Satya Prakash Pati

Structural, magnetic and hyperfine properties of single-phase SrFe12O19 nanoparticles prepared by a sol–gel route

Single-phase nanocrystalline strontium hexaferrite (SrFe12O19) powders have been synthesized by a sol–gel method and their structural, magnetic and hyperfine properties are discussed. The optimum annealing temperature of the as-prepared gel for formation of the single-phase SrFe12O19 structure has been found to be 800 °C. X-ray diffraction and Fourier transform infrared studies confirmed formation of the hexaferrite phase. The cation distribution at crystallographic inequivalent sites of the hexaferrite structure has been examined by 57Fe Mossbauer spectroscopy and found to be identical to that reported earlier. Single-phase SrFe12O19 nanoparticles (NPs) showed a magnetic hysteresis loop with high coercivity and saturation magnetization denoting an irreversible magnetization character. The temperature dependent magnetization measurements reveal a difference between the zero-field-cooled and field-cooled curves throughout the measurement range 5–300 K that is attributed to superparamagnetic relaxation of finer hexaferrite particles and disordered spins at the surface of the NPs. Both hyperfine and magnetic studies confirm that magnetic anisotropy plays a crucial role in hexaferrite NPs.

Signature of exchange bias and spin-glass like phenomena in Fe/CoO nanocomposite

Nanocomposite of Fe/CoO having 10 at. wt. % of α-Fe has been prepared by the high energy ball-milling method. The average particle size was estimated to be 15 nm from the results obtained by the rietveld refinement of x-ray diffraction pattern. The presence of exchange coupling at the interface of ferromagnetic and antiferromagnetic components was confirmed from the shift observed in field-cooled (FC) magnetic hysteresis loop. Irreversibility in temperature dependent zero field cooled (ZFC)-FC magnetization at different applied fields indicates the presence of spin-glass like (SGL) phase. Memory effect observed in the temperature dependent dc-magnetization at FC condition confirmed the existence of SGL state. It is argued that polydispersity and defects at the interfaces of nanoparticles are the possible causes of the observed SGL phase.

Morin transition temperature in (0001)-oriented α-Fe2O3 thin film and effect of Ir doping

The structural properties and Morin transition in c-plane-oriented α-Fe2O3 and Ir-doped α-Fe2O3 thin films have been investigated. The enhancement of the Morin transition temperature (TM) in α-Fe2O3 film by Ir doping has been demonstrated. The TM in the c-plane-oriented α-Fe2O3 thin film was determined from the temperature-dependent in-plane magnetization and change of coercivity (Hc); this TM value was found close to that of bulk α-Fe2O3. The spin directions of non-doped and Ir-doped α-Fe2O3 at room temperature were also estimated from conversion electron Mossbauer spectroscopy measurements. We confirmed that Ir doping dramatically enhances the TM of α-Fe2O3 thin film.

Observation of Enhancement of the Morin Transition Temperature in Iridium-Doped α-Fe2O3 Thin Film by 57Fe-Grazing Incidence Synchrotron Radiation Mössbauer Spectroscopy

The Morin transition of a (0001)-oriented iridium-doped α-Fe2O3 thin film deposited on an Al2O3(0001) substrate was studied by 57Fe-grazing incidence synchrotron radiation Mossbauer spectroscopy (GISRMS). Temperature-dependent spectra proved that the iridium doping markedly enhanced the Morin temperature of the α-Fe2O3 thin film; the iron spin directions were perpendicular to the film plane at temperatures below 100 °C, while they were in-plane at temperatures above 150 °C. The antiferromagnetic ordering was maintained far above 400 °C. The results demonstrated the availabilities of 57Fe-GISRMS, which enables a very quick evaluation of the magnetism in antiferromagnetic ultrathin films at high temperatures.

Néel temperature of Cr2O3 in Cr2O3/Co exchange-coupled system: Effect of buffer layer

The lattice parameter dependence of the Neel temperature TN of thin Cr2O3 in a Cr2O3/Co exchange-coupled system is investigated. Lattice-mismatch-induced strain is generated in Cr2O3 by using different buffer layers. The lattice parameters are determined from out-of-plane and in-plane X-ray diffraction measurements. The Neel temperature is detected by direct temperature-dependent magnetization measurement as well as the temperature-dependent interface exchange coupling energy. It is observed that in-plane lattice contraction can enhance TN in Cr2O3, which is consistent with theoretical calculations.

Enhancing the blocking temperature of perpendicular-exchange biased Cr2O3 thin films using buffer layers

In this study, we investigated the effect of spacer and buffer layers on the blocking temperature TB of the perpendicular exchange bias of thin Cr2O3 films, and revealed a high TB of 260 K for 20-nm-thick Cr2O3 thin films. By inserting a Ru spacer layer between the Cr2O3 and Co films and changing the spacer thickness, we controlled the magnitude of the exchange bias and TB. By comparing the TB values of the 20-nm-thick Cr2O3 films on Pt and alpha-Fe2O3 buffers, we investigated the lattice strain effect on the TB. We show that higher TB value can be obtained using an alpha-Fe2O3 buffer, which is likely because of the lattice-strain-induced increase of Cr2O3 magnetic anisotropy.

Magnetoelectric switching energy in Cr2O3/Pt/Co perpendicular exchange coupled thin film system with small Cr2O3 magnetization

We investigated perpendicular exchange bias switching by a magnetoelectric field cooling process in a Pt-spacer-inserted Cr2O3/Co exchange-coupled system exhibiting small Cr2O3 magnetization. Although higher magnetoelectric switching energies with decreasing Cr2O3 thickness due to the exchange bias were reported in Cr2O3/Co all-thin-film systems, in this study, we demonstrated low-energy switching in a magnetoelectric field cool process regardless of the exchange-bias magnitude; we balanced the exchange-bias energy with the Zeeman energy associated with finite magnetization in Cr2O3. We proposed a guideline for realizing low-energy switching in thin Cr2O3 samples.

Control of lateral ferromagnetic domains in Cr2O3/Pt/Co thin film system with positive exchange bias

We investigated the switching of perpendicular exchange bias (PEB) from a negative to a positive value in an exchange-coupled Cr2O3/Pt/Co heterostructure. With varying the Pt spacer layer thickness or the measurement temperature, two types of intermediate states were found during the switching process: a double hysteresis loop indicating a local PEB and a single hysteresis loop indicating an averaged PEB. We propose a way to control the lateral ferromagnetic domain, which is associated with the type of the intermediate state, through the control of the exchange coupling magnitude.

Large perpendicular exchange bias and high blocking temperature in Al-doped Cr2O3/Co thin film systems

We investigated the effect of Al doping on the perpendicular exchange bias in Cr2O3/Co thin film systems. By Al doping, a large unidirectional anisotropy energy (J K ~ 0.74 erg/cm2) and a high blocking temperature (T B) were obtained simultaneously. The variations in J K and T B induced by Al doping were discussed on the basis of Mauris model and explained from an increase in magnetic anisotropy and a decrease in the exchange stiffness constant of Cr2O3. Our first-principles calculations suggest a factor to improve the magnetic anisotropy of Cr2O3 markedly, which is different from the previously established lattice strain effect.

Inserted metals for low-energy magnetoelectric switching in a Cr2O3/ferromagnet interfacial exchange-biased thin film system

An inserted metal (IM) layer (such as Pt, Ru, or Cr) is typically fabricated between Cr2O3 and the ferromagnet in magnetoelectric switched antiferromagnet/ferromagnet exchange-biased thin film systems. The IM layer is used as a “spacer layer” to prevent oxidization of Co and enhance the blocking temperature. In this work, we found that the magnetic properties of these thin film systems vary depending on the type of IM, and they could be reproduced by bilayers or multilayers of Co and IM thin films. We also found that the IM layer was spin-polarized by the Co ferromagnetic layer based on X-ray magnetic circular dichroism data. These results suggest that the IM does not work separately as a “spacer layer”, but rather it is an integral component of Co-based stacked ferromagnetic films in magnetoelectric switched Cr2O3/ferromagnet exchange-biased systems. Furthermore, we found that some stacked ferromagnetic films easily exhibit a low unidirectional anisotropy energy (Jk), which may be a good way to decrease the magnetoelectric switching energy of antiferromagnets for practical device applications.