Congmian Zhen

Coexistence of reentrant-spin-glass and ferromagnetic martensitic phases in the Mn2Ni1.6Sn0.4 Heusler alloy

A giant exchange bias field of up to 1170 Oe was observed in the Mn2Ni1.6Sn0.4 Heusler alloy. A reentrant spin glass phase and a ferromagnetic martensitic phase coexist below the blocking temperature as confirmed by dc magnetization and ac susceptibility measurements. Exchange bias in Mn2Ni1.6Sn0.4 is thought to originate from the interface exchange interaction between the reentrant spin glass phase and the ferromagnetic martensitic phase. X-ray diffraction and selected area electron diffraction results demonstrate that excess Mn atoms occupy Ni and Sn sites randomly. In this way, Mn-Mn clusters are formed and constitute the reentrant-spin-glass phase.

Thickness-modulated optical dielectric constants and band alignments of HfOxNy gate dielectrics

Thickness-modulated optical dielectric constants and band alignments of HfOxNy films grown by sputtering have been investigated by spectroscopic ellipsometry (SE) and x-ray photoelectron spectroscopy. Based on SE measurements, it has been noted that an increase in optical dielectric constant and band gap has been observed as a function of the film thickness. Analyses of thickness-dependent band alignment of the HfOxNy/Si system indicate that the valence band offset increases, but only slight change in the conduction band offset, resulting from the thickness-induced change in the structure. The suitable optical dielectric constants and band offsets relative to Si make sputtering-derived HfOxNy film a promising candidate for high-k gate dielectrics.

Magnetization jumps and exchange bias induced by a partially disordered antiferromagnetic state in (FeTiO3)0.9-(Fe2O3)0.1

Magnetization jumps (MJs) and the exchange bias (EB) effect are simultaneously observed in the mixed-spin oxide (FeTiO3)0.9-(Fe2O3)0.1 at 2.0 K. Dc and ac susceptibility measurements confirm a reentrant spin glass phase with a partially disordered antiferromagnetic (PDA) state below the irreversibility temperature (Tir = 60 K). Antiferromagnetic (AFM) Fe3+ clusters are nested in AFM Fe2+ lattices forming a triangular lattice, in which 2/3 of the magnetic moments order antiferromagnetically with each other leaving the remaining 1/3 “confused.” This geometric frustration in the triangular lattice leads to a PDA state that is the ground state of the AFM triangular configuration. The PDA state, in the presence of a critical trigger field, evolves into a ferromagnetic (FM) state, and induces the AFM spins of the Fe2+ ions to enter a FM state, resulting in the MJs. Meanwhile, the FM spins of Fe2+ can serve as the pinned phase, and the AFM spins of Fe3+ can serve as the pinning phase, resulting in the EB effect....

Ferromagnetism in Ge/SiO2 multilayer films

The onset of room-temperature (RT) ferromagnetism (FM) has been experimentally observed in amorphous Ge/SiO2 multilayer films. Both the thickness of the individual layers of SiO2 and that of the Ge layers can influence the ferromagnetic order of the samples. The saturation magnetization (MS) reached a maximum of 18.3 emu/cm3 at RT for the film with structure [Ge(5 nm)/SiO2(8 nm)]3. The zero-field-cooled and field-cooled curves for the film show the coexistence of ferromagnetic and diamagnetic components. Obvious magnetic domains were observed in all of the samples. Ge forms mainly Ge–Ge bonds. In addition, Photoluminescence from interband indirect recombination and transitions between discrete energy levels in Ge nanostructures were observed. The FM in the Ge/SiO2 multilayer films can be attributed to both the quantum size effect and coupling of unpaired spins among the Ge nanostructures. The coupling tends to make the unpaired spins align in a ferromagnetic manner.

A Method for Preparation of Ordered Porous Silicon Based on a 2D SiO

A new method for fabricating ordered porous silicon is reported. A two-dimensional silica nanosphere array is used as a template with a hydrofluoric acid–hydrogen peroxide solution for etching the nanospheres. The initial diameter and distribution of the holes in the resulting porous silicon layer are determined by the size and distribution of the silica nanospheres. The corrosion time can be used to control the depths of the holes. It is found that the presence of a SiO2 layer, formed by the oxidation of the rough internal surface of the hole, is the primary reason allowing the corrosion to proceed. Ultraviolet reflection and thermal conductivity measurements show that the diameter and distribution of the holes have a great influence on properties of the porous silicon.

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Understanding the cation distribution and electronic transport properties of half-metallic NiCo2O4 (NCO) films is crucial to advancing their practical applications in optoelectronic materials. In this work, the specific range of growth temperature (TG) and oxygen pressure (Op) for the metallic NCO film is obtained. The concentration of Ni3+ and NiTet ↔ CoOct exchange interactions are the primary determinants of metallic behavior in NCO thin films, which is demonstrated by the experimental and computational results. Theoretical simulation shows that the electron–electron and electron–magnon scattering intrinsically determined the metallic behavior. The growth temperature has a great influence on the structure and lattice constant of the NCO film. The film with metallic behavior has a relatively small lattice constant, which results in a large saturation magnetization. Due to the disordered cation in the metallic film, two magnetic phases were observed. The formation of the low-valence oxides at low Op and the cation vacancies at high Op make the NCO film have high resistivity.

Template

Based on almost all the data from the literature on spontaneous exchange bias (SEB), it is expected that the system will show SEB if it meets two conditions simultaneously: (i) there are the coexistence and competition of antiferromagnetic (AFM) and ferromagnetic (FM) interactions and (ii) AFM interaction should dominate but not be too strong in this competition. In order to verify this view, a systematic study on SEB has been performed in this work. Mn50Ni40Sn10 with strong FM interaction and without SEB is chosen as the mother composition, and the negative chemical pressure is introduced by the substitution of Sn by Si to enhance AFM interaction. It is found that a long-range FM ordering window is closed, and a long-range AFM ordering window is opened. As a result, SEB is triggered and a continuous tuning of the spontaneous exchange bias field (HSEB) from 0 Oe to 1300 Oe has been realized in a Mn50Ni40Sn10−xSix system by the enhanced AFM interaction.

Insight into metallic behavior in epitaxial half-metallic NiCo2O4 films

The anomalous Hall effect in e-iron nitrides (e-Fe3-xN, 0 ≤ x ≤ 1) has been systematically investigated taking advantage of the fact that the exchange splitting of e-Fe3-xN can be continuously tuned through the nitrogen concentration. It has been found that the anomalous Hall conductivity, σxyAH, is proportional to the saturation magnetization MS, i.e., σxyAH=SHMS, across significant variations in the saturation magnetization (96–1146 emu/cc). This relationship is in excellent agreement with the intrinsic mechanism as well as with the unified theory of AHE. Our results also demonstrate that the anomalous Hall conductivity is sensitive to the exchange splitting of the band structure.

Tuning antiferromagnetic exchange interaction for spontaneous exchange bias in MnNiSnSi system

Cr/SmCo5/Cr films were fabricated by a DC facing targets magnetron sputtering. The influences of sputtered Ar pressure and substrate temperature on their microstructure and magnetic properties were investigated. Magnetic measurements indicate that the optimal substrate temperature was 450°C, and the film deposited at 2 Pa Ar pressure had the largest in-plane coercivity (2403.54 Oe). No SmCo5 diffraction peaks except Cr (110) peak with body-centered-cubic structure were seen in all the samples by X-ray diffusion. The Needle-like grains of the film deposited at low Ar pressure were observed by atomic force microscope. The domain pattern of the film fabricated at 2 Pa Ar pressure showed more uniform. When the sputtered Ar pressure was 2 Pa, the narrowest switching field distribution (0.57) was obtained, indicating a narrower grain size distribution. The δM value was nearly zero for the film deposited at 2 Pa Ar pressure, and this indicated that there was almost noninteraction between grains.