Ac Susceptibility Measurements Research Articles

Evidence of cluster-glass and Griffiths-like phases in partially ordered La2FeMnO6 double perovskite

The double perovskite La2FeMnO6 is ideally expected to be a ferrimagnet with a low saturation moment of 1 μB/f.u. Inhomogeneity in the Fe/Mn sites, along with other lattice disorders, can modify the exchange interactions in the material and result in a net saturation moment of more than 1 μB/f.u. Here, the origin of complicated magnetic behavior is examined of a pure phase La2FeMnO6 sample prepared by the sol-gel method. XRD analysis established that the material crystallizes in the orthorhombic Pbnm symmetry. The comprehensive analysis of x-ray photoelectron spectroscopy, x-ray near edge structure, and magnetic measurements acknowledge an antiferromagnetic coupling between Fe3+ and Mn3+ cations, thereby, resulting in the ferrimagnetic ground state for La2FeMnO6. The magnetic hysteresis loop obtained at 5 K shows a large coercivity of ∼820 Oe and a saturation moment of 1.6 μB/f.u., hinting at partial B-site ordering in La2FeMnO6. The ac susceptibility and dc magnetization measurements indicate the existence of magnetic glassy states (of cluster-glass type) with two distinct dynamical freezing points at ∼27 and 92 K, along with a Griffiths-like phase in the material. The experimental results are discussed taking several possible exchange interactions among Fe3+ and Mn3+ ions in the system into consideration. The magnetic complexity of this system makes it attractive for fundamental research and technological applications.

Non-equilibrium magnetic properties in bimorphic phases of ErIr3

In this work, we report the successful synthesis of a polycrystalline, binary compound ErIr3, forming a macroscopic coexistence of two different cubic structures (AuBe5-type and AuCu3-type). The magnetic properties of two polymorphs could be distinguished by means of different measurement techniques. The detailed study of dc magnetization and heat capacity behaviour confirm that the phase with the cubic AuBe5-type structure orders ferromagnetically below 5.2 K, while the AuCu3-type phase remains paramagnetic down to 2.5 K. The presence of a glassy state in this compound is manifested in the frequency dependent ac susceptibility measurement, magnetic relaxation and magnetic memory effect.

Emergence of weak pyrochlore phase and signature of field induced spin ice ground state in Dy2−xLaxZr2O7; x = 0, 0.15, 0.3

The pyrochlore oxides Dy2Ti2O7 and Ho2Ti2O7 are well studied spin ice systems and have shown the evidences of magnetic monopole excitations. Unlike these, Dy2Zr2O7 is reported to crystallize in a distorted fluorite structure. We present here the magnetic and heat capacity studies of La substituted Dy2Zr2O7. Our findings suggest the absence of spin ice state in Dy2Zr2O7 but the emergence of the magnetic field induced spin freezing near T ≈ 10 K in ac susceptibility measurements which is similar to Dy2Ti2O7. The magnetic heat capacity of Dy2Zr2O7 shows a shift in the peak position from 1.2 K in zero field to higher temperatures in the magnetic field, with the corresponding decrease in the magnetic entropy. The low temperature magnetic entropy at 5 kOe field is R ln2 − (1/2)R ln(3/2) which is the same as for the spin ice state. Substitution of non-magnetic, isovalent La3+ for Dy3+ gradually induces the structural change from highly disordered fluorite to weakly ordered pyrochlore phase. The La3+ substituted compounds with less distorted pyrochlore phase show the spin freezing at lower field which strengthens further on the application of magnetic field. Our results suggest that the spin ice state can be stabilized in Dy2Zr2O7 either by slowing down of the spin dynamics or by strengthening the pyrochlore phase by suitable substitution in the system.

Variation in Superconducting Symmetry Against Pressure on Noncentrosymmetric Superconductor Cd2Re2O7 Revealed by 185/187Re Nuclear Quadrupole Resonance

We performed $^{185/187}$Re nuclear quadrupole resonance (NQR) measurements under pressure to investigate the superconducting properties of noncentrosymmetric superconductor Cd$_{2}$Re$_{2}$O$_{7}$ under various crystal structures. The pressure dependence of superconducting transition temperature $T_{\rm c}$ determined through ac susceptibility measurements is consistent with the results of previous resistivity measurements [T. C. Kobayashi $et al$., J. Phys. Soc. Jpn. 80, 023715 (2011).]. Below 2.2 GPa, in the nuclear spin-lattice relaxation rate $1/T_{1}$, a clear coherence peak was observed just below $T_{\rm c}$, indicating conventional $s$-wave superconductivity. In contrast, the coherence peak disappears at 3.1 GPa, suggesting a change in superconducting symmetry to the $p$-wave dominant state against pressure.

The effect of Ni doping on the electrical and magnetic properties of Y1-xNixBa2Cu3O7−δ delta superconductors

The Y1-xNixBa2Cu3O7-δ superconducting samples with low level of Ni concentration were prepared by the standard solid-state reaction. The AC susceptibility, magnetoresistivity, microstructure and the critical current density, Jc, of samples as a function of temperature, magnetic field and Ni doping are investigated. The AC susceptibility measurements show that the displacement of the peak temperature Tp of the imaginary part of the susceptibility χ″ versus magnetic field was reduced strongly by Ni doping up to an optimum level, which shows the increasing of flux pinning by Ni doping. The Jc of samples was derived from AC susceptibility utilizing the Bean model. It shows an increasing in Jc by Ni doping, at any temperature or magnetic field, consistent with the results of Jc measurements. The magnetoresistance measurements were carried out under magnetic fields up to 1 T and explained by TAFC model. Utilizing this model, the vortex dynamic behaviour and the activation energy U (H) of the compound are investigated. We found that the U (H) increases and the resistive broadening is strongly reduced by Ni doping. The SEM measurements show that the grain sizes are clearly increased and the grain connections are improved by Ni doping. The results of all the observations taken from different measurements are consistent together and indicate the Ni doping has an effective role to improve the intergranular coupling and flux pinning.

Negative imaginary component of AC magnetic susceptibility, metastable states, and magnetic relaxation in La0.6Sr0.35MnTi0.05O3

Off-stoichiometric ([Formula: see text][Formula: see text])([Formula: see text]) MnO3 manganite was studied with in-phase (real, [Formula: see text]) and out-of-phase (imaginary, [Formula: see text]) components of ac susceptibility measurements. Short- and long-time relaxation effects, manifested by the revealed negative [Formula: see text], were observed. The effects are explained at the frame of Landau theory of phase transitions where a coexistence of stable and metastable states is considered. It is supposed that the appearance of negative [Formula: see text] is an intrinsic feature of magnetic materials with impurities, defects and vacancies blocking nucleation and growth of new phase.

Spin-glass mediated large exchange bias and coercivity in hexagonal Mn1+xCu1−xGa (x = 0.3–0.5) alloys

Ni2In-type MnMX alloys possess interesting noncollinear magnetic structures that may embed a variety of physical phenomena. In this work, magnetic and electrical transport properties of pure Ni2In-type structure Mn1 +xCu1−xGa (x = 0.3–0.5) alloys are investigated systematically. A cluster spin-glass phase is identified below the Curie temperature by magnetization and ac susceptibility measurements in x = 0.5 sample. Mediated by the exchange interaction between Mn-Mn clusters, significant exchange bias effect and coercivity are established, in which a giant exchange bias field of up to 0.323 T is observed in x = 0.5 sample. The exchange bias effect and coercivity, as well as the anomalous Hall effect are found to be greatly tuned via field cooling process. The large EB and coercivity identified in Mn–Cu–Ga alloys make them potentially applied in rare-earth-free magnets and spintronic devices.

Spin dynamics in two ReF6 -based single-molecule magnets from NMR and ac susceptibility measurements

Spin dynamics in two <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>ReF</mml:mi><mml:mn>6</mml:mn></mml:msub></mml:math> -based single-molecule magnets from NMR and ac susceptibility measurements

Coupling-decoupling of conducting topological surface states in thick Bi2Se3 single crystals

Sensitive ac susceptibility measurements of a topological insulator, Bi$_2$Se$_3$ single crystal, using mutual two coil inductance technique (Ref. 32) shows coupling and decoupling of high conducting surface states. The coupling of the surface states exists upto thickness of 70 microns, which is much larger than the direct coupling limit of thickness approximately 5 to 10 nanometers found in thin films. The high conducting topological surface states are coupled through the crystal via high electrically conducting channels, generated by Selenium vacancies. These conducting channels through the bulk disintegrate beyond 70 micron thickness and at high temperatures, thereby leading to decoupling of the topological surface states. We show the decoupled surface states persist upto room temperature in the topological insulator. Analysis of Nyquist plot of ac-susceptibility response of the TI using a resistor (R) Inductor (L) model shows an inductive nature of the coupling between surface states found in these Bi$_2$Se$_3$ crystals.

Structures, fluorescent properties and single-molecule-magnet behavior of two Ln4 (LnIII = Tb and Dy) clusters

Two coplanar tetranuclear Ln(III) clusters, [Ln4(acac)4(μ2-L)6(μ3-OH)2]·2C2H5OH (Ln = Tb (1) and Dy (2)) constructed by one Schiff base ligand (HL = 5-((4-methylbenzylidene)amino)quinolin-8-ol) and a β-diketonate ligand (Hacac = acetylacetone), have been synthesized and structurally and magnetically characterized. Alternating-current (ac) susceptibility measurements reveal that no significant out-of-phase (χ″) ac signals were observed in 1, however, the remarkable slow magnetic relaxation behavior were occurred for 2 with the anisotropic barrier (ΔE/kB) of 82.1 K and τ0 of 6.34 × 10−8 s. Interestingly, two maxima out-of-phase (χ″) ac signals are appeared in 2, which are indicative of the operation of more than one relaxation process. In addition, the luminescence properties of clusters 1 and 2 were studied.

The Synthesis of a Quasi-One-Dimensional Iron-Based Telluride with Antiferromagnetic Chains and a Spin Glass State.

The report on the superconductivity of the two-legged spin ladders BaFe2S3 and BaFe2Se3 has established 123-type iron chalcogenides as a novel subgroup in the iron-based superconductor family and has stimulated the continuous exploration of other iron-based materials with new structures and potentially novel properties. In this paper, we report the systematic study of a new quasi-one-dimensional (1D) iron-based compound, Ba9Fe3Te15, including its synthesis and magnetic properties. The high-pressure synthesized Ba9Fe3Te15 crystallized in a hexagonal structure that mainly consisted of face-sharing FeTe6 octahedral chains running along the c axis, with a lattice constant of a = 10.23668 Å; this led to weak interchain coupling and an enhanced one-dimensionality. The systematic static and dynamic magnetic properties were comprehensively studied experimentally. The dc magnetic susceptibility showed typical 1D antiferromagnetic characteristics, with a Tmax at 190 K followed by a spin glass (SG) state with freezing at Tf ≈ 6.0 K, which were also unambiguously proved by ac susceptibility measurements. Additionally, X-ray magnetic circular dichroism (XMCD) experiments revealed an unexpected orbital moment for Fe2+, i.e., 0.84 μB per Fe in Ba9Fe3Te15. The transport property is electrically insulating, with a thermal activation gap of 0.32 eV. These features mark Ba9Fe3Te15 as an alternative type of iron-based compound, providing a diverse candidate for high-pressure studies in order to pursue some emerging physics.

Low-temperature properties of pyrochlore Lu2Ir2O7

Low-temperature properties of Lu2Ir2O7 were studied by means of magnetization, ac-susceptibility and specific heat measurements. Our low-temperature measurements reveal a bifurcation of ZFC and FC magnetization data below 140 K, which is further pronounced as a broad anomaly in the temperature development of specific heat and ac-susceptibility. The anomaly can be attributed to the same physical mechanism as discussed for other A2Ir2O7 with A = magnetic rare-earth element, i.e. to short-range correlations among Ir4+ magnetic moments, rather than to a long-range order of Ir sublattice reported earlier. The ac-susceptibility data revealed a strong frequency dependence of the signal, a paradigmatic sign of a spin-glass transition. The analysis of low-temperature specific heat data brought important information on physical properties of Lu2Ir2O7, in striking agreement with our density functional theoretical (DFT) calculations, concluding relatively weak electron-phonon interaction and semimetallic ground state in Lu2Ir2O7.

Synthesis, characterization and magnetic studies of dinuclear lanthanide complexes constructed with a Schiff base ligand

A series of seven isostructural homometallic dinuclear lanthanide metal complexes having the general formula [Ln2(L)2(NO3)4]·2CH3 OH, where L = 6-methoxy-2-(2-pyridylmethyliminomethyl) phenol Ln = Tb (1), Dy (2), Ho (3), Er (4), Tm (5), Yb (6), and Lu (7), are described. Compounds 1–7 were obtained from the reactions of Schiff base ligand (L) with the respective metal nitrates in the presence of triethylamine in MeOH. The structure of 2 was determined by single-crystal X-ray diffraction and the remaining compounds were characterized from X-ray powder diffraction patterns and found to be isostructural. The magnetic properties of 1–7 have been investigated using dc and ac susceptibility measurements. All these compounds show antiferromagnetic behavior between adjacent paramagnetic centers. Out of these seven compounds, only 2 displays an out-of-phase ac susceptibility signal which is attributed to the presence of the anisotropic DyIII ions.

Reentrant spin-glass behaviour in highly frustrated Mn-rich spinel zinc manganate

The present work offers an insight into the magnetic properties of Mn-rich spinel zinc manganate. Rietveld refinement reveals the formation of Zn0.67Mn2.33O4, where Zn2+ and Mn2+ ions are randomly distributed in the tetrahedral sublattice. DC and AC susceptibility measurement of Zn0.67Mn2.33O4 infers the occurrence of two kinds of transition below 11 K. Paramagnetic to ferrimagnetic transition occur at 10.7 K and ferrimagnetic to spin glass-like transition occurs at 5.8 K. The long-range canted ferrimagnetic ordering is corroborated using modified Lotgering model and calculated the exchange interaction values (JAA = 5.2 K,JAB = 5.3 K, JBB = 14.8 K). Further, the observed shift in freezing temperature with DC magnetic field obeys Almeida–Thouless behaviour and frequency dependence of AC susceptibility follows Vogel–Fulcher law. However, the complete establishment of the canonical spin glass state is denied since the manganese ions occupied at the tetrahedral site is equal to the percolation threshold (=33%). Subsequently, the observed spin glass-like behaviour (5.8 K) below Curie temperature (10.7 K) evidences the reentrant spin glass nature. Similarly, the strong frustration in Zn0.67Mn2.33O4 lattice is observed through the substantial negative value of Curie–Weiss temperature (∼−599 K) and very high frustration factor (f = 56). Overall, the chosen Zn0.67Mn2.33O4 is a highly frustrated magnetic system revealing re-entrant spin-glass behaviour.

Synthesis and magnetic properties of Ba2Fe14O22 Y-type hexaferrite

Using the solid state reaction method, Y-type hexaferrite Ba2Fe2+2Fe3+12O22 (Fe2Y) polycrystalline ceramics were fabricated by calcining and sintering in a nitrogen atmosphere, and their magnetic properties were systematically investigated. The kink occurs at around 130 K in the zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves is independent of measuring frequencies, which is probably derived by the temperature-induced spin reorientation transitions of Fe2Y phase. While the kink at 50 K in the ZFC magnetization and the irreversibility between ZFC and FC curves were observed and ascribed to the spin-glass-like transition in the low temperature region. Both ac susceptibility and thermoremanent magnetization (TRM) measurements confirmed this assumption.

Effects of Mixed Valency in an Fe-Based Framework: Coexistence of Slow Magnetic Relaxation, Semiconductivity, and Redox Activity.

A 2-D coordination framework, (NEt4)2[Fe2(fan)3] (1·5(acetone); H2fan = 3,6-difluoro-2,5-dihydroxy-1,4-benzoquinone), was synthesized and structurally characterized. The compound is structurally analogous to a formerly elucidated framework, (NEt4)2[Fe2(can)3] (H2can = 3,6-dichloro-2,5-dihydroxy-1,4-benzoquinone), and adopts a 2-D (6,3) topology with the symmetrical stacking of [Fe2(fan)3]2- sheets that are held in position by the NEt4+ cations between the sheets. The investigation of the dc and ac magnetic properties of 1·5(acetone) revealed ferromagnetic ordering behavior and slow magnetization relaxation, as evinced from ac susceptibility measurements. Furthermore, the exposure of 1·5(acetone) to air led to the formation of a heptahydrate 1·7H2O which displayed distinct magnetic properties. The study of the redox state and extent of delocalization in 1·5(acetone) was undertaken via crystallography, in combination with Mössbauer and vis-NIR spectroscopy, to reveal the mixed-valence and delocalized nature of the as-synthesized material. As a result, the conductivity studies conducted on a pressed pellet showed a relatively high conductivity of 1.8 × 10-2 S cm-1 (300 K). In order to compare structurally related anilate-based structures, a relationship among the redox state, spectroscopic properties, and electronic properties was elucidated in this work. A preliminary investigation of 1·5(acetone) as a candidate anode material in lithium ion batteries revealed a high reversible capacity of 676.6 mAh g-1 and high capacity retention.

Unified phase diagram of F-doped LaFeAsO by means of NMR and NQR parameters

We present $^{75}$As Nuclear Magnetic and Quadrupole Resonance results (NMR, NQR) on a new set of LaFeAsO$_{1-x}$F$_x$ polycrystalline samples. Improved synthesis conditions led to more homogenized samples with better control of the fluorine content. The structural$\equiv$nematic, magnetic, and superconducting transition temperatures have been determined by NMR spin-lattice relaxation rate and AC susceptibility measurements. The so-determined phase diagram deviates from the published one especially for low F-doping concentrations. However, if the doping level is determined from the NQR spectra, both phase diagrams can be reconciled. The absence of bulk coexistence of magnetism and superconductivity and a nanoscale separation into low-doping-like and high-doping-like regions have been confirmed. Additional frequency dependent intensity, spin-spin, and spin-lattice relaxation rate measurements on underdoped samples at the boundary of magnetism and superconductivity indicate that orthorhombicity and magnetism originate from the low-doping-like regions, and superconductivity develops at first in the high-doping-like regions.

Pentanuclear Spirocyclic Ni4Ln Derivatives: Field Induced Slow Magnetic Relaxation in the Dysprosium and Erbium Analogues

Five pentanuclear heterometallic isostructural complexes, [Ni4Ln(L)2(LH)2(CH3CN)3Cl]·xH2O·yCH3OH {Ln = YIII (1), GdIII (2), TbIII (3), DyIII (4) and ErIII (5)} [for 1 and 2, x = 2, y = 1; for 3, x = 6, y = 2; for 4, x = 5, y = 1; for 5, x = 2, y = 2] were prepared by the reaction of (E)‐2‐(hydroxymethyl)‐6‐{[(2‐hydroxyphenyl)imino]methyl}‐4‐methylphenol (LH3) with LnCl3·6H2O and Ni(OAc)2·4H2O in the presence of tetrabutylammonium hydroxide (TBA‐OH) base. The structural characterization reveals that compounds 1–5 contain a spirocyclic pentanuclear core [Ni4Ln(µ3‐O)4(µ2‐O)4]3+ where two triangular motifs [Ni2Ln(µ3‐O)2(µ2‐O)2]3+ are fused together through a common vertex of the LnIII ion. The central LnIII ion forms an eight‐coordinated distorted triangular dodecahedron geometry, while the nickel(II) ions form a distorted octahedron geometry. Comprehensive dc magnetic studies reveal that antiferromagnetic exchange interaction exists between the NiII centres. The ac susceptibility measurement revels that dysprosium and erbium analogue shows field induced slow magnetic relaxation with an anisotropic barrier (Ueff) of 25.12 cm–1 and 22.13 cm–1 respectively.

Investigation of the phase separation property in [formula omitted][formula omitted][formula omitted][formula omitted] manganite

We report a comprehensive investigation of La0.2Pr0.4Ca0.4MnO3 to clarify the micrometre scale phase separation phenomenon in the mixed valent manganite (La,Pr,Ca)MnO3. The compound shows multiple magnetic transitions, in which the charge-ordered state is converted into a ferromagnetic state in steps with the application of a magnetic field. The ac susceptibility measurements show that the glassy transition at low temperatures does not depend on the frequency, thus indicating the absence of any spin glass behaviour. Magnetization as well as heat capacity measurements indicate that this low temperature transition is magnetic field dependent. The field dependent resistivity at 2 K shows a sharp drop indicating that the sample behaviour changes from a high resistive state to a low resistive state, corroborating the conversion of charge-ordered insulating (COI) phase to a ferromagnetic metallic (FMM) phase. Our results point towards the existence of phase separation, rigidity of the low temperature glassy-like phase as well as the conversion of COI phase to FMM phase by the application of magnetic fields.

Investigation of the crystal/magnetic structure, magnetic and optical properties of SrYxNbxFe12−2xO19 (x ≤ 0.05) hexaferrites

Series of Sr hexaferrites (HFs) co-substituted with Nb3+ and Y3+ ions (SrYxNbxFe12−2xO19 (x ≤ 0.05) were prepared via sol-gel combustion method. In addition to their optical, low temperature, magnetic properties and hyperfine interactions were investigated in detail. X-ray diffraction (XRD) patterns were considered proving both the formation and pureness of products. Mossbauer analysis indicated that the hyperfine field on iron nuclei at all sites decrease with substitution of Y3+–Nb3+ ions into Sr-hexaferrite as a result of changing the magnetic Fe3+ (5 μB) ions by nonmagnetic Y3+ and Nb3+ ions. This leads to decreasing of the magnetic super exchange interaction between the ions. Measurements of AC susceptibility and ZFC-FC magnetizations implemented in a range from room temperature (RT) down to around 2 K. The analysis of MFC–MZFC data proves the ferrimagnetic performance of different prepared HFs in the whole range of 2–325 K and a super-spin glass-like behavior was identified at minimal temperatures. The investigation of AC susceptibility revealed a weakening in the magnetic exchange interactions with the rise in the Nb3+ and Y3+ ions ratios.