Magnetic susceptibility of pure gadolinium as a function of the fluid in spin reorientation region

A.c. magnetic susceptibilities χ of a 99.99% nominal purity gadolinium polycrystal have been measured in the temperature range 110-295 K as a function of applied magnetic fields (0.3 G≤H≤2600 G). For weak fields and increasing temperature, χ(T) goes through a maximum at TM=208 K then it decreases slightly until it reaches the temperature T2=223 K where it falls precipitously. The temperature corresponding to the sharp fall of χ decreases when H is augmented but TM remains unaffected as long as H has not reached a critical field Hc. The shape of χ(T) changes drastically above Hc; for H=1040 G for example TM shifts to 141 K and T2 emerges as a local minimum in χ. We discuss these effects in terms of the variable angle between the ferromagnetic spins and the hexagonal c-axis of Gd. This fine structure of χ in the region of T2 has not been detected in a 99.9% pure polycrystal. (orig.)