Antiparallel Magnetization Configurations Research Articles

Giant magnetoresistance effect in a magnetic-electric barrier structure

A spin-independent giant magnetoresistance effect is demonstrated in a magnetically modulated two-dimensional (2D) electron gas, which can be realized by depositing two parallel ferromagnets on top of the heterostructure. The transmission for the parallel and antiparallel magnetization configurations shows a quite distinct dependence on the longitudinal wave vector of the incident electrons. Such a discrepancy results in a tremendous magnetoresistance (MR) ratio, which can be up to ${10}^{6}%$ for a realistic electron density. It is confirmed that the MR ratio can be further tuned by the inclusion of an electric barrier. In addition, studies indicate that the structure with antiparallel magnetization arrangement can be used to collimate the 2D electron beam.

Effect of disorder on the tunnel magnetoresistance: Lattice Green’s function method

The influences of random disorder in the barrier on tunnel conductance (TC) and magnetoresistance (TMR) in ferromagnet (FM)/insulator (semiconductor) (I(S))/FM junctions are investigated theoretically taking into account spin-orbit (SO) interaction. The TMR decreases significantly due to the spin-flip scattering (SF) caused by the SO interaction. We have found that the SF scattering is much stronger for anti-parallel configuration of magnetization and the TMR can even be inverted when the SF scattering is beyond some critical value. The numerical calculations are performed within the single-orbital tight-binding model using the recursive Green’s function method based on the extended Landauer–Büttiker formula.

Advanced synthetic ferrimagnetic media (invited)

The exchange coupling field (Hex) in two-layer synthetic ferrimagnetic media (SFM) is enhanced to extend the areal density capability of earlier structures. The thermal stability of SFM is improved by employing high magnetic anisotropy stabilizing layers without significantly affecting the required write fields. However, this necessitates larger Hex values than were earlier realized, to preserve the antiparallel layer magnetic configuration at remanence. Hex is improved by inserting thin high Co content hcp layers, called “E-layers,” between the Ru layer and the magnetic layers. Large Hex values from 1 to 4 kOe are obtained depending on the E-layer composition and thickness. As the intergranular coupling is affected by the Co-rich E-layers, a method is provided that improves Hex to values >1 kOe without degrading read-write properties.

Lorentz transmission electron microscopy and magnetic force microscopy characterization of NiFe/Al-oxide/Co films

Magnetization reversal process of NiFe/Al-oxide/Co junction films was observed directly using Lorentz transmission electron microscopy (LTEM) and magnetic force microscopy (MFM). In situ magnetizing experiments performed in both LTEM and MFM were facilitated by a pair of electromagnets, which were mounted on the sample stages. A two-stage magnetization reversal process for the junction film was clearly observed in LTEM with NiFe magnetization reversed first via domain wall motion followed by Co magnetization reversal via moment rotation and domain wall motion. Reversal mechanism and domain characteristics of the NiFe and Co layers showed very distinctive features. The magnetization curve of the junction film measured using alternating gradient force magnetometry showed a nonzero slope at the antiparallel magnetization configuration region, which implies that magnetization directions of the NiFe and Co layers were not exactly antiparallel due to Co moment rotation existed in that region. After the magnetization reversal of the Co was complete, MFM images revealed some magnetic contrast, which suggests that an out-of-plane magnetization component remained in the Co layer. Such magnetic contrast disappeared at higher magnetic fields when the Co moments further rotated and aligned parallel to the applied field direction.

ＡＦＣ媒体の記録再生シミュレーション

A read/write simulation based on the Poisson equation by a finite element method was performed on antiferromagnetically coupled (AFC) magnetic media composed of two laminated recording layers and a thin intervening Ru layer, and a thin-film head system. To estimate the thermal stability of the AFC media, a new medium model that introduces interlayer exchange coupling as proposed. This model explains well the ovserved minor loop shifts to the first quadrant in M-H loops due to the interlayer exchange coupling. However, the recorded magnetizations calculated by using this model are found to be in antiparallel magnetization configurations that are independent of the interlayer exchange coupling, and the thermal stability of the AFC media is found to be much improved in comparison with that of the conventional medium. This suggests that within the parameters used in the present simulation, the thermal stability of the AFC media is mainly enhanced by the magnetostatic interlayer coupling.

Magnon excitation of CoFe/Al-oxide/CoFe ferromagnetic tunnel junctions

Inelastic-electron-tunneling (IET) spectroscopy has been applied to investigate the magnon-induced inelastic tunneling process for Ta/Ni 80Fe 20/Cu/Ni 80Fe 20/IrMn/Co 75Fe 25/Al-oxide/Co 75Fe 25/Ni 80Fe 20/Ta ferromagnetic tunnel junctions. For the junction with the oxidation time, t ox =40 s , which is the exact time to oxidize the 8 Å Al, the subtraction spectrum of the IET spectra between the parallel and anti-parallel magnetization configurations clearly showed two peaks at ±3 and ±16 mV showed. On the other hand, the spectrum for the junction with t ox =120 s showed a broad peak at around 18 mV and a plateau at zero-bias. This was caused by the change of the correlation length of magnon inelastic excitation due to the over oxidation of the bottom electrode.

Magnon-assisted inelastic excitation spectra of a ferromagnetic tunnel junction

Inelastic-electron-tunneling spectroscopy (IETS) has been applied to investigate the spin dependent tunneling process for a Ta(50 Å)/Fe20Ni80(60 Å)/IrMn(300 Å)/Co(60 Å)/Al(13 Å)-oxide/ Co(40 Å)/Fe20Ni80(200 Å) spin-valve-type tunnel junction. IET spectra for both parallel and antiparallel magnetization configurations of ferromagnetic electrodes showed a strong peak at 12 mV. The subtraction spectrum defined by the difference between the spectra of both the configurations was calculated. Spin-independent inelastic excitation processes are not affected by an external magnetic field, and thus, the subtraction spectrum indicates the inelastic modes induced only by the magnetic origin. It showed a strong peak at 12 and 20 mV for the positively and negatively biased direction of the bottom electrode, respectively. The tendency of the tunneling magnetoresistance ratio to decrease with bias voltage agreed with the shape of the subtraction spectrum. By assuming the surface magnon excitation, we obtained the distributions of the correlation length and the Curie temperature for both ferromagnetic electrode surfaces faced on the insulator.

Voltage dependence of giant tunnel magnetoresistance in triple barrier magnetic systems

A quantum theory of the dependence on bias voltage of the tunnel magnetoresistance in a triple barrier system of the form MOMOMOM is presented. The Ms represent magnetic metallic layers and the Os are thin tunnel barriers. The two inner layers form spin-dependent quantum wells. The relative orientation of the magnetization in the successive magnetic layers can be changed from parallel to antiparallel. For a particular thickness of the inner metallic layers, a very large change of resistance occurs between the parallel and antiparallel magnetic configurations due to the spin dependence of the resonant tunnelling in these layers. It is shown that oscillations in the voltage dependence of the magnetoresistance amplitude take place associated with oscillations between resonant and antiresonant tunnelling as a function of the electrons' energy.

A low-temperature ultrahigh vacuum scanning tunneling microscope with a split-coil magnet and a rotary motion stepper motor for high spatial resolution studies of surface magnetism

We present the design of a new ultrahigh vacuum scanning tunneling microscope (STM) which operates at T<20 K inside the bore of a 2.5 T superconducting split-coil magnet. The tip/sample region can easily be controlled visually, thus allowing safe and fast exchange of samples and tips while the microscope stays at low temperatures. A newly developed rotary motion stepper motor is presented which allows rotation of the sample by >270° about an axis perpendicular to the tip axis. This feature allows metal or molecular beam evaporation normal to the sample surface. Even more important, by means of this device tip and sample can be brought into a parallel or antiparallel magnetic configuration thus opening a novel approach to the study of magnetic phenomena on an atomic length scale. In addition, measurements of the magneto-optical Kerr effect can be carried out without removing the sample from the STM. Also a new tip exchange mechanism is described. The microscopic and spectroscopic performance of the new instrument is illustrated on Au(111)/mica, on Tb(0001)/W(110), and on Gd(0001)/W(110).

Ｃｏ／Ａｌ／Ａｌ‐Ｏｘｉｄｅ／Ｃｏ接合界面のマグノン非弾性励起

Inelastic-electron-tunneling spectroscopy (IETS) was used to investigate the electron states of the Co/Al(dAl)/Al-oxide/Co interface in ferromagnetic tunnel junctions. The IET spectra for both parallel and anti-parallel magnetization configurations of ferromagnetic electrodes showed a strong positive peak for dAl = 0 A. The subtraction spectrum defined by the difference between the spectra of the two configurations was calculated. The subtraction spectrum indicates only a magnon-assisted inelastic tunneling process. The tendency of the tunneling magnetoresistance (TMR) ratio to decrease with bias voltage agreed with the shape of the subtraction spectrum. By assuming surface magnon inelastic excitation, we obtained the distributions of correlation length and Curie temperature for both ferromagnetic electrode surfaces on the insulator. If an Al layer was inserted between a ferromagnet and an insulator, the subtraction spectrum showed an asymmetry to the bias voltage and magnon inelastic excitation at the interface decreased.

Giant magnetoresistance effect of spin valve Co/Pd/Co/NiO

The possibility of the giant magnetoresistive (GMR) effect of Co/Pd multilayer was studied based on the prediction of the first principle band calculation. Anti-parallel magnetization configuration was achieved by fabricating spin valve structure, although the parallel magnetization is stable in the Co/Pd multilayer. Spin valve structures of Co/Pd/Co/NiO and Co/Cu/Co/NiO were fabricated on the glass substrate by dual ion beam sputtering. Anti-ferromagnetic NiO films were prepared at room temperature by oxygen reactive sputtering. The metal layers were deposited on the oxide layer subsequently. It is found that there exist a hexagonal phase and well-known cubic phase of NiO by analyzing XRD pattern. The exchange field of Cu/Co/hexagonal-NiO trilayer is larger than that of Cu/Co/cubic-NiO trilayer. The increase of the exchange field is proportional to the relative oxygen concentration. The GMR effect appeared in spin valve structure of Co/Pd/Co/hexagonal-NiO above critical Pd thickness, although GMR ratio is not so large. Anti-parallel magnetization configuration induced by a strong exchange field from NiO leads the GMR effect in the Co/Pd.

Oscillating tunneling magnetoresistance in magnetic double-tunnel-junction structures

Based on an extended Slonczewski model with double $\ensuremath{\delta}$-type potential barriers, we study spin-dependent resonant tunneling conductances in a double-tunnel-junction structure, in which two ferromagnetic electrodes are separated from a middle nonmagnetic layer of thickness a by two thin insulating layers, respectively. It is shown that as the thickness a is increased, the tunneling magnetoresistance, along with the tunneling conductance for parallel and antiparallel magnetization configurations, exhibits amplitude-varying oscillating behavior with a period of $a=\ensuremath{\pi}{/k}_{F} {(k}_{F}$ being the Fermi wave vector). This abnormal phenomenon is found to stem from the spin-dependent resonant transmission of electrons passing through the double-tunnel-junction structure.

Exchange-biased magnetic tunnel junctions fabricated with in situ natural oxidation

Exchange-biased magnetic tunnel junctions with a Ta/NiFe/FeMn/NiFe/Al–oxide/NiFe/Ta structure have been fabricated. The tunnel barrier was formed by the in situ natural oxidation of an Al metal layer under controlled oxygen pressure. Photolithography and ion milling were used to pattern the multilayer into junction structures of 2×2 μm2–20×20 μm2 dimensions. Magnetoresistance (MR) curves show spin-valve-like characteristics, in which an antiparallel configuration of magnetizations in both ferromagnetic layers is observed between 50 and 240 Oe, and the hysteresis loops for both the free and pinned layers exhibit sufficient separation. An evaluation of the MR curves shows the exchange-bias field to be 340 Oe and coercivity levels in the free layer to become as low as 13 Oe. At room temperature normalized junction resistance is 2×10−5 Ω cm2, with MR ratios still being maintained at 13%. This resistance value is much lower than previously reported values for junctions produced either with plasma oxidation or thermal oxidation in air. Maximum variation in junction resistance is only ±5% for 10×10 μm2 junctions over a 2 in. wafer. The MR ratio decreases by half when the bias voltage is raised from 0 to 440 mV, approximately the same ratio of decrease as has been previously reported for other successful junctions.

Effect of ferromagnetic layer structure on the magnetization process in NiFe/Co/Al–O/NiFe and Co/NiFe/Al–O/NiFe junctions

The effect of ferromagnetic layer structure on the magnetization reversal processes in (NiFe/Co)/Al–O/NiFe (S1) and (Co/NiFe)/Al–O/NiFe (S2) films was investigated. The films were fabricated by sputter deposition, and the Al–O layer was prepared by oxidizing an Al layer in air. Two distinct magnetization processes were observed by transmission Lorentz microscopy (TLM) with increasing in situ applied field. Reversal of the NiFe/Co bilayer in S1 occurs via moment rotation, while reversal of the Co/NiFe bilayer in S2 occurs by domain wall motion, in both cases at higher field than the top NiFe layer. The difference can be ascribed to the difference in the deposition order of the ferromagnetic bilayers (FMBs). High resolution electron microscopy shows that the grains in the top NiFe layer are randomly oriented in both films. In S1, the NiFe grains in the FMB are randomly oriented, with columnar grains present in the Co. In S2, a columnar grain structure of NiFe in the FMB is induced by the Co underlayer. The rough FMB/Al–O interface in S2 leads to weak biquadratic interlayer coupling, which will slightly reduce the field range of the antiparallel magnetization configuration. Hysteresis loops of S1 and S2 show two stage magnetization reversals in each sample, which are consistent with TLM results.

Spin-valve films using synthetic ferrimagnets for pinned layer

The magnetic behavior of spin-valve films using Co(t/sub 1/)/Ru(0.8 nm)/Co(t/sub 2/) synthetic ferrimagnets for the pinned layer were investigated, in which the Co(t/sub 2/) layer is exchange-biased by a CrMnPt antiferromagnetic layer. As compared with experimental results, the magnetic behavior of Co/Ru/Co/CrMnPt films could be qualitatively explained on the basis of a coherent rotation model and was roughly classified into two cases according to the thickness of t/sub 1/ and t/sub 2/. For t/sub 1/=t/sub 2/ the magnetization of the two Co layers changed the direction only in gentle rotation mode, which resulted in very large pinning fields. For t/sub 1//spl ne/t/sub 2/ the direction of the magnetization of the two Co layers rapidly reversed, maintaining an antiparallel magnetic configuration, at a relatively small applied field. The field where the rapid magnetization reversal was caused agreed with the exchange-bias field in the spin-valve film with a single Co pinned layer whose thickness was t/sub 1/-t/sub 2/.

Anisotropy and angular variation of the giant magnetoresistance in magnetic multilayers (invited)

The giant magnetoresistance (GMR) of magnetic multilayers is usually considered as isotropic, i.e., independent of the direction of the sensing current with respect to the applied field. In spin-valve samples of the form NiFe/Cu/NiFe/FeMn it is possible to accurately determine the amplitude of the GMR (without any contribution from the usual anisotropic magnetoresistance) for various direction of the current with respect to the direction of the magnetization of the two ferromagnetic layers, both in the parallel and antiparallel magnetic configurations. In three series of spin-valve samples of the composition F tF/Cu tCu/NiFe/FeMn, we have observed that the GMR amplitude is larger when the current is perpendicular to the magnetizations than when it is parallel to it. This intrinsic anisotropy in the GMR shows a pronounced maximum (relative amplitude of the anisotropy of the order of 10% at the maximum) for a thickness of the ferromagnetic layer of the order of 150 Å. In contrast, this anisotropy depends very weakly on the nonmagnetic spacer layer thickness. The results are compared with semiclassical calculations of Rijks et al. [Phys. Rev. B 51, 283 (1995)]. On another respect, we have measured the in-plane (CIP) and perpendicular to the plane (CPP) giant magnetoresistance of antiferromagnetically coupled (NiFe/Ag) multilayers. Particular attention has been paid on the variation of resistivity with the angle Δθ between the magnetization in the successive magnetic layers. While the CIP GMR varies almost linearly with cos(Δθ), the CPP GMR shows strong deviations from linearity especially at large NiFe thicknesses. The results are discussed in terms of relative role of s-like and d-like electrons in CIP and CPP transport.

Analysis of magnetization structure by calculating magnetostatic energy for multilayered film

The magnetization structure of multilayered magnetic films which are the stacking of magnetic layers at intervals of nonmagnetic layers, is analyzed by calculating the magnetostatic energy. The antiparallel magnetization configuration of the magnetic layer reduces magnetostatic energies. However, the irregular structure, such as an insertion of a pair of parallel magnetization arrangement which often appears experimentally, does not always increase a magnetostatic energy very much, compared to the antiparallel regular structure. The experimental magnetization structure of multilayered film can be understood by the magnetostatic energy.

An MHD simulation of the interaction of the solar wind with the outflowing plasma from a comet

The interaction between the solar wind and the outflowing plasmas from a comet has been studied by using a two‐dimensional time‐dependent magnetohydrodynamic (MHD) simulation. The model reproduced several features of the comet‐solar wind interaction predicted by earlier theories and observed on the recent cometary probes. These include the formation of the contact surface and the cometary magnetotail. For a constant interplanetary magnetic field (IMF) the cometary plasma captures field lines which drape over the comet to form an antiparallel magnetic field configuration in the tail and a thin plasma sheet. Eventually, tail magnetic reconnection begins to occur at several points. When the IMF orientation is reversed dayside magnetic reconnection occurs at the subsolar point and a large disturbance propagates down the tail.