Pressure effects on an S=1 / 2 Heisenberg two-leg ladder antiferromagnet Cu2(C5H12N2)2Cl4

The pressure effects on an S=1/2 Heisenberg two-leg ladder antiferromagnet (H2LLAF) Cu2(C5H12N2)2Cl4 have been investigated through magnetic and thermal measurements under pressures up to 10 kbar. The exchange interactions along the rung and leg hardly change under pressures, but the pressurization induces paramagnetic spins and magnetic order. This magnetic order is a pressure-induced one observed in a quantum spin system with an energy gap. The amount of induced paramagnetic spins increases almost in accordance with the square of pressure. The magnetic field dependence of the pressure-induced Schottky-type heat capacity suggests that the induced paramagnetic spins are not completely free, but weakly correlate with the H2LLAF system. A magnetic anomaly of the heat capacity has been observed around 2.6 K for P>~8.5 kbar, where more than 20% of the paramagnetic spins are induced. This anomaly is considered to be intrinsic to the magnetic order of the H2LLAF system, which seems to be triggered by the modulation of the staggered moment due to local defects. Even below the magnetic ordering temperature, the paramagnetic spins coexit with the magnetic order of the H2LLAF system. These pressure effects are similar to the impurity effects in another typical S=1/2 H2LLAF SrCu2O3 doped with nonmagnetic Zn2+ ions