This is a joint research project between The University of Texas at Austin (UT Austin) and the University of Alicante (UA) in Spain to study unexpected or emergent magnetism in metal nanocrystals. Magnetism has been observed in a variety of materials that are not expected to be magnetic, including the metal nanocrystals that will be studied in this research program. This research team will examine the underlying physics of emergent magnetic properties in gold and bismuth nanocrystals through a joint theoretical and experimental effort. The team at UT Austin will synthesize and measure the magnetic properties of gold and bismuth nanocrystals. The team at UA will perform model calculations of the electronic and magnetic properties of the nanocrystals, as well as experimental scanning tunneling spectroscopy (STS) and anisotropic magnetoresistance (AMR) measurements to measure the electronic properties of individual nanocrystals and to search for a definitive signature of ferromagnetism in these materials, if it exists.
The whole group, including students and post-docs involved in the project, meets twice a month by videoconference to discuss the project. These regular group meetings provide students with a significant amount of interaction during the course of the project. In addition, students from UT Austin spend one month in Alicante every year, learning the theory and how the STS and AMR measurements are performed and, vice-versa, students from UA visit Austin for one month to learn about the nanocrystal synthesis and magnetic measurements. This international research program provides exposure to a combination of theory and experiment?both synthesis and property measurement?that offers students an outstanding training opportunity in interdisciplinary materials science and engineering. This research project also provides a unique and exciting environment for students to learn how to cooperate scientifically across political, social and geographical borders.
This Materials World Network program supported a joint research program between The University of Texas at Austin (UT Austin) and the University of Alicante (UA) in Spain to study unexpected emergent magnetic properties in metal nanocrystals. The research team at UT Austin focused primarily on the synthesis and measurement of magnetic properties of gold, silver and bismuth nanocrystals, while rhe research team at UA performed model calculations of the electronic and magnetic properties of the nanocrystals, and state-of-the-art experimental scanning tunneling spectroscopy (STS) and anisotropic magnetoresistance (AMR) measurements. The STS and AMR measurements provided information about the electronic properties of individual nanocrystals in our search for a definitive signature of ferromagnetism in these materials. The research team included undergraduate and graduate students and post-docs, meeting twice per month by videoconference to discuss the project. These regular group meetings provided students with a significant amount of interaction during the course of the project. In addition, students from UT Austin spent one month in Alicante learning about the theory and how the STS and AMR measurements are performed. The NSF funding of the US investigators was also leveraged by funding from the Spanish government to the Spanish investigators involved in the research effort. Intellectual Merit. Magnetism has been observed in a variety of materials that are not expected to be magnetic, including the metal nanocrystals that were studied in this research program. An understanding of these properties requires a detailed analysis of electron-electron interactions within the core of the nanocrystal and on the surface. These calculations push the limits of what is currently understood about how to model these systems, and theoretical predictions must be verified by careful experiment. The research team performed experiments and theoretical calcuations focused on unraveling the underlying physics of emergent magnetic properties in gold and bismuth nanocrystals. The results of this research project helped extend fundamental understanding of how unusual magnetic properties arise in nanostructures and have helped explain many phenomena that have recently been observed in different materials during the last decade. Broader Impacts of the Proposed Activity. Magnetic properties of materials are widely used in many technologies, including hard disks for information storage and to increase the contrast in medical imaging techinques like magnetic resonance imaging. In condensed matter physics research, two relatively recent discoveries in magnetic properties of materials led to new ground-breaking technologies: giant magnetic resistance (GMR) and spin torque transfer (for rapidly developing magnetic random access memory, MRAM). This research project sought to answer the fundamental question: can nanoscale metals like Au (which is diamagnetic in the bulk) and Bi exhibit significant magnetism and how? And if a material like a gold nanocrystal can be magnetic, can it be exploited in new applications, much as GMR has been exploited for high density disk drives, enabling technologies like the mp3 player. Basically, emergent magnetic properties of nanoscale metals—and control over them—could enable new unexpected ways to utilize both spin and charge information in new technologies. The research effort showed that nanoscale gold nanocrystals can indeed produce magnetic properties that differ significantly from the bulk material. However, the magnetic signature was found to be relatively weak with magnetism equivalent to one Bohr magneton per nanocrystal at most. In the smallest nanocrystals, less than 2 nm or so in diameter, nanocrystals with an odd number of atoms yield an unpaired electron, which gives rise to the observe paramagentic signal. The research project also contributed significantly to student education and training. This international research program provided students with exposure to a combination of theory and experiment—both synthesis and property measurement—providing them with an outstanding training opportunity in materials science and engineering. This research project also provided a unique and exciting environment for students to learn how to cooperate scientifically across political, social and geographical borders. The value added by the international collaboration was the build an extremely strong team of researchers with the capacity to solve a very challenging scientific problem, while simultaneously providing students with participation in an eye-opening and life-changing international research experience.
