Advances in amorphous and nanocrystalline materials

A new amorphous alloy has been recently introduced which shows a saturation magnetic induction B_s of 1.64 T which is compared with B_s=1.57 T for a currently available Fe-based amorphous alloy and decreased magnetic losses. Such a combination is rare but can be explained in terms of induced magnetic anisotropy being reduced by the alloy's chemistry and its heat treatment. It has been found that the region of magnetization rotation in the new alloy is considerably narrowed, resulting in reduced exciting power in the magnetic devices utilizing the material. Efforts to increase B_s also have been made for nanocrystalline alloys. For example, a nanocrystalline alloy having a composition of Fe_80.5Cu_1.5Si₄B₁₄ shows B_s exceeding 1.8 T. The iron loss at 50 Hz and at 1.6 T induction in a toroidal core of this material is 0.46 W/kg which is 2/3 that of a grain-oriented silicon steel. At 20 kHz/0.2 T excitation, the iron loss is about 60% of that in an Fe-based amorphous alloy which is widely used in power electronics. Another example is a Fe₈₅Si₂B₈P₄Cu₁ nanocrystalline alloy with a B_s of 1.8 T, which is reported to exhibit a magnetic core loss of about 0.2 W/kg at 50 Hz and at 1.5 T induction. This article is a review of these new developments and their impacts on energy efficient magnetic devices.     

Properties of magnetocaloric La(Fe,Co,Si)13 produced by powder metallurgy

We present a comprehensive study of the magnetocaloric materials series La(Fe_1−xCo_x)_11.9Si_1.1 with 0.055<x<0.122. The ferromagnetic samples were manufactured using a novel powder metallurgy process by which industrial scale production is feasible. This new production method makes the materials more attractive as magnetic refrigerants for room temperature magnetic refrigeration. The Curie temperature of the compounds can be easily tuned by altering the Co content and all samples have little magnetic anisotropy and present a second-order magnetic transition so that thermal and magnetic hysteresis is absent. For all seven samples, we have calculated the magnetic entropy change, ΔS_M, from initial curve measurements and measured the adiabatic temperature change, ΔT_ad, directly. In addition, for two of the samples, we determined the heat capacity as a function of applied magnetic field and the thermal conductivity. Where relevant, the results are compared with those of Gd, the benchmark material for room temperature magnetic refrigeration.     

Review of the magnetocaloric effect in manganite materials

A thorough understanding of the magnetocaloric properties of existing magnetic refrigerant materials has been an important issue in magnetic refrigeration technology. This paper reviews a new class of magnetocaloric material, that is, the ferromagnetic perovskite manganites (R_1−xM_xMnO₃, where R=La, Nd, Pr and M=Ca, Sr, Ba, etc.). The nature of these materials with respect to their magnetocaloric properties has been analyzed and discussed systematically. A comparison of the magnetocaloric effect of the manganites with other materials is given. The potential manganites are nominated for a variety of large- and small-scale magnetic refrigeration applications in the temperature range of 100–375 K. It is believed that the manganite materials with the superior magnetocaloric properties in addition to cheap materials-processing cost will be the option of future magnetic refrigeration technology.     

Photo-induced Magnetic bubbles in the spin glass Alloy Fe2SnTe4

The ternary material Fe₂SnTe₄ is an amorphous metallic alloy that exhibits spin glass behavior with a freezing temperature of T_f = 12 K. When this material is cooled well below the spin glass freezing temperature (T = 5 K) and placed in a magnetization field, the zero-field-cooled spin glass state has been observed to absorb ultraviolet radiation with the concomitant generation of a magnetic bubble on the surface of the material. The photo-induced magnetic bubbles are detected via magnetization measurements with a superconducting SQUID susceptometer. Photomagnetic experiments are reported on the zero-fieldcooled and field-cooled specimens. Each of these experiments results in the generation of a magnetic bubble in the material.     

Influence of eddy currents on magnetic hysteresis loops in soft magnetic materials

In this paper an attempt has been made to extend the Jiles and Atherton (J–A) quasi-static hysteresis model to describe magnetisation of a material with an alternating magnetic field. In low – industrial – and medium frequency of magnetic field it is possible to ignore the magnetic relaxation and resonance. The field penetration is assumed to be uniform through the material. The influence of eddy currents on the hysteresis loop could be considered and calculated using the method of successive reactions of eddy currents, where a reaction is an additional magnetic field, called reaction H_d, induced in the material by the eddy currents according to rot J=γ∂B/∂t where γ is the electrical conductivity. The reaction field H_d was added to the basic field H₀∝Iz_1, where I is the current intensity in the magnetising coil of z₁ number of windings. By solving the J–A equation for the magnetic field H_w=H₀+H_d it has achieved an extension of the hysteresis loop at an increased frequency of the current, caused by increased losses of the eddy currents. At the frequency f→0 Hz , the hysteresis loop approaches the shape of the quasi-static one.     

Design of novel magnetic materials based on ZrCuSiAs-like semiconducting pnictide-oxides from first-principles calculations

We assumed that significant enlargement of the functional properties of the family of quaternary ZrCuSiAs-like pnictide-oxides, often called also 1111 phases, which are known now first of all as parent phases for new FeAs superconductors, may be achieved by replacement of non-magnetic ions by magnetic ions in semiconducting ZrCuSiAs-like phases. We checked this assumption by means of first-principles FLAPW–GGA calculations using a wide-band-gap semiconductor YZnAsO doped with Mn, Fe, and Co as an example. Our main finding is that substitution of Mn, Fe, and Co for Zn leads to drastic transformations of electronic and magnetic properties of the parent material: as distinct from the non-magnetic YZnAsO, the examined doped phases Y Zn_0.89Mn_0.11AsO, Y Zn_0.89Fe_0.11AsO, and Y Zn_0.89Co_0.11AsO behave as a magnetic semiconductor, a magnetic half-metal or as a magnetic gapless semi-metal, respectively.     

New type of biosensor based on magnetic nanoparticle detection

A new type of biosensor has been developed based on detection of nanosized superparamagnetic particles that serve as labels in bioreactions. The method is based on non-linear magnetic material detection by a magnetic field having components at two frequencies f₁ and f₂. The response is measured at the combinatorial frequencies f_i=mf₁+nf₂, where m and n are integers, e.g., f_i=f₁±2f₂. Several highly sensitive readers of superparamagnetic particles have been designed and used for development of various formats of immunoassays, including those compatible with immunoconcentration and magnetic enrichment of antigens.     

Ferromagnetic seeding for the magnetic targeting of drugs and radiation in capillary beds

A new implant assisted-magnetic drug targeting approach is introduced and theoretically analyzed to demonstrate its feasibility. This approach uses ferromagnetic particles as seeds for collecting magnetic drug carrier particles at the desired site in the body, such as in a capillary bed near a tumor. Based on the capture cross section (λ_c) approach, a parametric study was carried out using a 2-D mathematical model to reveal the effects of the magnetic field strength (μ₀H₀=0.01–1.0 T), magnetic drug carrier particle radius (R_p=20–500 nm), magnetic drug carrier particle ferromagnetic material content (x_fm,p=20–80 wt%), average blood velocity (u_B=0.05–1.0 cm/s), seed radius (R_s=100–2000 nm), number of seeds (N_s=1–8), seed separation (h=0–8R_s), and magnetic drug carrier particle and seed ferromagnetic material saturation magnetizations (iron, SS 409, magnetite, and SS 304) on the performance of the system. Increasing the magnetic field strength, magnetic drug carrier particle size, seed size, magnetic drug carrier particle ferromagnetic material content, or magnetic drug carrier particle or seed saturation magnetization, all positively and significantly affected λ_c, while increasing the average blood velocity adversely affected it. Increasing the number of seeds or decreasing the seed separation, with both causing less significant increases in λ_c, verified that cooperative magnetic effects exist between the seeds that enhance the performance. Overall, these theoretical results were encouraging as they showed the viability of this minimally invasive, implant assisted-magnetic drug targeting approach for targeting drugs or radiation in capillary beds.     

Pressure-induced changes in the magnetic properties of the single crystal of Mn3 single-molecule magnet

The magnetic measurements of the single crystal of Mn3 single-molecule magnet under high pressure at T=2 K have been performed.We find both the antiferromagnetic intermolecular coupling parameter J and the effective energy barrier to vary compared with the measurements at low pressure.The increase of|J|is estimated to be 12%at 0.7 GPa when compared with that of0 GPa,whereas the effective energy barrier becomes smaller with increasing pressure.Our results demonstrate that the intermolecular interaction of single-molecule magnet can be changed by pressure.Compared with the normal magnetic alloy,the effect of pressure on the magnetic properties of Mn3 is much more prominent,which implies that Mn3 may have great potential in magnetic multifunctional material.     

Possible room temperature ferromagnetism of Li-doped anatase TiO2: A first-principles study

The electronic structures and magnetic properties in substitutionally lithium-doped anatase TiO₂ (Li-TiO₂) have been investigated by density functional calculations. We found that while Li itself is nonmagnetic, it generates holes residing in O 2pπ orbitals that lead to magnetic moments in TiO₂. The magnetic moments are delocalized but inhomogeneous distributed in the lattice with long rang ferromagnetic interactions among them. The inhomogeneous distribution of the magnetic moments and the specific crystal symmetry results in distance as well as directional dependence of the ferromagnetism stability of the Li-TiO₂ system. Our calculations suggest Li-TiO₂ to be potential spintronics material.     

New perspective on the reciprocity theorem of classical electrodynamics

We provide a simple physical proof of the reciprocity theorem of classical electrodynamics in the general case of material media that contain linearly polarizable as well as linearly magnetizable substances. The excitation source is taken to be a point-dipole, either electric or magnetic, and the monitored field at the observation point can be electric or magnetic, regardless of the nature of the source dipole. The electric and magnetic susceptibility tensors of the material system may vary from point to point in space, but they cannot be functions of time. In the case of spatially non-dispersive media, the only other constraint on the local susceptibility tensors is that they be symmetric at each and every point. The proof is readily extended to media that exhibit spatial dispersion: For reciprocity to hold, the electric susceptibility tensor χ_{E_mn} that relates the complex-valued magnitude of the electric dipole at location r_m to the strength of the electric field at r_n must be the transpose of χ_{E_nm}. Similarly, the necessary and sufficient condition for the magnetic susceptibility tensor is χ_{M_mn} = χ^T_{M_nm}.     

Effects of a magnetic field on a modulated phase

The effects of a magnetic field on a modulated phase are studied. A modulated phase is found to have two critical fields H₁ and H₂. For a large enough magnetic field, H₁ and H₂ can be approximated by a linear law. As a result, the minimum magnetic field needed to destroy a modulated phase is a constant. The minimum magnetic field also greatly depends on the order of a commensurate phase. A very high order commensurate phase and an incommensurate phase cannot survive a magnetic field. The behaviour of a magnetoelastic chain in a magnetic field can be described by a harmless devil's staircase. The inverse temperature is found to play a role similar to that of a special magnetic field. The deeper physics underlying these new phenomena is the breaking of the left-right symmetry of a phase diagram. As a result a controllable path to a modulated phase is found.     

Ab initio studies of half-metallic ferromagnetism in carbon-doped CeO2

The electronic structures and magnetic properties in Carbon-doped CeO₂ have been investigated by means of first-principles calculations based on the LSDA+U scheme. The results demonstrate a magnetic moment of 2.00 μ_B per supercell with one Carbon dopant which mainly stems from Hund’s rule coupling based on rather localized 2p, 5d and 4f states. The hole-mediated long-range magnetic coupling between local magnetic moments can be attributed to the collective effects of the p–p, p–d, and p–f hybridizations between C and neighboring O or Ce atoms. Ferromagnetism and half-metallic characteristics of C-doped CeO₂ make it possible to be an ideal material for spintronic devices.     

Spin glass behavior and critical analysis across the magnetic phase transition in Gd2CoMnO6: dc magnetization and ac susceptibility study

We report here on critical analysis across magnetic phase transition and spin dynamics in Gd₂CoMnO₆. We found that this material behaves differently below and above the applied magnetic field of 20 kOe. The magnetic phase transition switches from nearly mean-field type to unusual class and T_c shifts towards the high temperature above 20 kOe field. The nature of the magnetic phase transition is explored by carrying out critical analysis at low as well as at high magnetic field. The critical exponents obtained at low field using Kouvel-Fisher method are β = 0.65 (2) γ = 0.90 (2), δ = 2.43 and T_c = 120 K. Apparently, these values of critical exponents appear close to mean-field model. For high field the critical exponents are β = 1.24 (2) γ = 0.64 (5), δ = 1.51 (3) and T_c = 128 K. The critical exponents show significant deviation from any universal class. This switchover in the nature of the magnetic phase transition is unique and not seen in many compounds. The formation of non-Griffiths-like clusters in this compound can be a reason for such unique behavior. Further, ac susceptibility has been measured to understand the spin dynamics in detail. The dispersion of frequency-dependent χ_ac below T_c confirms a spin glass state in this material. The observed value of τ_o and T_o indicate the slow dynamic spin which is caused by co-existence of Co/Mn spin magnetic moments. The magneto-caloric effect is also presented for Gd₂CoMnO₆ in this study. The magnetic study and critical analysis across the phase transition reveal a switchover in the nature of phase transition in this material. A non-Griffiths like cluster formation above T_c is found and dynamic susceptibility study reveals a spin glass state below T_c in Gd₂CoMnO₆.     

Effect of the spin short-range order on electron transport and neutron scattering in amorphous alloys Gd<Subscript>x</Subscript>Si<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>

Direct experimental evidence has been obtained for the existence of short-range ferromagnetic order in Gd_xSi_1?x alloys. Along with the standard magnetotransport measurements, a “local” experimental method of the elastic scattering of thermal neutrons on the magnetic moment of Gd is applied. The measurement results provide the conclusion that short-range ferromagnetic order is formed in nanoclusters with an increased content of Gd ions. We consider the amorphous alloy Gd_xSi_1?x as a new magnetic material with unique properties characteristic of both strongly inhomogeneous alloys and nanocomposite compounds.     

Spin glass like behavior in Cd0.42Mn0.58In2S4

The magnetic behavior of the diluted magnetic semiconductor Cd_0.42Mn_0.58In₂S₄ has been study by dc magnetization and ac susceptibility experiments. Zero field cooled and field cooled measurements reveal irreversibility below T_irr=2.60±0.15 K. Ac susceptibility data, performed as a function of the temperature and the frequency, confirm the spin-glass like behavior of the material with T_f=2.75±0.15 K. High temperature susceptibility data follow a typical Curie-Weiss law with θ=−74±1 K which suggests predominant antiferromagnetic interactions. The randomness of the magnetic ions, necessary to explain the magnetic behavior of the material, has been determined by X-ray powder diffraction experiments.     

Single-Lo-phonon and -plasmon recombination channels at narrow-bandgaps

Single phonon and plasmon recombination transitions have been observed in different Hg_1-xCd_xTe semi-metallic alloys. The recombination channels were switched on when energy resonance was magnetically adjusted between the effective bandgap and these elementary excitations. From experimental results of the magnetic field-dependent photoconductivity it can be concluded that these recombination mechanisms are very efficient and dominate the recombination behaviour near their resonance with the bandgap. The new recombination channels play an important role in narrow-gap material far-infrared devices.     

A new method for the production of atomic beams of highly refractory elements and first atomic beam magnetic resonances in Ta181

A new universal method for the production of atomic beams of highly refractory elements has been developed. The beams of free atoms are produced by locally heating a small area on a target consisting of the material to be evaporated. The local heating is achieved via the focussed electron beam of a commercial electron gun. The atomic beams so obtained are suitable for studying hyperfine structures by the atomic beam magnetic resonance method. The first observedrf resonances in the ground state⁴ F _3/2 and the first excited state⁴ F _5/2 of Ta¹⁸¹ are reported.     

First-order phase transition to strongly anisotropic ferromagnetism in Tl2Fe6Te6

The magnetic susceptibility, magnetization, specific heat and electrical resistivity were measured on the new material Tl₂Fe₆Te₆. All the data show a very sharp anomaly revealing a phase transition to ferromagnetism at T_c≈220 K. The presence in the structure of one-dimensional metallic clusters $\text{¦Fe}{}_3\text{¦}{}^1{}_{\mathrm{\infty }}$ is evidenced by a very strong magnetic anisotropy in the ferromagnetic state, showing an interesting intermediate situation between a pure linear chain and 3-dimensional iron. Furthermore, the unusual sharpness of the transition leads us to anticipate a first-order phase transformation, but the lack of detectable thermal hysteresis was surprising. The molecular field model proposed by Bean and Rodbell in order to account for the similar behavior of MnAs gives a close representation of our magnetic data. Thus, a pronounced dependence of the exchange interaction upon interatomic spacing may well be the dominant mechanism leading to the observed phenomena.     

Magnetism in the new full-Heusler compound,Zr2CoAl: A first-principles study

The electronic structure and magnetic properties of Zr₂CoAl bulk material were investigated within the Density Functional Theory (DFT) framework. The material, basically a complete spin polarized half-metallic ferromagnet in the ground state, crystallizes in the ordered full-Heusler inverse structure (Hg₂CuTi-type structure). The energy band gap, localized in minority spin channel is 0.48 eV at equilibrium lattice parameter, 6.54 Å. The total magnetic moment calculated, equal to 2 μ_B/f.u., is an integral, in agreement with the Slater-Pauling curve for full-Heusler alloys.