Ferroic Glasses: Magnetic, Polar and Strain Glass

Abstract

When a liquid phase is cooled below its freezing temperature, it usually transforms to a crystalline solid where the disorder-order transition is driven by the thermodynamic requirement of reducing the entropy in the corresponding thermodynamic potential of the system. However, some liquids do not crystallize because of the complex nature of the inter-atomic interactions as, for example, in multi-atom metallic materials when heterogeneous nucleation can be reduced during cooling from the liquid to the solid phase, which can be influenced by strain rates modifying the viscosity 1, 2. The resulting non-crystalline materials show a complex potential-energy landscape and the elastic properties of a cluster of atoms can vary significantly in space. Such local properties of metallic glasses are still subject of intensive ongoing research 3. But glass formation is also possible in a variety of other materials where the high-temperature state with chemical disorder of the atoms can freeze into a statically disordered phase showing local order. This is possible in martensitically transforming alloys. In such alloys the premartensitic region at temperatures above the martensitic transformation temperature is often subject to so-called tweed effects, which can be considered to arise from dynamically disordered lattice strains 4. These disordered lattice strains may arise from the compositional (chemical) disorder of the material or from doping the materials with additional point defects. Both effects can be thought to be responsible for the appearance of these ‘dynamically disordered lattice strains’, which below a critical temperature and above a critical degree of disorder or above a critical concentration of point defects may transform to a frozen disordered strain state, a so-called ‘strain glass’. Such strain glasses have recently been observed in some intermetallic Ni-Ti-Fe and Ni-Co-Mn-Ga Heusler alloys 5, 6. The experimental evidence of the non-ergodic behavior of these ‘new glasses’ comes from measuring ‘field-cooled- field-heated’ and ‘zero-field-cooled-zero-field-heated’ curves over the glass formation temperature down to low temperatures. Very early such non-ergodic behavior has already been observed in some relaxor ferroelectrics which show the effect of a polarization glass 7. The idea can be pursued to include besides the ferroelastic systems also magnetic metals with ferromangetic-antiferromagnetic interactions and chemical disorder giving rise to usual spin glass or cluster-spin glass phases. Famous examples are here the recently discovered Ni-Co-Mn-(In, Sn) intermetallics 8, 9. In this special issue we present a series of experimental and theoretical articles which deal with various and important physical aspects of such alloys in this new and exciting field of ‘strain glasses’ including articles with a discussion of the influence of magnetism on the glassy features. Of particular interest is that in some of these new materials we find both effects simultaneously, namely, the interaction of strain glass and cluster-spin glass formation. We expect that this rather new emerging field of ‘strain glasses’ in martensitically transforming alloys and the generalization of this concept to include other glass formation tendencies as in disordered ferroelectrics and disordered magnetic systems will attract many readers and might lead to a lot of activities in near future. Last not least we would like to mention that most alloys which are discussed in the different contributions to this special issue are smart materials showing shape-memory effects and magnetocaloric properties to name only two properties of technological relevance. It may turn out that ‘for the fi rst time’ glassy features of metals may be benefi cial for technologically relevant smart materials. In the due course and making of this special issue on fundamental research related to the new phases of ferroic glasses which appear now as well-established exciting experimental and theoretical observation in various martensites, ferroelectrics and other ferroic materials, the Guest Editor (P.E.) would like to thank the DFG Priority Programme "Change of microstructure and shape of solid materials by external magnetic fi elds" (SPP 1239, Dr. Sebastian Fähler, Chair) as well as the German multinational conglomerate Thyssen- Krupp Steel Europe AG (Klaus Bailer, Senior Vice President, Human Resources) for financial support. The Editors, Peter Entel, Raymundo Arroyave, Sebastian Fähler, Ryosuke Kainuma, Antoni Planes, Xiaobing Ren, and Avadh Saxena