Transcript ANOMALOUS MAGNETIC BEHAVIOR IN La A MnO

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
Introduction
Similarity and difference in manganites behavior.
Many aspects of the response of the manganite perovskites still remain unresolved,
including the interrelationship between the metal–insulator 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 (x<0.25) and 1 order
magnetic transitions (0.25<x<0.4) were observed.
the magnetic transition out
of the range 0.275 < x <
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(H=0,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))
Our detailed magnetization and susceptibility data on single crystal
La0.73Ca0.27MnO3 reveal a not previously predicted and hence
unexpected result—a 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
1. Introduction
2. Coincident 1 and 2 order magnetic
transitions in La0.73Ca0.27MnO3
3. Magnetic behavior of La0.73Ba0.27MnO3
4. Summary
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 χ(Ha,T) maxima) plotted against temperature.
La0.73Ca0.27MnO3
The S-shaped character
of isotherms for T >
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
.
La0.73Ca0.27MnO3
(1)
g+b=1.75
(2)
d = 4.8
(3)
g = 1.384
The observed behavior is
a characteristic signature
of a secondorder/continuous phase
transition.
(a) The ac susceptibility χ(Ha, T), measured in applied fields Ha increasing from 600Oe (top) to
2000 Oe;
the locus of these maxima—the crossover line—is 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.
La0.73Ba0.27MnO3
Magnetization isotherms at selected
temperatures below 100 K.
Below 50 K the magnetization in
the same field range—a qualitative
measure of the spontaneous
magnetization—falls.
No evidence of a metamagnetic
transition
Magnetization isotherms between
240 and 251 K (in 1 K steps).
Inset: the zero-field ac susceptibility
La0.73Ba0.27MnO3
t=(T-TC)/TC , h~H/TC
b= 0.364 ± 0.003, 2*10-3 < |t| < 3*10-2
g= 1.392 ± 0.005, 2*10-3 < t < 3*10-2
d= 4.83 ± 0.04, 200< H< 50 000 Oe
near-neighbour three-dimensional
Heisenberg model:
g=1.396, b=0.369, d=4.783.
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
La0.73Ba0.27MnO3
d=4.83±0.04
The critical isotherm, which gives TC=245 K
g=1.392±0.005
b=0.364±0.003
The spontaneous magnetization Ms
plotted against temperature.
The initial susceptibility ci
plotted against temperature
La0.73Ba0.27MnO3
A scaling plot of M/tb vs H/tg+b.
242.0 <T<247.5, 200<H<50 000 Oe
The solid lines drawn in this inset represent
the asymptotic forms of the scaling function.
60
a)
b)
40
20
0
-3
-2
-1
0
Energy (meV)
1
2
3
Q scan showing spin waves along the (001) direction in manganite single crystals.
a) 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)
b) 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).
La0.73Ba0.27MnO3
(a) The zero-field ac susceptibility measured on warming following cooling at zero (dc) field.
(b) 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).
La0.73Ba0.27MnO3
Arrott–Noakes 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).
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/4πD)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 dot–dashed line employs D = 0.45 meV as in (a) but
with D increased to 159 meV Å2.
La0.73Ba0.27MnO3
(
T
C
→
0
)
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The magnetization, M(H, T), measured on warming
following zero-field cooling in static applied fields.
The measured coercive field, HC(T).
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 < 5 Oe).
Summary
In La0.73Ca0.27MnO3 the transition at x = 0.27 displays features characteristic of both continuous
and discontinuous transitions that are—within experimental uncertainty—coincident.
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 magnetization—supplemented
by ac susceptibility data—indicate 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.