Surface Spin-glass Behavior Research Articles

Structural, morphological, and magnetic characterizations of (Fe0.25Mn0.75)2O3 nanocrystals: A comprehensive stoichiometric determination

This report aims to investigate in depth FeMnO3, a material of interest due to its fascinating magnetic and multiferroic properties and its many applications in fields including lithium-ion batteries, microwave devices, and catalysis. However, understanding the precise stoichiometry of the material is crucial for a better comprehension of its physical properties. A cheap, simple, and repeatable sol-gel process was used to fabricate the FeMnO3 nanocrystals. Comprehensive multi-technique characterization of the as-fabricated FeMnO3 indicates that the main phase (94 wt%) is Fe0.5Mn1.5O3, although hematite appears as the minority phase (6 wt%). Magnetic characterization shows core-shell spin-glass like behavior, as well as paramagnetic-ferrimagnetic transitions and a Griffiths phase regime. EPR measurements revealed a strong and broad resonance line across the temperature range of 4.3 K–300 K, primarily influenced by the majority phase. The g-value decreases monotonically from 2.93 at 50 K to 2.18 at 300 K. There is a notable change in the resonance field and linewidth between 40 and 50 K, attributed to surface spin glass behavior. The EPR data below 50 K are in line with the core-shell model of (Fe0.25Mn0.75)2O3 nanoparticles. Below 50 K, the shell's spin system undergoes a transition from paramagnetic to spin-glass-like, with a critical temperature around 43 K. Above 50 K, the superparamagnetic minority phase significantly affects the temperature dependence of the resonance linewidth. These results hold particular importance as they advance our understanding of the intricate magnetic interactions present in FeMnO3. For the best possible use of this material platform in new technologies, such insights are essential.

Nanomagnetism variation with fluorine content in Co(OH)F

Magnetic behaviors of cobalt hydroxide fluoride, Co(OH)F2-x Fx (x = 1.09 and 1.26), which are consisted with nanorods of ∼5 μm length and diameter ∼100 nm, obtained by a facile hydrothermal method, have been researched with the variation of fluorine content as a result of the different amount of reactant NH4F addition. Magnetic coupling performance are characterized based on the observation and analysis of magnetic susceptibility, X-ray Absorption Fine Structure (XAFS) and specific heat. Co(OH)F2-x is found to undergo an antiferromagnetic transition at 40 K. The peak temperature (T) on the transition curves shifts to lower temperature according with the increased magnetic field (H) following the rule of H2/3∞ (-T) because of the surface spin-glass behavior. The paramagnetic susceptibility can be fitted with the modified Curie–Weiss law between 100 K and 320 K. The negative Curie-Weiss temperature indicates that the antiferromagnetic coupling becomes stronger with the fluorine content. In addition, at temperatures below 5 K, the magnetic reordering was observed as spin-glass. Exchange bias behavior in Co(OH)F2-x was found after field cooling process demonstrating an exotic surface magnetic behavior generated with high fluorine contents due to the deformation of CoO6 octahedral induced large spin-glass behavior.

Disordered and Frustrated Magnetization in Coated MnFe<sub>2</sub>O<sub>4</sub> 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.

Evidence of surface spin-glass behavior in NiFe2O4 nanoparticles determined using magnetic resonance technique

Abstract Nanosized nickel ferrite (NiFe2O4) is successfully synthesized by the sol-gel method using citric acid (C6H8O7) as fuel. The X-ray diffraction pattern of the as-synthesized sample shows formation of NiFe2O4 nanoparticles (NPs) as the main phase, with mean crystalline size ∼50 nm. While transmission electron microscopy (TEM) reveals nearly spherical morphology with mean particle diameter of 41 ± 5 nm, high-resolution TEM shows inter-planar distance (2.55 A) corresponding to the (1 1 3) plane of the spinel structure. Additionally, the Raman spectrum exhibits five Raman active modes (two A1g at 575 cm−1 and 710 cm−1; two Eg at 345 cm−1 and 670 cm−1; and one T2g at 497 cm−1) typical of NiFe2O4. X-band magnetic resonance (MR) data reveal a single broad resonance line in the whole temperature range (3.8 K ≤ T ≤ 300 K), with g-value decreasing monotonically from 3.10 ± 0.01 at 3.8 K to 2.77 ± 0.01 at 300 K. The temperature dependence of both resonance field and resonance linewidth show a remarkable change in the range of 60–70 K, herein credited to surface spin-glass behavior. The model picture used to explain the MR data assumes NPs with a core-shell structure. Below about 60–70 K the shell’s spin system progressively reveals a paramagnetic to spin-glass-like transition upon cooling, with a freezing temperature estimated at 2.1 ± 0.1 K. However, above about 60–70 K spins in the paramagnetic shell align along the NP’s core dipolar field, resulting in an effective enlarged nanoparticle size.

Uniform wurtzite MnSe nanocrystals with surface-dependent magnetic behavior

Manganese selenide (MnSe) possesses unique magnetic properties as an important magnetic semiconductor, but the synthesis and properties of MnSe nanocrystals are less developed compared to other semiconductor nanocrystals because of the inability to obtain high-quality MnSe, especially in the metastable wurtzite structure. Here, we have successfully fabricated wurtzite MnSe nanocrystals via a colloidal approach which affords uniform crystal sizes and tailored shapes. The selective binding strength of the amine surfactant is the determining factor in shape-control and shape-evolution. Bullet-shapes could be transformed into shuttle-shapes if part of the oleylamine in the reaction solution was replaced by trioctylamine, and tetrapod-shaped nanocrystals could be formed in trioctylamine systems. The three-dimensional (3D) structure of the bullet-shaped nanorods has been demonstrated by the advanced transmission electron microscope (TEM) 3D-tomography technology. High-resolution TEM (HRTEM) and electron energy-loss spectroscopy (EELS) show that planar-defect structures such as stacking faults and twinning along the [001] direction arise during the growth of bullet-shapes. On the basis of careful HRTEM observations, we propose a “quadra-twin core” growth mechanism for the formation of wurtzite MnSe nanotetrapods. Furthermore, the wurtzite MnSe nanocrystals show lowtemperature surface spin-glass behavior due to their noncompensated surface spins and the blocking temperatures increase from 8.4 K to 18.5 K with increasing surface area/volume ratio of the nanocrystals. Our results provide a systematic study of wurtzite MnSe nanocrystals. Open image in new window

Surface spin-glass, large surface anisotropy, and depression of magnetocaloric effect in La0.8Ca0.2MnO3 nanoparticles

The surface magnetic behavior of La(0.8)Ca(0.2)MnO(3) nanoparticles was investigated. We observed irreversibility in high magnetic field. The surface spin-glass behavior as well as the high-field irreversibility is suppressed by increasing particle size while the freezing temperature T(F) does not change with particle size. The enhanced coercivity has been observed in the particles and we attributed it to the large surface anisotropy. We have disclosed a clear relationship between the particle size, the thickness of the shell, and the saturation magnetization of the particles. The large reduction of the saturation magnetization of the samples is found to be induced by the increase of nonmagnetic surface large since the thickness of the spin-disordered surface layer increases with a decrease in the particle size. Due to the reduction of the magnetization, the magnetocaloric effect (MCE) has been reduced by the decreased particle size since the nonmagnetic surface contributes little to the MCE. Based on the core-shell structure, large relative cooling powers RCP(s) of 180 J/kg and 471 J/kg were predicted for a field change of 2.0 T and 4.5 T, respectively, in the small particles with thin spin-glass layer.

Stabilization of surface spin glass behavior in core-shell Fe67Co33–CoFe2O4 nanoparticles

Magnetic properties of Co33Fe67–CoFe2O4 (core-shell) nanoparticles are presented. Both dc magnetization and ac susceptibility measurements indicate a spin glass (SG) like transition occurring at TF∼175 K. The SG nature of the transition is also confirmed by the field dependence of the freezing temperature (TF(H)) following the well known Almeida–Thouless line, δTF∼H2/3. Additionally, the particles exhibit a large exchange bias (HEB∼1357 Oe) arising from the core-shell (ferromagnetic-SG) coupling. The unusually high SG transition temperature and large exchange bias effects are attributed to a combination of several factors including the thickness of the amorphous oxide shell and large values of the exchange and anisotropy constants associated with the CoFe2O4 shell.

Surface spin glass behavior in sol–gel derived La0.7Ca0.3MnO3 nanotubes

We report the fabrication of La(0.7)Ca(0.3)MnO(3) nanotubes (LCMONTs) with a diameter of about 200 nm, by a modified sol-gel method utilizing nanochannel alumina templates. High resolution transmission electron microscopy confirmed that the obtained LCMONTs are made up of nanoparticles (8-12 nm), which are randomly aligned in the wall of the nanotubes. The strong irreversibility between zero field cooling (ZFC) and field cooling (FC) magnetization curves as well as a cusplike peak in the ZFC curve gives strong support for surface spin glass behavior.

Critical radius for exchange bias in naturally oxidized Fe nanoparticles

Details of synthesis and structural characterization of highly crystalline iron oxide nanoparticles are presented together with results on the magnetic investigation as a function of the temperature and applied field. Monodisperse Fe nanoparticles were prepared by thermal decomposition of iron pentacarbonyl in the presence of oleic acid. These iron nanoparticles were readily oxidized on exposure to air. The resulting nanocrystals have been identified as inverse spinels, being $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ the dominant phase of the small $5\text{\ensuremath{-}}\mathrm{nm}$ iron oxide nanocrystals, whereas the proportion of the ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ component gradually increases on increasing the particle size. The small particles volume resulted in finite-size effects, for instance, the magnetization deviates from the ${T}^{3∕2}$ Bloch's law. At the same time, high field irreversibility and shifted hysteresis loops after field-cooled processes have been detected, and attributed to a low-temperature surface spin-glass layer. Moreover, there is a critical diameter below which the surface spin-glass behavior and exchange bias effect abruptly disappear.

Surface spin glass and exchange bias inFe3O4nanoparticles compacted under high pressure

We have demonstrated a low temperature surface spin-glass layer in the ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ high-pressed nanocompacts. The related core-shell magnetic structure arises during the compaction, which is evident by the field dependence of the freezing temperature following the de Almeida--Thouless relationship and obvious exchange bias effect. Moreover, we have also found a critical cooling field, above which both the surface spin-glass behavior and the exchange bias effect abruptly disappeared.

Molten alkali metal nitrate flux to well-crystallized and homogeneous La 0.7Sr 0.3MnO 3 nanocrystallites

Nanocrystalline La 0.7Sr 0.3MnO 3 (LSMO) has been synthesized by an alkali metal nitrate flux process at a relatively low temperature of 600°C. Scanning electron and transmission electron microscopic images show that LSMO grains are of good crystallization and uniform size distribution. Inductively coupled plasma and elemental analyses indicate that the content of K + is no more than 0.6% in all the samples and the stoichiometry of La, Sr and Mn is very close to the expected value. A significant enhanced magnetoresistance is observed in the nanosized LSMO especially at low temperatures. By improving grain crystallization, the surface spin-glass behavior disappears, and the blocking temperature corresponding to the super-paramagnetic transformation shifts to a higher temperature.

Surface spin-glass behavior in La2/3Sr1/3MnO3 nanoparticles

The low-temperature magnetic and transport properties of La2/3Sr1/3MnO3 nanoparticles have been investigated. It is found that a surface spin-glass behavior exists in La2/3Sr1/3MnO3 nanoparticles, which undergo a magnetic transition to a frozen state below 45 K. The low-temperature surface spin-glass behavior exists even at the highest field used (H=50 kOe). Moreover, the spin-glass-like transition disappears for particles above 50 nm. In addition, the suppressed low-field magnetoconductivity (LFMC) observed at low temperature for nanosized La2/3Sr1/3MnO3 is obviously lower than the expected upper limit of LFMC, 1/3, for polycrystalline manganites, which is proposed to arise from the higher-order tunneling through the insulating spin-glass-like surface layers.