A magneto-resistance device is composed of an anti-ferromagnetic layer (5), a pinned ferromagnetic layer (20), a tunnel insulating layer (9) and a free ferromagnetic layer (21). The pinned ferromagnetic layer (20) is connected to the anti-ferromagnetic layer (5) and has a fixed spontaneous magnetization. The tunnel insulating layer (9) is connected to the pinned ferromagnetic layer (20) and is non-magnetic. The free ferromagnetic layer (21) is connected to the tunnel insulating layer (9) and has a reversible free spontaneous magnetization. The pinned ferromagnetic layer (20) has a first composite magnetic layer (6) to prevent at lest one component of the anti-ferromagnetic layer (5) from diffusing into tunnel insulating layer (9).