Insulating Ferromagnetic Spinels

Baltzer, P. K.; Lehmann, Hauke; Robbins, M.

doi:10.1103/physrevlett.15.493

Cited by 266 publications

(127 citation statements)

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“…more stable than non-magnetic solution, and the calculated moment (6.0 µ B /f.u) is in good agreement with experiments [20,21]. The electronic structures shown in Fig.1(a) and (b) suggest that the system can be approximately characterized as a "zero-gap half-metal" in the case without spinorbit coupling (SOC).…”

supporting

confidence: 72%

“…Its Curie temperature T c is high (around 106∼120 K), and its saturated moment is 5.64 µ B /f.u. [20,21], approaching the atomic value expected for high-spin Cr 3+ . Its transport behavior is different from other FM chalcogenide spinels, like CdCr 2 Se 4 and CdCr 2 S 4 which are clearly semiconducting.…”

mentioning

confidence: 84%

“…Its Curie temperature T c is high (around 106∼120 K), and its saturated moment is 5.64 µ B /f.u. [20,21] 22, 23]. The spinel structure (space group Fd3m) can be related to the zinc-blende and diamond structures in the following way.…”

mentioning

confidence: 99%

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Chern Semimetal and the Quantized Anomalous Hall Effect inHgCr2Se4

et al. 2011

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In 3D momentum space, a topological phase boundary separating the Chern insulating layers from normal insulating layers may exist, where the gap must be closed, resulting in a "Chern semimetal" state with topologically unavoidable band crossings at the fermi level. This state is a condensedmatter realization of Weyl fermions in (3+1) D, and should exhibit remarkable features, such as magnetic monopoles and fermi arcs. Here we predict, based on first-principles calculations, that such a novel quantum state can be realized in a known ferromagnetic compound HgCr2Se4, with a single pair of Weyl fermions separated in momentum space. The quantum Hall effect without an external magnetic field can be achieved in its quantum-well structure.PACS numbers: 71.20.-b, 73.20.-r, 73.43.-f Under broken time reversal symmetry, the topological phases of two-dimensional (2D) insulators can be characterized by an integer invariant, called Chern number [1], which is also known as the TKNN number [2] or the number of chiral edge states [3] in the context of the quantum Hall effect. 2D insulators can thus be classified as normal insulators or Chern insulators depending on whether or not the Chern number vanishes. Since the Chern invariant is defined only for 2D insulators, it is natural to ask what is its analog in 3D? Starting from a 2D Chern insulating plane (say at k z = 0), and considering its evolution as a function of k z , generally two situations may happen. If the dispersion along k z is weak, such that the Chern number remains unchanged, the system can be viewed as the simple stacking of 2D Chern insulating layers along the z direction. Such 3D Chern insulators are trivial generalization of the Chern number to 3D, which is quite similar to the weak topological insulators in systems with time reversal symmetry. However, if the dispersion along k z is strong, such that Chern number changes as the function of k z , the system will be in a nontrivial semimetal state with "topologically unavoidable" band crossings located at the phase boundary separating the insulating layers in k space with different Chern numbers [4,5]. This is due to the fact that the change of Chern number corresponds to a topological phase transition, which can happen only if the gap is closed. From the Kohn-Luttinger theorem, we can always expect that the band crossings appear at the fermi level at stoichiometry. This Chern semimetal state, if found to exist, can be regarded as a condensed matter realization of (3+1)D chiral fermions (or called Weyl fermions) in the relativistic quantum field theory, where the field can be described by the 2-component Weyl spinors [6] (either left-or righthanded), which are half of the Dirac spinors and must appear in pairs. The band-crossing points or Weyl nodes are topological objects in the following senses. First, since no mass is allowed in 2×2 Hamiltonian, the Weyl nodes should be locally stable and can only be removed when a pair of Weyl nodes meet together in the k space. Second the Weyl nodes are "topological def...

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Chern Semimetal and the Quantized Anomalous Hall Effect inHgCr2Se4

et al. 2011

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“…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).…”

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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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“…Kanamori [1] predicted that 90° ferromagnetic (FM) superexchange occurs between octahedrally coordinated 3d 3 orbital of one of Cr-ions with an empty e g (σ) level on the other Cr [2,3]. In the parent FM CuCr 2 Se 4 being a p-type metallic conductor with T C = 416 K and θ CW = 436 K [4], the chromium spins are coupled ferromagnetically via double exchange interaction involving the electrons jumping between Cr 3+ and Cr 4+ ions [5,6].…”

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confidence: 99%

Correlation between the negative magnetoresistance effect and magnon excitations in single-crystalline CuCr_1.6V_0.4Se₄

Malicka

Groń

Skrzypek

et al. 2010

Philosophical Magazine

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The structural, electrical and magnetic measurements as well as the electron spin resonance (ESR) spectra were used to characterize the singlecrystalline CuCr 1.6 V 0.4 Se 4 spinel and to study a correlation between negative magnetoresistance effect and magnon excitations. The ferromagnetic order below the Curie temperature T C ≈ 193 K, a p-type semiconducting behavior, the ESR change from paramagnetic to ferromagnetic resonance at T C , a big value of ESR linewidth and its temperature dependence in paramagnetic region were established. The electrical studies revealed the negative magnetoresistance, which can be enhanced with increasing magnetic field and decreasing temperature while a detailed thermopower analysis showed the magnon excitations at low temperatures. The spin-phonon coupling is explained within a

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Insulating Ferromagnetic Spinels

Cited by 266 publications

References 2 publications

Chern Semimetal and the Quantized Anomalous Hall Effect inHgCr2Se4

Chern Semimetal and the Quantized Anomalous Hall Effect inHgCr2Se4

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

Correlation between the negative magnetoresistance effect and magnon excitations in single-crystalline CuCr_1.6V_0.4Se₄

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Insulating Ferromagnetic Spinels

Cited by 266 publications

References 2 publications

Chern Semimetal and the Quantized Anomalous Hall Effect inHgCr2Se4

Chern Semimetal and the Quantized Anomalous Hall Effect inHgCr2Se4

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

Correlation between the negative magnetoresistance effect and magnon excitations in single-crystalline CuCr1.6V0.4Se4

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Correlation between the negative magnetoresistance effect and magnon excitations in single-crystalline CuCr_1.6V_0.4Se₄