Low-dimensional Magnetic Ordering Research Articles

Diverse Structures and Magnetic Properties in Nonlayered Monolayer Chromium Selenide.

Thickness-dependent magnetic behavior has previously been observed in chemical vapor deposition-grown chromium selenide. However, the low-dimensional structure in nonlayered chromium selenide, which plays a crucial role in determining the low-dimensional magnetic order, needs further study. Here, we report the structure-dependent magnetic properties in monolayer CrSe2 and Cr2Se3 grown by molecular beam epitaxy. In the monolayer CrSe2, 1T-CrSe2 with a lattice constant of 3.3 Å has a metallic character, coexisting with the 1T″ phase with 2 × 2 surface periodicity. Monolayer CrSe2 can be transformed into Cr2Se3 with a lattice constant of 3.6 Å by annealing at 300 °C. X-ray magnetic circular dichroism (XMCD) measurements combined with DFT calculations reveal that while the MBE-grown monolayer CrSe2 is antiferromagnetic, monolayer Cr2Se3 is ferromagnetic with a Curie temperature of ∼200 K. This work demonstrates the structural diversity in nonlayered chromium selenide and the critical effect of different structures on its electronic and magnetic properties.

Atomic origin of spin-valve magnetoresistance at the SrRuO3 grain boundary.

Defects exist ubiquitously in crystal materials, and usually exhibit a very different nature from the bulk matrix. Hence, their presence can have significant impacts on the properties of devices. Although it is well accepted that the properties of defects are determined by their unique atomic environments, the precise knowledge of such relationships is far from clear for most oxides because of the complexity of defects and difficulties in characterization. Here, we fabricate a 36.8° SrRuO3 grain boundary of which the transport measurements show a spin-valve magnetoresistance. We identify its atomic arrangement, including oxygen, using scanning transmission electron microscopy and spectroscopy. Based on the as-obtained atomic structure, the density functional theory calculations suggest that the spin-valve magnetoresistance occurs because of dramatically reduced magnetic moments at the boundary. The ability to manipulate magnetic properties at the nanometer scale via defect control allows new strategies to design magnetic/electronic devices with low-dimensional magnetic order.

On the “alpha-phase” of Ca2−xSrxMnO4 and extending the chemistry of Sr7−yCayMn4O15 to y>1

There has been renewed interest in the Ruddlesden-Popper phase (n=2) of composition Can+1MnnO3n+1 in the light of recent research that has highlighted the nature of the improper ferroelectric ground state, which arises due to the couplings between specific combinations of MnO6 octahedral rotations and tilts. A fruitful route to control these octahedral degrees of freedom, and hence such desired physical properties, is through chemical substitution on the A–site cation i.e. Ca2−xSrxMnO4 for n =1, and in light of this, we have reinvestigated the chemistry of this solid solution. Here we focus on a common impurity phase observed during this synthesis which has been termed the “alpha-phase” in the literature. We show that this impurity phase is actually comprised mainly of a structure related to Sr7Mn4O15 but is found here with significantly higher Ca substitution than previously believed possible. Sr7Mn4O15 is an interesting structural type in its own right, but has been mainly overlooked to date, exhibiting interesting physics related to low dimensional magnetic ordering and dimer interactions, and we show here that the structural type is a likely candidate for exhibiting a multiferroic ground state. The prospect of being able to tune the lattice and the exchange interactions through further chemical substitution is likely to lead to a renewed interest in this material.

One-dimensional magnetic order in the metal-organic framework Tb(HCOO)3

A combination of variable-temperature neutron scattering, reverse Monte Carlo analysis and direct Monte Carlo simulation is used to characterise the emergence of magnetic order in the metal--organic framework (MOF) Tb(HCOO)$_3$ over the temperature range 100 K to 1.6 K $=T_{\rm N}$. We show that the magnetic transition at $T_{\rm N}$ involves one-dimensional ferromagnetic ordering to a partially-ordered state related to the triangular Ising antiferromagnet. In this phase, the direction of magnetisation of ferromagnetic chains tends to alternate between neighbouring chains but this alternation is frustrated and is not itself ordered. In neutron scattering measurements this partial order gives rise to Bragg-like peaks, which cannot be interpreted using conventional magnetic crystallography without resort to unphysical spin models. The existence of low-dimensional magnetic order in Tb(HCOO)$_3$ is stabilised by the contrasting strength of inter- and intra-chain magnetic coupling, itself a consequence of the underlying MOF architecture. Our results demonstrate how MOFs may provide an attractive if as yet under-explored platform for the realisation and investigation of low-dimensional physics.

Magnetoelastic driven Negative Thermal Expansion in a Framework Antiferromagnet

Metal-organic frameworks (MOFs) have recently attracted great attention for their multiferroic, magnetocaloric and low dimensional magnetic order. These properties depend on the precise magnetic interactions in frameworks and a deeper understanding of the coupling between the lattice of these materials and their magnetic order is required to underpin the future developments of these promising compounds. Neutron diffraction is the ideal technique for such studies but its application to studies of the magnetic structure of MOFs has been limited to date due to challenges in solving the structure of these dilute yet complex magnetic compounds. We have recently examined the magnetic properties and structure of a new dicarboxylate framework, cobalt adipate, Co(C6H8O4), which adopts P21/c monoclinic symmetry.[1] This compound has been found to order antiferromagnetically at low temperature and fits to neutron diffraction data have shown it adopts Pb21/c magnetic symmetry. Its magnetic structure features sheets of Co cations coupled antiferromagnetically in two dimensions through carboxylate groups. The emergence of this order is accompanied by magnetoelastic coupling, which drives anisotropic negative thermal expansion along the a-axis at low temperature. This is the first evidence for such behaviour in a MOF and we find that both this behaviour and the spin orientation in this material are controlled by the presence of weak ferromagnetic dipole-dipole coupling between the layers of tetrahedral cobalt. Variable temperature high resolution synchrotron X-ray and neutron powder diffraction have also revealed that the monoclinic angle of Co adipate decreases on cooling, passing through a metrically orthorhombic phase without any indication of a phase transition. This unusual behaviour has been rationalised by examining the thermal expansion of the framework along its principal axis, highlighting the importance of such analysis in low symmetry materials.

Nuclear and incommensurate magnetic structure of NaFeGe2O6 between 5 K and 298 K and new data on multiferroic NaFeSi2O6

The compound NaFeGe2O6 was grown synthetically as polycrystalline powder and as large single crystals suitable for X-ray and neutron-diffraction experiments to clarify the low temperature evolution of secondary structural parameters and to determine the low temperature magnetic spins structure. NaFeGe2O6 is isotypic to the clinopyroxene-type compound aegirine and adopts the typical HT-C2/c clinopyroxene structure down to 2.5 K. The Na-bearing M2 polyhedra were identified to show the largest volume expansion between 2.5 K and room temperature, while the GeO4 tetrahedra behave as stiff units. Magnetic susceptibility measurements show a broad maximum around 33 K, which marks the onset of low-dimensional magnetic ordering. Below 12 K NaFeGe2O6 transforms to an incommensurately modulated magnetic spin state, with k = [0.323, 1.0, 0.080] and a helical order of spins within the M1-chains of FeO6 octahedra. This is determined by neutron-diffraction experiments on a single crystal. Comparison of NaFeGe2O6 with NaFeSi2O6 is given and it is shown that the magnetic ordering in the latter compound, aegirine, also is complex and is best described by two different spin states, a commensurate one with C2′/c′ symmetry and an incommensurate one, best being described by a spin density wave, oriented within the (1 0 1) plane.

Topotactic reduction as a synthetic route for the preparation of low-dimensional Mn(II) oxide phases: The structure and magnetism of LaAMnO4-x (A = Sr, Ba)

Reaction of LaSrMnO(4) with CaH(2) at 420 degrees C yields LaSrMnO(3.67(3)). Raising the temperature to 480 degrees C yields the Mn(II) phase LaSrMnO(3.50(2)). Neutron powder diffraction data show both phases adopt body-centred orthorhombic crystal structures (LaSrMnO(3.67(3)), Immm: a = 3.7256(1) A, b = 3.8227(1) A, c = 13.3617(4) A; LaSrMnO(3.50(2)), Immm: a = 3.7810(1) A, b = 3.7936(1) A, c = 13.3974(3) A) with anion vacancies located within the equatorial MnO(2-x) planes of the materials. Analogous reactivity is observed between LaBaMnO(4) and CaH(2) to yield body-centred tetragonal reduced phases (LaBaMnO(3.53(3)), I4/mmm: a = 3.8872(1)A, c = 13.6438(2) A). Low-temperature neutron diffraction and magnetisation data show that LaSrMnO(3.5) and LaBaMnO(3.5) exhibit three-dimensional antiferromagnetic order below 155 K and 135 K respectively. Above these temperatures, they exhibit two-dimensional antiferromagnetic order with paramagnetic behaviour observed above 480 K in both phases. The origin of the low dimensional magnetic order and ordering of the anion vacancies in the reduced phases is discussed.

Magnetic correlations in oxides: Neutron diffraction and neutron depolarization study

We have studied magnetic correlations in several oxide materials that belong to colossal magnetoresistive, naturally occurring layered oxide showing low-dimensional magnetic ordering, solid oxide fuel cell interconnect materials, and magnetic nanoparticles using neutron diffraction and neutron depolarization techniques. In this paper, an overview of some of these results is given.

Fine Structure of the Optical Spectrum and Multiparticle Excitations in Rb2MnCl4

Results of experimental investigations into the optical absorption spectrum of Rb2MnxCd1–xCl4 solid solutions corresponding to 6A1g → 4A1g4Eg transitions in manganese ions are presented. The spectra were measured at variable temperatures, magnetic fields, and concentrations x. The magnetic phase transitions accompanying variations of these parameters cause considerable changes in the spectrum. The exciton and exciton-magnon bands, their cold and hot magnon analogs, and the phonon satellite bands have been detected. The specific features of the optical absorption spectrum caused by the low-dimensional magnetic order are elucidated.

Low-dimensional magnetic ordering and Ising- and XY-like anisotropy of Er3+ in the system ErBa2Cu3Ox with 6

Magnetic ordering of ${\mathrm{Er}}^{3+}$ ions in ${\mathrm{ErBa}}_{2}$${\mathrm{Cu}}_{8}$${\mathrm{O}}_{\mathrm{x}}$ with 6.12\ensuremath{\le}x\ensuremath{\le}6.91 has been studied for both orthorhombic and tetragonal phases by means of magnetic-susceptibility and heat-capacity measurements. Superconductivity in the orthorhombic phase samples was confirmed by the onset of a diamagnetic susceptibility. The superconducting transition temperature ${T}_{c}$ was observed to decrease from 92 to 55 K as x was reduced from 6.91 to 6.31. The tetragonal samples with x=6.23 and 6.12 remained paramagnetic down to 1.2 K and showed no overt trace of diamagnetism. The states of ${\mathrm{Er}}^{3+}$ ions in ${\mathrm{ErBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6}$ and ${\mathrm{ErBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ were calculated by means of crystalline electric field (CEF) theory using both a point-charge model and scaled inelastic neutron scattering results for ${\mathrm{HoBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$.The calculated susceptibility is in excellent agreement with data on the tetragonal specimens for 1.2 K \ensuremath{\le}T\ensuremath{\le}160 K. For the orthorhombic samples, sharp heat-capacity peaks which resemble those for a two-dimensional Ising system and evidently correspond to the magnetic ordering of ${\mathrm{Er}}^{3+}$ moments are found. The transition temperature ${T}_{m}$ decreases smoothly from 0.604 to 0.545 K as x is varied from 6.91 to 6.31. In contrast, tetragonal samples show only a broad rounded heat-capacity anomaly having its maximum at 0.5 K and closely resembling that for a one-dimensional XY model. The change of ${\mathrm{Er}}^{3+}$ ion anisotropy from Ising-like to XY-like is explained by the CEF calculation which shows the ground state to be planar for the tetragonal phase but with the b axis slightly favored in the orthorhombic structure. Randomness in the coupling of ${\mathrm{Er}}^{3+}$ moments caused by structural disorder may effectively reduce the dimensionality of magnetic ordering as has been found in the case of random dilution. These results imply that the dominant magnetic interactions of ${\mathrm{Er}}^{3+}$ ions in these materials are of short range.

Mössbauer studies of the high-Tc superconductor YBa2(Cu1-xFex57)3O7- delta (x=0.033).

The high-${T}_{c}$ superconductor $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ doped with 3.3 at.% $^{57}\mathrm{Fe}$ has been investigated by M\"ossbauer spectroscopy. Measurements on samples prepared under various heat treatments show that Fe substitutes exclusively the Cu(1) sites with M\"ossbauer signals corresponding to tetragonal and orthorhombic local symmetry. The relative abundance of these signals allows the determination of the oxygen-deficiency parameter. M\"ossbauer measurements at low temperatures reveal magnetic hyperfine interactions which are tentatively attributed to low-dimensional magnetic ordering. Deoxygenation of the $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ phase results in the formation of the ${\mathrm{Y}}_{2}$BaCu${\mathrm{O}}_{5}$ phase (the green phase) in a reversible way. The M\"ossbauer properties of the $^{57}\mathrm{F}\mathrm{e}\ensuremath{-}\mathrm{d}\mathrm{o}\mathrm{p}\mathrm{e}\mathrm{d}$ phase are reported.