Magnetic properties of disordered Ni\u3csub\u3e3\u3c/sub\u3eC

The metastable Ni3C phase has been produced by mechanically alloying Ni and C. Ni3C particles of diameter 10 nm are produced after 90 h of mechanical alloying with no evidence of crystalline Ni in x ray or electron diffraction. Linear muffin-tin orbital band-structure calculations show that Ni3C is not expected to be ferromagnetic due to strong Ni-C hybridization in the ordered alloy; however, the introduction of even small amounts of disorder produces locally Ni-rich regions that can sustain magnetism. Mechanically alloyed Ni3C is ferromagnetic, with a room-temperature coercivity of 70 Oe and a magnetization of 0.8 emu/g at 5.5 T, although the hysteresis loop is not saturated. The theoretical prediction that interacting locally nickel-rich regions may be responsible for ferromagnetic behavior is supported by the observation of magnetically glassy behavior at low magnetic fields

Magnetic properties of disordered Ni\u3csub\u3e3\u3c/sub\u3eC

The metastable Ni3C phase has been produced by mechanically alloying Ni and C. Ni3C particles of diameter 10 nm are produced after 90 h of mechanical alloying with no evidence of crystalline Ni in x ray or electron diffraction. Linear muffin-tin orbital band-structure calculations show that Ni3C is not expected to be ferromagnetic due to strong Ni-C hybridization in the ordered alloy; however, the introduction of even small amounts of disorder produces locally Ni-rich regions that can sustain magnetism. Mechanically alloyed Ni3C is ferromagnetic, with a room-temperature coercivity of 70 Oe and a magnetization of 0.8 emu/g at 5.5 T, although the hysteresis loop is not saturated. The theoretical prediction that interacting locally nickel-rich regions may be responsible for ferromagnetic behavior is supported by the observation of magnetically glassy behavior at low magnetic fields

Oscillatory interlayer exchange coupling in [Pt/Co]\u3ci\u3e\u3csub\u3en\u3c/i\u3e\u3c/sub\u3e/NiO/[Co/Pt]\u3ci\u3e\u3csub\u3en\u3c/i\u3e\u3c/sub\u3e multilayers with perpendicular anisotropy: Dependence on NiO and Pt layer thicknesses

Interlayer exchange coupling has been studied in a series of [Pt( tPt Å)/Co(4 Å)]n/NiO( tNiO)/[Co(4 Å)/Pt( tPt Å)]n multilayers with perpendicular anisotropy. The coupling oscillates between antiferromagnetic and ferromagnetic as a function of tNiO with a period of ~5 Å, and the oscillatory behavior is related to the antiferromagnetic ordering of the NiO layer. This interlayer coupling between two Co/Pt multilayers is shown to occur domain by domain by magnetic force microscopy imaging. For the strongest antiferromagnetic coupling at tNiO = 11 Å, an oscillation with a period of ~6 Å is superposed onto the exponential decay of the coupling strength as a function of tPt. The exponential decay with tPt is ascribed to the exponential decay of the coupling between the Co layers across the Pt layers in each Co/Pt multilayer, and the superposed oscillatory behavior can be attributed to multiple reflections of electron waves at the Co/Pt interfaces and their interference. A linear dependence of the antiferromagnetic coupling strength on 1/n, (where n is the number of repeats), is suggestive of a surface interaction for this interlayer coupling

MFM studies of interlayer exchange coupling in Co/Ru/Co films: Effect of Ru layer thickness

Antiferromagnetically coupled magnetic thin films are promising candidates for the design of new magnetic storage and logic devices. The ability to control the interlayer thickness, therefore the magnetic reversal response, of exchange-coupled magnetic layers is of paramount importance in nanotechnology, especially in magnetic sensing element design and applications. In this work, magnetic force microscopy (MFM) with improved sensitivity and high spatial resolution probes was used to obtain a more detailed view of magnetization reversal behavior and domain evolution in the indirect exchange-coupled trilayer system: Co/Ru/Co. The effect of the variable Ru interlayer thickness on the exchange coupling and thus the magnetic domain structure during the ferromagnetic (FM)/antiferromagnetic (AF) coupling transition in Co/Ru/Co films is well demonstrated. The MFM images display a distinct signature of AF coupling for the films with Ru thickness of 0.4 nm. MFM has proven to be an effective tool for detecting FM/AF interlayer coupling and exploring magnetic domain structures in exchange-coupled layered thin films

Controlled short time large scale synthesis of magnetic cobalt nanoparticles on carbon nanotubes by flash annealing

Nanopatterned arrays of discrete cobalt nanostructures showing characteristic parameter-dependent sizes are formed from continuous thin films on a carbon nanotube substrate using millisecond pulsed intense UV light. The nanoparticles exhibit ferromagnetic behavior with magnetic remanence and coercivity depending on the particle size. The end-state particle size is shown to be a function of initial thin film thickness and excitation energy and is therefore tunable. The evolutionary process from continuous thin films to a discrete morphology is thermodynamically driven by the large surface energy difference between metastable thin films and the underlying carbon nanotube substrate. Evidence of the Danielson model of the dewetting process is observed. These arrays can find applications as platforms for the self-assembly of magnetically susceptible materials, such as iron or nickel nanostructures, into a conduction matrix for applications in energy extraction from a latent heat storage device

Topological phase transitions and Berry-phase hysteresis in exchange-coupled nanomagnets

Topological phase in magnetic materials yields a quantized contribution to the Hall effect known as the topological Hall effect, which is often caused by skyrmions, with each skyrmion creating a magnetic flux quantum ±h/e. The control and understanding of topological properties in nanostructured materials is the subject of immense interest for both fundamental science and technological applications, especially in spintronics. In this work, the electron-transport properties and spin structure of exchange-coupled cobalt nanoparticles with an average particle size of 13.7 nm are studied experimentally and theoretically. Magnetic and Hall-effect measurements identify topological phase transitions in the exchange-coupled cobalt nanoparticles and were used to discover a qualitatively new type of hysteresis in the topological Hall effect—namely, Berry-phase hysteresis. Micromagnetic simulations reveal the origin of the topological Hall effect—namely, the chiral domains, with domain-wall chirality quantified by an integer skyrmion number. These spin structures are different from the skyrmions formed due to Dzyaloshinskii–Moriya interactions in B20 crystals and multilayered thin films, and caused by cooperative magnetization reversal in the exchange-coupled cobalt nanoparticles. An analytical model is developed to explain the underlying physics of Berry-phase hysteresis, which is strikingly different from the iconic magnetic hysteresis and constitutes one aspect of 21st-century reshaping of our view on nature at the borderline of physics, chemistry, mathematics, and materials science

High energy product of MnBi by field annealing and Sn alloying

Permanent-magnet materials are one cornerstone of today’s technology, abundant in disk drives, motors, medical equipment, wind generators, and cars. A continuing challenge has been to reconcile high permanent-magnet performance with low raw-material costs. This work reports a Mn-Bi-Sn alloy exclusively made from inexpensive elements, exhibiting high values of Curie-temperature, magnetization, anisotropy, coercivity, and energy product. The samples are produced by field annealing of rapidly quenched Sn-containing MnBi alloys, where the improvement of the magnetic properties is caused by the substitutional occupancy of the 2c sites in the hexagonal NiAs structure by Sn. The substitution modifies the electronic structure of the compound and enhances the magnetocrystalline anisotropy, thereby improv- ing the coercivity of the compound. The energy product reaches 114 kJ/m3 (14.3 MGOe) at room temperature and 86 kJ/m3 (10.8 MGOe) at 200○C; this value is similar to that of the Dy-free Nd2Fe14B and exceeds that of other rare-earth-free permanent-magnet bulk alloys, as encountered in automotive applications

Texture development and coercivity enhancement in cast alnico 9 magnets

The effect of Y addition and magnetic field on texture and magnetic properties of arc-melted alnico 9 magnets has been investigated. Small additions of Y (1.5 wt.%) develop a (200) texture for the arc-melted alnico 9 magnet. Such a texture is hard to form in cast samples. To achieve this goal, we set up a high-field annealing system with a maximum operation temperature of 12500 C. This system enabled annealing in a field of 45 kOe with subsequent draw annealing for the solutionized buttons; we have been able to substantially increase remanence ratio and coercivity, from 0.70 and 1200 Oe for the Y-free alnico 0 to 0.90 and 1400 Oe for the Y-doped alnico 9, respectively. A high energy product of 7.3 MGOe has been achieved for the fully heat-treated Y-doped alnico 9. The enhancement of coercvity is believed to arise from the introduction of magnetocrystalline anisotropy from 80 nm Y2Co17- type grains, which are exchange-coupled to the main-phase alnico rods