Magnetic Anisotropy Measurement Based On Broadband Ferromagnetic Resonance And Its Application

Perovskite oxides,which were a big family of materials have a uniformed chemical formula of ABO₃,have a wide variety of magnetic and conduction properties dependent on the actual type of cations A and B.The various choice of A and B cations and the flexibility of structures makes it easy to modulate the properties of those materials by the construction of heterojunctions,and then introduce novel properties other than single layer film.As a typical perovskite material,strontium doped lanthanum manganate La_2/3Sr_1/3Mn O₃(LSMO)has a relatively high Curie temperature(T_C)among all the known perovskites.The room temperature ferromagnetic,spin half-metallic nature,colossal magneto-resistance effect and relatively low Gelbert damping fact(?)make the LSMO suitable for spintronic devices,and the recent study about oxide-based synthetic anti-ferromagnets further improve the usage of those materials.Very recently,unexpected inverse hysteresis loops(IHL)was found in epitaxial LSMO single layer film on Sr Ti O₃(STO)substrate,and then was attributed to the antiferromagnetic coupling between LSMO film and a spontaneously formed hard magnetic interfacial layer which was located between the STO substrate and the main phase of LSMO.The presence of IHL in LSMO as well as other interfacial coupling related phenomenon enhance the understanding about the magnetism of LSMO films.Ferromagnetic resonance(FMR)was an effectively methods to probe the magnetic anisotropy,interfacial coupling and high-frequency characteristics,however,the coexist of in-plane magnetic anisotropy and interfacial coupling in those oxide hetero-structures give rise to the difficulty on the FMR frequency spectrum analyses.This work proposes a new method to investigate the in-plane magnetic anisotropy by a broad-band FMR.By this method,only three different directions were taken into account during the measurement,hence the measurement will be greatly simplified.The magnetic anisotropic energies terms can be separated from each other by comparing the values of magnetic anisotropic effective field along the different axis,and then a rotational measurement no longer required to obtain the in-plane anisotropic energies.In particular,other than give an effectively method to investigate the in plane anisotropy of epitaxial films by CPW based FMR spectrometers,the new method using the frequency dependent signals to investigate both the in-plane magnetic anisotropy and the interlayer coupling or Gilbert damping.Hence,this method might be helpful to promote the researches about the interfacial coupling between epitaxial film and anisotropy damping.Additionally,the intrinsic magnetic parameters measured by this new designed method has been used to explain the IHL phenomenon found in LSMO film.Contrast analysis of minor loops and major loops confirmed the existing of hard magnetic interfacial layer,interfacial coupling between hard magnetic interfacial layer and the soft magnetic LSMO layer give rise to exchange bias in minor loops,and this effect will behave as IHL in major loops.Temperature dependent of the minor loops indicate that the coercivity of loops increasing with the temperature drops while the exchange-bias field kept constant,and give a clear explanation on the phenomenon that the IHL vanished in low temperature.Intrinsic magnetic parameters measured by FMR further explanation that the coercivity associated with the magneto-crystalline anisotropy,and the exchange-bias field was related to the bending field of exchange spring structure in soft magnetic LSMO film.