Tuning the magnetic ground state of a tetranuclear nickel(II) molecular complex by high magnetic fields

Electron spin resonance and magnetization data in magnetic fields up to $55\mathrm{T}$ of a multicenter paramagnetic molecular complex $\left[L_2{\mathrm{Ni}}_4\left({\mathrm{N}}_3\right){\left({\mathrm{O}}_2\mathrm{C}\mathrm{Ada}\right)}_4\right]\left(\mathrm{Cl}{\mathrm{O}}_4\right)$ are reported. In this compound, four Ni centers each having a spin $S=1$ are coupled in a single molecule via bridging ligands (including a ${\mu }_4$-azide) which provide paths for magnetic exchange. Analysis of the frequency and temperature dependence of the ESR signals yields the relevant parameters of the spin Hamiltonian, in particular the single ion anisotropy gap and the $g$ factor, which enables the calculation of the complex energy spectrum of the spin states in a magnetic field. The experimental results give compelling evidence for tuning the ground state of the molecule by magnetic field from a nonmagnetic state at small fields to a magnetic one in strong fields owing to the spin level crossing at a field of $\sim 25\mathrm{T}$.

Figure 1

Experimental scheme for ESR measurements in static (a) and pulsed magnetic fields (b), respectively. See text for details.

Figure 2

Molecular structure of ${\mathrm{Ni}}_4$-complex $\left[L_2{\mathrm{Ni}}_4\left({\mathrm{N}}_3\right){\left({\mathrm{O}}_2\mathrm{C}\mathrm{Ada}\right)}_4\right]\left(\mathrm{Cl}{\mathrm{O}}_4\right)$ The four Ni(II) ions are connected by the central azide, pyrazolate, and carboxylate bridges. See Ref. 7 for details.

Figure 3

(Color online) (a) ESR spectra of NiAz at $\nu =332\mathrm{GHz}$ for different selected temperatures. Four thermally activated resonance peaks are indicated by the arrows and numbered from 1 to 4, respectively. A sharp resonance signal at $11.8\mathrm{T}$ is the DPPH field marker. (b) Temperature dependence of the areas $I_i$ under the resonance peaks $i=1$ to 4 and the total area $I_{\mathrm{tot}}$, respectively.

Figure 4

Magnetic susceptibility $\chi$ and $\chi T$ of NiAz measured in an external field of $0.5\mathrm{T}$, filled and open circles, respectively (Ref. 7. The solid lines represent a numerical fit using the Hamiltonian (1). In this fit a temperature independent contribution ${\chi }_0=8.08\times {10}^{-4}\mathrm{emu}∕\text{mole}$ as well as a Curie-like impurity contribution corresponding to $\sim 2\%$ of “defect” Ni(II) paramagnetic sites which is responsible for a Curie tail below $20\mathrm{K}$ are included.

Figure 5

(Color online) ESR spectra of NiAz at $T=35\mathrm{K}$ at different selected excitation frequencies $\nu$. Four resonance modes are indicated by the arrows. Sharp lines are due to the DPPH field marker.

Figure 6

(Color online) Resonance branches at $T=35\mathrm{K}$ [including pulsed field ESR at $739\mathrm{GHz}$ $\left(35\mathrm{K}\right)$ and $1017\mathrm{GHz}$ $\left(4.2\mathrm{K}\right)$]. Branches 1 and 2 have a finite frequency offset ${\Delta }_1$ whereas branches 3 and 4 can be extrapolated to zero frequency at zero magnetic field. Note that resonance 2 at $1017\mathrm{GHz}$ is visible at $4.2\mathrm{K}$ owing to the spin level crossing. Solid lines are guides for the eye.

Figure 7

(Color online) ESR spectra of NiAz in pulsed magnetic field at $739\mathrm{GHz}$. Resonance modes 2, 3, and 4 indicated by thin lines are visible at elevated temperatures only.

Figure 8

(Color online) ESR spectra of NiAz in pulsed magnetic field at $1017\mathrm{GHz}$. Note that at $4.2\mathrm{K}$ a new broad line with a peak at about $32\mathrm{T}$ appears in the spectrum.

Figure 9

(Color online) Field dependence of the static magnetization $M\left(H\right)$ in pulsed magnetic field. Note a steplike increase of $M$ at $\sim 25\mathrm{T}$ and a linear background indicated by dot lines in the field range $0–15\mathrm{T}$ and $30–50\mathrm{T}$, respectively. The field derivative of the magnetization $dM∕dH$ for $T=1.45\mathrm{K}$ is shown in the inset.

Figure 10

Proposed exchange paths between Ni spin centers in NiAz. See text for details.

Figure 11

(Color online) Low lying energy levels, calculated for a single crystal with the magnetic field perpendicular to the z axis: the triplet $S=1$ and the quintuplet $S=2$ are well separated from the $S=0$ ground state by an activation energy of $E_{0,1}=37\mathrm{K}$ and $E_{0,2}\simeq 3E_{0,1}$, respectively. Both multiplets exhibit a zero field splitting due to the anisotropic CF. Note the level crossing at zero energy around $25\mathrm{T}$ which yields the change of the ground state.