LT 21 Proceedings of the 21st International Conference on Low Temperature Physics Prague, August 8-14, 1996

Part S1 - Quantum Fluids and Solids: Liquid Helium

Magnetism of Submicron Ag Powder: its Impact on the Kapitza-Resistance and its Applicability for Thermometry at Very Low Temperatures

R.KSnig a, Th.IterrmannsdSrfer a'b, W.Wendler a, W.Jansen c, and F.Pobell*

a Physikalisches Institut, Universit/it Bayreuth, D-95440 Bayreuth, Germany b Ilalm-Meitner-Institut Berlin, D-14109 Berlin, Germany c Dept. of Physics, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands

We have studied the magnetic properties of submicron Ag powder (400t[ and 700t~) in a magnetometer at 1.5K < T < 300K and in B < 6T. Once exposed to a magnetic field, the sinter keeps a temperature dependent magnetic moment whose magnitude strongly depends on the history of the sample. The dependence of the moment on the applied magnetic field is discussed in relation to the field dependence of the boundary resistance between an Ag sinter and liquid a He. - Doped with a small amount of magnetic impurities, the sinter may be used as an extremely sensitive, continuously working thermometer; its susceptibility exactly follows the differential Brillouin function down to at least 200/zK. The most important advantage of this sinter is its simultaneous applicability as thermometer and as heat exchanger in helium experiments.

1. I N T R O D U C T I O N

The investigation of the properties of liquid and solid helium at mK- and pK-temperatures requires the use of heat exchangers with large surface areas in order to reduce the temperature difference between the refrigerant and the helium. The nominal grain size of the Ag powder commonly used in low tempe- rature experiments is ~ 700/~ [1] leading to a specific surface area of about 2m2/g when pressed to a sin- ter sponge. Although the boundary resistance RK between a heat exchanger made of submicron metal powder and liquid helium was investigated in a num- ber of experiments, the mechanism for the cnergy transfer across the helium/metal interface is not yet fully understood. In particular, large deviations in the magnitude and discrepancies in the temperature dependence of RK are reported, and the origin for its magnetic field dependence is not clear either [2].

Apart from their application as heat exchangers, sintered metal powders are also used as substrate for investigations of two-dimensional systems [3], and - when pressed to a sinter body - they provide an important means to study the properties of liquid and solid helium in a restricted geometry [4].

This list of applications reveals the importarice of the Ag sinter for low temperature experiments', and was the motivation to study its magnetic properties at 1.5K _< T _< 300K using a commercial SQUID- magnetometer.

2. RESULTS

2.1. Magnetic Field Dependence of the Boun- dary Resistance

The existence of a magnetic channel for the energy transfer between metal powder heat exchan- gers and liquid aHe was experimentally confirmed by Osheroff et al. [5] who observed a strong dependence of RK on the magnetic field B for B < 0.3T. At hig- her fields, RK remained constant up to B = 9.4T. However, the origin of this field dependence of RK was unknown, although it was discussed that it might be attributed to magnetic impurities, particularly to oxygen-ions, on the sinter surface.

Fig.l shows the smoothed data of the B- dependence of RK of Ref.5. There is a striking agree- ment of this curve with the B-dependence of the per- manent magnetic moment the Ag powder kept after it was exposed to a magnetic field. This plot clearly shows that the origin of the field dependence of RK observed in Ref.5 has to be attributed to the magne- tic properties of the Ag powder. This polarizability remained unchanged even after the sinter was expo- sed to heat treatment and to chemical reduction with hydrogen. Therefore, we conclude that this anoma- lous magnetic behaviour is a property of the sinter itself rather an effect caused by surface impurities.

* New ~tdress: Fot~-Jmngszentrtun Rossendorf, D-01314 Dresden, Germany

Czechoslovak Journal of Physics, Vol. 46 (1996), Suppl. $1 ]61

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Figure 1: Permanent magnetic moment rn of the Ag sinter (*, left scale, measured at T = 7I 0 after ex- posure to a magnetic field given by the x-axis. - B- dependence of RK between an Ag sinter and liquid 3He at about l inK [5] (solid line, right scale).

2.2. T h e r m o m e t r y w i t h a Ag /Gla s s s ln ter

In order to use an Ag sinter for thermometry at mK- and p-temperatures, the 700It powder was mixed with pulverized Duran glass (mass ratio ~.. 15 : 1). The temperature dependence of the susceptibi- lity of the Ag/Duran sinter is then dominated by the magnetic properties of the glass which were separa- tely investigated in our SQUID-magnetometer. The magnetization of the Duran showed a Langevin pa- ramagnetic behaviour caused by ~180ppm magnetic impurities (mainly Fe3+-ions) with/~ = (5.5+0.4)#B and spin J = 2.5 4-0.2. Susceptibility measurements confirmed the expected behaviour for localized non- interacting magnetic moments down to ~ 0.4ink [6].

Figure 2 shows the low temperature data of the 16Hz-susceptibility X of a Ag/Duran sample. The T-dependence of X is well described by the differen- tial Brillouin function OM(J,p, B ,T) /OB with the above given values for J and p. The small size of the Duran particles embedded in the Ag sinter provides very short relaxation times of the order of tens of seconds even at the lowest temperatures achieved in this experiment.

3. C O N C L U S I O N S

The investigation of the magnetic properties of submicron Ag powder enables us to address the ma-

Figure 2: Temperature dependence of the ac- susceptibility of an Ag/Duran sinter. The upper curve was measured in B ~ 1G, the lower corre- sponds to B ..~ 2.5G.

gnetic field dependence of the boundary resistance between the Ag sinter and liquid 3He [5] to the intrin- sic magnetic properties of the metal powder. Mixed with submicron glass particles of known impurity content we have developped a very sensitive ther- mometer applicable in the inK- and pK-temperature range. In particular, the Ag/glass sinter might serve as a sensor for a direct measurement of the heat ex- changer temperature which in turn might be useful in further experiments on RK between metal sinter and liquid helium.

This work was supported by the "HCM: Large Scale Facility" program of the EC and by grants No. Po 88/13 and Po 88/18 of the Deutsche Forschungs- gemeinschaft.

R E F E R E N C E S [1] Ag powder, from Inabata Corp., Vacuum Me-

tallurgical Ltd., Japan [2] T.Nakayama, Prog. in Low Temp. Phys.,

D.F.Brewer (ed.), Vol.XII, p.l15, Els. Sci. Publ. B.V. (1989)

[3] see, e.g.D.S.Greywall and P.A.Busch, Phys. Rev. Left. 60, 1860 (1988)

[4] see, e.g.R.Schrenk et al., to be publ. in Phys. Rev. Lett. (1996)

[5] D.D.Osheroff and R.C.Richardson, Phys. Rev. Letl. 54, 1178 (1985)

[6] S.Rehmann et at., Cryogen. 35, 665 (1995)

]62. Czech. J. Phys. 46 (1996), Suppl. $1