Magnetic compensation phenomenon and the sign reversal in the exchange bias field in a single crystal of Nd _0.75 Ho _0.25 Al ₂

Abstract

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.

Self-magnetic compensation and shifted hysteresis loops in ferromagnetic samarium systems

“…One of the ferromagnetic Sm compounds is driven to yield a magnetic compensation characteristic by substitutional replacement of few percent of Sm 3+ by the Gd 3+ ions. In the compensated stoichiometry, the EB field is elucidated to change sign across T comp and echo the recent findings 12,23 . The behavior in the Sm compounds is generic, and has the potential for niche applications of magnetically ordered materials having large spin polarization, negligible self-stray field and the tunable exchange bias.…”

Exchange bias effect in alloys and compounds

“…UMn 2 was described as an AFM compound associated with a small parasitic FM component which was attributed to uncompensated sublattice moment or uncompensated domain wall magnetization, although absence of long range magnetic ordering was confirmed recently by 55 Mn NMR and NQR investigations [143]. Very recently, an interesting observation of EB effect has also been reported for R 1−x Gd x Al 2 [144,145] , exhibiting increasing trend of H E with decreasing temperature. Above a threshold field, the conduction electron polarization was also found to reverse its sign at the compensation temperature.…”

Analysis of the crystallographic and magnetic structures of the Tb0.1Pr0.9Al2 and Tb0.25Pr0.75Al2 magnetocaloric compounds by means of neutron scattering

“…Ferromagnetic (FM) coupling occurs if these rare earths are either light or heavy, while antiferromagnetic (AF) coupling is induced when A and B are light and heavy rare earth ions. Additionally, the inverse MCE can be observed, as in the case of (Pr,Gd)Al 2 [3], which in turn can induce compensation points, i.e., temperatures where the net magnetization is zero, as well as an exchange bias(EB)-like effect as in Ho 0.25 Nd 0.75 Al 2 [4] and Gd 0.25 Pr 0.75 Al 2 [5]. Consequently, other than magnetic refrigeration at low temperatures, these materials show a great potential for numerous applications such as information storage technology devices, magnetic random access memory, and magnetic field sensors [1,2,[6][7][8][9][10][11].…”