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Dependences of microwave surface resistance of HTS thin films on applied dc magnetic fields parallel and normal to the substrate

We investigated the dc magnetic field and temperature dependences of microwave surface resistance (Rs) of high-temperature superconductor (HTS) films. Previously, we reported that the surface resistance RsRs(n) under a dc magnetic field applied normaly to the substrate increased when increasing the applied magnetic field. For NMR application, we have to examine the Rs(p) under the dc magnetic field parallel to the substrate. We measured the Rs(p) of the YBCO and DyBCO thin films with a thickness of 500 nm deposited on a MgO (100) substrate using the dielectric resonator method at 21.8 GHz, and a dc magnetic field of up to 5 T. In a zero magnetic field, the values of Rs(n) and Rs(p) were 0.35 mQ at 20 K. Under the dc magnetic field, the Rs(n) and the Rs(p) also increased with increasing magnetic field, however, the Rs(p) had a lower magnetic field dependence and the value was about 1/10 of that of the Rs(n). The Rs(p) at 16.4 T and at 700 MHz could be estimated by the two-fluid model. The Rs(p) value was about 1/2600 compared with that of copper at 20 K. As a result, we clarified that 500 nm thick YBCO and DyBCO thin films could provide advantages for NMR application.


Abstract.
We investigated the dc magnetic field and temperature dependences of microwave surface resistance (R s ) of high-temperature superconductor (HTS) films. Previously, we reported that the surface resistance R s (n) under a dc magnetic field applied normaly to the substrate increased when increasing the applied magnetic field. For NMR application, we have to examine the R s (p) under the dc magnetic field parallel to the substrate. We measured the R s (p) of the YBCO and DyBCO thin films with a thickness of 500 nm deposited on a MgO (100) substrate using the dielectric resonator method at 21.8 GHz, and a dc magnetic field of up to 5 T. In a zero magnetic field, the values of R s (n) and R s (p) were 0.35 mΩ at 20 K. Under the dc magnetic field, the R s (n) and the R s (p) also increased with increasing magnetic field, however, the R s (p) had a lower magnetic field dependence and the value was about 1/10 of that of the R s (n) . The R s (p) at 16.4 T and at 700 MHz could be estimated by the two-fluid model. The R s (p) value was about 1/2600 compared with that of copper at 20 K. As a result, we clarified that 500 nm thick YBCO and DyBCO thin films could provide advantages for NMR application.

Introduction
The microwave surface resistance (R s ) of high-temperature superconductor (HTS) thin film is approximately two or three orders of magnitude smaller than that of the conducting metals usually used in the microwave frequency region. Because of this characteristic, passive microwave devices, such as filters [1] and antennas [2], have been widely investigated. Applications of HTS films with low R s under a high magnetic field have been proposed, such as pick-up coils for magnetic resonance imaging and nuclear magnetic resonance (NMR). In order to realize these microwave devices, it is necessary to select and evaluate HTS materials by precise R s measurement. For example, Honma et al. studied the R s of YBCO thin films under high magnetic fields up to 12 T [3]. According to their study [3], the R s of HTS films increases rapidly according to applied magnetic fields and thus it is thought that superconducting NMR pick up coils is difficult. However, in their experiment, a magnetic field was applied normaly to the substrate-plane, defined as R s (n) . In this study, a magnetic field was applied parallel to the substrate, defined as R s (p) . It is important to know how the R s (p) behaves in a high magnetic region. However, there were few experimentally reports. In this paper, we measured the R s (p) of YBa 2 Cu 3 O y (YBCO) and DyBa 2 Cu 3 O y (DyBCO) superconducting films under high dc magnetic fields up to 5 T and investigated whether or not these materials are valid for NMR pick-up coils.

Experimental
We measured the R s (p) in dc magnetic fields using the dielectric resonator method [4][5][6][7] and a superconducting magnet with NbTi (Suzuki Shokan Co., Ltd.: SSR-75). The dielectric resonator method is an international measurement standard for the R s of HTS superconducting films [8].The R s measurement system was described in detail in Ref. [7]. The resonator frequency of the TE 011 mode was set to 21.8 GHz. In the cavity, we set the HTS films as shown in Fig. 1. Fig. 1 (a) represents the measurement cavity for R s (n) , and Fig. 1 (b) represents the measurement cavity for R s (p) . Therefore, the field was (a) normal and (b) parallel to the substrate-plane of the HTS films. The film thickness and crystalline properties of the measured YBCO and DyBCO thin films are listed in Table 1. These films were deposited on MgO (001) substrates by the thermal co-evaporation method. The crystalline of the films was evaluated by θ/2θ and φ-scan X-ray diffraction. The δγ was the full-width half-maximum (FWHM) of the rocking curve of the (005) peak of the films. The δφ was the FWHM of the φ-scan of the (102) plane of the thin film. From table 1, the δγ values of the YBCO and DyBCO films were lower than 1.2 degree and the δφ values were lower than 1.7 degree, respectively. Therefore, the crystalline properties of the measured YBCO and DyBCO thin films were almost the same. Fig. 3 shows the temperature dependence of the R s (n) and R s (p) of 500 nm thick YBCO and DyBCO thin films in a zero magnetic field at 21.8 GHz. The R s (n) [3] and R s (p) values at 20 K were both 0.35mΩ. Measurement error between each cavity was less than 5 percent.   . These results could be explained by using the two-fluid model for high frequency and low magnetic field limits [10]. Under a low magnetic field, the R s is given as follows,

Temperature dependence of R s (n) and R s (p)
Under a high magnetic field, the R s is given as follows, Here, B is the magnitude of the applied dc magnetic field, d is the thickness of films, ω the angular frequency, Φ 0 the flux quantum, μ 0 the vacuum permeability, λ L the London penetration depth, the dynamic mobility and η the viscous drag coefficient which is related to the motion of the magnetic flux. We considered η as a function of the magnetic field and temperature [3]. In Fig. 4, solid lines represent the fitted data of R s under a low magnetic field calculated using Eq. (1). Dotted lines represent fitted data of R s under a high magnetic field calculated using Eq. (2). In low magnetic regions, R of various materials displayed extremely low magnetic field dependences up to 5 T. Therefore, in thin films with strong intrinsic pinning, motion of flux was reduced. DyBCO thin films could provide advantages for NMR pick-up coil materials. For NMR application, HTS materials need not only possess low R s , but also have a high critical current density (J c ). We will continue to consider HTS pick-up coils for NMR application in the future.

Conclusion
In this paper, we investigated the dc magnetic field and temperature dependences of microwave surface resistance (R s (n) , R s (p) ) in HTS films for NMR application. The R s (p) value shows low magnetic field dependence compared with R s (n) . This behavior is caused by the layered structure of YBCO and DyBCO thin films. The motion of flux was hindered. The estimated values of 500 nm thick YBCO and DyBCO thin films at f = 700 MHz, T = 20 K, and B = 16.4 T were 0.94 and 0.88 μΩ respectively. These values were lower than 25 μΩ, which was less than 1/1000 of the R s of copper. From these results, we clarified that YBCO and DyBCO thin films could provide advantages for NMR application.