Interfacial coupling effect of Cr2O3 on the magnetic properties of Fe72Ga28 thin films

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Interfacial coupling effect of Cr₂O₃ on the magnetic properties of Fe₇₂Ga₂₈ thin films

https://doi.org/10.21203/rs.3.rs-398847/v1

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Here it is investigated the effect of the antiferromagnet Cr₂O₃ on the magnetic properties of ferromagnetic Fe₇₂Ga₂₈ thin films. Although Fe₇₂Ga₂₈ layers have their magnetization almost in the sample plane, the interfacial coupling with Cr₂O₃ that has perpendicular magnetic moments enables to turn the Fe₇₂Ga₂₈ magnetization direction into the out of plane (OOP) direction. Cr₂O₃ has been obtained from Cr oxidation, whereas Fe₇₂Ga₂₈ has been deposited on top of it by sputtering in the ballistic regime. Although a uniaxial in-plane magnetic anisotropy is expected for Fe₇₂Ga₂₈ thickness above 100 nm, the interfacial coupling with Cr₂O₃ prevents this anisotropy. The formation of stripe domains in Fe₇₂Ga₂₈ above a critical thickness reveals the enhancement of the out of plane component of the Fe₇₂Ga₂₈ magnetization with respect to uncoupled layers. Due to the interface coupling, the Fe₇₂Ga₂₈ magnetization turns into the out-of-plane direction as its thickness is gradually reduced, and a perpendicular magnetic anisotropy of 3.4·10⁶ erg·cm^− 3 is inferred from experimental results. Eventually, the coupling between Cr₂O₃ and Fe₇₂Ga₂₈ promotes an exchange-bias effect that has been well fitted by means of the random field model.

Nanoscience

Materials Chemistry

perpendicular magnetic anisotropy

stripe domains

exchange-bias

Control of the magnetization is an important issue for the development of pioneering magnetic devices. Typically, thin films have the magnetization in the sample plane in order to reduce the energy of the system [1]. However, in many applications as for example: high density magnetic storage, spintronic devices, non-volatile random access memories, logic devices, skyrmions or sensors, materials with OOP magnetization are desirable [2–6].

Systems with large perpendicular magnetic anisotropy (PMA) such as L1₀ FePt and CoPt thin films are extensively investigated but, their large coercivity and switching fields can represent a drawback for their integration in devices [7–8]. Therefore, it is of interest the investigation of other materials with PMA, or the possibility of turning the magnetization into the OOP direction. Stripe domains appear above a critical thickness when a moderate PMA is present [9]. In permalloy films, stripes have been observed because of columnar growth [10], but they have also been promoted when coupled with NdCo [11]. Recently, the magnetization direction has been tuned in Fe-N layers by ion implantation and heat treatment conditions [9], and by annealing in Fe₈₇Si₉B₁₃ [12]. Even more notably, the control of the magnetic anisotropy by means of voltage has been observed at magnetic transition metal/oxide interfaces [2].

FeGa alloys are extensively studied because of their large magnetostriction constant and low coercivity [13–16]. Also interesting is the possibility in sputtered Fe₇₂Ga₂₈ layers of controlling the in-plane magnetic anisotropy by growth conditions [17–19] or by thermal treatments combined with mechanical stress [20]. Molecular beam epitaxy (MBE) FeGa can exhibit domain stripes due to a low PMA [21–23], but the general behavior observed by MFM in FeGa deposited either by electrodeposition [24], sputtering [25] or even MBE [26] is a magnetic ripple due to magnetic fluctuations with the magnetization almost in the sample plane.

In this work, we have explored the possibility of turning the Fe₇₂Ga₂₈ magnetization into the OOP direction when appropriately coupled with the Cr₂O₃, an antiferromagnet that has its magnetic moments in the perpendicular direction [27–29]. Although Cr₂O₃ has already been coupled with ferromagnetic metals [30–32], the interaction with Fe₇₂Ga₂₈ seems not a trivial problem. Fe₇₂Ga₂₈ has already been coupled with TbFe₂ in [Fe₇₂Ga₂₈/TbFe₂] multilayers to turn its magnetization into the OOP direction [33–34]. However, only a tilt of the magnetization of around 25° with respect to the sample plane was achieved despite the large PMA of TbFe₂ [33–34]. Here we show that thanks to an appropriate election of layer thickness and coupling material, it is possible to reach an effective interfacial interaction that enables to turn the Fe₇₂Ga₂₈ magnetization into the OOP direction. In addition, thanks to the interfacial interaction we have observed exchange-bias effect in the perpendicular direction being possible to fit the experimental exchange-bias fields (H_E) by means of the random field model [35–37].

30 nm-thick Cr₂O₃ was synthetized from the evaporation of Cr on glass substrates that was subsequently oxidized in oxygen atmosphere at 750 °C during 3 hours. Fe₇₂Ga₂₈ layers with a thickness ranging from 240 to 20 nm were grown by the DC magnetron sputtering technique in the ballistic regime at room temperature on top of the Cr₂O₃. The sputtering deposition was carried out in oblique incidence with an angle between the vapor beam and the perpendicular to the sample of about 25° and a distance of 9 cm between target and substrate. This direction of the vapor beam within the sample plane is taken as the reference direction to control the direction of the in-plane uniaxial anisotropy axis when created [16–17, 38]. Fe₇₂Ga₂₈ films were deposited from a target with a composition of Fe₇₂Ga₂₈ with a diameter of 5 cm and a thickness of 2 mm using an Ar pressure of 3·10^− 3 mbar and a growth power of 90 W in all cases. A 20-nm thick Mo layer was used as a capping layer to avoid FeGa oxidation. Mo was also deposited with a power of 90 W and with an Ar pressure of 3·10^− 3 mbar. Therefore, the structure of the studied samples is: glass/ Cr₂O₃/Fe₇₂Ga₂₈/Mo.

We have used X-ray diffractometry (XRD) in the Bragg-Brentano configuration to study the structural properties. Measurements were performed in a Philips X’Pert MPD using the Cu K_α wavelength (1.54056 Å). A Digital Instruments Nanoscope IIIa instrument was used to obtain Magnetic Force Microscopy (MFM) images. We monitored the cantilever’s phase of oscillation while the magnetic tip was scanning the sample surface to work in the phase detection mode. The distance between sample and surface was 40 nm on average (lift mode) [33]. The MFM measurements were performed without magnetic field after an OOP magnetic field of 10 kOe was applied. In-plane and OOP hysteresis loops were performed in a vibrating sample magnetometer (VSM) at room temperature. In the sample plane, we measured loops at different angles between the applied magnetic field and the in-plane reference direction. As a reference direction, we considered as 0° the beam incidence direction in the sample plane [17, 38]. Hysteresis loops at 5 K were measured in a SQUID magnetometer after a field-cooling process (FC) at 1 kOe from 360 K. In this case, only OOP loops were recorded.

First of all, we have analyzed the possibility of Fe₇₂Ga₂₈ oxidation because of its growth on top of Cr₂O₃ (Fig. 1). XRD measurements do not show evidences of oxidation within the resolution technique. The diffraction patterns are similar to what we have previously reported about sputtered Fe₇₂Ga₂₈ being the (110) the main Fe₇₂Ga₂₈ diffraction peak [16–19]. On the other hand, the diffraction peaks of Cr₂O₃ show no variations upon the deposition of Fe₇₂Ga₂₈ on top of it (Fig. 1).

Sputtered Fe₇₂Ga₂₈ layers deposited in the ballistic regime develop in-plane magnetic anisotropy above 100 nm [16, 18]. Nevertheless, the coupling with Cr₂O₃ completely eliminate this in-plane anisotropy even for thicknesses well above 100 nm (Fig. 2a). This can be understood considering that a sample will show PMA if two conditions are fulfilled: i) it is magnetically isotropic in the sample plane, and ii) the OOP direction is an easy axis in comparison to any direction in the sample plane. Therefore, the absence of in-plane magnetic anisotropy is a necessary condition for the PMA to be developed.

The VSM hysteresis loops also reveal the change of the magnetization direction from in-plane to OOP as the Fe₇₂Ga₂₈ thickness is reduced (Fig. 2b). We can quantitively monitor this evolution by means of the OOP squareness (M_r/M_max) defined as the ratio between the remanence (M_r) and the maximum magnetization (M_max) obtained in the perpendicular hysteresis loops. In table 1 we can observe how the squareness increases as the Fe₇₂Ga₂₈ thickness is reduced, mostly for a thickness below 80 nm.

MFM images can also be used to monitor the influence of the Cr₂O₃ on the Fe₇₂Ga₂₈ magnetic behavior (Fig. 3). When Fe₇₂Ga₂₈ is no coupled (Fig. 3a), it is observed the magnetic contrast known as magnetic ripple in agreement with previous works [24–26]. However, due to the interfacial coupling with Cr₂O₃, the OOP component of the magnetization of Fe₇₂Ga₂₈ is enhanced, and stripe domains start to be visible in the MFM images (Fig. 3b-d).

In ferromagnetic materials with PMA, the quality factor Q is defined as:

$$Q={K}_{FM}/2\pi {M}_{FM}^{2}$$

where K_FM is the perpendicular magnetic anisotropy, and ${M}_{FM}$ is the magnetization saturation [9, 39]. For materials with moderate or low PMA, Q < 1, and stripe domains appear above a critical thickness (${t}_{cr})$:

$${t}_{cr}=2\pi \sqrt{{A}_{ex}/{K}_{FM}}$$

where A_ex is the exchange energy per unit length. When Q > 0.1, stripe domains are wider than the layer thickness, whereas they exhibit a periodicity equals to the layer thickness if Q < 0.1.

We have inferred K_FM from the comparison of in-plane and OOP hysteresis loops following the work of Garnier et al. [9]. In average, we have obtained a K_FM of 3.4·10⁶ erg/cm³. Taking ${M}_{FM}$ as 1100 emu·cm^− 3 [26], it is inferred a Q of 0.3 in our samples and therefore, stripes are expected above a critical thickness. Considering A_ex = 1.7·10^− 6 erg·cm^− 1 from the literature [22–23, 40], and using Eq. (2), it is obtained an experimental t_cr of 44 nm. This is in agreement with our experimental findings in which stripes have only been observed for Fe₇₂Ga₂₈ thickness ≥ 40 nm. In fact, if we take this experimental value as t_cr, a K_FM of 4·10⁶ erg·cm^− 3 is calculated, pretty close to the value inferred from experimental hysteresis loops. Finally, from the MFM images we have obtained a stripe period of 125 nm by means of the power spectral density. This stripe periodicity higher than the layer thickness is consistent with the quality factor Q higher than 0.1 calculated in our samples.

In addition to the rotation of the Fe₇₂Ga₂₈ direction magnetization towards the perpendicular direction, we have observed an exchange-bias effect related to the Cr₂O₃/ Fe₇₂Ga₂₈interfacial coupling as indicated by the shift of the hysteresis loop in the horizontal axis (H_E) at 5 K after a FC process at 1 kOe (Fig. 4). This exchange-bias phenomenon in the perpendicular direction is related to the exchange-coupling between the antiferromagnetic Cr₂O₃, and the ferromagnetic Fe₇₂Ga₂₈ [41]. For a Fe₇₂Ga₂₈ thickness of 20 nm, H_E is -17 Oe, and -9 Oe for a thickness of 40 nm.

Since the perpendicular magnetic anisotropy inferred in this work for Fe₇₂Ga₂₈ (${K}_{FM}$ = 3.4·10⁶ erg·cm^− 3) is higher than the theoretical value of Cr₂O₃ (${K}_{AF}$ = 2·10⁵ erg·cm^− 3) [42], and some chemical roughness is expected at the Fe₇₂Ga₂₈/Cr₂O₃ interface, we have used the random field model proposed by Malozemoff [35–37] to calculate the theoretical H_E values. In this random field model, the AF layer breaks into domains, and the exchange-bias field is obtained thanks to the expression:

$${H}_{E}=\frac{2z\sqrt{{A}_{AF}{K}_{AF}}}{{\pi }^{2}{M}_{FM}{t}_{FM}}$$

where $z$ is generally taken as the unity, and ${A}_{AF}$ is the exchange stiffness of the antiferromagnet that takes a value of 4·10⁷ erg/cm [42]. With this expression (3) we obtain ${H}_{E}$ equals to -26 Oe and − 13 Oe for Fe₇₂Ga₂₈ thickness of 20 and 40 nm, respectively, that are pretty close to the experimental − 17 Oe and − 9 Oe, respectively. This well agreement confirms the possibility of using this model in Cr₂O₃-based exchange-biased systems as also previously reported [42].

Finally, from ${H}_{E}$ experimental values the interfacial exchange energy $\varDelta E$ can be calculated:

$$\varDelta E={H}_{E}2{M}_{FM}{t}_{FM}$$

For Fe₇₂Ga₂₈ of 20 nm and 40 nm, it is obtained a $\varDelta E$ of 0.08 erg·cm^− 2 that is higher than in previous works in which Cr₂O₃ has been coupled with other ferromagnets with reported values of 0.05 erg·cm^− 2 at 5 K [30, 41].

In summary, we have studied the effect of the interfacial coupling between an antiferromagnet with perpendicular magnetic moments (Cr₂O₃) and a ferromagnet (Fe₇₂Ga₂₈) with the magnetization almost in the sample plane. First of all, the in-plane magnetic anisotropy of Fe₇₂Ga₂₈ is completely vanished due to the coupling. Secondly, stripe domains are promoted due to the enhancement of the OOP component of the Fe₇₂Ga₂₈ magnetization. We have observed that the magnetization direction of Fe₇₂Ga₂₈ is gradually turned from in-plane to the OOP direction as the Fe₇₂Ga₂₈ thickness is reduced A perpendicular magnetic anisotropy of 3.4·10⁶ erg·cm^− 3 has been inferred from experimental results in the Fe₇₂Ga₂₈ layers. It has also been observed exchange-bias phenomena in the perpendicular direction fitting the experimental data to the random field model.

Acknowledgements.

“CAI Difracción Rayos-X” from UCM is also acknowledged for XRD measurements. This work has been financially supported through the projects RTI2018-097895-B-C43 of the Spanish Ministry of Science, Innovation, and Universities and MAT2017-87072-C4-3-P of the Spanish Ministry of Economy, Industry and Competitiveness.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Chikazumi, S. Physics of ferromagnetism. Oxford Science Publications(1997).
Fert, A., Reyren, A. N. & Cros, V. Nat. Rev. Mater. 2, 17031 (2017).
Dieny, B. & Chshiev, M. Rev. Mod. Phys. 89, 025008 (2017).
Sbiaa, R., Meng, H. & Piramanayagam, S. N. Phys. Status Solidi RRL. 5, 413 (2011).
Umadevi, K., Bysakh, S., Arout Chelvane, J., Kamat, S. V. & Jayalaskhmi, V. J. Alloys Compnd. 663, 430 (2016).
Sander, D. et al. J. Phys. D: Appl. Phys. 50, 363001 (2017).
Sbiaa, R., Law, R., Tan, E. L. & Liew, T. J. Appl. Phys. 105, 013910 (2009).
Krivorotov, I. N. et al. Science. 307, 228 (2005).
Garnier, L. C. et al. J. Phys. Mater. 3, 024001 (2020).
Romera, M., Ranchal, R., Ciudad, D., Maicas, M. & Aroca, C. J. Appl. Phys. 110, 083910 (2011).
Markó, D. et al. L. M. Álvarez-Prado, Appl. Phys. Lett. 115, 082401(2019).
Coïsson, M., Barrera, G. & Celegato, F. and P. Tiberto Appl. Surf. Sci. 476, 402 (2019).
Ranchal, R. & Maestre, D. J. Phys. D: Appl. Phys. 47, 355004 (2014).
Clark, A. E., Restorff, J. B., Wun-Fogle, M., Lograsso, T. A. & Schlagel, D. L. IEEE Trans. Magn. 36, 3238 (2000).
Clark, A. E., Wun-Fogle, M., Lograsso, T. A. & Cullen, J. R. IEEE Trans. Magn. 37, 2678 (2001).
Bartolomé, P., Begué, A., Muñoz-Noval, A., Ciria, M. & Ranchal, R. J. Phys. Chem. C. 124, 4717 (2020).
Muñoz-Noval, A., Ordóñez-Fontes, A. & Ranchal, R.Phys. Rev. B93,214408, (2016).
Muñoz-Noval, A., Fin, S., Salas-Colera, E., Bisero, D. & Ranchal, R. J. Alloys Compnd. 745, 413 (2018).
Muñoz-Noval, A., Salas-Colera, E. & Ranchal, R. J. Phys. Chem. C. 123, 13131 (2019).
Hontecillas, I., Figueruelo, I., Abad, S. & Ranchal, R. J. Magn. Magn. Mater. 494, 165771 (2020).
Barturen, M. et al. Appl. Phys. Lett. 101, 092404 (2012).
Tacchi, S. et al. Phys. Rev. B. 89, 024411 (2014).
Fin, S. et al. Phys. Rev. B. 92, 224411 (2015).
Ranchal, R., Fin, S. & Bisero, D. J. Phys. D: Appl. Phys. 48, 075001 (2015).
Bartolomé, P., Maicas, M. & Ranchal, R. J. Magn. Magn. Mater. 514, 167183 (2020).
Begué, A., Proiettia, M. G., Arnaudas, J. I. & Ciria, M. J. Magn. Magn. Mater. 498, 166135 (2020).
Kosub, T. et al. Nat. Comm. 8, 13985 (2017).
Fallarino, L., Binek, C. & Berger, A. Phys. Rev. B. 91, 214403 (2015).
He, X. et al. Nat. Mater. 9, 579 (2010).
Lin, K. W. & Guo, J. Y. J. Appl. Phys. 104, 123913 (2008).
Yuan, W. et al. Sci. Rep. 6, 28397 (2016).
Nozaki, T. et al. Appl. Phys. Lett. 105, 212406 (2014).
Bartolomé, P., Maicas, M., Biskup, N., Varela, M. & Ranchal, R. Phys. Status Solidi A. 215, 1800183 (2018).
Bartolomé, P. & Ranchal, R. Nanotechn. 31, 335715 (2020).
Malozemoff, A. P. Phys. Rev. B. 35, 3679 (1987).
Malozemoff, A. P. J. Appl. Phys. 63, 3874 (1988).
A. P.Malozemoff. Phys. Rev. B37,7673(1988).
Maicas, M., Ranchal, R., Aroca, C., Sánchez, P. & López, E. Eur. Phys. J. B. 62, 267 (2008).
Hubert, A. & Schäfer, R. Magnetic domains: the analysis of magnetic microstructures, Springer Science & Business Media(2008).
Gopman, D. B., Sampath, V., Ahmad, H., Bandyopadhyay, S. & Atulasimha, J. IEEE Trans. Magn. 53, 6101304 (2017).
Nogués, J. & Schuller, I. K. J. Magn. Magn. Mater. 192, 203 (1999).
Zheng, R. K., Liu, H., Wang, Y. & Zhang, X. X. Appl. Phys. Lett. 84, 702 (2004).

Table 1. Summary of the OOP squareness (M_r/M_max) calculated from the perpendicular hysteresis loops as a function of the Fe₇₂Ga₂₈ thickness.

Fe₇₂Ga₂₈ thickness (nm)	20	40	80	160	240
M_r/M_max (OOP direction)	0.5	0.3	0.2	0.2	0.2

No competing interests reported.

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Interfacial coupling effect of Cr₂O₃ on the magnetic properties of Fe₇₂Ga₂₈ thin films

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1. Introduction

2. Experimental Section

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4. Conclusions

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