A piece of magnetisable material when magnetised in the direction shown at A might represent the symbol 0, and when magnetised in the direction shown at B, the symbol 1. It is also well known that a piece of magnetisable material can be magnetised only by applying a magnetising force, technically termed a magnetising field or magnetic field. One way of applying a magnetic field is by surrounding the magnetisable material by a coil or loop wire carrying an electric current. The direction of the magnetisation is determined by the direction of the current, as indicated in Fig. 2. where the direction of the current is indicated by arrows.
Thus, if the current flows in the direction of the arrows (as shown at A), the material will be magnetised in the direction previously selected to represent the symbol 0, and if the current flows in the direction indicated at B, the direction of the magnetisation will he that selected to represent the symbol 1.
However, immediately after the magnetising field is removed by switching off the current, part of the magnetism which has been 'induced' in the material disappears, and only a portion, known as the 'remanent magnetism' is retained. With most magnetisable materials the magnetism H remains when the current is switched off is very much less than that existing in the material while the current is flowing. The relationship between the direction and strength of the resultant magnetism (denoted B) and the direction and strength of the magnetising x force (denoted H) is shown in the graphs termed hysteresis loops', reproduced in Fig. 3. For most magnetic materials this graph is of the form, indicated in Fig. 3a, and it is seen that a magnetising force of +Hp results in
a degree of magnetisation equal to +Bp, but when the magnetising force is removed the remanent magnetism is only +BR. Similarly, a magnetising force -Hp results in a degree of magnetisation -Bp and when the magnetising force is removed the remanent magnetism is only -BR.
Because, in magnetic memories, stored information takes the form of the remanent magnetism in one or other of the two states +BR.and -BR, one of them representing '0' and the other representing '1', it is important that there should be a very definite difference between these two states. To ensure this, a material is therefore chosen in which the value of BR does not differ greatly from the value of Bp. Such a material is 'ferroxcube 6', the magnetisation curve of which is of the form indicated in Fig. 3b. In actual practice, and for reasons which will appear later, straight bars of iron and complete loops or coils of wire are not used in magnetic memories. Instead, rings of the iron compound 'ferroxcube 6' are employed, and in place of loops, straight wires are threaded through these rings.
Back to my home page...