Denglu Hou

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.

Facing-target sputtered Fe-C granular films: Structural and magnetic properties

Structural and magnetic properties of as-deposited and 550°C annealed Fe–C granular films have been investigated. It is found that the as-deposited samples are composed of amorphous Fe granules dispersed in an amorphous C matrix, and transformed to α(bcc)∕γ(fcc)-Fe and graphitelike C after being annealed at 550°C for one hour. The morphologies of the as-deposited samples are composition dependent, and the phase segregation between Fe and C phases can be seen by high-resolution transmission electron microscopy. Magnetic measurements reveal that the as-deposited samples are superparamagnetic at Fe composition of xv=49,58,65, and soft ferromagnetic at xv=77,84. The in-plane coercivity of the annealed specimens decreases with increasing xv, which is attributed to the enhanced intra-particle interaction, as evidenced by the long-range domain structures observed using magnetic force microscope images. The magnetic reversal mechanism deviates from ideal domain wall motion mode and approaches Stoner–Wohlfarth rot...

Structure, magnetic and transport properties of polycrystalline Fe3O4–Ge nanocomposite films

Structure, magnetic and transport properties of polycrystalline Fe 3 Ο 4 -Ge nanocomposite films fabricated using a reactive cosputtering method have been examined. It is found that with the increase in Ge content, the Fe 3 Ο 4 grain size decreases and the crystallization of the grains becomes poor. The films are ferromagnetic at room temperature, and their saturation magnetization decreases with the increase in Ge content. The room-temperature resistivity, much higher than that of pure polycrystalline Fe 3 Ο 4 films, increases with increasing Ge content because of the poor crystallization and the appearance of Ge barriers at the grain boundaries. Meanwhile, the negative magnetoresistance also decreases from ∼4.4% to ∼1.9% as the Ge content increases from 0 to 38 at%, which can be explained by the fact that the crystallization of Fe 3 Ο 4 grains is disturbed by the increasing Ge content.

New insight into the effect of the formation environment of ferrihydrite on its structure and properties

Ferrihydrite (Fh) is a naturally occurring nanoscale iron oxyhydroxide mineral. It is of great interest in soil science and environmental science due to its extremely high surface area and reactivity. In this work, Fh samples were prepared by three procedures (named Fh-1, Fh-2, and Fh-3). The formation of Fh-1 went through a pH change from acidic to neutral, and the formation of Fh-2 went through a pH change from alkaline to neutral, while Fh-3 was formed at a constant neutral pH. The three Fhs were characterized by high-resolution transmission electron microscopy (HRTEM), terahertz (THz) spectroscopy, nitrogen adsorption isotherms, and low-temperature magnetic techniques. All these techniques indicate that the microstructure and formation process of Fh are strongly coupled. More importantly, the differences in microstructure among the three Fhs are reflected not only in their bulk structure but also in their surface properties. The adsorption and degradation of azo dye Mordant Yellow 10 (MY10) on the three Fhs were investigated. On the one hand, compared with Fh-1 and Fh-2, Fh-3 exhibits a high density of active sites per unit area, which leads to a large adsorption capacity. On the other hand, a strong affinity between Fh-3 and MY10 results in a more irreversible adsorption and a low degradation rate. The results from the current study shed new light on the synergetic effects of porosity and the variations of local structure on photocatalysis by iron oxide particles.

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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The properties of fine particles of α‐Fe2O3 (sample A), and α‐Fe2O3 coated with cobalt (sample B) and adsorbed with pyridine (sample C) have been investigated by means of magnetic measurements, Mossbauer spectra and x‐ray photoelectron spectroscopy (XPS). The size of these fine particles is about 500 A. The Morin transition temperatures Tm for samples A, B, and C are 191, <80, and 224 K, respectively. The Mossbauer spectra of the samples were measured in a temperature range from 80 to 300 K and the angles between 〈111〉 axis and the spin of iron ions for these samples have been calculated from these spectra. The XPS results show that the Fe 2p and 3d peaks of sample C shift to lower energy regions compared with those of sample A. It seems to be that some electrons may move from pyridine molecules to the iron atoms in sample C. The reasons for the Tm change of samples B and C were discussed on the basis of the experimental results.

Template

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.

Effects of surface adsorption on the properties of α-Fe2O3 fine particles

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.