Study on Quantum Phase Transition of Magnetic Materials induced by Very High Field

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We have developed in this study the magnet system to carry out the magnetization measurement up to 70 T as a usual manner and succeeded in observing various quantum phenomena induced by very high magnetic field. For example, we have succeeded in observation of wholel/4 plateau in BlP-TENO, which has been regarded as an S=1 antiferromagnetic spin ladder material. As the magnetism of this material comes from free radical, spin should be 1/2 but there is large ferromagnetic interaction between the spins that makes two spins resulted spin 1. From a crystal structure analysis, spin 1 form a spin ladder and the interaction between spin 1 is found to be antiferromagnetic from the magnetic measurements at low field. Up to now, high field magnetization measurement has showed that 1/4 plateau begins around 44 T but it is not obvious that the 1/4 plateau exists in this system. The end of 1/4 plateau has been observed for the first time in our study by extending the magnetization measurement up to 70 T and it also allow us to see the whole plateau region, which begins around 44 T and ends around 65 T. Another striking example is a of S=1. Recently, it has become clear that magnetism of the molecule disappears in a different manner as the antiferromagnetic ordering when it is adsorbed into micro-pores of some copper complex materials. We have succeeded in observation of transition from non-magnetic state to magnetic one by applying high field. It is also interesting point that there is no plateau though the S=1 molecules form spin ladder in the materials.