DSpace at VNU: Investigation of the Tunability of the Spin Configuration Inside Exchange Coupled Springs of Hard Soft Magnets

IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 2004906 Investigation of the Tunability of the Spin Configuration Inside Exchange Coupled Springs of Hard/Soft Magnets Thi Ngoc Anh Nguyen1,2, Vahid Fallahi3 , Quang Tuan Le1 , Sunjea Chung1 , Seyed Majid Mohseni1,4 , Randy K Dumas5, Casey W Miller6 , and Johan Åkerman1,5 Materials and Nano Physics, School of Information and Communication Technology, KTH Royal Institute of Technology, Stockholm 164 40, Sweden Spintronics Research Group, Laboratory for Nanotechnology, Vietnam National University, Ho Chi Minh City, Ho Chi Minh, Vietnam Department of Optics and Laser Engineering, University of Bonab, Bonab 5551761167, Iran Department of Physics, Shahid Beheshti University, Evin Tehran 19839, Iran Department of Physics, University of Gothenburg, Gothenburg 412 96, Sweden Department of Physics, University of South Florida, Tampa, FL 33620 USA Magnetic multilayer (ML) structures comprising a perpendicular magnetic anisotropy (PMA) layer coupled to an in-plane magnetic anisotropy (IMA) layer are promising materials for zero/low field operating spin-torque oscillators and bit patterned recording media The magnetization tilt angle can be easily tuned by varying the IMA layer thickness due to the competition between PMA and IMA layers To explore the underlying magnetization reversal mechanism and to further understand the control of tilt angle and uniformity of the magnetization, the IMA (NiFe, Co, and CoFeB)/PMA (Co/Pd MLs) exchange spring systems are systematically studied Experimental data obtained from magnetometry show good agreement with 1-D micromagnetic simulations, allowing us to design tunable exchange coupled spring as a function of IMA thickness Index Terms— Competing magnetic anisotropy, exchange spring, tilted anisotropy materials, tunable magnetization I I NTRODUCTION D EVELOPMENT of novel magnetic structures suitable for spintronic applications utilizing the spin-transfer torque (STT) effect [1]–[3], such as spin-torque oscillators (STOs) [4], [5] and STT-magnetoresistive random access memory (STT-MRAM) [6], [7] are currently receiving increased attention Tilted anisotropy materials have the potential to enhance devices of this sort A tilted magnetization has both inplane (IP) and out-of-plane (OOP) components, which, compared with purely IP and OOP magnetization directions, provide additional degrees of freedom to manipulate its static and dynamic states Relative to the standard IP and OOP systems, tilted systems may be useful for achieving higher-density recording, increased thermal stability, and faster switching in future storage devices [8]–[10], optimizing microwave signal generation [11], enhancing the spin-transfer efficiency [12], and controlling the static and dynamic magnetization states [13] We have recently shown that using the tilted magnetization in either the STO polarizer or free layer, one can simultaneously enable high output power and zerofield operation without the need for additional read-out layers [11], [14], [15] The recently discovered magnetic droplet soliton [16]–[20] also relies on a significant tilt angle of the STO fixed layer, so far only realized using large OOP fields Tilted anisotropies have traditionally been realized using collimated sputtering [21], depositing MLs on nanospheres [9], and exploiting crystallographic texture to control the magnetic Manuscript received November 10, 2013; accepted December 23, 2013 Date of current version June 6, 2014 Corresponding author: T N A Nguyen (e-mail: anhntn@kth.se) Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org Digital Object Identifier 10.1109/TMAG.2014.2299976 easy axis in alloys, such as (112)-textured D022 MnGa (with a tilt angle of 36°), and (111) or (101)-L10 FePt (with angles of 36° and 45°, respectively) [22]–[24] In contrast, hybrid anisotropy exchange springs combine strong PMA MLs with materials having IMA The magnetization profile, and in particular the angle of the IP anisotropy layer, can be varied by changing either the thickness of the IMA layer [25], [26], or the exchange interaction between the layers [27] Such tailoring of the magnetization profile is, e.g., highly effective in tuning the spin-wave spectrum [28], with particular advantages and freedom when designing magnonic devices [29]–[31] In this paper, we present a systematic experimental and numerical study of the spin configuration in hybrid anisotropy exchange spring magnets with different IMA materials (NiFe, Co, and CoFeB) grown on PMA [Co/Pd]5 MLs By taking advantage of the competition between the PMA and IMA in these systems we can tune the entire magnetization profile, both the magnetization angle and degree of nonuniformity, each of which can be estimated from a 1-D micromagnetic simulation II E XPERIMENTAL M ETHODS All film stacks were deposited at room temperature on thermally oxidized Si substrates using a confocal magnetron sputtering system under × 10−8 Torr base pressure as described in our previous works [25]–[27] All series were prepared on Ta (10 nm)/Pd (3 nm) seed layers which promote a strong PMA into our Co/Pd MLs, and then 10-nm thick Ta cap layer deposited to protect surface oxidization [32], [33] Historically, NiFe is the most attractive IMA magnetic material with wide use in magnetic devices, such as magnetoresistive sensors Recently, CoFeB (CFB) has become the equivalent preferred standard material for the soft magnetic 0018-9464 © 2014 IEEE Personal use is permitted, but republication/redistribution requires IEEE permission See http://www.ieee.org/publications_standards/publications/rights/index.html for more information 2004906 IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 layer in magnetic tunnel junctions (MTJs) Fe-rich CFB can lead to a low critical current for spin-torque induced switching, as well as enabling the observation of spin-torque induced RF oscillations at a low dc bias current (Idc ) [34] Ultralowcurrent-density and bias-field-free STOs have also been produced with the Fe-rich CFB free layers [35] In this paper, we prepared three series using different soft magnetic material: NiFe, Co, and CoFeB (the target composition of the Co–Fe–B was 20–60–20 at %, respectively) We kept a hard magnet structure with [Co(0.5 nm)/Pd(1.0 nm)]5 MLs in these series to investigate the tunability of the internal spin configuration Our three series samples were: Series A: [Co (0.5 nm)/Pd(1 nm)]5/NiFe (tNiFe nm); Series B: [Co (0.5 nm)/Pd(1 nm)]5 /Co (tCo nm); Series C: [Co (0.5 nm)/Pd(1 nm)]5 /CoFeB (tCFB nm) The soft IMA layers were deposited as wedges by an oblique deposition technique [26] This approach allows for a systematic study of how the soft layer thickness (tsoft ) affects the reversal and tilt angle, while minimizing the sample-tosample variations in the Co/Pd MLs The wedge thicknesses were varied from to 10 nm for NiFe and to nm for Co and CFB Individual samples were then cut perpendicular to the direction of the soft layer wedge While oblique deposition is a known technique to induce weak IP anisotropy in soft magnetic films [36]–[39], the angles used during our deposition were about 10°–15°, for which we expect less than 30 Oe induced anisotropy Since this is more than two orders of magnitude weaker than the PMA of the Co/Pd MLs and the demagnetizing field of the IMA layers, it will not be considered in the analysis Room temperature hysteresis loops were measured with the applied field OOP and IP using an alternating gradient magnetometer with a maximum applied field of 1.4 T III R ESULTS AND D ISCUSSION Fig shows the OOP and IP hysteresis loops for selected samples with different soft layer thicknesses (tsoft ) The data clearly reveal that the competition between IMA of the soft layer and the PMA of the Co/Pd MLs has a dramatic effect on the magnetization reversal as tsoft is varied When the soft layer is very thin (e.g., tNiFe = nm), a significant PMA is still maintained as the OOP loops shows a large OOP remanence, relatively small saturation field, and large coercivity, whereas the IP loop displays characteristic hard axis behavior This indicates that the thin soft layer is rigidly coupled to the Co/Pd MLs during reversal However, as tsoft increases, the IMA of the soft layer begins to dominate and the effective PMA is significantly reduced As is clearly observable in Fig 1(a), (c), and (e), the OOP remanence and saturation field increase dramatically with tsoft ; this increase is accompanied by a corresponding decrease in the OOP coercivity The coercivity is reduced from 770 Oe to 310 Oe for [Co/Pd]–NiFe system, to 295 Oe for [Co/Pd]–Co system, and to 440 Oe for [Co/Pd]–CFB system, respectively The coercivity field is reduced drastically by a factor of when the thickness of NiFe reaches 10 nm, Co reaches nm, and CFB reaches 3.8 nm This behavior is a typical two-phase system [25]–[28] The complementary trends are also observed for the IP loops; when tsoft is increased the IP loops turn from hard to easy axis behavior, as shown in Fig 1(b), (d), and (f) A clear Fig Hysteresis loops with various tsoft for the A, B, and C series, respectively (a), (c), and (e) OOP loops with the magnetic field applied perpendicular to the samples plane (b), (d), and (f) IP loops with the field applied in plane All measurements are normalized with their Msat decrease in the IP saturation field with tsoft is observed When the tsoft is further increased, the soft layer becomes mostly IP (e.g., tNiFe = 10 nm) We carried out micromagnetic simulations to gain further insight and quantitatively estimate the magnetization tilting in these three systems The calculations were based on a 1-D micromagnetic model The magnetic configuration of each layer was calculated by minimizing the system’s Gibbs free energy with respect to the local magnetization angle θ (z) In the continuous medium approximation the Gibbs free energy with magnetic field H Z applied perpendicular to the layer (i.e., along the z-axis) is given as follows: G= di (−1)i i=1 Ai o ∂θ (z) ∂z + K i − μo Mi2 sin2 θ (z) − μo Mi Hz cos θ (z) dz (1) in which i = refers to Co/Pd MLs and i = refers to the IMA layers The layer thickness, di ; exchange stiffness, Ai ; magnetocrystalline anisotropy, K i ; and saturation magnetization, Mi ; are used as material specific input parameters; θ (z) refers to the angle between the z-axis and the magnetization vector Positive K i values correspond to an intrinsic easy-axis perpendicular to the film plane We consider the anisotropy constants as effective values that include volume, surface, and interface contributions The interface between the Co/Pd MLs and the soft layers is located at z = The equilibrium state is determined by solving the Euler’s equation with the Weierstrass–Erdmann boundary conditions [40], [41] Material parameters used for simulation are shown in Table I The materials parameters for the Co/Pd ML, con- NGUYEN et al.: INVESTIGATION OF THE TUNABILITY OF THE SPIN CONFIGURATION 2004906 TABLE I M AGNETIC PARAMETERS U SED FOR S IMULATION Fig Normalized OOP remanence (Mr /M S ) from experiment (solid black, red, and blue symbols) and simulations (solid black, red, and blue lines) as functions of tNiFe , tCo , and tCFB , respectively sidered as a continuous single slab, and for NiFe were taken from previous studies, [25], [26]: K = 0.63 MJ/m3 , M1 = 0.365 MA/m, A1 = pJ/m, K = MJ/m3 , and A2 = 13 pJ/m We note that K = 0.45 MJ/m3 for Co since Co has a uniaxial IMA [42] One may also consider a uniaxial IMA for the CFB layers However, its value is typically low (K IP ≈ 103 J/m3 ) [43] and can in principle be ignored compared with the shape anisotropy (0.5 μo M S2 ≈ 106 J/m3 ) Based on PhaseFMRTM measurements of single NiFe and CFB films, both M S of NiFe and CFB were found to be strongly dependent on film thickness (not shown) The thickness dependence of M S is considered particularly in ultrathin CFB (tCFB ≤ 5.0 nm), reportedly attributed to a magnetically dead layer [12], [13], [44], [45], [48] In the thicker CFB films, the value of M2 = 0.83 MA/m was measured and in good agreement with prior reports, e.g., in [49] Addressing those parameters is critical for the simulations because of the strong dependence of the IMA on thickness and saturation magnetization The experimental and simulated OOP reduced remanence values (Mr /M S ) are shown in Fig as a function of the IMA soft layer thickness The graphs reveal a good quantitative match between theory and experiment for tNiFe > 4.8 nm, tCo > 2.3 nm, and tCFB > 1.1 nm in the three systems For tNiFe < 4.8 nm, tCo < 2.3 nm, and tCFB < 1.1 nm, the experimental Mr /M S is slightly smaller than the calculated one This discrepancy is likely due to finite temperature effects leading to a slight reduction in the experimentally measured Fig (a)–(c) Calculated tilt angle, θ (z), through the entire film thickness and that of the upper most soft layer, θTop (black solid lines), for series samples A, B, and C, respectively remanence [25], [53] As tsoft is increased, the IMA in the soft layer begins to compete with the PMA of the Co/Pd ML These results are consistent with the fact that thin soft layers (tNiFe < 4.8 nm, tCo < 2.3 nm, and tCFB < 1.1 nm) are rigidly coupled with Co/Pd ML, resulting in a dominant OOP remanent magnetization However, the tilt angle of the soft layers begins to deviate from OOP as tsoft further increases, resulting in a rapid reduction in Mr /M S The tNiFe = 4.8 nm, tCo = 2.3 nm, and tCFB = 1.1 nm are so-called critical thicknesses (tC ) The OOP remanence in all three systems shows a clear decrease when tsoft is larger than tC indicating significant tilting of the magnetization away from the film normal Of particular interest for our work is the angle of the magnetization through the entire film stacks, denoted θ (z) Fig 3(a)–(c) show the simulated θ (z) at remanence through the entire [Co/Pd]–NiFe, [Co/Pd]–Co, and [Co/Pd]–CFB stacks for different thicknesses of the IMA soft layers The magnetization in all three systems is highly tunable and can be continuously varied by simply changing tsoft For STOs with tilted spin polarizing fixed layer, the angle at the top of the IMA soft layer, θTop, is the most important, and its thickness 2004906 IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 These results reveal that using different soft materials results in a wide range of useful tilt angles, different transition regions, and different degrees of nonuniformity of the magnetization profile This can be easily understood by considering the different material parameters of the CFB and Co, in particular the larger magnetization M2 , as compared with the NiFe This difference results in a smaller effective anisotropy, the second term of (1) Therefore, the CFB and Co, having a relatively larger IMA, is far less susceptible to the influence of the [Co/Pd]5 PMA stack, making possible, a smaller tC , and a smaller transition region Additionally, the difference in exchange stiffness between the [Co/Pd]5 MLs and CFB is relatively large, which promotes a larger degree of nonuniformity of the magnetization profile of Co/Pd–CFB system As mentioned above, the thickness dependence of M S , and therefore the effective anisotropy, in IMA materials affects the range of tilt angle as well To quantify and illustrate the degree of nonuniformity in the PMA and IMA layers, respectively, Fig shows the magnetization tilt angle variation through the [Co/Pd]5 MLs (solid dots) and through the soft layer (empty dots) for all three series samples The solid lines are fits using the a ∗ [1 − exp(−(t − tC )/t O ] The model fits all three data sets well, which allows us to compute the difference between the angle variations through Co/Pd MLs and through the IMA soft layer The nonuniformity is strongly dependent on the IMA material The fits show that the angle variation through the [Co/Pd]5 MLs and through the IMA soft layer in Co/Pd–Co and Co/Pd–CFB present a larger difference, as compared to Co/Pd–NiFe In Co/Pd–Co and Co/Pd–CFB systems, the fitting coefficient, a, for Co/Pd MLs is about twice that of the IMA soft layer, however, it is nearly the same in the Co/Pd–NiFe system The critical thicknesses resulting from the fits are also in a good agreement with simulated values in Fig While the useful tCo and tCFB ranges are significantly narrower than that observed in the [Co/Pd]–NiFe material system, they are still well within the tolerances of standard MRAM and read head processes, where sub-Ångström thickness control is easily achieved IV C ONCLUSION Fig (a)–(c) Angle variation through the [Co/Pd]5 MLs (solid dots) and through the upper most soft layer (empty dots) for series samples A, B, and C, respectively The solid lines are fitted using the a ∗ [1 − exp(−(t − tC )/t O ] dependence is shown as a black solid line in Fig 3(a)–(c) Our calculations reveal that the magnetization in the IMA soft layers rotates from completely OOP (0°), at the [Co/Pd]5 interface, toward the film plane through the thickness as tsoft is increased We find that the tilt angle can be easily tuned and readily varied from 0° to 70° by simply changing tNiFe between 5.3 and 10 nm, while it varies from 0° to 68° for 2.3 nm < tCo < nm and from 0° to 72° for 1.1 nm < tCFB < nm This is a significant improvement over the tunable range than previously reported [25] The 5.3 nm < tNiFe < 10 nm, 2.3 nm < tCo < nm, and 1.1 nm < tCFB < nm are so-called transition regions in which the magnetization angle can be tunable In summary, we have carried out a systematic study of the spin configuration in OOP/IP exchange springs with various IP soft layer materials Taking advantage of the competition between the PMA and IMA in these systems we find that the entire magnetization profile can be tuned with respect to both magnetization angle and degree of nonuniformity Both experiments and simulations conclude that the spins in the soft layer remain essentially perpendicular to the film plane for tsoft ≤ tC However, a well defined tilt angle is achievable for tsoft > tC The range of angles and uniformity can be easily tuned by changing the soft material and its thickness These tailored exchange springs are thus particularly useful not only as the fixed but also as the free layers in spin-torque driven devices, which can broaden the scope of potential applications Specifically, the exchange spring magnets based on CFB soft layers can be applied in MgO-based MTJ systems This paper, therefore, provides meaningful insights for future utilization in STT-MRAM or STO devices NGUYEN et al.: INVESTIGATION OF THE TUNABILITY OF THE SPIN CONFIGURATION ACKNOWLEDGMENT This work was supported in part by the EC FP7 under Contract ICT-257159 “MACALO,” in part by the Swedish Foundation for Strategic Research, in part by the Swedish Research Council, and in part by the Knut and Alice Wallenberg Foundation The work of T N A Nguyen was supported in part by the National Foundation for Science and Technology Development of Vietnam under Project 03.02-2010.27 and in part by the KIST-IRDA under Alumni Project 2Z03750 The work of C W Miller was supported by NSF-ECCS The work of J Åkerman was supported by a grant through the Knut and Alice Wallenberg Foundation R EFERENCES [1] J C Slonczewski, “Current-driven excitation of magnetic multilayers,” J Magn Magn Mater., vol 159, nos 1–2, pp L1–L7, Jun 1996 [2] L Berger, “Emission of spin waves by a magnetic multilayer traversed by a current,” Phys Rev B, vol 54, no 13, pp 9353–9358, 1996 [3] D C Ralph and M D Stiles, “Spin transfer torques,” J Magn Magn Mater., vol 320, no 7, pp 1190–1216, 2008 [4] S Kaka, M R Pufall, W H Rippard, T J Silva, S E Russek, and J A Katine, “Mutual phase-locking of microwave spin torque nanooscillators,” Nature, vol 437, pp 389–392, Sep 2005 [5] J Slonczewski, “Excitation of spin waves by an electric current,” J Magn Magn Mater., vol 195, no 2, pp 261–268, 1999 [6] S Mangin, D Ravelosona, J A Katine, M J Carey, B D Terris, and E E Fullerton, “Current-induced magnetization reversal in nanopillars with perpendicular anisotropy,” Nature Mater., vol 5, no 3, pp 210–215, 2006 [7] J Åkerman, “Applied physics Toward a universal memory,” Science, vol 308, no 5721, pp 508–510, Apr 2005 [8] J P Wang, Y Y Zou, C H Hee, T C Chong, and Y F Zheng, “Approaches to tilted magnetic recording for extremely high areal density,” IEEE Trans Magn., vol 39, no 4, pp 1930–1935, Jul 2003 [9] M Albrecht, G Hu, I L Guhr, T C Ulbrich, J Boneberg, P Leiderer, et al., “Magnetic multilayers on nanospheres,” Nature Mater., vol 4, no 3, pp 203–206, 2005 [10] J.-P Wang, “Magnetic data storage: Tilting for the top,” Nat Mater., vol 4, pp 191–192, Mar 2005 [11] Y Zhou, C L Zha, S Bonetti, J Persson, and J Akerman, “Spintorque oscillator with tilted fixed layer magnetization,” Appl Phys Lett., vol 92, no 26, pp 262508-1–262508-3, 2008 [12] R Sbiaa, R Law, E.-L Tan, and T Liew, “Spin transfer switching enhancement in perpendicular anisotropy magnetic tunnel junctions with a canted in-plane spin polarizer,” J Appl Phys., vol 105, no 1, pp 013910-1–013910-6, 2009 [13] P.-B He, R.-X Wang, Z.-D Li, Q.-H Liu, A.-L Pan, Y.-G Wang, et al., “Current-driven magnetization dynamics in magnetic trilayers with a tilted spin polarizer,” Eur Phys J B, vol 73, no 3, pp 417–421, Jan 2010 [14] Y Zhou, C L Zha, S Bonetti, J Persson, and J Akerman, “Microwave generation of tilted-polarizer spin torque oscillator,” J Appl Phys., vol 105, no 7, pp 07D116-1–07D116-3, 2009 [15] Y Zhou, S Bonetti, C L Zha, and J Åkerman, “Zero-field precession and hysteretic threshold currents in a spin torque nano device with tilted polarizer,” New J Phys., vol 11, no 10, p 103028, 2009 [16] M Hoefer, T Silva, and M Keller, “Theory for a dissipative droplet soliton excited by a spin torque nanocontact,” Phys Rev B, vol 82, no 5, pp 054432-1–054432-14, 2010 [17] M Hoefer, M Sommacal, and T Silva, “Propagation and control of nanoscale magnetic-droplet solitons,” Phys Rev B, vol 85, no 21, pp 214433-1–214433-7, 2012 [18] S M Mohseni, S Chung, S R Sani, E Iacocca, R K Dumas, T N A Nguyen, et al., “Spin torque–generated magnetic droplet solitons,” Science, vol 339, no 6125, pp 1295–1298, 2013 [19] S M Mohseni, S R Sani, J Persson, T N A Nguyen, S Chung, Y Pogoryelov, et al., “Magnetic droplet solitons in orthogonal nanocontact spin torque oscillators,” Phys B, Condensed Matter, 2013, to be published 2004906 [20] S M Mohseni et al., “Spin transfer torque generated magnetic droplet solitons,” J Appl Phys., 2013, to be published [21] Y F Zheng, J P Wang, and V Ng, “Control of the tilted orientation of CoCrPt/Ti thin film media by collimated sputtering,” J Appl Phys., vol 91, no 10, pp 8007–8009, 2002 [22] C L Zha, R K Dumas, J Persson, S M Mohseni, J Nogu, and J Akerman, “Pseudo spin valves using a (112)-textured D022 Mn2.3 –2.4 Ga fixed layer,” IEEE Magn Lett., vol 1, p 2500104, Dec 2010 [23] C L Zha, J Persson, S Bonetti, Y Y Fang, and J Akerman, “Pseudo spin valves based on L10 (111)-oriented FePt fixed layers with tilted anisotropy,” Appl Phys Lett., vol 94, no 16, pp 163108-1–163108-3, 2009 [24] C L Zha, Y Y Fang, J Nogués, and J Akerman, “Improved magnetoresistance through spacer thickness optimization in tilted pseudo spin valves based on L10 (111)-oriented FePtCu fixed layers,” J Appl Phys., vol 106, no 5, pp 053909-1–053909-4, Sep 2009 [25] T N A Nguyen, Y Fang, V Fallahi, N Benatmane, S M Mohseni, R K Dumas, et al., “[Co/Pd]–NiFe exchange springs with tunable magnetization tilt angle,” Appl Phys Lett., vol 98, no 17, pp 172502-1–172502-3, 2011 [26] S Chung, S M Mohseni, V Fallahi, T N A Nguyen, N Benatmane, R K Dumas, et al., “Tunable spin configuration in [Co/Ni]-NiFe spring magnets,” J Phys D, Appl Phys., vol 46, no 12, p 125004, 2013 [27] T N A Nguyen, N Benatmane, V Fallahi, Y Fang, S M Mohseni, R K Dumas, et al., “[Co/Pd]4–Co–Pd–NiFe spring magnets with highly tunable and uniform magnetization tilt angles,” J Magn Magn Mater., vol 324, no 22, pp 3929–3932, 2012 [28] S Tacchi, T N A Nguyen, G Carlotti, G Gubbiotti, M Madami, R K Dumas, et al., “Spin wave excitations in exchange-coupled [Co/Pd]-NiFe films with tunable tilting of the magnetization,” Phys Rev B, vol 87, no 14, pp 144426-1–144426-5, 2013 [29] V V Kruglyak, S O Demokritov, and D Grundler, “Magnonics,” J Phys D, Appl Phys., vol 43, no 26, p 260301, 2010 [30] M Madami, S Bonetti, G Consolo, S Tacchi, G Carlotti, G Gubbiotti, et al., “Direct observation of a propagating spin wave induced by spintransfer torque,” Nature Nanotechnol., vol 6, no 10, pp 635–638, 2011 [31] J Bonetti and S Åkerman, “Nano-contact spin-torque oscillators as magnonic building blocks,” Topics Appl Phys., vol 125, pp 177–187, 2013 [32] L Gan, R D Gomez, C J Powell, R D McMichael, P J Chen, and W F Egelhoff, “Thin Al, Au, Cu, Ni, Fe, and Ta films as oxidation barriers for Co in air,” J Appl Phys., vol 93, no 10, p 8731, 2003 [33] B Cui, C Song, Y Y Wang, W S Yan, F Zeng, and F Pan, “Tuning of uniaxial magnetic anisotropy in amorphous CoFeB films,” J Phys Condensed Matter., vol 25, no 10, p 106003, 2013 [34] Y Masugata, S Ishibashi, H Tomita, T Seki, T Nozaki, Y Suzuki, et al., “Spin-torque induced rf oscillation in magnetic tunnel junctions with an Fe-rich CoFeB free layer,” J Phys., Conf Ser., vol 266, no 1, p 012098, 2011 [35] Z Zeng, G Finocchio, B Zhang, P K Amiri, J A Katine, I N Krivorotov, et al., “Ultralow-current-density and bias-fieldfree spin-transfer nano-oscillator,” Sci Rep., vol 3, p 1426, Mar 2013 [36] Z Zhang, X Fan, M Lin, D Guo, G Chai, and D Xue, “Optimized soft magnetic properties and high frequency characteristics of obliquely deposited Co–Zr thin films,” J Phys D, Appl Phys., vol 43, no 8, p 085002, 2010 [37] D O Smith, M S Cohen, and G P Weiss, “Oblique-incidence anisotropy in evaporated permalloy films,” J Appl Phys., vol 31, no 10, pp 1755–1762, 1960 [38] I Barsukov, P Landeros, R Meckenstock, J Lindner, D Spoddig, Z.-A Li, et al., “Tuning magnetic relaxation by oblique deposition,” Phys Rev B, vol 85, no 1, pp 014420-1–014420-6, 2012 [39] N N Phuoc and C K Ong, “Non-linear interplay between exchangebias-induced unidirectional anisotropy and oblique-deposition-induced uniaxial anisotropy,” J Appl Phys., vol 114, no 4, p 043911, 2013 [40] G Asti, M Ghidini, R Pellicelli, C Pernechele, and M Solzi, “Magnetic phase diagram and demagnetization processes in perpendicular exchange-spring multilayers,” Phys Rev B, vol 73, no 9, pp 094406-1–094406-16, 2006 2004906 [41] T Leineweber and H Kronrniiller, “Micromagnetic examination of exchange coupled ferromagnetic nanolayers,” J Magn Magn Mater., vol 176, nos 2–3, pp 145–154, 1997 [42] J Brandenburg, R Hühne, L Schultz, and V Neu, “Domain structure of epitaxial Co films with perpendicular anisotropy,” Phys Rev B, vol 79, no 5, pp 054429-1–054429-7, 2009 [43] M Raju, S Chaudhary, and D K Pandya, “Effect of interface on magnetic properties of Co20 Fe60 B20 in ion-beam sputtered Si/CoFeB/MgO and Si/MgO/CoFeB bilayers,” J Magn Magn Mater., vol 332, pp 109–113, Apr 2013 [44] K Oguz, P Jivrajka, M Venkatesan, G Feng, and J M D Coey, “Magnetic dead layers in sputtered Co40 Fe40 B20 films,” J Appl Phys., vol 103, no 7, pp 07B526-1–07B526-3, 2008 [45] P K Amiri, Z M Zeng, J Langer, H Zhao, G Rowlands, Y.-J Chen, et al., “Switching current reduction using perpendicular anisotropy in CoFeB-MgO magnetic tunnel junctions,” Appl Phys Lett., vol 98, no 11, p 112507, 2011 [46] S Ingvarsson, G Xiao, S S P Parkin, and W J Gallagher, “Thickness-dependent magnetic properties of Ni81 Fe19 , Co90 Fe10 and Ni65 Fe15 Co20 thin films,” J Magn Magn Mater., vol 251, no 2, pp 202–206, 2002 [47] Y Xu, D Zhang, Y Zhai, J Chen, J G Long, H Sang, et al., “FMR study on magnetic thin and ultrathin Ni-Fe films,” Phys Status Solidi (C), vol 1, no 12, pp 3698–3701, 2004 IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 [48] S Y Jang, C.-Y You, S H Lim, and S R Lee, “Annealing effects on the magnetic dead layer and saturation magnetization in unit structures relevant to a synthetic ferrimagnetic free structure,” J Appl Phys., vol 109, no 1, pp 013901-1–013901-5, 2011 [49] K Lee, J J Sapan, S H Kang, and E E Fullerton, “Perpendicular magnetization of CoFeB on single-crystal MgO,” J Appl Phys., vol 109, no 12, p 123910, 2011 [50] C J Kinane, A K Suszka, C H Marrows, B J Hickey, D A Arena, J Dvorak, et al., “Soft x-ray resonant magnetic scattering from an imprinted magnetic domain pattern,” Appl Phys Lett., vol 89, no 9, pp 092507-1–092507-3, 2006 [51] R S Iskhakov, N A Shepeta, S V Stolyar, L A Chekanova, and V Y Yakovchuk, “Spin-wave resonance in Co/Pd magnetic multilayers and NiFe/Cu/NiFe three-layered films,” JETP Lett., vol 83, no 1, pp 28–32, Mar 2006 [52] H Sato et al., “CoFeB thickness dependence of thermal stability factor in CoFeB/MgO perpendicular magnetic tunnel junctions,” IEEE Magn Lett., vol 3, p 3000204, Apr 2012 [53] R Skomski, P Kumar, G C Hadjipanayis, and D J Sellmyer, “FiniteTemperature Micromagnetism,” IEEE Trans Magn., vol 49, no 7, pp 3229–3232, Jul 2013  ... different soft layer thicknesses (tsoft ) The data clearly reveal that the competition between IMA of the soft layer and the PMA of the Co/Pd MLs has a dramatic effect on the magnetization reversal... systematic study of the spin configuration in OOP/IP exchange springs with various IP soft layer materials Taking advantage of the competition between the PMA and IMA in these systems we find that... plane All measurements are normalized with their Msat decrease in the IP saturation field with tsoft is observed When the tsoft is further increased, the soft layer becomes mostly IP (e.g., tNiFe