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45283-AC10
Porous and Biphasic Materials through Solid State Reactions

Ram Seshadri, University of California (Santa Barbara)

We are continuing our investigation of rapid, spontaneous routes to the self-assembly of mesoporous and biphasic materials. By utilizing novel chemical techniques and considering complex phase equilibria, we are establishing a toolkit for the production of composites with potential applications ranging from filtration and catalysis to magnetoelectric coupling and magnetic exchange bias. Here, we outline two of our approaches.

- Vapor-phase leaching

Last year, we described a hydrogen reduction process which converts dense, ceramic monoliths into hierarchically porous single-phase oxides. Extension of this procedure to ternary oxides such as NiMn₂O₄ yields two reduction processes below 700°C:

Since the starting material is homogeneous and the solid-state reaction occurs at relatively low temperatures, the pore size is around 100 nm and the Ni precipitates as 20-30 nm particles seen in Fig. 1(a). Most of these particles are encased within the MnO, rather than dotting the surface.

The metal nanoparticles and their intimate contact with the oxide monolith produce a highly interfacial composite. Because MnO is an antiferromagnet below its Néel temperature (T_N = 119 K) and Ni is a ferromagnet, we can probe the interface by characterizing the magnetic exchange bias between them.

Cooling below T_N in an applied 5 T magnetic field leads to irreversibility when the field direction is switched, and the observable result is a shift of the magnetic hysteresis loop along the field axis. A shifted loop can be seen in Fig. 1(b). The magnitude of the shift is the exchange bias field H_E. Fig. 1(c) shows the confirmation of exchange bias in our Ni/MnO composite: H_E appears at T_N and increases upon cooling. We can tailor H_E and H_C by choosing the starting composition of Ni_xMn_3-xO₄ with 0 < x ≤ 1.

Traditional routes to exchange biased nanocomposites have been uncontrolled partial reduction of oxide nanoparticles, high-energy ball milling, or high-cost thin film techniques with complex geometries and small volumes. Our process allows for rapid self-assembly of bulk exchange biased composites in pellet, film, or powder forms.

- Magnetically probing heterogeneous oxide solid solutions

Our investigations of highly interfacial oxide composites has led us to magnetically complex solid solutions containing Jahn-Teller distorting ions (mostly d⁴ Mn³⁺ or d⁹ Cu²⁺) in stretched MO₆ octahedra or MO₄ tetrahedra. Our focus lies on two spinel systems: (Cu_xMg_1-x)Cr₂O₄ and (Zn_xMn_1-x)Mn₂O₄, both with 0 ≤ x ≤ 1.

There is a strong preference for Cr to occupy the octahedral positions in the first spinel, so the solid solution ranges between zero and full occupation of the tetragonal sites by either Cu or Mg. The tetrahedral Cu²⁺ ions are Jahn-Teller active while the Mg²⁺ are not. CuCr₂O₄ is a tetragonal ferromagnet and MgCr₂O₄ is a cubic antiferromagnet. By changing the composition x, we can adjust the cubic-tetragonal transition from liquid helium temperatures to over 700°C.

Two opportunities are presented by this system: what is the magnetic response when two competing magnetic phases exist at low temperatures, and can we impart heterogeneity by quenching a two-phase sample to low temperatures?

Our preliminary neutron scatting studies observe the transition around 50 K in the composition (Cu_0.2Mg_0.8)Cr₂O₄. We hope to observe competing ferromagnetic Cu-rich regions and antiferromagnetic Mg-rich regions, which can produce exchange bias or glassy magnetism.

We are encouraged by our recent measurements of (Zn_xMn_1-x)Mn₂O₄, showing a strongly shifted exchange-biased magnetic hysteresis loop seen in Fig. 2. In this system, Mn occupies the octahedral sites and the tetrahedral sites vary from zero to full Zn or Mn occupation. Since the Jahn-Teller tendency on the tetrahedral site is suppressed, the driving force may be weaker for Zn clustering than for Cu clustering in the (Cu_xMg_1-x)Cr₂O₄ system.

- Personal Impact

Since receiving support from the PRF, Ram Seshadri has received tenure in the UCSB Materials Department. Eric S. Toberer has earned his PhD in Materials. He is currently in a postdoctoral position at Caltech and was awarded a Beckman Fellowship. Graduate student Daniel P. Shoemaker has been researching functional biphasic materials since Fall 2006 and will advance to candidacy in Fall 2008.

Figures

Fig. 1. (a) A Ni/MnO composite prepared by selective vapor-phase leaching shows Ni nanoparticles supported by mesoporous MnO. The magnetic hysteresis loop (b) is shifted and broadened along the field axis after field cooling below the Néel temperature (T_N) to 5 K. In (c), temperature dependence of the loop shift H_E and coercivity H_C indicates exchange bias below T_N = 119 K.

Fig. 2. Hysteresis loops of (Zn_xMn_1-x)Mn₂O₄ at 5 K show significant shifting along the field axis, indicative of exchange bias or glassy magnetism.

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