Submillimeter wave ESR measurements using the pulsed magnetic field is a powerful spectroscopic technique covering the wide frequency and magnetic field range with many advantages compared to the conventional X-band ESR measurements. Recent submillimeter wave ESR results showed that it is especially powerful to study the low dimensional antiferromagnets, that is, quantum spin systems. In accordance, the submillimeter wave ESR facilities in Japan are also presented. As an example of the study of quantum spin system, the investigation of the S=1/2 one dimensional antiferromagnets BaCu2(Si1-xGe x)2O7 (x=0, x=0.65), which have the staggered field effect in the system, is also presented in connection with the recent ESR theory by Oshikawa and Affleck (OA). Although both x=0 and x=0.65 systems show one dimensional behavior from the magnetic susceptibility, the temperature and frequency dependences of the mixed crystal x=0.65 system are well interpreted by the OA theory while they are not for the pure x=0 system. The antiferromagnetic resonance measurements of x=0 system (TN=8.9 K) have been performed up to 30 T at 1.8 K, and the magnetic phase transitions at 7 T and 16 T for H//a and H//c, respectively, are suggested for the first time.
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