15 research outputs found
Quenched disorder induced magnetization jumps in (Sm,Sr)MnO
Magnetic field induced step-like changes in magnetization and resistivity of
SmSr manganites were studied. A strong dependence of these features on the
cooling rate was observed. Magnetostriction, however, does not show the
presence of large strain in our samples. From all these features we can rule
out the conventional explanation of magnetization jumps as a consequence of
martensitic transition. We propose instead that quenched by fast cooling
disorder leads to the formation of an inhomogeneous metastable state and to
subsequent magnetization jumps.Comment: 4 pages, 4 figures, to be published in PRB Brief
Nonuniversal scaling behavior of Barkhausen noise
We simulate Barkhausen avalanches on fractal clusters in a two-dimensional
diluted Ising ferromagnet with an effective Gaussian random field. We vary the
concentration of defect sites and find a scaling region for moderate
disorder, where the distribution of avalanche sizes has the form . The exponents for size
and for length distribution, and the fractal dimension of
avalanches satisfy the scaling relation .
For fixed disorder the exponents vary with driving rate in agreement with
experiments on amorphous Si-Fe alloys.Comment: 5 pages, Latex, 4 PostScript figures include
Ising spin glass under continuous-distribution random magnetic fields: Tricritical points and instability lines
The effects of random magnetic fields are considered in an Ising spin-glass
model defined in the limit of infinite-range interactions. The probability
distribution for the random magnetic fields is a double Gaussian, which
consists of two Gaussian distributions centered respectively, at and
, presenting the same width . It is argued that such a
distribution is more appropriate for a theoretical description of real systems
than its simpler particular two well-known limits, namely the single Gaussian
distribution (), and the bimodal one (). The
model is investigated by means of the replica method, and phase diagrams are
obtained within the replica-symmetric solution. Critical frontiers exhibiting
tricritical points occur for different values of , with the possibility
of two tricritical points along the same critical frontier. To our knowledge,
it is the first time that such a behavior is verified for a spin-glass model in
the presence of a continuous-distribution random field, which represents a
typical situation of a real system. The stability of the replica-symmetric
solution is analyzed, and the usual Almeida-Thouless instability is verified
for low temperatures. It is verified that, the higher-temperature tricritical
point always appears in the region of stability of the replica-symmetric
solution; a condition involving the parameters and , for the
occurrence of this tricritical point only, is obtained analytically. Some of
our results are discussed in view of experimental measurements available in the
literature.Comment: 23 pages, 8 figures, accept for publication in Phys. Rev.
Magnetic hysteresis in the Cu-Al-Mn intermetallic alloy: experiments and modeling
We study isothermal magnetization processes in the Cu-Al-Mn intermetallic
alloy. Hysteresis is observed at temperatures below the spin-freezing of the
system. The characteristics of the hysteresis cycles as a function of
temperature and Mn content (magnetic element) are obtained. At low temperature
(5 K) a change from smooth to sharp cycles is observed with increasing Mn
content, which is related to the decrease of configurational disorder. We also
study a zero-temperature site-diluted Ising model, suitable for the description
of this Cu-Al-Mn system. The model reproduces the main features of the
hysteresis loops observed experimentally. It exhibits a disorder-induced
critical line separating a disordered phase from an incipient ferromagnetic
ground-state. The comparison between the model and the experiments allows to
conclude that the observed change in the experimental hysteresis loops can be
understood within the framework of the theory of disorder-induced criticality
in fluctuationless first-order phase transitions.Comment: 30 pages, 15 eps figures, 2 tables. To appear Phys. Rev. B 59 (June
1999
Magnetic hysteresis in the Cu-Al-Mn intermetallic alloy: experiments and modeling
We study isothermal magnetization processes in the Cu-Al-Mn intermetallic
alloy. Hysteresis is observed at temperatures below the spin-freezing of the
system. The characteristics of the hysteresis cycles as a function of
temperature and Mn content (magnetic element) are obtained. At low temperature
(5 K) a change from smooth to sharp cycles is observed with increasing Mn
content, which is related to the decrease of configurational disorder. We also
study a zero-temperature site-diluted Ising model, suitable for the description
of this Cu-Al-Mn system. The model reproduces the main features of the
hysteresis loops observed experimentally. It exhibits a disorder-induced
critical line separating a disordered phase from an incipient ferromagnetic
ground-state. The comparison between the model and the experiments allows to
conclude that the observed change in the experimental hysteresis loops can be
understood within the framework of the theory of disorder-induced criticality
in fluctuationless first-order phase transitions.Comment: 30 pages, 15 eps figures, 2 tables. To appear Phys. Rev. B 59 (June
1999
Local magnetic properties of antiferromagnetic FeBr2
The antiferromagnet FeBr2 has been studied by Mossbauer spectroscopy in external fields both in the metamagnetic region below the multicritical temperature TM&p and in the second-order transition region above. The local magnetization shows that the metamagnetic transition occurs by spin flips, as in simple models. However, in the second-order transition region, the local magnetization of the sublattice oriented antiparallel to the external field varies continuously but remains parallel to the c axis. This can only be understood if the external magnetic field induces strong transversal spin precession of the moments on the antiparallel sublattice. This shows that the anomalous maxima in the imaginary part y" recently found in the ac susceptibility [M.M. Pereira de Azevedo et a/. , J. Magn. Magn. Mater. 140-144, 1557] and denoted H below the critical field Hc(T), and H+ above, can be understood as being caused by noncritical transversal spin fiuctuations
Local Magnetic Properties of Antiferromagnetic FeBr2
The antiferromagnet FeBr2 has been studied by Mössbauer spectroscopy in external fields both in the metamagnetic region below the multicritical temperature TMCP and in the second-order transition region above. The local magnetization shows that the metamagnetic transition occurs by spin flips, as in simple models. However, in the second-order transition region, the local magnetization of the sublattice oriented antiparallel to the external field varies continuously but remains parallel to the c axis. This can only be understood if the external magnetic field induces strong transversal spin precession of the moments on the antiparallel sublattice. This shows that the anomalous maxima in the imaginary part χ\u27\u27 recently found in the ac susceptibility [M.M. Pereira de Azevedo et al., J. Magn. Magn. Mater. 140-144, 1557 (1995)] and denoted H_ below the critical field HC(T), and H+ above, can be understood as being caused by noncritical transversal spin fluctuations
Ising Universality in Three Dimensions: A Monte Carlo Study
We investigate three Ising models on the simple cubic lattice by means of
Monte Carlo methods and finite-size scaling. These models are the spin-1/2
Ising model with nearest-neighbor interactions, a spin-1/2 model with
nearest-neighbor and third-neighbor interactions, and a spin-1 model with
nearest-neighbor interactions. The results are in accurate agreement with the
hypothesis of universality. Analysis of the finite-size scaling behavior
reveals corrections beyond those caused by the leading irrelevant scaling
field. We find that the correction-to-scaling amplitudes are strongly dependent
on the introduction of further-neighbor interactions or a third spin state. In
a spin-1 Ising model, these corrections appear to be very small. This is very
helpful for the determination of the universal constants of the Ising model.
The renormalization exponents of the Ising model are determined as y_t = 1.587
(2), y_h = 2.4815 (15) and y_i = -0.82 (6). The universal ratio Q =
^2/ is equal to 0.6233 (4) for periodic systems with cubic symmetry.
The critical point of the nearest-neighbor spin-1/2 model is K_c=0.2216546
(10).Comment: 25 pages, uuencoded compressed PostScript file (to appear in Journal
of Physics A