Slow spin dynamics of cluster spin-glass spinel Zn(Fe1-xRux)2O4: role of Jahn-Teller active spin-1/2 Cu2+ions at B-sites.

We report the slow spin dynamics of cluster spin-glass (SG) spinel Zn(Fe1-xRux)2O4by means of detaileddc-magnetization andac-susceptibility studies combined with the heat capacity analysis. Two specific compositions (x = 0.5, 0.75) have been investigated in detail along with the substitution of Jahn-Teller (JT) active spin-1/2 Cu2+ions at B-sites. Measurements based on the frequency and temperature dependence ofac-susceptibility (χac(f,T)) and the subsequent analysis using the empirical scaling laws such as: (a) Vogel-Fulcher law and (b) Power law reveal the presence of cluster SG state below the characteristic freezing temperatureTSG(17.77 K (x = 0.5) and 14 K (x = 0.75)). Relaxation dynamics of both the compositions follow the non-mean field de Almeida-Thouless (AT)-line approach(TSG(H)=TSG(0)(1-AH2/ϕ)), with an ideal value ofφ = 3. Nevertheless, the analysis of temperature dependent high fielddc-susceptibility,χhf(2kOe ⩽ HDC ⩽ 20kOe,T) provides evidence for Gabay-Toulouse type mixed-phase (coexistence of SG and ferrimagnetic (FiM)) behaviour. Further, in the case of Cu0.2Zn0.8FeRuO4system, slowly fluctuating magnetic clusters persist even above the short-range FiM ordering temperature (TFiM) and their volume fraction vanishes completely across ∼6TFiM. This particular feature of the dynamics has been very well supported by the time decay of the thermoremanent magnetization and heat-capacity studies. We employed the high temperature series expansion technique to determine the symmetric exchange coupling (JS) between the spins which yieldsJS=-3.02×10-5 eV for Cu0.2Zn0.8FeRuO4representing the dominant intra-sublattice ferromagnetic interactions due to the dilute incorporation of the JT active Cu2+ions. However, the antiferromagnetic coupling is predominant in ZnFeRuO4and Cu0.2Zn0.8Fe0.5Ru1.5O4systems. Finally, we deduced the magnetic phase diagram in theHDC-Tplane using the characteristic parameters obtained from the field variations of bothac- anddc-magnetization measurements.

Magnetocaloric performance of the three-component Ho1-xErxNi2 (x = 0.25, 0.5, 0.75) Laves phases as composite refrigerants.

To date, significant efforts have been put into searching for materials with advanced magnetocaloric properties which show promise as refrigerants and permit realization of efficient cooling. The present study, by an example of Ho1-xErxNi2, develops the concept of magnetocaloric efficiency in the rare-earth Laves-phase compounds. Based on the magneto-thermodynamic properties, their potentiality as components of magnetocaloric composites is illustrated. The determined regularities in the behaviour of the heat capacity, magnetic entropy change, and adiabatic temperature change of the system substantiate reaching high magnetocaloric potentials in a desired temperature range. For the Ho1-xErxNi2 solid solutions, we simulate optimal molar ratios and construct the composites used in magnetic refrigerators performing an Ericsson cycle at low temperatures. The tailored magnetocaloric characteristics are designed and efficient procedures for their manufacturing are developed. Our calculations based on the real empirical data are very promising and open avenue to further experimental studies. Systems showing large magnetocaloric effect (MCE) at low temperatures are of importance due to their potential utilization in refrigeration for gas liquefaction.

Antiferromagnetic short-range order and cluster spin-glass state in diluted spinel ZnTiCoO4.

The nature of magnetism in the doubly-diluted spinel ZnTiCoO4= (Zn2+)A[Ti4+Co2+]BO4is reported here employing the temperature and magnetic field (H) dependence of dc susceptibility (χ), ac susceptibilities (χ' andχ″), and heat capacity (Cp) measurements. Whereas antiferromagnetic (AFM) Néel temperatureTN= 13.9 K is determined from the peak in the ∂(χT)/∂TvsTplot, the fit of the relaxation timeτ(determined from the peak in theχ″ vsTdata at different frequencies) to the Power law:τ=τ0[(T-TSG)/TSG]-zνyields the spin glass freezing temperatureTSG= 12.9 K,zν∼ 11.75, andτ0∼ 10-12s. Since the magnitudes ofτ0andzνdepend on the magnitude ofTSG, a procedure is developed to find the optimum value ofTSG= 12.9 K. A similar procedure is used to determine the optimumT0= 10.9 K in the Vogel-Fulcher law:τ=τ0 exp[Ea/kB(T-T0)] yieldingEa/kB= 95 K, andτ0= 1.6 × 10-13s. It is argued that the comparatively large magnitude of the Mydosh parameter Ω = 0.026 andkBT0/Ea= 0.115 (≪1) suggests cluster spin-glass state in ZnTiCoO4below TSG. In theCpvsTdata from 1.9 K to 50 K, only a broad peak near 20 K is observed. This and absence ofλ-type anomaly nearTNorTSGcombined with the reduced value of change in magnetic entropy from 50 K to 1.9 K suggests only short-range AFM ordering in the system, consistent with spin-glass state. The field dependence ofTSGshows slight departure (ϕ∼ 4.0) from the non-mean-field Almeida-Thouless lineTSG(H) =TSG(0) (1 -AH2/ϕ). Strong temperature dependence of magnetic viscositySand coercivityHCwithout exchange bias, both tending to zero on approach toTSGfrom below, further support the spin-glass state which results from magnetic dilution driven by diamagnetic Zn2+and Ti4+ions leading to magnetic frustration. Magnetic phase diagram in theH-Tplane is established using the high-field magnetization dataM(H,T) forTTN, the data ofχvsTare fit to the modified Curie-Weiss law,χ=χ0+C/(T+θ), withχ0= 3.2 × 10-4emu mol-1Oe-1yieldingθ= 4 K andC= 2.70 emu K mol-1Oe-1. This magnitude ofCyields effective magnetic moment = 4.65µBfor Co2+, characteristic of Co2+ions with some contribution from spin-orbit coupling. Molecular field theory with effective spinS= 3/2 of Co2+is used to determine the nearest-neighbor exchange constantJ1/kB= 2.39 K AFM and next-nearest-neighbor exchange constantJ2/kB= -0.66 K (ferromagnetic).

Determination of the tricritical point,H-Tphase diagram and exchange interactions in the antiferromagnet MnTa2O6.

Using the analysis of the temperature and magnetic field dependence of the magnetization (M) measured in the temperature range of 1.5 K to 400 K in magnetic fields up to 250 kOe, the magnetic field-temperature (H-T) phase diagram, tricritical point and exchange constants of the antiferromagnetic MnTa2O6are determined in this work. X-ray diffraction/Rietveld refinement and x-ray photoelectron spectroscopy of the polycrystalline MnTa2O6sample verified its phase purity. Temperature dependence of the magnetic susceptibilityχ(=M/H) yields the Néel temperatureTN= 5.97 K determined from the peak in the computed ∂(χT)/∂TvsTplot, in agreement with theTN= 6.00 K determined from the peak in theCPvsTdata. The experimental data ofCPvsTnearTNis fitted toCP=A|T-TN|-αyielding the critical exponentα= 0.10(0.13) forT>TN(T 25 K fits well with the modified Curie-Weiss law:χ=χ0+C/(T-θ) withχ0= -2.12 × 10-4emu mol-1 Oe-1yieldingθ= -24 K, andC= 4.44 emu K mol-1 Oe-1, the later giving magnetic momentµ= 5.96 µBper Mn2+ion. This yields the effective spinS= 5/2 andg= 2.015 for Mn2+, in agreement withg= 2.0155 measured using electron spin resonance spectroscopy. Using the magnitudes ofθandTNand molecular field theory, the antiferromagnetic exchange constantsJ0/kB= -1.5 ± 0.2 K andJ⊥/kB= -0.85 ± 0.05 K for Mn2+ions along the chainc-axis and perpendicular to thec-axis respectively are determined. TheχvsTdata when compared to the prediction of a Heisenberg linear chain model provides semiquantitative agreement with the observed variation. TheH-Tphase diagram is mapped using theM-Hisotherms andM-Tdata at differentHyielding the tricritical pointTTP(H,T) = (17.0 kOe, 5.69 K) separating the paramagnetic, antiferromagnetic, and spin-flop phases. At 1.5 K, the experimental magnitudes of the exchange fieldHE= 206.4 kOe and spin-flop fieldHSF= 23.5 kOe yield the anisotropy fieldHA= 1.34 kOe. These results for MnTa2O6are compared with those reported recently in the isostructural MnNb2O6.

Magnetic field-temperature phase diagram, exchange constants and specific heat exponents of the antiferromagnet MnNb2O6.

This work presents the magnetic field-temperature (H-T) phase diagram, exchange constants, specific heat (CP) exponents and magnetic ground state of the antiferromagnetic MnNb2O6polycrystals. Temperature dependence of the magnetic susceptibilityχ(=M/H) yields the Néel temperatureTN= 4.33 K determined from the peak in the computed ∂(χT)/∂TvsTplot in agreement with the transition in theCPvsTdata atTN= 4.36 K. The experimental data ofCPvsTnearTNis fitted toCP=A|T-TN|-αyielding the critical exponentα= 0.12 (0.15) forT>TN(T 50 K toχ=χ0+C/(T-θ) withχ0= -1.85 × 10-4emu mol-1Oe-1yieldsθ= -17 K, andC= 4.385 emu K mol-1Oe-1, the latter giving magnetic momentµ= 5.920µBper Mn2+ion. This confirms the effective spinS= 5/2 andg= 2.001 for Mn2+and the dominant exchange interaction being antiferromagnetic in nature. Using the magnitudes ofθandTNand molecular field theory (MFT), the exchange constantsJ0/kB= -1.08 K for Mn2+ions along the chainc-axis andJ⊥/kB= -0.61 K as the interchain coupling perpendicular toc-axis are determined. These exchange constants are consistent with the expectedχvsTvariation for the Heisenberg linear chain. TheH-Tphase diagram, mapped using theM-Hisotherms andM-Tdata at differentHcombined with the reported data of Nielsenet al, yields a triple-pointTTP(H,T) = (18 kOe, 4.06 K). The spin-flopped state aboveTTPand the forced ferromagnetism forH> 192 kOe are used to estimate the anisotropy energyHA≈ 0.8 kOe.

Entropy of Conduction Electrons from Transport Experiments.

The entropy of conduction electrons was evaluated utilizing the thermodynamic definition of the Seebeck coefficient as a tool. This analysis was applied to two different kinds of scientific questions that can-if at all-be only partially addressed by other methods. These are the field-dependence of meta-magnetic phase transitions and the electronic structure in strongly disordered materials, such as alloys. We showed that the electronic entropy change in meta-magnetic transitions is not constant with the applied magnetic field, as is usually assumed. Furthermore, we traced the evolution of the electronic entropy with respect to the chemical composition of an alloy series. Insights about the strength and kind of interactions appearing in the exemplary materials can be identified in the experiments.

ROS-generation and cellular uptake behavior of amino-silica nanoparticles arisen from their uploading by both iron-oxides and hexamolybdenum clusters.

The present work introduces combination of superparamagnetic iron oxides (SPIONs) and hexamolybdenum cluster ([{Mo6I8}I6]2-) units within amino-decorated silica nanoparticles (SNs) as promising design of the hybrid SNs as efficient cellular contrast and therapeutic agents. The heating generated by SNs doped with SPIONs (Fe3O4@SNs) under alternating magnetic field is characterized by high specific absorption rate (SAR = 446 W/g). The cluster units deposition onto both Fe3O4@SNs and "empty" silica nanoparticles (SNs) results in Fe3O4@SNs[{Mo6I8}I6] and SNs[{Mo6I8}I6] with red cluster-centered luminescence and ability to generate reactive oxygen species (ROS) under the irradiation. The monitoring of spin-trapped ROS by ESR spectroscopy technique indicates that the ROS-generation decreases in time for SNs[{Mo6I8}I6] and [{Mo6I8}I6]2- in aqueous solutions, while it remains constant for Fe3O4@SNs[{Mo6I8}I6]. The cytotoxicity is low for both Fe3O4@SNs[{Mo6I8}I6] and SNs[{Mo6I8}I6], while the flow cytometry indicates preferable cellular uptake of the former versus the latter type of the nanoparticles. Moreover, entering into nucleus along with cytoplasm differentiates the intracellular distribution of Fe3O4@SNs[{Mo6I8}I6] from that of SNs[{Mo6I8}I6], which remain in the cell cytoplasm only. The exceptional behavior of Fe3O4@SNs[{Mo6I8}I6] is explained by residual amounts of iron ions at the silica surface.

Fluorescent magnetic nanoparticles for modulating the level of intracellular Ca2+ in motoneurons.

This report introduces both synthesis and in vitro biological behaviour of dual magnetic-fluorescent silica nanoparticles. The amino group-decoration of 78 nm sized silica nanoparticles enables their efficient internalization into motoneurons, which is visualized by the red fluorescence arising from [Ru(dipy)3]2+ complexes encapsulated into a silica matrix. The internalized nanoparticles are predominantly located in the cell cytoplasm as revealed by confocal microscopy imaging. The magnetic function of the nanoparticles resulted from the incorporation of 17 nm sized superparamagnetic iron oxide cores into the silica matrix, enabling their responsivity to magnetic fields. Fluorescence analysis revealed the "on-off" switching of Ca2+ influx under the application and further removal of the permanent magnetic field. This result for the first time highlights the movement of the nanoparticles within the cell cytoplasm in the permanent magnetic field as a promising tool to enhance the neuronal activity of motoneurons.

Impression of magnetic clusters, critical behavior and magnetocaloric effect in Fe3Al alloys.

Herein, we report unusual magnetic behavior in arc melted bulk stoichiometric Fe3Al alloy with a D03 structure. The temperature variation in the magnetization measurements revealed two transitions, i.e. one at ∼763 K and another at ∼830 K. Below 763 K, it exhibits ferromagnetic ordering and the nature of the transition is second order. However, the second transition is more complex and detailed analysis of the magnetic data suggested the coexistence of ferromagnetic and paramagnetic phases (two-phase: α + D03/α + B2) and structural transitions triggered by temperature. We observed dual peaks in the magnetic entropy change curve, in accordance with the magnetization results, which corroborate the occurrence of a two-phase system. We believe that the concurrent magnetic ordering and the complex two-phase are associated with the evolution of short-range ordered magnetic clusters having a magnetic moment of ∼103µB in the host matrix. A cluster hypothesis is proposed to explain the observed intricate magnetic behavior of this alloy at high temperature. The estimated critical exponents using a modified Arrott plot, Kouvel-Fisher plot and critical isotherm analysis lie in between the 3D-Heisenberg and 3D-Ising model, indicating a short-range interaction and magnetic inhomogeneity, which again are consistent with the magnetization results. The obtained critical exponents follow the universal scaling behavior, which indicates the renormalization of interactions around the magnetic ordering transition (TC). Despite the obvious larger thermal entropy at very high temperature, our synthesized Fe3Al alloy showed enhanced magnetic entropy changes. The obtained magnetic entropy change for binary Fe3Al alloy shows twice the value of that of other binary/ternary Fe-based alloys.

Effects of Cu doping on the electronic structure and magnetic properties of MnCo2O4 nanostructures.

Reported here are the results and their analysis from our detailed investigations of the effects of Cu doping ([Formula: see text]) on the electronic structure and magnetic properties of the spinel [Formula: see text]O4. A detailed comparison is given for the [Formula: see text] and [Formula: see text] cases for both the bulk-like samples and nanoparticles. The electronic structure determined from x-ray photoelectron spectroscopy and Rietveld analysis of x-ray diffraction patterns shows the structure to be: ([Formula: see text])A [Formula: see text] [Formula: see text] [Formula: see text]]B [Formula: see text] i.e. [Formula: see text] substitutes for [Formula: see text] on the octahedral B-sites. For the bulk samples, the ferrimagnetic [Formula: see text] K for [Formula: see text] is lowered to [Formula: see text] K for the [Formula: see text] sample, this decrease being due to the effect of Cu doping. For the nanosize [Formula: see text] ([Formula: see text]) sample, the lower [Formula: see text] K ([Formula: see text] K) is observed using [Formula: see text] analysis, this lowering being due to finite size effects. For [Formula: see text], fits of dc paramagnetic susceptibility data of [Formula: see text] versus T in nanosize samples to the Néel expression are used to determine the exchange interactions between the A and B sites with exchange constants: [Formula: see text] K (4.1 K), [Formula: see text] K (16.3 K) and [Formula: see text] K (13.8 K) for [Formula: see text]. The temperature dependence of ac susceptibilities [Formula: see text] and [Formula: see text] at different frequencies shows that in bulk samples of [Formula: see text] and [Formula: see text], the transition at T C is the normal second order transition. But for the nanosize [Formula: see text] and 0.2 samples, analysis of the ac susceptibilities shows that the ferrimagnetic transition at T C is followed by a re-entrant spin-glass transition at lower temperatures [Formula: see text] K (138 K) for [Formula: see text] ([Formula: see text]). Analysis of the ac susceptibilities, [Formula: see text] and [Formula: see text], versus T data is done in terms of two scaling laws: (i) Vogel-Fulcher law [Formula: see text] [Formula: see text]; and (ii) power law of critical slowing-down [Formula: see text]. These fits confirm the existence of glassy behavior below T SG with the parameters [Formula: see text] (8.91), [Formula: see text] (9.6 × 10[Formula: see text]) and [Formula: see text] K (∼138 K) for the samples [Formula: see text] (0.2), with similar results obtained for other samples. The linear behavior of the peak maximum in [Formula: see text] versus [Formula: see text] (AT-line) further supports the existence of glassy states in nanosize samples. For [Formula: see text], the temperature and composition dependence of the hysteresis loop parameters are investigated; all the samples with x ⩾ 0.1 have the coercivity H C and remanence [Formula: see text]. Since the results reported here in these nanostructures are significantly different from those in bulk [Formula: see text] [Formula: see text], further investigations of their magnetic structures using neutron diffraction are warranted.