Preparation And Electrical And Magnetic Properties Of Cr_1-xTe Magnetic Thin Films

With the developing requirements of miniaturization,low power consumption and multi-functionality of electronic devices,spintronics has shown a very broad application prospect.The strongly correlated electron system related to electron spin is a research hotspot in the fields of condensed matter physics,electronics,materials,etc.There are many novel physical phenomena,such as quantum entanglement and topology,high-temperature superconductivity,fractional quantum Hall effect in two-dimensional electron gas,giant magnetoresistance effect in manganese oxides,etc.Cr-Te compound is a ferromagnetic material with the Ni As structure and a strong spin-orbit coupling system.Its obvious electromagnetic transport properties such as anomalous Hall effect,magnetoresistance effect and perpendicular magnetic anisotropy show attractive application prospects.In this paper,a series of Cr-Te thin film samples such as Cr₂Te₃,Cr Te,Cr_1-xTe with different component contents and doped Cr_2-xMn_xTe₃thin films were prepared on Sr Ti O₃（111）single crystal substrates by molecular beam epitaxy.Their electrical and magnetic properties were studied in detail.The research content of this paper mainly includes the following aspects:（1）The growth process of Cr₂Te₃thin films were explored on Sr Ti O₃（111）substrate by molecular beam epitaxy,and the optimal substrate temperature was275℃.Cr₂Te₃films with different thicknesses were synthesized at the optimum substrate temperature,and their electrical and magnetic properties were studied in detail.The results show that the samples with different thicknesses have obvious ferromagnetism,and the Curie temperature is around 175 K.Both the anomalous Hall signal and the out-of-plane hysteresis loop indicate that Cr₂Te₃has an obvious perpendicular magnetic anisotropy.The results show that the coercive force of the samples is very high,up to 10 k Oe,and the coercive force of the film decreases with the falling thickness.The study of the magnetoresistance effect shows that the Cr₂Te₃film has an obvious negative magnetoresistance effect,and the magnetoresistance reach a peak of over 40%near the Curie temperature.（2）Cr_2-xMn_xTe₃films with different Mn doping concentrations were grown on（111）oriented Sr Ti O₃single crystal substrates by molecular beam epitaxy adjusting the molecular beam currents of Cr,Te and Mn sources.The results show that by changing the temperature or beam current of the Mn source,Mn can be doped into the Cr₂Te₃thin film system in two different ways.One is doping in a way of replacing Cr or vacancy sites,and the other is that Mn and Te form Mn Te crystals which are embedded in the Cr₂Te₃thin film system.The electrical and magnetic properties of these two types of thin film samples show obvious differences.The curve of resistance versus temperature shows that the transition temperature of the system from the ferromagnetic metallic phase to the paramagnetic insulating phase is different.The Cr_2-xMn_xTe₃thin film samples all have obvious ferromagnetism,and the Curie temperature is around 175 K.With the increase of Mn doping concentration,the saturation magnetization and coercive force of the thin film samples decrease.The Cr_2-xMn_xTe₃thin film samples exhibit obvious negative magnetoresistance effect,and the magnetoresistance reach a peak of 60%near the Curie temperature.A clear topological Hall effect is observed at low temperature（2 K）for the sample with Mn source temperature of 700°C.（3）The effect of substrate temperature on Cr_1-xTe films was explored on Sr Ti O₃（111）single crystal substrates by molecular beam epitaxy,and the electrical and magnetic properties of the films were studied in detail.Various characterization results such as X-ray diffraction,Raman spectroscopy and energy spectroscopy patterns show that the substrate temperature has a significant effect on the formation of the phase structure of Cr_1-xTe thin films.The thin film samples grown at different substrate temperatures have an obvious ferromagnetism.As the substrate temperature increases,the Curie temperature of the thin films increase from 175 K to room temperature close to 300 K,and the magnetization perpendicular to the film plane gradually decrease,while the magnetization parallel to the film plane increase gradually.In addition,from the output curve of the anomalous Hall resistance signal,it is observed that the sign of the anomalous Hall signal of the Cr_1-xTe film is reversed in a specific temperature range,and the multi-resistance states of the material can be regulated by using this polar reversal.The study of the magnetoresistance effect shows that the Cr_1-xTe film has an obvious negative magnetoresistance effect,and the magnetoresistance is the largest near the Curie temperature.（4）The Cr Te film with high-quality crystalline and atomical flat surface was successfully synthesized on Sr Ti O₃（111）substrates by molecular beam epitaxy,and the microstructure and magnetic properties of the Cr Te film were studied in detail.The detailed magnetic study of the Cr Te film system shows that the Cr Te film sample has a room temperature ferromagnetism with a Curie temperature up to 330 K.The Cr Te thin film system exhibits an in-plane macroscopic magnetism of antiferromagnetic,ferromagnetic and paramagnetic phases at different temperature ranges,indicating its rich magnetic structure and the competitive relationship among various magnetic orders.When the magnetic field is enhanced,the Cr Te film system exhibits obvious out-of-plane magnetism:the magnetic moments of the Cr atoms are all turned out-of-plane,and the magnetization is much higher than the in-plane one.The Cr Te thin film sample exhibits a positive magnetoresistance at lower temperatures（below～90 K）,while the sample exhibits a negative magnetoresistance at higher temperatures（above～90 K）.The positive magnetoresistance at low temperature may be the result of the competition between the antiferromagnetic and ferromagnetic phases in the sample,or the strong electron-electron interaction in the strongly correlated Cr Te system.The Cr Te film also shows an anisotropic magnetoresistance effect,which is most pronounced in the direction of the easy axis of magnetization.Finally,the anomalous Hall effect,topological Hall effect and sign reversal of Hall signal are obviously observed in the Hall output signal.