Advancing magnetic sensing with Allegro MicroSystems’ Tunnel Magnetoresistance (TMR) technology
To overcome the performance limitations of these magnetic sensor solutions, engineers can opt for advanced magnetic technologies like tunnel magnetoresistance (TMR) sensors, which offer higher sensitivities, improved power efficiency, thermal stability, and robustness. How tunnel magnetoresistance (TMR) technology works Tunnel Magnetoresistance technology harnesses principles of quantum mechanics for accurate magnetic sensing. TMR sensors have a magnetic tunnel junction (MTJ) – a nanoscale structure that is composed of dual ferromagnetic layers separated by an ultra-thin insulating barrier. This insulating barrier, typically made of crystalline magnesium oxide (MgO) and a few atomic layers thick, allows electrons to tunnel through it quantum-mechanically. In classical physics, an electron cannot pass through an insulating barrier. However, in quantum mechanics, there is a slight probability that an electron can tunnel through the barrier, even if its energy is lower than the barrier height. This tunneling depends on the relative orientation of the magnetization in the two ferromagnetic layers of the MTJ.
Image credit: Allegro MicroSystems blog
When the magnetizations of the ferromagnetic layers are parallel, the electrons in the majority spin state of one layer can easily tunnel into the majority spin state of the other layer, resulting in a low-resistance state. Conversely, when the magnetizations are anti- parallel, electrons in the majority spin state of one layer must tunnel into the minority spin state of the other, leading to a high resistance. This change in resistance, called the “TMR effect,” is larger than the resistance change in other magnetic sensors, allowing TMR sensors to detect even the weakest magnetic fields precisely. The Magnetoresistance Ratio (MR) quantifies the performance of a TMR sensor by calculating the
percentage change in resistance between the parallel and antiparallel magnetization states. TMR sensors achieve MR ratios of over 200%, significantly higher than those of AMR and GMR sensors, with MR ratios of less than 10% and 20%, respectively. This high MR ratio translates to improved sensitivity, which allows TMR sensors to detect very weak magnetic fields.
Another advantage of TMR and GMR sensors are their compatibility with standard
semiconductor manufacturing processes. The sensors can be fabricated with advanced thin-film deposition techniques, such as sputtering and molecular beam epitaxy, and incorporated with other components on a single
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