Spin-glass Temperature Research Articles

Spin Glass Behavior in La0.7Ca0.23Sr0.07MnO3 Nanofibers Obtained by Electrospinning

This work contributes to the development of new nanostructured mixed valence manganites and to explore and optimize their magnetic properties at 1-D level. Nanofibers of La0.7Ca0.23Sr0.07MnO3 manganite were fabricated using the electrospinning method and three different heat treatments to determine how the nanostructure affects its thermomagnetic behavior. From scanning electron microscopy, nanofibers morphology was observed and average diameters of 75, 94, and 97 nm were identified after heat treatments of 973, 1073, and 1173 K, respectively. According to X-ray diffraction technique, a single-phase orthorhombic structure was defined for each sample. Average crystallite sizes were determined as 47, 49, and 58 nm. A ferromagnetic to paramagnetic transition with Curie temperatures of 297, 305, and 314 K were identified, respectively. Furthermore, a glassy state was induced by nanofibers agglomeration. The spin glass and irreversible temperatures diminished as the magnetic field was increased and a highly anisotropic state was evidenced for all samples. Thermomagnetic behavior in manganites showed to be significantly influenced by the one-dimensional structure and exposed how the dimensionality proportionated by the fabrication method can be used to adjust magnetic properties.

Effect of the paramagnetic to spin-glass phase transition on the fundamental absorption edge of MnIn2Se4 magnetic semiconducting compound

The temperature dependence of the indirect and direct fundamental band gaps of the layered compound MnIn2Se4,that crystallizes in a rhombohedral defect structure with space group R -3m (D5/3d), was studied by optical absorption spectra. The data were analyzedin terms of current theoretical models that take into account the contribution of antiferromagnetic exchange interaction between spins to the shift of the energy gap EG with T in the vicinity of the critical paramagnetic to spin-glass phase transition. It was established that short- and long-range effect spin correlations dominate the contribution to this shift in the critical region below about 20 K, near the spin-glass freezing temperature Tf, and noncritical one, between about 70 and 160 K, far from Tf, respectively. An intermediate temperature region, compatible with the behaviour expected for a cluster-glass transition where a gradual freezing of the magnetic moments occurs, was also observed.

Observation of multiple magnetic phases and complex nanostructures in Co implanted amorphous carbon films

Room temperature implantation of 30 keV Co ions into an amorphous carbon film with a high fluence of 1.2 × 1017 Co/cm2 results in formation of magnetic nanostructures displaying multiple magnetic phases. Cross-sectional TEM images show formation of Co containing nanoparticles at the surface and near-surface regions of the implanted films. EDXS measurements suggest the nanoparticles to be composed primarily of Co and O at the surface and Co and C in deeper regions. These nanoparticles with differing compositions were observed to be segregated by a thin layer devoid of Co. Magnetic measurements reveal the presence of superparamagnetic behavior from small CoxC nanoclusters with a blocking temperature of 5 K. There is a small fraction of larger CoxC nanoclusters that show magnetic hysteresis even at room temperature. The saturation magnetic moment is as high as 0.51 μB/Co at 2 K and 0.32 μB/Co at room temperature. Spin-disorder is seen with a range of spin glass temperatures below ∼70 K. Our high fluence Co implantation into amorphous carbon has resulted in the formation of complex magnetic nanostructures composed of cobalt, oxygen, and carbon. These nanostructures give rise to multiple magnetic phases such as superparamagnetism, spin glass, ferromagnetism, and possibly antiferromagnetism.

Lower lower-critical spin-glass dimension from quenched mixed-spatial-dimensional spin glasses

By quenched-randomly mixing local units of different spatial dimensionalities, we have studied Ising spin-glass systems on hierarchical lattices continuously in dimensionalities 1 =< d =< 3. The global phase diagram in temperature, antiferromagnetic bond concentration, and spatial dimensionality is calculated. We find that, as dimension is lowered, the spin-glass phase disappears to zero temperature at the lower-critical dimension d_c=2.431. Our system being a physically realizable system, this sets an upper limit to the lower-critical dimension in general for the Ising spin-glass phase. As dimension is lowered towards d_c, the spin-glass critical temperature continuously goes to zero, but the spin-glass chaos fully sustains to the brink of the disappearance of the spin-glass phase. The Lyapunov exponent, measuring the strength of chaos, is thus largely unaffected by the approach to d_c and shows a discontinuity to zero at d_c.

Dynamic variational study of chaos: spin glasses in three dimensions

We have introduced a variational method to improve the computation of integrated correlation times in parallel tempering dynamics, obtaining a better estimate (a lower bound, at least) of the exponential correlation time. Using this determination of the correlation times, we revisited the problem of the characterization of the chaos in temperature in finite dimensional spin glasses, by way of the study of correlations between various chaos indicators computed in the static and the correlation times of the parallel tempering dynamics. The sample-distribution of the characteristic time for the parallel tempering dynamics turns out to be fat-tailed, and to obey finite-size scaling.

Ferromagnetic order in dipolar systems with anisotropy: application to magnetic nanoparticle supracrystals

Single domain magnetic nanoparticles (MNP) interacting through dipolar interactions (DDI) in addition to the magnetocrystalline energy may present a low temperature ferromagnetic (SFM) or spin glass (SSG) phase according to the underlying structure and the degree of order of the assembly. We study, from Monte Carlo simulations in the framework of the effective one-spin or macrospin models, the case of a monodisperse assembly of single domain MNP fixed on the sites of a perfect lattice with fcc symmetry and randomly distributed easy axes. We limit ourselves to the case of a low anisotropy, namely the onset of the disappearance of the dipolar long-range ferromagnetic (FM) phase obtained in the absence of anisotropy due to the disorder introduced by the latter.

Reentrant phases in electron-dopedEuFe2As2: Spin glass and superconductivity

We report evidence for a reentrant spin glass phase in electron-doped $\text{EuFe}_2\text{As}_2$ single crystals and first traces of the superconductivity re-entrance in optics. In the close-to-optimal doped $\text{Eu}(\text{Fe}_{0.91}\text{Ir}_{0.09})_2\text{As}_2$ and $\text{Eu}(\text{Fe}_{0.93}\text{Rh}_{0.07})_2\text{As}_2$ samples two magnetic transitions are observed below the superconducting critical temperature $T_c \approx 21$~K: the canted $A$-type antiferromagnetic order of the $\text{Eu}^{2+}$ ions sets in around $17\,\text{K}$; the spin glass behavior occurs another $2\,\text{K}$ lower in temperature. In addition, strong evidence for an additional transition is found far below the spin glass temperature. Our extensive optical and magnetic investigations provide new insight into the interplay of local magnetism and superconductivity in these systems and elucidate the effect of the spin-glass phase on the reentrant superconducting state.

Probing the spin-glass and magnetoresistance in FeSr&lt;SUB align="right"&gt;2Y&lt;SUB align="right"&gt;1.7Ce&lt;SUB align="right"&gt;0.3Cu&lt;SUB align="right"&gt;2O&lt;SUB align="right"&gt;10-δ

We have measured the electronic transport and magnetic properties of the rare-earth doped iron-cuprate FeSr2Y2-xCexCu2O10−δ with x = 0.3 and compared the results with previous measurements on a less hole doped sample with x = 0.7. We find that there is a spin-glass and the spin-glass temperature (~23 K) has not changed with hole doping from x = 0.7 to x = 0.3. The conduction mechanism is not affected by hole doping and it is still variable range hopping (VRH) for x = 0.3, which indicates that there is still considerable disorder from Cu and Fe antisite disorder as well as oxygen site disorder in the FeO2-x planes. A large negative magnetoresistance of up to −10% is observed at 8 T but it is less than half of that reported for x = 0.7. The negative magnetoresistance can be accounted using the Nguen, Spivak, and Shklovskii (NSS) model where there is VRH quantum interference. The positive magnetoresistance at 2 K reported for x = 0.7 is not seen in x = 0.3 although the magnetoresistance becomes less negative below 5 K. A positive low temperature magnetoresistance can occur owing to scattering from free localised spins that is not accounted for in the NSS model and the difference in the low temperature magnetoresistance may be owing to a reduced concentration of localised free spins for x = 0.3 when compared with x = 0.7.

No percolation in low temperature spin glass

We consider the Edwards-Anderson Ising Spin Glass model for temperatures $T\geq 0.$ We define notions of Boltzmann-Gibbs measure for the Edwards-Anderson spin glass at a given temperature, and of unsatisfied (frustrated) edges, and recall the notion of ground states. We prove that for low positive temperatures, in almost every spin configuration the graph formed by the unsatisfied edges is made of finite connected components. Similarly, for zero temperature, we show that in almost every ground state the graph of unsatisfied edges is a forest all of whose components are finite. In other words, for low enough temperatures the unsatisfied edges do not percolate.

Role of Fe magnetic subsystems to form a magnetic spin glass state in RFeTi2O7

The experimental studies on R3+Fe3+Ti2O7 (R=Sm, Gd, Tb, Tm, Dy) magnetic properties evidence the low temperature spin glass state in all compounds. The possibility of rare-earth cation substitution allows the investigation of the role of magnetic iron Fe3+ ions and rare earth R3+ ions subsystems in a ground state formation in these oxide compounds.

Blocking temperature of interacting magnetic nanoparticles with uniaxial and cubic anisotropies from Monte Carlo simulations

The low temperature behavior of densely packed interacting spherical single domain nanoparticles (MNP) is investigated by Monte Carlo simulations in the framework of an effective one spin model. The particles are distributed through a hard sphere like distribution with periodic boundary conditions and interact through the dipole dipole interaction (DDI) with an anisotropy energy including both cubic and uniaxial symmetry components. The cubic component is shown to play a sizable role on the value of the blocking temperature Tb only when the MNP easy axes are parallel to the cubic easy direction ([111] direction for a negative cubic anisotropy constant). The nature of the collective low temperature state, either ferromagnetic or spin glass like, is found to depend on the ratio of the anisotropy to the dipolar energies characterizing partly the disorder in the system.

Displacive disorder and spin frustration hosted multiferroic orders in pyrochlore–spinel composites

Low temperature spin glass like behavior and high temperature dielectric relaxation have been observed in BNMO pyrochlore–spinel composites.

Strong Suppression of the Spin Hall Effect in the Spin Glass State.

We have measured spin Hall effects in spin glass metals, CuMnBi alloys, with the spin absorption method in the lateral spin valve structure. Far above the spin glass temperature T(g) where the magnetic moments of Mn impurities are randomly frozen, the spin Hall angle of a CuMnBi ternary alloy is as large as that of a CuBi binary alloy. Surprisingly, however, it starts to decrease at about 4T(g) and becomes as little as 7 times smaller at 0.5T(g). A similar tendency was also observed in anomalous Hall effects in the ternary alloys. We propose an explanation in terms of a simple model considering the relative dynamics between the localized moment and the conduction electron spin.

Quantum Optimization of Fully Connected Spin Glasses

The Sherrington-Kirkpatrick model with random $\pm1$ couplings is programmed on the D-Wave Two annealer featuring 509 qubits interacting on a Chimera-type graph. The performance of the optimizer compares and correlates to simulated annealing. When considering the effect of the static noise, which degrades the performance of the annealer, one can estimate an improvement on the comparative scaling of the two methods in favor of the D-Wave machine. The optimal choice of parameters of the embedding on the Chimera graph is shown to be associated to the emergence of the spin-glass critical temperature of the embedded problem.

Spin-glass behavior in single crystals of hetero-metallic magnetic warwickites MgFeBO4, Mg0.5Co0.5FeBO4, and CoFeBO4

Magnetic properties of heterometallic warwickites MgFeBO4, Mg0.5Co0.5FeBO4, and CoFeBO4 are presented, highlighting the effect of Co substitution on the magnetic properties of these compounds. The analysis of magnetization and heat capacity data has shown that these compounds exhibit a spin-glass transition below TSG=10, 20 and 22K, respectively. Using zero field ac susceptibility as entanglement witness we find that the low dimensional magnetic behavior above TSG show quantum entanglement behavior χ(Τ)∝T−α(Τ) up to TE≈130K. The α parameters have been deduced as a function of temperature and Co content, indicating the existence of random singlet phase in this temperature region. Above TE the paramagnetism is interpreted in terms of non-entangled spins giving rise to Curie–Weiss paramagnetism. The different intra- and inter-ribbon exchange interaction pathways have been calculated within a simple indirect coupling model. It is determined that the triangular motifs in the warwickite structure, together with the competing interactions, induce frustration. The spin-glass character is explained in terms of the substitutional disorder of the Mg, Fe and Co atoms at the two available crystallographic sites, and the frustration induced by the competing interactions. The Co substitution induces uniaxial anisotropy, increases the absolute magnetization and increases the spin-glass freezing temperature. The entanglement behavior is supported in the intermediate phase irrespective of the introduction of anisotropy by the Co substitution.

Low Temperature Spin-glass Behavior in Nonmagnetic Atom Disorder Compound Pr2CuIn3

We present the experimental results of ac and dc susceptibility, magnetization, magnetic relaxation, specific heat and electrical resistivity for hexagonal CaIn2-type polycrystalline Pr2CuIn3. Spin-glass state is confirmed to form in this system with a spin freezing temperature Tf∼5.4K. The frequency dependent cusp in ac susceptibility, the evident irreversible magnetism and long-time magnetic relaxation behavior, and the absence of visible anomaly in temperature dependences of specific heat and electrical resistivity are the typical features characteristic of the spin-glass behaviors. A dynamical analysis of the ac susceptibility data gives further evidence for the spin-glass state in Pr2CuIn3. Formation of spin-glass state in Pr2CuIn3 seems to originate from the continued site randomness of the non-magnetic elements, which introduce the random distribution of exchange interactions between Pr atoms.

Superspin glass behavior of self-interacting CoFe2O4 nanoparticles

Low-temperature magnetic properties of CoFe2O4 nanoparticles (3–16nm) have been investigated by AC and DC magnetic measurements. The saturation magnetization (MS) of ultra-small CoFe2O4 nanoparticles (3–9nm) sharply increases at low temperature (10K) compared to room temperature (RT) MS value. For example, the increment in MS value for 3nm CoFe2O4 nanoparticle is 22emu/g and is null for 12nm and larger sized nanoparticles. A similar trend of increment in MS is also seen in ultra-small size Fe3O4 and MnFe2O4 nanoparticles. However, the MS enhancement in ultra-small CoFe2O4 nanoparticles is found much higher as compared to Fe3O4 and MnFe2O4 nanoparticles. The ultra-small sized nanoparticles arrange with a high packing density to induce a strong exchange as well as dipolar interactions, which renders the enhanced low temperature MS with superspin glass (SSG) state. The exchange coupling strongly depends on magnetic anisotropy energy, which increases in the order Mn2+<Fe2+<Co2+ and thus the ultra-small CoFe2O4 nanoparticles show a large enhancement of MS at low temperature due to strong exchange coupling. A noticeable enhancement of spin glass temperature (Tg) for ultra-small sized CoFe2O4 nanoparticles also confirms the presence of strong exchange coupling in this case. Fitting of the ac susceptibility χ′(T, f) data to a power-law scaling and Vogel–Fulcher model shows a satisfactory fit and the dynamic critical exponent takes value between 8.9 and 11.9 which are in a range typical for the spin-glass systems. Memory behavior in ultra-small CoFe2O4 nanoparticles suggest that the frequency dependent blocking process of ultra-small sized nanoparticles can be better described by power law model, while the interaction regime present in the 12nm and above sized nanoparticles is ascribed to a Vogel–Fulcher model.

Exchange bias effect in Bi2Fe3AlO9 ceramics

The exchange bias (EB) effect is observed in Bi2Fe3AlO9 polycrystalline ceramics. The EB field (HEB), vertical magnetization shift, and coercive field show a strong dependence on the cooling fields. When a larger applied field is used to measure the hysteresis loop, the EB effect is suppressed. The induced spin-glass-like phase via Al3+ doping is responsible for the EB effect below the spin-glass temperature (TSG) of ∼25 K. Moreover, the HEB varies nonmonotonically with temperature above TSG, which is interpreted using a random field model with the exchange coupling between ferromagnetic clusters and an antiferromagnetic phase.

Magnetism in spin models for depleted honeycomb-lattice iridates: Spin-glass order towards percolation

Iridates are characterized by a fascinating interplay of spin-orbit and electron-electron interactions. The honeycomb-lattice materials ${A}_{2}{\mathrm{IrO}}_{3}$ $(A=\text{Na},\text{Li})$ have been proposed to realize pseudospin-1/2 Mott insulating states with strongly anisotropic exchange interactions, described by the Heisenberg-Kitaev model, but other scenarios involving longer-range exchange interactions or more delocalized electrons have been put forward as well. Here we study the influence of nonmagnetic doping, i.e., depleted moments, on the magnetic properties of experimentally relevant variants of the Heisenberg-Kitaev and Heisenberg ${J}_{1}\text{\ensuremath{-}}{J}_{2}\text{\ensuremath{-}}{J}_{3}$ models. We generically find that the zigzag order of the clean system is replaced, upon doping, by a spin-glass state with short-ranged zigzag correlations. We determine the spin-glass temperature as a function of the doping level and show that this quantity allows one to assess the importance of longer-range exchange interactions when the doping is driven across the site-percolation threshold of the honeycomb lattice.

Freezing of the octahedral tilt near ferromagnetic transition and appearance of a glassy phase at low temperature driven by the tilt instabilities in SrRuO3

The emergence of low temperature glassy phase in widely known itinerant ferromagnet SrRuO3 is remotely understood. In order to understand this aspect, we have undertaken a detailed temperature dependent (5–250 K) neutron diffraction study. We observe a freezing of the octahedral tilt near the ferromagnetic transition and an unusual deviation in the octahedral tilt near the onset of low temperature spin glass like phase. A reduction of the ordered magnetic moment and a decline in the total integrated magnetic intensity is observed around the temperature where the glassy behaviour starts to appear. The magnetotransport study also reveals the possibility for an additional magnetic ordering by demonstrating a peak in magnetoresistance at the low temperature side as well. The neutron diffraction study presented here provides useful information to understand the observed unusual low temperature magnetic phenomena in SrRuO3.