1,114 research outputs found
Mixed magnetic phases in (Ga,Mn)As epilayers
Two different ferromagnetic-paramagnetic transitions are detected in
(Ga,Mn)As/GaAs(001) epilayers from ac susceptibility measurements: transition
at a higher temperature results from (Ga,Mn)As cluster phases with [110]
uniaxial anisotropy and that at a lower temperature is associated with a
ferromagnetic (Ga,Mn)As matrix with cubic anisotropy. A change in the
magnetic easy axis from [100] to [110] with increasing temperature can be
explained by the reduced contribution of cubic anisotropy to the magnetic
properties above the transition temperature of the (Ga,Mn)As matrix
Effect of Ga irradiation on magnetic and magnetotransport properties in (Ga,Mn)As epilayers
We report on the magnetic and magnetotransport properties of ferromagnetic
semiconductor (Ga,Mn)As modified by Ga ion irradiation using focused ion
beam. A marked reduction in the conductivity and the Curie temperature is
induced after the irradiation. Furthermore, an enhanced negative
magnetoresistance (MR) and a change in the magnetization reversal process are
also demonstrated at 4 K. Raman scattering spectra indicate a decrease in the
concentration of hole carriers after the irradiation, and a possible origin of
the change in the magnetic properties is discussed
Ion Irradiation Control of Ferromagnetism in (Ga,Mn)As
We report on a promising approach to the artificial modification of
ferromagnetic properties in (Ga,Mn)As using a Ga focused ion beam (FIB)
technique. The ferromagnetic properties of (Ga,Mn)As such as magnetic
anisotropy and Curie temperature can be controlled using Ga ion
irradiation, originating from a change in hole concentration and the
corresponding systematic variation in exchange interaction between Mn spins.
This change in hole concentration is also verified using micro-Raman
spectroscopy. We envisage that this approach offers a means of modifying the
ferromagnetic properties of magnetic semiconductors on the micro- or nano-meter
scale.Comment: 4 pages, 4 figures, to appear in Jpn. J. Appl. Phys. (Part 2 Letters
Magnetic anisotropy switching in (Ga,Mn)As with increasing hole concentration
We study a possible mechanism of the switching of the magnetic easy axis as a
function of hole concentration in (Ga,Mn)As epilayers. In-plane uniaxial
magnetic anisotropy along [110] is found to exceed intrinsic cubic
magnetocrystalline anisotropy above a hole concentration of p = 1.5 * 10^21
cm^-3 at 4 K. This anisotropy switching can also be realized by post-growth
annealing, and the temperature-dependent ac susceptibility is significantly
changed with increasing annealing time. On the basis of our recent scenario
[Phys. Rev. Lett. 94, 147203 (2005); Phys. Rev. B 73, 155204 (2006).], we
deduce that the growth of highly hole-concentrated cluster regions with [110]
uniaxial anisotropy is likely the predominant cause of the enhancement in [110]
uniaxial anisotropy at the high hole concentration regime. We can clearly rule
out anisotropic lattice strain as a possible origin of the switching of the
magnetic anisotropy.Comment: 5 pages, 4 figures, to appear in Phys. Rev.
Dynamic relaxation of magnetic clusters in a ferromagnetic (Ga,Mn)As epilayer
A new scenario of the mechanism of intriguing ferromagnetic properties in
Mn-doped magnetic semiconductor (Ga,Mn)As is examined in detail. We find that
magnetic features seen in zero-field cooled and field cooled magnetizations are
not interpreted with a single domain model [Phys. Rev. Lett. 95, 217204
(2005)], and the magnetic relaxation, which is similar to that seen in magnetic
particles and granular systems, is becoming significant at temperatures above
the lower-temperature peak in the temperature dependence of ac susceptibility,
supporting the cluster/matrix model reported in our previous work [Phys. Rev.
Lett. 94, 147203 (2005)]. Cole-Cole analysis reveals that magnetic interactions
between such (Ga,Mn)As clusters are significant at temperatures below the
higher-temperature peak in the temperature dependent ac susceptibility. The
magnetizations of these films disappear above the temperature showing the
higher-temperature peak, which is generally referred to as the Curie
temperature. However, we suggest that these combined results are evidence that
the temperature is actually the blocking temperature of (Ga,Mn)As clusters with
a relatively high hole concentration compared to the (Ga,Mn)As matrix.Comment: 8 pages, 7 figures, to appear in Phys. Rev.
Search for Near-Infrared Pulsation of the Anomalous X-ray Pulsar 4U 0142+61
We have searched for pulsation of the anomalous X-ray pulsar (AXP) 4U 0142+61
in the K' band ( m) using the fast-readout mode
of IRCS at the Subaru 8.2-m telescope. We found no significant signal at the
pulse frequency expected by the precise ephemeris obtained by the X-ray
monitoring observation with RXTE. Nonetheless, we obtained a best upper limit
of 17% (90% C.L.) for the root-mean-square pulse fraction in the K' band.
Combined with i' band pulsation (Dhillon et al. 2005), the slope of the pulsed
component () was constrained to (90%
C.L.) for an interstellar extinction of .Comment: 11 pages, 3 figures, Accepted for publication in PAS
Inclination Effects and Beaming in Black Hole X-ray Binaries
We investigate the dependence of observational properties of black hole X-ray
binaries on the inclination angle i of their orbits. We find the following: (1)
Transient black hole binaries show no trend in their quiescent X-ray
luminosities as a function of i, suggesting that the radiation is not
significantly beamed. This is consistent with emission from an accretion disk.
If the X-rays are from a jet, then the Lorentz factor gamma of the jet is less
than 1.24 at the 90% confidence level. (2) The X-ray binary 4U1543-47 with i of
order 21 degrees has a surprisingly strong fluorescent iron line in the high
soft state. Quantifying an earlier argument by Park et al. (2004), we conclude
that if the continuum X-ray emission in this source is from a jet, then gamma <
1.04. (3) None of the known binaries has cos i 75 degrees. This
fact, plus the lack of eclipses among the 20 black hole binaries in our sample,
strongly suggests at the 99.5% confidence level that systems with large
inclination angles are hidden from view. The obscuration could be the result of
disk flaring, as suggested by Milgrom (1978) for neutron star X-ray binaries.
(4) Transient black hole binaries with i ~ 70-75 degrees have significantly
more complex X-ray light curves than systems with i < 65 degrees. This may be
the result of variable obscuration and/or variable height above the disk of the
radiating gas.Comment: 26 pages, to appear in The Astrophysical Journal, vol. 624, May 1,
200
Broad band X-ray spectroscopy of A0535+262 with SUZAKU
The transient X-ray binary pulsar A0535+262 was observed with Suzaku on 2005
September 14 when the source was in the declining phase of the August-September
minor outburst. The ~103 s X-ray pulse profile was strongly energy dependent, a
double peaked profile at soft X-ray energy band (<3 keV) and a single peaked
smooth profile at hard X-rays. The width of the primary dip is found to be
increasing with energy. The broad-band energy spectrum of the pulsar is well
described with a Negative and Positive power-law with EXponential (NPEX)
continuum model along with a blackbody component for soft excess. A weak iron
K_alpha emission line with an equivalent width ~25 eV was detected in the
source spectrum. The blackbody component is found to be pulsating over the
pulse phase implying the accretion column and/or the inner edge of the
accretion disk may be the possible emission site of the soft excess in
A0535+262. The higher value of the column density is believed to be the cause
of the secondary dip at the soft X-ray energy band. The iron line equivalent
width is found to be constant (within errors) over the pulse phase. However, a
sinusoidal type of flux variation of iron emission line, in phase with the hard
X-ray flux suggests that the inner accretion disk is the possible emission
region of the iron fluorescence line.Comment: 21 pages, 6 figures. Accepted for publication in the Astrophysical
Journal, 2008 January issu
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