ADVANCES IN ELECTRONICS AND ELECTRON PHYSICS VOLUME 71 EDITOR-IN-CHIEF PETER W. HAWKES Laboratoire d’Optique Electronique du Centre National de la Recherche Scientifique Toulouse, France ASSOCIATE EDITOR BENJAMIN KAZAN Xerox Corporation Palo Alto Research Center Palo Alto, California Advances in Electronics and Electron Physics EDITED BY PETER W. HAWKES Laboratoire d’Optique Electronique du Centre National de la Recherche Scientifique Toulouse, France VOLUME 71 ACADEMIC PRESS, INC. Harcourt Brace Jovanovich, Publishers Boston San Diego New York Berkeley London Sydney Tokyo Toronto COPYRIGHT 0 1988 B Y A BY TRANSLATING BEAM LASER BEAM ‘TRANSLATE BEAM FIG.6: Laser beam domain wall nucleation in NPP. 262 STEVEN W. MEEKS AND B. A . AULD portions of the crystal in tension, leading to twin nucleation in the upper right and lower left quadrants of the b plate of NPP in Fig. 6. Hoop stresses in the [loll direction favor twinning, but those in the [ f O l ] do not. Blade-like twins begin at the edges and grow into the crystal parallel to [loo] (the a direction), eventually coalescing into a pair of planar a-type walls. Curved twin boundaries straighten out to lower the elastic wall energy. Minimum wall energy occurs for (001) wall orientation (an a-type wall) where structures of the two twin states are well-matched (Weber et al., 1975). Once formed, the domain wall pair can be translated along the crystal by slowly shifting the position of the laser beam. This same translation can be obtained by slowly moving the laterally applied line forces used to inject the domain pattern in Fig. 4. A rough estimate (Kingery et al., 1976) of the thermal stresses obtained in laser injection can be obtained from the expression E a A T , where E is Young’s modulus, a is the linear thermal expansion coefficient, and AT is the temperature rise of the heated zone. For oxides, E is about 10l1 N/m2 and a is about lOP5K-l. If a temperature rise of 1°K due to laser heating is assumed, then the resulting thermal stress is 1 MN/mZ, comparable to the coercive stress in most ferroelastic crystals. Laser-induced twinning has been observed previously in ferroelasticferroelectric GMO and ferrobielastic quartz. However, the twins were much more difficult to induce in these materials, and even more difficult to control. The coercive stress wc required to induce mechanical twinning in NPP is much smaller than in quartz or GMO. Measurements (Weber et al., 1975) on NPP crystals at room temperature gave a, = 14 5 3 kN/m2 to induce a-type domain wall motion. For GMO (Keve et al., 1970) the coercive stress is about 1 MN/m2, and for quartz (Aizu, 1973), it is 500 MN/m2. Laser induced twinning in quartz is possible only at high temperatures where a, decreases to less than 10 MN/m2. In GMO, laser twinning was observed (Novak et al., 1977) at room temperature, but only for certain restricted geometries. When a c plate of GMO was exposed through a slit oriented parallel to [loo], numerous spike-like twins parallel to (110) and (TlO) were produced. It is much easier to generate twins in NPP because of its low coercive stress. A second advantage of NPP is the well-defined wall orientation. NPP belongs to the low symmetry ferroic species mmmF2/rn in which one-wall orientation is highly preferred. As pointed by Weber and associates (1975), the a-type twin-wall orientation is by far the easiest to nucleate and control. The situation is quite different for GMO and quartz. GMO is a ferroelastic, as NPP is, but its symmetry is higher. On cooling through its Curie temperature, GMO transforms from tetragonal to orthorhombic, APPLICATIONS OF FERROELASTIC CRYSTALS 263 corresponding to species 4/mmmFmmm. There are two symme