Magnetic Quantum Property Research Of Mn₃ Molecule Magnets

Macroscopic quantum effect,which bridges the mesoscopic and atomic scales by showing a quantum behavior through large objects,is usually hided by statisti-cal averaging.However,under specific extreme conditions,like low temperature,high magnetic fields and high pressure,the microscopic particles of large objects may couple with each other and form an entirety which exhibits the quantum property,such as the Bose-Einstein Condensation,superconductivity,super-fulid and Josephson effect.The research object of this thesis is Mn₃ molecule magnet-s.One part of this work focus on the quantum tunneling of magnetization,and the other part studies the magnetocaloric effect?MCE?of completely oxidated Mn₃ thus Mn₃???.We also prepared the Nb/Ti/Nb junction,aimed to obtain the Fermi velocity of Ti.Contrary to the widely studied single molecule magnets without intermolec-ular interaction,each Mn₃ molecule is coupled antiferromagnetically to its three neighbouring molecules by hydrogenbond,forming a 2D honeycomblike crystal structure in ab plane,and negligible interaction between layers.There have been interesting findings in this system,such as the equal interval splitting of the tunneling and the spin pair tunneling.In this work,we find direct mani-festation of quantum spin selection rules by means of low temperature magnetic measurements.According to the giant spin approximation Hamiltonian,we can calculate the Zeeman diagram,and determine the corresponding resonance of observed steps?dM/dH peaks?.Based on the analyses,four splitting steps of the resonance from |6>to |-5>should be absent,with two of them were absent clearly and the other two were demonstrated negligible contribution from |6>to|-5>confined by local spin environments,which were simulated by Monte Carlo method quantitatively.This phenomenon is firstly observed in exchange coupled molecule magnets,and it's the most radical phenomenon of destructive inter-ference.Since the quantum tunneling of SMMs with intermolecular exchange couplings is heavily dependent on LSE,and hence implies a feasibility of manip-ulating the quantum tunneling at macroscopic scale.It's know that the tunnel-ing process occurs in a definite window due to the various perturbations,such as the spin-phonon broadening effect.In this work,broadening effects of both single-spin tunneling and spin-pair tunneling are researched by ac-susceptibility measurements in high temperature regime.The results demonstrate that,the width of single-spin tunneling broadening by spin-phonon interactions increases with temperature apparently,whereas the broadening width of spin-pair tunnel-ing shows temperature independent behavior and consistent with the magnitude of non-phonon broadening.We think the spin-phonon interaction may be offset in some mechanisms in the spin-pair tunneling mechanism.The Mn₃???molecule magnet can be obtained by oxidating its parent com-pound Mn₃ in air for a certain time.The intensity of antiferromagnetic couplings in Mn₃???is three time larger than Mn₃,which leads to a higher phase transition temperature than block temperature.We studied the magnetocaloric effect of Mn₃???in this work,an inverse situation is observed at lower magnetic field,which is associated with the competition between external magnetic field and antif'erromagnetic couplings.However,the inverse MCE of Mn₃???vanishes and a standard pattern is obtained as the magnetic field increases,the critical value of magnetic field is 1.7 T,which is consistent with the intersection of these finite-temperature curves.This field-induced transition arises from the destruction of antiferromagnetism above the critical field value,since the intermolecular inter-actions in Mn₃???molecule magnets are too weak to transform the spin network into a classical antiferromagnet.In the previous research for the superconductivity of Ti,the Fermi velocity of Ti obtained from the measurement of critical field gave a value substantially lower than prediction of the band calculation.In this work,we try to distinguish the discrepancy through the transport property of Nb/Ti/Nb junction,and attempt to observe the McMillan-Rowell oscillations above the gap which will yield the renormalized Fermi velocity.The preliminary result shows that,a series of peaks below the gap is identified to be associated with the inhomogeneous property of large junction area.While above the gap,we also observe several peaks may be contributed to McMillan-Rowell oscillations.Analysis of the peaks yields a Fermi velocity in the barrier?Ti?of VF = 7.7×105 cm/s,which is one order lower than the result from critical field measurement.The mechanism of this lower Fermi velocity still needs further experimental investigations.