Tiejun Zhou

Anisotropy graded FePt–TiO2 nanocomposite thin films with small grain size

We reported the fabrication of (001)-textured FePt–TiO2 nanocomposite thin films composed of isolated grains of size of 5.3±1.1 nm. Anisotropy graded FePt–TiO2 thin films with hard layer thickness of 10 nm and soft layer thickness ranging from 0 to 8 nm were also prepared by cosputtering FePt and TiO2 at different substrate temperature. An exchange-coupled composite effect was demonstrated at a soft layer thickness of more than 3 nm.

Effects of Mn doping on temperature-dependent magnetic properties of L10 FeMnPt

We report the effect of Mn doping on temperature-dependent magnetic properties of L10 FeMnPt (001) epitaxial films. (001) textured L10 Fe50−xMnxPt50 (x = 0, 5, 10, 15, 20 at. %) films were prepared by cosputtering Fe, Pt, and Mn onto MgO single crystal substrates at 550 °C. θ–2θ XRD scans indicated the lattice parameter c increased whereas the ordering parameter S decreased with Mn doping. The thermal magnetic properties measured using a superconducting quantum interference device showed that Curie temperature TC could be reduced to 500 K with 15 at. % Mn doping, but Ku was decreased to 1.6 × 107 erg/cm3. The relation of temperature-dependent anisotropy and saturation magnetization showed that it did not obey the Callen–Callen theory.

Manipulation of magnetism by electrical field in a real recording system

We report an electrically controlled magnetism in a real recording system with CoCrPt–TiO2 nanocomposite thin films as recording medium. We show that in a spin-stand test, with a small voltage of 3 V applied across the head-media gap during recording, a 13% reduction of saturation current was observed and the read back waveforms showed sharper transitions. These account for the 3 dB improvement in read back signal-noise-ratio of the written magnetic information. The improved recording performance is mainly attributed to the reduction of anisotropy of the recording medium in the presence of electrical field. Simulations were carried out to understand the magnetization reversal process under applied electric and magnetic fields.We report an electrically controlled magnetism in a real recording system with CoCrPt–TiO2 nanocomposite thin films as recording medium. We show that in a spin-stand test, with a small voltage of 3 V applied across the head-media gap during recording, a 13% reduction of saturation current was observed and the read back waveforms showed sharper transitions. These account for the 3 dB improvement in read back signal-noise-ratio of the written magnetic information. The improved recording performance is mainly attributed to the reduction of anisotropy of the recording medium in the presence of electrical field. Simulations were carried out to understand the magnetization reversal process under applied electric and magnetic fields.

Observation of oscillatory resistance behavior in coupled Bernal and rhombohedral stacking graphene.

We report on the first observation of an anomalous temperature-dependent resistance behavior in coupled Bernal and rhombohedral stacking graphene. At low-temperature regime (<50 K) the temperature-dependent resistance exhibits a drop while at high-temperature regions (>250 K), the resistance increases. In the transition region (50-250 K) an oscillatory resistance behavior was observed. This property is not present in any layered graphene structures other than five-layer. We propose that the temperature-dependent resistance behavior is governed by the interplay of the Coulomb and short-range scatterings. The origin of the oscillatory resistance behavior is the ABCAB and ABABA stacking configurations, which induces tunable bandgap in the five-layer graphene. The obtained results also indicate that a perpendicular magnetic field opens an excitonic gap because of the Coulomb interaction-driven electronic instabilities, and the bandgap of the five-layer graphene is thermally activated. Potentially, the observed phenomenon provides important transport information to the design of few-layer graphene transistors that can be manipulated by a magnetic field.

The concept and fabrication of exchange switchable trilayer of FePt/FeRh/FeCo with reduced switching field

We report the concept and fabrication of exchange switchable trilayer of FePt/FeRh/FeCo with reduced switching field for heat assisted magnetic recording (HAMR). A thin layer of FeRh is sandwiched between L10 FePt and magnetically soft FeCo. At room temperature, FePt and FeCo are magnetically isolated by the antiferromagnetic FeRh layer. After the metamagnetic transition of FeRh layer by heating, FePt and FeCo are exchange-coupled together through ferromagnetic FeRh layer. Therefore, the switching field of FePt can be greatly reduced via exchange-spring effect. Simulation work was carried out to understand the exchange coupling strength and the FeCo thickness effects on the switching field reduction. It is found that switching field decreases with the increase of exchange coupling strength and FeCo thickness. The trilayer films were also successfully fabricated. A clear change of reversal mechanism from two-step to one-step switching upon heating was observed and a 3-time switching field reduction was dem...

3-D Sensitivity Function of Shielded Reader by Reciprocity Principle

The sensitivity function to recognize the gapped or shielded reader structure is critical for quantitative analysis of the readback waveform. Although the 3-D finite-element method (FEM) model can capture the full features of geometry and material properties of the reader, it is still preferred to have the precise analytical model as a fast tool for reader design and optimization. This work proposes an analytical model to describe the readback sensitivity function of a shielded reader by the reciprocity principle whereby the media flux absorptions by the sensor free layer and shields are based on the magnetic fields generated when they are treated as the writing elements. Using this approach, the reader geometry and also the material properties of shields and free layer can be included in the model. The proposed 3-D sensitivity function of the model allows us to examine the criteria for read head design and evaluate the reading performance through the analysis of readback waveforms.

Ferromagnetic nano-dot array fabricated by electron beam radiation induced nano-scale phase transition

We present a method of direct magnetic patterning of a nonmagnetic Co–C amorphousfilm by electron beamradiation induced nano-scale phase transition. Co–C alloy films with C concentration from 30 to 50 at % and thickness of 30–60 nm were prepared by alternately sputtering Co and C films onto C-buffered glass substrates. The films are amorphous and nonmagnetic with C concentration up to 40 at %. Due to their negative mixture entropy, as-deposited amorphous Co–C alloy films are metastable. Focused electron-beam irradiation causes localized phase segregation of the immiscible magnetic (Co-rich) and nonmagnetic (C-rich) phases. Ferromagnetic Co(C) nano-dot array was fabricated by subjecting the as-deposited Co 60 C 40 films to electron beamradiation using a beam current of 16 nA, a beam energy of 20 keV and a dwell time of 5 s per dot. Magnetic force microscopy images and magnetic measurements confirm the formation of the ferromagnetic phase. The present single-step nanolithography eliminates the cumbersome traditional processes and is potentially a new and flexible alternative for fabricatingpatterned magnetic nanostructures for submicron magnetic devices.

Electric field tuning of magnetocaloric effect in FeRh0.96Pd0.04/PMN-PT composite near room temperature

The first-order magnetic phase transition alloy, FeRh, exhibits a large magnetocaloric effect well above room temperature and works in a limited refrigeration temperature region, which hinders its application to some extent. In the present work, we report a remarkable electric-control magnetocaloric effect in the FeRh0.96Pd0.04/PMN-PT composite near room temperature through strain-mediated magnetoelectric coupling. By applying an electric field of 8 kV/cm, the metamagnetic phase transition temperature of the FeRh0.96Pd0.04 film shifts from 300 to 325 K. As a result, the refrigeration temperature region is broadened from 35 to 47 K. These results indicate that applying multiple driving fields is an effective method for tuning the magnetocaloric effect.

Room‐Temperature Ferromagnetism in ZnO‐Encapsulated 1.9 nm FePt3 Nanoparticle–Composite Thin Films with Giant Interfacial Anisotropy

As synthesized 1.9-nm FePt3 nanoparticles are superparamagnetic at room temperature. Coating those nanoparticles with ZnO renders them permanently ferromagnetic with coercivity field of 650 Oe at room temperature. First-principles calculations indicate that giant interfacial anisotropy, induced by the strong spin-orbit interaction of enhanced orbit momentum of Fe, overcomes the superparamagnetic limit, leading to exceptional room-temperature permanent ferromagnetism. The findings are important for the understanding of the origin of permanent ferromagnetism at ultrasmall size and critical for ultrahigh density recording and information processing.

Microstructure and magnetic properties of CoZr thin film

Annealing effects on the microstructure and magnetic properties of CoZr thin films are investigated in this article. It was found that a change in magnetic phase occurs by annealing the as-deposited films at temperatures of above 550 °C for 2 h. A much lower annealing temperature and shorter annealing time are needed to obtain a hard magnetic phase in thin films than in rapidly quenched CoZr bulk samples. Hard magnetic phase Co11Zr2 and ferromagnetic phase Co23Zr6 formed after annealing. All the annealed films show perpendicular magnetic anisotropy.