Hall effect sensors are the workhorses of magnetic sensing, but when you need to detect weak fields — the Earth's magnetic field for a compass, a tiny bias magnet on a wheel for ABS speed sensing, or current-induced fields below 1 mT — you reach for a magnetoresistive sensor instead. These devices change their electrical resistance in response to a magnetic field, and they're roughly 100× more sensitive than typical Hall elements.

There are three flavors you'll encounter:

• AMR (Anisotropic Magnetoresistance): A permalloy (NiFe) thin film whose resistance varies with the angle between current flow and magnetization direction. Sensitivity around 1–3 mV/V per Oersted. Saturates around 2–5 mT.
• GMR (Giant Magnetoresistance): Stacks of alternating ferromagnetic and non-magnetic layers (a few nm each). Resistance changes by 10–20% with field. The discovery won the 2007 Nobel Prize in Physics and revolutionized hard drive read heads.
• TMR (Tunneling Magnetoresistance): Magnetic tunnel junctions with an insulating barrier. Resistance changes 100%+ with field. Highest sensitivity, used in modern HDDs and MRAM.

The classic AMR sensor (like Honeywell's HMC1001 or NVE's AA-series) is built as a Wheatstone bridge of four magnetoresistive strips arranged so the field affects opposite arms differently. Power the bridge with 5 V, and you get a differential output of a few millivolts at full-scale field. Feed that into an instrumentation amp with a gain of 100–500, and you have a usable signal.

Real-world example: An automotive wheel speed sensor uses a GMR bridge facing a ferromagnetic tone wheel. Each tooth passing the sensor swings the local field by a few hundred microtesla, producing roughly 50 mV peak-to-peak at the bridge output. A Schmitt-triggered comparator converts this to clean digital pulses for the ABS controller — reliable down to 0 RPM, unlike inductive variable-reluctance sensors that go silent at low speed.

The flipping problem: AMR sensors suffer from polarity ambiguity. A strong field perpendicular to the easy axis can flip the magnetization domains, swapping the sensor's output polarity. The fix is a set/reset strap — an on-chip coil you pulse with ~3 A for a few microseconds to realign all domains before each measurement. This is essential for electronic compasses.

Rule of thumb: For sensitivity, TMR > GMR > AMR > Hall, roughly by factors of 10. For ruggedness and dynamic range, the order reverses. Pick AMR for ±2 mT precision work (compasses, low-current sensing), GMR for ±30 mT industrial speed/position sensing, and Hall for >10 mT applications where you don't need high resolution.