Patent GB-1227748-A (GB)
IPC: G02F1/00
Filed: 1967-09-08
Abstract: 1,227,748. Radiation detector. MULLARD Ltd. Dec.9, 1968 [Sept.8, 1967; March 19, 1968; Oct.18, 1968], Nos.41190/67, 13262/68 and 49556/68. Heading G1A. [Also in Divisions G4, G5, H1, H3 and H5] Electromagnetic radiation or changes therein is detected by directing the radiation on to an active element containing magnetic ions that can exist in first and second valence states, the valence of an ion being changeable by electron transfer, and the radiation providing energy which is absorbed by an ion of the first valence state causing an electron to be transferred across an energy threshold to an ion of the second valence state and observing a magnetic parameter change in the element due to the change in state of the ions. The magnetic change may be anisotropy, coercivity or permeability changes which can be measured. Anisotropy changes: These changes can be monitored by noting the change in ferromagnetic resonant frequency after irradiation of the element by near infrared radiation or the change in bias magnetic field required to maintain the same resonant frequency. Alternatively the torque change on the element when held in a fixed D. C. magnetizing field can be measured. Susceptibility changes in small A. C. fields or hysteresis loop changes in larger A. C. fields may be measured. Further methods may be by measuring the changes in the component of magnetization resolved along a given axis or change in the field necessary to reorient the magnetization along the easy axis to some other axis. Permeability changes: It is shown that when an active element is cooled down to 77‹ K in the dark and then erradiated the permeability decreases. Coercivity changes: The coercive force of the active element increases with the radiation impinging thereon. Reading/Writing: A toroidal active element may be used with a reading electromagnetic coil wound thereon so that an A.C. signal in the coil is used to determine the susceptibility changes in the toroid after illumination. If a second coil is wound on the toroid it may measure the hysteresis induced by the signal in the first coil. If the active element is in the form of a tape information is written into the magnetic material by passing the tape past a stationary magnetic field perpendicular to its plane and an irradiating source whereby the magnetization of the element is "frozen" in. The information may be read out using Faraday effect whereby the plane of polarization of a search radiation beam impinging on the element is rotated by an amount and dependent on the magnetization change and hence the anisotropy change. When the magnetic parameter which changes is coercivity the active element is in the form of a thin layer which is movable with respect to an electromagnetic radiation to be recorded, irradiated parts acquiring increased coercivity constituting the record. In order to read out the record a magnetic field is applied successively along the layer having a strength which lies between the coercive force of unexposed material and that of exposed material so that irradiated areas show a direction of magnetization different from that of surrounding areas. The areas are then read out using the Faraday effect. Photo-magnetic device: In this embodiment the active element is in the form of a thin layer which is relatively movable with respect to a first, recording, magnetic field and an electromagnetic radiation to be recorded which act successively on the element. A second magnetic field is then applied the direction of which is different from that of the first field and has a strength located between the first field and the coercive force of the exposed material. The coercive force of the material is locally increased by the radiation to form a record. Read out is by Faraday Effect. Binary and Analogue operation: If the active material has been subjected to a saturating magnetic field in a first direction it stores this direction if kept in the dark even though the field is subsequently re-orientated to a second direction. However if light is allowed to impinge on the material the anisotropy will be altered so that the direction of magnetization responds to the second direction of the field. The magnetization of the material may be reversed by a subsequent change in the polarization plane in the incident impinging radiation after an initial illumination even though the intensity of the magnetic field and irradiation do not alter. If the polarization change is 90 degrees binary operation is achieved or if continuous, analogue operation. Read out is again by Faraday effect. Active material: The active material may be ferromagnetic containing Fe ions, garnets, spinels or hexagonals having the formula M 3 Fe 5 <SP>3</SP>Œ#- #(A # Fe # <SP>2+</SP>)O 12  wherein M is one or more rare earths and A is Si, Ti, Ge or Sn, and # and # lie between 0 and 1, # may be zero, or RFe 2 <SP>3</SP>Œ#-  # A #  Fe # <SP>2+</SP>O 4  where R is Ni, Zn, Co, Mg or Mn or (LiFe) 0À5 and A, b and # are as above. If the material is a garnet or spinel it may have the formula EFe3+ 12-#-#  A# Fe<SP>2+</SP> # O 19  where E is Pb, Sr or Ba and A, b, # are as above. The magnetic ion may be Cr e.g. spinel structures such as Ga doped CdCr 2 Se 4 .