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EXCHANGE COUPLING INVESTIGATIONS ON EPITAXIAL DEPOSITED AND SUPERFICIALLY OXIDIZED Co AND Ni MONOCRYSTALLINE FILMS
H. Danan, H. Gengnagel, J. Steinert, A. Linzen
To cite this version:
H. Danan, H. Gengnagel, J. Steinert, A. Linzen. EXCHANGE COUPLING INVESTIGATIONS ON EPITAXIAL DEPOSITED AND SUPERFICIALLY OXIDIZED Co AND Ni MONOCRYSTALLINE FILMS. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-131-C1-133. �10.1051/jphyscol:1971142�.
�jpa-00214462�
EXCHANGE COUPLING INVESTIGATIONS ON EPITAXIAL DEPOSITED AND SUPERFICIALLY OXIDIZED Co AND Ni MONOCRYSTALLINE FILMS
by H. DANAN (*), H. GENGNAGEL
(**),J. STEINERT
(**)and A. LINZEN (**)
Rksum6.
- Le couplage ferro-antiferromagnktique a kt6 etudi6 sur des couches minces monocristallines de Co et de Ni preparees par epitaxie et oxyd&s superficiellement. L'anisotropie unidirectionnelle Ke et le deplacement des cycles d'hysttrkse He ont ete mesures pour differents 6chantillons et fournissent des valeurs en bon accord avec la theorie de Neel. L'ktude de Ke en fonction de l'kpaisseur de la couche ferromagnetique permet de calculer la constante d'kchange superficielle
A = (0.20 4 0,lO)erg. cm-2. Les propri6t6s observkes dans les differents plans cristaIlographiques ne prt- sentent pas de diffkrences notables entre elles.
Abstract.
-The ferro-antiferromagnetic exchange coupling has been studied on epitaxial deposited and superficially oxidized Co and Ni single crystal thin films. The unidirectional anisotropy Ke and hysteresis loop shifts He have been measured for various samples and give values which are in good agreement with Neel's theory. The variation of Ke with ferromagnetic film thickness give an average value for the superficial exchange constant A
=(0.20
&0.10) erg, cm-2.
No noticeable differences have been observed between the properties in the different crystallographic planes.
I. Introduction. - Various investigations based on the work of Meiklejohn and Bean [I] and the theory of NCel [2], have been recently performed about exchange coupled ferro-antiferromagnetic systems [3, 4, 5, 61, especially on Co-COO and Ni-NiO polycris:
talline thin films. The magnetic properties of these films depend essentially on size and crystallographic orientation of crystallites. Both are largely influenced by the deposition parameters. Consequently these results are very intricate and do not allow to explain the exchange coupling mechanism in detail.
The present investigations were carried out on superficially oxidized single-crystals Co and Ni thin films in order to get better experimental conditions.
11. Preparation of films.
-The Co and Ni films were vacuum deposited in the pressure range of 8 x to 2 x torr onto different monocrys- talline substrates : MgO 3,5 (AI,O,) cut in (100) - and (11 I) - planes or MgO and LiF cleaved in (100)
-planes. The deviations from the substrates lattice parameters to those of f. c. c. Co or Ni range from 5 to 15 %. The substrate temperature during the evaporation was at about 400 OC. Electron diffraction patterns and magnetic crystal anisotropy measure- ments show that there is a good epitaxial growth of Co and Ni on (100) -planes, which did not occur in (I I I) - planes in which the epitaxy was not perfect.
The thickness of the films ranges from 500 to 2 000 A.
These films have been oxidized in a torquemeter at different temperatures and various times in pure oxygen atmosphere. Immediately after the oxidation, the anisotropy and the magnetic moment were measu- red in magnetic fields up to 10 kOe. The films were oxidized at temperatures between 200 and 4000C, as it was not possible to get evidence of any oxidation below 2000C. From electron diffraction patterns it could be concluded that oxid layers with a thickness of about 100 A and more have a polycrystalline struc-
(*)
Laboratoire Pierre Weiss, Strasbourg, France.
(**)
Zentralinstitut
fiirFestkorperphysik
undWerkstoffor-
schungder
DAW,Institutsteil
furmagnetische Werkstoffe Jena,
DDR.ture. The electron diffraction equipment (electron reflection diffraction) was not adequate for investiga- tion on thinner oxid layers.
111. Experimental results. - Every partially oxidi- zed single crystal film shows, in addition to the magne- tocrystalline anisotropy (K,, K,), a unidirectional anisotropy (K,), a uniaxial anisotropy (K,) and a rotational hysteresis (KO). Measurements were carried out at 293 and 80 OK with a torquemeter for the deter- mination of the anisotropy constants (Fig. 1) and a
FIG. 1 . -
Torque curves
of Co-COO (sample NO4)- - - - mea-
sured at 293 OK,-
80 OK.50 cps-hysteresigraph for the observation of hysteresis loops. The samples were cooled from 300 OK (resp.
600% for Ni films) with and without an external magnetic dc field in order to investigate the influence of the saturation magnetization (M,) resp. spontaneous magnetization on the occurence of an unidirectional anisotropy. This anisotropy causes a hysteresis loop shift H, which was measured with the hysteresigraph.
If the Co films are cooled from 200OK to 80 OK without an external dc magnetic field loop shifts can be observed. However, it doesn't seem that these shifts could be attributed to the spontaneous magnetization since they always occur in the same direction as the shift observed during the latter cooling treatment
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971142
C 1
-
132 H. DANAN, H. GENGNAGEL, J. STELNERT AND A. LINZENwith dc field, even after cooling with ac field. These IV. Discussion. - The measurements of the cons- shifts disappear completely if the temperature before tant K, of magnetocrystalline anisotropy gave in cooling is about 330 OK or higher (Fig. 2). most cases (Table 1) the well known values [7, 81.
The uniaxial anisotropy of about lo4 erg. ~ m - ~ , already observed at room temperature on non-oxidi- zed or oxidized single crystal films, can be probably attributed to lattice distortions, which occur during the epitaxial growth [9]. This explains why it was not possible to determine the exact contribution of the antiferromagnetic oxid layer to the uniaxial anisotropy and to obtain any relation with the exchange coupling effects.
The constant of rotational hysteresis KO, and the constant of the unidirectional anisotropy K, are clearly related with the exchange coupling, both depending on the thickness of ferromagnetic layer (sample no 7).
Loop shifts calcuiations, according to the relation He
=K J M , f2, 61, are in good agreement with measured values (Table I).
Cooling from 330 OK to 800K with dc magnetic field along [loo] or [110] direction gives approxima- tely the same values for KO and K,, but significant differences are observed in the shape of the hysteresis loops. Spontaneous loop shifts do not appear if the temperature before cooling without dc field is high
FIG. 2.-
Hysteresis loops of sample N O 4, ac field N _//
11101enough to remove the latter with dc field
((frozen in
D(250 Oe'l
state.
. -
-,a) measured at 293 OK,
Nkel gives for the unidirectional anisotropy
b) at 80 OK after cooling in dc field H= = 250 Oe,
c) after cooling in ac field H, = 250 Oe.
Ke
=AM,
7Sample
Co onto Quartz polycryst.
Ni onto Quartz polycryst.
Co onto MgO monocryst. (100) Co onto spinel monocryst. (100) Co onto spinel monocryst. (100) Co onto LiF monocryst. (100) Co onto spinel monocryst. (1 11) before oxidation after 1 oxidation after 2 oxidation after 3 oxidation after 4 oxidation Ni onto LiF monocryst. (100)
A = K e . d ~ [erg. cm-21
T = measuring temperature ; d ~ = thickness of ferromagnetic layer ; d ~ - = thickness
of
ferromagnetic layer before oxidation.(*) Present values were determined in a magnetic field of 10 kOe, measurements in fields from 2.5 to 10 kOe show a decrease of KO with increasing field of about 10 %.
EXCHANGE COUPLING INVESTIGATIONS ON EPITAXIAL DEPOSITED C 1
-
133where A is the effective interface exchange coupling constant
;the dependence observed for K, on lid,
m
for all the Co films investigated is in good qualitative
7 -
k
+
agreement with this expression. The values of K,
plotted vs lid, (Fig.
3)give an average value for A of (0.20 + 0.10) e r g . ~ r n - ~ .
Creeping effects, generally observed on polycrystal- line films, could not be detected in all the single crystal
4 - o co on sp~nel C I 11) after
films investigated. Noticeable differences between
drfferent oxrdohon times
exchange coupling effects On films grown On (100) and (1 11) substrates, as reported by J. Bransky and
o Co on sprnel 11001
co-workers [9] on films grown on COO single crystal
x Coon LIF (1001
plates, could not be observed in the present investiga-
A co on M ~ O ( 1 0 0 ~
tions, probably because the superficial oxid layer has a polycrystalline structure.
v
Co on Quartz (polycristl- Acknowledgements. - The authors wish to thank o
I 2 3 to5 1
I ~ ~ - ~ I Dr. W. Brodkorb for helpful calculations and discus-
d~
sions, Mr. W. Schwab for the preparation of the subs-
FIG. 3.-
Variation of K~ with ferromagnetic thickness & oftrates and Dr. Heydenreich and Did. Phys. Miiller
Co films.
for performance of electron diffraction patterns.
References
[I] MEIKLEJOHN (W. H.), BEAN (C. P.), Phys. Rev., 1956 [6] BOYD (E. L.), IBM, J. Res. Develop., 1960, 4 , 116.
102,
