Spin-glass Formation Research Articles

The type of current carriers and the influence of prehistory on the dynamic magnetic characteristics in HoxMn1-xS

In a solid solution of HoxMn1-xS, the influence of the prehistory of a sample cooled without a field and in a magnetic field on the magnetic characteristics is investigated. An increase in the inductance of a coil with a sample cooled in a magnetic field is found. The type of current carriers, the temperature of the change of the sign of the current carriers, the concentration and mobility from the Hall constant are determined. The results are explained in the model of spin and orbital-charge glass formation.

A study of cluster spin-glass behaviour at the critical composition [formula omitted]NiGe

The solid solution Mn1-xFexNiGe with x~0.27 has been investigated via powder ray diffraction, dc magnetization, ac susceptibility, heat capacity, and magnetic relaxation measurements. The alloy Mn0.73Fe0.27NiGe crystallizes in a Ni2In-type hexagonal structure at room temperature. A coexistence of hexagonal and orthorhombic phases with a phase fraction of nearly 88:12 was found below 80 K. The dc susceptibility measurements reveal a paramagnetic to ferromagnetic transition at around Tt≃80 K and a thermal hysteresis observed during field-cooled-cooling and field-cooled-warming suggests the first order nature of the magnetic transition. The splitting between zero-field-cooled and field-cooled dc susceptibilities and the appearance of a frequency dependent peak in ac susceptibility indicate the onset of a spin-glass transition at Tg≃58 K. The relative shift in freezing temperature (δTf) is calculated to be ~0.03 and ~0.09, respectively from the real and imaginary parts of ac susceptibility indicating the formation of cluster spin-glass state. The analysis of frequency dependent Tf using power law yields a characteristic time for a single spin flip τ*≃1.5×10-10 s and critical exponent zν′≃5.5. Similarly, the analysis using the Vogel-Fulcher law results a characteristic time for a single spin flip τ0≃1.2×10-8 s, Vogel-Fulcher temperature T0≃54.8 K, and an activation energy Ea/kB≃72.8 K. The magnitude of τ* and τ0 together with a non-zero value of T0 add further evidence for the formation of cluster spin-glass. The magnetic relaxation and memory effect measurements also demonstrate the low temperature cluster spin-glass behaviour. The reason for the cluster spin-glass behaviour could be the difference in local environment of Mn atoms in the coexisting Mn-rich antiferromagnetic and Fe-rich ferromagnetic phases. Furthermore, Mn0.73Fe0.27NiGe shows the asymmetric response of magnetic relaxation with a change in temperature, belowTf, which can be explained by the hierarchical model. The low temperature heat capacity gives a large electronic coefficient γ≃25 mJ/mol K2 and the Debye temperature θD≃300 K. This value of θD is in close agreement with that calculated from the temperature variation of unit cell volume.

(Fe,Sn)4N alloy as a model spin-glass system with short-range competing interactions on a nonfrustrated simple cubic lattice

The origin of the spin-glass state in (Fe,Sn)4N alloys is studied on the basis of a Heisenberg Hamiltonian with parameters derived from first principles within the magnetic force theorem applied in the framework of the disordered local moments method and local spin-density approximation. We show that in the alloy concentration range where the spin-glass state is stable only one Fe sublattice is intrinsically magnetic and the interatomic exchange magnetic interactions are essentially short ranged due to effects of chemical and magnetic disorder. The magnetic Fe atoms with well-localized spin moments are randomly distributed over the nongeometrically frustrated simple cubic lattice. The magnetic frustration, which generally is believed to be an essential ingredient of the spin-glass state formation condition, may occur only due to the competition of the two nearest-neighbor interactions. We thus argue that (Fe,Sn)4N is a rare example of a spin-glass system where the mechanism of spin-glass state formation might be studied in the framework of the minimal random-site model on a simple cubic lattice with competing interactions, while the effects of the geometrical frustration can be excluded.

Tuning magnetism in 0.25BaTiO3-0.75CoFe2O4 hetero-nanostructure to control ferroelectric polarization

In multiferroic composites, the strain-mediated magnetoelectric coupling is strongly dependent on the characteristics of interface between two (ferroelectric/ferromagnetic) mechanically coupled phases. In this paper, we have studied the magnetic ordering in 0.25BaTiO3-0.75CoFe2O4 (BTO-CFO) nanoparticles and the results are compared with pure CoFe2O4 (CFO) nanoparticles. The cubic spinel structure of CFO and tetragonal BTO is analyzed with X-ray diffraction pattern. The magnetic hysteresis measured at room temperature for both multiferroic BTO-CFO and pure CFO are quite different, that indicate magnetic ordering, which in BTO-CFO composite is changeable. Comparatively higher coercivity in pure CFO than BTO-CFO nanoparticles is observed at room temperature which suggested freezing out of thermal excitations and the onset of stable ferromagnetic ordering. The value of blocking temperature (corresponding to superparamagnetic state), TB is 385 K is observed in pure CFO and 355 K of BTO-CFO. However, the magnetic curve of zero field cooling magnetization is reasonably different than that observed in pure CFO, which is further studied with frequency dependent ac magnetic susceptibility measurement. It results into spin-canting and spin-glass states formation. The spin-glass transition temperature is calculated with both dynamical scaling and Vogel-Fulcher methods. The ferroelectric hysteresis is observed at room temperature is reasonably varied with applied magnetic field of 2, 4, 5, 6 kOe and the variation in the values of polarization and coercive field might be consistent with the observed magnetic hysteresis.

Spin Glass in a Geometrically Frustrated Magnet of ZnFe2O4 Nanoparticles

We investigated the magnetic properties of a geometrically frustrated magnet of ZnFe2O4 nanoparticles by carrying out the comprehensive measurements of dc magnetization and ac susceptibility. Spin glass is confirmed to occur in this weakly Fe(Zn)-ionic-inversed sample. The common characteristics of spin glass including the aging, the memory, and the rejuvenation effects were observed. By using the critical-power law to study the spin dynamics, the spin-glass transition temperature Tg is obtained to be 18 K and the dynamic exponent zν is 8.0. The weak exchange interaction disorder introduced by the ferrimagnetic JAB interaction, together with the weak nonmagnetic dilute disorder induced by the ionic inversion, is responsible for the formation of spin glass in the ZnFe2O4 nanoparticles.

Hysteresis in magnetoresistance and formation of spin glass like structure in PVA capped Fe3O4

Here, we report the temperature dependent hysteresis in the magneto transport in PVA capped Fe3O4 nanoparticles. The synthesis of the nanoparticles of size 10–15 nm were carried out via co-precipitation method followed by heat treatment at different temperatures. Structural studies confirm the formation of polycrystalline Fd $$\bar 3{m}$$ phase of Fe3O4 when the samples were sintered up to 600 °C. FTIR studies show that PVA and magnetic grains were attached through hydrogen bonding between hydroxyl group of PVA and protonated surface of the oxide. Spin glass type transition was observed in the ZFC–FC plots at 125 K which is also the Verwey transition temperature TV as measured from the transport studies. A hysteresis in the MR plot was found which was explained in the realm of tunnel transport through ferromagnetic grain and antiferromagnetic surface of the ferrite particles. Tunnelling through the dielectric spacer layer of PVA has an overall impact to increase the resistance of the samples. The temperature dependence of the hysteresis gives a major hint to the origin of the spin glass type behaviour of the samples.

Orbital Dimer Model for the Spin-Glass State in Y_{2}Mo_{2}O_{7}.

The formation of a spin glass generally requires that magnetic exchange interactions are both frustrated and disordered. Consequently, the origin of spin-glass behavior in Y_{2}Mo_{2}O_{7}-in which magnetic Mo^{4+} ions occupy a frustrated pyrochlore lattice with minimal compositional disorder-has been a longstanding question. Here, we use neutron and x-ray pair-distribution function (PDF) analysis to develop a disorder model that resolves apparent incompatibilities between previously reported PDF, extended x-ray-absorption fine structure spectroscopy, and NMR studies, and provides a new and physical explanation of the exchange disorder responsible for spin-glass formation. We show that Mo^{4+} ions displace according to a local "two-in-two-out" rule on each Mo_{4} tetrahedron, driven by orbital dimerization of Jahn-Teller active Mo^{4+} ions. Long-range orbital order is prevented by the macroscopic degeneracy of dimer coverings permitted by the pyrochlore lattice. Cooperative O^{2-} displacements yield a distribution of Mo-O-Mo angles, which in turn introduces disorder into magnetic interactions. Our study demonstrates experimentally how frustration of atomic displacements can assume the role of compositional disorder in driving a spin-glass transition.

Evidence for Spin Glass Ordering Near the Weak to Strong Localization Transition in Hydrogenated Graphene.

We provide evidence that magnetic moments formed when hydrogen atoms are covalently bound to graphene exhibit spin glass ordering. We observe logarithmic time-dependent relaxations in the remnant magnetoresistance following magnetic field sweeps, as well as strong variances in the remnant magnetoresistance following field-cooled and zero-field-cooled scenarios, which are hallmarks of canonical spin glasses and provide experimental evidence for the hydrogenated graphene spin glass state. Following magnetic field sweeps, and over a relaxation period of several minutes, we measure changes in the resistivity that are more than 3 orders of magnitude larger than what has previously been reported for a two-dimensional spin glass. Magnetotransport measurements at the Dirac point, and as a function of hydrogen concentration, demonstrate that the spin glass state is observable as the zero-field resistivity reaches a value close to the quantum unit h/2e(2), corresponding to the point at which the system undergoes a transition from weak to strong localization. Our work sheds light on the critical magnetic-dopant density required to observe spin glass formation in two-dimensional systems. These findings have implications to the basic understanding of spin glasses as well the fields of two-dimensional magnetic materials and spintronics.

Optical observation of spin-glass formation in Cd1−XMnXTe compound

The behavior of spin glass (SG) is characteristic of different types of materials (semiconductor, metallic, superconductor, amorphous, etc). However, the nature of this magnetic spin state is still unclear. Usually the study of SG systems near the spin freezing temperature (Tf) is based on magnetic measurements. In this case a spin clustering model was used for the explanation of the obtained results. Here, we have studied the optical properties of the semi-insulating CdMnTe SG compound by using the localized exciton magnetic polarons (LEMPs) as nano-size probes for different microscopic magnetic spin states formed in SGs. This revealed spin cooperative behaviors at temperatures above Tf, which can be considered as presagers of spin freezing at Tf. We also observed an optical signature of the SG transition in diluted magnetic semiconductors (DMSs) and its dependence on the LEMP’s excitation intensity. A schematic of the temperature evolution of the spin’s cooperative behavior and formation of the SG phase based on the obtained results is presented. This opens novel opportunities for future investigations of the dynamical spin correlations in DMS SGs as very promising materials for spintronics using optical measurements.

Magnetic and thermodynamic properties of the spin-dimer systemLa2RuO5induced by Mn substitution

Magnetic properties as well as the specific heat of the spin-dimer system ${\mathrm{La}}_{2}{\mathrm{Ru}}_{1\ensuremath{-}y}{\mathrm{Mn}}_{y}{\mathrm{O}}_{5}$ were investigated for manganese concentrations $0\ensuremath{\le}y\ensuremath{\le}0.25$. The magnetic (dc) susceptibility of the unsubstituted ($y=0$) system shows a steplike decrease close to 160 K reflecting a magnetostructural transition into a dimerized ground state. With increasing manganese concentration this behavior (typical for singlet formation) becomes continuously suppressed and the susceptibility bears the signatures of the emergence of new magnetic ground states. The high-temperature Curie-Weiss susceptibility can be described by ${\mathrm{Ru}}^{4+}$ ($S=1$) and ${\mathrm{Mn}}^{4+}$ ($S=\frac{3}{2}$) spin moments, with a dramatic decrease of the Curie-Weiss temperatures by almost 30% close to $y=0.1$, indicating significant changes in the average mean magnetic exchange. Field-cooled and zero-field-cooled experiments as well as ac-susceptibility measurements provide clear evidence for the formation of a spin-glass state, well below the characteristic dimerization temperature. The relaxation dynamics can be described by a Vogel-Fulcher-Tammann behavior and indicates high fragility when characterized in terms of glassy dynamics of canonical supercooled liquids. Additional electron-spin resonance experiments indicate different spin-glass regimes and a rather dynamic nature of the dimerized phase. In the Mn-substituted compounds, a linear contribution to the heat capacity at low temperatures can be ascribed to the spin-glass formation. With increasing manganese concentration, the anomaly in the specific heat caused by the spin-singlet formation is shifted to lower temperatures and becomes continuously suppressed and smeared out. On the basis of these results, we propose a ($y,T$)-phase diagram indicating the competition of the spin-glass and the dimerized states. We stress the similarities with doped $\mathrm{Cu}\mathrm{Ge}{\mathrm{O}}_{3}$, the canonical inorganic spin-Peierls system.

Oxygen miscibility gap and spin glass formation in the pyrochlore Lu2Mo2O7

Abstract Rare earth ( R ) molybdate pyrochlores, R 2 Mo 2 O 7 , are of interest as frustrated magnets. Polycrystalline samples of Lu 2 Mo 2 O 7− x prepared at 1600 °C display a coexistence of cubic pyrochlore phases. Rietveld fits to powder neutron diffraction data and chemical analyses show that the miscibility gap is between a stoichiometric x =0 and an oxygen-deficient x ≈0.4 phase. Lu 2 Mo 2 O 7 behaves as a spin glass material, with a divergence of field cooled and zero field cooled DC magnetic susceptibilities at a spin freezing temperature T f =16 K, that varies with frequency in AC measurements following a Vogel–Fulcher law. Lu 2 Mo 2 O 6.6 is more highly frustrated spin glass and has T f =20 K.

Spin glass transition in the rhombohedral LiNi1/3Mn1/3Co1/3O2

Layered LiNi1/3Mn1/3Co1/3O2 has been synthesized by co-precipitation method, and the magnetic properties were comprehensively studied by dc and ac susceptibilities. The dc magnetization curves show the irreversibility and spin freezing behavior at 109K and 9K. The evolution of real and imaginary part of ac susceptibility under different frequencies indicates a spin glass transition at low temperature. Three models (the Néel–Arrhenius law, the Vogel–Fulcher law, and the power law) have been employed to study the relaxation behavior of the spin glass state. Both frustration and disorder play important role in the formation of spin glass.

ChemInform Abstract: Oxidation by Hydrogen in the Chemistry and Physics of the Rare‐Earth Metals

AbstractReview: 29 refs.

Suppression of Spin-Glass State in PrB6

The comprehensive study of transverse magnetoresistance (MR) and magnetization has been carried out on the high quality single crystals of PrB6 in the wide range of temperatures 2-40K and magnetic fields up to 80kOe. In order to estimate the role of boron vacancies in the formation of the new spin-glass (SG) phase detected by Alekseev et al. below 20K the experiments were carried out on the ordinary (initial state) and annealed single crystals of PrB6. The data obtained demonstrate the appearance of spontaneous magnetization below TSG21.3K with M~1.6 emu/mol for initial state and the absence of spontaneous magnetization for the annealed PrB6 samples. On the contrary, quite similar behavior of MR was detected for various samples of PrB6. Our results suggest the existence of the aggregated boron vacancies which provoke the new SG phase formation in PrB6 at TSG.

Oxidation by Hydrogen in the Chemistry and Physics of the Rare‐Earth Metals

Rare-earth metals (RE) easily react with hydrogen. For decades the bonding of hydrogen has been discussed controversially in terms of either the "proton model" or the "anion model". Detailed investigations of metal-rich compounds of the rare-earth metals provide clear evidence for the incorporation of hydrogen as a hydride anion. Several categories of compounds can be distinguished regarding their behavior towards hydrogen. Low-valence compounds with metal-metal bonding frequently provide their excess electrons to form hydride ions as found with the halide hydrides REXH(n). However, there are exceptions, such as, LaI which does not react with hydrogen as a result of special electronic and electrostatic conditions. The opposite is true with La(2)C(3) although this compound does not provide excess metal valence electrons. An amorphous phase La(2)C(3)H(1.5) forms at very low temperature, around 450 K. The presence of hydrogen strongly influences the electrical and magnetic properties, for example, spin-glass formation and colossal magneto resistance arising in the presence of 4f(n) cores with the lanthanoid elements.

Pyrochlore Range from Bi2O3–Fe2O3–TeO3 System for LTCC and Photocatalysis and the Crystal Structure of New Bi3(Fe0.56Te0.44)3O11

A new pyrochlore homogeneity field in Bi2O3–Fe2O3–TeO3 system is characterized with respect to its compositional range and phase relations to adjacent phases. The sintering temperature of these pyrochlores is 880°C and they do not react with metallic silver, which makes them suitable for low‐temperature cofired ceramics (LTCC) technology. Magnetic measurements indicate a spin‐glass formation. The spin freezing temperature that occurs at around 20 K shows a slight, yet systematic, variation with the sample composition. The dielectric constant increases with an increase in Bi content from 37(1) to 55(1). At cryogenic temperatures, they show the dielectric relaxation typical for displacive pyrochlores. The UV–Vis diffuse reflectance data were used for calculation of the electronic band gap. The lowest band gap was determined to be 1.965 eV. Next to the pyrochlore range, another new phase was discovered. Structural analysis showed that Bi3(Fe0.56Te0.44)3O11 crystallizes as KSbO3‐type in a displacive disordered cubic symmetry with a lattice parameter of 9.47045(15) Å.

Study of spin glass and cluster ferromagnetism in RuSr2Eu1.4Ce0.6Cu2O10-δ magneto superconductor

We report DC magnetization, detailed systematic linear and nonlinear AC magnetic susceptibility and transport for a single phase RuSr2Eu1.4Ce0.6Cu2O10-δ (EuRu-1222) magneto-superconductor. The studied sample is synthesized through standard solid state reaction route, which is crystallized in single phase tetragonal structure with space group I4/mmm. DC magnetic susceptibility measurements revealed that the studied EuRu-1222 is a magneto-superconductor with Ru spins ordering at around 110 K and superconductivity in the Cu-O2 planes below ≈ 30 K. Temperature dependence of AC susceptibility with different frequency and amplitude variations confirms spin-glass behavior with cluster ferromagnetism of the system. Change in the cusp position with frequency follows the Vogel-Fulcher law, which is commonly accepted feature for a spin-glass (SG) system with ferromagnetic clusters. The third harmonic of AC susceptibility (χ3) shows that the system undergoes a spin glass transition below 80 K. Superconducting transition temperature (Tc) onset and ρ = 0 are seen at around 30 and 18 K without any applied field and the same decreases to 10 and 2 K under 130 kOe applied field. Also low fields isothermal (MH) suggests that ferromagnetic clusters are embedded in spin-glass (SG) matrix. The magnetization versus applied field (MH) loops exhibited ferromagnetic (FM) like behavior with rather small coercive fields. Detailed AC magnetic susceptibility measurements are carried out to unearth the short range magnetic correlations. These results support the spin-glass (SG) formation followed by ferromagnetic clustering effects at low temperatures. Our detailed magnetization and magneto transport results will undoubtedly contribute to current understanding of the complex magnetism of the EuRu-1222 system.

The effect of chemical substitution inRh17S15

Rh17S15 has recently been shown to be a strongly correlated superconductor with a transitiontemperature of 5.4 K. In order to understand the nature of the strong correlations we studythe effect of replacement of some of the Rh and S atoms by other elements such as Fe, Pd,Ir and Ni on the Rh side and Se on the S side in this work. We find that while replacementsof Ir and Se lower the transition temperature considerably, those of Fe, Pd and Nidestroy the superconductivity down to 1.5 K. The resistivity data for these dopedsamples show a minimum which is presumably disorder induced. A reduction ofTc is always accompanied by a reduction of electron correlations, as deduced from heatcapacity and magnetization data. Interestingly, the Fe doped sample shows evidence of spinglass formation at low temperatures.

Cluster spin glass in concentrated alloys

Spin glass and structural states of Ni-Mn, Ni3Mn-Ni3V and Ni3Mn-Ni2Cr systems of alloys are investigated. The “freezing” temperature T g of spin glass and Curie temperatures are obtained from the temperature dependence of the differential magnetic susceptibility χac. Structural states of alloys are studied by the neutron diffraction technique. A cluster mechanism of spin glass formation is established.

Effect of self-organized quantum structures on spinglass and metal–insulator transition in CdMnTe single crystals

Vapour phase grown Cd 1− x Mn x Te crystals with x=0.2,0.3,0.4 and 0.5 were found to contain self-organized quantum structures as revealed by SEM micrographs. ac Susceptibility measurements were carried out in the temperature range 14–300 K at 313 Hz. Both the real and imaginary parts of susceptibility showed formation of spinglass phase at low temperatures. Dark and photo current studies in the temperature range 125–450 K indicated metal–semiconductor transitions. Further, photo excitation seemed to enhance the metal–semiconductor phase transition temperature. Both the spinglass formation and metal–semiconductor transition temperatures observed in the present CdMnTe samples were much higher than the earlier reported values and are attributed to the presence of self-organized microstructures.