Cluster Spin-glass State Research Articles

Alternating current susceptibility study on the cluster glass behavior in disordered β-LiFeO2

The static and dynamic magnetic behavior of β-LiFeO2 has been investigated by measuring dc and ac susceptibilities. The zero-field-cooled (ZFC) and field-cooled (FC) curves reveal an irreversibility behavior at about 156 K and a broad peak at 129 K, indicating a spin glass transition. However, the dc susceptibility of β-LiFeO2 in FC mode differs from a flat curve as expected for a homogeneous spin glass by an increase below Tirr, which is the characteristic magnetic behavior of a cluster spin glass. The real part of the ac susceptibility shows a broad maximum at 135 K, which shifts to higher temperatures and becomes less pronounced with increasing frequency. The corresponding parameter δ=ΔTf/TfΔlogω = 0.005 is calculated for β-LiFeO2, suggesting a spin glass state rather than superparamagnetism. In addition, three models (the Néel–Arrhenius law, the Vogel–Fulcher law, and the power law) have also been employed to analyze the frequency dependence of Tf, which confirm a cluster spin glass state in β-LiFeO2.

Ac magnetic susceptibility measurements in nanoparticulate powders of iron-doped ZnO

Low temperature (T) ac magnetic susceptibility (χac) measurements were performed in powder samples of the Zn1−xFexO (x=0–0.078) prepared by a combustion reaction method. The amplitude of the ac magnetic field was kept constant (1mT) while its frequency (f) varied in the range 10–104Hz for 5≤T≤300K. The diluted samples presented cusps with maxima distributed around Tf=17.5±0.5K while no such feature was observed for the pure sample (x=0). The cusp was found to become more pronounced with the increasing Fe content. Also it decreased and shifted to higher values of T by increasing f. The shift in Tf was found to follow a Vogel–Fulcher law with Ea/kB=317.6K, T0=4.65K and τ0=10−14s, for the activation energy, critical temperature and characteristic time-constant, respectively. The energy gap of the sample was measured and they were found to vary in the range 3.75–3.80eV. The overall χac-data resembles those obtained for cluster spin-glass state.

Unusual Magnetism Due to a Random Distribution of Cations in α-LiFeO2

The magnetic behavior of disordered α-LiFeO 2 is studied by Mossbauer spectroscopy, dc and ac susceptibilities. The 57 Fe Mossbauer spectrum of α-LiFeO 2 can be fitted by two doublets with similar isomer shifts but different quadruple splittings due to different electric field gradients at the Fe 3+ cores. The effective moment of α-LiFeO 2 is lower than that theoretical expected because of the formation of short-range antiferromagnetic clusters. The frequency dependence of the ac peaks at 88 K, irreversibility behavior and its evolution with the increasing strength of a superimposed dc field confirm a cluster spin glass state of α-LiFeO 2 .

A Kondo cluster-glass model for spin glass Cerium alloys

There are clear indications that the presence of disorder in Ce alloys, such as Ce(Ni,Cu) or Ce(Pd,Rh), is responsible for the existence of a cluster spin glass state which changes continuously into inhomogeneous ferromagnetism at low temperatures. We present a study of the competition between magnetism and Kondo effect in a cluster-glass model composed by a random inter-cluster interaction term and an intra-cluster one, which contains an intra-site Kondo interaction Jk and an inter-site ferromagnetic one J0. The random interaction is given by the van Hemmen type of randomness which allows to solve the problem without the use of the replica method. The inter-cluster term is solved within the cluster mean-field theory and the remaining intra-cluster interactions can be treated by exact diagonalization. Results show the behavior of the cluster glass order parameter and the Kondo correlation function for several sizes of the clusters, Jk, J0 and values of the ferromagnetic inter-cluster average interaction I0. Particularly, for small Jk, the magnetic solution is strongly dependent on I0 and J0 and a Kondo cluster-glass or a mixed phase can be obtained, while, for large Jk, the Kondo effect is still dominant, both in good agreement with experiment in Ce(Ni,Cu) or Ce(Pd,Rh) alloys.

Cluster spin glass and superparamagnetism in RuSr2Eu1.5Ce0.5Cu2O10- δ

We investigate in detail the dc magnetization and nonlinear ac susceptibility behavior of the superconducting ferromagnet RuSr2Eu1.5Ce0.5Cu2O10-δ (Ru1222) to develop a comprehensive understanding of the spin glass and superparamagnetism in this material. The structural properties of the system result in the formation of magnetic (ferromagnetic) clusters of different sizes, shapes and properties. The magnetic clustering of the system leads to observation of various features in dc magnetization and ac susceptibility consistent with superparamagnetism and cluster spin glass states, which can coexist or stand alone, depending on the temperature range considered. Experimental results of magnetic measurements in combination with their analysis have enabled us to explain and distinguish these phenomena, as well as to propose a temperature dependent scenario of the system behavior.

Magnetic states of lightly hole-doped cuprates in the clean limit as seen via zero-field muon spin spectroscopy

We have performed extensive zero-field $\ensuremath{\mu}\text{SR}$ experiments on pure ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{6+y}$ and diluted Y-rare-earth substituted ${\text{Y}}_{0.92}{\text{Eu}}_{0.08}{\text{Ba}}_{2}{\text{Cu}}_{3}{\text{O}}_{6+y}$ and ${\text{Y}}_{0.925}{\text{Nd}}_{0.075}{\text{Ba}}_{2}{\text{Cu}}_{3}{\text{O}}_{6+y}$ at light hole doping. A common magnetic behavior is detected for all the three families, demonstrating negligible effects of the isovalent Y-substituent disorder. Two distinct regimes are identified, separated by a crossover, whose origin is attributed to the concurrent thermal activation of spin and charge degrees of freedom: a thermally activated and a re-entrant antiferromagnetic regime. The peculiar temperature and hole density dependence of the magnetic moment $m(h,T)$ fit a model with a (spin) activation energy for the crossover between the two regimes throughout the entire investigated range. The magnetic moment is suppressed by a simple dilution mechanism both in the re-entrant regime $(0\ensuremath{\le}h\ensuremath{\le}0.056)$ and in the so-called cluster spin glass state coexisting with superconductivity $(0.056<h\ensuremath{\lesssim}0.08)$. We argue a common magnetic ground state for these two doping regions and dub it frozen antiferromagnet. Conversely either frustration or finite-size effects prevail in the thermally activated antiferromagnetic state, that vanishes at the same concentration where superconductivity emerges, suggesting the presence of a quantum critical point at ${h}_{c}=0.056(2)$.

Theoretical models for a complex magnetic system: The case of CeNi1-xCux

Special features in the phase diagram and in the low temperature hysteresis cycles have been experimentally observed in the complex CeNi1-xCux system. We present also here theoretical approaches, based firstly on a Kondo lattice model which describes the coexistence between a spin glass or a cluster glass state, the Kondo regime and the ferromagnetic ordering. The second model is a Monte Carlo simulation on a 3D lattice with clusters and random anisotropy and reproduces the existence of steps in the magnetizations cycles at very low temperatures. The theoretical results are compared with the experimental data of the very complex magnetic behaviour of CeNi1−xCux alloys. In particular, we can account for the existence of a cluster spin glass state which changes continuously into an inhomogeneous ferromagnetic phase at very low temperatures.

Enhanced room temperature magnetoresistance and cluster spin glass behavior in Mo doping La0.67Sr0.33MnO3

The structural, magnetic, and transport properties of Mo doping La0.67Sr0.33Mn1−xMoxO3 (x=0–0.04) manganite system have been investigated by x-ray diffraction, scanning electron microscopy, magnetization, and magnetoresistance measurements. The Mo doping in Mn site is found to lower the Curie temperature Tc slightly and induce the cluster spin glass behavior in ferromagnetic state of La0.67Sr0.33MnO3. The room temperature magnetoresistance of Mo doping La0.67Sr0.33Mn1−xMoxO3 is found to be 50% higher than that of La0.67Sr0.33MnO3 without Mo doping. The significant enhancement for room temperature magnetoresistance could likely be attributed to the presence of the cluster spin glass state caused by the Mo doping.

Cluster spin-glass state and Kondo behavior in Sm-based bulk metallic glasses

We investigate the magnetization and electrical transport properties of Sm-based ternary magnetic bulk metallic glasses at low temperatures. A cluster spin-glass phase is evidenced below Tf∼25 K, which is probably ascribed to competition among the multifold magnetic interactions and intrinsic structural inhomogeneity, and further demonstrated by the magnetic hysteresis under field-cooling conditions. It is shown that the short-range magnetic clusters play as scattering centers for conductive electrons and result in a Kondo-like transport behavior in highly spin disordered Sm50Al25Co25 at low temperature.

Spin glass state and its correlations with the magnetic and electrical properties as well as with the defectiveness in the spinels CuCr 2X 4 (X = S, Se) doped with Sb

High degree of defectiveness both in the stoichiometric CuCr 2− x Sb x S 4 ( x = 0.3, 0.4, 0.5) and nonstoichiometric Cu 1 + x Cr 1.5 + y Sb 0.5 + z Se 4 + t (where: −0.02 ≤ x ≤ 0.01, 0.03 ≤ y ≤ 0.35, −0.2 ≤ z ≤ −0.02, 0.01 ≤ t ≤ 0.08) spinels doped with Sb has been determined with the aid of the vacancy model. From the analysis carried out of the effective magnetic moments and the exchange integrals, it follows that in the spinels with sulphur mainly the cation subarrays are defected whereas in the spinels with selenium the defectiveness appears mainly in the anion subarrays. It turned out that in the spinels with sulphur the ferromagnetic cluster spin glass states appear accompanied by the large negative magnetoresistance, whereas in the spinels with selenium the corresponding clusters are antiferromagnetic and the giant magnetoresistance has been observed both positive and negative. A jump-like phase transition was here observed changing both the sign and the measured value of the magnetoresistance from −4433% at 153.2 K up to +980% at 154.24 K. Moreover, in the spinels with selenium both semiconductor–metal and metal–semiconductor phase transitions have been observed from the temperature dependence of the resistance.

Disorder-induced short-range ferromagnetism and cluster spin-glass state in sol-gel derivedLa0.7Ca0.3Mn1−xCdxO3(0⩽x⩽0.2)

Low temperature magnetic and transport properties of the sol-gel derived Cd doped ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Cd}}_{x}{\mathrm{O}}_{3}(0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.20)$ manganite system show ferromagnetism and cluster spin-glass behavior. Metal-insulator transition (MIT) is exhibited only by the samples with $x\ensuremath{\leqslant}0.1$; and for higher concentrations ($x=0.15$ and 0.20), samples are semiconductors. The MIT shifts to the lower temperature regime with increasing Cd content, indicating an increase of disorder in the system. Structural analysis shows local strain-induced small lattice distortion of the $\mathrm{Mn}{\mathrm{O}}_{6}$ octahedra with Cd doping. With increasing Cd content, the system undergoes interesting paramagnetic to ferromagnetic (with $xl0.10$) as well as paramagnetic to cluster spin-glass (for $x\ensuremath{\geqslant}0.10$) transitions. ac susceptibility $({\ensuremath{\chi}}_{\mathrm{ac}})$ and magnetic relaxation measurements confirm this cluster spin-glass behavior. ${\ensuremath{\chi}}_{\mathrm{ac}}$ is found to follow the critical slowing down law $(\ensuremath{\tau}∕{\ensuremath{\tau}}_{0}={\ensuremath{\epsilon}}^{\ensuremath{-}z\ensuremath{\nu}})$. The magnetic hysteresis loops indicate a field-induced irreversible ferromagnetic phase due to the presence of ``weak'' antiferromagnetic domains in the samples with $x\ensuremath{\geqslant}0.10$. This typical behavior is found to be most prominent in the sample with $x=0.15$. The observed cluster spin-glass state is explained by considering the interactions between the ferromagnetic and antiferromagnetic clusters in these doped systems having disorder and reduced geometrical tolerance factor $t$, which resulted from the random substitution of Mn with Cd ions.

Abnormality of Magnetic Behavior and Resistivity of La0.7xDyxSr0.3MnO3 (0.00x0.30) System at Low Temperature

By measuring M-T curves, -T curves and M R-T curves of the samples under different temperatures, the influence of Dy doping (0:00 x 0:30) on the magnetic and electric properties of La0.7xDyxSr0.3MnO3 has been studied. The experimental results show that, with the increase of the Dy content, the system undergoes a transition from long range ferromagnetic order to the cluster-spin glass state and further to antiferromagnetic order. For the samples with x=0.20 and 0.30, their magnetic behaviors are abnormal at low temperature, and their resistivities at low temperature have a minimum value. These peculiar phenomena not only come from the lattice effect induced by doping, but also from extra magnetic coupling induced by doping.

Nonlinear magnetic susceptibility and the spin glass state in the new nonstoichiometric spinel compounds Cu 1+ xCr 1.5+ ySb 0.5+ zSe 4+ t (where −0.02 ≤ x ≤ 0.01, 0.03 ≤ y ≤ 0.35, −0.2 ≤ z ≤ − 0.02, 0.01 ≤ t ≤ 0.08)

New nonstoichiometric spinel compounds Cu 1+ x Cr 1.5+ y Sb 0.5+ z Se 4+ t (where −0.02 ≤ x ≤ 0.01, 0.03 ≤ y ≤ 0.35, −0.2 ≤ z ≤ −0.02, 0.01 ≤ t ≤ 0.08) have been obtained with the aid of the solid-phase synthesis from the stoichiometric mixtures of the constituent elements. Six samples of these compounds with the different compositions have been chosen to the dc and ac magnetic susceptibility measurements. From the analysis of these measurements spin glass states have been revealed with the spin freezing temperature about 70 K for all the six samples. The magnetization measurements were performed for the same samples with the use of the induction method in the high stationary magnetic fields up to 14 T in the temperature range of 4.2–273 K. No sample under study reaches the saturation magnetization even at 14 T though they display a spontaneous magnetization, the latter existing at least up to 273 K. The temperature dependence of the dc inverse molar magnetic susceptibility reveals two different rectilinear parts which can be interpreted as related to two different Curie–Weiss temperatures: ∼68 and ∼45 K. It seems that these two temperatures correspond to two different types of the magnetically ordered clusters. The average effective magnetic moment is here equal to 2.35 μ B/ion pointing to the high defectiveness of both the crystal and magnetic structures. All the effects observed on both the magnetization and inverse molar magnetic susceptibility curves point to the appearance of the cluster spin glass states in the samples under study. Generally speaking, it is the statistical character of the substitution of the nonmagnetic antimony ions instead of the magnetic chromium ions which leads to the frustration of the orientations of the localized magnetic moments in the samples under study and in the consequence to the appearance of the spin glass states.

Coexistence of spin glass and superparamagnetism with ferrimagnetic order in polycrystalline spinel [formula omitted

We discuss our investigation of DC magnetization, as a function of time, temperature and magnetic field, and explore some distinct magnetic behaviour in the polycrystalline spinel Co 0.2Zn 0.8Fe 1.95Ho 0.05O 4. The system is essentially to be a ferrimagnet and the ferrimagnetic order (spontaneous magnetization) is identified using Arrot plot of M ( H ) data. Application of some scaling laws have shown the coexistence of spin glass (SG) and superparamagnetism (SPM) with ferrimagnetic order (FIMO) at low and high temperature, respectively, in the system. The coexistence of SG and SPM with FIMO shows paramagnetic to ferrimagnetic state at T C ≈ 225 K and ferrimagnetic to cluster spin glass state below T m ≈ 120 K . The non-linear increase, without hysteresis, of M ( H ) data above T C indicates the existence of short range interacting clusters in the paramagnetic state. In addition, the anisotropy effect of Ho 3+ has shown some interesting magnetic features, such as, spin glass of Ising nature and an unusual maximum in the thermoremanent magnetization about 180 K.

Susceptibility of the Paraprocess in Spinel Ferrites with a Frustrated Magnetic Structure

The temperature dependence of the susceptibility of the paraprocess χpara (T) is investigated for samples in the CuGaxAlxFe2−2xO4 (x = 0.2, 0.3, 0.4, 0.5, 0.6, 0.7), CuGaxAl2xFe2−3xO4 (x = 0.1, 0.2, 0.3, 0.4, 0.5), and GaxFe1− xNiCrO4 (x = 0.0, 0.2, 0.4, 0.6, 0.8) systems. It is found that long-range magnetic order arises in spinel ferrites at the temperature Ttran of the transition from a cluster spin-glass state to a frustrated magnetic structure with a maximum in the temperature dependence of the susceptibility of the paraprocess.

Influence of doped Dy on magnetic and electronic properties in La 0.67−xDy xSr 0.33MnO 3

In this paper, the reason for choosing dopant Dy with large magnetic moment ( μ ≈ 10.8 μ B ) in La 0.67− x Dy x Sr 0.33MnO 3 is to investigate how the magnetic structure of the system changes with Dy content increasing. It is found that all samples ( 0 ⩽ x ⩽ 0 . 3 ) undergo the transition from paramagnetism to ferromagnetism at T C. When T<T C, the doped samples enter the cluster-spin glass state, and the antiferromagnetism is exhibited at lower temperature. It is interesting that a novel magnetization peak is observed in the M– T curves for the highly doped samples at about T N. The experimental result of M– H indicates that the coercivity is weak. We suggest that the peak is attributed to the rotation of magnetic domain in the La(Dy) sublattice. The ρ∼T relations at T<T N are dominated by the antiferromagnetism for the doped samples and the ρ∼T ones at T>T C are determined by the phase separation for x=0.2 and by the Dy–O–Dy in La–O plane for x=0.3, respectively. The large magnetoresistance is caused by the sum of the applied and internal magnetic field.

Phase separation and abnormal transport behaviours in La 0.7 x Gd x Sr 0.3 MnO 3 system

The magnetic and transport behaviours of the La0.7−xGdxSr0.3MnO3 (0≤x≤0.70) system are investigated. The experimental results indicate that with increasing Gd doping content, the magnetism of the system changes from the long-range ferromagnetic order state to the cluster-spin glass state, then to the antiferromagnetic (AFM) state. It is interesting that the phase separation appears at x=0.30 and 0.40 and disappears for x≥0.50 where the AFM state occurs. At high doping content, the transport behaviours exhibit abnormality, e.g. there are two temperature ranges in which the ρ–T curves can be well fitted by a variable-range hopping (VRH) model. We suggest that the VRH does not come from the hopping of carriers between clusters, but from the different magnetic backgrounds in the clusters.

Fluctuations and depinning of stripes at acoustic frequencies in La 2− xSr xCuO 4

The low frequency elastic response of La 2− x Sr x CuO 4 displays an absorption process in correspondence with the freezing into the cluster spin glass state, that has been identified with fluctuations of the walls between the antiferromagnetic spin clusters. At these low temperatures (<10 K) the domain walls, which are thought to coincide with the charge stripes, are pinned by the Sr dopants. The process of depinning of the stripes is identified with an additional absorption maximum around 80 K, whose intensity: (1) drops at x>0.045, where the stripe order changes from parallel to diagonal; (2) has a local maximum at x=1/8; (3) vanishes in the overdoped region, x=0.2. The effective energy barrier for depinning has been deduced from the frequency dependence of the absorption, and exhibits the same dependence on doping as the logarithm of the absorption intensity. This is taken as evidence that pinning and stripe-strain interaction are mediated by the octahedral tilts, and larger atomic displacements result both in larger relaxation strength and larger pinning energy.

Cluster spin glass phase and charge stripe fluctuations in the high- Tc superconductor La 2− xSr xCuO 4

A rise of the acoustic absorption in correspondence with the freezing of the Cu 2+ spins into the cluster spin glass state has been identified with the fluctuations of the walls between the antiferromagnetic spin clusters, pinned by the Sr dopants. The depinning of the stripes is identified with an additional absorption maximum around 80 K , whose intensity and effective activation energy (identified with the pinning barrier) exhibit a dependence on doping consistent with pinning and stripe–strain interaction being mediated by the octahedral tilts.

Magnetic phase transitions in the anion-deficient La1−xBaxMnO3−x/2 (0 ≤ x ≤ 0.50)manganites

The crystal structure, magnetization and electrical resistivity properties of the anion-deficientLa1−xBaxMnO3−x/2 (0 ≤ x ≤ 0.50) perovskite manganiteswithout Mn4+ions have been investigated. It is established the reduced samples in the region 0 ≤ x ≤ 0.05 are O′-orthorhombic perovskites,in 0.10 ≤ x ≤ 0.25they are rhombohedric whereas in 0.27 ≤ x ≤ 0.50 they are cubic. Itis found that, as the xdoping level increases, the samples undergo a transition from a weak ferromagnetic state(x = 0)to an inhomogeneous ferromagnetic one, (x ≥ 0.03)being a mixture of antiferromagnetic and ferromagnetic phases. At x ≥ 0.12competition between antiferromagnetic and ferromagneticinteractions leads to a cluster spin glass state appearance with amagnetic moment freezing temperature of ∼ 45 K. The dominant magneticphase for x ≥ 0.22is supposed to be antiferromagnetic. All the reduced samples are semiconductorsand show considerable magnetoresistance over a wide temperature rangein a magnetically ordered state. The largest magnetoresistance (∼ 34%in a 9 kOe field at liquid nitrogen temperatures) is observed for an x = 0.30 sample. Themagnetic phase diagram of La1−x3+Bax2+Mn3+O3−x/22−manganites has been established by combining the results ofmagnetic and electrical measurements. The results obtained couldbe understood in terms of the phase separation and the 180°superexchange Mn3+–O–Mn3+interaction model.