Large Exchange Bias Research Articles

Electric field-controlled magnetization in exchange biased IrMn/Co/PZT multilayers

Electric-field modulating exchange bias and near 180° deterministic magnetization switching at room temperature are demonstrated in simple antiferromagnetic/ferromagnetic/ferroelectric (AFM/FM/FE) exchange-coupled multiferroic multilayers of IrMn/Co/PZT. A rather large exchange bias field shift up to ΔHex/Hex = 500% was obtained. This change governs mainly the electric-field strength rather than the applied current. It is explained as being realized through the competition between the electric-field induced uniaxial and unidirectional anisotropies. These results show good prospects for low-power spintronic devices.

Multiple magneto-functional properties of Ni46Mn41In13 shape memory alloy

Abstract The transport and magnetic properties of ferromagnetic shape memory alloy of nominal composition Ni46Mn41In13 are reported. The alloy undergoes martensitic phase transition (MPT) around 240 K. The region around this structural transition is found to be highly metastable and susceptible to the applied magnetic field. Giant negative magnetoresistance (∼ −64% at 70 kOe) is observed in the vicinity of the thermally driven MPT. The existence of large inverse magnetocaloric effect around the MPT is also confirmed by the direct measurement using modulated ac field technique. The sample shows large exchange bias at low temperature. The role of Mn–Mn intersite antiferromagnetic correlation is discussed for the above magneto-functional behaviors.

Exchange bias between multiferroic HoMnO3 and ferromagnetic SrRuO3 films

Orthorhombic HoMnO3 (o-HMO) thin films with a-axis orientation are epitaxially grown on ferromagnetic (FM) SrRuO3 (SRO) layers by using pulsed laser deposition. The structural and magnetic properties of o-HMO/SRO heterostructures are measured. The magnetic hysteresis loops of o-HMO/SRO show exchange bias at temperatures below the antiferromagnetic transition temperature of o-HMO. Furthermore, the large exchange bias field up to 815 Oe is observed at 5 K. These results demonstrate that the exchange bias between a FM oxide and the multiferroic o-HMO could be useful for future applications on spintronic devices.

Structural and magnetic properties of the Heusler compound Fe2MnGa

We report a structural and magnetization study of the Heusler compound Fe2MnGa. The sample shows a single cubic phase with a large Curie temperature (TC) of 750 K. The sample undergoes a disorder induced first-order ferromagnetic (FM) to antiferromagnetic (AFM) transition around the room temperature. This transition results in a mixed AFM and FM phases. A large exchange bias (EB) of 0.12 T is observed for a cooling field of 0.5 T. The EB decreases to a near saturation value of 0.07 T for a cooling field of 7 T. The ac-susceptibility measurements confirm that no spin-glass type of magnetic state is present in the sample.

Connection between orbital moment enhancement and exchange bias in a [Ni80Fe20/Mn]3multilayer

The overall and element specific magnetism in an exchange biased [Ni${}_{80}$Fe${}_{20}$/Mn]${}_{3}$ film system, with film layers 3.5 nm thick, has been studied to examine the magnetism between interfacial Mn, Ni, and Fe spins. Field-cooling the film results in large exchange bias loop shifts at temperatures from 2 to 30 K, and an enhanced coercivity remains until 75 K. The elemental magnetism ascertained from x-ray magnetic circular dichroism measurements shows that Mn appears to be a fully compensated antiferromagnet down to 20 K, and there is clear evidence of an enhanced orbital moment for both Fe and Ni from 20 to 300 K. This magnetism is likely due to overlapping $d$ orbitals between Fe-Mn and Ni-Mn, with this coupling increasing the local anisotropy resulting in the enhanced coercivity and enabling exchange bias.

Magnetic properties of CoO nanocrystals prepared with a controlled reaction atmosphere

In this work CoO nanocrystals with a cubic crystal structure were prepared by the thermal decomposition of cobalt(II) acetate tetrahydrate in a mixture of oleylamine and oleic acid with a controlled reaction atmosphere. The particle size is controlled by the binding capability of the ligands which is affected by the presence or absence of oxygen. The magnetic properties of the CoO nanocrystals of different particle sizes were studied and compared. Large exchange bias was found for the smaller CoO nanoparticles (58 nm) prepared in the presence of oxygen due to uncompensated spins on the surface. The magnetic behavior for the larger nanocrystals (275 nm) is dominated by the spins associated with the defects inside the CoO nanoparticles.

Controlled 3D-coating of the pores of highly ordered mesoporous antiferromagnetic Co3O4 replicas with ferrimagnetic FexCo3−xO4 nanolayers

The controlled filling of the pores of highly ordered mesoporous antiferromagnetic Co3O4 replicas with ferrimagnetic FexCo3-xO4 nanolayers is presented as a proof-of-concept toward the integration of nanosized units in highly ordered, heterostructured 3D architectures. Antiferromagnetic (AFM) Co3O4 mesostructures are obtained as negative replicas of KIT-6 silica templates, which are subsequently coated with ferrimagnetic (FiM) FexCo3-xO4 nanolayers. The tuneable magnetic properties, with a large exchange bias and coercivity, arising from the FiM/AFM interface coupling, confirm the microstructure of this novel two-phase core-shell mesoporous material. The present work demonstrates that ordered functional mesoporous 3D-materials can be successfully infiltrated with other compounds exhibiting additional functionalities yielding highly tuneable, versatile, non-siliceous based nanocomposites.

Watermelon-like iron nanoparticles: Cr doping effect on magnetism and magnetization interaction reversal

Cr-doped core-shell iron/iron-oxide nanoparticles (NPs) containing 0, 2, 5, and 8 at.% of Cr dopant were synthesized via a nanocluster deposition system and their structural and magnetic properties were investigated. We observed the formation of a σ-FeCr phase in 2 at.% of Cr doping in core-shell NPs. This is unique since it was reported in the past that the σ-phase forms above 20 at.% of Cr. The large coercive field and exchange bias are ascribed to the antiferromagnetic Cr2O3 layer formed with the Fe-oxide shell, which also acts as a passivation layer to decrease the Fe-oxide shell thickness. The additional σ-phase in the core and/or Cr2O3 in the shell cause the hysteresis loop to appear tight waisted near the zero-field axis. The exchange interaction competes with the dipolar interaction with the increase of σ-FeCr grains in the Fe-core. The interaction reversal has been observed in 8 at.% of Cr. The observed reversal mechanism is confirmed from the Henkel plot and delta M value, and is supported by a theoretical watermelon model based on the core-shell nanostructure system.

Direct observation of rotatable uncompensated spins in the exchange bias system Co/CoO–MgO

We have observed a large exchange bias field HE ≈ 2460 Oe and a large coercive field HC ≈ 6200 Oe at T = 2 K for Co/CoO core-shell nanoparticles (~4 nm diameter Co metal core and CoO shell with ~1 nm thickness) embedded in a non-magnetic MgO matrix. Our results are in sharp contrast to the small exchange bias and coercive field in the case of a non-magnetic Al2O3 or C matrix materials reported in previous studies. Using soft X-ray magnetic circular dichroism at the Co-L2,3 edge, we have observed a ferromagnetic signal originating from the antiferromagnetic CoO shell. This gives direct evidence for the existence of rotatable interfacial uncompensated Co spins in the nominally antiferromagnetic CoO shell, thus supporting the uncompensated spin model as a microscopic description of the exchange bias mechanism.

Magnetic field-induced martensitic transformation, superspin glass and exchange bias in Heusler alloys NiCoMnSn

The crystal structures and magnetic properties of Ni50-xCoxMn39Sn11 (8≤x≤10) Heusler alloys are investigated. As a result, we achieve the magnetic field induced martensitic transformation in Ni42Co8Mn39Sn11. It is found that the saturation magnetic moments of alloys increase with Co content increasing. Moreover, a superspin glass behavior and a large exchange bias effect are also found in samples with Co content being higher than 8. We confirm the coexistence of superspin glass of the martensite and ferromagnetic parent phase, which is the physical origin of the exchange bias effect. On the other hand, we propose that the origin of superspin glass in our NiMnCoSn system is due to the occurence of Mn-Mn cluster as reported by Ma et al. in Heusler Mn2Ni1.6Sn0.4 alloys [Ma L, Wang W H, Liu J B, Li J Q, Zhen M, Hou D L and Wu G H 2011 Appl. Phys. Lett. 99 182507].

In Situ Generated Gas Bubble‐Directed Self‐Assembly: Synthesis, and Peculiar Magnetic and Electrochemical Properties of Vertically Aligned Arrays of High‐Density Co3O4 Nanotubes

AbstractA novel and versatile gas bubble induced self‐assembly technique is developed for the one‐step fabrication of vertically aligned polycrystalline Co3O4 nanotube arrays (NTAs) by the rapid thermal decomposition of Co(NO3)2·6H2O on a flat substrate. In this protocol, the in situ generation and release of gas bubbles, which can be regulated by elaborately adjusting the kinetic factors such as reaction time, decomposition temperature and pressure as well as the content of the chemically adsorbed water, play a vital role in the formation of the Co3O4 NTAs. Due to the shape anisotropy, ordered hierarchically porous structure and high surface area, the as‐obtained Co3O4 NTAs show unique magnetic properties of a low Néel temperature and a large exchange bias field, as well as an initial discharge capacity up to 1293 mAh·g−1 at 35 mA·g−1 and the retention of a charge capacity as high as 895.4 mAh·g−1 after 10 cycles. This endows them with important potential use in magnetic shielding, magnetic recording media, and lithium ion batteries, etc. Due to the simplicity of the self‐assembly method, this process is applicable to the large‐scale production of the Co3O4 NTAs, and may be extended to other materials.

X-Ray Absorption and Mössbauer Spectroscopies Characterization of Iron Nanoclusters Prepared by the Gas Aggregation Technique

Partially oxidized iron nanoclusters have been prepared by the gas-phase aggregation technique with typical sizes of 2-3 nm. This preparation technique has been reported to obtain clusters with interesting magnetic properties such as very large exchange bias. In this paper, a sample composition study carried out by Mössbauer and X-ray absorption spectroscopies is reported. The information reached by these techniques, which is based on the iron short range order, results to be an ideal way to have a characterization of the whole sample since the obtained data are an average over a very large amount of the clusters. In addition, our results indicate the presence of ferrihydrite, which is a compound typically ignored when studying this type of systems.

Strong exchange bias with the (110)-oriented BiFeO3 films

Epitaxial BiFeO3 films were grown on the (001)-, (110)-, and (111)-oriented SrTiO3 substrates. Using CoFe as ferromagnetic layer, we have shown that both large exchange bias and coercive field enhancement can be observed in the (001)- and (110)-oriented BiFeO3. But no exchange bias was found in the (111)-oriented BiFeO3 films, which can be understood by the lack of either 109° or 71° ferroelectric domain walls and the preservation of the spatial modulated cycloid spin structure. The observation of large exchange bias field with the (110)-oriented BiFeO3 film may lead to an alternative choice towards reversible control exchange bias by electrical field.

Influence of exchange bias on magnetic losses in CoFeB/MgO/CoFeB tunnel junctions

The strength of the exchange bias field is found to influence the low-frequency magnetoresistive noise associated with the magnetic reference layer in sputtered-deposited and electron-beam-evaporated CoFeB/MgO/CoFeB tunnel junctions. The noise is due to magnetic losses arising in the reference layer. The losses are parameterized by a phase lag e which exhibits a nontrivial dependence on the externally applied field. The general trend found among all devices is that the losses are largest in the antiparallel state. The effect of exchange bias on the reference layer’s noise is investigated at a field corresponding to maximum resistance susceptibility, Href. Higher values for the phase lag at Href, eref, are found in devices having a large exchange bias field. We also observed that Href and eref are larger in devices having thicker seed layers. This characteristic is also evident in double-barrier magnetic tunnel junctions. Prolonged thermal annealing is found to decrease eref, reduce Href, and alter the field profile of the resistance susceptibility of the reference layer to resemble that of a more magnetically soft behavior. In addition to its impact on the magnetoresistive noise, the incorporation of exchange bias layers into the materials stack also affects the tunneling magnetoresistance ratio with higher values found at smaller exchange bias fields. We attribute the magnitude of the magnetic losses, and hence the magnetoresistive noise, from the reference layer to disorder in its magnetic microstructure. Our results indicate that the nature and degree of disorder are correlated to the strength of the exchange bias coupling. The origin of this correlation may be due to a competition between different microstructures among various layers, one that leads to coherent tunneling (large tunneling magnetoresistance) in MgO-based tunneling devices and the other which promotes strong exchange bias coupling. A decrease in the exchange bias either through degradation from thermal treatments or by varying the thickness of the underlying seed layer will lead to less magnetic disorder in the system. We show that the magnetoresistive noise can be used to probe magnetic disorder in exchange-biased devices through the determination of the magnetic losses.

Exchange bias effects in epitaxial Fe3O4/BiFeO3 heterostructures

High quality Fe3O4 (5–20 nm)/BiFeO3 (BFO) heterostructures have been grown by pulsed laser deposition on (001) SrTiO3 (STO) substrates. Large exchange bias (EB) effects have been observed below 200 K in Fe3O4/BFO/STO structures compared with Fe3O4/STO structures. The exchange bias field decreases when increasing temperature or the thickness of Fe3O4 layer, and does not change after annealing in 0.1 T field from 700 K to room temperature across Neel temperature of BFO. A possible mechanism was proposed based on the interface spin coupling between Fe3O4 and BFO. Our work shed new light on the study of exchange bias effect in BFO system as well as the application for magnetoelectric devices.

Influence of oxygen vacancies on the electronic structure and magnetic properties of NiFe2O4 thin films

We report stabilization of magnetic glassy state in non-stoichiometric nickel ferrite thin films prepared by pulse laser deposition. Details of electronic structure of the films are presented and compared with stoichiometric bulk counterpart. Hard x-ray photoelectron spectroscopy shows significant amount of oxygen vacancies and enhanced cationic inversion for thin films. Films show spin glass (SG) features which is contrary to the usual ferrimagnetic response of the bulk nickel ferrite. Films exhibit spin freezing temperature which is above room temperature in low fields (0.1 T) and shifts to lower temperature (∼250 K) in the presence of a large applied field of 3 T. An exceptionally large exchange bias (EB) of 170 Oe at a significantly higher temperature (∼50 K) is measured in cooling field of 3 T. In comparison, bulk samples do not show exchange bias and magnetic irreversibility vanishes in significantly weaker fields (i.e., few kOe). Role of oxygen vacancies is to induce spin canting by destabilizing indirect super exchange interaction. Consequently, the spin-glass like behavior occurs that is coupled with huge suppression in saturation magnetization in the thin films. Observation of exchange bias is explained to be due to oxygen vacancies (hence non-stoichiometry) which generates random anisotropy in exchange coupled grains.

Exchange Bias in Ni-Mn-Sn Heusler Alloy Films

We report a relatively large exchange bias e ect observed for the rst time in Ni Mn Sn thin lms with di erent microstructure and composition: a Ni50Mn36Sn14 epitaxial lm (A), a Ni50Mn43Sn7 lm which is phase decomposed (B), and a NiMn/Ni50Mn25Sn25 bilayer (C). Despite the samples di er markedly in both microstructure and composition HEB does not substantially di ers at 5 K. Exchange bias decreases with increasing T approximately as HEB(T ) ∝ HEB(5 K)/T with HEB(5 K) of 180 Oe and 60 Oe for sample B and C, respectively and almost linearly for sample A with HEB(5 K) = 65 Oe. Blocking temperature where the exchange bias vanishes is 40, 50 and 80 K for sample A, C and B, respectively. The results suggest that the role of AFM/FM interfaces is not substantial in formation of exchange bias in Ni Mn Sn Heusler alloy lms and exchange bias is rather related to AFM/FM interactions in nanoscale.

Exchange Bias in MultiferroicRFe2O4(R=Y, Er, Tm, Yb, Lu, and In)

An exchange bias (EB) behavior has been observed for multiferroic R Fe 2 O 4 ( R =Y, Er, Tm, Yb, Lu, and In). The materials with small R 3+ ions ( R =Tm, Yb, Lu, and In) exhibit large EB fields (\({\gtrsim}1\) kOe) below ∼100–150 K. This property is rooted in a magnetically glassy state, arising from the competition between ferromagnetic and antiferromagnetic domain interactions. In addition, the EB field tends to be more enhanced for smaller R 3+ ions. Hence, the EB is controlled by the substitution at the R -site in this series.

Exchange anisotropy in the nanostructured MnAl system

In this letter, we report on the achievement of exchange anisotropy magnitude in a nanostructured Mn55Al45 alloy fabricated by rapid solidification with large exchange bias values (HE ≈ 13 kOe at 10 K) and a blocking temperature of TB ∼ 95 K. Field-cooled magnetization loops show a prominent exchange bias for T < TB signaling the simultaneous presence of antiferromagnetic and ferromagnetic phases at these temperatures. Structural probes confirm a majority presence of the high-temperature metastable hexagonal ɛ-MnAl in the as-solidified state with an intriguing double-Bragg peak structure indicative of phase separation. The observed exchange bias is hypothesized to originate from an intimate mixture of antiferromagnetic and nanoscaled ferromagnetic phases or dual mictomagnetic phases, approximating a cluster glass with well-defined variations in the local Mn concentration of the composition and leading to Mn-rich and Mn-poor regions with antiferromagnetic and ferromagnetic characters, respectively.

Exchange bias in sputtered FM/BiFeO3 thin films (FM = Fe and Co)

Magnetic properties of sputter-deposited ferromagnetic (FM)/BiFeO3 (BFO) films on Pt/Ti/SiO2/Si(100) substrate (FM = Co and Fe) have been investigated. Isotropic perovskite BFO single phase is obtained for 200-nm-thick BFO films deposited at 300–450 °C and BFO films at 400 °C with thickness of 50–400 nm. Large exchange bias field (HEB) of 308–400 Oe and coercivity (Hc) of 1201–3632 Oe at RT are obtained for polycrystalline Co/BFO bilayers. The roughened surface induced by high deposition temperature and increasing thickness of BFO layer enhances localized shape anisotropy of FM layer, resulting in the increase of Hc the improved crystallinity and roughened surface of BFO/Co interface might be responsible for the HEB enhancement. Additionally, comparison on the HEB in polycrystalline Co/BFO and Fe/BFO systems is also discussed.