MAGNETIC AND STRUCTURAL CHARACTERIZATION OF Fe–Ga USING KERR MICROSCOPY AND NEUTRON SCATTERING

Title of dissertation: MAGNETIC AND STRUCTURAL CHARACTERIZATION OF Fe–Ga USING KERR MICROSCOPY AND NEUTRON SCATTERING Chaitanya Mudivarthi, Doctor of Philosophy, 2010 Dissertation directed by: Professor Alison B. Flatau and Professor Manfred Wuttig Dept. of Materials Science and Engineering Fe–Ga alloys belong to a class of smart materials called magnetostrictive materials. Magnetostrictive materials show dimensional (magnetostriction) and magnetization changes in response to magnetic and elastic fields. These effects can be utilized for transduction purposes. Most widely used magnetostrictive materials like Tb-Dy-Fe (Terfenol-D) show giant magnetostriction (∼ 2000 μǫ) but suffer from low modulus of elasticity, low tensile strength and are extremely brittle, limiting their usage to applications involving only axial loads. Fe–Ga alloys have recently been discovered to show an extraordinary enhancement in magnetostriction (from 36 μǫ to 400 μǫ) with the addition of the nonmagnetic element, Ga. Though their magnetostriction is less than that of Terfenol-D, they boast superior properties such as ductile-like behavior, high tensile strengths (∼ 400 MPa), low hysteresis, and low saturation fields (∼ 10 mT). Understanding the origin of the magnetostriction enhancement in these alloys is technologically and scientifically important because it will aid in our quest to discover alloys with higher magnetostriction (as Terfenol-D) and better mechanical properties (as Fe–Ga). With the goal of elucidating the nature of this unusually large magnetostriction enhancement, Fe–Ga solid solutions have recently been the focus of intense studies. All the studies so far, show the existence of nanoscale heterogeneities embedded in the cubic matrix but the experimental means to correlate the presence of nanoscale heterogeneities to the magnetostriction enhancement is lacking. In this work, Fe–Ga alloys of various compositions and heat treatments were probed at different length scales lattice level, nano-, micro-, and macro-scales. Neutron diffraction was used to probe the alloy at the lattice level to identify the existence of different phases. Small-Angle Neutron Scattering (SANS) experiments were used to study the nanoscale heterogeneities and their response to the applied magnetic and elastic fields. Ultra small-angle neutron scattering (USANS), magnetic force and Kerr microscopy were used to investigate the response of magnetic domains under externally applied magnetic and elastic fields. Piecing the results from lattice level, nano-, micro-, and macro-scales together with the macroscopic magnetostriction measurements, the nature of the magnetostriction in Fe–Ga alloys was uncovered. No evidence could be found that directly relates the presence of heterogeneities to the enhanced magnetostriction. Further, it was found that the observed heterogeneities were possibly of DO3 phase and are detrimental to the magnetostriction. MAGNETIC AND STRUCTURAL CHARACTERIZATION OF Fe–Ga USING KERR MICROSCOPY AND NEUTRON SCATTERING by Chaitanya Mudivarthi Dissertation submitted to the Faculty of the Graduate School of the University of Maryland, College Park in partial fulfillment of the requirements for the degree of Doctor of Philosophy 2010 Advisory Committee: Professor Alison B. Flatau, Chair/Advisor Professor Manfred Wuttig, Co-Advisor Professor James Cullen Dr. Mark Laver Professor Lourdes Salamanca-Riba Professor Norman M. Wereley c © Copyright by Chaitanya Mudivarthi 2010 g zb} œA g zEv Z , g zd vo mh ˜r, । g zsA "At̂ prB}œ t-m{ ™Fg rv nm, ॥ Gurur BrahmA Gurur Vishnu Gurur Devo MaheshvarA, Guru sAkshAt ParaBrahma tasmai Shri Guruve namah. Aadi Shankara (788 820 CE) “Guru (teacher) is truly a representative of Brahma the creator, Vishnu the protector, Maheshvara the destroyer because Guru creates, sustains knowledge and destroys the weeds of ignorance. Guru is the supreme spirit him/herself. I salute such a Guru.” This dissertation is dedicated to everyone who imparted knowledge and made me what I am today.