Decoupling the structural and magnetic phase transformations in magneto-optic MnBi thin films by the partial substitution of Cr for Mn

Article abstract of DOI:10.1063/1.121355

The first-order nature of the magnetic phase transformation at 360°C and the presence of a Bi-rich eutectic at 265°C have inhibited the application of MnBi thin films for high density magneto-optical data storage. It is suggested that partial substitution of Cr for Mn should both lower the Curie temperature, T_c, and decouple the lattice and magnetic transitions so as to allow reversible Curie point writing. It is found experimentally that 10% substitution of Cr for Mn reduces the apparent T_c to ～250°C while retaining a Kerr rotation angle greater than 1° at 633 nm, as measured through the silica glass substrate. Observations of increasing H_c with temperature in both MnBi and (Mn,Cr)Bi thin films suggest that the low-temperature phase is ferrimagnetic.

Full text of DOI:10.1063/1.121355

Decoupling the structural and magnetic phase transformations in magneto-optic MnBi
thin films by the partial substitution of Cr for Mn
Prabhakar R. Bandaru, Timothy D. Sands, Yukiko Kubota, and Ernesto E. Marinero
Citation: Applied Physics Letters 72, 2337 (1998); doi: 10.1063/1.121355
View online: http://dx.doi.org/10.1063/1.121355
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/72/18?ver=pdfcov
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APPLIED PHYSICS LETTERS
VOLUME 72, NUMBER 18
4 MAY 1998
Decoupling the structural and magnetic phase transformations in magneto-
optic MnBi thin films by the partial substitution of Cr for Mn
Prabhakar R. Bandaru,a) Timothy D. Sands, and Yukiko Kubota
Department of Materials Science and Mineral Engineering, University of California, Berkeley,
California 94720
Ernesto E. Marinero
IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, California 95120
͑Received 22 December 1997; accepted for publication 24 February 1998͒
The first-order nature of the magnetic phase transformation at 360 °C and the presence of a Bi-rich
eutectic at 265 °C have inhibited the application of MnBi thin films for high density magneto-optical
data storage. It is suggested that partial substitution of Cr for Mn should both lower the Curie
temperature, Tc , and decouple the lattice and magnetic transitions so as to allow reversible Curie
point writing. It is found experimentally that 10% substitution of Cr for Mn reduces the apparent Tc
to ϳ250 °C while retaining a Kerr rotation angle greater than 1° at 633 nm, as measured through
the silica glass substrate. Observations of increasing Hc with temperature in both MnBi and
͑Mn,Cr͒Bi thin films suggest that the low-temperature phase is ferrimagnetic. © 1998 American
Institute of Physics. ͓S0003-6951͑98͒01717-3͔
Since Curie-point writing with an electron beam was
first demonstrated in 1958,1 polycrystalline MnBi thin films
have been considered to be promising candidates for
magneto-optical ͑MO͒ data storage. In the quest for higher
density storage exploiting the anticipated availability of low-
cost, GaN-based blue lasers, MnBi assumes additional im-
portance as it has the largest Kerr response in the blue wave-
length regime of any known material at room temperature.
This Kerr response contributes to a large intrinsic value of
the figure of merit, R1/2⍜k , where R is the reflectivity and
⍜k is the Kerr rotation angle. Furthermore, large perpendicu-
lar anisotropy2 (K1ϳ106 J/m3) is readily achieved in MnBi
films as a consequence of uniaxial texture and a high intrin-
sic magnetocrystalline anisotropy.
ferromagnetic interactions predominate, possibly mediated
via an ‘‘exchange inversion’’6 superexchange mechanism
through the Bi 6p orbitals.7 During heating through 360 °C,
a coupled magnetic and first-order lattice transition results in
the formation of a paramagnetic high-temperature phase
͑HTP͒. This transformation is believed to involve the shuffle
of approximately 15% of the Mn to the five-fold-coordinated
bipyramidal interstices.3,8 The resulting orthorhombic struc-
ture is a superstructure of the hexagonal unit cell, with
pseudohexagonal lattice parameters of a0ϭ0.434 nm and
c0ϭ0.597 nm ͑extrapolated to room temperature͒.3 Upon
quenching from above 360 °C, the HTP can be retained. This
quenched HTP becomes ferrimagnetic below ϳ180 °C. It is
speculated3 that an antiferromagnetic exchange coupling be-
tween adjacent octahedral and bipyramidal Mn sites reduces
Despite these outstanding attributes, efforts to develop
MnBi for rewritable optical recording have been plagued by
a first-order transformation from the high-temperature para-
magnetic phase to the equilibrium low-temperature ferro-
magnetic phase.3 This coupling of the magnetic transition
with a first-order lattice transition at 360 °C leads to high
media noise and poor reversibility, consequences of the pres-
ence of both equilibrium and metastable phases ͑e.g., the
quenched high-temperature phase͒ following Curie-point
writing. Another issue of contention is the steady increase of
coercivity (Hc) with temperature4 which could be a potential
obstacle for Curie-point writing. This issue will be dealt with
in detail in a later paper.5 In this letter, an approach toward
decoupling the lattice and magnetic transitions through the
partial substitution of Cr for Mn is rationalized and prelimi-
nary results supporting this approach are presented.
the net magnetization per Mn ion from ϳ4 ␮ for the LTP to
B
ϳ2 ␮ for the quenched HTP.
B
From the perspective of reducing media noise and im-
proving reversibility, it is imperative that the magnetic tran-
The equilibrium low-temperature phase of MnBi ͑LTP͒
adopts the hexagonal NiAs structure with a0ϭ0.429 nm and
c0ϭ0.6126 nm ͑Fig. 1͒ at room temperature.3 In this struc-
ture, the Mn3ϩ ions are octahedrally coordinated by Bi, and
FIG. 1. Structure of low temperature MnBi. The octahedral sites are occu-
pied by ferromagnetically coupled Mn. In the high temperature phase, the
bipyramidal sites are partially occupied with Mn, which is antiferromagneti-
cally coupled to the octahedrally coordinated Mn in the quenched high
temperature phase.
a͒
Electronic mail: bpr@uclimk2.berkeley.edu
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0003-6951/98/72(18)/2337/3/$15.00 2337 © 1998 American Institute of Physics
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2338
Appl. Phys. Lett., Vol. 72, No. 18, 4 May 1998
Bandaru et al.
FIG. 2. Coupled ␪–2␪ x-ray ͑Cu K␣: 1.5406 Å͒ spectrum from a 111 nm
thick ͑Mn0.9Cr0.1͒Bi film on a silica glass substrate. The small peaks at
ϳ27° and at ϳ54° correspond to the K␤ ͑␭: 1.3922 Å͒ 0002 and 0004
reflections. X-ray rocking curve showing a FWHM of ϳ1° is inset.
FIG. 3. Curie temperature (Tc) as a function of film thickness for MnBi and
Mn0.9Cr0.1Bi.
sition to be used for MO recording be decoupled from any
first-order lattice transition. Furthermore, the difficulty in
completely eliminating Bi inclusions from MnBi films leads
to the practical requirement that the magnetic transition ͑ide-
ally second-order͒ involve a transition temperature below9
the Mn–Bi eutectic temperature of 265 °C.10 Previous at-
tempts at addressing the phase transformation problem have
endeavored to stabilize either the LTP or the quenched HTP
through alloying with Ti;11 Cu;12 Zn, Sb, Te, Cr, Fe, Co, Ni,
Ti, Sc;13 Al, Si;14 and Ru or Rh.15 Superheating of the LTP
and supercooling of the HTP using fast laser pulses compa-
rable to the conditions of thermomagnetic recording has also
been explored.16 In several of these studies, the LTP→HTP
transformation was suppressed,11,15 but at the expense of a
reduced MO figure of merit. For example, suppression of the
LTP yields a low magnetic transition temperature character-
istic of the quenched HTP, but with an associated reduction
in ⍜k by as much as 50%.11,12,13 Several alloying additions
Electron-beam evaporation (10Ϫ6 Torr) was used to se-
quentially deposit Bi, Mn, and Cr on silica glass disk sub-
strates ͑Hoya Industries͒ and silicon nitride ͑60 nm͒/silicon
substrates. The deposition of Bi first is essential to develop-
ing the required c-axis texture. The Bi is highly textured
as-deposited and this texture is imparted to MnBi as it nucle-
ates at the Mn/Bi interface during heating as was observed in
in situ transmission electron microscopy ͑TEM͒ studies.5
Layer thicknesses were adjusted to yield an average compo-
sition of Mn1ϪxCrxBi. It was found that very slow deposition
rates (Ͻ2 nm/min) were necessary to obtain reproducible
films. The thin films were then coated with a 60 nm SiO2
layer for protection from the ambient, followed by vacuum
annealing (10Ϫ7 Torr) at 360 °C for 30 min. X-ray diffrac-
tion ͑Siemens D5000͒ was employed for phase and texture
determination. The magnetic and magneto-optic property
measurements were obtained by the magneto-optic Kerr ef-
fect ͑MOKE͒. MOKE measurements utilized a He–Ne laser
probe ͑633 nm͒ of 1.5 mm diameter on the sample placed
within the pole pieces of a 20 kOe magnet. The remanent
Kerr rotation was recorded from the substrate side ͑for the
fused silica substrates͒ and from the film side ͑for the silicon
nitride/silicon substrates͒. MOKE measurements as a func-
tion of temperature were used for the analysis of magnetiza-
tion behavior and to determine the Tc . ͑Tc is defined here as
the temperature at which the remanent Kerr rotation extrapo-
lates to zero͒. The temperature was varied in 15 °C intervals
with heating and cooling rates around 40 °C/h. The films
were tested for magnetic reversibility by alternate heating
and cooling cycles. At least three measurements were taken
at each temperature to ensure accuracy and stability.
were reported to have resulted in new undesirable ternary
11–13
phases ͑e.g., Cu, Zn, Ti͒
or were found to inhibit the
reaction to form MnBi ͑e.g., Cr and Fe͒.13
The approach adopted in the present study was to reduce
the effective Curie temperature of the LTP by partial substi-
tution of Mn with an ion that would reduce the net ferromag-
netic coupling in the MnBi lattice. In the localized moment
picture of magnetism, the Curie temperature (Tc) is propor-
tional to the exchange integral J ͑JϾ0 for ferromagnetism
and JϽ0 for antiferromagnetism͒. By choosing a trivalent
ion that favors both antiferromagnetism and octahedral coor-
dination in the MnBi lattice, it was speculated that the Tc of
the LTP could be controllably reduced without the full pen-
alty in the figure of merit associated with the quenched HTP.
Although a previous alloying attempt13 was unsuccessful, Cr
was the obvious candidate. The affinity of an ion for a par-
ticular lattice position is measured by the relative crystal
field stabilization energy ͑CFSE͒ of the site—higher values
indicating greater stability. The excess octahedral CFSE of
Cr3ϩ ͑octahedral CFSE—tetrahedral CFSEϭ157.6 kJ/mol͒ is
the largest of the transition metal ions and is substantially
greater than that of Mn3ϩ ͑95.3 kJ/mol͒.17 Furthermore, par-
tial substitution of Cr for Mn should reduce the magnitude of
ferromagnetic coupling, J, due to the propensity for antifer-
MnBi and ͑Mn,Cr͒Bi films with a high degree of c-axis
texture were obtained upon annealing the as-deposited mul-
tilayers ͑Fig. 2͒. Based on calibration with the ͑kinematically
forbidden͒ 200 Si peak, the c0 parameter was found to be
0.609 nm for MnBi and 0.6064 nm for Mn0.9Cr0.1Bi. No
superstructure reflections were detected. A high degree of
c-axis fiber texture was evidenced by rocking curve full
width at half maximum ͑FWHM͒ values of of ϳ1° ͑see inset
of Fig. 2͒.
In accordance with the qualitative predictions above, it
was found that on substitution of 10% Cr for Mn, the Tc was
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romagnetic interactions between Cr ions.
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Appl. Phys. Lett., Vol. 72, No. 18, 4 May 1998
Bandaru et al.
2339
vation of domain pinning sites.4 The x-ray diffraction data
from the films of the present study, however, do not reveal
superstructure reflections, nor is there any evidence of ther-
mal irreversibility in the ͑Mn,Cr͒Bi as would be expected if
the film were to transform from a quenched phase to the
stable LTP. This magnetic behavior suggests competing fer-
romagnetic and antiferromagnetic interactions in the MnBi
and ͑Mn,Cr͒Bi thin films;5 the increasing Hc with tempera-
ture being indicative of a ferrimagnetic compensation point
at or near Tc , or a ferrimagnetically coupled interstitial frac-
tion that increases steadily with temperature.5,11 Preliminary
data show ferrimagnetic behavior but this has yet to be fully
confirmed.18
In conclusion, the partial substitution of Cr for Mn in
MnBi has been shown to reduce the apparent Curie tempera-
ture to around 250 °C, below the Bi melting point and inter-
mediate between the Curie temperatures of the LTP and the
quenched HTP. This has been accomplished with very little
degradation in the MO figure of merit. No indications of
phase separation, lattice phase transitions, or irreversibility
of the magnetic transition in ͑Mn,Cr͒Bi have been observed
in these preliminary studies, although future experiments us-
ing magneto-optic recording conditions are necessary before
it can be concluded that the magnetic transition in ͑Mn,Cr͒Bi
is not coupled with a first-order lattice transition.
This work has been supported by NSF Grant No. ECS-
9409730. The use of the facilities of the Berkeley Microlab,
the Berkeley Integrated Materials Laboratory ͑NSF DMR-
9214370͒ and facilities at the Lawrence Berkeley National
Laboratory are acknowledged. The magnetic measurements
were performed at the IBM Almaden Research Center, San
Jose.
FIG. 4. Remanent Kerr rotation (⌰k) and coercivity (Hc) as a function of
temperature for ͑a͒ Mn0.9Cr0.1Bi ͑111 nm thick͒ and ͑b͒ MnBi ͑96 nm thick͒
films on fused silica substrates. Data were acquired by measurement through
the silica glass substrates at a wavelength of 633 nm. Oscillations in ⌰k with
ϳ50 °C periods are attributed to Fabry–Perot interference arising from the
thermal expansion of the silica substrate. The curves representing the sinu-
soidal oscillations in the Kerr rotation are intended to be a guide to the eye.
1 L. Mayer, J. Appl. Phys. 29, 1454 ͑1958͒.
2 R. S. Tebble and D. J. Craik, Magnetic Materials ͑Wiley–Interscience,
London, 1969͒.
3 A. F. Andresen, W. Halg, P. Fischer, and E. Stoll, Acta Chem. Scand. 21,
1543 ͑1967͒.
decreased to 250Ϯ10 °C. A similar result was obtained on
substitution of 12.5% Cr and studies are in progress to ascer-
tain the compositional dependence. The constancy of Tc over
a range of film thicknesses suggests an intrinsic materials
characteristic ͑Fig. 3͒.
4 X. Guo, X. Chen, Z. Altounian, and J. O. Strom-Olsen, J. Appl. Phys. 73,
6275 ͑1993͒.
5 P. Bandaru, T. Sands, Y. Kubota, and E. Marinero ͑unpublished͒.
6 J. B. Goodenough, A. Wold, R. J. Arnott, and N. Menyuk, Phys. Rev. 124,
373 ͑1961͒.
7 P. W. Anderson, Phys. Rev. 115, 2 ͑1959͒.
Excellent figures of merit (R1/2⍜kϳ0.63) were obtained
with remanent Kerr rotation (⍜k) of 1.0°–1.2° and reflec-
tivities ͑R͒ of 25%–40%. Figure 4͑a͒ illustrates the remanent
8 B. W. Roberts, Phys. Rev. 104, 607 ͑1956͒.
9 The reduction of the Curie temperature to below the melting point of Bi is
seen as an additional advantage as it was found in in situ TEM experi-
ments that the incipient melting of Bi inclusions in MnBi films could
result in a deterioration of film properties and could result in irreversibility
͑see Ref. 5͒.
⍜
͑proportional to the magnetization͒ versus temperature
k
behavior for a 111 nm ͑Mn0.9Cr0.1͒Bi film on a silica glass
substrate. The oscillations in the ⍜k are consequences of the
thermal expansion of the flat silica glass substrates giving
rise to Fabry–Perot etalons. The mean Kerr rotation is found
by tracing a line through the inflection points of the oscilla-
tion ͑corresponding to zero interference͒. The ͑Mn,Cr͒Bi
films were found to be magnetically reversible after cycling
beyond the Tc in contrast to the unalloyed MnBi films ͓Fig.
4͑b͔͒ which suffered degradation in ⍜k , presumed to be due
in part to repeated segregation and melting of Bi.
10 Binary Alloy Phase Diagrams, edited by H. Okamoto and P. R. Subrama-
nian ͑ASM International, Materials Park, Ohio, 1990͒.
11 W. K. Unger, E. Wolfgang, H. Harms, and H. Haudek, J. Appl. Phys. 43,
2875 ͑1972͒.
12 A. Katsui, J. Appl. Phys. 47, 3609 ͑1976͒.
13 H. Gobel, E. Wolfgang, and H. Harms, Phys. Status Solidi A 34, 553
͑1976͒.
14 D. J. Sellmyer, R. D. Kirby, J. Chen, K. W. Wierman, J. X. Shen, Y. Liu,
B. W. Robertson, and S. S. Jaswal, J. Phys. Chem. Solids 56, 1549 ͑1995͒.
15 K. Lee, J. C. Suits, and G. B. Street, Appl. Phys. Lett. 26, 27 ͑1975͒.
16 K. Kempter and E. Bayer, IEEE Trans. Magn. MAG-12, 62 ͑1976͒.
17 A. R. West, Solid State Chemistry and its Applications ͑Wiley, New York,
1992͒.
Of particular interest is the steadily increasing coercive
field (Hc) with temperature for both MnBi and ͑Mn,Cr͒Bi
films. Previous observations of increasing Hc have been at-
18 The ⌰k vs temperature data suggest the presence of a ferrimagnetic com-
pensation point at a temperature near the Curie temperature. The implica-
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tributed to the nucleation of another phase11 or thermal acti-
tions of this behavior for Curie-point writing are not yet clear.
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