14 GHz longitudinally detected electron spin resonance using microHall sensors

Accepted Manuscript 14 GHz Longitudinally Detected Electron Spin Resonance using MicroHall sensors M. Bouterfas, S. Mouaziz, R. Popovic PII: DOI: Reference:

S1090-7807(17)30173-8 http://dx.doi.org/10.1016/j.jmr.2017.07.002 YJMRE 6128

To appear in:

Journal of Magnetic Resonance

Received Date: Revised Date: Accepted Date:

23 February 2017 8 July 2017 10 July 2017

Please cite this article as: M. Bouterfas, S. Mouaziz, R. Popovic, 14 GHz Longitudinally Detected Electron Spin Resonance using MicroHall sensors, Journal of Magnetic Resonance (2017), doi: http://dx.doi.org/10.1016/j.jmr. 2017.07.002

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14 GHz Longitudinally Detected Electron Spin Resonance using MicroHall sensors M. Bouterfas *, S. Mouaziz, R. Popovic . École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland. In this work we developed a home-made LOngitudinally Detected Electron Spin Resonance (LODESR) spectrometer based on a microsize Hall sensor. A coplanar waveguide (CPW)-resonator is used to induce microwave-excitation on the sample at 14 GHz. We used InSb cross-shaped Hall devices with active areas of

and

. Signal intensities of the longitudinal magnetization component of DPPH and YIG samples of volumes about and

, are measured under amplitude and frequency modulated microwave magnetic field generated

by the CPW-resonator. At room temperature,

sensitivity is achieved for

linewidth, a result

which is still better than most of inductive detected LODESR sensitivities.

Keywords: Electron Spin Resonance, Longitudinally Detected ESR, X-band spectroscopy, Coplanar waveguideresonator, Microsize Hall sensor.

I. INTRODUCTION The development of spectrometers at increasingly higher frequencies and static magnetic field has become a challenging trend in modern Electron Spin Resonance (ESR). This is because operating at higher frequency will result in a significant increase in sensitivity. The most advanced and versatile technique designed to observe continuous-wave electron spin resonance (CW-ESR) is the well known transverse detection method, which consists of modulating the external magnetic field whereas the frequency and amplitude of microwave magnetic field are kept constant. Longitudinally detected ESR (LODESR) spectroscopy is a different CW technique which consists of observing the change in the longitudinal magnetization under modulated microwave irradiation, whereas the external magnetic field is swept to cover the resonance. Since its investigation in 1952 [1], LODESR rapidly became an alternative method to the standard transverse magnetization detection in the conventional CW-ESR. A great advantage with LODESR technique is the reduction of the noise due to the frequency-modulation, since the LODERS signal is carried out using low-frequency detection electronics. In addition, larger microwave power levels can be applied for maximum sensitivity. In live animals, the coupling and tuning of the detection system is not affected by motion of the sample due to the much lower detection frequency and the Q-factor of the resonator used for the generation of the microwave field [2,3] . Therefore, no extra noise is added to the ESR signal.

1

With the longitudinal detection (LOD), the ESR signal is measured via the change in the longitudinal component,

,

of the magnetization [4,5]. The sample to be analyzed is placed in a high frequency resonator within a ramped magnetic field just as in conventional ESR. But, instead of modulating the static magnetic field

, the applied microwave field

is amplitude [6,7] or frequency [8], modulated at a much lower frequency causing the electron longitudinal magnetization

to oscillate at this frequency. Hence the instrument detects the modulation of the longitudinal

magnetization

induced by the chopped microwave field. This is a major advantage over the conventional

continuous-wave ESR which requires excitation and detection at the same microwave frequency. Compound to the famous spin-echo method, LODESR has shown to be useful to study short-lived radicals and measure short relaxation times [9-13]. In recent years, the LOD method has also extensively been applied in imaging [14-16].

In this paper, we report measurements of the longitudinal component of magnetization performed by a home-built spectrometer based on microsize Hall sensors and a coplanar waveguide (CPW) resonator at room temperature. In respect to [17], we attempt to improve sensitivity and spatial resolution using smaller sensor size (down to and higher microwave frequency (up to field

at about

corporation), with

)

). The coplanar waveguide resonator is tuned to generate the excitation

. We used InSb cross-shaped Hall devices (designed and fabricated in collaboration with Asahi and

active area, as sensitive elements. The Hall sensor, with the sample, is

placed closely to the CPW-resonator such that the sample lies in the region of maximum

. Results from microwave

field, amplitude and frequency, modulation spectroscopy on DPPH samples and YIG samples are presented. The proposed detection setup profits from the advantage of the LODESR method, the detection system is tuned to detect a frequency (few

in our case) very different from that of the microwave field, in other words, all the detection

electronics is independent of the resonant frequency and operates at low frequency. II. LODESR WITH MICROHALL SENSORS The measurement of the longitudinal component of magnetization about resonance, plus a perpendicular microwave field

results from a linearly swept external field

. This magnetization is made observable by amplitude or

frequency modulating the microwave field. The resulting changes

in

induce a voltage in a Hall sensor whose

sensitive area is perpendicular to the - direction. The Hall probe works then simply as a micro-magnetometer. The principle of the suggested detection scheme is illustrated in Figure 1(a). The Hall sensor, with the sample of interest (a paramagnetic substance), is placed in a static magnetic field

directed along the Z-axis. The sample grain should lie

in close proximity to the Hall sensor active area. The following assumptions are made in the analysis [18] 1.

The sensitive area of the probe is the approximate square region shown in Figure 1,

2.

The Hall probe has constant sensitivity over its active area,

2

3.

The Hall probe measures- or is sensitive to- only that field component normal to its surface,

4.

The output voltage is a measure of the average field across the sensitive area.

FIG.1. A Hall principle inspired LODESR experiment: the Hall plate is sensitive to the magnetization component parallel to . When is applied, the magnetization vector is no longer parallel to Z-axis.

Out of resonance and at the absence of the microwave field, the magnetization such as

, where

of the sample aligns parallel to

is the sample magnetic susceptibility. If the microwave magnetic field

oscillates at the Larmor frequency, is applied perpendicular to

, the magnetization

flips towards the

If the ESR irradiation is amplitude or frequency modulated, this modulation causes the magnetization the same frequency. The output voltage

, which plane.

to oscillate at

measured between the sensing contacts of the biased Hall device, induced by

the sample magnetization, is given by [19, 20] (1) where,

is the Hall sensor bias current we apply via the bias contacts,

the current related Hall sensitivity and

the

magnetic field perpendicular to the Hall active area due to the sample spins contribution. The magnetization motion affects the magnetic field

acting on the Hall sensor: The Hall device is sensitive to the average magnetic field

perpendicular to the Hall plate, which for a sample shape and placement is mainly due to the magnetization. In our case, one needs to evaluate the average magnetic field,

component of the sample

, induced by the unpaired electrons of a

paramagnetic sample at the surface of a Hall bar. The paramagnetic sample is placed on the top of the Hall cross area. When a rotating magnetic field longitudinal magnetization

with angular frequency

is applied in a direction perpendicular to

, the

along the field direction is described by the steady state solutions of Bloch equations [21,

22]. When resonance condition is met,

, the magnetization is reduced by

3

where

is the variation of the longitudinal component

gyromagnetic factor, and The Hall response

, as shown in Figure 1(b),

is the Larmor resonance frequency

depends on the average magnetic field

is the electron

. in the Hall cross junction, therefore, the changes

in magnetization of the paramagnetic sample can be detected. The average magnetic field characteristic area of the square cross junction. It scales as

is taken over the

, where the geometric Hall-form factor,

, differs from unity, it depends upon the geometry and the shape of the paramagnetic sample. Assuming that the noise is mainly due to the Hall device thermal noise generated by its resistance , the signal-to-noise ratio can be defined as

where

is the equivalent noise bandwidth of the detection electronics, and

the sample (and Hall device) temperature

(in ). At saturation, i.e.,

where, for a spin

, all the longitudinal magnetization is destroyed (all the spins lie on the X-Y plane),

system with

is the static magnetization (in

and

). The smallest amount of detectable spins within a frequency bandwidth of

,

represented by the spin sensitivity, is given by

where

is the number of spins contained in the sample given by

the sensitive volume of the sample (in

,

is the density of spins (in

),

is

). The factor 3 is introduced, quite arbitrarily, to ensure that the signal can be

readily distinguished from the noise. We can then write

where

.

4

Given the magnetic field resolution of the Hall sensor

where

,

can be expressed as

.

In the present work, the spin sensitivity estimation is based on simple assumptions about the effective volume of the sample that contributes to the ESR signal: only the spins contained in a cubic paramagnetic sample with base dimensions equal to the Hall active area (FIG.1.c) contribute to resonant signal. Thus, the effective sample volume scales as the cube of the Hall sensitive area length. The geometrical factor

in equations (3) and (4) is computed using

the paramagnetic Langevin expression, and the formula for the magnetic field amplitude induced at given point in space, and given temperature, by an elementary magnetic dipole [23]. We assume that the paramagnetic sample is composed of randomly oriented elementary free magnetic dipoles which align with an external magnetic field once applied. They contribute each in inducing a large magnetic component over the sample. We used a C-routine to compute the average magnitude of the perpendicular field

induced by both a cubic and a spherical paramagnetic

samples placed on the top of the Hall plate of given dimensions

. We assumed that both samples spatial

dimensions are comparable to that of the Hall cross section (cubic sample: performed calculations for

spherical sample:

values, down to few micrometers, show that an average

of about

). The is induced

over the characteristic area of the cross junction, if considering a cubic shape for the sample, which results in an average value for the geometric form factor, found to be about

, thus a

, of about of about

. Whereas, if considering a spherical sample, the average

is

.

Considering the above findings, the basic calculations for equation (3) show that, for a cubic paramagnetic sample at room temperature and sensitivity of about

microwave frequency, a , a resistance

cross area Hall sensor having a

, biased with a current of

, then a field resolution of about

at room temperature, should allow one to achieve a spin sensitivity of about temperature (Fig.2). Reducing the cross area by a factor

should result in a spin sensitivity of about

, if we maintain the same bias current for the Hall device.

5

at room

By using a micometer size Hall sensor, having at room temperature, a magnetic field resolution of would be possible to detect, at

and

, in a

sample, considering a geometrical form factor field resolution of detected in a

bandwidth, the

, it contained in a

. Using the same scaled Hall device with a magnetic

would improve the spin sensitivity by one order of magnitude, i.e.,

would be

bandwidth.

FIG.2. The ESR sensitivity versus the Hall-active area dimension calculated from equation (3). The sample is assumed to have a volume of sensor sensitive area. The Hall device characteristics:

at

frequency and at room temperature, , and positioned on the top of the Hall .

III. SPECTROMETER SETUP The main parts of our home-built Hall device-based LODESR spectrometer are shown in the block diagram in Figure 3. It consists of two main parts: the excitation module including all the microwave components, and the detection module (the Hall probe) with the radiofrequency components. Our sensitive elements are InSb cross-shaped Hall devices, designed and developed by Asahei Kasei Corporation-Japan [24, 25]. In Figure 4(a) we show a schematic of one of our working devices. Figure 4(b) shows a zoom at the center of the sensing area of the Hall device. A single crystal InSb film (1.0-micron- thick) is grown to shape a cross of width as small as

on a semi-insulating

large mobility of InSb

thick GaAs substrate, using molecular beam epitaxy. Due to the very , InSb/ GaAs combination has shown to produce a large magneto

resistance effect useful for high sensitive magnetic applications with low cost sensors (See Table 1). The fabricated Hall chips have a sheet carrier density, stable over a wide temperature range

, and electron mobility, . Macroscopic

pads are patterned on

, which are to contact

the current and the voltage leads to the probe as shown in figure 4(a). Compared to the Hall device used in ref [17], this

6

latter, also made of

, is more sensitive and has better magnetic field resolution, however it is not helpful for the

present work to perform ESR spectroscopy on the desired sample volumes, due to its sensing area designed to be as small as

, which is quite larger than the target dimensions for sample volumes down to . Moreover, and particularly, the Hall device used in [17] is well calibrated to work in a magnetic

field range up to to

, typically for

, it doesn't cover the desired magnetic field range for the present work, up

.

FIG.3. Detection electronics for continuous-wave LODESR setup. Both microwave field frequency modulation (FM), and amplitude modulation (AM) methods can be performed: 1. microwave–power source (Rhode & Schwarz SMR 20), 2. cristal diode (Sivers PM7520), 3. DC voltage source (Keithley K220) for B0-sweep, 4. Home-built solenoidal coils pair (unitary field 0.026 T/A), 5. DC current supplier (PTD 1706A) for micro-Hall bias current, 6. Hall sensor holding the sample to analyse, 7. Half-wavelength CPW-resonator made of 17 μm-copper cladding on epoxy-ceramic substrate ROGERS/RO4000 (resonant frequency about 13 GHz), 8. Low Noise Amplifier (INA 103EP), 9. Lock-in amplifier (EG&G 7260), 10. Acquisition interface: PCI data acquisition card (National Instruments PCI-6052E), general purpose interface bus (GPIB) card (National Instruments PCI-GPIB), Labview-based software (National Instruments) running on a Pentium III based PC.

Our on-wafer available InSb Hall sensors are diced, bonded on a ceramic substrate, and then mounted on a PCB probe with relevant lumped electronics to filter and amplify the Hall output voltage:

FIG.4. The InSb Hall plate on GaAs substrate: (a) schematic configuration of the cross shaped InSb sensitive region, (b) optical micrograph of our bonded micro-Hall device with sensitive area.

7

Hall output is fed to the entries of low noise amplifier, LNA - INA103EP (which acts as a differential amplifier) used to boost the measured ESR signal before demodulation. The gain is set to

. Before entering the LNA, the Hall voltage

passes a high pass filter to eliminate the linear (continuous) background component (a lumped high-pass filter: ceramic capacitor in parallel with

resistor, which provides a cut-off frequency of

), and to reduce the low

frequency noise. The PCB probe, holding the lumped devices (Hall sensor on the ceramic substrate, LNA+lumped electronic components for both low and high pass filters) is however shielded : the other side, the ground plane, is copper cladded. Table 1:

on

Hall characteristics determined experimentally at room temperature. The input resistance

is measured at

Active area

The external magnetic field

is provided by a home-built pair of solenoidal coils powered by a DC source (Keithley

K220) which allows field sweeping over the range of the resonance absorption. The micro-wave excitation field

is provided by an open-ended half-wavelength CPW-resonator (FIG.5). As the

fields distribution dictates for such planar resonator, the magnetic field is maximum at

. The CPW-resonator is

used only to supply microwaves. Our half-wavelength coplanar waveguide resonator is made of

-thick

copper cladding. It is structered on a commercially available epoxy-ceramic substrate, ROGERS/RO4000 with thick. The substrate material has a relative dielectric constant of the CPW-resonator are derived under the condition that the characteristic impedance

at

[26]. The dimensions of is

at

.

FIG.5. Schematic configuration of LODESR detection principle using a Hall sensor and a CoPlanar Waveguide resonator. Microwave power input from the source generator are fed to the CPW-resonator through a SMA connector using a semirigid 50Ω- coaxial line.

8

The resonator is about

long and

, a Q-factor

wide. The spacing is

. It has a resonant frequency of

and a microwave-conversion efficiency

coupled into the CPW-resonator from an adjacent

of about

-transmission line through a gap of

. The microwaves are . The investigation of

the EM characteristics and field’s distribution on the CPW-resonators was carried out using the full wave 3D−electromagnetic simulator, Ansoft–HFSS based on the FEM method. The outer conductor planes of the coplanar waveguide resonator are grounded and the signal-carrying center conductor is electrically connected to the external microwave feed line, a semirigid 50Ω- coaxial line, through a SMA connector. The microwave power is provided by a Rhode & Schwarz SMR20-source. This signal generator offers internal setting options for both amplitude and frequency modulations of the microwave power. The modulation signal is also used as a reference for the lock-in amplifier (EG&G 7260) for the demodulation of the filtered and then amplified LODESR signal. For best power coupling, the microwave source frequency is manually adjusted to always coincide with the resonant frequency of the CPW-resonator. To do so, we measure the spectra of the reflected waves from the CPW-resonator using a crystal detector (Sivers PM7520) and a power-meter (HP438A), then we tune the microwave source to minimize the reflected signal amplitude, indicating a maximum coupling of microwave power into the CPW-resonator. This checking is mainly useful for FM-LODESR because of the thermal drifts of the resonator frequency. The magnitude of the ESR signal depends on the local magnitude of

as a function of position along the resonator

(Fig.5). The Hall probe acts then as a magnetometer when scanning the

field along the resonator for maximum

absorption by the sample, hence a maximum Hall response. This maximum field position, as expected, is found to be at about mid-length of the resonator. The Hall sensor with the paramagnetic sample are placed such that the sample lies in the region of maximum

, as shown in Figure 6.

FIG.6. Side view of the experimental configuration for LODESR detection using a Hall sensor and a CPW-resonator. The magnetic field lines are almost parallel to the Hall active area where the paramagnetic sample lies.

9

IV. PROBE PERFORMANCE We first measured the LODESR spectra of a DPPH sample with active volume of about schemes using a

Hall active area, and

in both modulation

microwave input power. The obtained results are

shown in figures FIG.7 (a) and FIG. 7 (b). The ESR absorption signal amplitude is about

for both

-AM and -

FM methods. These measurements are within reasonable agreement with the theoretical predicted value ( ) calculated for a spherical DPPH sample (

at

using data from

Table 1. Figure 7 (b) confirms that the frequency modulation method acts in the same way as the CW-magnetic field modulation: the resonance spectra are recorded as the first derivative of the longitudinal magnetization

. These

spectra show to be free from baseline drift problems, and the noise rms value seems to be stable, compared to resonant cavities usually used to provide the microwave magnetic field

. The CPW-resonator shows to offer better coupling of

microwaves to the sample without generating any significant parasitic signal at the modulation frequency. Excitation and detection systems demonstrate complete decoupling. Previous ESR experiments, performed at the same frequency using the same Hall sensor and a rectangular cavity [27], gave one order of magnitude worse results: the Hall sensor, bonded on the ceramic substrate is inserted in the cavity through a hole, parallel to the magnetic field, so that the sample is subject to a maximum of

. The obtained spectra suffered from frequency shift due to thermal drifts of the cavity

resonance frequency (because of high used microwave power values, up to

). Also, the Hall ceramic substrate

lies on the cavity wall, forming a joint system. This physical contact between the cavity and the Hall probe contributed in adding more noise since all the whole system vibrations and instabilities are transmitted to the sensing probe.

FIG.7. A LODESR spectrum acquired with a DPPH sample of volume about using a Hall plate of with: (a) AM-method and (b) FM-method. Acquisition parameters : (a) AM-method: frequency , microwave power , ON/OFF modulation frequency , , number of sweeps = 10, number of step-points= 150, sweep rate , lock-in equivalent noise bandwidth , (b) FM-method: frequency , frequency modulation depth , microwave power , , number of sweeps = 10, number of step-points= 150, sweep rate , lock-in equivalent noise bandwidth .

10

Figure 8 shows the LODESR signal obtained from a DPPH grain using a Hall sensor of

and

microwave input power in the FM detection scheme. The corresponding signal amplitude is about experimental finding is in concordance with the predicted value (

at

. This

), calculated using data

from Table 1, given the assumption that, for this measurement, our DPPH sample is rather a cube ;

. The linewidth of the DPPH spectra is

linewidth of DPPH. The measured noise is

, a value compatible with the natural absorption

, slightly higher than the expected value,

. The

limiting noise source has not been firmly identified, but we assume that the dominant source of noise in our case is mainly determined by the thermal noise associated with the low-frequency detection Hall device and the amplification electronics (LNA). The overall noise value is obtained assuming the input noise of the low-noise amplifier (Burr-Brown INA103, is about

) and the thermal noise of the Hall sensor (about

). The measured signal-to-noise ratio

, which gives an experimental spin sensitivity of

the theoretical calculations given by equations (3) and (4) (about

, a result in good agreement with ).

Fig.8: A LODESR spectrum acquired with a DPPH sample of volume about using a Hall plate of with FM-method. Acquisition parameters: frequency , frequency modulation depth , microwave power , number of sweeps = 1, number of step-points= 200, sweep time= lock-in equivalent noise bandwidth .

We also used the present setup to carry-out AM and FM magnetization measurements on crystalline ferrimagnetic samples. Because of large magnetic field sweep, a pronounced drift is observed in the recorded LOD-FMR spectra. Base line corrected results for YIG samples with a volume of of microwave power input is used.

11

are shown in figures 9 (a) and 9 (b). About

FIG.9. A LODESR spectrum acquired with a YIG sample of about at a frequency with (a) AMmethod and (b) FM-method. Acquisition parameters: (a) AM-method:, microwave power , ON/OFF modulation frequency , , number of sweeps = 10, number of step-points = 200, sweep time , lock-in equivalent noise bandwidth . (b) FM-method: frequency modulation depth , microwave power , , number of sweeps = 1, sweep time = , number of step-points = 200, lock-in equivalent noise bandwidth .

V. CONCLUSIONS AND OUTLOOK In the present work, we used field-swept spectroscopy with microwave amplitude and frequency modulation to perform X-band LODESR spectroscopy via a microsize Hall sensor and a coplanar waveguide resonator. Rather than large rectangular cavities and their disperse lines, the CPW-resonator offers a smaller structure. Its compact geometry is suitable for small volumes since it concentrates the

field precisely at the sample. This approach offers better control

on the electromagnetic environment of the sample. Furthermore, because the resonator volume is much smaller than in conventional waveguide resonators, much less power is required to obtain a large

field in CPW-resonators. The

resonant cavities have much higher Q’s than CPW-resonators, and thus store more microwave energy. However, this energy is spread over a larger volume so that the average microwave magnetic field strength within the sample is small. Typical rectangular cavities at X-band frequencies require about an average

field of approximately

few tens of milli-Watts

of power incident upon a

mode to produce

, whereas, CPW-resonators developed in the present work require only a few a of microwave incident-power to generate the same

amplitude. In addition to

greater sensitivity, the CPW-resonators are much less sensitive to frequency changes and drift less than waveguide cavity resonators [27]. However, quantitative LODESR sensitivity data is often missing in the literature and no complete review about this approach is available, at the exception of its application in ESR imaging and in the determination of relaxation times. Furthermore, no commercially available spectrometer is intended for longitudinal detection, only works from home-built LOD setups are reported. The Hall-based LODESR setup presented in this work has currently a spin sensitivity of

achieved at room temperature for sensitive volumes of about

12

. Regardless, we believe that this result is one of the best results for ESR measurements reported in the literature to date [28-31]. The experimental findings in this paper leads us to the conclusion that the LODESR scheme we developed based on Hall sensors and CPW-resonators, presents a complementary ESR method that combines highsensitivity magnetization measurements with microwave absorption measurements. This method has proven once again to be a good alternative to the transverse method, since it can be used with comparable sensitivity at the same frequency, without the need of additional microwave electronic components. The Hall-based LODESR developed in the present work has the potential for substantial sensitivity gains at room temperature by enhancing the field resolution of the Hall sensor. It will be possible in the future to improve and widen its applicability for measurements of single micrometer-sized crystals and single cell tissues. One very desirable future improvement would be to analyze much smaller samples. We believe that an integrated solution, which would combine a very high sensitive Hall sensor and a resonator with high efficiency conversion factor, has further potential to conduct studies of the magnetization evolution of individual submicron and even nanometer sized samples, and to investigate spin dynamics in nanoscale systems through smaller probe. Also, we expect that, with samples much smaller than present volumes, several orders of magnitude improvement in sensitivity are still possible when operating at higher frequencies and lower temperature, which would fully benefit from the improved noise figure of the electronic components.

ACKNOWLEDGMENTS The authors would want to thank G. Boero from the MicroTechnique Institute (IMT) at EPFL for contribution and interesting discussions. They are grateful to I. Shibasaki (Asahi Kasei Co.) for supplying the microHall sensors and to the staff of the Atelier de Circuits Imprimés (ACI) of EPFL for their valuable help. They acknowledge technical assistance from Rogers Corporation, and from A. Skrivervik and J.-F. Zurcher from the LEMA laboratory at EPFL. They express a warm thought in memory of the late Prof. J. Perruisseau-Carrier for his help and support in the preliminary electrical characterization. This work was supported by Grants (200020-108162 and 200021-100585) from the Swiss National Science Foundation.

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REFERENCES

1.

N. Bloembergen and R. W. Damon, Relaxation effects in ferromagnetic resonance, Physical Review, vol. 85, no. 4 (1952) 699.

2.

I. Nicholson, F. J. L. Robb, and D. J. Lurie, Imaging paramagnetic species using radiofrequency longitudinally detected ESR (LODESR imaging), Physical Review, vol. 104, no. 3 (1994) 284.

3.

H. Yokoyama, T. Sato, T. Ogata, H. O. Nishiguchi, and H. Kamada, In vivo longitudinally detected ESR measurements at microwave regions of 300, 700, and 900 MHz in rats treated with a nitroxide radical , Journal of Magnetic Resonance, vol. 129 (1997) 201.

4.

G. Whitfield and G. Redfield, Paramagnetic resonance detection along the polarizing field direction, Physical Review, vol. 106, no. 5 (1957) 918.

5.

J. Pescia, Paramagnetic resonance detection along the polarizing field direction, Annales de Physique, vol. 10 (1965) 389.

6.

B. Vigouroux, J. C. Gourdon, and J. Pescia, CESR observation using an amplitude-modulated microwave field, Journal of Physics F: Metal Physics, vol. 6, no. 8 (1976).

7.

M. Fedin, I. Gromov, and A. Schweiger, Absorption line CW EPR using an amplitude modulated longitudinal field, Journal of Magnetic Resonance, vol. 171 (2004) 80.

8.

H. Hirata, T. Kuyama, M. Ono, and Y. Shimoyama, Detection of electron paramagnetic resonance absorption using frequency modulation, Journal of Magnetic Resonance, vol. 164 (2003) 233.

9.

A. Schweiger and R. Ernst, Pulsed electron spin resonance with longitudinal detection: a novel recording technique, Journal of Magnetic Resonance, vol. 77 (1985) 512.

10. J. Granwehr, J. Forrer, and A. Schweiger, Longitudinally detected EPR: improved instrumentation and new pulse schemes, Journal of Magnetic Resonance, vol. 151 (2001) 78. 11. F. Murányi, F. Simon, F. Fülöp, and A. Jánossy, A longitudinally detected high filed ESR spectrometer for the measurement of spin-lattice relaxation times, Journal of Magnetic Resonance, vol. 167 (2004) 221. 12. R. Lopez and S. K. Misra, Improvement in the measurement of spin-lattice relaxation time in electron paramagnetic resonance, in Computational and Instrumental Methods in EPR (C. J. Bender and L. J. Berliner, Eds.), vol. 25 of Biological Magnetic Resonance, Springer-Verlag Inc., New York, 2006, p. 31. 13. S. K. Misra, Microwave amplitude modualtion technique to measure spin-lattice and spin-spin (T2) relaxation times, in Computational and Instrumental Methods in EPR (C. J. Bender and L. J. Berliner, Eds.), vol. 25 of Biological MagneticResonance, Springer-Verlag Inc., New York, 2006, p. 1.

14

14.

F. Resmer, I. Nicholson, and J. M. S. Hutchison, A quadrature excitation coil for longitudinally detected electron spin resonance, Review of Scientific Instruments, vol. 72 (2001) 3073.

15. I. Nicholson, M. A. Foster, F. J. L. Robb, J. M. S. Hutchinson, and D. J. Lurie, In vivo imaging of nitrixidefree-radical clearance in the rat, using radiofrequency longitudinally detected ESR imaging,

Journal of

Magnetic Resonance, vol. 113 (1996) 256 . 16. I. Nicholson, F. J. L. Robb, S. J. McCallum, A. Koptioug, and D. J. Lurie, Recent developments in combining LODESR imaging with proton NMR imaging, Physics in Medicine and Biology, vol. 43 (1998) 1851. 17. G. Boero, P.-A. Besse, and R. S. Popovic, Hall detection of magnetic resonance , Applied Physics Letters, vol. 79, no. 10 (2001) 1498. 18. H. Koehler, B. Kostyshyn, and T. C. Ku, A note on Hall probe resolution, IBM Journal, (1961) 326. 19. E. Hall, "On a new action of the magnet on electric current, American Journal of Mathematics, vol. 2 (1879) 287. 20. R. Popovic, Hall Effect Devices, Bristol, UK: 2nd edition, Institute of Physics Publishing, 2004. 21. F. Bloch, Nuclear induction, Physical Review, vol. 70, no. 7-8 (1946) 460. 22. F. Bloch, Nuclear induction, Physical Review, vol. 70, no. 7-8 (1946) 474.

23. C. Kittel, Introduction to Solid State Physics, John Wiley & Sons, Inc., 8th edition New York, 2005. 24. A. Okamoto, H. Geka, I. Shibasaki, and K.Yoshida, Transport properties of InSb and InAs thin films on GaAs substrates, Journal of Crystal Growth, vol. 278 (2005) 604. 25. A. Okamoto, A. Ashihara, T. Akaogi, and I. Shibasaki, InSb thin films grown on GaAs substrate and their magneto-resistance effect, Journal of Crystal Growth, vol. 227–228 (2001) 619. 26. Rogers Corporation, RO4000r Series High Frequency Circuit Materials. Rogers Corporation, Advanced Circuit Materials: www.rogerscorporation.com. 27. M. Bouterfas, G. Boero, A. Skrivervik, J.-F. Zurcher, I. Shibasaki, A. Schweiger, and R. S. Popovic, Longitudinally detected ESR (LODESR) using miniaturized Hall sensors, in Advanced Techniques & Applications of EPR, The 38th Annual International Meeting, University of Bath, UK., March 2005. 28. R. Narkowicz, H. Ogata, E. Reijerse , D. Suter, A cryogenic receiver for EPR, Journal of Magnetic Resonance, vol. 237 (2013) 79. 29. K. Kawasaki, T. Sakurai, E. Ohmichi, S. Okubo, H. Ohta, K. Matsubayashi, Y. Uwatoko, Development of High-Pressure ESR System Using Micro-coil, Applied Magnetic Resonance, vol 46 (2015) 987. 30. Y. S.Yap, H.Yamamoto, Y.Tabuchi, M. Negoro, A. Kagawa, M. Kitagawa, Strongly driven electron spins using a K u band stripline electron paramagnetic resonance resonator, Journal of Magnetic Resonance, vol. 232 (2013) 62.

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31. Y. Twig, E. Dikarov, W.D. Hutchison, A. Blank, Note: High sensitivity pulsed electron spin resonance spectroscopy with induction detection, Review of Scientific Instruments, vol 82 (2011) 076105.

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Table 1: on Hall characteristics determined experimentally at room temperature. The input resistance is measured at

Active area

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Graphical abstract

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 A 14 GHz LOD-ESR spectrometer is presented.  A Microsize Hall device is used as sensitive element.  A coplanar waveguide (CPW) resonator induces the microwave-field on the sample.  LOD-ESR signal is measured using amplitude and frequency modulation.  A spin sensitivity of

is achieved at room temperature.

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