Magnetic element with switchable domain structure

1. A magnetic element comprises a closed loop of ferromagnetic material having an even number of magnetic domains of opposite sense, the magnetisation within the domains being in a circumferential direction, the domains having leading and trailing walls extending from the inside to the outside of the loop, wherein the magnetic element has a geometry such that there are at least two stable equilibrium domain configurations in which the domain walls are confined in predetermined portions of the loop and wherein the element is switchable between the stable configurations upon the application of a external magnetic field.

2. The element of claim 1 , wherein the loop is substantially circular.

3. The element of claim 1 or claim 2 , wherein there are notches in the loop of ferromagnetic material, the domain walls being confined in a stable equilibrium at positions corresponding to the notches.

4. The element of claim 3 , wherein the loop has two or four notches.

5. A Giant Magnetic Resistive (GMR) and Tunnel Magnetic Resistance (TMR) device comprising two magnetic elements according to any of the preceding claims placed one above the other and separated by a spacer.

6. A sensor comprising a first layer forming a magnetic element according to any of claims 1 to 4 , and a second layer forming by a magnetic element having a stable domain, wherein in use the resistance of the device can be measured to ascertain the alignment of domains of the first layer relative to the domain of the second layer, as a field sensor.

7. A write head comprising a magnetic element according to any of claims 1 to 4 , wherein a portion of the magnetic element containing a domain wall is arranged to pass, in use, near a magnetic storage medium such that the stray field generated at the domain walls aligns magnetic domains in the magnetic storage medium.