Manfred Reehuis

We have studied the magnetic properties of clinoatacamite Cu 2 Cl ( OH ) 3 , the parent compound of the quantum spin liquid candidate herbertsmithite and a long-standing puzzle among frustrated quantum magnets. As we reveal using density-functional theory, clinoatacamite belongs to the class of distorted kagome antiferromagnets with the kagome plane being embedded into a low-symmetry crystal structure. By means of thermodynamic measurements, muon spin rotation and relaxation, as well as neutron diffraction on single crystals, we find in zero magnetic field complex behavior below T I = 18.1 K which unfolds in three temperature regions I–III. We propose this complexity in multicritical clinoatacamite to arise from the competition of antiferromagnetic ordering modes from the underconstrained manifold of modes, which can lead to a metamagnetic texture in zero field.

The amorphous materials of the Ti₄₅Zr₃₈Ni₁₇ composition synthesized by mechanical alloying are widely recognized for their ability to store hydrogen with gravimetric densities above 2 wt.%. It is also known that those alloys can form a quasicrystalline state after thermal treatment and their structural and hydrogen sorption properties can be altered by doping with various elements. Therefore, in this paper, the results of the studies on the Ti₄₅Zr₃₈Ni₁₇ system with yttrium substituted for titanium and zirconium are presented. We demonstrate that these alloys are able to absorb hydrogen with a concentration of up to 2.7 wt.% while retaining their amorphous structure and they transform into the unique glassy-quasicrystal phase upon annealing. Furthermore, we demonstrate that the in-situ hydrogenation of those new materials is an effortless procedure in which the decomposition of the alloy can be avoided. Moreover, we prove that, in that process, hydrogen does not bind to any specific component of the alloy, which would otherwise cause the formation of simple hydrides or nanoclusters.

The mechanically alloyed amorphous alloys of the Ti₄₅Zr₃₈Ni₁₇ composition are known for their ability to form a quasicrystalline state after thermal treatment. It is also known that the amorphous and quasicrystal alloys belonging to the Ti₄₅Zr₃₈Ni₁₇ family are able to store hydrogen and yield gravimetric densities above 2 wt.%. In this contribution, we report the results of research on the Ti₄₅Zr₃₈Ni₁₇ system with vanadium doped instead of titanium. We found that the amorphous samples with moderate doping (x < 20) show the ability to absorb hydrogen while maintaining the amorphous state and they transform into the novel glassy-quasicrystal phase during annealing. Those materials with higher vanadium concentrations do not form entirely amorphous structures. However, they still can absorb hydrogen easily. It was also confirmed that the in situ hydrogenation of the amorphous alloys is a straightforward process without decomposition of the alloy. In this process, hydrogen does not attach to any particular constituent of the alloy, which would lead to the formation of simple hydrides or nanoclusters. Therefore, we were able to confirm the fully amorphous nature of the deuterides/hydrides of the Ti_45−xV_xZr₃₈Ni₁₇ with moderate V doping.

Spinels (AB ₂O₄) with magnetic ions occupying only the octahedral B sites have inherent magnetic frustration which inhibits magnetic long-range order (LRO) but may lead to exotic states. Here we report on the magnetic properties of the tetragonal spinel Zn_0.8Cu_0.2FeMnO₄, the tetragonality resulting from the Jahn–Teller active Mn³⁺ ions. X-ray diffraction and x-ray photoelectron spectroscopy of the sample yielded the composition ( Zn 0.8 2 + Cu 0.2 2 + ) _A [ Fe 0.4 2 + Fe 0.6 3 + M n 3 + ] _B O_4‒δ . Analysis of the temperature dependence of magnetization (M), ac magnetic susceptibilities (χ′ and χ′′), dc susceptibility (χ), heat capacity C_p , and neutron diffraction (ND) measurements show complex temperature-dependent short-range order (SRO) but without LRO. The data of χ vs. T fits the Curie–Weiss law: χ = C/(T ‒ θ) from T = 250 K to 400 K with θ ≃ 185 K signifying dominant ferromagnetic (FM) coupling with the FM exchange constant J/k _B = 17 K, and C = 3.29 emu K mol^‒1 Oe^‒1 yielding an effective magnetic moment µ _eff = 5.13 µ _B resulting from the high-spin states of Cu²⁺ (A site) and Fe²⁺ (B site), while the B site trivalent ions Mn³⁺ and Fe³⁺ are in their low-spin states. The extrapolated saturation magnetization obtained from the M vs. H data at T = 2 K is explained using the spin arrangement (Cu²⁺↓) _A [Fe²⁺↑, Fe³⁺↓, Mn³⁺↑] _B leading to FM clusters interact antiferromagnetically at low temperatures. Temperature dependence of d(χT)/dT shows the onset of ferrimagnetism below ∼100 K and peaks near 47 K and 24 K. The relaxation time τ obtained from temperature and frequency dependence of χ″ when fit to the power law and Vogel–Fulcher laws confirm the cluster spin-glass (SG) state. The magnetic field dependence of the SG temperature T SG H follows the equation: T SG H = T SG 0 1 − A H 2 / ϕ with T _SG (0) = 46.6 K, A = 8.6 × 10^‒3 Oe^‒0.593 and ϕ = 3.37. The temperature dependence of hysteresis loops yields coercivity H_C ∼ 3.8 kOe at 2 K without exchange-bias, but H_C decreases with increase in T becoming zero above 24 K, the T _SG(H) for H = 800 Oe. Variations of C_p vs. T from 2 K to 200 K in H = 0 and H = 90 kOe do not show any peak characteristic of LRO. However, after correcting for the lattice contribution, a broad weak peak typically of SRO becomes evident centered around 40 K. For T < 9 K, C_p varies as T² ; a typical signature of spin-liquids (SLs). Comparison of the ND measurements at 1.7 K and 79.4 K shows absence of LRO. Time dependence of thermo-remanent magnetization M _TRM(t) studies below 9 K reveal weakening of the inter-cluster interaction with increase in temperature. A summary of these results is that in Zn_0.8Cu_0.2FeMnO₄, ferromagnetic clusters interact antiferromagnetically without LRO but producing a cluster SG state at T _SG(0) = 46.6 K, followed by SL behavior below 9 K.

Non-graphitic carbons (NGCs), such as glass-like carbons, pitch cokes, and activated carbon consist of small graphene layer building stacks arranged in a turbostratic order. Both structure features, including the single graphene sheets as well as the stacks, possess structural disorder, which can be determined using wide-angle X-ray or neutron scattering (WAXS/WANS). Even if WANS data of NGCs have already been extensively reported and evaluated in different studies, there are still open questions with regard to their validation with WAXS, which is usually used for routine characterization. In particular, using WAXS for the damping of the atomic form factor and the limited measured range prevent the analysis of higher-ordered reflections, which are crucial for determining the stack/layer size (L_a, L_c) and disorder (σ₁, σ₃) based on the reflection widths. Therefore, in this study, powder WANS was performed on three types of carbon materials (glass-like carbon made out of a phenol-formaldehyde resin (PF-R), a mesophase pitch (MP), and a low softening-point pitch (LSPP)) using a beamline at ILL in Grenoble, providing a small wavelength and thus generating WANS data covering a large range of scattering vectors (0.052 Å⁻¹ < s < 3.76 Å⁻¹). Merging these WANS data with WANS data from previous studies, possessing high resolution in the small s range, on the same materials allowed us to determine both the interlayer and the interlayer structure as accurately as possible. As a main conclusion, we found that the structural disorder of the graphene layers themselves was significantly smaller than previously assumed.