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Cryogenic single-chip electron spin resonance detector Gabriele Gualco, Jens Anders, Andrzej Sienkiewicz, Stefano Alberti, László Forró, Giovanni Boero Ecole Polytechnique Federale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland article info Article history: Received 6 June 2014 Revised 12 August 2014 Available online 8 September 2014 Keywords: ESR Cryogenic CMOS abstract We report on the design and characterization of a single-chip electron spin resonance detector, operating at a frequency of about 20 GHz and in a temperature range extending at least from 300 K down to 4 K. The detector consists of an LC oscillator formed by a 200 lm diameter single turn aluminum planar coil, a metal–oxide–metal capacitor, and two metal–oxide–semiconductor field effect transistors used as nega- tive resistance network. At 300 K, the oscillator has a frequency noise of 20 Hz/Hz 1/2 at 100 kHz offset from the 20 GHz carrier. At 4 K, the frequency noise is about 1 Hz/Hz 1/2 at 10 kHz offset. The spin sensi- tivity measured with a sample of DPPH is 10 8 spins/Hz 1/2 at 300 K and down to 10 6 spins/Hz 1/2 at 4 K. Ó 2014 Elsevier Inc. All rights reserved. 1. Introduction Methods based on the electron spin resonance (ESR) phenome- non are used to investigate samples in a wide temperature range, ranging from above 1000 K [1–4] to below 1 K [5,6]. Low tempera- ture measurements are usually performed in large microwave cavities as well as with miniaturized conductive [7,8] or supercon- ducting [9–11] resonators. Miniaturized resonators are typically used to maximize the signal-to-noise ratio in experiments with mass-limited samples [7,12–17]. In Refs. [13,18,19] we presented single-chip integrated inductive ESR detectors, fabricated using complementary metal oxide semiconductor (CMOS) technologies, operating between 8 GHz and 28 GHz. The ESR phenomenon was detected as a variation of the frequency of an integrated LC-oscilla- tor due to an effective variation of its coil impedance caused by the resonant complex susceptibility of the sample. Operation in the temperature range from 300 K to 77 K was demonstrated in Refs. [18,19]. Here, we report on the implementation of a 20 GHz sin- gle-chip ESR detector, based on the same operating principle, but capable of operating from 300 K down to at least 4 K. Depending on the specific sample (and, in particular on the dependence of its relaxation times on temperature), the possibility to operate down to 4 K might represent a significant advantage in terms of spin sensitivity (larger polarization, lower thermal noise) as well as in terms of information richness. The fabricated device repre- sents also the first demonstration of a CMOS microwave oscillator operating down to 4 K. 2. Operating principle The principle of operation of the realized single-chip ESR detec- tor is identical to that reported in our previous work Refs. [13,18,19]. In typical experimental conditions, the oscillation frequency of an LC-oscillator coupled with an ensemble of electron spins is given by [19] x LCv x LC ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ gv 0 p ð1Þ where v 0 ¼ 1 2 ðx LCv x 0 ÞT 2 2 1 þ T 2 2 ðx LCv x 0 Þ 2 þ c 2 e B 2 1 T 1 T 2 x 0 v 0 ð2Þ is the real part of the magnetic susceptibility, x LC ¼ 1= ffiffiffiffiffi LC p is the unperturbed oscillator frequency, x 0 = c e B 0 , v 0 is the static mag- netic susceptibility, g is the filling factor (approximately given by (V s /V c ), where V s is the sample volume, and V c is the coil sensitive volume), c e is the electron gyromagnetic ratio, B 1 is the microwave magnetic field, T 1 and T 2 are the relaxation times. As discussed in Ref. [13], the oscillator frequency variation due to the electron spin resonance phenomenon in the sample is, in first approximation, given by Dx LCv ffið1=2Þx LC gv 0 . Consequently, the oscillator fre- quency variation is proportional to the real part v 0 of the sample complex susceptibility v ¼ v 0 jv 00 . The shape of the oscillator fre- quency variation is thus identical to that of the dispersion signal measured in conventional continuous wave experiments. However, the dependence on the microwave field B 1 is substantially different. In the conventional amplitude detection method the measured sig- nal is linearly proportional to the precessing magnetization (i.e., to the product v 00 B 1 for the absorption and v 0 B 1 for the dispersion). In http://dx.doi.org/10.1016/j.jmr.2014.08.013 1090-7807/Ó 2014 Elsevier Inc. All rights reserved. Corresponding author. Address: Ecole Polytechnique Fédérale de Lausanne (EPFL), Station 17, CH-1015 Lausanne, Switzerland. E-mail address: giovanni.boero@epfl.ch (G. Boero). Journal of Magnetic Resonance 247 (2014) 96–103 Contents lists available at ScienceDirect Journal of Magnetic Resonance journal homepage: www.elsevier.com/locate/jmr
Transcript
Page 1: 2014_JMR_Cryogenic Single-chip Electron Spin Resonance Detector

Journal of Magnetic Resonance 247 (2014) 96–103

Contents lists available at ScienceDirect

Journal of Magnetic Resonance

journal homepage: www.elsevier .com/locate / jmr

Cryogenic single-chip electron spin resonance detector

http://dx.doi.org/10.1016/j.jmr.2014.08.0131090-7807/� 2014 Elsevier Inc. All rights reserved.

⇑ Corresponding author. Address: Ecole Polytechnique Fédérale de Lausanne(EPFL), Station 17, CH-1015 Lausanne, Switzerland.

E-mail address: [email protected] (G. Boero).

Gabriele Gualco, Jens Anders, Andrzej Sienkiewicz, Stefano Alberti, László Forró, Giovanni Boero ⇑Ecole Polytechnique Federale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland

a r t i c l e i n f o a b s t r a c t

Article history:Received 6 June 2014Revised 12 August 2014Available online 8 September 2014

Keywords:ESRCryogenicCMOS

We report on the design and characterization of a single-chip electron spin resonance detector, operatingat a frequency of about 20 GHz and in a temperature range extending at least from 300 K down to 4 K. Thedetector consists of an LC oscillator formed by a 200 lm diameter single turn aluminum planar coil, ametal–oxide–metal capacitor, and two metal–oxide–semiconductor field effect transistors used as nega-tive resistance network. At 300 K, the oscillator has a frequency noise of 20 Hz/Hz1/2 at 100 kHz offsetfrom the 20 GHz carrier. At 4 K, the frequency noise is about 1 Hz/Hz1/2 at 10 kHz offset. The spin sensi-tivity measured with a sample of DPPH is 108 spins/Hz1/2 at 300 K and down to 106 spins/Hz1/2 at 4 K.

� 2014 Elsevier Inc. All rights reserved.

1. Introduction

Methods based on the electron spin resonance (ESR) phenome-non are used to investigate samples in a wide temperature range,ranging from above 1000 K [1–4] to below 1 K [5,6]. Low tempera-ture measurements are usually performed in large microwavecavities as well as with miniaturized conductive [7,8] or supercon-ducting [9–11] resonators. Miniaturized resonators are typicallyused to maximize the signal-to-noise ratio in experiments withmass-limited samples [7,12–17]. In Refs. [13,18,19] we presentedsingle-chip integrated inductive ESR detectors, fabricated usingcomplementary metal oxide semiconductor (CMOS) technologies,operating between 8 GHz and 28 GHz. The ESR phenomenon wasdetected as a variation of the frequency of an integrated LC-oscilla-tor due to an effective variation of its coil impedance caused by theresonant complex susceptibility of the sample. Operation in thetemperature range from 300 K to 77 K was demonstrated in Refs.[18,19]. Here, we report on the implementation of a 20 GHz sin-gle-chip ESR detector, based on the same operating principle, butcapable of operating from 300 K down to at least 4 K. Dependingon the specific sample (and, in particular on the dependence ofits relaxation times on temperature), the possibility to operatedown to 4 K might represent a significant advantage in terms ofspin sensitivity (larger polarization, lower thermal noise) as wellas in terms of information richness. The fabricated device repre-sents also the first demonstration of a CMOS microwave oscillatoroperating down to 4 K.

2. Operating principle

The principle of operation of the realized single-chip ESR detec-tor is identical to that reported in our previous work Refs.[13,18,19]. In typical experimental conditions, the oscillationfrequency of an LC-oscillator coupled with an ensemble of electronspins is given by [19]

xLCv ffixLCffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

1þ gv0p ð1Þ

where

v0 ¼ �12

ðxLCv �x0ÞT22

1þ T22ðxLCv �x0Þ2 þ c2

e B21T1T2

x0v0 ð2Þ

is the real part of the magnetic susceptibility, xLC ¼ 1=ffiffiffiffiffiffiLCp

is theunperturbed oscillator frequency, x0 = ceB0, v0 is the static mag-netic susceptibility, g is the filling factor (approximately given by(Vs/Vc), where Vs is the sample volume, and Vc is the coil sensitivevolume), ce is the electron gyromagnetic ratio, B1 is the microwavemagnetic field, T1 and T2 are the relaxation times. As discussed inRef. [13], the oscillator frequency variation due to the electron spinresonance phenomenon in the sample is, in first approximation,given by DxLCv ffi ð1=2ÞxLCgv0. Consequently, the oscillator fre-quency variation is proportional to the real part v0 of the samplecomplex susceptibility v ¼ v0 � jv00. The shape of the oscillator fre-quency variation is thus identical to that of the dispersion signalmeasured in conventional continuous wave experiments. However,the dependence on the microwave field B1 is substantially different.In the conventional amplitude detection method the measured sig-nal is linearly proportional to the precessing magnetization (i.e., tothe product v00B1 for the absorption and v0B1 for the dispersion). In

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Fig. 1. (a) Microphotograph of the single-chip ESR detector. Idc1 and Idc2: DC powersupply of the two oscillators (0.5–5 mA). VD: DC power supply for the mixer andfrequency-division module (1.5 V). OUTp and OUTn: differential output signal(ffi220 MHz). The circle indicates the area where the samples are placed. (b) Blockdiagram of the single-chip ESR detector. (c) Schematics of the frequency mixer(Vb1 ffi 0.9 V, Vb2 ffi 0.6 V). (d) Block diagram of the frequency divider based on D-latches. (e) Schematics of the D-latch (Vb ffi 0.6 V).

G. Gualco et al. / Journal of Magnetic Resonance 247 (2014) 96–103 97

the frequency detection case, for B1 below saturation the signalamplitude is independent of the value of B1 and decreases as 1=B2

1

in saturation (in the conventional method the signal is proportionalto B1 below saturation and decreases as 1/B1 in saturation).

Neglecting the noise contribution of the active devices in theoscillator feedback and assuming that the frequency noise spectraldensity (in Hz2/Hz) is only due to the thermal noise of the coilresistance and given by Sm ¼ kTRx2

LC=ð2pÞ2V20, the spin sensitivity

(in spins/Hz1/2) is given by [18]

Nmin ffi aT3=2 ffiffiffi

Rp

B20Bu

ffiffiffiffiffiT1

T2

s; ð3Þ

where V0 is the oscillation amplitude, R is the coil series resistance,Bu is the coil unitary field, T is the coil (and sample) temperature,and a ffi 20 m�1 kg5/2 s�4 K�3/2 A�3.

3. Description of the single-chip detector

Fig. 1 shows a photo and the block diagram of the realized chip,which consists of two LC-oscillators operating at about 21 GHz and17 GHz, a mixer, and a frequency division module. The chip is man-ufactured in a commercially available 130 nm CMOS technology(IBM 8RF). The total chip surface, including the bonding pads, isabout 1 mm2. The total power consumption of the chip is about20 mW at 300 K and about 6 mW at 4 K, with the oscillators biascurrents set to the minimum values which enable stable oscilla-tions. As in our previous designs [13,19], with the exception of thatreported in [18], we have integrated two oscillators to perform thefirst down-conversion of the oscillator frequency by a mixer. Theuse of two oscillators and a mixer as first step in the frequencydownconversion requires a frequency divider operating at 4 GHzinstead of 20 GHz, which is significantly less difficult to design.The excitation/detection octagonal coils of the oscillators have anexternal diameter of 200 lm, a metal width of 30 lm, a metalthickness of 7.5 lm (obtained by parallel connection of the threetop metal layers available in the process, two made of aluminumand one of copper), and an inductance of about 300 pH. Thecapacitor is realized using interdigitated aluminum fingers. Thecapacitance value for the two oscillators are 180 fF and 260 fF,respectively. The microwave magnetic field B1 can be varied from0.08 to 0.14 mT by changing the oscillator bias current Idc from 1to 5 mA. At low temperatures, a slightly lower bias current of about0.5 mA is sufficient to guarantee stable oscillations. The lower limitis determined by the minimum transconductance of the cross-cou-pled pair required for stable oscillation whereas the upper limit isgiven by the maximum voltage swing which can be applied acrossthe transistors in the cross-coupled pair without damaging theirthin gate oxides. The value of B1 generated by our single-chipdetector is estimated by measuring the voltage at the oscillatorbias node VIdc. In condition of stable oscillation the oscillationamplitude V0 ffi VIdc [20]. Hence, B1 ffi (1/2)Bu(VIdc/xLCL), whereBu ffi l0/d, d is the coil diameter, and L is the coil inductance. ThisB1 estimation is in agreement with saturation experiments withsamples of known relaxation times.

Fig. 1 shows also the schematics and block diagrams of themixer and frequency divider. The frequency mixer needed fordown-conversion of the oscillation frequency is based on a dou-ble-balanced Gilbert cell topology (see pages 368–370 of Ref.[21]). By mixing the two oscillator output voltages a signal at about4 GHz is obtained. The sum frequency component is filtered out bythe system parasitics (i.e., a low pass filter is not necessary). Thefrequency divider is realized by means of current model logic(CML) D-latches with resistive load (see pages 683–699 of Ref.

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98 G. Gualco et al. / Journal of Magnetic Resonance 247 (2014) 96–103

[21]). The on chip divide ratio is 16, by a cascade of four identicaldivide-by-two frequency dividers. The output buffer, realized asstandard source follower amplifier, is capable of driving an outputload of 5 pF with a voltage swing of 0.2 V at 200 MHz.

The detector reported in this paper is an improved version ofthe one we reported in Ref. [18]. In the previous work, cooling toabout 288 K was necessary to operate the detector, complicatingthe set-up and de facto preventing spectroscopy studies at samplestemperatures above 288 K. The improved version works properlyup to about 370 K. The total power consumption of the detectorreported here is an order of magnitude smaller with respect to thatreported in Ref. [18], allowing easier operation at low tempera-tures (the device reported in Ref. [18] can actually also operatedown to 4 K but it requires a more efficient cooling). In order toassure operation down 4 K, i.e., well below the impurity freeze-out temperature [22], we designed the integrated electronics usingexclusively metal–oxide–semiconductor and degenerately dopedstructures.

Fig. 3. Frequency noise spectral density of the integrated LC-oscillators at 300 Kand at 4.2 K.

4. Performance of the integrated detector

Fig. 2 shows an illustration of the setup used to characterize theperformance of the single-chip ESR detector. The chip is glued ontoa printed circuit board and electrically connected by wire bonding.On the same printed circuit board a commercial amplifier is usedas buffer to drive the coaxial cable carrying the chip output signalat about 200 MHz. The system operates properly in the range from300 K down to at least 4 K (no measurements below 4 K have beenperformed yet). The printed circuit board is inserted in a dynamiccontinuous flow cryostat where the temperature is controlled bymeans of a temperature controller unit working with a thin filmresistance temperature sensor. The signal at the output of the buf-fer is mixed, at room temperature, with a local oscillator signalhaving a frequency about 10 MHz higher. The signal at the outputof the mixer (with a carrier frequency of about 10 MHz) is fed intoa phase-locked loop (PLL). In most of the experiments, a magneticfield modulation at kHz frequencies is added to the static magneticfield to improve the signal-to-noise ratio as in ordinary ESR

Fig. 2. (a) Block diagram of the experimental set-up: (1) Electromagnet powersupply (Bruker), (2) electromagnet (Bruker, 0–1.5 T), (3) single chip ESR detector,(4) buffer (Texas Instruments THS4304D), (5) magnetic field modulation coils, (6)power amplifier (Rohrer PA508), (7) mixer (Mini-Circuits ZAD-3), (8) signalgenerator (Rohde–Schwarz SMR-20), (9) phase locked loop (PLL) circuitry (seeRef. [18]), (10) RF Switch (Mini-Circuits ZYSW-2-50DR), (11) selection mode switch,(12) lock-in of the EPR spectrometer (Bruker Elexsys-II E500), (13) cryostat (OxfordInstruments CF935), (14) temperature controller (Oxford Instruments ITC503), (15)heater, and (16) temperature sensor (Lake Shore Cernox CX1050).

spectrometers. The signal at the output of the frequency-to-voltageconverter is demodulated by a lock-in amplifier. The lock-ininternal reference signal is amplified and applied to the fieldmodulation coils. Some experiments are performed without fieldmodulation. In order to use the spectrometer lock-in also in thesemeasurements, the signal at the output of the frequency-to-voltageconverter is chopped using a switch controlled by the 100 kHz ref-erence signal of the lock-in amplifier, as shown schematically inFig. 2.

The operation of CMOS devices and circuits at temperaturesdown to 4 K and below has been already investigated in detail[22–33]. However, no phase noise characterizations of CMOSLC-oscillators at cryogenic temperatures have been reported sofar. Fig. 3 shows the square root of the measured frequency noisespectral density (i.e.,

ffiffiffiffiffiSmp

in Hz/Hz1/2) referred to the integratedLC-oscillator output. At 300 K, the noise at 100 kHz offset fromthe carrier is about 20 Hz/Hz1/2. At 4.2 K, a minimum noise of about

Fig. 4. Frequency difference between the two integrated LC-oscillators as a functionof the bias current Idc2 at 300 K and at 4.2 K with Idc1 ffi 1 mA.

Page 4: 2014_JMR_Cryogenic Single-chip Electron Spin Resonance Detector

G. Gualco et al. / Journal of Magnetic Resonance 247 (2014) 96–103 99

1 Hz/Hz1/2 is measured at 10 kHz offset from the carrier. AssumingR300K = 2 X, V0,300K = 1V R4.2K = 0.2 X, V0,4.2K = 1 V, the frequencynoise spectral density due to the coil series resistance, which isgiven by Sm ¼ kTRx2

LC=ð2pÞ2V20, are 2 Hz/Hz1/2 at 300 K and

0.1 Hz/Hz1/2 at 4.2 K, respectively. Consequently, the measured fre-quency noise is about an order of magnitude larger than the coilthermal noise limit at both temperatures, probably due to thenoise contribution from the cross-coupled transistor pair. The mea-sured frequency noise at low temperatures is highly dependent onthe parameters used in the temperature control loop and on the Hepump settings, going from a minimum value of 1 Hz/Hz1/2 to amaximum value of 100 Hz/Hz1/2. This is due to the influence oftemperature instabilities on the oscillator phase noise. Unfortu-nately, we are not yet able to find a strategy to reproduciblyachieve the minimum frequency noise of 1 Hz/Hz1/2 in all mea-surements. However, this minimum frequency noise is reproduc-ibly obtained when the chip is immersed in liquid helium.

Fig. 5. ESR spectra. The experimental ESR signal shown here (in kHz) is theamplitude of the component at the field modulation frequency of the LC-oscillatorfrequency. Experimental conditions notations: T is the sample temperature, B1 isthe amplitude of the microwave magnetic field, Bm is the amplitude of themodulation magnetic field, mm is the frequency of the magnetic field modulation, ts

is the time interval of the magnetic field sweep, Df is the equivalent noisebandwidth of the lock-in, xLC is the oscillator frequency far from the resonantmagnetic field. ESR spectra of a spherical crystal of ruby sample (Cr3+:Al2O3 samplewith 1% Cr3+ content) having a diameter of 122 lm placed on the 17 GHz oscillatorcoil and a DPPH sample having size of about (4 lm)3 placed on the 21 GHz oscillatorcoil at different temperatures. Experimental conditions: mm = 100 kHz, ts = 335 s,Df ffi 3 Hz, (a) B1 ffi 0.09 mT, Bm ffi 0.25 mT. (b) B1 ffi 0.11 mT, Bm ffi 0.25 mT. (c)B1 ffi 0.11 mT, Bm ffi 0.06 mT.

The oscillation frequency of the two integrated oscillators isdependent on temperature, increasing by about 2 GHz going from300 K to 4 K (i.e., the oscillation frequencies are about 23 GHz and19 GHz at 4 K). Additionally, the oscillation frequency depends onthe oscillator bias current but it is independent on the applied sta-tic magnetic field (up to at least 1.5 T). Fig. 4 shows the frequencydifference between the two integrated oscillators as a function ofthe oscillator bias current Idc2 at fixed bias current Idc1. Since thebias current of the oscillator at the lower frequency is fixed, thedecrease of the frequency difference Dfosc corresponds to adecrease of the frequency of the oscillator operating at the higherfrequency. This behavior is due to the larger effective gate-sourcecapacitance at higher bias currents [34,35]. For a bias current suf-ficiently far from the oscillator start-up the curves at 300 and 4.2 Kshow similar slopes (about 100 MHz/mA) but an otherwise differ-ent behavior. At 4.2 K, the oscillator frequency show sharp transi-tions as a function of the oscillator bias current, with relativelyflat regions between the sharp transitions. The amplitude of thesetransitions are in the order a few tens of kHz to a few tens of MHz.At the moment, we have no convincing explanation for this behav-ior. Since the ESR experiments are performed at fixed bias current,the sharp transitions observed at 4 K have no effect on the mea-sured ESR spectra, except if we operate at a bias current very closeto one of the sharp transitions.

To investigate the behavior of the integrated LC-oscillator aselectron spin resonance detectors we performed experiments withseveral different samples over the entire temperature range from300 K to 4 K. In order to demonstrate the versatility of the realizedsingle-chip ESR detector, we performed measurements with sam-ples having significantly different characteristics: an exchange

Fig. 6. ESR spectra of a DPPH sample having size of about (4 lm)3 placed onthe 21 GHz oscillator coil at different temperatures. Experimental conditions:Bm ffi 0.06 mT, mm = 100 kHz, ts = 42 s, Df ffi 3 Hz, (a) B1 ffi 0.09 mT, (b–d) B1 ffi 0.11 mT.See notations in Fig. 5.

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100 G. Gualco et al. / Journal of Magnetic Resonance 247 (2014) 96–103

narrowed standard system (DPPH), two hyperfine splitted systemshaving 100% and 1% concentrations (Cu2+ in TPP and in Ni(mnt)2), azero field splitted system (Cr3+ in Al2O3), and a very broad line Fe3+

system.Fig. 5 shows measurements performed with two different sam-

ples placed on the coils of the two integrated oscillators. A 122 lmdiameter spherical crystal of Cr3+:Al2O3 (Ruby G10, SaphirwerkIndustrieprodukt AG, Switzerland) having a concentration of 1%of Cr3+ is placed in the center of the 17 GHz oscillator coil. A singlecrystal of DPPH (1,1-diphenyl-2-picryl-hydrazyl) having a volumeof about (4 lm)3 is placed at the center of the 21 GHz oscillatorcoil. The DPPH sample is obtained by slow evaporation of a solu-tion of DPPH powder (Aldrich D9132) in diethyl ether at room tem-perature in air (0.13% in volume of DPPH), as previously reported inRef. [36]. As extensively investigated in Refs. [37,38], the rubyspectrum consists of six lines, with intensity and position stronglydependent on the orientation of the crystal with respect to theapplied static magnetic field. In the inset of Fig. 5a, a backgroundsignal at B0 ffi 0.63 T, which corresponds to g ffi 2 for the 17 GHzoscillator, is clearly visible (its amplitude is 5 kHz and its linewidthis 2.3 mT). The origin of the background signal, which is no morevisible at low temperatures, is unclear. Fig. 5 shows that the reso-nances are shifted towards higher magnetic fields at lower temper-atures. This is due to the shift towards higher frequencies of theoscillators at lower temperatures discussed above. Due to the tem-perature dependence of the oscillators frequencies mentionedabove, in Fig. 6 (as well as in Figs. 7 and 8) the obtained spectraare plotted as a function of magnetic field offset with respect tothe g ffi 2 condition. Fig. 6 reports spectra of the DPPH sampleobtained with narrow field sweeps about the resonance field at

Fig. 7. ESR spectra of single crystal of CuTPP having a volume of about (80 lm)3 atdifferent temperatures placed on the 17 GHz oscillator coil. Experimental condi-tions: Bm ffi 0.25 mT, mm = 100 kHz, ts = 84 s, Df ffi 6 Hz, (a) B1 ffi 0.09 mT, (b–e)B1 ffi 0.11 mT. See notations in Fig. 5.

different temperatures. The spectra in Fig. 5 are taken with a largerfield modulation amplitude, optimized for the ruby sample(0.25 mT instead of 0.06 mT), and a faster sweep rate (3 mT/sinstead of 0.07 mT/s) with respect to those in Fig. 6. Combined withthe fact that the two spectra are taken with the same lock-in timeconstant (about 80 ms), the DPPH spectra in Fig. 5 are distortedwhereas those in Fig. 6 are not.

The narrow sweeps in Figs. 6 are reported to show that theobtained signal shape corresponds, although only approximately,to the derivative of a dispersion signal as expected from field mod-ulation and Eq. (1). The opposite sign of the ruby and DPPH signalsis due to the fact that the two samples are placed on two differentoscillators and that the measured quantity is the differencebetween the two oscillators frequencies. An increase of the fre-quency of the oscillator operating at higher frequency correspondsto an increase of the frequency difference, whereas an increase ofthe frequency of the oscillator operating at the lower frequencycorresponds to a decrease of the frequency difference. Since DPPHhas a spin concentration of about 2 � 1027 spins/m3, the (4 lm)3

sample contains about 1011 spins. The ESR signal is about 20 kHzat 300 K and about 180 kHz at 30 K, as expected in the Curie-lawapproximation. The measured frequency noise spectral density is30 Hz/Hz1/2 at 300 K and 20 Hz/Hz1/2 at 30 K. Consequently, theexperimental spin sensitivities, as defined by Eq. (11) in Ref. [13],is about Nmin ffi 5 � 108 spins/Hz1/2 at 300 K and Nmin ffi 4 � 107

spins/Hz1/2 at 30 K. At lower temperatures the DPPH signalbecomes smaller and broader. As discussed above, in the tempera-ture range from 4 to 10 K, the frequency noise spectral densityassumes values from 1 Hz/Hz1/2 to 100 Hz/Hz1/2. This correspondsto spin sensitivities in the range from Nmin ffi 106 spins/Hz1/2 to

Fig. 8. ESR spectra of single crystal of Cu(mnt)2 in Ni(mnt)2 with a Cu concentrationof 1% having a volume of about (120 lm)3 at different temperatures placed on the21 GHz oscillator coil. Experimental conditions: Bm ffi 0.25 mT, mm = 100 kHz,ts = 84 s, Df ffi 6 Hz, (a) B1 ffi 0.09 mT, (b–e) B1 ffi 0.11 mT. See notations in Fig. 5.

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Fig. 9. ESR spectra acquired at different temperatures for a microcrystalline powderof synthetic b-hematin having a volume of about (200 lm)3 placed on the 17 GHzoscillator coil. Inset: blowup of the g ffi 2 region of the ESR spectrum. Experimentalconditions: ts = 84 s, Df ffi 13 Hz, (a) B1 ffi 0.09 mT, (c and d) B1 ffi 0.07 mT. Seenotations in Fig. 5.

G. Gualco et al. / Journal of Magnetic Resonance 247 (2014) 96–103 101

Nmin ffi 108 spins/Hz1/2. By means of Eq. (3), we can compute theexpected spin sensitivity of the single-chip ESR detector. Assumingan Al resistivity at 4.2 K of 10�9 X m (typical values are in therange 10�12 X m to 10�9 X m [39]), an Al thickness of 7.5 lm, acoil trace width of 30 lm, and a coil diameter of d = 200 lm, weobtain a coil microwave resistance R ffi 0.2 X and a unitary fieldBu ffi (l0/d) ffi 6 mT/A. Assuming T1 ffi T2, B0 ffi 1 T, and T = 4.2 K, weobtain Nmin ffi 104 spins/Hz1/2. The relatively large B1 produced bythe integrated oscillator saturates partially the DPPH sample,decreasing the frequency variation by approximately an order ofmagnitude with respect to the optimal non saturated conditions.As a consequence of the larger noise and the reduced signal ampli-tude, the experimentally achieved spin sensitivity is two orders ofmagnitude worse than the one achievable under the optimal con-ditions considered above (i.e., for c2B2

1T1T2 < 1 and with a fre-quency noise only due to the coil resistance thermal noise).

Fig. 7 shows spectra of a single crystal of non-diluted Cu(II)-tetraphenylporphine (CuTPP, Aldrich 25182) having a volume ofabout (80 lm)3. Fig. 8 shows spectra of a single crystal ofCu2+-doped tetramethylammonium-bis(maleonitriledithiolato)nickel with a Cu concentration of 1% (1% Cu(mnt)2) in Ni(mnt)2)and a volume of about (120 lm)3. All spectra show the hyperfinesplitting produced by the Cu nuclei (63Cu and 65Cu), having spinI = 3/2. The CuTPP spectra are similar in shape but the signal

amplitude does not increase following the Curie law. The 1%Cu(mnt)2 spectra have amplitudes which also do not follow theCurie law and have shapes which are highly temperature depen-dent. For both samples this behavior is probably due to the temper-ature dependence of their relaxation times.

Fig. 9 shows spectra of a (200 lm)3 microcrystalline powder ofsynthetic b-haematin (Fe(III)-protoporphyrin-IX)2, a synthetic ana-logue of haemozoin, a product of haemoglobin responsible formalaria [40,41]. Due to the very broad resonance lines (about0.2 T) with respect to the maximum magnetic field modulationachievable with our set-up (about 1 mT), a better signal-to-noiseratio is obtained by measuring the spectra without magnetic fieldmodulation. The sample is placed at the center of the 17 GHz oscil-lator. Fig. 9 shows measurements performed at different tempera-tures. No signal is measurable at room temperature. As reported inRef. [42] the ESR spectra of b-haematin consist of a strong signal atgeff ffi 4, and a weaker signal at geff ffi 2. These measurements aretaken by chopping the signal at the frequency-to-voltage converteroutput (see Fig. 2 and discussion above). The inset of Fig. 9d showsthe geff ffi 2 component after four averages. These measurements onsynthetic b-haematin show that our single-chip ESR detector has arelatively weak low frequency noise (about 300 Hz rms with abandwidth of 13 Hz and a sweep lasting 80 s) which allows formeasurements without field modulation, a desirable feature formeasuring resonance lines much broader than the maximumachievable magnetic field modulation amplitude.

5. Conclusion and outlook

In this work we have experimentally demonstrated that CMOSsingle-chip microwave LC-oscillators are a valid alternative to min-iaturized resonators [7,8,17] for high spin sensitivity ESR spectros-copy on mass limited samples in the entire temperature rangefrom 300 K down to at least 4 K. The spin sensitivity measuredwith a sample of DPPH is 108 spins/Hz1/2 at 300 K and down to106 spins/Hz1/2 at 4 K. These values are more than an order of mag-nitude better that recent results obtained with a sample of DPPHusing a miniaturized resonator [8], and similar to those obtainedwith a sample of E0 centers in SiO2 at 300 K and with a sample ofphosphorous doped silicon (28Si:P) at 10 K also measured with aminiaturized resonator [17]. Due to the dependence of the spinsensitivity on the relaxation times, the comparison with theremarkable results reported in [17] obtained with different sam-ples is only indicative.

In the following, we describe the main advantages and disad-vantages of single-chip microwave oscillators with respect to min-iaturized resonators. The small size of each chip and the on-chipdownconversion of the ESR signal into a robust frequency-encodedsignal might allow one to create dense arrays of independentdetectors that can be placed in the same magnet for simultaneousmeasurements of different samples. The integration of all compo-nents responsible for the spin sensitivity within a distance of100 lm from the detection coil reduce the signal losses to aminimum. This might be particularly important at frequenciesexceeding 100 GHz where conventional approaches requiresexpensive technologies to limit the losses and the degradation ofthe signal-to-noise ratio. CMOS LC-oscillators operating up to300 GHz have been already reported [43,44] and operation atTHz frequencies might be possible in the near future [45,46]. Thismeans that the single-chip microwave oscillator approach is suit-able up to the largest magnetic fields currently available. The localconversion of DC power into a near-field non-radiating microwavemagnetic field obtained with the single-chip microwave oscillatormight be interesting also for dynamic nuclear polarization (DNP)experiments.

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The main disadvantage of our single-chip approach is the diffi-culty in producing low B1 fields. This problem is particularly rele-vant for samples easily saturated (i.e., having a large relaxationtimes product T1T2). Currently, with a coil having a diameter ofthe order of 100 lm, we can hardly produce B1 fields significantlysmaller than 0.1 mT. As mentioned before, this is due to the factthat the microwave current in the integrated coil cannot be madearbitrarily small: a minimum value is required to sustain stableoscillations. We are currently studying oscillator topologiescapable to produce lower B1 fields. Alternatively, we are also inves-tigating the applicability of non steady-state techniques to ourdetection approach, such as the rapid scan of the magnetic field[47] or the oscillator frequency [48]. A rapid frequency scan canbe easily implemented by means of a voltage controlled capacitorplaced in parallel to the integrated oscillator coil. As discussed inSection 4, temperature instabilities in the cryostat affect signifi-cantly the oscillator frequency noise. This problem is caused bythe fact that, contrary to conventional approaches, the microwavesource is located in the cryostat. Although we believe that thisproblem can be solved by a careful optimization of the tempera-ture control system or by a less temperature sensitive oscillatordesign, the effectiveness of these solutions is not demonstrated yet.

In the near future we plan to investigate the possibility to oper-ate the CMOS single-chip detectors below 4 K. In particular, we aimto study the behavior of the oscillator at a temperature below thesuperconducting transition temperature of the miniaturized coil.Due to the low critical magnetic field of Al [49], we will considerthe possibility to replace the aluminum integrated coils with apost-processing integration of coils made of materials having a lar-ger critical field, such as thin films of Nb [9,50,51]. Additionally,operation at temperatures below 1 K with a frequency above100 GHz will allow us to investigate the behavior of the oscillatorin the condition kT < �hxLC. This condition combined with thedependence of the MOSFET gate-source capacitance on the oscilla-tion amplitude, which introduces an anharmonicity in the oscilla-tor behavior, might allow us to observe a non-trivial quantumbehavior of the LC-oscillator.

Acknowledgment

Financial support from the Swiss National Science Foundation isgratefully acknowledged.

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