Development of cylindrical resonator for millimeter-wave band ESR/NMR double magnetic resonance

doi:10.21203/rs.3.rs-7625537/v1

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Development of cylindrical resonator for millimeter-wave band ESR/NMR double magnetic resonance

https://doi.org/10.21203/rs.3.rs-7625537/v1

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published 25 Oct, 2025

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We have developed a magnetic resonance equipment for very low temperature and high magnetic field aiming at Dynamic Nuclear Polarization-Nuclear Magnetic Resonance (DNP-NMR) and Electron-Nuclear DOuble Resonance (ENDOR) of diluted spin system using with Electron Spin Resonance (ESR). In this study, we have developed a cylindrical resonator with a submicron-thick gold film for millimeter-wave band ESR/NMR double magnetic resonance by exploiting frequency dependence of skin depth. ESR measurements were performed using the fabricated resonator at around 130 GHz and in the temperature range of 3 K to 70 K of BDPA diluted to 100 mM in polystyrene. ESR sensitivity is obtained from the measurement. ¹⁹F-NMR signal from the sample holder is also successfully observed.

Nuclear Magnetic Resonance (NMR) and Electron Spin Resonance (ESR) are widely known as methods for investigating basic magnetism in a substance from microscopic viewpoints. NMR measures response of nuclear magnetic moments which provides information such as the structure of organic compounds and dynamics of surrounding electron spins [1, 2]. ESR technique can provide information on the environment of unpaired electrons which gives g value, as well as information on electron-electron or electron-nuclear spin interactions [3–5]. ESR and NMR are generally performed independently due to the difference in measurement targets. In recent years, with the progress of high-frequency light sources, the development of measurement methods that combine ESR and NMR has attracted much attention, especially in Dynamic Nuclear Polarization (DNP)-NMR measurement methods [6–9]. It has been reported that the sensitivity of NMR measurement is improved due to the DNP effect caused by irradiation of the millimeter wave of ESR frequency on the substance with hyperfine structure which is the interaction between electron and nucleus [10–14]. This method can improve NMR sensitivity especially for systems with dilute or small magnetization of nuclear spins. However, in order to perform this measurement, the ESR frequency needs to be known in advance. Most of DNP-NMR measurement system which have been developed so far have been designed to maximize NMR sensitivity without capability of ESR measurements.

It is generally known that the NMR sensitivity largely depends on the filling factor of the sample in the oscillating magnetic field created by the RF coil. In order to perform highly sensitive DNP-NMR measurement, it is necessary to bring the sample as close to the coil as possible without disturbing the millimeter wave mode for ESR. We have so far developed an ESR/NMR resonator in which a Helmholtz-type coil for NMR was installed in a Fabry-Pèrot-type resonator (FPR) for ESR, for planar samples [15–17]. This arrangement will be useful for electron-nuclear double resonance (ENDOR) measurements because of rather high sensitivity of ESR measurements. However, since the spatial arrangement of the Helmholtz type coil is limited in order to avoid disturbing the millimeter wave mode in the FPR, the filling factor of the sample for NMR cannot be high. Some years ago, two examples of ESR/NMR resonators using a cylindrical shape have been reported [18, 19]. In this double-magnetic-resonance resonator, a solenoid coil with a strip of conductor for NMR serves also as a cylindrical resonator for ESR at about 140 GHz using the TE₀₁₁ electromagnetic wave mode. Weis and coworkers reported the measurement results of ¹³C NMR signal and ²H Mims-ENDOR enhanced by the DNP effect by this resonator. This resonator shape has advantages that the filling factor of the sample in the coil for NMR can be as high as that for a normal solenoid and that one resonator can provide multimode for ESR, while the Q factor (Quality factor) of the resonator for ESR is deteriorated because the millimeter wave leaks between the turns of the coil. This low Q value is acceptable for pulsed ESR/ENDOR methods like Mims-ENDOR, while it is disadvantageous for cw-ESR measurements and for efficient irradiation of incident microwave power for DNP. It is favorable if one can measure in situ cw-ESR with an ESR/NMR dual resonator in order for precise tuning of ESR frequency for DNP measurement. In this study, in order to improve the sensitivity of DNP-NMR, we have developed a cylindrical resonator using a thin film made of gold (Au) that takes use of the different skin depths of ESR and NMR frequencies. By making an Au thin film with a thickness that reflects millimeter waves and transmits radio-frequency (RF) waves inside the resonator made of insulator resin, we can stop leaking of millimeter wave. This idea of utilizing gold layer for realizing a resonator for ESR/NMR double magnetic resonance was already used for a FPR [16, 17, 20]. Here, we report the development of a cylindrical type resonator for the ESR/NMR double magnetic resonance and the results of NMR and ESR measurements.

2.1 Relation between shape of cylindrical resonator and resonance frequency

The design of the resonator needs to fully consider the magnetic field and electric field patterns of various electromagnetic modes, that is, the H and E fields in the cavity and the current distribution I in the cavity wall. If patterns are carefully considered, unnecessary modes can be suppressed and desired modes can be excited. Further, it is possible to effectively position the sample insertion position and to appropriately set the sample insertion hole into the cavity and the electromagnetic wave insertion hole (iris). The resonance condition equation for the cylindrical resonator is given by the following equation [3],

$$\:{\left(Df\right)}^{2}=\:{\left(\frac{c{X}_{l,m}}{\pi\:}\right)}^{2}+{\left(\frac{nc}{2}\right)}^{2}{\left(\frac{D}{L}\right)}^{2}\:\:\:\:\:\:\:\:\left(1\right)$$

where f is the resonance frequency, c is the speed of light, and D and L are the diameter and the length of the cylinder, respectively. l, m, and n are integers representing the electromagnetic wave mode TE_lmn, and $\:{X}_{l,m}$ is m-th root of the l-th order Bessel function J_m’. For TE_0mn modes of a cylindrical resonator, the quality factor Q representing the sharpness of resonance has a broad maximum as a function of D/L ratio around D/L = 1.

2.2 Cylindrical ESR/NMR dual magnetic resonance resonator using Au thin film

In this subsection, the resonator for double magnetic resonance developed in this research is explained (Fig. 1(a)). This resonator uses TE₀₁₅ mode, and the oscillating magnetic field distribution becomes maximum at the center of the cylinder axis (Fig. 1(b)). Plungers at both ends of the resonator can change the length L of the cylindrical cavity, by which the resonance frequency can be adjusted. The base material of the cylindrical resonator is polyether ether ketone (PEEK), which is a highly insulating resin material, and oxygen-free copper is used for the coupling with the waveguide. The NMR coil is wound along the outside of the PEEK material. The Au thin film was sputtered to the inner wall of the resonance part using a coater (SC-701Mk II Advance, Sanyu Electron Co., Ltd). The deposited film thickness is estimated to be 0.47 µm from the sputtering time. This thickness was selected to be an intermediate value between the skin depths for ESR millimeter-wave (~ 100 GHz) and NMR RF wave (~ 100 MHz) in order that the film reflects the millimeter wave and transmits the RF.

2.3 ESR/NMR system

Here we describe the cryogenic ESR/NMR measurement system which is used for measurements in the following section. Figure 2 shows the ESR/NMR measurement system used in this study. A Millimeter Vector Network Analyzer (MVNA) (Model MVNA8-350-2, manufactured by ABmillimetre) was used both as a light source and as a detection system. The magnetic field was generated by a 9-T superconducting magnet (9-T SCM, Oxford Instruments) with the magnetic field homogeneity of 10^− 5/10 mm DSV. The sample was cooled with a variable temperature insert (VTI, Oxford Instruments). The usable temperature range of VTI is 1.5–200 K. A Cernox resistance thermometer of CX series (Lake Shore Cryotronics, Inc.) is installed just above the resonator to measure the temperature of the sample. We have performed cw-ESR by measuring reflected power from the resonator while sweeping the magnetic field.

We used a conventional pulsed NMR measurement system which is almost identical to that described elsewhere [21] except that the resonance circuit was not be installed in the cryogenic part. Alternatively, we utilized a top tuning system in which the capacitance of a variable capacitor and the length of coaxial cable connected to the coil on the resonator were adjusted at outside of the cryostat.

3.1 Manufacture and resonance characteristics of a cylindrical resonator

As shown in Eq. (1), the resonance frequency of the cylindrical resonator strongly depends on the diameter D and the length L of the cavity. Furthermore, in a high frequency range above 100 GHz, high processing accuracy is required because the wavelength becomes short. The base material of the resonator needs to be an insulating material. In this study, PEEK resin, which has higher strength and heat resistance than STYCAST1266, was used. The developed cylindrical resonator uses the TE₀₁₅ mode and is designed with a resonance frequency of 128 GHz and a cylinder inner diameter D of φ 6.5 mm. In order to prevent mixing of unwanted resonance modes, the resonator body was sliced into a few parts and the plunger diameter was made to be φ 6.2 mm which is a little smaller than D. This increases electrical resistance between the parts and suppresses the TM mode in which current flows in the longitudinal direction of the cylinder [3]. The resonance characteristics of this cylindrical resonator were measured at room temperature with MVNA. The measurement method is identical to that described in our previous work [16]. As shown in Fig. 3(a), a resonance mode was observed at a frequency of 127.51 GHz, and the Q value of this mode was estimated to be about 6500. The waves of the background are attributed to standing waves in a waveguide used to connect between MVNA and the resonator. Moreover, it was confirmed that the resonance frequency can be adjusted by moving the plungers as shown in Fig. 3(b). As expected, the resonance frequency increased as the distance between the plungers which equals L was decreased. The reason why the change in resonant frequency is not proportional to the change in L is probably due to the rattling of the screw that fixes the plunger. Such processing accuracy also affects Q value because the accuracy of parallelism between the plunger surfaces is important for TE_01n modes.

3.2 ESR measurement

In order to evaluate the developed resonator, ESR measurement of BDPA radical (α,γ-bisdiphenylene-β-phenylallyl) diluted to 100 mM in polystyrene was performed. The BDPA radical is a stable organic radical molecule and its crystal is known to have a sharp ESR line [22, 23]. BDPA is also utilized as a donor electron spin for causing ESR in DNP-NMR measurement [24]. Therefore, BDPA is suitable for evaluation in this study. Since this sample has one radical per molecule and the spin number can be adjusted by the dilution concentration, it can be used as a sample for a sensitivity evaluation. Figures 4, 5 and 6 show the temperature dependences of the ESR spectrum, integrated intensity of the ESR line, and the resonance frequency of the resonator. In each temperature, the ESR measurement frequency was adjusted to the resonant frequency of the resonator. Note that each measurement error in Figs. 5 and 6 is within the marker.

Figure 5. Temperature dependence of integrated intensity of ESR line.

In the vicinity of 130.85 GHz, one ESR line was observed over the entire temperature range from 5 to 70 K. Note that the ESR spectrum at each temperature was averaged over 5 sweeps. For the ESR spectrum at 5 K, the half width at half maximum was obtained to be 2 mT (= 20 G), and the signal to noise ratio (S/N ratio) was 6. No anomalies associated with the magnetic phase transition were observed in the temperature dependence of the integrated intensity, suggesting that it is paramagnetic in this temperature region. Regarding the temperature dependence of the resonance frequency, it was found that the resonance frequency increased as the temperature decreased. This is due to the contraction of the resonator. Next, the ESR measurement sensitivity of the resonator developed in this research is estimated. The measurement sensitivity of ESR can be expressed by the number of spins observable per Gauss. Considering that the sample used in this experiment was 34.66 mg and the density of diluted polystyrene was 1.05×10³ g/L, the number of spins contained in the sample was 2.0×10¹⁸ spins. The measurement sensitivity at about 5 K was calculated by (the number of spins)/((half-width)×(S/N ratio)), and was obtained to be 2 ×10¹⁶ spins/G.

3.4 ¹⁹F-NMR measurement

NMR measurement has been performed at 7 K using the developed cylindrical resonator with exactly the same sample as is used in ESR measurements. Measurement was performed with 117.55 MHz under the magnetic field 2.79 T, targeting ¹⁹F contained in a sample holder tube made of Teflon in order to prove that we can detect NMR signal of the sample inside of the resonator. Fourier transform (FT) spectrum of obtained ¹⁹F-NMR spin-echo signal is shown in Fig. 7. Here, the pulse width was 5 µs and the signal was averaged for 8 times. The practicality of the cylindrical resonator on NMR measurement in the very-low temperature region was confirmed. It is noteworthy that the sensitivity of NMR is possibly improved if the thickness of the Au film can be reduced without losing ESR sensitivity. The optimization of the film thickness is a future problem.

In this research, we have developed a cylindrical resonator for double magnetic resonance of ESR/NMR with the aim of increasing the sensitivity of DNP-NMR in the high frequency and very-low temperature regions. The resonator was made of PEEK with a submicron-thick gold film on the inner wall. This film thickness is such that the millimeter wave for ESR is reflected inside the resonator and the NMR RF wave generated by the coil wound outside the resonator is transmitted across the film due to the frequency dependence of the skin depth. The Q value representing the sharpness of resonance was estimated to be 6500 at 127.5 GHz for a designed mode TE₀₁₅. The frequency was variable at least in a range of 127.5–131.2 GHz at room temperature by moving the plungers at both ends of the resonator. As an evaluation in ESR, BDPA radical diluted in polystyrene at a concentration of 100 mM, was measured in the temperature range of 5 to 70 K and in the vicinity of 130.85 GHz. ESR measurement sensitivity was estimated to be 1.8×10¹⁶ spins/G. As an evaluation in NMR, NMR signal of ¹⁹F contained in a Teflon sample holder tube put in the resonator was observed at 7 K and 117.55 MHz. These results show that both ESR and NMR signals were successfully obtained at very low temperatures by using the cylindrical resonator made of submicron-thick gold film. From this result, the developed cylindrical resonator can be used for ESR/NMR dual magnetic resonance in high frequency and very-low temperature region with keeping acceptable sensitivities both for cw-ESR and for pulsed NMR. Better sensitivity will be possibly obtained by optimizing the thickness of the gold film and improving the processing accuracy.

Author Contribution

Yuya Ishikawa wrote the main manuscript text and prepared all of the figures and did all of these experiments.Yutaka Fujii gave us his foundation and did the experiment with Yuya Ishikawa.Kenta Ohya designed and fabricated the cavity and did the basic experiments.Kohei Hirosawa did the NMR measurements and measured resonant property.Jarno Järvinen and Sergey Vasiliev gave us the comments and support data for development of this cavity. They contributed to the succeeding the development.

Acknowledgement

One of the authors (Y. I.) would like to thank to Mr. Hidetomo Yamamori and everyone at the Advanced Development Center, University of Fukui, Dr. Takashi Furuya at Research Center for Development of Far-Infrared Region, University of Fukui, and Prof. Akira Fukuda at Hyogo College of Medicine for their technical support and useful discussions in carrying out this research.

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No competing interests reported.

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You are reading this latest preprint version

Development of cylindrical resonator for millimeter-wave band ESR/NMR double magnetic resonance

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

2. Principle and experimental

2.1 Relation between shape of cylindrical resonator and resonance frequency

2.2 Cylindrical ESR/NMR dual magnetic resonance resonator using Au thin film

2.3 ESR/NMR system

3. Results and discussion

3.1 Manufacture and resonance characteristics of a cylindrical resonator

3.2 ESR measurement

3.4 ¹⁹F-NMR measurement

4. Summary

Declarations

Author Contribution

Acknowledgement

References

Additional Declarations

Status:

Journal Publication

Version 1

Development of cylindrical resonator for millimeter-wave band ESR/NMR double magnetic resonance

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

2. Principle and experimental

2.1 Relation between shape of cylindrical resonator and resonance frequency

2.2 Cylindrical ESR/NMR dual magnetic resonance resonator using Au thin film

2.3 ESR/NMR system

3. Results and discussion

3.1 Manufacture and resonance characteristics of a cylindrical resonator

3.2 ESR measurement

3.4 19F-NMR measurement

4. Summary

Declarations

Author Contribution

Acknowledgement

References

Additional Declarations

Status:

Journal Publication

Version 1

3.4 ¹⁹F-NMR measurement