Cluster Glass Behavior Research Articles

Direct observation of the microstructure in cluster glass compoundU2IrSi3

We have examined the structure of aU2IrSi3 compound exhibiting ferromagnetic cluster glass behaviour by means of electrondiffraction observation and high-resolution electron microscopy. The structure ofU2IrSi3 has been proposedas a new one of the U2RuSi3-type with a short-range ordered double stacking sequence of theU2RuSi3-type structurealong the c-axis, and long-range ordered atomic arrangements in thea–b plane. The calculated patterns reproduce the characteristic features of observed electron patternswell. The Fourier-filtered high-resolution image clearly exhibits a micro-domain structure,which is considered to relate directly to the origin of the observed cluster glass behaviour inU2IrSi3.

Anion substitution effects on structure and magnetism of the chromium chalcogenide Cr 5Te 8—Part II: Cluster-glass and spin-glass behavior in trigonal Cr (1+x)Q2 with basic cells and trigonal Cr (5+x)Q8 with superstructures ( Q=Te, Se; Te:Se=6:2)

The effect of substitution of up to two Te atoms by Se atoms on the crystal structure, the magnetic and electronic properties has been studied in the system Cr (5+ x )Te 8− y Se y . Trigonal basic cells with space group P - 3 m 1 for Cr (1+ x ) Q 2 and trigonal supercells (doubling of the unit cell in all directions) with space group P - 3 m 1 for Cr (5+ x ) Q 8 ((1+ x)=1.27, 1.32, 1.36; (5+ x)=5.08, 5.28, 5.44; Q=Te, Se; Te:Se=6:2) have been identified in X-ray powder diffraction patterns and Rietveld refinements as high-temperature and low-temperature phases, respectively. The crystal structures are related to the hexagonal NiAs-type structure with metal vacancies in every second metal layer. The magnetic properties are closely related to the Cr content and the structure type. Cluster-glass and spin-glass behavior at low temperatures are observed for high and low Cr contents, respectively. For the same Cr content, the phases with trigonal basic cells have higher values for the Curie temperature T c and the freezing temperature T f, and larger magnetization than those for the phases with trigonal supercells. For the same structure type, the values for T c/ T f do not show a linear relationship with the change of Cr content but exhibit a V-shape fashion. Our experimental investigations were accompanied by spin-polarized relativistic Korringa–Kohn–Rostoker (SPR-KKR) electronic structure calculations. Cr deficiencies as well as atomic disorder on the chalcogen sites was accounted for using the coherent potential approximation (CPA) alloy theory. Calculation of the exchange coupling parameters J ij provided the basis for subsequent Monte Carlo simulations of the magnetic properties at finite temperatures.

Magnetic properties of RBzPy[Ni(α-tpdt)2] (R = H, Br, F): effects of cis–trans disorder

Three new salts RBzPy+[Ni(α-tpdt)2]− (α-tpdt = 2,3-thiophenedithiolate, BzPy = benzylpyridine, R = H, Br, F) were prepared and structurally and magnetically characterized. A common structural feature of these compounds is the arrangement of the anions with thiophenic sulfur atoms connecting to a coordinating sulfur atom of a neighbouring anion, placed almost perpendicular to each other. The cations are positioned with the pyridine rings inserted between thiophenic rings of anions, maximizing π–π interactions. Depending on the cation substitution structural differences and variable degrees of cis–trans disorder in the anions are observed, which affect the magnetic properties. HBzPy[Ni(α-tpdt)2] displays dominant ferromagnetic interactions and cluster glass behaviour at low temperatures, BrBzPy[Ni(α-tpdt)2] shows dominant antiferromagnetic interactions with a magnetic anomaly at ∼6 K and FBzPy[Ni(α-tpdt)2] magnetic behavior is dominated by weak ferromagnetic interactions but without magnetic ordering down to 1.5 K.

Spin dynamics in La 1−xSr xCoO 3 cobaltites

Magnetotransport properties, magnetization, AC susceptibilities and electron spin resonance (ESR) were measured to investigate internal spin dynamics of cobaltites La 1− x Sr x CoO 3 ( x = 0.1 , 0.2 , 0.3 ). Semiconducting behavior for x = 0.1 , 0.2 and semiconducting-metallic transition for x = 0.3 at ∼120 K were observed. With increasing Sr doping, the Curie temperature ( T C) increases. The spin-glass and cluster-glass behaviors were confirmed through the frequency-dependent AC susceptibility. ESR spectra of the samples in the temperature range studied could be divided into three regimes corresponding to different linewidths. This feature is due to both the presence of an anisotropic field arising from ferromagnetic clusters and the variation of spin-state transitions of Co ions. Lorentzian ESR signals at temperatures T< T min become Gaussian at T> T min. In a fashion similar to manganites, the peak-to-peak ESR linewidth vs. temperature, Δ H pp( T), for samples has a minimum value at temperature T min = ∼ 1.7 T C . To explain the physical mechanisms occurring in the samples, resistivity, AC susceptibility and Δ H pp( T) data were analyzed in detail.

Doping of Fe to In 2O 3

Model cluster calculations by discrete variational-Xα method for the Fe 3+ ions at 8 b and 24 d sites of the In 2O 3 crystal lattice demonstrated that the density of states of the Fe 3+ 3d electrons located around the center of the band gap, and the spin density of states for the Fe 3+ ions at 8 b and 24 d sites are asymmetric. For Fe-doped In 2O 3 powders, we found a cusp at ≈ 30 K overlapping cluster glass behavior in dc susceptibility, though the cusp showed no frequency dependence in ac susceptibility. The cusp at ≈ 30 K for the Fe-doped In 2O 3 powders can be attributed to superexchange interactions between Fe 3+ ions substituted for In 3+ ions of the In 2O 3 crystal lattice.

Study of the magnetization and transport properties of the La (2+ x)/3 Sr (1− x)/3 Mn 1− xCr xO 3 (0≤ x≤0.2) system

The samples with the Mn 3+/Mn 4+ ratio fixed at 2:1 La (2+ x)/3 Sr (1− x)/3 Mn 1− x Cr x O 3 (0≤ x≤0.20) have been prepared. The magnetic, electrical transport, and magnetoresistance properties have been investigated. Remarkable transport and colossal magnetoresistance (CMR) effect, as well as cluster glass (CG) behaviors have been clearly observed in the samples studied. It was found that the Curie temperature T c and insulator−metal transition temperature T p1 are strongly affected by Cr substitution. The experiment observations are discussed by taking into account the variety of tolerance factors t; the effects of A-site radius 〈 r A〉 and the A-site mismatch effect ( σ 2).

High-temperature cluster glass state and photomagnetism in Zn- and Ti-substituted NiFe2O4 films

We report on the cluster glass nature and photoinduced magnetization (PIM) of (Ni,Zn,Fe,Ti)3O4 thin films prepared using a pulsed-laser deposition (PLD) technique. The films exhibit cluster glass behaviors with a spin-freezing temperature Tf of approximately 230 K. It was found that the magnetization increases following light irradiation below Tf. The experimental results suggest that the electronic transitions induced by photons are intrinsic to the observed PIM rather than the thermal heating effects resulting from light irradiation. The change in the magnetization from the original dark value at 10 K was approximately 40% for the Ni0.7Zn0.3Fe1.7Ti0.3O4 film, and it was approximately 5% for the Ni0.4Zn0.6Fe2O4 film when the excitation intensity is 1.48mW∕mm2. The analysis of the excitation energy dependence of PIM suggests that an intervalence charge transfer Ti4++Fe2+→Ti3++Fe3+ contributes to the observed enhancement in the PIM of the Ti-substituted films.

AC susceptibility and specific heat of UAuAl

We present the AC susceptibility, DC magnetization and specific heat measurements on an orthorhombic polycrystalline UAuAl sample. The results show typical cluster glass behaviors at low temperatures. To characterize the spin freezing state of this system, the static spin freezing temperature T s , critical exponent z ν and activation energy E a are determined from the dynamical analyses of the AC susceptibility data.

Photocontrol of magnetization in Al-substituted Fe 3O 4 thin films

Thin films of (111)-oriented spinel ferrite Al 0.5Fe 2.5O 4 have been prepared by a pulsed-laser deposition (PLD) technique on α-Al 2O 3 (0001) substrates. The films exhibit cluster-glass behaviors with a spin-freezing temperature, T g, near or above room temperature. The magnetization was found to increase following light irradiation below T g, which indicates the photoinduced melting of cluster-glass states. An analysis comparing the dynamic behavior of magnetic response to light irradiation between zero-field-cooled (ZFC) states and field-cooled (FC) states at 10 K under various light intensities, I, revealed that the direct photoexcitation of spins occurs when I≤0.78 mW/mm 2, while the thermal heating effect following the light absorption of the samples also contributes to the enhancement of magnetization when I≥1.22 mW/mm 2. The magnetization of the films could be controlled by light irradiation even at room temperature. This suggests the possibility of utilizing these films in the development of novel magneto-optical memory devices.

Factors determining the effect of Co(ii) in the ordered double perovskite structure: Sr2CoTeO6

Double perovskites are of interest due to their diverse properties that are of potential use in technological applications. In order to determine the effect of Co(II) in the double perovskite structure in the absence of other paramagnetic species, mixed valency or mis-site disorder, we have studied the ordered perovskite Sr2CoTeO6 synthesised by the freeze-drying method. The room temperature monoclinic P21/n symmetry (a = 5.6437(2) A, b = 5.6096(2) A, c = 7.9271(2) A, β = 90.059(2)°) is maintained down to 4 K with structural transitions to I2/m then Fmm at 373 K (100 °C) and 773 K (500 °C). Below TN = 18 K, the Type I antiferromagnetic structure is observed with magnetic moments of magnitude 2.25(3) μB rotated 58° out of the ab plane. AC magnetic susceptibility and specific heat data show maxima at 19 K, associated with long-range antiferromagnetic order. EPR and diffuse reflectance spectroscopies confirm that Co is in the +2 oxidation state, in highly regular octahedral coordination and is highly ionic. A small magnetic irreversibility, with Hc and Mr of 36 Oe and 0.5 emu mol−1, is observed at low temperature but this is not due to conventional spin glass or cluster glass behaviour. Calculations from specific heat give a magnetic entropy of 4.2 J mol−1 K−1, close to the theoretical value for the S′ = ½ state of the Co2+ cation at low temperature. Distortions of the structure, demonstrated to be primarily rotations of highly regular octahedra, change the geometry of the magnetic exchange pathways but are insufficient to explain the variation in ordering temperatures and magnetic structure types observed, with orbital energies within the exchange pathways having a significant influence on the properties of these and similar technologically important materials.

Magnetic frustration effects in Fe-doped cobalt oxides

A thorough study of electron paramagnetic resonance (EPR) spectra of La 0.5Sr 0.5Co 1− x Fe x O 3 ( 0 ⩽ x ⩽ 0.6 ) compounds has been made. It is shown that, in the paramagnetic region, the EPR signal decreased considerably with increasing Fe-doped content. Cluster-glass (CG) behavior for 0 ⩽ x ⩽ 0.4 compositions was realized near the phase transition temperature T C, but almost disappeared with further iron addition. This feature is a direct consequence of the magnetic frustration of random competing interactions, namely the ferromagnetic Co 3+–Co 4+ double-exchange interactions and the antiferromagnetic interactions like Co–O–Fe and Fe–O–Fe. For x > 0.4 compositions, spin-glass (SG) behavior was observed and, this phase emerged and competed strongly with the CG phase. The results obtained from the EPR spectra are consistent with those obtained from the magnetization and resistivity measurements.

Anion substitution effects on structure and magnetism in the chromium chalcogenide Cr5Te8—Part I: Cluster glass behavior in trigonal Cr(1+x)Q2 with basic cell (Q=Te, Se; Te:Se=7:1)

The effect of substitution of the anion Te by Se in non-stoichiometric Cr5Te8 has been investigated with respect to its crystal structure, magnetic properties, and electronic structure. The compounds Cr(1+x)Q2 (Q=Te, Se; Te:Se=7:1; (1+x)=1.234(6), 1.264(6), 1.300(7)) were synthesized at elevated temperatures followed by quenching the samples to room temperature. The crystal structures have been refined with X-ray powder diffraction data with the Rietveld method in the trigonal space group P3̄m1 with lattice parameters a=3.8651(1)–3.8831(1)Å and c=5.9917(2)–6.0528(2)Å. The structure is related to the NiAs structure with full and deficient metal layers stacking alternatively along the c-axis. The irreversibility in the field-cooled/zero-field-cooled magnetization suggests that the substitution effects of one Te by one Se is strong enough to cause cluster-glass behavior, from ferromagnetic Cr5Te8 to cluster-glass Cr(1+x)Q2. Non-saturation magnetizations at 5.5T and the magnetic relaxation results further support the existence of cluster-glass behavior. Accompanying SPR-KKR (spin-polarized relativistic Korringa–Kohn–Rostoker) band structure calculations strongly support the observation that the Cr(1) sites are preferentially occupied by Cr atoms and predict that these compounds are metallic. Results for the spin-resolved DOS and magnetic moments on each crystallographic sites are presented.

Influence of the Mn intercalation on magnetic properties of TiSe2

Using the measurements of the magnetic susceptibility and magnetization we have investigated the magnetic properties of Mn-intercalated MnxTiSe2 compounds having a hexagonal crystal structure of a quasi-two-dimensional type. Unlike other 3d-transition metals, intercalation of Mn ions between Se–Ti–Se tri-layers increases the c-parameter of the lattice. All compounds of MnxTiSe2 exhibit a paramagnetic behavior above ∼15K. The effective magnetic moment μeff of Mn in MnxTiSe2 is found to be less than μeff for the free Mn2+ ion. In a low-temperature region a spin-glass or cluster-glass behavior depending on the intercalant concentration is observed. The paramagnetic Curie temperature θp takes negative values and decreases monotonically with increasing x, while the freezing temperature shows non-monotonous change at the intercalation. The obtained results are discussed taking into account Mn 3d–Ti 3d hybridization effects and variations of intralayer and interlayer exchange interactions with intercalation.

Study of spin dynamics, cluster glass and spin glass behaviors in La0.5Sr0.5Co1−xFexO3 (x=0, 0.1)

We present here the results of a detailed investigation on magnetic and transport properties of La0.5Sr0.5Co1−xFexO3 (x=0, 0.1) compounds. It is shown that the Curie temperature (TC) decreases slightly and the resistivity increases, even with a very small extent of Fe substitution. There was an evident coexistence of cluster glass (CG) and spin glass (SG) behaviors for x=0 composition. The Fe substitution induced additional antiferromagnetic Fe–O–Fe interactions that suppress strongly the ferromagnetism and conductivity, thus causing a phase separation for x=0.1 composition. We also figure out the nature of CG and SG behaviors and the influence of Fe doping on magnetic and transport properties of such a cobaltite system relying on electron paramagnetic resonance spectra.

Cluster glass behavior of La0.9Ca0.1CoO3 single crystals

The magnetic field and temperature dependence of magnetic susceptibility of La 0.9 Ca 0.1 CoO 3 single crystal has been measured using a SQUID magnetometer . The irreversibility line characteristic of spin-glass-like systems was determined on the H – T plane. It was shown that the magnetic moments in this lightly doped crystal freeze into a cluster glass state. Magnetic properties of the La 0.9 Ca 0.1 CoO 3 single crystals have been described in terms of a spin-state transition of the Co 3+ ions with increasing temperature from a LS ground state to an IS excited state with energy gap Δ =268 K.

On the structure and magnetic properties of CeNi with minor Cu substitutions

The introduction of Cu in the CeNi compound maintains the same crystallographic CrB-type structure up to CeNi 0.85Cu 0.15. The crystallographic structure of several compositions in the neighbourhood of this limit has been determined from single crystals in a 4-circle diffractometer. The thermal expansion coefficients of CeNi 0.9Cu 0.1 behave similar to those of CeNi. The Cu substitution disturbs the intermediate valence behaviour of CeNi, inducing non-zero magnetic moments at low temperatures and suggesting a cluster-glass behaviour.

Photoinduced magnetization in a thin Fe–CN–Co film

The magnetization of a thin Fe-Co cyanide film has been investigated from 5 K to 300 K and in fields up to 500 G. Upon illumination with visible light, the magnetization of the film rapidly increases. The original cluster glass behavior is further developed in the photoinduced state and shows substantial changes in critical temperature and freezing temperature.

Magnetic and transport measurements in Au(1−)Mn alloys in the cluster glass regime

Abstract A detailed study of magnetization and magnetoresistance has been carried out on a series of Au 1− x Mn x alloys (0.10⩽ x ⩽0.18) in the temperature range 4.2–300 K and in magnetic fields up to 9 T. The dc magnetization measurements show an irreversibility between the zero-field-cooled and field-cooled data while a fairly large negative magnetoresistance is observed in the alloys. These results are indicative of a cluster glass behaviour as has been observed in the AuFe alloys.

Cluster glass behavior of La0.9Ca0.1CoO3 single crystals

Abstract The magnetic field and temperature dependence of magnetic susceptibility of La 0.9 Ca 0.1 CoO 3 single crystal has been measured using a SQUID magnetometer. The irreversibility line characteristic of spin-glass-like systems was determined on the H – T plane. It was shown that the magnetic moments in this lightly doped crystal freeze into a cluster glass state. Magnetic properties of the La 0.9 Ca 0.1 CoO 3 single crystals have been described in terms of a spin-state transition of the Co 3+ ions with increasing temperature from a LS ground state to an IS excited state with energy gap Δ =268 K.

Anomalous cluster glass behavior in a quasi-one-dimensional organic-based magnet

The spin glass transition of the quasi-one-dimensional (1D) organic-based magnet [MnTPP]+[TCNE]− 2(1,3-C6H4Cl2) (TPP is tetraphenylporphyrin dianion, TCNE is tetracyanoethylene) is explored using both a.c. and d.c. measurements. The a.c. susceptibility shows a strong frequency dependent peak consistent with a clustered spin glass state. Application of a d.c. magnetic field suppresses the peak in the a.c. susceptibility. Magnetic relaxation data can be fit to a stretched exponential, with the results reflecting a spin glass transition near 4 K. The spin cluster dimension and the effective dimension of the system are calculated within a fractal cluster model of spin glasses with values for the spin cluster dimension ranging from ∼0.8 to over 1.5 as the glass transition is approached.