Magnetic Behavior

Magnetic Hyperthermia

Strain-Induced Ferromagnetism in Intermetallic Compounds

Introduction

Many intermetallics which are paramagnetic when well-annealed become ferromagnetic upon plastic straining, e.g. Fe3Al, FeAl, CoAl, CoGa, etc. Deformation induces disorders so that atoms of the ferromagnetic element are no longer isolated from each other but interact.

The strain-induced ferromagnetism in lightly-strained FeAl appears to arise mostly from APB tubes. However, the phenomenology of this behavior is not well understood. The purpose of this study is to examine this concept by quantitatively relating the observed defect structures in FeAl to the magnetic properties as a function of both alloy composition and temperature. The idea, if correct, should be applicable to any intermetallic compound which contains at least one magnetic element, and which deforms by the motion of APB-coupled dislocations.

Results

1. Deformation-induced paramagnetic to ferromagnetic transition

Rolling induces a paramagnetic to ferromagnetic transition in iron-rich FeAl single crystals (Fe-40Al to Fe-50Al). The ferromagnetic behavior becomes more apparent when deformation increases. Typical hysteresis loops of rolled Fe-40Al are shown in Figure 1. The hysteresis loops become bigger as the rolling strain increases.

Typical magnetization versus applied magnetic field curves for Fe-40Al
Figure 1. Typical magnetization versus applied magnetic field curves for Fe-40Al specimen.

2. The relation between saturation magnetization and rolling strain

The saturation magnetization (MS) first increases slowly with increasing of rolling strain up to 20%, then increases rapidly, as shown in Figure 2. This means that APB tube formation is controlled by thermally activated cross-slip of screw dislocations.

The saturation magnetization versus thickness reduction for Fe-40Al single crystals
Figure 2. The saturation magnetization versus thickness reduction for Fe-40Al single crystals.

3. Comparison of magnetic behavior of Fe-40Al and Fe-43Al alloys

In B2-structured FeAl, strain-induced ferromagnetism decreases as Al concentration increases. Fe-43Al has smaller MS than Fe-40Al under the same rolling strain. Since Fe-43Al has a higher APB energy than Fe-40Al alloy, the width (~1 nm) of APB tubes in Fe-43Al single crystal is only one half the width (~2 nm) of those in Fe-40Al. Thus, APB tubes in Fe-43Al need less time and lower temperature to anneal out strain induced MS. At 77 K, rolled Fe-40Al specimens still show some ferromagnetic behavior even after up to 973 K annealing in DSC. However, the remaining ferromagnetism is below the detection for Fe-43Al. Activation energies of APB tube annihilation in Fe-40Al and Fe-43Al are determined by DSC technique. Fe-43Al has lower activation energy (67 kJ/mol) than Fe-40Al (101 kJ/mol), hence, consistent with MS behavior of these two materials during annealing process.

4. APB tube observation under TEM

APB tubes are observed under TEM in lightly strained Fe-34Al, Fe-40Al and Fe-40Al-3Cu single crystals, but not in Fe-43Al specimens. Less dislocation dipoles are observed in Fe-43Al than Fe-40Al. In Fe-43Al, APB tubes would have a narrower width and lower density, thus they have lower contrast in the TEM. This is, presumably, why no APB tubes were observed in Fe-43Al specimens. A typical APB tube image in Fe-34Al is shown in Figure 3.

Bright field image of APB tubes in Fe-34Al single crystals
Figure 3. Bright field image of APB tubes (arrowed) in Fe-34Al single crystals compressed ~5%, g=200.

5. Theoretical calculations of saturation magnetization in FeAl

There are two sources of ferromagnetism in off-stoichiometric FeAl, that from the constitutional disorder from anti-site Fe atoms and that from APB tubes.

An APB tube encloses a region in which the lattice is displaced by an APB displacement vector with respect to the lattice outside the tube. The tube wall is therefore a continuous interface of APB. However, the bond arrangements inside the tube are still correct; as shown by ŒAl-Fe-Al1 line in Figure 4. Nearest neighbors of Fe-Fe atoms in APB tubes form zig-zag chains along the [ 00] direction.

Schematic diagrams showing the geometry of APB tubes
Figure 4. Schematic diagrams showing the geometry of APB tubes.

In our calculation, we considered ferromagnetism both from wrong bonds in the matrix and from APB tubes. The resulted MS values are in good agreement with experimentally determined values for Fe34Al, Fe40Al and Fe43Al single crystal specimens, see Figure 5.

The calculated and measured MS versus enthalpy associated with APB tube removal curves for three FeAl single crystals
Figure 5. The calculated and measured MS versus enthalpy associated with APB tube removal curves for three FeAl single crystals.

Strain-Induced Ferromagnetism References

Papers

  • "Annealing of Cold-Rolled Fe-40Al Single Crystals", Y. Yang and I. Baker, Proceedings of the Material Research Society, 460 (1997) p367.
  • "On the Mechanism of the Paramagnetic to Ferromagnetic Transition in FeAl", Y. Yang, I. Baker, and P. Martin, Philosophical Magazine B, 79 (1999) 449-461.
  • "Strain-Induced Ferromagnetism in FeAl Single Crystals", D. Wu and I. Baker, Proceedings of the Material Research Society, 646 (2001) N3.2.1-N3.2.6.
  • "Strain-Induced Ferromagnetism in FeAl Single Crystals", D. Wu and I. Baker, Materials Science and Engineering, A, 329-331 (2002) 334-338.
  • "The Activation Energy of APB Tube Annihilation in FeAl", D. Wu and I. Baker, Philosophical Magazine, 82 (2002) 2239-2248.
  • "Time and Orientation Dependence of the Ferromagnetism in Plastically Strained FeAl Single Crystals", D. Wu and I. Baker, Proceedings of the Fourth Pacific Rim International Conference on Advanced Materials and Processing (PRICM IV), Vol. I, (2001) 823-826.
  • "The Strain-Induced Paramagnetic to Ferromagnetic Transition in FeAl: Experiments and Calculations", D. Wu and I. Baker, Philosophical Magazine, 83 (2003) 295-313.
  • "The Effect of Hot Zone Velocity and Temperature Gradient on the Directional Recrystallization of Polycrystalline Nickel", J. Li, S.L. Johns and I. Baker, Acta Materialia, 50 (2002) 4491-4497.
  • "The Structure and Mechanical Properties of L21-structured Fe2AlMn", M. Wittmann, P.R. Munroe and I. Baker, Philosophical Magazine, 84 (2004) 3169-3194.

Presentations

  • "On the Mechanism of Deformation-Induced Magnetic Transition in FeAl", Y. Yang, I. Baker and P. Martin, Annual TMS meeting, San Diego, CA, March 1-5, 1999.
  • "The Strain-Induced Paramagnetic to Ferromagnetic Transition in FeAl", I. Baker*, Y. Yang, D. Wu and P. Martin, Annual TMS meeting, Nashville, TN, 12-16 March, 2000.
  • "On a Model for the Strain-Induced Paramagnetic to Ferromagnetic Transition in FeAl", D. Wu and I. Baker*, presented at the 5th International Conference on Structural and Functional Intermetallics, Vancouver, BC, 16-19 July, 2000.