Unidirectional Anisotropy Field Research Articles

Ａｌ／ＮｉＰディスク上に作製しためっき軟磁性裏打ち層の磁気特性と単磁区化

We propose a noble stacked soft magnetic underlayer (SUL) with the so-called top-type pinning structure, which consists of a soft magnetic layer (SM layer) fabricated by plating and an antiferromagnetic layer (AFM layer) deposited by sputtering, on an Al/NiP disk. Through investigation of the structure and magnetic properties of this stacked SUL, we clarified the following points: (1) the thermal stability of NiFeP plated on an Al/NiP disk is 280°C, which is the crystallization temperature of the amorphous NiP layer; (2) to induce large unidirectial magnetic anisotropy in the SUL, it is necessary to introduce a soft magnetic buffer layer and apply dry etching to the NiFeP surface; (3) a unidirectional magnetic anisotropy field can be induced in an SM layer fabricated by plating as well as in one fabricated by sputtering, which results in the whole disk having a single-domain structure.

Local Variation of Magnetic Parameters of the Free Layer in TMR Junctions

Local M-H loops have been measured on the free layer of a tunneling magnetoresistance (TMR) junction using the magneto-optical Kerr effect (MOKE) system, with an optical beam size of about 2 <TEX>$\mu$</TEX>m diameter. Tunnel junctions were deposited using the DC magnetron sputtering method in a chamber with a base pressure of 3<TEX>$\times$</TEX>10<TEX>$^{-9}$</TEX> Torr. The relatively irregular variations of coercive force H<TEX>$_c$</TEX>(∼17.5 Oe) and unidirectional anisotropy field H<TEX>$_{ua}$</TEX>(∼7.5 Oe) in the as-deposited sample are revealed. After <TEX>$200{^{\circ}C}$</TEX> annealing, He decreases to 15 Oe but H<TEX>$_{ua}$</TEX> increases to 20 Oe with smooth local variations. Two-dimensional plots of H<TEX>$_c$</TEX> and H<TEX>$_{ua}$</TEX> show the symmetric saddle shapes with their axes aligned with the pinned layer, irrespective of the annealing field angle. This is thought to be caused by geometric effects during deposition, together with a minor annealing effect. In addition, the variation of root mean square (RMS) surface roughness reveals it to be symmetric with respect to the center of the pinned-layer axis, with the roughness of 2.5 <TEX>$\AA$</TEX> near the edge and 5.8 <TEX>$\AA$</TEX> at the junction center. Comparison of surface roughness with the variation of H<TEX>$_{ua}$</TEX> suggests that the H<TEX>$_{ua}$</TEX> variation of the free layer is well described by dipole interactions related to surface roughness. As a whole, the reversal magnetization is not uniform over the entire junction area and the macroscopic properties are governed by the average sum of local distributions.

Temperature dependence of anisotropy in exchange biased Fe/KCoF3 system

We used molecular beam epitaxy to deposit a novel ferro-/antiferromagnet (Fe/KCoF3) system on gallium terminated GaAs (100) substrates. We varied the thicknesses of single crystal Fe (001) layers from 1.05 to 3 nm. The antiferromagnetic fluoride, with a thickness of 30 nm, was deposited either in a single-crystal or a polycrystalline form, depending on the deposition conditions. KCoF3 is an antiferromagnet with a Néel temperature of 114 K. Its cubic structure almost perfectly matches the Fe film structure. The growth was monitored by reflection high energy electro diffraction. The magnetic properties of the system were studied using ferromagnetic resonance. Uniaxial and unidirectional anisotropy fields, due to exchange bias, were measured at low temperatures in the field-cooled samples and were smaller than 45 and 72 Oe, respectively. Both anisotropy fields, unidirectional and uniaxial, decreased with increasing thickness of the Fe film or with increases in temperature. The temperature dependence of the fourfold anisotropy was studied for the samples with different thicknesses of Fe films and different crystalline states of the fluoride layer. Room temperature values of the fourfold anisotropy also increased with increasing thickness of the Fe layer and were in a fairly good agreement with previously reported values for single crystal Fe films. Also, the temperature characteristics of the fourfold anisotropy for the structures with single crystal fluoride seemed to reproduce low-temperature literature data for Fe (001) films. In contrast, the structures with polycrystalline KCoF3 demonstrated a significant enhancement of the fourfold anisotropy at low temperatures. For the thinnest sample (tFe=1.05 nm) the low temperature anisotropy is more than tripled compared to room temperature values. It is believed that the surface effects can be responsible for this enhancement.

Unidirectional anisotropy in NiO/Co and NiO/Ni80Fe20 layered structures

NiO/Co and NiO/Ni80Fe20 bilayers were prepared at 293 K onto SiO2/Si(111) and glass substrates using UHV RF/DC magnetron sputtering. Results on ex-situ magnetic measurements showed that the unidirectional anisotropy field is inversely proportional to the Co (Ni80Fe20) layer thickness. A maximal RT coupling energy for the NiO−Co and NiO−Py interface was estimated as 0.04 and 0.03 mJ/m2, respectively, for the samples prepared onto SiO2/Si(111) substrates. The above behaviour is consistent with greater interface roughness and/or interdiffusion for the samples prepared onto glass substrates.

Magnetic parameters of diluted magnetic semiconductors CdCr 2Se 4

Magnetic semiconductors of CdCr 2Se 4 diluted by indium belong to the class of soft magnetic materials. The magnetic parameters: magnetization M and unidirectional magnetic anisotropy field H an, and also field-cooled and zero field-cooled DC magnetization and the lattice parameter d are affected by the dilution level of indium. We have found from our experimental data, that these parameters depend on any one of the elements Cd or Cr that is substituted by In.

Exchange anisotropy in NiFe/FeMn bilayers studied by planar Hall effect

Planar Hall effect (PHE) in NiFe/FeMn films was experimentally measured and simulated using Boltzmann transport equation. PHE was found to be an effective method to characterize the exchange anisotropy in NiFe/FeMn films, from which the unidirectional exchange field and the magnetization reversal process of the sample can be determined. The effective uniaxial anisotropy field H Keff, the effective unidirectional anisotropy field H ua and the AF domain wall energy H w were obtained by fitting the experimental results. We found that in the NiFe/FeMn system, the parameters H Keff, H ua and H w had the same values in reversible and irreversible measurements, and the domain wall energy in AF layer was much larger than interfacial unidirectional anisotropy.

Planar Hall effect in NiFe/NiMn bilayers

The exchange anisotropy in NiFe/NiMn bilayers was studied by using the planar Hall effect. The sputtered NiFe/NiMn films were patterned into strips of 1 mm in length and 200 μm in width and with six terminals for anisotropy magnetoresistance and planar Hall voltage measurements by a photolithographic process. It is shown that the planar Hall effect is an effective method to characterize the exchange anisotropy in ferromagnetic/antiferromagnetic (AF) systems. It can be used to accurately determine the exchange field and describe the magnetization reversal processes. The effective uniaxial anisotropy field HK eff, the effective unidirectional anisotropy field Hud, and AF domain wall energy Hw can be obtained by fitting the experimental results. We found that in the NiFe/NiMn bilayer system, the parameters HK eff, Hud, and Hw have the same values in reversible and irreversible measurements, and the domain wall energy in AF layer is larger than interfacial unidirectional anisotropy.

A 90° ferromagnetic layer coupling in FM/AFM/FM structures

The magnetic anisotropy in the samples of Ta (5 nm)/NiFe (8 nm)/FeMn ( t AF)/NiFe (8 nm)/Ta (5 nm) with t AF=0–30 nm were studied by using a vibrating sample magnetometer (VSM) and ferromagnetic resonance (FMR). Unidirectional anisotropy field ( H B) and uniaxial anisotropy field ( H A) having strength comparable with literatures and easy axes in the direction of the field during deposition were observed in the bottom Py layers, a smaller H B with easy axis parallel to the deposition field in the top Py layer was also observed. An interesting finding was that an additional uniaxial anisotropy in the top Py layer appeared with the easy axis perpendicular to the direction of field during deposition. A possible explanation for the 90° coupling between the bottom and top Py layers through the intervening antiferromagnetic FeMn film is considered to be interface roughness.

Exchange Anisotropy in NiO/Co Bilayers

40 nm-NiO/dco-Co bilayers (0 < dco < 40 nm) were prepared onto SiO 2 (101)/Si(100) and glass substrates using UHV RF/DC magnetron sputtering. Results showed that the exchange coupling effects, i.e. unidirectional anisotropy field, HS (shift of the hysteresis loop), and broadening of the hysteresis loop, ΔH,, are inversely proportional to the Co thickness down to 2 nm. Furthermore, no significant difference in HS and ΔH, was observed between as deposited and field cooled bilayers. Practically no contraction of the successive hysteresis loops (training effects) was also observed at room temperature. On the other hand, the interface exchange coupling effects strongly depend on the preparation conditions. A maximal coupling energy for the NiO—Co interface was estimated as 0.04 mJ/m2 at room temperature for the samples prepared onto SiO2(101)/Si(100) substrates.

Effect of annealing field on asymmetric giant magnetoimpedance profile in Co-based amorphous ribbon

We prepared weak-field annealed amorphous samples in the open air and present the change of giant magnetoimpedance (GMI) profiles as functions of annealing field, H a. The profile for 0.1 MHz measuring frequency shows two typical peaks in the sample annealed in a field of H a<50 mOe. Increasing H a to 500 mOe, GMI peak in the region where the applied field is antiparallel to the annealing field decreases with H a, showing an asymmetric GMI profile. As the annealing field increases over 500 mOe, the GMI in the antiparallel field region disappears and a GMI-valve in the parallel field region is revealed due to the exchange coupling of domains in amorphous core with the unidirectional anisotropy field in a crystalline layer. However, in the sample with H a of 15 Oe, the GMI peak in the antiparallel region is the observable again.

Magnetic semiconductor thin films of CdCr 2−2 xIn 2 xSe 4 in spin glass state

Diluted by indium, the chalcogenide spinel CdCr 2−2 x In 2 x Se 4 is in an SG state. The temperature dependence of the magnetization ( M) with comparison to the ordered magnetic state is modified by non-zero density of states in the energy band gap. Also, the unidirectional magnetic anisotropy is attributed to the SG state due to the Dzyaloshinskii–Moriya (DM) interaction. The investigated samples of CdCr 2−2 x In 2 x Se 4 were obtained by an RF sputtering technique. We present the temperature (from 5 to 160 K) dependence of M, unidirectional magnetic anisotropy field H an and surface magnetic anisotropy constant K s t . The magnetic parameters were determined from the temperature dependence of the ferromagnetic resonance (FMR) and spin wave resonance (SWR) data.

Exchange bias relaxation in CoO-biased films

Because the memory of the bias direction is carried by the antiferromagnetic order in exchange biased films, the stability of the antiferromagnetic order is critical to the existence of the exchange bias field. Ferromagnetic resonance was used to measure the relaxation behavior of polycrystalline CoO films coupled to films of Ni80Fe20, probing the system on the time scale of the experiment, ≈103 s, and the time scale of the magnetic precession, ≈10−10 s. Unidirectional anisotropy (exchange biasing) and isotropic resonance field shifts are observed at the lowest temperatures. Above the apparent exchange bias blocking temperature, isotropic resonance field shifts persist. At still higher temperatures, diminishing resonance field shifts are accompanied by peaks in the FMR linewidth. The results highlight the effects of varying relaxation rates in the CoO relative to the two experimental time scales.

Magnetic Anisotropy of NiMn/Ni and NiMn/Cu/Ni Thin Films

The magnetic anisotropy of reentrant spin glass NiMn thili film, in the vicinity of the ferromagnetic Ni film, is investigated using NiMn/Ni, Cu/Ni/Cu (t = 10A and 500 A)/NiMn and NiMn/Ni/Cu (t = 10 5000 A)/Ni films. The unidirectional anisotropy field of NiMn layer in NiMn/Ni film is smaller than that of bulk NiMn. In Cu/Ni/Cu (t = 10A and 500 A)/NiMn film, the unidirectional anisotropy field is almost independent of the Cu spacer thickness. These results can be explained in terms of magnetic coupling between the magnetic layers in addition to the film structure of NiMn layer.

Influence of crystal structure and oxygen content on exchange-coupling properties of IrMn/CoFe spin-valve films

Investigations were made into the relationship between the exchange-coupling properties and the IrMn crystal grain size for CoFe/IrMn spin valves. The IrMn crystal grain size increased with increasing thickness and decreasing oxygen content of the IrMn layer. The increase in the IrMn grain size resulted in a higher blocking temperature (TB) but a lower unidirectional anisotropy field (HUA). This opposite dependence of TB and HUA on the IrMn grain size is different from those of other antiferromagnetic films such as FeMn, NiMn, and CrMnPt. An HUA of 400 Oe at room temperature and a TB of 300 °C are obtained for CoFe (2 nm)/IrMn (10 nm) spin valves by controlling the oxygen content of the IrMn layer.

Magnetic anisotropy of semiconductor thin films

The temperature and composition dependence of the unidirectional magnetic anisotropy field H an for thin films of magnetic semiconductors of CdCr 2Se 4: In (state with reentrant transition) and CdCr 2 − 2 x In 2 x Se 4 (spin glass state) have been determined by FMR technique (X-band). Also the temperature dependence of surface magnetic anisotropy energy constant K s was found from SWR spectrum (X-band).

超高真空成膜によるＰｄＰｔＭｎスピンバルブ膜のＭＲ特性

We investigated the relation between the background pressure (PB.G.) of the sputtering chamber and the unidirectional anisotropy field (Hua) of spin-valve films with PdPtMn antiferromagnetic layers fabricated by an ultrahigh vacuum (UHV) sputtering process. Hua increased with decreasing PB.G. until PB.G. reached 2 × 10-7 Pa. The UHV sputtering process helped to reduce the PdPtMn thickness from 25 nm, which was needed in the conventional high-vacuum (HV) sputtering process, to under 15 nm while preserving the magnitude of Hua. We fabricated a spin-valve structure with total thickness of 26.5 nm exhibiting a large MR ratio over 8.0%, sufficient Hua (>500 Oe), and large sheet resistance change (Δρ/ttotal = 1.6 Ω).

Field-anneal effects on magnetic and magnetoresistive properties of Fe-Mn, Ir-Mn and NiO spin valves

Spin valves with a Co film as a reference layer and with antiferromagnetic Fe-Mn, Ir-Mn and NiO films as pinning layers were deposited in a DC magnetron sputtering system, and annealed in a field parallel, perpendicular or antiparallel to the easy aids of the reference layer. After anneal, their ferromagnetic coupling and unidirectional anisotropy fields, as well as anisotropy and giant magnetoresistance coefficients were characterized at room temperature. Four critical temperatures were determined from variations of these properties with anneal temperatures. These spin valves were evaluated based on these properties and critical temperatures. Approaches to improvements of these spin valves for uses as read sensors were discussed.

Effect of rapid thermal heating on the unidirectional exchange anisotropy of Ni/sub 81/Fe/sub 19//FeMn system with very thin Ni/sub 81/Fe/sub 19/ layers

The unidirectional exchange anisotropy field, H/sub ua/, of NiFe(t/sub N/iFe)/FeMn(15 nm) with very small t/sub NiFe/ from 3 nm to 25 nm was studied by heating the NiFe films using an in-situ rapid thermal heater during sputtering. A critical NiFe film thickness of about 15 nm was found. When t/sub NiFe/ was below 15 nm, the H/sub ua/ of NiFe/FeMn films was enhanced by in-situ heating, compared with that of films deposited without heating. However, no significant difference of H/sub ua/ between the films with and without in-situ heating were observed for films with NiFe film thicker than 15 nm and the dependence of H/sub ua/ on NiFe film thickness roughly obeys 1/t/sub NiFe/ dependence. X-ray diffraction examination of these films suggests that a better epitaxial growth and improved crystallinity of the /spl gamma/-FeMn(111)/NiFe(111) might be responsible for the enhanced H/sub ua/ at NiFe/FeMn interface.

Spin valve films with PdPtMn antiferromagnetic layer fabricated using ultrahigh vacuum sputtering process

We investigated the background pressure (P/sub BG/) dependence of the unidirectional anisotropy field (H/sub ua/) of spin valve films with a PdPtMn antiferromagnetic layer fabricated by an ultrahigh vacuum (UHV) sputtering process. The H/sub ua/ increased with decreasing P/sub BG/ until P/sub BG/ reached 2/spl times/10/sup -7/ Pa. The UHV deposition, such as below 2.0/spl times/10/sup -7/ Pa of P/sub BG/, improved the fct [111] texture of PdPtMn, the fcc[111] of NiFe/CoFeB/Cu/CoFeB layers, enlarged the grains of spin valve films, and reduced an absorption of residual oxygen component into the spin valve films. Consequently, the exchange coupled CoFeB/PdPtMn layers exhibited large H/sub ua/ of 620 Oe, in spite of a quite thin PdPtMn (15 nm) as an ordered antiferromagnetic material, and the spin valve films show a large MR ratio of 7.8% and a sheet resistance change of 1.6 /spl Omega/.

Evidence of superparamagnetic behaviour in an amorphous Ni 62Mn 38 film by ESR measurements

Electron spin resonance (ESR) measurements were carried out on an amorphous Ni 62Mn 38 film in the temperature range of 4–300 K. The spectra comprise multi-peak resonance absorptions forming mainly two groups separated in the magnetic field; one of which corresponds to unresolved usual spin-wave resonance (SWR) modes, the other occurs at about H=3 kG for the applied field parallel to the film surface and shows a rather less dependence on the field direction compared to those of the main SWR modes. It has been observed that the latter peak becomes more and more apparent (the amplitude intensifies and the linewidth decreases) with decreasing temperature and then starts again to broaden as the temperature is further decreased. This overall behaviour leads us to suggest that the sample consists of a large number of smaller clusters coupled weakly to each other. Macroscopic ferromagnetic state averaged over all these clusters gives us the usual SWR peaks. However, each weakly coupled cluster behaves as an almost free individual magnetic entity and therefore the latter peak can be attributed to the superparamagnetic resonance absorbtions. By fitting the theoretical SWR modes simultaneously to the corresponding experimental ones for parallel and perpendicular geometries, the exchange stiffness constant D, the exchange unidirectional anisotropy field H exc, the bulk magnetic anisotropy constant 2 K eff/ M 0 and the surface pinning parameter QL have been obtained as a function of temperature. The results have been compared with the existing data for the NiMn alloys with different compositions. The role of the local spin fluctuations (LSFs) on the temperature dependence of the magnetic parameters mentioned above has been discussed.