Dilution-induced enhancement of the blocking temperature in exchange-bias heterosystems

The temperature dependence of the exchange bias field is investigated by superconducting quantum interference device magnetometry in Fe1-xZnxF2(110)/Fe14 nm/Ag35 nm, x=0.4. Its blocking temperature exhibits a significant enhancement with respect to the global ordering temperature TN=46.9 K, of the bulk antiferromagnet Fe0.6Zn0.4F2. The enhancement is attributed to fluctuations of the diamagnetic dilution which creates clusters on all length scales having a Zn dilution of 0\u3c~x\u3c~1. While the infinite clusters give rise to the well-known Griffiths phase, finite clusters also provoke a local enhancement of the exchange bias. The temperature dependence of the integral exchange bias effect is modeled by averaging all local contributions of the antiferromagnetic surface magnetization which exhibit a surface critical behavior

Cerebral differences in explicit and implicit emotional processing - An fMRI study

The processing of emotional facial expression is a major part of social communication and understanding. In addition to explicit processing, facial expressions are also processed rapidly and automatically in the absence of explicit awareness. We investigated 12 healthy subjects by presenting them with an implicit and explicit emotional paradigm. The subjects reacted significantly faster in implicit than in explicit trials but did not differ in their error ratio. For the implicit condition increased signals were observed in particular in the thalami, the hippocampi, the frontal inferior gyri and the right middle temporal region. The analysis of the explicit condition showed increased blood-oxygen-level-dependent signals especially in the caudate nucleus, the cingulum and the right prefrontal cortex. The direct comparison of these 2 different processes revealed increased activity for explicit trials in the inferior, superior and middle frontal gyri, the middle cingulum and left parietal regions. Additional signal increases were detected in occipital regions, the cerebellum, and the right angular and lingual gyrus. Our data partially confirm the hypothesis of different neural substrates for the processing of implicit and explicit emotional stimuli. Copyright (c) 2007 S. Karger AG, Basel

Models for the magnetic ac susceptibility of granular superferromagnetic CoFe/Al2_2O3_3

The magnetization and magnetic ac susceptibility, $\chi = \chi' - i \chi''$, of superferromagnetic systems are studied by numerical simulations. The Cole-Cole plot, $\chi''$ vs. $\chi'$, is used as a tool for classifying magnetic systems by their dynamical behavior. The simulations of the magnetization hysteresis and the ac susceptibility are performed with two approaches for a driven domain wall in random media. The studies are motivated by recent experimental results on the interacting nanoparticle system Co$_{80}$Fe$_{20}$/Al$_{2}$O$_{3}$ showing superferromagnetic behavior. Its Cole-Cole plot indicates domain wall motion dynamics similarly to a disordered ferromagnet, including pinning and sliding motion. With our models we can successfully reproduce the features found in the experimental Cole-Cole plots.Comment: 8 pages, 6 figure

Electric-field-induced phase transition of <001> oriented Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals

oriented 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 single crystals were poled under different electric fields, i.e. Epoling=4 kV/cm and Epoling=13 kV/cm. In addition to the temperature-dependent dielectric constant measurement, X-ray diffraction was also used to identify the poling-induced phase transitions. Results showed that the phase transition significantly depends on the poling intensity. A weaker field (Epoling=4 kV/cm) can overcome the effect of random internal field to perform the phase transition from rhombohedral ferroelectric state with short range ordering (microdomain) FESRO to rhombohedral ferroelectric state with long range ordering (macrodomain) FElRO. But the rhombohedral ferroelectric to tetragonal ferroelectric phase transition originating from to polarization rotation can only be induced by a stronger field (Epoling=13 kV/cm). The sample poled at Epoling=4 kV/cm showed higher piezoelectric constant, d33>1500 pC/N, than the sample poled at Epoling=13 kV/cm.Comment: 7 pages, 2 figure

Interface driven magnetoelectric effects in granular CrO2

Antiferromagnetic and magnetoelectric Cr2O3-surfaces strongly affect the electronic properties in half metallic CrO2. We show the presence of a Cr2O3 surface layer on CrO3 grains by high-resolution transmission electron microscopy. The effect of these surface layers is demonstrated by measurements of the temperature variation of the magnetoelectric susceptibility. A major observation is a sign change at about 100 K followed by a monotonic rise as a function of temperature. These electric field induced moments in CrO3 are correlated with the magnetoelectric susceptibility of pure Cr2O3. This study indicates that it is important to take into account the magnetoelectric character of thin surface layers of Cr2O3 in granular CrO2 for better understanding the transport mechanism in this system. The observation of a finite magnetoelectric susceptibility near room temperature may find utility in device applications.Comment: Figure 1 with strongly reduced resolutio

Random Field Models for Relaxor Ferroelectric Behavior

Heat bath Monte Carlo simulations have been used to study a four-state clock model with a type of random field on simple cubic lattices. The model has the standard nonrandom two-spin exchange term with coupling energy $J$ and a random field which consists of adding an energy $D$ to one of the four spin states, chosen randomly at each site. This Ashkin-Teller-like model does not separate; the two random-field Ising model components are coupled. When $D / J = 3$, the ground states of the model remain fully aligned. When $D / J \ge 4$, a different type of ground state is found, in which the occupation of two of the four spin states is close to 50%, and the other two are nearly absent. This means that one of the Ising components is almost completely ordered, while the other one has only short-range correlations. A large peak in the structure factor $S (k)$ appears at small $k$ for temperatures well above the transition to long-range order, and the appearance of this peak is associated with slow, "glassy" dynamics. The phase transition into the state where one Ising component is long-range ordered appears to be first order, but the latent heat is very small.Comment: 7 pages + 12 eps figures, to appear in Phys Rev