7 research outputs found
Chemical Manipulation of Magnetic Ordering in Mn<sub>1–<i>x</i></sub>Sn<sub><i>x</i></sub>Bi<sub>2</sub>Se<sub>4</sub> Solid–Solutions
Several compositions of manganese–tin–bismuth
selenide
solid–solution series, Mn<sub>1–<i>x</i></sub>Sn<sub><i>x</i></sub>Bi<sub>2</sub>Se<sub>4</sub> (<i>x</i> = 0, 0.3, 0.75), were synthesized by combining high purity
elements in the desired ratio at moderate temperatures. X-ray single
crystal studies of a Mn-rich composition (<i>x</i> = 0)
and a Mn-poor phase (<i>x</i> = 0.75) at 100 and 300 K revealed
that the compounds crystallize isostructurally in the monoclinic space
group <i>C</i>2/<i>m</i> (no.12) and adopt the
MnSb<sub>2</sub>Se<sub>4</sub> structure type. Direct current (DC)
magnetic susceptibility measurements in the temperature range from
2 to 300 K indicated that the dominant magnetic ordering within the
Mn<sub>1–<i>x</i></sub>Sn<sub><i>x</i></sub>Bi<sub>2</sub>Se<sub>4</sub> solid–solutions below 50 K switches
from antiferromagnetic (AFM) for MnBi<sub>2</sub>Se<sub>4</sub> (<i>x</i> = 0), to ferromagnetic (FM) for Mn<sub>0.7</sub>Sn<sub>0.3</sub>Bi<sub>2</sub>Se<sub>4</sub> (<i>x</i> = 0.3),
and finally to paramagnetic (PM) for Mn<sub>0.25</sub>Sn<sub>0.75</sub>Bi<sub>2</sub>Se<sub>4</sub> (<i>x</i> = 0.75). We show
that this striking variation in the nature of magnetic ordering within
the Mn<sub>1–<i>x</i></sub>Sn<sub><i>x</i></sub>Bi<sub>2</sub>Se<sub>4</sub> solid–solution series can
be rationalized by taking into account: (1) changes in the distribution
of magnetic centers within the structure arising from the Mn to Sn
substitutions, (2) the contributions of spin-polarized free charge
carriers resulting from the intermixing of Mn and Sn within the same
crystallographic site, and (3) a possible long-range ordering of Mn
and Sn atoms within individual {M}<sub><i>n</i></sub>Se<sub>4<i>n</i>+2</sub> single chain leading to quasi isolated
{MnSe<sub>6</sub>} octahedra spaced by nonmagnetic {SnSe<sub>6</sub>} octahedra
High‑<i>T</i><sub>c</sub> Ferromagnetism and Electron Transport in p‑Type Fe<sub>1–<i>x</i></sub>Sn<sub><i>x</i></sub>Sb<sub>2</sub>Se<sub>4</sub> Semiconductors
Single-phase
polycrystalline powders of Fe<sub>1–<i>x</i></sub>Sn<sub><i>x</i></sub>Sb<sub>2</sub>Se<sub>4</sub> (<i>x</i> = 0 and 0.13) were synthesized by a solid-state reaction
of the elements at 773 K. X-ray diffraction on Fe<sub>0.87</sub>Sn<sub>0.13</sub>Sb<sub>2</sub>Se<sub>4</sub> single-crystal and powder
samples indicates that the compound is isostructural to FeSb<sub>2</sub>Se<sub>4</sub> in the temperature range from 80 to 500 K, crystallizing
in the monoclinic space group <i>C</i>2/<i>m</i> (No. 12). Electron-transport data reveal a marginal alteration in
the resistivity, whereas the thermopower drops by ∼60%. This
suggests a decrease in the activation energy upon isoelectronic substitution
of 13% Fe by Sn. Magnetic susceptibility and magnetization measurements
from 2 to 500 K reveal that the Fe<sub>1–<i>x</i></sub>Sb<sub>2</sub>Sn<sub><i>x</i></sub>Se<sub>4</sub> phases exhibit ferromagnetic behavior up to ∼450 K (<i>x</i> = 0) and 325 K (<i>x</i> = 0.13). Magnetotransport
data for FeSb<sub>2</sub>Se<sub>4</sub> reveal large negative magnetoresistance,
suggesting spin polarization of free carriers in the sample. The high-<i>T</i><sub>c</sub> ferromagnetism in Fe<sub>1–<i>x</i></sub>Sn<sub><i>x</i></sub>Sb<sub>2</sub>Se<sub>4</sub> phases and the decrease in <i>T</i><sub>c</sub> of the Fe<sub>0.87</sub>Sn<sub>0.13</sub>Sb<sub>2</sub>Se<sub>4</sub> sample are rationalized by taking into account (1) the separation
between neighboring magnetic centers in the crystal structures and
(2) the formation of bound magnetic polarons, which overlap to induce
long-range ferromagnetic ordering
Geometrical Spin Frustration and Ferromagnetic Ordering in (Mn<sub><i>x</i></sub>Pb<sub>2–<i>x</i></sub>)Pb<sub>2</sub>Sb<sub>4</sub>Se<sub>10</sub>
Engineering the atomic structure
of an inorganic semiconductor
to create isolated one-dimensional (1D) magnetic subunits that are
embedded within the semiconducting crystal lattice can enable chemical
and electronic manipulation of magnetic ordering within the magnetic
domains, paving the way for (1) the investigation of new physical
phenomena such as the interactions between electron transport and
localized magnetic moments at the atomic scale and (2) the design
and fabrication of geometrically frustrated magnetic materials featuring
cooperative long-range ordering with large magnetic moments. We report
the design, synthesis, crystal structure and magnetic behavior of
(Mn<sub><i>x</i></sub>Pb<sub>2–<i>x</i></sub>)ÂPb<sub>2</sub>Sb<sub>4</sub>Se<sub>10</sub>, a family of three-dimensional
manganese-bearing main-group metal selenides featuring quasi-isolated
[(Mn<sub><i>x</i></sub>Pb<sub>2–<i>x</i></sub>)<sub>3</sub>Se<sub>30</sub>]<sub>∞</sub> hexanuclear
magnetic ladders coherently embedded and uniformly distributed within
a purely inorganic semiconducting framework, [Pb<sub>2</sub>Sb<sub>4</sub>Se<sub>10</sub>]. Careful structural analysis of the magnetic
subunit, [(Mn<sub><i>x</i></sub>Pb<sub>2–<i>x</i></sub>)<sub>3</sub>Se<sub>30</sub>]<sub>∞</sub> and
the temperature dependent magnetic susceptibility of (Mn<sub><i>x</i></sub>Pb<sub>2–<i>x</i></sub>)ÂPb<sub>2</sub>Sb<sub>4</sub>Se<sub>10</sub>, indicate that the compounds are geometrically
frustrated 1D ferromagnets. Interestingly, the degree of geometrical
spin frustration (f) within the magnetic ladders and the strength
of the intrachain antiferromagnetic (AFM) interactions strongly depend
on the concentration (<i>x</i> value) and the distribution
of the Mn atom within the magnetic substructure. The combination of
strong intrachain AFM interactions and geometrical spin frustration
in the [(Mn<sub><i>x</i></sub>Pb<sub>2–<i>x</i></sub>)<sub>3</sub>Se<sub>30</sub>]<sub>∞</sub> ladders results
in a cooperative ferromagnetic order with exceptionally high magnetic
moment at around 125 K. Magnetotransport study of the Mn<sub>2</sub>Pb<sub>2</sub>Sb<sub>4</sub>Se<sub>10</sub> composition over the
temperature range from 100 to 200 K revealed negative magnetoresistance
(NMR) values and also suggested a strong contribution of magnetic
polarons to the observed large effective magnetic moments
Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub>: A Lillianite Homologue with Promising Thermoelectric Properties
Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> crystallizes in the monoclinic space group <i>C</i>2/<i>m</i> (No. 12) with <i>a</i> = 13.991(3)
Ã…, <i>b</i> = 4.262(2) Ã…, <i>c</i> =
23.432(5) Å, and β = 98.3(3)° at 300 K. In its three-dimensional
structure, two NaCl-type layers A and B with respective thicknesses <i>N</i><sub>1</sub> = 5 and <i>N</i><sub>2</sub> = 4
[<i>N</i> = number of edge-sharing (Pb/Bi)ÂSe<sub>6</sub> octahedra along the central diagonal] are arranged along the <i>c</i> axis in such a way that the bridging monocapped trigonal
prisms, PbSe<sub>7</sub>, are located on a pseudomirror plane parallel
to (001). This complex atomic-scale structure results in a remarkably
low thermal conductivity (∼0.33 W m<sup>–1</sup> K<sup>–1</sup> at 300 K). Electronic structure calculations and
diffuse-reflectance measurements indicate that Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> is a narrow-gap semiconductor with an indirect
band gap of 0.23 eV. Multiple peaks and valleys were observed near
the band edges, suggesting that Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> is a promising compound for both n- and p-type doping. Electronic-transport
data on the as-grown material reveal an n-type degenerate semiconducting
behavior with a large thermopower (∼−160 μV K<sup>–1</sup> at 300 K) and a relatively low electrical resistivity.
The inherently low thermal conductivity of Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> and its tunable electronic properties point to a
high thermoelectric figure of merit for properly optimized samples
Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub>: A Lillianite Homologue with Promising Thermoelectric Properties
Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> crystallizes in the monoclinic space group <i>C</i>2/<i>m</i> (No. 12) with <i>a</i> = 13.991(3)
Ã…, <i>b</i> = 4.262(2) Ã…, <i>c</i> =
23.432(5) Å, and β = 98.3(3)° at 300 K. In its three-dimensional
structure, two NaCl-type layers A and B with respective thicknesses <i>N</i><sub>1</sub> = 5 and <i>N</i><sub>2</sub> = 4
[<i>N</i> = number of edge-sharing (Pb/Bi)ÂSe<sub>6</sub> octahedra along the central diagonal] are arranged along the <i>c</i> axis in such a way that the bridging monocapped trigonal
prisms, PbSe<sub>7</sub>, are located on a pseudomirror plane parallel
to (001). This complex atomic-scale structure results in a remarkably
low thermal conductivity (∼0.33 W m<sup>–1</sup> K<sup>–1</sup> at 300 K). Electronic structure calculations and
diffuse-reflectance measurements indicate that Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> is a narrow-gap semiconductor with an indirect
band gap of 0.23 eV. Multiple peaks and valleys were observed near
the band edges, suggesting that Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> is a promising compound for both n- and p-type doping. Electronic-transport
data on the as-grown material reveal an n-type degenerate semiconducting
behavior with a large thermopower (∼−160 μV K<sup>–1</sup> at 300 K) and a relatively low electrical resistivity.
The inherently low thermal conductivity of Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> and its tunable electronic properties point to a
high thermoelectric figure of merit for properly optimized samples
Coexistence of High‑<i>T</i><sub>c</sub> Ferromagnetism and <i>n</i>‑Type Electrical Conductivity in FeBi<sub>2</sub>Se<sub>4</sub>
The
discovery of <i>n</i>-type ferromagnetic semiconductors
(<i>n</i>-FMSs) exhibiting high electrical conductivity
and Curie temperature (<i>T</i><sub>c</sub>) above 300 K
would dramatically improve semiconductor spintronics and pave the
way for the fabrication of spin-based semiconducting devices. However,
the realization of high-<i>T</i><sub>c</sub> <i>n</i>-FMSs and <i>p</i>-FMSs in conventional high-symmetry semiconductors
has proven extremely difficult due to the strongly coupled and interacting
magnetic and semiconducting sublattices. Here we show that decoupling
the two functional sublattices in the low-symmetry semiconductor FeBi<sub>2</sub>Se<sub>4</sub> enables unprecedented coexistence of high <i>n</i>-type electrical conduction and ferromagnetism with <i>T</i><sub>c</sub> ≈ 450 K. The structure of FeBi<sub>2</sub>Se<sub>4</sub> consists of well-ordered magnetic sublattices
built of [Fe<sub><i>n</i></sub>Se<sub>4<i>n</i>+2</sub>]<sub>∞</sub> single-chain edge-sharing octahedra,
coherently embedded within the three-dimensional Bi-rich semiconducting
framework. Magnetotransport data reveal a negative magnetoresistance,
indicating spin-polarization of itinerant conducting electrons. These
findings demonstrate that decoupling magnetic and semiconducting sublattices
allows access to high-<i>T</i><sub>c</sub> <i>n</i>- and <i>p</i>-FMSs as well as helps unveil the mechanism
of carrier-mediated ferromagnetism in spintronic materials
Indium Preferential Distribution Enables Electronic Engineering of Magnetism in FeSb<sub>2–<i>x</i></sub>In<sub><i>x</i></sub>Se<sub>4</sub> p‑Type High-Tc Ferromagnetic Semiconductors
Single-phase samples
of the solid-solution series FeSb<sub>2–<i>x</i></sub>In<sub><i>x</i></sub>Se<sub>4</sub> (0
≤ <i>x</i> ≤ 0.25) were synthesized using
solid-state reaction of the elements to probe the effect of electronic
structure engineering on the magnetic behavior of the p-type semiconductor,
FeSb<sub>2</sub>Se<sub>4</sub>. Powder X-ray diffraction data suggest
that all samples are isostructural with FeSb<sub>2</sub>Se<sub>4</sub>. Rietveld refinements of the distribution of In atoms at various
metal positions indicate a preferential substitution of Sb at the
M1Â(<i>4i</i>) position within the magnetic layer A for In
concentration up to <i>x</i> = 0.1. FeSb<sub>2–<i>x</i></sub>In<sub><i>x</i></sub>Se<sub>4</sub> compositions
with higher In content show the distribution of In atoms at all metal
positions, except for the M3Â(<i>2d</i>), which is fully
occupied by Fe atoms. Interestingly, the ordering of Fe atoms within
the crystal structure of FeSb<sub>2–<i>x</i></sub>In<sub><i>x</i></sub>Se<sub>4</sub> remains essentially
unaffected by the degree of substitution (<i>x</i> values)
and is comparable to the distribution of Fe atoms reported in FeSb<sub>2</sub>Se<sub>4</sub>. X-ray photoelectron spectroscopy confirms
the oxidation states of various metal atoms InÂ(+3), SbÂ(+3), FeÂ(+2)
in the structure. Electronic transport properties indicate p-type
semiconducting behavior for all samples. The electrical conductivity
above 300 K first increases with In content, reaches the maximum value
for <i>x</i> = 0.1, then decreases with further increase
in In content. A reverse trend is observed for the thermopower. All
samples show drastically low thermal conductivity with room temperature
values ranging from 0.45 Wm<sup>–1</sup> K<sup>–1</sup> for <i>x</i> = 0 to 0.27 Wm<sup>–1</sup> K<sup>–1</sup> for the sample with <i>x</i> = 0.25. Magnetic
susceptibility data suggest ferromagnetic-like behavior from 2–300
K for all samples. The magnitude of the magnetic susceptibility rapidly
increases with In content, reaches a maximum for <i>x</i> = 0.1, and marginally decreases with further increase in In concentration.
The observed surprising change in the magnetic and electronic behavior
of samples with high In content (<i>x</i> > 0.1) is rationalized
using the concept of antiferromagnetic scattering of charge carriers
at the interfaces between overlapping bound magnetic polarons from
adjacent layers A and B