‘Off-centre’ ions in compounds with spinel structure

Grimes, N. W.

doi:10.1080/14786437208227375

Cited by 120 publications

(35 citation statements)

References 39 publications

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“…The possibility of this phase transition of this type has been the subject of previous speculation (but never experimentally demonstrated) for many spinels. 23,45 Our calculation reveals that both A 2u modes are quite hard with frequencies greater than 300 cm −1 . In addition, the frequencies of the other silent zone-center phonon modes, E u , T 1g , and T 2u , were all calculated to be greater than 100 cm −1 .…”

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Polar phonons and intrinsic dielectric response of the ferromagnetic insulating spinelCdCr2S4from first principles

Fennie

Rabe

2005

Phys. Rev. B

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We have studied the dielectric properties of the ferromagnetic spinel CdCr2S4 from first principles. Zone-center phonons and Born effective charges were calculated by frozen-phonon and Berry phase techniques within LSDA+U. We find that all infrared-active phonons are quite stable within the cubic space group. The calculated static dielectric constant agrees well with previous measurements. These results suggest that the recently observed anomalous dielectric behavior in CdCr2S4 is not due to the softening of a polar mode. We suggest further experiments to clarify this point.PACS numbers: 77.80. Bh, 61.50.Ks, 63.20.Dj Multiferroics (MFs) displaying simultaneous ferroelectric (FE) and magnetic order are receiving considerable attention today. 1 Basic questions on the nature of ferroelectricity required to coexist with magnetism have been addressed, 2 facilitating the search for new materials. 3,4 Much effort has been directed towards finding MFs displaying the magnetoelectric effect, due to both the desire to understand this particular fundamental manifestation of spin-lattice coupling and to the potential technological applications of controlling the magnetization (polarization) by an applied electric (magnetic) field. 5,6 One avenue for finding new MEs is to revisit known ferromagnetic (FM) insulators to look for ferroelectricity. CdCr 2 S 4 is a member of a family of semiconducting FM chalcogenide ACr 2 X 4 spinels with A = Cd, Hg and X = S, Se. 7,8 The preponderance of evidence shows that CdCr 2 S 4 crystallizes in the normal cubic spinel Fd3m space group over a wide temperature range, from 4K (lowest temperature measured) to decomposition temperature. CdCr 2 S 4 is a simple Heisenberg ferromagnet with Cr 3+ spins (S=3/2) ordering at T c =84K (Θ CW =155K). 9 The valence band consists of a relatively narrow Cr t 2g peak hybridized with mostly sulfur 2p states, while Cr e g states make up the lowest unoccupied states, across a gap of 1.6-1.8eV, in the conduction band. 10,11 Early interest in these materials was due to the observed coupling of the electronic structure and the lattice to the magnetic subsystem. 12,13,14,15 While the intrinsic nature of many of these effects has subsequently been questioned, 16,17,18,19 spin ordering has clearly been shown to have a relatively strong effect on the infraredactive and selected Raman-active phonon modes, 15,18 making the chalcogenide spinels an attractive system to study spin-lattice effects from first principles. 20 Recently, CdCr 2 S 4 was revisited to look for ferroelectricity, leading to the suggestion that CdCr 2 S 4 is in fact a relaxor FE displaying a particularly large magnetocapacitive effect. 21,22 A broad, frequency dependent dielectric peak in the real part of the dielectric constant, of the type associated with relaxor behavior, was observed. The dielectric constant at 10 Hz was shown to change by 500% by application of a 5 T magnetic field, with the change decreasing rapidly with increasing excitation frequency.Finally, hysteresis loops indicating non...

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Polar phonons and intrinsic dielectric response of the ferromagnetic insulating spinelCdCr2S4from first principles

Fennie

Rabe

2005

Phys. Rev. B

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“…Indeed the observation of polar displacements of the octahedrally coordinated ions has been reported for a number of spinels 22 . Guided by these results, we suggest that ferroelectricity in CdCr 2 S 4 results from an off-centre position of the Cr 3+ -ions.…”

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Relaxor ferroelectricity and colossal magnetocapacitive coupling in ferromagnetic CdCr2S4

Hemberger¹,

Lunkenheimer²,

Fichtl³

et al. 2005

Nature

500

317

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, 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...

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“…Este espectro fue refinado usando el método de Rietveld, obteniéndose una estrucutra cúbica espinela correspondiente a la ferrita de cobalto con un parámetro de celda a=8.3763(4)Å y un parámetro de oxígeno m=0.3821(8)Å. Estos datos son comparables a los reportados en la bibliografía (18).…”

Section: 4h 2 O (15) Feso 4 (Oh) [16] Y Feso 4 H 2 O (17)unclassified

Procesos de magnetización reversible-irreversible y propiedades de remanencia de ferrita de cobalto coprecipitada

Medina-Boudri¹,

Bueno-Baqués²,

Matutes-Aquino³

2001

Bol. Soc. Esp. Ceram. Vidr.

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Se obtuvo ferrita de cobalto por coprecipitación de sulfatos de hierro y cobalto en acetona. Usando el modelo móvil de Preisach se separaron las componentes reversible (M rev ) e irreversible (M irr ) de la magnetización. El parámetro η definido como η = (dM rev / dM irr ) H i y el parámetro móvil de Preisach k m , se evaluaron mediante la relación η = k m χ rev para investigar el comportamiento de M rev Se encontró que η no es proporcional a la susceptibilidad reversible, por lo tanto, k m es una función del campo interno. Se midieron las curvas de magnetización isotérmica remanente (MIR) y de desmagnetización (DCD). A partir de los datos MIR y DCD se construyó la curva δm y el gráfico de Henkel, el cual se encuentra por debajo de la línea de Wohlfarth indicando que domina el desorden local (efecto del tipo desmagnetizante) sobre el efecto de campo medio. Diferenciando las curvas de MIR y DCD se obtuvieron las curvas de distribución de campos de conmutación. Los valores pico de estas distribuciones de campos de conmutación difieren por un factor de 2.7.Palabras claves: reversibilidad, remanencia, ferrita de cobalto, coprecipitación. Reversible-irreversible magnetization processes and remanence properties of coprecipitated cobalt ferrite.Cobalt ferrite was obtained by coprecipitation of iron (II) sulphate and cobalt (II) sulphate in acetone medium. Using the moving Preisach model, the reversible (M rev ) and irreversible (M irr ) components of the magnetization were separated. The parameter η given by η = (dM rev / dM irr ) H i and the moving Preisach parameter k m , were evaluated according to the relation η = k m χ rev in order to investigate the behavior of M rev . It was found that η is not proportional to the reversible susceptibility, therefore k m is a function of the internal field. Isothermal remanent magnetization (IRM) and DC demagnetization (DCD) curves were measured . From these data the δm curve and the Henkel plot were obtained showing that all the points remain under the Wohlfath line indicating that the local disorder dominates (demagnetizing-like effect) over the mean-field effect. The switching field distribution curves were obtained by differentiation of the IRM and DCD curves. The peak values of these switching field distributions differ by a factor of 2.7.Keywords: reversibility, remanence, cobalt ferrite, coprecipitation. INTRODUCCIÓNLa ferrita de cobalto es una ferrita cúbica espinela. Esta ferrita posee una estabilidad química excelente, elevada resistencia a la corrosión, anisotropía magnetocristalina, magnetoestricción y propiedades magneto-ópticas, que la hacen un material útil como medio de grabación magnética con guías de onda ópticas (1-4). Sin embargo, no se han desarrollado estudios sobre las componentes reversible e irreversible de la magnetización, ni tampoco sobre las interacciones magnéticas interpartículas. En este trabajo se estudió el proceso de magnetización para separar las componentes reversible e irreversible de la magnetización. A partir de los datos de las curvas de ...

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‘Off-centre’ ions in compounds with spinel structure

Cited by 120 publications

References 39 publications

Polar phonons and intrinsic dielectric response of the ferromagnetic insulating spinelCdCr2S4from first principles

Polar phonons and intrinsic dielectric response of the ferromagnetic insulating spinelCdCr2S4from first principles

Relaxor ferroelectricity and colossal magnetocapacitive coupling in ferromagnetic CdCr2S4

Procesos de magnetización reversible-irreversible y propiedades de remanencia de ferrita de cobalto coprecipitada

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