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Register a new userMagnetization and spin dynamics of a Cr-based magnetic cluster: Cr$_{7}$Ni
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We study the magnetization and the spin dynamics of the Cr$_7$Ni ring-shaped magnetic cluster. Measurements of the magnetization at high pulsed fields and low temperature are compared to calculations and show that the spin Hamiltonian approach provides a good description of Cr$_7$Ni magnetic molecule. In addition, the phonon-induced relaxation dynamics of molecular observables has been investigated. By assuming the spin-phonon coupling to take place through the modulation of the local crystal fields, it is possible to evaluate the decay of fluctuations of two generic molecular observables. The nuclear spin-lattice relaxation rate $1/T_1$ directly probes such fluctuations, and allows to determine the magnetoelastic coupling strength.
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Using an elastic neutron scattering technique under a pulsed magnetic field up to 30 T, we determined the magnetic structure in the half-magnetization plateau phase in the spinel CdCr$_2$O$_4$. The magnetic structure has a cubic $P4_3$32 symmetry, which is the same as that observed in HgCr$_2$O$_4$. This suggests that there is a universal field induced spin-lattice coupling mechanism at work in the Cr-based spinels.
Significant discrepancies have been observed and discussed on the lattice stability of Cr between the predictions from the ab initio calculations and the CALPHAD approach. In the current work, we carefully examined the possible structures for pure Cr and reviewed the history back from how Kaufman originally determined the Gibbs energy of FCC-Cr in the 1970s. The reliability of Cr lattice stability derived by the CALPHAD and ab initio approaches was systematically discussed. It is concluded that the Cr lattice stability based on the CALPHAD approach has large uncertainty. Meanwhile, we cannot claim that the ab initio HFCC-Cr is error-free as FCC-Cr is an unstable phase under ambient conditions. The present work shows that the ab initio HFCC-Cr can be a viable scientific approach. As both approaches have their limitations, the present work propose to integrate the ab initio results into the CALPHAD platform for the development of the next generation CALPHAD database. The Fe-Cr and Ni-Cr binary systems were chosen as two case studies demonstrating the capability to adopt the ab initio Cr lattice stability directly into the current CALPHAD database framework.
Among the factors determining the quantum coherence of the spin in molecular magnets is the presence and the nature of nuclear spins in the molecule. We have explored modifying the nuclear spin environment in Cr$_7$Ni-based molecular nanomagnets by replacing hydrogen atoms with deuterium or the halogen atoms, fluorine or chlorine. We find that the spin coherence, studied at low temperatures by pulsed electron spin resonance, is modified by a range of factors, including nuclear spin and magnetic moment, changes in dynamics owing to nuclear mass, and molecular morphology changes.
A systematic investigation about the structure and magnetism of Fe75-xCr25Gax (11<x<33) and Fe50Cr50-yGay (0<y<33) series has been carried out in this work. It shows that the parent Fe50Cr25Ga25 phase has higher tolerance for Ga replacing Cr than replacing Fe atoms. An abrupt flip of Curie temperature and magnetization in the Fe50Cr50-yGay (0<y<33) series was observed at the composition of Fe50Cr25Ga25. We proposed an explanation concerning anti-sites occupation and magnetic structure transition in this series. The induced structure is proved energetically favorable from first-principles calculations. This work can help us to understand the dependences between the crystal structure and magnetism in Fe-based Heusler compounds, and provides a method to deduce the atomic configurations based on the evolution of magnetism.
We demonstrate by means of fully relativistic first principles calculations that, by substitution of Fe by Cr, Mn, Co, Ni or Cu in FePt-L10 bulk alloys, with fixed Pt content, it is possible to tune the magnetocrystalline anisotropy energy by adjusting the content of the non-magnetic species in the material. The changes in the geometry due to the inclusion of each element induces different values of the tetragonality and hence changes in the magnetic anisotropy and in the net magnetic moment. The site resolved magnetic moments of Fe increase with the X content whilst those of Pt and X are simultaneously reduced. The calculations are in good quantitative agreement with experimental data and demonstrate that models with fixed band structure but varying numbers of electrons per unit cell are insufficient to describe the experimental data for doped FePt-L10 alloys.
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