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Towards a unified description of dynamics and anisotropy in nanomagnets

Final Report Summary - DYNANIMAG (Towards a unified description of dynamics and anisotropy in nanomagnets)

The project aimed to study the physical properties of magnetic materials made from a few atoms up to thousand of atoms such as Molecular Nanomagnets (MNMs) and Magnetic Nanoparticles (MNPs). The understanding of their magnetic properties (magnetic anisotropy and magnetisation dynamics) and related phenomena is of fundamental importance for the validation of classical and quantum theories as well as for the exploitation of their functional properties in Nanotechnology. Therefore the objects of the project were: 1. to study the magnetic anisotropy in MNMs and MNPs with the aim to understand the physical origin of magnetic anisotropy and identify novel systems that lead to significantly improved magnetic data storage devices. 2. to perform spin dynamic experiments such as relaxation, quantum tunnelling and quantum coherence in MNMs and MNPs with the aim of understanding how Nature evolves upon the transition from classical to quantum world. In order to achieve the proposed objectives firstly a careful choice of the samples to be synthesised was made in collaboration with chemists across the world. The first part of the project was also dedicated to the study of the latest achievements and theories in the field of Nanomagnetism. In-depth characterisations of the magnetic properties of the samples obtained from the collaborators were performed by SQUID magnetometer measurements. Magnetic anisotropy and spin dynamics studies were performed in some of the most advanced centres of magnetic resonance in the UK (EPSRC CW-EPR service at the University of Manchester) and in Europe (University of Stuttgart and IFW Leibniz Institute of Dresden). Inelastic neutron scattering experiments were also performed at the neutron source at the ISIS large-scale facility (Oxford) and pulsed electron spin resonance experiments were performed at the Sir Peter Mansfield Magnetic Resonance Centre at the University of Nottingham as well as at the School of Chemistry of the University of Stuttgart. The main results achieved are: 1. Observation of surface effects and non-Gilbert relaxation of the ?Fe2O3 magnetisation in magnetoferritin. ?Fe2O3 magnetoferritin nanoparticles (Figure 1) with an average inorganic core diameter of 5.7 ± 1.6 nm were deeply investigated by and electron spin resonance (ESR) experiments. The analysis of ESR spectra measured at different temperatures, frequency ranges and magnetic field orientations (Figure 1) gave a complete and self-consistent characterisation of ?Fe2O3 magnetoferritin. The static and dynamic magnetic properties of magnetoferritin are strongly different from those of the corresponding bulk maghemite. The observed reduced magnetic moment per particle, increased magnetic anisotropy and particle uniaxial symmetry give evidence of surface effects (spin canting and reduced chemical coordination). The magnetisation dynamics studied by the resonance field – linewidth correlation extracted from the temperature dependence of the ESR spectra deviates from the classical Landau-Lifshitz-Gilbert (LLG) model. Further evidence for deviations of the magnetisation dynamics from the Gilbert-like relaxation is observed from the saturation of the frequency-dependent linewidth. An exponential decay function satisfactorily describes the magnetisation dynamics providing an overall relaxation time T2 of the order of sub-nanoseconds. In accordance with this conclusion no spin echoes were observed in pulsed-ESR measurements. 2. Magnetic resonance studies of bulk-like Mn17 SMMs. Magnetic properties and magnetisation dynamics were studied in [Mn17O8(N3)4(O2CMe)2(pd)10(py)10(MeCN)2(H2O)2](ClO4)3 in short Mn17 SMM (Figure 2) by electron spin resonance (ESR) spectroscopy. Temperature dependent and multifrequency ESR electron spin resonance measurements were performed. The temperature dependence of the ESR spectra showed the linewidth broadening and shifts of the resonance fields to lower magnetic fields upon cooling the sample from 150 K to 4 K typical for classical bulk magnets. The magnetic anisotropy (zero-field splitting) and the correlation between resonance field and external oscillating field (g value) were determined. The classical magnetic dynamics behaviour of Mn17 is interpreted as consequence of the small magnetic anisotropy and large spin ground state of this molecule which make their properties similar to bulk-like systems. The presented outcomes are expected to have a strong impact on the successful development of next-generation materials for ultrahigh-density magnetic data storage which is a multibillion euro market, highly sensitive microwave frequency detectors for astronomy applications, as well as for diagnostics and treatment of diseases in biomedicine. In addition this project will potentially have a strong impact on the fundamental physics stimulating the development of physical theories which try to merge classical and quantum properties for the description of the magnetic response of nanomagnets under microwave radiation. This is an urgent task that, because of the potential impact that nanomagnets are expected to have on our everyday life cannot be delayed any longer.