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Chapter 10

Low Dimensional Nanomaterials for Spintronics

Jinlong Yang and Hongjun Xiang

Abstract Moore’s Law in microelectronic technology will break down as the size

of individual bits approaches the dimension of atoms; this has been called the end of

the silicon road map. For this reason and also for enhancing the multifunctionality

of devices, the spin degree of freedom of electron is being investigated for magneto-

electronics applications, i.e., spintronics. Spin-based devices are closely connected

with the development of nanotechnology. In this chapter, recent developments of the

low-dimensional nanomaterials for spintronics are reviewed. In the ﬁrst section, the

main concepts of spintronics including nanospintronics are brieﬂy discussed. Exper-

imental studies on transition-metal-doped nanowires and nanotubes are summarized

in the second section. Extensive theoretical works in this ﬁeld are reviewed in the

third section. Finally, an outlook is given in the last section.

10.1 Spintronics

Spintronics (a neologism for “spin-based electronics”) [1, 2], also known as mag-

netoelectronics, is an emergent technology that exploits the quantum propensity of

electrons to spin as well as make use of their charge state. The spin itself is man-

ifested as a detectable weak magnetic energy state characterized as “spin up” and

“spin down”. Spintronics started attracting massive interest with the discovery of

giant magnetoresistance (GMR) in the 1980s, which has already been adopted as

the norm in the hard disk drive (HDD) manufacturing industry. Indeed, the impact

of spintronics in the HDD industry is a mere indication of things to come.

Spintronics has a number of potentially groundbreaking applications that are set

to drive next-generation electronics. Although GMR can arguably be considered the

driving force for spintronics at present, the biggest potential of spin-based devices

is in embedded memories. Magnetoresistive random access memory (MRAM) is

expected to revolutionize the memory market and contribute to the development

of advanced and versatile computing and personal devices. Advances in instantly

247

248 J. Yang, H. Xiang

bootable computers and MRAM could be the next big thing in sprintronics. Mag-

netic sensors have already found a wide market application, thereby establishing

the commercial viability of spin-based devices. Each spin could carry a bit of

information—in effect, a single-spin transistor—which could lead to faster comput-

ers that consume less electricity, alleviating the problem of heat dissipation. Quan-

tum computation is perhaps one of the most exciting potential applications of spin-

tronics. However, harnessing the power of the quantum states to enable information

processing and storage is not easy. The evolution of MRAMs and various spin-based

technologies could be critically important in facilitating the development of the ﬁrst

quantum computer.

To make a spintronic device, the primary requirement is to have a system that

can generate a current of spin polarized electrons, and a system that is sensitive to

the spin polarization of the electrons. Most devices also have a unit in between that

changes the current of electrons depending on the spin states. The simplest method

of generating a spin polarized current is to inject the current through a ferromagnetic

material. In particular, half metallic (HM) [3] ferromagnetic materials are ideal for

such applications since in these systems conduction electrons are 100% spin polar-

ized because of a gap at the Fermi level (E

) in one spin channel, and a ﬁnite density

of states (DOS) for the other spin channel.

10.1.1 Nano-Spintronics

Spin-based devices are closely connected with the development of nanotechnology.

The manipulation of spin calls for working at very small dimensions and this has

led to a high degree of miniaturization in these devices. Small is the way to go

in technology and miniaturization could potentially take spintronic devices to the

next level. The miniaturization of electronic devices represents an ongoing trend for

both industrial manufacture and academic research. Spintronics requires the fab-

rication of ferromagnetic nanostructures that, at room temperature, can transport

spin-polarized carriers, and that can be assembled into addressable hierarchies on a

macroscopic chip. Dimensionality and size are known to play a signiﬁcant role in

determining various properties of the systems.

Among many other possibilities, nanotubes (NTs) and nanowires are now being

actively explored as the possible building blocks for electronic devices with fea-

tures smaller than 100 nm. The controlled fabrication and fundamental under-

standing of low-dimensional ferromagnetic semiconductor nanostructures is thus

crucial to the development of semiconductor-based spintronic devices and spin-

based quantum-computation schemes. Meanwhile, one-dimensional diluted mag-

netic semiconductor (DMS) nanowires are advantageous to the integration of DMSs

toward a three-dimensional architecture of nanoscale spintronics chips for several

reasons. First, nanowires themselves are attractive building blocks for nanometer-

scale electronic and optoelectronic devices; second, magnetic nanowires could act

as spin ﬁlters to supply spin-polarized carrier currents and could also have large

10 Low Dimensional Nanomaterials for Spintronics 249

magnetic anisotropy energies; third, carriers could be conﬁned in the radial direction

of nanowires and, therefore, high carrier concentrations and efﬁcient injection of

spin-polarized carriers could be potentially achieved; ﬁnally, the single-crystalline

nanowires could be used as a model DMS system for exploring the origin of fer-

romagnetism in these semiconductors by excluding extrinsic effects such as sec-

ondary phases. It is reasonable that the combination of low dimensionality and room

temperature ferromagnetism in diluted magnetic oxides would generate new func-

tional nanomaterials useful for future miniaturized devices because of the strong

conﬁnement effect and the shape anisotropy. The availability of diluted magnetic

semiconductor nanowires with Curie temperatures above room temperature may

also open up new opportunities to realize nanometer-scale spintronic and optoelec-

tronic devices such as spin-LEDs, transistors, and ultradense nonvolatile semicon-

ductor memory, optical devices such as diodes and displays which can be operated

by a magnetic ﬁeld as well as spin-injection devices, e.g., ampliﬁers, frequency mul-

tipliers and square-law detectors based on 1D heterostructures.

10.2 Brief Overview of Experimental Works

With the advancement of experimental techniques, it has recently been possible to

synthesize low-dimensional nanostructures including nanowires and NTs. Recently,

several groups have succeeded in synthesizing single-crystalline Mn-doped GaN

nanowires free of defects with diameters ranging from 10–100nm and of lengths up

to tens of micrometers [4–6]. More importantly, Mn-doped GaN nanowires have

been found to be ferromagnetic up to 300 K and exhibit negative magnetoresis-

tance. Ham and Myoung found that the room-temperature ferromagnetism depends

on (Ga, Mn)N nanowires, which were converted from n-type to p-type with in-

creasing Mn concentration [7]. Cui et al. [8] used an electrochemical process to

grow Co-doped ZnO nanowire arrays at low temperature. The Co dopant introduces

room-temperature ferromagnetism in ZnO nanowires and affects their optical prop-

erties. FeCo-codoped ZnO nanowires were synthesized by in situ doping with iron

and cobalt using a chemical vapor deposition method [9]. No FeCo clusters and

other secondary phases were found in the nanowires. A Curie temperature higher

than 300 K was observed from the as-doped nanowires.

Very recently, Seong et al. reported room-temperature ferromagnetic properties

in single-crystalline AlGaN:Mn NTs [10]. Room temperature ferromagnetism in

Co-doped TiO

NTs prepared by sol–gel template synthesis was reported by Huang

et al. where the ferromagnetic behavior is related to the oxygen vacancies created by

the Co

substitution of Ti

site in TiO

[11]. The synthesis of room temperature

ferromagnetic Co-doped titanate NTs via a simple hydrothermal method was also

achieved in another study [12].

As discussed earlier, most efforts have been directed towards the mixing of

transition-metal (TM) atoms (such as Fe, Co, Ni, and Mn, which have perma-

nent magnetic moments) into semiconductor devices. To create nanostructures

250 J. Yang, H. Xiang

exhibiting room-temperature ferromagnetism, Krusin-Elbaum et al. synthesized

-alkylamine NTs through a combination of sol–gel and hydrothermal synthe-

sis [13]. They then “doped” the NTs, introducing foreign atoms into the gaps

between layers: lithium atoms, which act as electron donors to the vanadium lat-

tice; and iodine atoms, which act as electron acceptors, creating positive charge

carriers in the lattice called “holes”. In contrast to the TM devices mentioned earlier

(in which a 3d TM atom is added into a nonmagnetic host), here both guest atoms

are nonmagnetic but the host is magnetized by the elimination or addition of one ex-

tra charge (positive or negative). The bipolar nature of the NTs—the possibility of

making them conductors of either electrons or holes through appropriate doping—is

important for the fabrication of transistor structures and junctions (p-n junctions).

Experimentally, a carbon NT (CNT) based hybrid nanostructure can be achieved

either by ﬁlling the NT with foreign materials in its cavity or by coating the materi-

als on the tube surface. CNTs ﬁlled with TM have been extensively studied exper-

imentally [14–20]. In some experiments 1D crystal structure actually forms in the

cavity of the tube. Coating on the surface of CNTs can also be achieved with a va-

riety of materials. For example, titanium can form stable and continuous structures

on the NT surface. Recently, monocrystalline FeCo nanowires encapsulated inside

multiwalled carbon NTs was synthesized, which could be used in the fabrication of

high-density magnetic storage devices and magnetic composites [21].

10.3 Theoretical Studies

10.3.1 TM-Doped Nanowires

Using density functional theory (DFT), Wang et al. showed that the magnetic cou-

pling of Mn atoms in the nanowires, unlike that in the thin ﬁlm, is ferromagnetic

[22]. This ferromagnetic coupling, brought about because of the conﬁnement of

electrons in the radial direction and the curvature of the Mn-doped GaN nanowires’

surface, is mediated by N as is evidenced from the overlap between Mn 3d and

N 2p states. In addition, the magnitude of the magnetic moments can be tuned by

changing the wire thickness. They also carried out a theoretical study of the mag-

netic properties of Cr-doped GaN nanowires [23]. In contrast to the Mn-doped GaN

nanowires, the GaN/Cr nanowire is found to be FM irrespective of the distribution

of the Cr atoms. In addition, the binding energy of Cr to the GaN wire is larger than

that of Mn, suggesting that it may be possible to dope the GaN nanowire with Cr

more easily than with Mn. Thus, the GaN/Cr nanowire could prove to be a robust

system for applications. The magnetic moments at the Cr atoms are about 2.5

and these are coupled antiferromagnetically to a small moment at the N site. The

FM coupling results from a charge overlap between the N 2p and Cr 3d states and

the coupling is driven by a double exchange mechanism. Schmidt et al. [24] investi-

gated the electronic, structural, and magnetic properties of Mn-doped InP nanowires

10 Low Dimensional Nanomaterials for Spintronics 251

along the [111] direction with the dangling bonds on the surface saturated by hy-

drogen atoms. They found that the most energetically favorable position for the Mn

atom is near the surface. When the Mn atoms are in “bulklike” positions, the mag-

netic Mn–Mn coupling in the nanowire is ferromagnetic. In these cases, their results

are consistent with a host-p-Mn-d coupling mechanism. On the other hand, if at least

one of the Mn atoms is located near the surface, the coupling is antiferromagnetic.

10.3.2 TM-Doped Nanotubes

10.3.2.1 TM Doped C Nanotubes

Encapsulating ferromagnetic structures inside NTs has become an active research

topic. In 2003, Yang et al. [25] found through ab initio calculations that such TM/NT

hybrid structures exhibit substantial magnetism. In particular, cobalt atoms packed

inside a variety of CNTs offer strong spin polarization at the Fermi level as well as

considerable magnetic moments. They considered a (9,0) SWNT with six Co atoms

ﬁlled inside per unit cell, forming an ABAB staggered triangle packing. The nearest

neighboring distances between the Co atoms and C atoms range between 2.167 and

2.245

A. The calculated magnetic moment for the freestanding Co nanowire with

the same conﬁguration is about 2.08

. And the calculated Co-ﬁlled NT has an

average magnetic moment of 1.4

per Co atom. This moment is comparable to

that of hcp bulk Co, which is 1.58

. Deﬁne the binding energy as

= E(tube) + E(nanowire) −E(tube+ nanowire), (10.1)

was found to be 0.16 eV per Co atom. They also performed nonmagnetic cal-

culations for the same system and obtained a binding energy of 0.60 eV per Co

atom. Combining this with the substantial charge delocalization of the conduction

electrons, an average Co–C bond length of ∼2.19

A, and the observation that the

magnetic moment of the hybrid is different from the freestanding wire have led us

to the conclusion of a moderate covalent bonding between the Co and C atoms.

The reduction of moment of the nanowire inside the tube clearly indicates a strong

interaction between the magnetic metal and nonmagnetic carbon. The DOS shows

characteristic narrowing of d bands in the nanostructure owing to the reduced coor-

dination number. Band structure analysis shows that there are many more conduc-

tion bands crossing the Fermi level for the minority spin than those for the majority

spin for the hybrid system. Thus a spin-polarized transport process can be achieved

in such hybrid structures. Quantitatively, the calculated spin polarization P (85%)

is substantially higher than that calculated for the bulk Co (66%), where the spin

polarization P is calculated from the DOS N(E

) corresponding to the minority

(majority) spin at the Fermi level:

P =(N

↓

) −N

↑

))/(N

↓

)+N

↑

)). (10.2)

252 J. Yang, H. Xiang

They observed that the conduction electrons from the majority spin occupy mostly

the C sites, while those from the minority spin concentrate on the Co central wire.

There is also considerable delocalization of conduction electron density, which indi-

cates strong interaction between the Co and C atoms. Their studies of other similar

systems reveal that the qualitative results of magnetism and spin polarization do not

depend sensitively on the choice of the magnetic metal, the metal conﬁguration, or

the NT chirality. It is noteworthy that both semiconducting and metallic SWNTs

are transformed into good spin-polarized conductors with metal ﬁlling or coating.

Thus, there is no need to separate the semiconducting and metallic NTs in these

hybrid structures.

Durgun and Ciraci [26] presented a systematic analysis for the stability, atomic,

electronic, and magnetic properties of TM atomic chains adsorbed on the external

and internal wall of the (8,0) SWNT. They found that all adsorbed chains have ferro-

magnetic ground state. The coupling among the adsorbed TM atoms and the charge

transfer between adsorbed TM and nearest carbon atom of SWNT play an important

role in determining the resulting magnetic moment. Usually, the magnetic moment

of the free TM atom is reduced upon the adsorption. And high spin polarization

at the Fermi level can be obtained by the adsorption of V and Fe chains on the

(8,0) SWNT at speciﬁc geometries. Interesting variation of the magnetic moment

and binding energy with the number of ﬁlled d electrons of the adsorbate have been

revealed. The spin-dependent electronic structure and the net magnetic moment cal-

culated for ﬁnite-size systems are found to be different from inﬁnite and periodic

systems. Their results suggest that these ﬁnite-size tubes holding TM atoms can be

used as a nanomagnet and can perform as spin-valve or spin-dependent resonant-

tunneling devices when they are connected to the metal electrodes from both ends.

Titanium monomers and wires adsorbed on (8,0) CNTs was considered by Fagan

et al. [27,28]. The most stable conﬁgurations for monomers are found to be over the

centre of a C–C bond for inside and over the midpoint of the centre of the hexagonal

site for outside. They showed that the most stable conﬁguration is with the wire

inside the tube, and the resulting electronic structures show a metallic system with

high hybridization between the Ti and C atoms and a large charge transfer from Ti

to C atoms. For Ti wire adsorbed inside the tube the low spin conﬁguration is shown

to be more stable than high spin conﬁguration and the opposite behavior is observed

for the corresponding outside case.

10.3.2.2 TM Doped BN Nanotubes

As shown earlier, while TM nanowires ﬁlled CNTs can exhibit substantial mag-

netism, no HM behavior exists in such systems [25]. Since BN NTs are insulators

independent of their chiralities and are more chemically inert than CNTs, they

may serve as naturally insulating or protective shields for encapsulating conducting

metallic clusters, nanowires, and nanorods [29, 30]. The magnetic nanostructures

isolated by a nonmagnetic material, i.e., BN walls, could be efﬁciently used for

high density data storage, without the drawback of particle agglomeration and

10 Low Dimensional Nanomaterials for Spintronics 253

magnetic loss attributable to dipolar relaxation. This kind of protection also prevents

the oxidation of the metallic clusters, nanowires, and nanorods, which are prone

to be oxidized. Recently, BN NTs ﬁlled with 3d-TMs such as Fe-Ni Invar alloy,

Co nanorods, Mo clusters, Ni and NiSi

nanowires have been successfully syn-

thesized [31–34]. However, detailed electronic and magnetic properties of these

novel nano-structures are far from well understood. And the question how the

magnetic properties of TM/BN NTs differ from those of TM/CNTs is open. We

performed a comprehensive ﬁrst principles study on the electronic and magnetic

properties for these TM/BN NT hybrid structures. We show that the electronic and

magnetic properties of TM/BN NTs differ fundamentally from those of TM/CNTs

and HM ferromagnetism could be achieved in TM/BN NTs under appropriate

circumstances.

Our theoretical calculations are performed using the Vienna ab initio simula-

tion package (VASP) [67, 68]. We describe the interaction between ions and elec-

trons using the frozen-core projector augmented wave (PAW) approach [35, 69].

The overall framework is spin-polarized DFT [36, 37] in the generalized gradi-

ent approximation (PW91) [38, 39]. The basis set cut off is 400 eV. In a typical

calculation, a 1D periodic boundary condition is applied along the NT axis with

Monkhorst-Pack [40] k-point sampling. During structural relaxation, all atoms ex-

cept the ﬁxed ones are allowed to relax to reach the minimum energies until the

Hellmann-Feynman forces acting on them become less than 0.01 eV/

A. Since most

BN NTs are zig-zag type [41, 42], we choose BN zig-zag NTs with a smallest hcp

TM nanowire ﬁlled inside. Though the most stable bulk phase for Ni is fcc, the

stability for Ni nanowires depends not only on the structure, but also on the orien-

tation and radius of the nanowires. For example, the smallest hcp Ni nanowire is

found to be more stable by about 0.3 eV per Ni than the smallest fcc Ni nanowire in

the [111] orientation. The hcp TM nanowire is composed of six TM atoms per unit

cell with ABAB staggered triangle packing. BN(8,0), BN(9,0), and BN(10,0) NTs

ﬁlled with the Ni hcp nanowire are studied. Different initial guesses are used for

the local magnetic moments including ferromagnetic, antiferromagnetic (alternate

up-down spins on A and B TM atoms), and nonmagnetic spin conﬁgurations, which

are then fully relaxed to obtain the ﬁnal converged structures and spin alignments.

The free standing hcp Ni nanowire is also studied for comparison with the hybrid

structures.

Figure 10.1 shows the optimized structures for the free standing Ni nanowire and

the Ni encapsulated BN NTs. In the optimized structures Ni atoms almost lie in

the nitrogen planes of BN NTs, as can be seen from the side view for Ni encap-

sulated BN(9,0) NT (Fig. 10.1e). This relative position along axis between the BN

NT and metal nanowire results from the big electronegativity of the nitrogen atom.

The comparative calculation for Ni/C(9,0) shows that Ni atoms in Ni/CNTs do not

lie in any carbon planes but in the middle of two adjacent carbon planes since all

atoms are the same in carbon NTs. The symmetry of the Ni nanowire changes little

after coated by BN NTs. Generally, after being inserted, the Ni nanowire expands

a little, e.g., the Ni–Ni distance in a plane (A or B) changes from 2.27 to 2.32

Ain

Ni/BN(9,0), and the BN walls expand slightly, 0.01

AforBN(9,0)NT.