Jen-Hwa Hsu

Epitaxial growth mechanism of L10 FePt thin films on Pt/Cr bilayer with amorphous glass substrate

Ordered L10 FePt films with magnetic perpendicular anisotropy were fabricated with a Pt∕Cr bilayer. The squareness of the L10 FePt film with a Cr underlayer and a Pt buffer was close to one when a magnetic field was applied perpendicular to the film’s plane, because a semicoherent epitaxial growth was initiated from the Cr (002) underlayer; continued through the Pt buffer layer, and extended into the L10 FePt (001) magnetic layer. Without the Pt buffer layer, the Cr atoms may diffuse directly into the FePt magnetic layer. Consequently, an epitaxial barrier of the Cr-rich FePtCr alloy formed between the Cr underlayer and the FePt magnetic layer, degrading the magnetic performance and epitaxial growth of the latter.

Dielectric, magnetic and magnetoelectric properties of multiferroic BiFe0.5Cr0.5O3–NiFe2O4 composites

Multiferroic composites of NiFe2O4 (NFO) and BiFe0.5Cr0.5O3 (BFCO) are synthesized and their dielectric, magnetic, and magnetoelectric (ME) properties are measured at room temperature. NFO-BFCO forms a two-phase composite. The composite has a larger magnetization and dielectric constant than those of both parent compounds, due to the effects of interfacial strain on BFCO. Furthermore, the ME response in NFO-BFCO is about one time larger than that of BFCO, revealing the success of magnetic control of the dielectric response via the mechanical coupling, which can be exploited in the future applications of multiferroic composites.

Magnetoelectric response in lead-free multiferroic NiFe2O4–Na0.5Bi0.5TiO3 composites

Multiferroic composites that comprise NiFe2O4(NFO) and Na0.5Bi0.5TiO3(NBT) are synthesized using the sol-gel method. The results of XRD and scanning electron microscopy indicate the single-phase formation of NFO and NBT and the presence of two phases in the composites. The dielectric constant and dielectric loss are determined as functions of frequency over a wide range of frequencies from 40 Hz to 1 MHz. Room-temperature magnetization measurements show that these composites are soft magnetic. Further, the multiferroic character is confirmed by their magnetoelectric (ME) response at room temperature. The optimal ME response is 0.14% in a 67NFO–33NBT composite. The present study demonstrates that NFO–NBT could be effective as a lead-free multiferroic composite and provides an alternative for environmental-friendly ME devices.

Enhancement of perpendicular coercivity in L11 CoPt thin films by replacement of Co with Cu

Magnetic properties and microstructures of L11 (Co50−xCux)Pt50 films sputter-deposited at 350 °C on MgO(111) substrates are reported. The addition of Cu significantly improves the alignment of c-axis and chemical ordering. Perpendicular coercivity (Hc⊥) also increases markedly from 0.1 to 1.9 kOe while in-plane coercivity declines from 0.5 to 0.07 kOe for the 20 nm thick films at x=26. Similar phenomena are observed with larger effects for the 50 nm thick films. The coercive mechanism is attributed to domain-wall pinning produced by the compositional segregation of nanoscaled nonmagnetic Cu-rich and magnetic Co-rich regions within a coherent L11 crystal domain. Therefore, an intermediate value of Hc⊥ can be obtained from this hardening mechanism when further microstructure modifications are enforced, which largely increases the potential for the use in spintronic devices or patterned media.

Magnetic behaviour of Ni0.4Zn0.6Co0.1Fe1.9O4 spinel nano-ferrite

Nanoparticles of the spinel ferrite Ni0.4Zn0.6Co0.1Fe1.9O4, have been synthesized by a co-precipitation method. The x-ray diffraction patterns of the particles confirmed the formation of single-phase cubic spinel structure. The Langevin function fitting on M-H data at 300 K gives a log-normal particle size distribution with median diameter of 59.6 nm and standard deviation of 0.6. The isothermal dc magnetization studies have been performed using the superconducting quantum interface device and vibrating sample magnetometer in the temperature range of 5-300 K. These measurements show that the sample is superparamagnetic above the blocking temperature TB ∼ 253 K when an external magnetic field of 20 Oe is applied. The reduction in saturation magnetization in case of nanoparticles may be attributed to the fact that magnetic moments in the surface layers outside the core are in the state of frozen disorder. A doublet observed in the Mossbauer study also confirms the superparamagnetic behavior and nanocrystall...

Enhancement in Dielectric and Magnetic Properties of

Indium substituted nano nickel zinc ferrites Ni_0.5Zn_0.5In_xFe_2-xO₄ with <i>x</i>=0, 0.1, 0.2 and 0.3 are synthesized by a coprecipitation method. Magnetic and dielectric properties are studied over a frequency range 1-30 MHz. Initial permeability is found to be in the range of 9-19 with a magnetic loss tangent of 10^-1-10^-2 . The dielectric constant is noticed in the range of 11-17 along with low dielectric loss tangent (10^-1-10^-3). The composition with x=0.2 is found to have the highest specific saturation magnetization of 78.8 emu/g. Appreciable dielectric and magnetic values along with low losses can be correlated to small grain size and better compositional stoichiometry obtained as a result of processing by coprecipitation method. These fascinating dielectric and magnetic properties of these materials reveal a direction for high-frequency applications.

{\rm In}^{3+}

Fe top layers with a thickness t Fe between 1 and 15 nm were deposited on textured FePt(001)/Pt(001)/Cr(002) layers to study the associated exchange coupling effect and their switching behaviors. The deposition of a Fe layer does not change the crystalline structure of the FePt layer. Microstructural analysis reveals that the Fe(200) texture is formed epitaxially on the FePt(001) surface. When t Fe = 1 nm, the remanent coercivity (H cr) remains the same as that without a Fe top layer. The lack of reversible magnetization reveals that the Fe layer is thinner than the critical thickness; the strong coupling force throughout the whole Fe layer facilitates the cooperatively magnetic rotation of Fe and FePt layers. Significant exchange spring was observed in the sample with t Fe = 3 and 5 nm, reducing H cr by 14% and 43% (the maximum reduction), respectively. The results are consistent with the theoretical prediction. As t Fe was increased up to 8 and 12 nm, the exchange spring effect disappeared. Although the two films exhibited similarly reduced H cr values of about 1.5 kOe, the reduction of H cr is caused by the tilt of the magnetic moments rather than the exchange spring. This incline of the moments is caused by the weakened exchange coupling and the complex domain structure. The magnetic reversal was further examined by measuring the variation in longitudinal and transverse magnetizations during magnetic field sweeping. The results suggest that at t Fe = 1 and 3 nm, the reversal occurs cooperatively in a single direction. As t Fe is increased to 5 and 8 nm, the moments switch randomly, in a manner that may be related to the increase in the density of the domain wall, observed by MFM. This work demonstrates the feasibility of reducing coercivity through exchange-spring mechanism and provides information on detailed magnetic reversal mechanism in perpendicular exchange coupled FePt.

Substituted Ni-Zn Nano-Ferrites by Coprecipitation Method

The Agx–(Fe3O4)1−x and Agx–(Fe3O4)1−x composite films were prepared by dc sputtering on Si(100) substrates. The x-ray diffraction results show that the films contain essentially only the cubic inverse spinal phase from Fe3O4 and face-centered cubic phase from Ag or Al. The transmission electron microscopy images indicate that the metal granules are randomly distributed with Fe3O4 grains. The resistivity determined from the four-probe method decreases rapidly with increasing metal content. At x≒0.5, a percolation occurs. The conducting path is formed from metal granules in series with Fe3O4 grains. The magnetoresistance (MR) is defined to be {R(H=0.8 T)−R(H=0)}/R(H=0). It has been found that MR is isotropic and the appearance of Ag granules has significant impact on the MR effect. Furthermore, a positive MR region appears with 0.011<x<0.1 in Agx–(Fe3O4)1−x. On the contrary, the incorporation of Al granules does not have the same effect on MR as in Agx–(Fe3O4)1−x. A slow increase of MR with Al content might...

Magnetic Reversal Behaviors of Perpendicular Exchange-Coupled Fe/FePt Bilayer Films

A method of manufacturing granular Fe–Pb–O films is described. The temperature dependence of resistivity in these samples exhibits a semiconductorlike behavior indicating that the electronic transport takes place via a tunneling process. The magnetoresistance ratio has been found to be about 10% at room temperature. The enhancement of the tunneling magnetoresistance effect is believed to be due to spin-dependent tunneling through the tunneling barrier containing magnetic ferrite formed by PbO and α-Fe2O3.

Magnetoresistance effect in Ag–Fe3O4 and Al–Fe3O4 composite films

This study elucidates the magnetic properties of [Ta/Co60Fe20B20/MgO]5 as functions of thickness of each layer. Its perpendicular magnetic anisotropy (PMA) is found to depend strongly on the thickness of the MgO (tMgO) and Co-Fe-B (tCoFeB) layers. The Ta/CoFeB interface is critical to inducing PMA. A maximum room-temperature (RT) anisotropic energy (KuRT) of about 1 × 106 erg/cm3 and an anisotropic field (HkRT) of 4.7 kOe are obtained in the RT-prepared multilayered sample with tMgO = 1.0, tCoFeB = 1.3, and thickness of layer tTa = 10 nm. These values are comparable to the published values for Co60Fe20B20 after optimized field annealing. In this investigation, post-annealing has a more complicated effect on PMA in a multilayered structure than in a single or a double Co-Fe-B layer, both of which structure have been examined elsewhere. This result may be explained by the competing effects of the thermal process, which is an improvement of the crystallinity of Co-Fe-B and roughening of the interface.