Ferromagnetic-paramagnetic Transition Temperature Research Articles

Modeling the magnetocaloric effect and critical behavior near the ferromagnetic–paramagnetic transition temperature in Zn0.5-xNixMg0.5Fe2O4 (x = 0, 0.125 and 0.250) spinel ferrites

ABSTRACT The Landau model allows to estimate the Landau coefficients A(T), B(T) and C(T), which can be determined by fitting the Arrott plots in Zn0.5-xNixMg0.5Fe2O4 (x = 0, 0.125 and 0.250) ferrites. These coefficients are derived to determine − Δ S M ( T ) curves. A good correlation between − Δ S M ( T ) curves determined using Landau theory and the experimental curves estimated using the Maxwell relation was found. Then, the second-order magnetic phase transition in Zn0.5-xNixMg0.5Fe2O4 compounds was investigated. The critical properties of ferrites prepared by the double ceramic sintering process were investigated. Critical behavior near the FM–PM phase transition was studied from the magnetic data, through various techniques such as the modified Arrott plots (MAP), the Kouvel–Fisher method (KF) and critical isotherm (CI) analysis. The determined critical parameters were found to be close to the value of the mean field theory. On the other hand, the validity of the critical parameters was confirmed by the Widom scaling relation.

Optimized TCR and MR properties of La0.7-xSmxCa0.3MnO3 ceramics by Sm3+ doping

La0.7Ca0.3MnO3 (LCMO), with its high temperature coefficient of resistance (TCR) and large magnetoresistance (MR) effect, has emerged as a leading material in the field of new multi-functional ceramics. Enhancing the magnetoresistive effect of LCMO in low magnetic field and expanding its application range is one of the main research goals of this kind of materials. In this work, La0.7-xSmxCa0.3MnO3 (LSCMO) (0 ≤ x ≤ 0.09) ceramic targets with different doping amounts were successfully prepared by sol-gel method, and the influence mechanism of Sm3+ doping on the magnetoresistive effect and electrical transport performance of LSCMO was systematically analyzed. X-ray diffraction (XRD) results showed that the lattice parameters (a,b,c,V) of LSCMO decreased slightly, indicating that the doping amount of Sm3+ at the A-site increased. Scanning electron microscopy (SEM) showed that the grains were closely connected without obvious pores, the grain size decreased slightly. The ρ-T curve shows an obvious metal-insulator (M − I) transition, with the increase of Sm3+, the increase of resistivity, metal-insulator transition temperature (TMI) and ferromagnetic-paramagnetic (FM-PM) transition temperature (TC) move towards low temperature. When x = 0.015, the peak value of TCR reaches 41.94 %·K−1, and when x = 0.06, the peak value of MR reaches 80.25 %, both TCR and MR are improved. The results show that: Sm3+ doping can effectively enhance the electromagnetic transport performance of LSCMO. This material has a very wide application prospect in the fields of magnetic storage, and infrared detection.

Effect of Zn substitution on magnetic properties and magnetic entropy change in La0.7Ba0.25Nd0.05Mn1-xZnxO3 (x = 0.03 and 0.05) synthesized by using sol-gel method

This investigation explored the structural, magnetic, and magnetic entropy change characteristics of La0.7Ba0.25Nd0.05Mn1-xZnxO3 (x = 0.03 and 0.05) synthesized through the sol-gel method. The structural analysis revealed the crystallization of both samples in a Rhombohedral structure with the R-3c space group. The examination of magnetic properties indicated a suppression in magnetic behavior, with the ferromagnetic-paramagnetic transition temperature (TC) decreasing from 285 K to approximately 260 K with an increase in Zn content (x) from 0.03 to 0.05, respectively. The maximum value of magnetic entropy change (−ΔSM) under an applied magnetic field of 50 kOe was determined to be 3.96 J/kg·K for x = 0.03 and 2.27 J/kg·K for x = 0.05. Analysis of Arrot plots and the universal curve of magnetic entropy change confirmed that the ferromagnetic-paramagnetic phase transition observed in both samples exhibits a second-order phase transition (SOPT) transformation.

Thermodynamic Properties and Magnetocaloric Effect of Spins - 1/2 Square Lattice

This work aims to investigate the thermodynamic properties and magnetocaloric effect (MCE) of spins - 1/2 square lattice using Monte Carlo simulation. The considered Ising model is parameterized by the couplings J, [Formula: see text] and [Formula: see text]; where J is the ferromagnetic exchange interaction between first nearest-neighbor spins, a is the anisotropic parameter and [Formula: see text] is the ferromagnetic exchange interaction between second nearest-neighbor spins. Monte Carlo simulation under Metropolis algorithm allows us to study some thermodynamic properties and MCE of spins - 1/2 square lattice. The discussion of the obtained results allowed us to specify the conditions which can enrich the MCE in spins - 1/2 square lattice. The two-dimensional anisotropic Ising model presents clarifying the critical point of the ferromagnetic–paramagnetic transition, and we have shown that the maximum entropy variation increases when it occurs at the ferromagnetic–paramagnetic transition temperature regardless of the intensity of the applied magnetic field and the values of “a” and “b”.

Effect of Fe substituted on the monovalent La0.85Ag0.15Mn1-xFexO3 doped manganites : Their electromagnetic and microwave properties

New microwave absorber material, La0.85Ag0.15Mn1-xFexO3 (x = 0, 0.05, 0.10, 0.15 and 0.20) monovalent-based manganites with epoxy resin as a binder were prepared to investigate the effect of Iron (Fe) substitution on microwave absorption properties. The La0.85Ag0.15Mn1-xFexO3 (x = 0, 0.05, 0.10, 0.15 and 0.20) manganites were prepared using solid state method. The phase identification investigated by using X-ray diffraction (XRD) pattern, showed that all samples were single phase rhombohedral crystal structure. Fe substitution caused increase in room temperature resistivity and decrease in ferromagnetic–paramagnetic transition temperature, TC, indicating changes in carrier concentration and weakening of double exchange mechanism. Microwave reflection loss, RL measurements in the frequency range of 8 GHz to 18 GHz showed the highest reflection loss for x = 0 sample where value of reflection loss of − 57.2 dB at 16.41 GHz with a bandwidth of 2.67 GHz corresponding to reflection loss below − 10 dB was observed. However, Fe substitution produced lower reflection loss values with lowering of matching frequency with an upturn at x = 0.20. Permeability measurements showed real part of magnetic permeability increased but magnetic loss tangent decreased above 13 GHz with increasing Fe. Further analysis showed contribution of eddy current loss for most samples at high frequency region. On the other hand, permittivity measurements showed both real part of permittivity and dielectric loss tangent increased with Fe content at high frequencies indicates Fe substitution enhancing the dielectric loss component thus may dominantly contributes to the microwave absorption in the Fe substituted samples.

Sintering temperature effect on the magnetic interactions in Pr0.67Sr0.33MnO3 manganite

In this work, we report the effect of sintering temperature (TS) on the magnetic interactions and the critical behavior of Pr0.67Sr0.33MnO3 manganite prepared via sol–gel method (700 °C ≤ TS ≤ 1200 °C). The critical exponents study revealed that the 3D-Ising model is the best model to describe the magnetic interactions in the P1000 (TS = 1000 °C) and P1200 (TS = 1000 °C) samples. Consequently, a strong magnetic anisotropy characterizes these samples near the paramagnetic-ferromagnetic transition temperature, such anisotropy can be linked to the magnetic moment of Pr3+ ions (µ = 3.58 µB). For TS ≤ 850 °C, it becomes nearly impossible to speak about universality class due to the presence of great magnetic disorder induced by several factors like spin disordered shell in nanoparticles, magnetic frustration and Griffiths phase.

Magneto-functionalities of La1-xAxMnO3 (A = K; Ba) synthesized by flash combustion method

Abstract Due to their relatively large magnetocaloric effect, Ba and K doped LaMnO3 can be used in magnetic cooling systems. In this study, La1-xBaxMnO3 (x = 0.1; 0.2) and La0.9K0.1MnO3 powders were synthesized by the flash combustion method using glycine as a fuel and chelating agent. A subsequent calcination at 1000 °C was performed. Different characterization tools were used to carry out a detailed study of the structural, morphological, thermal, electrical, and magnetic behaviors of the doped materials. The structure of un-doped and doped samples was examined by X-ray Diffraction. According to magnetic data, La0.9K0.1MnO3 exhibits a ferromagnetic-paramagnetic phase transition at 271 K and a magnetic entropy variation under a magnetic induction of 4 T /ΔSM/ = 5.33 J kg−1K−1. On the other hand, the Curie temperature of La1-xBaxMnO3 increases with the Ba content from 157 K for x = 0.1 to 200 K for x = 0.2, whereas /ΔSM/ decreases from 2.56 J kg−1K−1 to 2.07 J kg−1K−1, respectively. Accordingly, it has been proven that the highest cooling performance corresponds to the K doped compound. The evolution from ferromagnetic to paramagnetic state was also determined with electron paramagnetic resonance spectroscopy. The temperature dependence of the resistivity shows a semiconductor character in La0.9Ba0.1MnO3 and 2 maxima of metal-insulator (M-I) transitions in the K doped sample. Enhanced magnetoresistance effects have been observed in La0.9K0.1MnO3 close to the ferromagnetic-paramagnetic transition temperature.

Impact of B-site doping on magnetic and magnetocaloric effect of Pr0.75Bi0.05Sr0.1Ba0.1Mn1-xTixO3 (0≤x≤0.04) manganites

Ceramic Pr0.75Bi0.05Sr0.1Ba0.1Mn1-xTixO3 (x = 0.0, 0.02 and 0.04) samples were prepared by the conventional sol-gel route. For a magnetic applied field of 0.05T, the magnetic measurements indicated that the ferromagnetic-paramagnetic (FM-PM) transition temperature TC decreased from 130 to 98 K with increasing titanium doping level from x = 0.0 to 0.04 respectively. The magnetic properties of these compounds are discussed relying on the magnetization measurements and the susceptibility analysis. The undoped compound displays a Griffiths phase at TG = 285 K which confirms the existence of ferromagnetic entities within the paramagnetic domain far above Tc. At μ0H = 5T, the maximum value of the magnetic entropy change varies from 2.33 to 1.88 J kg−1K−1 for x varying from 0.00 to 0.04 respectively. The master curve behavior for the temperature dependence of ΔSM(T), determined for various applied magnetic fields, revealed the second-order FM-PM phase transition of our compounds. Accordingly, important values of the relative cooling power (RCP) for all investigated materials were obtained proving that they can be proposed as good candidates for magnetic refrigeration at low temperatures.

Room temperature multiferroic properties of electrospun gallium ferrite nanofibers

Gallium ferrite (GaFeO3) is a promising multiferroic material for multifunctional device applications. Compared with bulk and thin film materials, nanofibers are possible to magnify the magnetostriction or piezoelectric effect due to their large length–diameter ratio, thus improving the performance of the material. In this work, GaxFe2 – xO3 (GFO) nanofibers have been synthesized by sol-gel based electrospinning. With the increasing Fe ion content, the room-temperature antiferromagnetic to ferromagnetic transition of GFO nanofibers has been confirmed by magnetic hysteresis loops; and the corresponding temperature dependent magnetization curves show that the ferromagnetic–paramagnetic transition temperature (Tc) is above room temperature and increases from around 292 K to above 400 K. Ferroelectricity of GFO nanofibers has been confirmed by second harmonic generation and piezoresponse force microscopy. Magnetoelectric (ME) coupling has been further measured by dual amplitude resonance tracking piezoresponse force microscopy and sequential excitation piezoresponse force microscopy under an in-plane external magnetic field. The newly developed SE-PFM method reduces the crosstalk of morphology, confirming that no obvious intrinsic ME coupling appears in the GFO nanofibers.

Substrate-induced strain effect on magnetic and electrical transport properties of La0·67Ca0·33MnO3 thin films with varying orientation

This paper carries out a detailed study on magnetic and electrical transport properties of La0·67Ca0·33MnO3 (LCMO) thin films deposited on SrTiO3 substrates with different orientations, which provide varying degrees of in-plane tensile strain. As confirmed by the structure and morphology analyses, the homogeneous LCMO thin films with columnar grains are epitaxially grown along the orientations of the substrates, and the suffered tensile strain in the film is increasing from (100) LCMO to (111) LCMO. The obtained data suggest that the magnetic and electrical transport properties of the thin films are sensitive to the substrate-induced tensile strain. The resistivity, temperature coefficient of resistivity (TCR) and magnetoresistance (MR) of the thin films were in ascending order of (1 0 0) < (1 1 0) < (1 1 1), while the ferromagnetic-paramagnetic transition temperature (TC), magnetization and metal-insulator transition temperature (TIM) were in descending order of (1 0 0) > (1 1 0) > (1 1 1).

Short and long range ordering in La1−xSrxCoO3 cobaltites

We have carefully studied the relation between the critical and electrical behavior of rhombohedral La1−xSrxCoO3 (x = 0.2–0.5) compounds prepared by solid-state reactions. The results obtained from analyzing M(T, H) data indicate that all compounds undergo a second-order phase transition and the ferromagnetic-paramagnetic (FM-PM) transition temperature increases from approximate 182 K for x = 0.2 to 253 K for x = 0.5. Basing on the modified Arrott and Kouvel-Fisher techniques, scaling hypothesis, and critical isotherm analysis, we have determined the critical exponent values characteristic of magnetic order of La1−xSrxCoO3, with β = 0.361–0.489, γ = 1.021–1.346, and δ = 2.99–4.72. Interestingly, though all compounds are metallic ferromagnets, we have found the phase segregation in magnetic order as x > 0.3, where magnetic interactions tend to change from long-to-short-range FM order together with the absence of the insulating-like behavior at temperatures below 100 K. These phenomena are related to the spatial coexistence of multiple electronic and magnetic phases due to the changes in concentration of cobalt ions, spin-state transitions, and the existence of poor-hole anti-FM and hole-rich FM regions.

Microstructural characterization of γ′ precipitates and their influence on the properties of Ni-Span C Elinvar alloy

Intensive microstructural characterizations were conducted on the γ′ precipitates in Ni-Span C, a typical precipitation hardened Fe-Ni-Cr based Elinvar alloy. The impact of precipitates on the physical and mechanical properties of the alloy was investigated. The results indicate that the aging generated nano-sized γ′ precipitates are coherent and coplanar with the matrix, and prolonging aging time leads to the increase in mismatch between γ′ and parent phase. It was found that the γ′ precipitates play key roles in promoting Elinvar behavior, reducing paramagnetic-ferromagnetic transition temperature and reinforcing the alloy.

Study on Conductive and Magnetic Properties of Ag-Doped La 0.67 Sr 0.33 MnO 3

Rare-earth doped manganese oxide La 0.67 Sr 0.33 MnO 3 (LSMO) has broad application prospects in the field of spin electronics, which has the paramagnetic-ferromagnetic transition temperature (T c ) above room temperature. Recent research shows that Ag-doping has effective improvements of grain size and conductivity of LSMO for improving the colossal magnetoresistance effect (CMR). In this paper, sol-gel method was used to prepare La 0.67 Sr 0.33 MnO 3 : Agx（x=0, 0.5 mol%） polycrystalline powder. By using X-ray diffraction (XRD), scanning electron microscopy (SEM), vibrating sample magnetometer (VSM) and X-ray photoelectron spectroscopy (XPS) technologies, the microstructure, crystalline phase structure, resistivity, magnetic properties, Ag and Mn valence states of LSMO were tested before and after Ag doping, respectively. The analysis shows that some Ag atoms exist at the grain boundary of LSMO in the form of element to enhance the conductivity; other Ag atoms go into the crystal lattice replacing A site La 3+ in the perovskite structure ABO3, which obtains a small amount of La 2 O 3 hybrid phase in LSMO and decreases the ratio of Mn 3+ /Mn 4+ . Ag doping weakens the double exchange effect and the magnetic property of LSMO. The substitution of La with Ag also makes minority spinon band of LSMO slightly reduced, which can weakly decrease the conductivity of LSMO.

Grain size effect on the electrical and magneto-transport properties of nanosized Pr0.67Sr0.33MnO3

Abstract In this study, nanosized of Pr0.67Sr0.33MnO3 prepared via sol-gel method followed by heat treatment at 600–1000 °C in intervals of 100 °C were synthesized. The structure, surface morphology, electrical, magneto-transport and magnetic properties of the samples were investigated. Rietveld refinements of X-ray diffraction patterns confirm that single phase orthorhombic crystal structure with the space group of Pnma (62) is formed at 600 °C. A strong dependence of surface morphology, electrical and magneto-transport properties on grain size have been observed in this manganites system. Both grain size and crystallite size are increases with the sintering temperature due to the congregation effect. Upon increasing grain size, the paramagnetic-ferromagnetic transition temperature increases from 278 K to 295 K. The resistivity drops and the metal-insulator transition temperature shifted from 184 K to 248 K with increases of grain size due to the grain growth and reduction of grain boundary. Below metal-insulator transition temperature, the samples fit well to the combination of resistivity due to grain or domain boundaries, electron-electron scattering process and electron-phonon interaction. The resistivity data above the metal-insulator transition temperature is well described using small polaron hopping and variable range hopping models. It is found that the negative magnetoresistance also increases with larger grain size where the highest %MR of – 26% can be observed for sample sintered at 1000 °C (245 nm).

Magnetic Criticality Enhanced Hybrid Nanodiamond Thermometer under Ambient Conditions

Nitrogen vacancy (NV) centres in diamond are attractive as quantum sensors owing to their superb coherence under ambient conditions. However, the NV centre spin resonances are relatively insensitive to some important parameters such as temperature. Here we design and experimentally demonstrate a hybrid nano-thermometer composed of NV centres and a magnetic nanoparticle (MNP), in which the temperature sensitivity is enhanced by the critical magnetization of the MNP near the ferromagnetic-paramagnetic transition temperature. The temperature susceptibility of the NV center spin resonance reached 14 MHz/K, enhanced from the value without the MNP by two orders of magnitude. The sensitivity of a hybrid nano-thermometer composed of a Cu_{1-x}Ni_{x} MNP and a nanodiamond was measured to be 11 mK/Hz^{1/2} under ambient conditions. With such high-sensitivity, we monitored nanometer-scale temperature variation of 0.3 degree with a time resolution of 60 msec. This hybrid nano-thermometer provides a novel approach to studying a broad range of thermal processes at nanoscales such as nano-plasmonics, sub-cellular heat-stimulated processes, thermodynamics of nanostructures, and thermal remanent magnetization of nanoparticles.

خواص مغناطیسی عایقهای توپولوژیکی مغناطیسی سه بعدی:اثرات انحنای شش گوشی

We have investigated the magnetic properties of three-dimensional topological insulators, doped with magnetic atoms. The spin-orbit locking of surface electrons results in interesting magnetic behaviors, absent in conventional insulators without spin-orbit coupling. In particular, the ferromagnetic-paramagnetic transition temperature has a strong dependence on different system parameters. Here, using the mean-field formalism we have studied the effects of hexagonal warping on the order parameter, critical temperature, and magnetic susceptibility of system.

Influence of Fe doped on the magnetocaloric behavior of LaCaMn1−xFexO3 compounds: a Monte Carlo simulation

The magnetocaloric behavior of LaCaMn1−xFexO3 for x = 0.00, 0.02, 0.03, 0.05, 0.07, 0.08 and 0.10 under the influence of an external magnetic field was simulated and analyzed. Simulations were carried out using the Monte Carlo method and the classical Heisenberg model under the Metropolis algorithm. These mixed valence manganites are characterized by having three types of magnetic ions corresponding to , which are bonded with , and and , related to . The ions were randomly included, replacing ions. With this model, the magnetic entropy change, , in an isothermal process was determined. showed maximum peaks around the paramagnetic-ferromagnetic transition temperature, which depends on doping. Relative cooling power was computed for different concentrations varying the magnetic applied field. Our model and results show that the doping decreases the magnetocaloric effect in the LaCaMn1−xFexO3, making this a bad candidate for magnetic refrigeration. The strong dependence of the magnetocaloric behavior on doping and the external magnetic field in LaCaMn1−xFexO3 can boost these materials for the future technological applications.

Effects of A-site cationic radius and cationic disorder on the electromagnetic properties of La0.7Ca0.3MnO3 ceramic with added Sr, Pb, and Ba

A series of polycrystalline ceramic samples of La0.7Ca0.25X0.05MnO3 (X = Sr, Pb, and Ba) were prepared using sol-gel method with methanol as the solvent. Resistivity-temperature curves show sharp metal-insulator transitions; furthermore, magnetization-temperature curves reveal a transition from ferromagnetism to paramagnetism. After doping with Sr, Pb, and Ba, both average ionic radius of A site and tolerance factor increase. Double-exchange also increases, which supports the metallicity of polycrystalline ceramic. However, larger lattice distortion is also introduced via substitution of larger ions at A site, and this tends to localize carriers. There is competing relationship between tolerance factor enhancement and lattice distortion; in combination, they determine electronic and magnetic properties of polycrystalline ceramics. As a result, after doping with Sr, Pb, and Ba, resistance temperature coefficient decreases, and metal-insulator transition temperature (TIM) increases. Meanwhile, ferromagnetic-paramagnetic transition temperature shows the same tendency as TIM.

Structural and magnetic properties of La2NiMnO6 ceramic prepared by ultra-high pressure sintering

A novel method, ultra-high pressure sintering (UHPS), for preparing highly-dense La2NiMnO6 (LNMO) ceramic at a ultra-high pressure (4 GPa) and a low sintering temperature (800 °C) over a short duration (10 min) had been developed. TEM images and Rietveld fitted XRD patterns confirmed the coexistence of monoclinic P21/n and hexagonal R3¯ phases in the ceramic. Compared with the samples prepared by conventional sintering, the grain size of the LNMO ceramic obtained by UHPS was remarkably decreased (from 3-10 μm to 300–500 nm) while the relative density was increased significantly (from 65-95% to 98.5%), which could be attributed to the unique UHPS sintering process. It was further noted that the ferromagnetic-paramagnetic transition temperature (TC1 ≈ 277 K) as well as the saturation magnetization (Ms≈3.6 μB/f.u.) were improved due to the denser structure and smaller grain size, as compared with the conventionally sintered LNMO samples.

Microstructure and some thermomagnetic properties of amorphous Fe-(Co)-Mn-Mo-B alloys

X-ray diffractometry, high resolution transmission electron microscopy (HREM) and transmission Mössbauer spectroscopy are used to study the microstructure and magnetic ordering in rapidly quenched amorphous Fe70-xCoxMn10Mo5B15 alloys with x = 0, 0.25, 0.5, 1 and 2. HREM images reveal the existence of medium range ordered (MRO) regions 2–4 nm in size in the as-cast alloys. The best fitting of the experimental, transmission Mössbauer spectra at 300 K is achieved applying the spectra decomposition into a set of symmetric quadrupole doublets and a single line for the paramagnetic at room temperature alloy (x = 0) or into a set of sextets and the single line for ferromagnetic alloys (x = 0.25, 0.5,1 and 2). The single line is described to the MRO regions which remain in paramagnetic state in the 77–300 K temperature range with small volume fraction in the ribbons. Within the experimental error the isomer shift of the single line for all alloys is the same indicating that MRO regions form similar paramagnetic phase independently of Co content. The magnetic properties of the studied alloys are determined mainly by the amorphous phases presented in the specimens. The Curie temperature rises with the increase of Co content and exhibits almost linear dependence on the average value of the hyperfine magnetic field induction. The maximum values of the magnetic entropy change occur near the ferromagnetic-paramagnetic transition temperature of the amorphous phase and are related to the Curie temperatures. An attempt to explain such feature is undertaken.