chemosensors Review Current Progress of Magnetoresistance Sensors Songlin Yang and Jin Zhang * Department of Chemical and Biochemical Engineering, Western University, 1151 Richmond St., London, ON N6A 5B9, Canada; [email protected] * Correspondence: [email protected] Abstract: Magnetoresistance (MR) is the variation of a material’s resistivity under the presence of ex- ternal magnetic fields. Reading heads in hard disk drives (HDDs) are the most common applications of MR sensors. Since the discovery of giant magnetoresistance (GMR) in the 1980s and the application of GMR reading heads in the 1990s, the MR sensors lead to the rapid developments of the HDDs’ storage capacity. Nowadays, MR sensors are employed in magnetic storage, position sensing, current sensing, non-destructive monitoring, and biomedical sensing systems. MR sensors are used to trans- fer the variation of the target magnetic fields to other signals such as resistance change. This review illustrates the progress of developing nanoconstructed MR materials/structures. Meanwhile, it offers an overview of current trends regarding the applications of MR sensors. In addition, the challenges in designing/developing MR sensors with enhanced performance and cost-efficiency are discussed in this review. Keywords: magnetoresistance (MR); graphene; magnetic nanoparticles; magnetic storage; position sensing; current sensing; non-destructive monitoring; biomedical sensing; magnetoresistance (MR) sensors; graphene nanocomposites Citation: Yang, S.; Zhang, J. Current Progress of Magnetoresistance Sensors. Chemosensors 2021, 9, 211. https://doi.org/10.3390/ 1. Introduction chemosensors9080211 Magnetoresistance is the variation of a material’s resistivity under the presence of external magnetic fields. In 1857, Lord Kelvin (William Thomson) first discovered and Academic Editors: Lorena Gonzalez- performed the investigation on magnetoresistance (MR) [1]. The observation of MR was Legarreta, David González-Alonso 40 years ahead of the experimental discovery of electrons. In the late 1980s, two scientists and Raffaele Velotta discovered that the multilayer system consisting of Fe/Cr thin layers displayed large resistance changes under low temperature [2]. The MR of this Fe/Cr multilayer system Received: 19 June 2021 under low temperature (4.2 K, 2 × 101~4 × 101 kOe) was later known as the giant magne- Accepted: 3 August 2021 toresistance (GMR) [3]. The discovery of GMR has dramatically boosted the development Published: 5 August 2021 of the magnetic recording industry, especially for the advancement of hard disk drive (HDD), since IBM introduced GMR reading heads to the HDDs [4]. Nowadays, MR sensors Publisher’s Note: MDPI stays neutral have been broadly applied in various fields including magnetic storage/recording devices, with regard to jurisdictional claims in position sensing, non-destructive monitoring, and MR biosensing platforms [4–7]. published maps and institutional affil- iations. In recent years, the demands on highly-sensitive and cost-effective MR sensors raised due to the growing concerns on further improving the data storage capacity/detection sensitivity of current MR sensors and cutting down the total cost of fabrication/production, which leads to the expansion of the application area of MR sensors. Specifically, with emerg- ing nanotechnology, the aim is to develop a novel nanomaterial that exhibits large resistance Copyright: © 2021 by the authors. change with minimum requirements on extreme conditions (e.g., low temperatures) and Licensee MDPI, Basel, Switzerland. large magnetic fields (>10 kOe). This work will provide a brief review of the research This article is an open access article background of magnetoresistances (MR) and the applications of MR sensors. Meanwhile, distributed under the terms and conditions of the Creative Commons this paper discussed current challenges in producing ideal MR sensors and the possible Attribution (CC BY) license (https:// solution to existing difficulties. In addition, this review also explored the applications of creativecommons.org/licenses/by/ the as-prepared materials in producing multifunctional MR sensors. 4.0/). Chemosensors 2021, 9, 211. https://doi.org/10.3390/chemosensors9080211 https://www.mdpi.com/journal/chemosensors Chemosensors 2021, 9, x FOR PEER REVIEW 2 of 23 Chemosensors 2021, 9, 211 2 of 23 1.1. Magnetoresistance Since the discovery of MR in 1856 by William Thomson and the observation of GMR in1.1. the Magnetoresistance 1980s, studies on investigating MR effects and materials/structures with MR are surgedSince [1,3 the]. MR discovery is defined of MR as in the 1856 phenomenon by William Thomsonthat the resistance and the observation of a material of GMR tends to changein the 1980s, when studies the external on investigating magnetic fields MR effects are applied. and materials/structures Currently, anisotropic with magnetore- MR are sistancesurged [ 1(AMR),,3]. MR giant is defined Magnetoresistance as the phenomenon (GMR), that and the tunneling resistance Magnetoresi of a materialstance tends (TMR) to havechange been when applied the external in commercial magnetic fieldsapplications are applied. of MR Currently, sensors anisotropic[8–12]. magnetoresis- tance (AMR), giant Magnetoresistance (GMR), and tunneling Magnetoresistance (TMR) 1.1.1.have beenAnisotropic applied inMagnetoresistance commercial applications (AMR)of MR sensors [8–12]. 1.1.1.In Anisotropic anisotropic Magnetoresistance magnetoresistance (AMR) (AMR), the resistance change (ΔR) depends on the angle (θ) between the electric current and the magnetization of the metal [13]. The AMR In anisotropic magnetoresistance (AMR), the resistance change (DR) depends on the originatesangle (q) between from the the effects electric of current magnetization and the magnetization and electron ofspin the-orbit metal interaction [13]. The AMR [14]. For positiveoriginates AMR, from thethe effectsprobability of magnetization of s-d scattering and electron of electrons spin-orbit is increased interaction when [14]. Forthe pos-current directionitive AMR, is theparallel probability to the ofmagnetiz s-d scatteringation direction of electrons [15 is,16 increased]. In ferromagnetic when the current metals di- (posi- tiverection AMR, is parallel such as to Ni), the the magnetization maximum resistance direction [15 appears,16]. In when ferromagnetic the magnetization metals (positive direction isAMR, parallel such to as the Ni), current the maximum direction, resistance and minimum appears resistance when the magnetizationappears when directionmagnetization is isparallel perpendicular to the current to the direction, current anddirection minimum (Figure resistance 1) [13]. appears AMR has when been magnetization widely applied is in theperpendicular recording toindustry the current since direction the 1970s. (Figure The1 )[magnitude13]. AMR hasof AMR been widelyof ferromagnetic applied in themetals canrecording be calculated industry by since Equation the 1970s. (1) The magnitude of AMR of ferromagnetic metals can be calculated by Equation (1) ρ -ρ rk−r?∥ ⊥ AMRAMR== (1) (1) r? ρ⊥ wherewhere r휌k∥ isis thethe resistivityresistivity when when current current is is parallel parallel to to the the magnetization, magnetization, and andr? the휌⊥ isthe is resistivity whenwhen the the current current is is perpendicular perpendicular to to the the magnetization. magnetization. Figure 1.1. SchematicSchematic diagram diagram of of anisotropic anisotropic magnetoresistance magnetoresistance (AMR). (AMR). 1.1.2. Giant MagnetoresistanceMagnetoresistance (GMR) (GMR) Giant magnetoresistancemagnetoresistance (GMR) (GMR) was was discovered discovered in thein the 1980s. 1980s. The The GMR GMR effect effect was was discovered in the multilayer system of Fe and Cr. The discovery of GMR initiated countless discovered in the multilayer system of Fe and Cr. The discovery of GMR initiated count- experimental or theoretical researches since GMR offers larger resistance change (DR) with less experimental or theoretical researches since GMR offers larger resistance change (ΔR) a relatively small magnetic field, although it may require extremely low temperatures [2,3]. with a relatively small magnetic field, although it may require extremely low tempera- As shown in Figure2, the general configuration of the magnetic multilayer system is turesthe ferromagnetic [2,3]. As shown layer/non-magnetic in Figure 2, the (or general non-ferromagnetic) configuration layer/ferromagnetic of the magnetic multilayer layer. systemGMR is is related the ferromagnetic to spin-dependent layer/non scattering,-magnetic which (or is triggerednon-ferromagnetic) by spin-polarization layer/ferromag- of neticelectrons layer. under GMR the is presence related ofto thespin external-dependent magnetic scattering, field [4 ,which17,18]. is The triggered high resistances by spin-po- larizationcan be observed of electrons when under the magnetization the presence of of the two external ferromagnetic magnetic layers fieldis [4,17,18] anti-parallel,. The high resistancesand the low can resistance be observed state can when be observed the magnetization when the magnetizations of two ferromagnetic are at a parallel layers state. is anti- parallel,The discovery and the of GMR low hasresistance significantly state increasedcan be observed the storage when capacity the magnetizations of magnetic storage are at a paralleldevices suchstate. as The HDDs discovery [4]. of GMR has significantly increased the storage capacity of magnetic storage