RESEARCH INTERESTS

My research interests lie primarily in magnetic materials, broadly defined.  I am currently funded in the long term (five years) to pursue two large projects: (i) Half-metallic ferromagnets, and (ii) room temperature magnetocaloric materials (this is a joint project with Professor Naushad Ali).  Each constitutes a very important class of materials that could have a profound impact on electronics (i.e., spintronics in the case of half-metals) and room-temperature solid-state refrigeration (in the case of magnetocaloric systems).  In addition, I am interested in solid-oxide fuel cell materials, ferromagnetic shape-memory alloys, and magnetic nanocomposites. 

 

I. In Search of Half-Metallic Alloys  (Funded by NSF:CAREER)

     In general, ferromagnetic materials have spin-polarized band structures.  This means that the spin-projected densities of states (DOS) for spin up (majority) and spin down (minority) electrons are unequal at the Fermi level.  In the extreme case where the density of states for one spin carrier is finite and the other one is zero, i.e., there is a gap at the Fermi level in the minority DOS, the material is referred to as being half-metallic (see the Figure).  Such materials can be used to create spin-polarized currents since, in the ideal case, electrons can conduct in only one spin channel.  There are many challenges to overcome in the attempt to produce ideal half-metallic materials, but the potential fruits of such an endeavor are overwhelming.  A new field is emerging which will thrive on the discovery of an industrially functional half-metal.  This field is known as spintronics, and it is thought to be the next evolutionary step in the development of electronics.

 

 

II. Near-Room-Temperature Magnetocaloric Materials (Funded by DOE PDF)

     In light of recent1-4 developments, interest in magnetocaloric materials has significantly expanded.  These materials are sought in order to realize “solid state” magnetic refrigeration systems that will have a multitude of advantages over their conventional, compressed gas counterparts.5 Magnetocaloric-based systems will be more environmentally friendly than conventional cooling technologies since they do not employ or produce ozone-depleting chlorofluorocarbons, noxious chemicals such as ammonia, or greenhouse gases.   Magnetic refrigeration systems have been shown to reach 60% of the theoretical Carnot efficiency limit, markedly outperforming the best compressed gas systems (efficiency ~40%), despite their relatively early stage of development.6 Large scale use of highly efficient magnetic cooling technology is paramount in addressing the projected energy and global warming crises: It will potentially reduce our consumption of and dependence on fossil fuels, and subsequently reduce the production of pollutants and greenhouse gases.  Magnetic cooling technology could become manifest in applications ranging from near-room-temperature devices, such as refrigerators and air conditioners, to low-temperature gas condensers.  This latter application is a significant crossover into another important energy technology: Hydrogen-based systems.  The efficient liquefying of hydrogen would be a vital step towards a hydrogen-based economy.

The magnetocaloric effect (MCE) occurs as the result of the alignment of magnetic moments with an external applied magnetic field.  This alignment reduces the magnetic randomness, or the magnetic component of the total entropy.  The reduction of magnetic entropy must be compensated by the increase in other components of the total entropy and, in the case of magnetocaloric materials, is channeled into electronic entropy and lattice entropy, or heat.  A full discussion of the thermodynamics of the MCE is presented in the work by Pecharsky, et al.7 This phenomenon can be exploited in devices by considering the following rudimentary magnetocaloric refrigeration cycle:  (1) The material is initially at ambient temperature and magnetically disordered;  (2) the material is subjected to a strong magnetic field causing the magnetic moments to order and, as a result of the decreased magnetic entropy, the material heats up due to the compensating increase in phonon entropy; (3) the excess heat is carried away via a thermal medium (water, air, etc.);  (4) the material is at ambient temperature and magnetically ordered; and (5) the magnetic moment randomize, the material cools below ambient temperature, and it can now be used to extract heat from a load.