C. M. Zhen

Magnetic properties of n-type Cu-doped ZnO thin films

A series of n-type Zn1−xCuxO (x=0.02, 0.06, 0.10, and 0.12) films was prepared using direct current reactive magnetron sputtering. Magnetic measurements indicate that all the films are ferromagnetic at room temperature and the moment per Cu ion decreases with increasing copper concentration and nitrogen doping. The observed magnetic moment was 1.8μB∕Cu for Zn0.98Cu0.02O film and the transition temperature of about 350K decreased to 320K due to nitrogen doping. It can be concluded that itinerant electrons play an important role in ferromagnetism. The resistivity increased with increasing copper concentration and nitrogen doping. The anomalous Hall effect has been found for ferromagnetic materials.

Room-temperature ferromagnetism in n-type Cu-doped ZnO thin films

A series of n-type Cu-doped ZnO thin films was prepared by magnetron sputtering. Such films have shown ferromagnetic properties at room temperature. The Cu ion is in a univalent state as identified by x-ray photoelectron spectroscopy. The moment per Cu ion decreases as the Cu concentration increases. The largest moment about 0.52u2002μB∕Cu was observed for Zn0.95Cu0.05O thin film. The Curie temperature about 360 K was observed for Zn0.95Cu0.05O:N film. To explore the relationship between ferromagnetism and carrier density, nitrogen was introduced into the samples which resulted magnetization and the transition temperature decreasing. These results indicate that the electron density plays an important role in ferromagnetism.

Martensitic and magnetic transformation in Mn50Ni50−xSnx ferromagnetic shape memory alloys

A martensitic transformation (MT) from a body-centered-cubic austenitic phase to a tetragonal martensitic phase has been found in Mn50Ni50−xSnx (0u2009≤u2009xu2009≤u200911) alloys. The martensitic transformation temperature can be decreased by about 71.6u2009K by increasing the Sn concentration by 1 at. %. For 9u2009≤u2009xu2009≤u200911, Mn50Ni50−xSnx ferromagnetic shape memory alloys are obtained. Due to the large magnetization difference (ΔMu2009=u200960u2009emu/g) and small thermal hysteresis (ΔTu2009=u20096u2009K) in the Mn50Ni40Sn10 alloy, a two-way magnetic-field-induced martensitic transformation is observed with dT/dHu2009=u20092u2009K/T.

Anomalous magnetic configuration of Mn2NiAl ribbon and the role of hybridization in the martensitic transformation of Mn50Ni50−xAlx ribbons

The magnetic configuration of Mn2NiAl ribbon has been investigated. In contrast to Ni2MnAl, the compound Mn2NiAl with considerable disorder does exhibit ferromagnetism and, due to exchange interaction competition, both ferromagnetic and antiferromagnetic moment orientations can coexist between nearest neighbor Mn atoms. This is unexpected in Heusler alloys. Regarding the mechanism of the martensitic transformation in Mn50Ni50−xAlx, it is found that increasing the Al content results in an unusual change in the lattice constant, a decrease of the transformation entropy change, and enhancement of the calculated electron localization. These results indicate that the p-d covalent hybridization between Mn (or Ni) and Al atoms gradually increases at the expense of the d-d hybridization between Ni and Mn atoms. This leads to an increased stability of the austenite phase and a decrease of the martensitic transformation temperature. For 11u2009≤u2009xu2009≤u200914, Mn50Ni50−xAlx ferromagnetic shape memory alloys are obtained.

Ferromagnetism in hydrogenated N-doped amorphous carbon films

Room temperature ferromagnetism has been observed in hydrogenated N-doped amorphous carbon films (a-CNx:H) prepared by plasma enhanced chemical vapor deposition. The magnetization of the films changed depending on the ratio (R) of the flow rate of nitrogen to that of methane during deposition and on the annealing temperature. The highest magnetization of the as-deposited samples was obtained with Ru2009=u20094. Annealing the films at 300u2009°C resulted in a significant increase in the magnetic moment compared to that of the as-deposited films. When the annealing temperature was above 500u2009°C, the magnetic moment of the samples decreased. No ferromagnetic impurities could be detected. The ferromagnetism of the a-CNx:H film is attributed mainly to spin-polarization of the p orbitals of the N adatoms on the surface or in the interlayers of irregular carbon.

Tuning exchange bias by Co doping in Mn50Ni41−xSn9Cox melt-spun ribbons

In Mn50Ni41−xSn9Cox ribbons, the exchange bias field is very sensitive to the Co content. Based on both theoretical and experimental studies, it has been found that with increasing Co content, the pinned phase (ferromagnetic phase) remains almost unchanged while the pinning phase is changed from a canonical spin glass to a cluster spin glass and finally to a ferromagnetic phase. Changing the Co content in Mn50Ni41−xSn9Cox alloys has been proven to be an effective way of tuning the magnetic anisotropy and the phase structure of the pinning phase. With different Co contents, a continuous tuning of the exchange bias field from 345u2009Oe to 3154u2009Oe is realized.