Submillimeter Wave ESR System Using the Pulsed Magnetic Field and Its Applications to One Dimensional Antiferromagnetic System

Abstract

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, are also presented. As an example of the study of quantum spin system, the investigation of the S=1/2 one dimensional antiferromagnets BaCu 2 (Si 1−x Ge x ) 2 O 7 (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 (T N =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.KEYWORDS: high field, high frequency, ESR, quantum spin system, line width, AFMR, BaCu 2 (Si 1−x Ge x ) 2 O 7 Submillimeter wave ESR and its facilities in JapanSubmillimeter wave ESR or, in other words, high field ESR measurements use higher frequency and higher magnetic field compared to the conventional X-band ESR measurement, which uses about 9.5 GHz and water cooling steady field up to 1 T. Due to these higher frequency and higher magnetic field, the submillimeter wave ESR has several advantages compared to the conventional X-band ESR. 1-3)(1) Stronger ESR absorption intensity (2) Higher resolution (3) Observation of a broad absorption line (4) Observation of ESR across the large energy splitting (5) Observation of ESR mode above the magnetic phase transition (6) Estimation of interchain exchange interaction in quasi one dimensional (1D) antiferromagnet The absorption intensity of ESR is proportional to the difference of Boltzman distribution between S Z =-1/2 and S Z =1/2 states, which are split by the Zeeman effect. Therefore, in principle, one will get stronger absorption intensity as one goes to the higher frequency because the Zeeman splitting will be larger. However, in reality, X-band ESR uses lock-in detection, a cavity etc. and has relatively higher sensitivity than submillimeter wave ESR. This means, on the contrary, there is still enough room for the submillimter wave ESR to gain its sensitivity, and we have to work on that. The magnetic field difference between the resonances with different gvalues becomes larger as the frequency is increased. If * E-mail address: ohta@phys.sci.kobe-u.ac.jp the field difference becomes larger than the line width, it will be easier to distinguish two resonances. Therefore, the submillimeter wave ESR gives higher resolution and this merit is also useful when analyzing the powder absorption.4) The line width of ESR is rel...

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