THE INFLUENCE OF COBALT ON CRYSTAL STRUCTURE AND MAGNETIC PROPERTIES OF nimnsbb2 HEUSLER ALLOYS

List of Figures Figure 1.1 – Schematic picture that shows the two basic processes of the magnetocaloric effect when a magnetic field is applied or removed in a magnetic system: the isoth

VNU UNIVERSITY OF SCIENCE

FACULTY OF PHYSICS

Vu Thi Huong

THE INFLUENCE OF COBALT ON CRYSTAL STRUCTURE AND MAGNETIC PROPERTIES OF

Submitted in partial fulfillment of the requirements for

the degree of Bachelor of Science in International Physics

( International Standard Program)

Supervisor: Prof.Dr Luu Tuan Tai

Hanoi - 2016

ACKNOWLEDMENT

First, I would like to say my sincere thanks to our teachers in the

Department of Low Temperature and Faculty of Physics, Hanoi university of

science, Vietnam National university, who have equipped me with valuable

knowledge in the past and help me a lots to have the best result

I would like to express my sincere thanks to Professor Luu Tuan Tai

who has enthusiastically instructed me to complete the thesis

After all, I would like to send many thanks to my friends, my colleges

and my closed people who did not hesitate to spend time on support, give

comments and help me during my study

Hanoi, May 25th,2016

Huong

Vu Thi Huong

List of Figures

Figure 1.1 – Schematic picture that shows the two basic processes of the magnetocaloric effect when a magnetic field is applied or removed in a magnetic system: the isothermal process, which lead to an entropy change, and the adiabatic process, which yields a variation in temperature 5

Figure 1.2 Theoretical molar magnetic entropy, SM, (left scale) and effective magnetic moment of free R3+ ions, peff, (right scale) of the lanthanide elements 8

Figure 1.3 The isothermal magnetization curves at different temperatures of a material with magnetic refrigeration 10

Figure 1.4 Analogy between magnetic refrigeration and vapor cycle or conventional refrigeration H = externally applied magnetic field; Q = heat quantity; P = pressure; ΔT ad = adiabatic temperature variation 11

Figure 1.5 Crystalline structure of Perovskite familiar ABO 3 14

Figure 1.6 Comparing the crystalline structure of Perovskite with another group 15

Figure 1.7 SEM image (a) and magnetic cooling curve(b) when magnetic field is applied (FC) and there isn’t magnetic applied field (ZFC) of

Figure 2.1 Acr meting system 28 Figure 2.2 Diagram describing operation principles X-ray diffraction method 30

Figure 2.3 X-ray measurement’s system at the Faculty of Physics, University of Sciences, VNU 31

Figure 3.1 X-ray diffraction of NiMnSbB 2 at 291K 33

Figure 3.2 X-ray diffraction of NiMnSbCo 1 B 2 at 77K 35 Figure 3.3 X-ray diffraction of NiMnSbCo 1 B 2 at 300K 34 Figure 3.4 The temperature dependence of magnetization under a magnetic field of 0.01 T (ZFC and FC) for the NiMnSbB 2 sample 36

Figure 3.5 The temperature dependence of magnetization under a magnetic field of 0.01 T (ZFC and FC) for the NiMnSbCo 1 B 2 sample 38

Figure 3.6 The temperature dependence of entropy’s variation under magnetic fields of 0.3 T, 0.5T and 0.8T of NiMnSbB2 39

Figure 3.7 The temperature dependence of entropy’s variation under magnetic fields of 0.3 T, 0.5T and 0.8T of NiMnSbCo 1 B 2 40

Contents

INTRODUCTION 1

CHAPTER I: OVERVIEW 4

1.1 Magnetocaloric effect 4

1.1.1 Concept of magnetocaloric effect 4

1.2.2 Magnetocaloric mechanism 4

1.1.3 Measurement of the magnetocaloric effect 8

1.1.4 Application of the magnetocaloric effect 10

1.2 Magnetocaloric materials 12

1.2.1 Some types of magnetocaloric materials 13

1.2.2 Applications of magnetocaloric materials 25

CHAPTER II: THE EXPERIMENTAL METHOD 28

2.1 Sample preparation using arc melting method 28

2.1.1 Weighting out samples 28

2.1.2 Melting samples 28

2.2 Measurements 29

2.2.1 Characteristic crystals measure 29

2.2 2 Magnetic characteristic and magnetic refrigeration measurement 32

CHAPTER III: RESULTS AND DISCUSSION 33

3.1 Crystalline structure of NiMnSbB2 and NiMnSbCo1B2 33

3.2 Magnetic properties of NiMnSbB2 and NiMnSbCoB2 alloys 35

CONCLUSION 40

INTRODUCTION

Although the magnetocaloric effect [4], which displays itself in the changing of a magnetic material temperature under magnetization in adiabatic conditions, was discovered a long time ago, in recent years it has attracted the attention of investigators and the amount of papers in this direction increases practically exponentially First of all this is related to practical application of the MCE and magnetic materials in refrigeration devices and, especially, in magnetic refrigerators, which work on magnetic refrigeration cycles instead

of conventional vapour-gas cycles Recently a series of acting magnetic refrigerator prototypes have been developed and created The particular importance is that the created prototypes work at room temperature and have significant potential to be incorporated into the marketplace Magnetic refrigerators are characterized by compactness, high effectiveness, low energy consumption and environmental safety The further development of such devices is related to progress in permanent magnets, which can replace such cumbersome sources of magnetic fields requiring liquid helium superconducting magnets It is expected that in the near future the energy product of permanent magnets will at least double We suggest that the first commercially available models of magnetic refrigerators will appear rather sooner in spite of essential competition which they will have to withstand from existing vapour–gas refrigeration technology [4] One of the most important parts of the magnetic refrigerator is its working body- a magnetic material- which should have high magnetocaloric properties, in particular, high MCE value The working body in ix many respects determines the characteristics of the whole refrigeration device At the present time the energies of many scientific groups working in the field of MCE are directed

on searching for the most effective magnetic working body This is the field where the scientific interests of investigators intersect with practical applications

In recent years, scientists have discovered giant magnetic refrigeration[7] (giant MCE) in the ambient temperature above room temperature material systems Gd5Si2Ge2, this material system opens up

possibilities for applications in set cooled at room temperature with environmental friendly technologies Magnetic refrigeration is near the critical state of magnetic transition Transition from second grade showed clear peaks of MCE and small MCE effects Transition from a point out of the top categories MCE sharp and big MCE effects The scientists also discovered the coexistence of Gd5Si1.7Ge2.3’s structure, phase and temperature of transition (239 K) Transfer in this phase has the phase transition temperature

of a kind with a delay of about 7.4 K, in about 1T The magnetic material is magnetic refrigeration is quite common, these are pretty good material for applications but the components Gd has a very high cost MnFeP1-xAsx system (0.25 <x <0.65) were found with a kind of phase transition and thermal lag of about 3.4 K, in the range of 0.5 T magnetic field and the material system is also quite popular This material system is suitable for many applications in the refrigeration equipment from such a large MCE effect, small thermal lag, the temperature is about 168 K and the cost of the components Mn, Fe , P are lower, however Arsenic is toxic elements and need to resolved

Currently, most of the research on the application of chillers from the focus on materials with magnetic refrigeration at room temperature, the material has enormous magnetic refrigeration (giant MCE) together with structural phase transitions (magneto-structural first-order) Some materials: Gd5 (SixGe1-x) 4 [9], MnFeP1-xAsx, MnAs1-xSbx, were studied showed giant magnetic refrigeration with structural phase transitions (FOMST) Moreover Ben , the Heusler alloy Ni - Mn - Sn [5] and of other alloys Ni - Mn - X (X =

Ga, in, Sb) materials are more attracted to the study of magnetic materials thermal technology and has large applications in refrigeration, by the special nature of this alloy offers such as shape-memory effect, magnetic refrigeration, from resistance and many other properties related to the martensitic phase transition (TM) These alloys are also the representative for the application in the magnetic refrigeration equipment because their low-cost materials and non-toxic property

With the purpose of research on the material from the high heat applications, with magnetic refrigeration in the room temperature and on the

basis of the preliminary findings of system Ni - Mn - Sb more elemental Boron and our group Cobalt In this thesis we refer to manufacturing technologies and do experiments of crystal structure, magnetic and magnetic refrigeration NiMnSbCo1B2 alloy and NiMnSbB2

The content of this thesis includes the following sections:

Introduction

Chapter 1: Overview

Chapter 2: The experimental method

Chapter 3: Results and Discussion

Conclusion

CHAPTER I: OVERVIEW

1.1 Magnetocaloric effect

1.1.1 Concept of magnetocaloric effect

The magnetocaloric effect (MCE) [2] is a magneto thermodynamic phenomenon in which a temperature change of a suitable material is caused

by exposing the material to a changing magnetic field This is also known by low temperature physicists as adiabatic demagnetization In that part of the refrigeration process, a decrease in the strength of an externally applied magnetic field allows the magnetic domains of a magnetocaloric material to become disoriented from the magnetic field by the agitating action of the thermal energy (phonons) present in the material If the material is isolated so that no energy is allowed to (re)migrate into the material during this time, (i.e., an adiabatic process) the temperature drops as the domains [1] absorb the thermal energy to perform their reorientation The randomization of the domains [2] occurs in a similar fashion to the randomization at the curie temperature of a ferromagnetic material, except that magnetic dipoles overcome a decreasing external magnetic field while energy remains constant, instead of magnetic domains being disrupted from internal ferromagnetism as energy is added

One of the most notable examples of the magnetocaloric effect is in the chemical element gadolinium and some of its alloys Gadolinium's temperature increases when it enters certain magnetic fields When it leaves the magnetic field, the temperature drops The effect is considerably stronger for the gadolinium alloy (Gd5Si2Ge2) Praseodymium alloyed with nickel (PrNi5) has such a strong magnetocaloric effect that it has allowed scientists

to approach to within one milliKelvin [4], one thousandth of a degree

of absolute zero And in this thesis, we concentrate into Heusler alloy

1.2.2 Magnetocaloric mechanism

In order to explain the origin of the magnetocaloric effect, we use thermo-dynamics, which relates the magnetic variables (magnetization and magnetic field) to entropy and temperature All magnetic materials

intrinsically show MCE, although the intensity of the effect depends on the properties of each material The physical origin of the MCE is the coupling of

the magnetic sub lattice to the applied magnetic field, H, which changes the

magnetic contribution to the entropy of the solid

When an object is in magnetic field, its magnetic moment tends to reorder following the magnetic direction This direct reduces entropy of magnetic moment system The isothermal process leads to an entropy change,

and in the adiabatic process, the sum of entropy is remain [1]

Figure 1.1 – Schematic picture that shows the two basic processes of the magnetocaloric effect when a magnetic field is applied or removed in a magnetic system: the isothermal process, which lead to an entropy change, and the adiabatic process, which yields a variation in temperature

When magnetic material is subject to a magnetic field changing by

∆H = Hf –Hi at constant pressure (the subscripts F and I indicate the final and the initial magnetic field strength, respectively), two different processes may occur in a magnetic material The first is the isothermal process that occurs when the magnetic field is altered but the material remains connected to the surroundings (heat sink/heat reservoir) and, therefore, remains at constant temperature The entropy of a magnetic solid is then changed by

∆SM (T )∆H is conventionally called magnetic entropy change The

magnetic entropy change of a solid directly characterizes the cooling capacity,

q, of the magnetic material

which indicates how much heat can be transferred from the cold end (at

T1) to the hot end (at T2) of the refrigerator in one ideal thermodynamic

cycle The second is an adiabatic process that occurs when the magnetic field

is modified but the material is isolated from the surroundings and, therefore, the total entropy of a solid remains constant The temperature of a magnetic material is then changed by

and ∆Tad(T ) ∆H is conventionally called adiabatic temperature change

The adiabatic temperature change indirectly characterizes both the cooling capacity and the temperature difference between the cold and the hot ends of

the refrigerator (generally a larger ∆T ad corresponds to a larger cooling

capacity of the material and to a larger temperature span of the refrigerator) It should be noted that the difference between the hot end and the cold end temperatures of a magnetic refrigerator greatly exceeds that of the maximum magnetocaloric effect in a properly designed active magnetic regenerator cycle If both the magnetization and entropy are continuous functions of the temperature and magnetic field, then the infinitesimal isobaric-isothermal

magnetic entropy change can be related to the magnetization (M), the magnetic field strength (H ), and the absolute temperature (T ) using one of

the Maxwell[7] relations

It is easy to show that the infinitesimal adiabatic (TdS = 0) temperature

rise for the reversible adiabatic-isobaric process is

Both ∆S M (T ) ∆H and ∆Tad (T )∆H depend on temperature and ∆H (Equations

1.5 and 1.9, respectively) and are usually studied and reported as functions of

temperature for a given ∆H, or as functions of ∆H for a given temperature The behavior of both characteristics of the magnetocaloric effect, i.e ∆S M (T

)∆H and ∆Tad (T )∆H, is material dependent, cannot be easily predicted from the first principles, and therefore, must be experimentally measured The heavy lanthanide metals and their compounds were always considered the best potential magnetocaloric materials because they have the largest magnetic

moments (and, therefore, the most favorable bulk magnetizattion and

potentially a large |∂M/∂T|; see Equations 1.4 and 1.9) and the largest

available magnetic entropy (Figure 1.2) In this review we are concerned with

Figure 1.2 Theoretical molar magnetic entropy, SM, (left scale) and effective magnetic moment of free R3+ ions, peff, (right scale) of the

lanthanide elements

only the magnetocaloric properties of materials in the temperature range away from absolute zero, which can be used for continuous cooling, e.g for air conditioning, household and commercial refrigeration/freezing, and gas liquefaction

1.1.3 Measurement of the magnetocaloric effect

The magnetocaloric effect can be measured (direct techniques) or calculated (indirect techniques) from the measured magnetization or heat capacity, both as a function of temperature and magnetic field The direct and indirect techniques have certain advantages and disadvantages

The direct techniques provide only one measure of the magnetocaloric effect, the adiabatic temperature change Since the temperatures are measured directly, no further processing of data except subtraction of the two numbers

is involved However, direct measurements are usually time-consuming and are difficult to perform at small temperature step intervals A comprehensive analysis of the errors is hardly possible, and the error estimate is usually based

on comparison of the measured data using any standard material If the direct measurement apparatus is not properly calibrated, or if the material is not properly isolated, large experimental errors become inevitable Furthermore, the magnetic field by definition must be changed as quickly as possible This may cause problems if the studied materials are poor conductors (this is almost always the case because magnetic materials near their ordering temperature have low thermal conductivities), or when phase transitions involved exhibit non-infinite kinetics

Unlike the direct MCE measurements, which yield only the adiabatic temperature change, the indirect experiments allow the calculation of both

∆T ad (T) ∆H and ∆S M (T ) ∆H from experimental heat capacity data, or ∆SM

(T) ∆H alone from experimental magnetization data Indirect techniques

provide results at practically any temperature interval However, considerable processing of experimental data is involved before the MCE is calculated Also on the positive side, indirect techniques allow comprehensive error analysis as long as the accuracy of experimental data This is the most common measurement by identifying the variation of entropy from ∆Sm to determine adiabatic temperature variation Manner of expression measurements based on the following equation:

magnetization curves at different temperatures, to determine the area of the curve and make sure the entropy from the effects of continuous area divided

by temperature variations (Figure 1.3)

Figure 1.3 The isothermal magnetization curves at different temperatures of a material with magnetic refrigeration

Figure 1.3 gives a system of isothermal magnetization curves of a material with large magnetic refrigeration is measured at Hanoi University of Sciences

1.1.4 Application of the magnetocaloric effect

The magnetic materials can be used in this field by two ways As known, magnetic materials make additional contributions to the heat capacity related to the magnetic subsystem [10], which is especially large near the magnetic phase transition points That is why the magnetic materials with the low ordering temperatures can provide high heat capacity in the low-temperature region where lattice and electronic heat capacities approach zero This circumstance allows the use of such materials in passive magnetic regenerators-the devices serving to expand a conventional refrigerator temperature span The described application of magnetic materials can be called ‘passive’ because here they only adsorb and desorb heat at different

stages of the refrigerator cycle, do not produce heat themselves and operate without application of a magnetic field In this way magnetic materials are used now in cryocoolers, essentially increasing their effectiveness But more interesting and perspective application is related to the magnetocaloric effect inherent in magnetic materials We mean the use of magnetic materials in magnetic refrigeration devices as working bodies, as a ‘heart’ of these devices Such an application can be called ‘active’ (here a magnetic material produces heat under magnetization) It is worth noting, however, that in some cases both ‘passive’ and ‘active’ properties of a magnetic material are used simultaneously- for example, in so-called magnetically augmented regenerators [5]

Figure 1.4 Analogy between magnetic refrigeration and vapor cycle or conventional refrigeration H = externally applied magnetic field; Q = heat quantity; P = pressure; ΔT ad = adiabatic temperature variation[10]

be further increased when the order-parameter of the phase transition changes strongly within the temperature range of interest The magnitudes of the magnetic entropy and the adiabatic temperature changes are strongly dependent upon the magnetic ordering process [12] The magnitude is generally small inantiferro-magnets, ferrimagnets and spin glass systems but can be much larger for ferromagnets that undergo a magnetic phase transition First order phase transitions are characterized by a discontinuity in the magnetization changes with temperature, resulting in a latent heat Second order phase transitions do not have this latent heat associated with the phase transition

In the late 1990s Pecharksy and Gschneidner reported a magnetic entropy change in Gd5(Si2Ge2) that was about 50% larger than that reported for Gd metal, which had the largest known magnetic entropy change at the time This giant magnetocaloric effect (GMCE) occurred at 270 K, which is lower than that of Gd (294 K) The MCE occurs below room temperature these materials would not be suitable for refrigerators operating at room temperature Since then other alloys have also demonstrated the giant magnetocaloric effect These include

Gd5(SixGe1−x)4, La(FexSi1−x)13Hx and MnFeP1−xAsx alloys Gadolinium and its alloys undergo second-order phase transitions that have no magnetic or thermal hysteresis However, the use of rare earth elements makes these materials very expensive Ni2Mn-X (X = Ga, Co, In, Al, Sb) Heusler alloys are also promising candidates for magnetic cooling applications because they

have Curie temperatures near room temperature and, depending on composition, can have martensitic phase transformations near room temperature These materials exhibit the magnetic shape memory effect and can also be used as actuators, energy harvesting devices, and sensors When the martensitic transformation temperature and the Curie temperature are the same (based on composition) the magnitude of the magnetic entropy change

is the largest In February 2014, GE announced the development of a functional Ni-Mn-based magnetic refrigerator The development of this technology is very material-dependent and will likely not replace vapor-compression refrigeration without significantly improved materials that are cheap, abundant, and exhibit much larger magnetocaloric effects over a larger range of temperatures Such materials need to show significant temperature changes under a field of 2 tesla or less, so that permanent magnets can be used for the production of the magnetic field [10]

1.2.1 Some types of magnetocaloric materials

1.2.1.1 Magnetocaloric materials combined from metal oxides Perovskite ceramic

Perovskite is a calcium titanium oxide mineral composed of calcium

titanate, with the chemical formula CaTiO3 The mineral was discovered in the Ural Mountains of Russia by Gustav Rose in 1839 and is named after Russian mineralogist Lev Perovskite (1792–1856)

It lends its name to the class of compounds which have the same type

of crystal structure as CaTiO3 (XIIA2+VIB4+X2−3) known as the perovskite structure The perovskite crystal structure was first described by Victor Goldschmidt in 1926, in his work on tolerance factors The crystal structure was later published in 1945 from X-ray diffraction data on barium titanate by Helen Dick Megaw [3]

a Perovskite’s structure

The perovskite structure is adopted by many oxides that have the chemical formula ABO3

In the idealized cubic unit cell of such a compound, type 'A' atom sits at cube corner positions (0, 0, 0), type 'B' atom sits at body center position (1/2, 1/2, 1/2) and oxygen atoms sit at face centered positions (1/2, 1/2, 0) (The diagram shows edges for an equivalent unit cell with A in body center, B at the corners, and O in mid-edge)

The relative ion size requirements for stability of the cubic structure are quite stringent, so slight buckling and distortion can produce several lower-symmetry distorted versions, in which the coordination numbers of A cations,

B cations or both are reduced Tilting of the BO6 octahedra reduces the coordination of an undersized A cation from 12 to as low as 8 Conversely, off-centering of an undersized B cation within its octahedron allows it to attain a stable bonding pattern The resulting electric dipole is responsible for the property of ferroelectricity and shown by perovskites such as BaTiO3 that distort in this fashion The orthorhombic and tetragonal phases are most common non-cubic variants Complex perovskite structures contain two different B-site cations This results in the possibility of ordered and disordered variants

Figure 1.5 Crystalline structure of Perovskite familiar ABO 3

Figure 1.6 Comparing the crystalline structure of Perovskite with

another group

b Material’s properties

Perovskites have a cubic structure with general formula of ABO3 In this structure, an A-site ion, on the corners of the lattice, is usually an alkaline earth or rare earth element B site ions, on the center of the lattice, could be 3d, 4d, and 5dtransition metal elements A large number of metallic elements

are stable in the perovskite structure, if the tolerance factor t is in the range of

Perovskite materials exhibit many interesting and intriguing properties from both the theoretical and the application point of view Colossal magnetoresistance, ferroelectricity, superconductivity, charge ordering, spin dependent transport, high thermo power and the interplay of structural, magnetic and transport properties are commonly observed features in this family These compounds are used as sensors and catalyst electrodes in certain types of fuel cells [5] and are candidates for memory devices and spintronics applications Many superconducting ceramic materials (the high temperature superconductors) have perovskite-like structures, often with

3 or more metals including copper, and some oxygen positions left vacant One prime example is yttrium barium copper oxide which can be insulating or superconducting depending on the oxygen content Chemical engineers are considering a cobalt-based perovskite material as a replacement for platinum

in catalytic converters in diesel vehicles

Figure1.7a

c Applications

Physical properties of interest to materials science among perovskites include superconductivity, magnetoresistance, ionic conductivity, and a multitude of dielectric properties, which are of great importance in microelectronics and telecommunication Because of the flexibility of bond angles inherent in the perovskite structure there are many different types of distortions which can occur from the ideal structure These include tilting of the octahedra, displacements of the cations out of the centers of their

coordination polyhedra, and distortions of the octahedra driven by electronic factors (Jahn-Teller distortions)

1.2.1.2 Magnetocaloric materials based on intermetallics alloys

Gadolinium is one of the most broadly studied magnetic refrigerant materials It is ferromagnetic below 292 K to 295 K depending on its purity, generally the cleaner metal orders at a higher temperature, entropy variation is

S mag= 4 (J/kg.K) in magnetic field 2 T The maximum entropy variation occurs at Tc, the value, bigger or equal 2 (J/kg.K), is called “giant” They have small heat capacities (property of metal)

The intermetallic FeRh is the first known intermetallic material reported to display the giant magnetocaloric effect near room temperature Direct measurements of the MCE of a heat-treated and quenched Fe51Rh49 alloy indicated that it could be as large as −13K at 307 K in a magnetic field

of 1.95T, but later it was reported to be −8.2K for ∆H= 2.5T The MCE

behavior in this material is unusual because of its sign (it is negative for a

positive ∆H), its extreme sensitivity to processing history, and its

irreversibility The MCE in FeRh displays an inverse skyscraper behavior

with the large relative cooling power, RCP(T ), of −166 K between 296 and

316 K The compound Cr3Te4 displays a conventional MCE (measured directly), which reaches 1.1 K at 316 K in 2T magnetic field The relative

cooling capacity, RCP(T) = 41 K is quite low The limited amount of

available experimental data indicates that, in general, the MCE in transition metal–based alloys is lower than that in lanthanide-based alloys for the same temperature range This is consistent with the lower available theoretical magnetic entropy in the magnetic transition metal atoms This makes them unlikely candidates for use as magnetic refrigerant materials below ∼300 K, but they may be useful materials for magnetic refrigerators/heat pumps

rejecting heat well above room temperature However, if new 3d-based alloys

are found, with a giant MCE similar in magnitude to that observed in FeRh, but reversible, they could be an excellent source of highly effective magnetic refrigerant materials [12]

Holtzberg have shown that when replacing Si by Ge in the compound Gd5(Si2Ge)4, the phase transition temperature decreased from 335K (at Gd5Ge4) to 295K (at Gd5(Si2Ge2)) This changing leads to change the structure of system, so the properties also change

By adding Fe, Co, Ni, Al, Ga or C into compound Gd5(Ge2Si2), we can change Tc and S mag of compound Gd5Si2Ge2

Figure 1.8 Variation of entropy following the changing of temperature

of Ga 5 Si 2 Ge 2[8]

b Gd 3 Al 2 and Er 3 AlC alloy systems

Gd3Al2 material have high Curie temperature at 281 K, but it’s still smaller 39K than room temperature This temperature is easy to get at