This review summarizes more than 100 years of research on spinel compounds, mainly focusing on the progress in understanding their magnetic, electronic, and polar properties during the last two decades. Many spinel compounds are magnetic insulators o
r semiconductors; however, a number of spinel-type metals exists including superconductors and some rare examples of d-derived heavy-fermion compounds. In the early days, they gained importance as ferrimagnetic or even ferromagnetic insulators with relatively high saturation magnetization and high ordering temperatures, with magnetite being the first magnetic mineral known to mankind. However, spinels played an outstanding role in the development of concepts of magnetism, in testing and verifying the fundamentals of magnetic exchange, in understanding orbital-ordering and charge-ordering phenomena. In addition, the A- site as well as the B-site cations in the spinel structure form lattices prone to strong frustration effects resulting in exotic ground-state properties. In case the A-site cation is Jahn-Teller active, additional entanglements of spin and orbital degrees of freedom appear, which can give rise to a spin-orbital liquid or an orbital glass state. The B-site cations form a pyrochlore lattice, one of the strongest contenders of frustration in three dimensions. In addition, in spinels with both cation lattices carrying magnetic moments, competing magnetic exchange interactions become important, yielding ground states like the time-honoured triangular Yafet-Kittel structure. Finally, yet importantly, there exists a long-standing dispute about the possibility of a polar ground state in spinels, despite their reported overall cubic symmetry. Indeed, over the years number of multiferroic spinels were identified.
Since the first observation of weak ferromagnetism in the charge-transfer salt kappa-(BEDT-TTF)2-Cu[N(CN)2]Cl [U. Welp et al., Phys. Rev. Lett. 69, 840 (1992)], no further evidence of ferromagnetism in this class of organic materials has been reporte
d. Here we present static and dynamic spin susceptibility measurements on kappa-(BEDT-TTF)2Hg(SCN)2Br revealing weak ferromagnetism below about TWF = 20 K. We suggest that frustrated spins in the molecular dimers suppress long-range order, forming a spin-glass ground state in the insulating phase.
The phase diagrams of EuFe$_{2-x}$Co$_x$As$_2$ $(0 leq x leq 0.4)$ and EuFe$_2$As$_{2-y}$P$_y$ $(0 leq y leq 0.43)$ are investigated by Eu$^{2+}$ electron spin resonance (ESR) in single crystals. From the temperature dependence of the linewidth $Delt
a H(T)$ of the exchange narrowed ESR line the spin-density wave (SDW) $(T < T_{rm SDW})$ and the normal metallic regime $(T > T_{rm SDW})$ are clearly distinguished. At $T > T_{rm SDW}$ the isotropic linear increase of the linewidth is driven by the Korringa relaxation which measures the conduction-electron density of states at the Fermi level. For $T < T_{rm SDW}$ the anisotropy probes the local ligand field, while the coupling to the conduction electrons disappears. With increasing substitution $x$ or $y$ the transition temperature $T_{rm SDW}$ decreases linearly accompanied by a linear decrease of the Korringa-relaxation rate from 8 Oe/K at $x=y=0$ down to 3 Oe/K at the onset of superconductivity at $x approx 0.2$ or at $y approx 0.3$, above which it remains nearly constant. Comparative ESR measurements on single crystals of the Eu diluted SDW compound Eu$_{0.2}$Sr$_{0.8}$Fe$_2$As$_2$ and superconducting (SC) Eu$_{0.22}$Sr$_{0.78}$Fe$_{1.72}$Co$_{0.28}$As$_2$ corroborate the leading influence of the ligand field on the Eu$^{2+}$ spin relaxation in the SDW regime as well as the Korringa relaxation in the normal metallic regime. Like in Eu$_{0.5}$K$_{0.5}$Fe$_2$As$_2$ a coherence peak is not detected in the latter compound at $T_{rm c}=21$ K, which is in agreement with the expected complex anisotropic SC gap structure.
We report on magnetic resonance studies within the magnetically ordered phase of the quasi-1D antiferromagnet LiCuVO_4. Our studies reveal a spin reorientational transition at a magnetic field H_c1 ~ 25 kOe applied within the crystallographical (ab)-
plane in addition to the recently observed one at H_c2 ~75 kOe [ M.G. Banks et al., cond-mat/0608554 (2006)]. Spectra of the antiferromagnetic resonance (AFMR) along low-frequency branches can be described in the frame of a macroscopic theory of exchange-rigid planar magnetic structures. These data allow to obtain the anisotropy of the exchange interaction together with a constant of the uniaxial anisotropy. Spectra of 7Li nuclear magnetic resonance (NMR) show that, within the magnetically ordered phase of LiCuVO_4 in the low-field range H < H_c1, a planar spiral spin structure is realized with the spins lying in the (ab)-plane in agreement with neutron scattering studies of B.J. Gibson et al. [Physica B Vol. 350, 253 (2004)]. Based on NMR spectra simulations, the transition at H_c1 can well be described as a spin-flop transition, where the spin plane of the magnetically ordered structure rotates to be perpendicular to the direction of the applied magnetic field. For H > H_c2 ~ 75 kOe, our NMR spectra simulations show that the magnetically ordered structure exhibits a modulation of the spin projections along the direction of the applied magnetic field H.
We present detailed ESR investigations on single crystals of the low-dimensional quantum magnet TiOCl. The anisotropy of the g-factor indicates a stable orbital configuration below room temperature, and allows to estimate the energy of the first exci
ted state as 0.3(1) eV ruling out a possible degeneracy of the orbital ground state. Moreover, we discuss the possible spin relaxation mechanisms in TiOCl and analyze the angular and temperature dependence of the linewidth up to 250 K in terms of anisotropic exchange interactions. Towards higher temperatures an exponential increase of the linewidth is observed, indicating an additional relaxation mechanism.
We report on the discovery of a novel triangular phase regime in the system La1-xSrxMnO3 by means of electron spin resonance and magnetic susceptibility measurements. This phase is characterized by the coexistence of ferromagnetic entities within the
globally paramagnetic phase far above the magnetic ordering temperature. The nature of this phase can be understood in terms of Griffiths singularities arising due to the presence of correlated quenched disorder in the orthorhombic phase.
We present angular dependent EPR measurements in NaV2O5 at X-band frequencies in the temperature range 4.2 K < T < 670 K. A detailed analysis in terms of the antisymmetric Dzyaloshinski-Moriya and the anisotropic exchange interactions yields the foll
owing scheme of charge order: On decreasing temperature a quarter-filled ladder with strong charge disproportions, existing for T > 100 K, is followed by zig-zag charge-order fluctuations which become long-range and static below T_SP=34 K.
Below a characteristic temperature, due to hybridisation effects Kondo insulators exhibit a gap in the electronic density of states and behave like semiconductors. By using Gd3+ electron spin resonance (ESR), the compound CeNiSn was investigated as a
representative of this class. In addition, the metal-to-insulator transition was studied as a function of doping for CeNi(1-x)Co(x)Sn and CeNi(1-y)Pt(y)Sn. The linewidth of the Gd resonance yields direct information about the density of states at the Fermi energy. So the size of the gap can clearly be estimated for the pure compound, and the closing of the gap by substitution of Ni by Co or Pt can be followed in detail. These results are compared to measurements of NMR, specific heat and susceptibility.
We measured X-band electron-spin resonance of single crystalline sodium vanadate doped with lithium, Na_{1-x}Li_xV_2O_5 for 0 < x < 1.3% . The phase transition into a dimerized phase that is observed at 34 K in the undoped compound, was found to be s
trongly suppressed upon doping with lithium. The spin susceptibility was analyzed to determine the transition temperature and the energy gap with respect to the lithium content. The transition temperature Tsp is suppressed following a square dependence of the lithium concentration while the energy gap is found to decrease linearly. At high temperatures (T>Tsp) the susceptibility remains nearly independent of doping.
