arXiv subjects

Harikrishnan S. Nair

Publications and source records attributed to Harikrishnan S. Nair.

At least 19 recordsLinked to original sources

arXiv2023

Sawtooth lattice multiferroic BeCr$_2$O$_4$: Non-collinear magnetic structure and multiple magnetic transitions

Noncollinear magnetic structures and multiple magnetic phase transitions in a sawtooth lattice antiferromagnet consisting of Cr$^{3+}$ are experimentally identified in this work, thereby proposing the scenario of magnetism-driven ferroelectricity in a sawtooth lattice. The title compound, BeCr$_2$O$_4$, displays three magnetic phase transitions at low temperatures, at $T_{N1}\approx$ 7.5 K, at $T_{N2}\approx$ 25 K and at $T_{N3}\approx$ 26 K, revealed through magnetic susceptibility, specific heat and neutron diffraction in this work. These magnetic phase transitions are found to be influenced by externally applied magnetic fields. Isothermal magnetization curves at low temperatures below the magnetic transitions indicate the antiferromagnetic nature of \bco\ with two spin-flop-like transitions occurring at $H_{c1}\approx$ 29 kOe and $H_{c2} \approx$ 47 kOe. Our high-resolution X-ray and neutron diffraction studies, performed on single crystal and powder samples unambiguously determined the crystal structure as orthorhombic $Pbnm$. By performing the magnetic superspace group analysis of the neutron diffraction data at low temperatures, the magnetic structure in the temperature range $T_{N3,N2} < T < T_{N1}$ is determined to be the polar magnetic space group, $P21nm.1^{\prime}(00g)0s0s$ with a cycloidal magnetic propagation vector $\textbf{k}_1$ = (0, 0, 0.090(1)). The magnetic structure in the newly identified phase below $T_{N1}$, is determined as $P21/b.1^{\prime}[b](00g)00s$ with the magnetic propagation vector $\textbf{k}_2$ = (0, 0, 0.908(1)). The cycloidal spin structure determined in our work is usually associated with electric polarization, thereby making \bco\ a promising multiferroic belonging to the sparsely populated family of sawtooth lattice antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2020

Nonmagnetic ground state, Crystal field effects and Heavy-fermion behaviour in the Remeika Phase: Pr$_3$Ir$_4$Ge$_{13}$

We report the magnetic, electronic and transport properties of the quasi-skutterudite compound Pr$_3$Ir$_4$Ge$_{13}$ by means of magnetic susceptibility $χ(T)$, electrical resistivity $ρ(T)$, specific heat $C_p(T)$, thermal conductivity $κ(T)$, thermoelectric power $S(T)$ and Hall coefficient $R_\mathrm{H}(T)$ measurements. Pr$_3$Ir$_4$Ge$_{13}$ does not show any phase transition down to 1.9 K. Magnetic, and specific measurements show that the system possesses a crystal electric field singlet ground state that is separated from the first excited state by about 37 K. $ρ(T)$ shows a negative temperature coefficient of resistance for the whole temperature range studied and which can be explained in terms of Mott's impurity band conduction mechanism. $R_\mathrm{H}(T)$ measurements show that Pr$_3$Ir$_4$Ge$_{13}$ is a low-carrier density semimetal and its transport properties indicate a metallic-non metallic cross over behaviour. Large Seebeck values were observed for the entire temperature range of investigation, and the analysis of temperature variation of $S$ and $S/T$ showed no sign of strong correlation between the Pr 4$f^2$ and conduction electron states near Fermi level. A large Sommerfeld coefficient, $γ= 150$~mJ/(mol K$^2$) indicates the formation of a moderate heavy-fermion state emerging from the dynamical crystal field fluctuations.

cond-mat.str-el↗

arXiv2020

Reentrant spin reorientation transition and Griffiths-like phase in antiferromagnetic TbFe$_{0.5}$Cr$_{0.5}$O$_3$

The perovskite TbFe$_{0.5}$Cr$_{0.5}$O$_3$ shows two anomalies in the magnetic susceptibility at $T_N$ = 257K and $T_{SR}$ = 190K which are respectively, the antiferromagnetic and spin reorientation transition that occur in the Fe/Cr sublattice. Analysis of the magnetic susceptibility reveals signatures of Griffiths-like phase in this compound. Neutron diffraction analysis confirms that, as the temperature is reduced from 350K, a spin reorientation transition from $Γ_2$ (F$_x$, C$_y$, G$_z$) to $Γ_4$ (G$_x$, A$_y$, F$_z$) occurs at $T_N$ = 257K and subsequently, a second spin reorientation takes place from $Γ_4$ (G$_x$, A$_y$, F$_z$) to $Γ_2$ (F$_x$, C$_y$, G$_z$) at $T_{SR}$ = 190K. The $Γ_2$ (F$_x$, C$_y$, G$_z$) structure is stable until 7.7K where an ordered moment of 7.74(1)$μ_\mathrm B$/Fe$^{3+}$(Cr$^{3+}$) is obtained from neutron data refinement. In addition to the long-range order of the magnetic structure, indication of diffuse magnetic scattering at 7.7K is evident, thereby lending support to the Griffiths-like phase observed in susceptibility. At 7.7K, Tb develops a ferromagnetic component along the crystallographic $a$ axis. Thermal conductivity, and spin-phonon coupling of TbFe$_{0.5}$Cr$_{0.5}$O$_3$ through Raman spectroscopy are studied in the present work. An antiferromagnetic structure with ($\uparrow \downarrow \uparrow \downarrow$) arrangement of Fe/Cr spins is found in the ground state through first-principles energy calculations which supports the experimental magnetic structure at 7.7K. The spin-resolved total and partial density of states are determined showing that TbFe$_{0.5}$Cr$_{0.5}$O$_3$ is insulating with a band gap of $\sim 0.12$ (2.4) eV within GGA (GGA+$U$) functionals.

cond-mat.mtrl-sci↗

arXiv2019

Electronic and magnetic properties of quasi-skutterudite PrCo$_2$Ga$_8$ compound

PrCo$_2$Ga$_8$ is an orthorhombic quasi-skutterudite type compound which crystallizes in the CaCo$_2$Al$_8$ structure type, with space group $Pbam$ (No. 55). The Pr$^{3+}$ ion has a site symmetry of $C_s$ which predicts a crystal electric field (CEF) level splitting into 9 singlets for $J$ = 4. However, a phase transition at $T_m$ = 1.28~K is observed in electrical resistivity and specific heat results and is reported in this paper. The electrical resistivity shows an upturn below $T_m$ due to the superzone-gap formation. This transition is tuneable in fields and is suppressed to lower temperatures with applied magnetic fields. The electronic specific heat $C_{p}(T)/T$ increases below $T_m$ and reaches a value of 7.37 J/(mol K$^2$) at 0.4~K. The Sommerfeld coefficient, $γ$ extracted from the low temperature analysis of $C_\mathrm{4f}(T)/T$ is 637 mJ/(mol K$^2$) indicating a possible mass enhancement of the quasiparticles. The calculated entropy value of 3.05 J/(mol K) is recovered around $T_m$ exhibiting almost 53% of Rln2, where R is the universal gas constant. Magnetic susceptibility results obeys the Curie-Weiss law for data above 100 K with an estimated effective magnetic moment, $μ_\mathrm{eff}$ = 3.37 $μ_B$/Pr and Weiss temperature, $θ_p$ = $-$124 K.

cond-mat.str-el↗

arXiv2017

Spin reorientation and disordered rare earth magnetism in Ho$_2$FeCoO$_6$

We report the experimental observation of spin reorientation in the double perovskite Ho$_2$FeCoO$_6$. The magnetic phase transitions in this compound are characterized and studied through magnetization and specific heat, and the magnetic structures are elucidated by neutron powder diffraction. Two magnetic phase transitions are observed in this compound - one at $T_\mathrm{N1} \approx$ 250~K, from paramagnetic to antiferromagnetic, and the other at $T_\mathrm{N2} \approx$ 45~K, from a phase with mixed magnetic structures to a single phase through a spin reorientation process. The magnetic structure in the temperature range 200~K - 45~K is a mixed phase of the irreducible representations $Γ_1$ and $Γ_3$, both of which are antiferromagnetic. The phase with mixed magnetic structures that exists in Ho$_2$FeCoO$_6$ gives rise to a large thermal hysteresis in magnetization that extends from 200~K down to the spin reorientation temperature. At $T_\mathrm{N2}$, the magnetic structure transforms to $Γ_1$. Though long-range magnetic order is established in the transition metal lattice, it is seen that only short-range magnetic order prevails in Ho$^{3+}$ - lattice. Our results should motivate further detailed studies on single crystals in order to explore spin reorientation process, spin switching and the possibility of anisotropic magnetic interactions giving rise to electric polarization in Ho$_2$FeCoO$_6$.

cond-mat.mtrl-sci↗

arXiv2017

Large magntocaloric effect and 3D Ising critical behaviour in Gd$_2$Cu$_2$In

The ternary intermetallic compound Gd$_2$Cu$_2$In crystallizes in Mo$_2$Fe$_2$B type structure with the space group $P4/mbm$ and we study critical behaviour and magnetocaloric effect near the ferromagnetic transition ($T_C$ $\approx$ 94 K) using the magnetic and heat capacity measurements. The maximum entropy change ($ΔS_m$) and adiabatic temperature change ($ΔT_{ad}$) for the field value of 7 T were observed to be 13.8 J/kg.K and 6.5 K respectively. We have employed modified Arrott plot (MAP), Kouvel-Fisher (KF) procedures to estimate the critical exponents near the FM-PM phase transition. Critical exponents $β$ = 0.312(2), $γ$ = 1.080(5) are self-consistently estimated from the non-linear fitting. The $β$ value is close to the three dimensional (3D) Ising model where as $γ$ and $δ$ values are close to mean field model. The estimated critical exponents for Gd$_2$Cu$_2$In suggest that the system may belong to different universal class. All the three critical exponent obey Widom scale and collapse the scaled magnetic isotherms into two distinct branches below and above $T_C$ in accordance with single scaling equation. Specific heat measurements show a $λ$ type peak near 94 K confirming the bulk magnetic ordering. The data near $T_C$ was fitted using the non-linear function $C_{P} = B + Cε+ A^{\pm}|ε|^{-α}(1 + E^{\pm} |ε|^{0.5})$ between -0.025$<ε<$0.025 which yielded the fourth critical exponent $α$ value to be 0.11 (3). The value indicates possible 3D-Ising behavior where Gd$^{3+}$ moments arranged uniaxially along long tetragonel axis 'c' as reported in literature.

cond-mat.str-el↗

arXiv2017

Pr-magnetism in the quasi-skutterudite compound PrFe$_2$Al$_8$

The intermetallic compound \PFA\ that possesses a three-dimensional network structure of Al polyhedra centered at the transition metal element Fe and the rare earth Pr is investigated through neutron powder diffraction and inelastic neutron scattering in order to elucidate the magnetic ground state of Pr and Fe and the crystal field effects of Pr. Our neutron diffraction study confirms long-range magnetic order of Pr below $T_N $ = 4.5~K in this compound. Subsequent magnetic structure estimation reveals a magnetic propagation vector $k$ = ($\frac{1}{2}$~0~$\frac{1}{2}$) with a magnetic moment value of 2.5~$μ_\mathrm{B}$/Pr along the orthorhombic $c$-axis and evidence the lack of ordering in the Fe sublattice. The inelastic neutron scattering study reveals one crystalline electric field excitation near 19~meV at 5~K in \PFA. The energy-integrated intensity of the 19~meV excitation as a function of |$Q$| $(A^{-1})$ follows the square of the magnetic form factor of Pr$^{3+}$ thereby confirming that the inelastic excitation belongs to the Pr sublattice. The second sum rule applied to the dynamic structure factor indicates only 1.6(2)~$μ_\mathrm{B}$ evolving at the 19~meV peak compared to the 3.58~$μ_\mathrm{B}$ for free Pr$^{3+}$, indicating that the crystal field ground state is magnetic and the missing moment is associated with the resolution limited quasi-elastic line. The magnetic order occurring in Pr in \PFA\ is counter-intuitive to the symmetry-allowed crystal field level scheme, hence, is suggestive of exchange-mediated mechanisms of ordering stemming from the magnetic ground state of the crystal field levels.

cond-mat.str-el↗

arXiv2016

Ferromagnetism and the Effect of Free Charge Carriers on Electric Polarization in Y_2NiMnO_6 Double Perovskite

The double perovskite Y_2NiMnO_6 displays ferromagnetic transition at Tc = 81 K. The ferromagnetic order at low temperature is confirmed by the saturation value of magnetization (M_s) and also, validated by the refined ordered magnetic moment values extracted from neutron powder diffraction data at 10 K. This way, the dominant Mn4+ and Ni2+ cationic ordering is confirmed. The cation-ordered P 21/n nuclear structure is revealed by neutron powder diffraction studies at 300 and 10 K. Analysis of frequency dependent dielectric constant and equivalent circuit analysis of impedance data takes into account the bulk contribution to total dielectric constant. This reveals an anomaly which coincides with the ferromagnetic transition temperature (T_c). Pyrocurrent measurements register a current flow with onset near Tc and a peak at 57 K that shifts with temperature ramp rate. The extrinsic nature of the observed pyrocurrent is established by employing a special protocol measurement. It is realized that the origin is due to re-orientation of electric dipoles created by the free charge carriers and not by spontaneous electric polarization at variance with recently reported magnetism-driven ferroelectricity in this material

cond-mat.str-el↗

arXiv2016

Magnetic, specific heat and electrical transport properties of Frank-Kasper cage compounds RTM$_2$Al$_{20}$ [R = Eu,Gd and La ; TM = V,Ti]

Single crystals of Frank-Kasper compounds RTM$_2$Al$_{20}$ (R = Eu, Gd and La; TM = V and Ti) were grown by self-flux method and their physical properties were investigated through magnetization ($M$), magnetic susceptibility ($χ$), specific heat ($C_P$) and electrical resistivity ($ρ$) measurements. Powder x-ray diffraction studies and structural analysis showed that these compounds crystallize in the cubic crystal structure with the space group $Fd\overline{3}m$. The magnetic susceptibility for the compounds EuTi$_2$Al$_{20}$ and GdTi$_2$Al$_{20}$ showed a sudden jump below the N{é}el temperature $T_N$ indicative of plausible double magnetic transition. Specific heat ($C_P$) and electrical resistivity ($ρ$) measurements also confirm the first-order magnetic transition (FOMT) and possible double magnetic transitions. Temperature variation of heat capacity showed a sharp phase transition and huge $C_P$ value for the (Eu/Gd)Ti$_2$Al$_{20}$ compounds Full width at half-maximum (FWHM) $<$ 0.2 K) which is reminiscent of a first-order phase transition and a unique attribute among RTM$_2$Al$_{20}$ compounds. We observed clear anomaly between heating and cooling cycle in temperature-time relaxation curve for the compounds GdTi$_2$Al$_{20}$ (2.38 $K$) and EuTi$_2$Al$_{20}$ (3.2 $K$) which is indicating a thermal arrest due to the latent heat. The temperature variation of $S_{mag}$ for GdTi$_2$Al$_{20}$ saturates to a value $0.95R\ln8$ while the other magnetic systems exhibited still lower entropy saturation values in the high temperature limit. Resistivity measurements showed that all the samples behave as normal Fermi liquid type compounds and $ρ(T)$ due to electron-phonon scattering follows Bloch-Gr$\ddot{\textrm u}$neisen-Mott relation in the paramagnetic region.

cond-mat.mtrl-sci↗

arXiv2016

Exchange bias-like effect in TbFeAl intermetallic induced by atomic disorder

Exchange bias-like effect observed in the intermetallic compound TbFeAl, which displays a magnetic phase transition at $T^h_c \approx$ 198~K and a second one at $T^l_c \approx$ 154~K, is reported. {\em Jump}-like features are observed in the isothermal magnetization, $M (H)$, at 2~K which disappear above 8~K. The field-cooled magnetization isotherms below 10~K show loop-shifts that are reminiscent of exchange bias, also supported by {\em training effect}. Significant coercive field, $H_c \approx$ 1.5~T at 2~K is observed in TbFeAl which, after an initial increase, shows subsequent decrease with temperature. The exchange bias field, $H_{eb}$, shows a slight increase and subsequent leveling off with temperature. It is argued that the inherent crystallographic disorder among Fe and Al and the high magnetocrystalline anisotropy related to Tb$^{3+}$ lead to the exchange bias effect. TbFeAl is recently reported to show magnetocaloric effect and the present discovery of exchange bias makes this compound a multifunctional one. The result obtained on TbFeAl generalizes the observation of exchange bias in crystallographically disordered materials and gives impetus for the search for materials with {\em exchange bias induced by atomic disorder.}

cond-mat.str-el↗

arXiv2016

Double-phase transition and giant positive magnetoresistance in the quasi-skutterudite Gd$_3$Ir$_4$Sn$_{13}$

The magnetic, thermodynamic and electrical/thermal transport properties of the caged-structure quasi-skutterudite Gd$_3$Ir$_4$Sn$_{13}$ are re-investigated. The magnetization $M(T)$, specific heat $C_p(T)$ and the resistivity $ρ(T)$ reveal a double-phase transition -- at $T_{N1}\sim$ 10~K and at $T_{N2}\sim$ 8.8~K -- which was not observed in the previous report on this compound. The antiferromagnetic transition is also visible in the thermal transport data, thereby suggesting a close connection between the electronic and lattice degrees of freedom in this Sn-based quasi-skutterudite. The temperature dependence of $ρ(T)$ is analyzed in terms of a power-law for resistivity pertinent to Fermi liquid picture. Giant, positive magnetoresistance (MR) $\approx$ 80$\%$ is observed in Gd$_3$Ir$_4$Sn$_{13}$ at 2~K with the application of 9~T. The giant MR and the double magnetic transition can be attributed to the quasi-cages and layered antiferromagnetic structure of Gd$_3$Ir$_4$Sn$_{13}$ vulnerable to structural distortions and/or dipolar or spin-reorientation effects. The giant value of MR observed in this class of 3:4:13 type alloys, especially in a Gd-compound, is the highlight of this work.

cond-mat.str-el↗

arXiv2016

Magnetic structures and magnetoelastic coupling of Fe-doped hexagonal manganites LuMn1-xFexO3 (0 < x < 0.3)

We have studied the crystal and magnetic structures of Fe-doped hexagonal manganites LuMn1-xFexO3 (x = 0, 0.1, 0.2, and 0.3) by using bulk magnetization and neutron powder diffraction methods. The samples crystalize consistently in a hexagonal structure and maintain the space group P63cm from 2 to 300 K. The Néel temperature TN increases continuously with increasing Fe-doping. In contrast to a single Γ4 representation in LuMnO3, the magnetic ground state of the Fe-doped samples can only be described with a spin configuration described by a mixture of Γ3 (P63'cm') and Γ4 (P63'c'm) representations, whose contributions have been quantitatively estimated. The drastic effect of Fe-doping is highlighted by composition-dependent spin reorientations. A phase diagram of the entire composition series is proposed based on the present results and those reported in literature. Our result demonstrates the importance of tailoring compositions in increasing magnetic transition temperatures of multiferroic systems.

cond-mat.str-el↗

arXiv2016

Field-insensitive heavy fermion features and phase transition in the caged-structure quasi-skutterudite Sm$_3$Ru$_4$Ge$_{13}$

The robust field-insensitive heavy fermion features in Sm$_3$Ru$_4$Ge$_{13}$ and the magnetic phase transition at $T_N \approx$ 5~K are studied using magnetization $M(T)$, specific heat $C_p(T)$, resistivity $ρ(T)$ and thermal conductivity $κ_T(T)$. The average crystal structure of Sm$_3$Ru$_4$Ge$_{13}$ conforms to the cubic space group $Pm\bar{3}n$ however, signatures of subtle structural distortions are obtained from the x ray data. The magnetic susceptibility, $χ(T)$, follows a modified Curie-Weiss law indicating the presence of crystal fields of Sm$^{3+}$ and the significance of van Vleck terms. No sign of ferromagnetism is observed in $M(H)$ of Sm$_3$Ru$_4$Ge$_{13}$ which yields only 0.025~$μ_\mathrm{B}$/f.u.-Sm at 2~K, 7~T. The Sommerfeld coefficient, $γ\approx$ 220~mJ/mol-Sm K$^2$, estimated from the analysis of low temperature specific heat suggests the formation of heavy quasi particles at low temperature. Though a ln$T$ dependence of $ρ(T)$ is observed till 60~K, the resistivity behavior is accounted for by assuming a two-band model for activated behavior of charge carriers. The field scans of resistivity, $ρ(H)$, below $T_N$ display significant nonlinearity while those above the $T_N$ are more metal-like. Low values of thermal conductivity, $κ_T(T)$, are observed in Sm$_3$Ru$_4$Ge$_{13}$ however, displaying an anomaly at $T_N$ which signifies magnetoelastic coupling. A fairly high value of Seebeck coefficient, $S \approx$ 40~$μ$V/K is observed at 300~K. We identify Sm$_3$Ru$_4$Ge$_{13}$ as a low charge carrier density system with unusual field-insensitive heavy fermion features very similar to the filled skutterudites.

cond-mat.str-el↗

arXiv2016

Magnetic structures and magnetic phase transitions in the Mn-doped orthoferrite TbFeO$_3$ studied by neutron powder diffraction

The magnetic structures and the magnetic phase transitions in the Mn-doped orthoferrite TbFeO$_3$ studied using neutron powder diffraction are reported. Magnetic phase transitions are identified at $T^\mathrm{Fe/Mn}_N \approx$ 295~K where a paramagnetic-to-antiferromagnetic transition occurs in the Fe/Mn sublattice, $T^\mathrm{Fe/Mn}_{SR} \approx$ 26~K where a spin-reorientation transition occurs in the Fe/Mn sublattice and $T^\mathrm{R}_N \approx$ 2~K where Tb-ordering starts to manifest. At 295~K, the magnetic structure of the Fe/Mn sublattice in TbFe$_{0.5}$Mn$_{0.5}$O$_3$ belongs to the irreducible representation $Γ_4$ ($G_xA_yF_z$ or $Pb'n'm$). A mixed-domain structure of ($Γ_1 + Γ_4$) is found at 250~K which remains stable down to the spin re-orientation transition at $T^\mathrm{Fe/Mn}_{SR}\approx$ 26~K. Below 26~K and above 250~K, the majority phase ($> 80\%$) is that of $Γ_4$. Below 10~K the high-temperature phase $Γ_4$ remains stable till 2~K. At 2~K, Tb develops a magnetic moment value of 0.6(2)~$μ_\mathrm{B}/$f.u. and orders long-range in $F_z$ compatible with the $Γ_4$ representation. Our study confirms the magnetic phase transitions reported already in a single crystal of TbFe$_{0.5}$Mn$_{0.5}$O$_3$ and, in addition, reveals the presence of mixed magnetic domains. The ratio of these magnetic domains as a function of temperature is estimated from Rietveld refinement of neutron diffraction data. Indications of short-range magnetic correlations are present in the low-$Q$ region of the neutron diffraction patterns at $T < T^\mathrm{Fe/Mn}_{SR}$. These results should motivate further experimental work devoted to measure electric polarization and magnetocapacitance of TbFe$_{0.5}$Mn$_{0.5}$O$_3$.

cond-mat.str-el↗

arXiv2015

Magnetic Ordering and Crystal Field Effects in Quasi Caged Structure Compound PrFe$_2$Al$_8$

The compound PrFe$_2$Al$_8$ possesses a three-dimensional network structure resulting from the packing of Al polyhedra centered at the transition metal element Fe and the rare earth Pr. Along the $c$-axis, Fe and Pr form {\em chains} which are separated from each other by the Al-network. In this paper, the magnetism and crystalline electric field effects in PrFe$_2$Al$_8$ are investigated through the analysis of magnetization and specific heat data. A magnetic phase transition in the Pr lattice is identified at $T^{Pr}_{N}\approx$ 4~K in dc magnetization and ac susceptibility data. At 2~K, the magnetization isotherm presents a ferromagnetic saturation, however, failing to reach full spin-only ferromagnetic moment of Pr$^{3+}$. Metamagnetic step-like low-field features are present in the magnetization curve at 2~K which is shown to shift upon field-cooling the material. Arrott plots centered around $T^{Pr}_{N}$ display "S"-like features suggestive of an inhomogeneous magnetic state. The magnetic entropy, $S_m$, estimated from specific heat outputs a value of $R$ ln(2) at $T_{N2}$ suggesting a doublet state for Pr$^{3+}$. The magnetic specific heat is modeled by using a 9-level Schottky equation pertinent to the Pr$^{3+}$ ion with $J$ = 4. Given the crystalline electric field situation of Pr$^{3+}$, the inference of a doublet state from specific heat and consequent long-range magnetic order is an unexpected result.

cond-mat.str-el↗

arXiv2015

Spin-Lattice Coupling and Frustrated Magnetism in Fe-doped Hexagonal LuMnO3

Strong spin-lattice coupling and prominent frustration effects observed in the 50$\%$ Fe-doped frustrated hexagonal ($h$)LuMnO$_3$ are reported. A Néel transition at $T_{\mathrm N} \approx$ 112~K and a possible spin re-orientation transition at $T_{\mathrm {SR}} \approx$ 55~K are observed in the magnetization data. From neutron powder diffraction data, the nuclear structure at and below 300~K was refined in polar $P6_3cm$ space group. While the magnetic structure of LuMnO$_3$ belongs to the $Γ_4$ ($P6'_3c'm$) representation, that of LuFe$_{0.5}$Mn$_{0.5}$O$_3$ belongs to $Γ_1$ ($P6_3cm$) which is supported by the strong intensity for the $\mathbf{(100)}$ reflection and also judging by the presence of spin-lattice coupling. The refined atomic positions for Lu and Mn/Fe indicate significant atomic displacements at $T_{\mathrm N}$ and $T_{\mathrm {SR}}$ which confirms strong spin-lattice coupling. Our results complement the discovery of room temperature multiferroicity in thin films of $h$LuFeO$_3$ and would give impetus to study LuFe$_{1-x}$Mn$_x$O$_3$ systems as potential multiferroics where electric polarization is linked to giant atomic displacements.

cond-mat.str-el↗

arXiv2015

Spin-Reorientation and Weak Ferromagnetism in Antiferromagnetic TbMn_{0.5}Fe_{0.5}O_3

Orthorhombic single crystals of TbMn0.5Fe0.5O3 are found to exhibit spin-reorientation, magnetization reversal and weak ferromagnetism. Strong anisotropy effects are evident in the temperature dependent magnetization measurements along the three crystallographic axes a, b and c. A broad magnetic transition is visible at T_N (Fe/Mn) = 286 K due to paramagnetic to AxGyCz ordering. A sharp transition is observed at T_SR (Fe/Mn) = 28 K, which is pronounced along c axis in the form of a sharp jump in magnetization where the spins reorient to GxAyFz configuration. The negative magnetization observed below TSR Fe/Mn along c axis is explained in terms of domain wall pinning. A component of weak ferromagnetism is observed in field-scans along c-axis but below 28 K. Field-induced steps-like transitions are observed in hysteresis measurement along b axis below 28 K. It is noted that no sign of Tb-order is discernible down to 2 K. TbMn0.5Fe0.5O3 could be highlighted as a potential candidate to evaluate its magneto-dielectric effects across the magnetic transitions.

cond-mat.mtrl-sci↗

arXiv2015

Spin Freezing in the Spin Liquid Compound FeAl2O4

Spin freezing in the $A$-site spinel FeAl$_2$O$_4$ which is a spin liquid candidate is studied using remnant magnetization and nonlinear magnetic susceptibility and isofield cooling and heating protocols. The remnant magnetization behavior of FeAl$_2$O$_4$ differs significantly from that of a canonical spin glass which is also supported by analysis of the nonlinear magnetic susceptibility term $χ_3 (T)$. Through the power-law analysis of $χ_3 (T)$, a spin-freezing temperature, $T_g$ = 11.4$\pm$0.9~K and critical exponent, $γ$ = 1.48$\pm$0.59 are obtained. Cole-Cole analysis of magnetic susceptibility shows the presence of broad spin relaxation times in FeAl$_2$O$_4$, however, the irreversible dc susceptibility plot discourages an interpretation based on conventional spin glass features. The magnetization measured using the cooling-and-heating-in-unequal-fields protocol brings more insight to the magnetic nature of this frustrated magnet and reveals unconventional glassy behaviour. Combining our results, we arrive at the conclusion that the present sample of FeAl$_2$O$_4$ consists of a majority spin liquid phase with "glassy" regions embedded.

cond-mat.dis-nn↗