**2. Principle of magnetic sensors**

perform numerous tasks automatically by way of sensitively and specifically detecting multiple targets from unprocessed sample material, thus creating a compact instrument in

With the phenomenal success of GMR-spin valve sensors and MTJ sensors in hard disc drives and magnetic memories, they have become an inspiration for testing their use in other areas including that of magnetic biodetection. Obviously, GMR and MTJ sensors take pride in finding themselves as one of the most widely investigated magnetic sensors for bioapplications [12-13]. They are also successful biosensors commercially as they offer high sensitivities, flexible sensor geometries and large bead-to-sensor ratio with well es‐ tablished integrated circuit fabrication technology. However, relatively low signal to noise ratio of these sensors may often leave scope for erroneous detection. The AMR sensors, in turn, offer greater ease of fabrication but the sensitivity of the AMR signal measured along the longitudinal direction is, however, limited by Johnson noise originating from thermal fluctuations at high frequencies, and by temperature drift at low frequencies [1]. However, the flaws associated with longitudinal AMR measurements can be greatly im‐ proved by measuring the voltage change in the transverse direction instead, a phenomen‐ on known as the planar Hall effect [14]. It has been shown that by using the PHE, the temperature drift was reduced by at least 4 orders of magnitude, and nano-Tesla sensitivi‐ ty has been exhibited [15]. In addition, compared with longitudinal AMR signals, PHE signals are more sensitive to local spin configuration and have much lower background

We propose here a planar Hall sensor array in exchange biased multilayer structure and demonstrate the performance of the sensor with the capability of detection of a single mag‐ netic bead. Also, the sensor is further shown to be capable of single biomolecule detection. Following a brief introduction on the need for exploring magnetic sensors, the book chapter describes the principle of magnetic sensing and highlights the merits of planar Hall sensor

In the experimental parts, the details of the general procedure for fabrication sequence of the sensor, its characterization and microarray integration were described. Subsequently, an ac‐ count on the theory and experiments of bead detection using planar Hall resistance (PHR) sensor in different multilayer structures and geometries leading to a complete evolution of novel PHR sensors is elaborately presented in the fourth section. Nevertheless, a hybrid AMR-PHR sensor in ring geometry has been identified for optimum sensor performance to‐

In the fifth section, apart from a brief description on the magnetic beads and their function‐ alization, a description of sensor performance and its capability for detection of magnetic beads including a single magnetic bead is given. This section also presents an account on the integration of microarray sensors with the aid of microfluidics for performing biomolecule experiments while showing the possibility of the planar Hall sensor for a sensitive detection of even single biomolecule. And, finally, it concludes the processes involved with a specific

in terms of field sensitivity and resolution in the second section.

mention on future trends to cater the needs of the society in general.

the form of ''lab-on-a-chip''.

198 State of the Art in Biosensors - General Aspects

voltage as well.

wards the end of this section.

The principle of detection employed by the magnetic sensors for magnetic bioassay in‐ volves a magnetic transduction mechanism which uses the magnetic micro- or nanoparti‐ cles as labels. The biomolecules are commonly detected by attaching them to highly specific magnetic labels that can, upon their binding, produce an observable quantitative electrical signal, as compared to the cumbersome detection of light signal from fluorescent labels. The specificity is traditionally achieved through a biomolecular recognition mecha‐ nism, such as antigen-antibody affinity which can be accomplished by label functionaliza‐ tion, as demonstrated in Fig.1.

**Figure 1.** Procedure for the immobilization of probe molecule on the sensor surface and hybridization of the target molecules through Streptavidin coated Dynabeads.

Magnetoresistance (MR) is the property of magnetic materials that results in a change of re‐ sistance with applied magnetic field. The MR materials are being developed for the applica‐ tions such as hard-disk read heads, magnetic random access memories and magnetic field sensors. The materials that display MR property with desired characteristics can replace the inductive coil sensors in a variety of applications including that of biomolecule recognition.

#### **2.1. Magnetoresistive materials for sensing**

Thin films and multilayer structures in different geometries show MR characteristics specific to their geometry and all these structures are found to be suitable for one or the other appli‐ cations. The ferromagnetic single layer exhibits an AMR which is measured in the current direction, while the planner Hall resistance (PHR) effect can be measured in current perpen‐ dicular direction in AMR materials [16]. The AMR and PHR effects are due to the anisotrop‐ ic magnetoresistivity in ferromagnetic layers. The magnetic thin film multilayer structures can give giant magnetoresistance [17] and tunneling magnetoresistance [18-19] effects and the MR property of these structures are superior to that of the AMR structure. The GMR structure consists of two layers of ferromagnetic metal separated by ultra-thin non-magnetic metal spacer layers. The TMR structures are similar to GMR except that they utilize an ultrathin insulating layer to separate two magnetic layers rather than a conductor. The GMR and TMR effects occur mainly due to the spin-dependent scattering as the current passes from one layer to the other through the spacer layer. The usual figure of merit of the MR ratio is traditionally defined as

$$\text{MR(\%)} = \frac{R\_{\text{max}} - R\_{\text{min}}}{R\_{\text{min}}} \times 100 \tag{1}$$

where *R*max and *R*min are the maximum and minimum resistance, respectively. The AMR ma‐ terials typically have MR ratios about 2 - 6 %, and GMR structures exhibit 10 - 50 % while the TMR structures commonly can achieve over 200 % of MR ratio using MgO tunnel barrier instead of the usual Al2O3.

A majority of the studies in MR effect in thin films are devoted to the research of multilay‐ ered structures showing the largest possible sensitivity of the resistivity for the magnetic field, and consequently a large number of transition metal-based multilayered structures ex‐ hibiting large MR ratios have been found. In connection with the technological problems to be solved, this book chapter devotes to a number of MR sensor designs using the planar Hall effect and tested to linearize the transducer signal, to enhance the resolution limited by the MR ratio.

#### **2.2. Relevant sensor characteristics**

Though there is a wide choice for sensor designs, optimum sensor performance in each de‐ sign can be ensured only when specific sensor characteristics are satisfactorily addressed. Among these characteristics, the field sensitivity and the sensor resolution are of utmost concern particularly for a PHR sensor and, thus, they are considered to be described here for elucidating their importance.

#### *2.2.1. Field sensitivity*

The field sensitivity of PHR sensor, *i.e*., the differential of measured PHR voltage versus ap‐ plied field, can be obtained as

$$\frac{\partial V\_{\rm PHR}}{\partial H} = \frac{I(\rho\_{\parallel} - \rho\_{\perp})}{t} \frac{\cos 2\theta \sin(\chi - \theta)}{H\_{\rm K} \cos 2\theta + H\_{\rm ex} \cos \theta + H \cos(\chi - \theta)}\tag{2}$$

where *I* is the current passing through the sensing layer, *t* is the thickness of the ferromag‐ netic layer, *γ* is the angle between applied magnetic field and easy axis, *H*K is the uniaxial anisotropy field and *ρ*// and *ρ*⊥ are the resistivity parallel and perpendicular to the magneti‐ zation, respectively. In Eq. (2), the field sensitivity depends not only on the intrinsic parame‐ ters *Δρ= ρ// - ρ HK*, and exchange bias field (*H*ex), but also on the extrinsic parameters such as the magnetization angle at an instant applied field, *θ,* and the applied magnetic field *H*.

In a typical curve of PHR signal with applied magnetic field, the maximum field sensitivity of PHR sensor is appeared at low (near zero) magnetic fields. Therefore, the PHR sensor can be used as low magnetic field sensors. Also, the PHR signal does not depend on the sensor size such as the width and length, and therefore, the micro or nano meter order of sensor size is possible by maintaining the same output signal. Thus, the PHR sensor is one of the good candidate bio-sensors for the micro- or nano- bead detector.

#### *2.2.2. Resolution (S/N)*

direction, while the planner Hall resistance (PHR) effect can be measured in current perpen‐ dicular direction in AMR materials [16]. The AMR and PHR effects are due to the anisotrop‐ ic magnetoresistivity in ferromagnetic layers. The magnetic thin film multilayer structures can give giant magnetoresistance [17] and tunneling magnetoresistance [18-19] effects and the MR property of these structures are superior to that of the AMR structure. The GMR structure consists of two layers of ferromagnetic metal separated by ultra-thin non-magnetic metal spacer layers. The TMR structures are similar to GMR except that they utilize an ultrathin insulating layer to separate two magnetic layers rather than a conductor. The GMR and TMR effects occur mainly due to the spin-dependent scattering as the current passes from one layer to the other through the spacer layer. The usual figure of merit of the MR ratio is

> max min min MR(%) 100 *R R R*

where *R*max and *R*min are the maximum and minimum resistance, respectively. The AMR ma‐ terials typically have MR ratios about 2 - 6 %, and GMR structures exhibit 10 - 50 % while the TMR structures commonly can achieve over 200 % of MR ratio using MgO tunnel barrier

A majority of the studies in MR effect in thin films are devoted to the research of multilay‐ ered structures showing the largest possible sensitivity of the resistivity for the magnetic field, and consequently a large number of transition metal-based multilayered structures ex‐ hibiting large MR ratios have been found. In connection with the technological problems to be solved, this book chapter devotes to a number of MR sensor designs using the planar Hall effect and tested to linearize the transducer signal, to enhance the resolution limited by

Though there is a wide choice for sensor designs, optimum sensor performance in each de‐ sign can be ensured only when specific sensor characteristics are satisfactorily addressed. Among these characteristics, the field sensitivity and the sensor resolution are of utmost concern particularly for a PHR sensor and, thus, they are considered to be described here for

The field sensitivity of PHR sensor, *i.e*., the differential of measured PHR voltage versus ap‐

K ex ( ) cos2 sin( )

q

*H tH H H*

cos2 cos cos( )

^ ¶ - - <sup>=</sup> ¶ + +- (2)

 q

 gq

q gq


traditionally defined as

200 State of the Art in Biosensors - General Aspects

instead of the usual Al2O3.

**2.2. Relevant sensor characteristics**

elucidating their importance.

plied field, can be obtained as

PHR //

r r

*V I*

*2.2.1. Field sensitivity*

the MR ratio.

A comparative study of the sensor's characteristics was made systematically and summar‐ ized in Table 1 for some of the sensors in practice [12]. It must be mentioned that all the compared sensors have similar active areas and were normally designed for detection of sin‐ gle or small number of micro-size particles.

In Table 1, the part (A) represents the dimensions and properties of different sensor devices compared. The represented thickness is that of the sensing volume used in 1/f noise calcula‐ tions. Whereas the part (B) shows calculated signals obtained from a single 2 *µ*m bead at the center and on the top of the sensor (the center of the bead is 1.2 *µ*m away from the sensing element), when a 15 Oe rms field is applied. Also represented are the 1/f noise and the ther‐ mal noise contributions, and the minimum detectable field as calculated from the expres‐ sions in the text, and the signal-to-noise ratio under the conditions described in the text.


**Table 1.** (from ref.12)

The comparison results have shown that the PH-AMR or PHE sensor has prominent advan‐ tages over others such as very high signal-to-noise ratio (S/Nf ) as well as very high resolu‐ tion (*µ*o*H*min) in the detection of the magnetic field. Furthermore, the voltage profile of a PHE sensor responds linearly to the magnetic field at the small values. This is a prominent ad‐ vantage in detection of small stray field induced from magnetic labels. Therefore, we have chosen and mainly focused on the development of the PHE sensors for bio-applications.
