8/8/2019 JP10-0326

1/5

Journal of the Korean Physical Society, Vol. 57, No. 4, October 2010, pp. 00

Radio-frequency Shot-noise Measurement in a Magnetic Tunnel Junctionwith a MgO Barrier

Mushtaq Rehman, Jung Hwan Park, Woon Song and Yonuk Chong

Korea Research Institute of Standards and Science, Daejeon 305-340

Yeon-Sub Lee, Byoung-Chul Min and Kyung-Ho Shin

Korea Institute of Science and Technology, Seoul 136-791

Sang-Wan Ryu

Department of Physics, Chonnam National University, Gwangju 500-757

Zheong G. Khim

Department of Physics and Astronomy, Seoul National University, Seoul 151-742

(Received 31 August 2010, in final form 2 September 2010)

We measured thenoise power of a magnetic tunnel junction in the frequency range of 710 1200 MHz. A low-noise cryogenic HEMT amplifier was used to measure the small noise signalat a high frequency with wide bandwidth. The MgO-barrier tunnel junction showed large tunnelmagnetoresistance ratio of 215% at low temperature, which indicates electronic transport throughthe tunnel barrier without any significant spin-flip scattering. In the bias-dependent noise mea-surement, however, the zero-bias shot noise was enhanced compared to the value expected from aperfect tunnel barrier or the value observed from a good Al-AlOx-Al tunnel junction. We assumethat this enhanced noise comes from inelastic tunneling processes through the barrier, which maybe related to the observed zero-bias anomaly in the differential resistance of the tunnel junctions.We present a simple phenomenological model for how the inelastic scattering process can enhancethe zero-bias noise in a tunnel junction.

PACS numbers: 85.75.-d, 73.40.Gk, 73.50.TdKeywords: Magnetic tunnel junction, Spintronics, NoiseDOI: 10.3938/jkps.57.0

I. INTRODUCTION

The tunnel magnetoresistance (TMR) effect [1], inwhich a magnetic tunnel junction (MTJ) shows a largechange in its resistance depending on the applied mag-netic field and, thus, the magnetization state of each elec-trode, has been studied extensively for the last decade.TMR has attracted much interest due to its potentialapplications to magnetic memories, sensors and logic de-vices. A large TMR value is required to enhance the de-vice performance. The MgO-based MTJ, a recent break-through, has demonstrated a very large TMR ratio ex-ceeding 600% at room temperature [2,3]. Beyond theconventional Jullieres model [4] that explains the TMRof a MTJ with an aluminum-oxide barrier, the large ob-served MR in the MgO-based system is thought to come

E-mail: yonuk@kriss.re.kr; Tel: +82-42-868-5782; Fax: +82-42-868-5018

from coherent tunneling due to symmetry [5,6].When a tunnel junction with metal electrodes is biased

at a finite voltage, discreteness of the electron transportresults in shot noise given by the formula [7]

SI(V) =2R

{f(E)[1 f(E eV)]

+f(E eV)[1 f(E)]} dE, (1)

where f(E) is the Fermi function that describes the elec-tron density of states of each metal electrode and R is thetunnel resistance of the junction. Evaluating the integralgives the result

SI(V, T) =2eV

Rcoth

eV

2kBT

. (2)

In the limit of the zero bias voltage, Eq. (2) reduces tothe thermal (Johnson) noise SI = 4kBT/R while at highbias voltages larger than the thermal energy kBT, the

-1-

8/8/2019 JP10-0326

2/5

-2- Journal of the Korean Physical Society, Vol. 57, No. 4, October 2010

Fig. 1. Schematic diagram of the measurement setup.Noise signal from the tunnel junction is amplified by 43 dBat low temperatures and then by 64 dB at room tempera-ture. The measurement frequency is chosen by using a band-pass filter. Finally, the noise power was measured by using asquare-law diode detector.

shot noise dominates, and Eq. (2) reduces to the stan-dard shot noise result SI = 2eI. Because MTJs areoperated at finite bias voltage, MTJ devices are usuallydominated by shot noise in their normal operating condi-tions. In the present paper, we report our measurementof the radio-frequency (RF) shot noise in an MgO-barriermagnetic tunnel junction with a large TMR. Althoughseveral noise studies have been reported in the MTJ,they mostly dealt with low-frequency noise in the rangeof tens of kHz [8,9]. Since the ferromagnetic resonance istypically in the GHz range, high frequency, broadbandnoise measurements should provide new information onthe device response that is more closely related to realoperation conditions.

II. EXPERIMENTS AND DISCUSSION

The noise measurement is performed in a He-3 refriger-ator with a precision low-temperature RF measurementchain. The system is designed to measure the shot noisein tunnel junctions for noise thermometry, in which thethermodynamic temperature is derived from the bias-dependent noise in a tunnel junction with metal elec-trodes. Figure 1 shows a schematic diagram of the mea-

surement setup. The DC bias is given through heavilylow-pass filtered lines, and the RF signal is measured

Fig. 2. Bias-dependent shot-noise measurement in an Al-AlOx-Al tunnel junction at 4.2 K. Open circles are measureddata, and the line is a fit to Eq. (1). The junction resistanceis 230 .

through a 50- matched transmission line. A low-noisecryogenic high electron mobility transistor (HEMT) am-plifier is used to amplify the small noise signal from thetunnel junction, and the signal is amplified again at theroom-temperature amplifier. The total system gain is110 dB. We choose the measurement bandwidth to befrom 710 MHz to 1200 MHz, but the frequency rangecan be extended up to >10 GHz if needed.

Figure 2 shows a typical noise measurement result inan Al-AlOx-Al tunnel junction. The measured noise fitsthe theoretical curve of Eq. (2) almost perfectly, andas a fitting parameter, the (electron) temperatures ofthe electrodes are given. For practical purpose, a 4-parameter fit function is used:

S= G

e(V Vo)

2kBcoth

e(V Vo)

2kBT

+ TN

, (3)

where T is the temperature, G is the system gain, TNis the noise temperature and Vo is a possible voltage off-set of the measurement electronics. The fit temperature

agrees well with the temperature measured by a cali-brated temperature sensor. The deviation is typicallyless than 1% at low temperatures. The details of themeasurement system and the noise thermometry resultwere presented in the previous paper [10].

The magnetic tunnel junction stack is depositedon an oxidized silicon substrate by using a mag-netron sputtering system. The MTJ stack con-sists of Si substrate/SiO2(300)/Ta(5)/Ru(30)/Ta(5)/NiFe(7)/IrMn(8)/CoFe(2.5)/Ru(0.85)/CoFeB(2.5)/MgO(2)/CoFeB(3)/Ti(3)/Ta(5)/Ru(5). The numbers in theparentheses indicate the thickness of the layers in nm.The MTJ stacks are patterned to a cross-junction geom-

etry by ion-beam etching and are then annealed at 270C. The details of the fabrication process are described

8/8/2019 JP10-0326

3/5

Radio-frequency Shot-noise Measurement in a Magnetic Tunnel Junction with a MgO Barrier Mushtaq Rehman et al. -3-

Fig. 3. Resistance of the tunnel junction at 4.2 K as a func-

tion of the applied magnetic field. The arrows indicate thedirections of the field sweep. The inset shows the structureof the stack that consists of the magnetic tunnel junction.

in the Ref. 11. Figure 3 shows the magnetoresistanceof a 20 20 m2 junction at 4.2 K. The TMR ratio isdefined as (Rap-Rp)/Rp, where Rp (Rap) is the tunnelresistance for a parallel (anti-parallel) alignment of themagnetization of the electrodes. At 4.2 K, a large TMRratio value of 215% is obtained.

Figures 4(a) and 4(b) show the differential resistanceof the tunnel junction as a function of the temperature.

As the temperature is lowered, the differential resistanceshows a sharp peak-like anomaly near zero-bias, knownas the zero-bias anomaly (ZBA). Zero-bias anomalieshave been reported in MTJs at low temperatures, typ-ically below 77K. There are several theoretical modelsfor the ZBA [12-15], and they involve inelastic scatteringin the barrier. A simple view of the ZBA is that addi-tional transport channels are involved in the tunnelingprocess due to the finite temperature and bias voltage.The models suggested for the ZBA include magnon- orphonon-assisted tunneling and two-step tunneling. Theobservation of the ZBA in our tunnel junction suggeststhat the electronic transport in our MTJ involves some

amount of inelastic process.Figure 5 shows a typical noise measurement in a MTJ.

The noise measurement is performed from the base tem-perature of 0.3 K up to 100 K, where a magnetic fieldbetween -0.5 kOe and +0.5 kOe is applied to definethe magnetization orientation. This MTJ device is notspecifically designed for RF measurements, but was orig-inally designed for DC measurements. The resistance ofthe junction changes from 120 to 400 , depending onmagnetization; hence, in the RF measurement, we needto consider an impedance mismatch. From our experi-ence with Al-tunnel junctions, this amount of impedancemismatch is not too large to measure the noise signal

from the tunnel junction. As verification, the Al tun-nel junctions in Fig. 2 has a junction resistance of 230

Fig. 4. Differential resistance of the tunnel junction at sev-eral temperatures in (a) the parallel (-0.5 kOe) and (b) theanti-parallel (+0.5 kOe) magnetization configurations. Be-low 4.2 K, a peak-like resistance increase is observed nearzero bias, which we call the zero-bias anomaly (ZBA). In theparallel configuration, the resistance decreases with decreas-ing temperature whereas in the anti-parallel configuration,the resistance increases as the temperature is lowered.

. However, the noise signal from this MTJ device is

attenuated by a factor of 100 (20 dB) compared to thatfrom an Al tunnel junction with a similar tunnel resis-tance. Several factors may suppress the RF noise signalfrom the device, and in this case, we attribute the sup-pression to either the RF loss of the silicon substrate orthe high resistance of the leads from the junction to themeasurement pads. Despite the attenuation, we can stillmeasure the bias-dependent noise signal from the device,and there is no significant difficulty in obtaining quanti-tative understandings of the junction behavior, but thesignal-to-noise ratio is degraded compared to the Al junc-tion measurement.

As shown in Fig. 5, we notice that the noise curve de-

viates from the theoretical curve of Eq. (2), especiallynear zero bias. The noise signal is larger than that pre-

8/8/2019 JP10-0326

4/5

-4- Journal of the Korean Physical Society, Vol. 57, No. 4, October 2010

Fig. 5. Measured noise power of the MTJ as a function of the bias voltage at 0.3 K in (a) the parallel (-0.5 kOe) and (b) theanti-parallel (+0.5 kOe) magnetization configurations. Open circles are the measured data. The solid curve is the best fit toEq. (3) with apparent fitting temperatures of (a) 7.60 K and (b) 13.87 K, which are far higher than the actual temperature of0.3 K. The dash-dot curve is the expected noise curve in an ideal tunnel junction at 0.3 K.

dicted from the theory. As a consequence, if we try to fitthis curve to Eq. (3), it will give an effective temperaturemuch higher than the actual temperature monitored bya calibrated temperature sensor. We assume that thisnoise enhancement is a result of an inelastic scatteringprocess. If electron transport through the barrier in-volves inelastic process, such as energy exchange with anexcitation or with a defect state in the barrier, the num-

ber of available final states in Eq. (1) increases, especiallynear zero bias. The existence of an inelastic process isinferred from the zero-bias anomaly in the differential re-sistance measurement, and we think this interpretationis quantitatively consistent with the noise measurement.

In order to account for the effect of the inelastic pro-cess on the shot-noise curve, we assume the amount ofthe energy exchange during the inelastic scattering to be. This will modify the final available states around theincident energy by

[1 f(E)] = f(E) =1

eE/kBT + 1

1

2

E/kBT+E/kBT

1

ey + 1dy. (4)

Hence, the Eq. (1) can be re-written as

SI(V, ) =1

R

f(E)

(EeV)/kBT+(EeV)/kBT

1

ey + 1dy + f(E eV)

E/kBT+E/kBT

1

ey + 1dy

dE. (5)

By evaluating Eq. (5), we can get the noise of the junc-tion, including the effect on the inelastic tunneling. How-ever, we can also quantitatively understand the noise en-hancement from Eq. (5). At high bias voltage, the noisecurve does not change much because the Fermi functionis close to either 0 or 1 over the range of the integral inEq. (4); hence, the integral reduces to the Fermi func-tion at energy E. On the other hand, near zero bias,the Fermi function changes value from 0 to 1; hence,more empty states become available by averaging avail-able states near the incident energy. As a consequence,

the noise is enhanced near zero bias.For further study, we need to increase the signal

to noise ratio by removing the unwanted RF attenu-ation in the chip. By designing a better impedance-matched junction and using a lower loss substrate andlow-resistance leads, we can reduce the attenuation. Amore detailed model for the effect of the inelastic processon the noise curve is under development, and the resultwill be presented elsewhere.

III. CONCLUSION

In conclusion, we present noise measurement resultsfor a magnetic tunnel junction in the frequency range

8/8/2019 JP10-0326

5/5

Radio-frequency Shot-noise Measurement in a Magnetic Tunnel Junction with a MgO Barrier Mushtaq Rehman et al. -5-

of 710 1200 MHz. The zero-bias noise is enhancedcompared with the thermal noise contribution. We as-cribe this increased noise to inelastic tunneling thoughthe barrier, which is also a possible cause of the zero-

bias anomaly observed in the differential resistance ofthe junctions.

ACKNOWLEDGMENTS

The authors would like to thank H. J. Lee and J. S.Choi for help in the measurements and the data analysis.The work of Yeon-Sub Lee and Byoung-Chul Min is sup-ported by the degree and research center (DRC) programfunded by the Korea Research Council of FundamentalScience and Technology (KRCF).

REFERENCES

[1] J. S. Moodera, L. Kinder, T. M. Wong and R. Meservey,Phys. Rev. Lett. 74, 3273 (1995).

[2] S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki andK. Ando, Nat. Mater. 3. 868 (2004).

[3] S. Ikeda, J. Hayakawa, Y. Ashizawa, Y. M. Lee, K.Miura, H. Hasegawa, M. Tsunoda, F. Matsukura andH. Ohno, Appl. Phys. Lett. 93, 082508 (2008).

[4] M. Julliere, Phys. Lett. A 54, 225 (1975).[5] W. H. Butler, Z. G. Zhang, T. C. Schulthess and J. M.

MacLaren, Phys. Rev. B 63, 054416 (2001).[6] J. Mathon and A. Umerski, Phys. Rev. B. 63, 220403(R)

(2001).[7] L. Spietz, K. W. Lehnert, I. Siddiqi and R. J. Schoelkopf,Science 300, 1929 (2003).

[8] E. R. Nowak, M. B. Weissman and S. S. P. Parkin, Appl.Phys. Lett. 74, 600 (1999).

[9] R. Guerrero, F. G. Aliev, T. Santos and J. S. Moodera,J. Magn. Magn. Mater. 316, e990 (2007).

[10] M. Rehman, J. H. Park, J. S. Choi, S. W. Ryu, Z.G. Khim, W. Song and Y. Chong, Submitted to IEEETrans. Instrum. Meas..

[11] I. J. Shin, B. C. Min, J. P. Hong and K. H. Shin, Appl.Phys. Lett. 95, 222501 (2009); IEEE Trans. Magn. 45,2393 (2009).

[12] S. Zhang, P. M. Levy, A. C. Marley and S. S. P. Parkin,

Phys. Rev. Lett.79

, 3744 (1997).[13] J. S. Moodera, J. Nowak and R. J. M. van de Veerdonk,Phys. Rev. Lett. 80, 2941 (1998).

[14] J. Zhang and R. M. White, J. Appl. Phys. 83, 6512(1998).

[15] L. Sheng, D. Y. Xing and D. N. Sheng, Phys. Rev. B 70,094416 (2004).

[16] G. Iannaccone, F. Crupi, B. Neri and S. Lombardo, IEEETrans. Electron Devices 50, 1363 (2003).