Mark Tondra

Low-frequency noise measurements on commercial magnetoresistive magnetic field sensors

Low-frequency noise was measured in the frequency range from 0.1Hzto10kHz on a variety of commercially available magnetic sensors. The types of sensors investigated include anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), and tunnel magnetoresistance (TMR) effect devices. The 1∕f noise components of electronic and magnetic origin are identified by measuring sensor noise and sensitivity at various applied magnetic fields. Commercial magnetometers typically consist of four elements in a Wheatstone bridge configuration and are biased with either a constant voltage or current. Voltage fluctuations at the sensor output are amplified by a pair of battery powered low-noise preamplifiers and input to a spectrum analyzer. A two-channel cross-correlation technique is used when the performance of a single preamplifier is not sufficient. For the AMR and GMR sensors investigated, both electronic and magnetic components contribute to the overall sensor noise. Maximum noise occurs at the bias field wh...

Picotesla field sensor design using spin-dependent tunneling devices

Pinned spin-dependent tunneling devices were fabricated and tested in a mode suited for low-field sensing. The basic structure of the devices was NiFeCo125/Al2O325/CoFe70/Ru9/CoFe70/ FeMn125 (in A). This structure had a tunneling resistivity of 110 MΩ μm2 and exhibited a 20% magnetoresistance when a field was swept along the easy direction of the soft electrode. High sensitivity, low hysteresis operation was achieved by applying a bias field orthogonal to the easy axis. A sensitivity of 3%/Oe with negligible hysteresis was observed using this mode of operation. A sensor using this type of material was designed to achieve a minimum resolvable field in the picotesla range. The sensor consists of a bridge with four elements, each having 16 tunnel junctions in series. A signal-to-noise ratio of 1:1 at 1 pT (10−8 Oe) is possible assuming achievable values for the tunneling resistivity, device size, bias level, and sensitivity.

Model for detection of immobilized superparamagnetic nanosphere assay labels using giant magnetoresistive sensors

Commercially available superparamagnetic nanospheres are commonly used in a wide range of biological applications, particularly in magnetically assisted separations. A new and potentially significant technology involves the use of these particles as labels in magnetoresistive assay applications. In these assays, magnetic bead labels are used like fluorescent labels except that the beads are excited and detected with magnetic fields rather than with photons. A major advantage of this technique is that the means for excitation and detection are easily integrable on a silicon circuit. A preliminary study of this technique demonstrated its basic feasibility, and projected a sensitivity of better than 10−12 molar [Baselt et al., Biosensors Bioelectronic 13, 731 (1998)]. In this article we examine the theoretical signal to noise ratio of this type of assay for the special case of a single magnetic bead being detected by a single giant magnetoresistive (GMR) detector. Assuming experimentally observed and reasona...

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.

Cobalt ferrite nanoparticles: Achieving the superparamagnetic limit by chemical reduction

An unanticipated superparamagnetic response has been observed in cobalt ferrite materials after thermal treatment under inert atmosphere. Cobalt ferrite particles were prepared via normal micelle precipitation that typically yields CoxFe3−xO4 nanoparticles (x=0.6−1.0). While samples thermally treated under oxygen show majority spinel phase formation, annealing in nitrogen gas yields materials consisting of Co-Fe alloy, FeS, and CoFe2O4 spinel. After thermal treatment, thermomagnetic studies reveal composition-insensitive, but highly treatment-sensitive, saturation magnetization, coercivity, blocking temperature, and Verwey transition temperature dependence. Extremely high saturation magnetization (159 emu/g) with low coercivity (31 Oe) was observed for one of the treated compositions, which drastically deviates from prototypical cobalt ferrite with large magnetocrystalline anisotropy. We attribute such unique magnetic response to Co-Fe alloy coexisting with FeS and CoFe2O4 spinel where the diameter of the...

Micromagnetic design of spin dependent tunnel junctions for optimized sensing performance

Pinned spin dependent tunneling devices have been fabricated into high sensitivity magnetic field sensors with many favorable properties including high sensitivity (∼10 μOe/Hz at 1 Hz and ∼100 nOe/Hz at >10 kHz), a linear bipolar output versus applied field, high processing yields, and high temperature stability and operability (over 200 °C). However, the performance of fabricated sensors has not yet approached the theoretical limit one calculates assuming ideal behavior of the sensors’ ferromagnetic layers’ magnetizations. Given a total magnetoresistive signal of 30%, and typical anisotropy fields and hard axis biasing conditions, there should be a region of linear nonhysteretic response at zero field with a slope of greater than 20%/Oe. Measured responses are 1%–3%/Oe, and exhibit some hysteresis. These less than desirable effects are the result of several factors including: (1) Self-demagnetizing fields of the soft (sensing) layer; (2) stray fields from the hard (pinned) layer; (3) imperfect pinning of...

Design of integrated microfluidic device for sorting magnetic beads in biological assays

On-chip manipulation of nanoscopic magnetic particles in microfluidic channels has been demonstrated. The particles, obtained from Bangs Labs, were 460 nm in diameter and contained 12% ferrite. They were moving in a 12 /spl mu/m wide by 4 /spl mu/m deep channel etched into a deposited layer of silicon nitride. An Al current strap was buried beneath the channel and shaped in such a way to generate an in-plane cross-channel field gradient when it was energized by an electrical current. An in-plane external field of 20 kA/m was applied in order to magnetize the particles. They were observed to move across the channel as either the current or field polarity was changed.

Pinhole analysis in magnetic tunnel junctions

Pinholes in the insulating layer of magnetic tunnel junctions are local shortcuts and cause malfunction of such devices. The need for reduction of the tunnel resistance by reduction of the insulator thickness will make this problem even more severe. Therefore, the development of low-resistance magnetic tunnel junctions requires analyzing the pinhole density. We developed a method for pinhole imaging using electrodeposition of copper. Selective nucleation at pinholes produces characteristic structures that can be visualized by conventional microscopy techniques. The experimental conditions were carefully chosen in order to avoid uncontrolled damage of the insulator layer.

Fabrication of BioInspired Inorganic Nanocilia Sensors

In nature, microscale hair-like projections called cilia are used ubiquitously for both sensing and motility. In this paper, biomimetic nanoscale cilia arrays have been fabricated through templated growth of Co in anodized aluminum oxide. The motion of arrays of Co cilia was then detected using magnetic sensors. These signals were used to prove the feasibility of two types of sensors: flow sensors and vibration sensors. The flow sensors were tested in a microfluidic channel. They showed the ability to detect flows from 0.5 ml/min to 6 ml/min with a signal to noise (SNR) of 44 using only 140 μW of power and no amplification. The vibration sensors were tested using a shake table in the low earthquake-like frequency range of 1-5 Hz. The vibration response was a mW signal at twice the frequency of the shake table.

Spin dependent tunnel/spin-valve devices with different pinning structures made by photolithography

Spin dependent tunnel and spin-valve devices were made using rf diode sputtering, with patterning done using standard semiconductor photolithography techniques. In order to tailor the pinning strength of the hard magnetic layers, three types of structures were tried: (1) NiFeCo/spacer/CoFe; (2) NiFeCo/spacer/CoFe/IrMn; and (3) NiFeCo/spacer/CoFe/Ru/CoFe/FeMn, with Al2O3 or Cu as spacers. The magnetoresistance of the spin dependent tunnel devices is up to 24% with a switching field of a few Oe for the free layer of NiFeCo. The saturation fields of the hard layers are a few tens, a few hundreds, and a few thousands of Oe for the three structures, respectively. The first structure is suitable for magnetic memory applications with the hard layer storing the information. The second structure is suitable for magnetic field sensors which must function after relatively high magnetic field excursions. The third structure makes use of the synthetic antiferromagnet of CoFe/Ru/CoFe in addition to the antiferromagnet (FeMn) to achieve the highest pinning field. It also reduces the fringing field to the free layer caused by the pinned layer, due to the flux closure of the two ferromagnetic layers in the synthetic antiferromagnet. This third structure is especially suitable for field sensor applications in environments with excursions of very high magnetic fields between sensing operations.Spin dependent tunnel and spin-valve devices were made using rf diode sputtering, with patterning done using standard semiconductor photolithography techniques. In order to tailor the pinning strength of the hard magnetic layers, three types of structures were tried: (1) NiFeCo/spacer/CoFe; (2) NiFeCo/spacer/CoFe/IrMn; and (3) NiFeCo/spacer/CoFe/Ru/CoFe/FeMn, with Al2O3 or Cu as spacers. The magnetoresistance of the spin dependent tunnel devices is up to 24% with a switching field of a few Oe for the free layer of NiFeCo. The saturation fields of the hard layers are a few tens, a few hundreds, and a few thousands of Oe for the three structures, respectively. The first structure is suitable for magnetic memory applications with the hard layer storing the information. The second structure is suitable for magnetic field sensors which must function after relatively high magnetic field excursions. The third structure makes use of the synthetic antiferromagnet of CoFe/Ru/CoFe in addition to the antiferromagnet ...