Symmetry breaking in magnetoresistive devices

Abstract

Detecting weak magnetic fields is paramount in areas such as scanning magnetometers and manipulation of magnetic nanoparticles, thus rendering it crucial to increase the weak-field sensitivity for developing nextgeneration magnetic sensors. The current approaches for high-sensitivity sensors, such as superconducting quantum interference devices, are complex and expensive. By contrast, magnetoresistive sensors and particularly extraordinary magnetoresistive sensors offer a simple operation at room temperature but, to date, at inferior sensitivity. To overcome these challenges, we induce device symmetry breaking to enhance the weak magnetic field sensitivity in semiconductor-metal hybrid structures exhibiting extraordinary magnetoresistance. Retaining the device mirror symmetry yields symmetric magnetoresistance curves with R(B) = R(−B), which results in inferior detection of weak magnetic fields as [dR/dB] B→0 = 0. Using finite element modeling, we study the change in device behavior as the symmetry is broken by varying the device geometry by spatially varying the constituent material properties or both. We show that symmetry breaking has three important implications: First, breaking the mirror symmetry causes an asymmetric sensor response with R(B) = R(−B), benefiting from a largely enhanced sensitivity to weak magnetic fields and detection of the magnetic field direction. Second, an interplay with the Hall effect causes a large negative magnetoresistance exceeding 79% at B = 1 T and room temperature without magnetic constituents or explicit optimization of this property. Third, the asymmetric geometries can be used as a key ingredient toward designing on-demand magnetoresistive characteristics such as linearity at low magnetic fields, step functions, and magnetic switchlike behavior. These implications pave the way for asymmetric topology optimization of magnetoresistive devices with unparalleled performance.

Universal material trends in extraordinary magnetoresistive devices

“…The effect of the contact resistance on the sensitivity evaluated at 10 mT and 0 T is depicted in figures 5(o) and S3, respectively. We observed only small variations in the sensitivity of the asymmetric device as the contact resistance was increased, however, the overall sensitivity value was high and consistent with previous studies [41]. For particular the bar-shaped and branched devices, the weak-field sensitivity is observed to have a non-monotonic dependence on the contact resistance, which arises from the complexity in figures 5(k)-(l).…”

“…The method is discussed in detail elsewhere [2,9,10]. In our work, we reimplement the approach with the use of Comsol Multiphysics (v. 5.6) as described in a previous publication [41] and apply it to the four key EMR geometries illustrated in figure 2. The concentric circular device (figure 2(a)) was based on the work by Solin et al [8] with a filling factor α = r i /r o = 12/16 where r i and r o = 1 mm are the radii of the inner metal and outer semiconductor circles, respectively.…”

“…The asymmetric circular geometry (figure 2(b)) was based on our previous work [41] using α = 12/16 with a 90% offset in the metallic inclusion. The dimensions used in the bar-shape device (figure 2(c)) were also based on previous work [35,38] where it was shown that a length of 45 µm, a semiconductor width of 3 µm, a metal width of 15 µm and a voltage probe distance of 8 µm results in a maximal magnetoresistance at all magnetic fields.…”

“…The material requirements sparked an interest in studying the behavior of EMR in various high-mobility materials, including InSb [8,[12][13][14][15][16][17], InAs [18][19][20], GaAs [21][22][23][24] and graphene [25][26][27][28][29][30]. Beyond investigating different material platforms and material parameters, there has also been a strong interest in exploring various EMR geometries [2,[31][32][33][34][35][36][37][38][39][40][41]. Four of the key geometries explored for EMR are displayed in figure 2, which include the concentric circular devices, bar-shaped devices, the asymmetric off-center circular devices and more exotic multi-branched devices.…”

“…The devices are generally symmetric, leading to an overall symmetric magnetoresistance where R(B) ≈ R(−B), yielding a sensitivity of dR/dB − → 0 for B − → 0 T. In these devices, several studies report the effect of changing geometric parameters such as the semiconductor width and the area of the metal region [8,18,33,35,38,42,43]. The device symmetry is broken in the off-centered geometry, where simulations predict highly asymmetric magnetoresistance curves and a non-zero sensitivity (dR/dB > 0) towards weak magnetic fields [41]. Lastly, a few variants of branched structures, such as the one shown in figure 2(d), have also been investigated computationally, yielding a magnetoresistance of up to 10 10 % at 5 T [29,38].…”