ANOMALOUS MAGNETIC BEHAVIOR IN La1-xAxMnO3 (A =Ca, Ba) SINGLE CRYSTALS

1
ANOMALOUS MAGNETIC BEHAVIOR IN La1-xAxMnO3 (A
Ca, Ba) SINGLE CRYSTALS
Ya. Mukovskii, A. Pestun, D.Shulyatev Moscow
State Institute of Steel and Alloys, Moscow,
119049, Russia Wei Li, H.P. Kunkel, X Z Zhou,
G.Williams University of Manitoba, Winnipeg, MB,
R3T2N2, Canada
2
Introduction Similarity and difference in
manganites behavior. Many aspects of the
response of the manganite perovskites still
remain unresolved, including the
interrelationship between the metalinsulator and
the paramagnetic to ferromagnetic (PFT)
transitions. A related question that has also
emerged concerns the order of the magnetic phase
transition, particularly the role played by
disorder. In some papers beginning from Imry
and Ma (PRL 35, 1399 (1975)) and Aharony and
Pytte (PRL 45, 1583 (1980)) it was shown that
various types of randomness destroy magnetic
long-range order and lead into glassy state
(also Burgy et al. PRL 87, 277202 (2001)). In
some recent studies of this question for
manganites (e.g. Rivadulla, Rivas, Goodenough
(Phys.Rev.B 70, 172410 (2004)) in La1-xCaxMnO3
regions with 2 order (xlt0.25) and 1 order
magnetic transitions (0.25ltxlt0.4) were observed.
3
the magnetic transition out of the range 0.275 lt
x lt 0.43 is not a true phase transition, but only
a change in the relative volume fractions of the
fluctuations that compete to develop below a
certain temperature, Tf,, c(H0,T) will saturate
when the correlation length x becomes comparable
to a cluster size L.
H/M vs. M2 plots for various compositions around
TC and Tf showing the change in the sign of the
slope. Tf is defined from the minimum in the
?M/?T measured at low field. (After F. Rivadulla,
J. Rivas, J. B. Goodenough, Phys.Rev.B 70, 172410
(2004))
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Our detailed magnetization and susceptibility
data on single crystal La0.73Ca0.27MnO3 reveal a
not previously predicted and hence unexpected
resulta combination of characteristics
associated with both first-order and second-order
transitions simultaneously namely, metamagnetic
behavior in magnetic isotherms occurring
coincidentally with a crossover line in the
field and temperature dependent
susceptibility. Single crystal La0.73Ba0.27MnO3
demonstrated drastically different behavior -
second-order transition into Heisenberg
ferromagnetic, - but with strange feature at low
temperature.

• Plan of the Talk
• Introduction
• Coincident 1 and 2 order magnetic transitions in
  La0.73Ca0.27MnO3
• Magnetic behavior of La0.73Ba0.27MnO3
• Summary

5
La0.73Ca0.27MnO3
The coercivity HC values do not exceed 10 Oe at
any temperature
(a) The zero-field ac susceptibility measured on
warming and on cooling. (b) The temperature
dependence of the coercive field Hc. (c) The
metamagnetic field plotted against temperature.
(d) The metamagnetic boundary as in (c) and the
crossover line (?) (the line of ?(Ha,T) maxima)
plotted against temperature.
6
La0.73Ca0.27MnO3
The S-shaped character of isotherms for T gt
237 K (metamagnetic transition)
H (kOe)
Magnetization isotherms, showing limited
hysteresis, for temperatures increasing from 234
K (top) to 245 K in 1 K steps. .
7
La0.73Ca0.27MnO3
d 4.8
gb1.75
g 1.384
The observed behavior is a characteristic
signature of a second-order/continuous phase
transition.
(a) The ac susceptibility ?(Ha, T), measured in
applied fields Ha increasing from 600Oe (top) to
2000 Oe the locus of these maximathe crossover
lineis shown by the dashed curve (b) the
susceptibility maxima (a test of equation (2))
against internal field Hi H,-NM (c) the
(reduced) peak temperature (a test of equation
(1)) against internal field, (d) the
susceptibility maxima against reduced peak
temperature (a test of equation (3)). The
solid lines show Heisenberg model exponents for
comparison.
8
La0.73Ba0.27MnO3
Magnetization isotherms at selected temperatures
below 100 K. Below 50 K the magnetization in the
same field rangea qualitative measure of the
spontaneous magnetizationfalls. No evidence of
a metamagnetic transition
Magnetization isotherms between 240 and 251 K
(in 1 K steps). Inset the zero-field ac
susceptibility
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La0.73Ba0.27MnO3
t(T-TC)/TC , hH/TC
b 0.364 0.003, 210-3 lt t lt 310-2 g 1.392
0.005, 210-3 lt t lt 310-2 d 4.83 0.04,
200lt Hlt 50 000 Oe
Modified Arrott plots M1/b vs (H/M)1/g using
Heisenberg model exponents for a selection of
magnetic isotherms at temperatures of 242 (top),
and 248 K (bottom), step 1 K. Inset
conventional Arrott plots M2 vs H/M
near-neighbour three-dimensional Heisenberg
model g1.396, b0.369, d4.783.
10
La0.73Ba0.27MnO3
The spontaneous magnetization Ms plotted against
temperature.
11
La0.73Ba0.27MnO3
A scaling plot of M/tb vs H/tgb. 242.0
ltTlt247.5, 200ltHlt50 000 Oe The solid lines
drawn in this inset represent the asymptotic
forms of the scaling function.
12
a)
b)

• Q scan showing spin waves along the (001)
  direction in manganite single crystals.
• A central quasielastic component to the
  fluctuation spectrum develops as T?TC for
  La0.7Ca0.3MnO3 (J.W.Lynn, C.P.Adams,
  Y.M.Mukovskii, et al. J. Appl. Phys. 89, 6846,
  2001)
• NO quasielastic central peak for La0.8Ba0.2MnO3
• (A.A.Arsenov, Ya.M.Mukovskii, J.W.Lynn et al.
  Phys.Stat.Sol.(a), 189, 673, 2002).

13
La0.73Ba0.27MnO3

• The zero-field ac susceptibility measured on
  warming following cooling at zero (dc) field.
• The ac susceptibility measured on warming
  following cooling at zero (dc) field in
• progressively increasing static (dc) applied
  fields of 200 Oe (top), and 1400 Oe (bottom).

14
La0.73Ba0.27MnO3
ArrottNoakes plots (M2 versus H/M with
mean-field exponents) at temperatures of,
sequentially, 50 K (top), and 230 K
(bottom). Inset data close to the Curie
temperature plotted using the same equation, but
with Heisenberg model exponents, at temperatures
of, sequentially, 241 K (top), 250 K (bottom).
15
La0.73Ba0.27MnO3
a)
b)
(a) Plots of the reduced spontaneous
magnetization (MS(T)/MS(0)) plotted against
temperature. The solid line represents a fit to
equation MS(T)/MS(0) 1 - (NS)-1(
kBT/4pD)3/2?(3/2,kBT) between 60 and 140 K
using D 65.7 meV Å2 and D 0.45 meV the
dashed line extends this fit below 60 K. (b)
Spontaneous magnetization data below 30 K the
dashed line uses the D and D values utilized in
(a) scaled to MS(T)/MS(0) 0.957(5) the solid
line utilizes the same value for D as in (a) but
with D increased to 2.35 meV, while the
dotdashed line employs D 0.45 meV as in (a)
but with D increased to 159 meV Å2.
16
La0.73Ba0.27MnO3
Influence of radiation stimulated disorder (fast
neutron irradiation with E 1 MeV, Trad 300
K, flux F 21019 neutron/cm2) on temperature
dependencies of AC susceptibility and electric
resistance of single crystals of layered
manganites La1.4Sr1.6Mn2O7 in magnetic field up
to 13.6 T. It was observed that the disorder
leads to suppression of the magnetic order (TC ?
0) and to disappearing of metallic character of
conductivity. In non magnetic state the CMR
effect remains, and its value exceeds an original
one. Also kinetics of the properties recovering
under annealing was studied. The work was
supported by the RFRB grant 02-02-16425 and ISTC
grant 1859
The magnetization, M(H, T), measured on warming
following zero-field cooling in static applied
fields.
The measured coercive field, HC(T). 
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While the physical origin of the underlying
magnetization processes in La0.73Ba0.27MnO3 may
be questionable, the estimate for MS(0)
0.96(NgµBS) is not. That this signifies an
effective moment reduction is unequivocal the
specific mechanism leading to this moment
reduction however cannot be identified
definitively. Spin canting - ? A spiral magnetic
structure - ? Structural changes - ? Does not
connect with technical processes (Hopkinson
maximum, HC lt 5 Oe).
Summary In La0.73Ca0.27MnO3 the transition at x
0.27 displays features characteristic of both
continuous and discontinuous transitions that
arewithin experimental uncertaintycoincident.
This behaviour is fundamentally different from
crossover effects from sequential second-order
to a first-order transition as T ? Tc, where the
first-order transition line would lie below that
for the continuous transition, a situation for
which the power-laws discussed above would be
expected to occur, as the transition is
approached from higher reduced temperatures. In
La0.73Ba0.27MnO3 estimates of the spontaneous
magnetizationsupplemented by ac susceptibility
dataindicate a spontaneous moment reduction
below 60 K. This reduction is not associated
with a (further) structural phase change nor
technical magnetic processes. Spin-wave stiffness
estimated with D 0 do not approach zero
temperature monotonically. Additional
experiments investigating the microscopic/atomic
spin arrangement in La1-xBaxMnO3 (0.2 x 0.3)
at low temperature might be appropriate.