Effect of 1.5 MeV Proton Irradiation on Superconductivity in FeSe 0.5 Te 0.5 Thin Films

: Raising the critical current density J c in magnetic ﬁelds is crucial to applications such as rotation machines, generators for wind turbines and magnet use in medical imaging machines. The increase in J c has been achieved by introducing structural defects including precipitates and vacancies. Recently, a low-energy ion irradiation has been revisited as a practically feasible approach to create nanoscale defects, resulting in an increase in J c in magnetic ﬁelds. In this paper, we report the effect of proton irradiation with 1.5 MeV on superconducting properties of iron–chalcogenide FeSe 0.5 Te 0.5 ﬁlms through the transport and magnetization measurements. The 1.5 MeV proton irradiation with 1 × 10 16 p/cm 2 yields the highest J c increase, approximately 30% at 5–10 K and below 1 T without any reduction in T c . These results indicate that 1.5 MeV proton irradiations could be a practical tool to enhance the performance of iron-based superconducting tapes under magnetic ﬁelds. MeV proton irradiation on superconducting properties of FST ﬁlms. Upon the irradiation up to 1 × 10 16 p/cm 2 dose, T c remains virtually unchanged in magnetization as well as in transport measurement. An approximately 30% enhancement of J c in the magnetic ﬁeld below 1 T is observed using 1.5 MeV proton irradiation with 1 × 10 16 p/cm 2 . Transport properties of a pristine ﬁlm and an irradiated ﬁlm with a ﬂuence of 1 × 10 15 p/cm 2 show a small anisotropy of J c in the applied magnetic ﬁeld range at 4.2 K. The enhancement of J c for almost all the ﬁeld orientations was accomplished by the irradiation at a dose of 1 × 10 15 p/cm 2 at 4.2 K and 1 T. These results indicate that 1.5 MeV proton irradiation is effective in providing less anisotropic pinning defects in the magnetic ﬁeld below 1 T in iron–chalcogenide superconducting ﬁlms. Additionally, by ﬁne tuning an irradiation ﬂuence of proton, superconducting properties can be further improved.


Introduction
Iron-based superconductors have a reasonably high superconducting transition temperature T c , very high upper critical magnetic fields H c2 , quite a small anisotropy γ and larger critical grain boundary angle than cuprate superconductors, which make them promising for high-field applications such as superconducting magnet and generators [1][2][3][4][5]. The use of superconducting materials for high field applications is limited by the critical current density J c in magnetic fields, which can be sustained by pinning the vortices (flux pinning) at structural defects with nano-meter sizes such as cracks, voids, grain boundaries and secondary phases [6,7]. The ion irradiation is a useful tool to generate the desired defect structure. Depending on the ion species, ion energy and the properties of the target materials, ion irradiation enables the creation of defects with well-controlled morphology and density, such as point [8], cluster [9][10][11][12] and columnar [13][14][15] defects. Early works on the ion irradiation of cuprate (Cu-O based) high-T c superconductors (HTS) for improving J c in the magnetic field have mostly focused on the high-energy, over hundreds of MeV, heavy ion irradiation [13][14][15]. At this energy range, the irradiation of superconducting materials by the swift heavy ion mainly causes electronic excitation and ionization of the target atoms. As a result, continuous amorphous tracks are formed in a process that can be described as the rapid melting and solidification of nm-sized columns in the path of an ion. Even though the heavy ion tracks proved to be very effective pinning defects, this approach has been limited to fundamental studies of the vortex matter.
Recently, ion irradiation of HTS with a low energy has received a renewed interest as a practical method for increasing J c in magnetic fields, due to the compact accelerator, lower radioactivity and less costly operation [9][10][11][12]. Low-energy ion irradiation utilizes a Quantum Beam Sci. 2021, 5, 18 2 of 8 different mechanism for the creation of vortex pinning defects. The electronic excitation and ionization are low enough so the heat can dissipate without damaging the materials. The low-energy ion irradiation leads to the collision of the ion with the target atom nuclei, resulting in cascade, point and cluster defects. Matsui et al. demonstrated that 3 MeV Au 2+ ion irradiation to 700 nm thick YBCO films yielded an enhancement in the in-field J c at 77 K of up to a factor of 4 [9]. Equally impressive results in YBCO commercial tape have been reported by Jia et al. using 4 MeV proton [10]. Recently, we reported a route to raise both T c and J c in iron-based superconducting FeSe 0.5 Te 0.5 (FST) thin films by low-energy (190 keV) proton irradiation [16,17]. The 190 keV proton irradiation yields the increase in T c due to the nanoscale compressive strain induced by cascade defects. The irradiation also induced a near doubling of J c at 4.2 K from the self-field to 35 T through strong vortex pinning by the cascade defects and surrounding nanoscale strain.
In this paper, we report the effect of 1.5 MeV proton irradiation on iron-chalcogenide FST superconducting films. We report the performance of irradiated samples at different temperatures in a magnetic field up to 9 T. We show that 1.5 MeV protons clearly enhance J c in magnetic fields <1 T with no subsequent reduction in T c . However, we did not observe a reproducible positive effect in the magnetic fields >1 T. The results are discussed in terms of the spatial distribution of defects produced by fast protons.

Materials and Methods
All films in this study were deposited by the pulsed laser deposition (PLD) method using a Nd:YAG laser (λ = 266 nm). We first grew a CeO 2 layer with a thickness of about 80-100 nm on SrTiO 3 single-crystal substrate at a substrate temperature of 600-650 • C and oxygen partial pressure of~115 mTorr. Then, 100-130 nm thick FST films were grown on CeO 2 buffer layers. During the deposition of FST films, the substrate temperature and oxygen partial pressure were kept at 300-360 • C and~1 × 10 -6 Torr, respectively.
Superconducting transport properties were measured using the conventional fourprobe method in a physical property measurement system (PPMS, Quantum Design). T c,10 and J c were determined from the ρT and I-V curves using 0.1 ρ n and 1 µV/cm criteria, respectively. Here, ρ n means the normal state resistivity above the transition temperature. The current was applied perpendicularly to the magnetic field. The magnetization was measured using a superconducting quantum interference device (SQUID, Quantum Design) magnetometer. Two FST films (sample A and B) were fabricated under the same deposition condition for different irradiation conditions. Each FST film was cut into 3 pieces: one for magnetization measurement before and after irradiation with same film, another for transport measurement before irradiation (pristine) and the other for transport measurement after irradiation (irradiated).
The FST films were irradiated with 1.5 MeV proton doses of 1 × 10 15 and 1 × 10 16 p/cm 2 in vacuum at room temperature using the 5 MV tandem accelerator of the Wakasa Wan Energy Research Center (WERC). The samples were mounted on a copper plate with a double-faced carbon tape. The incident angle of ions was set as normal to the film surface. The flux was kept around 3.2 × 10 12 p/cm 2 ·s, corresponding to a beam current density of 500 nA/cm 2 . The surface temperature was monitored by a thermocouple. The surface temperature during the irradiation remained below 40 • C.
Prior to the ion irradiation experiment, we ran Stopping and Range of Ions in Matter (SRIM) [18] to estimate ion range and damage profile in our experiment. Based on the simulation results, 1 × 10 15 and 1 × 10 16 p/cm 2 are estimated to be~3.2 × 10 -5 and 3.2 × 10 -4 dpa (displacement per atm), respectively. Figure 1a,b compare the temperature dependence of magnetic moment M with H//c for two FST films (film-A and film-B) before and after irradiation with 1 × 10 15 and 1 × 10 16 p/cm 2 dose, respectively. Both the zero-field-cooled (ZFC) and field-cooled (FC) magnetizations in 2 Oe magnetic field parallel to the c-axis indicate the appearance of superconductivity (obtained by the bifurcation of ZFC and FC) in pristine FST films at 16.8 K for film-A and 16.6 K for film-B. After the irradiation, the superconducting transitions occurred at 16.8 K for film-A and 16.8 K for film-B, indicating that 1.5 MeV proton irradiations with 1 × 10 15 and 1 × 10 16 p/cm 2 dose have little impact on T c mag . However, the diamagnetic signal was enhanced with a sharper superconducting transition in the FST film-B irradiated with 1 × 10 16 p/cm 2 dose. A degradation of T c after the ion irradiation is commonly reported in iron-based superconductors [19], although there have been a few reports on an increased T c in iron-based superconductors irradiated with proton and electron [16,20,21]. In previous work, the Fe(Se,Te) films were covered by Al foil with 80 µm thickness and irradiated with 3.5 MeV protons at doses of 2.68 × 10 16 and 5.35 × 10 16 p/cm 2 , corresponding to 2.30 × 10 -3 and 4.59 × 10 -3 dpa, respectively [22][23][24]. The average bombarding energy of the protons on the Fe(Se,Te) film was calculated to be 1.43 ± 0.07 MeV. As a result, the irradiations to doses of 2.68 × 10 16 and 5.35 × 10 16 p/cm 2 slightly suppressed T c from 17.7 K for pristine film to 17.3 K and 17.1 K, respectively. Given these results, the primary reason of the almost same T c s before and after the irradiation in our study would be a lower fluence than that in the previous works. Figure 1a,b compare the temperature dependence of magnetic moment M with H//c for two FST films (film-A and film-B) before and after irradiation with 1 × 10 15 and 1 × 10 16 p/cm 2 dose, respectively. Both the zero-field-cooled (ZFC) and field-cooled (FC) magnetizations in 2 Oe magnetic field parallel to the c-axis indicate the appearance of superconductivity (obtained by the bifurcation of ZFC and FC) in pristine FST films at 16.8 K for film-A and 16.6 K for film-B. After the irradiation, the superconducting transitions occurred at 16.8 K for film-A and 16.8 K for film-B, indicating that 1.5 MeV proton irradiations with 1 × 10 15 and 1 × 10 16 p/cm 2 dose have little impact on Tc mag . However, the diamagnetic signal was enhanced with a sharper superconducting transition in the FST film-B irradiated with 1 × 10 16 p/cm 2 dose. A degradation of Tc after the ion irradiation is commonly reported in iron-based superconductors [19], although there have been a few reports on an increased Tc in iron-based superconductors irradiated with proton and electron [16,20,21]. In previous work, the Fe(Se,Te) films were covered by Al foil with 80 μm thickness and irradiated with 3.5 MeV protons at doses of 2.68 × 10 16 and 5.35 × 10 16 p/cm 2 , corresponding to 2.30 × 10 -3 and 4.59 × 10 -3 dpa, respectively [22][23][24]. The average bombarding energy of the protons on the Fe(Se,Te) film was calculated to be 1.43 ± 0.07 MeV. As a result, the irradiations to doses of 2.68 × 10 16 and 5.35 × 10 16 p/cm 2 slightly suppressed Tc from 17.7 K for pristine film to 17.3 K and 17.1 K, respectively. Given these results, the primary reason of the almost same Tcs before and after the irradiation in our study would be a lower fluence than that in the previous works.  Figure 2 shows the magnetic field dependence of Jc for the FST film-B at 5, 8, 10 K before and after 1.5 MeV proton irradiation at a dose of 1 × 10 16 p/cm 2 . The Jc was estimated from the magnetization hysteresis (M-H) loops using the critical-state Bean model [25,26]. For a rectangular prism-shaped crystal of dimensions a < b, we obtained the in-plane critical current density Jc ab in the magnetic field parallel to the c-axis as Jc ab = 20ΔM/(a(1 − a/3b)), where ΔM is the difference in magnetization M(emu/cm 3 ) between the top and bottom branches of the M-H loop. In the inset of Figure 2, the M-H loop in FST film-B at 5 K before and after the irradiation of a dose of 1 × 10 16 p/cm 2 is plotted. A large irreversibility is noticeable up to around 4 T at 5 K. We attained a 30% increase in Jc in the magnetic field below 1 T, which indicates that the irradiation defects contribute to vortex pinning. In contrast, we observed almost no change in the in-field Jc above 1 T. Irradiation with MeV protons could produce mostly random point defects and nanocluster [27] due to ion-nucleus collisions. Sylva et al. reported that 3.5 MeV proton irradiation with 6.40 × 10 16 p/cm 2 dose (corresponding to 2.27 × 10 -3 dpa) yields Jc improvement of about 40% at 4.2 K and 7  Figure 2 shows the magnetic field dependence of J c for the FST film-B at 5, 8, 10 K before and after 1.5 MeV proton irradiation at a dose of 1 × 10 16 p/cm 2 . The J c was estimated from the magnetization hysteresis (M-H) loops using the critical-state Bean model [25,26]. For a rectangular prism-shaped crystal of dimensions a < b, we obtained the in-plane critical current density J c ab in the magnetic field parallel to the c-axis as J c ab = 20∆M/(a(1 − a/3b)), where ∆M is the difference in magnetization M(emu/cm 3 ) between the top and bottom branches of the M-H loop. In the inset of Figure 2, the M-H loop in FST film-B at 5 K before and after the irradiation of a dose of 1 × 10 16 p/cm 2 is plotted. A large irreversibility is noticeable up to around 4 T at 5 K. We attained a 30% increase in J c in the magnetic field below 1 T, which indicates that the irradiation defects contribute to vortex pinning. In contrast, we observed almost no change in the in-field J c above 1 T. Irradiation with MeV protons could produce mostly random point defects and nanocluster [27] due to ion-nucleus collisions. Sylva et al. reported that 3.5 MeV proton irradiation with 6.40 × 10 16 p/cm 2 dose (corresponding to 2.27 × 10 -3 dpa) yields J c improvement of about 40% at 4.2 K and 7 T with respect to the pristine film almost without a decrease in T c [22]. On the contrary, J c of 3.5 MeV proton irradiated Fe(Se,Te) films covered with 80 µm thick Al foil decreased by up to 80% after irradiation at 4.2 K. The in-field J c performance in the irradiated FST films in our study could be attributed to the small number of vortex pinning defects created by the irradiation at low fluence.

Magnetic Measurements
T with respect to the pristine film almost without a decrease in Tc [22]. On the contrary, Jc of 3.5 MeV proton irradiated Fe(Se,Te) films covered with 80 μm thick Al foil decreased by up to 80% after irradiation at 4.2 K. The in-field Jc performance in the irradiated FST films in our study could be attributed to the small number of vortex pinning defects created by the irradiation at low fluence.

Transport Measurement
In transport measurements, the current is forced to flow through the sample in a particular direction, enabling the direct characterization of superconductivity as a function of temperature, applied magnetic field and field angle. However, we observed an obvious degradation of superconducting properties in the transport measurement of the FST film-B. This could be due to sample degradation, sample handling during mounting and unmounting in a measurement system and possible damage by the laser cutting for patterning the bridge on FST films. In this section, we refer to the FST film-A. Figure 3 presents the temperature dependence of the electrical resistivity before and after irradiation for FST film-A with 1 × 10 15 p/cm 2 dose of 1.5 MeV proton. The FST films before and after the irradiation showed metallic behavior below 200 K. Additionally, 1.5 MeV proton irradiation with 1 × 10 15 p/cm 2 dose has little effect on normal-state resistivity due to the low dpa. On the contrary, the normal-state resistivity shows nearly upwards parallel-shift upon 6 MeV Au-ion irradiation with a dose of 1 × 10 12 Au/cm 2 , corresponding to 6.42 × 10 -3 dpa [11]. We observed no change in Tc,10 (=17.5 K) before and after the 1.5 MeV protons irradiation with 1 × 10 15 p/cm 2 dose. This could be due to the low fluence, i.e., low dpa. Figure 4 presents the magnetic field dependence of transport critical current density Jc with H//c for the FST film-A before and after irradiation with 1.5 MeV protons to a dose of 1 × 10 15 p/cm 2 at 4.2 K. Comparing Jcs obtained from magnetization and transport measurements, the values of Jc obtained from transport measurement are larger than those of Jc calculated from magnetization measurement. This would come from the difference of criterion to determine the Jc values. The positive effect of the proton irradiation on Jc at 4.2 K is unambiguous in the magnetic field below 1 T. As the magnetic field increased, the difference between pristine and the irradiated FST film became smaller. Similar behavior was observed in Jc(H) (calculated from magnetization measurement in Figure 2) for FST film-B irradiated with 1 × 10 16 p/cm 2 dose.
A more detailed representation of the pinning efficiency can be obtained from the angular dependence of Jc. We show Jc(θ) for the FST film-A irradiated with 1 × 10 15 p/cm 2

Transport Measurement
In transport measurements, the current is forced to flow through the sample in a particular direction, enabling the direct characterization of superconductivity as a function of temperature, applied magnetic field and field angle. However, we observed an obvious degradation of superconducting properties in the transport measurement of the FST film-B. This could be due to sample degradation, sample handling during mounting and unmounting in a measurement system and possible damage by the laser cutting for patterning the bridge on FST films. In this section, we refer to the FST film-A. Figure 3 presents the temperature dependence of the electrical resistivity before and after irradiation for FST film-A with 1 × 10 15 p/cm 2 dose of 1.5 MeV proton. The FST films before and after the irradiation showed metallic behavior below 200 K. Additionally, 1.5 MeV proton irradiation with 1 × 10 15 p/cm 2 dose has little effect on normal-state resistivity due to the low dpa. On the contrary, the normal-state resistivity shows nearly upwards parallel-shift upon 6 MeV Au-ion irradiation with a dose of 1 × 10 12 Au/cm 2 , corresponding to 6.42 × 10 -3 dpa [11]. We observed no change in T c,10 (=17.5 K) before and after the 1.5 MeV protons irradiation with 1 × 10 15 p/cm 2 dose. This could be due to the low fluence, i.e., low dpa.  Figure 5. The pristine film has a less-anisotropic Jc angular dependence at 1 and 3 T without a prominent Jc peak at H//c, which is often observed in YBa2Cu3Oy films [28]. A small Jc-anisotropy, γJc (Jc H//ab /Jc H//c ), of 1.7 is observed at 1 T. This value is smaller than the value of Fe(Se,Te) films grown on Febuffered MgO substrates (γJc = 2.6) [29] while it is larger than the value of Fe(Se,Te) films grown on CaF2 substrates [30,31]. These differences might arise from the difference of the substrate and buffer layer. Upon irradiation with 1.5 MeV proton, the Jc increases for most of the field orientations, retaining a small γJc of 1.7 at 1 T, indicating that the vortex pinning defects would be less anisotropic and randomly distributed. At 3 T, there is a significant decrease in Jc in the angular range ±30° from H//ab. Iron-based and cuprate high-temperature superconductors commonly possess inherent layered structures, consisting of alternating conducting and insulating atomic planes. In general, the strong Jc peak for H//ab could be ascribed to the vortex pinning by the intrinsic pinning and planar defects such as intergrowths and stacking faults, parallel to the ab plane [32][33][34][35]. In the iron-chalcogenide Fe(Se,Te) compound, which is composed of only the Fe-Se(Te) layer, Jc(θ) has a maximum at H//ab due to intrinsic pinning from the Fe-Se(Te) intralayer and Van der Waals interlayer couplings [29,34,35]. Hence, the Jc suppression at around H//ab would occur because of the reduction in the density of intrinsic pinning upon the irradiation.   Figure 4 presents the magnetic field dependence of transport critical current density J c with H//c for the FST film-A before and after irradiation with 1.5 MeV protons to a dose of 1 × 10 15 p/cm 2 at 4.2 K. Comparing J c s obtained from magnetization and transport measurements, the values of J c obtained from transport measurement are larger than those of J c calculated from magnetization measurement. This would come from the difference of criterion to determine the J c values. The positive effect of the proton irradiation on J c at 4.2 K is unambiguous in the magnetic field below 1 T. As the magnetic field increased, the difference between pristine and the irradiated FST film became smaller. Similar behavior was observed in J c (H) (calculated from magnetization measurement in Figure 2) for FST film-B irradiated with 1 × 10 16 p/cm 2 dose.  A more detailed representation of the pinning efficiency can be obtained from the angular dependence of J c . We show J c (θ) for the FST film-A irradiated with 1 × 10 15 p/cm 2 dose of 1.5 MeV proton beam under 1 and 3 T at 4.2K in Figure 5. The pristine film has a less-anisotropic J c angular dependence at 1 and 3 T without a prominent J c peak at H//c, which is often observed in YBa 2 Cu 3 O y films [28]. A small J c -anisotropy, γ Jc (J c H//ab /J c H//c ), of 1.7 is observed at 1 T. This value is smaller than the value of Fe(Se,Te) films grown on Fe-buffered MgO substrates (γ Jc = 2.6) [29] while it is larger than the value of Fe(Se,Te) films grown on CaF 2 substrates [30,31]. These differences might arise from the difference of the substrate and buffer layer. Upon irradiation with 1.5 MeV proton, the J c increases for most of the field orientations, retaining a small γ Jc of 1.7 at 1 T, indicating that the vortex pinning defects would be less anisotropic and randomly distributed. At 3 T, there is a significant decrease in J c in the angular range ±30 • from H//ab. Iron-based and cuprate high-temperature superconductors commonly possess inherent layered structures, consisting of alternating conducting and insulating atomic planes. In general, the strong J c peak for H//ab could be ascribed to the vortex pinning by the intrinsic pinning and planar defects such as intergrowths and stacking faults, parallel to the ab plane [32][33][34][35]. In the iron-chalcogenide Fe(Se,Te) compound, which is composed of only the Fe-Se(Te) layer, J c (θ) has a maximum at H//ab due to intrinsic pinning from the Fe-Se(Te) intralayer and Van der Waals interlayer couplings [29,34,35]. Hence, the J c suppression at around H//ab would occur because of the reduction in the density of intrinsic pinning upon the irradiation.

Conclusions
We conclude a study on the effect of 1.5 MeV proton irradiation on superconducting properties of FST films. Upon the irradiation up to 1 × 10 16 p/cm 2 dose, Tc remains virtually unchanged in magnetization as well as in transport measurement. An approximately 30% enhancement of Jc in the magnetic field below 1 T is observed using 1.5 MeV proton irradiation with 1 × 10 16 p/cm 2 . Transport properties of a pristine film and an irradiated film with a fluence of 1 × 10 15 p/cm 2 show a small anisotropy of Jc in the applied magnetic field range at 4.2 K. The enhancement of Jc for almost all the field orientations was accomplished by the irradiation at a dose of 1 × 10 15 p/cm 2 at 4.2 K and 1 T. These results indicate that 1.5 MeV proton irradiation is effective in providing less anisotropic pinning defects in the magnetic field below 1 T in iron-chalcogenide superconducting films. Additionally, by fine tuning an irradiation fluence of proton, superconducting properties can be further improved.

Conclusions
We conclude a study on the effect of 1.5 MeV proton irradiation on superconducting properties of FST films. Upon the irradiation up to 1 × 10 16 p/cm 2 dose, T c remains virtually unchanged in magnetization as well as in transport measurement. An approximately 30% enhancement of J c in the magnetic field below 1 T is observed using 1.5 MeV proton irradiation with 1 × 10 16 p/cm 2 . Transport properties of a pristine film and an irradiated film with a fluence of 1 × 10 15 p/cm 2 show a small anisotropy of J c in the applied magnetic field range at 4.2 K. The enhancement of J c for almost all the field orientations was accomplished by the irradiation at a dose of 1 × 10 15 p/cm 2 at 4.2 K and 1 T. These results indicate that 1.5 MeV proton irradiation is effective in providing less anisotropic pinning defects in the magnetic field below 1 T in iron-chalcogenide superconducting films. Additionally, by fine tuning an irradiation fluence of proton, superconducting properties can be further improved.