Frequency Dependence Of Ac Susceptibility Research Articles

Size-induced exchange bias in single-phase CoO nanoparticles

The tuning of exchange bias (EB) in nanoparticles has garnered significant attention due to its diverse range of applications. Here, we demonstrate EB in single-phase CoO nanoparticles, where two magnetic phases naturally emerge as the crystallite size decreases from 34.6 ± 0.8 to 10.8 ± 0.9 nm. The Néel temperature (T N) associated with antiferromagnetic ordering decreases monotonically with the reduction in crystallite size, highlighting the significant influence of size effects. The 34.6 nm nanoparticles exhibit magnetization irreversibility between zero-field cooled (ZFC) and field-cooled (FC) states below T N. With further reduction in size this irreversibility appears well above T N, resulting in the absence of true paramagnetic regime which indicates the occurnace of an additional magnetic phase. The frequency-dependent ac-susceptibility in 10.8 nm nanoparticles suggests slow dynamics of disordered surface spins above T N, coinciding with the establishment of long-range order in the core. The thermoremanent magnetization (TRM) and iso-thermoremanent magnetization (IRM) curves suggest a core–shell structure: the core is antiferromagnetic, and the shell consists of disordered surface spins causing ferromagnetic interaction. Hence, the EB in these CoO nanoparticles results from the exchange coupling between an antiferromagnetic core and a disordered shell that exhibits unconventional surface spin characteristics.

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.

Glassy magnetism of Tm2Cu2In in elevated pressures

Tm2Cu2In belonging to the family of R2T2X intermetallics is a ferromagnet with glassy features. This work describes results of high-pressure studies of its magnetic behavior. While TC exhibits only a weak and non-monotonous response to hydrostatic pressure, the pressure impact on frequency dependence of AC susceptibility can be interpreted as due to gradual increase of relaxation time and decrease of the activation energy barrier. Since the magnetic behavior is strongly related to the details of crystal structure, the temperature and pressure effects on the lattice are presented, as well. First principles calculations based on the density functional theory provide a background information.

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.

Nickel substitution induced reentrant spin glass behavior in EuGa4

Typically, atomic scale disorder is essential to show glassy behavior and amorphous materials can be the optimum candidates in this regard. Generating local disorder in an ordered material by substitution of another metal is a challenging task. In this paper, we successfully substituted Ni in ${\mathrm{EuGa}}_{4}$ (${\mathrm{EuNi}}_{0.37}{\mathrm{Ga}}_{3.63}$) to induce spin glass (SG) behavior. The compound was characterized by single-crystal x-ray diffraction, dc magnetization, frequency-dependent ac susceptibility, isothermal magnetization at various temperatures, spin relaxation, specific heat, and electric resistivity. Two distinct anomalies were observed in dc susceptibility $\ensuremath{\sim}16.5$ and 5.2 K corresponding to long-range antiferromagnetic (AFM) ordering and SG transition, respectively. Their characteristic peaks in specific heat capacity further support the AFM and SG behavior. The irreversibility temperature is fitted with the Almeida-Thouless equation, confirming the Ising SG. The SG behavior is further confirmed by shift of maxima to a high temperature in ac susceptibility with increasing frequency. Nonzero spin relaxation after 7200 s, fitted with stretched exponential model, provides the value of $\ensuremath{\beta}=0.41$. This material can be identified as a possible reentrant SG as AFM and SG behavior coexist in the low-temperature range. Additionally, a spin-flip transition is observed in the AFM region ($5.2\phantom{\rule{0.28em}{0ex}}\mathrm{K}<T<16.5\phantom{\rule{0.28em}{0ex}}\mathrm{K}$). Our first-principles calculations show Ni substitution induces the distortion in electronic band structure as compared with parent ${\mathrm{EuGa}}_{4}$. A minimal difference between the different magnetic configurations for Ni-substituted ${\mathrm{EuGa}}_{4}$ suggests the frustration in the lattice, pointing toward SG behavior.

Antiferromagnetic ordering to cluster-glass like transition behavior in DyVO[formula omitted

We herein present the detailed investigation of the structural and magnetic properties of the Jahn–Teller compound DyVO4 in otherwise reported as antiferromagnetic compound. The static and dynamic magnetic measurements reveal the emergence of cluster-glass like phase in DyVO4. The magnetization vs temperature measurement in field-cooling and zero-field-cooling shows irreversibility above Néel temperature as increasing the field above 1000 Oe, indicating the glass-like behavior in DyVO4. The frequency dependence of ac-susceptibility is analyzed by calculating the Mydosh parameter, which confirms the cluster-glass phase in DyVO4 at applied dc magnetic fields. The frequency dependence ac-susceptibility is also analyzed within the framework of dynamic scaling laws such as power law and Vogel–Fulcher law. The extracted parameters supports the fact that transitions are spin cluster-glass like. Further the memory effect establishes antiferromagnetic to spin cluster-glass like transition in DyVO4 conclusively.

Pinning potential in highly performant CaKFe4As4 superconductor from DC magnetic relaxation and AC multi-frequency susceptibility studies

We have investigated the pinning potential of high-quality single crystals of superconducting material CaKFe4As4 having high critical current density and very high upper critical field using both magnetization relaxation measurements and frequency-dependent AC susceptibility. Preliminary studies of the superconducting transition and of the isothermal magnetization loops confirmed the high quality of the samples, while temperature dependence of the AC susceptibility in high magnetic fields show absolutely no dependence on the cooling conditions, hence, no magnetic history. From magnetization relaxation measurements were extracted the values of the normalized pinning potential U*, which reveals a clear crossover between elastic creep and plastic creep. The extremely high values of U*, up to 1200 K around the temperature of 20 K lead to a nearly zero value of the probability of thermally-activated flux jumps at temperatures of interest for high-field applications. The values of the creep exponents in the two creep regimes resulted from the analysis of the magnetization relaxation data are in complete agreement with theoretical models. Pinning potentials were also estimated, near the critical temperature, from AC susceptibility measurements, their values being close to those resulted (at the same temperature and DC field) from the magnetization relaxation data.

Magnetic complexity, magnetodielectric effect and DFT calculations on correlation driven Gd2CoMnO6 insulator

In the family of Re2CoMnO6 manganite double perovskites, in contrast to parent La2CoMnO6 compound, Gd2CoMnO6 exhibits multiple magnetic transitions; ferromagnetic (FM) ordering, TC ∼ 112 K followed by AFM transition, TN ∼ 47 K, Gd spins ordering for T < 10 K and large isothermal entropy changes. A study of DC field-superimposed AC magnetic susceptibility measurements revealed the field-induced magnetic glassy behavior below TC and enhancement of FM correlations above TC. From the analysis of Almeida-Thouless behavior and dynamical power-law fit to frequency dependent AC susceptibility, Gd2CoMnO6 exhibits a volume spin glass-like nature below the freezing temperature, Tf ∼ 117.5 K. The isothermal field-dependent magnetic and dielectric permittivity data and temperature dependent Raman measurements (reported in ref. R. X. Silvaet al., J, Appl. Phys. 114 194,102 (2013)) confirms the spin-phonon coupling induced magnetodielectric effect. Further, the ground-state electronic structure and magnetic properties of Gd2CoMnO6 are investigated using DFT + U formalism with Vienna Ab-initio Simulation Package (VASP) code and predicted the material to be a correlation-driven insulator. The correlation value of the Hubbard U parameter at the 4f-Gd elements changes the stability of the magnetic state from Ferri to FM spin alignment for Ueff ≥ 3 eV and is correlated to the experimentally observed field-induced transformation of the short-range-order FiM/spin-glass-like phase into the long-range ordered FM phase.

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.

Magnetic properties and evidence of a new spin-glass transition induced by Mn-doping in Tb2NiSi3

It is extremely rare for the coexistence of two spin glass transitions in single phase materials. In the present paper, the structure and magnetic properties of Tb2Ni0.8Mn0.2Si3 were investigated. For comparison, the corresponding experimental results of Tb2NiSi3 compound were also provided. In the temperature dependence of dc susceptibility measurements, there are two anomaly transitions for both Tb2Ni0.8Mn0.2Si3 and Tb2NiSi3. Further studies on the frequency dependence of ac susceptibility show a new spin-glass transition at 14 K in Tb2Ni0.8Mn0.2Si3, except for the spin-glass transition at 8 K inheriting from Tb2NiSi3. This new spin-glass transition in Tb2Ni0.8Mn0.2Si3 is absent in Tb2NiSi3. It takes place below the long-range magnetic ordered phase transition and shows cluster spin glass behaviors, and it is attributed to the introduction of Mn ions into the intermetallic compound.

YBCO superconductor added with one-dimensional TiO2 nanostructures: Frequency dependencies of AC susceptibility, FC-ZFC magnetization, and pseudo-gap studies

This article reports the role of adding one-dimensional titanium dioxide nanowires (TiO2-NWs, having 30 nm in diameter and some micrometers in length) on FC-ZFC magnetization, frequency-dependent AC susceptibility, and electrical measurements of the YBCO bulk system. The non-added (A0) and added TiO2-NWs samples (A1) were prepared by solid-state reaction route. TiO2-NWs did not affect the crystalline structure of YBCO. SEM examinations revealed that TiO2-NWs preferred to localize at the grain boundaries filling the pores existing between YBCO grains. FC-ZFC magnetization and frequency-dependent AC susceptibility analysis showed an enhancement of flux pinning properties with TiO2-NWs addition. TiO2-NWs addition affected also the electrical and pseudo-gap analysis of the samples.

Effect of Fe substitution on the cation distribution and magnetic properties of CoCr2-xFexO4 (x = 0.6 to 1.0) nanoparticles

Evolution of the structure, microstructure, cation distribution, and magnetic properties of CoCr2-xFexO4 nanoparticles with increase in x from 0.6 to 1.0 synthesized through a coprecipitation technique is demonstrated. While X-ray diffraction shows a cubic phase with space group Fd3‾m, the cation distribution qualitatively examined by extended X-ray absorption fine structure is further quantified by Mössbauer spectroscopic measurements. We reveal that although Cr3+ ions have a strong preference for the B site, Fe3+ and Co2+ ions are found to be distributed among A and B sites. As a consequence, the normal spinel CoCr2O4 transforms to a mixed spinel type showing a decrease in the degree of inversion of Fe ions from 30% to 7% at the tetrahedral (A) site with increase in x from 0.6 to 1.0, whereas Fe3+ ions are found to be equally distributed among A and B sites for x = 0.3. The paramagnetic to ferrimagnetic transition temperature, TC, is increased from 263 K for x = 0.6 to above room temperature for x = 1.0 compared with 97 K obtained for x = 0. The maximum magnetization calculated from the M-H curve increases from 0.18 μB per formula unit for x = 0–0.32 μB per formula unit for x = 0.6 and reaches 3.71 μB per formula unit for x = 1.0. The frequency-dependent ac susceptibility confirms spin glass behavior irrespective of the composition, including x = 0.

Effect of R substitution in spin glass RFeTi2O7 compounds

Zirconolite oxides R3+Fe3+Ti2O7 (R rare earth element) are known to exhibit spin glass behaviour at low temperatures. Here we present a detailed study of these compounds for R = Eu, Gd, Dy, Ho, and Er, together with reviewed previous measurements on Sm, Tb, Tm, Yb and Lu, with the scope of determining the role played by the rare earth on their magnetic properties. They have been investigated using X-ray powder diffraction, and further characterized by magnetization, frequency dependent ac susceptibility and heat capacity measurements. RFeTi2O7 compounds are all isostructural showing orthorhombic structure, space group Pcnb at 300 K. Disorder of the magnetic ions in the RFeTi2O7 lattice induces spin glass behaviour at low temperatures, mainly due to the Fe sublattice. We show that magnetic rare earth ions participate in the spin glass state tuning its properties. The single ion anisotropy of the R3+ ions, excluding exchange interaction with other magnetic ions, has been calculated by ab initio methods, and expressed in terms of the g tensor of the ground doublet or quasi-doublet in Kramers (Sm, Dy, Er, Yb) and non-Kramers (Tb, Ho) ions, respectively, in an effective spin S* = 1/2 model. In the case of R with a singlet ground state (Eu, Tm) or a multiplet state (Gd), the ion is isotropic. We show that the relative increase in the spin-glass temperature ΔTSGR/TSGFe with respect to the LuFeTi2O7, where Lu is non-magnetic, correlates qualitatively with the product of the ratio gz/gJ (R = Tb, Dy, Ho, and Er), or g⊥/gJ (R = Sm), times the ratio of exchange interactions J~R,FeJFe,Fe determined from the paramagnetic room temperature susceptibility measurements. Besides, for increasing anisotropy the spin glass transition dynamics slows down to values typical of cluster glass. The coercive field below the transition is increased in the same trend. This paper explains the effect of the R-Fe exchange interaction and R single ion anisotropy on the spin-glass behaviour of these compounds.

Structural stability and magnetic properties of Mn2FeAl alloy with a β-Mn structure

The synthesized Mn2FeAl alloys crystallize in a geometrically frustrated cubic β-Mn structure (space group: P4132) with an antiferromagnetic ordering whereas the previous theoretical findings suggest for a Heusler structure (L21: regular and X: inverse). The experimental stability of the structure is verified by electronic structure calculations performed for various arrangements of Mn, Fe and Al atoms in the β-Mn-type crystal structure. When compared the energy of the β-Mn structure with the energy of L21 and X type structures, it is found that for an expansion of the lattice volume β-Mn structure becomes more preferable in total energy than L21 and X-type structures. The calculated theoretical equilibrium lattice parameter value for the β-Mn2FeAl is within the accuracy of the experimental value obtained in this work. Additional DFT + U calculations for the optimized crystal structure of the β-Mn2FeAl revealed that the electronic correlations in the Mn ions result in the increased total magnetic moment. In the X type structure, Mn2FeAl is a half metal, whereas the disordered arrangement of atoms in the β-Mn structure leads to the closure of the semiconductor gap. The β- Mn2FeAl alloys exhibit antiferromagnetic ordering (TN ≈ 42 K), which is in excellent agreement with our electronic structure calculations. The detailed analysis of the magnetic and heat capacity measurements suggests a short-range magnetic ordering in the Mn2FeAl alloys. Owing to the strong antiferromagnetic spin fluctuation caused by the geometric frustration in β-Mn, a large enhancement in the electronic heat capacity is noticed. Mn2FeAl shows the characteristic features of spin glass as verified from the frequency dependent AC susceptibility analysis using critical power law and Vogel-Fulcher law. To the best of our knowledge, this is the first ever report on the theoretically predicted lowest ground state configuration for Mn2FeAl with a β-Mn structure and the experimental realization of spin glass features in this geometrically frustrated antiferromagnet.

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.

Collective magnetic dynamics in artificial spin ice probed by ac susceptibility

We report on the study of the thermal dynamics of square artificial spin ice, probed by means of temperature- and frequency-dependent ac susceptibility. Pronounced influence of the interisland coupling strength was found on the frequency response of the samples. Through the subsequent analysis of the frequency- and coupling-dependent freezing temperatures, we discuss the phenomenological parameters obtained in the framework of the Vogel-Fulcher-Tammann law in terms of the samples' microscopic features. The high sensitivity and robust signal to noise ratio of ac susceptibility validate the latter as a promising and simple experimental technique for resolving the dynamics and temperature driven dynamics crossovers for the case of artificial spin ice.

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.

Nonequilibrium magnetic properties in oxygen-rich LaMnO3 nanoparticles

Nonequilibrium magnetic properties of oxygen-rich LaMnO3.21 nanoparticles have been investigated by comprehensive magnetic measurements. The composition falls in the metamagnetic canted spin region of the magnetic phase diagram. However, the zero-field-cooling memory effect and frequency-dependent AC susceptibility reveal a re-entrant glassy state at low temperature. In contrast to the super-spin glass or cluster glass that re-enter from the high-temperature ferromagnetic state in previous studies, analyses based on the power law and Vogel–Fulcher law indicate strongly a conventional spin glass nature. As the magnetic field increases, an anomalous enhancement of the irreversible temperature is observed, suggesting a field-induced nonequilibrium magnetic state. These results can be understood by considering the interaction between the antiferromagnetic clusters and the metamagnetic canted-spin matrix inside the nanoparticles.

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.