Nano Size Effect Study Of Spin-chain Compound ?-CoV₂O₆

The Ising spin-chain compound?-CoV₂O₆ is an interesting system without magnetic frustration due to regular triangular lattice but presenting 1/3 magnetization step.Meanwhile,it also possesses some other fascinating properties,such as large orbital moment,magnetodielectric coupling,field-induced spin-structural transition,giant magnetostriction and so on.Now,we aim to study the magnetism of?-CoV₂O₆ under the limited particle size,further in-depth study about metastable magnetism,spin-glass behavior and memory effect have also been done.Polycrystalline powder samples of?-CoV₂O₆ were prepared using the sol-gel method,and then a series of different nano-sized samples were obtained by mechanically milling for different times.XRD patterns,SEM images and EDS data confirm that all the samples are pure?phase with brannerite-like monoclinic crystal structure?space group C2/m?,as well as have different particle sizes.Magnetization measurements show that,compared with bulk sample,the ground state of the nano samples is still antiferromagnetic?AFM?,but at the AFM state below T_N,the magnetization is significantly enhanced.In addition,the 1/3 magnetization step is gradually suppressed in nanoparticle samples,the magnetization below H_c1 is significantly enhanced,but reduces at the ferromagnetic?FM?state above H_c2.Within the framework of core-shell model consisting of the AFM core spins and the uncompensated/disordered shell spins,the AFM enhancement can be attributed to enhanced shell disorder with weak ferromagnetism as the decrease of the grain size.This AFM enhancement,along with the suppression of saturation magnetization,results in the suppression of 1/3 magnetization step.Besides,Pulsed high field ESR measurements demonstrates the paramagnetism of the shell with FM correlations.The M?H?hysteresis and the zero-field remanence were observed in all samples,which inspires us to conduct further in-depth study about metastable magnetism by magnetic relaxation measurements.All the time-dependent magnetization curve can be well described by the stretch exponential relaxation formula.Spin-glass-like behavior exists in the nano samples,while the metastability of magnetization plateau in the bulk sample can be ascribed to a competition between magnetic,strain,and thermal energies.At the same time,we also observe magnetic memory effect in nano samples,which is consistent with the phenomenological hierarchical model,the relaxation at low temperatures hardly has any influence on the state at higher temperature,but not vice versa.