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Structural and magnetic properties of (Cr1−x Mn x )5Al8 solid solution and structural relation to hexagonal nanolaminates

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Electron microscopy is used to reveal the competitive epitaxial growth of bcc structure (Cr1−x Mnx )5Al8 and (Cr1−y Mny )2AlC [Mn+1AXn (MAX)] phase during both magnetron sputtering and arc deposition. X-ray diffraction θ–2θ measurements display identical peak positions of (000n)-oriented MAX phase and (Cr1−x Mnx )5Al8, due to the interplanar spacing of (Cr1−x Mnx )5Al8 that matches exactly half a unit cell of (Cr1−y Mny )2AlC. Vibrating sample magnetometry shows that a thin film exclusively consisting of (Cr1−x Mnx )5Al8 exhibits a magnetic response, implying that the potential presence of this phase needs to be taken into consideration when evaluating the magnetic properties of (Cr, Mn)2AlC.
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Structural and magnetic properties of (Cr
12x
Mn
x
)
5
Al
8
solid
solution and structural relation to hexagonal nanolaminates
A. Mockute P. O. A
˚. Persson J. Lu
A. S. Ingason F. Magnus S. Olafsson
L. Hultman J. Rosen
Received: 31 March 2014 / Accepted: 21 June 2014 / Published online: 15 July 2014
ÓSpringer Science+Business Media New York 2014
Abstract Electron microscopy is used to reveal the com-
petitive epitaxial growth of bcc structure (Cr
1-x
Mn
x
)
5
Al
8
and (Cr
1-y
Mn
y
)
2
AlC [M
n?1
AX
n
(MAX)] phase during both
magnetron sputtering and arc deposition. X-ray diffraction
h–2hmeasurements display identical peak positions of
(000n)-oriented MAX phase and (Cr
1-x
Mn
x
)
5
Al
8
, due to the
interplanar spacing of (Cr
1-x
Mn
x
)
5
Al
8
that matches exactly
half a unit cell of (Cr
1-y
Mn
y
)
2
AlC. Vibrating sample mag-
netometry shows that a thin film exclusively consisting of
(Cr
1-x
Mn
x
)
5
Al
8
exhibits a magnetic response, implying that
the potential presence of this phase needs to be taken into
consideration when evaluating the magnetic properties of
(Cr, Mn)
2
AlC.
Introduction
The M
n?1
AX
n
(MAX) phases constitute a class of layered
solids, where Mdenotes an early transition metal,
Adenotes an A-group element, Xdenotes C or N, and n=
1-3 [1]. These inherent atomic laminates have a hexagonal
structure, belonging to the P6
3
/mmc space group, which for
n=1 corresponds to a MXMAMXMAatomic
layer stacking in the c-direction. More than 60 MAX
phases have been synthesized to date, and the materials
have attracted attention primarily due to the unique com-
bination of metallic (good electrical and thermal conduc-
tors) and ceramic (hard, oxidation, and wear resistant)
characteristics, as well as extreme damage tolerance [2].
Research on MAX phase thin films has mostly been
concentrated on epitaxial growth with the c-axis perpen-
dicular to the substrate surface and subsequent materials
characterization. Phase identification and evaluation of the
film quality often relies on fast, simple, and nondestructive
X-ray diffraction (XRD) measurements, and in particularly
symmetric h–2hscans.
In a symmetric h–2hconfiguration, atomic planes per-
pendicular to the diffraction vector qare probed. For
polycrystalline samples, where grains are randomly ori-
ented, peaks from all crystallographic planes are observed
and form a unique XRD h–2hpattern, leading to straight-
forward phase identification. However, thin films are often
highly textured, such that a h–2hscan returns a limited set
of crystallographic planes. For the case of a sample con-
taining two epitaxial phases with identical out-of-plane
spacings, the XRD h–2hscan exhibits overlapping peaks at
exactly the same positions. Consequently, correct phase
identification is challenging. Hence, undiscovered com-
peting phases attained within the MAXelemental system
could have drastic consequences on stated sample phase
purity, interpretation of experimental data, and result in
divergence in reported results.
We present experimental evidence of the formation of
bcc (Cr
1-x
Mn
x
)
5
Al
8
(x=0.72) as a competing phase
during the synthesis of the (Cr, Mn)
2
AlC MAX phase.
Furthermore, it is shown that epitaxial films of (Cr, Mn)
2
AlC, Cr
5
Al
8
and Mn
5
Al
8
as well as (Cr, Mn)
5
Al
8
provide
A. Mockute (&)P. O. A
˚. Persson J. Lu
A. S. Ingason L. Hultman J. Rosen
Thin Film Physics Division, Department of Physics, Chemistry,
and Biology (IFM), Linko
¨ping University, 581 83 Linko
¨ping,
Sweden
e-mail: aurmo@ifm.liu.se
F. Magnus
Department of Physics and Astronomy, Uppsala University,
Box 530, 751 21 Uppsala, Sweden
S. Olafsson
Science Institute, University of Iceland, Dunhaga 3,
107 Reykjavı
´k, Iceland
123
J Mater Sci (2014) 49:7099–7104
DOI 10.1007/s10853-014-8416-8
identical XRD h–2hscans with respect to peak positions.
Magnetic properties have previously been theoretically
predicted for the (Cr, Mn)
2
AlC MAX phase [3]. In order to
investigate the possible influence of a (Cr, Mn)
5
Al
8
phase
on the magnetic response, (Cr, Mn)
5
Al
8
as well as Mn
5
Al
8
has also been characterized with respect to magnetic
properties. Moreover, we emphasize XRD pole measure-
ments as the correct approach to enable the detection of
competing phases.
Experimental details
Thin film synthesis of (Cr, Mn)
2
AlC MAX phase was
attempted using high current pulsed cathodic arc and DC
magnetron sputtering. For arc depositions, compound Cr/
Mn (50/50 at.%) as well as elemental Al and C cathodes
was used in alternating mode at a rate of 10 Hz. Pulse
lengths were set to 450, 250, and 1000 ls for Cr/Mn, Al,
and C cathodes, respectively. The films were deposited on
Al
2
O
3
(0001) substrates at a base pressure of 5 910
-7
Torr
and at a growth temperature of 625 °C. Magnetron sput-
tering was performed from Cr/Mn (50/50 at.%), Al, and C
targets at a base pressure of 3 910
-7
Torr at 600 8Con
Al
2
O
3
(0001) and MgO(111) substrates.
Cr
2
AlC, Cr
5
Al
8
,Mn
5
Al
8
,and(Cr
0.28
Mn
0.72
)
5
Al
8
films
weredepositedonAl
2
O
3
(0001) substrates by DC mag-
netron sputtering from elemental targets at a base pres-
sure of 9 910
-8
Torr at the growth temperature of
600 °C. All substrates were cleaned in acetone, methanol,
and isopropanol ultrasonic baths for 10 min each and
degassed for 10 min at the growth temperature prior to
deposition.
X-ray diffraction characterization was performed using
a Panalytical Empyrian MRD equipped with a line focus
Cu Kasource (k=1.54 A
˚) and a hybrid mirror optics on
the incident beam side for h–2hmeasurements. Pole scans
were acquired with point focus and X-ray lens optics.
Scanning electron microscopy (SEM) analysis was con-
ducted using a LEO 1550 SEM. Cross-sectional and plan-
view samples for transmission electron microscopy (TEM)
analysis were prepared by conventional mechanical meth-
ods followed by low-angle ion milling with a final fine-
polishing step at low acceleration voltage. TEM imaging,
electron diffraction (ED), and energy-dispersive X-ray
spectroscopy (EDX) for elemental analysis were performed
in a Tecnai G2 TF20 UT FEG instrument operated at
200 kV, equipped with an EDX detector, while scanning
(S)TEM imaging and EDX mapping were performed in the
doubly corrected Linko
¨ping Titan
3
60–300 equipped with a
Super-X EDX detector. Magnetic characterization was
carried out in a Cryogenic Ltd. vibrating sample magne-
tometer (VSM).
Results and discussion
Structural and compositional analysis
Figure 1a, b presents XRD h–2hscans of thin films aimed
for (Cr, Mn)
2
AlC MAX phase by cathodic arc and mag-
netron sputtering, respectively, on Al
2
O
3
(0001) substrates.
Only peaks corresponding to (000n)Cr
2
AlC positions are
observed, suggesting films consisting of phase pure epi-
taxial MAX phase. It was previously reported that the
c-lattice constant remains unaffected upon Mn incorpora-
tion, and thus, shifts in peak positions are not expected [4].
The cathodic arc-deposited sample was subjected to
high-resolution (HR) STEM imaging. Apart from a sig-
nificant component of (Cr, Mn)
2
AlC, the sample also
exhibits an unknown structure, which is shown in Fig. 2.
While the discrete atomic organization of the structure, as
shown in Fig. 2a, does not resemble a MAX phase, the
laminated appearance is identical to a MAX phase struc-
ture. The insets in both (a) and (b) show the intensity line
profile of the structure along the growth direction. As for a
MAX phase, the structure exhibits two bright layers of
heavier elements (compared with the M
2
X layers in a
MAX phase), which are interleaved by a lighter element
Intensity [arb. units]
(110)
(
220
)
(
550
)
(
660
)
(
330
)
(
440
)
(110)
(
220
)
(
330
)
(
440
)
(
550
)
(
660
)
Al2O3Al2O3
(a)
(b)
(c)
(d)
20 40 60 80 100
Fig. 1 XRD h–2hscans of thin films resulting from attempted (Cr,
Mn)
2
AlC MAX phase synthesis by acathodic arc and bmagnetron
sputtering. Indicated peak positions correspond to the (000n)MAX
phase. cCr
5
Al
8
and dMn
5
Al
8
h–2hscansfrom magnetron sputteredfilms
7100 J Mater Sci (2014) 49:7099–7104
123
layer (compared with the MAX phase Alayer). The in-
plane rotated sample, see Fig. 2b, also exhibits the same
layered structure.
The layered structure shown in Fig. 2is further inves-
tigated in Fig. 3. A low-magnification STEM image of a
deposited film particle is shown in Fig. 3a. Elemental
mapping of the particle reveals two domains of different
composition, where the composition of the left domain is
Cr/Mn/Al with approximate ratio of 9:22:69. The structure
of the left part of the particle corresponds to that shown in
Fig. 2, while the right domain corresponds to the (Cr,
Mn)
2
AlC MAX phase structure shown in Fig. 3bby
HRSTEM. The interface between the two domains is fur-
ther shown in (c). Note the transition where the atomic
layers bridge seamlessly between the domains, as indicated
by the arrows in Fig. 3c. Even the contrasts of the layers
are perfectly matched (bright–bright and dark–dark). The
only distinction between the domains can be made from the
discrete hexagonal (zigzag) atomic pattern of the MAX
phase (right) to the continuous lines of the competing phase
(left). The perfect transition between the domains, shown at
high magnification in Fig. 3c, is remarkable. Notably, the
atomic layers continue from one domain to the other
despite the significant change in composition. This
HRSTEM image provides a visual explanation why the
presence of this second commensurate phase may be
overlooked in the XRD measurements shown in Fig. 1a, b.
As distances along the diffraction vector qare probed in
h–2hconfiguration, equal distances give rise to peaks at the
same 2hangles and hence, peak overlap.
The domains in this sample proved too small to enable
ED exclusively on the unknown phase. Therefore, a thin
film aiming for the measured composition was synthesized.
TEM–EDX analysis of the resulting film proved the
attained composition of Cr/Mn/Al equal to 9:22:69.
HRTEM and ED were subsequently performed on the film
to obtain structural information and crystallographic
relation with the substrate. Figure 4a–f presents ED and
corresponding filtered HRTEM images along [001], [111],
and [113] zone axes. The structure could be identified as a
bcc crystal structure with lattice constant of 9.05 A
˚, with
an out-of-plane relation to the substrate as bcc(110)//
Al
2
O
3
(0001).
Plan-view TEM and ED analyses of the [(Cr, Mn)
2
AlC ?bcc structure] film grown on MgO(111), shown in
Fig. 5, reveal that the grains of the unknown phase acquire
three different in-plane orientations, with *120°in
between.
According to the literature, Cr
5
Al
8
and Mn
5
Al
8
are
phases exhibiting a bcc structure with a lattice constant
Fig. 2 a High-resolution cross-
sectional STEM imaging along
the 1
100½MAX phase zone
axis, including intensity line
profile, showing a laminated
appearance identical to a MAX
phase structure. bIn-plane
rotated sample (30°) to the
2
1
10½zone axis of the MAX
phase structure
Fig. 3 a STEM cross-sectional image from the cathodic arc-depos-
ited film with corresponding EDX elemental maps of Cr, Mn, Al, and
C, bHRSTEM image of the (Cr, Mn)
2
AlC MAX phase structure, and
cHRSTEM image of the interface between the domains
J Mater Sci (2014) 49:7099–7104 7101
123
of approximately 9 A
˚. Both have been experimentally
realized [5,6], and according to the phase diagram, they
can form a solid solution (Cr
1-x
Mn
x
)
5
Al
8
. Considering
the here-attained composition of the unknown bcc
structure,itwouldcorrespondto(Cr
0.28
Mn
0.72
)
5
Al
8
.Both
Cr
5
Al
8
and Mn
5
Al
8
were synthesized for further XRD
analysis, and in Fig. 1c, d, h–2hmeasurements of Cr
5
Al
8
and Mn
5
Al
8
, respectively, are presented. The resulting
XRD scans are strikingly similar to the (Cr, Mn)
2
AlC. In
fact, the peaks appear at exactly the same positions
(13.8°, 27.7°,42.2°,57.2°, 73.7°, and 92.1°) for all three
phases. It is obvious that by means of XRD h–2hmea-
surement, (Cr, Mn)
2
AlC and (Cr
1-x
Mn
x
)
5
Al
8
cannot be
distinguished, which implies that previous studies may
contain undiscovered competing phases coexisting with
the MAX phase, or, in the extreme case, a sample with
no actual MAX phase can be misinterpreted as phase
pure MAX.
Fig. 4 ED and corresponding
filtered HRTEM of the
unknown phase along a,
b[001], c,d[111], and e,
f[113] zone axes
7102 J Mater Sci (2014) 49:7099–7104
123
Pole measurements were taken to find an alternative
approach to distinguish between the MAX phase and
(Cr, Mn)
5
Al
8
.InCr
5
Al
8
and Mn
5
Al
8
as well as [(Cr,
Mn)
2
AlC ?(Cr, Mn)
5
Al
8
]//MgO, the bcc (411) planes
at 2h=42.3°were mapped for tilt angle w=[0–88]°.
In Fig. 6a–c, sixfold symmetry at 33.5°,60°, and 70.3°
is observed, corresponding to angles between (110) and
(411) planes in bcc crystal structure. However, the rel-
atively weak peaks at 70.3°are not visible in the Mn
5
Al
8
scan, which displays overall lower intensities. Corre-
sponding measurements of an epitaxial phase pure (Cr,
Mn)
2
AlC film would only provide a sixfold symmetric
10
13ðÞpeak at w&60°. Consequently, the middle peak
at w=60°in Fig. 6c is an overlap of (Cr, Mn)
2
AlC and
(Cr, Mn)
5
Al
8
. The high-intensity peaks at w=37.8°and
61°in Fig. 6a, b originate from Al
2
O
3
substrate, while in
Fig. 6c MgO substrate is visible as threefold symmetric
peak at w=54.4°. It is interesting to note that the peaks
are broadened when Al
2
O
3
is used as substrate, while for
MgO they are grouped in three, indicating better epi-
taxial match on MgO than on Al
2
O
3
and three in-plane
orientations, in agreement with observations in TEM.
Other MAX phase systems can also contain competing
phases involving Mand Aelements that crystallize in the
here-reported bcc structure. One such example is V
5
Al
8
.
Hence, it is recommended to interpret XRD h–2hmea-
surements of V
2
AlC with great care.
Magnetic properties
Recently, (Cr,Mn)
2
GeC [7,8], (Cr,Mn)
2
AlC [9], and
Mn
2
GaC [10] have been reported as the first magnetic
MAX phases. (Cr,Mn)
2
AlC has now been characterized.
Phase purity is extremely important for correct analysis of
magnetic characterization data, and the presence of
undiscovered magnetic impurity phases may lead to false
conclusions. Therefore, we took VSM measurements on
the (Cr, Mn)
5
Al
8
and Mn
5
Al
8
samples.
Figure 7shows the in-plane magnetization of the
(Cr
0.28
Mn
0.72
)
5
Al
8
and Mn
5
Al
8
thin films as a function of
magnetic field at 10 K. The magnetic signal from Mn
5
Al
8
is weak, with the saturation magnetization one-eighth that
of (Cr, Mn)
5
Al
8
. The more pronounced magnetic response
observed in (Cr, Mn)
5
Al
8
may be explained by the Cr–Mn
exchange interaction. The saturation magnetic moment per
M-atom at 10 K is 0.16 and 0.03 l
B
for (Cr, Mn)
5
Al
8
and
Mn
5
Al
8
, respectively, and did not change significantly in
the investigated temperature range of 10–300 K. As a
Fig. 5 ED of a plan-view [(Cr, Mn)
2
AlC ?bcc structure] sample on MgO(111). Diffraction pattern is recorded along [110] zone axis of the
unknown phase. Three different in-plane orientations of the bcc structure can be identified
Fig. 6 Pole figures of aCr
5
Al
8
(110)//Al
2
O
3
(0001), bMn
5
Al
8
(110)//
Al
2
O
3
(0001), and c[(Cr, Mn)
2
AlC(0001) ?(Cr, Mn)
5
Al
8
(110)]//
MgO(111) at 2h=42.3°corresponding to (411) planes for tilt angle
w=[0–88]°. Sixfold symmetry at 33.5°,60°, and 70.3°confirms the
bcc crystal structure of (Cr, Mn)
5
Al
8
J Mater Sci (2014) 49:7099–7104 7103
123
magnetic response is observed for (Cr, Mn)
5
Al
8
as well as
Mn
5
Al
8
, it is highly recommended to use complementary
analysis techniques for detailed evaluation of phases
present and nonlocal composition of the material, prior to
characterization of magnetism in (Cr, Mn)
2
AlC MAX
phases. It should be stressed that for the other reported Mn-
containing MAX phases (Cr, Mn)
2
GeC [7,8] and Mn
2
GaC
[10], corresponding bcc structures with a &9A
˚do not
exist. Furthermore, the formation of these MAX phases has
been confirmed by HRSTEM in combination with local
composition analysis by EDX. Consequently, the magnetic
response stated in these previous reports can be safely
ascribed to originate from the MAX phases.
Conclusions
We have presented evidence for a (Cr
1-x
Mn
x
)
5
Al
8
inter-
metallic phase of bcc structure and with an interplanar
spacing matching exactly half a unit cell of the (Cr
1-y
Mn
y
)
2
AlC MAX phase. Hence, routinely performed XRD
h–2hmeasurements display peak positions of (000n)-ori-
ented MAX phase and (Cr
1-x
Mn
x
)
5
Al
8
, which are identi-
cal. Complementary analysis techniques resolving both
structure and composition must therefore be used for
unambiguous phase identification, as proven here with
electron microscopy and diffraction, as well as X-ray pole
figure measurements. Furthermore, analysis of magnetic
properties of (Cr
0.28
Mn
0.72
)
5
Al
8
and Mn
5
Al
8
reveals that
they are both magnetic, and with a magnetic moment of
0.16 l
B
per M-atom at 10 K in (Cr
0.28
Mn
0.72
)
5
Al
8
. Con-
sequently, possible presence of these phases needs to be
taken into consideration when evaluating magnetic prop-
erties in both (Cr, Mn)
2
AlC and related MAX phases.
Acknowledgements The research leading to these results has
received funding from the European Research Council under the
European Communities Seventh Framework Programme (FP7/2007-
2013)/ERC Grant Agreement No. [258509]. J. R. acknowledges
funding from the Swedish Research Council (VR) Grant No.
642-2013-8020 and the KAW Fellowship program. P. O. A
˚. P. and L.
H. acknowledge the Swedish Research Council (VR) and Knut and
Alice Wallenberg Foundation for providing funding for the Linko
¨ping
double-corrected Titan
3
60–300 kV electron microscope. J. R. and P.
O. A
˚. P acknowledge support from the SSF Synergy Grant FUNCASE
Functional Carbides and Advanced Surface Engineering. F. M.
acknowledges financial support from the Carl Trygger Foundation.
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Fig. 7 Magnetic response of the (Cr
0.28
Mn
0.72
)
5
Al
8
and Mn
5
Al
8
thin
films measured by VSM at 10 K with the magnetic field applied
parallel to the film plane. The diamagnetic contribution from the
Al
2
O
3
substrates has been subtracted. The saturation magnetic
moment per M-atom in (Cr
0.28
Mn
0.72
)
5
Al
8
has been determined to
0.16 l
B
. The magnetic signal of Mn
5
Al
8
is considerably weaker
7104 J Mater Sci (2014) 49:7099–7104
123
... As the Fe concentration increases up to x = 0.2 (red and black curves), additional peaks appear, which we identified as Fe 5 Al 8 (ICSD CollCode 169545) or its solid solution with Cr 5 Al 8 (ICSD CollCode184444). These phases may also have peaks overlapping with the MAX phase, 26 as discussed further below. We note that the formation of the intermetallic compound (Fe 5 Al 8 ) is theoretically predicted to be a competitive phase in the Cr− Fe−Al−C system (see the Supporting Information, Table S1). ...
... These, however, cannot be distinguished by conventional XRD and pole figures since the interplanar spacing of Cr 5 Al 8 -based solid solution matches exactly half a unit cell of Cr 2 AlC. 26 Thus, we additionally perform pole figure measurements between ψ = 32°and 35° (Figure 5a). Peaks at this ψ angle position exist only for intermetallic Cr 5 Al 8 -based bcc structures and not for the MAX phase structure. ...
... Peaks at this ψ angle position exist only for intermetallic Cr 5 Al 8 -based bcc structures and not for the MAX phase structure. 26 The results presented in Figure 5a Figure 5a). Such a pattern suggests two in-plane orientations of MAX phase grains with respect to MgO(111). ...
... While Cr 5 Al 8 is nonmagnetic, the EPMA data demonstrate that an increasing amount of Mn incorporates into this phase as the nominal Mn composition of the sample increases (Table 2), and (Cr 1Àx Mn x )Al 8 has been shown to be magnetic. 40 However, as shown in Table 1, the nominal x = 0.10 sample actually shows a much larger fraction of Cr 5 Al 8 than the x = 0.15 sample (22 wt% vs. 6 wt%), despite the x = 0.15 sample having a larger overall magnetic signal. Therefore, we believe that the paramagnetic signal in these samples is coming from paramagnetic (disordered) Mn in the MAX phase. ...
Article
A few years after the theoretical prediction of magnetic MAX phases, a number of such materials have been experimentally reported, especially in the form of thin films. Yet, due to a relatively small number of studies, we have only just begun to discover the intriguing magnetic properties that are associated with this class of materials. The preparation of bulk MAX phases with later transition metals has been proven to be particularly challenging. Consequentially, there is a great need to develop synthetic strategies to obtain the respective materials in suitable quantities for magnetic investigations. Here, bulk Mn- and Fe-substituted Cr2AlC are prepared using non-conventional synthesis methods such as microwave heating and spark plasma sintering. Synchrotron X-ray diffraction coupled with detailed elemental analyses is used to confirm the successful doping of the MAX phase with the later transition metals as well as to elucidate the microstructure of the obtained dense materials. 57Fe Mössbauer spectroscopy data are presented showing signals of the doped MAX phase and Fe-containing secondary phases. Based on PPMS and SQUID measurements the non-trivial magnetic behavior of the obtained samples is discussed in the context of the existing studies.
... A minor amount of (Cr 1−y Mn y ) 5 Al 8 with y = 0.72 was also found in the samples by scanning (S)TEM and EDX analysis. This phase was later synthesized in pure form and studied further [107] in order to establish the possibility of it contributing to magnetic signal found for the MAX phase films. It was concluded that this could not be the case, even though the results showed that for y = 0.72 the magnetic moment per M atom in (Cr 1−y Mn y ) 5 Al 8 was 0.16 µ B and consequently this number is higher then measured for the (Cr 0.8 Mn 0.2 ) 2 AlC sample. ...
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This review presents MAX phases (M is a transition metal, A an A-group element, X is C or N), known for their unique combination of ceramic/metallic properties, as a recently uncovered family of novel magnetic nanolaminates. The first created magnetic MAX phases were predicted through evaluation of phase stability using density functional theory, and subsequently synthesized as heteroepitaxial thin films. All magnetic MAX phases reported to date, in bulk or thin film form, are based on Cr and/or Mn, and they include (Cr,Mn)2AlC, (Cr,Mn)2GeC, (Cr,Mn)2GaC, (Mo,Mn)2GaC, (V,Mn)3GaC2, Cr2AlC, Cr2GeC and Mn2GaC. A variety of magnetic properties have been found, such as ferromagnetic response well above room temperature and structural changes linked to magnetic anisotropy. In this paper, theoretical as well as experimental work performed on these materials to date is critically reviewed, in terms of methods used, results acquired, and conclusions drawn. Open questions concerning magnetic characteristics are discussed, and an outlook focused on new materials, superstructures, property tailoring and further synthesis and characterization is presented.
... [9] The possibility to achieve a stable ferromagnetic (FM) order at room temperature in nanolaminated MAX phases in bulk or thin film form makes these systems extremely interesting for both fundamental materials science and applications. [10] Recently, it has been demonstrated that magnetic order can be changed and stabilized up to room temperature by substituting Cr by Mn. [9,[11][12][13][14][15][16] Stable (Cr1-xMnx)2AC (A = Al, Ge or Ga) type MAX phase compounds can be considered as a new family of magnetic MAX phases, which exhibit FM properties. Furthermore, the Mn2GaC magnetic compound has been synthesized as a heteroepitaxial thin film containing Mn as the exclusive M element. ...
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Magnetic MAX phase (Cr0.5Mn0.5)2GaC thin films grown epitaxially on MgO(111) substrates were studied by ferromagnetic resonance at temperatures between 110 and 300 K. The spectroscopic splitting factor g = 2.00 ± 0.01 measured at all temperatures indicates pure spin magnetism in the sample. At all temperatures we find the magnetocrystalline anisotropy energy to be negligible which is in agreement with the identified pure spin magnetism.
... Recently, in a quest for the first magnetic MAX phase, and fuelled by theoretical predictions [3], Mn was added as a new element to the MAX phase family. Initially, it was an alloying element in (Cr1-xMnx)2AlC [4][5][6][7] and (Cr1-xMnx)2GeC [8], providing the first magnetic MAX phases. Subsequent experimental studies on magnetic properties of MAX phases have not only been done on alloys containing Mn [9,10] but the topic has also been addressed for other MAX phases like, Cr2AlC [11], Cr2GeC [12] and Cr2GaN [13]. ...
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Growth of (Cr0.5Mn0.5)2GaC thin films from C, Ga, and compound Cr0.5Mn0.5 targets is reported for depositions on MgO (111), 4H-SiC (0001), and Al2O3 (0001) with and without a NbN (111) seed layer. Structural quality is found to be highly dependent on the choice of substrate with MgO (111) giving the best results as confirmed by X-ray diffraction and transmission electron microscopy. Phase pure, high crystal quality MAX phase thin films are realized, with a Cr:Mn ratio of 1:1. Vibrating sample magnetometry shows a ferromagnetic component from 30 K up to 300 K, with a measured net magnetic moment of 0.67 μB per metal (Cr + Mn) atom at 30 K and 5 T. The temperature dependence of the magnetic response suggests competing magnetic interactions with a resulting non-collinear magnetic ordering.
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Prompted by the increased focus on MAX phase materials and their two-dimensional counterparts MXenes, a brief review of the current state of affairs in the synthesis of MAX phases as epitaxial thin films is given. Current methods for synthesis are discussed and suggestions are given on how to increase the material quality even further as well as arrive at those conditions faster. Samples were prepared to exemplify the most common issues involved with the synthesis, and through suggested paths for resolving these issues we attain samples of a quality beyond what has previously been reported. Impact Statement: We aim to address the quality of MAX phase thin films and suggest a more robust route for the synthesis of samples of consistent reproducible quality.
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Herein we present evidence for the coexistence of a Mn-rich ferromagnetic (FM) state and a Mn-poor reentrant cluster glass state in bulk, polycrystalline, layered (Cr1−x,Mnx)2GeC samples, where x is varied between 0.01 and 0.1. The Mn-poor regions form a reentrant cluster glass state below ∼30 K. The Mn-rich regions become FM at Curie temperatures that increase with increasing Mn content. The interface coupling between these two regions gives rise to exchange anisotropy and a change in sign at 20 K resulting in, rarely observed, inverted hysteresis loops.
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We report on first principles prediction and subsequent synthesis of Mn2GaC, a new member of the inherently nanolaminated Mn+1AXn (MAX) phase family. This phase, the first to include Mn as the sole M element, was synthesized as a heteroepitaxial thin film. The material was theoretically predicted to display magnetic ordering with ferromagnetic (FM) and antiferromagnetic configurations degenerate in energy within the computational accuracy. Vibrating sample magnetometer measurements show FM ordering with a saturation moment of ms=0.29 μB per Mn atom and remanent moment of mr=0.15 μB per Mn atom for temperatures≤230 K.
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We present an ab initio theoretical analysis of the temperature-dependent stability of inherently nanolaminated (Cr1−xMnx)2AlC. The results indicate energetic stability over the composition range x = 0.0 to 0.5 for temperatures ≥600 K. Corresponding thin film compounds were grown by magnetron sputtering from four elemental targets. X-ray diffraction in combination with analytical transmission electron microscopy, including electron energy-loss spectroscopy and energy dispersive x-ray spectroscopy analysis, revealed that the films were epitaxial (0001)-oriented single-crystals with x up to 0.16.
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The first experimental realization of a magnetic M_{n+1}AX_{n} (MAX) phase, (Cr_{0.75}Mn_{0.25})_{2}GeC, is presented, synthesized as a heteroepitaxial single crystal thin film, exhibiting excellent structural quality. This self-organized atomic laminate is based on the well-known Cr_{2}GeC, with Mn, a new element in MAX phase research, substituting Cr. The compound was predicted using first-principles calculations, from which a variety of magnetic behavior is envisaged, depending on the Mn concentration and Cr/Mn atomic configuration within the sublattice. The analyzed thin films display a magnetic signal at room temperature.
Article
(Cr1–xMnx)2AlC MAX phase thin films were synthesized by cathodic arc deposition. Scanning transmission electron microscopy including local energy dispersive X-ray spectroscopy analysis of the as-deposited films reveals a Mn incorporation of as much as 10 at% in the structure, corresponding to x = 0.2. Magnetic properties were characterized with vibrating sample magnetometry, revealing a magnetic response up to at least room temperature. We thus verify previous theoretical predictions of an antiferromagnetic or ferromagnetic ground state for Cr2AlC upon alloying with Mn. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
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The range of homogeneity of Ti1-xAl1+x was investigated by means of X-ray diffraction and metallography. This high-temperature phase is stable at the mole fraction xAl=0.64. The structure is homoeotypical with CuAu, , , c/a=0.981. The symmetry relationship to the homoeotypical phases as well as the structural coordination principles are discussed and the powder diffraction data are given.ZusammenfassungMit Hilfe der röntgenographischen und metallographischen Untersuchungen wurde der Homogenitätsbereich von Ti1-xAl1+x ermittelt. Diese Hochtemperaturphase ist beim Molenbruch xAl=0,64 stabil. Die Struktur ist zu CuAu nomöotyp, tP4, P4/mmm, , , c/a=0,981. Die Symmetriebeziehungen zu den verwandten Phasen sowie der Aufbau der Koordinationspolyeder werden diskutiert und die Pulveraufnahmedaten werden mitgeteilt.
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The high temperature phase Cr5Al8(h) (γ1) was prepared by splat cooling. The structure of this phase is of the same type as Cu5Zn8 (cI52; ; ). Powder diffraction data are given and compared with the electron diffraction patterns. The symmetry relationship between the high temperature phase Cr5Al8(h) and the low temperature phase Cr5Al8(1) is shown and the influence of the valence electron concentration on the formation of structural vacancies is discussed.ZusammenfassungDurch rasche Abkühlung der Schmelze wurde die Hochtemperaturphase Cr5Al8(h) (γ1) gewonnen. Sie ist zu Cu5Zn8 isotyp (cI52; ; ). Es werden Pulveraufnahmedaten mitgeteilt und mit Elektronenbeugungsaufnahmen verglichen. Ferner wird die Symmetriebeziehung zwischen der Hochtemperaturphase Cr5Al8(h) und der Raumtemperaturphase Cr5Al8(r) erläutert und der Einfluss der Valenzelektronenkonzentration auf die Leerstellenbildung diskutiert.
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A survey of some more recent results on the structural chemistry of compounds between transition elements and IVb group elements (carbon, silicon, germanium and tin) will be presented. There are essentially two large classes of compounds to be discussed, characterized by uniform structural principles, namely transition element carbides and related phases on the one hand and defect disilicide structure compounds and derivatives on the other.Starting with the problem of carbon ordering in transition element carbides and hydrogen containing carbides which reveal the significance of the octahedral [T6C]-group, numerous complex carbides of the general formula TxMyCz (T = transition element, M = another transition or B-group element) can be explained by means of a few common structural features. Perovskite carbides of formula T3MC, corresponding to the filled up Cu3Au-type or the filled up U3Si-type structures, β-Mn carbides of formula T3M2C, corresponding to the filled up β-Mn-type structure and K-carbides, related to the Mn3Al9Si-type structure are characterized by linking of the [T6C]-groups by corners. H-phase carbides of formula T2MC and carbides having Ti3SiC2-type structure exhibit linking of the [T6C]-groups by edges. A similar mode of linking also occurs for carbides with V3AsC-type or the filled Re3B-type structures, although in some cases such as VCr2C2 the trigonal prismatic [T6C]-group intervenes. Finally, the η-carbides having filled Ti2Ni-type and carbides of formula T5M3C with filled up Mn5Si3-type structure can be regarded as built up by linking of the octahedral [T6C]-groups by faces. The geometrical factor within the carbides is strongly supported by the short T-C-distances in the structural element [T6C], thus the formation and architecture of complex carbides may be understood from a topo-chemical point of view, for example, Ti2GeC (H-phase carbide) consists of the sum of TiC (octahedral group) and TiGe (trigonal prism).The second class of compounds, which are derived from the TiSi2-type structure, also belong to an uniform geometrical principle; however, some influence of the electronic concentration on the defect of the B-group element (Si,Ge) and the cell parameter will be recognized. The peculiar structural principle can be described by a partial lattice of the transition metal atom corresponding almost perfectly to that of the Ti-atoms of the TiSi2-type while the second partial lattice (Si,Ge or Sn) according to the defect of these atoms is expanding in one direction of the generating (110) plane. As a consequence of the mutual interference of T- and B-group element atoms a helicoidal structural element of the respective Si, Ge, etc., atoms results. Thus, the arrangement is characterized by a typical subcell and occasionally by extremely long c-axis. That also means, fairly complex compositions occur such as Mn11Si19, Mo13Ge23, V17Ge31 or Rh39 (Ga0.5Ge0.5)58. The problem of pseudo-homogenous domains of compounds arise in as far as within fairly small regions of composition a split according to different multiple of subcell will be observed, such as Mn11Si19(MnSi1.727); Mn26Si45(MnSi1.730); Mn15Si25(MnSi1.733) and Mn27Si47(MnSi1.741). A similar change in the multiple of subcells that means independent phases, takes place by substituting either the transition element by another or by substituting the B-group element by another B-group element such as Cr37Ge59As4 which corresponds to the Rh10Ga17-type while Cr11Ge19 is isotypic with Mn11Si19. In general, lowering of the overall electron concentration diminishes the defect of the B-group element in the compound.