Dependent Ac Susceptibility Research Articles

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.

Entropic topography associated with field-induced quantum criticality in a magnetic insulator DyVO4

Exploration of low temperature phase transitions associated with quantum critical point is one of the most mystifying fields of research which is under intensive focus in recent times. In this work, through comprehensive experimental evidences, we report the possibility of achieving quantum criticality in the neighborhood of a magnetic field-tuned tricritical point separating paramagnetic, antiferromagnetic and metamagnetic phases in a magnetic insulator, DyVO4. Magnetic susceptibility and heat capacity indicate to the presence of a long-range second order antiferromagnetic transition at TN ~ 3.2 K. Field variation of Magnetic susceptibility and heat capacity, along with differential magnetic susceptibility and DC field dependent AC susceptibility gives evidence of the modification of the antiferromagnetic structure below the tricritical point; implying the presence of a field-induced first order metamagnetic transition which persists down to 1.8 K. Further, the magnetic field dependence of the thermodynamic quantity − dM/dT, which is related to magnetic Gruneisen parameter, approaches a minimum, followed by a crossover near 5 kOe to a maximum; along with a hyperbolic divergence in temperature response of dM/dT in the critical field regime. Temperature response of heat capacity at 5 kOe also shows a deviation from the conventional behavior. Entropic topography phase diagram allows tracking of the variation of the entropy, which indicates towards the emergence of the peak at quantum critical point into a V-shaped region at high temperatures. Our studies yield an inimitable phase diagram describing a tricritical point at which the second-order antiferromagnetic phase line terminates followed by a first order line of metamagnetic transition, as the temperature is lowered, leading to metamagnetic quantum critical end point.

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.

Bulk magnetization, temperature - Dependent ac susceptibility and dc resistivity study on Mn0.7+xZn0.3SixFe2-2xO4 (x = 0.0–0.3) spinel ferrites

Polycrystalline samples of spinel ferrite series, Mn0.7+x Zn0.3SixFe2-2xO4 (x = 0.0, 0.1, 0.2, 0.3), were characterized by X-ray powder diffractometry, bulk magnetization at 300 K and 80 K, temperature dependence of low field ac susceptibility (χac (T)) (T = 300–525 K) and dc electrical resistivity (ρdc (T)) (T = 300–772 K) measurements. The variation of intrinsic and extrinsic magnetic parameters as a function of Mn–Si concentration (x) has been discussed. It is found that the system possesses a non-collinear spin arrangement and the canting angle increases with x. The Néel temperatures determined experimentally from susceptibility measurement adequately reflected the values calculated theoretically. The χac (T) curves revealed normal ferrimagnetic behavior while ρdc (T) depicts characteristic semiconducting behavior. The activation energy is found to increase with x that can be explained in light of the structural and microstructural parameters.

Surface driven exchange bias in nanocrystalline CoCr2O4

This article reports exchange bias (EB) in nanocrystalline cobalt chromite CoCr2O4, - a phenomenon manifested by loop shift followed by increase in coercivity after cooling the sample in static magnetic field. The average particle size of the nanocrystalline CoCr2O4 is 30 nm, confirmed from transmission electron microscopy images. Thermal variation of magnetization data M(T), in zero field cooling (ZFC) and field cooling (FC) modes depict long range order below the lock-in transition at TL≈16 K, spin-spiral transition at TS≈28 K and a collinear ferrimagnetic state at TC≈100 K. Magnetic hysteresis loops are recorded in ZFC and FC modes at different cooling fields (). In addition, hysteresis loops are recorded in FC mode for a wide temperature range. In both cases, a clear signature of conventional exchange bias effect is observed. Analysis of the training effect in magnetic hysteresis loop and temperature dependent ac susceptibility (χac) at different frequencies suggest that the surface spins of CoCr2O4 nanoparticles play an important role in controlling the magnetic properties, and enhance the EB effect in collaboration with competition between non-collinear spiral spin state and collinear long range ferrimagnetic order.

Exotic magnetic behaviour and evidence of cluster glass and Griffiths like phase in Heusler alloys Fe2-xMnxCrAl (0\u2009\u2264\u2009x\u2009\u2264\u20091)

We present a detailed study of structural, magnetic and thermodynamic properties of a series of Heusler alloys Fe2-xMnxCrAl (x = 0, 0.25, 0.5, 0.75 and 1). Structural investigation of this series is carried out using high resolution synchrotron X-ray diffraction. Results suggest that with increasing Mn concentration, the L21 structure of Fe2CrAl is destabilized. The DC magnetization results show a decrement in paramagnetic (PM) to ferromagnetic (FM) phase transition temperature (TC) with increasing Mn concentration. From the systematic analysis of magnetic memory effect, heat capacity, time dependent magnetization, and DC field dependent AC susceptibility studies it is observed that, Fe2CrAl exhibits cluster glass(CG)-like transition approximately at 3.9 K (Tf2). The alloys, Fe1.75Mn0.25CrAl and Fe1.5Mn0.5CrAl exhibit double CG-like transitions near Tf1 ~ 22 K, Tf2 ~ 4.2 K and Tf1 ~ 30.4 K, Tf2 ~ 9.5 K respectively, however, in Fe1.25Mn0.75CrAl, a single CG-like transition is noted at Tf2 ~ 11.5 K below TC. Interestingly, FeMnCrAl shows the absence of long ranged magnetic ordering and this alloy undergoes three CG-like transitions at ~22 K (Tf*), 16.6 K (Tf1) and 11 K (Tf2). At high temperatures, a detailed analysis of temperature response of inverse DC susceptibility clearly reveals the observation of Griffiths phase (GP) above 300 K (T*) in Fe2CrAl and this phase persists with Mn concentration with a decrement in T*.

Study of A-site disorder dependent structural properties and magnetic ordering in polycrystalline perovskite Sm0.5Ca0.5−xSrxMnO3

This paper reports the studies on the effect of A-site substitution by strontium on the structural properties and magnetic ordering in polycrystalline perovskite Sm0.5Ca0.5−xSrxMnO3 (x = 0.1, 0.2 and 0.3). The investigated samples are prepared by conventional solid-state reaction technique. XRD analysis at room temperature has confirmed orthorhombic structure of the sample with space group Pnma. The dependence of structural parameter, Curie temperature and coercivity on Sr doping content has been thoroughly investigated. It is observed that substitution of Sr2+ for Ca2+ increases lattice parameter, tolerance factor and the Curie temperature. However, the coercivity (Hc) decreases with increasing Sr content while the charge ordering process is weakened with increasing Sr content. Field cooled (FC) and zero-field cooled (ZFC) dc magnetizations measurements at low field and low temperature indicate that there is a spin-glass (SG) like state occurred. Temperature dependent ac susceptibility at different frequency indicates a spin-glass-like transition of the sample.

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.

Disordered and Frustrated Magnetization in Coated MnFe&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt; Nanoparticles Prepared by Microwave Plasma Synthesis

Disordered and frustrated magnetization of different surface coated (Cr2O3, Co3O4, ZrO2, and SiO2) MnFe2O4nanoparticles have been studied using SQUID-magnetometry. Magnetic measurements, such as ZFC/FC and ac-susceptibility evidence surface spin-glass behavior. ZFC/FC curves were also compared with numerical simulation to get information about effective anisotropy constants. Frequency dependent ac susceptibility results were analyzed by using Arrhenius, Vogel Fulcher and dynamic scaling laws to further confirm the spin-glass behavior. It is observed that the strength of surface spins disorder and frustration strongly depends upon the type of the coating material. All these analyses signify that disordered and frustrated surface magnetization in MnFe2O4nanoparticles greatly depend on the type of the surface coating materials and are useful for controlling the nanoparticle’s magnetism for different practical applications.

The Frequency Dependence of AC Susceptibility for Bi-2223/Ag Tapes With Varying Lengths

An experimental investigation on ac susceptibility χ = χ′ − j χ′′ of Bi-2223/Ag superconducting tapes with different lengths L is carried out under a perpendicular ac magnetic field. A series of n -value is deduced from the ac susceptibility scaling law, which is used to systematically analyze the induced current in Bi-2223/Ag superconducting tapes. Based on the experiment results, a critical length L c is defined for the tapes as ten times the width of them. AC susceptibility χ shows conspicuous frequency dependence when the length L is smaller than L c. This phenomenon is attributed to the induced transverse current in silver. In addition, the proportion of transverse current to total induced current increases with the decreasing length dimension in our research. The results help understand the electromagnetic properties of Bi-2223/Ag superconducting tapes better and lay down important experimental foundation for practical applications.

Coexistence of cluster ferromagnetism and cluster spin-glass like behaviour in melt-quenched Cu2Mn0.5Fe0.5Al Heusler alloy

We investigated the structural stability and magnetic properties of the melt-quenched Heusler alloy-Cu2Mn0.5Fe0.5Al using X-ray diffraction (XRD), scanning electron microscopy (SEM), dc magnetization and ac susceptibility measurements respectively. The structural analysis by Rietveld refinement showed that the alloy crystallised in L21 structure with no trace of secondary phase in the XRD pattern. The dc magnetic measurements exhibited a thermal irreversibility between field-cooled (FC) and zero field cooled (ZFC) magnetization curves at an irreversible temperature Tirr. A deviation from de Almeida –Thouless like line for Tirrvs. H data was observed, suggesting a possibility of cluster glass (CG) like phase in the sample. To confirm the CG behaviour, frequency dependent ac susceptibility (χac) measurements were carried out on the sample and was analysed by Vogel-Fulcher (VF) and Critical slowing down models. We obtained large relaxation time constants (τ0) from the fit to both the phenomenological models which indicate that the origin of glassy nature may not be from randomly oriented atomic spins like in spin-glasses (SG) but of magnetic clusters. The analysis of nonlinear susceptibilities of the sample confirmed the coexistence of ferromagnetic interactions and glassy nature in the sample. Further confirmation for the glassy nature in the sample was provided by ZFC memory experiments. No anomaly was found corresponding to long-range order in the heat capacity (CP(T)) measurement and a T3/2 dependence was also seen in the electrical resistivity which indicates the magnetic glassiness in the sample.

Surface effects and spin glass state in Co3O4 coated MnFe2O4 nanoparticles

Surface effects and spin glass state have been studied in Co3O4 coated manganese ferrite (MnFe2O4) nanoparticles by using AC and DC magnetic measurements. Average crystallite size of Co3O4 coated MnFe2O4 nanoparticles was 7 nm as calculated by Debye–Scherrer’s formula. Simulated ZFC/FC revealed higher value of effective anisotropy (Keff) = 5 × 106 erg cm−3 of these nanoparticles as compared to bulk MnFe2O4 due to enhanced surface effects. Temperature dependent saturation magnetization followed the Bloch’s law. Temperature dependent coercivity showed sharp increase below 25 K due to strong surface anisotropy and exchange coupling at interface between ferrimagnetic core and antiferromagnetic surface. For frequency dependent AC-susceptibility, dynamic scaling law fit confirmed the spin glass behavior in Co3O4 coated MnFe2O4 nanoparticles. DC field dependent AC-susceptibility showed the suppression of energy barrier, reduced activation energy and decreased strength of interparticle interactions with increasing DC field. Slow spin relaxation in ZFC protocol further confirmed the presence of spin-glass behavior. All this analysis confirmed the existence of spin-glass behavior as attributed to disordered surface spins and interparticle interactions in these nanoparticles, which gets suppressed after the application of moderate DC field.

Complex magnetic behaviour and evidence of a superspin glass state in the binary intermetallic compound Er5Pd2

The binary intermetallic compound Er5Pd2 has been investigated using dc and ac magnetic susceptibilities, magnetic memory effect, isothermal magnetization, non-linear dc susceptibility, heat capacity and magnetocaloric effect studies. Interestingly, even though the compound does not show geometrical frustration it undergoes glassy magnetic phase transition below 17.2 K. Investigation of dc magnetization and heat capacity data divulged absence of long-ranged magnetic ordering. Through the magnetic memory effect, time dependent magnetization and ac susceptibility studies it was revealed that the compound undergoes glass-like freezing below 17.2 K. Analysis of frequency dependence of this transition temperature through scaling and Arrhenius law; along with the Mydosh parameter indicate, that the dynamics in Er5Pd2 are due to the presence of strongly interacting superspins rather than individual spins. This phase transition was further investigated by non-linear dc susceptibility and was characterized by static critical exponents γ and δ. Our results indicate that this compound shows the signature of superspin glass at low temperature. Additionally, both conventional and inverse magnetocaloric effect was observed with a large value of magnetic entropy change and relative cooling power. Our results suggest that Er5Pd2 can be classified as a superspin glass system with large magnetocaloric effect.

Low temperature ferromagnetic behavior and temperature dependent anomalous dielectric relaxation of Zn0.90Ni0.05Mn0.05O diluted magnetic semiconductor

We report structural, magnetic and temperature dependent dielectric properties of diluted magnetic semiconductor Zn0.90Ni0.05Mn0.05O prepared by solid state reaction technique. X-ray diffraction analysis revealed formation of single phase hexagonal wurtzite structure. Scanning electron microscopy and atomic force microscopy images indicated increase in grain size and roughness respectively with increasing sintering temperature. Field dependent DC magnetization at low temperature exhibited ferromagnetic ordering with coercivity ~ 6 × 104 A/m and remanence ~ 17 A/m. Complex initial permeability values were found to be positive for the measurement frequency range (1 kHz–120 MHz) with a relaxation at lower frequency. Temperature dependent DC magnetization and AC susceptibility followed curie law with curie temperature below 65 K. Temperature dependent dielectric constants ($$\varepsilon ^{\prime}\,\& \,\varepsilon ^{\prime\prime}$$) and loss tangents ($$tan\delta$$) measured for selected frequencies were found to be an increasing function of temperature and decreasing function of frequency. AC conductivity ($${\sigma }_{ac}$$) values were found to be an increasing function of frequency and temperature. Clear signatures of relaxations were observed in $$\varepsilon ^{\prime},\,\varepsilon ^{\prime\prime},$$ $$tan\delta$$ and $${\sigma }_{ac}$$ for temperatures above 200 °C.

Study of activation energy and AC-susceptibility for nano-ZnO doped MgB2 superconductor in presence of varying amplitude of applied field

AC-susceptibility dependence of temperature measurements with varying applied magnetic field is studied for nano-ZnO doped MgB2 superconductors at different doping levels along with pristine material. The home-made susceptometer is used to measure the AC-susceptibility at different varying field (H ac = 0.2, 0.8, 4.0, 20.0 and 40.0 A/cm). The AC-susceptibility of the material shows the magnetic properties of that material either attracting or repelling in nature. The purpose of choosing the MgB2 superconductor is that because it has simple crystal structure with profound properties such as two band gaps. Generally, the AC-susceptibility has two parts, real and imaginary. The peak height of the imaginary part of the AC-susceptibilities is enhancing by increasing the applied field and shifted to the lower temperature. The real part of AC-susceptibility as a function of temperature under the presence of varying applied fields illustrates the minutely drop of the transition temperature, T c. All the samples were cooled down from 300 to 4 K in presence of applied field. The relaxation time is calculated using by optimum frequency (f = 98 Hz). The activation energy as a function of current density is measured with different doped and pure MgB2 samples. In addition, this activation energy as a function of current density is also determined under the effect of the applied field. The collective pinning model was adopted in order to calculate the activation energy as a function of current density. The current density is found to be increased as the activation energy increases in both the cases.

Evolution of structure and magnetic properties of stoichiometry and oxygen rich LaMnO3 nanoparticles

In this report, the evolution of structure and magnetic properties in stoichiometry and oxygen rich LaMnO3 samples synthesized by a simple, cost effective, conventional co-precipitation technique are systematically studied. While X-ray powder diffraction reveals a monoclinic, I2/a structure in both samples, Reitveld refinement demonstrates higher John-Teller (J-T) distortion in the former one. Higher J-T distortion is accompanied with a large deviation in MnO bond length. FESEM (Field Emission Scanning Electron Microscopy) analysis combined with crystallite size calculated from XRD peak profile suggest that the particles are of 50–60nm and are polycrystalline in nature. The antiferromagnetism observed in bulk LaMnO3 with TN=140K evolves towards a ferromagnetic order in both stoichiometry and oxygen rich samples. However, Curie temperature, TC, is found to be higher in oxygen rich sample due to higher MnOMn bond angle. The large effective magnetic moment calculated from Curie-Weiss plot is found to be higher in stoichiometry sample. Higher moment is ascribed to high oxygen vacancies evidenced from XPS analysis. Dispersion behavior observed in frequency dependent ac susceptibility (χ) indicates the spin-glass, cluster-glass and interacting superparamagnetic behavior. Fitting the susceptibility data with phenomenological models such as Neel-Arrhenius, Vogel-Fulcher and Power law, while canonical spin-glass, cluster-glass and interacting superparamagnetism is ruled out, the spin-glass behavior with a higher relaxation time in oxygen rich sample is confirmed.

Role of SiO2 coating in multiferroic CoCr2O4 nanoparticles

Effect of silica (SiO2) coating concentration on structural and magnetic properties of multiferroic cobalt chromite (CoCr2O4) nanoparticles have been studied. The nanoparticles with average crystallite size in the range 19 to 28 nm were synthesised by sol-gel method. X-ray diffraction (XRD) analysis has verified the composition of single-phase cubic normal spinel structure of CoCr2O4 nanoparticles. The average crystallite size and cell parameter decreased with increasing SiO2 concentration. TEM image revealed that the shape of nanoparticles was non-spherical. Zero field cooled/field cooled (ZFC/FC) curves revealed that nanoparticles underwent a transition from paramagnetic (PM) state to collinear short-range ferrimagnetic (FiM) state, and this PM–FiM transition temperature decreased from 101 to 95 K with increasing SiO2 concentration or decreasing crystallite size. A conical spin state at Ts = 27 K was also observed for all the samples which decreased with decreasing average crystallite size. Low temperature lock-in transition was also observed in these nanoparticles at 12 K for uncoated nanoparticles which slightly shifted towards low temperature with decreasing average crystallite size. Saturation magnetization (Ms) showed decreasing trend with increasing SiO2 concentration, which was due to decrease in average crystallite size of nanoparticles and enhanced surface disorder in smaller nanoparticles. The temperature dependent AC-susceptibility also showed the decrease in the transition temperature (Tc), broadening of the Tc peak and decrease in magnetization with increasing SiO2 concentration or decreasing average crystallite size. In summary, the concentration of SiO2 has significantly affected the structural and magnetic properties of CoCr2O4 nanoparticles.

Magnetoelectric Coupling, Ferroelectricity, and Magnetic Memory Effect in Double Perovskite La3Ni2NbO9.

We observe ferroelectricity in an almost unexplored double perovskite La3Ni2NbO9. Ferroelectricity appears below ∼60 K, which is found to be correlated with the significant magnetostriction. A reasonably large value of spontaneous electric polarization is recorded to be ∼260 μC/m(2) at 10 K for E = 5 kV/cm, which decreases signifi- cantly upon application of a magnetic field (H), suggesting considerable magnetoelectric coupling. The dielectric permittivity is also influenced by H below the ferroelectric transition. The magnetodielectric response scales linearly to the squared magnetization, as described by the Ginzburg-Landau theory. Meticulous studies of static and dynamic features of dc magnetization and frequency dependent ac susceptibility results suggest spin-glass state below 29 K. Intrinsic magnetic memory effect is observed from zero-field cooled magnetization and isothermal remanent magnetization studies, also pointing spin-glass state below 29 K. Appearance of ferroelectricity together with a significant magnetoelectric coupling in absence of conventional long-range magnetic order is promising for searching new magnetoelectric materials.