magnetic particles detection by using spin valve sensors and magnetic traps

Due to the device design, the magnetic field created by the current lines allows the control of the sensor operating point, to magnetize the particles and also to concentrate the particl

Magnetic particles detection by using spin valve sensors and magnetic traps

Andrei Jitariu, Crina Ghemes, Nicoleta Lupu, and Horia Chiriac

Citation: AIP Advances 7, 056616 (2017); doi: 10.1063/1.4973745

View online: http://dx.doi.org/10.1063/1.4973745

View Table of Contents: http://aip.scitation.org/toc/adv/7/5

Published by the American Institute of Physics

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of not requiring an external magnetic field source such as permanent magnet to magnetize the particles or to bias the spin-valve sensor Due to the device design, the magnetic field created by the current lines allows the control of the sensor operating point, to magnetize the particles and also to concentrate the particles in the sensor active area in order to be detected by the spin valve sensor Detection experiments using FeCrNbB magnetic particles show that the device is capable

to detect and quantify the particles in a linear scale over a concentration range

of 0.1 to 1 mg/ml © 2017 Author(s) All article content, except where

other-wise noted, is licensed under a Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/ ) [http://dx.doi.org/10.1063/1.4973745]

I INTRODUCTION

Magnetoresistive devices were proposed and continuously developed nowadays to be used as biosensors for biological applications.1 4 For these applications, the targeting biomolecules are labeled with magnetic micro or nanoparticles, captured by target-probe biomolecular recognition and then detected by magnetoresistive sensors To be successfully used in biosensing applications,

a magnetoresistive sensor must be capable to detect low particles concentrations and to quantify them in a linear scale Magnetic traps were proposed to be used in order to facilitate the magnetic transport and to concentrate them over the sensing region to maximize the target-probe hybridization and detection efficiency.5 9 Most of these devices use a combination of AC currents through the current lines and external magnetic fields sources (such as permanent magnets or coils) to trap and magnetize the particles in order to be detected by the MR sensors, which increase the final device size and complicates the setup, requiring more complex electronics and signal processing

We are proposing a simple magnetoresistive device with integrated current lines as magnetic traps for magnetic particles transport, concentration and detection Moreover, due to the device design, the magnetic field created by the current lines is also used to bias the sensor and to magnetize the particles Our device has the advantage of not requiring any other external magnetic field source to magnetize the particles or to bias the spin-valve sensor By tuning the DC current intensity through the current lines, the sensor operating point can be tuned in order to bring the sensor in the maximum sensitivity point Measurements using FeCrNbB particles were performed to demonstrate the device capability to concentrate and detect magnetic particles Different magnetic particles concentrations were tested in order to investigate the sensor detection limits

II MAGNETORESISTIVE DEVICE MICROFABRICATION

Spin-valves with the multilayer structure of: Ta (2 nm)/NiFe (5 nm)/CoFe (2 nm)/Cu (2.7 nm)/CoFe (3 nm)/IrMn (10 nm)/Ta (2 nm) were deposited by rf/dc magnetron on an 18×18 mm2 Si/SiO2substrate Using DWL (direct write laser) lithography and lift-off, the spin-valves were pat-terned in rectangular stripes with the lateral dimensions of 150 µm × 3 µm The spin-valves stripes

056616-2 Jitariu et al. AIP Advances 7, 056616 (2017)

FIG 1 Design of the microfabricated chip (a) and a optical picture of the sensor active area (b).

were defined with the exchange bias direction (sensor sensitive axis) along the short axis of the stripe On the same substrate, 8 individual spin-valves sensors were microfabricated using the design presented in figure1(a) The sensors electrical contacts were defined by lithography, Al (100 nm) deposition and lift-off After electrical contact fabrication a 200 nm thick SiO2insulating layer was deposited on the chip surface except the contact pads area The magnetic traps that consists in current lines defined by DWL lithography, Al (300 nm) deposition and lift-off are positioned on the top of the spin valve sensors as indicated in figure1 In the sensor area, the current lines have a width of

6 µm, therefore, the spin valve sensors (with a width of 3 µm) are fully covered by the Al current lines This design assures the accumulation of the magnetic particles in the sensor region In the end,

in order to protect the chip from corrosion, a final passivation layer of SiO2(200 nm) was deposited

on the chip surface Characterization of the spin-valve sensors was performed measuring the sensor output voltage with a bias current of 1 mA During the measurements, the magnetic field was applied along the short axis of the spin-valve stripe (sensor sensitive direction)

III RESULTS AND DISCUSSIONS

The fabricated sensors show a hysteresis-free response and a magnetoresistive ratio of 7.1 % From the sensor transfer curve, presented in figure2(a), we can observe a shift of the transfer curve

to a magnetic field of 34 Oe This offset arises from the ferromagnetic coupling between the free and pinned layer and can be tailored by tuning the Cu layer thickness.10Moreover, we can also observe that the maximum sensitivity (0.88 mv/Oe) is obtained for a magnetic field of 34 Oe while a much lower sensitivity (0.22 mv/Oe) is obtained in zero magnetic fields As a result, in order to bring the spin-valve sensor to its most sensitive operating point we need to compensate the offset field

of the sensor This can be achieved using the magnetic field created by the current line By passing

FIG 2 Sensor output voltage and sensitivity dependence on the applied magnetic field (a) Normalized MR curves for different current intensities through the current line (b) and sensor offset field dependence on the current intensity (inset).

also set to 60 mA.

Due to the device design, the magnetic field created by the current line is also used to magnetize the particles and to attract them to the sensor region to be detected by the spin valve sensor To demonstrate the sensor functionality, detection experiments were performed using FeCrNbB particles having a mean diameter of 1 µm Due to their relatively high saturation magnetization and soft magnetic properties, these particles, initially developed for magnetic hyperthermia,11might be also used as magnetic labels in biodetection applications The magnetic particles were dispersed in water

by ultrasonication obtaining solutions with concentrations between 0.1 mg/ml and 10 mg/ml For detection measurements the sensor output voltage was recorded with a time step of 1 second After a waiting time of 100 seconds a volume of 1µL from the prepared solutions was drop-casted over the sensor active area and the sensor output voltage was monitored further

An example of a typical curve obtained for a 0.1 mg/ml magnetic particles concentration is presented in figure 3 We can observe that as soon as the droplet is placed on the sensor surface (100 s), the sensor output voltage starts to increase till a given moment (tsat) when it saturates The increase of the sensor signal is associated to magnetic particles accumulation in the sensor region, being attracted by the magnetic field gradient created by the current line Accumulation of the particles

to the sensor position can be also observed in the SEM images obtained after the natural drying of the droplet (figure3(b)) It has to be mentioned that the particles far from the sensor position cannot be attracted by the magnetic field gradient, therefore, the saturation of the sensor signal can be explained taking into account that only the particles at a certain distance from sensor position can be attracted

to the sensor area As can be seen in the inset of the fig3(b), in the sensor vicinity, most of the particles were collected by the current line As a result, no more particles can reach the sensor area

to contribute to the total fringe field sensed by the sensor, leading to the sensor signal saturation

In order to determine the detection limits of the device, measurements for various magnetic particles concentration were performed The sensor output voltage measurements results for concen-trations from 0.1 mg/ml to 10 mg/ml are presented in figure4(a) Regardless of the magnetic particle

FIG 3 Sensor output voltage versus time trace for a 0.1 mg/ml magnetic particles concentration (a) and SEM images of the sensor after the measurement (b) A higher magnification image of the sensor region is presented in the inset.

056616-4 Jitariu et al. AIP Advances 7, 056616 (2017)

FIG 4 Sensor output voltage versus time trace (a) and optical microscopy images of the sensor area after measurements with different concentrations (b) Sensor output voltage change (c) and saturation time (d) dependence on particles concentration.

concentration, the sensor signal shows the same behavior as mentioned above: the sensor voltage start to increase when the droplet is placed on the sensor surface and then it saturates However, the voltage change (∆V) and the sensor signal saturation time are highly dependent on magnetic particles concentration The voltage change dependence on the magnetic particles concentration is shown in figure4(c) For low concentration (0.1 – 1 mg/ml), a linear dependence of the output voltage on the particle concentration can be observed, followed by a saturation tendency of the sensor signal for higher concentrations (1 – 10 mg/ml) This behavior can be explained taking into account the sensor covered area with magnetic particles As can be observed from the optical picture of the sensor after measurements with 0.1, 1 and 10 mg/ml, presented in figure4(b), different coverage of the current line in sensor region are obtained depending on particles concentration The increase of the sensor covered area as the particles concentration increase can be understood if we consider that the magnetic field gradient created by the current line is able to collect only the particles from a certain area around the sensor position and by increasing the particle concentration, the particles density on the same area is increasing As can be seen from figure4(b), for the 0.1 mg/ml particles concentration, the current line surface is partially covered with magnetic particles while for a concentration of 1 mg/ml the surface is almost fully covered with magnetic particles Since the total magnetic field sensed by the spin-valve sensor is proportional with the number of particles on the sensor surface, the sensor output voltage will increase linearly with the particles concentration For concentrations higher than 1 mg/ml, this linear dependence is not valid anymore By increasing the concentration further, particles will agglomerate in the outside area of the sensor, as can be observed in figure4(b)for the case of a

10 mg/ml concentration These particles will have a lower contribution on the total field sensed by the sensor, as a result, the sensor signal will tend to saturate at the same value, regardless of particles concentration, as the sensor surface tends to be fully covered by magnetic particles

As we mentioned above, the time required for the sensor signal to saturate (∆ts) is also highly dependent on the magnetic particles concentration Figure4(d)shows the saturation time dependence

on the magnetic particle concentration Here the ∆tsis considered the time between the moment of droplet placing (t=100 s) and signal saturation moment (tsat) We can observe a decrease of the saturation time as the particle concentration increases The sensor signal saturation time depends

on how fast the particles near the sensor region are collected by the current line and how fast the sensor area is covered by particles It is straightforward to understand that for higher concentration the sensor area will be rapidly covered by particles leading to short signal saturation time while by for low concentrations a longer time will be required for the particles to be collected to the sensor area

IV CONCLUSION

A magnetoresistive device with integrated magnetic traps that use only the magnetic field created

by a current line to concentrate, to magnetize the particles and to bias the spin valve sensor to its

Research and Innovation in the framework of NUCLEU Programme, project PN 16 37 02 02.

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