Electrical Transport Properties Of Oxide-based Ferromagnetic Semiconductor

Spintronics is a rapidly growing field focusing on phenomena and related devices essentially dependent on spin transport.Spin injection,spin transport,and spin detection by electrical or optical methods are critical for the spintronics.Spin injection is suffering from the problems:1) lack of magnetic semiconductors with high Curie temperature and high spin polarization;2) low efficiency of injection which is due to the resistance mismatch between metal and semiconductor materials.Spin related transport,including magnetoresistance,anomalous Hall effects and so on,is relatively less known and different experimental results have been reported by different research groups.So we focus on electrical transport properties of oxide ferromagnetic semiconductors with high Curie temperature and high transition metal concentrations which are recently observed by our group,and hope we can get better understanding of the spin transport which is supposed to give out some valuable suggestion on the spin controlling and spintronics device design. Our studies contain several parts:first of all,we studied the variable range hopping in wide-band-gap oxide ferromagnetic semiconductor by considering Coulomb interaction, exchange interaction and "hard gap" energy in the same frame;secondly,we studied the electrical transport property and positive magnetoresistance of singlecrystal diluted oxide magnetic semiconductor grown by molecular beam epitaxy;the third,we studied the origination of positive magetoresistance in oxide ferromagnetic semiconductor nanocluster system grown by gas-aggregation-sputtering system.Finallly,we have done some work torward magnetic field tunable Ge_1-xMn_x/Ge heterojunctions.In the following are main results we obtained so far:1) Based on our experimental results of Zn_1-xCo_xO_1-v and Ti_1-xCo_xO_2-v,we for the first time propose a spin-dependent variable range hopping model which takes into account the electron-electron Coulomb interaction and the spin-spin exchange interaction in the same frame.The linear relation of lnp versus T^-1/2 observed at low temperature which shows different slopes and interactions at different magnetic field is well explained by this model. From theoretical fitting we can get the information about spin polarization,and the relative magnitude of exchange interaction and Coulomb interaction.2) Based on systematically study of electrical transport properties of Zn_1-xFe_xO_1-v ferromagnetic semiconductors,we found that the low temperature transport mechanism changes from Efros variable range hopping to "hard gap" resistance with increasing Fe concentration.As the energy which determines the hopping probability between two localization states is additive,we improved our spin dependent variable range hopping by combining the contribution of the "hard gap" energy.Detailed analysis reveals that the "hard gap" is a non-magnetic one which is due to the multi-electron relaxation effects followed by a long distance single electron hopping.3) We found Coulomb screening effects in the(In_1-xFe_x)₂O_3-v ferromagnetic semiconductors,which showed Mott variable range hopping at low temperature.This is because that In₂O₃ is much easier to be n-type doped.As a result these samples have a high carrier concentration which consists with the experimental results,i.e.the resistivity of (In_1-xFe_x)₂O_3-v samples is comparable to some "bad metal".Dues to the high carrier concentration,the Coulomb screening length is relatively small,while the hopping distance is large at low temperature,the interaction between initial and final states can be negelected.4) We for the first time propose that Al₂O₃ can be turned into magnetic semiconductors by introducing impurity levels inside its band gap through transition metal doping.We found similar crossover behavior from Efros variable range hopping to "hard gap" resistance in our(Al_1-xCo_x)₂O_3-v samples as in other wide band gap oxide semiconductors.The transport properties of(Al_1-xCo_x)₂O_3-v semiconductor and corresponding Co-Al₂O₃ granular system are studied.5) Step by step we finally realized that there exists a universal relationship between the various interactions of the ferromagnetic semiconductors and the transport properties.It is found that different materials with different doping elements show common electrical transport feature:Mott variable range hopping in the lower resistivity range;Efros variable range hopping in the middle resistivity range;and "hard gap" resistance in the higher resistivity range.This reveals that the electrical transport of oxide ferromagnetic semiconductors is mainly controlled by the concentration of oxygen vacancies. Experimental results are well described by considering the relative magnitude of the Coulomb screening length,localization length of the carriers,and optimal hopping distance. The contributions of "hard gap" energy,Coulomb interaction energy,correlation energy, and exchange interaction energy to the cartier hopping are also quite different.6) From the magnetic field and temperature dependence of the magnetoresistance,we conclude that large positive magnetoresistance observed in our single crystal Co-doped ZnO prepared by molecular beam epitaxial device originates from the wave function shrinkage of the carriers which is an orbital effect rather than spin effect.No spin- dependent transport behavior was observed.This is understood as a result of the relative small magnetic energy compared with electrical energy during the hopping process.7) We report large positive magnetoresistance as high as 800%in the Co-doped ZnO nanoclusters prepared by gas-aggregation-sputtering-cluster system.From the field dependence of the magnetoresistance,which means linear dependence on magnetic field at low magnetic field and saturate quickly in the high magnetic field,we know that the large positive magnetoresistance results from the large Zeeman splitting effects in the Co-doped ZnO nanoclusters.On the contrary,Co-doped Cu₂O samples only show a small positive magnetoresistance(5%) with similar magnetic field and temperature dependence as Co-doped ZnO nanoclusters,because the p-d exchange interaction in the Co-doped Cu₂O is small.8) For the first time we report single crystal Ge_1-xMn_x/Ge magnetic diodes formed by p-type Ge_1-xMn_x ferromagnetic semiconductor and intrinsic(or slightly n-type doping) Ge. They have amazing magnetic tunable properties except good rectifying properties.For example,room temperature magnetoresistance for Ge_0.95Mn_0.05/Ge is 440%,which makes it good candidate for high temperature spintronics applications.Furthermore,the magnetoresistance has a maximum value at Curie temperature of Ge_1-xMn_x.It is possible to shift the magnetoresistance maximum to high temperature by increasing the Curie temperature of Ge_1-xMn_x.