Preparation,Electronic,and Magnetic Properties Of Mn₂SB Epitaxial Thin Films

With the channel widths of semiconductor field-effect transistors（FETs）approaching quantum scales,Moore’s Law no longer applies,and the road to controlling the switching characteristics of logic circuits by charge appears to be coming to an end.Therefore,researchers have sought to harness both the charge and spin of electrons to construct new magnetoelectric devices,thus giving birth to spintronics in solid-state physics.It is a hot research topic in spintronics to study the electrical transport of electron intrinsic spin and its associated magnetic moment in complex magnetic and magnetic phase change materials,based on which prototype devices,such as magnetic tunnel junctions,magnetic spin valves,and spin-transfer-torque magnetic memory have been built.Mn₂Sb is an interesting magnetic material system with strong spin-lattice coupling.The two Mn atoms in the chemical formula belong to different crystallographic sites,and the exchange between their electron orbitals makes the rearrangement of spin and magnetic moments easy in the magnetic ordering temperature range,so that the magnetic and electrical transport of the material are strongly associated with multiple physical properties.Transition metal elements doping could significantly change the electron spin arrangement and magnetic moment-related properties of Mn₂Sb,such as magnetic ordered structure,magnetic domain structure,magnetostriction,and magnetoelectric transport.The multi-dimensional coupling and tunability between magnetism and other physical properties in Mn₂Sb provides an ideal platform for room-temperature electronic/spintronic applications,as well as to investigate the competing interactions between physical parameters.In the past few decades,research on Mn₂Sb has mainly confined to crystalline bulk materials,however,in order to realize device applications,a comprehensive and systematic study of Mn₂Sb epitaxial thin films is urgently needed.Unfortunately,the report on the epitaxy of Mn₂Sb on oxide single crystal substrates is still absent.In this paper,we have grown high-quality Mn₂Sb and Mn_2-xCr_xSb epitaxial thin films on SrTiO₃ substrates for the first time.The crystal structure,microscopic morphology,elemental composition,electrical-transport and magnetic properties of the thin films were characterized in multiple dimensions,and the results corroborate with each other.The magnetic order structure of the films is analysed,the relationship between spin orientation and magnetoelectric transport properties is studied,and the internal mechanism of the coupling between the physical phenomena of Mn₂Sb films and the substrate interface is discussed in depth.This work is mainly divided into the following three parts:（1）High-quality Mn₂Sb（001）epitaxial films were grown on SrTiO₃（001）single crystal substrates using molecular beam epitaxy.After repeated optimization experiments,the optimal process parameters were determined:the flux rate of the Mn source was 1.8-1.9（?）/min,the flux rates ratio of the Mn source and the Sb source was about 1:1.1,and the substrate temperature was about 330 ^oC.The grown samples were high-quality single-oriented epitaxial films without any impurity phase,and the full width at half-maximum（FWHM）of the XRD rocking curve taken on the Mn₂Sb（001）diffraction peak was only 0.1^o.Through electric-transport and magnetic measurements,an unexpected antiferromagnetic phase was observed in Mn₂Sb epitaxial films at low temperatures,which has never been reported in the study of Mn₂Sb crystalline materials.The change of the lattice structure of the film with temperature was analysed by low-temperature in-situ XRD,and it was found that the c-direction lattice of the film contracted abnormally due to the interface clamping,so that the distance between Mn atoms was narrowed and their exchange effect was enhanced,resulting in the transition from ferrimagnetic to antiferromagnetic phase.In addition,spin reorientation was observed in the ferrimagnetic phase of the thin film,and the spin magnetic moment was switched from in-plane to out-of-plane of the thin films,which changed the transport properties of the magnetic anisotropy of the thin films.（2）Mn₂Sb epitaxial films with different thicknesses were grown on SrTiO₃substrates.By studying the effect of thickness on the spin moments and electrical transport properties of the films,it is found that there are two independent and competing mechanisms at the interface of the film and the substrate,namely,the clamping effect caused by the crystal epitaxy and the stress effect caused by the substrate lattice mismatch.The former leads to a ferrimagnetic-antiferromagnetic transition in the Mn₂Sb film,while the latter causes the spin moment canted in the film plane,resulting in a spin canted layer.For thinner films,the magnetization of the spin-canted layer contributes to the main magnetoelectric properties,and there is a very large saturation magnetization that decrease the antiferromagnetic stability of the film,while increasing film thickness will dilute the effect of the canted antiferromagnetic.In addition,Mn₂Sb film with a thickness of 32 nm exhibits a huge negative magnetoresistance of-51.8%（81.4 K,14 T）.The Mn₂Sb thin film with a thickness of16 nm has the largest anomalous Hall conductivityσ_AH of 24.2Ω^-1·cm^-1（150 K）,and a very respectable room-temperature anomalous Hall conductivityσ_AH of 8.5Ω^-1·cm^-1,its room-temperatureσ_AH is about 2-3 times that of Mn₂Sb bulk.（3）For the room temperature application,we modified the films by Cr doping,and grown Mn_2-xCr_xSb epitaxial films on SrTiO₃ substrates.At the room temperature of 300 K,Mn_1.89Cr_0.11Sb film was obtained.A negative magnetoresistance of-13.6%was obtained at B 5 T,and it decreased to-17.2%when B increased to 14 T.Compared with the undoped Mn₂Sb film at room temperature and B 5 T（-0.27%）,the magnetoresistance effect is improved by more than 50 times.Simultaneously,doping can make the largest anomalous Hall resistivityρ_AH of the films appear near room temperature,and theρ_AH at 280 K increased to 0.89μΩ·cm.In addition,the Cr doping extends the antiferromagnetic phase range of the films towards the higher temperature range,while the temperature range of ferrimagnetic spin reorientation is not affected.Thus,the antiferromagnetic phase will constrict the spin reorientation of the films,so that the orientations of the spin moment in the antiferromagnetic phase gradually switches from in-plane to out-of-plane.