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?

Exchange bias (EB) phenomena have been observed in Nd 2/3 Ca 1/3 MnO 3 colossal magnetoresistance perovskite below the Curie temperature T C ~ 70 K and attributed to an antiferromagnetic (AFM) -ferromagnetic (FM) spontaneous phase segregated state of this compound. Field cooled magnetic hysteresis loops exhibit shifts toward negative direction of the magnetic field axis. The values of exchange field H EB and coercivity H C are found to be strongly dependent of temperature and strength of the cooling magnetic field H cool . These effects are attributed to evolution of the FM phase content and a size of FM clusters. A contribution to the total magnetization of the system due to the FM phase has been evaluated. The exchange bias effect decreases with increasing temperature up to T C and vanishes above this temperature with disappearance of FM phase.Relaxation of a non-equilibrium magnetic state of the compound manifests itself through a training effect also observed while studying EB in Nd 2/3 Ca 1/3 MnO 3 .

Specific heat and ac magnetic susceptibility measurements, spanning low temperatures (T ≥ 40 mK) and high magnetic fields (B ≤ 14 T), have been performed on a two-dimensional (2D) antiferromagnet Cu(tn)Cl2 (tn = C3H10N2). The compound represents an S = 1/2 spatially anisotropic triangular magnet realized by a square lattice with nearest-neighbor (J/kB = 3 K), frustrating nextnearest-neighbor (0 < J ′ /J < 0.6), and interlayer (|J ′′ /J| ≈ 10 −3 ) interactions. The absence of long-range magnetic order down to T = 60 mK in B = 0 and the T 2 behavior of the specific heat for T ≤ 0.4 K and B ≥ 0 are considered evidence of high degree of 2D magnetic order. In fields lower than the saturation field, Bsat = 6.6 T, a specific heat anomaly, appearing near 0.8 K, is ascribed to bound vortex-antivortex pairs stabilized by the applied magnetic field. The resulting magnetic phase diagram is remarkably consistent with the one predicted for the ideal square lattice, except that Bsat is shifted to values lower than expected. Potential explanations for this observation, as well as the possibility of a Berezinski-Kosterlitz-Thouless (BKT) phase transition in a spatially anisotropic triangular magnet with the Néel ground state, are discussed. PACS numbers: 75.40.-s, 75.10.JmRecently, Cu(tn)Cl 2 has been identified as a potential model system for the realization of the spatially anisotropic triangular lattice from the collinear Néel phase (J ′ /J < 0.6). 15 For Cu(tn)Cl 2 studied in B = 0, no evidence for long-range magnetic order was observed down to 60 mK, and the data suggested intralayer inter-action strengths of J/k B = 3 K and 0 < J ′ /J < 0.6, while the interlayer coupling is |J ′′ /J| ≈ 10 −3 . These interactions are described by the Hamiltonianwhere i, j label intralayer spins and k labels the interlayer ones. J J a b c FIG. 1: Realization of Heisenberg model of a spatially anisotropic triangular lattice within a single bc layer in Cu(tn)Cl2. The layers are stacked along the a direction. The full circles denote Cu 2+ ions.

A detailed study of the low-temperature magnetic state and the relaxation in the phase-separated colossal magnetoresistance Nd 2/3 Ca 1/3 MnO 3 perovskite has been carried out. Clear experimental evidence of the cluster-glass magnetic behavior of this compound has been revealed. Well defined maxima in the in-phase linear ac susceptibility χ′(T) were observed, indicative of the magnetic glass transition at T g ~ 60 K. Strongly divergent zero-field-cooled and field-cooled static magnetizations and frequency dependent ac susceptibility are evident of the glassy-like magnetic state of the compound at low temperatures. The frequency dependence of the cusp temperature T max of the χ′(T) susceptibility was found to follow the critical slowing down mechanism. The Cole-Cole analysis of the dynamic susceptibility at low temperature has shown extremely broad distribution of relaxation times, indicating that spins are frozen at "macroscopic" time scale. Slow relaxation in the zerofield-cooled magnetization has been experimentally revealed. The obtained results do not agree with a canonical spin-glass state and indicate a cluster glass magnetic state of the compound below T g , associated with its antiferromagnetic-ferromagnetic nano-phase segregated state. It was found that the relaxation mechanisms below the cluster glass freezing temperature T g and above it are strongly different. Magnetic field up to about m 0 H ~ 0.4 T suppresses the glassy magnetic state of the compound.

Thermodynamic and transport properties of the CaRu1–xTixO3 system with x = 0, 0.03, 0.07, 0.10, and 0.15 in magnetic field up to 9 T have been studied. The unconventional temperature dependences of magnetic susceptibility, specific heat, and electrical resistivity observed for CaRuO3 are typical for non‐Fermi liquids and they support the assumption about the proximity of the system to the quantum critical point. The analysis of the experimental results suggests the electronic phase separation in CaRuO3 into ferromagnetic itinerant regions coexisting with strongly correlated ones. Substitution by titanium seems to push the system towards ferromagnetism, and drives it away from the quantum criticality.