Giant intrinsic magnetic hardness in RFe5−x Ni x (R=rare earth,x=4 to 5)

Giant low temperature intrinsic magnetic hardness is observed in structurally homogeneous CaCu₅ type compounds RFe_5−x Ni _x . In SmFe_5−x Ni _x , this magnetic hardness peaks approximately at a composition SmFe_0.2Ni_4.8, with an extrapolated coercive force of 230 kOe at absolute zero. The transition metal sublattice is not anisotropic. Thus, the rare earth alone creates giant coercivity. Only compounds withc-axis preference exhibit substantial magnetic hardness (Sm, Er, Tm). Partial substitution of a tetravalent rare earth to produce crystal field anisotropy fluctuations apparently increases coercivity somewhat in the axis-preference compound SmFeNi₄, but has no effect in the plane-preferred compound TbFeNi₄.     

Influence of partial rare-earth substitution for Sm in the giant intrinsic magnetic hardness compound SmCo2Ni3

The phenomenon of giant intrinsic magnetic hardness is investigated in compounds R_1−x Sm _x Co₂Ni₃ with R=Y, Pr, Gd, Tb, Er. Partial Er substitution for Sm actually increases magnetic hardness while all other substitutions decrease magnetic hardness. The strength of coercivity is thus dependent on both the sign and magnitude of the crystal field interaction. The temperature dependence of coercivity is complex in the case of Pr substitution as a result of competing effects from thermal activation and a decrease in anisotropy at low temperatures. This study has been supported by a grant from the National Science Foundation.     

Magnetization switching of CoFeSiB free-layered magnetic tunnel junctions

Magnetic tunnel junctions (MTJs) comprising amorphous Co_70.5Fe_4.5Si₁₅B₁₀, possessing low saturation magnetization of 560 emu/cm³, as a free layer have been investigated. The switching behaviours were confirmed for the micrometer-sized elements experimentally, using the scanning magneto-optical Kerr effect (scanning MOKE). A micromagnetic modelling study was also carried out for the submicrometer-sized elements. By using either a CoFeSiB single or a synthetic antiferromagnetic free-layer structure, the magnetization switching field became much lower than conventionally used CoFe free layered MTJs.     

Giant magnetic moments in dilute magnetic semiconductors

The concentration dependence of the specific magnetic moment value at room temperature in dilute semiconductor titanium oxides doped with either Co or Fe has been investigated. This value was found to increase sharply at small concentrations of magnetic impurity. The magnetic moment of 22.9 μ_B per impurity atom has been revealed for TiO₂ doped with 0.15 at% Co, not yet reported in any semiconductor oxide systems. We conclude the observed giant magnetic moments are caused by the crystal lattice polarization at small impurity concentrations. The comparison with published data point to different types of the magnetization concentration dependence for various semiconductor matrixes that is probably related to the dielectric permittivity of the environment.     

Equilibrium magnetic properties of dipolar interacting ferromagnetic nanoparticles

We use Monte Carlo simulations to study the influence of dipolar interaction on the equilibrium magnetic properties of monodisperse single-domain ferromagnetic nanoparticles. Low field magnetizations simulated in zero field cooling (ZFC)/field cooling (FC) procedures and field-dependent magnetization curves above the blocking temperatures show strong dependence on the concentration and the spatial arrangement (cubic or random) of the magnetic particles. The field-dependent magnetizations can not be simply described by the T^* model at relative low temperatures due to the interplay between anisotropy and dipolar interactions, as well as the spatial arrangement effect.     

Giant magnetic susceptibility enhancement in field-structured nanocomposites

We demonstrate through experiment and simulation that when mono-domain Fe nanoparticles are formed into chains by the application of a magnetic field, the susceptibility of the resulting structure is greatly enhanced (11.4-fold) parallel to the particle chains and is much larger than transverse to the chains. Simulations show that this significant enhancement is expected when the susceptibility of the individual particles approaches 5 in MKS units, and is due to the spontaneous magnetization of individual particle chains, which occurs because of the strong dipolar interactions. This large enhancement is only possible with nanoparticles, because demagnetization fields limit the susceptibility of a spherical multi-domain particle to 3 (MKS). Experimental confirmation of the large susceptibility enhancement is presented, and both the enhancement and the susceptibility anisotropy are found to agree with simulation. The specific susceptibility of the nanocomposite is 54 (MKS), which exceeds the highest value we have obtained for field-structured composites of multi-domain particles by a factor of four.     

Magnetism, magnetocaloric effect and positive magnetoresistance in Fe-doped antipervoskite compounds SnCMn3−xFex (x=0.05-0.20)

The effects of Fe substitution on the structure, magnetic properties, magnetocaloric effect and positive magnetoresistance (MR) effect in antipervoskite compounds SnCMn_3−xFe_x (x=0.05-0.20) have been investigated systematically. Partial substitution of Fe for Mn leads to the monotonic reduction in both the Curie temperature T_C and saturated magnetization (M_S). It can be attributed to the reduction of electronic density of state at the Fermi energy by Fe-doping. The maximum values of magnetic entropy change (−ΔS_M) and positive MR gradually decrease as x increases, due to the broadening of magnetic phase transition. The refrigerant capacity increases initially with x≤0.05, then decreases gradually as x increases further, which is suggested to originate from the competition between the decreasing −ΔS_M and broadening temperature span. Our result indicates that the chemical doping on Mn site is an effective method for manipulating the properties of antiperovskite compounds AXMn₃.     

Magnetic tunnel junctions comprising amorphous NiFeSiB and CoFeSiB free layers

Magnetic and magneto-transport behaviors of magnetic tunnel junctions (MTJs) incorporating newly developed amorphous NiFeSiB and CoFeSiB as free layers were investigated. Both experimental and theoretical approaches were carried out to understand the details of switching characteristics. The MTJs with traditional free layer materials such as NiFe and CoFe were also fabricated and compared. The studied amorphous ferromagnets appeared beneficial for reducing switching fields without loosing tunnelling magnetoresistive outputs. Further layer thickness, composition, and MTJ structure optimization are expected.     

Magnetocaloric effects in (Gd1−xTbx)Co2

The structures and magnetocaloric effects of (Gd_1−xTb_x)Co₂ (x=0, 0.25, 0.4, 0.5, 0.6, 0.7, 0.8, and 1) pseudobinary compounds were investigated by X-ray powder diffraction and magnetic properties measurement. The results show that the T_c of the alloy is near room temperature when X=0.6. The magnetic entropy changes of the compounds increase from 1.7 to 3.6 J/kg K with increasing the content of Tb under an applied field up to 2 T. All the compounds exhibit second order magnetic change. As a result, the values of their ΔS_M are lower than that of some large magnetocaloric effect materials.     

Magnetization processes of iron whiskers in small magnetic fields

The magnetization behaviour of iron whiskers in de magnetic fields has been investigated using the enhanced longitudinal magneto-optic Kerr effect [6]. The experimental results were compared with the phenomenological membrane model [1] and also with magneto static calculations.     

The stress effect on magnetic domain structure of Fe-Co amorphous alloys

The stress effect on magnetic domain structure has been studied for the Fe-Co amorphous alloy system. The domain structure was dominated by the magnetostriction-internal stress coupling. By applying the tensile stress, the domain structure was changed significantly according to the magnitude and sign of magnetostriction. The value of internal stress estimated was about 3×10⁸ dyn/cm² and nearly constant for all the alloy composition.     

High-throughput screening of magnetic properties of quenched metallic-alloy thin-film composition spreads

A novel method for capturing and characterizing high-temperature phases of metallic-alloys in thin-film composition spreads has been developed. A high-vacuum (10⁻⁸ Torr) high-temperature annealing and quenching furnace system has been developed that allows formation of high-temperature phases in thin-film composition-spread samples deposited on 3 in. silicon wafers. Scanning SQUID microscopy and magneto-optical Kerr effect measurements were used to map the remnant magnetization and the hysteresis loops of the spreads of magnetic materials quenched at high temperatures. The combination of these techniques can be used to obtain comprehensive information on the magnetic properties of various metallic-alloy systems.     

Barkhausen effect in magnetic thin films: Experimental noise spectra

In this paper Barkhausen noise spectra are shown that were measured on thin uniaxial 83-17 Ni?Fe films and these spectra were compared with the known magnetic behaviour of these films. On these types of sample the Barkhausen effect had been investigated by Lambeck [1], but their Barkhausen noise spectra have not been investigated so far. The film thicknessd _m in our experiments ranged between 400 and 2400 Å. In this range a change occurs in the dynamics of the magnetization behaviour caused by transitions of the type of domain wall. The results presented here show the very strong dependence of the Barkhausen noise spectra on the type of domain wall. Films with Bloch walls always show a frequency dependence off ^?1.7 in the higher frequency range. Samples in the thickness range where the transition occurs from the Néel wall via the cross-tie wall to the Bloch wall, have exponents between ?1 and ?2. The frequencyf _c above which the measured noise intensity begins to decrease varies very much for the different films. The curve off _c versus film thicknessd _m has the same form as the curve of the domain wall mobilitym versusd _m.     

Scratch-modified magnetic domain patterns in garnet films

If a loaded Al₂O₃ sphere is passed over SmGa and LuTb garnet crystalline films, the domain pattern is modified. The width of the changed pattern is proportional to the diameter of the zone of contact between the sphere and the garnet. The pattern depends on load: a herring-bone type is found at low loads, a parallel pattern at high loads. After a magnetic field has disturbed the pattern, a narrow region of straight domains is left, which persists unless the material is heated above 1100°C. The patterns are independent of the sign of the magnetostriction constants.     

The influence of planar defects on the coercive field of hard magnetic materials

Planar defects are found to act as strong pinning centres in hard magnetic materials. The interaction of the domain wall with planar defects is assumed to originate in a local perturbation of exchange coupling and crystalline anisotropy. The coercive field is calculated within the framework of micromagnetic continuum theory as well as with a discrete lattice theory which accounts for the interaction between the individual atomic spins. The theory has been applied to discuss the influence of grain boundaries and antiphase boundaries in the intermetallic compound Co₅Sm.     

A simple model for magnetic after-effects in magnetite at room temperature

Magnetic after-effects in vacancy-doped magnetite at room temperature are considered on the basis of a statistical thermodynamic treatment. In this paper we study the kinetics of a very small vacancy concentration distributed on the four non-equivalent octahedral sites of the inverse spinel structure. It is to be noted that satisfactory agreement is found between recent experimental results and the theoretical model.     

Switching field dependence on heating pulse duration in thermally assisted magnetic random access memories

The minimum applied field H_SW required to reverse the magnetic moment of the ferromagnetic/antiferromagnetic storage layer of a thermally assisted magnetic random access memory (TA-MRAM) device during the application of a heating electric pulse is investigated as a function of pulse power P_HP and duration δ. For the same power of the heating pulse P_HP (or, equivalently, for the same temperature of the storage layer), H_SW increases with decreasing heating time δ. This behavior is consistently interpreted by a thermally activated propagating domain-wall switching model, corroborated by a real-time study of switching. The increase of H_SW with decreasing pulse width introduces a constraint for the minimum power consumption of a TA-MRAM where writing combines heating and magnetic field application.     

The domain structure and magnetic anisotropy in Co-based amorphous ribbons after annealing in a helical magnetic field: Part II

The interaction of stray fields and the eddy currents inherent in a sandwich domain structure is studied. This interaction allows us to control the existence of a sandwich domain structure in an amorphous ribbon. The formation of non-uniform magnetic anisotropy in an amorphous ribbon as a result of its annealing in a helical magnetic field is also investigated.     

Studies on the magnetic viscosity and the magnetic anisotropy of γ-Fe2O3 powders

2 O₃ powders, acicular γ-Fe₂O₃, and CoFe–γ-Fe₂O₃ powders are prepared by different methods. Particle shapes and mean particle sizes of samples are determined by transmission electron microscopy (TEM). Magnetic parameters are measured by a vibrating sample magnetometer (VSM) at different temperatures. Effective magnetic anisotropy constants K_E of granular γ-Fe₂O₃ powders at different temperatures are obtained by using the law of approach to saturation (LATS). K_E values of acicular γ-Fe₂O₃ and CoFe–γ-Fe₂O₃ powders are measured by a magnetotorquemeter. It is found for the first time that the variation tendency of K_E with temperature for granular γ-Fe₂O₃ is about the same as that of shape magnetic anisotropy K_sh. Fluctuation field H_f and activation volume V_f of samples are measured. A theoretical expression of V_f is derived. For granular γ-Fe₂O₃ powders, calculated activation volumes are consistent with experimental ones at different temperatures. But as for acicular γ-Fe₂O₃ powders, calculated activation volumes are larger than experimental ones. Experimental results show that magnetization reversal of granular γ-Fe₂O₃ at different temperatures is close to homogeneous rotation. Received: 27 November 1996/Accepted: 18 April 1997     

In-plane magnetic surface anisotropies in Fe(110)

In-plane magnetic surface anisotropies have been detected for Fe(110) on W(110) using in situ Conversion Electron Mössbauer Spectroscopy (CEMS). The phenomenon used for the determination of this anisotropy was a switching of the spontaneous magnetizationJ _s from [001] to [1¯10] with decreasing thickness. Analysis of the data is performed using a homogeneous magnetization approximation for competing surface and bulk anisotropies, which is justified by a micromagnetic analysis and established experimentally by CEMS. In-plane surface anisotropy constants for the clean Fe(110) surface, the Fe metal-interface and the FeGaAs interface are determined toK _s,p ^FeUHV =0.065 erg·cm^–2,K _s,p ^FeMetal =0.040 erg ·cm^–2, andK _s,p ^FeGaAs =0.047 erg ·cm^–2, all with an estimated accuracy of the order of 10%.