M. Brando

Magnetic properties and specific heat of R MnO 3 ( R = Pr , Nd)

The ground-state properties of the perovskites PrMnO 3 and NdMnO 3 were investigated by magnetization, magnetic ac susceptibility, and specific heat measurements. A strongly anisotropic behavior has been detected for temperatures below the antiferromagnetic phase transition T N 100 K. The susceptibility and the weak spontaneous ferromagnetic moment appear to be different in both compounds due to different anisotropic rare-earth contributions. The specific heat shows strong Schottky-type contributions at low temperatures, which for NdMnO 3 strongly depend on the magnetic field. A spin reorientation phase transition (spin-flop type) induced by a magnetic field along the b axis was observed in NdMnO 3 at H∼110 kOe and T= 5 K. All results can consistently be explained by anisotropic contributions of the rare-earth ions: In PrMnO 3 the electronic ground state is determined by a low-lying quasidoublet split by the crystal field (∼19 K). In NdMnO 3 the Kramers doublet is split by the Nd-Mn exchange field (∼20 K).

An unusual interplay among disorder, Kondo-effect and spin-glass behavior in the Kondo lattices, Ce2Au1−xCoxSi3

Abstract We report the results of magnetic measurements for the solid solution Ce2Au1−xCoxSi3. The results reveal that this solid solution is characterized by a magnetic phase diagram (plot of magnetic transition temperature versus x) unusual for Kondo lattices. In particular, the spin-glass freezing induced by disorder is observed only for the compositions at the weak coupling limit; as one approaches the quantum critical point by a gradual replacement of Au by Co, this disorder effect is surprisingly suppressed in favor of long range antiferromagnetic ordering in contrast to expectations. This unusual interplay between disorder, spin-glass freezing and the Kondo-effect calls for further refinement of theories on competition between magnetism and the Kondo effect.

Magnetic behavior of a new series of ternary compounds of the type, R2PtSi3 (R=La, Ce, Pr, Nd, Gd and Y)

Abstract We report here the formation of a new series of ternary intermetallic compounds R 2 PtSi 3 , where R=La, Ce, Pr, Nd, Gd and Y, crystallize in an AlB 2 hexagonal structure. The results of electrical resistivity, heat capacity, magnetic susceptibility and isothermal magnetization ( M ) measurements are also reported. The results reveal that the Ce, Nd and Gd compounds order magnetically around ( T C =) 5, 18 and 24xa0K respectively. While most of them are classified as ferromagnets on the basis of the observation of hysteresis loops, isothermal M behavior and positive paramagnetic Curie temperature ( θ p ), and Pr exhibits a complex type of magnetic behavior. La and Y compounds remain non-magnetic as expected down to 2xa0K. There is a breakdown of de Gennes scaling of magnetic ordering temperature for Ce and Nd compounds. For the Ce compound, the Kondo effect at low temperatures is negligible, as the characteristic resistivity minimum is not observed and the low temperature θ p is positive. It is stressed that there is a prominent minimum in the temperature dependence of ρ around 40xa0K (well above T C ) in Gd 2 PtSi 3 , and this observation is particularly notable considering that no such minimum even for the Ce compound is observed, and this is the third of this interesting class of Gd compounds, viz., those exhibiting magnetic precursor effects mimicking the Kondo effect, known till date.

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.

Heavy fermions in LiV2O4: Kondo compensation versus geometric frustration

7Li nuclear magnetic resonance and heat capacity measurements were performed on the metallic spinel LiV2O4 down to 30 mK. The temperature dependencies of the linewidth, the Knight shift and the spin-lattice relaxation rate were investigated in the temperature range 30 mK≤T≤280 K and at applied magnetic fields of 4.6, 10, 44 and 83 kOe. The longitudinal nuclear magnetization was found to relax following a stretched-exponential form with a stretching exponent in the range 0.5<β<1. For temperatures T<1 K and at the lowest applied magnetic field of 4.6 kOe, we observe a spin-lattice relaxation rate which slows down exponentially, exhibiting a hindering barrier of the order of 1 K. This excitation energy separates different configurations of a highly degenerate ground state of a completely frustrated magnet.

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.

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.

Field Induced Metal-Insulator Transition in (Pr:Ca:Sr)MnO3

In the system Pr0.65Ca0.28Sr0.07MnO3 (PCSM) the field induced metal-insulator transition exhibits changes of the resistivity up to 11 orders in magnitude. In order to study this colossal magnetoresistance effect (CMR) we carried out measurements on a PCSM single crystal, applying a broad range of experimental methods. Resistivity, magnetization and magnetic AC-susceptibility were measured in magnetic fields up to 140 kOe and in the temperature range 1.5 K T 800 K. With decreasing temperature and in zero magnetic field PCSM undergoes a sequence of phase transitions from a paramagnetic into a charge ordered insulating and finally antiferromagnetic phase. The conductivity in these phases is dominated by variable range hopping processes. By applying magnetic fields H ≥ 10 kOe a ferromagnetic metallic phase is induced. This transition exhibits an extremely marked hysteresis behavior. Below T ≈ 100 K metastable states can be generated, in which ferromagnetic metallic and insulating phases coexist.