Micromagnetic Study Of Soft/hard Magnetic Composite Particles Based On The Exchange Coupling

L1₀-Fe Pt with high uniaxial magnetocrystalline anisotropy constant?K_u?has been currently considered as one of the promising materials to realize the ultrahigh density magnetic recording.However,the coercivity of L1₀-Fe Pt exceeds the highest writing field supplied by current magnetic heads,which is needed to solve in the field of magnetic recording.The exchange coupled soft/hard magnetic medium can effectively reduce the magnetic coercive force and ensure the thermal stability.In addition,the noise of magnetic storage can be effectively reduced by the bit patterned media.In this paper,the soft/hard magnetic composite particles based on L1₀-Fe Pt were studied by the micromagnetic simulation.The soft/hard magnetic composite particles with various structures and the microwave-assisted magnetization reversal of the double layer magnetic composite media are investigated.For the multilayer structure with K_u?L1₀-Fe Pt?less than 5×10⁶ J/m³,the coercivity of L1₀-Fe Pt can be reduced and the hysteresis loops are characteristic of uniform reversal due to the strong exchange coupling interaction between soft and hard magnetic phase.When K_u?L1₀-Fe Pt?is more than 5×10⁶ J/m³,the additional step appears in the hysteresis loops of composite particles,which causes the two-phase reversal behavior.As the soft phase thickness increases,the coercivity can be reduced further.However,the step cannot be eliminated as the soft phase thickness increases.In order to eliminate the additional step in the hysteresis loops,the core-shell composite particles are studied.The coercivities of the structure B and C can be reduced effectively,which is smaller than the highest writing field.In addition,the hysteresis loops are characteristic of uniform reversal.Compared the simulations of the structure B and C,the remanent magnetization can be enhanced in the structure C.As the soft phase thickness increases,the coercivity can be reduced further and the whole system transforms from the rigid coupling state to the exchange spring state.For the structure B and C,the stability of composite particles under the stray field from the magnetic head is investigated.With the increasing of K_u?L1₀-Fe Pt?,the magnetic reversal process transforms from the Stoner-Wohlfarth model to the Kondorsky model.For the structure B with K_u?L1₀-Fe Pt?=1×10⁶ J/m³,the magnetic reversal process transforms from the Stoner-Wohlfarth model to the Kondorsky model as the soft phase thickness increases.Microwave-assisted magnetization can also reduce the coercivity by the ferromagnetic resonance phenomenon.For the double layer Co/L1₀-Fe Pt composite particle,there are three resonance peaks in the magnetic spectrum,respectively corresponding to the soft phase,the soft and hard magnetic interface,the hard phase.The magnetic moment of the double layer composite particle can be switched under the microwave-assisted field with appropriate frequency and field strength.With the increase of the soft magnetic thickness,the resonance peak of the soft magnetic layer moves to the low frequency direction.Hence,magnetization switching occurs for much lower frequency and field strength in the phase diagram of microwave-assisted magnetization reversal.