Final Report Summary - DMH (Nonlinear dynamic hysteresis of nanomagnetic particles with application to data storage and medical hyperthermia)
The main thrust of the project was a theoretical study of the magnetodynamics of single-domain particles driven by a strong ac field. Due to the large magnitude of the magnetic dipole moment (~10000–100000 Bohr magnetons) giving rise to a relatively large Zeeman energy even in moderate external magnetic fields, the magnetization relaxation process has a pronounced field dependence causing nonlinear effects in the dynamic susceptibility and field induced birefringence, stochastic resonance, dynamic magnetic hysteresis, etc. However, the nonlinear response to an external field represents an extremely difficult task even for dilute systems because it always depends on the precise nature of the stimulus. Thus, no unique response function valid for all stimuli exists unlike in linear response. The subject of prime interest to us was the dynamic magnetic hysteresis (DMH), i.e. the magnetic response of nanoparticles to an ac field of arbitrary amplitude with/without a dc bias field. In spite of numerous publications, progress in this method is hampered by the lack of a reliable understanding of the laws governing the nonlinear magnetodynamics of solid and liquid suspensions of magnetic nanoparticles. Presently, two aspects of DMH theory require unification. The first concerns DMH in an individual nanoparticle and that in an assembly of nanoparticles in suspensions. The nonlinear ac stationary response hitherto has been calculated for uniaxial nanomagnets either (i) by assuming the energy of a nanomagnet in external fields is much less than the thermal energy kT so that the response may be evaluated via perturbation theory or (ii) by assuming that strong external fields are directed along the easy axis of the particle so that axial symmetry is preserved. Thus, the results are in reality very restricted. In particular, the conventional assumption of axial symmetry is hardly realizable in nanoparticle systems under experimental conditions because the easy axes of the particles are randomly oriented in space. Furthermore, many interesting nonlinear phenomena (such as the damping dependence of the response and the interplay between precession and thermoactivation) cannot be included because in axial symmetry no dynamical coupling between the longitudinal and transverse (or precessional) modes of motion exists. The objective of the planned studies was to gain an understanding of the magnetodynamics of assemblies of fine magnetic particles of diverse nature (volume, saturation magnetization, etc.) placed in solid or liquid environmentы subjected to magnetic fields. The diversity of physical properties of particles and solid and liquid matrices, as well as that of the magnetic agitation modes renders the scope of the research very wide.
In the context of the project, we have developed accurate theoretical methods for evaluating the nonlinear ac stationary response of an individual single domain nanoparticle with various magnetic anisotropy potentials (uniaxial, biaxial, etc.) of both surface and volume origin in the presence of strong dc and ac magnetic fields both in the high and low damping limits. We have also extended the methods to calculating the nonlinear ac stationary response of an assembly of magnetic nanoparticles in in superimposed dc and ac magnetic fields. Furthermore, we have developed effective methods of calculation of the DMH in an individual nanoparticle and assemblies of nanoparticles with randomly oriented easy axes and we have analyzed the dependence of the area of the DMH loops on the temperature, frequency, and ac and dc bias field magnitude and orientation. By generalizing these results to the calculation of the nonlinear susceptibility and energy absorption of ferrofluids, we have also elaborated efficient methods of calculation of nonlinear DMH in liquid suspensions of magnetic nanoparticles. Theoretical predictions have been compared with relevant experimental data comprising the magnetization dynamics of nanoparticles driven by a strong ac field with particular application in magnetic hyperthermia. Hence, we have achieved all the anticipated objectives set out in our project.
The results obtained may have many applications, two of the most important being: magnetic moment switching (under pulsed fields) and heat generation (under oscillating fields). The first is important from the viewpoint of magnetic data storage, the second - for the development of magnetically induced hyperthermia not exclusively for medical applications. Besides the medical uses, DMH can also be used to characterize the recording density, signal-to-noise ratio, etc., in a given nanogranular medium. Furthermore, by accounting for the effect of thermally activated magnetization reversal (superparamagnetism), we can model switching processes for any desired field- temperature-thermomagneticprotocol, e.g. heat-assisted or hybrid magnetic recording techniques. As far as biomedical applications are concerned, one of the most promising thermal approaches which has recently received considerable attention is local magnetic hyperthermia. This utilizes ac magnetic field energy absorption by nanosized ferromagnetic particles syringed into the tumor. In addition to the above mentioned applications to ferromagnetic hyperthermia and data storage technologies we expect that our studies have substantially increased our knowledge concerning such promising trends in experimental techniques as microrheometry of complex (including biological) liquids employing small magnetic particles.
In the context of the project, we have developed accurate theoretical methods for evaluating the nonlinear ac stationary response of an individual single domain nanoparticle with various magnetic anisotropy potentials (uniaxial, biaxial, etc.) of both surface and volume origin in the presence of strong dc and ac magnetic fields both in the high and low damping limits. We have also extended the methods to calculating the nonlinear ac stationary response of an assembly of magnetic nanoparticles in in superimposed dc and ac magnetic fields. Furthermore, we have developed effective methods of calculation of the DMH in an individual nanoparticle and assemblies of nanoparticles with randomly oriented easy axes and we have analyzed the dependence of the area of the DMH loops on the temperature, frequency, and ac and dc bias field magnitude and orientation. By generalizing these results to the calculation of the nonlinear susceptibility and energy absorption of ferrofluids, we have also elaborated efficient methods of calculation of nonlinear DMH in liquid suspensions of magnetic nanoparticles. Theoretical predictions have been compared with relevant experimental data comprising the magnetization dynamics of nanoparticles driven by a strong ac field with particular application in magnetic hyperthermia. Hence, we have achieved all the anticipated objectives set out in our project.
The results obtained may have many applications, two of the most important being: magnetic moment switching (under pulsed fields) and heat generation (under oscillating fields). The first is important from the viewpoint of magnetic data storage, the second - for the development of magnetically induced hyperthermia not exclusively for medical applications. Besides the medical uses, DMH can also be used to characterize the recording density, signal-to-noise ratio, etc., in a given nanogranular medium. Furthermore, by accounting for the effect of thermally activated magnetization reversal (superparamagnetism), we can model switching processes for any desired field- temperature-thermomagneticprotocol, e.g. heat-assisted or hybrid magnetic recording techniques. As far as biomedical applications are concerned, one of the most promising thermal approaches which has recently received considerable attention is local magnetic hyperthermia. This utilizes ac magnetic field energy absorption by nanosized ferromagnetic particles syringed into the tumor. In addition to the above mentioned applications to ferromagnetic hyperthermia and data storage technologies we expect that our studies have substantially increased our knowledge concerning such promising trends in experimental techniques as microrheometry of complex (including biological) liquids employing small magnetic particles.