Magnetization and torque measurements on Nd2Fe14B single crystals

Solid State Communications, Printed in Great Britain.

MAGNETIZATION K.

AND

Vol.56,No.4,

TORQUE

Tokuhara

;985.

MEASUREMENTS

, Y. M.

pp.333-336,

Ohtsu,

F.

Sagawa + and

0038-|098/85 $3.00 + .00 Pergamon Press Ltd.

ON Nd2FeI4B Ono Y.

, O.

SINGLE

CRYSTALS

Yamada,

Matsuura +

Department of Physics, Faculty of Science, Okayama University, 3-1-1 Tsushlma-Naka, Okayama 700, Japan. ** College of Liberal Arts and Sciences, Okayama University, 2-1-1 Tsushima-Naka, Okayama 700, Japan. + Sumltomo Special Metals Co. Ltd., Egawa, Shimamoto-cho, Mishima-gun, (Recieved

Osaka 618, Japan.

i0 July 1985 by J. Kanamori)

Magnetization and torque measurements on single crystal specimens of Nd2FeI4B have been carried out. The magnetization values measured always in the direction of easy magnetization and those in the [001] direction have been precisely determined at temperatures from 4.2K to 600K with a superconducting magnet up to 52.65 kOe. Below the spin reorlentatlon temperature 135K, the magnetization value of the direction of easy magnetization increases anomalously with decreasing temperature. The direction of easy magnetization tilts from the [001] axis to the [Ii0] axis and this tilt angle has been also precisely determined by torque measurement in the temperature range below the spin reorientation temperature. The four-fold symmetry in torque curve for the (001) plane is continuously observed at even up to near room temperature and the [ii0] direction of easy magnetization and the [i00] direction of hard magnetization do not change below and above the spin reorientation temperature.

i.

Torque measurements have been carried out by the conventional method with an electric magnet of 16.9 kOe maximum magnetic field. The specimens consist of two type disks of (001) and (ii0) planes, of which the sizes are 2 ~m in diameter and 150Dm in thickness and i mm in diameter and 160~m in thickness, respectively.

Introduction

Recently, Sagawa et al.[l] have developed a low-cost, high-performance, permanent magnet comprising Nd2FeI4B as the main phase. Soon after that, Herbst et al.[2] and Givord et al.[3] have clarified the crystal structure of Nd2FeI4B. Givord et al.[4] and Sagawa et al.[5] have found that the Nd2FeI4B crystal exhibits a spin reorientation at about i40K and the direction of the easy magnetization tilts from the [001] axis to the (001) plane with the decrease of temperature. This paper reports magnetization and torque measurements of Nd2FeI4B below and above the spin reorientation temperature. 2.

3.

Resets

(i) Magnetization

Measurements

From the present measurements, the spontaneous magnetization for Nd2FeI4B at 4.2K is determined to be 195.9 emu/g (37.9~B/F.U.). To estimate the effects of Nd and Fe atoms to the total magnetization separately, it is needed to measure the magnetization for Y2FeI4B, in which yttrium atoms are considered to be nonmagnetic and the contribution of Fe atoms can be obtained. Therefore, measurements for this material have also been made. The determined spontaneous magnetization of Y2FeI4B is 179.2 emu/g (31.1 ~B/F.U.) at 4.2K. If the contribution for the total magnetization is only from the Fe atoms, the average magnetic moment per iron atom is 2.22U B, which is very close to the value 2.2~ B for pure iron. Accordingly, when we use the value of magnetic moment for the iron atom in Y2FeI4B as it is, the average magnetic moment per Nd atom is obtained as 3.4~B , which is less than the value 3.75~ B obtained by Givord et al. [4] and which is close to 3.27~ B for the Nd +3 ion. Figure i shows the magnetization vs. temperature curve obtained with the specimen put in a rotatable state in external magnetic field of

Experimental

Magnetization measurements have been carried out by the induction method in the temperature range from 4.2K to 600K with a superconducting magnet of 52.65 k0e maximum magnetic field. Single crystal specimens were spheres about 5 mm in diameter. The measurements have been carried out in two ways, in one of which the external magnetic field is applied in the [001] direction of easy magnetization at room temperature and in the other of which the specimen is put in a freely rotatable state so that the external magnetic field can be applied to the specimen always in the direction of easy magnetization even if the direction of easy magnetization changes. *Present address: Sumitomo Special Metals Co. Ltd., Egawa, Shimamoto-cho, Mishima-gun, Osaka 618, Japan. 333

334

MEASUREMENTS ON Nd2Fel4B SINGLE CRYSTALS 200

Vol. 56, No. 4

1-0!

A

o ~o

¢:=:

~E

|

0.8

p-

d z

9

190

~ 0.6

i-

He,=52~5 kOe

.~0.4 1800

•~0

I

100 TEMPERATURE

I

150

I

200

Fig. i The magnetization vs. temperature c u r v e obtained with the single crystal specimen of Nd2FeI4B put in a rotatable state in external field of 52.65 kOe.

I

0

(i)

From eq.(1),! the tilt angles of the direction of easy magnetization, from the [001] axis are 30.7 and 26.9 degrees at 4.2K and 77.3K~ respectlvely, as llsted in Table 1. Figure 2 shows the temperature dependence of spontaneous magnetization for Nd2FeI4B and Y2FeI4B. The Brillouln function with J=l is shown for comparison alongside of the curves. The curve for Nd2FeI4B comes under that for Y2FeI4 B by the Table i. The tilt angle e determined by the measured magnetization values using eq.(1). M(O,T)0ol(emu/g)

M(0,T)Free(emu/g)

168.4

195.9

30.7

77.3

172.6

193.6

26.9

amount of the increase of magnetization due to the spin reorlentatlon. However, the two curves approximately coincide when we subtract the anomalously increased part of magnetization from the magnetization vs. temperature curve of Nd2FeI4B. (2) Torque Measurements Figure 3 shows some observed torque curves for the (001) plane at 77.3K. Since the magnetic anlsotropy is very large, the magnetization cannot

I

I

I

I

I

Nd2Fe,- B (001) plune ot T=77.3 K --. 4

Hex= 8 kOe

u

~0 cp

~-4

Z

uJ I--

ZN

4

o n-

O (deg.) I--

4.2

I

0.2 0.4 0.6 0.8 1.0 REDUCED TEMPERATURE TI Tc

Fig.2 The temperature dependence of the spontaneous magnetization for Nd2FeI4B and Y2FeI4 B. The Brillouin function with J=l is also shown for comparison alongside of the curves.

O

T(K)

Brittouin function with J=l

a w

52.65 kOe. Although the magnetization increases gradually with decreasing temperature, the magnetizatlcm curve shows a clear kink at 135K. This kink is thought to be ascribed to the spin reorientatlon which Givord et al.[4] and Sagawa et al. [5] have pointed out. It would be of great interest that the magnetization begins to increase anomalously from 135K with decreasing temperature. The cause of this behavior remains still unknown; however~ the problem would be important in considering that the spin structure would take part in the anomalous increase of the magnetization. By using the spontaneous magnetization, M(0,T)Free , measured always in the direction of easy magnetization and that of the [001] axis, M(O,T)001, the tilt angle e is expressed as, e = cos-l( M(0,T)001/ M(0,T)Free ).

IE

"0.2

T (K)

-4

\ 0

I

I

I

90

180 O (degree)

270

Fig.3 The t o r q u e Nd2Fel4B obtained

curves for at 77.3K.

the

(001) p l a n e

360 of

be saturated even at the maximum external magnetic field of 16.9 kOe and the value of torque increases with external magnetic field. It can be found, however, in Fig.3 that the direction of easy magnetization in the (001) plane is the [ii0] axis and that of hard magnetization is the [I00] axis. This fact implies that, below the spin reorientatlon temperature, the direction of the easy magnetization

MEASUREMENTS ON Nd2FeI4B SINGLE CRYSTALS

Vol. 56, No. 4

tilts over from the [001] axis in the (Ii0) plane. From the temperature dependence measured at the external field of 16.9 kOe, the following facts have been found: the value of torque decreases rapidly with increasing temperature and, when compared to the value at 77.3K, it becomes for example about 1/2, 1/15 and 1/35 at I16.4K, 160.1K and 197.5K, respectively; the four-fold symmetry is continuously observed even up to remarkably high temperature which is close to room temperature and the directions [ii0] and [i00] of easy and hard magnetization respectively do not change, as seen from Fig.4. I

I

NdzFeN B (001) plane 2

335

I

I

I

Nd2Fe,~B (110) plane

at T=293K

1

% o

%

0 -1

0

I

>-

at Hex=16.gkOe

-1 Z LIJ t-Z

T=t28-6K

IJJ :::) 0 nO I--

1 0 -1

-2

>. 0

!

I

i=..

0

zLLI

90

149.4 K

!

180 e (degree)

2?0

360

FIg.5 The torque curves obtained for the (ii0) plane of Nd2FeI4B at room temperature.

w oQ:

160.1

K

I

-

I

Ndz Fe,4 B

(1 10) plane 0

|

!

!

90

180

270

e

(degree)

at T=4.2K

1 360

Fig. 4 The torque curves for t h e (001) plane under a external field of 16.9 kOe at temperatures below and above the spln reorlentation temperature. Figures 5 and 6 show the torque curves obtained for the (ii0) plane including the [001] and [ii0] axises at room temperature and 4.2K, respectively. At room temperature, the [001] axis Is the direction of easy magnetization and the [ii0] the direction of hard magnetization. At 4.2K, the [Ii0] axls is still the direction of hard magnetlzatlon; however, the direction of easy magnetization tilts ~ 30 degrees from the [001] axis. The curves signify that the direction of easy magnetization tilts from the [001] axis to the [ii0] axis below the spln reorlentatlon temperature. From the temperature dependence of torque curve obtained at the external field of 16.9 kOe, It was found that the value of tllt angle 0 decreases gradually with increasing temperature from 4.2K and the value of 0 becomes zero at about 133K. Figure 7 shows the temperature dependence of the tllt angle 0. The values of 0 are 30.3 and 26.5 degrees at 4.2K and 77.3K, . respectively, which approximately coincide with

Hex=6kOe

~

-1

12 kOe

0

"'

0

0 0 I.--1

-2 I

0

90

I

180 e (degree)

I

270

360

Flg.6 The torque curves obtained for the (ii0) plane of Nd2FeI4B at 4.2K.

336

MEASUREMENTS ON Nd2Fe|4B SINGLE CRYSTALS I

!

I

Vol. 56, No. 4 I

..30 NdzFe14B

g, "0 v

\

20 U.I ..J

\

\

Z

I--

0 i

0

50

I

100 TENIPERTURE

lq~o--o-o--o-o--o--e-o-I I

150 T (K)

200

Fig.7 The tilt angle 8 of the direction of easy magnetization from the [001] axis to the [ii0] axis determined by the torque measurement for the (ii0) plane of Nd2FeI4B at temperatures below and above the spin reorientatlon temperature.

those obtained from magnetization measurements. WRen we determine the spin reorientatlon temperature from the kink point of the magnetization vs. temperature curve, it is 135K. When we determine the temperature, however, as the start point where the direction of easy magnetization begins to tilt

from the [001] axis in torque measurements, the temperature is found to be 133K. A rather good agreement exists between these two temperature values taking into consideration the fact that we cannot bring the thermocouple into contact with the specimen in torque measurements.

References [i] M. Sagawa, S. Fujimura, N. Togawa, N. Yamamoto and Y. Matsuura; J. Appl. Phys., 5--52083 (1984). [2] J.F. Herbst, J.J. Croat, F.E. Pinkerton and W.B. Telon; Phys. Rev. B 2 9 4 1 7 6 (1984). [3] D. Givord, H.S. Li and J.M. Moreau; Solid State Commun., 50 497 (1984).

[4] D. Givord, H.S. Li, R. Perrier de la B~thle; Solid State Commun., 51 857 (1984). [5] M. Sagawa, S. Fujimura, H. Yamamoto, Y. Matsuura and S. Hirosawa; Conf. Magn. Magn. Mat. (San Diego, 1984).