Non-collinear states in magnetic sensors

Certain materials have an electrical conductivity that is extremely sensiti ve to an applied magnetic field; this phenomenon, termed 'giant magnetoresi stance'(1-3), can be used in sensor applications. Typically, such a device comprises several ferromagnetic layers, separated by non-magnetic spacer la yer(s)-a so-called 'super-lattice' geometry(1-3). In the absence of a magne tic field, the ferromagnetic layers may be magnetized in opposite direction s by interlayer exchange coupling, while an applied external magnetic field causes the magnetization directions to become parallel. Because the resist ivity depends on the magnetization direction, an applied field that changes the magnetic configuration may be detected simply by measuring the change in resistance. In order to detect weak fields, the energy difference betwee n different magnetization directions should be small; this is usually achie ved by using many non-magnetic atomic spacer layers. Here we show, using fi rst-principles theory, that materials combinations such as Fe/V/Co multilay ers can produce a non-collinear magnetic state in which the magnetization d irection between Fe and Co layers differs by about 90 degrees. This state i s energetically almost degenerate with the collinear magnetic states, even though the number of non-magnetic vanadium spacer layers is quite small.