16
1 Introduction
a
b
E
r
δ
(a)
(b)
Fig. 1.5 a Schematic diagram of the interactions involved in the Dzyaloshinskii-Moriya interaction,
involving the magnetic ions (blue circle) and the superexchange-mediating oxygen ion (red circle),
showing the displacement of the oxygen ions r away from the vector connecting adjacent spins
δ. b Schematic diagram of the exchange-striction mechanism for electromagnons in rare-earth
manganites, depicting four Mn ions (blue) and four oxygen ions (purple) in one ab layer. An
electric field applied in the a-direction causes a uniform displacement of the oxygen ions, which
modulates the nearest-neighbour exchange interactions
cycloid, hence if the plane of the cycloid is changed (such as by a magnetic fieldinduced spin flop) the selection rules also change accordingly. The same does not
hold for the ES electromagnons, which are linked to the crystal lattice, and as such
do not rotate with the plane of the cycloid.
The first experimental observations of electromagnons were made by Pimenov
et al. in 2006, when they were observed in the rare-earth manganates TbMnO 3
and GdMnO 3 [40]. Since this pioneering paper, much progress has been made in
understanding electromagnons in rare-earth manganates (RMnO 3 ) and manganites
(RMn 2 O 5 ) from both experimental [41–44] and theoretical perspectives [49, 50].
Multiferroic phases, and hence electromagnons, tend to be observed at temperatures
below ∼ 70 K in these materials. As mentioned previously, electromagnons that occur
at higher temperatures would be highly desirable for any potential applications. An
IR and Raman-active electromagnon has recently been observed at up to 250 K in
a z-type hexaferrite [51]. However the material system focused on in this thesis is
Cu 1−x Zn x O alloys, in which multiferroicity and electromagnons have been observed
between 213 and 230 K in x = 0 [8] and between 159 and 190 K in x = 0.05 [52].
A particularly intruiging prediction from theoretical investigations is that the introduction of non-magnetic impurities into CuO may stabilize the multiferroic phase
at higher temperatures than the pure case [53], and that hydrostatic pressure can
broaden the multiferroic phase above room temperature [54]. The following section
will explore the current literature on multiferroicity and electromagnons in CuO.
1 Introduction
a
b
E
r
δ
(a)
(b)
Fig. 1.5 a Schematic diagram of the interactions involved in the Dzyaloshinskii-Moriya interaction,
involving the magnetic ions (blue circle) and the superexchange-mediating oxygen ion (red circle),
showing the displacement of the oxygen ions r away from the vector connecting adjacent spins
δ. b Schematic diagram of the exchange-striction mechanism for electromagnons in rare-earth
manganites, depicting four Mn ions (blue) and four oxygen ions (purple) in one ab layer. An
electric field applied in the a-direction causes a uniform displacement of the oxygen ions, which
modulates the nearest-neighbour exchange interactions
cycloid, hence if the plane of the cycloid is changed (such as by a magnetic fieldinduced spin flop) the selection rules also change accordingly. The same does not
hold for the ES electromagnons, which are linked to the crystal lattice, and as such
do not rotate with the plane of the cycloid.
The first experimental observations of electromagnons were made by Pimenov
et al. in 2006, when they were observed in the rare-earth manganates TbMnO 3
and GdMnO 3 [40]. Since this pioneering paper, much progress has been made in
understanding electromagnons in rare-earth manganates (RMnO 3 ) and manganites
(RMn 2 O 5 ) from both experimental [41–44] and theoretical perspectives [49, 50].
Multiferroic phases, and hence electromagnons, tend to be observed at temperatures
below ∼ 70 K in these materials. As mentioned previously, electromagnons that occur
at higher temperatures would be highly desirable for any potential applications. An
IR and Raman-active electromagnon has recently been observed at up to 250 K in
a z-type hexaferrite [51]. However the material system focused on in this thesis is
Cu 1−x Zn x O alloys, in which multiferroicity and electromagnons have been observed
between 213 and 230 K in x = 0 [8] and between 159 and 190 K in x = 0.05 [52].
A particularly intruiging prediction from theoretical investigations is that the introduction of non-magnetic impurities into CuO may stabilize the multiferroic phase
at higher temperatures than the pure case [53], and that hydrostatic pressure can
broaden the multiferroic phase above room temperature [54]. The following section
will explore the current literature on multiferroicity and electromagnons in CuO.