Enhanced perpendicular coercivity of ultrathin perpendicularly magnetized tbefeecofilms on silicon substrates using a thin pt underlayer

We have demonstrated that the presence of a thin Pt underlayer can enhance the perpendicular coercivity of ultrathin TbFeCoﬁlms with thickness down to 1 nm while preserving their low sat

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Original article

Enhanced perpendicular coercivity of ultrathin perpendicularly

a thin Pt underlayer

Bang Doa,b,*, Hiroyuki Awanoa

a Toyota Technological Institute, Nagoya 468-8511, Japan

b Institute of Materials Science, VAST, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Viet Nam

a r t i c l e i n f o

Article history:

Received 14 March 2016

Accepted 23 March 2016

Available online 11 April 2016

Keywords:

Perpendicular coercivity

TbeFeeCo

Perpendicular magnetic anisotropy

a b s t r a c t

We have studied the effect of an additional thin Pt underlayer on the magnetic properties of ultrathin perpendicularly magnetized TbFeCoﬁlms grown on silicon substrates We have demonstrated that the presence of a thin Pt underlayer can enhance the perpendicular coercivity of ultrathin TbFeCoﬁlms with thickness down to 1 nm while preserving their low saturation magnetization This can be attributed to the hybridization of Pt and Co orbitals, which induces the strong interfacial perpendicular anisotropy of Co/Pt and/or CoFe/Pt These characteristics are potentially useful to reduce the critical current density induced magnetization switching and enhance the thermal stability of a magnetic tunnel junction and the current-induced domain wall motion in a nanowire made out of the ultrathin TbFeCo magnetic layer

article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

Perpendicular magnetic anisotropy materials have been studied

for spintronics applications due to their excellent thermal stability

and low critical current density of spin-torque-induced

magneti-zation switching [1e5] An effective way to reduce the critical

current density is reducing thickness of the magnetic layer while

maintaining its desirable perpendicular anisotropy for thermal

stability [6] For practical applications, further reduction to the

critical current density must be reached, representing an important

but difﬁcult task Amorphous TbFeCo alloys, which have low

satu-ration magnetization and large perpendicular anisotropy[7,8], are

expected to fulﬁll the aforementioned requirements[9e11] It has

been reported that the perpendicular anisotropy can still be

real-ized in TbFeCo ﬁlms with thickness of just a few nanometers

[10,11] In order to reduceﬁlm thickness while preserving a large

perpendicular coercivity of the TbFeCoﬁlm, underlayers, such as Ti,

SiN [12], Al [12,13], and Ag [14] have been added onto Si/SiO2

substrates In particular, Lee et al.[12]have shown that the

pres-ence of Al and Ti underlayers can enhance the perpendicular

coercivity of ultrathin TbFeCoﬁlms with thickness down to 3 nm

This enhancement of the perpendicular coercivity is attributed to the formation of a Ti, Al island-like surface which effectively pre-vents oxidation of Tb atoms It has been otherwise shown that the amount of Tb is reduced in TbFeCo alloy, leading to the formation of in-plane anisotropy of the FeCo phase while reducing the perpen-dicular anisotropy of the ultrathin TbFeCoﬁlm[15]

According to the recent studies, the hybridization of Pt and Co orbitals can induce the interfacial perpendicular anisotropy of Co/Pt [16] and CoFe/Pt [17] multilayers, where the thickness of the magnetic layers is less than 1 nm The enhanced interfacial anisotropy is attributed to the strong spineorbit coupling and the least reactive metal of Pt These studies have motivated us to perform a systematic investigation into the effect of an additional Pt underlayer on the magnetic properties of TbFeCoﬁlms grown on silicon substrates In this study, we demonstrate the enhancement

of perpendicular coercivity in Pt/TbFeCo bilayerﬁlms with thick-ness down to 1 nm while preserving their low saturation magnetization

2 Experimental The Tb26Fe66.8Co7.2 films were directly grown on naturally oxidized Si(100) substrates without and with Pt underlayers by using RF/DC magnetron sputtering During the deposition, highly purified Ar gas was kept at a fixed pressure of 1 mTorr The layer

* Corresponding author Toyota Technological Institute, Nagoya 468-8511, Japan.

E-mail address: dobang@toyota-ti.ac.jp (B Do).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

http://dx.doi.org/10.1016/j.jsamd.2016.03.001

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thicknesses of the TbFeCo and Pt underlayer were varied in the

ranges of tTbFeCo¼ 0.5e27 nm and tPt¼ 0.5e2 nm, respectively A

2 nm-thick Ptﬁlm was subsequently capped on the TbFeCo layer to

prevent oxidization The magnetic properties of the ﬁlms were

investigated using a magneto-optical Kerr effect (MOKE) and an

alternating gradient force magnetometer (AGFM) in out-of-plane

ﬁelds of up to ±15 kOe The surface topologies of the oxidized Si

substrate and the 2 nm-thick underlayer were observed by atomic

force microscopy (AFM)

3 Results and discussion

Fig 1(a,b) shows the MOKE hysteresis loops of the TbFeCoﬁlms

with different thicknesses without and with the 2 nm-thick Pt

underlayer It can be seen inFig 1(a) that while the TbFeCoﬁlms

with thickness down to 3 nm, grown directly on the oxidized Si

substrates possess well-deﬁned square hysteresis loops, the

thinnerﬁlms show a dominant in-plane loop of the CoFe phase,

which has been suggested to be formed due to reduction of Tb

content in the alloy[11] To prevent oxidization of Tb, a 2-nm-thick

Pt underlayer wasﬁrst grown on the oxidized Si substrate.Fig 1(b)

shows the MOKE hysteresis loops of TbFeCoﬁlms grown on the Pt

underlayers As one can see clearly in thisﬁgure, the presence of the

Pt underlayer enables the well-deﬁned square loops in the TbFeCo

films with thickness reduced to 1.5 nm In this case, the Pt under-layer is much more efficient in preventing the oxidation of Tb than the Al and Ti underlayers as reported in Ref.[12] It has been re-ported[12,13]that the island-like surfaces caused by the Al or Ti underlayer can increase number of pinning sites that restrict mo-tion of domain walls and hence increase the perpendicular coer-civity The same mechanism is expected to be responsible for the case of our Pt/TbFeCofilms

Fig 2shows the AFM images of the naturally oxidized Si sub-strate, without and with the 2-nm-thick Pt underlayer The mean surface roughnesses are determined to be 0.156 and 0.301 nm for the oxidized Si and Pt, respectively Similar to the Al and Ti underlayers[12,13], the large and high island-like structure of the

Pt surface is expected to reduce contact areas between the TbFeCo layer and the oxidized Si substrate, resulting in the observed magnetic properties of the TbFeCoﬁlm We also grown 2-nm-thick TbFeCo layers on much thinner Pt underlayers and measured their MOKE hysteresis loops (data not shown) and found that the Pt underlayer of 0.5 nm thickness can also improve the perpendicular coercivity of the 2-nm-thick TbFeCo layer

InFig 3, one can see that the perpendicular coercivities of the TbFeCoﬁlms grown on the Pt underlayers are larger than those grown directly on the oxidized Si substrate The enhancement of perpendicular coercivity was kept for all theﬁlm thicknesses of

Fig 1 Magneto-optical Kerr effect hysteresis loops for TbFeCo ﬁlms grown on the oxidized Si(100) substrate (a) without and (b) with the 2-nm-thick Pt underlayer.

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2e27 nm, except for the 12-nm-thick ﬁlm This enhancement can

be attributed to the hybridization of Pt and Co orbitals, which

in-duces the strong interfacial perpendicular anisotropy of Co/Pt[16]

and CoFe/Pt[17]multilayers, where the thickness of the magnetic

layers is less than 1 nm In particular, Fang et al.[17]suggested that

the nanoscaled interface structure of a metal layer and an TbFeCo

layer will decrease the exchange interaction between the Tb atom

and the Fe/Co atom locally since the non-magnetic metal is

inser-ted The magnetic pinning sites will be introduced at these weak

exchange interaction locations, giving rise to the enhanced

perpendicular coercivity Gadetsky et al [18] reported that the

TbFeCoﬁlm exhibited a good perpendicular anisotropy when the

Tb content varied between 15 and 38 at.% The authors also showed

that with increasing Tb content, the coercivity ﬁrst increased,

reached a maximum at 23 at.% Tbe which is a compensation

composition, and then decreased for higher Tb contents For our

samples, we used an alloy target of 26 at.% Tb We observed a

remarkable change in the polarity of Kerr rotation angle for both

the TbFeCoﬁlms grown on the oxidized Si substrate without and

with the Pt underlayer, when the thickness of the ﬁlm was

increased from 7 to 9 nm This switched polarity is attributed to the

change of ﬁlm magnetization conﬁguration from the CoeFe

transitional-metal (TM)-rich to Tb-rare-earth (RE)-rich

composi-tion It can be seen that the enhancement of corecivity is larger for

the TbFeCoﬁlms with thicknesses less than 8 nm This provides

solid evidence for the usefulness of the Pt underlayer in preventing

oxidization of Tb from residual oxygen on the bare oxidized Si

substrate It is worth mentioning here that the increase of

perpendicular anisotropy can be an effective way to increase the

thermal stability and reduce the intrinsic critical current density for

magnetization switching with decrease in the saturation

magne-tization of the magnetic layer in a magnetic tunnel junction

Fig 4shows the thickness dependence of saturation

magneti-zation (Ms) for both TbFeCoﬁlms grown without and with the

2-nm-thick Pt underlayer It can be seen that Msstrongly depends

on thickness for the ﬁlms grown without the 2-nm-thick Pt

underlayer The Mswas largest for the thinnestﬁlm, which had the

smallest Tb content due to oxidization as noted above As compared

to the case of theﬁlm grown without the Pt underlayer, Msof the

TbFeCoﬁlm grown with the Pt underlayer (~95 emu/c.c.) was found

to be almost unchanged with respect to variation inﬁlm thickness,

because the presence of the Pt underlayer prevents oxidization of

Tb These results are in good agreement with those reported in Refs

[15]and[18] In those studies, the Mswas reported to decrease with the increase of Tb content for the TM-rich composition An important consequence that emerges from our study is that we demonstrate the addition of a thin Pt underlayer can effectively enhance the perpendicular coercivity of ultrathin TbFeCo ﬁlms while preserving their low saturation magnetization These char-acteristics are desirable for reducing the critical current density induced switching magnetization and domain wall motion, as well

as for enhancing the thermal stability of the TbFeCo magnetic layer

4 Conclusions

In summary, we studied the effect of adding a Pt underlayer on the magnetic properties of ultrathin TbFeCo ﬁlms The Pt under-layer was formed as an island-like structure, which created mag-netic pinning sites The presence of the Pt underlayer effectively enhanced the perpendicular coercivity of ultrathin TbFeCo ﬁlms with thicknesses down to 1 nm while preserving their low satu-ration magnetization The improved characteristics have potential

to reduce the critical current density induced switching magneti-zation and enhance the thermal stability of the TbFeCo magnetic layer in a magnetic tunnel junction and the current-induced domain wall motion in a nanowire made out of the TbFeCo thin ﬁlm

Acknowledgments This work wasﬁnancially supported by the Ministry of Educa-tion, Culture, Sports, Science and Technology, Japane KAKENHI No

26630137 (2014-2016)

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