Magnetic and electronic properties of the magnetoresistive
perovskites La
Bi Ca
MnO
0.67~x x 0.33
3
M.C. Walsh1!, M. Foldeaki", A. Giguere", D. Bahadur#, S.K. Mandal#, R.A. Dunlap!,*
! Department of Physics, Dalhousie University, Halifax, Nova Scotia, Canada B3H 3J5
" Institut de Recherche sur l+Hydroge% ne, Universite& du Que& bec a% Trois-Rivie% res, Trois-Rivie% res, Que& bec, Canada G9A 5H7
# Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Powai, Bombay 400 076 India
Abstract
Magnetotransport, magnetic susceptibility and ferromagnetic resonance (FMR) studies of La
Bi Ca
MnO
0.67~x x 0.33
3
perovskites with x"0.03 to 0.16 are reported. The magnetoresistivity ratio shows a peak in all samples corresponding to
the Curie transition. In samples with x*0.13 a second peak at lower temperature is seen in the magnetoresistivity.
Magnetic susceptibility measurements show Curie like behavior for all samples. Samples with low Bi content show
a difference between field cooled and zero-field cooled measurements at low temperature. This difference diminishes with
increasing x. FMR spectra show a single resonance above ¹ . Samples with small Bi content show two resonances just
#
below ¹ but these resonances cannot be clearly resolved at lower temperatures due to line broadening. These two
#
resonances also become less resolved with increasing x. No anomalous behavior corresponding to the low-temperature
peak in the magnetoresistivity is seen in the magnetic susceptibility or FMR measurements. Results are interpreted in
terms of the effects of Bi on the structure of grain boundaries.
Keywords: Giant magnetoresistance; Perovskites; Magnetotransport; Ferromagnetic resonance; Magnetic susceptibility
1. Introduction
The giant magnetoresistance (GMR) effect in
polycrystalline manganese-based perovskite oxides
has attracted considerable interest in recent
years [1—4]. Much of this work has dealt with
compounds of the composition ABO such as
3
LaMnO . It is known that LaMnO is an antifer3
3
romagnetic insulator and that it transforms to
a metallic ferromagnet as a function of substitution
at lanthanum sites by divalent alkali ions such as
Ca2` or Sr2`. Doped charge carriers (e.g. holes)
mediate the ferromagnetic interaction between localized spins through the double exchange mechanism [5—8]. The ferromagnetic Curie temperature
is related to the strength of the exchange integral
between Mn3`@4` ions and this influences the
104
electronic conductivity. Crystallographic distortion
is introduced by substitution of different sized ions
at the A sites [1]. This may result in the simultaneous
presence of canted ferromagnetic and antiferromagnetic spin structures and is believed to be the
principal mechanism responsible for the GMR effect in these oxides [9—12]. It is known that small
changes in composition and stoichiometry can
have remarkable effects on the magnetoresistive
properties of these perovskites and that it is often
possible to relate this behavior to changes in the
lattice properties and in the Mn3`/Mn4` ratio
[9—13]. In recent work, we have investigated
La—Er—Ca—Mn—O-based perovskites with small
variations in erbium and calcium content [13] and
La M
Ca
MnO perovskites with M"Er,
0.6 0.07 0.33
3
Yb and Bi [14], in order to understand these properties. The Bi containing system shows a particularly high value of the Curie temperature and
a substantial GMR effect. We have, therefore,
continued these investigations and report here on
magnetotransport, magnetic susceptibility and ferromagnetic resonance (FMR) studies of La
0.67~x
Bi Ca
MnO with x"0.03 to 0.16.
x 0.33
3
were field cooled (fc) and for those which were
zero-field cooled (zfc). In the former case the cooling field was 0.01 T. Ferromagnetic resonance
(FMR) studies have been performed on rectangular
samples with approximate dimensions of 2.0]
1.0]0.4 mm at a frequency of 9.1 GHz using
a Varian Associates EPR spectrometer over the
temperature range of 77 to 298 K. The samples
were mounted at the center of a rectangular TE
waveguide and FMR spectra were recorded for two
different applied field geometries; parallel to and
perpendicular to the long axis of the sample.
3. Results
3.1. X-ray diffraction measurements
X-ray diffraction studies of all samples showed
them to be of single perovskite structure. Samples
with x up to 0.07 showed a gradual decrease in the
lattice parameter as would be expected for Bi substitutions in the La-based material. No clear trends
were observed for higher Bi contents.
2. Experimental methods
3.2. Magnetoresistance measurements
Samples of La
Bi Ca
MnO with x"
0.67~x x 0.33
3
0.03, 0.07, 0.10, 0.13 and 0.16 were prepared by
mixing the required amount of either oxides or
carbonates of the different constituent elements and
heating slowly to 1225 K and holding at that temperature for a period of 6 h. The decomposed powder was again mixed and was pelletized and heated
to 1725 K for six hours in air and then cooled
slowly to room temperature. X-ray diffraction patterns were recorded with a Siemens D-500 scanning
diffractometer using Cu K radiation.
a
The magnetoresistance was measured as a function of temperature between 4.2 and 300 K and in
applied magnetic fields up to 5.0 T using conventional four-point techniques. Temperature-dependent magnetic susceptibility measurements were
performed between 4.2 and 330 K using a conventional DC SQUID (Superconducting Quantum Interference Device) magnetometer in an applied field
of 0.01 T. Data were obtained for samples which
Results of magnetoresistance studies are illustrated in Fig. 1. All of the resistivity measurements
have several characteristics in common. All of the
samples show at least one well-defined peak in
resistance as a function of temperature. Below the
peak the resistivity drops sharply as has been observed previously for single crystal and thin film
samples [15]. As the magnetic field is increased the
value of peak resistance decreases monotonically.
Also, the temperature of the maximum resistance
generally increases with applied field but not necessarily monotonically. Perhaps, the most significant
overall trend in the data is the gradual appearance
of a second peak in resistivity at lower temperature
with increasing bismuth content. In general, the
second peak is broader than the higher temperature
peak and has a lower peak resistivity. It is useful to
analyse these data in terms of their normalized
response to applied field. The magnetoresistance
ratio (MRR) defined as (o (0 T)!o (5 T))/o (5 T) is
105
Fig. 1. Resistivity as a function of temperature and applied
magnetic field for La
Bi Ca
MnO with (a) x"0.03,
0.67~x x 0.33
3
(b) x"0.07, (c) x"0.10, (d) x"0.13 and (e) x"0.16. For
each composition decreasing resistivity curves correspond to
increasing applied magnetic field. Applied field values are (a) 0,
0.5, 1, 2 and 5 T, (b) 0, 1, 2, 3, 4 and 5 T, (c) 0, 1, 2, 3, 4 and 5 T
(d) 0, 1, 2, 3, 4 and 5 T and (e) 0, 1, 2, 3 and 5 T.
shown in Fig. 2. These plots contrast resistivity at
0 and 5 T and thus give an indication of the sensitivity of the resistivity to applied field. These results
reveals that for A site doping by bismuth of less
than 10% the magnetoresistance is not substantially affected. Bismuth concentrations of 13% or
Fig. 2. Magnetoresistance ratio for La
Bi Ca
MnO
0.67~x x 0.33
3
with (a) x"0.03, (b) x"0.07, (c) x"0.10, (d) x"0.13 and
(e) x"0.16.
greater show a magnetoresistance peak for each
resistivity peak revealing the magnetic origin of the
lower temperature peak observed in the resistivity
plots. This behavior is most apparent in the case of
the x"0.16 sample. Perhaps more interesting is
the magnitude of the observed effect which (by
comparing peak values in Figs. 1 and 2) is dramatically larger. In all cases a significant magnetoresistance effect is observed over the entire temperature
range and at lower temperatures this effect is
106
Table 1
Magnetic transition temperatures in La
Bi Ca
MnO
0.67~x x 0.33
3
as determined by the maximum in the MRR (¹ ) and by the
.
extrapolation of measured magnetic susceptibilities (¹ )
#
x
¹ (K) ($5 K)
.
¹ (K) ($4 K)
#
0.03
0.07
0.10
0.13
0.16
270
266
259
235
254
266
265
258
231
250
maximized for x"0.16. For single crystals, the
magnetoresistivity effect is found only in the vicinity of the transition temperature and at low temperatures it is not significant. For example, in single
crystal La
(Pb,Ca)
MnO a magnetoresitivity
0.65
0.35
3
effect of only 0.2% is observed at 5 K in a field of
5 T [15]. However, in many polycrystalline materials a substantial effect has been reported even at
low temperatures [4,16—18]. It is customary to intrepret the temperature of the highest temperature
peak in the MRR in terms of the magnetic transition
temperature. This temperature, defined as ¹ , is
.
given as a function of x in Table 1. For samples with
small concentrations of Bi the results obtained here
are consistent with those for La
Ca
MnO
0.67 0.33
3
reported in the literature [e.g. 3,4].
3.3. Magnetic susceptibility measurements
Typical results of magnetic susceptibility measurements on the samples studied here are illustrated
in Fig. 3. In all cases except the x"0.16 sample,
a substantial difference between fc and zfc results are
observed. The fc data show ferromagnetic like behavior and the Curie temperature, ¹ , may be inter#
preted as the temperature at which the magnetic
susceptibility extrapolates to zero. These values are
compared with the results from the magnetotransport studies in Table 1. Also a single magnetic phase
is observed in these measurements for all samples.
3.4. FMR measurements
Fig. 4 shows typical FMR spectra for samples
with x"0.07, 0.10, and 0.16 both in the parallel
Fig. 3. Magnetic susceptibility as a function of temperature for
La
Bi Ca
MnO with (a) x"0.03, (b) x"0.10 and
0.67~x x 0.33
3
(e) x"0.16.
and perpendicular configuration recorded at
298 K. In Figs. 5 and 6 the temperature dependence of FMR spectra for the samples with x"0.07
and 0.13, respectively, are shown. These spectra
shown here are close to the transition temperature.
At 298 K, a single resonance is observed corresponding close to the free spin g-value. However,
a small difference in resonance field persists in parallel and perpendicular configuration even at room
temperature, which is significantly above the
transition temperature. The FMR line width, *H, is
essentially constant above ¹ , but below the
#
transition considerable line broadening is observed.
With further lowering of temperature, the two resonances cannot be distinguished due to substantial
broadening. It may be pointed out that the presence of the second resonance is more obvious in
samples with x)0.10. For samples with x"0.13
and 0.16, only broadening and shifting of the line
are seen, although the presence of a second weaker
resonance cannot be ruled out. The low-field resonance in such cases may be much weaker than the
high-field resonance. Fig. 7 shows the temperature
variation of the line width for the samples with
107
Fig. 5. FMR spectra for La
Bi Ca MnO with x"0.07 at
0.67~x x 0.33
3
different temperatures; (a) 298 K, (b) 258 K, (c) 248 and (d) 173 K.
Fig. 4. Typical ferromagnetic resonance (FMR) spectra recorded at 298 K in parallel and perpendicular configuration for
La
Bi Ca
MnO ; (a) x"0.07, (b) x"0.10, and
0.67~x x 0.33
3
(c) x"0.16.
x"0.07 to 0.16. In all cases, the line width increases significantly with lowering of temperature
below ¹ but is essentially constant above ¹ .
#
#
4. Discussion
The results of the previous sections indicate that
the properties of La
Bi Ca
MnO change
0.67~x x 0.33
3
significantly as x is varied from 0 to 0.16. The
primary evidence for this assertion is the appearance of a significant magnetoresistance below
¹ and the gradual appearance of a second peak in
#
the zero-field resistivity at lower temperatures with
increasing x. Unlike the higher temperature peak,
however, there is no clearly associated corresponding feature in the magnetic susceptibility or FMR
measurements. In single crystal samples, it
is known that the magnetoresistivity effect is
insignificant below ¹ , although in polycrystalline
#
Fig. 6. FMR spectra for La
Bi Ca
MnO with x"0.13
0.67~x x 0.33
3
at different temperatures; (a) 263 K, (b) 238 K, (c) 223 and
(d) 118 K.
108
Fig. 7. Temperature dependence of the FMR line width in the
parallel configuration for La
Bi Ca
MnO
with
0.67~x x 0.33
3
(a) x"0.07 and (b) x"0.16. The splitting of the curve in (a)
near ¹ is due to the presence of two resonances with different
#
line widths.
samples there are several reports of substantial lowtemperature effects [4,16—18]. On the basis of several reports, the possible origin of such behavior
could be (1) very fine particles as suggested by
Mahesh et al. [18]; (2) the existence of a second
magnetic phase (3) the appearance of a second
magnetic phase transition or (4) the effect of grain
boundaries.
The samples used here have been prepared by the
ceramic route and the SEM micrographs taken on
some samples confirm that the grain size is in the
micron range and this rules out the first possibility
above. The X-ray diffraction along with the magnetic susceptibility results demonstrate that these
are single-phase materials. Hence, such effects
coming from secondary magnetic phase is not possible. The third and fourth possibilities as given
above are discussed in detail.
Behavior similar to that observed here has been
seen previously in single phase polycrystalline samples and has been discussed in the context of grain
boundary effects by Ju et al. [16,17]. Although
these authors have given convincing arguments in
favor of grain boundary effects, it is important to
consider the reasons why this behavior is not seen
in all polycrystalline samples. We suggest that these
characteristics in the magnetoresistivity are depen-
dent on the thickness and structure of the grain
boundary. This is consistent with the effects of spinpolarized tunneling between grains on the magnetoresistivity as suggested by Hwang et al. [19].
Bismuth oxide due to its low melting behavior, is
known to segregate at the grain boundaries in
many oxide systems, thicken the grain boundary
and dramatically change the electrical behavior.
The most important example is ZnO-based varistor
materials where 1—2% of Bi O is added and segre2 3
gates at grain boundaries. This is associated with
the highly nonlinear I—» characteristics observed
in these materials [20]. We believe that for small
concentrations, bismuth ions, which are compatible
with lanthanum in terms of charge and ionic radii)
substitute for La in the perovskite structure, thus
decreasing the lattice constant. With increased bismuth concentration, a small amount of excess bismuth may migrate to the grain boundary region
yielding a thicker boundary region. Such an effect
would either create oxygen vacancies or enhance
the Mn4`/Mn3` ratio. Since all samples were prepared the samples under identical conditions, it is
reasonable to assume that the oxygen stoichiometry remains the same and that the
Mn4`/Mn3` ratio marginally increases. It is established that a thick grain boundary in these oxide
systems is characterized by a greater degree of
disorder and strain. Subsequently the Mn4` and
Mn3` spins within the grain boundary region
would be more random. Charge carrier mobility
due to hopping is decreased, resulting in higher
resistivity. With the application of the magnetic
field, Mn spins within grain boundary are aligned
resulting in increased carrier mobility and decreased resistivity. Domain walls, in principle, exhibit the same type of effect as discussed in earlier
papers [4,13]. Within the domain wall, spins are
not parallel, but change direction slowly with respect to adjacent domains. In these regions the
electron transfer between Mn3` and Mn4` is not
as easy as within the domain itself leading to a higher resistivity. However, with the application of the
magnetic field the density of domain walls decreases resulting in a lower resistivity. A thick grain
boundary produces analogous effects although
with the application of a magnetic field (near saturation) domain walls are nonexistent unlike grain
109
boundary. It is known in general that the grain
boundary resistivity is very high in several oxide
systems. Ju et al. [17] have calculated the grain and
grain boundary resistivity and found that the grain
boundary resistivity is much larger than the bulk
grain resistivity and that the peak in the grain
boundary resistivity occurs below ¹ .
#
The magnetic susceptibility curves show substantial differences between fc and zfc results and
this difference, in general, diminishes as a function
of increasing bismuth content. For 0.03(x(0.13,
the zfc curve remains below the fc results. When
the samples were cooled through their transition
temperatures under an applied field, the samples
exhibit conventional ferromagnetic behavior. Analogous to the situation in a spin glass, samples
cooled without an applied field show reduced magnetization, indicating the freezing in of some degree
of spin disorder.
The difference in the FMR resonance field between parallel and perpendicular field configuration is due to the presence of the demagnetizing
field in the samples, which is proportional to magnetization. This difference, though small, persists
even above ¹ . This has also been discussed earlier
#
on the basis of FMR and magnetization studies
[14,16,21]. The value of Curie constant was found
to be enhanced by a factor of 2 from the spin only
value for temperatures just above ¹ and the tail in
#
the M versus ¹ plot persists beyond ¹ [16]. This is
#
a clear indication that some short range order continues to exist above ¹ .
#
The magnetic ions present in the system are
Mn3`(3d4) and Mn4`(3d3). A single resonance is
observed at room temperature indicating that there
is no magnetic interaction present. However, hopping between these ions would still occur. If the
hopping frequency is greater than the larmor frequency, it may lead to a single resonance corresponding to the mean field value. At lower temperatures, below ¹ , the magnetic interaction is
#
present. Two interactions are feasible between
Mn3` and Mn4`. These are ferromagnetic interactions or canted ferromagnetic/antiferromagnetic
interactions which sensitively depend upon the
distance and angle of Mn—O—Mn bond and the
Mn4`/Mn3` ratio. Mn ions responsible for
canted FM/AFM interaction can be driven to FM
interactions by the application of a magnetic field.
As a result of these two possible types of interactions, two clear and broader resonances are seen
just below ¹ for samples up to x"0.10. The
c
low-field resonance seems to arise from Mn ions
participating in canted ferromagnetic interactions,
while the high-field resonance is presumably due to
Mn ions participating in FM interactions. However, above x"0.10 (i.e. for x"0.13 and 0.16),
there is no clear evidence of two resonances just
below ¹ . However, significant broadening of the
#
resonance and asymmetry is always seen below ¹ ,
#
as has been reported in other manganates [22,23]
and in other polycrystalline systems [24]. This has
been explained on the basis of a random orientation of crystallites and a distribution of demagnetizing fields.
5. Conclusions
In conclusion, we have shown that the present
measurements exhibit a low-temperature peak in
the resistivity and, correspondingly, in the MRR for
samples with x'0.10. There is no discernible
anomaly in the magnetic susceptibility or FMR
data which corresponds to this feature in the magnetotransport studies. The general behavior seen
here is analogous to that seen in high-pressure
studies of certain GMR’s. Neumeier et al. [25] have
reported magnetotransport and magnetic studies
for La
Ca
MnO , which show a lower tem0.60 0.40
3
perature peak in the resistivity which increases in
magnitude with increasing applied pressure. For
higher pressures the results reported by Neumeier
et al. [25] are similar to those reported in the
present work for x"0.16. High-pressure studies of
La
Ca
MnO and La
Ca
MnO did
0.79 0.21
3
0.67 0.33
3
not show the appearance of an anomalous lowtemperature peak. The effects of applied pressure
and Bi doping may, at least in some case, be expected to show similarities. The substitution of
smaller Bi ions for La [26], results in a slight
decrease in the lattice parameter [14,27]. This has
a substantial effect on the double exchange interactions responsible for the ferromagnetic coupling in
these materials as a result of changes in the carrier
concentration. Although the double exchange
110
interaction alone has been shown to be insufficient
to explain giant magnetoresistance [28], Neumeier
et al. [25] have suggested that those compositions
which are closer to the boundary between ferromagnetic and antiferromagnetic behavior may be
most sensitive to the large magnetovolume effects
seen in these materials. According to this argument,
pressure effects may be manifested at low temperature by the introduction of spin canting and this
type of transition would result in an increase in
resistivity, although it is unclear how these effects
would influence the magnetic and FMR measurements. Neutron diffraction studies would help to
clarify the role of magnetic interactions in the lowtemperature magnetoresistivity in these materials.
If grain boundary effects are responsible for the
observed magnetoresistivity behavior, it is not necessarily expected that corresponding features
would be seen in the magnetic properties. As well,
applied pressure may influence resistivity as a result of large magnetovolume effects within grain
boundaries yielding increased resistivity at low
temperature. Thus, although definitive conclusions
concerning the origins of the observed behavior may be difficult to draw on the basis of the
present results alone, the view of magnetoresistivity
influenced by grain boundary effects is consistent
with expectations for Bi-doped materials.
Acknowledgements
This work was supported in part by a grant from
the Natural Sciences and Engineering Research
Council of Canada. The authors are grateful to J.
O’Brien and D.A. Eelman for assistance with
manuscript preparation.
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