Magnetic impurities in glasses and gelatine

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Cryogenics 35 (1995) 665-647 0 1995 Elsevier Science Limited Printed in Great Britain. All rights reserved

H

001 I-2275/95/$10.00

Magnetic impurities in glasses and gelatine S. Rehmann, T. Herrmannsdiitfer Pobell Physikalisches Institut, Universitat 95440 Bayreuth, Germany Received

7 March

and F.

Bayreuth,

D-

1995

We have measured the d.c. magnetization at 4.2 K 5 TS 300 K and the a.c. susceptibility at 0.3 mK I TS 100 mK of a standard glass pipette, of Duran and Suprasil glasses, as well as of a gelatine capsule. Whereas the concentration of magnetic impurities in the first two glasses is -0.4 and 0.2%, and -0.01% for the gelatine capsule, it is below 1 ppm for the Suprasil glass (assuming pimpurih/= 2 /J~~,,~).This makes the latter glass very suitable as a sample holder with a very small, constant susceptibility background in investigations of the magnetic properties of materials.

Keywords: glasses; magnetic bility

impurities;

suscepti-

In studies of the magnetic properties of materials, very often sample holders are needed which are non-magnetic or, at least, contain a negligible concentration of magnetic impurities. We were repeatedly confronted with this problem in our studies of weakly magnetic materials in a SQUID magnetometer of very high sensitivity (S600, Cryogenic Consultants Ltd., London, UK) and in several a.c. susceptometers. Whereas the magnetic contribution of a sample holder can be avoided in motion SQUID magnetometers by mounting the sample in the centre of a holder with homogeneously distributed magnetic impurities, there is no method to null the background in conventional a.c. susceptometers with one pick-up coil. In the search for sample holders with suitable properties, we have measured the d.c. magnetization and the a.c. susceptibility of some glasses and of a gelatine capsule, as detailed in Table I, over a very wide temperature range (0.3 mK 5 T I 300 K). From the results shown in Figures l-5 and summarized in Table 2, we conclude that Suprasil is a glass essentially free of magnetic impurities (concentration
temperature range have been performed at a magnetic field of 1 T in a commercial SQUID magnetometer (S600, Cryogenic Consultants). The d.c. susceptibility of the samples has been obtained by dividing the measured magnetization by the applied field. The measurements of the a.c. susceptibility at very low temperatures have been performed in the ‘field-free’ region of our nuclear refrigerators’. The experimental arrangements for sample, primary and secondary coils are similar to the ones described in references 2 and 3. The set-ups were surrounded by superconducting Nb cylinders to shield against magnetic fields. The excitation field of 16 Hz had an amplitude of a few microtesla. The signals were detected using commercial mutual induction bridges (LR 400 and LR 700, Linear Research Inc., San Diego, CA, USA). Information about the samples is given in Table 1. From the measured inductance L(T) we obtain the susceptibility, according to

(1) The filling factors VsampJVcoilwere between 0.24 and 0.31; values for the high temperature inductance L(m) varied between 1.3 and 4.9 mH. All samples show diamagnetic behaviour at high temperatures with diamagnetic susceptibilities xZ = -0.345 +m.u. g-’ for the alkali (AR) glass, and x= = -0.400 to -0.406 pe.m.u. g-’ for the other three materials (see Figures I and 2 and Table 2). On decreasing the temperature we observe, in addition, a typical paramagnetic contribution to the susceptibilities following a CurieWeiss law and originating from magnetic impurities in the AR and the Duran glass, as well as in the gelatine capsule. The signal from the Suprasil glass, however, stays at its diamagnetic value to within the accuracy of our measurements. These results are shown for the kelvin temperature range in Figures I and 2. The 16 Hz a.c. susceptibilities measured at millikelvin temperatures are shown in Figures 3-5. The saturation of the signals of AR glass below 40 mK and of Duran glass below 6 mK (see Figures 3 and 4) is caused by thermal decoupling of the samples from the refrigerator. This interpretation was confirmed by repeating the measurement with a powdered Duran sampie, which was mixed and pressed at 20 kbar* with 700 A? Ag powder (Ag:Duran ratio = 3: 1) for better thermal coupling. This mixed sample shows an increase in its paramagnetic susceptibility to the lowest temperature of the experiment, 0.4 mK. The too low value for the Curie constant C of Duran glass obtained from the a.c. susceptibility measurement compared to the d.c. measurement (see Table 2) is probably caused by a small remanent field of the order of 0.1 mT in the a.c. experiment.

* 1 bar = lo5 N m-2 tlA=lO-‘Om

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Research and technical note Table 1

Properties of samples

Sample

Origin

Shape

Density (g cm-Y

Alkali (AR) glass Duran glass Suprasil glass

Chemistry pipette Glass-blower workshop Heraeus Quarzglas GmbH, D63450 Hanau, Germany Bal-Tee, EM-Technologie und Applikation, D-65396 Walluf, Germany

Tube, 6 mm o.d., 5 mm i.d. Tube, 6 mm o.d., 4 mm i.d. Tube, 6 mm o.d., 4 mm i.d.

2.51 2.23 2.20

Gelatine

Table 2

Magnetic

properties

Sample

T range

AR glass

>4K cl K >5K co.1 K >5K co.1 K >7K

Duran glass Suprasil glass Gelatine aconcentration

Capsule, 5 mm o.d., 0.1 mm wall thickness, 25 mg

of samples

of magnetic impurities,

Frequency mode

,y_ (pe.m.u.

g-l)

d.c. ac. dc.

-0.345 f 0.01

x:

-0.402 f 0.01

::“,I

-0.406 f 0.01

-0.400 f 0.01

assuming

8 (K)

C (pe.m.u. g-1 )

(-0.4 f 1.0) (-0.05 + 0.03) (-0.9 * 1 .O) (-0.02 rt 0.05) mK (-1 fl) Not detectable (-1.5fl)

36fl 29f3 15.8 f 0.5 (5.7f 0.6) 0.01 f 0.01 SO.003 1.01 f 0.03

K

P 0.43% 0.35% 0.17% (Ifl) ppm CO.3 ppm 0.01%

CL,= 2 psoh,

-5.

. *

x

m

20-

3 8

15-

g c 3

IO-

AR-Glass n

5-

n

d ()_ -_ ._..--____.-____.._____..L __

_____.B___

Suprasil

. . . . ..I

-2

-5 ..‘..,

100

%qmiture

0.001

.,,....1

,,.....I

0.01

0.1

[K]

,,.....I , ,‘...,,, . ,, 1

10

Temperature [K]

Figure 1 D.c. susceptibility of AR, Duran and Suprasil glasses (see Table 7) as a function of temperature in the kelvin temperature range. Measurements were performed in a field of 1 T

Figure 3 Inductance (proportional to a.c. susceptibility; see text) of AR glass as a function of temperature. The saturation at low temperatures is due to thermal decoupling of the sample

b

b

Suprasil .A

-0.45

!

10

100

i

Temperature [K]

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1995 Volume

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Temperature [mK]

Figure 2 D.c. susceptibility of gelatine and of Suprasil glass as a function of temperature in the kelvin temperature range. Note the sign and expanded scale of the vertical axis. Measurements were performed in a field of 1 T

666

A A

AALA

I

’ .I

10

Figure 4 Inductance (proportional to a.c. susceptibility; see text) of Duran and Suprasil glasses as a function of temperature in the millikelvin temperature range. The saturation of the signal from Duran glass at T< 6 mK is due to thermal decoupling of the sample (see Figure 5)

Research and technical note with Curie constants

s

c

150-

=

NOCLO x(Nerr~d

[K e.m.u. g-’ ]

47m,,,,

DLlran

(in CGS units)

(3)

3ks

OJ

2

5 :

loo-

0 0

SO-

0.1 -

I

10

1000

Temperature [mK] Figure 5 Inductance (proportional to ac. susceptibility; see text) of Duran glass as a function of temperature in the millikelvin temperature range. Note the compressed vertical axis compared to Figure 4. The filled circle data points were taken on a bulk Duran glass, and the open circle data points on a sample made from a powder of Duran glass mixed with 700 A Ag powder in the ratio 1:3 for better thermal coupling

In Equation (3) x is the concentration of magnetic impurities and N,, is their magnetic moment in units of the Bohr magneton pB. The magnetic impurities are very probably predominantly Fe, and we assume N,, = 2. The Curie temperatures 8 are zero within the accuracy of our measurements. Our data show that AR glass used for pipettes in chemistry laboratories as well as Duran glass from glass-blower shops contain a rather high concentration of magnetic impurities. Gelatine capsules may be suitable as sample holders for magnetic measurements of not too high sensitivity. Most important, Suprasil glass is essentially free of magnetic impurities (less than 1 ppm) and therefore very suitable as a sample holder in magnetic investigations.

References Such remanent fields are known to reduce substantially the absolute values of ax. susceptibilities4. The susceptibility of the samples can be written as

Gloos, K., Smeibidl, P., Kennedy, C., Mueller, R.M. et al. J Low Temp Phys (1988) 73 101 Herrmannsdiirfer, T., Uniewski, H. and Pobell, F. J Low Temp Phys (1994)

(2)

97 189

Pobell, F. Materials and Methods at Low Temperatures SpringerVerlag, Berlin, Germany ( 1992) Casimir, H.B. and du Pti, F.K. Physica (1938) 5 507

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