Materials Science & Engineering
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Item In situ Manipulation of Magnetization via Direct Mechanical Interaction in Magnetostrictive Thin Films(2014) Nero, Paris Noelle Alexander; Cumings, John P; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The pursuit of a universal memory- possessing fast write/read times, nonvolatile and unlimited data endurance, low operating power, low manufacture costs, high bit density, as well as being easily integrable with on-trend complementary metal-oxide semiconductor (CMOS) devices- has reenergized research in the field of multiferroic and magnetoelectric materials. Such materials simultaneously exhibit ferroelectricity and ferromagnetism, and allow for the coupling of the two order parameters, known as magnetoelectric coupling. This coupling is enhanced in magnetostrictive/piezoelectric bilayer systems where applied electrical bias can modify magnetic order via strain-mediation, a mechanism that can reduce the power demands in emerging magnetic random access memory (MRAM) technologies. We have previously investigated this relationship in an Fe0.7Ga0.3/BaTiO3 bilayer structure using magnetic contrast imaging techniques with in situ applied electric fields. The goal of this thesis was to explore methods to better control magnetoelectric effects in order to enhance local magnetic response to external stimuli. Specifically, we investigated magnetoelastic response of freestanding, magnetostrictive Fe0.7Ga0.3 thin films via direct mechanical interaction with an external probe, as the well known strain-mediated mechanism in magnetoelectric devices depends on the lesser known magnetoelastic nature of strain transfer between the distinct material phases. Magnetoelastic effects are directly associated with both external magnetic field and stress via Lorentz-force transmission electron microscopy (LTEM) contrast techniques, and the hysteresis of magnetic order was charted with respect to both stimuli. For relevant application to MRAM devices, we have initiated studying these effects in patterned media as well, where individual, nanoscale magnetic geometries represent bistable bits for memory. We demonstrate static pure stress effects on the magnetoelastic response in continuous thin films, as well as real-time mechanical "writing" of stable domain states. The external probe is directed into the film, inducing a non-uniform, radially symmetric local strain. Micromagnetic simulation reveals that the strength of observed magnetoelastic effects is offset by small, undulating variations in magnetization characteristic of polycrystalline thin films, known as magnetization ripple. Imposing a threshold function on the effective anisotropy of the film describes the spontaneous onset of these effects and the differences in magnetic order for films with hysteresis solely due to stress, or with both field and stress. Thus, a method to achieve bistable logic for MRAM applications using direct uniform stress, in lieu of external fields, is proposed.Item FABRICATION AND CHARACTERIZATION OF MAGNETOSTRICTIVE THIN FILMS USING THE COMBINATORIAL METHOD(2011) Hunter, Dwight Denroy; Takeuchi, Ichiro; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Magnetostrictive materials are smart materials which show dimensional and magnetization changes in response to magnetic fields. Presently, there is growing interest to find magnetostrictive thin films for many microsystem applications, especially in microelectromechanical systems (MEMS) as powerful transducers for microactuators. But to exploit their capabilities and meet the stringent needs of microactuator and sensor applications, small driving magnetic fields on the order of mT are desirable. Current magnetostrictive materials such as rare-earth containing Terfenol, despite exhibiting giant magnetostriction, is at an extreme disadvantage due to the high saturation field (H > 0.1 T) imposed by its large magnetocrystalline anisotropy. Recently, large magnetostriction was observed in Fe-Ga alloys, and has sparked widespread research into other Fe-based alloys for possible replacements of Terfenol. Furthermore, it is becoming increasingly important to find rare-earth free compounds from a cost and availability point of view. In this thesis, we investigated the composition dependent magnetostrictive and micro-structural properties of several binary (Fe-Ga, Co-Fe, Fe-Zn, Fe-W, Fe-Mo) and ternary (Fe-Ga-Zn, Fe-Co-Al) Fe-based thin film alloys prepared using a co-sputtering based composition spread approach. This technique facilitates synthesis and screening of large compositional landscapes in individual studies and allows rapid identification of compositions with enhanced physical properties. Magnetostriction measurements were performed on as-deposited and on some annealed composition spread films, which were fabricated on arrays of micro-machined cantilevers substrates. From this study, binary Co-Fe thin film alloys emerged as a large magnetostrictive material with effective magnetostrictive values in excess of 260 ppm at a low saturation field ¡Ö 10 mT, which were quenched following a vacuum anneal at 800 oC for 1 hour. This substantial increase in magnetostriction was observed for compositions near the (fcc+bcc)/bcc phase boundary (Co0.65Fe0.5), and was found to depend on the cooling rate from the annealing temperature. Structural characterization by synchrotron micro-diffraction and transmission electron microscopy (TEM) reveals that this large increase in magnetostriction is associated with the presence of an equilibrium Co-rich fcc phase that precipitates into a Fe-rich bcc host phase upon annealing. The Co-Fe system is compared with Fe-Ga alloys, in which DO3 nanoprecipitates dispersed in the host A2 matrix were observed at compositions (Fe0.8Ga0.2), which displays enhanced magnetostriction. The DO3 nanoprecipitates in the Fe-Ga alloys are believed to behave as tetragonal defects in the matrix and their orientations can be changed by the application of a magnetic field, leading to magnetostriction. It is speculated that the Co-rich precipitates in our Co-Fe films function in much the same way as the DO3 precipitates in the Fe-Ga alloys, implying that the mechanisms which give rise to magnetostriction in both systems are similar. The results on the as-deposited Fe-Ga-Zn and Fe-Co-Al ternary thin film spreads are somewhat encouraging from the point of view of finding new magnetostrictive materials. In the Fe-Ga-Zn alloys, we found that the magnetostriction value around the Fe0.6Ga0.1Zn0.3 compositions was reasonably high, &lambdaeff ~ 80 ppm. This could be interesting from an application and cost points of view: this means that a less expensive metal such as Zn could be substituted for Gab, while still preserving the magnetostriction. For the Fe-Co-Al ternary, the highest effective magnetostriction, &lambdaeff ~ 80 ppm, was observed near the Fe0.5Co0.25Al0.25 composition.Item MAGNETIC AND STRUCTURAL CHARACTERIZATION OF Fe-Ga USING KERR MICROSCOPY AND NEUTRON SCATTERING(2010) Mudivarthi, Chaitanya; Flatau, Alison B; Wuttig, Manfred; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)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 ppm) 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 ppm to 400 ppm) 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 (&sim 400 MPa), low hysteresis, and low saturation fields (&sim 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.Item Combinatorial Investigation of Magnetostrictive Materials(2007-08-24) Hattrick-Simpers, Jason Ryan; Takeuchi, Ichiro; Wuttig, Manfred; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Combinatorial materials synthesis is a research methodology, which allows one to study a large number of compositionally varying samples simultaneously. We apply this technique in the search for novel multifunctional materials. The work presented here will discuss the combinatorial investigation of novel magnetostrictive materials. In particular, binary Fe-Ga and the ternary Fe-Ga-Al, Fe-Ga-Pd systems are studied. Magnetron co-sputtered composition spread samples of the alloys have been fabricated to study composition dependent trends in magnetostriction. Magnetostriction measurements on all systems studied here have been carried out by optically measuring the deflection of micro-machined cantilever arrays. Measurements of the magnetostriction on binary Fe-Ga thin-films show similar compositional trends as had been reported in bulk systems. The maximum value of magnetostriction observed is 220 ppm, which is comparable to bulk values. A previously unreported minor maximum in magnetostriction as a function of composition has been found for Ga contents of about 4 at%. It is believed that the origin of this minor maximum is related to a peak in the magnetic moment of Fe atoms in Fe-Ga alloys at this composition. We have mapped the Fe-Ga-Pd and Fe-Ga-Al ternary systems. Large regions of the phase diagrams have been mapped out in a single experiment, and the observed magnetostrictive dependence on Ga content matches trends seen in bulk. It was found that the trend of magnetostriction deviated from that of bulk with the inclusion of as little as 1 at% Pd. The addition of up to 10 at % Al to Fe70Ga30 was possible without severe degradation of its magnetostriction.Item Elasticity in Ferromagnetic Shape Memory Alloys(2004-11-23) Dai, Liyang; Wuttig, Manfred; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Ferromagnetic shape memory alloys (FSMAs) are a new class of active materials, which combine the properties of ferromagnetism with those of a diffusionless, reversible martensitic transformation. These materials are technologically interesting due to the possibility of inducing large shape changes with an external applied magnetic field; either inducing the austenite/ martensite transformation or rearranging the martensitic variant structure with an applied field will induce a reversible shape change. The dependence of a solid's elastic properties on temperature in the vicinity of a structural transformation provides insight into the nature of the transition. Therefore, the elasticity of Ni2MnGa and Fe3Pd were studied. The temperature dependence of the elastic constants of the austenitic Ni0.50Mn0.284Ga0.216 and Ni0.49Mn0.234Ga0.276 were studied by an ultrasonic continuous wave method. Anomalous behavior in austenite was observed, which indicates a premartensitic transition. The temperature dependence of the elastic constants in martensitic Ni0.50Mn0.284Ga0.216 indicates a structural phase change from the tetragonal to a second phase at lower temperature. Modeling this phase change as a reentrance transition reproduces the major aspects of the temperature dependence of the shear elastic constant, (C11-C12)/2. The elasticity as a function of temperature and magnetic field of Fe3Pd was studied as well. An abrupt change of the elastic constants at around 45OC indicates a possible premartensitic transformation. The magnetic field dependence of elastic constants also indicates a probably magnetic field induced transition.