Large Exchange Bias Research Articles

Exchange bias on polycrystalline BiFeO3/Co2Fe(Al0.5Si0.5) heterostructures

Polycrystalline antiferromagnetic BiFeO3 (BFO) thin films were grown on Si/SiO2/Ti/Pt (111) substrates by pulsed laser deposition and then ferromagnetic films Co2Fe(Al0.5Si0.5) (CFAS) by magnetron sputtering. After fabrication, the films were vacuum-annealed under a 0.1-T magnetic field at different temperatures from 150 to 500 °C. The exchange bias field can be tuned by the annealing temperature for the heterostructures, and the electric domain size can be controlled by the crystal grain size. A large exchange bias of about 5 × 10−3 T is observed in one of the samples. It can be speculated that the crystal grain sizes are the key element in determining the exchange bias and coercivity of the films annealed at the temperature of higher than Neel temperature (TN) of BFO. Furthermore, it is possible to extend spin theories from single-crystal BFO system to polycrystalline BFO system.

Coexistence of Perpendicular and Parallel Exchange Bias Effects on Ni/CoO Bilayer

We present exchange bias properties of Ni/CoO bilayer field cooled in parallel and perpendicular directions. Magnetic moments of cobalt atoms in CoO magnetic unit cells are known to be slightly tilted with respect to their easy axes. Coexistence of both is attributed to the tilt of magnetic moments. CoO layer thickness was kept constant in this study, whereas Ni thickness was varied in a range in order to adjust its perpendicular magnetization. MOKE and VSM were exploited to determine their magnetic characteristics. After field cooling, Ni(22 A)/CoO(20 A) bilayer exhibited a large exchange bias of 3 kOe in parallel direction and a relatively small (0.5 kOe) in perpendicular direction at 10 K.

Multiferroic tunnel junction of Ni50.3Mn36.9Sb12.8/BiFeO3/Ni50.3Mn36.9Sb12.8 for magneto-electric random access memory devices

A multiferroic tunnel junction composed of two ferromagnetic shape memory alloy electrodes separated by a multiferroic barrier was fabricated from a Ni50.3Mn36.9Sb12.8/BiFeO3/Ni50.3Mn36.9Sb12.8 trilayer. A large exchange bias field (HEB) of ∼59 Oe at room temperature was found for this trilayer. Besides the exchange bias effect in this multiferroic tunnel junction, one of the most interesting results was the magnetoelectric effect, which is manifested by the transfer of strain from the Ni50.3Mn36.9Sb12.8 electrodes to the BiFeO3 tunnel barrier. The magnetic field dependence of the junction resistance was observed at room temperature after aligning the ferroelectric polarization of the BiFeO3 barrier with the poling voltage of ±3 V. A change in junction resistance was also observed between the magnetic parallel and antiparallel states of the electrodes, suggesting an entire flip of the magnetic domains against the magnetic field. After reversing the polarization of the BiFeO3 barrier between the two directions, the entire R-H curve was shifted so that both parallel and antiparallel resistances switched to different values. Hence, after applying positive and negative voltages, two parallel and two antiparallel states, i.e., four distinct states were observed. These four states will encode quaternary information by both ferromagnetic and ferroelectric order-parameters, to read non-destructively by resistance measurement. These findings may be helpful towards reconfigurable logic spintronics architectures in next generation magneto-electric random access memory devices.

Large Perpendicular Exchange Bias in CoFeB/MgO Systems Pinned by a Bottom IrMn Layer via an Interfacial CoFe/Ta Composite Layer

CoFeB/MgO films with perpendicular magnetization pinned by an antiferromagnetic IrMn layer have been explored. CoFeB/MgO films directly grown on an antiferromagnetic IrMn layer exhibit insufficient perpendicular magnetic anisotropy (PMA) to overcome shape anisotropy. With a Ta dusting layer inserted into the CoFeB layer, perpendicular magnetization and perpendicular exchange bias (PEB) are simultaneously acquired in IrMn/CoFeB/MgO films. More significantly, when the CoFeB sublayer between IrMn and Ta is replaced by a thin CoFe layer, the PEB is drastically enhanced with large PMA sustained. Through optimizing the thicknesses of CoFe and Ta, a PEB field of about 540 Oe is achieved in the CoFeB films with an effective perpendicular anisotropy field of about 2.5 kOe. The present results suggest that the interfacial CoFe/Ta composite layer insertion paves the way to establish large PEB in the IrMn/CoFeB/MgO system.

Large exchange bias in polycrystalline MnN/CoFe bilayers at room temperature

We report on the new polycrystalline exchange bias system MnN/CoFe, which shows exchange bias of up to 1800Oe at room temperature with a coercive field around 600Oe. The room temperature values of the interfacial exchange energy and the effective uniaxial anisotropy are estimated to be $J_\mathrm{eff} = 0.41\,\mathrm{mJ}/\mathrm{m}^2$ and $K_\mathrm{eff} = 37\,\mathrm{kJ}\,/\,\mathrm{m}^3$. The thermal stability was found to be tunable by controlling the nitrogen content of the MnN. The maximum blocking temperature exceeds $325^\circ$C, however the median blocking temperature in the limit of thick MnN is $160^\circ$C. Good oxidation stability through self-passivation was observed, enabling the use of MnN in lithographically defined microstructures. As a proof-of-principle we demonstrate a simple GMR stack exchange biased with MnN, which shows clear separation between parallel and antiparallel magnetic states. These properties come along with a surprisingly simple manufacturing process for the MnN films.

Manipulation of perpendicular exchange bias effect in [Co/Ni]N/(Cu, Ta)/TbCo multilayer structures

With the demand for increasing storage density in spintronic applications, extensive work has been devoted to searching for perpendicular magnetic material systems with strong exchange bias effect. In this study we have investigated the exchange bias effect in perpendicular magnetized heterostructures of [Co/Ni]N/(Cu, Ta)/TbCo. An interlayer of 0.8 nm Cu is capable of achieving separate magnetization switching, showing a quite large exchange bias field over 2.9 kOe. With increasing the interlayer thickness, both the Co/Ni bias field and TbCo switching field decrease much more rapidly for the samples with a Ta interlayer as compared to the Cu case, due to the better coverage ability of the amorphous nature. The influence of layer thickness and composition of the FM and FI layers has also been investigated and the variation tendencies are well interpreted.

Room Temperature Instability of Exchange Anisotropy in FeMn/FeCo System

Ferromagnetic (FM) FeCo is investigated in exchange bias systems. The ferromagnetic layer is grown on a FeMn antiferromagnetic (AFM) layer. Partial superficial oxidation of FeCo is observed. The standard field cooling procedure results in a large room temperature exchange bias effect. However, the training effect observed when the hysteresis loops are repeated does not have a saturating trend. This behavior is related to the evolution of pinned moments at the FM/AFM interface. X-ray circular magnetic dichroism technique is used to clarify this mechanism.

Large exchange bias in polycrystalline ribbons of Ni56Mn21Al22Si1

Large exchange bias (EB) effect is demonstrated in a Ni56Mn21Al22Si1 polycrystalline ribbon that shows spin-glass behaviour below 68K and superparamagnetic (SPM) behaviour above this temperature. The average magnetic moment μ of the SPM clusters is estimated to be considerably smaller than that reported for other Heusler alloy systems that demonstrate spin glass behaviour and EB effect. The M–H loops measured in field-cooled (FC) conditions show significant asymmetry that is observed to be strongly field and temperature dependant. The maximum EB field (HEB ~2.6kOe at 20kOe and 2K) is much larger than that reported for any metallic alloy system with conventional EB effect. The dependence of HEB on temperature shows an exponential decrease. An approach towards enhancement of the EB effect in this conventional EB system can be realized via manipulation of SPM or superferromagnetic (SFM) cluster size with variation of cooling magnetic field.

Giant exchange bias behavior and training effect in spin-glass-like NiCr2O4/NiO ceramics

NiCr2O4/NiO ceramic has been synthesized successfully through co-precipitation method to study the magnetic properties. Field cooling (FC) magnetic hysteresis loops measured at low temperature show significant shift in both coercive field and remnant magnetization. It has been plotted that the exchange bias (EB) field can be as large as 11.86 kOe at 10 K followed with an enhanced coercive field. The variation of EB field and shift of remnant magnetization after FC processes shows a regular tendency with increasing temperature and cooling field. Known as training effect, the EB behavior also presents a strong dependence on loop cycle. The cycle dependence of large EB field, vertical shift of magnetization, and coercive field at low temperature can be ascribed to the pinned spins in spin-glass-like (SGL) phase of the system interface. The SGL phase has been proved by Almeida–Thouless line and high field relaxation effect. The disorder structure is responsible for the frustration interface and the formation of SGL phase in the interface.

Formation of BiFeO3(110) films on ferromagnetic CoPt(111) electrode layer on glass substrates at reduced temperatures

Structure, ferroelectric, and magnetic properties of BiFeO3 (110) films grown on 20-nm-thick ferromagnetic CoPt(111) buffered glass substrate at 350–550 °C have been studied. (110)-texture of BFO films is developed at the reduced temperature as low as 400–550 °C, but isotropic orientation is found at higher temperature of 600 °C. Low temperature deposition results in dense microstructure, fine grains, and smooth surface morphology, which are favorable for applications. BFO(110) films on CoPt(111) underlayer exhibit desired ferroelectric and magnetic properties. Electrical polarization (2Pr) of 96–137 μC/cm2 and coercive field (Ec) of 495–618 kV/cm for studied BFO (110) films are comparable to those grown on single crystal substrates. Moreover, exchange bias between BFO and CoPt is observed after a field cooling from 370 °C to room temperature (RT) at 2 kOe. Large exchange bias field of 155 Oe at RT and coercivity of 1631 Oe are obtained. The presented results provide useful information for the applications based on electric-magnetic interactions.

High coercivity in large exchange-bias Co/CoO-MgO nano-granular films**Project supported by the National Basic Research Program of China (Grant No. 2012CB932304), the National Natural Science Foundation of China (Grant Nos. U1232210, 91122035, 11174124, and 11374137), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant No. 14KJB140003), and the Priority Academic Program Development of Jiangsu Higher Education Institutions,

We present a detailed study on the magnetic coercivity of Co/CoO-MgO core-shell systems, which exhibits a large exchange bias due to an increase of the uncompensated spin density at the interface between the CoO shell and the metallic Co core by replacing Co by Mg within the CoO shell. We find a large magnetic coercivity of 7120 Oe around the electrical percolation threshold of the Co/CoO core/shell particles, while samples with a smaller or larger Co metal volume fraction show a considerably smaller coercivity. Thus, this study may lead to a route to improving the magnetic properties of artificial magnetic material in view of potential applications.

Observation of giant exchange bias in bulk Mn50Ni42Sn8 Heusler alloy

We report a giant exchange bias (EB) field of 3520 Oe in bulk Mn50Ni42Sn8 Heusler alloy. The low temperature magnetic state of the martensite phase has been studied by DC magnetization and AC susceptibility measurements. Frequency dependence of spin freezing temperature (Tf) on critical slowing down relation and observation of memory effect in zero field cooling mode confirms the super spin glass (SSG) phase at low temperatures. Large EB is attributed to the strong exchange coupling between the SSG clusters formed by small regions of ferromagnetic order embedded in an antiferromagnetic (AFM) matrix. The temperature and cooling field dependence of EB have been studied and related to the change in unidirectional anisotropy at SSG/AFM interface. The training effect also corroborates with the presence of frozen (SSG) moments at the interface and their role in EB.

Colossal increase in negative magnetization, exchange bias and coercivity in samarium chromite due to a strong coupling between Sm3+–Cr3+ spins sublattices

We report giant temperature dependent negative magnetization (magnetization reversal) along with a large exchange bias and large coercivity in SmCrO3. The static magnetization measurements show the negative magnetization below ~192 K, due to competition between the external field, thermal activation energy and antiparallel Sm3+–Cr3+ spin interaction. At further lower temperatures, Sm3+ spins show an increased alignment due to the internal induced field of Cr3+ spins with minimum magnetization ~ − 0.037 emu g−1. The temperature dependent exchange bias shows non-monotonic behavior. At 35 K, the exchange bias ceases to exist due to the orientation of Sm3+ moments with respect to canted Cr3+ moments. The crossover temperature decreases from ~191 K at 100 Oe to ~153 K at 250 Oe. The training effect further confirms the exchange bias in SmCrO3. The dynamic magnetization measurements exhibit anomalies around spin reorientation transition (TSR ~ 34 K) and Nèel transition (TN ~ 192 K) which is consistent with static measurement and no frequency dependence was observed. The room temperature Raman spectra of SmCrO3 show peaks at ~364, ~375 and ~456 cm−1 suggesting O-Cr-O bending modes within the octahedral.

Large exchange bias and magnetocaloric effect in TbMn2Si2

We report multiple first order magnetic transitions in TbMn2Si2 as evidenced by the thermal hysteresis in the M-T data and the Arrott plots. Metamagnetic transitions are observed at various temperatures as a result of the antiferromagnetic to ferromagnetic transition of the Mn sublattice. Very interestingly, the compound shows significant exchange bias field of about 600 Oe at 5 K, which is attributed to the formation of small domains or regions with ferromagnetic and antiferromagnetic interactions. Furthermore, a large magnetocaloric effect has been found at relatively low fields at both the transition regions. Maximum magnetic entropy changes (−ΔSM) of 7.2 and 5.4 J kg−1 K−1 have been observed at 68 K and at 48 K, respectively, at 20 kOe.

Observation of Large Exchange Bias Effect in Bulk Mn<sub>50</sub>Ni<sub>41</sub>Sn<sub>9</sub> 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.

Structural evolution from Bi4.2K0.8Fe2O9+δ nanobelts to BiFeO3 nanochains in vacuum and their multiferroic properties.

In this paper, we report the structural evolution of Bi(4.2)K(0.8)Fe(2)O(9+δ) nanobelts to BiFeO3 nanochains and the related variations in multiferroic properties. By using in situ transmission electron microscopy with comprehensive characterization, it was found that the layered perovskite multiferroic Bi(4.2)K(0.8)Fe(2)O(9+δ) nanobelts were very unstable in a vacuum environment, with Bi being easily removed. Based on this finding, a simple vacuum annealing method was designed which successfully transformed the Bi(4.2)K(0.8)Fe(2)O(9+δ) nanobelts into one-dimensional BiFeO(3) nanochains. Both the Bi(4.2)K(0.8)Fe(2)O(9+δ) nanobelts and the BiFeO3 nanochains showed multiferroic behavior, with their ferroelectric and ferromagnetic properties clearly established by piezoresponse and magnetic measurements, respectively. Interestingly, the BiFeO(3) nanochains had a larger magnetization than the Bi(4.2)K(0.8)Fe(2)O(9+δ) nanobelts. Moreover, the BiFeO(3) nanochains exhibited a surprisingly large exchange bias with small training effects. This one-dimensional BiFeO(3) multiferroic nanostructure characterized by a relatively stable exchange bias offers important functionalities that may be attractive for device applications.

Investigation of multifunctional properties of Mn50Ni40−xCoxSn10 (x = 0–6) Heusler alloys

A series of Co doped Mn50Ni40−xCoxSn10 (x=0, 1, 2, 2.5, 3, 4 and 6) Heusler alloys has been investigated for their structural, magnetic, magnetocaloric and exchange bias properties. The martensitic transition temperatures are found to decrease with the increase in Co concentration due to the decrease in valence electron concentration (e/a ratio). The Curie temperature of austenite phase increases significantly with increasing Co concentration. A large positive magnetic entropy change (ΔSM) of 8.6 and 10.5J/kgK, for a magnetic field change of 50kOe is observed for x=0 and 1 alloys, and ΔSM values decreases for higher Co concentrations. The relative cooling power shows a monotonic increase with the increase in Co concentration. Large exchange bias fields of 920 Oe and 833 Oe have been observed in the alloys with compositions x=0 and 1, after field cooling in presence of 10kOe. The unidirectional anisotropy arising at the interface between the frustrated and ferromagnetic phases is responsible for the large exchange bias observed in these alloys. With increase in Co, the magnetically frustrated phase diminishes in strength, giving rise to a decrease in the exchange bias effect for larger Co concentration. The exchange bias fields observed for compositions x=0 and 1, in the present case are larger than that reported for Co doped Ni–Mn–Z (Z=Sn, Sb, and Ga) alloys. Temperature and cooling field dependence of the exchange bias field and coercivity have been related to the change in the unidirectional anisotropy. This study shows that an optimum Co doping results in the tuning and enhancement of the multifunctional properties in Mn rich Ni–Mn–Sn alloys.

Exchange bias in UO2/Fe3O4 thin films above the Néel temperature of UO2

Magnetic exchange bias (EB) has been investigated on antiferromagnetic/ferrimagnetic heterostructures consisting of a stoichiometric UO2 film grown epitaxially on a LaAlO3 substrate and covered with highly textured Fe3O4 layers of variable thickness (tFe3O4 = 90–700 Å). EB values reaching ∼2.6 kOe were observed at lowest temperatures in UO2/Fe3O4 with the thinnest Fe3O4. Magnetization measurements demonstrate that a remarkably large exchange bias in UO2/Fe3O4 is present at temperatures higher than the bulk Néel temperature of UO2 (TN = 30.8 K). Our study reveals a complex nature of the effect: we suggest that it is provided both by strong exchange coupling between UO2 and Fe3O4 and by exchange biasing within magnetite (up to 25% of the total effect at 5 K in the UO2/Fe3O4 sample with the largest EB). The former can be caused by proximity effects of Fe3O4 on TN combined with a magnetic anisotropy of UO2, persisting above a continuous magnetic-ordering transformation. For magnetite, the results indicate the presence of pinned moments up to 120 K that correlates with the change in the easy magnetization axis of Fe3O4 across the Verwey transition.

Superspin glass state and exchange bias in amorphous Fe/Fe-O core/shell nanoparticles

Nanoparticles of iron and iron oxide are widely explored in several biomedical and technological applications. We report on the magnetic properties of amorphous Fe/Fe–O core/shell nanoparticles compared to those of a reference system with crystalline Fe–O nanoparticles. These nanoparticles are prepared by thermal decomposition of iron precursor, where the amorphous and crystalline nature of core and shell is determined by the choice and concentration of the ligand. The crystalline system exhibits a blocking temperature higher than 300 K and negligible exchange bias effect. In contrast, the amorphous systems display large exchange bias, and collective magnetic behavior at low temperatures, with features of magnetic frustration and disorder reminiscent of those observed in spin glass and superspin glass systems. We discuss the origin of the dynamical magnetic behavior of the amorphous particles and study the dependence of the exchange bias field on the cooling field.

Unexpected magnetism, Griffiths phase, and exchange bias in the mixed lanthanidePr0.6Er0.4Al2

We report an unusual coexistence of ferromagnetism and ferrimagnetism, and metamagnetism in ${\text{Pr}}_{0.6}$${\text{Er}}_{0.4}$${\text{Al}}_{2}$. In addition, this compound retains a clear Griffiths phase behavior even at 1 kOe magnetic field and shows a large exchange bias after field cooling from the paramagnetic state. The crystal-field excitations and opposite exchange interactions between nearest-neighbor and next-nearest-neighbor rare earth sites explain these behaviors.