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Domain aspects of multi-fe rroics are reviewed, i.e. of materials, in which two or all three of 1he properties ·rerroeleclricily.' •fcrromagnelism' and ·fe rroelasticity' occur simultaneo usly in the same phase, and in which the magnelic poinl group has been reliably established by magneiOelectric, optical, dielectric, magnetic and related studies on single crystals and single domains. Nearly o nly members of lhc boracitc cryslal famil y are concem ed, whereas fo r 1he pcrovskile fami ly and other classes of material there is great pauci1y of data on single crystals. Polarized lighl microscopy is shown to be an indispensable tool for the s1udy of multi-ferroics. in particular when ferroelasticity is involved. Potenlial multi-fcrroic magnetoclectrics. so far only kno wn in ceramic form . are excluded from the survey.

Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. We report on extensive experimental studies on thin film, single crystal, and ceramics of multiferroic bismuth ferrite BiFeO 3 using differential thermal analysis, high-temperature polarized light microscopy, hightemperature and polarized Raman spectroscopy, high-temperature x-ray diffraction, dc conductivity, optical absorption and reflectivity, and domain imaging, and show that epitaxial ͑001͒ thin films of BiFeO 3 are clearly monoclinic at room temperature, in agreement with recent synchrotron studies but in disagreement with all other earlier reported results. We report an orthorhombic order-disorder ␤ phase between 820 and 925 ͑Ϯ5͒°C, and establish the existence range of the cubic ␥ phase between 925 ͑Ϯ5͒ and 933 ͑Ϯ5͒°C, contrary to all recent reports. We also report the refined Bi 2 O 3 -Fe 2 O 3 phase diagram. The phase transition sequence rhombohedral-orthorhombic-cubic in bulk ͓monoclinic-orthorhombic-cubic in ͑001͒BiFeO 3 thin film͔ differs distinctly from that of BaTiO 3 . The transition to the cubic ␥ phase causes an abrupt collapse of the band gap toward zero ͑insulator-metal transition͒ at the orthorhombic-cubic ␤-␥ transition around 930°C. Our band structure models, high-temperature dc resistivity, and light absorption and reflectivity measurements are consistent with this metal-insulator transition.

Domains are of unparalleled technological importance as they are used for information storage and for electronic, magnetic and optical switches. They are an essential property of any ferroic material. Three forms of ferroic order are widely known: ferromagnetism, a spontaneous magnetization; ferroelectricity, a spontaneous polarization; and ferroelasticity, a spontaneous strain. It is currently debated whether to include an ordered arrangement of magnetic vortices as a fourth form of ferroic order, termed ferrotoroidicity. Although there are reasons to expect this form of order from the point of view of thermodynamics, a crucial hallmark of the ferroic state--that is, ferrotoroidic domains--has not hitherto been observed. Here ferrotoroidic domains are spatially resolved by optical second harmonic generation in LiCoPO4, where they coexist with independent antiferromagnetic domains. Their space- and time-asymmetric nature relates ferrotoroidics to multiferroics with magnetoelectric phase control and to other systems in which space and time asymmetry leads to possibilities for future applications.

The symmetry conditions for the occurrence in a same phase of one or more of the four primary ferroic properties, i.e., ferroelectricity, ferromagnetism, ferrotoroidicity and ferroelasticity, are discussed. Analogous conditions are outlined for the admission of so-called secondary and tertiary ferroic effects, such as magnetoelectric, piezoelectric, piezomagnetic, piezotoroidic, etc. Formerly postulated 'magnetotoroidic' and 'electrotoroidic' effects are found to be describable as tertiary ferroic magnetoelectric effects. For understanding ferroic and multiferroic domains and their possibilities of switching, knowledge of the pairs of prototype point group/ferroic phase point group (so-called 'Aizu species') is decisive. A classification into ensembles of species with common properties, recently extended to ferrotoroidic crystals, allows distinguishing between full, partial or no coupling between order parameters and understanding domain patterns and poling procedures. The switching by reorientation with angles other than 180 • of ferromagnetic, antiferromagnetic and ferroelectric domains by magnetic fields, electric fields or by stress requires the ferroic phase to be ferroelastic. For ferromagnetic/ferrotoroidic and antiferromagnetic/ferrotoroidic phases, the ferrotoroidic domains are found to be identical with the ferromagnetic and antiferromagnetic ones, respectively. As a consequence and depending on symmetry, ferrotoroidic domains can be switched by crossed electric and magnetic fields, by collinear electric and magnetic fields or by a magnetic field alone. Examples of ferrotoroidic domains are discussed for Fe 2−x Ga x O 3 , Co 3 B 7 O 13 Br and LiCoPO 4 . Recent new results on symmetry and domains of the antiferromagnetic incommensurate phase of BiFeO 3 are also discussed.

Ferroelectricity and weak ferromagnetism have been found to set on simultaneously in NiaB 7 0 13 I at about 64°K. This is evidenced by dielectric hysteresis, spontaneous Faraday effect, quadratic magnetoelectric hysteresis, etc. The strong coupling between the mutually perpendicular spontaneous polarization-[001]-and spontaneous magnetization-[110]-is such that, when the former is reversed, the latter turns by 90°.The magnetic point group is most probably m'm2'. Dielectric constant, magnetic, and magnetoelectric susceptibilities and magnetic coercive field are shown as a function of temperature.

Campylobacter is among the most important agents of enteritis in developed countries. We have described the potential environmental determinants of the seasonal pattern of infection with campylobacter in Europe, Canada, Australia and New Zealand. Specifically, we investigated the role of climate variability on laboratory-confirmed cases of campylobacter infection from 15 populations. Regression analysis was used to quantify the associations between timing of seasonal peaks in infection in space and time. The short-term association between weekly weather and cases was also investigated using Poisson regression adapted for time series data. All countries in our study showed a distinct seasonality in campylobacter transmission, with many, but not all, populations showing a peak in spring. Countries with milder winters have peaks of infection earlier in the year. The timing of the peak of infection is weakly associated with high temperatures 3 months previously. Weekly variation in campylobacter infection in one region of the UK appeared to be little affected by short-term changes in weather patterns. The geographical variation in the timing of the seasonal peak suggests that climate may be a contributing factor to campylobacter transmission. The main driver of seasonality of campylobacter remains elusive and underscores the need to identify the major serotypes and routes of transmission for this disease.

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