Matti Irjala

Modeling flux pinning in thin undoped and BaZrO3-doped YBCO films

A simple model based on distributions of twin boundaries, dislocations, and BaZrO3 nanorods is presented to describe the Jc properties of undoped and BaZrO3 (BZO)-doped YBa2Cu3Ox thin films. The model accurately describes the shape of Jc(B,T) curves of the films, when the pinning site distributions are taken from distributions of twin spacings and BZO nanorods from transmission electron microscope images. Thus, assuming that the model can be used for prediction of the Jc properties, we conclude that for enhancement of undoped films more crystalline defects are needed and for doped films a dopant that would create slightly larger rods would be optimal.

Greatly decreased critical current density anisotropy in YBa2Cu3O6+x thin films ablated from nanocrystalline and BaZrO3-doped nanocrystalline targets

The anisotropy of the critical current density Jc was determined from angular critical current measurements for YBa2Cu3O6+x (YBCO) thin films laser deposited from a nanograined and a BaZrO3-doped nanograined target. In quantitative analysis using Blatter scaling the effective Jc anisotropy was found to be much smaller than for standard YBCO films.

Influence of BaZrO3 dopant concentration on properties of YBa2Cu3O6+x films in magnetic fields up to 30 T

The effect of BaZrO3 (BZO) dopant concentration on superconducting properties in YBa2Cu3O6+x thin films is investigated in a wide magnetic field range up to 30 T. Based on the magnetization and resistivity measurements, the optimal BZO concentration for flux pinning is found to be 4% up to 20 T and increasing up to around 8% in higher fields. This result is qualitatively explained by a model where more BZO is needed in order to organize optimal columnar defect structure at high fields because the amount of single BZO particles acting as isotropic point pinning centers is increased, especially in high BZO concentrations. This means that for applications of thin film or coated conductor structures carried out at high magnetic fields, even higher dopant concentration than used in a low-field range should be produced in order to attain the optimal flux pinning conditions.

Optimization of the

A series of studies has been carried out on thin films prepared by pulsed laser deposition (PLD) from micro-grained targets of YBa₂Cu₃O_7-<i>x</i> (YBCO) to determine the effect of doping them with BaCeO₃ (BCO) in concentrations between 0-10 wt%. BCO has been found to increase <i>J</i>_c in e.g. melt-textured and bulk samples, but no systematic study with PLD thin films has been performed. Magnetometric measurements up to 8 T and down to 10 K as well as structural measurements with XRD and AFM were performed. <i>J</i>_c increased with doping and the highest <i>J</i>_c s were achieved with low dopant levels (2% and 4%), which varied according to the external field and temperature. <i>T</i>_c decreased when the dopant concentration was increased, similarly as in BaZrO₃ (BZO) doped films. However, the BCO growth process in the films was found to be different from the BZO and, furthermore, an interesting effect where the YBCO twin structure is altered was found around 6 wt% of BCO. The <i>J</i>_c results and a comparison with structural properties with increasing dopant level and optimally BZO-doped YBCO thin films are presented.

{\rm BaCeO}_{3}

The influence of the BaZrO₃ (BZO) dopant concentration on structural and superconducting properties of YBa₂Cu₃O₆₊<i>x</i> (YBCO) thin films deposited on single crystal SrTiO₃ (STO) and buffered NiW metal substrates is investigated. Depending on the applied temperature and external magnetic field, the optimal BZO content <i>n</i>_opt is found to vary so that irrespective of substrate used, in higher magnetic fields also higher <i>n</i>_opt is needed. Moreover, the <i>n</i>_opt of 4wt.% in YBCO films grown on STO is increased up to 7.5wt.% in films on NiW, especially in fields above 1 T. This phenomenon is qualitatively explained by the difference in size, shape and the distribution of BZO pinning centers and other structural disorders in the YBCO matrix. The results substantially affect to the development of the optimal flux pinning structure in future applications of high-temperature superconductors.

Concentration in YBCO Films Prepared by Pulsed Laser Deposition

A comparative study has been performed on Pr-doped Y1-x PrxBCO (x=0-0.20) thin films deposited by pulsed laser deposition on MgO and buffered NiW substrates to study the effect of Pr-doping on the grain boundary critical current density (Jcgb). Our earlier work on bulk materials and SrTiO3 substrates indicated that, whereas Pr increases Jc in bulk samples, it does not increase Jc in film samples without grain boundaries. In this work, we present increased Jc in low concentrations of Pr3+-doping (x< 0.04) at temperatures above 60 K in film samples on MgO substrates and at all temperatures and fields in film samples on buffered NiW substrates. Results indicate that Pr segregates into grain boundary regions, improving the local hole concentration and carrier density, hence, increasing Jcgb.

Optimal BZO Doping in YBCO Films Grown on Single Crystal STO and Buffered NiW Substrates

The accommodation field, B* , is generally defined to be the field at which the cross over from single vortex pinning to collective pinning occurs. It is determined from magnetization curves as the point where the J c plateau ends and it used as a convenient way of comparing the pinning properties of superconducting films. Similarly, the characteristic field, B c, can be obtained from magneto-optical (MO) images from when the flux fronts meet in the middle of the film. The matching field, B phi, at which there is one vortex line per pinning site, is sometimes thought to be the same as B*, but in BaZrO3-doped YBa2Cu3O7 films the calculated B phi is much higher than the observed B*. B phi can be determined from angular dependent transport measurements. All of the field values correspond to some special case in the flux pinning in the film and relate to J c. In this work we have determined B c, B* and B phi for different kinds of YBCO films using MO, magnetization and transport measurements to reveal the deeper meaning of the special fields.

Increased grain boundary critical current density Jcgb by Pr-doping in pulsed laser–deposited Y1−xPrxBCO thin films

Superconducting YBa2Cu3O6+x (YBCO) films are grown by pulsed laser deposition (PLD) in the orientation where its c-axis is lying in the plane of the film using (110) oriented SrTiO3 (STO) substrates with PrBCO template layer. By optimization of deposition temperature and rate the easily formed (103) orientation was avoided to get smooth and completely (110) oriented YBCO films. To improve the superconducting properties, we have also optimized the BaZrO3 (BZO)-doping in the (110) YBCO films. This gives the possibility to apply the current along the YBCO CuO2 planes where the critical current is much higher in thin film multilayers or to investigate the anisotropy and the effect of BZO pinning structure in different film orientations.

Defining , and for YBCO Thin Films

The accommodation field, B* , is generally defined to be the field at which the cross over from single vortex pinning to collective pinning occurs. It is determined from magnetization curves as the point where the J c plateau ends and it used as a convenient way of comparing the pinning properties of superconducting films. Similarly, the characteristic field, B c, can be obtained from magneto-optical (MO) images from when the flux fronts meet in the middle of the film. The matching field, B phi, at which there is one vortex line per pinning site, is sometimes thought to be the same as B*, but in BaZrO3-doped YBa2Cu3O7 films the calculated B phi is much higher than the observed B*. B phi can be determined from angular dependent transport measurements. All of the field values correspond to some special case in the flux pinning in the film and relate to J c. In this work we have determined B c, B* and B phi for different kinds of YBCO films using MO, magnetization and transport measurements to reveal the deeper meaning of the special fields.

Effect of

The accommodation field, B* , is generally defined to be the field at which the cross over from single vortex pinning to collective pinning occurs. It is determined from magnetization curves as the point where the J c plateau ends and it used as a convenient way of comparing the pinning properties of superconducting films. Similarly, the characteristic field, B c, can be obtained from magneto-optical (MO) images from when the flux fronts meet in the middle of the film. The matching field, B phi, at which there is one vortex line per pinning site, is sometimes thought to be the same as B*, but in BaZrO3-doped YBa2Cu3O7 films the calculated B phi is much higher than the observed B*. B phi can be determined from angular dependent transport measurements. All of the field values correspond to some special case in the flux pinning in the film and relate to J c. In this work we have determined B c, B* and B phi for different kinds of YBCO films using MO, magnetization and transport measurements to reveal the deeper meaning of the special fields.