High field specific heat, Cp, and magnetic susceptibility, c{hi}, measurements were performed on the quasi-two dimensional Heisenberg antiferromagnet [Cu(pyz)2(pyO)2](PF6)2. While no Cp anomaly is observed down to 0.5 K in zero magnetic field, the ap
plication of field parallel to the crystallographic ab-plane induces a lambda-like anomaly in Cp, consistent with Ising-type magnetic order. On the other hand, when the field is parallel to the c-axis, Cp and c{hi} show evidence of XY-type antiferromagnetism. We argue that it is a small but finite easy-plane anisotropy in quasi-two dimensional [Cu(pyz)2(pyO)2](PF6)2 that allows the unusual observation of field induced XY and Ising-type magnetic states.
A new calorimeter for measurements of the AC heat capacity and magnetocaloric effect of small samples in pulsed magnetic fields is discussed for the exploration of thermal and thermodynamic properties at temperatures down to 2 K. We tested the method
up to mu 0H = 50 Tesla, but it could be extended to higher fields. For these measurements we used carefully calibrated bare chip Cernoxtextregistered and RuO2 thermometers, and we present a comparison of their performance. The monotonic temperature and magnetic field dependences of the magneto resistance of RuO2 allow us to carry on precise thermometry with a precision as good as pm 1mK at T = 2 K. To test the performance of our calorimeter, AC heat capacity and magnetocaloric effect for the spin-dimer compound Sr3Cr2O8 and the triangular lattice antiferromagnet RbFe(MoO4)2 are presented.
The electrical resistivity (Rxx) and Hall resistivity (Rxy) of LaFeAsO1-xFx have been measured over a wide fluorine doping range 0 =< x =< 0.14 using 60 T pulsed magnets. While the superconducting phase diagram (Tc, x) displays the classic dome-shape
d structure, we find that the resistive upper critical field (Hc2) increases monotonically with decreasing fluorine concentration, with the largest Hc2 >= 75 T for x = 0.05. This is reminiscent of the composition dependence in high-Tc cuprates and might correlate with opening of a pseudo-gap in the underdoped region. Further, the temperature dependence of Hc2(T) for superconducting samples can be understood in terms of multi-band superconductivity. Rxy data for non-superconducting samples show non-linear field dependence, which is also consistent with a multi-carrier scenario.
The electrical resistivity and Hall coefficient of LaFeAsO0.95F0.05 polycrystalline samples were measured in pulsed magnetic fields up to m0H = 60 T from room temperature to 1.5 K. The resistance of the normal state shows a negative temperature coeff
icient (dr/dT < 0) below 70 K for this composition, indicating insulating ground state in underdoped LaFeAsO system in contrast to heavily doped compound. The charge carrier density obtained from Hall effect can be described as constant plus a thermally activated term with an energy gap DE = 630 K. Upper critical field, Hc2, estimated from resistivity measurements, exceeds 75 T with zero-field Tc = 26.3 K, suggesting an unconventional nature for superconductivity.
