A method of preparing and reading information which is permananently recorded on a magnetic layer

Claim 1

A method of reading information which is permanently recorded on a magnetic layer (1) containing magnetically anisotropic particles, selected first regions (A) of said layer containing particles whose easy axes are predominantly orientated in a selected first orientation direction, said first regions being spaced from one another by second regions (B) containing particles whose easy axes are predominantly orientated in a selected second orientation direction, said permanently recorded information being represented by the positions of transitions which define the boundaries between adjacent first and second regions, said transitions lying substantially parallel to one another, said information being prepared for read-out by the application and subsequent removal of a uni-directional developing field, characterised in that neither of said first and second orientation directions is parallel to said transitions, and said developing field is applied in a direction lying between said first and second orientation directions such that the components of remanant magnetisation at right angles to the transitions in adjacent first and second regions are in opposite directions, field sensing means (P) being used to detect the positions of said transitions.

Claim 2

A method of reading information which is permanently recorded on a magnetic layer (1) containing magnetically anisotropic particles, selected first regions (A) of said layer containing particles whose easy axes are predominantly orientated in a selected first orientation direction, said first regions being spaced from one another by second regions (B) containing particles whose easy axes are predominantly orientated in a selected second orientation direction, said permanently recorded information being represented by the positions of transitions which define the boundaries between adjacent first and second regions, said transitions lying substantially parallel to one another, said information being prepared for read-out by the application and subsequent removal of a uni-directional developing field, characterised in that neither of said first and second orientation directions is parallel to said transitions, and said developing field is applied in a direction lying between said first and second orientation directions such that the components of remanant magnetisation at right angles to the transitions in adjacent first and second regions are in opposite directions, field sensing means (P) being used to detect the positions of said transitions.

Claim 4

A method of verifying a recording medium purporting to represent a magnetically structured medium (1), said medium being prepared for verification by the application and subsequent removal of uni-directional developing field to form a developed track in said medium, characteristed in that field sensing means (R) are used to detect the fluctations in remanent magnetic pole strength along an edge of said track, thereby establishing whether said medium is genuine.

Claim 5

A method of verifying a recording medium purporting to represent a magnetically structured medium (1), said medium being prepared for verification by the application and subsequent removal of uni-directional developing field to form a developed track in said medium, characteristed in that field sensing means (R) are used to detect the fluctations in remanent magnetic pole strength along an edge of said track, thereby establishing whether said medium is genuine.

Claim 9

A method according to Claims 3, 4, 5 or 6 in which said medium is of elongate strip-like form characterised in that said developing field is applied in a direction substantially parallel to a long edge of said medium.

Claim 10

A method according to any preceding claim in which said field sensing means comprises a magnetoresistive transducer (P,R) characterised in that horizontal and vertical bias fields are applied such that said transducer is least sensitive to magnetic fields directed at substantially 45 0 or 225 0 to the easy axis of said transducer and is most sensitive to magnetic fields directed at substantially 135° or 315° to said easy axis in the same rotational sense.