Abstract. The phenomenology of exchange bias effects observed in structurally single-phase alloys and compounds but composed of a variety of coexisting magnetic phases such as ferromagnetic, antiferromagnetic, ferrimagnetic, spin-glass, clusterglass, disordered magnetic states are reviewed. The investigations on exchange bias effects are discussed in diverse types of alloys and compounds where qualitative and quantitative aspects of magnetism are focused based on macroscopic experimental tools such as magnetization and magnetoresistance measurements. Here, we focus on improvement of fundamental issues of the exchange bias effects rather than on their technological importance.

Magnetite nanoparticles of nearly uniform size have been prepared by precipitating ferrous ions in the presence of two different polyelectrolytes, viz., poly(acrylic acid) and the sodium salt of carboxymethyl cellulose at high pH (∼13). The size of the magnetite nanoparticles can be controlled easily by varying the concentration of the polyelectrolyte in the medium. Transmission electron microscopy study indicates that the average particle size varies from 5 to 15 nm, depending on the concentration and the nature of the polyelectrolyte. X-ray diffraction study shows the presence of only magnetite phase. FTIR spectroscopy and thermogravimetric analysis confirmed the presence of polyelectrolyte on the magnetite surface. The magnetization and Mössbauer studies were performed on two samples with mean diameters 7.0 and 14.7 nm. Magnetization measurements suggest that both of these particles are of single magnetic domain. The measurements also estimate the superparamagnetic blocking temperature, T B = 145 K for smaller particles, while T B > 300 K for the larger particles. Mössbauer spectra at 300 K show only a quadrupole doublet for the smaller particles and mostly a magnetically separated sextet for the larger ones, indicating also the marked size dependence of moment dynamics.

The ground state properties of the ferromagnetic shape memory alloy of nominal composition Ni2Mn1.36Sn0.64 have been studied by dc magnetization and ac susceptibility measurements. Like few other Ni-Mn based alloys, this sample exhibits exchange bias phenomenon. The observed exchange bias pinning was found to originate right from the temperature where a step-like anomaly is present in the zero-field-cooled magnetization data. The ac susceptibility study indicates the onset of spin glass freezing near this step-like anomaly with clear frequency shift. The sample can be identified as a reentrant spin glass with both ferromagnetic and glassy phases coexisting together at low temperature at least in the field-cooled state. The result provides us an comprehensive view to identify the magnetic character of various Ni-Mn-based shape memory alloys with competing magnetic interactions.PACS numbers: 75.50. Lk, 75.60.Nt,75.47.Np Recently Ni 2 Mn 1+x Z 1−x (Z = In, Sn, and Sb) based ferromagnetic shape memory alloys (FSMAs) have attracted considerable attention due to their multifunctional properties, which include magnetic sueprelasticity, giant magnetoresistance, large inverse magnetocaloric effect and magnetic memory effect [1,2,3,4,5]. The observed phenomena are primarily related to the magnetic field (H) induced reverse transition across the martensitic transformation (MT) [6]. The stoichiometric Heusler compositions (Ni 2 MnZ) are all ferromagnetic with the Curie point (T C ) lying just above room temperature. The excess Mn doping at the expense of Z atoms induces structural instability in the system leading to the ferromagnetic shape memory effect. Short range antiferromagnetic (AFM) interaction between the excess Mn (at the Z site) and the original Mn atoms has been predicted for the doped alloys [7,8]. Although, predominantly ferromagnetic (FM) character is present in the Mn doped alloys with T C around 300 K, the AFM correlation is evident from the gradual decrease of saturation moment with increasing amount of excess Mn [7,9]. Diffuse peaks observed in the powder neutron diffraction data of Ni-Mn-Sn alloys also indicate the existence of incipient AFM coupling [10].Evidently, the magnetic nature of the ground state of the alloys may not be very simple. A fascinating evidence for the complex ground state of the alloys is the recently observed exchange bias (EB) phenomenon in bulk samples [9,11,12]. EB is referred to the shift of the center of the magnetic hysteresis loop from the origin when the sample has been cooled from high temperature in presence of magnetic field [13]. The origin of EB is gener- * Electronic address: sspsm2@iacs.res.in ally ascribed to the presence of FM and AFM interfacial coupling in a heterogeneous sample. EB has also been observed in materials having FM/spin-glass (SG) and FM/Ferrimagnet interfaces [14,15] other than FM/AFM systems. However in all the cases, it is required that the ordering temperature (T N F ) for the non-ferromagnetic phase (may be AFM, SG or ferrimagnet) should be l...

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