Journal of Low Temperature Physics, Vol. IS, Nos. 1/2, 1974 Magnetothermal Properties of Nb-W, Nb-Ti, and Nb-Mo Alloys in the Mixed State S. V. Subramanyam* and V. Choprat Clarendon Laboratory, University of Oxford, Oxford (Received October 10. 1973) Flux jumps and magnetothermal effects in single crystals of niobium and niobium doped with tungsten, titanium, and molybdenum have been studied in the mixed state in a vacuum calorimeter. It is observed that (a) the field at which the firstfluxjump occurs is independent of the sweep rate at low sweep rates (1-10 Oe sec) but increases sharply at higher sweep rates (,,-100 Oe/sec) of the magnetic field, (b) the heat evolved during the flux jump becomes larger as the sample's temperature is reduced, (c) the temperature fluctuations of the type observed by Zebouni et al. and Levy et al. are evidently due to the imperfect adiabatic conditions of experiments, and (d) the interval between the succes- sive flux jumps is in good agreement with that expected from the critical stat'e model of Swartz and Bean. I. INTRODUCTION The magnetothermal behavior of niobium has been investigated by various workers 1-5 and shows several interesting features in its mixed state. The magnetocaloric cooling, predicted and observed earlier by Mendelssohn and Moore, 6 Yakub, 7 and others, is found at temperatures between 2 and 4 K on sweeping the magnetic field. This is due to the negative surface energy of the mixed state boundaries. In addition, the creation and destruction of flux vortices by the magnetic field causes the heating of the material. For non- ideal materials the imperfections hinder the motion of these vortices, caused by the sweeping magnetic field, and the heat generated is larger. The flux jumps have often been found to be periodic in a magnetic field. Several new magnetothermal effects observed in niobium 3'9 have not been satisfactorily understood so far. *Commonwealth AcademicStaff Fellow (on leave from Department of Physics, Indian Institute of Science, Banalore, India). tPresent address: Bhabha Atomic Research Centre, Physics Division, Trombay, Bombay, India. 71 © 1974 Plenum Publishing Corporation, 227 West 17th Street. New York. N.Y. 1001I. No part of'this publicalion may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical. photoCopying, microfilming, recording:, or otberwiss, without written permission of the publisher.