Desmond J. Mapps

Some new horizons in magnetic sensing: high-Tc SQUIDs, GMR and GMI materials

Driven by rapid progress in microelectronics and thin film technologies, magnetic sensors development continues to expand. This paper presents issues related to the principles, categorisation and applications of magnetic sensors. Special attention is paid to two types of sensors and their present and future impact: superconducting quantum interference devices (SQUID) magnetometers with their unsurpassed sensitivity, and giant magnetoresistance (GMR)/giant magnetoimpedance (GMI) based sensors as the most promising technology. A review of recent developments in SQUID technology and a discussion of limitations and aspects that could contribute to the wider acceptance of this technology, are presented. The giant magnetoresistance and giant magnetoimpedance effects have already found applications in magnetic sensing and have promise in other applications. Their unique characteristics and miniaturisation potential have contributed to the rapid acceptance of these technologies. The article describes the principles of the GMR and GMI effects along with recent developments in these technologies particularly in manufacturing techniques and materials.

Asymmetrical magnetoimpedance in as-cast CoFeSiB amorphous wires due to ac bias

Asymmetrical giant magnetoimpedance (AGMI), which utilizes a high frequency bias field hb, is realized in a Co-based amorphous wire having a circumferential anisotropy in the outer region. No asymmetry in the dc magnetic configuration is needed in this case. AGMI is discussed in terms of the surface impedance tensor, demonstrating that the effect of hb is related to the role of the off-diagonal component of the impedance in the voltage response measured across the wire. This effect is important for developing autobiased linear magnetic sensors. Using two oppositely biased wires, a near-linear voltage output (±4 mV) is obtained in the range of ±5 Oe for the sensed dc field at a frequency of 8 MHz.

One-sided fluxes in planar, cylindrical, and spherical magnetized structures

In planar structures, magnetization patterns exist that have the surprising property that all the external fringing field of flux emerges from one side of the structure, with precisely none appearing on the other side. In the last 25 years, this fact has led to a number of useful applications. More recently, the same concept has been found to hold for both cylindrical and spherical hollow structures, where all the fringing flux emerges either inside or outside the structure, with precisely none appearing on the other side. Again, this has led to a number of important practical applications. While many of the concepts have been published previously, this paper contains, for the first time, a uniform mathematical analysis of these one-sided flux configurations. Additionally, the paper reviews the principal applications of one-sided flux structures.

Feasibility of ultra-dense spin-tunneling random access memory

Three-dimensional (3-D) finite element models have been utilized to simulate electromagnetic behaviors in spin-tunneling random access memory (STram). The most significant contributors have been identified. Compared with conventional current-in-plane (CIP) giant magneto-resistive (GMR) memory, whose signal level is inversely proportional to the square root of the storage density, these current-perpendicular-to-plane (CPP) STram elements provide an excellent readout property in that their signal level is independent of their cross-section area. This result is so attractive that the density of STram should not be limited by signal degradation. Moreover, a magnetic flux closure design was found to reduce the crossfeed by about a factor of five, compared with conventional keeperless design, which is the most favored approach for achieving 10/sup 9/ bits/cm/sup 2/ areal density. Although the storage mechanism described in this paper is made of STram, the flux-closure design could be generally applicable to other magnetic solid state memories.

THICKNESS AND GRAIN-SIZE DEPENDENCE OF THE COERCIVITY IN PERMALLOY THIN FILMS

This paper reports the effect of thickness and grain size on the coercivity of Ni81Fe19 permalloy thin films (2.5–30 nm) sputtered on glass substrates for their application in magnetoresistive sensors. Coercivity was systematically investigated as a function of underlayer materials, thickness, and substrate temperature. Lateral grain size of the sputtered films was investigated. The grain size reduced very quickly in the thinner films. It was also found that the coercivity of the films with very small lateral grain size is much lower than those with a normal grain size. The lowest coercivity (Hc=0.8 Oe) was observed in 7.5 nm thick film and having a grain size of 4 nm. When an underlayer is used, its crystallinity affects the lateral grain size in the permalloy and correlates with the observed coercivity variation [P. Galtier, R. Jerome, and T. Valet, Mater. Res. Soc. Symp. Proc. 313, 417 (1993).] It was also observed that the coercivity of the permalloy is dependent on the nature of the underlayer. It wa...

Measurement of field-dependent surface impedance tensor in amorphous wires with circumferential anisotropy

The surface impedance tensor final_sigmâ is found for 120 μm diameter Co-based amorphous wires as a function of a dc axial magnetic field Hex. Contrary to the usual practice of giant magneto-impedance experiments, the voltage response is measured not only across the wire but also in the external coil (secondary coil), and the wire is subjected to an ac current flowing through it or the coil (primary coil), which allows all the components of the impedance tensor final_sigmazz, final_sigmaφφ and final_sigmazφ=final_sigmaφz to be determined. Under certain conditions, they all are very sensitive to Hex (especially final_sigmaφφ: 100–250%/Oe) but exhibit a different field behavior: the diagonal terms final_sigmazz and final_sigmaφφ are symmetrical with respect to Hex whereas the off-diagonal terms final_sigmazφ=final_sigmaφz are antisymmetrical, and at high frequencies, final_sigmazz has a minimum at Hex=0 where final_sigmaφφ has a maximum. The effect of a moderate dc bias current Ib is demonstrated to be favo...

Approximate three-dimensional head fields for perpendicular magnetic recording

This paper proposes a method for obtaining approximate, but very accurate, three-dimensional (3-D) head fields for perpendicular magnetic recording heads. The method uses an assumed form for the scalar magnetic potential variation between a pole or shield and the underlayer to give the potential in the head/shields-face plane. It then uses this approximate air-bearing surface potential to find the potential or field components at any other position of interest. The approach is illustrated here by applying it to a symmetric double-shielded single-pole head with an underlayer and with side shields.

Magnetic Properties Of NdFeB Thin Films On Platinum Underlayers

A series of NdFeB (Nd/sub 12/Fe/sub 82/B/sub 6/) films of different thicknesses were r.f. sputtered on 1000 /spl Aring/-thick Pt underlayers for information storage application. The perpendicular coercivity has a maximum of 3000 Oe for a 1000 /spl Aring/ - thick film annealed at 625/spl deg/C for fifteen minutes. By using Transmission Electron Microscopy (TEM) we have found that the film is tetragonal with an additional fcc structure. The crystallographic texture is established during annealing at 625/spl deg/C. The TEM micrographs show some voids and a grain size in the range 30 to 50 nm. For films annealed at 700/spl deg/C for fifteen minutes, the voids disappeared and the grain size was found to be 100 nm. X-Ray Diffraction analysis of the annealed sample indicates that the NdFeB film is amorphous. Annealing at 625/spl deg/C produces the 2:15:1 phase in the films.

An analysis of a shielded magnetic pole for perpendicular recording

The new solution of an idealized mathematical model of the field due to a symmetrically shielded magnetic pole suitable for perpendicular recording is presented. Accurate Fourier coefficients and corresponding magnetic fields are obtained for a representative pole dimension and a range of pole/shield separation distances, and comparisons are made with the field of an unshielded isolated pole. It is found that the presence of a shield may significantly increase both the peak field strength and the field gradient. The highest field gradients presented are predicted for in-contact recording with certain thin-film pole/shield geometries. >

Exploitation of multilayer coatings for infrared surface plasmon resonance fiber sensors

We demonstrate surface plasmon resonance (SPR) fiber devices based upon ultraviolet inscription of a grating-type structure into both single-layered and multilayered thin films deposited on the flat side of a lapped D-shaped fiber. The single-layered devices were fabricated from germanium, while the multilayered ones comprised layers of germanium, silica, and silver. Some of the devices operated in air with high coupling efficiency in excess of 40 dB and an estimated index sensitivity of Delta lambda/Delta n=90 nm from 1 to 1.15 index range, while others provided an index sensitivity of Delta lambda/Delta n=6790 nm for refractive indices from 1.33 to 1.37.