R. Fichtl

Colossal Magnetocapacitance and Colossal Magnetoresistance in HgCr2S4

We present a detailed study of the dielectric and charge transport properties of the antiferromagnetic cubic spinel HgCr2S4. Similar to the findings in ferromagnetic CdCr2S4, the dielectric constant of HgCr2S4 becomes strongly enhanced in the region below 60-80 K, which can be ascribed to polar relaxational dynamics triggered by the onset of ferromagnetic correlations. In addition, the observation of polarization hysteresis curves indicates the development of ferroelectric order below about 70 K. Moreover, our investigations in external magnetic fields up to 5 T reveal the simultaneous occurrence of magnetocapacitance and magnetoresistance of truly colossal magnitudes in this material.

Orbital freezing and orbital glass state in FeCr2S4.

Broadband dielectric spectroscopy has been performed on single-crystalline FeCr2S4 revealing a transition into a low-temperature orbital glass phase and on polycrystalline FeCr2S4 where long-range orbital order is established via a cooperative Jahn-Teller transition. The freezing of the orbital moments is revealed by a clear relaxational behavior of the dielectric permittivity, which allows a unique characterization of the orbital glass transition. The orbital relaxation dynamics continuously slows down over six decades in time, before at the lowest temperatures the glass transition becomes suppressed by quantum tunneling.

Relaxation dynamics and colossal magnetocapacitive effect in CdCr2S4

A thorough investigation of the relaxational dynamics in the recently discovered multiferroic CdCr2S4 showing a colossal magnetocapacitive effect has been performed. Broadband dielectric measurements without and with external magnetic fields up to 10 T provide clear evidence that the observed magnetocapacitive effect stems from enormous changes of the relaxation dynamics induced by the development of magnetic order.

Multiferroicity and colossal magneto-capacitance in Cr-thiospinels

The sulfur based Cr-spinels RCr2S4 with R = Cd and Hg exhibit the coexistence of ferromagnetic and ferroelectric properties together with a pronounced magnetocapacitive coupling. While in CdCr2S4 purely ferromagnetic order is established, in HgCr2S4 a bond-frustrated magnetic ground state is realized, which, however, can easily be driven towards a ferromagnetic configuration in weak magnetic fields. This article shall review our recent investigations of both compounds. Besides the characterization of the magnetic properties, the complex dielectric permittivity was studied by means of broadband dielectric spectroscopy as well as measurements of polarization hysteresis and pyro-currents. The observed colossal magneto-capacitive effect at the magnetic transition seems to be driven by an enormous variation of the relaxation dynamics.

Orbital physics in sulfur spinels: ordered, liquid and glassy ground states

Measurements of magnetization M(T, H), heat capacity C(T), NMR lineshift K(T) and linewidth Δ(T), neutron scattering S(Q, ω, T) and broadband dielectric spectroscopy (ω, T) provide experimental evidence of the different orbital ground states in the cubic sulfur spinels under investigation. In all compounds, the tetrahedrally coordinated Jahn–Teller ions Fe2+ are characterized by a degeneracy of the orbital degrees of freedom. Particularly, we found a long-range orbital ordering in polycrystalline (PC) FeCr2S4, and a glassy freezing of the orbital degrees of freedom in FeCr2S4 (single crystals) (SCs). In contrast, FeSc2S4 belongs to the rare class of spin–orbital liquids, where quantum fluctuations accompanying the glassy freezing of the orbitals suppress long-range magnetic order.

Multiferroic behavior in CdCr2X4(X=S,Se)

Abstract The recently discovered multiferroic material CdCr 2 S 4 shows a coexistence of ferromagnetism and relaxor ferroelectricity together with a colossal magnetocapacitive effect. The complex dielectric permittivity of this compound and of the structurally related CdCr 2 Se 4 was studied by means of broadband dielectric spectroscopy using different electrode materials. The observed magnetocapacitive coupling at the magnetic transition is driven by enormous changes of the relaxation dynamics induced by the development of magnetic order.

Magnetoelectrics: Is CdCr2S4 a multiferroic relaxor? (reply)

Catalan and Scott propose an alternative interpretation for our findings for CdCr2S4 in terms of Maxwell–Wagner effects. They also quote related isomorphs, such as HgCr2S4 (ref. 3), which has been discussed elsewhere. As we have shown, Maxwell–Wagner relaxations can indeed strongly affect the dielectric properties of transition-metal oxides and semiconductors. We do not find Catalan and Scotts arguments about CdCr2S4 convincing, however, and think that our experiments provide ample evidence for multiferroic relaxor behaviour.

Multiferroic behavior in

Abstract The recently discovered multiferroic material CdCr 2 S 4 shows a coexistence of ferromagnetism and relaxor ferroelectricity together with a colossal magnetocapacitive effect. The complex dielectric permittivity of this compound and of the structurally related CdCr 2 Se 4 was studied by means of broadband dielectric spectroscopy using different electrode materials. The observed magnetocapacitive coupling at the magnetic transition is driven by enormous changes of the relaxation dynamics induced by the development of magnetic order.

Multiferroic behavior in CdCr2X4(X=S,Se)CdCr2X4(X=S,Se)

Abstract The recently discovered multiferroic material CdCr 2 S 4 shows a coexistence of ferromagnetism and relaxor ferroelectricity together with a colossal magnetocapacitive effect. The complex dielectric permittivity of this compound and of the structurally related CdCr 2 Se 4 was studied by means of broadband dielectric spectroscopy using different electrode materials. The observed magnetocapacitive coupling at the magnetic transition is driven by enormous changes of the relaxation dynamics induced by the development of magnetic order.

Is CdCr2S4 a multiferroic relaxor? (reply): Magnetoelectrics

Catalan and Scott propose an alternative interpretation for our findings for CdCr2S4 in terms of Maxwell–Wagner effects. They also quote related isomorphs, such as HgCr2S4 (ref. 3), which has been discussed elsewhere. As we have shown, Maxwell–Wagner relaxations can indeed strongly affect the dielectric properties of transition-metal oxides and semiconductors. We do not find Catalan and Scotts arguments about CdCr2S4 convincing, however, and think that our experiments provide ample evidence for multiferroic relaxor behaviour.