A Study of Reversible and Irreversible Magnetization Behaviour in Conventional Superconductors

  • S. Ramakrishnan
  • Ravi Kumar
  • C. V. Tomy
  • A. K. Grover
  • S. K. Malik
  • P. Chaddah


The complete expulsion of magnetic flux on cooling a superconductor in a dc magnetic field (Meissner-Oschenfeld effect) ensures that the magnetic response of a superconductor is reversible and that it is a thermodynamic property. Incomplete flux expulsion is, however, a common occurrence in both type I and type II superconductors and such materials display a path-dependent or hysteretic magnetic response. Detailed studies in the high temperature superconductors (HTSC) have resulted in the observation1 that there exists a thermodynamic reversible region in (H,T) space just below the normal-superconductor phase boundary (T c (H) line). Here H is the applied field, T is the temperature of the sample. Hysteretic behaviour sets in only below the irreversibility line, (T r , H r ), and different techniques have been used to determine this (T r,Hr) line in the HTSC. We have searched for such an irreversibility line in specimens of conventional type I and type II superconductors by both dc and ac magnetization techniques. The following four different procedures were employed to determine the (H r , T r ) line in HTSC:
  1. (a)

    Field-cooled (FC) and zero field-cooled (ZFC) magnetization curves in a constant dc field H were measured as a function of temperature. The temperature at which these merge [T r (H)] gives a point on the irreversibility line.

  2. (b)

    Hysteretic isothermal magnetizaton, i.e, M vs H, curves were measured where M is the sample’s magnetization per unit volume and H is the applied field. The field, [H r (T)], at which the hysteresis vanishes gives a point on the (T r , H r ) line.

  3. (c)

    The complex ac magnetic susceptibility (χ = χ′ H + iχ″ H ) was measured in constant dc field H as a function of temperature. A thermoaynamic response requires that χ′ H (T) be positive just below T c and this is called the Differential Paramagnetic Effect (DPE).2 The temperature at which χ′ H (T) switches from positive to negative is identified with T r (H).

  4. (d)

    From the simultaneous measurements of χ″ H (T) as a function of temperature in a constant dc field, the temperature at which it peaks is identified as T p (H).



Hysteresis Loss Perpendicular Orientation Disc Plane Irreversibility Line Conventional Superconductor 
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Copyright information

© Springer Science+Business Media New York 1991

Authors and Affiliations

  • S. Ramakrishnan
    • 1
  • Ravi Kumar
    • 1
  • C. V. Tomy
    • 1
  • A. K. Grover
    • 1
    • 2
  • S. K. Malik
    • 1
  • P. Chaddah
    • 1
    • 3
  1. 1.Tata Institute of Fundamental ResearchBombayIndia
  2. 2.Department of PhysicsPunjab UniversityChandigarhIndia
  3. 3.Bhabha Atomic Research CentreBombayIndia

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