11 research outputs found
Conversion of glassy antiferromagnetic-insulating phase to equilibrium ferromagnetic-metallic phase by devitrification and recrystallization in Al substituted PrCaMnO
We show that PrCaMnO with 2.5% Al substitution and
LaCaMnO (LCMO) exhibit qualitatively similar and
visibly anomalous M-H curves at low temperature. Magnetic field causes a broad
first-order but irreversible antiferromagnetic (AF)-insulating (I) to
ferromagnetic (FM)-metallic (M) transition in both and gives rise to soft FM
state. However, the low temperature equilibrium state of
PrCaMnAlO (PCMAO) is FM-M whereas that
of LCMO is AF-I. In both the systems the respective equilibrium phase coexists
with the other phase with contrasting order, which is not in equilibrium, and
the cooling field can tune the fractions of the coexisting phases. It is shown
earlier that the coexisting FM-M phase behaves like `magnetic glass' in LCMO.
Here we show from specially designed measurement protocols that the AF-I phase
of PCMAO has all the characteristics of magnetic glassy states. It devitrifies
on heating and also recrystallizes to equilibrium FM-M phase after annealing.
This glass-like AF-I phase also shows similar intriguing feature observed in
FM-M magnetic glassy state of LCMO that when the starting coexisting fraction
of glass is larger, successive annealing results in larger fraction of
equilibrium phase. This similarity between two manganite systems with
contrasting magnetic orders of respective glassy and equilibrium phases points
toward a possible universality.Comment: Highlights potential of CHUF (Cooling and Heating in Unequal Fields),
a new measurement protoco
Coexisting tuneable fractions of glassy and equilibrium long-range-order phases in manganites
Antiferromagnetic-insulating(AF-I) and the ferromagnetic-metallic(FM-M)
phases coexist in various half-doped manganites over a range of temperature and
magnetic field, and this is often believed to be an essential ingredient to
their colossal magnetoresistence. We present magnetization and resistivity
measurements on Pr(0.5)Ca(0.5)Mn(0.975)Al(0.025)O(3) and Pr(0.5)Sr(0.5)MnO(3)
showing that the fraction of the two coexisting phases at low-temperature in
any specified measuring field H, can be continuously controlled by following
designed protocols traversing field-temperature space; for both materials the
FM-M fraction rises under similar cooling paths. Constant-field temperature
variations however show that the former sample undergoes a 1st order transition
from AF-I to FM-M with decreasing T, while the latter undergoes the reverse
transition. We suggest that the observed path-dependent phase-separated states
result from the low-T equilibrium phase coexisting with supercooled glass-like
high temperature phase, where the low-T equilibrium phases are actually
homogeneous FM-M and AF-I phases respectively for the two materials
Ferromagnetic ground state of the robust charge-ordered manganite Pr(0.5)Ca(0.5MnO(3)obtained by minimal Al-substitution
We show that minimal disturbance to the robust charge ordered
Pr(0.5)Ca(0.5)MnO(3) by 2.5% Al substitution on Mn-site drives the system
towards ferromagnetic ground state. The history-dependent coexisting phases
observed are explained as an outcome of a hindered first order transition with
glass like arrest of kinetics resulting in irreversibility. Consistent with a
simple phase diagram having ferromagnetic ground state, it is experimentally
shown that these coexisting phases are far from the equilibrium.Comment: This version is Accepted in Physical Review
Anomalous First-order transition in Nd<SUB>0.5</SUB>Sr<SUB>0.5</SUB>MnO<SUB>3</SUB>: an interplay between kinetic arrest and thermodynamic transitions
A detailed investigation of the first-order antiferromagnetic insulator (AFI) to ferromagnetic metal (FMM) transition in Nd0.5Sr0.5MnO3 is carried out by resistivity and magnetization measurements. These studies reveal several anomalous features of thermomagnetic irreversibility across the first-order transition. We show that these anomalous features cannot be explained in terms of the supercooling effect alone and the H-T diagram based on isothermal M-H or R-H measurements alone does not reflect the true nature of the first-order transition in this compound. Our investigations reveal glass-like arrest of kinetics at low temperature which plays a dominant role in the anomalous thermomagnetic irreversibility observed in this system. The interplay between kinetic arrest and supercooling is investigated by following novel paths in the H-T space. It is shown that coexisting FMM and AFI phases can be tuned in a number of ways at low temperature. These measurements also show that kinetic arrest temperature and supercooling temperature are anticorrelated, i.e. regions which are arrested at low temperature have higher supercooling temperature and vice versa
First order antiferro-ferromagnetic transition in Fe49(Rh0.93Pd0.07)51 under simultaneous application of magnetic field and external pressure
The magnetic field-pressure-temperature (H-P-T) phase diagram for first order
antiferromagnetic (AFM) to ferromagnetic (FM) transition in
Fe49(Rh0.93Pd0.07)51 has been constructed using resistivity measurements under
simultaneous application of magnetic field (up to 8 Tesla) and pressure (up to
20 kbar). Temperature dependence of resistivity ({\rho}-T) shows that with
increasing pressure, the width of the transition and the extent of hysteresis
decreases whereas with the application of magnetic field it increases.
Consistent with existing literature the first order transition temperature (TN)
increases with the application of external pressure (~ 7.3 K/ kbar) and
decreases with magnetic field (~ - 12.8 K/Tesla). Exploiting these opposing
trends, resistivity under simultaneous application of magnetic field and
pressure is used to distinguish the relative effect of temperature, magnetic
field and pressure on disorder broadened first order transition. For this a set
of H and P values are chosen for which TN (H1, P1) = TN (H2, P2). Measurements
for such combinations of H and P show that the temperature dependence of
resistivity is similar i.e. the broadening (in temperature) of transition as
well as extent of hysteresis remains independent of H and P. The transition
width decreases exponentially with increasing temperature. Isothermal
magnetoresistance measurement under various constant pressure show that even
though the critical field required for AFM-FM transition depends on applied
pressure, the hysteresis as well as transition width (in magnetic field) both
remains independent of pressure, consistent with our conclusions drawn from
{\rho}-T measurements.Comment: 10 pages, 6 figure
Relating supercooling and glass-like arrest of kinetics for phase separated systems: studies on doped CeFe and (La,Pr,Ca)MnO
Coexisting ferromagnetic and antiferromagnetic phases over a range of
temperature as well as magnetic field have been reported in many materials of
current interest, showing disorder-broadened 1st order transitions. Anomalous
history effects observed in magnetization and resistivity are being explained
invoking the concepts of kinetic arrest akin to glass transitions. From
magnetization measurements traversing novel paths in field-temperature space,
we obtain the intriguing result that the regions of the sample which can be
supercooled to lower temperatures undergo kinetic-arrest at higher
temperatures, and vice versa. Our results are for two diverse systems viz. the
inter-metallic doped CeFe which has an antiferromagnetic ground state, and
the oxide La-Pr-Ca-Mn-O which has a ferromagnetic ground state, indicating the
possible universality of this effect of disorder on the widely encountered
phenomenon of glass-like arrest of kinetics
Effect of pinning and driving force on the metastability effects in weakly pinned superconductors and the determination of spinodal line pertaining to order-disorder transition
We explore the effect of varying drive on metastability features exhibited by
the vortex matter in single crystals of 2H-NbSe and CeRu with varying
degree of random pinning. An optimal balance between the pinning and driving
force is needed to view the metastability effects in typically weakly pinned
specimen of low temperature superconductors. As one uses samples with larger
pinning in order to differentiate the response of different metastable vortex
states, one encounters a new phenomena, viz., the second magnetization peak
(SMP) anomaly prior to the PE. Interplay between the path dependence in the
critical current density and the non-linearity in the electromagnetic response
determine the metastability effects seen in first and the third harmonic
response of the ac susceptibility across the temperature regions of the SMP and
the PE. The limiting temperature above which metastability effects cease can be
conveniently located in the third harmonic data, and the observed behavior can
be rationalized within the Beans Critical State model. A vortex phase diagram
showing the different vortex phases for a typically weakly pinned specimen has
been constructed via the ac susceptibility data in a crystal of 2H-NbSe
which shows the SMP and the PE anomalies. The phase space of coexisting weaker
and stronger pinned regions has been identified. It can be bifurcated into two
parts, where the order and disorder dominate, respectively. The former part
continuously connects to the reentrant disordered vortex phase pertaining to
the small bundle pinning regime, where the vortices are far apart, interaction
effects are weak and the polycrystalline form of flux line lattice prevails.Comment: Submitted to the Special Volume on Vortex State Studies, Pramana J.
Phy