An experimental study of paramagnetic susceptibility at low temperatures

We have investigated the microwave absorption spectra of cerium, praseodymium, neodymium and samarium in magnesium double nitrate at 4.2°K. We have also measured the bulk susceptibility of a nitric oxide clathrate and of potassium manganicyanide from 4.2°K to 300°K.
APPARATUS:
We used a 3 cm reflex klystron microwave spectrometer, obtaining temperatures down to 1.5°K by the use of fee liquid helium. A large water-cooled electromagnet produced homogeneous fields of up to 15 kilogauss which were usually measured by an ammeter showing the current through the coils. In order to measure the field accurately very close to the specimen at 4.2°K, but it was too broad to be very useful.
The bulk susceptabilities were measured by means of a Sucksmith ring balance which covered the temperatures range from 2°K to 300°K by use of the free liquid helium, hydrogen and oxygen. The ring deflections were measured by means of two opposing selenium photocells in series with a sensitive galvanometer used as a null detector. The force on the specimen was produced by the imhomogeneous field of an air-cooled electromagnet. We were unable to obtain a field of suitable symmetry to measure the highly anisotropoc crystals.
RESULTS
THE DOUBLE NITRATES:
Cerium:
An absorption line was observed with g&bot;=1.82 &pm; 0.02 and g//= 0.25 &pm; 0.05, and interpreted as a single electronic transition between the states cos&ominus; | 1, - > - sin&ominus; | 0 + > - cos&ominus; | -1, + > + sin&ominus; | 0, - &half; > in L,s quantizization where &ominus; = 17&half;°.
Praseodymium:
Six absorption lines were observed with the r.f. and d.c. fields parallel. They are interpreted as resulting from a single electronic transition having g// = 1.55 &pm; 0.02 split by the nuclear spin of 5⁄2 into six hyperfine lines. The transition is ot between the states of a Kramers' doublet. A crystal field of C3h symmetry will give a singlet and J doubts, which will be split by the Jahn Teller effect. We propose that the transition occurs between states described by Jz quantizization as cos&ominus; | &pm; 2 > + sin &ominus; | &mp; 4 > with &ominus; = 22°.
The hyperfine parameters are interpreted in terms of the spin Hamiltonian H = βg// HzSz + AIzSz giving A = 0.078 &pm; 0.001 cm-1.
Neodymium:
A main line flanked by 15 resolved hyperfine lines was observed. They are interpreted as resulting from a single electronic transition having g&bot; = 2.72 &pm; 0.02 and g// = 0.45 &pm; 0.05 split by the nuclear spins of 7⁄2 of two odd isotopes, Nd143 and Nd145 into sixteen hyperfine lines, one of which was not resolved from the main line. The hyperfine parameters are interpreted in terms of the usual spin Hamiltonian H=β[g//HzSz + g&bot;(HxSx + HySy)] + AIzSz + Β(IxSx + IySy) giving
B143 = 0.0372 cm-1
B145 = 0.0194 cm-1
The ratio of the nuclear magnetic moment of Nd143 to that of Nd145 is 1.61.
The transition is proposed to occur within the Kramers' doublet in Jz quantization of cos&ominus; | &pm; 7⁄2 > + sin&ominus; | &mp; 5⁄2 > where ∈ = 47°
The line width of the main line was 170 gauss which is wider than theory predicts.
Several weak unidentified lines were observed.
Samarium:
A main line flanked by 16 fully resolved hyperfine lines was observed. They are interpreted as resulting from a single electronic transition having g&bot; = 0.40 &pm; 0.05, g// = 0.76 &pm; 0.01 split by the nuclear spins of 7⁄2 of two odd isotopes, Sm147. The hyperfine parameters are interpreted from the same spin Hamiltonian as (c) to be
| A147 = 0.0346 &pm; 0.0005 cm-1
A149 = 0.0287 &pm; 0.0005 cm-1
B &leq; 0.009 cm-1 for both isotopes
MN147
MN149  = 1.22
The transition is proposed to occur within the Kramers' doublet in JJz quantization of cos &ominus; | 5⁄2 &pm; &half; > + sin&ominus; | 7⁄2 &pm; &half; > where &ominus; ~ 6°.
The observed line width of 75 gauss is very much wider than theory predicts, but this may be due to a short spin lattice relaxation time.
POTASSIUM MANGANICYANIDE:
The effective magnetic moment,
n = √ 3XKT
Nβ2
approaches zero as the temperature is decreased, showing that the ground level is diamagnetic. From the temperature variation of n we can conclude that the covalent forces upon the Mn+++ ion cannot be accurately represented by an electrostatic crystal field of cubic symmetry.
At 293°K the affective magnetic moment of n = 3.6.
NITRIC OXIDE CLATHRATE
The effective magnetic moment of nitric oxide molecules trapped in the quinol lattice approaches zero as the temperature is decreased, showing that the ground level is diamagnetic.
The first exited level is not far above the ground level as it is in the free molecule, and we conclude that there is a small interaction between the NO molecules and the quinol lattice. At 293°K, however, the susceptability of the NO in the clathrate is that of fee NO within the experimental error, viz. 47 x 10-6eml per gram.