Relaxation dynamics and colossal magnetocapacitive effect inCdCr2S4

Abstract

A thorough investigation of the relaxational dynamics in the recently discovered multiferroic CdCr 2 S 4 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.PACS numbers: 75.80.+q, 77.22.Gm Multiferroic materials, showing the simultaneous occurrence of dielectric and magnetic order, are a "hot" topic in recent solid state research [1,2]. Especially the strong variation of electric (magnetic) properties under application of a magnetic (electric) field found in some of these materials, make them highly attractive not only from an academic point of view, but also for potential applications in microelectronics. Among the recently discovered multiferroics, the spinel compound CdCr 2 S 4 is exceptional, showing ferromagnetic and relaxor ferroelectric behavior with sizable ordering temperatures and, most remarkably, a colossal magnetocapacitive (MC) effect of nearly 500% in an external magnetic field H = 5 T [2]. However, so far the origin of the strong MC coupling in this compound is unknown. In [2], it was suggested that it may arise from the magnetic field affecting the relaxor dynamics of the polar moments, but alternatively also effects resulting from the hopping dynamics of localized charge carriers were discussed. In the present letter, from broadband dielectric measurements, we provide detailed information on the relaxation dynamics in CdCr 2 S 4 , both in the paramagnetic and ferromagnetic state. The results clearly reveal that it is the radical change of the relaxational dynamics driven by the magnetization that leads to the observed colossal MC effects in this compound.For the present investigation, CdCr 2 S 4 single crystals of the same batch as in [2] were used, with sputtered goldcontacts being applied on opposite sides of the plate-like samples. Dielectric constant and loss were measured over a broad frequency range of 12 decades (3 mHz < ν < 3 GHz) using frequency-response analysis and a reflectometric technique [3]. A conventional 4 He bath-cryostat and a cryomagnet allowed measurements at temperatures down to

Colossal magnetocapacitance and scale-invariant dielectric response in phase-separated manganites

“…We model C*(ω) using a Maxwell-Wagner (MW) circuit equivalent 16 shown in related dielectric studies of transition metal oxides 17 , spinels 4,5,18 and multiferroics 7 where the material under investigation is the 'insulator' (I) of a MIM structure rather than the base electrode as discussed here.…”

Colossal Magnetocapacitance and Colossal Magnetoresistance inHgCr2S4

“…Especially, despite this material exhibits no long-range FM order, its magnetocapacitance exceeds by far that observed in the FM spinel compounds CdCr 2 S 4 [3,4] and CdCr 2 Se 4 [5]. The reason for the exceptional behavior of HgCr 2 S 4 must be found in the strong magnetic frustration as revealed by magnetic and specific-heat studies [ 6].…”

“…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].…”

“…Concerning the origin of the polar moments, currently only speculations are possible, e.g. about an offcenter position of the Cr 3+ -ions [3], similar to the situation in perovskite ferroelectrics (e.g., BaTiO 3 ), an intrinsic surface-related Maxwell-Wagner mechanism [ 4,12], or more exotic scenarios as, e.g., electronic ferroelectricity driven by charge order and/or coupling of the electron system to the lattice [13,14]. For CdCr 2 S 4 , the occurrence of ferroelectricity due to the softening of a polar mode has been excluded in recent LSDA+U calculations [15].…”

“…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.…”