Relaxor ferroelectricity and colossal magnetocapacitive coupling in ferromagnetic CdCr2S4

Abstract

, 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...

Magnetocapacitance without magnetoelectric coupling

“…This is the socalled magnetodielectric ͑or magnetocapacitance͒ effect, which has been reported for a wide range of materials. 11,[15][16][17][18][19][20] The problem with this approach, however, is that magnetoelectric coupling is not the only way to produce magnetocapacitance: as shown in this letter, magnetoresistive artifacts can also give rise to an apparently large magnetodielectric effect. Thus, while multiferroicity may imply magnetocapacitance, the converse is not true.…”

“…28,29 It is worth mentioning that the present model requires only that within a material system there exist regions with different MR responses, which happens not only in boundary layers, but also when there are phase-separated clusters, as in some mixed-valence manganites, 20 and possibly also in relaxor-like selenides. 16 In the present calculations we have assumed a core resistivity of ϳ10 5 ⍀ m ͑typical of undoped compounds such as BiMnO 3 13 and YMnO 3 15 ͒, with boundary layers having a resistivity 100 times higher and t i / t b = 0.1. The intrinsic ͑di-polar͒ dielectric constant is in principle the same for interface and bulk, r ϳ 25 being a representative figure.…”

Magnetoimpedance spectroscopy of epitaxial multiferroic thin films

“…[1][2][3][4][5][6] A main aspiration is to design charge storage devices, where information could be written electrically onto a data bit and retrieved magnetically. For such challenges new multiferroic materials are required in thin-film form, where the magnetic properties can be modified by electric fields and vice versa.…”

Large Coupled Magnetoresponses in EuNbO2N

“…Large magneto-resistive and capacitive effects have recently been discovered in sulfide spinels such as CdCr 2 S 4 14 and HgCr 2 S 4 . 8 A mechanism based on intrinsic multiferroic relaxor behavior has been developed, 15 but an alternative explanation is that the magnetocapacitance is a microstructural effect due to Maxwell-Wagner polarization, 9,10 an interfacial polarization arising from charge depleted grain boundaries.…”