, which reveal magnetic and electric order, are in the focus of recent solid state research [1][2][3][4] . Especially the simultaneous occurrence of ferroelectricity and ferromagnetism, combined with an intimate coupling of magnetization and polarization via magneto-capacitive effects, could pave the way for a new generation of electronic devices. Here we present measurements on a simple cubic spinel with unusual properties: It shows ferromagnetic order and simultaneously relaxor ferroelectricity, i.e. a ferroelectric cluster state, reached by a smeared-out phase transition, both with sizable ordering temperatures and moments. Close to the ferromagnetic ordering temperature the magneto-capacitive coupling, characterized by a variation of the dielectric constant in an external magnetic field, reaches colossal values of nearly 500%. We attribute the relaxor properties to geometric frustration, which is well known for magnetic moments, but here is found to impede long-range order of the structural degrees of freedom.The coexistence of ferroelectricity and ferromagnetism would constitute a mile stone for modern electronics and functionalised materials. The most appealing applications are new types of storage media using both magnetic and electric polarization and the possibility of electrically reading/writing magnetic memory devices (and vice versa). However, it is clear now that ferroelectric ferromagnets are rare 5,6 and mostly exhibit rather weak ferromagnetism. Spinel compounds are an important class of materials and their electronic properties are in the focus of research since the famous work of Verwey on magnetite 7 . Recent reports on geometrical frustration of the spin and orbital degrees of freedom 8 , and the observation of an orbital-glass state 9 in sulpho spinels, demonstrate the rich and complex physics, characteristic of these compounds. Here we report on another interesting experimental observation in a spinel system: R elaxor ferroelectricity in ferromagnetic CdCr 2 S 4 and the occurrence of colossal magnetocapacitive effects.CdCr 2 S 4 crystallizes in the normal cubic spinel structure (space group Fd3m, a = 1.024 nm), with Cr 3+ octahedrally surrounded by sulphur ions, yielding a half-filled lower t 2g triplet with a spin S = 3/2. Ferromagnetism in CdCr 2 S 4 is well known 10 , but early experimental observations of a number of mysterious features have fallen into oblivion: For example, reports of an anomalous expansion coefficient at low temperatures 11,12 , an unexpected concomitant broadening of the d iffraction lines 11 , a strong blue shift of the absorption edge on passing the ferromagnetic phase transition 13 , the observation of anomalously large phonon shifts and damping effects close to T c 14 and the observation of large magneto-resistance effects 15 . Figure 1a shows the inverse magnetic susceptibility χ -1 and the low-temperature magnetization M. The straight line indicates a fit to the paramagnetic susceptibility, which results in a Curie-Weiss temperature of 155 K and a paramagneti...

The dielectric properties of CaCu 3 Ti 4 O 12 , a material showing colossal values of the dielectric constant, were investigated over a broad temperature and frequency range extending up to 1.3 GHz. A detailed equivalentcircuit analysis of the results and two crucial experiments, employing different types of contacts and varying the sample thickness were performed. The results provide clear evidence that the apparently high values of the dielectric constant in CaCu 3 Ti 4 O 12 are nonintrinsic and due to electrode polarization effects. The intrinsic properties of CaCu 3 Ti 4 O 12 are characterized by charge transport via hopping of localized charge carriers and a relatively high dielectric constant of the order of 100.Recent reports on the observation of colossal dielectric constants (CDCs) in CaCu 3 Ti 4 O 12 (CCTO) reaching values up to 10 5 (Refs. 1-3) have generated considerable interest in this material and related compounds. 4-14 The stunning observation in CCTO was a high and almost temperatureindependent dielectric constant Ј at elevated temperatures and a steep decrease by almost three orders of magnitude at low temperatures. The steplike decrease of Ј as function of temperature and an associated peak in the dielectric loss Љ, strongly depend on the measuring frequency and roughly follow an Arrhenius behavior. This relaxational behavior was ascribed to the slowing down of highly polarizable relaxational modes 2 or to the slowing down of dipolar fluctuations in nanosize domains. 3 Similar effects were obtained in thin films of CCTO making this system a good candidate for many applications. 7 However, shortly after the reports of the CDCs, their intrinsic nature has been questioned and arguments have been put forth that extrinsic effects as contributions from the electrode/sample interface, from grain boundaries in polycrystalline materials, or from twin boundaries in single crystals may be the sources of the giant dielectric constant. 4,10,12,14,15 Indeed, as it is known since decades, 16,17 the above outlined characteristics of the dielectric permittivity exactly corresponds to what is expected in case of interfacial polarization. Similar behavior, termed MaxwellWagner relaxation, has been observed in numerous materials (see, e.g., Ref. 15). It was pointed out that, even if the CDCs in CCTO are not intrinsic, this material could be a possible candidate for commercial applications as internal barrierlayer capacitor (IBLC). In IBLCs, internal barriers at grain boundaries lead to the observed high values of Ј (Ref. 5). Thus, clearly there is an urgent need for clarification of the true origin of the observed CDCs, which in case of an intrinsic nature would have important theoretical implications and in case of an IBLC scenario still would be of high technical relevance.As has been pointed out in Ref. 15, intrinsic (or IBLC) and electrode effects can well be separated using different contacts and sample geometries. Consequently, Ramirez et al. 8 remeasured CCTO using a different type of contact preparation. ...

We present a detailed study of the dielectric and charge transport properties of the antiferromagnetic cubic spinel HgCr 2 S 4 . Similar to the findings in ferromagnetic CdCr 2 S 4 , the dielectric constant of HgCr 2 S 4 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.PACS numbers: 75.80.+q, 77.22.Gm The detection of c olossal magnetoresistance in a number of perovskite-related manganites [1] maybe was the most notable discovery in solid state physics since the emergence of high-T c superconductors. Very recently another "colossal" effect with tremendous prospects for applications in modern microelectronics attracted considerable attention: In several, partly quite different materials an extremely strong coupling of magnetic and dielectric properties, termed "magnetocapacitive" or "magnetoelectric" effect, was found [2,3]. Among these, the cubic spinel CdCr 2 S 4 stands out by showing simultaneous ferromagnetic (FM) and relaxor ferroelectric behavior [3] and by setting a record value of the magnetocapacitive effect with an increase of the dielectric constant ε' of up to 3000% in a magnetic field of 10 T [4]. At the FM transition at T c ≈ 84 K, the dielectric constant shows a strong increase, which was ascribed to a speeding up of relaxational dynamics under the formation of magnetic order [ 4]. However, c learly the microscopic origin of the extraordinary behavior of CdCr 2 S 4 is far from being understood. A similar, but weaker effect was recently reported in isostructural CdCr 2 Se 4 [5] exhibiting FM order below T c ≈ 125 K. In the present letter, we provide dielectric data on HgCr 2 S 4 , which in contrast to CdCr 2 S 4 does not show any long-range FM order but exhibits a complex antiferromagnetic (AFM) type of order below 22 K [ 6]. Nevertheless, it shows a colossal magnetocapacitance (CMC) of even larger amplitude than in CdCr 2 S 4 and, m ost remarkably, the simultaneous occurrence of colossal magnetoresistance (CMR). In addition, our experiments indicate that in HgCr 2 S 4 , similar to our findings in CdCr 2 S 4 , short range ferroelectric order develops at low temperatures.All measurements were performed on single crystals of HgCr 2 S 4 , grown by chemical transport. Measurements with silver paint and sputtered gold contacts applied to opposite sides of the samples were performed. Details on crystal preparation and experimental methods are given in Refs. [3,6]. A thorough characterization of the magnetic properties of HgCr 2 S 4 [6] revealed an AFM state with non-collinear spin configuration below 22 K [ 7]. In moderate external magnetic fields up to 0.5 T, the AFM transi...

Low-temperature specific heat measurements and dielectric spectroscopy have been performed on polycrystalline and single-crystalline FeCr2S4, the single crystals showing a transition into a low-temperature orbital glass phase. The freezing of the orbital moments is revealed by a glasslike specific heat anomaly and by a clear relaxational behavior of the dielectric permittivity, exhibiting several hallmark features of glassy dynamics. 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.

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