Ultra-sharp magnetization steps in perovskite manganites
R. Mahendiran,1 A. Maignan,2 S. Hébert,2 C. Martin,2 M. Hervieu,2 B. Raveau,2 J. F.
Mitchell,3 and P.Schiffer1
1Department of Physics and Materials Research Institute, Pennsylvania State University,
University Park, PA 16802
2Laboratoire CRISMAT, ISMRA, 6 Boulevard du Maréchal Juin, 14050 Caen Cedex,
3Materials Science Division, Argonne National Laboratory, Argonne, IL 60439
We report a detailed study of step-like metamagnetic transitions in the magnetization and
resistivity of polycrystalline Pr0.5Ca0.5Mn0.95Co0.05O3. The steps have a sudden onset
below a critical temperature, are extremely sharp (width < 2 × 10-4 T), and occur at
critical fields which are linearly dependent on the absolute value of the cooling field in
which the sample is prepared. Similar transitions are also observed at low temperature in
non-Co doped manganites, including single crystal samples. These data show that the
steps are an intrinsic property, qualitatively different from either previously observed
higher temperature metamagnetic transitions in the manganites or metamagnetic
transitions observed in other materials.
Metamagnetic phase transitions, those induced by a magnetic field, have an
extended history of investigation in long range ordered antiferromagnets . Much
recent research has also focused on a wide range of other material systems including
clean metals , magnetic cluster compounds , and geometrically frustrated magnets
, all of which display transitions induced by a magnetic field. Included in this
category are many of the perovskite “colossal magnetoresistance” manganites  which
display first order field-induced transitions in an applied field at low temperatures from
an antiferromagnetic charge-orbital ordered insulating (COOI) phase to a ferromagnetic
metallic phase. In the case of the manganites, the phases typically coexist in low
magnetic fields  (particularly in the presence of Mn site doping [7,8,9,10,11]), and the
metamagnetic transitions progress through an increasing fraction of the ferromagnetic
phase in the phase-separated materials .
Metamagnetism in the manganites has been investigated primarily at temperatures
above 5 K, and the transitions are rather broad in that temperature regime as may be
expected from the inhomogeneous nature of the low field phase-separated state. There
has, however, been a recent report of unusual step-like magnetic field dependence of the
magnetization and other properties measured at temperatures below 5 K in a particular
series of compounds, Pr0.5Ca0.5MnO3 doped with a few percent of other cations such as
Sc or Ga on the Mn site . We present here a detailed investigation of the
temperature and field dependence of reproducible steps in the magnetization and
resistivity of Pr0.5Ca0.5Mn1-xCoxO3 (x = 0.05). We find that the onset of the steps is
extremely sharp in temperature and that the steps themselves are extraordinarily sharp
with a width of ∆H/H ~ 10-4 even at temperatures immediately below the onset
temperature. Surprisingly, the field (Hc) at which the steps occur increases after field
cooling, and Hc itself is linear in the magnitude of the cooling field. We also observe a
similar step transition in manganites which are not doped on the Mn site, demonstrating
that the low temperature steps comprise a qualitatively new class of metamagnetic
transitions in the manganites which also differs significantly from those seen in other
We have studied high quality polycrystalline samples of Pr0.5Ca0.5Mn1-xCoxO3
prepared by a standard solid state technique (sample preparation and characterization
details are published elsewhere ). We measured magnetization (M) on a Quantum
Design SQUID magnetometer and resistivity (ρ) by a standard four-probe technique.
Data are shown primarily for x = 0.05 (chosen because it is unaffected by thermal cycling
unlike previously studied compounds exhibiting steps  or Co-doped materials with x
< 0.04), but the results are qualitatively equivalent for x = 0.06 and 0.07 . The results
have been qualitatively reproduced in three separate series of samples grown under
slightly different conditions and in two different sample preparation laboratories.
In figure 1 we show the high-temperature behavior of this material through the
temperature dependence of the magnetization, M(T), measured on heating after the
sample was either zero-field cooled (ZFC) or field cooled (FC). As seen in the inset to
figure 1, there is a kink in H/M(T) around 220 K, which is a signature of the onset of
charge-orbital ordering . Both the ZFC and FC M(T) show a ferromagnetic transition
at TC ~ 60 K in low fields. We presume, as in the case of many other manganites, that
charge ordering coexists with ferromagnetism in the low temperature phase. The
coexistence of the two phases in an inhomogeneous state at low temperatures is
evidenced  by a divergence of the ZFC M(T) at H = 1 T from the FC curve below Tirr
~ 30 K (Tirr is not suppressed to zero even at H = 7 T, implying that this is not a
homogeneous spin glass phase).
The two-phase nature of the low temperature phase is also evident in figure 2a
where we plot M(H) at T = 5 K. There is an initial increase in M(H), attributable to
alignment of the ferromagnetic regions of the sample, and then a kink at H ~ 2 T
followed by a second increase at higher fields associated with conversion of the charge-
ordered regions to a ferromagnetic conducting phase (similar behavior has been observed
in other mixed phase manganites such as Pr0.5Ca0.5Mn0.98Cr0.02O3  and
La0.250Pr0.375Ca0.375MnO3 ). The magnetization appears to saturate at 7 T, with a
saturation moment of 3.3 µB/formula unit -- close to the 3.4 µB/formula unit expected for
Pr0.5Ca0.5Mn3+0.40Co2+0.05Mn4+0.55O3. When the field is reduced from 7 T, M(H) behaves
like a long range ferromagnet with little change down to 2 T and then a rapid decrease as
H → 0.
The breadth of the 5 K field-induced transition from a predominantly COOI to
predominantly ferromagnetic metallic state suggests a wide distribution of critical fields
which drive this transition in different parts of the sample. This is consistent with the
presumption of phase separation and the behavior in other manganite materials in which
such a transition has been observed [11,12,15,16]. The behavior changes dramatically at
lower temperatures, however, as shown in figures 2b-d. There is no qualitative change in
M(H) down to 4.7 K, as shown in figure 2b, but at 4.6 K M(H) shows an abrupt step near
Hc1 = 2.5 T. The relative magnitude of the step can be written as g = ∆M/Msat = 0.18 at
4.6 K. After the step, M(H) increases linearly with a relatively small slope until it merges
with the M(H) curve recorded at T = 4.7 K. Such steps are also observed at lower
temperatures, and additional steps appear with decreasing temperature. The ZFC M(H) at
T = 4 K (figure 2c) again shows a single step at Hc1 = 2.4 T of higher magnitude (g =
0.43). At T = 3 K, however, there are two sharp step transitions in the ZFC data, at Hc1 =
2.3 T (g = 0.41) and at Hc2 = 4.5 T (g = 0.22), as seen in figure 2d. At all temperatures
below 5 K, the saturation magnetization at H = 7 T is close to the theoretical limit and
the magnetization behaves like a homogeneous ferromagnet upon reducing the field from
7 T and on subsequent field sweeps.
The measurements in figure 2a-d were performed with a field interval of 0.2 T,
and therefore do not set a strict limit on the width of the steps. We also measured with
much smaller field intervals of 0.2 mT (2 Oe) for T = 2 and 3 K. As shown in the inset to
figure 2d, the step width is even narrower than this interval, indicating that the transition
across the entire sample happens at essentially the same magnetic field. These
extraordinarily sharp transitions are quite surprising given the polycrystalline nature of
the sample and the presumed inhomogeneous nature of the phase-separated low
temperature state in these materials.
Since field-cooling changes the relative fraction of ferromagnetic metallic and
COOI phases, we also studied samples which were field-cooled with H > 0 T from T =
120 K (> TC). After stabilization at the measurement temperature, the field was reduced
to zero, and then M(H) was measured up to 7 T and back down to zero field. At T = 5 K,
such field cooling apparently increases the ferromagnetic fraction of the sample at low
fields and thus results in a larger low field magnetization. This behavior is consistent
with conventional understanding of phase separation in the manganites, and similar
behavior has been seen in Nd0.5Ca0.5Mn0.98Cr0.02O3 . Field cooling similarly increases
the low field magnetization at temperatures below the onset of the step transitions, but the
step transitions are shifted to higher fields as a result of field-cooling, which is surprising
since field cooling should enhance ferromagnetic tendencies. The shift of Hc is strikingly
linear in the cooling field (as shown in the inset to figure 4b from resistivity data
discussed below), but field-cooling in a sufficiently high magnetic field converts the
sample into an almost fully ferromagnetic state, eliminating the steps. The steps after
field cooling are not measurably broadened beyond our limit of 0.2 mT, implying that the
magnetic phase of the sample is uniformly affected by the cooling field. Furthermore, as
shown in figure 3, cooling in a negative field and then raising the field continuously to its
maximum positive value results in exactly the same shift of Hc.
In figure 4a we plot the field dependence of the FC and ZFC resistivity, ρ(H), in
analogy to the magnetization data in figure 2d. Note that ρ(H) shows downward steps at
the same fields as the M(H) data show upward steps. These data, along with the sizable
magnitude of the magnetization steps, indicate that the entire sample is involved in the
step transitions, rather than isolated regions. In figure 4b we plot ZFC ρ(H) at several
temperatures, and we see that a third step appears in the T = 2 K data at Hc3 ~ 7.36 T.
Surprisingly, although the steps appear suddenly below a certain temperature, there is
very little temperature dependence to Hc1 or Hc2. This is clearly seen in the inset to figure
3b where we demonstrate the linear dependence of Hc1 and Hc2 on the cooling field, since
the data for all of the different measurement temperatures lie on top of each other. A
careful measurement of the temperature dependence of Hc1 indicates that it does decrease
slightly (by ~ 0.2 T) from 4.5 K to 2 K .
Our data show that the step-like transitions are a qualitatively new phenomenon in
the manganites. While previously observed field-induced transitions at higher
temperatures are rather broad (width ~ 1 T) and sharpen somewhat with decreasing
temperature , the steps we observe are extremely sharp, even at the highest
temperature at which they are observed. These step-like transitions are not unique to
Pr0.5Ca0.5MnO3 doped on the Mn site, but appear to be a generic feature of a broad range
of manganites. We also observe the onset of a similar step-like transition in single crystal
Pr0.7Ca0.3MnO3 at 2 K (figure 4 inset) and also in both single crystal and polycrystalline
Pr0.65(Ca1-ySry)0.35MnO3 and polycrystalline Sm0.5Sr0.5MnO3 although the same cooling
field dependence of Hc is not observed in all these materials and the observation of the
step at 2 K in Pr0.7Ca0.3MnO3 is also depended on the field sweep rate. The charge-orbital
ordered antiferromagnetic domains in the Co doped Pr0.5Ca0.5MnO3 are smaller than in
the undoped compounds [7-11], suggesting a possible explanation for why we can
observe step transitions at lower fields and higher temperatures in these materials than in
the undoped compounds.
There are examples of step-like metamagnetic transitions in other magnetic
materials, but those transitions are either not as sharp (i.e. their width depends strongly on
temperature), or they occur only in single crystal samples when the applied field is along
a particular direction [1,17,18]. Neither case provides a good model for the present
behavior, where the steps are extraordinarily sharp even just below the onset temperature
and the samples are polycrystalline. One possible explanation for the step transitions in
the manganites is that they are due to martensitic effects associated with strain between
the phase-separated regions , but such a mechanism would have difficulty accounting
for the presence of multiple steps which are reproduced in different samples (given the
structural and magnetoelectronic inhomogeneity intrinsic to polycrystalline manganites).
The sharpness of the steps suggests that they reflect an intrinsic thermodynamic property
of the charge-ordered phase, i.e. phase transitions through which the charge-ordered
regions are becoming successively more polarized in steps between different canted
antiferromagnetic  or ferrimagnetic (as in FeCl2·2H2O ) arrangements. Orbital
orbiting in the charge-ordered domains may result in such behavior by stabilizing certain
types of spin structures with increasing field . Alternatively, it is possible that the
dipole interactions between Mn spins provide the anisotropy typically associated with
such transitions. One may be able to explain the shift of the steps with cooling field by
postulating that field cooling alters the orbital configurations in the COOI domains,
making them more difficult to transform into a highly polarized state . The field
cooling effect may also be explained as a softening of the antiferromagnetic exchange
interaction, which would increase the polarization of the charge-ordered regions and
reduce the Zeeman energy advantage of the ferromagnetic state to which the system is
The important question raised by our data is how the steps can be so sharp and
have such a sudden onset temperature in polycrystalline samples with an inhomogeneous
phase-separated ground state, since such sharp steps in other metamagnetic systems
require a high degree of crystalline anisotropy [1,17]. The reproducibility of the
observed phenomena in a range of compounds make it clear that these step transitions in
the manganites comprise a robust and qualitatively new sort of metamagnetism,
presenting a challenge to the theoretical community working on these materials.
Acknowledgements: The authors gratefully acknowledge the support of NSF
grant DMR-0101318 and MENRT (France). The submitted manuscript has been created
in part by the University of Chicago as Operator of Argonne National Laboratory
("Argonne") under Contract No. W-31-109-ENG-38 with the U.S. Department of Energy.
The U.S. Government retains for itself, and others acting on its behalf, a paid-up,
nonexclusive, irrevocable worldwide license in said article to reproduce, prepare
derivative works, distribute copies to the public, and perform publicly and display
publicly, by or on behalf of the Government.
Figure 1. Temperature dependence of the magnetization of Pr0.5Ca0.5Mn0.95Co0.05O3
while warming from 2 K after zero field cooling and field cooling. The inset shows
H/M(T) in the high temperature range at H = 1 T, where a kink indicates TCO, the charge-
orbital ordering temperature.
Figure 2. Field dependence of the magnetization at different temperatures measured
while stepping from H = 0 T → H = 7 T → H = 0 T in 0.2 T steps. (a). T = 5 K, (b). T =
4.7 K & 4.6 K, (c). T = 4 K, and (d). T = 3 K. Note that the smooth metamagnetic
transition at T = 4.7 K changes to include an abrupt jump at T = 4.6 K. Data are taken
either after zero-field-cooling (ZFC) or after field cooling (FC) from 120 K (> TC) in the
indicated magnetic fields (the FC data are taken on increasing field, after the field is first
reduced to zero at the measurement temperature). The inset in figure 2(d) shows data
taken with field interval of 0.2 mT, placing an upper limit on the width of the steps.
Figure 3. Comparison of M(H) at T = 2K after cooling in positive and negative fields.
Positive field-cooled data were taken in the same manner as the data in figure 2.
Figure 4. (a). Field dependence of the resistivity at T = 3 K in ZFC and FC modes
showing steps which correspond to those observed in the magnetization (figure 2d). The
inset shows the onset of a similar step transition in single crystal Pr0.7Ca0.3MnO3 at T = 2
K (sample preparation details given in ). (b). Field dependence of the resistivity for
2 K ≤ T ≤ 5 K. The inset shows the cooling field dependence of Hc at different
temperatures where the color of the symbols corresponds to the different temperatures in
the main figure.
0 50 100150
Figure 1 (Mahendiran et al.)
2.474 2.475 2.476
T = 5 K
FC (0.5 T)
FC (3 T)
Magnetic Field (tesla)
T = 4 K
Figure 2 (Mahendiran et al.)
T = 3 K
Magnetic Field (tesla)
Figure 3 (Mahendiran et al.)
resistivity (Ω cm)
Magnetic Field (T)
Figure 4 (Mahendiran et al.)
Cooling Field (T)
resistivity (Ω cm)
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