IrMn Layer Research Articles

Exchange bias properties of 140 nm-sized dipolarly interacting circular dots with ultrafine IrMn and NiFe layers

We studied the exchange bias effect in an array of IrMn(3nm)/NiFe(3nm) circular dots (size ~140nm and center-to-center distance ~200nm, as revealed by microscopy analyses), prepared on a large area (3×3mm2) by electron beam lithography and lift-off, using dc sputtering deposition. Hysteresis loops were measured by SQUID magnetometer at increasing values of temperature T (in the 5–300K range) after cooling from 300K down to 5K in zero field (ZFC mode) and in a saturating magnetic field (FC mode). The exchange bias effect disappears above T~200K and, at each temperature, the exchange field HEX measured in ZFC is substantially lower than the FC one. Micromagnetic calculations indicate that, at room temperature, each dot is in high-remanence ground state, but magnetic dipolar interactions establish a low-remanence configuration of the array as a whole. Hence, at low temperature, following the ZFC procedure, the exchange anisotropy in the dot array is averaged out, tending to zero. However, even the FC values of HEX and of the coercivity HC are definitely smaller compared to those measured in a reference continuous film with the same stack configuration (at T=5K, HEX~90Oe and HC~180Oe in the dots and HEX~1270Oe and HC~860Oe in the film). Our explanation is based on the proven glassy magnetic nature of the ultrathin IrMn layer, implying the existence of magnetic correlations among the spins, culminating in a collective freezing below T~100K. We propose, also by the light of micromagnetic simulations, that the small dot size imposes a spatial constraint on the magnetic correlation length among the IrMn spins so that, even at the lowest temperature, their thermal stability, especially at the dot border, is compromised.

Reduction of magnetic 1/f noise in miniature anisotropic magnetoresistive sensors

The magnetic 1/f noise in miniature anisotropic magnetoresistive sensors has been studied quantitatively, and a linear correlation between sensitivity and magnetic 1/f noise parameter has been observed. The noise level can be effectively reduced by having an IrMn exchange bias layer adjacent to the permalloy layer, as sensing stripes of 2 μm width exhibit 16 times smaller magnetic 1/f noise parameter upon introduction of an IrMn layer. This simple method energetically stabilizes the magnetization fluctuation in permalloy, and lowers the 1/f noise without degrading the sensitivity. The geometry dependences of both noise and sensitivity have also been investigated to better guide future magnetoresistive sensor design.

Effect of L12 ordering in antiferromagnetic Ir-Mn epitaxial layer on exchange bias of FePd films

Two series of samples of single-layer IrMn and IrMn/FePd bilayer films, deposited on a single-crystal MgO substrate at different IrMn deposition temperatures (Ts = 300–700 °C), were investigated using magnetron sputtering. L12 ordering was revealed for the 30 nm-thick IrMn epitaxial (001) films with Ts ≥ 400 °C, determined by synchrotron radiation x-ray diffractometry (XRD). XRD results also provide evidence of the epitaxial growth of the IrMn films on MgO substrate. Increasing Ts from 400 to 700 °C monotonically increases the ordering parameter of L12 phases from 0.17 to 0.81. An in-plane exchange bias field (Heb) of 22 Oe is obtained in a 10 nm-thick FePd film that is deposited on the disordered IrMn films. As the L12 ordering of the IrMn layers increases, the Heb gradually decreases to 0 Oe, meaning that the exchange bias behavior vanishes. The increased surface roughness, revealed by atomic force microscopy, of the epitaxial IrMn layers with increasing Ts cannot be the main cause of the decrease in Heb due to the compensated surface spins regardless of the disordered and ordered (001) IrMn layers. The change of antiferromagnetic structure from the A1 to the L12 phase was correlated with the evolution of Heb.

Role of the antiferromagnetic pinning layer on spin wave properties in IrMn/NiFe based spin-valves

Brillouin light scattering (BLS) was exploited to study the spin wave properties of spin-valve (SV) type samples basically consisting of two 5 nm-thick NiFe layers (separated by a Cu spacer of 5 nm), differently biased through the interface exchange coupling with an antiferromagnetic IrMn layer. Three samples were investigated: a reference SV sample, without IrMn (reference); one sample with an IrMn underlayer (10 nm thick) coupled to the bottom NiFe film; one sample with IrMn underlayer and overlayer of different thickness (10 nm and 6 nm), coupled to the bottom and top NiFe film, respectively. The exchange coupling with the IrMn, causing the insurgence of the exchange bias effect, allowed the relative orientation of the NiFe magnetization vectors to be controlled by an external magnetic field, as assessed through hysteresis loop measurements by magneto-optic magnetometry. Thus, BLS spectra were acquired by sweeping the magnetic field so as to encompass both the parallel and antiparallel alignment of the NiFe layers. The BLS results, well reproduced by the presented theoretical model, clearly revealed the combined effects on the spin dynamic properties of the dipolar interaction between the two NiFe films and of the interface IrMn/NiFe exchange coupling.

마이크로 코일-채널과 GMR-SV 소자를 이용한 적혈구-자성비드 검출 특성연구

광 리소그래피 공정을 이용하여 GMR-SV(Giant magnetoresistance-Spin valve) 다층박막 위에 마이크로 크기의 바이오센서용 소자와 코일-채널을 적층으로 각각 제작하였다. 직경 <TEX>$1{\mu}m$</TEX> 크기인 여러 개의 자성비드가 결합된 적혈구가 마이크로 채널로 지나갈 때 코일에 인가하는 AC 신호로 정지 또는 통과하는 적혈구 움직임을 조절하였다. GMR-SV 소자 위에 포획된 적혈구-자성비드가 갖는 미세자기장은 자기저항비를 변화시켜 검출용 출력신호 특성으로 나타났다. 이것을 이용하여 자성비드를 결합한 적혈구의 막 변형에 따른 운동 특성을 분석하는 바이오센서로 활용할 수 있음을 보여주었다. The micro device, coil, and channel for the biosensor integrated with the GMR-SV device based on the antiferromagnetic IrMn layer was fabricated by the light lithography process. When RBCs coupled with several magnetic beads with a diameter of <TEX>$1{\mu}m$</TEX> passed on the micro channel, the movement of <TEX>$RBC+{\mu}Beads$</TEX> is controlled by the electrical AC input signal. The <TEX>$RBC+{\mu}Beads$</TEX> having a micro-magnetic field captured above the GMR-SV device is changed as the output signals for detection status. From these results, the GMR-SV device having the width magnitude of a few micron size can be applied as the biosensor for the analysis of a new magnetic property as the membrane's deformation of RBC coupled to magnetic beads.

Direct imaging of thermally-activated grain-boundary diffusion in Cu/Co/IrMn/Pt exchange-bias structures using atom-probe tomography

Magnetic devices are often subject to thermal processing steps, such as field cooling to set exchange bias and annealing to crystallize amorphous magnetic electrodes. These processing steps may result in interdiffusion and the subsequent deterioration of magnetic properties. In this study, we investigated thermally-activated diffusion in Cu/Co/IrMn/Pt exchange biased polycrystalline thin-film structures using atom probe tomography. Images taken after annealing at 400 °C for 60 min revealed Mn diffusion into Co grains at the Co/IrMn interface and along Pt grain boundaries for the IrMn/Pt stack, i.e., a Harrison type C regime. Annealing at 500 °C showed further Mn diffusion into Co grains. At the IrMn/Pt interface, annealing at 500 °C led to a type B behavior since Mn diffusion was detected both along Pt grain boundaries and also into Pt grains. The deterioration of the films' exchange bias properties upon annealing was correlated to the observed diffusion. In particular, the topmost Pt capping layer thickness turned out to be crucial since a faster deterioration of the exchange bias properties for thicker caps was observed. This is consistent with the idea that Pt acts as a getter for Mn, drawing Mn out of the IrMn layer.

Effects of interfacial roughness on the planar Hall effect in NiFe/Cu/IrMn multilayers

This paper reports that the planar Hall effect in NiFe/Cu/IrMn multilayers was strongly influenced by the Cu spacer thickness (t Cu), which was due to the variation of interfacial roughness. With t Cu increasing, a peculiar change of planar Hall voltage was observed. The reason for the voltage behaviors was that the interfacial roughness influenced the spin-asymmetry of spin-polarized electrons in ferromagnetic metals. The diffuse scattering to the electrons turned to specular scattering when the interface became flat, leading to the variation of resistivity change (Δρ). As the increase in t Cu, the extremum field was reduced because of the weaken exchange coupling between NiFe and IrMn layers.

Self-referenced multi-bit thermally assisted magnetic random access memories

The feasibility of 3-bits per cell storage in self-referenced thermally assisted magnetic random access memories is demonstrated both by macrospin simulations and experiments. The memory dot consists of a storage layer where CoFe/CoFeB magnetization direction is pinned by an IrMn layer using the ferromagnet/antiferromagnet interfacial exchange coupling, separated by an MgO tunnel barrier from a CoFeB sense layer whose magnetization direction is free to rotate. Writing is performed by heating the antiferromagnet above its blocking temperature by sending a current pulse through the magnetic tunnel junction, with the application of an in-plane field during the subsequent cooling phase, thus setting the new storage layer pinning direction. This pinning direction actually carries the information stored in the storage layer. Reading is performed by applying a rotating field, inducing a coherent rotation of the sense layer, and subsequently locating the field angle associated with the minimum measured resistance. This angle corresponds to the parallel magnetic configuration of the magnetic tunnel junction and therefore allows determining the pinning direction established during the write operation. The number of distinguishable pinning angles defines the total number of bits that can be stored in a single dot.

Perpendicular exchange bias effect in sputter-deposited CoFe/IrMn bilayers

CoFe/IrMn bilayers with perpendicular magnetization for various IrMn layer thicknesses exhibit unusual two-step hysteresis loops with both positive and negative loop shifts. Observed at room temperature in the as-grown state, they provide direct evidence of large antiferromagnetic domain formation at the IrMn interface. The exchange bias field reaches 100 mT with an IrMn layer thickness of 4 nm after field annealing at 200 °C–300 °C in 800 mT, which is at least three times as large as the coercivity, and may be useful for reference layers of spin-valves or magnetic tunnel junctions with perpendicular magnetic anisotropy.

Study on the occurrence of spontaneously established perpendicular exchange bias in Co49Pt51/IrMn bilayers

In this study, perpendicular exchange bias (PEB) effect in the as-grown Co49Pt51/IrMn bilayers was demonstrated at room temperature using single-layered Co49Pt51 alloy thin film as ferromagnetic (FM) layer. Several unusual features were observed in this system, viz.,: (i) the PEB was spontaneously established without any external magnetic field treatments, (ii) single-shifted loops were obtained rather than double-shifted ones, and (iii) the spontaneous PEB effect was accompanied by a reduction in perpendicular coercivity, Hc⊥ from 1024 to 632 Oe. The results of x–ray diffraction revealed the formation of IrMn (111) texture. Training effect studies indicate that the PEB effect is stable in this system with less than 5% variation in PEB value within 15 repetitive scans. Significant reduction in the PEB effect was found for the CoPt/IrMn films either grown or subjected to post-annealing under external magnetic field (Hind). The thickness dependence of PEB effect with respect to the FM and antiferromagnetic layers were also investigated and a largest PEB value of 533 Oe was obtained for the sample grown with 3-nm thick CoPt and 10-nm thick IrMn layers. The results of present study thus establish an opportunity to realize PEB effect in the absence of external field during fabrication.

Sign change of exchange bias in [Pt/Co]3/IrMn multilayer

The properties of exchange bias in a multilayer of [Pt(1.0 nm)/Co(1.0 nm)]2/Pt(tPt nm)/Co(1.0 nm)/ IrMn(12.0 nm) were investigated with a variation of Pt layer thickness, tPt. For tPt ≤ 1.6 nm, it was typically observed that Co layers were ferromagnetically coupled while IrMn layer exhibited negative exchange bias. With increasing Pt thickness, antiferromagnetic (AF) interlayer coupling strength increased and caused AF spin configuration between the Co layers. With further increasing of Pt thickness (tPt = 2.5 nm), the exchange bias between Co and IrMn layers was changed from negative to positive. Therefore, a large enhancement of AF interlayer coupling induced the sign change of exchange bias from negative to positive and resulted in a drastic change of switching behavior in a magnetization reversal. Both extraordinary Hall-effect and magnetoresistance were measured to verify the exchange bias direction and spin configurations upon magnetization reversal.

Characterization of Microstructural and Morphological Properties in As-deposited Ta/NiFe/IrMn/CoFe/Ta Multilayer System

Surface morphology and its relationship with microstructure in Ta/NiFe/IrMn/CoFe/Ta multilayer system deposited by pulsed DC magnetron sputtering have been investigated in dependence of Ta buffer and NiFe seed layer thicknesses using atomic force microscopy. The structural parameters such as grain size, dislocation density, texture and strain were calculated. For each surface, a self-affinity behavior with mean fractal dimensions in the range of 2.03–2.18 was found. Additionally, it was also observed that the surface of all samples has locally smooth textured surface structure in the short range. The texture aspect parameter and texture direction index have been obtained for isotropy/anisotropy surface texture. A significant relationship between the surface texture and the strength of the texture in IrMn layer has been found. The analysis indicated that the surface roughness is strongly affected by the thicknesses of the NiFe seed and Ta buffer layers.

Suppression of superconductivity in Nb by IrMn in IrMn/Nb bilayers

Effect of antiferromagnet on superconductivity has been investigated in IrMn/Nb bilayers. Significant suppression of both transition temperature (Tc) and lower critical field (Hc1) of Nb is found in IrMn/Nb bilayers as compared to a single layer Nb of same thickness; the suppression effect is even stronger than that of a ferromagnet in NiFe/Nb bilayers. The addition of an insulating MgO layer at the IrMn-Nb interface nearly restores Tc to that of the single layer Nb, but Hc1 still remains suppressed. These results suggest that, in addition to proximity effect and magnetic impurity scattering, magnetostatic interaction also plays a role in suppressing superconductivity of Nb in IrMn/Nb bilayers. In addition to reduced Tc and Hc1, the IrMn layer also induces broadening in the transition temperature of Nb, which can be accounted for by a finite distribution of stray field from IrMn.

Detection of the dynamic magnetic behavior of the antiferromagnet in exchange-coupled NiFe/IrMn bilayers

The magnetothermal behavior of antiferromagnetic IrMn layers of different thickness (3, 6, 10 nm) has been studied by exploiting the exchange coupling with a ferromagnetic 5 nm-thick NiFe layer. A procedure has been devised for the measurement of the magnetization of the NiFe/IrMn bilayers as a function of temperature and time at different values of an external magnetic field, Hinv, antiparallel to the unidirectional exchange anisotropy. This analysis allows one to probe the effective distribution of anisotropy energy barriers of the antiferromagnetic phase, as sensed by the ferromagnetic layer. Two magnetic regimes have been distinguished. At temperature T < 100 K, the interfacial IrMn spins are frozen in a glassy state and are collectively involved in the exchange coupling with the NiFe spins. At T ∼ 100 K the collective state breaks up; thus, above this temperature, only the interfacial IrMn spins which are tightly polarized by the IrMn nanograins, forming the bulk of the layer, are effectively involved in the exchange coupling mechanism. Due to that, for T > 100 K the exchange coupling is ruled by the anisotropy energy barriers of the bulk IrMn nanograins, namely by the layer thickness. The thermal evolution of the exchange field and of the coercivity in the three samples is coherently explained in the framework of this description of the dynamic magnetic behavior of the IrMn phase.

Enhanced blocking temperature in (Pt/Co)3/IrMn/Co and (Pd/Co)3/IrMn/Co trilayers with ultrathin IrMn layer

IrMn blocking temperature (TB) is compared between IrMn/Co bilayers and (Pt/Co)3/IrMn/Co and (Pd/Co)3/IrMn/Co trilayers for different IrMn thicknesses. Exchange bias field (Hex) is measured from 5 to 400 K. Trilayers show a more concave thermal decrease of Hex(T) compared to bilayers and, for thin IrMn layers, larger TB and sharper peak of coercive field Hc around TB. This Hex(T) behaviour presents improved characteristics for thermally assisted-MRAM (TA-MRAM) applications, with large Hex/Hc ratio. Two physical explanations are proposed: an indirect IrMn intergrain coupling through the (Pt(Pd)/Co)3 layer and a reduction of IrMn/Co interfacial coupling due to out-of-plane canting of the IrMn spins.

The role of the (111) texture on the exchange bias and interlayer coupling effects observed in sputtered NiFe/IrMn/Co trilayers

Magnetic properties of sputtered NiFe/IrMn/Co trilayers grown on different seed layers (Cu or Ta) deposited on Si (100) substrates were investigated by magnetometry and ferromagnetic resonance measurements. Exchange bias effect and magnetic spring behavior have been studied by changing the IrMn thickness. As shown by X-ray diffraction, Ta and Cu seed layers provoke different degrees of (111) fcc-texture that directly affect the exchange bias and indirectly modify the exchange spring coupling behavior. Increasing the IrMn thickness, it was observed that the coupling angle between the Co and NiFe ferromagnetic layers increases for the Cu seed system, but it reduces for the Ta case. The results were explained considering (i) different anisotropies of the Co and IrMn layers induced by the different degree of the (111) texture and (ii) the distinct exchange bias set at the NiFe/IrMn and IrMn/Co interfaces in both systems. The NiFe and Co interlayer coupling angle is strongly correlated with both exchange bias and exchange magnetic spring phenomena. It was also shown that the highest exchange bias field occurs when an unstressed L12 IrMn structure is stabilized.

Direction and Size of Ir Magnetic Moment Induced in MnIr/Co1-xFexExchange Bias Bilayers from Resonant X-ray Magnetic Scattering Experiments at the Ir L3Absorption Edge

Interface magnetism was investigated for Ir atoms in Mn73Ir27/Co70Fe30 and Mn77Ir23/Fe bilayers which showed an exchange bias effect. Scattering angle and X-ray energy dependent resonant X-ray magnetic scattering patterns were measured at a small angle region around the Ir L3 absorption edge to determine the induced magnetic structure of the Ir 5d magnetic moment. It is found that the induced Ir magnetic moment which follows applied field reversal is concentrated at the first atomic layer of the antiferromagnetic MnIr layer for both MnIr/CoFe and MnIr/Fe samples. The direction of the Ir moment is parallel to the magnetic moment of the ferromagnetic layer independent of the composition of the ferromagnetic layer. This behavior is different from that reported on the uncompensated 3d magnetic moment of Mn atoms. The average size of the Ir moment in the MnIr/CoFe bilayer is estimated to be about 1.4 times larger than that in the MnIr/Fe bilayer. These results are consistent with a first-principles band calcul...

Tuning of perpendicular exchange bias for magnetic memory applications

AbstractPerpendicular exchange bias (PEB) between [Co 0.3 nm/Pd 0.8 nm]5 multilayers and IrMn antiferromagnetic (AFM) layer is studied as functions of thickness of the interface layer and the AFM layer. It is found that increasing the thickness of a CoFe interface layer up to 2.1 nm could effectively improve the PEB. The achieved PEB field (Hbias) is more than 500 Oe. On the other hand, the coercivity (Hc) exhibits an opposite trend as a function of CoFe interface layer thickness, which might promote the integration of PEB structure with the perpendicular magnetic memory stack. It is also found that PEB is sensitive to the thickness of the AFM layer. The thickness window is only around 2 nm to achieve the largest Hbias. Moreover, for a very thin IrMn layer, a ferromagnetic nature is observed at low magnetic fields, which is likely owing to the net spins at the surface that might have been magnetized by the CoFe interface layer.

Effect of Interface Structure on Exchange Biased Heusler Alloy Films

We report on the effects of grain size in antiferromagnetic IrMn layers exchange bias to Co2FeSi. We also report on an enhanced effect where Mn layers are inserted in the interface. The exchange-biased IrMn/Co2FeSi samples were grown by a HiTUS system, which allows us to control the grain size. The smaller IrMn grains were too small to give a large Hex while an Mn layer 0.5 nm thick dramatically increased Hex. This significant increase is attributed to optimization of the Mn concentration at the interface. This grain-size and interface tuning offers a way to control the exchange bias in such systems.

Interdependence of reversal asymmetry and training effect in Ir22Mn78/Ni81Fe19 bilayers probed with magnetoresistance

Using magnetoresistance as a probe we demonstrate the correlation between reversal asymmetry and training effect in ion-beam sputtered IrMn/NiFe bilayers. During the training procedure, both exchange bias field and the degree of asymmetry decrease monotonically following a very similar trend. The analysis of the magnetoresistance behaviour establishes that the two distinct training mechanisms are operative. The first one is exhibited by an abrupt single cycle training effect and an accompanying pronounced reversal asymmetry, attributed to the presence of biaxial anisotropy in the IrMn layer. The second one displays a gradual cycling dependence due to thermal depinning of uncompensated antiferromagnetic spins.