Cluster Glass Research Articles

Anomalous magnetism in half doped cobaltite Eu0.5Sr0·5CoO3

Half doped cluster glass compound Eu0.5Sr0·5CoO3 is prepared in polycrystalline form and is characterized by using x-ray diffraction, x-ray absorption spectroscopy, x-ray photoelectron spectroscopy and magnetization studies. The magnetic properties of this compound have been reinvestigated using DC magnetization studies. Magnetization versus Temperature (M-T) measurements revealed that the sample is ferromagnetic below 110 K with an additional transition observed around 230 K, which has not been reported in the previous studies on this compound. After analyzing the 1/M versus temperature data, the new transition was attributed to the onset of short range ferromagnetic interactions in the sample at ~230 K. Based on magnetization versus applied magnetic field (M − H) measurement, the sample has been projected as a coexistent state of ferromagnetic and antiferromagnetic clusters below 110 K. In view of the coexistent magnetic state, the possibility of the presence of exchange bias in the system has also been explored and it was established that this compound exhibits a negative exchange bias of about −4683Oe, making it technologically important.

Effect of (Cu/Fe)O5 bipyramid size and separation on magnetic and dielectric properties of rare earth layered perovskite LaBaCuFeO5 and LuBaCuFeO5

We report structural, magnetic, and dielectric properties of layered perovskite materials LnBaCuFeO5 (Ln = La and Lu). LaBaCuFeO5 shows magnetic cluster glass behavior below 60 K owing to the competing ferromagnetic and antiferromagnetic exchange interactions. Glassy dynamics of electric dipoles has also been observed in the vicinity of the magnetic glass transition temperature. The presence of significant coupling between spin and polar degrees of freedom results in the multiglass feature in LaBaCuFeO5. The LuBaCuFeO5 compound undergoes YBaCuFeO5 like commensurate to incommensurate antiferromagnetic transition at 175 K. Large magnetic irreversibility below 17 K in this compound suggests the presence of strong spin anisotropy. In addition, in this compound, the interaction between the dipoles is not strong enough, which results in the absence of glassy dynamics of electric dipoles. The contrasting behavior of two compounds is possibly due to variations in the ferromagnetic and antiferromagnetic interactions along the c-axis, which is the manifestation of structural modification arising out of the difference in the ionic radii of La and Lu.

Structural, magnetic and electric-polarization properties of geometrically frustrated YBaCo4O7and DyBaCo4O7cobaltites

In the RBaCo4O7 (R = Ca, Y and rare earth) cobaltite family, only CaBaCo4O7 shows 3D long-range ferrimagnetic and spin-driven electric-polarization properties. In the present study, we have investigated the structural, magnetic and electric polarization properties in YBaCo4O7 and DyBaCo4O7 members of this family and the obtained results are compared with the CaBaCo4O7. The compound YBaCo4O7 showed a series of magnetic transitions in agreement with the reported literature, additionally, a cluster glass behavior below 5 K is observed. Powder neutron diffraction studies on DyBaCo4O7 cobaltite showed an orthorhombic Pbn21 symmetry and signature of short-range 120° magnetic correlations of kagome layer and a spin glass behavior below 65 K. Dielectric measurements on both the samples showed strong frequency dispersion at high-temperature and frequency-independent behavior at low temperature without dielectric anomaly. Pyroelectric current measurement has showed a broad peak around 50 K in both the samples; however, a careful analysis relates the peak to thermally stimulated depolarization current. This study signifies that the giant structural distortions and cobalt charge ordering at the kagome layer are the key factors to drive both long-range magnetic ordering and spin-driven electric polarization in this cobaltite family.

Field induced nature and nonlinear DC susceptibility studies of Ni substituted Dy5Pd2 cluster glass system

We report a systematic investigation of the magnetic properties of Ni substituted Dy5Pd2 cluster glass system. Magnetic susceptibility result shows that both the compounds undergo paramagnetic to cluster glass transition. Remarkably, below this transition, they exhibit an additional field induced magnetic phase transition. This field induced nature of the magnetic phase transition is also clearly visible in the applied field response of magnetization and isothermal magnetic entropy change. Additionally, the cluster glass magnetic state is also analyzed by the temperature dependent magnetization memory effect and nonlinear DC susceptibility studies. Both the studies have confirmed the presence of two glassy transitions in these intermetallic compounds.

Magnetic correlations in mixed valent layered La1.2Sr0.8CoO4

We present here investigations on a hole doped layered perovskite, La1.2Sr0.8CoO4. In these systems concentration of the holes in host matrix decides ratio of the Co2+/Co3+ and magnetic phase diagram. In addition, spin-state of the Co3+ ions has an important role in magnetic correlations. Magnetization measurements show the signature of some exotic phases like cluster glass or canonical spin-glass and ferromagnetic ordering (Tc ∼ 211 K). In addition, calculated temperature variation of order parameter by also including the spin-state variations of Co3+ manifests that the orbital magnetic moments are not quenched here and has an important role in overall magnetic moment at low temperatures.

B-site disorder driven multiple-magnetic phases: Griffiths phase, re-entrant cluster glass, and exchange bias in Pr2CoFeO6

The magnetic spin ordering and the magnetization dynamics of a double perovskite Pr2CoFeO6 have been investigated by employing the (dc and ac) magnetization and neutron powder diffraction techniques. The study revealed that Pr2CoFeO6 adopted a B-site disordered orthorhombic structure (Pnma). Furthermore, ab initio band structure calculations suggested an insulating antiferromagnetic ground state. Magnetization measurements revealed that the system possesses a spectrum of competing magnetic phases, viz., long range canted antiferromagnetic (AFM) spin ordering (TN ∼ 269 K), Griffiths-like phase, re-entrant cluster glass (TG ∼ 34 K), and exchange bias effects. The neutron diffraction study divulged the exhibition of a long range G-type of canted AFM spin ordering. The random nonmagnetic dilution of magnetic Fe3+ (high spin) ions by Co3+ (low spin) ions due to B-site disorder essentially played a crucial role in manifesting such magnetic properties of the system.

Nonequilibrium magnetic properties in oxygen-rich LaMnO3 nanoparticles

Nonequilibrium magnetic properties of oxygen-rich LaMnO3.21 nanoparticles have been investigated by comprehensive magnetic measurements. The composition falls in the metamagnetic canted spin region of the magnetic phase diagram. However, the zero-field-cooling memory effect and frequency-dependent AC susceptibility reveal a re-entrant glassy state at low temperature. In contrast to the super-spin glass or cluster glass that re-enter from the high-temperature ferromagnetic state in previous studies, analyses based on the power law and Vogel–Fulcher law indicate strongly a conventional spin glass nature. As the magnetic field increases, an anomalous enhancement of the irreversible temperature is observed, suggesting a field-induced nonequilibrium magnetic state. These results can be understood by considering the interaction between the antiferromagnetic clusters and the metamagnetic canted-spin matrix inside the nanoparticles.

Strain-Induced Cluster Glass State in LaMnO3 Films

Strain effect has great influence on the magnetic properties of manganite films. Here, we report a new phenomenon in which the biaxial tensile strain can induce the formation of the cluster glass s...

Magnetocaloric properties of cluster glass compound Pr2Ni0.95Si2.95

In this work, we report the magnetocaloric properties of a hexagonal intermetallic compound Pr2Ni0.95Si2.95 that exhibits spin cluster glass behaviour below 3.3 K. At lower temperature, the compound undergoes magnetic field induced metamagnetic transition beyond 5 kOe applied field value as inferred from the temperature dependence of magnetization at various fields as well as isothermal magnetization measurements. The temperature dependence of magnetic entropy change (−ΔSM) has been calculated from the magnetization data and the maximum value is found to be 7.9 J/kg-K for a magnetic field change of 70 kOe. Interestingly, although the compound does not undergo any long range magnetic ordering, it exhibits considerable magnetocaloric effect in a temperature region much above the spin freezing due to the presence of short range magnetic correlation that persists up to much higher temperature. The suppression of short range interaction with the application of magnetic field has been argued to be responsible for such magnetic entropy change and adiabatic temperature change over a large temperature scale.

Strain coupling and acoustic attenuation associated with glassy magnetic phase transitions in the disordered double perovskite La2FeMnO6

Elastic and anelastic anomalies in a ceramic sample of ${\mathrm{La}}_{2}{\mathrm{FeMnO}}_{6}$ have been characterized by resonant ultrasound spectroscopy in order to understand the strength and form of magnetoelastic coupling that accompanies the glassy magnetic transitions of a double perovskite with no long-range order of the B-site cations. The first transition, to a cluster glass below $\ensuremath{\sim}280\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, does not appear to involve any significant coupling with strain. The second glassy transition, near 55 K, appears to conform to Vogel-Fulcher dynamics in which magnetic dissipation and acoustic loss peaks arise from freezing driven by interactions between ferromagnetic clusters, with an activation energy of $\ensuremath{\sim}0.03\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ and time constant ${\ensuremath{\tau}}_{\mathrm{o}}\ensuremath{\sim}{10}^{\ensuremath{-}9}\phantom{\rule{0.16em}{0ex}}\mathrm{s}$. The magnetoelastic coupling mechanism appears to involve local spin states with strain relaxation enhanced by changes in local electronic structure. Mediation of the coupling via strain also ensures that local heterogeneity in the strain state, such as at ferroelastic twin walls, will contribute to the magnetic heterogeneity of these materials.

A revisit to the effect of annealing temperature on magnetic properties of LaFe0.5Mn0.5O3

We report an experimental study for the structural and magnetic properties of highly pure LaFe0.5Mn0.5O3 perovskite phase. The impurity free LaFe0.5Mn0.5O3 has been prepared by sol-gel technique at 500 °C and annealed at different temperatures up to 1000 °C. Previous works on LaFe0.5Mn0.5O3 revealed presence of secondary phases along with contradicting magnetic properties. Such as, Bhame et al (2005 Phys. Rev. B 72 054426–7) reported the superparamagnetic or spin-glass like behavior for 200 °C treated sample that persisted even at 700 °C sample. However, Wei et al (2012 Mater. Chem. Phys. 136 755–61) claimed room temperature ferromagnetism in all the samples annealed in the range of 600 °C–700 °C where the saturation magnetization decreases with the increase in temperature. These contradicting results lead us to revisit the effect of annealing temperature on the magnetic properties of LaFe0.5Mn0.5O3. We noticed a gradual increase in magnetization with increase in annealing temperatures without any signature of long range spin ordering for pure single phase samples. The increased magnetic moment with annealing temperatures has been attributed to the suppression of surface contribution of disordered spin. The low temperature magnetic behaviors can be explained by the interacting cluster glass behavior for the pristine as well as for 1000 °C annealed samples.

Experimental Evidence for a Cluster Glass Transition in Concentrated Lysozyme Solutions.

Lysozyme is known to form equilibrium clusters at pH ≈ 7.8 and at low ionic strength as a result of a mixed potential. While this cluster formation and the related dynamic and static structure factors have been extensively investigated, its consequences on the macroscopic dynamic behavior expressed by the zero shear viscosity η0 remain controversial. Here we present results from a systematic investigation of η0 using two complementary passive microrheology techniques, dynamic light scattering based tracer microrheology, and multiple particle tracking using confocal microscopy. The combination of these techniques with a simple but effective evaporation approach allows for reaching concentrations close to and above the arrest transition in a controlled and gentle way. We find a strong increase of η0 with increasing volume fraction ϕ with an apparent divergence at ϕ ≈ 0.35, and unambiguously demonstrate that this is due to the existence of an arrest transition where a cluster glass forms. These findings demonstrate the power of tracer microrheology to investigate complex fluids, where weak temporary bonds and limited sample volumes make measurements with classical rheology challenging.

Slow dynamics coupled with cluster formation in ultrasoft-potential glasses.

We numerically investigate the slow dynamics of a binary mixture of ultrasoft particles interacting with the generalized Hertzian potential. If the softness parameter, α, is small, the particles at high densities start penetrating each other, form clusters, and eventually undergo the glass transition. We find multiple cluster-glass phases characterized by a different number of particles per cluster, whose boundary lines are sharply separated by the cluster size. Anomalous logarithmic slow relaxation of the density correlation functions is observed in the vicinity of these glass-glass phase boundaries, which hints the existence of the higher-order dynamical singularities predicted by the mode-coupling theory. Deeply in the cluster glass phases, it is found that the dynamics of a single particle is decoupled from that of the collective fluctuations.

Cluster glass induced unconventional exchange bias in epitaxial LaMnO3/SrMnO3 bilayers

Unconventional exchange bias (EB) has been observed in epitaxially grown manganite bilayers composed of ferromagnetic LaMnO3 and G-type antiferromagnetic SrMnO3, with the Curie temperature TC of LaMnO3 lower than the Néel temperature TN of SrMnO3. A maximum EB field of 254 Oe at 2 K is achieved and the unconventional EB appears below a blocking temperature of about 150 K, very close to the TC of the LaMnO3 layer. Such an effect is believed to arise from an interfacial cluster-glass-like state due to the spin frustration at the interface in the LaMnO3/SrMnO3 bilayer.

Spatially limited antiferromagnetic order in a cluster glass compound Tb2Ni0.90Si2.94

In the present study, the synthesis of a new ternary intermetallic compound Tb2Ni0.90Si2.94 has been reported. The detailed studies on structure, static and dynamical magnetic properties of the compound have been investigated by means of powder x-ray diffraction, compositional analysis, dc & ac magnetization, non-equilibrium dynamics, heat capacity and neutron diffraction measurements. The dc & ac magnetic susceptibility reveal that the compound undergoes spin cluster-glass behaviour below 9.9 K. The frequency dependence of the freezing temperature have been analysed on the basis of dynamic scaling laws such as power-law divergence and Vogel-Fulcher law, which further confirm the cluster-glass state formation for the compound. A detailed study on non-equilibrium dynamical behaviour associated with cluster-glass state has been carried out through magnetic relaxation behaviour along with magnetic memory effect in zero-field-cooled (ZFC) as well as field-cooled (FC) conditions and associated aging effect. The zero-field neutron diffraction study reveals the presence of a spatially limited antiferromagnetic phase in addition to the magnetically frustrated cluster-glass state. This result has also been supported through zero-field heat capacity studies. The variation in local electronic environment among the magnetic rare-earth ions caused by the structural disorder associated with Ni/Si ions have been argued to be responsible for the coexistence of different magnetic phases.

Interface-induced transition from a cluster glass state to a spin glass state in LaMnO3/BiFeO3 heterostructures

The LMO/BFO interface induces the transition from a cluster glass state to a spin glass state in the LMO/BFO heterostructure.

B-Site Disorder and Spin States Driven Multiple-Magnetic Phases: Griffiths Phase, Re-Entrant Cluster Glass and Exchange Bias in Pr <sub>2</sub>CoFeO <sub>6</sub>

The magnetic spin ordering and the magnetization dynamics of a double perovskite Pr2CoFeO6 have been investigated by employing the (dc and ac) magnetization, neutron powder diffraction (NPD) and X-ray magnetic circular dichroism (XMCD) techniques. Structural study revealed that Pr2CoFeO6 adopts a B-site disordered orthorhombic structure (Pnma). Furthermore, ab initio band structure calculations suggested an insulating anti-ferromagnetic ground state. Magnetization measurements revealed the system to possess a spectrum of competing magnetic phases viz., long range antiferromagnetic (canted) spin ordering (TN ~269 K), Griffiths phase, re-entrant cluster glass (RCG) (TG ~34 K) and exchange bias effects. The Neutron diffraction divulged the exhibition of a long range canted G-type spin ordering by Fe spins. Moreover, the electronic structure probed by the X-ray absorption spectroscopy (XAS) study suggested a nominal valance state of +3 for both the B-site ions (Co/Fe). The random non-magnetic dilution of magnetic Fe3+ (HS) ions by Co3+ (LS) ions due to B-site disorder essentially played a crucial role in manifesting the magnetic properties of the system.

Coexistence of cluster ferromagnetism and cluster spin-glass like behaviour in melt-quenched Cu2Mn0.5Fe0.5Al Heusler alloy

We investigated the structural stability and magnetic properties of the melt-quenched Heusler alloy-Cu2Mn0.5Fe0.5Al using X-ray diffraction (XRD), scanning electron microscopy (SEM), dc magnetization and ac susceptibility measurements respectively. The structural analysis by Rietveld refinement showed that the alloy crystallised in L21 structure with no trace of secondary phase in the XRD pattern. The dc magnetic measurements exhibited a thermal irreversibility between field-cooled (FC) and zero field cooled (ZFC) magnetization curves at an irreversible temperature Tirr. A deviation from de Almeida –Thouless like line for Tirrvs. H data was observed, suggesting a possibility of cluster glass (CG) like phase in the sample. To confirm the CG behaviour, frequency dependent ac susceptibility (χac) measurements were carried out on the sample and was analysed by Vogel-Fulcher (VF) and Critical slowing down models. We obtained large relaxation time constants (τ0) from the fit to both the phenomenological models which indicate that the origin of glassy nature may not be from randomly oriented atomic spins like in spin-glasses (SG) but of magnetic clusters. The analysis of nonlinear susceptibilities of the sample confirmed the coexistence of ferromagnetic interactions and glassy nature in the sample. Further confirmation for the glassy nature in the sample was provided by ZFC memory experiments. No anomaly was found corresponding to long-range order in the heat capacity (CP(T)) measurement and a T3/2 dependence was also seen in the electrical resistivity which indicates the magnetic glassiness in the sample.

Effect of nickel substitution on magnetism in the layered van der Waals ferromagnet Fe3GeTe2

We have grown a series of nickel substituted single crystals of the layered ferromagnet (FM) Fe$_3$GeTe$_2$. The large single crystalline samples of (Fe$_{1-x}$Ni$_x$)$_3$GeTe$_2$ with $x = 0-0.84$ were characterized with single crystal X-ray diffraction, magnetic susceptibility, electrical resistance and muon spin spectroscopy. We find Fe can be continuously substituted with Ni with only minor structural variation. In addition, FM order is suppressed from $T_\mathrm{C}=212$~K for $x=0$ down to $T_\mathrm{C}=50$~K for $x=0.3$, which is accompanied with a strong suppression of saturated and effective moment, and Curie-Weiss temperature. Beyond $x=0.3$, the FM order is continuously smeared into a FM cluster glass phase, with a nearly full magnetic volume fraction. We attribute the observed change in the nature of magnetic order to the intrinsically disordered structure of Fe$_3$GeTe$_2$ and subsequent dilution effects from the Ni substitution.

Exchange bias and spin-phonon coupling in complex glassy layered perovskite SrLaMn0.5Ni0.5O4

In the present work, single-layered polycrystalline Ruddlesden-Popper oxide, SrLaMn0.5Ni0.5O4 (SLMNO) has been synthesized and characterized. X-ray diffraction and Raman spectroscopy revealed tetragonal crystal structure (I4/mmm) with lattice parameters of a = b = 3.8388 (4) Å and c =12.5593 (2) Å. XPS analysis confirms Mn4+/Ni2+ as the major valance state in SLMNO. Temperature evolution of dc magnetization suggests a ferromagnetic ordering below 130 K followed by a glassy like behaviour below 13 K. The ac susceptibility measurement corroborates the dc magnetization data and confirms cluster glass behavior at lower temperatures. Interestingly, SLMNO exhibits a negative exchange bias of ∼ 0.075 kOe at 5 K under field cooling of 50 kOe and also shows the existence of spin-phonon coupling.