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NiO layered structures
Journal of Magnetism and Magnetic Materials 226}230 (2001) 1764}1766
Magnetization reversal and anisotropy in CoO/permalloy/Cu/permalloy/NiO layered structures C. Prados *, E. Pina , P. Crespo , M. Alonso-San udo, F. Cebollada, J.M. GonzaH lez , A. Hernando Instituto de Magnetismo Aplicado, Universidad Complutense, P. O. Box 155, 28230 Las Rozas, Madrid, Spain Departamento de Fn& sica Aplicada, EUITT-UPM, Ctra. de Valencia km. 7, 28031 Madrid, Spain Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain
Abstract This work presents results on the temperature dependence of the hysteretic processes of glass /CoO/permalloy/Cu/permalloy/NiO layered structures. The antiferromagnetic behavior of both oxides below their di!erent blocking temperatures leads to the apparition of unidirectional anisotropy and enhancement of coercivity in the permalloy layers. The magnetization reversal is in#uenced by the exchange coupling between the permalloy layers, which is controlled by the Cu thickness. Both ferromagnetic layers remain uncoupled for 7.5 nm Cu thickness and are coupled for 1.5 nm. The hysteresis loops measured along di!erent directions of the samples at room temperature point out the di!erent features in the magnetization processes of systems with uniaxial and/or unidirectional anisotropies. 2001 Elsevier Science B.V. All rights reserved. Keywords: Spin valve; Exchange biasing; Thin "lm multilayers
The aim of this work is to study the magnetization reversal in a multilayered system exhibiting unidirectional anisotropy and a variable exchange coupling between the di!erent ferromagnetic components. The studied samples were deposited by RF magnetron sputtering onto glass substrates held at room temperature. The thickness and composition of the di!erent layers were CoO (20 nm)/permalloy (20 nm)/Cu (1.5}7.5 nm)/permalloy (20 nm)/NiO (20 nm). Magnetic characterization was performed by measuring the hysteresis loops in the temperature range from 80 up to 375 K with a vibrating sample magnetometer, after cooling the samples from 400 K under an applied magnetic "eld of 1000 Oe. Longitudinal and transverse hysteresis loops were measured at room temperature by using a Kerr e!ect loop tracer.
* Corresponding author. Tel.: #34-1#630-4278; fax: #34 91-300-7176. E-mail address: [email protected] (C. Prados).
Fig. 1 displays the thermal evolution of the coercivity and the loop shift for the structure with 7.5 nm of Cu. The hysteresis loops of individual permalloy (Py) layers cannot be distinguished at the lowest temperatures, but a stepped loop with two independent processes appears above 150 K. In this case, the Cu interlayer is thick enough to uncouple the Py layers. A possible reason for the apparition of a single loop at low temperature could be that in the range both Py layers exhibit similar coercivity and shift values. In agreement with previous results on exchange biasing with Ni and Co oxides [1], the loop shift disappears at 200 K in the case of the Py layer biased by CoO (Py/CoO), and at 350 K in the Py biased by NiO (Py/NiO). The coercivity of the Py/CoO remains around 10 Oe down to 200 K and then increases with the decrease of temperature. In the case of Py/NiO, the coercivity steadily increases with the decrease of temperature from the initial value at 375 K (which is also around 10 Oe as in the case of the Py/CoO at that temperature). The coercivity of the structure at 80 K is around 140 Oe.
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 1 0 3 7 - 4
C. Prados et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1764}1766
1765
Fig. 1. Thermal evolution of the coercivity (a) and loop shift (b) for the sample with 7.5 nm Cu thickness.
Fig. 3. Longitudinal and transverse hysteresis loops at room temperature (a) for the sample with 6 nm Cu thickness. Panel (b) displays an outline of the two magnetization processes composing the measured transverse hysteresis loop.
Fig. 2. Thermal evolution of the coercivity (a) and loop shift (b) for the sample with 1.5 nm Cu thickness.
Fig. 2 shows the thermal dependence of coercivity and loop shift for the structure with 1.5 nm of Cu interlayer. As a di!erence from the previous case, this sample exhibits a smooth hysteresis loop in the measured temperature range, which indicates that both Py layers are coupled through the Cu interlayer. It is worth noting that, while the value of coercivity at low temperatures is similar for both, uncoupled and coupled structures, the shift of the hysteresis loop is half in the case of the coupled system. The dependence of the loop shift, H with the thickness of the ferromagnetic layer, t , is # $+ given by the following expression [1]: H " , # M t $+ $+ where is the net interfacial exchange energy density, and M is the magnetization of the ferromagnetic (FM) $+ layer. Thus, although both Py layers are exchange biased through their respective ferromagnetic/antiferromagnetic (AFM) interfaces, the exchange coupling between them through the thin Cu layer doubles their e!ective
thickness. This total thickness becomes the relevant parameter in order to determine the shift of the loop in this coupled system. Also it is observed that, as a di!erence with the uncoupled system, the exchange bias is decreased (and almost erased) at room temperature. This is probably due to the large e!ective thickness and weak interfacial exchange energy density at that temperature. The hysteretic behavior of the exchange-coupled system has been related to antiferromagnetic grains switching together with the ferromagnetic layer, at least in the case of rotational hysteresis [2]. However, the increase of coercivity observed in the present systems needs a deeper analysis, and also could be related to a change in the magnetization reversal mechanism [2,3], and even di!erent reversal mechanisms for increasing and decreasing magnetic "elds [4]. Figs. 1 and 2 show that the value of coercivity remains similar at low temperature for both coupled and uncoupled systems. This indicates that, once the FM layer is exchange biased by the AFM, the enhancement of the coercivity is mainly related to the properties of the FM layer. This could be the case of a change of the magnetization process in biased "lms [2,3]. Fig. 3a shows in-plane longitudinal and transverse hysteresis loops measured at room temperature for the sample with a Cu interlayer of 6 nm. The loops displayed in Fig. 3a was obtained by measuring the magnetization component along a direction rotated by 203 with respect
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C. Prados et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1764}1766
to the cooling "eld direction (which also is the easy axis of the sample). The longitudinal loop exhibits two independent magnetization processes. The loop corresponding to the Py/NiO layer is shifted by the e!ect of the exchange bias, while the Py/CoO is not displaced. The transverse loop shows di!erent features in each Py layer. Fig. 3b shows an outline of the two magnetization processes composing the transverse loop. They have been sketched by taking into account the coercivity values of both Py layers in the longitudinal loops. The transverse magnetization of the unbiased Py layer exhibits the typical behavior of a uniaxial system in which the magnetic "eld is applied along a direction rotated with respect to the easy axis. However, the transverse magnetization of the biased Py indicates that the magnetic moments of this layer rotate in the same direction regardless of the magnetization switching direction (towards positive or negative applied "elds). In summary, di!erent dependence of coercivity and loop shift on the degree of coupling between two Py
layers separated by a Cu layer has been observed. Each Py layer is biased by AFM NiO and CoO layers with di!erent blocking temperatures. Transverse hysteresis loops points out the occurrence of di!erent magnetization processes in biased and unbiased layers. This work was supported by the Spanish CICyT agency under project MAT98-0965-C04. References [1] J. NogueH s, I.K. Schuller, J. Magn. Magn. Mater. 192 (1999) 203. [2] M.D. Stiles, R.D. McMichael, Phys. Rev. B 59 (1999) 3722. [3] D.V. Dimitrov, S. Zhang, J.Q. Xiao, G.C. Hadjipanayis, C. Prados, Phys. Rev. B 58 (1998) 12090. [4] V.I. Nikitenko, V.S. Gornakov, A.J. Shapiro, R.D. Shull, Kai Liu, S.M. Zhou, C.L. Chien, Phys. Rev. Lett. 84 (2000) 765.
Magnetization reversal and anisotropy in CoO/permalloy/Cu/permalloy/NiO layered structures C. Prados *, E. Pina , P. Crespo , M. Alonso-San udo, F. Cebollada, J.M. GonzaH lez , A. Hernando Instituto de Magnetismo Aplicado, Universidad Complutense, P. O. Box 155, 28230 Las Rozas, Madrid, Spain Departamento de Fn& sica Aplicada, EUITT-UPM, Ctra. de Valencia km. 7, 28031 Madrid, Spain Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain
Abstract This work presents results on the temperature dependence of the hysteretic processes of glass /CoO/permalloy/Cu/permalloy/NiO layered structures. The antiferromagnetic behavior of both oxides below their di!erent blocking temperatures leads to the apparition of unidirectional anisotropy and enhancement of coercivity in the permalloy layers. The magnetization reversal is in#uenced by the exchange coupling between the permalloy layers, which is controlled by the Cu thickness. Both ferromagnetic layers remain uncoupled for 7.5 nm Cu thickness and are coupled for 1.5 nm. The hysteresis loops measured along di!erent directions of the samples at room temperature point out the di!erent features in the magnetization processes of systems with uniaxial and/or unidirectional anisotropies. 2001 Elsevier Science B.V. All rights reserved. Keywords: Spin valve; Exchange biasing; Thin "lm multilayers
The aim of this work is to study the magnetization reversal in a multilayered system exhibiting unidirectional anisotropy and a variable exchange coupling between the di!erent ferromagnetic components. The studied samples were deposited by RF magnetron sputtering onto glass substrates held at room temperature. The thickness and composition of the di!erent layers were CoO (20 nm)/permalloy (20 nm)/Cu (1.5}7.5 nm)/permalloy (20 nm)/NiO (20 nm). Magnetic characterization was performed by measuring the hysteresis loops in the temperature range from 80 up to 375 K with a vibrating sample magnetometer, after cooling the samples from 400 K under an applied magnetic "eld of 1000 Oe. Longitudinal and transverse hysteresis loops were measured at room temperature by using a Kerr e!ect loop tracer.
* Corresponding author. Tel.: #34-1#630-4278; fax: #34 91-300-7176. E-mail address: [email protected] (C. Prados).
Fig. 1 displays the thermal evolution of the coercivity and the loop shift for the structure with 7.5 nm of Cu. The hysteresis loops of individual permalloy (Py) layers cannot be distinguished at the lowest temperatures, but a stepped loop with two independent processes appears above 150 K. In this case, the Cu interlayer is thick enough to uncouple the Py layers. A possible reason for the apparition of a single loop at low temperature could be that in the range both Py layers exhibit similar coercivity and shift values. In agreement with previous results on exchange biasing with Ni and Co oxides [1], the loop shift disappears at 200 K in the case of the Py layer biased by CoO (Py/CoO), and at 350 K in the Py biased by NiO (Py/NiO). The coercivity of the Py/CoO remains around 10 Oe down to 200 K and then increases with the decrease of temperature. In the case of Py/NiO, the coercivity steadily increases with the decrease of temperature from the initial value at 375 K (which is also around 10 Oe as in the case of the Py/CoO at that temperature). The coercivity of the structure at 80 K is around 140 Oe.
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 1 0 3 7 - 4
C. Prados et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1764}1766
1765
Fig. 1. Thermal evolution of the coercivity (a) and loop shift (b) for the sample with 7.5 nm Cu thickness.
Fig. 3. Longitudinal and transverse hysteresis loops at room temperature (a) for the sample with 6 nm Cu thickness. Panel (b) displays an outline of the two magnetization processes composing the measured transverse hysteresis loop.
Fig. 2. Thermal evolution of the coercivity (a) and loop shift (b) for the sample with 1.5 nm Cu thickness.
Fig. 2 shows the thermal dependence of coercivity and loop shift for the structure with 1.5 nm of Cu interlayer. As a di!erence from the previous case, this sample exhibits a smooth hysteresis loop in the measured temperature range, which indicates that both Py layers are coupled through the Cu interlayer. It is worth noting that, while the value of coercivity at low temperatures is similar for both, uncoupled and coupled structures, the shift of the hysteresis loop is half in the case of the coupled system. The dependence of the loop shift, H with the thickness of the ferromagnetic layer, t , is # $+ given by the following expression [1]: H " , # M t $+ $+ where is the net interfacial exchange energy density, and M is the magnetization of the ferromagnetic (FM) $+ layer. Thus, although both Py layers are exchange biased through their respective ferromagnetic/antiferromagnetic (AFM) interfaces, the exchange coupling between them through the thin Cu layer doubles their e!ective
thickness. This total thickness becomes the relevant parameter in order to determine the shift of the loop in this coupled system. Also it is observed that, as a di!erence with the uncoupled system, the exchange bias is decreased (and almost erased) at room temperature. This is probably due to the large e!ective thickness and weak interfacial exchange energy density at that temperature. The hysteretic behavior of the exchange-coupled system has been related to antiferromagnetic grains switching together with the ferromagnetic layer, at least in the case of rotational hysteresis [2]. However, the increase of coercivity observed in the present systems needs a deeper analysis, and also could be related to a change in the magnetization reversal mechanism [2,3], and even di!erent reversal mechanisms for increasing and decreasing magnetic "elds [4]. Figs. 1 and 2 show that the value of coercivity remains similar at low temperature for both coupled and uncoupled systems. This indicates that, once the FM layer is exchange biased by the AFM, the enhancement of the coercivity is mainly related to the properties of the FM layer. This could be the case of a change of the magnetization process in biased "lms [2,3]. Fig. 3a shows in-plane longitudinal and transverse hysteresis loops measured at room temperature for the sample with a Cu interlayer of 6 nm. The loops displayed in Fig. 3a was obtained by measuring the magnetization component along a direction rotated by 203 with respect
1766
C. Prados et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1764}1766
to the cooling "eld direction (which also is the easy axis of the sample). The longitudinal loop exhibits two independent magnetization processes. The loop corresponding to the Py/NiO layer is shifted by the e!ect of the exchange bias, while the Py/CoO is not displaced. The transverse loop shows di!erent features in each Py layer. Fig. 3b shows an outline of the two magnetization processes composing the transverse loop. They have been sketched by taking into account the coercivity values of both Py layers in the longitudinal loops. The transverse magnetization of the unbiased Py layer exhibits the typical behavior of a uniaxial system in which the magnetic "eld is applied along a direction rotated with respect to the easy axis. However, the transverse magnetization of the biased Py indicates that the magnetic moments of this layer rotate in the same direction regardless of the magnetization switching direction (towards positive or negative applied "elds). In summary, di!erent dependence of coercivity and loop shift on the degree of coupling between two Py
layers separated by a Cu layer has been observed. Each Py layer is biased by AFM NiO and CoO layers with di!erent blocking temperatures. Transverse hysteresis loops points out the occurrence of di!erent magnetization processes in biased and unbiased layers. This work was supported by the Spanish CICyT agency under project MAT98-0965-C04. References [1] J. NogueH s, I.K. Schuller, J. Magn. Magn. Mater. 192 (1999) 203. [2] M.D. Stiles, R.D. McMichael, Phys. Rev. B 59 (1999) 3722. [3] D.V. Dimitrov, S. Zhang, J.Q. Xiao, G.C. Hadjipanayis, C. Prados, Phys. Rev. B 58 (1998) 12090. [4] V.I. Nikitenko, V.S. Gornakov, A.J. Shapiro, R.D. Shull, Kai Liu, S.M. Zhou, C.L. Chien, Phys. Rev. Lett. 84 (2000) 765.