Novel Bilayer Structures for Short Wavelength High Density Magneto-Optical Data Storage

We report a novel bi-layer thin film structure for high density magneto-optical (MO) data storage, which combines the advantages of blue wavelength and magnetically induced superresolution (MSR) recording. A double-layer system of exchange-coupled light rare-earth (LRE) element doped NdGdFeCo and traditional TbFeCo is used as the recording medium. The experimental results demonstrate that this NdGdFeCo/TbFeCo double layer has large Kerr rotation under blue wavelength. Centre aperture detection (CAD) MSR effect with temperature rising is also observed. Theoretical calculation is also carried out to verify the experimental results. These results collectively suggest that the new bilayer structure is very promising in next generation high density MO data storage.     

Micromagnetic Simulation of Transfer Curve in Giant-Magnetoresistive Head

The transfer curve of the giant-magnetoresistive （GMR） magnetic head represents its most important property in applications, and it is calculated by the micromagnetic modeling of the free layer and the pinned layer in the heart of the GMR head. Affections of the bias hard magnetic layer and the anti-ferromagnetic pinning layer are modeled by effective magnetic fields. The simulated transfer curve agrees with experiment quite well, therefore the values of these effective magnetic fields can be determined by the model. A synthetic antiferromagnetic spin valve structure GMR head is also analyzed for comparison.     

Influence of surface roughness and chemisorption on magnetic hysteresis curves of a Ni(110) surface observed by spin-resolved inverse photoemission

Using the high spin asymmetry in inverse photoemission of the Ni d-band just above the Fermi level as an indicator of surface magnetization, we have measured hysteresis curves of the (110) surface of nickel. Nearly rectangular hysteresis loops indicate a well-defined behavior of the surface magnetization of the picture-frame single crystal with sides along 110 directions. The influence of geometrical order and chemisorption of O, S, and CO on the shape of the hysteresis loops has been investigated. We found a significant reduction of the coercive force (about 10%) if the clean high-quality (110) surface is disordered on an atomic scale by ion bombardment or low-coverage chemisorption.     

Numerical simulation of magnetization process in antiferromagnetic–ferromagnetic bilayer with compensated interface

The properties of antiferromagnetic (AFM)–ferromagnetic (FM) bilayer have been studied using self-consistent mean-field approximation for Heisenberg Hamiltonian. The perpendicular exchange coupling has been revealed in a bilayer with a compensated interface. For a uniform AFM film a symmetrical hysteresis loop has been calculated, because the transverse instability develops within the AFM film at certain critical value of external magnetic field. On the other hand, shifted hysteresis loop with a finite exchange bias field has been obtained for a non-uniform AFM film consisting of various AFM domains with perpendicular directions of the easy anisotropy axes.     

Asymmetrically shaped hysteresis loop in exchange-biased FeNi/FeMn film

The magnetization reversal of the bilayer polycrystalline FeNi(50 Å)/FeMn(50 Å) film sputtered in a magnetic field has been studied by magnetic and magneto-optical techniques. The external magnetic fields were applied along the easy or hard magnetization axis of the ferromagnetic permalloy layer. The asymmetry of hysteresis loop has been found. Appreciable asymmetry and the exchange bias were observed only in the field applied along the easy axis. The specific features of magnetization reversal were explained within the phenomenological model that involves high-order exchange anisotropy and misalignment of the easy axes of the antiferromagnetic and ferromagnetic layers. It has been shown that the film can exist in one of three equilibrium magnetic states in the field applied along the easy axis. The transitions between these states occur as first-order phase transitions. The observed hysteresis loop asymmetry is related to the existence of the metastable state.     

Asymmetric Magnetization Reversal Probed by Recoil Loop Measurements inan Exchange Biased La0.67Sr0.33MnO3/La0.33Ca0.67MnO3 Bilayer Film

We exploit the recoil loop measurements to study La0.07 Sr0.33 Mn O3/La0.33 Ca0.67 Mn O3 bilayer film. the asymmetric magnetization reversal in an exchange-biased It is found that the recoil curve encloses a marked area only in the second quadrant of the hysteresis loop, and the recoil susceptibility in the descending branch of the major loop is evidently higher than that in the ascending branch. The study indicates that the exchange anisotropy of a unidirectional nature and an orientation deviated from the easy axis of the ferromagnetic layer plays a crucial role in creating the reversal asymmetry.     

Analysis of exchange bias effect of NiO+NiFe2O4 composites

Exchange bias (EB) and magnetic properties of ferrimagnetic (FI) NiFe₂O₄ and antiferromagnetic (AFM) NiO bulk composites, prepared by a chemical co-precipitation and post-thermal decomposition method from Fe-doped NiO matrix, have been investigated. Enhanced coercivities and shifted hysteresis loops are still observed for these samples after field cooling. But the vertical magnetization shifts are not observed. In comparison with the bulk samples, a NiO/10% NiFe₂O₄ nanocomposite was also prepared via direct mixture, in which both the horizontal and vertical shift in the hysteresis loops are observed at 10 K. The observed phenomena are explained in terms of interfacial exchange interaction between the two phases and the finite-size effect, respectively.     

Dependence of the driving current on the harmonic behavior of giant magneto-impedance voltage of Co-based amorphous wires

The frequency and the amplitude of the driving AC current dependence on the harmonic behavior of the giant magneto-impedance (GMI) voltage of Co-rich amorphous wire have been studied. Single-peak, two-peak and three-peak behavior in the GMI characteristics was studied with the change of amplitude and frequency of the AC current. GMI ratio was calculated from the first harmonic signals which were initially increased and then decreased monotonically with the increase of frequency (f) and the amplitude (I_AC) of the driving current. The response of the GMI voltage was found non-linear with the generation of higher harmonics at higher amplitude of the driving current. The second and third harmonic signals were also evaluated and their dependence on the driving current amplitude and frequency were presented in the paper. The experimental results were analyzed using single domain rotational model. The voltage harmonics were calculated through Fourier transform of the time derivative of the estimated circular magnetization of the sample.     

Phase-Transition and Magnetic Moment of the Gd3+ Ion in the Gd2Fe17 Compound

The structure and magnetic phase transitions of the Gd2Fe17 compound are investigated by using a differential thermal/thermogravimetric analyzer, x-ray diffraction, and magnetization measurements. The result shows that there are two phase structures for the Gd2Fe17 compound： the hexagonal Th2Nilr-type structure at high temperatures （above 1243℃）, and the rhombohedral Th2Zn17-type structure, respectively. A method to measure the magnetic moments of the Gd-sublattice and the Fe-sublattice in the Gd2Fe17 compound is presented. The moments of the Gd-sublattice and the Fe-sublattice in the Gd2Fe17 compound from 77 to 500 K are measured in this way with a vibrating sample magnetometer. A detailed discussion is presented.     

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.     

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.     

Investigations of the first order magnetic phase transition in dysprosium

The first order magnetic phase transition in polycrystalline sample of dysprosium (Dy) has been investigated in detail by dc-resistance vs. temperature (R-T) measurements. Distinct signature of coexistence of two magnetically ordered phases (i.e. ferromagnetic and antiferromagnetic), originating as a result of the metastable behavior associated with the first order phase transition (FOPT) is found near 90 K. By successfully recording the minor hysteresis loops within the hysteretic R-T regime of Dy, a varying degree of phase coexistence in the FOPT is demonstrated.     

Temperature dependence of the saturation magnetization of ion-implanted YIG films

The temperature dependence of the saturation magnetization of a series of ionimplanted YIG films is presented. The films were implanted with neon ions at an energy of 450 keV; the dose ranged from 2 to 5*10¹⁴ ions/cm². The experimental data can be described by the molecular field theory showing that the ion-implanted part of the film can be approximated as consisting of two regions each having their own magnetization and Curie temperature. The values of these magnetic parameters vary as a function of dose and differ strongly from the values for pure YIG.     

Study of magnetization process in ordered Fe nanowire arrays

Fe polycrystalline nanowires were electrodeposited in nanoporous anodized alumina membranes. Their magnetic properties were studied at 10 K. The behavior of the isothermal remanence _Mr$M_{\mathrm{r}}$ and the demagnetization remanence _Md$M_{\mathrm{d}}$ was determined. The corresponding ΔM$\mathrm{\Delta }M$ plot revealed the character demagnetizing of the dominant interactions. A simple analysis suggests that curling is the reversal magnetization mode in these samples. The forms of the _Mrev(_Mirr)Hi$M_{\mathrm{rev}}(M_{\mathrm{irr}}{)}_{H_{\mathrm{i}}}$ curves were different from those encountered until now, and it seems that these forms are associated with the curling reversal mode.     

Surface electronic structure of Gd(0001) films on W(110)

Received: 1 September 1996/Accepted: 6 January 1997     

The role of the dopant in the magnetism of Fe-doped SnO2 films

Transparent pure and Fe-doped SnO₂ thin films were grown by pulsed laser deposition technique on LaAlO₃ substrates. X-ray diffraction shows that the films are polycrystalline and have the rutile structure. Surprisingly, the pure film presents magnetic-like behavior at room temperature with a saturated magnetization of almost one-third of the doped film (∼3.6 and 11.3 emu/g, respectively) and its magnetization could not be attributed to any impurity phase. Taking into account the magnetic moment measured in the pure film, the effective contribution of the impurity in the doped one can be inferred to be ∼2 μ_B per Fe atom. A large magnetic moment was also predicted by an ab initio calculation in the doped system, which increases if an oxygen vacancy is present near the Fe impurity.     

Hysteresis of two interacting magnetic pairs with inequivalent perpendicular anisotropy

We consider a model for magnetic memory that consists of strongly coupled dipolar or antiferromagnetic (AF) pairs with inequivalent perpendicular anisotropy _K1$K_1$ and _K2$K_2$. For appropriate parameter values, determined in this work, they have two inequivalent storage states with zero net magnetic moment. Both analytical and numerical calculations are performed, in some cases yielding different results because of relaxation effects (i.e., a dependence on the damping parameter α$\alpha$). Hysteresis loops for a wide variety of parameter values are obtained, both for the AF case and the dipole case. An Appendix gives analytic results for slightly non-collinear spins in an applied field, which were used to test the numerical results.     

Magnetic and structural study of Cu-doped TiO2 thin films

Transparent pure and Cu-doped (2.5, 5 and 10 at.%) anatase TiO₂ thin films were grown by pulsed laser deposition technique on LaAlO₃ substrates. The samples were structurally characterized by X-ray absorption spectroscopy and X-ray diffraction. The magnetic properties were measured using a SQUID. All films have a FM-like behaviour. In the case of the Cu-doped samples, the magnetic cycles are almost independent of the Cu concentration. Cu atoms are forming CuO and/or substituting Ti in TiO₂. The thermal treatment in air promotes the CuO segregation. Since CuO is antiferromagnetic, the magnetic signals present in the films could be assigned to Cu substitutionally replacing cations in TiO₂.     

Magnetic properties of Nd–Fe–B film magnets prepared by RF sputtering

Highly oriented films of ∼6 μm in thickness consisting of the Nd₂Fe₁₄B compound phase were obtained by a three-dimensional sputtering method at room temperature and the subsequent crystallization by annealing. The c-axis orientation and coercivity of film samples were sensitive to the sputtering parameters and annealing conditions. The optimum temperature and time for annealing were 650 °C and 30 min to show the highest coercivity without any deterioration for the orientation of Nd₂Fe₁₄B grains, and furthermore the degree of c-axis orientation was increased by decreasing the Ar gas pressure or input power for sputtering. The resultant film magnets with good magnetic properties of B_r=∼1.06 T, H_C=∼371 kA/m, and (BH)_max=∼160 kJ/m³ were obtained under the optimized parameters for sputtering.     

Optical and magnetic properties of (Ga1−xMnx)N/GaN digital ferromagnetic heterostructures

(Ga_1−xMn_x)N/GaN digital ferromagnetic heterostructures (DFHs) and (Ga_1−xMn_x)N/GaN grown on GaN buffer layers by using molecular beam epitaxy have been investigated. The photoluminescence (PL) spectra showed band-edge exciton transitions. They also showed peaks corresponding to the neutral donor-bound exciton and the exciton transitions between the conduction band and the Mn acceptor, indicative of the Mn atoms acting as substitution. The magnetization curves as functions of the magnetic field at 5 K indicated that the saturation magnetic moment in the (Ga_1−xMn_x)N/GaN DFHs decreased with increasing Mn mole fraction and that the saturation magnetic moment and the coercive field in the (Ga_1−xMn_x)N/GaN DFHs were much larger than those in (Ga_1−xMn_x)N thin films. These results indicate that the (Ga_1−xMn_x)N/GaN DFHs hold promise for potential applications in spintronic devices.