Synthetic ferrimagnets based on Co and Gd bear promise for directly bridging the gap between volatile information in the photonic domain and nonvolatile information in the magnetic domain, without the need for any intermediary electronic conversion. Specifically, these systems exhibit strong spin–orbit torque effects, fast domain wall motion, and single-pulse all-optical switching of the magnetization. An important open challenge to bring these materials to the brink of applications is to achieve long-term stability of their magnetic properties. In this work, we address the time-evolution of the magnetic moment and compensation temperature of magnetron sputter grown Pt/Co/Gd trilayers with various capping layers. Over the course of three months, the net magnetic moment and compensation temperature change significantly, which we attribute to quenching of the Gd magnetization. We identify that intermixing of the capping layer and Gd is primarily responsible for this effect, which can be alleviated by choosing nitrides for capping as long as reduction of nitride to oxide is properly addressed. In short, this work provides an overview of the relevant aging effects that should be taken into account when designing synthetic ferrimagnets based on Co and Gd for spintronic applications.

Ferrimagnetic spintronics is an active contemporary research field.1,2 One major motivation for investigating these magnetic systems is to combine the easy characterization and manipulation of the magnetic order of ferromagnets with the fast exchange-driven dynamics and information robustness associated with antiferromagnetic coupling. One particular class of ferrimagnets that is of interest here is the so-called transition metal (TM) rare earth (RE) ferrimagnets, characterized by a composition consisting of one or more TMs (Co, Fe, and Ni) and one or more REs (e.g., Gd and Tb). These systems exhibit various fascinating phenomena, which have attracted the attention of the scientific community, such as single-pulse all-optical switching of the magnetization,3–9 effective spin–orbit torque (SOT)-driven manipulation of the magnetic order,10–13 the presence and efficient motion of skyrmions,14–17 and exchange torque driven current-induced domain wall motion with velocities over 1000 m/s.17–19 The combination of these phenomena in one material system makes them a promising candidate to bridge the gap between photonics and spintronics.19–22 

Within this class of materials, often a distinction is made between alloys, with an approximately uniform distribution of TM and RE elements, and synthetic ferrimagnets consisting of discrete layers of the RE and TM. The latter category has a few distinct advantages in comparison with the alloy systems. The layered structure of these synthetic ferrimagnets allows for easier adaptation to wafer scale production. Also, contrary to alloys, a much wider composition range between the 3d and 4f metal exhibits single-pulse all-optical switching.23,24 Combined with the increased access to interfacial engineering, this leads to more flexibility and tunability of its magnetic properties. Moreover, the Pt/Co/Gd trilayer displays strong interfacial spintronic effects, such as perpendicular magnetic anisotropy, the spin-Hall effect,25 and the interfacial Dzyaloshinskii–Moriya interaction,26 which are essential ingredients for applications based on efficient domain wall motion or SOT-driven manipulation.2,27

Nonetheless, one important challenge that yet needs to be addressed, before applications of spintronic devices based on synthetic RE-TM systems can be considered, is their long-term stability. The aging and sample structure of RE-TM alloys have been extensively studied in the past, where several factors gave rise to changing magnetic properties over time, like the capping layer,28,29 compositional inhomogeneities at the scale of several nm,30,31 and thermodynamically driven intermixing.13,28,32 Despite this knowledge, aging effects on basic magnetic properties in the ultrathin synthetic ferrimagnets with perpendicular magnetic anisotropy have been poorly studied. Therefore, in this work, we present a systematic VSM-SQUID study of the magnetic moment and compensation temperature of a Co/Gd bilayer over time. Here, based on the earlier work in the alloys, we expect three main contributions to changes in the balance between Co and Gd magnetization, the origin of which is illustrated schematically in Fig. 1: intermixing between the capping layer and the Gd, intermixing between the Co and Gd, and oxidation of the Gd via grain boundaries in, or partial oxidation of, the capping layer. Answering the question of which of these contributions is dominant will be addressed in this work.

FIG. 1.

Schematic illustration of the three main aging effects that can impact the magnetization of the Gd subsystem: intermixing of Gd with the Co layer, intermixing of Gd with the capping layer, and oxidation of the Gd layer. The illustration is not to scale, nor an exact representation of the (expected) intermixing profiles.

FIG. 1.

Schematic illustration of the three main aging effects that can impact the magnetization of the Gd subsystem: intermixing of Gd with the Co layer, intermixing of Gd with the capping layer, and oxidation of the Gd layer. The illustration is not to scale, nor an exact representation of the (expected) intermixing profiles.

Close modal

Ta/Pt/Co/Gd/X samples were fabricated by DC magnetron sputtering on thermally oxidized Si/SiOx substrates, where X denotes the different capping layers used. The precise layer structure used in this study is summarized in Table I (for more details about the sputtering process, see supplementary material 1). These full sheet samples were diced into 4.54.5 mm2 samples for SQUID characterization. Out-of-plane (OOP) SQUID measurements as a function of OOP applied field and temperature were performed, using a commercially available MPMS3 VSM-SQUID system. A typical room-temperature measurement of the area-normalized net OOP magnetic moment as a function of the OOP applied field is shown in Fig. 2(a). We choose the area-normalized moment, m ̃, as defining the magnetization requires us to define a thickness by which to normalize. However, in that case, coarse assumptions have to be made about the intermixing and magnetization profile in Gd,33 making area normalization the more consistent choice. The resultant square hysteresis loop in m ̃ for OOP SQUID indicates perpendicular magnetic anisotropy, as expected for thin films of Co on Pt.7,19

TABLE I.

Overview of the stack structures of the samples under investigation in this work.

Substrate (μm) Main stack (nm) Capping layer (nm)
Si(500)/SiOx(0.1)  Ta(4)/Pt(4)/Co(1)/Gd(3)  Pt(4) 
Si(500)/SiOx(0.1)  Ta(4)/Pt(4)/Co(1)/Gd(3)  Ta(4) 
Si(500)/SiOx(0.1)  Ta(4)/Pt(4)/Co(1)/Gd(3)  TaN(4) 
Si(500)/SiOx(0.1)  Ta(4)/Pt(4)/Co(1)/Gd(3)  TaN(4)/Pt(4) 
Substrate (μm) Main stack (nm) Capping layer (nm)
Si(500)/SiOx(0.1)  Ta(4)/Pt(4)/Co(1)/Gd(3)  Pt(4) 
Si(500)/SiOx(0.1)  Ta(4)/Pt(4)/Co(1)/Gd(3)  Ta(4) 
Si(500)/SiOx(0.1)  Ta(4)/Pt(4)/Co(1)/Gd(3)  TaN(4) 
Si(500)/SiOx(0.1)  Ta(4)/Pt(4)/Co(1)/Gd(3)  TaN(4)/Pt(4) 
FIG. 2.

VSM-SQUID characterization of Ta/Pt/Co/Gd systems. (a) Characteristic OOP hysteresis loop measured at room temperature with VSM-SQUID. (b) Characteristic temperature-dependence of the OOP magnetic moment. Arrows schematically indicate the relative balance between Co (blue) and Gd (orange) magnetization. (c) Time evolution of the OOP moment, as indicated by the red arrow in (a), for various capping layers. (d) Time evolution of the compensation temperature, as indicated by the red circle in (b), for various capping layers. Solid lines are a guide to the eye. (e) Comparison of the OOP magnetic moment as a function of temperature of Pt-capped samples at different time intervals after growth: as grown (black), seven days old stored in ambient conditions (red), and seven days old stored in high vacuum (±5 × 10−10 mBar, blue).

FIG. 2.

VSM-SQUID characterization of Ta/Pt/Co/Gd systems. (a) Characteristic OOP hysteresis loop measured at room temperature with VSM-SQUID. (b) Characteristic temperature-dependence of the OOP magnetic moment. Arrows schematically indicate the relative balance between Co (blue) and Gd (orange) magnetization. (c) Time evolution of the OOP moment, as indicated by the red arrow in (a), for various capping layers. (d) Time evolution of the compensation temperature, as indicated by the red circle in (b), for various capping layers. Solid lines are a guide to the eye. (e) Comparison of the OOP magnetic moment as a function of temperature of Pt-capped samples at different time intervals after growth: as grown (black), seven days old stored in ambient conditions (red), and seven days old stored in high vacuum (±5 × 10−10 mBar, blue).

Close modal

A typical plot of the temperature dependence of m ̃ is shown in Fig. 2(b). At room temperature, and at these thicknesses of Co and Gd, the magnetization of the Co dominates the magnetic balance.7 For decreasing temperature, the magnetization of the Gd increases more rapidly than that of the Co, as is typical for the Gd S = 7/2 spin system. This manifests in Fig. 2(b) as the decrease in m ̃ with temperature. The temperature at which m ̃ crosses zero corresponds to the compensation temperature T comp; here, the moment of the Co and Gd atoms exactly cancels each other [ T comp 200 K in Fig. 2(b)]. We note that the magnetization compensation and angular momentum compensation point are not the same in these 3d/4f ferrimagnets due to the different Landé g-factor of Co34 and Gd;35 in the remainder of this work, we will refer to the magnetization compensation only. Finally, at even lower temperature [ 105 K in Fig. 2(b)], m ̃ largely vanishes due to a transition from OOP to in-plane magnetization owing to the increased shape anisotropy generated by the Gd magnetization. This overcomes the total perpendicular magnetic anisotropy, which mostly originates from the bottom Pt/Co interface.

These hysteresis loops and temperature sweeps were repeated over an extended period of time for two different capping layers, which are typically used to prevent oxidation: X = Pt(4), Ta(4). From each measurement, the remanent area-normalized net OOP magnetic moment at room temperature, m ̃ 0, and T comp were extracted as indicated by the red arrow and dot in Figs. 2(a) and 2(b), respectively. We plot the temporal evolution of m ̃ 0 and T Comp in Figs. 2(c) and 2(d), respectively. For these monatomic capping layers, we observe an increase (decrease) of m ̃ 0 ( T Comp) with time, which indicates a decrease in the Gd magnetic moment, since m ̃ 0 is dominated by the Co at room temperature. We hypothesize this to be a consequence of the intermixing between the capping layer and Gd, something that is known to occur for several metallic capping layers on RE/TM alloys.28 As the capping layer intermixes with the Gd, the induced magnetization in the Gd is quenched, causing the lower temperature needed to raise the Gd-magnetization to compensate that of the Co. The fact that this effect occurs more severely for Pt than for Ta may be a consequence of the larger number of intermetallic compounds between Pt and Gd vs Ta and Gd, 6 vs 0, respectively,36 which is typically assumed to impact the species mutual solid solubility with their alloy components. We have repeated this study on a second identically prepared batch of samples, which confirms these trends (see supplementary material 2 for details).

We recognize that the increase (decrease) in m ̃ 0 ( T Comp) shown in Figs. 2(c) and 2(d) may also be explained by oxidation of the Gd. To investigate the relevant importance of intermixing and oxidation during the aging process, we investigated the temperature dependence of m ̃ of two identical samples of the same structure as those listed in Table I with a Pt(4) capping layer. These were stored in vacuum ( 5 × 10 9 mBar) and under ambient conditions and compared to the OOP SQUID measurements on the as-grown sample. The resulting data are plotted in Fig. 2(e). It can be seen that the decrease in T Comp occurs irrespective of whether the sample has been exposed to ambient conditions or not. This suggests that the dominant mechanism for the changes in magnetic properties at these early stages of the aging process is indeed magnetization quenching by intermixing between the Gd and the monatomic capping layer.

To further test the hypothesis of the capping layer intermixing with the Gd layer, we move from a monatomic capping layer to the ceramic capping layer TaN, deposited by reactive sputtering from a pure Ta target in a mixed Ar:N2 environment. Nitrides like SiNx and AlNx are often used for the capping of RE-TM alloys, as they form a covalently bonded compound, rendering them very stable toward interdiffusion.18,29 When inspecting Figs. 2(c) and 2(d), we first note that a significantly decreased (increased) m ̃ 0 ( T Comp) is observed for both the as-deposited and aged samples capped with TaN, which is consistent with earlier observations.33 The origin of this change has not been unequivocally established. One explanation could, thus, be that intermixing processes between Gd and capping layer play a smaller role for the ceramic TaN than for the monatomic capping layers, leading to a reduced quenching of the Gd magnetization. However, we also note that due to exposure to the N2 gas during the reactive sputtering of TaN, formation of the ferromagnetic semiconductor GdN37 may also play a role here. The full characterization of the role potential Gd nitrification plays in the magnetic balance of this system would, however, require high-resolution depth-resolved magnetometry experiments (e.g., neutron scattering), which are beyond the current scope of this manuscript.

When inspecting the time evolution of m ̃ 0 and T Comp for the sample capped with TaN(4) in Figs. 1(c) and 1(d), we find that m ̃ 0 and T comp are passivated effectively for the first ∼35 days, after which m ̃ 0 ( T Comp) start to increase (decrease) similarly to the monatomic capping layers, which we hypothesize is due to (partial) oxidation of the TaN under ambient conditions, as reported in earlier work.38 To verify this, scanning transmission electron microscopy (STEM) and energy dispersive x-ray spectroscopy (EDX) measurements were performed on a sample with the same structure as the ones characterized with SQUID and a capping layer of TaN (see supplementary material 3 for experimental details). A bright field STEM image is presented in Fig. 3(a), where a contrast shift of the TaN at the TaN/air interface is observed. In the elemental depth profile obtained from the EDX characterization shown in Fig. 3(b), we find that this contrast shift indeed follows from a reduction of TaN to TaOx.

FIG. 3.

Chemical analysis of Ta/Pt/Co/Gd/TaN samples (see Table I). (a) Bright field scanning transmission electron microscopy image, where the different layers are labeled. (b) Part of the elemental profile of the sample investigated in (a) obtained by EDX. Relative peak intensities do not represent the atomic percentages. Shaded areas indicate the nominal layer thicknesses as listed in Table I and serve as a guide to the eye. The full profile of this lamella, including the C-contribution and the rest of the substrate and SiOx(C) layer, can be found in supplementary material. (c) Comparison of XPS survey spectra for an as-grown and three-month-old sample. (d) Comparison of the Gd4d photoelectron peak for an as-grown and three-month-old sample. Dashed lines indicate characteristic changes in the spectrum.

FIG. 3.

Chemical analysis of Ta/Pt/Co/Gd/TaN samples (see Table I). (a) Bright field scanning transmission electron microscopy image, where the different layers are labeled. (b) Part of the elemental profile of the sample investigated in (a) obtained by EDX. Relative peak intensities do not represent the atomic percentages. Shaded areas indicate the nominal layer thicknesses as listed in Table I and serve as a guide to the eye. The full profile of this lamella, including the C-contribution and the rest of the substrate and SiOx(C) layer, can be found in supplementary material. (c) Comparison of XPS survey spectra for an as-grown and three-month-old sample. (d) Comparison of the Gd4d photoelectron peak for an as-grown and three-month-old sample. Dashed lines indicate characteristic changes in the spectrum.

Close modal

This observation is corroborated by x-ray photoelectron spectroscopy measurements (XPS, see supplementary material 5 for experimental details). Figure 3(c) shows a significantly increased (decreased) oxygen (nitrogen) contribution to the spectrum when comparing the spectrum of a three-month-old sample to an as-grown sample, for samples with a TaN cap as listed in Table I. The gradual replacement of the TaN with TaOx could also explain the eventual disappearance of the passivating properties of TaN as a capping layer, as the oxygen in the TaOx reacts with the Gd or facilitates diffusion of oxygen to the Gd. The oxidation of the Gd can be investigated by comparison of the main 4d-photoemission peak complex of Gd for the same aged and fresh sample, as shown in Fig. 3(d). This yields two key differences: An 0.7 ± 0.2 eV shift of the peak at a binding energy of 141 eV and the repression of the peak at 147 eV. Both of these changes match the earlier reports on the change of the 4d-complex of Gd upon exposure to O2,39,40 confirming at least partial oxidation of the Gd at the Gd/TaN interface.

Considering how the results discussed above suggest that monatomic capping layers quench the Gd moment by intermixing and TaN cap is not as effective against oxidation, we suggest using a TaN(4)/Pt(4) capping layer to passivate the magnetic properties of Gd, where the Pt layer is introduced to protect the TaN from oxidizing. Here, in Figs. 2(c) and 2(d), within the measurement window of three months, we actually observe a small decrease (increase) of m ̃ 0 ( T Comp), suggesting a net increase in the Gd moment. Multilayers of Co and Gd are known to interdiffuse spontaneously,41,42 so it stands to reason that over time thermodynamically driven intermixing between the Co and Gd can be expected. Since the magnetization of Gd at these specific thicknesses occurs from the exchange interaction with the Co, a larger amount of Co neighbors will lead to a larger net moment in the Gd magnetic system. We conjecture that this has also occurred in the samples with the other capping layers but was offset by the dominant effect of simultaneous quenching of the Gd magnetization due to intermixing between Gd and the capping layer. We finally note that the as-grown measurement of the TaN/Pt sample is absent due to practical limitations related to the subsequent growth of the samples discussed in Figs. 2(c) and 2(d). However, since in this work, we are mainly interested in the trends over longer time periods, this does not affect the main conclusions of this work.

In conclusion, we have demonstrated that care has to be taken when considering layered synthetic ferrimagnets based on Gd for applications in spintronics. Significant aging effects were observed in the compensation temperature and the net moment of Co/Gd bilayer samples, which suggest a gradual quenching of the induced Gd magnetization over time. The aging effects driven by gradual intermixing described here are expected to not be restricted to the bilayer samples investigated in this work, but also to more complex synthetic ferrimagnetic multilayers based on Pt/Co/Gd-multilayers, which have recently garnered attention from the research community. Although we expect no direct effect on the fundamental properties of the layered systems, small changes in the net magnetic moment may be detrimental for applications, which require precise control over the magnetization. Ultimately, this work provides important guidelines for stack design involving Gd-terminated Co/Gd-based synthetic ferrimagnets and demonstrates that with proper care of the capping layer, their magnetic properties can be effectively passivated.

See the supplementary material for (1) details about the sputter deposition process; (2) a repeat study of the key experiment in the main text; (3) experiments regarding the EDX/STEM characterization; (4) a full range version of the EDX profile in Fig. 3(b); and (5) technical details of the XPS characterization.

This work was part of the research program Foundation for Fundamental Research on Matter (FOM) and Gravitation program “Research Center for Integrated Nanophotonics,” which are financed by the Dutch Research Council (NWO). This work was supported by the Eindhoven Hendrik Casimir Institute (EHCI). This project has also received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant Agreement No. 860060. Solliance and the Dutch province of Noord Brabant are acknowledged for funding the TEM facility. Peter Graat (Eurofins Materials Science Netherlands) is gratefully acknowledged for discussions on the TEM-EDX quantification.

The authors have no conflicts to disclose.

Thomas Kools: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (lead); Validation (lead); Visualization (lead); Writing – original draft (lead); Writing – review & editing (lead). Youri L. W. Van Hees: Conceptualization (equal); Data curation (supporting); Formal analysis (supporting); Investigation (supporting); Supervision (supporting); Validation (supporting); Writing – review & editing (supporting). Kenneth Poissonnier: Conceptualization (supporting); Data curation (supporting); Formal analysis (supporting); Investigation (supporting); Methodology (supporting); Validation (supporting); Writing – review & editing (supporting). Pingzhi Li: Conceptualization (supporting); Formal analysis (supporting); Investigation (supporting); Methodology (supporting); Writing – review & editing (supporting). B. Barcones Campo: Methodology (supporting); Writing – review & editing (supporting). M. A. Verheijen: Formal analysis (supporting); Investigation (supporting); Methodology (supporting); Writing – review & editing (supporting). Bert Koopmans: Conceptualization (equal); Funding acquisition (equal); Supervision (equal); Writing – review & editing (supporting). Reinoud Lavrijsen: Conceptualization (equal); Funding acquisition (equal); Supervision (equal); Writing – review & editing (equal).

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

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