Ac Susceptibility Measurements Research Articles

Competition between ferromagnetism and frustrated antiferromagnetism in quasi 2D Ce2.15(Pd1−xAgx)1.95In0.9 alloys

Low temperature thermal and magnetic measurements performed on ferro-magneticl (FM) alloys of composition Ce2.15(Pd1−xAgx)1.95In0.9 are presented. Pd substitution by Ag depresses from 4.1 K down to 1.1 K for x = 0.5, which is related to the increase of band electrons, with a critical concentration extrapolated to . The decrease is accompanied by a weakening of the magnetization of the FM phase. At high temperature (T > 30 K) the inverse magnetic susceptibility reveals the presence of robust magnetic moments ( ), whereas the low value of the Curie–Weiss temperature K excludes any relevant effect from Kondo screening. The specific heat jump at decreases accordingly, while an anomaly emerges at a fixed temperature K. This unexpected anomaly does not show any associated sign of magnetism checked by AC-susceptibility measurements. Since the total magnetic entropy (evaluated around ) practically does not change with Ag concentration, the transference of degrees of freedom from the FM component to the non-magnetic T* anomaly is deduced. The origin of this anomaly is attributed to an arising magnetic frustration of the ground state and the consequent entropy bottleneck produced by the divergent increasing of density of excitations at low temperature.

Lanthanide dinuclear complexes constructed by 8-hydroxyquinoline Schiff base showing magnetic refrigeration and slow magnetic relaxation

Two dinuclear lanthanide(III) complexes based on 8-hydroxyquinoline Schiff base derivatives, [Ln2(hfac)6L2]·0.5C7H16 (Ln=Gd (1) and Dy (2), HL=5-(4-methylbenzylidene)-8-hydroxylquinoline and hfac=hexafluoroacetylacetonate), have been synthesized, structurally and magnetically characterized. Both 1 and 2 are phenoxo-O bridged dinuclear complexes, and center Ln(III) ions are eight-coordinated with three bidentate hfac and two μ2-O bridging 8-hydroxyquinoline Schiff base ligands. Magnetic studies reveal that 1 exhibits antiferromagnetic exchange interaction between nearby Gd(III) centers and shows magnetic refrigeration (−ΔSm=14.9Jkg−1K−1 for ΔH=7 T at 2K). Ac susceptibility measurements reveal that 2 exhibits slow magnetic relaxation behavior with the anisotropic barriers (ΔE/kB) of 3.0K.

Synthesis and characterization of a mixed-valent Mn11–La2 aggregate with benzoate

A new mixed-valent Mn–Ln complex with an Mn 9 III Mn 2 II La2 core $$ [{\text{Mn}}_{9}^{\text{III}} {\text{Mn}}_{2}^{\text{II}} {\text{La}}_{ 2} (\mu_{ 4} {-}{\text{O}})_{7} (\mu_{ 3} {-}{\text{O}})(\mu_{ 3} {-}{\text{OH}})_{ 2} ({\text{benzoate}})_{ 1 8} ({\text{NO}}_{3} )({\text{MeCN}})] $$ has been synthesized. Its magnetic characterization reveals a complex with dominating slow relaxation. In addition, alternating current (ac) susceptibility measurements reveal a frequency-dependent out-of-phase (χ′′M) signal below 4 K, suggesting the complex to be a single-molecule magnet.

Magnetic flux motion in (PrxY1−x)Ba2Cu3O7−δ polycrystal samples sintered in Ar and O2 atmospheres

We present a comparative study of the magnetic flux motion in ceramic pellets made of (PrxY1−x)Ba2Cu3O7−δ as a function of their composition and morphology. Samples produced in Ar or O2 atmosphere present noticeable differences in their magnetic response that we explain in terms of their structural parameters. The material’s parameters that most influence the flux dynamics are morphology and oxygen content, that change dramatically with the sintering atmosphere. Moderate changes are also observed as a function of the Pr content. Magnetic pinning efficiency is discussed in terms of intergranular couplings and effective activation energies, estimated from AC-susceptibility and magnetoresistance measurements.

Phase diagram and magnetic relaxation phenomena inCu2OSeO3

We present an investigation of the magnetic-field-temperature phase diagram of Cu2OSeO3 based on dc magnetization and ac susceptibility measurements covering a broad frequency range of four orders of magnitude, from very low frequencies reaching 0.1 Hz up to 1 kHz. The experiments were performed in the vicinity of Tc=58.2 K and around the skyrmion lattice A phase. At the borders between the different phases the characteristic relaxation times reach several milliseconds and the relaxation is nonexponential. Consequently the borders between the different phases depend on the specific criteria and frequency used and an unambiguous determination is not possible.

Improvement of superconducting properties of (Bi, Pb)-2223phase by TlF3 doping

In this work, the superconducting properties of thallium fluoride (TlF3)doped Bi1.8−xTlxPb0.35Sr2Ca2Cu3OyF3x(x=0–0.15)compounds are presented. The X-ray diffraction analysis shows that the proportion of (Bi, Pb)-2223phase is higher than the secondary (Bi, Pb)-2212one in all samples and its highest value is about 82.74%for x=0.05. From the resistivity curves, the highest values of the onset critical transition temperature (Tc.on), the offset critical transition temperature (Tc.off) are seen for x=0.10. The calculation of activation energy (U0) in the TAFF (thermally activation flux flow) region proves the positive effect of TlF3 doping on the dissipative behavior of energy near Tc.off. Flux pinning strength is enhanced by increasing TlF3content up to x=0.10, and decreased by the application of a magnetic field. The other superconducting parameters like T(Hc2), T(Hirr), ξ(0) and μ0Hc2(0) are also improved significantly by doping. The obtained results of Ac susceptibility measurements show that the onset temperature of diamagnetism is improved by TlF3 doping, in accordance with the resistivity results.

Highly Sensitive Third-Harmonic Detection Method of Magnetic Nanoparticles Using an AC Susceptibility Measurement System for Liquid-Phase Assay

Magnetic sensing techniques have been recently developed to detect biomarkers using magnetic nanoparticles (MNPs) in liquid-phase assays. In the case of alternating current (ac) magnetic susceptibility when detecting a low number of MNPs, the diamagnetic signal of water becomes a major problem at a low signal-to-noise ratio. Therefore, we developed a high-temperature superconducting-quantum-interference-device-based ac susceptibility measurement system that can detect third-harmonic signals from MNPs. On the basis of the nonlinear characteristic of MNPs with large ac magnetic excitation (1.06 kHz), the third-harmonic signal detection (3.18 kHz) of MNPs leads to the elimination of the diamagnetic signal of water. The system consists of excitation and gradiometer pickup coils and a multifunctional circuit device. To improve the signal-to-noise ratio of the third-harmonic measurement, the system has two cancelation functions regarding the fundamental magnetic field. The MNPs are magnetized by using the fundamental magnetic field using the excitation coil. Third-harmonic signals from the MNPs were then detected during the up-and-down movement of the MNPs. We evaluated the detection sensitivity of our system using MNPs. We confirmed that the limit of detection consistency of MNPs was 10 ng by using the third-harmonic measurement method.

Vortex Flux Dynamics and Harmonic ac Magnetic Response of Ba(Fe0.94 Ni0.06)2As2 Bulk Superconductor

Vortex dynamics and nonlinear ac response are studied in a Ba(Fe0.94 Ni0.06)2As2(T c = 18.5 K) bulk superconductor in magnetic fields up to 12 T via ac susceptibility measurements of the first ten harmonics. A comprehensive study of the ac magnetic susceptibility and its first ten harmonics finds shifts to higher temperatures with increasing ac measurement frequencies (10 to 10,000 Hz) for a wide range of ac (1, 5, and 10 Oe) and dc fields (0 to 12 T). The characteristic measurement time constant t 1 is extracted from the exponential fit of the data and linked to vortex relaxation. The Anderson-Kim Arrhenius law is applied to determine flux activation energy E a /k as a function dc magnetic field. The de-pinning, or irreversibility lines, were determined by a variety of methods and extensively mapped. The ac response shows surprisingly weak higher harmonic components, suggesting weak nonlinear behavior. Our data does not support the Fisher model; we do not see an abrupt vortex glass to vortex liquid transition and the resistivity does not drop to zero, although it appears to approach zero exponentially.

Evidence of cluster-glass-like state at low temperature in anti-site disordered La1.5Ca0.5CoMnO6 double perovskite

We have investigated structural and magnetic properties having cluster-glass (CG) like ordering at low temperature in a polycrystalline La1.5Ca0.5CoMnO6 double perovskite sample. In this anti-site disordered double perovskite, ferromagnetic (FM) ordering is observed at ∼157 K. From temperature variation linear ac susceptibility measurements, a CG state is clearly observed below a freezing temperature of ∼50.1 K, the origin of which can be best explained through (1) structural disorder and (2) consequence of the coexistence and competition of antiferromagnetic and FM interactions. The linear complex ac susceptibility data near the freezing temperature are best fitted to a critical slowing down model characterized by the dynamic exponents zυ ∼ 4.95(5) and τ0 ∼ 1.23 × 10−7 s. These values are very similar to what is generally observed in a CG system. The glassy transition temperature versus Hdc23 behaviour follows the Almeida-Thouless line. The time dependent slow relaxation dynamics and aging effect have also been investigated to confirm a CG-like state formation in this compound.

Giant exchange bias in Mn2FeGa with hexagonal structure

In this study, we present the experimental observation that polycrystalline Mn2+xFe1−xGa (x = −0.2, 0, 0.2, 0.4) compounds can be synthesized to be D019-type (Ni3Sn-type) hexagonal structure with space group P63/mmc. A giant exchange bias field up to 1.32 kOe was achieved in hexagonal Mn2FeGa alloy at 5 K. A cluster glass state is confirmed by ac susceptibility measurement under different driving frequencies. Interestingly, robust horizontal and vertical shifts in magnetic hysteresis loop were simultaneously observed at 5 K under high cooling field up to 90 kOe. The large exchange bias is originated from the large exchange anisotropy between cluster glass phase and ferrimagnetic matrix. The vertical shift is thought to be attributed to the incomplete reversal of frozen cluster spins.

Magnetic and structural phase diagram of the solid solutionLaCoxMn1−xO3

We present a structural and magnetic phase diagram of the solid solution ${\text{LaCo}}_{x}{\text{Mn}}_{1\ensuremath{-}x}{\text{O}}_{3}$. We show by neutron diffraction that the monoclinic structure previously observed for the elpasolite form ${\text{La}}_{2}{\text{CoMnO}}_{6}$ (${\text{LaCo}}_{0.5}{\text{Mn}}_{0.5}{\text{O}}_{3}$) is also observed for another member of the solid solution $x\phantom{\rule{0.28em}{0ex}}=\phantom{\rule{0.28em}{0ex}}0.35$. We also present the transition temperatures for the orthorhombic/monoclinic structures of the series to the rhombohedral structure and determine the expected transition temperatures from rhombohedral to cubic symmetry. We present the magnetic structures as determined by neutron diffraction for materials with lower cobalt content and provide evidence, including ac and dc susceptibility measurements, for the possible glassy nature of the magnetism in the cobalt-rich materials in the series. Based on high-resolution neutron diffraction, we also suggest that there is a limit to the ${\text{LaCo}}_{x}{\text{Mn}}_{1\ensuremath{-}x}{\text{O}}_{3}$ solid solution at $x=0.85$. Finally we present a possible, previously unreported, low-temperature monoclinic structure for the sample ${\text{LaCo}}_{0.75}{\text{Mn}}_{0.25}{\text{O}}_{3}$.

Cobalt(II) and copper(II) covalently and non-covalently dichlorido-bridged complexes of an unsymmetrical tripodal pyrazolyl-pyridyl amine ligand: Structures, magnetism and cytotoxicity

The reaction of a methanolic solution containing MIICl2 (M=Co or Cu) with bis(3,5-dimethyl-1H-pyrazol-1-yl-1-ethyl)(2-pyridylmethyl)amine (bedmpzp) in the presence of NH4PF6 afforded the dinuclear doubly bridged-dichlorido complex [Co2(bedmpzp)2(μ-Cl)2](PF6)2 (1) and the mononuclear [Cu(bedmpzp)Cl]PF6 (2) one. The complexes were structurally and magnetically characterized. The weak ferromagnetic exchange and the axial type of magnetic anisotropy found in 1 is associated with slow relaxation of magnetization as revealed by AC susceptibility measurements. This finding puts 1 into a class of polynuclear single-molecule magnets based on 3d metals. X-ray structure of 2 revealed mononuclear nature of the complex, forming supramolecular dimers in the solid state. The non-covalent interactions of the Cu⋅⋅⋅Cl type present in its crystal structure induced a weak antiferromagnetic exchange. The results of magnetic analysis were also supported by DFT and CASSCF/NEVPT2 calculations. The in vitro cytotoxicity of the complexes against MCF7 and HeLa human cancer cell lines were also tested. The best cytotoxicity was achieved for complex 2 on HeLa, with IC50=2.5(0.9)μM.

Multiple Magnetization Reversal Channels Observed in a 3d-4f Single Molecule Magnet

The present study discusses the magnetic dynamics of a previously reported cyanide bridged 3d-4f dinuclear DyIIICoIII complex. Following the axial anisotropy suggested by previous Electron Paramagnetic Resonance spectroscopy (EPR) analysis, the complex turned out to show slow relaxation of the magnetization at cryogenic temperature, and this was studied in different temperature and field regimes. The existence of multichannel relaxation pathways that reverse the magnetization was clearly disclosed: a tentative analysis suggested that these channels can be triggered and controlled as a function of applied static magnetic field and temperature. Persistent evidence of a temperature independent process even at higher fields, attributable to quantum tunneling, is discussed, while the temperature dependent dynamics is apparently governed by an Orbach process. The broad distribution of relaxation rates evidenced by the ac susceptibility measurements suggest a relevant role of the intermolecular interactions in this system.

Porous Coordination Polymers Based on {Mn6} Single-Molecule Magnets.

In this paper, three isostructural porous coordination polymers, namely, [Mn6(μ3-O)2(sao)6(DMF)4(L(1))2/3]·4DMF·2H2O·2CH3OH (1), [Mn6(μ3-O)2(sao)6(DMF)4(L(2))2/3]·4DMF·2H2O·2CH3OH (2), and [Mn6(μ3-O)2(sao)6(DMF)4(L(3))2/3]·4DMF·4H2O·2CH3OH (3) (DMF = dimethylformamide, H2sao = salicylaldoxime, H3L(1) = benzene-1,3,5-trisbenzoic acid, H3L(2) = 4,4',4″-s-triazine-2,4,6-triyltribenzoic acid, and H3L(3) = 2,4,6-tris(4-carboxyphenoxy)-1,3,5-s-triazine), based on the oximato-bridged {Mn6} single-molecule magnet (SMM) and tricarboxylic acid ligands, were designed and synthesized. X-ray structural analysis shows that they possess a two-dimensional layered structure, where the {Mn6} moieties are linked by the corresponding (L(x))(3-) carboxylate ligands (x = 1, 2, 3) forming a huge honeycomb layer. These compounds not only show the SMM behavior as confirmed by alternative current susceptibility measurements but also show selectivity for CO2 over N2 at 273 K. On the basis of the magnetic fitting to the magnetic susceptibilities and the field dependence of magnetization for complexes 1-3, the spin ground states are S = 4. Compared with isolated {Mn6} SMMs with S = 4, the out-of-phase susceptibilities of 1-3 show obvious peaks only under the external direct-current field of 2 kOe. However, no peaks in χm″ are observed in the partially desolvated sample of compound 1.

Study of the magnetic properties of the Cex La1−x Pt alloy system: Which interaction establishes ferromagnetism in Kondo systems?

In order to study Kondo ferromagnetism, particularly of the CePt compound, we investigate the magnetic properties of the CexLa1−xPt alloy system in the temperature range from 1.8K to 320K. The results of these investigations can be summarized as follows: dc-susceptibility can be described by the Curie–Weiss law at higher temperatures down to about 100K, but also at the low temperatures above the phase transition. At higher temperatures, the extracted Curie–Weiss constant, θp, is negative in contrast to the low temperatures, where θC is positive. The extracted effective magnetic moment from the higher temperatures is the same for all the alloys and is close to the theoretical value of the isolated Ce3+ ion, μ=2.54μB, indicating the hybridization is weak and, and consequently, Kondo interaction is weak. These observations confirm the main important conclusions inferred from an earlier transport properties investigation of this alloy system. The Curie temperature extracted by various approaches was compared to the extraction from the ac-susceptibility measurements. We show that its concentration dependence is not consistent with Doniach's diagram. Hence, RKKY interaction is not responsible for the ferromagnetism in this alloy system.

Robust Magnetic Properties of a Sublimable Single-Molecule Magnet.

The organization of single-molecule magnets (SMMs) on surfaces via thermal sublimation is a prerequisite for the development of future devices for spintronics exploiting the richness of properties offered by these magnetic molecules. However, a change in the SMM properties due to the interaction with specific surfaces is usually observed. Here we present a rare example of an SMM system that can be thermally sublimated on gold surfaces while maintaining its intact chemical structure and magnetic properties. Muon spin relaxation and ac susceptibility measurements are used to demonstrate that, unlike other SMMs, the magnetic properties of this system in thin films are very similar to those in the bulk, throughout the full volume of the film, including regions near the metal and vacuum interfaces. These results exhibit the robustness of chemical and magnetic properties of this complex and provide important clues for the development of nanostructures based on SMMs.

Flux-creep activation energy for a BaFe1.9Ni0.1As2 single crystal derived from alternating current susceptibility measurements

Systematic ac susceptibility measurements have been performed to investigate the vortex dynamics in a BaFe1.9Ni0.1As2 single crystal as a function of temperature, frequency, ac field amplitude, and dc magnetic field. The complex activation energy U(T,B,j) is derived in the framework of thermally activated flux creep theory and can be expressed in one simple formula. A power law dependence of U∼Bα with α = −0.46 is observed. The activation energy reaches 104 K at low fields, suggesting strong pinning in the material. The nonlinear function of the activation energy vs. the current density is determined, which has the expression of U∝j−0.1.

Fragile singlet ground-state magnetism in the pyrochlore osmatesR2Os2O7(R=Yand Ho)

The singlet ground state magnetism in pyrochlore osmates Y$_2$Os$_2$O$_7$ and Ho$_2$Os$_2$O$_7$ is studied by DC and AC susceptibility, specific heat, and neutron powder diffraction measurements. Despite the expected non-magnetic singlet in the strong spin-orbit coupling (SOC) limit for Os$^{4+}$ ($5d^4$), Y$_2$Os$_2$O$_7$ exhibits a spin-glass (SG) ground state below 4 K with weak magnetism, suggesting possible proximity to a quantum phase transition between the non-magnetic state in the strong SOC limit and the magnetic state in the strong superexchange limit. Ho$_2$Os$_2$O$_7$ has the same structural distortion as occurs in Y$_2$Os$_2$O$_7$. However, the Os sublattice in Ho$_2$Os$_2$O$_7$ shows long-range magnetic ordering below 36\,K. The sharp difference of the magnetic ground state between Y$_2$Os$_2$O$_7$ and Ho$_2$Os$_2$O$_7$ signals the singlet ground state magnetism in $R_2$Os$_2$O$_7$ is fragile and can be disturbed by the weak $4f-5d$ interactions.

Investigation of magnetic interactions and transport mechanism in Al−substituted La0.4Bi0.6MnO3 manganites

In this paper, the effect of Al substitution on structural, magnetic, electrical and magnetotransport properties of the polycrystalline La0.4Bi0.6Mn1−xAlxO3 (where, x = 0.01, 0.05 and 0.1) have been investigated. Powder X-ray diffraction measurement and Rietveld refinement technique revealed that all the samples crystallize into orthorhombic structure having Pnma space group. The X-Ray photoemission spectra (XPS) measurements have been carried out on the samples to understand the core level states of Mn, Al and O in the samples. The lattice parameters and grain size are found to decrease with increasing Al concentration which is also evident from scanning electron micrographs. The magnetic measurements suggest the ferromagnetic nature along with trivial charge ordered phase present in the samples. Interestingly the suppression of charge ordering is observed with increasing Al substitution at Mn site. Frequency dependent ac susceptibility measurements reveal dynamic fluctuations due to cooperative interactions between antiferromagnetic domain pinning effects giving rise to spin-glass property. Electrical resistivity measurements reveal the semiconducting behavior of the sample in the entire temperature range studied. Analysis of resistivity data with temperature suggests that for T>(θD/2, the Debye temperature) the electrical transport is governed by non-adiabatic small polaron hopping mechanism. Whereas for T≤θD/2 the conduction process follows Shklovskii-Efros variable range hopping mechanism. Further the MR studies explain the grain contribution on the observed electrical transport properties.

Investigation of magnetic interactions, electrical and magneto-transport properties in Ga-substituted La0.4Bi0.6MnO3 perovskite manganites

We report on magnetic, electrical and magneto-transport properties in La0.4Bi0.6Mn1−xGaxO3 (with x=0.05 and x=0.1) samples. The phase purity and microstructural analysis of the samples have been carried out using X-ray diffraction and scanning electron microscopy technique. From the magnetization data, the studied samples show paramagnetic to ferromagnetic transition followed with glassy behavior at low temperatures. Frequency dependent ac susceptibility measurements reveal dynamic fluctuations due to cooperative interactions between antiferromagnetic domain pinning effects giving rise to glassy behavior in the studied samples. Electrical resistivity measurements suggest semiconducting behavior for the studied samples in the measured temperature range (100–300K). Our study reveals that resistivity data for T≤ (θD/2, the Debye temperature) follow power law and for T>θD/2 Shklovskii-Efros variable range hopping transport mechanism. The MR% vs. magnetic field has been studied to understand the simultaneous behavior of FM and insulating nature around T=100K existing in the samples.