We report magnetic-field and angular-dependent high-resolution specific-heat measurements of the organic superconductor β''-(BEDT-TTF)2SF5CH2CF2SO3. When the magnetic field is aligned precisely within the conducting BEDT-TTF layer, at low temperatures a clear upturn of the upper critical field beyond the Pauli limit of 9.73 T is observed, hinting at the emergence of a Fulde-Ferrell-Larkin-Ovchinnikov state. This upturn disappears when the field is oriented out of plane by more than ∼0.5  deg. For smaller out-of-plane angles, the specific-heat anomaly at T(c) sharpens and a second peaky phase transition appears within the superconducting state.

Ultrasound and magnetization studies of the frustrated spinel ZnCr 2 Se 4 are performed as a function of temperature and magnetic field up to 14 T. In zero field, the sound velocity and attenuation reveal significant anomalies at the antiferromagnetic transition at T N ≈ 21 K indicating strong spin-lattice coupling. External magnetic fields shift these anomalies to lower temperatures concomitantly with the reduction of the Néel temperature. At 2 K, the sound velocity as a function of magnetic field manifests three pronounced anomalies: a deep minimum at 5.4 T related to an inflection point of the magnetization followed by two plateaus with distinct stiffness at fields above 7 and 10 T. The first plateau is ascribed to a transformation from a tetragonal to a cubic phase, while the second one corresponds to a state with fully polarized magnetization. The evolution of magnetic and structural states is discussed within a H -T phase diagram and compared with related frustrated magnetic spinels with strong spin-lattice coupling.

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The magnetic-field and temperature dependencies of ultrasound propagation and magnetization of single-crystalline CoCr2O4 have been studied in static and pulsed magnetic fields up to 14 T and 62 T, respectively. Distinct anomalies with significant changes in the sound velocity and attenuation are found in this spinel compound at the onset of long-range incommensurate spiral-spin order at Ts = 27 K and at the transition from the incommensurate to the commensurate state at T l = 14 K, evidencing strong spin-lattice coupling. While the magnetization evolves gradually with field, steplike increments in the ultrasound clearly signal a transition into a new magneto-structural state between 6.2 and 16.5 K and at high magnetic fields. We argue that this is a high-symmetry phase with only the longitudinal component of the magnetization being ordered, while the transverse helical component remains disordered. This phase is metastable in an extended H-T phase space. Multiferroic materials, which exhibit concomitant magnetic and ferroelectric order, are of great current interest, both from a fundamental as well as applicationoriented view. They challenge our understanding of ordering phenomena, but in addition provide new functionalities in spintronics, since these dielectric and magnetic polarizations can be tuned either by external magnetic or electric fields [1][2][3][4]. Among multiferroics magnetic AB 2 X 4 compounds with spinel structure attracted considerable interest revealing colossal magnetocapacitance and spontaneous dielectric polarization in the magnetically ordered state [5][6][7][8][9]. The appearance of dielectric polarization is associated either with a non-collinear arrangement of spins, with charge order, or with magnetic ions moving off-center from their symmetric site positions in the lattice due to strong magneto-elastic effects.Significant spin-lattice coupling and magnetic frustration are important features of spinels, specifically, of chromium oxides and chalcogenides. Despite quenched orbital moments these compounds reveal structural instabilities which are governed explicitly by ordering of spins, e.g., giant magnetostriction, negative thermal expansion [10], and spin Jahn-Teller instabilities [11][12][13][14][15]. In Cr spinels with only one magnetic sublattice, where the Cr 3+ ions are located solely on the pyrochlore lattice of the B sites, it is well known that the oxides with dominating antiferromagnetic (AFM) exchange reveal strong geometrical frustration, while in the sulfides and selenides ferromagnetic (FM) exchange becomes important. ZnCr 2 S 4 and ZnCr 2 Se 4 are strongly frustrated due to competing AFM and FM interactions, with the ground state still being antiferromagnetic [16]. At low temperatures, the geometrically frustrated oxide spinels exhibit magnetization plateaus as function of an external magnetic field, with a 3-up 1-down spin configuration [17][18][19]. Theoretically, it has been suggested that these plateaus are stabilized by lattice distortions [20]. Indeed, in high-fiel...