IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 56, NO. 4, APRIL 2009 665

Modeling and Characterization of Smart LSMOFerromagnetic Thin-Film Tunable Resistance

Mahmoud Al Ahmad, Member, IEEE, Robert Plana, Member, IEEE, Chae Il Cheon, and Eui-Jung Yun

Abstract—This paper addresses the potential use of the(La0.67, Sr0.33)MnO3 (LSMO) ferromagnetic thin-film materialsas microwave tunable enabling technology. LSMO thin-film mate-rials have a strong interaction between their electrical and mag-netic properties that could be translated into innovative tunablemicrowave components. The knowledge of the electroactive andmagnetoactive properties of these materials is essential for model-ing and design of novel microwave devices. The activity of thesematerials can be described by their electro/magnetoresistanceand magnetocapacitance. The 400-nm-thick LSMO thin film isformed by the chemical solution deposition. Interdigital capacitoris chosen due to its high sensitivity for tunability and the factthat the metallization and patterning by photolithography are keyissues in the fabrication of such structures which are formed bythe deposition of metal on a single side of the LSMO thin film. TheI–V relation, measured by tuning the bias voltage, is nonlinearand symmetric with respect to the polarity of the applied field.The switching to high resistive state is definite and sharp. Underelectrostatic bias field, it is assumed that electric current flowingthrough narrow low-resistance path induces intense local magneticfield, resulting in the decrease of film resistance. This paper takes aclose look in modeling the change of the LSMO resistance in termsof both working frequency and applied bias.

Index Terms—Electroresistance (ER), ferromagnetic, magne-toresistance (MR), material parameters, tunability, tunable cir-cuit, tunable inductor, tunable resistor, (La0.67, Sr0.33)MnO3

(LSMO).

I. INTRODUCTION

INTEGRATED adaptive/tunable microwave components, in-cluding filters and low-noise power amplifiers, are of great

importance for future systems. Tunability is being driven by anumber of very interesting enabling technologies. However, thedesign and fabrication of tunable microwave structures/systemswith sufficient tuning range and stable performance are not triv-ial and have not yet been satisfactorily achieved. High-qualitytuning RF performance, compactness, miniaturization, reliabil-ity, good power efficiency, and excellent temperature stabilitycall for a joint effort in materials development, processing

Manuscript received July 15, 2008; revised December 1, 2008. First pub-lished February 24, 2009; current version published March 25, 2009. Thereview of this paper was arranged by Editor V. R. Rao.

M. Al Ahmad and R. Plana are with the Laboratoire d’Analyse etd’Architectures des Systèmes, Centre National de la Recherche Scientifique,31077 Toulouse Cedex 4, France (e-mail: al-ahmad.mahmoud@ieee.org).

C. I. Cheon is with the Department of Materials Science and Engineering,Hoseo University, Choognam 336-795, Korea.

E.-J. Yun is with the Department of Semiconductor and Display Engineer-ing and Department of System and Control Engineering, Hoseo University,Choognam 336-795, Korea (e-mail: ejyun@hoseo.edu).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TED.2009.2014191

technologies, and device concepts. To address the requirementsof high-quality tuning RF performance, engineers are usingnew techniques and materials as well as getting more fromexisting technologies. The potential of tunable technologiescan be leveraged advantageously into existing or innovateddesigns and later having the ability to enable a new class ofagile adaptive systems that will aid propel future terminals forwireless communications.

Recently, manganite heterojunctions have attracted consid-erable attention due to their potential applications. Manganiteperovskite oxides of the general formula La1−x Ax MnO3 (A =Ca, Sr, Ba, Pb, etc.) exhibit a very large magnetoresistance(MR) effect in a temperature range centered around the fer-romagnetic ordering of manganese spins [1]. These mangan-ites are ferromagnetic above a certain value of x (or Mn4+

content) and become metallic at temperatures below the Curietemperature Tc. This behavior is commonly attributed to dou-ble exchange, but recent studies clearly indicate that it alonecannot explain the many fascinating features of these materials.(La0.67, Sr0.33)MnO3 (LSMO) [2]–[4] is generally consideredas a double-exchange perovskite compound with a highermetal–insulator transition temperature [5]–[12]. On the otherhand, this kind of junction also possesses plentiful physicalmeanings, such as positive colossal MR [13], [14], resistiveswitching between two or multilevel resistance states [15], andcolossal electroresistance (ER) [16]. Sun et al. [17] have in-vestigated the electronic transport behavior of LSMO epitaxialthin films with different thicknesses under various applied dccurrents. Their films with low thickness show a giant negativeER, whereas the films with 100-nm thickness show unusualgiant positive ER, which can reach 30% at room temperature.Debnath and Lin [18] have investigated the current effect inpolycrystalline films of LSMO, which shows large MR at lowtemperatures. They concluded that the enhancement of the ERvalue in LSMO thin films is correlated with the coexistenceof metallic and insulating phases, and they attributed it to thephonon-assisted delocalization mechanism. Furthermore, a dif-ferential resistance (DR) (dV/dI) at the ambient temperaturedue to MR effect has been presented in [19]. FerromagneticLSMO thin films have been used in the context of microwavetunability or to assist tunability in other thin-film materials suchas tunable barium strontium titanate paraelectric films [20].

This paper takes a close look at the potential use of this ap-proach. After summarizing some of the pervious LSMO works,the preparation of our LSMO thin films and the fabricationprocess of their devices are explained in Section II. The dcheterostructure characterizations and analysis are explored inSection III. The modeling of the smart resistance is presented

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666 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 56, NO. 4, APRIL 2009

TABLE IVARIATION OF RESISTIVITY OF LSMO ON THE

ANNEALING TEMPERATURE

in Section IV. Section V shows the frequency measurementsand the modeling of the tunable component. The equivalentcircuit and competing technologies are discussed in Section VI.Section VII summarizes the results of this paper.

II. LSMO THIN-FILM PREPARATIONS AND

STRUCTURE FABRICATIONS

The preparation of the heterostructure is performed as fol-lows: A 425-μm-thick silicon substrate with dielectric constantof 11.8 and resistivity of 10 Ω · cm is first coated with anITO (90 : 10 wt%) layer with thickness of 100 nm at ambienttemperature. The ITO was sputtered with 10% O2 + 90% Ar.The 400-nm-thick LSMO thin film is formed by the chemicalsolution deposition. La acetate hydrate, Sr acetate, and Mn ac-etate tetrahydrate were dissolved and refluxed at 80 ◦C for 3 h inthe mixed solution of acetic acid and distilled water (in volumeratio of 3 : 1). The mixed solution is dropped on ITO/SiO2/Si substrate, spun off at 6000 r/min for 30 s, and dried at 350 ◦Cfor 5 min on a hot plate. The dried films were crystallized by an-nealing at 800 ◦C for 1 h in RTA furnace. Table I shows the dataof electronic transport in LSMO films. An interdigital junctionis designed to study the behavior of LSMO thin-film materialswhen a dc electrostatic field bias is applied. The fabricationof these several structures was performed as follows: Prior tothe electrode deposition, the sample’s surface is cleaned by asoft O2 plasma treatment and baked in oven (200 ◦C, 30 min).Then, the pattern of top electrodes is designed by lithographyinsolation of a photoresist resin through a shadow mask. The400-nm-thick Au electrodes were deposited by evaporation. Toensure the adherence of the noble metal film, a Ti layer (50 nm)is deposited prior to Au deposition. After resin removal, ananneal step was performed in a furnace (250 ◦C/20 min). Thefinal device is shown in Fig. 1.

III. LSMO THIN-FILM DC CHARACTERIZATIONS

The structure of (La0.67, Sr0.33)MnO3 (LSMO) films pre-pared in this paper were analyzed using X-ray diffraction mea-surements as shown in Fig. 2. Fig. 2 shows clearly the presenceof the LSMO(012), LSMO(110), LSMO(104), LSMO(202),LSMO(006), LSMO(024), LSMO(116), and LSMO(214) peaksfor the LSMO films.

As shown in Fig. 3, a well-behaved M−H curve was ob-served for the LSMO thin films prepared at 900 ◦C. Fromthe slope near zero magnetic field in Fig. 3, we can also getsusceptibility k as follows:

k =M

H(1)

=119.17181.8

= 0.66emu

cm3 · Oe. (2)

Fig. 1. Fabricated device: The IDC has ten fingers with width of 5 μm, lengthof 100 μm, and spacing of 5 μm. The CPW access in IDC is 100 μm long.The dc bias is applied between pad1 and pad2.

Fig. 2. X-ray plots of LSMO films to analyze their structures.

Fig. 3. M−H plots of (La0.67, Sr0.33)MnO3 (LSMO) films.

AL AHMAD et al.: MODELING AND CHARACTERIZATION OF LSMO FERROMAGNETIC THIN-FILM RESISTANCE 667

Fig. 4. Room-temperature I–V measurements. Inset: The DR.

Then, initial relative permeability μr = 1 + 4πk is equalto 9.24.

The inherent sheet resistance (Rs) of ITO layer is found tobe 5 × 106 Ω/sq. The dc characterization of the interdigitedcapacitor (IDC) device (I–V measurements) has been per-formed using an Agilent 4142 B. The measurements have beenperformed from −5 to 5 V. The measured electrical currentinduced in the LSMO device as a function of the applied dcvoltage is shown in Fig. 4.

The I–V relation, measured by tuning the bias voltage, isnonlinear and symmetric with respect to the polarity of theapplied field. The dc current induced increase of ferromag-netic metallic phase and current-induced lattice distortion viaelectron wind force under high current density [18]. The DR(dV/dI) is shown in the inset of Fig. 4. It is interesting to notethat the electric current can also change the MR of the films.According to the inset of Fig. 4, the switching to high resistivestate is definite and sharp. This result also indicates that theresistance change varies with bias voltage. It is assumed thatelectric current flowing through a narrow low-resistance pathinduces intense local magnetic field, resulting in the decreaseof film resistance. Zhang and Wang [21] have reported thatthe grain size of the ITO is much larger than that of theLSMO, i.e., the LSMO grains are embedded into the ITO layer.Furthermore, LSMO behaves like a ferromagnetic metal belowthe Tc and changes into a p-type semiconductor above the Tc

[22], while the ITO is an n-type semiconductor with higherconductance. When both are connected, a p-n heterostructurecan form at the interface. A further analysis indicates that theI–V relations can be well described by a simple equation

I = σV + kV n (3)

where σ, k, and n are constants to be determined. By fitting theI–V curve with (3) (see Fig. 4), σ, k, and n are 0.00238 Ω−1,6.04415 × 10−4 Ω−1 · V −1, and 3, respectively. This modelgives us a way to calculate the relative current in the structure.The expected behavior will depend on the physical nature of theelectrodes, the barrier, and the presence or absence of scatteringin the barrier regions [23].

Fig. 5. Normalized DR superimposed with the fitted. tV = 0.4.

Fig. 6. Real part of the impedance of device. Af1, Af2, tf1, and tf2 are of400, 83, 1 225, and 52 744 760, respectively.

IV. TUNABLE RESISTANCE MODELING

For modeling of the resistance versus voltage characteristic,the DR is normalized and fitted with exponential decays in firstorder, and it is shown in Fig. 5.

The resistance varying with the voltage is approximately welldescribed by the following simple equation:

R(V ) = exp(−V/tV ) kΩ. (4)

Now, to model the resistance in terms of working frequency,the measurements in low frequency range have been performedfrom 40 Hz up to 100 MHz using an Agilent 4294 A impedanceanalyzer. The measurements of the resistance of the deviceare shown in Fig. 6. The measurement and the fitting aresuperimposed and are shown in Fig. 6. The measurements werefitted with the exponential decays in second order, i.e.,

R(f) ≈ {Af1 exp(−f/tf1) + Af2 exp(−f/tf2)} kΩ (5)

where f is the frequency and A{f1,f2} and t{f1,f2} are con-stants. A{f1,f2} depends on several parameters, namely, thethickness of the ITO layer underlaying the LSMO materials,

668 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 56, NO. 4, APRIL 2009

Fig. 7. Tunability in resistance at 1 kHz, 100 kHz, and 10 MHz.

TABLE IITUNABILITY VERSUS VOLTAGE

and the thickness and composition of LSMO thin-film materi-als. The measured tunability at different working frequencies isshown in Fig. 7.

The tunability is the ability to change effective material prop-erties with externally applied tuning action. Here, tunability isdefined as Rmax/Rmin, where the material resistance at zerobias and at higher or defined fields are represented by Rmax

and Rmin, respectively. The tunability of the resistance can beapproximately well described by the following simple equation:

T = A + B ∗ V + C ∗ V 2 + D ∗ V 3 + E ∗ V 4 (6)

where V is the applied voltage bias and {A,B,C, and E}are constants which depend also on the thickness of theITO layer underlaying the LSMO materials and the thicknessand composition of LSMO thin-film materials, and their valuesare reported in Table II.

The effect of the ITO layer thickness is explored for twodifferent thicknesses: 50 and 100 nm. The data are presentedin Table III. The measurements show that the induced currentdecreases as the thickness increases. Therefore, the powerconsumption increases as the thickness gets smaller. On thebasis of the fact that the resistance of the ITO film (RITO)is given by RITO = rl/(tw), where r is the resistivity of theITO film and l, t, and w are the length, thickness, and widthof the ITO film, respectively, it is also worth mentioning thatthe induced current and the power consumption in the devicesdecrease as RITO decreases due to the increase of t.

It is evident that the indium tin oxide thickness plays the mostimportant role in such a tunable heterostructure.

TABLE IIICOMPARISON VERSUS ITO THICKNESS

TABLE IVTUNED RESISTANCE PERFORMANCE VERSUS CYCLING RV /R0

Fig. 8. IDC performance measured at low frequency.

In measurements, the dc bias is switched periodically(one cycle) to record possible hysteresis effects in R–V per-formance. The small hysteresis is assumed to be caused bysporadic ferromagnetic phases, i.e., local microdomains ofspontaneous polarization. On the other hand, ferromagneticmaterials exhibit degradation of switched polarization duringthe voltage cycling, known as polarization fatigue. Table IVsummarizes the performance of the circuit for two differentcycling dc biases.

V. FREQUENCY MEASUREMENTS AND ANALYSIS

The low dynamic frequency measurements show that thedevice exhibits a parallel resonance peak of around 33 kΩcentered around 62 MHz as shown in Fig. 8.

The tunability in both reactance and resistance was mea-sured at 50 MHz. The voltage has been applied from −40 up

AL AHMAD et al.: MODELING AND CHARACTERIZATION OF LSMO FERROMAGNETIC THIN-FILM RESISTANCE 669

Fig. 9. Tunability at 50 MHz. (a) Tunability of resistance. (b) Tunability ofreactance.

to 40 V. The measured tunability is shown in Fig. 9. Withincreasing the bias voltage, the resistance changes. The low-impedance measurements also indicate that the heterostructureexhibits a capacitive behavior at zero bias. Under the appliedfield (above 0.65 V), the heterostructure becomes inductive.Similar to our work, Wang et al. have investigated the low-frequency negative capacitance in La0.8Sr0.2MnO3/Nb-dopedSrTiO3 heterojunction [24]. Negative capacitance was observedat low frequencies under positive dc biases. This phenomenonwas found to result from the combinational contributions fromthe Maxwell–Wagner interfacial relaxation and the dipolar re-laxation related to detrapped carriers which give rise to induc-tive effect under an applied electric field.

The IDC device has been measured from 200 MHz up to1 GHz. The measured reflection/transmission coefficients asa function of frequency for zero bias and 5-V dc bias areshown in Fig. 10. It is evident that tunability of the DR for thematerials is also observed at the gigahertz range. As the voltageincreases, the DR of the LSMO film decreases, and therefore,the scattering parameters change. The return loss changes by2 dB, whereas the insertion loss changes by 10 dB.

Fig. 10. Measured scattering parameters versus frequency for different ap-plied bias for the IDC.

Fig. 11. Variable component model.

TABLE VCOMPARISON WITH OTHER TECHNOLOGIES

VI. EQUIVALENT CIRCUIT AND

COMPETING TECHNOLOGIES

The corresponding equivalent circuit has been constructedbased on the physical behavior of the structure and is shownin Fig. 11. The value of the single-element circuit is found byfitting the circuit performance with measured scattering param-eters. R is tuned from 1.3 kΩ to 300 Ω. The heterostructureexhibits a resistor that could be tuned from 1.3 kΩ to 300 Ω withthe application of 5 V. The tunability is sensitive for low biasvalues, i.e., 45 : 1 @ 1 V, whereas smaller sensitivity for higherapplied voltages. The tunability has been limited to 300 Ωwhich represents the total resistance in the device that includesthe resistances of contact, metallization, and the ITO layer. Weemphasize here that the contact resistance from LSMO andITO contact will decrease with increasing the bias voltage,resulting in the increase of tunability of the device.

Finally, we compare the present data with other solid-statetunable microwave enabling technologies, namely, the BSTand the semiconducting technologies as shown in Table V.

670 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 56, NO. 4, APRIL 2009

The presented work illustrates the highest tunability with thelowest applied bias that ever reported. The tunable componentis a resistance that could be varied with low bias voltage. Thistunable resistor is a high-temperature process which can beused in high-temperature technologies relative to CMOS basedsuch as cofired ceramics. The presented tunable LSMO is verywell suited for low-frequency applications.

VII. CONCLUSION

An original work in tunable ferromagnetic is presented.In summary, the ferromagnetic LSMO thin film is depositedby chemical solution deposition on indium tin oxide coatedSiO2/Si substrate. Interdigited capacitor structures are formedon the top of LSMO materials. The structure exhibits a widetunable resistance with low bias. With the applied dc biasvoltage, the DR for the thin film decreases. The potential of thepresented tunable technology can be leveraged advantageouslyinto existing or innovated designs to enable a new class of agilecomponents.

REFERENCES

[1] S. Jin, O. Bryan, T. H. Tiefel, M. McCormack, and W. W. Rhodes, “Largemagnetoresistance in polycrystalline La–Y–Ca–Mn–O,” Appl. Phys. Lett.,vol. 66, no. 3, pp. 382–384, Jan. 1995.

[2] N. K. Todd, N. D. Mathur, S. P. Isaac, J. E. Evetts, and M. G. Blamire,“Current-voltage characteristics and electrical transport properties ofgrain boundaries in La(Sr/Ca)MnO,” J. Appl. Phys., vol. 85, no. 10,pp. 7263–7266, May 1999.

[3] K.-K. Choi and Y. Yamazaki, “Substrate-dependent microstructure andmagnetoresistance of La–Sr–Mn–0 thin films grown by RF sputtering,”Jpn. J. Appl. Phys., vol. 38, no. 1A, pp. 56–60, Jan. 1999.

[4] H. Y. Hwang, S.-W. Cheong, N. P. Ong, and B. Batlogg, “Spin polarizedintergrain tunneling in LSMO,” Phys. Rev. Lett., vol. 77, no. 10, pp. 2041–2044, Sep. 1996.

[5] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitu, G. Kido, andY. Tokura, “Insulator-metal transition and giant magnetoresistance inLa1−x Srx MnO3,” Phys. Rev. B, Condens. Matter, vol. 51, no. 20,pp. 14 103–14 109, May 1995.

[6] A. J. Millis, T. Darling, and A. Migliori, “Quantifying strain dependencein colossal magnetoresistance manganites,” J. Appl. Phys., vol. 83, no. 3,pp. 1588–1591, Feb. 1998.

[7] H. S. Wang and Q. Li, “Strain-induced large low-field magnetoresistancein Pr0.67Sr0.33MnO3 ultrathin films,” Appl. Phys. Lett., vol. 73, no. 16,pp. 2360–2362, Oct. 1998.

[8] O. Pietzsch, A. Kubetzka, M. Bode, and R. Wiesendanger, “Real-spaceobservation of dipolar antiferromagnetism in magnetic nanowires by spin-polarized scanning tunneling spectroscopy,” Phys. Rev. Lett., vol. 84,no. 22, pp. 5212–5215, May 2000.

[9] M. Bode, M. Getzlaff, and R. Wiesendanger, “Spin-polarized vacuumtunneling into the exchange-split surface state of Gd(0001),” Phys. Rev.Lett., vol. 81, no. 19, pp. 4256–4259, Nov. 1998.

[10] S. Heinze, M. Bode, A. Kubetzka, O. Pietzsch, X. Nie, S. Blügel, andR. Wiesendanger, “Real-space imaging of two-dimensional antiferromag-netism on the atomic scale,” Science, vol. 288, no. 5472, pp. 1805–1808,Jun. 2000.

[11] J.-H. Park, E. Vescovo, H.-J. Kim, C. Kwon, R. Ramesh, andT. Venkatesan, “Direct evidence for a half-metallic ferromagnet,” Nature,vol. 392, no. 6678, pp. 794–796, Apr. 1998.

[12] A. Gaur and G. D. Varma, “Electrical and magnetotransport properties ofLa0.7Sr0.3MnO3/TiO2 composites,” Cryst. Res. Technol., vol. 42, no. 2,pp. 164–168, Jan. 2007.

[13] B. Lu, G. Z. Yang, Z. H. Chen, S. Y. Dai, Y. L. Zhou, K. J. Jin,B. L. Cheng, M. He, L. F. Liu, H. Z. Guo, Y. Y. Fei, W. F. Xiang,and L. Yan, “Positive colossal magnetoresistance in a multilayer p-nheterostructure of Sr-doped LaMnO3 and Nb-doped SrTiO3,” Appl. Phys.Lett., vol. 84, no. 24, pp. 5007–5009, Jun. 2004.

[14] K. J. Jin, H. B. Lu, Q. L. Zhou, K. Zhao, B. L. Cheng, Z. H. Chen,Y. L. Zhou, and G. Z. Yang, “Positive colossal magnetoresis-tance from interface effect in p-n junction of La0.9Sr0.1MnO3 and

SrNb0.01Ti0.99O3,” Phys. Rev. B, Condens. Matter, vol. 71, no. 18,pp. 184 428-1–184 428-6, 2005.

[15] A. Sawa, T. Fujii, M. Kawasaki, and Y. Tokura, “Hystereticcurrent-voltage characteristics and resistance switching at a rectify-ing Ti/Pr0.7Ca0.3MnO3 interface,” Appl. Phys. Lett., vol. 85, no. 18,pp. 4073–4075, Nov. 2004.

[16] X. P. Zhang, B. T. Xie, Y. S. Xiao, B. Yang, P. L. Lang, andY. G. Zhao, “Current-induced colossal electroresistance in p-n het-erostructure of La0.67Ca0.33MnO3- and Nb-doped SrTiO3,” Appl. Phys.Lett., vol. 87, no. 7, pp. 072 506-1–072 506-3, Aug. 2005.

[17] Y. H. Sun, Y. G. Zhao, X. L. Zhang, S. N. Gao, P. L. Lang, X. P. Zhang, andM. H. Zhu, “Electric current-induced giant electroresistance in epitaxialLa0.67Sr0.33MnO3 thin films,” J. Magn. Magn. Mater., vol. 311, no. 2,pp. 644–650, Apr. 2007.

[18] A. K. Debnath and J. G. Lin, “Current-induced giant electroresistancein La0.7Sr0.3MnO3 thin films,” Phys. Rev. B, Condens. Matter, vol. 67,no. 6, pp. 064 412-1–064 412-5, Feb. 2003.

[19] Y. W. Xie, J. R. Sun, D. J. Wang, S. Liang, and B. G. Shen, “Reversibleelectroresistance at the Ag/La0.67Sr0.33MnO3 interface,” J. Appl. Phys.,vol. 100, no. 3, pp. 033 704–033 708, Aug. 2006.

[20] E.-J. Yun and C. I. Cheon, “High frequency tunable LC devices withferroelectric/ferromagnetic thin film heterostructure,” Phys. Stat. Sol. (B),vol. 241, no. 7, pp. 1625–1628, Jun. 2004.

[21] N. Zhang and M. Wang, “Giant positive magnetoresistance in compositeLSMO ITO,” Appl. Phys. Lett., vol. 88, no. 12, p. 122 111, Mar. 2006.

[22] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, andY. Tokura, “Insulator-metal transition and giant magnetoresistance inLa1−xSrxMnO3,” Phys. Rev. B, Condens. Matter, vol. 51, no. 20,pp. 14 103–14 109, May 1995.

[23] M. Paranjape, J. Mitra, and A. K. Raychaudhuri, “Nonlinear electricaltransport through artificial grain-boundary junctions in La0.7Ca0.3MnO3

epitaxial thin films,” Phys. Rev. B, Condens. Matter, vol. 68, no. 14,p. 144 409, Oct. 2003.

[24] C. C. Wang, G. Z. Liu, M. He, and H. B. Lu, “Low-frequency negative ca-pacitance in La0.8Sr0.2MnO3/Nb-doped SrTiO3 heterojunction,” Appl.Phys. Lett., vol. 92, no. 5, pp. 052 905-1–052 905-3, Feb. 2008.

Mahmoud Al Ahmad (S’01–M’06) was born inJenin, West Bank, in 1976. He received the B.A.degree in electrical engineering from Birzeit Uni-versity, Ramallah, Palestine, in 1999, and the M.Sc.and Dr.-Ing. degrees in microwave engineeringfrom Technische Universitaet Mnuechen, Munich,Germany, in 2002 and 2006, respectively.

He was with Siemens Corporate Technology,Munich, while working toward the Ph.D. degreewhich focused on the design and fabrication of widetunable passive microwave components for software-

defined radio, combining two ceramic technologies: low-temperature cofiredceramics and piezoelectric actuation technologies. In 2005, he was with theInstitut d’Electronique de Microélectonique et de Nanotechnologie, Lille,France, as a Postdoctoral Fellow. He has been engaged in barium strontiumtitanate tunable capacitor loss compensation using active negative circuittechniques. Since September 2006, he has been a Research Scientist with theLaboratoire d’Analyse et d’Architectures des Systèmes, Centre National dela Recherche Scientifique, Toulouse France. Currently, his work involves thedesign and fabrication of tunable active/passive microwave components forthe next wireless generations. His research interests include the design andthe fabrication of integrated millimeter-wave and microwave circuits basedon barium strontium titanate, piezoelectric material, ferromagnetic material(LSMO) thin films, and carbon nanotubes/nanowires technologies. Moreover,he is engaged in the characterization of bulk and thin material characterizationfor microwave applications employing microwave techniques.

Dr. Al Ahmad is also a member of the IEEE Microwave Theory andTechniques Society.

AL AHMAD et al.: MODELING AND CHARACTERIZATION OF LSMO FERROMAGNETIC THIN-FILM RESISTANCE 671

Robert Plana (M’94) was born in Toulouse, France,in March 1964. He received the Ph.D. degree whichfocused on noise modeling and characterization ofadvanced microwave devices (HEMT, PHEMT, andHBT), including the reliability, from Laboratoired’Analyse et d’Architectures des Systèmes, Cen-tre National de la Recherche Scientifique (LAAS-CNRS) and Paul Sabatier University, Toulouse,France, in 1993.

In 1993, as an Associate Professor with LAAS-CNRS, he started a new research area concerning

the investigation of millimeter-wave capabilities of silicon-based technologies.More precisely, he focused on the microwave and millimeter-wave propertiesof SiGe devices and their capabilities for low-noise circuits. In 1995, hestarted a new project concerning the improvement of the passives on siliconthrough the use of MEMS technologies. In 1999, he was involved with SiGeSemiconductor, Ottawa, where he was working on the low-power and low-noise integrated circuits for RF applications. In 2000, he was a Professor withPaul Sabatier University and Institut Universitaire de France, Paris, France,and he started a research team at LAAS-CNRS in the field of micro- andnanosystems for RF and millimeter-wave communications. He has built a net-work of excellence in Europe in this field “AMICOM,” regrouping 25 researchgroups. He is currently heading a research group at LAAS-CNRS in the fieldof micro- and nanosystems for wireless communications. He has authored andcoauthored more than 200 international journals and conferences. In 2004, hewas appointed as the Deputy Director of the Information and CommunicationDepartment, CNRS Headquarter. From January 2005 to January 2006, he wasappointed as the Director of the Information and Communication Department,CNRS. His main research interests include the technology, design, modeling,test, characterization, and reliability of RF MEMS for low-noise and high-power millimeter-wave applications and the development of the MEMS ICconcept for smart microsystem.

Dr. Plana received a special award from CNRS, in 1999, for his works onsilicon-based technologies for millimeter-wave communications.

Chae Il Cheon received the M.S. and Ph.D. degrees in materials scienceand engineering from Korea Advanced Institute of Science and Technology,Daejeon, Korea, in 1987 and 1991, respectively.

From 1991 to 1992, he was a Senior Researcher with Samsung CorningCompany, Ltd., Seoul, Korea. In 1996, he was a Visiting Scientist with theMicroelectronic Research Center, University of Texas, Austin, and in 2004,he was with the Liquid Crystal Institute, Kent State University, Kent, OH.He is currently a Professor with the Department of Materials Science andEngineering, Hoseo University, Chungnam, Korea. His research area includesferroelectric ceramics/thin films, piezoelectric ceramics/thin/thick films, andliquid crystal composites with ferroelectric nanoparticles.

Eui-Jung Yun received the B.S. degree in electronics engineering from KoreaUniversity, Seoul, Korea, in 1985, and the M.S. and Ph.D. degrees in electricaland computer engineering from the University of Texas, Austin, in 1988 and1994, respectively.

From 1994 to 1996, he was a Postdoctoral Fellow with the University ofTexas, Austin. In 1996, he joined the faculty of the Department of Systemand Control Engineering, Hoseo University, Chungnam, Korea, where he iscurrently a Professor. He was a Visiting Scholar with the ECE Department,University of Texas, Austin, in 2004. His research area includes micromagneticdevices (thin-film inductors and transformers) operating at RF frequency.