Rich magnetic phase diagram in the Kagome-staircase compound Mn3V2O8
Rich magnetic phase diagram in the Kagome-staircase compound Mn3V2O8
Rich magnetic phase diagram in the Kagome-staircase compound Mn3V2O8
E. Morosan,1 J. Fleitman,1 T. Klimczuk,2,3 and R. J. Cava1
1Department of Chemistry, Princeton University, Princeton NJ 08544 
2Division of Thermal Physics, Los Alamos National Laboratories, Los Alamos NM 87545 
3Faculty of Applied Physics and Mathematics, Gdansk University of Technology, Narutowicza 
11/12, 80-952 Gdansk, Poland 
Mn3V2O8 is a magnetic system in which S = 5/2 Mn2+ is found in the 
kagomé staircase lattice. Here we report the magnetic phase diagram for 
temperatures above 2 K and applied magnetic fields below 9 T, characterized by 
measurements of the magnetization and specific heat with field along the three 
unique lattice directions. At low applied magnetic fields, the system first orders 
magnetically below Tm1 ≈ 21 K, and then shows a second magnetic phase 
transition at Tm2 ≈ 15 K. In addition, a phase transition that is apparent in specific 
heat but not seen in magnetization is found for all three applied field orientations, 
converging towards Tm2 as H → 0. The magnetic behavior is highly anisotropic, 
with critical fields for magnetic phase boundaries much higher when the field is 
applied perpendicular to the Kagomé staircase plane than when applied in-plane. 
The field-temperature (H - T) phase diagrams are quite rich, with 7 distinct phases 
observed.  
Geometrically frustrated magnetic materials have recently emerged as the focus of 
intense study. Among these, compounds based on the kagomé net, a regular planar lattice made 
from corner sharing of equilateral triangles, are of particular interest due to the very high 
degeneracy of energetically equivalent magnetic ground states. Breaking the ideal triangular 
symmetry of the kagomé net typically favors one particular magnetically ordered state above 
others. For the particular case of the kagomé-staircase geometry, however, in which the 
symmetry breaking occurs via buckling of the kagomé plane (see inset to Fig. 1), an exquisitely 
close competition between different magnetically ordered states has been observed, resulting in 
complex temperature-applied field magnetic phase diagrams. The kagomé-staircase lattice is 
observed in the transition metal vanadates T3V2O8 (with T = Co, Ni, Cu, and Zn) [1-11], and the 
Ni [2-4,7-8,11] and Co [1-3,5-6] variants have been widely studied. Simultaneous long-range 
ferroelectric and magnetic order have been found in Ni3V2O8 [11], allowing its classification as a 
multiferroic compound. 
Orthorhombic symmetry Mn3V2O8 (MVO) is isostructural with Co3V2O8 (CVO) and 
Ni3V2O8 (NVO), but its physical properties have been only marginally studied [12]. The t2g3eg2, 
isotropic spin, S = 5/2 L = 0 configuration of Mn2+ in MVO presents an interesting contrast to the 
t2g5eg2 S = 3/2 Co2+ and t2g6eg2 S = 1 Ni2+ cases of CVO and NVO. Here we report the 
observation of rich anisotropic magnetic field-temperature (H - T) phase diagrams for MVO, as 
determined from magnetization and specific heat measurements on single crystals. Two distinct 
magnetic phase transitions, at 21 K and 15 K, are observed for fields applied in all three principal 
crystallographic directions. A phase transition that is apparent in specific heat but not seen in 
magnetization is found for all three applied field orientations, converging towards the 15 K 
transition as H → 0. The magnetic behavior is highly anisotropic, with critical fields for 
magnetic phase boundaries much higher when the magnetic field is applied perpendicular to the 
kagomé-staircase plane (H parallel to the b crystallographic axis) then when applied in-plane (H 
parallel to the a and c crystallographic axes). The field-temperature (H - T) phase diagrams are 
quite rich, with 7 distinct magnetic/structural phases observed. The magnetic phase diagrams are 
distinctly different from what is observed for CVO and NVO.   
Single crystals of MVO were grown out of a MoO3/V2O5/MnO flux as previously 
described [12]. The starting oxides (MnO 99% Aldrich, V2O5 99.6% Alfa Aesar, MoO3 99.95% 
Alfa Aesar) were packed in an alumina crucible, which was then heated in a vertical tube furnace 
under flowing Argon gas. Sacrificial MnO powder was placed in an alumina crucible above the 
MoO3/V2O5/MnO flux to create an oxygen partial pressure that would neither oxidize the Mn2+ 
nor reduce the V5+[ref 12]. The vertical furnace was heated to 1200 oC at 200 oC/hr, held at 1200 
oC for 1 hour, cooled to 900 oC at 5 oC /hr, then cooled to room temperature at 300 oC/hr. After 
the heat treatment red-brown platelet crystals were extracted from the flux using a bath of 1 part 
glacial acetic acid (Fisher) and 3 parts deionized water. The crystals were found to be single 
phase by single crystal and powder X-ray diffraction, with the orthorhombic Cmca structure and 
lattice parameters a = 6.2672(3) Å, b = 11.7377(8) Å and c = 8.5044(5) Å. Field- and 
temperature-magnetization measurements were performed in a Quantum Design Physical 
Properties Measurement System (PPMS). The specific heat data were also collected in a PPMS, 
using a relaxation technique with fitting of the whole calorimeter (sample with sample platform 
and puck). 
 The H = 0.5 T inverse magnetization data for MVO (Fig.1) indicates the presence of long 
range magnetic ordering below Tm1 ≈ 21 K. Previous low-field magnetization data [12] suggest 
the existence of an additional magnetic phase transition near 40 K, but the feature observed is 
most likely due to the presence of an Mn2V2O7 impurity phase. A high-temperature fit of the 
susceptibility (dotted line, Fig.1) to the Curie-Weiss law χ = χ0 + C/(T-θW) yields an effective 
moment µeff = 5.94 µB, in excellent agreement with the theoretical value µeff = 5.92 µB expected 
for high-spin S = 5/2 Mn2+. The Weiss temperature θW = -320 K indicates the dominance of 
antiferromagnetic exchange interactions. Given the kagomé staircase magnetic lattice (inset in 
Fig. 1), it is not surprising that |θW / Tm1|  ≈ 15, characteristic of a strongly frustrated 
antiferromagnetic spin system. Deviations from Curie-Weiss behavior, typical of magnetically 
frustrated materials, are observed to begin on cooling at approximately 70 K.  
The easy magnetization axis lies close to the kagomé-staircase ac-plane, where the 
magnetization is largest (Fig.1). Upon further inspection of the behavior of the magnetization in 
different applied fields, two magnetic phase transitions can be identified in the M(T) data for all 
field orientations. Fig. 2a and c illustrate the field dependence of these transitions for H || a and 
H || c respectively. The competition between the antiferromagnetic spin coupling and the 
anisotropy associated with the kagomé staircase structure precludes the system from attaining a 
zero net magnetization ground state. This is suggested by the rapidly increasing magnetization as 
the system enters the high temperature, low field state (HT1) upon cooling below Tm1 ≈ 21 K. A 
net ferromagnetic component can probably be associated with the HT1 phase. Subsequent 
cooling of the sample gives rise to a sharp cusp followed by a local minimum around Tm2 =15 K, 
where a second magnetic phase transition, from HT1 to a low-T, low-H state (LT1) occurs. 
Increasing magnetic field (Fig.2a,c) has almost no effect on the long-range magnetic ordering 
temperature Tm1, but it broadens the cusp and slowly drives the second transition down in 
temperature. Above H = 0.04 T the H || a low temperature magnetization plateaus at a finite 
value, which strongly suggests a canted spin configuration even for the LT1 state, with a smaller 
ferromagnetic component along a than in the HT1 state. Very similar behavior occurs for the 
other in-plane orientation H || c (Fig.2c), with the two distinct transitions persisting up to slightly 
higher field H = 0.1 T. The insets in Fig. 2a,c represent examples of how the critical 
temperatures for the magnetic phase transitions at constant field are determined: as shown, the 
vertical arrows mark Tm1 and Tm2, H = 0.01 T, and correspond to local minima in the 
temperature-derivative of magnetization dM/dT. The two low-field phases that are observed in 
the magnetically ordered state are possibly a result of the ordering of the spins on one or both of 
the distinct Mn2+ ions (inset Fig. 1) in the ac-plane, similar to the transitions encountered in 
NVO [4]. Fewer magnetic phases are distinguishable at low fields in MVO, however, than are 
seen in either NVO or CVO. 
A more complex scenario is revealed in MVO at finite fields. Fig. 2b shows a selection of 
the H || b M(H) isotherms (full symbols), with the T = 2 K field-derivative dM/dH (open 
diamonds) as an example of how the critical field values were determined. At T = 2 K, the 
magnetization is low and linear with field for H < 2 T, which corresponds to the LT1 phase. This 
behavior is consistent with the antiferromagnetic spins slowly rotating from the easy axis in the 
ac plane, closer to the direction of the applied field H || b. A sharp step in magnetization around 
Hc1 = 2.1 T marks the transition from LT1 to LT2, possibly due to a spin-flop transition on one 
or both of the Mn2+ sites. Although the magnetization increases linearly with field above this 
transition, as expected for the spin-reorientation subsequent to a spin-flop, another transition 
occurs just below 3 T, where M(H) changes slope (full diamonds, Fig.2b) and the system enters 
the state LT3. The spin-flop transition yields a sharp peak in dM/dH (open diamonds, Fig.2b); 
the higher critical field value Hc2 is determined using an on-set criterion for dM/dH. Both 
transitions are marked by small vertical arrows in Fig.2b. 
As the temperature is raised, the initial slope of the M(H) curves increases in the 
magnetically ordered phase (Fig.2b) such that the magnetization jump at the spin-flop transition 
becomes indistinguishable. The two magnetic phase transitions move slightly down with field, 
and are hard to identify in the magnetization measurements above 16 K. Specific heat 
measurements complement the magnetization data, by confirming the magnetic phase lines, but 
also by revealing another phase transition that was not visible in the M(T,H) data. A selection of 
the specific heat curves, plotted as Cp/T vs. T, is shown in Fig. 3a, for H || b and applied fields up 
to 9 T. For H = 0 (full squares) a sharp peak associated with long range magnetic ordering is seen 
around Tm1 = 21 K, with a second peak at the lower phase transition temperature Tm2. After 
subtracting the lattice contribution to the specific heat as measured for the non-magnetic 
analogous compound Zn3V2O8 [2] (solid line, Fig.3b) one can estimate the magnetic specific 
heat Cm for MVO (open symbols, Fig.3b). The temperature dependence of the magnetic entropy 
can then be calculated and is shown in the inset in Fig.3b for H = 0 (open circles): only about 
50% of the R ln6 entropy expected for a S = 5/2 state is accounted for between 2 and 40 K. This 
could be an indication that additional phase transitions may exist below 2 K. Another possible 
explanation, given the observed departure from Curie-Weiss behavior below ~ 70 K (Fig.1), is 
that more entropy is associated with short range order below 70 K. No additional entropy is 
recovered with the application of magnetic field, as the H = 9 T temperature-dependent entropy 
(crosses, inset Fig.3b) differs only slightly from the H = 0 data. However, as the field is turned 
on, very different behavior is observed for the two peaks in Cp (Fig.3a): the one just below 16 K 
is affected little in temperature by the increasing magnetic field, but the higher-temperature one 
moves down in field. Concurrently, a third, broader peak emerges above ~ 1.5 T and is driven 
higher in temperature with increasing field. It is likely that both phase transitions exist at finite 
fields even for H < 1.5 T, and converge at Tm1 for H → 0, but their proximity in temperature 
makes it impossible to discern two separate peaks. For H > Hbc1 the lower temperature peak is 
not associated with any phase transition observed in the magnetization data. Given its invisibility 
in the magnetization, and the relative insensitivity of the transition temperature to applied field, 
we speculate that this transition may have a structural component, though the fact that the 
amount of entropy in the transition is suppressed by the field indicates that there must be a 
magnetic component as well. 
Based on our extensive magnetization and specific heat measurements, we present the H 
– T magnetic phase diagrams for MVO, for magnetic fields applied along the unique structural 
directions, in Figs. 4 and 5. As the temperature is lowered in zero field, MVO orders 
magnetically at Tm1 = (20.7 ± 0.2) K, entering first a high temperature phase (HT1) and then a 
low temperature phase (LT1) at Tm2 = (15.2 ± 0.5) K. The response of the system to applied 
magnetic field is highly anisotropic. For H || a (Fig. 4a), in finite field, two distinct phase 
boundaries emerge at Tm2: one represents the lower temperature magnetic phase transition, which 
moves down in temperature as H increases, and the second is an almost vertical line, which is 
only visible in the specific heat data. The intermediate temperature phase delineated by these two 
phase boundaries is LT4, which extends in field up to about 0.04 T. An almost horizontal phase 
line cuts across the phase diagram at Hac1 ≈ 0.04 T. It separates the low field, low temperature 
(LT1) and a high temperature (HT1) phases from two different states (LT3 and HT3) at higher 
fields.  
For the other field orientation close to the plane (H || c, Fig. 4b), the low field phase 
diagram is similar to that for H || a, with the HT1, LT4 and LT1 phases extending up in field up 
to a much higher critical value Hcc1 = 0.3 T. In the T → 0 limit, a second magnetic phase 
transition occurs at Hcc2 = 2.6 T, and the critical field value is slowly reduced with temperature. 
The two almost horizontal phase lines at Hcc1 and Hcc2 separate a low temperature (LT2) and a 
high temperature (HT2) phase at intermediate field values from the high field states LT3 and 
HT3.  
When field is applied perpendicular to the kagomé planes (Fig. 5) the phase diagram is 
analogous to the in-plane ones. The most noticeable difference is that the critical field values are 
much higher: Hbc1 = 2.2 T and Hbc2 = 3.0 T respectively for T → 0. This is expected given the 
observed anisotropy, which constrains the magnetic moments to lie closer to the ac-plane: 
stronger fields are needed to pull the moments towards the “hard” axis b. In addition, the LT4 
phase is missing, and the phase line that starts at Hbc1 at T → 0 converges at Tm2 in the H = 0 
limit. As a consequence, the HT2 phase merges with HT1 just below the magnetic ordering at 
Tm1. 
The temperature-field magnetic phase diagram for Mn3V2O8 is quite different from those 
seen in Ni3V2O8 and Co3V2O8. In all three compounds, the competition between the crystalline 
anisotropy and the antiferromagnetic interactions in the kagomé staircase structure gives rise to 
strong geometric frustration. In NVO and CVO, differences in the magnetically ordered states 
have been found to involve differences in the ordering of the moments on the two kinds of 
magnetic ion sites, the so-called spine and crosstie sites. The same will no doubt prove true for 
MVO, with the present measurements revealing that the magnetic moments on the two distinct 
Mn2+ sites lie close to the ac-plane when in the H = 0 magnetically ordered states. For magnetic 
fields applied in-plane, the magnetic states in MVO are much more sensitive to applied field than 
they are in NVO and CVO, with fig. 4 showing for example that the LT1 and HT1 phases 
disappear in applied fields in the a direction as low as 0.04 T. The complexity of the anisotropic 
H – T phase diagrams in MVO appears to be derived from competition between nearly balanced 
magnetic interactions, leading to canted spin configurations or field-induced spin-flop 
transitions. An integration of the entropy observed under the H = 0 phase transitions between 2 
and 40 K does not yield the expected Rln6 for Mn2+, suggesting that there may be more magnetic 
phase transitions below 2 K, or that additional entropy is associated with short-range order below 
70 K. Detailed neutron scattering measurements are desirable in order to elucidate the nature of 
the different states observed in MVO, and also to clarify whether the almost field independent 
phase boundary at Tm2 is associated with a structural phase transition. Investigation of possible 
multiferroic phases will also be of considerable interest.  
Acknowledgements 
 This research was supported by the US Department of Energy, Division of Basic Energy 
Sciences, grant DE-FG02- 98-ER45706. We thank G. Lawes for providing the specific heat data 
for Zn3V2O8.  
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Figure Captions 
Fig 1. Anisotropic inverse susceptibility data for H = 0.5 T (symbols) and linear fit of the high-
temperature data (dotted line). Insert: kagomé staircase structure of the Mn2+ array in Mn3V2O8. 
Crystallographic axes are shown. Spine sites are shown in purple and crosstie sites are shown in 
pink.  
Fig. 2. (a) H || a M(T) data for H = 0.015 T, 0.025 T, 0.04 T, 0.06 T, 0.08 T, 0.1 T and 5.0 T. 
Inset: the H = 0.01 T dM/dT curve; small vertical arrows indicate the position of the minima in 
the derivative, from which the critical temperature values are determined. (b) H || b M(H) 
isotherms for T = 2 K, 15 K, 18 K, 20 K and 30 K (full symbols, left axis); the T = 2 K dM/dH 
curve (open symbols, right axis) illustrates how the critical field values Hc1 and Hc2, marked by 
vertical arrows, are determined. (c) H || c M(T) data for H = 0.02 T, 0.05 T, 0.5 T, 1.0 T, 2.0 T, 
3.0 T and 5.0 T. Inset: the H = 0.01 T dM/dT curve; small vertical arrows indicate the position of 
the minima in the derivative, from which the critical temperature values are determined. 
Fig 3. (a) H || b Cp/T vs. T data for H = 0, 1.5 T, 2.0 T, 6.0 T and 9.0 T. (b) Cp/T data for MVO 
(full symbols) and Zn3V2O8 (solid line) (right axis) used to determine the magnetic specific heat 
Cm of MVO (open symbols, left axis) plotted as Cm/T. Inset: the temperature-dependence of the 
magnetic entropy Sm for H = 0 and 9 T. 
Fig 4. (a) H || a and (b) H || c H – T phase diagrams: points are determined from M(T) data 
(orange symbols), M(H) data (wine symbols) or Cp(T)|H data (open symbols). The solid lines are 
guides connecting the points determined experimentally; extrapolations of these phase 
boundaries in regions where measurements were missing or critical H and T values were difficult 
to determine are represented by dotted lines. 
Fig 5. H || b H – T phase diagrams: points are determined from M(T) data (orange symbols), 
M(H) data (wine symbols) or Cp(T)|H data (open symbols). The solid lines are guides connecting 
the points determined experimentally; extrapolations of these phase boundaries in regions where 
measurements were missing or critical H and T values were difficult to determine are represented 
by dotted lines. 
Fig.1. 
0 50 100 150 200 250
    H = 0.5 T
 H || c
 H || b
0 10 20 30 40 50 60 70
0 1 2 3 4 5
0 10 20 30 40 50 60
H || c
0.02T
0.05T
0.5 T
H || b
H || a
0.025T
0.015T
0.04T
0.06T
0.08T
0 10 20 30
H = 0.01 T
0 10 20 30
H = 0.01 T
Fig. 2. 
Fig. 3. 
8 12 16 20 24 28 32 36 40
0 10 20 30 40
0 10 20 30 40
10(a) (b)Mn
 H = 0
 H = 1.5 T
 H = 2.0 T
 H = 6.0 T
 H = 9.0 T
 (J/m
H = 0
 H = 0
 H = 9 T
R ln2
R ln3
Fig.4. 
0 4 8 12 16 20 24 28
 T(K)
PMLT3 HT3
HT2LT2
H || c 
LT1 HT1
H || a 
Fig.5. 
0 4 8 12 16 20 24
H || b 
HT3LT3
LT1 HT1
