Anomalous magnetism in decagonalAl69.8Pd12.1Mn18.1

Abstract

We report the results of measurements of 27 Al and 55 Mn NMR spectra, the related spin-lattice relaxation rates, and the dc magnetic susceptibility of the stable decagonal quasicrystal Al 69.8 Pd 12.1 Mn 18.2 . The temperature variation of the magnetic susceptibility (T) reveals that the Mn ions carry only an effective magnetic moment of approximately 2 B , and confirms a spin-glass-type freezing of the Mn moments at T f ϭ12 K. The NMR spectra reveal two partially resolved lines for the 27 Al-nuclei, indicating that there are two different sets of environments for the Al-sites. The integrated intensity of the Mn line in the spectra suggests that about half of the Mn ions carry no magnetic moment. Below 50 K, and upon decreasing the temperature, the 27 Al NMR linewidth w and the spin-lattice relaxation rate T 1 Ϫ1 both grow with an increasingly negative slope, as it is often observed and interpreted as a critical ''slowing down'' of magnetic moments in systems approaching a spinglass transition. Various features, such as a broad maximum in T 1 Ϫ1 (T) and a slope change in the w() plot, both around 120 K, suggest a gradual reduction of the number of Mn moments with decreasing temperature below 120 K.

Inhomogeneous Mn-magnetism in Al–Pd–Mn quasicrystals

“…1 displays DC susceptibility measurements in an applied field of 100 G at temperatures between 4.5 and 350 K: The data for the decagonal sample confirms a previously observed spin-glass freezing at T f;deca ¼ 12 K [1]. From high-temperature susceptibility data and NMR-spectra recorded for the same material, we conclude that roughly half of the Mn-atoms carry an average effective magnetic moment of p ¼ 3 m B [2]. This average moment is not compatible with any of the possible ionization states of Mn.…”

“…Annealing the resulting tapes for 100 h at 820 C transforms them into polycrystalline flakes of the stable decagonal phase [1]. We confirmed the high perfection of the quasicrytalline phases for both, the icosahedral and the decagonal sample, by selected area electron diffraction, and by scanning electron microscopy [2]. Fig.…”

“…However, we found evidence that below 100 K; the concentration of magnetic Mn-ions in the decagonal sample is reduced by 30% [2], but we have not observed any indication yet, that the same feature also occurs in the icosahedral material.…”

“…From high-temperature Curie-Weiss-fits of the form wðTÞ ¼ C=ðT À yÞ þ w 0 to the data, we extracted the Curie-constant C and the Curie-temperature y; with w 0 as the constant diamagnetic background. In comparison with C ¼ 0:54 emu=mol for the decagonal material [2], the Curie constant of the icosahedral alloy, C ¼ 0:95 emu=mol; is by almost a factor of two larger and the spin-glass freezing temperature, estimated from the irreversibility point of the wðTÞ-data in Fig. 1, T f;ico ¼ 18 K; is higher by 50%.…”

Electrical properties of lead nitrate single crystals

“…In a ferroelectric phase transition, the relaxation process exhibits critical behaviour in the vicinity of the dielectric peak temperature [40]. The phenomena related to the critical slowing down were observed in many condensed matters having meta-stability, such as nanosized ferroelectric particles [41], quasicrystalline systems [42], superconductors having non-equilibrium phenomena [43] and granular ferromagnets [44], by using dynamical measurements such as dielectric dispersion, NMR measurements, Mössbauer spectra and light scattering. The observation in the lead nitrate crystal gives new information about the phenomena of the critical slowing down in condensed matter having meta-stability.…”