Spin-glass Features Research Articles

Anisotropic spin-glass-like behavior in single crystal GdCo2B2

A high-quality single crystal of GdCo2B2 was successfully synthesized and subjected to a comprehensive investigation into its spin-glass characteristics. We conducted a systematic analysis of magnetization, aging effects, and alternating current (AC) susceptibility. Interestingly, a clear separation was observed between the zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves under low magnetic field of 0.01 T. Furthermore, the crystal exhibited a long-time relaxation behavior at 4 K. The freezing temperature increases with frequency increasing. Additionally, we explored the response of the crystal to DC magnetization and AC susceptibility under different pressure conditions, up to 1.58 GPa. The crystal exhibits three anomalies TC = 22.0 K, T1 = 20.5 K and TN = 11.0 K under ambient pressure. An analysis of the anisotropy of the spin-glass-like behavior in the crystal was conducted, with a specific focus on the a-axis as the preferred direction of magnetization. All of the results provide evidence for the coexistence of mixed ordering states in the crystal.

Magnetic properties of Dy[formula omitted](Co,Ni) compounds at ambient and high pressures

Dy3Co, in addition to two antiferromagnetic (AFM) transitions (35 K and TN∼ 44 K), exhibits a Griffiths-like phase around 65 K and it persists up to ∼1 GPa pressure. External pressure alters the magnetic state significantly and results in increase (decrease) of TN (TAF) by 6.5% (∼13%) with dTN/dP ∼ 3 K/GPa (dTAF/dP ∼ −4.5 K/GPa). Increase in metamagnetic critical field and other changes with pressure indicate the strengthening of AFM interactions in Dy3Co. On the other hand, Dy3Ni shows three magnetic transitions at To∼ 55 K, T1∼ 39 K (AFM) and T2∼ 24 K (AFM). Experimental evidence points to the magnetic ordering temperature to be at To∼55 K instead of 39 K as claimed by certain reports. External pressure reduces (increases) the transition temperature To (T1) by ∼7.9% (7.6%) at a rate of −3.5 K/GPa (2.5 K/GPa). Pressure has only marginal effect on the other low temperature transition at T2. Additionally, ac-susceptibility data exhibit spin-glass like features, which seem to arise from domain wall movements and probably random freezing of small components of spins, coexisting with dominant long-ranged non-collinear AFM structure in these compounds.

Magnetic frustration and glassiness in an icosahedral i−Tb−Cd quasicrystal

The dynamical magnetic properties of the icosahedral i−Tb−Cd quasicrystal (QC) were investigated by means of squid magnetometry. At low temperatures, below T∼4.5 K, we observe conventional spin glass features including aging, memory, and rejuvenation. Interestingly, from T∼4.5 to 10 K, the spin configuration is also found to evolve with time (i.e., age as in spin glass), yet without memory. This suggests a reentrantlike behavior for the i−Tb−Cd QC, where upon cooling from high temperatures, the system first displays a magnetically frustrated phase before turning into a spin glasslike one at the lowest temperatures. We discuss the nature and possible origin of this magnetic behavior, in the light of the results obtained in the i−Gd−Cd QC, and ternary Tb-based quasicrystal approximants. Published by the American Physical Society 2024

Signatures of nearly compensated magnetism and spin glass behavior in highly frustrated β-Mn-type Mn50Fe25+xAl25-x Heusler alloys.

The present study examines the effect of Fe/Al concentration on the structural and magnetic properties of Mn-rich Mn50Fe25+xAl25-x (x = 5, 10, 15) Heusler alloys through x-ray diffraction, temperature- and field-dependent DC magnetization, thermoremanent magnetization, magnetic memory effect, AC susceptibility measurements, and DFT calculations. The samples crystallize in a cubic β-Mn structure. The trend shows a reduction in lattice parameters (unit cell volume) with the increasing Fe proportion. These alloys exhibit strong antiferromagnetic interactions with large frustration parameters, indicating the presence of competing magnetic interactions. The DC magnetization data reveal spin glass-like features with a peak at spin glass freezing temperature (Tf). The observation of bifurcation in temperature-dependent zero-field-cooled and field-cooled magnetization curves, exponential dependence of the temperature variation of remanence and coercivity, magnetic relaxation, and magnetic memory effect below Tf support the spin-glass character of these alloys. The frequency dependence of Tf is also examined in the context of dynamic scaling laws, such as the Vogel-Fulcher law and critical slowing down model, which further supports the presence of spin glass behavior. In the theoretical DFT calculations, the electronic structure is found to be metallic and similar for both spin projections. Moreover, the antiferromagnetic arrangement of the magnetic moments, in line with the experimental observations, is stabilized by exchange interactions, resulting in an almost compensated total magnetic moment of 0.02-0.38µB/f.u. This is probably caused by the frustrated structure and non-stoichiometric compositions of Mn50Fe25+xAl25-x.

Probing the strongly correlated magnetic state of Co2C nanoparticles at low temperatures using μSR

Co2C nanoparticles (NPs) are amongst transition metal carbides whose magnetic properties have not been well explored. An earlier study (Roy et al 2021 J. Phys.: Condens. Matter 33 375804) showed that a pellet made from Co2C NPs exhibits exchange bias (EB) effect below a temperature, T EB = 50 K and a spin glass (SG) feature emerges below T SG = 5 K. In the current study we use magnetic, electrical transport, specific heat, and muon spin rotation (μSR) measurements to explore further the magnetic properties of a pellet made with 40 nm diameter pure Co2C NPs. We uncover the onset of Kondo localization at Kondo temperature T K (= 40.1 K), which is close to the onset temperature (T EB) of the EB effect. A crossover from the Kondo-screened scenario to the Ruderman–Kittel–Kasuya–Yosida interaction-dominated regime is also observed for T < T K. Temperature-dependent specific heat measurement further supports the Kondo localization scenario in the pellet and shows the heavy fermionic nature of the strongly correlated electronic state in Co2C. The zero field μSR asymmetry spectra in the low-temperature regime are characterized by two distinct fast and slow relaxation rates. The spectra show the absence of long-range magnetic order in the sample. However, our analysis suggests the NPs-pellet shows the presence of a dominant magnetically disordered fraction and a smaller fraction with short-range order. Muons in the disordered fraction exhibit a slower relaxation rate, while muons in the smaller fraction with short-range order exhibit a faster relaxation rate. We observe an increase in this fast relaxation rate between T EB and T SG. This increase below T EB ∼ 50 K suggests a slowing down of the fluctuating local magnetic environment around muons. Transverse field-μSR asymmetry spectra show the emergence of a stable, multi-peaked local magnetic field distribution in the pellet below T EB. Longitudinal field μSR spectra shows distinct changes in the dynamics of fluctuations suggesting the presence of a frozen glassy like state below 6 K. Based on our results, we suggest that below T EB, the pellet of Co2C NPs develops a magnetic interface that separates the two magnetic fractions; one is a disordered fraction, and the other is a fraction with short-range order. The exchange interaction that sets in below T EB at the interface couples the two fractions, leading to a suppression of the fluctuations. With the suppression of magnetic fluctuations below T EB, strong correlation effects in the electronic state of Co2C lead to Kondo localization.

Reentrant canonical spin-glass dynamics and tunable field-induced transitions in (GeMn)Co2O4 Kagomé lattice

We report on the reentrant canonical semi spin-glass characteristics and controllable field-induced transitions in distorted Kagomé symmetry of (GeMn)Co2O4. This B-site spinel exhibits complicated, yet interesting magnetic behaviour in which the longitudinal ferrimagnetic (FiM) order sets in below the Néel temperature T FN ∼ 77 K due to uneven moments of divalent Co (↑ 5.33 μ B) and tetravalent Mn (↓ 3.87 μ B) which coexists with transverse spin-glass state below 72.85 K. Such complicated magnetic behaviour is suggested to result from the competing anisotropic superexchange interactions (J AB/k B ∼ 4.3 K, J AA/k B ∼ −6.2 K and J BB/k B ∼ −3.3 K) between the cations, which is extracted following the Néel’s expression for the two-sublattice model of FiM. Dynamical susceptibility (χ ac (f, T)) and relaxation of thermoremanent magnetization, M TRM (t) data have been analysed by means of the empirical scaling-laws such as Vogel–Fulcher law and Power law of critical slowing down. Both of which reveal the reentrant spin-glass like character which evolves through a number of intermediate metastable states. The magnitude of Mydosh parameter (Ω ∼ 0.002), critical exponent zυ = (6.7 ± 0.07), spin relaxation time τ 0 = (2.33 ± 0.1) × 10−18 s, activation energy E a/k B = (69.8 ± 0.95) K and interparticle interaction strength (T 0 = 71.6 K) provide the experimental evidences for canonical spin-glass state below the spin freezing temperature T F = 72.85 K. The field dependence of T F obtained from χ ac (T) follows the irreversibility in terms of de Almeida–Thouless mean-field instability in which the magnitude of crossover scaling exponent Φ turns out to be ∼2.9 for the (Ge0.8Mn0.2)Co2O4. Isothermal magnetization plots reveal two field-induced transitions across 9.52 kOe (H SF1) and 45.6 kOe (H SF2) associated with the FiM domains and spin-flip transition, respectively. Analysis of the inverse paramagnetic susceptibility after subtracting the temperature independent diamagnetic term (=−3 × 10−3 emu mol−1 Oe−1) results in the effective magnetic moment = 7.654 μ B/f.u. This agrees well with the theoretically obtained = 7.58 μ B/f.u. resulting the cation distribution in support of the Hund’s ground state spin configuration and of Mn4+ and Co2+, respectively. The H–T phase diagram has been established by analysing all the parameters (T F(H), T FN(H), H SF1(T) and H SF2(T)) extracted from various magnetization measurements. This diagram enables clear differentiation among the different phases of the (GeMn)Co2O4 and also illustrates the demarcation between short-range and long-range ordered regions.

Exchange bias studies of CoFe2O4 coated BiFeO3 nanoparticles

The study of exchange bias effects in inverted core-shell nanoparticles has unearthed rich and varied physical phenomena. In this paper, we have studied the exchange bias effects in inverted BFO/CFO core-shell nanoparticles by varying the crystallinity of the CFO shell. The M-T measurements have shown distinct features of the BFO core and CFO shell. AC susceptibility and memory effect measurements indicated that the core-shell nanoparticles with the less crystalline CFO shell displayed spin-glass characteristics. The training effect displayed by the nanoparticles with the spin-glass shell can be explained using the spin-glass and Binek’s models. Exponential decay of coercivity and exchange bias with temperature is in conformance with the spin-glass behavior of the shell, and the cooling-field dependence of the exchange bias field is explained based on the Niebieskikwiat model. In the case of core-shell nanoparticles with the more crystalline CFO shell, the anisotropy field is greater than the maximum applied field. The exchange bias value has the contributions from the minor loop effects, as evident from the forced minor loop measurements. The extrapolated value of the exchange bias increases moderately compared with similar inverted core-shell structures, even though the coercivity increased considerably. The out-diffusion of Bi ions from the BFO core changed the pinning strength of the BFO core, affecting the exchange bias and coercivity.

Superparamagnetic and spin glass characteristics with Griffiths phase in Ni50Mn30−xFexSn20−ySby (1 ≤ x ≤ 4 and 2 ≤ y ≤ 8) Heusler alloys

Superparamagnetic and spin glass characteristics with Griffiths phase in Ni50Mn30−xFexSn20−ySby (1 ≤ x ≤ 4 and 2 ≤ y ≤ 8) Heusler alloys

A stretched exponential-based approach for the magnetic properties of spin glasses

The spin glasses show intriguing characteristic features that are not well understood yet, as for instance its aging, rejuvenation and memory effects. Here a model based on a stretched exponential decay of its magnetization is proposed, which can describe the main magnetic features of spin glasses observed in experiments as the time-decay of thermoremanent magnetization, the relaxation of zero field cooled magnetization, the ac and dc magnetization as a function of temperature and others. In principle, the here proposed model could be adapted to describe other glassy systems.

Heterogeneous magneto-electric response in Mn-doped SrTiO3

The consecutive spin and structural glass features were observed in polycrystalline Sr0·98Mn0·02TiO3 previously. Here, the dielectric spectra with higher harmonics have been analyzed by different perturbing electric fields cooling under various magnetic fields across these transitions. Two relaxations are present below the structural glass transition Tg. It is inferred that the high-frequency saddle-like relaxation occurs due to the reorientation of almost antiferroelectrically coupled Mn-doped dipoles (Mn2+Sr) associated with TiO6 octahedra, and the Arrhenius-like low-frequency response comes from individual reorientation of Mn2+Sr. They merge above the Tg, and Mn2+Sr respond collectively with induced polar cluster reversal. High-frequency relaxation is glassy, whereas low-frequency relaxation is relaxor-like due to the presence of second harmonics. Subtracted spectra from high-field to low-field show that the energy absorption is non-uniform. Below Tg, the energy absorption becomes minimum with reducing permittivity under an intermediate magnetic field, indicating a decrease in electric potential barrier by magnetic ordering.

Direct observation of a dynamical glass transition in a nanomagnetic artificial Hopfield network

Spin glasses, generally defined as disordered systems with randomized competing interactions, are a widely investigated complex system. Theoretical models describing spin glasses are broadly used in other complex systems, such as those describing brain function, error-correcting codes, or stock-market dynamics. This wide interest in spin glasses provides strong motivation to generate an artificial spin glass within the framework of artificial spin ice systems. Here, we present the experimental realization of an artificial spin glass consisting of dipolar coupled single-domain Ising-type nanomagnets arranged onto an interaction network that replicates the aspects of a Hopfield neural network. Using cryogenic x-ray photoemission electron microscopy (XPEEM), we performed temperature-dependent imaging of thermally driven moment fluctuations within these networks and observed characteristic features of a two-dimensional Ising spin glass. Specifically, the temperature dependence of the spin glass correlation function follows a power law trend predicted from theoretical models on two-dimensional spin glasses. Furthermore, we observe clear signatures of the hard to observe rugged spin glass free energy in the form of sub-aging, out of equilibrium autocorrelations and a transition from stable to unstable dynamics.

Evidence for canonical spin glass behaviour in polycrystalline Mn1.5Fe1.5Al Heusler alloy

Magnetic properties of magnetically frustrated polycrystalline Mn1.5Fe1.5Al Heusler type alloy with a β-Mn structure were systematically studied using dc magnetization, ac susceptibility, magnetic relaxation and magnetic memory effect measurements to understand its magnetic ground state. The Mn1.5Fe1.5Al alloy system exhibits a sharp peak at low temperature around 34.5 K with strong antiferromagnetic correlations (θCW ∼ −639 K) and a high degree of frustration (fp ∼ 18.7) which suggest the presence of competing magnetic interactions. Despite of strong antiferromagnetic correlations, this peak has spin-glass character as confirmed by the observations of bifurcation of the zero-field-cooled and field-cooled magnetization curves below the irreversibility temperature, field dependent shift of the irreversibility temperature, unidirectional exchange bias effect on the field-cooled M-H hysteresis loop, exponential decay of the temperature variation of remanence and coercivity, frequency dependent shift of the spin glass freezing temperature (Tf) as per the dynamic scaling laws given by the critical slowing down model and Vogel-Fulcher law, magnetic relaxation and magnetic memory effect below Tf. The observation of a clear asymmetric response with respect to the temperature cycling in the magnetic relaxation measurements validates the hierarchical model of the spin-glass magnetic relaxation in Mn1.5Fe1.5Al alloy. Since the behaviour exhibited by the Mn1.5Fe1.5Al alloy has perfect similarity with the well-known atomic spin glasses like CuMn, AuFe, therefore, it may be considered as canonical spin-glass system.

Studies on nanoconfinement effect of NiO-SiO2 spin glass within mesoporous Al2O3 template

Nanoconfinement studies in the glass system xNiO-(1-x)SiO2 having x values as 0.2 and 0.4 were carried out by preparing nanocomposites using mesoporous alumina template. These samples were studied using wide and small angle xrd data. BET analysis was conducted to find the surface area and pore size distribution of the virgin and nanocomposite samples. The microstructure and distribution of transition metal ions in the system were delineated using STEM-EDAX mapping. XPS data provide the quantitative analysis of Ni2+/Ni3+ species present in the system. Theoretical modeling using DFT simulations were implemented to understand the nanoconfinement effect which was in corroboration with the experimental findings. The theoretical model was validated using UV-Vis absorption spectrum. The magnetodielectric results were explained considering Catalan’s model having positive magnetoresistance. Nyquist plot studies were employed to visualize the realistic circuit model with contributions from different layers present in the nanocomposites. The temperature dependent resistivity data were explained using small polaron hopping model and the inter-site separation between Ni2+/Ni3+ ions was found to be larger in nano confined system than the corresponding bulk glass. Existence of more conducting species manifest a significant enhancement in percentile change of MD, MR & conductivity values for NC-2 compared to NC-1. Study of ZFC-FC magnetization curves effectively substantiate spin glass characteristics which exhibit freezing temperature at 4.1 K. The existence of spin glass behavior are well interpreted in terms of memory effect and relaxation dynamics study.

Charge-transfer driven ferromagnetism in a disordered three-dimensional 3d-5d spin system

A three-dimensional disordered spin system consisting of 3d transition metal ion Mn4+ with strong electronic correlations and heavy 5d transition metal ion Ir4+ has been investigated in form of a perovskite thin film. The studied compound of the composition SrMn0.5Ir0.5O3 does not exist as bulk or single crystal, but can be stabilized by epitaxy onto SrTiO3 single crystals as fully disordered double perovskite by pulsed laser deposition. As measured by X-ray circular dichroism, the ground state of this material is ferromagnetic with both, Mn and Ir, spins aligned in parallel. This unusual ground state can be qualitatively explained by charge-transfer driven magnetic exchange involving Mn eg-state and Ir t2g-state, with no evidence of spin–orbit coupling in Ir. Due to the coexistence of competing magnetic interactions and randomness in the system, spin-glass features are observed at low temperatures.

Low-field-induced spin-glass behavior and controllable anisotropy in nanoparticle assemblies at a liquid-air interface

Stacking nanoscale-building blocks into one-dimensional (1D) assemblies with collective physical properties is a frontier in designing materials. However, the formation of 1D arrays using weak magnetic fields and an in-depth understanding of their magnetic properties remain challenging. Here, low-dimensional assemblies of iron oxide nanocubes with a disordered arrangement are fabricated at the diethylene-glycol/air interface in the presence of assembly fields (0/1/3/5/30/50 mT). Ring-shaped assemblies gradually transform as the assembly field increases from 0 to 50 mT, first to a porous network consisting of elongated assemblies and then to an aligned array of filaments, in which the aligned filaments are formed when the assembly field is ≥3 mT and duration t > 14 min. Spin-glass characteristics and static (dynamic) anisotropy factors ∼2(3) are achieved by tuning the strength of the assembly field. In the presence of a relatively weak assembly field, the interplay between dipolar interactions and disorder with respect to magnetic easy axis alignment leads to spin-glass characteristics. The alignment of the magnetic easy axes and the strength of the dipolar interactions increase with increasing assembly field, resulting in the disappearance of spin-glass characteristics and enhancement of the magnetic anisotropy. This study presents a strategy for obtaining magnetic assemblies with spin-glass behavior and controllable anisotropy while shedding light on the magnetic interactions of low-dimensional assemblies.

Spin glass feature and exchange bias effect in metallic Pt/antiferromagnetic LaMnO3 heterostructure

Emergent phenomena at interfaces have been investigated intensely in pursuit of the next generation spintronics. In this work, we have integrated heterostructure consisting of paramagnetic (PM) metallic Pt and antiferromagnetic (AFM) insulator LaMnO3 (LMO). High-quality Pt (3 nm)/LMO (100 nm) heterostructure has been obtained by pulsed laser deposition. The structure, lattice strain and magnetic properties of epitaxial Pt/LMO heterostructure are fully studied. Due to the high sensitivity of synchrotron radiation and the high quality of epitaxial layer, the reflection intensity of the 3 nm-thick ultrathin Pt layer and LMO layer can be detected, and then lattice strain can be calculated. The LMO layer is under relative large tensile strain (2.13%), while the Pt layer is under relative small compressive strain (−0.46%). Magnetization measurements suggest that unexpected ferromagnetic behavior is observed clearly in the PM-Pt/AFM-LMO heterostructure. Moreover, spin glass (SG) state and exchange bias (EB) is also observed in this heterostructure. SG state is observed as a result of competing magnetic orders and spin frustration at the Pt/LMO interface. The heterostructure shows the EB effect below blocking temperature (T B), which is much lower than the Néel temperature (T N) of LMO, suggesting that the EB is strongly related to the SG state. The EB originates from the coupling between the SG and AFM phases.

Competing magnetic interactions and magnetoresistance anomalies in cubic intermetallic compounds, Gd4RhAl and Tb4RhAl , and enhanced magnetocaloric effect for the Tb case

We report complex magnetic, magnetoresistance (MR), and magnetocaloric properties of ${\mathrm{Gd}}_{4}\mathrm{RhAl}$ and ${\mathrm{Tb}}_{4}\mathrm{RhAl}$, forming in the ${\mathrm{Gd}}_{4}\mathrm{RhIn}$-type cubic structure. Though the synthesis of the compounds was reported long ago, to our knowledge, no attempt was made to investigate the properties of these compounds. The present results of ac and dc magnetization, electrical resistivity, and heat-capacity measurements down to 1.8 K establish that these compounds undergo antiferromagnetic order initially, followed by complex spin-glass features with decreasing temperature. These characteristic temperatures are as follows: for the Gd case, ${T}_{N}=\ensuremath{\sim}46\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${T}_{G}=\ensuremath{\sim}21\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and for Tb, \ensuremath{\sim}32 and \ensuremath{\sim}28 K, respectively. Additionally, there are field-induced magnetic effects, interestingly leading to nonmonotonic variations in MR. There is a significant MR over a wide temperature range above ${T}_{N}$, similar to the behavior of the magnetocaloric effect (MCE) as measured by isothermal entropy change (\ensuremath{\Delta}S). An intriguing finding we made is that \ensuremath{\Delta}S at the onset of magnetic order is significantly larger for the Tb compound than that observed for the Gd analog near its ${T}_{N}$. On the basis of this observation in a cubic material, we raise the question of whether the aspherical nature of the $4f$ orbital can play a role to enhance MCE under favorable circumstances---a clue that could be useful to find materials for magnetocaloric applications.

Removal of Ferrimagnetic ZnFe2O4 Impurity Phase by Li Co-Doping in Fe Doped ZnO

A simple approach is presented to remove the impurity phase of ZnFe2O4 from Fe doped ZnO. Gradual incorporation of non-magnetic Li ions reduced 5.6% phase contamination of ZnFe2O4 to negligible limit (<0.01%) from 4 at% (atomic %) Fe doped ZnO. X-ray absorption near edge spectroscopy (XANES) revealed that the most of the Fe ions were in Fe3+ state. The optical absorption spectra of Fe doped ZnO samples exhibited d-d transition between Fe ions. It also indicated the signature of ZnFe2O4 phase, which reduced gradually on Li increment. ZnFe2O4 contained samples exhibited strong magnetic ordering with spin-glass character at low temperature and weak ferromagnetism (FM) at the room temperature with disordered inverse spinel structure. However, 6 at% Li incorporated sample did not show any significant FM in the entire temperature range of 2–300 K. ZnFe2O4 is supposed to be the possible source of the observed strong magnetic ordering i.e., FM has extrinsic origin. The gradual removal of impurity phase ZnFe2O4 by increasing Li doping without damaging any intrinsic property of Fe doped ZnO is primarily due to gradual increase of tetrahedral coordination of Fe ions.

Structural stability and magnetic properties of Mn2FeAl alloy with a β-Mn structure

The synthesized Mn2FeAl alloys crystallize in a geometrically frustrated cubic β-Mn structure (space group: P4132) with an antiferromagnetic ordering whereas the previous theoretical findings suggest for a Heusler structure (L21: regular and X: inverse). The experimental stability of the structure is verified by electronic structure calculations performed for various arrangements of Mn, Fe and Al atoms in the β-Mn-type crystal structure. When compared the energy of the β-Mn structure with the energy of L21 and X type structures, it is found that for an expansion of the lattice volume β-Mn structure becomes more preferable in total energy than L21 and X-type structures. The calculated theoretical equilibrium lattice parameter value for the β-Mn2FeAl is within the accuracy of the experimental value obtained in this work. Additional DFT + U calculations for the optimized crystal structure of the β-Mn2FeAl revealed that the electronic correlations in the Mn ions result in the increased total magnetic moment. In the X type structure, Mn2FeAl is a half metal, whereas the disordered arrangement of atoms in the β-Mn structure leads to the closure of the semiconductor gap. The β- Mn2FeAl alloys exhibit antiferromagnetic ordering (TN ≈ 42 K), which is in excellent agreement with our electronic structure calculations. The detailed analysis of the magnetic and heat capacity measurements suggests a short-range magnetic ordering in the Mn2FeAl alloys. Owing to the strong antiferromagnetic spin fluctuation caused by the geometric frustration in β-Mn, a large enhancement in the electronic heat capacity is noticed. Mn2FeAl shows the characteristic features of spin glass as verified from the frequency dependent AC susceptibility analysis using critical power law and Vogel-Fulcher law. To the best of our knowledge, this is the first ever report on the theoretically predicted lowest ground state configuration for Mn2FeAl with a β-Mn structure and the experimental realization of spin glass features in this geometrically frustrated antiferromagnet.

Structural characterizations and dielectric, magnetic, and optical properties of Ba2FeVO6 double perovskite ceramics

Here, we present the structural characterization and physical properties of Ba2FeVO6 (BFVO) ceramics prepared by the solid-state ceramic method, which is regarded as potential Mott multiferroics. Structural studies demonstrate that the BFVO ceramic samples crystallize in a hexagonal structure with R3c symmetry. SEM images show a spherical morphology of the ceramic grains with an average composition of Bi:Fe:V close to 2.0:1.0:1.0 determined by x-ray energy dispersive spectroscopy. Infrared spectra revealed the vibrational modes of [FeO6] and [VO6] octahedrons in the ceramics, and XPS spectra confirmed the dual chemical valence states of iron (Fe2+ and Fe3+) and vanadium (V3+ and V4+) and two states of oxygen species in the BFVO ceramics, while barium was present as Ba2+. The BFVO ceramics exhibit relaxor-like dielectric behavior with an activation energy of 0.62 eV. The antiferromagnetic (AFM) nature was confirmed in the BFVO ceramics, whereas a weak magnetism was observed at 2 K. The temperature dependence of magnetizations measured with (and without) field cooling across 2 K–300 K under external magnetic fields indicates the spin glass characteristics in the present system. A strong variation of the dielectric behavior observed close to the AFM transition temperature was due to the induced dielectric polarization via the magnetostrictive effect driven by AFM transition. The bandgaps (Eg) of the investigated ceramic samples were deduced from their UV–visible spectra. The Eg values were about 1.15 eV and 1.50 eV, which were ascribed to the d–d electronic transitions of Fe 3d t2g → Fe 3d eg and 6A1g → 4T1g, respectively.