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The technological demand to push the gigahertz switching speed limit of today’s magnetic memory/logic devices into the terahertz (1THz=1ps−1) regime underlies the entire field of spin–electronics and integrated multi- functional devices. This challenge is met by all–optical magnetic switching based on coherent spin manipulation By analogy to femto–chemistry and photosynthetic dynamics where photo-products of chemical/biochemical re- actions can be influenced by creating suitable superpositions of molecular states, femtosecond (fs) laser–excited coherence between spin/orbital/charge states can switch magnetic orders, by “suddenly” breaking the delicate balance between competing phases of correlated materials, e.g., the colossal magneto–resistive (CMR) manganites suitable for applications. Here we discuss femtosecond (fs) all-optical switching from antiferro- to ferromagnetic ordering via establishment of a magnetization increase within ∼100 fs, while the laser field still interacts with the system. Such non-equilibrium ferromagnetic correlations arise from quantum spin–flip fluctuations corre- lated with coherent superpositions of electronic states. The development of ferromagnetic correlations during the fs laser pulse reveals an initial quantum coherent regime of magnetism, clearly distinguished from the pi- cosecond lattice-heating regime characterized by phase separation. We summarize a microscopic theory based on density matrix equations of motion for composite fermion Hubbard operators, instead of bare electrons, that take into account the strong spin and charge local correlations. Our work merges two fields, femto-magnetism in metals/band insulators and non–equilibrium phase transitions of strongly correlated electrons, where local interactions exceeding the kinetic energy produce a complex balance of competing orders.
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