Low-temperature Magnetic Transition Research Articles

Antiferromagnetic resonance in the (K1-xNax)MnF3 and (K1-xLix)MnF3 systems

The X-band antiferromagnetic resonance (AFMR) spectrum of KMnF3 with Li+ and Na+ impurities was studied at temperatures from 40 K to TN. From comparison of the AFMR measurements with results of X-ray studies for the same good-quality crystals it was found that complete coincidence between the magnetic and structural phase transitions does not occur. It was also ascertained that, for T<TN, low-temperature magnetic phase transitions are highly sensitive to the presence of even small quantities of impurities. Consequently, these transitions depend markedly on the quality of the samples analysed.

The effects of substitutional impurities on the magnetic, electronic, and hyperfine interactions in ilvaite, CaFe3Si2O7O (OH), a mixed-valence iron silicate (abstract)

Ilvaite, a mixed-valence iron silicate, rivals magnetite in the complexity of the phenomena exhibited in connection with the simultaneous occurrence of Fe2+ and Fe3+ on equivalent sites. Inasmuch as the effects of impurities on the several phase transitions and electron delocalization have been explored little, if at all, an investigation of the effect of manganese impurities by means of measurements on a large suite of samples has been undertaken. Significant variations in magnetic properties are observed even for low impurity levels. For a Mn2+ content of 0.5 wt %, the low temperature magnetic phase transition is at ∼40 K, for a Mn2+ content of 1.2%, the transition temperature is less than 20 K and for a Mn2+ content of 4.5%, the transition appears to be well below 10 K. In addition to the quantitative variations, the 1/χ versus T curves are qualitatively different for samples with different concentrations of manganese. The 57Fe Mössbauer spectra are also qualitatively different at 298 K with pronounced differences in the resolution of the two Fe2+ patterns and in their relative intensities. In agreement with earlier reports, no evidence for magnetic ordering at 120 K has been observed in the susceptibility curves; the ordering is, however, exhibited in the 57Fe Mössbauer spectra obtained at 82 K. The exploration of the effect of Mn2+ impurities has permitted rare insights to be gained on the relationship between the bulk and local, site-specific properties of ilvaite. Further progress is expected to result from electrical conductivity and x-ray diffraction measurements.

Investigation of ternary transition-metal nitride systems by reactive cosputtering

A reactive dc cosputtering technique has been used to evaluate compound formation in bimetallic transition-metal nitride systems. A wide range in M-M[prime] composition can be studied in a single deposition run, and the method is applicable to nonalloying metal combinations. Using this technique, it was found that [delta]-NbN and YN form a continuous (Nb,Y)N solid solution of the NaCl type, in which the superconducting transition temperature varies with the Nb:Y ratio. In the Gd-Cr-N system only limited solid solubility of CrN and GdN was found, and evidence was obtained for a new Gd-Cr-N species centered around the GdCr[sub 2] composition. This species shows a low-temperature magnetic transition. 12 refs., 5 figs.

Observation of a low temperature magnetic phase transition in nonsuperconducting La 2− xSr xCuO 4+ y by macroscopic magnetization and electron spin resonance

We show that the transition to the low temperature ( T ≲ 12 K) magnetic phase (LTMP) of La 2 − x Sr x CuO 4 + y (LSCO) for 0.02 ≲ x ≲ 0.03 is clearly observed via bulk magnetometry in vacuum-annealed samples. The transition temperature T f depends on the strontium concentration and is insensitive to the vacuum treatment. Measurements of an electron spin resonance (ESR) signal are also described. The temperature dependent amplitude of this signal displays a clear anomaly at T f, suggesting that the observed spins participate in the LTMP. Arguments are presented for the intrinsic nature of the observed ESR signal and the LTMP. We compare temperature dependent effects in both probes to expectations for spin glasses, and relate our results to consideration of the nature of the LTMP, and of the origin of the spin system from which the LTMP is formed.

Heat capacity and Na+ ion disorder in Nasicon-type solid electrolytes Na3M2P3O12 (M2= Fe2, Cr2, ZrMg) in the temperature range 10 to 300 K

Heat-capacity measurements on Nasicon-type solid electrolytes have been performed by adiabatic calorimetry. The lattice contribution is estimated from the usual model using spectroscopic data and the excess heat capacities are discussed in terms of low-temperature magnetic transitions in iron and chromium derivatives and of Na+ ion disorder in the three compounds at higher temperatures. The entropy excess at 300 K is related to the relative ionic conductivity and the occupancy factors of Na+-available sites.

The magnetic structure in the antiferromagnetic phase of Ce(Fe1-xCox)2

The nature of the low-temperature (T approximately=90 K) magnetic phase transition in cubic Laves-phase Ce(Fe1-xCox)2 compounds with x=0.15, 0.20 has been determined using powder neutron diffraction measurements. Below this temperature, ferromagnetism is replaced by an antiferromagnetic phase, which consists of (111)-type sheets with parallel spins and antiparallel coupling of spins between adjacent (111) sheets. The orientation of the magnetic moments could not be uniquely determined; however, they are oriented within a cone that is defined by sin2 phi =2/3 relative to the (111) sheets and includes the (Fe, Co) sublattice nearest-neighbour directions. No difference in magnitude between the (Fe, Co) sublattice moments of the two phases could be detected, nor was any significant magnetic moment detected at the Ce sites in either phase. High-resolution neutron diffraction measurements show that the application of large magnetic fields ( approximately=4T) in the antiferromagnetic phase restores ferromagnetism and removes the associated structural distortion.

Elastic and thermal properties of CsNiF 3

Measurements of the thermal expansion coefficients and the elastic constants are presented for the temperature range 1.8 K to 300 K for CsNiF 3. The anomalies observed at the low temperature magnetic phase transition are correlated through the Ehrenfest relations to predict the longitudinal and shear stress dependence of the Neel temperature T N. A new anomaly is observed for the first time at a temperature of approximately 190 K in the thermal expansion coefficients both for in-plane and c-axis measurements. It is suggested that this anomaly is related to a structural phase transformation of the kind known to be induced by pressure in CsNiF 3.

Low-Temperature Magnetic and Structural Transitions of the Quasi-One-Dimensional Magnets TMMC and DMMC at High Pressures

The magnetic and structural phase transitions of the quasi-one-dimensional magnetic systems TMMC ((CH3)4NMnCl3; TN=0.84 K) and DMMC ((CH3)2NH2MnCl3; TN=3.60 K) have been studied by the heat capacity measurements at hydrostatic pressures. From the pressure dependence of the Néel temperature, the magnetic Grüneisen constant for the interchain interaction through –Mn– (CH3)4N–Mn– bridge has been conjectured to be much larger than 10 for the intrachain super-exchange interaction through –Mn–Cl3–Mn– bridge. A pressure-induced structural transition has been found in TMMC at pressure p=3.2±0.1 kbar at 4He temperatures. DMMC has been also suggested to experience a structural change in the range 3.2 kbar<p<3.7 kbar.

Hall effect anomalies at the magnetic phase transitions in MnAs1-xPxcompounds

The specific Hall resistivity rho H of MnAs1-xPx crystals (x=0.1, 0.123) has been measured at stationary conditions for temperatures between 50 and 250K using the double-AC method with Bpeak<or=0.4 T. The combination of experimentally determined magnetisation values with the isothermal curves rho H(B) allowed for the separation of the normal and anomalous Hall constants R0 and RS, which show different anomalies at the phase transition temperatures TN, Tl and Tk. The dependences on R0(T) are interpreted in terms of a two-band model leading to the concept of charge-carrier localisation at Tk i.e. at the low-temperature magnetic phase transition beta to or from gamma .

Transition from antiferromagnetism to ferromagnetism in the superconducting mixed ternary system (Sm1?x Er x )Rh4B4

The mixed ternary system (Sm1−x Er x )Rh4B4 has been investigated by means of measurements of ac electrical resistance in various applied magnetic fields, ac magnetic susceptibility, and specific heat. The low-temperature superconducting and magnetic transition temperature versus Er concentrationx phase diagram reveals the occurrence of a single transition to long-range magnetic order for all concentrations, changing from antiferromagnetism for SmRh4B4 to ferromagnetism for ErRh4B4 atx ≈ 0.2. The upper critical magnetic field versus temperature curves for various Er concentrations exhibit combined features characteristic of both SmRh4B4 and ErRh4B4, rather than a gradual evolution from one type to the other. A model for such a mixed magnetic system is discussed, and comparisons with other mixed ternary systems are made.

Observations of phase transitions in the mixed crystal system Dy pGd 1-pVO 4

The low temperature crystallographic and magnetic phase transitions in the mixed crystal system Dy p Gd 1- p VO 4 (0 ⩽ p ⩽ 1) have been studied using thermal expansion measurements and Mössbauer spectroscopy. The introduction of magnetic Gd 3+ ions into the DyVO 4 host causes an anomalous dip in the Néel temperature T N ( p), and a rapid reduction of the distortion temperature T D ( p) which differs from results on similar mixed systems. A number of pseudo-spin Hamiltonians that model these observations are discussed.

58. Neutron scattering experiments on well-staged graphite-FeCl 3

This paper reports results of a neutron diffraction study on both the magnetic and the lattice dynamic structure of graphite-FeCl/sub 3/. The FeCl/sub 3/ system was selected for this study because of the interesting magnetic properties previously reported for this system. Of particular interest is the low temperature magnetic phase transition, which has been shown to be stage dependent. Special emphasis was given to the study of the low temperature magnetic state of the Fe/sup 3 +/ spins and to the longitudinal phonon modes for k/sub z/ axis acoustic branches. All experiments reported here were carried out on a stage 2 sample of graphite-FeCl/sub 3/. The sample was prepared by a two-zone vapor growth technique and was characterized by (00l) x-ray and neutron diffraction patterns to be > 95% stage 2, with only minor inclusions of other stages.

The role of imperfections on magnetic and thermodynamic properties of cerium magnesium nitrate in the low-temperature magnetic phase transition region

We present the measurement of the low-temperature properties of an imperfect crystal of the salt cerium magnesium nitrate and find that imperfections have a profound influence on the low-temperature behavior of the salt; in particular, (1) an anomalously high x′/C is observed at very low temperatures, (2) there is a maximum in x′ which enables one to explore the critical properties of the substance, and (3) there is an indication that relaxation times are decreased by the imperfections.

Low-temperature magnetic phase transitions of CeBi and CeSb

Specific heat, low- and high-field magnetic susceptibility, thermal expansion, and magnetostriction measurements have been made on single crystals of CeBi and CeSb in order to investigate the low-temperature magnetic phase transitions of these compounds. We have observed structural transitions at the respective Neel temperatures. The maximum tetragonal distortions below TN are of the order of 10−3 in both cases. Specific heat results confirm the unusually small overall splitting of the ground-state multiplet. Applications of external magnetic fields induce a variety of interesting effects and provide insight into the magnetic phase diagram of these substances. The results are discussed qualitatively.

Specific heat of TMMC in the region of low temperature magnetic phase transition

Specific heat measurements on the one-dimensional linear chain antiferromagnet TMMC in the temperature range 0.4–4.0 K show an anomaly at 0.83 K indicative of a three-dimensional magnetic phase transition. Above 0.83 K, that part of the specific heat not related to the phase transition, shows a linear and quadratic dependence on temperature in agreement with measurements at higher ( T > 1.5 K) temperatures. The interchain coupling does not appear to be responsible for the observed low value of the linear term. Application of a 15 kG magnetic field along the c-axis broadens the specific heat anomaly and shifts the maximum value from 0.83 K to about 1.0 K.

Low temperature magnetic susceptibilities and phase transitions in two linear Ni2+ salts: TMNC and TMNB

We studied the magnetic susceptibilities parallel and perpendicular to the chain axis in TMNC in the temperatures ranges 20–13 K and 4.2–0.4 K. These experiments lead to TN=(1.195±0.005) K, J/kB=(1.09±0.10) K, D/kB=(2.10±0.46) K. Because of the ferromagnetic character of the intrachain exchange parameter, the dipolar interchain interaction is much more important in TMNC than in isostructural TMMC, as shown by our calculations. In TMNB, a transition to a 3-d antiferromagnetic ordered state was found at TN=(2.680±0.010) K.

Cooperative Jahn-Teller Phase Transition in the Nickel-Zinc-Chromite System

Results of thermal expansion and sound velocity measurements are presented for the Nickel-Zinc-Chromite system (Ni x Zn 1- x Cr 2 O 4 ). It is found that for x >0.6 this system exhibits a cooperative Jahn-Teller phase transition which occurs at a temperature T a higher than the magnetic phase transition at T N . Both transitions at T a and T N are of first order. The temperature dependence of the soft mode c 11 – c 12 could only be measured for T > T a . From its temperature dependence, we conclude that the electronic state of the Ni 2+ ion couples predominantly to the macroscopic strains. A comparison between our sound velocity data and the Kataoka-Kanamori theory gives only fair agreement for T > T a . It is found that the low temperature magnetic phase transition influences the cooperative J-T-transition considerably.

Geomagnetic secular variation recorded in the stable magnetic remanence of recent sediments

Continuous cores of sediment covering the period 13 000 yr BP to the present day have been collected for magnetic investigations from Lake Windermere using a pneumatically operated piston corer. The total remanent magnetization of one 6 metre core and six 1 metre cores has been measured using a computerized slow-speed spinner magnetometer. The stability of the remanence has been confirmed by alternating magnetic field tests and repeat measurements of the remanence. The magnetic mineral content of the sediments has been deduced from thermo-magnetic and rotational hysteresis studies. Low-temperature magnetic transition experiments on the NRM of the sediment shows that it least half of the remanence is carried by haematite. The inclinations of the magnetic vectors are consistently low. This is attributed to compaction of the sediment after deposition. The time variations of the geomagnetic field are discussed and their correlations with the results from the Lake Windermere sediments are indicated. A comparison is also made of the results from the Pleistocene Narita bed (Japan) with those from Lake Windermere.

Low temperature magnetic phase transition in ErCrO 3

A ferromagnetic-antiferromagnetic transition was observed at 9.82 ± 0.02°K in addition to a Schottky anomaly with a maximum splitting of Δ = 13.5 ± 0.5° K.

Pressure dependence of the low-temperature magnetic transition in α-Fe 2O 3

The effect of the hydrostatic pressure up to 1.5 kbar on the low-temperature magnetic transition of α-Fe 2O 3 was studied by neutron diffraction. The transition temperature increases linearly with increasing pressure by 3.7 ± 0.2 deg/kbar.