E. Ohmichi

Magnetic detection of high-resolution electron spin resonance using a microcantilever in the millimeter-wave region up to 240 GHz

Highly sensitive magnetic detection of electron spin resonance (ESR) using a microcantilever is presented. By combining a modulation technique with the use of a piezoresistive cantilever, we successfully observed ESR signals of a tiny single crystal (mass<1 microg) of Co Tutton salt, Co(NH(4))(2)(SO(4))(2) x 6 H(2)O, in the frequency region of 80-240 GHz. The achieved spin sensitivity was approximately 10(9) spins/G at 4.5 K, providing promising applications to high-resolution and high-sensitivity terahertz ESR.

High-frequency electron spin resonance system using a microcantilever and a pulsed magnetic field

A novel technique of high-frequency electron spin resonance (ESR) in a pulsed magnetic field is presented. Our technique is based on the magnetic detection of a magnetization change associated with the ESR absorption using a microcantilever. We successfully observed ESR signals of a microcrystal (mass approximately 1 microg) in the millimeter-wave region up to 130 GHz in pulsed magnetic fields of up to 2.4 T. This result corresponds to the spin sensitivity of approximately 10(11) spins/G, which is four orders of magnitude better than that of conventional transmission-type ESR techniques.

Design of in situ sample rotation mechanism for angle-dependent study of cantilever-detected high-frequency ESR

Cantilever detected electron spin resonance (ESR) technique is combined with a precise rotation stage for angle-dependent ESR study of a tiny single crystalline sample on the order of 1 μg. Generally speaking, sample rotation in transmission-type ESR measurement is quite difficult. However, high angle resolution Δθ(min)∼0.2° and compact design of our stage allow in situ sample rotation in strong magnetic fields at low temperatures. As a result, a systematic study of ESR spectra for different field orientations can be easily obtained without sample extraction from a cryostat. As an example, angle-dependent ESR study of Co Tutton salt in the millimeter wave region is demonstrated at liquid helium temperature.

Multi-frequency ESR using a microcantilever in the millimeter wave region

Low-temperature magnetic properties of condensed matter system is investigated microscopically with high-frequency electron spin resonance (ESR). Experiments are carried out conventionally with a transmission method, but its sensitivity is not often sufficient to detect ESR signals of systems with small number of spins such as newly synthesized microcrystals. To attain better sensitivity, we focus on a novel technique of multi-frequency ESR system utilizing a microcantilever. In this method, ESR signal is detected as a torque change associated with the absorption of electromagnetic wave. Its sensitivity is greatly increased due to mechanical resonance when the modulation frequency of electromagnetic wave coincides with the eigenfrequency of the cantilever. In this study, we have succeeded in mechanical detection of ESR signals in the millimeter wave region up to 240 GHz. The achieved signal-to-noise ratio was greater than 103 for 1-μg sample of Co-Tutton salt, corresponding to the spin sensitivity of 109 spins/G.

Cantilever-detected high-frequency ESR measurement using a backward travelling wave oscillator

Our cantilever-detected electron spin resonance (ESR) technique is motivated for terahertz ESR spectroscopy of a tiny single crystal at low temperature. In this technique, ESR signal is detected as deflection of a sample-mounted cantilever, which is sensitively detected by built-in piezoresistors. So far, ESR detection at 315 GHz was succeeded using Gunn oscillator. In this study, we combine our ESR technique with a backward traveling wave oscillator (BWO), which can cover a wide frequency range 120-1200 GHz, to achieve better spectral resolution. Experiments were carried out at 4.2 K for a single crystal of Co Tutton salt with a newly constructed optical system. We successfully observed two ESR absorption lines in BWO frequencies up to 370 GHz. From multi-frequency measurements, the observed ESR lines shifted linearly with BWO frequency, being consistent with paramagnetic resonance. The estimated g values are g1 = 3.00 and g2 = 3.21. The spin sensitivity was estimated to ~1012 spins/gauss at 370 GHz.

Development of High-Sensitivity Cantilever-Detected ESR Measurement Using a Fiber-Optic Interferometer

Cantilever-detected high-frequency electron spin resonance (ESR) is a powerful method of sub-terahertz and terahertz ESR spectroscopy for a tiny magnetic sample at low temperature. In this technique, a small magnetization change associated with ESR transition is detected as deflection of a sample-mounted cantilever. So far, we have succeeded in ESR detection at 370 GHz using a commercial piezoresistive microcantilever. The spin sensitivity was estimated to ~10 12 spins/gauss. In order to further increase the sensitivity, we adopt a fiber-opticbased detection system using a Fabry-Perot interferometer in place of piezoresistive system. Fabry-Perot cavity is formed between an optical-fiber end and microcantilever surface, and a change in the interference signal, corresponding to the cantilever deflection, is sensitively detected. This system is suitable for low-temperature and high-magnetic-field experiments because of its compact setup and less heat dissipation. In this study, performance of Fabry-Perot interferometer is evaluated, and its application to cantilever-detected ESR measurement is described.

Magnetic susceptibility measurement under high pressure and magnetization measurement of S = 1/2 dioptase lattice antiferromagnet

Natural mineral Dioptase (Cu6Si6O18 ⋅ 6H2O) is an S = 1/2 antiferromagnet with a unique spin network which consists of helical chains along the c axis connected by inter chain exchange interactions. In order to study the ground state of dioptase, we have performed the magnetization measurement for B//c (easy axis) at 1.5 K and have found the spin-flop transition at 13 T. Moreover, the pressure effect of the magnetic susceptibility is investigated up to 1.78 GPa using the SQUID magnetometer where TN increased while the broad maximum reflecting the low dimensionality of the system decreased by pressure. The results will be discussed in connection with our previous antiferromagnetic resonance (AFMR) measurement results and the phase diagram obtained by Gros et al.