Tuning the coercive field of CoFe2 in hard/soft CoFe2O4/CoFe2 bilayers

The sensitivity of magnetoresistive devices, i.e. their ability to be switched by very small fields, depends on the softness of their soft electrode. In this paper, we show the possibility to tune the coercive field of the CoFe2 alloy, commonly used as a soft electrode, from intrinsic values down to zero, by varying the pulsed laser ablation conditions of CoFe2/CoFe2O4 bilayers. This tuning possibility relies on the existence of a frustration of the spins of the CoFe2 layer originating from both the ferrimagnetic nature of the CoFe2O4 layer and the oxide/metal interface roughness.


Introduction
Spintronics-based devices have attractive applications in sensors and magnetic random access memories (MRAM). Their operation requires two electrodes of different magnetic hardness. In the majority of the studies presented in the literature [1,2], this is obtained by the use of electrodes of different chemical compositions. Still, some works use electrodes of the same nature for the soft and the hard magnetic layers [3]- [7]. The different hardnesses are then obtained either by simply using different thickness for each electrode or by using a pinning layer in the form of an antiferromagnetic material [3,4]. The hardness of the electrode may thus be increased but not decreased below its intrinsic value. It is, however, of crucial interest to control the softness of the electrodes, as well as their hardness because this softness controls the ability of the electrode to be sensitive to magnetic fields as weak as possible. In this paper, we show the possibility to tune the hardness of a magnetic CoFe 2 layer in the CoFe 2 O 4 /CoFe 2 system from an ultra soft magnetic material to a hard magnetic material by varying the elaboration conditions. One major advantage of this system is that the CoFe 2 O 4 layer, being insulating, will not shunt the current away from the active layers in current-in plane configurations.

Experimental
The CoFe 2 O 4 /CoFe 2 systems were grown on Si (100) substrates heated at 300 • C by pulsed laser deposition using an excimer laser: XeCl (λ = 308 nm) (samples OM1) and KrF (λ = 248 nm) (samples OM2) with a fluency of about 2 and 3 J cm −2 respectively. The pulse length of the laser was 20 ns at a repetition rate of 10 Hz. The two layers CoFe 2 and CoFe 2 O 4 were deposited by ablating a CoFe 2 metallic target. The oxide was first deposited under O 2 : N 2 (20 : 80) at a deposition pressure of 6.5 Pa for the samples (OM1) [8] and 5 Pa for the samples (OM2), respectively [9]. The laser was stopped during the evacuation of the reactive atmosphere down to a ground pressure of around 10 −6 Pa for the deposition of the CoFe 2 layer. The thickness of the CoFe 2 O 4 layer was fixed at 150 nm while that of the metal was varied between 5 and 54 nm. The bilayers were characterized by cross-sectional transmission electron microscopy. The magnetization of the films was measured by using a commercial Quantum Design superconducting quantum interference device (SQUID) magnetometer.

Results and discussion
A magnetization hysteresis loop typical for all samples is shown in figure 1. We observe two magnetic contributions of different hardness corresponding to the soft metallic layer and the hard oxide one. The samples were polarized in the plane of the bilayer to +50 kOe. The insert corresponds to the minor loop, where the field is traced from +1 kOe to −1 kOe and back to +1 kOe. This minor loop witnesses the reversal of the magnetization of the soft CoFe 2 layer without affecting the hard CoFe 2 O 4 layer.
The hardness of the CoFe 2 layer may be evaluated from the coercive field of these minor loops. Its variation with the inverse CoFe 2 thickness for all samples is shown in figure 2. It is linear, which indicates that the coercive field variation has an interfacial origin. However, two different behaviors are observed: the coercive field either increases with the inverse of the CoFe 2 thickness for samples (OM1) (figure 2(a)) or decreases for samples (OM2) (figure 2(b)).  The effective anisotropy K of the CoFe 2 layer of thickness t may be written as the sum of a volume K V and interface K S terms according to [10]: and may be estimated from the linear variation of the coercive field with the inverse thickness of the metallic layer through the following relationship [11]: where M S is the saturation magnetization of the CoFe 2 layer (the measured value is about 1600 emu cm −3 , close to the 1800 emu cm −3 observed for the bulk CoFe 2 alloy). The values of the volume and surface anisotropies deduced from figures 2(a) and (b) are K V = 170 kerg cm −3 and K S = −0.6 erg cm −2 for samples (OM1) and K V = 150 kerg cm −3 and K S = −0.04 erg cm −2 for samples (OM2). For all samples (OM1 and OM2), the product K S K V is negative. This means that the two anisotropy contributions are perpendicular to each other. It is therefore possible to observe a thickness value (36 and 3 nm extrapolated from figures 2(a) and (b) respectively) for which the surface and volume terms cancel. The anisotropy then vanishes and the material, having a zero coercive field, is ultra soft. A perpendicular anisotropy term can be found at surfaces or interfaces, but, when observed, it is pointing out of the sample plane [12]- [14]. In our measurements, the magnetization curves are performed in the plane of the samples, therefore only the in-plane anisotropies are probed. As a consequence, the anisotropy term that is shown to be perpendicular to the applied field is not actually pointing out of plane but is confined in the plane of the film. However, it must be remembered that our samples are polycrystalline and therefore are not supposed to show welldefined anisotropy directions. This leads us to think that the observed perpendicular anisotropy is not a real anisotropy but reflects the tendency of the magnetization to align in a direction perpendicular to the direction of the applied magnetic field. This can be explained if we take into account the establishment of magnetic frustrations at the CoFe 2 O 4 /CoFe 2 interface. Indeed, CoFe 2 O 4 is a ferrimagnetic layer constituted by two sub-lattices having opposite magnetization directions. Therefore, the roughness of the CoFe 2 O 4 layer, observed on cross-sections by transmission electronic microscopy ( figure 3), results in the creation of frustrations: a part of the ferromagnetic layer is willing to be parallel with the net magnetization of the ferrimagnet while another part is willing to be antiparallel. For the ferromagnetic layer the only way to solve these frustrations is to align perpendicularly to the ferrimagnet magnetization direction (as schematized in figure 4), which results in a perpendicular coupling [15]. However, due to the ferrimagnetic nature of CoFe 2 O 4 , the interface is not magnetically compensated, therefore a purely perpendicular coupling is not observed but is represented by the surface anisotropy term K S that is perpendicular to the applied field direction. When the interface is very rough, the value of K S is large because the degree of magnetic frustration is high. This is the case for samples (OM1) (figure 3(a)). For samples (OM2) deposited with the energetic excimer laser (KrF), the interface roughness is much smaller and the CoFe 2 O 4 layer, while still being polycrystalline, presents a strong crystallographic (111) texture ( figure 3(b)). This leads to lower frustrations and to a smaller value of K S . The magnitude of the K S value is therefore a measure of the degree of magnetic frustration in the CoFe 2 layer.

Conclusion
We have shown that in the CoFe 2 O 4 /CoFe 2 system, the coercive field of the soft layer CoFe 2 may be tuned and even decreased to zero. That may be explained by a magnetic frustration existing in the soft metallic layer and originating from the ferrimagnetic nature of the oxide layer coupled to the roughness of the oxide/metal interface. This frustration induces the fact that the surface anisotropy term is perpendicular to the bulk term. The total soft layer anisotropy term may then be tuned by the choice of its thickness. The values measured for the surface and bulk terms of the soft layer anisotropy are such that the total anisotropy may even be completely cancelled. This CoFe 2 O 4 /CoFe 2 system therefore offers a precious way to obtain an ultra soft magnetic layer.