Research

The Berry Group has synthesized an unusual heterotrimetallic Mo≡Mo-Ni chain complex for which the following effect is observed: ferromagnetic alignment of δ-symmetry electrons at both the Mo₂ and Ni centers arising from delocalization of a σ-symmetry electron across the Mo≡Mo-Ni chain .^[1]

Heterometallic chain compounds Mo₂Ni(dpa)₄Cl₂ (compound 1) and [Mo₂Ni(dpa)₄Cl₂]OTf (dpa = 2,2’-dipyridylamine) (compound 2) were prepared and studied with SQUID magnetometry.

Crystal structures of compound 1 (left) and 2 (right.) Hydrogen atoms and any solvent molecules have been removed for clarity.

Magnetic susceptibility data for compound 1 and 2 are shown in the figure below. The χT value for 1 at 300 K, 1.09 emu*K*mol^-1, agrees with the spin-only value for an S=1 system of 1.0 emu*K*mol^-1. In the case of 2 the high temperature limit in our data is 1.75 emu*K*mol^-1, for which the only reasonable assignment is to an S=3/2 system (spin only value of 1.875 emu*K*mol^-1). Thus, the data for 1 and 2 were modeled as S=1 or S=3/2 systems, respectively. For both 1 and 2, a sharp downward feature is seen in the χT versus T plot below 50 K, indicative of zero field splitting. The data for 1 and 2 were fitted with g_iso= 2.053(3); |D| =9.19(3) cm^-1, and g_iso=1.916(3); |D| = 2.86(4) cm^-1, respectively.

Variable temperature magnetic susceptibility data for compound 1 (black squares) and 2 (red triangles).

Thus, in reporting the first example of a one-electron oxidized Mo₂M_B(dpa)₄Cl₂ monocation in which M_B represents a first-row transition metal, they find that the cation displays unusual, strongly ferromagnetic coupling affording an S=3/2 ground state that persists to room temperature. This conclusion is supported through EPR spectroscopy, SQUID magnetometry, as well as DFT computations. The ferromagnetism arises via a new mechanism in which a delocalized itinerant electron couples ferromagnetically to electrons in orthogonal orbitals in neighboring spin centers. The resulting magnitude of the coupling (J≥150 cm^-1) suggests promise for the use of this effect in the design of novel magnetic materials.

Jin Group reported a method for the low pressure, low temperature synthesis of both FeGe and Fe_1−xCo_xGe alloys with the cubic B20 structure. AC magnetic susceptibility analysis of Fe_0.95Co_0.05Ge provided clear evidence for the existence of a skyrmion phase observed in any Fe_1−xCo_xGe alloy compound for the first time. These results not only introduce a more facile synthetic method to make high quality cubic B20 FeGe and Fe_1−xCo_xGe materials without the need for a high pressure apparatus but also enable the construction of Fe_0.95Co_0.05Ge’s magnetic phase diagram. ^[2]

Magnetic property analysis and magnetic phase determination of Fe_0.95Co_0.05Ge. (a) DC and (b) AC magnetic field sweeps of Fe_0.95Co_0.05Ge. Transitions in the (b) AC field sweeps are characteristic of a skyrmion phase, and the relevant magnetic transitions have been highlighted with dashed lines. (c, d) Magnetic phase diagrams for (c) Fe_0.95Co_0.05Ge and (d) FeGe illustrating the pocket of stability for the skyrmion phase as well as the existence of the other relevant magnetic phases: helimagnetic, conical, field polarized, and paramagnetic (above T_c).

References:

[1] Chipman, Jill A., and John F. Berry. “Extraordinarily Large Ferromagnetic Coupling (J≥ 150 cm− 1) by Electron Delocalization in a Heterometallic Mo≣ Mo− Ni Chain Complex.” Chemistry-A European Journal 24.7 (2018): 1494-1499.

[2] Stolt, Matthew J., et al. “Chemical Pressure Stabilization of the Cubic B20 Structure in Skyrmion Hosting Fe1-xCoxGe Alloys.”Chemistry of Materials (2018): 1146-1154.