼17.25 mA). It was clearly confirmed that the Co and the CoFe diffusion barriers effectively control the Cu spacer inter-diffusion resulting in a smaller reduction in both GMR ratio and exchange bias field of the EBGMR SVSDs. Furthermore, it was obviously observed that the effects of CoFe diffusion barrier on controlling the Cu spacer inter-diffusion are more significant than that of Co. The effectively reduced Mn atomic inter-diffusion at the NiFe/FeMn interface and the well-maintained interfacial spin-dependent scattering resulted from the control of EM and TM-induced Cu spacer inter-diffusion were the main physical reasons for the significant improvement of magnetic and electrical degradation of top EBGMR SVSDs.

Thermomigration-induced magnetic degradation of current perpendicular to the plane giant magnetoresistance spin-valve read sensors operating at high current density

The theoretically analyzed physical mechanism of thermomigration (TM)-induced magnetic degradation that occurred in the current perpendicular to the plane (CPP) Ir20Mn80 exchange biased giant magnetoresistance spin-valve (EBGMR SV) read sensors is presented. The device size was changed from 60×60 to 140×140 nm2 at the fixed aspect ratio of 1(L):1(W), and the operating current density was varied from J=1×108 A/cm2 to J=5×108 A/cm2 in a current control mode. It was numerically confirmed that the Mn atomic interdiffusion through the Ir20Mn80/Co80Fe20 interface due to the thermally induced mass transport and the “Villari magnetic reversal” of the CoFe pinned layer due to the thermally induced stress are mainly responsible for the serious degradation of exchange bias and magnetoresistance. Furthermore, the TM-induced magnetic degradation of CPP EBGMR SV read sensors was found to become severe by increasing the operating current density. However, interestingly, this undesirable magnetic degradation was dramatically diminished by reducing the read sensors below 100(L)×100(W) nm2.

Magnetization relaxation and structure of CoFeGe alloys

The magnetic relaxation of 10 and 50 nm thin films of (CoFe)100−xGex (0 at. %≤x≤35 at. %) alloys was investigated by broadband ferromagnetic resonance (FMR) experiments. 10 nm thin films exhibit a significant two magnon contribution to the FMR linewidth. The 50 nm films exhibit very low damping constants of α≈0.0025 and relaxation rates as low as 33 MHz in the composition range of 20 at. %≤x≤30 at. % Ge after annealing. Structural characterization revealed B2 order for these compositions. First principles calculations confirm a pseudogap in the minority channel for B2 ordered (CoFe)75Ge25 which may cause the low damping parameters and high ΔRA in CoFeGe based current perpendicular to the plane giant magnetoresistance spin valves.

Negative Magnetoresistance in Dual Spin Valve Structures With a Synthetic Antiferromagnetic Free Layer

Giant magnetoresistance (GMR) in spin valves is due to spin-dependent scattering occurring at ferromagnet/normal metal (F/N) interfaces and/or in the ferromagnetic layers. In a spin valve with a typical F/N/F structure where the spin scattering asymmetry factor (alpha) of both F/N interfaces is the same (more or less than 1), the GMR is expected to be positive. If alpha is greater than one at one F/N interface and less than one at the other F/N interface, however, the GMR is expected to be negative. Here, we show that the F1/Cu/SAF/Cu/F2/IrMn dual spin valve structure exhibits negative GMR, where F1 and F2 are CoFe and SAF = CoFe/Ru t/CoFe, due to both opposite electron spin scattering asymmetry factor at the CoFe/Ru/CoFe interfaces as well as the electrical separation of the overall structure into two GMR spin valves connected in parallel. A GMR of 6% is observed in the structure without the Ru spacer layer, insertion of a 0.6 nm thick Ru in the SAF results in a negative GMR ratio of -3% , which becomes positive again at the Ru thickness of 0.8 nm, the oscillation from positive to negative MR is consistent with interlayer exchange coupling period across the Ru spacer.

Magnetic instability of giant magnetoresistance spin-valves due to electromigration-induced inter-diffusion

Electromigration-induced magnetic failures in NiFe/(Co)/Cu/(Co)/NiFe spin-valve (SV) multilayers have been investigated. Electrically stressed NiFe/Cu/NiFe SV multilayers showed a dramatic reduction of magnetic moment up to 41%, and a shift of interlayer coupling characteristics, while no obvious magnetic degradation was observed in the NiFe/Co/Cu/Co/NiFe SV multilayers. It was experimentally confirmed that the magnetic degradation of the NiFe/Cu/NiFe SV multilayers is primarily due to the formation of Ni–Cu intermixing caused by the electromigration-induced Cu spacer inter-diffusion. Furthermore, it was demonstrated that an ultra thin Co diffusion barrier at the NiFe/Cu interface is promisingly effective to improve the magnetic stability of NiFe/(Co)/Cu/(Co)/NiFe SV multilayers against electromigration.

Anisotropy dispersion in the exchange-biased pinned layer of a spin valve prepared by 550 eV hydrogen-ion irradiation

By investigating angular dependence of resistance and applying the Boltzmann distribution to the anisotropy dispersion of the magnetization in an exchange-biased pinned layer, we quantized the intrinsic anisotropy dispersion σγ of spin valves. The σγ was estimated to be 0.412° for the as-deposited spin valve and 0.183° for the ion-irradiated spin valve. This indicates that the dispersion indeed narrowed when the spin valve was field-annealed or irradiated by 550 eV hydrogen ions under a magnetic field, which is consistent with our previous attribution to the significant improvement in both exchange anisotropy and giant magnetoresistance of spin valves thus treated. Our methodology can be applied for other spin devices characterized by angular dependence of resistance to determine useful device properties such as the intrinsic anisotropy dispersion and the exchange bias of the exchange-biased reference layer.

Magnetoresistance calculation in current-perpendicular-to-plane giant magnetoresistance spin valves with current-confined paths

We compared the magnetoresistance (MR) ratios of current-confined-path (CCP) current-perpendicular-to-plane (CPP) giant MR spin valves (SVs) calculated using the two-current-model. The simplified two-current model, which places the parasitic resistance external to the two-current network, has been inaccurate in determining the overall resistance of a SV in parallel magnetization configuration. The characteristics of the error in MR depend on the parasitic resistance value, the spin asymmetry coefficient, and the spacer layer resistivity. In comparison to the complete two-current model, we found that although the simplified model could generally predict the trend of various MR behaviors, it also overestimates MR for different CCP densities. A more comprehensive calculation on the CCP SVs was then performed using the spin-drift-diffusion equations based on the Valet–Fert model. It was found that the MR behavior upon considering spin relaxation in the ferromagnetic layers has a closer match with results predicted by the simplified model due to the strong reduction in spin accumulation in the parasitic layers. Spin relaxation in the spacer layer, however, shows a competing effect that tends to reduce the MR ratios. Our results in this paper are useful for accurate evaluation of MR performance in a CPP SV sensor with current-confined paths.

Current perpendicular to the plane spin-valves with CoFeGe magnetic layers

The magnetotransport properties of current perpendicular to the plane giant magnetoresistive spin-valves utilizing (CoFe)100−xGex alloys are investigated. The composition range of 21≤x≤32 at. % is determined to be the alloy composition that maximizes magnetoresistance. ΔRA values of 2.6 mΩ–μm2 are measured for spin-valves with CoFeGe in the free and reference layer resulting in magnetoresistance values of greater than 6%, which is almost twice the value of similar spin-valves with CoFeAl alloys. An analysis of the physical properties of the CoFeGe thin films in the range of interest is also presented.

Current-perpendicular-to-plane giant magnetoresistance in spin-valve structures using epitaxial Co2FeAl0.5Si0.5/Ag/Co2FeAl0.5Si0.5 trilayers

A current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) spin valve using epitaxial layers of Co2FeAl0.5Si0.5 (CFAS) Heusler alloy as ferromagnetic electrodes is reported. A multilayer stack of Cr/Ag/CFAS/Ag/CFAS/Co75Fe25/Ir22Mn78/Ru was deposited on a MgO (001) single crystal substrate. Epitaxial growth of the Cr, Ag, and CFAS layers in the (001) orientation up to the top CFAS layer was confirmed. Large MR ratios of 6.9% at room temperature and 14% at 6 K were observed for the CPP-GMR device. High spin polarization of epitaxial CFAS is the possible reason for high MR ratios.