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Structure and magnetic properties of the self-assembled Co52Pt48 nanowire arrays

March 2008
Applied Physics Letters 92(10):102501-102501-3

March 2008
92(10):102501-102501-3

DOI:10.1063/1.2894199

Authors:

Da-Li Sun

North Carolina State University

Qingfeng Zhan

Chinese Academy of Sciences

Show all 7 authorsHide

Co52Pt48 nanowire arrays with diameter of about 10 nm have been fabricated by electrodeposition into the anodic aluminum oxide templates. The as-deposited nanowire arrays exhibit fcc structure with [110]-preferred texture along the wire axes. The nanowire arrays present both large magnetocrystalline anisotropy along the [111] direction and strong shape anisotropy along the wire axis, resulting in high coercive fields parallel and perpendicular to the wire axes, respectively. From experimental results and micromagnetic simulation, the coercive field variation as a function of the angles evidenced that the [111] magnetocrystalline anisotropy plays important role besides shape magnetic anisotropy.

(a) Magnetic hysteresis loops of the Co 52 Pt 48 nanowire arrays measured by SQUID magnetometer with magnetic field parallel and perpendicular to wire axis at 5 K . (b) The simulated loops by micromagnetic simulation with magnetic field parallel to the wire axes for different magnetocrytalline axes distribution. The solid triangles and open circles correspond to magnetocrytalline axes in fixed direction and distributes randomly in the cross section.

…

X-ray diffraction spectrum of the as-deposited Co 52 Pt 48 nanowire arrays with the wire diameter of 10 nm .

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Structure and magnetic properties of the self-assembled Co52Pt48

nanowire arrays

Jian-Hua Gao, Da-Li Sun, Xiang-Qun Zhang, Qing-Feng Zhan, Wei He,

Young Sun, and Zhao-Hua Chenga兲

State Key Laboratory of Magnetism and Beijing National Laboratory for Condensed Matter Physics,

Institute of Physics, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China

共Received 13 January 2008; accepted 18 February 2008; published online 10 March 2008兲

Co52Pt48 nanowire arrays with diameter of about 10 nm have been fabricated by electrodeposition

into the anodic aluminum oxide templates. The as-deposited nanowire arrays exhibit fcc structure

with 关110兴-preferred texture along the wire axes. The nanowire arrays present both large

magnetocrystalline anisotropy along the 关111兴direction and strong shape anisotropy along the wire

axis, resulting in high coercive ﬁelds parallel and perpendicular to the wire axes, respectively. From

experimental results and micromagnetic simulation, the coercive ﬁeld variation as a function of the

angles evidenced that the 关111兴magnetocrystalline anisotropy plays important role besides shape

In the recent years, magnetic nanowire arrays have re-

ceived growing interest due to not only strong potential can-

didates for high density magnetic recording media but also

the important role in the fundamental research. Among them,

transition metal alloys such as Co–Pt and Fe–Pt alloys, due

to high magnetic perpendicular anisotropy 关⬃2⫻106J/m3

for Co–Pt 共Ref. 1兲and ⬃6⫻106J/m3for Fe–Pt 共Ref. 2兲兴,

are considered promising candidates for potential application

in magnetic storage media. It is only very recently that Co–Pt

and Fe–Pt nanowire arrays have been fabricated.3–13 Huang

et al.3ﬁrstly fabricated Co–Pt and Fe–Pt nanowires by elec-

trodeposition. Yasui et al.5have also electrodeposited Co–Pt

nanowires and try to adjust perpendicular magnetic aniso-

tropy by textured underlayer. Mallet et al.6and Dahmane

et al.14 and Min et al.15 have reported that the as-deposited

nanowires exhibit fcc structure with soft magnetic properties,

an annealing treatment is necessary to obtain L10phase with

large coercive ﬁeld higher than 10 kOe. Unfortunately, the

nanowires usually lose uniaxial magnetic anisotropy and ex-

hibit isotropic anisotropy after the postannealing. Another

disadvantage of annealed nanowires is that it is difﬁcult to

obtain excellent single phase.12,14 In addition, most reports

are concentrated on the fabrication, and little investigation

focused on the magnetization reversal mechanism, in which

magnetic anisotropy is the heart of understanding the mag-

netization reversal process. In the present work, magnetic

properties of the electrodeposited Co52Pt48 nanowire arrays

have been investigated. The contribution of shape magnetic

anisotropy and magnetocrystalline anisotropy to magnetiza-

tion reversal has been simulated by micromagnetic simula-

tion.

The fabrication of nanowire arrays began with nano-

porous anodic aluminum oxide 共AAO兲templates prepared by

two-step anodic anodization process.16 Co52Pt48 nanowires

are electrodeposited into the AAO templates by ac deposition

from the electrolytes consisted of 0.01MCoSO4and 0.01M

Pt共NO2兲2共NH3兲2. Using inductively coupled plasma analysis,

the composition of the nanowires is determined with atomic

percentage Co52Pt48. The crystal structure of the nanowire

arrays is characterized by x-ray diffraction 共XRD兲. The mag-

netic hysteresis loops have been measured by a supercon-

ducting quantum interference device 共SQUID兲magnetometer

and vibrating sample magnetometer 共VSM兲. The micromag-

netic simulation has been performed using the object ori-

ented micromagnetic computing framework 共OOMMF兲

code.17

Figure 1shows the XRD pattern of the Co52Pt48 nano-

wire arrays with the Cu K

␣

radiation. From the diffraction

peaks, the constant lattice can be determined and the value is

a=3.806 Å, corresponding to fcc structure of the Co–Pt al-

loy. Furthermore, fcc structure is the only phase and prefer-

ential growth of the 关110兴orientation along wire axes is ob-

served for the as-deposited nanowires. Therefore, the

Co52Pt48 as-deposited nanowire arrays have fcc structure

with strong 关110兴-preferred texture along the wire axes.

Magnetic hysteresis loops measured by SQUID magne-

tometer at 5 K and calculated by micromagnetic simulation,

is shown in Fig. 2. Figure 2共a兲is the measured loops with

magnetic ﬁeld parallel and perpendicular to the wire axis.

For the applied ﬁeld parallel to the nanowire arrays, the hys-

teresis loop is relatively square with coercive ﬁeld HC

储

=4.52 kOe and remanence squareness S

储

=0.76. While when

the applied ﬁeld is perpendicular to the wire axes, the smaller

coercive ﬁeld HC⬜=2.98 kOe and the remanence squareness

a兲Author to whom correspondence should be addressed. Electronic mail:

zhcheng@aphy.iphy.ac.cn.

FIG. 1. X-ray diffraction spectrum of the as-deposited Co52Pt48 nanowire

arrays with the wire diameter of 10 nm.

APPLIED PHYSICS LETTERS 92, 102501 共2008兲

Downloaded 10 Mar 2008 to 159.226.36.156. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

S⬜=0.28 are obtained. Therefore, we can conclude that the

as-deposited Co52Pt48 nanowire arrays exhibit a hard ferro-

magnetic behavior, with easy magnetization axes along the

wire axes. Similar results were reported by Mallet et al. for

the CoxPt1−xnanowire arrays.4

According to the chain spheres of symmetric fanning

mechanism,18 the coercive ﬁeld parallel to the wire axes

should be HC

储

共sy兲=共

␲

MS/6兲共6Kn−4Ln兲=1.49 kOe, where

MS⬇920 emu/cc is the saturation magnetization for bulk

Co–Pt alloy within the error range.19 The coercive ﬁeld per-

pendicular to the wire axes should be zero. However, the

experimental coercive ﬁelds in the two directions are HC

储

=4.52 kOe and HC⬜=2.98 kOe, respectively, which are

much larger than the chain sphere prediction. This large dis-

crepancy is considered to be the magnetocrystallline aniso-

tropy contribution of the Co52Pt48 nanowire arrays. It has

been predicted by calculation that Co50Pt50 ﬁlms with fcc

structure exhibit large intrinsic magnetocrystallline aniso-

tropy along the 关111兴direction.20,21 Since the nanowire ar-

rays are textured along the wire axes, i.e., 关110兴direction,

therefore, the shape anisotropy should be along the 关110兴

direction. The easy magnetocrystalline axes is along the

关111兴direction, which makes angles of about 35° with the

wire axis. According to the calculation prediction, the mag-

nitudes of the two anisotropies are comparable, so this mag-

netocrystalline anisotropy should be taken into account.

In order to further conﬁrm the magnetocrystalline aniso-

tropy contribution, micromagnetic simulation is used to cal-

culate the hysteresis loops. The simulated hysteresis loops

with different magnetocrystalline anisotropy conﬁgurations

are shown in Fig. 2共b兲, for comparison, the measured loop

is also shown. As dissussed in details for Fe3Pt nanowires,22

the dipolar interaction between nanowires is very important

for nanowire arrays. In fact, the dipolar interaction is taken

into account in the OOMMF code during the simulation when

the wire diameter, length, and distance of nanowire array are

given. In the simulation, a hexagonal cell of 4⫻4 array of

nanowires is chosen, with the unit cell size 2.5⫻2.5

⫻5nm

3. The geometry parameters come from the experi-

ment, with the wire diameter d=10 nm and length L

=400 nm. Since the wire diameter is up to 10 nm, the satu-

ration magnetization MSand exchange stiffness Aare close

to corresponding bulk values, with MS=9.2⫻105and A

=2.65⫻10−12 A/m and magnetocrystalline anisotropy con-

stant Ku=2.0⫻105J/m3. If we assume that the magneto-

crystalline anisotropy axes are ﬁxed in a certain direction,

the simulated coercive ﬁeld of 5.5 kOe is much higher than

the experimental results. Although the nanowires have strong

关110兴-preferred texture along the wire axes, the 关111兴direc-

tion of crystallites distributes randomly with the azimuthal

angle in the cross section perpendicular to the wire axis ow-

ing to the polycrystalline nanowires 关inset of Fig. 2共b兲兴.

Therefore, we simulated the hysteresis loops with the 关111兴

magnetocrystalline axis randomly distributed in the cross

section. The simulated coercive ﬁeld of 4.59 kOe is in good

agreement with measured coercivity of HC

储

=4.52 kOe. The

relatively large deviation in remanence squareness S

储

is re-

lated to the surface magnetic moment deviating from the

axes of the nanowires.

Figure 3shows the angle dependence of the coercive

ﬁelds 关HC共

␪

兲兴 for Co52Pt48 nanowire arrays obtained by ex-

periment and micromagnetic simulation. The solid squares

are the experimental points measured by VSM magnetometer

at room temperature. There is a platform in the HC共

␪

兲curve

for the angles below 20° and the coercive ﬁelds decrease

rapidly with further increasing angles. The HC共

␪

兲variation

exhibits a bell-type shape as reported for Ni nanowires.23

This HC共

␪

兲variation shape is determined by rotation reversal

mechanism instead of curling mechanism,24 which is not sur-

prising since the diameter of the Co52Pt48 nanowire arrays is

smaller than the critical diameter about 25 nm 共dcoh

=冑24A/

␮

0MS

2兲.25 Also, we should keep in mind that the

HC共

␪

兲variation is different from the inﬁnite cylinders with

uniaxial anisotropy. As we discussed above, due to the 关111兴

magnetocrystalline easy axes, making an angle of 35° with

FIG. 2. 共Color online兲共a兲Magnetic hysteresis loops of the Co52Pt48 nano-

wire arrays measured by SQUID magnetometer with magnetic ﬁeld parallel

and perpendicular to wire axis at 5 K. 共b兲The simulated loops by micro-

magnetic simulation with magnetic ﬁeld parallel to the wire axes for differ-

ent magnetocrytalline axes distribution. The solid triangles and open circles

correspond to magnetocrytalline axes in ﬁxed direction and distributes ran-

domly in the cross section.

FIG. 3. 共Color online兲Angle dependence of the coercive ﬁelds for the

Co52Pt48 nanowire arrays. The solid squares indicate experimental points

obtained by VSM magnetometer at 300 K. The solid triangles and open

circles denote the simulation results with magnetocrytalline axis in ﬁxed

direction and distributed in the cross section with the randomly azimuthal

angles, respectively.

102501-2 Gao et al. Appl. Phys. Lett. 92, 102501 共2008兲

Downloaded 10 Mar 2008 to 159.226.36.156. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

the wire axis, the Co52Pt48 nanowire arrays are not ideal sys-

tem with uniaxial anisotropy. Using micromagnetic simula-

tion, we have calculated the coercive ﬁelds at different

angles, taking magnetocrytalline anisotropy into account

with different conﬁgurations, as shown in Fig. 3. The open

circles are the simulation results with magnetocrytalline axis

ﬁxed in a certain direction. There appears a peak around 15°

in the curve which exihibits large deviation with the experi-

ment results. We assume that the 关111兴magnetocrytalline

axes distribute randomly in the cross section perpendicular to

the wire axis and the simulated curve is shown by the solid

triangles. For this magnetocrytalline axes conﬁguration, the

HC共

␪

兲variation in coercive ﬁelds has improved better at

small angles, even there exists relative large deviation at the

large angles. We can conclude that besides shape anisotropy

along the wire axis, magnetocrytalline anisotropy also plays

an important role for fcc Co52Pt48 nanowire arrays.

In summary, the structure and magnetic properties of

Co52Pt48 nanowire arrays with the diameter 10 nm have been

investigated. The XRD pattern shows that as-deposited nano-

wire arrays exhibit fcc structure with the 关110兴orientation

along the wire axis. Due to shape anisotropy along the wire

axes and magnetocrystalline anisotropy along the 关111兴di-

rection, the nanowire arrays exhibit large coercive ﬁelds both

parallel and perpendicular to the wire axis. Magnetocrystal-

line anisotropy plays important role is evidenced by the ex-

periment and micromagnetic simulation.

This work was supported by the State Key Project of

Fundamental Research and the National Natural Science

Foundation of China.

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May 2002

In this study we have fabricated by electrodeposition CoPt(FePt) nanowires embedded inside an array of empty holes in anodized aluminum disks. By adjusting the current density and solution composition, one can control the composition of the film to 50:50 in order to obtain the high anisotropy CoPt and FePt face centered tetragonal (L10) phases. The as-made films are magnetically soft. Magnetic hardening is developed after annealing at 700 °C with coercivity typically in the range of 3–6 kOe. The scanning electron microscopy and transmission electron microscopy studies showed that nanowire structured CoPt and FePt with near stoichiometric composition inside an array of holes which are about 25–100 nm in diameter. A preferred perpendicular anisotropy is observed in the CoPt nanowires. © 2002 American Institute of Physics.

Angular dependence of the coercivity and remanence of ferromagnetic nanowire arrays

Article

Full-text available

Jun 2003

Magnetization properties of magnetic nanowire arrays are studied on various ferromagnetic materials grown in anodic alumina (alumite) and track etched polycarbonate (PCTE) membranes by pulsed electrodeposition. Magnetization curves were measured as functions of wire material, field orientation, and wire length. The coercivity (Hc) and remanent squareness (S) of the various wire arrays were derived from hysteresis loops as a function of angle (θ) between the field and wire axis. For PCTE membranes, Hc(θ) curves for CoNiFe, NiFe, and Co nanowire arrays all show an otherwise-bell-type variation, while they change shapes from the otherwise bell to bell type for Ni nanowire arrays as the wire diameter decreases to 30 nm. These characteristics can be understood based on different magnetization reversal mechanisms of small wires. The effect of magnetostatic interaction among wires on the magnetic properties was examined by changing the wire lengths in alumite membranes. It is found that the interaction reduces Hc and S values significantly and may cause the overall easy axis change from parallel to perpendicular to the wire axis. However, the interaction is much weaker than expected from an independent precession theory. The strong coupling among the wire may also induce a change of magnetization reversal mechanism. © 2003 American Institute of Physics.

Magnetic Properties of Metals: d-Elements, Alloys and Compounds

Book

Jan 1991

H. P. J. Wijn

During the last decades the knowledge of the magnetic properties of the d transition elements and of their metallic alloys and compounds has increased widely. The improvement of preparation techniques for well-defined substances, the development of sophisticated measuring methods and above all the drive to obtain more insight in the origin of magnetic interactions in solids have resulted in the publication of many specific magnetic properties for an abundance of all kinds of metallic materials. The data assembled in this booklet are selected from the comprehensive compilation of magnetic and related properties of metals in the Landolt-Bornstein New Series Group III sub volumes 19a, band c. It has been attempted to include preferentially those properties which are of a basic character and which therefore are most often needed by scientists active in the field of solid state magnetism. In the field of magnetism, there is a gradual transition from the use of cgs/emu units to SI units. It was, however, not intended to represent all data in the units of one system, regardless of how nice this would have been from a systematic point of view. Instead, mostly preference was given to the system of units that was originally used by the authors whose work is quoted. Thus cgs/ emu units occur most frequently. Of colirse the user of the tables and figures is helped in several ways to convert the data to the units which he is most familiar with, see, e. g.

Composition-dependent structural and magnetic properties of Co1−xPtx (0.09 ⩽ x ⩽ 0.86) nanowire arrays

Article

Apr 2006

Highly ordered 14 nm diam cobalt platinum alloy nanowire arrays with Pt content ranging from 9 to 86 at. % have been electrodeposited into nanoporous anodic aluminum oxide templates. The crystal structure of the as-deposited Co1−xPtx nanowire arrays changes progressively from hcp to a mixture of hcp and fcc phases and finally to pure fcc with increasing Pt content. The coercivity, squareness, and saturation magnetization of the as-deposited nanowire arrays have been investigated as a function of nanowire composition.

Criterion for Uniform Micromagnetization

Article

Mar 1957

William Fuller Brown

Current theories of "single-domain" ferromagnetic particles compare the free energy in a state of uniform magnetization with that in an arbitrarily chosen state of nonuniform magnetization. In this paper, the comparison is made between an initial uniform state and all neighboring states, uniform or nonuniform, as an initially large applied field decreases. The initial state becomes unstable when, for some choice of the varied magnetization, the second variation of the free energy changes from positive to negative. This instability criterion leads to a boundary-value problem; the relative magnitudes of certain eigenvalues determine whether the deviation from the initial state occurs by uniform rotation or by development of nonuniform magnetization. Formulas for the critical radius are found in simple cases; they agree, except for a numerical factor, with formulas of Kondorskii.

Ab initio theoretical description of the dependence of magnetocrystalline anisotropy on both compositional order and lattice distortion in transition metal alloys

Article

Jun 2001

Recently, we outlined a scheme to investigate the effects of both short-ranged and long-ranged compositional order on the magnetocrystalline anisotropy of alloys from a first-principles electronic structure point of view [Phys. Rev. Lett. 82, 5369 (1999)] and showed that in the Co0.5Pt0.5 alloy compositional order enhances the magnitude of magnetocrystalline anisotropy energy (MAE) by some two orders of magnitude. Here we describe our scheme in detail and study some more transition metal alloys. In the Co0.25Pt0.75 alloy we find the perfect L12 structure to be magnetically soft whereas imposition of directional order greatly enhances its MAE. We also present the effect of lattice distortion (tetragonalization) on MAE on the same footing and find that in the Co0.5Pt0.5 alloy it accounts for only about 20% of the observed enhancement, thus confirming that compositional order is the major player in this effect. Tetragonalization of the lattice has also a modest effect on the MAE of the Fe0.5Co0.5 alloy. We also examine the electronic effects which underpin the directional chemical order that is produced by magnetic annealing of permalloy which we study within the same framework.

Work function of (8,0) single-walled boron nitride nanotube at the open tube end

Article

May 2006

The effects of hydrogen (H) and fluorine (F) chemisorption on the work function (Φ) and polarization field of (8,0) boron nitride nanotubes (BNNT) have been investigated using periodic density functional theory. The binding energies of coadsorbed H and F atoms on B and N sites in BNNT are much higher than that of single adsorption case. The work function at the open tube end can be affected by adsorption on the tube wall due to changes in the macroscopic polarization field along the tube, as well as the shifting of the Fermi level. Among the various adsorption patterns on the wall, we found that adsorption of H,F pairs, or H,H pairs, on the tube wall to form a continuous, armchair chain along the tube axis, produces strong enhancement of the intrinsic polarization field. With wall adsorption of (H,H) pairs, the Φ at the B terminal becomes 3.25 eV, compared with 5.40 eV of the pristine tube, and with (H,F) pairs wall adsorption, the Φ at the B terminal is reduced to 3.56 eV.

Thick-film permanent magnets by membrane electrodeposition

Article

Jun 2005

Thick films of nearly equiatomic Co–Pt produced by electrodeposition into nanoporous membranes exhibit an isotropic magnetic phase with square hysteresis loops (Mr/Ms = 0.95) and coercivity of up to 1.3 T. The initial magnetization curve is characteristic of pinning-type magnets. The coercivity can be controlled by the choice of annealing temperature or alloy composition, which depends on the chemical composition of the plating bath, properties which can be exploited to produce fine-pitch pole patterns.