****NON-TECHNICAL ABSTRACT**** Electrons have two properties that make them the prime elements in many electronic devices. These properties are their electrical charge and what is known as electron spin, the fact that they individually produce a magnetic field. It is when all or most of the electron spins are aligned or anti-aligned in an electrically conducting material, a metal, that properties of that material known as ferromagnetism or anti-ferromagnetism exists. This project will investigate how long the spins take to become ordered or disordered from a magnetic state while diffusing through layers of different types of metals. The studies will determine the rate at which the magnetic state relaxes or diffuses through the sample layers. These properties of the material are of high importance to "spintronics". Spintronics is the name of a new type of device operating on the properties of the electron spin and charge rather than just on the charge as is currently used in semiconductor electronics. This project seamlessly integrates educational activities into the research program at all levels of education. In addition outreach efforts to faculty members from historically black colleges and universities (HBCUs) through an existing workshop during the summer months will continue.
The magnetization relaxation in magnetic multilayer structures is of crucial importance for their application in spintronic devices including read-heads, magnetic random access memories and spin-torque oscillators. A better understanding of the relaxation mechanisms will enable further optimization of the magnetic multilayer structures for spintronic applications. This work will combine the techniques of vector-network analyzer ferromagnetic resonance (VNA-FMR) and shorted waveguide ferromagnetic resonance to obtain information about the magnetization dynamics and the magnetization relaxation in magnetic multilayer structures over a wide frequency range. The three main thrusts of the proposed work are the investigation of the influence of doping on the spin-diffusion in thin copper films, the determination of the interfacial spin-flip probability at metal/metal interfaces and the investigation of spin-diffusion in metallic antiferromagnets. This project seamlessly integrates educational activities into the research program at all levels of education. In addition outreach efforts to faculty members from historically black colleges and universities (HBCUs) through an existing workshop during the summer months will continue.
The results of this project are as follows: We have developed a broadband ferromagnetic resonance system based on coplanar waveguides (CPWs), which enables measurements from below 1 GHz all the way up to 50 GHz. In collaboration with Prof. Y. Bao from the department of Chemical and Biological Engineering we have shown that this new broadband approach is suitable to measure the relaxation of magnetic nanoparticles in solution. The instrument developed under this grant has enabled various investigations of ferrimagnetic single crystal materials, which were synthesized by the group of Prof. Y.-K. Hong at the University of Alabama. We have investigated the losses and the anisotropy in Zr-substituted Zn-Y hexaferrite single crystals. Our investigations of SrBa-Y hexaferrite single crystals indicate that extrinsic relaxation mechanisms contribute significantly to the overall linewidth observed in those samples. Using a series of multilayer samples we were able to determine the spin diffusion length of Cu94Pt6 to be 7 nm at room temperature. These experiments also show that the approach to determine the spin diffusion length, which was outlined in the proposal, was successfully implemented. We also investigated the properties of (CoFe)100-xGex alloys in collaboration with Hitachi GST. This work resulted in the discovery of a very low relaxation rate for (CoFe)70Ge30. This finding can be understood in the framework of our theoretical work on the origin of low Gilbert damping in half metals, which we carried out in collaboration with a theory group at UA (C.K.A. Mewes, M. Chshiev, W.H. Butler and C. Liu). We have further utilized the instrument to investigate the magnetization relaxation in multilayers containing metallic antiferromagnets. This work, done in collaboration with R.L. Stamps, C.K.A. Mewes and S. Gupta, has resulted in a number of important new findings: (a) Spin-pumping contributes significantly to the magnetization relaxation in multilayers containing antiferromagnets, direct exchange coupling is not required for this contribution. (b) In exchange biased multilayers we found a unidirectional contribution to the magnetization relaxation, which is at least in part caused by two-magnon scattering at the interface. In collaboration with C.K.A. Mewes we have developed M3, a Matlab based micromagnetics code, which has been made available for download (for details see: http://bama.ua.edu/~tmewes/Mcube/Mcube.shtml). This project provided research opportunities for two high school students, four undergraduate students and five graduate students. Of the four undergraduate students three have since started graduate studies. Undergraduate students and graduate students have presented their results at conferences. The PI has presented project results at a number of colloquia and international conferences.
