Distribution of blocking temperatures in nano-oxide layers of specular spin valves

Ventura, J.; Araújo, J.P.; Sousa, J.B.; Veloso, A.; Freitas, P. P.

doi:10.1063/1.2736290

Cited by 18 publications

(20 citation statements)

References 22 publications

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“…Electrical resistance and magnetoresis-tance were measured with a standard four-point dc method, with a current stable to 1 : 10 6 and applied fields of up to 8 kOe and an automatic control and data acquisition system. Temperature dependent measurements were performed in a closed cycle cryostat down to 15 K. 32 To study the training effect, several experimental procedures were used. First, we performed field cooling runs ͑from 320 K͒ under different cooling fields H 0 ͑always applied along the MnIr/CoFe exchange bias direction͒.…”

Section: Methodsmentioning

confidence: 99%

“…28 These NOL spin valves can more than double the MR ratio of simpler stacks because of specular reflection of electrons at the FM/NOL interfaces. 29 We recently showed that the pinned layer NOL is formed by CoFe oxides that have a paramagnetic/antiferromagnetic transition below room temperature ͑T B Ϸ 175 K͒ that can strongly affect transport properties, particularly MR. [30][31][32] In this work, we present a detailed study on the training effect in MnIr/CoFe/NOL/CoFe/Cu/CoFe/NOL/Ta specular spin valves by using MR͑H͒ measurements ͑in the 300-15 K temperature range͒ and different experimental procedures. The training effect was never observed in common nonspecular spin valves ͑without NOLs, but otherwise similar to the studied specular SV͒.…”

Section: Introductionmentioning

confidence: 99%

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Training effect in specular spin valves

et al. 2008

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Specular spin valves show an enhanced giant magnetoresistive ͑GMR͒ ratio due to specular reflection in nano-oxide layers ͑NOLs͒ formed by the partial oxidation of CoFe pinned and free layers. The oxides that form the ͑pinned layer͒ NOL were recently shown to antiferromagnetically order at T ϳ 175 K. Here, we study the training effect ͑TE͒ in MnIr/CoFe/NOL/CoFe/Cu/CoFe/NOL/Ta specular spin valves in the 300-15 K temperature range. The exchange bias direction between the MnIr and CoFe layers impressed during annealing is taken as the positive direction. The training effect is observed in antiferromagnetic ͑AFM͒/ferromagnetic ͑FM͒ exchange systems and related to the rearrangement of interfacial AFM spin structure with the number of hysteretic cycles performed ͑n͒, resulting in the decrease of the exchange field ͑H exch ͒. Here, in the studied specular spin valve, TE was only observed for T Ͻ 175 K and is thus related to the pinned layer NOL-AFM ordering and to the evolution of the corresponding spin structure with n. We show that FM spins that are strongly coupled to AFM domains do not align with the applied positive magnetic field ͑H͒, giving rise to a residual MR at H 0. Such nonsaturating MR will be related with a spin-glass-like behavior of the interfacial magnetism induced by the nano-oxide layer. The observed dependence of the training effect on the field cooling procedure is also likely associated with the existence of different spin configurations available in the magnetically disordered oxide. Furthermore, anomalous magnetoresistance cycles measured after cooling runs under −500 Oe are here related to induced NOL exchange bias/applied magnetic field misalignment. The temperature dependence of the training effect was obtained and fitted by using a recent theoretical model.

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Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Training effect in specular spin valves

et al. 2008

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“…Temperature-dependent measurements were performed in a closed cycle cryostat down to 20 K. 26,27 We define here the relative MTJ-resistance change ͑for parallel and antiparallel states͒ between 300 and 20 K as ␥ P,AP = ͑R 300 K P,AP − R 20 K P,AP ͒ / R 300 K P,AP , so that ␥ Ͻ 0 ͑Ͼ0͒ indicates tunnel-͑metallic-͒ like transport. 24…”

Section: Methodsmentioning

confidence: 99%

Pinholes and temperature-dependent transport properties of MgO magnetic tunnel junctions

et al. 2008

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Magnetic tunnel junctions ͑MTJs͒ with thin crystalline MgO͑001͒ barriers displaying large tunnel magnetoresistance ͑TMR͒ and low resistance-area product ͑RA͒ will likely be used as the next generation sensors in read heads of ultrahigh-density hard drives. However, the thin insulating barrier may result in the presence of metallic pinholes joining the two electrodes. Here we study the transport properties of thin MgO-based low resistance MTJs ͑barrier thickness t = 7.5 Å͒, deposited by magnetron sputtering, with RA values of ϳ40 ⍀m 2 , reaching TMR values of ϳ60-75% at room temperature. We performed temperature-dependent ͑300-20 K͒ resistance ͑R͒ measurements and observed different behaviors for different magnetic states: positive dR / dT for the parallel ͑P͒ state, attributed to the presence of pinholes in the barrier, but a mixed character in the antiparallel ͑AP͒ state, with dR / dT changing from negative to positive with decreasing temperature. This indicates an interesting competition between tunneling and metallic transport in the studied samples. To explain this transport behavior, we developed a simple model with the two conducting channels, tunnel and metallic, in parallel. The model assumes a linear variation of the electrical resistance with temperature for both conducting channels and its dependence on the MTJ magnetic state ͑P and AP͒. The modeled results show that the sign of dR / dT does not give an indication of the dominant conductance mechanism and that the crossover temperature at which dR / dT changes sign depends strongly on the linear electrical resistance-temperature coefficients. We performed fits to our experimental R͑T͒ data, using the proposed model, and observed that such fits reproduced the data quite well, illustrating the validity of the model.

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“…A gradual freezing of the interfacial frustrated spins at low T has been proposed as an explanation for the low-T sharp increase in the thermal variation in EB field, as observed in several F/AF bilayers. 5,[13][14][15] We believe that as a result of the spatial fluctuations in interfacial roughness and correlatively in frustration of exchange interactions across the F/AF interface, a distribution of J F/AF interfacial coupling also exists, which we write D͑J F/AF ͒. This other factor also contributes to the thermal dependence of exchange bias.…”

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confidence: 96%