Possibility of type II magnetism in some mixed magnets and spin glasses

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Abstract
A phenomenological Ginzburg-Landau free energy is introduced to describe mixed magnets or spin glasses with two order parameters, the bulk magnetisation and the spin glass order parameter. The susceptibility has a cusp at the onset of the spin glass phase. To the two order parameters are associated two coherence lengths and, depending on their ratio, two types of mixed magnetic states can occur: (i) the conventional mixed magnetism with macroscopic domains, and (ii) a new type of magnetism for which the energy of the domain walls becomes negative, in strong analogy with type II superconductivity. It is argued that MnSi and the alloys PdMn and PdFe in limited ranges of concentration are magnets of the second type.