106
5 High-Field Terahertz Time-Domain Spectroscopy …
electric field of a THz pulse. Ultrafast magnetic dynamics have been theoretically
predicted to occur via excitation of an electromagnon in RMnO 3 at electric field
strengths ∼14 MVcm
−1 [78]. Experimentally, modification of the magnetic structure
in TbMnO 3 by excitation of an electromagnon by THz pulses with electric fields of
300 kVcm
−1 has been demonstrated [79]; in this work the authors suggest that THz
pulses with an electric field on the order of 1–2 MVcm
−1 may be sufficient to cause
a 90
◦ rotation of the spin-cycloid in TbMnO 3 , an order of magnitude lower than the
theoretical prediction in reference [78].
As discussed previously in Sect. 1.3.3, CuO is a magnetically-induced multiferroic that exhibits electromagnons at significantly higher temperatures than rare-earth
manganates such as TbMnO 3 ; hence understanding the behaviour of CuO during
interaction with intense THz electric fields may be an important step towards room
temperature, optical electric field control of magnetic order and domain switching
for device applications. The coercive field of CuO, around 55 kVm
−1 at 220 K in the
multiferroic phase [80], is also considerably lower than the coercive field of TbMnO 3 ,
around 2 MVm
−1 at 24 K in the multiferroic phase [81]; a lower coercive field could
suggest that CuO will be more susceptible to control by electric fields and nonlinear
processes could be induced at lower electric field strengths. The following section
will present results of preliminary investigations into the interaction of strong THz
electric fields with CuO and its electromagnons, and suggest future experimental
avenues of research.
5.5.1 Electric Field-Dependence of the Electromagnon
Response in CuO
As an initial exploration into the electric field control of magnetic order in CuO,
the dependence of the electromagnon response on the driving electric field strength
was investigated. A single crystal sample of CuO oriented with the [101]- and [010]directions in-plane, identical to that used in Chap. 3, was mounted in an optical
cryostat in the sample space of the high-field THz spectrometer shown in Fig. 5.4.
The electric field strength was varied using the same method as in Sects. 5.3 and 5.4,
with the electric field strength incident on the sample varying from 38 to 354 kV cm
−1
due to the ∼95 % THz transmission of the TPX cryostat windows.
As described in Sect. 1.3.3 and shown in Fig. 1.7c, while the resonant frequency of
the A
3
u and B
3
u phonon modes in CuO (12.5 THz and 16 THz, respectively) are outside
the bandwidth of the high-field THz pulses generated in this spectrometer, due to their
broad linewidth both phonon modes make a significant contribution to the absorption
at low THz frequencies [45, 82]. In order to see the features of the electromagnon
clearly, following the method of Jones et al. in reference [45] and the method used in
Chap. 3 of this thesis, the change in absorption coefficient due to multiferroicity α
is presented in Fig. 5.10a for various THz electric field strengths. Reference scans
were taken at 210 K in the AF1 phase and sample scans taken at 216 K in the AF2
5 High-Field Terahertz Time-Domain Spectroscopy …
electric field of a THz pulse. Ultrafast magnetic dynamics have been theoretically
predicted to occur via excitation of an electromagnon in RMnO 3 at electric field
strengths ∼14 MVcm
−1 [78]. Experimentally, modification of the magnetic structure
in TbMnO 3 by excitation of an electromagnon by THz pulses with electric fields of
300 kVcm
−1 has been demonstrated [79]; in this work the authors suggest that THz
pulses with an electric field on the order of 1–2 MVcm
−1 may be sufficient to cause
a 90
◦ rotation of the spin-cycloid in TbMnO 3 , an order of magnitude lower than the
theoretical prediction in reference [78].
As discussed previously in Sect. 1.3.3, CuO is a magnetically-induced multiferroic that exhibits electromagnons at significantly higher temperatures than rare-earth
manganates such as TbMnO 3 ; hence understanding the behaviour of CuO during
interaction with intense THz electric fields may be an important step towards room
temperature, optical electric field control of magnetic order and domain switching
for device applications. The coercive field of CuO, around 55 kVm
−1 at 220 K in the
multiferroic phase [80], is also considerably lower than the coercive field of TbMnO 3 ,
around 2 MVm
−1 at 24 K in the multiferroic phase [81]; a lower coercive field could
suggest that CuO will be more susceptible to control by electric fields and nonlinear
processes could be induced at lower electric field strengths. The following section
will present results of preliminary investigations into the interaction of strong THz
electric fields with CuO and its electromagnons, and suggest future experimental
avenues of research.
5.5.1 Electric Field-Dependence of the Electromagnon
Response in CuO
As an initial exploration into the electric field control of magnetic order in CuO,
the dependence of the electromagnon response on the driving electric field strength
was investigated. A single crystal sample of CuO oriented with the [101]- and [010]directions in-plane, identical to that used in Chap. 3, was mounted in an optical
cryostat in the sample space of the high-field THz spectrometer shown in Fig. 5.4.
The electric field strength was varied using the same method as in Sects. 5.3 and 5.4,
with the electric field strength incident on the sample varying from 38 to 354 kV cm
−1
due to the ∼95 % THz transmission of the TPX cryostat windows.
As described in Sect. 1.3.3 and shown in Fig. 1.7c, while the resonant frequency of
the A
3
u and B
3
u phonon modes in CuO (12.5 THz and 16 THz, respectively) are outside
the bandwidth of the high-field THz pulses generated in this spectrometer, due to their
broad linewidth both phonon modes make a significant contribution to the absorption
at low THz frequencies [45, 82]. In order to see the features of the electromagnon
clearly, following the method of Jones et al. in reference [45] and the method used in
Chap. 3 of this thesis, the change in absorption coefficient due to multiferroicity α
is presented in Fig. 5.10a for various THz electric field strengths. Reference scans
were taken at 210 K in the AF1 phase and sample scans taken at 216 K in the AF2