Magnetic structure - Handbook on the Physics and

2.1. Magnetic structure and interfacial magnetism

The magnetic structure of artificially deposited multilayers containing heavy R, which couples antiferromagnetically with T in the interface region, is shown schematically in fig. 3. In fig. 3a, which represents relatively thicker layers (say >50.~), the R will be disordered at room temperature, the T (e.g. Fe) will be ordered ferromagnetically and there will be an interface region with scattered R moments because of the disordered structure and random magnetic anisotropy. In the interior T region, the magnetic moment is usually parallel to the film plane. Thus multilayers with thick individual layer thicknesses will show a two-phase characteristic of R and T because the interface contribution is negligibly small. In the case of thinner individual layers (say 5-10,~) as shown in fig. 3b, because of the mixing between R and T layers the X-ray diffraction data indicate an amorphous structure with sinusoidal compositional modulation (fig. 3c). The interfaces may give the major contribution to the magnetic moments as the nominal layer thicknesses of R and T decrease until the interface dominates and then the net magnetic moment may be perpendicular to the film plane.

The rest of this chapter will be focused on R/T multilayers with thin layers where the interfacial magnetism plays a major role. Many of the outstanding properties, such as large PMA, controlled coercivity and compensation point in multilayers, are associated with the interracial magnetism. The temperature dependence of magnetic properties is controlled by the interracial magnetism, which also is the key to obtain the desired properties for practical applications.

The interfaces may be regarded as compositionally modulated disordered R-T alloys because X-ray diffraction shows an amorphous structure in the interface region. In the next subsection a brief overview of the interactions and structures of R and R-T alloys is given, on the basis of which the magnetic structure of interfaces and the origin of PMA may be well understood.

NANOSCALE R/T MULTILAYERS 87

R T R

I I J I

i tll I i

Inf..

moment-/ LR moment

(o)

(b)

C R C T

.'-',

• V \ / V ~,' \ / \ / ^~,

..'h/',,_,'t3

('~ I I I ~ I I I I

z Axis

(c)

Fig. 3. Schematic diagrams of magnetic structure for (a) thick (Int. = interface regions) and (b) thin R/T multilayers;

(e) compositional modulation for the thin case (after Shan and Sellmyer 1990b).

2.2. Magnetic interactions in R and R - T alloys

In an amorphous R material it is important to account for the possibility of fluctuation and randomness in both exchange interactions and directions of local single-ion anisotropy and this leads to the Hamiltonian (Sellmyer and Naris 1985)

H = - ~ (Jo + AJo')Si" Sj - O ~ (hi" Si) 2,

~i i

(1)

in which the first term represents the exchange and the second term the random magnetic anisotropy (RMA). The local easy axes (hi) are taken as random from site to site. Si (Sj) is the atomic spin at i site ( j site), Jo is an average ferromagnetic exchange, AJij represents the exchange fluctuations, and D an average local uniaxial anisotropy arising from electric field gradients of neighboring atoms. I f AJij are taken to be zero, eq. (1) becomes the Harris-Plischke-Zuckermann Hamiltonian in which the effect of the first term is to align the spins (if J0 > 0) while the RMA term tends to scatter them. Figure 4 presents a schematic phase diagram in which t = kBT/Jo, a =D/Jo and 6 = (AJ)/Jo. Clearly in the a = 0 plane one has the possibility for ferromagnetism (F) for small 6 values, a spin-glass-like phase (SG) for large 6 values and there also is the possibility of a mixed phase (M) in the intermediate region. On the other hand in the 6 = 0 case a speromagnetic (spin scattered) structure (SM) exists for large a values

t

Z.S. SHAN and D.J. SELLMYER

Fig. 4. Schematic phase diagram of possible magnetic states in the presence of RMA and exchange fluctuations (after Sellmyer and Naris 1985). See the text for the identification of the various symbols and acronyms.

MCP = multi-critical point.

and a correlated speromagnetic structure (CSM) exists for small a values where there is short-range ferromagnetic order. It can be seen that in the presence o f all three terms in the Hamiltortian there will be a complicated three-dimensional phase to be considered.

Figure 5 shows some examples discussed above in which there are speromagnetic, asperomagnetic and ferrimagnetic structures depending on the presence o f R M A in the first two cases and its absence in the third case. When there are both lanthanide and transition metals such as Fe and Co present, i.e. R - T disordered alloys, one can obtain even more complex structures as illustrated in the lower part o f fig. 5. An important point is that the h e a v y lanthanide moments tend to couple antiparallel whereas the light lanthanides couple parallel to the T moments.

On the basis o f the above consideration the magnetic structure o f heavy R - T alloys in the interface region may have sperimagnetic features with random orientation o f the local easy axes. For the material to show a macroscopic anisotropy, there must be an orientational coherence to these local anisotropy axes. The interfaces in R/T multilayers may have such desired structure to offer this orientational coherence. In sect. 3, the experimental evidence is presented and in sect. 4 a detailed model, which clearly shows such orientational coherence, is discussed.

SPEROMAGNET ASPEROMAGNET FERRIMAGNET

~ F e IHERAVY

~FAe, /

LIGHT

^{~ + W}

HEAVY

Fig. 5. Schematic diagram and definitions of phases possible in one- and two- component RMA systems (after Sellmyer and O'Shea 1983).

NANOSCALE R/T MULTILAYERS 89 3. E x p e r i m e n t a l properties o f R/T multilayers

In this section we give a brief review of the experimental results including the layered or compositionally modulated structure for R/T multilayers, the layer thickness and temperature dependencies of magnetic properties for Dy/Fe, Dy/Co and Tb/Fe multilayers.

We will not provide a comprehensive review of all recent work in this field. Rather, our aim is to focus on discussing the above fundamental properties, and only limited references which are closely related to this discussion are cited. The role that the interfacial magnetism plays in determining magnetic properties will be emphasized.

Mrssbauer measurements, which are used to determine the local Fe moment and its direction at different temperatures for Tb/Fe, Dy/Fe and Nd/Fe, are presented in this section. Magnetic circular X-ray dichroism (MCXD) is a relatively new element-selective technique, which can be used to obtain information on magnetic properties for both R and T atoms; one example of MCXD studies of Tb/Fe multilayers is given.

3.1. Layered structure

The layered structure of R/T multilayers can be investigated by X-ray diffraction and transmission electron microscopy (TEM). One example of small angle X-ray diffraction for 3.7 .~ Tb/2.5 ,~ Fe and 16 A Tb/3 0 ]~ Fe multilayers prepared by Sato (1986) is shown in figs. 6a and b with only first and up to fourth order of diffraction peaks, respectively.

Two TEM micrographs for 4.8ATb/10.5 ]~Fe and 16]~Tb/30]~Fe are shown in fig. 7 and the periodic distributions of Tb and Fe are observed (Sato 1986). A brief summary of the small-angle X-ray diffraction peaks for R/T (R = Tb, Dy, Nd; T=Fe, Co, Ni) is listed in Table 1.

100

8O 8"

> ,

~ 4c

I I I I

5 10 15 20

20 (deq.) ( e ) 2C

~ 4o

c 2o

0 I t | I

0 5 10 15 20

2e(deg3 (b)

Fig. 6. The small-angle X-ray diffraction pattems for (a) 3.7 ~. Tb/2.5 ~.Fe and (b) 16.~Tb/30A Fe (after Sato 1986).

90 ^Z.S.SHAN and D.J. SELLMYER

(o) (b)

Fig. 7. Transmission electron micrographs of the cross-section for (a) 4.8,~Tb/10.5 ,~ Fe and (b) 16ATb/

30,~Fe (after Sato 1986).

It is concluded from table 1 that for most of the samples only the first diffraction peak, which implies sinusoidal composition modulation, is observed for the bilayer thicknesses

~, < 30 ]k. This implies that R/T multilayers with thin layers have diffuse interfaces and

Table 1

A summary of the small X-ray diffraction peaks for various R/T multilayers. ~ is bilayer thickness and d is the individual layer thickness

R/T 1st Peak 2nd Peak 3rd Peak 4th Peak Ref.

Tb/Fe 3.7A Tb/2.5,~ Fe 16,~ Tb/30,4, Fe 1

Tb/Fe ), ~< 12 ,~ 2

Tb/Fe /1, ~< 15/k 3

Tb/FeCo ),=21A, ~,=31.5A 4

Tb/Co d~b, dco ~> 15 ,~ 30A Tb/30,~, Co 5

Tb/Co ~,=37/~ ),=72A 6

Dy/Fe a 7

Dy/Fe ,~/> 10,~ 8

Dy/Fe 14,~ Dy/40/k Fe 9

Nd/Fe X>/10,4, 10

a No peaks were observed for ~3.~ Dy/11-22/k Fe and 17A Dy/-2,~ Fe.

References

(1) Sato (1986) (6) Honda et al. (1987)

(2) Yamauchi et al. (1988) (7) Yoden et al. (1988) (3) Shah and Sellmyer (1990a) (8) Sato and Habu (1987) (4) Shin et al. (1987) (9) Shan and Sellmyer (1990b) (5) Ertl et al. (1992) (10) Mibu et al. (1989)

NANOSCALE R/T MULTILAYERS 91

c 2c

I I I I 0 I I I I

3 6 9 12 0 3 6 9 12

20 (deg.) 2e (aeg.)

( o ] (b)

Fig. 8. Small-angle X-ray diffraction patterns for 4.8 ,~Tb/10.5 A Fe: (a) as deposited; (b) after annealing at 250°C for 5 hours (after Sato 1986).

these are sometimes called compositionally modulated films (CMF) instead of multilayers.

We notice that R/T CMFs have much more diffuse interfaces compared with the Co/Pt, Pd and Au multilayers which usually show 3rd-4th order peaks (Shan et al. 1994) and up to the 7th order peak for Co/Au multilayer (Zhang et al. 1993).

An example of the annealing effect on the small angle X-ray diffraction peak of 4.8 A Tb/10.5 A Fe before and after annealing at 250°C for 5 hours is illustrated in fig. 8a and b. The peak intensity decreases because of the intermixing between Tb and Fe atoms at the interface boundaries; however, the layered structure still exists.

3.2. Magnetic properties of Dy/Fe, Dy/Co and Tb/Fe 3.2.1. Dy/Fe

3.2.1.1. Layer-thickness dependence of magnetic properties at room temperature.

Figure 9a (Shan and Sellmyer 1990b) shows a detailed Fe layer-thickness dependence of hysteresis loops for 5 A Dy/X A Fe as the Fe layer thickness varies from 2.5 .~ 1:o 40/~; note especially that the interval is only 1.25 A asX ranges from 2.5 to l0 ,~. The layer-thickness dependences of magnetization and anisotropy determined from fig. 9a are summarized in fig. 10.

Several results about the magnetization can be found from figs. 9a and 10. To understand the layer-thickness dependence of magnetization, both the antiferromagnetic coupling of Dy and Fe moments and the modulated distribution of composition have to be taken into account. (i) Sample 5 A Dy/6.25 ,~ Fe is in a state close to the compensation point: the Dy moment dominates for X < 6.5 A and the Fe moment dominates for X > 6.5 A. (ii) As X increases from 2.5 to 6.5 ,~, the magnetization magnitude of Dy/Fe, I g I, first increases, then decreases. This feature is due to the existence of two competitive

92 Z.S. S H A N and D.J. SELLMYER

' 1 " 1 ' 1 ' m l / m

>- II

• I " I " ! "

>- II

' " I " I " I "

• I . 1 , 1 , , I . l , l l , 1 , I , I . , I , l , f I 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ~

0 0 0 0 0 0 0 0 0 0 0 0 0

(O-olnuJe) I/~

/ ~ ~ ~\~ ~_~

',,,~ ~ I~ ~1~ ~ ~'~ ~

( 8 / n w ~ ) .0

o , r -

.... o % o O - Q ~ o o

~4-r

g e

~°~

o ~ ~

Q o< o

0 °

0 ~" .~ o

T N ~

i a

• "~ .~

o "~

0 , - ~

• . ~,~,

0O _ _ , . . ~

NANOSCALE R/T MULTILAYERS 93

120

100 " ~ l 8O

20 ~ f

I , I

0 10 20

6O 4 0 b

t I

t , I ,

i i I I

50 40

LAYER THICKNESS OF Fe (,~) 2 x 106

' o ~

' o

-10 6 -2x10 6 -5x106 -4x106

Fig. 10. Fe layer-thickness dependence of magnetization and measured anisotropy fo r 5 ~ Dy/X ]k Fe at 300 K (after Shan and Sellmyer 1990b).

processes: the enhancement of the Dy moments by the exchange interactions between Fe and Dy subnetwork moments as the Fe atomic fraction increases in this range, and the antiferromagnetic coupling between the Dy and Fe subnetwork moments. The former dominates as X is close to 2.5 ]~ and the latter prevails as X approaches 6.5 A. (iii) As X increases from 12 to 20 A the magnetization of ~rll and cr± changes very little, where

II

and A_ represent measurements parallel or perpendicular to the film plane. This may be attributed to the fact that the pure amorphous Fe is disordered magnetically; as the Fe layer thickness ranges between 10 ]k and 20 ]~, the Fe atomic fraction in the central region of the Fe layer is close to unity and its structure is amorphous which gives no contribution to the moments. Therefore the magnetization exhibits a "kink" there. Honda et al, (1991a) also reported such behavior in Fe/DyFe multilayers. They fotmd that when the Fe layer thickness dFe decreases, the crystallographic structure of the Fe layer changes from body- centered-cubic to amorphous at around 20 A. At this critical thickness, the spontaneous magnetization and the Kerr rotation angle are nearly zero and the magnetization curve does not show hysteresis or saturation indicating its spin-glass-like or superparamagnetic nature. (iv) ForX > 20 A, the Fe has crystalline structure and its moment increases rapidly as shown in the figs. 9a and 10. (v) It is noticed that the four samples of 5 A Dy/X ~, Fe (2" = 2.5, 6.25, 15, 20) exhibit the character of smaller magnitude of magnetization and this can be interpreted as follows. Sample 5 ,~Dy/2.5 ,~Fe is just ordered weakly by the exchange interactions between Dy and Fe moments and has an almost homogeneous distribution of Fe and Dy constituents because both the Fe and Dy layers are very thin. Sample 5 A Dy/6.25 A Fe displays almost zero magnetization since the Dy moment dominates in the Dy region and the Fe moment dominates in the Fe region and they

94 z.s. SHAN and D.J. SELLMYER

compensate each other to produce a nearly zero net magnetization. However, sample 5 ]k Dy/15 A Fe (or 5 ]~ Dy/20 A Fe) has a much thicker Fe layer and the central Fe region gives no magnetization contribution as mentioned above. (vi) As the Dy layer becomes thicker (not shown in fig. 10) the compensation point will move toward larger Fe layer thickness and this is understandable from the antiferromagnetic coupling of Dy and Fe moments.

The layer-thickness dependence of the magnetic anisotropy, as shown in figs. 9a and 10, exhibits the following features. (i) Figure 9a shows that the hysteresis loops of Crll change their shapes very little as the Fe layer thickness varies from 5 to 20.~, whereas the hysteresis loops of or± change their characteristics noticeably in the same range. When the Fe layer is thicker than 30A, the hysteresis loops of Crll also change noticeably. This implies that the magnetic properties in the direction perpendicular to the film plane strongly depend on the "interface" which is characterized by the anisotropic distribution of constituent atoms, but the magnetic properties in the parallel direction are mainly determined by the "inner part" of the Fe layer. (ii) The samples with the layer thickness thinner than about 12A usually have perpendicular anisotropy for both the 2.5 .~ < X < 6.25 A region where the Dy moment dominates and the 6.25 .~ < X < 12 ]k region where the Fe moment dominates. This suggests that the source of the perpendicular anisotropy is related to the anisotropic distribution and constituent magnetization rather than their net magnetization. (iii) The behavior of the uniaxial anisotropy, Ku, at the compensation point is uncertain. Sato and Habu (1987) reported that Ku=0 at the compensation point for Dy/Fe, Tb/Fe, and Gd/Fe CME But in the case of nominally homogeneous R-T films van Dover et al. (1985) and Egami et al. (1987) claimed that Ku changes smoothly through the compensation point. This problem will be discussed in more detail in sect. 5.2.

Figure 9b (Shen 1994) shows the Dy layer-thickness dependence of hysteresis loops of Y.ADy/5.AFe as the Dy layer thickness varies from 6.5A to 5 A; note especially (i) although the thickness interval is only 0.5 A, the coercivity and magnetization are very strongly dependent on thickness as the Dy layer thickness Y approaches 5 A where sample 5 A Dy/5 _A Fe is in a state close to the compensation point. (ii) Compared with the loops in fig. 9a, the loops in fig. 9b illustrate much better squareness because these samples were coated with a 500 A SiO layer to protect from oxidation.

3.2.1.2.

Temperature dependence of magnetic properties.

Figure 11 is an example of the temperature

dependence

of the magnetic properties for

Y~,Dy/6AFe

(Y=3, 5, 8, and 14). The discussion is simplified as follows. (i) The magnetization at 4.2 K is much larger than at 300 K since Dy is magnetically ordered at 4.2 K. (ii) At 4.2 K the samples usually possess a giant

coercivity,

e.g. He± = 59 kOe for 5 A Dy/6 A Fe and He± = 37 kOe for 14 ,~ Dy/6 ~, Fe which is very weakly magnetically ordered at 300 K. The coercivity He of pure Dy film is about 10kOe at 4.2K. Therefore all the samples in fig. 10 have a coercivity larger than that of pure Dy films at 4.2 K. (iii) For all four samples, although H~± >Hell at 4.2 K, the anisotropy value is difficult to measure by torque magnetometry

NANOSCALER/TMULTILAYERS

b

i i i

4.2K

3ADy/6AFe

' ~J_ 6E

5ADy/6AFe ~ --'-~ ~-'i

f / i --~

a l ~ / / -

/ / 1 - + / - /

/ / " . / - - J .

~,~ l o 3

' 8 A D y / 6 ~ .

~

^{3.7x10 4}

I I I

Dalam dokumen Handbook on the Physics and (Halaman 95-104)