Nguyen T. Nam

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...

Exchange bias of ferromagnetic/antiferromagnetic in FePt/FeRh bilayers

A systematic investigation of structural and magnetic properties of FePt/FeRh bilayers has been carried out. The transition temperature of single FeRh layer is higher than that of FePt/FeRh bilayer. A sharp decrease in coercivity is observed at transition temperature for FePt/FeRh bilayer. The thickness FePt dependence of exchange bias field and unidirectional anisotropy constant are discussed. Of interest is the exchange bias effect that appears in FePt/FeRh bilayer and the unidirectional anisotropy constant, which is very large (up to 0.8 erg/cm2), suggesting that this system can be applied for spin-valve sensors and heat assisted magnetic recording medium. It may also provide some useful information for better understanding of the mechanism of exchange bias.

First-order magnetic phase transition in FeRh–Pt thin films

The first-order antiferromagnetic/ferromagnetic phase transition in ordered FeRh100−XPtX (0≦X≦15) thin films grown onto MgO(100) substrate was investigated by temperature dependent magnetization measurements. It is shown that the phase transition temperature increases with increasing Pt content. The field dependence of transition temperature was also measured and a shift of −8 to −3.3 K/T is observed for FeRh100−XPtX thin films with increasing Pt contents. In addition, the entropy changes associated with the magnetic phase transition were studied, and it can be proposed that the change in electronic entropy associated with the magnetic moment of Rh atoms is the main mechanism for the first-order magnetic phase transition in ordered FeRh-based alloys.

Temperature Dependence of Magnetic Properties in Epitaxial FePt/FeRh Bilayer

A systematic investigation of structural and magnetic properties of FePt/FeRh bilayers on dependence of temperature has been carried out. The transition temperature of single FeRh layer was higher than that of FePt/FeRh bilayer. A sharp decrease in coercivity is observed at transition temperature for FePt/FeRh bilayer. The thickness FePt and FeRh dependence of exchange bias field and unidirectional anisotropy constant are discussed. Of interest is that the exchange bias effect appears in FePt/FeRh bilayer and unidirectional anisotropy constant is very large up to 1 erg/cm 2, suggesting that this system can be applied to spin-valve sensors and heat assisted magnetic recording medium.

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.

Magnetic Properties and Phase Transition Kinetics of Fe

The magnetic properties and phase transition kinetics during antiferromagnetic (AFM) to ferromagnetic (FM) phase transition in epitaxial-grown Fe50(Rh1-XPtX)50 thin films were investigated in the present study. X-ray diffraction results show a change of ~0.6% in c lattice parameter during AFM to FM phase transition in Fe50(Rh0.95Pt0.05)50 thin film. The substitution of Pt is very effective not only for tuning the first-order AFM/FM phase transition behavior but also for decreasing the thermal hysteresis. The time dependent magnetization measurement indicates that the clusters of the FM phase nucleate in the AFM matrix heterogeneously and vice versa during the first-order phase transition in ordered FeRh-Pt thin films. The nucleation and growth kinetics of the first-order phase transition in Fe50(Rh1-XPtX)50 thin films was described by the classical Kolmogorov-Johnson-Mehl-Avrami model.