APPLIED PHYSICS LETTERS
VOLUME 72, NUMBER 12
23 MARCH 1998
Synthesis of high-temperature superconductive and colossal
magnetoresistive surfaces on insulating particles
D. Kumar,a) James Fitz-Gerald, and Rajiv K. Singh
Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611-6400
~Received 23 October 1997; accepted for publication 23 January 1998!
The surfaces of insulating alumina particles have been coated with high-temperature
superconducting YBa2Cu3O72x and colossal magnetoresistive Pr0.65Ba0.05Ca0.3MnO32x films.
These coatings on particulate surfaces have been realized using a technique which is based on
laser-assisted generation of a homogeneous flux of ablated materials in front of a fluidized bed of
host particles. The coated particulates have been characterized using scanning electron microscopy,
energy dispersive x-ray analysis, Auger electron spectroscopy, and superconducting quantum
interference device magnetometer. © 1998 American Institute of Physics.
@S0003-6951~98!00512-9#
High-temperature superconductors ~HTSCs! and colossal magnetoresistance ~CMR! materials have attracted unprecedented research efforts in the area of electronic materials during the last decade.1–3 The structures of both of these
materials are pervoskite-like.4,5 The superconducting properties of HTSCs are associated with Cu–O chains and planes6
whereas the magnetoresistive properties of CMR materials
are associated with Mn–O planes.7 Particulate systems
coated with HTSC and CMR materials having unique functional properties are well suited for electrical applications
that demand both good magnetic and transport properties.8,9
It may be noted here that although the nanoparticles themselves may be equally suitable for applications, the synthesis
of nanoparticles of these multielemental compounds is very
expensive and complex.8 Therefore, it appears to be more
practical to coat commercially available nanoparticles such
as alumina, which are core particles in the present study,
with materials of interest to realize the desired nanofunctional properties. For example, superconducting surfaces on
particulate materials can be used in electromagnetic shielding and particulates coated with colossal magnetoresistive
material have great potential in ultra-high-density data storage due to their extremely high sensitivity in magnetic fields.
Particulates coated with these materials can also be excellent
soft magnets and yet, because electrons are scattered at the
grain boundaries, they would have relatively high electrical
resistivity, which may result in reduced eddy current losses
in electrical applications.
In this letter, we report a method to synthesize insulating
particulates
with
high-temperature
superconducting
YBa2Cu3O72x ~YBCO! and colossal magnetoresistive
Pr0.65Ba0.05Ca0.3MnO32x ~PBCMO! surfaces. As indicated
by their unit chemical formulae, both of these materials involve multielements. The reproduction of the exact stoichiometry in the coating is very difficult if more commonly
used techniques such as sputtering, thermal evaporation, or
e-beam evaporation are applied.10,11 It is in this context that
we have used a nanosecond pulsed laser, which has emerged
as the most successful technique to deal with the problems
associated with multielemental stoichiometry and homogeneous distribution of the condensate on the substrate
materials.12 The results obtained in the present work have, in
fact, shown that not only the stoichiometry of the elements in
coatings are similar to that of target but the overall structural
and physical properties of the coatings are also very good.
The coating experiments were conducted in a vacuum
chamber where an oxygen partial pressure of 200–300
mTorr was maintained during the coating process. In-house
fabricated YBCO and PBCMO bulk targets were used for
coating alumina particles with superconductive and magnetoresistive materials, respectively. A small amount of Ag ~5
wt %! was added to the YBCO target as Ag addition was
found to improve the superconducting properties of the target as well the films prepared using that target.13 The alumina particles were placed in an aluminum crucible in front
of the laser-induced plume and were constantly fluidized
during the coating process using a mechanical vibration
method. The schematic of the coating process is shown in
Fig. 1. In the present work, all the coating experiments have
been done using 10 mm size alumina particles. The coating
thickness was controlled by the number of the laser pulses
and the core particle size used in the process. Alumina particles coated with YBCO and PBCMO were annealed at
a!
Electronic mail: dhkum@mse.ufl.edu
0003-6951/98/72(12)/1451/3/$15.00
FIG. 1. Schematic diagram of laser-assisted particulate coating process.
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© 1998 American Institute of Physics
1452
Appl. Phys. Lett., Vol. 72, No. 12, 23 March 1998
Kumar, Fitz-Gerald, and Singh
FIG. 2. ~a! Scanning electron micrograph of YBCO/alumina particles and
~b! EDX spectrum obtained from a single YBCO/alumina particle whose
scanning electron micrograph is shown in ~a!.
900 °C for half an hour in an oxygen ambient. The microstructure of the coatings were subsequently studied using
scanning electron microscopy, x-ray diffraction ~XRD!, energy dispersive x-ray ~EDX! analysis, and Auger electron
spectroscopy ~AES!. The superconducting and magnetic
characteristics of the coating materials were determined using a superconducting quantum interference device magnetometer.
Shown in Fig. 2~a! is the scanning electron micrograph
of alumina particles coated with YBCO superconducting material. The EDX spectrum @Fig. 2~b!#, obtained from a single
alumina particle coated YBCO superconductor, shows approximately a correct stoichiometry of metallic species in the
coating. Based on AES depth profile measurements, the average thickness of the coating was found to be approximately
30 nm. Microstructural characterization carried out on
PBCMO/alumina particles have shown that the nature of PBCMO coating on alumina from stoichiometry and thickness
point of view was similar to that of YBCO coating on alumina particles. Due to the weight of the coating materials
~HTSC and CMR! being smaller than the detection limit of
the XRD machine, XRD measurements carried out on the
coated particles have not yielded concrete phase-related information.
The magnetic susceptibility measurements carried out on
YBCO/alumina showed that the critical temperature for transition to the superconducting state was 92 K @Fig. 3~b!#.
Figure 3~c! shows the magnetic hysteresis loop from YBCO/
alumina composite particles measured at 10 K. The critical
FIG. 3. ~a! Ac susceptibility of YBCO/alumina particles as a function of
temperature showing a T c 592K of the YBCO coating and ~b! magnetic
hysteresis loop of YBCO/alumina particles at 10 K.
current density (J c ) of the YBCO/alumina particles was calculated using the Beans formula: DM 5J c t/30, where DM is
the change in magnetization per unit volume and t is diameter of the sphere.14 The calculated value of J c was found to
be ;6.03106 A/cm2 at 10 K. In determination of J c , the
volume of the YBCO coating ~required to estimate DM per
unit volume! was calculated assuming the coating to be uniform and the core alumina particles to be perfectly spherical.
Though a possible combination of nonuniformity in the coating and deviation in the shape of alumina particles from a
perfect sphere may cause a slight error in the estimation of J c
using Bean’s model, this seems to be the most suitable
method available for the estimation of magnetic J c in such
cases. Nevertheless, the realization of high T c and J c clearly
indicates that the quality of the YBCO coating on alumina
particles is very good.
When a magnetic field is applied to the PBCMO material, the spins of electrons in one Mn–O layer become parallel to spins in the other Mn–O layer, and consequently,
magnetization increases. Shown in Fig. 4~a! is the variation
of magnetization of PBCMO/alumina particles as a function
of temperature. This figure provides evidence of ferromagnetic behavior of this phase at low temperature with a transition to the paramagnetic phase around 100 K and saturation
Appl. Phys. Lett., Vol. 72, No. 12, 23 March 1998
Kumar, Fitz-Gerald, and Singh
1453
zero and applied fields, the magnetoresistance ratio ~MR!,
DR/R H , of the PBCMO film was calculated and the same is
plotted in Fig. 4~b!. As seen in this figure, a colossal MR
ratio of the order of 109 % has been obtained. It is very
interesting to note from this figure that an application of
magnetic field causes such a huge change in the film’s resistance. This feature of the materials is extremely desirable for
the fabrication of sensitive magnetic sensors and magnetic
read heads.
In summary, a method has been developed to synthesize
particulate materials with superconducting and magnetoresistive surfaces. The efficacy of the process has been demonstrated by coating alumina particles with multielement highT c superconducting ~YBa2Cu3O72x ) and colossal magnetoresistive (La0.65Ba0.05Ca0.3MnO32x materials.
The funding for this research from the National Science
Foundation ~Grant No. 94-02989! and the Department of Energy ~Grant No. DE-FG 05-95ER45533! are gratefully acknowledged.
1
FIG. 4. ~a! Magnetization of PBCMO/alumina particles as a function of
temperature at 0.5 T. ~b! Resistance as function of temperature of a 1000 Å
thick PBCMO film on a ~100! LaAlO3 substrate in 0 and 5 T magnetic field.
Shown also in this figure is the variation of the MR ratio as a function of
temperature.
magnetization around 60 K. Since a direct measurement of
magnetotransport property of PBCMO/alumina particles is
not possible due to noncontinuous coating, we measured the
magnetotransport property of a PBCMO film deposited on a
LaAlO3 flat surface. Figure 4~b! shows the variation of resistance of the PBCMO film as a function of temperature in
zero and applied fields ~5 T!. Using the resistance values in
D. L. Lowndes, D. B. Geohegan, A. A. Purretsky, D. P. Norton, and C. M.
Rouleau, Science 273, 898 ~1996!.
2
S. Jin, T. H. Tifel, M. McCormack, R. A. Fastnatch, R. Ramesh, and L. H.
Chen, Science 264, 413 ~1994!.
3
R. K. Singh and D. Kumar, Mater. Sci. Eng. Rep. ~in press!.
4
J. E. Greedan, A. O’Reilly, and C. V. Stager, Phys. Rev. B 35, 6716
~1987!.
5
E. W. Wollan and W. C. Koehler, Phys. Rev. 100, 545 ~1955!.
6
W. I. F. David, W. T. A. Harrison, J. M. F. Gunn, O. Moze, A. K. Soper,
P. Day, J. D. Jorgensen, D. G. Hinks, M. A. Beno, L. Soderholm, D. W.
Capone II, I. K. Schuller, C. U. Serger, K. Zhang, and J. D. Grace, Nature
~London! 327, 310 ~1987!.
7
G. H. Jonker and J. H. Van Santen, Physica ~Amsterdam! 16, 337 ~1950!.
8
J. Ouellette, Ind. Physicist 3, 15 ~1997!.
9
M. Naito, A. Kondo, and T. Yokoyama, Iron Steel Int. J. 33, 915 ~1993!.
10
Y. Kousaka, Y. Endo, M. Alsono, H. Ichitoubo, and A. Fukui, Adv. Powder Technol. 6, 11 ~1995!.
11
I. Saito and M. Sena, Kona 13, 191 ~1993!.
12
R. K. Singh and J. Narayan, Phys. Rev. B 43, 8843 ~1990!.
13
D. Kumar, M. Sharon, R. Pinto, P. R. Apte, S. P. Pai, S. C. Purandare, L.
C. Gupta, and R. Vijayaraghavan, Appl. Phys. Lett. 62, 3522 ~1993!.
14
C. P. Bean, Rev. Mod. Phys. 36, 31 ~1964!.