Non-collinear states in magnetic sensors

Certain materials have an electrical conductivity that is extremely sensitiveto an applied magnetic field; this phenomenon, termed ‘giant magnetoresistance’1,2,3, can be used in sensor applications. Typically, such a devicecomprises several ferromagnetic layers, separated by non-magnetic spacer layer(s)—aso-called ‘super-lattice’ geometry1,2,3. In theabsence of a magnetic field, the ferromagnetic layers may be magnetized inopposite directions by interlayer exchange coupling, while an applied externalmagnetic field causes the magnetization directions to become parallel. Becausethe resistivity depends on the magnetization direction, an applied field thatchanges the magnetic configuration may be detected simply by measuring thechange in resistance. In order to detect weak fields, the energy differencebetween different magnetization directions should be small; this is usuallyachieved by using many non-magnetic atomic spacer layers. Here we show, usingfirst-principles theory, that materials combinations such as Fe/V/Co multilayerscan produce a non-collinear magnetic state in which the magnetization directionbetween Fe and Co layers differs by about 90°. This state is energeticallyalmost degenerate with the collinear magnetic states, even though the numberof non-magnetic vanadium spacer layers is quite small.