The low-temperature, magnetic entropy of certain single-crystal paramagnetic materials, eg DyPO 4 changes dramatically as the crystal rotates in a magnetic field. A new magnetic refrigerator design based on the anisotropic nature of such materials is presented. The key advantages of the rotational-cooling concept are: a single, rotary motion is required; magnetic field shaping is not a problem because the entire working material is in a constant field," and the size of the refrigerator is smaller than other comparable magnetic refrigerators because the working material is entirely inside the magnet at all times. The main disadvantage of the rotational-cooling concept is that small-dimension single crystals are required.
A 4 K to 20 K rotational-cooling magnetic refrigerator capable of I mW to > 1 W operation
J.A. Barclay
Jeffries and Abragam suggested in 1963 that nuclei could be polarized simply by rotating a suitable crystal in a magnetic field at low temperatures./a The proposed technique involves critically transferring a large paramagnetic-ion polarization to nuclear spins without the use of microwaves and with no constraint on the uniformity of the magnetic field. It requires a very anisotropic g-factor for the paramagnetic ion. The rotational-cooling method has successfully produced polarized proton targets. 3
In the magnetic refrigerator to be described in this report, the temperature changes associated with changes in the large paramagnetic-ion polarization are used directly to heat or cool a heat-exchange fluid.