V. V. Bruk, V. V. Eremenko, N. I. Makedonskaya, Yu. A. Shabakayeva, and V. A. Sirenko, Investigation of magnetic flux gradients in hard superconductors, Low Temp. Phys. 27, (2001) [Fiz. Nizk. Temp. 27, 419-424, (2001)].