Ac Susceptibility Studies Research Articles

New structural types of Mn16 single-molecule magnets: W-shaped topology from reductive aggregation.

Two new Mn16 clusters are reported: [Mn16O10(OH)3(OMe)8(O2CPhBut)17(MeOH)5] (2) and [Mn16O16(OMe)6(O2CPh)12(NO3)4(MeOH)2(H2O)4] (3). The complexes were obtained by reductive aggregation of MnO4– in CH2Cl2/MeOH, and oxidation of MnII and preformed (NnBu4)[Mn4IIIO2(O2CPh)9(H2O)] with CeIV, respectively. The core of 2 has a Mn16III core with an unusual 1:2:3:4:3:2:1 layer structure and a W-shaped pleated topology, whereas 3 contains a central 2 × 3 Mn6IV planar grid held within a nonplanar Mn10III loop and is a rare example of a complex with nitrate ions bridging like carboxylate ions. Variable-temperature, solid-state dc susceptibility, and ac susceptibility studies reveal that 2 and 3 possess S = 12 and S = 8 ground states, respectively. Fits of dc magnetization data collected over a temperature range of 1.8–4.0 K and a magnetization range of 0.1–4 T were fit to give S = 12, D = −0.16(2) cm–1, g = 1.98(3) for 2 and S = 8, D = −0.22(1) cm–1, g = 1.99(2) for 3, where D is the axial zero-field splitting parameter. The ac in-phase (χM′T) susceptibility below 15 K confirmed the ground-state spin values of 2 and 3, as determined from dc data, and the appearance of frequency-dependent out-of-phase (χM″) signals revealed that both complexes are new single-molecule magnets (SMMs). Fits of the ac data gave Ueff = 49.7(1) K and τ0 = 4.32 × 10–9 s for 2 and Ueff ≈ 14.0 ± 2 cm–1 and τ0 ≈ 3.2 ± 0.5 × 10–8 s for 3, where Ueff is the effective barrier to magnetization relaxation and τ0 is the pre-exponential factor. Thus, complexes 2 and 3 are two new members of a growing family of Mn16 clusters, and two new examples of high-nuclearity SMMs, with the Ueff for 2 approaching the value for the prototypical SMM family, [Mn12O12(O2CR)16(H2O)4].

Manifold relaxation processes in a mononuclear Co(II) single-molecule magnet

Mononuclear Co(II) complex [Co(Me6tren)Cl]ClO4 has been investigated in terms of the AC susceptibility studies at various frequencies, temperatures, and external magnetic fields. In addition to the slow magnetic relaxation present even in an absence of the external field, the application of the BDC=0.2, 0.4, and 0.8T induces a second, slower relaxation path. The effective barrier to spin reversal is little sensitive to the magnetic field and stays approximately U/kB∼20K. The extrapolated relaxation time for the faster process varies between 3×10−9 and 6×10−8s. At T=2.0K the detected relaxation time is (1–3)×10−4s for the faster branch and 0.06–0.3s for the slower branch, respectively.

Effect of sintering temperature on the nature of weak links and flux pinning mechanism in La1.85Sr0.15CuO4 superconductor

AC and DC susceptibility study is carried out to investigate the granular nature of La1.85Sr0.15CuO4 superconductor. Presence of weak links and flux pinning phenomena are two important implications of granular type II high temperature superconductors. Weak links manifested as Superconductor-Insulator-Superconductor (SIS) / Superconductor-Normal metal-superconductor (SNS) Josephson Junctions are investigated from the temperature and field dependence study of AC susceptibility. On the other hand, DC susceptibility measurement is used to study the flux pinning mechanism. Mechanism of flux pinning is dependent on the nature and size of the pinning centres as well as on the microstructure wavelength. Thus, the nature of grain boundaries plays an important role in determining the nature of pinning mechanism of flux lines. In the present work, effect of sintering temperature on the nature of weak links and flux pinning mechanism in the bulk polycrystalline sample of La1.85Sr0.15CuO4 superconductor is studied.

AC Losses in YBCO-Added Bi1.6Pb0.4Sr2Ca2Cu3O δ Superconductors

The effect of YBCO adding on the superconducting of BPSCCO system with nominal starting composition of (Bi1.6Pb0.4Sr2Ca2Cu3O δ )1−x(YBCO) x with x = 0.00−0.10 was studied. The preparation methods used to prepare the samples are the conventional solid-state techniques. From the XRD data, it is observed that the percentage of the Bi-2223 phase increases and Bi-2212 decreases for addition x = 0.02–0.04, respectively. The effect of YBCO addition on the BPSCCO system also has been investigated in term of AC susceptibility study. We have studied the various applied field dependence of the AC susceptibility in polycrystalline samples. The AC susceptibility measurements in the range 0.05 to 2.00 Oe show that as the applied fields increases, the intergranular AC loss peaks move to lower temperature, and also height decreases and width increases. The effect of YBCO addition for the intergranular critical current J cm, and the presence of weak links that coupled the superconducting grains were defined.

Reticular three-dimensional 3d-4f frameworks constructed through substituted imidazole-dicarboxylate: syntheses, luminescence and magnetic properties study.

A family of heteronuclear metal organic frameworks (MOFs) using lanthanide and transition metal ion based on a rigid ligand of substituted imidazoledicarboxylic acid, namely, {[Ln2Zn2 (μ3-Hmimda)2 (μ3-mimda)2·4H2O]n·mn H2O}, Ln = Nd, m = 2 (1), Ln = Ho, m = 3 (2), Ln = Er, m = 2 (3), Ln = Yb, m = 3 (4), [Tb2Co2(μ3-Hmimda)2 (μ3-mimda)2·4H2O]n·2nH2O} (5) and [Dy2Co2 (μ3-Hmimda)2 (μ3-mimda)2·4H2O]n·nH2O} (6), (H3mimda = 1H-2-methyl-4,5-imidazole-dicarboxylic acid), have been developed and characterized. All the complexes (except for a little disparity for 1) are isostructural, and all crystallize in the monoclinic system. They possess an extended reticular-like porous structure constructed from corrugated 2D layers. Direct current (dc) magnetic susceptibility measurements for 1-5 indicated depopulation of the Stark components at low temperature and/or possible weak anti-ferromagnetic interactions within hetero-nuclear MOFs. Alternating current (ac) susceptibility studies reveal that the Dy(III)-Co(II) complex exhibits both possible ferromagnetic couplings and frequency-dependent out-of-phase signals, behaviour of single-ion magnet nature. Fluorescence properties of series complexes both in near-infrared (NIR) and visible regions have been comparatively studied.

Encapsulation of methotrexate loaded magnetic microcapsules for magnetic drug targeting and controlled drug release

We report on the development and evaluation of methotrexate magnetic microcapsules (MMC) for targeted rheumatoid arthritis therapy. Methotrexate was loaded into CaCO3-PSS (poly (sodium 4-styrenesulfonate)) doped microparticles that were coated successively with poly (allylamine hydrochloride) and poly (sodium 4-styrenesulfonate) by layer-by-layer technique. Ferrofluid was incorporated between the polyelectrolyte layers. CaCO3-PSS core was etched by incubation with EDTA yielding spherical MMC. The MMC were evaluated for various physicochemical, pharmaceutical parameters and magnetic properties. Surface morphology, crystallinity, particle size, zeta potential, encapsulation efficiency, loading capacity, drug release pattern, release kinetics and AC susceptibility studies revealed spherical particles of ~3µm size were obtained with a net zeta potential of +24.5mV, 56% encapsulation and 18.6% drug loading capacity, 96% of cumulative drug release obeyed Hixson-Crowell model release kinetics. Drug excipient interaction, surface area, thermal and storage stability studies for the prepared MMC was also evaluated. The developed MMC offer a promising mode of targeted and sustained release drug delivery for rheumatoid arthritis therapy.

Complex Magnetic Behavior of the Heusler Alloy—Cu2Mn0.75Fe0.25Al

We investigated the structural and magnetic properties of Cu 2 Mn 1-x Fe x Al (x = 0.25, 0.5, 0.75) alloys. Among the three compositions, only x = 0.25 composition crystallizes in single phase with L2 1 structure but its magnetic response indicates a complex magnetic structure. The magnetic isotherms were fitted to modified Langevin equation which indicates the presence of clusters with large magnetic moment. The ac susceptibility studies showed the presence of a superparamagnetic phase in the system over and above ferromagnetic order. To determine the nature of the interactions among the clusters, the variation of freezing temperature (T f ) with applied ac frequency was analyzed using different models (Neel-Arrhenius, Vogel-Fulcher, and critical slowing down model).

Interactions and magnetic relaxation in boron doped Mn3O4 nanoparticles

We have studied magnetic interactions in phase pure Mn3O4 and composite Mn2O3/Mn3O4 nanoparticle systems having different interparticle separations between the Mn3O4 ferrimagnetic cores. We characterized the morphology and structure of these nanoparticles using x-ray diffraction and transmission electron microscopy. We find that the incorporation of boron stabilizes the Mn3O4 spinel structure resulting in the formation of phase pure nanoparticles, while in the absence of boron, the sample consists of both Mn3O4 and antiferromagnetic Mn2O3 nanoparticles. We correlate the morphology of these systems with their magnetic properties using ac susceptibility studies. The low temperature frequency dependent relaxation exhibits larger magnetic interactions in the phase pure Mn3O4 nanoparticles as compared to the Mn3O4/Mn2O3 composites, which we attribute to differences in the separation between the ferrimagnetic cores in these two samples.

Susceptibility, high-field magnetization and ESR studies in a spin-5/2 triangular-lattice antiferromagnet Ba3MnSb2O9

The polycrystalline Ba3MnSb2O9 system as a spin-5/2 quasi-two dimensional triangular-lattice antiferromagnet (TLAF), is investigated by means of magnetic susceptibility, specific heat, high field magnetization and electron spin resonance (ESR) measurements. The ac susceptibility and specific heat studies reveal that it undergoes an AFM transition at TN=10.2K rather than the spin-glass one. High-field magnetization at 4.2K exhibits a weak 1/3 magnetization step near ~16.5T indicative of the field-induced quantum phase transition behavior. The analysis of temperature dependence of ESR spectra suggests that the short-range antiferromagnetic (AFM) correlation appear at temperatures well above TN. Further high-field ESR measurement up to 25T at 4.2K shows a broad resonance line, and the obtained g value is 2.094±0.012. Combined with the magnetic susceptibility and specific heat, the magnetic state below TN is explained by an AFM long rang order or a well developed AFM short range order with small anisotropy.

Glassy dynamics in the low-temperature inhomogeneous ferromagnetic phase of the quantum spin ice Yb2Sn2O7

Here we present a detailed muon spin relaxation ($\ensuremath{\mu}$SR), neutron diffraction, and ac-susceptibility study of Yb${}_{2}$Sn${}_{2}$O${}_{7}$ that shows that despite the onset of ferromagnetic long-range order at ${T}_{c}\ensuremath{\approx}0.140$ K, the low-temperature phase of this material remains highly disordered with glassy dynamics that span various frequency decades. We suggest that the observed disordered ground state might be made of infinite, percolating clusters coexisting with smaller ones. This mesoscopic cluster glass state would be the result of the enhanced effect of even a small amount of disorder, capable of disrupting the ``clean'' ordered state near a quantum critical point.

Synthesis, diffraction experiments, transmission electron microscopy and magnetic properties of α-Fe2O3/SiO2 nanocomposite sample

The sol-gel method was successfully utilized for the synthesis of the highly concentrated α-Fe2O3/SiO2 nanocomposite containing 45 wt% of hematite. TEM micrograph showed that spherical particles of 10 nm average size were evenly dissolved in glassy silica matrix. Both DC magnetization and AC susceptibility measurements indicated superparamagnetic behaviour of the system with the blocking temperature TB ≈ 65 K. Magnetization vs. field data at 300 K was successfully fitted to a weighted Langevin's curve for superparamagnetic system and the particle size obtained from this fit was in excellent agreement with the TEM results. Several features evinced in the magnetic data pointed to the presence of non-negligible magnetic interaction between hematite nanoparticles: the weak field dependence of the magnetization as a function of temperature, narrowing of the hysteresis loop at low applied field below TB, and high value of TB for hematite nanoparticles of 10 nm in size. On the other hand, features like continuous increase of the ZFC magnetization with decreasing temperature and the absence of a plateau or a peak in the FC branch showed that no strong magnetic interactions were present in the system. The AC susceptibility study showed that the Vogel-Fulcher model for singledomain inter-acting particles was applicable, while the empirical parameters C1 = 0,055 and C2 = 0,57 that lie between values for strongly interacting systems and non-interacting systems obtained have pointed to the presence of the interactions of intermediate strength. On the basis of the above listed results, we can conclude that the system under consideration consists of interacting single-domain hematite nanoparticles with magnetic properties that are located between weakly and strongly interacting systems.

Effect of the capping ligand on luminescent erbium(iii) β-diketonate single-ion magnets

Erbium complexes featuring β-diketonate ligand 2,4-hexanedione (Hh) and N,N-donor-ligands 2,2'-bipyridine (bipy), 5-nitro-1,10-phenanthroline (5NO2phen) and bathophenanthroline (bath) have been synthesized. The structures of the ternary complexes [Er(h)3(bipy)], [Er(h)3(5NO2phen)] and [Er(h)3(bath)] have been determined by single crystal X-ray diffraction. Excitation of the complexes in the ultraviolet region (337 nm) led to near infrared (NIR) luminescence at 1532 nm characteristic of the trivalent erbium ion in the three compounds, with an improved antenna effect in the 5-nitro-1,10-phenanthroline complex. The AC susceptibility studies conducted at frequencies ranging from 33 to 9995 Hz and at temperatures in the 1.7 to 10 K range revealed that the application of a static magnetic field induces a slow magnetic relaxation in all three compounds. The complex with the bulkier capping ligand (bathophenanthroline) exhibits the highest energy barrier U/k(B) = 23 K.

Revisiting Heat Capacity of Bulk Polycrystalline YBa2Cu3O7−δ

In this letter, we present the superconducting property characterization of a phase pure, reasonably good quality YBa2Cu3O7−δ sample. Studied compound is crystallized in orthorhombic Pmmm space group with lattice parameters a, b, and c being 3.829(2), 3.887(1) and 11.666(3) A, respectively. Bulk superconductivity is observed below 90 K as evidenced by resistivity and dc/ac magnetization measurements. The resistivity under magnetic field (ρTH) measurements showed clearly both the intra-grain and inter-grain transitions, which are supplemented by detailed (of varying frequency and amplitude) ac susceptibility studies as well. The upper critical field at 0 K, i.e. H c2(0) being determined from ρTH measurements with 50 % criteria of resistivity drop, is ∼70 T. Studied polycrystalline YBa2Cu3O7−δ is subjected to detailed heat capacity (C P ) studies. C P exhibited well-defined anomaly at below 90 K, which decreases with applied field. Although the C P anomaly/peak at T c reduces with applied field, the same is not completely suppressed in high applied fields of up to 12 T. The Sommerfeld constant (γ) and Debye temperature (Θ D), as determined from low temperature fitting of C P (T) data to Sommerfeld–Debye model, are 10.65 mJ/mole-K2 and 312.3 K, respectively. The results are compared with existing literature on bulk polycrystalline superconducting YBa2Cu3O7−δ sample.

Single‐Ion Magnetism in a Luminescent Er3+ β‐Diketonato Complex with Multiple Relaxation Mechanisms

AbstractIn the crystal structure of tris(2,2,6,6‐tetramethyl‐3,5‐heptanedionato)mono(bathophenanthroline)erbium(III), C57H73ErN2O6, the Er3+ ion is in an antiprismatic environment, surrounded by two N atoms and six O atoms. The β‐diketonato ligands show structural disorder. The ac susceptibility studies conducted at frequencies from 33 to 9995 Hz and at temperatures from 1.7 to 10 K revealed that the application of a static magnetic field induces a slow magnetic relaxation. Two relaxation modes (fast relaxation = FR; slow relaxation = SR) could be distinguished with energy barriers UFR/kB = 15.6 K and USR/kB = 22.4 K and relaxation times of 10–6 and 10–4 s, respectively. The Er3+ compound is luminescent in the solid state, emitting in the near IR region.

AC Susceptibility Studies of Phase Transitions and Magnetic Relaxation: Conventional, Molecular and Low-Dimensional Magnets

The paper presents the review of AC susceptibility measurements in study of magnetic phase transitions and spin dynamics. Examples of several types of phase transitions in conventional and molecular magnets are shown. The phenomenon of magnetic relaxation is brie y ascribed and illustrated with results on relaxation in low-dimensional molecular magnets, such as single molecule magnets, single chain magnets and quasi 1-dimensional systems showing magnetic phase transition to the long-range ordered phase. The following experimental possibilities of the AC technique, which allow to get insight into properties of novel magnetic materials are mentioned: (i) change of the AC eld amplitude (in order to distinguish ferro-(ferri) and antiferromagnets); (ii) use of a static eld HDC (to conclude on the type of magnetic order and to study the phase diagram); (iii) measurement of nonlinear susceptibilities (to conclude on the type of phase transition); (iv) probing the dependence of both components of susceptibility on frequency of the AC eld (to study magnetic relaxation); (v) tests of the DC eld e ect on the frequency dependence of χAC (to analyze the possibility of blocking or reentrant spin-glass transition).

Correspondence between neutron depolarization and higher order magnetic susceptibility to investigate ferromagnetic clusters in phase separated systems

It is a tough task to distinguish a short-range ferromagnetically correlated cluster-glass phase from a canonical spin-glass-like phase in many magnetic oxide systems using conventional magnetometry measurements. As a case study, we investigate the magnetic ground state of La0.85Sr0.15CoO3, which is often debated based on phase separation issues. We report the results of two samples of La0.85Sr0.15CoO3 (S-1 and S-2) prepared under different conditions. Neutron depolarization, higher harmonic ac susceptibility and magnetic relaxation studies were carried out along with conventional magnetometry measurements to differentiate subtle changes at the microscopic level. There is no evidence of ferromagnetic correlation in the sample S-2 attributed to a spin-glass phase, and this is compounded by the lack of existence of a second order component of higher harmonic ac susceptibility and neutron depolarization. A magnetic relaxation experiment at different temperatures complements the spin glass characteristic in S-2. All these signal a sharp variance when we consider the cluster-glass-like phase (phase separated) in S-1, especially when prepared from an improper chemical synthesis process. This shows that the nonlinear ac susceptibility is a viable tool to detect ferromagnetic clusters such as those the neutron depolarization study can reveal.

Spin dynamics, short-range order and superparamagnetism in superconducting ferromagnet RuSr2Gd1.4Ce0.6Cu2O10−δ

We report structural, detailed DC and linear/non-linear AC, isothermal and thermoremanent magnetization study of the rutheno-cuprate superconducting ferromagnet RuSr2Gd1.4Ce0.6Cu2O10−δ (GdRu-1222). Structural analysis, by employing Rietveld refinement of X-ray diffraction pattern, reveals that GdRu-1222 crystallizes in tetragonal phase with I4/mmm space group. GdRu-1222 is a reported superconducting ferromagnet with Ru spins magnetic ordering at around 110K and superconductivity below 40K in Cu–O2 planes. Detailed linear/non-linear first and higher order harmonic of AC susceptibility studies unveiled the complex magnetism of GdRu-1222. A frequency dependent cusp is observed in AC susceptibility (χac) vs. T measurements. The change in cusp position with applied frequency followed the well known Vogel–Fulcher law, which is a feature to describe a spin-glass (SG) system with possibility of embedded homogeneous/non-homogeneous magnetically interacting/non-interacting ferromagnetic clusters. Such an interpretation is also supported by thermoremanent magnetization (TRM) study at T=60K. Detailed interpretation of AC magnetization results revealed the formation of magnetic (ferromagnetic) homogenous/non-homogenous clusters of different sizes embedded in spin-glass (SG) matrix. The magnetization vs. applied field loops do not saturate, even at high applied fields (50kOe), resulting in the short-range magnetic ordering in the system, which causes the formation of clusters that freeze at low temperatures. Temperature variation of first- and third-order susceptibility harmonics show good agreement with the Wohlfarth's model (WM), leading to the superparamagnetism (SPM) state. Detailed magnetization (DC and AC both) results and their analysis helped in explaining the temperature dependent magnetism of the GdRu-1222 system.

Magnetic properties of NdMn1−xFexO3+δ (0≤x≤0.3) system.

We have studied the effect of Fe for Mn substitution on magnetic properties of NdMn1−xFexO3+δ compounds. Our heat capacity, AC susceptibility and magnetization measurements revealed that the transition from paramagnetic to magnetically ordered state of the Mn sublattice decreases linearly from TN=74.9K to TN=49K with doping up to x=0.25. The ordering temperature T1 of Nd sublattice varies in a non-monotonous way in the temperature range 11–16K. For x=0.3, the ordering of the Mn/Fe sublattice is residual, while the magnetism in Nd sublattice seems to be intact. The broad maximum in heat capacity C(T) at about 10K was ascribed by 2-level Schottky contribution (Δ=21(1)K) for x=0, but this description failed for Fe-doped samples. An upturn of C(T) at temperatures below 0.5K was observed on NdMnO3 and can be accounted to nuclear contribution. A more detailed study of AC susceptibility performed on samples with x=0.1 and 0.2 revealed a shift of both peaks to higher temperature with increasing frequency. The compensation temperature and a large negative field-cooled magnetization was observed for concentrations x=0.2 and x=0.25.

Enhanced Vapor-Phase Processing in Fluorinated Fe4Single-Molecule Magnets

A new tetrairon(III) single-molecule magnet with enhanced volatility and processability was obtained by partial fluorination of the ancillary β-diketonato ligands. Fluorinated proligand Hpta = pivaloyltrifluoroacetone was used to assemble the bis(alkoxido)-bridged dimer [Fe2(OEt)2(pta)4] (1) in crystalline form, from which the new tetranuclear complex [Fe4(L)2(pta)6] (2) was synthesized in a one-pot reaction with H3L = 2-hydroxymethyl-2-phenylpropane-1,3-diol, NaOEt, and FeCl3 in a Et2O:EtOH solvent mixture. The structure of compound 2 was inferred from (1)H NMR, mass spectrometry, magnetic measurements, and DFT calculations. Direct current magnetic data are consistent with the expected metal-centered triangular topology for the iron(III) ions, with an antiferromagnetic coupling constant J = 16.20(6) cm(-1) between the central iron and the peripheral ones and consequent stabilization of an S = 5 spin ground state. Alternating current (ac) susceptibility measurements in 0 and 1 kOe static applied fields show the presence of a thermally activated process for magnetic relaxation, with τ0 = 2.3(1) 10(-7) s and U(eff)/kB = 9.9(1) K at zero static field and τ0 = 2.0(2) 10(-7) s and U(eff)/kB = 13.0(2) K at 1 kOe. At a pressure of 10(-7) mbar, compound 2 sublimates at (440 ± 5) K vs (500 ± 10) K for the nonfluorinated variant [Fe4(L)2(dpm)6] (Hdpm = dipivaloylmethane). According to XPS, ToF-SIMS, and ac susceptibility studies, the chemical composition, fragmentation pattern, and slow magnetic relaxation of the pristine material are retained in sublimated samples, suggesting that the molecular structure remains totally unaffected upon vapor-phase processing.

Slow spin relaxation induced by magnetic field in [NdCo(bpdo)(H2O)4(CN)6]⋅3H2O

We report on a comprehensive investigation of the magnetic properties of [NdCo(bpdo)(H2O)4(CN)6]⋅3H2O (bpdo=4, 4′-bipyridine-N,N′-dioxide) by use of electron paramagnetic resonance, magnetization, specific heat and susceptibility measurements. The studied material was identified as a magnet with an effective spin S = 1/2 and a weak exchange interaction J/kB = 25 mK. The ac susceptibility studies conducted at audio frequencies and at temperatures from 1.8 to 9 K revealed that the application of a static magnetic field induces a slow spin relaxation. It is suggested that the relaxation in the magnetic field appears due to an Orbach-like process between the two lowest doublet energy states of the magnetic Nd3+ ion. The appearance of the slow relaxation in a magnetic field cannot be associated with a resonant phonon trapping. The obtained results suggest that the relaxation is influenced by nuclear spin driven quantum tunnelling which is suppressed by external magnetic field.