Patent GB-1160057-A (GB)
Title: Improvements in and relating to Magneto-Thermal Recording
IPC: G11B11/105
Filed: 1968-03-12
Abstract: 1,160,057. Magneto - thermal recording. AMPEX CORP. 12 March, 1968 [13 March, 1967], No. 12050/68. Heading G5R. An apparatus or process for magneto-thermal recording of magnetic information in a magnetic recording medium comprises locally heating the medium and applying a magnetic field to it, the medium being such that it is responsive to the field in a temperature range below its Curie point, and varying either the applied magnetic field, or the heating, or both, to record information. A source of electrons or heat 12 is provided, Fig. 1, the beam of particles or radiation from which is passed through appropriate modulating means 14, to which information signals may be applied at 20, before impinging on the surface of a magnetic recording medium 18. A magnetic writing head 22 is disposed on the other side of the medium 18 and is connected to a current supply 24 whereby a magnetic field 26 may be applied in either direction along the easy axis of magnetization of the medium. Information signals to modulate the current supply may be applied at 28, simultaneously with, or instead of, the signals applied at 20. The magnetic recording medium 18 may be a material whose magnetocrystalline anisotropy decreases with increasing temperature and passes through zero at a particular temperature substantially below the Curie temperature of the material. In the region of the particular temperature referred to, the coercive force becomes very low, and its rate of change with temperature there is greater than in regions above and below that region. Examples of materials are nickel, hexagonal cobalt, iron boride, and neodymium-cobalt alloy. Alternatively, the magnetic recording medium 18 may be a material which is in an antiferromagnetic state at one temperature and in a ferromagnetic state at a higher temperature. The inversion temperature is substantially lower than the Curie temperature, and in the region of it the coercive force drops rapidly to a low value. In the same region the magnetization moment increases sharply from a low to a high value. Examples of such materials are manganese antimonide, and an iron-rhodium alloy. As a further alternative, a layer of ferromagnetic material 40, Fig. 4, may be disposed on a layer of antiferromagnetic material 38, on a substrate 36 to form a composite medium 34. The material 38 has a much higher coercive force than the material 40, and in addition has a Neel temperature (at which it becomes paramagnetic) which is lower than the Curie temperature of the material 40. The medium 34 may be pre-magnetized in certain regions in one direction to represent "0" binary digits as shown at 42, 44, 46. If a " 1 " digit is to be written, the medium is heated at 16<SP>1</SP> to a temperature above the Neel temperature of layer 38, and a field Hw of reverse direction is applied. The direction of magnetization is thereupon reversed at 48 upon cooling. Even if the magnetization of the ferromagnetic layer 40 is changed by an external field, as during some types of readout, it will be returned to its original direction upon removal of the field, by the magnetization of the layer 38. In a variant of this embodiment the antiferromagnetic material is dispersed as particles throughout. a layer of ferromagnetic material located directly on the substrate. Examples of pairs of materials 38, 40 are: nickel oxide/nickel, iron sulphide/iron, and cobalt oxide/cobalt.