Magnetocaloric Study of Spin Relaxation in `Frozen' Dipolar Spin Ice Dy2Ti2O7

The magnetocaloric effect of polycrystalline samples of pure and Y-doped dipolar spin ice Dy2Ti2O7 was investigated at temperatures from nominally 0.3 K to 6 K and in magnetic fields of up to 2 T. As well as being of intrinsic interest, it is proposed that the magnetocaloric effect may be used as an appropriate tool for the qualitative study of slow relaxation processes in the spin ice regime. In the high temperature regime the temperature change on adiabatic demagnetization was found to be consistent with previously published entropy versus temperature curves. At low temperatures (T < 0.4 K) cooling by adiabatic demagnetization was followed by an irreversible rise in temperature that persisted after the removal of the applied field. The relaxation time derived from this temperature rise was found to increase rapidly down to 0.3 K. The data near to 0.3 K indicated a transition into a metastable state with much slower relaxation, supporting recent neutron scattering results. In addition, magnetic dilution of 50 % concentration was found to significantly prolong the dynamical response in the milikelvin temperature range, in contrast with results reported for higher temperatures at which the spin correlations are suppressed. These observations are discussed in terms of defects and loop correlations in the spin ice state.Comment: 9 figures, submitted to Phys. Rev.

Nonlinear excitations in CsNiF3 in magnetic fields perpendicular to the easy plane

Experimental and numerical studies of the magnetic field dependence of the specific heat and magnetization of single crystals of CsNiF3 have been performed at 2.4 K, 2.9 K, and 4.2 K in magnetic fields up to 9 T oriented perpendicular to the easy plane. The experimental results confirm the presence of the theoretically predicted double peak structure in the specific heat arising from the formation of nonlinear spin modes. The demagnetizing effects are found to be negligible, and the overall agreement between the data and numerical predictions is better than reported for the case when the magnetic field was oriented in the easy plane. Demagnetizing effects might play a role in generating the difference observed between theory and experiment in previous work analyzing the excess specific heat using the sine-Gordon model.Comment: 6 pages, 5 figures, submitted to Phys. Rev.

Cu3(tmen)3(tma)2(H2O)2Cu_3(tmen)_3(tma)_2(H_2O)_2 · 6.5H2O6.5H_2O - New S = 1/2 "Sawtooth" Chain?

The results of the investigation of magnetic susceptibility, magnetization and specific heat of $Cu_3(tmen)_3(tma)_2(H_2O)_2$ · $6.5H_2O$ (tmen = N,N,N',N'-tetramethylethane-1,2 diamine; $H_3$ tma = 1,3,5-benzenetricarboxylic acid) are reported. The spatial arrangement of magnetic Cu(II) ions and network of covalent bonds suggest that the studied material might be a representative of S = 1/2 sawtooth chain with moderate exchange coupling J/$k_{B}$. The investigation of the temperature dependence of susceptibility and magnetic field dependence of magnetization yielding J/$k_{B}$ ≈ - 0.63 K is consistent with the structural features. In addition, specific heat data reveal short-range correlations in milikelvin temperature range and indicate long-range ordering below 150 mK

Experimental Study of Magnetocaloric Effect in Dipolar Magnet KEr(MoO4)2KEr(MoO_4)_2

We report on magnetothermodynamic properties of single crystal $KEr(MoO_4)_2$ which were investigated from nominally 0.4 K to 20 K in magnetic fields up to 4 T. Using the available specific heat data the diagram of temperature and magnetic field dependence of the total entropy was constructed. Predictions for temperature dependences of the relative temperature variations during the adiabatic demagnetization and the entropy variation during the isothermal demagnetization were calculated from the specific heat data. The obtained results suggest that unlike common refrigerants, in $KEr(MoO_4)_2$ the quantities describing magnetic cooling remain nearly constant in the temperature range 2-10 K

EPR Study of the Two-Dimensional Quantum System Cu(en)(H2O)2SO4Cu(en)(H_2O)_2SO_4

The angular dependence of electron paramagnetic resonance spectra of $Cu(en)(H_2O)_2SO_4$ single crystals was studied in the X-band frequency range at temperatures 4 and 300 K. Analysis of the linewidth at 300 K revealed nice agreement with the angular variation of the g-factor. This coincidence is the manifestation of the symmetric and antisymmetric exchange coupling, as main broadening mechanisms in $Cu(en)(H_2O)_2SO_4$ at high temperatures. The radical change of the angular dependence of the linewidth observed at 4 K can be ascribed to dipolar coupling

Frustrated zig-zag spin chains formed by hydrogen bonds in [Cu(H₂O)(OH)(tmen)]₂[Pd(CN)₄]·2H₂O

The magnetic properties of the novel dimeric compound [Cu(H₂O)(OH)(tmen)]₂[Pd(CN)₄]·2H₂O (tmen=N,N,N',N'-tetramethylethylenediamine) with modulated crystal structure were studied in the temperature range from 95 mK to 300 K. Magnetic measurements revealed a presence of weak antiferromagnetic exchange coupling in the compound. The temperature dependence of specific heat is characterized by the presence of a Schottky-like maximum at 0.47 K and a λ-anomaly at 0.28 K, indicating the formation of long-range order in the system. The comparison of the experimental data with theoretical predictions revealed the presence of antiferromagnetic intradimer exchange coupling J/k_{B}= -1.2 K and interdimer coupling of a similar strength mediated via hydrogen bonds between dimeric units forming a frustrated magnetic zig-zag chain structure

Thermal Conductivity of a Layered CsGd(MoO4)2CsGd(MoO_4)_2 Crystal

The thermal conductivity of $CsGd(MoO_4)_2$ has been studied in the temperature range from 2 to 50 K in zero magnetic field. The analysis of the data performed within the Debye model with the relaxation-time approximation revealed the presence of the scattering of phonons by critical fluctuations. The behaviour of phonon mean free path at the lowest temperatures is discussed

CuBr2(C10H8N2)−S=1//2CuBr_2(C_{10}H_8N_2) - S = 1//2 Two-Dimensional Rectangular Heisenberg Antiferromagnet

Magnetic susceptibility and X-band electron spin resonance study of a two-dimensional Heisenberg antiferromagnetic system $CuBr_2$(bipy), where bipy = $C_{10}H_8N_2$ is 4,4'-bipyridyl, has been performed in the temperature range from 300 K down to 2 K. A rhombic anisotropy of the g-factor was obtained from X-band EPR measurements with $g_{x}$ = 2.037, $g_{y}$ = 2.100 and $g_{z}$ = 2.219. The temperature dependence of the magnetic susceptibility with a round maximum observed at 28 K suggests antiferromagnetic type of short-range order. No signature of the magnetic long-range order in studied compound was observed. The comparison of the magnetic susceptibility data with a two-dimensional rectangular Heisenberg model yields the values of intralayer exchange couplings J/$k_{B}$ = - 47 K and J'/$k_{B}$ = - 9.4 K