Erasing magnetically recorded Data

Characteristic values of hard magnetic materials used in storage media are

• the size of their coercivity H_c
• the size of their remanent magnetisation J_rem

Both properties are measured after a previous magnetisation to the magnetic saturation J_sat .
The size of the coercivity H_c is important for the conservation of the recorded data and for the preventative of unintentional deletions / changes by superimposed magnetic fields.
A high remanence J_rem is decisive for a strong reproducing signal and thus for a clear readability of the stored data.

In the following diagram are represented

• J(H)-magnetisation-characteristic
• J(H)-hysteresis-loop

of a ferro-magnetic material.
(The loop was not measured at a hard magnetic disc. For data storage, a rectangular shape of the loop is preferred.)

For the magnetic recording of data smallest volumes of the magnetic layer are sequentially magnetised. Their magnetic moments m = {J_rem * V} are aligned or arranged according to a specific code in parallel or anti-parallel.

That stored data are erased by an external field Ha if all these magnetic moments m = {J_rem * V}

• are forced in the same direction/ orientation and
• are approximately equal.

This is shown in the next picture by the example of volumes V with antiparallel magnetic moments.
Their magnetic initial state with the different remanent magnetisations {_Jrem} and {-J_rem} is characterized by red dots.
The magnitude and direction of the external field Ha are represented by the blue arrow.

Effect of the magnetic field on volumes with remanence {J_rem} > 0:

The magnetisation changes are described by the green loop section.The volumes of interest are magnetised in their initial state in direction of the external field H_a.
They are more magnetised by this field Ha to the end point of the loop with the value J ≈ J_sat
If the external field is returned to the value H_a = 0, the magnetisation J returns to the original remanence {J_rem}.

Effect of the magnetic field on volumes with remanence {J_rem} < 0:

In this case the magnetisation changes are described by the red loop section.
The magnetic moments m = {J_rem * V} of these volumes are directed against the external field Ha. in the initial state. This remains up to the coercive field H_c. Above this value, the direction change takes.
Then these volumes are magnetised by the field H_a along the red loop section to the endpoint of the J(H)-loop .

Changes of magnetisation by the magnetic field

Both volumes are equal after magnetisation by the magnetic field H_a and have got the same value {J_rem} > 0 (= upper red dot).
They can not be discriminated from each other, they do not store any information.

The procedures are illustrated schematically in the following diagram.

Criteria for a complete erasure of magnetically recorded data

For a total erasure of magnetically stored data the used field strength H_a has to be stronger than the saturation strength H_sat of the hardmagnetic layer.
The saturation field strength H_sat is required for achieving the (material-specific) saturated magnetisation J_sat .

In the case of a rectangular shape of a J(H)-loop

• the saturation field strength H_sat and the coercive force H_c
• the corresponding remanent magnetisations J(H_sat) rem and J(H_c)_rem.

approach each other.

The erasure of data is not complete, if the magnetic field Ha is greater than the coercive force H_c and smaller than the saturation strength H_sat of the material of hard magnetic layer. In this case

• all magnetic moments {J_rem*V} have the same direction
• the amount of the remanence {J_rem} depends on the initial magnetic state.

This is shown in the next picture.

Such differences in the remanent magnetisations J_rem are detected in an atomic force microscope MFM (MFM = magnetic atomic force microscope ) and marked as non-unique data erasure.

If in the read-write head only a field strength H_{read-write-head} < H_sat below the saturation field strength H_sat is reached, for a safe data erasure a field force H_a ≥ H_{read-write-head} f would be sufficient. One assumes generally H_{read-write-head} ≥ H_sat is valid.