Superferromagnetic State Research Articles

Magneto-Optical Spectroscopy of Nanocomposites (CoFeB)x(LiNbO3)100 – x with Concentrations up to the Percolation Threshold: From Superparamagnetism and Superferromagnetism to Ferromagnetism

Nanocomposites (CoFeB)x(LiNbO3)100 – x with x = 17–48 at % have been synthesized by ion beam sputtering of a composite target comprised of Co40Fe40B20 and LiNbO3 onto silicon substrates, and the tran-sitions from the superparamagnetic state to the superferromagnetic and ferromagnetic states with an increase in the concentration of the magnetic component are studied by magneto-optical methods. The magneto-optical properties have been investigated in the geometry of the equatorial (transverse) Kerr effect (TKE). Magneto-optical spectra are recorded in the range of 0.5–4.0 eV in fields up to 2.5 kOe at 20–300 K, field and temperature dependences of the TKE at certain wavelengths are obtained, and the domain structure during magnetization reversal is visualized using a magneto-optical Kerr microscope. It is shown that the sample with x = 17 at % is superparamagnetic at temperatures above the blocking temperature (about 30 K). The interaction between the granules is considerable already at x = 20 at %, the transition to the superferro-magnetic state occurs at x ≈ 32–36 at %, and the transition to the ferromagnetic state occurs at x ≈ 44 at %near the metal–dielectric transition, i.e., at a concentration below the percolation transport threshold.

Magneto-Optical Spectroscopy of Nanocomposites (CoFeB)x(LiNbO3)100 –x with Concentrations up to the Percolation Threshold: From Superparamagnetism and Superferromagnetism to Ferromagnetism

Nanocomposites (CoFeB)x(LiNbO3)100 –x with x = 17–48 at % have been synthesized by ion beam sputtering of a composite target comprised of Co40Fe40B20 and LiNbO3 onto silicon substrates, and the transitions from the superparamagnetic state to the superferromagnetic and ferromagnetic states with an increase in the concentration of the magnetic component are studied by magneto-optical methods. The magneto-optical properties have been investigated in the geometry of the equatorial (transverse) Kerr effect (TKE). Magneto-optical spectra are recorded in the range of 0.5–4.0 eV in fields up to 2.5 kOe at 20–300 K, field and temperature dependences of the TKE at certain wavelengths are obtained, and the domain structure during magnetization reversal is visualized using a magneto-optical Kerr microscope. It is shown that the sample with x = 17 at % is superparamagnetic at temperatures above the blocking temperature (about 30 K). The interaction between the granules is considerable already at x = 20 at %, the transition to the superferromagnetic state occurs at x ≈ 32–36 at %, and the transition to the ferromagnetic state occurs at x ≈ 44 at % near the metal–dielectric transition, i.e., at a concentration below the percolation transport threshold.

DC magnetization of titania supported on reduced graphene oxide flakes

Abstract DC magnetization of a series of titania nanocomposites modified with reduced graphene oxide (rGO) has been investigated. Hysteresis loops observed at room temperature disappeared at low temperatures. At a temperature of about 100 K, a phase transition to the superferromagnetic order state was observed, probably due to the linear expansion and self-reorientation of the magnetic moments. Processes associated with magnetic moment reorientation can cause a hysteresis loop to disappear at low temperatures as well as superferromagnetic ordering. It was suggested that the isolated nanoparticle in the nanopore could be used to create a “compass” at a nanometer-sized level that would be many times more sensitive than the conventional one. Measurements of the zero-field cooling and field cooling modes do not exclude the possibility of the coexistence of a superparamagnetic state.

Manipulation of dipolar magnetism in low-dimensional iron oxide nanoparticle assemblies.

The manipulation of magnetic states in nanoparticle supercrystals promises new pathways to design nanocrystalline magnetic materials and devices. Trench-patterned silicon substrates were used as templates to guide the self-assembly of iron oxide nanoparticles. Grazing incidence small angle X-ray scattering shows that the nanoparticles form a long-range ordered structure along the trench direction while in the direction perpendicular to the trenches, no coherent structure is observable. Electron holography provides evidence of an ordered magnetic state of nanoparticle moments in the remanent state after the application of a saturation magnetic field parallel to the trenches. However, a disordered magnetic state was observed in a perpendicular geometry. Hysteresis loops indicate that the nanoparticle moments form a superferromagnetic state for the geometry parallel to the trenches. Memory effect investigations reveal that the disordered magnetic state corresponds to a collective superspin glass state in the perpendicular geometry, while the superferromagnetic state in the parallel geometry suppresses the superspin glass state.

Superposition of conventional and spontaneous exchange bias in a Ni50Mn34In13Fe3 magnetic shape memory alloy

The conventional exchange bias and spontaneous exchange bias are two interesting but fundamentally different effects in Mn rich Ni-Mn-Z magnetic shape memory alloys. Previous investigations show that these two phenomena appear separately. Here, we report that the conventional and spontaneous exchange bias phenomena coexist in a Ni50Mn34In13Fe3 alloy and the superposition of them increases the total exchange bias of the alloy. Our results suggest that the canonical spin glass and super spin glass may coexist and are embedded in the antiferromagnetic matrix of the martensite, through properly tuning the temperature-field history, which further results in the superposition of conventional and spontaneous exchange bias for this alloy. The interfacial interaction between the canonical spin glass and the antiferromagnetic matrix forms during field cooling process, which results in the conventional exchange bias. Moreover, the application of large magnetic field during the isothermal magnetization process changes the superspin glass into a superferromagnetic state, which interacts with the antiferromagnetic matrix at the interface, causing the spontaneous exchange bias. The superposition of exchange bias provides an effective way to achieve large exchange bias in materials.

Monte-Carlo calculation of the coercive force and phase transitions in ensembles of Stoner-Wohlfarth particles with exchange interactions

A Monte-Carlo (MC) method is used to study the magnetization of an ensemble of Stoner-Wohlfarth particles with interparticle interactions of a ferromagnetic type. A modification of the procedure for the MC calculation is proposed to speed up the calculations. The magnetization of the ensemble is studied at different temperatures and for different ratios of the exchange field of the interparticle interaction and the uniaxial magnetic anisotropy field of the particles. It is shown that Neel relaxation has a significant influence on the coercive properties of the ensemble if the interparticle exchange field does not exceed the anisotropy field of the particles. It is also found that, at low temperatures, the coercive properties of Stoner-Wohlfarth particles with exchange interactions are related to the loss of stability of the orientation of the average magnetic moment vector of the particles. At high temperatures, the coercivity of the particle ensemble is associated with the complete loss of stability of the superferromagnetic state. The critical fields and critical temperatures for an ensemble of Stoner-Wohlfarth particles with exchange interactions are analyzed.

Demagnetizing Field Induced Instability in a Superferromagnetic (CoFe2O4)20 Ag80 Nanocomposite

The magnetic properties of a (CoFe2O4)20Ag80 nanocomposite packed into a cylindrical sample holder were investigated. The nanoparticle system becomes superferromagnetic (SFM) below 223 K. The SFM state was investigated by measuring the temperature dependence of the zero-field-cooled (ZFC) and field-cooled (FC) magnetizations and by recording hysteresis loops. For magnetic field ( $H$ ) applied along the axis of the sample holder, the ZFC and FC magnetizations and the hysteresis loops showed typical behaviors of a SFM phase yielding a remanence of 49.2 emu/cm3 and a coercivity of 9.65 kOe at 10 K. The demagnetizing field for $H$ applied transverse to the axis of the sample holder induced an instability that reduced the magnitude of the remanence to 12.7 emu/cm3 and reduced the coercivity to 31 Oe at 10 K. The overall results are explained in terms of macroscopic demagnetizing fields.

Interparticle interactions mediated superspin glass to superferromagnetic transition in Ni-bacterial cellulose aerogel nanocomposites

The interparticle interactions in the magnetic nanocomposites play a dominant role in controlling phase transitions: superparamagnetic to superspin glass and to superferromagnetic. These interactions can be tuned by controlling the size and number density of nanoparticles. The aerogel composites, 0.3Ni-BC and 0.7Ni-BC, consisting of Ni nanoparticles distributed in the bacterial cellulose have been used as a model system to study these interactions. Contrary to conventional approach, size of Ni-nanoparticles is not controlled and allowed to form naturally in bacterial cellulose template. The uncontrolled growth of Ni results in the formation of nanoparticles with 3 different size distributions—<10 nm particles along the length of fibrils, 50 nm particles in the intermediate spaces between the fibrils, and >100 nm particles in voids formed by reticulate structure. At room temperature, the composites exhibit a weakly ferromagnetic behaviour with a coercivity of 40 Oe, which increases to 160 Oe at 10 K. The transition from weakly ferromagnetic state to superferromagnetic state at low temperatures is mediated by the superspin glass state at intermediate temperatures via the interparticle interactions aided by nanoparticles present along the length of fibres. A temperature dependent microstructural model has been developed to understand the magnetic behaviour of nanocomposite aerogels.

Spin-glass-like dynamics of ferromagnetic clusters in La0.75Ba0.25CoO3

We report a magnetization study of the compound La0.75Ba0.25CoO3 where the Ba2+ doping is just above the critical limit for percolation of ferromagnetic clusters. The field cooled and zero-field cooled (ZFC) magnetization exhibit thermomagnetic irreversibility and the ac susceptibility shows a frequency dependent peak at the ferromagnetic ordering temperature (TC ≈ 203 K) of the clusters. These features indicate the presence of a non-equilibrium state below TC. For the non-equilibrium state, the dynamic scaling of the imaginary part of the ac susceptibility and the static scaling of the nonlinear susceptibility clearly establish a spin-glass-like cooperative freezing of ferromagnetic clusters at 200.9(2) K. The assertion of the occurrence of spin-glass-like freezing of ferromagnetic clusters is further substantiated by ZFC ageing and memory experiments. We also observe certain dynamical features which are not present in a typical spin glass, such as: the initial magnetization after ZFC ageing first increases and then decreases with the waiting time; and there is an imperfect recovery of relaxation in negative temperature cycling experiments. This imperfect recovery transforms to perfect recovery for concurrent field cycling. Our analysis suggests that these additional dynamical features have their origin in the inter-cluster exchange interaction and cluster size distribution. The inter-cluster exchange interaction above the magnetic percolation level gives a superferromagnetic state in some granular thin films, but our results show the absence of a typical superferromagnetic-like state in La0.75Ba0.25CoO3.

Study about the possibility to control the superparamagnetism–superferromagnetism transition in magnetic nanoparticle systems

This paper presents a study about the possibility to switch a magnetic dense nanoparticle system from a superparamagnetic state to an ordered state, called superferromagnetism. We start from the idea that the magnetic state of the system is influenced not only by the magnetic interactions between the nanoparticles, but also by the distribution of the nanoparticle sizes and the distribution of the effective anisotropy constants. By using stochastic methods and models, we superpose these conditions and we search, in accordance with the effects, the parameters for controlling the transition from the superparamagnetic to the superferromagnetic state.

Superferromagnetism and coercivity in Co-Al2O3 granular films with perpendicular anisotropy

Magnetic properties of nano-granular Co0.55–(Al2O3)0.45 films have been studied by vibrating sample magnetometry, ferromagnetic resonance, and magnetic force microscopy (MFM). The films possess a growth-induced perpendicular anisotropy related to the shape anisotropy of the granules. Being unpercolated physically, the films demonstrated a clear magnetic percolation behavior in the temperature dependence of the coercivity and MFM scans. The temperature dependence of the coercivity measured along the easy magnetization axis of the granules indicates the presence of a collective long-range magnetic state sustained by a ferromagnetic interparticle interaction. This interaction is supposed to form a homogeneously magnetized superferromagnetic state, however, the perpendicular anisotropy and the competition between the dipole-dipole and ferromagnetic interactions lead to the emergence of an inhomogeneous labyrinth-like magnetic stripe-domain structure with the averaged domain width being about two orders of magnitude larger than the averaged granule size. The temperature evolution of the spontaneous magnetization inside the stripe domains has been measured by the MFM up to the temperature of superferromagnetic ordering (420 K). The formerly developed mean-field-based model of the coercivity of an interacted ensemble has been tested and found to yield adequate quantitative predictions.

Size dependent dipolar interactions in iron oxide nanoparticle monolayer and multilayer Langmuir–Blodgett films

The dipolar interactions in monolayer and multilayer assemblies of iron oxide nanoparticles have been investigated as a function of the nanoparticle size. The magnetic properties of iron oxide nanocrystals of various sizes have been measured for particles as powders and assembled in mono- and multilayers by the Langmuir–Blodgett technique, and compared to the behavior of non-interacting nanoparticles. It is shown that increasing dipolar interactions lead to higher blocking temperatures and to deviation from the Néel–Brown law. Dipolar interactions are found to be stronger for particles assembled in thin films compared to powdered samples. The effect of interactions increases strongly with the nanoparticle size in agreement with simulations, leading to an unusual behaviour for the larger particles assembled in monolayer, which could be a signature of a superferromagnetic state.

Single-particle blocking and collective magnetic states in discontinuous CoFe/Al2O3 multilayers

Discontinuous metal–insulator multilayers (DMIMs) of [CoFe(tn)/Al2O3]m containing soft ferromagnetic (FM) Co80Fe20 nanoparticles embedded discontinuously in a diamagnetic insulating Al2O3 matrix are ideal systems to study interparticle interaction effects. Here the CoFe nanoparticles are treated as superspins with random size, position and anisotropy. At low particle density, namely nominal layer thickness tn = 0.5 nm, single-particle blocking phenomena are observed due to the absence of large enough interparticle interactions. However at 0.5 nm < tn < 1.1 nm, the particles encounter strong interactions which give rise to a superspin glass (SSG) phase. The SSG phase has been characterized by memory effect, ageing, dynamic scaling, etc. With further increase in particle concentration (1.1 nm < tn < 1.4 nm) and, hence, smaller interparticle distances, strong interactions lead to a FM-like state which is called superferromagnetic (SFM). The SFM state has been characterized by several techniques, e.g. dynamic hysteresis, Cole–Cole plots extracted from ac susceptibility, polarized neutron reflectometry, etc. Moreover, the SFM domains could be imaged by x-ray photoemission electron microscopy and magneto-optic Kerr effect microscopy. At tn > 1.4 nm physical percolation occurs between the particles and the samples are no longer discontinuous and then termed as metal insulating multilayers. Competition between long- and short-ranged dipolar interactions leads to an oscillating magnetization depth profile from CoFe layer to CoFe layer with an incommensurate periodicity.

Effects of interparticle interactions in magneticFe/Si3N4granular systems

An experimental evidence of the progressive modification in the magnetic behavior of granular $\text{Fe}/{\text{Si}}_{3}{\text{N}}_{4}$ samples due to interaction effects between particles is reported. Microstructural features and local structure were determined by x-ray absorption spectroscopy and transmission electron microscopy to select granular samples with predetermined cluster size. $\text{Fe}/{\text{Si}}_{3}{\text{N}}_{4}$ systems have been characterized by ac- and dc-magnetization measurements to study the gradual evolution of magnetic properties of granular systems, where three different behaviors have been observed. As-deposited samples with Fe thickness layers of 2.5 nm, present a modified superparamagnetic behavior, due to very weak interactions between very small Fe clusters separated by a nonmagnetic FeN phase. An evolution of the average blocking temperature at intermediate fields $({T}_{\text{B}}\ensuremath{\sim}{H}^{3/2})$ is observed, similar to noninteracting systems, but first signatures of a frozen spin state at low temperatures appear. Annealed samples exhibit a noticeable modification from the multilayer character to a random three-dimensional organization of Fe clusters embedded in a ${\text{Si}}_{3}{\text{N}}_{4}$ matrix. After annealing, samples with initial Fe layer thickness of 0.7 nm provide iron cluster in the range of 1.3 nm and exhibit a superspin-glass state, with a de Almeida-Thouless evolution of the energy barriers $({T}_{\text{B}}\ensuremath{\sim}{H}^{2/3})$ that is explained in terms of increasing interparticle interactions. Moreover, annealed samples, with initial layer thickness of 1.3 nm, supply iron cluster of near 3 nm that present stronger interactions and yield a superferromagnetic state, likely provided by residual ultrasmall particles between the blocked clusters.

Coercivity anomaly in the superferromagnetic state of an ensemble of nanoparticles with oriented anisotropy

An anomalous angular dependence of the coercive field of physically nonpercolated granular CoFeB–SiO2 films with oriented anisotropy of the granules, where the presence of the superferromagnetic phase has been established earlier, is observed experimentally at temperatures above the blocking temperature. The angular dependence of the coercive field is similar to that typically observed in the case of magnetization reversal through an inhomogeneous magnetic state of single granules in the ensemble. As the temperature becomes lower than the blocking temperature, the angular dependence changes to that typical of the magnetization reversal through a coherent rotation of granules’ magnetic moments. It is shown theoretically that the anomaly is determined by a singularity of the angular dependence of the critical field causing the absolute loss of the superferromagnetic phase stability in the ensemble of weakly interacting ferromagnetic nanoparticles.

SSG or SFM state in [formula omitted] nano-agglomerates fabricated by micro-emulsion method

We report on magnetic properties of CoFe 2 O 4 nano-agglomerates obtained by micro-emulsion technique under specific conditions. The samples form a unique morphology as observed by transmission electron microscopy and scanning electron microscopy investigations. Concerning magnetic properties, they exhibit a considerable coercivity of almost 1 T at 2 K, which continuously decreases up to the characteristic temperature, T g = 350 K . The temperature dependence of the zero-field cooled (ZFC) and field cooled (FC) magnetization, respectively, is furcated at the T g , and the temperature dependencies of the a.c. susceptibility exhibit a frequency-dependent maximum at ∼ T g . The observed behavior is discussed in terms of the super-spin-glass (SSG) and the super-ferromagnetic (SFM) regime considering the morphology of the nano-agglomerates.

Magnetoresistive telegraph noise in Langmuir-Blodgett films of colloidal magnetite nanocrystals as seen via scanning tunneling microscopy

Temperature-dependent fluctuations in the local current passing through close-packed magnetite nanocrystal (NC) films were probed by scanning tunneling microscopy. This phenomenon, which peaked near the blocking temperature $({T}_{\text{b}})$, reflects spin-polarized tunneling fluctuations due to NC magnetization switching events. The current exhibited telegraph noise patterns, switching between low and high states. Above ${T}_{\text{b}}$ both states occurred with equal probability while below it the high current state dominated, which is consistent with a superferromagnetic ground state where the NC moments are aligned.

Thermomagnetic transitions and coercivity mechanism in bulk composite Nd 60Fe 30Al 10 alloys

The thermomagnetic behaviour (within the temperature range 553–300 K) for the bulk composite Nd 60Fe 30Al 10 alloy is described in terms of a transition from paramagnetic to superferromagnetic state at T=553 K, followed by a ferromagnetic ordering for T<473 K. For the superferromagnetic regime, the alloy thermomagnetic response was associated to a homogeneous distribution of magnetic clusters with mean magnetic moment and size of 1072 μ B and 2.5 nm, respectively. For T<473 K, a pinning model of domain walls described properly the alloy coercivity dependence with temperature, from which the domain wall width and the magnetic anisotropy constant were estimated as being of ≈8 nm and ≈10 5 J/m 3, typical values of hard magnetic phases. Results are supported by microstructural and magnetic domain observations.

The influence of intergranular interaction on the magnetization of the ensemble of oriented Stoner–Wohlfarth nanoparticles

The influence of interparticle interaction on the processes of magnetization reversal is considered for an ensemble of oriented Stoner–Wohlfarth nanoparticles. This is done through a solution of a kinetic equation describing the relaxation of the total magnetization to its equilibrium value in an effective mean field which includes a term proportional to the instantaneous value of the magnetization. It is shown that the interparticle interaction influences the temperature dependence of a coercive field. Under certain conditions, the presence of the interparticle interaction can lead to the formation of the so-called superferromagnetic state with the correlated directions of the magnetic moments of the particles. If the system is unable to come to the equilibrium during the time interval necessary to perform measurements, some measured quantities become dependent on the measurement time. It is shown that the blocking temperature Tb and the temperature dependence of coercive field at T&amp;lt;Tb are strongly dependent on a measurement time. At T&amp;gt;Tb, however, the coercivity, if exists, does not depend on the measurement time. The data of magnetostatic measurements, carried out on the (CoFeB)x–(SiO2)1−x nanogranular films with the concentration of ferromagnetic particles slightly lower than a percolation threshold, are in compliance with the results of the calculations.

Effect of Interaction in the Magnetization Reversal Relaxation of Superparamagnetic Granular CoFeB -SiO&lt;sub&gt;2&lt;/sub&gt; Films

In order to study the magnetic interactions influence on the relaxation processes in superparamagnetic media, the magnetostatic measurements were carried out for the granular (CoFeB)x-(SiO2)1-x films with x values near percolation threshold. The observed temperature dependences of the coercivity for the samples magnetized along in-plane easy axis have shown two parts linear on with different inclination angles, which cross each other at the blocking temperature (Tb). The first part of the curve, which corresponds to the temperature range below Tb, depends on the measuring time and is related to the thermal activation nature of magnetization reversal process in the blocked superparamagnetic state. Second part of the curve, which corresponds to the temperature range above Tb, is practically independent on the measuring time and is related to existence of correlated state of superparamagnetic grains moments, i.e. to superferromagnetic state. The method of the magnetization reversal curves simulation based on the numerical solving of kinetic equation has been applied to explain the experimental results.