Study On Magnetic Behavior In VO₂ And Effects Due To Doping By Transition Metals

Vanadium dioxide(VO2),as a typical strongly correlated materials,has been attracting a lot of attentions because of the metal-to-insulator(M-I)transition occurring at a temperature Tc ~ 340 K.One reason for the attentions is that the M-I transition characterized by the sharp variation of several orders of magnitude in resistivity occurs near room temperature,proposing a positive prospect of application to various areas,such as switching devices,sensor and smart windows.On the other hand,the understanding of the origins responsible for the M-I transition involves many fundamental physical theories in strong correlation physics.So far,various approaches based on a Spin-Peierlsor Mott-Hubbard-type insulator as well as an ordinary band theory have been proposed,which stress,to a different degree,the role of lattice instabilities,electron-phonon interaction and electron-electron correlations.Both the experiments and mechanisms have been advanced,however,it is still a matter of controversy for the origin responsible for the M-I transition.In this paper,pure and transition-metal-doped VO2 bulks are taken as the object of the researches to find a clue to understand the origin of M-I transition.The temperature dependence of magnetic properties are investigeated systematically.Based on the quantified analyses to the susceptibility above and below Tc,the image of magnetic states during the transition is proposed.Along this route,the influence on the magnetic behaviors caused by 3d and 4d transition-metal-doping is studied.The main research contents and results are listed as followed:(1)To get the high-quality pure sample which is the key factor of the researches,the preparation technology of pure A-type VO2 sample is explored.Pure B-type VO2 powders are prepared by hydrothermal method.Then the powders are respectively annealed at Ar-atmosphere and in vacuum.The results show that although annealing at Ar-atmosphere can transform B-type VO2 into A-type VO2,but the impurity phases including V6O13 or V8O15 in samples can’t be eliminated.As a contrast,the sample annealed above the temperature of 700 oC in vacuum can suppress the formation of V6O13.Further increasing the annealing temperature upon 1000 oC,the impurity of V8O15 is still absence.The results show that annealing B-type VO2 in vacuum can get the Pure A-type VO2.Based on the preparation technology of pure A-type VO2 powders,the preparation technology of pure 3d transition-metal Cr-,Fe-doped and 4d transition-metal Nb-doped VO2 samples is explored.(2)The temperature dependence of resistivity in zero magnetic field and susceptibility in low magnetic field for pure VO2 sample is measured.In the variational temperature process,the measuring results show the simultaneous occurrence of both the M-I transition characterized by the sharp variation of more than two orders of magnitude in resistivity and the magnetic transition characterized by the sharp variation of susceptibility.By comparing the temperature dependence of resistivity and susceptibility measured in different modes,the onset temperature,transition temperature region,thermal relaxation and extent of thermal relaxation in M-I transition and magnetic transiton are shown to be highly similar,suggesting that both the transitions originate from a same physical mechanism.(3)Quantified analyses and researches about the origins of both the high-temperature paramagnetism and the low-temperature paramagnetism are displayed.The paramagnetic behavior above Tc is shown to originate from the Pauli paramagnetism of V4+ ions.While the anomalous temperature-dependent paramagnetic behavior below Tc is due to the Curie paramagnetism of dimers with a effective magnetic moment μeff.Proposing every dimer is formed by spin pairing of two V4+ ions at an angle θ close to 180o,further considering co-contributions of Pauli paramagnetism of residual V4+ ions and the Curie-type paramagnetism of dimers with effective spin,a phenomenological expression which can quantificationally explain the temperature-dependent susceptibility below Tc is proposed.Based on this model and analyses,the observed magnetic transition in VO2 is argued to be due to a transition from high-temperature Pauli paramagnetic state of V4+ ions to low-temperature Curie-type paramagnetism state of dimers.While the M-I transition is argued to be due to a transition from high-temperature near-free electron state of V4+ ions to low-temperature strongly localization state of dimers.(4)The 3d transition-metal(Cr and Fe)doping effects in VO2 are experimentally researched and analysed.As shown in the measuring curves of temperature dependence of susceptibility,the doped samples display two distinct characteristic-temperatures Tm1 and Tm2.Samples respectively undergo two magnetic transitions characterized by the sharp variation of susceptibility near the two temperatures in variational temperature process.Doping does not change the crystalline structure characterized by the periodic arrangement of V4+ ions along the c-axis to form a straight chain and the Pauli paramagnetic behavior in the high-temperature R phase above Tm1.The doped samples undergo the first magnetic transition when the temperature decreases to near Tm1.Structural analysis indicates that the doped samples belong to M2 structure at room temperature.There are two kinds of V4+ ions chains in the M2 phase,one is a dimerized chain while the other is a zig-zag Heisenberg chain which is consist of periodically arranged V4+ ions.Based on the experimental results,we then propose that the observed magnetic transition near Tm1 originates from the transition from the straight chain of V4+ ions to the dimerized chain in M2 phase.Further decreases the temperature to Tm2,the doped samples undergo the second magnetic transition which originates from the transition from the Heisenberg chain in M2 phase to dimerized chain in low-temperature M1 phase.Our results indicate that the anomalous temperature dependence of susceptibility below Tm2 in doped samples can be quantificationally explained based on the contribution of Curie-type paramagnetism of dimers with effective magnetic moment μeff.(5)The 4d transition-metal(Nb)doping effects in VO2 are experimentally researched and analysed.The Nb-doping hardly changes the Pauli paramagnetic behaviors in high-temperature R phase except for the increasing of susceptibility.However,comparing with the pure VO2,the doped samples show different paramagnetic behaviors in low-temperature M1 phase.By considering the Curie paramagnetic susceptibility of V3+ ions introduced by Nb-doping,the observed temperature dependence of susceptibility at low-temperature in doped samples obtains quantified explanation.More importantly,the temperature of magnetic transition characterized by the sharp variation of susceptibility decreases obviously with the increasing of doping content.The transition temperature can be lower to room-temperature even below.Due to the simultaneous occurrence of both magnetic transition and M-I transition,the measuring results indicate that the M-I transition temperature can be effectively manipulated by changing the Nb doping content.Based on the analyses for the experimental results,we propose that doping with Nb5+ ions with larger radius can not only increases the distance between V4+ ions,but also introduces V3+ ions,which resulting in the dimerization temperature moves to low-temperature.With the increasing of doping content,the suppression of dimerization becomes more effective.Therefore,the temperature of magnetic transition(as also M-I transition)evidently decreases with the increasing of Nb doping content.