Magnetism and ferroelectricity are essential to many forms of current technology, and the quest for multiferroic materials, where these two phenomena are intimately coupled, is of great technological and fundamental importance. Ferroelectricity and magnetism tend to be mutually exclusive and interact weakly with each other when they coexist. The exciting new development is the discovery that even a weak magnetoelectric interaction can lead to spectacular cross-coupling effects when it induces electric polarization in a magnetically ordered state. Such magnetic ferroelectricity, showing an unprecedented sensitivity to ap plied magnetic fields, occurs in 'frustrated magnets' with competing interactions between spins and complex magnetic orders. We summarize key experimental findings and the current theoretical understanding of these phenomena, which have great potential for tuneable multifunctional devices.

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Multiferroics: a magnetic twist for ferroelectricity

Magnetocaloric effect: From materials research to refrigeration devices

Shape memory alloys (SMAs) with high transformation temperatures can enable simplifications and improvements in operating efficiency of many mechanical components designed to operate at tem...

High temperature shape memory alloys

We report the discovery of a large negative thermal expansion (NTE) up to α=−25×10−6K−1 (α: coefficient of linear thermal expansion) in Ge-doped anti-perovskite manganese nitrides Mn3AN (A=Cu,Zn,Ga). This gigantic NTE is several to ten times higher than that of commercially available NTE materials. The discontinuous lattice expansion seen in pure Mn3AN is broadened by Ge substitution over a wide temperature window, at widest ΔT∼100K, around room temperature. Such a large, isotropic and nonhysteretic NTE is desirable for practical applications.

Giant negative thermal expansion in Ge-doped anti-perovskite manganese nitrides

This paper provides a detailed overview of developments in transducer materials technology relating to their current and future applications in micro-scale devices. Recent advances in piezoelectric, magnetostrictive and shape-memory alloy systems are discussed and emerging transducer materials such as magnetic nanoparticles, expandable micro-spheres and conductive polymers are introduced. Materials properties, transducer mechanisms and end applications are described and the potential for integration of the materials with ancillary systems components is viewed as an essential consideration. The review concludes with a short discussion of structural polymers that are extending the range of micro-fabrication techniques available to designers and production engineers beyond the limitations of silicon fabrication technology.

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New materials for micro-scale sensors and actuators An engineering review

The stress–strain behavior of two single crystalline Ni–Mn–Ga alloys with martensitic transformation temperatures above the Curie temperature has been studied during uniaxial compression along the [001] and [110] axes. The superelastic effect associated to the reversible stress-induced martensitic transformation has been obtained in these high-temperature shape memory alloys. The experimental values of the critical stress to induce the martensitic transformation, σc, depend linearly on the test temperature. This dependence is described well by the thermodynamics of the stress-induced martensitic transformation through a Clausius–Clapeyron-type equation. However, the experimental values of slopes dσc/dT obtained in the two compression directions can only be interpreted well if it is assumed that different martensitic phases are formed for each compression axis. Good agreement with the thermodynamic model exists if tetragonal martensites with c/a 1 are induced in the [001] and [110] axes, respectively.

Superelasticity in high-temperature Ni–Mn–Ga alloys

A consistent consideration of ferromagnetic alloys undergoing a cubic-tetragonal martensitic transformation on cooling is carried out within the framework of Landau theory. The concept of two different Curie temperatures, corresponding to the parent (austenitic) and resultant (martensitic) phases, is substantiated by taking into account the volume magnetostriction of an alloy. The difference in these two temperatures is evaluated for different alloys from the Ni-Mn-Ga family typifying ferromagnetic martensites. A statistical dispersion of the local Curie temperatures reflecting the heterogeneity of martensitic states is introduced. The qualitatively different temperature dependencies of the magnetization are predicted theoretically and observed experimentally under an applied magnetic field of about 10 kOe. Quantitative agreement between the theoretical and experimental curves is achieved.

Ferromagnetism of thermoelastic martensites: Theory and experiment

The superelastic effect under constant magnetic fields exhibited by the Ni-Mn-Ga ferromagnetic thermoelastic martensites with single-variant and polyvariant microstructure is studied experimentally and theoretically. The formerly proposed statistical model of the magnetostrain effect in the ferromagnetic martensite is modified for the theoretical description of the superelastic stress-strain dependence under magnetic field. The mechanical stress, which is equivalent to the internal stress induced by the magnetic field, is determined by fitting the theoretical stress-strain curves to the experimental results. A quadratic dependence of the field-induced stress on the value of the magnetization of martensite is found. This dependence supports the model assumption that a magnetoelastic interaction causes the magnetostrain effect.

Magnetic-field-induced superelasticity of ferromagnetic thermoelastic martensites: Experiment and modeling

Inhomogeneous magnetoelectric effect

The ferromagnetism of the inhomogeneous crystalline structure arising below the temperature of the martensitic phase transition is considered using a phenomenological expression for the Helmholtz free energy of a cubic crystal. The magnetic anisotropy of the martensitic structure is related to the order parameter of the ferroelastic phase transition. The expressions for the static magnetic susceptibility and for the magnetization of the martensite are derived. The theoretical results are compared with the experimental data known for alloy. The magnetoelastic constant and the constant of strain-induced magnetic anisotropy of this alloy are estimated from the experimental magnetic field dependence of the magnetization.

http://iopscience.iop.org/article/10.1088/0953-8984/10/21/015/pdf

Victor A. L'vov

Papers

Superelasticity in high-temperature Ni–Mn–Ga alloys

Ferromagnetism of thermoelastic martensites: Theory and experiment

Magnetic-field-induced superelasticity of ferromagnetic thermoelastic martensites: Experiment and modeling

Inhomogeneous magnetoelectric effect

A phenomenological model of ferromagnetic martensite