First Principle Theoretical Study On Magnetic Properties Of Monolayer Ni₂Fe-LDHs And Ni₂Fe-OH

Our density functional theory plus Hubbard U calculations reveal that the magnetic structure of unilamellar Ni₂Fe hydroxide nanosheets is sensitive to the experimental conditions,containing cation ordering and surface species and elastic stress.Whether the interlayer anion is present or absent affects the local magnetic moments and valence states of the Ni and Fe ions of the layer,while cation ordering does not seem to have the same effect.All of these facilitate the formation of a variety of magnetic superlattices in Ni-Fe hydroxide nanosheets.The valence state change of the Fe2+/Fe3+ ions is related to the absent of interlayer anion and further affects the sequence of breaking of the O-H bond and of release of the hydroxyl group,which will promote different reactions.Cation ordering and surface species do not change the superexchange nature of coupling between Ni and Fe.The relationships of magnetic property and catalytic activity of Ni₂Fe-LDHs and Ni₂Fe-OH can be established by the valence state change of the Fe ions as reactive sites,which will be helpful to open a way for the design of LDHs-based magnetic catalyst.Moreover,external stress will lead to the magnetic transitions of monolayer Ni₂Fe-LDHs and Ni₂Fe-OH from ferromagnetic to antiferromagnetic.Firstly,we study the cation ordering effects on the magnetic properties of monolayer Ni₂Fe-LDHs.The results show that the Ni-OH-Ni is Ferromagnetic coupling?FM?,the Ni-OH-Fe and the Fe-OH-Fe is Anti ferromagnetic coupling?AFM?.The ordering or disordering of the cations does not change the magnetic coupling between adjacent Ni atoms,adjacent Ni and Fe atoms,but allows for the energy-degenerated magnetic domains with different spin alignments.Secondly,we investigate the magnetic properties and stability of the monolayer Ni₂Fe-LDHs,as the interlayer anion Cl-is absent.The Bader analysis,the bond valence calculation and the atomic magnetic moment show that the valence state of Fe ions is reduced from +3 to +2.In the meanwhile,however,the superexchange nature of coupling between Ni and Fe do not change.In addition,the induced impurity states in the gap not only lead to the spin splitting of TM-3d electrons,but also affect the catalytic activity by promoting the cleavage of the O-H bond,rather than the release of hydroxyl group.Finally,we study the magnetic properties of the monolayer Ni₂Fe-LDHs and Ni₂Fe-OH under external stress and find magnetic transformation is occurred in Ni₂Fe-LDHs and Ni₂Fe-OH.