Distribution Of Blocking Temperatures Research Articles

Exploring induced microstructural changes in magnetically modified crude oils through nonlinear rheology and magnetometry

Adsorptive phenomena involving dispersed iron oxide superparamagnetic nanoparticles and asphaltenes in crude oil have been profiled as promising technological alternatives, particularly since these interactions can induce significant structural changes within the oil matrices, effectively inhibiting the formation of complex long-range viscoelastic structures. Furthermore, the effect of adsorbed asphaltenes on magnetic dipolar interactions among particles has been proven, showing the formation of multiple asphaltene layers that stimulate a steric repulsive barrier. Despite the discussed hindering phenomena, this research demonstrated the effectiveness of the sequence of physical processes framework to provide intra-cycle structure-rheological interpretations in large amplitude oscillatory shear of a ferrofluid-modified heavy oil, upon the application of an external magnetic field. The analysis proved that disordered nanoparticle/asphaltene aggregates are highly extended and naturally formed in the absence of magnetic forces. In contrast, in the presence of a perpendicular field applied by a controlled rate magneto-rheometer, the formation of interacting structural aggregates of several hundred nanometers was observed, analogous to magnetorheological fluids. These results were validated by adjusting a phenomenological model that effectively represented the intricate processes involved in the formation and reorientation of aggregates, based on the experimental data acquired from zero-field-cooled and field-cooled magnetization curves. This revealed a distinct blocking temperature distribution at around 274 K, which was linked to Brownian relaxation phenomena exhibited by nanoparticle aggregates. In this regard, this research provided a precise extended description of the effect of magnetic fields on the microstructural organization of complex fluids using nonlinear rheology and magnetometry.

Engineering buffer layers to improve temperature resilience of magnetic tunnel junction sensors

Improving the thermal resilience of magnetic tunnel junctions (MTJs) broadens their applicability as sensing devices and is necessary to ensure their operation under harsh environments. In this work, we are address the impact of temperature on the degradation of the magnetic reference in field sensor stacks based on MgO-MTJs. Our study starts by simple MnIr/CoFe bilayers to gather enough insights into the role of critical morphological and magnetic parameters and their impact in the temperature dependent behavior. The exchange bias coupling field (H ex), coercive field (H c), and blocking temperature (T b) distribution are tuned, combining tailored growth conditions of the antiferromagnet and different buffer layer materials and stackings. This is achieved by a unique combination of ion beam deposition and magnetron sputtering, without vaccum break. Then, the work then extends beyond bilayers into more complex state-of-the-art MgO MTJ stacks as those employed in commercial sensing applications. We systematically address their characteristic fields, such as the width of the antiferromagnetic coupling plateau ΔH, and study their dependence on temperature. Although, [Ta/CuN] buffers showed higher key performance indications (e.g. H ex) at room temperature in both bilayers and MTJs, [Ta/Ru] buffers showed an overall wider ΔH up to 200 °C, more suitable to push high temperature operations. This result highlights the importance of properly design a suitable buffer layer system and addressing the complete MTJ behavior as function of temperature, to deliver the best stacking design with highest resilience to high temperature environments.

Evaluation of blocking temperature and its distribution for L10-type FePt granular films

The blocking temperature (T B) and blocking temperature distribution (ΔT B) of magnetic grains were evaluated for L10-type FePt granular films with various grain boundary materials (GBMs). T B and ΔT B were measured from remanent magnetization. The relationship between the degree of order and Curie temperature (T C) for the L10-type FePt phase was separately evaluated using alloy films. According to actual measurements, the obtained mean T B, mean T B/T C and ΔT B for granular films were 620 K, 0.83 K, and 200 K, respectively, for SnO GBM film and 450 K, 0.61 K, and 260 K, respectively, for TiO2 GBM film. Based on these values, mean T B, mean T B/T C, and ΔT B were calculated assuming that τ = 2 × 10−7 s, which is the relaxation time, corresponds to the HAMR writing time. These were 737 K, 0.98 K, and 24 K, respectively, for SnO GBM film and 685 K, 0.93 K, and 72 K, respectively, for TiO2 GBM film. These results for granular films can be useful for designing a hard disk drive with HAMR.

A detailed study of magnetic properties and room temperature memory effect in wet chemically prepared [formula omitted][formula omitted] nanostructured alloy

We present the magnetic properties of nanostructured Ni0.2Cu0.8 granular alloy prepared by a chemical reduction process. Zero-field-cooled and field cooled magnetization studies show branching due to the presence of superparamagnetism. The large particle size distribution in our sample is reflected in a broad blocking temperature distribution which on the higher side extends to 390 K. Magnetic field dependence of magnetization shows a superparamagnetic component accompanied with a ferromagnetic part at low temperature with prominent hysteresis loops exhibited at temperatures 2–50 K. The average magnetic moment of nickel (0.5μB) is smaller than its expected ferromagnetic saturation value (0.6μB) indicating that not all Ni atoms contribute to ferromagnetism. The magnetization data are analyzed in terms of an existing core–shell nanoparticle model. The study demonstrates that the overall behavior of the system is that of an assembly of non-interacting superparamagnetic (SPM) particles exhibiting the magnetic memory effect in relaxation dynamics, which has been observed using different experimental protocols, and persists even up to room temperature.

Role of the Fraction of Blocked Nanoparticles on the Hyperthermia Efficiency of Mn-Based Ferrites at Clinically Relevant Conditions

To investigate the role of magnetic anisotropy on magnetic hyperthermia heating efficiency at low field conditions, Mn, MnZn, and MnCo-ferrite nanoparticles were synthesized using the hydrothermal method. The coercive field temperature dependence method was used to determine the blocking temperature distribution of the particles by considering the temperature dependence of anisotropy and magnetization and the random anisotropy axis configuration. The data allowed one to estimate the room-temperature quasi-static superparamagnetic diameter, which was found to be lower than the theoretical value. Magnetic hyperthermia experiments of the magnetic nanocolloids at 522 kHz indicated that soft nanomagnets heat more efficiently at clinically relevant conditions. The heating performance was found to decrease at the higher fraction of blocked nanoparticles. For instance, samples with similar size distribution and mean diameter of 10 nm, at a field amplitude of only 120 Oe (9.6 kA m–1), showed a decrease of specific...

Influence of citric acid and oleic acid coating on the dc magnetic properties of Fe3O4 magnetic nanoparticles

Abstract The properties of two samples of Fe3O4 magnetic nanoparticles coated with citric acid and oleic acid respectively, have been studied by means of DC magnetic measurements. The analysis has been performed by measuring the Zero Field Cooling (ZFC) and Field Cooling (FC) magnetization curves as a function of the temperature, at 0.01 T applied field. In this framework, the blocking temperature (TB) distribution and the effect of the coatings on the experimentally evaluated samples properties have been analysed. Moreover, the magnetization as a function of the field has been measured at temperatures above TB and the comparison between the results obtained for the two samples has been discussed.

Impact of blocking temperature distribution on the thermal behavior of MnIr and MnPt magnetoresistive stacks

Abstract Ensuring a reliable reference in magnetoresistive devices requires robust exchange biased antiferromagnetic/ferromagnetic (AF/FM) bilayers with strong resilience to high temperatures. In this work, we studied the thermal stability of MnPt and MnIr by investigating the exchange bias effect in AF/CoFe bilayers and tunneling magnetoresistance (TMR) full stacks. In bilayers, the obtained blocking temperature ( T b ) of 380 °C for MnPt is higher than T b of 340 °C for MnIr, indicating a better thermal stability. Based on Malozemoff’s model, we developed a comprehensive model to describe the exchange bias field ( H ex ) behavior at high temperature by considering the T b distribution and temperature dependence of FM magnetization, giving a good agreement with the experimental data. With increasing temperature, a coercivity ( H c ) enhancement peak was observed in MnIr bilayer, which shows a good agreement with T b distribution. This evidences the H c peak is dominated by the thermal activation of AF grains, indicating that the H c peak only appears at T b center , which is lower than overall T b . The structural properties and grain size of AF layers were studied by X-ray diffraction (XRD), where a mean grain size of 10.6 and 12.6 nm were obtained for MnPt and MnIr, respectively. Compared to bilayers, a more stable thermal behavior was observed in full stacks. In conclusions, our study compared the thermal stability of exchange bias coupling using MnPt and MnIr and developed a comprehensive model to study their temperature dependence. This proposed approach can be used for theoretical studies and experimental designs of AF-based materials for spintronic applications, such as magnetoresistive devices and high density memories.

EPR and Magnetization Studies of Polymer-Derived Fe-Doped SiCN Nanoceramics Annealed at Various Temperatures: Blocking Temperature, Superparamagnetism and Size Distributions

X-band EPR spectra on SiCN ceramics, doped with Fe(III) ions, annealed at 800 °C, 1000 °C, 1100 °C, 1285 °C, and 1400 °C have been simulated to understand better their magnetic properties, accompanied by new magnetization measurements in the temperature range of 5–400 K for zero-field cooling (ZFC) and field cooling (FC) at 100C. The EPR spectra reveal the presence of several kinds of Fe-containing nanoparticles with different magnetic properties. The maxima of the temperature variation of ZFC magnetization were exploited to estimate (i) the blocking temperature, which decreased with annealing temperature of the samples and (ii) the distribution of the size of Fe-containing nanoparticles in the various samples, which was found to become more uniform with increasing annealing temperature, implying that more homogenous magnetic SiCN/Fe composites can be fabricated by annealing at even higher temperatures than 1400 °C to be used as sensors. The hysteresis curves showed different behaviors above (superparamagnetic), below (ferromagnetic), and about (butterfly shape) the respective average blocking temperatures, 〈TB〉. An analysis of the coercive field dependence upon temperature reveals that it follows Stoner–Wohlfarth model for the SiCN/Fe samples annealed above 1100 °C, from which the blocking temperatures was also deduced.

Effect of shell thickness on the exchange bias blocking temperature and coercivity in Co-CoO core-shell nanoparticles

The exchange bias blocking temperature distribution of naturally oxidized Co-CoO core-shell nanoparticles exhibits two distinct signatures. These are associated with the existence of two magnetic entities which are responsible for the temperature dependence of an exchange bias field. One is from the CoO grains which undergo thermally activated magnetization reversal. The other is from the disordered spins at the Co-CoO interface which exhibits spin-glass-like behavior. We investigated the oxide shell thickness dependence of the exchange bias effect. For particles with a 3 nm thick CoO shell, the predominant contribution to the temperature dependence of exchange bias is the interfacial spin-glass layer. On increasing the shell thickness to 4 nm, the contribution from the spin-glass layer decreases, while upholding the antiferromagnetic grain contribution. For samples with a 4 nm CoO shell, the exchange bias training was minimal. On the other hand, 3 nm samples exhibited both the training effect and a peak in coercivity at an intermediate set temperature Ta. This is explained using a magnetic core-shell model including disordered spins at the interface.

Monte Carlo investigation of how interfacial magnetic couplings affect blocking temperature distributions in exchange bias bilayers

Exchange bias in ferromagnetic (F)/antiferromagnetic (AF) bilayers is a function of both the bulk properties of the AF layer and the interfacial properties determining the effective interfacial couplings between the F and AF layers. The distinction between bulk and interface can be clearly revealed in blocking temperature distributions, where AF grain volume distribution results in a high-temperature peak while disordered interfacial magnetic phases produce a low-temperature contribution. However, the coupling conditions producing such bimodal blocking temperature distributions remain to be specified. In this article, we use a granular model which accounts for the disordered interfacial phases by considering small magnetic grains (SGs) with weaker anisotropy and coupling with the F grains at the F/AF interface. The SG are included in the AF material. The coupling conditions producing bimodal blocking temperature distributions were determined. Then, using Monte Carlo simulations, these conditions were validated and the effect of interfacial F-SG coupling on distributions was investigated. We next determined how the ratio between F-SG and F-AF couplings could be used to estimate the surface coverage of the disordered interfacial phases from experimental data.

Exchange-biased Py/CoO vortex structures: Magnetization reversal, cooling-field dependence, and training

In this paper, the magnetization reversal of exchange-biased Py/CoO vortex structures was investigated. The vortex structures were prepared by film deposition onto closely packed arrays of silica particles. By zero field cooling below the N\'eel temperature of the antiferromagnetic CoO, a so called circular exchange bias can be imprinted. By completing the same cooling process in an applied magnetic field, either a displaced vortex or even a $c$ state can be stabilized, which depends strongly on the CoO layer thickness. A strong athermal training effect mainly on the descending branch after the second loop is observed, which is accompanied by a strong reduction in loop shift, indicating a rearrangement of the CoO spin configuration during reversal. By analyzing the nucleation and annihilation field of the exchange-biased loops, a strong asymmetry is observed, which hints to a magnetization reversal via formation of a distorted viscous vortex. This asymmetry gets more pronounced with increasing CoO layer thickness. Furthermore, we investigated the blocking temperature distribution of the Py/CoO sample series. Below the mean blocking temperature, an increased coercive field is observed when the vortex is cooled in a field larger than the annihilation field compared to zero field cooling, revealing the competition between the vortex state formation and the alignment of spins along the cooling-field direction.

Thermally activated processes and superparamagnetism in Bi12MnO20 nanoparticles: A comparative study

Manganese sillenite (Bi12MnO20) nanoparticles having average particle size between 22 and 43nm were synthesized by a low temperature soft chemical route under refluxing conditions. A careful structural and microstructural characterization by means of high resolution X-ray diffraction experiments and transmission electron microscopy is presented. The as-cast powder displayed an isotropic superparamagnetic (SPM) behavior with a blocked state for temperatures below TB∼13.0K. We used three different measurement techniques to extract and compare the Bi12MnO20 blocking temperatures. First, we extracted TB with the modified Bean–Livingstone model from the coercive field temperature dependence obtained from hysteresis curves measured as a function of temperature. Then, the blocking temperature distribution function, f(TB), was obtained by deriving the zero field-cooled/field-cooled curves difference. For each applied field, the maximum of the distribution function gave us the mean blocking temperature value. Finally, the maximum of the magnetic susceptibility imaginary part as a function of frequency was used, combined with the Néel–Brown equation, to extract the blocking temperature. All measurement techniques yield an equivalent dependence of TB with H of the Bi12MnO20 superparamagnetic nanoparticles.

Intrinsic dependence of the magnetic properties of CoFe2O4 nanoparticles prepared via chemical methods with addition of chelating agents

In this work, the effect of addition of different chelating agents on the magnetic properties of cobalt ferrite nanoparticles produced by the combining of both co-precipitation and hydrothermal methods is reported. The Rietveld analyses of X-ray diffraction patterns reveal that our samples are single phase (space group: Fd-3m) with small average sizes. The weight losses observed in the thermogravimetric measurements together with the M×H curves show that the organic contamination coming from chelating agent decomposition can give rise to misinterpretation of the magnetization measurements. Besides, analyses of the zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements and the M×H curves measured at room temperature allows us to state that both the average blocking temperature and particles size distribution are sensitive to the kind of chelating agent.

Bimodal distribution of blocking temperature for exchange-bias ferromagnetic/antiferromagnetic bilayers: a granular Monte Carlo study with less stable magnetic regions spread over the interface

In exchange bias based devices, ferromagnetic/antiferromagnetic disordered interfacial spins which exhibit low freezing temperatures contribute to the alteration of the exchange bias properties. In particular, the blocking temperature distributions earlier observed experimentally show two contributions: the common high-temperature peak due to the antiferromagnetic grain volume distribution and a less usual low-temperature contribution presumably attributed to disordered interfacial spins. Here, in order to test this assumption Monte Carlo simulations based on a granular level model are used to calculate blocking temperature distribution. Small magnetic grains with weaker anisotropy and interfacial coupling are introduced at the ferromagnetic/antiferromagnetic interface to account for the disordered interfacial spins. As a result, the bimodal character of blocking temperature distributions is reproduced and varying the amount of these smaller magnetic grains tunes the low-temperature contribution of the distribution. These simulations therefore validate the assumption that less stable magnetic regions spread over the ferromagnetic/antiferromagnetic interface may originate the bimodal character of blocking temperature distributions, as earlier brought forward by experimentalists to explain their results. In addition, because of the heterogeneities of the interface in the presence of less stable small grains, our simulations show that such bilayers cannot be described using a simple model of uniform rotation of the ferromagnetic layer interacting with an average interfacial coupling.

Hyperfine and magnetic properties of a YxLa1 − xFeO3 series (0 ≤ x ≤ 1)

A series of orthoferrites YxLa1−xFeO3 in the entire range of composition was synthesized at room temperature by mechanochemical activation of oxide mixtures. Phase composition, structure and microstructure of the obtained powder materials were characterized by X-ray diffraction and field-emission scanning electron microscopy. Hyperfine interactions and magnetic properties were determined by Mössbauer spectroscopy, SQUID and vibrating sample magnetometry. Two magnetic contributions could be identified in the series of materials: a paramagnetic state, associated with a fraction of the smallest particles and a ferromagnetic state, attributed to the larger particles. The results showed that the relative proportion of both contributions is very dependent on x, the Y content of samples. From M vs T measurements, it was possible to estimate the blocking temperature distribution for the end members of the series. Annealing of samples produced the elimination of the superparamagnetic behavior and the formation of Y3Fe5O12 impurities.

Strong memory effect at room temperature in nanostructured granular alloy Co0.3Cu0.7

Non-interacting magnetic CoCu nanoparticles with a blocking temperature distribution show strong magnetic memory effect even at room temperature.

Synthesis, characterization and magnetic properties of Co Cu1− (x∼0.01–0.3) granular alloys

Abstract Nanostructured CoCu granular alloys have been prepared by borohydride reduction of CuCl2 and CoCl2 salt solutions using cetyltrimethylammonium bromide (CTAB) as a surfactant. Characterization by inductively coupled plasma optical emission spectroscopy (ICPOES), X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies yields spherical particles of CoxCu1−x ( x ∼ 0.01 – 0.3 ) of average size 8–25 nm formed in a face-centered-cubic (fcc) lattice as in copper. Studies of zero-field-cooled/field-cooled (ZFC/FC) magnetization and thermoremanent magnetization (TRM) have been performed in the temperature range 4–300 K, and the results have been analyzed by independent particle model. At the lowest cobalt concentration ( x ∼ 0.01 ), the alloy is superparamagnetic and there is no blocking of magnetization down to 4 K. For all other samples, the magnetization at low magnetic field is characterized by a blocking temperature distribution which is not influenced by the Co content in samples. Study of hysteresis loops shows that the magnetization at any temperature 4–300 K is a sum of ferromagnetic (FM), superparamagnetic (SPM) and paramagnetic (PM) contributions. The FM part increases and SPM part decreases with increase in Co content. However, the values of coercivity and magnetic anisotropy constant do not depend on Co content. The results suggest that CoxCu1−x alloys are formed in a spherical core-shell type structure with cobalt being concentrated near the core of particles.

Exchange bias and blocking temperature distribution of Fe-film/CoO-nanoparticle hybrid bilayers

We fabricated and characterized Fe-film/CoO-nanoparticle hybrid bilayers and studied their temperature-dependent magnetic properties. The magnetic nanoparticles are uncoupled or weakly coupled in the particle films, which essentially resemble the antiferromagnetic grains in the deposited thin films. The size and size distribution of the nanoparticles can be well-controlled by chemical synthesis prior to assembling, providing additional route for engineering the antiferromagnetic layer in exchange-bias systems. Using cycled annealing-cooling measurements, we observed a unimodal blocking-temperature distribution in our samples that is a reflection of the intrinsic antiferromagnetic monodisperse “grain” size distribution. The absence of the low-temperature contribution is likely attributed to both the fabrication process and the nanoparticle morphology.

Magnetization Study of CoxCu1-x Nanoparticles

Nanostructured CoCu granular alloys have been prepared by chemical reduction method using NaBH4 as a reducing agent and characterized by inductively coupled plasma optical emission spectroscopy (ICPOES), x-ray diffraction (XRD) and transmission electron microscopy (TEM) studies. Zero-field cooled and field cooled (ZFC/FC) magnetization study shows a blocking temperature distribution resulting from particle size distribution which is a characteristic of superparamagnetism. Magnetization study reveals that at low Co concentration, the alloy particles are superparamagnetic. At higher Co concentration the magnetization is combination of ferromagnetism (FM) and superparamagnetism (SPM). The coercive field is independent of Co concentration.

Benefit of inserting a (Cu/Pt) intermixing dual barrier for the blocking temperature distribution of exchange biased Co/(Cu/Pt)/IrMn stacks

Exchange bias based spintronics devices involve ferromagnetic/antiferromagnetic interfaces and concomitant layers intermixing. As a consequence, interfacial spin-glass-like phases with reduced properties and increased dispersions form and lower the device performance. It is therefore necessary to limit intermixing by introduction of diffusion barriers. One of the major difficulties is that the barrier must be inert. This paper uses blocking temperature distributions to quantify the interfacial quality of Co/IrMn based stacks. Inserting a (Cu/Pt) dual barrier fulfils the manifold requirements of limiting Co-Mn, Co-Pt, and Cu-Mn intermixing, which takes place when using either no or single Pt and Cu barriers, respectively.