Cubic symmetry and magnetic frustration on the fccfcc spin lattice in K2_2IrCl6_6

Cubic crystal structure and regular octahedral environment of Ir$^{4+}$ render antifluorite-type K$_2$IrCl$_6$ a model fcc antiferromagnet with a combination of Heisenberg and Kitaev exchange interactions. High-resolution synchrotron powder diffraction confirms cubic symmetry down to at least 20 K, with a low-energy rotary mode gradually suppressed upon cooling. Using thermodynamic and transport measurements, we estimate the activation energy of $\Delta\simeq 0.7$ eV for charge transport, the antiferromagnetic Curie-Weiss temperature of $\theta_{\rm CW}\simeq -43$ K, and the extrapolated saturation field of $H_s\simeq 87$ T. All these parameters are well reproduced \textit{ab initio} using $U_{\rm eff}=2.2$ eV as the effective Coulomb repulsion parameter. The antiferromagnetic Kitaev exchange term of $K\simeq 5$ K is about one half of the Heisenberg term $J\simeq 13$ K. While this combination removes a large part of the classical ground-state degeneracy, the selection of the unique magnetic ground state additionally requires a weak second-neighbor exchange coupling $J_2\simeq 0.2$ K. Our results suggest that K$_2$IrCl$_6$ may offer the best possible cubic conditions for Ir$^{4+}$ and demonstrates the interplay of geometrical and exchange frustration in a high-symmetry setting.Comment: 9 page

Exploring the spin-1/2 frustrated square lattice model with high-field magnetization measurements

We report on high-field magnetization measurements for a number of layered vanadium phosphates that were recently recognized as spin-1/2 frustrated square lattice compounds with ferromagnetic nearest-neighbor couplings (J_1) and antiferromagnetic next-nearest-neighbor couplings (J_2). The saturation fields of the materials lie in the range from 4 to 24 T and show excellent agreement with the previous estimates of the exchange couplings deduced from low-field thermodynamic measurements. The consistency of the high-field data with the regular frustrated square lattice model provides experimental evidence for a weak impact of spatial anisotropy on the nearest-neighbor couplings in layered vanadium phosphates. The variation of the J_2/J_1 ratio within the compound family facilitates the experimental access to the evolution of the magnetization curve upon the change of the frustration magnitude. Our results support the recent theoretical prediction by Thalmeier et al. [Phys. Rev. B, 77, 104441 (2008)] and give evidence for the enhanced bending of the magnetization curves due to the increasing frustration of the underlying spin system.Comment: Brief Report: 4 pages, 3 figures, 1 tabl

Completely compensated ferrimagnetism and sublattice spin crossing in the half-metallic Heusler compound Mn1.5FeV0.5Al

The Slater-Pauling rule states that L21 Heusler compounds with 24 valence electrons do never exhibit a total spin magnetic moment. In case of strongly localized magnetic moments at one of the atoms (here Mn) they will exhibit a fully compensated half-metallic ferrimagnetic state instead, in particular, when symmetry does not allow for antiferromagnetic order. With aid of magnetic and anomalous Hall effect measurements it is experimentally demonstrated that Mn1.5V0.5FeAl follows such a scenario. The ferrimagnetic state is tuned by the composition. A small residual magnetization, that arises due to a slight mismatch of the magnetic moments in the different sublattices results in a pronounced change of the temperature dependence of the ferrimagnet. A compensation point is confirmed by observation of magnetic reversal and sign change of the anomalous Hall effect. Theoretical models are presented that correlate the electronic structure and the compensation mechanisms of the different half-metallic ferrimagnetic states in the Mn-V-Fe-Al Heusler system.Comment: Under revie

Frustrated couplings between alternating spin-1/2 chains in AgVOAsO4

We report on the crystal structure and magnetic behavior of the spin-1/2 compound AgVOAsO4. Magnetic susceptibility, high-field magnetization, and electron spin resonance measurements identify AgVOAsO4 as a gapped quantum magnet with a spin gap Delta ~ 13 K and a saturation field H_s ~ 48.5 T. Extensive band structure calculations establish the microscopic magnetic model of spin chains with alternating exchange couplings J ~ 40 K and J' ~ 26 K. However, the precise evaluation of the spin gap emphasizes the role of interchain couplings which are frustrated due to the peculiar crystal structure of the compound. The unusual spin model and the low energy scale of the exchange couplings make AgVOAsO4 a promising candidate for an experimental investigation of Bose-Einstein condensation and other exotic ground states in high magnetic fields.Comment: 10 pages + supplementary information and cif files, 7 figures, 6 table

Magnetic interactions and high-field properties of Ag(2)VOP(2)O(7): frustrated alternating chain close to the dimer limit

We report on high-field magnetic properties of the silver vanadium phosphate Ag(2)VOP(2)O(7). This compound has a layered crystal structure, but the specific topology of the V-P-O framework gives rise to a one-dimensional spin system, a frustrated alternating chain. Low-field magnetization measurements and band structure calculations show that Ag(2)VOP(2)O(7) is close to the dimer limit with the largest nearest-neighbor interaction of about 30 K. High-field magnetization data reveal the critical fields \mu_0H_{c1} of about 23 T (closing of the spin gap) and \mu_0H_{c2} of about 30 T (saturation by full alignment of the magnetic moments). From H_{c1} to H_{c2} the magnetization increases sharply similar to the system of isolated dimers. Thus, the magnetic frustration in Ag(2)VOP(2)O(7) bears little influence on the high-field properties of this compound.Comment: 4 pages, 2 figures. A paper for the proceedings of the HFM 2008 conferenc

Magnetoelectric effect and phase transitions in CuO in external magnetic fields

Apart from being so far the only known binary multiferroic compound, CuO has a much higher transition temperature into the multiferroic state, 230 K, than any other known material in which the electric polarization is induced by spontaneous magnetic order, typically lower than 100 K. Although the magnetically induced ferroelectricity of CuO is firmly established, no magnetoelectric effect has been observed so far as direct crosstalk between bulk magnetization and electric polarization counterparts. Here we demonstrate that high magnetic fields of about 50 T are able to suppress the helical modulation of the spins in the multiferroic phase and dramatically affect the electric polarization. Furthermore, just below the spontaneous transition from commensurate (paraelectric) to incommensurate (ferroelectric) structures at 213 K, even modest magnetic fields induce a transition into the incommensurate structure and then suppress it at higher field. Thus, remarkable hidden magnetoelectric features are uncovered, establishing CuO as prototype multiferroic with abundance of competitive magnetic interactions.Comment: 26 pages, 5 figure

Role of alkaline metal in the rare-earth triangular antiferromagnet KYbO2_2

We report crystal structure and magnetic behavior of the triangular antiferromagnet KYbO$_2$, the A-site substituted version of the quantum spin liquid candidate NaYbO$_2$. The replacement of Na by K introduces an anisotropic tensile strain with 1.6% in-plane and 12.1% out-of-plane lattice expansion. Compared to NaYbO$_2$, both Curie-Weiss temperature and saturation field are reduced by about 20% as the result of the increased Yb--O--Yb angles, whereas the $g$-tensor of Yb$^{3+}$ becomes isotropic with $g=3.08(3)$. Field-dependent magnetization shows the plateau at 1/2 of the saturated value and suggests the formation of the up-up-up-down field-induced order in the triangular AYbO$_2$ oxides (A = alkali metal), in contrast to the isostructural selenides that exhibit the 1/3 plateau and the up-up-down field-induced order

Design of compensated ferrimagnetic Heusler alloys for giant tunable exchange bias

The discovery of materials with improved functionality can be accelerated by rational material design. Heusler compounds with tunable magnetic sublattices allow to implement this concept to achieve novel magnetic properties. Here, we have designed a family of Heusler alloys with a compensated ferrimagnetic state. In the vicinity of the compensation composition in Mn-Pt-Ga, a giant exchange bias (EB) of more than 3 T and a similarly large coercivity are established. The large exchange anisotropy originates from the exchange interaction between the compensated host and ferrimagnetic clusters that arise from intrinsic anti-site disorder. We demonstrate the applicability of our design concept on a second material, Mn-Fe-Ga, with a magnetic transition above room temperature, exemplifying the universality of the concept and the feasibility of room-temperature applications. Our study points to a new direction for novel magneto-electronic devices. At the same time it suggests a new route for realizing rare-earth free exchange-biased hard magnets, where the second quadrant magnetization can be stabilized by the exchange bias.Comment: Four figure