Formation Of Magnetic Clusters Research Articles

Spin-Glass States Generated in a van der Waals Magnet by Alkali-Ion Intercalation.

Tuning magnetic properties in layered van der Waals (vdW) materials has captured significant attention due to the efficient control of ground states by heterostructuring and external stimuli. Electron doping by electrostatic gating, interfacial charge transfer, and intercalation is particularly effective in manipulating the exchange and spin-orbit properties, resulting in a control of Curie temperature (TC) and magnetic anisotropy. Here, an uncharted role of intercalation is discovered to generate magnetic frustration. As a model study, Na atoms are intercalated into the vdW gaps of pristine Cr2Ge2Te6 (CGT) where generated magnetic frustration leads to emerging spin-glass states coexisting with a ferromagnetic order. A series of dynamic magnetic susceptibility measurements/analysis confirms the formation of magnetic clusters representing slow dynamics with a distribution of relaxation times. The intercalation also modifies other macroscopic physical parameters including the significant enhancement of TC from 66 to 240K and the switching of magnetic easy-hard axis direction. This study identifies intercalation as a unique route to generate emerging frustrated spin states in simple vdW crystals.

Evolution of ferrimagnetism against Griffiths singularity in calcium ruthenate

The magnetism in the correlated metal CaRuO3 is enigmatic as it is poised near a triple point among the ferromagnetic, antiferromagnetic, and paramagnetic ground states. Here we report a detailed work on structural, spectroscopic, magnetic, and transport properties in CaRu 1−x Cr x O3. We find that Cr doping reduces the orthorhombicity in CaRuO3. Surprisingly, a tiny (x = 0.01) amount of Cr-doping drives the magnetic ground state from ‘paramagnetic-like’ to ferrimagnetic. Slightly higher Cr-doping (x = 0.05) results formation of magnetic clusters which gives rise to Griffiths singularity and power law divergence in magnetic susceptibility. The magnetism in CaRu 1−x Cr x O3 is explained in terms of ‘seven atom’ ferrimagnetic clusters. Electrical transport shows a gradual evolution of a non-metallic state upon Cr-doping. In particular, for x ⩾ 0.1, the temperature-dependent resistivity follows Mott-VRH conduction. The XPS study also supports significant role of disorder and electron correlation which effectively reduces the itinerant character of electrons. Finally, a new T-x phase diagram is constructed depicting the evolution of electronic and magnetic state in CaRu 1−x Cr x O3.

P and Fe doping, a strategy to develop light and magnetic responsive multifunctional materials: The case of LiMn2O4

The current work reports an unprecedented multifunctional material with optical activity and a modified magnetic response by a unique combination of doping with P and Fe into the spinel LiMn2O4. Through inductively coupled plasma – optical emission spectroscopy, X-ray absorption near-edge spectroscopy, X-ray diffraction and scanning transmission electron microscopy, the chemical composition, oxidation state and the crystalline structure are determined. Solid-state UV-Vis spectroscopy, magnetic susceptibility and electronic conductivity reveal the critical importance of the interaction between iron and phosphorus when simultaneously doping the crystalline structure of LiMn2O4. The presence of Fe and P considerably increases charge carrier concentration as a mechanism for enhancing electronic conductivity. Fe and P doping also creates Fe-Fe spin interactions that allow double electron optical excitations. This opens a pathway to create multifunctional materials for light-assisted charging lithium-ion batteries. P doping also induces the formation of magnetic clusters arising from the Fe-O-Fe, Fe-O-Mn and Mn-O-Mn spin exchange interactions. The magnetic response of the materials is strongly influenced by the relative amount of Fe in octahedral or tetrahedral sites of the spinel structure. Such ferrimagnetic behaviour has not been reported before LiMn2O4 doped with Fe or P separately. The potential applicability of this newly identified magnetic feature was demonstrated by a significant capacity gain when a lithium-ion cell is exposed to a static external magnetic field.

Synthesis of a magneto-chromatic tunable Fe3O4-HPAM colloid

Colloids of soft magnetic (Fe3O4) submicrometer particles, formed by magnetic nanoparticle clusters, coated with a polyelectrolyte (hydrolyzed polyacrylamide; HPAM) were fabricated successfully in situ with a highly controllable chemical approach. The formation mechanism of submicron particles was analyzed under the amounts of HPAM, urea, citrate, and metallic precursors. This mechanism was followed by SEM. The results showed that the physicochemical properties of the HPAM favored physical interactions with the reactants, promoting the formation of dense-packed magnetic micrometer clusters in one-step synthesis. The results showed that the physicochemical properties of the HPAM favored physical interactions with the reactants, promoting the formation of dense-packed magnetic micrometer clusters in one-step synthesis. The different estimated radii of these particles in solution (hydrodynamic and gyration) and dried (internal radius) decrease when the HPAM amount increases. Moreover, the particles exhibited a superparamagnetic behavior with a magnetization of 48.2 emu/g, 50.3 emu/g, and 54.3 emu/g when the amounts were 0.5 g, 1 g, and 2g of HPAM, respectively. Consequently, the optical response of the colloids to the application of a rotating magnetic field (RMF) parallel to the horizontal plane where they were located exhibited a behavior similar to photonic structures since their reflection spectrum showed a periodic diffraction color that closely matches the frequency of the RMF.

Spiral spin cluster in the hyperkagome antiferromagnet Mn3RhSi

Local spin correlation orders emerge in a paramagnetic state, with notable examples such as the partial order, cooperative paramagnetism, and soliton spin liquid. The noncentrosymmetric intermetallic antiferromagnet Mn3RhSi also exhibits the local spin correlation order in the paramagnetic state as magnetic short-range order in a wide temperature range. Here, we show that the local spin correlation order has a spiral structure by neutron scattering measurement of a Mn3RhSi single crystal. The possible origins of the magnetic cluster formation are discussed in terms of the Lifshitz invariant and the Griffiths phase, and compared with the room-temperature skyrmion phase of Co7Zn7Mn6 and non-Fermi liquid behavior of β-Mn.

Colossal piezoresistance in narrow-gap Eu5In2Sb6

A remarkably large reduction in the electrical resistivity of the quantum material Eu${}_{5}$In${}_{2}$Sb${}_{6}$ is discovered as a function of applied uniaxial pressure. This phenomenon, called colossal piezoresistance, is linked to a colossal magnetoresistance effect previously observed in this material. There is a common origin for the two phenomena, namely the interaction-driven formation of magnetic metallic clusters in semiconducting Eu${}_{5}$In${}_{2}$Sb${}_{6}$. These results will aid our understanding of colossal magnetoresistance and piezoresistance effects in a broad set of quantum materials.

Relationship between the Formation of Magnetic Clusters and Hexagonal Phase of Gold Matrix in AuxFe1-x Nanophase Thin Films.

AuxFe1−x nanophase thin films of different compositions and thicknesses were prepared by co-deposition magnetron sputtering. Complex morpho-structural and magnetic investigations of the films were performed by X-ray Diffraction, cross-section Transmission Electron Microscopy, Selected Area Electron Diffraction, Magneto Optical Kerr Effect, Superconducting Quantum Interference Device magnetometry and Conversion Electron Mössbauer Spectroscopy. It was proven that depending on the preparation conditions, different configurations of defect α-Fe magnetic clusters, i.e., randomly distributed or auto-assembled in lamellar or filiform configurations, can be formed in the Au matrix. A close relationship between the Fe clustering process and the type of the crystalline structure of the Au matrix was underlined, with the stabilization of a hexagonal phase at a composition close to 70 at. % of Au and at optimal thickness. Due to different types of inter-cluster magnetic interactions and spin anisotropies, different types of magnetic order from 2D Ising type to 3D Heisenberg type, as well as superparamagnetic behavior of non-interacting Fe clusters of similar average size, were evidenced.

Magnetocaloric properties of Fe-substituted La–Sr-manganites

The effect of Fe doping on the structural, magnetic and magnetocaloric properties was studied on La1-xSrxMn1-yFeyO3 (x = 0.10, 0.20; y = 0.00, 0.10, 0.15) manganites prepared by solid state reaction method. X-ray structural analysis showed that the substitution of manganese by iron does not change the phase state in the rhombohedral series (x = 0.20). In the orthorhombic series (x = 0.1), the substitution of manganese by iron leads to a phase transition from the Jahn-Teller O′ phase to the ordinary O phase. We explain this transition by a decrease in the concentration of Mn3+ ions, which are Jahn-Teller centers. In addition, we recorded an increase in the unit cell parameters in the iron-containing samples of both series. We explain this effect by the presence of a certain amount of Fe4+ ions. Magnetic measurements showed that all samples undergo a second order FM–PM transition at the Curie temperature (Tc). The Curie temperature TC and the Curie-Weiss temperature (Θ) decrease with increasing Fe content. The relationship between the variation of TC and structural parameters the Me–O bond length and the bond angle of Me–O–Me with increasing Fe content has been established. In the paramagnetic temperature range, the formation of magnetic clusters was suggested from isothermal magnetization data. For a magnetic field variation of 5 T, the entropy change is evaluated.

Flow behavior of cementitious-like suspension with nano-Fe3O4 particles under external magnetic field

The flow behavior of cementitious-like (limestone powder) suspension containing nano-Fe3O4 particles at constant shear rate of 10 s−1, characterized by the evolution of apparent viscosity over time, is investigated under various magnetic fields. Results show that the limestone powder suspension at flow-state exhibits remarkable magneto-rheological responses, reflected by a significant increase in the apparent viscosity after applying an external magnetic field. A higher field strength corresponds to a more rapid and pronounced response. The apparent viscosity experiences a sudden alteration with the stepwise change of the magnetic field due to the formation or disintegration of magnetic clusters. Linearly increasing magnetic field strength at low ranges (e.g. 0 T–0.3 T) shows less influences on the evolution of apparent viscosity, while at relatively high magnetic field, the apparent viscosity gradually increases with the magnetic field strength and the increase rate is comparable to that obtained under constant high magnetic field of 0.75 T. When the magnetic field is removed, the apparent viscosity exhibits a sharp reduction. If the magnetic field strength linearly decreases to zero, however, the apparent viscosity continuously increases until reaching a peak and then gradually decreases. This research shows in different ways how a desired apparent viscosity level of a cementitious-like suspension can be reached by means of an external magnetic field.

Evidence for magnetic clusters in stoichiometric quantum critical CeRu2Si2

Systems that have been prepared to undergo a second-order phase transition at zero Kelvin, the so-called quantum critical systems, appear to fall into two categories: (chemically) heavily doped systems where the unusual properties can be related to a disorder-induced distribution of Kondo shielding temperatures, and (almost) stoichiometric systems where the departures from Fermi-liquid theory have been attributed to intrinsic instabilities. Here we show that this distinction is not as clear-cut and that magnetic clusters associated with a distribution of Kondo shielding temperatures are also present in ${\mathrm{CeRu}}_{2}{\mathrm{Si}}_{2}$, a system close to a quantum critical point. By revisiting the published data on this system and comparing them to the results for heavily doped quantum critical $\mathrm{Ce}{({\mathrm{Ru}}_{0.755}{\mathrm{Fe}}_{0.245})}_{2}{\mathrm{Ge}}_{2}$, we show that clusters exist in both systems at low temperatures and that the moments of the Ce ions within these clusters have all lined up with their neighbors. This implies that the dominant physics that drives heavily doped systems, namely, the spontaneous formation of magnetic clusters, should also play a leading role in the response of homogenous systems. This represents a notable departure from how the physics that governs quantum critical points is treated in the literature.

Rheological behavior of cement paste with nano-Fe3O4 under magnetic field: Magneto-rheological responses and conceptual calculations

The magneto-rheological responses of cement paste with nano-Fe3O4 particles are experimentally investigated. The estimated magneto-dynamic force between two neighboring nanoparticles and equilibrium movement velocity of the nanoparticles in cement-based suspensions are calculated. Results show that the nanoparticles have a potential to move to form magnetic clusters when a magnetic field is applied, which creates a sort of agitation effect breaking down the early C-S-H links between cement particles, and thus the corresponding suspensions exhibit liquid-like behavior immediately after applying the magnetic field. The solid-like property of the studied suspensions becomes more dominant with magnetizing time due to the formation of magnetic clusters and the reconstruction of C-S-H bridges. The rheological properties of paste medium exert significant influences on the magneto-rheological responses of cement paste containing nano-Fe3O4 particles. It is revealed that the calculated magnetic yield parameter and nanoparticle movement velocity are useful relevant indicators to evaluate the magneto-rheological effect of cementitious paste.

Mössbauer and Magnetic Study of Lanthanum Manganite La1 – xCaxMn0.98Fe0.02O3 + δ (x = 0.05, 0.10, 0.20): Nonstoichiometric and Stoichiometric Composition

Mossbauer and magnetic study of calcium-doped lanthanum manganites La1 – xCaxMn0.98Fe0.02O3 + δ (x = 0.05, 0.10, 0.20) of nonstoichiometric (LCM–NS) and stoichiometric (LCM–S) compositions have been carried out in a wide temperature range. Phase structural separation is observed in the samples of stoichiometric composition (δ = 0). With a decrease in temperature all investigated lanthanum manganite samples exhibit superparamagnetic behavior, caused by the formation of small magnetic clusters in the structure. An expansion of the concentration range toward lower dopant concentrations in the LCM–S samples allowed us to establish the competing processes occurring at a low Ca content, including the Jahn‒Teller effects (orbital order) and the presence of Mn4+ ions (violation of orbital order), and study the dynamics of the structure and magnetic properties as functions of the Ca content.

Transport in two-dimensional Rashba electron systems doped with interacting magnetic impurities

We study the transport properties of two dimensional electron systems with strong Rashba spin-orbit coupling (SOC) doped with interacting magnetic impurities. Interactions between magnetic impurities cause the formation of magnetic clusters with temperature dependent mean sizes (CMSs) distributed randomly on the surface of the system. Treating magnetic clusters as scattering centers, by employing a generalized relaxation time approximation we obtain the non-equilibrium distribution functions of Rashba electrons in both regimes of above and below the band-crossing point (BCP) and present the explicit forms of the conductivity in terms of effective relaxation times. We demonstrate that the combined effects of SOC and magnetic clusters cause the system to be anisotropic and the magneto-resistance strongly depends on both the clusters' mean size and spin, the strengths of SOC and the location of Fermi energy with respect to the BCP. Our results show that there are many contrasts between the transport properties of the system in the two regimes of above and below the BCP. By comparing the anisotropic magneto-resistance (AMR) of the two dimensional Rashba systems with the surface AMR of three dimensional magnetic topological insulators, we also point out the differences between these systems.

Iron oxidation state effect on the Mg-Al- Si-O glassy system

Mg-Al-Si-O glassy systems have a great importance in a wide range of industrial applications, specifically as an electrolyte for molten oxide electrolysis processes in steelmaking. Understanding how the iron oxidation state of the raw material (Fe2+/Fe3+) and its corresponding amount influence this glassy system's properties will be the aim of the current work. Iron oxides (as Fe2O3 or Fe3O4) were used to dope Mg-Al-Si-O system obtaining amorphous materials through an unconventional method: Laser Floating Zone (LFZ). Above 8% mol of Fe formation of magnetic phases or iron clusters, were observed in the glass matrix. Samples with Fe2O3 showed a higher crystal concentration, when compared with Fe3O4. The electron paramagnetic resonance measurements show a strong dependence on the iron source (Fe3O4 or Fe2O3). In addition, the magnetization decreases linearly with iron content, independently of iron oxidation state, except for samples with a higher concentration of Fe2O3(15% mol), due to sample crystallization. Moreover, with Fe3O4 as raw material there is an improvement (~250 times) of the electrical conductivity when compared with Fe2O3. The results show that the presence of Fe2+ on the glass influences the electrical conductivity, which could have impact in the efficiency of molten oxide electrolysis process.

Toward the Mott state with magnetic cluster formation in heavily Cu-doped NaFe1−xCuxAs

Recent neutron scattering measurements indicate that NaFe$_{1-x}$Cu$_{x}$As forms an antiferromagnetic stripe phase near $x\approx 0.5$ in a Mott insulating state. This copper concentration is well in excess of that required for superconductivity, $x < 0.04$. We have investigated the development of magnetism in this compound using $^{23}$Na nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation measurements performed on single crystals ($x$ = 0.13, 0.18, 0.24, and 0.39). We find multiple inequivalent Na sites, each of which is associated with a different number of nearest neighbor Fe sites occupied by a Cu dopant. We show that the distribution of Cu substituted for Fe is random in-plane for low concentrations ($x = 0.13$ and 0.18), but deviates from this with increasing Cu doping. As is characteristic of many pnictide compounds, there is a spin pseudo gap that increases in magnitude with dopant concentration. This is correlated with a corresponding increase in orbital NMR frequency shift indicating a change in valence from Cu$^{2+}$ to a Cu$^{1+}$ state as $x$ exceeds 0.18, concomitant with the change of Fe$^{2+}$ to Fe$^{3+}$ resulting in the formation of magnetic clusters. However, for $x\leq 0.39$ there is no evidence of long-range static magnetic order.

Magneto-optical imaging of stepwise magnetic domain disintegration at characteristic temperatures in EuB6

Prior to the onset of the ferromagnetic transition in semimetallic EuB_6, unusual magnetic and electric behavior have been reported. Using a highly sensitive magneto-optical imaging (MOI) technique, we visualize the behavior of magnetic domains in a EuB_6 single crystal.The transformation from a paramagnetic to a ferromagnetic state is shown to be non-Curie Weiss like and proceeds via multiple breaks in the curvature of the temperature-dependent local magnetization. From our experiments, we identify three characteristic boundaries, T^*(H),T^*_c1(H) and T_c2(H),in a field - temperature magnetic phase diagram.Using scaling and Modified Arrott plot analysis of isothermal bulk magnetization data, we determine the critical exponents $\beta$ = 0.22 $\pm$ 0.01, $\gamma$ = 0.88 $\pm$ 0.05 and $\delta$= 5.0 $\pm$ 0.1 and a critical transition temperature = 12.0 $\pm$ 0.2 K which is found to be equal to T_c2. The critical exponents are close to those associated with the universality class of tricritical mean field model. The absence of a model with which the exponents correspond with directly, suggests the presence of large critical fluctuations in this system. The critical fluctuations in this system are sensitive to the applied magnetic field, which leads to field dependence of boundaries in the magnetic phase diagram. With increasing T the magnetic domains disintegrate into finger-like patterns before fragmenting into disjoint magnetized puddles at T^*_c1 and ultimately disappearing at T*. At T^*_c1 we observe a significant increase in the spatial inhomogeneity of the local magnetic field distribution associated with the magnetic domain structure disintegrating into smaller magnetized structures. We explain our results via the formation of magnetic polaronic clusters and their coalescing into larger domains.

Magnetic clustering, half-moons, and shadow pinch points as signals of a proximate Coulomb phase in frustrated Heisenberg magnets

We study the formation of magnetic clusters in frustrated magnets in their cooperative paramagnetic regime. For this purpose, we consider the $J_1$-$J_2$-$J_3$ classical Heisenberg model on kagome and pyrochlore lattices with $J_2 = J_3=J$. In the absence of farther-neighbor couplings, $J=0$, the system is in the Coulomb phase with magnetic correlations well characterized by pinch-point singularities. Farther-neighbor couplings lead to the formation of magnetic clusters, which can be interpreted as a counterpart of topological-charge clusters in Ising frustrated magnets [T. Mizoguchi, L. D. C. Jaubert and M. Udagawa, Phys. Rev. Lett. {\bf 119}, 077207 (2017)]. The concomitant static and dynamical magnetic structure factors, respectively $\mathcal{S}({\bm{q}})$ and $\mathcal{S}({\bm{q}},\omega)$, develop half-moon patterns. As $J$ increases, the continuous nature of the Heisenberg spins enables the half-moons to coalesce into connected `star' structures spreading across multiple Brillouin zones. These characteristic patterns are a dispersive complement of the pinch point singularities, and signal the proximity to a Coulomb phase. Shadows of the pinch points remain visible at finite energy, $\omega$. This opens the way to observe these clusters through (in)elastic neutron scattering experiments. The origin of these features are clarified by complementary methods: large-$N$ calculations, semi-classical dynamics of the Landau-Lifshitz equation, and Monte Carlo simulations. As promising candidates to observe the clustering states, we revisit the origin of "spin molecules" observed in a family of spinel oxides $AB_2$O$_4$ ($A=$ Zn, Hg, Mg, $B=$ Cr, Fe).

Features of electronic paramagnetic resonance in the ErAl3(BO3)4 single crystal

An additional resonance line was found in the EPR spectrum of the ErAl3(BO3)4 crystal at a temperature of 4.2 K, along with the main line. The appearance of an additional line is associated with the possible formation of magnetic clusters in the crystal.

Phonon properties of ZnSnSb2 + Mn semiconductors: Raman spectroscopy

AbstractDoping of II–IV–V2 semiconductors opens up new opportunities for wide application. Addition of Mn to these materials induces formation of magnetic clusters, which are responsible for high‐temperature ferromagnetism. Our aim was to examine how the addition of Mn influences the optical and structural properties of ZnSnSb2 by micro‐Raman spectroscopy. For four samples of Zn1 − xMnxSnSb2 synthesized using the direct fusion method, with x = 0.027, 0.066, 0.076, and 0.086, Raman spectra were measured at room temperature in spectral range from 60 to 300 cm−1. The obtained results indicate that these are multiphase materials. Based on the size and shape of complex microstructures, which consist of different phases and clusters, dispersive and duplex or triplex types of microstructures can be identified. Existence of ZnSb, SnSb, and MnSb phases was confirmed. By analyzing the Raman spectra, phonons of ZnSb and SnSb are determined and they are consistent with the data from literature. Phonon properties of ZnSnSb2, as well as of MnSb, are experimentally obtained for the first time. On the basis of a shift of the ZnSnSb2 phonons, we found that some amount of Mn entered lattice and form Zn1 − xMnxSnSb2. Microstructures affect the physical properties and behavior of a material. Analysis of this complex semiconductors and obtained results are important for their optimization and customization for possible applications.

Magnetization reversal process and evaluation of thermal stability factor in Cu doped granular L1 FePt films

Curie temperature TC distributions and magnetization reversal mechanism in Cu doped L10 FePt granular films is investigated as a function of film thickness in the range of ∼5–12 nm with Cu mol. % varying in the range of 0%–6%. It is shown that Cu doping increases the FePt tetragonality and chemical ordering. For Cu doped FePt-X films, coercivity (HC) exhibits a non-monotonic behavior with increasing film thickness, i.e., HC increases initially up to tcr ∼ 7 nm, and decreases thereafter. We attribute this behavior to the change in magnetization reversal mechanism from coherent to an incoherent (domain-wall driven) mode. While in un-doped films, the domain-walls nucleate at the grain boundaries, in doped films the Cu atoms may act as domain-wall nucleation and pinning sites, isolating magnetic spin clusters of reduced dimensionality with respect to the physical grain size. This is experimentally supported by a much poorer dependence of the AC susceptibility (both, real and imaginary components) on the film thickness above 7 nm than in the case of un-doped films. The formation of magnetic spin clusters inside the grains as a consequence of the reduced coupling between Fe-Fe and Fe-Pt-Fe atoms with increasing Cu doping can explain the experimentally evidenced reduction of both, the film Curie temperature, TC, and intrinsic anisotropy energy density, KC, with increasing Cu doping.