Cluster Glass Research Articles

Revealing the impact of ball milling as an intermediate stage in solid-state reaction synthesis of SmFeO3 particles

In this work, the impact of milling time on the structural, morphological, and magnetic properties is investigated in SmFeO3 (SFO) powders obtained by solid state reaction. Specifically, the ball milling process is incorporated as an intermediate step during the synthesis and their magnetic properties vary when the samples are milled for 4 and 10 h. Based on X-ray diffraction, the resulting samples crystallized in Pbnm space group, with a slight decrease in crystallite size and cell volume with milling time. These results are also supported by confocal Raman microscopy with a slight shift towards lower wavenumbers. In this sense, magnetic characterization demonstrates a weak ferromagnetic contribution coexisting with the dominant antiferromagnetic order. Butterfly shaped hysteresis loops are observed in both samples for T > 200 K while a single shaped loop is achieved for T < 200 K. Zero field cooled, and field cooled magnetization curves shows a broad maximum centered at 140 K and bifurcation of the two curves because of the competing interaction between the Sm+3 and Fe3+ ions into the SFO crystal structure. Interestingly, exchange bias (EB) effect is found up to 200 K with a field cool of 20 kOe for both samples. The existence of Fe–Fe, Fe–Sm, and Sm–Sm interactions in a cluster glass state induced by ball milling processes might be the main cause of such EB effect.

Observation of the multiple magnetic phases in double perovskite Pr1.8La0.2CoFeO6

The structural and magnetic properties of the double perovskite Pr1.8La0.2CoFeO6 have been investigated. The X-ray diffraction study shows that the system acquires room-temperature orthorhombic phase with the Pnma space group. The X-ray photoemission spectroscopy (XPS) measurement confirmed that the B-site ions are present in mixed valence states i.e. Co has been found in Co2+ and Co3+ valance states whereas Fe has been found in Fe3+/Fe4+. Magnetic measurements of the system confirm the existence of several fascinating magnetic behaviors, such as long-range canted antiferromagnetism withTN ∼ 271 K, giant exchange bias, and re-entrant cluster glass phase (TG ∼ 33 K). Field-dependent magnetization shows a large spontaneous exchange bias; HSEB∼ 1.8 kOe and giant conventional exchange bias, HCEB∼ 2.4 kOe at 5 K. Antiferromagnetic materials with large exchange bias can be utilized in high-density spintronic devices. Temperature-dependent Raman study demonstrates that the observed phonon mode exhibits anomalous behavior close to the magnetic transition temperature. Analysis of anomalous softening of phonon mode below TN, clarifies the presence of significant spin-phonon coupling while the magnetostriction effect does not play any significant role in the observed phonon anomaly.

Construction of a cryogenic dual scanner magnetic force microscope equipped with piezoresistive cantilever

We present a low-temperature magnetic force microscope (MFM) incorporating a piezoresistive cantilever and a dual-range scanner for experiments across a wide temperature range from cryogenic levels to room temperature. The piezoresistor-based MFM eliminates the need for optical readjustment, typically required due to thermal expansion at varying temperatures, thereby providing a more stable and precise measurement environment. The integration of a dual scanner system expands the versatility of scanning operations, enabling accurate sample positioning for detailed exploration of magnetic and superconducting properties under diverse thermal conditions. To demonstrate the capabilities of our MFM, we show detailed imaging of Fe3GaTe2, a van der Waals ferromagnet, and Yb0.7Y0.3CuAs2, a ferromagnetic cluster glass material. These studies demonstrate the potential of our MFM in revealing intricate details of magnetic domain dynamics and contribute to our understanding of materials exhibiting the anomalous Hall effect as well as superconducting phenomena.

Correlation between the exchange bias effect and antisite disorder in Sr2−xLaxCoNbO6 (x=0, 0.2)

We unravel the effect of La substitution and hence antisite disorders on the exchange bias (EB) mechanism in Sr2−xLaxCoNbO6 (x=0, 0.2) double-perovskite samples using the detailed analysis of the field cooled magnetization isotherms (M−H) and training effect. The field dependence of the freezing temperature deviates from both Gabay-Toulouse and de Almeida-Thouless lines and the analysis suggests that the x=0 sample follows a different universality class with moderate anisotropy in the frozen spins. Interestingly, we find that the EB effect is significantly suppressed in the x=0.2 sample due to increase [decrease] in the size of ferromagnetic (FM) [cluster glass (CG)] domain, which reduces the effective disordered interface responsible for the EB. The changes in fraction of FM, antiferromagnetic, and CG-like interactions with the La substitution and applied magnetic field are found to be crucial in governing the EB effect in these samples. Furthermore, the training effect measurements show the unequal shift in the left and right branches of the M−H loops and their different evolution with the field cycles (n). The analysis reveals that the rotatable spins relax approximately one order of magnitude faster than the frozen spins at the disordered interface. We find a possible correlation between the observed EB effect and the antisite disorders in these samples. Published by the American Physical Society 2024

Magnetic studies of ultrafine CoFe2O4 nanoparticles with different molecular surface coatings.

Surface functionalized ultrafine CoFe2O4 nanoparticles (NPs), with mean diameter ∼5 nm, were investigated by means of DC magnetization and AC susceptibility over the temperature range of 4-400 K. All NPs present the same CoFe2O4 core, with different molecular surface coatings, increasing gradually the number of carbon atoms in the coating layer: glycine (C2H5NO2), alanine (C3H7NO2), aminobutanoic acid (C4H9NO2), aminohexanoic acid (C6H13NO2), and aminododecanoic acid (C12H25NO2). Samples were intentionally fabricated in order to modulate the core-core magnetic dipolar interaction, as the thickness of the coating layer increases with the number of carbon atoms in the coating molecule. The magnetic data of the uncoated CoFe2O4 NPs were also collected for comparison. All investigated CoFe2O4 NPs (coated and uncoated) are in a magnetically blocked state at room temperature as evidenced by ZFC/FC measurements and the presence of hysteresis with ∼700 Oe coercivity. Low temperature magnetization scans show slightly constricted hysteresis loops with coercivity decreasing systematically with a decreasing number of carbon atoms in the coating molecule, possibly resulting from differences in magnetic dipole coupling between NPs. Large thermomagnetic irreversibility, slow monotonic increase in the FC magnetization and non-saturation of the magnetization give evidence for the cluster glass (CG) nature in the CoFe2O4 NPs. The out of phase part (χ'') of AC susceptibility for all samples shows a clear frequency dependent hump which was analyzed to distinguish superparamagnetic (SPM), cluster glass (CG) and spin glass (SG) behavior by using Néel-Arrhenius, Vogel-Fulcher, and power law fittings. These analyses rule out the SPM state and suggest the presence of significant inter-cluster dipolar interaction, giving rise to CG cooperative freezing in the high-temperature region. In the low-temperature range, however, the disordered spins on the nanoparticle's surface play an important role in the formation of the SG-like state, as evidenced by Arrott plots and temperature dependency of dM/dH in the initial magnetization curves. In summary, the magnetic measurements showed that undercooling the system evolves from a SPM state of weakly interacting spin clusters, through the CG state induced by strong dipolar interaction, to the SG state resulting from the frustration of the disordered surface spins.

Epitaxial strain manipulation of the cluster glass state in LaMnO3 films.

As a new-type magnetic state, the cluster glass state in manganite is arousing considerable attention due to its important theoretical value and extensive application prospects in condensed matter physics and spintronics. Due to the complex magnetic interactions, the cluster glass state is difficult to form and regulate in single films. Studies report a new phenomenon that epitaxial strain can regulate the formation of the cluster glass state in LaMnO3 (LMO) films. Comparing LMO thin films with different thicknesses grown on a (001)-oriented LaAlO3 (LAO) single crystal substrate, we found that the 20-nm-thick LMO film is more likely to form the cluster glass state than the 60-nm-thick and 120-nm-thick films. This can be attributed to the uneven distribution of strain and Mn ions in the depth profile. Our work demonstrates that thickness is an important method for regulating the formation of the cluster glass state in LMO films.

Phase transitions and slow spin dynamics of slightly inverted A-site spinel CoAl2−x Ga x O4

We report the properties of an A-site spinel magnet, CoAl2−x Ga x O4, and analyze its anomalous, low-temperature magnetic behavior, which is derived from inherent, magnetically frustrated interactions. Rietveld analysis of the x-ray diffraction profile for CoAl2−x Ga x O4 revealed that the metallic ions were randomly distributed in the tetrahedral (A-) and octahedral (B-) sites in the cubic spinel structure. The inversion parameter η could be controlled by varying the gallium (Ga) composition in the range 0.055 ⩽ η ⩽ 0.664. The composition-induced Néel-to-spin-glass (NSG) transition occurred between 0.05 ⩽ η ⩽ 0.08 and was verified by measurements of DC-AC susceptibilities χ and thermoremanent magnetization (TRM) below the Néel transition temperature T N. The relaxation rate and derivative with respect to temperature of TRM increased at both T N and the spin glass (SG) transition temperature T SG. The TRM decayed rapidly above and below these transitions. TRM was highly sensitive to macroscopic magnetic transitions that occurred in both the Néel and SG phases of CoAl2−x Ga x O4. In the vicinity of the NSG boundary, there was a maximum of the TRM relaxation rate at T max < T N. With increasing inversion η(x), the anomaly at T max merged with that of the Néel transition at a tricritical point (η tc, T tc) = (0.08, 4.0 K), where the paramagnetic, Néel, and SG states met. We successfully extracted the relaxation time τ and other characteristic parameters from the TRM isothermal temporal evolution based on the Weron function derived for a purely stochastic process. To distinguish the magnetic states, we compared our results with previously studied inversion-free A-site spinel, CoRh2O4, and CoGa2O4 cluster glass. We generated an inversion-temperature phase diagram based on the comprehensive measurements of DC and AC susceptibilities, TRM, and specific heat in the range 0.055 ⩽ η ⩽ 0.664 for CoAl2−x Ga x O4. Based on this phase diagram, we speculate that a NSG quantum critical phase transition occurred at η = 0.050(6). Our findings are consistent with suppression of the long-range order antiferromagnetic state in CoAl2O4 revealed through neutron diffraction studies, even at T << T N.

Existence of complex magnetic ground state and topological Hall effect in centrosymmetric silicide DyScSi

Topological Hall effect (THE) originating from non-trivial spin arrangements in magnetic materials has been extensively investigated in recent years. In this context, a centrosymmetric ternary silicide, DyScSi, is explored. Here we show that, a complex magnetic ground state drives THE in a centrosymmetric system. Temperature dependent magnetisation and neutron diffraction results establish the presence of commensurate antiferromagnetic (AFM) phase around 92 K, followed by an incommensurate AFM phase below 40 K. Additionally, two cluster glass transitions near 20 and 8 K, are also noted. These observed features arise due competing AFM and FM interactions. In conjunction with this, a finite contribution of THE is also observed in the intermediate field regime (8–30 kOe), at low temperature in DyScSi. The behaviour of this silicide appears to be fascinating in terms of interplay between complex magnetic ground state and THE in centrosymmetric structure.

Cluster Formation in Solutions of Polyelectrolyte Rings.

We use molecular dynamics simulations to explore concentrated solutions of semiflexible polyelectrolyte ring polymers, akin to the DNA mini-circles, with counterions of different valences. We find that the assembly of rings into nanoscopic cylindrical stacks is a generic feature of the systems, but the morphology and dynamics of such a cluster can be steered by the counterion conditions. In general, a small addition of trivalent ions can stabilize the emergence of clusters due to the counterion condensation, which mitigates the repulsion between the like-charged rings. Stoichiometric addition of trivalent ions can even lead to phase separation of the polyelectrolyte ring phase due to the ion-bridging effects promoting otherwise entropically driven clustering. On the other hand, monovalent counterions cause the formation of stacks to be re-entrant with density. The clusters are stable within a certain window of concentration, while above the window the polyelectrolytes undergo an osmotic collapse, disfavoring ordering. The cluster phase exhibits characteristic cluster glass dynamics with arrest of collective degrees of freedom but not the self-ones. On the other hand, the collapsed phase shows arrest on both the collective and single level, suggesting an incipient glass-to-glass transition, from a cluster glass of ring clusters to a simple glass of rings.

Magnetic phase diagram of Ge1–x–y(SnxMny)Te multiferroic semiconductors: Coexistence of ferromagnetic and cluster glass ordering

Noncentrosymmetric IV-VI semiconductors doped with magnetic ions offer exciting possibilities such as carrier mediated ferromagnetism, ferroelectric-controlled spin texture and integration of semiconducting features with ferromagnetic order. Also, coupling between more than one ferroic orders and giant Rashba spin splitting can be studied to develop materials for spintronic applications. In that context, we report the structural and magnetic results of polar α–GeTe doped with Sn and Mn from x = 0.185–0.841 and y = 0.02–0.086, respectively. The magnetic results of Ge1–x–y(SnxMny)Te (GSMT) crystals identify Mn-clustering effect with scaling parameter, R = 0.033 for x ≈ 0.2 and y = 0.06. The excessive Sn ions are assumed to drive the inception of short range ferromagnetic clusters. For the crystals revealing glassy magnetic behavior, the irreversibility temperature, Tirr shifts at high dc magnetic field which is well described by de Almeida-Thouless equation, δTirr ∝ HΦ/2 yielding Φ values of 1.55 and 1.7 that validates the formation of Mn clusters. The spin relaxation time, τ0 ∼ 10–9 s, activation energy, Ea/kB ∼ 5 TF where TF is freezing temperature and Vogel-Fulcher temperature, T0 ∼ TF also signify intracluster interactions. Also, the Mn-hole magnetic exchange constant, Jpd drops from 0.24 eV for the samples with smaller Sn content of x ≈ 0.2 to 0.16 eV for Sn rich alloy with x ≈ 0.8. Consequently, we present a magnetic phase diagram for GSMT bulk crystals as a function of Mn ions.

Disorder induced cluster spin glass like state in MnFeSb

Hexagonal MnSb is a ferromagnet with Curie temperature and saturation magnetization values of 587 K and 110 emu/g, respectively. In this paper, the origin of the complex magnetic behavior of a single phase MnFe0.20Sb prepared using solid-state reaction method is examined. A thorough investigation of temperature dependent magnetization and magnetic relaxation measurements reveals the presence of a glassy like phase below 15 K. AC susceptibility measurements revealed the existence of a cluster glass-like state with a freezing temperature of 18 K. The presence of competing magnetic interactions and random occupation of Fe atoms at 2d site leads the system to exhibit a cluster glass state. Furthermore, resistivity measurements indicate that the material exhibits a Kondo-like behavior below 30 K and can be attributed to the interaction of conduction electrons with localized Mn moments.

Cluster-glass freezing and antiferromagnetic phase transitions in corundum structure Mg3−xCoxTeO6

We investigate polycrystalline powder samples of the Co-substituted Mg3−xCoxTeO6 (x = 0.0–2.4) by X-ray diffraction, magnetic susceptibilities, and specific heat. The non-magnetic parent compound Mg3TeO6 crystallizes in the trigonal R3¯ space group. All Co-substituted samples have a similar crystal structure as the parent compound. The Rietveld refinements of synchrotron X-ray diffraction data reveal distorted (Mg/Co)O6 and TeO6 octahedra. Remarkably, we find the evolution of a magnetic phase from a non-magnetic ground state to an antiferromagnetic order through a cluster glass. The antiferromagnetically ordered state emerges above the percolation threshold (x ≥ 1.0), while the cluster glass state is stabilized for x = 0.8.

Experimental observation of spin glass state in the highly disordered quaternary Heusler alloy FeRuMnGa

The realization of the spin-glass (SG) state in Heusler alloys is very rare despite the presence of inherent structural and elemental disorder in those compounds. Although a few half- and full-Heusler alloys are known to exhibit the SG state, there is hardly any manifestation of the same in cases of quaternary Heusler compounds. Here we report the observation of a SG state in a highly disordered equiatomic quaternary Heusler compound: FeRuMnGa where the SG state is in between the canonical SG and the cluster glass. Different intricate features of the SG state including nonequilibrium magnetic dynamics at low temperatures in the compound are unveiled through our comprehensive magnetic, heat capacity, and neutron-diffraction studies. The structural disorder in the sample is neither conventional $A2$- nor $B2$ -type whereas those two types are commonly observed for Heusler compounds. The presence of disorder also plays a significant role in electron transport properties of the alloy, which is reflected in its exhibition of semimetallic behavior and anomalous Hall effect at low temperatures.

Disorder-driven cluster glass state in a geometrically frustrated hexagonal perovskite

We report the observation of cluster glass-like properties in a double perovskite ruthenate Ba$_2$CoRuO$_6$ through structural (neutron and synchrotron X-ray diffraction), magnetic and transport measurements. The system exhibits classic glassy characteristics like a frequency dependence in ac Susceptibility, aging and memory effects along with persistance of short-range correlations upto room temperature. The significant ($\sim30\%$) anti-site disorder on the dimer sites, coupled with the inherent geometrical frustration, allows a variety of exchange (both antiferro- and ferromagnetic) interactions to be distributed randomly across the lattice. On cooling, locally dominant interactions cause spins to nucleate and form local, short-range ordered clusters which grow in size until a global freezing occurs at about $T_f \sim 43K$.

Short-term memory capacity analysis of Lu3Fe4Co0.5Si0.5O12-based spin cluster glass towards reservoir computing

Reservoir computing is a brain heuristic computing paradigm that can complete training at a high speed. The learning performance of a reservoir computing system relies on its nonlinearity and short-term memory ability. As physical implementation, spintronic reservoir computing has attracted considerable attention because of its low power consumption and small size. However, few studies have focused on developing the short-term memory ability of the material itself in spintronics reservoir computing. Among various magnetic materials, spin glass is known to exhibit slow magnetic relaxation that has the potential to offer the short-term memory capability. In this research, we have quantitatively investigated the short-term memory capability of spin cluster glass based on the prevalent benchmark. The results reveal that the magnetization relaxation of Co, Si-substituted Lu3Fe5O12 with spin glass behavior can provide higher short-term memory capacity than ferrimagnetic material without substitution. Therefore, materials with spin glass behavior can be considered as potential candidates for constructing next-generation spintronic reservoir computing with better performance.

Evidence of cluster glass phase, exchange bias, reversible magnetocaloric effect and study of electrical properties in Ni1−xCdxCr2O4 (x = 0–0.30) spinel

Evidence of cluster glass phase, exchange bias, reversible magnetocaloric effect and study of electrical properties in Ni1−xCdxCr2O4 (x = 0–0.30) spinel

Low temperature cluster glass behavior in nanosized La0.5-xHoxCa0.5MnO3 (0 ≤ x ≤ 0.15) manganites

Dynamic magnetic properties response of La0.5-xHoxCa0.5MnO3 (0 ​= ​x ​≤ ​0.15) nanosized samples have been investigated. Temperature dependence of AC magnetic susceptibility measurements shows a broad maximum at freezing temperature Tf, suggesting the presence of spin or cluster glass behavior. For the substituted samples, a second peak can be detected below Tf and is more pronounced for x ​= ​0.15, which indicates the appearance of cluster glass behavior. The analysis of the frequency dependent peak obeys to the Vogel-Fulcher and power law, which rules out the superparamagnetic state. An unusual decrease of Tf was observed for Ho-substituted samples with increasing frequency.

Two states of magnetic frustration in La0.7Sr0.3MnO3 amorphous films with fractal structures: New knowledge about clusters and cluster ensembles

In amorphous La0.7Sr0.3MnO3 films with a fractal structure, two states of magnetic frustration were found, which are the characteristic of cluster spin glass (CSG) and spin glass (SG) states and are associated with the presence of competing, FM and AFM, magnetic interactions, and the geometry of fractal formations. A high density of clusters in them provides effective magnetic interactions of magnetic moments without the participation of free charge carriers. It has been established that the formation of CSG begins with the transition of the central parts of the clusters to the FM state (at T &amp;lt; 98 K) and ends with the formation of the AFM order in the peripheral areas of the clusters, at T &amp;lt; 64 K. Increasing the field to H = 1.2 kOe, which stimulates AFM ordering of peripheral areas, strengthens the state of the cluster glass. A further decrease in temperature (T &amp;lt; 26 ) and an increase in the field (H &amp;gt; 2.5 kOe) causes the transition of peripheral areas from the AFM to the SG state. A phenomenological description of the frustration process is made. The dependences of the order parameter and barrier height on temperature and magnetic field have been studied. The consequences of the CSG ⇒ SG phase transition for the magnetism of the samples and the direct influence of the external field on their magnetic structure are discussed.

Relationship between magnetoresistance behavior and magnetic states in intercalated compounds FexTiS2

Temperature and field-dependent neutron powder diffraction (NPD) measurements have been performed to reveal the nature of the unusual evolution of the magnetoresistance behavior with increasing Fe content in the intercalated compounds ${\mathrm{Fe}}_{x}{\mathrm{TiS}}_{2}$ (with $x=0.25,0.33,0.50,0.55$) synthesized by solid-phase reaction method with prolonged homogenization heat treatment. As derived from neutron diffraction measurements, both the ${\mathrm{Fe}}_{0.25}{\mathrm{TiS}}_{2}$ and the ${\mathrm{Fe}}_{0.50}{\mathrm{TiS}}_{2}$ compound exhibit an antiferromagnetic (AFM) order below their respective N\'eel temperatures ${T}_{N}\ensuremath{\approx}52$ K and ${T}_{N}\ensuremath{\approx}140\phantom{\rule{4pt}{0ex}}\mathrm{K}$, which results in the presence of a large magnetoresistance accompanying the field-induced phase transition from AFM to the ferromagnetic (FM) state. At low temperatures, this AFM-FM transition is irreversible, confirmed by the irreversibility of changes in the NPD patterns and the presence of remnant magnetoresistance. In contrast, ${\mathrm{Fe}}_{0.33}{\mathrm{TiS}}_{2}$ shows short-range magnetic order at ${T}_{f}\ensuremath{\approx}44$ K due to a triangular network of intercalated Fe atoms and frustrations of exchange interactions with a field-induced FM alignment of Fe magnetic moments in an applied magnetic field as revealed by NPD measurements. The field-induced transformations of the cluster glass magnetic state in this compound lead to a significant decrease in electrical resistivity. According to NPD data, a reduced impact of an external magnetic field on the electrical resistivity of the compound ${\mathrm{Fe}}_{0.55}{\mathrm{TiS}}_{2}$ can be ascribed to the presence of ferromagnetic or ferrimagnetic order in compounds with the Fe concentrations above $x=0.50$. The results obtained indicate that the distribution of the Fe atoms, along with their concentration in ${\mathrm{Fe}}_{x}{\mathrm{TiS}}_{2}$ layered compounds, plays a decisive role in the formation of the magnetic state and the behavior of the magnetoresistance.

The Magnetic Susceptibility of Alloys Below the Percolation Threshold

Within the theory of random interaction fields, the possibility of determining the Curie point and the paramagnetic Curie point corresponding to the appearance of short-range order is shown. In ferromag-netic alloys, there is a concentration range in which the long-range order is destroyed, but the short-range order persists. This leads to the appearance of a cluster glass phase, which is characterized by a time depen-dence of the magnetic susceptibility and the appearance of viscous magnetization. For an AuFe alloy as an example, the behavior of the initial magnetic susceptibility as a function of temperature and concentration is studied and compared with experimental data.