Nontraditional NMR Spectroscopy of Magnetics
Nontraditional NMR Spectroscopy of Magnetically Ordered Materials with Applications to Materials Research and NMR Instrumentation
Tech Area / Field
- MAT-SYN/Materials Synthesis and Processing/Materials
- PHY-SSP/Solid State Physics/Physics
3 Approved without Funding
Tbilisi State University, Georgia, Tbilisi
- Georgian Technical University, Georgia, Tbilisi\nTbilisi State University / Institute of Physics (Ge), Georgia, Tbilisi
- San Jose State University / College of Chemistry, USA, CA, San Jose\nUniversity of Uppsala / Department of Physics, Sweden, Uppsala\nCNRS / Laboratoire de Magnétisme, France, Grenoble\nFlorida State University / Department of Chemistry and Biochemistry, USA, FL, Tallahassee\nRoosevelt University, USA, IL, Chicago\nRheinisch-Westfalische Technische Hochschule / Institute of Technical Chemistry and Macromolecular Chemistry, Germany, Aachen
Project summaryResonance properties of electron-nuclear magnetic system of magneto-ordered materials (magnetics) attract researcher's attention due to a combination of different physical phenomena. Here Hahn and multilevel nuclear spin echo signals are observed. At low temperatures the motion of nuclear magnetization becomes essentially nonlinear. The NMR method is widely used for the investigation of electron magnetic structure, nuclear spin-echo signals find there application in functional electronics. All this shows the importance of development of the theoretical and experimental methods of nuclear spin dynamics investigations in magnetics and the search of new resonance effects. We will only restrain ourselves by some of them which are directly related with the aims of our Project.
During seventies a new direction in the physics of magnetics developed – the investigation of nonlinear properties and mechanisms of nuclear spin echo formation. At low temperatures in weakly anisotropic magnetics a coupling develops between the oscillations of nuclear and electron magnetization resulting in a characteristic shift of resonant NMR frequencies known as the dynamic frequency shift (DFS). In this case the NMR frequency depends on the amplitude of oscillations of the nuclear magnetization which is a characteristic for nonlinear systems. For this reason the classical Hahn mechanism in such systems is not effective and the nuclear spin echo is formed by the new so-called frequency modulated mechanism (FM). By this mechanism the nuclear spin echo signals are formed in such interesting systems as superfluid 3He, and solid 3He, plasma and so on. A similar effect is observed in multidomain magnetics at low temperatures in the conditions of coupled oscillations of domain walls (DW) and nuclei what also results in the DFS effect for nuclei arranged in DW. It turned out that properties of spin echo signals of nuclei, arranged in DW, at low temperatures (so-called auxiliary echo) were not described by known mechanisms of spin echo formation. Theoretical and experimental investigations of this phenomena made so far among others also by our group, were not resulted in the full understanding of this important phenomenon and further experimental and theoretical investigations are necessary what could make it possible to improve essentially the present level of understanding of nonlinear phenomena in dynamics of nuclear spin magnetization at low temperatures in magnetics. The investigation of nuclear spin system dynamics revealed that one of the essential futures of FM echo was the possibility of its nonresonant excitation.
The essential progress obtained in the understanding of properties of two-pulse echo (TPE), single-pulse echo (SPE) and secondary echo signals formed by FM mechanism stimulated intensive investigations of SPE and secondary echo signals in Hahn systems with a large inhomogeneous broadening of NMR line where generally accepted point of view was not exist till the late ninetieth. The first theoretical mechanism of SPE in inhomogeneously broadened Hahn systems was only presented in 1979 – so-called nonresonant mechanism. The mechanism of its formation turned out to be the edge type connected with an nonadiabatically fast change of effective field Heff direction in the rotating coordinate system (RCS) when RF pulse adges were acting as two resonant RF pulses in the two-pulse Hahn echo method. As the development of this approach a possibility was shown by us of excitation the single-pulse analogs of multipulse Hahn echo excitation and the generation of new type echo signals (so-called inversion echo) at application only magnetic pulses (videopulses of magnetic field) and so-called magnetic echo at combined excitation by RF and magnetic pulses. Besides, the possibility of echo signals generation was shown at stepwise change of Heff direction in RCS changing the amplitude or frequency of RF pulse in the limits of its duration. In the frames of Project we suppose to carry out systematic experimental and theoretic al investigations of the nature of these echo-signals, particularly of the magnetic echo, and to establish the possibility of its practical applications for the materials research and functional electronics. The edge-type SPE signals can appear also due to transient processes in radiotechnical circuits, causing stepwise changes of Heff direction on the edges of RF pulse, in the frames of distortion mechanism. Our group was among the first ones to study the role of transient processes in the SPR formation. In the present Project we will continue the investigations of distortion mechanism's role in the SPE signal formation. Besides the edge-type mechanisms like nonresonant and distortion ones, the SPE in inhomogeneously broadened Hahn systems can be formed by the nonlinear mechanism of intrinsic nature so-called multipulse mechanism. The possibility of new intrinsic type SPE mechanism formation in magnetics was pointed for the first time by us in 1996. We will continue its further investigations in lithium ferrite and other magnetics. It turned out that by this mechanism the secondary echo signals of SPE and two-pulse echo (TPE) were also formed. Let us note that the understanding of SPE and secondary echo signals nature is important not only from the theoretical point of view but also it is of a great practical meaning for radiotechnical devices using the spin echo phenomenon.
Another most important application of NMR in magnetics is, first of all, its use for investigation of the DW structure. Besides, one more important application is connected with the use of relaxation processes in magnetics for the control of technology of their optimal synthesis for the aims of present project. A number of our works in this direction make it possible to use this effects for a practical value.
In the course of many years research since seventies our group has developed a number of original and sensitive methods of NMR investigations in magnetics. Among them let us note the pulsed method of investigation of NMR spectra basing on the use of two RF linearly FM pulses considerably simplifying and accelerating the process of obtaining the NMR spectra and, besides this, the simple and sensitive original continuous method of multidomain magnetics NMR spectra investigation which is particularly sensitive at low temperatures. The complex of devices for obtaining of strong pulsed and low frequency magnetic fields for investigations of DW dynamics and NMR spectra in multidomain magnetics was developed. In the course of this Project this complex of equipment will be modernized possibly resulting in some commercially interesting applications. Spin processor's prototype using the magnetic echo effect will be developed. As a further development and application of presented methods the investigations of magnetoacoustic echo phenomena will be performed in ferrites and HTSC ceramics. For the purposeful development of synthesis technology to study the memory in these systems specialists of the Silicate Technology Chair of GTU and the Scientific and Research Laboratory of "Superconductive Microelectronics" of the MIET (TU) will be involved. The role of DW and Abrikosov vortex lattice in the formation of magnetoacoustic echo signals and also the effect of magnetic field pulses on their formation will be studied. EPR methods including nonresonant microwave absorption will be used to control of HTSC properties. Along with experimental studies the theoretical investigations involving quantum-statistical methods developed by the Georgian school in the Magnetic Resonance Theory will be conducted. More then half of Project's participants are former weapon scientists. Among them 7 have got the D.Sc, (Phys.-Math.) degrees. The participation in this Project will be an alternative for their defence activity and makes it possible to restore their normal scientific activity and renew former contacts with leading specialists in Magnetic Resonance from Moscow, S.Petersburg, Ekaterinburg and so on and strengthen existing scientific contacts with our collaborators and establish new ones.
The main aims of present project could be summarized as follows:
1. Investigations involving both theoretical and experimental studies of nuclear spin echo phenomena in magnetics. These phenomena include magnetic and multipulse echoes and problems of nonlinear dynamics of electron-nuclear spin-systems including the case of simultaneous excitations by alternating and pulsed magnetic fields.
2. Development of NMR and EPR methods for characterization of magnetics and HTSC ceramics. These techniques will allow an improvement of the technology available for the application of magnetics and HTSC materials in functional electronics devices that employ spin echo phenomenon.
3. Development of new types of NMR spectrometers in magnetics, including new types of spin and acoustic processors using magnetic and domain-acoustic phenomena.