Structure And Magnetic Properties Of REAlX(RE=rare Earth,X=Si,Ge)Weyl Semimetal

REAlX is a Weyl semimetal with unique physical properties,which has potential applications in the field of environmentally and friendly electronic devices.At present,the interest of REAlX(RE=rare earth,X=Si,Ge)system are mainly focused on the CeAlGe and PrAlGe,and the structural model of PrAlSi is controversial.Thus,the study of REAlX system is extended to the other rare earth-aluminum-germanium and rare earth-aluminum-silicon intermetallics in this work,including Nd and Sm based intermetallic compounds.The crystal structures of REAlX are refined by Rietveld refinement,and their magnetic properties are systematically studied with solid and molecular empirical electron theory(EET)according to their valence electron structure.REAlX(RE=La,Ce,Pr,Nd,Sm;X=Si,Ge)and(La,Pr)AlGe were prepared by non-consumable vacuum arc melting.The phase and structure analyses were carried out by X-ray powder diffraction.The diffraction peaks were initially indexed by Semi-Exhaustive Trial-and-Error(TREOR)program and the structures of REAlX were then refined from X-ray powder diffraction data by using Rietveld powder diffraction profile fitting technique.The temperature and field dependences of magnetizations for REAlX were measured by superconductor quantum interference device(SQUID)-vibrating sample magnetometer(VSM).X-ray diffraction analyses illustrate that REAlGe and(La,Pr)AlGe are single phases with non-centerosymmetrically body centered tetragonal structure,and its space group is I4lmd.In each unit cell,RE,Al,Ge occupy three inequivalent 4a(0,0,z)crystal positions,respectively.The structure consists of alternating stacks of A/B layers along the a axis,RE3,A13,and Gd3 constitute isosceles triangles ? in the ? layer and reversal isosceles triangle ? in the B layer.The non-centrosymmetrically magnetic structure is built by RE3? in the A layer and its reversal RE3 ? in the B layer.The magnetic triangle prism(RE6)is constructed by linking the two parallel RE3 ? between A and A layers.X-ray diffraction patterns of REAlSi(RE=La,Ce,Pr)show the coexistence of ordered phase with space group of 14lmd and site-disordered phase with space group of I4l/amd.The ordered phase is the main phase.SEM morphologies and EDS spectra of REAlX show that the grains sizes ranges from 200 nm to 7 ?m,the surface of the grains is smooth.EDS measurements show the element compositions of REAlX alloys are close to the stoichiometrically equiatomic REAlX.CeAlGe and PrAlGe order ferromagnetically at TC=5.7 K and TC=16.8 K,respectively.The temperature dependence of susceptibility for REAlGe(RE=Ce,Pr)follows the Curie-Weiss law in the paramagnetic region,and their effective magnetic moments are close to the theoretic magnetic moments of RE3+.It implies that the magnetism of REAlGe originates from RE3+ion.The exchanging interaction RE3??-RE3?? induces the ferromagnetism in REAlGe.REAlSi shows various magnetism.CeAlSi and PrAlSi order ferromagnetically at TC=8.2 K and Tc=18.9 K,respectively,PrAlSi shows low temperature spin-glass behavior,NdAlSi shows paramagnetic character,SmAlSi shows metamagnetic transition at 10 K and ferromagnetic order at 143 K,respectively.SmAlSi follows the Van-Vleck paramagnetism in the paramagnetic region.The effective moments of paramagnetic REAlSi are close to the theoretic moments of trivalent RE3+ion,it implies that the contribution to moment originates from trivalent RE3+ions.The effective magnetic moment of SmAlSi is lower than that of Sm3+ion,it might be due to the existence of Sm2+.The moment of Sm2+is zero,the existence of Sm2+in SmAlSi alloy dilutes the moment and causes the decrease of moment.La0.3Pr0.7AlGe,La0.5Pr0.5AlGe and La0.8Pr0.2AlGe order ferromagnetically at 11.2 K,8.0 K and 20.9 K,respectively.A weak magnetic transition occurs at 227 K in(La,Pr)AlGe.REAlX can hardly be magnetized and the magnetization cannot reach the saturation under a maximum field of 50 kOe,which might be related to the special magnetic structures.The ferromagnetism of REAlX originates from the parallel arrangement of RE3?? and RE3 ?? in A/A layer along a axis.In polycrystalline REAlX system,the magnetization direction is between the easy c axis and hard ab basal plane,thus,the saturation is very difficult for the magnetization.The correlation between the valence electron structure and magnetic moment of rare earth is studied by empirical electron theory of solids and molecules(EET).The study shows that the theoretical moment and bond lengths agree with the experimental ones well,the magnetic property of REAlX depends strongly on its electron structure.The cohesive energy of REAlSi is higher than that of REAlGe,the hybrid state of Al prefers to occupy the t state.The valence electrons number increases along with RE=La to RE=Pr,the magnetic 4f electron contributes to the magnetic properties.