Magnetism and magneto-optics of hexaferrite layers

of magnetism and magnetic materials

,A•Journai ELSEVIER

Journal of Magnetism and Magnetic Materials 175 (1997) 79 89

Magnetism and magneto-optics of hexaferrite layers R. Gerber a'*, R. Atkinson b, Z. Sim~a c Department of Physics, Joule Laboratoo', Unic,ersi~ oJ'Sa!/'ord, Sa(/brd, M5 4 WT, UK bDepartment oJ'Pure and Applied Physics, 7"he Queen's, UniversiO, oJBel/ast, Bel/ast, BT7 1NN, UK cInstitute of Physics ASCR, Cukrovarnick~i 10, 16200 Prague 6, Czech Republic

Abstract Recent contributions to research in magnetism and magneto-optics of hexaferrite layers, resulting from the collaboration between the above-mentioned institutions, are comprehensively reviewed. The pulsed laser deposition (PLD) technique is described and its main features, relying on the plume diagnostics and correct oxygen pressure, both being important for the deposition of hexaferrites of complex stoichiometry, are highlighted. The fabricated layers were investigated structurally and it was found that they are highly textured with the c-axis perpendicular to the film plane. Their magnetization was measured over a wide temperature range, 4.2 300 K, and in fields up to 12 T. Its dependence upon the cobalt content x in BaFelz-x-~,CoxTiyO19 was also determined in the interval 0 ~< x ~< 0.8. The results were interpreted in terms of N6el theory and this, when combined with our results of M6ssbauer spectra measurements, led to the formulation of a consistent model for the cation distribution in Co-Ti-substituted barium hexaferrites. The hysteresis-loop measurements provided data for obtaining values of anisotropy, which are in agreement with those of the bulk materials. The domain structure of thin hexaferrite layers was also studied, particularly the domain period dependence upon the sample thickness and cobalt content. The domain period dependence was found to be in very good agreement with theoretical micromagnetic calculations. Ellipsometry, reflectance photometry and Kerr/Faraday polarimetry were used to determine the optical and magneto-optical properties of hexaferrite platelets and thin layers. The complex refractive index and magneto-optic parameter were determined over the spectral range 350 850 nm and the reliability of the data was tested by comparison with photometric measurements of reflectance. The Faraday rotation and absorption spectra of substituted hexaferrite thin layers were measured in the 500,2000 nm wavelength range at room temperature and 80 K. The results obtained are interpreted in terms of single-ion optical electron transitions belonging to either cobalt or iron cations occupying the tetrahedral and/or octahedral positions in the spinel block of hexagonal ferrite crystal lattice. Keywords: Magnetism; Magneto-optics; Hexaferrites; Layers

1. Introduction

* Corresponding author. Tel.: + 44-161-745-5635; fax: + 44161-745-5903; e-mail: [email protected],

B a r i u m a n d s t r o n t i u m hexaferrites have attracted considerable interest in recent years as low-loss

ferrimagnets suitable for high-tech applications. At

0304-8853/97/$17.00 ~ 1997 Elsevier Science B.V. All rights reserved PII S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 0 1 5 1-0

80

R. Gerber et al. /Journal of Magnetism and Magnetic Materials 175 (1997) 79-89

present they are utilised in both high-density longitudinal and perpendicular magnetic recording and are seen promising candidate for use as media in archival and/or near-field magneto-optic recording. Furthermore, at very high frequencies, they may be suitable as elements integrated into electro-optic waveguide systems. This is because hexaferrites are highly anisotropic, chemically inert, mechanically resilient and last but not least, willing to accept substitutional elements that can enhance their magneto-optical properties at certain wavelengths. It has often been assumed that hexaferrites, being old established materials, are well-characterised and understood. This is far from being true, particularly, when these ferrimagnetic compounds have been produced in the form of thin layers. There have been significant gaps in our interpretation of the magnetization process, anisotropy and domain structure. Also, the quantitative values of the hexaferrite MO parameters have been virtually unknown and, at the microscopic level, the cation distribution of the substituted species has not been fully established. Moreover, the contributions of such species to MO effects in hexaferrites have not been determined and explained in terms of quantum-mechanical transitions. This situation was perceived by us for some time in the past. Therefore, in 1992 we have initiated a collaboration between scientists at the Universities of Salford and Belfast, and the Institute of Physics, Prague, to advance our knowledge in the area of magnetism and magneto-optics of hexaferrite thin layers. The purpose of this paper is to review this research effort and highlight its results to date.

axis from each crystal, their surfaces were then polished, etched in warm (54C) orthophosphoric acid and finally cleaned using lens tissue and acetone. These platelets were used for some of the magnetic measurements [5, 6, 10, 13] and for some of the optical and magneto-optical measurements [1, 3, 4, 9] that were carried out in the reflectance mode. Some of the platelets were thinned down to 25 Jam thickness to carry out optical and magnetooptical measurements [-2, 4] in the transmission mode.

2.2. Thin layers The pulsed laser deposition (PLD) technique was used for the preparation of a large number of thin films and layers of BaFe12019, SrFex2019, BaFe12_x_~.CoxTiyO19 and BaFe12 xAlxO19. These films and layers were used for further magnetic, optic and magneto-optic measurements [11, 12, 14-18, 20] carried out in both the reflectance and transmission modes. A schematic diagram of the pulsed laser deposition (PLD) system [9] is shown in Fig. 1. The output beam of a KrF excimer laser (wavelength ). = 248 nm, 200 n J/pulse, 23 ns pulse width and pulse repetition frequency of 20 Hz) was passed through an aperture and focused onto a sintered hexaferrite target through an off-axis lens (focal length f = 230 mm) at an angle of incidence of 30 °. The target, lens and substrate were all rotated to improve the uniformity and texture of the deposited material. The radiation flux density at the target surface was 2 x 104 J m - 2 with ablated

2. The fabrication of thick and thin layers

2.1. Single-crystal platelets (thick layers) To have a reference to the values of magnetic and magneto-optic parameters of bulk hexaferrites, single crystals of BaFe12-x yCo~TiyO19, for x ~ y = 0-0.78, were grown by a slow-cooling flux method [1]. Small platelets about 130 gm thick were cut perpendicularly to the hexagonal (0 0 0 1)

Fig. 1. Schematic view of the P L D system. E: excimer laser, A: aperture, L: rotating lens, W: UV window, P: plume, S: substrate and heater, T: target [91.

R. Gerber et al. /Journal of Magnetism and Magnetic Materials 175 (1997) 79-89

material being ejected in a direction normal to the plane of the target. Typically, 30000 pulses were required to deposit a film 250 nm thick. Hexaferrite films were grown [11, 12, 21] on the basal (0 0 0 1) plane of heated single crystal A1203 substrates in order to determine the potential of PLD for producing high-quality films. A1203 (0 0 0 1) was a suitable substrate since ferrite can be grown heteroepitaxially in spite of small lattice mismatch. A systematic investigation [14-16] of the influence of deposition temperature and oxygen pressure on the structural, magnetic and MO properties of the films was initiated, leading to the optimization of the deposition process and giving insight into the parameters that control grain size, phase purity, stoichiometry, orientation and ultimately magnetic properties. Owing to the fact that the magnetic anisotropy of hexaferrites results from their anisotropic structure, the control of crystal orientation during film growth was of paramount importance for achieving improved magnetic properties. Results demonstrated that polycrystalline textured films of optically smooth surface with perpendicular anisotropy could be prepared under a wide range of oxygen pressures (0.01 0.3 mbar) and relatively high temperatures (700-840°C), sufficient to synthesise the material. However, an almost exclusive c-axis orientation normal to the film plane could only be obtained for a narrow window centred at 0.1 mbar and 840°C.

8I

Fig. 2. Time integrated CCD image of temporally evolving barium hexaferrite plasma plume 3 ~s after beam impact [21, 22].

2. a slowing of the plume relative to propagation in vacuum resulting in spatial confinement. Films with the best properties were deposited under conditions that generated the most energetic plume, leading to the conclusion that energetic and ionised species are important to high-quality film synthesis.

2.3. Plume diagnostics

3. Structure and morphology

The dynamics of light-emitting species produced by the excimer laser ablation of hexaferrite targets into vacuum and ambient oxygen pressures used for film growth have been studied [21, 22] by space-time resolved optical emission spectroscopy and ultrafast CCD photography of the evolving plume shown in Fig. 2. Effective particle velocities were obtained from the time dependence of the optical emission intensity. The combined measurements indicate that a rise in the background oxygen pressure results in the following effects: 1. a marked increase in the emission intensity from all the species due to collisions on the leading edge of the expansion,

The PLD system produced films on sapphire substrates of optically smooth surface with the hexagonal c-axis well oriented perpendicularly to the film plane [7, 9, 11, 12, 14, 15]. A representative XRD pattern [11, 14] is shown in Fig. 3. Strong (000 1) reflections of the magnetoplumbite [8] phase confirm that the film is highly aligned with the c-axis perpendicular to the film plane. The values of FWHM of the rocking curves scanned around the (0 0 0 8) line are typically of 0.3. AFM micrographs (Fig. 4) revealed a columnar microstructure with a characteristic pyramidal growth pattern for each hexagonal column. This is believed to result from a screw dislocation at its

R. Gerber et al. /'Journal of Magnetism and Magnetic Materials 175 (1997) 79 89

82

Log'(Intensit'y)

I (008)

10

20

30

I

I

~

50

60

I

,

'-(

t

(AI203 )

40

70 80 90 20 (degrees)

Fig. 3. XRD pattern of an oriented film of SrFea2Ot9 deposited on (0 0 0 1) sapphire substrate at a temperature of 840'C and a background oxygen pressure of 0.1 mbar [11, 14],

velopment of sharp intense (0 0 0 1) peaks and increase in crystallite size. The patterns of AFM micrographs indicate that not only the c-axis is oriented but also that there is a partial alignment of the hexagonal a- and b-axis in the film plane. This textttte has been confirmed by preliminary torque measurements [23] showing a sixfold symmetry component in the plane of the film. Hexaferrite films deposited on the YSZ (1 0 0) substrates [15] were also perpendicularly oriented with the c-axis normal to the film plane and grew by the spiral mechanism. However, these films were characterized by more open in plane hysteresis loops as a consequence of the poorer structural matching of the Ba SrM/YSZ system.

4. Magnetic properties -1.00

4.1. Saturation magnetization -0,75

-0,50

The saturation magnetization, M,, of BaFe12_x yCoxTiyO19, for x ~ y , was established [5] by VSM measurements on single-crystal platelets in fields up to 120 kOe and in the temperature range 5 300 K. At room temperature, M~ may be described [17] as M s = ( - - 1 5 8 x 3 + 5 5 x 2 --

-0.25

44x + 355) gauss,

(1)

for x within 0 ~< x ~< 0.8. 4.2. Cation distribution

o

o, 9'5

o. 50

o. 75

0 1, oo IJN

Fig. 4. AFM image of the surface of a barium hexaferrite film grown at 0.1 mbar at 8 4 0 C [14, 21].

centre and each growth step has height equal to the c-parameter of a unit cell. The spiral growth mechanism is initiated in the early stages of the film growth because of the lattice mismatch between the hexaferrite and the sapphire causing strain in the film at the interface which is relieved by forming misfit dislocations and crystalline defects. The crystalline quality improved with increasing deposition temperature as evidenced by the de-

The cation distribution in BaFet2 x_yCoxTiyO19 was determined [13] by transmission M6ssbauer spectroscopy. Typical measurements of the 57Fe M6ssbauer spectra are shown in Fig. 5. A combined analysis of the M6ssbauer spectra, magnetic [53, optic [4] and magneto-optic [2] investigations leads to the cation distribution shown in Table 1. 4.3. Magnetization process and anisotropy Typical hysteresis loops [17] of a ( 0 0 0 1)oriented Co Ti-substituted barium hexaferrite film (x~-y = 0.53) measured in the perpendicular and parallel directions to the film plane are shown in Fig. 6.

R. Gerber et al. /Journal of Magnetism and Magnetic Materials 175 (1997) 79-89

-11

-9

i

i

-7

-5

83

!

-3

-1

7

9

11

velocity (mm/s) Fig. 5. 57Fe M6ssbauer spectra of BaFe~2_.~ ~CoxTiyOl9 single-crystal samples (x = 0, 0.11, 0.58 and 0.78). The full lines are the best fits to the spectra El3].

Table 1 The distribution of Co and Ti concentrations x and y respectively on the crystallographic sites of barium hexaferrite lattice Co

x

Ti

y

12k 2a 2b 4ft 4Q

0 0 0 0.2 0.8

12k 2a 2b 4fl 4fz

0.857 0 0.143 0 0

4OO

Parallel j Perpendicular

200 0

Y

-200 -400-1.5

-10

-0.5

0

0.5

1.0

1.5

Happ (104 Oe) The anisotropy K may be calculated either from the anisotropy field HA as K = HAMs~2 or from the initial susceptibility MII/H as K =(1/2)M2/(MII/H), where MII is the in-plane magnetization when H is applied parallel to the film surface. If only rotational processes were involved, then both methods of calculation would lead to the same value of K. The values of K as

Fig. 6. Typical hysteresis loops of a (000 l)-oriented BaFet0.87Co0.53Ti0.60019 film, measured parallel and perpendicular to the film plane [17].

a function of x for single-crystal platelets (full circles), films using HA (open circles) and films using M j H (open squares) are shown in Fig. 7. It is clear that K evaluated for films using HA agrees well with

84

R. Gerber et al. / Journal o f Magnetism and Magnetic Materials 175 (1997) 79-89

3

,

,

,

,

,

,

,

,

,

,

,

,

,

,

,

~o J

0

0.2

0.4

0.6

/ I

0.8 x

/

Fig. 7. Magnetic anisotropy, K, as a function of cobalt content, x. Full circles (Q) are single crystal values, open circles (©) are film values resulting from HA, and open squares (V1) are film values resulting from MII/H [17].

the bulk value of K pertinent to single crystals, while the 'apparent' values of K evaluated for films using MII/H are much lower. The explanation of this difference is connected with the magnetization reversal in Bloch walls [24]. In thick layers (single-crystal platelets) the volumes of domains are much larger than those of Bloch walls. In this case the magnetization process consists almost entirely from the rotation of magnetic moments within domains, while the contribution from the walls is negligible. Hence, the initial susceptibility and HA are of the same rotational origin and this leads to the same values of K. By contrast, in thin layers, the volumes of domains and Bloch walls are comparable (see Fig. 8). In this case the magnetization reversal in Bloch walls is not negligible and, in fact, it is responsible for the appearance of a small hysteresis loop at low applied fields. Thus, the initial susceptibility and HA are of different origin and this results in the 'apparent' values of K being lower than the true ones.

Beside magnetocrystalline anisotropy also induced anisotropy, which gives rise to disaccommodation losses, has recently attracted attention. It manifests itself in perminvar-like loops of some

HA

H

Fig. 8. Schematic diagram explaining the Bloch wall magnetisation reversal.

hexaferrites and this phenomenon is a subject of investigations [19] at present.

4.4. Domains Hexagonal ferrite layers are ideal for testing the theories of stripe domain structures. When a simple stripe domain structure is established, and this in the case of barium hexaferrite is for layers of thicknesses h < 4 ~am, the domain period, p, decreases at first with decreasing h according to the half-power law 1-25]; p = a l l / 2 h 1/2 ,

(2)

where l = aw/(poM2) is the characteristic material length, aw is the Bloch wall-energy density and a is a numerical factor, depending upon the rotational permeability. However, the calculations by Mfilek and Kambersk~, [26] showed for the first time that as h

R. Gerber et al. /' Journal o f Magnetism and Magnetic Materials 175 (1997) 79-89

85

2000 ',, :

BaFem01+

v [] +

I

R:', E

! '~",, 1000 •: --\',,,

wedges ' ,,,ms

- -.......

t

J

theory (i=32 nm) 'rigid' theory .

, ~ r ~

Q_

0 a)

"" :',

i+i "',

f~

.+i

2

500 400

5

100

1000

2500

thickness h [nm] Fig. 9. The theoretical and experimentaldomain period, p, plotted as a function of thickness, h, of BaFe12019 thin layers and wedges [is].

decreases the domains become less favourable, and the thickness reaches eventually a critical value, ho, below which p starts increasing with further decrease of h. In the end, the layer becomes uniformly magnetized perpendicularly to its surface. The M & K 'rigid' theory [26] has been refined many times since 1958 but it presents an essentially correct physical picture which is in acceptable agreement with the latest micromagnetic computations [,18]. The experimental confirmation [-6, 10, 18] of the theory, in particular, for the first time the rise of p for h < ho, is shown using our pulsed laser deposited layers in Fig. 9.

where ~o = 10-9/(36re) F m 1 is the permittivity of free space, n = n ' + in" is the complex refractive index and Q = Q' + iQ" is the complex magnetooptic parameter. Both n and Q are material parameters. If, for instance, the normal polar Kerr effect is considered and the magneto-optical interaction is due to radiation reflected at a single plane interface between air and a magnetic layer, whose thickness is very much greater than the skin depth, then the relation [-27] between experimental quantities and material parameters is OK + ie,K = i n Q / ( n 2 --

5. Magneto-optic properties 5.1.

The p e r m i t t i v i t y t e n s o r

An important factor in the description of all first-order magneto-optical effects is the permittivity tensor

1),

(4)

where 0K and e,K are the measured Kerr rotation and ellipticity, respectively. It is clear from expression (4) that in order to obtain the magneto-optical material parameters it is necessary to measure also independently the optical parameters of the layers involved. 5.2. O p t i c a l p a r a m e t e r s

1 [~] = ~o n2

-iQ

0

iQ

1

0

0

0

1

,

(3)

Optical properties of hexaferrite layers were studied in Refs. [2-4, 9, 20]. As an example, the dispersion of the real, n', and imaginary, n', parts of

R. Gerber et al. /,'Journal o f Magnetism and Magnetic Materials" 175 (1997) 79 89

86

3,2

!

F 12,L

A

0+3

Q'xl

3.0

2,8

2,6

/, tl,t I I 2.4

i/l~I / /

2,2

300

,

i

i

I

i

400

500

600

700

800

Wevelength 1,4 1.2

•

_41 900

300

i

i

i

r

500

600

700

800

(nm)

Wavelength

],.] r t

900

(nm)

Q " x 1 0 +3

"-,,,"" • 4

1,0 0.8

~ 400

x=O

,.)\\ x=O.78

-~" ~ ", ---,, \ \ ,,\ \

0.6

\'W~x=o.3~ x=0.35

~'.::.,~ " "-,

.~

. ~

x=0.58

"'....,~-_ /

o.,

-2

0.2

0.0 300

×=0

I 400

I 500

; 600

i

I

I

700

800

Wavelength

r

-4

900

(nm)

300

i

i

400

i

i

500

i

i

600

i

i

700

Wavelength

i

i

800

900

(nm)

Fig. 10. D i s p e r s i o n of the real, n', a n d i m a g i n a r y , n", parts of the refractive i n d e x of single crystal C o T i - d o p e d b a r i u m h e x a ferrites [ 9 ] .

Fig. 1 l . D i s p e r s i o n of the real, Q', a n d i m a g i n a r y , Q", parts of the m a g n e t o - o p t i c p a r a m e t e r o f single crystal C o - T i - d o p e d bari u m hexaferrites [ 9 ] .

the refractive index of BaFe12_x_yCoxTiyO19, x ~ y, determined by ellipsometry using single-crystal platelets is shown as a function of wavelength and composition, x, in Fig. 10. These dependences were used in conjunction with expression (4) to obtain Q' and Q" from the magneto-optical measurements.

parameter of BaFe~2_x_sCoxTiyO19, x~-y, is shown as a function of wavelength and composition, x. These results were obtained from the measurements of the complex polar Kerr rotation, OK + ieK, using expression (4) in combination with the above presented optical data in Fig. 10. The results shown in Fig. 11 indicate that Q varies systematically with wavelength and composition, x. It should be noted that a peak in magneto-optical activity occurs at short wavelengths (380 nm), where the next generation of magneto-optic devices are expected to operate; this is most likely due to transitions associated with the Fe 3 + ions in the lattice. The fact that this peak is

5.3. The Kerr effect Hexaferrite layers of various compositions were subjected to Kerr effect measurements [-1, 3, 9, 11, 14], Typical results are presented in Fig. 11, where the dispersion of the real, Q', and imaginary, Q", parts of the magneto-optic

R. Gerber et al. / Journal of Magnetism and Magnetic Materials 175 (1997) 79 89

reduced as the Fe 3 + ions are replaced by Co 2 + ions supports this premise. At the same time there is the onset of a noticeable increase in magnetooptic activity at 720 nm that is clearly identified with increasing Co 2+ ion concentration. The unambiguous systematic variation of Q with wavelength and cobalt substitution level is clearly an indicator of the reliability of the data. In addition, these data provide excellent theoretical predictions of the behaviour of thin films of the same material.

87

200

o

7o -200

g eel

'~ -400 r./}

o

S~ of Sample 1

~

J

-6~ E (eV)

5.4. The Faraday effect The Faraday effect was investigated [2, 16, 20] in the 500-2000 nm wavelength range at room and low temperatures using both thick and thin layers of Co-Ti-doped barium hexaferrites. The results of these investigations are also supported by measurements of optical absorption in the 800 2400 nm wavelength region. The main results are shown in Figs. 12 and 13, where the Faraday coefficients Sco and SFe per one cobalt and one iron cation, respectively, are plotted versus the photon energy, E, of incident radiation. The observed spectra of Sco and Sv~ may be expressed by a series of paramagnetic and diamagnetic type electron transitions that can be

,-~ r)

10000

[]

80 K

o

300 K

0

3 -10000

-20000 1

2 E (eV)

Fig. 12. The Faraday coefficient, Sco, at 80 and 300 K plotted versus the photon energy, E, of incident radiation. Symbols represent the averaged experimental data, the solid curves represent the best fits to expression (5), which yields the parameters listed in Ref. 1-20].

Fig. 13. The Faraday coefficient, Sve, at 300 K plotted versus the photon energy, E, of incident radiation. Symbols represent the experimental data, the solid curve represents the best fit to expression (5), which yields the parameters listed in Ref. [20]. The dashed curve represents the reversed sign normalized Sve of YIG at 300 K.

written as

Sco/Fe

K +

E2(E 2-

E 2 - Ci)

v'@ Ki (E{ - E 2 + Ci)2 + 4E2Ci

+ ~ Kj E(E)

- E) 2 - C j

[(E~ - E) 2 + C~]2'

(5)

where the second term (with the summation over i) and the third term (summation over j) on the right-hand side of expression (5) represent paramagnetic and diamagnetic electron transitions, respectively. In expression (5), K i , K~ are the magnitudes and Ci, Cj are the line widths of the paramagnetic and diamagnetic transitions occurring at the respective photon energies El, Ej. A detailed list of the paramagnetic and diamagnetic transitions of cobalt and iron cations is to appear in Ref. 1-203. Here, referring to the single-ion cobalt contributions (Fig. 12), most of the features in the range from 0.73 to 0.89 eV come from two paramagnetic lineshapes assigned to the 4 A 2 ~ 4 T 1 and 4 T e ( 4 F ) crystal-field transitions of tetrahedral Co 2 + but the two less prominent diamagnetic contributions of 5E --~ 5T 2 of the tetrahedral Co 3 + cannot be ruled out.

88

R. Gerber et al. / Journal o/'Magnetism and Magnetic Materials 175 (1997) 79 89

A n o t h e r large t r a n s i t i o n is found to be a b o u t 1.8 eV and, in a c c o r d a n c e with the garnet system [28], it is p r i m a r i l y ascribed to the 4A 2 ~ 4T1(4P) electron t r a n s i t i o n of t e t r a h e d r a l C o 2 +. The single ion c o n t r i b u t i o n of F e 3 + cations to the o b s e r v e d F R s p e c t r a constitutes a m u c h m o r e difficult p r o b l e m t h a n the c o b a l t ion c o n t r i b u t i o n . C o m p a r i n g o u r results with those of the m o r e thoroughly investigated case of y t t r i u m iron g a r n e t [28], it a p p e a r s that the first significant t r a n s i t i o n s due to iron ions in hexaferrites also occur only in the visible p a r t of the spectra (i.e. for p h o t o n energies a b o v e 1.6 eV). T h e salient feature in Fig. 13 is the occurrence of the line at 1.78 1.8eV. As this line is enc o u n t e r e d only in hexaferrites, we can assign it to the single-ion t r a n s i t i o n s of F e ions in the 2bh e x a g o n a l positions. In C o - s u b s t i t u t e d films this subtle t r a n s i t i o n (note the difference in the vertical scales of Figs. 12 a n d 13) is o v e r l a y e d by m o r e intense transitions of C o - i o n s centred a r o u n d 1.8 eV.

6. Conclusions A c o m p r e h e n s i v e a c c o u n t of c o n t r i b u t i o n s to research in m a g n e t i s m a n d m a g n e t o - o p t i c s of hexaferrite layers, resulting from the c o o p e r a tion between Belfast, P r a g u e a n d Salford research g r o u p s for the last four years, has been presented. C o n s i d e r a b l e a d v a n c e m e n t has been achieved in the P L D of hexaferrite layers a n d in better unders t a n d i n g of physics related to their magnetic, optic a n d m a g n e t o - o p t i c properties. This, together with a significant b o d y of new data, p r o v i d e s m e a n s for engineering novel recording m e d i a a n d c o m p o n e n t s for m a g n e t o - o p t i c applications.

Acknowledgements T h e a u t h o r s gratefully a c k n o w l e d g e the g r a n t s that were p r o v i d e d by the E P S R C , British Council a n d Czech G r a n t Agency to s u p p o r t , in part, the research w o r k described in this paper.

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