In this paper, MnCl,.4H,O crystals with monoclinic
structure, for the study of magnetic hyperfine field in magnetic
crystal at low temperature, have been grown. To study the effect
of impurities on T, (spin-lattice relaxation time), crystals with
different percent of "Co impurities, from saturated mixed
solution of MnCl,.4H,O and CoCl,.6H,O at temperature of
30Q'K, have been grown. The percent of cobalt impurities in the
solution and crystal was determined by using the radioactive
isotopes of "Mn, 6OCO and "Co as probes. The result indicated
that the ratio of "Co/"Mn was about 6.2% which has a good
correspondence with the results of the NMRON spectrum of
54Mn in low temperatures (T<lOOmK).