Coupling magnetic ions

Research on multi-qubit molecules starts with the synthesis and characterization of systems that seem to embody more than one qubit, for example, systems with weakly coupled electron spins. Indeed, many molecular structures include several weakly coupled magnetic ions [76-78]. On a smaller scale, the capability of implementing a Controlled-NOT quantum logic gate using molecular clusters... 

Crystalline heavy lanthanide elements and many compounds have been particularly instructive and appropriate for magnetization studies due to their well defined 4f local moment where the net moment is generally that of the ground state spin-orbit or Russell-Saunders coupled magnetic ion. A thorough discussion of these magnetic properties is found in other chapters of this book, ch. 6 sections 1-4 and ch. 14 sections 1-5. 

In summary, NMR techniques based upon chemical shifts and dipolar or scalar couplings of spin-1/2 nuclei can provide structural information about bonding environments in semiconductor alloys, and more specifically the extent to which substitutions are completely random, partially or fully-ordered, or even bimodal. Semiconductor alloys containing magnetic ions, typically transition metal ions, have also been studied by spin-1/2 NMR here the often-large frequency shifts are due to the electron hyperfine interaction, and so examples of such studies will be discussed in Sect. 3.5. For alloys containing only quadrupolar nuclei as NMR probes, such as many of the III-V compounds, the nuclear quadrupole interaction will play an important and often dominant role, and can be used to investigate alloy disorder (Sect. 3.8). 

The values of the electron relaxation rates of the coupled metal ion strongly depend both on the relative electron relaxation rates of the isolated ions and on the value of the magnetic coupling constant J. When the absolute value of J (expressed as J /K) is smaller than both electronic relaxation rates, no effect on the electronic relaxation of the pair is expected. When J /H > (electron relaxation rate of the first ion) but smaller than T[ 2) (electron relaxation rate of the second ion), from first order perturbation... 

The TSP interface is typically combined with quadrupole MS, but coupling with ion-trap (91) or magnetic sector MS (92) has been also reported. Drawbacks of LC-TSP-MS are the requirements for volatile modifiers and the control of temperature, particularly for thermolabile compounds. Lack of structural information from LC-TSP-MS applications can be overcome by the use of LC-TSP-MS-MS. Use of this tandem MS approach provides enhanced selectivity, generally at the cost of a loss of sensitivity as a consequence of a decreased ion transmission. 

The band at 22000 cm-1 (anomalous because other Cr complexes of Cl- and Br- do not absorb strongly here) has been assigned to simultaneous excitation of a pair of antiferro-magnetically coupled Cr11 ions so that the absorption is at ca. 2x11500 cm-1. 

Axial symmetry according to Bleaney s approach is maintained in polymetallic lanthanide complexes when the metals lie on the molecular threefold or fourfold axes. For n magnetically non-coupled lanthanide ions packed along the symmetry axis, contact and pseudo-contact contributions can be considered as additive and the original model-free equation (eq. (47)) is transformed into eq. (61) in which the sum runs over the n paramagnetic centres, each being located at the origin of its own reference frame associated with a specific set of axial coordinates O 1 and r" (the z axis corresponds to the molecular symmetry axis, fig. 54),... 

In the Linnett, weak-field model of an octet, the spin-densities off opposite comers of the octet are of opposite signs, Fig. 32. This suggests that two, identical magnetic ions coordinated colinearly by an oxide ion might be coupled antiferromagnetically, Fig. 33, as, in fact, has been... 

Figure 3.16 Energy level diagram for ferric iron matched to spin-forbidden crystal field transitions within Fe3+ ions, which are portrayed by the polarized absorption spectra of yellow sapphire (adapted from Ferguson Fielding, 1972 Sherman, 1985a). Note that the unassigned band at -17,600 cm-1 represents a paired transition within magnetically coupled Fe3+ ions located in adjacent face-shared octahedra in the corundum structure.

The azide compound exhibits a slightly reduced magnetic moment (/iejr = 4.78 BM) 535 this suggests the presence of antiferromagnetic coupled Mn1" ions (via an bridge) in this compound. 

In these equations S is the total electron spin of the paramagnetic ion, r is the electron-nuclear distance, co is the electron resonance frequency, and x i - and x 2 are the rate constants for the reorientation of the coupled magnetic moment vectors. They are related to other rate constants by the expressions ... 

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