353 research outputs found
XYZ-polarisation analysis of diffuse magnetic neutron scattering from single crystals
Studies of diffuse magnetic scattering largely benefit from the use of a
multi-detector covering wide scattering angles. Therefore, the different
contributions to the diffuse scattering that originate from magnetic, nuclear
coherent, and nuclear spin-incoherent scattering can be separated by the
so-called XYZ-polarization analysis. In the past this method has been
successfully applied to the analysis of diffuse scattering by polycrystalline
samples of magnetic disordered materials. Single crystal studies that exploit
the vector properties of spin correlations are of particular interest for
furthering our understanding of frustration effects in magnetism. Based on the
symmetry properties of polarised scattering a suitable extension of the
conventional XYZ method has been derived, which allows for the complete
separation and the analysis of features of diffuse magnetic scattering from
single crystals.Comment: 6 pages 2 figures, revised as published, one Eq. removed, minor
corrections, typos correcte
Magnetisation distribution in the tetragonal phase of BaFe2As2
We have determined the spatial distribution of the magnetisation induced by a
field of 9 T in the tetragonal phase of BaFe2As2 using polarised neutron
diffraction. Magnetic structure factors derived from the polarisation
dependence of the intensities of Bragg reflections were used to make a maximum
entropy reconstruction of the distribution projected on the 110 plane. The
reconstruction shows clearly that the magnetisation is confined to the region
around the iron atoms and that there is no significant magnetisation associated
with either the As or Ba atoms. The distribution of magnetisation around the Fe
atom is significantly non-spherical with a shape which is extended in the
directions in the projection. These results show that the electrons which give
rise to the paramagnetic susceptibility are confined to the Fe atoms their
distribution suggests that they occupy 3d t_2g type orbitals with about 60% in
those of xy symmetry
Magnetization distribution and orbital moment in the non-Superconducting Chalcogenide Compound K0.8Fe1.6Se2
We have used polarized and unpolarized neutron diffraction to determine the
spatial distribution of the magnetization density induced by a magnetic field
of 9 T in the tetragonal phase of K0.8Fe1.6Se2. The maximum entropy
reconstruction shows clearly that most of the magnetization is confined to the
region around the iron atoms whereas there is no significant magnetization
associated with either Se or K atoms. The distribution of magnetization around
the Fe atom is slightly nonspherical with a shape which is extended along the
[0 0 1] direction in the projection. Multipolar refinement results show that
the electrons which give rise to the paramagnetic susceptibility are confined
to the Fe atoms and their distribution suggests that they occupy 3d t2g-type
orbitals with around 66% in those of xz/yz symmetry. Detail modeling of the
magnetic form factor indicates the presence of an orbital moment to the total
paramagnetic moment of Fe2+Comment: 7 pages, accepted for publication in Physical Review
Approaching the Ground State of Frustrated A-site Spinels: A Combined Magnetization and Polarized Neutron Scattering Study
We re-investigate the magnetically frustrated, {\it
diamond-lattice-antiferromagnet} spinels FeAlO and MnAlO using
magnetization measurements and diffuse scattering of polarized neutrons. In
FeAlO, macroscopic measurements evidence a "cusp" in zero field-cooled
susceptibility around 13~K. Dynamic magnetic susceptibility and {\it memory
effect} experiments provide results that do not conform with a canonical
spin-glass scenario in this material. Through polarized neutron scattering
studies, absence of long-range magnetic order down to 4~K is confirmed in
FeAlO. By modeling the powder averaged differential magnetic neutron
scattering cross-section, we estimate that the spin-spin correlations in this
compound extend up to the third nearest-neighbour shell. The estimated value of
the Land\'{e} factor points towards orbital contributions from Fe.
This is also supported by a Curie-Weiss analysis of the magnetic
susceptibility. MnAlO, on the contrary, undergoes a magnetic phase
transition into a long-range ordered state below 40~K, which is
confirmed by macroscopic measurements and polarized neutron diffraction.
However, the polarized neutron studies reveal the existence of prominent
spin-fluctuations co-existing with long-range antiferromagnetic order. The
magnetic diffuse intensity suggests a similar short range order as in
FeAlO. Results of the present work supports the importance of spin-spin
correlations in understanding magnetic response of frustrated magnets like
-site spinels which have predominant short-range spin correlations
reminiscent of the "spin liquid" state.Comment: 10 pages, 10 figures, double-column, accepted in Phys. Rev. B, 201
Magnetic Excitations in the Ground State of
We report an extensive study on the zero field ground state of a powder
sample of the pyrochlore . A sharp heat capacity anomaly
that labels a low temperature phase transition in this material is observed at
280 mK. Neutron diffraction shows that a \emph{quasi-collinear} ferromagnetic
order develops below with a magnetic moment of
. High resolution inelastic neutron scattering
measurements show, below the phase transition temperature, sharp gapped
low-lying magnetic excitations coexisting with a remnant quasielastic
contribution likely associated with persistent spin fluctuations. Moreover, a
broad inelastic continuum of excitations at meV is observed from the
lowest measured temperature up to at least 2.5 K. At 10 K, the continuum has
vanished and a broad quasielastic conventional paramagnetic scattering takes
place at the observed energy range. Finally, we show that the exchange
parameters obtained within the framework of linear spin-wave theory do not
accurately describe the observed zero field inelastic neutron scattering data.Comment: 11 pages, 9 figures, Phys. Rev. B. (accepted
Magnetic and structural transitions in LaNaFeAs single crystals
LaNaFeAs single crystals have been grown out of an
NaAs flux in an alumina crucible and characterized by measuring magnetic
susceptibility, electrical resistivity, specific heat, as well as single
crystal x-ray and neutron diffraction. LaNaFeAs single
crystals show a structural phase transition from a high temperature tetragonal
phase to a low-temperature orthorhombic phase at T\,=\,125\,K. This
structural transition is accompanied by an anomaly in the temperature
dependence of electrical resistivity, anisotropic magnetic susceptibility, and
specific heat. Concomitant with the structural phase transition, the Fe moments
order along the \emph{a} direction with an ordered moment of
0.7(1)\, at \emph{T}\,=\,5 K. The low temperature stripe
antiferromagnetic structure is the same as that in other
\emph{A}FeAs (\emph{A}\,=\,Ca, Sr, Ba) compounds.
LaNaFeAs provides a new material platform for the
study of iron-based superconductors where the electron-hole asymmetry could be
studied by simply varying La/Na ratio.Comment: 9 pages, 7 figures, to appear in Physical Review
Structural and magnetic properties of ErTbmultilayers
Abstract.: We have investigated the structural and magnetic properties of Er|Tb multilayers by different scattering methods. Diffuse X-ray scattering under grazing incidence reveals the interface structure in Er|Tb bilayers and trilayers, indicating vertically correlated roughness between the Er and Tb interfaces. The magnetic properties of ErnEr|TbnTb superlattices have been studied as a function of the superlattice composition (indices denote the number of atomic layers). Coupled ferromagnetic structures exist in all investigated samples. The phase transition temperature varies with the Tb layer thickness. Modulated magnetic order is short range for all samples beside the Er20|Tb5 superlattice, the sample with the smallest Tb layer thickness. We observe dipolar antiferromagnetic coupling between single ferromagnetic Tb layers in all samples, with the onset of this ordering depending on the Tb layer thickness. Due to competing interactions, exchange coupling is limited to the interface near region. Therefore long range modulated magnetic order is observed in the Er20|Tb5 superlattice only, where the interface regions overlap. The distinct differences to the magnetic structure of an Er0.8Tb0.2 alloy film are explained by a highly anisotropic arrangement of neighbouring atoms due to the correlated roughnes
PENGARUH PENAMBAHAN POTONGAN KARUNG PLASTIK SEBAGAI BAHAN CAMPURAN UNTUK MEMPERKUAT TIMBUNAN TANAH LEMPUNG
Terbatasnya lahan untuk pembangunan yang diperlukan mengakibatkan tidak dapat dihindarinya
pembangunan di atas tanah lempung seperti tanah yang ada di Indonesia. Oleh karena itu, sangat
penting untuk mencampurkan bahan tambahan sebagai campuran ke dalam tanah lempung tersebut
untuk meningkatkan kualitas daya dukung tanah pada tanah lempung. Karena banyaknya
permintaan karung plastik, limbah karung plastik menjadi semakin banyak, maka peneliti mencoba
untuk memanfaatkan serat karung plastik tersebut sebagai bahan campuran pada tanah lempung
benda uji. Dengan mencampurkan potongan serat karung plastik ke dalam tanah lempung,
diharapkan tanah lempung campuran serat karung plastik tersebut dapat meningkatkan kepadatan
tanah dan dapat memperkuat kuat geser tanah.
Studi ini meliputi, studi literatur dan penelitian di Laboratorium Mekanika Tanah ITN Malang
pada tanggal 7 April – 19 Juli 2019. Tanah benda uji dalam penelitian ini berasal dari Ds.
Argotirto, Kec. Sumbermanjing Wetan, Kab. Malang dan bahan campuran potongan serat karung
plastik sepanjang ≥ 1cm - ≤ 3cm dengan variasi kadar campuran; 1%, 1,5%, 2%, 2,5%, 3% dari
berat total tanah saat kadar air optimum. Penelitian ini meliputi pengujian sifat fisik dan sifat
teknis. Pengujian sifat fisik meliputi; Kadar Air Tanah Asli (w), Berat Jenis (Gs), Analisa
Saringan, Analisa Hidrometer, Uji Plastisitas (LL, PL, SL), Tingkat Pengembangan, dan Tingkat
Keaktifan (Activity). Pengujian sifat teknis meliputi; Compaction Standard dan Modified, Triaxial
UU, dan Unconfined Compression, masing-masing pengujian terdiri dari 3 sampel. Dengan
dibantu uji hipotesis data untuk mengetahui adanya pengaruh atau tidak adanya pengaruh dalam
pengujian tanah lempung campuran serat karung plastik tersebut.
Berdasarkan hasil pengujian, tanah benda uji penelitian tersebut termasuk dalam tanah MH (Lanau
anorganik atau pasir halus diatomac, atau lanau diatomac, lanau yang elastis). Nilai hasil
pengujian campuran serat karung plastik cenderung meningkat dibandingkan tanah tanpa
campuran. Nilai tertinggi untuk bahan timbunan mendapatkan prosentase kadar campuran serat
karung plastik sebanyak 3% pada pengujian Modified Compaction sebesar; 1,528g/cm3, dengan
nilai kadar air (w) sebesar; 26,89%. Pada pengujian Triaxial UU, mendapatkan nilai Kohesi (c)
sebesar; 0,350kg/cm², dengan nilai Sudut Geser (φ) sebesar; 6,277 pada kadar campuran 3%. Dan
pada pengujian Unconfined Compression, mendapatkan nilai Tegangan Normal (qu) sebesar;
0,681kgf/cm2 pada kadar campuran 1%
Soft X-ray resonant scattering study of single-crystal LaSrMnO
Soft X-ray resonant scattering studies at the Mn - and
the La - edges of single-crystal LaSrMnO are
reported. At low temperatures, below K, energy scans
with a fixed momentum transfer at the \emph{A}-type antiferromagnetic (0 0 1)
reflection around the Mn -edges with incident linear
and polarizations show strong resonant enhancements. The
splitting of the energy spectra around the Mn -edges may
indicate the presence of a mixed valence state, e.g., Mn/Mn. The
relative intensities of the resonance and the clear shoulder-feature as well as
the strong incident and polarization dependences strongly
indicate its complex electronic origin. Unexpected enhancement of the charge
Bragg (0 0 2) reflection at the La -edges with
polarization has been observed up to 300 K, with an anomaly appearing around
the orbital-ordering transition temperature, K,
suggesting a strong coupling (competition) between them.Comment: Accepted by European Physical Journal
Topological magnons driven by the Dzyaloshinskii-Moriya interaction in the centrosymmetric ferromagnet MnGe
The phase of the quantum-mechanical wave function can encode a topological
structure with wide-ranging physical consequences, such as anomalous transport
effects and the existence of edge states robust against perturbations. While
this has been exhaustively demonstrated for electrons, properties associated
with the elementary quasiparticles in magnetic materials are still
underexplored. Here, we show theoretically and via inelastic neutron scattering
experiments that the bulk ferromagnet MnGe hosts gapped topological
Dirac magnons. Although inversion symmetry prohibits a net
Dzyaloshinskii-Moriya interaction in the unit cell, it is locally allowed and
is responsible for the gap opening in the magnon spectrum. This gap is
predicted and experimentally verified to close by rotating the magnetization
away from the -axis with an applied magnetic field. Hence, MnGe
realizes a gapped Dirac magnon material in three dimensions. Its tunability by
chemical doping or by thin film nanostructuring defines an exciting new
platform to explore and design topological magnons. More generally, our
experimental route to verify and control the topological character of the
magnons is applicable to bulk centrosymmetric hexagonal materials, which calls
for systematic investigation.Comment: 24 pages, 4 figures. Accepted in Nature Communication
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