A change in electrical resistance when a magnetic field is applied. It is how the read head of a hard disk senses its bits, and in some layered magnets the change is enormous – a stack can go from barely conducting to conducting – which is how the magnetic state of a few atomic layers is read out electrically.
Going deeper
When layers of a magnetic stack point in opposite directions, each one blocks the spin the other passes, and the resistance is high. A field that aligns them opens the path – in CrI3 stacks the two states differ by orders of magnitude, which is how magnetic order in a few atomic layers is read electrically.
Several effects with one name
Any change of resistance in a magnetic field is magnetoresistance, but the mechanisms differ enormously in size. Ordinary orbital magnetoresistance, from the curving of carrier paths, is positive and grows with . It is usually small, but in clean with nearly equal numbers of it becomes enormous – millions of percent in WTe2. magnetoresistance, a few percent in metals, depends on the angle between current and magnetisation.
The large effects come from -dependent transport through more than one magnetic layer. Giant magnetoresistance, discovered in 1988 in Fe/Cr multilayers and recognised with the 2007 Nobel Prize, arises because electrons of one spin scatter less in a layer magnetised parallel to their spin. Tunnelling magnetoresistance, across an insulating barrier, works the same way through the spin-dependent and is the basis of magnetic memory.
Spin filters made of van der Waals magnets
A layered magnet such as CrI3 offers something the metallic multilayers could not: each atomic layer is itself a magnet, and in thin neighbouring layers couple antiferromagnetically. A tunnel junction with a of CrI3 as the barrier therefore passes current very differently depending on how many layers are aligned, because each layer filters one spin.
As a magnetic field flips the layers one at a time, the resistance falls in steps, by factors that can reach thousands or more. This turned a magnetic state that is nearly invisible to into a simple transport measurement, and it made layer-by-layer magnetic order – including electrical switching of it by – accessible in devices.
Reading the numbers
Magnetoresistance is quoted as a ratio, but with two different conventions: the change divided by the low-resistance value, or by the high-resistance value. The first can exceed 100 % without difficulty and is the source of headline figures in the millions of percent; the second cannot exceed 100 %. Comparisons need to know which was used, along with the temperature and bias, since tunnelling magnetoresistance falls steeply with both.
For junctions there are further caveats: pinholes short the barrier, the measured switching fields depend on sweep rate and history, and most of these magnets order only well below room temperature – so a spectacular ratio at 2 K says little about a device at 300 K.
For specialists
The field dependence of . Ordinary orbital magnetoresistance is positive and usually small, though enormous in compensated semimetals such as WTe2; giant and tunnelling magnetoresistance come from spin-dependent transport across magnetic layers; colossal magnetoresistance from a field-driven change of electronic state. In a few layers of CrI3 act as a spin filter whose resistance changes by orders of magnitude as adjacent layers align, which is how layer-by-layer magnetic order became measurable in transport.