Thickness dependence of exchange anisotropy in NiFe/IrMn bilayers studied by Planar Hall Effect

Planar Hall Effect (PHE) in NiFe(t)/IrMn(10.0 nm) thin film structures has been experimentally investigated as a function of NiFe thickness in the range from 3 to 20 nm, under the applied magnetic field perpendicular to the easy axis. The PHE voltage change and its field sensitivity increase with NiFe thickness, but the field interval of two voltage maxima decreases with the thickness. There are good agreements between measured and calculated PHE voltage profiles, where the parameters of exchange-biased and effective anisotropy fields have been characterized to decrease with NiFe thickness. However, an anisotropic resistivity change increases as the NiFe thickness increases. These analyses suggest that PHE is the effective method, inferred to single domain, to determine the electrical and magnetic parameters in magnetic devices.     

A study of the induced anisotropy in a ferromagnetic grain from an exchange coupled antiferromagnetic grain with uniaxial anisotropy

Approximate analytic solutions for the energy of an antiferromagnetic (AF) grain, which experiences an external exchange torque from a ferromagnetic grain have been obtained for a wide range of AF thickness. The accuracy of the analytic expression is within 2.0% of the exact solution, which do not have a closed analytical form. The model predicts that there are two critical AF grain thickness for each particular exchange energy strength. Below the first critical thickness, the induced anisotropy energy is well approximated by an uniaxial anisotropy term. Above the second critical thickness the induced anisotropy is unidirectional. In the intermediate range the induced anisotropy can not be expressed simply as uniaxial or unidirectional. The exchange bias and coercivity in NiFe/IrMn films have been studied as a function of the IrMn thickness, and the results are consistent with the proposed theory.     

The study of exchange coupling in NiFe/Cu/IrMn trilayer structures by MOKE and FMR measurements

The exchange coupling strength of NiFe/Cu/IrMn trilayer films was examined with both a new magneto optical Kerr effect (MOKE) method developed for the exchange coupling field determination and ferromagnetic resonance (FMR) measurements. We found that the value for exchange coupling field obtained by the MOKE technique coincided with FMR result with high accuracy. Other peculiarities of FMR measurements due to interlayer exchange coupling such as angular dependence of resonance field on Cu spacer thickness are also shown in the article.     

Electrical measurement of antiferromagnetic moments in exchange-coupled IrMn/NiFe stacks

We employ antiferromagnetic tunneling anisotropic magnetoresistance to study the behavior of antiferromagnetically ordered moments in IrMn exchange coupled to NiFe. Experiments performed by common laboratory tools for magnetization and electrical transport measurements allow us to directly link the broadening of the NiFe hysteresis loop and its shift (exchange bias) to the rotation and pinning of antiferromagnetic moments in IrMn. At higher temperatures, the broadened loops show zero shift, which correlates with the observation of fully rotating antiferromagnetic moments inside the IrMn film. The onset of exchange bias at lower temperatures is linked to a partial rotation between distinct metastable states and pinning of the IrMn antiferromagnetic moments in these states. The observation complements common pictures of exchange bias and reveals an electrically measurable memory effect in an antiferromagnet.     

Effect of Ta buffer and NiFe seed layers on pulsed-DC magnetron sputtered Ir20Mn80/Co90Fe10 exchange bias

A systematic investigation has been done on the correlation between texture, grain size evolution and magnetic properties in Ta/Ni₈₁Fe₁₉/Ir₂₀Mn₈₀/Co₉₀Fe₁₀/Ta exchange bias in dependence of Ta buffer and NiFe seed layer thickness in the range of 2-10 nm, deposited by pulsed DC magnetron sputtering technique. A strong dependence of 〈1 1 1〉 texture on the Ta/NiFe thicknesses was found, where the reducing and increasing texture was correlated with exchange bias field and unidirectional anisotropy energy constant at both NiFe/IrMn and IrMn/CoFe interfaces. However, a direct correlation between average grain size in IrMn and H_ex and H_c was not observed. L1₂ phase IrMn₃ could be formed by thickness optimization of Ta/NiFe layers by deposition at room temperature, for which the maximum exchange coupling parameters were achieved. We conclude finally that the coercivity is mainly influenced by texture induced interfacial effects at NiFe/IrMn/CoFe interfaces developing with Ta/NiFe thicknesses.     

A Decrease in the Exchange Bias Caused by an Increase in the Effective Thickness of the Copper Layer in the NiFe/Cu/IrMn Heterostructures

Physics of the Solid State - In a series of NiFe/Cu/IrMn structures with a variable effective thickness of the nonmagnetic copper interlayer (up to its absence in the NiFe/IrMn sample), we observed...     

Magnetoresistance and magnetization studies through a Cu interlayer in spin valves of NiFe/Cu/NiFe/FeMn

The influence of the Cu layer thickness on the magnetic and magnetotransport properties has been investigated in Ta/NiFe/Cu/NiFe/FeMn spin valves. The magnetization and magnetoresistance measurements were carried out for magnetic field applied along the easy-axis direction. A phenomenological model, which assumes formation of a planar domain wall at the anti-ferromagnetic side of the interfaces as well as bilinear coupling between the ferromagnetic layers, was used to derive the anisotropy characteristics and orientation of each NiFe layer magnetization. The anisotropy and spin valve magnetoresistance were simulated numerically and compared with the experiment. It was found that the anisotropy magnetoresistance is negligible and that there is a poor agreement for the spin-valve one, which was attributed to the model (valid for ferromagnetic layers in single-domain state only) used for its calculation. It was found that the increase of the Cu layer thickness provokes a decrease of the interdiffusion between the NiFe and FeMn layers, and, as consequence, changes of the uniaxial anisotropy of the pinned NiFe layer, of the exchange interaction between the pinned NiFe layer and the FeMn ones, as well as of the exchange-bias field of the pinned NiFe layer.     

Exchange Interactions in NiFe/Ta/IrMn Heterostructures under Conditions of Tantalum Deficiency

Physics of the Solid State - In the NiFe/Ta/IrMn structures, the contributions of two types of exchange interactions at the NiFe/IrMn interface, partially separated by sections of the nonmagnetic...     

Antiferromagnet IrMn thickness dependence in exchange-biased perpendicular magnetic anisotropy based on CoFe/Pt/CoFe/[IrMn(tIrMn)] multilayers

The effect of the antiferromagnetic IrMn thickness upon the magnetic properties of CoFe/Pt/CoFe/[IrMn(t_IrMn)] multilayers is studied. An oscillatory interlayer coupling (IEC) has been shown in pinned CoFe/Pt(t_Pt)/CoFe/IrMn multilayers with perpendicular anisotropy. The period of oscillation corresponds to about 2 monolayers of Pt. The oscillatory behavior of IEC depends on the nonmagnetic metallic Pt thickness and is thought to be related to the antiferromagnetic ordering induced by the IrMn layer. From the extraordinary Hall voltage amplitude (EHA) curves as function of IrMn thickness, we report that the oscillation dependence of IEC for the [CoFe/Pt/CoFe] multilayer system induced by IrMn with spacer-layer thickness is a important features of perpendicular exchange biased system.     

Magnetic anisotropy in IrMn/Co structures with an alternative sequence of deposition of antiferromagnetic and ferromagnetic layers

The preparation conditions and the magnitudes of the uniaxial and unidirectional magnetic anisotropies of IrMn/Co structures with an alternative sequence of deposition of antiferromagnetic and ferromagnetic layers upon heat treatment and cooling in an external magnetic field have been investigated. It has been revealed that the unidirectional anisotropy (exchange bias) arises in the structure with an antiferromagnetic layer deposited on a ferromagnetic layer (TS structure) at an annealing temperature of higher than 100°C. In structures with a ferromagnetic layer deposited on an antiferromagnetic layer (BS structure), the exchange bias does not arise in the annealing temperature range under investigation. The possible factors responsible for this effect and the ratio between the temperature of the appearance of the exchange bias and the Néel temperature have been discussed.     

具有条纹磁畴结构的NiFe薄膜的制备与磁各向异性研究

具有条纹磁畴结构的磁性薄膜表现出面内转动磁各向异性,对于解决高频电子器件的方向性问题起着至关重要的作用.本文采用射频磁控溅射的方法,研究了NiFe薄膜的厚度、溅射功率密度、溅射气压等制备工艺参数对条纹磁畴结构、面内静态磁各向异性、面内转动磁各向异性、垂直磁各向异性的影响规律.研究发现,在功率密度15.6 W/cm~2与溅射气压2 mTorr(1 Torr=1.33322×102Pa)下生长的NiFe薄膜,表现出条纹磁畴的临界厚度在250 nm到300 nm之间.厚度为300 nm的薄膜比250 nm薄膜的垂直磁各向异性场增大近一倍,从而磁矩偏离膜面形成条纹磁畴结构,并表现出面内转动磁各向异性.高溅射功率密度可以降低薄膜出现条纹磁畴的临界厚度.在相同功率密度15.6 W/cm~2下生长300 nm的NiFe薄膜,随着溅射气压由2 mTorr增大到9 mTorr,NiFe薄膜的垂直磁各向异性场逐渐由1247.8 Oe(1 Oe=79.5775 A/m)增大到3248.0 Oe,面内转动磁各向异性场由72.5 Oe增大到141.9 Oe,条纹磁畴周期从0.53μm单调减小到0.24μm.NiFe薄膜的断面结构表明柱状晶的形成是表现出条纹磁畴结构的本质原因,高功率密度下低溅射气压有利于柱状晶结构的形成,表现出规整的条纹磁畴结构,高溅射气压会导致柱状晶纤细化,面内转动磁各向异性与面外垂直磁各向异性增强,条纹磁畴结构变得混乱.     

Microscopic domain structures in unidirectional and isotropic exchange-coupled NiO/NiFe bilayers

The dependence on nickel oxide thickness in unidirectional and isotropic exchange-coupled NiO/NiFe bilayer films was investigated by magnetic force microscopy to better understand exchange biasing at microscopic length scales. As the NiO thickness increased, the domain structure of unidirectional biased films formed smaller and more complex in-plane domains. By contrast, for the isotropically coupled films, large domains generally formed with increasing NiO thickness including a new cross type domain with out-of-plane magnetization orientation. The density of the cross domain is proportional to exchange biasing field, and the fact that the domain mainly originated from the strongest exchange coupled region was confirmed by imaging in an applied external field during a magnetization cycle.     

Sharp Change in the Exchange Bias and the Magnetic Anisotropy Symmetry at a Subthreshold Interlayer Copper Content in NiFe/Cu/IrMn Heterostructures

Journal of Experimental and Theoretical Physics - The NiFe/Cu/IrMn heterostructures with a variable number of interlayer Cu atoms exhibit a sharp change in the exchange-bias field, the coercive...     

Parameters of the transition layer in the exchange-biased NiFe/DyCo film structure

The parameters of the transition layer in exchange-biased film structures are necessary agents to understand the mechanism of formation of unidirectional anisotropy. The layer thickness in NiFe/DyCo films has been determined by comparison of signals of the polar magneto-optical Kerr effect from a reference DyCo film and a hard magnetic layer of the exchange-biased structure. The layer thickness obtained is one order of magnitude larger than that characteristic of ferromagnet-antiferromagnet bilayer films. The mechanism of magnetization reversal of the structure under study has been explained within the model suggesting the formation of 180° boundaries in the interface.     

Unidirectional anisotropy in (<Emphasis Type="Italic">RE-TM</Emphasis>)/NiFe film systems

The quasi-static magnetic parameters of TbFe/NiFe and DyCo/NiFe bilayer exchange-biased films characterized by unidirectional anisotropy are studied. The characteristic temperatures are determined at which the unidirectional anisotropy disappears due to processes in the magnetically hard layer. The mechanisms responsible for the appearance of unidirectional anisotropy associated with the heterophase property of the magnetically hard layer are analyzed.     

Microstructural and magnetic characteristics of IrMn exchange-biased tunnel junctions

Tunneling magnetoresistance values above 20% and 40% were obtained for as-deposited and annealed tunnel junctions, Ta/NiFe/Cu/NiFe/IrMn/CoFe/Al-oxide/CoFe/NiFe/Ta, respectively. Exchange biasing field increased from 270 to 550 Oe after annealing resulting from sharpening of the IrMn/CoFe interface. dV/dI vs. V curves showed asymmetric profiles, which were due to asymmetry of the CoFe/Al-oxide interfaces and difference in microstructure of the CoFe layers.     

High temperature magnetoresistance in (Co,CoFe and NiFe)/Cu/CoFe/IrMn spin-valves

The MR characteristics of temperature variations ranging from room temperature to 570 K for IrMn spin-valves which consist of free layer Co, CoFe and NiFe were studied. Co-SV had the highest MR value in all temperature ranges among them and even at 510 K the MR ratio held more than half of room temperature value. Whereas CoFe-SV had a slightly higher MR than NiFe-SV below 450 K, above 450 K CoFe-SV showed a lower MR ratio than NiFe-SV. MR loops of large coercivity such as CoFe free magnetic layer collapsed in the lower elevated temperature compared to NiFe and Co. Small coercivity of the free magnetic layer would be preferable to spin-valves.     

NiFe/FeMn双层膜的交换耦合

采用平面霍尔效应测量方法，对NiFe/FeMn双层膜的交换耦合进行了研究. 结果表明，在NiFe/FeMn体系中不存在spin-flop模型给出的单轴各向异性场. 而导致交换耦合场可逆与不可逆测量结果之间较大差异的原因是反铁磁颗粒的不稳定性或铁磁层的分畴现象. 关键词： 反铁磁/铁磁双层膜 交换偏置场 可逆与不可逆测量     

81NiFe/Cr多层膜磁电阻单向各向异性与交换耦合

采用电子束蒸发法制备的８１ＮｉＦｅ／Ｃｒ多层膜具有单向各向异性，磁滞回线非轴对称，相当存在１Ｏｅ数量级的交换偏场。磁电阻回线的上升和下降两枝差别明显，其中以横向磁电阻回线最为显著。有的试样上升枝与下降枝相应的最大横向磁电阻率之差已高达０．７８％，其最大各向异性磁电阻率（Ｒ_ｍａｘ－Ｒ_ｍｉｎ）／Ｒ为４．７２％，甚高于Ｍｉｙａｚａｋｉ等^[１］的优质８２ＮｉＦｅ合金单层膜的报道值３％。可以认为这类多层膜的磁电阻效应除主要来源于自旋－轨道耦合机制外， 关键词：     

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.