Miller6, and Johan Åkerman1,5 1Materials and Nano Physics, School of Information and Communication Technology, KTH Royal Institute of Technology, Stockholm 164 40, Sweden 2Spintronics Re
Trang 1Investigation 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
1Materials and Nano Physics, School of Information and Communication Technology,
KTH Royal Institute of Technology, Stockholm 164 40, Sweden
2Spintronics Research Group, Laboratory for Nanotechnology, Vietnam National University, Ho Chi Minh City, Ho Chi Minh, Vietnam
3Department of Optics and Laser Engineering, University of Bonab, Bonab 5551761167, Iran
4Department of Physics, Shahid Beheshti University, Evin Tehran 19839, Iran
5Department of Physics, University of Gothenburg, Gothenburg 412 96, Sweden
6Department 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 INTRODUCTION
DEVELOPMENT 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
atten-tion Tilted anisotropy materials have the potential to enhance
devices of this sort A tilted magnetization has both
in-plane (IP) and out-of-in-plane (OOP) components, which,
com-pared 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
achiev-ing higher-density recordachiev-ing, 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
mag-netization 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
zero-field 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)-L10FePt (with angles
of 36° and 45°, respectively) [22]–[24]
In contrast, hybrid anisotropy exchange springs combine strong PMA MLs with materials having IMA The magneti-zation 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 EXPERIMENTALMETHODS
All film stacks were deposited at room temperature on thermally oxidized Si substrates using a confocal magnetron sputtering system under 3 × 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 mag-netoresistive sensors Recently, CoFeB (CFB) has become the equivalent preferred standard material for the soft magnetic
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Trang 2layer 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]
Ultralow-current-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
compo-sition 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-to-sample variations in the Co/Pd MLs The wedge thicknesses
were varied from 0 to 10 nm for NiFe and 0 to 5 nm for
Co and CFB Individual samples were then cut
perpendicu-lar 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 RESULTS ANDDISCUSSION
Fig 1 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 = 3 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
accom-panied 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 2 when the
thickness of NiFe reaches 10 nm, Co reaches 5 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 1. 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:
2
i=1
(−1) i
d i
o
A i
∂θ(z)
∂z
2
+
K i −1
2μ o M i2
sin2θ(z) − μ o M i H zcosθ(z)
dz (1)
in which i = 1 refers to Co/Pd MLs and i = 2 refers to
the IMA layers The layer thickness, d i; exchange stiffness,
A i ; magnetocrystalline anisotropy, K i; and saturation
magne-tization, Mi; are used as material specific input parameters;
θ(z) refers to the angle between the z-axis and the magneti-zation 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 = 0
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,
Trang 3con-TABLE I
M AGNETIC P ARAMETERS U SED FOR S IMULATION
Fig 2 Normalized OOP remanence (M r /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]: K1 = 0.63 MJ/m3,
M1 = 0.365 MA/m, A1 = 6 pJ/m, K2 = 0 MJ/m3, and
A2 = 13 pJ/m We note that K2= 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
(KIP ≈ 103 J/m3) [43] and can in principle be ignored
compared with the shape anisotropy (0.5 μ o M S2≈ 106J/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
thick-ness 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 (M r /M S) are shown in Fig 2 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 M r /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 3 (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 M r /M S The tNiFe = 4.8 nm,
tCo = 2.3 nm, and tCFB = 1.1 nm are so-called critical
thick-nesses (tC) The OOP remanence in all three systems shows a
clear decrease when tsoftis larger than tCindicating 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 simulatedθ(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 mag-netization 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,θ , is the most important, and its thickness
Trang 4Fig 4 (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 − t C )/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 < 5 nm and from 0° to 72° for 1.1 nm < tCFB<
5 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 < 5 nm, and 1.1 nm < tCFB < 5 nm are
so-called transition regions in which the magnetization angle
can be tunable
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 magneti-zation 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 nonuni-formity 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 4 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 − t C )/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 depen-dent 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 3 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 CONCLUSION
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
Trang 5This 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
Wallen-berg 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
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