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Large voltage modulation in superconducting quantum interference devices with submicron-scale step-edge junctions
Physica C: Superconductivity and its applications 540 (2017) 20–25
Contents lists available at ScienceDirect
Physica C: Superconductivity and its applications journal homepage: www.elsevier.com/locate/physc
Large voltage modulation in superconducting quantum interference devices with submicron-scale step-edge junctions Simon K.H. Lam CSIRO Manufacturing Flagship, PO Box 218, Lindfield, NSW 2070, Australia
a r t i c l e
i n f o
Article history: Received 28 April 2017 Accepted 12 July 2017 Available online 13 July 2017
a b s t r a c t A promising direction to improve the sensitivity of a SQUID is to increase its junction’s normal resistance value, Rn , as the SQUID modulation voltage scales linearly with Rn . As a first step to develop highly sensitive single layer SQUID, submicron scale YBCO grain boundary step edge junctions and SQUIDs with large Rn were fabricated and studied. The step-edge junctions were reduced to submicron scale to increase their Rn values using focus ion beam, FIB and the measurement of transport properties were performed from 4.3 to 77 K. The FIB induced deposition layer proves to be effective to minimize the Ga ion contamination during the FIB milling process. The critical current–normal resistance value of submicron junction at 4.3 K was found to be 1–3 mV, comparable to the value of the same type of junction in micron scale. The submicron junction Rn value is in the range of 35–100 , resulting a large SQUID modulation voltage in a wide temperature range. This performance promotes further investigation of cryogen-free, high field sensitivity SQUID applications at medium low temperature, e.g. at 40–60 K. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Superconducting quantum interference device (SQUID) is the most sensitive magnetic flux detector with a large frequency bandwidth. The figure of merit for SQUID devices is the magnetic flux noise, Sφ , which depends on Sv , the voltage noise spectral density and Vφ , the flux to voltage transfer function, i.e. Sφ = Sv /Vφ 2 . Assuming Sv is dominated by the Johnson noise of the SQUID normal resistance, Rn , Sv 1/2 = 4 (kB TRn )1/2 . On the other hand, Vφ ∼ Rn /L when the SQUID parameter, β L (2LIc /φ o ) and hysteresis parameter, β c (4π Ic Rn C/o ) are both equal to one [1]. Here, L is the SQUID inductance; Ic and C are the junction critical current and capacitance respectively. For a sinusoidal voltage modulation in an applied flux, the peak-to-peak modulation voltage, V is related to Vφ by V = Vφ (o /π ). Theoretically, one would design a SQUID with a low value of L to achieve a large Vφ and small Sφ . After the first low-temperature superconducting, LTS thin film SQUID was demonstrated in 1963, the fabrication processes had been evolved to produce more robust and sensitive devices. Nowadays, a commercial LTS SQUID (L ∼ 50pH) of field sensitivity, Sf in the order of 2–3 fTHz−1/2 , operating at liquid helium temperature of 4.2 K is available [1]. In many non-laboratory applications, the SQUID has to be operated stably in an unshielded environment. Reducing the junction size has been found to improve the SQUID
E-mail address: [email protected] http://dx.doi.org/10.1016/j.physc.2017.07.004 0921-4534/© 2017 Elsevier B.V. All rights reserved.
performance even after the SQUID has been exposed to a medium field [2]. Furthermore, most SQUID applications require a large inductance pick-up loop to detect the magnetic signal and couple into the SQUID. So the SQUID would require a large value of inductance, L to minimize the inductance mismatch with the pickup loop. As the junction size is reduced, the voltage modulation of a SQUID with a large value of L will be enhanced significantly due to the larger value of Rn and a smaller value of C. By using submicron sized low-temperature superconducting junctions, a LTS SQUID with L = 300pH and Sf = 0.3 fTHz−1/2 has been demonstrated [2]. High-temperature superconducting (HTS) SQUIDs based on grain boundary junctions (GBJs) have been studied over the past 25 years [1]. In HTS SQUIDs, the simplest way to implement a magnetometer is to use a large pickup loop on a typical substrate of 10 mm × 10 mm and directly inject the external flux signal into the SQUID (directly coupled). The typical value of L is chosen to be 50–100 pH to comprise between flux coupling and noise performance [1]. Bi-crystal and step-edge are the two most commonly used HTS superconducting GBJ technologies for both research and commercial HTS SQUID system. CSIRO developed a step-edge GBJ technology in 1990s [3,4]. Various devices have been fabricated for different applications [5–8]. In the case of SQUID applications, the junction width is 2–3 μm to obtain Ic and Rn value in the range of 20–100 μA and 3–10 respectively, for L = 65 pH (β L ∼ 1). These SQUIDs have the values of V ∼ 30 μV, Sφ ∼ 6 μo Hz−1/2 and field noise, Sf , ∼ 50 fTHz−1/2 at 77 K [8]. To further improve the SQUID’s
S.K.H. Lam / Physica C: Superconductivity and its applications 540 (2017) 20–25
field sensitivity, i.e. reduce the value of Sf , increasing the value of L would enhance the flux coupling. However, V will decrease and thus increasing the flux noise [9]. For example, SQUIDs with L = 130 pH was found to have a value of V = 12 μV [10]. To overcome the deterioration of V with a large value of L, increasing the value of Rn while keeping β L ∼ 1 is the most obvious way to improve Sφ and ultimately Sf . For a particular HTS junction technology, Rn can be increased by reducing the junction width. In addition, the previous study of SQUID with submicron HTS junction had shown Vφ would not be suppressed by external fields up to 300 μT, indicating the advantage of submicron junction operating stably in earth’s field environment [11]. It is well known that the higher operating temperature actually imposes an intrinsic limit of HTS SQUID noise performance [2] compared with its LTS counterpart. However, the commercial availability of cryocoolers makes it possible to operate HTS SQUIDs in the temperature range of 40–60 K using a single stage minicryocooler instead of at 77 K using liquid nitrogen. Also, a recent advance in technology greatly reduces the magnetic disturbances on the SQUIDs due to the motion of the cryocooler [12]. Ultimately, our aim is to develop a cryogen-free directly coupled SQUID system, operating in a temperature range of 40–60 K with magnetic field noise, Sf <10 fTHz−1/2 , which can be used in many non-laboratory applications where using both liquid helium and nitrogen is not feasible. As a first step toward this goal, an HTS junction technology with submicron size junction size and a large resistance is required, the analogy to the recent development of LTS submicron cross-type junction [2]. In this work, submicron wide HTS step-edge junctions and dc SQUIDs were fabricated and characterized. The junction width is chosen in the range of 30 0−50 0 nm to give a large value of Rn while keeping the SQUID parameter, β L ∼ 1 to maximize V. In the case of our step-edge GBJ, reducing w from 2 μm to 0.5 μm should increase Rn by a factor of 5–30 . There were few reports on fabricating and characterization of submicron wide junctions [13–18]. FIB milling has been used in fabricating grain boundary junctions and SQUIDs [17–19]. To protect the YBCO surface from gallium contamination during FIB milling, single or double passivation layers were deposited on top of the YBCO surface. The device geometry was then defined by standard lithography and argon ion milling before reducing the junction size using FIB milling. Here, our submicron step-edge GBJs were patterned in a similar way but without the passivation layer during the standard lithography. Therefore, they can then be tested to determine the junction width after milling. This is particularly important for the optimization of HTS SQUID performance due to the intrinsic variation of grain boundary junction parameters [20]. In our process, the passivation layer was deposited by the FIB induced deposition, only covered a few microns square across the junction area, in which the FIB milling was conducted. Here, the impact of this local passivation layer on the junction’s properties before and after FIB milling is also reported. SQUIDs with L = 135 pH using this junction have also been fabricated and characterized. This moderate large value of L is chosen to illustrate the improvement of SQUID modulation voltage on large inductance SQUID. 2. Experimental procedure The 220 nm thick YBCO films for both junction and SQUID chips were deposited in Ceraco, GmBH. The magnetometer design has a SQUID loop, directly coupled to a single layer of 5 mm × 5 mm pick-up loop [10]. The SQUID inductance, L = 135 pH was determined by the fast Henry finite element modeling. The samples were patterned using photolithography and ion beam etching. A typical value of junction width, w is 2–3 μm. The junction width
21
was reduced to submicron size using FIB milling to increase its Rn value. Before milling, a platinum layer of ∼ 200 nm thick and 1 μm × 2 μm in size was deposited over the step-edge YBCO as a protective layer using the ion beam induced deposition, IBID (Fig. 1a). During this deposition, a low beam current of 10 pA at 30 kV was used to minimize the poisoning of the YBCO by Ga ions. The same beam current and accelerating voltage were also used for milling the junction to the desired width (Fig. 1b). The equipment is a cross-beam station of Zeiss Auriga. Using this technique, both individual junctions and dc SQUIDs were fabricated (Table 1). All the SQUID measurements were performed using a dip-stick probe within a single-layer mu-metal shield. The residual field inside the shield is in the order of 0.5 μT. The magnetic field was applied perpendicular to the plane of the sample using a coil wound from copper wire. The probe was inserted into a liquid helium dewar. The sample temperature was controlled by varying the distance of the sample from the surface of liquid helium and the temperature was measured once the sample had reached thermal equilibrium. The maximum current being used through the copper coil was 2 μA with 50 μW power, while the junction power dissipation was of the order of 100 nW. Variations in power dissipation induced temperature fluctuations of less than 100 mK. Multiple measurements of the two junctions, X and Y were also performed at 77 K using the same dip-stick probe which was inserted into a liquid nitrogen dewar positioned inside a six layer mu-metal shield. Between measurements, the probe was warmed up and kept dry by immersion in flowing room temperature nitrogen gas. 3. Results and discussions Fig. 2 shows the current–voltage characteristics, IVCs of two junctions X and Y (on the same chip) before and after the deposition of the Pt passivation layer. Both junctions are 3 μm wide with Ic Rn ∼ 200 μV at 77 K before Pt deposition. It was noticed that the Ic value of junction X reduced by 45% right after the Pt deposition but then recovered and became similar (< 5% difference) to the pre-deposition value after few days, while there was no significant change in Rn value. Few studies of grain boundary junctions indicated that Ic and Rn are always interrelated, e.g. after oxygen plasma treatment or baking [21–23]. After the treatment, the junction’s Ic and Rn would change in the opposite way, i.e. one of them increases while the other would decrease. Therefore, the observation of Ic changes while Rn stays constant in our experiment, is unlikely due to the oxygen content variation on the grain boundary, introduced by the FIB induced deposition. For junction Y, it was noticed that the Ic value right after the Pt deposition is similar to the pre-deposition value (Fig. 2b). No significant change was found for few days but then increased by about 100% after few more days, while there is no significant change in Rn value. It shall be noticed that junction Y has a larger value of Ic (smaller value of Rn ) than junction X before the Pt deposition. The exact reason for the changes in Ic value is not clear. It could be related to the strain on the junction [24] and the Ic value restoration is possibly related to the stress/strain relaxation process in the initial days after Pt deposition. The width of junction X was reduced to 500 nm and measured inside a helium dewar at different temperatures. The IVCs are shown in Fig. 3a. It becomes hysteretic at a temperature <20 K, similar to its micron sized counterpart [4]. A voltage criterion of 5 μV is used to define the critical current value, while the Rn value is defined at a bias current greater than 3 times the value of Ic . As our measurement set-up does not have a feedback control of temperature, the temperature was found to increase slightly (<0.5 K) with bias current > 2 Ic . The measurement at 4.3 K was performed in liquid helium in which the temperature was stable for all bias current. The temperature dependences of Ic and Rn are shown in
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S.K.H. Lam / Physica C: Superconductivity and its applications 540 (2017) 20–25
(a)
(b)
Fig. 1. Scanning electron micrographs of the two step-edge GBJs (a) with platinum 1 μm × 2 μm deposited on top of the YBCO step-edge as protective layer before the FIB milling (b) front view of the submicron junction, with w ∼ 400 nm. Table 1 A summary of the devices. All the five devices have been deposited with Pt protective layer. The width of the junction Y has not been reduced. Device
Type
w (μm) Before FIB
X Y A B C
Junction Junction SQUID SQUID SQUID
3 3 2 2 2
Rn per junction ()
Ic Rn (mV) at 4.3 K
After FIB
Before FIB
After FIB
After FIB
0.5 – 0.3 0.4 0.5
3.2 2.6 6.5 7.2 5.9
37 – 120 72 48
2.95 – 1.05 2.26 2.74
Fig. 3b. Rn equals 37 and is almost temperature independent. The Ic value increases linearly with decreasing temperature and has a range of a few to a few tens of micro-ampere between 4.3 and 70 K. As demonstrated in the next section, these values are well suited for SQUIDs application with β L (2LIc /φ o ) ∼ 1 in a wide temperature range while the large Rn value is expected to give a large voltage modulation for SQUID with large L (Vφ ∼ Rn /L). Three SQUIDs A, B, and C with w = 30 0, 40 0 and 50 0 nm respectively were fabricated and characterized. Fig. 4 shows a few of the IVCs of the devices at different temperatures. The Rn values for junctions A, B, and C are 120, 72 and 48 respectively, assuming the pairs of SQUID junctions are identical in width. The presence of the Pt passivation patch also provides external resistive shunting to the junction. The IBID-Pt resistivity (ion beam energy of 30 kV) is in the order of 15 μ m [25] and is dependent on the deposition condition, including ion beam voltage and current density. The estimated value of resistance on our micrometer size Pt patch is about 300 , 3–5 times larger than the measured Rn of the submicron junctions. Therefore, it is expected that the Pt patch did not reduce the submicron junction’s intrinsic Rn value significantly and thus its presence did not suppress the modulation voltage. The SQUID voltage-magnetic field characteristics, VBCs, were measured at different bias currents. The maximum modulation voltage, V, at each temperature was found at a bias current of between 1.3 and 1.5Ic . Fig. 5 shows the VBCs at maximum V at different temperatures. For all three devices tested, the device VBCs became increasingly asymmetric as the temperature decreased. The exact reason is not clear but possibly due to the asymmetry of the Ic and Rn values between the two junctions on each SQUID [2629]. The magnetic field required to induce one ο on the VBC was found to have a slight inverse temperature dependency. Over the full range of temperatures, the field required varied by less than 15%. The cause is believed to be due to the decrease of the penetration depth with decreasing temperature [30]. A shift of the pattern along the magnetic field axis was often observed when the probe position was moved inside the dewar to change the sample tem-
perature. This shift was attributed to a change in background field, the probe only had one layer of mu-metal shield. Fig. 5d shows one of the VBCs illustrating a change of the background field induces a shift of the VBC pattern as the applied magnetic field changes. Fig. 6a shows the temperature dependence of V for the three SQUIDs. SQUID B (w = 400 nm) had the largest operating temperature range and useful value of V while device C (w = 500 nm) became hysteretic at T < 30 K due to the large Ic value, and the Ic value of device A was too small to allow SQUID operation for T > 40 K. Compared with the typical value of V ∼ 12 μV before the junction’s size was reduced, V for these SQUIDs was significant larger. This is attributed to the much larger value of Rn for the junctions after trimming (Table 1). Submicron YBCO SEJs have also been fabricated using LaAlO3 substrates and a micro-SQUID (L = 46 pH) with a V of 0.83 mV at 4.3 K and a Rn value 36 has also been fabricated [31]. It is useful to compare these parameters against our device B with similar V at low temperature, in which L = 135 pH while Rn ∼ 72 . Although our devices have a much larger L, V is comparable to the device with small L due to the larger Rn value (Vφ ∼ Rn /L), which is promising for low flux noise device with large L value, for better coupling to its input circuit. The temperature dependence of the SQUID parameter, β L (2LIc /o ) is shown in Fig. 6b for all three SQUIDs and they all show an inverse dependency on temperature. This is attributed to the known inverse dependency of junction Ic versus temperature (Fig. 3b). For devices B and C, β L >1 at T < 60 K while V keeps increasing at a lower temperature (Fig. 6a). For a sinusoidal voltage modulation in an applied flux, V is related to the transfer function Vφ by V = (o /π )Vφ . Assuming the SQUID’s two junction has the same junction parameters, Enpuku et al. found [32]
Vφ =
4
IC Rn kB T L exp −3.5π 2 2 . o (1 + βL ) o
(1)
S.K.H. Lam / Physica C: Superconductivity and its applications 540 (2017) 20–25
Fig. 2. The IVCs of junctions X (a) and Y (b) before and after the deposition of Pt passivation on top of the YBCO step-edge.
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Fig. 3. (a) The IVCs of junctions X at different temperatures after its width was reduced to 500 nm. (b) The temperature dependence of Ic and Ic Rn .
The left hand side of (1) can then be written in terms of V as,
V (1 + βL )π 4IC Rn
= exp −3.5π 2
kB T L
2o
(2)
The dotted line and data points in Fig. 6c show the calculation of the R.H.S. with L = 135pH and the L.H.S. for the three SQUIDs at different temperatures respectively. The discrepancy between the data and the theory appears to increase with reducing junction size. This may indicate an effect from the variation of individual junction critical current, which is expected to increase with reducing junction size [20]. This is consistent with the observation of asymmetrical VBC at a lower temperature as shown in Fig. 5. 4. Conclusion FIB induced deposition of local passivation layer was used in the milling of HTS step-edge grain boundary junctions to submicron scale. The junctions were found to have comparable values of Ic Rn with those submicron junctions using passivation overlayer [17]. The local passivation layer protects the junction surface during FIB milling while preserving the intrinsic large value of junction normal resistance. Therefore, this technique provides the advantage of fabrication without using any lithography mask and one can evaluate the device performance through multiple fabrication and characterization steps. Using these junctions, SQUIDs with
Fig. 4. The IVCs measured at different temperatures for the three SQUIDs with (device A) and without (devices A, B and C) an applied external flux, = 0 and 0.5o .
large inductance have been studied from 4.3 to 72 K. Our results indicate large modulation voltage can be achieved on SQUIDs with large inductance over this wide temperature range due to the large value of normal resistance. This is promising for applications on maximum field sensitivity where coupling with large inductance input circuit is required. The choice of junction width will be determined by the target operating temperature. For the integration of
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S.K.H. Lam / Physica C: Superconductivity and its applications 540 (2017) 20–25
Fig. 6. (a) A plot of the maximum modulation voltage, (b) β L and (c) normalized V (left hand side of Eq. (2)) against temperature for SQUIDs A, B and C.
Fig. 5. The VBCs of SQUIDs A (a), B (b) and C (c) at different temperatures. Pattern asymmetry was noticed when the temperature decreased. The shift of the VBCs along the field axis is believed to be due to a change of the background field. (d) A change of SQUID voltage (SQUID B at 55 K) after an externally applied field cycle illustrates the change of the background field during the VBC measurement.
devices onto a single stage cryocooler with operating temperature of ∼ 50 K, the present study indicates a junction width of ∼ 500 nm would be appropriate in which the Rn and V value would be in the order of 30 and 0.1 mV respectively. These results shall stimulate the development of a cryogen-free; directly coupled SQUID system with magnetic field noise, Sf <10 fTHz−1/2 , which can be used in an unshielded environment for industrial and clinical applications.
S.K.H. Lam / Physica C: Superconductivity and its applications 540 (2017) 20–25
Acknowledgments The author would like to thank J. Du for the SQUID fabrication and the inductance calculation by S.T. Keenan. Stimulated discussions with S.T. Keenan and K.E. Leslie are also acknowledged.
References [1] J. Clarke, A.I. Braginski, The SQUID Handbook, Wiley-VCH, 2006. [2] M. Schmelz, R. Stolz, V. Zakosarenko, T. Schonau, S. Anders, L. Fritzsch, M. Muck, H.-G. Meyer, Supercond. Sci. Technol. 24 (2011) 065009. [3] C.P. Foley, E.E. Mitchell, S.K.H. Lam, B. Sankrithyan, Y.M. Wilson, D.L. Tilbrook, S.J. Morris, IEEE Trans. Appl. Supercond. 9 (1999) 4281. [4] E.E. Mitchell, C.P. Foley, Supercond. Sci. Technol. 23 (2010) 065007. [5] C.P. Foley, K.E. Leslie, R. Binks, C. Lewis, W. Murray, G.J. Sloggett, S.K.H. Lam, B. Sankrithyan, N. Savvides, A. Katzaro, K.-H. Muller, E.E. Mitchell, J. Pollock, J. Lee, D.L. Dart, R.R. Barrow, M. Asten, A. Maddever, G. Panjkovic, M. Downey, C. Hoffman, R. Turner, IEEE Tran. Appl. Supercond. 9 (1999) 3786. [6] J. Du, J.C. Macfarlane, S.K.H. Lam, R. Taylor, Supercond. Sci. Technol. 22 (2009) 105013. [7] S.T. Keenan, J.A. Young, C.P. Foley, J. Du, Supercond. Sci. Technol. 23 (2010) 025029. [8] S.K.H. Lam, R. Cantor, J. Lazar, K.E. Leslie, J. Du, S.T. Keenan, C.P. Foley, J. Appl. Phys. 113 (2013) 123905. [9] K. Enpuku, G. Tokita, T. Maruo, T. Minotani, J. Appl. Phys. 78 (1995) 3498. [10] J. Du, D.L. Tilbrook, J.C. Macfarlane, K.E. Leslie, D.S. Ore, Physica C 411 (2003) 18. [11] V.N. Glyantsev, Y. Tavrin, W. Zander, J. Schubert, M. Siegel, Supercond. Sci. Technol. 9 (1996) A105.
25
[12] C. Becker, A. Steppke, T. Koettig, J. Gerster, L. Dorrer, M. Thurk, F. Schmidl, P. Seidel, J. Phys. 97 (2008) 012039. [13] M. Kawasaki, P. Chaudhari, T.H. Newman, A. Gupta, APL 58 (1991) 2555. [14] P. Larsson, A.Y. Tzalenchuk, Z.G. Ivanov, Appl. Phys. Lett. 90 (2001) 3450. [15] M. Vaupel, G. Ockenfuss, R. Wordeneber, APL 68 (1996) 3623. [16] J. Nagel, K.B. Konovalenko, M. Kemmler, M. Turad, R. Werner, E. Kleisz, S. Menzel, R. Klingeler, B. Buchner, R. Kleiner, D. Koelle Supercond, Sci. Technol. 24 (2011) 015015. [17] G. Testa, Q.A. Monaco, E. Sarneli, A. D’Agostino, D.-J. Kang, E.J. Tarte, S.H. Mennema, C. Bell, M.G. Blamire, Supercond. Sci. Technol. 17 (2004) 287. [18] T. Schwarz, R. Wolbing, C.F. Reiche, B. Muller, M.J. Martinez-Perez, T. Muhl, B. Buchner, R. Kleiner, D. Koelle, Phys. Rev. Appl. 3 (2015) 044011. [19] Ijsselsteijn, H. Elsner, W. Morgenroth, V. Schultze, H.-G. Meyer, IEEE Tran. Supercond. 9 (1999) 3933. [20] S.K.H. Lam, J. lazar, J. Du, C.P. Foley, Supercond. Sci. Technol. 27 (2014) 055011. [21] J.P. Sydow, M. Berninger, R.A. Buhrman, B.H. Moeckly, IEEE Trans. Appl. Supercond. 9 (1999) 2993. [22] J. Du, C.P. Foley, Physica C 391 (2003) 31. [23] F. Dillmann, V.N. Glyantsev, M. Siegel, Appl. Phys. Lett. 69 (1996) 1948. [24] D.C. van der Laan, T.J. Haugan, P.N. Barnes, Phys. Rev. Lett. 103 (2009) 027005. [25] V. Gopal, V.R. Radmilovic, C. Daraio, S. Jin, P. Yang, E.A. Stach, Nano Lett. 4 (2004) 2059. [26] G. Testa, S. Pagano, E. Sarnelli, C.R. Calidonna, M.M. Furnari, Appl. Phys. Lett. 79 (2001) 2943. [27] Y.S. Greenberg, Physica C 383 (2003) 354. [28] M. Rudolph, J. Nagel, J.M. Meckbach, M. Kemmler, M. Siegel, K. llin, D. Koelle, R. Kleiner, Appl. Phys. Lett. 101 (2012) 052602. [29] I.L. Novikov, Y.S. Greenberg, V. Schultze, R. Ijsselsteijn, H.-G. Meyer, Physica C 469 (2009) 30. [30] L. Hao, J.C. Macfarlane, J.C. Gallop, S.K.H. Lam, J. Appl. Phys. 99 (2006) 123916. [31] K. Chen, P. Magnelind, P. Larsson, A.Y. Tzalenchuk, Z.G. Ivanov, Physica C 372–376 (2002) 63. [32] K. Enpuku, G. Tokita, T. Maruo, T. Minotani, J. Appl. Phys. 78 (1995) 3498.
Contents lists available at ScienceDirect
Physica C: Superconductivity and its applications journal homepage: www.elsevier.com/locate/physc
Large voltage modulation in superconducting quantum interference devices with submicron-scale step-edge junctions Simon K.H. Lam CSIRO Manufacturing Flagship, PO Box 218, Lindfield, NSW 2070, Australia
a r t i c l e
i n f o
Article history: Received 28 April 2017 Accepted 12 July 2017 Available online 13 July 2017
a b s t r a c t A promising direction to improve the sensitivity of a SQUID is to increase its junction’s normal resistance value, Rn , as the SQUID modulation voltage scales linearly with Rn . As a first step to develop highly sensitive single layer SQUID, submicron scale YBCO grain boundary step edge junctions and SQUIDs with large Rn were fabricated and studied. The step-edge junctions were reduced to submicron scale to increase their Rn values using focus ion beam, FIB and the measurement of transport properties were performed from 4.3 to 77 K. The FIB induced deposition layer proves to be effective to minimize the Ga ion contamination during the FIB milling process. The critical current–normal resistance value of submicron junction at 4.3 K was found to be 1–3 mV, comparable to the value of the same type of junction in micron scale. The submicron junction Rn value is in the range of 35–100 , resulting a large SQUID modulation voltage in a wide temperature range. This performance promotes further investigation of cryogen-free, high field sensitivity SQUID applications at medium low temperature, e.g. at 40–60 K. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Superconducting quantum interference device (SQUID) is the most sensitive magnetic flux detector with a large frequency bandwidth. The figure of merit for SQUID devices is the magnetic flux noise, Sφ , which depends on Sv , the voltage noise spectral density and Vφ , the flux to voltage transfer function, i.e. Sφ = Sv /Vφ 2 . Assuming Sv is dominated by the Johnson noise of the SQUID normal resistance, Rn , Sv 1/2 = 4 (kB TRn )1/2 . On the other hand, Vφ ∼ Rn /L when the SQUID parameter, β L (2LIc /φ o ) and hysteresis parameter, β c (4π Ic Rn C/o ) are both equal to one [1]. Here, L is the SQUID inductance; Ic and C are the junction critical current and capacitance respectively. For a sinusoidal voltage modulation in an applied flux, the peak-to-peak modulation voltage, V is related to Vφ by V = Vφ (o /π ). Theoretically, one would design a SQUID with a low value of L to achieve a large Vφ and small Sφ . After the first low-temperature superconducting, LTS thin film SQUID was demonstrated in 1963, the fabrication processes had been evolved to produce more robust and sensitive devices. Nowadays, a commercial LTS SQUID (L ∼ 50pH) of field sensitivity, Sf in the order of 2–3 fTHz−1/2 , operating at liquid helium temperature of 4.2 K is available [1]. In many non-laboratory applications, the SQUID has to be operated stably in an unshielded environment. Reducing the junction size has been found to improve the SQUID
E-mail address: [email protected] http://dx.doi.org/10.1016/j.physc.2017.07.004 0921-4534/© 2017 Elsevier B.V. All rights reserved.
performance even after the SQUID has been exposed to a medium field [2]. Furthermore, most SQUID applications require a large inductance pick-up loop to detect the magnetic signal and couple into the SQUID. So the SQUID would require a large value of inductance, L to minimize the inductance mismatch with the pickup loop. As the junction size is reduced, the voltage modulation of a SQUID with a large value of L will be enhanced significantly due to the larger value of Rn and a smaller value of C. By using submicron sized low-temperature superconducting junctions, a LTS SQUID with L = 300pH and Sf = 0.3 fTHz−1/2 has been demonstrated [2]. High-temperature superconducting (HTS) SQUIDs based on grain boundary junctions (GBJs) have been studied over the past 25 years [1]. In HTS SQUIDs, the simplest way to implement a magnetometer is to use a large pickup loop on a typical substrate of 10 mm × 10 mm and directly inject the external flux signal into the SQUID (directly coupled). The typical value of L is chosen to be 50–100 pH to comprise between flux coupling and noise performance [1]. Bi-crystal and step-edge are the two most commonly used HTS superconducting GBJ technologies for both research and commercial HTS SQUID system. CSIRO developed a step-edge GBJ technology in 1990s [3,4]. Various devices have been fabricated for different applications [5–8]. In the case of SQUID applications, the junction width is 2–3 μm to obtain Ic and Rn value in the range of 20–100 μA and 3–10 respectively, for L = 65 pH (β L ∼ 1). These SQUIDs have the values of V ∼ 30 μV, Sφ ∼ 6 μo Hz−1/2 and field noise, Sf , ∼ 50 fTHz−1/2 at 77 K [8]. To further improve the SQUID’s
S.K.H. Lam / Physica C: Superconductivity and its applications 540 (2017) 20–25
field sensitivity, i.e. reduce the value of Sf , increasing the value of L would enhance the flux coupling. However, V will decrease and thus increasing the flux noise [9]. For example, SQUIDs with L = 130 pH was found to have a value of V = 12 μV [10]. To overcome the deterioration of V with a large value of L, increasing the value of Rn while keeping β L ∼ 1 is the most obvious way to improve Sφ and ultimately Sf . For a particular HTS junction technology, Rn can be increased by reducing the junction width. In addition, the previous study of SQUID with submicron HTS junction had shown Vφ would not be suppressed by external fields up to 300 μT, indicating the advantage of submicron junction operating stably in earth’s field environment [11]. It is well known that the higher operating temperature actually imposes an intrinsic limit of HTS SQUID noise performance [2] compared with its LTS counterpart. However, the commercial availability of cryocoolers makes it possible to operate HTS SQUIDs in the temperature range of 40–60 K using a single stage minicryocooler instead of at 77 K using liquid nitrogen. Also, a recent advance in technology greatly reduces the magnetic disturbances on the SQUIDs due to the motion of the cryocooler [12]. Ultimately, our aim is to develop a cryogen-free directly coupled SQUID system, operating in a temperature range of 40–60 K with magnetic field noise, Sf <10 fTHz−1/2 , which can be used in many non-laboratory applications where using both liquid helium and nitrogen is not feasible. As a first step toward this goal, an HTS junction technology with submicron size junction size and a large resistance is required, the analogy to the recent development of LTS submicron cross-type junction [2]. In this work, submicron wide HTS step-edge junctions and dc SQUIDs were fabricated and characterized. The junction width is chosen in the range of 30 0−50 0 nm to give a large value of Rn while keeping the SQUID parameter, β L ∼ 1 to maximize V. In the case of our step-edge GBJ, reducing w from 2 μm to 0.5 μm should increase Rn by a factor of 5–30 . There were few reports on fabricating and characterization of submicron wide junctions [13–18]. FIB milling has been used in fabricating grain boundary junctions and SQUIDs [17–19]. To protect the YBCO surface from gallium contamination during FIB milling, single or double passivation layers were deposited on top of the YBCO surface. The device geometry was then defined by standard lithography and argon ion milling before reducing the junction size using FIB milling. Here, our submicron step-edge GBJs were patterned in a similar way but without the passivation layer during the standard lithography. Therefore, they can then be tested to determine the junction width after milling. This is particularly important for the optimization of HTS SQUID performance due to the intrinsic variation of grain boundary junction parameters [20]. In our process, the passivation layer was deposited by the FIB induced deposition, only covered a few microns square across the junction area, in which the FIB milling was conducted. Here, the impact of this local passivation layer on the junction’s properties before and after FIB milling is also reported. SQUIDs with L = 135 pH using this junction have also been fabricated and characterized. This moderate large value of L is chosen to illustrate the improvement of SQUID modulation voltage on large inductance SQUID. 2. Experimental procedure The 220 nm thick YBCO films for both junction and SQUID chips were deposited in Ceraco, GmBH. The magnetometer design has a SQUID loop, directly coupled to a single layer of 5 mm × 5 mm pick-up loop [10]. The SQUID inductance, L = 135 pH was determined by the fast Henry finite element modeling. The samples were patterned using photolithography and ion beam etching. A typical value of junction width, w is 2–3 μm. The junction width
21
was reduced to submicron size using FIB milling to increase its Rn value. Before milling, a platinum layer of ∼ 200 nm thick and 1 μm × 2 μm in size was deposited over the step-edge YBCO as a protective layer using the ion beam induced deposition, IBID (Fig. 1a). During this deposition, a low beam current of 10 pA at 30 kV was used to minimize the poisoning of the YBCO by Ga ions. The same beam current and accelerating voltage were also used for milling the junction to the desired width (Fig. 1b). The equipment is a cross-beam station of Zeiss Auriga. Using this technique, both individual junctions and dc SQUIDs were fabricated (Table 1). All the SQUID measurements were performed using a dip-stick probe within a single-layer mu-metal shield. The residual field inside the shield is in the order of 0.5 μT. The magnetic field was applied perpendicular to the plane of the sample using a coil wound from copper wire. The probe was inserted into a liquid helium dewar. The sample temperature was controlled by varying the distance of the sample from the surface of liquid helium and the temperature was measured once the sample had reached thermal equilibrium. The maximum current being used through the copper coil was 2 μA with 50 μW power, while the junction power dissipation was of the order of 100 nW. Variations in power dissipation induced temperature fluctuations of less than 100 mK. Multiple measurements of the two junctions, X and Y were also performed at 77 K using the same dip-stick probe which was inserted into a liquid nitrogen dewar positioned inside a six layer mu-metal shield. Between measurements, the probe was warmed up and kept dry by immersion in flowing room temperature nitrogen gas. 3. Results and discussions Fig. 2 shows the current–voltage characteristics, IVCs of two junctions X and Y (on the same chip) before and after the deposition of the Pt passivation layer. Both junctions are 3 μm wide with Ic Rn ∼ 200 μV at 77 K before Pt deposition. It was noticed that the Ic value of junction X reduced by 45% right after the Pt deposition but then recovered and became similar (< 5% difference) to the pre-deposition value after few days, while there was no significant change in Rn value. Few studies of grain boundary junctions indicated that Ic and Rn are always interrelated, e.g. after oxygen plasma treatment or baking [21–23]. After the treatment, the junction’s Ic and Rn would change in the opposite way, i.e. one of them increases while the other would decrease. Therefore, the observation of Ic changes while Rn stays constant in our experiment, is unlikely due to the oxygen content variation on the grain boundary, introduced by the FIB induced deposition. For junction Y, it was noticed that the Ic value right after the Pt deposition is similar to the pre-deposition value (Fig. 2b). No significant change was found for few days but then increased by about 100% after few more days, while there is no significant change in Rn value. It shall be noticed that junction Y has a larger value of Ic (smaller value of Rn ) than junction X before the Pt deposition. The exact reason for the changes in Ic value is not clear. It could be related to the strain on the junction [24] and the Ic value restoration is possibly related to the stress/strain relaxation process in the initial days after Pt deposition. The width of junction X was reduced to 500 nm and measured inside a helium dewar at different temperatures. The IVCs are shown in Fig. 3a. It becomes hysteretic at a temperature <20 K, similar to its micron sized counterpart [4]. A voltage criterion of 5 μV is used to define the critical current value, while the Rn value is defined at a bias current greater than 3 times the value of Ic . As our measurement set-up does not have a feedback control of temperature, the temperature was found to increase slightly (<0.5 K) with bias current > 2 Ic . The measurement at 4.3 K was performed in liquid helium in which the temperature was stable for all bias current. The temperature dependences of Ic and Rn are shown in
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S.K.H. Lam / Physica C: Superconductivity and its applications 540 (2017) 20–25
(a)
(b)
Fig. 1. Scanning electron micrographs of the two step-edge GBJs (a) with platinum 1 μm × 2 μm deposited on top of the YBCO step-edge as protective layer before the FIB milling (b) front view of the submicron junction, with w ∼ 400 nm. Table 1 A summary of the devices. All the five devices have been deposited with Pt protective layer. The width of the junction Y has not been reduced. Device
Type
w (μm) Before FIB
X Y A B C
Junction Junction SQUID SQUID SQUID
3 3 2 2 2
Rn per junction ()
Ic Rn (mV) at 4.3 K
After FIB
Before FIB
After FIB
After FIB
0.5 – 0.3 0.4 0.5
3.2 2.6 6.5 7.2 5.9
37 – 120 72 48
2.95 – 1.05 2.26 2.74
Fig. 3b. Rn equals 37 and is almost temperature independent. The Ic value increases linearly with decreasing temperature and has a range of a few to a few tens of micro-ampere between 4.3 and 70 K. As demonstrated in the next section, these values are well suited for SQUIDs application with β L (2LIc /φ o ) ∼ 1 in a wide temperature range while the large Rn value is expected to give a large voltage modulation for SQUID with large L (Vφ ∼ Rn /L). Three SQUIDs A, B, and C with w = 30 0, 40 0 and 50 0 nm respectively were fabricated and characterized. Fig. 4 shows a few of the IVCs of the devices at different temperatures. The Rn values for junctions A, B, and C are 120, 72 and 48 respectively, assuming the pairs of SQUID junctions are identical in width. The presence of the Pt passivation patch also provides external resistive shunting to the junction. The IBID-Pt resistivity (ion beam energy of 30 kV) is in the order of 15 μ m [25] and is dependent on the deposition condition, including ion beam voltage and current density. The estimated value of resistance on our micrometer size Pt patch is about 300 , 3–5 times larger than the measured Rn of the submicron junctions. Therefore, it is expected that the Pt patch did not reduce the submicron junction’s intrinsic Rn value significantly and thus its presence did not suppress the modulation voltage. The SQUID voltage-magnetic field characteristics, VBCs, were measured at different bias currents. The maximum modulation voltage, V, at each temperature was found at a bias current of between 1.3 and 1.5Ic . Fig. 5 shows the VBCs at maximum V at different temperatures. For all three devices tested, the device VBCs became increasingly asymmetric as the temperature decreased. The exact reason is not clear but possibly due to the asymmetry of the Ic and Rn values between the two junctions on each SQUID [2629]. The magnetic field required to induce one ο on the VBC was found to have a slight inverse temperature dependency. Over the full range of temperatures, the field required varied by less than 15%. The cause is believed to be due to the decrease of the penetration depth with decreasing temperature [30]. A shift of the pattern along the magnetic field axis was often observed when the probe position was moved inside the dewar to change the sample tem-
perature. This shift was attributed to a change in background field, the probe only had one layer of mu-metal shield. Fig. 5d shows one of the VBCs illustrating a change of the background field induces a shift of the VBC pattern as the applied magnetic field changes. Fig. 6a shows the temperature dependence of V for the three SQUIDs. SQUID B (w = 400 nm) had the largest operating temperature range and useful value of V while device C (w = 500 nm) became hysteretic at T < 30 K due to the large Ic value, and the Ic value of device A was too small to allow SQUID operation for T > 40 K. Compared with the typical value of V ∼ 12 μV before the junction’s size was reduced, V for these SQUIDs was significant larger. This is attributed to the much larger value of Rn for the junctions after trimming (Table 1). Submicron YBCO SEJs have also been fabricated using LaAlO3 substrates and a micro-SQUID (L = 46 pH) with a V of 0.83 mV at 4.3 K and a Rn value 36 has also been fabricated [31]. It is useful to compare these parameters against our device B with similar V at low temperature, in which L = 135 pH while Rn ∼ 72 . Although our devices have a much larger L, V is comparable to the device with small L due to the larger Rn value (Vφ ∼ Rn /L), which is promising for low flux noise device with large L value, for better coupling to its input circuit. The temperature dependence of the SQUID parameter, β L (2LIc /o ) is shown in Fig. 6b for all three SQUIDs and they all show an inverse dependency on temperature. This is attributed to the known inverse dependency of junction Ic versus temperature (Fig. 3b). For devices B and C, β L >1 at T < 60 K while V keeps increasing at a lower temperature (Fig. 6a). For a sinusoidal voltage modulation in an applied flux, V is related to the transfer function Vφ by V = (o /π )Vφ . Assuming the SQUID’s two junction has the same junction parameters, Enpuku et al. found [32]
Vφ =
4
IC Rn kB T L exp −3.5π 2 2 . o (1 + βL ) o
(1)
S.K.H. Lam / Physica C: Superconductivity and its applications 540 (2017) 20–25
Fig. 2. The IVCs of junctions X (a) and Y (b) before and after the deposition of Pt passivation on top of the YBCO step-edge.
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Fig. 3. (a) The IVCs of junctions X at different temperatures after its width was reduced to 500 nm. (b) The temperature dependence of Ic and Ic Rn .
The left hand side of (1) can then be written in terms of V as,
V (1 + βL )π 4IC Rn
= exp −3.5π 2
kB T L
2o
(2)
The dotted line and data points in Fig. 6c show the calculation of the R.H.S. with L = 135pH and the L.H.S. for the three SQUIDs at different temperatures respectively. The discrepancy between the data and the theory appears to increase with reducing junction size. This may indicate an effect from the variation of individual junction critical current, which is expected to increase with reducing junction size [20]. This is consistent with the observation of asymmetrical VBC at a lower temperature as shown in Fig. 5. 4. Conclusion FIB induced deposition of local passivation layer was used in the milling of HTS step-edge grain boundary junctions to submicron scale. The junctions were found to have comparable values of Ic Rn with those submicron junctions using passivation overlayer [17]. The local passivation layer protects the junction surface during FIB milling while preserving the intrinsic large value of junction normal resistance. Therefore, this technique provides the advantage of fabrication without using any lithography mask and one can evaluate the device performance through multiple fabrication and characterization steps. Using these junctions, SQUIDs with
Fig. 4. The IVCs measured at different temperatures for the three SQUIDs with (device A) and without (devices A, B and C) an applied external flux, = 0 and 0.5o .
large inductance have been studied from 4.3 to 72 K. Our results indicate large modulation voltage can be achieved on SQUIDs with large inductance over this wide temperature range due to the large value of normal resistance. This is promising for applications on maximum field sensitivity where coupling with large inductance input circuit is required. The choice of junction width will be determined by the target operating temperature. For the integration of
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S.K.H. Lam / Physica C: Superconductivity and its applications 540 (2017) 20–25
Fig. 6. (a) A plot of the maximum modulation voltage, (b) β L and (c) normalized V (left hand side of Eq. (2)) against temperature for SQUIDs A, B and C.
Fig. 5. The VBCs of SQUIDs A (a), B (b) and C (c) at different temperatures. Pattern asymmetry was noticed when the temperature decreased. The shift of the VBCs along the field axis is believed to be due to a change of the background field. (d) A change of SQUID voltage (SQUID B at 55 K) after an externally applied field cycle illustrates the change of the background field during the VBC measurement.
devices onto a single stage cryocooler with operating temperature of ∼ 50 K, the present study indicates a junction width of ∼ 500 nm would be appropriate in which the Rn and V value would be in the order of 30 and 0.1 mV respectively. These results shall stimulate the development of a cryogen-free; directly coupled SQUID system with magnetic field noise, Sf <10 fTHz−1/2 , which can be used in an unshielded environment for industrial and clinical applications.
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Acknowledgments The author would like to thank J. Du for the SQUID fabrication and the inductance calculation by S.T. Keenan. Stimulated discussions with S.T. Keenan and K.E. Leslie are also acknowledged.
References [1] J. Clarke, A.I. Braginski, The SQUID Handbook, Wiley-VCH, 2006. [2] M. Schmelz, R. Stolz, V. Zakosarenko, T. Schonau, S. Anders, L. Fritzsch, M. Muck, H.-G. Meyer, Supercond. Sci. Technol. 24 (2011) 065009. [3] C.P. Foley, E.E. Mitchell, S.K.H. Lam, B. Sankrithyan, Y.M. Wilson, D.L. Tilbrook, S.J. Morris, IEEE Trans. Appl. Supercond. 9 (1999) 4281. [4] E.E. Mitchell, C.P. Foley, Supercond. Sci. Technol. 23 (2010) 065007. [5] C.P. Foley, K.E. Leslie, R. Binks, C. Lewis, W. Murray, G.J. Sloggett, S.K.H. Lam, B. Sankrithyan, N. Savvides, A. Katzaro, K.-H. Muller, E.E. Mitchell, J. Pollock, J. Lee, D.L. Dart, R.R. Barrow, M. Asten, A. Maddever, G. Panjkovic, M. Downey, C. Hoffman, R. Turner, IEEE Tran. Appl. Supercond. 9 (1999) 3786. [6] J. Du, J.C. Macfarlane, S.K.H. Lam, R. Taylor, Supercond. Sci. Technol. 22 (2009) 105013. [7] S.T. Keenan, J.A. Young, C.P. Foley, J. Du, Supercond. Sci. Technol. 23 (2010) 025029. [8] S.K.H. Lam, R. Cantor, J. Lazar, K.E. Leslie, J. Du, S.T. Keenan, C.P. Foley, J. Appl. Phys. 113 (2013) 123905. [9] K. Enpuku, G. Tokita, T. Maruo, T. Minotani, J. Appl. Phys. 78 (1995) 3498. [10] J. Du, D.L. Tilbrook, J.C. Macfarlane, K.E. Leslie, D.S. Ore, Physica C 411 (2003) 18. [11] V.N. Glyantsev, Y. Tavrin, W. Zander, J. Schubert, M. Siegel, Supercond. Sci. Technol. 9 (1996) A105.
25
[12] C. Becker, A. Steppke, T. Koettig, J. Gerster, L. Dorrer, M. Thurk, F. Schmidl, P. Seidel, J. Phys. 97 (2008) 012039. [13] M. Kawasaki, P. Chaudhari, T.H. Newman, A. Gupta, APL 58 (1991) 2555. [14] P. Larsson, A.Y. Tzalenchuk, Z.G. Ivanov, Appl. Phys. Lett. 90 (2001) 3450. [15] M. Vaupel, G. Ockenfuss, R. Wordeneber, APL 68 (1996) 3623. [16] J. Nagel, K.B. Konovalenko, M. Kemmler, M. Turad, R. Werner, E. Kleisz, S. Menzel, R. Klingeler, B. Buchner, R. Kleiner, D. Koelle Supercond, Sci. Technol. 24 (2011) 015015. [17] G. Testa, Q.A. Monaco, E. Sarneli, A. D’Agostino, D.-J. Kang, E.J. Tarte, S.H. Mennema, C. Bell, M.G. Blamire, Supercond. Sci. Technol. 17 (2004) 287. [18] T. Schwarz, R. Wolbing, C.F. Reiche, B. Muller, M.J. Martinez-Perez, T. Muhl, B. Buchner, R. Kleiner, D. Koelle, Phys. Rev. Appl. 3 (2015) 044011. [19] Ijsselsteijn, H. Elsner, W. Morgenroth, V. Schultze, H.-G. Meyer, IEEE Tran. Supercond. 9 (1999) 3933. [20] S.K.H. Lam, J. lazar, J. Du, C.P. Foley, Supercond. Sci. Technol. 27 (2014) 055011. [21] J.P. Sydow, M. Berninger, R.A. Buhrman, B.H. Moeckly, IEEE Trans. Appl. Supercond. 9 (1999) 2993. [22] J. Du, C.P. Foley, Physica C 391 (2003) 31. [23] F. Dillmann, V.N. Glyantsev, M. Siegel, Appl. Phys. Lett. 69 (1996) 1948. [24] D.C. van der Laan, T.J. Haugan, P.N. Barnes, Phys. Rev. Lett. 103 (2009) 027005. [25] V. Gopal, V.R. Radmilovic, C. Daraio, S. Jin, P. Yang, E.A. Stach, Nano Lett. 4 (2004) 2059. [26] G. Testa, S. Pagano, E. Sarnelli, C.R. Calidonna, M.M. Furnari, Appl. Phys. Lett. 79 (2001) 2943. [27] Y.S. Greenberg, Physica C 383 (2003) 354. [28] M. Rudolph, J. Nagel, J.M. Meckbach, M. Kemmler, M. Siegel, K. llin, D. Koelle, R. Kleiner, Appl. Phys. Lett. 101 (2012) 052602. [29] I.L. Novikov, Y.S. Greenberg, V. Schultze, R. Ijsselsteijn, H.-G. Meyer, Physica C 469 (2009) 30. [30] L. Hao, J.C. Macfarlane, J.C. Gallop, S.K.H. Lam, J. Appl. Phys. 99 (2006) 123916. [31] K. Chen, P. Magnelind, P. Larsson, A.Y. Tzalenchuk, Z.G. Ivanov, Physica C 372–376 (2002) 63. [32] K. Enpuku, G. Tokita, T. Maruo, T. Minotani, J. Appl. Phys. 78 (1995) 3498.