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Ultrafast optical modification of exchange interactions in iron oxides.

Mikhaylovskiy RV, Hendry E, Secchi A, Mentink JH, Eckstein M, Wu A, Pisarev RV, Kruglyak VV, Katsnelson MI, Rasing T, Kimel AV - Nat Commun (2015)

Bottom Line: Here we propose a scenario for coupling between the electric field of light and spins via optical modification of the exchange interaction, one of the strongest quantum effects with strength of 10(3) Tesla.We demonstrate that this isotropic opto-magnetic effect, which can be called inverse magneto-refraction, is allowed in a material of any symmetry.From its strength we estimate that a sub-picosecond modification of the exchange interaction by laser pulses with fluence of about 1 mJ cm(-2) acts as a pulsed effective magnetic field of 0.01 Tesla.

View Article: PubMed Central - PubMed

Affiliation: School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK.

ABSTRACT
Ultrafast non-thermal manipulation of magnetization by light relies on either indirect coupling of the electric field component of the light with spins via spin-orbit interaction or direct coupling between the magnetic field component and spins. Here we propose a scenario for coupling between the electric field of light and spins via optical modification of the exchange interaction, one of the strongest quantum effects with strength of 10(3) Tesla. We demonstrate that this isotropic opto-magnetic effect, which can be called inverse magneto-refraction, is allowed in a material of any symmetry. Its existence is corroborated by the experimental observation of terahertz emission by spin resonances optically excited in a broad class of iron oxides with a canted spin configuration. From its strength we estimate that a sub-picosecond modification of the exchange interaction by laser pulses with fluence of about 1 mJ cm(-2) acts as a pulsed effective magnetic field of 0.01 Tesla.

No MeSH data available.


Related in: MedlinePlus

Determination of the absolute sign of the change of D/J in TmFeO3.(a) The signal emitted just below the spin reorientation temperature at 60 K (black) is shown together with its low-frequency (blue) and high-frequency part (brown). The latter part is in phase with the signal measured at 40 K which describes a quasi-antiferromagnetic (q-AFM) mode only (magenta). The low-frequency part corresponds to the quasi-ferromagnetic mode (q-FM). The first half-cycle of the quasi-antiferromagnetic mode has a different sign compared with the first half-cycle of the quasi-ferromagnetic mode (see dashed line). (b) During the spin reorientation the spin configuration of TmFeO3 continuously rotates in the (xz) plane, while keeping the weak ferromagnetic moment in the same plane. At low temperatures, the magnetization is oriented along the x axis. So, due to the laser-induced reorientation at 60 K, the x-component of the magnetization initially decreases. (c) Since the first half-cycle of the quasi-antiferromagnetic mode has a different sign, due to the exchange-driven torque the magnetization initially moves so as to have a positive x-component which implies an increase of D/J.
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f6: Determination of the absolute sign of the change of D/J in TmFeO3.(a) The signal emitted just below the spin reorientation temperature at 60 K (black) is shown together with its low-frequency (blue) and high-frequency part (brown). The latter part is in phase with the signal measured at 40 K which describes a quasi-antiferromagnetic (q-AFM) mode only (magenta). The low-frequency part corresponds to the quasi-ferromagnetic mode (q-FM). The first half-cycle of the quasi-antiferromagnetic mode has a different sign compared with the first half-cycle of the quasi-ferromagnetic mode (see dashed line). (b) During the spin reorientation the spin configuration of TmFeO3 continuously rotates in the (xz) plane, while keeping the weak ferromagnetic moment in the same plane. At low temperatures, the magnetization is oriented along the x axis. So, due to the laser-induced reorientation at 60 K, the x-component of the magnetization initially decreases. (c) Since the first half-cycle of the quasi-antiferromagnetic mode has a different sign, due to the exchange-driven torque the magnetization initially moves so as to have a positive x-component which implies an increase of D/J.

Mentions: To determine whether laser excitation leads to a decrease or an increase of the ratio D/J we take advantage of the strong temperature dependence of the magnetic anisotropy, which is characteristic for many orthoferrites. For instance, heating of TmFeO3 from 80 to 90 K leads to a change of the equilibrium orientation of the weak magnetic moment from the x to the z axis. If the equilibrium orientation is changed as a result of a sudden heating by a femtosecond laser pulse, such a change is followed by oscillations of the weak magnetic moment in the (xz) plane at the frequency of the quasi-ferromagnetic mode (∼100 GHz)3839. As discussed in ref. 26 in the range between 55 and 68 K, such low-frequency oscillations corresponding to the quasi-ferromagnetic mode are observed in THz emission spectra together with the high-frequency quasi-antiferromagnetic oscillations (see Fig. 6). We applied a low pass filter to the data (cutoff frequency 250 GHz) to isolate the quasi-ferromagnetic mode and a high-frequency filter (cutoff frequency 650 GHz) to isolate the quasi-antiferromagnetic mode. Such a choice of the cutoffs ensures the filtering out of the impurity modes which complicate the dynamics26. It is seen from Fig. 6 that the high-frequency mode measured at 60 K is in phase with that observed at 40 K. One can also see that the initial phases of the low-frequency quasi-ferromagnetic and high-frequency quasi-antiferrimagnetic modes are ∼180° apart. Note that for the z-cut TmFeO3 sample, with a net magnetic moment oriented upwards, a laser-induced spin-reorientation transition should trigger the quasi-ferromagnetic mode in such a way that the Mx component of the magnetization decreases. The observed difference in the phases between the two oscillations shows that the quasi-antiferromagnetic mode is triggered in such a way that the Mx component increases, which means that the canting angle becomes larger. Such a behaviour can only be explained by assuming that the quasi-antiferromagnetic oscillations are triggered by an increase of the ratio of the exchange parameters D/J. If this conclusion is true, in the x-cut sample the initial phases of the two modes must be the same, since the spin reorientation in this sample proceeds in the opposite direction. Measurements in the vicinity of the spin-reorientation temperature in ErFeO3 cut perpendicular to the x axis confirm this conclusion (see Supplementary Fig. 8; Supplementary Note 3). Interestingly, the increase of the ratio D/J cannot be explained on the basis of the simplistic model defined by the Fe–O–Fe cluster that predicts an increase of J and does not evaluate the change of D. However, the calculation of ΔJ demonstrates the plausibility of the proposed mechanism of optical manipulation of the symmetric exchange interaction in principle.


Ultrafast optical modification of exchange interactions in iron oxides.

Mikhaylovskiy RV, Hendry E, Secchi A, Mentink JH, Eckstein M, Wu A, Pisarev RV, Kruglyak VV, Katsnelson MI, Rasing T, Kimel AV - Nat Commun (2015)

Determination of the absolute sign of the change of D/J in TmFeO3.(a) The signal emitted just below the spin reorientation temperature at 60 K (black) is shown together with its low-frequency (blue) and high-frequency part (brown). The latter part is in phase with the signal measured at 40 K which describes a quasi-antiferromagnetic (q-AFM) mode only (magenta). The low-frequency part corresponds to the quasi-ferromagnetic mode (q-FM). The first half-cycle of the quasi-antiferromagnetic mode has a different sign compared with the first half-cycle of the quasi-ferromagnetic mode (see dashed line). (b) During the spin reorientation the spin configuration of TmFeO3 continuously rotates in the (xz) plane, while keeping the weak ferromagnetic moment in the same plane. At low temperatures, the magnetization is oriented along the x axis. So, due to the laser-induced reorientation at 60 K, the x-component of the magnetization initially decreases. (c) Since the first half-cycle of the quasi-antiferromagnetic mode has a different sign, due to the exchange-driven torque the magnetization initially moves so as to have a positive x-component which implies an increase of D/J.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4595597&req=5

f6: Determination of the absolute sign of the change of D/J in TmFeO3.(a) The signal emitted just below the spin reorientation temperature at 60 K (black) is shown together with its low-frequency (blue) and high-frequency part (brown). The latter part is in phase with the signal measured at 40 K which describes a quasi-antiferromagnetic (q-AFM) mode only (magenta). The low-frequency part corresponds to the quasi-ferromagnetic mode (q-FM). The first half-cycle of the quasi-antiferromagnetic mode has a different sign compared with the first half-cycle of the quasi-ferromagnetic mode (see dashed line). (b) During the spin reorientation the spin configuration of TmFeO3 continuously rotates in the (xz) plane, while keeping the weak ferromagnetic moment in the same plane. At low temperatures, the magnetization is oriented along the x axis. So, due to the laser-induced reorientation at 60 K, the x-component of the magnetization initially decreases. (c) Since the first half-cycle of the quasi-antiferromagnetic mode has a different sign, due to the exchange-driven torque the magnetization initially moves so as to have a positive x-component which implies an increase of D/J.
Mentions: To determine whether laser excitation leads to a decrease or an increase of the ratio D/J we take advantage of the strong temperature dependence of the magnetic anisotropy, which is characteristic for many orthoferrites. For instance, heating of TmFeO3 from 80 to 90 K leads to a change of the equilibrium orientation of the weak magnetic moment from the x to the z axis. If the equilibrium orientation is changed as a result of a sudden heating by a femtosecond laser pulse, such a change is followed by oscillations of the weak magnetic moment in the (xz) plane at the frequency of the quasi-ferromagnetic mode (∼100 GHz)3839. As discussed in ref. 26 in the range between 55 and 68 K, such low-frequency oscillations corresponding to the quasi-ferromagnetic mode are observed in THz emission spectra together with the high-frequency quasi-antiferromagnetic oscillations (see Fig. 6). We applied a low pass filter to the data (cutoff frequency 250 GHz) to isolate the quasi-ferromagnetic mode and a high-frequency filter (cutoff frequency 650 GHz) to isolate the quasi-antiferromagnetic mode. Such a choice of the cutoffs ensures the filtering out of the impurity modes which complicate the dynamics26. It is seen from Fig. 6 that the high-frequency mode measured at 60 K is in phase with that observed at 40 K. One can also see that the initial phases of the low-frequency quasi-ferromagnetic and high-frequency quasi-antiferrimagnetic modes are ∼180° apart. Note that for the z-cut TmFeO3 sample, with a net magnetic moment oriented upwards, a laser-induced spin-reorientation transition should trigger the quasi-ferromagnetic mode in such a way that the Mx component of the magnetization decreases. The observed difference in the phases between the two oscillations shows that the quasi-antiferromagnetic mode is triggered in such a way that the Mx component increases, which means that the canting angle becomes larger. Such a behaviour can only be explained by assuming that the quasi-antiferromagnetic oscillations are triggered by an increase of the ratio of the exchange parameters D/J. If this conclusion is true, in the x-cut sample the initial phases of the two modes must be the same, since the spin reorientation in this sample proceeds in the opposite direction. Measurements in the vicinity of the spin-reorientation temperature in ErFeO3 cut perpendicular to the x axis confirm this conclusion (see Supplementary Fig. 8; Supplementary Note 3). Interestingly, the increase of the ratio D/J cannot be explained on the basis of the simplistic model defined by the Fe–O–Fe cluster that predicts an increase of J and does not evaluate the change of D. However, the calculation of ΔJ demonstrates the plausibility of the proposed mechanism of optical manipulation of the symmetric exchange interaction in principle.

Bottom Line: Here we propose a scenario for coupling between the electric field of light and spins via optical modification of the exchange interaction, one of the strongest quantum effects with strength of 10(3) Tesla.We demonstrate that this isotropic opto-magnetic effect, which can be called inverse magneto-refraction, is allowed in a material of any symmetry.From its strength we estimate that a sub-picosecond modification of the exchange interaction by laser pulses with fluence of about 1 mJ cm(-2) acts as a pulsed effective magnetic field of 0.01 Tesla.

View Article: PubMed Central - PubMed

Affiliation: School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK.

ABSTRACT
Ultrafast non-thermal manipulation of magnetization by light relies on either indirect coupling of the electric field component of the light with spins via spin-orbit interaction or direct coupling between the magnetic field component and spins. Here we propose a scenario for coupling between the electric field of light and spins via optical modification of the exchange interaction, one of the strongest quantum effects with strength of 10(3) Tesla. We demonstrate that this isotropic opto-magnetic effect, which can be called inverse magneto-refraction, is allowed in a material of any symmetry. Its existence is corroborated by the experimental observation of terahertz emission by spin resonances optically excited in a broad class of iron oxides with a canted spin configuration. From its strength we estimate that a sub-picosecond modification of the exchange interaction by laser pulses with fluence of about 1 mJ cm(-2) acts as a pulsed effective magnetic field of 0.01 Tesla.

No MeSH data available.


Related in: MedlinePlus