V. Tsurkan

Relaxor ferroelectricity and colossal magnetocapacitive coupling in ferromagnetic CdCr2S4

Materials in which magnetic and electric order coexist—termed ‘multiferroics’ or ‘magnetoelectrics’—have recently become the focus of much research. In particular, the simultaneous occurrence of ferromagnetism and ferroelectricity, combined with an intimate coupling of magnetization and polarization via magnetocapacitive effects, holds promise for new generations of electronic devices. Here we present measurements on a simple cubic spinel compound with unusual, and potentially useful, magnetic and electric properties: it shows ferromagnetic order coexisting with relaxor ferroelectricity (a ferroelectric cluster state with a smeared-out phase transition), both having sizable ordering temperatures and moments. Close to the ferromagnetic ordering temperature, the magnetocapacitive coupling (characterized by a variation of the dielectric constant in an external magnetic field) reaches colossal values, approaching 500 per cent. 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 that drive the formation of the ferroelectric state.

Néel-type skyrmion lattice with confined orientation in the polar magnetic semiconductor GaV4S8.

Following the early prediction of the skyrmion lattice (SkL)--a periodic array of spin vortices--it has been observed recently in various magnetic crystals mostly with chiral structure. Although non-chiral but polar crystals with Cnv symmetry were identified as ideal SkL hosts in pioneering theoretical studies, this archetype of SkL has remained experimentally unexplored. Here, we report the discovery of a SkL in the polar magnetic semiconductor GaV4S8 with rhombohedral (C3v) symmetry and easy axis anisotropy. The SkL exists over an unusually broad temperature range compared with other bulk crystals and the orientation of the vortices is not controlled by the external magnetic field, but instead confined to the magnetic easy axis. Supporting theory attributes these unique features to a new Néel-type of SkL describable as a superposition of spin cycloids in contrast to the Bloch-type SkL in chiral magnets described in terms of spin helices.

Colossal Magnetocapacitance and Colossal Magnetoresistance in HgCr2S4

We present a detailed study of the dielectric and charge transport properties of the antiferromagnetic cubic spinel HgCr2S4. Similar to the findings in ferromagnetic CdCr2S4, the dielectric constant of HgCr2S4 becomes strongly enhanced in the region below 60-80 K, which can be ascribed to polar relaxational dynamics triggered by the onset of ferromagnetic correlations. In addition, the observation of polarization hysteresis curves indicates the development of ferroelectric order below about 70 K. Moreover, our investigations in external magnetic fields up to 5 T reveal the simultaneous occurrence of magnetocapacitance and magnetoresistance of truly colossal magnitudes in this material.

Spin and orbital frustration in MnSc2S4 and FeSc2S4.

Crystal structure, magnetic susceptibility, and specific heat were measured in the normal cubic spinel compounds MnSc2S4 and FeSc2S4. Down to the lowest temperatures, both compounds remain cubic and reveal strong magnetic frustration. Specifically the Fe compound is characterized by a Curie-Weiss (CW) temperature ThetaCW = -45 K and does not show any indications of order down to 50 mK. In addition, the Jahn-Teller ion Fe2+ is orbitally frustrated. Hence, FeSc2S4 belongs to the rare class of spin-orbital liquids. MnSc2S4 is a spin liquid for temperatures T>TN approximately 2 K.

NMR study in the iron-selenide Rb0.74Fe1.6Se2: determination of the superconducting phase as iron vacancy-free Rb0.3Fe2Se2.

77Se and 87Rb nuclear magnetic resonance (NMR) experiments on Rb0.74Fe1.6Se2 reveal clearly distinct spectra originating from a majority antiferromagnetic (AF) and a minority metallic-superconducting (SC) phase. The very narrow NMR line of the SC phase evidences the absence of Fe vacancies and any trace of AF order. The Rb content of the SC phase is deduced from intensity measurements identifying Rb(0.3(1))Fe2Se2 as the actual composition of the SC fraction. The resulting estimate of 0.15  electrons/Fe brings this class of superconductors 245 family closer to the other Fe-based superconductor families.

Spin-phonon coupling in antiferromagnetic chromium spinels

The temperature dependence of eigenfrequencies and intensities of the infrared (IR) active modes has been investigated for the antiferromagnetic chromium spinel compounds CdCr2O4, ZnCr2O4, ZnCr2S4, ZnCr2Se4, and HgCr2S4 by IR spectroscopy for temperatures from 5 to 300?K. At the transition into the magnetically ordered phases, and driven by spin-phonon coupling, most compounds reveal significant splittings of the phonon modes. This is true for geometrically frustrated CdCr2O4, and ZnCr2O4, for bond frustrated ZnCr2S4 and for ZnCr2Se4, which is also bond frustrated, but dominated by ferromagnetic (FM) exchange. The pattern of splitting is different for the different compounds and crucially depends on the nature of frustration and of the resulting spin order. HgCr2S4, which is almost FM, exhibits no splitting of the eigenfrequencies, but shows significant shifts due to FM spin fluctuations.

Orbital freezing and orbital glass state in FeCr2S4.

Broadband dielectric spectroscopy has been performed on single-crystalline FeCr2S4 revealing a transition into a low-temperature orbital glass phase and on polycrystalline FeCr2S4 where long-range orbital order is established via a cooperative Jahn-Teller transition. The freezing of the orbital moments is revealed by a clear relaxational behavior of the dielectric permittivity, which allows a unique characterization of the orbital glass transition. The orbital relaxation dynamics continuously slows down over six decades in time, before at the lowest temperatures the glass transition becomes suppressed by quantum tunneling.

Superconductivity at Tc = 44 K in LixFe2Se2(NH3)y

Following a recent proposal by Burrard-Lucas et al. [arXiv:1203.5046] we intercalated FeSe with Li in liquid ammonia. We report on the synthesis of new LixFe2Se2(NH3)y phases as well as on their magnetic and superconducting properties. We suggest that the superconducting properties of these new hybride materials appear not to be influenced by the presence of electronically-innocent Li(NH2) molecules. Indeed, high onset temperatures of 44 K and shielding fractions of almost 80% were only obtained in samples containing exclusively Lix(NH3)y moieties acting simultaneously as electron donors and spacer units. The c-axis lattice parameter of the new intercalated phases is strongly enhanced when compared to the alkali-metal intercalated iron selenides A1−xFe2−ySe2 with A = K, Rb, Cs, Tl with Tc = 32 K.

Spin-driven phonon splitting in bond-frustrated ZnCr2S4.

Magnetic susceptibility, specific heat, thermal expansion, and IR spectroscopy provide experimental evidence that the two subsequent antiferromagnetic transitions in ZnCr2S4 at TN1 = 15 K and TN2 = 8 K are accompanied by significant thermal and phonon anomalies. The anomaly at TN2 reveals a temperature hysteresis typical for a first-order transformation. Because of strong spin-phonon coupling, both magnetic transitions at TN1 and TN2 induce a splitting of phonon modes. The anomalies and phonon splitting observed at TN2 are suppressed by strong magnetic field. Regarding the small positive Curie-Weiss temperature Theta approximately 8 K, we argue that this scenario of two different magnetic phases with different magnetoelastic couplings results from the strong competition of ferromagnetic and antiferromagnetic exchange.

Ac susceptibility studies of ferrimagnetic FeCr2S4 single crystals

Ac linear and nonlinear susceptibilities, χ0 and χ2, of ferrimagnetic FeCr2S4 single crystals were measured in the temperature range from 4.2 to 300 K for different driving ac and applied dc magnetic fields in the frequency range of 10−1–103 Hz. For high driving ac fields the real part of χ0 exhibits a cusp at around Tm≈60 K correlated with the onset of dc magnetization irreversibilities. The imaginary part of χ0 shows a strong increase below 100 K and nonmonotonic temperature dependence with a maximum shifted toward low temperatures with an increase in the driving field. Both real and imaginary parts of the linear susceptibility, χ0′ and χ0″, show a pronounced frequency dependence between 90 and 20 K with a maximal difference at around 60 K. Below the Curie temperature the real part of the nonlinear susceptibility, χ2, exhibits a broad negative peak which is strongly shifted towards low temperatures with an increase in the driving field. No clearly divergent behavior of χ2′ around Tm is observed. The fie...