1. MgB2 Thin Films for Doping and Flux Pinning
M.A. Susner
M.D. Sumption
and E.W. Collings

2. Acknowledgements
This work is funded by the Unites States Department of Energy, Office of High Energy Physics, The Ohio State University Institute for Materials Research, the AFRL, and the State of Ohio. A portion of this work derives from data collected at the ENCOMM facility at Ohio state as well as at the NHMFL in Florida

3. Chemical and Structural Modifications to MgB2
Discovered to be superconducting 12 years ago, MgB2 has been the subject of much investigation with respect to its two-gap nature, increasing the value of Bc2 by chemical substitution, or by increasing electrical connectivity through decreasing the amount of deleterious “supercurrent blocking” impurities
A more subtle, refined approach is needed to create MgB2 with high critical current densities at high magnetic fields:
New and innovative ways to introduce dopants and/or second phases for pinning
Investigation of the formation of heterostructures with isostructural materials to increase flux pinning or activate new pinning type
High pressure/ high temperature synthesis to simplify superconductive characterization and to understand the fundamental limits of changing the chemistry of MgB2

4. MgB2 thin films Focusing lens KrF excimer laser, λ=248 nm
to turbo
Focusing lens
KrF excimer laser,
λ=248 nm
MgB2 target mounted on rotating platform
Resistive heater (1050oC max.)
SiC (0001) substrate
Nom. 25 ns laser pulse, λ=248 nm
400 mJ max. energy (typically >10 J/cm2 energy density) impinges upon the MgB2 target, producing significant, rapid heating of target material
A laser-induced plasma forms on the target surface
Plasma forwardly and supersoniacally ejected from interaction volume, forms “plume”
Plume impinges upon the substrate, depositing plume species on surface

5. PLD synthesis routes
PLD synthesis routes for MgB2

6. Growth of Undoped MgB2 thin films
We have created granular MgB2 thin films with a three-step process: (1) deposition of MgB2, (2) deposition of Mg cap layer, and (3) anneal at ~700oC at 1 atm highly purified Ar
Process optimized to drive up Tc to maximum seen in PLD-synthesized MgB2 films

7. Glancing X-ray analysis of MgB2 thin films
MgB2 thin films are deposited on SiC substrates. They are c-axis oriented and are randomly oriented in-plane

8. TEM of Undoped MgB2 SiC MgB2 Pt
MgB2 thin films are ~200 nm thick with an interface layer of Mg2Si formed by reaction with the SiC substrate
MgB2
200 nm Pt

9. ZrB2, TiB2, and other additions via PLD
We have worked to develop high quality MgB2 thin films by Pulsed Laser Deposition (PLD) to understand transport and upper critical field limits in MgB2 materials (Superconductors with Tcs of 39 K and Bc2s from T)
MgB2 thin films difficult to fabricate due to high Mg vapor pressure and large chemical differences between Mg and B
Also, the proclivity of either species (especially the Mg) to form an oxide necessitates careful control of impurities
Several methods have been used in the literature: MBE, HPCVD, and PLD

10. Addition of ZrB2 by Superstructure formation + annealing
ZrB2/MgB2
Nominal %ZrB2
Tc,onset, K
(magnetic)
MgB2-164
30.4
MgB2-165
5/200
2.4
25.3
MgB2-166
10/200
4.8
22.5
MgB2-167
20/200
9.1
18.1
MgB2-168
30/200
13.0
15.0
MgB2-169
50/200
20.0
9.99
MgB2-172
70/200
25.9
5.88

11. Magnetic Superconductive Properties
B//c axis
See monotonic decrease in Tc with increasing Zr addition
Also see change in dBc2/dT slope, indicating some doping effect
Small amounts of Zr expected to be beneficial at low T (probable crossover)

12. Structural Analysis of Zr addition
Grazing incident diffraction (both in-plane and out-of plane) used to elucidate structure
a-lattice parameter increases with amount of ZrB2 added
c parameter more or less constant

13. STEM-EDS analysis Mg
EDS
Region 2
EDS
Region 3
MgB2
SiC
50 nm
STEM-EDS analysis shows the presence of Zr in the MgB2 grains
Also, a concentration gradient from sample surface to the SiC interface

14. Resistively measured data
Resistively measured data shows high values of Bc2 together with a large reduction in anisotropy with increasing Zr addition
B//ab plane

15. Further considerations and new directions
Using either PLD we can investigate the limits of chemical substitutions in MgB2
What is needed is a thoughtful, considered approach with respect to the structure of MgB2 , i.e. the formation of MgB2 heterostructures to increase properties like flux pinning
By combining the fundamental structural and chemical lessons of high pressure, high temperature synthesis developed at OSU (see Bohnenstiehl et al.) with the non equilibrium synthesis procedure of PLD, we can create heterostructures similar to those seen in 2G HTS for enhancing the superconductive properties of MgB2
What kind of interesting properties/materials can be found?
Let’s take a look at some previous work done by our group in collaboration with AFRL, in this case in YBCO

16. Heterostructures and flux pinning in high-T Superconductors
High Tc superconductors like YBa2Cu3O7 need nano-scale defects to enhance the pinning of fluxons for high J, high B applications
Anything in which the YBCO lattice is disturbed over a nm scale has been suggested (e.g. precipitates, dislocations)
The most successful experiments involve the formation of heterostructures:
Layered and 3D random nano dispersions that create stress fields
Self assembled nano-columns
T.Haugan, P.N. Barnes, R. Wheeler, F. Meisenkothen, and M.D. Sumption, Nature 430, (2004)
35 layer YBa2Cu3O7/ Y2BaCuO5 nano-dispersion array
50 nm
Various additives induce the formation of different heterostructure morphologies
T.Haugan, F.Baca, M. Mullins, N.Pierce, T. Campbell, E. Brewster, P. Barnes, H. Wang, and M.D. Sumption, IEEE Trans. Appl. Supercond. 19(3), (2009)

17. YBCO heterostructures
Nano dispersions:
Generally occur when strain is large
e.g. Y2BaCuO5 (Y211) in YBa2Cu3O7 (Y123)
lattice mismatch a211/2x(a,b)avg-123=-7.4%;
c211, rip-45deg/(a,b)avg-123=+4.5%; b211/c123= +4.5%
T.Haugan, P.N. Barnes, R. Wheeler, F. Meisenkothen, and M.D. Sumption, Nature 430, (2004)
50 nm
Nano columns and nano dots:
Generally occur when strain is smaller
e.g. BaZrO3 in YBa2Cu3O7 (Y123)
Epitaxial nano columns: aBZO//cYBCO
Randomly oriented nano dots: random orientation relationships
Lattice strain: can be engineered by the amount of BZO present which distorts the orthorhombic lattice with distortions ranging from 0.8-2%
A. Goyal et al. Supercond. Sci. Tech. 18, (2005)

18. Possible Additions for MgB2 heterostructure synthesis
Space group
a (Å)
c (Å)
Δa, % Ti
P63/mmc
2.951
4.684
-4.38
AlB2
P6/mmm
3.009
3.262
-2.46
TiB2
3.038
3.239
-1.52
MgB2
3.085
3.523 --
NbB2
3.111
3.266
0.84
UB2
3.130
3.988
1.46
ZrB2
3.169
3.531
2.72 Zr
3.230
5.145
4.70
CdS
P63mc
4.142
6.724
34.3
Ag (along (111) plane)
Fm-3m
64.86
Possibility of impurity dispersion
Large amount of materials available in which to modify the effect of strain on the resulting microstructures-
Possibility of complex heterostructure formation
Difficult to find materials that are chemically compatible with MgB2
Some targets are already made or purchased (Ti, TiB2, MgB2, ZrB2); others will be synthesized via high-pressure, high T apparatus
2400oC Tmax at ambient Pressure,
1700oC at 10 MPa (1500 psi)

19. Questions?
Thank you!