4 research outputs found
Magnetization steps in Zn_(1-x)Mn_xO: Four largest exchange constants and single-ion anisotropy
Magnetization steps (MST's) from Mn pairs in several single crystals of
Zn_(1-x)Mn_xO (0.0056<=x<=0.030, and in one powder (x=0.029), were observed.
The largest two exchange constants, J1/kB=-18.2+/-0.5K and J1'/kB=-24.3+/-0.6K,
were obtained from large peaks in the differential susceptibility, dM/dH,
measured in pulsed magnetic fields, H, up to 500 kOe. These two largest J's are
associated with the two inequivalent classes of nearest neighbors (NN's) in the
wurtzite structure. The 29% difference between J1 and J1' is substantially
larger than 13% in CdS:Mn, and 15% in CdSe:Mn. The pulsed-field data also
indicate that, despite the direct contact between the samples and a
superfluid-helium bath, substantial departures from thermal equilibrium
occurred during the 7.4 ms pulse. The third- and fourth-largest J's were
determined from the magnetization M at 20 mK, measured in dc magnetic fields H
up to 90 kOe. Both field orientations H||c and H||[10-10] were studied. (The
[10-10] direction is perpendicular to the c-axis, [0001].) By definition,
neighbors which are not NN's are distant neighbors (DN's). The largest DN
exchange constant (third-largest overall), has the value J/kB=-0.543+/-0.005K,
and is associated with the DN at r=c. Because this is not the closest DN, this
result implies that the J's do not decrease monotonically with the distance r.
The second-largest DN exchange constant (fourth-largest overall), has the value
J/kB=-0.080 K. It is associated with one of the two classes of neighbors that
have a coordination number z=12, but the evidence is insufficient for a
definite unique choice. The dependence of M on the direction of H gives
D/kB=-0.039+/-0.008K, in fair agreement with -0.031 K from earlier EPR work.Comment: 12 pages, 10 figures. Submitted to PR
Valency configuration of transition metal impurities in ZnO
We use the self-interaction corrected local spin-density approximation to
investigate the ground state valency configuration of transition metal (TM =
Mn, Co) impurities in n- and p-type ZnO. We find that in pure Zn1-xTMxO, the
localized TM2+ configuration is energetically favored over the itinerant
d-electron configuration of the local spin density (LSD) picture. Our
calculations indicate furthermore that the (+/0) donor level is situated in the
ZnO gap. Consequently, for n-type conditions, with the Fermi energy eF close to
the conduction band minimum, TM remains in the 2+ charge state, while for
p-type conditions, with eF close to the valence band maximum, the 3+ charge
state is energetically preferred. In the latter scenario, modeled here by
co-doping with N, the additional delocalized d-electron charge transfers into
the entire states at the top of the valence band, and hole carriers will only
exist, if the N concentration exceeds the TM impurity concentration