Strength Of Exchange Bias Research Articles

The magnetic interfacial properties of an exchange biased nanocrystalline Ni80Fe20/α-Fe2O3 bilayer studied by polarized neutron reflectometry and Monte Carlo simulation

The strength of exchange bias can be influenced by interface roughness and antiferromagnetic morphology. Here, we studied the interface profile of an exchange biased, nanocrystalline Ni80Fe20/α-Fe2O3 bilayer. Magnetometry determined the bilayer’s exchange bias is observed below a blocking temperature of 75 K. Polarized neutron reflectometry measurements revealed the Ni80Fe20 layer was fully saturated to yield a net-moment of 0.95 μB/atom, while the majority of the Fe2O3 layer exhibited zero net-magnetization with the exception of the interfacial region with an uncompensated moment between 0.5 and 1.0 μB/Fe2O3. Monte Carlo simulations of a ferromagnetic/antiferromagnetic bilayer incorporating a granular antiferromagnet indicate that an extrinsic uncompensated moment of ∼1.0 μB/Fe2O3 can arise from grain boundary disorder. The size of the modeled moment is equivalent to the experimental value, and comparable with previous calculations. Furthermore, unlike intrinsic uncompensated spins, it is found that the disorder-induced moment in the granular antiferromagnet is not destroyed by interface roughness.

The exchange bias effect on single layer of Fe-rich FeRh thin film

Thin films of Fe rich FeRh were prepared by using DC magnetron sputtering from a Fe67Rh33 target onto MgO (0 0 1) substrate. In particular, here we identify a unique exchange-bias phenomenon. This is one of the homogenous system to have an exchange bias phenomena without the presence of two layer system with well-defined interface of ferromagnetic (FM)/antiferromagnetic (AFM) layers. Meanwhile, annealing at higher temperatures resulted in ordering of FeRh alloy which in turn gives rise to a higher exchange bias strength. Possible scenario for the exchange bias should have occurred from the boundaries between AFM/FM domains which could be due to the formation of AFM domain inside of FM matrix or vice versa in our single layer thin film.

Controlling the magnetic reversal mechanism of exchange biased MnxOy/Ni80Fe20 bilayers through O+ implantation

Abstract This work investigates the different magnetic reversal mechanisms of as-grown and oxygen-ion (O+) implanted MnxOy/Ni80Fe20 bilayers. A MnxOy/Ni80Fe20 bilayer was prepared by ion-beam sputter deposition in an Ar+ atmosphere using a partially oxidised Mn target. Oxygen implantations were then performed using 8 keV O+ ions at fluences of 1016, 1017 and 1018 ions/cm2 to modify the exchange bias strength at the MnxOy/Ni80Fe20 interface. The magnetic, crystallographic and chemical properties of the bilayers before and after O+ implantation were studied using transmission electron microscopy, X-ray reflectometry, magnetometry and polarised neutron reflectometry. The results show an overall improved exchange bias and coercivity for the O+ implanted bilayers. The 1017 ions/cm2 implanted sample shows the greatest improvement in exchange bias and was further studied for its detailed spin-reversal behaviour using polarised neutron reflectometry. Data analysis in the magnetically-trained state reveals a coexistence of coherent rotational and non-coherent magnetic spin reversal in the as-grown sample and a solely coherent spin rotation reversal mechanism for the O+ implanted MnxOy/Ni80Fe20 bilayer.

Reduction in magnetic exchange bias in CoFe/FeMn/CoFe trilayers due to reduced pinned uncompensated moments in AFM layer

Abstract The M-H hysteresis curves of field cooled CoFe/FeMn bilayers and CoFe/FeMn/CoFe trilayers were studied to understand the exchange bias phenomena in these systems. The measured data revealed that the values of the exchange bias corresponding to a bottom CoFe layer reduced by about 23.5% with an addition of another CoFe layer at the top of bilayer stack. It was also observed that, while this reduction in exchange bias of a bottom CoFe layer (calculated in %) depends on thicknesses of a top CoFe layer and an antiferromagnetic FeMn layer, it is independent of the thickness of bottom CoFe layer. As the strength of exchange bias depends on the presence of pinned uncompensated moments in an antiferromagnetic layer, our observations indicate that the FeMn layer consists comparatively lower amount of pinned uncompensated moments in trilayers. This reduction in pinned uncompensated moments of FeMn layer in trilayers is then co-related with the domain wall suppression in the FeMn layer in CoFe/FeMn/CoFe trilayers.

Tailoring exchange bias in ferro/antiferromagnetic FePt3 bilayers created by He+ beams

We report on artificial exchange bias created in a continuous epitaxial FePt3 film by introducing chemical disorder using a He+ beam, which features tailorable exchange bias strength through post-irradiation annealing. By design, the ferromagnetic (FM)/antiferromagnetic (AF) heterostructure exhibits stratified degrees of chemical order; however, the chemical composition and stoichiometry are invariant throughout the film volume. This uniquely allows for a consideration purely of the magnetic exchange across the FM/AF interface without the added hindrance of structural boundary parameters which inherently affect exchange bias quality. Annealing at 840 K results in the strongest exchange biased system, which displays a cross-sectional morphology of fine (<10 nm) domain structure composed of both of chemically ordered and chemically disordered domains. A magnetic model developed from fitting the characteristic polarised neutron reflectometry spectral features reveals that dual interactions can be attributed to the observed exchange bias: magnetic coupling at the FM/AF interface and also between neighbouring FM (chemically disordered) and AF (chemically ordered) domains within the nominally FM layer. Our results indicate that exchange bias is hindered at interfaces which are both chemically and magnetically perfect, while annealing can be used to balance the volume proportions of interfacial FM and AF domains to enhance the magnetic interface roughness for customisable exchange bias in mono-stoichiometric FM/AF heterostructures crafted by ion beams.

Influence of interface roughness on the exchange bias of Co/CoO multilayers

The effect of interface roughness on the magnetic properties of ferromagnetic/antiferromagnetic multilayer has been studied by comparing the interfaces within identical multilayer (lower vs. upper) or from different Co/CoO multilayers (different deposition sequence). It has been found that for identical multilayer, the upper Co/CoO interfaces grow rougher and show stronger exchange bias than the lower Co/CoO interfaces. Structural analyses indicate that the successive layer deposition gives rise to a cumulative roughness at the upper interface, which affects the magnetic properties of ferromagnetic layer and its coupling to the antiferromagnetic layers. The interface roughness strengthens the interfacial exchange coupling by the increase of defect-generated uncompensated antiferromagnetic spins; such spins form coupling with the spins from Co layer at the interface. Due to the different coupling strength of each Co layer to the neighboring CoO layers, the multilayer showed distinct switching fields during the magnetization reversal process. Our results imply the possibility to control the strength of exchange bias, coercivity, and related magnetic properties by tuning the interface roughness.

The Exchange Bias of a Permalloy and Ordered Epitaxial IrMn Thin Film Prepared From the Growth of Ultrathin [Ir/Mn] Multilayer

The influence of Ir <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> composition and post-annealing on structure and exchange bias of epitaxial Ir <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> (~10 nm)/Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">80</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> (7 nm) films were studied. The Ir <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> layer was prepared by MBE growth of [Mn(0.4 nm <; X nm <; 0.7 nm)/Ir(0.2 nm)] <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</sub> multilayer films. The optimal H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ex</sub> increases to ~190 Oe after annealing at 533 K for 3 h. X-ray diffraction indicates that the strength of exchange bias is increased with the order parameter of Ir <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> alloy.

Angular dependence of the exchange bias direction and giant magnetoresistance on different cooling-field strengths

We studied the effect of different cooling-field strengths on the exchange bias by measuring the angular-dependent sheet resistance and the giant magnetoresistance of exchange-biased spin valves using a PtMn antiferromagnetic and a synthetic antiferromagnetic layer. When we annealed the spin valve at a cooling-field of 100 Oe, the exchange bias was antiparallel to the cooling-field. As we increased the cooling-field to 4000 Oe, the exchange bias direction gradually rotated and it ended up parallel to the cooling-field direction. The giant magnetoresistance also changed with the cooling-field strength. In the cooling-field range between 100 and 4000 Oe, the magnetoresistance ratios measured along the cooling-field direction were significantly reduced. However, the magnetoresistance ratios measured along the exchange bias direction increased, although still remaining smaller than those of the spin valve annealed at 100 or 4000 Oe. On the other hand, the exchange bias strength did not change significantly with the cooling-field strength.

Linear correlation between uncompensated antiferromagnetic spins and exchange bias in Mn–Ir/Co100−xFex bilayers

A correlation between uncompensated (UC) antiferromagnetic (AF) spins and exchange bias (EB) strength was investigated for Mn74Ir26/Co100−xFex bilayers. The EB strength had a maximum at x=25 at. %, and the x-ray magnetic circular dichroism of Mn, representing the UC-AF spins, changed systematically in sign and magnitude with respect to x. A linear correlation was found between the EB strength and the root mean square magnitude of the UC-Mn spins. The hidden parameter connecting these two quantities might be exchange coupling energy at the heterointerface, which varies as a function of the ferromagnetic layer composition.

Current-Induced Reorientation of Exchange Bias on a Nanoscale

We demonstrate how local heating by an electric current can induce and reorient the exchange bias on a nanoscale. In our experiments we use point contacts ~10 nm in size to inject current densities as high as 10 into F/N/F/AFM exchange-biased spin valves (EBSV) where two ferromagnetic (F) layers are separated by a nonmagnetic (N) metal spacer and one of the Fs is biased by an adjacent antiferromagnetic (AFM) layer. At low currents the spin valves exhibit the usual giant magnetoresistance (GMR) when two F layers switch from parallel to antiparallel orientation. At highest applied currents the Joule heating in the contact becomes significant and in combination with static magnetic field can induce and repeatedly reorient the exchange bias in a small contact volume . The strength of exchange bias induced in the point contact was found to depend on the polarity of the applied current. We tentatively attribute this polarity dependence to spin-transfer torques arising near F/AFM interface at high currents.

Micromagnetic modeling of the magnetization dynamics in a circularly exchange-biased and exchange-coupled ferromagnetic multilayer

The magnetization dynamics of a magnetically coupled multilayer structure have been studied by analytical and numerical methods. The simulated multilayer is disk-shaped and consists of a circularly exchange-biased ferromagnetic permalloy (Py) layer coupled to an unbiased Py layer, each in a magnetic vortex configuration, separated by a thin nonmagnetic spacer. The sign and strength of the interlayer exchange coupling was varied, leading to either parallel or antiparallel vortex chiralities in the two Py layers. The magnetization dynamics after the application of an external magnetic field pulse normal to the plane of the disks were investigated. Both analytical and numerical models show two branches of frequency response of circularly symmetric eigenmodes for both parallel and antiparallel configurations. However, the upper branch mode in the antiparallel configuration is severely damped in the numerical simulations due to coupling with short-wavelength spin waves. The frequency of the modes can be tuned independently with interlayer exchange coupling strength and exchange-bias strength. The good agreement between the mode frequencies obtained from the analytical and numerical models confirms that the main driving forces for the eigenmodes are the magnetostatic field from the radial motion of the magnetization, and also the interlayer exchange coupling field. In addition, themore » vortex cores, which are neglected in the analytical model, are found to play no significant role in the dynamic response.« less

Correlation between exchange bias field and domain size of ferromagnetic layer in Mn–Ir/Co–Fe bilayers

In order to clarify the correlation between the exchange bias strength and magnetic domain structure in ferromagnetic/antiferromagnetic (AF) bilayers, ferromagnetic domain structure was observed at Co-L3 edge for polycrystalline Mn–Ir/Co–Fe bilayers by using the x-ray magnetic circular dichroism photoelectron emission microscopy technique. From the observation for four different samples prepared with the respective thermal annealing procedures, a positive correlation was found between the strength of exchange bias field and the ferromagnetic domain size. Within the framework of single spin ensemble model, it was indicated that the larger size of AF domain, which means small distribution of fixing direction of AF spins, can induce the larger exchange bias field.

Polarized current changes the exchange bias in a current-in-plane spin valve

A spin-polarized current changes the strength and direction of the exchange bias in spin valves with a current-in-plane geometry. The exchange bias can be manipulated and systematically changed by applying current pulses. The changes are nonmonotonic and asymmetric with respect to the directions of the applied field and current pulses. For different current pulses, different exchange-bias fields can be achieved in the same sample. Furthermore, for samples with different exchange bias, the bias field exhibits a dependence on the applied pulse. Since the strength of exchange bias is highly correlated to the micromagnetic state distribution of the antiferromagnet, we explain our observations by the spin torque exerted on the interfacial antiferromagnetic moments, excluding Joule heating and training effects.

Magnetic characterization of magnetic tunnel junction devices using circle transfer curves

We describe new characterization methods that allow an accurate determination of all of the magnetic parameters that govern the behavior of magnetoresistive devices. These characterization methods are explained and used to measure the magnetic properties of MgO-based magnetic tunnel junction (MTJ) devices with magnetoresistance values of over 150%. We will show that the analysis of so-called “circle transfer curves,” which are measurements of the device magnetoresistance in a rotating, constant-magnitude applied field, can accurately determine the magnitude and direction of the free layer anisotropy as well as the pinned layer orientation and exchange bias strength. We also show how a measurement of the MTJ’s remnant resistance curve, obtained by saturating the MTJ at different field angles and then removing the applied field, can provide additional information on the free layer anisotropy characteristics. We will also compare our results with values extracted from traditional Stoner-Wohlfarth asteroid curves. Finally, we show that the extracted parameters can accurately predict the shape of traditional MTJ transfer curves.

Micromagnetic modeling of spin-wave dynamics in exchange-biased permalloy disks

The magnetization dynamics in exchange-biased $12\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ thick micron-sized permalloy disks have been studied using micromagnetic modeling. The magnetization in the permalloy and in the adjacent antiferromagnetic layer are set in a vortex configuration, and the disk is equilibrated in an applied field, which is then released. The behavior of the magnetization has been modeled as a function of both exchange bias strength and applied field, in both the time and frequency domains. We show that the exchange bias increases the curvature of the effective potential confining the vortex and that the gyrotropic frequency of the vortex core motion increases linearly with exchange bias. The eigenmodes of the spin waves to which the field couples are either azimuthal (for an in-plane field) or circularly symmetric (for a perpendicular field), with several orders of modes being visible. For the cicularly symmetric modes, the increase in frequency with exchange bias is in good agreement with an analytical model.

The influence of the interfacial FeMn insertion layer on the pinning effect of IrMn/CoFe exchange coupled bilayers

The exchange bias and thermal stability of CoFe/IrMn films with and without thin FeMn layer at the interface were investigated. Very thin FeMn insertion layer was found to increase the pinning field largely. An optimum pinning field of ∼220 Oe was obtained for the film CoFe (100 Å)/FeMn (1.5 Å)/IrMn (70 Å), while only 140 Oe was obtained for the film CoFe (100 Å)/IrMn (70 Å). Their coercivities were not much different. The two films’ pinning field dependences of temperature were also almost the same and their blocking temperatures were both about 250 °C. This is ascribed that the local structure deformations at the interface of CoFe/IrMn increase the net spins of the interfacial antiferromagnetic atoms, and hence the exchange bias strength.

Dynamic magnetic anisotropy at the onset of exchange bias: TheNiFe∕IrMnferromagnet/antiferromagnet system

The strength of exchange bias and rotatable anisotropy in polycrystalline $\mathrm{Ni}\mathrm{Fe}\ensuremath{-}\mathrm{Ir}\mathrm{Mn}$ ferromagnet/antiferromagnet systems is quantified from dc down to the picosecond time scale by regular quasistatic and microwave magnetometry, as well as magnetic domain observation. A transition from superparamagnetic to antiferromagnetic behavior with increasing IrMn thickness is derived from the magnetic resonance frequency and the effective magnetic damping parameter. A discrepancy between magnetic loop shift and dynamically obtained exchange bias strength is explained by asymmetric rotatable anisotropy contributions with different relaxation times in the antiferromagnetic layer. The time-dependent relaxation is directly confirmed by magnetic domain observations. Partially switching in the IrMn layer even with strong exchange bias is concluded. The increase of coercivity rises solely from the rotatable anisotropy contribution.

FeMn thickness effect on the domain structure of exchange-biased permalloy films

We have investigated the FeMn thickness dependence of the exchange bias in the Py/FeMn/Py structure. The exchange biases of lower and upper Py layers show a difference in strength. The exchange bias of the lower Py is larger than that of the upper Py. The coercivity of the upper Py is larger than that of the lower Py. There are qualitative differences in MFM images according to the strength of exchange bias. The pinning points of MFM images are strongly related with the magnitude of exchange bias and coercivity is a less important parameter.

Nature of magnetization reversal in exchange-coupled polycrystalline NiO-Co bilayers

The nature of magnetization reversal in exchange-coupled NiO-Co polycrystalline bilayers was investigated. As-deposited bilayers exhibit a moderate value of exchange bias ${H}_{E}$ (=-0.9 mT) and a significantly enhanced coercivity ${(H}_{c}^{\mathrm{N}\mathrm{i}\mathrm{O}\ensuremath{-}\mathrm{C}\mathrm{o}}=12.4\mathrm{mT}),$ which is roughly 5 times the coercivity of a reference Co single film ${(H}_{c}^{\mathrm{Co}}=2.7\mathrm{mT}).$ Real time investigation of magnetization reversal in exchange-coupled NiO-Co bilayers shows that reversal is highly local and nonuniform in nature. It is preceded by the formation of precursors or embryos of reversed domains as the applied field reaches a critical value \ensuremath{\cong}8.8--9.0 mT. Once this critical applied field value is reached, numerous reversed domains are formed. Growths of such reversed domains occur primarily by the abrupt nucleation and the subsequent coalescence together of reversed domains; wall motion is not the dominant growth mode. Clear evidence is presented which shows that the strength of exchange bias varies at the microscopic scale across the sample. This manifests itself as different microscopic regions switching abruptly at different fields, and a given microscopic area switching at different fields in the positive and negative field directions. When the applied field is along the unidirectional anisotropy, reversal of a given strongly coupled microscopic region is aided by exchange bias, and such a region switches first; the same region undergoes reversal last when the polarity of the applied field is changed to oppose unidirectional anisotropy. Significantly, it was found that, locally, the measured value of exchange bias may vary by a factor of 3 or more from the macroscopically measured value of ${H}_{E}$ (=-0.9 mT) obtained from the shift of the M-H loop. High-resolution transmission electron microscopy (HRTEM) shows that that the local variation in ${H}_{E}$ may be explained by considering the underlying microstructure and interfacial topography of the NiO-Co interface. HRTEM results show that the NiO surface parallel to the interface may be a fully compensated (200) plane (low ${H}_{E}),$ an uncompensated plane (111) (high ${H}_{E}),$ or a partially compensated plane (moderate ${H}_{E})$ of antiferromagnetic NiO. Clear evidence of partial compensation of the moments of antiferromagnetic NiO at the interface is found. Partial compensation (and hence a reduced value of ${H}_{E})$ results from a highly faceted NiO surface at the atomic level, with different facets having different orientation (different sublattices). Finally, a magnetic effect is observed, which suggests that it may be energetically more favorable to have steps on the NiO surface that are even multiples of the unit cell dimensions of NiO ${a}_{0}{(=2\mathrm{ma}}_{0},m\ensuremath{\in}\mathrm{integer}&gt;0)$ and energetically unfavorable to have steps that are odd multiples of the unit cell dimension ${a}_{0}[=(2m\ensuremath{-}{1)a}_{0},m\ensuremath{\in}\mathrm{integer}&gt;0].$