Ac Susceptibility Measurements Research Articles

Pinning Energy and Evidence of Granularity in the AC Susceptibility of an YBa2Cu3O7-x Superconducting Film

The study of granularity in superconducting films by using AC susceptibility has a crucial role in the development of and improvement in the ReBCO-coated conductors, which are a constantly evolving reality in the modern power applications of superconductivity. Specifically, the study of the granularity is essential because the ReBCO superconducting wires and tapes are far from the regularity of a single crystal while they often present an inter- and intragranular contribution to the critical current density. On the other hand, the AC susceptibility is a key part of the characterization of a granular sample because this technique is very sensitive to the presence of granularity in the superconductors and, moreover, the study of its first harmonic allows for determining pivotal properties such as the pinning energy as well as the dissipation processes acting in the sample. The pinning energy values and the granularity of an YBCO thin film have been studied by means of AC susceptibility measurements as a function of the AC amplitude, temperature, and DC field. In particular, the first harmonic imaginary component of the AC susceptibility χ1″ related to the dissipation processes of the sample has been studied. First, starting from the Brandt approach, the critical current density Jc and the pinning energy U of the sample have been extracted at 77 K by using the χ1″ measurements as a function of the AC amplitude at different AC frequencies and DC fields. From these measurements, a first signal of granularity appears. In order to confirm it, the temperature dependence of the χ1″ at different DC fields has been studied and a contribution deriving from the inter- and intragranular part of the sample has emerged. By taking the temperature corresponding to the crossover between the two contributions at the different DC fields, the intergranular and intragranular response has been separated. Successively, the temperature has been fixed to 77 K, together with an AC frequency equal to 1597.9 Hz, and the χ1″ as a function of the DC field at different AC amplitudes has been analyzed showing a clear presence of granularity in all the curves. By drawing the contour plot of the χ1″ with the DC and AC values, it was possible to determine the best parameters to put at 77 K in order to exploit the material for applications.

Effects of Mn and Ru substitutions on the electrical properties and AC susceptibility of Tl(BaSr)CaCu2O7 superconductor

In this paper, the effects of M = Mn and Ru substitutions at the Tl site of Tl[Formula: see text]Mx(BaSr)CaCu2O7 (Tl-1212) on the electrical properties and AC susceptibility were investigated. The X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) methods were used to examine the phase and microstructure, respectively. All samples showed diffraction patterns with Tl-1212 as the major phase and an increase in the Tl-1223 phase with increasing Mn and Ru substitutions. The microstructure showed partial melting and randomly oriented grains in all samples. The electrical resistance measurements showed metallic normal state behavior for all samples. The zero-resistance transition temperature [Formula: see text] was the highest for x = 0.4 where [Formula: see text] was 107[Formula: see text]K and 90[Formula: see text]K for Mn and Ru substitutions, respectively. The superconducting transition, [Formula: see text] obtained from AC susceptibility measurements increased with Mn and Ru substitutions up to x = 0.4. Using Bean’s model, the current density at the peak temperature of the imaginary part of the AC susceptibility, [Formula: see text]([Formula: see text] was determined to be between 16 and 22[Formula: see text]A[Formula: see text]⋅[Formula: see text]cm[Formula: see text] for Mn, and between 15 and 19[Formula: see text]A[Formula: see text]⋅[Formula: see text]cm[Formula: see text] for Ru-substituted samples. Mn substitution showed a higher transition temperature compared with Ru. This study showed that optimization of the average Cu valence was achieved with Mn and Ru substitutions for x = 0.4 at the Tl site. The ionic radii of the substituting and substituted elements are important factors in enhancing the transition temperature of Tl(BaSr)CaCu2O7 superconductor.

Physical properties of MnFeAl-based alloys affected by Mn content and annealing

The present study is devoted to the Mn2.4Fe0.8Al0.8 and Mn2FeAl alloys prepared by induction melting and studied in their original and subsequently annealed states. The annealing was carried out at 773 K/100 h and 1073 K/100 h in the argon atmosphere. The microstructure, phase composition, magnetic properties, and atom arrangement are followed with regard to Mn content and annealing conditions.The scanning electron microscopy completed by the energy dispersive X-ray spectroscopy and neutron activation analysis has detected single-phase alloys with compositions close to the nominal ones. Their structure, analyzed by X-ray diffraction, was found to be primitive cubic β-Mn with the lattice parameters of 0.6359(2) nm for Mn2.4Fe0.8Al0.8 and 0.6339(1) nm for Mn2FeAl. The coherent potential approximation calculations and positron annihilation spectroscopy have allowed obtaining an overview to the arrangement of Mn, Fe, and Al atoms in the β-Mn structure and formation of the open volume defects. It is shown that Mn atoms occupy predominantly 8c Wyckoff sites and remaining Mn, Fe and Al atoms occupy 12d sites in equal 1/3 proportion. The open volume defects, vacancies and vacancy clusters, occur in both alloys and both annealed states in a very low concentration. From the magnetic viewpoint, both alloys in the as-prepared state and after annealing at 773 K/100 h are paramagnetic at room temperature with transition to antiferromagnetic state at about 30-40 K. The ac susceptibility measurements have indicated spin glass nature of the Mn2FeAl alloys. The annealing at a higher temperature, 1073 K/100 h, has affected mainly Mn2.4Fe0.8Al0.8 alloy manifesting a weak ferro-/ferrimagnetic contribution at room temperature contributing to a strong magnetic ordering below 42 K.

Magnetic Anisotropy and Relaxation in Four-Coordinate Cobalt(II) Single-Ion Magnets with a [CoIIO4] Core.

A mononuclear four-coordinate Co(II) complex with a [CoIIO4] core, namely, PPN[Li(MeOH)4][Co(L)2] (1) (PPN = bis(phosphoranediyl)iminium; H2L = perfluoropinacol), has been studied by X-ray crystallography, magnetic characterization, and theoretical calculations. This complex presents a severely distorted coordination geometry. The O-Co-O bite angle is 83.42°/83.65°, and the dihedral twist angle between the O-Co-O chelate planes is 55.6°. The structural distortion results in a large easy-axis magnetic anisotropy with D = -104(1) cm-1 and a transverse component with |E| = +4(2) cm-1. Alternating current (ac) susceptibility measurements demonstrate that 1 exhibits slow relaxation of magnetization at zero static field. However, the frequency-dependent out-of-phase (χ"M) susceptibilities of 1 at 0 Oe do not show a characteristic maximum. Upon the application of a dc field or the dilution with a diamagnetic Zn matrix, the quantum tunneling of magnetization (QTM) process can be successfully suppressed. Notably, after dilution with the Zn matrix, the obtained sample exhibits a structure different from that of the pristine complex. In this altered sample, the asymmetric unit does not contain the Li(MeOH)4+ cation, resulting in an O-Co-O bite angle of 86.05° and a dihedral twist angle of 75.84°, thereby leading to an approximate D2d symmetry. Although such differences are not desirable for magnetic studies, this study still gives some insights. Theoretical calculations reveal that the D parameter is governed by the O-Co-O bite angle, in line with our previous report for other tetrahedral Co(II) complex with a [CoIIN4] core. On the other hand, the rhombic component is found to increase as the dihedral angle deviates from 90°. These findings provide valuable guidelines for fine-tuning the magnetic properties of Co(II) complexes.

Quantum spin liquid ground state in the trimer rhodate Ba4NbRh3O12

Frustrated magnets offer a plethora of exotic magnetic ground states, including quantum spin liquids (QSLs), in which enhanced quantum fluctuations prevent a long-range magnetic ordering of the strongly correlated spins down to lowest temperature. Here we have investigated the trimer based mixed valence hexagonal rhodate Ba4NbRh3O12 using a combination of dc and ac magnetization, electrical resistivity, specific heat, and muon spin rotation/relaxation (μSR) measurements. Despite the substantial antiferromagnetic exchange interactions, as evident from the Weiss temperature (θW∼−35 to −45K), among the Rh-local moments, neither long-range magnetic ordering nor spin freezing is observed down to at least 50 mK, in ac-susceptibility, specific heat, and zero-field μSR measurements (down to 0.26 K). We ascribe the absence of any magnetic transition to enhanced quantum fluctuations as a result of geometrical frustration arising out of the edge-sharing equilateral Rh-triangular network in the structure. Our longitudinal-field μSR result evidences persistent spin fluctuations down to 0.26 K, thus stabilizing a dynamic QSL ground state in Ba4NbRh3O12. Furthermore, the magnetic specific heat data at low T reveal a significant T-linear contribution plus a quadratic T dependence, which may indicate the gapless Dirac QSL phenomenology of the spinon excitations with a linear dispersion. Published by the American Physical Society 2024

Unraveling the origin of the Griffiths phase in Ho2NiMnO6: perspectives from linear and nonlinear AC susceptibility

In this study, we investigated the magnetic ordering and underlying mechanism of the Griffiths phase, observed in Ho2NiMnO6 through AC susceptibility measurements. Our results indicate that the transition around 86 K corresponds to a paramagnetic to ferromagnetic transition characterized by classical magnetic ordering. Notably, nonlinear AC susceptibility measurements revealed the existence of ferromagnetic clusters within a paramagnetic background well above the transition temperature, establishing this as the origin of the Griffiths-like phase within the Ni/Mn sublattice of Ho2NiMnO6. Our study on the Ho2NiMnO6 system provides insight into the intricate magnetic phenomena common to various other strongly correlated electron systems.

Incidence of the Brownian Relaxation Process on the Magnetic Properties of Ferrofluids.

Ferrofluids containing magnetic nanoparticles represent a special class of magnetic materials due to the added freedom of particle tumbling in the fluids. We studied this process, known as Brownian relaxation, and its effect on the magnetic properties of ferrofluids with controlled magnetite nanoparticle sizes. For small nanoparticles (below 10 nm diameter), the Néel process is expected to dominate the magnetic response, whereas for larger particles, Brownian relaxation becomes important. Temperature- and magnetic-field-dependent magnetization studies, differential scanning calorimetry, and AC susceptibility measurements were carried out for 6 and 13.5 nm diameter magnetite nanoparticles suspended in water. We identify clear fingerprints of Brownian relaxation for the sample of large-diameter nanoparticles as both magnetic and thermal hysteresis develop at the water freezing temperature, whereas the samples of small-diameter nanoparticles remain hysteresis-free down to the magnetic blocking temperature. This is supported by the temperature-dependent AC susceptibility measurements: above 273 K, the data show a low-frequency Debye peak, which is characteristic of Brownian relaxation. This peak vanishes below 273 K.

Heptanuclear Mixed-Valence Co4IIICo3II Molecular Wheel─A Molecular Analogue of Layered Double Hydroxides with Single-Molecule Magnet Behavior and Electrocatalytic Activity for Hydrogen Evolution Reactions.

We present a bifunctional heptanuclear cobalt(II)/cobalt(III) molecular complex formulated as [Co7(μ3-OH)4(H2L1)2(HL2)2](NO3)6·6H2O (1) (where H5L1 is 2,2'-(((1E,1'E)-((2-hydroxy-5-methyl-1,3-phenylene)bis(methanylylidene))bis(azanylylidene))bis(propane-1,3-diol)) and H2L2 is 2-amino-1,3-propanediol). Compound 1 has been characterized by single-crystal X-ray diffraction analysis along with other spectral and magnetic measurements. Structural analysis indicates that 1 contains a mixed-valence Co7 cluster where a central Co(II) ion is connected to six different Co centers (four CoIII and two CoII ions) by four μ3-OH groups, giving rise to a planar heptanuclear cluster that resembles a molecular fragment of a layered double hydroxide (LDH). Two triply deprotonated (H2L1)3- ligands form the outer side of the cluster while two singly deprotonated (HL2)- ligands are located at the top and bottom of the central heptanuclear core. Variable temperature magnetic measurements indicate the presence of weak ferromagnetic CoII···CoII interactions (J = 3.53(6) cm-1) within the linear trinuclear CoII cluster. AC susceptibility measurements show that 1 is a field-induced single-molecule magnet (SMM) with τ0 = 8.2(7) × 10-7 s and Ueff = 11.3(4) K. The electrocatalytic hydrogen evolution reaction (HER) activity of 1 in homogeneous phase shows an overpotential of 455 mV, with a Faradaic efficiency of 81% and a TOF of 8.97 × 104 μmol H2 h-1 mol-1.

Signatures of nearly compensated magnetism and spin glass behavior in highly frustrated β-Mn-type Mn50Fe25+xAl25-x Heusler alloys.

The present study examines the effect of Fe/Al concentration on the structural and magnetic properties of Mn-rich Mn50Fe25+xAl25-x (x = 5, 10, 15) Heusler alloys through x-ray diffraction, temperature- and field-dependent DC magnetization, thermoremanent magnetization, magnetic memory effect, AC susceptibility measurements, and DFT calculations. The samples crystallize in a cubic β-Mn structure. The trend shows a reduction in lattice parameters (unit cell volume) with the increasing Fe proportion. These alloys exhibit strong antiferromagnetic interactions with large frustration parameters, indicating the presence of competing magnetic interactions. The DC magnetization data reveal spin glass-like features with a peak at spin glass freezing temperature (Tf). The observation of bifurcation in temperature-dependent zero-field-cooled and field-cooled magnetization curves, exponential dependence of the temperature variation of remanence and coercivity, magnetic relaxation, and magnetic memory effect below Tf support the spin-glass character of these alloys. The frequency dependence of Tf is also examined in the context of dynamic scaling laws, such as the Vogel-Fulcher law and critical slowing down model, which further supports the presence of spin glass behavior. In the theoretical DFT calculations, the electronic structure is found to be metallic and similar for both spin projections. Moreover, the antiferromagnetic arrangement of the magnetic moments, in line with the experimental observations, is stabilized by exchange interactions, resulting in an almost compensated total magnetic moment of 0.02-0.38µB/f.u. This is probably caused by the frustrated structure and non-stoichiometric compositions of Mn50Fe25+xAl25-x.

Experimental studies on noncentrosymmetric Nd3Se4: Strongly correlated noncollinear ferromagnet

We have synthesized polycrystalline Nd3Se4 by the solid state sealed tube reaction route followed by structural characterization and magnetic property measurements. The temperature-dependent magnetization studies reveal the existence of competing magnetic interactions below the ferromagnetic ordering (TC ≈ 48.4 K). The observation of hysteresis in the isothermal magnetization M(H) data with the coercive field of ≈ 0.25 T at 2 K indicates dominant ferromagnetic interactions in the applied field. The nonsaturating behavior of M(H) data at higher applied field and kinks in corresponding dM/dHvs. H plots suggest the coexistence of multiple magnetic interactions at low temperatures. Using the M(T,H=2 T) data, the Rhodes-Wohlfarth ratio (qc/qs) has been estimated to be ≈ 3.6, which supports the itinerant magnetism in Nd3Se4. These competing interactions have been further investigated by AC-susceptibility measurements at various excitation frequencies. The χ′ reduces drastically with an increase in the applied DC field, as observed from isothermal field-dependent AC-χ studies. Additionally, the strong correlation between delocalized charge and localized magnetic moments is evident from ρ(T) measurements at H = 0 and 5 T. The ρ(T,H=0 T) data shows a clear drop having quadratic temperature dependence, below TC. Furthermore, the inter-dependence between electrical transport and magnetism has been established firmly through negative magnetoresistance, in which the maxima correspond to the coercive field obtained from M(H) data.

Spin-Cluster Glassy and Long-Range Ordered Magnetic States in Honeycomb-Layered Compositionally Complex Oxides Na3-xLixT2SbO6 (T = Cu1/3Ni1/3Co1/3).

The concept of high-entropy oxides has triggered extensive research of this novel class of materials because their numerous functional properties are usually not mere linear combinations of those of the components. Here, we introduce the new series of compositionally complex honeycomb-layered magnets Na3-xLixT2SbO6 (T = Cu1/3Ni1/3Co1/3). An unusual feature of the system is its nonmonotonous dependences of the monoclinic lattice parameters b and β on x. Rietveld refinement of the crystal structures of the Na and Li end members reveals apparent Sb-T site inversion in the former and considerable Li-Cu site inversion in the latter. The materials are characterized by measurements of specific heat Cp, magnetization M, and ac and dc magnetic susceptibility χ. Na3T2SbO6 exhibits sharp long-range antiferromagnetic order (TN = 10.2 K) preceded by noticeable correlation effects at elevated temperatures. The magnetic phase diagram of Na3T2SbO6 is established. Introduction of Li, just at x = 0.8, destroys AFM order, resulting in spin-cluster glass behavior attributed to Li/Cu inversion, with TG growing with x to 10.4 K at x = 3.

Exploration of Griffiths phase, spin glass behaviour and memory effect in a frustrated Al2MnCoO7 pyrochlore compound

Al2MnCoO7 pyrochlore compound has been synthesized following the multistep solid-state ceramics route. The compound exhibits a monoclinic structure, which was confirmed through the X-ray diffraction pattern analysis. The magnetization measurements in both zero-field-cooled (ZFC) and field-cooled (FC) modes indicate that the compound undergoes a phase transition from paramagnetic (PM) to antiferromagnetic (AFM) state at the Néel transition temperature TN ∼31 K. Notably, below TN, there is a substantial discrepancy between the ZFC and FC curves, which can be ascribed to the existence of competing ferromagnetic (FM) and AFM interactions, resulting in a glassy behaviour. Additionally, the compound demonstrates a Griffiths phase above TN, signifying the presence of ferromagnetic clusters within the PM region of the material. The spin-glass behaviour is confirmed by the shift of maxima towards higher temperatures with increasing frequency in the ac susceptibility measurement. The memory effect also confirms the spin-glass nature of the compound. The Arrott's plots confirmed that the magnetic phase transition in the material to be a second-order type. Additionally, the material exhibited a maximum magnetic entropy change (−ΔS) value of 1.08 J/kg.K and a relative cooling power of 36.60 J/kg under an applied magnetic field of 5 T.

AC susceptibility measurement of magnetic nanoparticles using an optically pumped magnetometer and a flux transformer

Magnetic particle imaging (MPI) is used to detect small magnetic fields in magnetic nanoparticles (MNPs). As a first step in MPI systems, we proposed an AC susceptibility measurement system with an optically pumped magnetometer (OPM) based on a flux transformer. First, the magnetic frequency response of the OPM was obtained for calibration. Second, the AC susceptibility of the Resovist MNPs in liquid and solid phases was obtained using the calibration data. These results are consistent with those of previous studies. Therefore, the proposed method is useful for detecting weak MNP magnetic signals in relatively strong magnetic fields and is expected to be applicable to MPI.

Magnetic ac susceptibility of superconducting Ta films for quantum computing

Recently, breakthroughs were reported in the long coherence time (>0.5 ms) transmon qubit using tantalum films (Ta). Identifying the loss mechanism in Ta films will help further improve the performance of Ta-based superconducting qubits. Here, we report a study of superconducting properties in two sets of Ta films grown on c-cut and a-cut sapphires respectively using contact transport and noncontact magnetic ac susceptibility measurements. Although the resistive transition appears to be similar in both films, we found strikingly different responses in their complex magnetic susceptibilities . in the c-cut films exhibits a sharp peak at superconducting transition T c and becomes featureless below T c indicating a strongly coupled superconducting state. In contrast, in the a-cut films exhibits a broad peak near T c and a second peak appears below T c, indicating granular superconductivity behavior. This second peak in is associated with the hysteresis loss at weak-link-type grain boundaries, which is believed to be a leading source of decoherence in the a-cut Ta films. The derived magnetic loss tangent in the a-cut films is about one order of magnitude higher than in the c-cut films. This study demonstrates that ac susceptibility is an excellent probe for characterizing bulk superconducting properties of the constituent superconducting materials used in qubits.

Current induced effect on the resistivity in La0.8−xDyxNa0.2MnO3 monovalent doped manganites

The observation of electroresistance, ER effect in several substituted doped manganites which characterized the sensitivity of sample’s resistivity to the change of applied current was suggested can be enhanced by the presence of magnegtic inhomogenity. However, such effect is not well understood but the magnetic inhomogenity may influence the strength of ER related mechanism, hence current induced effect on the resistivity in monovalent doped La0.8−xDyxNa0.2MnO3 (x = 0.00–0.20) manganites prepared using solid state reaction method have been investigated to elucidate the matter. The ρ vs T curves shown that all samples exhibit smaller resistivity under higher applied current of 10 mA as compared to 5 mA of applied current in the wide temperature range of 30 K–300 K which led to ER effect. Measurement of ac susceptibility show that x = 0 sample exhibit ferromagnetic behaviour while both Dy substituted samples exhibit ferromagnetic to paramagnetic transition behavior with reduction in Tc values. Large ER effect with value of 130% at 300 K observed in ER vs. T curve for x = 0.1 sample, while for x = 0 the ER value is 9% indicates improved sensitivity of the Dy substituted sample towards electric field. While x = 0.20 sample exhibit double ER peaks which may related to presence of impurity phases. It is suggested Dy substitution induces magnetic inhomogeneities which favors formation of more filamentary conductive paths under the presence of high applied current thus enhanced the conduction process of charge carriers. The finding on the enhancement of ER effect indicates that the Dy substituted sample as potential candidates for spintronic-based devices.

Honey-like Odor Meets Single-Ion Magnet: Synthesis, Crystal Structure, and Magnetism of Cobalt(II) Complex with Aromatic Trans-Cinnamic Acid

The hexacoordinate Co(II) complex [Co(neo)2(cin)][BPh4]·½Me2CO (1·½Me2CO) containing trans-cinnamic acid (Hcin) and neocuproine (neo) was prepared. The compound 1·½Me2CO was characterized via single-crystal X-ray analysis, FT-IR spectroscopy, and magnetic measurements. The coordination polyhedron of the complex cation adopts a deformed octahedron shape, and cinnamate exhibits a bidentate mode of coordination, which is unusual for mononuclear Co(II) cinnamate complexes. The analysis of DC magnetic measurements with zero-field splitting (ZFS) spin Hamiltonian revealed large magnetic anisotropy defined by the axial ZFS parameter D = +53.2 cm−1. AC susceptibility measurements revealed the slow relaxation of magnetization under the applied field; thus, 1·½Me2CO behaves as a field-induced single-molecule magnet. The analysis of magnetic properties was also supported by CASSCF/NEVPT2 calculations.

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.

Table-like magnetocaloric effect due to field-induced inter-cluster interactions in Eu0.5Sr0.5MnO3–EuMnO3 composites

The table-like behavior of the magnetic entropy change is generally observed in rare-earth-based alloys (e.g. La(Fe,Si)13, Gd50Co45Fe5, Gd56Ni15Al27Zr2, GdCo x Al y etc). Here, we report similar table-like behavior in a bulk perovskite Eu0.5Sr0.5MnO3–EuMnO3 composite system with a working temperature span of ∼80 K for a field change of ∼70 kOe. AC susceptibility and time-dependent DC magnetization relaxation measurements confirm the presence of interacting ferromagnetic clusters with a Vogel–Fulcher freezing temperature of 42.7 K. The Langevin function fittings of the isothermal magnetization curves show a non-monotonous variation of cluster size with temperature. We propose a model for the high-field dependency of the ferromagnetic cluster coupling in the composite system. This widens the transition temperature region of long-range order formation and hence gives rise to the table-like magnetocaloric effect in Eu0.5Sr0.5MnO3–EuMnO3 composite systems.

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.

Effect of incoherent interaction between A-site Cu2+ and Mn3+ on magnetic properties of B-site non-magnetic A′A3B4O12 quadruple perovskite

The magnetic state of A-site Cu2+ sublattice is sensitive to the non-magnetic B-site cations in A'A3B4O12 as revealed from ferromagnetic CaCu3M4O12 (M = Sn and Ge) and antiferromagnetic CaCu3Ti4O12. We report the sensitivity of Cu2+ ordering when part of it is substituted by Mn3+ keeping the B-site cations non-magnetic. We observed that mixed occupancy of Cu2+ and Mn3+ in A-site, long-range ordering of Cu2+ is remarkably destabilized in (Ca/La/Y)Cu3-xMnxTi4-yAlyO12 (x/y = 0, 0.5 and 1.0). Samples are characterized by powder X-ray diffraction, FE-SEM imaging, EDX analysis, XPS analysis, Raman spectroscopy and magnetization. The antiferromagnetic ordering of CaCu3Ti4O12 is drastically suppressed in CaCu2MnTi3AlO12. Introduction of Mn3+ in Cu2+ sublattice largely hampers the Cu2+—Cu2+ interaction. This result is in strong contrast to the system with magnetic cations at B-site. This may be attributed to the incoherent interaction between Cu2+ and Mn3+ spins though they occupy the same crystallographic sites. LaCu3Ti3AlO12 and La2/3Cu2.5Mn0.5Ti3.5Al0.5O12 show rather similar behaviour. LaCu3Ti3AlO12 shows prominent difference in magnetization compare to La2/3Cu3Ti4O12, whereas YCu3Ti3AlO12 does not with respect to Y2/3Cu3Ti4O12. The differential magnetic behaviour is possibly due to the size difference between La and Y. The ac-susceptibility measurement in LaCu3Ti3AlO12 suggests the formation of short-range ordered magnetic clusters above TN in the paramagnetic phase and there is no evidence of spin glass-like behaviour at low temperature.