Characterization of Domain Switching Behavior of MTJ Cells Using Magnetic Force Microscopy (MFM) and R–H Loop Analysis

Correlation between electrical and magnetic properties of magnetic tunnel junctions (MTJ) for magnetic random access memory (MRAM) was studied. The MTJ (Ta/NiFeCr/ PtMn/CoFe/Ru/CoFe/Al₂O₃/CoFe/NiFe/Ta) was analyzed by utilizing R-H loops and MFM images. We verified that a kink in an R-H loop comes from a vortex domain of free layer. In addition, we also observed a close relationship between a domain switching behavior and an irregular R-H curve. These results would be useful for the characterization of the MTJ cell, thereby optimizing the process to realize an ultrahigh density MRAM.     

Analysis due to vector magnetic field for recording characteristic of metal evaporated tape

Metal-evaporated tape (ME tape) is one of the most promising materials for high-density magnetic recording. In a previous study, we reported that the recording characteristics of ME tape are directly affected by the columnar microstructure of the magnetic layer (Co-Ni-O magnetic film). In this study, we have conducted the theoretical analysis of the effects of both head motion and the vector magnetic field Hθ, which is essentially parallel to the columnar longitudinal axis. The results of this analysis offer an effective explanation of the recording characteristics of ME tape.     

Spin-transfer switching current distribution and reduction in magnetic tunneling junction-based structures

Spin transfer switching current distribution within a cell and switching current reduction were studied at room temperature for magnetic tunnel junction-based structures with resistance area product (RA) ranged from 10 to 30 /spl Omega/-/spl mu/m/sup 2/ and TMR of 15%-30%. These were patterned into current perpendicular to plane configured nanopillars having elliptical cross sections of area /spl sim/0.02 /spl mu/m/sup 2/. The width of the critical current distribution (sigma/average of distribution), measured using 30 ms current pulse, was found to be 3% for cells with thermal factor (KuV/k/sub B/T) of 65. An analytical expression for probability density function p(I/I/sub c0/) was derived considering a thermally activated spin transfer model, which supports the experimental observation that the thermal factor is the most significant parameter in determining the within-cell critical current distribution. Spin-transfer switching current reduction was investigated through enhancing effective spin polarization factor /spl eta//sub eff/ in magnetic tunnel junction-based dual spin filter (DSF) structures. The intrinsic switching current density (J/sub c0/) was estimated by extrapolating experimental data of critical current density (J/sub c/) versus pulse width (/spl tau/), to a pulse width of 1 ns. A reduction in intrinsic switching current density for a dual spin filter (DSF: Ta/PtMn/CoFe/Ru/CoFeB/Al2O3/CoFeB/spacer/CoFe/PtMn/Ta) was observed compared to single magnetic tunnel junctions (MTJ: Ta/PtMn/CoFe/Ru/CoFeB/Al2O3/CoFeB/Ta). J/sub c/ at /spl tau/ of 1 ns (/spl sim/J/sub c0/) for the MTJ and DSF samples were 7/spl times/10/sup 6/ and 2.2/spl times/10/sup 6/ A/cm/sup 2/, respectively, for identical free layers. Thus, a significant enhancement of the spin transfer switching efficiency is seen for DSF structure compared to the single MTJ case.     

不同缓冲层和保护层对NiFe/PtMn双层膜磁性的影响

制备了以Ta和非磁性NiFeCr作为缓冲层和保护层的NiFe/PtMn双层膜,研究了它们的结构和物性,结果表明:用NiFeCr做缓冲层和保护层的NiFe/PtMn双层膜,其交换偏置场比以Ta为缓冲层和保护层的NiFe/PtMn的交换偏置场有了10%的提高;退火处理以后,以NiFeCr为缓冲层和保护层的样品的磁性层NiFe的磁矩降低要小于以Ta为缓冲层和保护层的样品的磁性层NiFe的磁矩降低.同时,XRD测量计算发现,以NiFeCr为缓冲层生长的NiFe/PtMn样品比Ta为缓冲层生长的NiFe/PtMn样品具有更好的织构,更大的平均晶粒尺寸,因而具有更好的热稳定性.NiFeCr比Ta更适合做基于Mn合金为反铁磁层的缓冲层和保护层.     

Exchange-biased soft underlayers for perpendicular recording

We inserted NiFe/CoFe/antiferromagnetic-MnIr/CoFe layers between two CoTaZr soft layers to enhance the exchange-bias field (H/sub eb/) and then evaluated the effect of this lamination on the spike noise and recording characteristics of CoCrPt-SiOx media with an exchange-biased soft magnetic underlayer (SUL). The two CoTaZr layers were exchange-biased radially throughout the disk, and a higher H/sub eb/ was obtained for the upper CoTaZr layer. By using the laminated SUL, spike noise was suppressed even when the total thickness of the CoTaZr layers was increased to 300 nm. Although the medium had a high H/sub c/ of 7.0 kOe, a fairly good overwrite and signal-to-noise ratio were obtained. As another application of exchange biasing, we also examined the possibility of combining exchange biasing and antiparallel-coupled (APC) soft layers; i.e., a pinned APC SUL. An exchange-bias field from the pinning layers to the lower CoTaZr layer and an exchange-coupled field between the two CoTaZr layers were successfully applied. The medium with the pinned APC SUL showed no spike noise throughout the disk, and wide-area adjacent track erasure was effectively suppressed.     

Soft magnetic properties of hybrid ferromagnetic films with CoFe, NiFe, and NiFeCuMo layers

Two-layered ferromagnetic alloy films (NiFe and CoFe) with intermediate NiFeCuMo soft magnetic layers of different thicknesses were investigated to understand the relationship between coercivity and magnetization process by taking into account the strength of hard-axis saturation field. The thickness dependence of H_EC (easy-axis coercivity), H_HS (hard-axis saturation field), and χ (susceptibility) of the NiFeCuMo thin films in glass/Ta(5 nm)/[CoFe or NiFe(5 nm-t/2)]/NiFeCuMo(t = 0, 4, 6, 8, 10 nm)/[CoFe or NiFe(5 nm-t/2)]/Ta(5 nm) films prepared using the ion beam deposition method was determined. The magnetic properties (H_EC, H_HS, and χ) of the ferromagnetic CoFe, NiFe three-layers with an intermediate NiFeCuMo super-soft magnetic layer were strongly dependent on the thickness of the NiFeCuMo layer.     

Insertion of a specular reflective and transmissive nano-oxide layer into giant magnetoresistance spin-valve structure

We have studied the influence of the insertion of a nano-oxide layer (NOL) into a magnetic GMR spin-valve. It was found that the spin-valve with NOL has a higher GMR ratio than that of the normal spin-valve without NOL. Naturally formed NOL without vacuum break shows a uniform layer, which effectively suppresses the current shunt, resulting in the reduction of the sheet resistance of GMR. The NOL spin-valve also shows a lower interlayer coupling (Hin) than that of the optimal normal spin-valve, which is consistent with AFM measurement showing lower roughness of NOL formed CoFe surface. Based on the advantage of NOL, we succeeded in lowering Hin while maintaining GMR ratio by insertion of NOL inside the CoFe free layer, where the free layer consists of CoFe/NOL/CoFe/NOL/Capping layer.     

Asymmetry of Magnetization Reversal of Pinned Layer in NiFe/Cu/NiFe/IrMn Spin-Valve Structure

Two types of asymmetry in giant magnetoresistance (GMR) are observed which are not related to a training effect, but indicate different mechanisms of magnetization reversal of the pinned layer in spin-valve (SV) structures for ascending and descending field scans. GMR, exchange bias and coercivity in Si/Ta/NiFe/Cu/NiFe/IrMn/Ta SV-structures were investigated as functions of the thickness of the nonmagnetic spacer. The spacer thickness effects are discussed in correlation with layers microstructure and interfaces morphology variations.     

磁控溅射法制备IrMn底钉扎自旋阀研究

采用高真空直流磁控溅射的方法,在玻璃衬底上制备了结构为Ta／buffer layer／IrMn／CoFe／Cu／CoFe／NiFe／Ta的IrMn底钉扎自旋阀。研究了NiFe和Cu作为缓冲层对自旋阀磁性能的影响,并对缓冲层厚度进行了参数优化,当缓冲层厚度为2nm时自旋阀各项性能达到最佳。研究了退火制度对底钉扎自旋阀性能的影响,得到了30000e强磁场下200℃保温1h为最佳处理条件。通过结构的改善和工艺的优化,得到的底钉扎自旋阀的磁电阻率8．51％,矫顽场为0．50e,交换偏置场超过8000e。最后对自旋阀的底钉扎和顶钉扎结构进行了比较。     

Synthetic ferrimagnetic media: effects of thermally assisted writing

Thermally assisted writing on high-coercivity synthetic ferrimagnetic media (SFM) was demonstrated using a conventional spin stand equipped with an optical head for commercial magnetooptical drives. The laser light (/spl lambda/ = 685 nm) was focused through a glass substrate onto a recording layer. The optical spot size was 1.1 /spl mu/m and a commercial magnetic head had a writer width of /spl sim/0.25 /spl mu/m. The recording properties were measured as a function of the writing current (I/sub w/) and the laser power (P/sub w/). For the thermally stable medium with H/sub c/ = 6 kOe, a laser irradiation with an optimum power significantly improved the overwrite performance and the signal-to-noise ratio (SNR) values. The SNR values were improved by optimizing P/sub w/ over a wide writing current range. The improvements with the assist were found in both the signal and the noise. The media with a large dynamic coercivity value or with thick magnetic layers clearly showed the advantages with thermal assist.     

RIE对巨磁电阻自旋阀磁性能的影响

对巨磁电阻自旋阀磁场传感器制作中的关键技术之一：自旋阀薄膜的反应离子刻蚀(RIE)工艺,进行了试验研究。自旋阀结构为：Ta(3．5nm)／Cu(0．7nm)／NiFe(4．5nm)／CoFe(1nm)／Cu(3nm)／CoFe(2nm)／Ru(0．7nm)／CoFe(2nm)／MnIr(8nm)／Ta(4nm),刻蚀气体为氢氯碳氟化合物(HCFC：Hydro—chloro—fluoro—carbon),气体流量为10．5seem,RF功率为180W,时间为27min。结果表明：RIE技术可以加工出理想的巨磁电阻自旋阀薄膜图形,且加工过程对自旋阀的磁性能影响不大,这些结果对于巨磁电阻自旋阀型集成磁传感器的批量制作具有积极意义。     

High tunnel magnetoresistance in epitaxial Co/sub 2/Cr/sub 0.6/Fe/sub 0.4/Al/MgO/CoFe tunnel junctions

We have fabricated fully epitaxial magnetic tunnel junctions (MTJs) using a Co-based full-Heusler alloy Co/sub 2/Cr/sub 0.6/Fe/sub 0.4/Al (CCFA) thin film and an MgO tunnel barrier. The CCFA thin film for the lower ferromagnetic electrode was deposited by magnetron sputtering on an MgO-buffered MgO single-crystal substrate, and the MgO tunnel barrier was formed by electron beam evaporation. The microfabricated epitaxial CCFA/MgO/CoFe MTJs showed high tunnel magnetoresistance ratios of 42% at room temperature and 74% at 55K.     

Thermal stability improvement for longitudinal magnetic recording media

This paper reports and discusses the effects of M/sub r/t cancellation and bottom/top magnetic layer thickness ratio on thermal stability and recording characteristics of low-noise synthetic antiferromagnetic (SAF) longitudinal recording media. It is shown that by proper design and process, SAF media with low M/sub r/t cancellation can deliver excellent overall recording performance with acceptable thermal stability. We present a unique approach to significantly improve the thermal stability of SAF media with low M/sub r/t cancellation by adjusting the bottom/top magnetic layer thickness ratio. Increasing the bottom/top magnetic layer thickness ratio significantly improves amplitude decay rate and media coercivity temperature dependence with little sacrifice in recording performance.     

Role of bottom layer of double recording layer structure for antiferromagnetically coupled longitudinal media

Double recording layers stacked on a Ru spacing layer are commonly used to obtain good recording properties for antiferromagnetically coupled (AFC) longitudinal media. In this paper, we report on a study that clarified the role of the bottom recording layer (BRL). We measured physical magnetic properties such as magnetic anisotropy field (H/sub k/), magnetocrystalline anisotropy energy (K/sub u/), intergranular interaction, and thermal agitation of the AFC media, with and without BRL. Results showed that: 1) even a thin BRL (1 nm thick) significantly increases remanent coercivity H/sub cr/; 2) this increase in H/sub cr/ is mainly caused by an increase in H/sub k/ due to elimination of the low K/sub u/ region at the bottom portion of the top recording layer; and 3) judging from magnetic cluster size, insertion of a BRL can reduce the intergranular exchange coupling at the initial growth region of the recording layer.     

Transverse recording using thin film recording heads

This paper presents the results of an investigation of a high density magnetic recording technique utilizing a thin film recording head and a transverse mode of recording on thin media. The significant results of this investigation are as follows. 1) Densities as high as 18 500 transitions per inch were experimentally written in a 300-Å thick FeCr medium having an Hcof 70 oersteds. 2) These densities were written with a thin film, vapor-deposited, recording head having a MATED-FILM® structure with a 0.4-mil etched gap. 3) Track widths of 1-mil on 2-mil centers were experimentally achieved. 4) Optical readout of a 0.2-mil wide transition (width of beam) region corresponding to 5000 transitions per inch was achieved using a laser beam and a linear motion transport system under ideal experimental conditions. 5) The magnetic field from the Néel wall separating recording domains was detected using a MATED-FILM Etched Gap head making this a possible readout method. Maximum achieved linear bit densities as a function of recording media coercivities are given.     

Effect of Cu surface segregation on properties of NiFe/FeMn bilayers

The films of NiFe/FeMn with Ta and Ta/Cu buffer layers were prepared by magnetron sputtering. Results show that the exchange bias field of NiFe/FeMn films with Ta/Cu buffer is lower than that of the films with Ta buffer. The crystalline texture, surface roughness and element distribution of these two sets of samples were examined, and there is no apparent difference for the texture and roughness. However, the segregation of Cu atoms on the surface of NiFe in the trilayer of Ta/Cu/NiFe has been observed by using the angle-resolved X-ray photoelectron spectroscopy. The decrease of the exchange bias field for NiFe/FeMn films with Ta/Cu buffer layers is mainly caused by the diffusion of Cu atoms through NiFe layer, which stayed at the interface of NiFe/FeMn film or even intruded into FeMn layer. The present results indicate that Cu segregation through NiFe layer should be suppressed in order to improve the exchange bias field in giant magnetoresistance spin valves with Cu spacer.     

Synthesis of nano-sized spherical barium-strontium ferrite particles

Magnetic recording media requires good particle dispersion, a smooth surface, and small interparticle interaction to make an adequate signal-to-noise ratio (SNR). Well dispersed 50-60 nm sized spherical barium-strontium ferrite (S-Ba/Sr-Fe) nanoparticles were successfully prepared with 40 nm sized hematite precursor particles and BaCO/sub 3//SrCO/sub 3/ colloid. The coercivity and saturation magnetizations of S-Ba/Sr-Fe nano-particles were 1568 Oe and 48.6 emu/g, respectively. In order to evaluate magnetic interaction, magnetic tape was prepared using an Eiger mill with binder and organic solvent. /spl Delta/M measurement showed the S-Ba/Sr-Fe nanoparticles in the tape had negative magnetic particle-to-particle interaction.     

基于非晶软磁层的巨磁电阻单元性能研究

采用剥离工艺制备了单元大小为10μm×18μm的CoNbZr/Co/Cu/Co和NiFe/Co/Cu/Co多层膜结构的3×3自旋阀单元阵列,并测试了自旋阀单元的静态和动态巨磁电阻特性.结果表明CoNbZr层对快速磁场变化具有良好的线性响应特性.与NiFe/Co/Cu/Co自旋阀单元相比,微米尺度的CoNbZr/Co/Cu/Co自旋阀单元具有更良好的自旋电子特性,可以应用到包括MRAM器件在内的自旋电子器件中.     

Effect of strain on ferroelectric and magnetic behavior in Pb(Zr0.52Ti0.48)O3-based magnetoelectric heterostructures

In this paper, the "sandwich" structured magnetoelectric composite films of Pb(Zr0.52Ti0.48)O3/ NiFe2O4/Pb(Zr0.52Ti0.48)O3 and Pb(Zr0.52Ti0.48)O3/CoFe2O4/Pb(Zr0.52Ti0.48)O3 are epitaxially grown on SrRuO3/SrTiO3 substrates by pulsed-laser deposition. The crystalline quality and microstructures of these heterostructures are investigated by X-ray diffraction technique. The effects of strain on the ferroelectric, magnetic and magnetoelectric coupling properties of these thin films are systematically studied. The results show that the strain effect induced by lattice mismatch between the ferroelectric/ferromagnetic layers plays an important role in the ferroelectric and magnetic properties of these composite films. Compared to the strained Pb(Zr0.52Ti0.48)O3/ CoFe2O4/Pb(Zr0.52Ti0.48)O3 heterostructure, improved ferroelectric properties with an out-of-plane polarization (2P(r)) of 34.2 microC/cm2 and electric coercivity field of 158 kV/cm are obtained in the strain-free Pb(Zr0.52Ti0.48)O3/NiFe2O4/Pb(Zr0.52Ti0.48)O3 heterostructure. The ME measurement results not only show that the strain induced by lattice mismatch has great influence on the ME behavior, but also provide an understanding of the multilayers with full control over the interface structure at the atomic-scale.     

Film optimization of laminated antiferromagnetically coupled media

Longitudinal media with multiple isolated magnetic layers (laminated media) have been shown to have a significant signal-to-noise ratio (SNR) advantage over conventional media. However, the application of laminated media has been hindered by reduced overwrite and wider magnetic pulsewidth compared to conventional media. Some of the major causes for such degradation in recording properties are poor writing of transition in the magnetic layer farther from the head and an offset in the transition position in the multiple magnetic layers resulting from the decrease in head field magnitude with spacing. We find that the transition writing and transition alignment in the multiple magnetic layers of the laminated antiferromagnetically coupled (AFC) media can be optimized by adjusting the magnetic anisotropy of the relevant magnetic layers to compensate for the reduction of the head field magnitude with spacing. This optimization leads to significant improvements in media recording performance, such as an increase of overwrite, reduction of magnetic pulsewidth, and further increase of SNR. Such adjustment should also be applicable to laminated conventional (nonAFC) media.