A NEW METHOD TO MEASURE THE PERPENDICULAR ANISOTROPY OF RE-TM AMORPHOUS FILMS USING SPONTANEOUS HALL EFFECT

(1)

HAL Id: jpa-00229033

https://hal.archives-ouvertes.fr/jpa-00229033

Submitted on 1 Jan 1988

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of

sci-entific research documents, whether they are

pub-lished or not. The documents may come from

teaching and research institutions in France or

abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est

destinée au dépôt et à la diffusion de documents

scientifiques de niveau recherche, publiés ou non,

émanant des établissements d’enseignement et de

recherche français ou étrangers, des laboratoires

publics ou privés.

A NEW METHOD TO MEASURE THE

PERPENDICULAR ANISOTROPY OF RE-TM

AMORPHOUS FILMS USING SPONTANEOUS HALL

EFFECT

K. Okamoto, H. Zimmermann, H. Hoffmann

To cite this version:

(2)

JOURNAL DE PHYSIQUE

Colloque C8, Supplement au no 12, Tome 49, dBcembre 1988

A NEW METHOD TO MEASURE THE PERPENDICULAR ANISOTROPY OF

RE-TM AMORPHOUS FILMS USING SPONTANEOUS HALL EFFECT

K. Okamotol, H. Zimmermann and H. Hoffmann

Institut fiir Angewandte Physzk, Universitatht Regensburg, UniversitWsstraj3e 31 Postfach 397, 8400 Regensburg,

F. R. G.

Abstract.

-

A new method to measure the effective magnetic anisotropy field (H:) of amorphous RETM films with perpendicular magnetic anisotropy is described. In this method, H; is determined by the measurement of spontaneous Hall effect. The experimental data for TbFe sputtered films agreed with the theoretical prediction.

R E T M amorphous films with perpendicular magnetic anisotropy, such as GdCo, GdFe, TbFe, GdTbFeCo, etc., have now attracted special interest because of their promissing possibility as magneto- optical recording material, and intensive studies have been concentrating on these materials. However, it has not yet been clearly explained why these amorphous films have the perpendicular anisotropy. By the way, it is not so easy to investigate the magnetic anisotropy of such R E T M films by conventional torque method. The reasons are: 1) they are ferrimagnetic thin films and have compensation temperature,

T,,,,,

in the vicinity of room temperature, so that the value of saturation magnetization M, is very small, on the other

hand, 2) the magnetic anisotropy field Hk is very large.

Okarnoto, the first author of this paper, has recently developed a new method to measure the effective mag-

/

netic anisotropy field Hk = Hk-4rM, using the spon-

taneous Hall effect, which is very large in R E T M films independent of their M, values [I]. According to his

method, if the film has a rectangular Hall hysteresis loop (VH - H loop) with magnetic remanence Hall volt-

age VH=

-

this means the film has a rectangular M

-

H

/

loop with remanence Mr - then Hk can be determined

from the VHr

-

HII curve. Where, HII is the magnetic

field applied in the direction of film plane. And if the film has a linear type of VH

-

H loop as shown in figure 1, in which VH,? 0 due to the linear magnetization characteristics of the film, H; is determined from the Hall loop for the obliquely applied magnetic field in the direction a = 45' as illustrated in figure 2. In figure 1,

a predicted Hall loop for a = 45' is also shown. In this paper, we deal with the latter case more detailed. Let us consider the case of figure 2, where the film has become a single domain state due to the sufficiently large applied field H. We define the effective uniaxial

magnetic anisotropy energy K as 1 K = K, - 2 a ~ : = -M,Hk. 2 (1) a= 4 5 O V H S / D -- --

-

4

- -

---

I multi- single domain 1 domain region

I

region

Fig. 1. - Transverse Hall loop (a = 0') and expected Hall loop for oblique magnetic field (a = 45') for a film with linear M

-

H loop.

fllm normal

(easy axis)

Fig. 2.

-

Application of oblique magnetic field to a film

with perpendicular magnetic anisotropy.

Here K, is the uniaxial magnetic anisotropy energy and 2 n ~ : is the demagnetization energy for a thin film. Under a single domain condition, the energy per unit volume of the film is given by

E = -M,H cos (8

-

a)

+

K sin2 8. ₍₂₎ The energy minimum condition with respect to 8 is

aE

/

38 = M,H sin (0

-

a)

+

K sin 28 = 0. ₍₃₎

If the film is an ideal perpendicular magnetic anisotropy film and the ordinary Hall effect is negli- gibly small

-

this holds true for RE-TM amorphous films - angle 8 can be expressed by

' o n leave from Kagawa University, Faculty of Education, 1-1, Saiwaicho, Takarnatsu, Kagawa 760, Japan.

(3)

C8 - 1722 JOURNAL DE PHYSIQUE

Here is the saturation Hall voltage for perpendic-

-

a=

o0

ular magnetic field, namely in the case of transverse Hall effect, and VH is a Hall voltage.

Letting a = 45', then from equations (I), (3) and

(4), we obtain

H; =

4'

- (vH /vHs

12-'

/(VH /VHs )

l

/

_Fig._4._-_Transverse_{Hall loop and oblique Hall loop mea-}

(5) sured in a TbFe sputtered film. Using equation (5), we can determine the value of

H; of the film.

The process of the experiment is as follows. Firstly, measure the Hall loop for cr = 0' and read the saturation Hall voltage Vis. Secondly, measure the Ball loop for a = 45" - this can be done by using a rotatable sample holder - then read an arbitrary value of field H and the corresponding value of VH from the hysteresis- free portion of this Hall loop in higher field, i.e. the portion for the single domain region in figure 1.

By substituting the values of VH,, VH / ~ d H into equation (5), we can obtain the value of Hk.

Of course, equation (5) is not applicable t o the mul- tidomain region in figure l. However, when once the film become a single domain by a sufficiently large oblique magnetic field, the Hall curve is considered t o follow a hysteresis-free curve which is described by equation (5). Figure 3 is a normalized theoretical Hall curve calculated from equation (5). The curve in figure 3 is considered t o be the asymptote of the actual VH - H loop for a = 45' shown in figure 1.

H 1 H i

Fig. 3. - Theoretical Vj

-

H curve for the oblique field

CY = 45'. VH and H are normalized by the saturation Hall

voltage VH, for the vertical field a = '0 and the effective magnetic anisotropy field Hk, respectively.

for a = 0' and a = 45' measured in a TbFe film. The Hall loop for a = 45' agrees with the conjectured loop shown in figure 1 and also corresponds with the theoretical curve in figure 3 for higher field. Figure 5 shows the relationship between applied field H and H; values which were calculated from equation (5) for two TbFe films. It is seen in figure 5, as the applied

I

field increases, Hk approaches t o a constant value in each film. These H; values are consistent with those

obtained by the torque method. This supports the validity of equation (5).

The above mentioned method will be effective also for films with rectangular M

-

H loop.

,

Fig. 5. - Dependence of calculated Hk on the intensity of applied field for two TbFe films.

Acknowledgment

This work was supported by the Alexander von Humboldt-Foundation, Jean-Paul-Straf3e 12, 5300 Bonn 2, F.R.G.