Perpendicular Magnetic Anisotropy and Hydrogenation-Induced Magnetic Change of Ta/Pd/CoFeMnSi/MgO/Pd Multilayers

The perpendicular magnetic anisotropy (PMA) has been achieved in Ta/Pd/CoFeMnSi (CFMS)/MgO/Pd film, in which the Heusler compound CoFeMnSi is one of the most promising candidates for spin gapless semiconductor (SGS). The strong PMA, with the effective anisotropy constant Keff of 5.6 × 105 erg/cm3 (5.6 × 104 J/m3), can be observed in the Ta/Pd/CFMS (2.3 nm)/MgO (1.3 nm)/Pd films annealed at 300 °C. In addition, it was found that the magnetic properties of Ta/Pd/CFMS/MgO/Pd films are sensitive to hydrogen (H2) under a weak magnetic field (< 30 Oe), whose residual magnetization (Mr) decreased from 123.15 to 30.75 emu/cm3 in the atmosphere with H2 concentration of 5%.


Background
Nowadays, hydrogen (H 2 ) as one of the new clean and efficient energy source has attracted more attentions, and thus, ensuring the safety of its usage becomes more and more significant. Solid state conductometric gas sensor is commonly used to detect the hydrogen, but it lacks the chemical selectivity and humidity sensitivity [1]. Recently, the magnetic sensors have been proved to be a useful way to detect the various gases, especial hydrogen, in which the film structures containing palladium (Pd) layer are currently under the intense study because the Pd possesses the high sensitivity [2] and selectivity [3] to hydrogen. Thus, the Pd-containing films can be used as an effective catalyst for hydrogen dissociation and absorption [4]. To date, many studies have reported the hydrogenation-induced magnetic change in Pd-rich magnetic alloy film and Pd/ferromagnetic layer (Pd/FM) multilayer films, such as Co 17 Pd 83 [1], Pd/Fe [5], [Co/Pd] 12 [6], and Pd/Co/Pd [7] films. The hydrogenation-induced magnetic change can be attributed to the swell of Pd lattice due to the hydrogen absorption, which could contribute to about 2-3% volume expansion.
On the other hand, Pd as a noble metal is commonly used for realizing the perpendicular magnetic anisotropy (PMA) owing to the d-d electron orbital hybridization at the interfaces of Pd/ferromagnetic layer. This critical interfacial effect of electron orbital hybridization is very sensitive to the interfacial strain or stress [8], which could be brought in through the volume evolution of noble metal. Therefore, high sensitivity of hydrogen-induced magnetic change could be expected from the PMA film with Pd layer by making use of the strong interfacial dependences of perpendicular magnetic anisotropy.
So far, a huge number of studies of PMA have been reported, which have been originated from the d-d or d-p electron orbital hybridizations of ferromagnetic layer and noble metal (Pt, Pd) or oxygen of oxides at the interfaces [9][10][11][12]. Moreover, Heusler quaternary compound CoFeMnSi (CFMS) has been proved to be a spin gapless semiconductor (SGS) [13][14][15], which is also very sensitive to the external field [16], showing the potential advantages of being a sensor. In this work, the Ta/Pd/CoFeMnSi/MgO/Pd-structured films were designed to achieve the strong PMA by the interfacial effect, and the hydrogenation-induced magnetic change was explored. Different from the above reports [1,[5][6][7], the perpendicular magnetic anisotropic film structure and SGS-like CoFeMnSi ferromagnetic layer are all sensitive to the extrinsic effects, such as interfacial stress or strain. Thus, the highly sensitive change of magnetism could be expected from the films under a low magnetic field.

Methods
Four sets of samples were prepared as follows: Ta The magnetic properties were characterized by vibrating sample magnetometer (VSM: Lakeshore 7404). The electric transport property measurement system (ET Chen, ET9000) was used to monitor the Hall resistivity with the change of hydrogen absorption and desorption in real time. All measurements were carried out at room temperature and atmospheric pressure. The total gas flow rate was fixed at 3.5 L/ min for the sensitivity of hydrogen gas. The hydrogen concentration was tuned by controlling the gas flow rate of the mixed gas (H 2 :Ar = 5:95) and nitrogen gas (N 2 ).

Results and Discussion
To understand the effect of the MgO layer thickness on PMA, Fig. 1 shows magnetic hysteresis loops measured along the in-plane and out-of-plane directions for Ta/ Pd/CFMS (2.3 nm)/MgO (t MgO )/Pd films annealed at 300°C with varied thicknesses t MgO . All samples are easily magnetized along the out-of-plane direction, and large saturation fields are needed along the in-plane direction, exhibiting PMA behaviors. The strength of PMA firstly increases with increasing t MgO and reaches the maximum value with the squareness (M r /M s ) close to 1 when t MgO = 1.3 nm while obviously decreases with further increasing t MgO .
In order to elucidate the influence of annealing temperature on PMA, Fig. 2 (Fig. 2b). The easy magnetization axis of the sample annealed at 300°C shifted to out-of-plane direction, showing strong PMA (Fig. 2c). The PMA could be maintained after T an rising to 350°C, but the squareness decreased. With further increasing the T an , the PMA was destroyed and the easy magnetization axis shifted back to in-plane orientation (Fig. 2e, f ). The results indicate that the strong PMA can only be achieved at the proper annealing temperature and is easy to be deteriorated at a higher annealing temperature. This is because high annealing temperature could give rise to the intensified inter-diffusion of the atoms at the interface and deteriorate the electron orbital hybridization, which is consistent with our previous reports [9,12,17,18].
In order to clarify the interfacial effect on the PMA in the Ta/Pd/CFMS/MgO/Pd films, the M-H loops of different film stacks were given in Fig. 3a-c. As shown in Fig. 3a, the film without MgO layer presents strong IMA behavior. But for the film without the bottom Pd layer, the easy magnetization axis of the Ta/CFMS/MgO/Pd sample exhibits a slight shift from the in-plane direction, showing the weak IMA (Fig. 3b). The strong PMA is observed in the film after inserting the Pd and MgO layers (i.e., Ta/Pd/CFMS/MgO/Ta) as seen in Fig. 3c, implying that both the Pd/CFMS and CFMS/MgO interface are essential for realizing PMA, and the contribution of CFMS/MgO interface to the PMA plays a major role [12,17]. That is, an appropriate amount of Co-O bonds at the CFMS/MgO interface is helpful for achieving the optimum PMA. The thin MgO layer makes CFMS/MgO underoxidized (Fig. 1a, b), and thick MgO layer makes CFMS/MgO overoxided (Fig. 1d), which both weaken the PMA [11]. As shown in Fig. 1c, the sample with t MgO = 1.3 nm has the proper Co-O bonds in the CFMS/ MgO interface to obtain strong PMA.
To quantify the PMA strength in the Ta/Pd/CFMS/ MgO/Pd films, the effective anisotropy constant K eff is given by where K V and K S are the bulk and interface anisotropy, respectively. K eff is determined by the differences of magnetization energy between the hard and easy magnetization directions. The positive K eff represents PMA, and the negative K eff represents IMA. The product of K eff × t CFMS as a function of t CFMS for the Ta/Pd/ CFMS (t CFMS )/MgO (1.3 nm)/Pd films annealed at different temperatures is shown in Fig. 3d. All as-deposited films present the negative K eff , implying the absence of PMA. The PMA of the films annealed at 250°C can only be observed with t CFMS = 1.9 nm. For the films annealed at 300°C, the PMA can be maintained within a wide t CFMS range (below 2.7 nm). The largest K eff value of the sample is 5.6 × 10 5 erg/cm 3 (5.6 × 10 4 J/m 3 ) with t CFMS = 2.3 nm. As shown above, the PMA is very sensitive to the interfacial environment, which could be also affected through the gas absorption or desorption of noble metal Pd. Thus, the hydrogenation-induced magnetic change was investigated on the Ta/Pd/CFMS (2.3 nm)/MgO (1.3 nm)/Pd films annealed at 300°C. The M-H loops were checked under different gas atmospheres through varying the H 2 concentration as shown in Fig. 4a. Noted here, the M-H loops cannot be affected by the pure nitrogen N 2 and pure argon Ar atmospheres (data did not show here). After introducing H 2 , the M-H loop significantly changes, and the easy magnetization axis shifts away from the out-of-plane direction, showing a large saturation field of the out-of-plane magnetic curve. It is found that the saturation field increases with increasing H 2 concentration. The sample exhibits an excellent hydrogen sensitivity under a small applied magnetic field (< 30 Oe). Figure 4b shows the M-H loops measured under the air atmosphere before and after the addition of H 2 . It can be seen that the M-H loop is well back to the initial state after removing H 2 . As shown in Fig. 4c, M r decreases from 123.15 to 30.75 emu/cm 3 (decreased 75%), and the saturation field (H k ) increases from 5.5 to 18 Oe with increasing H 2 concentration from 0 to 5%. Figure 5 shows the dependence of the Hall resistivity on time for H 2 absorption and desorption in Ta/Pd/ CFMS (2.3 nm)/MgO (1.3 nm)/Pd annealed at 300°C. As shown in Fig. 5, H 2 absorption rate is faster than the desorption rate. The Hall resistivity gradually increased to be saturated in 70 min after being exposed to H 2 . However, through introducing N 2 to expel H 2 , the Hall resistivity only decreases 60% due to undesorpted H 2 . The Hall resistivity increased/decreased quickly at the beginning (first 10 min) under the processes of H 2 absorption/desorption, since the hall resistivity is mainly related to the magnetic layer (CoFeMnSi). Thus, it can be deduced that the resistivity changes at the beginning are majorly originated from the interfacial variations between Pd and CoFeMnSi layers due to H 2 absorption/ desorption. The resistivity change at the later stage could be the intrinsic changes of multilayer films due to the absorbed H 2 . In comparison to Fig. 4b, the magnetic detection of multilayered films could be very reproducible owing to the well recovering of the magnetic performance in comparison to the resistivity variations.
As mentioned above, the hydrogenation-induced magnetic change mainly comes from the stress acting on the  [19]. It is known that Pd is an effective catalyst for dissociating the hydrogen molecule [4]. Hydrogen molecules are adsorbed and dissociated to hydrogen atoms on the surface of the Pd layer. The lattice of Pd could be expanded with the absorption of hydrogen atoms [20], which in turn has tensile stress to its adjacent MgO and CFMS layer, leading to the controllable magnetism of CoFeMnSi. After discharging H 2 , the hydrogen atoms can escape from the Pd membrane surface [21], causing the recovering of the magnetic performance.

Conclusions
We demonstrated the strong PMA and the hydrogenation-induced magnetic change in the Ta/Pd/ CFMS/MgO/Pd films. The loop squareness (M r /M s ) is close to 1 for the sample with t CFMS = 2.3 nm and t MgO = 1.3 nm after annealing at 300°C, obtaining a high perpendicular magnetic anisotropy K eff value of 5.6 × 10 5 erg/cm 3 . Owing to the hydrogen absorption of Pd, the annealed Ta/Pd/CFMS/MgO/Pd film at 300°C exhibited excellent hydrogen sensitivity; the residual magnetization (M r ) decreased 75% under the atmosphere with H 2 of 5%.