Does magnetization in thin-film manganates suggest the existence
of magnetic clusters?
J. Z. Sun,a) L. Krusin-Elbaum, A. Gupta, and Gang Xiaob)
IBM T. J. Watson Research Center, Yorktown Heights, New York 10598
S. S. P. Parkin
IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120
~Received 29 April 1996; accepted for publication 30 May 1996!
We report results of quantitative magnetization studies on epitaxial thin films of
La0.60Y0.07Ca0.33MnO32 d and La12x h x MnO32 d , with h standing for vacancy on the La site. The
magnetization in all samples shows significant field dependent broadening of the ferromagnetic
transition. Classical mean-field analysis of magnetization data in applied fields above a few Tesla
leads to a magnetic cluster size of the order of a few unit cells. The density of such a magnetic
cluster correlates with the doping concentration x in the compound series La12x h x MnO32 d . In thin
films of La0.60Y0.07Ca0.33MnO32 d , a magnetic field dependent, non-Curie–Weiss susceptibility was
found in the paramagnetic state, indicating local ferromagnetic cluster formation above macroscopic
ferromagnetic ordering temperature. © 1996 American Institute of Physics.
@S0003-6951~96!01533-1#
One important aspect of the ferromagnetic La–Ca–
Mn–O material is the strong field dependence of their magnetic properties. In an applied field of several tesla, the ferromagnetic transition broadens significantly, the lowtemperature saturation magnetic moment increases, and a
large reduction of electrical resistivity appears.1–4 Von Helmolt et al.5 have suggested, based on Zener’s double exchange picture,6–9 that magnetic inhomogeneity around cation doping sites might locally create a ferromagnetic
arrangement of Mn spins. Possible existence of magnetic polarons, mediated by the strong Jahn–Teller phonons, was
suggested by Millis et al.10,11 Recent neutron diffraction data
also point to the existence of magnetic inhomogeneity over
the length scale of tens to hundreds of angstroms.12,13 This
length scale leads to a ferromagnetic cluster size of the order
of tens to hundreds of effective Mn moment, and could
therefore give the observed field dependence.
In this letter, we report some systematic trends found in
the magnetic properties of epitaxial manganate thin films.
Results from two types of films are presented. One type is
the compound La0.60Y0.07Ca0.33MnO32 d , 4,14 another type the
self-doped La12x h x MnO32 d , with h standing for vacancy
on the La site.15 Our measurements and analysis point to a
plausible picture where nanoclusters of ferromagnet exist,
with the cluster density of the same order as the dopant concentration. For La0.60Y0.07Ca0.33MnO32 d , these ferromagnetic clusters start to form around 200 K, above the macroscopic Curie temperature which lies somewhere around 120
K at low fields.
Thin films used for this study were in situ grown on
SrTiO3 ~100! substrates using laser deposition. A typical set
of deposition parameters include a substrate temperature of
700 C, an oxygen pressure of 300 mTorr, and a laser fluence
of 3 J/cm2 . Films are about 1000 Å thick. X-ray diffraction
confirms the nature of c-axis orientation of the resulting film,
a!
Electronic mail: jonsun@watson.ibm.com
Also at Physics Department, Brown University, Providence, RI 02912.
b!
with a c-axis lattice constant of c/253.87Å for
La0.60Y0.07Ca0.33MnO32 d , and c/253.89 and 3.86 Å, respectively for La12x h x MnO32 d at x50 and x50.15.15 No postdeposition heat treatment was performed. Transport properties of these films have been published elsewhere.4,15
Magnetization of the films was measured using a Quantum
Design MPMS superconducting quantum interference device
~SQUID! magnetometer. The applied field was parallel to the
film surface and along the ~100! direction of the SrTiO3 substrate.
To analyze the magnetization data a simple classical
mean-field expression16 was used, in which
M
5L ~ a !
MS
~1!
with
L ~ a ! 5coth~ a ! 21/a
~2!
and
a5
TC M
mH
13
.
k BT
T MS
~3!
Three independent parameters are needed for the construction of the model: M S , the mean-field saturation moment, T C , the mean-field Curie temperature and m / m B , the
mean-field magnetic moment associated with the strength of
individual magnetic cluster whose internal degrees of freedom are considered frozen. The high-temperature Curie–
Weiss susceptibility x 5C/(T2T C ) can be derived from
these expressions with C5 m M S /3k B .
Figure 1 is a summary of the temperature dependence of
magnetization
in
an
epitaxial
thin
film
of
La0.60Y0.07Ca0.33MnO32 d . Points are data, lines are threeparameter fits using Eqs. ~1!–~3!. The inset shows the magnetic field dependence of the effective cluster size m / m B as
well as the mean-field Curie temperature T C as a function of
applied field. Individually, each M (T) curve at a given field
can be well reproduced by the simple mean-field model. The
1002
Appl. Phys. Lett. 69 (7), 12 August 1996
0003-6951/96/69(7)/1002/3/$10.00
© 1996 American Institute of Physics
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FIG. 1.
Temperature dependence of magnetization of a
La0.60Y0.07Ca0.33MnO32 d thin film at different applied fields. Magnetization
data taken from full hysteresis sweeps of field between 65/T. Points are
data, solid lines are three-parameter fits using Eqs. ~1!–~3!.
parameters m, M S , and T C , however, are strongly field dependent. Taken at face value, these data suggest that in
higher magnetic field, the system has stronger ferromagnetic
ordering, resulting in a higher saturation moment M S , as
well as an increased mean-field transition temperature T C .
The effective magnetic cluster moment, m / m B at high field
saturates into a value of around 16, corresponding to the total
moment of about three pseudo cubic cells of Mn. The lowfield rise of m / m B indicates an M (T) transition that is
broader than what simple mean-field theory would predict in
small field. This may in part have to do with macroscopic
inhomogeneities of the material. The high-field saturation
value of m / m B is probably more interesting, especially because the cluster density calculated from this value coincides
with Ca dopant concentration.
To further investigate the possible relation between the
magnetic cluster density and dopant concentration, we examined another series of data, taken from a set of self-doped
La12x h x MnO32 d epitaxial thin films. The results are summarized in Fig. 2. Here the same mean-field model is used to
fit M (T) data taken at 4 T. We note that a systematic trend,
FIG. 2. Dependence of T C and magnetic moment m / m B on composition for
thin films of La12x h x MnO32 d . T C and m / m B were obtained from threeparameter fittings to measured M (T) data using Eqs. ~1!–~3!. Measurements
of M (T) were done in 4 T of field applied parallel to the film surface and
along the ~100! axis of SrTiO3 substrate.
FIG. 3. Left y axis: magnetization measured as a function of temperature
under constant field values. Points are data, curves are three-parameter fits.
Low-field, low-temperature end of the data appears to be somewhat different
from that shown in Fig. 1 because this is a constant field measurement, and
a small hysteresis effect exists that made data in this region slightly history
dependent. Right y axis: susceptibility as a function of temperature as calculated from the magnetization. Inset, a blowup of the low-field susceptibility data, showing the change of slope in 1/x data around 200 K.
both in T C and in effective magnetic cluster size m / m B , can
be observed as a function of dopant concentration x. The size
of magnetic cluster as suggested by this set of data lies between 1 and 6 pseudo-cubic cells of Mn ions. The cluster
size decreases with increasing x, and the cluster density increases correspondingly. These data suggest the possible existence of ferromagnetic nanoclusters, and the density of
which correlates with dopant concentration.
If ferromagnetic clusters do exist, one should expect to
see a separate ferromagnetic to paramagnetic transition that
unlocks the internal degree of freedom within individual
clusters at a temperature higher than the mean-field global
ferromagnetic ordering temperature T C . This is indeed the
case. Figure 3 shows a summary of the high-temperature
magnetic
susceptibility
data
for
a
film
of
La0.60Y0.07Ca0.33MnO32 d measured at several static field values. Two features in this data set are worth noting. First,
there appears to be a large amount of magnetic field dependence in the high-temperature susceptibility in the field range
between 0.01 and 4 T. This is not expected from simple
mean-field ferromagnetism, and is indicative of a ferromagnetic moment whose size is strongly field dependent, just as
we observed in the magnetic field dependence of the effective saturation magnetic moment M S as determined from the
mean-field estimate discussed before. This is consistent with
a magnetic cluster whose total magnetic moment is field dependent. In fact it could indeed be the same magnetic cluster
that is responsible for the low-temperature M (T). In other
words, the mean-field transition T C observed here is a T C for
the disordering of such ferromagnetic clusters, with each individual cluster retaining a large magnetic moment.
The second feature noticeable in Fig. 3 is the change of
slope in low-field 1/x (T) around 200 K. While the linear
1/x (T) below 200 K reproduces the mean-field Curie temperature T C , the slope of 1/x (T) above 200 K suggests a
magnetic state with much weaker moment, and a possible
ferrimagnetic ~or canted spin! arrangement @in that the linear
extrapolation of the 1/x (T) line leads to negative tempera-
Appl. Phys. Lett., Vol. 69, No. 7, 12 August 1996
Sun et al.
1003
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tures#. This is consistent with a change in local spin arrangement, possibly a change in the canting angle between neighboring Mn ions, induced by a phase transition within each
individual magnetic cluster. It may also be interesting to notice that, while the overall ferromagnetic transition of these
films are reduced to 120 to 150 K, the change of slope in
susceptibility occurs at a temperature of 200 K, not too different from the Curie temperature one would expect from a
fully oxygenated pure La0.60Ca0.33MnO3 bulk sample.
In summary, quantitative examination of temperature
and field dependent magnetization in thin-film manganates
leads us to suggest a nanocluster picture. The model associates the relatively low-field scale involved in these systems
for causing significant change in magnetic state with that of
a moderate magnetic cluster size, of the order of a few to
tens of Mn ions. These magnetic nanoclusters first develop a
ferromagnetic moment within itself at a relatively high temperature ~200 K in the case of La0.60Y0.07Ca0.33MnO32 d !.
The scattering of conduction electrons at the boundaries of
such nanoclusters is likely to be strongly spin orientation
dependent, and the understanding of which may led to new
insights into the cause of colossal magnetoresistance in this
class of materials.
The authors wish to thank David Divincenzo, John Slonczewski, Peter Duncombe, Bill Gallagher, Roger Koch,
Chang Tsuei, John Kirtley, Kathryn Moler, Bob Laibowitz,
Robin Altman, and Steve Brown at IBM Research, and Mark
Rubinstein at NRL for helpful discussions and help during
the experiment. G.X. wishes to acknowledge support from
National Science Foundation under Grant No. DMR9414160.
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Appl. Phys. Lett., Vol. 69, No. 7, 12 August 1996
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