## LSU Historical Dissertations and Theses

1989

Dissertation

#### Degree Name

Doctor of Philosophy (PhD)

#### Department

Physics and Astronomy

Roy G. Goodrich

#### Abstract

The development of an automated small sample (0.5 to 50mg) calorimeter is discussed and evaluated. With this calorimeter, measurements of the specific heat of Gd$\sb1$Ba$\sb{\rm 2-x}$Sr$\sb{\rm x}$Cu$\sb3$O$\sb7$ for x = 0, 0.25, and 0.5 in the temperature range 0.6 to 3 K are reported. Measurements on the x = 0 sample agree with data collected by other groups. In addition, measurements in magnetic fields from 0 to 3 T for the x = 0 sample are reported. In each case a peak in the specific heat at the antiferromagnetic ordering temperature (the Neel temperature) is recorded and analyzed. Strontium doping does not appreciably change the Neel temperature, but drastically changes the shape of the specific heat curve below the ordering temperature. The Neel temperature is reduced proportional to the square of the applied magnetic field, becoming no longer observable for fields greater than 3.0 T. Mean field theory is applied to the Neel temperature shift in order to gain a qualitative description for this behavior. The model calculation of the spin wave contribution to the low temperature specific heat discussed in the paper indicates that strong couplings along the c axis and weak dipolar couplings in the a-b plane are responsible for the antiferromagnetic order. The strontium doping further substantiates this argument by showing that the removal of the barium ions weakens the gadolinium coupling in the c direction by removing the super-exchange path.

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