Development of Millimeter-Wave Electron-Spin- Resonance Measurement Apparatus for Ultralow Temperatures and Its Application to Measurements of CuPzN

We have developed a millimeter-wave electron spin resonance (ESR) measurement system using a
³
He-
⁴
He dilution refrigerator for the ultralow-temperature range below 1 K. The currently available frequency range is 125-130 GHz. The frequency can be changed by adjusting the distance between the mirrors using a piezoelectric actuator. Using this apparatus, we performed ESR measurements on a Heisenberg quantum-spin chain -copper pyrazine dinitrate (CuPzN for short), Cu(C
₄
H
₄
N
₂
)(NO
₃
)
₂
-over the temperature range from 6.6 down to 0.25 K. The ESR lines shift continuously with decreasing temperature. The results suggest that the ESR spectrum of CuPzN is possibly useful as a temperature sensor below 1 K.