Hnin Yu Yu Ko

Magnetization behaviors for FeRh single crystal thin films

The structural and magnetic phase transition in FeRh thin films are investigated. The (001) oriented single crystal FeRh thin films fabricated onto MgO (100) substrate possess lattice parameter a smaller than c, because of the compressive stress along the a axis from MgO substrate. With increasing annealing temperature, paramagnetic FeRh thin films transform into ferromagnetic and then antiferromagnetic-ferromagnetic (AF-FM) stages. The transition temperature of AF-FM increases with annealing temperature, while the thermal hysteresis width decreases. The M‐H loops of the film annealed at 700°C show an opening at high magnetic fields during the transition state. Note that the opening in M‐H loops disappears when AF phase has transformed into FM phase. The origin of the opening is not known but possibly due to the combination of hysteresis loops of FM and AF phases, where FM phase is soft magnetic and the AF phase FeRh shows a hysteresis behavior with coercivity due to the reduction of magnetic anisotropy a...

Magnetic Properties of Single-Crystalline FeRh Alloy Thin Films

The structural and magnetic properties of single-crystalline Fe100-XRhX (26lesXles63) grown on MgO (100) substrates were systematically studied as a function of composition, temperature and applied magnetic field. The first-order magnetic phase transition of Fe100-XRhX was observed with the range of 51lesXles60. The temperature dependent magnetization behavior for all the samples under investigation is presented. The transition temperature for the change from anti-ferromagnetic (AFM) to a ferromagnetic (FM) state vary between 70 to 150degC. The change in entropy, DeltaS, estimated by the change in transition temperature with applied magnetic field during cooling process, decreases with increasing X and at Xcong62, it become zero. The observed hysteresis loop performed P-MOKE shows shaper transition as compared to VSM, indicates that surface magnetization distribution is different from the overall film.

Synthesis and magnetic properties of self-organized FeRh nanoparticles

Nanoparticles with various compositions of Fe–Rh have been fabricated by a solution phase chemical method and their magnetic properties characterized. After refining and size selection, the average particle size is about 2–5nm by high-resolution transmission electron microscopy observation. From x-ray diffraction results, as-deposited FeRh nanopaticles reveal a typical chemically disordered fcc structure which can be transformed into CsCl-type structure through thermal annealing. Under selected annealing conditions, the coercivities, Hc, for FeRh nanoparticles measured at room temperature are increased, compared with that of their as-deposited states. This increasement in Hc is due to the crystallization of CsCl-type structure after annealing. Nanoparticles of Fe36Rh64 and Fe38Rh62 annealed at 600°C for 6h show the presence of the magnetic phase transitions, while those of Fe75Rh25 particles did not exhibit such a transition.

Fabrication and characterization of FeRh nanoparticles

The magnetic and structural properties of FexRh100−x (x=35–80) nanoparticles fabricated by a solution phase chemical method have been studied systematically. From x-ray diffraction results, as-deposited nanoparticles of FeRh reveal a chemically disordered fcc structure which can be transformed into CsCl-type structure through thermal annealing. The high-resolution transmission electron microscopy observations of FeRh nanoparticles indicate that the average size of 3–5nm in as-deposited state and larger size (10–15nm) particles are also distributed in the annealed sate. Nanoparticles with x=35–55 exhibit the behaviors of magnetization with temperature, which suggests the transition from the AFM to FM phases at around 100–200°C, whereas those with x=80 do very little change of magnetization with temperature. It is also found that there seems an effect of applied magnetic field on transition, that is, the transition temperature is shifted to a lower side upon the application of magnetic field during the tran...

Fabrication and characterization of granular-type (FePt∕Fe3Pt)- and (FePt∕fcc FePt)-tilted magnetic recording media

The magnetic properties of tilted media with a film stack structure of [(fccFePtxnm) or (Fe3Ptynm)]∕FePt∕MgO∕FePt∕MgO4nm∕SiO24nm∕glass were investigated. The angular dependence of coercivity from the film normal to the in plane exhibits a broad peak in the case of Fe3Pt, implying the tilt of the easy axis from the film normal. The coercivity of the media with a hard layer decreases by about 50% from its initial value with an increase in Fe3Pt layer thickness. However, the media with fcc FePt exhibit about a 70% decrease in coercivity for layer thickness up to 3 nm. There is not much change in the α[=4π(dM∕dH)H=Hc] value (α∼2–3) with an increase in the Fe3Pt layer thickness.

Self-Assembly and Magnetic Properties of FePt, FeRh Nanoparticles, and FePt/FeRh Nanocomposite Particles

FeRh nanoparticles with the composition of Fe₇₅Rh₂₅ and Fe₃₆Rh₆₄ have been fabricated by a solution phase chemical method. After refining and size selection, a particle size is about 2 to 5 nm by HRTEM observation. From XRD results, as-deposited FeRh nanopaticles reveal a typical chemically disordered fcc structure which can be transformed into CsCl-type structure through thermal annealing. In the case of annealed Fe₆₄Pt₃₆ nanoparticles, the perpendicular coercivity increases comparing with that of as-deposited particles. It may be the results of the crystal orientation of fct-phase

Structural and magneto-optical properties of FeRh thin films

A systematic investigation of the magneto-optical properties in conjunction with structure and magnetic properties of FeRh thin films epitaxially deposited onto MgO(100) substrates by rf sputter-deposition system was performed. The result, for the first time, shows a (negative) maximum peak of Kerr rotation at around 3.8eV when FeRh thin films are in ferromagnetic state. The polar Kerr rotation angle is found to increase at temperatures above 100°C, which corresponds to the antiferromagnetic-ferromagnetic transition of FeRh thin films.

Magnetic Properties of Nanocomposite Particles of FePt/FeRh

The magnetic properties of selected compositions of FePt, FeRh, and their composites, prepared by the solution phase method, were investigated. A high-resolution transmission electron microscopy study revealed that the average size of monodispersed FePt and FeRh are 3-5 nm in the prepared state while average sizes are larger in the annealed state. In the case of FeRh, X-Ray diffraction studies show the disordered fcc phase in the as prepared which transforms into the CsCl structure through annealing. Similarly, in the case of FePt, X-Ray diffraction studies show the fee structure in as a deposited case with the L10 phase in the annealed state. The temperature-dependent magnetization of composite FePt/FeRh shows a significant decrease in the coercivity which starts at around 30degC and decreases rapidly with increasing temperature. This result reveals a strong exchange coupling between ferromagnetic (FePt) and antiferromagnetic (FeRh) phases. The thermal hysteresis width; Delta H of annealed FePt/FeRh shows about 5degC while that of FeRh in the single particles state is 135degC. The shift in transition temperature toward the lower side, in the presence of the magnetic field, implies the role of an applied magnetic field. The entropy change associated with this transition is estimated to be 185 mJ/Kcc.