Geometry‐driven Magnetoresistance

Abstract

Magnetoresistance (MR) in materials arises from two properties of the system. One is the physical MR that is dependent on the intrinsic properties of the materials in the physical system. The second source originates in the geometric structure and distribution of the materials in the physical system. In this review, we focus on the geometric MR and show how it can be altered and dramatically enhanced in two cases. First, in metal–semiconductor hybrid (MSH) structures, the judicious distribution of metal and semiconductor leads to a dramatic increase in the MR due to the rerouting of the current passing preferentially through the metallic regions at low fields, due to their high conductivity, into the semiconductor regions in the presence of a magnetic field. This phenomenon is now known as extraordinary magnetoresistance (EMR), with observed room‐temperature EMR in excess of 100% at 0.05 T and reaching values of 10 6 % at 5 T. This property of the MSH has been shown to be scalable down to structure dimensions on the order of ∼50 nm with a measured EMR of 35% at a signal field of 0.05 T. The details of the diffusive transport mechanism leading to the enhancements in MR are presented. Second, in nanocontacts between ferromagnetic wires, the quantum point contact leads to a ballistic magnetoresistance (BMR) that is due to the development of a domain wall (DW) between the wires when they are magnetized in an antiparallel configuration. The scattering of electrons by the DW in the nanochannels between the wires changes the conductance of the channel, and BMR of ∼300–3000% and higher have been observed. Here again the geometric origin of the MR is evident. The various aspects of the nanochannel governing BMR are discussed. Both geometric enhancements show great promise for technological applications in the area of reliable nanoscale sensors for computer read heads, magnetic actuators and switches, and biological sensors.

Extraordinary Magnetoresistance in Semiconductor/Metal Hybrids: A Review

“…There are many factors contributing to the MR effect, which can be divided into physical and geometric contributions. Physical contributions arise from the magnetic field dependence of carrier mobility, energy-band structure, or spin-spin interactions [ 8 , 9 ], while geometric contributions stem from the shape dependence, the placements of the contacts, or any inhomogeneities of conductivity in the structure [ 10 , 11 , 12 , 13 ]. The majority of common MR sensors utilize physical contributions, also called intrinsic contributions, such as giant magnetoresistance (GMR) [ 14 ] and tunneling magnetoresistance (TMR) [ 15 ].…”

Extraordinary magnetoresistance in two and three dimensions: Geometrical optimization

“…The geometric contribution can include the size and shape of the metallic regions and the device as a whole, the number of metallic regions, and even the orientation of the current and potential contacts. 17 We can see this especially in the geometry of the square region with centered contacts, where the highest MR is obtained for an inner metal width of 8 lm at fields of H ¼ 61 T and also for the 10 lm square with two metal regions and contacts centered along x for 1 T. However, we see an even greater MR in the 10 lm square with two metal regions and contacts centered along x for 1 T. If we compare these two cases, when we have the same amount of metal we can achieve a greater MR when the separation width is larger. Even for lower values of the field, we see in most cases a very large MR (10 4 to 10 5 percent for two regions at 640 mT) which means that EMR read heads would be more sensitive than the currently used TMR read-heads.…”

“…The sensitivity of the device is based on intrinsic contributions from physical properties such as carrier mobility and energy band structure. 17 However, there is also a geometric contribution to the MR, which can play an even more important role. The geometric contribution can include the size and shape of the metallic regions and the device as a whole, the number of metallic regions, and even the orientation of the current and potential contacts.…”