High resolution ultrasonic velocity measurements have been used to determine the temperaturemagnetic-field phase diagram of the monoclinic multiferroic CuO. A new transition at TN3 = 230 K, corresponding to an intermediate state between the antiferromagnetic non-collinear spiral phase observed below TN2 = 229.3 K and the paramagnetic phase, is revealed. Anomalies associated with a first order transition to the commensurate collinear phase are also observed at TN1 = 213 K. For fields with B b, a spin-flop transition is detected between 11 T -13 T at lower temperatures. Moreover, our analysis using a Landau-type free energy clearly reveals the necessity for an incommensurate collinear phase between the spiral and the paramagnetic phase. This model is also relevant to the phase diagrams of other monoclinic multiferroic systems.PACS numbers: 75.30.Kz, 75.85.+t, 75.30.Gw Multiferroic phenomena have been a subject of intense interest in recent decades arising from opportunities to explore new fundamental physics as well as possible technological applications [1][2][3]. Coupling between different ferroic orders has been proven to be driven by several different types of mechanisms. In particular, multiferroics with a spiral spin-order-induced ferroelectricity have revealed high spontaneous polarization and strong magnetoelectric coupling [4,5]. Cupric oxide (CuO), the subject of this letter, was characterized as a magnetoelectric multiferroic four years ago when it was shown that its ferroelectric order is induced by the onset of a spiral antiferromagnetic (AFM) order at an unusually high temperature of 230 K [3]. Thus far, two AFM states have been reported, a low temperature (T N 1 ∼ 213 K) AF1 commensurate collinear state with the magnetic moments along the monoclinic b axis and an AF2 incommensurate spiral state with half of the magnetic moments in the ac plane (T N 2 ∼ 230 K) [3,6,7]. However, the authors of the neutron diffraction measurements [6] Encouraged by recent experiments on other multiferroic systems using ultrasonic measurements [11], we measured the temperature and field dependence of the velocity of transverse modes in order to determine the magnetic phase diagram of CuO. A new transition is detected at T N 3 = 230 K just above the AF2 spiral phase observed at T N 2 = 229.3 K, while the first order transition is observed at T N 1 = 213 K. Furthermore, dielectric constant measurements confirm that only the spiral phase (between T N 1 and T N 2 ) supports a spontaneous electric polarization. In addition, we report on a spin-flop transition in the low temperature AF1 collinear phase when B b. Thus, based on these findings, a new magneticfield vs temperature phase diagram is proposed for CuO. In order to elucidate the possible nature of the AFM states observed in CuO, a non-local Landau-type free energy is also developed for CuO and similar monoclinic multiferroics. This approach has been very successful in explaining the magnetic phase diagrams of other multiferroic systems [12][13][14]. In contrast w...

Ultrasonic velocity measurements on the magnetoelectric multiferroic compound CuFeO2 reveal that the antiferromagnetic transition observed at TN1 = 14 K might be induced by an R3m C2/m pseudoproper ferroelastic transition [1]. In that case, the group theory states that the order parameter associated with the structural transition must belong to a two dimensional irreducible representation Eg (x 2 − y 2 , xy). Since this type of transition can be driven by a Raman Eg mode, we performed Raman scattering measurements on CuFeO2 between 5 K and 290 K. Considering that the isostructural multiferroic compound CuCrO2 might show similar structural deformations at the antiferromagnetic transition TN1 = 24.3 K, Raman measurements have also been performed for comparison. At ambient temperature, the Raman modes in CuFeO2 are observed at ωE g = 352 cm −1 and ωA g = 692 cm −1 , while these modes are detected at ωE g = 457 cm −1 and ωA g = 709 cm −1 in CuCrO2. The analysis of the temperature dependence of modes shows that the frequency of all modes increases down to 5 K. This typical behavior can be attributed to anharmonic phonon-phonon interactions. These results clearly indicate that none of the Raman active modes observed in CuFeO2 and CuCrO2 drive the pseudoproper ferroelastic transition observed at the Néel temperature TN1. Finally, a broad band at about 550 cm −1 observed in the magnetoelectric phase of CuCrO2 below TN2 could be attributed to a magnon mode.

Using high resolution sound velocity measurements we have obtained a very precise magnetic phase diagram of Ba3CoSb2O9 a material that is considered to be an archetype of the spin 1/2 triangular-lattice antiferromagnet. Results obtained for the field parallel to the basal plane (up to 18 T) show three phase transitions, consistent with predictions based on simple 2D isotropic Heisenberg models and previous experimental investigations. The phase diagram obtained for the field perpendicular to the basal plane clearly reveals an easy-plane character of this compound and in particular, our measurements show a single first order phase transition at Hc1 = 12.0 T which can be attributed to a spin-flop between an umbrella-type configuration and a coplanar V-type order where spins lie in a plane perpendicular to the ab-plane. A low temperatures softening of the lattice within some of the ordered phases is also observed and may be a result of residual spin fluctuations.

We present 13C NMR lineshapes and spin lattice relaxation measurements of (TMTTF),Br and (TMTSF),PF6 in the spin density wave (SDW) state. We c o n f m that (TMTSF),PF6 has an incommensurate SDW and show that the phason mode is dominant in the relaxation. We prove that (TMTTF)2Br is commensurate and estimate the SDW amplitude. No phason contribution to the relaxation is observed in this compound. The implications for theory are discussed.