I would think that the hyperfine field at 50K is smaller than at 18K. Because the field is proportional to the strength of the magnetic field, which gets stronger the lower the temperature goes.
Magnetic moments tend to align themselves in the same direction to lower the overall energy. But the higher the temperature the more the random vibrations of the nuclei and therefore making it harder to align different nuclei. If the temperature drops, the nuclei “shake” less and it becomes easier for the magnetic moments to align in the same direction, therefore strengthening the overall magnetic field.