Cluster Glass Research Articles

Magnetic cluster glass behavior and grain boundary effect in Nd0.7Ba0.3MnO3 nanoparticles

The magnetic and transport properties of Nd0.7Ba0.3MnO3 nanoparticles were explored by transmission electron microscopy, x-ray powder diffraction, resistivity, magnetoresistance, thermopower (S), and magnetic measurements. The metal-insulator transition behavior of the temperature dependence of resistivity for the sample with the largest particle size changes to insulating nature with the decrease in the particle size due to the enhancement of the grain boundary effect. The magnetoresistance of the nanoparticles is analyzed in the light of a phenomenological model based on the spin polarized tunneling at the grain boundaries. The thermopower of the samples shows a crossover from negative to positive values and at high temperatures S follows adiabatic small polaron hopping theory. The zero field cooled and field cooled (FC) magnetizations display broad ferromagnetic transition. The Curie temperature (TC) and the irreversibility temperature (Tirr) decrease considerably with the decrease in the particle size. During cooling the ac susceptibility of the nanoparticles exhibits two magnetic phase transitions with paramagnetic, ferromagnetic, and glassy phases. The frequency dependent peak in the out of phase part (χ″) of the ac susceptibility is the signature of cluster glass behavior. Large thermomagnetic irreversibility, monotonic increase in the FC magnetization, nonsaturation of the magnetization, and the observation of two distinct magnetic transitions in ac susceptibility give evidence for the cluster glass nature of the nanoparticles.

Effects of nonstoichiometry on the properties of electron-doped manganite Pr 0.1Ca 0.9MnO 3

Mn 4+-rich perovskite manganites (nominal composition: Pr 0.1Ca 0.9MnO 3) were synthesized by using a citric acid method and sintered at different temperatures (800, 900, 1000, and 1300 °C) to adjust the concentration of charge carrier. All the samples are found to be in the cluster glass state at low temperature, as indicated by dc and ac magnetization. With the increase of sintering temperature, the ferromagnetic component in samples increases at first, and attains a maximum at 1000 °C, then decreases again at 1300 °C, which could be well interpreted by the change of charge carrier concentration induced by nonstoichiometry, consistent with the trend on the phase diagram. However, nonstoichiometry breaks long-range Mn–O–Mn interaction, resulting in the variance from the phase diagram.

Dynamics of the Magnetic Susceptibility of Fe$_{x}$Al$_{100 - {\rm x}} ({\rm x} = 70, 71)$ Alloys

The magnetic dynamics of Fe71 Al29(x = 71) and Fe70 Al30(x = 70) are analyzed. The alloys have been prepared by milling and further annealing. AC-susceptibility has been measured from 5 K to 300 K at several frequencies between 100 and 10 kHz with an alternating field of 0.1 Oe. In x = 70, additional susceptibility data fixing the frequency to 500 Hz and varying the field between 0.1 les h les 10 Oe were obtained. The results display real components similar to those reported in DC-magnetization, with two transitions in x = 70 and a single transition in x = 71. The complex component is much more clear with a well-defined peak for the low temperature transition ascribed to a spin freezing. The frequency variation gives 0.078(5) in x = 70 and critical zv = 7.2(5) and 0.066(9) and zv = 9.8(5) for x = 71, related to a cluster glass behavior. However the parameter beta ap 1 for both alloys which is high in the framework of a conventional slowing down. This is related to the clustered nature of the magnetic structure lacking of defined interfaces.

Cluster-glass wave function in the two-dimensional extendedt−Jmodel

The recent observation of an electronic cluster glass state composed of random domains with unidirectional modulation of charge density and/or spin density on ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{CaCu}}_{2}{\text{O}}_{8+\ensuremath{\delta}}$ reinvigorates the debate of existence of competing interactions and their importance in high-temperature superconductivity. By using a variational approach, here we show that the presence of the cluster glass state is actually an inherent nature of the model based on the antiferromagnetic interaction $(J)$ only, i.e., the well-known $t\text{\ensuremath{-}}J$ model. There is no need yet to introduce a competing interaction to understand the existence of the cluster glass state. The long-range pairing correlation is not much influenced by the disorder in the glass state which also has nodes and linear density of states. In the antinodal region, the spectral weight is almost completely suppressed. The modulation also produces subgap structures inside the ``coherent'' peaks of the local density of states.

Response to “Comment on ‘Cluster glass induced exchange biaslike effect in the perovskite cobaltites’ ” [Appl. Phys. Lett. 93, 176101 (2008)

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Evidence for cluster glass behavior inFe2VAlHeusler alloys

Detailed magnetic measurements on intermetallic ${\text{Fe}}_{2}\text{VAl}$ Heusler type alloys are presented. Annealed ${\text{Fe}}_{2}\text{VAl}$ sample is structurally better ordered and exhibits higher magnetization, resistivity and magnetoresistance values compared to as cast samples. From the systematic analysis of magnetization and electrical transport data it has been established that the ${\text{Fe}}_{2}\text{VAl}$ behaves more like a cluster glass. Magnetization as a function of magnetic field at various temperatures suggests the presence of superparamagnetic clusters along with paramagnetic background. Arrot plots confirm the absence of spontaneous magnetic moment and hence long-range ferromagnetic order down to 5 K. The field cooled and zero-field-cooled magnetization curves suggest the superparamagnetic entities freeze below 10 K. Our magnetoresistance data corroborate the magnetization results and confirm the presence of magnetic clusters.

Magneto-electric Behaviors of La<SUB>0.67-x</SUB>Nd<SUB>x</SUB>Sr<SUB>0.33</SUB>MnO<SUB>3</SUB> System

The magneto-electric behaviors of La_(0.67-x)Nd_xSr_(0.33)MnO_3 system were studied by the measurement of M-T curve,ESR curve,infrared spectra,Raman spectra,p-T curve and MR-T curve of the La_(0.67-x)Nd_xSr_(0.33)MnO_3 system(x=0.00,0.10,0.20,0.30,0.40).The experiment result shows that with the Nd doping increasing,the magnetic structure of the system transforms from the long-range ferromagnetic order to spin cluster glass state;for the samples with x=0.30 and 0.40,the phase separation occurs at the temperature high above T_c;The transport property exhibits change with the Nd doping increasing.Magneto-electric behaviors of the system and CMR effect attributes to lattice effect and interface tunneling effect relating to the spin.

Structure and properties of the kagome compoundYBaCo3AlO7

Large single crystals of ${\text{YBaCo}}_{3}{\text{AlO}}_{7}$ were synthesized in a floating-zone furnace. Powder and single-crystal x-ray data confirm that the structure belongs to the swedenborgite compound family ($P{6}_{3}mc$, $a=6.28098(3)\text{ }\text{\AA{}}$, and $c=10.21200(5)\text{ }\text{\AA{}}$). ${\text{YBaCo}}_{3}{\text{AlO}}_{7}$ can reversibly absorb/desorb oxygen up to about ${\text{O}}_{7.5}$ according to thermo gravimetric measurements. The magnetic substructure of corner-sharing $(\text{Co}/\text{Al}){\text{O}}_{4}$ tetrahedra forms kagome-like layers, resulting in a geometric frustration. The magnetization data indicate a highly degenerate spin state with a freezing temperature of ${T}_{f}=16.8\text{ }\text{K}$. Relaxation of magnetic remanence, including aging effects, frequency dependence of ac susceptibility near the transition temperature, and lack of entropy change at ${T}_{f}$ allow the conclusion that ${\text{YBaCo}}_{3}{\text{AlO}}_{7}$ is a spin glass or a ``cluster glass'' with randomly frozen domains of antiferromagnetically interacting spins at low temperatures. The origin of this glassy spin state is a combination of Co and Al disorder at the tetrahedral sites and the high frustration between the sites caused by kagome geometry.

Evidence of intrinsic exchange bias and its origin in spin-glass-like disorderedL0.5Sr0.5MnO3manganites (L=Y,Y0.5Sm0.5, andY0.5La0.5)

Exchange-bias (EB) phenomena have been observed in the ${L}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}$ ($L=\mathrm{Y}$, ${\mathrm{Y}}_{0.5}{\mathrm{Sm}}_{0.5}$, and ${\mathrm{Y}}_{0.5}{\mathrm{La}}_{0.5}$)-type manganites showing cluster-glass-like and spin-glass-like behavior. The field cooled magnetic hysteresis loops exhibit shifts both in the field and magnetization axes. The values of exchange field $({H}_{E})$, coercivity $({H}_{C})$, remanence asymmetry $({M}_{E})$, and magnetic coercivity $({M}_{C})$ of ${L}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}$ are found to depend strongly on temperature, measuring field, as well as strength of cooling field ${H}_{\mathrm{cool}}$. ${H}_{E}$ increases sharply with the magnitude of ${H}_{\mathrm{cool}}$ $(\ensuremath{\leqslant}3\phantom{\rule{0.3em}{0ex}}\mathrm{T})$, but for larger ${H}_{\mathrm{cool}}$ $(g3\phantom{\rule{0.3em}{0ex}}\mathrm{T})$, it decreases due to the growth of the ferromagnetic cluster size. This observed behavior has been explained in terms of interfacial exchange coupling between coexisting ferromagnetic cluster glass and the disordered spin-glass-like phases, where the spin configurations are strongly affected by the cooling field strength. Below the spin and/or cluster glass freezing temperature $({T}_{f})$, both ${H}_{E}$ and ${M}_{E}$ decrease exponentially with temperature. The value of ${H}_{C}$ increases almost exponentially with increasing magnetic-field step size $(\ensuremath{\Delta}H)$. In addition, the observed training effect of the EB behavior has been explained well in terms of the existing relaxation model developed for other classical EB systems. In view of the use of manganites in spintronics, the present observation of EB-like shift even in the mixed valent polycrystalline manganites such as ${L}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}$ is of paramount importance.

Two isotypic diphosphates Li M2H 3(P 2O 7) 2 ( M=Ni, Co) containing ferromagnetic zigzag MO 6 chains

Two new isotypic phosphates LiNi 2H 3(P 2O 7) 2 ( 1) and LiCo 2H 3(P 2O 7) 2 ( 2) have been hydrothermally synthesized and structurally characterized by the single-crystal X-ray diffraction technique. They crystallize in the monoclinic space group C2/ c with the lattice: a=10.925(2) Å, b=12.774(3) Å, c=8.8833(18) Å, β=123.20(3)° for 1 and a=10.999(2) Å, b=12.863(3) Å, c=8.9419(18) Å, β=123.00(3)° for 2. The transition metal atoms are octahedrally coordinated, whereas the lithium and phosphorus atoms are all tetrahedrally coordinated. As the lithium-induced derivatives of MH 2P 2O 7 ( M=Ni, Co), 1 and 2 possess the same structure with MH 2P 2O 7 in terms of topology, comprising the MO 6 zigzag chains and P 2O 7 as the interchain groups. The magnetisms of 1 and 2 could be interpreted by adopting a quasi-one-dimensional (1D) zigzag chain model as that in their parent compounds: both 1 and 2 have ferromagnetic (FM) NiO 6/CoO 6 chains; 1 shows a FM cluster glass behavior at low temperatures, which is originated from the possible antiferromagnetic (AFM) next-nearest-neighbour intrachain interactions; 2 shows a AFM ordering at T N=2.6 K and a metamagnetic transition at H C=4.2 kOe at 1.8 K.

Structure and magnetic order in La 0.7Ca 0.3Mn 0.5Co 0.5O 3 and La 0.8Sr 0.2Mn 0.5Co 0.5O 3 perovskites

In this paper we report the study of the perovskites La 0.7Ca 0.3Mn 0.5Co 0.5O 3 and La 0.8Sr 0.2Mn 0.5Co 0.5O 3 by neutron powder diffraction at various temperatures and magnetization measurements in zero applied field and at low cooling regimes. The replacement of half Mn by Co in La 0.7Ca 0.3MnO 3 and La 0.8Sr 0.2MnO 3 destroys their long-range ferromagnetism exhibiting a cluster glass ferromagnetic order similar to the one observed in many cobaltites.

Photoemission and x-ray absorption studies of valence states in (Ni,Zn,Fe,Ti)3O4 thin films exhibiting photoinduced magnetization

By means of photoemission and x-ray absorption spectroscopy, we have studied the electronic structure of (Ni,Zn,Fe,Ti)3O4 thin films, which exhibits a cluster glass behavior with a spin-freezing temperature Tf of ∼230K and photoinduced magnetization (PIM) below Tf. The Ni and Zn ions were found to be in the divalent states. Most of the Fe and Ti ions in the thin films were trivalent (Fe3+) and tetravalent (Ti4+), respectively. While Ti doping did not affect the valence states of the Ni and Zn ions, a small amount of Fe2+ ions increased with Ti concentration, consistent with the proposed charge-transfer mechanism of PIM.

Magnetic and photomagnetic studies in nanocrystalline Ni0.5Zn0.5Fe1.7Co0.3O4

This work presents magnetic and photomagnetic studies on the nanocrystalline form of the spinel Ni0.5Zn0.5Fe1.7Co0.3O4. Field cooled, zero field cooled, and hysteresis have been recorded for this compound with and without irradiation. Kinetics of relaxation of the magnetization in the dark and after irradiation has also been studied. An explanation of these results is given in the framework of freezing/melting of cluster glass behavior.

Electronic Localization and Two-Dimensional Metallic State in UPt2Si2

We present a detailed study of the physical properties of single crystalline UPt 2 Si 2 . By neutron diffraction we establish the presence of frozen-in strain disorder on the Pt(2)/Si(2) ligand sites within the tetragonal crystal structure. Further, the system undergoes an antiferromagnetic transition at T N =32 K. In the range T N < T < T irr ≃36 K we observe a cluster glass like phase as result of the disorder. The highly anomalous behavior of the electronic transport properties–metallic along the tetragonal a and localized along the c axis–is a result of the specific type of disorder. Altogether, UPt 2 Si 2 emerges as a model compound to study disorder effects in f electron intermetallics.

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.

Size effects on charge ordering and magnetic properties in La0.25Ca0.75MnO3

The size effects on the charge ordering (CO) and magnetic properties in La(0.25)Ca(0.75)MnO(3) with mean particle size ranging from 40 to 2000 nm were studied. With decreasing particle size the CO transition temperature shifts to lower temperature and the transition width becomes increasingly wide, indicating the weakening of the CO state. Meanwhile the ferromagnetic (FM) cluster glass state appears and the magnetization at low temperature increases significantly. The behaviour is due to the increasing uncompensated surface spins which weaken the antiferromagnetic interaction and disfavour the formation of the CO state. The suppression of the CO state and appearance of the FM cluster glass state are also found in La(0.25)Ca(0.75)MnO(3) nanowires fabricated by a sol-gel template method. These results indicate that the CO state can be modulated effectively by varying particle size, which has an important implication for nano-device applications of manganites. (c) 2007 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Magnetoresistive properties of Zn 1−xCo xFe 2O 4 ferrites

The magnetic and magnetoresistive properties of spinel-type Zn 1− x Co x Fe 2O 4 ( x=0, 0.2 and 0.4) ferrites are extensively investigated in this study. A large negative magnetoresistance (MR) effect is observed in Zn 1− x Co x Fe 2O 4 ferrites of spinel structure. These materials are either ferrimagnetic or paramagnetic at room temperature, and show a spin-(cluster) glass transition at low temperatures, depending on the chemical compositions. The MR curves as a function of magnetic fields, MR( H), are parabolic at all temperatures for paramagnetic polycrystalline ZnFe 2O 4. The MR for ZnFe 2O 4 at 110 K in the presence of 9 T applied magnetic field is 30%. On the other hand, MR( H) are linear for x=0.2 and 0.4 ferrimagnetic Zn 1− x Co x Fe 2O 4 samples up to 9 T. The MR effect is independent of the sintering temperatures, and can be explained with the help of the spin-dependent scattering and the Yafet–Kittel angle of Zn 1− x Co x Fe 2O 4 mixed ferrites.

Particle size effects on interplay between charge ordering and magnetic properties in nanosizedLa0.25Ca0.75MnO3

The charge ordering (CO) and magnetic properties of ${\mathrm{La}}_{0.25}{\mathrm{Ca}}_{0.75}\mathrm{Mn}{\mathrm{O}}_{3}$ compounds with average particle sizes ranging from $40\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}2000\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ have been studied. With decreasing particle size, the CO transition gradually shifts to lower temperature and becomes increasingly wide and weak; meanwhile, the ferromagnetic cluster glass state appears, and the temperature ${T}_{G}$ for the appearance of ferromagnetic clusters shows a nonmonotonous variation with a maximum at about $100\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The magnetization at low temperature shows a relatively complex change with particle size. A core-shell model was proposed to explain these anomalous behaviors, and the temperature-particle size phase diagram of ${\mathrm{La}}_{0.25}{\mathrm{Ca}}_{0.75}\mathrm{Mn}{\mathrm{O}}_{3}$ was established. These results are helpful in deeply understanding the coupling between charge and spin degrees of freedom in manganites.

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.