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Magnetic Clusters in Ilmenite-Hematite Solid Solutions

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Magnetic Clusters in Ilmenite-Hematite Solid Solutions PHYSICAL REVIEW B 81, 224423 ͑2010͒ Magnetic clusters in ilmenite-hematite solid solutions C. Frandsen,1,* B. P. Burton,2 H. K. Rasmussen,1 S. A. McEnroe,3 and S. Mørup1 1Department of Physics, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark 2NIST, 100 Bureau Drive, Gaithersburg, Maryland 20878, USA 3Geological Survey of Norway, N-7491 Trondheim, Norway ͑Received 22 May 2010; published 16 June 2010͒ We report the use of high-field 57Fe Mössbauer spectroscopy to resolve the magnetic ordering of ilmenite- ͓ ͑ ͒ ͔ Ͼ hematite xFeTiO3 − 1−x Fe2O3 solid solutions with x 0.5. We find that nanometer-sized hematite clusters exist within an ilmenite-like matrix. Although both phases are antiferromagnetically ordered, the hematite clusters show ferrimagnetic behavior due to superexchange coupling with Fe2+ in ilmenite. For ilmenite-rich samples ͑x=0.95͒, the clusters are isolated and superparamagnetic. For more hematite-rich samples with x =0.80 and x=0.70, the clusters interact to form a cluster glass. DOI: 10.1103/PhysRevB.81.224423 PACS number͑s͒: 75.50.Tt, 76.80.ϩy, 91.25.FϪ I. INTRODUCTION diagrams see Refs. 6 and 7͒ but at the atomic scale, the 7,9,15 The ilmenite-hematite ͓xFeTiO −͑1−x͒Fe O ͔ solid solu- magnetic ordering remains largely unresolved. For 0.5 3 2 3 Շ Ͻ tion series has attracted significant attention because of its x 1.0 magnetization measurements show that solid solu- complex magnetic and electronic properties. Both ilmenite tions are significantly magnetic, up to more than ͑ ͒ ͑␣ ͒ ϳ 2 / xϳ 1–3 FeTiO3 and hematite -Fe2O3 are antiferromagnetic and 30 A m kg for compositions of 0.6–0.7, due to insulating but intermediate compositions can be ferrimag- Fe3+ ͑or Fe2+͒ ions substituting for Ti4+ in the B layers, re- netic and semiconducting.1–15 The system is of significant sulting in a ferrimagnetic structure.2,4,17 Neutron diffraction interest as a source of natural remnant magnetism on the has shown that for 0.5ՇxϽ1.0 essentially all Ti4+ is in the B earth, the moon, and the planet Mars.8–11 It is also considered layers but the cation distribution within layers is promising for spintronics applications.13–15 The semicon- inhomogenous.5 Magnetization measurements suggest that ducting properties can be controlled as p or n type by vary- small magnetic clusters exist, with long-range magnetic cor- ing the composition, and it has been demonstrated that for relations beyond a percolation threshold of xՅ0.87.7,17 x=0.6 the carrier spins are polarized at room temperature.13 However, the details of Fe2+/Fe3+ charge-ordered cation dis- A detailed understanding of the magnetic and Fe2+/Fe3+ tribution and their implications for magnetic ordering are yet charge ordering in ilmenite-hematite is therefore of great im- to be clarified. portance. To determine the local structure and magnetic properties, Ilmenite and hematite both have rhombohedral crystal we performed high-field 57Fe-Mössbauer studies of ilmenite- structures. The complex magnetic properties of their solid rich solid solutions with x=0.95, x=0.80, and x=0.70. High- solutions derive from incompatibilities between their differ- field Mössbauer studies can elucidate both cation distribu- ent modes of cation and magnetic ordering. In ilmenite, Fe2+ tions in ferrimagnets18 and superparamagnetic relaxation A layers alternate with Ti4+ B layers along the rhombohedral phenomena.19–21 ͓111͔ axis. Adjacent Fe2+ A layers have opposite magnetiza- tion directions parallel to ͓111͔, and the Néel temperature of 3+ ilmenite is around 55 K.2 In hematite, Fe occupies both A II. EXPERIMENTAL DETAILS and B layers with antiferromagnetic coupling between adja- cent layers. Hence the antiferromagnetic modulation length Samples with nominal compositions of x=0.95, 0.80, and of hematite is about half that of ilmenite. The Néel tempera- 0.70 were prepared by heating mixtures of Fe2O3 and TiO2 ture of hematite is about 955 K.2 Above 263 K ͑the Morin in Ag foils, sealed into silica tubes, at 900–920 °C followed transition temperature͒, or in samples with small grain size by rapid cooling ͑for details see Refs. 7 and 22͒. The samples ͑Ͻ20 nm͒, or with ͑Ͼ1%͒ Ti4+ substitution,16 the sublattice are referred to as Ilm95, Ilm80, and Ilm70. The x-ray diffrac- magnetization directions of hematite are perpendicular to tion analysis given in Ref. 7 shows that Ilm95 is a pure ͓111͔ with a small canting of the two sublattices. This results rhombohedral phase of ilmenite-hematite while Ilm70 is al- in weak ferromagnetism ͑canted antiferromagnetism͒ of most purely rhombohedral with a small impurity of an uni- ϳ0.4 A m2 /kg, with the net magnetization nearly perpen- dentified phase. Unpublished data of Ilm80 indicates a pure dicular to the sublattice magnetization directions. In pure ilmenite-hematite sample. In Ref. 7, Ilm95 and Ilm70 com- bulk hematite below 263 K the sublattice magnetization is positions have been redetermined by use of x-ray diffraction along ͓111͔. Owing to immiscibility of ilmenite and hematite and corresponding samples are listed as Ilm97.4 and Ilm71.0. below ϳ1000 K, exsolution of near end-member phases oc- Mössbauer spectra were obtained using a conventional curs upon slow cooling.8–11 Rapid cooling quenches meta- constant acceleration spectrometer with a source of 57Co in stable solid solutions.1–7 rhodium. Isomer shifts are given relative to ␣-Fe at 295 K. Previous measurements have revealed the overall mag- Spectra with and without a magnetic field of 6 T, applied netic properties of the ilmenite-hematite system ͑for phase parallel to the gamma-ray propagation direction, were ob- 1098-0121/2010/81͑22͒/224423͑5͒ 224423-1 ©2010 The American Physical Society FRANDSEN et al. PHYSICAL REVIEW B 81, 224423 ͑2010͒ Ilm95Ilm80 Ilm70 100 100 100 Data 96 Fe3+ (hematite) (a) 99 (e) 96 (i) B 92 A 2+ - - Fe s 98 si i te t 92 88 e s s Fe2+ (ilmenite) 6K 97 6K 6K 84 0T 88 0T 96 0T Sum of fit 100 100 100 96 (b) 98 (f) 96 (j) 92 92 (%) 6K 96 6K 6K 88 6T 6T 6T 100 100 100 90 (c) 96 (g) 98 (k) Transmission 80 92 96 60 K 180 K 180 K 88 70 0T 0T 0T 100 100 100 98 95 (d) 96 (h) (l) 96 90 92 94 60 K 180 K 295 K 85 6T 88 6T 92 0T 12-8-40 4 8 12 12-8-40 4 8 12 12-8-40 4 8 12 Velocity(mms-1) Velocity(mms-1) Velocity(mms-1) FIG. 1. ͑Color online͒ Mössbauer spectra of Ilm95, Ilm80, and Ilm70 at different temperatures and in zero ͑0T͒ or 6 T applied field as indicated. Colored lines represent fits to data. tained using a liquid-helium cryostat with a superconducting surprising because application of large applied fields to anti- coil. ferromagnetic hematite at temperatures below TM results only in some line broadening in the Mössbauer spectra.25 Moreover, high-field Mössbauer studies20 of canted antifer- III. RESULTS AND DISCUSSION romagnetic hematite nanoparticles have shown that the small Figure 1 shows Mössbauer spectra of Ilm95, Ilm80, and net magnetic moment due to canting was aligned with the Ilm70. Zero-field spectra obtained at 6 K ͓Figs. 1͑a͒, 1͑e͒, applied field, such that the sublattice magnetization direc- tions become nearly perpendicular to the field, and the spec- and 1͑i͔͒ show the presence of a magnetic ͑six-line͒ compo- tra therefore consist of a single sextet with relatively narrow nent ͑red fit͒ with a magnetic hyperfine field of 50.0 T in lines and a magnetic splitting close to that of the zero-field Ilm95, 51.0 T in Ilm80, and 51.2 T in Ilm70. In Ilm80 and spectra.20 Thus, the behavior of the hematite components in Ilm70 the isomer shifts of this sextet are 0.53 mm s−1 and ͑ ͒ ͑ ͒ ͑ ͒ −1 the high-field spectra in Figs. 1 b , 1 f , and 1 j is clearly the quadrupole shifts are −0.11 mm s , which are very different from that of antiferromagnetic and canted antiferro- 3+ close to the values for Fe in pure hematite with the sublat- magnetic hematite, and it indicates that the hematite in the 21 tice magnetization perpendicular to ͓111͔. The hyperfine three samples responds ferrimagnetically to an applied field. ͑ fields are slightly smaller than that of pure hematite Bhf In Ilm95 the magnetic hyperfine fields of these sextets are ϳ53 T͒.20 This may be explained by the presence of Ti4+ in 56.3 and 44.7 T, in Ilm80 they are 56.6 and 44.3 T, and in the hematite or by magnetic relaxation effects.20,21,23 It is Ilm70, 56.8 and 44.6 T. Within uncertainty ͑ϳϮ0.5 T͒ reasonable to assume that these components in Figs. 1͑a͒, these magnetic hyperfine fields correspond to the values in 1͑e͒, and 1͑i͒ are all due to Fe3+ in hematite-like environ- the zero-field spectra Ϯ6 T. Also, lines 2 and 5 of the zero- ments, and hence that their isomer shifts are nearly identical. field spectra are much less intense in the 6 T spectra ͓Figs. By comparing the line positions of lines 1 and 6 of the he- 1͑b͒, 1͑f͒, and 1͑j͔͒. This shows that the sublattice magneti- matite sextets in Figs. 1͑a͒, 1͑e͒, and 1͑i͒ we can estimate the zations of the hematite components are either nearly parallel quadrupole shift in Ilm95 as ␧=+0.2Ϯ0.02 mm s−1.
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