Thermophysical Properties of Ferromagnetic Materials

NON-TECHNICAL ABSTRACT

When heated, all materials change their volume, and the celebrated discovery of the low thermal expansion of Fe65Ni35 "Invar" led to a Nobel prize. Only recently has this Invar behavior become understood as a delicate competition between atom vibrations and magnetic spins in the material. This competition likely occurs in all magnetic materials. Besides competing, vibrations and spins can influence each other through "spin-phonon interactions."

There is some evidence that spin-phonon interactions can improve the efficiency of magnetic refrigerators, which are not yet practical as household appliances. The "magnetocaloric" alloy La1Fe11.5Si1.5, with chemical substitutions for La and Fe, holds promise for practical refrigerators. Perhaps spin-phonon interactions in a variant of La1Fe11.5Si1.5 can couple the large heat capacity of atom vibrations to the change in magnetism with temperature.

This research is an experimental study of how magnetism changes the thermal expansion of metals such as Fe-Pd, La1Fe11.5Si1.5, and perhaps EuO. Labs at Caltech are used to measure heat capacity, magnetization, and thermal expansion at temperatures from 2-400 K in applied magnetic fields. Three different types of X-ray scattering measurements at the Advanced Photon Source in Argonne, IL, provide a detailed understanding of vibrations and spins.

The work is part of a Ph.D. thesis at Caltech. The graduate student and the principal investigator are mentoring a summer undergraduate research fellow, and mentoring high school students in a new summer program at Caltech to show them first-hand how scientific research is done. TECHNICAL ABSTRACT

Thermal expansion and magnetocaloric properties of ferromagnetic metals are studied with measurements of heat capacity and magnetization at temperatures from 2-400 K and magnetic fields up to 9 T. Materials of interest include Fe-Pd, chemical variants of La1Fe11.5Si1.5, and perhaps Tb-Dy and EuO. Measurements with nuclear resonant scattering of synchrotron radiation provide entropies from both atom vibrations and magnetism.

The data are acquired with the samples under pressure in diamond anvil cells. Thermodynamic Maxwell relations convert the pressure dependences of vibrational and magnetic entropies to thermal expansion. For the overall thermal expansion, the vibrations and spins can work for or against each other.

Less understood are interactions between spins and atomic vibrations. Some spin-phonon interactions in iron alloys are large enough to have measurable effects on thermophysical properties. Assessments of spin-phonon interactions use nuclear resonant inelastic X-ray scattering measurements of phonon spectra as pressure drives the material through its Curie transition.

The entropy of the Curie transition is obtained from heat capacity measurements and is used to determine magnetocaloric properties of interest for magnetic refrigeration. There is evidence that interactions between spins and phonons can pull some of the large phonon entropy into the magnetic transition, which could either enhance or diminish magnetocaloric efficiency.

The work is research for a Ph.D. student at Caltech. A summer undergraduate research fellow is also mentored and supported. Through the ongoing Caltech Summer Research Connection, a new program, "Thinking Like a Scientist," is being developed, where three high school students embedded in our research group learn how to formulate a hypothesis in applied physics or materials science and write a proposal to test it.

In six weeks, they learn how materials scientists plan and do their work and develop confidence that they can do it, too.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.