Structural, magnetic and transport study of DBPLD fabricated magnetic semiconductors 3

6.2 Magnetic Properties of Zn1-xCoxO Thin Films 6.2.1 M-H Loops of Zn 1-xCoxO Thin Films at Room Temperature Figure 6-1a illustrates the M-H curves of Zn 1-xCoxO films at room tempera

CHAPTER 6 DEPENDENCE OF PROPERTIES OF Zn1-xCoxO THIN FILMS GROWN ON (0001) SAPPHIRE SUBSTRATES ON

Co CONCENTRATION

6.1 Introduction

Today, the fabrication of room temperature DMSs is still a great challenge For the purpose of fabrication of room temperature DMSs, Zn1-xCoxO materials have widely studied, and many experimental results have been reported However it is hard to see these results show consistency, especially discrepancies were reported by different groups for the magnetic properties Beside ferromagnetic behaviours, paramagnetic or spin glass behaviours were all reported [1-6] For example, Zn1-xCoxO thin films were

reported to be paramagnetic for x < 0.12 in Ref [5]; in contrast, M-H hysteresis loops

were observed for Zn1-xCoxO thin films with similar Co concentrations [7]

In our view, the differences are probably due to different experimental conditions and the sensitivities of the properties for Zn1-xCoxO thin films Hence it is necessary to prepare the films under the identical experimental conditions with a large range of Co

concentrations and study the properties dependence on Co concentration x

In our study, the DBPLD was used to synthesize Zn1-xCoxO thin films, hence the films could be prepared under the identical experimental conditions with a large range

of Co concentrations Magnetic behaviors and semiconductive properties are the most

important properties for DMSs In this chapter, the magnetic and transport behaviors

of the films were characterized From our experimental results, we conclude that

Zn1-xCox O with x < 0.1 is a candidate material exhibiting both magnetic and

semiconductive properties at room temperature That is, the magnetism can be realized with Co doped into ZnO However, the improvement was limited The transport behaviors of the Zn1-xCoxO thin films can be explained by a hopping mechanism, whose electrical states of Co ions is suggested to be localized

In this chapter, we are only concerned with magnetic behaviors The magnetic mechanisms will be discussed in a specific chapter (see Chapter 7)

6.2 Magnetic Properties of Zn1-xCoxO Thin Films

6.2.1 M-H Loops of Zn 1-xCoxO Thin Films at Room Temperature

Figure 6-1(a) illustrates the M-H curves of Zn 1-xCoxO films at room temperature

with Co concentration x ranging from 0.015 to 0.27 by VSM Even in the presence of

some noise due to weak signals, hysteresis loops were observed The coercivity measured was around 100 Oe, including the curve 1 of Fig 6-1 Our experimental results showed that magnetism could be realized by even a very low Co concentration From curve 1-3 of Fig 6-1, there are not apparent differences between them It seems that with increasing Co concentration, the improvement of magnetism was limited Further increase in Co concentration, magnetism may increase, as shown in curve 5 of Fig 6-1(a), and magnetic anisotropy may even be observed [Fig 6-1(b)] However, the

XPS Co (2p) spectrum indicates presence of Co–Co binding, as shown in the inset of

Fig 6-1(b) A peak centered at 778.9 eV was observed outside the binding energy range of Co–O (779.4– 780.2 eV) [8] Hence, the magnetic anisotropy may be due to the Co precipitates in the film

-40

-20

0 20

40

Excimer Laser

10 Hz, 3J/cm 2 5x10 -5 Torr

650 o C

1.Co 0.015 2.Co 0.02 3.Co 0.05 4.Co 0.16 5.Co 0.27 5

4 3 2

40

Co 0.27

775 780 785 790 14000 16000 18000 20000 22000

BE (eV) 778.9 eV

Fig 6-1(a) Magnetic loops of thin films with different Co concentrations measured at

room temperature by VSM; (b) Magnetic loops of thin films with Co concentration of

x = 0.27 at different angles by VSM Inset shows the XPS Co 2p3/2 spectrum

Figure 6-2 depicts the saturation magnetization per Co atom dependence on Co

concentration When the Co concentration is very low (x ~ 0.01), the film reveals a

relatively high effective number of Bohr magnetons, around 3 µB /Co, agreeing with the value in reference [9] It is consistent with the calculated value for the effective number of Bohr magnetons under the condition of quenched orbital moments for Co2+

with the configuration of 3d7 [10] It is worth noting that the expected value of

magnetic moment per Co atom in [9] is obtained under the condition of p-type ZnO

situation It was also found that the magnetic moment per Co atom decreased with

increasing Co concentration for x < 0.1 The decrease of magnetic moment per Co atom with x suggests the presence of antiferromagnetic interactions in the Zn 1-xCoxO system The details of antiferromagnetic coupling mechanism between magnetic ions will be discussed in the section 7.4.2 Here only a brief description is given as follows

It is reasonable to ascribe the antiferromagnetic coupling between magnetic ions to superexchange interaction [11] With increase in Co concentration, the mean distance between Co atoms is reduced, and thus the antiferromagnetic interaction is reinforced When the Co concentration is over 0.1, magnetic moment per Co stops decreasing The reason maybe related to the Co cluster induced which is the results of Co concentration over the percolation limit This result corresponds to those of structure studies in section 5.6, in which Zn1-xCoxO films showed good crystallinity, good lattice without

obvious clusters with x < 0.1 Similar results will be discussed in the section 6.2.3,

where the magnetic moment obtained at 1000 Oe was observed to decrease when the

Co concentration increases from 0.02 to 0.09, as shown in Fig 6-5(b)

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.0

0.5 1.0 1.5 2.0 2.5 3.0

Fig 6-2 Ms per Co atom dependence on Co concentration at room temperature

6.2.2 Magnetic Properties of a Zn 0.98 Co 0.02 O Film

A SQUID magnetometer was used to precisely characterize the magnetic properties

of the Zn1-xCox O thin films with the applied field parallel to the film plane The M-H

curve of Zn1-xCox O film at 300 K with the Co concentration of x = 0.02 is shown in Fig

6-3(a) A clear hysteresis loop was observed It is obvious that the Zn1-xCoxO thin film

is magnetic up to 300 K At room temperature, the coercivity (H c) is about 90 Oe and the saturation magnetization is about 12.5 emu/cm3, in agreement with VSM results obtained at room temperature The coercivity of the film increased with decreasing temperature, as shown in the inset of Fig 6-3(a) According to our experimental results, magnetic hysteresis loops were observed at least up to room temperature, and

magnetism could be realized with Co doped into ZnO However, the improvement was limited It is experimentally shown that most of the specimens revealed small

coercivity, and the maximum H c obtained without perceptible precipitates was less than 300 Oe

Figure 6-3(b) shows a typical H c dependence on temperature for Zn1-xCoxO thin

films with Co concentration x = 0.02 We observed that H c remains at about 100 Oe near room temperature, and it increases with decreasing temperature When temperature goes below 100 K, it reaches about 200 Oe

-4000 -3000 -2000 -1000 0 1000 2000 3000 4000 -15

-10

-5 0 5 10 15

-2 0

100 120 140 160 180 200 220

T (K)

(b)

Fig 6-3(a) Magnetic loop of the thin film with Co concentration of x = 0.02 measured

at 300 K by SQUID Inset: Enlarged view of the low field region to show the presence

of hysteresis and remanence for the samples measured at 300, 100 and 30 K; (b) Hc dependence on temperature of the film with Co concentration of x = 0.02

6.2.3 Magnetic Properties of Zn1-xCoxO Thin Films Dependence on Temperature

In order to obtain more details of the magnetic behaviors of Zn1-xCoxO thin films,

magnetization (M) dependence on temperature was measured via a SQUID The

specimen was placed in an applied field parallel to the specimen plane at a temperature

ranging from 5 to 400 K To obtain the temperature dependence of the magnetization,

zero-field-cooled (ZFC) and field-cooled methods were applied For ZFC

magnetization measurement, the sample was first cooled down to 5 K in the absence of

an applied magnetic field, and the magnetization of the sample was measured in the

temperature range up to 400 K On the other hand, for FC magnetization measurement,

the sample was cooled down to 5 K in an applied magnetic field, and measurements of

magnetic moment at each intermediate temperature were carried out at constant

applied fields

By Figure 6-4(a) we describe the temperature dependence of magnetization of the

Zn1-xCox O film with Co concentration x = 0.05 The film was measured by the applied

field parallel to the specimen planes by means of ZFC and FC methods with the

magnetic field of 100, 1000 and 2000 Oe Curie temperature for this film could not be

reached due to the operation limit of the SQUID equipment used We estimate that the

T c is higher than 400 K The curve obtained in a magnetic field of 100 Oe shows a

nonzero magnetization up to room temperature, which is in accordance with the M-H

curves Discrepancybetween the ZFC and the FC curves at a lower magnetic field was

observed The magnetization decreases slowly with decreasing temperature for

temperatures up to 50 K This is not concluded a rule in most DMS materials [12] but rather an exception References [13-15] attributed it to the effect of randomness and disorder on the percolating FM clusters In our view, it can be explained by spin glass behaviors Antiferromagnetic coupling leads to the decrease in magnetism under the condition of ZFC Comparing the curves under different magnetic fields, the point at which ZFC and FC starts to deviate tends to shift toward low temperatures for large magnetic fields This is one of the features for spin glass [16,17]. The magnetization abruptly increases when the temperature is lower than 40K, exhibiting a low temperature tail

Using Figure 6-4(b) we can make out the difference, denoted by D, in

magnetization of FC and ZFC for Zn1-xCox O thin films with Co concentration x = 0.05

measured in 100, 1000 and 2000 Oe It is clear that, under the three different magnetic

fields, the difference D decreases with increasing temperature, and D is not zero at

temperatures close to 300 K We can observe a ferromagnetic contribution in the film

up to 300 K, which is agreement with our M-H results It is interesting to note that D

increases when the field is increased from the smallest value at 100 Oe to 1000 Oe, but decreases again when the magnetic field increases beyond 2000 Oe This is the result

of competition of positive magnetic moment from the film and negative magnetic moment from the diamagnetic sapphire substrate

0 50 100 150 200 250 300 350 400 450 0

1 2 3 4

Fig 6-4(a) ZFC (open) and FC (solid) curves and (b) temperature dependence of the

difference magnetization between FC and ZFC magnetizations for Zn1-xCoxO thin films with Co concentration 0.05 measured in 100 Oe, 1000 Oe and 2000 Oe

c3 0.05 c4 0.09

Fig 6-5 ZFC/FC curves measured at (a) 100 Oe, (b) 1000 Oe and (c) 2000 Oe,

respectively; and temperature dependence of the difference magnetization between FC and ZFC magnetizations measured at (e) 100 Oe, (f) 1000 Oe and (g) 2000 Oe, respectively, for Zn1-xCox O thin films with Co concentration x = 0.02, 0.05 and 0.09,

denoted by squares, circles and triangles, respectively Here, solid symbols denote FC and open ones denote ZFC for (a-c)

Figure 6-5(a-c) shows the ZFC and FC curves of Zn1-xCoxO films with different Co concentrations at respective magnetic fields of 100, 1000 and 2000 Oe All show positive curvatures The discrepant point at which ZFC and FC start to deviate can be observed for all the curves, and these points decreases with increasing magnetic field From them, the discrepant points between the ZFC and the FC curves were estimated and listed in Table 5-1 It revealed the behaviour of the discrepant point shifting The above properties are typical features for a fine particle system, such as superparamagnetic or spin glass system

To estimate the size of the clusters in a supermagnetic system, we proceed as follows The highest blocking temperature is reached when the magnetic field is much smaller than the so-called anisotropy field [17] Hence, when a very small magnetic field is applied, the discrepancy is caused by a large anisotropy of the system which is due to the structure formed by widely separated chains of Co atoms The blocking temperature ( ) obtained at low magnetic field was used to estimate the mean size of clusters [17] using the formula

B T

B B K

T k

V = 25

,

where V is the mean volume of the cluster and k B is the Boltzmann constant

From the Table 6-1, the blocking temperature for the film with Co concentration x =

0.09 is estimated to be about 380 K The anisotropy constant ( ) of fine Co particle

is 7×10

u K

5

J/m3 [17] Hence the cluster size is deduced to be about 4 nm We should observe such size of cluster under a HRTEM observation However we did not observe clusters in the Zn CoO with x < 0.1, suggesting that it should be explained using

another system, such as spin glass system

Table 6-1 The discrepant points (K) estimated from Fig 5-5 (a-c)

0.02 >400 >400 0.05 >400 400 360 0.09 380 360 350

From Fig 6-5(b), we can see that when Co concentration increases from 0.02 to 0.09, the magnetization obtained at 1000 Oe was observed to decrease, which coincides with the previous results in Fig 6-2

Figure 6-5(d-f) show the difference between FC and ZFC of the Zn1-xCoxO films for different Co concentrations at a magnetic field of 100, 1000 and 2000 Oe, respectively

For all of the curves, D decreases with increasing temperature, and films with lower

Co concentrations exhibit larger magnetization difference between FC and ZFC

0 100 200 300 400 0.0

0.2 0.4 0.6 0.8 1.0

Figure 6-6 displays magnetic moment per Co atom, denoted by m, dependence on temperature T for the films with different Co concentrations at a magnetic field of

1000 Oe and 100 Oe, respectively The shapes of m-T curves are similar to those of

M-T curves, as shown in Fig 6-6(a) To show the features of m-T curves for the

Zn1-xCox O films with small Co concentrations, here we also present a m-T curve of the

Zn1-xCox O film with x = 0.27 The m-T curve exhibits a typical ferromagnetic

behaviour [10], as shown in Fig 6-6(b) For the film with a relatively lower Co concentration, the value of magnetic moment per Co is higher It agrees with our previous result in Fig 6-2

6.2.4 Overview Studies of Magnetic Properties of Zn1-xCoxO Thin Films

In this section, we will show some results of samples which were obtained under different experimental conditions and different Co concentrations All these data were

obtained from M-H loops from AGM

Figure 6-7(a-c) present an overview of the coercivity H c , magnetization M and magnetic moment per Co atom (m) as a function of the Co concentration x The data

scattered in the Fig 6-7(a) figures indicates that there is no apparent dependence of

coercivity H c on Co concentration x From Fig 6-7 (b), the contribution of Co to

magnetization trends to decrease with increasing Co concentration, which coincides with our previous results in Fig 6-2 The inset of Fig 6-7(c) shows a linear relationship between the inverse of magnetic moment per Co atom and Co concentration Using the effective concentration (x ), we can explain it by the formula s