Magnetic circular dichroism: Adapting an electromagnet to accommodate a standard cryostat

An electromagnet was modified for measurements of the magnetic circular dichroism of samples held at cryogenic temperatures using a standard laboratory cryostat. The external dimensions of the cryostat are too great to permit its insertion in the air gap between the poles of the magnet without an unacceptable reduction in the strength of the magnetic field at the sample. This problem was overcome by designing new pole caps which become an integral part of the vacuum system of the cryostat. The ends of the new pole caps project into the body of the cryostat so that the gap between them is 1 in. or less, thus achieving a magnetic field exceeding one Tesla at the sample. No permanent alterations of the cryostat are required. The chief advantages of this design are economy and flexibility since a general purpose cryostat is used instead of a special unit designed to fit in the small space between the poles of an unmodified magnet. The cryostat used in this design cools the sample by conduction; thus the problem of optical distortions resulting from bubbling of liquid nitrogen or other cryogen is avoided and the temperature can be varied continuously using standard auxiliary equipment. Extra windows at 90° with respect to the optical beam permit inspection of the sample in situ and could be used for experiments such as fluorescence-detected magnetic circular dichroism which require optical access perpendicular to the direction of the magnetic field.