Presence Of Exchange Bias Research Articles

Analytical studies of the magnetic domain wall structure in the presence of non-uniform exchange bias

The pinning phenomena of the domain wall in the presence of exchange bias are studied analytically. The analytic solution of the domain wall spin configuration is presented. Unlike the traditional solution, which is symmetric, our new solution could exhibit the asymmetry of the domain wall spin profile. Using the solution, the domain wall position, its width, its stability, and the depinning field are discussed analytically.

Controllable Exchange Bias Effect in (Ga, Mn)As/(Ga, Mn)(As, P) Bilayers With Non-Collinear Magnetic Anisotropy

We have investigated the exchange bias (EB) effect occurring in (Ga, Mn)As/(Ga, Mn)(As, P) bilayers, in which the (Ga, Mn)As layer has an in-plane (IP) magnetic anisotropy, and the magnetic anisotropy of the (Ga, Mn)(As, P) layer is out-of-plane (OP). Planar Hall resistance (PHR) measured during magnetization reversal in this system showed a clear shift of the hysteresis loops, indicating the presence of EB in the (Ga, Mn)As layer. The EB significantly depends on the direction of the field used in the field-cooling process. The observed EB occurring in a bilayer in which the constituent layers have orthogonal magnetic anisotropies can be understood in terms of the formation of magnetic closure domains in the (Ga, Mn)(As, P) layer during field cooling. We show that such EB can be effectively controlled by the process of field cooling, suggesting the possibility of memory device applications based on bilayer systems with orthogonal magnetic anisotropy.

Room temperature exchange bias in antiferromagnetic composite BiFeO3-TbMnO3

The magnetic property of a 0.7BiFeO3-0.3TbMnO3 composite has been studied in detail and compared with that of 0.8BiFeO3-0.2TbMnO3. The magnetic properties in 0.7BiFeO3-0.3TbMnO3 are improved in manifold compared to BiFeO3. An exchange bias (HEB) is observed in both the compositions of these antiferromagnetic composites, which varies between 5 and 180 Oe with a maximum at ∼275 K. Isothermal remanent magnetization measurements at room temperature indicate the presence of an interfacial layer of a 2-dimensional dilute uniaxial antiferromagnet in a uniform magnetic field (2D DAFF). The presence of exchange bias can be explained on the basis of a strong strain-mediated magnetoelectric coupling induced exchange interaction and the creation of a 2D DAFF layer at the interface. The properties of this layer are defined by canting and pinning of BiFeO3 spins at the interface with TbMnO3 due to Fe and Mn interaction. X-ray magnetic circular dichroism confirms the presence of canted antiferromagnetic ordering of BiFeO3, charge transfer between Mn ions, and different magnetically coupled layers that play a vital role in the exchange bias.

Exchange bias in ferromagnetic bilayers with orthogonal anisotropies: the case of GaMnAsP/GaMnAs combination

We report the observation of exchange bias in a ferromagnetic Ga0.94Mn0.06As0.77P0.23/ Ga0.94Mn0.06As bilayer, in which the easy axis in one layer is oriented out-of-plane, and in the other in-plane. Magnetization reversal in this system is explored using planar Hall effect (PHE) measurements under various initial conditions and with various field-cooling orientations. Our results show that the two magnetic layers are ferromagnetically exchange-coupled, and that such coupling results in pronounced exchange-bias-like shifts of magnetic hysteresis loops during reversal of in-plane magnetization. The presence of exchange bias in this system can be understood on the basis of magnetic closure domains formed in the layer with the out-of-plane easy axis.

Complicated Magnetic Phases and Presence of Exchange Bias in Off-Stoichiometric Mn50Ni25 + xIn25−x Heusler Alloys

The martensitic transformation (MT), magnetic phases, and exchange bias (EB) effect in Mn50Ni25 + xIn25−x (x = 14.5∼18) Heusler alloys have been investigated. The MT and magnetic state can be tuned significantly by modifying the Ni/In ratio, while the Curie temperature (TC) remains almost unchanged. With increase of the Ni/In ratio, the ferromagnetic components in the studied samples decrease continuously, while the antiferromagnetic components increase and it therefore enhances the exchange interaction as well. All of the samples exhibit negative horizontally shifted loops after field-cooled procedure. In addition, the samples with the compositional range of 15 ≤ x ≤ 16 also present the EB effect in zero field-cooled condition. To evaluate the causes of this scenario, EB field and coercive field as a function of temperature and cooling field have been investigated. The experimental results imply that the spin reconfiguration of superspin glass (SSG) clusters and the competition between antiferromagnetic and ferromagnetic interactions can account for the behavior of EB effect.

Magnetic behavior of nanostructured NiTiO3 and NiO material: Anomalous increase in coercivity

In this paper we present an interesting magnetic property of ilmenite NiTiO3 material prepared by sol-gel mediated slow evaporation induced self-assembly (EISA) method using Pluronic F127. Two oxide samples were prepared using Ni:Ti 1:1 and 1:2M ratios under similar reaction conditions. It is observed from powder XRD analysis that sample NTO11 (Ni:Ti=1:1) is a predominating NiTiO3 phase with small percentage of NiO and TiO2, whereas NTO12 (Ni:Ti=1:2) is a mixture of anatase TiO2 and NiO structures. Both materials reveal, presence of exchange bias (in hysteretic M-H loop) decreasing with increase in temperature. Both samples show an anomalous increase of magnetic coercivity with increase in temperature. The anomalous coercivity increase in NTO11 is much more pronounced than in NTO12 (by a factor of 6). We explain this large anomaly in NTO11, by invoking layer structure and weak interlayer exchange coupling between Ni moments in NiTiO3, which allows a transition near the surface of nanoparticles from antiferromagnetically coupled ferromagnetic planes of Ni moments to uncoupled set of ferromagnetic layers on the surface (each layer can be thought of as single ferromagnetic plane), blocked along different directions. The increased coercivity at higher temperature is due to excess coercivity of the nanosized ferromagnetic layers.

Structural and magnetic studies on perovskite rare-earth manganites (Nd1−xGdx)0.55Sr0.45MnO3 (x=0, 0.1, 0.3 and 0.5): Observation of two-step magnetization and negative magnetization behavior

The effect of smaller and heavy rare-earth cation Gd3+(rGd3+=1.107Å and SGd3+=7/2) on the magnetic and structural properties of Nd0.55Sr0.45MnO3 has been investigated. The orthorhombic structure (space group, Pnma) evolves from orbital distorted-O” type symmetry (c>a) for x=0 to orbital ordered-O’ type (a>c) for x≥0.1. Diffraction data shows the crucial role of the shear strains involved in the structure. The magnetization measurement shows two magnetic transitions occurring in compounds x≥0.1. Spectacular feature was observed in the heavily Gd-doped sample (x=0.5) such as: the continuous increase of M with high magnetic field, the low value of saturation magnetization at low temperature, the metamagnetic transition, and the presence of exchange bias. These findings are interpreted as the fingerprints of the phase separation. Our result indicates that the martensitic accommodation strain, in addition to quenched disorders, may be an alternative approach to understand the physical phase separation. The interesting feature for x=0.5 sample is the negative magnetization observed at low field. Result is interpreted in terms of two interacting networks, with negative exchange interaction between the (Gd+Nd) and the ferromagnetic Mn lattices. The disappearance of the negative magnetization for magnetic field well below the coercive field (~0.047T) suggest that the magnetic properties of x=0.5 behaves as an inhomogeneous ferrimagnet at low field.

Martensitic transition, magnetic, magnetocaloric and exchange bias properties of Fe-substituted Mn–Ni–Sn Heusler alloys

Abstract In this report, effect of Fe substitution on martensitic transition, magnetic, magnetocaloric and exchange bias (EB) properties of Mn50Ni40−xFexSn10 (x=0, 0.5, 1, 1.5, 2 and 3) Heusler alloys series has been investigated systematically. Fe substitution has been found to affect the ferromagnetic/antiferromagnetic interactions significantly in both the martensite and austenite phases. Martensitic transition temperature decreases with increasing Fe content, which is attributed to the decrease in number of average valence electrons per atom (e/a ratio) of these alloys. Large magnetic entropy change (ΔSM) and refrigerant capacity (RC) have been observed in these alloys, as a maximum ΔSM of ~12.6 J/kg. K is observed for composition x=0.5. Present alloys have also been found to show large exchange bias properties, as maximum exchange bias fields (HEB) of 890 Oe and 810 Oe are observed for x=0 and 0.5, respectively at 5 K. Composition and temperature dependencies of EB are associated with the change in exchange anisotropy at interfaces of competing magnetic phases. Study of minor loop and training effect also corroborates with the presence of EB in these alloys.

Surface effects in uncoated and amorphous SiO2 coated cobalt ferrite nanoparticles

Surface effects in uncoated and amorphous silica (SiO2) coated cobalt ferrite nanoparticles have been studied by using dc magnetization. Both uncoated and SiO2 coated nanoparticle samples were synthesized by using sol–gel method. SiO2 coated nanoparticles exhibit smaller average crystallite size as compared to uncoated nanoparticles. Saturation magnetization (Ms) revealed decreasing trend at low temperatures for uncoated nanoparticles as compared to coated nanoparticles which may be due to magnetically dead nanoparticle's surface layer for uncoated nanoparticles. A step-like behavior near remanent field was observed for both the samples but more pronounced for SiO2 coated nanoparticles. This fast magnetic relaxation near remanent field in coated nanoparticles is due to lesser dipolar (interparticle) interactions among nanoparticles. Bloch's law shows a good fit for coated nanoparticles. SiO2 coated nanoparticles showed larger values of coercivity at low temperatures due to their enhanced surface anisotropy as compared to uncoated nanoparticles. Kneller's law for temperature dependent coercivity shows a good fit for SiO2 coated nanoparticles and is attributed to lesser dipolar interactions among coated nanoparticles as compared to uncoated nanoparticles. The exchange bias phenomenon was also observed for both the samples, which signify the presence of core–shell interactions. However, SiO2 coated nanoparticles exhibit larger values of exchange bias at low temperatures due to their strong surface anisotropy as compared to uncoated nanoparticles. The presence of exchange bias in uncoated nanoparticles indicates that their surface layer is not totally magnetically dead. In summary, (i) the SiO2 coated nanoparticles have lesser dipolar interactions due to SiO2 coating and (ii) surface layer in uncoated nanoparticles is not magnetically dead but may have different preferred ordering at low temperatures as compared to SiO2 coated nanoparticles.

Inverse spin Hall effect of antiferromagnetic MnIr in exchange biased NiFe/MnIr films

Antiferromagnetic Mn3Ir, which is widely employed in exchange-biased applications, has attracted much attention recently due to its predicted and subsequently observed large spin Hall effect, therefore increasing its potential for spintronic devices in place of conventional paramagnetic 5d spin Hall metals. (Pt, Ta and W) Via the electrical detection of ferromagnetic resonance, we study a series of exchange biased NiFe/MnIr films for various MnIr thicknesses. In these systems, spin-pumped spin currents from NiFe are converted into dc voltages within MnIr via the inverse spin Hall effect (ISHE), which mixes with spin rectification voltages generated from NiFe. Through angular measurements, we separate these different voltage contributions to qualitatively detect non-zero ISHE in MnIr, which coexists with a non-zero unidirectional anisotropy. We find significant extrinsic damping contributions which prevent the accurate quantification of spin pumping-induced ISHE in MnIr films. The results show that spin currents may propagate and dissipate in MnIr films through ISHE in the presence of exchange bias.

Interfacial magnetic phenomena of mechanosynthesized Fe nanoparticles in MnO matrix

Pure phase Fe/MnO nanocomposites are prepared by high energy ball-milling. Effect of crystallite size and milling induced defects on magnetic properties of nanocomposites are investigated. Mossbauer spectra have shown that Fe atoms in the nanocomposites are in ordered state up to 20h milling duration. However, the appearance of a minor doublet in addition to a sextet in the Mössbauer spectrum of sample milled for 30h has indicated polydispersity nature of the sample. The presence of exchange bias in the samples milled for higher durations are confirmed by magnetic hysteresis measurements. Temperature dependent ac and dc magnetization has indicated the presence of spin-glass like relaxation in the sample. This possibility has been confirmed by field cooled memory effect and temperature cycling in ZFC magnetization.

Memory effects in exchange coupled Fe/Co3O4 nanocomposites

Nanocomposites comprising a ferromagnet and an antiferromagnet have drawn attention recently because of their interesting physical properties and variety of technological applications. In the present work, structural, hyperfine and magnetic properties of Fe/Co3O4 nanocomposites prepared by a chemical route having 10–70 wt.% of Fe, have been investigated. XRD and TEM measurements confirmed polycrystalline nature of the samples having grain size in the nanometer regime. FTIR measurements show the presence of two bands of Co–O corresponding to Co2+ and Co3+. Mössbauer spectra recorded at room temperature confirm the presence of Fe in the blocked state. Presence of exchange bias at Fe–Co3O4 interfaces is confirmed by the magnetization measurements. Irreversibility in temperature dependent FC–ZFC measurements points to interface effect. Frequency dependent ac susceptibility measurements as well as memory effect observed in dc magnetization measurements indicate the superspin glass nature of the nanocomposites.

Structural, dielectric, and magnetic properties of La0.8Bi0.2Fe1−xMnxO3 (0.0≤x≤0.4) multiferroics

Polycrystalline multiferroic La0.8Bi0.2Fe1−xMnxO3 (0.0≤x≤0.4) samples were synthesized by the conventional solid state reaction method. Reitveld refinement of the x-ray diffraction patterns confirms the single phase character of all the compositions with orthorhombic structure having space group Pnma (No. 62). Dielectric properties of the samples at temperatures 200–475 K and frequencies 500 kHz–1 MHz authenticate the stabilization of ferroelectric phase with Mn substitution. Dielectric responses of these multiferroics have been analyzed carefully, in the light of “universal dielectric response” model. While cooling from room temperature to 20 K, systematic shifts in magnetization hysteresis loops indicate the presence of exchange bias (EB) phenomenon in the system. Magnetic behavior of these samples has been briefly discussed on the basis of “EB” model for granular systems. Temperature and magnetic field dependent magnetization data demonstrate enhanced magnetization due to the Mn substitution. Magnetocapacitance measurement reveals the magnetoelectric coupling for wide range of temperature (180–280 K) and decrease in dielectric loss at high magnetic field (3 T).

Observation of enhanced exchange bias behaviour in NiCoMnSb Heusler alloys

We report the observation of large exchange bias (EB) in Ni50−xCoxMn38Sb12 Heusler alloys with x = 0, 2, 3, 4, 5, which is attributed to the coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) phases in the martensitic phase. The phase coexistence is possibly due to the supercooling of the high temperature FM phase and the predominant AFM component in the martensitic phase. The presence of EB is well supported by the observation of the training effect. The EB field increases with Co concentration. The maximum value of 480 Oe at T = 3 K is observed in x = 5 after field cooling in 50 kOe, which is almost double the highest value reported so far in any Heusler alloy system. Increase in the AFM coupling after Co substitution is found to be responsible for the increase in the EB.

Magnetic characterization of Co 1−xNi xFe 2O 4 (0⩽ x⩽1) nanoparticles prepared by co-precipitation route

Magnetic nanoparticles of Ni-doped cobalt ferrite [Co 1− x Ni x Fe 2O 4 (0⩽ x⩽1)] synthesized by co-precipitation route have been studied as a function of doping concentration ( x) and particle size. The size of the particles as determined by X-ray diffractometer (XRD) and transmission electron microscope (TEM) analyses was found in the range 12–48 nm. The coercivity ( H C) and saturation magnetization ( M S) showed a decreasing behavior with increasing Ni concentration. M S of all the samples annealed at 600 °C lies in the range 65.8–13.7 emu/gm. Field-cooled (FC) studies of the samples showed horizontal shift (exchange bias) and vertical shift in the magnetization loop. Strong decrease in exchange bias ( H b) and vertical shift ( δM) was found for low Ni concentrations while negligible decrease was found at higher concentrations. The presence of exchange bias in the low Ni-concentration region has been explained with reference to the interface spins interaction between a surface region (with structural and spin disorder) and a ferrimagnetic core region. M( T) graphs of the samples showed a decreasing trend of blocking temperature ( T b) with increasing Ni concentration. The decrease of T b with increasing Ni concentration has been attributed to the lower anisotropy energy of Ni +2 ions as compared to Co +2 that increases the probability of the jump across the anisotropy barrier which in turn decreases the blocking temperature of the system.

Inhomogeneous ferrimagnetic-like behavior in Gd 2/3Ca 1/3MnO 3 single crystals

We present a study of the magnetic properties of Gd 2/3Ca 1/3MnO 3 single crystals at low temperatures, showing that this material behaves as an inhomogeneous ferrimagnet. In addition to small saturation magnetization at 5 K, we have found history dependent effects in the magnetization and the presence of exchange bias. These features are compatible with microscopic phase separation in the clean Gd 2/3Ca 1/3MnO 3 system studied.

Comparison of the crystalline structure, morphology, and magnetic properties ofγ-phaseMn∕Cu3Au(100)ultrathin films by varying the growth temperature

The structural and magnetic properties of room-temperature (RT: $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$)-grown and low-temperature (LT: $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$)-grown $\mathrm{Mn}∕{\mathrm{Cu}}_{3}\mathrm{Au}(100)$ thin films were investigated. Mn films deposited at RT and LT demonstrate very different behaviors in the crystalline structure, morphology, and magnetism. $\mathrm{RT}\text{\ensuremath{-}}\mathrm{Mn}$ films reveal apparent layer-by-layer growth for $0--2\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$ (monolayer) followed by reduced oscillations. Although the medium-energy electron diffraction (MEED) oscillation is reduced, the intensity of specular spot increases monotonically after $6--7\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$, inferring the tendency of smooth morphology. The study of scanning tunneling microscopy also shows that even in $19\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$ $\mathrm{Mn}∕{\mathrm{Cu}}_{3}\mathrm{Au}(100)$, the surface morphology is composed of large terraces with the size up to hundreds of nanometers. The $\mathrm{LT}\text{\ensuremath{-}}\mathrm{Mn}$ films reveal apparent layer-by-layer growth for $0--5\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$ followed by the reduced oscillations, and then the MEED intensity remains at low intensity, inferring the rough surface. The RT- and $\mathrm{LT}\text{\ensuremath{-}}\mathrm{Mn}$ films exhibit a thickness-dependent structural transition from a face-centered cubic to a face-centered tetragonal structure at different critical thicknesses, $\ensuremath{\sim}12--14$ and $\ensuremath{\sim}8\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$, respectively. Significant exchange bias is observed in $\mathrm{Fe}∕\mathrm{RT}\text{\ensuremath{-}}\mathrm{Mn}$ bilayers. It increases monotonously with Mn thickness. The exchange bias coupling in $\mathrm{Fe}∕\mathrm{LT}\text{\ensuremath{-}}\mathrm{Mn}$ is much weaker than $\mathrm{Fe}∕\mathrm{RT}\text{\ensuremath{-}}\mathrm{Mn}$ and drastically varies with Mn film thickness. The presence of exchange bias in the $\mathrm{Fe}∕\mathrm{Mn}$ bilayers also indicates the antiferromagnetism of $\ensuremath{\gamma}$-phase $\mathrm{Mn}∕{\mathrm{Cu}}_{3}\mathrm{Au}(100)$.