Electron spin resonance insight into broadband absorption of the Cu3Bi(SeO3)2O2Br metamagnet

Abstract

Metamagnets, which exhibit a transition from a low-magnetization to a high-magnetization state induced by the applied magnetic field, have recently been highlighted as promising materials for controllable broadband absorption. Here we show results of a multifrequency electron spin resonance(ESR) investigation of the Cu3Bi(SeO3)2O2Br planar metamagnet on the kagome lattice. Its mixed antiferromagnetic/ferromagnetic phase is stabilized in a finite range of applied fields around 0.8 T at low temperatures and is characterized by enhanced microwave absorption. The absorption signal is non-resonant and its boundaries correspond to two critical fields that determine the mixed phase. With decreasing temperature these increase like the sublattice magnetization of the antiferromagnetic phase and show no frequency dependence between 100 and 480 GHz. On the contrary, we find that the critical fields depend on the magnetic-field sweeping direction. In particular, the higher critical field, which corresponds to the transition from the mixed to the ferromagnetic phase, shows a pronounced hysteresis effect, while such a hysteresis is absent for the lower critical field. The observed hysteresis is enhanced at lower temperatures, which suggests that thermal fluctuations play an important role in destabilizing the highly absorbing mixed phase.

Full text

AIP ADVANCES 6, 056210 (2016)
Electron spin resonance insight into broadband
absorption of the Cu3Bi(SeO3)2O2Br metamagnet
A. Zorko,1,aM. Gomilšek,1M. Pregelj,1M. Ozerov,2,bS. A. Zvyagin,2
A. Ozarowski,3V. Tsurkan,4,5 A. Loidl,4and O. Zaharko6
1Jožef Stefan Institute, Jamova c. 39, SI-1000 Ljubljana, Slovenia
2Dresden High Magnetic Field Laboratory, Helmholtz-Zentrum Dresden-Rossendorf,
01328 Dresden, Germany
3National High Magnetic Field Laboratory, Florida State University, Tallahassee,
Florida 32310, USA
4Experimental Physics V, Center for Electronic Correlations and Magnetism, Institute
of Physics, University of Augsburg, D-86135 Augsburg, Germany
5Institute of Applied Physics, Academy of Science of Moldova,
MD-2028 Chisinau, Republic of Moldova
6Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institute,
CH-5232 Villigen PSI, Switzerland
(Presented 12 January 2016; received 6 November 2015; accepted 21 December 2015;
published online 3 March 2016)
Metamagnets, which exhibit a transition from a low-magnetization to a high-
magnetization state induced by the applied magnetic field, have recently been
highlighted as promising materials for controllable broadband absorption. Here we
show results of a multifrequency electron spin resonance (ESR) investigation of
the Cu3Bi(SeO3)2O2Br planar metamagnet on the kagome lattice. Its mixed anti-
ferromagnetic/ferromagnetic phase is stabilized in a finite range of applied fields
around 0.8 T at low temperatures and is characterized by enhanced microwave
absorption. The absorption signal is non-resonant and its boundaries correspond to
two critical fields that determine the mixed phase. With decreasing temperature these
increase like the sublattice magnetization of the antiferromagnetic phase and show
no frequency dependence between 100 and 480 GHz. On the contrary, we find that
the critical fields depend on the magnetic-field sweeping direction. In particular,
the higher critical field, which corresponds to the transition from the mixed to the
ferromagnetic phase, shows a pronounced hysteresis effect, while such a hysteresis
is absent for the lower critical field. The observed hysteresis is enhanced at lower
temperatures, which suggests that thermal fluctuations play an important role in
destabilizing the highly absorbing mixed phase. C2016 Author(s). All article content,
except where otherwise noted, is licensed under a Creative Commons Attribution 3.0
Unported License. [http://dx.doi.org/10.1063/1.4943534]
I. INTRODUCTION
Broadband absorption of electromagnetic radiation that spans over several decades in fre-
quency is a material’s property that is highly praised in modern electronics. Its applications include
RF/microwave filtering,1optical signal processing,2,3electromagnetic interference shielding,4,5etc.
The common weakness of materials that are used nowadays is a rather narrow absorption range,
typically covering only a few decades in frequency.4Moreover, the absorption of these materials
is in general not controllable via external stimuli. In this respect, the recently discovered control-
lable broadband absorption in metamagnets is a very promising phenomenon that pledges novel
functionality of these materials.6
aAuthor to whom correspondence should be addressed. Electronic address: andrej.zorko@ijs.si
bPresent address: FELIX Laboratory, Radboud University, 6525 ED Nijmegen, The Netherlands
2158-3226/2016/6(5)/056210/6 6, 056210-1 ©Author(s) 2016

056210-2 Zorko et al. AIP Advances 6, 056210 (2016)
Metamagnets are magnetic materials that undergo a magnetic-field induced phase transition
from a state with low magnetization [typically an antiferromagnetic (AFM) state] into a state with
high magnetization [typically a ferromagnetic (FM) state].7Because of demagnetization fields, the
AFM-FM transition is often more complicated, as an intermediate mixed phase – a phase where
both the AFM and FM phases coexist – is stabilized in a finite field range.7,8Due to various possible
AFM-FM domain configurations8a broad excitation spectrum of the mixed phase is anticipated.
Indeed, it has been recently demonstrated that in the Cu3Bi(SeO3)2O2Br planar metamagnet the
range of excitations, i.e. absorption, extends over at least ten decades in frequency (from 100 Hz to
a few hundreds of GHz).6Importantly, as the broadband absorption is limited to the mixed phase of
the material, it is thus controllable by the external magnetic field.6
The mineral francisite9Cu3Bi(SeO3)2O2Br features two-dimensional pseudo-kagome layers
of Cu2+S=1/2 spins.10 These are coupled by competing nearest-neighbor FM and next-nearest
neighbor AFM exchange interactions within the layers, as well as by an additional, much weaker
AFM coupling between adjacent layers.11 In zero magnetic field the compound undergoes Néel
ordering at TN=27.4 K, where a canted, non-collinear ferrimagnetic spin arrangement within
individual layers alternates antiferromagnetically between neighboring layers.11 Such a complicated
order is a consequence of competing intralayer exchange interactions and is stabilized by magnetic
anisotropy of the Dzyaloshinskii-Moriya type.12 Magnetic fields perpendicular to the layers that
exceed ∼0.8 T (at T≪TN) induce FM order of neighboring layers.11 In a finite field range around
the mean transition field a mixed AFM/FM phase exists.6It is stabilized by the demagnetization
field of the FM component compensating for any increase of the external field and thus resulting
in a constant total internal magnetic field until a homogeneous FM order is established. Therefore,
its width is determined by the sample shape and is typically of the order of 0.1 T for plate-like
samples.6The mixed phase is characterized by strong absorption, as detected through an enhanced
imaginary part of the magnetic susceptibility in the kHz range (ac susceptibility measurements) and
in the GHz range (electron spin resonance measurement).6In order to further characterize the highly
absorbing mixed phase, to determine the dependence of the critical fields on the frequency, and
to study possible hysteresis effects, we performed a comprehensive electron spin resonance (ESR)
investigation.
II. EXPERIMENTAL DETAILS
All measurements were performed on the same single crystal samples as the ones used in
our previous study.6A typical sample dimension was 3 ×2×0.2 mm3. The samples were grown
at 500–550◦C by a chemical-transport-reaction method with bromine as the transport agent, using
powders prepared by a solid-state reaction.
The ESR experiments were performed on custom-made transmission-type ESR spectrome-
ters14,15 at the High Magnetic Field Laboratory at the Helmholtz-Zentrum Dresden-Rossendorf,
Germany and at the National High Magnetic Field Laboratory, Tallahassee, USA. The investiga-
tions were conducted in the temperature range 2.3 – 35 K and frequency range 100 – 480 GHz in
the Faraday configuration, with the applied magnetic field perpendicular to the kagome layers. The
standard field-modulation technique was used to enhance the signal-to-noise ratio.
III. RESULTS AND DISCUSSION
A typical ESR spectrum recorded in the magnetically ordered state of Cu3Bi(SeO3)2O2Br is
shown in the inset of Fig. 1. It consists of several components. At 240 GHz and 5 K a sharp signal is
centered around 0.8 T, while two much broader components are found at 1.8 T and 3.0 T, the latter
being by far the most dominant in intensity. With decreasing frequency the two broad components
shift to higher fields, while the position of the narrow component remains unchanged (Fig. 1).
Clearly, the origin of the narrow and the broad components is different, which is also indicated by
the different ESR signal phase of these components (inset in Fig. 1).
The broad ESR components represent FM resonance modes, which were studied before by Wang
et al.13 However, their study was limited to frequencies in the range 300 – 490 GHz, therefore, only the

056210-3 Zorko et al. AIP Advances 6, 056210 (2016)
FIG. 1. The frequency-field diagram of Cu3Bi(SeO3)2O2Br showing various different modes (different symbols). Open
symbols represent measurements at 2 K published in Ref. 13, while solid symbols are new measurements performed at
5 K. The vertical dashed lines show the boundaries of the highly absorbing mixed phase characterized by a non-resonant
absorption mode. The solid lines go through the positions on the dominant FM resonance mode in both sets of branches. The
inset shows the derivative ESR absorption spectrum with three spectral components (arrows), as recorded at 240 GHz and
5 K.
modes where the frequency increases by increasing magnetic field could be detected. Our measure-
ments reveal new FM resonance branches at ν < 300 GHz. At these frequencies we observe one
dominant mode and additional weaker modes. This is similar to observations in Ref. 13, where four
different branches were observed at ν > 300 GHz, again one being by far the most dominant in
intensity. The multiple resonance modes can be due to different sublattices having slightly different
exchange anisotropies,13 or due to surface anisotropy of ferromagnets, which can cause additional
modes with slightly different energies.16
The main focus of this paper is on the sharp ESR component, which represents a non-resonant
absorption mode. There is no dependence of this mode on frequency, as found in the entire fre-
quency range (100 – 480 GHz) of our investigation (Figs. 2and 3). The critical fields B1and B2,
which we define as the lower and the higher borders of the absorption signal, respectively, exhibit
temperature dependence resembling that of an order parameter of the AFM phase (Fig. 3) and
coincide with the boundaries of the mixed AFM/FM phase of the investigated compound.6The
FIG. 2. A collection of non-resonant absorption spectra recorded at 20 K and at various frequencies. Each spectrum is offset
vertically by the valued of the corresponding frequency. The vertical lines show the lower B1and the upper B2critical fields,
corresponding the the boundaries of the highly absorbing mixed phase in Cu3Bi(SeO3)2O2Br.

056210-4 Zorko et al. AIP Advances 6, 056210 (2016)
FIG. 3. The temperature dependence of the lower B1and the upper B2critical fields in Cu3Bi(SeO3)2O2Br at two selected
frequencies behaving like an order parameter of the AFM phase. The solid lines are a guide to the eye.
absorption properties of this phase are notably different from the properties of the two neighboring
homogeneous phases and results in the observed non-resonant mode.
In order to better characterize the transition from the AFM into the mixed phase at B1and
from the mixed into the FM phase at B2we measured the non-resonant absorption signal upon
sweeping the magnetic field up and down. A pronounced hysteresis is found in the value of the
higher critical field B2, which depends significantly on the direction of the field sweep (Fig. 4).
Moreover, this hysteresis effect is strongly temperature dependent. It corresponds to a 2.0% increase
of B2in the sweep-up experiment compared to the sweep-down experiment at 20 K, while this
difference increases to 5.5% at 5 K (Fig. 5). At this temperature the total field width of the mixed
phase is as much as 30% larger when increasing the field than when decreasing it. In contrast,
within experimental errorbars there is no hysteresis observed in the lower critical field B1(Fig. 4) at
either of the two temperatures.
Our ESR investigation of Cu3Bi(SeO3)2O2Br thus confirms that the first-order transition from
the low-field AFM phase into the high-field FM state is spread over a range of applied fields,
which defines the mixed AFM/FM phase that is highly absorptive. Both boundaries shows an AFM
order-parameter-like temperature dependence. Any hysteresis effects are absent in the value of B1,
as expected on the border between the AFM phase and the mixed phase with the FM-phase fraction
FIG. 4. The frequency dependence of both critical fields at two temperatures (separated by the dashed line). The direction of
the field sweep in the experiments is indicated by vertical arrows.

056210-5 Zorko et al. AIP Advances 6, 056210 (2016)
FIG. 5. The hysteresis of the upper critical field between the mixed and the ferromagnetic phase for experimental field sweep
up and down at two selected temperatures. The horizontal lines indicate the average value.
being zero at this field. On the contrary, the transition from the mixed phase into the FM phase
at the critical field B2shows a hysteresis. Its presence is unlikely related to usual mechanisms
encountered in FM materials, where the hysteresis corresponds to changing domain structure in
multiple-domain ferromagnets or reorientation effects in single-domain ferromagnets. Moreover,
the width of the hysteresis is not proportional to the value of the magnetization in the FM phase,6as
the former increases with decreasing temperature much more profoundly. This experimental obser-
vation suggests that thermal fluctuations play an important role in destabilizing the mixed phase. At
higher temperatures, where these are more pronounced, the mixed phase is less stable to increasing
magnetic field, the history effect becomes less important and, consequently, the hysteresis in B2
tends to decrease.
IV. CONCLUSIONS
In summary, we performed a systematic ESR study of the Cu3Bi(SeO3)2O2Br metamagnet,
focusing on the non-resonant absorption that is associated with the mixed AFM/FM phase of this
material. Our multifrequency experiments have shown that the enhanced absorption in this phase
is frequency independent in the entire region of our investigation (100 - 480 GHz). Moreover, we
found that the phase boundary between the high-field FM phase and the mixed phase shows a
hysteresis effect, while the boundary between the mixed and the low-field AFM phase does not.
The hysteresis is suppressed by temperature, implying that thermal fluctuations may play a role in
effectively destabilizing the mixed phase. This should be considered in any future modeling of the
mixed phase of Cu3Bi(SeO3)2O2Br and thus in the ultimate search of a microscopic mechanism of
enhanced absorption of metamagnets.
ACKNOWLEDGMENTS
We acknowledge the financial support of the Slovenian Research Agency (Program No. P1-0125
and Project BI-US/14-15-039), the Swiss National Science Foundation (SCOPES project IZ73Z0_15
2734/1), and the Deutsche Forschungsgemeinschaft (DFG, Germany) and HLD at HZDR, member
of the European Magnetic Field Laboratory (EMFL). NHMFL is supported by the NSF through the
cooperative agreement DMR-1157490, the State of Florida and the Department of Energy.
1J.-S. G. Hong and M. J. Lancaster, Microstripfilters for RF/microwave applications, 2nd ed. (Wiley-VCH, Weinheim, 2011).
2D. D. Stancil and A. Prabhakar, Spin Waves: Theory and Applications (Springer, New York, 2009).
3B. Lenk, H. Ulrichs, F. Garbs, and M. Münzenberg, Phys. Rep. 507, 107 (2011).

056210-6 Zorko et al. AIP Advances 6, 056210 (2016)
4S. Geetha, K. K. Satheesh Kumar, C. R. K. Rao, M. Vijayan, and D. C. Trivedi, J. App. Polym. Sci. 112, 2073 (2009).
5Z. Chen, C. Xu, C. Ma, W. Ren, and H.-M. Cheng, Adv. Mater. 25, 1296 (2013).
6M. Pregelj, O. Zaharko, A. Zorko, M. Gomilšek, O. Sendetskyi, A. Günther, M. Ozerov, S. A. Zvyagin, H. Luetkens, C.
Baines, V. Tsurkan, and L. A., Adv. Funct. Mater. 25, 3634 (2015).
7E. Stryjewski and N. Giordano, Adv. Phys. 26, 487 (1977).
8N. S. Kiselev, C. Bran, U. Wolff, L. Schultz, A. N. Bogdanov, O. Hellwig, V. Neu, and U. K. Rößler, Phys. Rev. B 81, 054409
(2010).
9A. Pring, B. M. Gatehouse, and W. D. Birch, Am. Mineral. 75, 1421 (1990).
10 P. Millet, B. Bastide, V. Pashchenko, S. Gnatchenko, V. Gapon, Y. Ksari, and A. Stepanov, J. Mater. Chem. 11, 1152 (2001).
11 M. Pregelj, O. Zaharko, A. Günther, A. Loidl, V. Tsurkan, and S. Guerrero, Phys. Rev. B 86, 144409 (2012).
12 I. Rousochatzakis, J. Richter, R. Zinke, and A. A. Tsirlin, Phys. Rev. B 91, 024416 (2015).
13 Z. Wang, M. Schmidt, Y. Goncharov,V. Tsurkan, H.-A. K. von Nidda, A. Loidl, and J. Deisenhofer, Phys. Rev. B 86, 174411
(2012).
14 S. A. Zvyagin, J. Krzystek, P. H. M. Van Loosdrecht, G. Dhalenne, and A. Revcolevschi, Physica B: Cond. Matter 346–347,
1 (2004).
15 A. . K. Hassan, L. A. Pardi, J. Krzystek, A. Sienkiewicz, P. Goy, M. Rohrer, and L.-C. Brunel, J. Magn. Reson. 142, 300
(2000).
16 C. Kittel, Introduction to solid state physics, 8th ed. (John Wiley & Sons, New York, 2005).