jats:titleAbstract</jats:title> jats:pThe flux-pinning landscape in type-II superconductors determines the response of the flux line lattice to changing magnetic fields. Typically, the flux vortex behaviour is hysteretic and well described within the framework of the Bean critical-state model and its extensions. However, if the changing magnetic field does not move the flux vortices from their pinning sites, their response remains linear and reversible. The vortex displacement, then, is characterised by the Campbell penetration depth, which itself is related directly to the effective size of the pinning potential. Here, we present measurements of the Campbell penetration depth (and the effective size of the pinning potential) as a function of magnetic field in a single-grain bulk GdBajats:sub2</jats:sub>Cujats:sub3</jats:sub>Ojats:inline-formula jats:tex-math

</jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> mml:msub <mml:mrow /> mml:mrow mml:mn7</mml:mn> mml:mo−</mml:mo> mml:miδ</mml:mi> </mml:mrow> </mml:msub> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="sustab5b53ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> superconductor using a pick-up coil method. Hence, the hysteretic losses, which take into account the reversible vortex movement, are established.</jats:p>