Top and Bottom Spin Valves With Ni-Fe-Mn Antiferromagnetic Layer

Structure, magnetic and magnetoresistive properties of spin valves with Ni-Fe-Mn antiferromagnet as a pinning layer have been studied. A technique of fabrication of spin valves with an enhanced thermal stability and improved hysteretic characteristics has been elaborated.


Introduction
Bilayers of permalloy/antiferromagnetic triple Ni-Fe-Mn alloy have been studied, as well as magnetic and magnetoresistive properties of spin valves (SVs) with Ni-Fe-Mn antiferromagnet (AF) as a pinning layer. A technique of fabrication of bottom spin valves based on Ni-Fe-Mn ordered AF with an enhanced thermal stability and improved hysteretic characteristics has been elaborated.
To apply the ordered Ni-Fe-Mn AF phase in spin valves, a definite deposition order of permalloy and manganese layers is necessary. Notably, the permalloy layer should be deposited on manganese or the manganese containing alloy [1]. With this deposition order, the authors of [2] fixed the high value H ex = 110 Oe after the annealing of the FeMn(15nm)/NiFe(15nm) bilayers. It should be noted that the formation of this ordered system is only possible with the thermal treatment of the films that contain the permalloy and manganese layers or the layers of the manganese containing alloy. The annealing of Ni 48 Fe 12 Mn 40 and Ni 32 Fe 8 Mn 60 films which after magnetron sputtering from the targets of corresponding compositions, were in the state of a homogeneous ternary solid solution did not result in the formation of an ordered antiferromagnetic phase. After annealing at 400°C, the decomposition of a ternary solid solution into the nickel based solid solution and almost pure manganese was revealed. A similar result was found in [3].

Experimental details
The samples were made by DC magnetron sputtering and by electron-beam evaporation on glass (Corning) substrates and single-crystalline sapphire (1012). To form a unidirectional anisotropy the magnetic field of 110 Oe was applied in the process of nanostructures growth, and the thermal-magnetic treatment was performed at pressure of 10 -4 Pa in permanent magnetic field of 2 kOe applied in the sample plane at 260 ºC for 4 h. The exchange bias field temperature dependence was measured in the temperature range of 20-260 ºC. Etching of samples for preparation of bottom SVs was carried out in a device for reactive ion-plasmic etching PlasmaPro NGP 80 RIE Oxford.

Results and discussion
According to the results of the studies carried out, the bilayers with antiferromagnetic triple alloy Ni 14 Fe 6 Mn 80 are characterized by medium blocking temperature T b = 150 ºС ( Fig. 1) and moderate energy of exchange interaction J ex = 0.05 erg/cm 2 [4].
Higher T b = 270 ºС ( Fig. 1) and J ex = 0.27 erg/cm 2 were obtained for the annealed manganese/permalloy bilayers. In this case, an ordered AF phase of Ni-Fe-Mn is formed [5]. The ordered AF phase formation is testified by an appearance of super-structural Debye rings (100), (110), (210), (211) (indicated by arrows in Fig. 2a) in electron diffraction patterns of sample Al 2 O 3 /Mn(50 nm)/Ni 77 Fe 23 (30 nm)/Ta(5 nm) after its annealing in the magnetic field at T ann = 260 о С for 4 h. In this case in the electron-microscope images one can see the columnar structure, and an intermediate layer between manganese and permalloy layers is absent (Fig.  2b). To study the magnetoresistance dependence of a spin valve on the copper layer thickness the samples with t AF = 25 nm were prepared.
With increasing copper layer thickness the magnetoresistance ΔR/R s at first increases and then decreases [4]. The dependence obtained is nonmonotone and demonstrates qualitative agreement with the data published in [6].
The maximal value of ΔR/R s = 7.30 % corresponds to t Cu = 2.8 nm (Fig. 3). The magnetoresistance sensitivity determined as an average value for the ascending and descending hysteresis loop of the free layer Ni 80 Fe 20 /Co 90 Fe 10 is Δ(ΔR/R s )/ΔH = 0.75 %/Oe. The data obtained demonstrate possibility of using disordered alloy Ni 14 Fe 6 Mn 80 as a pinning layer in top spin valves. To fabricate a bottom SV with the ordered AF phase Ni-Fe-Mn a technological cycle has been worked out, including the following operations: 1) Formation of the ordered AF phase Ni-Fe-Mn in a sample Al 2 O 3 /Ni 77 Fe 23 (5 nm)/Mn(50 nm)/Ni 77 Fe 23 (30 nm)/Ta(5 nm) by the thermal-magnetic treatment at 260 о С for 4 h.
2) Ion etching for 20 min of the annealed sample for the surface layer removing.
The ion etching duration was chosen to assure the removal of the contaminated surface layer and retaining the ferromagnetic area together with the ordered AF phase required for the formation of the unidirectional anisotropy in the FM layer sputtered on the sample after etching.
3) Magnetron sputtering of the layered structure consisting of ferromagnetic Co 90 Fe 10 layers separated by Cu on the as-prepared sample Al 2 O 3 /Ni-Fe-Mn.
The magnetoresistance of the as-fabricated spin valve Al 2 O 3 /Ni-Fe-Mn/Co 90 Fe 10 (5.5 nm)/Cu(3.6 nm)/Co 90 Fe 10 (5.5 nm)/Ta(5 nm) is ΔR/R s = 3.8 % (Fig. 4). This value is significantly higher than the effect obtained in [1], since the replacement of permalloy in the free and pinning layer by Co or the Co 90 Fe 10 alloy leads to an increase in spindependent scattering and an increase in the giant magnetic resistance effect (GMR) in the spin valves [7].

Conclusion
A possibility of application of the Ni-Fe-Mn AF for SV devices is demonstrated on a sample of composition Ta/Ni 80 Fe 20 /Co 90 Fe 10 /Cu/Co 90 Fe 10 /Ni 14 Fe 6 Mn 80 /Ta. A technology of fabrication of a spin valve based on the ordered AF phase Ni-Fe-Mn has been worked out. A spin valve with giant magnetic resistance exceeding the GMR of previously known structures has been created,