Weak Exchange Bias Research Articles

Reversible exchange bias in epitaxial V2O3/Ni hybrid magnetic heterostructures

In this work we present a temperature and angular dependent study of the structural and magnetic properties in highly crystalline V2O3/Ni/Zr magnetic heterostructure films. Our investigation focuses on the coupling between the ferromagnetic Ni layer and V2O3 layer which undergoes an antiferromagnetic/paramagnetic phase transition coupled to the structural phase transition of the material at around 150 K. Structural investigations using x-ray diffraction reveal highly crystalline films of a quality which has previously not been reported in the literature. The Ni layers display an absence of in-plane magnetic anisotropy owing to the highly textured (1 1 1) layering of the Ni films on the underlying V2O3(0 0 0 1) oriented layer. During the transition we observe a strain related enhancement of the coercivity and the onset of a weak exchange bias for cooling under an external magnetic field. Heating the films to above the transition temperature, the exchange bias in the Ni is removed and can be reversed upon subsequent cooling under an inverted external magnetic field. Using temperature dependent polarized neutron reflectometry we investigate the film structure at the interface, capturing the magnetic and nuclear profiles.

Structural characterization and magnetic properties of single‐crystalline (La0.6Pr0.4)0.67Ca0.33MnO3 nanowires

AbstractHerein, structural and magnetic properties of single‐crystalline (La0.6Pr0.4)0.67Ca0.33MnO3 nanowires synthesized via a hydrothermal process are reported. The nanowires are crystallized in an orthorhombic structure (Pnma space group). Their lattice parameters follow the relationship a ≈ c ≈ b/√2 These nanowires exhibited a clean and smooth surface with diameters of 60‐120 nm and an average length of approximately 2.0 μm. High‐resolution transmission electron microscopy images confirmed the single‐crystalline nature of the nanowires growing along the [100] direction. The nanowires demonstrated magnetic hysteresis loops at low temperatures and a weak exchange bias (EB) effect. Paramagnetic (PM)–ferromagnetic (FM) phase transition occurred at a Curie temperature (TC) of 224 K, and strong irreversibility between zero‐field‐cooled (ZFC) and field‐cooled (FC) magnetization (MZFC and MFC, respectively) curves was observed at 273 K. The MZFC curve exhibited a significantly broad peak with a maximum at a freezing temperature (Tf) of 134 K. Relative difference between MFC and MZFC in the nanowires [(MFC − MZFC)/MFC] rapidly increased below Tf and reached approximately 50% below 35 K. The effective magnetic moment deduced from the Curie constant is larger than the theoretical value, indicating short‐range FM interactions in the nanowires. A positive PM TC (θp) implies dominant FM interactions in the nanowires, and θp > TC observed herein indicates the existence of short‐range ordered states above TC.

Weak ferromagnetism and exchange bias effect in Co1+yAl2-yO4/SiO2 nanocomposites

Nanocomposites made of magnetic spinel oxide Co1+yAl2-yO4 nanoparticles (NPs) with y = 0.93, 1.20 and 1.34 dispersed in an amorphous SiO2 matrix were prepared by a sol-gel method. The average crystallite size DXRD of about 25 nm determined by X-ray diffraction (XRD) agrees well with the average particle size observed in transmission electron microscopy (TEM). Dynamic magnetic susceptibility measurements indicate a spin-glass-like behavior of the nanoparticles below the spin-glass (SG) transition temperature TC < 10 K. Field dependent magnetization exhibits weak ferromagnetism and shift of the magnetic hysteresis loop. On the basis of the observed magnetic properties, we propose a model of the core-shell structure of the Co1+yAl2-yO4 nanoparticles with the spin-glass phase in the core and weak ferromagnetism in the shell. The exchange bias field disappears above the spin-glass transition temperature TC, whereas the weak ferromagnetism persists also at temperatures above TC. The component of weak ferromagnetism is associated with the presence of Co vacancies in the nanoparticle spinel crystal lattice.

Magnetic properties and thermal stability of polyvinylidene fluoride—Fe2O3 nanocomposites

Abstract

One-step synthesis of polyethyleneimine-coated magnetite nanoparticles and their structural, magnetic and power absorption study.

Magnetic nanoparticles (NPs) are especially interesting for several biomedical applications due to their chemical surface, especially for targeted cancer imaging and therapeutics. In order to realize these applications, it is important to know their magnetic properties among other complementary properties that help to improve the understanding of the synthesis process. In this work, we report the magnetic properties of polyethyleneimine-coated magnetite (PEI-Fe3O4) NPs synthesized by a one-step method via the co-precipitation method and using PEI as a stabilizer. Transmission electron microscopy (TEM) images revealed agglomerated magnetic nanoparticles with an average size of ∼10 nm; meanwhile, the X-ray diffraction (DRX) analysis confirmed a pure magnetite phase. The study of magnetic properties shows a superparamagnetic system with coexistence of non-interacting single NPs with a low blocking temperature (∼35 K) and interacting NPs in the aggregates with a higher blocking temperature (>150 K), in which the interparticle interactions of magnetic cores dominate over surface spin disorder. The interaction between the surface spin-disorder layer and NP core was found to be weak, related to a weak exchange bias effect. A maximum specific loss power (SLP) value of 70 W g−1 was obtained (f = 571 kHz and H = 23.87 kA m−1) indicating that the magnetic response plays a crucial role in determining the heating efficiency for future applications.

Uncompensated-spins induced weak ferromagnetism in Ca3Mn2O7: Magneto-conductive and dual magneto-capacitive effects

Temperature dependent magnetization study on single phase orthorhombic Ruddlesden-Popper manganite Ca3Mn2O7 evidences antiferromagnetic (AFM) ordering below 123 K. Field-dependent magnetization M(H) depicts off-centered hysteretic loops below ~110 K—confirming the existence of both weak ferromagnetism (WFM) and exchange bias, whose development upon cooling is examined in detail. WFM is attributed to the formation of uncompensated-spin-clusters within the AFM-matrix, stabilized by the high-anisotropy in the manganite, favoring Dzyaloshinskii-Moriya (D-M) interaction. Temperature dependence of an evaluated average canting angle from the M(H) loops provides a quantitative measure of the WFM-evolution. Dielectric measurements exhibit magneto-conductive effects concurrent to the WFM-onset. Temperature dependence of the low-frequency/dc-limit of Jonscher-fits to the conductivity isotherms confirms the presence of relatively more-conductive clusters, embedded in the insulating-bulk. Nyquist impedance-analysis of complex impedance reveals two relaxations, yielding dual magnetic-field effects; the lumped circuit parameters feature anomalies across the magnetic phase changes.

Perpendicular exchange bias at room temperature in (Pt/Co) multilayers coupled to NiO antiferromagnetic layer

(Pt/Co) multilayers coupled to NiO exhibit a small exchange bias field HEB which is consistent with a NiO blocking temperature TB of about the room temperature. Direct deposition of nickel oxide on cobalt increased coercivity and induced a weak exchange bias field. The reversal process has also been affected. It changes from a reversal clearly governed by wall propagation initiated by only a few domains to a reversal dominated by wall propagation that is initiated by a larger number of domains leading to an important role in the nucleation. The domains are larger in the sample without NiO. The insertion of a platinum layer with thickness range 0–0.6 nm between Co and NiO acts on the coercivity and the exchange field. The optimal values of HC and HEB are observed for tpt = 0.2 nm. We found that the magnetization reversal process changes from a wall propagation-dominated process for tpt ≤0.2 nm to a domain nucleation dominated process for tpt> 0.2 nm.

Thermal annealing induced competition of oxidation and grain growth in nickel thin films

The interface of Ni-NiO thin films was developed by thermal evaporation of nickel and subsequent annealing in oxygen atmosphere at 400 °C at varying duration of time. The evolution of layer thicknesses with annealing time was studied using rutherford backscattering spectrometry. The structural characterization showed grain growth stagnation for Ni at higher duration of annealing. The Ni phase had more crystallinity compared to the NiO phase. The surface was studied using atomic force microscope. The magnetic domains were also imaged. Magnetic stripe domain patterns were observed for selected films. Variation in saturation magnetisation and coercivity with annealing time was observed. The observation of weak exchange bias shows the importance of antiferromagnetic phase in determining the exchange bias properties. Thermal annealing of Ni films caused a competition among oxidation and grain growth.

Antiphase boundary in antiferromagnetic multiferroic LuMn0.5Fe0.5O3: anomalous ferromagnetism, exchange bias effect and large vertical hysteretic shift

The emergence of exchange bias effect in Fe3O4 thin films has been since attributed to the presence of anti phase boundary (APB) growth defects despite lack of direct experimental evidence. In the present report, APB induced anomalous weak ferromagnetism and exchange bias property of single-phase antiferromagnetic (AFM) system LuMn0.5Fe0.5O3 (LMFO) is discussed and 57Fe Mössbauer spectroscopy and high resolution transmission electron microscopy (HRTEM) measurements are used to probe the origin of the observed effect. In addition to the sextet component corresponding to the long range AFM ordering, the measured Mössbauer spectra reveal the presence of a small component (10%–12%) near zero velocity with unusually small internal field. This indicates the presence of APB defects. From micro structural investigations using HRTEM, presence of APB type defects and dislocations are confirmed. In addition to the exchange bias effect, upon field cooling, hysteresis loop exhibits large vertical shift due to strong pinning effect of the APB. Finally we further annealed the optimally sintered sample LMFO and studied the evolution of defects, and their influence on weak ferromagnetism and exchange bias properties. Our present experimental findings may pave the way in creating new functionalities in materials using APB-type growth defects.

Monte Carlo study of the exchange bias effect in Co/CoO core–shell nanowires

We study the magnetic properties of cylindrical ferromagnetic core—antiferromagnetic shell nanowires using Monte Carlo simulations and a classical Heisenberg Hamiltonian in order to elucidate the impact of the oxidized shell on the magnetic properties and the magnetization reversal mechanism. We find that the coupling to the antiferromagnetic shell leads to suppression of the coercivity and emergence of a weak exchange bias effect. Comparison of the magnetization reversal mechanism in the bare and the surface-oxidized nanowire reveals that the domain wall propagation and annihilation remains the dominant reversal mechanism in surface oxidized nanowires as in their ferromagnetic counterparts. However, the interface exchange coupling introduces a secondary reversal mechanism activated in the central part of the wire with characteristics of coherent rotation, which acts in synergy to wall propagation leading to enhancement of the wall mobility. This effect is more pronounced in nanowires with large exchange bias values and is attributed to the uncompensated interface moments that act as nucleation centers for magnetization reversal. Our results are in good agreement with recent measurements in Co and Co/CoO nanowires.

Magnetic properties, exchange bias, and memory effects in core-shell superparamagnetic nanoparticles of La0.67Sr0.33MnO3

The static magnetic properties and memory and exchange bias effects have been studied in sol-gel prepared La0.67Sr0.33MnO3 (LSMO) nanoparticles. Transmission electron microscopy (TEM) micrographs and static magnetization show log-normal particle and magnetic size distributions with a core-shell structure. Analysis of the magnetization measurements indicates the presence of a magnetic structure with a 7.8 nm core radius and a magnetic dead layer of thickness 1.6 nm in the LSMO nanoparticles, which comprises about 40% of the volume. The disordered spins in the shell freeze at lower temperatures than the core and produce a surface spin glass state exhibiting a weak exchange bias effect. Field cooled and zero-field cooled magnetization measurements have been carried out to study the slow dynamics of the sample and associated magnetic memory effects; the results reveal the superparamagnetic behavior of LSMO nanoparticles described in terms of the magnetic size distribution rather than a superspin glass state.

Magnetic Phase Separation in Diluted Magnetic System: Zn1−x Fe x O

Single-phase diluted magnetic systems Zn1−x Fe x O have been prepared by chemical route. Structural and spectroscopic (UV–Vis and Mössbauer) studies indicate the incorporation of Fe3+ ions in the lattice sites. The UV–Vis results point to a systematic increase in the band gap with increasing Fe doping. The room temperature magnetization of Zn1−x Fe x O indicates a paramagnetic behavior which is in accordance with the Mössbauer results, illustrating quadrupolar doublet. At low temperature, the zero-field-cooled (ZFC) magnetization shows a cusp and this temperature increases systematically with decreasing particle size. The weak exchange bias effect manifested by a M–H loop shift is observed for x = 0.03. This shift is accompanied by the enhancement of coercivity. The dc magnetization results suggest the coexistence of ferromagnetic and antiferromagnetic exchange interactions for low doping of Fe, i.e., for x = 0.03.

The abnormal electrical and optical properties in Na and Ni codoped BiFeO3 nanoparticles

Bi0.97Na0.03Fe1−xNixO3 (x = 0, 0.005, 0.01, 0.015) nanoparticles are prepared via a sol-gel method. Weak ferromagnetism and exchange bias phenomenon without field cooling are observed in the samples. The oxygen vacancy concentration and leakage current density are increased with increasing the Ni content. However, with the increase of Ni content, the band gap of Bi0.97Na0.03Fe1−xNixO3 nanoparticles first decreases and then increases. To explain the abnormal phenomenon, the interplay of oxygen vacancy donor and hole acceptor is analyzed and a phenomenological qualitative model based on the electronic energy band is proposed. Additionally, the threshold switching behavior appears in Bi0.97Na0.03Fe1−xNixO3 samples with x = 0.01, 0.015 and the effect is qualitatively explained by introducing a conducting channel model based on the high-density mobile charges.

Unusual ferromagnetic YMnO3 phase in YMnO3/La2 / 3Sr1 / 3MnO3 heterostructures

By means of first-principles density functional calculations, we study the structural, magnetic and electronic properties of YMnO3/MnO3 heterostructures. Although in the bulk the ground state of YMnO3 is an antiferromagnet, the YMnO3/MnO3 heterostructure stabilizes the ferromagnetic (FM) phase in YMnO3 in the interface region over a wide range of Coulomb repulsion parameters. The hypothetical FM phase of bulk YMnO3 is dielectric and due to substantial differences between the lattice constants in the ab plane, a strong magnetocrystalline anisotropy is present. This anisotropy produces a high coercivity of the unusual FM YMnO3 that can explain the large vertical shift in the hysteresis loops observed in recent experiments (Paul et al 2014 J. Appl. Crystallogr. 47 1054). The correlation between weak exchange bias and the vertical shift is proposed, which calls for reinvestigation of various systems showing vertical shifts.

Exchange bias effect in Ni80Fe20/CoO bi-component nanowires

We investigate the magnetic and transport behavior of exchange biased Ni80Fe20(Py)/CoO bi-component nanowires (NWs) consisting of Py NWs with top surface fully or partially covered with CoO and lateral exchange coupled Py/CoO structure. The exchange bias field and magnetoresistance (MR) behavior of the bi-component NWs are found to be markedly dependent on the Py/CoO interface area. We observed that when the Py/CoO interface area is reduced to half, the exchange bias field decreases significantly due to the reduced interfacial anisotropy. We found that the degree of asymmetry of the ascending and descending MR branches is very sensitive to the Py/CoO interface area. For lateral exchange coupled Py/CoO structure, we observed a weak exchange bias effect below 50 K due to a much smaller effective interfacial anisotropy.

Exchange bias in iron oxide nanoclusters

Iron oxide nanoclusters have been prepared by the gas-phase aggregation technique to form thin film structures with very high exchange bias values (up to 3000 Oe at low temperatures). Composition has been analysed by x-ray absorption and Mössbauer spectroscopies in order to elucidate the actual origin of the observed magnetic behaviour. The formation of a metal–oxide core–shell arrangement to explain the observed exchange bias has to be discarded since results show no metallic iron content and the main presence of α-Fe2O3. The observed weak ferromagnetism and exchange bias are in agreement with the obtained size of α-Fe2O3 nanoparticles: weak ferromagnetism because of the well-known spin canting in this antiferromagnetic structure and exchange bias because of the interaction between different spin sublattice configurations promoted by the modification of iron coordination in α-Fe2O3 nanoparticles. Moreover, the preparation method is proposed for tuning both magnetization and exchange bias values by modification of the preparation conditions of α-Fe2O3 nanoparticles, which open new possibilities in the design of new materials with required properties.

Anomalous Weak Ferromagnetism in Electron-doped Nd1-xSrxMnO3 (0.50 ¡? x ¡? 0.62) Thin Films

In the present work, we demonstrate that an anomalous weak ferromagnetism occurs in the composition range 0.50 <= x <= 0.62 in nanostructured thin films of Nd1-xSrxMnO3 (NSMO). Nanocrystalline thin films of overdoped manganite Nd1-xSrxMnO3 (x similar to 0.50, 0.55, 0.60, and 0.62) are grown on single-crystal LaAlO3 (001) substrates by using a nebulized chemical spray pyrolysis technique. These single phase films possess an average crystallite size similar to 15 nm, and the width of the grain boundaries is similar to 1 - 2 nm. In the composition range 0.50 <= x <= 0.62, the ground state of NSMO is an A-type antiferromagnetic (AFM) metallic state. However, in the present films, a broad paramagnetic (PM)-to-ferromagnetic (FM) transition is observed in all the compositions, at T-C similar to 226 K for x = 0.50 and at 235 K for rest of the samples. All the films show a well-defined M-H hysteresis loop at 5 K. The coercivity (H-C) of these films is much larger than those having compositions in the range 0.35 < x < 0.45. For x = 0.50 (0.62), the films H-C = -1210 (-1484) Oe and +1162 (+1476) Oe. The asymmetry in the coercivity suggests the presence of a weak exchange bias effect in these films. The FM ground state in these films is observed to have a smaller magnetic moment per Mn atom than the expected full moment from the rigid model; we term this as a weak ferromagnetic (WFM) state arising due to the destabilization of the AFM ordering. We propose a possible scenario based on the combined effect of spin reorganization and enhanced orbital disordering in nanosized manganites to explain the observed anomalous weak ferromagnetism in the A-type AFM spin ordered region.

Nanocrystallite interface effect and exchange bias phenomenology in Cr 2O 3 and NiO nanoparticles

Nanocrystalline Cr 2O 3 and NiO are prepared using high-energy ball milling. Average sizes of the particles obtained from Scanning Electron Microscopy and crystallite sizes obtained from X-ray diffraction are larger for Cr 2O 3 than NiO particles. At low temperature, large high-field magnetization and small coercivity lead to a weak exchange bias for Cr 2O 3, whereas small high-field magnetization and large coercivity lead to a considerable exchange bias for NiO. The training effect is observed for NiO at 4 K which could be described with a recursive formula constructed in the framework of the spin configurational relaxation model. The results suggest that the pinning mechanism at the interface between the antiferromagnetic and the weak ferromagnetic component ascribed to uncompensated spins leads to the exchange bias effect.

Synthesis and magnetic properties of antiferromagnetic Li2MnO3 nanoribbons

Single-crystalline Li2MnO3 nanoribbons have been synthesized via the precursor template Na0.44MnO2 nanoribbons in LiNO3–LiCl eutectic molten salt. The as-prepared Li2MnO3 nanoribbons are characterized by a range of methods including X-ray diffractometer, scanning electron microscope, transmission electron microscope, energy dispersive X-ray spectroscopy, and selected-area electron diffraction techniques. Magnetization measurements show that the Li2MnO3 nanoribbons present weak ferromagnetism, spin-glass-like behavior, and exchange bias effect at low temperature. The magnetic behaviors of Li2MnO3 nanoribbons can be interpreted based on a core-shell model.

Optimization of the Multilayer Structures for a High Field-Sensitivity Biochip Sensor Based on the Planar Hall Effect

We have investigated the planar Hall effect (PHE) in three multilayer structures such as a bilayer, a spin-valve and a weak exchange bias coupling bilayer structure introduced a very thin Cu spacer layer between the antiferromagnetic and ferromagnetic layers. These thin films are Ta(3)/NiFe(10)/IrMn(10)/Ta(3) (nm), Ta(3)/NiFe(10)/Cu(1.2)/NiFe(2)/IrMn(10)/Ta(3) (nm), and Ta(3)/NiFe(10)/Cu(0.2)/IrMn(10)/Ta(3) (nm), respectively. The active layers in all three structures were kept constant. The field-sensitivity of the fabricated PHE sensors obtained for the respected structures are about 1.6 muV middot Oe-1, 5 muV middot Oe-1, and 12 muV middot Oe-1, respectively. The results suggest that the sensor based on a weak exchange bias coupling structure has the highest field-sensitivity compared with the others. The proposed weak exchange bias coupling structure emphasizes for the development of the PHE sensor materials.