In the Nd0.75Ho0.25Al2 alloy system, the magnetic moments of Nd and Ho occupying the same crystallographic site randomly are antiferromagnetically coupled via long-range indirect exchange interaction mediated by the conduction electrons. A single crystal grown at this stoichiometry displays a magnetic compensation behavior (Tcomp∼24 K) in all orientations. In the close vicinity of Tcomp, the magnetization hysteresis loops measured for H || [100] assume an asymmetric shape, and the notion of an exchange bias field (Hexch) surfaces. Hexch changes sign across Tcomp as the left shift of the loops transforms to the right shift. This phase reversal appears to correlate with the corresponding reversal in the directions of the local magnetic moments of Nd3+ and Ho3+ ions together with that of the conduction electron polarization (CEP). Near Tcomp, where the opposing contributions to the net magnetization from local magnetic moments are nearly equal, the contribution from CEP assumes an accentuated significance. Interestingly, the width of the M-H loop shows a divergence, followed by a collapse on approaching Tcomp from high- as well as low-temperature ends. The observed behavior confirms a long-standing prediction based on a phenomenological model for ferrimagnetic systems. The field-induced changes in the magnetization data leave an imprint of a quasi-phase transition in the heat capacity data. Magneto-resistance (ΔR/R vs. T) has an oscillatory response, in which onset of magnetic ordering and phase reversal in magnetic orientations can be recognized.

The crystallographic structure and magnetic anisotropy have been investigated in Co/Pt multilayers deposited by direct current magnetron sputtering on two different buffer layers, Ta and Pt. Detail theoretical and experimental investigations reveal the presence of three effects: magneto-elastic, interface and shape anisotropies, and their competition results in three distinct regions based on the Co layer thickness (tCo). In the region I, with tCo< 6 Å, the coherent lattice strain modifies the volume anisotropy through magneto-elastic contribution and it leads to overall positive volume anisotropy energy. With further increase in tCo, lattice mismatch initiates and this results in a decrease in magneto-elastic effect and it enhances the interface anisotropy. The presence of both diminished magneto-elastic effect and enhanced interface anisotropy results in an intermediate region (region II) with negative volume anisotropy energy which is relatively wider (6 Å<tCo< 12 Å) for multilayer stacks with Ta underlayer. In the region III, with tCo> 12 Å, the magneto-elastic effect emerges as an interface phenomena and the shape anisotropy becomes dominant.

Exchange bias effect is an important attribute in several device applications.Traditionally, it is discussed as a form of exchange anisotropy at the interface between the ferromagnetic/antiferromagnetic layers of two distinct systems. We report here on the magnetically ordered alloys possessing the feature of unidirectional anisotropy, which reverses its sign across a characteristic temperature. These are admixed rare earth intermetallics, comprising two dissimilar rare earth (RE) ions, belonging to the two different halves of the 4f-series and imbibing the notion of ferromagnetic coupling between the spins of all 4f-rare earth ions and that of the conduction electrons. Magnetic moments of dissimilar RE ions in such admixed alloys, however, get antiferromagnetically coupled and they display magnetic compensation feature such that the field induced reversal in the orientation of the magnetic moments of dissimilar rare earth ions happens across the compensation temperature (T comp ). Above a threshold field, the conduction electron polarization also reverses its sign across T comp , this is argued to correlate with the observed phase reversal in the exchange bias field.

We report the observation of positive magnetization on field cooling (PMFC) in low applied magnetic fields (H < 100 Oe) in a single crystal of Ca 3 Rh 4 Sn 13 near its superconducting transition temperature (T c ≈ 8.35 K). For 30 Oe < H < 100 Oe, the PMFC response crosses over to a diamagnetic response as the temperature is lowered below 8 K. For 100 Oe < H < 300 Oe, the diamagnetic response undergoes an unexpected reversal in its field dependence above a characteristic temperature (designated as T * VL = 7.9 K), where the field-cooled cool-down magnetization curves intersect. The in-phase and out-of-phase ac susceptibility data confirm the change in the superconducting state across T * VL . We ascribe the PMFC response to a compression of magnetic flux caused by the nucleation of superconductivity at the surface of the sample. In very low fields (H < 20 Oe), the PMFC response has an interesting oscillatory behaviour which persists up to about 7 K. The oscillatory nature underlines the interplay between competing responses contributing to the magnetization signal in PMFC regime. We believe that the (i) counterintuitive field dependence of the diamagnetic response for H > 100 Oe and above T * VL (lasting up to T c ), (ii) the oscillatory character in PMFC response at low fields and (iii) the PMFC peaks near 8.2 K in 30 Oe ≤ H ≤ 100 Oe provide support in favour of a theoretical scenario based on the Ginzburg-Landau equations. The scenario predicts the possibility of complex magnetic fluctuations associated with transformation between different metastable giant vortex states prior to transforming into the conventional vortex state as the sample is cooled below T * VL .