Strain-Mediated Control Of Magnetic And Electric Properties In Ferromagnetic Hybrid Structures

Strain-mediated magnetism means the materials,which possess ferromagnetism,paramagnetism or anti-ferromagnetism are grown on specific substrates,may change their magnetism under strain.The phenomenon has drawn lots of interests.In this thesis,we have systematically studied the strain-mediated magnetism and its physical mechanism by growing two types of heterostructures on[Pb(Mg1/3Nb2/3)O3]0.7-(PbTiO3)0.3(PMN-PT),which is a typical piezoelectric material.Then,we designed a new type of flexible spin valve and studied the strain-mediated giant magnetoresistance(GMR)of the spin valve grown on a flexible substrate.The main results of my thesis are listed below.(1)We have studied the in-plane electric field controlled magnetism of perovskite LaSrMnO3(LSMO)films,which were epitaxially deposited on different single crystal PMN-PT substrates by pulsed laser deposition.By applying uniform electric fields on the PMN-PT substrates,the LSMO films on the substrates suffer strains because PMN-PT is a typical piezoelectric material.The in-plane coercivity,remanence,Curie temprature and anisotropic magnetoresistance of the LSMO films show regular change with the in-plane electric fields applied on the substrates,which can be used to generate magnetic signals.The experimental results can be well explained by Stoner-Wohlfarth model and our first principle calculations.(2)The in-plane electric field controlled perpendicular magnetic anisotropy of L10-FePt films,which were deposited on polarized PMN-PT(001)single crystal substrates under 650 ℃ by magnetron sputtering,is studied.The out-of-plane coercivity Hc of the FePt films changes with the in-plane electric field applied on the substrates.The experimental change of Hc is approximately 21.3%while the electric field varies from-10 to 10 kV/cm.The magnetization direction can be controlled by switching the electric field.The remanence of the FePt films can also be manipulated by the electric field,so that a "W"-shaped sawtooth magnetic signal can be generated by a sawtooth electric field.(3)The strain-mediated GMR of a top pinned spin valve with the stack structure of Pt/Co90Fe10/Cu/Co90Fe10/IrMn are deposited on a flexible polyethyleneterephthalate substrate by magnetron sputtering.The strain transverse to the magnetic easy axis can manipulate the magnetism of the spin valve and it results in a large reversible GMR variation from 0.64%to 2.08%.The phenomenon is repeatable,even after bending the substrate 500 times.It can be explained by Stoner-Wohlfarth model.The heterostructure could also be used to generate rectangle or sawtooth wave GMR signals.