Mössbauer Study of FeCl2·4H2O in the Temperature Range 4.2° to 0.025°K

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Abstract
The Mössbauer spectrum of 57Fe in FeCl2·4H2O has been examined in the temperature range 4.2° to 0.025°K. The temperature dependence of the internal magnetic field indicates that the spin ordering starts at 1.3°K. The spectrum below the Néel temperature is well‐explained with the parameters H = 266 kOe, e2qQ/2 = 3.10 mm/sec, η= −0.2 and ∠Hq = 25°, where H, e2qQ/2, and η have the conventional meanings and ∠Hq is the angle between the internal magnetic field and the electric field gradient at the iron nucleus. The spectrum for a single crystal with gamma rays being parallel to the monoclinic crystal axis b determines ∠Hb = 15°. Thus, below the Néel temperature the directions of the internal magnetic field at the two sites of iron nuclei in this material make an angle of 30° with each other.