Noise Characterization of Vortex-State GMR Sensors with Different Free Layer Thicknesses
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The spin valve principle is the most prominent sensor design among giant- (GMR) and tunneling (TMR) magnetoresistive sensors. A new sensor concept with a disk shaped free layer enables the formation of a flux-closed vortex magnetization state if a certain relation of thickness to diameter is given. The low frequency noise of current-in-plane GMR sensing elements with different free layer thicknesses at different external field strengths has been measured. The measurements of the 1/f noise in external fields enabled a separation of magnetic and electric noise contributions. It has been shown that while the sensitivity is increasing with a decreasing element thickness, the pink noise contribution is increasing too. Still the detection limit at low frequencies is better in thinner free layer elements due to the higher sensitivity.Keywords:
Spin valve
Giant magnetoimpedance
The magnetoresistance effect, especially the giant magnetoresistance (GMR) effect, has received much attention in recent years. In this study, we discuss the magnetoresistance behavior in Cu–Ni–Fe thin films with Cu content varied from 40 to 90 at.%, prepared by the cosputtering of both Cu and Fe50Ni50 targets. Films with low Cu content, for example, Cu50Ni25Fe25 and Cu40Ni30Fe30, exhibit a mixed behavior of GMR and anisotropic magnetoresistance (AMR). The electrical resistivity of these films substantially increases once the field is applied due to the anisotropic magnetoresistance contribution, and then decreases again at higher fields, which is believed to be related to the giant magnetoresistance effect. As a result of a compromise between both the GMR and the AMR effects, the MR ratios of these low Cu content films are only minus 1%–2% both at room temperature and at 4.2 K. However, it is found that the giant magnetoresistance contribution dominates magnetoresistance behavior in films with Cu content higher than 50 at.%. There exists a large drop in resistance at low fields followed by a long tail at high fields in the MR curves for these high Cu content films. The MR ratios of these films show an increasing tendency as temperature decreases, for example, from 3% at room temperature to 11% at 4.2 K for Cu90Ni5Fe5 film. The GMR effect in these high Cu content films is ascribed to the spin-dependent scattering at the two-phase interface and in the ferromagnetic phase(s), similar to that of the GMR in multilayers, although the contribution from the magnetic fluctuations cannot be excluded.
Colossal Magnetoresistance
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By extending a previous semi-classical model, we investigate the effects of inserting a second ferromagnetic material at the interface or in the interior of the ferromagnetic layer in NiFe/Cu/NiFe and/or Co/Cu/Co sandwiched structures on their giant magnetoresistance (GMR). The calculated GMR was found to be consistent with experimental results, indicating that our model is applicable even for complicated spin-valve multilayers. Moreover, some theoretical predictions are given in this letter that allow us to propose ways to optimize spin-valve structures.
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The giant magnetoimpedance (GMI) and giant magnetoresistance (GMR) of amorphous ribbon/Cu/amorphous ribbon trilayer microstructures, based on Metglas™ 2714a ribbons and Cu foils, is measured and analyzed. We obtain GMI and GMR ratios of 830% and 2630%, respectively, in the 0.2–20 MHz frequency range. These very large GMI and GMR values are a direct consequence of the large effective relative permeability due to the closed magnetic flux path in the trilayer structure. We study the effect of magnetocrystalline and shape anisotropy, and analyze our experimental results in terms of the model of Makhnovskiy et al. [Sens. Actuators 81, 91 (2000)].
Giant magnetoimpedance
Metglas
Ribbon
Magnetocrystalline anisotropy
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The giant magnetoresistance (GMR) effect has extensively been used to manufacture magnetic recording heads. such as spin valve sensors. A conventional spin valve sensor consists of a sandwich structure, i.e., a free layer, a spacer, and a pinned layer, whose pinning field is achieved by exchange coupling with an antiferromagnetic layer.
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A new sensitive magnetic sensor was developed by utilizing the giant magnetoresistance effect of spin-valve film. The sensor element was composed of a spin-valve layer patterned to a micro size, and a bias current layer which generates a square wave AC magnetic field. High sensitivity was attained using only the edge part of the step shape MR loop.
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Giant magnetoimpedance
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NiMn is an interesting material for achieving a high exchange bias in spin valve systems. We investigated the influence of a nano-oxide layer (NOL) inserted in the pinned Co layer on the magnetotransport properties of NiMn/Co/Cu/Co spin valve sensors. The samples were annealed at 350 °C for 10 min to achieve the antiferromagnetic L10 ordered structure of NiMn. The NOL has been characterized by small angle x-ray reflectivity, transmission electron microscopy (TEM), and energy filtered TEM. The inclusion of the NOL leads to an increase in the giant magnetoresistance (GMR) by 20 % indicating a high degree of specular reflection at the NOL. For NOL positions close to the NiMn/Co interface, a decrease in the exchange bias field (Hex) is observed. The best combination of high GMR value and large Hex was found when the NOL was inserted in the center of the pinned Co layer.
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Recently it has been found that metallic multilayer films exhibit novel phenomena, particularly giant magnetoresistance in Fe/Cr, Co/Cu, Co/Cu/NiFe/Cu, and other multilayers. In this review, we explicate the theory of the giant magnetoresistance effect, placing emphasis on its mechanism and dependence on the constituent materials of multilayers. Some relations between the giant magnetoresistance, the electrical resistivity of the magnetic alloys, and the anisotropic magnetoresistance are also discussed.
Colossal Magnetoresistance
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The giant magnetoimpedance effect of the nanocrystalline ribbon Fe84Zr2.08Nb1.92Cu1B11 (atom fraction in %) was investigated. There is an optimum annealing temperature (TA≈ 998 K) for obtaining the largest GMI (giant magneto impedance) effect in the ribbon Fes4Zr2.08Nb1.92Cu1B 11. The ribbon with longer ribbon length has stronger GMI effect, which may be connected with the demagnetization effect of samples. The frequencyfmax, where the maximum magnetoimpedance GMI(Z)max = [(Z(H) - Z(0))/Z(0)]max occurs, is near the intersecting frequency fi of the curves of GMI(R), GMI(X), and GMI(Z) versus frequency. The magnetoreactance GMI(X) decreases monotonically with increasing frequency, which may be due to the decrease of permeability. In contrast, with the AC (alternating current) frequency increasing, the magnetore sistance GMI(R) increases at first, undergoes a peak, and under then drops. The increase of the magnetoresistance may result from the enhancement of the skin effect with frequency. The maximum magnetoimpedance value GMI(Z)max under H = 7.2 kA/m is about -56.18% at f= 0.3 MHz for the nanocrystalline ribbon Fe84Zr2.08Nb1.92Cu1B11 with the annealing temperature TA= 998 K and the ribbon length L = 6 cm.
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Nanocrystalline material
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