Magnetic Interactions Between Mesoscopic Ferromagnetic Metallic Nanoparticles: Aftereffect Measurements and Preisach Modeling of Magnons Lawrence H. Bennett, George Washington University, DMR 0733526

The goal of this project is to - advance the understanding of the Bose-

Einstein condensation of magnons in mesoscopic nanopartices

-attempt to measure the macroscopic quantum entanglement of the magnons.

Magnetic aftereffect measurements were made (on a SQUID magnetometer) for Co80Ni20 nanoparticles with 0.4%wt concentration in diamagnetic PVC matrix

-at a wide range of temperatures from room temperature to very low temperatures

-for holding fields near remanent coercivity. The curve of normalized maximum

decay rate of magnetization as a function of temperature

-the peak, could be accounted for by a Bose-Einstein condensation temperature, at ~50K

-normal decay curves demonstrate monotonic Arrhenius behavior.

Maximum Decay Rate, R, vs. Temperature for Co80Ni20 nanoparticles , quasi-spherical with 65±9 nm diameter, with 0.4%wt concentration in diamagnetic PVC matrix (Inset: Environmental Scanning Electronic Microscope micrographic magnetic Co80Ni20 nanoparticles prepared by polyol process)

Magnetic Interactions Between Mesoscopic Ferromagnetic Metallic Nanoparticles: Aftereffect Measurements and Preisach Modeling of Magnons

Lawrence H. Bennett, George Washington University, DMR 0733526

We have attributed peaks in the decay coefficient with temperature to Bose-Einstein condensation such as seen in the previous slide. We have seen this non-Arrhenius behavior in a variety of materials, such as perpendicular Co/Pt multilayer medium, in metal particle tapes and in nanograin iron powders.

The present samples have different concentrations of magnetic materials in a diamagnetic matrix offering the opportunity to measure the effect of interactions between the particles for the first time.

Collaborations: - The nanoparticles we are studying were provided to us by Prof.

Manuel Vazquez of the Instituto de Ciencia de Materiales de Madrid, Spain.

- Mr. Shuo Gu, graduate student, has been doing measurements at Chemistry Dept. at GWU and NIST and has been interacting with Prof. Michael Wagner and Dr. Paul Morrows. Gu has also consulted Dr. Virgil Provenzano and Dr. Cindi Dennis at NIST.

Contributions within discipline: Advanced the understanding of magnetic nanoparticles. Advanced experimental and theoretical knowledge of Bose-Einstein condensation of magnons

Contributions to other disciplines: - The Institute for Magnetics Research at GWU holds weekly

meetings for discussion of research accomplishments. All of the students present their accomplishments for the week. Many visitors attend on occasion, including Post-Docs from NIST, Scientists from NIH, Professors from other disciplines and out-of-town visitors.

- A PI was the general chairman of the International Hysteresis and Micromagnetic Modeling Symposium which held a meeting in May (HMM2009). This involves a broad community of physicists, engineers, and mathematicians. Modeling the magnetic properties of nanoparticles and the importance of the Bose-Einstein condensation was an important part of this meeting.

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Comparison with aftereffect measurements in other materials: