Abstract
We propose a new model for interpreting the magnetic interactions in crystals with mosaic texture called the mosaic anisotropy (MA) model. We test the MA model using hematite as a model system, comparing mosaic crystals to polycrystals, single crystal nanoparticles, and bulk single crystals. Vibrating sample magnetometry confirms the hypothesis of the MA model that mosaic crystals have larger remanence (Mr/Ms) and coercivity (Hc) compared to polycrystalline or bulk single crystals. By exploring the magnetic properties of mesostructured crystalline materials, we may be able to develop new routes to engineering harder magnets.
Highlights
One such class of architectures is that of mesostructured crystalline materials, including mosaic crystals, whose crystalline order falls between the perfect order of a single crystal and the random order of a polycrystal.[7,8]
Though many examples have reported mesostructured crystals made from magnetic materials, there has been limited characterization of their bulk magnetic properties and there is no general framework for understanding the long-range magnetic interactions in these materials.[21,22,23]
We describe a generalized model for the magnetic interactions within one type of hierarchical architecture, mosaic crystals, in which the adjacent crystalline domains are separated by small angle (
Summary
Biomineralization produces materials with hierarchical architectures that impart improved functionality compared to their non-biogenic counterparts.[1,2] Much research has focused on translating biomineral growth strategies into the laboratory to produce crystalline materials with complex architectures.[3,4,5,6] One such class of architectures is that of mesostructured crystalline materials, including mosaic crystals, whose crystalline order falls between the perfect order of a single crystal and the random order of a polycrystal.[7,8] Numerous studies make the critical connection between control of the crystallographic texture and manipulation of the mechanical, catalytic, or electronic properties.[8,9,10,11,12,13]. Hematite makes an excellent model system, both because it is a well-studied magnetic material and because mosaic crystals of hematite have already been prepared synthetically and characterized thoroughly.[46,47,48,49] The two key features of the magnetic properties of bulk hematite are its canted antiferromagnetic order at room temperature and its spin-flop transition to collinear antiferromagnetism (AFM), called the Morin transition, at ∼260 K.50 The canted antiferromagnetic order at room temperature endows hematite with a large pseudo-uniaxial magnetocrystalline anisotropy.[28] due to the canting, the net ferromagnetic moment in hematite is tiny (∼0.01 uB/Fe3+), meaning there is little driving force to break into domains and Lex is ∼10-100 μm.[51,52] This huge ferromagnetic exchange length means that only large-grained polycrystalline hematite will not experience reduced effective anisotropy as described by the random anisotropy model.
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