Size-dependent magnetization fluctuations in NiO nanoparticles

2017

The influence of the particle size on magnetic behaviors of nickel oxide nanoparticles (NiO NPs) was reported. NiO NPs with a uniform particle size were synthesized via a facile sol-gel method, and various sizes of NiO NPs (11, to 49 nm) were achieved by calcination at various temperatures (400, to 700 °C). X-ray diffraction (XRD) analysis revealed that increasing the calcination temperature increased the crystallite size. TEM observations and XRD analysis were used to determine the particle size of the NiO NPs. The field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images showed flake-like morphologies, which consisted of interconnected nanoparticles with a porous channel. The magnetic properties of NiO NPs with different size were studied using vibrating sample magnetometer (VSM). The results suggested that the particle size plays an important role in magnetic property of NiO nanoparticles.

IEEE Transactions on Magnetics, 2017

The magnetization of noninteracting metallic nanoparticles is investigated by comparing the particle-size and temperature dependences of the magnetization for several mechanisms. The nanoparticle magnetization deviates from that of the underlying bulk materials due to zero-temperature and thermal effects, and on a mean-field level, the corresponding surface-core interaction is described by a Landau-Ginzburg approach. A major factor is the reduced atomic coordination at the surface, which has often, but not always, opposite effects on the zero-temperature magnetization and Curie temperatures. The coordination effect is particularly pronounced for very weak itinerant ferromagnets and for strongly exchange-enhanced Pauli paramagnets. With regard to external magnetic fields, the nanoparticle magnetization involves several "superparamagnetic" phenomena, namely Néel relaxation, Brownian relaxation, and Langevin macrospin paramagnetism. Many or most applications of magnetic nanoparticles exploit the magnetization of the nanoparticles, alongside other magnetic properties, such as anisotropy. A variety of factors contributes to the magnetization, including interparticle interactions [1, 2], micromagnetic processes (domain-wall formation and propagation) . In this paper, we investigate magnetization contributions in isolated metallic magnetic nanoparticles smaller than about 10-15 nm, where domain-wall propagation is negligible and whose magnetization is wellapproximated in lowest order by a single "macrospin" m = N µ B .O u rf o c u si so nmetallic nanoparticles, especially on Co based alloys, whose magnetization is generally much higher than that of oxides, such as the widely used Fe 3 O 4 . The paper compares various mechanisms contributing to the magnetization of nanoparticles. Some of the mechanisms, such as superparamagnetism, are well-known individually, but others involve an application of complicated physics to nanoparticles. A conceptually very complicated concept is the Curie temperature of nanoparticles. In one-dimensional magnets (nanowires) and in zero-dimensional magnets (nanoparticles), the Curie temperatures are exactly zero. The proof is very easy for nanoparticles, where the partition function Z = p exp(-E p /k B T ) is a sum over real exponential functions. The free energy F and the magnetization M also exhibit a smooth temperature dependence, because they are derivatives of Z . However, by definition, the Curie point is a singularity, and it is therefore not possible to define a well-defined sharp T c in a nanoparticle. On the other hand, nanoparticles often contain thousands of atoms, and at some point the behavior

Journal of Magnetism and Magnetic Materials, 2005

A study of the magnetic properties of an assembly of non-interacting iron oxide-based nano-particles is presented. The measured average particle size is found to approach the expected thickness of the spin disordered layer. In this regime, the magnetization is characterized by two parameters: T b above which the particle acts as a single domain, and T E below which intra-particle coupling between pinned and unpinned spins become important. r

RSC Advances, 2020

Nickel oxide nanoparticles having a mean particle size of 19.5 nm were synthesized by a simple chemical method. Those nanoparticles exhibited a spin glass like behaviour at a temperature around 9 K. The samples showed electronic conduction arising out of small polaron hopping between the Ni 2+ and Ni 3+ species present in the material. A large magnetodielectric parameter with a maximum value of 52.2% was observed in the sample at room temperature which resulted from the Maxwell-Wagner polarization effect. This was explained as arising due to a large negative magnetoresistance caused by spin polarized electron hopping between Ni 2+ and Ni 3+ sites with the consequential formation of space charge polarization at the interfaces of the NiO nanoparticles. This was substantiated by direct measurement of magnetoresistance of the samples which gave identical results. It is believed that negative magnetoresistance after direct measurement occurred due to the interaction between ferromagnetic and antiferromagnetic phases and the value was 37%, the highest reported in the literature so far. As a result of the presence of Ni 3+ ions, antiferromagnetic phase and ferromagnetic like behaviour of NiO nanoparticles gave higher magnetization than other reported nanoparticles. Such large values of magnetoresistance of the samples will make the material useful as an ideal magnetic sensor.

Journal of Alloys and Compounds, 2011

We have observed unusual magnetic properties of NiO (nickel oxide) nanoparticles embedded in a silica matrix. The sample was synthesized by a method based on the contribution of sol-gel and combustion processes. X-ray powder diffraction (XRPD) of the sample shows the formation of the nanocrystalline NiO phase whereas transmission electron microscope (TEM) reveals spherical-shaped nanoparticles of about 4 nm diameter. Moreover, HRTEM images show lattice fringes of the nanoparticles and defects in the crystal structure. The temperature and field dependence of the magnetization are also measured. The zero-field-cooled (ZFC) measurements show two maximums, one sharp and narrow at low temperatures ∼6.5 K and an other broad one at higher temperature ∼64 K. The FC magnetization shows a continuous increase upon lowering the temperature. The M(H) measurements reveal that NiO nanoparticles display anomalous hysteretic behaviors at low temperatures (below the low temperature maximum in the ZFC curve, 2 K and 5 K) showing that the magnetization initial curve lies below the hysteresis loop for a certain field range. Moreover, jump of the magnetization at low temperatures (2 K and 5 K) are also observed. These features represent novel magnetic properties for nanosized NiO which may be attributed to the surface spins. Moreover, these results indicate that the NiO nanoparticle consists of magnetically disorder shell and antiferromagnetically order core with an uncompensated magnetic moment.

Applied Physics Letters, 2008

Applied Physics A, 2019

NiO nanoparticles with different sizes were synthesized at different temperatures from 300 to 700 • C to the study of behavior related to size. The nanoparticles show dimensions in the range 5.07-68.29 nm, which were determined by transmission electron microscopy images. The nanoparticles present an irregular morphology from 300 to 600 • C, while the observed structure is a truncated octahedron with FCC structure only for samples at 700 • C. X-ray diffraction measurements and Rietveld analysis verify this crystal structure, the crystal size, and the lattice parameters. Raman spectroscopy of the nanoparticles shows the normal modes of proposed truncated octahedrons related to longitudinal optical, transverse optical phonons, and a combination of both. Optical properties were measured by UV-visible spectroscopy to analyze the variation of the band gap in function of the size. In addition, magnetic measurements, magnetization versus temperature, and magnetization versus magnetic field present ferromagnetic behavior. M-H hysteresis curves show the coercive field with anisotropic characteristics that we related to the competition between two magnetic orders coexisting in the samples.

Journal of Physical Chemistry C, 2011

An important reason to prepare magnetic nanoparticles of uniform size and shape is to ensure uniform magnetic properties. However, here, we demonstrate that magnetic iron oxide crystals of 20 nm or less with a low polydispersity of the geometric size can nevertheless have a strikingly broad distribution of the magnetic dipole moment. A comparative study was performed on nanoparticles with

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Materials Research Bulletin, 2012