Frequency Dependence Of Ac Susceptibility Research Articles

Weak Ferromagnetism and Dynamic Magnetic Behavior in a Single End‐to‐End Azide‐Bridged Nickel(II) Chain

An isolated 1D NiII chain containing single end-to-end azide bridges has been constructed (see figure). The non-centrosymmetric character of the three-atom single-bridge leads to a weak ferromagnetic behavior. Surprisingly, it shows a strong frequency dependence of AC susceptibility and magnetic hysteresis loops even at 10 K. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2089/2006/c0253_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Ac susceptibility and critical-current densities in sintered YBa2Cu3O7−δ superconductors

The field-amplitude and frequency dependent complex ac susceptibility of a well sintered YBa2Cu3O7−δ disk has been measured at 77K, which shows that its intra- and intergranular critical-current densities are determined by the collective flux creep inside the grains and the maximum Josephson currents across the grain boundaries, respectively. While the former is widely recognized, the latter contrasts to a popular belief of a mechanism of Josephson-vortex pinning and creep.

Ac magnetic susceptibility of the self-doped manganites La 0.9− xSr 0.1MnO 3 ( x = 0.02 and 0.03)

The real ( χ′) and imaginary ( χ″) components of the ac susceptibility of the self-doped manganites La 0.9− x Sr 0.1MnO 3 ( x = 0.02 and 0.03), have been studied as functions of temperature, ac field amplitude and frequency. Both χ′ and χ″ of the samples are dependent on the ac magnetic field parameters. The influence of magnetic field amplitude and frequency on the ac susceptibility increases by increasing La-vacancy. Increasing La-vacancy, the inhomogeneity of sample increases more, resulting in the formation of strong domain wall pinning centers. The observation of low temperature anomaly and amplitude and frequency dependence of ac susceptibility in the ferromagnetic region is determined mainly by the domain wall dynamics, rather than due to the glassy spin configuration or spin frustration. Activation energy, extracted from Arrhenius-like expression for the frequency and temperature in the experimental ranges could be presented with the following equation: U ( H , T ) = U 0 1 − T / T 0 H − 0.45 .

Power-law E(J) characteristic converted from field-amplitude and frequency dependent ac susceptibility in superconductors

The power-law E(J) characteristic of a superconductor may be determined by ac susceptibility (χ=χ′−jχ″) measurements from the field amplitude at which χ″ takes maximum, Hm(χm″), and frequency f. The relation between E vs J and Hm(χm″) vs f obtained from direct calculations of χ for a long cylinder and a thin disk with a power-law E(J) provides a firm base for the ac susceptibility technique of Jc determination of advanced high-temperature superconductors.

Searching for magnetic frustration-like properties in tetrahedral spin systems Cu 2Te 2O 5(Br 1−xCl x) 2

The magnetic frustration in pure and substitutionally disordered compounds from the Cu 2Te 2O 5(Br 1− x Cl x ) 2 ( x = 1 , 0.75, 0.5, 0) family of low-dimensional quantum magnets was studied experimentally by the methods of zero-field-cooled (zfc) versus field-cooled (fc) DC magnetic susceptibilities, frequency-dependent AC susceptibility and DC thermoremanent magnetization (TRM) decay. The absence of zfc–fc susceptibility splitting, frequency-independence of the AC susceptibility and zero TRM down to the temperature 2 K, that is much below the temperatures of spin ordering in these compounds, demonstrate that the spin system is different from classical magnetically frustrated systems—site-disordered spin glasses and geometrically frustrated pure antiferromagnets.

Brownian magnetic relaxation of water-based cobalt nanoparticle ferrofluids

Following the synthesis of monodispersed 20nm cobalt nanoparticles via a thermal decomposition method we have successfully transferred these hydrophobic nanoparticles into the water phase using tetramethylamonium hydroxide pentahydrate as phase transfer agent and 12-aminododecanoic acid as a stabilizing agent. Frequency dependent ac susceptibility of water-based cobalt nanoparticle ferrofluids was measured at room temperature in the frequency range of 0.01–1000Hz. In addition to the “high” frequency magnetic relaxation peak at 200Hz, which is determined by the solvent viscosity and hydrodynamic volume of the nanoparticles, a lower frequency relaxation peak attributed to the interaction of surface coatings is observed. The position of the latter varies with the solution pH in the range of 0.02–0.05Hz. This variation is explained as due to the change of chemical and charge state of the surfactant molecules. The peak at low frequency may be potentially used to study protein configuration changes in solution or other dynamic properties of biomolecules.

New Valence-Sandwich [MnII4MnIII4MnII4] Aggregate Showing Single-Molecule Magnet Behavior

The reaction of manganese(II) acetate, 1,1,1-tris(hydroxymethyl)methane (H3thme), and triethylamine in methanol leads to the formation of [Mn12O2(OMe)2(thme)4(OAc)10(H2O)4].2MeOH. The [Mn(III)4Mn(II)8] core consists of a central [Mn(III)4O6] rhombus sandwiched by two [Mn(II)4O7] fragments. Frequency-dependent ac susceptibility and hysteresis loops in the magnetization indicate single-molecule magnet behavior with a pure quantum-tunneling regime of relaxation below 0.2 K.

Spin dynamics in La 1−xSr xCoO 3 cobaltites

Magnetotransport properties, magnetization, AC susceptibilities and electron spin resonance (ESR) were measured to investigate internal spin dynamics of cobaltites La 1− x Sr x CoO 3 ( x = 0.1 , 0.2 , 0.3 ). Semiconducting behavior for x = 0.1 , 0.2 and semiconducting-metallic transition for x = 0.3 at ∼120 K were observed. With increasing Sr doping, the Curie temperature ( T C) increases. The spin-glass and cluster-glass behaviors were confirmed through the frequency-dependent AC susceptibility. ESR spectra of the samples in the temperature range studied could be divided into three regimes corresponding to different linewidths. This feature is due to both the presence of an anisotropic field arising from ferromagnetic clusters and the variation of spin-state transitions of Co ions. Lorentzian ESR signals at temperatures T< T min become Gaussian at T> T min. In a fashion similar to manganites, the peak-to-peak ESR linewidth vs. temperature, Δ H pp( T), for samples has a minimum value at temperature T min = ∼ 1.7 T C . To explain the physical mechanisms occurring in the samples, resistivity, AC susceptibility and Δ H pp( T) data were analyzed in detail.

Magnetism of sol-gel fabricated CoFe2O4∕SiO2 nanocomposites

Details of synthesis and characterization of sol-gel-produced CoFe2O4 nanoparticles embedded in the amorphous SiO2 matrix are presented together with results of an extended magnetization study of these materials. The particle size was found to increase from 6to15nm by varying the temperature of a subsequent annealing from 800to1100°C. All samples exhibited superparamagnetic behavior with values of the blocking temperature TB increasing with the particle size. At temperatures above TB the magnetization curves follow the expected Langevin scaling of M vs H∕T, which is consistent with the formation of the superparamagnetic state. For T&amp;lt;TB, the coercivity field Hc was found to be proportional to T and the frequency-dependent ac susceptibility was found to obey the Néel–Arrhenius law. Both observations are compatible with a model of noninteracting randomly oriented single-domain particles.

Frequency Response of AC Susceptibility in Melt Textured YBCO Superconductors

We have reviewed the scaling relations for frequency dependent AC susceptibility proposed by Lee. et al. and other contributions. Based on vortex glass analysis, we derived a scaling equation for the peak temperature of the imaginary part and ac magnetic field frequency f. The peak temperature Tp is found to obey: Tp = Cf1/(v(z−1))+Tg, where C is a constant, and Tg is the vortex glass temperature. The scaling relationship was applied to our melt textured YBCO superconductors, which was found in good agreement with the experimental data.

Increase in Tc and spin glass transition in La 0.9Ca 0.1Mn 1− yCr yO 3 ( y=0, 0.1 and 0.2)

We report on the structural, frequency dependent ac susceptibility, dc magnetization and magnetoresistance (MR) measurements on polycrystalline samples of La 0.9Ca 0.1Mn 1− y Cr y O 3 ( y=0, 0.1 and 0.2) prepared by sol–gel technique. For y=0, a paramagnetic to ferromagnetic transition is observed at T c=136 K. Both for y=0.1 and 0.2, T c increases from 136 to 180 K. For y=0, the imaginary part of the ac susceptibility shows a broad transition at T f< T c which does not depend very much on the frequency. However, for y=0.1 and 0.2, the frequency dependence resembles that of a spin glass. Though all the three samples show a semi-conducting behavior between 300 and 5 K, a negative MR is observed corresponding to T c and T f. The value of MR decreases for the Cr substituted samples.

Synthesis, structure, and magnetic properties of a [Mn22] wheel-like single-molecule magnet.

The synthesis and magnetic properties of the compound [Mn(22)O(6)(OMe)(14)(O(2)CMe)(16)(tmp)(8)(HIm)(2)] 1 are reported. Complex 1 was prepared by treatment of [Mn(3)O(MeCO(2))(6)(HIm)(3)](MeCO(2)) (HIm = imidazole) with 1,1,1-tris(hydroxymethyl)propane (H(3)tmp) in MeOH. Complex 1.2MeOH crystallizes in the orthorhombic space group Pbca. The molecule consists of a metallic core of 2 Mn(IV), 18 Mn(III), and 2 Mn(II) ions linked by a combination of 6 micro(3)-bridging O(2)(-) ions, 14 micro(3)- and micro(2)-bridging MeO(-) ions, 16 micro-MeCO(2)(-) ligands, and 8 tmp(3)(-) ligands, which use their alkoxide arms to bridge in a variety of ways. The metal-oxygen core is best described as a wheel made from [Mn(3)O(4)] partial cubes and [Mn(3)O] triangles. Variable-temperature direct current (dc) magnetic susceptibility data were collected for complex 1 in the 1.8-300 K temperature range in a 1 T applied field. The chi(M)T value steadily decreases from 56 cm(3) K mol(-)(1) at 300 K to 48.3 cm(3) K mol(-)(1) at 30 K and then increases slightly to reach a maximum value of 48.6 cm(3) K mol(-)(1) at 15 K before dropping rapidly to 40.3 cm(3) K mol(-)(1) at 5 K. The ground-state spin of complex 1 was established by magnetization measurements in the 0.1-2.0 T and 1.80-4.00 K ranges. Fitting of the data by a matrix-diagonalization method to a model that assumes only the ground state is populated and incorporating only axial zero-field splitting (DS(z)()(2)), gave a best fit of S = 10, g = 1.96 and D = -0.10 cm(-)(1). The ac magnetization measurements performed on complex 1 in the 1.8-8 K range in a 3.5 G ac field oscillating at 50-1000 Hz showed frequency-dependent ac susceptibility signals below 3 K. Single-crystal hysteresis loop and relaxation measurements indicate loops whose coercivities are strongly temperature and time dependent, increasing with decreasing temperature and increasing field sweep rate, as expected for the superparamagnetic-like behavior of a single-molecule magnet, with a blocking temperature (T(B)) of approximately 1.3 K.

Enhancement of Tc and the re-entrant spin glass transition inLa0.86Ca0.14Mn1−yCryO3 (y = 0,0.1 and 0.2)

We report on structural, frequency dependent ac susceptibility, dc magnetizationand magnetoresistance (MR) measurements on polycrystalline samples ofLa0.86Ca0.14Mn1−yCryO3 (y = 0, 0.1 and 0.2) prepared by the sol–gel technique. Fory = 0, a paramagnetic to ferromagnetic transition was observed atTc = 185 K.Fory = 0.1, the value ofTc = 200 K, an increase of15 K, and for y = 0.2, Tc = 195 K,anincrease of 10 K. The imaginary part of the ac susceptibility for all three samples shows a secondary transitionat Tf<Tc.For y = 0, there is no definite law accounting for the frequency dependence ofTf and it is attributed to a transition arising out of a canted structure. However, in the cases ofy = 0.1 and 0.2, the frequency dependence indicates the presence of a re-entrant spin glass transition atTf. Although all three samples show a semiconducting behaviourbetween 300 and 5 K, a negative MR was observed corresponding toTc and Tf. The value of MR decreased for the Cr substituted samples.

Characterisation and magnetic behaviour of nickel nanoparticles encapsulated in carbon

In the previous papers, we reported the synthesis of nickel nanocrystals encapsulated in an amorphous carbon matrix by a method that consists in the sonochemical decomposition of the Ni(COD) 2 organometalic precursor followed by a heating treatment in vacuum. Both as-prepared and heated samples have been further characterised in the present paper by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) to determine the microstructure and the stability of the nickel nanocrystals for long periods of air storage. It has been shown that carbon is protecting the nickel nanocrystals from oxidation leading to a magnetic behaviour of the sample congruent with the superparamagnetic properties of single domain particles. The analysis of the frequency-dependent ac susceptibility leads to two well-defined distributions of particle size consistent with those observed in TEM experiments. The effective anisotropy constant has been determined for the two particle sizes of nickel nanoparticles.

Static and dynamic magnetic properties of mechanically alloyed Fe9Cu5Ag86

The static and dynamic magnetic behaviors of mechanically alloyed Fe9Cu5Ag86 have been investigated by field-dependent DC magnetization and frequency-dependent AC susceptibility. The results reveal the coexistence of small Fe particles and ferromagnetic particles of larger size. Rietveld refinement of the X-ray diffraction patterns confirms the existence of nanometric Fe grains (similar to 6 nm) and of Ag (similar to 18 nm). Despite the low Fe content, the existence of interactions among the magnetic particles is evident, as deduced from Henkel plots. (C) 2003 Elsevier B.V. All rights reserved.

Structural disorder and magnetic frustration inALaMnMoO6(A=Ba,Sr)double perovskites

The crystal and magnetic structures of the $A{\mathrm{LaMnMoO}}_{6}$ $(A=\mathrm{B}\mathrm{a},\mathrm{S}\mathrm{r})$ double perovskites have been studied. The Sr compound is monoclinic (space-group ${P2}_{1}/n)$ and does not exhibit long-range magnetic order. The crystal lattice of the Ba compound is pseudocubic, although detailed analysis of neutron diffraction spectra together with the synchrotron diffraction data reveal a slight triclinic distortion (space-group $I1\ifmmode\bar\else\textasciimacron\fi{}).$ The magnitude of the tilting distortion is significantly reduced compared to the Sr compound. Ferrimagnetic order is observed below 80(10) K. The refined Mn moment is $1.35(7){\ensuremath{\mu}}_{B}$ at 4 K, suggesting that the ordered moment is reduced by strong frustration effects. This frustration is discussed in terms of competition between nearest-neighbor and next-nearest-neighbor superexchange interactions. Frequency dependent ac susceptibility measurements rule out the existence of a spin-glass state.

Observation of spin-glass behavior in homogeneous (Ga,Mn)N layers grown by reactive molecular-beam epitaxy

We present a detailed study of the magnetic properties of (Ga,Mn)N layers grown directly on 4H-SiC substrates by reactive molecular-beam epitaxy. X-ray diffraction and transmission electron microscopy demonstrates that homogeneous (Ga,Mn)N alloys of high crystal quality can be synthesized by this growth method up to a Mn-content of 10--12 %. Using a variety of magnetization experiments (temperature-dependent dc magnetization, isothermal remanent magnetization, frequency and field dependent ac susceptibility), we demonstrate that insulating (Ga,Mn)N alloys represent a Heisenberg spin-glass with a spin-freezing temperature around 4.5 K. We discuss the origins of this spin-glass characteristics in terms of the deep-acceptor nature of Mn in GaN and the resulting insulating character of this compound.

Strong reduction of thermally activated flux jump rate in superconducting thin films by nanodot-induced pinning centers

From frequency-dependent ac susceptibility studies of (Cu,T1)BaSrCa2Cu3Oy superconducting thin films, with and without nanodot-induced artificial pinning centers, we estimated the activation energy of flux jumps. The result was that, in the film with nanodots, the pinning potential is several times higher, leading to a probability of thermally activated flux jumps several orders of magnitude lower than in the film without artificial pinning centers. We suggest that our no cost straightforward method for creating extended defects can be successfully employed for the reduction of thermal noise in superconducting electronic devices.

Superparamagnetic-like ac susceptibility behavior in the partially disordered antiferromagnetic compoundCa3CoRhO6

We report the results of dc and ac magnetization measurements as a function of temperature (1.8 - 300 K) for the spin chain compound, Ca$_3$CoRhO$_6$, which has been recently reported to exhibit a partially disordered antiferromagnetic (PDAF) structure in the range 30 - 90 K and spin-glass freezing below 30 K. We observe an unexpectedly large frequency dependence of ac susceptibility in the T range 30 - 90 K, typical of superparamagnets. In addition, we find that there is no difference in the isothermal remanent magnetization behavior for the two regimes below 90 K. These findings call for more investigations to understand the magnetism of this compound.

Anomaly in cluster glass behaviour of Co0.2Zn0.8Fe2O4 spinel oxide

We report cluster glass behaviour mixed with short-range ferrimagnetic ordering in Co0.2Zn0.8Fe2O4 spinel oxide. The AC susceptibility data show a maximum at Tf1≈110K and a shoulder at Tf2≈212K. The DC magnetization data also show a maximum at Tm1≈110K and a shoulder at Tm2≈260K. The frequency dependence of AC susceptibility and field dependence of DC magnetization data suggest a cluster spin-glass freezing at Tf1≈110K. The appearance of shoulder in AC susceptibility data at Tf2≈212K can be explained in terms of freezing of larger size clusters associated with anisotropy distribution. This is a pseudo-phase transition related to cluster size effect, rather than spin-glass-like freezing. The AC susceptibility, DC magnetization and room temperature Mössbauer spectroscopy measurements suggest that above 260K the magnetic phase is dominated by short-range interacting clusters in paramagnetic state, called cluster-paramagnetic phase and at 110K<T⩽260K short-range ferrimagnetic phase coexist with cluster spin-glass phase, called mixed magnetic phase, in the present system.