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J. Phys. Chem. C 2008, 112, 7936–7938
Magnetic Ground State and Phase Diagram, Hc(T), for Magnetically Ordered [Ru2(O2CMe)4]3[Cr(CN)6]† William W. Shum, Jason N. Schaller, and Joel S. Miller* Department of Chemistry, UniVersity of Utah, Salt Lake City, Utah 84112-0850 ReceiVed: December 17, 2007; ReVised Manuscript ReceiVed: January 23, 2008
The magnetic phase diagram for [Ru2(O2CMe)4]3[Cr(CN)6] was determined from temperature dependent isothermal magnetization studies below its 33-K Tc. Below ∼25 K, the change in slope of the M(H) for ∼1100 ( 400 Oe increases with increasing temperature, as does the initial slope at lower applied field, whereas above ∼25 K, the slope decreases with increasing temperature. The M(H) below its 25-K tricritical point is characteristic of a first order transition, whereas above 25 K, it is characteristic of a second order transition. The magnetic phase diagram of [Ru2(O2CMe)4]3[Cr(CN)6] is characteristic of a class 1 metamagnet and has an antiferromagnetic ground-state arising from antiferromagnetically coupled ferrimagnetic lattices. Introduction Molecular materials with controllable optical, mechanical, electrical, and magnetic properties, particularly in combination with other important modulatable properties, are anticipated to be technologically important for future applications. One research trajectory led to the discovery of [Fe(C5Me5)2]•+[TCNE]•- (TCNE ) tetracyanoethylene), the first organic-based magnet (T ) 4.8 K).1,2 Expansion of these studies led to roomtemperature magnets of V[TCNE]x composition.3 Additionally, ferri- and in a few cases ferromagnetic ordering has been established for several octahedral hexacyanometalate-based, [M(CN)6]n-, materials4 that possess the cubic Prussian blue structure.5 Use of the dimeric ruthenium acetate monocation, [Ru2(O2CMe)4]+, led to the formation of a new family of moleculebased magnets of [Ru2(O2CR)4]3[MIII(CN)6] (M ) Cr, Mn, and Fe) composition.6 [Ru2(O2CMe)4]3[Cr(CN)6] (1) has a body centered cubic structure with two interpenetrating lattices and orders as a ferrimagnet below 33 K. It also exhibits an anomalous (a) hysteresis, (b) saturation magnetization, (c) outof-phase, χ′′(T), AC susceptibility, and (d) zero field cooledfield cooled temperature-dependent magnetization data with respect to either the material prepared from acetonitrile,6b or layered (2-D) [Ru2(O2CBut)4]3[Cr(CN)6] · 2H2O7 prepared from water. These unexpected behaviors are attributed to the presence of interpenetrating lattices;8 however, 1 also exhibits an unexpected pressure dependent hysteretic behavior.9 The hysteresis is unusual due to it being constricted as the initial field dependent magnetization, M(H), data rises gradually prior to a more rapid increase with increasing applied field before reaching saturation of 20 800 emuOe/mol.6b Thus, the shape of the initial isothermal magnetization data suggest that a transition occurs upon application of a small applied magnetic field. This field-induced magnetic transition is reminiscent of metamagnetic behavior.10 Some commonly known metamagnets include FeCl2, FeBr2, and Ni(NO3)2 · 2H2O, which antiferromagnetically order with two-sublattices below the Tc. Likewise, the organic-based magnet [Fe(C5Me5)2][TCNQ] (TCNQ ) 7,7,8,8-tetracyano-p-quinodimethane) exhibits metamagnetic † Part of the “Larry Dalton Festschrift”. * Corresponding author. E-mail
[email protected].
behavior.11 Metamagnets, however, do not exhibit either a remanant magnetization or a coercivity,10 as 1 exhibits (i.e., 3840 emuOe/mol and 470 Oe at 2 K, repsectively.6 Metamagnets undergo a second order transition when cooled in zero field. Just below Tc they undergo second order magnetic transition from antiferromagnetic to paramagnetic states with an external applied magnetic field. However, at lower temperature, the transition is a first order transition, as it is a discontinuous-like as function of the applied field due to two regimes with different dM/dH slopes.10 To understand the magnetic behavior of 1, the temperature dependence of its M(H) was studied. The temperature dependence of the metamagnetic-like critical transition field, Hc, was obtained to identify the magnetic phase diagram and magnetic ground state. Herein, we report the magnetic phase diagram, Hc(T), for [Ru2(O2CMe)4]3[Cr(CN)6] at ambient pressure, and identify its magnetic ground state. Experimental Methods 1 was prepared via the literature method.6 The field dependencies of the magnetization were obtained by cooling in zero field at various temperatures, and then data were collected isothermally up to a 5 kOe external magnetic field using a Quantum Design MPMS-5XL 5 T SQUID magnetometer equipped with standard transport measurement system, low field option, and continuous low-temperature control with enhanced thermometry features, as previously described.12 1 (13 mg) was placed in a gelatin capsule and mounted with standard transport inside the magnetometer. To achieve zero-field for cooling, the
10.1021/jp711835f CCC: $40.75 2008 American Chemical Society Published on Web 04/29/2008
Magnetically Ordered [Ru2(O2CMe)4]3[Cr(CN)6]
J. Phys. Chem. C, Vol. 112, No. 21, 2008 7937
Figure 1. 5-33-K M(H) for 1.
Figure 3. Temperature dependence of the slope between 1100 ( 400 Oe for 1 showing two discontinuous regions. The lines are guides for the eye.
Figure 2. Representative examples of M(H) and extrapolation to the internal critical transition field, Hc, for the 10 and 31 K data for 1. At 10 K, the data is characteristic of a first order transition, whereas the 31-K data is characteristic of a continuous second order antiferromagnetic-paramagnetic transition.10
MPMS fluxgate option for ultra low field was used. 1 was cooled in approximate zero magnetic field ((0.002 Oe). Results and Discussion The isothermal field-dependent magnetization, M(H), for 1 between 5 and 33 K is presented in Figure 1. At 5 K, the M(H) shows typical metamagnetic behavior with an initial shallow slope for H < ∼700 Oe that increases with increasing field (Figure 1 inset) and is characteristic of an antiferromagnet.10 The 10-K M(H) data (Figure 2) is characteristic of the discontinuous-like nature observed and analyzed for metamagnetic Ni(NO3)2 · 2H2O and FeCl2 as a function of the field and
is referred to as a first order transition.10 Note that metamagnetic FeCl2 exhibits a more abrupt (discontinuous) transition than 1 partly due to the study of a single crystal aligned parallel to the [0001] basal plane, while 1 is a polycrystalline powder with a cubic unit cell that lacks a preferable easy axis. Hence, two regimes with two different slopes are observed. As the temperature is increased these two regions persist until ∼20 K. Above ∼20 K the regimes merge and becomes a continuous transition between antiferromagnetic and paramagnetic states indicating a second order transition.10 The change in nature of the transition, from first to second order with increasing temperature, is also reflected in the temperature dependence of the slope for intermediate applied fields (∼1100 ( 400 Oe) that initially increases with increasing temperature (Figure 3). This is in contrast to typical metamagnetic behavior.10 As the temperature increases there is a discontinuity between 20 and 25 K in the slopes that indicates a change from first to a second order transition (see Supporting Information). At higher temperatures, approaching the 33-K Tc, the slope becomes more typical of a metamagnet as it decreases, Figure 2. In addition to the discontinuous region observed from the change in slope in the ∼1100 ( 400 Oe region, the slope of the initial M(H) data (Figure 1 inset) is linear below ca. 25 K.
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Figure 4. Hc(T) magnetic phase diagram of 1. The line is a guide for the eye. The dashed line indicates a first order transition, whereas the solid line indicates a second order transition, and 2 is the tricritical point, Tt.
In both regimes extrapolation to zero magnetization from the linear transition region of the M(H) data between 1100 ( 400 Oe led to the determination of the critical transition field, Hc, at various temperatures, as exemplified in Figure 2 for both regimes, and established for FeCl2, etc.10 The temperature dependence of Hc is plotted in Figure 4. The so-called tricritical point, Tt (∼25 K), is the temperature at which the first order transitions (dashed line, Figure 4) intersects the second order transitions (solid line, Figure 4).10,13 At 5 K, Hc is 760 Oe and decreases with increasing temperature (Figure 4). At 33 K, the Hc is 4.6 Oe. The Hc(T) data is the magnetic phase diagram for 1 showing the different magnetic regions, and is similar to that reported for FeCl2 or Ni(NO3)2 · 2H2O, etc.10 The magnetic phase diagram of 1 is characteristic of a class 1 metamagnet, which is highly anisotropic with a field-induced first order phase transition below the tricritical point (T < Tt) and second order phase transition between tricritical point and critical temperature (Tt < T < Tc) arising from the reversal of the local spin directions.10,14 For historical reasons this sudden reversal of the local spins is called a metamagnetic phase transition in class 1 metamagnets. Since 1 ferrimagnetically orders at 33 K, its ground-state best described as antiferromagnetically coupled ferrimagnetic lattices. Conclusion The field-induced magnetic phase transition of [Ru2(O2CMe)4]3[Cr(CN)6] has been investigated by isothermal magnetization measurement below Tc (33 K). The magnetic phase diagram is characteristic of class 1 metamagnetic or antiferromagnetic behavior. This is attributed to an antiferromagnetic ground-state arising from antiferromagnetic coupling of the ferrimagnetically ordered interpenetrating lattices. The study of the specific heat and magnetic phase diagram at applied pressure is in progress to obtain a deeper understanding of this unusual and novel material whose properties arise from the interpenetrating lattices. Acknowledgment. We gratefully acknowledge the continued partial support by the U.S. National Science Foundation (Grant No. 0553573). Supporting Information Available: The χ(H,T) and representative 33-K dχ/dH data for 1. This material is available free of charge via the Internet at http://pubs.acs.org.
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