Brebis Bleaney

Anomalous paramagnetism of copper acetate

The paramagnetic resonance spectrum of copper acetate is anomalous in that it resembles that of an ion of spin 1, and its intensity decreases as the temperature is lowered. The latter is correlated with the decreasing susceptibility found by Guha (1951). The following hypotheses are suggested: (1) the crystalline field acting on each copper ion is similar to that in other salts such as the Tutton salts; (2) isolated pairs of copper ions interact strongly through exchange forces, each pair forming a lower singlet state and an upper triplet state, the latter only being paramagnetic. On this basis both the fine structure and the hyperfine structure of the spectrum have a simple explanation, and the theory also predicts a small initial splitting of the triplet state of the same order as that found experimentally. The unit cell of the crystal contains two differently oriented pairs of ions, and, using an empirical value for the exchange parameter, fair agreement with the susceptibility measurements of Guha is obtained.

The paramagnetic resonance spectra of two salts of manganese

The magnetic resonance spectra of manganese fluosilicate and manganese ammonium sulphate have been analyzed using centimetre wave-lengths. It is shown that the splitting of the 6S state is due mainly to the non-cubic symmetry of the crystalline field; the effect of the cubic field, hitherto assumed to be dominant, appears only as a small correction. The spectra show that the splitting is consistent with a mechanism suggested by Abragam & Pryce (1951), where it is attributed to magnetic interaction between the five electrons constituting the 6S state. An extensive hyperfine structure is resolved, and the nuclear spin (5/2) of 55Mn is confirmed. The spectroscopic splitting factor, g, is 2·000±0·001, very close to the free-spin value. The energy levels in zero magnetic field are evaluated for manganese ammonium sulphate.

Paramagnetic Resonance in Diluted Copper Salts. III. Theory, and Evaluation of the Nuclear Electric Quadrupole Moments of

The previous theory of Abragam & Pryce (1951) is extended to a special case of rhombic symmetry, and to include higher-order terms. The theory gives satisfactory agreement between the observed anisotropies of the g-tensor and the magnetic hyperfine structure, and shows that the size of the hyperfine structure is about 15 % smaller than would be expected on the assumption that the magnetic electron is confined to the copper ion. Comparison with the optical absorption spectrum suggests that the spin-orbit coupling is reduced by about the same amount, and the effects are explained by an overlap of the wave function of themagnetic electron on to neighbouring water molecules. The nuclear electric quadrupole moments are found to be (in units of 10-24 cm2) ; 63Cu, — 0.16; 65Cu, — 0.15, with an estimated maximum error in the theory of 20 % .

^{63}

The principal values of the g-tensor and the hyperfine structure (due to interactions with the magnetic dipole and electric quadrupole moments of the stable copper isotopes) have been determined in several copper Tutton salts diluted with the isomorphous zinc salts. At high dilution the residual line width is mainly due to interaction with the nuclear magnetic moments of the protons in the water of crystallization, and increased resolution is obtained by replacing these by deuterons. Crystals of the diluted copper potassium salt have been grown from heavy water, and a detailed study made of the electric quadrupole interaction.

Cu and

The paramagnetic resonance spectra of ferric rubidium sulphate and ferric potassium selenate alums (diluted with the corresponding aluminium alums) have been analyzed in detail. It is found that the splitting of the 6S state of the ferric ion can be explained in terms of a spin Hamiltonian containing trigonal terms of the second and fourth degree, and a cubic term. The measurements show also that the axes of the cubic crystalline field are rotated through a small angle from those of the crystal unit cell, as expected from X-ray analysis of the structure (Lipson 1935). No hyperfine structure due to 57Fe could be resolved in a sample containing 40% abundance of this isotope. From analysis of the line shape, an upper limit of 0·0006 cm-1 could be placed on the overall width of any such structure, corresponding to a nuclear moment of 0·05 n.m.

^{65}

Paramagnetic resonance has been observed in Mn2+, Eu2+ and Gd3+ ions in single crystals of calcium fluoride grown from the melt. The Mn2+ ion has a very small cubic-field Stark splitting (a = + 0.6 ± 0.4 x 10-4 cm-1), and a fluorine hyperfine structure of overall splitting of about 60G due to some covalent bonding with the fluorine ligands. The Eu2+ spectrum has cubic symmetry with splitting parameters b4 = 57.9 + 0.2, b6 = 0.5 + 0.2 ( x 10-4 cm-1), but the Gd3+ spectrum has tetragonal symmetry with much larger Stark splittings; neither ion shows a resolved fluorine structure. The manganese and europium hyperfine structures are closely the same as in other salts with small covalent bonding.

Cu

The paramagnetic resonance spectra of cobalt ammonium sulphate, cobalt potassium sulphate, cobalt fluosilicate and cobalt sulphate, each diluted with the isomorphous zinc salt, have been analyzed. These salts exhibit great anisotropy both in the g-values (spectroscopic splitting factor) and the hyperfine structure. The results show that the ground state of the cobalt ion is an electronic doublet, and confirm the nuclear spin of 59Co as 7/2. The theoretical interpretation of the results by crystalline field theory has been carried out by Abragam & Pryce (1951 a). The contribution to the specific heat from the hyperfine structure is computed for each salt. In addition, the measurements of the specific heat and susceptibility of cobalt sulphate by Fritz & Giauque (1949) are given a new interpretation consistent with that of other cobalt salts. This requires that the magnetic entropy of cobalt sulphate be taken as R loge 2, instead of R loge 4, as assumed by Fritz & Giauque.

Paramagnetic Resonance in Diluted Copper Salts. I. Hyperfine Structure in Diluted Copper Tutton Salts

An interesting transition in the paramagnetic resonance spectrum has been observed for a number of cupric salts with trigonal symmetry. At room temperature only a single nearly isotropic line is observed, with a small hyperfine structure. Below a certain temperature, which depends on the salt, the spectrum is that of three kinds of copper ion each with nearly axial symmetry about one of three mutually perpendicular directions. The behaviour of this spectrum is similar to that of the copper Tutton salts in the anisotropy of the (/-tensor and magnetic hyperfine structure, and in the presence of a nuclear electric quadrupole interaction. A careful analysis of the spectrum is made for (Cu, Mg)3La2(NO3)12.24D2O.

Paramagnetic resonance spectra of some ferric alums, and the nuclear magnetic moment of 57Fe

Accurate measurements of the paramagnetic resonance spectra of gadolinium and neodymium ethyl sulphates have been made both in strong magnetic fields at a wave-length of 3 cm, and in weak fields at wave-lengths between 6 and 22 cm. (i) The Gd3+ ion is in an 8S state, whose levels are split by the action of the crystalline electric field, which is assumed to have C3h symmetry. The results are consistent with this supposition, except for some discrepancies in the position of the zero field lines, whose origin is not certain. The main parameters in the spin Hamiltonian are evaluated, and the spectroscopic splitting factor is found to be isotropic at 1.990±0.002. (ii) The Nd3+ ion is in a 4I2/3 state, which is split by the crystal field leaving a Kramers doublet as the ground state. The hyperfine structure due to the two odd isotopes 143, 145 has been measured and gives the ratio of the nuclear magnetic moments (143/145) as 1.6083±0.0012. Some small discrepancies in the positions of the hyperfine lines in zero field are found, which prevent the determination of accurate values of the nuclear electric quadrupole interaction. The theory of Elliott & Stevens (1953b) leads to values of 1.0 and 0.62 nuclear magnetons for the moments of isotopes 143 and 145 respectively, with an uncertainty of about ±25% arising from lack of a precise value for r–3¯¯¯, where r is the electron-nuclear distance.

Paramagnetic resonance of S-state ions in calcium fluoride

The paramagnetic resonance spectra of five chromic sulphate alums have been examined at temperatures down to 20° K and wave-lengths 3 and 10 cm. Their behaviour is markedly different as follows. (a) Rubidium, caesium and methylamine alums show the normal spectrum associated with a trigonal splitting at all temperatures, but the splitting decreases as the temperature is lowered, becoming constant below 90° K. The change in the splitting is most marked for rubidium, but substantially zero for methylamine alum. (b) Ammonium alum shows the normal spectrum at high temperatures, with a steadily decreasing splitting down to 80° K; here a crystallographic transition takes place, and the spectrum becomes abnormal, with two splittings. (c) Potassium alum behaves like ammonium alum down to 160° K, below which an anomalous spectrum appears with lines whose intensity increases as the temperature falls. The two splittings in this region are difficult to reconcile with previous measurements of the specific heat of the spin system by other methods.