1. Gifford, W.E., “The Gifford-McMahon Cycle,” Advances in Cryogenic Engineering, Vol. 11, 1966 (K.D. Timmerhaus, ed.). Plenum Press, New York, p. 152.
2. Kuriyama, T., Hakamada, R., Nakagome, H., Tokai, T., Sahashi, M., Li, R., Yoshida, O., Matsumoto, K., and Hasimoto, T., “High-efficiency two-stage GM refrigerator with magnetic material in the liquid helium temperature region,” Advances in Cryogenic Engineering, Vol. 35B, 1990 (R.W. Fast, ed.), Plenum Press, New York, p. 1261.