Neutron Diffraction Study On The Evolution Of Spin Structure And Related Physical Properties In MM'X Alloys

The MM'X alloys(M,M'=transition element,X=main element)with Ni₂In-type hexagonal structure show rich magnetic and crystal properties,and hence many physical effects are derived,such as negative thermal expansion(NTE),magnetocaloric effect(MCE),barocaloric effect(BCE)and skyrmion,etc.Compared to the X-ray diffraction(XRD),neutron powder diffraction(NPD)has many advantages,such as spin structure analysis,distinguishing neighbor elements,in-situ external field environments,etc.Therefore,NPD is considered to be one of the most important characterization methods in materials research.In this dissertation,the MM'X alloys are taken as the research objects.Though combining the in-situ neutron powder diffraction and first-principles calculations,the spin structure,lattice distortion,and the spin structure evolution driven by temperature,magnetic and pressure field were systematically studied.As a result,giant negative thermal expansion(NTE)and baromagnetic effect(BME)were achieved,and the underlying mechanism of the pressure regulated MCE was disclosed from atomic level.The main researches are shown as follows:1)The incommensurate cone-spiral magnetic ordering related lattice distortion,texture effect and giant negative thermal expansion were discovered in the Fe-doped MnNiGe alloys.Utilizing the GPPD diffractometer of China Spallation Neutron Source(CSNS)and the BT-1 diffractometer of National Institute of Standards and Standards(NIST),the spin structure of Fe-doped Mn NiGe alloys were comparably studied.The refinement results revel that the both Mn_0.87Fe_0.13NiGe and Mn_0.89Fe_0.11NiGe with slightly different Fe doping show similar incommensurate cone-spiral magnetic structure in the martensite,where the spiral axis l poses parallel to a-axis,and the angle between the magnetic moment and spiral axis l is less than 90°.First-principles calculations were performed to demonstrate the stability and related mechanism of the cone-spiral magnetic structure.For the Fe-doped Mn_0.87Fe_0.13NiGe alloy,the energy of the cone-spiral magnetic structure is 0.8 me V/f.u lower than the spiral antiferromagnetic(AFM)structure,which supports the NPD results.Further research indicated that Mn_0.87Fe_0.13NiGe with incommensurate cone-spiral magnetic ordering shows significantly lattice distortion,compared to MnCoGe_0.99In_0.01 with linear ferromagnetic(FM)ordering.Due to the distinct magnetic coupling,the differences of Mn-Mn(d1)and Mn-Mn(d2)distances in the martensite between the two alloys reach3.61%and 2.60%,respectively.However,there are almost no difference of the Mn-Mn distances in the paramagnetic austenite.Therefore,the two alloys show distinct lattice distortions during the martensitic transformation.Quantitative calculations show that the lattice distortion degree(?_ani)of Mn_0.87Ni_0.13Fe Ge(?_ani?8.68%)is 15%larger than that of MnCoGe_0.99In_0.01(?_ani?7.49%).The larger lattice distortion in the former motivates the cleavage breaking of hexagonal phase along c-axis,and controllable texture effect appears,which greatly enhances the in-plane NTE behaviour.The measured maximal linear thermal expansion of Mn_0.87Fe_0.13NiGe with cone-spiral magnetic ordering reaches?L/L?-23690×10^-6,which is 3.3 times larger than that of corresponding average crystallographical contribution(-7121×10^-6).This work reveals the incommensurate cone-spiral magnetic ordering of Fe-dopedMnNiGe for the first time.This is also the first incommensurate spiral magnetic structure detected and resolved by the newly built GPPD diffractometer.The comparative neutron diffraction experiment between the GPPD diffractometer and the internationally advanced BT-1 diffractometer verified the scientific validity and important value of the GPPD diffractometer of CSNS in the research of materials science.Moreover,this is the first time to utilize the magnetic structure related lattice distortion and texture effect to gain giant NTE,which paves a new way for exploring NTE materials2)A new cycloidal spiral antiferromagnetic(AFM)structure(Cy S-AFM^b)was discovered in Fe-doped Mn_0.87Fe_0.13NiGe alloys.Though combining the in-situ neutron powder diffraction and first-principles calculations,the spin structure configuration was resolved,and its evolution with temperature,magnetic field and pressure field and related mechanisms were revealed.As a result,a giant BME was achieved.Previous research indicated that the AFM coupling of the stoichiometric Mn NiGe is so robust that the atomic local environment changes by a pressure as high as 8 kbar is unable to alter the AFM state.The substitution of Mn with Fe introduces Fe-Mn ferromagnetic(FM)coupling,which can help to establish a cone-spiral magnetic coupling.Hence,a new cycloidal spiral AFM structure(Cy S-AFM^b)was discovered in Fe-doped Mn_0.87Fe_0.13NiGe below 150 K,in which the spiral axis l is along the b-axis and the magnetic moment is perpendicular to the l.The new spin structure behaves susceptible against atomic local environment changes,and can be easily changed by either magnetic field or pressure.NPD with in-situ magnetic field indicated that,at 5 K,the Cy S-AFM^bevolves into the conical spiral FM(70°-Co S-FM^b)at around 0.6T,in which the spiral axis l is still along the b-axis,but the conical angle turn to be 70°.As H>1 T,the 70°-Co S-FM^bgradually evolves into a linear FM structure.More importantly,a pressure higher than 4 kbar can transform the Cy S-AFM^b into an easily magnetized conical spiral FM(45°-Co S-FM^a).Although the Ti Ni Si-type orthorhombic structure(space group:Pnma)remains unchanged under pressure,the reduced magnetic moment can be as much as 22%through altering the Mn-Mn(d1)and Mn-Mn(d2)distances.As a result,giant BME was demonstrated.The maximal baromagnetic coefficient(BMC)appears to be 5.34 emu·g^-1·kbar^-1under 0.35 T within temperature window?150 K(0-150 K),while 9.03 emu·g^-1·kbar^-1 under 2-5 T within temperature window?250 K(0-250 K),which all far exceed those of the previous reports.First-principles calculations provide theoretical explanations for the pressure-driven spin structure evolution and the shortened magnetic moment by pressure.With the lattice compressed,the overlap of Mn 3d orbitals becomes stronger and makes the 3d orbitals of Mn atoms more itinerant.As a result,the d-d hybrid bands become broader and hence leading to reduction of magnetic moments.The new spiral spin structure found in Mn_0.87Fe_0.13NiGe behaves sensitivity to atomiclocal environments,and shows complex evolution with external fields,which not only introduce a giant BME,but also provides an excellent platform for exploring other novel physical effect,such as topological magnetism.3)MnNi_1-xFe_xSi_1-yGe_y alloys were prepared,and the mechanism of pressure regulated magnetostructural transition(MST)and MCE were clarified by NPD with in-situ pressure.The MM'X alloys with magnetostructural transition usually show a giant MCE.Enhanced and diminished MCEs by hydrostatic pressure were both reported previously,but the underlying mechanism is unclear.The decoupling of MST was usually proposed to be the origin.In this dissertation,Mn Ni_0.47Fe_0.53Si_0.46Ge_0.54was taken as the research object,and the pressure regulated MST and MCE were studied by combining NPD with in-situ pressure with magnetic measurements.Careful refinements indicate that the martensitic orthorhombic phase possesses a linear FM structure with the magnetic moments confined on Mn sites and the direction along b axis.Although a pressure P?5 kbar can somewhat compress the Mn-Mn(d1)bond lengths in a small range 3.080(?)?d1?3.086(?),the linear FM structure and the magnetic moment on Mn atoms remain unchanged.It suggests that the altered magnitude of Mn-Mn(d1)bond lengths is not enough to affect the spin structure and magnetic moment on Mn atoms.The refined results from NPD patterns indicate that the MST notably slows down as the pressure reaches 5 kbar,which originates from the extended temperature region of the coexistent FM orthorhombic and the non-magnetic hexagonal phase,rather than the decoupling of MST.Furthermore,the lattice entropy change(?S_Latt)of Mn Ni_0.47Fe_0.53Si_0.46Ge_0.54 under different pressures was calculated based on Debye approximation.The NPD refinements indicate that the maximal?V/V across the MST reduces by 7%from 2.84%to 2.63%as the pressure increases from 0kbar to 2 kbar.Accordingly,the estimated lattice entropy change?S_Latt from Debye approximation reduces by 10%from 37.1 J·kg^-1K^-1 to 33.5 J·kg^-1K^-1.The magnetic entropy change?S driven by magnetic field is calculated by using Maxwell relationship based on series of isothermal magnetization(M-H curves).For a field change of 0-5 T,the-?S peaks around?250 K with maximum value?33.3 J·kg^-1K^-1 under 0 kbar.An application of 2.1 kbar pressure shifts the-?S peak to?219 K with a nearly unchanged peak value 32.4 J·kg^-1K^-1.These?S driven by 5 T magnetic field are somewhat smaller than the lattice entropy change?S_Latt driven by temperature.This result may reflect that the MST driven by a magnetic field up to 5 T cannot be completed.This work discloses the mechanism of the pressure regulated MST and MCE fromatomic level,which is of great significance for understanding the regulated MCE by multiple fields for the materials with MST.