X-ray diffraction, magnetic susceptibility, electron-spin resonance, and heat-capacity investigations were performed on MAl 2 O 4 compounds with M = Co, Fe, Mn, and Zn. All compounds crystallize in cubic spinel structure AB 2 O 4 with minor inversion between A and B sites. CoAl 2 O 4 and FeAl 2 O 4 reveal spin-glass-like ground states with freezing temperatures well below the Curie-Weiss temperatures indicating strong geometric frustration. Below the freezing temperatures, the heat capacities show a T 2 temperature dependence for T → 0 K as has recently been observed in some related geometrically frustrated magnets, in contrast to the linear dependence expected for common spin glasses. The heat capacity of FeAl 2 O 4 exhibits an additional orbital contribution. With the absence of a cooperative Jahn-Teller distortion, this points toward an orbital freezing. MnAl 2 O 4 orders antiferromagnetically below T N = 40 K with a reduced value of the ordered moment and a large paramagnetic component.

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Using ferromagnetic resonance spectroscopy at 34 GHz we explored the magnetic anisotropy of single-crystalline GaVS in the field-polarized magnetic state. We describe the data in terms of an easy-axis type uniaxial anisotropy with an anisotropy constant [Formula: see text] erg cm at 2 K, corresponding to a relative exchange anisotropy [Formula: see text]%, and about [Formula: see text]erg cm near 11 K, i.e. at temperatures where the skyrmion-lattice phase was recently discovered. The relatively large value of K explains the confinement of the skyrmion tubes to the [Formula: see text] easy axes. A distinct set of resonances in the spectra is attributed to the co-existence of different rhombohedral domains. Complementary broadband spectroscopy demonstrates that non-collinear spin states may sensitively be detected by electron spin resonance techniques.

We report on structural, susceptibility, conductivity and heat-capacity studies of FeSe 0.5 Te 0.5 single crystals with 2% substitution of Mn for Fe. Mn-doped samples show a higher onset temperature, a narrower width of the superconducting transition and a higher magnitude of the jump in the specific heat at T c in comparison to undoped samples. The normal-state susceptibility exhibits a quasi-linear increase up to about 130 K. From the resistivity data in magnetic fields parallel to the c axis we derived an upper critical field H c2 of ∼420 kOe for doped samples compared to 370 kOe for pure samples. Using a single-band BCS model we can describe the electronic specific heat in the superconducting state with a gap (T = 0) = 31 K for the Mn-doped sample in comparison to 26 K for pure FeSe 0.5 Te 0.5 .

La2RuO5 shows a magneto-structural phase transition at 161 K with spin dimerization and concomitant formation of a non-magnetic singlet ground state. To gain a deeper insight into the origin of this transition systematic substitution of Ru by Ti has been carried out. Polycrystalline samples have been synthesized by thermal decomposition of citrate precursors leading to La2Ru(1-y)Ti(y)O5 (0 ≤ y ≤ 0.45). The crystal structure was investigated by x-ray powder diffraction at room temperature and at 100 K. The valences of Ti and Ru were obtained from x-ray absorption near edge structure spectroscopy at the Ti-K and the Ru-LIII absorption edges, respectively. The magnetic phase transition was investigated by magnetic susceptibility measurements as a function of Ti substitution, revealing a decreasing transition temperature on increasing the level of substitution. The step-like feature in the magnetic susceptibility reflecting the Ru-Ru spin dimerization transition becomes smeared out close to y = 0.3 and completely vanishes at y = 0.45, indicating complete suppression of spin-dimer formation. Additional specific-heat measurements show a continuous decrease of the magnetic entropy peak with increasing Ti substitution mirroring the reduced number of spin dimers due to the magnetic dilution. A magnetic anomaly of the dimerization transition can hardly be detected for y ≥ 0.3. Density functional theory calculations were carried out to study changes of the electronic band structure caused by the substitution. A possibly preferred distribution of Ti and Ru and the magnetic interactions as well as the change of the density of states close to the Fermi level are investigated. Based on these experimental results a detailed (y,T) phase diagram is proposed.