A way to search for interesting new magnetic materials with first order phase transformations

Transcription

1 A way to search for interesting new magnetic materials with first order phase transformations Richard James Aerospace Engineering and Mechanics Joint work with Jerry Zhang, graduate student AEM Also: Tom Shield (AEM), Chris Palmstrom (CEMS), Jon Messier (AEM) Supported by ARO ( materials with unprecedented properties ), AFOSR, NSF, MURI

2 Subject of this talk: first order phase transformations + magnetism Why? Magnetic properties are extremely sensitive to the lattice parameters First order phase transformations have a change of lattice parameters: martensitic transformations Can switch back and forth between completely different materials Most first order phase transformations are not reversible Key point: reversibility. What governs reversibility of martensitic transformations? Generalization: multiferroic materials

3 Sensitivity to changes of lattice parameters It is well-known in both ferromagnetism, ferroelectricity that magnetic and electric properties are extremely sensitive to the lattice parameters. Exchange energy is extremely sensitive to lattice distances Heusler alloys, Mn in Ni 2 MnGa, Diffusion of N 2 into rare earth magnets Epitaxial growth of magnetic materials (e.g., BiFeO 3 ) Ferroelectricity is extremely sensitive to lattice distances R. E. Cohen (2001): Properties of ferroelectrics are extremely sensitive to volume (pressure), which can cause problems since small errors in volume can result in large errors in computed ferroelectric properties. D. Schlom (Jan. 26, 2006). Epitaxy of BaTiO 3, SrTiO 3

4 Martensitic phase transformation Ni Ga Mn Ni 2 MnGa N S

5 Example of this sensitivity: ferromagnetic shape memory materials: Ni 2 MnGa austenite martensite Courtesy: T. Shield

6 Example, continued, Ni 2 MnGa magnetization curves austenite martensite 60 c-axis a-axis M (emu/g) M (emu/g) H (Oe) H (Oe) 12000

7 Ferromagnetic shape memory Two ways to field-induce a shape change: T 1) Field-induce the austenite-martensite transformation 2) Rearrange variants of martensite below transformation temperature. picture below drawn with measured lattice parameters of Ni 2 MnGa H

8 Strain vs. field in Ni 2 MnGa H (010) (100) 30 times the strain of giant magnetostrictive materials

9 Ni 2 MnGa cantilever H(t) Energy stored is proportional to h (because of the micromagnetic term ) rather than h 3 m h dx picture drawn with measured lattice parameters of Ni 2 MnGa (Electromagnetic force on the cantilever is zero; it is driven by configurational force)

10 H(t) A machine that makes the field do this: (a) (b) Rotating Poles Magnets Specimen here (c) (d)

11 Multiferroic materials by phase change: seek a reversible first order phase transformation between ferroelectric and ferromagnetic phases Also Rarity predicted by QM circumvented The volume fraction of ferroelectric vs. ferromagnetic phases could be changed E&M property Lattice parameter Key scientific issues: 1) Understand the sensitivity of properties to lattice parameters. 2) What governs reversibility of phase transformations?

12 Main themes in science on hysteresis in structural phase transformations Pinning of interfaces by defects System gets stuck in an energy well on its potential energy landscape

13 Experimental viewpoint Alloys that undergo highly reversible phase transformations: What do they have in common? The most highly reversible big first order phase transformations occur in shape memory alloys. Here, highly reversible = low hysteresis Almost all work done (by far) done on only a few systems: and, among these, some transformations have incredibly low hysteresis, given the size of the transformation strain

14 FCC monoclinic orthorhombic cubic Otsuka, Sakamoto, Shimizu

15 Distortion matrices: general forms 3.

16 Cubic to monoclinic, <110> polarized

17 Cubic to monoclinic, <100> polarized

18 Alloys in red have unusually low hysteresis

19 Alloys in red have unusually low hysteresis

20 The typical mode of transformation when : austenite 10 µm two variants of martensite, finely twinned

21 Consider To make solve for the shear : Take, e.g. +. Substitute measured, for Cu 68 Zn 15 Al 17 :

22 An additional restriction... Theorem. Suppose in addition to, we have, for a twin system a,n Then, there are infinitely many austenite/martensite interfaces, with any volume fraction between 0 and 1.

23 Pictures corresponding to

24 There are distortion matrices that satisfy all three conditions: det U 1 = 1, middle eigenvalue of U 1 is 1, layered structures with any volume fraction of martensite. 1. Cubic to orthorhombic. There are exactly two matrices: 1 and 2 1/ 2 2. Cubic to monoclinic <100> polarized. There are two one-parameter families of matrices passing through the identity: 1 3. Cubic to monoclinic <110> polarized. There are 14 one-parameter families more difficult to write down explicitly (several work for many twin systems)

25 Pictures corresponding to 1

26 Tuning composition to satisfy and det U 1 = 1 A slice of NiTiCuPd at constant Ti composition Cu Ni Pd Jerry Zhang

27 Measurements of the width of the hysteresis loop by Quandt and by Miyazaki B. Winzek & E. Quandt, Proc. Mat. Res. Soc. Symp. 604, p S. Miyazaki NiTiPd Middle eigenvalue equals 1 here (= )

28 Width of the hysteresis for various alloys, from US Patent 5,951,763 Closest composition to our surface λ 2 = 1 and det U 1 = 1

29 Recent alloy development NiTi Pt NiTiAu Hysteresis( o C) A f - M f A s + A f - M s - M f Hysteresis( o C) A f - M f A s + A f - M s - M f Pt at. % Au at. %

30 Summary Electromagnetic properties of materials are often lattice parameter sensitive. Thus, structural phase transformation can lead to coexistence of unlikely properties. The key: reversibility. Accepted ideas of origins of hysteresis in phase transformations seem to be flawed. The existence of many simple ways to fit the phases together seems to relate to reversibility. Possible connections with energy barriers. Proposed rules for reversibility

31 Summary, continued Recent work of Mounmi: correlation between fatigue life and hysteresis. The idea is potentially applicable to other lattice-parametersensitive pairs of properties High -- low solubility for H 2 High band gap -- low band gap semiconductor Conductor -- insulator (electrical or thermal) Opaque -- transparent (at various wavelengths) High -- low index of refraction ( also nonlinear optical properties) Luminescent nonluminescent Solid state batteries New thermoelectric and thermomagnetic materials