Coercive-force measurements on specimens of new high-coercive-force materials with Hall transducers

Abstract

The coercive-force H c of permanent magnets is measured by applying to the test magnet an external magnetic field having an intensity vector opposite to that of the specimen. The intensity of the external magnetic field that reduces the magnetization of the test magnet to zero is then determined. A convenient indicator for zero magnetization when making measurements in an open magnetic circuit is a Hall transducer placed parallel to the magnetic axis of the specimen at one of its ends [1]. The source of the external magnetic field can be a solenoid or an electromagnet with a gap in which the test specimen is positioned. Recently new materials for permanent magnets have appeared which are made from intermetallic compounds of the rare-earth elements Sin, Ce, and Pr with cobalt which have very high coercive forces, in some cases up to 4000 kA/m [2, 3]. It turns out to be difficult to measure coercive forces onthe order of 1000 to 4000 kA/m when using germanium or silicon Hall transducers as a zero indicator because a longitudinal emf is produced in the transducer when fields are higher than 1000 kA/m. The interference shifts the zero value of the emf and makes the transducer unsuitable as a zero indicator. The use of a system of two transducers of the same type permits the interference to be eliminated. One of the transducers is a zero indicator and is placed parallel to the magnetic axis of the test specimen :at one of its ends. Owing to its longitudinal disposition with respect to the direction of the lines of force of the demagnetizing field this transducer registers the reduction in the magnetization of the magnet and its passage through zero only when the intensity of the external field goes up to 1000 kA/m. For demagnetizing fields from 1000 to 4000 kA/m a longitudinal Hall emf appears in the transducer which increases rapidly with field strength. In order to eliminate this signal, which complicates the measurement of He, a second correcting transducer is placed near the measuring transducer. It must be positioned so that the relatively weak field of the specimen does not evoke a Hall emf in the transducer while a strong demagnetizing field, as with the first transducer, produces a longitudinal Hall emf. The nature of the variation of the Hall emf from both transducers with field strength is the same and they can be mutually balanced by means of the control currents in the transducers. Hence by connecting the transducers' potential electrodes in series a system is created that does not react to longitudinal magnetic fields of any magnitude. At the same time the sensitivity of such a system for the useful Hall emf, which is proportional to the magnetization of the test specimen, is maintained at the same level as when a single transducer is employed. The second transducer can be positioned in various ways: when measuring the coercive force of small magnets in a uniform field of sufficient volume the transducer is positioned along the lines of foice of the demagnetizing field ata distance from the testspecimen large enough so that the specimen's field does not produce a Hail ernf; where the uniform region of the demagnetizing field is restricted, it can be set close to the specimen's surface but the plane of the transducer should be left parallel with the lines of force of the external field and be in the same plane as the specimen's magnetic axes. The transducers are supplied from separate current sources.