A Versatile High Field Calorimeter for Condensed Matter Research: GR/R06540

Transcription

1 : GR/R Introduction We received funding through the 2000 Joint Research Equipment Initiative (GR/R06540) to procure a Quantum Design Physical Properties Measurement System (QD PPMS) with Heat Capacity option. The purchase included a large discount from Elliot Scientific and Quantum Design (40% of the list price) and a significant contribution from the University of Warwick (10% of the list price). This apparatus is based in the laboratories of the Superconductivity and Magnetism Group in the Physics Department, University of Warwick. This report details the work carried out between October 2001 and October 2004 using this piece of equipment. 2. Quantum Design Physical Properties Measurement System (PPMS) The Quantum Design PPMS is an open architecture variable temperature-field system that can be used to make a variety of measurements. The system at Warwick is configured to make heat capacity measurements. The machine uses a relaxation technique with a two tau model to accurately simulate the heat flow between the calorimeter platform and the sample as well as between the sample and the supporting stage. The apparatus has the following specifications: a system capable of making heat capacity measurements over a wide range of temperatures (400 mk and 400 K) and magnetic fields (0 to 9 tesla). a turnkey system that is relatively easy to use and maintain. a system that can measure small as well as large samples (sample mass mg) with a resolution of 10 nj / K at 2 K. a system based on a low loss helium-nitrogen dewar set with minimal cryogen consumption. 3. The PPMS at Warwick University 3.1 Operation of the system The PPMS was delivered in October The system has proved to be very reliable and there have been no significant difficulties associated with the commissioning, or day-to-day operation of this system. The system performs to specification. The machine was set up as a multi-user facility and was made available to research workers based here at Warwick and at other HEI s within the U.K. To date, the greater proportion of the available instrument time has been taken up by research workers based here at Warwick. The majority of the samples studied in the PPMS were prepared through our EPSRC funded single crystal growth programme (GR/M33327). Complementary measurements of the magnetic and transport properties of the materials studied have been performed using a range of instruments available in-house. Neutron scattering measurements have been carried out using different neutron techniques including elastic and inelastic scattering measurements at both continuous and pulsed neutron sources. The PPMS has been used almost continuously since its delivery and is now an integral part of our ongoing research programmes. 3.2 Beneficiaries, training and research outcomes The PPMS has made a significant contribution to our EPSRC funded research programmes. Dr. Vincent Hardy worked in our laboratories under an EPSRC Visiting Fellowship (GR/R94299/01) between September 2002 and August He made extensive use of the PPMS. His expertise using this system helped to optimise the operation of this new apparatus and allowed us to develop a suite of data analysis packages. The system was also used to collect data that contributed to the work for the grant entitled The Mixed State in Exotic Superconductors (GR/N38060/01). The system has been used by two EPSRC funded post doctoral fellows (Dr. Subham Majumdar and Dr. Le Duc Tung); the PPMS has also played a part in the training of several EPSRC funded Ph.D. students including Dr. Emma Chung ( ), Dr. Andrew Bebb ( ), Sonya Crowe ( ), and Jennifer Wooldridge ( ). The apparatus has also been used by 14 undergraduate students to collect data for their final year M.Phys. or B.Sc. research projects. To date, the data collected by the PPMS has been used in 8 published papers, 3 accepted papers and 2 submitted papers, all in refereed international journals. 5 papers are in the final stages of preparation with several others planned. IGR Final Report. Page 1 of 6

2 4. Research programmes carried out using the PPMS with heat capacity option 4.1 Introduction In the following we describe the results of some of the research carried using our PPMS. These examples help to highlight the quality of the work carried out and emphasise the range of different materials that we have studied. These examples also give a flavour of the capabilities of the system. 4.2 A specific heat study of Ca 3 Co 2 O 6, a member of a family of novel 1D magnetic materials [1] The cobalt oxides form a large family of compounds with fascinating structural and physical properties. The different possible oxidation states of cobalt, together with its various spin configurations are responsible for numerous original phenomena. Amongst these cobalt oxides, the compound Ca 3 Co 2 O 6 presents two additional features well-known to generate exotic physical properties: low-dimensional magnetism and geometrical frustration. Recent studies on this compound have revealed novel, complex magnetic behaviour. The rhombohedral structure of Ca 3 Co 2 O 6 consists of Co 2 O 6 infinite chains running along the c axis of the corresponding hexagonal cell, and separated by Ca cations. Each chain is built of alternating CoO 6 trigonal prisms [site Co(1)] and CoO 6 octahedra [site Co(2)]. The 1D magnetic character results from the interchain separation which is about twice the intrachain Co-Co distance. Each Co 2 O 6 chain is surrounded by 6 chains forming a triangular lattice in the a-b plane. Previous studies of Ca 3 Co 2 O 6 have established several basic features about the Co-Co interactions in this compound: (i) a strong Ising character, with the spins oriented along the chain axis; (ii) a ferromagnetic (FM) intrachain coupling; (iii) an antiferromagnetic (AFM) nearest-neighbour interchain coupling. Such features, combined with the triangular arrangement of the chains on the ab plane, give rise to a prototypical situation of geometrical frustration. On the other hand, many questions remain open. For example, the form of the magnetic phase diagram including the nature of the Partially Disordered Antiferromagnetic (PDA) or Frozen Spin (FS) states seen at low T is unclear. There is an unusual multi-metamagnetic behaviour found in M(H) at low-t accompanied by a large hysteresis, and a sharp ferri- to ferromagnetic transition at T~10 K that are not well understood. We carried out the first combined specific heat/magnetisation study on single crystals of Ca 3 Co 2 O 6. Our work revealed several features that complemented previous studies. We have shown that there is a peak at T N ~25 K in the magnetic component of the heat capacity, C M (T), under an applied field of 0 and 2 T. This feature clearly demonstrates that the occurrence of a long-range ordering is associated with the AFM interchain coupling. In 5 T, this peak is absent, as expected for a FM transition in a large magnetic field. In an applied field of 2 T, the ordered state is ferrimagnetic and the peak at T N is found to be very pronounced. In zero magnetic field, the peak at T N is significantly reduced; the magnetic entropy displays a smooth, continuous evolution versus T below T N, that is interrupted when entering the frozen spin state around T FS ~7 K. One observes a pronounced crossover in the T dependence of C M (T) around T FS T 2 (K 2 ) Below T FS, the specific heat has a linear term (γt) that can be associated with frozen magnetic disorder. No additional peaks were detected in C M (T) below T N, which appears to rule out a ferrimagnetic transition as proposed by some PDA scenarios. Our magnetisation data reveal a noticeable time dependence in the intermediate T range between T FS and T N. The specific heat and magnetisation results demonstrate that the magnetic state below T N in zero field is still highly disordered and evolves continuously with both temperature and time. This probably derives from a combination of geometric frustration and the slow spin dynamics of FM chains. C M (T) exhibits a broad maximum at high T which is related to 1D short-range ordering along the FM chains. The fact that only a small fraction (~0.18) of the total magnetic entropy [Rln(2S+1)] is released up to T N is consistent with the expectations for a 1D system. C / T (J K -2 mol -1 ) 1.0 (C /T) (C L /T) C / T (J K -2 mol -1 ) T(K) Figure 1. Total specific heat (C) and lattice contribution (C L ) in Ca 3 Co 2 O 6 under zero field. The inset shows the low-t C/T versus T 2 data. 4.3 Comparative study of the specific heat of Ca 3 Co 2 O 6 and Ca 3 CoRhO 6 [2] We extended this work by comparing the properties of Ca 3 Co 2 O 6 and Ca 3 CoRhO 6. Previous magnetisation and neutron diffraction studies have shown that many of the properties of these compounds are similar, as expected from their related magnetic chain structure. We have shown that in contrast to Ca 3 Co 2 O 6, where a clear maximum in C(T) is seen at the onset of the interchain ordering, there is no such peak in the heat capacity data of Ca 3 CoRhO 6. The crossover to the FS states, characterised by a linear term in C(T) of amplitude close to 10 mj/ mol K 2, is visible in the specific heat of both materials. In contrast to the situation for Ca 3 Co 2 O 6, the specific heat data for Ca 3 CoRhO 6 is not consistent with a well defined ferrimagnetic-to-paramagnetic (PM) transition around T N in intermediate fields (e.g. 2 T). Ca 3 Co 2 O 6 and Ca 3 CoRhO 6 have significantly different intrachain and interchain coupling constants, which IGR Final Report. Page 2 of 6

3 take larger values in the latter compound. Our heat capacity studies have revealed that despite their structural similarity, there are several important differences in the behaviour of these two materials. Further work is required to understand the origin of these differences. 4.4 Specific heat studies in Sr 3 CuPtO 6 [3] We continued this work with a study of the isostructural compounds in the Sr 3 MPtO 6 and Sr 3 MIrO 6 families (M being a 3d transition element), which exhibit a great variety of magnetic behaviour including random spin chain paramagnetism. We have performed a detailed investigation of the magnetic susceptibility and the heat capacity behaviour of the compound Sr 3 CuPtO 6. The use of a nonmagnetic iso-structural compound (Sr 3 ZnPtO 6 ) has enabled us to extract the magnetic contribution to C. Both the magnetic susceptibility and the heat capacity were found to be consistent with the Johnston and the Bonner-Fisher models for an S=½ AFM spin chain, with similar values of the intra-chain coupling parameters (J~25.5 K). Based on the fact that there is no long range magnetic order observed in this system, at least down to 2 K, we estimate the ratio of the intra- to inter-chain coupling parameters to be ~130. In contrast to previous claims, these observations clearly identify Sr 3 CuPtO 6 as an S=½ spin chain compound with 1D magnetic character; deviations from the 1D uniform S=½ Heisenberg models observed at low T indicate the existence of a gap in the spin excitation spectrum of this material. C mag /T (JK -2 mol -1 ) T (K) Figure 2. The C M (T) data of Sr 3 CuPtO 6 along with the curve (solid line) predicted by the Johnston model (down to 1 K) and the curve (dotted line) obtained by fitting to C = γ Texp( / T) M M Exp. data Spin gap model Johnston model C mag (JK -1 mol -1 ) T (K) between 2 and 5 K. The inset shows the quasi-linear behaviour of CM(T) 4.5 Other low dimensional magnetic oxides [4-5] Heat capacity data was used in a study aimed at understanding the magnetic properties of quasi-1d cuprate Li 2 CuO 2. Single-crystal neutron diffraction at 2 K indicated the presence of a large ordered oxygen moment of 0.11(1)µ B. The magnetization densities of the Cu and O atoms were shown to be highly aspherical, forming quasi-1d ribbons of localized Cu and O moments. Magnetic structure refinements, magnetisation, and heat capacity measurements all suggest that the magnetic structure of Li 2 CuO 2 at 2 K may be canted. We have investigated the magnetic properties of the tetravalent praseodymium (Pr 4+ ) compound Sr 2 PrO 4. The compound shows signatures of long range magnetic order at T N =3 K in the heat capacity and magnetisation data. C(T) data show that a significant part of the magnetic entropy is released above T N. This reveals that short range magnetic correlations among the Pr moments exist well above T N. The magnetic entropy obtained from the C(T) data indicates that only the crystal field ground state of Pr with effective spin-½ is significantly populated at low T. 4.6 Heat capacity study of the martensic transition in Gd 5 Ge 4 [6] The pseudobinary system Gd 5 (Si x Ge 1-x ) 4 has attracted a growing interest in recent years owing to the wealth of interesting physical properties it displays including a giant magnetocaloric effect and colossal magnetostriction. These striking phenomena are related to a strong interplay between the magnetic and the structural features in this system. These compounds have a layered structure made up of sub nanometric slabs connected via covalent-like bonds. The degree of interslab connectivity not only depends on x, but also on the magnetic state. For instance, with x=0, the slabs are completely interconnected in the FM state, whereas all the bonds are broken in both the AFM and PM states. FM and AFM domains can co-exist in Ge 5 Ge 4 and the transformation between the two phases has a pronounced martensitic character. We have investigated the heat capacity behaviour of the polycrystalline sample of Gd 5 Ge 4 down to 2 K. Our C(H) data show a sharp step at an applied field slightly below 2 T, which corresponds to the AFM-FM martensitic transition seen in the magnetisation data. We have also measured the zero field heat capacity of Gd 5 Ge 4 as a function of T for the zero-field cooled virgin state and the field induced treated state (obtained after applying a 50 koe field for 15 minutes at 2 K and then removing the field). The C(T) data for the virgin state and the field treated state deviate from one another at 25 K. Our investigation identified a zero-field FM critical temperature at 25 K for the field induced state, where an order-order transition occurs between the high T AFM phase and the low T FM phase. 4.7 Other intermetallic systems [7-8] We have studied the heat capacity of a single crystal of PrCoAl 4. The data indicate Pr 3+ in this compound has a nonmagnetic singlet ground state. The compound becomes antiferromagnetically ordered below T N =17 K. The magnetocrystalline anisotropy in PrCoAl 4 is relatively strong. Up to 300 K, the entropy in the specific heat is 15.2 J/ K mol, ~83% of the entropy expected for the 2J+1-fold degeneracy of the ground multiplet of the Pr 3+ ion. IGR Final Report. Page 3 of 6

4 Nonresonant ferromagnetic x-ray diffraction was used to separate the spin and orbital contribution to the magnetisation density of the proposed zero-moment ferromagnet Sm Gd Al 2. The alignment of the spin and orbital moments relative to the net magnetisation shows a sign reversal at 84 K, the compensation temperature. Below this T the orbital moment is larger than the spin moment, and vice versa above it. This result implies that the compensation mechanism is driven by the different temperature dependencies of the 4 f spin and orbital moments. Specific heat data indicate that the system remains ferromagnetically ordered throughout. 4.8 Magnetism and superconductivity in Na x CoO 2 [9] The layered transition metal oxide, Na x CoO 2, displays many interesting characteristics including unusual thermoelectric properties as well as charge and spin ordering; for x=0.3 the intercalation of water drives the system superconducting with a T C of 4.5 K. These properties reflect the strong electron correlations within the system. The structure is highly anisotropic; 2D layers of edge-sharing CoO 6 octahedra assembled incoherently along the c-axis are separated by layers of partially occupied Na sites. The system has a complex phase diagram; two PM metallic phases are separated by a charge ordered insulating phase at x=0.5. The samples with higher Na doping exhibit magnetic ordering below 22 K in the form of a spin density wave (SDW), with the magnetic moments ordered AFM along the c-axis. At lower T the magnetic ground state is further modified by the Figure 3. The phase diagram of Na x CoO 2. A charge ordered phase is located between a PM and a Curie-Weiss metallic state. The hydrated superconducting state is also shown. appearance of a weak FM moment (also along c) and finally a pinning of the SDW onto the underlying Co lattice resulting in a glassy ground state at around 4 K. These samples also show Na ordering transitions at ~340 K. We have measured the heat capacity of Na 0.71 CoO 2 single crystals from 400 mk to 380 K (see figure 4). Over the whole temperature range studied satisfactory agreement with the data can be obtained by fitting using a combined Debye-Einstein function giving Θ D of 410 K and Θ E of 820 K weighted in the ratio 4/5. This appears to be consistent with a mixture of acoustic and optical modes expected from a combination of light and heavy elements. The high temperature data contains features at T 1 = 307 K and T 2 = 336 K. We attribute the transition at T 2 to a real space ordering of the Na within the system. At this transition, the Na is rearranged from random ordering on the 6h(2x,x,¼) to the higher symmetry 2c(⅔,⅓,¼) site. The entropy associated with this transition is ~1 J/ mol K. This corresponds to 10% of the configurational entropy expected for Na ordering over the two available sites, which is calculated to be 1.2R. This may indicate that considerable Na disorder persists below T 2. Between a limited temperature range of 22 K to 30 K the data can be explained by assuming that C comprises of two terms, an electronic term and a phonon component, which in the low temperature limit, can be characterised using the Debye model giving C = γt+βt 3. A linear fit to the data Figure 4. Specific heat capacity data for Na 0.71 CoO 2. Three transitions are visible; two high temperature peaks are attributed to Na ordering within the structure and the SDW is visible as a peak at 22 K (panel A). The solid line in the main figure corresponds to the combined Debye/Einstein model; the line in panel A is a fit to γt+ βt 3 above T SDW. Low temperature C/T vs. T 2 data (panel B) shows a hump centered at 7 K, indicating a glassy state. (inset B of fig. 4) gives a γ of 35 mj/ mol K 2 and Θ D =550 K. This value of Θ D is slightly higher than those calculated using the high temperature data but is in line with previous estimates. Using this value we find that above 40 K the experimental data deviates substantially from the Debye theory. This is consistent with a smaller than expected value for the measured heat capacity at high temperature with [C(380 K)/(3.7 3R)]=0.7 while the expected ratio using the Debye theory is 0.9. There is a lambda like anomaly at 22 K (inset A of fig. 4) indicating the onset of magnetic ordering. This feature is associated with the development of a SDW within this material. The transition is rather broad extending over at least 7 K, whereas the minimum sample heat pulse used around the transition was 0.1 K. This feature occurs at the same T in heating and cooling runs with no discernable hysteresis indicating that this is a second-order phase transition. The jump at T SDW is 0.4 J/ mol K (25% of the signal at this T). Using the BCS expression C/T SDW =1.43 gives γ=12.7 mj/ mol K 2 which is consistent with the values obtained from the low T heat capacity. Below T SDW there is a considerable reduction in γ. An extrapolation of the low T data produces a value of IGR Final Report. Page 4 of 6

5 15 mj/ mol K 2. It is interesting that the γ value below the SDW transition also agrees with estimates for γ made for the superconducting material Na 0.3 CoO 2 1.3H 2 O. Assuming the γ term arises solely from charge carriers we can use π 2 k 2 N(E F ) to calculate the free electron density of states. N(E F ) is ev/ mol just above T mag reducing γ= ( ) 2 B to ev/ mol at 2 K. The ratio of γ above and below the transition suggests that 50 60% of the Fermi surface is removed by the opening up of a gap at T mag. The entropy associated with this feature can be obtained by subtracting from the total specific heat, C, an estimate of the background specific heat, C, made up of a contribution from the ungapped electrons plus the phonon contribution which we assume remains unchanged between base and 30 K (see figure 4). The entropy associated with the anomaly alone (i.e. from T=14 to 25 K) equals J/ mol K and corresponds to only 5% (0.3Rln(2S+1)=1.73 J/ mol K) of the entropy expected for an x=0.71 sample with a Co 3+ (spin 0)/Co 4+ (spin ½) system in the ratio of 3:1. Including the excess entropy down to 2 K this value rises to 0.16 J/ mol K. A low T γ of 15 mj/ mol K 2 suggests that the total Sommerfeld electronic entropy at T=22 K should be 0.33 J/ mol K. The entropy associated with the transition is approximately 50% of this value. We can model C=C-C using δc=aexp( /T) between 15 and 22 K and obtain an estimate for =163 K (=7.4 T mag ). This value is much larger than the weak coupling BCS value and indicates that strong electron-electron interactions are important in this system. It would be interesting to compare the value of obtained here with estimates made using other techniques such as inelastic neutron scattering. There is also an additional contribution (inset B of figure 4) to the heat capacity at 7 K, which is clearly visible in the C/T vs. T 2 plot as a broad bump (extending from 4-15 K). This low T feature is consistent with the notion of a glassy state, indicated by complementary χ ac (T) data. The broad peak is a result of the pinning of SDW domains to over an extended T range. Below 4 K the C versus T dependence can once again be described by C=γT+βT 3. In contrast to previous work we observe no upturn in C/T at low T. 4.9 Magnetic phase diagram of Gd 2 Ti 2 O 7, an antiferromagnetic pyrochlore [10-12] Amongst the members of the titanium pyrochlore oxide family with the general formula R 2 Ti 2 O 7, the magnetic properties of which are all presently the subject of intense investigations, gadolinium titanium oxide (GTO) holds a unique position. In contrast to the other members of this series of compounds, where magnetic anisotropy plays a crucial role and results in the appearance of some most unusual ground states including the spin-ice state, GTO has a negligible single-ion anisotropy. The Gd 3+ ions are in an S=7/2 and L=0 state. The magnetic ions are antiferromagnetically coupled. Therefore, apart from the ever present dipole-dipole interactions, GTO may be regarded as an ideal Heisenberg AFM on a frustrated pyrochlore lattice. We have measured C(T) and C(H) for a single crystal of GTO. C(T)/T, measured in zero field is shown in Fig. 5. Sharp peaks of nearly the same amplitude observed at T N1 =1.02(2) K and T N2 =0.74(2) K mark consecutive phase transitions. The two data sets shown here demonstrate the high degree of reproducibility in these measurements. Several features reported in previous studies are absent in out data sets; the absolute values of C(T) also differ significantly from some reports. These differences are attributable to the sample form, quality, and impurity levels and emphasise the need to study high quality single crystals of a suitable mass and geometry. The inset in Fig. 5 shows the temperature dependence of the magnetic entropy, obtained by extrapolating the C(T)/T to 0 K. Approximately 90% of the expected entropy is recovered by 5 K with almost all the entropy being recovered by 10 K, a T which corresponds to the reported Curie-Weiss temperature of GTO. At 10 K the magnetic contribution to the specific heat Figure 5. C(T)/T versus T of single crystal Gd 2 Ti 2 O 7 measured in zero field. The inset shows the temperature dependence of the magnetic entropy. becomes negligible. The fact that a majority of the entropy is recovered only at T>T N1 signifies the importance of short-range correlations in GTO. We suggest that at T N1 GTO undergoes a transition from a PM to a P state consisting of kagome planes with q=0 -type order with interstitial sites carrying a zero magnetic moment, while at T N2 the transition is to an ordered 4 sublattice structure (the F state). Our lowest experimentally accessible temperature of 0.39 K is about 50% of T N2. At these low temperatures C(T) exhibits a T 2 dependence. The application of a small external field causes the convergence of the transition temperatures, T N1 and T N2, regardless of the field orientation. In a field of about 1.5 T, the two transitions merge. In a small field and low-t, C(T) still has a T 2 dependence; above 1.5 T, however, there is no T 2 dependence in C(T). For fields above 1.5 T, significant differences in the positions of the peaks in the C(T) curves for GTO are clearly visible. The H-T phase diagrams for GTO obtained by plotting the temperatures of the various phase transitions observed in both the C(H) IGR Final Report. 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6 and C(T) data are shown in Fig. 6. These phase diagrams highlight the anisotropic nature of the magnetic behaviour of GTO. For H [111] the field favours the onset of magnetic order. For example, in 4 T T N1 =1.14(2) K compared to 1.02(2) K in zero field, while the transition to a saturated phase measured at 0.57 K occurs at about 5.7 T. When H [112], an external field inhibits the onset of long range magnetic order; at 4 T the transition is reduced to 0.82 K and the saturation field at 0.57 K does not exceed 5.2 T. The application of the field along the [110] direction has an intermediate effect. Even more remarkably, an additional phase transition induced by the external field at about half of the saturation field is clearly present at low T for H [111], but is absent for H [112]. The origin of this phase transition is not clear, although the fact that it happens at H sat /2 strongly suggests that it is related to a collinear spin state, where 3 spins on Figure 6. Magnetic phase diagrams of Gd 2 Ti 2 O 7 for three different orientations of applied magnetic field. each tetrahedra are aligned with the field and the fourth is pointing in the opposite direction. Our phase diagrams differ significantly from those published previously and from theoretical predictions based on a simple mean-field model. These observations strongly suggest the need for further development of the theoretical model, which should include some degree of magnetic anisotropy Other magnetic oxides [13] We have shown that for CuFeO 2 a Heisenberg model with a relatively weak anisotropy gives a much better description of all the data available than the generally accepted 2D Ising model. In zero field, CuFeO 2 orders via an incommensurate phase at T N1 =14 K into a collinear 4-sublattice structure at T N2 =11 K. The first-order phase transition at T N2 is marked by a sharp peak in the specific heat data. This feature, which is well defined in our PPMS data, is absent in data collected by some other groups. A field-induced phase transition at H c1 to a non collinear structure is clearly visible in the C(H) curves collected in magnetic fields up to 9 T and is marked by a pronounced hysteresis. Remarkably, below 4 K, C decreases with increasing H at H c1 and increases with increasing field at T>4 K. This confirms that the T dependence of the magnetic excitations in the 2 phases are entirely different, an observation which is consistent with the conjecture that phase II is a non collinear spin-flop phase. 5 Concluding Remarks We believe that this grant represented excellent value for money. The science carried out using the PPMS was exciting and topical. We anticipate that the PPMS will have a useful life of at least ten years. The apparatus will continue to make a valuable contribution to many of our future research programmes. For example, we have recently been awarded an EPSRC grant to study the physics of layered cobalt oxides (EP/C000757/1); the PPMS will be used throughout the course of this project. References [7] Specific heat studies of PrCoAl 4 single crystal, L.D. Tung, [1] Specific heat and magnetization study on single crystals of the D.M. Paul, M.R. Lees, P. Schobinger-Papamantellos, frustrated quasi-one-dimensional oxide Ca 3 Co 2 O 6 V. Hardy, S. K.H.J. Buschow, J. Magn. Magn & Mater. 281, 378 (2004). Lambert, M.R. Lees and D.M. Paul, Phys. Rev. B 68, [8] Temperature dependence of the spin and orbital magnetization (2003). density in Sm Gd Al 2 around the spin-orbital compensation [2] Specific heat investigation of the magnetic ordering in two point J.W. Taylor, J.A. Duffy, A.M. Bebb, M.R. Lees, L. frustrated spin-chain oxides: Ca 3 Co 2 O 6 and Ca 3 CoRhO 6, V. Bouchenoire, S.D. Brown and M.J. Cooper, Phys. Rev. B 66. Hardy, M.R. Lees, A. Maignan, S Hébert, D. Flahaut, C. Martin (2002). and D.M. Paul, J. Phys.: Condens. Matter 15, 5737 (2003). [9] Investigation of the spin density wave in NaxCoO 2, [3] Magnetic susceptibility and heat capacity investigations of the J. Wooldridge, D.M. Paul, G. Balakrishnan and M.R. Lees, to unconventional spin-chain compound Sr 3 CuPtO 6, S. Majumdar, V. appear J. Phys.: Condens. Matter (2005). Hardy, M.R. Lees, D.M. Paul, H Rousselière and D. Grebille, [10] Magnetic phase diagram of the antiferromagnetic pyrochlore Phys. Rev. B 69, (2003). Gd 2 Ti 2 O 7, O.A. Petrenko, M.R. Lees, G. Balakrishnan, D.M. [4] Magnetic behaviour of the tetravalent praseodymium Paul, Phys. Rev. B 70, (2004). compound Sr 2 PrO 4, S. Majumdar, M.R. Lees, G. Balakrishnan and [11] Adiabatic demagnetisation of a pyrochlore antiferromagnet D.M. Paul, J. Phys.: Condens. Matter 15, 7585 (2003). Gd 2 Ti 2 O 7, S.S. Sosin, L.A. Prozorova, A.I. Smirnov, O.A. [5] Oxygen moment formation and canting in Li 2 CuO 2, E.M.L. Petrenko, G. Balakrishnan and M.E. Zhitomirsky, to appear in J. Chung, G.J. McIntyre, D.M. Paul, G. Balakrishnan and M.R. Lees, Magn. Magn & Mater. (2005). Phys. Rev. B 68, (2003). [12] Magnetocaloric effect in a strongly frustrated pyrochlore [6] Investigation of the magnetic field induced state in Gd 5 Ge 4, S. magnet Gd 2 Ti 2 O 7, S.S. Sosin, L.A. Prozorova, A.I. Smirnov, O.A. Mujumdar, V. Hardy, S. Crowe, M.R. Lees, D.M. Paul, C. Yaicle, Petrenko, G. Balakrishnan, A.I. Golov, I.B. Bertukov and M.E. A. Maignan, C. Martin and S. Hébert, submitted to J. Phys.: Zhitomirsky, to appear in Phys. Rev. B (2005). Condens. Matter (2004). [13] Revised magnetic properties of CuFeO 2 - a case of mistaken identity O.A. Petrenko, M.R. Lees, G. Balakrishnan S. de Brion and G. Chouteau, submitted to Eur Phys. J. B (2004). IGR Final Report. Page 6 of 6