Structure And Magnetostriction Of Pseudo-binary RT₂ Compounds

Pseudobinary Laves phase compounds RT2(R:rare earth;T:transition metals)have attracted intensive attention due to their giant low-field magnetostriction around the second-order ferromagnetic-ferromagnetic transition,'for instance spin reorientation transition(SRT).Previous study has shown that the crystal symmetry remains unchanged and the average magnetic moment varies continuously across such transition.However,recent high resolution x-ray diffraction(XRD)measurement has revealed that RT2 ferromagnets involves a first order structure transformation.Consequently,the SRT can also be regarded as a magnetic morphotropic phase boundary(M-MPB),which separates two ferromagnetic phases with different crystal symmetries.Both enhanced and weakened magnetostriction effects in pseudobinary RT2 systems have been found from the magnetic MPB.However,there remains another question:whether the local magnetization changes or not across such transition or not?.In this work,two pseudobinary systems Dy(Co1-xFex)2 and Tb(Co1-yFey)2 were developed.For the former,the two components DyFe2 and DyCo2 possess the same tetragonal symmetry(Ms//[001])but different tetragonalities(c/a<1 for DyCo2,T1 with Tc ? 140 K and c/a>1 for DyFe2,T2 with Tc = 635 K)below Tc.For the latter,TbCo2 and TbFe2 exhibit the same rhombohedral crystal symmetry below Tc but much difference in the magnetic ordering(Tc is 235 K for TbCo2 and 697 K for TbFe2).Using in-situ high resolution synchrotron XRD,neutron powder diffraction(NPD)and magnetometry measurements,the phase constituents,local magnetic moments and magnetostriction across the ferromagnetic-ferromagnetic transition were investigated.Main results are as follows:Magnetostriction sign change effect from the ferromagnetic MPB is found in the pseudobinary system Dy(Co1-xFex)2.The in-situ synchrotron XRD data reveal that two tetragonal phases with c/a<1 and c.a>1 coexist over a broadening temperature range,and their preferable variant reorientations result in contraction and subsequent elongation behaviors.Upon cooling below TC,the high temperature T2 phase with c/a>1 transforms into MPB region within which two tetragonal phases coexist and further into low temperature T1 phase c/a<1.Accordingly,the ?//-H curve changes from the "?" type to the "W' type and finally to the"?" type due to the magnetic domain switching.Especially within the MPB region,the magnetostriction magnitude as well as the critical field at which magnetostriction sign changes are highly temperature dependent due to the gradual evolution of the constituting tetragonal phases and the magnetocrystalline anisotropy.Further analysis suggests that the magnetostriction sign change effect originates from preferable domain switch of the T1 phase with lower magnetocrystalline anisotropy than the T2 phase with larger magnetocrystalline anisotropy.This finding may complete the study of intermetallic-rare earth metallic alloys and may bring important implications.Local magnetization changes across the ferro.-ferro.transition in the pseudobinary system Tb(Co1-yFey)2.Magnetometry measurements reveal that a composition independent ferromagnetic-ferromagnetic transition occurs at?100 K despite the increased Curie temperature Tc with raised Fe concentration in the Tb(Co1-yFey)2.Temperature-dependent synchrotron XRD data reveal that the lattice strain along the<111>direction reaches a peak at 100 K,accompanied with experimentally observed magnetostriction maximum.In-situ neutron powder diffraction(NPD)results show that the local magnetic moments of T atoms exhibits an abrupt change across the ferro.-ferro.transition at 100 K.Such findings indicate close correlations between the magnetic structure and the magnetostrictive effect in the pseudobinary RT2 compounds.