# Presentation: 341662

## Targeted Antiferromagnetism to Ferromagnetism in Complex Metal Borides

**Rational Design and Quantum Chemistry of Complex Itinerant Intermetallic Magnets (DMR 0502671)**
**Gordon J. Miller, Iowa State University****	**

**GOAL:** establish structure-bonding-property relationships for complex intermetallic compounds in order to
- tailor new compounds with desired features, especially magnetic properties.
**HIGHLIGHTS** from 1-Oct-2006 – 30-Sep-2007

- Two series of complex intermetallic borides, Sc2M(Ru1xRhx)5B2 and Ti9M2Rh18B8 (M = Cr, Mn, Fe, Co, Ni; M = Fe, Co, Ni) were prepared, structurally and magnetically characterized, and studied by theoretical calculations.  Systematic changes from antiferromagnetism to ferromagnetism could be tuned by changing the electron count through changes in chemical composition.  New theoretical tools for estimated Curie and Neél temperatures were developed.

**Sc****2****M(Ru****1********x****Rh****x****)B****2**

**Chain of M atoms**

**Ti****9****M****2****Rh****18****B****8**

**Ladder of M atoms**

- _Collaboration_ through MWN
- with R. Dronskowski, B. Fokwa,
- RWTH-Aachen, Germany

**Notes:**
The Miller Group at Iowa State University studies complex intermetallic compounds using experiment and theory, which include synthesis, structural characterization and physical property measurements (resistivity and magnetization) coupled with ab initio electronic band structure calculations.  Theory helps us identify new targets with possibly interesting properties.  In this slide, I highlight three important aspects of our work this past year.

(Left) Flux synthesis of rare-earth/Ta/Al compounds in Al fluxes led to the growth of large single crystals of TaAl3. Calculations on TaAl3 shows a pseudogap in the density of states at the Fermi level and gave us motivation to prepare the pseudobinary, TaAlGa2, which shows semiconducting behavior (small band gap) and may have interesting magnetoresistive behavior.  Semiconductivity in close-packed metals is very unusual, but, in this case, is related to the nature of the main group element clusters formed in this structure. (Red = Ta; Al and Ga form pyramids.)  These results may provide interesting new materials for metal-semiconductor junctions.  One paper is currently in press in Inorganic Chemistry; the work was performed by graduate student, Cathie Condron.

(Center) We are studying materials with applications to magnetic refrigeration – these include LnFe13-xSix and LnFe13-xAlx, which form 3D networks of mixed-metal icosahedra.  As the variable x increases, there is a tendency toward increasing order so that repulsive Si-Si or Al-Al interactions are minimized.  At sufficiently high x, the structure changes from cubic to tetragonal.  Magnetic measurements suggest that only the cubic phases are candidates for magnetic refrigeration.  This work is part of the Ph.D. thesis of Mi-Kyung Han.

(Right) Examination of structures containing icosahedra led us to an examination of Mn-containing intermetallics.  GaCr, GaMn and GaFe form a series of isostructural compounds based on chains of icosahedra.  Again, calculations revealed a change in magnetic behavior and experiment confirmed that GaCr is antiferromagnetic at low temperatures, GaFe is ferromagnetic and GaMn shows a complex magnetic behavior.  Thus, we have a series of isostructural compounds with different band fillings to study the relationship between valence electron concentration and magnetic behavior.  One paper was published in the Journal of Solid State Chemistry; one is being reviewed by the Journal of the American Chemical Society.  This work was performed by post-doctoral associate Olivier Gourdon. 

## Linking Intermetallics and Zintl PhasesGordon J. Miller, Iowa State University

**Education and Mentoring**
**	***Undergraduate Participation:*
- 	Ruth Cink (NSF-REU Program, Summer, 2007)
- 	Josh Weber (DOE-SULI Program, Summer, 2007)
- 	*Graduate Participation:*
- 	Tae-Soo You; Fei Wang
**International Activities**
- 	Visiting Scientist at Max-Planck-Institut for Chemical Physics of Solids, Dresden, Germany (Jan-Jul,2006);
- 	Advanced Masters School in Materials, TU-München, Invited Lecturer
- 	Agreement with Wiley-Interscience to write a monograph
- 		*Chemistry and Physics of Intermetallic Compounds*

**Structure-Composition-Property Relationships in Complex Intermetallic Phases** **(DMR 0605949)**
**Rational Design and Quantum Chemistry of Complex Itinerant Intermetallic Magnets (DMR 0502671)**
**Gordon J. Miller, Iowa State University**

**Notes:**
This project has involved two undergraduates over the past year.  One is now a graduate student in Chemistry at Florida St. University and it seems he will pursue research with a solid state chemist there.  Another undergraduate was supported via a REU program at Iowa
State, and returned to his undergraduate institution.

There are three graduate students involved with this project.