Helicon Resonator based Strong Magnetic Field Sensor

Transcription

1 1.48/v MEASUREMENT SCIENCE REVIEW, Volume 11, No., 11 Helion Resonator based Strong Magneti Field Sensor. aurinavičius Department of Eletrial Engineering Vilnius Gediminas Tehnial University, Saulėtekio av.11, T-4, Vilnius, ithuania, The effets of dimensional resonane of magnetoplasmi waves in semiondutors were investigated. It was demonstrated that these effets ould be used to measure the strong magneti fields. Possible ways to design ontatless magneti field sensors operating at room or at ryogeni temperature, suh as that of liquid nitrogen are disussed. It has been shown that for strong field sensor a semiondutor material of high arrier mobility μ, density N and ompat RF system for registration of dimensional resonane of magnetoplasmi waves in helion resonator is needed. Keywords: Semiondutors, magnetoplasmi waves, dimensional resonator, magneti field, sensor M 1. INTRODUCTION AGNETIC FIED STRENGTH is measured using different tehnologies. Eah magneti field measurement method has unique properties that make it more suitable for partiular appliations. The magneti sensors an be lassified aording to low, medium, or high-field sensing range and operating temperatures. Strong field or bias field sensors detet fields that are larger than the Earth's field [1]. Conventional sensors an detet a physial property (pressure, temperature and other) diretly but magneti sensors detet hanges in magneti fields and from them derive information on physial properties. The output signal of these sensors requires some signal proessing for translation into the desired parameter. Mine high-field sensor tehnologies are magnetoresistane, Hall and GMR (giant magnetoresistane) effets. Some sensors, suh as magnetoresistors, are apable of measuring fields up to several teslas, others, suh as GMR devies, an detet fields smaller than the Earth's field []. Operational temperature range of strong magneti field meters is only at room temperature. Sometimes, in pratie, high pulsed magneti fields with amplitudes up to T an be generated at room or ryogeni temperatures. The existing magneti field measurement methods are appliable in ase of known magneti field diretion and the auray of suh methods is low when the diretion of the magneti field is not determined in advane or it is hanging during the experiment []. Possible ways to design ontatless strong magneti field sensors operating at room or ryogeni temperatures, suh as that of liquid nitrogen will be disussed.. PHYSICA BACKGROUND OF INVESTIGATION If semiondutor speimen is plaed in an external magneti field (Fig.1), exited mirowaves an propagate in semiondutor along the diretion of magneti indution B. The propagation of magnetoplasmi wave may be deteted from the harateristi equation [4]. k ' '' + ± i ± = 1 p [( ± ) + iν ], (1) where k is wave vetor, is light veloity, is exiting frequeny, ± is omplex permittivity, is lattie onstant of semiondutor, = eb m is ylotron / 1/ frequeny, e N = p m is plasmas frequeny, ν = 1/τ is frequeny of arrier s ollisions, is dieletri onstant, e, m and N are harge, effetive mass and density of free harge arriers, respetively. The subsripts ( ± ) refer to right and left irularly polarized magnetoplasmi waves. d = λ H / B Fig.1. Semiondutor speimen is plaed in external magneti field B. The onditions when magnetoplasmi waves an propagate in magneti material are as follows: d y x z 149

2 MEASUREMENT SCIENCE REVIEW, Volume 11, No., 11 ± >> ν ; >> ; τ μb >>1, () where μ is mobility of free harge arriers (eletrons). The helion waves (left polarized, diretion of irular polarization oinides with ylotron rotation of free harge arriers) are only of interest for pratial appliation (for measurement of the strong magneti fields). The equations of real and imaginary parts of omplex permittivity for helion wave in the ase of strong magneti field μ B >> 1 looks as follows []: ' = 1 = p en, () B '' pν en = =. (4) B μ The length of helion waves equation λ H is determined by 1/ 1 / B λ = = H. () ' en It means that it is possible to determine the density of free harge arriers N by observation of resonanes of magnetoplasmi mirowaves in magneti materials [6]. When mine (half length) dimensional resonane is observed, the value of density N is determined by simple equation N A B d = (6) Where R d = λ H /, d is thikness of the speimen and A = onst. Physial bakground of the operation of magneti field sensor is based on the effet of the dimensional resonane of magnetoplasmi waves in known semiondutor plate (free harge arriers N and μ ) plaed in the stati magneti field. f is resonane frequeny ( ) f R. RESUTS The tehnial possibility to realize the magneti field indution B sensor with the help of high frequeny Hall effet in semiondutors was proposed by the author []. A urrent-arrying InSb semiondutor plate is kept in a magneti field and has two eletrodes. The Hall voltage inreases with applied field to several teslas. The temperature dependene of the voltage is governed by the temperature dependene of the arrier mobility μ. Different semiondutor materials and different doping levels result in trade-offs between sensitivity and temperature dependene of sensor., The main objetive of the present paper is to show that it is possible to determine the magneti indution B by observation of dimensional resonanes of magnetoplasmi mirowaves in small size semiondutors. In semiondutors the phase veloity of magnetoplasmi mirowaves is 1 4 times less than the speed of light. Thus the dimensions of resonators (thikness of plane-parallel plates, Fig.1) for mine dimensional resonane are less than 1mm in high frequeny (HF) range (wavelength in the free spae λ < 1 m). A semiondutor sample (helion resonator) should be plaed into DC magneti field B and AC mirowave field b. Then, by satisfying the onditions of helion origination () we may obtain oupling of oils of a resonant harater. A non-ontating high field sensor may be developed by applying two perpendiular miro-strip lines (oils) to a semiondutor layer (plate) of finite dimensionality. Fig. shows the priniple onstrution of a helion resonator based magneti field sensor. Two inner ondutors oming from the inlet and output port ross under an angle of 9 degrees are onneted to the outer ondutor at the other end. They are isolated from eah other where they ross eah other. A semiondutor disk helion resonator is situated between the inner and outer ondutors. A HF urrent in inner ondutor (1) generates a HF magneti field in the semiondutor disk. When the onstant magneti field B =, the osillating magneti field has only one omponent (for example b x ) and there is no signal in the outlet of the sensor. If field B, the perpendiular b y omponent will appear and magnetoplasmi helion waves will oupling lumped elements. The interation between magneti fields of the windings and the average field of the standing wave in semiondutor has resonant harater. Outer ondutor Semiondutor disk (helion resonator) Inner ondutors 1 Outer ondutor Fig.. Priniple onstrution of helion resonator based magneti field sensor. The magneti indution B an be easily alulated from typial amplitude frequeny response in Fig.: B = N f R d. () 1

3 MEASUREMENT SCIENCE REVIEW, Volume 11, No., 11 The rate of oupling between the perpendiular miro-strip lines depends on quality Q of the dimensional resonator and on the filling of miro-strip lines by semiondutor ore. In Fig. it is a measure of magnitude of the output as a funtion of frequeny, in omparison to the input, when filling of miro-strip lines is different (amplitude vs. frequeny dependenes 1-4). Comparison of amplitude frequeny responses shows, that amplitude of peak (quality Q of the dimensional resonator) is lower for partly filling sensor (urve 4). U/U, Relative units, 1 A magneti field sensor (1) is put in a magneti field generated by axial solenoids (). Fast swithing RF signal generator () is onneted through a oaxial able to exiting miro-strip line (oil) (4): inner ondutor of oaxial able is onneted to inner ondutor of sensor and outer ondutor (metalli shield) is onneted to outer ondutor of sensor. The helion wave is exited in loal area of a semiondutor ore. Propagating aross semiondutor helion waves is indiated by reeiving miro-strip line (oil) (), situated perpendiularly to the exiting line. A reeiving signal is registered by reorder (6). The desribed magnetoplasmi magneti field meter has also a high frequeny detetor (), ryogeni system, signal analyser and system for ontrolling radio frequeny output, whih are not shown., 4 1 4, f R 1 1 f, MHz Fig.. A typial amplitude frequeny response. Here U and U are voltages of generated and indiated (propagated through semiondutor plate) high frequeny signals. 6 For the development of strong magneti field sensors a semiondutor material of high arrier mobility μ and high density N of main free harge arriers is needed. The most suitable semiondutor material for the magneti field sensor is narrow-gap Te-doped n-insb, where the τ = 1 ondition may be ahieved in the magneti fields of. Tesla (at room or ryogeni temperature) and density of eletrons more times exeeding density of holes. For sensors operating at room or liquid nitrogen temperatures the semiondutor alloy Cd x Hg 1-x Te with high temperatureindependent density of eletrons (x =.1-.1) is suitable. There are some possibilities to observe the dimensional resonanes of magnetoplasmi mirowaves in the semiondutor layer and to determine the resonant frequeny. Sine the resonant frequeny of the objets under test is mainly in the radio frequeny (RF) range (-1 MHz) and small sensor dimensions are required, we have employed a RF system. The helion waves are exited by transmitting antenna. The sattered fields are then reeived by the reeiving antenna, whih has the apability of reording waveforms with bandwidth up to 1 GHz. To extrat the resonant frequenies from the waves it is neessary to employ various data proessing methods. A simplified blok diagram of high magneti field meter is shown in Fig.4. Fig.4. Blok diagram of high magneti field meter The operation of the offered high magneti field sensor has been demonstrated by measurement of magneti indution (flux density) of magneti systems with rare earth permanent magnet material: dis shape SmCo magnets (diameter mm and thikness mm; max. energy (BH) max. = MGOe; axially magnetized). The sensor is held at fixed distane ( mm) from the fae of the magnet or in the middle of 1 mm gap between two iron-iruit magnets (Fig. ). The spae between the poles of a magnet is filled with wood or any other non-magneti material with a hole in the middle. The sensor an be fixed at any plae of the sensor loation hole and an san the gap and provide flux density measurements at different points. Results of measurements using different magneti field measurement tehniques are shown in Fig.. The measurements were repeated several times. 11

4 fr 9 MEASUREMENT SCIENCE REVIEW, Volume 11, No., 11 R B,T,6,4, SmCo Hall sensor Helion sensor R, m Fig.. A omparison of the measured magneti field strength on the ative surfaes of rare earth neodymium (NdFeB) or (SmCo) dis-type permanent open-iruit and iron-iruit magnets using Hall (magneti field strength meter FH ) and helion sensor. A omparison of the urves produed by the Hall tehnology sensor (magneti field strength meter FH ) and the helion resonator based magneti field sensor shows rather good agreement of measurements in the entre of magnet gap (R < + m). At the orners of disk magnets ( R = +. m ) results of measurement are different. The misalignment relates to the sensor dimensions. Magneti field strength meter FH is equipped with Hall tehnology probe with small ative area of.4 mm (magneti indution range T; probe temperature orretion K). Ative area of helion resonator based sensor is several times higher (. mm) than the magnet edge impat observed earlier. For the measurement of magneti fields lower than T, the most suitable semiondutor material for the sensors is alloy Cd x Hg 1-x Te or Te-doped n-insb of high arrier mobility and high mine arrier density N >. 1 m -, together with the eletron density far exeeding that of the holes. At ryogeni temperatures it is advisable to make helion resonator from the anisotropi alloy Bi 1-x Sb x (x =.1). The mobility of eletrons in this alloy is several times higher than in the mentioned semiondutors (Tab. 1). For the magneti fields exeeding T, pratially all semiondutor materials of high arrier mobility ( μ >.1 m V -1 s -1 ) are suitable for sensors, beause τ >. Other materials should be studied for their magneti sensitivity and thermal stability. At ryogeni and at room temperatures f R (B) dependene is linear (Fig.6, ). Tab.1. Eletrial and temperature parameters of semiondutor material for strong magneti field sensor Semiondutor material Density N, m - Mobility μ, m V -1 s -1 Temperature T, K n- InSb 1. n- InSb n- InSb Cd x Hg 1-x Te (x=.1) Bi 1-x Sb x (x=.1) f R, MHz d=. mm N=1 1 m - T= K B, T Fig.6. Resonane frequeny f R dependene from magneti indution B at liquid nitrogen (T= K) temperature: alulation (dashed line); experiment (points). 1

5 fr 9 MEASUREMENT SCIENCE REVIEW, Volume 11, No., 11 f R, MHz 9 d=. mm N= 1 m - T= K B, T Fig.. Resonane frequeny f R dependene from magneti indution B at room (T= K) temperature: alulation (dashed line); experiment (points). 4. CONCUSION The measurements of magneti indution with the help of helion waves in semiondutors an be provided in ontatless mode. Pratially all semiondutor materials of high arrier mobility an be put into pratie for measurement high magneti field (~ tesla). Magneti field sensing with the help of semiondutor helion resonator an be used at ryogeni temperatures as well. It was demonstrated that frequeny dependene between the measurement of magneti field indution and the main resonator is linear in a wide temperature range. Priniple onstrution of helion resonator based small size magneti field sensor, using RF system was demonstrated. Comparison of results obtained from measurement permanent magnets using different tehniques was done. REFERENCES [1] enz, J.E. (199). A review of magneti sensors. Proeedings of the IEEE, 8 (6), [] Mihael, J.C., Bratland, T., Smith, C.H., Shneider, R. (1998). A new perspetive on magneti field sensing. Sensors Magazine, 1 (1), [] Bartkevičius, S. et al. (6). Data proessing of manganite sensors array for measurements of nonhomogeneous pulsed magneti field. Eletronis and Eletrial Engineering, 4 (68), 69-. [4] Palik, E.D., Furdina, J.K. (19). Infrared and mirowave magnetoplasma effets in semiondutors. Report on Progress in Physis,, [] Jankauskas, Z., aurinaviius,. (). Magneti and eletri exitation of magnetoplasmi waves. Eletronis and Eletrial Engineering, (), -4. [6] Jankauskas, Z., Kvedaras, V., aurinaviius,. (). The measurements of arrier density and mobility in magnetised materials by the help of helion maser. Measurement Siene Review, (), [] aurinavičius,. et al. (199). Strong Magneti Field Sensor. U.S.S.R. Patent 1,84,66 A1. Reeived June 1, 11. Aepted Otober 1, 11. 1