Youyong Dai

Giant magnetoimpedance effect in sandwiched films

The giant magnetoimpedance (GMI) effect in films with a sandwiched structure has been studied. FeNiCrSiB/Cu/FeNiCrSiB sandwiched films with a magnetic closed-loop structure were deposited onto a glass substrate by rf sputtering and then annealed at an optimum temperature. GMI ratios as large as 63% and 77% were obtained at 13 MHz in longitudinal and transverse fields, respectively. The ratios are almost twice as large as those obtained in FeNiCrSiB single films.

Dependence of the magnetic properties on the alignment magnetic field for NdFeB bonded magnets made from anisotropic HDDR powders

The dependence of the hard magnetic properties on the alignment magnetic field for Nd(Fe,Co)B bonded magnets made from anisotropic HDDR powders is studied. The experimental results demonstrate that addition of a little Ga can induce a strong magnetic anisotropy in the HDDR magnetic powders. The application of an alignment magnetic field while the powders are bonded can increase the remanence, the coercivity and the maximum energy product in different degrees and the hard magnetic properties of the magnet are obviously improved with increasing alignment field.

Spin memristive magnetic tunnel junctions with CoO-ZnO nano composite barrier

The spin memristive devices combining memristance and tunneling magnetoresistance have promising applications in multibit nonvolatile data storage and artificial neuronal computing. However, it is a great challenge for simultaneous realization of large memristance and magnetoresistance in one nanoscale junction, because it is very hard to find a proper spacer layer which not only serves as good insulating layer for tunneling magnetoresistance but also easily switches between high and low resistance states under electrical field. Here we firstly propose to use nanon composite barrier layers of CoO-ZnO to fabricate the spin memristive Co/CoO-ZnO/Co magnetic tunnel junctions. The bipolar resistance switching ratio is high up to 90, and the TMR ratio of the high resistance state gets to 8% at room temperature, which leads to three resistance states. The bipolar resistance switching is explained by the metal-insulator transition of CoO1−v layer due to the migration of oxygen ions between CoO1−v and ZnO1−v.

Long-ranged and high temperature ferromagnetism in (Mn,C)-codoped ZnO studied by first-principles calculations

We theoretically investigated the electronic structures and magnetic properties of (Mn,C)-codoped ZnO system based on the first-principles calculations within a GGA+Up,d approach. We found that codoping C is a promising approach to enhance the ferromagnetic coupling between the nearest-neighboring Mn ions due to the strong hybridization between the C:2p and the Mn:3d states. Moreover, the spin-polarized states induced by C are so extended that they can mediate long-ranged ferromagnetic exchange interactions beyond the nearest-neighboring case. Therefore, (Mn,C)-codoped ZnO is expected to be a ferromagnetic semiconductor with long-ranged ferromagnetism and high Curie temperature.

Strong anisotropy of magnetization and sign reversion of ordinary Hall coefficient in single crystal Ge1−xMnx magnetic semiconductor films

Epitaxial single-crystal Ge1−xMnx ferromagnetic-semiconductor films were fabricated on Ge(001) substrates by molecular beam epitaxy. All the samples are ferromagnetic and have strong magnetic anisotropy indicated by different magnetization in plane and out of plane. The electrical transport of the films obeys Efros variable range hopping law in the low temperature range. Interestingly, a negative coefficient of the ordinary Hall effect of p-type carriers was found in the variable range hopping at low temperature. Anomalous Hall effect in Ge0.949Mn0.051 film was observed below the Curie temperature, indicating the carrier-mediated intrinsic ferromagnetism.

Spin polarization of Zn1−xCoxO probed by magnetoresistance

The spin polarization of Zn0.32Co0.68O1−v (v means oxygen vacancies) concentrated magnetic semiconductor (CMS) films was extracted from measurements of tunneling magnetoresistance (TMR), and spin-dependent variable range hopping, respectively. A TMR ratio of 19.1% was observed at 2 K in Co/ZnO/Zn0.32Co0.68O1−v magnetic tunnel junctions, which gives a low limit of the spin polarization of 25% in the Zn0.32Co0.68O1−v CMS. The TMR decreases with increasing temperature and bias voltage mainly due to the tunneling via localized impurity states in the barrier. By contrast, the spin polarization of the Zn0.32Co0.68O1−v CMS was estimated to be 36.1% by spin-dependent variable range hopping.

Tunable rectification and giant positive magnetoresistance in Ge1−xMnx/Ge epitaxial heterojunction diodes

Ge1−xMnx/Ge single-crystal heterojunction diodes with p-type Ge1−xMnx ferromagnetic semiconductor were grown, respectively on Ge substrates of p-type, n-type, and intrinsic semiconductors by molecular beam epitaxy. The I-V curve of the p-Ge0.95Mn0.05/intrinsic-Ge diode can be greatly tuned by a magnetic field, which was indicated by a large positive magnetoresistance. The magnetoresistance shows a peak value of 700% under a +2 V bias voltage around the Curie temperature of 225 K of the Ge0.95Mn0.05 magnetic semiconductor, and it remains as high as 440% at room temperature. The origin of the positive magnetoresistance is discussed.

Controllable spin-polarized electrical transport in wide-band-gap oxide ferromagnetic semiconductors

A family of wide-band-gap ternary oxide ferromagnetic semiconductor films with high transition metal concentration was prepared. The resistivity of these films can be changed up to four orders of magnitude by varying the composition or the concentration of the oxygen vacancies. Moreover, all these films show common features in electrical transport, i.e., Mott variable range hopping (VRH) in the lower resistivity range, Efros VRH in the middle resistivity range, and “hard gap” resistance in the higher resistivity range. The above phenomena are well understood by considering the relative magnitude of three characterization lengths, i.e., Coulomb screening length, localization length of the carriers, and optimal hopping distance. Furthermore, spin polarization ratio of these magnetic semiconductors was obtained by fitting the experimental results of electrical transport. Therefore, the wide gap oxide ferromagnetic semiconductors with controllable spin-polarized electrical transport are expected to have appli...

Engineering optical mode ferromagnetic resonance in FeCoB films with ultrathin Ru insertion

Ferromagnetic resonance (FMR) in soft magnetic films (SMFs) to a large extent determines the maximum working frequency of magnetic devices. The FMR frequency (fr) in an optical mode is usually much higher than that in the corresponding acoustic mode for exchange coupled ferromagnet/nonmagnet/ferromagnet (FM/NM/FM) trilayers. In this study, we prepared a 50 nm FeCoB film with uniaxial magnetic anisotropy (UMA), showing a high acoustic mode fr of 4.17 GHz. When an ultrathin Ru spacer was inserted in the very middle of the UMA-FeCoB film, the zero-field FMR was abruptly switched from an acoustic mode to an optical one with fr dramatically enhanced from 4.17 GHz to 11.32 GHz. Furthermore, the FMR mode can be readily tuned to optical mode only, acoustic mode only, or double mode by simply varying the applied filed, which provides a flexible way to design multi-band microwave devices.

Magnetic Properties and Giant Magneto-Impedance in Nanocrystalline FeCuCrVSiB Films

The giant magneto-impedance (GMI) effect has been observed in nanocrystalline FeCuCrVSiB films prepared by rf sputtering followed by an annealing treatment. The magnetic properties and the longitudinal and transverse GMI have been studied in detail. Field dependence and frequency spectra of GMI effect have been studied in the frequency range from 100 kHz to 13 MHz. Maximum GMI ratios as large as 50% and 25% were obtained at 13 MHz in the longitudinal and transverse cases, respectively. The field sensitivity of the longitudinal GMI is about 0.11% m/A at 13 MHz. The dependence of effective permeability on the frequency and magnetic field has been investigated.