Cluster-glass Phase Research Articles

Giant exchange bias in Mn2FeGa with hexagonal structure

In this study, we present the experimental observation that polycrystalline Mn2+xFe1−xGa (x = −0.2, 0, 0.2, 0.4) compounds can be synthesized to be D019-type (Ni3Sn-type) hexagonal structure with space group P63/mmc. A giant exchange bias field up to 1.32 kOe was achieved in hexagonal Mn2FeGa alloy at 5 K. A cluster glass state is confirmed by ac susceptibility measurement under different driving frequencies. Interestingly, robust horizontal and vertical shifts in magnetic hysteresis loop were simultaneously observed at 5 K under high cooling field up to 90 kOe. The large exchange bias is originated from the large exchange anisotropy between cluster glass phase and ferrimagnetic matrix. The vertical shift is thought to be attributed to the incomplete reversal of frozen cluster spins.

Ferromagnetic Cluster Glass Phase Embedded in a Paramagnetic and Metallic Host in Non-Uniform Magnetic System CaRu1−xScxO3

We have investigated both static and dynamic magnetic properties of polycrystalline CaRu$_{1-x}$Sc$_{x}$O$_{3}$ system in order to clarify the role of Sc ions as a disorder for magnetic ordering. We have observed typical features of a ferromagnetic cluster glass state below around 40 K: (i) a broad, frequency-dependent peak in the ac magnetic susceptibility, (ii) a slow relaxation of the magnetization, and (iii) a continuous increase in the dc magnetic susceptibility in field cooling process. The composition dependence of characteristic parameters for the cluster glass state suggests that chemical segregation can hardly explain the clustering mechanism. We propose a possible picture that the ferromagnetic clusters are distributed uniformly and form the glassy state embedded in the paramagnetic and metallic host of CaRuO$_{3}$.

The coexistence of cluster glass behavior and long-range ferromagnetic ordering in La0.7Sr0.25Na0.05Mn0.7Ti0.3O3 manganite

The electron-doped La0.7Sr0.25Na0.05Mn0.7Ti0.3O3 (LSNMTi0.3) sample was synthesized by a conventional solid-state reaction. Rietveld analysis of the X-ray diffraction (XRD) data showed that the compound crystallized in the space group R3¯c. Magnetic characterization present a signature of a coexisting AFM–FM ordering and a cluster-glass phase. The M2 vs. H/M curves prove that the samples exhibit a second-order magnetic phase transition and the critical properties near the ferromagnetic–paramagnetic phase transition temperature have been analyzed from data of the static magnetization measurements for the sample, through various techniques such as the modified Arrott plot and the critical isotherm analysis. The critical exponent values estimated from the isothermal magnetization measurements are found to be consistent and comparable to those predicted by the long-range mean-field theory.

Structural, electric and magnetic properties of La1−xSrxCo1−xRuxO3 (0 ≤ x ≤ 0.6) solid solution

Structural, electrical transport and magnetic properties of La1−xSrxCo1−xRuxO3 (0≤x≤0.6) have been investigated. Sr and Ru co-doped LaCoO3 (LCO) at La and Co sites reveals a structural phase transition from rhombohedral to monoclinic with increase of x. X-ray photoelectron spectroscopy shows that Ru is present in the mixed valence state, Ru4+ and Ru5+ accompanied with Co3+ and Co2+ valencies. Pure LCO reveals a change from charge gap at higher temperature to spin gap at lower temperature in charge transport process. Higher spin configuration of Co3+ stabilizes ferromagnetic ground state in LCO. Temperature dependence of electrical resistivity indicates a semiconducting behavior for the entire composition range (0≤x≤0.6). High degree of disorder of Co and Ru ions leads to spin glass and cluster glass phase for x≤0.2. The observation of Curie temperature at about 154 K indicates the phase segregation into ferromagnetic SrRuO3 (SRO) for x≥0.3. A critical behavior study for the ferromagnetic phase suggests that the ferromagnetism can be described by mean field model. The co-existence of semiconducting and ferromagnetism can be attributed to random distribution of four cations, La, Sr, Co and Ru.

Evidence for magnetic clusters in Ni1-xVx close to the quantum critical concentration

The d-metal alloy Ni1-xVx undergoes a quantum phase transition from a ferromagnetic ground state to a paramagnetic ground state as the vanadium concentration x is increased. We present magnetization, ac-susceptibility and muon-spin relaxation data at several vanadium concentrations near the critical concentration xc ~ 11.6 % at which the onset of ferromagnetic order is suppressed to zero temperature. Below xc, the muon data reveal a broad magnetic field distribution indicative of a long-range ordered ferromagnetic state with spatial disorder. We show evidence of magnetic clusters in the ferromagnetic phase and close to the phase boundary in this disordered itinerant system as an important generic ingredient of a disordered quantum phase transition. In contrast, the temperature dependence of the magnetic susceptibility above xc is best described in terms of a magnetic quantum Griffiths phase with a power-law distribution of fluctuation rates of dynamic magnetic clusters. At the lowest temperatures, the onset of a short-range ordered cluster-glass phase is recognized by an increase in the muon depolarization in transverse fields and maxima in ac-susceptibility.

Exchange bias effect in polycrystalline SR2IrO4

We report the presence of an exchange bias effect in a layered iridate compound, Sr2IrO4, in polycrystalline form. The Rietveld refinement of the x-ray diffraction pattern shows that Sr2IrO4 crystallizes in a tetragonal structure (I41acd). The temperature dependence of zero-field cooled (ZFC) and field cooled (FC) magnetization measurements show a magnetic ordering temperature of ~240 K, which is considered as its canted antiferromagnetic ordering temperature. An irreversibility between ZFC and FC curves is observed at Tirr, which decreases with an increasing field (H). The Tirr vs. H2/3 plot follows the Almeida-Thouless (AT) line. The presence of an AT line and a significant amount of coercivity (~0.3 T) at 5 K hint a cluster-glass like behaviour in Sr2IrO4 polycrystals. A negative exchange bias effect has been observed at 5 K with an exchange bias field of 0.018 T, which is attributed to an interfacial exchange interaction between a canted antiferromagnetic phase and a probable cluster-glass phase. This is the first report on the observation of an exchange bias effect in Sr2IrO4. Further studies are underway for a detailed understanding.

Correspondence between neutron depolarization and higher order magnetic susceptibility to investigate ferromagnetic clusters in phase separated systems

It is a tough task to distinguish a short-range ferromagnetically correlated cluster-glass phase from a canonical spin-glass-like phase in many magnetic oxide systems using conventional magnetometry measurements. As a case study, we investigate the magnetic ground state of La0.85Sr0.15CoO3, which is often debated based on phase separation issues. We report the results of two samples of La0.85Sr0.15CoO3 (S-1 and S-2) prepared under different conditions. Neutron depolarization, higher harmonic ac susceptibility and magnetic relaxation studies were carried out along with conventional magnetometry measurements to differentiate subtle changes at the microscopic level. There is no evidence of ferromagnetic correlation in the sample S-2 attributed to a spin-glass phase, and this is compounded by the lack of existence of a second order component of higher harmonic ac susceptibility and neutron depolarization. A magnetic relaxation experiment at different temperatures complements the spin glass characteristic in S-2. All these signal a sharp variance when we consider the cluster-glass-like phase (phase separated) in S-1, especially when prepared from an improper chemical synthesis process. This shows that the nonlinear ac susceptibility is a viable tool to detect ferromagnetic clusters such as those the neutron depolarization study can reveal.

Dramatic effect of A-site substitution upon the structure and magnetism of the “114” CaBaCo4O7cobaltite

Electron and hole doping at the cobalt sites of CaBaCo${}_{4}$O${}_{7}$ has been successfully realized by heterovalent substitution on A sites (Ln at Ca sites and K at Ba sites) with a narrow range. Both electron and hole doping have a dramatic impact upon the structure and magnetism, decreasing the orthorhombic distortion and weakening considerably the ferrimagnetism at the benefit of magnetic frustration. The hole doping very drastically kills the ferrimagnetism of the parent phase leading to a cluster-glass phase at lower temperatures, whereas the electron doping with a smaller cation like yttrium leads to an admixture of cluster and spin glass along with the preservation of weak ferrimagnetism. The frequency dependence of the peak position in ${\ensuremath{\chi}}^{\ensuremath{'}}(T)$ curves was quantitatively analyzed using the power law, $\ensuremath{\tau}={\ensuremath{\tau}}_{0}{({T}_{f}/{T}_{g}\ensuremath{-}1)}^{\ensuremath{-}z\ensuremath{\nu}}$. Moreover, the electron doping with larger lanthanides (Pr and Nd) leads to spin-glass states, which are manifested at two temperatures vis-\`a-vis \ensuremath{\sim}60 K and \ensuremath{\sim}110--115 K. The appearance of magnetic frustration at the expense of ferrimagnetism in both cases is interpreted as the result of deviation of the ${\mathrm{Co}}^{2+}/{\mathrm{Co}}^{3+}$ ratio from unity and cationic disordering on cobalt sites, even if the crystal remains orthorhombic.

Evidence of two distinct dynamical freezing processes in single-layered perovskiteLa0.7Sr1.3CoO4

The static and dynamic features of magnetization have been investigated in terms of dcmagnetization, ac susceptibility, and memory effects on single-layered perovskiteLa0.7Sr1.3CoO4. The results indicate that short-range ferromagnetic clusters coexistwith the glassy magnetic state, i.e., a cluster–glass phase betweenTf andTG and a spin-glass-likephase below Tf. The clear evidence of memory effects in the dc magnetization is also observed belowTf andTG, respectively. These two glassy states with cooperative relaxation processes mayderive from the different interaction processes among nanoscale ferromagneticclusters mediated by the matrix, which is influenced by changing the spin state ofCo3 + ions in spontaneously phase-separated cobaltite.

Quantum Griffiths Phase in the Weak Itinerant Ferromagnetic AlloyNi1−xVx

We present magnetization (M) data of the d-metal alloy Ni(1-x)V(x) at vanadium concentrations close to x(c) approximately = 11.4% where the onset of long-range ferromagnetic (FM) order is suppressed to zero temperature. Above x(c), the temperature (T) and magnetic field (H) dependencies of the magnetization are best described by simple nonuniversal power laws. The exponents of M/H approximately T(-gamma) and M approximately H(alpha) are related by 1-gamma = alpha for wide temperature (10 < T < or = 300 K) and field (H < or = 5 T) ranges. gamma is strongly x dependent, decreasing from 1 at x approximately = x(c) to gamma < 0.1 for x = 15%. This behavior is not compatible with either classical or quantum critical behavior in a clean 3D FM. Instead it closely follows the predictions for a quantum Griffiths phase associated with a quantum phase transition in a disordered metal. Deviations at the lowest temperatures hint at a freezing of large clusters and the onset of a cluster glass phase.

10.1007/s11451-008-1013-4

The temperature dependences of the magnetic susceptibility χ(T) and the electrical resistivity ρ(T) of ceramic samples of La1 − x Ca x MnO3 with x = 0.67 (LCMO) and La1 − x Ca x Mn1 − y Fe y O3 with x = 0.67 and y = 0.05 (LCMFO) are investigated in magnetic fields B = 50–105 G and the temperature range T = 4.2–400 K. Both samples undergo a transition from the paramagnetic state to a state with charge (orbital) ordering (CO) at temperatures T CO ≈ 272 K for LCMO and T CO ≈ 222 K for LCMFO. The behavior of the paramagnetic phase in the temperature range 320–400 K for LCMO and 260–400 K for LCMFO is described by the Curie-Weiss law with effective Bohr magneton numbers p eff = 4.83 μB (LCMO) and 4.77 μB (LCMFO), respectively. The disagreement between the observed positive Weiss temperatures (θ ≈ 175 K (LCMO) and θ ≈ 134 K (LCMFO)) and negative Weiss temperatures required for the antiferromagnetic ground state can be explained by the phase separation and transition to the charge-ordered state. The magnetic irreversibility for T < T CO is accounted for by the existence of a mixture of the ferromagnetic and antiferromagnetic phases, as well as the cluster glass phase. At low temperatures, doping with iron enhances the frustration of the system, which manifests itself in a more regular behavior of the decay rate of the remanent magnetization with time. The temperature dependence of the electrical resistivity in the range of the charge-ordered phase conforms to the variable-range hopping model. The behavior of the electrical resistivity is governed by the complex structure of the density of localized states near the Fermi level, which includes a soft Coulomb gap Δ = 0.464 eV for LCMO and 0.446 eV for LCMFO. It is established that the ratio between the localization radii of charge carriers a for LCMFO and a und for LCMO is a/a und = 0.88.

Specific heat of the dilute antiferromagnetic system FexZn1-xF2

The specific heat (cp) of the dilute antiferromagnet FexZn1-xF2 has been measured in absence of external magnetic fields for x = 0, 0.26, 0.31, 0.34, 0.36, 0.38, 0.41, 0.45, 0.56, 0.88, 0.97, and 1.0. For x > 0.45, a sharp peak associated to the antiferromagnetic (AF) phase transition at TN(x) is the only observed feature. For 0.31 ≤ x ≤ 0.41, this peak becomes smaller with decreasing x and a rounded bump appears at higher temperatures T. Closer to the percolation concentration (xp = 0.24), the peak characteristic of the AF phase transition disappears and the rounded bump becomes the only observed feature. The low-T cp behavior confirms a crossover from AF long range order at large x to a spin glass behavior close to xp. At intermediate x, the low-T joint signature of both phenomena indicates a coexistence of AF order and a cluster-glass phase. The x-dependence of the Néel temperature was accounted for using a simple phenomenological model.

Complex magnetic phases in [formula omitted

DC magnetization and AC susceptibility measurements on LuFe 2O 4 single crystals reveal a ferrimagnetic transition at 240 K followed by additional magnetic transitions at 225 K and 170 K, separating cluster glass phases, and a kinetically arrested state below 55 K. The origin of giant magnetic coercivity is attributed to the collective freezing of ferrimagnetic clusters and enhanced domain wall pinning associated with a structural transition at 170 K. Magnetocaloric effect measurements provide additional vital information about the multiple magnetic transitions and the glassy states. Our results lead to the emergence of a complex magnetic phase diagram in LuFe 2O 4.

A theoretical study of the cluster glass-Kondo-magnetic disordered alloys

Abstract The physics of disordered alloys, such as typically the well known case of CeNi 1 - x Cu x alloys, showing an interplay among the Kondo effect, the spin glass state and a magnetic order, has been studied firstly within an average description like in the Sherrington–Kirkpatrick model. Recently, a theoretical model [S.G. Magalhaes, F.M. Zimmer, P.R. Krebs, B. Coqblin, Phys. Rev. B 74 (2006) 014427] involving a more local description of the intersite interaction has been proposed to describe the phase diagram of CeNi 1 - x Cu x . This alloy is an example of the complex interplay between Kondo effect and frustration in which there is in particular the onset of a cluster-glass state. Although the model given in Magalhaes et al. [Phys. Rev. B 74 (2006) 014427] has reproduced the different phases relatively well, it is not able to describe the cluster-glass state. We study here the competition between the Kondo effect and a cluster glass phase within a Kondo-lattice model with an inter-cluster random Gaussian interaction. The inter-cluster term is treated within the cluster mean-field theory for spin glasses [C.M. Sokoulis, Phys. Rev. B 18 (1978) 3757], while, inside the cluster, an exact diagonalisation is performed including inter-site ferromagnetic and intra-site Kondo interactions. The cluster glass order parameters and the Kondo correlation function are obtained for different values of the cluster size, the intra-cluster ferromagnetic coupling and the Kondo intra-site coupling. We obtain that the increase of the Kondo coupling tends clearly to destroy the cluster glass phase.

Magnetic and electric transport properties of Nd0.75Sr1.25Co1−xMnxO4

We present the results of a systematic investigation of crystal structure, dc magnetization, ac susceptibility, resistivity, and magnetoresistance (MR) of Nd0.75Sr1.25Co1−xMnxO4 (0≤x≤0.3) polycrystals. All synthesized specimens are indexed in the same tetragonal space group I4/mmm with random occupation of Co and Mn ions at the identical site. The refinement result confirms the tetragonal distortion of the CoO6 octahedron with elongation along the c axis. The substitution of the Mn ions at Co site brings about the suppression of ferromagnetism as well as the enhancement of antiferromagnetism. The coexistence of ferromagnetic double exchange interactions and antiferromagnetic superexchange interactions and the suppression of ferromagnetism with increasing Mn doping are substantiated by the isothermal magnetization hysteresis loops. The result suggests that the substitution creates more antiferromagnetic bonds with superexchange interactions at the expense of the existing Co–O–Co bonds with ferromagnetic double exchange interactions. At low temperature, a crossover from ferromagnetic cluster-glass phase to spin-glass phase is shown in the dc and ac magnetic measurements. For all the specimens, the resistivity ρ(T) follows semiconducting behavior (dρ/dT&amp;lt;0) in the whole measured temperature region. The substitution induces an obvious increase in resistivity, which originates from the diminishing of ferromagnetic double exchange interactions and the localization of charge carriers caused by the disorder for the substitution. The system presents negative MR due to tunneling effect at low temperatures and positive MR at high-temperature range.

Doping effect on magnetic and transport properties ofPr2−xSrxCoO4

We present the results of a systematic investigation of the dcmagnetization, ac susceptibility, resistivity and thermopower ofPr2−xSrxCoO4 (x = 1.2, 1.3 and1.5). The x = 1.2 specimen undergoes a paramagnetic phase above 201.57 K, Griffiths phase between201.57 and 129.24 K, ferromagnetic cluster-glass phase between 129.24 and 16.19 Kand spin-glass state reentrance below 16.19 K. With increasing Sr content, theGriffiths phase and the spin-glass state are suppressed. The specimen shows ap-type semiconducting conduction changing from thermally activated conductionto variable range hopping conduction with increasing Sr doping level. The richmagnetic and transport behaviors are suggested due to the complicated magneticinteractions and Jahn–Teller distortion of the cobalt ions associated with smallpolarons.

Changeover from glassy ferromagnetism of the orbital domain state to long-range ferromagnetic ordering inLa0.9Sr0.1MnO3

An attempt is made to resolve the controversy related to the low temperature phase (ground state) of the low-doped ferromagnetic (FM)-insulator manganite through bulk magnetic measurements on ${\mathrm{La}}_{0.9}{\mathrm{Sr}}_{0.1}\mathrm{Mn}{\mathrm{O}}_{3}$ sample. It is shown that the FM phase, formed out of well defined transition in the low-doped system, becomes inhomogeneous with decrease in temperature. This inhomogeniety is considered to be an outcome of the formation of orbital domain state of ${e}_{g}$ electrons having hole rich (metallic) walls separating the hole deficient (insulating) regions. The resulting complexity brings in metastability and glassy behavior within the FM phase at low temperature, however, with no resemblance to spin glass, cluster glass, or reentrant phases. It shows aging effect without memory but magnetic relaxation shows signatures of intercluster interaction. The energy landscape picture of this glassy phase is described in terms of hierarchical model. Further, it is shown that this inhomogeneity disappears in ${\mathrm{La}}_{0.9}{\mathrm{Sr}}_{0.1}\mathrm{Mn}{\mathrm{O}}_{3.08}$ where the orbital domain state is destroyed by self-doping, resulting in reduction of ${\mathrm{Mn}}^{3+}$ and hence ${e}_{g}$ electrons. The ferromagnetic phase of the nonstoichiometric sample does not show glassy behavior. It neither follows ``hierarchical model'' nor ``droplet model'' generally used to explain glassy or inhomogeneous systems. Its magnetic response can be explained simply from the domain wall dynamics of otherwise homogeneous ferromagnet.

Electrical conductivity and magnetic properties of La1 − x Ca x Mn1 − y Fe y O3 ceramic samples (x = 0.67, y = 0, 0.05)

The temperature dependences of the magnetic susceptibility χ(T) and the electrical resistivity ρ(T) of ceramic samples of La1 − xCaxMnO3 with x = 0.67 (LCMO) and La1 − xCaxMn1 − yFeyO3 with x = 0.67 and y = 0.05 (LCMFO) are investigated in magnetic fields B = 50–105 G and the temperature range T = 4.2–400 K. Both samples undergo a transition from the paramagnetic state to a state with charge (orbital) ordering (CO) at temperatures TCO ≈ 272 K for LCMO and TCO ≈ 222 K for LCMFO. The behavior of the paramagnetic phase in the temperature range 320–400 K for LCMO and 260–400 K for LCMFO is described by the Curie-Weiss law with effective Bohr magneton numbers peff = 4.83 μB (LCMO) and 4.77 μB (LCMFO), respectively. The disagreement between the observed positive Weiss temperatures (θ ≈ 175 K (LCMO) and θ ≈ 134 K (LCMFO)) and negative Weiss temperatures required for the antiferromagnetic ground state can be explained by the phase separation and transition to the charge-ordered state. The magnetic irreversibility for T < TCO is accounted for by the existence of a mixture of the ferromagnetic and antiferromagnetic phases, as well as the cluster glass phase. At low temperatures, doping with iron enhances the frustration of the system, which manifests itself in a more regular behavior of the decay rate of the remanent magnetization with time. The temperature dependence of the electrical resistivity in the range of the charge-ordered phase conforms to the variable-range hopping model. The behavior of the electrical resistivity is governed by the complex structure of the density of localized states near the Fermi level, which includes a soft Coulomb gap Δ = 0.464 eV for LCMO and 0.446 eV for LCMFO. It is established that the ratio between the localization radii of charge carriers a for LCMFO and aund for LCMO is a/aund = 0.88.

Quantum Critical Behavior of the Cluster Glass Phase

In disordered itinerant magnets with arbitrary symmetry of the order parameter, the conventional quantum critical point between the ordered phase and the paramagnetic Fermi liquid (PMFL) is destroyed due to the formation of an intervening cluster glass (CG) phase. In this Letter, we discuss the quantum critical behavior at the CG-PMFL transition for systems with continuous symmetry. We show that fluctuations due to quantum Griffiths anomalies induce a first-order transition from the PMFL at T = 0, while at higher temperatures a conventional continuous transition is restored. This behavior is a generic consequence of enhanced non-Ohmic dissipation caused by a broad distribution of energy scales within any quantum Griffiths phase in itinerant systems.

Enhanced grain surface effect on magnetic properties of La0.5Gd0.2Sr0.3MnO3 nanoparticles: A comparison with bulk counterpart

Magnetization studies on La0.5Gd0.2Sr0.3MnO3 (LGSMO) nanoparticles (∼20nm) reveal superparamagnetic phase associated with this system and thereby contrasting from cluster glass (CG) phase of its bulk counterpart. Doping of Gd on La sites and its antiferromagnetic coupling with Mn lattices are expected to induce random magnetic disorder in the magnetic lattice of LGSMO system. Study reveals that random magnetic disorder, which results in CG phase in an otherwise long range ordered ferromagnetic host matrix of bulk, does not have similar significant effect when the uniformity of the host matrix reduces to nanosize. On the contrary, analysis brings out that magnetic properties of LGSMO nanoparticles are primarily decided by its nanodimension having physical size of ∼20nm, which yield single domain magnetic entities of dimension of ∼12nm surrounded by a magnetic dead layer of ∼4nm.