ANISOTROPIC EXCHANGE INTERACTION IN A SPIN-CANTED SINGLE CHAIN MAGNET BASED ON Co(II) IONS
Andrei Palii, Oleg Reu, Sergei Ostrovsky, Sophia Klokshner
Institute of Applied Physics, Academy of Sciences of Moldova, Kishinev, Moldova
Kim Dunbar
Department of Chemistry, Texas A&M University, College Station, TX, USA
Boris Tsukerblat
Chemistry Department, Ben-Gurion University of the Negev,
Beer-Sheva, Israel
We report a model of a single-chain magnet Co(H2L)(H2O) (L = 4-Me-C6H4-CH2N(CPO3H2)2) based on Co(II) ions with unquenched orbital angular momenta. The model includes a tetragonal crystal field, spin-orbit interaction acting within each Co(II) ion, an antiferromagnetic Heisenberg exchange between the Co(II) ions and the titling of the tetragonal axes of the neighboring Co units in the zigzag structure. The titling of the anisotropy axes gives rise to a spin canting and consequently to a non-vanishing magnetization of the chain. The effective pseudo-spin-1/2 Hamiltonian for the interaction between the Co ions in their ground Kramers doublet states proved to be of the Ising type. The model provides a perfect fit to the experimental data for static and dynamic magnetic properties of the chain.
1. INTRODUCTION
One-dimensional systems, exhibiting magnetic bistability, commonly referred to as single chain magnets (SCMs) are of high interest due to their unusual physical properties and potential importance for high-density data storage and quantum computing [1,2]. During last years this branch of molecular molecular magnetism dealing with the 1D magnets is being intensively developed [2-13]. The background for the description of SCM is provided by the Glauber’s stochastic approach [14]. Glauber predicted the presence of a slow relaxation of magnetization in a chain composed of ferromagnetically coupled spins that can be described by the Ising Hamiltonian:
, (1)
where is the -component of the pseudo-spin operator. In Glauber’s theory of the thermal variation of the relaxation time is described by the Arrhenius low
(2)
in which the barrier to reverse the magnetization direction represents the energy loss in one spin flip-flop process, that is
. (3)
An Ising spin chain can behave as SCM if the constituent magnetic moments are not canceled. In majority of known SCMs this condition is satisfied either due to ferromagnetic interaction between identical spins or due to alternation of different antiferromagnetically coupled canted spins. Recently an unusual SCM composed of antiferromagnetically coupled metal ions with the canted spin structure has been reported [13]. This system represents the cobalt(II) diphosphonate Co(H2L)(H2O) compound (L = 4-Me-C6H4-CH2N(CPO3H2)2) in which the Co(II) ions are linked through a bridging phosphonate oxygen atom to create a 1D chain of corner-sharing octahedra which propagate in a zigzag fashion (Fig.1). This compound is the first SCM based on antiferromagnetically coupled homospin centers in which non-vanishing magnetization appears due to non-collinear spin structure (spin canting).
Fig. 1. A zig-zag chain structure of the Co(H2L)(H2O) compound. The 4-Me-C6H4-CH2-groups of the diphosphonate ligands have been omitted for the sake of clarity. The Co octahedral and CPO3 tetrahedral units are shaded in green and pink, respectively.The aim of this article is to give a quantum-mechanical description of the magnetic anisotropy and spin-canting phenomena in this system [15]. We elaborate a relatively simple model that incorporates the main sources of the magnetic anisotropy, namely strong axial crystal fields acting on the Co(II) ions, spin-orbital interaction and the topology of the chain. All these factors give rise to a canted spin structure and subsequently to an uncompensated magnetic moment. Finally, we demonstrate that the model perfectly agrees with the experimental data on the static and dynamic susceptibility.
2. THE MODEL
The two lowest terms of a free Co(II) ion arising from the 3 configuration are (ground) and separated from by the gap 15B (Racah parameter) that is typically about 15,000 cm-1 . Octahedral ligand field splits atomic level into two orbital triplets terms , and the orbital singlet in such a way that the orbital triplet becomes the ground state meanwhile the excited state results in the . In addition two terms (momenta ) are mixed by the cubic ligand field but the ground state that is mainly of character and contains also an admixture of .
Since the spin canting is closely related to the magnetic anisotropy the model should incorporate both spin-orbital interaction and local non-cubic crystal field. Let us first consider the Co(H2L)(H2O) unit (Fig. 2). The Co-ligand distances in the first coordination sphere of the Co(H2L)(H2O) unit (Fig. 2) are collected in the Table [13].
Fig. 2. ORTEP representation of the Co(H2L)(H2O) unit.
Table . Bond lengths [Å] and angles [deg] for the nearest Co surrounding in Co(H2L)(H2O) (data from ref. [13]).
______Co(1)-O(22)#1 2.035(5) Co(1)-O(1W) 2.098(5)
Co(1)-O(13) 2.134(5) Co(1)-O(23)#2 2.139(5)
Co(1)-O(23) 2.176(5) Co(1)-N(1) 2.282(6)
The nearest octahedral surrounding of the Co(II) ions is strongly distorted so that the actual symmetry is very low. But considering the fragment of the structure involving Co(II) and the nearest surrounding one can see that a simplifying assumption about tetragonal character of distortion can be made with a reasonable accuracy. The tetragonal axis is expected to coincide with the N-Co-O diagonal in the distorted hetero-ligand coordination sphere that can be approximately described by C4v point symmetry group. Let us assign the indices A and B to two octahedrally coordinated Co(II) ions which occupy non-equivalent crystallographic positions in a 1D chain. Let us introduce two local frames of reference (Fig. 3) related to ions A and B in the chain. We assume that the tetragonal local and axes subtend an angle , the and axes are chosen to be parallel to each other and perpendicular to -plane, while the axes and lie in the -plane. The local axes for B
center can be obtained from those related to A center axes by the turn through angle around or axis. A tetragonal component of the ligand field splits the ground term of the Co(II) ion in C4v symmetry into the orbital singlet and the orbital doublet . The axial ligand field and the spin orbit interaction is described by the following single-ion Hamiltonian [16, 17] :
(4)
where are the projections of the orbital angular momentum operators onto the local -axes, and the orbital reduction factor takes into account both the covalence effects and the mixing of and terms by the cubic crystal field. The factor in eq. (4) is conventionally introduced in the matrix of the angular momentum operator due to the fact that the matrix of within manifold coincides with the matrix of defined in the atomic -basis.
Fig. 3. Local and molecular co-ordinates.The tetragonal ligand field defined by the first term of eq. (4) stabilizes the term (state with ) in the case of positive tetragonal field and the term when . The spin- orbit coupling produces further splitting of these levels into the Kramers doublets.
In general the axial crystal field and spin-orbital interaction are to be taken account simultaneously. In order to elaborate a model that would incorporate these main physical factors and at the same time would be relatively simple we note that the geometry of a strongly distorted hetero-ligand surrounding of Co(II) ion in Co(H2L)(H2O) (Table) seems to provide an argument in favor of a simplified model based on the assumption of a strong axial field. In this context we will assume that the fields exceeds considerably the spin-orbital interaction, . We have to discuss which sign of is relevant . In the strong positive axial field limit the ground term is orbitally non-degenerate (conventionally, spin system) so that the first-order orbital angular momentum is quenched. However, the observed room temperature value can be considered as an indication of the presence of an unquenched orbital momentum of the Co(II) ion in the Co(H2L)(H2O) compound. Actually, the observed value of at 300K is 3.2 emu·mol-1 K is higher than the expected value = 1.875 emu·mol-1 K for a spin system. The case of seems to be incompatible with the observed SCM properties of the compound. In fact, such type of behavior can be observed in the chains composed of the magnetically coupled Ising spins. This means that the case of positive axial field can be excluded from the further consideration. Another argument in favor of relevance of the case of is discussed below (see Section 4). For this reason we focus on the case of when the ligand field stabilizes the orbital doublet .
Assuming that the axial field considerable exceeds the spin orbital coupling (axial limit) and neglecting the spin orbital mixing of the and terms we arrive at the scheme of the energy levels shown in Fig. 3. The spin orbit interaction takes an axial form with the only non-vanishing Z- component
within the ground term and leads to the splitting of this term into four equidistant Kramers doublets , ,, as shown in Fig. with the with state being the ground one. This is an immense simplification but we will show that the experimental data can be perfectly explained within a model so far discussed.
Along with the local frames we will also use the molecular coordinate frame chosen in such a way that the molecular Z axis is directed along the bisector of the angle formed by the local and axes while the Y axis of the molecular system coincides with the local and axes (Fig. 2). The full Hamiltonian of the Co(II) pair includes the intracenter interactions described by eq. (4) and the exchange interaction between Co(II) ions. In general, the interaction between orbitally degenerate ions is described by the so-called orbitally dependent exchange Hamiltonian Following the the approximation proposed by Lines [16,17] and discussed in the subsequent papers [18] dealing with the orbitally dependent superexchange in Co(II) we assume here that exchange interaction is represented by the isotropic Heisenberg-Dirac- Van Vleck (HDVV) Hamiltonian
, (5)
where J is the exchange parameter, the single ion spin operators and ( are the spins of a Co(II) ion) and their projections and refer to the molecular frame. It is convenient to pass from the operators , to the operators defined in the local frames [18].
(6)
Using these matrices one can present the exchange Hamiltonian as follows:
.(7)
The Hamiltonian in Eq. (7) acts within the full basis set formed by the ground state basis of the two Co(II) ions, i.e. direct product of two bases (144x144 matrix). The energy gap between the ground Kramers doublet with and the first excited one () is assumed to exceed the exchange splitting so at low temperatures we can restrict ourselves by considering only the ground
Fig. 3. Splitting of the ground cubic 4T1(3d7) term of the Co(II) ion by a tetragonal crystal field and spin-orbit coupling in the limit of strong negative tetragonal field (spin-orbital mixing of 4B2 and 4E terms is neglected).Kramers doublet for each Co ion All matrix elements of the operators , , and are vanishing within the basis set of the ground Kramers doublet.
(8)
and hence the Hamiltonian, eq. (7) dealing with the “true” Co(II) spins (s=3/2) is reduced to the Ising form for the pseudo-spins :
(9)
Although in the adopted model the mixing of the Kramers doublets by the exchange interaction is also assumed to be small this mixing is taken into account as the correction in the evaluation of the effective exchange parameter :
. (10)
The term proportional to in eq. (10) represents the second order correction arising from the mixing of the ground and excited manifolds of the cobalt pairs by the exchange interaction. One can see that the new exchange parameter reflects the geometry of the zig-zag chain through the angle meanwhile in the adopted approximation it is independent of axial crystal field. It is worthwhile to note that in the framework of the assumption so far adopted the effective exchange vanishes if the local axes are orthogonal and reach the maximum value in the linear geometry when the local axes coincide. This provides a possible receipt for a chemical control of the magnetic properties of these kind of 1D compounds. While deriving eq. (9) we passed from the true spin-3/2 operators , to the pseudo-spin-1/2 operators , . The pseudo-spin-1/2 basis is chosen in such a way that the component of the ground Kramers doublet level with corresponds to the projection of the pseudo-spin-1/2. Providing such choice the pseudo-spin-1/2 Hamiltonian in the presence of the external magnetic field is found to be [19]:
, (11)
where etc are the components of the magnetic field in the local frames. The principal values of the effective g –tensor for the Co(II) ion in their local surroundings are found as:
(12)
where the is related to the local Z-axes (along tetragonal axes) and to the local XY planes. One can see that the system is highly anisotropic and in particular first order Zeeman splitting disappears in the perpendicular field. The values
(13)
are the principal components of the tensor of the Van-Vleck temperature independent paramagnetism (TIP). Using these results we can write down the following total Hamiltonian for a chain including exchange and Zeeman terms:
, (14)
where index i numbers the pairs and the TIP contribution will be added later on.
In eq. (14) both the pseudo-spin operators and the components of the magnetic field are defined in the local frames. To get insight on the spin structure of the system one can pass to with the aid of the following relations
(15)
where , , , . Then the exchange Hamiltonian takes on the form