Subrojati Bosu

Large Interface Spin-Asymmetry and Magnetoresistance in Fully Epitaxial Co2MnSi/Ag/Co2MnSi Current-Perpendicular-to-Plane Magnetoresistive Devices

Current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) characteristics were investigated in a Co2MnSi (CMS)/Ag/CMS fully epitaxial device and compared to those in a CMS/Cr/CMS device systematically. Reflection high-energy electron diffraction and transmission electron microscopy images showed the two samples had no remarkable differences and little interdiffusion. The large spin-asymmetry of electron scattering was found at the CMS/Ag inteface compared with that at the CMS/Cr interface. Finally, the largest magneto-resistance (MR) ratio of 28.8% was observed at room temperature in the CMS/Ag/CMS CPP-GMR device.

Temperature dependence of spin-dependent transport properties of Co2MnSi-based current-perpendicular-to-plane magnetoresistive devices

The origin of magnetoresistance (MR) ratio reduction below ~80 K in Co2MnSi (CMS)/Ag/Co2MnSi current-perpendicular-to-plane giant magnetoresistive devices was investigated. The temperature dependence of ΔRA was independent of the CMS layer thickness, indicating that the spin-diffusion length in the CMS layers is unimportant for the reduction in the MR ratio at low temperatures. A small 90° interlayer exchange coupling, which originated from inter-diffused Mn impurities in the Ag spacer, was observed only at low temperatures from 5 to ~100 K. A possible origin for the reduction in the MR ratio below ~80 K is the drastic reduction in the spin-diffusion length of the Ag spacer due to magnetic ordering of the Mn impurities.

Magnetic characterization of thin Co50Fe50 films by magnetooptic Kerr effect

Magnetic properties of thin Co50Fe50 films on MgO(0 0 1) prepared with different annealing temperatures between room temperature and 300 °C are studied by vectorial magnetometry based on the magnetooptic Kerr effect. Independent from the annealing, the Co50Fe50 films reveal some similar magnetic properties, e.g. no quadratic magnetooptic Kerr effect and a magnetic reversal process which is completely in-plane with magnetic easy axes in Co50Fe50〈1 1 0〉 directions. If the alignment of the external magnetic field is close to the magnetic hard axes, incoherent rotation of magnetic moments between saturation and remanence occurs instead of coherent rotation as for the other directions. If the magnitude of the magnetization is polar plotted with respect to the azimuthal magnetization angle, sequential switching of magnetic moments from one magnetic easy axis to another can be proved by the course of the magnetization. Here, a two-domain switching process can be distinguished from a four-domain switching process generated by the incoherent rotation between saturation and remanence. Furthermore, both the uniaxial magnetic anisotropy constants and the domain wall pinning energies are determined from the magnetic switching fields using the Stoner–Wohlfarth model while the cubic magnetic anisotropy constants are obtained from fitting the magnetization curves. The domain wall pinning energies obtained from these analyses decrease with increasing annealing temperature due to fewer defects in the film. The cubic magnetic anisotropy also decreases slightly which can be attributed to relaxation of the crystal lattice for increasing annealing temperature due to a small change of spin–orbit coupling. Compared with the cubic magnetic anisotropy the uniaxial magnetic anisotropy is very small. This may be attributed to the reduction of strain in the film caused by a buffer stack Cr/Au/Cr between the Co50Fe50 film and MgO(0 0 1).

Biquadratic Exchange Coupling in Epitaxial Co

Interlayer exchange coupling (IEC) behavior in epitaxial Co2MnSi (20 nm)/Cr/Fe (7 nm) trilayers has been investigated against Cr spacer thickness (tCr= 0.3-6.3 nm). High-quality trilayer samples have grown on a Cr (5 nm)/Au (30 nm)/Cr (15 nm) buffer layer on a MgO substrate by ultrahigh vacuum (UHV) compatible dc sputtering method. A simple numerical simulation model has been used to explain magnetization process, which shows good agreement with the experimental M-H curves. The values of bilinear and biquadratic coupling energy (J1 and J2) and the cubic anisotropy energy of Co2MnSi and Fe (KCMS and KFe) have been determined uniquely from the simulations. As a result, we have found a dominating contribution of biquadratic (90deg) coupling and absence of bilinear (180deg) coupling in all the samples with nonferromagnetic coupling. It has also been found that, the energetical competition between 90deg coupling energy and anisotropy energy largely affects the magnetization process due to different easy directions of bottom Co2MnSi (110) and Fe (100).

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Spin Seebeck effect (SSE) was investigated in Heusler compound Co2MnSi (CMS) and ferromagnetic binary alloy Ni80Fe20 (Py) thin films deposited on MgO substrates at different ambient temperatures, T = 30 K to 295 K, by maintaining a finite temperature gradient along the film plane. Low temperature enhancement of voltage signal and a peak around T = 70 K were detected in a Pt wire placed on the higher temperature end of CMS (or Py) film. It has been found that the low temperature enhancement was dominated by a thermal artifact, which was generated by the anomalous Nernst effect with an additional perpendicular temperature gradient across the film plane. The remarkable mismatch between thermal conductivities of MgO and CMS (or Py) at low temperatures is considered to give rise to the perpendicular temperature gradient causing the thermal artifact. We showed that consideration of thermal conductivity matching between thin film and substrate is essential for performing experiments of SSE correctly; moreover, t...

MnSi/Cr/Fe Trilayers

Interlayer exchange coupling (IEC) in epitaxially grown Co2MnSi/X/Co2MnSi (X=Cr and V) trilayers has been studied for different thicknesses of X spacer layers. The samples were grown by UHV-compatible dc-sputtering on an MgO (001) substrate. Strong biquadratic coupling with an unusual large oscillation period (3.3–3.5 nm) was observed for X=Cr while no evidence for non-ferromagnetic type IEC was found for X=V.

Thermal artifact on the spin Seebeck effect in metallic thin films deposited on MgO substrates

This work reports on the investigation of structure-property relationships in thin CoFe films grown on MgO. Because of the very similar scattering factors of Fe and Co, it is not possible to distinguish the random A2 (W-type) structure from the ordered B2 (CsCl-type) structure with commonly used x-ray sources. Synchrotron radiation based anomalous x-ray diffraction overcomes this problem. It is shown that as grown thin films and 300 K post annealed films exhibit the A2 structure with a random distribution of Co and Fe. In contrast, films annealed at 400 K adopt the ordered B2 structure.

Interlayer coupling in epitaxial Co2MnSi/X/Co2MnSi (X=Cr and V) trilayer structures

The spacer layer thickness dependence of interlayer exchange coupling has been investigated in the fully epitaxial trilayers of the Co2MnAl (CMA)/Cr/CMA structure. A series of high-quality samples of CMA (20 nm)/Cr (tCr=0.3–8.1 nm)/CMA (10 nm) trilayers was prepared on a MgO substrate by ultrahigh vacuum compatible dc sputtering. Comparison of the results of the experiments and the simulations of magnetization curves revealed novel behavior, dominating the 90° coupling and the absence of 180° coupling. No clear oscillation, only a peak of the 90° coupling strength (J2∼−0.68 erg/cm2), was observed at tCr=1.2 nm.

Structure determination of thin CoFe films by anomalous x-ray diffraction

We model the charge, spin, and heat currents in ferromagnetic metal?normal metal?normal metal trilayer structures in the two current model, taking into account bulk and interface thermoelectric properties as well as Joule heating. The results include the temperature distribution as well as resistance-current curves that reproduce the observed shifted parabolic characteristics. Thin tunneling barriers can enhance the apparent Peltier cooling. The model agrees with the experimental results for wide multilayer pillars, but the giant effects observed for diameters <100 nm are still under discussion

Interlayer thickness dependence of 90° exchange coupling in Co2MnAl/Cr/Co2MnAl epitaxial trilayer structures

The magnetic structure of a thin B2 ordered Co50Fe50 film is determined by vectorial magnetometry using magnetooptic Kerr effect including different polarizations of the incident light and varied directions of the external magnetic field. Both the magnetic reversal process and the magnetic remanence reveal two in-plane magnetic easy axes of different strengths which are not orthogonal. Atypical magnetization curves including multidomain states in magnetic remanence with some magnetic moments providing antiparallel alignment to the direction of the previously applied external field (if projected to the direction of the external field) confirm the appearance of different strong magnetic easy axes. This magnetic structure can be explained by a cubic magnetic anisotropy (CMA) induced by the crystalline film structure superimposed by an additional uniaxial magnetic anisotropy (UMA) which is not parallel to one of the magnetic easy axes of the CMA. The results are compared with the regular magnetic behaviour of a thin A2 ordered Co50Fe50 film with UMA parallel to one of the magnetic easy axes of the CMA.