Inverse Magnetic Susceptibility Research Articles

Magnetocaloric effect and critical exponents at near room temperature of CrTe

In this work, I investigated and studied the CrTe compound, which has a spin amplitude of about 2, to determine its critical behaviors at near room temperature, magnetocaloric cooling, and magnetic characteristics. For the purpose of this study, I used the Metropolis algorithm with the Monte Carlo approach. The Curie temperature of CrTe is around 326 K, which is determined through the magnetization M(T) and the inverse magnetic susceptibility (χ−1). The Arrott curves (H/M vs. M2) and the isothermal magnetization M(H) demonstrate a second-order ferromagnetic (FM) to paramagnetic (PM) state phase transition. The magnetic entropy change (-ΔSM), relative cooling power (RCP), and refrigeration capacity (Q) at 8 T are 1.31 J kg⁻1 K⁻1, 106 J kg⁻1, and 79 J kg⁻1, respectively. To calculate the critical exponents of CrTe, I used the Widom scaling relation and the modified Arrott plot (MAP). The critical exponent values were found to be in good agreement with those estimated using the renormalization group approach for a 3D-Ising model.

ESR and magnetization studies of double perovskite Sr[formula omitted]FeNbO6

By pyrolysis of nitrate-organic mixtures, we synthesized double perovskite Sr2Fe0.6NbO5.4. We performed the study of the temperature dependencies of the EPR spectra, DC and AC magnetization, and magnetization isotherms in a wide temperature range of 4–300K. The EPR spectrum is described by the sum of two contributions below 125 K and indicates phase separation of the sample. The paramagnetic part of the inverse magnetic susceptibility is well approximated by the Curie–Weiss law with ΘCW≈−60K. The exchange integrals between iron spins, calculated from the temperature dependence of the ESR integral intensity and the Curie–Weiss temperature, coincide and are equal to J/kB≈10K. The negative sign of the Curie–Weiss temperature and the hysteresis loops indicate the antiferromagnetic canted nature of exchange interactions, which is further confirmed by the presence of a peak in the real part of the AC magnetization and its absence in the imaginary part.

Improved magnetic properties of Fe substituted La2CoMnO6 for spintronic applications

A detailed study of magnetic behaviour of double perovskites was carried out by developing La2Co1-xFexMnO6; (x = 0.0, 0.2, 0.5, 0.8 and 1.0) via sol–gel technique. All samples showed single magnetic transition except pure LCMO (x = 0.0) and LCFMO2 (x = 0.5). Temperature dependent inverse magnetic susceptibility (χ-1) was fitted in accordance with Curie-Weiss law. μeff (effective magnetic moment) was determined for all the developed samples. Pure LCMO showed a downward divergence from Curie-Weiss law below 230 K but above transition temperature which indicates the characteristics of Griffiths phase. Saturation magnetization (Ms) increased initially and then decreased, whereas coercive field (Hc) was found to be monotonous decreased with increase in Fe content. LCFMO1 (x = 0.2) was noticed to have maximum Ms (38.87 emu/g) which indicated that the magnetic properties of pure LCMO were improved with Fe substitution, but for lower substitution only. Magnetization vs. field (M−H) curves were not well saturated even at a magnetic field of 60 kOe, therefore, values of Ms for all the samples were estimated using law of approach to saturation. With increase in Fe content, an increased anti-site disorder among Mn and Co strengthened antiferromagnetic (AFM) super exchange interactions and weakened ferromagnetism (FM). Therefore, substitution of Fe on Co-site was responsible for systematic decrease in saturation magnetization when × exceeded 0.2.

Revival of Ferromagnetic-Metallic Phase and Emergence of Griffiths Phase via Ni and Cr Substitution in Charged-Ordered Pr0.75Na0.25MnO3 Manganite

Polycrystalline samples of Pr0.75Na0.25Mn1-xAxO3 (A = Ni, Cr; x = 0, 0.03) were prepared using conventional solid-state method and their structural and magnetic properties were investigated. Monovalent-doped Pr0.75Na0.25MnO3 exhibits insulating behavior which is related to the strong localization of charge carriers due to the presence of charge ordering (CO). The substitution of different types of magnetic ions, A, which are Ni2+ and Cr3+, at a concentration of x = 0.03 at the Mn site in the CO Pr0.75Na0.25Mn1-xAxO3 manganite, has been found to dramatically modify its structural and magnetic properties. X-ray diffraction patterns showed that all samples were present in single phase and crystallized in orthorhombic structure with Pnma space group. The Rietveld refinement analysis showed that the unit cell volume increased due to Ni2+ substitution, while it decreased due to Cr3+ substitution, which may be attributed to the different ionic radii of both ions. The suppression of the CO state and the induction of the ferromagnetic-metallic state in the Ni-substituted and Cr-substituted samples are suggested to be due to the induction of double-exchange interaction involving Ni2+–O–Mn4+ and Mn3+–O–Cr3+, respectively. The variation in magnetic properties between the Ni-substituted and Cr-substituted samples is suggested to be due to the existence of different strengths of ferromagnetic interaction between Ni2+–O–Mn4+ and Mn3+–O–Cr3+. In addition, the deviation of the temperature dependence of the inverse magnetic susceptibility curves from the Curie-Weiss law suggests the existence of Griffith’s phase-like behaviour for both substitution samples. The Ni-substituted sample induced a greater GP effect than the Cr-substituted sample due to the larger difference of TC and TG value (TC-TG).

Surfactant-free synthesis and magnetic property evaluation of air-stable cobalt oxide nanostructures

We report the synthesis of metastable cobalt oxide (CoO) nanostructures via the low-temperature microwave-assisted solvothermal (MAS) process. An alcoholic solution of cobalt (II) acetylacetonate in a sealed vessel was irradiated with microwaves at a temperature <150 °C and a pressure below 100 psi. As-synthesized powder material was characterized in terms of its structure and morphology. X-ray diffractometry (XRD) indicates the formation of well-crystallized CoO nanoparticles without the need for post-synthesis annealing. The mean crystallite size of the nanoparticles was estimated to be 41 nm. The morphology of the as-prepared powder sample was evaluated by field-emission scanning electron microscopy (FESEM), which revealed the formation of densely packed nanospheres of diameter <100 nm. The CoO nanospheres were obtained without the need for any surfactants or capping agents; they were found to be quite resistant to oxidation in ambient air over several months. We attribute the stability of CoO nanospheres to their dense packing, the driving force being the minimization of surface energy and surface area. Fourier-transform infrared (FT-IR) spectroscopy and Raman spectroscopy confirm the formation of phase-pure CoO nanostructures. The deconvolution of the active modes in Raman spectra obtained at room temperature reveals the Oh symmetry in rock-salt CoO produced by the MAS route. We have analyzed its effect on the magnetic characteristics of the CoO nanostructures. Isothermal field-dependent magnetization (MH) and inverse magnetic susceptibility measurements show a phase transition from antiferromagnetic to ferromagnetic interactions in the CoO nanostructures at around 10 K. The results indicate that the phenomenon of magnetic phase transition as a function of temperature is unique to CoO nanoparticles. This finding reveals the magnetic behavior of CoO nanostructures and presents opportunities for its possible application as an anisotropy source for magnetic recording.

The significant magnetic attenuation with submicrometer scale magnetic phase separation in tensile-strained LaCoO3 films

It is well known that the epitaxial strain plays a vital role in tuning the magnetic states in transition metal oxide LaCoO3 films. Here, we reported a robust long-range ferromagnetic (FM) ground state in a tensile-strained perovskite LaCoO3 film on a SrTiO3 (STO) substrate, which has a very significant attenuation when the thickness ranges from 10 to 50 nm. It is speculated that such attenuation may be caused by the appearance of the cross-hatched grain boundary, which relaxes the tensile strain around the crosshatch, resulting in the local non-FM phases. Magnetic force microscope observation reveals non-FM patterns correlated with the structural crosshatches in the strain-relaxed film even down to a temperature of 2 K and up to a magnetic field of 7 T, suggesting the phase separation origin of magnetization attenuation. Furthermore, the investigations of the temperature-dependent inverse magnetic susceptibility show a deviation from the Curie–Weiss law above the transition temperature in a 50-nm-thick LaCoO3/STO film but not in the LaCoO3/LaAlO3 film, which is ascribed to the Griffiths phase due to the crosshatch-line grain boundaries. These results demonstrated that the local strain effect due to structural defects is important to affect the ferromagnetism in strain-engineered LaCoO3 films, which may have potential implications for future oxide-based spintronics.

Particle size-dependent structural and magnetic properties of Pr0.6-xBixSr0.4MnO3 (x = 0.10 and 0.25)

Nanoparticles of Pr0.6-xBixSr0.4MnO3 (x = 0.10 and 0.25) have been prepared using the swift and cost-effective top-down approach in a high-energy planetary ball mill without any chemical processing and environmental impacts resulting due to pollutants arising from nanoparticle synthesis. The work investigates the role of particle size reduction on the structural and magnetic properties. An increase in milling time from 0 to 240 min results in the drastic reduction of particle size which drops significantly in the initial phase of ball milling i.e., 0–60 min while the microstrain and dislocation densities show nearly monotonic increase. Interestingly the magnetism and size reduction demonstrate a clear one to one correspondence. In both compositions, the paramagnetic (PM) to ferromagnetic (FM) transition, TC decreases and the magnetic transitions broaden both as function Bi and size reduction. The Curie–Weiss analysis of the inverse magnetic susceptibility ({chi }^{-1}) shows a deviation from linearity indicating the presence of the short-range FM interactions above TC which has been understood as Griffith’s phase like singularity. The size reduction can effectively reduce TC, enabling a significant control over tuning of transition temperature and net magnetization. The field dependent magnetization for x = 0.10 demonstrate dominant FM state whereas for x = 0.25, metamagnetic magnetization highlights competitive coexistence of FM and antiferromagnetic (AFM) interactions in the system. With ball milling, the net magnetization decreases, and the metamagnetic behaviour is suppressed. The M–H loops of ball milled nanoparticles are highly unsaturated indicating the enhanced surface disorder in the nanoparticles which can be understood using the core–shell structure.

Structural and magnetic characterization of Co2+ substituted Ni-Zn ferrite nanoparticles synthesized by refluxing co-precipitation and their potential application as gas sensors

This work investigates the effect of Co2+ content on the structural, magnetic, and gas sensing properties of Ni0.3Zn0.7-xCoxFe2O4 (x=0.00, 0.05, 0.15, 0.25, 0.35) nanoferrites synthesized by refluxing co-precipitates of metal chlorides. X-ray diffraction and electron microscopy were used to study the structure and morphological aspects of the materials. X-ray diffraction (XRD) patterns revealed single-phase nanomaterial with shrinking lattice constants from 8.41 to 8.38 Å with increasing Co2+ content. The crystallite sizes varied slightly between 9.95 and 10.06 nm. The vibrating sample magnetometer (VSM) was used to extract information of the magnetization of the materials at room temperature. Saturation magnetization increased from 39.5 to 48.5 emu/g from x = 0 to 0.25 and dropped at x = 0.35 to 46.2 emu/g. Furthermore, the coercivity, effective anisotropy constant and remnant magnetization increased from 170 to 400 Oe, 696 to 1925 erg/Oe, and 1.15 to 2.72 emu/g respectively. A distinct correlation between the inverse initial magnetic susceptibility and unit cell volume of the materials was found. Room temperature 57Fe Mössbauer spectra revealed a magnetic transition from paramagnetic to ordered magnetic phase with increased Co2+ content. Except for sample with x = 0 and 0.15, where suspected co-existence of Fe3+ and Fe2+ was observed Fe3+ ions were observed. Zn2+ ions were found to largely occupy the B sites. Specific surface area and the average pore size distribution increased from 61 – 85 m2 g−1 and 22 – 25 nm respectively, with the addition of Co2+ content. The composition x= 0 showed exceptional response to propanol suggesting good suitability for sensor fabrication. All samples revealed the selectivity preferences in the order of propanol > ethanol > methanol.

Formation of 2/1 and 1/1 approximants in a quaternary Al–Pd–Au–Yb system

In this study, 2/1 and 1/1 approximants in a quaternary Al–Pd–Au–Yb system were synthesized by substituting Au into original Al–Pd–Yb quasicrystals. In the Al50Pd24Au10Yb16 and Al52Pd22Au10Yb16 alloys, 2/1 and 1/1 phases were formed, respectively, and the lattice constants were relatively small—2.314 and 1.431 nm for the 2/1 and 1/1 phases, respectively. Scanning transmission electron microscopy revealed that both approximants comprise Tsai-type atomic clusters. Inverse magnetic susceptibility for the approximants obeyed the Curie–Weiss law, indicating the presence of paramagnetism. The effective magnetic moments were estimated to be 3.20 μB and 2.94 μB for the 2/1 and 1/1 phases, respectively, indicating that Yb in both approximants adopts an intermediate valence state.

Magnetic and transport behaviors of Co substitution in La0.7Sr0.3MnO3 perovskite

The magnetic and electrical properties of the substituted La0.7Sr0.3Mn1-xCoxO3 perovskite (0 ≤ x ≤ 1) were studied. Analysis of the average spin value obtained from inverse magnetic susceptibility using Curie–Weiss law for the samples suggested Co4+ ions were in the high spin state, whereas the spin state of Co3+ ions transformed from high spin to intermediate-spin state at x= 0.8. The metallic conduction behavior of the x = 0 and x = 1 samples was analyzed using the empirical equation ρt=ρ0+ρ2T2+ρ4.5T4.5, reflecting the importance of electron–magnon scattering effects similarly to other manganites. All the other samples displayed semiconductor-like behavior throughout the studied temperature range. The resistivity data in semiconducting La0.7Sr0.3Mn1-xCoxO3 could be fitted to the model of variable range hopping, and both hopping distance and average hopping energy reduced with increasing doped Co concentration.

Observation of Griffith like phase and large magnetocaloric effect in nanocrystalline La0.7Ag0.2Bi0.1MnO3

In this paper, we present structural, magnetic, magnetocaloric, and critical study of perovskite La0.7Ag0.2Bi0.1MnO3 (LABMO) nanocrystalline compound synthesized by the sol–gel method. Temperature dependent magnetization measurements reveal the significant suppression of ferromagnetism in the LABMO sample upon Bi-doping on a La-site. The downturn in inverse magnetic susceptibility (χ−1) observed just above TC (236 K) in the paramagnetic regime corroborates the presence of short-range ferromagnetic correlations, which is the characteristic of the Griffith like phase below 270 K. The deviation from linear paramagnetic behavior in χ−1 implies the strong Griffith singularity. Furthermore, we have employed an integrated Maxwell's thermodynamic relation numerically and used isothermal magnetization data to determine the change in magnetic entropy at various magnetic fields. For a magnetic field change of 5 T, the value of maximum magnetic entropy change is found to be ∼6 J kg−1 K−1. We have also explored the critical behavior of the LABMO sample at transition temperatures using different theoretical models. The value of exponents β, γ, and δ does not fall into any known universality class. Despite this, the scaling relations show that interactions are renormalized around the Curie temperature (TC). The exponent n ≤ 2 extracted from field dependency on the magnetic entropy change confirms the second-order phase transition in LABMO.

Anomalous electrical transport behavior in the vicinity of the first-order magnetostructural transition in the giant magnetocaloric Gd4ScGe4

Magnetic, specific heat, and electrical transport measurements of ${\mathrm{Gd}}_{4}{\mathrm{ScGe}}_{4}$ single crystals reveal sharp, discontinuous, nearly anhysteretic first-order magnetostructural transformation at ${T}_{\mathrm{C}}=63\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. The electrical resistivity exhibits two distinct regions where it increases with decreasing temperature: between ${T}_{\mathrm{C}}$ and 120 K, as well as below 3 K; electronic transport remains conventionally metallic at all other measured temperatures, up to 325 K. The dispersion of charge carriers due to electron-paramagnon scattering is the likely reason for the observed anomalous transport above ${T}_{\mathrm{C}}$. Additionally, the existence of intermediate lattice states near the transition recognized by the spike in the interslab Ge-Ge distances is expected to reduce the mean free path of the electrons contributing to the unusual behavior of the electrical resistivity between ${T}_{\mathrm{C}}$ and 120 K. Beyond conventional electronic and lattice terms, the third component of likely magnetic origin contributes to the low-temperature heat capacity; the presence of spin waves may be responsible for the increased electron-magnon scattering below 3 K. Minor magnetocrystalline anisotropy is observed with the $b$ axis as the easy magnetization axis in ${\mathrm{Gd}}_{4}{\mathrm{ScGe}}_{4}$. A negative deviation from linearity in the temperature dependence of the inverse magnetic susceptibility is detected below 150 K.

The Study of Diluted Magnetic Semiconductor: The Case of Manganese Doped Gallium Nitride

The diluted magnetic semiconductor, (Ga, Mn) N has recently attracted intense research interest for the purpose of spintronics application. The material is believed to circumvent the difficulty of combining data processing and mass storage facilities in a single crystal besides solving non-volatility problems. The concentration x, of Mn that substitutes for a fraction of Ga in the compound is thought to contribute a large concentration of magnetic moments and holes. The material studied is focused on dilute magnetic semiconductors (DMS) like <i>Ga_1-XMn<SUB>X</SUB>N</i> that play a key role in semiconductor spintronics. Due to their ferromagnetic properties they can be used in magnetic sensors and as spin injectors. The basic problems for applications are, however, the relatively low Curie temperatures of these systems. Therefore, we focus on the understanding of the magnetic properties and on a reliable calculation of Curie temperatures from first principles. We have developed a theoretical framework for calculating critical temperatures by combining first principles calculations and in terms of the Ruderman–Kittel–Kasuya–Yosida quantum spin model in Green’s function approach. Magnetic properties of the group-III nitride semiconductors are introduced here with basic material parameters (temperature, concentration, heat capacity, etc. Temperature dependencies of the spin wave specific heat, inverse magnetic susceptibility and reduced magnetization are determined. Therefore, the dependence of the Neel temperature on the manganese ion concentration is linear thus for our calculation the highest Neel temperature obtained T=146.3k within the concentration of 0.2.

The magnetic properties of multiferroic Ba5Fe3F19−δ

In order to break the contradictory requirements of the outer shell electronic structure for multiferroism in oxide perovskites, multiferroic fluorides were selected to be studied, in which the ferroelectricity and ferromagnetism may both come from the same magnetic ions. Pure phase Ba5Fe3F19−δ powders were prepared by solid state reaction. Energy dispersive X-ray spectroscopy results indicate the deficiency of Fe. The mixed valence states of Fe3+ and Fe2+ are confirmed by X-ray photoelectron spectroscopy. The observation of amplitude and phase hysteresis loops measured by piezoresponse force microscopy indicates the ferroelectricity in Ba5Fe3F19−δ. The temperature dependent magnetization curves show a continuous increase with decreasing temperature, without clear phase transition. However, the temperature dependent inverse magnetic susceptibility (χ) follows the Curie–Weiss law above 150 K with Curie–Weiss temperature of −55 K, suggesting predominantly antiferromagnetic interactions between the Fe ions. The χT decreases with the temperature decreasing from 300 K, indicating the onset of weak antiferromagnetic interaction from temperature above 300 K. No hysteresis loop was observed for the magnetic dependent magnetization curves down to 5 K, indicating the lacking of long-range ordering. A peak was observed in the temperature dependent dielectric constant and dielectric loss at around 257 K, which was strongly suppressed by the application of 5 T magnetic field. The observation of magnetocapacitance effect confirms the magnetoelectric coupling, which originates from the magnetic and structural coupling.

Tailoring the Griffiths-like cluster formation in the insulator ferromagnet spin-glass Gd2NixCo1−xMnO6 double perovskite

In this paper we present the effects of the composition on structural, vibrational, magnetic, and electronic properties of double perovskite 3d based manganese oxides ${\mathrm{Gd}}_{2}{\mathrm{Ni}}_{x}{\mathrm{Co}}_{1\ensuremath{-}x}\mathrm{Mn}{\mathrm{O}}_{6}$ (GNCMO) powder (for $0&lt;x&lt;1$) prepared by high temperature solid state chemistry. These compounds have a great research interest due to their specific physical properties. We have shown that the synthesized compounds exhibit ferromagnetic interaction between the spins of (${\mathrm{Mn}}^{4+}, {\mathrm{Co}}^{2+}$), and (${\mathrm{Mn}}^{4+}, {\mathrm{Ni}}^{2+}$) cations via oxygen ions and antiferromagnetic interactions between (${\mathrm{Gd}}^{3+}, {\mathrm{Co}}^{2+}$), (${\mathrm{Gd}}^{3+}, {\mathrm{Ni}}^{2+}$) and (${\mathrm{Gd}}^{3+}, {\mathrm{Gd}}^{3+}$) and present a paramagnetic to ferromagnetic phase transition. The paramagnetic Curie-Weiss temperature of GNCMO (${T}_{\mathrm{c}}$) increases from 121to 128K as the nickel content increases from 0.2 to 0.8. Further, temperature dependent inverse magnetic susceptibility $\ensuremath{\chi}{(T)}^{--1}$ shows deviation from Curie Weiss behavior around ${T}_{G}$ which depicts that Griffiths phase already observed in several perovskite compounds. The value of the GP percentage and the exponent of ($\ensuremath{\lambda}&lt;1$) are found to increase as the nickel content increases. The hysteresis curve at $T=20\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for $x=0.2$ displays a metamagnetic transition explained by the reorientation of the Gd spin moments towards the Mn, Co, and Ni spin moments. The imaginary part of susceptibility ${\ensuremath{\chi}}^{\ensuremath{'}\ensuremath{'}}$ shows a small shift of the peak $\ensuremath{\sim}1\phantom{\rule{4pt}{0ex}}\mathrm{K}$ indicating the presence of spin or cluster glass behavior.

Synthesis, Optical, Magnetic and Thermodynamic Properties of Rocksalt Li1.3Nb0.3Mn0.4O2 Cathode Material for Li-Ion Batteries

Since the discovery of the reversible intercalation of lithium-ion materials associated with promising electrochemical properties, lithium-containing materials have attracted attention in the research and development of effective cathode materials for lithium-ion batteries. Despite various studies on synthesis, and electrochemical properties of lithium-based materials, fairly little fundamental optical and thermodynamic studies are available in the literature. Here, we report on the structure, optical, magnetic, and thermodynamic properties of Li-excess disordered rocksalt, Li1.3Nb0.3Mn0.4O2 (LNMO) which was comprehensively studied using powder X-ray diffraction, transient absorption spectroscopy, magnetic susceptibility, and low-temperature heat capacity measurements. Charge carrier dynamics and electron–phonon coupling in LNMO were studied using ultra-fast laser spectroscopy. Magnetic susceptibility and specific heat data are consistent with the onset of long-range antiferromagnetic order at the Neel temperatures of 6.5 (1.5) K. The effective magnetic moment of LNMO is found to be 3.60 μB. The temperature dependence of the inverse magnetic susceptibility follows the Curie–Weiss law in the high-temperature region and shows negative values of the Weiss temperature 52 K (3), confirming the strong AFM interactions.

Synthesis, Optical, Magnetic and Thermodynamic Properties of Rocksalt Li1.3Nb0.3Mn0.4O2 Cathode Material for Li-Ion Batteries

Since the discovery of the reversible intercalation of lithium-ion materials associated with promising electrochemical properties, lithium-containing materials have attracted attention in the research and development of effective cathode materials for lithium-ion batteries. Despite various studies on synthesis, and electrochemical properties of lithium-based materials, fairly little fundamental optical and thermodynamic studies are available in the literature. Here, we report on the structure, optical, magnetic, and thermodynamic properties of Li-excess disordered rocksalt, Li1.3Nb0.3Mn0.4O2 (LNMO) which was comprehensively studied using powder X-ray diffraction, transient absorption spectroscopy, magnetic susceptibility, and low-temperature heat capacity measurements. Charge carrier dynamics and electron–phonon coupling in LNMO were studied using ultra-fast laser spectroscopy. Magnetic susceptibility and specific heat data are consistent with the onset of long-range antiferromagnetic order at the Néel temperatures of 6.5 (1.5) K. The effective magnetic moment of LNMO is found to be 3.60 μB. The temperature dependence of the inverse magnetic susceptibility follows the Curie–Weiss law in the high-temperature region and shows negative values of the Weiss temperature 52 K (3), confirming the strong AFM interactions.

Effect of nanoscale size reduction on the magnetic properties of Pr0.6Sr0.4MnO3

The ferromagnetic Pr0.6Sr0.4MnO3 sample has been subjected to time bound high energy planetary ball milling with an aim to reduce the particle size. When the particle size is analyzed as a function of milling time, it is found that particle size decreases with increasing milling time and then saturates at a certain value. The implications of the decrease in particle size on magnetic properties have been carefully studied by analyzing the magnetization data in detail. With a decrease in particle size, the Curie temperature, TC and the magnitude of magnetization drops. A deviation in inverse magnetic susceptibility from the Curie-Weiss behaviour has been noticed for nanoparticles which suggest the presence of quenched disorder due to particle size reduction. The strength of magnetic interactions switches from long range to short range with a systematic decrease in particle size. The observed properties can be explained considering the core–shell structure of the nanoparticle where the surface defects strongly influence the degree of disorder in the system.

Thermomagnetic Correlation in La0.85-xBixNa0.15MnO3 Soft Ferromagnet due to Nonmagnetic Bi3+ Substitution

We report the structural, magnetic, and magnetocaloric properties of Bismuth (Bi)-substituted manganite La0.85-xBixNa0.15MnO3 (x=0, 0.1, 0.2, 0.25, and 0.3). X-ray diffraction data implicates the rhombohedral structure with Roverline{3}c space group. Bi2O3 has helped in ensuring phase pure, densified compounds even at low sintering temperature and hence avoiding the evaporation of volatile sodium. The increase in grain size and decrease in magnetic transition temperature (TC) are due to the Bi chemical activity and electronic structure. The samples have shown indirect magnetic transformation from soft ferromagnet to canted ferromagnet/antiferromagnet with Bi. Griffiths phase-like behavior in the inverse magnetic susceptibility was observed for x=0.1; with further increase in Bi, the samples are found to develop the antiferromagnetic competing phase. The phenomenological model was used to model the thermomagnetic behavior of all the samples. The sample with x=0.1 shows an increase in magnetic entropy change upon Bi substitution and the maximum of magnetic entropy change is seen at 275K emphasizing its potential in room temperature magnetic refrigeration.

Simulated tempering with irreversible Gibbs sampling techniques.

We present here two novel algorithms for simulated tempering simulations, which break the detailed balance condition (DBC) but satisfy the skewed detailed balance to ensure invariance of the target distribution. The irreversible methods we present here are based on Gibbs sampling and concern breaking DBC at the update scheme of the temperature swaps. We utilize three systems as a test bed for our methods: a Markov chain Monte Carlo simulation on a simple system described by a one-dimensional double well potential, the Ising model, and molecular dynamics simulations on alanine pentapeptide (ALA5). The relaxation times of inverse temperature, magnetic susceptibility, and energy density for the Ising model indicate clear gains in sampling efficiency over conventional Gibbs sampling techniques with DBC and also over the conventionally used simulated tempering with the Metropolis-Hastings (MH) scheme. Simulations on ALA5 with a large number of temperatures indicate distinct gains in mixing times for inverse temperature and consequently the energy of the system compared to conventional MH. With no additional computational overhead, our methods were found to be more efficient alternatives to the conventionally used simulated tempering methods with DBC. Our algorithms should be particularly advantageous in simulations of large systems with many temperature ladders, as our algorithms showed a more favorable constant scaling in Ising spin systems as compared with both reversible and irreversible MH algorithms. In future applications, our irreversible methods can also be easily tailored to utilize a given dynamical variable other than temperature to flatten rugged free energy landscapes.