Shift Of Hysteresis Loop Research Articles

Magnetic anisotropy of crystalline Fe films grown on (001) GaAs substrates using Ge buffer layers

Magnetic anisotropy of Fe films grown on (001) GaAs substrates using Ge buffer layers were investigated by planar Hall effect measurements. In addition to phenomena arising from dominant cubic symmetry of the Fe specimen, the study of angular dependence of magnetization reversal revealed breaking of this symmetry in the form of systematic asymmetric shifts of magnetic hysteresis loops around the <110 > crystallographic directions. We ascribe such symmetry breaking to an admixture of uniaxial anisotropy associated with the [100] direction in the Fe film. To determine the parameters associated with this uniaxial anisotropy, we quantitatively analyze the asymmetric shifts of the hysteresis loop centers from the <110 > directions. Even though the value of these parameters turns out to be relatively small compared to that of the cubic anisotropy (by about two orders of magnitude), they survive up to room temperature.

Origins of the Exchange-Bias Phenomenology, Coercivity Enhancement, and Asymmetric Hysteretic Shearing in Core-Surface Smart Nanoparticles

We have used a spin-1 Ising model Hamiltonian with dipolar (bilinear,J), quadrupolar (biquadratic,K), and dipolar-quadrupolar (odd,L) interactions in pair approximation to investigate the exchange-bias (EB), coercive field, and asymmetric hysteretic shearing properties peculiar to core/surface (C/S) composite nanoparticles (NPs). Shifted hysteresis loops with an asymmetry and coercivity enhancement are observed only in the presence of the odd interaction term in the Hamiltonian expression and their magnitudes show strong dependence on the value ofL. The observed coercivity and EB inC/SNPs originated from nonzero odd coupling energies and their dependence on temperature (T) and particle size (R) are also discussed in relation to experimental findings.

Controlling disorder-mediated exchange bias in (Mn,Zn,Fe)3O4 thin films

We report exchange bias in (Mn,Zn,Fe)3O4 thin films that are compositionally homogeneous. We show that exchange bias in these Mn–Zn ferrite (MZFO) films can be tuned quite easily through annealing of the as-deposited films. The annealing process increases the crystallinity, as measured by X-ray diffraction (XRD). This improvement in crystallinity is accompanied by lower coercive fields, lower exchange bias fields, and higher saturation magnetizations. Exchange bias in these nominally homogeneous ferrite films is correlated with the degree of both structural and magnetic disorder. Based on the annealing experiments, we believe that these MZFO films may consist of crystalline regions that are separated from one another by disordered regions of the same nominal composition. The disordered regions serve to exchange bias the more structurally and magnetically ordered crystalline MZFO grains, leading to a shift of the magnetic hysteresis loop. Together these results indicate that the magnitude of the exchange bias can be controlled by tuning the degree of crystallinity in the system.

Exchange Bias Effect Determined by Anisotropic Magnetoresistance in CoxNi1−xO/Ni0.8Fe0.2 Bilayer System

Exchange bias effect is the unidirectional anisotropy induced by the interface between ferromagnetic (FM) and antiferromagnetic (AFM) layers below the Neel temperature of antiferromagnetic materials, leading to a shift of hysteresis loop. The effect of exchange bias has been studied for many years because of its possible application in spintronics, especially in spin valves for magnetic recording and sensor devices [1]. The essentials of exchange bias effect are not fully understood yet. It is generally accepted that the uncompensated moments in the AFM layer play an important role in pinning the spins at the interface and determine the strength of exchange bias field [2]. We prepared bilayer systems composed of the FM layer Ni 0.8 Fe 0.2 and the AFM layer Co x Ni 1−x O (x = 0.3, 0.4, 0.5, and 0.6) by using the DC/RF magnetron sputtering method. Exchange bias field H EB , the shift field in hysteresis loop, was observed in all the Ni 0.8 Fe 0.2 /Co Ni 1−x O bilayer systems. The changes of H EB were explicitly studied for various parameters such as the composition of AFM material x, the measured temperature T, and the angle θ of applied magnetic field. We measured anisotropic magnetoresistance (AMR) and analyzed the AMR data to extract the H EB , since the peak structure in AMR is not exactly same to the coercive field H C , unlike the magnetization data. We propose a new approach to analysis for AMR in determining H EB and H C along the field angle θ with respect to the field-cooling direction. The results were compared with the variations of H EB and H C simulated by Mauri model and spin-glass model [3].

The ageing and de-ageing behaviour of (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 lead-free piezoelectric ceramics

Ageing behaviour usually occurs in acceptor-doped piezoelectric materials (e.g., hard lead zirconate titanate) and exhibits the development of a pinched or shifted hysteresis loop over time. Although no pinched hysteresis loop was observed for lead-free (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 material, this study showed that the piezoelectric properties change over time in the poled state. The shift of the hysteresis loop along the electric field axis and the development of asymmetry in strain and permittivity hysteresis loop were observed during the ageing process. The origin of this ageing behaviour is proposed to be local defect dipoles and the migration of the charged defects to the grain boundaries. The reorientation of the defect dipole contributes to a fast but unstable ageing mechanism in this material while the migration of the charged defects contributes to a slow but more stable mechanism.

Temperature effect on exchange coupling and magnetization reversal in antiferromagnetically coupled (Co/Pd) multilayers

Magnetization reversal of antiferromagnetically coupled (AFC) soft and hard (Co/Pd) multilayers was studied as a function of temperature. While the hard [Co(0.3 nm)/Pd(0.8 nm)]×10 was kept unchanged, the softness of the [Co(t)/Pd(0.8 nm)]×3 was controlled by varying the thickness t of the Co sublayer. Clear two-step hysteresis loops were observed for all the investigated multilayers with t ranging between 0.4 and 1 nm. The spin reorientation of the soft layer magnetization from in-plane direction to out-of-plane direction was investigated from 50 to 300 K. The antiferromagnetic field HAFC measured from the shift of the minor hysteresis loop reveals a good agreement to the quantum-well model. From the out-of-plane hysteresis loop of the uncoupled soft layer, its magnetization shows an in-plane orientation for t ≥ 0.6 nm. The strong HAFC helps to induce an out-of plane orientation of the soft layer with a linear decrease of its coercivity with temperature. These investigated structures show the possibility to reduce the unwanted stray field and improving the out-of-plane anisotropy even for relatively thicker soft layer.

Exchange bias effect in CaMn0.9Nb0.1O3

The exchange bias effect in CaMn0.9Nb0.1O3 has been investigated. It was found that both vertical and horizontal shifts in magnetic hysteresis loop of field cooled sample decrease monotonously with increasing temperature and vanish above 70 K, while the coercivity disappears only above 90 K, upon approaching the Néel temperature. An exponential decay successfully describes variation of the shifts with temperature. The exchange bias phenomenon is in general attributed to low-temperature phase separation into ferromagnetic clusters immersed in the G-type or C-type antiferromagnetic matrix and the resulting appearance of interfaces between coexisting different magnetic phases. Our results indicate that the exchange bias effect in CaMn0.9Nb0.1O3 may be induced, at least partially, by Dzyaloshinsky–Moriya interactions. Measurements of hysteresis loops at various cooling fields from the range −90 < H < 90 kOe show that magnetization at magnetic field H > 20 kOe exhibits a nonlinear increase and a sort of metamagnetic transition that appears to be responsible for a complete suppression of the exchange bias effect in CaMn0.9Nb0.1O3 at high magnetic field.

Exchange bias effect in spin glass CoCr2O4 nanoparticles

CoCr2O4 nanoparticles are about 5.4nm in diameter synthesized by a hydrothermal technique. Magnetization measurements reveal that the nanoparticles exhibit a spin glass behavior below glass transition temperature. Signature of memory effect is clear in reheating curve where the step-like shape increasing with the increase of temperature is recovered after cooling process. Magnetic relaxation is performed to prove memory effect. Ageing effect is also detected in CoCr2O4 nanoparticles to verify the spin glass behavior. As temperature decreases to 5K, which is far below the glass transition temperature, exchange bias effect can be observed clearly accompanied with a shift in field-cooled hysteresis loop. As particle size decreases to 5.4nm, spin glass behavior appears due to the increased spin disorder effect. The spin glass phase providing a pinning force from some frozen spins to the rotatable spins gives the key to explain the exchange bias effects.

Modulation of unidirectional anisotropy for Co-based amorphous ribbons by longitudinal pulse field

Field-induced unidirectional anisotropy of Co-based amorphous ribbons was discussed by magnetization measurement. The shifted hysteresis loops of Co58Fe5Ni10Si11B16 amorphous ribbons were obtained by annealing the samples in longitudinal magnetic field. Here, the feasibility of employing the longitudinal pulse field to tailor the anisotropy characteristic is demonstrated. It is found that the shifted loops can be technically controlled by enhancing the pulse field to modulate the magnetic anisotropy from unidirectional to uniaxial, and even back to unidirectional. The surface domains scan gives strong evidence that the pulse field can be one of the skillful methods to navigate the unidirectional anisotropy in the amorphous ribbons for potential applications.

Degradation of Magnetic Properties in Transformer Steel: Role of Prior Elastic Deformation

Transformer steel, fully processed Hi-B (high permeability) grade of cold rolled grain oriented (CRGO), was subjected to relatively minor, 0 to -55 MPa, stresses. Detailed magnetic and microstructural characterizations were then carried out. Shift of hysteresis loop and clear degradation in magnetic properties, namely increase in core loss and drop in permeability, were noted. Though absence of mesoscopic plastic strains was confirmed, possibilities of minor local plastic deformation cannot be ruled out. Prior deformations: 1) modified residual strain/stress signatures in both CRGO laminates and Forsterite (Mg <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) coating and 2) caused corresponding degradation in magnetic performance. Redistribution of residual stresses through local plastic deformation appears to be the plausible justification for these observations. This paper brought out clear correlations between degradation in magnetic properties with introduced residual stress/strain.

Observation of Large Exchange Bias Effect in Bulk Mn&lt;sub&gt;50&lt;/sub&gt;Ni&lt;sub&gt;41&lt;/sub&gt;Sn&lt;sub&gt;9&lt;/sub&gt; Heusler Alloy

An experimental study on exchange bias (EB) and magnetic properties of bulk Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">41</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sub> Heusler alloy has been performed using magnetization measurements. A large value of EB field of 1165 Oe in the martensite phase has been observed after 10 kOe field cooling process. The observed double shifted hysteresis loop after zero field cooling and a large training effect strongly suggest the presence of EB phenomenon in this alloy. The reason for the large EB field may be an additional antiferromagnetic exchange interaction present between Mn atoms on Ni sites and the Mn atoms on the regular sites. The effect of various parameters, such as temperature and cooling field, on the EB properties has also been studied.

Study of Exchange Bias in All Ferromagnetic Fe/Co Soft/Hard Bilayer

We report the unusual and exotic phenomenon of positive exchange bias observed in all ferromagnetic layered soft/hard Fe/Co spring magnet systems. Interestingly, the results are well reproducible both at low (5 K) and room temperatures (300 K). The effect of applied magnetic pinning field of 7 and -5 T reveals a noticeable shift of magnetic hysteresis loops along the field direction. We argue that the reason behind observed positive exchange bias is the establishment of antiferromagnetic (AFM) exchange coupling at the interface as a result of intermixing of Fe into the Co layer. The collective coupling of this intermix layers is AFM which facilitates positive shift of magnetic hysteresis loop.

Magnetic properties and the mechanism of formation of the uncompensated magnetic moment of antiferromagnetic ferrihydrite nanoparticles of a bacterial origin

The magnetic properties of the superparamagnetic ferrihydrite nanoparticles that form as a result of the vital activity of Klebsiella oxytoca bacteria are studied. Both an initial powder with an average number of iron atoms N Fe ∼ 2000–2500 in a particle and this powder after annealing at 140°C for 3 h in air are investigated. The following substantial modifications of the magnetic properties of the ferrihydrite nanoparticles are detected after annealing: the superparamagnetic blocking temperature increases from 23 to 49.5 K, and the average magnetic moment of a particle increases (as follows from the results of processing of magnetization curves). The particles have antiferromagnetic ordering, and the magnetic moment resulting in the superparamagnetism of the system appears due to random spin decompensation inside the particle. For this mechanism, the number of uncompensated spins is proportional to the number of magnetically active atoms raised to the one-half power, and this relation holds true for the samples under study at a good accuracy. The possible causes of the detected shift of magnetic hysteresis loops at low temperatures upon field cooling are discussed.

Angular dependence of hysteresis shift in oblique deposited ferromagnetic/antiferromagnetic coupled bilayers

The angular dependence of the hysteresis shift has been investigated in ferromagnetic/antiferromagnetic (NiFe/IrMn) bilayers grown by oblique deposition under the influence of a static magnetic field applied perpendicular to the uniaxial anisotropy direction induced during the growth process. It was found that at low oblique deposition angles, the unidirectional anisotropy field is much greater than the uniaxial anisotropy field and the corresponding anisotropies directions are noncollinear. In these conditions, the angular dependence of the hysteresis loop shift exhibits the well know cosine like shape but demanding a phase shift. Contrary to this, at high oblique deposition angle (70°), the uniaxial anisotropy plays the fundamental role and the anisotropies directions are collinear. In this case, the exchange bias displays a jump phenomenon. The numerical calculations are consistent with the experimental data obtained from magneto-optical Kerr effect and ferromagnetic resonance.

Phase-coexistence and thermal hysteresis in samples comprising adventitiously doped MnAs nanocrystals: programming of aggregate properties in magnetostructural nanomaterials.

Small changes in the synthesis of MnAs nanoparticles lead to materials with distinct behavior. Samples prepared by slow heating to 523 K (type-A) exhibit the characteristic magnetostructural transition from the ferromagnetic hexagonal (α) to the paramagnetic orthorhombic (β) phase of bulk MnAs at Tp = 312 K, whereas those prepared by rapid nucleation at 603 K (type-B) adopt the β structure at room temperature and exhibit anomalous magnetic properties. The behavior of type-B nanoparticles is due to P-incorporation (up to 3%), attributed to reaction of the solvent (trioctylphosphine oxide). P-incorporation results in a decrease in the unit cell volume (∼1%) and shifts Tp below room temperature. Temperature-dependent X-ray diffraction reveals a large region of phase-coexistence, up to 90 K, which may reflect small differences in Tp from particle-to-particle within the nearly monodisperse sample. The large coexistence range coupled to the thermal hysteresis results in process-dependent phase mixtures. As-prepared type-B samples exhibiting the β structure at room temperature convert to a mixture of α and β after the sample has been cooled to 77 K and rewarmed to room temperature. This change is reflected in the magnetic response, which shows an increased moment and a shift in the temperature hysteresis loop after cooling. The proportion of α present at room temperature can also be augmented by application of an external magnetic field. Both doped (type-B) and undoped (type-A) MnAs nanoparticles show significant thermal hysteresis narrowing relative to their bulk phases, suggesting that formation of nanoparticles may be an effective method to reduce thermal losses in magnetic refrigeration applications.

Bimodal switching field distributions in all-perpendicular spin-valve nanopillars

Switching field measurements of the free layer element of 75 nm diameter spin-valve nanopillars reveal a bimodal distribution of switching fields at low temperatures (below 100 K). This result is inconsistent with a model of thermal activation over a single perpendicular anisotropy barrier. The correlation between antiparallel to parallel and parallel to antiparallel switching fields increases to nearly 50% at low temperatures. This reflects random fluctuation of the shift of the free layer hysteresis loop between two different magnitudes, which may originate from changes in the dipole field from the polarizing layer. The magnitude of the loop shift changes by 25% and is correlated to transitions of the spin-valve into an antiparallel configuration.

Formation of ordered NiFeMn antiferromagnetic phase in permalloy/manganese bilayers in the course of thermomagnetic treatment

The formation of the ordered NiFeMn antiferromagnetic phase in the course of thermomagnetic treatment of manganese-permalloy bilayers has been investigated. The influence of the type of substrate, number of layers, and modes of thermomagnetic treatment on the magnetic properties of the films is studied. It has been shown that the maximal effect of unidirectional anisotropy associated with the presence of the ordered NiFeMn ferromagnetic phase in ferromagnetic layers is attained at an annealing temperature of 260°C for 4 h. The shift in the magnetic hysteresis loop with a thickness of the switched layer of about 40 nm is 380 Oe, while the blocking temperature is ≌270°C.

Exchange bias in room-temperature epitaxial Fe3O4 film

We have grown 100-nm-thick epitaxial Fe3O4(111) film on α-Al2O3(0001) substrate at room temperature by the laser molecule beam epitaxy method. Exchange bias effect is observed once the film is cooled from 300K to 5K in the presence of a magnetic field, characterized by the shift and expansion of magnetic hysteresis loops. The observed exchange bias effect is related to the exchange coupling between antiferromagnetic Fe–O–Fe superexchange interactions at antiphase domain boundaries and ferromagnetic spins in the neighboring Fe3O4 domains. The magnitudes of the exchange bias field and coercive field show strong dependence on magnetic and thermal histories.

Enhanced exchange bias effect in size modulated Sm0.5Ca0.5MnO3 phase separated manganite

The effect of grain size modulation on exchange bias effect in CE-type antiferromagnetic Sm0.5Ca0.5MnO3 phase separated manganite is reported here. With the reduction of particle size, ferromagnetic clusters are found to form in the charge ordered antiferromagnetic matrix and gradually become larger. The horizontal and vertical shifts of the magnetic hysteresis loops in the field cooled magnetization process clearly indicate the size dependent exchange bias effect and it can be tuned with the reduction of particle sizes. The values of exchange bias parameter, i.e., exchange bias field (HE), coercivity (HC), remanence asymmetry (ME), and magnetic coercivity (MC) are found to depend strongly on the particle size. The variations of HE follow non-monotonic dependencies with reduction in particle size and show maximum (1205 Oe) at particle size of 150 nm at T = 5 K, which can be ascribed due to the changes in uncompensated surface spins. The values of HE and ME are found to decrease exponentially with increasing temperature below the spin- or cluster-glass like freezing temperature. The spin relaxation model has been employed for analysis of large magnetic training effect. The linear relationship between HE and ME further confirms the role of uncompensated surface spins. In view of spintronics application of manganites, the present observation of large exchange bias shift in this half-doped manganite may have great technological importance.

Exchange Bias Properties in Bulk Ni&lt;sub&gt;45&lt;/sub&gt;Co&lt;sub&gt;5&lt;/sub&gt;Mn&lt;sub&gt;38&lt;/sub&gt;Sn&lt;sub&gt;12&lt;/sub&gt; Heusler Alloy

We report the exchange bias properties in the bulk Ni45Co5Mn38Sn12 quaternary Heusler alloy. The ferromagnetic (FM) –antiferromagnetic (AFM) interactions get reinforced after the Co substitution for Ni in the Ni-Mn-Sn alloy, which increase the exchange bias field (HE). A maximum shift in hysteresis loops of 306 Oe was observed in the 10 kOe field cooled sample. The origin of this large exchange bias field has been discussed. Magnetic hysteresis loop obtained in the zero field cooled (ZFC) mode shows double-shifted loop, and the reason of this phenomenon has been explained in detail.