510 research outputs found
Absence of room temperature ferromagnetism in bulk Mn-doped ZnO
Structural and magnetic properties have been studied for polycrystalline
Zn_1-xMn_xO (x=0.02, 0.03, 0.05). Low-temperature (~500 oC) synthesis leaves
unreacted starting ZnO and manganese oxides. Contrary to a recent report, no
bulk ferromagnetism was observed for single-phase materials synthesized in air
at temperatures above 900 oC. Single-phase samples show paramagnetic
Curie-Weiss behavior.Comment: Accepted for publication in J. Appl. Phys., RevTeX, 3 pages, 4
figure
Absence of ferromagnetism in Co and Mn substituted polycrystalline ZnO
We discuss the properties of semiconducting bulk ZnO when substituted with
the magnetic transition metal ions Mn and Co, with substituent fraction ranging
from = 0.02 to = 0.15. The magnetic properties were measured as a
function of magnetic field and temperature and we find no evidence for magnetic
ordering in these systems down to = 2 K. The magnetization can be fit by
the sum of a Curie-Weiss term with a Weiss temperature of 100 K and
a Curie term. We attribute this behavior to contributions from both \textit{t}M
ions with \textit{t}M nearest neighbors and from isolated spins. This
particular functional form for the susceptibility is used to explain why no
ordering is observed in \textit{t}M substituted ZnO samples despite the large
values of the Weiss temperature. We also discuss in detail the methods we used
to minimize any impurity contributions to the magnetic signal.Comment: 6 pages, 4 figures (revised
Structural and physical properties of perovskites
We combine the results of magnetic and transport measurements with neutron
diffraction data to construct the structural and magnetic phase diagram of the
entire family of SrMnRuO ()
perovskites. We have found antiferromagnetic ordering of the C type for lightly
Ru-substituted materials () in a similar manner
to SrMnO (=La, Pr), due to the generation of Mn in
both families of manganite perovskites by either -site substitution of
Ru for Mn or -site substitution of for Sr.
This similarity is driven by the same ratio of / ions in both
classes of materials for equivalent substitution level. In both cases, a
tetragonal lattice distortion is observed, which for some compositions () is coupled to a C-type AF transition and results in
a first order magnetic and resistive transition. Heavily substituted
SrMnRuO materials are ferromagnetic due to dominating
exchange interactions between the Ru ions. Intermediate substitution
() leads to a spin-glass behavior instead of a
quantum critical point reported previously in single crystals, due to enhanced
disorder.Comment: 9 pages, 10 figures, accepted for publication in Physical Review
The long-term variations of critical frequency in the F2 region of ionosphere and solar activity
Tuning of magnetic and electronic states by control of oxygen content in lanthanum strontium cobaltites
We report on the magnetic, resistive, and structural studies of perovskite
LaSrCoO. By using the relation of synthesis
temperature and oxygen partial pressure to oxygen stoichiometry obtained from
thermogravimetric analysis, we have synthesized a series of samples with
precisely controlled . These samples show three structural
phases at , , , and two-phase
behavior for other oxygen contents. The stoichiometric material with
is a cubic ferromagnetic metal with the Curie temperature K. The increase of to 0.15 is followed by a linear decrease of
to 160 K and a metal-insulator transition near the
boundary of the cubic structure range. Further increase of results in
formation of a tetragonal phase for
and a brownmillerite phase for . At low
temperatures, these are weak ferromagnetic insulators (canted antiferromagnets)
with magnetic transitions at and 120 K, respectively. At
higher temperatures, the phase is -type
antiferromagnetic between 230 K and 360 K. Low temperature magnetic
properties of this system for can be described in terms of a
mixture of Co ions in the low-spin state and Co ions in the
intermediate-spin state and a possible spin transition of Co to the
intermediate-spin state above . For , there appears to
be a combination of Co and Co ions, both in the high-spin state
with dominating antiferromagnetic interactions.Comment: RevTeX, 9 pages, 7 figures, to be published in Physical Review
Carrier-induced ferromagnetism in p-Zn1-xMnxTe
We present a systematic study of the ferromagnetic transition induced by the
holes in nitrogen doped Zn1-xMnxTe epitaxial layers, with particular emphasis
on the values of the Curie-Weiss temperature as a function of the carrier and
spin concentrations. The data are obtained from thorough analyses of the
results of magnetization, magnetoresistance and spin-dependent Hall effect
measurements. The experimental findings compare favorably, without adjustable
parameters, with the prediction of the Rudermann-Kittel-Kasuya-Yosida (RKKY)
model or its continuous-medium limit, that is, the Zener model, provided that
the presence of the competing antiferromagnetic spin-spin superexchange
interaction is taken into account, and the complex structure of the valence
band is properly incorporated into the calculation of the spin susceptibility
of the hole liquid. In general terms, the findings demonstrate how the
interplay between the ferromagnetic RKKY interaction, carrier localization, and
intrinsic antiferromagnetic superexchange affects the ordering temperature and
the saturation value of magnetization in magnetically and electrostatically
disordered systems.Comment: 14 pages, 10 figure
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