Associate Professor of Physics
- B.S. College of William and Mary
- M.S. Southern Illinois University-Carbondale
- Ph. D. Southern Illinois University-Carbondale
- Magnetic properties of materials
- Phase transitions
- Rare earth-transition metal intermetallics
- Magnetic neutron scattering
My research on magnetic materials has focused on the preparation and characterization of new magnetic alloys. Samples are first prepared by arc melting the elements in the proper atomic ratios in an argon atmosphere. Then they are cut into pieces with a diamond saw. X-ray measurements are made to determine the quality of the alloy, and if the sample is of acceptable quality then magnetic and resistivity measurements are made. The aim of all studies is to determine the factors that control the magnetic state of the material, then to be able to design materials with the desired magnetic characteristics. Some of the systems I have studied so far are the Kondo lattice system CeSix, the rare earth pyrochlore compounds R2Mo2O7 (R = rare earth), the reentrant magnetic materials CeMn2(GexSi1-x)2, and most recently the R5Si3 series. Currently I am working on the hard magnetic materials R2Fe17-xMx (M = metal) and their carbides. It is hoped that we can identify some materials suitable for development into the much-needed high temperature permanent magnets.
Research is carried on in collaboration with researchers at Southern Illinois University at Carbondale, where we have access to arc melting and powder sample preparation facilities as well as a wide array of sample characterization equipment, including a Lakeshore Model 7000 ac susceptometer, Quantum Design SQUID magnetometer with a 5.5 Tesla magnet, and a capacitance dilatometer, for relative thermal expansion measurements. X-ray diffraction measurements are carried out here at Southeast in collaboration with Dr. Michael Aide in Geosciences. I am currently developing a sensitive resistivity measurement system which will yield the resistivity of magnetic alloys from temperatures of 4 K to 300 K and later also as a function of applied field. This system with be computer interfaced for automated data acquisition and control.
- P. Hill, Naushad Ali, A.J.A. de Oliveira, W.A. Ortiz, P.C. de Camargo and Eric Fawcett, Local Moments in the Paramagnetic Phase of Dilute CrV Alloys, J. Phys. Condens. Matter 6 (1994) 1761.
- A. Fernandez-Baca, Peggy Hill, B. C. Chakoumakos and Naushad Ali,Neutron Diffraction Study of the Magnetic Structures of CeMn2Ge2 and CeMn2Si2, J. Appl. Phys.79 (1996) 5398.
- P. Hill and Lance L. Miller, Magnetic Properties of the Rare Earth Silicide Ho5Si3, J. Appl. Phys.,87 (2000) 6034.
- Kanishka Marasinghe, Weerasinghe Priyantha, Kishore Kamaraju, William James, William Yelon, Igor Dubenko, Peggy Hill, and Naushad Ali, Mixed rare-earth effects in (Sm/Gd)2(Fe/Si)17 intermetallics, IEEE Trans. Mag., 37 (2001) 2599.
- I. S. Dubenko, P. Hill, N. Ali, Magnetic Properties of LaCr1-xMxSb3 (M = V, Mn, Fe, Cu and Al), J. Appl. Phys.,89 (2001) 7326.
Supervised Student Research Projects
- Harken, Andrew, Mechanical Alloying as a Technique to Alter Magnetic Properties of Materials, junior, B.A. Physics Major, University of Northern Iowa, colloquium, Oct. 2, 1997.
Andrew prepared samples of CeFe2 and measured magnetization to show that good quality polycrystalline samples could be prepared. He helped set up glove box and performed mechanical alloying experiments on samples of CeMn2(Si1-xGex)2 to test these new instruments.
- S. Malaise, P. Hill, and L. Miller, Magnetic Properties of the Rare Earth Silicide Er5Si3, 45th Annual Midwest Solid State Conference, Manhattan, KS, Oct. 3-4, 1997.
Steve prepared and measured the magnetization and ac susceptibility of Er5Si3. He attended and presented his poster at this conference.
- Podalefsky, Noah, A System for Measuring Resistivity, junior, Physics major, University of Northern Iowa, colloquium, Oct. 29, 1998.
Noah helped design and machine a resistivity sample probe. He then wired it and interfaced it to the computer for automated data acquisition.
- Moeller, Julia, Development of a Resistivity System for the Study of R5Si3 Alloys, sophomore, B.S. Physics major, University of Northern Iowa, colloquium, Oct. 13, 1999.
Julia made some modifications to the resistivity measurement system and used it to measure the resistivity of R5Si3 alloys.
- Moeller, Julia, Synthesis and Magnetic Characterization of Tb5Si3, UNI Sigma Xi 6th Annual Sigma Xi Student Research Conference, April 10, 1999.
Julia prepared the Tb5Si3 alloy and made magnetization and ac susceptibility measurements on it.
- Peggy Hill, Julia Moeller and Lance L. Miller, Magnetic and Electrical Properties of Er5Si3 and Tb5Si3, APS Meeting, Minneapolis, MN, March 20-24, 2000.
Julia made further measurements on her samples, including resistivity. She helped in the analysis of results and in beginning to identify property trends in the R5Si3 system.
Possible Future Projects and Research
Effects of Non-magnetic ion substitution on the magnetic and electrical properties of R5Si3 compounds
We have previously shown that Er5Si3 and Ho5Si3 show two magnetic transitions at low temperatures, most likely due to ordering on the two independent magnetic sublattices. We have also shown that Tb5Si3 undergoes only one magnetic transition at low temperatures, but also shows a discontinuous volume change at this temperature, leading us to suspect that both lattices order at the same time in this compound, and that the magnetic transition is accompanied by a change in crystal structure. This has not been fully investigated, and would be an interesting topic to pursue now. One way we might investigate the magnetic transitions in these compounds further is to try to separate the influences of the magnetic ions on each lattice site. This may be accomplished by substituting a nonmagnetic rare earth for the magnetic one. For example, if we wanted to look at the two magnetic transitions in Ho5Si3 we might substitute a small amount of lanthanum (La) or yttrium (Y) for the Ho. (Since all rare earths are chemically similar it is usually easy to substitute small amounts of one for the other.) Addition of a nonmagnetic ion for the magnetic one will tend to dilute the magnetic coupling between ions and will tend to lower the magnetic transition temperature. Since the two Ho sites are different, and the La or Y ion we are substituting is a different size from the Ho, it will be interesting to see if the nonmagnetic ion substitutes preferentially at one of the two magnetic sites. This might show up as the lowering of one magnetic transition temperature more than the other. As for the Tb5Si3it would be interesting to see if we can decouple the magnetic order on the two sites and see two independent magnetic transitions rather than the single transition we see in the pure Tb compound.
Study of Ferromagnetic Chromium Ordering in RCrSb3 Compounds
Intermetallic alloys combining a 4f metal (R), the source of large magnetocrystalline anisotropy, with a 3d transition metal (Cr), the source of large magnetic exchange coupling, are an interesting class of materials for the study of the basic magnetic behaviors of materials having possible technological applications. In addition, much current research in magnetism has turned to bulk materials consisting of layers and chains of atoms exhibiting reduced dimensionality in magnetic and transport properties. Examples of such materials include the high temperature superconducting materials and the layered colossal magnetoresistance (CMR) materials. Substitution of different elements in these compounds can allow one control of the magnetic aspects of these materials through variations in the inter- and intra-layer couplings.
Studies of Layered Perovskite Cobaltites: RBaCo2O5+d
Recently, due to the similarity in structure to the CMR manganites, attention has been drawn to the interesting magnetic and electrical behaviors of the layered perovskite cobaltites having the chemical formula LnBaCo2O5+d (Ln = lanthanide and 0 = d = 1). Though these materials do not show as large a magnetoresitance as the manganites they do exhibit a wealth of interesting magnetic, electrical and structural behaviors. These behaviors are strongly correlated with the oxygen content (d) which appears to be governed by the mean radius of the lanthanide and by the heat treatment used during synthesis. Some of the interesting behaviors observed to occur as a function of temperature in various members of this series include magnetic ordering, structural transitions, charge ordering, metal-insulator transitions and spin-transitions. As this is a new area of study there is much yet to be learned about the interplay between the structure and bonding in these materials and their physical properties.