Single Free Layer Research Articles

비정질 CoFeSiB 단일 및 합성형 반강자성 자유층을 갖는 자기터널접합의 자기저항 효과

To obtain low switching field () we introduced amorphous ferromagnetic single and synthetic antiferromagnet (SAF) free layers in magnetic tunnel junctions (MTJs). The switching characteristics for MTJs with structures 45/Ru 9.5/IrMn 10/CoFe 7/AlOx/CoFeSiB 7 or CoFeSiB (t)/Ru 1.0/CoFeSiB (7-t)/Ru 60 (in nm) were investigated and compared to MTJs with and free layers. CoFeSiB showed a lower saturation magnetization of and a higher anisotropy constant of than CoFe and NiFe, respectively. An exchange coupling energy () of was observed by inserting a 1.0 nm Ru layer in between CoFeSiB layers. In the CoFeSiB single and SAF free layer MTJs, it was frond that the size dependence of the originated from the lower experimentally and by micromagnetic simulation based on the Landau-Lisfschitz-Gilbert equation. The CoFeSiB SAF structures showed lower than that of NiFe, CoFe and CoFeSiB single structures. The CoFeSiB SAF structures were proved to be beneficial far the switching characteristics such as reducing the coercivity and increasing the sensitivity in micrometer to submicrometer-sized elements.

Improved selectivity of synthetic anti-ferromagnetic free Layer in high-density MRAM array

We investigate switching characteristics of the patterned submicrometer magnetic tunnel junction (MTJ) cells incorporating with a NiFe single and a synthetic anti-ferromagnetic (SAF) as a free layer. The MTJs with single NiFe free layer show the increased probability of vortex appearance with increase of NiFe thickness. Even though the free layer thickness is decreased, single NiFe free layer shows large variation of switching field. However, the employment of the SAF free layer dramatically improves switching characteristics with the suppression of vortex states and single domain-like tendency.

The switching behaviors of submicron magnetic tunnel junctions with synthetic antiferromagnetic free layers

The switching behaviors in elliptic shaped (aspect ratio=2) submicron magnetic tunnel junctions using CoFeB single free layer and CoFeB∕Ru∕CoFeB synthetic antiferromagnetic (SAF) free layers are studied. It is found that under considerable stray fields originating from pinned layers, junctions with single free layer show complex switching behaviors with larger Hc variations. In contrast, junctions with SAF free layers exhibit kink-free R‐H loops and less Hc variations. The Hc of junctions with SAF free layers is less dependent on the junction size than that with a single free layer. Furthermore, for junctions smaller than a critical size the SAF free layers have a smaller Hc than single free layers.

Suppression of bias voltage dependence in double-barrier magnetic tunnel junctions comprised of freelayers with an amorphous layer insertion

A double-barrier magnetic tunnel junction (DMTJ) comprising an amorphous freelayer was investigated. The typical sample structure consists of Ta42∕CoZrNb9.2∕IrMn15.5∕CoFe3.3∕AlOx2∕freelayer∕AlOx2∕CoFe7.5∕IrMn15.5∕Ru60(nm). The freelayer includes CoFe2.3∕CoZrNb3.4∕CoFe3. A CoFe 5nm single freelayer was also prepared for reference purpose. Although TMR ratio and RA product appear similar, less attenuation in the TMR ratio was observed at 0.4V for the junction with CoZrNb layer. Coercivity and interlayer coupling were reduced while squareness was improved. The top portion of the DMTJ including top barrier became more uniform after amorphous layer insertion.

Switching field distribution in magnetic tunnel junctions with a synthetic antiferromagnetic free layer

By replacing the traditional single magnetic free layer in magnetic random access memory (MRAM) with a synthetic antiferromagnetic (SAF) layer composed of two antiferromagnetically coupled layers, the possibility of the kink generation has been suppressed to a minimum level and the normalized free-layer shift has been reduced from 15% to less than 5%. These phenomena are thought to be from the reduction of the effective thickness and magnetic moment by introducing a SAF free layer. Since the SAF free layer decreases the magnetostatic interaction with the pinned layer, free-layer shift is also decreased. A SAF free layer forms a closed magnetic loop between two antiferromagnetically coupled layers. Therefore, it increases a tendency to become a single domain and its switching field distribution is enhanced. With the optimized SAF free layer, array quality factor [AQF, σ(Hc)∕Hc] is increased to more than 10 and switching window could be obtained which is the area to be selectively switched in the memory cell array. When adopted in a 64-kb MRAM, it is confirmed that cells can be selectively switched without the disturbance to the other cells.

Reduction of NÉel Coupling in Magnetic Tunnel Junctions Containing an Artificial Ferrimagnet in the Free Layer

Magnetic tunnel junctions containing an artificial ferrimagnet (AFi) in the free layer are investigated. A lower offset field for the free layer is found compared to tunnel junctions with a single free layer of equal net magnetic moment. A reduction in Ne/spl acute/el coupling up to 36% is achieved. The Ne/spl acute/el coupling is interpreted in terms of the roughness parameters of the tunnel barrier. The combination of an AFi in the free layer with a robust exchange-biased hard layer, including an artificial antiferromagnet, leads to even lower values for the offset field. The sign of the offset field depends on the order of the layers inside the (free) AFi. The coupling strength of the AFi is further investigated in terms of layer thicknesses.

Spin flop switching for magnetic random access memory

By replacing the traditional single magnetic free layer in magnetic random access memory (MRAM) with a free layer composed of two antiferromagnetically coupled layers which switch by spin flop switching, the half select and activation energy problems that have hindered progress in MRAM to date can be essentially eliminated. By analytically and numerically solving a single domain model, the theory of spin flop switching for MRAM is developed here, including a discussion of the various types of easy axis hysteresis loops, the critical switching curve as a function of word and bit line fields, the role of thickness asymmetry, and the dependence of activation energy on applied field. The theory is developed for arbitrary thickness, length, width, magnetization, intrinsic anisotropy, and exchange coupling. Exact analytic formulas are given for the relevant switching fields and the activation energy. In particular, it is shown that the field required to switch the bit under half select can be many times larger than the field required for full select, and furthermore that the activation energy initially increases under application of a half select field.

スピンバルブ構造をもつ極薄ＣｏＦｅフリー層の磁歪

An ultrathin CoFe free layer is very important for future high density recording head such as those using specular spin-valve films, because a CoFe free layer gives a higher MR ratio than a conventional NiFe/CoFe free layer. In order to realize a single CoFe free layer, we investigated the magnetostriction (λs) of an ultrathin CoFe free layer in spin-valve films. We compared the magnetostriction dependence on the underlayer structure among Cu, Ru, and Ru/Cu underlayers on a Ta buffer layer. For an Ru/Cu double-layered underlayer, λs becomes positively large when the Ru underlayer thickness is increased; on the other hand, it becomes negatively large when the Cu underlayer thickness is increased. This tendency can be explained by a model in which the λs is changed by the lattice elastic strain, where the volume ratio of Ru/Cu changes the lattice strain of an ultrathin CoFe free layer. The sensitivity of λs to the Cu underlayer thickness becomes large with increasing Co concentration in the CoFe free layer. This indicates that λs of a CoFe free layer becomes sensitive to the elastic strain with increasing Co concentration.

Spin-filter spin-valve films with an ultrathin CoFe free layer

The concept of spin-filter spin-valve (SFSV) films is reviewed. The dependence of the free layer structure on the spin-filter effect was investigated by using model films and two types of free layers were compared, a single CoFe layer and a conventional NiFe/Co free layer. At the same magnetic thickness of the free layer, the SFSV films with a CoFe free layer showed a larger magnetoresistance (MR) ratio than those with a NiFe/Co free layer. This is partly attributed to the thinner CoFe free layer thickness, which is due to the fact that the CoFe free layer has higher Bs than the NiFe/Co free layer. Moreover, SFSV films with a CoFe free layer still showed a larger MR ratio when the free layer thickness was the same. It suggests that other factors contribute to the high MR performance, such as the quality of the interface between a free layer and a high conductance layer. Film performance of MR 9% to 10%, ΔRs 1.5–2.0 Ω, Hcea∼3 Oe, and λs⩽±0.5 ppm was obtained with a single CoFe free layer and synthetic antiferromagnetic pinned structure.

ＴＭＲ素子の磁化反転磁界抑制の検討

Two types of laminated structures for free layers were investigated to avoid the increase in the switching field of the free layer caused by the reduction of the size of the TMR element. Laminated free layers of NiFe(5)/Ru(0.7)/NiFe(3) and NiFe(4)/Ta(3)/NiFe(4) showed a lower switching field than a single free layer of NiFe(8). The low switching field of the synthetic free layer is due to the decrease in the net moment, which results in a decrease of the demagnetizing field. In the case of the NiFe(4)/Ta(3)/NiFe(4) free layer, there is a possibility that magnetostatic coupling plays an important role in the decrease in the demagnetizing field.