Ferrimagnetic–ferromagnetic phase transition in Mn4N films favored by non-magnetic In doping

The ferrimagnet Mn4N forms a family of compounds useful in spintronics. In a compound comprising non-magnetic and magnetic elements, one basically expects the compound to become ferromagnetic when the proportion of the magnetic element increases. Conversely, one does not expect ferromagnetism when the proportion of the non-magnetic element increases. Surprisingly, Mn4N becomes ferromagnetic at room temperature when the Mn content is decreased by the addition of In atoms, a non-magnetic element. X-ray magnetic circular dichroism measurement reveals that the magnetic moment of Mn atoms at face-centered sites, Mn(II), reverses between x = 0.15 and 0.27 and aligns parallel to that of Mn atoms at corner sites, Mn(I), at x = 0.27 and 0.41. The sign of the anomalous Hall resistivity also changes between x = 0.15 and 0.27 in accordance with the reversal of the magnetic moment of the Mn(II) atoms. These results are interpreted from first-principles calculation that the magnetic moment of Mn(II) sites which are the nearest neighbors to the In atom align to that of Mn(I) sites.

Molecular-Field Approximation in the Theory of Ferromagnetic Phase Transition in Diluted Magnetic Semiconductors

In this pedagogical paper, the comparative analysis of two common approaches describing the ferromagnetic phase transition in diluted magnetic semiconductors (DMS) is expounded in terms of the Weiss field approximation. Assuming a finite spin polarization of the magnetic ions, the treatment of carrier-ion exchange interaction in the first order evokes a homogeneous Weiss molecular field that polarizes the spins of free carriers. In turn, this spin polarization of the free carriers exerts the effective field that may stabilize the DMS spin polarization belowa critical temperature TC. The treatment of such self-consistent spontaneous DMS magnetization can be done in terms of the spin-spin interaction independent of the inter-ion distance and the infinitesimal in thermodynamic limit. On the other hand, by additionally accounting for the second-order effects of the carrier-ion exchange interaction, we can treat a Weiss field in terms of the Ruderman–Kittel–Kasuya–Yosida indirect spin-spin interaction, which oscillates and does not disappear at finite inter-ion distances in the case of a finite concentration of carriers. These both approaches result in the same Curie temperature TC provided a non-correlated homogeneous random distribution of the localized spin moments over the sample volume. We discuss the origin of such coincidence and show when this is not a case in other more realistic models of the conducting DMSs.

Electric-field control of spin dynamics during magnetic phase transitions

Controlling magnetization dynamics is imperative for developing ultrafast spintronics and tunable microwave devices. However, the previous research has demonstrated limited electric-field modulation of the effective magnetic damping, a parameter that governs the magnetization dynamics. Here, we propose an approach to manipulate the damping by using the large damping enhancement induced by the two-magnon scattering and a nonlocal spin relaxation process in which spin currents are resonantly transported from antiferromagnetic domains to ferromagnetic matrix in a mixed-phased metallic alloy FeRh. This damping enhancement in FeRh is sensitive to its fraction of antiferromagnetic and ferromagnetic phases, which can be dynamically tuned by electric fields through a strain-mediated magnetoelectric coupling. In a heterostructure of FeRh and piezoelectric PMN-PT, we demonstrated a more than 120% modulation of the effective damping by electric fields during the antiferromagnetic-to-ferromagnetic phase transition. Our results demonstrate an efficient approach to controlling the magnetization dynamics, thus enabling low-power tunable electronics.

Driving the polar spin reorientation transition of ultrathin ferromagnets with antiferromagnetic–ferromagnetic phase transition of nearby FeRh alloy film

We show that in-plane to out-of-plane magnetization switching of a ferromagnetic layer can be driven by antiferromagnetic–ferromagnetic phase transition in a nearby FeRh system. For FeRh/Au/FeAu trilayers, the impact of the magnetic phase transition of FeRh onto the perpendicular magnetization of monoatomic FeAu superlattices is transferred across the Au spacer layer via interlayer magnetic coupling. The polar spin reorientation process of the FeAu spins driven by the magnetic phase transition in the FeRh reveals its major features; namely it is reversible and displays hysteresis.