Ac Susceptibility Measurements Research Articles

Observation of low temperature metastable states in complex CaMn7O12

The low temperature magnetic behaviour of the multiferroic quadruple perovskite CaMn7O12 is investigated. The magneto-caloric effect in this material is presented for the first time. Along with the established magnetic transitions, TN1 ~ 90 K and TN2 ~ 45 K, an anomaly at 20 K is observed in our dc magnetization, ac susceptibility and dielectric measurements; below which, an inverse magnetocaloric effect is also observed in our −ΔSM (T) plots. The neutron scattering measurements show minimal change between 10 K and 30 K in static correlations, but a clear change in energy and linewidth of the magnetic excitations is evident. The results suggest that only dynamic correlations change across TM ~ 20 K. The existence of multiple magnetic interactions below 45 K, with significant coupling between them, is demonstrated using an Arrott plot analysis of our magnetic data. Compatible conclusions are drawn from magnetocaloric plots. The peak change in isothermal magnetic entropy (−ΔSM) is 1.3 JK−1 kg−1 and the value of refrigeration capacity in CaMn7O12 is 34.5 J · kg−1 at 7 T.

Magnetic Properties of 4f Localized Ferromagnet CeAgSb2 under Transverse Magnetic Fields

We performed dc-magnetization and ac-susceptibility measurements to clarify how the magnetic properties of CeAgSb2, which shows ferromagnetic order along the c-axis below 9.6 K, evolve with the application of a transverse magnetic field (Hab). The Curie temperature (TC) is suppressed to zero by the application of Hab, and the ac-susceptibility measured along the c-axis \(\chi '_{c}\) under Hab continuously shows a critical-divergence-like anomaly at TC, indicating the presence of a field-induced ferromagnetic quantum critical point in this compound. On the other hand, the magnitude of \(\chi '_{c}\) at TC rapidly decreases with increasing Hab up to around 0.5 T. This result suggests that the magnetic properties around TC rapidly change with Hab in the relatively lower Hab range.

AC Susceptibility and Superconducting Properties of Graphene Added YBa2Cu3O7−d

The effect of graphene (G) addition on YBa2Cu3O7−δ(G)x (x = 0 − 0.03 wt%) has been studied using the X-ray diffraction method, scanning electron microscopy, electrical resistance versus temperature, transport critical current density Jc, and AC susceptibility measurements. XRD patterns showed single-phase YBa2Cu3O7−δ (YBCO) for all samples. SEM micrographs showed filling of the voids between YBa2Cu3O7−δ grains as graphene was added. The temperature-dependent electrical resistance curves showed metallic normal state behavior and onset transition temperature Tconset between 90 and 92 K for all samples. AC susceptibility measurement showed transition temperature $T_{\mathrm {c} \chi ^{\prime }}$ between 90 and 93 K. $T_{\mathrm {c} \chi ^{\prime }}$ was maintained or improved slightly as graphene was added. The x = 0.001 wt% showed the highest Jc, i.e., 2750 A cm−2 at 77 K and 5570 mA cm−2 at 30 K which was 13 and 40 times higher than that of the non-added YBCO, respectively. The peak temperature Tp of the imaginary part of the susceptibility χ″ was around 78–82 K for all samples indicating grain coupling was not weakened as graphene was added.

Heterometallic hexanuclear Ni4M2 (M = Dy, Y) complexes: structure and single-molecule magnet for the Dy(iii) derivative.

To achieve a heterometallic single-molecule magnet with novel topologies, two isostructural hexanuclear clusters [Ni4Ln2(μ3-OH)2L4(OAc)8]·H2O [Ln = Dy, 1; Y, 2] were prepared from the reactions of 1,3-diamine-2-propanol (HL) with MCl3·6H2O (M = Dy, Y) and Ni(OAc)2·4H2O in acetonitrile, in which the studies for 2 with the diamagnetic metal ion of Y(iii) are just for further understanding of the magnetic properties of 1. Single-crystal X-ray diffraction measurements indicate that both heterometallic hexanuclear complexes possess an S-shaped double-pocket skeleton with the two M(iii) ions separated by a chair-like Ni4O4 moiety. The temperature- and frequency-dependent alternating-current (ac) susceptibility measurements under zero dc fields revealed a single-molecule magnet for 1. Its magnetic properties can be further understood based on the magnetic analysis of 2.

Field-dependent dynamic responses from dilute magnetic nanoparticle dispersions.

The response of magnetic nanoparticles (MNPs) to an oscillating magnetic field outside the linear response region is important for several applications including magnetic hyperthermia, magnetic resonance imaging and biodetection. The size and magnetic moment are two critical parameters for the performance of a colloidal MNP dispersion. We present and demonstrate the use of optomagnetic (OM) and AC susceptibility (ACS) measurements vs. frequency and magnetic field strength to obtain the size and magnetic moment distributions including the correlation between the distributions. The correlation between the size and the magnetic moment contains information on the morphology and intrinsic structure of the particle. In OM measurements, the variation of the second harmonic light transmission through a dispersion of MNPs is measured in response to an oscillating magnetic field. We solve the Fokker-Planck equations for MNPs with a permanent magnetic moment, and develop analytical approximations to the ACS and the OM signals that also account for the change in the curve shapes with increasing field strength. Further, we describe the influence of induced magnetic moments on the signals, by solving the Fokker-Planck equation for particles, which apart from the permanent magnetic moment may also have an induced magnetic moment and shape anisotropy. Using the results from the Fokker-Planck calculations we fit ACS and OM measurements on two multi-core particle systems. The obtained fit parameters also describe the correlations between the magnetic moment and size of the particles. From such an analysis on a commercially available polydisperse multicore particle system with an average particle size of 80 nm, we find that the MNP magnetic moment is proportional to the square root of the hydrodynamic size.

Glassy behavior of diluted Cu-Zn ferrites

The magnetic behavior of Zn substituted Cu-Zn spinel ferrites having chemical formula Cu1−xZnxFe2O4 (x = 0.7, 0.8, 0.9 and 1.0) has been studied by SQUID magnetometry, by means of magnetic hysteresis, field-cooled (FC) and zero-field-cooled (ZFC) magnetization, memory effect and low field ac susceptibility measurements. These measurements suggest that the ferrimagnetic phase of the x ≤ 0.8 samples is gradually turned into a spin glass (x ≥ 0.9). The compound with x = 0.9 exhibits the typical dynamical behavior of spin glasses, with indication of aging, rejuvenation and memory effects. The evolution of the magnetic properties of Cu-Zn spinel ferrites with substitution of Zn for Cu is discussed.

The effect of TM doping on the superconducting properties of ZrNi2-xTMxGa (TM = Cu, Co) Heusler compounds

We have explored the structural and superconducting properties of ZrNi2-xTMxGa (TM = Cu, Co) Heusler compounds via x-ray diffraction, scanning electron mi croscopy, electrical resistivity, dc magnetization and ac susceptibility measurements. All samples crystallized in the cubic L21 structure at room temperature. For x ≤ 0.25, all the ZrNi2-xCuxGa compounds showed superconducting properties and a decrease in TC with increasing Cu concentration. The dc magnetization data suggested type-II superconductivity for all the Cu-doped compounds. Contrary to the ZrNi2-xCuxGa compounds, no superconductivity was observed in the ZrNi2-xCoxGa compounds. Substitution of Ni by a small concentration of Co destroyed superconductivity in the Co-doped compounds. The experimental results are discussed and possible explanations are provided.

Investigation of Structural and Magnetic Properties on Mg1−xZnxFe2−xAlxO4 (0.0 ≤ x ≤ 0.8) Nanoparticles

A mixed spinel ferrite nanoparticle, Mg1−xZnxFe2−xAlxO4 NPs (0.0 ≤ x ≤ 0.8), were synthesized effectively by co-precipitation method and sintered at 600 °C for 10 h. The structural and magnetic properties of the products were studied through X-ray powder diffraction (XRD), scanning electron microscopy, transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and vibrating sample magnetometer. The cubic spinel phase was confirmed by XRDs with particle size between 24.5 and 40.2 nm. The lattice parameters for the products are increased with increasing the Zn2+ and Al3+ ratio due to the successfully integrated into the cubic system without changing the original structure. Although it was observed from the cation distributions, that the cubic phase is was an inverse spinel, wherein which the Fe3+ and Mg2+ ions occupied both the tetrahedral A and octahedral B- sites, the Zn2+ ions preferred to occupy the A- sites and Al3+ occupy preferred the B -sites. The morphology of the nanoparticles NPs detailed was using TEM, HR-TEM, and SAED in selected area confirmed the particle size and crystalline spinel structure. Magnetization results at room temperature presents a narrow hysteresis loop for all ratios, which is specific of the soft magnetic materials. Also, we noticed that the increase in the magnetization with increasing the ratio of Zn2+ and Al3+ consistent with the enhancement of crystallinity. Moreover, we found that the saturation magnetization, coercively and remanent for Mg1−xZnxFe2−xAlxO4 where x = 0.6 sample is the highest, indicating the potential of Zn and Al substitution in enhancing the magnetic properties of magnesium ferrite. According to AC magnetic susceptibility measurements, the nanoparticles exhibit superparamagnetic/spin glassy behaviour with a very strong inter-nanoparticles interaction. Additionally, AC susceptibility measurements indicated a relative sensitivity of samples to the variation of applied frequency, which is an important result for the applications in hyperthermia based therapy. This is the first study in which both Zn2+ and Al3+ ions with varying concentration were tried to substitute into MgFe2O4 simultaneously and their effects on magnetic properties of MgFe2O4 was investigated.

Inhomogeneities and superconductivity in poly-phase Fe-Se-Te systems

Abstract The impact of synthesis conditions, post-preparation heating procedure, aging and influence of pressure on the superconducting properties of FeSe 0.4 Te 0.6 crystals is reported. Two FeSe 0.4 Te 0.6 single crystals were used in the study, prepared from stoichiometric melt but cooled down with very different cooling rates, and investigated using magnetic bulk and electrical-resistivity methods. The fast-cooled crystal contains large inclusions of F e 3 Se 2.1 Te 1.8 and exhibits bulk superconductivity in its as-prepared state, while the other is homogeneous and shows only traces of superconductivity. AC susceptibility measurements under hydrostatic pressure show that the superconducting transition temperature of the inhomogeneous crystal increases from 12.3 K at ambient pressure to Tsc = 17.9 K at 9 kbar. On the other hand, neither pressure nor mechanically-induced stress is sufficient to induce superconductivity in the homogeneous crystal. However, an additional heat treatment at 673 K followed by fast cooling down and/or long-term aging at ambient conditions leads to the appearance of bulk superconductivity also in the latter sample. This sample remains homogeneous on a scale down to few μm but shows an additional magnetic phase transition around 130 K suggesting that it must be inhomogeneous. For comparison also F e 3 Se 2.1 Te 1.8 polycrystals have been prepared and their magnetic properties have been studied. It appears that this phase is not superconducting by itself. It is concluded that nano-scale inhomogeneities that appear in the F e S e x Te 1 − x system due to a spinodal decomposition in the solid state are necessary for bulk superconductivity, possibly due to minor changes in the crystal structure and microstructure. Macroscopic inclusions quenched by fast cooling from high temperatures lead obviously to strain and hence variations in the lattice constants, an effect that is further supported by application of pressure/stress.

Multifunctional PEG-carboxylate copolymer coated superparamagnetic iron oxide nanoparticles for biomedical application

Biocompatible magnetite nanoparticles (MNPs) were prepared by post-coating the magnetic nanocores with a synthetic polymer designed specifically to shield the particles from non-specific interaction with cells. Poly(ethylene glycol) methyl ether methacrylate (PEGMA) macromonomers and acrylic acid (AA) small molecular monomers were chemically coupled by quasi-living atom transfer radical polymerization (ATRP) to a comb-like copolymer, P(PEGMA-co-AA) designated here as P(PEGMA-AA). The polymer contains pendant carboxylate moieties near the backbone and PEG side chains. It is able to bind spontaneously to MNPs; stabilize the particles electrostatically via the carboxylate moieties and sterically via the PEG moieties; provide high protein repellency via the structured PEG layer; and anchor bioactive proteins via peptide bond formation with the free carboxylate groups. The presence of the P(PEGMA-AA) coating was verified in XPS experiments. The electrosteric (i.e., combined electrostatic and steric) stabilization is efficient down to pH 4 (at 10 mM ionic strength). Static magnetization and AC susceptibility measurements showed that the P(PEGMA-AA)@MNPs are superparamagnetic with a saturation magnetization value of 55 emu/g and that both single core nanoparticles and multicore structures are present in the samples. The multicore components make our product well suited for magnetic hyperthermia applications (SAR values up to 17.44 W/g). In vitro biocompatibility, cell internalization, and magnetic hyperthermia studies demonstrate the excellent theranostic potential of our product.

Syntheses, crystal structures and magnetic properties of sandglass DyIII9 and irregular tetrahedron DyIII4 complexes

The synthesis, crystal structures, and magnetic properties of a nonanuclear dysprosium complex [Dy9(μ3-OH)8(μ4-OH)2(N3)8{(py)2C(OCH3)O}8](OH)·4H2O (1, (py)2C(OCH3)OH = the hemiacetal form of di-2-pyridyl ketone) and a tetranuclear dysprosium complex [Dy4(μ4-O)(μ-N3)2(μ-OCH3)2(L)4](OH)2·3.5CH3OH (2, HL = 2-(benzothiazol-2-yl-hydrazonomethyl)-6-methoxyphenol) have been prepared and structural characterization. Nine DyIII ions in 1 form a sandglass with the eight DyIII atoms at the apexes and one DyIII ion located at the center of a square antiprism formed by eight DyIII ions, while four DyIII ions in 2 form an irregular tetrahedron. Complexes 1 and 2 have been investigated by direct current (dc) and alternating current (ac) susceptibility measurements. The ac susceptibility studies revealed that complexes 1 and 2 exhibit slow magnetic relaxation. Moreover, complex 2 shows two-step thermal magnetic relaxation with their energy barriers of about 15 K and 2.68 K, respectively. The orientations of the magnetic anisotropy of DyIII ions in 1 and 2 were also estimated by electrostatic calculations.

Effects of alkali metals on the formation of Tl-1212 type phase (Tl0.85Cr0.15)Sr2−x A x CaCu2O7 with A = Li, Na and Rb

The effects of alkali metals A = Li, Na and Rb on (Tl0.85Cr0.15)Sr2−x A x CaCu2O7 (Tl-1212) superconductors with x = 0–0.1 were studied. The samples were prepared via the solid-state reaction method. X-ray diffraction patterns showed that the formation of Tl-1212 phase was favorable when the ionic radius of the alkali metal was closer to the radius of Tl1+. Ca0.3Sr0.7CuO2 was present as a major or minor phase in some of the samples. Scanning electron micrographs showed that the grain size increased with Li, Na and Rb substitutions. The temperature-dependent electrical resistance measurements showed metallic normal state behavior for all samples with onset transition temperature, T c onset between 96 and 105 K and zero-resistance temperature, T c zero between 92 and 99 K. AC susceptibility measurement showed superconducting transition, \({T_{{c_{{{\upchi}}}}{{'}}}}\) between 92 and 97 K. The peak temperature T p of the imaginary part of the susceptibility χ″ was the highest for Na and Rb with x = 0.1 samples (T p = 92 K). The inter-grain critical current density, J c at T p for x = 0 sample was J c (T p = 68 K) = 14 A cm−2 and for Li substituted sample with x = 0.1, J c (T p = 76 K) = 22 A cm−2. This work showed that Rb greatly enhanced the Tl-1212 phase formation due to the proximity of the Rb1+ and Tl1+ ionic radius. Na and Rb improved the transition temperature. Li, Na and Rb improved the inter-grain critical current density and increased the grain size of the samples.

Heterometallic Heptanuclear [Cu5Ln2] (Ln = Tb, Dy, and Ho) Single-Molecule Magnets Organized in One-Dimensional Coordination Polymeric Network.

The reaction of a multisite coordination ligand, LH3, with Cu(II) salts and Ln(NO3)3·nH2O in a 1:2:1 stoichiometric ratio in the presence of triethylamine was found to afford a series of one-dimensional heterometallic [{Cu5Ln2(L)2(μ3-OH)4(ClO4)(NO3)3(OH2)5}(ClO4)2(H2O)x]∞ [Ln = Tb(1), Dy(2) and Ho(3), x = 4.25, 5.5, and 5 for 1-3, respectively] coordination polymers. Complexes 1-3 have been characterized by single crystal X-ray crystallography. The detailed study of the magnetic properties has also been performed and compared with the parent [Cu5Ln2] molecular analogues. The ac susceptibility measurements for complexes 1-3 confirm their SMM behavior with the following characteristics: Δeff/kB = 23.4 K, τ0 = 1.1 × 10-6 s and Δeff/kB = 27.9 K, τ0 = 6.6 × 10-7 s under 0 and 1200 Oe dc fields, respectively for 1; Δeff/kB = 8.3 K, τ0 = 3.1 × 10-6 s for 2 under 0 dc field. For 3, the fast QTM below 4 K prevents the estimation of the SMM energy barrier. Remarkably, the magnetic and SMM properties of the previously reported molecular [Cu5Ln2] analogues are preserved after their assembly in these coordination networks.

Glassy magnetic ground state and Kondo-like behaviour in Mn10FeGe8 alloy

We report a detailed investigation of the ground-state magnetic properties of newly synthesized alloy. The sample can be thought of being derived by substituting one Mn atom by Fe of the parent compound . Fe-substitution leads to a drastic change in the magnetic ground state as well as to the magneto-transport properties of the parent alloy. On cooling below 250 K, undergoes a transition from paramagnetic phase to a state having significant ferromagnetic correlations. The ground state is found to be canonical spin glass (CSG) type in nature as evident from the dc magnetization and ac susceptibility measurements. Interestingly, the resistivity data shows an upturn at low temperature below about 30 K, mimicking Kondo-like behaviour. turns out to be a rare example among 3d transition metal alloys, where a Kondo-like state coexists within a CSG phase.

Structural and magnetic short-range order in fluorite Yb2TiO5

We studied structural and magnetic ordering in ${\mathrm{Yb}}_{2}{\mathrm{TiO}}_{5}$ using synchrotron x-ray diffraction, neutron diffraction and total scattering, ac and dc susceptibility, and inelastic neutron scattering. Diffraction measurements reveal an average disordered fluorite structure with additional diffuse scattering features, which are caused by structural short-range orthorhombic order, as evidenced by the neutron pair distribution function measurements. The ac susceptibility measurements show a broad peak at ${T}_{f}\ensuremath{\approx}0.35$ K that displays Arrhenius behavior with an activation energy of 2.51(5) meV. Zero-field neutron scattering measurements show broad magnetic diffuse scattering in the elastic channel with an antiferromagnetic-type gapless excitation extending to 1.5 meV. A polarized state with partial spin order is induced with an applied magnetic field which opens a gapped excitation that increases monotonically with field strength.

Superconductivity in single crystalline ThBe13 and LuBe13

Single crystals of ThBe13 and LuBe13 have been prepared using aluminum flux method. The crystals structure of both compounds was confirmed to be cubic of the NaZn13-type with the lattice parameters a = 10.183(6)Å and a = 10.395(3)Å for LuBe13 and ThBe13, respectively. The low temperature measurements of electrical resistance and ac susceptibility have revealed a BCS-type of superconductivity in ThBe13 below Tc = 125mK. LuBe13 has been found to be superconducting below Tc = 630mK.

Superconductivity in the ternary compound SrPt10P4 with complex new structure

We report superconductivity at 1.4 K in the ternary ${\mathrm{SrPt}}_{10}{\mathrm{P}}_{4}$ with a complex new structure. ${\mathrm{SrPt}}_{10}{\mathrm{P}}_{4}$ crystallizes in a monoclinic space-group C2/c (#15) with lattice parameters $a=22.9151(9)\phantom{\rule{0.16em}{0ex}}\text{\AA{}}, b=13.1664$(5)$\phantom{\rule{0.16em}{0ex}}\text{\AA{}}, c=13.4131$(5)$\phantom{\rule{0.16em}{0ex}}\text{\AA{}}$, and $\ensuremath{\beta}=90.0270(5){}^{\ensuremath{\circ}}$. Bulk superconductivity in the samples has been demonstrated through resistivity, ac susceptibility, and heat capacity measurements. High pressure measurements have shown that the superconducting ${\mathrm{T}}_{C}$ is systematically suppressed upon application of pressure, with a ${\mathrm{dT}}_{C}/\mathrm{dP}$ coefficient of $\ensuremath{-}0.016$ K/GPa.

Magnetic Core-Size Distribution of Magnetic Nanoparticles Estimated From Magnetization, AC Susceptibility, and Relaxation Measurements

Magnetic Core-Size Distribution of Magnetic Nanoparticles Estimated From Magnetization, AC Susceptibility, and Relaxation Measurements

Magnetic compensation-induced sign reversal of exchange bias in a multi-glass perovskite SmFeO3

We report an unusual sign reversal of exchange bias (EB) across a magnetic compensation point in an orthorhombic perovskite SmFeO3. A conventional negative EB with a positive vertical magnetization shift is observed below a cluster-glass freezing temperature (Tg ∼ 150 K). Upon further lowering of the temperature, the EB disappears at the magnetic compensation point before reversing its sign to a positive exchange bias below 4 K. The EB effect originates from an interfacial exchange interaction within a cluster glass phase, whereas its sign reversal arises from the reversal of the direction of the net magnetic moment as a result of dominance of Sm3+ over Fe3+ below the compensation temperature. The existence of a multi-glass state is demonstrated by ac-susceptibility and electrical permittivity measurements. A phenomenological model is presented to understand the EB effect and its sign reversal across the compensation point.

Heterometallic Zn3 Ln3 Ensembles Containing (μ6 -CO3 ) Ligand and Triangular Disposition of Ln3+ ions: Analysis of Single-Molecule Toroic (SMT) and Single-Molecule Magnet (SMM) Behavior.

Two new heterometallic Zn3 Ln3 (Ln3+ =Dy, Tb) complexes, with a double triangular topology of the metal ions, have been assembled from the polytopic Mannich base ligand 6,6'-{[2-(dimethylamino)ethylazanediyl]bis(methylene)}bis(2-methoxy-4-methylphenol) (H2 L) with the aid of an in situ generated carbonate ligand from atmospheric CO2 fixation. Theoretical calculations indicate axial ground states for the Ln3+ ions in these complexes, with their local magnetic moments being almost coplanar and tangential to the Ln3+ atoms that define the equilateral triangle. Therefore, they can be considered as single-molecule toroics (SMTs) with almost zero total magnetic moment. Micro-SQUID measurements on the Dy3+ counterpart show hysteresis loops below 3 K that have an S-shape, with large coercive fields opening upon cooling. This behavior is typical of a single molecule magnet (SMM) with very slow zero-field relaxation. At around ±0.35 T, the loops have a broad step, which is due to a direct relaxation process and corresponds to an acceleration of the relaxation of the magnetization, also observed at this magnetic field from ac susceptibility measurements. Simulations suggest that the broad step corresponds to two level avoidance of crossing points where the spin chiral Kramers doublet meets excited states of the coupled manifold, whose position is defined by exchange and dipole interactions. The Tb3+ counterpart does not exhibit SMM behavior, which is due to the fact that the degeneracy of the ground state of the exchange coupled system is lifted at zero field, thus favoring quantum tunneling of magnetization (QTM).