M. Nicklas

Ionic transport and heat capacity of glass-forming metal-nitrate mixtures

Abstract Ionic transport of the glass-forming metalnitrate mixtures [Ca(NO 3 ) 2 ] 0.44 [KNO 3 ] 0.56 (MKN), and [Mg(NO 3 ) 2 ] 0.44 [NaNO 3 ] 0.56 (MNN) was investigated near the glass transition using broadband spectroscopy of the complex conductivity to 300 MHz. The real part of the conductivity exhibits a transition from frequency independent to power law behavior as found in most ionic conductors. At high frequencies the frequency exponent approaches unity and becomes larger at low temperatures. In the real part of the dielectric constant, a relaxation step could be observed in CRN. A detailed evaluation of the results within the modulus formalism is presented. The imaginary part of the electrical modulus exhibits well developed peaks which are described by the Kohlrausch—Williams—Watts function. All materials studied can be characterized as fragile glass formers. The results are compared to findings in [Ca(NO 3 ) 2 ] 0.4 [KNO 3 ] 0.6 (CKN). Low temperature heat capacity, for three nitrate glass-formers, including CKN, exhibits weak excess contributions near 10 K.

The evolution from long-range magnetic order to spin-glass behaviour in PrAu2(Si1-xGex)2

We have studied the magnetic behaviour of PrAu2(Si1-xGex)2 by means of magnetic susceptibility, resistivity, and heat capacity measurements, and x-ray and neutron powder diffraction. All compounds are isostructural and crystallize in the well known ThCr2Si2-type structure. PrAu2Si2 shows the characteristic features of a canonical spin glass with a freezing temperature of TF = 3 K. Completely unexpectedly, on introducing/increasing the atomic disorder by alloying with Ge, the spin-glass transition is suppressed as is first evident from a slight decrease of the freezing temperature TF up to x = 0.10. Long-range magnetic order sets in for x0.12. Within the range 0.15 x1, all compounds show the same simple AF-I-type antiferromagnetic structure with a monotonic increase of both the Neel temperature and the ordered magnetic moment for increasing Ge concentration. The magnetic phase diagram of PrAu2(Si1-xGex)2 is explained by the presence of weak disorder at a constant level at the Au sites. Clear indications of crystal-field effects have been observed.

Magnetic properties of lithium phthalocyanine

Abstract The magnetic susceptibility and specific heat of the organic semiconductor lithium phthalocyanine (LiPc) are studied for temperatures 1.5 K ≤ T ≤ 300 K. LiPc has one unpaired electron on the inner ring of the macrocycle. These electrons are localized along the molecular stack and behave like spin chains. The spin susceptibility, as determined by electron spin resonance measurements, and the magnetic contribution to the heat capacity are well described by a model of a S = 1/2 antiferromagnetic Heisenberg chain. The exchange constant is given by J ≈ 40 K. In comparison the iodinated compound LiPcI is ESR silent. Due to oxidizing with iodine the unpaired electrons have been removed from the macrocycle.

MAGNETIC PROPERTIES OF THE SPIN GLASS PRAU2SI2

Abstract We examined the magnetic properties of pure PrAu2Si2 and present results of linear and nonlinear magnetic susceptibility measurements. The data show the characteristic features of a canonical spin glass, e.g. the critical behavior of the nonlinear susceptibility at the glass transition and the frequency dependence of the complex linear susceptibility near the freezing temperature, which is determined near T F ≈3 K . It is the first time that spin glass behavior was found in a pure rare-earth compound with well-localized magnetic moments.

LiV2O4: a heavy-fermion transition metal oxide?

Abstract We present electron-spin resonance ESR and magnetic susceptibility results and in pure and doped LiV 2 O 4 which provide experimental evidence that this transition-metal oxide is not a pure heavy-fermion system but has to be characterized as a spin-fluctuation compound where magnetic degrees of freedom play an essential role.

Ground state properties of Ce(Pd1−xNix)2Ge2 and CeNi2(Ge1−ySiy)2

Abstract We present the results of a systematic study of transport, magnetic and calorimetric properties of the Kondo–lattice compounds Ce(Pd 1− x Ni x ) 2 Ge 2 and CeNi 2 (Ge 1− y Si y ) 2 . Due to the substitution of Pd by Ni and Ge by Si the hybridization strength changes continously, yielding a transition from local moment magnetism to intermediate valence behavior. Special attention is given to the regime where the magnetic order is suppressed characterized by significant deviations from Fermi-liquid behavior.

Non-Fermi liquid behavior in Ni–Pd

Abstract Ni–Pd is investigated at the borderline from strongly enhanced paramagnetism to itinerant ferromagnetism. At approximately 2.6 at.% nickel a ferromagnetic quantum-critical point exists and the temperature dependence of the resistivity follows Δ ρ ∝ T 5/3 . In this paper we focus on the magnetic field dependence of the electrical resistivity close to this zero-temperature phase transition. The electrical resistivity reveals a transition from a non-Fermi liquid regime to a Landau Fermi-liquid, characterized by ρ = ρ 0 + AT 2 in a magnetic field of 2 T. At higher temperature long-range ferromagnetic order is induced.