Low-temperature Magnetic Phase Transition Research Articles

Low-temperature heat capacity and magnetic phase transition of TbB2

The heat capacity of TbB2 prepared by high-temperature elemental synthesis has been measured from 5 to 300 K. The Cp(T) curve has a broad anomaly near 100 K, due to the Schottky contribution, and a sharp peak at 143.28 K, arising from the ferromagnetic ordering of TbB2. The total heat capacity of terbium diboride has been represented as the sum of magnetic, lattice, and Schottky contributions, and their parameters have been determined.

Magnetoelastic effects and the magnetic phase diagram of multiferroicDyMn2O5

The magnetoelastic coupling in multiferroic $DyMn_2O_5$ is investigated by magnetostriction measurements along the three crystallographic orientations. Strong lattice anomalies as a function of the magnetic field are detected at the low temperature magnetic and ferroelectric phase transitions. The sign and magnitude of the magnetostrictive coefficient as well as the lattice anomalies at the transitions are correlated with the Dy moment order and with the sharp changes of the dielectric constant and the ferroelectric polarization. With the magnetic field applied along the c-axis a new high-field phase has been detected the magnetic structure of which has yet to be explored.

Frustrated magnetism and the effects of titanium substitution in the mixed-valence oxideBaV10−xTixO15

The low temperature magnetic phase transitions in $\mathrm{Ba}{\mathrm{V}}_{10}{\mathrm{O}}_{15}$ at $\ensuremath{\sim}40\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $\ensuremath{\sim}25\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ are investigated in detail. The topology of the V sublattice, which consists of linked ${\mathrm{V}}_{10}$ clusters formed by four tetrahedra sharing edges and corners, leads to geometric magnetic frustration and a depression of the ordering temperature characterized by the frustration index, $f=\ensuremath{\mid}\ensuremath{\theta}\ensuremath{\mid}∕{T}_{c}\ensuremath{\sim}30$. Specific heat data show the coexistence of both long range and short range magnetic order at low temperature. The position of a $\ensuremath{\lambda}$ peak, identified with ${T}_{c}$ for long range antiferromagnetic order, depends on the ${\mathrm{V}}^{2+}∕{\mathrm{V}}^{3+}$ ratio and varies from $43.0\phantom{\rule{0.3em}{0ex}}\mathrm{K}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}41.6\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for samples with the highest ${\mathrm{V}}^{2+}$ content but is suppressed to zero for highly oxidized samples. A broad maximum in the specific heat occurs at $\ensuremath{\sim}25\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and is present in all samples regardless of the level of oxidation. For samples with a well defined ${T}_{c}$, estimates of the entropy removal over the range $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}55\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ are only 11% of that expected for $8\phantom{\rule{0.3em}{0ex}}{\mathrm{V}}^{3+}(S=1)$ and $2\phantom{\rule{0.3em}{0ex}}{\mathrm{V}}^{2+}(S=3∕2)$ ions per formula unit, suggesting the presence of considerable short-range order. Neutron diffraction data support the presence of long range antiferromagnetic (AF) order with ${T}_{c}\ensuremath{\sim}40\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The magnetic structure below ${T}_{c}$ is complex: there are five crystallographically independent vanadium ions in the unit cell, leading to 40 magnetic ions in the crystal unit cell and 80 spins per magnetic cell. Furthermore, the magnetic structure is of the multi-$k$ type involving the two unrelated wave vectors $\mathbf{k}=(0\phantom{\rule{0.3em}{0ex}}0\phantom{\rule{0.3em}{0ex}}0)$ and $\mathbf{k}=(1∕2\phantom{\rule{0.3em}{0ex}}0\phantom{\rule{0.3em}{0ex}}0)$. Zero field muon spin relaxation data show spin freezing below $30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. ac susceptibility results are anomalous. Two peaks, at $\ensuremath{\sim}0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $\ensuremath{\sim}25\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ are seen in ${\ensuremath{\chi}}^{\ensuremath{'}}$ but the maxima are frequency independent as is the prominent maximum in ${\ensuremath{\chi}}^{\ensuremath{''}}$ near $25\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, behavior atypical of spin glasses. Ti-doped samples, $\mathrm{Ba}{\mathrm{V}}_{10\ensuremath{-}x}{\mathrm{Ti}}_{x}{\mathrm{O}}_{15}$, show remarkable results as only 5% Ti doping is sufficient to destroy the AF long range order and induce conventional spin glass behavior in the dc and ac susceptibility, with freezing temperatures ranging from $15\phantom{\rule{0.3em}{0ex}}\mathrm{K}--5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for doping levels from 5% to 50%. The frustration indices reach enormous values to $f=130$ for $x=3$. The spin glass state is quenched for 70% Ti doping and only paramagnetic behavior is seen.

Magnetic properties of Nd-deficient manganites Nd 0.9−xCa xMnO y

X-ray diffraction and magnetic studies of neodymium deficient Nd 0.9− x Ca x MnO y (0⩽ x⩽0.4) systems have been carried out depending on oxygen or calcium content. The Nd 0.9MnO y samples have been prepared in the 2.85⩽ y⩽2.93 concentration range. According to differential thermal analysis (DTA) data the sample with y = 2.85 exhibits a first-order phase transition at T = 1200 K , whereas in y = 2.93 composition an anomaly of elastic properties evidences the transition nearby T = 500 K . The transitions are supposed to be caused by a cooperative ordering of e g-orbitals of manganese ions. Composition with y = 2.85 is antiferromagnet with T N = 85 K , whereas for more oxidized Nd 0.9MnO y samples a coexistence of antiferromagnetic and ferromagnetic phases is suggested. Low-temperature magnetic phase transition which is accompanied by a negative magnetization appearance has been found in the Nd 0.9MnO 2.90 compound. Magnetic behavior of Nd 0.9− x Ca x MnO y (0.1⩽ x⩽0.4; 2.85⩽ y⩽2.89) solids strongly resembles that observed for stoichiometric Nd 1− x Ca x MnO 3 series. Properties of the Nd 0.9− x Ca x MnO y (0⩽ x⩽0.4) solid solutions are in agreement with a hypothesis according to which a part of Nd ions can be substituted by Mn ions.

TmZn: A possible regenerator material for low-temperature cryocoolers

The intermetallic compound TmZn has two low-temperature magnetic phase transitions and is a potential regenerator material. Application of a magnetic field to regulate the phase-transition temperature by influencing the specific heat cp(T) dependence in TmZn was investigated. The low-temperature heat capacity of a monocrystalline TmZn sample was measured as a function of magnetic field (up to 30 kOe). The anisotropy of the transition temperature in the applied magnetic field was modeled using ac susceptibility measurements. From the measured values, the TmZn regenerator specific heat was calculated taking into account both the temperature regulation and the anisotropy. A numerical simulation study of the TmZn regenerator was made. The results show an improvement in TmZn regenerator performance by the application of a magnetic field and employment of the transition anisotropy. The minimal cold chamber temperature was lowered from 11.76 to 9.77 K by the application of a magnetic field of 30 kOe. The performance of the TmZn regenerator is compared with the existing regenerator materials, Er3Ni, Pb, HoCu2, and Er50Pr50.

Magnetic Phase Transitions in Substituted Spinels of Zn1−x Cu χ Cr2 Se4; 0.0 ≤ x ≤ 0.3

Magnetic and crystallographic properties have been studied by neutron powder diffraction and measurements of magnetization and magnetization hysteresis-loops for substituted spinels of Zn1−xCuxCr2Se4 with 0.0≤x≤0.3. It is found that the Zn0.85Cu0.15Cr2Se4 spinel has two magnetic phase transitions at 23.0 K (Néel temperature; T N) and 410 K (Curie temperature; T C) and that the Zn0.70Cu0.30Cr2Se4 spinel has magnetic transitions at 24.5 K (T N) and 415 K (T C) on heating. The low-temperature magnetic phase transition is from a spiral antiferromagnet to a ferromagnet, and the high-temperature magnetic phase transition is from a ferromagnet to a paramagnet, while ZnCr2Se4 shows a magnetic phase transition only from a spiral antiferromagnet to a paramagnet at about 21.0 K. From neutron powder diffraction, it is also found that the spinels of Zn1−x Cu x Cr2Se4; 0.0 ≤ x ≤ 0.3. show satellite-like magnetic reflection having indexes (h ± Q, k, l) with Q = 0.470 below T N and short-range order of spins (spin glass-like) above T N. The incommensurate antiferromagnetic phase below T N results from a spiral long-range order of the spins of Cr3+. The intermediate ferromagnetic phase between T N and T C is related not to the spiral spin order but to double-exchange magnetic interaction among Cr3+ and Cr4+ mediated by current carriers, positive holes, which is made by the substitution of Zn2+ ions with Cu1+ ions in Zn1−x Cu x Cr2Se4.

SPONTANEOUS AND FIELD-INDUCED MAGNETIC PHASE TRANSITIONS IN THE INTERMETALLIC COMPOUND DyMn2Ge2

The temperature dependence of lattice constants a and c of intermetallic compund DyMn2Ge2 is measured in the temperature range 10-800K by using the X-ray method. The magnetoelastic anomaly of lattice constants a is observed at spontaneous magnetic phase transition from paramagnetic state to antiferromagnetic state. The ac magnetic susceptibility of DyMn2Ge2 is measured in the temperature range 4.2-200K. Low temperature magnetic phase transitions in DyMn2Ge2 are discussed in the molecular field approximation by taking into account of next nearest Mn-Mn interlayer exchange interaction. The temperature dependence of magnetization and magnetic curves at different temperatures of DyMn2Ge2 single crystal are calculated. Theoretical calculation shows that the observed field-induced magnetic phase transition in DyMn2Ge2 single crystal at temperature lower than 33K is the transition from ferrimagnetic state into intermediate state.

Low-temperature magnetic phase transitions of the geometrically frustrated isosceles triangular Ising antiferromagnetCoNb2O6

Low-temperature magnetic phase transitions of the geometrically frustrated isosceles triangular Ising antiferromagnet ${\mathrm{CoNb}}_{2}{\mathrm{O}}_{6}$ have been investigated by means of neutron diffraction down to $T=0.2\mathrm{K}$ under applied fields up to ${H}_{\ensuremath{\parallel}c}=4.4 \mathrm{kOe}.$ Below $T\ensuremath{\sim}0.6\mathrm{K},$ the relaxation time of the system becomes extremely long compared with our observation time, being responsible for all the anomalous low-temperature magnetic properties observed in the bulk measurements [T. Hanawa et al., J. Phys. Soc. Jpn. 63, 2706 (1994)]. In addition to confirmation of the triple point where the antiferromagnetic, field-induced ferrimagnetic, and incommensurate phases meet together in the ${H}_{\ensuremath{\parallel}c}--T$ magnetic phase diagram, we also found various ordered phases that are field induced between the ferrimagnetic and saturated paramagnetic phases.

Low Temperature Magnetic Behaviour of γ-Fe<sub>2</sub>O<sub>3</sub> Nanoparticles

γ-Fe 2 O 3 magnetic nanoparticles exhibit both strong exchange anisotropy and magnetic training effect together with high field irreversibility in M(H) curves and zero field cooling-field cooling (ZFC-FC) processes. A low temperature magnetic phase transition is evidenced by the existence of a freezing process with strong irreversibility between the ZFC and the FC branches even at H= 55 kOe. The cusp of the ZFC branch moves as a function of the magnetic field following the well known de Almeida-Thouless line δT F ∞ H 2/3 . Strong exchange anisotropy is observed after cooling the sample in high field through the freezing temperature T F ≃40 K. The temperature dependence of the exchange anisotropy field HE has been determined and analyzed observing approximately a linear falloff (1-T/T crit ) at a critical temperature T crit ≃25 K slightly below the spin glass transition temperature T F in good agreement with the predictions of the random-field model of exchange anisotropy. These experimental results, together with the high field magnetic relaxation processes, clearly point to the existence of a surface spin glass-like layer that undergoes a magnetic transition to a frozen state below T F . A close relation between H C and HE is observed, which would imply that the increase of H C observed in nanoparticles at low temperatures may be due to the pinning effect of the blocked spin glass-like surface layer upon the single-domained core.

Thermal properties at the low-temperature structural and magnetic phase transitions in Pr2NiO4 crystals.

The coefficients of the thermal expansion and the specific heat of ${\mathrm{Pr}}_{2}$${\mathrm{NiO}}_{4}$ single crystals have been measured in order to study the thermodynamic and structural properties of the low-temperature structural and magnetic phase transitions. We find large, strongly anisotropic length changes and first-order specific heat anomalies at the low-temperature tetragonal (LTT) to low-temperature orthorhombic (LTO) transition at ${\mathit{T}}_{\mathrm{LT}}$. These anomalies imply huge anisotropic uniaxial pressure dependences of ${\mathit{T}}_{\mathrm{LT}}$. The anisotropy and size of the length changes indicate that the LTT phase occurs in order to minimize the effects of the ionic size mismatch. The discontinuities in both properties at ${\mathit{T}}_{\mathrm{LT}}$ in ${\mathrm{Pr}}_{2}$${\mathrm{NiO}}_{4}$ are orders of magnitude larger than those in the high-${\mathit{T}}_{\mathrm{c}}$ superconducting cuprates and the pressure dependences differ even qualitatively. Besides the anomalies at ${\mathit{T}}_{\mathrm{LT}}$ we find two first-order phase transitions in the thermal expansion of ${\mathrm{Pr}}_{2}$${\mathrm{NiO}}_{4}$ at T\ensuremath{\approxeq}90 K due to the reorientation of the Ni spins in the quasi-two-dimensional antiferromagnet. No corresponding anomalies could be resolved in the specific heat, which implies a very large and anisotropic pressure dependence of these magnetic transitions and thus shows an intimate coupling between structural and magnetic degrees of freedom in ${\mathrm{Pr}}_{2}$${\mathrm{NiO}}_{4}$. \textcopyright{} 1996 The American Physical Society.

The low-temperature phase diagram of UCu5 determined from magnetization and magnetoresistance measurements

The low-temperature phase diagram of polycrystalline UCu5 has been determined from magnetization and magnetoresistance measurements. This system undergoes two magnetic transitions at low temperatures. The lower-temperature phase transition at T2=1.16 K, which manifests itself as a sudden jump in the magnetization and electrical resistance with decreasing temperature, shows a temperature hysteresis of about 40 mK for fields up to 12 T. The magnetic properties of this low-temperature phase show irreversible and metastable behaviour, which becomes more dramatic at the lower fields. The critical temperature T2 is observed to increase proportionally with the square of the magnetic field. Our results are discussed in comparison with available specific heat, neutron scattering and nuclear magnetic resonance data, with the aim of determining the actual nature of the low-temperature magnetic phase transition.

New possibilities offered by high resolution Fourier transform spectroscopy in studying magnetic phase transitions

High resolution optical absorption spectra of an intrinsic R3+ ion or Er3+ probe were measured in cuprates R2Cu2O5 and R2BaCuO5 related to high Tc superconductors of the 123 type and in low-dimensional nickelates R2BaNiO5. We have registered for the first time phase transitions, some of them not detectable by other methods, in R2BaCuO5 with R=Y, Lu, Nd, Dy, Yb, and Lu, in R2BaNiO5, with R=Nd, Sm, Eu, Dy, Ho, Tm, and Lu. Coexistence of two different magnetic phases in Y2BaCuO5 and Lu2BaCuO5 in a broad temperature range was demonstrated. Fine splitting of the low temperature magnetic phase transition, possibly caused by the interaction of critical fluctuations, has been observed in Dy2BaCuO5. Low-dimensional correlations at temperatures higher than the temperature of three-dimensional (3-D) magnetic ordering have been analyzed in quasi-2D compounds R2Cu2O5 and quasi-1-D R2BaCuO5. We developed the method of the Er3+ probe in a series of isostructural f–d compounds to select the right magnetic structure of an ordered d subsystem from several structures that fitted neutron scattering data equally well.

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.

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.

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.

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 .

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.