Pt structure with different thicknesses

Journal of Magnetism and Magnetic Materials 456 (2018) 22–30

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Current-induced switching in CoGa/L10 MnGa/(CoGa)/Pt structure with different thicknesses R. Ranjbar ⇑, K.Z. Suzuki, S. Mizukami WPI-Advanced Institute for Materials Research, Tohoku University, 980-8577 Sendai, Japan

a r t i c l e

i n f o

Article history: Received 23 October 2017 Received in revised form 9 January 2018 Accepted 3 February 2018 Available online 5 February 2018 Keywords: MnGa film Spin orbit torque Spin Hall effect

a b s t r a c t In this paper, we present the results of our study into current-induced spin-orbit torque (SOT) switching in perpendicularly magnetized CoGa/MnGa/Pt trilayers with different thicknesses of MnGa and Pt. The SOT switching was observed for all films that undergo Joule heating. We also investigate SOT switching in the bottom (CoGa)/MnGa/top(CoGa/Pt) films with different top layers. Although both the bottom and top layers contribute to the SOT, the relative magnitudes of the switching current densities JC in the top and bottom layers indicate that the SOT is dominant in the top layer. The JC as a function of thickness is discussed in terms of the magnetic properties and resistivity. Experimental data suggested that the MnGa thickness dependence of JC may originate from the perpendicular magnetic anisotropy thickness product K eff u t value. On the other hand, J C as a function of the Pt thickness shows weak dependence. This may be attributed to the slight change of spin-Hall angle hSH value with different thicknesses of Pt, when we assumed that the SOT switching is primarily due to the spin-Hall effect. Ó 2018 Elsevier B.V. All rights reserved.

1. Introdution Tetragonal Heusler-like Mn-based alloys and their derivatives, such as MnGa and Mn3 Ge, have been extensively studied for spintronic applications. These alloys have very low net-magnetic moments due to their ferrimagnetism, high perpendicular magnetic anisotropy (PMA), low Gilbert damping, and high spin polarization [1–9]. Materials with these unique properties are promising because they can be used as free magnetic layers in the perpendicular-magnetic tunnel junctions (p-MTJs) used in magnetic random access memory (MRAM) devices. The p-MTJs consist of an insulator sandwiched between two ferromagnetic (FM) pinned and free layers, which exhibit different resistances in the presence of a magnetic field depending on the magnetization configuration. This so-called tunnel magnetoresistance (TMR) effect can be used to store information in memory devices. In order to write the information to the memory, several methods have recently been developed to switch the magnetization of the free layer, including switching the magnetization of magnetic films using either magnetic fields or spin transfer torques [10–12]. Yet another method leverages current-induced spin-orbit torque

⇑ Corresponding author at: Max-Planck-Institute for Chemical Physics of Solids, Nöthnitzer Straße 40, Dresden 01187, Germany. E-mail address: [email protected] (R. Ranjbar). https://doi.org/10.1016/j.jmmm.2018.02.007 0304-8853/Ó 2018 Elsevier B.V. All rights reserved.

(SOT) switching to realize high performance and low-power consumption in three-terminal p-MTJ devices [13,14]. In-plane longitudinal electrical current flows in heavy metals (HMs) and FM structures can generate spin currents perpendicular to the film plane due to the spin-Hall effect caused by large spin orbit coupling in HMs. Since the spin current exerts torque on the magnetic layers in HMs/FM structures, current-induced SOT magnetization switching will occur if the spin current is large enough. Most studies on the SOT in ultrathin films with an interfacial PMA, such as HMs/FM/oxide layers, found that the SOT originates from both the spin-Hall and Rashba effects [14–23]. Generally speaking, interfacial PMA is relatively weaker than bulk PMA; therefore, it is also important to clarify the SOT in films with bulk PMA for potential application in MRAMs scaled below the 20nm technology node. Until now, only a few reports have discussed the SOT in films with bulk PMA, such as amorphous rare-earth transition metals magnets [24,25]. Recently, we discovered a novel low-temperature method of chemically growing 1–3-nm-thick MnGa films with a wellordered crystalline structure and c-axis orientation that utilize B2 ordered CoGa as a buffer layer [26]. We were thereby able to develop ultrathin MnGa/MgO p-MTJs [27], and a current-induced spin-orbit torque (SOT) method of switching the magnetization in MnGa films with PMA [28]. For the first time, we reported current-induced SOT magnetization switching in an ultrathin CoGa/MnGa/Pt film with bulk PMA. Independently, other groups

R. Ranjbar et al. / Journal of Magnetism and Magnetic Materials 456 (2018) 22–30

have also reported the SOT effect on Ta-capped MnGa grown on GaAs [29,30]. Very recently, we demonstrated that SOT-switching was possible even in CoGa/MnGa/MgO film without any HMs [31]. This work demonstrated SOT-switching due to the spin-Hall effect for paramagnetic CoGa with a positive spin-Hall angle hSH . This resulted in increased interest in the SOT effect on ultrathin ordered Mn-based alloys films with PMA. In previous reports, we showed that current-induced SOT magnetization switching was possible in CoGa/MnGa/Pt and CoGa/ MnGa/MgO films with a large PMA and moderate switching current density J C while the experimental value of J C was two orders of magnitude smaller than the theoretical value [28,31]. However, many issues remain, such as the thickness effect of the magnetic layer, the role of the Pt and CoGa layers on the SOT switching, and the origin of the small J C , which is still unclear. Therefore, in this article, we report on the results of our study into the effects of the MnGa (Pt) thickness on current-induced SOT magnetization switching in CoGa/MnGa/Pt trilayers with bulk PMA in order to gain more insight into the above-mentioned issues. In addition, we study effect of bottom and top layers on current-induced SOT magnetization switching in bottom(CoGa)/MnGa/top(CoGa/Pt) films with different top layers.

2. Experimental procedure Sample films were fabricated using ultra-high vacuum magnetron sputtering with a base pressure of < 10
7 Pa onto an MgO (1 0 0) single crystal substrate. The stacking structures from the substrate side were CoGa(15)/MnGa-(tMnGa )/Pt(2) and CoGa(15)/ MnGa(2.5)/Pt(t Pt ) (thickness in nm), as shown in Fig. 1(a) and (b), respectively. The MnGa (Pt) thicknesses were 2, 2.5, 3, and 3.5 nm (2, 3, 4, and 5 nm). We also fabricated CoGa(15)/MnGa(2) film with four different top layers of CoGa(2), CoGa(1)/Pt(1), CoGa(0.5)/

23

Pt-(1.5) and Pt(2) to investigate effect of bottom and top layers on current-induced SOT magnetization switching. The film compositions of MnGa and CoGa were Mn59 Ga41 and Co60 Ga40 , respectively, as estimated using inductively coupled plasma spectroscopy. The Mn59 Ga41 film composition corresponded to the L10 structure, as shown in Fig. 1(c). In addition, three films with stacking structures of CoGa(15), CoGa(15)/MnGa(2.5), and CoGa(15)/MnGa(2)/Cu(5) were fabricated as control samples. The Cr/CoGa bilayer was much better than CoGa in terms of the MnGa epitaxial growth; however, a Cr buffer could not be used because Cr is known to shunt the electrical current I. These film structure were validated by X-ray diffraction (XRD) with Cu K a radiation. Although MnGa peaks were not observed in the XRD patterns due to the thinness of the MnGa, the B2-ordered paramagnetic CoGa buffer with peak orientations of (0 0 1) and (0 0 2) was clearly observed in the XRD patterns for both series stacking structures. Note that the XRD patterns are not shown. This CoGa buffer may promote the c-axis-oriented epitaxial growth of L10-ordered MnGa without significant interfacial reaction, as we reported in our previous paper [26]. The magnetic properties were characterized by a vibrating sample magnetometer (VSM) and the polar magneto-optical Kerr effect (p-MOKE). The films were patterned into Hall devices using conventional ultraviolet photo-lithography and Ar ion milling. In this study, Ti (20)/Au(150) films were used as electrodes. The length and width of the main Hall bar through which the current I flows were 30 and 6 lm, respectively. A photograph of the smallest Hall device and a schematic illustrating the measurement geometry is shown in Fig. 2. The anomalous Hall resistance RH was measured with the standard DC four terminal method by applying a longitudinal magnetic field HX or perpendicular magnetic field H, where H, HX , and HY refer to the out-of-plane, in-plane longitudinal, and in-plane transverse magnetic fields, respectively. The X-Y-Z coordinate system is shown in Fig. 2. In the case of current-induced

Fig. 1. Stacking structure of MnGa/Pt films with different thicknesses of (a) MnGa, and (b) Pt. (c) Illustration of the crystalline structure of L10 MnGa.

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Fig. 2. Photograph of the Hall device [28] and a schematic of the measurement geometry. The terms H, HX , and HY refer to the out-of-plane, in-plane longitudinal, and inplane transverse magnetic fields, respectively.

switching measurements, the DC electrical current I in the channel was swept from
20 to 20 mA under a static HX that varied from
1500 to 1500 Oe. All measurements were performed at room temperature. 3. Experimental result and discussion Fig. 3 displays the anomalous Hall resistance measurements for the various Hall bars with different magnetic applications. Fig. 3(a) and (c) show the anomalous Hall resistance ðRH Þ as a function of a magnetic field applied perpendicular to the sample plane (H) for MnGaðtMnGa Þ=Ptð2Þ and MnGað2:5Þ=PtðtPt Þ films with varying MnGa and Pt thicknesses, respectively. An electrical current of 1 mA was applied in the X direction, and the RH was measured for the voltage channel in the Y direction. The square hysteresis loops shown in the figures for all films indicate that PMA was realized in the MnGa

films. These hysteresis loops are identical to the magnetization hysteresis loop measured by VSM (not shown here). As the MnGa thickness increased, the RH magnitude increased, an effect that partially originated from the larger M S . The switching field also increased as the MnGa thickness increased, which is in good agreement with the magnetization hysteresis loop. On the other hand, the magnitude of RH decreased as the Pt thickness increased for the MnGað2:5Þ=PtðtPt Þ films. In this case, the magnetic properties were found to be independent of the Pt thickness; therefore, the reduction observed in the magnitude of the RH may have been due to the smaller electrical current in the MnGa layer. The RH as a function of HX was measured by applying a 0.1 mA electrical current using a physical properties measurement system for MnGaðt MnGa Þ=Ptð2Þ and MnGað2:5Þ=Ptðt Pt Þ films with varying MnGa and Pt thicknesses, as shown in Fig. 3(b) and (d), respectively. The Heff k value can be estimated from RH -H X hysteresis loop. When the

Fig. 3. Anomalous Hall resistance RH as a function of (a) H, and (b) HX for MnGaðt MnGa Þ=Ptð2Þ films. The anomalous Hall resistance RH as a function of (c) H and (d) HX for MnGað2:5Þ=Ptðt Pt Þ films. The electrical currents were I = 1 and 0.1 mA for H and HX , respectively.

R. Ranjbar et al. / Journal of Magnetism and Magnetic Materials 456 (2018) 22–30

HX was large enough, i.e., HX > Heff k , the magnetization of the films was aligned in-plane and parallel to the magnetic field. The value of RH was constant and close to zero for this regime. Then, the magnetization direction changed gradually from in-plane to perpendicular as the value of HX decreased to zero. This is indicated by the increase in the magnitude of RH . Fig. 4 shows the magnetic properties obtained using AHE and VSM measurements. The saturation magnetization M S and switching field HSW as function of the MnGa and Pt thicknesses are shown in Fig. 4(a) and (b). The values of M S and HSW were independent of the Pt thickness for MnGað2:5Þ=Ptðt Pt Þ films. On the other hand, the M S and HSW were enhanced by the increasing MnGa thickness for the MnGaðtMnGa Þ=Ptð2Þ films. The maximum value of M S was less than 200 emu/cm3, which was smaller than that for the MnGa film prepared on a Cr/CoGa buffer layer and capped with Mg/MgO film [26]. This reduction in M S primarily originated from the atomic mixing of the MnGa and Pt capping layers. We estimated the thickness of the dead layer by using a p-MOKE measurement. The pMOKE signal uk was plotted as a function of MnGa thickness and it showed linear dependency. By extrapolation of this data regarding thickness axis, the thickness of the dead layer was found to be approximately 1.5 nm, whereas no dead layer was observed for the MnGa films with a Mg/MgO capping layer. This suggests that the Pt film diffused into the MnGa layer. Another reason may be that the MnGa layer was slightly less ordered than that grown on the Cr/ CoGa buffer layer. eff Fig. 4(c) and (d) show the Heff k and PMA thickness product ðK u tÞ as function of the thickness for both series samples, respectively. eff eff eff Here, K eff u is equal to M S Hk =2. The values of H k and K u t as a function of the Pt thickness are constant for MnGað2:5Þ=Ptðt Pt Þ films, which indicates that the PMA properties were independent of the Pt thickness. However, for MnGaðt MnGa Þ=Ptð2Þ films, a linear enhancement was observed with increasing MnGa thicknesses. This suggests that the PMA properties were enhanced by

25

increasing the MnGa thickness due to the improved chemical ordering of the L10 MnGa. We then measured the anomalous Hall resistance as a function of the electrical current (RH -I) under various in-plane longitudinal magnetic fields to investigate the SOT in MnGa/Pt films. Some typical RH -I data for MnGa(2)/Pt(2) is shown in Fig. 5(a) and illustrates a clear magnetization hysteresis loop when positive and negative HX were applied, whereas it does not show a magnetization hysteresis loop for HX ¼ 0 Oe. This measurement also shows the magnetization hysteresis changed from clockwise to anticlockwise when HX changed from negative to positive, which is in agreement with the finding that Pt had a positive hSH . The RH magnitude was similar to the RH -H data, which indicates that a complete switching process occurred in this film. These properties demonstrate that current-induced magnetization SOT switching is possible for MnGa/Pt films. The same measurement was performed for the CoGa(15)/MnGa(2)/Cu(5) control film; however, no SOT switching was observed. This is because Cu is not an HM and only exhibits weak spin orbit coupling. We also estimated the magnitude of the magnetic field, i.e., the so-called Oersted’s field, generated from the current at 31 Oe at I = 15 mA. This value is quite small, and therefore cannot be responsible for the observed magnetization switching. The anomalous Hall resistance as a function of the electrical current (RH -I) under various in-plane transverse magnetic fields HY was measured in order to investigate the SOT in MnGa/Pt films due to the Rashba effect. No current-induced magnetization SOT switching was observed with this measurement. Nevertheless, we were not able to conclude that the Rashba effect [14] was negligible because we did not determine the SOT due to the Rashba effect in this study. In order to obtain more insight in SOT due to the Rashba effect, a harmonic measurement would be required [22,24,32,33] which is the subject of future study. Therefore, in this paper, we just assumed that the spin-Hall effect as an only source

Fig. 4. Thickness dependence of (a) the saturation magnetization MS , (b) switching field HSW , (c) perpendicular magnetic anisotropy field Heff k , and (d) perpendicular magnetic anisotropy thickness product K eff u t for MnGaðt MnGa Þ=Ptð2Þand MnGað2:5Þ=Ptðt Pt Þ films.

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R. Ranjbar et al. / Journal of Magnetism and Magnetic Materials 456 (2018) 22–30

Fig. 5. Anomalous Hall resistance RH as a function of the electrical current I measured at different in-plane longitudinal magnetic fields HX for (a) CoGa(15)/ MnGa(2)/Pt(2) (b) CoGa(15)/MnGa(2)/CoGa(2) films. Data points near I = 0 mA have been removed because those was noisy.

for the current-induced magnetization SOT switching in MnGa/Pt film. As reported in a previous study [34], J C for films with perpendicular magnetization where the SOT originated from the spin-Hall effect is:

Fig. 6. Phase diagram of the switching current density J C vs. the in-plane magnetic field HX for (a) MnGaðtMnGa Þ=Ptð2Þ and (b) MnGað2:5Þ=PtðtPt Þ films. (c) Illustration of the magnetization rotation corresponding to numbers 1, 2, 3 and 4.

" # 2e M S t Heff HX k
pffiffiffi ; JC ¼ h hSH 2 2

ð1Þ

R. Ranjbar et al. / Journal of Magnetism and Magnetic Materials 456 (2018) 22–30

where e,  h, and hSH are the electron charge, Planck constant, and spin-Hall angle for an HM layer, respectively, and MS , Heff k , and t are the saturation magnetization, effective PMA field, and thickness of the magnetic layer, respectively. In this model, it is assumed that the magnetization uniformly rotates without thermal fluctuations in HX , which is much smaller than Heff k . Phase diagrams of the switching current IC vs. the in-plane magnetic field HX for MnGaðtMnGa Þ=Ptð2Þ and MnGað2:5Þ=Ptðt Pt Þ films are shown in Fig. 6 (a) and (b), respectively, and are in good agreement with Eq. (1). As HX increased, the absolute value of IC decreased for all samples, as was expected from Eq. (1). In Fig. 6(a) and (b), four areas are marked with numbers 1, 2, 3, and 4. An illustration of the magnetization rotation corresponding to these numbers is shown in Fig. 6 (c). The magnetization switched from downward to upward when both I and HX were in the same direction (i.e., X and -X), as in numbers 1 and 3, respectively. However, the magnetization switched from upward to downward when I and HX were antiparallel, as in numbers 2 and 4, respectively. These results are consistent with those from previous studies. Liu et al. [21] and Fukami et al. [17] reported the opposite symmetry. The magnetization switched from upward to downward (downward to upward) when the direction of I was parallel (antiparallel) to HX , in Pt/Co/Al and PtMn/CoNi films compared to that observed in our film. This is because we placed Pt heavy metal on top of the magnetic film in our films, whereas the Pt and PtMn layers were placed below the magnetic layer in the other films. This symmetry difference can be explained by the fact that the direction of pure spin current generated from the spin-Hall effect is antisymmetric with respect to the current path above or below the magnetic layer. Both the Pt and PtMn layers in previous studies exhibited a positive hSH , which suggests that the hSH in this work was also positive [17,21]. To clarify the effects of thermal fluctuations, such as Joule heating in magnetization switching, DC and pulse measurements were performed. The value of RH as a function of H with different DC values and I pulses of 2, 5, 10, and 15 mA for the MnGa(2.5)/Pt(2) film are shown in Fig. 7(a) and (b), respectively. A pulse duration time s of 50 ls was used for the pulse measurement. The values of RH for a magnetic field of zero or HSW were the same for both the DC and pulse measurements when I was 2 mA. By increasing I, clear differences, especially in the value of HSW , were observed for both the DC and pulse measurements. The value of HSW was significantly reduced for the DC measurements when I was 15 mA, which is close to the magnetization switching current of the MnGa(2.5)/Pt (2) film. This may have originated from the larger Joule heating in the DC measurement compared to that for the pulse measurement, and suggests that the enhancement of the SOT-switching may have been due to the thermal effect in the DC measurement. Here, we should also mention that the Curie temperature of the

27

bulk Mn60 Ga40 and Co60 Ga40 alloys are about 640 K and 200 K, respectively [35,36]. This means that the MnGa and CoGa alloys are ferromagnetic and paramagnetic, respectively, at room temperature. If the Joule heating is large enough it can change structural and magnetic properties of MnGa(2.5)/Pt(2) film. Therefore, we measured this sample again with applying I of 2 mA after measuring of RH as a function of H with different I of 5, 10 and 15 mA. The RH as a function of H was recovered without any significant changes. This suggests that the Joule heating is not large enough to change magnetic properties of MnGa(2.5)/Pt(2) film because of relatively large Curie temperature of MnGa. 4. Discussion In this section, we discuss the role of the Pt and CoGa layers on the SOT switching, the magnitude of J C and the influence of the thickness of the Pt and MnGa layers with respect to the differences between the SOT theory and experimental results. The resistances of the CoGa, MnGa, and Pt layers in the main and control samples were evaluated, and the current ratios flowing into the Pt ðIPt =IÞ and CoGa ðICoGa =IÞ layers were evaluated based on the resistance measurements. The values of IPt =I and ICoGa =I as a function of the MnGa and Pt thicknesses for both series films are shown in Fig. 8(a). The resistance of a film is proportional to the inverse of the thickness ðR / 1=tÞ. Therefore, smaller resistances are expected for thicker MnGa and Pt films. As the MnGa thickness increased, IPt =I and ICoGa =I slightly decreased for the MnGaðtMnGa Þ=Ptð2Þ films due to the reduction of the MnGa resistance. However, the MnGa resistance was fixed for MnGað2:5Þ=Ptðt Pt Þ films, although the Pt resistance decreased as the Pt thickness increased. This caused larger and smaller currents to flow through the Pt and CoGa layers, respectively. We also evaluated the resistivity q for the CoGa, MnGa, and Pt layers. The resistivity of bottom CoGa was approximately 225 lX  cm, which is consistent with the value for Co60 Ga40 reported by Whittle et al. [37]. For the MnGaðtMnGa Þ= Ptð2Þ films, the MnGa resistivity did not change relative to the MnGa thickness, and the values were between 160 to 190 lX  cm (Fig. 8(b)). These values were in good agreement with those reported in the previous study [38]. The evaluated resistivity for the 2-nm-thick Pt film was found to be 39 lX  cm, which is very close to the value reported by Miren Isasa et al. [39]. The resistivity was found to decrease linearly for thicker Pt films (Fig. 8(b)), and this trend was also observed in Ta=Ptðt Pt Þ=Co=MgO=Ta multilayers [40]. The authors reported that the thickness dependence of the Pt resistivity was due to surface scattering at the Pt/Co interface. Yu et al. reported the efficient enhancement or cancellation of the SOT in HMs1/CoNi/HMs2 film, depending on the combination of the two HMs [41]. They demonstrated the enhancement

Fig. 7. Anomalous Hall resistance RH as a function of H with electrical currents of 2, 5, 10, and 15 mA for (a) DC, and (b) pulse measurements for the MnGa(2.5)/Pt(2) film.

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R. Ranjbar et al. / Journal of Magnetism and Magnetic Materials 456 (2018) 22–30

CoGa/MnGa/Pt film. However, clear SOT switching was observed for this film. In order to clarify more this point, we estimated switching current densities J C in the top (J top ) and bottom (J bottom ) layers. The J top and J bottom were separately estimated using the following equations, as reported in the previous study [41]:

J top ¼

1 t bottom þ tMnGa þ t top

qtop

J bottom ¼

t bottom

qbottom

1

qbottom

t

þ qtMnGa þ qtop MnGa

Jt ;

ð2Þ

top

t bottom þ t MnGa þ ttop t bottom

qbottom

t

þ qtMnGa þ qtop MnGa

Jt ;

ð3Þ

top

where J t is the total current density of the bottom layer/MnGa/top layer films. Here, we assume that:

J t At ¼ J bottom Abottom þ J MnGa AMnGa þ J top Atop ;

Fig. 8. (a) Thickness dependence of the electrical current ratio flows in Pt and CoGa layers for MnGaðt MnGa Þ=Ptð2Þ (square symbols) and MnGað2:5Þ=Ptðt Pt Þ (circle symbols) films. (b) Thickness dependence of the resistivity of MnGa and Pt layers for MnGaðtMnGa Þ=Ptð2Þ and MnGað2:5Þ=PtðtPt Þ films.

(cancellation) of the effective SOT by choosing HMs with the opposite (same) sign as hSH . In a recent study, SOT switching was observed in CoGa/MnGa/MgO film without any HMs [31]. A positive hSH of 0.04–0.12 for CoGa film was obtained, and these values were comparable with those reported for Pt film [42]. This might originate from the spin-orbit interaction of Ga 4p orbitals in CoGa comparable with that of Pd 4d orbitals where the large SOT and current-induced switching were observed for ferromagnetic material in contact with Pd film [31,43,44]. Since the CoGa and Pt films have positive hSH , one can expect the cancellation of the SOT in

ð4Þ

where the A is the area of film which electrical current flows through it. Different physical parameters such as resistivity for top layer qtop , IC , Jtop and J bottom and as well as Jt while the HX ¼ 1000 Oe are summarized in Table 1 for CoGa(15)/MnG(2)/Pt (2) film. Both CoGa and Pt film in this study shows positive hSH , therefore, we expect cancellation of SOT when the J C value of bottom and top layers are same. However, this film exhibits different and larger JC value for top layer compared with bottom layer. This means that the SOT from top layer overcomes to that from the bottom layer. We also examined the SOT in CoGa(15)/MnGa(2)/CoGa(0. 5)/Pt(1.5), CoGa(15)/MnGa(2)/CoGa(1)/Pt(1) and CoGa(15)/MnGa (2)/CoGa(2) films. All the films exhibit larger J C value for top layer compared with bottom layer as is shown in Table 1. As the thickness of top Pt layer decreases the resistivity of the top layer becomes larger, therefore, smaller electrical current ratio passes through the top layer and this causes to a smaller difference between Jtop and Jbottom . This suggests that the resultant SOT becomes smaller when the Pt top layer is replaced with CoGa film. On the other hand, even though the total thickness is same for all the samples, the smaller J t value for the CoGa(15)/MnGa(2)/Pt(2) film compared with other films showed that the SOT should be larger in the MnGa(2)/Pt(2) film. However, the different between Jtop and Jbottom in CoGa(15)/ MnGa(2)/CoGa(2) film is quite small and it is difficult to explain the SOT from top layer is dominant. In order to gain more insight into the role of top and bottom layers, we examined the SOT in CoGa/MnGa/Cu films. But the SOT was not observed in this film even though the CoGa was used as a bottom layer. This indicates the important role of the top layer on SOT in our film. Therefore we can conclude that the SOT is dominant in CoGa and Pt top layers. The SOT switching is clearly observed in CoGa(15)/MnGa(2)/ CoGa(2) film as can be seen from Fig. 5(b). Since both CoGa and Pt films having positive hSH and the J C is also larger in top layers the symmetry becomes same between CoGa(15)/MnGa(2)/Pt(2) and CoGa(15)/MnGa(2)/CoGa(2) films. However, the RH value for the negative range of Hx is smaller than that value for the positive range in CoGa(15)/MnGa(2)/CoGa(2) film compared with CoGa (15)/MnGa(2)/Pt(2) film. This is due to slight partial switching

Table 1 Summary of different physical parameters such as resistivity for top layer qtop , IC , J top and J bottom and as well as J t while the HX ¼ 1000 Oe for CoGa(15)/MnGa(2) films with different top layers. Sample

CoGa(15)/MnGa(2)/Pt(2) CoGa(15)/MnGa(2)/CoGa(0.5)/Pt(1.5) CoGa(15)/MnGa(2)/CoGa(1)/Pt(1) CoGa(15)/MnGa(2)/CoGa(2)

qtop ðlX  cmÞ

IC at HX ¼ 0 Oe

J bottom

J top

Jt

(mA)

ð1011 A=m2 Þ

ð1011 A=m2 Þ

ð1011 A=m2 Þ

39 47 57 180

12.5 13.5 14.5 16.0

0.70 0.84 0.94 1.31

4.10 3.94 3.72 1.64

1.09 1.18 1.27 1.40

R. Ranjbar et al. / Journal of Magnetism and Magnetic Materials 456 (2018) 22–30

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Fig. 9. Thickness dependence of the switching current density in Pt and CoGa for (a) MnGaðtMnGa Þ=Ptð2Þ and (b) MnGað2:5Þ=PtðtPt Þ films.

which might be originated from the existence of un-switched domains. Fig. 9(a) and (b) show J C in the Pt and CoGa layers as function of thickness for the MnGaðt MnGa Þ=Ptð2Þ and MnGað2:5Þ=PtðtPt Þ films respectively, while HX is 1500 Oe. The value of J C in the CoGa layer did not change significantly as the thicknesses of Pt and MnGa were varied. The values were also one order of magnitude smaller than those of the Pt film, which means the SOT switching primarily originates from the Pt film. This causes magnetization switching in MnGa film, even though we expect the cancellation of the SOT in our films. The value of J C in the Pt layer increased linearly as the thickness of the MnGa and Pt increased for both the MnGaðtMnGa Þ=Ptð2Þ and MnGað2:5Þ=Ptðt Pt Þ films. It seems hSH for the MnGaðt MnGa Þ=Ptð2Þ films is constant when the Pt thickness is fixed. Therefore, changes in the magnetic properties of the MnGa film were responsible for this linear increase. According to Eq. eff (1), J C is proportional to K eff u t. As we observed in Fig. 4(d), K u t exhibited linear enhancement with the MnGa thickness. Thus, the increase in J C for thicker MnGa films was due to the larger

K eff u t value. On the other hand, the magnetic properties were found to be independent of the Pt thickness for the MnGað2:5Þ=PtðtPt Þ films, which suggests that the enhancement of the J C may have originated from the smaller value of hSH for the thicker Pt film. The value of hSH was proportional to the resistivity, as reported in previous studies [40,45]. As can be seen from Fig. 8(b), the resistivity of the Pt film decreased linearly as the Pt thickness increased in the MnGað2:5Þ=Ptðt Pt Þ films, which indicates that the value of hSH was smaller for thicker Pt films. As reported in the previous study, hSH depends on the thickness of the non-magnetic layer, and can be expressed as[46]:

hSH ðtÞ ¼ 1
sechðt=ksf Þ; hSH ð1Þ

ð5Þ

where ksf and t are the spin-diffusion length and thickness of the Pt layer, respectively. Thus, the magnitude of hSH became large as the thickness increased, and saturated at a thickness larger than the spin-diffusion length in order to obtain a full spin accumulation in the Pt. On the other hand, the hSH became increasingly large as the scattering frequency increased, due to various imperfections in the metals. As Sagasta et al. reported in the previous study [47] the hSH is related to resistivity by:

hSH  rint SH qPt ;

ð6Þ

where the rint SH is intrinsic spin Hall conductivity. They demonstrated that the rint SH is constant in Pt. Therefore, the hSH exhibited a trend similar to that of the resistivity. The resistivity increased

with decreasing Pt layer thickness, indicating that hSH increased in the thinner regime of the Pt, which was opposite to the thickness dependence of the magnitude of hSH . The tradeoff between the thickness dependence of hSH and the spin accumulation causes the weak thickness dependence of JC . As seen in Fig. 9(a), the smallest experimental value of J C in the Pt layer was obtained for the MnGa(2)/Pt(2) film. This value was approximately 3:9  1011 A=m2 for the HX of 1500 Oe. This value is comparable with that reported for Pt/Co/AlO film [21] and smaller than that of Pt/Co/MgO film [16]. In comparison to that for CoFeB film, the J C value is still one order of magnitude larger [23]. We also estimated JC theoretically using Eq. (1). Here, the experimental values of MS and Heff k were used for the estimation of JC . The values of hSH , t, and HX were set to 0.11, 2 nm, and 1500 Oe, respectively. The value of hSH was the largest among the various reported values of hSH for Pt, which range from 0.004 to 0.1 [42]. The estimated value of JC for the MnGa(2)/Pt(2) film based on Eq. (1) was approximately 3:8  1013 A=m2 , which is two orders of magnitude larger than that of the experimental value. Here, we assumed that the magnetization switches based on the coherent rotation model. However, the magnetization of ferromagnetic material can also switch based on the domain wall motion model [21] which needs much less energy than coherent rotation model. Therefore, it seems domain wall motion also has a contribution in magnetization switching and it is one the reasons for small J C value. In addition, this reduction in the experimental value of JC may be attributed to the thermal fluctuations of the magnetization and Joule heating effect [16,48,49]. As discussed in this paper, the amount of Joule heating appears to be large for DC measurements. To reduce the Joule heating effect, a pulse measurement is needed; however, this was beyond the scope of this study. The experimental results showed that the MnGaPt alloys were formed at the interface of MnGa/Pt. Therefore, the hSH may be larger than the value used in Eq. (1). Until now, according to our best knowledge, there is no report on the hSH of MnPtGa. Interestingly, Ou et al. reported that the hSH is 0.24 for material with an antiferromagnetic alignment that contains Mn and Pt elements [50]. Therefore, the Mn-Pt termination could be one of the reasons for a larger value of hSH in MnPtGa alloy.

5. Conclusions In this work, current-induced spin-orbit torque (SOT) switching was studied in CoGa/MnGa/Pt trilayers with bulk PMA with different thicknesses of MnGa and Pt. The experimental results showed that the magnetic properties were dependent (independent) of the MnGa (Pt) thickness. Clear current-induced magnetization SOT switching was observed for all the samples patterned with micron

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size Hall devices. Although we did not determine the currentinduced magnetization SOT switching due to Rashba effect, it seems the spin-Hall effect is the source for current-induced magnetization SOT switching in our films. A comparison between the results of the DC and pulse measurements suggested that the SOT-switching was enhanced due to Joule heating in CoGa/ MnGa/Pt films. When the smaller experimental value of J C was compared with the theoretical value, it was thought to have originated from domain wall motion, the Joule heating effect and larger hSH due to the mixing of the Pt layer into the MnGa. We also investigated the effect of bottom and top layers in SOT switching in CoGa/MnGa/CoGa/Pt film with different top layers and we found that both the bottom (CoGa) and top (CoGa and Pt) layers contributed to the SOT switching, but the SOT was dominant in the top layers. We estimated the value of J C as function of the MnGa (Pt) thickness, and identified a strong (weak) thickness dependency. These trends were discussed in terms of the magnetic properties and resistivity. The experimental results showed that the linear enhancement of J C with increasing MnGa (Pt) thickness was possible due to the larger (smaller) value of K eff u t ðhSH Þ. Acknowledgment This work was partially supported by a Grant-in-Aid for Scientific Research (No. 16H03846 and No. 16K14244), the ImPACT Program, and the Asahi Glass Foundation. The authors thank Y. Kondo for technical assistance and T. Miyazaki for valuable comments. S. M. thanks CSRN. References [1] B. Balke, G.H. Fecher, J. Winterlik, C. Felser, Appl. Phys. Lett. 90 (2007) 152504. [2] H. Kurt, K. Rode, M. Venkatesan, P. Stamenov, J.M.D. Coey, Phys. Rev. B. 83 (2011), 020405(R). [3] J. Winterlik, S. Chadov, A. Gupta, V. Alijani, T. Gasi, K. Filsinger, B. Balke, G.H. Fecher, C.A. Jenkins, F. Casper, J. Kubler, G.D. Liu, L. Gao, S.S.P. Parkin, C. Felser, Adv. Mater. 24 (2012) 6283. [4] S. Mizukami, F. Wu, A. Sakuma, J. Walowski, D. Watanabe, T. Kubota, X. Zhang, H. Naganuma, M. Oogane, Y. Ando, T. Miyazaki, Phys. Rev. Lett. 106 (2011) 117201. [5] Q. Ma, A. Sugihara, K. Suzuki, X. Zhang, T. Miyazaki, S. Mizukami, Spin 4 (2014) 1440024. [6] L. Zhu, J. Zhao, Appl. Phys. A 111 (2013) 379. [7] S. Mizukami, T. Kubota, F. Wu, X. Zhang, T. Miyazaki, H. Naganuma, M. Oogane, A. Sakuma, Y. Ando, Phys. Rev. B 85 (2012), 014416. [8] J. Jeong, Y. Ferrante, S.V. Faleev, M.G. Samant, C. Felser, S. Parkin, Nat. Commun. 7 (2016) 10276. [9] S. Mizukami, A. Sakuma, A. Sugihara, K.Z. Suzuki, R. Ranjbar, Scr. Mater. 118 (2016) 70. [10] T. Kawahara, K. Ito, R. Takemura, H. Ohno, Microelectron. Reliab. 52 (2012) 613–627. [11] J.M. Slaughter, Annu. Rev. Mater. Sci. Res. 39 (2009) 277–296. [12] S. Ikeda, J. Hayakawa, Y.M. Lee, F. Matsukura, Y. Ohno, T. Hanyu, H. Ohno, IEEE Trans Electron Dev. 54 (2007) 5. [13] A. Chernyshov, M. Overby, X. Liu, J.K. Furdyna, Y. Lyanda-Geller, L.P. Rokhinson, Nat. Phys. 5 (2009) 656–659.

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