High-magnetic-field magnetization and magnetoresistance of some amorphous ferromagnets

Measurements of isothermal magnetization and magnetoresistance at T = 4.0-4.2 K and 15 ? H ? 132 kG on the amorphous ferromagnets Fe₈₀B₂₀, Fe₇₈Mo₂B₂₀, Fe₄₀Ni₄₀P₁₄B₆, and Fe₃₂Ni₃₆Cr₁₄P₁₂B₆ indicate a relatively large H-induced increase (3.6%) in the high-magnetic-field magnetization of the latter alloy, and marked differences in the magnitude and sign of the high-field magnetoresistance of the four alloys. The results are qualitatively interpreted in terms of internal effective field distributions which include a small fraction of atomic spins in negative field sites.     

Magnetorefractive effect in granular films with tunneling magnetoresistance

The magnetorefractive effect, consisting in a change of reflectivity under magnetization, has been observed in granular Co-Al-O metal-insulator magnetic films with tunneling magnetoresistance in the IR region. It is shown that this effect manifests itself most clearly in interference conditions, is even in magnetization, and reaches as high as 0.8% at frequencies of about 1100 cm^?, which exceeds both the linear equatorial Kerr and the even orientational magneto-optic effects by an order of magnitude.     

Magnetorefractive effect in granular alloys with tunneling magnetoresistance

The magnetorefractive effect and optical reflectivity are studied in granular Co-Al-O, Co-Si-O, and Co-Ti-O metal-insulator alloys exhibiting tunneling magnetoresistance for compositions close to the percolation threshold. The dependences of these effects on frequency, angle of incidence, and light polarization were measured. The experimental data obtained suggest that the major MRE mechanism in these systems is spin-dependent tunneling at optical frequencies.     

Long-term relaxation of magnetoresistance in a granular ferromagnet

Long-term relaxations (of the logarithmic type) are revealed in the tunnel magnetoresistance of Fe/SiO₂ nanocomposites, which are due to variation of the magnetization of the nanocomposites. Good qualitative agreement between experimental results and the recently developed concepts of the behavior of magnetization of granular ferromagnets [7] proves that the revealed relaxations are associated with the spin-glass nature of the magnetic state of such systems. It is further demonstrated that it is, in principle, impossible to observe such relaxations using the anomalous Hall effect (proportional to magnetization) because of physical reasons, i.e., mesoscopic fluctuations of the Hall voltage as a result of the magnetic field effect and variation of magnetization.     

Negative tunneling magnetoresistance by canted magnetization in MgO/NiO tunnel barriers

The influence of the insertion of an ultrathin NiO layer between the MgO barrier and the ferromagnetic electrodes in magnetic tunnel junctions has been investigated from measurements of the tunneling magnetoresistance and via x-ray magnetic circular dichroism (XMCD). The magnetoresistance shows a high asymmetry with respect to bias voltage, giving rise to a negative value of up to -16% at 2.8 K. We attribute this effect to the formation of noncollinear spin structures at the interface of the NiO layer as inferred from XMCD measurements. The magnetic moments of the interface Ni atoms tilt from their easy axis due to exchange coupling with the neighboring ferromagnetic electrode, and the tilting angle decreases with increasing NiO thickness. The experimental observations are further supported by noncollinear spin density functional calculations.     

Nonconcavity of the magnetization in Ising ferromagnets

Some Ising ferromagnets having nonconcave magnetization are presented as counterexamples to the often assumed case of concave magnetization.     

Systematic studies of tunneling magnetoresistance in granular films made from well-defined Co clusters

The tunneling magnetoresistance (TMR) of granular films prepared by the co-deposition of well-defined ferromagnetic Co clusters and insulating inert-gas matrix atoms has been studied as function of matrix atoms (Kr, Xe), cluster volume fraction, temperature (4–40 K) and cluster size (4.2–5.2 nm). Tunneling samples with resistivities that differ by about five orders of magnitude have a TMR that is found to be independent of matrix and cluster volume fraction, i.e. is independent of both tunneling barrier height and width. All samples show a characteristic TMR(T)-dependence that can be explained by a model which takes into account that magnetic moments at the cluster surface are becoming misaligned with increasing temperature. The fraction of misaligned moments at a fixed temperature is increasing with decreasing clusters size.     

Role of interparticle coupling on the tunneling magnetoresistance of granular films in interacting superparamagnetic system

A phenomenological model is employed to explain the departure of H/T scaling of tunneling magnetoresistance (TMR) in Co_0.43(ZrO₂)_0.57 granular films (see [B.J. Hattink, M. García del Muro, Z. Konstantinovi?, X. Batlle, A. Labarta, Phys. Rev. B 73 (2006) 045418]). The model bases on the modification of Langevin function by taking into account the field-dependent correlation length, in the framework of Inoue–Maekawa model. Both TMR–H and TMR–H/T curves can be reproduced very well by this model, indicating that the interparticle coupling plays an important role on TMR in the interacting superparamagnetic systems. The influence of anisotropy and spin polarization of the magnetic particles on TMR–H/T curves is also discussed.     

Low-temperature heat capacity and magnetization of ferromagnets

The change in the magnetization of a ferromagnet due to disordering in the system of atomic magnetic moments with increasing temperature is put in correspondence with the change in the entropy of the magnetic subsystem. A relationship is obtained for calculating the magnetic component of the heat capacity of the ferromagnet, which satisfactorily describes the temperature dependence of the heat capacity for Fe, Co, TbB₂, and DyB₂ at low temperatures.     

Stress effect on the magnetoresistance of amorphous ferromagnets

To investigate the magnetization in amorphous ferromagnetic alloys we measured the change in the magnetoresistance (H⊥j and H 6j) simultaneously applying a tensile stress along the direction of the ribbons. With increasing stress we observed a complete alignment of the magnetic domain vectors M_s parallel andantiparallel (for λ_s > 0) or perpendicular (for λ_s < 0) to the stress axis. It is found that on average a value of σ_s = 10 kg/mm² is sufficient in all measured samples to produce this effect. The change in the magnetoresistance by stress annealing indicates that a preferred domain orientation in stress direction is induced.     

Spontaneous magnetization of randomly dilute ferromagnets

We consider Ising ferromagnets on random subgraphs of the square lattice. These are obtained by independent random selections either of sites or of bonds. We assume that for each site (or, respectively, bond) the probability of being selected exceeds the critical percolation probability. Then, at sufficiently low temperatures and zero external field, spontaneous magnetization occurs. Some further related results are obtained.     

Magneto-transport properties of dilute granular ferromagnets

We present magnetoresistance (MR) measurements performed on quench-condensed granular Ni thin films which are on the verge of electric continuity. In these systems, the electric conductivity is believed to be governed by the resistance between a very small number of grains. The films exhibit sharp resistance jumps as a function of magnetic field. We interpret these findings as being the result of magneto-mechanical distortions that occur in single grains which act as bottlenecks in the dilute percolation network. The observed features provide a unique measure of magnetostriction effects in nano-grain structures as well as being able to shed light on some of the properties of regular granular magnetic films.     

Resonant inversion of tunneling magnetoresistance

Resonant tunneling via localized states in the barrier can invert magnetoresistance in magnetic tunnel junctions. Experiments performed on electrodeposited Ni/NiO/Co nanojunctions of area smaller than 0.01 microm(2) show that both positive and negative values of magnetoresistance are possible. Calculations based on Landauer-Büttiker theory explain this behavior in terms of disorder-driven statistical variations in magnetoresistance with a finite probability of inversion due to resonant tunneling.     

The spatial fluctuations of the magnetization in amorphous ferromagnets

The spatial variation of the magnetization of amorphous ferromagnets containing transition metal and metalloid atoms is shown to depend explicitly on that of the alloy concentration and of the nearest neighbour distance. This variation will thus be in general on an atomic scale for these materials.     

On the continuity of the magnetization and energy in Ising ferromagnets

We investigate the phase diagram of ferromagnetic Ising spin systems satisfying the G.H.S. inequality. We show that they cannot have a normal first-order phase transition as the temperatureT is varied, i.e., one where the magnetization is discontinuous and there are three coexisting phases. Furthermore, for n.n. interactions, discontinuity in the magnetization at 0 <T ₀ T _c implies an uncountable infinity of pure phases atT ₀.