Magnetoabsorption in Single-Crystal Semiconducting Ferromagnetic Spinels

Harbeke, G.; Pinch, H. L.

doi:10.1103/physrevlett.17.1090

Cited by 164 publications

(36 citation statements)

References 8 publications

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“…35 In CdCr 2 S 4 at room temperature the band gap has been estimated as 1.57 eV exhibiting an unusual blue shift on decreasing temperatures. 36 These band gaps could not explain the observed temperature dependence of ǫ ∞ of the two compounds under consideration which are very similar and suggest a red shift of the band edges from room temperature down to 100 K and a subsequent blue shift in the ferromagnetic phase (CdCr 2 S 4 ) or in the state dominated by ferromagnetic fluctuations (HgCr 2 S 4 ). And indeed, it has been suggested, that these reports on the band gaps in the chromium spinels are heavily influenced by the Cr 3+ crystal field excitations and do not represent the true electronic band edges.…”

Section: Analysis and Discussionmentioning

confidence: 85%

Polar phonons and spin-phonon coupling inHgCr2S4andCdC

et al. 2007

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Polar phonons of HgCr2S4 and CdCr2S4 are studied by far-infrared spectroscopy as a function of temperature and external magnetic field. Eigenfrequencies, damping constants, effective plasma frequencies and Lyddane-Sachs-Teller relations, and effective charges are determined. Ferromagnetic CdCr2S4 and antiferromagnetic HgCr2S4 behave rather similar. Both compounds are dominated by ferromagnetic exchange and although HgCr2S4 is an antiferromagnet, no phonon splitting can be observed at the magnetic phase transition. Temperature and magnetic field dependence of the eigenfrequencies show no anomalies indicating displacive polar soft mode behavior. However, significant effects are detected in the temperature dependence of the plasma frequencies indicating changes in the nature of the bonds and significant charge transfer. In HgCr2S4 we provide experimental evidence that the magnetic field dependence of specific polar modes reveal shifts exactly correlated with the magnetization showing significant magneto-dielectric effects even at infrared frequencies.

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Section: Analysis and Discussionmentioning

confidence: 85%

Polar phonons and spin-phonon coupling inHgCr2S4andCdC

et al. 2007

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“…The straightforward effect of compression is the reduction of the Cr-Cr distances, which in turn causes the enhancement of the AFM J 1 interactions over the FM J 2 ones. Besides from tuning the magnetic interactions, pressure can also lead to structural [18][19][20][21][22][23] , electronic 21,24,25 , and even magnetic 21 27 . A red-shift of E g was detected below T C 27 , but without any apparent complementary structural effect 5,28 .…”

Section: Introductionmentioning

confidence: 99%

Pressure-induced phase transitions in the CdCr2Se4 spinel

et al. 2016

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We have conducted high-pressure x-ray diffraction and Raman spectroscopic studies on the CdCr 2 Se 4 spinel at room temperature up to 42 GPa. We have resolved three structural transitions up to 42 GPa, i.e. the starting Fd3 m phase transforms at ~11 GPa into a tetragonal I4 1 /amd structure, an orthorhombic distortion was observed at ~15 GPa, whereas structural disorder initiates beyond 25 GPa. Our ab initio DFT studies successfully reproduced the observed crystalline-to-crystalline structural transitions. In addition, our calculations propose an anti-ferromagnetic ordering as a potential magnetic ground state for the high-pressure tetragonal and orthorhombic modifications, as compared to the starting ferromagnetic phase. Furthermore, the computational results indicate that all phases remain insulating in their stability pressure range, with a direct-to-indirect band gap transition for the Fd3 m phase taking place at 5 GPa.We attempted also to offer an explanation behind the peculiar first-order character of the Fd3 m (cubic)→I4 1 /amd (tetragonal) transition observed for several relevant Cr-spinels, i.e. the sizeable volume change at the transition point, which is not expected from space group symmetry considerations. We detected a clear correlation between the cubic-tetragonal transition pressures and the next-nearest-neighbor magnetic exchange interactions for the Crbearing sulphide and selenide members, a strong indication that the cubic-tetragonal transitions in these systems are principally governed by magnetic effects.

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“…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 paramagnetic moment of 3.88 µ B , close to the theoretically expected value of 3.87 µ B per Cr…”

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confidence: 99%

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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Magnetoabsorption in Single-Crystal Semiconducting Ferromagnetic Spinels

Abstract: ferring to terminate on the grid wires rather than pass close by them. Hence, it seems that those rings which disappear from the beam are destroyed by interacting strongly with a line, and the charge given up by the ring has good probability of being trapped by the line.

Cited by 164 publications

References 8 publications

Polar phonons and spin-phonon coupling inHgCr2S4andCdC

Polar phonons and spin-phonon coupling inHgCr2S4andCdC

Pressure-induced phase transitions in the CdCr2Se4 spinel

Relaxor ferroelectricity and colossal magnetocapacitive coupling in ferromagnetic CdCr2S4

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