Continuous-wave far-infrared ESR spectrometer for high-pressure measurements

We present a newly-developed microwave probe for performing sensitive high-field/multi-frequency electron spin resonance (ESR) measurements under high hydrostatic pressures. The system consists of a BeCu-made pressure-resistant vessel, which accommodates the investigated sample and a diamond microwave coupling window. The probe’s interior is completely filled with a pressure-transmitting fluid. The setup operates in reflection mode and can easily be assembled with a standard oversized microwave circuitry. The probe-head withstands hydrostatic pressures up to 1.6 GPa and interfaces with our home-built quasi-optical high-field ESR facility, operating in a millimeter/submillimeter frequency range of 105–420 GHz and in magnetic fields up to 16 T. The overall performance of the probe was tested, while studying the pressure-induced changes in the spin-relaxation mechanisms of a quasi-1D conducting polymer, KC₆₀. The preliminary measurements revealed that the probe yields similar signal-to-noise ratio to that of commercially available low-frequency ESR spectrometers. Moreover, by observing the conduction electron spin resonance (CESR) linewidth broadening for KC₆₀ in an unprecedented microwave frequency range of 210–420 GHz and in the pressure range of up to 1.6 GPa, we demonstrate that a combination of high-pressure ESR probe and high-field/multi-frequency spectrometer allows us to measure the spin relaxation rates in conducting spin systems, like the quasi-1D conductor, KC₆₀.     

ESR spectrometer with a wide frequency range using a gyrotron as a radiation power source

The development of a high frequency electron spin resonance (ESR) spectrometer with a wide frequency range using a gyrotron as the radiation power source is described.GYROTRON FU-E, optimized for use in an ESR spectrometer in the millimeter wave range, was developed in Fukui University. In order to test the normal operation of the spectrometer, the ESR of two standard samples, single crystal and polycrystalline DPPH, has been measured, in the pulsed mode over the frequency range from 65 GHz to 135 GHz.     

Distance determination in human ubiquitin by pulsed double electron-electron resonance and double quantum coherence ESR methods

Recently, distance measurements by pulsed ESR (electron spin resonance) have been obtained using pulsed DEER (double electron-electron resonance) and DQC (double quantum coherence) in SDSL (site directed spin labeling) proteins. These methods can observe long range dipole interactions (15-80A). We applied these methods to human ubiquitin proteins. The distance between the 20th and the 35th cysteine was estimated in doubly spin labeled human ubiquitin. Pulsed DEER requires two microwave sources. However, a phase cycle is not usually required in this method. On the other hand, DQC-ESR at X-band ( approximately 9GHz) can acquire a large echo signal by using pulses of short duration and high power, but this method has an ESEEM (electron spin echo envelope modulation) problem. We used a commercial pulsed ESR spectrometer and compared these two methods.     

Multipurpose High-Frequency ESR Spectrometer for Condensed Matter Research

We describe a multifrequency quasi-optical electron spin resonance (ESR) spectrometer operating in the 75–225 GHz range and optimized at 222.4 GHz for general use in condensed matter physics and chemistry. The quasi-optical bridge detects the change of millimeter wave polarization at the ESR condition. A controllable reference arm maintains a millimeter wave bias at the detector. The sensitivity of 2 × 10¹⁰ spin/(G Hz^0.5), measured on a dilute Mn:MgO sample in a non-resonant probe head at 222.4 GHz and 300 K, is comparable to that of commercial high-sensitive X-band spectrometers. The spectrometer is robust, easy to use and may be operated by undergraduate students. Its performance is demonstrated by examples from various fields of condensed matter physics.     

A longitudinally detected high-field ESR spectrometer for the measurement of spin-lattice relaxation times

We describe a high-field longitudinally detected electron spin resonance (LOD-ESR) spectrometer operating at 35 and 75 GHz. The lack of resonant microwave circuits facilitates operation at different microwave frequencies without changing the probehead. A very low noise radio frequency detection compensates partially the resulting low sensitivity. The major elements of the LOD-ESR spectrometer are commercially available and may be adapted to usual high frequency spectrometers. The instrument allows field and frequency dependent spin lattice relaxation time (T1) studies. T1 in the range of 2-80 ns can be determined from the phase sensitively detected LOD-ESR spectra. We demonstrate the performance of the apparatus by the measurement of T1 in the normal state of RbC60, an electrically conducting alkaline fulleride polymer.     

Variable Coupling Scheme for High-Frequency Electron Spin Resonance Resonators Using Asymmetric Meshes

The sensitivity of a high-frequency electron spin resonance (ESR) spectrometer depends strongly on the structure used to couple the incident millimeter wave to the sample that generates the ESR signal. Subsequent coupling of the ESR signal to the detection arm of the spectrometer is also a crucial consideration for achieving high spectrometer sensitivity. In previous work, we found that a means for continuously varying the coupling was necessary for attaining high sensitivity reliably and reproducibly. We report here on a novel asymmetric mesh structure that achieves continuously variable coupling by rotating the mesh in its own plane about the millimeter-wave transmission-line optical axis. We quantify the performance of this device with nitroxide spin label spectra in both a lossy aqueous solution and a low-loss solid-state system. These two systems have very different coupling requirements and are representative of the range of coupling achievable with this technique. Lossy systems, in particular, are a demanding test of the achievable sensitivity and allow us to assess the suitability of this approach for applying high-frequency ESR, e.g., to the study of biological systems at physiological conditions. The variable coupling technique reported on here allows us to readily achieve a factor of ca. 7 improvement in the signal-to-noise ratio at 170 GHz and a factor of ca. 5 at 95 GHz over what has previously been reported for lossy samples.     

Development of High-Pressure ESR System Using Micro-coil

We have developed the high-pressure electron spin resonance (ESR) system using a micro-coil in the frequency region up to around 2 GHz and potentially 10 GHz. The hybrid-type piston-cylinder pressure cell whose maximum pressure reaches 4 GPa was used. In this study, we obtained ESR spectra at 2.3 GPa successfully, which can never be obtained by the single-layer piston-cylinder pressure cell. The minimum detectable spin number was estimated to be the order of 10¹² spins/G. Moreover, it is shown that the sensitivity can be improved by two orders of magnitude using the field modulation technique. This high-pressure ESR technique is a promising one to achieve the sensitivity and the high pressure simultaneously.     

High frequency tunable continuous wave ESR spectroscopy

We describe the ESR spectrometer we developed. Our aim was twofold: i) to reach the highest possible frequency and ii) to devise a frequency tunable spectrometer. The tunable source is an optically pumped far infrared laser which has been used from 160 GHz up to 525 GHz with magnetic fields of up to 19 T. We present measurements performed in semiconductor physics and in molecular chemical physics. These measurements allowed us to distinguish electric dipolar transitions from magnetic dipolar transitions. The increase ing-factor resolution was used to discriminate between entities withg-factors differing by a few 10^?5. This property together with the study of the line-width frequency dependence was used in geophysics. We studied the spin relaxation mechanisms of the model system Phosphorus doped Silicon. The variation of the spin relaxation time with temperature shows the importance of two-phonon mechanisms. High frequency tunable ESR makes possible the study of compounds with large zero field splitting which are ESR silent at standard frequencies.     

Detection of electron paramagnetic resonance absorption using frequency modulation

A frequency modulation (FM) method was developed to measure electron paramagnetic resonance (EPR) absorption. The first-derivative spectrum of 1,1-diphenyl-2-picrylhydrazyl (DPPH) powder was measured with this FM method. Frequency modulation of up to 1.6 MHz (peak-to-peak) was achieved at a microwave carrier frequency of 1.1 GHz. This corresponds to a magnetic field modulation of 57microT (peak-to-peak) at 40.3 mT. By using a tunable microwave resonator and automatic control systems, we achieved a practical continuous-wave (CW) EPR spectrometer that incorporates the FM method. In the present experiments, the EPR signal intensity was proportional to the magnitude of frequency modulation. The background signal at the modulation frequency (1 kHz) for EPR detection was also proportional to the magnitude of frequency modulation. An automatic matching control (AMC) system reduced the amplitude of noise in microwave detection and improved the baseline stability. Distortion of the spectral lineshape was seen when the spectrometer settings were not appropriate, e.g., with a lack of the open-loop gain in automatic tuning control (ATC). FM is an alternative to field modulation when the side-effect of field modulation is detrimental for EPR detection. The present spectroscopic technique based on the FM scheme is useful for measuring the first derivative with respect to the microwave frequency in investigations of electron-spin-related phenomena.     

脉冲强磁场高频电子自旋共振装置的研制

国内第一套脉冲强磁场高频电子自旋共振(ESR)装置由武汉国家脉冲强磁场科学中心研制成功.该装置的频率范围为210-370 GHz,样品温度范围为2-300 K, 磁场强度为0-50 T.在脉冲强磁场ESR装置上进行了红宝石ESR实验,获得了Cr³⁺ 离子的ESR谱.     

ESR spectrometer with a loop-gap resonator for cw and time resolved studies in a superconducting magnet

The design and performance of an electron spin resonance spectrometer operating at 3 and 9 GHz microwave frequencies combined with a 9-T superconducting magnet are described. The probehead contains a compact two-loop, one gap resonator, and is inside the variable temperature insert of the magnet enabling measurements in the 0-9T magnetic field and 1.5-400 K temperature range. The spectrometer allows studies on systems where resonance occurs at fields far above the g approximately 2 paramagnetic condition such as in antiferromagnets. The low quality factor of the resonator allows time resolved experiments such as, e.g., longitudinally detected ESR. We demonstrate the performance of the spectrometer on the NaNiO2 antiferromagnet, the MgB2 superconductor, and the RbC60 conducting alkaline fulleride polymer.     

Electron spin resonance on a two-dimensional electron gas in a single AlAs quantum well

Direct electron spin resonance (ESR) on a high mobility two-dimensional electron gas in a single AlAs quantum well reveals an electronic g factor of 1.991 at 9.35 GHz and 1.989 at 34 GHz with a minimum linewidth of 7 G. The ESR amplitude and its temperature dependence suggest that the signal originates from the effective magnetic field caused by the spin-orbit interaction and a modulation of the electron wave vector caused by the microwave electric field. This contrasts markedly with conventional ESR that detects through the microwave magnetic field.     

Educational experiments on ESR imaging with a portable ESR spectrometer

A light-weight portable electron spin resonance (ESR) spectrometer with the permanent magnet of Nd-B-Fe (Neomax) and field sweep coils was used as a demonstration tool of ESR microscopy for student’s experiments. Educational experiments on one-and two-dimensional ESR imaging are made by inserting field gradient coils or field scanning wire arrays into the cavity using test samples of DPPH. A preliminary work on double frequency modulation is described for two-dimensional imaging using microwire arrays.     

Microwave electric field induced spin transitions of AlAs 2D electrons

We present the first direct electron spin resonance (ESR) on a 2D electron gas in a III–V semiconductor. ESR on a high mobility 2D electron gas in a single AlAs quantum well reveals an electronic g-factor of 1.991 at 9.35 GHz and 1.989 at 34 GHz with a minimal linewidth of 7 Gauss. Both the signal amplitude and its dependence on the position and orientation of the sample in the cavity unambiguously demonstrate that the spin transitions in our experiment are caused by the microwave electric field. We present a model that ascribes the spin transitions to the effective magnetic field acting on the electron spins that arises from (Bychkov–Rashba) spin-orbit interaction and the modulation of the electron wavevector around k_F induced by the microwave electric field.     

Comparison of 2-D ESR imaging techniques in two frequency ranges

Two-dimensional pulsed electron spin resonance (ESR) imaging was developed in two frequency ranges (radio frequency at 370 MHz and X-band at 9.5 GHz) using the Fourier imaging (FI) and filtered back projection (FBP) technique. A comparison and evaluation of FI and FBP images in both frequency ranges shows the superiority of the FBP method under the given conditions. Benefits and drawbacks of the different frequency ranges are analyzed with respect to the signal-to-noise ratio and the disturbing influences of the skin effect. Examples for the mapping of dynamic properties (e.g., relaxation times, diffusion coefficient) are given. Functional imaging is illustrated using variable temperature for discriminating radiation-induced differences in physical sample properties.     

Electron spin resonance study of the Ba-doping manganite Nd0.5Sr0.5MnO3

We have performed magnetization measurements and electron spin resonance （ESR） on polycrystalline manganites of Nd0.5Sr0.5-xBaxMnO3 （x = 0.1）. Phase separation and phase transitions are observed from the susceptibility and the ESR spectra data. Between 260 K （～ Tc） and 185 K （～ TN）, the system coexists of the paramagnetic phase and the ferromagnetic （FM） phase. Between 185 K and 140 K, the system coexists of the FM phase and the antiferromagnetic （AFM） phase. These results indicate that the system has a very complex magnetic state due to the origin of the instability stemming from manganite Nd0.5Sr0.4Ba0.1MnO3 by partially substituting the larger Ba^2＋ ions for the smaller Sr^2＋ ions.     

Magnetic properties of the charge ordered Nd0.75Na0.25MnO3

Nd_0.75Na_0.25MnO₃ polycrystalline ceramic is prepared via sol-gel process and its magnetic properties and electron spin resonance (ESR) spectra have been investigated experimentally. As the compound is cooled from room temperature, a charge-ordered state first develops below 170 K. A high magnetic field melts the charge ordered state and stabilizes a ferromagnetic (FM) state below 170 K. A field induced transition, analogous to a spin flip transition, is observed between 40 and 170 K. The critical temperature for spin flip increases with increasing temperature. Below 130 K, the compound tends to be intrinsically inhomogeneous, i.e. FM clusters and paramagnetic domains coexist in this system at least, which is confirmed by ESR measurements. When the external magnetic field is zero, long range FM interaction is not developed in this system; however, a tendency of re-entrant FM transition is observed in this compound.     

Recent Advances in High-Frequency Electron Spin Resonance Detection Using a Microcantilever

Our recent developments in highly sensitive high-frequency electron spin resonance (ESR) using a microcantilever are reviewed. ESR signals of a Co Tutton salt microcrystal (<1 μg) have been detected at low temperature at frequencies up to 315 GHz under a static magnetic field using a microcantilever and a modulation technique. The achieved sensitivity is about 10⁹ spins/G at 4.5 K. Moreover, we have shown that similar ESR detection using a microcantilever is possible up to 130 GHz under a pulsed magnetic field without using a modulation technique. The achieved sensitivity is about 10¹¹ spins/G at 1.7 K. These results suggest that the ESR detection using a microcantilever is promising for applications to high-resolution and high-sensitivity terahertz ESR.     

High-field ESR in the metallic fullerides RbC60 and CsC60

We discuss field-dependent electron spin resonance (ESR) experiments on the orthorhombic phase of RbC₆₀ and CsC₆₀. X-and W-band measurements have been performed to determine electronic scattering rates in the metallic temperature regime from 50 to 350 K. The insulating low-temperature phase (T < 50 K) has been investigated by X-, Q-, and W-band ESR, i.e., at 9, 34 and 95 GHz, in order to clarify whether collective resonance modes of a possibly magnetic ground state can be observed.     

L波段三维ESR成像系统的研制（Ⅱ）——L波段三维ESR成像系统

叙述了一个L波段（1.05 GHz）用于ESR和ESR成像的装置，用这套自制装置实现了3D ESR成像. 该装置由L波段ESR谱仪、三组梯度场线圈及控制单元和PC机数据采集系统组成. 样品腔是一个3-环2-缝再进入式谐振腔，可放入直径为20 mm、 长30 mm的H₂O样品，空谐振腔的频率是1.05 GHz. 微波振荡频率用自动频率控制（AFC）的方法自动锁在有载腔的频率上. 梯度场线圈沿X-，Y-和Z-轴产生线性梯度场，在中心40 mm球形范围内梯度场强度为2 mT/cm. 依照Lauterbur's方法进行3D ESR 图像重建. 用该系统检测了样品中TEMPO氮氧自由基的3D空间分布. 得到了TEMPO的2D、3D ESR图像、用像素灰度表示的自旋密度分布图及3D ESR-CT图像.