Mössbauer Spectroscopy of Cobalt Ferrites Andrew Watson Dr. Kelly Krieble γ Fe 57 γ

Outline The Mössbauer Effect: What is it and how does it work? Materials (Cobalt Ferrites) Applications & Production The Spinel Crystal Structure Current and Future Data Conclusions

The Mössbauer Effect: What is it? Absorption / emission “[J]ust like a gun recoils when firing a bullet” [1], an atom must recoil when emitting / absorbing a γ-ray. Atoms, etc. have “excited states”, which require an exact amount of energy to attain, too much or too little will not do. If a γ-ray strikes an atom, it must have an energy greater than the energy of this transition, due to the recoil of the atom. Conversely, due to recoil, a γ-ray emitted from said atom will have slightly less than this transition energy. E0E0 E1E1 E2E2 ΔEΔE ΔE+E R [2]

The Mössbauer Effect: What is it? Classical vs. Quantum Mechanical Models Classical Model - γ-rays need an exact energy to be absorbed into / emitted from an atom. ZERO overlap. Quantum Mechanical Model – wider acceptable range of energies due to Heisenberg’s Uncertainty Principle However, this “overlapping” region is very small and very rare. Recoilless emission / absorption almost never happens with a lone atom. Resonant absorption / emission E+E R E-E R E E+E R Classically Q-Mechanically γ Fe 57 γ γ γ γ

The Mössbauer Effect: How does it work? Crystal Lattices… …relative to a single atom, have effectively an infinite mass. When a single γ-ray strikes a crystal, energy can still be lost in recoil, but it must be lost in discrete quanta called “phonons”, and the recoil is distributed to the entire solid. This recoil is sometimes so small that not even a single phonon is released. This is known as a “recoilless emission / absorption.” γ E=E γ γ

The Mössbauer Effect: Definition The Mossbauer effect is the recoilless, resonant emission and absorption of gamma rays

The Mössbauer Effect: Can any crystal exhibit it? To successfully exhibit the effect, a crystal lattice needs to meet a few requirements: “As resonance only occurs when the transition energy of the emitting and absorbing nucleus match exactly, the effect is isotope specific” [1]. Essentially, what is needed is a “favorable” ratio of recoil energy to excitation energy [1]

The Mössbauer Effect: Why is it significant? Mössbauer spectroscopy gives us information about a few types of nuclear interactions: Isomer Shift Hyperfine Splitting

The Mössbauer Effect: Isomer Shift “For a source made of compound s, and an absorber of compound a, the peak of the absorption curve will move from zero velocity of source relative to absorber, found if s and a are the same compound, to a value given by… a γ s

The Mössbauer Effect: Isomer Shift “δ(v) = (4/5)πe 2 R 2 (ΔR/R)[|Ψ(0)| s 2 -|Ψ(0)| a 2 ] Where R = (R e +R g )/2 and ΔR = R e -R g … Hence δ(v) = K(|Ψ(0)| s 2 -|Ψ(0)| a 2 ). K depends only on the nuclear characteristics and is a constant for a given Mössbauer transition. It can be seen that the magnitude of this shift, called the isomer shift …is determined by the difference in the electron densities at the nuclei of the atoms containing the Mössbauer nuclei in the source and absorber.” [5]

The Mössbauer Effect: Zeeman Splitting Nuclear energy levels Magnetic effect, dipoles align

Materials (Cobalt Ferrites) Sample Creation Cardiff University, Wales: Powders of 99.9% pure Co 3 O 4, 97%+ pure Mn 3 O 4, and Fe-oxide mixed to desired ratios for 30 minutes to thoroughly blend powders Samples are ground together, pressed into a ½” dia. ¾” slug, then heated in a furnace at 1000°C for an hr., cooled for 24 hrs., repeat Samples ground again, pressed into a slug, then heated at 1350°C for 24 hrs., cooled again for several hours

Named after the mineral “spinel” - MgAl 2 O 4 Basic structure: AB 2 O 4 Includes A-site (tetrahedral) and B-site (octahedral) atoms Preferences for different sites affect magnetic properties Materials (Cobalt Ferrites) Spinel Structure

Allows two atoms which are not touching to interact through an intermediate atom Two magnetic cations interact through a non-magnetic anion Electrons act as dipoles, dipoles align Materials (Cobalt Ferrites) Superexchange

Applications energy and transportation industries, actuators noncontact stress and torque sensors to help address energy efficiency, environmental, and safety issues If used as torque sensors for electronic power steering assists a 5% improvement in fuel efficiency would be realized over hydraulic power steering…” [6]

Fitting Data ‘Lor’= FWHM of intrinsic Lorentzian ‘z0’= Center of Hyperfine Field Gaussian Distribution ‘dz’= FWHM of HFD Gaussian ‘shift0’= shift value when z0 = 0 ‘rel area’ = area above the curve “The full width at half maximum (FWHM) is a parameter commonly used to describe the width of a ‘bump’ on a curve or function. It is given by the distance between points on the curve at which the function reaches half its maximum value.” [3] [4]

Data Ge-series: 0.1, 0.2, 0.3, 0.4, 0.6 (Co[1+x]Ge[x]Fe[2-2x]O[4]) Co-series: 0.2, 0.6, 0.9, 1.1, 1.2, 1.8, 2.2 (Co[x]Fe[3-x]O[4]) Quenched samples: from 600°C, 800°C, 1000°C, 1200°C, 1400°C Al-series: 0.08, 0.25, 0.54, 0.73, 0.92 approx. (CoAl[x]Fe[2-x]O[4])

More Data

Conclusions Ge-series: “Bump” in data was normalized with new samples Co-series: 3-site pattern suggests the samples are a 2-phase mixtures of spinel structure and another phase Quenched: Differences are subtle. “By other measurements, we believe that we see the evidence of site occupancy changes that you might expect. The ones quenched from the lowest temperature appear to have the most Co in octahedral (B) sites, and least in tetrahedral (A) sites, and the ones quenched from the higher temperatures appear to approach a more random arrangement…” – Dr. John Snyder

Sources! 1.Mössbauer Spectroscopy Group, Royal Society of Chemistry (RSC) website, Introduction to Mössbauer Spectroscopy Part 1. Accessed July International Board on the Applications of the Mössbauer Effect (IBAME) and Mössbauer Effect Data Center (MEDC), Mössbauer Effect website. Accessed July Weisstein, Eric W. "Lorentzian Function." From MathWorld --A Wolfram Web Resource. 4."Gaussian Distribution." Wikipedia. Wikimedia, 08 July Web. 9 July Maddock, Alfred. Mössbauer Spectroscopy: Principles and Applications. 1st ed. Chichester: Horwood Publishing, Print. 6.Snyder, Ph.D., J.E., and D.C. Jiles. New Substituted Cobalt Ferrite-Based Magnetoelastic Materials: Understanding a New Class of Oxide-Based Smart Materials. The Wolfson Centre for Magnetics, Print.