Temperature-driven growth of antiferromagnetic domains in thin-film FeRh.

The evolution of the antiferromagnetic phase across the temperature-driven ferromagnetic (FM) to antiferromagnetic (AF) phase transition in epitaxial FeRh thin films was studied by x-ray magnetic linear and circular dichroism (XMLD and XMCD) and photoemission electron microscopy. By comparing XMLD and XMCD images recorded at the same temperature, the AF phase was identified, its structure directly imaged, and its evolution studied across the transition. A quantitative analysis of the correlation length of the images shows differences between the characteristic length scale of the two phases with the AF phase having a finer feature size. The asymmetry of the transition from FM to AF upon cooling and AF-FM upon heating is evidenced: upon cooling the formation of AF phase is dominated by nucleation at defects, with little subsequent growth, resulting in a small and non-random final AF domain structure, while upon heating, heterogeneous nucleation at different sites followed by significant domain size growth of the FM phase is observed, resulting in a non-reproducible final FM large domain structure.

Electron-mediated ferromagnetic behavior in CoO/ZnO multilayers.

CoO/Al-doped ZnO (AZO) multilayers exhibit ferromagnetism up to ~300 K. The magnetic behavior oscillates with odd vs even number of Co layers in the insulating antiferromagnetic CoO and (separately) with the thickness of the AZO layers and vanishes if AZO is replaced by intrinsic ZnO. Magnetization is due to uncompensated (111) ferromagnetic planes of insulating CoO for odd numbers of atomic planes per layer that are coupled together via RKKY exchange mediated by electron carriers in the nonmagnetic AZO layers. The period of the oscillation with AZO thickness qualitatively matches the Fermi wave vector calculated from the carrier concentration measured by ordinary Hall effect. Magnetic polarization of the AZO carriers is confirmed via an anomalous Hall effect that is proportional to the magnetization.

Fe spin reorientation across the metamagnetic transition in strained FeRh thin films.

A spin reorientation accompanying the temperature-induced antiferromagnetic (AFM) to ferromagnetic (FM) phase transition is reported in strained epitaxial FeRh thin films. (57)Fe conversion electron Mössbauer spectrometry showed that the Fe moments have different orientations in FeRh grown on thick single-crystalline MgO and in FeRh grown on ion-beam-assist-deposited (IBAD) MgO. It was also observed, in both samples, that the Fe moments switch orientations at the AFM to FM phase transition. Perpendicular anisotropy was evidenced in the AFM phase of the film grown on IBAD MgO and in the FM phase of that grown on regular MgO. Density-functional theory calculations enabled this spin-reorientation transition to be accurately reproduced for both FeRh films across the AFM-FM phase transition and show that these results are due to differences in strain.

Electronic structure changes across the metamagnetic transition in FeRh via hard X-ray photoemission.

Stoichiometric FeRh undergoes a temperature-induced antiferromagnetic (AFM) to ferromagnetic (FM) transition at ~350 K. In this Letter, changes in the electronic structure accompanying this transition are investigated in epitaxial FeRh thin films via bulk-sensitive valence-band and core-level hard x-ray photoelectron spectroscopy with a photon energy of 5.95 keV. Clear differences between the AFM and FM states are observed across the entire valence-band spectrum and these are well reproduced using density-functional theory. Changes in the 2p core levels of Fe are also observed and interpreted using Anderson impurity model calculations. These results indicate that significant electronic structure changes over the entire valence-band region are involved in this AFM-FM transition.

Thermodynamic measurements of Fe-Rh alloys.

FeRh undergoes an unusual antiferromagnetic-to-ferromagnetic (AFM-FM) transition just above room temperature (T(AFM>FM)) that can be tuned or even completely suppressed with small changes in composition. The underlying temperature-dependent entropy difference between the competing AFM and FM states that drives this transition is measured by specific heat as a function of temperature from 2 to 380 K on two nearly equiatomic epitaxial Fe-Rh films, one with a ferromagnetic ground state (Fe-rich) and the other with an antiferromagnetic ground state (Rh-rich). The FM state shows an excess heat capacity near 100 K associated with magnetic excitations that are not present in the AFM state. The integrated entropy and enthalpy differences between the two alloys up to T(AFM>FM) agree with the previously measured entropy of the transition (ΔS = 17 ± 3 J/kg/K) and yield a T=0 energy difference of 3.4 J/g, consistent with literature calculations and experimental data; this agreement supports the use of the Fe-rich FM sample as a proxy for the (unstable) FM state of the AFM Rh-rich sample. From the low-temperature specific heat, along with sound velocity and photoemission measurements, the lattice contribution to the difference (ΔS(latt) = -33 ± 9 J/kg/K) and electronic contribution (ΔS(el) = 8 ± 1 J/kg/K) to the difference in entropy are calculated, from which the excess heat capacity in the FM phase and the resulting entropy difference are shown to be dominated by magnetic fluctuations (ΔS(mag) = 43 ± 9 J/kg/K). The excess magnetic heat capacity is dominated by the magnetic heat capacity of the FM phase, which can be fit to a Schottky-like anomaly with an energy splitting of 16 ± 1 meV and a multiplicity of 1 per unit cell.

Evidence for nanoscale two-dimensional Co clusters in CoPt3 films with perpendicular magnetic anisotropy.

The length scale of the local chemical anisotropy responsible for the growth-temperature-induced perpendicular magnetic anisotropy of face-centered cubic CoPt(3) alloy films was investigated using polarized extended x-ray absorption fine structure (EXAFS). These x-ray measurements were performed on a series of four (111) CoPt(3) films epitaxially grown on (0001) sapphire substrates. The EXAFS data show a preference for Co-Co pairs parallel to the film plane when the film exhibits magnetic anisotropy, and random chemical order otherwise. Furthermore, atomic pair correlation anisotropy was evidenced only in the EXAFS signal from the next neighbors to the absorbing Co atoms and from multiple scattering paths focused through the next neighbors. This suggests that the Co clusters are no more than a few atoms in extent in the plane and one monolayer in extent out of the plane. Our EXAFS results confirm the correlation between perpendicular magnetic anisotropy and two-dimensional Co segregation in CoPt(3) alloy films, and establish a length scale on the order of 10 Å for the Co clusters.