ORBITALLY DEPENDENT SUPEREXCHANGE IN MIXED-VALENCE CYANO-BRIDGED Mn(III)-Mn(II) DIMER. A NEW PERSPECTIVE FOR SINGLE MOLECULE MAGNETS

A.V.Palii

Institute of Applied Physics,

Academy of Sciences of Moldova, Academy Str. 5, MD-2028 Kishinev, Moldova

e-mail:

The model of the orbitally dependent magnetic exchange in the mixed-valence bioctahedral Mn(III)-CN-Mn(III) dimer is developed. The kinetic exchange mechanism involves the electron transfer from the single occupied t2 orbitals of the Mn(II) ion ( ground state) to the single occupied t2 orbitals of the Mn(III) ion (ground state) resulting in the charge transfer state of the pair. The deduced effective exchange Hamiltonian leads to an essentially non-Heisenberg energy pattern. The energy levels are shown to be dependent on both spin and orbital quantum numbers providing thus the direct information about the magnetic anisotropy of the system. Along with the magnetic exchange the model includes the axial component of the crystal field and the spin orbit coupling operating within the ground 3T1(t24) cubic term of the Mn(III) ion. We have shown that under some conditions the interplay between these three interactions leads to the appearance of the barrier for the reversal of magnetization, so the results obtained can be regarded as a first step in the explanation of the magnetic bistability exhibiting by the recently synthesized trigonal bipyramidal cyanide cluster {[MnII(tmphen)2]3[MnIII(CN)6]2} (tmphen = 4,5,7,8-tetramethyl-1,10-phenantroline).

  1. INTRODUCTION

Molecules that exhibit magnetic bistability, commonly referred to as Single-Molecule Magnets (SMM), are of high interest due to their unusual physical properties and potential importance for high-density data storage and quantum computing (1). To date, almost all the molecules firmly established as displaying SMM behavior incorporate oxide-based bridging ligands that mediate the magnetic exchange coupling between metal centers. A remarkable feature of these systems is that in them all orbital angular momenta are quenched by the local low-symmetry crystal fields, so the oxo-bridged SMMs can be referred to as pure spin systems. Such molecules possess a large total spin ground state (S) formed by the isotropic Heisenberg magnetic exchange, which, when combined with a negative axial zero-field splitting (DS <0), leads to the appearance of the energy barrier for spin reversal.

Recently in the interest of producing clusters with larger spin reversal barriers, the trigonal bipyramidal cyano-bridged cluster [MnIII(CN)6]2[MnII(tmphen)2]3 (tmphen = 4, 5, 7, 8 – tetramethyl–1, 10–phenanthroline) was synthesized and characterized, ref. (2). The observed ac-susceptibility signal indicates that this cluster (hereunder abbreviated as Mn5-cyanide cluster) represents a new SMM.

The Mn(III) ions in the Mn5-cyanide cluster occupy almost perfect octahedral sites. As a result the strong cubic crystal field produced by six carbon ions leads to the orbitally degenerate ground term of the Mn(III) ion. Insofar as this state possesses the unquenched orbital angular momentum the system under consideration is drastically different from the classical oxo-bridged SMMs consisting of orbitally non-degenerate ions. The first difference is that the conventional Heisenberg-Dirac-Van-Vleck Hamiltonian fails when one deals with the Mn5-cyanide cluster containing the Mn(III) ions with unquenched orbital angular momenta. In fact, the exchange Hamiltonian of the Mn5-cyanide cluster should involve not only spin but also orbital operators, i. e., such magnetic exchange proves to be orbitally dependent. The most important feature of the orbitally dependent exchange is that it is highly anisotropic, refs. (3-5). The second difference is a significant (first order) single ion anisotropy that can be expected in the Mn5-cyanide cluster. Such anisotropy represents a first order effect with respect to the spin-orbit coupling and axial component of the crystal field acting on each Mn(III) ion, i. e., this single ion anisotropy can not give rise to a global anisotropy described by the second order Hamiltonian . One can expect that both exchange anisotropy and single ion anisotropy contributes to the global magnetic anisotropy responsible for the formation of the barrier for the reversal of magnetization in the Mn5-cyanide cluster.

In this paper we endeavor to develop a model that would be able to qualitatively explain the existence of the barrier for the reversal of magnetization in the Mn5-cyanide cluster. In order to avoid complications implied by the consideration of polynuclear the cluster entire and to make the results more clear and transparent we will restrict ourselves by considering the Mn(III)-CN-Mn(II) pair, that seems to retain the main peculiarities inherent in the entire Mn5-cyanide cluster. In fact, the analysis of the structure of the Mn5-cyanide cluster shows that only the superexchange interaction between Mn(II) and Mn(III) ions through the cyanide bridges can be significant, meanwhile the interactions between two Mn(II) ions and two Mn(III) ions are negligible due to the large intermetallic distances. The model includes the orbitally dependent superexchange mediated by cyanide bridge, as well as the spin-orbit coupling and axial crystal field operating within the ground state of the Mn(III) ion. The results obtained in the framework of this model can be considered as the first step in the understanding of the magnetic behavior of the Mn5-cyanide cluster.

2. HAMILTONIAN OF THE KINETIC EXCHANGE BETWEEN ORBITALLY DEGENERATE IONS

Let us consider the kinetic exchange between two octahedrally coordinated transition metal ions A and B assuming that both ions are in the ground states ( and terms). We focus on the particular case when one or both ground terms are orbitally degenerate. Kinetic exchange appears as a second order contribution with respect to the following intercenter one-electron transfer operator playing a role of perturbation:

(1)

where the operator creates (annihilates) electron on the orbital of the ion B (A) with spin projection , or e, label the one-electron basis, and is the hopping integral. Hereunder we will consider a corner shared bioctahedral dimer and use the real one-electron cubic basis related to the cubic local coordinate frames, with zA (zB) axes being directed along C4 axis of the pair. This means that and will run over (t2-basis) and , (e-basis). The operator, eq. (1), connects the ground state with the excited charge transfer (CT) states arising from the electronic configurations in which one electron is transferred from the site A(B) to the site B(A).

The general approach to the problem of the magnetic exchange between orbitally degenerate ions is outlined in our recent papers, refs. (3)-(5) and will not be repeated here. Application of this approach leads to the following general expression for the kinetic exchange Hamiltonian operating within the ground manifold of the pair:

(2)

In eq. (2) are the Clebsch-Gordan coefficients for the group, are the cubic irreducible tensor operators acting within the orbital manifold of the center i (i = A, B ) and are the single ion spin operators. The operators are defined in such a way that their reduced matrix elements , where is the dimension of , so the matrix elements of these operators coincide with the Clebsch-Gordan coefficients appearing in the Wigner-Eckart theorem , ref. (12).Finally, the parameters and are expressed in terms of the energies of the CT states, the general formulas for these parameters are given in ref. (4).

  1. EXCHANGE MODEL FOR A Mn(III)-CN-Mn(II) PAIR

Hereunder we will apply the general formalism outlined in the previous Section to the cyano-bridged Mn(III)-CN-Mn(II) pair in which the Mn(III) ion is surrounded by six carbon ions, and the Mn(II) ion is surrounded by six nitrogen ones. As a first step we assume that both metal ions are in a perfect octahedral ligand fields. The ground term of the Mn(III) ion in a strong cubic field produced by the carbon atoms is expected to be the low-spin orbital triplet 3T1(t24). On the contrary, weak crystal field induced by the nitrogen atoms gives rise to a ground high-spin orbital singlet of the Mn(II) ion, so the ground state of the bioctahedral corner-shared dimer (overallsymmetry) will be .

In order to apply the general formula for the exchange Hamiltonian, eq.(2), to the Mn(III)Mn(II) pair we will assign the indices A and B to the Mn(II) and Mn(III) ions, respectively, so that and . Insofar as the transfer of the electron from the site B to the site A leads to the CT states with very high excitation energies we will neglect such processes and consider only the electron transfer. There are two possibilities for the electron transfer, namely, the transfer from the single occupied orbitals of the Mn(II) ion to the single occupied t2 orbitals of the Mn(III) ionthrough the bonding and antibonding orbitals of the cyanide ion, and the transfer from the single occupied e orbitals of Mn(II) to the empty e orbitals of Mn(III) through the cyanide -orbitals (the hopping parameters corresponding to the transfer are expected to be negligible due to the orthogonality of t2 and e orbitals). At the same time recent density functional theory calculations of the exchange parameters in cyano-bridged species (6) demonstrated that the interaction through the the cyanide -orbitals was significantly smaller compared to the interaction through the and orbitals (see also (7) and refs. therein). That is why only transfer processes are assumed to be important and will be taken into account in our exchange model. It is easy to see that the overlap between -type t2 orbitals of Mn(II) and Mn(III) through the and orbitals of cyanide bridge is strong, and the same overlap takes place between orbitals. So there are two equivalent hopping parameters associated to these overlaps (Fig.1). At the same time the integral can be omitted because there is no effective overlap between orbitals. Note that the transfer can not affect

the subshell of the ion A ( -state). At the same time this transfer decreases the spin of the ion A by . The analysis of the Tanabe-Sugano diagrams (8) for and ions shows that the only appropriate state for the oxidized configuration of the ion A is the state . Analogously, the reduced configuration of the ion B gives rise to the only state . We thus arrive at the conclusion that the transfer results in the only CT state . It is remarkable that the single-ion states involved in this CT state are the “pure” states, each resulting from the only electronic configuration. For this reason, no complications implied by the Coulomb mixing of different electronic configuration can appear in the kinetic exchange problem under consideration.

The orbital schemes for the [Mn(II)]A[Mn(III)]B pair (ground state) and[Mn(III)]A[Mn(II)]B pair (CT state), and the electron transfer process connecting these states are shown in Fig.2. It is to be noted that each orbital scheme depicts only one Slater determinant (microstate) of the many-electron open shell wave-function. For example, the only determinant involved in the two-determinant wave-function of the low-spin Mn(III) ion is shown in Fig.2. On the contrary, the state is represented by the only microstate, so the corresponding orbital scheme in this case shows the full wave-function of the high-spin Mn(II) ion. The jumping electron does not change its spin projection and selects the initial and final microstates as exemplified in Fig.2. Now it is a straightforward work to adapt the general expression for the kinetic exchange Hamiltonian, eq.(2), to the Mn(III)-CN-Mn(II) pair under consideration. Substituting the relevant values of the Clebsch-Gordan coefficients

into eq.(2) one finds:

(3)

While deducing eq. (3) it has been taken into account that and for all , so (only spin operators act within the orbitally non-degenerate ground state of the Mn(II) ion). The orbital operators and are represented by the following matrices in the cubic basis :

(4)

The parameters in eq. (3) can be found with the aid of the approach developed in (3-5); the results are the following:

(5)

where is the energy of the excitation.

The T-P-isomorphismmakes it possible to consider the ground term of the Mn(III) ion (ion B) as a state possessing fictitious orbital angular momentum (8). This allows us to express the cubic irreducible tensor in eq. (3) in terms of the orbital angular momentum operator acting within the basis ( is the projection of the fictitious orbital angular momentum) as follows:

(6)

Substituting eqs. (5) and (6) into eq. (3) we arrive at the following final formula for the kinetic exchange Hamiltonian of the Mn(III)-CN-Mn(II) pair:

.(7)

This Hamiltonian is essentially non-Heisenberg and includes both spin and orbital angular momenta operators (orbitally-dependent exchange).

4. RESULTS AND DISCUSSION

The Hamiltonian, eq. (7), proves to be isotropic in the spin subspace and axially symmetric in the orbital subspace, so that (total spin of the pair and its projection) and are the good quantum numbers. The eigenvalues of are calculated as follows:

(8)

The energy pattern (formed by the magnetic exchange) of the Mn(III)-CN-Mn(II) pair contains two superimposed groups of the energy levels with and (see Fig.3). The total spin of the pair takes the values , and the energy levels within each group obey the Lande’s rule. The exchange splitting of both and multiplet proves to be antiferromagnetic . The conclusion about the antiferromagnetic exchange splitting in each group of the energy levels is in agreement with the underlying ideas of Anderson, ref. (9) and Goodenough and Kanamori (see ref. (10) and refs. therein). In fact, these authors indicated that the electron hopping between the half-occupied orbitals should result in the antiferromagnetic exchange coupling.

It is to be underlined that the Lande’s rule is not valid for the whole energy pattern, particularly, the non-monotonic alternation of the levels with and takes place. Another important result is that the energy levels depend not only on the total spin

quantum number but also on . This leads to the interesting peculiarities in the magnetic behavior of the system. In the magnetic field applied parallel to the axis of the bioctahedron the orbital contribution to the Zeeman splitting of the ground level is significant

(first order effect) because the operator possesses the following nonvanishing matrix elements within the ground level:

(9)

In eq. (9) is the orbital reduction factor, sign “minus“ appears due to the fact that the matrix elements of within and bases are of the opposite signs, ref. (8). On the contrary, the orbital contribution to the Zeeman splitting of the ground level in a perpendicular field is much smaller because it appears as a second order effect due to the mixing of the ground level with the second excited level (, ) by the operator (Van Vleck paramagnetism). Therefore, as distinguished from the Heisenberg magnetic exchange, the orbitally dependent exchange interaction described by the Hamiltonian, eq. (7), produces the strong magnetic anisotropy of the pair.

Now in order to make our consideration more realistic we will take into account the fact that the nearest surrounding of the Mn(III) ion is axially distorted. In this case the operator of the axial crystal axial crystal field acting on the Mn(III) ion should be added to the magnetic exchange Hamiltonian. The operator can be defined as follows:

,(10)

where is the parameter of the axial field. This interaction splits the ground term of the Mn(III) ion into the orbital doublet (orbital basis ) and the orbital singlet () in such a way that the orbital singlet (orbital doublet) becomes the ground state providing . Finally, one should also take into account the spin orbit (SO) coupling acting within the term of the Mn(III) ion, the corresponding operator is given by

,(11)

where is the many-electron SO coupling parameter for the term , ref. (11).

We will consider the most typical situation when the axial field is strong significantly exceeding both the magnetic exchange and the SO coupling ( ). Provided that the strong axial crystal field totally removes the orbital degeneracy

giving rise to the pure spin system with the Heisenberg-type pattern of the low-lying energy levels. We will not consider this trivial situation and focus on the analysis of more interesting

(from the point of view of the barrier for the reversal of magnetization) case when . Strong negative axial field that results in a strong destabilization (by the value ) of thesubset of theenergy levels (formed by the magnetic exchange) with . The SO coupling splits the levels belonging to the low-lying subset with and mixes thelevels with different values belonging to this subset. In addition the SO coupling mixes the