Jean-Pierre Rivera

Observation of ferrotoroidic domains

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.

On definitions, units, measurements, tensor forms of the linear magnetoelectric effect and on a new dynamic method applied to Cr-Cl boracite

In this paper, definitions, units (SI and Gaussian) and some measurement techniques of the magnetoelectric (ME) effects are clarified. The tensor forms of the linear ME effect are given for the 58 ME point groups, comprising also numerous corrections with respect to errata in former published tables. A new dynamic technique for detecting the bilinear ME effect with strikingly enhanced sensitivity is proposed and applied to the measurement of β333(T) of Cr-Cl boracite, allowing two magnetic phase transitions to be clearly evidenced, at 9.7 K (±0.1 K) and at 13.5 K (±0.1 K). It is proposed that in that range of temperature, the ferroelectric phase of Cr-Cl boracite is antiferromagnetic with Shubnikov point group mm2.

Uniaxial stress induced ferroelastic detwinning of YBa2Cu3O7−δ

Abstract Evidence for ferroelastic domain switching of the orthorhombic phase of YBa 2 Cu 3 O 7−δ has been obtained by the in situ observation under a polarizing microscope of the transformation of “puzzle” type lamellar polydomain single crystals into large orthorhombic single domains by means of the application of uniaxial stress of about 5 × 10 7 Nm -2 at 325°C. Good remanence is observed both at the detwinning temperature and at room temperature.

Polarized light and X-ray precession study of the ferroelastic domains of YBa2Cu3O7-δ

The ferroelastic domains of the orthorhombic phase of YBa2Cu3O7-σ have been observed in polarized light on ceramics and single crystals. By combining polarized light microscopy with the X-ray precession technique, the correlation of the orientation of the orthorhombica- andb-axes with that of thea-b-plane bireflectance, reflection dichroism, transmission dichroism (at a thickness of about 1μm), reflection tints generated with compensators and upon uncrossing of polars, as well as the orientation of etch pits has been realized on ferroelastic single domains, bi-domains and more complicated domain patterns. Four ferroelastic orthorhombic domain states have been identified, at variance with former group theoretical considerations, predicting only two states. Ensembles of lamellar domains beyond optical resolution generate strong bireflectance with principal axes rotated by 45° relative to the truea, b-directions.

Structural phase transition and high temperature phase structure of Friedels salt, 3CaO.Al2O3.CaCl2.10H2O

Friedels salt, the chlorinated compound 3CaO · Al2O3 · CaCl2 · 10H2O (AFm phase), presents a structural phase transition at about 30°C from a monoclinic to a rhombohedral phase. It has been studied by X-ray powder diffraction and optical microscopy in transmitted light with crossed polarisers on single crystals prepared by hydrothermal synthesis. The high temperature phase was determined at 37°C from X-ray single crystal diffraction data. The compound crystallises in the space group Rc with lattice parameters of . The refinement of 498 independent reflections with I > 2σ(I) led to a residual factor of 7.1%. The Friedels salt can be described as a layered structure with positively charged main layers of composition [Ca2Al(OH)6]+ and negatively charged layers of composition [Cl−,2H2O]. The chloride anions are surrounded by 10 hydrogen atoms, of which six belong to hydroxyl groups and four to water molecules. The structural phase transition may be related to the size of the chloride anions, which are not adapted to the octahedral cavity formed by bonded water molecules.

Relativistic nature of a magnetoelectric modulus of Cr2O3 crystals : A four-dimensional pseudoscalar and its measurement

The magnetoelectric effect of chromium sesquioxide Cr2O3 has been determined experimentally as a function of temperature. One measures the electric field-induced magnetization on Cr2O3 crystals or the magnetic field-induced polarization. From the magnetoelectric moduli of Cr2O3 we extract a four-dimensional relativistic invariant pseudoscalar alpha-tilde. It is temperature dependent and of the order of ~10−4/Z0, with Z0 as vacuum impedance. We show that the new pseudoscalar is odd under parity transformation and odd under time inversion. Moreover, alpha-tilde is for Cr2O3 what Tellegens gyrator is for two port theory, the axion field for axion electrodynamics, and the PEMC (perfect electromagnetic conductor) for electrical engineering.

On the birefringence of magnetoelectric BiFeO3

Abstract The birefringence dispersion of a ferroelastic single domain of magnetoelectric BiFeO3 crystal has been measured at room temperature (24°C) in the rhombohedral phase. A thin (11μm) (110)cubic cut was used although it was not a principal cut. The optical axis (and spontaneous polarization) formed an angle of 35° with the normal to the cut. Due to anomalous interference colors, the right order was deduced with the help of conoscopy. The birefringence of this cut shows normal dispersion and its value is very large (dn′ = 0.13 at λ = 550 nm to 0.08 at λ = 850 nm). The principal birefringence (dn = 0.34 at A = 550 nm) has been evaluated with the help of the computed (Gladstone-Dale relationship) mean refractive index ([nbar] = 2.62). From dn′ (T), (8K < T < 820 K) the Neel temperature has been found for the first time optically, TN =653K (380°C), and a critical exponent has tentatively been derived near TN .

Magnetoelectric effect of Cr2O3 in strong static magnetic fields

Abstract The magnetoelectric effect of Cr2O3 single crystals has been studied in magnetic fields up to 20 T in the range of liquid helium to room temperature In the antiferromagnetic phase for low magnetic fields the well known results for the magnetoelectric effect below the Neel temperature were reproduced. In the spin-flop phase, for magnetic fields above a critical field (H > H crit ≃ 10 T), the magnetic field induced electric polarization has been measured along all three crystallographic directions. A magnetoelectric effect, linear in the magnetic field was found for the three orientalions leading to the conclusion that the magnetic ground state symmetry of the spin-flop phase is either 1′ or 2/m′, where 2′/m can be excluded.

Ferroelectric domains, birefringence and absorption of single crystals of BiFeO3

Abstract The domain structure of BiFeO3 has been studied by transmission (4–800K) and reflection (300–1100K) microscopy. The rhombohedral phase transforms at ∼ 1083K to a non-cubic twinned phase, stable up to the decomposition at 1185K. Owing to a strong negative temperature shift (∼ -5. 9 cm-1K-1) of an absorption edge BiFeO3 turns opaque upon heating. At 15600cm-1 the 6Alg → 4T2g crystal field transition of Fe3+ occurs. The maximal spontaneous birefringence of the rhombohedral phase is very high (Δnγ, α = 0. 58 at Λ = 644nm and R. T.) and has been measured between 10 and 800K.

Commensurate-Incommensurate Magnetic Phase Transition in Magnetoelectric Single Crystal LiNiPO4

Neutron scattering studies of single crystal LiNiPO4 reveal a spontaneous first-order commensurate-incommensurate magnetic phase transition. Short- and long-range incommensurate phases are intermediate between the high temperature paramagnetic and the low temperature antiferromagnetic phases. The modulated structure has a predominant antiferromagnetic component, giving rise to satellite peaks in the vicinity of the fundamental antiferromagnetic Bragg reflection, and a ferromagnetic component, giving rise to peaks at small momentum transfers around the origin at (0,+/-Q,0). The wavelength of the modulated magnetic structure varies continuously with temperature. It is argued that the incommensurate short- and long-range phases are due to spin-dimensionality crossover from a continuous to the discrete Ising state.