Spin Arrangement Research Articles

Strong Magnetocaloric Coupling in Oxyorthosilicate with Dense Gd3+ Spins.

Searching for working refrigerant materials is the key element in the design of magnetic cooling devices. Herein, we report on the thermodynamic and magnetocaloric parameters of an X1 phase oxyorthosilicate, Gd2SiO5, by field-dependent static magnetization and specific heat measurements. An overall correlation strength of |J|S2 ≈ 3.4 K is derived via the mean-field estimate, with antiferromagnetic correlations between the ferromagnetically coupled Gd-Gd layers. The magnetic entropy change -ΔSm is quite impressive, reaches 0.40 J K-1 cm-3 (58.5 J K-1 kg-1) at T = 2.7 K, with the largest adiabatic temperature change Tad = 23.2 K for a field change of 8.9 T. At T = 20 K, the lattice entropy SL is small enough compared to the magnetic entropy Sm, Sm/SL = 21.3, which warrants its potential in 2 -20 K cryocoolers with both the Stirling and Carnot cycles. Though with relatively large exchange interactions, the layered A-type spin arrangement ultimately enhances the magnetocaloric coupling, raising the possibilities of designing magnetic refrigerants with a high ratio of cooling capacity to volume.

Direct correlation between spin states and magnetic torques in a room-temperature van der Waals antiferromagnet

Explorations of van der Waals (vdW) antiferromagnets have revealed new avenues for understanding the fundamentals of highly anisotropic magnetism and realizing spin-based functional properties. However, there is a serious limitation to the feasibility of spintronic applications at room temperature owing to the lack of suitable materials. In this work, we examined the anisotropic magnetic characteristics of Co-doped Fe5GeTe2, a high-TN antiferromagnet with TN = 350 K in which magnetic multilayers are intrinsically formed. Our spin-model calculations with uniaxial anisotropy quantify the magnetocrystalline anisotropy energy and visualize the specific spin arrangements varying in the presence of rotating magnetic fields at room temperature. We further show that the spin configurations can be profoundly relevant to the distinctive evolution of magnetic torques in different magnetic phases. Our advanced approach offers a high-TN vdW antiferromagnet as a magnetic platform to establish room-temperature spin-processing functionalities.

Ground States of Heisenberg Spin Clusters from a Cluster-Based Projected Hartree–Fock Approach

Recent work on approximating ground states of Heisenberg spin clusters by projected Hartree–Fock theory (PHF) is extended to a cluster-based ansatz (cPHF). Whereas PHF variationally optimizes a site–spin product state for the restoration of spin- and point-group symmetry, cPHF groups sites into discrete clusters and uses a cluster-product state as the broken-symmetry reference. Intracluster correlation is thus already included at the mean-field level, and intercluster correlation is introduced through symmetry projection. Variants of cPHF differing in the broken and restored symmetries are evaluated for ground states and singlet-triplet gaps of antiferromagnetic spin rings for various cluster sizes, where cPHF in general affords a significant improvement over ordinary PHF, although the division into clusters lowers the cyclical symmetry. In contrast, certain two- or three-dimensional spin arrangements permit cluster groupings compatible with the full spatial symmetry. We accordingly demonstrate that cPHF yields approximate ground states with correct spin- and point-group quantum numbers for honeycomb lattice fragments and symmetric polyhedra.

Structural, Magnetic and Vibrational Properties of Van Der Waals Ferromagnet CrBr3 at High Pressure

The crystal and magnetic structures of van der Waals layered ferromagnet CrBr3 were studied using X-ray powder diffraction and neutron powder diffraction at pressures up to 23 GPa at ambient temperature and up to 2.8 GPa in the temperature range 6-300 K, respectively. The vibration spectra of CrBr3 were studied using Raman spectroscopy at pressures up to 23 GPa at ambient temperature. The anomalous pressure behavior of structural parameters and vibrational modes was observed, associated with a gradual isostructural phase transition in the pressure range 2.5-7 GPa. The Curie temperature TC reduced rapidly with a pressure coefficient dTC/dP=-4.1(4) K/GPa. A full suppression of the ferromagnetic state was expected at PC~8.4 GPa, where onset of the antiferromagnetic spin arrangement or magnetically disordered state may take place. Anomalies in Raman spectra at P~15 GPa point to another possible phase transformation in CrBr3, which may be related to the proximity of metallization of this van der Waals ferromagnet.

Electronic and thermal transport properties of the metallic antiferromagnet MnSn2.

We report the structure, magnetic and electrical/thermal transport properties of the antiferromagnet MnSn2. Importantly, the existence of the two antiferromagnetic states below TN2 (∼320 K) is confirmed by magnetism and electrical transport measurements. An unsaturated positive magnetoresistance up to 150% at ∼9 T was observed at 5 K, whereas the magnetoresistance becomes negative in the whole range at high temperatures (T > 74 K). Systematic investigations of the Hall transport and thermoelectric properties reveal that the hole-type carriers are the majority carriers in MnSn2. The kink around 320 K in the Seebeck coefficient originates from the effect of the antiferromagnetic phase on the band structure, while the pronounced peak around 231 K is attributed to the phonon-drag effect. The results suggest that the spin arrangement plays a vital role in the magnetic, electrical, and thermal transport properties in MnSn2.

Oxygen octahedra distortion‐induced multiferroic properties of Er and Zr co‐doped BiFeO3 nanoparticles

AbstractThe present work unveiled the distortion of oxygen octahedra influencing magnetic and magnetoelectric properties of novel Bi1−xErxFe1−yZryO3 (x = 0, .05, .1, y = .02, .05) polycrystalline nanoparticles by sol–gel route. X‐ray diffraction patterns analysis reveals that pristine BiFeO3 and doped BiFeO3 are crystalized in the rhombohedral structure (R3c). The Fe–O–Fe bond angle of Bi1−xErxFe1−yZryO3 (x = 0, .05, .1, y = .02, .05) varies between 141° and 159.62° as the concentration of Er (via Bi site) and Zr (via Fe site) ions increases in BiFeO3. As a result, the tilt angle of oxygen octahedra and the canting angle of spiral spin arrangement increase. Hence, the maximum magnetization varies between .03144 and .37558 emu/g in Er and Zr co‐doped BiFeO3 system. The number of electrons per unit cell of Bi1−xErxFe1−yZryO3 (x = 0, .05, .1, y = .02, .05) lies between 733.38 and 831, respectively. Further, the number of coherently diffracting domains increases from 3.07 to 5.21, and then it decreases when Er and Zr are increased in BiFeO3. Consequently, the magnetoelectric coupling coefficient varies between .0265 and .2511 mV/cm Oe, respectively. Particularly, Bi0.95Er0.05Fe0.98Zr0.02O3 shows enhanced magnetic and magnetoelectric behaviors compared to other samples.

Alternating Magnetic Field Induced Magnetic Heating in Ferromagnetic Cobalt Single-Atom Catalysts for Efficient Oxygen Evolution Reaction.

Alternating magnetic field (AMF) is a promising methodology for further improving magnetic single-atom catalyst (SAC) activity toward oxygen evolution reaction (OER). Herein, the anchoring of Co single atoms on MoS2 support (Co@MoS2), leading to the appearance of in-plane room-temperature ferromagnetic properties, is favorable for the parallel spin arrangement of oxygen atoms when a magnetic field is applied. Moreover, field-assisted electrocatalytic experiments confirmed that the spin direction of Co@MoS2 is changing with the applied magnetic field. On this basis, under AMF, the active sites in ferromagnetic Co@MoS2 were heated by exploiting the magnetic heating generated from spin polarization flip of these SACs to further expedite OER efficiency, with overpotential at 10 mA cm-2 reduced from 317 mV to 250 mV. This work introduces a feasible and efficient approach to enhance the OER performance of Co@MoS2 by AMF, shedding some light on the further development of magnetic SACs for energy conversion.

Magnetic ground states of honeycomb lattice Wigner crystals

Lattice Wigner crystal states stabilized by long-range Coulomb interactions have recently been realized in two-dimensional moiré materials. We employ large-scale unrestricted Hartree-Fock techniques to unveil the magnetic phase diagrams of honeycomb lattice Wigner crystals. For the three lattice filling factors with the largest charge gaps, n=2/3,1/2,1/3, the magnetic phase diagrams contain multiple phases, including ones with non-collinear and non-coplanar spin arrangements. We discuss magnetization evolution with external magnetic field, which has potential as an experimental signature of exotic spin states. Our theoretical results could potentially be validated in moiré materials formed from group VI transition metal dichalcogenide twisted homobilayers.

Easily exfoliable monolayer of GdTe[formula omitted]: ab initio study

A recently synthesized material GdTe3 has shown very interesting properties like superconductivity and high carrier mobility in its bulk and flake form. Here, we have shown the formation of a monolayer of GdTe3 from its bulk parent material by means of exfoliation. We have also studied magnetic ground state and electronic properties of bulk and two-dimensional (2D) GdTe3, using ab initio calculations. We find both of them to have the same anti-ferromagnetic spin arrangement in their ground state. Our calculations further reveal that both bulk and 2D-GdTe3 are metallic, mainly because of the electronic contribution arising from the Te-atoms. The magnetic ground state does not change on the inclusion of the Hubbard-U parameter, calculated self-consistently to have a value of 5.7 eV for the Gd-f electrons. The spin–orbit coupling (SOC) calculations in 2D-GdTe3, for both U = 0 and 5.7 eV, show that the system prefers an in-plane anti-ferromagnetic ordering and has an easy magnetization axis along the crystallographic a-direction.

Chiral Spin Textures for Next-Generation Memory and Unconventional Computing

The realization of chiral spin textures - comprising myriad distinct, nanoscale arrangements of spins with topological properties - has established pathways for engineering robust, energy-efficient and scalable elements for non-volatile nanoelectronics. Particularly, current-induced manipulation of spin textures in nanowire racetracks and tunnel junction based devices are actively investigated for applications in memory, logic and unconventional computing. In this article, we paint a background on the progress of spin textures, as well as the relevant state-of-the-art techniques used for their development. In particular, we clarify the competing energy landscape of chiral spin textures, such as skyrmions and chiral domain walls, to tune their size, density and zero-field stability. Next, we discuss the spin texture phenomenology and their response to extrinsic factors arising from geometric constraints, inter-wire interactions and thermal-electrical effects. Finally, we reveal promising chiral spintronic memory and neuromorphic devices, and discuss emerging material and device engineering opportunities.

Cuprate Oxyfluorides La2Cu0.8Ni0.2O3F2 and La2CuO3F2 with "Channel-like" Anion Ordering.

Highly fluorinated cuprate Ruddlesden-Popper oxyfluorides La2Cu0.8Ni0.2O3F2 and La2CuO3F2 were obtained by topochemical reaction between poly(vinylidene fluoride) (PVDF) and the corresponding oxides La2Cu0.8Ni0.2O4 and La2CuO4 prepared by citrate-based soft chemistry synthesis. The crystal structures of both oxyfluorides were investigated by powder diffraction techniques. The structure of La2Cu0.8Ni0.2O3F2 was solved based on combined neutron and X-ray powder diffraction. It crystallizes in a new monoclinic distorted version [C2/c a = 13.1880(3) Å, b = 5.7244(1) Å, c = 5.6007(1) Å, and β = 90.85(1)°] of the anionic ordered structure lately reported for La2NiO3F2. For La2CuO3F2, an even less symmetrical triclinic structure was derived from X-ray powder diffraction data [P1̅ a = 5.6180(5) Å, b = 5.7316(6) Å, c = 7.1978(9) Å, α = 113.32(1)°, β = 90.89(9)°, and γ = 90.16(11)°]. For both compounds, an additional tilt component of the partially Jahn-Teller elongated (Cu,Ni)O4F2 octahedra was found as the origin for the lowered symmetry. The formation reaction of La2CuO3F2 was studied by in situ XRD measurements. In these investigations, two new reaction intermediates were identified. The magnetic properties of both oxyfluorides La2Cu0.8Ni0.2O3F2 and La2CuO3F2 were characterized by field- and temperature-dependent measurements. An antiferromagnetic ordering with TN = 240 K was found for La2Cu0.8Ni0.2O3F2. In La2CuO3F2, additional weak ferrimagnetism was observed, resulting in a pronounced hysteresis but a weak saturation moment, which was attributed to result from a canted antiferromagnetic spin arrangement.

Orbital Angular Momentum of Magnons in Collinear Magnets.

We study the orbital angular momentum of magnons for collinear ferromagnet (FM) and antiferromagnetic (AF) systems with nontrivial networks of exchange interactions. The orbital angular momentum of magnons for AF and FM zigzag and honeycomb lattices becomes nonzero when the lattice contains two inequivalent sites and is largest at the avoided-crossing points or extremum of the frequency bands. Hence, the arrangement of exchange interactions may play a more important role at producing the orbital angular momentum of magnons than the spin-orbit coupling energy and the resulting noncollinear arrangement of spins.

Nanosheet fabrication from magnon thermal conductivity cuprates for the advanced thermal management

Spin-chain–spin-ladder cuprates, such as La5Ca9Cu24O41, have notable electronic and thermal properties because of their unique electron spin arrangement. Among them, magnon thermal conductivity, which originates from the excitation of paired electron spins, is promising for the advanced thermal management applications that enable dynamic control of heat flow. This is because of its high, anisotropic thermal conductivity at room temperature and its dynamic controllability. In this study, we report nanosheet fabrications from polycrystalline La5Ca9Cu24O41 to enhance the control width. We obtained that the nanosheets with a thickness of about 3 nm are obtained via immersion of the polycrystals in a NaOH aqueous solution. We propose the exfoliation model based on the chemical reaction between the (La/Ca)2Cu2O3 subsystem and NaOH solution. The nanosheet fabrications can also lead to new research development on spin-ladder system and other strongly correlated cuprates.

Comparative Study on the Magnetic and Transport Properties of B-Site Ordered and Disordered CaCu3Fe2Os2O12.

The B-site Fe/Os ordered and disordered quadruple perovskite oxides CaCu3Fe2Os2O12 were synthesized under different high-pressure and high-temperature conditions. The B-site ordered CaCu3Fe2Os2O12 is a system with a very high ferrimagnetic ordering temperature of 580 K having the Cu2+(↑)Fe3+(↑)Os5+(↓) charge and spin arrangement. In comparison, the highly disordered CaCu3Fe2Os2O12 has a reduced magnetic transition temperature of about 350 K. The Cu2+Fe3+Os5+ charge combination remains the same without any sign of changes in the valence state of the constituent ions. Although the average net moments of each sublattice are reduced, the average ferrimagnetic spin arrangement is unaltered. The robustness of the basic magnetic properties of CaCu3Fe2Os2O12 against site disorder may be taken as an indication of the tendency to maintain the short-range order of the atomic constituents.

Nuclear magnetic resonance diffraction with subangstrom precision

We have combined ultrasensitive force-based spin detection with high-fidelity spin control to achieve NMR diffraction (NMRd) measurement of ~2 million [Formula: see text]P spins in a [Formula: see text] volume of an indium-phosphide (InP) nanowire. NMRd is a technique originally proposed for studying the structure of periodic arrangements of spins, with complete access to the spectroscopic capabilities of NMR. We describe two experiments that realize NMRd detection with subangstrom precision. In the first experiment, we encode a nanometer-scale spatial modulation of the z-axis magnetization of [Formula: see text]P spins and detect the period and position of the modulation with a precision of <0.8 Å. In the second experiment, we demonstrate an interferometric technique, utilizing NMRd, to detect an angstrom-scale displacement of the InP sample with a precision of 0.07 Å. The diffraction-based techniques developed in this work extend the Fourier-encoding capabilities of NMR to the angstrom scale and demonstrate the potential of NMRd as a tool for probing the structure and dynamics of nanocrystalline materials.

Exploration of structural and magnetic aspects of biocompatible cobalt ferrite nanoparticles with canted spin configuration and assessment of their selective anti-leukemic efficacy

Single phase CoFe2O4 nanoparticles (CF NPs) were prepared by a simple co-precipitation method. The cation distribution over A and B sites was estimated from the Rietveld analysis. In addition, different structural parameters, viz, oxygen positional parameter, bond angle, bond length, and strength of different interactions were also determined. The M-H loop at room temperature with moderate coercivity (∼204 Oe) and remanent magnetization (∼0.85 emu/gm) implies that the sample is ferromagnetic in nature and suitable for drug delivery/biomedical applications. The magnetization does not change appreciably with temperature due to the Yafet-Kittel spin arrangement on the B sublattice. For the evolution of the anti-leukemic activity of CF NPs, cell viability assay was performed against the K562 cell line, as well as the cytotoxicity was determined against healthy human blood cells. Significant anti-leukemic efficacy was found in a time and concentration dependent manner (IC50: 27.77 µg/ml); whereas in the case of normal cells, a higher level of IC50 value (87.03 µg/ml) was observed. Oxidative stress was induced after treatment with CF NPs in cancer cells, which was reported by measuring cellular redox status, oxidative stress marker, and antioxidant enzyme activity. As evident by fluorescence imaging, the increased level of ROS produced DNA damage and ultimately leads to the death of the cancer cells.

Magnetic structure and properties of the vanthoffite mineral Na6Mn(SO4)4

A detailed analysis of the magnetic properties of a vanthoffite type mineral Na6Mn(SO4)4 basedon dc magnetization, low temperature neutron powder diffraction and theoretical calculations is reported. The mineral crystallizes in a monoclinic system with space group P21/c, where MnO6 octahedra are linked via SO4 tetrahedra. This gives rise to super-exchange interaction between two Mn2+ ions mediated by two nonmagnetic bridging anions and leads to an antiferromagnetic ordering below 3 K. The magnetic structure derived from neutron powder diffraction at 1.7 K depicts an antiferromagnetic spin arrangement in the bc plane of the crystal. The magnetic properties are modelled by numerical calculations using exact diagonalization technique, which fits the experimental results and provides antiferromagnetic ground state of Na6Mn(SO4)4.

Implications of electron and hole doping on the magnetic properties of spin–orbit entangled Ca4IrO6 from DFT calculations

We investigate the electronic structure and magnetic properties of a Jeff=1/2 iridate Ca4IrO6 and the implications of doping electrons and holes using ab initio density functional theory. Our calculations considering spin–orbit interaction reveal that although the Mott-insulating parent compound transforms into a conductor upon doping, antiferromagnetism sustains in the doped system, albeit with a grossly noncollinear arrangement of the spins. We find a strong spin–orbit interaction and magneto-crystalline anisotropy, causing frustration in the system, possibly leading to the highly noncollinear arrangement of spins upon non-magnetic doping. Our results may be important from the viewpoint of spintronics using iridates or other 5d materials.

Excitonic transverse and amplitude fluctuations in noncollinear and charge-ordered RbFe2+Fe3+F6

RbFe$^{2+}$Fe$^{3+}$F$_{6}$ is an example of an antiferromagnet with charge ordering of the octahedrally coordinated Fe$^{2+}$ and Fe$^{3+}$ ions. As well as different spin values, Fe$^{2+}$ ($S=2$) and Fe$^{3+}$ ($S={5\over2}$) possess differing orbital ground states with Fe$^{2+}$ having an orbital degeneracy with an effective orbital angular momentum of $l=1$. The resulting low temperature magnetic structure is non collinear with the spins aligned perpendicular to nearest neighbors (S. W. Kim \textit{et al.} Chem. Sci. {\bf{3}}, 741 (2012)). The combination of an orbital degeneracy and non collinear spin arrangements introduces the possibility for unusual types of excitations such as amplitude modes of the order parameter. In this paper we investigate this by applying a multi-level analysis to model neutron spectroscopy data (M. Songvilay \textit{et al.} Phys. Rev. Lett. {\bf{121}}, 087201 (2018)). In particular, we discuss the possible origins of the momentum and energy broadened continuum scattering observed in terms of amplitude fluctuations allowed through the presence of an orbital degree of freedom on the Fe$^{2+}$ site. We extend previous spin-orbit exciton models based on a collinear spin structure to understand the measured low-energy excitations and also to predict and discuss possible amplitude mode scattering in RbFe$^{2+}$Fe$^{3+}$F$_{6}$.

Orthogonal antiferromagnetism to canted ferromagnetism in CaCo3Ti4O12 quadruple perovskite driven by underlying kagome lattices

AA′3B4O12 quadruple perovskites, with magnetic A′ and non-magnetic B cations, are characterized by a wide range of complex magnetic structures. These are due to a variety of competing spin-exchange interactions up to the fourth nearest neighbours. Here, we synthesize and characterize the magnetic behaviour of the CaCo3Ti4O12 quadruple perovskite. We find that in the absence of an external magnetic field, the system undergoes antiferromagnetic ordering at 9.3 K. This magnetic structure consists of three interpenetrating mutually orthogonal magnetic sublattices. Under an applied magnetic field, this antiferromagnetic structure evolves into a canted ferromagnetic structure. In explaining these magnetic structures, as well as the seemingly unrelated magnetic structures found in other quadruple perovskites, we suggest a crucial role played by the underlying kagome lattices in these systems. All observed magnetic structures of these materials represent indeed one of the three possible ways to reduce spin frustration in the A′ site kagome layers. More specifically, our survey of the magnetic structures observed for quadruple perovskites AA′3B4O12 reveals the following three ways to reduce spin frustration, namely to make each layer ferromagnetic, to adopt a compromise 120° spin arrangement in each layer, or to have a magnetic structure with a vanishing sum of all second nearest-neighbour spin exchanges.