Current-perpendicular-to-the-plane Giant Magnetoresistance Research Articles

Free Layer Thickness Dependence of the Stability in Co2(Mn0.6Fe0.4)Ge Heusler Based CPP-GMR Read Sensor for Areal Density of 1 Tb/in2.

Current-perpendicular-to-the-plane giant magnetoresistance (CPP-GMR) read sensors based on Heusler alloys are promising candidates for ultrahigh areal densities of magnetic data storage technology. In particular, the thickness of reader structures is one of the key factors for the development of practical CPP-GMR sensors. In this research, we studied the dependence of the free layer thickness on the stability of the Co2(Mn0.6Fe0.4)Ge Heusler-based CPP-GMR read head for an areal density of 1 Tb/in2, aiming to determine the appropriate layer thickness. The evaluations were done through simulations based on micromagnetic modelling. The reader stability indicators, including the magnetoresistance (MR) ratio, readback signal, dibit response asymmetry parameter, and power spectral density profile, were characterized and discussed. Our analysis demonstrates that the reader with a free layer thickness of 3 nm indicates the best stability performance for this particular head. A reasonably large MR ratio of 26% was obtained by the reader having this suitable layer thickness. The findings can be utilized to improve the design of the CPP-GMR reader for use in ultrahigh magnetic recording densities.

Optimal Sizing of CPP-GMR Read Sensors for Magnetic Recording Densities of 1–4 Tb/in²

Several studies have confirmed that current-perpendicular-to-the-plane giant magnetoresistance (CPP-GMR) technology is appropriate for next-generation read sensors for ultrahigh areal densities (ADs) of data storage applications. Since the physical dimension of the read sensor is a crucial factor for developing the reader to overcome its limitations, this paper proposes an optimal sizing prediction of the CPP-GMR read heads for ADs of 1-4 Tb/in2. Micromagnetic modelling was performed in the simulations. The appropriate length of the stripe height (SH) and the read width (RW) of the readers was estimated based on a consideration of sensor outputs including the readback signal, asymmetry parameter, dibit response and power spectral density (PSD) profile. It was found that a variation of SH and RW lengths had an influential impact on the readback signal waveform. Those affectations were further characterized through the echoes of dibit response showing that shortening the SH length or increasing the RW length could improve the resolution and reduce the distortion occurring in the readback signal. Moreover, the PSD profile indicated that the reader operation became more stable at shorter SH lengths or longer RW lengths. The head response spectrum was also examined. In addition, the magnitude of the bias current was studied in relation to the head response. Lastly, the optimal physical dimension (SH $\times $ RW) of the CPP-GMR readers for ADs of 1-4 Tb/in2 was predicted to be ( $40 \times 48$ ) nm, ( $28 \times 29$ ) nm, ( $25 \times 26$ ) nm and ( $19 \times 20$ ) nm, respectively. The results can be utilized to design the CPP-GMR sensors at ultrahigh magnetic recording capacities.

Unstable Playback Response of CPP-GMR Read Head Induced by Electromagnetic Interference: Structural Dependence

Although several studies have claimed that the current perpendicular-to-the-plane giant magnetoresistance (CPP-GMR) technology is the most attractive technology to achieve ultrahigh capacities of data storage applications, there are still recent studies indicating the inaccuracy of the reading head response caused by the electromagnetic interference (EMI). To improve the precision of the head response, we report a dependence of the physical dimension of CPP-GMR read head on the readback signal distortion due to the EMI. The simulations were based on micromagnetic modeling. The EMI impacts on the head response were analyzed as a function of structural parameters of the head, including the length of stripe height (SH) and read width (RW). Then, the readback characteristics of the head, including the spin-transfer induced noise, power spectral density, and bit-error-rate (BER), were discussed. It was found that an alternation of either SH or RW influences the noise profile. Adjusting the length of RW demonstrates a more influential impact on the noise than that of SH. The frequency spectrum of the head response was also carried out. The physical explanations regarding the results found in this article were given through the behavior of the demagnetization field. The effects of bias current on the head output were additionally investigated. Moreover, the BER of the head with varying the head dimensions was characterized. From findings, the appropriate sizing of the CPP-GMR sensors for an areal density of 1 Tb/in2 was predicted. The results can be utilized to design CPP-GMR read heads at ultrahigh areal densities.

Spin transfer torque switching of hybrid memory layers with low Curie temperature CoPd/Pd multilayers

Thermally assisted spin transfer torque (STT) switching of hybrid memory stacks with a low Curie temperature (TC) CoPd/Pd multilayers (MLs) and high TC Co/Pd MLs were investigated. Current-perpendicular-to-the-plane giant-magneto-resistance (CPP-GMR) structures with a stack of [Co/Pt]6/Cu/[Co/Pd]M/[CoPd/Pd]N (M + N = 3) were fabricated. The intrinsic critical current density (Jc0) and thermal stability factor Δ of the hybrid memory layer were evaluated from the STT-switching of the CPP-GMR. Jc0 and Δ were found to be almost independent of the pillar diameter, and the effective reversal size contributing to Δ was found to be much smaller than the physical pillar diameter. At room temperature, slight decreases of Jc0 and Δ were confirmed with increasing the thickness ratio of the low TC MLs (N) in the hybrid memory layer. At high temperature of ~150 °C, Jc0 and Δ were found to significantly reduce, suggesting the thermally assisted STT switching by using the low TC/high TC hybrid memory layer.

Playback signal distortion in CPP-GMR read heads due to induced electromagnetic interference

Electromagnetic interference (EMI) induced magnetic instabilities are an influential factor which can degrade the performance of magnetic read heads. In this paper, we investigate the influences of EMI on the playback response of the current perpendicular-to-the-plane giant magnetoresistance (CPP-GMR) read heads via micromagnetic simulations. The readback signal and the noise spectral density of the heads when subjected to the EMI were characterized. It was found that the readback signal could be significantly distorted, or become inaccurate, from the EMI through the induced spin transfer torque effect. Also, the incorrect readback signal results in a notable expansion of the noise spectral density which further impacts the head stability. Furthermore, the bit-error-rate of the head signal processing could be markedly increased by multiple EMI occurrences. Hence, the playback response distortion of the CPP-GMR read heads due to the EMI is another concerned factor for the read sensors at higher storage capacities.

Improved Signal-to-Noise Ratio in Current-Perpendicular-to-Plane Giant Magnetoresistance Sensors Using Strong Exchange-Biased Reference Layers

We present experimental studies of simple-pinned current-perpendicular-to-the-plane giant magnetoresistance (CPP-GMR) sensors demonstrating large gains in signal-to-noise ratio from improvements in reference layer exchange pinning strength J k and magnetoresistance ratio AR/R. Significant increases in J k from 0.7 to 1.25 erg/cm 2 and AR/R from 7.5% to 13% are achieved by progressive improvements in the sensor structure. The improved sensors show lower spin transfer torque-induced noise due to stronger pinning and higher damping of the reference layer, both the consequence of increases in J k . This enables larger operational bias voltage, which along with higher AR/R, results in signal-to-noise gains of up to 6 dB in the recording heads. Our findings provide important insights for further development of CPP-GMR sensors in hard disk drives and other magnetic sensing applications.

Temperature-dependence of current-perpendicular-to-the-plane giant magnetoresistance spin-valves using Co2(Mn1−xFex)Ge Heusler alloys

The properties of Co2(Mn1−xFex)Ge (CMFG) (x = 0–0.4) Heusler alloy magnetic layers within polycrystalline current-perpendicular-to-the plane giant magnetoresistance (CPP-GMR) spin-valves are investigated. CMFG films annealed at 220–320 °C exhibit partly ordered B2 structure with an order parameter SB2 = 0.3–0.4, and a lower SB2 was found for a higher Fe content. Nevertheless, CPP-GMR spin-valve devices exhibit a relatively high magnetoresistance ratio of ∼13% and a magnetoresistance-area product (ΔRA) of ∼6 mΩ μm2 at room temperature, which is almost independent of the Fe content in the CMFG films. By contrast, at low temperatures, ΔRA clearly increases with higher Fe content, despite the lower B2 ordering for increasing the Fe content. Indeed, first-principles calculations reveal that the CMFG alloy with a partially disordered B2 structure has a greater density of d-state at the Fermi level in the minority band compared to the Fe-free (Co2MnGe) alloy. This could explain the larger ΔRA measured on CMFG at low temperatures by assuming that s-d scattering mainly determines the spin asymmetry of resistivity as described in Mott's theory.

Electromagnetic interference-induced instability in CPP-GMR read heads

Electromagnetic interference (EMI) has been a significant issue for the current perpendicular-to-the-plane giant magnetoresistance (CPP-GMR) read heads because it can cause magnetic failure. Furthermore, the magnetic noise induced by the spin transfer torque (STT) effect has played an important role in the CPP read heads because it can affect the stability of the heads. Accordingly, this work proposed an investigation of the magnetic instabilities induced by EMI through the STT effect in a CPP-GMR read head via micromagnetic simulations. The magnetization fluctuation caused by EMI was examined, and then, magnetic noise was evaluated by using power spectral density analysis. It was found that the magnetization orientation can be fluctuated by EMI in close proximity to the head. The results also showed a multimode spectral density. The main contributions of the spectral density were found to originate at the edges of the stripe height sides due to the characteristics of the demagnetization field inside the free layer. Hence, the magnetic instabilities produced by EMI become a significant factor that essentially impacts the reliability of the CPP-GMR read heads.

High signal output in current-perpendicular-to-the-plane giant magnetoresistance sensors using In–Zn–O-based spacer layers

We report current-perpendicular-to-the-plane giant magnetoresistance (CPP-GMR) sensors with a conductive Ag/In–Zn–O (IZO)/Zn trilayer as the spacer layer. Magnetoresistance ratios as high as 26% at resistance–area product (RA) = 60–120 mΩ µm2 were obtained in thin, polycrystalline spin valves suitable for modern read sensors of hard disk drives (HDDs). The large CPP-GMR values are attributed to the large spin-dependent scattering at the interfaces of the ferromagnet/IZO-based spacer junctions. The maximum voltage output of sensors with Ag/IZO/Zn spacers was ΔVmax = 11.3 mV, significantly larger than that observed for metallic AgSn spacers (2.3 mV). Such improved properties are important for HDD read heads with recording densities larger than 1 Tbit/in.2.

Effect of Nanowire Diameter and Period Number on Magnetic Properties and CPP-GMR of Ni-Fe/Cu/Co/Cu Multilayer Nanowire Arrays

Ni-Fe/Cu/Co/Cu multilayer nanowire arrays were electrodeposited into the pores of AAO template by using dual-bath method. The morphology and structure of the nanowire arrays were characterized by scanning electron microscopy and transmission electron microscopy. Vibrating sample magnetometer and physical property measurement system revealed that the magnetic properties and current perpendicular to the plane-giant magnetoresistance (CPP-GMR) of Ni-Fe/Cu/Co/Cu nanowire arrays were affected by the changes of nanowire diameter and period number. Along with the increase of the diameter, the coercivity and remanence ratio increased first, and then decreased. The properties of nanowire arrays were optimum and the maximum room temperature CPP-GMR ratio of 45.24% was achieved when the nanowire diameter was 80 nm and the period number was 300.

CPP GMR Through Nanowires

All-metal current perpendicular to the plane (CPP) giant magnetoresistance (GMR) layers have been made within insulating matrices by direct growth to avoid sidewall damage that is caused by lithographical patterning in current vacuum-deposited devices. These insulating matrices can be made to have nanostructures with multiscale order to allow photolithographical alignment of contacts to 10-500 nm devices. This alignment was demonstrated with 100-200 nm diameter structures to prove feasibility. Next, trilayers of [Co(15 nm)/Cu(5 nm)/Co(10 nm)] with 10 nm diameters were made by electrochemical deposition with 30 Ω resistance and 19% magnetoresistance. These parameters are desirable for read head sensors, especially because the nanowires described here have 1:1 aspect ratios, 10× smaller areas, and 100× lower resistances than conventional read sensors based on lithographically-produced magnetic tunnel junctions. Finally, the potential application of closely spaced arrays of CPP GMR sensors for enabling one-pass two dimensional recording as well as a new technique called cross recording will be discussed.

Giant Magnetoresistance and Coercivity of electrodeposited multilayered FeCoNi/Cu and CrFeCoNi/Cu

The effect of Cr addition on electrodeposited multilayered nanowires CrFeCoNi/Cu was investigated from a magnetic property perspective: current perpendicular to the plane-Giant Magnetoresistance (CPP-GMR) and Coercivity (BH loops). The magnetic behavior of multilayered nanowires of CrFeNiCo/Cu was also affected by the alloy deposition potential, alloy pulsing time (layer thickness) and number of bilayers. Furthermore, the addition of Cr influenced both the nanowires GMR and Coercivity. Cr addition to the ferromagnetic FeCoNi layer induced a reduction in the room temperature GMR from 10.64% to 5.62%; however, the magnetic saturation field decreased from 0.45 to 0.27T. The increase in the number of bilayers, from 1000 to 2500, resulted in a higher GMR value, 14.56% with 0.35T magnetic saturation field. Addition of Cr to the ferromagnetic layer decreased the coercivity from 0.015 to 0.0054T. Low saturation field CPP-GMR nanowires showing low coercivity at room temperature opens a new door for magnetic sensing devices. To the best of our knowledge, this is the first study on electrodeposited CrFeCoNi/Cu multilayered nanowires.

Magnetoresistance and spin transfer torque in electrodeposited Co/Cu multilayered nanowire arrays with small diameters

Multilayered Co/Cu nanowires with 10 nm diameter were fabricated as arrays in anodic aluminum oxide templates. Magnetization hysteresis (MH) loops and current perpendicular to the plane giant magnetoresistance (CPP-GMR) were measured as functions of Cu layer thicknesses. The largest CPP-GMR values (11%, or 19.5% for multilayers alone) were found in nanowire arrays that had identical MH loops whether the field was applied parallel or perpendicular to the wires. Spin transfer torque (STT) switching was measured in the 10 nm diameter nanowires and also in 60 nm diameter wires. For the 10 nm diameter nanowires, these STT curves were overlaid with small jumps (1%–2%) that were attributed to individual layers flipping and larger jumps (6.25%) that were due to collective spin flips between interacting Co layers. For larger diameter nanowires, the individual spin flips were not present, and the curves were similar to typical STT results. The current densities required to switch the Co layers from antiparallel to parallel and back (JAP-P/JP-AP) were 2.7×108/1.3×108 and 3.2×107/−1.6×107 A/cm2, respectively, for the 10 and 60 nm diameter nanowires. The ability to construct large area arrays of 10 nm CPP-GMR and STT structures makes these first results exciting for eventual use in magnetic random access memory.

Effect of Current-Confined Path on Spin-Torque Noise in CPP-GMR Heads With Current Screen Layer

We investigated the effect of current-confined path on spin-torque noise in current perpendicular to the plane giant magnetoresistive (CPP-GMR) heads with a current screen layer (a nano-oxide layer having current-confined paths). We found that both in measurement and calculation spin-torque noise of heads with a resistance area (RA) product of 1.0 Omega middot mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> was smaller than that with RA of 0.25 Omega middot mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . We also found that the calculated spin-torque noise decreased as the exchange stiffness constant in the free layer increased. These behaviors indicate that the magnetization fluctuation in conductive areas can be reduced by the magnetic coupling with the magnetization in nonconductive areas. Accordingly, the current screen structure is effective in suppressing the spin-torque noise. To reduce spin-torque noise in CPP-GMR heads with a current screen layer, it is effective to reduce the area of each metallic nano-hole while keeping the total conductive area constant since the magnetic coupling between the magnetization in conductive and nonconductive areas keeps strong.

A theoretical study of a spin polarized transport and giant magnetoresistance: The effect of the number of layers in a magnetic multilayer

This study presents a quantum-mechanical free electron model for analyzing a spin polarized transport and current-perpendicular-to-the-plane giant magnetoresistance (CPP-GMR) in a more realistic way. The CPP-GMR is evaluated by using three spin resolved conductance parameters based on the Landauer conductance formula. In a ballistic regime, a transfer-matrix method is used to calculate the spin dependent transmission probability as a function of the transverse mode. A spin dependent conduction band structure is constructed by extracting parameters of the free electron model, such as the atomic magnetic moments and the conduction electron densities, from the spin dependent layer-decomposed density of states of the Cu and Co interfacial layers in a Cu5/Co11 slab; these calculations are derived from the density functional theory. As a result, this study shows that the CPP-GMR in a [Cu(5ML)∕Co(11ML)]n magnetic multilayer (n=2–5) with a 35ML×35ML cross section is in the range of 60%–111%. It is qualitatively comparable to the calculation results of first principles. This study also uses transmission probability to explain the increase of spin dependent scattering and CPP-GMR as a function of the number of layers in the [Cu∕Co]n magnetic multilayer. Moreover, the study confirms that modification of the free electron model by quantum-mechanical methods can be applied to calculations of a spin polarized transport and CPP-GMR in a specific material system.

Spin-Torque Noise in CPP-GMR Heads With Current Screen Layer

We investigated the head noise in current perpendicular-to-the-plane giant-magnetoresistive (CPP-GMR) heads with a current screen layer (nano oxide layer having a confined current path) to experimentally evaluate spin-torque noise in the head application. Depending on the direction/magnitude of sensing current, a significant increase in head noise was observed. Some features in noise profiles and a correlation between noise and peak asymmetry are qualitatively reasonable from the perspective of the spin-torque theory. This fact indicates that this extraordinary noise was induced by spin-transfer torque. The spin-torque noise was not observed when the heads were well biased with an orthogonal direction for the free and the reference layer, regardless of whether sensing current was positive or negative. This suggests that the spin-torque noise is not a fatal issue in current screen type CPP-GMR heads with small peak asymmetry.

Influence of ferromagnetic current screen layer on current perpendicular to the plane-giant magnetoresistance

We investigated the influence of the ferromagnetic current screen layer on a current perpendicular to the plane-giant magnetoresistance (CPP-GMR). We prepared a bottom-type synthetic-pinned spin-valve film with a ferromagnetic current screen layer of naturally oxidized CoFe, which was separated magnetically from the free and pinned layers. When the CoFe layer thickness increased to above 1.0nm before oxidation, we observed a magnetic moment corresponding to the unoxidized CoFe. We also found that the MR ratio abruptly increased from 1.6% to 7.2% with the increase in the CoFe layer thickness from 1.0to3.7nm before oxidation, which was the same tendency observed in the magnetic moment. The MR ratio seemed to be strongly dependent on the magnetic moment of the current screen layer. The MR curve with the high MR ratio had two peaks near the zero magnetic fields. Based on the analysis of the magnetization configuration of the free, pinned, and current screen layers, we found that the ferromagnetic properties of the current screen layer contributed to the high MR ratio. From transmission electron microscopy observation and magnetic properties of the current screen layer, we estimated that the metal CoFe bridge exists in the current screen layer. We believe that the metal bridge made from the ferromagnetic metal in the current screen layer is important for enhancing the MR ratio of CPP-GMR for both current screening and spin dependent scattering.

Ultrathin CoPt-pinned current perpendicular to the plane spin valves

The magnetics and magnetotransport of current perpendicular to the plane (CPP) giant magnetoresistive (GMR) spin valves utilizing thin CoPt hard magnet pinning layers on Cr seed layers are investigated. 50-Å-thick CoPt18 films grown on &amp;gt;20Å Cr seed layers exhibit a coercivity of ∼1.5kOe and a resistivity of 31μΩcm. The low critical thickness of the CoPt hard magnet compared to IrMn or PtMn antiferromagnets makes it an attractive pinning material for small gap CPP sensors required for high magnetic recording densities. Furthermore, CoPt∕Ru∕CoFe trilayers exhibit strong antiparallel coupling and the low CoPt resistivity minimizes parasitic resistance yielding a higher magnetoresistance. Moreover, free layers with coercivities and coupling fields in a range of 5–10 Oe have been measured, demonstrating that free layer softness does not suffer from the proximity to the CoPt hard magnet due to the relatively thick spacer layers compatible with CPP GMR sensors. In particular we deposited CoPt18 pinned antiferromagnetically coupled spin with a variety of reference layers. The films were patterned into pillars with diameters ranging from 50 to 200 nm by using a combination of electron beam lithography and ion milling. The devices exhibit a magnetoresistance up to 3.6% and a resistance area product of only 20mΩμm2.

CPP-GMR with oxidized CoFe layer on various lower-electrode materials

We investigated current perpendicular to the plane-giant magnetoresistive (CPP-GMR) films with a current screen layer of oxidized CoFe. The magnetoresistance (MR) ratio increased monotonically with an increment of resistance-area product (RA) when the thickness of the precursor CoFe layer was increased. We were able to obtain a high MR ratio of 6.7% at an RA of about 2 /spl Omega//spl mu/m/sup 2/. Based on transmission electron microscopic observations, the current screen layer was crystalline, and some metal portions were locally observed in the current screen layer. We also investigated the dependence of CPP-GMR properties on he crystalline structure by changing lower-electrode materials. Decreasing RA without a degradation of MR ratio was observed with increasing grain sizes by changing the lower-electrode materials. Fabricated read heads for perpendicular recording, which had a track-width of 80 nm with an MR ratio of 3% and an RA of 0.6 /spl Omega//spl mu/m/sup 2/, showed a high output voltage of 2.2 mV and a good head-amp signal-to-noise ratio of 30 dB, at an operating voltage of 120 mV. Therefore, an improved CPP-GMR film up to a high MR ratio of 6% at the low RA of 0.3 /spl Omega//spl mu/m/sup 2/ with good soft magnetic properties is promising for future read heads of over 200 Gb/in/sup 2/.

Sensing-current dependence of peak asymmetry in CPP-GMR heads

The read performance, especially peak asymmetry, of current-perpendicular-to-the-plane giant-magnetoresistive (CPP-GMR) heads with a current screen layer (nano oxide layer having a confined current path) was investigated through a spin stand test and a micromagnetic simulation. We fabricated CPP-GMR heads having a magnetoresistive (MR) ratio of 3%. The head-amplifier signal-to-noise ratio of the CPP-GMR heads with longitudinal media was 18-27 dB with a bandwidth of 200 MHz. The peak asymmetry of these heads has a complicated dependence on the sensing current. This complicated behavior can be explained in terms of the particular transfer curve for the CPP-GMR head. The transfer curve calculated from the micromagnetic simulation was "stair-like," having kinks caused by the current-induced field. Increasing the sensing current changes the positions of the kinks on the transfer curve. This stair-like transfer curve directly influences the peak asymmetry through the sensing current. Accordingly, the longitudinal bias field, sensor size, and current-induced biasing should be considered when controlling the peak asymmetry of the CPP-GMR heads.