Interplay between mesoscopic phase separation and bulk magnetism in the layered NaxCoO2

Specific heat of the layered NaxCoO2 (x=0.65, 0.70 and 0.75) oxides has been measured in the temperature range of 3-360 K and magnetic field of 0 and 9 T. The analysis of data, assuming the combined effect of inter-layer superexchange and the phase separation into mesoscopic magnetic domains with localized spins embedded in a matrix with itinerant electronic character, suggests that the dominant contribution to the specific heat in the region of short-range ordering is mediated by quasi-2D antiferromagnetic clusters, perpendicular to the CoO2 layers

Dynamical crossover in 'hot' spin ice

The magnetic dynamics of the spin ice material Ho2Ti2O7 in its paramagnetic ('hot') phase have been investigated by a combination of neutron spin echo and ac-susceptibility techniques. Relaxation at high temperatures (T > 15 K) is proved to occur by a thermally activated single-ion process that is distinct from the process that dominates at lower temperatures (1 K < T < 15 K). It is argued that the low-temperature process must involve quantum mechanical spin tunnelling, as quasi-classical channels of relaxation are exhausted in this temperature range. Our results resolve a mystery in the physics of spin ice: why has a 15 K ac-susceptibility peak been observed in Dy2Ti2O7 but not in Ho2Ti2O7 or Ho2Sn2O7?NRC publication: Ye

Thermodynamic properties of Na0.65CoO2

Specific heat of the layered Na0.65CoO2 powder sample over the temperature region of 3-360 K has been studied. Features hinting at charge ordering below room-temperature were observed. Magnetic phase transition at 28 K as well as signs of short-range ordering confirm the 2D character of magnetism with weak inter-planar coupling in the system. Comparison to different 2D antiferromagnetic models suggests that the magnetic lattice is more square- than triangular-like, what could be favoured by the charge ordering.12th Czech and Slovak Conference on Magnetism, Jul 12-15, 2004, Kosice, Slovaki

Magnetic Phase Diagram of Geometrically Frustrated Magnetic System ErB4 under High Pressure

Rare earth tetraborides REB4 crystallize in tetragonal crystal structure, where the sublattice of rare earth ions forms in ab plane a geometrically frustrated Shastry Sutherland lattice. In this work we constructed a detailed TN vs. B phase diagram of ErB4 from data of temperature field dependencies of resistance. We have applied hydrostatic pressure up to 2.88 GPa using a piston cylinder pressure cell and performed sensitive ac resistance measurements in temperature range 1.8 K 300 K and in magnetic fields up to 6 T. The obtained results exhibit shifts of ordering temperature TN to higher values as well as shifts of boundaries between different magnetic phases. The effect of pressure on the interaction between magnetic ions in this compound is discussed and compared with the previous results obtained on TmB

Spin anisotropy in Cu en H2O 2SO4 A quasi two dimensional S 1 2 spatially anisotropic triangular lattice antiferromagnet

We have studied in detail the effect of the spin anisotropy on the electron paramagnetic resonance spectra and magnetic properties of Cu en H2O 2SO4, an S 1 2 spatially anisotropic triangular lattice antiferromagnet. The angular and temperature dependence of the resonance fields as well as the magnetization and magnetic susceptibility reflect the exchange and g factor anisotropy with Jz Jx,y lt; 1 and gz gx,y gt; 1, respectively. The exchange anisotropy and Dzyaloshinskii Moriya interaction are responsible for the main broadening mechanism at higher temperatures while spin diffusion effects prevail at helium temperatures. The ratio of the uniform susceptibilities calculated along the three crystal directions suggests an easy axis anisotropy with the a axis as the magnetic easy axis. Its impact on the physical properties of the title compound is discusse

Magnetic and thermal properties of the two-level magnetic system, KTm(MoO4)2

The magnetic susceptibility and specific heat of single crystals of KTm(MoO4)2 in the temperature interval 0.1-300 K are investigated. It is shown that the ground state of the rare-earth magnetic system of the Tm3+ ions can be represented by two close-lying singlet levels (a non-Kramers quasidoublet with M-J = \ +/- 6>) separated by a rather large energy interval from the excited levels. The effect of the ''freezing' of energy levels on magnetic susceptibility and the Schottky anomaly on magnetic specific heat have been observed in the low-temperature region T < 2.5 K. These results show that this magnetic system can be considered as a simple two-level quantum system over a wide temperature range

Neutron and EPR Study of Cu tn Cl 2 a Two Dimensional Spatially Anisotropic Triangular Lattice Antiferromagnet

We have studied the temperature dependence of the lattice parameters and the influence of spin anisotropy on the electron paramagnetic spectra of $Cu(tn)Cl_2$, an S=1/2 quasi-two-dimensional spatially-anisotropic triangular-lattice Heisenberg antiferromagnet. The variation of the resonance fields with temperature reflects the presence of an easy-plane exchange anisotropy with $J_{z}$/$J_{x,y}$1

Magnetism and magnetocaloric effect in S=7/2 Heisenberg antiferromagnet Gd-2(fum)(3)(H2O)(4)center dot 3H(2)O

Magnetic properties of Gd-2(fum)(3)(H2O)(4)center dot 3H(2)O were investigated by static and alternating susceptibility, specific heat and adiabatic magnetization. The material is characterized by weak antiferromagnetic interactions, arising predominantly from dipolar couplings that leads to magnetic ordering at T-N = 0.19 K. A pronounced change in the magnetic entropy was found near the critical temperature. The magnetocaloric behavior estimated from specific heat data is confirmed by adiabatic magnetization above 0.2 K. In addition, adiabatic magnetization and demagnetization near the critical temperature revealed only a small hysteresis. Finally, the magnetothermal response and resulting refrigeration capacity of Gd-2(fum)(3)(H2O)(4)center dot 3H(2)O is compared with other magnetic refrigerants proposed for low-temperature applications. (C) 2009 Elsevier B.V. All rights reserved