ALBERT

Notes: A detailed study of the temperature and field dependence of the dc magnetization near the antiferromagnetic (AF) transition in Gd2CuO4 shows the presence of field-induced weak ferromagnetism (WF) well above the Néel temperature TN. ac susceptibility measurements indicate TN=266(3) and 282(3) K, for single crystals and ceramic samples, respectively. The shift of the electron-spin-resonance line of the Gd3+ ions as a function of temperature T, δHr(T), is compared with the static internal field Hi(T), which describes the coupling between the paramagnetic Gd lattice and the WF component of the Cu moments. Both δHr(T) and Hi(T) show that the WF persists up to ≈50 K above TN. The initial susceptibility (for H≤100 G) presents a contribution in excess of the paramagnetism of Gd ions. For T≥TN, it decays as K(T−TN)−1, with K=0.20(3) emu K mol. The relationship between this result and the strong two-dimensional AF correlations in these cuprates is discussed.

Notes: Polycrystalline samples of the tubular phase Bi4Sr8Cu5O19+x with Fe ions substituting the Cu positions have been prepared up to a maximum content of 2 Fe ions per unit formula. The electrical and magnetic properties of these oxides have been examined and it is shown that they are insulating and present a complex magnetic behavior. It is found that the Fe ions have a localized magnetic moment displaying a Curie–Weiss law at high temperatures while below T≈100 K antiferromagnetic correlations are detected. From the evaluation of the effective paramagnetic moment and its composition dependence it is deduced that Fe doping induces a paramagnetic moment in some Cu ions which then contribute also to the temperature dependent paramagnetism. Finally, at lower temperatures (T〈25 K) a susceptibility peak is observed signaling a freezing of the magnetic moments. We suggest a spin-glass behavior associated either to a clustering of Fe ions or to the development of correlated domains interacting through the strongly correlated copper ions. We point out that the weak sensitivity of the resistivity on the Fe concentration gives further support to the hypothesis of some Fe clustering within the tubular crystal structure.

Notes: Polycrystalline Tb2CuO4 with tetragonal (T') structure was synthesized under high pressure. The in-phase component of the alternating-current (ac) susceptibility χ'(T) presents a peak whose onset at TN ≈ 290 K, we associate with antiferromagnetic ordering of the Cu moments. The shape of this peak is strongly dependent on the amplitude of the excitation field, indicating nonlinearity of the magnetization as a function of the applied magnetic field. The frequency dependence of the peak observed in χ'(T) and the nonzero out-of-phase component χ‘(T) in the same temperature range indicate the existence of relaxation processes associated with the development of a weak ferromagnetic component. Below ∼100 K, the relaxation becomes much slower than the frequency of the oscillating field and the ac susceptibility is dominated by the paramagnetic response of Tb ions, which follows a Curie–Weiss law. The direct-current (dc) magnetization presents a nonlinear dependence on the applied field for Ha (approximately-less-than) 2 kOe and shows thermomagnetic irreversibility below a second characteristic temperature Tirr 〈 TN. An additional peak at TL ≈ 10 K indicates a long range magnetic order of the Tb ions with a possible spin reorientation of the Cu moments. The thermal irreversibility and nonlinearity of the dc magnetization below Tirr, the relaxation processes revealed by the ac susceptibility, and the typical logarithmic decay of the remanent magnetization, point to the existence in this material of a weak ferromagnetic state with spin-glass-like characteristics.

Notes: Under field conditions, a plot sown with Forrajero-Baer, a rye (Secale cereale L.) cultivar exuding hydroxamic acids through its roots, resulted in 83% less weed biomass than wheat (Triticum aestivum L.) and 76% less than forage oats (Avena strigosa L.). Forage oats reduced broadleaf weed biomass by 40% compared to wheat but failed to suppress wild oats (Avena fatua L.). Rye interference reduced wild oat biomass by 84% and 86% compared to wheat and forage oats, respectively. Bioassays carried out with 2-O-ß-glucopyranosyl-4-hydroxy-l,4-benzoxazin-3-one (DIBOA-glc), the main hydroxamic acid found in rye, its aglucone 2,4-dihydroxy-1,4-benzoxazin-3-one (DIBOA), and its breakdown product benzoxazolin-2-one (BOA) showed inhibition of root and coleoptile growth of A. fatua seedlings at concentrations above 0–25 mM l−1. These results indicate that hydroxamic acids from Forrajero-Baer rye probably play a significant role in the weed-suppressing ability of this cultivar. Interférence entre la croissance des adventices et les cereales de zones tempérées: l'effet d'un cultivar de seigle (Secale cereale L.) secrétant des acides hydroxamiquesSous conditions de plein champ, une parcelle semée avec Forrajero-Baer, un cultivar de seigle (Secale cereale L.) secrétant des acides hydroxamiques à travers ses racines, a aboutit à une biomasse en adventices de 83% de moins qu'un blé (Triticum aestivum L.) et 76% de moins que de l'avoine fourragère (Avena strigosa L.). Les avoines fourragères réduisent la biomasse des adventices dicotylédones de 40% en comparaison du blé, mais n'éliminent pas les folles avoines (Avena fatua L.). L'inter-férence du seigle a réduit la biomasse de folle avoine par 84% et 86% en comparaison du blé et de l'avoine fourrageé respectivement. Des bioessais conduits avec du 2-O-ß-glucopyrano-glucopyranosyl- 4 -hydroxy- 1,4- benzoxazin -3-one (DIBOA-glc), le principal acide hydroxamique trouvé dans le seigle, son aglycone 2,4-dihydroxy-1-4 benzoxazin 3-one (DIBOA) et son produit de dégradation benzaxazolin-2-one (BOA) ont mis en évidence une inhibition de la croissance des racines et du coléoptile des plantules d'A. fatuaà des concentrations supérieures à 0,25 mM 1−1. Ces résultats montrent que les acides hydroxamiques du seigle Forrajero-Baer jouent un rôle significatif dans l'aptitude à combattre les adventices de ce cultivar. Beeinträchtigung des Unkrautwachstums durch Getreide: die Wirkung einer Hydroxamsäuren ausscheidenden Sorte des Roggens (Secale cereale L.)In einem Feldversuch war unter der Roggen-Sorge ‘Forrajero-Baer’ (Secale cereale L.), die öber die Wurzeln Hydroxamsäuren ausscheidet, die Unkrautbiomasse 83% geringer als unter Weizen (Triticum aestivum L.) und 76% geringer als unter Sand-Hafer (Avena strigosa L.). Der Sand-Hafer reduzierte die Biomasse dikotyler Unkrautarten im Vergleich zum Weizen um 40%, konnte Flug-Hafer (Avena fatua L.) aber nicht unterdrücken. Der Roggen reduzierte die Biomasse von Flug-Hafer im Vergleich zu Weizen und Sand-Hafer um 84 bzw. 86%. In Biotests hemmten 2-O-β-Gluco-pyranosyl - 4 - hydroxy -1,4- benzoxazin - 3 - on (DOBIA-glc), die hauptsächlich in Roggen gefundene Hydroxamsäure, sein Aglukon 2,4-Hydroxy-l,4-benzoxazin-3-on (DIBOA) und sein Abbauprodukt Benzoxazolin-2-on (BOA) in Konzentrationen über 0,25 mm1−1 das Wachstum der Wurzeln und der Koleoptile von Avena-fatua-Keimpflanzen. Diese Ergebnisse zeigen, daß Hydroxamsäuren aus der Roggen-Sorte ‘Forrajero-Baer’ offensichtlich eine wichtige Rolle bei der unkrautunterdrückenden Wirkung dieser Sorte spielen.