Coexistence and interconvertibility of ferromagnetic and antiferromagnetic phases in the single crystal of Mn$_3$ single-molecule magnet

0.47 MB·English

by  Yan Cui

Coexistence and interconvertibility of ferromagnetic and antiferromagnetic phases in
the single crystal of Mn single-molecule magnet
3
Yan Cui, Yan-Rong Li, Rui-Yuan Liu, Zhen-Yu Mi and Yun-Ping Wang
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics,
Chinese Academy of Sciences, Beijing 100190, People’s Republic of China
(Dated: January 23, 2015)
We report the coexistence of ferromagnetic and antiferromagnetic phases in the single crystal of
5
Mn3 single-molecule magnet. The coexistent state appears within a certain period of time in the
1
progress of either oxidation or reduction during a reversible oxidation-reduction process, when the
0
sample is exposed in the air (oxygen) or the methyl vapor. We noticed an apparent change in the
2
molecular structure,which is also reversible in termsof that themethylgroup is droppedor added

n to the molecules during the oxidation or reduction. The absence or presence of the methyl group
a in the molecules exert an essential impact upon the intermolecular exchange interaction, and the
J ferromagnetic phase comes from theheterogenous intermolecular bondsbetween pairs of molecules

2 of which one molecule has a methyl group whereas the other has lost the methyl group. The
2 reversiblechange in molecular structuresuggests themagnetic structureof Mn3 might bedesigned
and modulated at molecular scale, which implies Mn3 has a great application potential.
]
l
l PACSnumbers: 75.50.Xx,75.30.-m,75.45.+j,75.25.+z
a
h
-
s The coexistence of multiple magnetic phases has at- The first term represents the zero-field splitting energy,
e tracted considerable interest in recent years, due to the which produces a uniaxial anisotropy barrier separat-
m
potentialfor technologicalapplications[1–3] andtheoret- ing degenerateopposite projectionsofthe spin alongthe
. ical researches[4, 5]. In the reported researches, so far, magnetic easy axis[12]. The second term is the Zeeman
t
a the coexistent systems of multiple magnetic phases are energy resulting fromthe interactionof the spin with an
m
polycrystals or crystals with apparent defects[1, 3, 6–8] appliedmagneticfield. The thirdtermisthe intermolec-
- in which the system structure as well as the mechanism ular exchange interaction energy between one molecule
d
of the formation of the coexistent state are complicated, and its three neighboring molecules.
n
making it difficult to anticipate feasible applications. In
o The measurements are performed using 7T SQUID-
this paper, for the first time, we report the coexistence
c
VSM (Quantum Design), with three differently sized
[ and interconvertibility of ferromagnetic (FM) and anti-
fresh samples preserved in the air or oxygen. The
ferromagnetic(AFM)phasesinthesinglecrystalofMn3
1 sizes of sample #1 to #3 are 2.0mm×2.0mm×0.8mm,
single-molecule magnet (SMM)[9–11] which is without
v 1.8mm×1.5mm×0.5mm, and 1.8mm×1.0mm×0.5mm,
4 apparentdefectanddisplaysquantumtunneling ofmag-
respectively. Each of the sample is well oriented and
8 netization (QTM) at low temperature[12, 13]. We have
fixed on a home-made Teflon cubic, which is glued on
4 observed the coexistent state of FM and AFM phases
5 duringanoxidationprocesswhenthesampleispreserved the sample holder. The magnetic easy axis is ensured to
0 be parallel to the applied field. Fig.1a presents the nor-
intheairoroxygen,aswellasthereductionprocesswhen
1. the oxidized sample is preserved in methanol gas. It is malizedmagnetizationvstemperature(M/Ms-T)curves
of sample #1 preserved in the air on different days. It
0 predicted that the oxidation-reduction process may be
5 controlledbyelectricalstimuli[14]ofscanning-tunneling- is seen that, FM phase starts appearing as time goes by,
1 microscopy(STM)tip,whichimpliesthemagneticstruc- andbecomes apparentonDay10. InFig.1b,the M/Ms-
: T curve at 1Oe indicates an apparent spontaneous mag-
v turemaybedesignedatmolecularscale,suggestingMn3
i may have a great potential in extensive applications of netization at Tc=6.5K shown. The M/Ms-T curves of
X sample #1 on Day 10 shown in Fig.1b can be well fit-
nanodevicesformagneticstorageandspintronics[15–19].
r tedbyM(T,H)=k(T)H+C(T)atlowmagneticfields,
a The single crystals of single-molecule magnet
wherethefield-independenttermC(T)isfromFMphase
Mn3([Mn3O(Et-sao)3(ClO4)(OH)3]) used in our ex-
whilethefield-proportionaltermk(T)H includesthecon-
perimentaresynthesizedaccordingtothecrystalgrowth
tributions of AFM phase and a small amount of isolated
procedures reported by Inglis et al[9]. Mn3 is known
FM occurrence. The fitting curves are indicated with
to be a SMM with AFM intermolecular exchange
the dash lines. It should be noted that, there is no field-
interaction which only exists in ab plane with a honey-
independenttermC(T)whenthefieldisappliedperpen-
comb structure[11], and hence may be regarded as a
dicularto the easy axisofthe crystalwhile the magnetic
two-dimensional magnetic system. The Hamiltonian of
moment of FM phase is parallel to the easy axis of the
a Mn3 molecule can be described as:
crystal. WiththevalueofC(T)atT=2K,theproportion
3 of FM phase can be figured out. For all samples that we
Hˆ =−DSˆz2+gµ0µBH·S−JXSˆizSˆz, (1) have measured, the proportion of FM phase is not more
than 0.2%. Meanwhile, AFM phase is also observed by
i=1
2

a%) 00..2204 H//c C (J/K)46P00H//c 25000 O00e00 OOee DDDDaaaayyyy 14710 a 1.0 H//c
(M/M S 00..1126 205 6Temp7eratDu8rae y(K )91010H=2 4D aOy e31 M/Ms00..05 T=1.6k
0.08
Day 1
0.04 -0.5 Day 3
b 0.00 C 1 Oe Day 6
(%) S 0.6 H//c kC*+Hk*H 2359144449 4OOOO Oeeeee b -11.50 Day 36
M
M/ 0.4 Ms12
/
H) 9
0.2
d
/
M 6
d
0.0 (
c 0.24 H//c Month 5-oxidized 3
Day 2-reduced
%) 0.20 0
(M S 0.16 -3 -2 -1 0 1 2 3
M/ 0.12 FIG. 2: (Color online) (a), N0oHrm (Tal)ized magnetization hys-
H=24 Oe
0.08 teresis loops (M/Ms-H) of Mn3 sample #2 preserved in the
air on different days. The sweep rate is 50Oe/s. (b), Deriva-
0.04
tivecurves from -3T to 3T which are shifted along y axis for
0.00 clarity. TheQTMpeaksfrom |-6>to|6> and|-6>to|5>of
2 4 6 8 10 12 14 16 18
Mn3(I)(blackline)andMn3(II)(blueline)arebothindicated
Temperature (K) bydark cyan arrows and pink arrows, respectively.
FIG. 1: (Color online) (a), The normalized magnetization
vs temperature (M/Ms-T) curves of sample #1 preserved in
theairondifferentdays. Theappliedfieldis24Oe. Theinset energy barrier is reducing. In Fig.2b, for the derivative
showstheheatcapacity(CP)curvesofsample#1onthe10th curves, the magnetic moment is saturated at Hz=-3T,
dayatdifferentfields. (b),TheM/Ms-T curvesofsample#1 and the magnetic field is swept from -3T to 3T. It
onthe10thdayatdifferentmagneticfields.Theymaybewell
is seen that, the first QTM peak deviates from zero
fittedbyM(T,H)=k(T)H+C(T)markedbydashlines. (c),
magnetic field, which is expected in the system with
The M/Ms-T curves of the fully oxidized sample #3 before
intermolecular-exchange-coupling [20]. The resonant
and after 24-hourreduction arepresented in thick lines. The
sample #3 is fully oxidized by being preserved in the air for field of the QTM from |-S> to |S − l> spin state
5 months. The applied field is 24Oe. in single-molecule magnets with identical exchange
interaction can be described as[21]:
Hz =lD/gµ0µB+(n↓−n↑)JS/gµ0µB, (2)
heatcapacitymeasurementwhichisperformedonPPMS
(Quantum Design). The inset in Fig.1a shows the heat where n and n represent the number of a tunneling
↓ ↑
capacity vs temperature curves of sample #1 at several molecule’s neighboring molecules which occupy the |-6>
given magnetic fields measured on Day 10. The peak and |6> state respectively.
moves to low temperature with increasing field, indicat- Therefore, the series of QTM peaks of Mn3(I) (black
ing the typical characteristic of AFM phase transition. line)indicatedbydarkcyanarrowsandpinkarrows,orig-
Therefore, it is evident that FM and AFM phases coex- inate from the QTM from |-6> to |6> and |-6> to |5>
ist during the oxidation process. respectively. These peaks are succeeded by new series of
The normalized magnetization hysteresis loops QTM peaks (in the red, green and blue lines) as time
(M/Ms-H) on different days of sample #2 preserved in goes by. The QTM from |-6> to |6> and |-6> to |5>
the air measured at T=1.6K and the derivatives of the of Mn3(II) (blue line) are indicated by dark cyan arrows
M/Ms-H curves with increasing field in the loops are andpinkarrowsaswell. Apparently,Mn3(I)andMn3(II)
presented in Fig.2. Black loops/curves are of the fresh moleculesarecoexistentduringthe oxidationprocess,as
Mn3 (hereafter Mn3(I)), blue loops/curves are of the theQTMpeaksofbothMn3(I)andMn3(II)areobserved
fully oxidized Mn3 (hereafter Mn3(II)), red and green in both red and green curves. On the other hand, the
loops/curves are of Mn3 in the states between fresh and QTM peaks in all the curves are well defined, indicating
fully oxidized. In Fig.2a, the hysteresis loops become thatthe easy magnetizationaxesofallthe moleculesare
narrower as time goes by, indicating the anisotropy all parallel to the applied field, which means the sample
3
(a) (b)
(c) (d)
FIG. 4: (Color online) Sketch map of magnetic structure in
ab plane of Mn3 during the oxidation process. Gray dots
represent Mn3(I) molecules. Black dots represent Mn3(II)
molecules. The intermolecular exchange interaction only ex-
(e) ists in ab planes. Magenta lines represent the intermolecu-
lar bonds with AFM exchange interaction between Mn3(I)
molecules. Redlinesrepresenttheintermolecularbondswith
AFM exchange interaction between Mn3(II) molecules. Blue
lines represent the intermolecular bonds with FM exchange
interactionbetweenMn3(I)moleculesandMn3(II)molecules.
FIG.3: (Coloronline) (a),moleculestructureofMn3(I).(b),
molecule structure of Mn3(II). (c), hydrogen bonds between #3 right after the 24-hour reduction is close to that of
Mn3(I) molecules. (d), hydrogen bonds between Mn3(II) sample #1 on Day 1. Since oxygen seems to play a key
molecules. e, hydrogen bonds between Mn3(I) molecule and role in the process, a new fresh sample is preserved in
Mn3(II) molecule. Color code: Mn, purple; N, cyan; O, red; oxygenfor further test, and it is seen that, the M/Ms-T
Cl, green; C, black; H, blue. The ethyl groups on Et-sao
curveonDay6issimilartothatofsample#1onDay31
ligands are omitted for clarity. The hydrogen bonds are in-
in Fig.1a which is preserved in the air. The result indi-
dicated by dark green arrows. The methyl groups near the
catesthattheoxidationprocessisacceleratedbyoxygen.
hydrogen bondsare circled by red dashes.
Additionalexperimentswiththefreshsamplespreserved
in argon or nitrogen for two weeks and covered by AB
glue for one year indicate that, the M/Ms-T curves of
isstillasinglecrystalastheorientationremainsthesame these samples remains unchanged when the samples are
during the oxidationprocess. Accordingto equation(2), isolated from oxygen and methanol gas. Therefore, it
the value of anisotropy parameter D and intermolecular is safe to conclude that, the oxidation-reduction process
interaction J for Mn3(I) and Mn3(II) may be calculated maybe manuallycontrolledby favorablyapplyingeither
outrespectively. ForMn3(I), D=0.98K,J=-0.041K;and oxygen or methanol gas to the sample to obtain the de-
for Mn3(II), D=0.925K, J=-0.132K. The negative value sired magnetic state of the sample.
of J indicates the AFM exchange interaction in both
In order to understand the nature of the magnetic
Mn3(I) and Mn3(II). The AFM coupling parameter J of characteristics during this process, we have monitored
Mn3(II) is over three times of Mn3(I), and the coupling the crystal structure[9] of the sample before and after
hascontributedtotheAFMphasetransitionobservedin
the oxidation. The crystal structures of fresh and fully
the inset of Fig.1a. It should be noted that, FM phase
oxidized samples are characterized by four-circle X-ray
also appears in the same period of time during the pro-
diffractometer (Bruker SMART APEX-CCD) and the
cess, but since it has a low concentration in the sample,
molecule structures are figured out by SHELXTL. The
the QTM peaks of FM phase are not observed.
lattice structure and space group of the sample before
We’ve also observed that, another fresh sample (sam- and after the oxidation remain the same. The only ap-
ple #3) becomes fully oxidized after it is preserved in parent change seen in Fig.3a and Fig.3b is that, each
the air for 5 months, with the proportion of FM phase Mn3(II) molecule has lost three methyl groups. The lat-
not more than 0.001%. Remarkably, this oxidized sam- tice constant only differs slightly with this change. For
pleturnsbacktothefreshsamplewhenitispreservedin both Mn3(I) and Mn3(II), two intermolecular hydrogen
methanolgas(0.10bar)formorethan24hours,whichin- bonds[9] are formed between the oxygen and hydrogen
dicates the oxidation process can be reversed. M/Ms-T atoms as shown in Fig.3c and Fig.3d, and these inter-
curves of sample #3 before and after the reduction pro- molecularhydrogenbondsdeterminestheintermolecular
cess are shownin Fig.1c. The M/Ms-T curves of sample exchangeinteraction. The presence orabsenceofmethyl
4
groupsresultsinobviouschangesofthedistanceandthe it are oxidized, hence the AFM phase is formed in these
angleofthesehydrogenbonds,andhencesignificantlyaf- area. After that, the FM phase will gradually dimin-
fects the magnitude and even the sign of the intermolec- ish and disappear when more and all molecules become
ular exchange interaction parameter J[10, 20, 22]. oxidizedeventually,andthesamplewillexhibitonlyanti-
With regard to FM phase observed in the process, it ferromagnetismintheend. Ontheotherhand,whenthe
may be well understood as the following. As shown in oxidizedsampleispreservedinmethanolgas,themethyl
Fig.4, the fresh sample is soaked in the oxygen, and groupsofmethanolwillgraftonMn3(II)molecules,then
hence, the Mn3(I) molecules have the probability to a reduction process will take place, the oxidized sample
be oxidized which results in the random distribution of will change back to the fresh sample.
Mn3(II) molecules. Black dots connected by red lines
It is very clear, the oxidation-reduction process is es-
represent Mn3(II) molecules with strong AFM exchange
sentially of that, the Mn3 molecule loses or gains methyl
interaction (J=-0.132K) which is the origin of the AFM
groups. We have demonstrated that the process is re-
phase,whereasthegreydotsconnectedbymagentalines
versible, and may be controlled by chemical stimuli of
represent Mn3(I) molecules which are not oxidized and
applyingfavorablyoxygenormethanolgastothesample
hence with weak AFM interaction (J=-0.041K). The
so that the methyl groups may be added to or dropped
bluelinesindicatetheheterogenousintermolecularbonds
from the Mn3 molecule. It might be also possible that,
(HIBs)betweenMn3(I)andMn3(II)molecules(eachHIB
the process could be controlled by photo-irradiation[23]
between the two different molecules consists of two hy-
orevenelectricalstimuli[14]. Weareespeciallyattracted
drogen bonds connecting the two molecules as shown
tothethoughtofelectricalstimuli,withtheideaofgraft-
in Fig.3e). As mentioned above, the sample always
ing the monomolecular layer or thin film of Mn3 on con-
maintains the monocrystalline structure. Therefore, it
ducting substrate (the method which has been tried for
may be convinced that the FM phase observed during
Mn3[24]), placing the device in a mixture of oxygen and
the process should come from the HIBs between Mn3(I)
methanolgaswithproperdosage,andsettingalocalvolt-
and Mn3(II). The presence of FM phase indicates that
age between a STM tip and the conducting substrate
the exchange interaction[10, 20, 22] between Mn3(I) and
to determine oxidation or reduction process so that the
Mn3(II) (shown in Fig.3e) is FM interaction (J>0). In
intermolecularexchangeinteractionmaybeswitchedbe-
actual scenario,there are relatively fewer HIBs with FM
tweenFMandAFMbytuningthevoltage,whichimplies
interaction at the beginning of the process, these HIBs
the magnetic structure of Mn3 may be designed or mod-
appear in small proportion, isolated and noncoherent.
ified at molecular scale. This also suggests a possibility
With the oxidation process developing, there are more
ofextending single-moleculespintronics device[15, 19]to
and more HIBs appearing, and some of them will cor-
two-dimensional spintronics device as the magnetic cou-
relate with each other by dipolar interaction to form
pling network may be modulated as desired.
”pieces” of FM phase which will gradually accumulate
and then the FM phase will become manifested. Mean- This work was supported by the National Key Basic
while,therearesomeareathatmostMn3(I)moleculesin Research Programof China (grant 2011CB921702).
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