Spontaneous Exchange Bias Research Articles

Dense inter-particle interaction mediated spontaneous exchange bias in NiO nanoparticles

The observed spontaneous exchange bias in NiO nanoparticles is induced by the dense inter-particle interactions.

Perpendicular exchange bias behaviors of CoPt/IrMn and CoPt/FeMn bilayers: A comparative study

In this study, FeMn was introduced as an antiferromagnetic (AFM) layer to couple with a single-layered Co49Pt51 alloy thin film, and it was compared with a Co49Pt51/IrMn bilayer system in exchange bias (EB) effect, to explore the mechanism of spontaneous perpendicular exchange bias (PEB), which has been recently observed in CoPt/IrMn bilayers. Bilayers of CoPt/IrMn and CoPt/FeMn were prepared under the same conditions by sputtering at room temperature without any inducing field. Although PEB was observed in as-grown CoPt/FeMn bilayers, the loop shape and PEB behavior were found to exhibit different characteristics from those of CoPt/IrMn bilayers. The CoPt (5 nm)/FeMn (10 nm) bilayer has a sheared loop that is similar to a double-shifted loop and a much lower squareness ratio (SQR = 0.52) and exchange bias field (He = 180 Oe) than the CoPt (5 nm)/IrMn (10 nm) system, which has a rectangular loop shape and a high SQR of 0.97 and large He of 290 Oe. The two systems present entirely different dependences of PEB on the thickness of the AFM layer. CoPt/IrMn exhibits behavior that is typical of most EB systems, but for CoPt/FeMn, this dependence is more complicated with an unusual peak at an AFM layer thickness of 10 nm. Based on the dissimilar loop shapes and dependences of PEB on AFM thickness, the mechanisms of the spontaneously established PEB in these two systems are considered to differ. Investigations of cross-sectional transmission electron microscopy revealed no apparent difference between the interfacial microstructures of the two systems. X-ray diffraction studies demonstrated the ⟨111⟩ texture of both systems. Therefore, different interfacial spin configurations may be responsible for the dissimilar PEB behaviors in these two FM/AFM bilayer systems.

Strain-driven fractional spontaneous exchange bias in ferromagnetic/antiferromagnetic thin films with composition-graded ferromagnetic layer

We experimentally demonstrate that the stress-induced magnetic anisotropy in composition-graded ferromagnetic NiFeTa layers can be employed to trigger exchange bias in NiFeTa/IrMn bilayers without using any deposition field or field cooling procedure. In particular, we found that the NiFeTa/IrMn bilayers exhibit double-shifted loops being composed of both negative and positive exchange biases when the deposition angle is low. As the deposition angle is increased, the magnetization curves change into a single-shifted loop with spontaneous exchange bias aligned along negative or positive direction. The results can be explained by the formation of magnetic domain state under the presence of a strong stress-induced magnetic anisotropy and this domain state is pinned upon the deposition of an antiferromagnetic layer. We also studied the dynamic permeability spectra of the films with respect to temperature and discussed the results in relation with the static magnetic properties.

Strongly enhanced antiferromagnetism and giant spontaneous exchange bias in Ni50Mn36Co4Sn10 Heusler alloy

An enormous spontaneous exchange bias (EB) was obtained after zero-field cooling in Ni50Mn36Co4Sn10 Heusler alloy. It can be attributed to the enhancement of antiferromagnetism, which induces a strong interaction with superferromagnetism, superspin glass, and superparamagnetism. In addition, the EB with an abrupt drop below the lower temperature of 5 K was observed, which can be interpreted as the fact that the rotation of the antiferromagnetic spins from a disordered state to a more ordered state is hindered by domain wall pinning.

Giant zero field cooled spontaneous exchange bias effect in phase separated La1.5Sr0.5CoMnO6

We report a giant zero field cooled exchange bias (ZEB) effect (∼0.65 T) in La1.5Sr0.5CoMnO6 sample. Magnetic study has revealed a reentrant spin glass ∼90 K, phase separation to spin glass and ferromagnetic phases below 50 K and canted antiferromagnetic transition ∼10 K. A small conventional exchange bias (CEB) is established with the advent of spontaneous phase separation down to 10 K. Giant ZEB and enhanced CEB effects are found only below 10 K and are attributed to the large unidirectional anisotropy at the interface of isothermally field induced ferromagnetic phase and canted antiferromagnetic background.

Stacked Nanosheets of Pr1–xCaxMnO3 (x = 0.3 and 0.49): A Ferromagnetic Two-Dimensional Material with Spontaneous Exchange Bias

Two-dimensional (2D) planar architectures of inorganic materials have attracted great attention owing to their fascinating physical properties. However, nanosheets of the technologically relevant doped rare-earth manganites are still elusive. Stacked 10–14 nm thick nanosheets of phase pure Pr1–xCaxMnO3 (PCMO; x = 0.3 and 0.49) were obtained by decomposition of carbon coated CaCO3/MnCO3 microsheets and Pr2O2CO3 aggregates, the latter synthesized under pressure at 500–800 °C. The PCMO nanosheets had flatter Mn–O–Mn tilt angles in the MnO6 octahedra and ferromagnetic (FM) moments at 5 K. Spontaneous exchange bias (SEB) coupling between the antiferromagnetic (AF)/FM spins was observed at 5 K under zero-field cooling. High FM moments and SEB were possible due to the long-range magnetic interactions in the stacked 2D arrangement of Mn3+/Mn4+ d-electron spins, where charge ordering was completely suppressed. The SEB behavior was dependent on the initial magnetization process and the direction of rotation of the random spins at the AF/FM interface.

Magnetic phase separation and exchange bias in off-stoichiometric Ni-Mn-Ga alloys

The magnetic phase separation and exchange bias effect in off-stoichiometric Ni2Mn1.4Ga0.6 alloy were reported. Spontaneous exchange bias after zero-field cooling was observed in this alloy. The magnetic nature of ground state was characterized as phase separation with non-percolated ferromagnetic domains in spin glass matrix due to the spatial composition fluctuation and competing ferromagnetic and antiferromagnetic interactions. A mechanism of field-induced growth of ferromagnetic domains changing from non-percolating to percolating state was proposed for the appearance of spontaneous exchange bias.

Room temperature spontaneous exchange bias in (La,Sr)MnO3/PbZr0.8Ti0.2O3/(La,Sr)MnO3 sandwich structure

We report on a room temperature spontaneous exchange bias (SEB) effect in La0.67Sr0.33MnO3/PbZr0.8Ti0.2O3/La0.67Sr0.33MnO3 (LSMO/PZT/LSMO) sandwich structure, where PZT is a discontinuous film producing somewhat the direct touch of two LSMO layers. The exchange coupling between the top and bottom LSMO, which, respectively, behaves as the antiferromagnetic and ferromagnetic layer, is demonstrated to be responsible for the SEB effect. The formation of antiferromagnetic LSMO is ascribed to the large strain imposed through the PZT seed layer, whose thickness profoundly affects the SEB behavior. Thus, our finding provides an appealing alternative way to generate exchange bias by strain engineering.

Spontaneous exchange bias in a nanocomposite of BiFeO3-Bi2Fe4O9

We have observed a large as well as path-dependent spontaneous exchange bias (HSEB) (∼30–60 mT) in a nanocomposite of BiFeO3-Bi2Fe4O9 across 5–300 K when it is measured in an unmagnetized state following zero-field cooling and appropriate demagnetization. The path dependency yields a variation in the exchange bias depending on the sign of the starting field and the path followed in tracing the hysteresis loop. The asymmetry thus observed—ΔHSEB—is found to be decreasing nonmonotonically across 5–300 K with a peak around 200 K. The ΔHSEB together with large HSEB could have significant ramification in tuning the exchange bias driven effects and consequent applications.

Spontaneous exchange-bias in Fe/Mn thin multilayers

Fe/Mn multilayers were grown by means of a molecular beam epitaxy system onto quartz substrates changing the thickness of the elemental layers. A spontaneous unidirectional anisotropy develops for thickness of Fe or Mn layer of about 35 Å. Since the samples were no subjected to field cooling treatments during or after the growth, this kind of anisotropy can be explained considering besides the exchange coupling at the Fe/Mn interface, the structural disorder due to dislocations and defects. In effect, the appearance and strength of the exchange-bias field are depending on the surface roughness of the samples and are significantly enhanced by the formation of a structure constituted by islands showing a snake-like morphology. The fitting of the angular dependence of the exchange-bias field indicates that the associated anisotropy is due to the superposition of two contributions, the principal one with unidirectional symmetry and the other showing uniaxial characteristics.

Exchange bias in LaFeO3 nanoparticles

Nanoparticles of antiferromagnetic LaFeO3 were prepared by the sol–gel method. An exchange bias effect has been observed and is attributed to the exchange coupling between the ferromagnetic shell and antiferromagnetic core of the particles. The results provide clear evidence of the presence of spontaneous exchange bias in this system. After field cooling from room temperature, the exchange bias increases while the coercivity decreases with decreasing temperature. Taking into account the role of thermal activation, the temperature dependence of exchange bias and coercivity has been interpreted in terms of the spontaneous exchange bias mechanism proposed recently.

Isothermal low-field tuning of exchange bias in epitaxial Fe∕Cr2O3∕Fe

Moderate dc magnetic fields of less than 1T allow tuning the exchange bias in an epitaxially grown Fe 10nm∕Cr2O3 2.7nm∕Fe 10nm trilayer between negative and positive bias fields. Remarkably, this tunable exchange bias is observed at least up to 395K which exceeds the Néel temperature of bulk Cr2O3 (307K). The presence of spontaneous exchange bias and the absence of training effects at room temperature suggest the existence of stable interface moments independent of antiferromagnetic long range order in Cr2O3. Furthermore, the coercivity remains constant, independent of the exchange bias field. In contrast, large training associated with nonequilibrium spin configurations of antiferromagnetically ordered Cr2O3 appears below 50K.

Spontaneous chemical ordering and exchange bias in epitaxial Mn0.52Pd0.48/Fe(001) bilayers prepared at room temperature

We report spontaneous chemical ordering of Mn0.52Pd0.48 films, grown by molecular-beam epitaxy on body-centered-cubic Fe(001) films. The bilayers were grown at room temperature onto a seeding structure of Ag(001)/Fe(001)/GaAs(001). X-ray, neutron, and electron diffraction studies confirm extensive chemical ordering of the MnPd to the L10 antiferromagnetic phase which grows as a twinned film with the tetragonal c axis in the film plane. The Fe film exhibits a unidirectional exchange bias and we find no pronounced difference in magnetic structure of the biased Fe layer at the ascending and descending crossing fields where the net moment along the applied field axis is zero.