Albrecht Jander

Magnetoresistive sensors for nondestructive evaluation

New high-sensitivity solid-state magnetoresistive (MR) sensor technologies offer significant advantages in nondestructive evaluation (NDE) systems. A key advantage of MR sensors is a flat frequency response extending from dc to hundreds of MHz, making them particularly attractive for low-frequency and multi-frequency eddy current detection for deep-flaw detection and depth profiling. MR sensors are mass produced by thin film processing techniques similar to integrated circuit manufacturing, dramatically reducing the cost per sensor. The fabrication process is compatible with silicon circuit technology, allowing integration of sensors with on-chip signal processing. MR sensors can easily be produced in dense arrays for rapid, single-pass scanning of large areas. The small size and low power consumption of these solid-state magnetic sensors enable the assembly of compact arrays of sensors on a variety of substrates as well as on-chip sensor arrays. Arrays have been fabricated with sensor spacing as small as 5 μm. This paper presents a review of the state of the art in MR sensors and applications in NDE. The physical principles, manufacturing process, and performance characteristics of the three main types of MR devices, anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR) and tunneling magnetoresistance (TMR) are discussed. Their performance is compared to other magnetic sensor technologies for NDE applications. Finally, we provide a comprehensive review of the literature on NDE applications of MR sensors.

Domain Structure in CoFeB Thin Films With Perpendicular Magnetic Anisotropy

Domain structures in CoFeB-MgO thin films with a perpendicular easy magnetization axis were observed by magneto-optic Kerr-effect microscopy at various temperatures. The domain-wall surface energy was obtained by analyzing the spatial period of the stripe domains and fitting established domain models to the period. In combination with superconducting quantum interference device measurements of magnetization and anisotropy energy, this leads to an estimate of the exchange stiffness and domain-wall width in these films. These parameters are essential for determining whether domain walls will form in patterned structures and devices made of such materials.

Reduction of spin transfer by synthetic antiferromagnets

Synthetic antiferromagnetic layers (SAF) are incorporated into spin transfer nanopillars giving a layer composition [Cobottom/Ru/Cofixed]/Cu/Cofree, where square brackets indicate the SAF. The Cobottom and Cofixed layers are aligned antiparallel (AP) by strong indirect exchange coupling through the Ru spacer. All three magnetic layers are patterned, so this AP alignment reduces undesirable dipole fields on the Cofree layer. Adding the Cobottom/Ru layers reduces the spin polarization of the electron current passing through the nanopillar, leading to a decreased spin-torque per unit current incident on the Cofree layer. This may be advantageous for device applications requiring a reduction of the effects of a spin-torque, such as nanoscale current-perpendicular-to-plane magnetoresistive read heads.

Giant magnetoresistance monitoring of magnetic picodroplets in an integrated microfluidic system

This letter describes the integration of giant magnetoresistance (GMR) sensors with a microfluidic system for the velocity and size monitoring, and enumeration of flowing magnetic entities. We have fabricated a microdevice that enables: (1) controlled formation of picoliter-sized droplets of a ferrofluid separated by a nonmagnetic oil; and (2) continuous-flow sensing of these ferrofluid droplets. It is shown that the flow velocity, droplet size, and droplet-formation frequency can readily be determined from the GMR response. These results are validated by comparisons to fluorescence microscopy data.

Chopping techniques for low-frequency nanotesla spin-dependent tunneling field sensors

Three chopping techniques to address 1/f noise in spin-dependent tunneling magnetoresistive sensors are investigated. These include modulation of the sensitivity using orthogonal fields, modulation, and second-harmonic generation using the nonlinear response of the magnetoresistive element and modulation of the flux concentrator permeability. Of these, only the second technique resulted in a slight reduction in low-frequency noise. In order to achieve significant noise reduction by chopping, domain noise will have to be reduced.

Impact of new magnetoresistive materials on magnetic recording heads (invited)

Advances in magnetoresistive materials have recently enabled magnetic recording heads to achieve higher levels of performance. This article describes why higher signal outputs are necessary for improvements to be made in areal density. The requirements for recording at an areal density of 16 Mb/mm2 (10 Gb/in.2) are discussed with regards to both the channel and the head design. Increased output from new multilayer magnetoresistive materials is required to counteract the decrease in output due to the reduction in the size of the head geometry. An areal density of 16 Mb/mm2 is shown to be feasible with spin valve recording heads using materials with magnetoresistance ratios of 10%. Fabrication issues relating to the manufacturing of these materials are shown to be more stringent than previously required.

A model for predicting heating of magnetoresistive heads

A simplified geometric model is presented for analyzing the thermal conduction problem in magnetoresistive heads. The model leads to an approximate analytical solution for the thermal resistance as a function of the key geometric and material parameters. The model reveals trends that will be helpful in designing the next generation of high resolution recording heads.

3-Axis Magnetometers Using Spin Dependent Tunneling: Reduced Size and Power

A 3-axis magnetometer has been constructed using 3 Spin Dependent Tunneling (SDT) magnetic field sensors as transducers. This magnetometer has been designed for use in Unattended Ground Sensor (UGS) applications. As such, there has been an emphasis on low cost, size, and power. The present version is smaller than previous versions, and is ready for prototype sampling. This paper describes the basic properties of the SDT 3-axis magnetometer, including size, power, and noise floor.

Inkjet printing of magnetic materials with aligned anisotropy

3-D printing processes, which use drop-on-demand inkjet printheads, have great potential in designing and prototyping magnetic materials. Unlike conventional deposition and lithography, magnetic particles in the printing ink can be aligned by an external magnetic field to achieve both high permeability and low hysteresis losses, enabling prototyping and development of novel magnetic composite materials and components, e.g., for inductor and antennae applications. In this work, we report an inkjet printing technique with magnetic alignment capability. Magnetic films with and without particle alignment are printed, and their magnetic properties are compared. In the alignment-induced hard axis direction, an increase in high frequency permeability and a decrease in hysteresis losses are observed. Our results suggest that unique magnetic structures with arbitrary controllable anisotropy, not feasible otherwise, may be fabricated via inkjet printing.

Surface Acoustic Wave Magnetic Sensor using Galfenol Thin Film

The performance of a surface acoustic wave (SAW) magnetic field sensor using galfenol (FeGa) thin film is investigated in this effort. We measure the change in the SAW velocity as a function of an externally applied magnetic field for galfenol films of varying thicknesses. A maximum change of 0.64%, greater than results reported for similar sensors, is obtained with a 500 nm thick film.