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Interfacial exchange coupling between transition metals and Mn1.5Ga studied by X-ray magnetic circular dichroism
Accepted Manuscript Interfacial exchange coupling between transition metals and Mn1.5Ga studied by x-ray magnetic circular dichroism Jun Okabayashi, Kazuya Suzuki, Shigemi Mizukami PII: DOI: Reference:
S0304-8853(18)30098-2 https://doi.org/10.1016/j.jmmm.2018.03.060 MAGMA 63831
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
12 January 2018 21 February 2018 25 March 2018
Please cite this article as: J. Okabayashi, K. Suzuki, S. Mizukami, Interfacial exchange coupling between transition metals and Mn1.5Ga studied by x-ray magnetic circular dichroism, Journal of Magnetism and Magnetic Materials (2018), doi: https://doi.org/10.1016/j.jmmm.2018.03.060
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Interfacial exchange coupling between transition metals and Mn1.5Ga studied by x-ray magnetic circular dichroism Jun Okabayashi,1* Kazuya Suzuki,2 and Shigemi Mizukami2 1 Research Center for Spectrochemistry, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan 2 WPI-Advanced Institute for Materials Research, Tohoku University, Sendai 980-8579, Japan
(March 26, 2018) *e-mail address: [email protected]
We investigated the element-specific magnetic properties at interfaces of Fe (1 nm)/Mn1.5Ga and Co (1 nm)/Mn1.5Ga using x-ray magnetic circular dichroism (XMCD). We found that the spins in the Fe and Co layers were coupled with and parallel to those in the Mn1.5Ga under the as-grown conditions, while the post-annealing at 350 °C changed the interface magnetic coupling to antiferromagnetic in Co. The element-specific hysteresis curves at each XMCD absorption edge revealed large coercive fields in Fe and Co through the exchange coupling with Mn1.5Ga. After the annealing process, XMCD spectral line shapes indicate that the interfacial reactions promote the formation of Mn2CoGa-like secondary alloys, which influences the performance of tunnel magnetoresistance using the interfaces between transition metals and MnGa.
Magnetic ordered alloys have attracted significant attention for use as spintronics materials because they are highly likely to exhibit perpendicular magnetic anisotropy (PMA), which can be utilized in applications such as hard drives and spin-transfer torque magnetic random access memory (STT-MRAM) [1,2]. As promising spintronics materials, tetragonal L10-type MnGa and D022-type Mn3Ga alloys and their mixed alloy Mn3-Ga have been extensively investigated. These alloys exhibit high PMA energies of over 1.0 MJ/m3, low magnetic damping constants, and low saturation magnetizations originating from their ferrimagnetic properties that cause antiferromagnetic coupling at the different Mn sites [3-8]; thus, they have high potentials for use in magnetic tunnel junctions integrated in STT-MRAM [9-12]. The origin of strong magnetism 1
in Mn3-Ga compounds has been investigated by using first-principles band-structure calculations [13,14] and spectroscopies [12,15-17] and has been shown to result in distortion at different Mn sites, causing the appearance of PMA and spin polarizations as large as 88% [7]. Furthermore, Co substitution products such as Mn2CoGa or Co2MnGa [18,19], and the creation of magnetic tunnel junctions for tunneling magnetoresistance (TMR) using Mn3Ga/MgO/Mn3Ga stacks of over 600% and L10-type CoGa/MgO/MnGa with an epitaxial strain was also predicted [20-22]. Using the advantage that Mn3-Ga is a hard magnetic film, the deposition of other ferromagnetic materials on Mn3-Ga layers can be used to induce perpendicular magnetization through exchange interactions [23-27]. Ultra-thin Fe1-xCox layers deposited on Mn3-Ga couple ferromagnetically
or
antiferromagnetically,
depending
on
their
compositions
[26].
Antiferromagnetic coupling has been demonstrated in high-Co-concentration regions, while low-Co-concentration regions have been shown to exhibit ferromagnetic coupling. However, the abruptness and element-specific magnetic properties at the interfaces between Mn3-Ga and transition-metal (TM) layers must be clarified explicitly. In particular, TMR ratios higher than those in as-grown samples have been found to appear after annealing at 350 °C in order to maintain the highly crystallized body-centered-cubic structures of FeCo alloys [26]. X-ray magnetic circular dichroism (XMCD) can be a powerful tool for investigating the element-specific spin and orbital magnetic moments in the lower layers by probing beneath the 5-nm-thick surface layers wih falling the signal intensities exponentially, and it enables measurement of element-specific hysteresis curves. In a previous report, an XMCD study that was used to examine an FeCoB/Mn3Ga interface was presented, and the ferromagnetic coupling in the as-grown conditions was discussed [15]. The Co/Mg1.5Ga interfaces with and without annealing processes are also examined by XMCD hysteresis curves. Antiparallel coupling between Co and Mn1.5Ga can be detected by XMCD. The importance of Ga termination was also anticipated [28].
Rode et al. performed XMCD analysis to investigate Mn3-Ga [12]. Because
two kinds of Mn sites exist, the separation of XMCD spectra into components with the standard magneto-optical sum rule analysis is difficult. Another means of deducing the element-specific 2
magnetic moments involves using the interfacial exchange coupling effect, which was the focus of this investigation. In this paper, we present our study in which XMCD was employed to investigate the element-specific magnetic properties at TMs/Mn1.5Ga interfaces. In particular, we discuss the interfacial coupling, which may be ferromagnetic or antiferromagnetic depending on the annealing of the samples. The samples were prepared by magnetron sputtering. The 40-nm-thick Cr buffer layers were deposited on single-crystal MgO (001) substrates at room temperature, and in situ annealing at 700 °C was performed. Subsequently, 30-nm-thick L10-type Mn1.5Ga layers were grown at room temperature with in situ annealing at 500 °C. Fe and Co were sputtered at room temperature to form a layer 1 nm thick and were capped with a 2-nm-thick MgO layer. A piece of an “as-grown” sample of MgO (2 nm)/TMs (1 nm)/Mn1.5Ga (30 nm)/Cr (40 nm)/MgO (001) was annealed at 350 °C for 10 min to prepare the “annealed” sample. The composition of Mn1.5Ga was determined by the Inductively Coupled Plasma mass spectrometry measurements. The details of the employed sample preparation method were reported in Ref. [26]. The magneto-optical polar Kerr effect (MOKE) measurement was carried out with a laser wavelength of about 400 nm and the maximum magnetic field of ±2 T. The XMCD analysis was performed at BL-7A in the Photon Factory at the High-Energy Accelerator Research Organization (KEK). The photon helicity was fixed, and a magnetic field of ±1.2 T was applied parallel to the incident polarized soft X-ray beam and defined as + and spectra. The total electron yield mode was adopted, and all measurements were performed at room temperature. The x-ray absorption spectroscopy (XAS) and XMCD measurement geometries were set to normal incidence, so that both the photon helicity and the magnetic field were normal to the surface, enabling measurement of the absorption processes involving the normal components of the spin and orbital angular momenta. Figure 1 shows the hysteresis curves for the as-grown and annealed samples that were measured by using the MOKE for Co/Mn1.5Ga and Fe/Mn1.5Ga. In this method, the magnetization for both TM and Mn1.5Ga within the light penetration depth was observed, and the 3
components of the magnetization normal to the sample surface were detected via the Kerr rotation. The remanent magnetizations at zero magnetic field clearly decrease after the annealing, and the Kerr rotation angles increase gradually with the magnetic field in the case of Co, suggesting that the unsaturated in-plane magnetization components appear together with the PMA components. Cross-sectional transmission electron microscope (TEM) images of the annealed sample in Fe and Co on Mn3-Ga were reported [20,21]. All of the layers are crystallized in epitaxial forms in the lattice image. The interfaces between the TMs and Mn1.5Ga layers are also identifiable in this image, although inter-diffusion might be expected in some local areas. Figure 2 shows the polarization dependences of the XAS and XMCD results for the Mn and Co L2,3-edges in Co/Mn1.5Ga. Clear metallic spectral line shapes are observable for the Fe and Co L-edges [29,30]. The broad line shape with the shoulder structures in the XAS results for the Mn L-edge is quite similar to those shown in previous reports because the two kinds of Mn components overlap [12,15,17]. The spins of Mn and Co are coupled ferromagnetically. Because of the ferrimagnetic nature of Mn1.5Ga, the spins of the Mn sites almost cancel, and the XMCD intensity of the Mn L-edge is smaller than that of Co. After the annealing, the XMCD intensities in both Mn and Co L-edges become small, and the signs at the L3 and L2 edges in Co change. By comparing these spectral line shapes with those in the as-grown case, it is evident that the XAS intensity ratios between Mn and Co are also modulated, which suggests that the Co atoms diffuse chemically into the MnGa layer, or vice versa, within the probing depth. In the Mn L-edge XAS results, little difference between the + and - XAS peaks due to the annealing process is evident, which indicates that the other antiferromagnetic Mn compounds are formed by interfacial reactions. The changes of XAS intensities are shown in Fig. S1. The Mn L-edge XMCD line shape shows two chemically shifted components, although the sign of the XMCD remains unchanged by the annealing. Generally, 1-nm-thick Co layers capped with 2-nm-thick MgO exhibit clear XMCD signals identical to those shown in the as-grown case. The fact that the differences between the + and - XAS peaks of the Co L-edges are only slight suggests that the spin magnetic moments of Co are also 4
suppressed. Opposite XMCD signs are observed in Co, and the XMCD intensities are different from those in the as-grown case. The Co XAS line shapes exhibit small shoulder structures at the higher-energy side, which do not influence the XMCD line shapes. It might be originated from the Co-based Heusler alloy formation. Antiferromagnetic coupling between Co and Mn1.5Ga is consistent with the previous studies [26]. The element-specific hysteresis curves for the as-grown and annealed samples are shown in the insets of Fig. 2. The photon energies at the Mn and Co L3-edges were fixed in the normal incidence setup. In the as-grown sample, the clear hysteresis loops with a coercive field (Hc) of 0.4 T indicate PMA. In the high magnetic field regions, the intensities of Mn are saturated and become constant with almost square while Co L-edge exhibits different in the high magnetic field regions with unsaturated slope. This suggests that there is no intermixing between Co and Mn1.5Ga at as-grown stage. In fact, the MOKE hysteresis curve shown in Fig. 1 also exhibits slightly increasing features in high field regions. Thus, the element-resolved hysteresis curves can clearly reveal the details of the components canted from the surface normal direction in Co. After the annealing, interestingly, the hysteresis loops for both elements change drastically. Because of the small XMCD intensities, the statistics of the hysteresis loops are noisier than those in the as-grown cases. The characteristic features not only of the PMA, but also of the unsaturated slopes, are clearly observable for both elements and indicate the appearance of secondary phases at the interfaces with hard-axis components. The hysteresis loop of the Co L-edges is opposite to that of Mn, causing the XMCD signs in Co to be different. On the other hand, the slopes of the hard-axis components and the Hc values are common for both elements. For Mn, both the PMA and the in-plane components are overlapped, with the same spin direction. The PMA contributions result from the bulk Mn1.5Ga. The slopes of Co hysteresis curves are influenced by the interfacial secondary alloy phases with anti-parallel coupling. Next, the XAS and XMCD in the interface between Fe and Mn1.5Ga are shown in Fig. 3 in the same manner as those in Co/Mn1.5Ga. In the case of as-grown Fe/MnGa, the line shapes of Mn XAS and XMCD is quite similar to those in Co/MnGa, which suggests that there is no interfacial reaction at the as-grown stage. Clear XAS and XMCD in Fe L-edge are observed 5
which is similar to the bulk Fe. However, hysteresis curve shown in the inset of Fig. 3 exhibits the summation of exchange coupling from MnGa and in-plane anisotropy, which is saturated at ±0.7 T and is consistent with the MOKE results. After the annealing, the XMCD intensities of both Mn and Fe are suppressed although the line shapes remain unchanged, which is quite different from the case of Co/Mn1.5Ga. XAS intensities also modulated as shown in Fig. S2. Element-specific hysteresis curves become similar, which suggests the Fe-Mn-Ga alloy formation by the annealing with the parallel coupling between Mn and Fe. Contrary to the case of Co/Mn1.5Ga, there is no slope unsaturated at ±1.0 T. The suppression of XMCD intensities might originate from the non-magnetic Fe-Mn-Ga alloy formation. These element-specific interfacial analyses cannot be detected by the TEM technique because of nearly equal scattering amplitudes from Mn and Fe. Considering the above results, we can discuss the magnetic properties at the interfaces before and after the annealing in terms of three aspects: the exchange coupling at the interfaces, the PMA at the TMs/MgO interfaces, and the chemical reactions at the interfaces. When parallel coupling is dominant, the strong perpendicular magnetization in the TM layers is induced by exchange coupling with Mn1.5Ga accompanied by the large Hc values of the Fe and Co hysteresis curves. In the as-grown stages, the ratio of spin (mspin) and orbital (morb) magnetic moments morb/mspin was found to be 0.09 and 0.12 for Fe and Co, respectively, which is consistent with the cases of bulk Fe and Co [31]. It suggests that the interfacial reaction does not occur and PMA and large Hc is induced by the exchange coupling from Mn1.5Ga layer. By annealing, anti-parallel exchange coupling becomes dominant in Co. Since the common slopes appear for both Mn and Co hysteresis curves, Co-Mn-Ga alloy formation becomes a possible candidate. Ternary-alloy phase diagram suggests the formations of Heusler-type Co2MnGa or Mn2CoGa [32]. In case of Co-based Heusler alloy; Co2XY where X and Y represent TMs and non-magnetic elements, respectively, Co XAS line shapes possess the shoulder structure [33,34]. The Co XAS and XMCD line shapes of CoGa and MnCo are also different from those in Fig. 2 [22,35]. Therefore, Mn2CoGa-like compounds are most likely to the interfacial layer where the Mn has two kinds of inequivalent sites and is coupled antiferromagnetically with Co [36]. In order to confirm the 6
alloy formation by annealing, angle dependence of XAS intensity ratios is plotted in Fig. 4. As shown in Fig. 4(a), by tilting the angle between beam incidence and sample surface normal directions, the Co intensities are enhanced in as-grown case because the Co layer is stacked on Mn1.5Ga. After the annealing, the ratios almost remain unchanged due to uniform alloy formation within the probing depth of XAS. On the other hand, Fe-Mn-Ga case is different. Ternary-alloy phase diagram of Fe-Mn-Ga exhibits little mixing between Fe and MnGa [36]. It suggests that the annealing promotes the interfacial coupling between Fe and MnGa, which is evident from the similar hysteresis curves after the annealing and maintaining the similar XMCD line shapes. The suppression of XMCD intensities might be derived from the non-magnetic interfacial layer formation. Furthermore, the annealing promotes the strong chemical bonding between Fe and MgO, resulting in interfacial PMA [38]. The exchange coupling at the Fe/Mn1.5Ga interface is also sustained by the interfacial PMA in Fe/MgO . After the annealing, there is a small slope in Fig. 4(b), which is evident of Fe/MgO formation at the upper interface in Fe. We emphasize that the interfacial secondary phases formed by annealing are not observable in a TEM image but clearly detected by the XMCD line shapes and element-specific hysteresis curves. Although the annealing process at 350 °C is necessary to obtain high quality TMs/MgO interfaces, it inevitably produces an interfacial layer at the Mn1.5Ga/TMs boundaries, which contributes slightly to the TMR properties [20,21]. Furthermore, we performed similar measurements for Cr/Mn1.5Ga case as a reference. The results, shown in Fig. S3, clearly exhibit the no-interfacial reaction or small reactivity by the annealing, which argues that the interfacial reaction depends on the elements. Next, we analyze the changes of Mn L-edge XMCD line shapes by annealing using the spectral deconvolution in Co/Mn1.5Ga. Broad L3 peaks, compared with the metallic Mn case [35], are deconvoluted at least into two components. Figure 5 exhibits the deconvoluted Mn L3,2-edge XMCD spectra for as-grown and annealed cases except the intermediate regions at 647 eV. In many cases of Mn XMCD, there appear small multiplet structures in the opposite sign with L3 at around 637 eV. One component keeps unchanged in the peak positions by annealing. The other shifts higher energies in both L3 and L2 edges, suggesting that the alloy component appears 7
through the hybridization with Co. Our fitting procedure suggests one of the possibilities for spectral analysis. The contributions from other components with the opposite sign also have to be considered. The unchanged peak originates from the Mn site which consists of L10-type ordered MnGa and the other deconvoluted peak exhibits the different Mn site which reacts with Co. The changes of intensity ratios suggest that the annealing promotes the L10-type MnGa formation with the Mn2CoGa-like alloy. Although recent investigations of Co/MnGa discuss the interfacial termination of Mn or Ga facing on Co [28,39], the real case is governed by the interfacial atomic diffusion because of high reactivity between Mn and Co atoms. In summary, by using XMCD, we found that the spins in TMs were coupled with and parallel to those in Mn1.5Ga under the as-grown conditions, while the post-annealing at 350 °C changed the interface magnetic coupling to antiferromagnetic in Co and ferromagnetic in Fe. The element-specific hysteresis curves at each XMCD absorption edge revealed large coercive fields in Fe and Co that obeyed the magnetic properties of Mn1.5Ga. After the annealing, alloy formation was observed from the XMCD spectral line shapes, and hysteresis curves in each absorption edge are also modulated through the interfacial alloy formation.
The authors thank Professor Masafumi Shirai for the fruitful discussion. This work was partly supported by the QPI program, the Asahi Glass Foundation. This work was partly supported by a Grant-in-Aid for Scientific Research (B) (Grant No. 15H03562) and (S) (Grant Nos 25220604, 16H06332) from the Japan Society for the Promotion of Science and Center for Spintronics Research Network (CSRN) of Japan. The synchrotron radiation experiments were performed with the approval of the Photon Factory Program Advisory Committee, KEK (Nos. 2015G090 and 2017G060).
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Figure captions:
Fig. 1 (Color online). MOKEs of as-grown and annealed samples of (a) Co/Mn1.5Ga and (b) Fe/Mn1.5Ga. Fig. 2 (Color online). XAS and XMCD spectra of Mn and Co L-edges in Co/Mn1.5Ga. (a) as-grown and (b) annealed cases. Insets show the element-specific hysteresis curves taken at L3-edges. The units of magnetic field in horizontal axis are Tesla. 11
Fig. 3 (Color online). XAS and XMCD spectra of Mn and Fe L-edges in Fe/Mn1.5Ga. (a) as-grown and (b) annealed cases. Insets show the element-specific hysteresis curves taken at L3-edges. The units of magnetic field in horizontal axis are Tesla.
Fig. 4 (Color online). XAS intensity ratio of TM to Mn L-edges depending on angle for as-grown and annealed cases in (a) Co/Mn1.5Ga and (b) Fe/Mn1.5Ga. Inset shows the sample geometry and definition of angle. Incident beam and magnetic field directions are fixed parallel. The regions displayed in all vertical axes are adjusted in the same ranges.
Fig. 5 (Color online). XMCD spectra of Mn L-edge with deconvoluted components in Co/Mn1.5Ga. (a) as-grown and (b) annealed cases.
Fig.1
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Fig.2
Fig. 3
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Fig. 4
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Interfacial exchange coupling between transition metals and Mn1.5Ga studied by x-ray magnetic circular dichroism Jun Okabayashi,1* Kazuya Suzuki,2 and Shigemi Mizukami2 1 Research Center for Spectrochemistry, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan 2 WPI-Advanced Institute for Materials Research, Tohoku University, Sendai 980-8579, Japan
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We investigated the element-specific magnetic properties at interfaces of Fe (1 nm)/Mn1.5Ga and Co (1 nm)/Mn1.5Ga using x-ray magnetic circular dichroism (XMCD). We found that the spins in the Fe and Co layers were coupled with and parallel to those in the Mn1.5Ga under the as-grown conditions, while the post-annealing at 350 °C changed the interface magnetic coupling to antiferromagnetic in Co. The element-specific hysteresis curves at each XMCD absorption edge revealed large coercive fields in Fe and Co through the exchange coupling with Mn1.5Ga. After the annealing process, XMCD spectral line shapes indicate that the interfacial reactions promote the formation of Mn2CoGa-like secondary alloys, which influences the performance of tunnel magnetoresistance using the interfaces between transition metals and MnGa.
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S0304-8853(18)30098-2 https://doi.org/10.1016/j.jmmm.2018.03.060 MAGMA 63831
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
12 January 2018 21 February 2018 25 March 2018
Please cite this article as: J. Okabayashi, K. Suzuki, S. Mizukami, Interfacial exchange coupling between transition metals and Mn1.5Ga studied by x-ray magnetic circular dichroism, Journal of Magnetism and Magnetic Materials (2018), doi: https://doi.org/10.1016/j.jmmm.2018.03.060
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Interfacial exchange coupling between transition metals and Mn1.5Ga studied by x-ray magnetic circular dichroism Jun Okabayashi,1* Kazuya Suzuki,2 and Shigemi Mizukami2 1 Research Center for Spectrochemistry, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan 2 WPI-Advanced Institute for Materials Research, Tohoku University, Sendai 980-8579, Japan
(March 26, 2018) *e-mail address: [email protected]
We investigated the element-specific magnetic properties at interfaces of Fe (1 nm)/Mn1.5Ga and Co (1 nm)/Mn1.5Ga using x-ray magnetic circular dichroism (XMCD). We found that the spins in the Fe and Co layers were coupled with and parallel to those in the Mn1.5Ga under the as-grown conditions, while the post-annealing at 350 °C changed the interface magnetic coupling to antiferromagnetic in Co. The element-specific hysteresis curves at each XMCD absorption edge revealed large coercive fields in Fe and Co through the exchange coupling with Mn1.5Ga. After the annealing process, XMCD spectral line shapes indicate that the interfacial reactions promote the formation of Mn2CoGa-like secondary alloys, which influences the performance of tunnel magnetoresistance using the interfaces between transition metals and MnGa.
Magnetic ordered alloys have attracted significant attention for use as spintronics materials because they are highly likely to exhibit perpendicular magnetic anisotropy (PMA), which can be utilized in applications such as hard drives and spin-transfer torque magnetic random access memory (STT-MRAM) [1,2]. As promising spintronics materials, tetragonal L10-type MnGa and D022-type Mn3Ga alloys and their mixed alloy Mn3-Ga have been extensively investigated. These alloys exhibit high PMA energies of over 1.0 MJ/m3, low magnetic damping constants, and low saturation magnetizations originating from their ferrimagnetic properties that cause antiferromagnetic coupling at the different Mn sites [3-8]; thus, they have high potentials for use in magnetic tunnel junctions integrated in STT-MRAM [9-12]. The origin of strong magnetism 1
in Mn3-Ga compounds has been investigated by using first-principles band-structure calculations [13,14] and spectroscopies [12,15-17] and has been shown to result in distortion at different Mn sites, causing the appearance of PMA and spin polarizations as large as 88% [7]. Furthermore, Co substitution products such as Mn2CoGa or Co2MnGa [18,19], and the creation of magnetic tunnel junctions for tunneling magnetoresistance (TMR) using Mn3Ga/MgO/Mn3Ga stacks of over 600% and L10-type CoGa/MgO/MnGa with an epitaxial strain was also predicted [20-22]. Using the advantage that Mn3-Ga is a hard magnetic film, the deposition of other ferromagnetic materials on Mn3-Ga layers can be used to induce perpendicular magnetization through exchange interactions [23-27]. Ultra-thin Fe1-xCox layers deposited on Mn3-Ga couple ferromagnetically
or
antiferromagnetically,
depending
on
their
compositions
[26].
Antiferromagnetic coupling has been demonstrated in high-Co-concentration regions, while low-Co-concentration regions have been shown to exhibit ferromagnetic coupling. However, the abruptness and element-specific magnetic properties at the interfaces between Mn3-Ga and transition-metal (TM) layers must be clarified explicitly. In particular, TMR ratios higher than those in as-grown samples have been found to appear after annealing at 350 °C in order to maintain the highly crystallized body-centered-cubic structures of FeCo alloys [26]. X-ray magnetic circular dichroism (XMCD) can be a powerful tool for investigating the element-specific spin and orbital magnetic moments in the lower layers by probing beneath the 5-nm-thick surface layers wih falling the signal intensities exponentially, and it enables measurement of element-specific hysteresis curves. In a previous report, an XMCD study that was used to examine an FeCoB/Mn3Ga interface was presented, and the ferromagnetic coupling in the as-grown conditions was discussed [15]. The Co/Mg1.5Ga interfaces with and without annealing processes are also examined by XMCD hysteresis curves. Antiparallel coupling between Co and Mn1.5Ga can be detected by XMCD. The importance of Ga termination was also anticipated [28].
Rode et al. performed XMCD analysis to investigate Mn3-Ga [12]. Because
two kinds of Mn sites exist, the separation of XMCD spectra into components with the standard magneto-optical sum rule analysis is difficult. Another means of deducing the element-specific 2
magnetic moments involves using the interfacial exchange coupling effect, which was the focus of this investigation. In this paper, we present our study in which XMCD was employed to investigate the element-specific magnetic properties at TMs/Mn1.5Ga interfaces. In particular, we discuss the interfacial coupling, which may be ferromagnetic or antiferromagnetic depending on the annealing of the samples. The samples were prepared by magnetron sputtering. The 40-nm-thick Cr buffer layers were deposited on single-crystal MgO (001) substrates at room temperature, and in situ annealing at 700 °C was performed. Subsequently, 30-nm-thick L10-type Mn1.5Ga layers were grown at room temperature with in situ annealing at 500 °C. Fe and Co were sputtered at room temperature to form a layer 1 nm thick and were capped with a 2-nm-thick MgO layer. A piece of an “as-grown” sample of MgO (2 nm)/TMs (1 nm)/Mn1.5Ga (30 nm)/Cr (40 nm)/MgO (001) was annealed at 350 °C for 10 min to prepare the “annealed” sample. The composition of Mn1.5Ga was determined by the Inductively Coupled Plasma mass spectrometry measurements. The details of the employed sample preparation method were reported in Ref. [26]. The magneto-optical polar Kerr effect (MOKE) measurement was carried out with a laser wavelength of about 400 nm and the maximum magnetic field of ±2 T. The XMCD analysis was performed at BL-7A in the Photon Factory at the High-Energy Accelerator Research Organization (KEK). The photon helicity was fixed, and a magnetic field of ±1.2 T was applied parallel to the incident polarized soft X-ray beam and defined as + and spectra. The total electron yield mode was adopted, and all measurements were performed at room temperature. The x-ray absorption spectroscopy (XAS) and XMCD measurement geometries were set to normal incidence, so that both the photon helicity and the magnetic field were normal to the surface, enabling measurement of the absorption processes involving the normal components of the spin and orbital angular momenta. Figure 1 shows the hysteresis curves for the as-grown and annealed samples that were measured by using the MOKE for Co/Mn1.5Ga and Fe/Mn1.5Ga. In this method, the magnetization for both TM and Mn1.5Ga within the light penetration depth was observed, and the 3
components of the magnetization normal to the sample surface were detected via the Kerr rotation. The remanent magnetizations at zero magnetic field clearly decrease after the annealing, and the Kerr rotation angles increase gradually with the magnetic field in the case of Co, suggesting that the unsaturated in-plane magnetization components appear together with the PMA components. Cross-sectional transmission electron microscope (TEM) images of the annealed sample in Fe and Co on Mn3-Ga were reported [20,21]. All of the layers are crystallized in epitaxial forms in the lattice image. The interfaces between the TMs and Mn1.5Ga layers are also identifiable in this image, although inter-diffusion might be expected in some local areas. Figure 2 shows the polarization dependences of the XAS and XMCD results for the Mn and Co L2,3-edges in Co/Mn1.5Ga. Clear metallic spectral line shapes are observable for the Fe and Co L-edges [29,30]. The broad line shape with the shoulder structures in the XAS results for the Mn L-edge is quite similar to those shown in previous reports because the two kinds of Mn components overlap [12,15,17]. The spins of Mn and Co are coupled ferromagnetically. Because of the ferrimagnetic nature of Mn1.5Ga, the spins of the Mn sites almost cancel, and the XMCD intensity of the Mn L-edge is smaller than that of Co. After the annealing, the XMCD intensities in both Mn and Co L-edges become small, and the signs at the L3 and L2 edges in Co change. By comparing these spectral line shapes with those in the as-grown case, it is evident that the XAS intensity ratios between Mn and Co are also modulated, which suggests that the Co atoms diffuse chemically into the MnGa layer, or vice versa, within the probing depth. In the Mn L-edge XAS results, little difference between the + and - XAS peaks due to the annealing process is evident, which indicates that the other antiferromagnetic Mn compounds are formed by interfacial reactions. The changes of XAS intensities are shown in Fig. S1. The Mn L-edge XMCD line shape shows two chemically shifted components, although the sign of the XMCD remains unchanged by the annealing. Generally, 1-nm-thick Co layers capped with 2-nm-thick MgO exhibit clear XMCD signals identical to those shown in the as-grown case. The fact that the differences between the + and - XAS peaks of the Co L-edges are only slight suggests that the spin magnetic moments of Co are also 4
suppressed. Opposite XMCD signs are observed in Co, and the XMCD intensities are different from those in the as-grown case. The Co XAS line shapes exhibit small shoulder structures at the higher-energy side, which do not influence the XMCD line shapes. It might be originated from the Co-based Heusler alloy formation. Antiferromagnetic coupling between Co and Mn1.5Ga is consistent with the previous studies [26]. The element-specific hysteresis curves for the as-grown and annealed samples are shown in the insets of Fig. 2. The photon energies at the Mn and Co L3-edges were fixed in the normal incidence setup. In the as-grown sample, the clear hysteresis loops with a coercive field (Hc) of 0.4 T indicate PMA. In the high magnetic field regions, the intensities of Mn are saturated and become constant with almost square while Co L-edge exhibits different in the high magnetic field regions with unsaturated slope. This suggests that there is no intermixing between Co and Mn1.5Ga at as-grown stage. In fact, the MOKE hysteresis curve shown in Fig. 1 also exhibits slightly increasing features in high field regions. Thus, the element-resolved hysteresis curves can clearly reveal the details of the components canted from the surface normal direction in Co. After the annealing, interestingly, the hysteresis loops for both elements change drastically. Because of the small XMCD intensities, the statistics of the hysteresis loops are noisier than those in the as-grown cases. The characteristic features not only of the PMA, but also of the unsaturated slopes, are clearly observable for both elements and indicate the appearance of secondary phases at the interfaces with hard-axis components. The hysteresis loop of the Co L-edges is opposite to that of Mn, causing the XMCD signs in Co to be different. On the other hand, the slopes of the hard-axis components and the Hc values are common for both elements. For Mn, both the PMA and the in-plane components are overlapped, with the same spin direction. The PMA contributions result from the bulk Mn1.5Ga. The slopes of Co hysteresis curves are influenced by the interfacial secondary alloy phases with anti-parallel coupling. Next, the XAS and XMCD in the interface between Fe and Mn1.5Ga are shown in Fig. 3 in the same manner as those in Co/Mn1.5Ga. In the case of as-grown Fe/MnGa, the line shapes of Mn XAS and XMCD is quite similar to those in Co/MnGa, which suggests that there is no interfacial reaction at the as-grown stage. Clear XAS and XMCD in Fe L-edge are observed 5
which is similar to the bulk Fe. However, hysteresis curve shown in the inset of Fig. 3 exhibits the summation of exchange coupling from MnGa and in-plane anisotropy, which is saturated at ±0.7 T and is consistent with the MOKE results. After the annealing, the XMCD intensities of both Mn and Fe are suppressed although the line shapes remain unchanged, which is quite different from the case of Co/Mn1.5Ga. XAS intensities also modulated as shown in Fig. S2. Element-specific hysteresis curves become similar, which suggests the Fe-Mn-Ga alloy formation by the annealing with the parallel coupling between Mn and Fe. Contrary to the case of Co/Mn1.5Ga, there is no slope unsaturated at ±1.0 T. The suppression of XMCD intensities might originate from the non-magnetic Fe-Mn-Ga alloy formation. These element-specific interfacial analyses cannot be detected by the TEM technique because of nearly equal scattering amplitudes from Mn and Fe. Considering the above results, we can discuss the magnetic properties at the interfaces before and after the annealing in terms of three aspects: the exchange coupling at the interfaces, the PMA at the TMs/MgO interfaces, and the chemical reactions at the interfaces. When parallel coupling is dominant, the strong perpendicular magnetization in the TM layers is induced by exchange coupling with Mn1.5Ga accompanied by the large Hc values of the Fe and Co hysteresis curves. In the as-grown stages, the ratio of spin (mspin) and orbital (morb) magnetic moments morb/mspin was found to be 0.09 and 0.12 for Fe and Co, respectively, which is consistent with the cases of bulk Fe and Co [31]. It suggests that the interfacial reaction does not occur and PMA and large Hc is induced by the exchange coupling from Mn1.5Ga layer. By annealing, anti-parallel exchange coupling becomes dominant in Co. Since the common slopes appear for both Mn and Co hysteresis curves, Co-Mn-Ga alloy formation becomes a possible candidate. Ternary-alloy phase diagram suggests the formations of Heusler-type Co2MnGa or Mn2CoGa [32]. In case of Co-based Heusler alloy; Co2XY where X and Y represent TMs and non-magnetic elements, respectively, Co XAS line shapes possess the shoulder structure [33,34]. The Co XAS and XMCD line shapes of CoGa and MnCo are also different from those in Fig. 2 [22,35]. Therefore, Mn2CoGa-like compounds are most likely to the interfacial layer where the Mn has two kinds of inequivalent sites and is coupled antiferromagnetically with Co [36]. In order to confirm the 6
alloy formation by annealing, angle dependence of XAS intensity ratios is plotted in Fig. 4. As shown in Fig. 4(a), by tilting the angle between beam incidence and sample surface normal directions, the Co intensities are enhanced in as-grown case because the Co layer is stacked on Mn1.5Ga. After the annealing, the ratios almost remain unchanged due to uniform alloy formation within the probing depth of XAS. On the other hand, Fe-Mn-Ga case is different. Ternary-alloy phase diagram of Fe-Mn-Ga exhibits little mixing between Fe and MnGa [36]. It suggests that the annealing promotes the interfacial coupling between Fe and MnGa, which is evident from the similar hysteresis curves after the annealing and maintaining the similar XMCD line shapes. The suppression of XMCD intensities might be derived from the non-magnetic interfacial layer formation. Furthermore, the annealing promotes the strong chemical bonding between Fe and MgO, resulting in interfacial PMA [38]. The exchange coupling at the Fe/Mn1.5Ga interface is also sustained by the interfacial PMA in Fe/MgO . After the annealing, there is a small slope in Fig. 4(b), which is evident of Fe/MgO formation at the upper interface in Fe. We emphasize that the interfacial secondary phases formed by annealing are not observable in a TEM image but clearly detected by the XMCD line shapes and element-specific hysteresis curves. Although the annealing process at 350 °C is necessary to obtain high quality TMs/MgO interfaces, it inevitably produces an interfacial layer at the Mn1.5Ga/TMs boundaries, which contributes slightly to the TMR properties [20,21]. Furthermore, we performed similar measurements for Cr/Mn1.5Ga case as a reference. The results, shown in Fig. S3, clearly exhibit the no-interfacial reaction or small reactivity by the annealing, which argues that the interfacial reaction depends on the elements. Next, we analyze the changes of Mn L-edge XMCD line shapes by annealing using the spectral deconvolution in Co/Mn1.5Ga. Broad L3 peaks, compared with the metallic Mn case [35], are deconvoluted at least into two components. Figure 5 exhibits the deconvoluted Mn L3,2-edge XMCD spectra for as-grown and annealed cases except the intermediate regions at 647 eV. In many cases of Mn XMCD, there appear small multiplet structures in the opposite sign with L3 at around 637 eV. One component keeps unchanged in the peak positions by annealing. The other shifts higher energies in both L3 and L2 edges, suggesting that the alloy component appears 7
through the hybridization with Co. Our fitting procedure suggests one of the possibilities for spectral analysis. The contributions from other components with the opposite sign also have to be considered. The unchanged peak originates from the Mn site which consists of L10-type ordered MnGa and the other deconvoluted peak exhibits the different Mn site which reacts with Co. The changes of intensity ratios suggest that the annealing promotes the L10-type MnGa formation with the Mn2CoGa-like alloy. Although recent investigations of Co/MnGa discuss the interfacial termination of Mn or Ga facing on Co [28,39], the real case is governed by the interfacial atomic diffusion because of high reactivity between Mn and Co atoms. In summary, by using XMCD, we found that the spins in TMs were coupled with and parallel to those in Mn1.5Ga under the as-grown conditions, while the post-annealing at 350 °C changed the interface magnetic coupling to antiferromagnetic in Co and ferromagnetic in Fe. The element-specific hysteresis curves at each XMCD absorption edge revealed large coercive fields in Fe and Co that obeyed the magnetic properties of Mn1.5Ga. After the annealing, alloy formation was observed from the XMCD spectral line shapes, and hysteresis curves in each absorption edge are also modulated through the interfacial alloy formation.
The authors thank Professor Masafumi Shirai for the fruitful discussion. This work was partly supported by the QPI program, the Asahi Glass Foundation. This work was partly supported by a Grant-in-Aid for Scientific Research (B) (Grant No. 15H03562) and (S) (Grant Nos 25220604, 16H06332) from the Japan Society for the Promotion of Science and Center for Spintronics Research Network (CSRN) of Japan. The synchrotron radiation experiments were performed with the approval of the Photon Factory Program Advisory Committee, KEK (Nos. 2015G090 and 2017G060).
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Figure captions:
Fig. 1 (Color online). MOKEs of as-grown and annealed samples of (a) Co/Mn1.5Ga and (b) Fe/Mn1.5Ga. Fig. 2 (Color online). XAS and XMCD spectra of Mn and Co L-edges in Co/Mn1.5Ga. (a) as-grown and (b) annealed cases. Insets show the element-specific hysteresis curves taken at L3-edges. The units of magnetic field in horizontal axis are Tesla. 11
Fig. 3 (Color online). XAS and XMCD spectra of Mn and Fe L-edges in Fe/Mn1.5Ga. (a) as-grown and (b) annealed cases. Insets show the element-specific hysteresis curves taken at L3-edges. The units of magnetic field in horizontal axis are Tesla.
Fig. 4 (Color online). XAS intensity ratio of TM to Mn L-edges depending on angle for as-grown and annealed cases in (a) Co/Mn1.5Ga and (b) Fe/Mn1.5Ga. Inset shows the sample geometry and definition of angle. Incident beam and magnetic field directions are fixed parallel. The regions displayed in all vertical axes are adjusted in the same ranges.
Fig. 5 (Color online). XMCD spectra of Mn L-edge with deconvoluted components in Co/Mn1.5Ga. (a) as-grown and (b) annealed cases.
Fig.1
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Fig.2
Fig. 3
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Interfacial exchange coupling between transition metals and Mn1.5Ga studied by x-ray magnetic circular dichroism Jun Okabayashi,1* Kazuya Suzuki,2 and Shigemi Mizukami2 1 Research Center for Spectrochemistry, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan 2 WPI-Advanced Institute for Materials Research, Tohoku University, Sendai 980-8579, Japan
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We investigated the element-specific magnetic properties at interfaces of Fe (1 nm)/Mn1.5Ga and Co (1 nm)/Mn1.5Ga using x-ray magnetic circular dichroism (XMCD). We found that the spins in the Fe and Co layers were coupled with and parallel to those in the Mn1.5Ga under the as-grown conditions, while the post-annealing at 350 °C changed the interface magnetic coupling to antiferromagnetic in Co. The element-specific hysteresis curves at each XMCD absorption edge revealed large coercive fields in Fe and Co through the exchange coupling with Mn1.5Ga. After the annealing process, XMCD spectral line shapes indicate that the interfacial reactions promote the formation of Mn2CoGa-like secondary alloys, which influences the performance of tunnel magnetoresistance using the interfaces between transition metals and MnGa.
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