Frequency Dependent Ac Susceptibility Measurements Research Articles

Observation of weak ferromagnetism in the antiferromagnetic host and large electronic heat capacity in β-Mn-type structured Mn3Al alloy

Numerous interesting features of antiferromagnets open up the possibility of designing new antiferromagnetic materials that can be useful for spintronic device applications. Here we present the structural, magnetic, thermal and electrical transport properties of polycrystalline Mn3Al alloys prepared by arc melting. Rietveld refinement of the room temperature X-Ray diffraction patterns reveals that the arc-melted Mn3Al crystallizes in a β Mn phase (space group 213). Detailed analysis of the magnetic data suggests for a dominant antiferromagnetic ordering. However, an in-homogenous magnetic disorder is observed in the system having a weak ferromagnetic phase in the antiferromagnetic host. In spite of the strong spin fluctuations in this system, the signature of spin glass-like nature in these alloys are not observed as probed by the frequency-dependent AC-Susceptibility measurements. Furthermore, the heat capacity was insensitive to the applied field of 3 Tesla with a negligible magnetic contribution suggesting a large electronic contribution to the heat capacity in these alloys. This work provides a useful insight into the tunable physical properties of the Heusler alloys for exploring antiferromagnetic based spintronic applications.

Vortex Dynamics and Pinning in CaKFe4As4 Single Crystals from DC Magnetization Relaxation and AC Susceptibility

Among various “families” of iron-based superconductors, the quite recently discovered AeAFe4As4 (where Ae is an alkali-earth metal and A is an alkali metal) has high critical current density, a very high upper critical field, and a low anisotropy, and has recently received much interest for the possibility of high magnetic field applications at the liquid hydrogen temperature. We have performed DC magnetization relaxation and frequency-dependent AC susceptibility measurements on high-quality single crystals of CaKFe4As4 with the aim of determining the pinning potential U*. The temperature dependence of U* displays a clear crossover between elastic creep and plastic creep. At temperatures around 27–28 K, U* has a very high value, up to 1200 K, resulting in an infinitesimally small probability of thermally activated flux jumps. From the dependence of the normalized pinning potential on irreversible magnetization, we have determined the creep exponents in the two creep regimes, which are in complete agreement with theoretical models. The estimation of the pinning potential from multifrequency AC susceptibility measurements was possible only near the critical temperature due to equipment limitations, and the resulting value is very close to the one that resulted from the magnetization relaxation data. Magnetic hysteresis loops revealed a second magnetization peak and very high values of the critical current density.

Griffiths phase, re-entrant spin-glass behaviour and schottky anomaly in anti-site disordered double perovskite Pr2MnNiO6

In the present study, the effect of anti-site disorder on the magnetic properties of Pr2MnNiO6 is explored. Due to anti-site disorder, a reduced T C preceded by a Griffith phase has been observed. At low temperatures, we also report the development of the spin glass phase in co-existence with the cluster-like ferromagnetic order. The signature of the re-entrant spin glass phase, where the material is ferromagnetic below T c, and undergoes a transition to the spin glass state, is revealed by the frequency-dependent ac-susceptibility measurements. The spin glass behavior is also supported by the slow decay of thermo-remanent magnetization. The low temperature specific heat shows a small field dependent behaviour which may be attributed to Schottky effects.

Kink distortion of the pseudo-S4 axis in pseudotetrahedral [N2O2] bis-chelate cobalt(II) single-ion magnets leads to increased magnetic anisotropy.

Two new mononuclear cobalt(II) complexes with the general formula [Co(L1,2)2] (1 and 2) were synthesized using bidentate Schiff base ligands with NO donor set, namely, 2-(benzothiazole-2-ylimino)methyl-5-(diethylamino)phenol (HL1) and its methyl substituted derivative 2-(6-methylbenzothiazole-2-ylimino)methyl-5-(diethylamino)phenol (HL2). X-ray structure analysis reveals a distorted pseudotetrahedral coordination sphere at the cobalt(II) ion, that cannot be described by a simple twisting of the two ligand chelate planes with respect to each other, which would imply a rotation about the pseudo-S4 axis of the complex. Such a pseudo-rotation axis would approximately be colinear with the two vectors defined by the cobalt ion and the two centroids of the chelate ligands, where the angle κ between the two vectors would be 180° in an ideal pseudotetrahedral arrangement. For complexes 1 and 2, the observed distortion can be characterized by a significant bending at the cobalt ion with angles κ of 163.2° and 167.4°, respectively. Magnetic susceptibility and FD-FT THz-EPR measurements together with ab initio calculations reveal an easy-axis type of anisotropy for both complexes 1 and 2, with a spin-reversal barrier of 58.9 and 60.5 cm-1, respectively. For both compounds, frequency-dependent ac susceptibility measurements show an out-of-phase susceptibility under applied static fields of 40 and 100 mT, which can be analyzed in terms of Orbach and Raman processes within the observed temperature range.

Pinning Potential of the Self-Assembled Artificial Pinning Centers in Nanostructured YBa2Cu3O7-x Superconducting Films.

For high-field power applications of high-temperature superconductors, it became obvious in recent years that nano-engineered artificial pinning centers are needed for increasing the critical current and pinning potential. As opposed to the artificial pinning centers obtained by irradiation with various particles, which is a quite expensive approach, we have studied superconducting samples having self-assembled defects, created during the sample fabrication, that act as effective pinning centers. We introduced a simple, straight-forward method of estimating the frequency-dependent critical current density by using frequency-dependent AC susceptibility measurements, in fixed temperatures and DC magnetic fields, from the positions of the maxima in the dependence of the out-of-phase susceptibility on the amplitude of AC excitation magnetic field. The results are compatible with a model that stipulates a logarithmic dependence of the pinning potential on the probing current. A mathematical derivation allowed us to estimate from the experimental data the pinning potentials in various samples, and in various DC magnetic fields. The resulted values indicate large pinning potentials, leading to very small probability of magnetic flux escaping the pinning wells, hence, leading to very high critical currents in high magnetic fields.

Structure and magnetic phase transitions in (Ni1−xCox)Cr2O4 spinel nanoparticles

NiCr2O4 and CoCr2O4 are two distinct members of the chromite family (ACr2O4) having unique structural and magnetic properties. This paper is a continuation of previous work [Mohanty et al., J. Magn. Magn. Mater.451(2018)20] and is focused on the modification of ferrimagnetic properties in NiCr2O4 by substituting Co ions at Ni sites. In order to do so (Ni1−xCox)Cr2O4 (x = 0, 0.50, 0.75, 1.00) powder samples were synthesized using chemical co-precipitation techniques. Interest is primarily on (Ni0.5Co0.5)Cr2O4 and (Ni0.25Co0.75)Cr2O4 as characterization results on the end members have been reported. It was determined with in-situ temperature dependent X-ray diffraction measurements that these compositions become fully crystallized cubic spinels with space group Fd3-m above 700 °C. All samples were subsequently calcined at 900 °C to ensure phase purity and uniform crystallinity. Transmission electron microscopy showed non-uniform distribution of particle sizes with the majority of the particles having bi-pyramidal structures with flat tops for both samples. A comparative account of different ferrimagnetic Curie temperature, θf or TC, estimations are presented as determined from measured temperature dependences of the following techniques: Fits of the magnetic susceptibility data indicated a minimum value for the (Ni0.5Co0.5)Cr2O4 sample at 56.1 ± 0.9 K, followed by an increase to 76.8 ± 0.4 K for the (Ni0.25Co0.75)Cr2O4 sample; Neutron diffraction measurements gave values of 80 ± 1 K and 87 ± 1 K; Their paramagnetic Curie points, θp, were determined to be 73.3 and 84.9 K, respectively. The magnetic frustration index f = ΘCW/TC was found to decrease continuously over the series from 11.9 for NiCr2O4 to 7.1 and 6.5 for x = 0.5 and 0.75 respectively with (Ni0.5Co0.5)Cr2O4 being more frustrated than (Ni0.25Co0.75)Cr2O4. Magnetization as a function of applied magnetic field measured at 3 K demonstrates unusually high coercivity for (Ni0.5Co0.5)Cr2O4, supporting the notion for the higher degree of magnetic frustration in this sample. No exchange bias was observed for M (μ0H) in the (Ni0.25Co0.75)Cr2O4 sample measured at 3 K, but did however show a significant anomaly in its hysteresis loop at this temperature. The anomaly in hysteresis behaviour disappeared when measured at 1.7 K and also at higher temperatures. Frequency dependent ac-susceptibility measurements revealed no shift in the peak position with frequency refuting the existence of spin-glass like behavior in both these samples.

Synthesis and temperature dependent magnetic properties of nanocrystalline Ni0.5Zn0.5Fe2O4 ferrites

nanoparticles were successfully synthesized by modified citrate gel method. X-ray diffraction (XRD) analysis confirmed the development of single-phase cubic spinel structure with Fd m space group. The cation distribution was estimated by Rietveld refinement of XRD pattern and found that the prepared sample has a mixed spinel structure. The high field regions of magnetic hysteresis loops were modeled using the Law of Approach to Saturation to determine the saturation magnetization () and magnetic anisotropy constant () for the prepared nanoparticles. It is found that the values of and increases significantly with decreasing temperature. However, temperature dependence of saturation magnetization () and coercivity () were found to have considerable deviations from the Bloch’s and Kneller’s laws, respectively. This behavior is elucidated in terms of finite size effects, disordered surface spins and interparticle interactions in the prepared nanoparticles. Furthermore, the effects of interparticle interactions were investigated by using Vogel–Fulcher law for frequency dependent AC susceptibility measurements.

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.

Magnetic nature of the austenite–martensite phase transition and spin glass behaviour in nanostructured Mn2Ni1.6Sn0.4 melt-spun ribbons

Nanocrystalline ribbons of inverse Heusler alloy Mn2Ni1.6Sn0.4 have been synthesised by melt spinning of the arc-melted bulk precursor. The single-phase ribbons crystallize into a cubic structure and exhibit very fine crystallite size of <2 nm. Temperature-dependent magnetization (M–T) measurements reveal ferromagnetic–austenite (FM-A)–antiferromagnetic–martensite (AFM-M) phase transition that begins at MS ≈ 249 K and finishes at Mf ≈ 224 K. During warming, the reverse AFM-M to FM-A transitions begins at As ≈ 240 K and finishes at Af ≈ 261 K. A re-entrant FM transition is observed in the M-phase at \(T_{\text{CM}}\) ≈ 145 K. These transitions are also confirmed by temperature-dependent resistivity (ρ–T) measurements. The hysteretic behaviour of M–T and ρ–T in the temperature regime spanned by the A-M transition is a manifestation of the first-order phase transition. M–T and ρ–T data also provide unambiguous evidence in favour of spin glass at \(T < T_{\text{CM}}\). The scaling of the glass freezing temperature (Tf) with frequency, extracted from the frequency-dependent AC susceptibility measurements, confirms the existence of canonical spin glass at \(T < T_{\text{CM}}\) ≈ 145 K. The occurrence of canonical spin glass has been explained in terms of the nanostructuring modified interactions between the coexisting FM and AFM correlations in the martensitic phase.

High-Field Pinning Potential in YBCO Films with Nanoengineered Pinning Centres

Frequency-dependent AC susceptibility measurements have been used to estimate the critical current densities and pinning potential of two 1.1-μm-thick YBCO films with nanoengineered pinning centres. The films were grown by pulsed laser deposition using a target with 4 % BaZrO3 nanoinclusions. Data were analysed in the framework of the Anderson-Kim model, collective pinning model, and a model due to Zeldov that implies a logarithmic dependence of pinning potential on the current density. It was found that the latter describes better our experimental data, and that the resulting pinning potential has reasonable high values in high magnetic field up to 9 T, indicating a high current-carrying capability in high magnetic fields.

Memory effects in exchange coupled Fe/Co3O4 nanocomposites

Nanocomposites comprising a ferromagnet and an antiferromagnet have drawn attention recently because of their interesting physical properties and variety of technological applications. In the present work, structural, hyperfine and magnetic properties of Fe/Co3O4 nanocomposites prepared by a chemical route having 10–70 wt.% of Fe, have been investigated. XRD and TEM measurements confirmed polycrystalline nature of the samples having grain size in the nanometer regime. FTIR measurements show the presence of two bands of Co–O corresponding to Co2+ and Co3+. Mössbauer spectra recorded at room temperature confirm the presence of Fe in the blocked state. Presence of exchange bias at Fe–Co3O4 interfaces is confirmed by the magnetization measurements. Irreversibility in temperature dependent FC–ZFC measurements points to interface effect. Frequency dependent ac susceptibility measurements as well as memory effect observed in dc magnetization measurements indicate the superspin glass nature of the nanocomposites.

Magnetic properties of Co1+yAl2−yO4–SiO2 nanocomposites synthesized by sol–gel method

Nanocomposites made of magnetic spinel oxide Co1+yAl2−yO4 particles dispersed in an amorphous SiO2 matrix were synthesized by a sol–gel technique. Samples with various mass fractions of Co1+yAl2−yO4 in composite x, and with various Co concentrations in spinel y were studied. The grain sizes were below 100 nm with a large size distribution. The paper represents thorough investigation of magnetic properties, but special attention was given to peculiar effects and low temperature magnetic frozen states. Magnetic measurements reveal multiple magnetic ground states at the lowest temperature. The frequency dependent ac susceptibility measurements indicate spin-glass behavior of these ground states at low temperatures. The samples, with higher concentration x of Co1+yAl2−yO4 in SiO2, showed very similar Curie–Weiss paramagnetic behavior with antiferromagnetically interacting Co ions (θ ≈ −100 K). Other samples, with lower x, showed complex behavior, spin-glass magnetic freezing at the lowest temperatures and lower absolute value of θ and consequently lower exchange constant.

Relaxation phenomena in ensembles of CoFe2O4 nanoparticles

Collective magnetic behavior of CoFe2O4 nanoparticles with diameters of 76, 16, 15 and 8nm, respectively, prepared by different chemical methods has been investigated. Particle composition, size and structure have been characterized by inductive coupled plasma (ICP), transmission electron microscopy (TEM) and powder X-ray diffraction (PXRD). Basic magnetic properties have been determined from the temperature dependence of magnetization and magnetization isotherms measurements. The three samples exhibit characteristic of a superparamagnetic system with the presence of strong interparticle interactions. Magnetic relaxation phenomena have been examined via frequency-dependent ac susceptibility measurements and aging and memory effect experiments. For the particles coated with oleic acid, it has been demonstrated that the sample reveals all attributes of a super-spin glass (SSG) system with strong interparticle interactions.

Coexistence of spin glass behavior and long-range ferrimagnetic ordering in La- and Dy-doped Co ferrite

The structure, dc magnetization and ac susceptibility characteristics of the rare-earth (R = La,Dy) ionsubstituted cobalt-ferrites (CoO.Fe1.925La0.075O3 and CoO.Fe1.925Dy0.075O3) are evaluated. R-substituted Co-ferrites crystallize in the cubic inverse spinel phase. The irreversible temperature (Tirr) between zero field cooled (ZFC) and field cooled (FC) magnetization for CoO.Fe1.925La0.075O3 and CoO.Fe1.925Dy0.075O3 determined from the temperature variation of magnetization measurements are 283 and 292 K, respectively. The broadening of ZFC magnetization and more than one maximum indicates the coexistence of short-range ferrimagnetic clusters of different size with a long-range ferrimagnetic phase. Magnetization curves indicate no saturation up to 30 kOe suggesting the canted spin structure inside the clusters. The relaxation times of spin clusters calculated using theVogel−Fulcher law for the frequency-dependent ac susceptibility measurements are on the order of ∼10−6 s.

Site determination of Zn doping in protein encapsulated ZnxFe3−xO4 nanoparticles

The x-ray absorption spectra of the Fe and Zn L edges for 6.7nm Fe3O4 nanoparticles grown inside 12nm ferritin protein cages with 10%, 15%, 20%, and 33% zinc doping show that Zn is substitutional as Zn2+ within the iron oxide host structure. A Neel–Arrhenius plot of the blocking temperature in frequency dependent ac-susceptibility measurements shows that the particles are noninteracting and that the anisotropy energy barrier is reduced with Zn loading. X-ray magnetic circular dichroism of the Fe displays a linear decrease with Zn doping in sharp contrast to the initial increase present in the bulk system. The most plausible explanation for the decrease in moment is that Zn substitutes preferentially into the tetrahedral A site as a Zn2+ cation, generating a mixed spinel.

The dynamics of magnetic ordering in a natural hemo-ilmenite solid solution

SUMMARY We investigated the micromagnetic properties of hemo-ilmenite particles in an alluvial soil. All magnetic accessory minerals except the weathering resistant hemo-ilmenite grains were removed from the soil matrix by chemical treatment with concentrated acid followed by magnetic separation. X-ray diffraction revealed hemo-ilmenite grains with single crystal properties and an ilmenite mole fraction of y = 0.86. Magnetization versus applied magnetic field plots in a temperature range between 6 and 300 K were recorded in order to study the hysteresis and the exchange properties. In addition, field and frequency-dependent AC susceptibility measurements were performed with and without a DC bias field in order to analyse the dynamic magnetization of the sample down to 3 K. The hemo-ilmenite particles are considered as a mixed system with nano-sized cationordered areas (COA) and cation-disordered areas (CDA), which differ in their local Fe (III) concentration. Ferrimagnetic single-domain order in the Fe (III) -enriched CDA started at about 220 K. Upon cooling gradual transdomain transformation generates multidomain order. A maximum in the blocking distribution was reached at 44 K, followed by the onset of spin-glass dynamics. At lower temperature, blocking of superparamagnetic clusters in the COA created antiferromagnetic (AFM) ordering, which became more prominent with decreasing temperature. The interaction between the spin-glass like CDA and the AFM areas was documented by the onset of exchange bias at T < 20 K. The occurrence of exchange bias as well as spin-glass dynamics in the hemo-ilmenite grains is probably an effect of the inhomogeneity of the local Fe (III) concentration. This effect leaves a magnetically competitive regime with areas showing ilmenite-like magnetic properties, and Fe (III) rich disordered areas with magnetic long-range ordering up to 220 K and frustration near the ordering temperature of ilmenite.

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.

A charge-transfer salt magnet based on a non-cyanocarbon acceptor, 1,4,9,10-anthracenetetrone and decamethylferrocene

The charge-transfer (CT) salt approach to the synthesis of magnetic solids has been shown to be an attractive and general route to the creation of new families of molecule-based magnetic solids. It is based on reactions between building block donors and acceptors, where new candidates for the latter role can be readily identified by their shape and by their electrochemical properties. In particular, acceptors should be flat (to facilitate stacking) and exhibit reversible electrochemical reduction at potentials that are not too negative (for promoting the creation of the radical anion). Fortunately, these same criteria have previously been used to identify one-electron acceptors for the synthesis of CT salt conductors. Although one such molecule, 1,4,9,10-anthracenetetrone has not yielded good organic metals in its reactions with alkali metals or tetrathiafulvalene and related molecules, it was found to give a magnet ( T c = 3 K ) in its reaction with decamethylferrocene. Frequency-dependent ac susceptibility measurements indicate glassiness. Significantly, this result shows that nitrile functional groups on the acceptor are not necessary to obtain magnetic ordering in CT salts.

Carrier-induced magnetic properties in small-gal semiconductors

In the small-gap semiconductor Pb1-x-ySnyMnxTe the magnetic phase transition from a ferromagnetic to a spin-glass state at high carrier concentrations is studied. The authors present new data on magnetization, AC susceptibility and specific heat for various carrier concentrations showing the transition. They also present frequency-dependent AC susceptibility measurements. For the spin-glass sample there is a strong dependence on the measuring frequency, which follows the Vogel-Fulcher law. Furthermore, they calculate the phase boundary in the x-p magnetic phase diagram using the mean random field model. There is a satisfactory agreement between the experimental data and the calculations.