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Suppression of 1/f noise in tunnel magnetoresistance (TMR) sensors is a central issue in the realization of magnetic field sensors with ultrafine magnetic field detectivity. Although AC modulation with an external magnetic field has been proposed as a method to shift the operating frequency of a sensor to a high frequency and substantially suppress 1/f noise, its effects on the two types of 1/f noise, that is, magnetic and electrical 1/f noise, are not well understood. In this study, we investigated the noise characteristics and signal detection performance of TMR sensors with an even-function resistance-magnetic field curve operated by the AC modulation method. For one TMR device in which the magnetic 1/f noise was dominant, AC modulation degraded the magnetic field detectivity owing to the additional noise induced by the AC modulation field. However, in another TMR device, in which the electrical 1/f noise was artificially enhanced by introducing lattice defects in the MgO tunnel barrier, AC modulation effectively suppressed the 1/f noise and improved the magnetic field detectivity by one order. This demonstrates that the AC modulation method using an external magnetic field is effective for magnetic field sensors in which electrical 1/f noise is dominant. [ABSTRACT FROM AUTHOR]

We report on tunnel magnetoresistance (TMR) sensors with two ferromagnetic free layers (FLs) on both sides of a MgO tunnel barrier. The magnetizations of these FLs are weakly (softly) pinned in an anti-parallel configuration by the exchange bias of IrMn antiferromagnetic layers through ferromagnetic and/or antiferromagnetic interlayer couplings of non-magnetic spacer layers. These dual soft-pinned FL (SPFL) sensors show a symmetric resistance (R) change with respect to the polarity of the external magnetic field (H) applied to the hard axis (HA) of the SPFLs and TMR ratios of over 200%, which is much higher than those of the single-SPFL sensors, where one side of the ferromagnetic layers is an SPFL and the other side is a strongly pinned reference layer. When the applied H exhibits an angular misalignment from the HA, dual-SPFL sensors show much-reduced asymmetry in the R–H curves than single-SPFL sensors. Macrospin simulations of the magnetization behavior and R–H curve for both dual- and single-SPFL sensors revealed that the small asymmetry in the R–H curves of dual-SPFL sensors was owing to their significantly smaller dependence of relative magnetization angle between the SPFLs on the magnetic field misalignment from the HA. These results suggest that dual-SPFL TMR sensors are promising for use in magnetic sensors such as encoder sensors for position and rotation detections. [ABSTRACT FROM AUTHOR]

Recent advances in the study of materials with topological electronic band structures have revealed magnetic materials exhibiting giant anomalous Hall effects (AHEs). The giant AHE has not only attracted the research interest in its mechanism but also opened up the possibility of practical application in magnetic sensors. In this article, we describe simulation-based investigations of AHE magnetic sensors for applications to read head sensors (readers) of hard disk drives. With the shrinking of magnetic recording patterns, the reader technology, which currently uses multilayer-based tunnel magnetoresistance (TMR) devices, is associated with fundamental challenges, such as insufficient spatial resolution and signal-to-noise ratio (SNR) in sensors with dimensions below 20 nm. The structure of an AHE-based device composed of a single ferromagnetic material is advantageous for magnetic sensors with nanoscale dimensions. We found that AHE readers using topological ferromagnets with giant AHE, such as Co2MnGa, can achieve a higher SNR than current TMR readers. The higher SNR originates from the large output signal of the giant AHE as well as from the reduced thermal magnetic noise, which is the dominant noise in TMR readers. We highlight a major challenge in the development of AHE readers: the reduction in the output signal due to the shunting of the bias current and the leakage of the Hall voltage through the soft magnetic shields surrounding the AHE reader. We propose reader structures that overcome this challenge. Finally, we discuss the scope for future research to realize AHE readers. [ABSTRACT FROM AUTHOR]

The combination of magnetoresistive (MR) element and magnetic flux concentrators (MFCs) offers highly sensitive magnetic field sensors. To maximize the effect of MFC, the geometrical design between the MR element and MFCs is critical. In this paper, we present simulation and experimental studies on the effect of the geometrical relationship between current-in-plane giant magnetoresistive (GMR) element and MFCs made of a NiFeCuMo film. Finite element method (FEM) simulations showed that although an overlap between the MFCs and GMR element enhances their magneto-static coupling, it can lead to a loss of magnetoresistance ratio due to a magnetic shielding effect by the MFCs. Therefore, we propose a comb-shaped GMR element with alternate notches and fins. The FEM simulations showed that the fins of the comb-shaped GMR element provide a strong magneto-static coupling with the MFCs, whereas the electric current is confined within the main body of the comb-shaped GMR element, resulting in improved sensitivity. We experimentally demonstrated a higher sensitivity of the comb-shaped GMR sensor (36.5 %/mT) than that of a conventional rectangular GMR sensor (28 %/mT). [ABSTRACT FROM AUTHOR]

Large magnetoresistance (MR) with a linear response to the magnetic field is an important feature in MR devices for magnetic sensor applications. Following the largest MR ratio realized in a current-in-plane (CIP) giant magnetoresistance (GMR) device using a metastable body-centered cubic (bcc)-Cu spacer, we developed a new CIP-GMR device with a combination of two spacer materials, i.e. CoFe/Rh/CoFe (sensing layer)/bcc-Cu/CoFe asymmetric structure and realized a large linear sensitivity due to large MR ratio (21%) and linear response through a biquadratic coupling, high sensitivity of 0.047% mT−1, reduced nonlinearity of 0.98% full scale along with a substantially wider operation field range of 220 mT. Our numerical simulation analyses of magnetization curves clearly establish that the middle CoFe sensing layer is well-controlled by interlayer exchange coupling through the Rh spacer, giving rise to a linear response without losing the high MR ratio through the bcc-Cu spacer. These findings demonstrate that CIP-GMR devices are promising for wide dynamic range magnetic sensors. [ABSTRACT FROM AUTHOR]

We investigated the degree of B2 order (S B2) of 4-10 nm-thick polycrystalline Co 2 (Mn 0.6 Fe 0.4)Ge (CMFG) Heusler alloy films using anomalous X-ray diffraction (AXRD) and its correlation to the magnitude of current-perpendicular-to-plane giant magnetoresistive (CPP-GMR) effect. We found that S B2 of the CMFG films is degraded for the film thickness below 6 nm, which is an issue for practical applications of CPP-GMR devices such as read head sensors of hard disk drives. Since S B2 has a thickness-dependence, the bulk spin polarization (β) of the CMFG layers is also expected to vary depending on the thickness; thus, the deduction of β using the Valet-Fert model, where β is assumed to be thickness-independent, is not valid for these CPP-GMR devices. Image, graphical abstract [ABSTRACT FROM AUTHOR]

We report on the direct observation of the magneto-Peltier effect in a current-in-plane giant magnetoresistive (CIP-GMR) spin valve. By means of the recently developed thermoelectric imaging technique based on lock-in thermography, we demonstrate that thermoelectric cooling and heating are generated by applying a local magnetic field to the CIP-GMR spin-valve film, confirming the different Peltier coefficients of the spin valve between the parallel and antiparallel magnetization configurations. The cooling and heating positions are found to be tuned simply by changing the magnitude of the local magnetic field. This versatile and reconfigurable thermoelectric conversion functionality may provide a thermal management method for CIP-GMR magnetic sensors. [ABSTRACT FROM AUTHOR]

The near-future ultra-high-density magnetic recording with an assist recording technology requires a substantial evolution of the read-head technology. A read sensor with a small sensor resistance-area product ( $RA$ ), a large magnetoresistance ratio ( \Delta R/R$ ), and a thin total sensor thickness is required for the low bit error rate and high-spatial resolution of reading. We developed current-perpendicular-to-plane giant magnetoresistance sensors using highly spin-polarized Heusler alloy ferromagnetic layers with the spin polarization of the conducting electron \beta \sim 0.8$ . A reasonably large \Delta R/R$ up to 14% was obtained by a practical bottom-pinned spin-valve structure with a conventional metallic spacer layer of an Ag90Sn10 (AgSn) alloy. By replacing the spacer layer with a conductive oxide-based AgSn/InZnO bilayer, a large \Delta R/R , which is optimal for the read sensor application. A large voltage output up to 6 mV was obtained for a sensor utilization of ~33% at a bias voltage of 100 mV. [ABSTRACT FROM PUBLISHER]

• CoFe/Rh/CoFe/bcc-Cu/CoFe based epitaxial current-in-plane -GMR devices were fabricated. • The effect of CoFe and Rh thicknesses on the nature of magnetic couplings was studied. • Numerical simulation confirmed both biquadratic and bilinear couplings, offering linear MR. • CIP-GMR devices with the asymmetric structure are promising for the next generation linear magnetic sensors. Giant magnetoresistance (GMR) devices known for their widespread application as magnetic field sensors face challenges owing to their nonlinear resistance response with the magnetic field. Here, we have developed CoFe(3 nm)/Rh(t R h nm)/CoFe(t m i d C o F e nm)/bcc Cu(1.6 nm)/CoFe(t t o p C o F e nm)/MgO(2 nm) based current in plane (CIP)-GMR devices and demonstrated linear MR curves with tunable characteristics. The MR ratio and saturating magnetic field were tunable between 3.9% and 23.5%, and 0.013 T and 1.5 T. The numerical simulation modeling confirmed biquadratic coupling between the bottom and middle CoFe layers through Rh and antiferromagnetic coupling between the middle and top CoFe layers through a Cu spacer, providing the linear MR. The fine-tuning of thicknesses of the middle and top CoFe layers provides supplementary routes for optimizing the sensitivity and operation field range of the CIP-GMR devices to specific magnetic sensor applications in the low and medium magnetic field range. The above results demonstrate that CIP-GMR devices with the asymmetric structure demonstrated in this study are promising for the next generation linear magnetic sensors. [ABSTRACT FROM AUTHOR]