We have studied the magnetic field dependence of far-infrared active magnetic modes in a single ferroelectric domain BiFeO3 crystal at low temperature. The modes soften close to the critical field of 18.8 T along the [001] (pseudocubic) axis, where the cycloidal structure changes to the homogeneous canted antiferromagnetic state and a new strong mode with linear field dependence appears that persists at least up to 31 T. A microscopic model that includes two Dzyaloshinskii-Moriya interactions and easy-axis anisotropy describes closely both the zero-field spectroscopic modes as well as their splitting and evolution in a magnetic field. The good agreement of theory with experiment suggests that the proposed model provides the foundation for future technological applications of this multiferroic material. Due to the coupling between electric and magnetic properties, multiferroic materials are among the most important yet discovered. With a multiferroic material used as a storage medium, information can be written electrically and then read magnetically without Joule heating [1]. Hence, applications of a room-temperature multiferroic would radically transform the magnetic storage industry. Because it is the only known roomtemperature multiferroic, BiFeO 3 continues to attract intense interest.Although its ferroelectric transition temperature[2] T c ≈ 1100 K is much higher than its Néel transition temperature [3][4][5] T N ≈ 640 K, the appearance of a longwavelength cycloid [3,[6][7][8] with a period of 62 nm enhances the ferroelectric polarization below T N . The induced polarization has been used to switch between magnetic domains with an applied electric field [4,5,9].Progress in understanding the microscopic interactions in BiFeO 3 has been greatly accelerated by the recent availability of single crystals for both elastic and inelastic neutron-scattering measurements. By fitting the spin wave frequencies above a few meV, recent arXiv:1302.2491v2 [cond-mat.str-el]

In-plane angular magnetoresistivity Deltarho(anis)(ab) measurements were made on Y(1-x)Pr(x)Ba(2)Cu(3)O(7-delta) single crystals in the pseudogap region. For x>/=0.2 single crystals, Deltarho(anis)(ab)(theta) displays a deviation from the typical quasiparticle contribution (proportional, sin((2)theta) for temperatures smaller than a certain value T(phi) in the pseudogap region. This deviation is consistent with a flux-flow type contribution to angular magnetoresistivity, indicating the presence of vortexlike excitations above the zero-field critical temperature in the pseudogap region.

We investigated in-plane angular magnetoresistivity ⌬ ab / ab ͑0°͒ below and above the zero-field critical temperature T c0 of Y 1−x Pr x Ba 2 Cu 3 O 7−␦ single crystals with 0.0ഛ x ഛ 0.53. The angular dependence of in-plane magnetoresistivity shows three temperature regimes: ͑a͒ a low T regime which extends from below to above the critical temperature T c0 , up to an x-dependent temperature ͑e.g., up to 1.15ϫ T c0 for x = 0.46͒, displaying a two-dimensional ͑2D͒ scaling ͑consistent with pancakelike excitation͒, ͑b͒ an intermediate T regime, corresponding to vortex line excitations, which extends up to T Ͼ T c0 , and ͑c͒ a high T regime in which the quasiparticles dominate the dissipation. The constructed electronic phase diagram shows that T ͑x͒ departs from T c0 ͑x͒ for x ജ 0.20 and reaches a maximum at x = 0.46.

The magnetic response of the strongly underdoped Y 0.47 Pr 0.53 Ba 2 Cu 3 O 7−␦ was investigated. We found the presence of superconducting and magnetic orders deep into the paramagnetic state, up to 200 K, which manifest as diamagneticlike response and hysteresis, respectively. We propose that the main source of irreversibility in this T range is the softening and melting of the glassy state into a viscous liquid of entities that behave like superparamagnetic particles with antiferromagnetic cores.

The nature of the out-of-plane dissipation was investigated in underdoped Y0.54P r0.46Ba2Cu3O 7−δ single crystals at temperatures close to the critical temperature. For this goal, temperature and angle dependent out-of-plane resistivity measurements were carried out both below and above the critical temperature. We found that the Ambegaokar-Halperin relationship [V. Ambegaokar, and B. I. Halperin, Phys. Rev. Lett. 22, 1364Lett. 22, (1969] depicts very well the angular magnetoresistivity in the investigated range of field and temperature. The main finding is that the in-plane phase fluctuations decouple the layers above the critical temperature and the charge transport is governed only by the quasiparticles. We also have calculated the interlayer Josephson critical current density, which was found to be much smaller than the one predicted by the theory of layered superconductors. This discrepancy could be a result of the d-wave symmetry of the order parameter and/or of the non BCS temperature dependence of the c-axis penetration length.

Using magnetization measurements, we report the existence of an irreversibility in the magnetization in Yo.47Pro.53Ba2Cu 3 0 7. 5 single crystals above the critical temperature (T c = 13 K) that is visible up to 200 K. Magnetization vs temperature displays irreversibility up to an applied field of 500 Oe. We noticed a weak irreversibility at high temperatures and a stronger one at low temperatures. In the weak irreversibility regime, both zero-field-and field-cooled magnetization follow similar Curie-Weiss (C-W) dependences, with slightly different parameters. The C-W parameters, decrease fast with increasing field. The C-W temperature is negative, suggesting that the average interaction between spins is antiferromagnetic. All these data suggest that the paramagnetic state is in fact a free spin liquid containing floating antiferromagnetic clusters as well as superconducting phase fluctuations.

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