Research Of Magnetic Quantum Tunneling In Single-molecule Magnets And Metal-organic Frameworks

Many macroscopic quantum effects?hereafter,MQE?,such as Josephson effect,quantum Hall effect and A-B effect,have been discovered with the devel-opment of experimental techniques and appearance of new materials.The MQE could not only promote the development of new physical theory but also play a key role in practical application.The Magnetic Quantum Tunneling?hereafter,MQT?in Single-Molecule Magnets?hereafter,SMMs?and Metal-Organic Frame-works?hereafter,MOFs?studied in this thesis also belongs to MQE.The thesis includes two parts.The first one focuses on the research of magnetic relaxation in SMMs and preparation of films with SMMs.The second part introduces the effect of hydrogen bond on MQT in MOFs and the development of experimental technique involved.The previous studies show that dipolar interaction,hyperfine interaction and disorder play a key role in the magnetic relaxation of SMMs.Mn₁₂-tBuAc,a carboxylate derivative of Mn₁₂-Ac,is synthesized by straightforward carboxylate substitution methodology.Although lattice constants both a and c in the former are larger by 20%than that in the latter,the two SMMs have the same ratio of c/a and magnetic core,which makes them a perfect couple to study MQE.The mag-netic relaxation rates of Mn₁₂-Ac and Mn₁₂-tBuAc are obtained by measuring DC magnetic susceptibility at low temperature.The results shows that the magnetic relaxation rate in Mn₁₂-Ac is slower than that in Mn₁₂-tBuAc in which magnetic dipolar field is smaller.“Hole digging”experiments give the information of hyperfine interaction and indicate that the strength of hyperfine interaction in Mn₁₂-Ac and Mn₁₂-tBuAc are close to each other.To compare the magnetic dipolar field quantitatively,numerical calculation is done and demonstrates that the component of dipolar field along the easy axis H_dip,zand the component of dipolar field perpendicular the easy axis H_dip,xin Mn₁₂-Ac are both twice that in Mn₁₂-tBuAc.The following experiment shows that the decreasing of intermolec-ular separation makes the“detuning”effect of H_dip,zexceed the“promoting”effect of H_dip,xand results in a slower magnetic relaxation rate?^?in Mn₁₂-Ac.The magnetic property of a single molecule in SMMs is identical to that of the bulk,which makes them good candidates in ultra-high information stor-age and spintronics devices.The very first step to realized these applications is fabrication of low dimensional systems of SMMs.In this thesis,the films of Mn₁₂-tBuAc are successfully prepared on a substrate of Si?100?by Langmuir-Blodgett technique.The further characterization confirms the films are monolayers.Be-sides,the films are perfectly smooth and homogeneous.The anisotropic hysteresis loops of the films at low temperature indicate that the preferential orientation of the molecules in the films and the molecules still own anisotropic barriers.AC magnetic susceptibility results shows that two kinds of molecules with different anisotropic barriers exist in the films.The molecules with higher barrier are the same with that in bulks.The appearance of the lower barrier leads to a faster relaxation rate.The molecules keep the property of SMMs and the preparation of the films lay the foundation of single molecule device fabrication.The coexistence of magnetoelectric coupling and MQT in Metal-Organic Frameworks[?CH₃?₂NH₂]Fe?HCOO?₃?hereafter,DMFeF?provides a perfect plat-form to tune the MQT by nonmagnetic means.Samples with different disorders of hydrogen bond in DMFeF are prepared with different cooling rates at the antiferroelectric transition.Different disorders of hydrogen bond lead to dif-ferent magnetic properties both in DC and AC magnetic susceptibility at low temperature?<20 K?.It turns out that the hydrogen bonds effects not only the amount of single ion magnets component but also the anisotropic barriers.The distribution of electrons in oxygen atoms surrounding Fe²⁺changes after partic-ipating in the hydrogen bond.As a result,the crystal fields around Fe²⁺vary as well as the anisotropic barriers.The research above provides a powerful tool to tune magnetic quantum tunneling by nonmagnetic means.AC magnetic susceptibilities at low frequency?<1 Hz?are needed in the research on MQT.PPMS,the commercial instrument of Quantum Design,could not provide measurements below 10 Hz.To fulfill the need of AC magnetic sus-ceptibility measurement at low frequency,a new device based on Hall effect is fabricated.Compared with the coil detection based on Faraday law,the sensitiv-ity of our device is unsensitive to frequency.After optimizing the parameter in measurement,a sensitivity of 10^-9emu is obtained at 0.7 Hz.What's more,to e-liminate the large background at high frequency,we introduce a coherent double AC measurement to the AC magnetic susceptibility measurement.The back-ground of measurement is suppressed effectively due to the different frequencies between lock-in and magnetic field,which extends the application of the device to high frequencies.