16 research outputs found
Dipolar ground state of planar spins on triangular lattices
An infinite triangular lattice of classical dipolar spins is usually
considered to have a ferromagnetic ground state. We examine the validity of
this statement for finite lattices and in the limit of large lattices. We find
that the ground state of rectangular arrays is strongly dependent on size and
aspect ratio. Three results emerge that are significant for understanding the
ground state properties: i) formation of domain walls is energetically favored
for aspect ratios below a critical valu e; ii) the vortex state is always
energetically favored in the thermodynamic limit of an infinite number of
spins, but nevertheless such a configuration may not be observed even in very
large lattices if the aspect ratio is large; iii) finite range approximations
to actual dipole sums may not provide the correct ground sta te configuration
because the ferromagnetic state is linearly unstable and the domain wall energy
is negative for any finite range cutoff.Comment: Several short parts have been rewritten. Accepted for publication as
a Rapid Communication in Phys. Rev.
Single-domain versus two-domain configuration in thin ferromagnetic prisms
Thin ferromagnetic elements in the form of rectangular prisms are
theoretically investigated in order to study the transition from single-domain
to two-domain state, with changing the in-plane aspect ratio p. We address two
main questions: first, how general is the transition; second, how the critical
value p_c depends on the physical parameters. We use two complementary methods:
discrete-lattice calculations and a micromagnetic continuum approach. Ultrathin
films do not appear to split in two domains. Instead, thicker films may undergo
the above transition. We have used the continuum approach to analyze recent
Magnetic Force Microscopy observations in 30 nm-thick patterned Permalloy
elements, finding a good agreement for p_c.Comment: 22 pages, 5 figure
Selection rules for Single-Chain-Magnet behavior in non-collinear Ising systems
The magnetic behavior of molecular Single-Chain Magnets is investigated in
the framework of a one-dimensional Ising model with single spin-flip Glauber
dynamics. Opportune modifications to the original theory are required in order
to account for reciprocal non-collinearity of local anisotropy axes and the
crystallographic (laboratory) frame. The extension of Glauber's theory to the
case of a collinear Ising ferrimagnetic chain is also discussed. Within this
formalism, both the dynamics of magnetization reversal in zero field and the
response of the system to a weak magnetic field, oscillating in time, are
studied. Depending on the geometry, selection rules are found for the
occurrence of slow relaxation of the magnetization at low temperatures, as well
as for resonant behavior of the a.c. susceptibility as a function of
temperature at low frequencies. The present theory applies successfully to some
real systems, namely Mn-, Dy-, and Co-based molecular magnetic chains, showing
that Single-Chain-Magnet behavior is not only a feature of collinear ferro- and
ferrimagnetic, but also of canted antiferromagnetic chains.Comment: 15 pages, 6 figure
Dipolar interaction between two-dimensional magnetic particles
We determine the effective dipolar interaction between single domain
two-dimensional ferromagnetic particles (islands or dots), taking into account
their finite size. The first correction term decays as 1/D^5, where D is the
distance between particles. If the particles are arranged in a regular
two-dimensional array and are magnetized in plane, we show that the correction
term reinforces the antiferromagnetic character of the ground state in a square
lattice, and the ferromagnetic one in a triangular lattice. We also determine
the dipolar spin-wave spectrum and evaluate how the Curie temperature of an
ensemble of magnetic particles scales with the parameters defining the particle
array: height and size of each particle, and interparticle distance. Our
results show that dipolar coupling between particles might induce ferromagnetic
long range order at experimentally relevant temperatures. However, depending on
the size of the particles, such a collective phenomenon may be disguised by
superparamagnetism.Comment: 11 pages, 5 figure
Strongly hybridized dipole-exchange spin waves in thin Fe-N ferromagnetic films
International audienc
Complement Regulatory Protein Crry/p65-mediated Signaling in T Lymphocytes: Role of Its Cytoplasmic Domain and Partitioning into Lipid Rafts
Crry/p65 is a type I glycoprotein, which protects mouse T cells from complement attack. We have previously shown that complement receptor I-related protein Crry/p65 (Crry) ligation has a costimulatory effect on mouse CD4+ T cell activation. Here, we have examined the mechanisms responsible for Crry costimulation, addressing the question of whether Crry potentiates signal transduction starting at the T cell receptor (TCR)/CD3 complex or promotes distinct costimulatory signals. We show that Crry increases early TCR-dependent activation signals, including p56lck-, zeta-associated protein-70 (ZAP-70), Vav-1, Akt, and extracellular signal-regulated kinase (ERK) phosphorylation but also costimulation-dependent mitogen-activated protein kinases (MAPK), such as the stress-activated c-Jun N-terminal kinase (JNK). It is intriguing that Crry costimulus enhanced p38 MAPK activation in T helper cell type 1 (Th1) but not in Th2 cells. A fraction of Crry is found consistently in the detergent-insoluble membrane fraction of Th1 or Th2 cells or CD4+ lymphoblasts. Crry costimulation induced clustering of lipid rafts, increasing their content in Crry, CD3epsilon, and p59-60 forms of p56lck, and caused actin polymerization close to the site of activation in Th2 cells. Such events were inhibited by wortmannin, suggesting a role for phosphatidylinositol-3 kinase in these effects. The Crry cytoplasmic domain was required for JNK activation and interleukin-4 secretion but not for the presence of Crry in rafts or activation of p56lck, ZAP-70, Akt, Vav-1, or ERK. This suggests that Crry costimulation involves two different but not mutually exclusive signal transduction modules. The dual function of Crry as a complement regulatory protein and as a T cell costimulator illustrates the importance of complement regulatory proteins as links between innate and adaptive immunity
Stripe domains reorientation in ferromagnetic films with perpendicular magnetic anisotropy
International audienceFerromagnetic thin films with moderate perpendicular magnetic anisotropy (PMA) are known to support weak stripe domains provided film thickness exceeds a critical value. In this work, we performed both an experimental and theoretical investigation of a peculiar phenomenon shown by weak stripe domains: namely, the stripe domains reorientation when a dc magnetic field is applied in the film plane along the direction perpendicular to the stripes axis. We focus on bct α ′-Fe 8 N 1−x thin films obtained by N 2 irradiation of α-Fe films epitaxially grown on ZnSe/ GaAs(001). By using different ion implantation and heat treatment conditions, we show that it is possible to tune the PMA values. Magnetic force microscopy and vibrating sample magnetometer measurements prove the existence of weak stripe domains at remanence, and of a threshold field for the reorientation of the stripes axis in a transversal field. Using a one-dimensional model of the magnetic stripe domains, where the essential parameter is the maximum canting angle of the stripe magnetization out of the film plane, the various contributions to the magnetic energy can be separately calculated. A linear increase of the reorientation threshold field Stripe domains reorientation in ferromagnetic films with PMA 2 on the PMA is obtained, in qualitative agreement with experimental data in our Fe-N films, as well as in other thin films with weak stripe domains. Finally, we find that also the rotatable anisotropy field linearly increases as a function of the PMA magnitude
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