Associated Scientist: Marcelo Jaime <mjaime@lanl.gov> or (505) 667-7625

Experimental research of materials at low temperatures has advanced our knowledge of solid state physics in many ways. In any practical attempt to study low-temperature phenomena, the question of specific heats crops up immediately, in connection with the refrigeration needed to take care of the thermal capacity of the apparatus. Apart from this, knowledge of specific heats forms a powerful tool in many other areas, such as lattice vibrations, electronic distributions, energy levels in magnetic materials, and order-disorder phenomena.

Thermal measurements in magnetic fields are challenging and require experimental setups specially designed for the task. Mechanic vibrations, that result in undesired heat dissipation in metallic components and wires, must be minimized. Special care has to be paid to thermometry calibration, which usually varies sensibly in the presence of magnetic fields. Additionally, specific heat determinations are long requiring the magnetic field to be constant and stable over extended periods of time. Because of these reasons, specific heat measurements have been limited in the past to magnetic fields below 20 Tesla produced with state of the art superconducting solenoids.

In the NHMFL we have accomplished the first specific heat measurements in magnetic fields up to 60 Tesla in the 60 Tesla Long Pulse (60TLP) magnet, a magnet that is unique in its type. For this experiment we designed a calorimeter made out completely of non metallic parts that includes epoxy embedded fiber glass, home- made He-tight epoxy seals, and Si crystal platforms and frames which eliminates the problem of heat dissipation in the environment of pulsed magnetic fields. The specific heat is measured using the adiabatic method during the 100 ms magnetic field plateau produced by the 60TLP magnet. For this experiment samples must be at least a few mg big, and need to be polished as a flat slab that allows for a large surface of contact with the Si platform. This minimizes the thermal equilibrium time with thermometry and heater and also minimizes the area exposed to changing magnetic field responsible for Eddy-current heating in metals. The addenda of our calorimeter consists of approximately 10 mg Si plus 2 mg Cu, and can be easily subtracted from the measured data. Due to the unfortunate and premature failure of the 60TLP last summer, specific heat experiments at 60T are temporarily* unavailable.

At the NHMFL we can also measure the specific heat using the standard thermal relaxation techniques in superconducting solenoids (H is less than/equal to 18T), resistive Cu solenoids (H is less than/equal to 33T) and the recently comissioned hybrid magnet (H is less than/equal to 45T). A new isothermal AC technique, known as 3-omega technique, designed for use in pulsed magnets and Si-nitride membrane microcalorimeters are at present being developed.

Researchers involved in this project: Marcelo Jaime, Albert Migliori, David Miller, Kee-Hoon Kim, Guillermo Jorge (MST-NHMFL, LANL), Roman Movshovich (MST-10, LANL), Ward Beyerman (UC-Riverside), Greg Stewart (UF-Gainesville), Scott McAll, James Brooks (NHMFL, Tallahassee, FL), Frances Hellman (UC-San Diego).

* A new 60T magnet is presently under construction, and will be aavailable again in early 2003.