174 research outputs found
Coexistence of ferro- and antiferromagnetic order in Mn-doped NiMnGa
Ni-Mn-Ga is interesting as a prototype of a magnetic shape-memory alloy
showing large magnetic field induced strains. We present here results for the
magnetic ordering of Mn-rich Ni-Mn-Ga alloys based on both experiments and
theory. Experimental trends for the composition dependence of the magnetization
are measured by a vibrating sample magnetometer (VSM) in magnetic fields of up
to several tesla and at low temperatures. The saturation magnetization has a
maximum near the stoichiometric composition and it decreases with increasing Mn
content. This unexpected behaviour is interpreted via first-principles
calculations within the density-functional theory. We show that extra Mn atoms
are antiferromagnetically aligned to the other moments, which explains the
dependence of the magnetization on composition. In addition, the effect of Mn
doping on the stabilization of the structural phases and on the magnetic
anisotropy energy is demonstrated.Comment: 4 pages, 3 figure
A two dimensional model for ferromagnetic martensites
We consider a recently introduced 2-D square-to-rectangle martensite model
that explains several unusual features of martensites to study ferromagnetic
martensites. The strain order parameter is coupled to the magnetic order
parameter through a 4-state clock model. Studies are carried out for several
combinations of the ordering of the Curie temperatures of the austenite and
martensite phases and, the martensite transformation temperature. We find that
the orientation of the magnetic order which generally points along the short
axis of the rectangular variant, changes as one crosses the twin or the
martensite-austenite interface. The model shows the possibility of a subtle
interplay between the growth of strain and magnetic order parameters as the
temperature is decreased. In some cases, this leads to qualitatively different
magnetization curves from those predicted by earlier mean field models.
Further, we find that strain morphology can be substantially altered by the
magnetic order. We have also studied the dynamic hysteresis behavior.
The corresponding dissipation during the forward and reverse cycles has
features similar to the Barkhausen's noise.Comment: 9 pages, 11 figure
Non-equilibrium emission of complex fragments from p+Au collisions at 2.5 GeV proton beam energy
Energy and angular dependence of double differential cross sections
d/ddE was measured for reactions induced by 2.5 GeV protons
on Au target with isotopic identification of light products (H, He, Li, Be, and
B) and with elemental identification of heavier intermediate mass fragments (C,
N, O, F, Ne, Na, Mg, and Al). It was found that two different reaction
mechanisms give comparable contributions to the cross sections. The
intranuclear cascade of nucleon-nucleon collisions followed by evaporation from
an equilibrated residuum describes low energy part of the energy distributions
whereas another reaction mechanism is responsible for high energy part of the
spectra of composite particles. Phenomenological model description of the
differential cross sections by isotropic emission from two moving sources led
to a very good description of all measured data. Values of the extracted
parameters of the emitting sources are compatible with the hypothesis claiming
that the high energy particles emerge from pre-equilibrium processes consisting
in a breakup of the target into three groups of nucleons; small, fast and hot
fireball of 8 nucleons, and two larger, excited prefragments, which
emits the light charged particles and intermediate mass fragments. The smaller
of them contains 20 nucleons and moves with velocity larger than the CM
velocity of the proton projectile and the target. The heavier prefragment
behaves similarly as the heavy residuum of the intranuclear cascade of
nucleon-nucleon collisions. %The mass and charge dependence of the total
production cross %sections was extracted from the above analysis for all
observed %reaction products. This dependence follows the power low behavior
%(A or Z)
Study of the 3He-eta system in d-p collisions at COSY-11
We present preliminary results from dp --> 3He X (X=pi0,eta) measurements
near the eta production threshold. The data were taken during a slow ramping of
the COSY internal deuteron beam scattered on a proton target. The 3He ejectiles
were registered with the COSY-11 detection setup. The ongoing data analysis
should deliver high precision data for the dp --> 3He eta total and
differential cross sections for the excess energies in the range from threshold
up to 9 MeV. The preliminary excitation function for the reaction dp --> 3He
pi0 does not show any structure which could originate from the decay of 3He-eta
bound state. We present also a threshold excitation curve for the dp --> 3He X
channel. Contrary to corresponding results from SATURNE we see no cusp in the
vicinity of the eta threshold.Comment: 8 pages; 5 figures; Talk at Eta05 Workshop on Production and Decay of
eta and eta-prime Mesons, Cracow, Poland, 15-18 Sep 2005; Acta Phys. Slovaca,
in prin
Drift chamber with a c-shaped frame
We present the construction of a planar drift chamber with wires stretched
between two arms of a c-shaped aluminium frame. The special shape of the frame
allows to extendthe momentum acceptance of the COSY-11 detection system towards
lower momenta without suppressing the high momentum particles. The proposed
design allows for construction of tracking detectors covering small angles with
respect to the beam, which can be installed and removed without dismounting the
beam-pipe. For a three-dimensional track reconstruction a computer code was
developed using a simple algorithm of hit preselection.Comment: submitted to Nucl. Instr. & Meth
Structural, thermal, and magnetic properties of Ni[sub 2]MnGa
The two main effects underlying the magnetic shape memory effect in Ni2MnGa are martensitic transformations and magnetic anisotropy energies. Both issues are addressed here with first-principles calculations. First, we examine how the tetragonality in the martensitic phase varies with the composition. Then, the actual transformation is investigated by comparing the free energies of different phases. The transition from the cubic structure to the tetragonal structure with c/a=1.27 is driven by the vibrational free energy and occurs at a temperature of 200 K which is in the experimental range. Finally, we focus on the magnetic anisotropy energy for the tetragonal structure with c/a=0.94. It is shown to be a magnetically nearly ideal uniaxial system determined by the first-order anisotropy constant. However, it is estimated that the twinned microstructure can cause higher-order anisotropies to show up in the measured anisotropy.Peer reviewe
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