Y. Öner

The role of Pt impurities on both bulk and surface anisotropies in amorphous NiMn films

Abstract Electron spin resonance (ESR) measurements have been carried out on amorphous Ni76Mn24 and Ni74Mn24Pt2 films in the temperature range 4–300 K. The spectra at each temperature have been taken for both parallel geometry, where the applied magnetic field is in the surface of the films and perpendicular geometry with the magnetic field perpendicular to the surface. Using computer simulations, the bulk (K) and surface anisotropy constants (Q and ks), the exchange stiffness constants (D) and the induced exchange fields (Hexc) were obtained for both films as a function of temperature. At lower temperatures, we were not able to analyse the ESR spectra due to very broad lines, which are associated with the frustration of spins. The frustration manifests itself at higher temperatures for Ni74Mn24Pt2 than for Ni76Mn24 and becomes more severe with Pt impurities. The induced exchange anisotropy, Hexc, which is created during cooling of the sample rotates freely towards the applied field. The temperature behaviour of this anisotropy is in agreement with the exponential law (exp(-α/T) for both samples. The exchange stiffness constants, D, of both samples show similar behaviour, D increases with decreasing temperature, slowly at first above certain temperature values (100 K for Ni76Mn24 and ∼ 150 K for Ni74Mn24Pt2) and then more rapidly at lower temperatures, being related to the frustration of the system. Furthermore, the deduced bulk and surface anisotropy coefficients from the ESR data analysis revealed that Pt impurities have a considerable influence on the domain structure and, accordingly, the surface anisotropies of the NiMn system.

A spin-wave resonance study on reentrant Ni77Mn23 thin films

Abstract The magnetic properties of thin (300 A) reentrant Ni 77 Mn 23 films grown on a quartz substrate using the electron beam evaporation technique have been investigated. The spin-wave resonance (SWR) technique has been used to study the exchange parameter D and magnetic anisotropies as a function of temperature between 4 and 300 K. The SWR spectra exhibit directional character upon cooling the sample in an external field H c , which is a sign of a spin-glass property. At higher temperatures, in addition to the dominant surface SWR mode, there is a much weaker bulk mode on the lower field side. Using a uniaxial surface and effective bulk anisotropy together we simulated the experimental spectra and deduced the surface anisotropy and magnetic stiffness parameters as a function of temperature. The sample shows an easy plane surface anisotropy. Both the exchange and anisotropy parameters increase when the temperature is in the spin-glass regime. Furthermore, the surface anisotropy energy dominates the overall magnetic behavior of this system.

Anomalous anisotropy of re-entrant film

Re-entrant polycrystalline films, 600 A thick, grown on a quartz substrate by using an electron beam evaporation technique, have been investigated. A spin-wave resonance (SWR) technique has been used to study the magnetic properties of the films. The SWR spectra exhibit highly anisotropic behaviour with respect to the film normal. When the sample is rotated around the film normal in a fixed applied field, both the resonance fields and the line-shapes are changed. This unexpected behaviour - for a polycrystalline film - has been attributed to a growth-induced geometric (oblique) anisotropy. The SWR spectra have been successfully analysed by using this oblique anisotropy term in addition to usual magnetocrystalline effective bulk, surface, and unidirectional anisotropy terms in the magnetic free energy. A substantial and temperature-dependent growth-induced geometric anisotropy has been obtained, beside the usual bulk and surface anisotropies. A strong correlation between the surface and oblique anisotropy has been found. This correlation has been attributed to a manifestation of the influence of easy-plane surface anisotropy along the fibre axes of individual grains on the effective magnetic anisotropy energy.

Resistivity minimum in Ni76−xMn24Ptx and Ni72−xMn28Ptx amorphous alloys

Abstract The electrical resistivity ϱ as a function of temperature has been measured for amorphous thin films of Ni 76 − x Mn 24 P x ( x = 0, 2, 5) and Ni 72 − x Mn 28 Pt x ( x = 0, 4, 10) alloys in the temperature range 1.5–400 K. The resistivity for every sample exhibits a low-temperature minimum, T min , following the logarithmic dependence. The results show the presence of two effects: one of them, associated with Kondo-like scattering, exists for all samples, the other, the electron-localized spin-wave scattering mechanism is dominant for the samples with higher Mn concentrations. In addition, the resistivity shows quadratic temperature behaviour in the intermediate temperature range. The examination of the quadratic behaviours suggests that the scattering mechanism with magnetic origin is not negligible compared with that of the electron-structural disordered scattering. Furthermore, the present data permits us for the first time to compare the nature of the resistivity minimum at lower temperatures for each sample of amorphous Ni 76 − x Mn 24 P x and Ni 72 − x Mn 28 Pt x alloys with that of the corresponding alloys in the polycrystalline phase.

Localization in amorphous films

The electrical resistivity has been measured as a function of temperature between 1.5 - 300 K for amorphous alloys with x = 0.176, 0.22, 0.26. The resistivities all show square-root temperature dependences below the minima temperatures. The high-field magnetoresistance (H varies between 0 - 120 kOe) can be accounted for in theoretical models of localization in the presence of strong spin - orbit interaction. In addition, the spin-flip scattering rate due to local spin fluctuations decreases with increasing temperature and then levels off at about T = 50 K in a manner consistent with the magnetic state of the sample, while the inelastic scattering rate in this range remains almost of the same order. Furthermore, the magnetic anisotropy of the resistivity together with the magnetization data show that the magnetic order is progressively suppressed with increasing Fe content.

Magnetism of new semi-Heusler compounds FeVSn and CoVSn

Magnetic properties of new intermetallics FeVSn and CoVSn are reported. Both materials belong to the group of semi-Heusler compounds, with lattice parameters of 0.593 and 0.598 nm respectively in a cubic MgAgAs structure. X-ray investigations indicate partial atomic ordering in both cases. Ferromagnetic order was observed in FeVSn with T/sub c/=86 K. The saturation moment obtained by M versus H measurements at 4 K has a value of 0.45 Gig, indicating that FeVSn is a weak itinerant ferromagnet. CoVSn was found to be paramagnetic with no magnetic ordering down to 2 K. Room-temperature /sup 51/V NMR spectra were used as a local probe of the magnetic behavior of these materials. The results Indicate that V d orbitals play a strong role in the magnetism of FeVSn. Results for CoVSn are consistent with predicted semiconducting or semimetallic behavior for this material.

Effect of exchange bias on the electrical resistivity of Pd doped NiMn thin films: Two-channel Kondo system

Electrical resistivity measurements have been carried out for both flash-evaporated reentrant spin glasses (RSGs) (Ni76−xPdx)Mn24 and Ni74.5Mn23.5Pd2, as well as Ni75Mn23Pd2, a pure SG. These measurements were carried out at temperatures down to 4K. We observed a very deep resistivity minimum at about 75K for Ni74Mn24Pd2. It was found previously [Oner et al., J. Appl. Phys. 89, 7044 (2001)] that this sample shows the largest coercivity and exchange unidirectional anisotropy among these films. In addition, magnetization measurements show that this takes place just on the border of the RSG such that it could be handled as a superparamagnetic sample. Previously it was assumed that the exchange bias created in the sample between the domains plays the dominant role in the resistivity minimum. On the other hand, in order to account for the temperature dependence of the resistivity below the minimum we have analyzed these data using the Kondo, two-channel Kondo, weak localization, and Cochrane models for structu...

Influence of palladium on the unidirectional anisotropy field in NiMn disordered systems

Abstract We have carried out AC susceptibility, hysteresis cycle and torque measurements on (Ni 76− x Pd x )Mn 24 , with 0 ⩽ x ⩽ 5, bulk alloys in the temperature and field ranges of 4.2–260 K and 0–20 kG, respectively. The AC susceptibility curves, χ ′( T ), present a maximum at the same freezing temperature, T g , as the isoelectronic alloy Ni 76 Mn 24 . These alloys exhibit also the same Curie temperature, T c , and the same saturated magnetization, M s . However, both the unidirectional anisotropy field, H a , and the coercivity field, H c , increase considerably with the palladium content. For instance, H a is 1.5 times larger for (Ni 74 Pd 2 )Mn 24 than for Ni 76 Mn 24 . The torque measurements give approximately the same H a at 4.2 K as the hysteresis cycle. We conclude that Pd impurities enhance unidirectional anisotropy but have practically no effect on T c , T g and M s . This is ascribed to the greater spin-orbit coupling of palladium.

Magnetic disorder in Ti doped ErCo2: High-magnetic-field study

Magnetization measurements have been carried out on ErCo(2−x)Tix (x=0, 0.01, 0.02, 0.03, 0.04, and 0.05) alloys in the temperature range of 4–300 K under high dc magnetic fields of up to 23 T. The substitution of Ti for Co causes no change in the crystal structure of ErCo2 (cubic Laves phase C15 with space group Fd3¯m). The lattice parameter a decreases almost linearly with increasing x up to x=0.04 and then increases slightly for the largest Ti concentration. The Curie temperature TC shows a similar change with Ti concentration. Above TC all the studied samples show a small but unambiguous hysteresis associated with a metamagnetic transition. The observed hysteresis is attributed to the coexistence of the paramagnetic and the field-induced ferrimagnetic states. Linear temperature extrapolations of the critical fields Hc1 (where the hysteresis sets in) and Hc2 (the highest field where the hysteresis is observed) were done for each sample. Two different critical temperatures were found suggesting the exist...

High-field study of the magnetization in Ti doped ErCo2 compound

The magnetization of powdered MgCu2 structure-type intermetallic ErCo1.95Ti0.05 was studied in the temperature range of 4.2–123K in magnetic fields up to 23T. A field-cooled magnetization jump at certain temperatures (TC) corresponds to a first-order magnetic phase transition from a paramagnetic to a ferrimagnetic state indicating cobalt moment formation. We observe that TC approach asymptotically T=50K with the cooling field Hc. The magnetization above TC stays constant (temperature independent), but does not saturate even in the field of 23T. The moment per unit formula was found to be approximately 7.25μB for this field. This behavior may be attributed to domain structures with the enhanced magnetocrystalline anisotropy field with titanium impurities. Magnetization versus magnetic field curves has also been recorded for both zero-field-cooled and field-cooled cases at various temperatures below and above TC. These curves below TC show typical ferrimagnetic phase; in the intermediate temperature range o...